Ion-enhanced etch yields of poly-Si and WSi1.66 With beams of molecular and atomic chlorine and oxygen are reported as a function of neutral-to-ion-flux ratios. The ions used are 500 and 1000 eV Ar+ and 500 eV O-2(+)/O+. Measurements were made in a vacuum chamber with chlorine and oxygen molecules/atoms effusing from an external discharge source as a beam and impacting the selected surface. The etch rates were measured using quartz-crystal microbalances onto which poly-Si and WSi1.66 were deposited. In the ion-flux limited regime, atomic chlorine in comparison to molecular chlorine enhances the etch yield of poly-Si by a factor of 4 and of WSi1.66 by a factor of 9. Therefore, the etch yield selectivity of WSi1.66 over poly-Si, which is 0.7 with molecular chlorine, increased to 1.5 with atomic chlorine in the presence of energetic Arf ions. The effect of neutral O/O-2 on both molecular and atomic chlorine/Ar+ etching of poly-Si and WSi1.66 is negligible. However, with 500 eV oxygen ions, the etch yields of poly-Si with both Cl and Cl-2 are greatly reduced in comparison to those with 500 eV Arf. The etch yields of WSi1.66 are also lower with oxygen ions, but the reduction is smaller than for the poly-Si case. Therefore, the etch yield selectivity of WSi1.66 over poly-Si increases to 10 with Cl/oxygen ions.