【作者】 丁芳；

【导师】 朱晓东；

【作者基本信息】 中国科学技术大学 ， 等离子体物理， 2010， 博士

【摘要】 高气压(100Torr-760Torr)直流辉光放电等离子体,由于气压高,粒子间碰撞频率大,具有较高的温度而接近热平衡状态,使这种等离子体具有较高的气体分子离解效率,可以获得高浓度的活性粒子,在等离子体工艺中特别是均匀功能薄膜快速沉积应用中具有独特的优势。同时这种等离子体的放电装置结构简单,气体消耗低,易于实现规模化工业应用。但是随着气压升高,等离子体气体温度也大幅上升,宏观上各种不均匀性如电场梯度、温度梯度等在高气压下会变得非常明显,容易诱发辉光放电向弧放电转变,这实质上是等离子体中各种微观不稳定性演化的结果。对大面积高气压直流辉光等离子体放电物理和技术的研究对发展新型等离子体源及其应用具有重要意义。本文建立了一套高气压脉冲直流辉光等离子体产生装置,对大功率稳定辉光等离子体的放电技术、放电物理及其在功能材料方面的应用进行了系统的研究。通过对放电电极不断的设计改造和放电参数的优化,目前装置可在气压为100Torr-200Torr的范围内,在氢气、甲烷和氩气的混合气体中产生功率约15kW,最大直径80mm的辉光放电等离子体,并可连续稳定运行600小时以上。利用发射光谱和示波器,研究了等离子体特性随多工艺参数的演变;对等离子体进行了空间分辨诊断,获得了电子温度、气体温度和主要反应粒子浓度的三维空间分布。研究表明,高气压等离子体中,气体加热非常显著,电极温度的控制对于等离子体的稳定性至关重要。热效应成为一种重要粒子激发和离解机制,它同电子碰撞机制共同作用,有时则相互竞争,甚至成为主要机制,影响着等离子体中活性粒子浓度和空间分布以及放电的稳定性。不同的脉冲占空比和频率能够调节等离子体的非平衡程度,影响不同粒子的相对浓度。空间分辨诊断结果显示,紧贴阴极表面的一层阴极辉光区具有较大的电位降和电场,消耗了总放电功率中的大部分,是维持整个放电等离子体稳定最重要的区域,其较高的电子温度使该区域的光谱具有丰富的谱线结构。正柱区是整个等离子体的主体,较低的电子温度和较高的气体温度使该区域具有较低的非平衡性,热效应比较明显。示波器诊断结果显示,典型的等离子体衰减特征时间约为65微秒,远低于低气压时的衰减时间,而等离子体密度上升特征时间在几百微秒的量级,随放电参数的变化而不同。电流、占空比、频率、电极间距和气压变化可以影响等离子体的衰减和增长速率。而气体流量、甲烷和氩气浓度对等离子体的衰减速率影响不大。利用这种等离子体在甲烷和氢气中进行了金刚石厚片的制备研究,目前可以生长出直径75mm,厚度达5mm的金刚石厚片。金刚石片的质量和厚度均匀性受到气压、甲烷浓度、电流、基底大小等多个参数既相互关联又相互制约的影响。高气压碳氢等离子体中过多的碳团簇不利于金刚石厚片的生长。在高气压直流碳氢等离子体中首次观察到一种厘米尺度、具有三维空间分布的金刚石树枝状结构的自组织生长,等离子体中大量的带电团簇在金刚石树的生长过程中起到了关键作用。该发现被《Nature China》选为当年来自中国大陆和香港的突出科学研究成果并作为研究亮点报道。直流等离子体中实现金刚石结构的三维空间快速生长为CVD方法合成复杂结构的金刚石提供了一个可能性。此外,在高气压等离子体中还观察到两种具有自组织特性的辉纹结构,一个是出现在等离子体正柱区的条状辉纹,其大小与气体温度有关。另一个是在阳极边缘出现的规则辉点分布。更多还原

【Abstract】 Due to high collision frequency between particles, direct current (dc) glow discharge plasmas with high pressure (100Torr-1atm) are characterized by high gas temperature and close to thermal equilibrium states. Owing to and high dissociation efficiency of gas molecules, the plasmas are abundant in active radicals, which make them very suitable for the rapid deposition of functional films. Furthermore, the simple device structure and low gas consumption make the plasmas meet the requirement of industrial application in large scale. Nevertheless, the high gas temperature of the plasmas can lead to serious inhomogeneity of macro-parameters, such as electric field, temperature of plasma, which apt to induce the glow to arc transition (GAT). Essentially the GAT is the result of development of micro-instabilities. The research of discharge physics and technology of high pressure dc glow plasmas in large area is significant for the development and application of new plasma sources.In the dissertation a set of apparatus for producing pulsed dc glow discharge plasma under high pressure was built. The discharge technique, mechanism and application in functional materials synthesis were studied systematically.After massive reformations of electrodes and optimization of discharge parameters, the apparatus now can produce glow discharge plasma with the max diameters of 80mm and the power of 15 kW under the gas pressure between 100 Torr and 200 Torr, In addition, the plasma can remain stable more than 600 h.By virtue of OES (optical emission spectroscopy) and oscillograph, the feature evolutions of the plasma with multiple parameters were investigated. Through the spatially resolved OES diagnosis, three-dimensional distributions of electron temperature, gas temperature and the main reactive particles were obtained. The research indicates that, in the high gas pressure plasmas, the gas heating is remarkable, the control of electrode temperature is crucial for the discharge stability. Thermal effect becomes one of important mechanisms for particles excitation and dissociation. It influences not only the concentrations and spatial distributions of active particles in plasma but also the discharge stability, sometimes combined with, in other times competing with, even dominating over electron collision effects. Different pulse frequency and duty cycle can lead to different non-equilibrium degree, influencing the radical concentrations. The spatially resolved diagnosis shows the cathode region constricts into a thin layer sticking to the surface of cathode in which a large voltage drop and electric field consist. The cathode region consumes most of the total discharge power, is the most important region in charge of the stability of discharge plasma. Due to the relatively high electron temperature, abundant spectrum lines present in the OES of the region. Positive column region occupies the major bulk of plasma. Because of relative lower electron temperature and higher gas temperature, the region is characterized by small non-equilibrium degree and strong thermal effect. By virtue of oscillograph, the typical decay time of plasma is found to be 65μs, far lower than that at low gas pressure. The rise time of plasma density is about hundreds of microseconds depending on the discharge parameters. The rise and decay rates of plasma are associated with discharge current, interelectrode gap, gas pressure, pulse frequency and duty circle, but seem to be irrelevant to gas flow rate, methane and argon concentration.The plasmas also were used to deposit thick diamond wafers in the mixture of hydrogen and methane. The diamond wafer with diameter of 75 mm and thickness of 5 mm was successfully synthesized. The diamond wafer’s uniformity in quality and thickness is influenced by the joint and interacting effects of gas pressure, discharge current, size of substrate and proportion of methane. The excessive carbon powders in hydrocarbon plasmas are detrimental to the growth of thick diamond wafers.A self-organization three dimensions growth of centimeter-scale treelike diamond structures was firstly found in the high pressure dc hydrocarbon plasma. Numerous powders with negative charges formed in the dc plasma play a crucial role in the growth process of diamond tree. The findings were featured on Nature China as one of outstanding research results from China’s mainland and Hong Kong in 2009. The findings demonstrate the possibility of using CVD for the rapid growth of complex diamond structures. In addition, two other self-organization glow structures in the plasma were also observed and discussed. One is the glow striations in the positive column region, whose size is related with the gas temperature. The other is the pearllike glow spots around the anode edge.更多还原