Biomedical Engineering Reference
In-Depth Information
molecular hydrogen dissociates into atomic hydrogen and the H atoms are able to convert meth-
ane into methyl radicals, which are generally regarded as the active diamond precursor species, and
thereafter into acetylenic species and other hydrocarbons, which are stable at these elevated tempera-
tures. Atomic hydrogen and the neutral and radical hydrocarbon species then diffuse to the substrate
surface. Although the gaseous species generated at the filament are in equilibrium at the filament tem-
perature, the species are at a super-equilibrium concentration when they arrive at the much cooler sub-
strate. The reactions, which generate these high-temperature species close to the filament where there is
a high concentration of hydrogen atoms, proceed faster than any other reactions which decompose these
species during the transit time from the filament to the substrate. Consider the equilibrium between
methane and acetylene:
•
CH
H
CH
H
C H
2
H
1
2
4
3
2
2
2
2
At the filament surface, the reaction is immediately driven to the right, creating methyl radicals
and thereafter acetylene. Subsequently, the CH
3
radicals and other hydrocarbon species diffuse to the
substrate. Thermodynamic equilibrium at the substrate temperature of ∼1,100 K calls for the forma-
tion of methane, but the reverse reaction proceeds much slower. Solid carbon precipitates on the sub-
strate to reduce the super-equilibrium concentration of species such as CH
3
in the gas phase. The
allotrope diamond is “stabilized” relative to graphite by a concurrent super-equilibrium concentra-
tion of atomic hydrogen. This simple explanation emphasizes the importance of reaction kinetics in
diamond synthesis by HFCVD. The mechanism of diamond CVD is summarized in
Figure 15.5
. Not
Reactants
H
2
+ CH
4
Activation
C
′
+ Heat
H2
CH
4
+ H
2H
•
CH
3
•
+ H
2
Transport and reaction
Surface diffusion
SiC
Substrate
Bulk diffusion
Si
FIGURE 15.5
Schematic of the CVD diamond growth mechanism.
