handbook of carbon, graphite, diamond and fullerenes

CVD Diamond 319
(see Ch. 2, Sec. 2.1). The deposition of 12 C diamond is accomplished by
the decomposition in a microwave plasma of methane in which the carbon
atom is 99.97% 12 C. The resulting polycrystalline deposit is crushed,
processed to a single crystal with molten iron and aluminum at high
temperature and pressure, and recovered by leaching out the metalsl 37 ^ 38 ]
(SeeCh. 12, Sec. 3.0).
In diamond, thermal conductivity occurs by aflow of phonons (see Ch.
11, Sec. 5.2). These phonons are scattered by imperfections such as
isotopic imperfections and the scattering varies as the fourth power of the
phonon frequency. Thus, the exclusion of 13 C should result in a significant
increase in thermal conductivity. This increase was confirmed experimentally
as 12 C diamond is reported to have a 50% higher thermal conductivity
than natural diamond.I 37 " 38 ' It is also reported to be harder than natural
diamond by a few percent as determined by the relation between hardness
and the elastic coefficients (see Ch. 11, Sec. 10.2).
3.8 Nucleation and Structure
CVD diamond is a polycrystalline material with grain size ranging from
a few nanometers to a few micrometers. The polycrystallinity results from
the formation of separate nuclei on the deposition surface. As deposition
proceeds, these nuclei eventually coalesce to form acontinuousfilm. Some
grow in size while other are crowded out and the average crystal size often
becomes larger as thickness increases. Some degree of crystal orientation
(texture) can be achieved. A cross-section of a coating is shown in Fig. 13.7.
As mentioned in Ch. 11, the most prevalent crystal surfaces in CVD
diamond are the (111) octahedral and the (100) cubic. Cubo-octahedral
crystals combining both of these surfaces are also common. Twinning
occurs frequently at the (111) surface. The 6H structure may also be
present.
3.9 Substrate Preparation and Adhesion
The nature of the substrate and its predeposition treatment play a
major role in determining the surface nucleation rate but not necessarily the
rate of subsequent growth (after the immediate surface layer is deposited)
.t 13 !! 39 ! The most widely used substrate is still silicon, but other materials
perform successfully such as the refractory metals (W, Ta, Mo), carbides