Vapor Deposited Diamond Thin Films

1. Vapor Deposited Diamond Thin Films Review by: Vivek Krishnan Materials Research and Education Center Auburn University, AL

2. Presentation Outline • Introduction – diamond and graphite • Diamond Synthesis • Nucleation and growth • Substrate choice • Applications • Conclusions

3. Introduction • Diamond – treasured for centuries as a gemstone • Extreme properties – “biggest and the best”. 'Synthetic Diamond - Emerging CVD Science and Technology', Spear and Dismukes, Wiley, NY, 1994

4. Questions • Why is atomic hydrogen important for growth of diamond thin films by CVD? • What is Bias Enhanced Nucleation?

5. Properties • Extreme mechanical hardness (~90 GPa). • Strongest known material, highest bulk modulus (1.2 x 1012 N/m2), lowest compressibility (8.3 x 10-13 m2/ N). • Highest known value of thermal conductivity at room temperature (2 x 103 W/m/ K). • Thermal expansion coefficient at room temperature (0.8 x 10-6 K) is comparable with that of invar. • Broad optical transparency from the deep UV to the far IR region of the electromagnetic spectrum. http://www.chm.bris.ac.uk/pt/diamond/end.htm

6. Properties contd. • Good electrical insulator (room temperature resistivity is ~1016 ohm cm). • Diamond can be doped to change its resistivity over the range 10-106 ohm cm, so becoming a semiconductor with a wide bad gap of 5.4 eV. • Very resistant to chemical corrosion. • Biologically compatible. • Exhibits low or 'negative' electron affinity. http://www.chm.bris.ac.uk/pt/diamond/end.htm

7. Carbon Chemistry • Ability of carbon to bond with itself. • Allotropes – diamond, graphite and amorphous carbon. • Single element carbon atoms arranged in a lattice. • Directional covalent bonds. http://invsee.asu.edu/nmodules/Carbonmod/bonding.html

8. Structures - Graphite http://www.nyu.edu/pages/mathmol/modules/carbon/carbon1.html

9. Structures - Diamond http://www.nyu.edu/pages/mathmol/modules/carbon/carbon1.html

10. Carbon Phase Diagram F.P. Bundy, The P,T Phase and Reaction diagram for elemental Carbon, 1979; J. Geophys. Res.85 (B12) (1980) 6930

11. Diamond Synthesis • High Pressure High Temperature synthesis (HPHT)- ASEA and General Electric (1950’s) • Graphite- hydraulic press at high temperatures and pressures- metallic catalyst - converts to diamond over a period of a few hours. • P~50-100 kbar and T~1800-2300K. • mm sized crystals • unsuitable as gemstones • used as cutting tools and drill bits. http://www.chm.bris.ac.uk/pt/diamond/end.htm

12. Low Pressure Diamond • Metastable diamond phase at low pressures first deposited by William Eversole of the Union Carbide Corp. (1962) • Ability to produce polycrystalline diamond films with excellent properties using simple hydrocarbon mixtures. • Techniques present economical alternative to HPHT and the promise of synthesizing other metastable phases. “Thin Film Diamond Growth Mechanisms”, James E. Butler and Richard L. Woodin, Thin Film Diamond, Ed. Lettington and Steeds, Pub. Chapman and Hill, 1994

13. Chemical Vapor Deposition • Involves a gas-phase chemical reaction occurring above a solid surface, which causes deposition onto that surface. • Requires gas-phase activation. • Thermal, plasma activation or combustion flame. http://www.sandia.gov/1100/CVDwww/cvdinfo.htm

14. Vapor Phase Diamond Deposition P.W. May,"Diamond Thin Films: A 21st Century Material" Phil. Trans. R. Soc. Lond. A, 358 (2000) 473-495.

15. Growth of Carbon Lattice Reduction of carbon from hydrocarbons to form diamond. P.W. May,"Diamond Thin Films: A 21st Century Material" Phil. Trans. R. Soc. Lond. A, 358 (2000) 473-495.

16. Film deposition • Compromise between growth rate and quality of film. • Quality- ratio between sp2 and sp3 species, crystallinity. • Combustion methods - high rates (upto 200 µm/hr, respectively), very small, localized areas, poor quality films. • The hot filament and plasma methods have much slower growth rates (0.1-10 µm/hr), but produce high quality films. • Microwave – deposition rate increases linearly with microwave power. P.W. May,"Diamond Thin Films: A 21st Century Material" Phil. Trans. R. Soc. Lond. A, 358 (2000) 473-495.

17. Common Gas- Activation Methods Hot Filament CVD Pressure of 20-30 torr using rotary pump. Gas flow rates of few hundred sccm. Substrate heated to 700-900C. Filament (W) heated to more than 2200C. P.W. May, "Diamond Thin Films: A 21st Century Material" Phil. Trans. R. Soc. Lond. A, 358 (2000) 473-495.

18. Microwave Plasma CVD Microwave power coupled to chamber via quartz window to create discharge. Microwaves couple their energy to gas phase electrons, energy transferred to gas which dissociates to form reactive species. Using 2.45GHz frequency. 20-200 mbar pressure P.W. May, "Diamond Thin Films: A 21st Century Material" Phil. Trans. R. Soc. Lond. A, 358 (2000) 473-495.

