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Diamond thin films 87

shows the initial stages of nucleation, with individual diamond crystallites
growing in scratches on the surface. Another, better-controlled version of
this is to use ultrasonic agitation to abrade the substrate immersed in a
slurry of diamond grit in water. Whatever the abrasion method, however,
the need to damage the surface in such a poorly-defined manner prior to
deposition may severely inhibit the use of diamond for applications in, say,
the electronics industry, where circuit geometries are frequently on a sub-
micron scale.

5.7 The CVD diamond film

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 colum-
nar 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 sub-
strate 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 particu-
lar 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 CH in H the crystalline morphology disappears
4 2
altogether (see Figure 5.5(f)). Such a film is referred to as ‘nanocrystalline’