Experimental investigation of plasma recovery during the pulse-off time in plasma source ion implantation

• Plasma recovery process after a high-voltage pulse is turned off is investigated.
• The recovery time is proportional to the sheath thickness at the switch-off time.
• For large sheaths, the recovery times are shorter than theoretically calculated ones.
• Reduction of recovery time is caused by a non-uniform density distribution.

Experimental results pertaining to the plasma recovery process during the pulse-off time in plasma source ion implantation are presented. A series of experiments conducted in low-pressure argon plasma with various pulse voltages and pulse-on times reveals that the plasma recovery time is almost linearly proportional to the sheath thickness at the switch-off time. However, for sheaths larger than several centimeters, the plasma recovery times are observed to be shorter than those predicted by a simple theoretical model, expressed as 5 s/uB where uB and s are the Bohm speed and the sheath thickness at the switch-off time, respectively. The reduction of plasma recovery time is found to be the consequence of realistic plasma parameters such as non-uniform density and ion drift velocity distributions in front of a target, which were not considered in the theoretical model.