19. Plasma Jet Methods • Gas at high flow rates passes through electrical discharge- forms jet of ionized particles. • Strike substrate in secondary chamber (100 torr to 1 atm). • DC arc jet used to pass high currents through ionized flowing process gases. • Exceedingly high growth rates (900 microns/hr). • Very small deposition area P.W. May, "Diamond Thin Films: A 21st Century Material" Phil. Trans. R. Soc. Lond. A, 358 (2000) 473-495.

20. Summary of Deposition Techniques “Low Pressure Diamond Synthesis: Techiques and Results”, Peter K. Bachmann, Thin Film Diamond, Ed. Lettington and Steeds, Pub. Chapman and Hill, 1994

21. Film growth and nucleation • Phase diagram shows diamond is metastable. • Atomic hydrogen – crucial role. • Undergo H abstraction reactions, yield reactive methyl radicals, help propagate diamond lattice. • Terminate dangling carbon bonds, stop graphitization.

22. Effect of mixture concentration and temperature • Low CH4 partial pressure and low temperature – microcrystalline (111) triangular faces with twins. • At higher concentrations and/or temperature rectangular (100) faces- columnar growth P.W. May, "Diamond Thin Films: A 21st Century Material" Phil. Trans. R. Soc. Lond. A, 358 (2000) 473-495.

23. Parameter effects contd. • At higher partial pressures crystalline morphology disappears – disordered graphite with nano crystalline diamond. • Gas composition – film quality from acetylene mixtures inferior to that from methane mixtures.

24. C-H-O composition diagram Synthetic Diamond: Emerging CVD Science and Technology, Edited by K.E. Spear and J.P. Dismukes (Wiley, 1994)

25. Choice of Substrate • Single crystal Si wafers are most common. • Should have high melting point. • Good carbide formers. Mo, W, Ti • Low C diffusion. Synthetic Diamond: Emerging CVD Science and Technology, Edited by K.E. Spear and J.P. Dismukes (Wiley, 1994)

26. Enhanced Nucleation • Substrate preparation – Scratching surface using abrasive film – enhances nucleation. W. A. Yarbrough, J. Am. Ceram. Soc., 75[12] 3179-200 (1992)

27. Bias Enhanced Nucleation • Deposition conditions altered by biasing the substrate. • Negative potential(100-200V) applied for the first few minutes. • Carbon rich layer on top- high nucleation rate and preferred orientation. Huimin Liu and David S. Dandy, Diamond Chemical Vapor Deposition, Noyes Pubilications, New Jersey, USA, 1995 M. Katoh, M. Aoki and H. Kawarada, Jpn. J. Appl. Phy., 33(2A):L194-L196 (1994)

28. Characterization • Difficult to distinguish between diamond, graphite and diamond like carbon • X Ray Diffraction – show pattern for cubic diamond. • Raman Spectroscopy – shows characteristic Raman absorption peaks – unique signature- best technique. W. A. Yarbrough, J. Am. Ceram. Soc., 75[12] 3179-200 (1992)

29. Some Other Reactions and Models • Diamond deposited without using hydogen using fluorocarbon chemistry. • CS2 reacted with fluorine to give diamond with SF6. • Thermodynamics proves mechanism is possible with no necessary activation stages although further experimentation needed. D.E. Patterson, C.J.Chu, B.J. Bai, N.J. Komplin, R.H. Hauge and J.L. Hargrave; pp. 569-576 in Applications of Diamond Films and Related Materials. Ed. Y.Tzeng, M. Yoshikawa, M. Murukawa and A. Feldman, Elsevier, Amsterdam, 1991

30. Charged Cluster Model • Cluster formation during nucleation could also explain diamond formation. • Capillary pressures during cluster formation could result in high pressures stabilizing diamond over graphite. • Charge- most important factor in low pressure diamond synthesis. • Inhibit Brownian coagulation between clusters. • Form an electrical double layer at the cluster surface stabilizing dielectric diamond over conducting graphite clusters. I.D. Jeon, C. J. Park, D. Y. Kim, N.M. Hwang, J. Crys. Growth 223 (2001) 6-14

31. Thermodynamic Paradox

32. Nucleation of Clusters

33. Applications • Thermal dissipation in microelectronic packages – high thermal conductivity. • Composite reinforcement – extreme stiffness – metal-matrix composites. W. A. Yarbrough, J. Am. Ceram. Soc., 75[12] 3179-200 (1992) http://www.chm.bris.ac.uk/pt/diamond/end.htm

34. Conclusions • Diamond is being widely researched because of its excellent properties and wide applications. • Plasma Chemical Vapor Deposition promises to be a commercial technique to grow industrial diamond. • Thermodynamic and kinetic studies are needed not only to improve diamond deposition but also to understand deposition of other metastable phases.

35. Reference List of Deposition Techniques “Low Pressure Diamond Synthesis: Techiques and Results”, Peter K. Bachmann, Thin Film Diamond, Ed. Lettington and Steeds, Pub. Chapman and Hill, 1994

36. Answers • Atomic hydrogen is thought to etch graphite and aid in diamond deposition. It satisfies the dangling bonds at the surface and stops the forming of double bonds which result in graphite. • Substrate is given a bias which accelerates charged particles like ions and electrons to produce reactive species and to increase nucleation density.

37. Discussion