Cryogenic deep etching of silicon is investigated using SO 2 for passivating the sidewalls of the etched features. The passivating efficiency of SO 2 in a SF 6 /SO 2 inductively coupled plasma is assessed comparatively with the traditional SF 6 /O 2 chemistry by means of mass spectrometry and optical emission spectroscopy diagnostics. Emphasis is placed on the evolution of the density of various neutral species (e.g. SiF 4 , F, O, SO x F y , SF x ). These measurements allow us to determine the SO 2 /SF 6 and O 2 /SF 6 gas flow ratios above which a passivation layer forms and inhibits silicon etching. Furthermore, different reaction schemes are proposed to explain the variations in relative densities measured for the two plasma chemistries. In SF 6 /SO 2 plasmas, surface reactions involving SOF and SO 2 species with F radicals are favoured, providing a greater number of SOF 2 and SO 2 F 2 molecules in the gas phase. In SF 6 /O 2 plasmas, a higher rate of O radicals available for reacting with SF x species can account for the greater concentration in SOF 4 molecules. However, these trends are significant for high passivating gas concentrations only. This is consistent with the similar etch results obtained for both chemistries when etching silicon at cryogenic temperatures with a low percentage of passivating gas.
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Duluard et al. (2008) studied this question.