Machining of CVD diamond film by rie. Laser ablation and thermo-chemical polishing

Abstract

Diamond, having many advanced physical and mechanical properties, is one of the most important materials used in the mechanical, telecommunication and optoelectronic industry. However, high hardness value and extreme brittleness have made diamond a very difficult material to be machined by conventional mechanical grinding and polishing. In the present study, the microwave CVD method was employed to produce epitaxial diamond films on silicon single crystal. Reactive ion etching (RIE). laser ablation and thermo-chemical polishing experiments were then conducted on the obtained diamond films. The underlying material removal mechanisms, microstracture of the machined surface and related machining conditions were also investigated. It was found that during the laser ablation, peaks of the diamond grains were removed mainly by the photo-thermal effects introduced by excimer laser. The diamond structures of the protruded diamond grains were transformed by the laser photonic energy into graphite, amorphous diamond and amorphous carbon which were removed by the subsequent laser shots. Laser ablation could improve the surface roughness from above ljiim to around 0.1 μm in few minutes' time in this study. However scanning would be required to cover a large area. and. as a consequence, it could be very time consuming. Thermo-chemical polishing, in the other hand, was proved to be able to remove the diamond film very . . . ° effectively (4.8μm deep of diamond film was removed in 30 minutes when polishing at 550 °C and 5.7m s) and the removal rate increased with polishing speed, temperature and pressure. Gases such as 02. 02 / CF4. 02 / SF6 ...were used as the reactive gases in the RIE experiments and it was found that 02 and 02 / SF6 offered better results. However, the obtained etch rates were higher at areas like grain boundary and sharp corners which made further improvement of surface roughness very difficult.