Nanomechanical Testing of Thin Polymer Films

1
Nanomechanical Testing of Thin Polymer Films
Kyle Maner and Matthew Begley
Structural and Solid Mechanics Program Department
of Civil Engineering University of Virginia
Uday Komaragiri (UVA)
Special thanks to Dr. Warren C. Oliver (MTS)
Prof. Marcel Utz (UConn)
2
Why test thin polymer films?

• Improve thermomechanical stability via
  self-assembly of nanostructure
• Establish connections between the nanostructure
  mechanical properties
• Determine the size scale of elementary
  processes of plastic deformation

3
Overview

• Traditional nanoindentation of thin films bonded
  to thick substrates
• A novel freestanding film microfabrication
  procedure
• A novel method to probe freestanding films

4
Do polymers exhibit scale dependence?
Is traditional nanoindentation sensitive enough
to detect such behavior?
5
3 Pure, amorphous polymers Poly(styrene) (PS)
Mw 280 kD Poly(methyl methacrylate) (PMMA) Mw
350 kD Poly(phenylene oxide) (PPO) Mw 250
kD 2 Block co-polymers Poly(methyl
methacrylate)-ruthenium (PMMA-Ru) Mw 56 kD
(a metal-centered block co-polymer) Poly(styrene)-
poly(ethylene propylene) (PS-PEP) (a
lamellar microphase separated block co-polymer)
6
Experimental Procedure

• Calibrate the tip discard data for depths
  where the calibration is inaccurate
• Indent polymer films on PS substrates 16
  indents per sample to a depth of 1.0 mm
• Discard rogue tests due to surface debris
• Average data to determine elastic modulus and
  hardness curves as a function of penetration
  depth

7

• The Berkovich diamond tip does not come to a
  perfect point
• The radius of the tip gradually increases with
  use
• The shape change alters the contact area of the
  indenter for a given depth
• A tip calibration determines the best-fit
  coefficients for the area function describing the
  tip

8
Quartz, E 72 GPa
9
(No Transcript)
10
(No Transcript)
11
Nanostructured lamellar block co-polymer
12
Conclusions from traditional nanoindentation

• Substrate effects can be dramatically reduced if
  elastic mismatch is minimized
• A tip calibration can be accurate for depths
  greater than 5 nm
• Scale effects indicate that elementary processes
  of deformation occur at depths less than 200 nm

13
Overview

• Traditional nanoindentation of thin films
  bonded to thick substrates
• A novel freestanding film microfabrication
  procedure
• A novel method to probe freestanding films

14
A new microfabrication procedure should be

• applicable to a wide range of materials
• easily prepared on any wet-bench
• easily integrated with existing test equipment
• easily interpreted with relatively simple
  mechanics models

The experimental testing of the sample created
should be
15
The short answer
Spin-casting
Etching
Testing
16
Spin-cast polymer film onto glass plate with
etchable fibers
17
The short answer
Spin-casting
Etching
Testing
18
2 HCl
BACK-LIGHTING
FRONT-LIGHTING
19
Mechanical properties via nanoindentation before
and after acid bath
20
The short answer
Spin-casting
Etching
Testing
21
Overview

• Traditional nanoindentation of thin films bonded
  to thick substrates
• A novel freestanding film microfabrication
  procedure
• A novel method to probe freestanding films

22
An overview of the test method

• constant harmonic oscillation superimposed on a
  ramp loading
• at contact, stiffness of sample causes drop in
  harmonic oscillation
• mechanical properties can be extracted from
  load-deflection response

23
Probing of freestanding films surface find
24
Probing of freestanding films test flow
25
(No Transcript)
26
Stiffness scan
27
(No Transcript)
28
With the given parameters (thickness span),
what is the anticipated response??
Linear plate
Transition
Membrane
29
PMMA
Mw 120 kD thickness 350 nm span 30 mm
30
Finite element study of PPO plasticity

• Load-deflection response generated via finite
  elements
• Elastic-perfectly plastic stress-strain
  relationship
• Varied values of yield strength, elastic
  modulus, and pre-stretch

31
PPO
Mw 250 kD thickness 750 nm span 30 mm

32
Conclusions

• Approximated size scale over which elementary
  processes of plastic deformation occur in
  polymers
• Developed a new microfabrication technique to
  create submicron freestanding polymer films
• Developed a new testing method to probe thin
  freestanding films and illustrated its
  repeatability
• Successfully used numerical models to extract
  mechanical properties from submicron films

33
Questions?
Thank you.
34

• Introduction and motivation
• Description of the MTS Nanoindentation System
• Traditional nanoindentation of thin films bonded
  to thick substrates
• A novel freestanding film microfabrication
  procedure
• A novel method to probe freestanding films

35
Traditional methods of testing thin films

• Wafer curvature
• Bulge testing
• Nanoindentation of thin films bonded to thick
  substrates
• Microfabrication probing of freestanding
  films

36
Nanoindentation Probe
37
Special features of the MTS Nanoindentation System
DCM (dynamic contact measurement) module
ultra-low load indentation head with
closed-loop feedback to control dynamic
motion CSM (continuous stiffness measurement)
approach measures the stiffness of the contact
continuously during indentation as a function of
depth by considering harmonic response of head
38

• Introduction and motivation
• Description of the MTS Nanoindentation System
• Traditional nanoindentation of thin films
  bonded to thick substrates
• A novel freestanding film microfabrication
  procedure
• A novel method to probe freestanding films

39
The research on submicron films

• Metals, metals, and more metals deformation
  and scale-dependent behavior is well understood
• Plasticity in polymers how it occurs but not
  how big
• Minimization of substrate effects via elastic
  homogeneity of film and substrate
• Probing of freestanding Si-based brittle and
  metal structures

40
The question of contact
41
Film thickness before and after acid bath
42
A novel method to probe freestanding films should
combat the problems facing experimental testing
of compliant films.

• Tip calibration errors can produce inaccurate
  measurements
• The surface of compliant materials is difficult
  to find
• Mechanics to extract properties is very complex

43
Sensitivity of the Method
PMMA 350 nm thick, 30 mm span
E 3.0 GPa
e0 0.0026
44
(No Transcript)
45
Tip Calibration Equations

• Stiffness as a function of depth, S(d), is
  measured
• The area function, A(d), is determined from the
  following equation

• Elastic properties of calibration sample and
  indenter tip must be know to calculate,

• The calculated area function is a series with
  geometrically decreasing exponents

46
Standard method Nanoindentation of
film/substrate system

• CSM stabilizes harmonic motion of the indenter
  head
• Probe begins to move towards surface
• Contact (1) occurs when stiffness increases
• Load (2) to a prescribed displacement
• Hold (3) at maximum load to assess creep
  behavior
• Unload (4) 90 of the way
• Hold (5) at 90 unload to assess thermal drift

47
Parameters of Spin-Casting
48
Surface Characterizations
PS substrate
PMMA film on PS substrate
49
(No Transcript)
50
Illustrative Theory, i.e. Math for non-Udays
Strain-displacement
, where
Stress-strain
Equilibrium
51
By combining the strain-displacement,
stress-strain, and equilibrium equations, the
following equation can be found
0
For small deflections, , thus
The equation for load becomes
Due to small deflections, the denominator goes to
1, and load as a function of deflection is
52
Sensitivity of the method very shallow depths
PMMA 350 nm thick, 30 mm span
E 3.0 GPa
e0 0.0026