advanced materials integration and thermal operation …
TRANSCRIPT
ADVANCED MATERIALS INTEGRATION AND THERMAL OPERATION OF
HEATED MICROCANTILEVERS AND MICROCANTILEVER ARRAYS
BY
HOE JOON KIM
DISSERTATION
Submitted in partial fulfillment of the requirements
for the degree of Doctor of Philosophy in Mechanical Engineering
in the Graduate College of the
University of Illinois at Urbana-Champaign, 2015
Urbana, Illinois
Doctoral Committee:
Professor William King, Chair and Director of Research
Professor Joseph Lyding
Assistant Professor Sanjiv Sinha
Assistant Professor SungWoo Nam
ii
ABSTRACT
This dissertation presents the development and characterization of novel heated
microcantilevers and microcantilever arrays for atomic force microscope (AFM) applications.
AFM microcantilevers with integrated solid-state resistive heaters enable heat flow
measurements, manufacturing, and material characterization at the nanometer scale. However,
current heated microcantilevers need to be improved in several areas, such as scan speed and
scan area, tip stability, and thermal operation reliability for industrial applications. This
dissertation focuses on tackling these issues by integrating advanced materials with heated
microcantilevers and microcantilever arrays, along with detailed analysis of their thermal
operation under various conditions. The microcantilevers and the microcantilever arrays
demonstrate outstanding throughput and stability for applications in nanomanufacturing and
nanometrology.
Firstly, this work presents the design and fabrication of heated microcantilever arrays to
improve throughput of heated cantilever AFM techniques. The microcantilever array consists of
five identical independently-controlled heated cantilevers. The thermal crosstalk between the
cantilevers is analyzed in steady and transient operating conditions when the array is either in
contact with a substrate or freely suspended in air. Results show that the thermal crosstalk
between neighboring cantilevers induces non-negligible increases in cantilever temperature,
depending upon the operating conditions.
Secondly, this work investigates the long term operation and reliability of heated
microcantilevers. The electrical and thermal operation of heated microcantilevers is sensitive to
the distribution of dopants within the silicon cantilever. For long term operation, or for operation
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at very high temperatures, the cantilever electro-thermal properties can change due to dopant
diffusion from the high-doped region towards the low-doped heater. Such changes in cantilever
properties are closely monitored and analyzed to understand the reliability of heated
microcantilevers under harsh operating conditions.
Thirdly, this work presents the integration of ultrananocrystalline diamond (UNCD) and
multilayer graphene onto microcantilevers using conventional microfabrication processes. First,
an ultrasharp and wear-resistant UNCD tip is integrated onto the heated AFM microcantilevers
and microcantilever arrays. The UNCD tips are batch-fabricated and have apex radii of about 10
nm and heights up to 7 µm. The UNCD tips, used for thermal topography imaging and tip-based
nanomanufacturing, demonstrate much improved tip stability compared to silicon tips. In
addition, this work reports the direct integration of chemical vapor deposition (CVD) grown
graphene with microcantilevers. A method of transfer-free graphene synthesis is developed,
optimized, and applied to the batch fabrication of graphene-coated microcantilevers.
Finally, a heated microcantilever is used to measure the temperature-dependent viscosity
of water and ethylene glycol solutions. An applied AC voltage in the presence of an external
magnetic field simultaneously heats and actuates the microcantilever. By monitoring the changes
of resonant frequencies of the heated microcantilever inside liquid at different heater
temperatures, the temperature-dependent liquid viscosities can be measured. The measurements
match well with the finite difference model that predicts the dynamic response of the cantilever
in the frequency domain.
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Acknowledgement
First of all, I would like to sincerely thank my advisor, Professor William King, for his
support and guidance for my PhD study. Professor King was not only a great research advisor,
but also was a good mentor who gave many invaluable lessons that shaped me into a more
complete student and a researcher. I will surely miss working with Professor King and the
research environment that he provided. I would also like to thank my doctoral committee
members, Professor Joseph Lyding, Professor Sanjiv Sinha, and Professor Nam, for serving on
my committee and for their advices on my research.
Second, I would like to thank my collaborators and friends at University of Illinois at
Urbana-Champaign. I am especially grateful for my current and former lab mates in the King
research group, James, Suhas, Sezer, Matt, Patty, Huan, Hyunkyu, Sihan, Katarina, Patrick,
Johnny, Bikram, Beomjin, Nick, Kyle, Byeonghee, Hanna, and Ting. I would also like to thank
staff members of Micro-Nano-Mechanical Systems and the Micro & Nanotechnology Laboratory
cleanrooms, where I spent many hours for the device fabrication.
Finally, I would like to thank my family for their endless support, sacrifice, and love.
Without them, none of this would have been possible. In particular, I would like to thank my
father, who has been my lifetime advisor, for instilling the importance of education and for
guiding me to achieve my dreams.
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TABLE OF CONTENTS
List of Symbols ............................................................................................................................ vii
Chapter 1: Introduction ............................................................................................................... 1
1.1 Atomic Force Microscopy.............................................................................................. 1
1.2 Heated Microcantilevers ................................................................................................ 4
1.3 Applications of Heated Microcantilevers ...................................................................... 7
1.3.1 Data Storage ............................................................................................................ 7
1.3.2 Thermal Sensing ..................................................................................................... 7
1.3.3 Nanofabrication....................................................................................................... 9
1.3.4 Thermal Operation Reliability .............................................................................. 11
1.4 Dissertation Overview .................................................................................................. 12
1.5 References .................................................................................................................... 13
Chapter 2: Thermal Crosstalk in Heated Microcantilever Arrays ........................................ 19
2.1 Introduction .................................................................................................................. 19
2.2 Microcantilever Array Fabrication and Characterization ............................................ 21
2.3 Experiments and Results .............................................................................................. 26
2.4 Discussion .................................................................................................................... 37
2.5 Conclusion ................................................................................................................... 38
2.6 References .................................................................................................................... 39
Chapter 3: A Study of Long Term Operation and Reliability of
Heated Microcantilevers ............................................................................................................ 42
3.1 Introduction .................................................................................................................. 42
3.2 Experiment ................................................................................................................... 44
3.3 Numerical Modeling .................................................................................................... 47
3.4 Results .......................................................................................................................... 49
3.5 Discussion .................................................................................................................... 57
3.6 Conclusion ................................................................................................................... 58
3.7 References .................................................................................................................... 59
Chapter 4: Diamond Tip Heated Microcantilevers and Microcantilever Arrays ............... 61
4.1 Introduction .................................................................................................................. 61
4.2 Cantilever Design and Fabrication ............................................................................... 62
4.3 Cantilever Characterization .......................................................................................... 67
4.4 Thermal Topography Imaging and Thermal Nanolithography .................................... 73
4.5 Conclusion ................................................................................................................... 78
4.6 References .................................................................................................................... 79
vi
Chapter 5: Batch Fabrication of Graphene-Coated Microcantilevers .................................. 82
5.1 Introduction .................................................................................................................. 82
5.2 Transfer-free Graphene Synthesis ................................................................................ 83
5.3 Cantilever Fabrication .................................................................................................. 87
5.4 Conclusion ................................................................................................................... 90
5.5 References .................................................................................................................... 91
Chapter 6: Measurement of Temperature-Dependent Liquid Viscosity Using Heated
Microcantilevers .......................................................................................................................... 93
6.1 Introduction .................................................................................................................. 93
6.2 Experiment ................................................................................................................... 95
6.3 Dynamic Response Modeling ...................................................................................... 99
6.4 Results and Discussion ............................................................................................... 103
6.5 Conclusion ................................................................................................................. 111
6.6 References .................................................................................................................. 112
Chapter 7: Conclusions and Future Work ............................................................................. 115
7.1 Future Work ............................................................................................................... 117
7.1.1 Thermal Crosstalk in Heated Microcantilever Arrays ........................................ 117
7.1.2 A Study of Long Term Operation and Reliability of Heated Microcantilevers . 118
7.1.3 Diamond Tip Heated Microcantilevers ............................................................... 118
7.1.4 Graphene-Coated Microcantilevers .................................................................... 119
7.1.5 Direct Measurement of Temperature-Dependent Liquid Viscosities using Heated
Microcantilevers ................................................................................................. 120
7.2 References .................................................................................................................. 121
Appendix A: Heated Microcantilever Array Fabrication Process ....................................... 123
Appendix B: Diamond Tip Heated Microcantilever Fabrication Process ........................... 133
Appendix C: Graphene-Coated Microcantilever Fabrication Process ................................ 138
vii
LIST OF SYMBOLS
AC Alternating Current
a-C Amorphous Carbon
AFM Atomic Force Microscope
Ar Argon
C Doping Concentration
Cu Copper
CVD Chemical Vapor Deposition
D Diffusion Coefficient
DC Direct Current
DI Deionized
DRIE Deep Reactive Ion Etching
E Young's Modulus
E0 Activation Energy
EG Ethylene Glycol
FDM Finite Difference Model
FEM Finite Element Model
Fh Hydrodynamic Force
Fm Magnetic Driving Force
Ftip Tip-Substrate Interaction Force
G Tip-to-Tip Thermal Conductance
viii
I Bending Moment of Inertia
ICP Inductively Coupled Plasma
k Thermal Conductivity
L Length
LTA Local Thermal Analysis
MEMS Microelectromechanical Systems
NEMS Nanoelectromechanical Systems
Ni Nickel
PMMA poly(methyl methacrylate)
RC,RT Cantilever Resistance at Room Temperature
Rsense Sense Resistor
SEM Scanning Electron Microscope
Si Silicon
SiO2 Silicon Dioxide
SOI Silicon-on-Insulator
SPM Scanning Probe Microscopy
SThM Scanning Thermal Microscopy
STM Scanning Tunneling Microscopy
T Temperature
t thickness, time
tCNL Thermal Chemical Nanolithography
tDPN Thermal Dip Pen Nanolithography
ix
TH Steady Cantilever Heater Temperature
UNCD Ultrananocrystalline Diamond
W Width
x Displacement
γ Thermal Crosstalk Ratio
δ Viscous Penetration Depth
ΔTheated Temperature Rise in Heated Cantilever
ΔTunheated Temperature Rise in Unheated Cantilever
η Viscosity
ρ Density
ω Cantilever Oscillation Frequency
ωr Cantilever Resonant Frequency
1
CHAPTER 1: INTRODUCTION
Microcantilever is one of the most widely used microelectromechanical systems (MEMS),
especially for sensing applications. Scanning Probe Microscopy (SPM) uses microcantilevers
with a sharp tip to interact with a substrate at the nanometer-scale. Among numerous SPM
modes, Atomic Force Microscopy (AFM) [1] is widely used for probing mechanical [2-4],
chemical [5, 6], electrical [7-10], and thermal [11-13] phenomena at the nanometer scale.
Temperature is an intrinsic material property and AFM microcantilever having an integrated
heater [11] allows for the control and the measurement of temperature and heat flows. Such
heated microcantilevers are used for high density data storage [12, 14], nano-manufacturing [15-
17], material property measurements [13, 18, 19], and nanometer-scale imaging [20]. Further
development and understanding of heated microcantilevers and microcantilever arrays could
improve the throughput, tip stability, and thermal operation reliability. An integration of
advanced materials and a study of thermal operation of the microcantilever could advance the
current state of heated microcantilevers and their applications.
1. 1 Atomic Force Microscopy
SPM uses a microcantilever with a sharp tip in close proximity or in contact with a
surface to study nanometer-scale structures and properties. Piezoelectric actuators control the
relative positions of the tip and surface with picometer resolution using an appropriate feedback
mechanism. The interaction between the tip and the surface is recorded as the tip raster scans the
surface, generating two or three dimensional images of the surface. Scanning probe tips could be
used to manipulate or fabricate a wide range of materials [9, 21, 22]. SPMs could also be used
2
for nanometer scale measurements of mechanical [2-4], chemical [5, 6], electrical [7-10], and
thermal [11-13] properties of the surface.
The first type of SPM is the scanning tunneling microscope (STM) [23], which measures
surfaces with atomic resolution using the principle of quantum tunneling of electrons. A control
unit monitors the tunneling current between the surface and the conducting tip, which is placed
few angstroms above while scanning the surface. The STM operates in constant height or
constant current mode, depending on the application requirements. In constant height mode, the
distance between the STM tip and the surface is maintained and the change in tip current is
measured. In constant current mode, the tunneling current between tip and the surface is kept
constant and the tip position is monitored to generate topographic image. Although the SPM
provides excellent spatial resolution, it requires a conductive surface and a vacuum environment.
One of the most widely used modes of SPM is the atomic force microscope (AFM) [1],
which measures short-range forces. Figure 1.1 shows a diagram of an AFM. An AFM uses a
microcantilever with a sharp tip at the cantilever free end. A microcantilever bends as the tip
reacts to force between the tip and the surface. The deflection of the cantilever beam is then
optically measured using a laser which reflects off the end of the cantilever into a photodiode.
The two most popular imaging modes of AFM are contact mode and intermittent contact mode.
In contact mode, the tip makes contact with the substrate while the control electronics move the
cantilever using a piezoelectric actuator to keep the cantilever at a constant position. In
intermittent contact mode, also called tapping mode, a piezoelectric element oscillates the
cantilever near the resonant frequency of the cantilever [24]. The interactions between the tip
and surface modulate the cantilever amplitude, which is in the order 10 - 100 nanometers. The
piezoelectric actuator controls the height of the cantilever to maintain constant oscillation
amplitude.
3
Figure 1.1 Schematic of an atomic force microscope [25]
Most AFM microcantilevers have a low spring constant (0.01 – 10 N/m) to improve data
acquisition speeds and to minimize the damage to the tip or the surface. Typical AFM
microcantilevers are fabricated on the micrometer scale using compliant materials such as silicon
or silicon nitride using cleanroom microfabrication processes [26]. Microfabrication allows
wafer scale batch fabrication of AFM microcantilevers at reasonable price.
One of few drawbacks of AFM is the low throughput, which results from the slow scan
rates and the small measurement areas. One way to improve the throughput of AFM is using an
array of microcantilevers for parallel AFM operations [27-31]. Widely used optical-lever system
is not suitable for multi-cantilever operation, as the optical setup cannot be scaled to arrays of
microcantilevers [32]. In contrast, the microcantilevers with the embedded topography sensor,
such as piezoresistive [33], piezoelectric[34], and heater-thermometer [11, 14] sensors, suite
multi-cantilever operations. Especially, heater-thermometer integrated microcantilevers provides
superior topographic resolution compared to microcantilevers with piezoresistive sensors [35].
4
Another important requirement for AFM microcantilevers is the tip stability, as the tip
radius directly determines the resolution of AFM imaging. However, maintaining a constant tip
shape can be problematic when the tip, typically when the tip is made out of silicon or silicon
nitride, is scanned over a hard surface. The tip wear or fouling limits the use of AFM
microcantilevers for under harsh operating conditions. The tip stability improved by a tip coating
or the use of novel tip materials [36-38]. Diamond is an attractive material to be used as a tip,
since it has high hardness, chemical stability, low friction coefficient, low work function, and
low adhesion [39]. Batch fabrication of diamond tips using RIE is a simple process [40, 41] that
can provide good control over tip dimensions, such as tip radius and height, and thus generating
sharper and taller diamond tips than other fabrication methods such as molding [42] and tip-
coating [37].
1. 2 Heated Microcantilevers
AFM microcantilevers with an integrated heating element can generate and sense the
nanometer scale heat. Such heated microcantilevers enabled a variety of applications in
nanometrology and nanomanufacturing. The first thermal probes used metal heating elements,
such as a loop of Wollaston wire [43, 44] and a metal thermocouple [45-47] tips. These metal
tips can apply heat to surface and measure the material response at the nanometer scale [48].
Silicon cantilevers with solid state heater-thermometers [11, 49, 50] were originally developed
for data storage, in which a heated tip at about 400 °C softens the polymer substrate to write or
erase data bits [14, 49]. The use of heated cantilevers has expanded into many areas including
nano-manufacturing [13, 16, 51-53], material property measurements [54-56], and nanometer-
scale imaging [11, 56]. Compared to metal cantilevers, silicon cantilevers allow higher operating
5
temperatures, and thus more resistant to wear and fatigue. Moreover, silicon cantilevers are more
batch fabrication friendly with sharp tips.
Figure 1.2 A scanning electron microscope (SEM) image of a silicon heated AFM cantilever.
The cantilever has an integrated solid-state resistive heater at the cantilever free end.
Figure 1.2 shows Scanning Electron Microscope (SEM) images of the single crystal
silicon heated microcantilever. The cantilever is U-shaped and consists of two regions: a
cantilever heater and legs. The cantilever free end is low doped (~1017 cm-3) with phosphorus
and acts as a heater or thermometer. The legs are high doped with phosphorus (~1020 cm-3) to
carry current to the heater region. Upon passing the current through the cantilever, more than 95%
of the cantilever power dissipates from the low doped resistive heater region, causing a localized
heating at the cantilever free end [57]. The cantilever has an oxidation sharpened tip [58] at the
cantilever free. The tip radius is about 20 nm and the tip height is about 2 µm. The cantilever
allows rapid heating and cooling with microsecond time constants [59]. The cantilever electrical
resistance is a function of a cantilever temperature due to the temperature-dependent carrier
mobility and concentration [60]. The cantilever temperature is calibrated from Stokes peak shift
50 µm
2 µm
LowDopedSilicon High Doped
Silicon
6
using Raman spectroscopy [61-64]. Detailed information on the design, fabrication, and
characterization of heated cantilevers has been reported elsewhere [11, 57].
Figure 1.3 shows the heat transfer within and from a heated microcantilever. The
dominant mode of heat transfer from the cantilever is thermal conduction rather than natural
convection or radiation [65-67]. There are three conduction modes for heat dissipation from the
cantilever heater. First, majority of the generated heated from the cantilever heater conducts
through the silicon cantilever legs [68]. Then the remaining heat conducts trough the gap
between the cantilever and the surface and through the silicon tip-surface contact. Less than 1%
of the generated heat conducts through the cantilever tip due to the large thermal resistances of
the tip-surface interface [66, 69]. Convective heat transfer from the cantilever heater to air is
negligible due to high surface area to volume ratio and small thermal capacitance [70, 71].
Radiative heat transfer from the cantilever heater to surrounding air environment is predicted to
be less than 1% of the overall heat loss when the cantilever temperature is of about 700 K [72].
Figure 1.3 Heat transfer from a heated cantilever [11]. Most of heat from the cantilever heater
conducts within the cantilever and the cantilever tip, and through the air around the cantilever.
Air
conductionRadiation
Conduction
through contact
7
1. 3 Applications of Heated Microcantilevers
1.3.1 Data Storage
AFM microcantilevers with integrated heaters enable high density data storage [14, 73,
74], where a small heated AFM probe was used to thermomechanically deform a polymer film
with pits that served as data bits. An array of thousands of heated cantilever tips, named
Millipede, demonstrated ultrahigh density with rapid read-and-write rate [14]. When the
cantilever is hot, it sinks into the film and forms an indentation that serves as a data bit. The
cantilever can then sense the bits without erasing them by measuring the heat transfer between a
warm cantilever and the surface. By heating the probe slightly above the polymer surface, the
film melts and the bit is erased. The technology came out of the Millipede project has enabled
much research into nanometer scale microscopy and manufacturing that extends far beyond data
storage.
1.3.2 Thermal Sensing
Heated AFM microcantilevers have been used to measure the nanometer scale thermal,
mechanical, and electrical properties of materials. The spatial resolution of tip-based thermal
sensing is limited by the contact area between probe tip and sample. The heat transfer between
tip and sample is measured and fit to models to extract material properties.
Variations in thermal conductivity and heat capacity across the sample influence the tip
temperature. Qualitative measurements of thermal interaction of micro heater-thermometers with
the sample have been made for a variety of probes and substrates.[66, 75-78] Combining this
heater temperature with a tip-sample heat transfer model allows calculations of thermal
8
conductivity. Local thermal conductivity measurements have been reported for different
substrates using heated microcantilevers with active heating [77-80].
Heated microcantilevers can be used to measure the thermomechanical properties of
materials, such as Glass transition, melting, recrystallization and thermal decomposition
temperatures were recorded for a number of polymers [81-83]. Glass transition temperatures are
measured by detecting tip height changes when the heated tip sinks into the surface [83]. In
addition to glass transition measurements, oscillating the thermal probe at resonance while in
constant contact with a surface enables measurement of sample elastic moduli, damping factors,
and thermal coefficients of expansion as a function of temperature [83-85].
Figure 1.4 The concept of thermal topography imaging using a heated microcantilever. The
substrate topography is mapped by tracking cantilever thermal conductance changes that occur
due to changes in the distance between the cantilever and the substrate.
9
Figure 1.4 shows the concept of thermal topography imaging using a heated AFM
microcantilever [68, 86]. Most of the heat being generated at the heater region of the cantilever
flows into the substrate such that the heat flow from the cantilever is inversely related to the
distance between the cantilever and the substrate [20]. The substrate topography is measured by
tracking changes in the cantilever power while cantilever temperature is held constant in a
regime of positive temperature coefficient of resistance. The quality of thermal topography
imaging is quantified by topography sensitivity and noise-limited vertical resolution. Thermal
topography sensitivity is defined as the change in cantilever voltage for a unit change in
topography. Resolution is defined as the smallest change in topography that can be detected and
is calculated as the thermal signal noise divided by the sensitivity.
1.3.3 Nanofabrication
Heated microcantilevers take advantage of the thermomechanical and thermochemical
properties of materials to fabricate a wide variety of nanostructures. They have been used to
mechanically deform and remove material underneath the probe tip, chemically modify surfaces,
deposit material onto surfaces, and thermally grow material on the probe end. A wide variety of
materials have been patterned with heated microcantilevers, including organics, metals,
nanoparticles, biomaterial, carbon nanotubes, and graphene.
Heated microcantilevers were first used to thermomechanically form pits on a polymer
surface for data storage applications [12]. Heated microcantilevers can also be used to make
three dimensional patterns in thin films that undergo thermal sublimation [13]. The spatial
resolution of such technique is in the order of few nanometers with the writing speed up to 500
kHz, when the microcantilever is operated with thermal feedback [87].
