Simulation of pressure effects in a multipulsed nitrogen plasma source ion implantation system

Abstract

Plasma source ion implantation is typically performed at low pressures (10−4 Torr), but pressure plays an important role in the control of the deposited ion energies and the location of the time dependent sheath edge. At high pressures (10−2 Torr) with energy-dependent secondary electron production rate, the sheath edge advances to a nearly fixed position for most of the voltage pulse, and the deposited ion energy spectrum appears collisional, consistent with the ion-mean-free path being shorter than the sheath length. This results in high TiN surface concentration with a broad tail into the sample. Nitrogen dissociation increases the amount of monatomic N gas and its ions with each system pulse. At low pressures (10−3 and 10−4 Torr) the sheath edge advance into the bulk plasma throughout the pulse, instead of reaching a quasistatic limit as it does at 10−2 Torr. The energy spectrum of ions arriving at the cathode shows a larger proportion of high energy ions, which produces a deep TiN subsurface band. In this one-dimensional implantation system simulation (N2 plasma and Ti target), we have shown that pressure can be used to tailor the desired TiN profile and that the TiN deposition rate drops as the number of voltage pulses increases.