Fig. 6. Classification diagram for amorphous carbon films (e.g., [70]).

The majority of a-C films contain mainly sp2 carbon, but the sp3 carbon content can be varied over the range 5-55%; the hardness of the films increase with sp3 content. The H content of a-C:H films can be varied over a wide range and the hardness of a-C:H films is inversely related to the hydrogen content. For both types of films a high sp3 content is produced by ion beam deposition. Films with a very high sp3 content (-80-90%) and a correspondingly high hardness have been called tetrahedrally-bonded amorphous carbon films, ta-C films [70]. Hard a-C:H films were called 'diamond-like carbon', DLC, and this term has been used as a generic name for all amorphous carbon films. Thus, as a general rule, hardness increases with sp3 carbon content, as the proportion of 'diamondlike' carbon increases. Conversely, the films become softer as the sp2 carbon content and/or the hydrogen content increases, reflecting the increasing content of 'graphite-like' carbon or 'polymer-like' carbon respectively. Clearly, there is considerable scope for varying properties of the carbon films by careful control of processing parameters.

There is evidence for segregation of sp2 and sp3 bonded carbon in a-C and a-C:H films. The structure of a-C films with a high sp2 carbon content is envisaged as clusters of warped graphitic domains bounded by sp3 carbon [71]. In a-C:H films the extent of segregation of sp2 and sp3 carbon decreases with increasing carbon content. The sp2 carbon content of both a-C and a-C:H films increases on heat-treatment in the range 300-600 °C, i.e., there is thermal transformation to graphitic structures; ta-C films are thermally stable to -1000 °C.

4.2 CVD Diamond Films

The standard free energy changes for the process Cgas -> graphite and Cgas -> diamond are -671.26 and -668.36 kJ-mol"1 at 298 K respectively. This comparison shows that the thermodynamic driving forces for forming graphite and diamond from the vapour phase are similar. Also, the work on amorphous carbon films shows that the proportions of sp2 and sp3 bonded carbon in films formed from the vapour phase can be varied by careful control of processing conditions, Fig. 6. Taken together these considerations suggest that it may be possible to produce films consisting entirely of sp3 carbon, i.e., diamond films, by low pressure CVD processes. This objective was first realised in the early 1980s by Russian and Japanese workers [75-78] and since that time there have been considerable international efforts made to develop and improve the quality of CVD diamond films.

A large number of CVD diamond deposition technologies have emerged; these can be broadly classified as: thermal methods (e.g., hot filament methods) and plasma methods (direct current, radio frequency, and microwave) [79]. Film deposition rates range from less than 0.1 (im-h"1 to ~1 mm-h"1 depending upon the method used. The following are essential features of all methods.

a) A carbon source gas that is rich in hydrogen but dilute (typically ~1.0 vol%) in the carbon-containing gas. The gas mixture may also contain molecular oxygen or oxygen-containing molecules, e.g., CO. Some experiments have also included halogen-containing gases.

b) A means of activating the gas to produce free radicals, excited molecular species, or a plasma (see above).

c) A temperature-controlled substrate (typically at 500-900 °C).

CVD diamond films can be deposited on a wide range of substrates (metals, semi-conductors, insulators; single crystals and polycrystalline solids, glassy and amorphous solids). Substrates can be abraded to facilitate nucleation of the diamond film.

The majority of CVD diamond films are polyerystalline, although single crystals can be grown. The chief impurities are non-diamond carbon at grain boundaries in polyerystalline films and hydrogen (from 300 to 2000 ppm). Other impurities can be avoided by using clean conditions and CVD diamond films with purities similar to natural type Ila diamonds can be grown. CVD diamond films contain a high concentration of dislocations and stresses associated with crystal defects and impurities that can adversely affect the adhesion of the film to the substrate. Emerging applications for CVD diamond films include heat sinks for electronic devices, optical windows and coatings, and wire drawing dies .

The mechanism of growth of diamond films is not understood in detail. A useful perspective is provided by the triangular CHO diagram of Bachmann et al [80], Fig. 7. This diagram shows that gas compositions for diamond growth for a very wide range of thermal and plasma CVD experiments are defined by a narrow triangular field denoted as the 'diamond domain'. Gas compositions richer in carbon than those in the diamond domain resulted in non-diamond carbon growth, e.g., pyrocarbon. No carbon films form if gas compositions are richer in hydrogen and oxygen than those in the diamond domain. Bachmann et al [80]were also able to link gas compositions in the diamond domain to the quality of the diamond films obtained. Subsequently, Prijaya et al [81] used thermochemical considerations to rationalise the Bachmann diagram. They showed that the diamond domain includes gas compositions that are close to the carbon solubility limit for the excitation temperatures used to activate the gas. Supersaturation occurs on cooling towards the substrate temperature, so creating conditions for carbon deposition.

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