10
Figure 1.5 shows schematic of a heated tip depositing polymer features using the thermal
Dip Pen Nanolithography (tDPN) method. In tDPN, a heated tip is coated with an ink which is
solid at room temperature. The tip is then placed in contact with a surface and heated, causing
the ink to melt and flow onto the surface. Heated tips have deposited polymers [37], metals [52],
and polymer-nanoparticle composites [15]. The ink flow rate from tip to surface is sensitive to
tip temperature, tip geometry, and tip wettability, since surface tension effects drive ink flow
[88]. This method is not limited to materials that are mobile at room temperature in an aqueous
environment.
Figure 1.5 Schematic of thermal dip-pen nanolithography.
In addition to being able to thermomechanically deform surfaces and deposit a wide
variety of materials, heated probes are ideally suited to induce nanometer scale thermochemical
transitions in films. This technique is known as thermochemical nanolithography (TCNL).
Many of the applications for TCNL use heat to break a chemical bond in a thin film, which
removes a part of the molecule and changes the physical properties of the material. TCNL has
been used to create nanostructures in semi-conducting material. For example, graphene oxide
was reduced by local heating to produce conducting graphene nanoribbons [51]. Additionally,
ferroelectric structures were fabricated on a variety of electronic compatible substrates [16]. In
all cases it was shown that features on the order of 100 nm could be produced using this method.
11
1.3.4 Thermal Operation Reliability
Most of the aforementioned applications of heated microcantilevers require accurate
cantilever temperature control and temperature measurement. Moreover, reliable thermal
operations of heated microcantilevers enable repeatable and consistent use of cantilevers for
nanometrology and nanomanufacturing. Figure 1.6 shows the range of operating temperatures of
the cantilever for various applications. For emerging applications of heated cantilevers, the
operating temperature range has increased up to 1100 °C while a cantilever is often used for long
times [51, 52]. In addition, many recent applications of heated microcantilevers require
unconventional operating environments such as high vacuum [89, 90] and liquid [11, 91]. Thus,
it is necessary to develop a novel heated microcantilever and instrumentation to suit the specific
needs of each application, along with a detailed analysis of cantilever thermal operations.
Figure 1.6 Operating temperatures of the cantilever for various applications
Op
era
tin
gT
em
pe
ratu
re(o
C)
0
200
400
600
800
1000
DataStorage
LTA ThermalImaging
TDPN TCNL
Op
era
tin
gT
em
pe
ratu
re(o
C)
0
200
400
600
800
1000
MaterialSynthesis
12
1. 4 Dissertation Overview
The further development of heated AFM microcantilevers and microcantilever arrays will
not only improve the performance of current applications, but also enable new nanometrology
and nanofabrication applications. This dissertation focuses on fabrication, material integration,
characterization, and application of novel microcantilevers having integrated heaters.
Specifically, this thesis contains the following 7 chapters.
This chapter (chapter 1) introduces relevant work, gives a brief literature review, and
presents the motivation for this work.
Chapter 2 describes fabrication, characterization, and thermal crosstalk analysis of
heated microcantilever arrays, which consists of five independently controlled heated
microcantilevers.
Chapter 3 presents a detailed study on long term operation and reliability of heated
microcantilevers under extreme operating conditions.
Chapter 4 presents the integration of ultra-sharp ultrananocrystalline diamond tips
onto heated microcantilevers and microcantilever arrays for imaging and
nanomanufacturing applications.
Chapter 5 focuses on development of transfer-free graphene growth and its
application to wafer level batch fabrication of graphene coated microcantilevers.
Chapter 6 introduces a novel heated microcantilever metrology, which directly
measures the temperature dependent viscosities of water and ethylene glycol solution.
Chapter 7 concludes this thesis with a summary and future work.
13
1. 5 References
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[2] W. A. Ducker, T. J. Senden, and R. M. Pashley, "Direct measurement of colloidal forces
using an atomic force microscope," Nature, vol. 353, pp. 239, 1991.
[3] G. Meyer and N. M. Amer, "Simultaneous measurement of lateral and normal forces with
an optical‐beam‐deflection atomic force microscope," Applied Physics Letters, vol. 57,
pp. 2089, 1990.
[4] S. Sundararajan, B. Bhushan, T. Namazu, and Y. Isono, "Mechanical property
measurements of nanoscale structures using an atomic force microscope,"
Ultramicroscopy, vol. 91, pp. 111, 2002.
[5] Y. Sugimoto, P. Pou, M. Abe, P. Jelinek, R. Perez, S. Morita, and O. Custance,
"Chemical identification of individual surface atoms by atomic force microscopy,"
Nature, vol. 446, pp. 64, 2007.
[6] L. Gross, F. Mohn, N. Moll, P. Liljeroth, and G. Meyer, "The Chemical Structure of a
Molecule Resolved by Atomic Force Microscopy," Science, vol. 325, pp. 1110, 2009.
[7] H.-K. Lyeo, A. A. Khajetoorians, L. Shi, K. P. Pipe, R. J. Ram, A. Shakouri, and C. K.
Shih, "Profiling the Thermoelectric Power of Semiconductor Junctions with Nanometer
Resolution," Science, vol. 303, pp. 816, 2004.
[8] A. Olbrich, B. Ebersberger, and C. Boit, "Conducting atomic force microscopy for
nanoscale electrical characterization of thin SiO2," Applied Physics Letters, vol. 73, pp.
3114, 1998.
[9] D. J. Wold and C. D. Frisbie, "Fabrication and Characterization of
Metal−Molecule−Metal Junctions by Conducting Probe Atomic Force Microscopy,"
Journal of the American Chemical Society, vol. 123, pp. 5549, 2001.
[10] T. W. Kelley, E. Granstrom, and C. D. Frisbie, "Conducting Probe Atomic Force
Microscopy: A Characterization Tool for Molecular Electronics," Advanced Materials,
vol. 11, pp. 261, 1999.
[11] W. P. King, B. Bhatia, J. R. Felts, H. J. Kim, B. Kwon, B. Lee, S. Somnath, and M.
Rosenberger, "Heated Atomic Force Microscope Cantilevers and Their Applications,"
Annual Review of Heat Transfer, pp. 287, 2013.
[12] G. Binnig, "Ultrahigh-density atomic force microscopy data storage with erase
capability," Appl. Phys. Lett., vol. 74, p. 1329, 1999.
[13] D. Pires, J. L. Hedrick, A. De Silva, J. Frommer, B. Gotsmann, H. Wolf, M. Despont, U.
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19
CHAPTER 2: THERMAL CROSSTALK IN HEATED
MICROCANTILEVER ARRAYS
2. 1 Introduction
The atomic force microscope (AFM) [1] is widely used for nanometer-scale
measurements. Silicon AFM cantilevers with integrated solid-state resistive heaters were
developed for data storage [2-4] but have also been used for material property measurements [5-
7], thermal topography imaging [8, 9], and manufacturing [10-12] at the nanometer-scale. Heated
AFM cantilevers can be scaled to large arrays for high throughput imaging and nano-writing [3,
13, 14]. In an array of heated AFM cantilevers, each cantilever can be individually addressed
through an electro-thermal element integrated into each cantilever. These elements can operate
as heaters for actuation or thermistors for temperature measurement. Heat flow between neighbor
cantilevers in the array, referred to here as thermal crosstalk, can induce temperature increase in
these neighbor cantilever and potentially induce errors in the cantilever thermal operation [14,
15]. Such thermal crosstalk could hinder accurate cantilever temperature control and
measurement. The cantilever temperature measurement and control is particularly important in
applications that require precise cantilever temperature, for example material property
measurements and scanning thermal microscopy (SThM) [16, 17]. An understanding of thermal
crosstalk is important for achieving the highest possible accuracy and precision in these uses of
heated AFM cantilevers. However there is lack of published articles that carefully explore the
flows of heat between neighboring cantilevers in an array of resistively-heated doped silicon
20
cantilevers. Here we report detailed characterization and analysis of thermal crosstalk in a heated
microcantilever array.
Cantilever temperature measurement and control is important for nearly every application
of heated AFM cantilevers. For thermomechanical data writing, accurate temperature control of
the heated AFM tip enables the writing and erasing of data bit indents on polymer substrates,
while monitoring the cantilever heater temperature change allows data reading [2, 4]. Precise
temperature control and calibration are required for nanometer-scale heat transfer measurements
[16-19] and to set the cantilever temperature for nanometer-scale thermal material property
measurements [5-7, 20, 21]. Cantilever temperature control allows fabrication of nanostructures
in different shapes and sizes via surface modification [10, 11] or material deposition [12], and
high-temperature materials synthesis [22, 23]. Heated tips can measure surface topography by
monitoring the change in the cantilever heater power dissipation [9]. All of these applications
could be scaled to cantilever arrays; however they also require precise temperature control of
each cantilever. Thus there is a need to understand cantilever temperature control within a heated
cantilever array.
Only a little work has been published on the analysis of thermal crosstalk in cantilever
arrays. Thermal crosstalk was studied in an array of silicon nitride cantilevers with application to
nano-manufacturing [14, 24]. These silicon nitride cantilevers had metal heater integrated into
the cantilever base for thermomechanical actuation. Thermal conduction through the cantilever
solid chip array caused unanticipated bending of unheated cantilevers. In this application, the
cantilever heater temperature was as high as 65 °C. In another study, finite element analysis
calculated the thermal crosstalk in a polyimide-based thermal cantilever array. In these
cantilevers, the heating occurred in a resistive metal heater near the cantilever free end which
21
was positioned 15 µm above a substrate [15]. The cantilever temperature was as high as 110 °C.
These publications did not consider thermal crosstalk between neighboring silicon heated
cantilevers, nor did they consider thermal crosstalk when the array is in contact with a substrate,
which is the case for many applications, such as thermal imaging, nano-manufacturing, and
nanometer-scale thermal material property measurements. In addition, the operating temperature
of heated cantilevers, as high as 900 °C for nano-manufacturing [22], is much higher than both
polyimide-based thermal probe [15] and thermomechanically-actuated silicon nitride cantilever
[24].
This chapter reports the analysis of thermal crosstalk in a heated microcantilever array in
either steady or transient operating conditions while the array is either in contact with a substrate
or freely suspended in air. In addition, a simple model simulates thermal crosstalk as functions of
array dimensions: the cantilever-to-cantilever distance, the cantilever width, the cantilever length,
and the cantilever thickness. The studied cantilever heater temperatures range over 25 – 900 °C.
2. 2 Microcantilever Array Fabrication and Characterization
Figure 2.1 shows schematics and scanning electron microscope (SEM) images of the
heated cantilever array, which consists of five identical heated cantilevers. The cantilevers were
fabricated from doped single crystal silicon. The legs are highly doped to act as electrical leads
while the cantilever free end is low doped to act as a resistive heater. The electrical resistance in
the heater region is about 1.95 kΩ and the electrical resistance of each leg is about 60 Ω, and
thus over 90 % of the power dissipated in the cantilever occurs near the cantilever free end [25].
The spacing between adjacent cantilevers is 110 μm. Hereafter the cantilevers are referred to as
C1, C2, C3, C4, or C5 as shown in Fig. 2.1(c).
22
Figure 2.1: Schematics (a-b) and SEM images (c-d) of the heated cantilever array. Each array
consists of five identical doped silicon cantilevers having an integrated heater. The temperature
of each cantilever can be independently controlled. The typical tip radius is about 20 nm.
Figure 2.2 shows the heated cantilever array fabrication process, which is similar to a
previously published process [25] but modified and improved to accommodate the array.
Fabrication begins with a 100-mm-diameter silicon-on-insulator wafer, which has a 5-µm-thick
silicon device layer, a 1-µm-thick buried oxide layer, and a 400-µm-thick silicon handle layer.
First, the cantilever anchor beams and the tip cylinders are formed using an inductively coupled
plasma (ICP) deep reactive ion etching (DRIE). The tip cylinder is then sharpened with the
anisotropic wet etch followed by an oxidation sharpening [26]. The fabricated tips have an
average radius of curvature near 20 nm and height up to 1.5 µm. After the tip formation, the
cantilever free end is low doped n-type with phosphorous (~1017 cm-3) and then the cantilever
legs are high doped n-type with phosphorous (~1020 cm-3). For electronic connection to the
cantilever legs, a 1.5-µm-thick aluminum layer is evaporated and then etched to form the metal
leads. Finally, a silicon handler layer is backside etched via ICP-DRIE and the buried oxide layer
is etched with hydrofluoric acid solution, releasing the final device. The cantilever thickness is
200 μm
(c)
4 μm
(d)
(b)(a)
C5
C4
C3C2
C1
23
about 1 µm with a heater size of 14 × 20 µm2. Each cantilever leg is 20 µm in width and 135 µm
in length. A single 100 mm wafer produces about 400 cantilever arrays with a yield of 85%.
Figure 2.2 Summary of fabrication steps. Fabrication begins with silicon-on-insulator wafer. (a)
Tip and anchor formation with inductively coupled plasma (ICP) deep reactive ion etching
(DRIE). The tip is sharpened via thermal oxidation. (b) Low dosage and high dosage phosphorus
implantation. (c) Deposition of aluminum contacts for electronic connection to the highly doped
silicon. (d) Backside through wafer etch using the buried oxide layer as an etch stop. (e) Final
device release.
(a)
(c)
(e)
(b)
(d)
Intrinsic silicon
Silicon dioxide
Aluminum
High-doped silicon (N+)
Low-doped silicon (N+)
24
The electrical and thermal properties of the cantilever array were characterized using
established techniques [25]. Figure 2.3(a) shows the cantilever electrical resistance as a function
of the applied steady excitation voltage. Figure 2.3(b) shows the cantilever electrical resistance
as a function of the cantilever heater temperature for all five cantilevers in the array. The steady
cantilever heater temperature was measured using Raman spectroscopy [25, 27]. The cantilever
electrical resistance varies with temperature due to the temperature-dependence of the carrier
concentration and carrier mobility. The heated cantilever resistance increases with temperature as
the carrier mobility decreases with temperature. At about 560 °C, the thermally generated
intrinsic carriers dominate the doping concentration of the cantilever, causing a sharp drop in
cantilever resistance. This “thermal runaway” effect in heated cantilevers is well understood and
expected from previously published research [28]. For all five cantilevers in an array, the
cantilever resistance values are around 2 kΩ at room temperature and show similar temperature
dependencies over the range 25 − 900 °C.
25
Figure 2.3 (a) Cantilever resistance as a function of the steady excitation voltage. The cantilever
was operated in series with a 10 kΩ sense resistor. (b) Cantilever resistance as a function of the
heater temperature, measured using Raman spectroscopy.
Cantilever Heater Temperature (oC)
Ca
nti
lev
er
Re
sis
tan
ce
(k
)
0 200 400 600 800 10001
2
3
4
5
6
Total Voltage (V)
Ca
nti
lev
er
Re
sis
tan
ce
(k
)
0 5 10 15 20 25 301
2
3
4
5
6
C1
C2
C3
C4
C5
(a)
(b)
26
2. 3 Experiments and Results
The dominant mode of heat transfer from the cantilever is thermal conduction rather than
natural convection or radiation [18, 29, 30]. Heat flows directly from the heater into the nearby
air, or along the legs and then into the air. When the cantilever operates near a substrate, nearly
all of the heat flows into the substrate either from the heater or from the legs [4]. Thermal
conduction through the cantilever array chip is much smaller than thermal conduction into the
environment from the cantilever heater and legs [8]. Thermal crosstalk within the cantilever
array is mostly due to the heat flow between the cantilevers through the surrounding medium.
Here we consider thermal crosstalk between the cantilevers during either steady or transient
heating. We performed all studies at a room temperature of 21 °C and at atmospheric pressure.
Figure 2.4 shows the experimental setup to study thermal crosstalk between the
cantilevers, where one cantilever is resistively heated and the other cantilevers act as temperature
sensors. All of the cantilevers were temperature calibrated from the measured cantilever
resistances, shown in Fig. 2.3(b). Using this method, the cantilever temperature can be measured
to within an accuracy of about 1 °C [27, 31]. The temperature measurement precision is much
less than 1 °C. To help validate the measurements, the cantilevers were recalibrated several times
during the experiments, using Raman spectroscopy. The cantilever temperature calibration and
current-voltage properties were nearly constant from one experiment to the next, from which
conclude that any changes in the cantilever electro-thermal properties were small. The heated
cantilever operates in series with a power supply and sense resistor (10 kΩ). By monitoring the
voltage drop on a resistor in series with the cantilever, the heated cantilever resistance can be
calculated and thus the heater temperature can be measured and controlled [27]. The temperature
rise of the unheated neighbor cantilever was calibrated from its electrical resistance, which was
27
measured using a digital multimeter with an applied current of 10 µA. For transient operation, a
function generator applied a 10 ms square voltage to the heated cantilever and a digital
oscilloscope monitored the voltage drop on the heated cantilever as a function of time. To
capture the voltage drop on the neighboring unheated cantilever using a digital oscilloscope, a
digital sourcemeter applied a steady current of 10 µA to the unheated cantilever. For both steady
and transient measurements, the applied test current caused a temperature rise in the unheated
cantilever of about 0.1 °C, which is much smaller than the temperature rise due to thermal
crosstalk.
Figure 2.4 Schematic of experiment for the thermal crosstalk analysis of a heated cantilever
array. The electrical resistance of the unheated cantilever was measured while the heated
cantilever was resistively heated under steady or pulsed conditions.
To validate and understand the measurements, we performed 3D transient finite element
simulations of the cantilever temperature distribution using COMSOL. The cantilever array
operation in air or on substrates was modeled using the temperature-dependent electrical and
thermal properties of highly doped silicon [32, 33]. For boundary conditions, an outer surface of
an air was treated as a continuous medium, while the bottom of the substrate and the end of the
anchor was assumed to be at room temperature, 21 °C [8, 34]. Since the key parameter of interest
28
is heat flow from the heater region, the mesh was denser at the heater and was coarser away from
the heater, ranging from 0.5 µm to 25 µm. There were approximately 2×104 elements in the
mesh, and the convergence criterion was 10-3 °C.
Steady Operation Figure 2.5(a-c) shows the DC response of the unheated cantilevers
when only the leftmost cantilever (C1) was heated while the array was freely suspended in air, in
contact with a 1-mm-thick glass substrate, or in contact with a 500-µm-thick silicon substrate.
Physical contact of all five cantilevers to the substrate was maintained throughout the
measurement. For all cases, the measured temperature rise in the unheated cantilever was
proportional to the heated cantilever heater temperature rise. When the cantilever array was in
contact with either substrate, the temperature rise of the unheated cantilevers was much lower
compared to the air operation, since more heat flowed to the substrate rather than to the
neighboring unheated cantilevers through air. Both silicon and glass substrates serve as heat
sinks, although there is a higher thermal conductance into the silicon compared to the glass due
to the difference in their thermal conductivities. Thermal crosstalk is higher in the absence of a
nearby heat sink.
29
Figure 2.5 Steady-state thermal crosstalk between cantilevers in the array. C1, the leftmost
cantilever was heated while the temperature rise of the neighbor cantilevers was monitored. (a)
Cantilever array suspended in air. Array in contact with (b) a glass substrate or (c) a silicon
substrate.
Cantilever 1 Temperature (oC)
Ca
nti
lev
er
Te
mp
era
ture
,T
(oC
)
0 200 400 600 800 1000
21
22
T3
T2
T5
T4
Silicon
Cantilever 1 Temperature (oC)
Ca
nti
lev
er
Te
mp
era
ture
,T
(oC
)
0 200 400 600 800 100020
22
24
26
28
T3
T2
T5
T4
Glass
Cantilever 1 Temperature (oC)
Ca
nti
lev
er
Te
mp
era
ture
,T
(oC
)
0 200 400 600 800 100020
30
40
50
60
T3
T2
T5
T4
Air
(a)
(b)
(c)
30
Figure 2.6(a) shows measured and predicted steady state temperature distribution along
the cantilever heater regions of an array suspended in air. Cantilever C3 was heated to 300, 600,
or 900 °C, while the temperatures of the other cantilevers were measured. Measurements and
simulations of the cantilever temperatures agree to within 2% with no fitting parameters. The
predicted air temperature near the unheated cantilever is higher than the unheated cantilever
heater temperature, indicating that heat is flowing from the air into the unheated cantilever.
Figure 2.6(b) shows the thermal crosstalk ratio, γ = ΔTunheated / ΔTheated, where ΔTunheated is a
temperature rise of the unheated cantilever caused by ΔTheated, a temperature shift in the heated
cantilever. Figure 2.6(c) shows measured and predicted tip-to-tip thermal conductance, G, while
C1 was maintained at 300 °C for either suspended in air, or in contact with either glass or silicon.
The thermal conductance is G = q/(Theated - Tunheated) where G is the tip-to-tip thermal conductance,
q is the power transferred from the heated cantilever to the unheated cantilever, Theated is the
heated cantilever heater temperature, and Tunheated is the unheated cantilever heater temperature. q
was measured by monitoring the heated cantilever power shift when the neighboring cantilever
was heated to the same temperature and then turned off. When the array was suspended in air
away from a substrate, the thermal conductance between neighbor cantilevers was as high as
0.61 µW/°C. In contrast, the thermal conductance was nearly zero when the cantilever array was
in contact with silicon substrate. Obtained thermal conductance values can be used to predict the
heater temperature of both heated and unheated (or heated to lower-temperature) cantilevers in
various operating circumstances.
31
Figure 2.6 Temperature distribution, thermal crosstalk ratio, and thermal conductance within a
heated cantilever array. (a) Calculation (lines) and measurement (points) of temperature across
the cantilever heater regions in the array while the only C3, the middle cantilever, is heated. (b)
Thermal crosstalk ratio and (c) Tip-to-tip thermal conductance of the heated cantilever array
when the array is either in air or in contact with a substrate.
Cantielver
Th
erm
alC
on
du
cta
nc
e,G
(W
/oC
)
2 3 4 5
0
0.2
0.4
0.6
GSilicon
GAir
GGlass
Cantilever
Th
erm
alC
ros
sta
lkR
ati
o,
(%)
2 3 4 5-1
0
1
2
3
4
5
Air
Silicon
Glass
Distance (m)
Te
mp
era
ture
,T
(oC
)
0 100 200 300 400 500 6000
200
400
600
800
1000
1200
1400
Experiment
Calculation
T3
= 300oC
T3
= 900oC
T3
= 600oC
(c)
(b)
(a)
32
It is possible to use the cantilever thermal conductances to estimate the thermal crosstalk,
even when more than one cantilever is heated. The finite element simulation calculates the
thermal crosstalk ratio, γ, which is the ratio of the temperature of the unheated cantilever to the
temperature of the heated cantilever. The thermal crosstalk ratio is different for each cantilever in
the array. The temperature rise in the unheated cantilever can be predicted by summing the
product of each thermal crosstalk ratio with the temperature of its corresponding cantilever.
