High Resolution Quantitative Kelvin Probe Force Microscopy-Principles and Applications
PeakForce KPFM
Chunzeng Li, Ph.D
Applications Scientist
Bruker Nano Surfaces Division
Outline
• KPFM Background
• A New Mode: PeakForce KPFM
• Improved Spatial Resolution and Accuracy
• Improved Repeatability
• Compatible with Quantitative NanoMechanical property mapping (PF-QNM)
• Summary
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Many Ways of Doing KPFM FM and PeakForce scaling do not compete
AFM
KPFM Tapping PeakForce
AM TP-AM PeakForce
KPFM-AM
FM TP-FM PeakForce
KPFM
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Both
AM
& F
M K
PF
M
impro
ves w
ith low
er
k
Tapping is limited to high k levers due to
adhesive forces. PeakForce Tapping k is not.
*Except TP-FM, all are done in lift-mode.
Two Known KPFM Modes FM Provides high resolution and accuracy
AM Amplitude-Modulation
FM Frequency-Modulation Better spatial resolution
Better accuracy
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Physical Review B 2005, 71(12) 125424
KPFM measures the work function difference of tip/sample.
Probe Modeling and Assumptions Electrostatic Forces are Long Range - cantilever geometry matters
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L W
∆L
l
The probe body-cone and lever-is an equal
potential body.
Charges are only present on the surface
(holds for any good conductor).
The conical body surface is a stack-up of
rings, each ring contributes to the total
electric field in proportion to their capacitance
(assumption).
Cantilever Simulation Contribution to AM and FM
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um
H
WL
zH
WL
z
C
H
WL
zH
WL
z
C
zH
WLdl
L
l
zH
WC
L
l
zH
LWC
Lever
Lever
L
Lever
l
/375.3
2
1
)(2
1
2
1
875.16
4
1
)(4
1
2
1
2
1
332
2
22
0
um10 H um, 225L um,30 W :PIT-SCM
um10 H um, 225L um,30 W :PIT-SCM
force on deflection per end Tip to d(normalize )
L W
∆L
l
Tip Cone Simulation Contribution to AM and FM
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h H
z
Ө
z
zh
z
hz
dhzh
hC
hzh
h
d
AC
h
h
h
ln)tan(2
)tan(2
)tan(2
0
h. height to tip cone from ecapacitanc of nIntegratio
Tip Cone Contribution in KPFM FM gradient detection isolates contribution from tip
• FM-KPFM:
• The foremost 0.3% of the tip cone accounts for half of the signal in.
• FM can achieve a lateral resolution better than 50nm.
• AM-KPFM
• The contribution from the tip cone never reaches 50%.
• Its lateral resolution is dictated by the um-scale lever.
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0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
0% 0% 1% 10% 100%
Co
ne
Co
ntr
ibu
tio
n%
Height Inclusion (h/H)%
FM z=10 nm
FM z=50 nm
AM z=10 nm
AM z=50 nm
Based on SCM-PIT Geometry:
W=30um, L=225um, H=10um, Cone Angle=45
Frequency Modulation-Core Concept
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Force Gradient changes the effective spring
constant.
m
k
:oscillator harmonic driven simple a For
z
F
kk
k
el
22
:gradient force electric by caused shift Frequency
PeakForce KPFM Retains FM-KPFM’s High Resolution
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PeakForce KPFM-AM
PeakForce KPFM
PeakForce KPFM High Resolution Example
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2nm
30nm
30mV
105mV
Single Strand Carbon Nanotube (~2nm)
PeakForce KPFM VS FM-AM FM detection advantage maintained
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240 mV
PeakForce KPFM
97 mV
PeaForce KPFM-AM
FM sees larger and more localized contrast leading to better accuracy.
AM contrast smaller and more convoluted.
