Interactions between plasma and ionization gauge in plasma immersion ion implantation

Abstract

During plasma processes such as plasma immersion ion implantation-deposition (PIII-D), proper monitoring of the working gas pressure is essential as the pressure has a large influence on the discharge behavior and plasma parameters (for example, plasma density, ion distribution, electron temperature, etc.), and consequently the surface properties of the treated samples. The pressure is generally monitored using thermal gauges at low vacuum and ionization gauges at high vacuum. An ionization gauge detects the pressure by measuring the collected ion current that can be affected by the presence of an externally sustained plasma in the vacuum chamber. Its existence can introduce errors in the ion current measured by the ion gauge leading to an erroneous pressure reading. This is also true for PIII experiments conducted using high-voltage glow discharge, that is, with plasma created by biasing the samples with a high negative voltage. The plasma in the ion gauge can work as a seed plasma to help ignite the glow-discharge plasma with a relatively small time delay. At a nitrogen pressure of 2.5 mtorr and voltage of −20 kV, the ionization gauge can expedite the formation of the glow discharge plasma by 10s of μs. These discrepancies must be understood and accounted for in practical applications, as a different gas pressure impacts the plasma chemistry, ion mean free path, and so on.