Figure 2.7(a) shows the thermal crosstalk ratios and their application for unheated cantilever C1,
and Fig. 2.7(b) shows the thermal crosstalk ratios and their application for unheated cantilever
C3. These figures compare the superposition of the thermal crosstalk from individual cantilevers,
as well as the result from the finite element simulation under the same circumstances. For both
Fig. 2.7(a) and 2.7(b), the finite element results agree with the superposition of thermal
crosstalks to within 5%. The small error is due to the temperature-dependence of the thermal
conductivity of doped silicon, which is small for this temperature range [27]. The results of Fig.
7 are for all of the heated cantilevers held at the same temperature. Other combinations of
cantilevers heated at different temperatures were tested, achieving good agreement between the
superposition approach and finite element simulations. In conclusion, this approach is generally
useful in predicting thermal crosstalk in heated cantilever arrays.
33
Figure 2.7 Predictions for thermal crosstalk when multiple cantilevers were heated in an array.
Two different unheated cantilever temperatures were predicted to validate the superposition
principle: 1) temperature from the superposition of the thermal crosstalk from individual
cantilevers (Tsuperposition) and 2) temperature from the finite element simulation when the other
four cantilevers were simultaneously heated (Tall heated). (a) Thermal crosstalk when all
cantilevers except C1 were heated. (b) Thermal crosstalk when all cantilevers except C3 were
heated.
Heated Cantilever Temperature (oC)
Ca
nti
lev
er
1T
em
pe
ratu
re,T
1(o
C)
0 200 400 600 800 100020
30
40
50
60
70
80
T1,superposition
T1,all heated
Heated Cantilever Temperature (oC)
Ca
nti
lev
er
3T
em
pe
ratu
re,T
3(o
C)
0 200 400 600 800 100020
40
60
80
100
120
T3,superposition
T3,all heated
γ2 γ3 γ4 γ5
0.0370 0.0107 0.0038 0.0017
C1 C2 C3 C4 C5
ΔT1 ΔT2 ΔT3 ΔT4 ΔT5
(a)
(b)
C1 C2 C3 C4 C5
ΔT1 ΔT2 ΔT3 ΔT4 ΔT5
γ1 γ2 γ4 γ5
0.0106 0.0311 0.0311 0.0016
34
Figure 2.8 Predicted thermal crosstalk for various cantilever array designs. The thermal crosstalk
ratio is the ratio of the unheated cantilever temperature rise to the temperature rise of its nearest
neighbor. (a) Predicted thermal crosstalk ratio as functions of the distance between the
cantilevers and the cantilever width. (b) Predicted thermal crosstalk ratio as functions of the
cantilever thickness and the cantilever length.
Cantilever Width, W (m)
Cantilever Distance, D (m)
Th
erm
alC
ros
sta
lkR
ati
o(%
)
10 20 30 40 50
20 40 60 80 100
2
4
6
8
10
12
Cantilever Length, L (m)
Cantilever Thickness, t (m)
Th
erm
alC
ros
sta
lkR
ati
o(%
)
50 100 150 200 250
0 0.5 1 1.5 2 2.5 3
0
4
8
12
16
20
Heated Unheated
DL
W
(a)
(b)
35
The validated model can help us to understand the thermal crosstalk performance of
future cantilever array designs. Finite element simulations predicted thermal crosstalk in air as
functions of cantilever dimensions: the distance between the cantilevers (D), the cantilever width
(W), the cantilever thickness (t), and the cantilever length (L). The baseline cantilever design has
dimensions D = 50 µm, W = 20 µm, L = 100 µm, t = 1 µm, and varied one parameter at a time.
Figure 2.8(a) shows thermal crosstalk ratios with varying D or W. Figure 2.8(b) shows thermal
crosstalk ratios with varying L and t. The variation in each cantilever dimension resulted change
in the array thermal crosstalk. Thermal crosstalk increased when L increased or when D, W, or t
decreased.
Transient Operation Figure 2.9 shows measured and predicted transient cantilever
array response to square pulses of 10 ms duration at 10.25 V. The cantilever thermal time
constant, Tt, is the time required to reach the 66% of the time needed to heat or cool the
cantilever including the legs [35]. The measured thermal time constant of the heated cantilever is
Tt = 220 µs, which is consistent with previously published experimental [25] and computational
results [36]. For the unheated cantilever adjacent to the heated cantilever, the cantilever thermal
time constant is 1.3 ms. The time for the unheated cantilever temperature to respond to the
heated cantilever temperature increases with the distance between cantilevers. As the
temperature change is faster for the heated cantilevers than the unheated cantilevers, thermal
crosstalk can be mitigated if the heated cantilever is heated with a shorter pulse, especially if the
pulse is less than 1 ms.
Along with thermal crosstalk, thermoelectric and electrical couplings are present in the
heated cantilever array. Cantilever heating generates a temperature gradient in the nearby
unheated cantilevers, which induces a thermoelectric voltage on the order of 0.1 mV, due to the
36
Seebeck effect in doped silicon [29, 37]. In addition, pulse heating of a cantilever induces a
current on the order of 1 µA in the unheated cantilevers, which is assumed to be a capacitive
coupling. Both thermoelectric and capacitive couplings yield negligible cantilever heating. In
addition, the temperature error arising from the induced thermoelectric voltage of 1 mV and the
induced capacitive coupling current of 1 µA is only about 0.09 °C and 0.14 °C, respectively.
Thus both thermoelectric and capacitive couplings have negligible impact in controlling the
cantilever heater temperature.
Figure 9: Thermal crosstalk during transient cantilever heating. Calculation (lines) and
measurement (points) of the array transient response to a voltage of 10.25 V on C1. The signal
from C5 was too small to observe.
Cantilever Heating Duration (msec)
He
ate
dC
an
tile
ve
rT
em
pe
ratu
re(o
C)
Un
he
ate
dC
an
tile
ve
rT
em
pe
ratu
re(o
C)
0 1 2 3 40
100
200
300
400
500
20
25
30
35
40
45
50
T5
T1
T2
T3T
4
37
2. 4 Discussion
Thermal crosstalk is inevitable in operating heated AFM cantilever arrays. The approach
described in this chapter can help to anticipate and mitigate thermal crosstalk, through careful
measurements can calibrations. When the cantilever is operated in closed-loop operation that
controls cantilever resistance, the cantilever temperature can be controlled to within 0.02 °C [9].
For open loop operation, the thermal crosstalk should be accounted for during calibration.
Understanding and correcting thermal crosstalk is important for some applications of
heated cantilever arrays. Thermal crosstalk could cause significant uncertainties in material
property measurements and SThM where the accurate temperature measurement is essential. In
local thermal analysis of polymers [5, 6], the typical operating temperature at 150 °C could
induce temperature error of about 2 °C to neighboring cantilevers. For thermogravimetry [38],
the measurement temperature is about 200 °C, for which thermal crosstalk could induce an error
as large as 10 °C. In nanometer-scale SThM [16, 17], the power measurement resolution is in the
order of 1 µW. For a measurement at 100 °C, heat flow from a neighbor cantilever could be as
large as 49 µW, which is significant relative to the measurement resolution. For thermal imaging
[9], a typical cantilever heater temperature is 200 °C, which can induce a temperature rise in a
neighbor cantilever of about 2 °C, which could result in a measurement artifact of 0.8 nm,
depending upon the control algorithm being used. For nano-manufacturing [10-12] , the
cantilever resistance can be controlled to reach a desired cantilever temperature. For this
application, a typical cantilever operating temperature is 250 °C [12] and the estimated
temperature rise of the unheated cantilever due to thermal crosstalk is about 3 °C.
38
2. 5 Conclusion
In conclusion, this chapter reports detailed characterization and analysis of thermal
crosstalk in a heated microcantilever array. Thermal crosstalk is analyzed under steady or pulse
heating, while the cantilever array is either in contact with a substrate or freely suspended in air
In all operating conditions, the temperature rise of the unheated cantilever is proportional to the
heated cantilever heater temperature. In addition, thermal crosstalk is less significant for
operation near the substrate compared to away from the substrate. In transient operation, it takes
several milliseconds to achieve thermal equilibrium in the entire cantilever array. Thermoelectric
and electrical couplings are present, but have no significance in the array thermal operation. We
also predicted thermal crosstalk as functions of various array dimensions and the results can be
used in designing the future heated cantilever arrays. By understanding and correcting for this
thermal crosstalk, it is possible to increase the accuracy of temperature calibration in heated
cantilever arrays.
39
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surrounding air environment," Applied Thermal Engineering, vol. 29, pp. 1631, 2009.
[37] T. H. Geballe and G. W. Hull, "Seebeck Effect in Silicon," Physical Review, vol. 98, pp.
940, 1955.
[38] R. Berger, H. P. Lang, C. Gerber, J. K. Gimzewski, J. H. Fabian, L. Scandella, E. Meyer,
and H. J. Guntherodt, "Micromechanical thermogravimetry," Chemical Physics Letters,
vol. 294, pp. 363, Sep 1998.
42
CHAPTER 3: A STUDY OF LONG TERM OPERATION AND
RELIABILITY OF HEATED MICROCANTILEVERS
3. 1 Introduction
The atomic force microscope (AFM) [1] is widely used for probing mechanical, chemical,
electrical, and thermal phenomena at the nanometer scale. Silicon AFM cantilevers with
integrated solid-state resistive heaters [2] were developed for data storage, in which a heated tip
at about 400 °C softens the polymer substrate to write or erase data bits [3, 4]. The use of heated
cantilevers has expanded into many areas including nano-manufacturing [5-9], material property
measurements [10-13], and nanometer-scale imaging [2]. For emerging applications of heated
cantilevers, the operating temperature range has increased up to 1100 °C while a reliable
cantilever thermal operation is required for long times [5, 6]. However, the cantilever behavior at
such high temperatures and long heating times is not yet fully understood. This paper reports on
a detailed analysis of long term reliabilities and operating limits of heated cantilevers for various
operational conditions.
Precise calibration of cantilever properties is critical for accurate temperature control and
temperature measurements using heated cantilevers. Accurate control of the tip-substrate
interface temperature allows nanometer-scale deposition and thermomechanical or
thermochemical surface modification in a wide range of materials, including, polymers, metals,
ferroelectric/piezoelectric materials and graphene [5-9]. For example, heating the polymer or
metal coated tip near the substrate allows the direct deposition of materials in various sizes and
shapes [2, 6]. In addition, heated cantilevers can be used to measure temperature-dependent
43
material properties, such as glass transition and melting temperatures, thermal conductivity,
elastic modulus, and thermal expansion coefficient [12-15], by controlling the tip-substrate
interface temperature and then measuring the cantilever or sample material responses. For
reproducible results for all of above applications, reliable and consistent cantilever thermal
operation for long times is critical.
Previous publications on the characterization and the temperature calibration of heated
cantilevers report methods for accurate temperature control and temperature measurements using
heated cantilevers over a wide temperature range of 25 – 800 °C [2, 16]. However, the effects of
long heating durations or high heating temperatures on heated cantilevers has not yet been
studied in detail. For applications in nanomanufacturing, heated cantilevers are often used at
temperatures of above 800 °C for more than several hours. For example, heated tips at high
temperatures, up to 1060 °C, can be used to reduce graphene oxide into electronics-grade
graphene, with nanometer-scale resolution [5]. In another example, a heated tip can be used to
deposit nanometer-scale structures of indium, which requires a cantilever heater temperature as
high as 1030 °C p [6]. Thus, it is necessary to understand and account for changes in cantilever
behaviors for applications that require high heater temperatures and long heating times. This
paper presents an analysis of long term operation and reliability of heated cantilevers at a wide
range of cantilever heater temperatures ranging from 25 – 1100 °C under steady, periodic, and
pulse heating conditions.
44
3. 2 Experiment
Figure 3.1(a) shows Scanning Electron Microscope (SEM) images of a fabricated doped
single crystal silicon heated cantilever. The cantilever is U-shaped and consists of two regions: a
cantilever heater and legs. The cantilever free end is low doped (~1017 cm-3) with phosphorus
and acts as a heater or thermometer. The legs are high doped with phosphorus (~1020 cm-3) to
carry current to the heater region. Upon passing the current through the cantilever, more than 90%
of the cantilever power dissipates from the low doped resistive heater region, causing a localized
heating at the cantilever free end [16]. Detailed information on the design, fabrication, and
characterization of heated cantilevers has been reported elsewhere [16]. Using the established
calibration techniques, the electro-thermal properties of a heated cantilever are characterized [16].
Figure 3.1(b) shows the experimental setup for the cantilever operation. The circuit consists of a
power supply, series resistor (RS =10 kΩ), and heated cantilever. From the measured voltage
drop across RS, the cantilever electrical resistance can be calculated. The steady cantilever heater
temperature (TH) is determined by calibrating the temperature dependence of the Stokes peak
position using a Raman spectroscopy [16, 17]. Figure 3.1(c) shows cantilever resistance and TH
as functions of power dissipated in the cantilever. The cantilever electrical resistance is a
function of a cantilever temperature due to the temperature-dependent carrier mobility and
concentration [18]. The cantilever can be heated up to TH above 1100 °C with an intrinsic
cantilever resistance (RC,RT) of about 2 kΩ at the room temperature.
45
Figure 3.1 The heated cantilever and its measured electro-thermal properties. (a) A scanning
electron microscope (SEM) image of a silicon heated AFM cantilever. The cantilever has an
integrated solid-state resistive heater-thermistor at the cantilever free end. (b) Experiment setup
for the characterization of the cantilever electro-thermal properties. (c) Cantilever electrical
resistance and cantilever heater temperature (TH) as functions of cantilever power. TH is
measured and calibrated using Raman spectroscopy.
Cantilever Power (mW)
Ca
nti
lev
er
Re
sis
tan
ce
(k
)
Ca
nti
lev
er
He
ate
rT
em
pe
ratu
re(o
C)
0 1 2 3 4 5 6 71
2
3
4
5
6
7
8
9
0
200
400
600
800
1000
1200
(a)
(b)
(c)
46
For the long term operation and reliability studies of heated cantilevers, cantilevers were
heated with three different electrical heating schemes of steady, periodic, and pulse heating. First,
cantilevers were steady heated for long durations, up to 108 hours, to study the effect of long
heating times on cantilever electro-thermal properties. For steady heating, a digital source meter
supplied a DC voltage to maintain a constant TH while a digital multimeter measured the RC,RT in
every 10 minutes. In addition, cantilever properties were characterized in every 6 hours to
monitor the shift in cantilever electro-thermal properties. After each property characterization,
the input DC voltage was adjusted to maintain a constant TH. The shift in cantilever properties
under periodic heating was also monitored for more than 20 million heating cycles. For periodic
heating studies, a function generator supplied square pulses at 500 Hz with 50% duty cycle to the
cantilever while measuring the RC,RT in every 10 minutes. The heating frequency of 500 Hz was
chosen to ensure that TH reaches the target maximum temperatures, as the thermal time constants
of heated cantilevers are in the order of 100 µsec [16]. In addition to the steady and periodic
heating studies, the cantilevers were intentionally failed by applying a square pulse at high
voltage amplitudes to understand the cantilever failure mechanisms. Cantilevers were heated at
high voltage amplitudes, up to 120 V, with wide ranges of pulse durations of from 5 µsec to 5 ms.
During the pulse heating, a digital oscilloscope was used to measure the transient response of
cantilever resistances, which later calibrated into TH. The studied TH ranges from 20 – 1100 °C
and all experiments were carried out in ambient environment with the room temperature of 20 °C.
47
3. 3 Numerical Modeling
Figure 3.2 shows a schematic of the heated AFM cantilever free end, which consists of a
low-doped (~1017 cm-3) cantilever heater and high-doped (~1020 cm-3) cantilever legs. Upon
heating the cantilever at high temperatures, phosphorus dopants from cantilever legs diffuse into
cantilever heater by the silicon self-interstitial mechanism [19]. Dopant diffusion at the heater-
leg junction induces unwanted redistribution of the phosphorus dopants thus changing cantilever
electro-thermal properties. Such changes in cantilever properties need to be analyzed and
accounted for accurate temperature control and temperature measurements using heated
cantilevers. A detailed numerical modeling predicts the changes in cantilever properties induced
by uncontrollable dopant diffusion at the heater-leg junction under various heating conditions.
Figure 3.2 Schematic of the cantilever heater-leg junction. Dopant concentration gradient
induces dopant diffusion across the heater-leg junction upon heating the cantilever, thus affecting
the electro-thermal properties of heated cantilevers.
48
Fick’s second law predicts the time-dependent dopant concentration across the cantilever
[20]. The dopant concentration changes only along the length of the cantilever, making that the
one-dimensional calculation is appropriate. This is a valid assumption because the implanted
phosphorus dopants are fully diffused into silicon toward the direction of the cantilever thickness
after the high-temperature annealing step during cantilever fabrication. For heated cantilevers,
the dopant diffusion across the heater-leg junction can be treated as an interdiffusion between
two semi-infinite specimens of different dopant concentrations [20-22]. In addition, the dopant
diffusion is assumed to be intrinsic, because more than 90% of the total cantilever electro-
thermal property depends on the cantilever heater [2], where the phosphorus dopant
concentration is much lower than the intrinsic carrier concentration of silicon. The transient
dopant concentration distribution for the intrinsic interdiffusion can be expressed as:
𝐶(𝑥, 𝑡) = (𝐶𝑙+𝐶ℎ
2) 𝑒𝑟𝑓𝑐 (
𝑥
2√𝐷𝑡) + 𝐶ℎ (eq. 3.1)
where Cl and Ch are initial dopant concentrations at the cantilever leg and cantilever heater, x is
location along the cantilever length, D is the intrinsic phosphorus diffusion coefficient, and t is
time. TH is set at constant value while the steady-state temperature distribution along the
cantilever leg is calculated from a one-dimensional finite-element analysis. Once the cantilever
temperature distribution is known, D is calculated using the following Arrhenius’ equation:
𝐷 = 𝐷𝑜𝑒𝑥𝑝 (−𝐸𝑜
𝑘𝐵𝑇) (eq. 3.2)
where Do = 3.84 cm2/s and Eo = 3.66 eV are diffusion coefficient extrapolated to infinite
temperature and experimentally determined activation energy for phosphorus, respectively [20].
The calculated dopant concentration distribution predicts the electrical resistivity across
the cantilever by using the published conversion equation that has been fitted to the experimental
49
data of Thurber et al. [23]. Finally, RC,RT is determined from the calculated electrical resistivity
values and the cantilever dimensions.
3. 4 Results
Long heating durations at high TH induce the diffusion of phosphorus dopants across the
cantilever heater-leg junction of the cantilever. Figure 3.3(a) shows the calculated dopant
concentration along the cantilever after 10 hours of steady heating at different heater
temperatures of from 900 °C to 1100 °C. The inset in figure 3.3(a) shows the calculated intrinsic
dopant diffusivity as a function of temperature. The intrinsic dopant diffusivity increases by
several orders of magnitudes from 800 °C to 1200 °C. Figure 3.3(b) shows the calculated dopant
concentration along the cantilever for various heating durations at TH of 1000 °C. The amount of
phosphorus dopants diffused from the leg to the cantilever heater increase with both TH and
heating times. This dopant diffusion decreases the size of the cantilever heater, and thus
decreasing RC,RT and ultimately shifting the cantilever electro-thermal properties.
Both measurements and predictions show that after the steady heating, the change in
cantilever electro-thermal properties is significant. Figure 3.4(a) shows predicted and measured
RC,RT as a function of a heating duration at different TH of 600 °C, 800 °C, and 1000 °C. The
room temperature cantilever resistance RC,RT decreased with both TH and heating duration. The
rate of change in RC,RT increases with TH, which is expected from the diffusion model. After 54
hours of steady heating, the cantilever electrical resistances decreased by 0.5% for 600 °C, 2.7%
for 800 °C, and 17.5% for 1000 °C. Noticeably, RC,RT decreased by as much as 24% after 108
hours of steady heating at the heater temperature of 1000 °C. Such large shifts in cantilever
properties are important to understand for accurate control of cantilever temperature.
50
Figure 3.3 Predicted dopant concentration distribution (a) at various TH after 10 hours of steady
heating and (b) at various heating durations when the cantilever is steady heated at TH of 1000 °C.
Distance (m)
Do
pa
nt
Co
nc
en
tra
tio
n(c
m-3)
-4 -2 0 2 4 6 810
16
1017
1018
1019
1020
1021
t = 10 hr t = 108 hr
t = 2 hr
t = 0 hrt = 24 hr
t = 54 hr
T = 1000oC
Distance (m)
Do
pa
nt
Co
nc
en
tra
tio
n(c
m-3)
-4 -2 0 2 4 6 810
16
1017
1018
1019
1020
1021
T = 900oC
T = 1100oC
T = 950oC
T = 1000oC
T = 1050oC
CantileverLeg
CantileverHeater
Temeprature (oC)
Dif
fus
ivit
y(c
m-2/s
)
800 1000 120010
-17
10-16
10-15
10-14
10-13
10-12
10-11
(a)
(b)
51
Figure 3.4(b) shows the measured cantilever electrical resistance as a function of TH after
heating the cantilever at 1000 °C for various heating durations of up to 108 hours. Below TH of
about 750 °C, the temperature dependent cantilever resistance decreases as the heating duration
increases. However, the shift in cantilever resistance becomes negligible for TH above 750 °C.
This is because the thermally generated intrinsic carrier density, not the extrinsic phosphorus
dopant density, dominates the electro-thermal properties of a cantilever at high temperatures [18].
As the cantilever electro-thermal properties change upon heating at high temperatures, timely
and frequent characterizations of cantilever properties are required for reliable and accurate
thermal operations using heated cantilevers.
The cantilever heater oxidized after heating at high temperatures in ambient air, as shown
in the inset SEM image of Fig. 3.4(b). For 108 hours of heating at TH =1000 °C, silicon dioxide
grew to a thickness of 110-nm on the surface of the cantilever heater. The silicon dioxide
thickness was measured using Nanometrics NanoSpec AFT 4000, which has spatial resolution of
about 3 µm and the measurement precision of about 0.2 nm. The formation of silicon oxide on
the cantilever heater increased the tip apex radius from 20 nm to about 55 nm. Such an increase
in tip radius could affect the resolution of the tip-based measurement and nanofabrication. The
original tip apex radius of 20 nm was recovered after the wet etching of the silicon oxide layer
with buffered HF [24].
