Transient poly-Si etching behavior in CF4 + D-2 electron cyclotron resonance plasmas containing different D-2 proportions was investigated. Higher D2 proportions resulted in lower atomic F and higher CF2 concentration in the plasma, as evidenced by optical emission spectroscopy (OES), and in. greater oxide-to-Si etch selectivity, A high initial poly-Si etch rate that declined very rapidly to a finite-steady-state value was observed for plasma etching under conditions giving low (3:1) oxide-to-Si etch selectivity. In contrast, a lower initial etch rate that declined to approximately zero over a longer (similar to 45 s) period was observed for poly-Si etching under plasma conditions giving (similar to 15:1) selectivity. In the latter case, Si consumption during overetching would be significantly underestimated if calculated on the basis of the conventional 60 s selectivity ration. X-ray photoelectron spectroscopy analysis indicated that a thick, more F-deficient fluorocarbon film was deposited on Si under the high-selectivity etching conditions. Real-time SiF4 and atomic F signals, which were measured during SiO4 etching using OES and mass spectroscopy, respectively, evidenced significantly different end-point trends for the high- and low-selectivity etching conditions. These trends are interpreted in light of the transient etching behavior observed for poly-Si under equivalent plasma conditions.