Reactor Characterization for an Si02 Plasma Etchback Process with Response-Surface Methodology: Effects of Interelectrode Spacing

The characteristics of CF4/O ~ plasmas which are used to etch composite layers of sacrificial photoresist and underlying SiO2 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 rates for CF4 (150-200 sccm) and 02 (0-50 sccm), reactor pressure (900-2000 mtorr), and interelectrode spacing (4-6 ram) on the etch rates of SiO2 and photoresist were explored at fixed radio frequency power (500W at 13.56 MHz) at 22 ~-+ 2 ~ Variation in spacing shows dif-ferent effects for these two materials; whereas the SiO2 etch rates generally increase with increasing spacing as the other parameters are varied, the photoresist etch rates exhibit more complex behavior. At the lower spacing of 4 mm, neither film has a measurable etch rate for low settings of the other variables. The SiO2 etch rates are virtually constant with in-creasing O2 flow rate for low, middle, and high settings of the other variables. At low settings of the other variables, the photoresist etch rates are zero over the range of 02 flow rates but increase with increasing 02 flow rates for middle and high settings of the other variables. At a spacing of 4 ram, the etch rates of both films are negligible over the range of CF4 flow rates and the range of reactor pressure. At larger spacings the etch rates of photoresist increase with increasing CF4 flow rate, while the etch rates of SiO2 are similar for middle and high settings of the other variables over the range of CF4 flow rates. Process optimization using the empirical models to attain suitable etch rates for both films for planarization of