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 Among other properties, diamond is the hardest known material, very low wear resistance has the highest thermal conductivity at room temperature, is transparent over a very wide wavelength range, is the stiffest material, the least compressible, and is inert to most chemical reagents.

What is diamond thin film Nano-crystalline diamond (NCD) Grain sizes from 30nm to 100nm Lot of sp3 carbon (99%) Grown in hydrogen rich gass Ultra-nano-crystalline diamond (UNCD) Fine grained (5-15nm) Less of sp3 carbon (90%) Grown in argon rich gass

CVD Processes Substrate temp. 700–900 ◦C Filament temp. 2200 ◦C. Hot filament CVD ( HFCVD) Microwave plasma CVD (MWCVD) Substrate temp. 700–900 ◦C Filament temp. 2200 ◦C. Process gasses: 99% H2, 1% CH4 Substrate temp 700◦C Process gasses: 99% H2, 1% CH4 All CVD techniques for producing diamond films require a means of activating gas phase carbon-containing precursor molecules. , filament material usually tungsten or tantalum. Grow rate about 1-10 um per hour. the precursor gas (usually CH4) is diluted in excess of hydrogen, in a typical mixing ratio of 1%vol methane HFCVD pressure 20 Torr Plasma torch

Diamond thin film growth Although the bulk of diamond is fully sp3 bonded, at the surface there is effectively a ‘dangling bond’ Atomic H is known to etch graphitic sp2 carbon many times faster than diamond- like sp3 carbon. H atoms react with neutral species such as CH4 to create reactive radicals, such as CH3, which can then attach to suitable surface sites.

Problems High temperature Thermal expansion coefficient Substrate must be able to produce thin carbide layer Solubility issues -substrate must withstand about 700 C, this excludes plastics, aluminum, some glasses. -Thus, the diamond film will experience significant compressive stresses from the shrinking substrate, leading to bowing of the sample, and/or cracking, flaking or even delamination of the entire film. -Cu, Sn, Pb, Ag and Au,

Applications Cutting tools Thermal management in electronics Optics Structural materials in MEMS Diamond coated fibres -non ferrous tools, tungsten carbide tools. -devices can be packed more tightly without overheating. -optical components, especially as a protective coating for harsh environments. (ZnS), ZnSe -find uses as reinforcing agents in metalmatrix composites, allowing stronger, stiffer and lighter loadbearing structures to be manufactured for use in, say, aerospace application^.

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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 (2-5nm) with sp2 carbon between the particles. UNCD is grown in argon rich, hydrogen poor CVD environments. UNCD contains about 95% sp3 carbon. [1], [2] NCD materials diamond particles are columnar grains usually below 100nm. NCD is grown in hydrogen rich conditions and contain far less sp2 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 21st-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.