Biomedical Engineering Reference
In-Depth Information
Table 15.3 Selected Physical Data for Prospective Filament Materials [50]
Material
Melting Point (K)
Resistivity ( Ω m)
Density (kg m 3 )
39  10 8
Tungsten
3,695
19,254
63  10 8
Tantalum
3,293
16,670
A wide range of literature on HFCVD-based diamond deposition places the low filament
temperature limit at 2,200 K and the upper limit at 2,700 K for successful diamond growth.
The filament carburization process [47,51] proceeds according to the following sequence of steps:
l Transport of CH 4 from the gas phase to the metal surface and subsequent physical adsorption
l Decomposition of CH 4 on the metal surface where the resulting carbon and hydrogen atoms are
chemisorbed
l Liberation of gaseous H, H 2 , and CH X
l Transformation of the adsorbed C atoms into the dissolved state
l Diffusion of carbon atoms into the metal lattice to form sub-carbides (M 2 C) and carbides (MC).
If the filament temperature falls below a critical value, depending on the carbon concentration, a
carbon film forms on the filament. This film deactivates the filament surface (i.e., the filament tem-
perature decreases) and the production of atomic hydrogen is rapidly reduced. If the filament power
is not increased immediately to dissolve or vaporize the carbon film, then it will grow thicker leading
to a further decrease in the filament temperature. Spectral emissivity measurements aim at measur-
ing the spectral emissivity of the filaments. The filament activity is highest if the filament surface is
totally free of carbon. The changes in filament emissivity, resistance, and power consumption have
been attributed to the deposition and etching of carbon occurring at the surface.
15.3.2.3 Diamond Nucleation Process
For obvious reasons the initial stage of diamond nucleation on a nondiamond substrate is critical par-
ticularly when considering the application of the process to dental burs [51-57] . It affects the uni-
formity and morphology of the diamond films deposited by CVD.
Seeding or abrading with diamond powder or immersing in diamond paste containing small crys-
tallites processed in an ultrasonic bath has been used for many years for diamond film deposition
[52-55] . The method promotes nucleation of diamond crystals onto the substrate surface by creating
a high density of nucleation sites, which reduces the induction time. This process can be carried out
mechanically. The substrate is polished with abrasive grit. Typically, diamond powder of 0.1-10 μm
particle sizes is employed.
Bias-enhanced nucleation is another method for the growth of heteroepitaxial diamond films and
is much more controllable than mechanical abrasion [47,56] . In this method the substrate is nega-
tively biased relative to the filament from 0 to 500V. Intense plasma is created because diamond has
a negative electron affinity and emits electron. At the same time the positively charged ion, generated
as a result of electron bombardment, bombard the substrate resulting in the creation of nucleation
sites for subsequent growth. The higher density of diamond nucleation sites initiate diamond growth.
As the bias voltage is increased, the crystal size is decreased.
 
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