handbook of carbon, graphite, diamond and fullerenes

CVD Diamond 329
It should be noted that beryllia is being phased out because of the
acute health problems associated with its use.
Heat sinks in the form of thin slices prepared from single-crystal
natural diamond are already used commercially but are limited in size to
approximately 3x3x1 mm. These single-crystal diamonds are gradually
being replaced by CVD diamond which is now available in shapes up to 15
cm in diameter.t 30 !! 31 ! CVD-diamond heat sinks may be at the threshold of
large-scale commercialization and could be the most promising application
of the material. 139 !
5.6 Optical Applications
A CVO diamond coating is yet to be produced with the optical clarity
of single-crystal diamond and, for that reason, CVD diamond has found
little application so far in optics. Typically CVD diamond coatings are
translucent. Some are nearly white, some have a black tingeJ 54 '
The rough surface of CVD diamond is also a drawback since an
optically smooth coating is an essential requirement for most optical
applications. The surface can be smoothed to some degree by increasing
the nucleation density and by deposition strongly textured coatings
where the surface consists of flat {100} faces more or less parallel to the
substrate.1 55 ]
Diamond for IR Windows. A new optical-brazing process bonds a
thin layer of CVD diamond on a zinc sutfide or zinc selenide IR window.
This thin layer is obtained by depositing diamond on a silicon substrate and
dissolving the silicon. The diamond layer is then brazed to the IR window
with the smooth interface surface on the outside. The brazing material is
an index-matching chalcogenide glassJ 56 ' This process is a possible
solution to the optical-roughness problem mentioned above. The process
is shown schematically in Fig. 13.9. In such an application, diamond would
compete directly with diamond-like carbon (DLC) (see Ch. 14, Sec. 5.2).
Advances in deposition techniques have now made possible the
fabrication of free-standing shapes such as the 7.6 cm (3") diameter
hemispherical dome for IR-window applications shown in Fig. 13.10 which
may eventually replace the present IR materials.