The role of ion bombardment in plasma etch selectivity of organosilicate glass to etch stop layers of silicon carbide and silicon nitride has been investigated in a C4F8/N-2/Ar plasma using a method that produces a narrow ion energy distribution (IED) at the substrate surface. The effects of the narrow IED are compared with those of the broad, bimodal IED produced by the conventional sinusoidal bias voltage wave form (at 13.56 MHz). A comparison of etch rate versus average ion bombardment energy shows a higher ion energy threshold for etching, a larger gap between the thresholds for the two materials, and high selectivity over a wider range of bias voltage with the narrow IED. A physical explanation of the observed phenomena is proposed. (C) 2004 American Vacuum Society.