Polycrystalline diamond for optical thin films

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, freestanding 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(exp 10) cm(exp -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) cou ld be reduced by enforcing secondary nucleation. However, the resulting nanocrystalline films have considerable amounts of nondiamond 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.