Metals diamond film substrates

C fi3 diamond films can be deposited on a wide range of substrates (metals, semi-conductors, insulators single crystals and polycrystalline solids, glassy and amorphous solids). Substrates can be abraded to facilitate nucleation of the diamond film. 

The substrate materials are metals (W, Mo, Ti), silicon (e.g. mirror-polished wafers used in the production of semiconductor devices), glassy carbon, graphite [15], etc., depending on manufacturer or user preferences. Diamond nanocrystals are used as seed-crystals on the substrate surface to enhance the nucleation and make the film growth more uniform. The silicon substrate can be then etched off, and a freestanding diamond film is thus produced. 

Ti, Ta, Zr, and Nb are preferred, because of their ability of forming stable compact oxides film during anodic polarization. Compared with other metals, titanium has the lowest density, easy machinability, high anticorrosion and quick repassivation electrochemical performance, and relatively low cost (Leyens and Peters 2003). It becomes a preferred choice as the substrate material of diamond film electrode (Drory and Hutchinson 1994 Chen andLin 1995 Chen et al. 2003 Hian et al. 2003 Gerger et al. 2004 Chen et al. 2005 Guo and Chen 2007a). 

The primary difficulty inherent in this issue is the small niunber of materials with suitable crystal structures and lattice constants. Some transition metals and ceramics, such as Ni, Cu, Fe, and cBN (Table 5, Ch. 3), are the few isostructural materials with sufficiently similar lattice constants (mismatch <5%). In addition, the extremely high surface energies of diamond (ranging from 5.3 to 9.2 J m for the principle low index planes) and the existence of interfacial misfit and strain energies between diamond films and non-diamond substrates constitute the primary obstacles in forming oriented two-dimensional diamond nuclei. Earlier attempts to grow heteroepitaxial diamond on the transition metals were not successful. The reasons may be related to the high solubility/ mobility of C in/on the metals (for example, Fe, Co, or the... 

D diamond nuclei and single-crystal growth. The high solubility/mobility of C in/ on the metals (Fe, Co, Ni), or the formation of an intermediate layer (carbides or graphite) may inhibit the possible development of an orientational, epitaxial relationship between diamond films and the substrates. 

Yehoda et al. ° presented a method to catalyze the nucleation and growth of diamond films in MW PACVD without seeding substrates. A thin film of Fe, Cu, Ti, Nb, Mo, or Ni was abraded or deposited onto SiC-coated substrate surfaces. The metal films resulted in varying degrees of diamond nucleation enhancement. A qualitative ordering of the best to the worst nucleating metals was established to be Fe, Cu, Ti, Ni, Mo to Nb at the substrate center (hot area), and Fe, Nb, Cu, Mo, Ti, to Ni away fi om the center (cold Fe exhibiting the most pronoimced effect on diamond... 

The most conventional non-equilibrium plasma-chemical systems that produce diamond films use H2-CH4 mixture as a feed gas. Plasma activation of this mixture leads to the gas-phase formation of hydrogen atoms, methyl radicals (CH3), and acetylene (C2H2), which play a major role in further film growth. Transport of the gas-phase active species to the substrate is mostly provided by diffusion. The substrate is usually made from metal, silicon, or ceramics and is specially treated to create diamond nucleation centers. The temperature of the substrate is sustained at the level of 1000-1300 K to provide effective diamond synthesis. The synthesis of diamond films is provided by numerous elementary surface reactions. Four chemical reactions in particular describe the most general kinetic features of the process. First of all, surface recombination of atomic lydrogen from the gas phase into molecular hydrogen returns back to the gas phase ... 

Figure 5.3 Different experimental arrangements used by researchers when working with BDD electrodes, (a) For thin-film BDD still attached to the growth substrate an glass electrochemical cell can be used. (Adapted from Ref. [35].) (i) Cu or Al metal current-collecting plate (ii) the diamond film electrode (ill) the Viton 0-ring seal (iv) the input for nitrogen purge gas (v) carbon rod or Pt counterelectrode and (vi) reference electrode, (b) Preparation of BDD free-standing electrodes. (Adapted from Ref. [36] with permission.)...

The poor adhesion of diamond films to metal substrates has been a long-standing issue that prevents practical applications of the excellent properties of synthetic diamond. Analysis [97] of Ni surface chemisorption indicated that the C-Ni(OOl) bond experiences strong compression and the surface is covered with Ni" and Ni ", while the N-Ni(OOl) bond undergoes slight tension and the surface comprises Ni" and Ni P alternatives. XRD measurement [15] has confirmed the prediction that carbon turns the tensile stress of Ti surface to be compressive. Comparison of the surface morphology of TiC with that of TiN indicates that the surface stresses are different in nature [98]. 

The development of low-pressure synthesis methods for diamond, such as the chemical vapor deposition (CVD) technique, has generated enormous and increasing interest and has extended the scope of diamond applications. Highly efficient methods have been developed for the economical growth of polycrystalline diamond films on non diamond substrates. Moreover, these methods allow the controlled incorporation of an impurity such as boron into diamond, which in this case forms a ptype semiconductor. By doping the diamond with a high concentration of boron (B/C = O.Ol), conductivity can be increased, and semi-metallic behavior can be obtained, resulting in a new type of electrode material with all of the unique properties of diamond, such as hardness, optical transparency, thermal conductivity and chemical inertness [1,2]. 

The substrate should be desirably conductive, because otherwise the back contact must be established on the periphery of the diamond film, which results in large polarization losses under high current densities. Silicon is often used as a substrate and consequently boron-doped Si is preferable, since it minimizes the phenomenon of cross contamination of the dopant atom, and it would probably form a non-blocking contact with the top diamond film [3]. Refractory metals, such as Ti, Ta, Nb or W are also used as suitable substrates for diamond electrodes, but so far the best quality films are obtained on Si substrates. Columnar growth of the crystallites in the diamond film leads to coalescence of the crystallites as the film thickness increases. To reduce the contribution of the grain boundaries on the electrochemical processes, crystallites of a few pm in size, and hence film thickness of this order, are preferable. 

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