Effects of discharge frequency in plasma etching and ultrahigh-frequency plasma source for high-performance etching for ultralarge-scale integrated circuits

A low-temperature, uniform, high-density plasma is produced by applying ultrahigh-frequency (UHF) power through a spokewise antenna. The plasma is uniform within ±5% over a diameter of 30cm. No magnetic field is needed to maintain the high-density plasma. Consequently, the plasma source is fairly simple and lightweight. This plasma creates a high electron density and a low degree of dissociation of the feed gas at the same time because the electron energy distribution function is not Maxwellian (bi-Maxwellian distributions). The plasma characteristics are highly suitable for the precise etching of Al and gate electrodes. Additionally, by the combination of bi-Maxwellian electron energy distribution in the UHF plasma and new fluorocarbon gas chemistries (C₂F₄, CF₃I), selective radical generations of CF₂ and CF₃ could be realized for high-aspect contact hole patterning of SiO₂. A high ion density and a high-energy tail in the electron energy distribution can also be maintained over a wide range of pressure (from 3 to 20mTorr), whereas in conventional inductively coupled plasma (ICP: 13.56 MHz), the ion density and number of high-energy electrons are drastically reduced when the gas pressure is increased. This indicates that the ionization in the UHF plasma does not depend significantly on gas pressures between 3 and 20mTorr because the discharge frequency is higher than the frequency of electron collisions in the plasma. As a result, the UHF plasma provides a process window for high-performance etching that is wider than the one provided by an ICP.