spm_overview for nsci 434_2013 (1)

Scanning Probe Microscopy

College of Nanoscale Science and Engineering,

University at Albany, Albany, NY 12203

Scanning Probe Microscopy (SPM)

• A family of microscopy forms where a sharp probe is scanned across a surface and some tip/sample interactions are monitored

• Scanning tunneling Microscopy (STM)• Atomic Force Microscopy (AFM)

– contact mode– non-contact mode– Tapping Mode

• Other forms of SPM– Lateral force– Force modulation – Magnetic or electric force– surface potential– scanning thermal– phase imaging

STM: Basic Configuration

Xe on Ni

Kanji for atom

General AFM “Beam Deflection” Detection

• Used for Contact Mode, Non-contact and TappingMode AFM

• Laser light from a solid state diode is reflected off the back of the cantilever and collected by a position sensitive detector (PSD). This consists of two closely spaced photodiodes. The output is then collected by a differential amplifier

• Angular displacement of the cantilever results in one photodiode collecting more light than the other. The resulting output signal is proportional to the deflection of the cantilever.

• Detects cantilever deflection <1A

Solid State

Laser Diode

Cantilever and Tip

A

B

Piezoelectric Scanners• SPM scanners are made from a piezoelectric material

that expands and contracts proportionally to an applied voltage.

• Whether they expand or contract depends upon the polarity of the applied voltage. Digital Instruments scanners have AC voltage ranges of +220 to -220V.

0 V - V+ V

No applied voltage Extended Contracted

In some versions, the piezo tube moves the sample relative to the tip. In

other models, the sample is stationary while the scanner moves the tip.

AC signals applied to conductive areas of the tube create piezo

movement along the three major axes.

AFM: Basic Configuration

Highly oriented pyrolytic

graphite (HOPG): 2 nm x 2

nm scan

OUTLINE• AFM principle and structure

• Tip-surface interactions

• Force-distance curves and modes of operation

• Feedback techniques

• The scanner

• Cantilevers and tips

• Tip convolution and resolution

• Linewidth and sidewall metrology

• Other Imaging Modes

14

Mask surfaces:

Cleaning recipe:

UV UPW SPM UPW SC1 UPW Dry

Experimental – Sample preparation

15

0 100 200 300 400 500

-120

-80

-40

0

(6)

(5)

(4)

(3) (2) (1)

Fadhesion

SiO2 interaction with quartz in air

Forc

e F

[nN

]

Displacement z [nm]

Approach

Retraction

1. Start of force cycle

2. Non-touching line

3. Jump-in point

4. Touching line

5. Touching line

6. Jump-out point

AFM-based adhesion measurements in air

0 50 100 150 200 250

115

116

117

118

119

Fa

dh

esio

n [

nN

]

Cycle number

250 Samples for Statistical Accuracy

114 115 116 117 118 119 1200

10

20

30

40

50

60

Fadhesion,average

= 117.0 nN

= 0.5 nN

Fre

quency o

f occure

nce

Fadhesion

[nN]

16

Mask blank Micromanipulator

AFM scanner head

AFM flow cell

AFM-based adhesion measurements in fluid flow

Inlet

Outlet

Position of AFM chip

with cantilever

Cell designed to confine flow at MB

surface

17

0 100 200 300 400 500

0.0

0.4

0.8

1.2

Forc

e F

[n

N]

Displacement z [nm]

Approach

Retraction

SiO2 interaction with quartz in UPW

AFM-based adhesion measurements in fluids

Data analysis

• Conversion of the z-distance into a probe-surface separation

• Conversion of the laser deflection into an interaction energy

• Averaging of up to 40 single measurements

• Curve fitting (DLVO theory) under the conditions of constant surface

charge or constant surface potential (van der Waals + electric double

layer interaction)

Overall repulsive behavior

No adhesion

0 50 100 150 200 25010

-6

10-5

10-4

10-3

Experimental data

Const. surf. charge: q0 = -0.65 mC/m

2

Const. surf. potential: U = -68 mV

SiO2 interaction with quartz in UPW

Inte

raction e

nerg

y W

[J/m

2]

Separation z [nm]

Advantages and Disadvantages of the 2 main types of AFM

•Contact Mode: Advantages:• High scan speeds• The only mode that can obtain “atomic resolution” images• Rough samples with extreme changes in topography can sometimes be

scanned more easily

•Contact Mode: Disadvantages:• Lateral (shear) forces can distort features in the image• The forces normal to the tip-sample interaction can be high in air due

to capillary forces from the adsorbed fluid layer on the sample surface.• The combination of lateral forces and high normal forces can result in

reduced spatial resolution and may damage soft samples (i.e. biological samples, polymers, silicon) due to scraping

•Tapping Mode AFM: Advantages:• Higher lateral resolution on most samples (1 to 5nm)• Lower forces and less damage to soft samples imaged in air• Lateral forces are virtually eliminated so there is no scraping

•Tapping Mode AFM: Disadvantages:• Slightly lower scan speed than contact mode AFM

Constant Height and Constant ForceConstant Height:

• In constant-height mode, the spatial variation of the cantilever deflection can be used directly to generate the topographic data set because the height of the scanner is fixed as it scans.

