Anisotropic etching of n+ polycrystalline silicon with high selectivity using a chlorine and nitrogen plasma in an ultraclean electron cyclotron resonance etcher

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 of n+ poly-Si. With the addition of more than about 10% N2, highly anisotropic etches of n+ poly-Si can be obtained simultaneously with selectivities as high as 160 to SiO2 in 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 the n+ poly-Si surface during etching and it is proposed that this layer protects the sidewall against Cl radicals in a N2/Cl2 plasma. The suppression of SiO2 etching by O2 addition to a N2/Cl2 plasma has also been demonstrated.