Abstract

We have studied pulsed laser polishing of faceted CVD-diamond films as a function of laser fluence and angle of incidence. A fluence above the ablation threshold leads to polishing by graphitization and subsequent sublimation. The more inclined the laser beam the smoother the surface. Reduction of roughness R_a from 1 μm to below 0.1 μm is achieved. At an angle of incidence α larger than a critical angle α_crit, however, a surface instability occurs. This is revealed by the formation of a periodic grating structure which perpetuates itself across the surface irradiated with overlapping pulses. The spacing of the ripples is typically 10 μm. They are oriented parallel to the plane of incidence, irrespective of the laser wavelength or the polarization. Neither diffraction nor interference effects can therefore be the cause for the rippling. Rather, a shadowing effect is shown to initiate the corrugation. The occurrence of permanent ripples poses a limit to the laser polishing of rugged diamond films. However, rippling is avoided and significantly smoother films result (with an average roughness of 33 nm compared to the 0.1 μm mentioned above) by rotation of the diamond film during laser processing.