Comparison of etch rates of silicon nitride, silicon dioxide, and polycrystalline silicon upon O2 dilution of CF4 plasmas

Abstract

The etch rates of Si3N4, SiO2, and doped polycrystalline silicon with CF4 plasmas have been examined as a function of O2 dilution and radio frequency (rf) (13.56 MHz) power density at constant residence time (25 mTorr and 25 sccm total process gas flow rate) at 22±2 °C in a batch reactor. At fixed reactor pressure and process gas composition, the etch rates of these materials increase linearly, with different slopes, with increasing rf power density. Under the conditions studied, the etch rates of SiO2 are lower than those of the other two materials and are primarily a function of rf power for CF4 plasmas that are diluted with 0%–25% O2 by flow rate. In comparison, the etch rates of Si3N4 and doped polycrystalline silicon are functions of both rf power density and gas composition, although at each rf power level (200, 300, and 400 W) the Si3N4 etch rate reaches a plateau at an O2 dilution of ∼10%–15% by flow rate. Although the doped polycrystalline silicon etch rate exhibits similar behavior at 200 and 300 W, a steady increase in etch rate is noted at 400 W over the range of O2 dilution examined in this work. At low O2 dilutions, the Si3N4 etch rate is greater than the doped polycrystalline silicon etch rate. However, with increasing O2 dilution, the doped polycrystalline silicon etch rate exceeds the Si3N4 etch rate, with the intersections of these etch rate curves shifting to greater O2 dilution at higher rf power settings.