We have examined the effect of CO addition to C4F8 or C4F8 /Ar plasmas for selective etching of organosilicate glass (OSG) over SiC etch stop layers. The variation of important gas phase species, thin film etching rates and surface chemistry with feedgas composition was determined. CO addition exhibits dramatically different consequences on OSG/SiC etching selectivity when added to C4F8 or C4F8 /Ar plasmas containing a high proportion of Ar. An improvement of the OSG/SiC etching selectivity results from CO addition to C4F8. We observe little CO dissociation in this case, which is plausible considering the lower dissociation energy, threshold of C4F8 relative to CO. X-ray photoelectron spectroscopy (XPS) analysis of OSG and SiC surfaces shows that the etching selectivity improvement for C4F8 /CO may be explained not only by an increase of the thickness and a reduction of the F/C ratio of the steady-state fluorocarbon surface layer on the SiC surface during etching, but little incorporation of CO into deposited fluorocarbon films. Adding CO to C4F8 /Ar discharges with a high proportion of Ar leads to a reduction of the OSG/SiC etching selectivity. Significant dissociation of CO in Ar-rich C4F8 /Ar/CO discharges is observed, consistent with the fact that the dissociation energy threshold of CO is lower than the Ar ionization and metastable energies. Oxygen incorporation in deposited fluorocarbon films and a reduction of the steady-state fluorocarbon, surface layer thickness on SiC are observed by XPS in this case, explaining the loss of OSG/SiC etching selectivity for C4F8 /Ar/CO discharges. (C) 2004 American Vacuum Society.