Nanoscale mechanical property measurements in AFM modes with direct force controlPart I: PeakForce Tapping and Force Volume mechanical property mappingBede Pittenger, Senior Applications Scientist

A brief review of AFM imaging technology

• Mapping topography -> More information

• Contact mode (1986)

• Tapping Mode (1992)

• Force-Volume Mapping (~1992)

• Contact Resonance (AFAM, UAFM~1996)

• Peak Force Tapping/PeakForce QNM (2009)• PeakForce TUNA (2011)• PeakForce KPFM (2012)• PeakForce IR (2014)• PeakForce XYZ (…)

7/16/2014 2

• Sinusoidal ramping (not linear): no piezo resonance, no overshoot

• Real feedback loop force control: benefits from prior curves

• Fast ramping (~kHz): faster images, even with more pixels

• Linear ramping: abrupt turn-around at high speed -> ringing, overshoot

• Discrete force triggers at each ramp: attempts to turn around at trigger. At high speeds, it can’t reverse fast enough, so it overshoots.

• Ramping rate is limited (<~0.01kHz for best curves)

7/16/2014 3

Force Volume (FV)Z motion

Deflection

PeakForce Tapping (PF-QNM)

PeakForce QNM vs. Force VolumeMechanical property mapping modes

The result combines:• Best force control• Fastest ramp speed• Highest imaging resolution• Excellent force stability

The result, three options:1. Fast & high resolution, but poor

force control, poor z accuracy2. Good force control & high res.,

but slow imaging (>1h)3. Fast & good force control, but

low resolution (few pixels)

7/16/2014 4

Force Volume (FV)Z motion

Deflection

PeakForce Tapping (PF-QNM)

PeakForce QNM vs. Force VolumeMechanical property mapping modes

PFT Provides Excellent Spatial Resolution & Force Control

7/16/2014 5

PF-QNM & FV calculate sample properties directly from force curves

(ii)

The complete force curve from every interaction between tip and sample is analyzed in real-time, allowing:

• Feedback based on the peak force, protecting the tip and sample. • Peak Force, Adhesion, Young’s Modulus, Deformation, Dissipation

mapped simultaneously with topography.• Individual curves can be examined and analyzed offline (PeakForce

Capture)

7/16/2014 6

High resolution PF-QNMNew information revealed

Heat sealed bag: Barrier and Tie layers are incompatible, so we expect a relatively abrupt interphase.

• Single scan line has a clear step in modulus over a distance of ~75nm.

• Lamella do not cross the interface, but grow epitaxially from the Barrier layer – can see in averaged profile.

• Lamella are highly ordered and perpendicular to interface ~250nm into the Tie layer.

(a)

(b)

(a)

(c)

(b)

DMTModulus

Barrier layerNylonStrength & gas impermeability

Tie layerULDPEPreserves layer adhesion

Sealant layerMetallocene PE/LDPE blendAdheres to itself when heated

7/16/2014 7

Height

High resolution PF-QNMNew information revealed

Tie and Sealant layers are relatively compatible = wider interphase.

• Single scan line: the variation in modulus is dominated by individual lamella.

• Collectively: modulus varies over a much wider range ~250nm to ~1um.

• Lamella from Tie layer act as nucleation sites or penetrate into the Sealant: more ordered region to ~1um from the interface.

Barrier layerNylonStrength & gas impermeability

Tie layerULDPEPreserves layer adhesion

Sealant layerMetallocene PE/LDPE blendAdheres to itself when heated

7/16/2014 8

(a)

(b)

(a)

(c)

(b)

DMTModulus

Variation in viscoelastic responseVisible in Dissipation map

• Dissipation in Barrier<Tie• Demonstrated both by images and simultaneous force curves extracted

from HSDC

• Hysteresis in contact part of force curves suggests an inelastic deformation mechanism is active

9

Dissipation

7/16/2014

Signature of viscoelasticity in force curvesVEDA simulation: ramping at different rates

