High voltage ionization during plasma immersion ion implantation

During the plasma immersion ion implantation (PIII) process, ions are typically created by an external plasma source, such as electron impact glow discharge using hot filaments, radio frequency, electron cyclotron resonance, metal arc, and so on. There is, however, a less obvious ion formation mechanism by the high voltage itself, especially for a long pulse duration or at high working pressure, as shown by the implantation current not decreasing monotonically as predicted by the Child-Langmuir law. A proof of this secondary phenomenon is that the measured total current sometimes increases dramatically in a low vacuum sustained by RF glow discharge. Another example is that the current can gradually rise after a short delay during long pulse, hot filament glow discharge PIII. These phenomena can be attributed to high voltage ionization during the PIII process and are related to the gas pressure, high voltage pulse duration, target size, target materials, and so on. In this paper, we will present supporting experimental data in addition to a theoretical analysis.