• Constant-height mode is often used for taking atomic-scale images of atomically flat surfaces, where the cantilever deflections and thus variations in applied force are small. Constant-height mode is also essential for recording real-time images of changing surfaces, where high scan speed is essential.

Constant Force:

• In constant-force mode, the deflection of the cantilever can be used as input to a feedback circuit that moves the scanner up and down in z ,responding to the topography by keeping the cantilever deflection constant. In this case, the image is generated from the scanner’s motion. With the cantilever deflection held constant,the total force applied to the sample is constant.

• In constant-force mode,the speed of scanning is limited by the response time of the feedback circuit, but the total force exerted on the sample by the tip is well controlled. Constant-force mode is generally preferred for most applications.

Dimensional Metrology via Direct Profiling

• Direct profiling metrology utilizes a micromachined stylus to determine dimensional size via direct contact– Atomic Force Microscope (AFM)– High Resolution Profilometer (HRP)

• Advantages– Elimination of e-beams and lenses

• Disadvantages– Increased measurement time– Convolution of tip shape with feature size– Challenges for use with HAR structures

NanoProbe Geometries

Linewidth metrology: Probe optimized

for sidewall slope determination

V. Mancevski, Xidex Corp. 2000

Trenches

Ver

tica

l

Can

tile

ver

AFM CD Profiles: Sidewall angle metrology

V. Mancevski, Xidex Corp. 2000

NanoProbe Geometries

Height metrology: Probe optimized for depth

information

HAR nanoprobes used for via depth metrology

www.tmmicro.com

DEFECT_AFM.PDF

134 nm

Direct Profiling for

Defect Metrology

Image nanoscale

dimensional defects via

direct profiling (AFM/HRP)

Automated image processing

software used for defect

sizing and topographical

analsyis

Pacific Nanotechnology Corp. 2000

Additional Force Microscopy Techniques

• Scanning capacitance microscopy (SCM)

• Scanning thermal microscopy (SThM)

• Near field scanning optical microscopy (NSOM)

• Kelvin probe microscopy

• Scanning spreading resistance microscopy

• Contact Potential Difference (CPD)

• Pulsed force mode (PFM)

• Ultrasonic Force Microscopy (UFM)

Nanoscale Electrical Metrology

Scanning Probe Microscope Platform

• Scanning capacitance microscopy

• Scanning spreading resistance microscopy

• Electrostatic force microscopy

Applications

• Dopant profiling in semiconductors

• Static charge distributions in micro/nanostructures

Advantages: Provides direct imaging of electrostatic materials properties

Disadvantages: Requires destructive cross-sectioning for depth profiling

www.tmmicro.com

Comparison of SPM based Techniques

Conductive-AFM

(C-AFM)

Scanning Capacitance

Microscopy (SCM)

Kelvin Probe Force

Microscopy (KPFM)

Signal: Electric Current

Operate: Contact Mode

difficult to differentiate

carrier type

Signal: Surface Potential

Operate: Lift Mode

Relative surface potential

difference across sample

Signal: Capacitance

Operate: Contact Mode

Require top oxide layer

to form MOS capacitor

Veeco Instruments Inc. Application Modules: Dimension and MultiMode Manual

P. Breitschopf et al., Microelectronics Reliability 45 (2005) 1568-1571

William Thomson Lord Kelvin, Phil. Mag. 46 (1898) 82 62

180 nm

source drain

gate

R. Kleiman - Lucent

Rau and Ourmazd et al., IHP

SCM Image of CMOS transistor

(cross-section)

• p-type substrate visible

• n-type regions visible

• Insulators visible

TEM-based holography of

transistor (cross-section)

• Source, drain and gate regions

visible

SCM Image of DRAM cell (cross-section)

