Investigation of long-pulse plasma immersion ion implantation

In conventional plasma immersion ion implantation (PIII), the voltage pulse duration is typically on the order of a few tens of microseconds. However, a longer pulse width brings a number of advantages to the hydrogen PIII / Ion-Cut and water Pill processes, for example. They include reduction of surface hydrogen, implantation damage, and metallic contamination. In this paper, we present our results on long-pulse PIII. The simulation and experiments are conducted using a conducting grid separating the vacuum chamber into two zones. The grids confines the plasma sheath and allow long-pulse PIII experiments without premature plasma extinction. We measure the expansion of the plasma sheath and plasma recovery time by monitoring the electron saturation current using a Langmuir probe. We also determine the in-depth distribution and uniformity of the implanted hydrogen as well as the voltage / current waveforms. Our investigation indicates that long-pulse PIII can be realized in a conventional Pill chamber and the practical pulse width can be higher than 500 microseconds because the grid stops the expansion of the plasma sheath. It implies that the dimension of the vacuum chamber does not constitute a limiting factor. The calculated maximum pulsing frequency is about 1 kHz that is limited by the plasma recovery time. A practical pulse duration is between 100 and 500 microseconds. Our results also show improvement in the ion energy distribution under this operation mode.