Sn60Pb40 Alloy
Work functions: Sn 4.42 eV; Pb 4.25 eV
PeakForce KPFM Offers Simultaneous Mechanical Information
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Height 100 nm
Deformation 25 nm
Adhesion 5 nN
Modulus 10 MPa
Potential 150 mV
PS=PolyStyrene LDPE=Low Density PolyEthylene
PS
LDPE
PeakForce KPFM vs TP-FM No spring constant conflicts, yields superior mechanical data in PeakForce Tapping Mechanical Property Maps
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240 mV
PeakForce KPFM
235 mV
TP-FM
Sn-Pb Alloy PeakForce
Adhesion
Tapping
Phase
Improve Repeatability Through Tight Parameter Control and Probe Design
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Sources of Uncertainty Bruker Solution
Operating Frequency Tight parameter control (ScanAsyst-KPFM):
• Thermal tune for resonance frequency
• Fixed oscillation amplitude
• Optimal phase setting
Tip-Sample Separation
Tip work function change
due to tip wear
Probe Design:
• Single tip material
• Proprietary way to limit DC current flow Electrochemical reaction
under bias
PeakForce KPFM Repeatability 5x improvement over traditional KPFM
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-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
0.95
Au-Al 0.825 0.019 0.847 0.796
Au 0.639 0.018 0.617 0.670
Al -0.185 0.020 -0.159 -0.222
Average Std Dev Maximum Minimum
9 KPFM Porbes
Bruker AFM + MBraun Glovebox -Integrated, Turnkey
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Sturdy
Support
Vibration
Isolation
Table
<1ppm
O2/H2O
It Must Be Artifact-free to be quantitative But sometimes, Artifact can be deceivingly beautiful.
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Height 5 nm
Deformation 5 nm
Adhesion 2.5 nN
Potential 600 mV
Brush Polymer on Mica
PeakForce KPFM Lift Mode Offers a Means to Avert Artifact
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Brush Polymer
250 nm x 250 nm
22 nm
23 nm
24 nm
25 nm
27 nm
28 nm
While small tip-sample distance is desirable for
high spatial resolution, it is sometimes necessary
for tip to completely clear surface to avoid
artifacts due to tip-sample direct contact.
Artifacts PeakForce-KPFM: can be identified and avoided with
Tapping KPFM-FM: can be unavoidable sometimes
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KPFM-FM (Tapping mode, single-pass
PeakForce KPFM
Lift Height Test
Height Phase Potential
Lift-Height 35nm Lift-Height 40nm
Solar Cell MultiCrystal Si
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Organic Photovoltaic Applications: PCBM Crystals on MDMO-PCBM Matrix
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Work function downshifts 535 mV under 300-sun illumination.
Height Adhesion Potential
Da
rk | L
igh
t
Particles are PCBM crystals on matrix of MDMO-PCBM blend, ITO substrate.
Sample courtesy of Dr. Philippe Leclere, University of Mons
Scaling Topography and Potential PeakForce KPFM to Take Us Further
• But Tapping Mode Requires :
• k to be not too small
• Q not to be too big
Tapping and KPFM scaling in conflict.
• Peak Force Tapping Mode Allows Freedom to use:
• Smaller k (10x or more)
• Big Q (10x or more)
PeakForce Tapping and KPFM scaling aligned.
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Summary
PeakForce KPFM- A New Way of doing KPFM.
• It retains the high spatial resolution and accuracy of FM-KPFM.
• It acquires high repeatability through tight parameter control and probe design.
• It leverages PeakForce QNM to give simultaneous mechanical and electrical information.
• It bears the promise to further enhance FM-KPFM sensitivity.
We have come to a point that we can begin to call KPFM Quantitative, and High Resolution (spatial).
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Work Function Table
PeakForce KPFM-HV Measuring High Voltage the Soft Way-Principle
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2)(2
1V
z
C
z
UFel
)sin( tV
eVV mACDC
Term 2
Term
Term DC
)2cos(4
1
)sin()(
)2
1)((
2
1
2
22
tVz
C
tVe
Vz
C
Ve
Vz
C
F
AC
ACDC
ACDC
el
2
2
2
4
4
1
AC
ac
V
A
A
AV
z
C
0dcV
PeakForce KPFM-HV Electric Potential of Static Charge
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-99 V
158 V
PeakForce-HV modes measures up to 200 V electrostatic potential with <15%
relative error.
PDMS
Electrostatic Potential on PDMS
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Summary
PeakForce KPFM-that
• is Quantitative:
High Resolution (spatial)
Accurate
Repeatable
• gives simultaneous mechanical and electrical information.
• bears the promise to further enhance FM-KPFM sensitivity.
PeakForce KPFM-HV extends potential range beyond ±200V.
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