High Resolution Quantitative Kelvin Probe Force … · High Resolution Quantitative Kelvin Probe...

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

8/22/2012 2 Bruker Nano Surfaces Division

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


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


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


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


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


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.


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

9 Bruker Nano Surfaces Division 8/22/2012

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


PeakForce KPFM-AM

PeakForce KPFM

PeakForce KPFM High Resolution Example


2nm

30nm

30mV

105mV

Single Strand Carbon Nanotube (~2nm)

PeakForce KPFM VS FM-AM FM detection advantage maintained


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


Height 100 nm

Deformation 25 nm

Adhesion 5 nN

Modulus 10 MPa

Potential 150 mV

PS=PolyStyrene LDPE=Low Density PolyEthylene

PS

LDPE

//upload.wikimedia.org/wikipedia/commons/f/f7/Polyethylene-repeat-2D-flat.png

//upload.wikimedia.org/wikipedia/commons/6/60/Polystyrene.svg

PeakForce KPFM vs TP-FM No spring constant conflicts, yields superior mechanical data in PeakForce Tapping Mechanical Property Maps


240 mV

PeakForce KPFM

235 mV

TP-FM

Sn-Pb Alloy PeakForce

Adhesion

Tapping

Phase

Improve Repeatability Through Tight Parameter Control and Probe Design


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


-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

17 Bruker Nano Surfaces Division

Sturdy

Support

Vibration

Isolation

Table

<1ppm

O2/H2O

It Must Be Artifact-free to be quantitative But sometimes, Artifact can be deceivingly beautiful.


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


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


KPFM-FM (Tapping mode, single-pass

PeakForce KPFM

Lift Height Test

Height Phase Potential

Lift-Height 35nm Lift-Height 40nm

Solar Cell MultiCrystal Si


Organic Photovoltaic Applications: PCBM Crystals on MDMO-PCBM Matrix


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.


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).



Work Function Table

PeakForce KPFM-HV Measuring High Voltage the Soft Way-Principle

26 Bruker Nano Surfaces Division

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


-99 V

158 V

PeakForce-HV modes measures up to 200 V electrostatic potential with <15%

relative error.

PDMS

//upload.wikimedia.org/wikipedia/commons/6/6a/Pdms.png

Electrostatic Potential on PDMS


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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