The temporal evolution of the negative ion density in a low duty cycle pulsed reactive magnetron discharge

Abstract

Time-resolved O− density measurements have been made in the bulk plasma of a unipolar pulsed-DC magnetron using laser photodetachment. The magnetron was operated with a titanium target at a total pressure of 1.3Pa and a fixed oxygen-to-argon partial pressure ratio of 10%. The duty cycle was maintained at 5% but the peak on-time discharge power was varied from 120 to 720 W.For all discharge powers, both the electron (ne) and negative ion (n−) densities increase during the plasma on-time, with the electron density reaching a maximum at the end of this phase. In the off-time, the electron density initially decreases at a rapid rate (characteristic decay time ~25μs) for the first 50μs, followed by a slower rate (~150μs) for the remainder of the off-time, however, the negative ion density continues to increase in this phase, reaching a maximum at about ~150μs after the termination of the discharge power. Both the electron and negative ion densities increase with discharge power. The maximum negative ion density was 2.5×1016m−3 for a peak power of 720W, corresponding to a negative ion-to-electron density ratio n−/ne (α) of about 3. In the long afterglow, this ratio reaches a maximum value of 12 as the electron density decreases faster than the negative ion density. This shows that in the afterglow the plasma is highly electronegative.