Radical Kinetics in a Fluorocarbon Etching Plasma

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.