• Trench capacitors visible

• Contact regions visible

SCM Imaging of DRAM Cell

SCM

AFM

www.tmmicro.com

SCM Imaging of Transistor Drain SCM Scan Geometry

• Polished cross-section

• Scan on B-implanted p+ drain

region

SCM Image of B-implanted n-type Si

• Dashed line: silicon surface

• Solid and short-dashed line: transition

region to n-type Si

Rennex et al, Characterization and Metrology

for ULSI Tech. 2000

Doping profile ranges from 1017 cm-3 to 1019 cm-3

Spatial resolution ~ 10 nm

SCM ImageConverted Doping Contours

SCM Imaging of Transistor Drain

Rennex et al, Characterization and Metrology

for ULSI Tech. 2000

Surface Potential Measurement Based on Metallic Probes

d

Probe & sample electrically isolated

• Energy of electrons in tip and

sample (w.r.t. Evac) determined

by work function difference

• Tip and sample Fermi energies

not equal, in general

d

DV = VCPD

Probe & sample electrically connected

• Tip & sample Fermi energies

equalize

• Charge redistribution results in

contact potential difference:

VCPD=DF

• Resultant electric field between tip

and sample 71

( ) sinDC AC

dCF V V t

dz DF

2

2

1cos 2

4AC

dCF V t

dz

Kelvin Probe Force Microscopy Principle

21( )

2U C V D

21( )

2

dU dCF V

dz dz D

21( sin )

2DC AC

dCF V V t

dz DF

221

( )2 2

ACDC DC

dC VF V

dz

DF

• Force on tip is derivative of energy

with separation distance:

• Split force into three spectral

components:

72

• Total force at applied voltage:

H. O. Jacobs et al., Ultramicroscopy 1997,. 69, 39. ΔΦ : contact potential difference

ω : resonant frequency of cantilever

VCPD

VACsin(t)

VDC

• Introduce DC and AC voltage

between tip and sample:

• Treat tip-sample as a simple

capacitor

tVVV ACDC sinDFD

Tip oscillation at modulated by DF-VDC

KPFM Working Principle

1st pass: Topography 2nd pass: Surface Potential

M. Nonnenmacher et al., Appl. Phys. Lett. 58, 2921 (1991) http://www.ntmdt.com/spm-principles/view/kelvin-probe-microscopy

T Machleidt et al., Meas. Sci. Technol. 20 (2009) 084017 http://www.kelvinprobe.info/technique-theory.htm

Lift Height

73

21( sin )

2DC AC

dCF V V t

dz DF

• Total force at applied voltage:

Tip oscillation at modulated by

DF-VDC

( ) sinDC AC

dCF V V t

dz DF

VCPD

VACsin(t)

VDC

Experimental Setup

VCPD

VACsin(t)

VDC

SiO2

SiO2 POLY SiO2 POLY SiO2

SiO2

Si Substrate

Graphene flake

V1

V2

74

• Gate voltages were applied from

external power sources

• Graphene was grounded with

respect to KPFM probe

• Micromanipulators were used to

contact electrode extensions

G1= 0V, G2= 0V G1= -10V, G2= +10V G1= +10V, G2= -10V

Ambipolar p-n junction in exfoliated pristine graphene by electrostatic doping

Exfoliated Graphene p-n Junction

75

SiO2

Section profile from exfoliated

graphene on 200nm gate spacing

Fitting width: 307 ± 16 nm

Direct Measurement of Junction Profile

0 1 2 3 4

-200

-175

-150

-125

25

50

75

100

G1=-10V; G2=+10V

pn_erf (User) Fit of Sheet1 Potential

Surf

ace

Pote

ntial (m

V)

Position

G1=+10V; G2=-10V

pn_erf (User) Fit of Sheet1 Potential

p-regions

n-regions p-regions

n-regions

• Switchable p-n junction behavior observed

Junction Profiles: Exfoliated Graphene

0 5 10 15

0

50

100

Gate 4Gate 3Gate 2Gate 1

No

Gate

-5V-5V-5V

-5V

+5V+5V+5V

+5V, -5V

-5V, +5V

Su

rfa

ce

Po

ten

tia

l (m

V)

Distance (m)

+5V

Switchablep-n junction

Switchablep-n junction

Distance (m)

+5V -5V

-5V

+5V

No Bias

77

Lateral Force Microscopy• The probe is scanned sideways. The

degree of torsion of the cantilever is used as a relative measure of surface friction caused by the lateral force exerted on the probe.

• Identify transitions between different components in a polymer blend, in composites or other mixtures

• This mode can also be used to reveal fine structural details in the sample.

LFM measures lateral deflections (twisting) of the cantilever that arise from

forces on the cantilever parallel to the plane of the sample surface.

LFM images variations in surface friction, arising from inhomogeneity in surface

material, obtaines edge-enhanced (slope variations) images of any surface.

To separate the effects AFM and LFM should be used simultaneously.

Lateral deflection by slope variations

Lateral Force Microscopy

Magnetic recording head. Al oxide

grains and contamination

800nm scan

Natural rubber/EDPM blend: 20 micron scan

Polished poly-crystal silicon carbide film. Grain structures

30 micron scan

Images/photo taken with NanoScope® SPM, courtesy Digital Instruments, Santa Barbara ,CA

Phase Imaging

• Accessible via Tapping Mode• Oscillate the cantilever at its resonant frequency. The amplitude is used as

a feedback signal. The phase lag is dependent on several things, including composition, adhesion, friction and viscoelastic properties.

