Reduction of sidewall roughness during dry etching of SiO2

Abstract

The appearance of striations on dry etched semiconductor laser mesas is a common feature of these structures. We describe a number of different methods of reducing the extent of this roughness, including the choice of dielectric etch chemistry, modification of the initial resist processing, and deposition of a SiN sidewall to prevent additional roughening during the plasma etch step. SF6 is found to be preferable to CF4 for dielectric etching because of an absence of polymer formation. This produces smoother SiO2 sidewalls. Flood exposure of theinitial photoresist mask and optimization of the postbake temperature also produces smoother sidewalls on the subsequently etched SiO2. The sidewall can also be protected from roughening that occurs during the dry etch step by coating it with a low temperature SiN layer. A combination of all of these methods produces sidewalls with morphological variations of ≤500 Å.