Abstract: Thin film semiconductor devices such as amorphous silicon alloy p-i-n diodes and the like which utilize mesa-like semiconductor structures having vertical sidewalls are formed by a process which eliminates overhangs and neutralizes contaminants on the sidewalls that can result in short circuits or degradation of device performance. Smooth vertical sidewalls free of overhangs and voids are created by: successively depositing the desired semiconductor layers on a substrate, then depositing and patterning a top metal contact mask on the semiconductor layers, followed by removing the unwanted portions of the semiconductor layers by reactive ion etching. The disclosed reactive ion etching provides controlled vertical etching with virtually no lateral etching, thereby providing smooth sidewalls.