Tungsten reactive ion etching formulations obtained from CF2Cl2in combination with CF4/O2, and from CF3Cl with O2, are known to exhibit distinct etch rate and etch selectivity advantages relative to other chemistries used for tungsten patterning. Within this work, it is found that CF3Cl in place of CF2Cl2causes a similar result and that it is the balanced presence of atomic F, Cl, and O in the discharge that governs the etch rate of tungsten. Limiting case approximations suggest that the mechanism of tungsten etch enhancement does not depend upon the potential for tungsten removal as a volatile chloride. The participation of tungsten oxide is also studied. Finally, the novel chemistries are extended to silicon etching, resulting in a similarly beneficial etch rate and selectivity advantage.