Page 91 - Visions of the Future Chemistry and Life Science

80 P. W. MAY

systems, microwave power is coupled into the chamber in order to create
a discharge or plasma. This leads to heating and fragmentation of the gas
molecules, resulting in diamond deposition onto a substrate which is
immersed in the plasma. The most common type of microwave reactor in
use is shown in Figure 5.2(b). Nowadays, microwave powers of up to 60 kW
can be utilised in such systems giving growth rates well in excess of
0.1mm per hour. As well as high powers and hence higher growth rates,
other advantages of microwave systems over other types of reactors are
that they can use a wide variety of gas mixtures, including mixtures with
high oxygen content, or ones composed of chlorine- or fluorine-containing
gases. The fact that no filament is involved makes microwave systems
inherently cleaner than hot filament systems, and so they have become the
system of choice for making diamond for electronic applications.
A number of other deposition methods have been used for growing
diamond, with varying degrees of success. These include oxyacetylene
welding torches, arc jets and plasma torches, laser ablation and liquid
phase crystallisation, but none of these yet realistically compete with the
hot filament or microwave systems for reliability and reproducibility.

5.4 The chemistry of CVD diamond growth
The complex chemical and physical processes which occur during
diamond CVD are comprised of a number of different but inter-related fea-
tures, and are illustrated in Figure 5.3. At first sight, this may seem like a
daunting array of physical and chemical reactions which need to be grasped
if diamond CVD is to be understood. But over the past 10 years there have
been a large number of studies of the gas phase chemistry, and we are now
beginning to obtain a clearer picture of the important principles involved.
The first clue was that diamond growth appeared to be independent of the
chemical nature of the gas phase precursors – it was only the total number
of carbons, hydrogens and oxygens in the reactant molecules that mat-
tered. This meant that the gas phase chemistry is so rapid that it simply
and effectively breaks down the constituent gases to smaller, reactive
components.
It is now believed that the most critical component in the gas phase
mixture is atomic hydrogen, and indeed, this reactive atom drives the
whole chemical system. Two hydrogen atoms are made when a hydrogen
molecule (H ) splits apart. In a hot filament system, the thermal energy
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