Once individual diamond crystallites have nucleated on the surface, growth proceeds in three dimensions until the isolated crystals meet their neighbours and coalesce. At this point a continuous film is formed, and the only way growth can then proceed is upwards. The resulting film is 'poly-crystalline' with many grain boundaries and defects, and exhibits a columnar structure extending upward from the substrate. Furthermore, as the film becomes thicker, the crystal size increases whilst the number of defects and grain boundaries decreases. This means that the outer layers of thicker films are often of much better quality than the initial nucleating layers. For this reason, if the diamond film is to be used as a heat spreader or optical window (applications where good quality and small number of grain boundaries are paramount), the film is often separated from its substrate and the lower 50-100 ^m are removed by mechanical polishing.
The surface morphology of the diamond film obtained during CVD depends critically upon the various process conditions, especially the gas mixing ratio. Depending upon the ratio of methane to hydrogen, the film can be randomly oriented (Figure 5.5(b)) or have some degree of preferred orientation, such as triangular- (Figure 5.5(c)) or square-faceted films (Figure 5.5(d)). By employing growth conditions which favour one particular orientation, highly textured films can be produced which are very closely aligned to the structure of the underlying substrate (Figure 5.5(e)). The ultimate aim, for electronic applications, is to produce diamond films which are essentially single crystal, but although a number of groups have recently made significant progress, this goal still hasn't been achieved. With increasing methane concentrations, the crystal sizes decrease, until above about 3 per cent CH4 in H2 the crystalline morphology disappears altogether (see Figure 5.5(f)). Such a film is referred to as 'nanocrystalline'
or 'ballas' diamond, and may be considered to be an aggregate of diamond nanocrystals and disordered graphite. Although this type of film might be considered inferior to the more crystalline and therefore better quality diamond films, it still possesses many of the desirable properties of diamond while being much smoother and considerably faster to deposit. Thus, by the simple expedient of changing the growth conditions, films can be deposited with properties ranging from almost graphitic to essentially those of natural diamond. This allows the quality, appearance and properties of a diamond film, as well as its growth rate and cost, to be easily tailored to suit particular applications. With the advent of high power microwave deposition systems, it is now possible to produce CVD diamond films over areas up to 8 inches in diameter and of thicknesses exceeding 1mm (see Figure 5.6).
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