52
Figure 3.4 Shifts in the cantilever electro-thermal properties upon heating at high temperatures.
(a) Calculation (lines) and measurement (points) of changes in cantilever resistance as a function
of cantilever heating duration at various TH. (b) Cantilever resistance as a function of TH after
various heating durations while the cantilever is steady heated at 1000 °C. The inset SEM image
shows the formation of a 110-nm-thick SiO2 layer on a cantilever heater after 108 hours of
heating in ambient air.
Heating Duration (hr)
Ca
nti
lev
er
Re
sis
tan
ce
Ch
an
ge
(%)
0 10 20 30 40 50-25
-20
-15
-10
-5
0
5
Experiment
Prediction
Tcant
= 1000oC
Tcant
= 600oC
Tcant
= 800oC
Cantilever Heater Temperature (oC)
Ca
nti
lev
er
Re
sis
tan
ce
(k
)
0 200 400 600 800 10001
2
3
4
5
6
7
8
9
108 hr
0 hr
18 hr
54 hr
10µm
Cantilever Heater Temperature (oC)
Ca
nti
lev
er
Re
sis
tan
ce
(k
)
0 200 400 600 800 10001
2
3
4
5
6
7
8
9
108 hr
0 hr
18 hr
54 hr
10µm
(a)
(b)
53
We also studied the effect periodic heating at high TH on cantilever electro-thermal
properties. Figure 3.5(a) shows the measured cantilever resistance at different pulse heating
voltages at the heating frequency of 500 Hz with 50% duty cycle. In a given heating pulse, the
cantilever resistance changed with time and then reached a constant value as the cantilever heater
reached a steady temperature. This cantilever response during heating and cooling was expected
from a previously published research [16]. Figure 3.5(b) shows the change in RC,RT as a function
of the number of heating cycles at different AC voltage amplitudes of 14 V, 17 V, and 25 V,
which corresponds to the maximum heater temperatures of about 600 °C, 800°C, and 1000 °C,
respectively. The cantilever electrical resistance RC,RT decreased with the number of heating
cycles and the amplitude of applied voltages. Although the periodic heating at high TH indeed
shifted the cantilever electro-thermal properties, cantilevers did not fail after tens of millions of
heating cycles.
54
Figure 3.5 Response of heated cantilevers to the periodic heating. (a) Response of cantilever
resistances while pulse heated at 500 Hz with 50% duty cycle. (b) Changes in room temperature
cantilever resistance as a function of the number of heating cycles. Input voltage amplitudes are
14 V, 17 V, and 25 V, which corresponds to the maximum TH of 600 °C, 800 °C, and 1000 °C,
respectively.
Time (sec)
Ca
nti
lev
er
Re
sis
tan
ce
(k
)
-500 0 500 1000 1500
2
3
4
5
6
7
25 V
14 V
17 V
Pulse Heating
Number of Heating Cycles
Ca
nti
lev
er
Re
sis
tan
ce
Ch
an
ge
(%)
-3
-2
-1
0
2x1070 5x10
61x10
71.5x10
7
25 V
14 V
17 V
(a)
(b)
55
To understand the cantilever failure mechanism, we excited heated cantilevers with a
square pulse at high voltages until the cantilevers failed. Figure 3.6(a) shows responses of the
cantilever resistance to a 100 µsec square pulse at different excitation voltage amplitudes. The
cantilever resistance reached a steady value faster for higher input voltage amplitudes. At the
input voltage amplitude of 70 V, it took less than 3 µsec for a cantilever to reach a peak
resistance value, which corresponds to TH of about 600 °C. However, a cantilever failed at the
voltage amplitude of 90V or higher. Figure 3.6(b) shows the cantilever failure voltages as a
function of applied pulse durations. Cantilevers failed at higher voltages as the pulse heating
duration decreased. The inset in Fig. 3.6(b) shows SEM images of failed cantilevers, which
failed at the heater-leg junction. In every case, the failure occurred at the first heater-leg junction
against the direction of the flow of electrons, which are the charge carriers for phosphorus doped
n-type silicon. We believe that the cantilevers failed at this location because of the electrical
resistance at the junction induced localized heating. The first heater-leg junction against the
current flow can be treated as a forward-biased n-n+ homojunction, which heats up under high
voltages in the presence of electrical resistance at the junction. Melting of the heater-leg junction
caused an irreversible mechanical breakage of the cantilevers. Although heated cantilevers can
quickly reach very high temperatures, a careful selection of heating parameters is required to
prevent the cantilever failure.
56
Figure 3.6 A square pulse excitation of heated cantilevers. (a) Changes in cantilever resistance at
various heating voltage amplitudes with a pulse duration of 100 µsec. (b) Required input
voltages to break the cantilever for various pulse durations. The insets show SEM images of
broken cantilevers, where the failure occurred right at the cantilever heater-leg junction.
Applied Pulse Heat Duration (sec)
Ca
nti
lev
er
Fa
ilu
reV
olt
ag
e(V
)
0 500 1000 1500 200060
70
80
90
100
110
10 µm
e- e-
Pulse Heating Duration (sec)
Ca
nti
lev
er
Re
sis
tan
ce
(k
)
0 20 40 60 80 1000
1
2
3
4
5
6
7
8
20 V
70 V50 V
40 V
30 V
26 V
14 V
10 V
(a)
(b)
57
3. 5 Discussion
Operating temperatures and heating times are important factors that determine the
lifetime and reliability of heated cantilevers. Our study shows that the change in cantilever
electro-thermal properties is negligible when the cantilever heating temperature TH is lower than
about 600 °C. There are many applications for which the cantilever operates below this
temperature, for example data storage, local thermal analysis (LTA), calorimetry, and thermal
topography imaging [2]. Thus heated cantilevers can be used for long times for aforementioned
applications. However, the condition of the cantilever tip should be closely monitored, as the
blunted or contaminated tip may hinder the cantilevers from making consistent measurements.
For high temperature applications, above TH of about 600 °C, the change in cantilever electro-
thermal properties increase with both heating temperatures and heating times. Examples of
applications that that use this operating range include nanomanufacturing and materials synthesis.
Changes in cantilever properties can be problematic for read-write-read operations [4], where the
cantilever is heated to a high temperature for tip-based nanomanufacturing and then cooled to a
lower temperature for thermal nanoimaging. Thus a timely and frequent calibration of the
cantilever properties is required for high temperature applications and read-write-read operations.
For the next generation of heated cantilevers, few design modifications could improve the
cantilever reliability. First, increasing the volume of low-doped heater region could alleviate the
effect of the dopant diffusion on the overall cantilever electro-thermal properties. For example,
doubling the heater volume, while maintaining the constant cross sectional area of the heater-leg
junction, would nearly halve the change in overall cantilever electrical resistances. This is
because the amount of diffused dopants is the functions of heater temperatures and heating times
not the low-doped heater volume. Secondly, decreasing the cross section area of the heater-leg
58
junction, or narrowing a thermal constriction, could limit the amount of diffused dopants. Since
the dopant diffusion across the cantilever is one-dimensional, the amount of dopants diffuse into
the heater depends on the cross section area of the heater-leg junction. In addition, a narrower
thermal constriction increases the thermal resistance of the cantilever heater-leg junction, and
thus more heat from the cantilever heater to dissipate to the substrate rather than the cantilever
legs. As TH depends more on the heat transfer between the cantilever and substrate, a narrower
thermal constriction would improve thermal measurement sensitivities in thermal topography
imaging [2].
3. 6 Conclusion
In summary, this chapter reports on a detailed study of long term operation and reliability
of silicon heated cantilevers. High heating temperatures of above 800 °C and long heating times
induce the diffusion of phosphorus dopants across the cantilever heater-leg junction, and thus
causing the undesired shift in cantilever properties. At high heating voltages, the localized
heating at the cantilever heater-leg junction induces an irreversible breakage of the cantilever.
For the high temperature and long heating duration applications, the changes in cantilever
properties should be closely monitored and recalibrated if necessary to achieve the highest
accuracy possible in temperature control and temperature measurements. Lastly, the findings in
this chapter can be applied to understand the reliability of other types of MEMS with a doped
silicon heater-thermometer.
59
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61
CHAPTER 4: DIAMOND TIP HEATED MICROCANTILEVERS
AND MICROCANTILEVER ARRAYS
4. 1 Introduction
The atomic force microscope (AFM) [1] is widely used for nanometer-scale
measurements. AFM cantilevers with integrated heaters have been used for data storage [2],
topography imaging [3], material property measurements [4, 5], and manufacturing [6, 7] at the
nanometer-scale. A key challenge for AFM measurements is tip wear and fouling, which can be
mitigated by a tip coating or the use of novel tip materials [8, 9]. This chapter reports on the
design, fabrication, and application of heated AFM microcantilevers having ultrananocrystalline
diamond (UNCD) tips, which display high mechanical robustness and resistance to fouling.
Various materials, coatings, and surface treatments have been proposed to mitigate tip
wear, including platinum silicide tips [10], silicon dioxide tip encapsulation [11], silicon carbide
tips [12], and diamond or diamond-like carbon tips [8, 9, 13-15]. Diamond tips are attractive as
they are mechanically robust due to high hardness and are fouling resistant due to chemical
inertness, low surface energy, and low adhesion [16]. Moreover, diamond, which is normally
insulating, can be made electrically conductive via doping [17]. Diamond tips can also be
robustly functionalized with chemical groups via direct C-C linkages [18], which is a more stable
attachment method than thiols and silanes which are conventionally used. AFM cantilevers with
diamond tips can be fabricated by various methods. Focused ion bean milling can generate sharp
diamond tips, but is expensive and relatively slow [19]. It is possible to batch-fabricate diamond
AFM tips by depositing diamond into a mold as part of the fabrication process [8, 15, 20], but
62
this process typically results short tips, may not be well-suited to integrating electronic devices
into the tip and cantilever, and often generates wedge shaped defects. Diamond-coated silicon
tips can be batch-fabricated [9, 13, 21], but the radius of curvature (>50 nm) may be somewhat
larger than is desired, since the diamond layer increases the radius of the final tip. Diamond tip
formation and sharpening using reactive ion etching (RIE) is a simple process for batch
fabrication of diamond tips [22, 23] that can provide good control over tip radius, height, and
aspect ratio, thus generating taller and sharper diamond tips than other fabrication methods.
This chapter presents the integration of UNCD tips onto a heated AFM cantilever and
cantilevers arrays using UNCD coating and RIE sharpening. This work investigates tip wear,
cantilever thermal function, and the use of these heated tips for thermal imaging and
nanolithography.
4. 2 Cantilever Design and Fabrication
Figure 4.1 shows the fabrication process for the heated AFM cantilever having a UNCD
tip. Heated cantilever arrays follow similar fabrication processes with some modifications to the
mask designs. The fabrication starts with a 100-mm-diameter silicon-on-insulator wafer having a
5-µm-thick silicon device layer, a 1-µm-thick buried oxide layer, and a 400-µm-thick silicon
handle layer. First, the cantilever structure is formed using inductively coupled plasma (ICP)
deep RIE followed by two separate n-type phosphorous doping steps at different concentrations.
The cantilever free end is low doped (~1017 cm-3) forming a resistive heater region, while
cantilever legs are highly doped (~1020 cm-3) to carry current. After annealing to drive in the
dopants, the cantilever is electrically active and ready for tip formation.
63
Figure 4.1 Fabrication process of the single heated cantilever with an all-UNCD tip. Fabrication
begins with a silicon-on-insulator wafer. (a) Anchor and cantilever formation with inductively
coupled plasma (ICP) deep reactive ion etching (DRIE). (b) Low dosage and high dosage
phosphorus implantation. (c) Conformal UNCD growth via hot-filament chemical vapor
deposition (CVD). (d) ICP-DRIE oxygen plasma etching of UNCD by differential etch and slope
amplification using plasma enhanced CVD (PECVD) silicon dioxide as a precursor cap. (e)
Metallization of the device via a 1.5-µm-thick aluminum deposition and etching. (f) Backside
ICP-DRIE of 400-μm-thick silicon handle layer followed by the sacrificial oxide layer etching
using the hydrofluoric acid solution. Finally, the device is released. The heated cantilever array
fabrication follows similar steps with a small modification in the mask design.
64
The silicon wafer substrates were seeded by ultrasonication with ultra-disperse diamond
nanoparticle suspension. A 7-µm-thick UNCD layer with roughness of 30 – 50 nm (RMS, as
measured with a 2-µm-tip-radius stylus profilometer on 5 points on the wafers) was grown by
hot filament chemical vapor deposition. The wafer is treated with hot piranha to converse the H-
terminated carbon bonds into more adhesion-favorable -OH bonds, which improves the adhesion
between the UNCD layer and a protective silicon dioxide mask [22]. A protective silicon dioxide
mask was patterned to form 3-µm-diameter precursor caps, and the exposed UNCD was etched
with ICP-RIE in O2/SF6 plasma [30]. The differential etching rate of UNCD to silicon dioxide of
approximately 15:1 allows the gradual etching of the protective mask until it is completely
removed, resulting in ultrasharp UNCD tips. Although, direct etching often forms feather-like
whiskers during RIE, the whiskers do not affect the overall performance of diamond tips [23].
After tip formation, a 1.5-µm-thick aluminum layer was evaporated and then etched for electrical
contacts. Finally, the silicon handle layer was backside-etched via ICP-RIE to form the handling
chips, and the buried oxide layer was etched with hydrofluoric acid solution, releasing the final
device. Each 100 mm wafer could produce 178 single heated cantilevers and 356 cantilever
arrays with a yield of 80%.
65
Figure 4.2 shows schematic and scanning electron microscope (SEM) images of a heated
AFM cantilever with a UNCD tip. UNCD tips are integrated into the heater region, 14 × 20 µm2
in size, located at the cantilever free end. Fabricated UNCD tips have a typical tip radius of 10
nm and an overall aspect ratio of 4:1 or better. The tip is sharper than diamond-coated tips and
taller than most tips fabricated onto heated AFM cantilevers using other techniques [2, 9], thus
can image taller topography features.
Figure 4.2 Schematic (a) and SEM images (b-d) of silicon heated AFM cantilevers with
integrated solid state heaters and UNCD tips. The cantilevers were batch-fabricated with a
typical tip height of 6 µm and a tip radius of 10 nm.
Figure 4.3 shows an SEM image of an array of heated AFM cantilevers with UNCD tips.
The cantilever array consists of five individually-controllable identical cantilevers, with a tip-to-
tip distance of 110 µm. The cantilevers in the array have nearly identical tip profiles with tip
heights of 6 µm and typical apex radii of 10 nm.
66
Figure 4.3 An SEM image of the heated AFM cantilever array with UNCD tips. An array
consists of five identical heated cantilevers, which are 110 µm apart from each other and
controlled independently. All five cantilevers have similar electrothermal properties over the
range 25 – 1000 ºC.
Figure 4.4 shows a bright-field transmission electron microscope (TEM) image of a
fabricated UNCD tip after the release. Rounded features circled with dotted lines in figure 4(b)
are consistent with grain sizes of UNCD, known to be in the 3 − 10 nm range. Therefore, these
are likely single crystal grains. Typical grain sizes less than 10 nm were observed on UNCD
deposited with the same recipe in molded monolithic UNCD tips [15] and diamond-coated
silicon tips [9]. These RIE-made UNCD tips are similar in sharpness to the best UNCD tips
made by molding [15]. Selected area diffraction in the inset of figure 4.4(a) confirmed that the
tip consists of randomly oriented nanoscale UNCD grains. The image indicates that the UNCD
grains may have been smoothened by the RIE processing, reducing further the low intrinsic
roughness of the UNCD from the as deposited range 30 − 50 nm (rms), to about 13.5 nm (rms).
The later value was obtained by digitizing the profile of the tip presented in figure 4.4(a) and
data analysis This reduction in roughness is consistent with known smoothing action of RIE
67
the initial UNCD
thickness, showing that a sizable amount of UNCD was removed from above the tip, to reach
this shape.
Figure 4.4 Representative TEM images of a UNCD tip. (a) A TEM image of the tip apex. The
inset shows the selected area diffraction pattern of the same tip, which is typical of high-quality
UNCD. (b) A TEM image of UNCD grains (dotted lines) near the tip surface. The typical grain
diameter was measured to be less than 10 nm, with a tip radius of approximately 10 nm.
4. 3 Cantilever Characterization
The cantilever electrical and thermal properties were characterized [24]. The low-doped
heater region dissipates over 90% of total cantilever power and its steady temperature was
calibrated from Stokes peak shift, measured using a Renishaw InVia Raman spectroscopy with a
488 nm argon laser [24]. This Raman microscope has spatial resolution of approximately 1 µm,
accuracy of approximately 1%, and spectral resolution of 0.1 cm-1 [25]. Figure 4.5(a) shows the
cantilever current and the cantilever electrical resistance as functions of cantilever voltage under
steady heating. Figure 5(b) shows the cantilever heater temperature and the cantilever electrical
68
resistance as functions of the electrical power dissipated in the cantilever. The cantilever has a
positive temperature coefficient of resistance (TCR) as the cantilever electrical resistance varies
with temperature due to the temperature-dependent carrier concentration and carrier mobility
[26]. The cantilever can be heated up to 600 °C, above which the diamond tip burns in an
oxidizing environment such as air [22]. A previous publication reported a UNCD film on a
silicon tip completely oxidized and removed at 750 °C [9]. However, below 600 °C the
cantilever could operate for long periods of time with essentially zero detectible wear. All
cantilevers in the cantilever array demonstrated similar electro-thermal properties in the
measured temperature range. The tip-substrate interface temperature strongly depends on the
thermal conductivity of the substrate as the thermal resistance of the tip-substrate contact is much
higher than the thermal resistance of the tip itself [27, 28]. Consequently, the estimated
temperature of the tip-substrate interface ranges from 10% to 90% of the cantilever heater
temperature [27]. The spring constant of 10 different cantilevers was measured to be 0.81 ± 0.42
N/m using a commercial AFM system (MFP-3D, Asylum research) [24]. The uncertainties in
spring constant measurement are due to slight cantilever thickness variations across the wafer
since the cantilever spring constant varies as thickness cubed.
69
Figure 4.5 Representative electrical and thermal characterization of the device. (a) Current and
cantilever resistance as functions of cantilever voltage show the temperature-dependent resistivy
of a doped silicon. (b) Cantilever resistance and heater temperature as functions of the cantilever
power, measured using Raman spectroscopy.
70
A UNCD tip and a silicon tip were scanned with high contact forces for long times to
compare tip wear. In a previous publication, a diamond-coated silicon tip showed negligible
wear under harsh operating conditions of high temperature, high force, high speed, and long
scans [9]. In the present study, tips were scanned for 1.2 m at 10 µm/s speed over 10 × 10 µm2 of
a single-crystal silicon grating with a feature height of 100 nm and a pitch of 3 µm. Following
the protocol established in other wear studies ([8] and [12], among many others), the applied
load was held constant, in this case at 200 nN. The corresponding contact pressure will vary with
tip radius; we estimate it ranges from 8 - 17 GPa based on Hertzian contact mechanics assuming
tip the radius varying from 10 nm (for the upper pressure limit) to 30 nm. The adhesive load will
increase the contact pressure even further. While this is a crude estimate, it nonetheless shows
the tip exhibits low wear despite experiencing very high stresses. Pull-off force measurements
were recorded every 1000 µm of tip travel in order to detect changes associated with tip blunting
or tip fouling.
Figure 4.6 compares the wear of a UNCD tip and a silicon tip. Figure 4.6(a-b) shows
SEM images of the UNCD tip before and after wear testing, resulting in a minor increase in tip
radius from 28.0 ± 3.4 nm to 36.7 ± 3.5 nm. By comparison, figure 4.6(c-d) shows SEM images
of the silicon tip before and after wear testing, revealing major wear and fouling. Figure 4.6(e)
shows pull-off force data for both the UNCD tip and the silicon tip. The UNCD tip pull-off force
increased slightly at the beginning of the test but remained steady at approximately 40 nN,
showing negligible wear throughout the test duration. In contrast, the silicon tip pull-off force
increased dramatically from 20 nN to 200 nN within the first 200 mm of travel. Since pull-off
force correlates to tip radius, this indicates that the silicon tip began significantly wearing at the
beginning of the wear test. The decrease in pull-off force after 200 mm is attributed to the
71
accumulation of surface contamination which is visible in figure 4.6(d). While the fractional
increase in tip radius is larger than was demonstrated for a diamond-coated tip (from 47 nm to 49
nm) under similar testing conditions [9], the present UNCD tip remains sharper after scanning
than the pre-wear diamond-coated tip. The estimated contact pressure ratio of a UNCD tip to a
diamond-coated tip is approximately 1.4:1. These results demonstrate the robustness of UNCD
tips under harsh scanning conditions.
72
Figure 4.6 Wear test comparison between the UNCD tip and the silicon tip. Each tip was
-crystal silicon
grating. SEM images of (a-b) the UNCD tip and (c-d) the silicon tip before and after wear testing.
(e) Pull-off force measurements for both tips performed every 1000 µm.
73
4. 4 Thermal Topography Imaging and Thermal Nanolithography
Wear-resistant UNCD tips expand the uses of heated AFM cantilevers under harsh
conditions. With UNCD tips, cantilevers can scan faster and for longer distances, meaning higher
throughput and longevity for thermal topography imaging and nanofabrication. For material
analysis, tip stability is an important requirement for consistent and repeatable measurements.
Thus integration of wear resistant UNCD tip onto a heated AFM cantilever can improve the
overall performance in such applications. To demonstrate the utility of the UNCD tip, heated
AFM cantilevers with UNCD tips were used for thermal topography imaging and thermal
nanolithography.
Figure 4.7 shows the concept of thermal nanotopography imaging using a heated AFM
cantilever. Most of the heat being generated at the heater region of the cantilever flows into the
substrate such that the heat flow from the cantilever is inversely related to the distance between
the cantilever and the substrate [3]. The substrate topography is measured by tracking changes in
the cantilever power while cantilever temperature is held constant in a regime of positive
temperature coefficient of resistance. The quality of thermal topography imaging is quantified by
topography sensitivity and noise-limited vertical resolution. Thermal topography sensitivity is
defined as the change in cantilever voltage for a unit change in topography. Resolution is defined
as the smallest change in topography that can be detected and is calculated as the thermal signal
noise divided by the sensitivity. In this experiment, the cantilever was mounted in an Asylum
Research MFP-3D AFM with a holder modified to interface with the cantilever’s electrical
contact pads. The cantilever scanned a silicon substrate with 100 nm tall gratings having a pitch
of 3 µm at 10 µm/s. The cantilever scanned in contact mode and in intermittent contact mode at
74
75.67 kHz. The cantilever was operated at around 410 °C with an approximate tip force of 100
nN.
