Characterizing fast electrons at the onset of cathode voltage reversal of high-power impulse magnetron sputtering of a graphite target

Abstract

High-power impulse magnetron sputtering (HiPIMS) with cathode voltage reversal is one popular method to produce better quality films, yet understanding the underlying physics is still necessary. This study investigated spatial and temporal profiles of the electron energy distribution function (EEDF) and plasma potential using a Langmuir probe in a HiPIMS with cathode voltage reversal using a graphite target. The time-resolved EEDFs and potentials were measured across an axial distance of 16-104 mm from the target, and the amplitude of the positive reversal voltage was 0-60 V, with a minimum time resolution as small as 180 ns. The measured EEDF changed from Maxwellian during the main pulse to sub-Druyvesteyn during the voltage reversal on the cathode. At the onset of the positive voltage pulse, it was observed at different positions that the effective electron temperature increased rapidly to 5-10 eV while the electron density temporarily decreased approximately by half. As the amplitude of the positive reversal voltage increased, the effective temperature was also raised and the plasma potential at the probing location increased faster. It is inferred that the electric field is mostly localized near the target, and a temporary potential inclination exists downstream across the axial distance after the onset of the positive voltage pulse. Electrons are drawn and accelerated by this temporary potential inclination, exhibiting a rise in the electron temperature and a dip in the density. Controlling these fast electrons may also contribute to the propagation of the potential and ion diffusion, which can subsequently be employed to optimize the sputtering process further when using cathode voltage reversal in HiPIMS.