Highly selective etching for polysilicon and etch-induced damage to gate oxide with halogen-bearing electron-cyclotron-resonance plasma

Abstract

The investigations of polysilicon etching with three halogen-bearing plasmas (SF6, Cl-2, and HBr) in an electron-cyclotron-resonance reactor have been made. We examine the etching characteristics which include etching rate, anisotropy, and selectivity based on the discussions of the chemical and electrical properties of the F, Cl, and Br radicals, It was found that the degree of anisotropy strongly depends on the deposition of a silicon halide film on sidewall and on the spontaneous etching properties of three halogen plasma mixtures. The selectivity to oxide is related to the amount of silicon halides that are produced during etching poly-Si and the electrically polarized level of these products when they adsorb on the oxide surface. We find that the selectivity on oxide is approximately infinite for the HBr system and over 90 for the Cl-2 system. In addition, the effects of oxygen addition, microwave power, and rf power are also investigated. It was found that the maximum etching rate and selectivity of SF6, Cl-2, and HBr plasmas occurred at concentrations of added oxygen at about 7.5%, 5%, and 3% total flow rates, respectively. The suitable range for the microwave power applied is from 250 to 300 CV and the most respectable value of applied rf power is about 35 W. Concurrently, the local surface charging causing damage to the gate oxide during the overetching period is studied. It is observed that low device yield, large flatband voltage V-fb shift, and high interface-state density D-it are correlated with longer overetch and high antenna ratio structure of the device. A more obvious degradation is observed in the SF6 gaseous system than in Cl-2 and HBr ones. Finally, the influence of magnetic coil current on device damage is also addressed. (C) 1996 American Institute of Physics.