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I. INTRODUCTION

Because of its rarity and beauty as a gem stone, diamond is a material that attracts more than casual nontechnical interest. However, diamond also has considerable technological importance. Its unique hardness makes it very useful, if not essential, for grinding and machining operations on hard materials such as ceramics. An unusual combination of high thermal conductivity and low electrical conductivity (in the pure state) also makes diamond a perfect substrate for high-power solid-state electronic components, and the band gap of diamond makes it attractive for use in high-temperature electronic components. Finally, the optical properties of diamond offer the potential to improve greatly the effectiveness of systems that rely heavily on optical components, for example infrared detectors, radones, and lasers. For many years the application of very high pressures and temperatures using the "metal solvent-catalyst method"1 was the only well-known method for synthesizing diamond. However, as early as 1911 there were indications that diamond can be formed by chemical vapor deposition under conditions where graphite, not diamond, is the stable carbon phase. A description of the early work in this area is given by DeVries.2 In the late 1950s and early 1960s work carried out by Deryagin and co-workers (see, for example, Refs. 3 and 4), Eversole,5 and Angus and co-workers (see, for example, Ref. 6) verified that diamond can indeed be deposited from the pyrolysis of hydrocarbon gases under conditions where it is metastable. In particular, Deryagin's 1480

J. Mater. Res., Vol. 5, No. 7, Jul 1990

work, which supplied the basis for much of the deposition technology used today, was especially important. This early work was followed by a series of programs at the National Institute for Research in Inorganic Materials in Japan, where Matsumoto, Setaka, Kamo, Sato, and co-workers (see, for example, Refs. 7 and 8) developed techniques for deposition of diamond at rates of several microns per hour. This increase in deposition rates from those attained in earlier investigations moved diamond deposition into the realm of a potentially viable commercial process. As evidenced by the large number of subsequent papers, this initial work spurred an intense effort on the deposition of diamond from the vapor phase, which remains unabated today. A number of recent articles have been written reviewing the literature on low-pressure diamond deposition. Among these articles are ones by DeVries,2 Badzian and DeVries,9 Angus and Hayman,10 and Angus et al.u The elements of diamond deposition discussed below will be confined to the morphology of the asdeposited diamond surface and the microstructure of diamond because these are the subjects of this paper. Even in the early work, it was recognized that the morphology of the as-deposited diamond surface varied widely with deposition conditions.4'8 These studies found that under some deposition conditions the deposit appears as a collection of hemispheres, but under other conditions i