CVD DIAMOND PROCESSES

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. 

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... 

Born in London, Paul May grew up in Redditch, Worcestershire. He went on to study at Bristol University, where he graduated with a first class honours in chemistry in 1985. He then joined GEC Hirst Research Centre in Wembley where he worked on semiconductor processing for three years, before returning to Bristol to study for a PhD in plasma etching of semiconductors. His PhD was awarded in 1991, and he then remained at Bristol to co-found the CVD diamond research group. In 1992 he was awarded a Ramsay Memorial Fellowship to continue the diamond work, and after that a Royal Society University Fellowship. In October 1999 he became a full-time lecturer in the School of Chemistry at Bristol. He is currently 36 years old. His scientific interests include diamond films, plasma chemistry, interstellar space dust, the internet and web technology. His recreational interests include table-tennis, science fiction, and heavy metal music. 

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. 

Activation and conductivity at room temperature are problems that can be addressed by the incorporation of other electronic structures that increase carrier transport. Crystal morphology is an important parameter in the boron doping process to determine uncompensated acceptors (Aa-Ad) for different crystal facets as a function of doping concentration. The temperature coefficient of resistance for a CVD diamond film can be changed by boron doping. As conductivity depends on the crystal phase, the combined electromechanical properties can be exploited in sensor applications both for resistive temperature detectors and for pressure transdu-cers. " As electrical conductivity is related linearly with boron concentration, a better-controlled process may allow for the development of better semiconductor devices improving crystal quality and operating limits. ... 

It was argued that the most celebrated violation of thermodynamics is the fact that CVD (chemical vapor deposition) diamond forms under thermodynamically unstable pressure - temperature conditions. A careful analysis of thermodynamic data led to the conclusion that certain equilibria are taking part in the process, so that the process no longer appears as a thermodynamic paradox [18]. The key idea was to assume that CVD diamond formation is a chemical process consisting in accretion of polymantane macromolecules. Thus, violations of thermodynamic principles could be avoided. 

Figure 14. Example of a CVD diamond hemispherical dome, 70 mm in diameter and over 1.0 mm thick that has been semi-processed on both surfaces.

Figure 22. Interferometric surface profile of a CVD diamond window showing that by the use of an improved process, a flatness of 0.5 fringe can be obtained over a 48 mm diameter.

The ability to process CVD diamond to controlled form shapes has also allowed the production of samples with wedge angles up to 1 degree, as required to eliminate interferenee fringes for broad band IR transmission, as is the case in synchrotron beam lines [4,33]. 

As discussed in 2.3.2.2, it is possible to process CVD diamond plates to accuracy close to one visible fringe over 25 mm. This is equivalent to a deviation from flatness... 

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