Identify two-phase structure

of polymer blends

Identify surface

contaminants that are not

seen in height images

Less damaging to soft

samples than lateral force

microscopy

Phase Imaging

Composite polymer imbedded in a matrix 1 micron scan

Bond pad on an integrated circuit: Contamination

1.5 micron scan

MoO3 crystallites on a MoS2 substrate

6 micron scan

Image/photo taken with NanoScope® SPM, courtesy Digital Instruments, Santa Barbara ,CA

Magnetic Force Microscopy• Special probes are used for MFM. These are magnetically

sensitized by sputter coating with a ferromagnetic material.

• The cantilever is oscillated near its resonant frequency (around 100 kHz).

• The tip is oscillated 10’s to 100’s of nm above the surface

• Gradients in the magnetic forces on the tip shift the resonant frequency of the cantilever .

• Monitoring this shift, or related changes in oscillation amplitude or phase, produces a magnetic force image.

• Many applications for data storage technology

Magnetic Force Microscopy

Overwritten tracks on a textured hard disk, 25 micron scan

Domains in a 80 micron garnet film

Image/photo taken with NanoScope® SPM, courtesy Digital Instruments, Santa Barbara ,CA

Nanoscale Elastic Modulus Metrology

Potential Applications in Nanomaterials:

Mechanical Property Control in Nanomaterials

• Mechanical responses poorly understood for nanoscale

materials

• Polymeric materials

• Nanoporous materials

• Nanoscale mechanical defects difficult to image

• Explore nanoscale reliability of materials and structures

Investigate Scanning Probe approach for Elastic Metrology

Nanoporous Dielectric

(Cross-section)

SiC Fiber/SiO2 Composites

(Cross-section)

1 m

PMMA

Rubber Inclusions

Polymer/Polymer Composites

Why is nanoscale mechanical metrology required?

Si

SiGe

Strained Si Transistor

(Cross-section)


Mechanical Defects Limits Electrical Functionality of NTs

Zhao et al. Sci. and App. of Nanotubes p.195 (2000)


sample piezo

SOD Trench

Epoxy

quadrant PD

AFM tip

S

Nanoscale Elastic Imaging: Ultrasonic Force

Microscopy (UFM) Experimental Configuration

• Sample mounted in

conventional SPM system

• Ultrasonic oscillation provided

by piezoelectric transducer

• CS-mode: underside piezo

used

• Operating parameters

fultrasonic = 2.2 MHz

fmod = 13 kHz

Tip force : 5-30 nN

Scan rate: 3 m/s

Elastic Imaging: Delamination of Ta on Low-k SiLK

Polymer

3D Plot for Topography (left) and Elasticity (right) highlights differences

in information content between AFM and UFM

200 nm SOD Trenches

in SiO2: Topography and CS-UFM

SiN Liner (12 nm)

TOPOGRAPHY

500 nm

CS-UFM

500 nm SOD SiO2

• Cross-section of

SOD-filled

trenches

• TOP reveals little

structure

• CS-UFM reveals:

• SiN sidewall

• Gap fill

mechanical

variations

• Estimated Depth Sensitivity ~ 25 nm

• Estimated In-plane resolution < 10 nm

Muthuswami et al, IITC, 2002

Densification in Gap-Fill

PMD: Corner Defects

Low resistance of SOD to HF etch often

observed in corner area of large

trenches (> 200nm)

SEM reveals low contrast at corner

(void or density variation

AFM Topography and CS-UFM reveal

low-mechanical compliance in corner

region

ESOD/ESiO2 1.0 0.05

1 mSEM

1 mTOP

1 mCS-UFM

0.00 0.25 0.50 0.75 1.00

0

2

4

He

igh

t (n

m)

x (m)

0.9

1.0

2S

iOE

/E

• Calibrated CS-UFM relative to SiO2

reduced modulus (vertically-averaged profile in area defined by white rectangle)

• EPMD/ESiO2 0.90 – 0.93

• EPMD 55 ± 2 GPa

• Data not spatially deconvoluted

Profile Average:

Relative Modulus Variation

Densification in Gap-Fill PMD: Intra-Trench

Defects

150 nm trenches filled with PMD.

