Numerical investigation of plasma recovery in plasma source ion implantation

In plasma source ion implantation (PSII), a plasma sheath which has been expanded by a negative, high-voltage pulse recovers to its initial state after the pulse is turned off. In this paper, the recovery process of the plasma sheath during the pulse-off time for low-pressure, non-uniform plasma is investigated with a one-dimensional fluid code in plane geometry. The numerical simulations reveal that recovery of plasma during the pulse-off time is a very slow process accompanied by the propagation of a compression wave after the floating sheath formation in front of a target. For relatively small-sized sheaths, the characteristic time for the recovery process is linearly proportional to 5s/uB, where uB and s are the Bohm speed and the sheath size at the switch-off time, respectively. However, the plasma recovery time is enhanced by increasing the sheath size at the pulse-off time since the propagation speed of the compression wave increases with time. The present study is expected to be useful in determining the pulse frequency and duty in the PSII process operated in a highly repetitive mode since the maximum operating frequency is mainly limited by the plasma recovery process.