A laser dry etch process for smooth continuous relief structures in InP

A laser induced etch process is described which uses a pulsed 248 nm KrF excimer laser and Cl₂ atmosphere for the fabrication of monolithic continuously curved reliefs in InP substrate. In a bakeable processing chamber with low base pressure a wide range of laser fluences is available for damage-free etching. Especially, by photothermal heating far above the melting point, mirrorlike smooth surfaces are obtained. The etch rate characteristics are correlated to the maximum surface temperature reached during the laser pulse. The etch rate is independent of pressure and gas flux in the ranges 0.1–10 mbar and 20–300 sccm, respectively. It increases, however, with the background substrate temperature. Etch rates of up to 3.6 nm/pulse or 4.3 lm/min are possible at 20 Hz pulse repetition rate without visible surface damage. The process exhibits a smooth increase of the etch rate from 1 to 3 nm/pulse between 200 and 300 mJ/cm², which could be used for making curved reliefs by optical transmission variations on the projection mask.