Numerical study of rotating spokes in RF magnetron discharges

DC and RF magnetron discharges are used in a variety of important applications, especially in thin film deposition processes. Notwithstanding wide-spread applications of these devices, a lot of relevant physics is not well understood. Magnetron plasmas are subject to various instabilities resulting in oscillations of plasma parameters with frequencies ranging from kHz to MHz for all magnetron types: DC, pulse, or RF-driven magnetrons. Rotating spokes are prominent low-frequency (on the order of kHz) oscillations in these discharges. In this work, we study the dynamics of rotating spokes in the RF magnetrons using the two-dimensional axial-azimuthal fully kinetic particle-in-cell/Monte Carlo collision code 2D-EDIPIC. Our simulations show that the spokes are excited above the powered electrode and rotate in the azimuthal (+ExB) direction with the propagation speed that is in good agreement with the recent experimental observation in RF magnetrons [M. Panjan, J. Appl. Phys. 125, 203303 (2019)]. We find that the spoke is driven by the density-gradient-drift instability. The high frequency (on the order of MHz) oscillations - electron cyclotron drift instabilities (ECDI) are also found to be excited. The synergistic effects of the ECDI and spokes on electron heating and anomalous transport are carefully analyzed and interpreted.