• Generalized Maxwell model using Prony fit of storage & loss modulus data

• Apparent modulus changes with ramp rate• Hysteresis in curve also changes with ramp

rate

7/16/2014 10

Simulated force vs. separation curves on isotacticpolypropylene. Blue: sinusoidal ramps. Violet: linearramp at 100Hz

10MHz

100Hz

100KHz

1KHz

Tip-sample separationFo

rce

Read, B. E. (1989)

Isotactic polypropylene

D. Kiracofe; J. Melcher; A. Raman; S.n Balasubramaniam; S. D. Johnson; S. Hu (2012), "VEDA: Virtual Environment for Dynamic AFM," https://nanohub.org/resources/veda. (DOI: 10.4231/D3222R54G).

PFQNM uncovers variation in modulus

7/16/2014 11

PSLDPE

0.5Hz

2000Hz

Ram

p ra

te

PF-QNMextends

freq. range

ForceVolume

PS modulus roughly constantLDPE modulus increasing with ramp rate

FV

Contact Resonance AFMExtending Modulus measurements to even higher frequencies

• Access higher frequencies ~100-1000kHz• Better sensitivity for very stiff samples• Can use multiple eigenmodes with the same probe• Viscoelastic properties can also be measured (e.g. Yuya, Hurley, &

Turner, J. Appl. Phys. 2008)

7/16/2014 12

Contact Resonance AFMContact Mode issues

• Problem: Contact Mode• Tip/Sample damage: limits resolution for soft/delicate samples• Unstable property signal due to tip-sample contact area variation

• Could CR be combined with PFT?

7/16/2014 13

Contact mode PeakForce Tapping

1/29/2014 14Bruker Nano Surfaces Division

Integrating PeakForce Tapping with TUNA (2011)

• PFTUNA is built on PF-QNM• Additionally, PF-TUNA module has improved bandwidth:

PeakForce Tapping Frequency : 1 kHz -2 kHzTUNA : >10x faster

Height

Phase

IR Reflection

IR Absorption

1736 cm-1

PeakForce Tapping Mapping BreadthStable, nondestructive imaging with simultaneous mechanical properties

7/16/2014 15Bruker Nano Surfaces

PeakForce IR: sSNOM imaging of PS-

PMMA at 1736cm-1, allowing identification of

blend components based on CO stretch through either IR reflection or

absorption. 4000nm image.

PeakForce QNM nanomechanical imaging with atomic defect resolution, shown here on calcite. 10nm image.

Could you combine PFT and CR using our open signal access?

PeakForce KPFM work function imaging, here

shown for reduced graphene oxide. Revealing <20nm

potential variations due to chemical heterogeneity.

750nm image.

PeakForce TUNA conductivity imaging, shown here on vertically standing carbon nanotubes. Impossible with contact mode. 1000nm image.

Height Conductivity Work function

Stiffness IR Absorption

Summary

Force Volume and PeakForce QNM both allow analysis of individual force spectra

• Wide range of properties can be covered

• Multiple properties can be mapped simultaneously

Ramp rate for PFQNM >> FV • Allows high resolution mapping

• Expands accessible range of frequency significantly

Time dependent mechanical properties can be investigated by observing modulus and dissipation at different ramp rates

• DMT, Sneddon models do not include viscoelasticity

• Further work required to make a quantitative connection between ramp observations and models

• Contact Resonance may be able to help

PeakForce Tapping is a great candidate for integration with other AFM techniques

16

Tie-Sealant interface in heat sealed bag composite

7/16/2014

7/16/2014 17

contact me at:

Bede Pittenger, PhDbede.pittenger@bruker-nano.com www.bruker.com/service/education-

training/webinars/afm.html

Also check out Nanoscale world community and SPM Digest Forum:nanoscaleworld.bruker-axs.com/

