SO(2) passivating chemistry for silicon cryogenic deep etching

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.