CVD diamond deposition

A large number of CVD diamond deposition technologies have emerged these can be broadly classified as thermal methods (e.g., hot filament methods) and plasma methods (direct current, radio frequency, and microwave) [79]. Film deposition rates range from less than 0.1 pm-h to 1 mm-h depending upon the method used. The following are essential features of all methods. 

Ramesham, R. and Rose, M. F. (1997), Electrochemical characterization of doped and undoped CVD diamond deposited by microwave plasma. Diam. Relat. Mater., 6(1) 17-27. 

Unlike the activation of the CVD-diamond deposition which is a chemical process, the activation of DLC deposition is a physical phenomenon in which the sp configuration is stabilized by ion bombau dment of the growing film instead of (or in addition to) atomic hydrogen. The process is known in general terms as physical vapor deposition (PVD). 

The feed gases are 99.8% pure CBU and 99.999% pure hydrogen. The flows are controlled by MKS mass flow controllers at 0.4 seem for CH4 and 99.6 seem for H2. The pressure of the chamber is controlled by the flow rate and the pumping speed through a valve. Typical chamber pressures are 30 to 100 torr. During CVD diamond deposition, we observed soot at the back of the substrate holder where the temperature was low. 

Figure 15 shows the variation of diamond deposition rates by various activated CVD techniques as well as the HP—HT technique (165). It can be seen that the highest growth rate of activated CVD diamond synthesis is stiU an order of magnitude lower than the HP—HT technique. However, CVD has the potential to become an alternative for diamond growth ia view of the significantly lower cost of activated CVD equipmeat and lower miming and maintenance costs. 

Fig. 15. Variation of diamond deposition rates by various activated CVD techniques as well as the HP—HT technique (165).

The activated CVD diamond techniques can be mote attractive in cases where the huge capital investment (several hundred million dollars) requited for the HP—HT technology is not available or where the high level of technical knowledge requited for HP—HT synthesis is not available. In addition, most wear-resistant apphcations requite diamond coatings only of the order of a few micrometers thick. Such coatings can be deposited ditecdy on the finished product without the need for further finishing if CVD techniques are employed. 

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. 

CVD deposition except for the special and important case of diamond deposition, a topic which is reviewed in Ch. 7.P H34]... 

These compounds generally decompose into two stable primary species the methyl radical (CH3) and acetylene (C2H2).P 1 The methyl radical is considered the dominant compound in generating the growth of CVD diamond.P2][23] Direct deposition from acetylene, although difficult experimentally, has been accomplished, with a marked increase in the crystallinity of the diamond deposit.P" ... 

Most CVD-diamond processes require a plasma (see Ch. 5. Sec. 9). Two types of plasma are currently used for the deposition of diamond microwave plasma (non-isothermal) and arc plasma (isothermal). 

DLC has properties similar to CVD diamond and it is easier to process without the high-temperature substrate requirements and with little restriction on size. However, it has several disadvantages low deposition rate, high internal stress, and availability only in thin coatings. A number of important applications have been developed with a promising future. 

DLC coatings are already in production in several areas (optical and IR windows) and appear particularly well-suited for abrasion and wear applications due to their high hardness and low coefficient of friction. They have an extremely smooth surface and can be deposited with little restriction of geometry and size (as opposed to CVD diamond). These are important advantages and DLC coatings will compete actively with existing hard coatings, such as titanium carbide, titanium nitride, and other thin film... 

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... 

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. 

CVD 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. 

There are numerous materials, both metallic and ceramic, that are produced via CVD processes, including some exciting new applications such as CVD diamond, but they all involve deposition on some substrate, making them fundamentally composite materials. There are equally numerous modifications to the basic CVD processes, leading to such exotic-sounding processes as vapor-phase epitaxy (VPE), atomic-layer epitaxy (ALE), chemical-beam epitaxy (CBE), plasma-enhanced CVD (PECVD), laser-assisted CVD (LACVD), and metal-organic compound CVD (MOCVD). We will discuss the specifics of CVD processing equipment and more CVD materials in Chapter 7. 

FIGURE 3.10 (a) Chemical-vapour deposition reactor (b) cross section of a 100 pm-thick CVD diamond film grown by DC arc jet. The columnar nature of the growth is evident, as is the increase in film quality and grain size with growth time. (Courtesy of Dr. Paul May and Prof. Mike Ashfold, Bristol University.)... 

Using the temperature gradient method it was also possible to deposit c-BN on various substrates, e.g., on diamond crystals or CVD-diamond sheets [156, 180]. 

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