Gheorghe Stan

gheorghe.stan@nist.gov

Nanoscale mechanical property measurements in AFM modes with direct force control

Part II: Force Control in Contact-Resonance Atomic Force Microscopy

2

Mode 2

free

contact

Mode 1

Contact resonance AFM: cantilever dynamics

Clamped‐spring‐coupled beam,  Rabe et al., Rev. Sci. Instrum. 67, 3281, 1996

Atomic force acoustic microscopy (AFAM)  Rabe et al., Rev. Sci. Instrum. 67, 3281, 1996

Ultasonic Atomic force microscopy (UAFM)  Yamanaka et al., Japan. J. Appl. Phys. 35, 3787, 1996

3

Contact resonance AFM: contact mechanics

Tip

Sample

NF

2r

testreference

TS MME /1/1/1

T

n

S

R

R

n

S

R

S Mkk

Mkk

M1111

nRSRS kkEE )/( n = 1 –flat punch

n = 3/2 –spherical indenter

*2rEFkz

NN

)1/( 2 EM

Hertz model(sphere on flat) 

One reference

Two references

uncertainty 20%U. Rabe et al., Ultrasonics 38, 430 (2000)

21*

2

*1

12*

*1

*2

*1

1111

1

RR

n

R

R

RR

n

S

R

n

R

R

S

MMkk

MMkk

kk

M

uncertainty 5%G. Stan and W. Price, Rev. Sci. Instrum. 77, 103707 (2006)

TEST SAMPLEREFERENCE 1 REFERENCE 2TEST SAMPLEREFERENCE 1 REFERENCE 2

A. M. Jackob et al., Nanoscale, 6, 6898, (2014): “… in accordance to,20, 21less effort in probe modeling is still sufficient for quantitative nanomechanical analysis of conventional materials as long as a multi‐reference sample approach is used. 

Quasi‐static vs dynamic contact stiffness: measurement sensitivityAFM spectroscopy: Force‐distance curves

,111

ct

kkk

2t

cfd )1(

11

kk

S

c/ kk

0.01

0.1

1

10

100

Nor

mal

ized

sen

sitiv

ity, S

/S(k

*=0)

(%)

100806040200Normalized contact stiffness, k*/kc

S1 / S1(k*=0), CR-AFM first eigenmode S2 / S1(k*=0), CR-AFM second eigenmode Sfd / Sfd(k*=0), force-distance AFM

),(1

1

n

nn kFf

fS

CR‐AFM spectroscopy: Resonance spectra

4

F‐d versus CR‐AFM with a medium‐stiff cantilever, kc=35.5 N/m, on low‐k dielectric films

5

Samples from Sean King (Intel Corp.)

6

CR‐AFM imaging on Cu/low‐k dielectric interconnects

Si substrate

Low‐kILD

Cu

Stan et al., Nanotechnology 23, 215703 (2012)

On a Cu‐low‐k interconnection structure, CR‐AFM provided contrast in contact stiffness and damping between the Cu lines and the surrounding dielectric material and the intervening Ta/TaN barrier layer. By varying the load applied to the probe, clear differences in the decay of damping with depth beneath the surfaces of the Cu and the dielectric were revealed. 

Ta/TaN

7

CR‐AFM imaging on granular Au filmTopography (STM)                  Contact resonance (CR‐AFM)

Indentation modulus

Single‐asperity contact Multiple‐asperity contact

F = 100 nN

F = 250 nN

F = 350 nN

10

20

3010

20

30

nm

10

10

20

30

0

10

20

3010

2030

nm

1020

0

10

20

30

0

10

20

3010

2030

nm

1020

0

10

20

30

0

10

20

3010

2030

nm

1020

0

10

20

30

100 nm 100 nm

100 nm

Stan and Cook, Nanotechnology 19, 235701 (2008)

Both topography and contact stiffness maps were self‐consistentlycorrelated to reveal nanoscale variations of the indentation modulus atthe grain level as well as across the grains.

