Breakdown in planar magnetron discharges of argon on copper

In magnetrons the magnetic field confines partially the electron swarm in a trap near the cathode increasing sputtering yield and deposition rates. The magnetic field also decreases the ignition voltage even at relatively low pressures thus allowing sputtering at lower pressures. Under this conditions fewer collisions, less diffusion and less thermalization of the sputtered atoms occur during the transport to the substrates. Atoms arrive at substrates with larger energy and at higher rates.We present an experimental study of the magnetic field influence on the breakdown voltage in planar magnetron abnormal glow discharges for argon on copper. A magnetron cathode was constructed, with a finely tuneable magnetic configuration. The experimental curves of the breakdown voltage as a function of the discharge pressure at constant “confinement power” show minima similar to those occurring in Paschen's law. At lower pressures the breakdown voltage has a strong dependence on magnetic configuration and changes from 750 to 250 V can be found. At higher pressures the breakdown voltage is less sensitive to the magnetic field.Paschen's law could not be fitted to the experimental results and as alternative an empirical expression is proposed and its parameters discussed.The results can be understood by the increased length of the average electron path both in helical and cycloid type trajectories near the cathode and by the reduction of the electron drift towards the anode and the walls of the chamber. In the Townsend regime, before breakdown and fixed voltage the electron density near the cathode increases with the confinement power. This causes a higher flux and energy of the ions that strike on the cathode for the same applied voltage. The voltage needed to get the self-sustained discharge is strongly reduced.