Anisotropic etching of polycrystalline silicon with a hot Cl2 molecular beam

Abstract

A hot Cl2 molecular (Cl*2) beam was successfully applied to achieve highly anisotropic, highly selective, and almost damage-free etching of polycrystalline Si. The anisotropy, the ratio of etch rates in vertical and horizontal directions, was larger than 25. The selectivity, the ratio of polycrystalline Si and SiO2 etch rates, was larger than 1000. The Cl*2 beam was produced by free jet expansion of a Cl2 gas heated in a graphite furnace. The furnace temperature was 830 °C. The substrate temperature was 180 °C. The average total energy (0.38 eV) of a Cl*2 molecule impinging on a substrate surface is much lower than the critical energy (approximately 10 eV) to displace the atoms of the etched material and to cause surface damage. This is the essential reason why this highly selective and almost damage-free etching has been achieved. The highly anisotropic etching mechanism is explained by a model taking into account the directional incidence of Cl*2 molecules to the surface, and the deactivation process of the Cl*2 molecules on a cold surface.