Polysilicon etch rates and polysilicon‐silicon dioxide selectivities have been measured in a collimated electron cyclotron resonance (ECR) chlorine discharge as a function of microwave power, pressure, gas flow rate, O2addition, and substrate position. High chlorine flow rates, moderate input microwave power, and reduced wafer separation from the ECR plane were found to provide simultaneous high polysilicon etch rates and high polysilicon‐oxide selectivities. The addition of a small amount of oxygen to the chlorine plasma establishes an etch‐deposition competition which can increase the polysilicon‐oxide etch selectivity. Polysilicon etch rates of 4000–5000 Å/min are obtained in combination with stable polysilicon‐oxide etch selectivity values of 50–150 and polysilicon‐resist selectivities of 13–15, using 150 sccm Cl2, 0.6% O2, 3 mTorr total pressure, and 700 W input microwave power. The etched profiles are anisotropic with residue‐free surfaces.