Diamond by CVD

To deposit diamond by CVD, the carbon species must be activated since, at low pressure, graphite is thermodynamically stable and without activation only graphite would be formed. Activation is obtained by two energy-intensive methods high temperature and plasma. CVD processes based on these two methods are continuously expanded and improved and new ones are regularly proposed. 

A large variety of carbon-containing gas species have been employed to synthesize diamond by CVD, These include methane, aliphatic and aromatic hydrocarbons, alcohols, ketones, amines, ethers, and carbon monoxide, with methane being the most frequently used reagent. In... 

Diamond by CVD is formed as crystals or as films from various gaseous hydrocarbons or other organic molecules in the presence of activated, atomic hydrogen. It consists of sp -hybridized carbon atoms with the three-dimensional crystalline structure of the diamond lattice. 

CVD diamond or "low-pressure diamond are synonyms of the term diamond by CVD. Diamond by CVD can be prepared in a variety of ways. Deposition parameters are total (low) pressure, partial hydrogen pressure, precursor molecules in the gas phase, temperature for activation of the hydrogen and that of the surface of the underlying substrate. The energy supply for the hydrogen activation may be, for instance heat, radio frequency, microwave excitation (plasma deposition) or accelerated ions (e.g. Ar ions). CVD diamond has also been obtained at atmospheric pressure from oxyacetylene torches... 

Another approach is to coat the cutting tool material with a carbide former, such as titanium or siUcon or their respective carbides by CVD and deposit diamond on top of it. The carbide layer may serve as an iaterface between diamond and the cemented carbide, thus promoting good bonding. Yet another method to obtain adherent diamond coatings is laser-iaduced microwave CVD. By ablating the surface of the substrate with a laser (typically, ArF excimer laser) and coating this surface with diamond by microwave CVD, it is possible to improve the adhesion between the tool and the substrate. Partial success has been achieved ia this direction by many of these techniques. 

Glaze coatings (58) are appHed to dry or bisque-fired clay ceramics to form a strong, impermeable surface that is aesthetically pleasing. Protective ceramic coatings can also be deposited by CVD (68,90). Plasma activated CVD has been used extensively to produce diamond and diamondlike films. Diamond films can also be used to make optical coatings with a tailored refractive index. 

Also noted is the rapid expansion of a number of materials produced by CVD, which include copper, tungsten, diamond, silicon carbide, silicon nitride, titanium nitride, and others. The coverage of the chemistry and deposition techniques of these materials has been greatly expanded. 

This chapter is a review of the two major allotropes graphite and diamond, which are both produced extensively by CVD. The properties of these two materials can vary widely. For instance, diamond is by far the hardest-known material, while graphite can be one of the softest. Diamond is transparent to the visible spectrum, while graphite is opaque diamond is an electrical insulator, while graphite is a conductor. 

Diamond is obtained as a polycrystalline material by CVD with properties similar to these of natural diamond. Efforts to produce single crystal thin films have so far been largely unsuccessful. 

Like diamond, DLC can be obtained by CVD by plasma action in a hydrocarbon atmosphere. Its deposition process differs from that of diamond in as much as the activation is not so much chemical (i.e., the use of hydrogen atoms) but physical. This physical activation is usually obtained by colliding accelerated ions produced by a high-frequency discharge. 

Like synthetic diamond, C-BN is normally obtained by high-pressure processing. Efforts to synthesize it by CVD at low pressure are promising. It is deposited in an electron-cyclotron-resonance (ECR) plasma from a mixture of BF3 and either ammonia or nitrogen at 675°C on an experimental basis.F l Like CVD diamond, it is also deposited by the hot-filament method using diborane and ammonia diluted with hydrogen at 800°C.P 1... 

Many problems must be solved before practical, reliable, and cost-effective diamond semiconductor devices become available. Yet, the prospects are good, particularly if epitaxial single-crystal or highly oriented poly crystalline diamond can be effectively produced by CVD.P5]... 

CVD plays an increasingly important part in the design and processing of advanced electronic conductors and insulators as well as related structures, such as diffusion barriers and high thermal-conductivity substrates (heat-sinks). In these areas, materials such as titanium nitride, silicon nitride, silicon oxide, diamond, and aluminum nitride are of particular importance. These compounds are all produced by CVD. 1 1 PI... 

Heat sinks, in the form of thin slices prepared from single-crystal natural diamond, are already used commercially but are limited in size to approximately 3x3x1 mm. These single-crystal diamonds are gradually being replaced by CVD diamond, which is now available in shapes up to 15 cm in diameter. P6]-[28] gQg - gf cVD diamond may remain a... 

Whereas a microwave plasma is most commonly used for the PE-CVD of diamond films, an ECR is the only plasma that is used for diamond deposition below 1 Torr [27-29]. Although Bozeman et al. [30] reported diamond deposition at 4 Torr with the use of a planar ICP, there have been a few reports that describe the synthesis of diamond by low-pressure ICP. Okada et al. [31-33] first reported the synthesis of nanocrystalline diamond particles in a low-pressure CH4/CO/H2 ICP, followed by Teii and Yoshida [34], with the same gas-phase chemistry. 

Cubic boron nitride (cBN) has a zinc blende-type crystal structure with a lattice constant of 3.615 A, which is very close to that of diamond (3.567 A). The difference is only about 1.3%. According to RHEED measurements with the electron beam parallel to the 111 layer of cBN, a growth of diamond by DC plasma CVD on cBN(lll) [150] using c = 0.5%CH4/H2, T = 900°C, and F=180Torr led to a result that a smooth (111) layer of diamond was epitaxially deposited in such a way that the [110] direction of diamond was parallel to that of cBN. Namely, D 111 //cBN(lll and D[110]//cBN[110]. In the RHEED pattern, however, extra spots were observed, which were presumably due to the twinnings of (111 diamond layers. In the Raman spectra, there were two lines due to cBN at 1054.5 and... 

In Figure 10.8, reference spectra of AES and XPS-EELS from various materials observed by Stoner et al. [2] are shown. Based on these data, it is clearly seen that the specimen spectra of Figure 10.7 exhibit a transition process from P-SiC formed by CVD to diamond. This transition process was also confirmed by Raman spectroscopy. According to an XTEM observation for the specimen after a 1-h biasing followed by a 5-h diamond CVD, an a-SiC layer of 6-nm (maximum 10-nm) thickness was present between the Si substrate surface and the diamond layer. An HRTEM indicated that diamonds nucleated within the interfacial layer but above the Si substrate. From the observed data, a model of diamond nucleation by BEN was proposed, as shown in Figure 10.9. 

Diamond is electrically a good insulator because of its large band gap (5.47 eV), but the diamond surface grown by CVD was found to be p-type and conducting. 

Under these circumstances, it would be of significance to review the articles on oriented and heteroepitaxial growth of diamond films by CVD, and particularly summarize the processing conditions for the readers to further develop and elaborate the science and technology of diamond CVD. It is expected that this monograph would be useful for such purposes. 

Boron is an acceptor in diamond with an activation energy of 0.37 eV. Natural type II b diamond is an example of a boron doped p-type diamond. There are many commercial sources of synthetic boron doped-diamond. Polycrystalline boron doped diamond films, made by CVD, have been made for electrode applications. High-pressure high-temperature diamonds with boron doping are also available. ... 

Coatings are made by CVD, MT (medium temperature)-CVD, PVD, and plasma-activated CVD. The latter technique was recently successful in producing adherent diamond layers. The keenest edges are now produced by PVD coating. 

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