Radical Kinetics in a Fluorocarbon Etching Plasma

Abstract

Neutral free radicals, CF₃ and CF₂, in a radio-frequency CF₄/H₂ discharge were detected using appearance mass spectrometry. The spatial density distributions of these radicals were obtained with the mixing ratio of H₂/CF₄ as a key parameter. Adding a 10% H₂ gas to CF₄ enhanced the CF₂ density by a factor of 50 and flattened the spatial profile. By the injection of the H₂ gas abruptly into the CF₄ discharge, the temporal transition to the CF₄/H₂ discharge was investigated. When the percentage of H₂≥5%, a strange time variation is found for the CF₃ radical: its density sharply rises by a factor of 10 and slowly falls to a value close to the initial one. The slow time response was attributed to the H₂-induced polymer deposition. The surface loss probability of CF₂ and CF₃ was measured in various conditions which suggested the importance of surface chemistry of radicals.