摘要
电容耦合等离子体(capacitively coupled plasma CCP)因其可以产生大面积等离子体,装置简单,容易控制,已广泛应用于薄膜沉积与等离子体刻蚀；等离子体增强化学气相沉积(plasma enhanced chemical vapour deposition PECVD)技术是借助于辉光放电等离子体使含有薄膜组份的气态物质发生化学反应,从而实现薄膜材料生长的一种制备技术。在新兴的光伏产业中,硅基薄膜扮演着举足轻重的角色,特别是综合了SiO2膜和Si3N4膜的优点的氮氧化硅(silicon oxynitride SiOxNy)薄膜具有优异的光学性能,力学性能,化学稳定性及低的应力,已成功运用于光电领域,并且作为太阳能电池的减反射膜,可以有效提高太阳能电池的利用率。所以,氮氧化硅薄膜有着广阔的应用前景,对其放电研究具有重要的意义。然而在连续放电的条件下,产生较高的离子能量会对基片造成损伤,并且在这种条件下生成的薄膜光电性能较差。与此相比,脉冲调制放电的参数调节更加灵活,可以获得高质量的沉积膜。对于硅烷特别是脉冲射频等离子体放电,由于混合气体放电的化学组分及反应过程的复杂性,仍需要进行深入的描述和研究。本文采用方波对射频电压进行了脉冲调制,采用流体力学模型对SiH4/N2/O2混合气体的容性耦合等离子体进行了数值模拟。
第一章和第二章分别介绍了本文研究的相关背景知识和所采用的一维流体力学模型,并对计算中用到的相关参数进行了详细概括。
第三章着重讨论了脉冲放电参数对容性耦合等离子体SiH4/N2/O2放电特性的影响。结果表明等离子体参数受占空比和调制频率的影响很大。相同的沉积功率密度下,放电中心的电子密度与占空比成反比,脉冲放电下的等离子体密度更高；相同的射频电压下，电子密度则与占空比成正比,与调制频率成反比。电子温度的周期性变化则因占空比和调制频率的不同发生了明显的变化,在较小的占空比或调制频率下,电子温度变化比较剧烈,脉冲开启的瞬间就会增加至最大值。脉冲调制放电下,等离子体区内的电负性几乎保持不变,但是在鞘层边界的变化比较大。在电源关闭时,较低的电子温度使得负离子从等离子体区逃离,造成了下极板处的电负性增大。此外,在相同的沉积功率密度下,减小占空比和调制频率都可以获得较低的离子轰击能量,减少对基片的损失。与此同时,正离子的密度和平均速度、平均通量的改变进一步调节了沉积功率密度,使得薄膜的沉积过程更加灵活可变。在相同的占空比和调制频率的基础上,研究了其他的放电参数如气压、极板宽度和射频电压幅值对等离子体各宏观物理量的影响。提高电压幅值,增加气体压强或者增加极板宽度均能有效地提高等离子体的密度。极板中心处电子温度的周期性变化受气压和极板宽度的影响很小。加大气压或者增大极板间距,会促进各粒子之间的碰撞,造成离子能量的减少,但是提高射频电压可以为离子提供更多的能量。
以上模拟结果表明,在脉冲调制放电中,通过改变占空比和调制频率,以及其它放电参数,可以获得高密度的等离子体,并能提高沉积功率密度,有效地降低离子能量,从而降低离子轰击对基片的损伤,提高沉积薄膜的质量。
Capacitively coupled plasma(CCP) has been widely used in thin film deposition and plasma etching due to its simple device, which can be easily controlled and could produce large plasma. Plasma enhanced chemical vapour deposition(PECVD) is a preparation technique to achieve the growth of thin film materials, in which chemical reactions among gases containing film components are produced by glow discharge plasma. In photovoltaic industry, silicon-based thin films play a very important role, especially silicon oxynitride(SiOxNy) thin film, which integrated the advantages of silicon oxide(SiO2) thin film and silicon nitride (Si3N4) thin film. Due to their excellent properties, including good photoelectrical characteristics, mechanical property, chemical stability and low stress, SiOxNy films have been widely applied in optical and microelectronic fields as graded index films or antireflection coatings. Meanwhile, SiOxNy films are also used as the reflection film of solar cells in order to get high utilization. So, SiOxNy thin film has broad application prospects and the research on it has the vital significance. However, in continuous discharges, high-energy ions produced will damage to the substrate and the photoelectric of the film is poor in this condition. By contrast, high quality of SiOxNy film can be obtained in pulse modulation discharge with more flexible parameters. Nevertheless, apart from pure gas discharges, such studies on mixture gas discharge is relatively insufficient due to the complexity of both the chemical components and the reaction process. There is still an increasing need for deep descriptions and predictions of silicon discharge, especially for pulsed radio frequency plasma. In this paper, the behavior of silicon plasma mixed with SiH4, N2 and O2 driven by time modulated voltage have been carried out by using a one-dimensional fluid model.
Chapter I and Chapter II introduce the related background knowledge and the one dimensional fluid mechanics model separately, and give a detailed summary of the related parameters in the calculation.
Chapter III emphasizes on the effect of pulse parameters on SiH4/N2/O2 capacitive coupling plasma discharge characteristics. Our results demonstrate great influence of pulse duty cycle and modulation frequency on the plasma parameter. When the averaged density of the deposition power is kept constant, the electron density at the center of the discharge is inversely proportional to the duty cycle, and more higher plasma density can be achieved. If the rf voltage is fixed, the electron density is proportional to the duty cycle, but decreases with the increasing modulated frequencies. Meanwhile, the electron temperature changes obviously under different duty cycles or modulated frequencies, and shows a quite dynamic behavior when the duty cycle or the modulated frequency is smaller. The electronegativity of the plasma keeps almost constant in the bulk plasma and shows great change near the plates. Furthermore, lower ion energy will be obtained under the same deposition power density by reducing the modulated frequency of duty cycle, leading to less damage to the substrate. The change of mean velocity and flux of the positive ions at the powered plate under pulsed discharge will re-adjusted the other prameters, making the film deposition process more flexible. The impacts of other discharge parameters on the plasma, such as air pressure, plate width and rf voltage are investigated at the fixed duty cycle and modulation frequency. The plasma density can be effectively improved with higher rf voltage, larger pressure or more wide plate width. Small periodic change of electron temperature at the center of the plates occurs with the various of pressure and plate width. Furthermore, ion energy can be reduced because of the intensified collisions between particles when the pressure or the width of the plates increases. Meanwhile, higher rf voltage can offer more energy to ions.
It is concluded that while choosing proper duty cycle, modulated frequency or other discharge parameters, high plasma density and deposition power density can be provided in pulse modulated discharge, and the ion energy can be lowered effectively. Thus, damage on the substrate caused by ion bombardment can be reduced in order to get high quality thin films.
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