PMD exhibits marginal modulus reduction; PMD exhibits

isolated defects at center of trench (no corner nucleation)

TOP CS-UFM

Intra-Trench SOD

Defects: Resistance

to HF Etch• SOD cross-sections

exposed to HF etch

(200:1 H20: HF for 90

seconds)

• Regions of low

mechanical compliance in

CS-UFM correspond

exactly to SOD regions

with low resistance to HF

etch

CS-UFM TOP – Post Etch

CS-UFM TOP – Post Etch

250 nm 250 nm


Intra-Trench SOD Defects: Calibrated Response

• Mechanical defects in SOD: ESOD/ESiO2 0.65

• Approximately 30% reduction in effective modulus

• Attributed to incomplete densification due to diffusion constraints during cure

cycle


Summary: Scanning Probe Microscopy

Summary

Topographic Imaging

AFM and STM offer atomic resolution and are primarily nondestructive

AFM subject to various imaging artifacts – most notably tip-sample interaction

Well characterized technique suitable for metrology application

Other Imaging Modes

Greatest utility for SPM lies in transduction of signals to tip-displacement

Wide array of imaging modes – most of these techniques less ‘well-

characterized’ compared to topography

Future Directions

SPEED!

CALIBRATION!

Near-field Optical Microscopy

• Aperture and aperture-less approaches

• Nano-Raman microscopy

• Selected examples: Stress profiling in Si

Diffraction of Light from an Aperture > λ

• The diffraction of light from an aperture > λ can be represented as follows

)(

0zx kkti

in eEE

Aperture Δx

x

zkxkti

xscatterd dkezkFE

E zx )(0 )0,(2

kz

kx

k

222 2

zx kk

where dkx ≈ 2π/Δx and F

is the Fourier function

of the aperture

• For Δx > λ the scattered field is given as:

• The scattered field represents propagating EM waves in space

• k is the wave vector and the dispersion relation is:

Incident light Scattered lightscatteredE

Diffraction & Near-Field Optics

• But, when Δx < λ: 222 2

kk x

• Therefore, the dispersion relation requires 22kkik xz

221)(/0 where)0,(2

kkddkeezkFE

E xpx

xktidz

xscatteredxp

• Scattered wave propagates in x direction and decays exponentially in z

• Non-propagating (evanescent) wave: A local illumination source

kz

kx

k

222 2

zx kk

xkx D

2

Near-field optics: incident plane waves passing through apertures << λ

)(

0

zkti

inzeEE

Aperture Δx ~ 2/k

scatteredEIncident light

• The wave uncertainty principle requires:

dkx ≈ 2π/ΔxF = aperture Fourier function

Aperture-Based Near-Field Optical Probe

• Near-field light: Non-propagating light localized at a sub-wavelength aperture – Aperture typically illuminated by a far-field

source– Near-field light at aperture is evanescent in

nature (exponentially decays along the aperture normal)

– Decay length ~ /2

• Near-field Optical Probe– Aperture is brought in close proximity to

sample (~ /2)– Interaction of evanescant field with sample

results in emitted photons– Emitted photons directed to far-field

detector

Near Field Optical Microscope NSOM: Experimental geometry

A tapered end (metallized)

optical probe serves for:

optical imaging

topographic imaging (AFM)

The resolution is determined

mainly by the aperture size.

Various illumination geometries are used.

Illumination Mode: The probe illuminates the object in the

NF region and the signal is collected by detector in the

FF

Collection Mode: The probe collects signals in the NF of a

specimen illuminated by FF

Illumination-Collection mode: The probe illuminates and

collects the signal in the NF region

Advantage: Lateral resolution better than the diffraction limit is achieved

Overview: What is Raman Scattering?

Laser source

Freq. n0

sample

n

n0 n

n0 n

Notch filter

Vibrating molecule

(or phonon excitation)

in the sample

Raman

scattered

photons

detector

Rayleigh scattered photons

• Raman scattered light refers to that portion of light which undergoes inelastic

scattering

• Monitoring the inelastically scattered photons provides a vibrational (or

phonon) spectrum of a molecule (or crystal) which is referred to as its Raman

spectrum

• In general only about 1 in 107 photons undergoes Raman Scattering

n n

cou

nts

Raman shift(cm-1)

Optical Si Stress Metrology: Micro-Raman Spectroscopy

Drawbacks of Micro-Raman spectroscopy for Si stress metrology

• Typical spatial resolution is 1 micron (200nm reported for UV Raman)

• Penetration depth in Si is about 300 nm: Implies an averaging of stress over a

significantly varying stress profile

Raman spectrum from Si (neutral and stressed) Raman frequency shift from stress-free value

Ingrid De Wolf (1996) Semicond. Sci. Technol. 11139-154

cou

nts

ωofrequency (cm-1)

Δω>0: compressiveΔω<0: tensile

ωcωt

Alternate Approach to NSOM-based Optical Spectroscopy: Apertureless NSOM

• Utilize a sharp conducting tip as a local

scatterer

• FF illumination excites surface plasmons at

conducting surface resulting in evanescent

field (‘optical tunneling’)

• Collect photons emitted from sample surface

in FF with large NA optics

• Spatial resolution of ANSOM is determined by

tip radius, and collection angle can be largeANSOM:

More

Efficient

Photon

Collection

NSOM:

Low Optical

Throughput

of Aperture

Metallized

Probe Tip

Evanescent

field

sample

Far-Field

Illumination

Tip-mediated field enhancement simulation results:

(Left) Using p-polarized light

(Right) Using s-polarized light (right).

