Biomedical Engineering Reference
In-Depth Information
only do the H atoms play a crucial role in diamond CVD by driving the formation of active hydro-
carbon species, but they also prevent both the reconstruction of the growing diamond surface and the
formation of graphitic nuclei. The deposition of high quality diamond at appreciable growth rates
depends on the ratio of C:H in the precursor gas mixture. Increasing the proportion of carbon results
in higher concentrations of CH
3
at the growing diamond surface and hence increased growth rates.
However, higher concentrations of carbon also lead to poorer quality films, as there is insufficient
H to etch away nondiamond carbon deposits. If the concentration of H at the growing surface is too
high, then diamond will be etched, resulting in very low growth rates or no diamond growth at all.
In HFCVD, the transport of the active species and their mixing with the surrounding gas takes
place as a result of the interaction of free and forced convection as well as diffusion. A combination
of concentration and thermal gradients is the major driving force. In order to achieve uniform film
coating and uniform thickness, the temperature of the substrate and the local specie concentration
close to the substrate surface need to be uniform.
In diamond HFCVD, the mechanism of heat transfer also plays a major role. In addition to con-
duction, convection, and radiation, heat transfer is also achieved via atomic hydrogen. This is as
expected since the formation of atomic hydrogen at or near the filament surface is highly endother-
mic. Atomic hydrogen readily recombines on solid surfaces to form molecular hydrogen with the
recombination reaction being highly exothermic. Thus, atomic hydrogen acts as a carrier of heat from
the filament to the growth surface. Since the quality, morphology, and defect density of diamond films
are sensitive to temperature, a uniform substrate temperature distribution is crucial for the deposi-
tion of uniform diamond films with consistent properties. An important feature of HFCVD is that the
growth step is separate from the film activation step, which ultimately allows independent control of
both growth and activation temperatures.
15.3.2.2 Filament Characteristics
It is obvious that in a process such as HFCVD, the filament plays a critical role. The key features of
the filament characteristics and assembly have been summarized
[47,48]
. The most commonly used
filament materials are tantalum and tungsten due to their high melting point and high electron emis-
sivity. Refractory metals which form carbides (e.g., tungsten and tantalum) typically must carburize
their surface before supporting the deposition of diamond films. The process of filament carburization
results in carbon consumption from the hydrocarbon precursor gas. Hence, there is a specific incuba-
tion time for the nucleation process which produces diamond films. Therefore, this process may affect
the early stages of film growth, although it is insignificant over longer growth periods. Furthermore,
the volume expansion produced by carbon incorporation results in cracks along the length of the wire.
The development of these cracks is undesirable as it reduces the lifetime of the filament but does not
adversely affect the quality of the resulting films.
As well as having a high melting point, suitable filament materials must also have an appreciable
electron emissivity to cause dissociation of molecular hydrogen and ultimately initiate the growth pro-
cess. For these reasons, tungsten (W, melting point 3,695 K) and tantalum (Ta, melting point 3,293 K)
filaments are typically used in HFCVD processes. The physical data for tungsten and tantalum is
shown in
Table 15.3
. In this research work, Ta filaments are used because there is less contamination
of the diamond films than when W filaments are used. As the differences in the chemical natures and
physical properties of the filament materials have a direct influence on gas activation and decomposi-
tion, diamond deposition may vary slightly given otherwise constant deposition parameters.
