Microstructured Vertically Aligned Carbon Nanotube Composites

1. Microstructured Vertically Aligned Carbon Nanotube Composites Dr. Robert Davis & Group Brigham Young University Physics and Astronomy davis@byu.edu (presented by Prof. David Allred, BYU)

2. Outline Vertical aligned carbon nanotube (VACNT) growth Dense nanotube structures Filling in with Si by LPCVD Resistivity Crystallinity of filler material Density, Elastic Modulus Constraints/Engineering Design Rules

3. Vertical Nanotube Growth

4. 30 nm Al2O3 (Barrier Layer) Si VACNT growth details • Pattern Fe by lift-off • Heat to 750 °C in H2: Ar • Ar : H2 : C2H4 at 70 : 500 : 700 sccm 1-15 nm Fe

5. SEM - VACNT forest Can top surface and edge roughness be improved? 100X – 50 µm holes 4500X – 3 µm holes 30000X – edge

6. Light Photoresist 1-15 nm Fe 30 nm Al2O3 (Barrier Layer) Si VACNT growth process • Photolithography & lift-off Photoresist Photoresist

7. Dependence on Fe Thickness 15 nm 5.5 nm 2 nm < 1 nm Good repeatability Rate strongly dependant on thickness

8. 2 nm Fe on Al2O3 • Small voids (< 200 nm across) • Sharp features (few stray tubes); Sidewall roughness < 200 nm • High growth rate ~50 µm/min

9. TEM Grid

10. Bistable Mechanisms

11. Nanotube “forest” growth Height: up to 1mm+ Feature size: a few microns Speed: 10-100µm/min Density:

12. Low density, weakly bound material As-grown forests are flimsy and tear off the surface at the slightest touch

13. Dense Nanotube Structures

14. Liquid Induced Densification Dried structure Submerged nanotube structures

15. Vapor Condensation Induced Densification -- Lines Longer exposure to vapor Shorter exposure to vapor

16. Surface forces dominates Unequal angles become equal angles. Final structure depends on initial structure surface forces Difficult to control what results!

17. Horizontal Aligned CNT films

18. AIST Japan group working on in-plane aligned CNT MEMS Yuhei Hayamizu, Takeo Yamada, Kohei Mizuno, Robert C. Davis, Don N. Futaba, Motoo Yumura, & Kenji Hata Nature Nanotechnology 3, 241 (2008).

19. Microstructured VACNT Composites

20. Leave the nanotubes vertical?

21. Filling in with Si by LPCVD

22. VACNT Composite MEMS Process “floor layer”

23. Solid High Aspect Ratio Structures

24. A variety of materials? Filled with amorphous Si: Filled with amorphous C:

25. High aspect ratio structures in a variety of materials? FROM: Chemical Vapor Deposition, ed. Jong-Hee Park, ASM International (2001)

26. Properties: Resistivity Isolated nanotubes: Can exhibit ballistic conduction over distances of several microns Undoped poly-Si: ρ ~ 102Ω cm Si-coated nanotubes: ρ ~ ? Coat tubes with insulator  Conductive MEMS made from insulating materials?

27. Properties: Resistivity R0 K1= 42.6 Ωm K2= 0.04 μm

28. Properties: Resistivity R0 ρforest ~ 4 Ω cm ρpoly-Si alone ~ 10000 Ω m

29. Properties: Resistivity R0 ρforest ~ 4 Ω-cm Approximately the same resistivity as previously reported for other CNT-composites

30. Microstructure: Poly-Si Between tubes, dark field

31. Properties: Poly-Si Alumina layer Iron Si Substrate Between tubes, dark field

32. Properties: Poly-Si Between tubes, dark field

33. Mechanical Characterization Filled forests are solid and well adhered (can withstand the scotch tape test)

34. Beam Bending Measurement of Elastic Modulus 1000um 20um 38um Eop = 120 GPa Reported bulk polySi modulus ~ 140-210 GPa, dependent on deposition conditions

35. Actuated device: thermomechanical in-plane microactuator (TIM)

36. Developing Engineering Design Rules • Height to width ratio for dimensional stability • Maximum feature width for filling of forest interior • Role of geometry LPCVD fill-factor

37. Researchers BYU Physics David Hutchison Brendan Turner Katherine Hurd Matthew Carter Nick Morrill Dr. Richard Vanfleet BYU Mechanical Engineering • Quentin Aten • Dr. Brian Jensen • Dr. Larry Howell Nanocarbon Research Center AIST Japan Dr. Kenji Hata Funded by a Brigham Young University Environment for Undergraduate Mentoring Grant Partial funding provided by Moxtek Inc.

38. High Aspect Ratio Micromachining Deep Reactive Ion Etching (Si) MARIO Process (Titanium) “High-aspect-ratio bulk micromachining of titanium,” Aimi et al., Nature Mat. 3, 103-5 (2004) “Vertical Mirrors Fabricated by DRIE for Fiber-Optic Switching Applications,” C. Marx et al., J. MEMS 6, 277, (1997) LIGA process (photoresist) SU-8 / C-MEMS (photoresist / carbon) “C-MEMS for the Manufacture of 3D Microbatteries,” Wang et al., Electrochem. Solid-State Lett. 7 (11) A435-8 (2004) “Micromechanisms,” H. Guckel, Phil. Trans. R. Soc. Lond. 353, 355-66 (1995)

39. VACNT Growth Studies Objective: Dimensional Control • Stable uniform features • Smooth straight sidewalls and top surface • Minimum void dimension Variables: • Barrier layer material • Fe catalyst thickness

40. Barrier layer study: • Al2O3 – 30 nm • Native - SiO2 • Thermally grown SiO2 – 30 nm • Titanium – 30 nm Early growth 2 sec. of C2H4 flow

41. Barrier Layer Study

42. TEM analysis of barrier layers Both oxides form barrier to Fe diffusion, Al2O3 results in smallest particles

43. Quantitative particle size analysis of barrier layers Alumina has far more of the smallest particles.

44. Effects of Al2O3 barrier layer • Reduce Fe loss through diffusion • Controls Fe particle size during anneal • Reduces CNT diameter • Increases CNT density and growth rate • Indicates reduces Fe surface diffusion