Heavily phosphorus‐doped polycrystalline silicon films (n+poly‐Si) were etched in a pure chlorine plasma using an ultraclean electron cyclotron resonance etcher. Compared against undoped polycrystalline etching, horizontal etch rates were too high to allow anisotropic etching ofn+poly‐Si. With the addition of more than about 10% N2, highly anisotropic etches ofn+poly‐Si can be obtained simultaneously with selectivities as high as 160 to SiO2in a 4 mTorr plasma. These results are significant to lower submicron fabrication. X‐ray photoelectron spectroscopy studies show that Si—N bonds are formed on then+poly‐Si surface during etching and it is proposed that this layer protects the sidewall against Cl radicals in a N2/Cl2plasma. The suppression of SiO2etching by O2addition to a N2/Cl2plasma has also been demonstrated.