Plasma-sheath expansion during plasma immersion ion implantation of insulating materials

Due to the high throughout and capability of treating objects with regular geometries, plasma immersion ion implantation (PIII) has large industrial potential. PIII has mainly been applied to metals and semiconductors because that of insulating materials is not as straightforward. In this work, two-dimensional (2-D) numerical simulation on plasma implantation of insulating samples is carried out. The simulation is conducted on an insulating sample 100 mm in diameter placed on a cylindrical target holder with the dimension of φ150 mm× 50 mm biased to the high negative bias. The voltage reduction induced by the capacitance effect decreases both the ion implantation energy and incident dose. Lateral nonuniformity in the ion dose may result due to variation in the surface potential and reduction of the incident dose is also observed due to the reduced bias voltage. Our results suggest that the nonuniformity and dose reduction are not determined exclusively by the variation in the surface potential. The effects are worsened by the self-response (distortion) of the plasma sheath to the surface potential. For instance, when the surface potential changes from V=1/5 to V=5/5 (normalized to applied potential), the incident dose increases by 183% at t=8 μs, while the sheath thickness changes by only 47%. Our preliminary experimental results corroborate the distortion of plasma sheath as predicted by our 2-D model. In order to increase the ion energy, mesh-assisted PIII should be employed.