Numerical Study Of Sheath Characteristics With Dielectric Target In Plasma Source Ion Implantation

Plasma source ion implantation (PSII) is one of the most important technology for surface modification of materials, and it has mainly been applied to metals and semiconductors. PSII possesses many advantages such as low costs, the high processing efficiency and capability of processing complex-shaped objects. It has been extended to the treatment of dielectric materials such as polymers, ceramics, etc. recently. However, treatment of dielectric materials by using PSII has been severely hampered by the fact that the conductivity of dielectric materials is extremely poor. The major obstacle is the charging problems resulting from the positive charge accumulation on the surface. This leads to that the surface potential is lower than the applied voltage, the implanted ions thus can not reach the desirable energy. On the other hand, the charging problems must be alleviated to reduce technical difficulties such as electrical arcing in the plasma chamber. In order to solve these problems it is necessary to investigate theoretically the sheath expansion next to the dielectric objects during the PSII 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 PSII, the study on sheath evolution can reveal the physical mechanism of ion implantation and provide some guidance for the applications of PSII to dielectric materials.In this thesis the fluid model is mainly adopted to numerically study the characteristics of the sheath expanding in PSII with dielectric materials. The influences of experimental parameters on the surface charging and sheath expansion are then discussed. The thesis is organized as:Chapter 1 briefly reviews the characteristics and applications of PSII, the investigation methods for sheath expansion in PSII, and introduces the current research state and significance of the sheath expansion in PSII with dielectric materials.In Chapter 2, one- dimension analytical model is used to investigate the characteristics of the sheath expansion during PSII with dielectric materials. The influence of experimental parameters on the sheath expansion is analyzed and discussed. It shows that the sheath near the dielectric surface is thinner than that near the metal surface, and the surface potential of the dielectric materials and the dose of ions implantation decrease owing to the surface charging. The influence of charging effect on the sheath depends strongly on the experimental parameters. Among these parameters the plasma density, the pulse voltage and the thickness of the dielectric materials play an important role. The sheath would collapse when the plasma density is too high. Low plasma density and thin dielectric materials can reduce charging effects. Short pulse duration is also beneficial to the process of PSII with dielectric materials.In Chapter 3, a one-dimensional self-consistent fluid sheath model is developed to study the expansion dynamics of plasma sheath next to dielectric materials in PSII. The influences of the secondary electron emission (SEE), rise time of applied pulse voltage and geometry of target on the charging effect are discussed. The model includes the ion-fluid equations of continuity and motion, and Poisson's equation with one assumption that the electron density obeys the Boltzmann distribution, in which the potential and the electric field distribution in the sheath is evaluated by solving Poisson's equation. An extra equation of surface charging with SEE is coupled up to the model. The spatiotemporal variations of the potential, the electric field, ion density and electron density etc. inside the sheath are studied numerically. The instantaneous sheath thickness, the surface potential at the dielectric substrate and charge density accumulated on dielectric substrates are given. Additionally, the ion energy distributions arriving at dielectric substrates are calculated. It finds that the SEE increases the surface charging effects significantly. The role of SEE can not been ignored in the application of PSII to dielectric substrate. The pulse rise time also plays an important role in PIII process with dielectric substrates. The longer pulse rise time would decrease the effect of surface charging. The curvature of the target surface has some influence on the outcome of implantation.In Chapter 4, a two-ion fluid model describing nitrogen molecular ions N₂⁺ and atomic ions N ⁺ is developed to investigate the influence of ion species ratio on the expansion dynamics of plasma sheath with dielectrics during the PSII. The numerical results demonstrate that more atomic ions N ⁺ in the plasma can increase the charging effects. Controlling nitrogen atomic ions N ⁺ and raising the nitrogen molecular ions N₂⁺ in PSII with dielectric materials will weaken the effect of surface charging and improve the result of ion implantation.In Chapter 5, a two-dimensional self-consistent fluid sheath model in planar target is developed to study the charging effects and the characteristics of the sheath expansion during a dielectric processed in PSII. The much attention is paid to the dose nonuniformity. The spatiotemporal evolution of the surface potential, the charge density accumulated and the ion impact angle at the surface of dielectrics are calculated. It shows that the evolution of sheath around the target is not uniform, and that the distributions of physical parameters along the dielectric surface are also nonuniform. The nonuniformity is more severe next to but not at the edge of the dielectric target. The nonuniformity, is mainly related to the target geometry. And the surface charging effects makes it more severe. To lengthen the size of the metal electrode located below the dielectric target, one can improve the uniformity of the dose on the dielectric surface.