Self-organizing plasma behavior in RF magnetron sputtering discharges

Abstract

Self-organized plasma structures, called spokes, have been previously reported for continuous and pulsed magnetron discharges. Here, we demonstrate that spokes also form in the oscillatory RF magnetron sputtering (RFMS) regime. We used an intensified charge-coupled device camera and electrical probes to investigate the rotating plasma patterns. The spokes in RFMS were compared to the spokes in classical DC magnetron sputtering (DCMS) at similar discharge conditions. In both regimes, stable plasma patterns were observed for a wide range of discharge parameters. For similar gas pressures and discharge powers, the number of spokes in the RFMS regime was always larger than that in the DCMS regime. The number of spokes changed sequentially when the working gas pressure was increased for both magnetron operations. In the DCMS regime, a single spoke was observed at the lowest argon pressure (0.25 Pa), and a second spoke was observed only at the highest pressure (2 Pa). In the same pressure range, the plasma in the RFMS regime displayed four spokes at the lowest pressure and six or seven spokes at the highest pressure. The influence of discharge power on the number of spokes was less pronounced for both magnetron regimes. We analyzed the spoke patterns by examining the inelastic collisions between electrons and argon atoms. For this purpose, we simulated the dissipation of electron energy in the drift direction and compared the calculations to the length and number of spokes for particular discharge conditions. Overall, the simulations agree well with the observed plasma patterns in DCMS and RFMS.