Modeling and experiment on plasma source ion implantation

The critical plasma parameters for plasma source ion implantation (PSII) are the ion implantation current, the sheath expansion characteristic, the energy of the implanted ions, and the electric field intensity on the electrode surface. These quantities have been calculated for spherical electrodes using a fluid dynamic model. The results show that for the implantation time larger than ten ion plasma periods, both the ion current and the sheath expansion characteristic of the fluid dynamic model are consistent with that of the previous analytic models. But for implantation time less than ten ion plasma periods, the results are quite different. As the sphere radius gets smaller, the difference in the results becomes greater. In our experiments, the measured cathode current and the sheath expansion characteristic of the spherical cathode are shown to fit the fluid dynamic model better. The negative high voltage pulse employed in our experiments has a rising edge of less than 1 μs and a flat top approaching the theoretical model. The secondary electron emission coefficients for stainless steel and copper under PSII conditions have also been estimated using two different methods: retarding static electric field energy analyzer and the cathode current measurement. It is found that under PSII conditions, secondary electron emission coefficients are in general larger than the values taken under the condition of atomically clean surfaces in ultrahigh vacuum.