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Today’s SPM in Nanotechnology
An introduction for Advanced Applications
Qun (Allen) Gu, Ph.D., AFM Scientist, Pacific Nanotechnology
IEEE Bay Area Nanotechnology Council, August, 2007
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Content
- AFM fundamentals: Principle, instrument, applications
- Field Modes
EFM
KPM
MFM
- Shark Modes (C-AFM, I-V)
- Lithography
LAO
Scratch
DPN
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What is an SPM
• An SPM is a mechanical imaging instrument in which a small, < 10 nm in radius, probe is scanned over a surface. By monitoring the motion of the probe, the surface topography and/or surface physical properties are measured with an SPM.
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Forces
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AFM System
Computer
Software for gathering and
processing images resides
on the computer.
Electronic Controller
Generates electronic
signals that control all
functions in the stage
Stage
Scanner (laser, PD, PZ),
Optical Microscope,
sample stage.
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AFM System
Force
Transducer
Z Piezo
X-Y Piezo
Sample
Feedback
Controller Compare
Set Force
XY Raster Electronics
Image Out
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AFM Light Lever
Sample
So
Photo detector
Cantilever
Laser
When the cantilever moves up and down, the position of the
laser on the photo detector moves up and down.
Differential Amplifier
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Feedback Control
• A control system which monitors its
effect on the system it is controlling
and modifies its output.
• Measure, Compare, Update
Car on a road
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Comparison
Non-destructive; 3D Magnification; Ambient air;
Surface physical property
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AFM Applications
• Life Sciences
• Cells, Bio-molecules, Biomaterials
• Material Sciences
• Semiconductors, Ceramics, Polymers
• High Technology
• Data Storage, Optics, Semiconductors,
Biotech.
• Low Technology
• Paper, Steel, Plastics, Automobile
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Life Sciences
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Material Sciences
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High Technology
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Low Technology
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AFM Modes: Advanced Applications
• Field Modes • KPM/EFM
• Magnetic Force
• Electrical Modes (Shark)
• Lithography • LAO
• Scratching
• DPN
• Material Sensing Modes • Lateral Force
• Vibrating Phase
• Mechanical • Force/Distance
• Indenting
• Liquid
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Field Modes
1st Scan
2nd Scan
Charged/magnetic samples
F = Fsurface + Felectrostatic + Fmagnetic + Fother
- Electrostatic force/magnetic force interaction (> tens of nm)
- Qualitative/Quantitative
- Resolution depends on sample, probe coating
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Probe/Surface Interaction
A Free oscillations
B Oscillation damped by a surface
Frequency
Am
plit
ude
Tip-sample interaction = A spring in series with cantilever
(Linear approximation)
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INSTRUMENTATION
1. Lock-in technique: at constant fD, cantilever Δf results in ΔA (a) and
Δφ (b), which can be interpreted as a force signal. (No FB)
2. Frequency Modulation (servo controller): Measure Δf: at the constant
phase (phase lag is zero in a phase locked loop).
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KPM/EFM
Sample
Laser
Photo Detector
Nano-R / Nano-I
Controller(ω0)
Lock-In Amp (ω)
SPT-20
Surface Potential of the sample VSVDC
VAC cosωt
Feedback control adjusts to minimize
the amplitude and records VDC.
This recorded VDC is equal to VS.
KPM Image
T-B Signal
(ω+ω0)
Bimorph Piezo(ω0)
Amplitude of frequency ω
EFM Image
V0cosω0t
Schematic of the EFM・KPM Mode
Sample
Laser
Photo Detector
Nano-R / Nano-I
Controller(ω0)
Lock-In Amp (ω)
SPT-20
Surface Potential of the sample VSVDC
VAC cosωt
Feedback control adjusts to minimize
the amplitude and records VDC.
This recorded VDC is equal to VS.
KPM Image
T-B Signal
(ω+ω0)
Bimorph Piezo(ω0)
Amplitude of frequency ω
EFM Image
V0cosω0t
Schematic of the EFM・KPM Mode
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EFM
Calculate the change in the resonant frequency(ω): Use Equations
for fields above a surface and calculate the derivative of the field.
++++
Vibration
Amplitude
Charged region
Electric field
Cantilever
Vibration
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Electric Forces (EFM)
Topography Electric Force
+v -v -v
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KPM
Vs: Contact potential difference or work function difference
- DC component: static attractive force between electrodes (topo)
- component: a force between charges induced by AC field (KPM)
- 2 component: a force induced to capacitors only by AC voltage (SCM)
Lock-in Amp detects the signal at , feedback control minimizes this
component by adjusting VDC, so VS+VDC = 0
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EFM/KPM
Surface potential distribution
Capacitance (C-z, C-V)
Polarization of adsorbed molecules
Polarization or piezo effect of ferroelectric
Charge distribution
Carrier distribution in semiconductor
Local work function
others
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EFM/KPM
10X10 um topography and KPM images of a DVD-RW surface
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Corrosion Study
Surface potential mapping for a metal alloy surface: enhanced
corrosion (higher cathodic reaction) observed in the boundaries.
