Transcript Slide 1
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