Calculation of the current-voltage-pressure characteristics of dc diode sputtering discharges

Abstract

The relationship between current density, applied voltage, and pressure in dc diode discharges used for sputter deposition has been calculated using a model which considers the electron and ion currents within the cathode dark space. Electrons emitted from the target cause ionization primarily at the interface between the dark space and negative glow and ions crossing the dark space suffer symmetric charge exchange collisions with neutrals. The resulting expression for the characteristic is of the form J = B−2/3 (Va−V0)3/2, where V0 is the minimum voltage at which the discharge can be maintained. The values of B and V0 have been obtained by comparison with experimental data for argon and oxygen discharges with a Ta sputtering target. They vary approximately as (pressure)−4/3 and (pressure)−1, respectively. Values of B calculated from the experimental data for J and Va in argon and oxygen discharges agree with the values obtained from the model and measured values for the dark space distance. Comparison of the model with experimental data gives an estimate of the average value of Townsend’s ionization coefficient.