Silicon dioxide reactive ion etching dependence on sheath voltage

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Abstract
Previous experiments using different chlorinated Freons and CF4 have shown the etch rate of SiO2 to have a stronger dependence on the sheath voltage than on the nature or concentration of the chemical species, as if dominated by physical sputtering. To better understand the etching mechanism for SiO2, the etch rate, sheath voltage, and wafer temperature were measured as a function of the percentage of CF4 in Ar and as a function of the total pressure. The experiments were done in a reactive ion etching (RIE) system. It was observed that at high pressures or at high percentages of CF4 in Ar, the etch rate depended only on the sheath voltage. This is probably due to saturation of etching species on the surface. As the pressure and CF4 percentage are lowered, the system slowly becomes unsaturated. At low pressures, where the sheath voltage is nearly independent of the CF4 percentage, the etch rate steadily decreases with Ar addition, thus showing a dependence of the etching on the concentration of etching species. At low percentages of CF4, the etch rate depends on the pressure as well as on the sheath voltage. These results explain why physical sputteringlike behavior of the etching of SiO2 had been observed in other experiments in which saturation conditions predominated. A model for SiO2 etching that explains most of the results is proposed.

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