Page 95 - Visions of the Future Chemistry and Life Science

84 P. W. MAY

5.5 The substrate material
Most of the CVD diamond films reported to date have been grown on single
crystal silicon wafers, mainly due to its availability, low cost, and favour-
able properties. However, this is by no means the only possible substrate
material – although any candidates for diamond growth must satisfy a
number of important criteria. One requirement is obvious – the substrate
must have a melting point higher than the temperature required for
diamond growth (normally 700°C). This precludes the use of existing
CVD techniques to coat low-melting point materials, like plastics, alumin-
ium, some glasses, and electronic materials such as gallium arsenide.
Another criterion is that the substrate material should expand by the
same amount as diamond when heated. This is because at the high growth
temperatures currently used, a substrate will tend to expand, and thus the
diamond coating will be grown upon, and bonded directly to, an expanded
substrate. Upon cooling, the substrate will contract back to its room tem-
perature size, whereas the diamond coating will be relatively unaffected by
the temperature change. Thus, the diamond film will experience signifi-
cant compressive stresses from the shrinking substrate, leading to bowing
of the sample, and/or cracking and flaking of the entire film.
Another issue is that at the high deposition temperatures many sub-
strate materials react with carbon directly to form a carbide. The presence
of a thin carbide layer is not a problem – in fact it is desirable, since the
carbide layer can be pictured as the ‘glue’ which aids the adhesion of the
diamond layer by (partial) relief of stresses at the interface. Without this
carbide glue, any diamond layer will not adhere well to the surface, and the
films will often readily delaminate after deposition. This can be utilised as
one method to make free-standing diamond films, using non-carbide-
forming substrate materials such as copper, tin, silver and gold.
Conversely, if the substrate material is too reactive toward carbon, then
the deposited carbon (even when it’s in the form of diamond) simply dis-
solves into the surface forming a solid solution. This can result in large
quantities of carbon being transported into the bulk, rather than remain-
ing at the surface where it can promote diamond growth. Metals where this
is significant include titanium, nickel and iron. The latter metal is of par-
ticular concern, because this means that at present all industrially impor-
tant ferrous materials (such as iron and stainless steel) cannot be diamond
coated using simple CVD methods.