Effects of discharge frequency on plasma characteristics and etching characteristics in high density Cl2 plasma: comparison of ultrahigh-frequency plasma and radio-frequency plasma

We investigated the effects of discharge frequency on the characteristics of polycrystalline-silicon etching rates and on the etching selectivity on the gate oxide (SiO₂). An ultrahigh-frequency (UHF) plasma excited at 500 MHz was found to possess a wider process window for highly selective polycrystalline silicon etching than did an inductively coupled plasma excited at 13.56 MHz. The ionization rate in the UHF plasma is nearly constant at discharge pressures from 3-20 mtorr because the discharge frequency is higher than the electron-collision frequency in that plasma