The importance of bias pulse rise time for determining shallow implanted dose in plasma immersion ion implantation

The composition of the low-energy ions arising from the rise and fall time periods of the voltage pulse in plasma immersion ion implantation (PIII) are simulated by particle-in-cell (PIC) modeling. It is shown that more than 70% of the low-energy ions with an energy corresponding to less than half of the applied voltage come from the short rise time period. Although the fall time period is typically 30 times longer than the rise time, less than 25% of the low-energy ions originate from it. Based on the PIC results, the depth profile of the implanted ions is derived using the Monte-Carlo code SRIM2000 [J. F. Ziegler, The Stopping and Range of Ions in Solids (Pergamon, New York, 1985)]. The low-energy ions are found to be implanted to a much shallower depth than ions introduced during the fall time period the concentration profile which decays more sharply into the bulk. These results indicate that the most effective way to reduce or increase the surface concentration is by adjusting the rise time of the PIII voltage pulse. This will require a power supply capable of fast rise times and good matching between power supply and load.