Study of etch rate characteristics of SF/sub 6//He plasmas by response-surface methodology: effects of interelectrode spacing

The characteristics of SF/sub 6//He plasmas which are used to etch Si/sub 3/N/sub 4/ have been examined with experimental design and modeled empirically by response-surface methodology using a Lam Research Autoetch 480 single-wafer system. The effects of variations of process gas flow rate (20-380 sccm), reactor pressure (300-900 mtorr). RF power (50-450 W at 13.56 MHz), and interelectrode spacing (8-25 mm) on the etch rates of LPCVD (low-pressure chemical vapor deposition) Si/sub 3/N/sub 4/, thermal SiO/sub 2/, and photoresist were examined at 22+or-2 degrees C. Whereas the etch rate of photoresist increases with interelectrode spacing between 8 and 19 mm and then declines between 19 and 25 mm, the etch rate of Si/sub 3/N/sub 4/ increases smoothly from 8 to 25 mm, while the etch rate of thermal SiO/sub 2/ shows no dependence on spacing between 8 and 25 mm. The etch rates of all three films decrease with increasing reactor pressure. Contour plots of the response surfaces for etch rate and etch uniformity of Si/sub 3/N/sub 4/ as a function of spacing and flow rate at constant RF power (250 W) display complex behavior at fixed reactor pressures. A satisfactory balance of etch rate and etch uniformity for Si/sub 3/N/sub 4/ is predicted at low reactor pressure ( approximately 300 mtorr), large electrode spacing (12-25 mm), and moderate process gas flow rates (20-250 sccm).<
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