Page 89 - Visions of the Future Chemistry and Life Science

78 P. W. MAY

Figure 5.1. In diamond, every
carbon atom is bonded to four
others in a strong, rigid
tetrahedral structure.

growing film, leaving only pure diamond behind. This process became
known as ‘chemical vapour deposition’ (CVD), since it involves a chemi-
cal reaction occurring within a vapour over a surface, leading to deposition
of a thin coating onto that surface. Over the next few years more break-
throughs were made which allowed diamond films to be grown at signifi-
cant rates on many useful materials. This series of discoveries stimulated
world-wide interest in diamond CVD, in both academia and industry,
which continues to the present day.

5.3 Methods for production of CVD diamond

All CVD techniques for producing diamond films require a means of ‘acti-
vating’ gas phase carbon-containing precursor molecules. This activation
can involve heating (e.g. a hot filament), or an electric discharge, such as a
plasma. Figure 5.2 illustrates two of the more popular experimental
methods. While each method differs in detail, they all share a number of
features in common. For example, growth of diamond (rather than graph-
ite) normally requires that the precursor gas (usually methane, CH ) is
4
diluted in excess of hydrogen – typically the mixing ratio is 1 per cent
methane to 99 per cent hydrogen. Also, the temperature of the substrate is
usually greater than 700°C in order to ensure the formation of diamond
rather than amorphous carbon.
Hot Filament CVD (see Figure 5.2(a)) is relatively cheap and easy to
operate and produces reasonable quality polycrystalline diamond films at