UNSTABLE ARC OPERATION AND CATHODE SPOT MOTION IN A MAGNETICALLY FILTERED VACUUM-ARC DEPOSITION SYSTEM

Arc discharges were established on a Ti cathode in vacuum in an are deposition system with a toroidal macroparticle filter. A magnetic field with a radial component of up to 3 mT on the cathode surface at the distance of 20 mm from the cathode center was applied in order to drive the cathode spots in an azimuthal motion on the front surface of the cathode, and an axial component of the field of up to 30 mT was applied parallel to the walls of the plasma ducts leading from the cathode region to the substrate in order to collimate the plasma beam. Cathode spot motion was observed by means of a television camera and VCR via a window installed at the substrate holder flange and a mirror located in the quarter tonus. Ion current convected by the plasma beam was measured with a negatively biased probe. It was shown that the magnetic field of the coils located on the plasma duct has a strong influence on cathode spot behavior. If they produce a field stronger than 4 mT at the cathode surface, the spots move off the cathode surface, and the probability of their return decreases with stronger fields. The spots moving off the cathode surface cause an increase of the are voltage and its fluctuations, and the are becomes unstable and tends to extinguish. Location of the cathode spots on the side of the cathode becomes increasingly likely with increasing duct magnetic field, and results in a decrease in the plasma flux through the plasma duct. An optimal duct field of 16 mT maximized the plasma output flux. (C) 1995 American Vacuum Society.