Contaminant is removed from holes by etching in a gaseous plasma by first removing contaminant from the vicinity of the edges of the hole. Next, a mask is provided in the vicinity of the edges to prevent etching by contacting with a gaseous plasma which is different from the gaseous plasma employed in the first etching step. The holes are then etched in a gaseous plasma to remove contaminant from the interior of the holes in the vicinity of the center of the holes, whereby the mask protects the edges from being etched.