Macroparticle contamination in cathodic arc coatings: generation, transport and control

The cathode spot of a vacuum arc generates a spray of liquid droplets directed almost parallel to the cathode surface as well as a highly ionized plasma jet directed normal to the cathode surface. Theories include droplet ejection by the reaction force of back-streaming ions on the underlying microscopic liquid pool and formation from explosive debris. The droplets have an exponentially decreasing size distribution and velocities ranging from 10 to 800 m s^-1. During their motion in the arc, the macroparticles can be further accelerated and deflected, obtain a negative charge, be heated to temperatures of around 2000 °C and evaporate. The macroparticle mass emission rate from the cathode increases with increasing arc current and average cathode surface temperature and decreases with increasing cathode material melting temperature. Cathodes with gaseous surface layers have less macroparticle erosion than clean cathodes. Droplet production can be reduced by maintaining as low a temperature as possible on the cathode surface near the cathode spots by providing effective cooling, by operating at low cathode current densities, by using magnetic fields to provide for directed rapid cathode spot movement and, in reactive deposition, by operating a poisoned cathode. Macroparticle inclusions can be reduced by substrate biasing and by concentrating the plasma flow with magnetic fields and can be eliminated completely by using a curved magnetic plasma duct.