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Highly selective etching of Si3N4 to SiOC film has been studied, using dual frequency superimposed (DFS) rf capacitively coupled plasma, which controlled the self-bias voltage (-V dc) independently of electron density (N e). The SiOC etch rate was effectively suppressed by fluorocarbon polymer deposition, which was controlled by the additive H2 gas flow rate ratio and carbon-to-silicon ratio of the SiOC film, without reduction in Si3N4 etching rate. Infinitely highly selective etching could be realized with a H2 gas flow rate ratio of above 50%. Stringent V dc control was required for line-and-space pattern etching in order to suppress the erosion of the SiOC pattern edge, even when a highly selective etching condition was used. The V dc range that was available for selective etching of Si3N4 to SiOC was as small as 150 V, which was approximately 50 V lower than that for selective etching of SiOC to Si3N. However, a much lower -V dc, such as 100 V, caused excess deposition and taper etching. It was also found that lower N e, which was controlled by a 100 MHz rf power, was required to suppress the excess deposition.

A dual-frequency superimposed (DFS) 100 MHz and 3.2 MHz rf capacitive-coupled plasma etch process for sub-90 nm devices has been developed. The electron density of DFS reactive ion etching (RIE) plasma at 40 mTorr was controlled from 4.0×1010 to 3.6×1011 cm-3 by adjusting the 100 MHz rf power, and the self-bias voltage (-V dc) was controlled from 20 to 760 V by adjusting the superimposed 3.2 MHz rf power. DFS RIE demonstrated independent control of electron density and self-bias voltage in a wide range. In the damascene etch process of SiOC film using Si3N4 as an etch mask, it was found that mask edge erosion is dependent on ion energy regardless of the selectivity of SiOC to Si3N4. DFS RIE offers the most suitable process for damascene etching of SiOC, which requires precise ion energy control.