Simulation methods of ion sheath dynamics in plasma source ion implantation

Abstract

Progress of the theoretical studies on the ion sheath dynamics in plasma source ion implantation (PSII) is reviewed in this paper. Several models for simulating the ion sheath dynamics in PSII are provided. The main problem of nonuniform ion implantation on the target in PSII is discussed by analyzing some calculated results. In addition, based on the relative researches in our laboratory, some calculated results of the ion sheath dynamics in PSII for inner surface modification of a cylindrical bore are presented. Finally, new ideas and tendency for future researches on ion sheath dynamics in PSII are proposed.Keywords: plasma sheath simulation, plasma source ion implantation, fluid model, particle simulation, inner surface plasma source ion implantation.Plasma source ion implantation (PSII) has been a well-known technique to modify surface properties of materials. It has many advantages over conventional ion beam implantation, i.e. it is non-line-of-sight accessible, cost-effective and it can be easily operated. In PSII, a target is immersed in a plasma. When a series of negative high-voltage ( 10 100 kV) pulses are applied to the target, ions are extracted directly from the plasma and accelerated to the target for implantation. Since the technique was proposed by Conrad in 1986 [1,2] , many experiments have performed and shown its superiorities in improving the surface hardness, the resistance to wear and corrosion of materials [3 6] .The ion dose and impact energy are the most important factors to determine the result of PSII, and the two factors depend mainly on the dynamics of ion sheath evolution. Therefore, theoretical studies on the ion sheath dynamics emerged and have been continuously developing, giving guidance for the PSII techniques. The physical model of ion sheath forming and expanding in PSII (Fig. 1) is: When a sudden negative voltage is applied to the target, initially, in the time scale of the inverse electron plasma frequency ( pe 1 ), electrons near the target are driven away, on this time scale the ion motion is negligible so that as the electrons recede, they leave behind a region of nearly uniform ion space charge around the target, which is called the "ion-matrix sheath". Subsequently, on the time scale of the inverse ion plasma frequency ( pi 1 ), ions within the sheath are accelerated into the target as they fall through the ion-matrix sheath. This, in turn, drives the sheath-plasma edge further away, exposing new ions that are extracted. Finally, on a longer time scale, the system evolves toward a steady-state Child law sheath. Fig. 1. Schematic diagram of ion sheath evolution in PSII.Based on the above physical model, before the sheath expansion, the ion density in the sheath can be assumed uniform. Thus the sheath thickness, the electric field and potential distributions within the ion-matrix sheath can be calculated by Poisson's equation [1,8 10] . During the period of sheath propagation, the ion velocity, energy, flux, dose and incidence angle at different positions of the target...