Individual elastically‐deformed asperitiesmake a non‐conforming contact with thespherical smooth end of theCR‐AFM probe.

Contact geometry: tip wear

Si tips, especially the sharp ones, wear when they are used in contact spectroscopy and scanning modes. Great care is required to monitor the tip wear during use. This can be done from repeated contact stiffness measurements on the same material as the contact stiffness is in direct relationship with the change in contact area,                . Consequently, reliable contact stiffness measurements required stable tip shapes, which, most likely, will be the ones that are pre‐wear.

Fresh sharp tip(NanoSensors) Tip used in CR‐AFM spectroscopy

(Stan and Price, Rev. Sci. Instrum. 77, 103707 (2006))

Tip used in contact AFM scanning mode(Killgore, Geiss, and Hurley, Small 7, 1018 (2012))

rEk 2

8

Flattened tip used in CR‐AFM measurements on low‐k dielectric thin films

Credit for SEM images: Kavuri Premsagar Purushotham (NIST)

9

Load‐dependent CR‐AFM measurements

1000

800

600

400

200

0

Appl

ied

forc

e (n

N)

50403020100Piezo displacement (nm)

710

705

700

695

690

685

680Res

onan

ce fr

eque

ncy

(kH

z)

Load-dependent CR-AFM on Si(100)Flattened-tip: RT=150 nm, r=32.5 nm

The measurements consist of recording simultaneouslyboth the deflection and resonance frequency of anAFM cantilever as the probe is gradually brought in andout of contact.

G. Stan et al., J. Mater. Res. 24, 2960 (2009)

10

Real‐time load‐dependent CR‐AFM on a 1.6 GPa low‐k dielectric

1STcc )/1/1(22 MMrErk

Flat‐punch approximation:

11

12

Intermittent Contact Resonance AFM (ICR‐AFM)

The measurements consist of recording simultaneously thedeflection, resonance frequency, and amplitude of the AFMcantilever as the probe is gradually brought in and out ofcontact at any point in the scan.

Key points for quantitative nanoscalemechanical property characterization by3D ICR‐AFM:

‐force control during intermittentcontacts (Force Volume, Peak ForceTapping)

‐adhesive force measurement

‐force‐resonance frequencycorrelation during contacts

‐imaging at a non‐eigenmode frequency(lower than f 1 )

13

Intermittent Contact Resonance AFM (ICR‐AFM) in Force‐Volume AFM

0.200.150.10

0.200.150.10

0.200.150.10

0.200.150.10

0.200.150.10

0.200.150.10

0.30.20.1

0.50.40.3

0.500.450.40

690680670

690680670

690680670

680670

670665660

665660

665660

662660658

660659658 = - 5.0 nm

= - 2.5 nm

= - 1.0 nm

= - 0.5 nm

= 0.0 nm

= 0.5 nm

= 1.0 nm

= 2.5 nm

= 5.0 nm

670665660

670665660

670665660

670665660

680

660

680

660

680

660

680

660

680

660

0.20.1

0.20.1

0.20.1

0.20.1

0.20.1

0.30.20.1

0.30.20.1

0.40.2

0.40.2

= - 5.0 nm

= - 2.5 nm

= - 1.0 nm

= - 0.5 nm

= 0.0 nm

= 0.5 nm

= 1.0 nm

= 2.5 nm

= 5.0 nm

Approach RetractResonance frequency Resonance amplitude Resonance frequency Resonance amplitude

f1air = 104.9 kHz, f2air = 657.6 kHz; kc = 9.5 ± 0.5 N/m

80604020

nm

ICR‐AFM on PS‐PP blend: 128x128 ramps over 2 µm x 2 µm area; 1 s per ramp.  