The incident angle is 45o and the observation plane is /200 below the tip

end.

q

kTip

Schematic tip diagram for

simulations.

q: incident angle of far-field

illumination

k: optical wave vector

Tip length = and tip width = /80.

† W.X. Sun, Z.X. Shen, Ultramicroscopy, 94, 237 (2003).

Apertureless NSOM: Fundamental Concepts

• Tip-induced enhancement factor of scattered p-polarized optical field

theoretically approaches 10,000†

• A-NSOM employs the combination of two base systems:

• Veeco Aurora-3 NSOM (shear-force feedback system)

• Renishaw RM-100 Raman spectrometer

?Ti

p

Sam

Optical fiber

?Ti

p

Sample

Turning fork

laser

laser

source

detector

Apertureless NSOM: Experimental Setup

Veeco Aurora-3 NSOMRenishaw RM-100

Raman Spectrometer

Holographic Notch

Filter (used as beam

splitter)

Shear-force, tuning-

fork-based feedback

system

200 m

200 nm

Ag-coated W

tip used to

acquire

TERS data

100 m

Tip

Probe radius:

≤≤ 5 nmSample

Spectrometer

(Raman)

laser

STM tip on

Turning

fork

Tip

Probe radius:

≤≤ 5 nmSample

Spectrometer

(Raman)

laser

STM tip on

Turning

fork

• Tips fabricated from 250 mm and 125 mm diameter W wire

– Reduced W wire diameter yields improvement in phase feedback of Aurora-3 NSOM

– Nominal changes in tip radius from electropolishing

Apertureless NanoRaman: Tip Fabrication and TERS for Fixed Tip (45º Incidence -Veeco Aurora)

200 nm

Ag-coated W tip

used to acquire

TERS data Tip

radius

~ 17 nm

TERS Enhancement for W Tips: Coated vs. Noncoated

TERS Enhancement investigated as a function of average tip radius

– No enhancement measured for non-coated tips of any radius

– Ag-coating (5-10 nm) used for TERS tips fabrication

0.001 0.010 0.100

100

1000

10000 Enhancement Factor

Power law exponent 2.03 +/- 0.2

Observed dependence

of enhancement on

tip radius

En

ha

nce

me

nt

Fa

cto

r

(Tip Radius)-1

(nm-1

)

TERS Enhancement:

Tip-enhancement from first-pass

testing was documented

Enhancement determined from relative

increase in normalized Raman

intensity.

TERS Enhancement as a Function of Tip

Radius

200 nm

Ag-coated W tip used

to acquire TERS data

Tip radius

~ 17 nm

Horizontal

tuning fork

Bent tip

Sample

Illumination/

Collection

Optic

Hologrpahic

Notch

Filter

GratingDetector

Ar-ion

Laser

Apertureless NSOM: Experimental Setup

200 m

Ag-coated W

100 m

Ag-coated W

2 m

200 nm

Current Probe:

Au-coated Ag

microparticle on

micro-pipette

Application of TERS to Strained-Si Stack

High-res reciprocal space mapping of sSOI (Special thanks to Prof. R. J. Matyi)

• Two streaks seen at 004:

Vertical streak ║ to 00l: dynamic diffraction peak

• Intensity at l = 4.023 from the sSOI corresponds to Dc/c = 0.00575 or 0.575%

• Assuming biaxial strain and n = 0.278, in-plane strain = 0.746%

SOITEC Stack

Buried oxide

Si (001)

Strained Si (001)63 nm

140 nm

Application of TERS to Strained-Si Stack (Aurora NSOM)

Raman spectra of SOI stack (tip out of feedback)

• Strained peak: 514.40 cm-1

• Bulk peak: 520.37 cm-1

• Lorentzian widths ~ 3.3 cm-1

• Corresponds to in-plane strain of 0.00767 ~ error of 3% with XRD

SOITEC Stack

Buried oxide

Si (001)

Strained Si (001)

(tensile)63 nm

140 nm

510 520 530 5400.00

0.75

1.50

2.25

Data: TIP2RETRACT_D

Model: Lorentz

Equation: y = y0 + (2*A/PI)*(w/(4*(x-xc)^2 + w^2))

