Simulation Of Growth Mechanism And Transport Of Nano-Scale Particles In Capacitively Coupled Silane Discharges

Dusty plasmas, defined as complex plasmas which contain electrons, ions, neutral particles and dust particles, are ubiquitous in space plasmas. In recent years, formation of dust particles has been found to be important to the plasma and material surface properties in material processing discharges. Especially, for the polysilicon thin film techniques, it seems that the creation of nanoparticles on the thin films can result in the increasing duration time of light in the film, improving the photoelectric conversion efficiencies. Therefore, meticulous investigation of the formation and transport of dust particles and their effects on the plasma properties is very necessary.In this work, the well-known fluid model is used to study the formation, charging and transport of nanoparticles in dual-frequency (DF) and very-high-frequency (VHF) capacitively coupled silane discharges, as well as the silane discharge processes in the thin film deposition techniques.In Chapter 1, the corresponding physics theory of the laboratory dusty plasma and applications of nanoparticles in the thin film solar cell technology are briefly introduced. We mainly concern on the recent progresses in the theoretical and experimental studies of the formation and growth of dust particles in the silane discharges, and put forward some problems unsolved in this research area. Finally, the outline of this thesis is present.In Chapter 2, the basic principles of the dust plasma are presented. Major aspects including the formation, growth, and charging mechanisms, as well as the forces acting on the dust particles are discussed in detail.In Chapter 3, the first stage of particle formation, which is the gas phase nucleation by growth of negative ion or neutral clusters, is carefully studied by using a self-consistent one-dimensional (ID) fluid model in DF capacitively coupled silane discharges. The effects of dual-frequency sources on the behavior of dust particles are carefully discussed. It is found that the nanoparticle density and charge are mainly influenced by the voltage and frequency of the high-frequency source, while the voltage of the low-frequency source may exert a limited effect on the nanoparticle formation. Furthermore, the influence of the formation of dust particles on the plasma properties is also discussed.In Chapter 4, the second stage of particle formation, a more rapid growth process by coagulation of clusters, is systematically studied, by using the self-consistent ID fluid model coupled with an aerosol dynamics model. The density and charge distribution profiles are presented for particles ranging in size between 1 and 50 nm. The effect of forces on the location of nanoparticle growth is studied. Moreover, the effect of the high-and low-frequency electric sources on the particle density and charge distribution is also discussed. The results show that the high frequency voltage has evident effect on the nanoparticle density in the coagulation phase, which will greatly improved the deposition rate of silicon.In Chapter 5, a two-dimensional (2D) fluid model extended from the above 1D fluid model is used to study the VHF capacitively coupled silane discharges, with the special concern on the influence of controlled phase shift between two VHF (50 MHz) voltages applied to the powered electrodes on the uniformity of the silane discharge. The results show that, by controlling the phase-shift, not only the uniformity of the electron and dust particle densities, but also that of the deposition rate can be improved considerably.In Chapter 6, a 1D fluid model is developed to investigate the behavior of plasma in the low-frequency pulsed voltage modulated SiH4/NH3/N2 discharges. We mainly focus on the effect of the duty cycle on the plasma density and ion energy that bombardment to the substrate. It is found that, a rapic decrease of the ion energy and relatively slow decrease of ion density can be observed as the duty ratio decreases, implying that proper decrease of the duty ratio can be good to the film properties. Futhermore, the behavior of plasma in DF capacitively coupled SiH4/NH3/N2 discharges is investigated based on a self-consistent 2D fluid model. The effect of high- and low-frequency voltages, and mixed-gas component on the plasma density and ion energy on the substrate are carefully discussed.