Retrograde Motion Velocity of Graphite Cathode Spot in Vacuum Arc Deposition Apparatus

Abstract

Knowledge of cathode spot motion on a graphite cathode in a vacuum arc deposition apparatus is important for producing a diamond-like carbon film from the view point of droplet suppression. In this paper, the motion of graphite cathode spot is observed and its velocity is measured. The experimental conditions are as follows; cathode dediameter: 64mmφ, hydrogen gas flow rate: 10ml/min, arc current: 50A, pressure: 0.2 and 0.05Pa and magnetic flux density: 3-25mT.
The results are summarized as follows; (1) The cathode spot is driven to retrograde direction by applying radial magnetic field on the cathode surface. (2) The retrograde velocity increases as the magnetic flux density increases and as the pressure decreases. (3) The velocity is less than 10^-2 of that for Ti or Al cathode under the same experimental conditions. (4) When the magnetic flux density exceeds about 8mT, the cathode spot extinguishes on the way of moving or immediately after ignition. (5) The maximum velocities obtainable are 20 and 25mm/s at pressures of 0.2 and 0.05Pa, respectively.
Those measurement results are interpreted as follows; thermoelectronic emission is required for the formation of graphite cathode spot and it takes some time for the surface to be heated up to emit enough electrons. As a result, the velocity is very slow and limited by a certain value.