Radio frequency plasma etching of Si/SiO2 by Cl2/O2: Improvements resulting from the time modulation of the processing gases

Abstract

The etching of poly-Si and its selectivity over gate oxide SiO2 were measured for both Cl2 and Cl2/O2 plasmas and the experimental data indicate that modulation of the processing gases increases the Si/SiO2 selectivity while decreasing the unwanted deposition. The best results to date have been Si etch rates of ∼1000 Å/min and Si/SiO2 selectivities of ∼50. It has also been demonstrated that the Si/SiO2 selectivity is strongly dependent on the presence of photoresist. A qualitative model for these results is proposed, which appears to fit these observations. In this model, hydrocarbon fragments from sputtered photoresist reduces the SiO2 to Si, with a corresponding decrease in the Si/SiO2 etching selectivity. This carbonaceous material can be removed by the oxygen before it causes etching damage to the SiO2. Oxygen, unfortunately, can also react with the etching reaction products (e.g., SiCl4) to form SiO2 and the Si oxychlorides (Si2OCl6, Si3OCl8), which deposit on the wafer. The experimental evidence indicates, however, that this unwanted deposition can be minimized by alternating the Cl2 and O2 gases, rather than by running them both at a constant pressure, as is typically done. Of most immediate interest is the fact that this improvement by modulation can be achieved on time scales (≳1 s), which are attainable with the fairly long residence times in present generation hex etchers.