handbook of carbon, graphite, diamond and fullerenes

3.3 Glow-Discharge (Microwave) Plasma Deposition
CVD Diamond 311
A glow-discharge (non-isothermal) plasma is generated in a gas by a
high-frequency electric field such as microwave at relatively low pressure.
In such a plasma, the following events occur:
• In the high-frequency electric field, the gases are ionized
into electrons and ions. The electrons, with their very
small mass, are quickly accelerated to high energy levels
corresponding to 5000 K or higher.
• The heavier ions with their greater inertia cannot respond
to the rapid changes in field direction. As a result, their
temperature and that of the plasma remain low, as
opposed to the electron temperature (hence the name
non-isothermal plasma).
• The high-energy electrons collide with the gas molecules
with resulting dissociation and generation of reactive
chemical species and the initiation of the chemical reaction.
The most common frequencies in diamond deposition are the microwave
(MW) frequency at 2.45 GHz and, to a lesser degree, radio frequency (RF) at
13.45 MHz (the use of these frequencies must comply with federal regulations).
Deposition Process. A typical microwave plasma for diamond
deposition has an electron density of approximately 10 20 electrons/m 3 , and
sufficient energy to dissociate hydrogen. A microwave-deposition reactor
is shown schematically in Fig. 13.2J 16 H 22] The substrate (typically a silicon
wafer) is positioned at the lower end of the plasma. Gases are introduced
at the top of the reactor, flow around and react at the substrate, and the
gaseous by-products are removed into the exhaust. The substrate must be
heated to 800 - 1000°C for diamond to form. This can be done by the
interaction with the plasma and microwave power but this is difficult to
regulate and, more commonly, the substrate is heated directly by radiant or
resistance heaters which provide more accurate temperature control.
Typical microwave deposition conditions are the following:
Incident Power: 600 W
Substrate Temp.: 800 -1000°C
Gas mixture H2/CH4: 50/1 to 200/1
Pressure: 10 to 5000 Pa
Total gas flow: 20 - 200 scm 3 /min