Nanomechanical Testing of Thin Polymer Films

Kyle Maner and Matthew Begley Structural and Solid Mechanics Program Department of Civil Engineering University of Virginia thanks to: Uday Komaragiri (UVA) Special 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

Overview

•

Traditional nanoindentation of thin films bonded to thick substrates

• A novel freestanding film microfabrication procedure • A novel method to probe freestanding films

Do polymers exhibit scale dependence?

Is traditional nanoindentation sensitive enough to detect such behavior?

3 Pure, amorphous polymers: Poly(styrene) (PS) – M w = 280 kD Poly(methyl methacrylate) (PMMA) – M w = 350 kD Poly(phenylene oxide) (PPO) – M w = 250 kD 2 Block co-polymers: Poly(methyl methacrylate)-ruthenium (PMMA-Ru) – M w = 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 m • Discard rogue tests due to surface debris • Average data to determine elastic modulus and hardness curves as a function of penetration depth

S

(  ) 

dP d

 

C

1

E r A

(  ) • 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

Quartz, E = 72 GPa

S

(  ) 

dP d

 

C

1

E r A

(  )

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

Overview

• Traditional nanoindentation of thin films bonded to thick substrates •

A novel freestanding film microfabrication procedure

• A novel method to probe freestanding films

A new microfabrication procedure should be: • applicable to a wide range of materials • easily prepared on any wet-bench The experimental testing of the sample created should be: • easily integrated with existing test equipment • easily interpreted with relatively simple mechanics models

The short answer…

Spin-casting Etching Testing

Spin-cast polymer film onto glass plate with etchable fibers

The short answer…

Spin-casting Etching Testing

2% HCl FRONT-LIGHTING BACK-LIGHTING

Mechanical properties via nanoindentation before and after acid bath

The short answer…

Spin-casting Etching Testing

Overview

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

A novel method to probe freestanding films

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 Transition Membrane

PMMA

M w = 120 kD thickness = 350 nm span = 30 m 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

M w = 250 kD thickness = 750 nm span = 30 m 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

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

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 Method

PMMA: ~350 nm thick, 30 m m span E = 3.0 GPa e 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:

S

(  )  2 

E r A

(  ) • Elastic properties of calibration sample and indenter tip must be

E r

1

E r

  1  

s

2

E s

  1  

E i i

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

A

(  ) 

C

1  2 

C

2  

C

3  1 / 2  ...

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: Stress-strain: Equilibrium:  ˆ 2  1  1  e 0 , where  ˆ  

L

 

E

e 

F y

 0  

P

 2 (

F

sin  )

F



P

2 sin 

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

P

 2

EA

  ˆ 2  1  1  e 0  ˆ  ˆ 2  1  ˆ 2  1  1  1 2  ˆ 2  0 (  ˆ 3 0 )  ...

The equation for load becomes:

P



EA

 ˆ 3  1  2

EA

e 0  ˆ 1 2  ˆ 2 Due to small deflections, the denominator goes to 1, and load as a function of deflection is:

P

(  ˆ )  (  ˆ 3  2  ˆ e 0 )

EA

Sensitivity of the method: very shallow depths PMMA: ~350 nm thick, 30 m m span E = 3.0 GPa e 0 = 0.0026