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Diamond thin films 79

Figure 5.2. Two of the more common types of low pressure CVD reactor. (a) Hot
Filament Reactor – these utilise a continually pumped vacuum chamber, while
process gases are metered in at carefully controlled rates (typically a total flow rate
of a few hundred cubic centimetres per minute). Throttle valves maintain the
pressure in the chamber at typically 20–30 torr, while a heater is used to bring the
substrate up to a temperature of 700–900°C. The substrate to be coated – e.g. a
piece of silicon or molybdenum – sits on the heater, a few millimetres beneath a
tungsten filament, which is electrically heated to temperatures in excess of
2200°C. (b) Microwave Plasma Reactor – in these systems, microwave power is
coupled into the process gases via an antenna pointing into the chamber. The size
of the chamber is altered by a sliding barrier to achieve maximum microwave
power transfer, which results in a ball of hot, ionised gas (a plasma ball) sitting on
top of the heated substrate, onto which the diamond film is deposited.

a rate of around 1–10 m per hour, depending upon exact deposition condi-
tions (1 m is one thousandth of a millimetre). However, it also suffers
from a number of major disadvantages. The hot filament is particularly
sensitive to oxidising or corrosive gases, and this limits the variety of gas
mixtures which can be employed. It is also very difficult to avoid contam-
ination of the diamond film with filament material. For diamond to be used
in mechanical applications, metallic impurities at the tens of parts per
million level are not a significant problem, but it becomes unacceptable for
electronic applications.
Microwave Plasma CVD reactors use very similar conditions to hot fil-
ament reactors, and despite being significantly more expensive, are now
among the most widely used techniques for diamond growth. In these