Numerical Study Of Sheath Characteristics With Inner Surface Of A Hollow Cylindrical Dielectric Tube In Plasma Immersion Ion Implantation

Plasma immersion ion implantation (Pâ…¢) is a new ion implantation processing, which possesses many advantages such as low costs, simple operation and the capability of processing complex-shaped objects etc. Presently, plasma immersion ion implantation as a new method is regarded by some material engineers and researchers. And it has been applied extensively in industry, such as the modification of materials, doping in the course of the manufacture of CMOS chip and silicon on insulating (SOI). In the actual application, the modification of the inner surface of a cylindrical dielectric tube is very complex. On the one hand, there exist the charging effects as a result of the charge accumulation at the dielectric surface because of low electrical conductivity of the dielectric. On the other hand, owning to overlapping of the converging plasma sheaths from the surfaces inside the hollow cylindrical dielectric tube in the plasma, thus makes the implant dose and energy decrease. In order to solve above problems, it is of necessity to investigate theoretically the sheath evolution next to the inner surface of a hollow cylindrical dielectric tube during the Pâ…¢processing. Because the characteristics of the plasma sheath formed near the surface of the processed materials directly affect the final properties of the target materials in Pâ…¢. And then, theoretical research results can reveal the physical mechanism of ion implantation and provide some guidance for the actual processing of ion implantation optimization.In this thesis, we mainly adopt the fluid model to study the principles of Pâ…¢and the expansion of the sheath in the inner surface of a hollow cylindrical dielectric tube. The influences of many parameters on the Pâ…¢are then discussed.First of all, we apply the one-dimensional collision-less fluid sheath model to study the expansion dynamics of plasma sheath in the inner surface of a cylindrical dielectric tube. Then we can get the characteristics of sheath evolution, and the influence of other parameters including the thickness of dielectric-film, the shapes of pulse etc. on the ion implantation is disclosed in detail. It is disclosed that in the course of Pâ…¢into the cylindrical dielectric tube, the sheath expands towards the auxiliary central electrode in the pulse duration, and in the pulse fall time, the sheath collapses. Under the same parameters, the sheath near the dielectric surface is thinner than that near the metal surface. There exist the charging effects because of charge accumulation at the surface of dielectric, which decrease the dielectric surface potential and the ion implant energy. Various parameters have certain influence on the ion implantation. In the actual processing, we can thin the dielectric film and at the same time adopt suitably longer pulse rise time and shorter pulse fall time in order to reduce the influence of the charging effects.In the next place, we develop a two-ion fluid model describing nitrogen molecular ions N2+ and atomic ions N+ to investigate the influence of ion species ratio on the expansion dynamics of plasma sheath with dielectrics during Pâ…¢. The numerical results demonstrate that more atomic ions N+ in the plasma can make the charging effects increase. In the actual ion implantation, we had better raise the nitrogen molecular ions N2+ in Pâ…¢with dielectric-film in order to weaken the charging effects and improve the dose of ion implantation.In the end, we study the sheath evolution when applying strong magnetic field in the axis of the cylindrical dielectric tube, and the effects of strong magnetic field on the characteristics of sheath evolution are investigated. The results indicate that the ion energy would remain unchanged when introducing the magnetic field in the axis of a cylindrical dielectric tube. However, the magnetic field makes the ion energy shift between the radius direction and angle direction. The introduction of magnetic in the axis makes the ion velocity in the angle direction appear. At the same time, with increasing the strength of magnetic field, the high energy peak of ion energy distribution in the radius direction would move towards the low energy section.