We investigated the mechanism for differences in etched depth between isolated and dense via holes of a damascene structure using a SiOCH film (k=2.8). in Ar/CHF3/N-2 and Ar/C4F6/N-2 plasmas, the depth of an isolated (4.8 mu m pitch) hole was more than 40% varied from a dense (200 nm pitch) hole. The difference between dense and isolated holes was found to become larger with increasing air exposure time of the wafer. The amount of H2O in the SiOCH film also increased with increasing air exposure time based on thermal desorption spectroscopy (TDS) analysis. Therefore, we, investigated the effects of H2O addition to the plasmas. The investigation revealed that the etched depths of dense holes became similar to those of isolated holes with H2O addition. Based on these results, we concluded that the mechanism for the difference between isolated and dense holes was as follows. When the capping layer on SiOCH is etched, the H2O contained in SiOCH desorbs in etched holes and affects etching reactions. The etching characteristics of isolated holes are significantly affected, since the amount of desorbed H2O in isolated holes is larger than that in dense holes. (c) 2006 American Vacuum Society.