Measured radial and angular distributions of sputtered atoms in a planar magnetron discharge

Abstract

The effect of current, voltage, and magnetic field on the radial profile of ions sputtering a flat cathode has been studied in a hollow cathode enhanced magnetron discharge. The measurements of sputtering rate were made using a directional quartz crystal microbalance, translated parallel to the cathode. We find the radial sputtering profiles are determined by the plasma sheath thickness, which can be calculated using the Child-Langmuir law for collisionless space-charge limited current flow. A hollow cathode electron source was used to control the sheath thickness by increasing the current to the magnetron cathode at a given voltage and constant pressure (0.07 Pa). Uniformity of the radial profiles increases with decreasing sheath thickness (i.e., at lower voltages and higher current densities). The angular distribution of sputtered atoms was measured by changing the polar angle of the microbalance. The angular distributions are asymmetric on either side of the radial position where the ion current density to the cathode is maximum. We believe this is due to ions which impact the cathode at angles up to ≊10°from the normal, having been radially accelerated away from the position of maximum plasma density.