Effects of target voltage during pulse-off period in pulsed magnetron sputtering on afterglow plasma and deposited film structure

In this study, we investigated the effect of the target bias voltage during the pulse-off period in the pulsed magnetron sputtering (PMS) of repetition frequency 200 Hz and duty ratio 5-20%. Copper films were deposited by the PMS for Ar gas pressures of 1-5 Pa (with and without the pulse-off target voltage) and compared with those deposited by DC magnetron sputtering (dcMS). The time-averaged power was set to 100 W. Compared to films deposited by dcMS, those deposited by PMS showed smooth surface and a ductile nature. These properties became more evident when the positive pulse-off voltage was applied. To clarify the reason for film structure modification, the plasma was investigated by time resolved Langmuir probe measurement. It was confirmed that electron density was as high as 10(17) m(-3) in PMS mode during the pulse-on period. On applying +40 V of pulse-off target voltage, the plasma potential was found to be raised by about 30 V in comparison to the zero voltage case. We suggest that this increase in the plasma potential leads to an increase in impinging energy of Cu ions on the substrate, and modifies the deposited film structure. Though the pulsed plasma we used was in the modest power range, we expect this approach is applicable to high-power impulse sputtering (HiPIMS) mode with high degree of ionization. (C) 2012 Elsevier Ltd. All rights reserved.