HIGH-SPEED CLEANING OF BORONIZED WALL WITH A CF4 CONTAINING PLASMA

Rapid cleaning of boron-thin-film deposited walls with use of a CF4 containing plasma was demonstrated. An extremely high removal rate (approximately 40 angstrom/min) of the films was obtained by a dc glow discharge of a CF4 and H2 mixture. Moreover, a 2-3% addition of oxygen enhanced the removal rate by a factor of two, which is about two orders of magnitude larger than that obtained by a conventional H2 discharge. The mass spectroscopic measurement revealed that the film is removed as BF3. The time evolution of gas composition during the cleaning suggests that the oxygen addition suppresses the surface coverage with carbon layers.