【作者】 刘相梅；

【导师】 宋远红；

【作者基本信息】 大连理工大学 ， 等离子体物理， 2011， 博士

【摘要】 尘埃等离子体是由电子、离子、中性粒子以及尘埃微粒组成的复杂等离子体,它广泛存在于空间等离子体中。最近的研究表明：在用于材料表面处理的等离子体工艺中,尘埃颗粒的产生对等离子体性质及材料表面性质有着重要的影响。特别是对于等离子体沉积多晶硅薄膜工艺,沉积到表面上的纳米颗粒可以增加薄膜中的太阳光光程,保证充分的光吸收,从而能够提高薄膜太阳能电池的光电转化效率。因此,非常有必要对气体放电过程中尘埃颗粒的产生和输运机理以及它对等离子体特性的影响进行深入细致的理论研究和计算机模拟。本文利用流体力学模型,对双频及甚高频容性耦合硅烷等离子体中尘埃颗粒形成机理进行分析,模拟了尘埃颗粒的充电和输运过程,并对薄膜沉积工艺中的硅烷放电过程展开研究。第一章简述了实验室尘埃等离子体中发生的重要物理现象以及尘埃颗粒的形成及生长在薄膜太阳能电池工艺中的应用,并着重介绍了目前对硅烷放电中纳米颗粒的形成及生长过程的理论和实验研究进展,指出了硅烷放电中尚未解决的问题,最后给出本文的内容安排。第二章介绍了尘埃等离子体的相关基本理论。主要包括如下三个方面：尘埃颗粒的形成和生长、尘埃颗粒的充电过程以及尘埃颗粒在等离子体中的受力情况。在第三章中,采用硅烷气体放电—维流体力学模型,详细地研究了双频容性耦合等离子体(Capacitively coupled plasma,即CCP)中尘埃颗粒形成的第一阶段,即通过负离子生长起来的成核阶段。文中主要讨论了双频电源参数对等离子体及尘埃颗粒的影响。结果表明：尘埃颗粒密度及电荷分布主要受高频电源参数的影响,增加高频电源的频率及电压可以有效地提高尘埃颗粒的密度及带电荷数,低频电源参数的影响较小。同时,本章还讨论了尘埃颗粒的形成对等离子体特性的影响。在第四章中,利用一维流体力学模型及气态动力学模型相结合的方法,进一步研究了纳米颗粒形成的第二阶段,即通过团簇凝结的快速生长阶段。给出了直径为1nm到50nm的尘埃颗粒密度及电荷分布情况,分析了尘埃粒子受力对其密度分布的影响,讨论了双频源电压对凝聚过程中尘埃颗粒密度及电荷分布的影响。结果表明：增加高频源电压可以有效地提高整个凝聚阶段尘埃粒子的密度,进而提高硅薄膜的沉积速率。在第五章中,利用二维流体力学模型研究了甚高频容性耦合硅烷放电特性,重点讨论了两个甚高频电源相位差对尘埃颗粒密度及沉积速率均匀性的影响。结果表明：通过调节相位差可以有效改善电子密度、电子温度及尘埃颗粒密度的空间均匀性,进而提高了薄膜沉积速率的均匀性。在第六章,针对等离子体沉积氮化硅薄膜工艺,首先采用一维流体力学模型研究了氮气(N2)、硅烷(SiH4)和氨气(NH3)混合气体低频脉冲放电的物理特性,着重讨论了占空比对等离子体密度及离子轰击能量的影响。研究发现在一定的脉冲放电占空比下,可以获得较高的等离子体密度,而此时离子的轰击极板能量大幅度降低。在此基础上,利用二维流体力学模型研究了SiH4/NH3/N2混合气体在双频CCP中特性。深入分析了高、低频电压以及混合气体组分比对等离子体密度空间分布、离子的轰击极板能量及离子通量径向分布的影响。更多还原

【Abstract】 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.更多还原