Figures 4.7(b-c) show the topography images of the substrate generated from the
conventional laser-deflection signal and the cantilever voltages for both contact mode and
intermittent contact mode of operation. The thermal topography signals compare well to the
laser-deflection derived height signals and the thermal signals are inverted due to the inverse
relation between the cantilever voltage and the distance between the cantilever and the substrate.
The intermittent contact mode thermal image shows sharp topographical features since the
oscillation frequency is several orders of magnitude higher than the frequency at which the
cantilever scans the grating. The topography sensitivity of the thermal signals is 0.079 ± 0.001
mV/nm via contact mode and 0.060 ± 0.001 mV/nm via intermittent contact mode imaging. The
sensitivity uncertainties are due to voltage measurement uncertainties and environmental
disturbances. The noise limited vertical resolution is 3.154 ± 0.062 nm via contact mode and
1.911 ± 0.036 nm via intermittent contact mode imaging. Overall, the sensitivity and resolution
are comparable to heated AFM cantilevers with silicon tips [3]. Since the lateral resolution
during imaging is directly proportional to the tip radius, and the radius of curvature of the UNCD
tips (10 nm) is smaller when compared to silicon tips (20 nm radius) or diamond-coated silicon
tips (50 nm radius) integrated onto heated cantilevers, the heated cantilevers with UNCD tip will
have higher lateral resolution in thermal imaging.
75
Figure 4.7 Thermal topography sensing using the heated cantilever with a UNCD tip. (a) The
substrate topography is mapped by tracking cantilever thermal conductance changes that occur
due to changes in the distance between the cantilever and the substrate. (b-c) Three-dimensional
topography maps of a 100-nm-tall silicon grating comparing the laser-deflection-based
measurement with the thermally derived signals while the cantilever was operated in both (b)
contact and (c) intermittent contact mode.
76
Figure 4.8(a) shows schematic of a heated tip depositing polymer features using the
thermal Dip Pen Nanolithography (tDPN) method. In tDPN, a heated tip is coated with an ink
which is solid at room temperature. The tip is then placed in contact with a surface and heated,
causing the ink to melt and flow onto the surface. Heated tips have deposited polymers [9],
metals [29], and polymer-nanoparticle composites[30]. The ink flow rate from tip to surface is
sensitive to tip temperature, tip geometry, and tip wettability, since surface tension effects drive
ink flow [31]. While polymer typically wets diamond-coated tips more than silicon tips, it was
unknown if the height of the UNCD tip or the whisker structures formed during UNCD tip
fabrication would significantly impact surface tension driven ink flow.
Figure 4.8(b) shows 100 dots of polyethylene deposited from the UNCD tip. Each feature
was written by dwelling the UNCD tip for 1 sec at a temperature of 200 °C. A topographical
profile image, shown in figure 4.8(c), reveals that the features is approximately 100 nm tall and
approximately 600 nm in diameter. These results are similar to results obtained by depositing
polyethylene dots with a 1- -tall diamond-coated tip, indicating that the tip height and
whisker formations on the tip do not significantly impact polymer mass flow down the tip.
77
Figure 4.8 (a) Schematic of thermal dip-pen nanolithography. (b) An AFM image of 100
polyethylene nanostructures deposited with heated UNCD tips. (c) Topography profile of one
nanostructure. The nanostructure is 100 nm tall and 600 nm in diameter.
Position (m)
He
igh
t(n
m)
0 0.2 0.4 0.6 0.8 10
50
100
150
(c)
(b)
(a)
4 µm
78
4. 5 Conclusion
This work reports design, fabrication, characterization, and applications of heated AFM
microcantilevers with ultrasharp UNCD tips. The UNCD tips were fabricated by direct mask RIE
technique, which is relatively simple and has high throughput he UNCD tip showed almost no
wear after more than a meter of scanning while a silicon tip scanned under the same conditions
was completely blunted with much fouling. We demonstrated the use of UNCD tips for thermal
topographic imaging with nanometer resolution, and also nanolithography of hundreds of
polymer nanostructures at a writing speed of 1 Hz. The UNCD tips with exceptional tip stability
can expand the uses of heated AFM cantilevers to imaging and tip-based nanofabrication under
harsh conditions.
79
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[29] B. A. Nelson, W. P. King, A. R. Laracuente, P. E. Sheehan, and L. J. Whitman, "Direct
deposition of continuous metal nanostructures by thermal dip-pen nanolithography,"
Applied Physics Letters, vol. 88, p. 033104, 2006.
[30] W. K. Lee, Z. T. Dai, W. P. King, and P. E. Sheehan, "Maskless Nanoscale Writing of
Nanoparticle-Polymer Composites and Nanoparticle Assemblies using Thermal
Nanoprobes," Nano Letters, vol. 10, pp. 129, 2010.
[31] J. R. Felts, S. Somnath, R. H. Ewoldt, and W. P. King, "Nanometer-scale flow of molten
polyethylene from a heated atomic force microscope tip," Nanotechnology, vol. 23, p.
215301, 2012.
82
CHAPTER 5: BATCH FABRICATION OF
GRAPHENE-COATED MICROCANTILEVERS
5.1 Introduction
Graphene [1] micro/nano electromechanical systems (MEMS/NEMS) and electronics
have demonstrated superior performance for applications in sensing and signal processing [2, 3].
Although graphene can be grown at large-scale via chemical vapor deposition [4, 5], batch
fabrication of graphene integrated MEMS still remains a challenge. Typically, graphene
integration requires the wet solution transfer of graphene grown on a catalytic metal surface to
the desired substrates [6-8]. Graphene transfer is not suitable for wafer-scale graphene device
fabrication, especially for delicate free-standing or suspended MEMS devices. Graphene growth
using solid carbon sources does not require such a transfer process [9-13], and thus can be
suitable for batch fabrication of graphene integrated micro/nano-devices. This chapter describes
the fabrication of graphene-coated microcantilevers with a transfer-free method, using graphene
growth from a solid carbon source.
Graphene has drawn much attention in recent years with its fascinating electrical and
mechanical properties [1] such as quantum electronic transport [14], a tunable bandgap,
extremely high mobility, and outstanding thermal conductivity [15]. Many graphene synthesis
methods have been developed in recent years, such as the mechanical exfoliation [1], reduction
of graphene oxide [16] or silicon carbide [17] via high temperature annealing, and chemical
vapor deposition (CVD) growth on catalytic metals [4-6]. Among the aforementioned methods,
CVD graphene growth method is most widely used due to its outstanding scalability and film
83
quality at a reasonable price [4, 5]. However, the CVD graphene needs to be transferred to the
desired substrate for graphene device fabrication [6-8]. This transfer process is tedious and
cannot be scaled up to meet industrial application demands. To overcome these issues, several
synthesis methods of transfer-free graphene have been developed using solid carbon sources
such as carbon, self-assembled monolayers, and polymers [9-13]. Such a transfer-free graphene
synthesis method gives a good control over the number of graphene layers and can be scaled up
to large area. Although transfer-free graphene synthesis is a promising fabrication technique for
integrating graphene onto MEMS/NEMS, there is a lack of published research on graphene
device fabrication.
This chapter presents the wafer scale batch fabrication of multilayer graphene-coated
microcantilevers using the transfer-free graphene growth from solid carbon sources along with
conventional cleanroom microfabrication processes.
5.2 Transfer-free Graphene Synthesis
For transfer-free graphene synthesis, two carbon sources of poly(methyl methacrylate)
(PMMA) and amorphous carbon was used. Figure 5.1(a) shows the transfer-free graphene
synthesis process. First, a thin layer of PMMA or amorphous carbon was applied to the clean
silicon dioxide/silicon substrate via spin coating and thermal evaporation, respectively. Then a
200 nm thick nickel layer was deposited via electron beam evaporation as a catalytic metal for
graphene growth. Then the sample went through a high temperature annealing process, as shown
in figure 5.1(b). After the thermal annealing, quick O2 plasma was performed to remove the
graphene film synthesize on topside of the nickel layer. Finally, the nickel layer was wet etched
using nickel etchant TFB (Transene, Inc.) leaving graphene directly on the silicon dioxide layer.
84
Figure 5.1 (a) A schematic diagram of the transfer-free graphene synthesis using solid carbon
sources. (b) Thermal annealing conditions for the transfer-free graphene synthesis.
The effect of various synthesis parameters was studied to optimize the graphene thickness
and quality. Figure 5.2(a) shows Raman spectra of graphene films acquired from two different
solid carbon sources of spin coated PMMA (0.2 wt % in anisole) and 5-nm-thick evaporated
amorphous carbon. Both solid carbon sources resulted in multilayer graphene. However, PMMA
resulted in better graphene quality compared to amorphous carbon-assisted graphene, and thus
PMMA was used as the solid carbon source for subsequent growth study and cantilever
fabrication.
The quality of the graphene film strongly depended on the annealing temperature; the
best quality graphene film, characterized by Raman D-band intensity (at ~1,350 cm-1), was
synthesized at an annealing temperature of 1000 °C, as shown in Fig. 5.2(b). The graphene
quality did not significantly change for various annealing times between 1 – 20 minutes.
However, annealing times longer than 10 minutes resulted in dewetting of the nickel film, and
limited the formation of continuous graphene. We further tested three different cooling rates of
85
natural cooling (~0.1 °C/sec), rapid cooling (~20 °C/sec), and instant cooling (~200 °C/sec). The
rapid cooling resulted in high-quality graphene films, consistent with earlier literature [6]. In
contrast, the natural cooling formed graphene only on the topside of a nickel layer, and the
instant cooling synthesized highly defective graphene.
PMMA concentration in anisole between 0.1 – 0.4 weight percent (wt %) controlled the
amount of the solid carbon source. Figure 5.2(c) shows Raman spectra acquired from graphene
films grown from different PMMA concentrations. The number of synthesized graphene layers
increased with the amount of solid carbon source, characterized by Raman 2D (at ~2680 cm-1) to
G (at ~1590 cm-1) band ratio. PMMA concentration of 0.1 wt % synthesized bilayer graphene;
however, the graphene coating was not continuous. PMMA concentration of 0.2 and 0.4 wt %
resulted in good coverage of high-quality multilayer graphene and graphite, respectively. Figure
5.2(d) shows the 2D-peak to G-peak ratio for different PMMA concentrations. By accurately
controlling the PMMA concentration, the graphene thickness was tuned from two to many
layers.
86
Figure 5.2 Raman spectroscopic analysis of graphene. (a) Raman spectra of multilayer graphene
films grown from different solid carbon sources of PMMA and amorphous carbon. Raman
spectra of multilayer graphene acquired from various (b) annealing temperatures and (c) PMMA
concentrations. (d) I2D/IG of multilayer graphene synthesized from varying PMMA
concentrations.
87
5.3 Cantilever Fabrication
Figure 5.3 shows the process for fabricating the graphene-coated microcantilevers. First,
a 50 nm thick thermal SiO2 layer was grown on a released silicon cantilever to prevent the
silicide formation between silicon and a catalytic metal during high-temperature graphene
synthesis. Then 495K PMMA (0.2 wt % in anisole) was spin coated at 5000 rpm as a solid
carbon source. Next, a 200 nm thick nickel layer was deposited. After being annealed at 1000 °C
for 5 minutes with Ar/H2 gases at 720 mTorr, the sample was rapidly removed from the hot zone
of the annealing tube at a cooling rate of about 20 °C/min. After thermal annealing, 2 minute O2
plasma at 100 W etched graphene film on the topside of the nickel layer. Finally, a nickel wet
etch released the multilayer graphene coated microcantilevers.
Figure 5.3 Fabrication process of graphene-coated microcantilevers. (a) Thermal oxide growth
on a released silicon microcantilever, (b) spin coat PMMA, (c) nickel deposition, (d) high-
temperature annealing, and (e) nickel wet etch.
Using the optimized transfer-free graphene growth process, we fabricated multilayer
graphene coated microcantilevers at wafer-scale. Figure 5.4(a) shows an image of 2 × 2.5 cm
wafer with 26 fully-released cantilever chips. Each chip has 4 microcantilevers in various sizes
of 25-40 µm in width and 200-400 µm in length. The wafer area was limited to 2 × 2.5 cm to fit
the size of the sample stage inside the annealing tube.
88
Figure 5.4 Raman spectroscopy and scanning electron microscopy of graphene coated
microcantilevers. (a) Image of 2×2.5 cm silicon wafer with 26 released cantilever chips. Each
chip has 4 microcantilevers. (b) Raman spectra acquired from 4 different microcantilevers. (c)
SEM image of a graphene coated cantilever. (d) ID/IG and I2D/IG Raman mapping of a cantilever.
89
We analyzed the graphene coverage on microcantilevers using Raman spectroscopy and
scanning electron microscopy (SEM). Figure 5.4(b) shows Raman spectra of four
microcantilevers randomly chosen from the same wafer. Consistent Raman measurements
indicate that both the graphene quality and its thickness are conformal over the entire wafer.
Figure 5.4(c) shows an SEM image of a 40-µm-wide and 275-µm-long graphene coated
microcantilever. Figure 5.4(d) shows Raman mapping results of the same microcantilever. More
than 94% of the cantilever substrates had a continuous coating of multilayer graphene with ID/IG
= 0.18 ± 0.08 and I2D/IG = 0.44 ± 0.25. Raman analysis and SEM of all microcantilevers showed
that more than 75% of cantilevers have a conformal coating of multilayer graphene, which more
than 90% graphene coverage per cantilever. The coverage and the quality of graphene film on
microcantilevers did not change after subsequent DI-water/IPA solution cleaning, demonstrating
that our fabrication approach is highly compatible with conventional microfabrication processes.
We analyzed the graphene coverage on microcantilevers using Raman spectroscopy and
scanning electron microscopy (SEM). Figure 5.4(b) shows Raman spectra of four
microcantilevers randomly chosen from the same wafer. Consistent Raman measurements
indicate that both the graphene quality and its thickness are conformal over the entire wafer.
Figure 5.4(c) shows an SEM image of a 40-µm-wide and 275-µm-long graphene coated
microcantilever. Figure 5.4(d) shows Raman mapping results of the same microcantilever. More
than 94% of the cantilever substrates had a continuous coating of multilayer graphene with ID/IG
= 0.18 ± 0.08 and I2D/IG = 0.44 ± 0.25. Raman analysis and SEM of all microcantilevers showed
that more than 75% of cantilevers have a conformal coating of multilayer graphene, which more
than 90% graphene coverage per cantilever. The coverage and the quality of graphene film on
microcantilevers did not change after subsequent DI-water/IPA solution cleaning, demonstrating
90
that our fabrication approach is highly compatible with conventional microfabrication processes.
5.4 Conclusion
This chapter reports transfer-free graphene synthesis from solid carbon sources and its
application for wafer-scale fabrication of multilayer graphene coated microcantilevers. The
fabrication technique presented here could enable batch fabrication of other types of MEMS and
NEMS, with the key advantage being the ability to fabricate graphene without a transfer process.
91
5.5 References
[1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V.
Grigorieva, and A. A. Firsov, "Electric Field Effect in Atomically Thin Carbon Films,"
Science, vol. 306, pp. 666, October 22, 2004 2004.
[2] J. S. Bunch, A. M. van der Zande, S. S. Verbridge, I. W. Frank, D. M. Tanenbaum, J. M.
Parpia, H. G. Craighead, and P. L. McEuen, "Electromechanical Resonators from
Graphene Sheets," Science, vol. 315, pp. 490, 2007.
[3] Y.-M. Lin, C. Dimitrakopoulos, K. A. Jenkins, D. B. Farmer, H.-Y. Chiu, A. Grill, and P.
Avouris, "100-GHz Transistors from Wafer-Scale Epitaxial Graphene," Science, vol. 327,
p. 662, 2010.
[4] S. Bae, H. Kim, Y. Lee, X. Xu, J.-S. Park, Y. Zheng, J. Balakrishnan, T. Lei, H. Ri Kim,
Y. I. Song, Y.-J. Kim, K. S. Kim, B. Ozyilmaz, J.-H. Ahn, B. H. Hong, and S. Iijima,
"Roll-to-roll production of 30-inch graphene films for transparent electrodes," Nat Nano,
vol. 5, pp. 574, 2010.
[5] X. Li, W. Cai, J. An, S. Kim, J. Nah, D. Yang, R. Piner, A. Velamakanni, I. Jung, E.
Tutuc, S. K. Banerjee, L. Colombo, and R. S. Ruoff, "Large-Area Synthesis of High-
Quality and Uniform Graphene Films on Copper Foils," Science, vol. 324, pp. 1312, June
5, 2009 2009.
[6] K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H. Ahn, P. Kim, J.-Y.
Choi, and B. H. Hong, "Large-scale pattern growth of graphene films for stretchable
transparent electrodes," Nature, vol. 457, pp. 706, 2009.
[7] X. Li, Y. Zhu, W. Cai, M. Borysiak, B. Han, D. Chen, R. D. Piner, L. Colombo, and R. S.
Ruoff, "Transfer of Large-Area Graphene Films for High-Performance Transparent
Conductive Electrodes," Nano Letters, vol. 9, pp. 4359, 2009/12/09 2009.
[8] Y. Lee, S. Bae, H. Jang, S. Jang, S.-E. Zhu, S. H. Sim, Y. I. Song, B. H. Hong, and J.-H.
Ahn, "Wafer-Scale Synthesis and Transfer of Graphene Films," Nano Letters, vol. 10, pp.
490, 2010/02/10 2010.
[9] Z. Yan, Z. Peng, Z. Sun, J. Yao, Y. Zhu, Z. Liu, P. M. Ajayan, and J. M. Tour, "Growth
of Bilayer Graphene on Insulating Substrates," ACS Nano, vol. 5, pp. 8187, 2011.
[10] R. Hirano, K. Matsubara, G. Kalita, Y. Hayashi, and M. Tanemura, "Synthesis of
transfer-free graphene on an insulating substrate using a solid phase reaction," Nanoscale,
vol. 4, pp. 7791, 2012.
[11] J. Kwak, J. H. Chu, J.-K. Choi, S.-D. Park, H. Go, S. Y. Kim, K. Park, S.-D. Kim, Y.-W.
Kim, E. Yoon, S. Kodambaka, and S.-Y. Kwon, "Near room-temperature synthesis of
transfer-free graphene films," Nat Commun, vol. 3, p. 645, 2012.
[12] G. Pan, B. Li, M. Heath, D. Horsell, M. L. Wears, L. Al Taan, and S. Awan, "Transfer-
free growth of graphene on SiO2 insulator substrate from sputtered carbon and nickel
films," Carbon, vol. 65, pp. 349, 2013.
[13] H.-J. Shin, W. M. Choi, S.-M. Yoon, G. H. Han, Y. S. Woo, E. S. Kim, S. J. Chae, X.-S.
Li, A. Benayad, D. D. Loc, F. Gunes, Y. H. Lee, and J.-Y. Choi, "Transfer-Free Growth
of Few-Layer Graphene by Self-Assembled Monolayers," Advanced Materials, vol. 23,
pp. 4392, 2011.
[14] Y. Zhang, Y.-W. Tan, H. L. Stormer, and P. Kim, "Experimental observation of the
quantum Hall effect and Berry's phase in graphene," Nature, vol. 438, pp. 201, 2005.
92
[15] A. K. Geim and K. S. Novoselov, "The rise of graphene," Nat Mater, vol. 6, pp. 183,
2007.
[16] V. C. Tung, M. J. Allen, Y. Yang, and R. B. Kaner, "High-throughput solution processing
of large-scale graphene," Nat Nano, vol. 4, pp. 25, 2009.
[17] C. Berger, Z. Song, X. Li, X. Wu, N. Brown, C. Naud, D. Mayou, T. Li, J. Hass, A. N.
Marchenkov, E. H. Conrad, P. N. First, and W. A. de Heer, "Electronic Confinement and
Coherence in Patterned Epitaxial Graphene," Science, vol. 312, pp. 1191, May 26, 2006
2006.
93
CHAPTER 6: MEASUREMENT OF
TEMPERATURE-DEPENDENT LIQUID VISCOSITY
USING HEATED MICROCANTILEVERS
6.1 Introduction
The atomic force microscope (AFM) [18] has a wide range of applications in the fields of
nanotechnology, including the force measurement and sensor applications [19-21]. AFM
microcantilevers have drawn much attention to rheological measurements on fluids, since the
microcantilever sensors enable rapid and sensitive measurements with a small volume
requirement of fluids [21-23]. Especially, AFM microcantilevers have demonstrated outstanding
performance in measuring the liquid viscosity [21-30]. However, measuring the temperature-
dependent viscosity of the liquid using microcantilevers can be challenging as it uses an external
heating element, and thus requiring major modifications to AFMs [27, 30]. This chapter presents
the direct measurement of the temperature-dependent viscosity of liquids using a heated AFM
microcantilever and a commercially available AFM.
In liquid, the frequency response of the microcantilever depends strongly on the density
and the viscosity of the surrounding medium [22, 31-34]. Especially, analyzing the variations in
resonant frequency of the microcantilever enables the measurement of the viscosity of liquids,
which is an important parameter for many applications that take place in liquid environments
[21-27]. In addition, the microcantilever sensors have numerous advantages over conventional
viscometers, such as high sensitivity and short measurement time while requiring a small liquid
volume [21-23]. However, the viscosity measurement using microcantilevers has limitations in
94
that it requires a) a commercial AFM with major modifications or b) a home built apparatus
similar in design to the AFM [24-28, 35, 36]. Moreover, an external heater, such as the Peltier
element and a micro-fabricated resistive heater, is required to measure the temperature-
dependent viscosity of liquid, which is a crucial property of liquids for biological and heat
transfer applications [27, 30].
