A process for depositing smooth polycrystalline layers, especially diamond layers, comprises two steps. In a first step the crystal direction of an octahedron (8, 38) is grown with a growth parameter alpha of about 3, preferably over a layer thickness of between 400 and 600 times the mean seed distance on the substrate. In a second step, by reducing the growth parameter alpha , surfaces are generated perpendicularly to this crystal direction and enlarged until they completely cover the surface of the polycrystalline layer. Here, the growth parameter alpha is given by alpha = V100/V111 2ROOT 3, the growth rates of the crystal on {100} and {111} surfaces being given by V100 and V111. This two-step process makes it possible to produce smooth polycrystalline diamond layers in millimetre thicknesses with a roughness in the 100 nanometre range.

Polycrystalline diamond (PCD) films have been grown on silicon substrates by thermally activated (hot filament) chemical vapour deposition (CVD) and microwave plasma-assisted CVD. In addition, free-standing PCD wafers have been prepared by removing the substrate. A requirement for optical applications of these films is a relatively smooth surface, i.e. a small surface roughness as compared to the optical wavelength. A reduction of surface roughness of thin PCD films has been achieved by increasing the nucleation density to above 10(10) cm-2 using an appropriate substrate pretreatment. This high nucleation density allows the deposition of thin films with thicknesses between 0.1 and 0.6-mu-m showing intense interference colours. By moving silicon wafers 3 in (7.6 cm) in diameter under the hot filament during the deposition process, large area diamond coatings with very uniform film thicknesses were obtained.The roughness of thick PCD films (thickness greater than 1-mu-m) could be reduced by enforcing secondary nucleation. However, the resulting nanocrystalline films have considerable amounts of non-diamond phases. Using microwave plasma-assisted CVD, relatively smooth PCD films with a surface consisting of (100) faces oriented parallel to the substrate were prepared. These films exhibit a pronounced 100 texture.

The applicability of CVD diamond films in optics is investigated. Emphasis is put on the role of the surface roughness as the most critical parameter influencing the optical quality of polycrystalline diamond films. In particular we report on the preparation of optically smooth, thin diamond films, on the in situ analysis of the optical quality, on the coating of large area substrates, and on the growth of thick, free-standing diamond wafers with flat surfaces.