Nanoscale mechanical property measurements in AFM modes … · 2014-07-16 · The measurements...
Transcript of Nanoscale mechanical property measurements in AFM modes … · 2014-07-16 · The measurements...
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 (…)
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• 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)
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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)
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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
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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)
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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
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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
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(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
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Dissipation
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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
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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
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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)
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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?
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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
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contact me at:
Bede Pittenger, [email protected] 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
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, [email protected] www.bruker.com/service/education-
training/webinars/afm.html
Also check out Nanoscale world community and SPM Digest Forum:nanoscaleworld.bruker-axs.com/