Study on the mechanism of silicon etching in HNO3-rich HF/HNO3 mixtures

The wet chemical etching of silicon using HNO3-rich HF/HNO3 mixtures has been studied. The effect of different parameters on the etch rate of silicon, for example, the HF/HNO3 mixing ratio, the silicon content of the etchant, temperature, and stirring speed in these solutions, has been examined and discussed in light of a previous study on etching in HF-rich HF/HNO3 mixtures. Nitrogen(HI) intermediates are generated owing to the dissolution of silicon and the decomposition if the solution is exposed to air. The nitrite ion concentration, measured in diluted etchant solution by ion chromatography, acts as a sum parameter for the reactive N(III) species in the concentrated etchant. The etch rate shows two different correlations to the nitrite concentration. In the region of high nitrite concentrations, the etch rate decreases slightly with decreasing nitrite concentration, whereas at lower nitrite concentrations, the etch rate increases linearly with further decreasing nitrite concentration. Stirring experiments and the determination of activation energies show that the etching of silicon in HNO3-rich etchants is controlled by diffusion. X-ray photoelectron spectroscopy measurements of the silicon surface after etching revealed a hydrogen termination independent of the concentration of reactive species and the content of HNO3 in the etchant. Si-O containing surface species were not found. A combined electrochemical (injection of holes into the valence band of silicon) and chemical (Si-Si back-bond breaking by an attack of HF) reaction mechanism of silicon etching without generation of SiO2 is proposed.