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Diamond has always been an outstanding and desirable material. With the
invention of synthetic growth techniques at high pressures and
temperatures in the fifties, it became technical material,
especially for mechanical applications. However, it was the
advent of low pressure deposition techniques that made
accessible the excellent mechanical, thermal, optical and
electronic properties. With these chemical vapour
deposition (CVD) techniques diamond became available in
the form of extended thin films and free-standing plates or
windows. Doping during deposition could be realized, making
diamond a p-type semiconductor. With CVD-diamond a wealth of new
applications opened up.
The fundamental problem of diamond synthesis is the
allotropic nature of carbon. Under ordinary conditions graphite,
not diamond, is the thermodynamically stable crystalline phase
of carbon. Hence, the main requirement of diamond CVD is to
deposit carbon and simultaneously suppress the formation of
graphitic sp2-bonds. This can be realized by
establishing high concentrations of non-diamond carbon etchants
such as atomic hydrogen. Usually, those conditions are achieved
by admixing large amounts of hydrogen to the process gas and by
activating the gas either thermally or by a plasma.
Hence, a common feature of all diamond CVD techniques is a
gas-phase nonequilibrium, i.e. a high supersaturation of atomic
hydrogen and of various hydrocarbon radicals. Typical deposition
conditions are: 1 % methane in hydrogen as source gas,
700-1000°C deposition temperature and gas pressures in the range
30-300 Torr.
The various diamond CVD techniques differ mainly in the way
of gas phase activation and dissociation. The most common
techniques are microwave plasma assisted CVD and thermally
assisted CVD, usually realized by gas activation with a hot
filament, Each of these techniques has its pros and cons. The
distinguishing features are the deposition rate, the deposition
area and the quality of the deposited diamond. The maximum
growth rate reported so far amounts to almost 1 mm/h. However,
those high growth rates are usually limited to very small
deposition areas (« 1 cm2). In general there is an
inverse relationship between film quality and growth rate.
Optically transparent films with high thermal conductivities are
usually deposited at rates not exceeding 10 µm/h, regardless of
the deposition technique. The excellent optical properties of
diamond have been known for a long time. However, optical
applications require extended discs or thin coatings not
provided by natural diamond crystals. With the development of
CVD techniques the situation has changed completely. |
