Biomedical Engineering Reference
In-Depth Information
crystals and thin films using inductively coupled RF plasma methods
[37,38]
and capacitively cou-
pled methods
[39,40]
has been widely reported. A high power in the discharge leading to greater elec-
tron densities is necessary for efficient diamond growth. However, the use of higher power results in
physical and chemical sputtering from the reactor walls, leading to contamination of the diamond
films. The advantage of RF plasmas is that they can easily be generated over much larger areas com-
pared to microwave plasmas, but the method is not routinely used for the deposition of diamond films
because the quality of the diamond films is inferior.
15.3.1.3 DC Plasma-Enhanced CVD
In this method, plasma in an H
2
-hydrocarbon mixture is excited by applying a DC bias across two
parallel plates, one of which is the substrate
[41-43]
. DC plasma-enhanced CVD has the advantage
of being able to coat large areas as the diamond deposition area is limited by the electrodes and the
DC power supply. In addition, the technique has the potential for very high growth rates. However,
diamond films produced by DC plasmas were reported to be under high stress and contain high
concentrations of hydrogen as well as impurities resulting from plasma erosion of the electrodes.
15.3.2
Hot Filament CVD
In this chapter we consider the hot filament CVD (HFCVD) method in detail because it has particular
advantages over the other methods mentioned earlier. Atomic hydrogen can be produced by the pas-
sage of H
2
over a refractory metal filament, such as tungsten, molybdenum, and tantalum, heated to a
temperature between 2,000 and 2,500 K. When atomic hydrogen was added to the hydrocarbon typi-
cally with a C/H ratio of
0.01, it was observed that diamond could be deposited while graphite for-
mation was suppressed
[43-48]
. The generation of atomic hydrogen during diamond HFCVD enables
the following to occur:
A dramatic increase in the diamond deposition rate to
1 μm h
1
l
l
The nucleation and growth of diamond on nondiamond substrates.
Due to its inherent simplicity and comparatively low operating cost, HFCVD has become widely
used in industry.
Table 15.2
outlines typical deposition parameters used in the growth of diamond
films by HFCVD.
A wide variety of refractory materials have been used as filaments including tungsten, tantalum,
and rhenium due to their high melting point and high electron emissivity.
15.3.2.1 Growth Mechanisms
Various mechanisms for diamond growth
[45-49]
have been postulated. Thermodynamic near-
equilibrium is established in the gas phase at the filament surface. At temperatures of around 2,300 K,
Table 15.2
Typical Deposition Parameters Used in the Growth of Diamond Films by HFCVD
Gas Mixture
Total Pressure (Torr)
Temperature (K)
Substrate
Filament
CH
4
(0.5-2.0%)/H
2
10-50
1,000-1,400
2,200-2,500