Weighting:

y Statistical

Chi^2/DoF = 5.53081

R^2 = 0.87152

y0 31.5174 ±0.25459

xc1 520.36811 ±0.02381

w1 3.32971 ±0.061

A1 11311.65341 ±151.93658

xc2 514.40385 ±0.0281

w2 3.07531 ±0.07163

A2 7877.05405 ±140.21919

Blanket SOI Point Spectra

Inte

nsity (

10

3co

un

ts)

Raman Shift (cm-1)

2 = 5.53081

y0 31.52 ± 0.25

xc1 520.368 ±0.024

w1 3.33 ± 0.06

A1 11311 ± 152

xc2 514.404 ± 0.028

w2 3.08 ± 0.07

A2 7877 ±140

TERS : Vertical Incidence

500 520

0

900

1800

2700

Data: FARFIELD1_B

Model: TwoLorentz

Weighting:

y Statistical

Chi^2/DoF = 5.1279

R^2 = 0.98229

y0 -43.21965 ±1.1297

A1 6873.31779 ±320.81741

A2 11659.05714 ±360.95508

w1 3.69592 ±0.10873

w2 3.10291 ±0.12422

xc1 515.0956 ±0.12679

xc2 519.92405 ±0.0489

Tip Retract

Tip in Feedback

inte

nsity

(C

ts)

Raman shift cm-1

Data: FARFIELD1_I

Model: TwoLorentz

Weighting:

y Statistical

Chi^2/DoF = 0.99226

R^2 = 0.99119

y0 500.10642 ±4.97657

A1 5223.97914 ±526.70146

A2 6856.70588 ±616.33092

w1 3.90736 ±0.25597

w2 3.46875 ±0.39295

xc1 514.97443 ±0.24832

xc2 519.79103 ±0.14241

Ratio of strained Si peak to

unstrained Si peak:

Tip Retract: 0.59

Feedback: 0.762

Clearly indicates enhancement,

but coupled to other effects –

shadowing and background

increase

Peak at 520cm-,from Si substrate

is not enhanced as expected

Tip placement optimization: Scan

for minimization of shadowing

Ratio of strained Si peak to

unstrained Si peak:

Tip Retract: 0.53

Feedback: 1.42

Twofold mprovement in relative

count rate between Si and sSi500 510 520 530 540

0

1000

2000

3000

4000

5000

Data: Data1_RetractBG11mod

Model: Lorentz

Equation: y = y0 + (2*A/PI)*(w/(4*(x-xc)^2 + w^2))

Weighting:

y No weighting

Chi^2/DoF = 13686.15933

R^2 = 0.98208

y0 56.22984 ±14.07886

xc1 520.18229 ±0.04699

w1 3.33338 ±0.15636

A1 20121.0089 ±905.00841

xc2 515.32884 ±0.10665

w2 4.24811 ±0.34446

A2 13559.13075 ±1030.40208

Inte

nsity

(Cts

)

Shift (cm-1)

Feedback

Retract

• Image reversal approach used with positive PR

– Cr hardmask

– Wet hardmask etch

– TMAH Si etch (visual endpointing)

– Cr strip and piranha clean

Apertureless NanoRaman:

Application to Patterned SOI Test structures

Faceting due to anisotropic etch

rate of TMAH

Investigate 2 m diameter ‘mesas’

of strained Si

Substantial relaxation to

investigate TERS spatial

resolution

AFM with TERS probe provides

reasonable spatial accuracy

No evidence of erosion in strained

Si layer – possible erosion in BOX

(< 10 nm)

2.521.510.50

60

50

40

30

20

10

0

X[µm]

Z[n

m]

600nm

• Map-merge of nanoRaman and Topography

– Asymmetric strain distribution – (111) axis

– Spatial offset observed – requires additional calibration from tip-scanning

– Substantial local variation in observed strained Si peak position

– Convolution, process-induced relaxation

Apertureless NanoRaman:

First Pass Mapping of SOI Test structures

Topography

2 m

Raman Map+ Topo

2 m

TERS Enhancement• Evaluate TERS with and without

device layer

• Enhancement observed for bulk Si and device layer Si

• Consistent with tip radius (probe depth ~ 2Rc)

1

2

3

4

500 510 520 530 540-500

0

500

1000

1500

2000

2500

3000

3500

Inte

nsity (

Cts

)

Raman Shift (cm-1)

Point 1

Point 2

Point 3

Point 4

Differential Raman Intensity

Device Layer

Enhancement

Bulk

Enhancement

0 1 2 3 4 50

1000

2000

3000

4000

5000 MP Tip in Feedback

MP Tip Retracted

Differential (TERS Signal)

Ra

ma

n P

ea

k In

ten

sity

Position (m)

Green squares indicate total Raman signal from strained-Si mesa (far-field + TERS).