AFM microcantilevers with an integrated heater-thermometer can generate and sense the
nanometer scale heat [37]. Heated AFM microcantilevers have many applications, such as data
storage, heat flow measurements, nanomanufacturing, material characterization, and
nanoimaging [37]. Although heated microcantilevers have a wide range of applications, the
microcantilever is seldom used in the liquid medium. Previous research reported characterization
and calibration of heated microcantilevers in deionized (DI) water [38]. Another work used the
heated microcantilever as a micro-sized heat source to study the fluid convection around the
cantilever [39]. However, heated microcantilevers were not used for sensing or material
characterization applications in the liquid environment, majorly due to the lack of understanding
in dynamic response of the heated microcantilever immersed in liquid. Thus it is necessary to
develop a theoretical model that predicts the dynamic response of the heated microcantilever in a
viscous fluid.
This chapter presents novel metrology applications of the heated microcantilever to
measure temperature-dependent viscosity of liquids. In addition, this work develops a finite-
difference model that calculates the frequency response of the heated microcantilever in various
liquids and cantilever heater temperatures.
95
6.2 Experiment
Figure 6.1(a) shows an Scanning Electron Microscope (SEM) image of the heated
microcantilever used in this paper. The cantilever is U-shaped and fabricated from single-crystal
doped silicon. The cantilever free end is lightly-doped (~1017cm3) and the cantilever legs are
heavily-doped (~1020 cm3) with phosphorus. The selective doping of the cantilever allows the
lightly-doped cantilever heater to dissipate more than 95% of the total cantilever power. The
spring constant of the cantilever is about 0.7 N/m and the resonant frequency in air is 107 kHz.
Detailed information on the design, fabrication, and characterization of heated microcantilevers
has been reported elsewhere [37, 40]. Figure 6.1(b) shows a circuit used to operate heated
microcantilevers. The circuit consists of a heated microcantilever, sense resistor (2 kΩ), and a
DC or an AC power supply. Measuring the voltage drop across the sense resistor enables the
calibration of the electrical resistance of the cantilever, which is a function of the heater
temperature [40, 41].
We studied the dynamic response and thermal operation of the heated microcantilever in
aqueous solutions of DI water, 50 wt % ethylene glycol/water (EG 50%), and ethylene glycol
(EG 100%). Figure 6.1(c) shows the experimental setup for the cantilever operation in liquid.
The heated microcantilever was mounted in a droplet holder kit of a commercial AFM (Asylum
Research Cypher S). A gap between the flat glass on the droplet holder and the silicon substrate
formed a liquid meniscus, where the cantilever was fully immersed and placed 300 µm above the
substrate. A built-in laser and optical components measured the cantilever deflection. A
permanent magnet beneath the silicon substrate generated a magnetic field, which actuated the
AC heated cantilever via Lorentz force [33, 42]. In water, using the Lorentz actuation generated
clearer resonant peak at 39.35 kHz compared to using the piezoelectric shaker in the AFM, as
96
shown in figure 6.1(d). This is because the Lorentz actuation provided a magnetic force directly
on the cantilever free end, and thus suitable for actuating the heated microcantilever in viscous
liquid. All experiments were carried out in ambient conditions, with the room temperature of
about 20 °C.
Figure 6.1 The operation of heated microcantilever operation in liquid. (a) Scanning electron
microscope (SEM) micrograph of the heated microcantilever used in the experiment. (b) A
circuit used for the cantilever operation. (c) The schematic of the experiment. An applied AC
voltage in the presence of an external magnetic field simultaneously heats and actuates the
microcantilever, which is placed inside the liquid. (d) Cantilever responses in water when
actuated by either piezoelectric shaker or Lorentz actuation. The Lorentz actuation results a
clearer response than piezoelectric actuation.
Magnet
Glass
Silicon
HeatedCantilever
Laser
Liquid
Photodiode
ACSource
50 µm
Frequency (kHz)
No
nd
ime
ns
ion
alA
mp
litu
de
20 30 40 50 600.0
0.2
0.4
0.6
0.8
1.0
1.2
Piezo-actuated
f0
= 38.39 kHz
Frequency (kHz)
No
nd
ime
ns
ion
alA
mp
litu
de
20 30 40 50 600.0
0.2
0.4
0.6
0.8
1.0
1.2
Lorentz-actuated
f0
= 39.35 kHz
Q = 2.7
Water
RSense
HeatedCantilever
V
(a)
(c)
(b)
(d)
97
We characterized the electro-thermal properties of the cantilever in air and liquid [38, 40].
Figure 6.2(a) shows the cantilever heater temperature as a function of the applied DC voltage.
For DC heating, the cantilever heater temperature is calibrated from the Stokes peak position
using a Raman spectroscopy [40]. Figure 6.2(b) shows the steady heater temperature as a
function of the applied AC voltage amplitude. A digital function generator applied a sine wave at
105 kHz, which is the resonant frequency of the cantilever in air. For AC heating, the steady
cantilever temperature is determined from the measured cantilever resistance using an
oscilloscope. The inset in figure 6.2(b) shows a periodic temperature of the cantilever heater as a
function of amplitude of the driving AC voltage. The periodic temperature of the heated
cantilever is measured using the 3ω method [43]. The periodic temperature is small, less than
2 °C in liquid, as the thermal time constant of the cantilever is in the order of 100 µsec [40]. For
both DC and AC heating, the heat transfer from the cantilever depends on the thermal properties
of the surrounding medium. In air, the temperature rise of the cantilever heater is larger
compared to liquid as the thermal conductivity of air is one order of magnitude smaller than that
of water and ethylene glycol solutions.
98
Figure 6.2 Electro-thermal characterization of the heated microcantilever in air and liquid. (a)
Cantilever heater temperature as a function of the applied DC voltage. (b) Cantilever heater
temperature as a function of the applied AC voltage. The inset shows the periodic temperature of
the cantilever heater. At the same input voltage, the rise of the cantilever heater temperature in
air is much larger compared to liquid for both DC and AC heating schemes.
Voltage Amplitude (V)
Ste
ad
yH
ea
ter
Te
mp
era
ture
(oC
)
0 2 4 6 8 10 120
50
100
150
200
250
300
Water
Air
EG 100%
EG 50%
AC Heating (105 kHz)
Voltage Amplitude (V)
Pe
rio
dic
Te
mp
era
ture
(oC
)
0 2 4 6 8 10 12-1
0
1
2
3
4
5
Total Voltage (V)
Ca
nti
lev
er
He
ate
rT
em
pe
ratu
re(o
C)
0 1 2 3 4 5 60
50
100
150
200
250
300
Air
Water
EG 50%
EG 100%
DC Heating
(a)
(b)
99
6.3 Dynamic Response Modeling
The frequency response of an oscillating microcantilever is very sensitive to the
properties of the surrounding fluid [21, 22, 31]. Especially, the resonant frequency of the
cantilever largely depends on the liquid viscosity (η) and is a good parameter to study of the
viscous damping acting on the cantilever [21, 23]. In this section, we develop a theoretical model
that analyzes the dynamic response of the cantilever immersed in liquid. In more detail, we
implement the Viscous Model developed by Sader et al. [31] and modify the model to account
for the local heating of the cantilever.
Figure 6.3 Diagram of the one-dimensional finite difference model showing nodes, vertical
displacement (y), and applied forces (Fh : hydrodynamic damping force, Fm : magnetic force, Fs :
shear force). The model predicts the dynamic response of the cantilever.
L2
w1
L1
w2
b
CantileverHeater
Cantilever Leg
yi ynyi+1yi-1y0
Fm
y
x
Fh,i
Δx Fs,i+
Fs,i-
100
Figure 6.3 shows the one-dimensional finite difference model (FDM) used to formulate
the dynamic response of the cantilever. The U-shaped cantilever is modeled as a rectangular
beam that consists of the cantilever leg and the cantilever heater. The governing equation for the
cantilever beam oscillation in viscous fluid can be expressed as [31]
𝐸𝐼𝑑4𝑦(𝑥,𝑡)
𝑑𝑥4+ 𝜌𝑤𝑏
𝑑2𝑦(𝑥,𝑡)
𝑑𝑡2= 𝐹(𝑥, 𝑡) (6.1)
where the first term represents the reaction force due to the shear forces (Fs) and the second term
represents the cantilever mass force acting on each node. For Eq. (6.1), y(x,t) is the deflection, w
is the width, and b is the thickness, E is the Young’s modulus, I = wb3/12 is the moment of
inertia, and ρ is the density of the cantilever. Table 6.1 presents the dimensions and material
properties of the cantilever. F(x,t) is the external force acting on each node of the cantilever, and
consists of the hydrodynamic force (Fh), the magnetic driving force (Fm), and the tip-substrate
interaction force (Ftip). Thus, F(x,t) can be expressed as
𝐹(𝑥, 𝑡) = 𝐹ℎ(𝑥, 𝑡) + 𝐹𝑚(𝑥, 𝑡) + 𝐹𝑡𝑖𝑝(𝑥, 𝑡) (6.2)
For the heated cantilever, we assume that Fm only applies to the end of the cantilever, the last
node in our FDM model, and neglect Ftip because the cantilever is placed far enough (300 µm)
from the substrate [44]. Fh can be approximated as [44, 45]
𝐹ℎ(𝑥, 𝑡) = −𝛼1𝑑2𝑦(𝑥,𝑡)
𝑑𝑡2− 𝛼2
𝑑𝑦(𝑥,𝑡)
𝑑𝑡 (6.3)
where the first term represents the force exerted by the additional fluid mass moving along with
the cantilever and the second term represents the viscous damping force of the liquid. In equation
3, α1 is the added fluid mass per unit length and α2 is the liquid damping coefficient. For α1 and
α2, we used the expression derived by Hosaka et al. [46], which considers a rectangular cantilever
beam as a string of spheres with the diameter that is same to the width of the cantilever..
𝛼1 =1
12𝜋𝜌𝑙𝑖𝑞𝑤
2 +3
4𝜋𝑤√2𝜌𝑙𝑖𝑞𝜂 𝜔⁄ (6.4)
101
𝛼2 = 3𝜋𝜂 +3
4𝜋𝑤√2𝜌𝑙𝑖𝑞𝜂𝜔𝑟 (6.5)
where ρliq is the density and η is the dynamic viscosity of the liquid, and ω is the resonating
frequency and ωr is the resonant frequency of the cantilever. Using Eq. (6.1) through (6.5), the
governing equation now becomes
𝐸𝐼𝑑4𝑦(𝑥,𝑡)
𝑑𝑥4+ 𝛼2
𝑑𝑦(𝑥,𝑡)
𝑑𝑡+ (𝜌𝑤𝑏 + 𝛼1)
𝑑2𝑦(𝑥,𝑡)
𝑑𝑡2= 0 (6.6)
The magnetic actuation force is sinusoidal, so we assume a sinusoidal solution of the form
𝑦(𝑥, 𝑡) = 𝑌(𝑥) · 𝑒−𝑖𝜔𝑡 (6.7)
Finally, we get the governing equation in the frequency domain as
𝐸𝐼𝑑4𝑌(𝑥)
𝑑𝑥4+ 𝛼2𝑖𝜔𝑌(𝑥) − (𝜌𝑤𝑡 + 𝛼1)𝜔
2𝑌(𝑥) = 0 (6.8)
where the magnitude of Y(x) gives the vertical displacement of the cantilever, y(x). The boundary
conditions for Eq. (6.8) are the usual clamped-free conditions.
Table 6.1 Dimensions and material properties of the heated microcantilever
Parameter Heated microcantilever
Leg length (L1) 113 µm
Leg width (w1) 37 µm
Heater length (w1) 30 µm
Heater width (w1) 18 µm
Thickness (b1) 1.6 µm
Density (ρ) 2330 kg/m3
Young’s Modulus (E) 169 GPa
102
Our FDM model can account for different cantilever dimensions and liquid properties for
each node, and thus can model the dynamic response of the heated microcantilever at different
liquid temperatures. From Eq. (6.8), the only parameters changing with the liquid temperature
are ρliq and η. However, the change in ρliq is small, i.e. from 20 °C to 50 °C , the change in ρliq is
only about 1% and 2% for water and ethylene glycol, respectively. Therefore, our model uses ρliq
at the temperature of 20 °C for each liquid throughout the modeling. Table 6.2 presents the
properties of the tested liquids at different temperatures [47]. When the cantilever is heated, the
temperature of the liquid around the cantilever heater and the cantilever leg are different. Figure
6.4 shows the heat diffusion from the heated cantilever to the surrounding water at the heater
temperature of 50 °C. The temperature drops sharply at the heater-leg junction, as much heat
from the cantilever heater flows to the surrounding liquid medium rather than the cantilever legs.
Thus, for FDM, we apply only two different liquid temperatures, one for the cantilever heater
and one for the cantilever leg, and ignore the temperature transition across the heater-leg junction.
For calculation, the variation in liquid temperature in vertical direction from the cantilever is
neglected as the oscillation amplitude of the cantilever is very small, in the order of 1 – 10 nm.
Table 6.2 Properties of the tested liquid mediums
DI Water EG 50% EG 100%
at 20 °C at 50 °C at 20 °C at 50 °C at 20 °C at 50 °C
Viscosity, η (kgm-1s-1) 0.0010 0.0005 0.0039 0.0017 0.0213 0.0075
Density, ρliq (kg/m3) 998 988 1056 1039 1116 1095
Thermal conductivity, k
(W/mK) 0.604 0.644 0.426 0.449 0.250 0.258
Viscous penetration
depth at 30 kHz, δ (µm) 3.3 2.3 6.3 4.2 14.2 8.5
103
Figure 6.4 Heat diffusion from the heated microcantilever to the surrounding water at the heater
temperature of 50 °C. The temperature drops sharply from the cantilever heater to cantilever legs,
because much heat from the cantilever heater dissipates to the surrounding liquid.
6.4 Results and Discussion
Figure 6.5(a) shows both measurement and prediction of the oscillation amplitude of the
cantilever as a function of the oscillation frequency in ambient environment. The peak amplitude
of the measurement is normalized with respect to the peak amplitude of the prediction. Applied
AC voltage amplitude was 1 V, which is enough to actuate the cantilever magnetically but
induces less than 1 °C rise in cantilever heater temperature. The inset shows the dynamic
response of the cantilever in air. The cantilever thickness in the model is fitted to match the
resonant frequency of the cantilever in air at 107 kHz, and is used for all other calculations in
this paper. The measurement shows that the resonant frequency of the cantilever largely depends
on the viscosity of the medium. Both the resonant frequency and the peak oscillation amplitude
104
increases as the liquid viscosity decreases. However, the percentage error between the measured
and the predicted resonant frequencies of the cantilever increased with the liquid viscosity; and
reached 40% for EG 100% solution.
We also studied the cantilever response at elevated liquid temperatures by heating the
liquid medium using an external Peltier element, which was placed between the magnet and the
silicon substrate. Figure 6.5(b) shows the dynamic response of the cantilever as a function of the
cantilever oscillation frequency at the liquid temperatures of 25 and 40 °C. For all solutions, both
the resonant frequency of the cantilever and the peak amplitude of the oscillation increased with
the temperature of the liquid medium. Figure 6.5(c) shows the shift in resonant frequency of the
cantilever as a function of the liquid viscosity. For each solution, we measured the resonant
frequency of the cantilever at five different liquid temperatures of 25, 30, 35, 40, and 45 °C. The
measurement and the model matched well for water and EG 50% solution (η = 0.0005 – 0.005
kgm-1s-1). In contrast, the measurement of the cantilever resonant frequency was about 40%
larger than the predicted value for EG 100% solution. Previous study [23] suggests that stiffer
cantilevers could be more sensitive to the change in viscosity at higher ranges. The cantilever
used in this paper would be suitable for measuring the variation in liquid viscosity of 0.005 kgm-
1s-1 or lower.
105
Figure 6.5 Dynamic response of the microcantilever in water and ethylene glycol solutions. (a)
Cantilever oscillation amplitude as a function of frequency. The inset shows the dynamic
response of the cantilever in air. (b) Cantilever oscillation amplitude as a function of frequency at
the elevated temperatures. The entire liquid was heated using a Peltier element. (c) Measured and
predicted resonant frequency of the cantilever as a function of the liquid dynamic viscosity. For
each liquid, the measurement was made at five different liquid temperatures of 25, 30, 35, 40,
and 45 °C.
Measurement
Model
Frequency (kHz)
Os
cilla
tio
nA
mp
litu
de
(a.u
.)
0 20 40 60 80 100 1200.0
0.2
0.4
0.6
0.8
1.0
1.2
EG 100% (40oC)
Water (40oC)
Water (25oC)
EG 50% (25oC)
EG 50% (40oC)
EG 100% (25oC)
Frequency (kHz)
Am
plitu
de
(a.u
.)
0 20 40 60 80 100 1200
1
2
3
4
5
EG 100%
Water
EG 50%
Measurement
Model
Frequency (kHz)
Os
cilla
tio
nA
mp
litu
de
(a.u
.)
0 20 40 60 80 100 1200.0
0.2
0.4
0.6
0.8
1.0
1.2
EG 100% (40oC)
Water (40oC)
Water (25oC)
EG 50% (25oC)
EG 50% (40oC)
EG 100% (25oC)
Frequency (kHz)
Am
plitu
de
(a.u
.)
90 100 110 1200
50
100
150
AirMeasurement
Model
Frequency (kHz)
Os
cilla
tio
nA
mp
litu
de
(a.u
.)
0 20 40 60 80 100 1200.0
0.2
0.4
0.6
0.8
1.0
1.2
EG 100% (40oC)
Water (40oC)
Water (25oC)
EG 50% (25oC)
EG 50% (40oC)
EG 100% (25oC)
Measurement
Model
Measurement
Model
Measurement
Model
Dynamic Viscosity (kgm-1s
-1)
Re
so
na
nt
Fre
qu
en
cy
(kH
z)
0.000 0.005 0.010 0.015 0.0200
10
20
30
40
50
EG 100%
Water
EG 50%
(a)
(b)
(c)
106
Heating the cantilever in liquid shifted the resonant frequency of the cantilever. Figure
6.6 shows the frequency response of the heated microcantilever in water, EG 50% and EG 100%
solutions at input AC voltage amplitudes ranging from 1 – 7 V. The heating from the cantilever
was the only heat source. For all mediums, the resonant frequency of the cantilever increased
with the heater temperature. In addition, the oscillation amplitude of the cantilever increased
with the input AC voltage, as the magnetic driving force is a linear function of the applied
current.
The FDM verifies our measurements. Figure 6.7(a) shows the frequency response of the
cantilever in EG 50% solution at selected AC voltage amplitudes. The peak amplitude of the
measurement is normalized with respect to the peak amplitude of the prediction. For EG 50%
solution, the measurement and the prediction of the resonant frequency of the cantilever agree to
within 7%. Figure 6.7(b) shows measurements and predictions of the resonant frequency of the
cantilever as a function of the cantilever heater temperature. When the cantilever heater
temperature increases from 21°C to 50 °C, the resonant frequency of the cantilever increases by
4%, 12%, and 26% for water, EG 50%, and EG 100% solutions, respectively. The results show
that the cantilever heating induces significant change in the dynamic response of the cantilever.
Figure 6.8(a) shows the measured and the theoretical value of liquid viscosities as a
function of the liquid temperature. The measured value of the liquid viscosity is determined by
fitting the viscosity used in the model, so that the calculated and the measured resonant
frequencies match. Figure 6.8(b) shows the percentage error of our viscosity measurements as a
function of the liquid temperatures ranging from 20 to about 50 °C. The average of the error is
about 7%, 9%, and 32% for water, EG 50%, EG 100% solutions, respectively. Although our
measurement does not fit well for EG 100% solution, the error is less than 10% for water and EG
107
50% solution, demonstrating that the heated microcantilever is suitable for measuring the liquid
viscosities over a wide range of liquid temperatures. We assume the error in measurement is
greater for more viscous fluid as the viscous penetration depth (𝛿 = √2𝜂 𝜔𝜌⁄ ) increases with the
liquid viscosity, and thus the temperature variation away from the cantilever become more
important for highly viscous fluids. The heated microcantilever used in this paper is suited for
measuring liquid viscosities ranging from 0.0005 to 0.005 kgm-1s-1.
108
Figure 6.6 Dynamic response of the cantilever in liquids, when the cantilever is heated.
Cantilever oscillation amplitude as a function of frequency in (a) water and (b) ethylene glycol
solutions. The oscillation amplitude increases almost linearly to the applied AC voltage
amplitude, because the Lorentz force is a linear function of the applied current.
Frequency (kHz)
Os
cilla
tio
nA
mp
litu
de
(nm
)
0 10 20 30 40 50 600
5
10
15
20
25
30
7 V (48oC)
6 V (40oC)
5 V (34oC)
4 V (29oC)
3 V (26oC)
2 V (23oC)
1 V (21oC)
Water
Frequency (kHz)
Os
cilla
tio
nA
mp
litu
de
(nm
)
-20 0 20 40 600
5
10
15
20
EG 100%
7 V (55oC)
6 V (44oC)
5 V (36oC)
4 V (31oC)
3 V (27oC)
2 V (23oC)
1 V (21oC)
Ethylene Glycol
Frequency (kHz)
Os
cilla
tio
nA
mp
litu
de
(nm
)
-20 0 20 40 600
5
10
15
20
EG 50%
7 V (52oC)
6 V (44oC)
5 V (36oC)
4 V (30oC)
3 V (26oC)
2 V (23oC)
1 V (21oC)
(a)
(b)
109
Figure 6.7 (a) Dynamic response of the cantilever in EG 50% solution when the cantilever is
heated. The measurement and the FDM calculation show good agreement. (b) Measured and
predicted resonant frequencies of the cantilever as a function of the cantilever heater temperature.
Frequency (kHz)
No
rma
lize
dA
mp
litu
de
0 10 20 30 40 500.5
1.0
1.5
2.0
7 V (52oC)
EG 50%
1 V (21oC)
3 V (26oC)
5 V (36oC)
6 V (44oC)
Measurement
Model
Cantilever Heater Temperature (oC)
Re
so
na
nt
Fre
qu
en
cy
(kH
z)
10 20 30 40 50 600
10
20
30
40
50
EG 100%
Water
EG 50%
Measurement
Model
(a)
(b)
110
Figure 6.8 (a) Measured and theoretical values of the liquid dynamic viscosities as a function of
the liquid temperature. The measured value of the liquid viscosity is determined by fitting the
liquid viscosities used in the model to the measured resonant frequencies for each cantilever
heater temperature. (b) Error of the measured viscosity in comparison to the theoretical value of
liquid viscosities as a function of the liquid temperature.