out o

f con

tact

in con

tact

14

0.4

0.2

0.0

nm

690680670660

kHz

nm

-5

0

5

700 nm

2000 nm

690680670660

kHz

Tomographic ICR‐AFM

Frequency Approach Frequency Retract

Amplitude Retract

80604020

nmTopographyx

y

0.4

0.2

0.0

nm

Amplitude Approach

xy

z

G. Stan, S. D. Solares, B. Pittenger, N. Erina, and C. Su, Nanoscale 6. 962 (2014)

15

Fast dynamic indentation on elastomers

5

4

3

2

1

0

-1Inde

ntat

ion

dept

h,

(nm

)

35302520151050Contact stiffness, k* (N/m)

PS: Approach; RetractPP: Approach; Retract 1

= 0

.0 1

= 0

.5 1

= 1

.0

PP

PS

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

/ k

*1/2

250200150100500

k*3/2

1 =

0.0

1 =

0.5

1 =

1.0

PP

PS

In the case of a fast dynamic indentation of an elastomer (Wahl et al., J. Colloid Interface Sci., 2006, 296, 178), due to viscoelastic effects, the contact area remains approximately constant during oscillations and the contact geometry resembles that of a ”flat punch” configuration, with                  rather than                          . c~* rk /* Fk

With Schwarz contact model (U. D. Schwarz, J. Colloid Interface Sci., 2003, 261, 99), a transition model between DMT and JKR models, the depth‐dependence of the contact stiffness in the “ dynamic flat‐punch” limit is given by:

2Ta2

1

12T

2 */*24

*4/* ERFkERk

2/3* */ kk

2a1 8/2 F

JKR 1, DMT ,01

DMT ‐long‐range attractive force outside the contact area.

JKR ‐short‐range attractive force inside the contact area.

T4/1* RE

16

Intermittent Contact Resonance AFM (ICR‐AFM)

6420 GPa

10-1

Nor

mal

ized

mea

sure

men

ts

1.51.00.50.0-0.51

Approach Retract

PP

PS

Indentation modulus, 

Transition parameter,N

orm

aliz

ed m

easu

rem

ents

654321Indentation modulus (GPa)

Approach Retract

PP

PS

*E

1

17

ICR‐AFM: Frequency, amplitude, and dissipated energy

6050403020100.40.30.20.1

-8

-4

0

4

690680670660

2000 1500 1000 500 02000 1500 1000 500 0Distance (nm)

-8

-4

0

4

2000 1500 1000 500 0

Resonance frequency (kHz) Resonance amplitude (nm) Dissipated power (fW)

0.5

0.4

0.3

0.2

0.1

Res

onan

ce a

mpl

itude

(nm

)

-8 -4 0 4Indentation depth, (nm)

60

40

20

0Dis

sipa

ted

pow

er (f

W)

-8 -4 0 4

685

680

675

670

665

660

Res

onan

ce fr

eque

ncy

(kH

z)

-8 -4 0 4

PS Approach RetractPP Approach Retract

a b c

d e f

g h i

Inde

ntat

ion

dept

h (n

m)

Inde

ntat

ion

dept

h (n

m)

PPPS PS PP PS PP

PS

PP

18

Intermittent Contact Resonance AFM (ICR‐AFM) in Peak Force Tapping

Signal processor: Force, displacement, frequency, andamplitude vs time during individual oscillations.

XYZPiezoscanner

Photodiodedetector

Force vs time andDisplacement vs timeSetpoint

PI Controller

Z Modulation

PLL

Laserdiode

Frequency vs time andAmplitude vs time

Piezoshaker

Peak Force Tapping @ 2 kHz

500 µs

f1air = 107.3 kHz, f2air = 670.5 kHz, f3air = 1,865.5 kHz; kc = 9.12 ± 0.07 N/m

‐force control during intermittent contacts (Force Volume, Peak Force Tapping)‐adhesive force measurement‐force‐resonance frequency correlation during contacts‐imaging at a non‐eigenmode frequency (lower than f 1 )

