Secondary electron target escape probability : a parametric study for conventional magnetron sputtering discharges

Abstract

A magnetic trap in front of a magnetron sputtering cathode (target) effectively confines plasma but also leads to the partial recapture of secondary electrons (SEs) emitted from the target, reducing the effective secondary electron yield. Non-recaptured electrons are counted as having escaped from the target and further joining the discharge. A three-dimensional Monte Carlo simulation was developed to quantify the SE escape probability. The electric field was obtained from a validated two-dimensional analytical description of the plasma potential, and all plasma parameters were assumed homogeneous along the third dimension. Most computations were performed for a nearly balanced magnetic field configuration; however, unbalanced configurations were also considered. The SE escape probability is highly non-uniform across the target and depends on both the electric and magnetic fields. The target-averaged escape probability exhibits a non-monotonic dependence on the discharge voltage and current, with a local maximum determined by a critical combination of the local sheath thickness and electric field. This study provides a method for estimating the SE escape probability in a magnetron sputtering discharge that covers a wide range of relevant parameters, including discharge voltage and current, SE energy, magnetic field strength, the electron reflection coefficient at the target surface, and working pressure. The overall average indicates an escape probability of approximately 0.35–0.40 for conventional magnetron sputtering operation, with higher values at higher pressures or higher electron reflection.