(a)
(b)
Temperature (oC)
Dy
na
mic
Vis
co
sit
y(k
gm
-1s
-1)
10 20 30 40 50 6010
-4
10-3
10-2
Water
EG 100%
EG 50%
Temperature (oC)
Err
or
inV
isc
os
ity
(%)
10 20 30 40 50 600
10
20
30
40
50
Water
EG 100%
EG 50%
Measurement
Theory
Measurement
Linear fit
111
6.5 Conclusion
We report on a detailed analysis on dynamic response of the heated microcantilever
immersed in viscous liquid, and the application of the cantilever to measure temperature-
dependent liquid viscosities. The finite-difference model predicts the frequency response of the
cantilever when the entire liquid medium is heated or when only the cantilever is heated. Our
measurement fits well with the model for the liquid viscosity range of 0.0005 – 0.005 kgm-1s-1.
The model is used to calibrate the liquid viscosity from the measured variations in the resonant
frequency of the cantilever at different heater temperatures. This measurement technique could
pave the way for a fast and simple analysis of the temperature-dependent liquid properties
without any modification to commercially available AFMs.
112
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115
CHAPTER 7: CONCLUSIONS AND FUTURE WORK
This dissertation develops novel AFM microcantilevers via the integration of advanced
materials along with detailed studies on cantilever thermal operations. In more detail, the present
work develops three novel microcantilevers: heated microcantilever arrays, diamond tip
microcantilevers, and graphene coated microcantilevers. In addition, this work investigates the
thermal crosstalk within heated microcantilever arrays and the reliability of heated
microcantilevers in extreme heating conditions. Lastly, we investigate the dynamic response of
the heated microcantilever immersed in viscous fluid, and develop a novel metrology application
that measures the temperature-dependent liquid viscosity.
In chapter 1, we designed, fabricated and characterized the array of five heated
microcantilevers [1]. Thermal crosstalk was analyzed under steady or pulse heating, while the
cantilever array is either in contact with a substrate or freely suspended in air. In all operating
conditions, the temperature rise of the unheated cantilever was proportional to the heated
cantilever heater temperature. In addition, thermal crosstalk was less significant for operation
near the substrate compared to away from the substrate. Thermoelectric and electrical couplings
were present, but had no significance in the array thermal operation. We also predicted thermal
crosstalk as functions of various array dimensions and the results can be used in designing the
future heated cantilever arrays [2]. By understanding and correcting for this thermal crosstalk, it
is possible to increase the accuracy of temperature calibration in heated cantilever arrays.
Chapter 2 reports on a detailed study of long term operation and reliability of silicon
heated cantilevers. High heating temperatures of above 800 °C and long heating times induce the
diffusion of phosphorus dopants across the cantilever heater-leg junction, and thus causing the
116
undesired shift in cantilever properties. At high heating voltages, the localized heating at the
cantilever heater-leg junction induces an irreversible breakage of the cantilever. For the high
temperature and long heating duration applications, the changes in cantilever properties should
be closely monitored and recalibrated if necessary to achieve the highest accuracy possible in
temperature control and temperature measurements. Lastly, the findings in this chapter can be
applied to understand the reliability of other types of MEMS with a doped silicon heater-
thermometer .
Chapter 3 demonstrated design, fabrication, characterization, and applications of heated
AFM microcantilevers with ultrasharp UNCD tips [3]. The UNCD tips were fabricated by direct
mask RIE technique [4], which is relatively simple and has high throughput he UNCD tip
showed almost no wear after more than a meter of scanning while a silicon tip scanned under the
same conditions was completely blunted with much fouling. We demonstrated the use of UNCD
tips for thermal topographic imaging with nanometer resolution, and also nanolithography of
hundreds of polymer nanostructures at a writing speed of 1 Hz. The UNCD tips with exceptional
tip stability can expand the uses of heated AFM cantilevers to imaging and tip-based
nanofabrication under harsh conditions.
Chapter 4 reports transfer-free graphene synthesis from solid carbon sources and its
application for wafer-scale fabrication of multilayer graphene coated microcantilevers. The
fabrication technique presented here could enable batch fabrication of other types of MEMS and
NEMS, with the key advantage being the ability to fabricate graphene without a transfer process.
Chapter 5 reports on a detailed analysis on dynamic response of the heated
microcantilever immersed in viscous liquid, and the application of the cantilever to measure
temperature-dependent liquid viscosities. The finite-difference model predicts the frequency
117
response of the cantilever when the entire liquid medium is heated or when only the cantilever is
heated. Our measurement fits well with the model for the liquid viscosity range of 0.0005 –
0.005 kgm-1s-1. The model is used to calibrate the liquid viscosity from the measured variations
in the resonant frequency of the cantilever at different heater temperatures. This measurement
technique could pave the way for a fast and simple analysis of the temperature-dependent liquid
properties without any modification to commercially available AFMs.
7.1 Future Work
7.1.1 Thermal Crosstalk in Heated Microcantilever Arrays
The novel heated microcantilever array and the thermal crosstalk analysis and presented
in this dissertation could improve the throughput in many AFM applications. In particular, the
heated microcantilever array can be used for parallel thermal topography imaging and
nanofabrication [5, 6]. In addition, the array can also be used to batch fabricate nanodevices via
thermochemical nanolithography (TCNL) [6-9] and thermal Dip Pen Nanolithography (tDPN)
[10-12], and map material properties rapidly [13].
The number of heated microcantilevers in an array could be increased to achieve higher
throughput in AFM operations. The recent generation of the heated cantilever array carries 30
individually addressable cantilevers [2]. This array demonstrates more than two orders of
magnitude improvement in thermal imaging throughput compared to conventional AFM
measurements.
118
7.1.2 A Study of Long Term Operation and Reliability of Heated Microcantilevers
Improving the cantilever reliability can enable long term use of heated microcantilevers
for many applications, specifically for applications that require a high cantilever heater
temperatures, above 1000 °C. Such applications include graphene electronics fabrication, metal
deposition, and material synthesis[9, 14, 15]. For the next generation of heated cantilevers, few
design modifications could improve the cantilever reliability. First, increasing the volume of
low-doped heater region could alleviate the effect of the dopant diffusion on the overall
cantilever electro-thermal properties. For example, doubling the heater volume, while
maintaining the constant cross sectional area of the heater-leg junction, would nearly halve the
change in overall cantilever electrical resistances. This is because the amount of diffused dopants
is the functions of heater temperatures and heating times not the low-doped heater volume. In
addition, decreasing the cross section area of the heater-leg junction, or narrowing a thermal
constriction, could limit the amount of diffused dopants. Since the dopant diffusion across the
cantilever is one-dimensional, the amount of dopants diffuse into the heater depends on the cross
section area of the heater-leg junction.
7.1.3 Diamond Tip Heated Microcantilevers
This work presents the integration of the ultrasharp UNCD tip onto a heated cantilever.
The UNCD tip is used for thermal imaging and tip-based nanofabrication. In future, heated
microcantilevers with the UNCD tip can be used for many novel applications. Doping UNCD
with nitrogen during chemical vapor deposition makes the UNCD to become conductive [16]. A
conducting UNCD tip on a heated microcantilever can be used for making simultaneous electro-
thermal measurements [17]. The amount of electron field emission from UNCD tips depends on
119
the tip aspect ratio [18], which can be controlled using the tip fabrication process described in
this dissertation. Thus the correlation between the tip aspect ratio and the electron field emission
can be studied at various cantilever heater temperatures. Lastly, the UNCD tips can be
functionalized to become more hydrophobic or hydrophilic [19], and thus can be used for
biological or chemical applications.
7.1.4 Graphene-Coated Microcantilevers
This work presents a scalable synthesis of transfer-free multilayer graphene and its
application for the batch fabrication of graphene-coated microcantilevers. This synthesis process
can be suitable for the direct integration of graphene on micro/nano-devices. Heated
microcantilevers can synthesis carbon nanotubes in room temperature, using the cantilever heater
as the only heat source [14]. Similarly, heated microcantilevers can be used for transfer-free
graphene synthesis, and thus coating the cantilever tip with graphene. Such graphene-coated tips
can be used to study mechanical properties of graphene, and to make electrical measurements.
7.1.5 Direct Measurement of Temperature-Dependent Liquid Viscosities using Heated
Microcantilevers
This work develops a model that predicts the frequency response of the heated
microcantilever in viscous liquid. The models along with measurements indicate that the
cantilever heating induces significant change in the dynamic response of the cantilever. This
information can be used for the accurate mechanical and thermal operation of heated
microcantilevers in liquid environments for thermal imaging, and material characterization
applications. Moreover, the heated microcantilever can be used to study the temperature
120
dependency of various liquid properties. Microcantilevers have been used for the measurement
of not only the viscosity, but also the density, the local concentration, the temperature , and the
velocity of the liquid [20]. Heated microcantilevers can be used to study the temperature-
dependency of all aforementioned parameters of liquids.
121
7.2 References
[1] H. J. Kim, Z. Dai, and W. P. King, "Thermal crosstalk in heated microcantilever arrays,"
Journal of Micromechanics and Microengineering, vol. 23, p. 025001, 2013.
[2] M. Seong, S. Somnath, H. J. Kim, and W. P. King, "Parallel nanoimaging using an array
of 30 heated microcantilevers," RSC Advances, vol. 4, pp. 24747, 2014.
[3] H. J. Kim, N. Moldovan, J. R. Felts, S. Somnath, Z. Dai, T. D. B. Jacobs, R. W. Carpick,
J. A. Carlisle, and W. P. King, "Ultrananocrystalline diamond tip integrated onto a heated
atomic force microscope cantilever," Nanotechnology, vol. 23, p. 495302, 2012.
[4] N. Moldovan, R. Divan, H. Zeng, and J. A. Carlisle, "Nanofabrication of sharp diamond
tips by e-beam lithography and inductively coupled plasma reactive ion etching," J. Vac.
Sci. Technol. B, vol. 27, pp. 3125, 2009.
[5] S. Somnath, H. J. Kim, H. Hu, and W. P. King, "Parallel nanoimaging and
nanolithography using a heated microcantilever array," Nanotechnology, vol. 25, p.
014001, 2014.
[6] K. M. Carroll, X. Lu, S. Kim, Y. Gao, H.-J. Kim, S. Somnath, L. Polloni, R. Sordan, W.
P. King, J. E. Curtis, and E. Riedo, "Parallelization of thermochemical nanolithography,"
Nanoscale, vol. 6, pp. 1299, 2014.
[7] D. Wang, V. K. Kodali, W. D. Underwood Ii, J. E. Jarvholm, T. Okada, S. C. Jones, M.
Rumi, Z. Dai, W. P. King, S. R. Marder, J. E. Curtis, and E. Riedo, "Thermochemical
Nanolithography of Multifunctional Nanotemplates for Assembling Nano-Objects,"
Advanced functional materials, vol. 19, pp. 3696, 2009.
[8] R. Szoszkiewicz, T. Okada, S. C. Jones, T.-D. Li, W. P. King, S. R. Marder, and E. Riedo,
"High-Speed, Sub-15 nm Feature Size Thermochemical Nanolithography," Nano Letters,
vol. 7, pp. 1064, 2007/04/01 2007.
[9] Z. Wei, D. Wang, S. Kim, S.-Y. Kim, Y. Hu, M. K. Yakes, A. R. Laracuente, Z. Dai, S.
R. Marder, C. Berger, W. P. King, W. A. de Heer, P. E. Sheehan, and E. Riedo,
"Nanoscale Tunable Reduction of Graphene Oxide for Graphene Electronics," Science,
vol. 328, pp. 1373, 2010.
[10] B. Nelson, W. King, A. Laracuente, P. Sheehan, and L. Whitman, "Direct deposition of
continuous metal nanostructures by thermal dip-pen nanolithography," Applied Physics
Letters, vol. 88, p. 033104, 2006.
[11] P. Sheehan, L. Whitman, W. King, and B. Nelson, "Nanoscale deposition of solid inks
via thermal dip pen nanolithography," Applied Physics Letters, vol. 85, p. 1589, 2004.
[12] H. Hu, P. K. Mohseni, L. Pan, X. Li, S. Somnath, J. R. Felts, M. A. Shannon, and W. P.
King, "Fabrication of arbitrarily shaped silicon and silicon oxide nanostructures using tip-
based nanofabrication," Journal of Vacuum Science & Technology B, vol. 31, 2013.
[13] B. A. Nelson and W. P. King, "Measuring material softening with nanoscale spatial
resolution using heated silicon probes," Review of Scientific Instruments, vol. 78, pp.
023702, 2007.
[14] E. O. Sunden, T. L. Wright, J. Lee, W. P. King, and S. Graham, "Room-temperature
chemical vapor deposition and mass detection on a heated atomic force microscope
cantilever," Applied Physics Letters, vol. 88, pp. 033107, 2006.
[15] B. A. Nelson, W. P. King, A. R. Laracuente, P. E. Sheehan, and L. J. Whitman, "Direct
deposition of continuous metal nanostructures by thermal dip-pen nanolithography,"
Applied Physics Letters, vol. 88, p. 033104, 2006.
122
[16] S. Bhattacharyya, O. Auciello, J. Birrell, J. A. Carlisle, L. A. Curtiss, A. N. Goyette, D.
M. Gruen, A. R. Krauss, J. Schlueter, A. Sumant, and P. Zapol, "Synthesis and
characterization of highly-conducting nitrogen-doped ultrananocrystalline diamond
films," Applied Physics Letters, vol. 79, pp. 1441, 2001.
[17] P. C. Fletcher, B. Lee, and W. P. King, "Thermoelectric voltage at a nanometer-scale
heated tip point contact," Nanotechnology, vol. 23, p. 035401, 2012.
[18] A. R. Krauss, O. Auciello, M. Q. Ding, D. M. Gruen, Y. Huang, V. V. Zhirnov, E. I.
Givargizov, A. Breskin, R. Chechen, E. Shefer, V. Konov, S. Pimenov, A. Karabutov, A.
Rakhimov, and N. Suetin, "Electron field emission for ultrananocrystalline diamond
films," Journal of Applied Physics, vol. 89, pp. 2958, 2001.
[19] J. Wang, M. A. Firestone, O. Auciello, and J. A. Carlisle, "Surface Functionalization of
Ultrananocrystalline Diamond Films by Electrochemical Reduction of Aryldiazonium
Salts," Langmuir, vol. 20, pp. 11450, 2004/12/01 2004.
[20] S. Kim, K. Kihm, and T. Thundat, "Fluidic applications for atomic force microscopy
(AFM) with microcantilever sensors," Experiments in Fluids, vol. 48, pp. 721, 2010.
123
APPENDIX A: HEATED MICROCANTILEVER ARRAY
FABRICATION PROCESS
0. Wafer Material
Wafer specification: SOI Wafer <100>, Diameter: 100mm (or 4 inch)
Device layer thickness: 5±0.5 µm
Handle thickness: 500 µm,
Resistivity: 1-10 Ω·cm, Doping: N/Ph
Box layer thickness: 1 µm
Dehydration bake: temperature >120°C, time > 5min
Spin deceleration: less than 1000rpm/sec
1. Tip formation
1.1 Spin Photoresist (NR7-1500P)
Equipment: Spinner (MNMS)
Recipe: dehydration bake
Spin NR7-1500P – 5000rpm for 45sec (acceleration 1000rpm/sec)
Softbake: hotplate 150 °C, 1min without Al ring (or 90 sec with Al ring)
Thickness = ~1.5 µm
1.2 Photolithography of Mask #1 (2.7 um Tip Structures)
Equipment: EV420 (MNMS)
Recipe: Exposure
Mode: Hard contact (6 µm separation)
Time: 40sec (this step needs to be overexposed)
Post-exposure bake: hotplate 100 °C, 1 min without Al ring
Development: RD-6 13 sec (if not full developed, dip for 1 extra second each time)
Rinse with DI Water and dry w/ N-gun
1.3 Hard Bake
Equipment: Hot Plate
Recipe: 120 °C for 10 min with an Al ring
1.4 Topside Silicon Etch (Anisotropic)
Equipment: ICP-DRIE (MMS)
Recipe: Program Name: Bosch-1
Estimated Number of Cycles = 8 to 9
Etch thickness: 1.7 µm to 2.3 µm
(Please try a practice wafer first to determine the number of cycles )
1.5 Wafer clean (Acetone)
Equipment: Solvent Bench
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Recipe: Acetone, Time = 10 min
Rinse with DI water and dry with N-gun
1.6 Piranha Clean
Equipment: Acid Bench
Recipe: Piranha Solution (H2SO4:H2O2 : 70%:30%)
Temperature: 120 °C, Time = 10 min
Rinse with DI water and dry with N-gun
1.7 Topside Silicon Etch (Isotropic)
Equipment: Wet Bench - HNA
Recipe: HNA - 2%, HF, 3% CH3COOH, 95% HNO3
Time = ~2min
(use SEM to observe the etch rate, stop when the size of the pillar is around 0.5 µm)
1.8 Oxidation Sharpen (Dry)
Equipment: Oxidation Furnace (MNMS)
Recipe: Furnace - 1000 °C, 15 Hours, Gas: O2, flow rate: 6 sccm
Expected thickness = ~0.3 μm
Note: the temperature should always be lower than 1050 °C
1.9 Isotropic Silicon Dioxide Wet Etch
Equipment: Wet Bench
Recipe: BOE dip around 3.5 min
Note: Surface will change from hydrophilic to hydrophobic
1.10 SEM
Equipment: SEM
Recipe: Measure tip curvature (if tips are blunt, go back to 1.8)
2. Cantilever formation
2.1 Spin photoresist (NR7-1500P)
Equipment: Spinner
Recipe: dehydration bake
Spin NR7-1500P at 1000rpm for 45sec (acceleration 200rpm/sec)
Softbake - 150°C for 1.5min without ring, (or 2 min with ring)
Thickness = ~2.7 μm
2.2 Photolithography of Mask #2 (Beam Structure)
Equipment: EV420 (MMS)
Mode: hard contact
Expose: 25sec
Post-exposure bake: 100 °C for 1 min without ring
Development: RD-6, 40sec
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Rinse with DI Water and dry w/ N-gun
Note: Alignment is very important. We use 10X lens in this step. Be very careful during
the lens changing process.