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Semiconductor
Puntambekar et al., Appl. Phys. Lett., Vol. 83, No. 26, 29 December 2003
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Semiconductor
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Nanowires embedded in alumina matrix. (Right) EFM
images show the electrical discontinuity of the nanowires.
C. A. Huber; Science, 263, 1994), pp. 800-802.
Nanomaterials
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MFM
A magnetically sensitive cantilever interacts with the magnetic stray field
of the sample. Resulting changes in the status of the cantilever are
measured by the deflection sensor, and recorded to produce an image.
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MFM
F = Fmag + Felec + Fvan
Fvan = AHR/6z2 Felec = V2R/z2
R=10 nm, Z=50 nm, F’elec, F’van ~ 10-6 N/m
F’mag ~ 1/(a+z)2 a: domain width; z: distance
Sharp tip and small V, F’elec F’van << F’mag (at Z > 20 nm)
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MFM
Constant frequency mode:
Maintain the frequency by adjusting z
Topography convolution; AC+DC
Felec as servo force
Lift-mode:
Monitoring fr or the phase shift
during 2nd pass
Constant height:
Applying small bias to compensate
Felec by work function difference;
Highest S/N (no FB noise)
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MFM
A topography (left) and MFM image (right) for a hard disk.
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MFM
A topography (left) and MFM image (right) for a degassed
hard disk. MFM image acquired by raising the magnetic tip
~80 nm above the surface. The bit microstructures were
never found on this sample surface.
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MFM
A topography (left) and MFM image (right) for a Magnetic recording tape
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SHARK Mode
• Monitor Current Between Tip and Sample while
scanning in contact mode
• Measure current map and Topography
Simultaneously
v
A/D
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SHARK Example
Glass Substrate
Gold
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Electrical Test
LPM Software allows probing the sample; SP, Voltage ramping, holding time
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SHARK Example
Conducting Substrate
Insulating Matrix
NanoTube
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Nano-Lithography
-Change surface chemical composition
-Deposit materials on a surface
-Physically scratch surface
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Nano-Lithography
-Draw Line as vector
- Draw an array of dots “dot matrix”
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Local Anodic Oxidation
Silicon
50 nm Lines
Si + H2O SiO2+ H+
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Local Anodic Oxidation
Humidity
Linewidth SP/Voltage
Scan rate
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Local Anodic Oxidation
The formation of a single (tunneling) barrier within a thin metal
film is shown. The tip repeatedly scans along a single line,
monitoring the conductance through the device while oxidising.
The image shows the 70nm wide metallic wire(black), defined by
AFM induced oxide barrier, 21 nm wide.
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Scratching
When Scratching With the AFM, there is a torsion on the cantilever so the probe area changes.
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DIP-PEN NANOLITH
Transfer ink materials (small molecules) onto substrate
in a pre-defined pattern
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DIP-PEN NANOLITH
(Left) amino-modified polystyrene particles onto carboxylic acid
alkanethiol (-) template. (Right) Opposite electrostatic assembly of
citrate-stabilized gold nanoparticles onto carboxylic acid alkanethiol
(-) surrounding a hydrophobic, uncharged dot array (ODT).
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DIP-PEN NANOLITH
High Resolution and Accuracy: 14 nm linewidths, 5 nm spatial resolution; Automated registry
Versatile Chemical and Material Flexibility: Alkylthiols (e.g. ODT & MHA), Fluorescent dye, Silazanes, Alkoxysilanes,
Conjugated polymer, DNA, Proteins, Sols, Colloidal particles, Metal salts
Simple Operation and Experimental Procedures: Can deposit direct-write, without need for resists; Operates in ambient
conditions (no UHV); Patterning and imaging by the same instrument.
Efficient and Scalable: Patterning and imaging routines are automated via InkCAD;
parallel pen arrays scale to 52 parallel pens;
2D nano PrintArrays™ in development: 2D arrays of 55,000 pens.
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Summary
AFM is a Hot Instrument in nanotechnology applications
- High Resolution: a few nm X/Y, A in Z
- Versatile: Measure electrical/magnetic field, tens of nm
I-V Curve measurement, conductive mapping
Nanolithography (LAO/DPN); Force measurement
- Non-destructive, Ambient/water environments, affordable
- Weakness: Limited Z, Low speed, …