Diamond thin films by CVD process Miro Koskinen

Diamond thin film properties • Excellent • Mechanical properties • Thermal properties • Optical properties • Inert • Bio compability • Very good electrical insulator • Can be doped, becoming semiconductor

What is diamond thin film • Nano‐crystalline diamond (NCD) • Grain sizes from 30 nm to 100 nm • Lot of sp 3 carbon ( 99%) • Grown in hydrogen rich gass • Ultra‐nano‐crystalline diamond (UNCD) • Fine grained (5‐ 15 nm) • Less of sp 3 carbon ( 90%) • Grown in argon rich gass

CVD Processes Hot filament CVD ( HFCVD) Microwave plasma CVD (MWCVD) • Substrate temp. 700– 900 ◦C • Filament temp. 2200 ◦C. • Process gasses: 99% H 2, 1% CH 4 • Substrate temp 700◦C • Process gasses: 99% H 2, 1% CH 4

Diamond thin film growth

Problems • High temperature • Thermal expansion coefficient • Substrate must be able to produce thin carbide layer • Solubility issues

Applications • Cutting tools • Thermal management in electronics • Optics • Structural materials in MEMS • Diamond coated fibres

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Information slides • Diamond films are classified to ultra‐nano‐crystalline diamond (UNCD) and nano‐crystalline diamond (NCD) thin films, based on their microstructure properties and growth environment. UNCD materials are composed of small diamond particles (2‐ 5 nm) with sp 2 carbon between the particles. UNCD is grown in argon rich, hydrogen poor CVD environments. UNCD contains about 95% sp 3 carbon. [1], [2] • NCD materials diamond particles are columnar grains usually below 100 nm. NCD is grown in hydrogen rich conditions and contain far less sp 2 carbon than UNCD films. [1], [2]

CVD processes • Hot filamnet CVD uses vacuum chamber, while process gasses are metered in the chamber. The substrate to be coated is few millimeters beneath the filament. The filament is heated to about 2200 C. The filament is usually made from metals that can withstand such conditions such as tungsten and tantalum. HFCVD is relatively cheap and easy to use and produces good quality diamond films. Disadvantages are that some filament material usually ends up in the film as well. In many electronic applications even small inpurities are unacceptable. [3], [4] • Microwave plasma CVD use very similar conditions to HFCVD. MWCVD is more expensive than HFCVD, but is now one of the most widely used techniques for diamond growth. Microwaves create a discharge[3], [4]

References • [1] Butler, James E. , and Anirudha V. Sumant. "The CVD of nanodiamond materials. " Chemical Vapor Deposition 14. 7‐ 8 (2008): 145‐ 160. • [2] Williams, Oliver A. , and Miloš Nesládek. "Growth and properties of nanocrystalline diamond films. " physica status solidi (a) 203. 13 (2006): 3375‐ 3386. • [3] May, Paul W. "Diamond thin films: a 21 st‐century material. " Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences 358. 1766 (2000): 473‐ 495 • [4] Ashfold, M. N. R. , et al. "Thin film diamond by chemical vapour deposition methods. " Chem. Soc. Rev. 23. 1 (1994): 21‐ 30.