2.3 Hard Bake
Equipment: Hot Plate
Recipe: 120 °C for 10 min with Al ring
2.4 Topside Silicon Etch (Anisotropic)
Equipment: ICP-DRIE (MNMS)
Recipe: Program Name: Bosch-1
Estimated Number of Cycles = 12
Note: you will see the uniform oxide layer exposed,
2.5 Wafer clean (Acetone soaked)
Equipment: Solvent Bench
Recipe: soaked in Acetone, Time = 10 min
Rinse with DI water and dry with N-gun
2.6 Piranha Clean
Equipment: Wet Bench (MNMS)
Recipe: Piranha Solution (H2SO4:H2O2 : 70%:30%) at 120°C
Time = 10 min
Rinse with DI water and dry with N-gun
3. Implantation (low dosage)
3.1 Spin photoresist (Shipley 1827)
Equipment: Spinner (MMS)
Recipe: dehydration bake
Spin Shipley 1827 at 3000rpm for 45sec (acceleration 300rpm/sec)
Softbake: hotplate 120°C for 1.5min without ring, or 2min with ring
Thickness = ~3 μm
3.2 Photolithography of Mask #3 (Low Dose Implantation)
Equipment: EV420 (MMS)
Recipe: Hard contact mode
Expose: around 20 sec
Development: MF319, around 55 sec
Rinse with DI Water and dry w/ N-gun Note: 4X lens or 10X lens
3.3 Hard Bake
Equipment: Hot Plate (MMS)
Recipe: 120 °C for 25 min with ring
3.4 Ion Implantation of Entire Beam (low dosage)
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Equipment: Outside Vendor - Core Systems, CA 3.2.3
Recipe: Phosphorus /2.51e13 atoms/cm2 / 200 KeV/tilt 7°
3.5 Photoresist stripping
Equipment: Solvent Bench (MMS)
Recipe: soak in Acetone for 10min
3.6 Piranha Clean
Equipment: Acid Bench (MMS)
Recipe: Piranha Solution (H2SO4:H2O2 : 70%:30%)
Time = 10 min
Rinse with DI water and dry with N-gun
3.7 Oxide Deposition
Equipment: PlasmaLab PECVD (MNTL)
Recipe: high deposition rate
Thickness: 300nm
Time: 12 min
3.8 Diffusion
Equipment: Anneal Furnace (MMS)
Recipe: Furnace - 1000 °C, 0.5 Hour
N2 flow at 2 sccm
3.9 BOE
Equipment: Wet Bench
Recipe: BOE
Time = ~3min (until oxide is fully removed, check the edge surface change from
hydrophilic to hydrophobic, then give 30 sec more)
4. Implantation (high dosage)
4.1 Shipley 1827
Equipment: Spinner
Recipe: dehydration bake
Spin Shipley 1827 at 3000rpm for 45sec (acceleration 300rpm/sec)
Softbake: hotplate 120°C for 1.5min without Al ring, or 2min with Al ring
Thickness = ~3μm
4.2 Photolithography of Mask #4 (High Dose Implantation)
Equipment: EV420 (MNMS)
Recipe: Exposure
Hard contact mode
Expose: around 20 sec
Development: MF319, around 55sec
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Rinse with DI Water and dry w/ N-gun
Note: 4X or 10X lens
4.3 Hard Bake
Equipment: Hot Plate
Recipe: 120 °C for 25 min with ring
4.4 Ion Implantation (high dosage)
Equipment: Outside Vendor - Core Systems, CA
Recipe: Phosphorus /2.51e16 atoms/cm2 / 200 keV / 45° tilt
/Orientation 180°
4.5 Photoresist stripping (acetone soaking)
Equipment: solvent bench
Recipe: soak the wafer in Acetone at room temperature, overnight
Then place the wafer in ultrasound cleaner for less than 5 minutes
4.6 Photoresist striping (Piranha Clean)
Equipment: Acid Bench
Recipe: Piranha Solution (H2SO4:H2O2 : 70%:30%) at 120°C
Time = 10 min
Rinse with DI water and dry with N-gun
4.7 Photoresist striping (Oxygen Plasma)
Equipment: Axic RIE (MMS)
Recipe: Program Name: ztdai_O2
Time: 5 min (5min more if necessary)
4.8 Oxide Deposition
Equipment: PlasmaLab PECVD (MNTL)
Recipe: high deposition rate
Thickness: 300nm
Time: 12 min
4.9 Diffusion
Equipment: Anneal Furnace (MNMS)
Recipe: Temperature = 1000°C
N2 flow at 2 sccm Time = 2 hr
5. Via and Metallization
5.1 Shipley 1827
Equipment: Spinner (MNMS)
Recipe: dehydration bake
Spin Shipley 1827 at 3000rpm for 45sec (acceleration 300rpm/sec)
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Softbake: hotplate 120°C for 1.5 min without Al ring, or 2 min with Al ring
Thickness = ~3μm
5.2 Photolithography of Mask #6 (Open Vias for Metal Contact)
Equipment: EV420 (MNMS)
Recipe: Exposure
Hard Contact (6 μm separation)
Expose: 20sec
Development: MF319, around 55 sec
Rinse with DI Water and dry w/ N-gun
Note: 4X or 10X lens
5.3 Hard Bake
Equipment: Hot Plate
Recipe: 110 °C for 10 min with ring
5.4 Topside Oxide Etch
Equipment: Acid hood (MNMS)
Recipe: BOE dip for 75 sec
Note: make sure do a dummy first
5.5 Photoresist stripping
Equipment: Solvent Bench (MNMS)
Recipe: soak in Acetone for 10min
5.6 Piranha Clean
Equipment: Acid Bench (MNMS)
Recipe: Piranha Solution (H2SO4:H2O2 : 70%:30%) at 120°C
Time = 10 min
5.7 BOE Dip
Equipment: Acid Bench
Recipe: BOE
Time = 15 sec
5.8 Topside Aluminum Deposition (Evaporation)
Equipment: CHA Evaporator (MNTL)
Recipe: 1.5 μm Aluminum
5.9 Spin Shipley 1827
Equipment: Spinner (MMS)
Recipe: dehydration bake
Spin Shipley 1827 at 3000rpm for 45sec (acceleration 300rpm/sec)
Softbake: hotplate 120°C for 1.5 min without Al ring, or 2 min with Al ring
Thickness = ~3 μm
129
5.10 Photolithography of Mask #6 (pattern Al)
Equipment: EV420 (MMS)
Recipe: Exposure
Hard contact mode
Expose: 35 sec
Development: MF319 around 70sec (can be longer do 50 sec in one bath then transfer to
second bath,for total 90-120 seconds)
Check in between anchors for PR, continue developing until completely removed—OK to
overdevelop
Rinse with DI Water and dry w/ N-gun
Note: 4X or 10X lens, this step needs to be over-exposed
***HARD BAKE 10 minutes at 120
5.11 Aluminum etch
Equipment: Acid Bench
Recipe: Aluminum Etchant Type A, at 60°C (heat for ~20 minutes firstly)
Estimated Etch Rate = 3000 Å/min
Time = 4 to 5min ~ (6 to 9 minutes)
Note: you will see the shiny Al is remove and the color (blue) of SiO2 shows up, after Al
layer is fully etched, give it 30 sec more etch time
5.12 Photoresist Striping
Equipment: Solvent Bench (MMS)
Recipe: Soak in Acetone for 10 min
Check Resistance if around 2kΩ do not need to sinter
If greater than 40 or 50 kΩ then
5.13 Check Device Resistance with Multimeter
Note: the resistance of type II device is around 2 kΩ
5.14 Sintering Equipment: Vacuum Annealer (MMS)
Recipe: Temperature = 400 °C Time = 0.5 hr
5.15 Oxide Deposition
Equipment: Plasma PECVD (MNTL)
Recipe: High deposition rate
Oxide thickness = 400nm
Time = 16min
Note: this layer of Oxide just to keep the final device clean
130
6. Cantilever Release
6.1 Apply Thick Photoresist (PR) to Topside
Equipment: Spinner (MNMS)
Recipe: dehydration bake
Spin Shipley 1827 at 3000rpm for 45sec (acceleration 300rpm/sec)
Softbake: hotplate 120°C for 1.5 min without Al ring, or 2 min with Al ring
Thickness = ~3 μm
6.2 Hard Bake
Equipment: Hot Plate 3.2.5
Recipe: 110 °C for 10 min
6.3 BOE Dip
Equipment: Acid Bench (MNMS)
Recipe: BOE
Time: 10 sec, until oxide on the back side is fully removed
6.4 Cleave Wafer
Equipment: Work Bench
Recipe: Cleave wafer into 4 quadrants
6.5 Apply Thick Photoresist (PR) to Bottom of the Wafer
Equipment: Spinner (MNMS)
Recipe: dehydration bake
Spin NR5-8000 at 1000rpm for 40sec (acceleration 200rpm/sec)
Softbake: 150°C for 6min
Thickness = ~ 17 μm
Note: cover the hotplate with aluminum foil to avoid leaving photoresist residue on the
hotplate. Cool down on Alphawipe before exposed in EV420
6.6 Photolithography of Mask #8 (Backside Openings)
Equipment: EV420 (MNMS)
Recipe: Exposure
Hard Contact Mode or proximity mode (50um of separation)
Time = 30 sec
Post exposure bake: 100°C for 2min
Development: RD-6, around 70 sec
Rinse with DI Water and dry w/ N-gun
Note: use default backside alignment lens
6.7 Hard Bake
Equipment: Hot Plate (covered with Al foil)
Recipe: 120 °C for 10 min
6.8 Apply Thick Photoresist (PR) to Topside of Carrier Wafer
131
Equipment: Spinner (MMS)
Recipe: dehydration bake
Spin NR5-8000 at 1000rpm for 40sec (acceleration 200rpm/sec)
6.9 Attach 1/4 Wafer to Carrier Wafer
Equipment: By Hand
Recipe: N/A
6.10 Hard Bake
Equipment: Hot Plate (MMS)
Recipe: 120 °C for 10 min, 140 °C for 20 min with Al ring
6.11 Backside Silicon Etch
Equipment: ICP-DRIE (MNMS)
Recipe: Bosch-1
Estimated Number of Cycles = 800
Note: you will see the cantilever structure from the etched trench; the 1 μm thick box SiO2
layer is transparent
6.12 Backside Oxide Etch
Equipment: Freon RIE (MNTL)
Recipe: CF4
Time = 30 min (~600nm etch)
6.13 Soak to Separate Wafers
Equipment: Wet Bench
Recipe: Photoresist Stripper 1165 at 80C°
Time = overnight
After this step, we can’t clean the wafer with DI water gun and dry the wafer with
N2 gun.
6.14 Wafer clean (optional, for dirty wafers)
Equipment: Acid Bench (MMS)
Recipe: Piranha Solution (H2SO4:H2O2 : 70%:30%)
Temperature: 120C° Time = 5 sec
Rinse in DI water and change DI water 5 times, then dry on a hot plate at 110C°.
6.15 HF Dip
Equipment: Wet Bench 3.2.5
Recipe: Pad Etch
Box oxide Thickness = 1 μm Time = around 20 min
Rinse in DI water and change DI water 5 times, then dry on a hot plate at 110 °C.
Note: check devices under optical microscope to see cantilevers are not bending, use
multi-meter to check device resistance. Make sure the cantilever is not bent
6.16 Device final check (SEM)
132
Equipment: Hitachi SEM S4800 (MNTL)
Recipe: N/A
Final check
133
APPENDIX B: DIAMOND TIP HEATED MICROCANTILEVER
FABRICATION PROCESS
0. Wafer Material
Wafer specification: SOI Wafer <100>, Diameter: 100mm (or 4 inch)
Device layer thickness: 5±0.5 µm
Handle thickness: 500 µm,
Resistivity: 1-10 Ω·cm, Doping: N/Ph
Box layer thickness: 1 µm
Dehydration bake: temperature >120°C, time > 5min
Spin deceleration: less than 1000rpm/sec
1. Wafer thinning
1.1 Spin Photoresist (NR7-1500P)
Equipment: Spinner (MNMS)
Recipe: dehydration bake
Spin NR7-1500P – 5000rpm for 45sec (acceleration 1000rpm/sec)
Softbake: hotplate 150 °C, 1min without Al ring (or 90 sec with Al ring)
Thickness = ~1.5 µm
1.2 Hard Bake
Equipment: Hot Plate
Recipe: 120 °C for 10 min with an Al ring
1.3 Topside Silicon Etch (Anisotropic)
Equipment: ICP-DRIE (MMS)
Recipe: Program Name: Bosch-1
Estimated Number of Cycles = 12~15
Etch thickness: 4 µm
(Please try a practice wafer first to determine the number of cycles )
1.4 Wafer clean (Acetone)
Equipment: Solvent Bench
Recipe: Acetone, Time = 10 min
Rinse with DI water and dry with N-gun
1.5 Piranha Clean
Equipment: Acid Bench
Recipe: Piranha Solution (H2SO4:H2O2 : 70%:30%)
Temperature: 120 °C, Time = 10 min
Rinse with DI water and dry with N-gun
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2. Cantilever formation
2.1 Spin photoresist (NR7-1500P)
Equipment: Spinner
Recipe: dehydration bake
Spin NR7-1500P at 1000rpm for 45sec (acceleration 200rpm/sec)
Softbake - 150°C for 1.5min without ring, (or 2 min with ring)
Thickness = ~2.7 μm
2.2 Photolithography of Mask #2 (Beam Structure)
Equipment: EV420 (MMS)
Mode: hard contact
Expose: 25sec
Post-exposure bake: 100 °C for 1 min without ring
Development: RD-6, 40sec
Rinse with DI Water and dry w/ N-gun
Note: Alignment is very important. We use 10X lens in this step. Be very careful during
the lens changing process.
2.3 Hard Bake
Equipment: Hot Plate
Recipe: 120 °C for 10 min with Al ring
2.4 Topside Silicon Etch (Anisotropic)
Equipment: ICP-DRIE (MNMS)
Recipe: Program Name: Bosch-1
Estimated Number of Cycles = 12
Note: you will see the uniform oxide layer exposed,
2.5 Wafer clean (Acetone soaked)
Equipment: Solvent Bench
Recipe: soaked in Acetone, Time = 10 min
Rinse with DI water and dry with N-gun
2.6 Piranha Clean
Equipment: Wet Bench (MNMS)
Recipe: Piranha Solution (H2SO4:H2O2 : 70%:30%) at 120°C
Time = 10 min
Rinse with DI water and dry with N-gun
3. Implantation (low dosage): Identical to Appendix A
4. Implantation (high dosage): Identical to Appendix A
135
5. Diamond Tip Fabrication
5.1 UNCD Deposition
Equipment: CVD chamber
Recipe: Proprietary (sonicate wafer in diamond nanoparticle colloidal suspension,
growth in hot-filament CVD chamber), thickness ~ 6 µm
Note: UNCD Tip was fabricated by Advanced Diamond Technologies Inc.,
www.thindiamond.com, Tel) 815-293-0900
5.2 Silicon Dioxide Mask Formation
Equipment: PECVD, Mask Aligner
Recipe: Proprietary (Advanced Diamond Technology Inc.)
Note: silicon dioxide precursor cap (~3 µm diameter) protects UNCD during tip etching
5.3 UNCD Tip Etch
Equipment: ICP-RIE
Gas: O2/SF6 plasma
Recipe: Proprietary (Advanced Diamond Technology Inc.)
Note: The differential etching rate of UNCD to silicon dioxide of approximately 15:1
allows the gradual etching of the protective mask until it is completely removed, resulting
in ultrasharp UNCD tips.
5.2 Check UNCD Tip Quality
Equipment: SEM
Recipe: Check UNCD Tip quality.
5.3 Oxide Deposition
Equipment: PlasmaLab PECVD (MNTL)
Recipe: Slow deposition rate
Thickness: 600 nm
Time: 55:33 min:sec
Note: PECVD Oxide protects the tip from the following metallization/release process
Do not perform O2 plasma beyond this point, as the O2 plasma etches UNCD
6. Via and Metallization: Identical to Appendix A
6.1 Shipley 1827
Equipment: Spinner (MNMS)
Recipe: dehydration bake
Spin Shipley 1827 at 3000rpm for 45sec (acceleration 300 rpm/sec)
Softbake: hotplate 120°C for 1.5 min without Al ring, or 2 min with Al ring
Thickness = ~3μm
6.2 Photolithography of Mask #6 (Open Vias for Metal Contact)
Equipment: EV420 (MNMS)
136
Recipe: Exposure
Hard Contact (6 μm separation)
Expose: 20sec
Development: MF319, around 55 sec
Rinse with DI Water and dry w/ N-gun
Note: 4X or 10X lens
6.3 Hard Bake
Equipment: Hot Plate
Recipe: 110 °C for 10 min with ring
6.4 Topside Oxide Etch
Equipment: Acid hood (MNMS)
Recipe: BOE dip for 75 sec
Note: make sure do a dummy first
Do not use O2 Plasma, as it will etch UNCD
6.5 Photoresist stripping
Equipment: Solvent Bench (MNMS)
Recipe: soak in Acetone for 10min
6.6 Piranha Clean
Equipment: Acid Bench (MNMS)
Recipe: Piranha Solution (H2SO4:H2O2 : 70%:30%) at 120°C
Time = 10 min
6.7 BOE Dip
Equipment: Acid Bench
Recipe: BOE
Time = 15 sec
6.8 Topside Aluminum Deposition (Evaporation)
Equipment: CHA Evaporator (MNTL)
Recipe: 1.5 μm Aluminum
6.9 Spin Shipley 1827
Equipment: Spinner (MNMS)
Recipe: dehydration bake
Spin Shipley 1827 at 3000rpm for 45sec (acceleration 300rpm/sec)
Softbake: hotplate 120°C for 1.5 min without Al ring, or 2 min with Al ring
Thickness = ~3 μm
6.10 Photolithography of Mask #6 (pattern Al)
Equipment: EV420 (MNMS)
Recipe: Exposure
Hard contact mode
137
Expose: 35 sec
Development: MF319 around 70sec (can be longer do 50 sec in one bath then transfer to
second bath,for total 90-120 seconds)
Check in between anchors for PR, continue developing until completely removed—OK to
overdevelop
Rinse with DI Water and dry w/ N-gun
Note: 4X or 10X lens, this step needs to be over-exposed
***HARD BAKE 10 minutes at 120
6.11 Aluminum etch
Equipment: Acid Bench
Recipe: Aluminum Etchant Type A, at 60°C (heat for ~20 minutes firstly)
Estimated Etch Rate = 3000 Å/min
Time = 4 to 5min ~ (6 to 9 minutes)
Note: you will see the shiny Al is remove and the color (blue) of SiO2 shows up, after Al
layer is fully etched, give it 30 sec more etch time
6.12 Photoresist Striping
Equipment: Solvent Bench (MMS)
Recipe: Soak in Acetone for 10 min
Check Resistance if around 2kΩ do not need to sinter
If greater than 40 or 50 kΩ then
6.13 Check Device Resistance with Multimeter
Note: the resistance of type II device is around 2 kΩ
6.14 Sintering Equipment: Vacuum Annealer (MMS)
Recipe: Temperature = 400 °C Time = 0.5 hr
6.15 Oxide Deposition
Equipment: Plasma PECVD (MNTL)
Recipe: High deposition rate
Oxide thickness = 400nm
Time = 16min
Note: this layer of Oxide just to keep the final device clean
7. Cantilever Release: Identical to Appendix A
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APPENDIX C: GRAPHENE-COATED MICROCANTILEVER
FABRICATION PROCESS
0. Wafer Material
Wafer specification: SOI Wafer <100>, Diameter: 100mm (or 4 inch)
Device layer thickness: 5±0.5 µm
Handle thickness: 500 µm,
Resistivity: 1-10 Ω·cm, Doping: N/Ph
Box layer thickness: 1 µm
Dehydration bake: temperature >120°C, time > 5min
Spin deceleration: less than 1000rpm/sec
1. Wafer thinning
1.1 Spin Photoresist (NR7-1500P)
Equipment: Spinner (MNMS)
Recipe: dehydration bake
Spin NR7-1500P – 5000rpm for 45sec (acceleration 1000rpm/sec)
Softbake: hotplate 150 °C, 1min without Al ring (or 90 sec with Al ring)
Thickness = ~1.5 µm
1.2 Hard Bake
Equipment: Hot Plate
Recipe: 120 °C for 10 min with an Al ring
1.3 Topside Silicon Etch (Anisotropic)
Equipment: ICP-DRIE (MMS)
Recipe: Program Name: Bosch-1
Estimated Number of Cycles = 12~15
Etch thickness: 4 µm
(Please try a practice wafer first to determine the number of cycles)
1.4 Wafer clean (Acetone)
Equipment: Solvent Bench
Recipe: Acetone, Time = 10 min
1.5 Piranha Clean
Equipment: Acid Bench
Recipe: Piranha Solution (H2SO4:H2O2 : 70%:30%)
Temperature: 120 °C, Time = 10 min
Rinse with DI water and dry with N-gun
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2. Cantilever formation
2.1 Spin photoresist (NR7-1500P)
Equipment: Spinner
Recipe: dehydration bake
Spin NR7-1500P at 1000rpm for 45sec (acceleration 200rpm/sec)
Softbake - 150°C for 1.5min without ring, (or 2 min with ring)
Thickness = ~2.7 μm
2.2 Photolithography of Mask #MR-01 (Beam Structure)
Equipment: EV420 (MMS)
Mode: hard contact
Expose: 25sec
Post-exposure bake: 100 °C for 1 min without ring
Development: RD-6, 40sec
Rinse with DI Water and dry w/ N-gun
Note: Alignment is very important. We use 10X lens in this step. Be very careful during
the lens changing process.
2.3 Hard Bake
Equipment: Hot Plate
Recipe: 120 °C for 10 min with Al ring
2.4 Topside Silicon Etch (Anisotropic)
Equipment: ICP-DRIE (MNMS)
Recipe: Program Name: Bosch-1
Estimated Number of Cycles = 12
Note: you will see the uniform oxide layer exposed,
2.5 Wafer clean (Acetone soaked)
Equipment: Solvent Bench
Recipe: soaked in Acetone, Time = 10 min
Rinse with DI water and dry with N-gun
2.6 Piranha Clean
Equipment: Wet Bench (MNMS)
Recipe: Piranha Solution (H2SO4:H2O2 : 70%:30%) at 120°C
Time = 10 min
Rinse with DI water and dry with N-gun
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3. Cantilever Release
3.1 Hard Bake
Equipment: Hot Plate 3.2.5
Recipe: 110 °C for 10 min
3.2 BOE Dip
Equipment: Acid Bench (MNMS)
Recipe: BOE
Time: 10 sec, until oxide on the back side is fully removed
3.3 Cleave Wafer
Equipment: Work Bench
Recipe: Cleave wafer into 4 quadrants
3.4 Apply Thick Photoresist (PR) to Bottom of the Wafer
Equipment: Spinner (MNMS)
Recipe: dehydration bake
Spin NR5-8000 at 1000rpm for 40sec (acceleration 200rpm/sec)
Softbake: 150°C for 6min
Thickness = ~ 17 μm
Note: cover the hotplate with aluminum foil to avoid leaving photoresist residue on the
hotplate. Cool down on Alphawipe before exposed in EV420
3.5 Photolithography of Mask #MR-02 (Backside Openings)
Equipment: EV420 (MNMS)
Recipe: Exposure
Hard Contact Mode or proximity mode (50um of separation)
Time = 30 sec
Post exposure bake: 100°C for 2min
Development: RD-6, around 70 sec
Rinse with DI Water and dry w/ N-gun
Note: use default backside alignment lens
3.6 Hard Bake
Equipment: Hot Plate (covered with Al foil)
Recipe: 120 °C for 10 min
3.7 Apply Thick Photoresist (PR) to Topside of Carrier Wafer
Equipment: Spinner (MMS)
Recipe: dehydration bake
Spin NR5-8000 at 1000rpm for 40sec (acceleration 200rpm/sec)
3.8 Attach 1/4 Wafer to Carrier Wafer
Equipment: By Hand
Recipe: N/A
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3.9 Hard Bake
Equipment: Hot Plate (MMS)
Recipe: 120 °C for 10 min, 140 °C for 20 min with Al ring
3.10 Backside Silicon Etch
Equipment: ICP-DRIE (MNMS)
Recipe: Bosch-1
Estimated Number of Cycles = 800
Note: you will see the cantilever structure from the etched trench; the 1 μm thick box SiO2
layer is transparent
3.11 Backside Oxide Etch
Equipment: Freon RIE (MNTL)
Recipe: CF4
Time = 30 min (~600nm etch)
3.12 Soak to Separate Wafers
Equipment: Wet Bench
Recipe: Photoresist Stripper 1165 at 80C°
Time = overnight
After this step, we can’t clean the wafer with DI water gun and dry the wafer with
N2 gun.
3.13 Wafer clean (optional, for dirty wafers)
Equipment: Acid Bench (MMS)
Recipe: Piranha Solution (H2SO4:H2O2 : 70%:30%)
Temperature: 120C° Time = 5 sec
Rinse in DI water and change DI water 5 times, then dry on a hot plate at 110C°.
3.14 HF Dip
Equipment: Wet Bench 3.2.5
Recipe: Pad Etch
Box oxide Thickness = 1 μm Time = around 20 min
Rinse in DI water and change DI water 5 times, then dry on a hot plate at 110 °C.
Note: check devices under optical microscope to see cantilevers are not bending, use
multi-meter to check device resistance. Make sure the cantilever is not bent
3.15 Device final check (SEM)
Equipment: Hitachi SEM S4800 (MNTL)
Recipe: N/A
Final check
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4. Graphene Synthesis
4.1 PMMA deposition (Source #1)
Equipment: Spinner (MNMS)
Recipe: Diluted PMMA (450K PMMA 4A: Anisole = 1:39)
Spin at 5500 rpm for 3 minutes, thickness ~ 10 nm
4.2 Carbon Deposition (Source #2)
Equipment: Thermal evaporator (Beckman ITG)
Recipe: 5 – 30 nm amorphous carbon
4.3 Nickel Deposition (Catalytic metal)
Equipment: CHA E-beam evaporator (MNTL)
Recipe: 100 – 300 nm Nickel
4.4 Graphene Synthesis
Equipment: High Temperature CVD (Nam Group)
Recipe: 5 min at 1000 °C (H2 = 50 sccm, Pressure = 150 mTorr)
4.5 Nickel Wet Etch
Equipment: Acid Bench
Recipe: 10 min using Nickel Etchant TFB
Do not agitate the etchant container
4.6 Sample Clean
Equipment: Wet Bench
Recipe: Rinse with IPA/DI Water (1:1)
Do not use N2 gun