Role of oxygen in ion-enhanced etching of poly-Si and WSi_x with chlorine

Ion-enhanced etch yields of poly-Si and WSi_1.66 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 WSi_1.66 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 WSi_1.66 by a factor of 9. Therefore, the etch yield selectivity of WSi_1.66 over poly-Si, which is 0.7 with molecular chlorine, increased to 1.5 with atomic chlorine in the presence of energetic Ar⁺ ions. The effect of neutral O/O₂ on both molecular and atomic chlorine/Ar⁺ etching of poly-Si and WSi_1.66 is negligible. However, with 500 eV oxygen ions, the etch yields of poly-Si with both Cl and Cl₂ are greatly reduced in comparison to those with 500 eV Ar⁺. The etch yields of WSi_1.66 are also lower with oxygen ions, but the reduction is smaller than for the poly-Si case. Therefore, the etch yield selectivity of WSi_1.66 over poly-Si increases to 10 with Cl/oxygen ions.