Blue circles represent same scan with tip retracted from surface.

Red triangles represent the differential nanoRaman TERS signal.

Dramatically increases spatial resolution.

mesa

Differential Raman Mappping: Separating near-field signal from far-field signal

Apparent reduction of

TERS signal at edges of

mesa structure

Width of signal

reduction region ~ 275

nm

Total internal reflection

exclusion zone at edge

~ 250-300nm

Intensity reduction not

expected to affect peak

position for TERS

Differential Raman Mappping:

Edge-induced signal Raman signal reduction

1 2 3 4

0

1

2

3

4

0

50

100

Inte

nsity (

10

3 C

ts)

Position (m)

H

eig

ht

(nm

)

Tip

Evanescent Field of Tip

Illumination

Strained Si

Tip

Evanescent Field of Tip

Illumination

Strained Si

Quantitative Stress Profiling via NanoRaman: FEA of strain relaxation in sSi mesas

• Finite element analysis of relaxation

in strained Si mesas for comparison

with nanoRaman measurements

• Intellisuite mechanical FEA package

• 0.15 m mesh (in plane): Si (0.5 m x

6m x 6 m); Box (0.14 m x 6m x 6

m); sSi (0.063 m x 2m x 2 m

[octagonal])

• Initial (biaxial strain) 0.0075

2 3 4

0.0

0.4

0.8

1.2

1.6

2.0

2.4

0.0

0.3

0.6

0.9

1.2

1.5

0

50

Differential Raman Intensity

Gaussian Step

Diffe

ren

tia

l R

am

an

In

ten

sity (

10

3 C

ts)

Position (m)

Measured Stress

Stre

ss (G

Pa

)

Heig

ht (n

m)

Quantitative Stress Profiling via NanoRaman: FEA of strain relaxation in sSi mesas

• Biaxial stress assumed

• FEA results vertically

averaged over mesa

structure

• Simple model in excellent

agreement with measured

differential signal peak

positions

• Poor agreement with peak

positions determined from

near-field+far-field data

FE Simulation

AFM/Raman Spectral and Spatial Resolution:

Measuring Residual Si Strain Under Strained Si Islands

Strained Si ‘Mesa’

Current Probe

Size

Target

Probe

Size

2 m

200 nm

Current Probe:

Au-coated Ag

microparticle on

micro-pipette

Figure 1. (Left) SEM micrograph of current nanoRaman probe consisting of a Au-coated Ag particle on a SiO2 micropipette. (Right) Schematic of target size for new metal-tip carbon fiber probe. Reduction of probe size to 50 nm diameter should enable sufficient spatial resolution for nanoRaman stress profiling in strain-engineered Si devices.

• Patterning of strained Si layer on SOITEC SOI wafers results in change in near-surface stress profile of the underlying Si wafer.

– Strain in device layer Si > 1GPa (tensile)

– Strained-Si layer removed via wet etch

– Modification of mechanical environment on removal of the strained-Si layer has a discernible effect on Si underlayer

– Preliminary analysis: Strain relaxation in island leads to strain profile in underlying Si (through buried oxide).



Spectral mapping of Si underlayer stress profile

AFM/Raman spectra near the bulk Si Raman peak

(~ 521 cm-1) were acquired along dashed line in the

AFM topograph (above)

Collection at every other 4 pixels

Si below strained-Si island shows residual

compressive stress (~ 13 MPa)

Gaussian width of ‘transition region’ ~ 150 nm0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

521.44

521.46

521.48

521.50

521.52

521.54

521.56

521.58

Ra

ma

n S

hift

(cm

-1)

Position (m)

13 +/- 6 MPa

Si area under

strained-Si island

Si area under open BOx

2 m

Strained Si ‘Island’

Buried Oxide

Residual Si Strain - Model

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0521.44

521.46

521.48

521.50

521.52

521.54

521.56

521.58

Ra

ma

n S

hift

(cm

-1)

Position (m)

13 +/- 6 MPa

Si area under

strained-Si island

Si area under open BOx

Topography

2 m

Strained Si (0.75% tensile)

Bulk Si

BOx

Little or no strain in Bulk Si

Bulk Si

BOx

Strain Relaxation in Island

Residual compression in Bulk Si under Island



Bulk Si

Area under mesa

Preliminary FEA qualitatively

supports observation

Compressive strain observed at

below BOx interface (>> 18

MPa)

0.0 0.4 0.8 1.2 1.6 2.0

521.5

521.6

521.7 Bulk Si Raman Shift

Guide to the Eye

Ram

an

Shift (c

m-1)

Position (m)

18 +/- 6 MPa

Si area under

strained-Si

islandSi area under

open BOx

FE Model

spm_overview for nsci 434_2013 (1)

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