Dependence of electron energy distributions on discharge pressure in ultrahigh-frequency and inductive-coupled Cl₂ plasmas

In the ultrahigh frequency (UHF) plasma, a high ion density and a high energy tail in the electron energy distributions can be maintained in a wide pressure range from 3 to 20 mTorr, whereas in the inductively coupled plasma (ICP) these characteristics are drastically reduced when the gas pressure is increased. This indicates that the ionization in the UHF plasma does not depend significantly on the gas pressure from 3 to 20 mTorr because the discharge frequency is higher than the electron collision frequency in the UHF plasma. As a result, the UHF plasma possesses a wider process window for highly-selective polycrystalline silicon etching than the ICP.