Corinne Duluard

Neutral species in inductively coupled SF6/SiCl4 plasmas

Inductively coupled SF6/SiCl4 plasmas interacting with a bulk silicon substrate and a SiO2-coated substrate have been investigated. Mass spectrometry and optical emission spectroscopy diagnostics were used to characterize the neutral population in the diffusion chamber. SiF4 molecules were detected as the dominant species, and their formation has been attributed to the high reactivity of F radicals with SiClx species. In a complementary experiment, a silicon chloride layer was deposited on the reactor walls during a SiCl4 plasma step and subsequently etched by a SF6 plasma. Time-resolved measurements of the neutral densities during the SF6 plasma step showed the importance of heterogeneous reactions between impinging F radicals and SiClx species deposited on the reactor walls. In SF6/SiCl4 plasmas, these reactions lead to a depletion in F radicals, which results in a decrease in the silicon substrate etch rate. Furthermore, this impacts on the concentration of SFx species and on the creation of new species, such as ClF, SF5Cl and S2Cl2.

SO2 passivating chemistry for silicon cryogenic deep etching

Cryogenic deep etching of silicon is investigated using SO2 for passivating the sidewalls of the etched features. The passivating efficiency of SO2 in a SF6/SO2 inductively coupled plasma is assessed comparatively with the traditional SF6/O2 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. SiF4, F, O, SOxFy, SFx). These measurements allow us to determine the SO2/SF6 and O2/SF6 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 SF6/SO2 plasmas, surface reactions involving SOF and SO2 species with F radicals are favoured, providing a greater number of SOF2 and SO2F2 molecules in the gas phase. In SF6/O2 plasmas, a higher rate of O radicals available for reacting with SFx species can account for the greater concentration in SOF4 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.

Alternating SiCl4/O2 passivation steps with SF6 etch steps for silicon deep etching

Deep etching of silicon has been investigated in an inductively coupled plasma etch reactor using short SiCl4/O2 plasma steps to passivate the sidewalls of the etched structures. A study was first carried out to define the appropriate parameters to create, at a substrate temperature of −20 °C, a passivation layer by SiCl4/O2 plasma that resists lateral chemical etching in SF6 plasma. The most efficient passivation layer was obtained for a SiCl4/O2 gas flow ratio of 2:1, a pressure of 1 Pa and a source power of 1000 W. Ex situ analyses on a film deposited with these parameters show that it is very rich in oxygen. Silicon etching processes that alternate SF6 plasma etch steps with SiCl4/O2 plasma passivation steps were then developed. Preliminary tests in pulsed-mode conditions have enabled etch rates greater than 2 µm min−1 with selectivities higher than 220. These results show that it is possible to develop a silicon deep etching process at substrate temperatures around −20 °C that uses low SiCl4 and O2 gas flows instead of conventional fluorocarbon gases for sidewall protection.