Surface roughness of silicon oxynitride film was examined as a function of process parameters. Etching was conducted in a C2F6 inductively coupled plasma and surface roughness was measured by atomic force microscopy. The parameters involved include radio frequency source power, bias power, pressure, and C2F6 flow rate. For optimization, parameter effects on the deviations of etch rate, surface roughness and DC bias were related. The source power-induced DC bias was strongly correlated to the surface roughness. This was similarly observed for the bias power variations. Complex effects of pressure or C2F6 flow rate were reasonably explained by combining DC bias and reported [F] and [CFx] variations. It is noticeable that the dominance of [F] or [CFx] was similar for both variations in pressure and C2F6 flow rate. In other words, surface roughness seemed to be initially dominated by fl-driven chemical etching and then by [CFx]-driven polymer deposition. A useful clue identified for optimization is that for all parameter variations the smallest deviation of surface roughness occurred at the largest DC bias deviation. (C) 2007 Elsevier Ltd. All rights reserved.