High rate and highly selective anisotropic etching for tungsten polycide (WSix/poly-Si) has been developed by fully utilizing such advantages of the electron cyclotron resonance plasma etcher, as high plasma density and independent control of ion energy and plasma density. Highly anisotropic etching with a WSix/poly-Si etch rate of 400 nm/min and a poly-Si/SiO2 selectivity of 50 was realized by adding O-2 to Cl-2 and reducing the ion energy. O-2 addition increases the WSix etch rate and reduces the SiO2 etch rate, keeping the poly-Si etch rate nearly constant. This leads to the same etch rate for WSix and poly-Si, and a higher selectivity for poly-Si/SiO2. The decrease in the SiO2 etch rate was found to be mainly caused by a deposition of SiOx on the surface. The role of the O-2 was found to be not only increasing the WSix etch rate and the poly-Si/SiO2 selectivity but forming a sidewall protection him to achieve an anisotropic etching.