Effects of discharge frequency on the ion-current density and etching characteristics in high-density Cl2 plasmas

We investigated the effects of discharge frequency on the characteristics of polycrystalline silicon (poly-Si) etching rates and on the etching selectivity on the gate oxide (SiO₂). An ultrahigh-frequency (UHF) plasma excited at 500MHz was found to possess a wider process window for highly selective polycrystalline silicon etching than a radio frequency (RF) plasma excited at 13.56MHz. This result has a strong correlation to the pressure dependence of the ion current density in both plasmas. In the UHF plasma, high ion-current density can be maintained in a wider pressure range, from 3 to 20 mTorr, whereas in the RF plasma the current density is drastically decreased with an increase in pressure. Thus, because the discharge frequency in the UHF plasma is higher than the electron collision frequency, the ionization in the UHF plasma does not depend significantly on the discharge pressure from 3 to 20 mTorr.