handbook of carbon, graphite, diamond and fullerenes

148 Carbon, Graphite, Diamond, and Fullerenes
2.5 Deposition Systems and Apparatus
A common CVD apparatus for the deposition of pyrolytic graphite is the
so-called cold-wall reactor. This reactor does not require a furnace since the
substrate to be coated is heated directly, usually by induction heating.
The decomposition reactions for the deposition of pyrolytic graphite are
endothermic, i.e., they absorb heat. As a result, deposition takes place
preferentially on the surfaces where the temperature is the highest, in this case
the substrate, while the cooler walls of the reactor remain essentially uncoated.
A simple laboratory type reactor for pyrolytic-graphite deposition is
shown in Fig. 7.2 J 31 The substrate is a molded-graphite disk which is rotated
to improve deposition uniformity. It is heated by a high-frequency (450 kHz)
induction coil and deposition occurs at low pressure (500 Pa). Temperature
is monitored and controlled by a sheathed thermocouple and corroborated
by an optical pyrometer.
Production systems of a similar basic design now reach considerable
size, with CVD furnaces 1.2 m in diameter and over 2 m high commercially
available.
Thermocouple —'
Quartz
Reactor
Tube
Substrate
Exhaust
Figure 7.2. Schematic of a cold-wall reactor for the production of pyrolytic
graphite. PI