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

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.