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)

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

•Traditional nanoindentation of thin films bonded to thick substrates

• A novel freestanding film microfabrication procedure

• A novel method to probe freestanding films

Overview

Do polymers exhibit scale dependence?

Is traditional nanoindentation

sensitive enough to

detect such behavior?

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)

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 m

• Discard rogue tests due to surface debris

• Average data to determine elastic modulus and hardness curves as a function of penetration depth

• 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

)()( 1

AECd

dPS r

Quartz, E = 72 GPa

)()( 1

AECd

dPS r

Nanostructured lamellar block co-polymer

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

• Traditional nanoindentation of thin films bonded to thick substrates

• A novel freestanding film microfabrication procedure

• A novel method to probe freestanding films

Overview

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:

The short answer…

Spin-casting Etching Testing

Spin-cast polymer film onto glass

plate with etchable fibers

The short answer…

Spin-casting Etching Testing

FRONT-LIGHTING BACK-LIGHTING

2% HCl

Mechanical properties via nanoindentation before and after acid bath

The short answer…

Spin-casting Etching Testing

•Traditional nanoindentation of thin films bonded to thick substrates

• A novel freestanding film microfabrication procedure

• A novel method to probe freestanding films

Overview

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

Probing of freestanding films: surface find

Probing of freestanding films: test flow

Stiffness scan

With the given parameters (thickness & span), what is the anticipated response??

Linear plate

Membrane

Transition

PMMA

Mw = 120 kD thickness = 350 nm span = 30 m

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

PPO

Mw = 250 kD thickness = 750 nm span = 30 m

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

Questions?

Thank you.

• 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

Traditional methods of testing thin films

• Wafer curvature

• Bulge testing

• Nanoindentation of thin films bonded to thick substrates

• Microfabrication & probing of freestanding films

Nanoindentation Probe

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

• 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

• 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

The research on submicron films

The question of contact

Film thickness before and after acid bath

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

Sensitivity of the MethodPMMA: ~350 nm thick, 30 m span

E = 3.0 GPa 0 = 0.0026

Tip Calibration Equations

• Stiffness as a function of depth, S(), is measured

• The area function, A(), is determined from the following equation:

)(2

)(

AES r

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

i

i

s

s

r EEE

22 111

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

...)( 2/132

21 CCCA

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

Parameters of Spin-Casting

Surface Characterizations

PS substrate

PMMA film on PS

substrate

Illustrative Theory, i.e. Math for non-Uday’s

Strain-displacement:0

2 11ˆ

Stress-strain: E

Equilibrium:

sin2

)sin(20

PF

FPFy

L

ˆ, where

By combining the strain-displacement, stress-strain, and equilibrium equations, the following equation can be found:

ˆ

1ˆ

11ˆ

22

02

EAP

For small deflections, , thus:1ˆ ...)ˆ(0ˆ2

111ˆ 322

0

The equation for load becomes:

2

03

ˆ21

1

ˆ2ˆ

EAEAP

Due to small deflections, the denominator goes to 1, and load as a function of deflection is:

EAP )ˆ2ˆ()ˆ( 03

E = 3.0 GPa = 0.0026

Sensitivity of the method: very shallow depths

PMMA: ~350 nm thick, 30 m span