G. Stan and R. Gates, Nanotechnology 25, 245702 (2014)

19

ICR‐AFM as Amplitude Modulation – Frequency Modulation of Peak Force Tapping

40

20

0

nm

40200500 nm

7654

8642

nN

400

300

200

400300200

eV

4

3

2

432

GPa

6

5

4500040003000200010000

Distance (nm)

654

kHz

2.1

1.9

1.7

2.22.01.81.6

kHz

Dis

sipa

tion

(eV)

H

eigh

t (nm

) A

dhes

ion

(nN

) M

odul

us (G

Pa)

Fre

q. s

hift

(kH

z)

Fre

q. s

hift

(kH

z)

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(l)

Topography (PFT)

Adhesion (PFT)

Dissipation (PFT)

DMT Elastic Modulus (PFT)

Resonance frequency shift f3 (ICR) slow PLL ‐ 100 µs time constant

Resonance frequency shift f3 (ICR) fast PLL ‐ 1 µs PLL time constant

PS‐PMMA

20

15 15

10 10

5 5

0 0

15 15

10 10

5 5

0 0

40

30

20

10

0

-10

473472471470469468

40

30

20

10

0

-10

495494493492491490 Time (ms)

Appl

ied

forc

e (n

N)

Res

onan

ce fr

eque

ncy

shift

(kH

z)

Res

onan

ce fr

eque

ncy

shift

(kH

z)

Appl

ied

forc

e (n

N)

Time (ms)

ICR‐AFM: Individual oscillations

Slow PLL detection

Fast PLL detection

With a slow detection, the changes in the contact resonance frequency are averaged over the entire period of an oscillation including out-of-contact intervals.

With a fast detection, the averaging time is reduced, so the changes in the contact resonance frequency during individual oscillations are momentarily tracked.

6

4

2

0

(F+F

a)1/

2 (nN

1/2 )

14012010080604020

k*3/2

(N/m)3/2

PMMA retract data PMMA fit by eq.

prediction band for PMMA fit PS retract data PS fit by eq.

prediction band for PS fit

21

nm

40

20

0500 nm

PS-PMMA

40

30

20

10

0

-10

Forc

e (n

N)

860840820800780Time (ms)

12

8

4

0Freq

uenc

y (k

Hz)

860840820800780Time (ms)

PS PSPMMA

50

40

30

20

10

0

-10

k* (N

/m)

403020100-10Applied force, F (nN)

Approach on PSRetract on PSApproach on PMMARetract on PMMA

EPS = 3.20 GPa, 1=0.27 EPS = 3.20 GPa, 1=0.72 EPMMA = 2.77 GPa, 1=0.08 EPMMA = 2.77 GPa, 1=0.66

3/2

aa21

13/12T 3

4*6*

FFFERk

03.066.0 GPa, 09.083.201.072.0 GPa, 10.040.3

:region slopeNegative

04.008.0 GPa, 09.083.204.027.0 GPa, 10.040.3

:region slopePositive

1PS

1PS

1PMMA

1PS

EE

EE

3a *kFF

2a1 3/2 F

2T6/1* RE

Conclusions

22

‐Depth‐dependence of contact stiffness in force‐controlled CR‐AFM. 

‐Load‐dependent CR‐AFM elastic modulus measurements with flattened tips.

‐ICR‐AFM (in force volume and peak‐force tapping) is a new 3D high‐speed nanomechanicalproperty measurement AFM technique. 

‐Improved quantitative elastic modulus measurements were demonstrated by using ICR‐AFM on PS/PP and PS/PMMA blends. 

‐Transition contact models (e.g. Schwarz model) provide a self‐adaptive analysis for the heterogeneous mechanical properties of elastomeric surfaces at the nanoscale.

7/16/2014 23

contact me at:

Bede Pittenger, PhDbede.pittenger@bruker-nano.com www.bruker.com/service/education-

training/webinars/afm.html

Also check out Nanoscale world community and SPM Digest Forum:nanoscaleworld.bruker-axs.com/