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

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

26. Stiffness scan

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

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

50. Illustrative Theory, i.e. Math for non-Uday’s Strain-displacement: , where Stress-strain: Equilibrium: