碰撞对非对称射频鞘层特性的影响

对于大多数活性射频等离子体刻蚀工艺，由于放电室中两个电极的面积不等，使得两个电极附近的等离子体鞘层是非对称性的.考虑离子与中性粒子的碰撞效应，建立了一种描述这种非对称射频鞘层动力学特性的自洽动力学模型.数值结果显示碰撞效应对极板上的瞬时电压降、瞬时电子鞘层厚度、鞘层内的离子密度和动能的空间分布以及两个极板上电压之差等物理量的影响. 关键词： 射频鞘层 非对称电极 碰撞效应 离子     

尘埃等离子体鞘层的玻姆判据

采用流体方程和自洽电荷变化模型研究了尘埃等离子体鞘层的玻姆判据. 讨论了离子临界马赫数和尘埃粒子临界马赫数随尘埃密度变化的关系, 以及尘埃表面势随尘埃密度变化的趋势. 由于鞘边尘埃粒子的存在, 离子必需以大于声速的速度进入鞘层; 随尘埃密度的持续增加, 离子的临界马赫数增加到一个最大值后开始逐渐减小. 数值计算得到的结果满足Sagdeev势的定性分析. 关键词： 等离子体 鞘层 玻姆判据     

电子发射对稳态等离子体鞘层影响的理论研究和数值模拟

 建立了一套非线性自洽的流体力学方程组,研究了电子发射对稳态等离子体鞘层的影响,并分别考虑了发射电子及离子同中性原子的弹性和非弹性碰撞过程。数值结果表明：发射电子的初始束流密度及中性气体密度是描述鞘层空间演化的两个最关键的物理量,特别是发射电子的存在使得鞘层厚度变薄。     

低气压感性耦合等离子体源模拟研究进展

感性耦合等离子体源具有放电气压低、等离子体密度高、装置结构简单等优点,因此常被用于材料刻蚀及表面处理工艺中.为了深入了解感性耦合等离子体的特性及其与表面的相互作用,数值模拟成为了目前人们普遍采用的研究手段之一.针对具体问题,可以选择不同的模拟方法,如整体模型、流体力学模型、流体力学/蒙特卡罗碰撞混合模型、偏压鞘层模型、粒子模拟/蒙特卡罗碰撞混合模型等.其中,整体模型计算效率最高,常被用于模拟复杂的反应性气体放电.但整体模型无法给出各物理量的空间分布,因此二维及三维的流体力学模型,也得到了人们的广泛关注.在低气压等极端的放电条件下,由于电子能量分布函数显著偏离麦氏分布,则需要耦合蒙特卡罗碰撞模型,来精确地描述等离子体内部的动理学行为.此外,通过耦合偏压鞘层模型,还可以自洽地模拟鞘层的瞬时振荡行为对等离子体特性的影响.对于等离子体中的非局域及非热平衡现象,则需要采用基于第一性原理的粒子模拟方法来描述.最后对目前感性耦合放电中的前沿问题进行了展望.     

等离子体鞘层附近尘埃颗粒特性的数值模拟

为了研究尘埃等离子体中尘埃颗粒以及鞘层中粒子密度分布等特性,对尘埃颗粒存在条件下等离子体鞘层结构的采取数值模拟.采用稳态无碰撞的尘埃等离子体鞘层模型,对玻姆判据、尘埃颗粒的荷电性质、平板鞘层区域的电势分布及鞘层内粒子分布特性进行了系统的数值模拟研究.计算结果显示,鞘层边缘尘埃颗粒数密度的增加、尘埃温度的升高,将引起孤立尘埃颗粒对电子吸附能力的减弱,集体效应也受到一定程度的影响;二者同时对离子玻姆速度以及鞘层厚度的增加都有着极大的促进作用.鞘层电势在靠近下极区处降落迅速,主要聚集在接近阴极极板的鞘层区域,各种微粒数密度的空间分布满足准中性条件.     

脉冲束流引出时的等离子体鞘层变化

采用一维无碰撞的动力学鞘层模型计算了脉冲等离子体在恒压引出时的等离子体鞘层厚度变化,分别对短脉冲和长脉冲放电时的离子源发射面演变进行了分析。结果表明:对于短脉冲放电,发射面位置的变化相对等离子体密度的变化存在一定时间的延迟;对于长脉冲的上升沿和直流放电的开启阶段,鞘层厚度变化的速度与离子初始速度相关,稳定后发射面的位置与离子初始速度和等离子体密度的乘积相关。     

碰撞效应对入射到射频偏压电极上离子能量分布和角度分布的影响

考虑了离子与中性粒子的弹性碰撞和电荷交换碰撞效应,建立了一套描述射频等离子体鞘层动力学特性的自洽模型,并利用MonteCarlo模拟方法研究了入射到电极上的离子的能量分布和角度分布.数值结果表明:随着放电气压增加,入射到电极上离子的能量分布逐渐地由双峰分布变成单峰分布,而且低能离子的数目也逐渐地增加.入射到电极上的离子呈小角分布,而且放电气压等参数对角度分布的影响不是太明显. 关键词： 射频放电 等离子体 离子 鞘层     

斜磁场作用下的射频等离子体平板鞘层结构

建立一个一维坐标空间、三维速度空间的斜磁场作用下的射频等离子体平板鞘层模型，讨论了磁场对射频鞘层结构及其参数特性的影响.研究结果表明：磁场对鞘层结构有不可忽略的影响，特别是能够使鞘层边界附近的离子速度分布和密度分布产生明显的变化.此外，虽然磁场不能改变离子总的能量密度分布，却能改变离子的运动状态，并同时影响着基板上离子在各个方向上的能量分布和入射偏移角度. 关键词： 射频 鞘层 磁场     

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基于OOPIC软件,对平面直流磁控溅射放电等离子体进行了二维自洽粒子模拟,重点研究了磁场、阴极电势和气压等工作参数对磁控放电特性的影响。模拟发现,在一定的工作参数范围内,随着磁场的增强,鞘层厚度变窄,鞘层电势降减小,阴极离子密度增大,但是分布变窄;随着阴极电势的增加,鞘层厚度稍微变窄,鞘层电势降增大,阴极离子密度增大,分布变宽;随着气压的升高,鞘层厚度基本不变,鞘层电势降会增大,阴极离子密度先增大后减小,分布略微变宽。     

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采用PIC-MC自洽模型,模拟了氮气电容性耦合射频放电的微观等离子体过程及带电粒子(e,N2+,N+)的行为。结果表明,离子(N2+,N+)的运动状态滞后瞬时射频电场的变化;在两极附近,N2+具较高密度,但能量较低,N+具较低的密度但能量较高,两者的密度差6倍左右。两种离子轰击射频电极的能量分布变化规律类似,随放电参数变化,离子(N2+,N+)能量变化显著,其密度变化不明显。模拟的电子能量几率分布与测量结果一致。     

Reactor modeling of magnetically enhanced capacitive RF discharge

Magnetic and collisional effects on capacitive radio frequency (RF) discharges for magnetically enhanced reactive ion etching (MERIE) are investigated. Using simplified plasma and sheath models, a collisional magnetic-sheath equation that governs the sheath dynamics under a de magnetic field crossed with a sinusoidal RF electric field is obtained. The sheath equation includes global effects of the bulk plasma. Together with the power-balance equation and the particle-conservation equation, the sheath equation is used to extract a circuit model and predict the electrical behavior of MERIE reactors. Numerical results on the plasma density and the power in MERIE reactors agree well with reported experimental results and the circuit model describes the repeated discharge properties well     

Negative Ion Kinetics in RF Glow Discharges

Using temporally and spatially resolved laser spectroscopy, we have determined the identities, approximate concentrations, effects on the local field, and kinetics of formation and loss of negative ions in RF discharges. Cl- and BCl3- are the dominant negative ions found in low-frequency discharges through Cl2 and BCl3, respectively. The electron affinity for Cl is measured to be 3.6118 ± 0.0005 eV. Negative ion kinetics are strongly affected by application of the RF field. Formation of negative ions by attachment of slow electrons in RF discharges is governed by the extent and duration of electron energy relaxation. Similarly, destruction of negative ions by collisional detachment and field extraction is dependent upon ion energy modulation. Thus, at low frequency, the anion density peaks at the beginning of the anodic and cathodic half-cycles after electrons have attached but before detachment and extraction have had time to occur. At higher frequencies, electrons have insufficient time to attach before they are reheated and the instantaneous anion density in the sheath is greatly reduced. When the negative ion density is comparable to the positive ion density, the plasma potential is observed to lie below the anode potential, double layers form between sheath and plasma, and anions and electrons are accelerated by large sheath fields to electrode surfaces.     

Dependence of resonance condition on pressure in an RF resonancemethod

The plasma density is shown as functions of pressure and magnetic flux density in an RF resonance method using the XPDP1 simulation code. The RF resonance method has the unique feature that a strong electric field in bulk controls the plasma density. Owing to the balance between the electric field decrease and the collision rate increase, the plasma density in the RF resonance method has a peak with respect to pressure. The plasma density with respect to the magnetic flux density depends on the condition of the RF resonance method, and the dependence is strong at low pressure because of the strong resonance. Sheath thickness is the most important parameter that determines the strength of the resonance induced. It is shown that the sheath thickness s is related to the plasma density n as a function of ns, obtained from a dispersion relation at constant external parameters. The magnetic flux density which induces the strong resonance is determined from sheath thickness. The plasma density in the RF resonance method can be predicted from discharge parameters using the relation between plasma density and sheath thickness     

ANALYSIS OF DUAL FREQUENCY CONFINED CAPACITIVELY COUPLED PLASMA AT LOW POWER

Confined dual frequency hydrogen plasma discharge has been investigated with microwave interferometer method and radial profiles are taken by Abel inversion technique. Dual radio-frequency sources, operating at 27.12MHz and 1.94MHz, are coupled to each other through the plasma. 27.12MHz RF power is used to enhance plasma density and 1.94MHz power is used to enhance ion acceleration energy to the electrode. Radial density profiles has been taken for comparing the effects of low frequency source that the secondary RF source causes reduction in plasma density due to the sheath expansion. Instead radial density profile is assumed as flat by most of the models, there is about 2.5eV of potential drop occurs from centre to boundary at 40W of primary source power. It has been observed that increasing sheath width (increasing the secondary source power to primary source power) reduces the bulk plasma volume and makes potential profile flattening in y direction. While the high frequency power is dissipated by electrons in the bulk plasma; low frequency power is mostly dissipated by ions in the sheath region. Using both high and low frequency power, we may control plasma density and ion acceleration energy to the electrode simultaneously.     

Numerical study on discharge characteristics influenced by secondary electron emission in capacitive RF argon glow discharges by fluid modeling

A one-dimensional(1D) fluid model of capacitive RF argon glow discharges between two parallel-plate electrodes at low pressure is employed. The influence of the secondary electron emission on the plasma characteristics in the discharges is investigated numerically by the model. The results show that as the secondary electron emission coefficient increases,the cycle-averaged electric field has almost no change; the cycle-averaged electron temperature in the bulk plasma almost does not change, but it increases in the two sheath regions; the cycle-averaged ionization rate, electron density, electron current density, ion current density, and total current density all increase. Also, the cycle-averaged secondary electron fluxes on the surfaces of the electrodes increase as the secondary electron emission coefficient increases. The evolutions of the electron flux, the secondary electron flux and the ion flux on the powered electrode increase as the secondary electron emission coefficient increases. The cycle-averaged electron pressure heating, electron Ohmic heating, electron heating, and ion heating in the two sheath regions increase as the secondary electron emission coefficient increases. The cycle-averaged electron energy loss increases with increasing secondary electron emission coefficient.     

Tonks-Langmuir problem for a bi-Maxwellian plasma

An analytical solution of the Tonks-Langmuir (TL) problem with a bi-Maxwellian electron energy distribution function (EEDF) is obtained for a plasma slab. The solution shows that the ambipolar potential, the plasma density distribution, and the ion flux to the wall are mainly governed by the cold electrons, while the ionization rate and voltage drop across the wall sheath are governed by the hot electrons. The ionization rate by direct electron impact is found to be spatially rather uniform, contrary to the T-L solution where it is proportional to the plasma density distribution. The temperature of hot electrons defined by the ionization balance is found to be close to that of the T-L solution for a mono-Maxwellian EEDF, and is in reasonable agreement with experiments carried out in a low pressure capacitance RF discharge. The energy balance for cold electrons in this discharge shows that their heating by hot electrons via Coulomb interaction is equalized by the cold electrons' escape to the RF electrodes during collapse of the RF sheath     

Nonlinear dynamics of radio frequency plasma processing reactorspowered by multifrequency sources

The source frequency has a strong influence on plasma characteristics in RF discharges. Multiple sources at widely different frequencies are often simultaneously used to separately optimize the magnitude and energy of ion fluxes to the substrate. In doing so, the sources are relatively independent of each other. These sources can, however, nonlinearly interact if the frequencies are sufficiently close. The resulting plasma and electrical characteristics can then be significantly different from those due to the sum of the individual sources. In this paper, a plasma equipment model is used to investigate the interaction of multiple frequency sources in capacitively and inductively coupled RF excited plasmas. In capacitively coupled systems, we confirmed that the plasma density increases with increasing frequency but also found that the magnitude of the DC bias and DC sheath voltage decreases. To produce a capacitively coupled discharge having a high plasma density with a large DC bias, we combined low and high frequency sources. The plasma density did increase using the dual frequency system as compared to the single low frequency source. The sources, however, nonlinearly interacted at the grounded wall sheath, thereby shifting both the plasma potential and DC bias. In inductively coupled plasmas (ICP), the frequency of the capacitive substrate bias does not have a significant effect on electron temperature and density. The DC bias and DC sheath voltage at the substrate were, however, found to strongly depend on source frequency. By using additional RF sources at alternate locations in ICP reactors, it was found that the DC bias at the substrate was varied without significantly changing other plasma parameters, such as the substrate sheath potential     

Matching an RF sheath model to a bulk plasma model

In this paper, we present a combined plasma-sheath model designed for the study of high density discharges, or other systems with thin sheaths. Sheaths in high density plasmas are typically less than 1 mm in thickness. When modeling multidimensional discharges, fully resolving the sheaths can be prohibitively expensive computationally, especially when RF power is coupled capacitively into the discharge. However, the sheath impedance often strongly affects instantaneous and period-averaged plasma potential, which in turn can strongly influence crucial processing characteristics such as the ion energy and angular distributions impacting surfaces. In the combined plasma-sheath model we present, the sheaths are treated independently from the plasma region, and different length scales are employed for each. The Godyak-Sternberg sheath model [Phys. Rev. A, 42, 2299 (1990)] is used to represent the sheaths. The bulk plasma portion of the discharge is represented using a fluid model. Boundary conditions at the plasma-sheath interfaces transfer information dynamically between the sheath and bulk plasma portions of the model. Results from the combined plasma-sheath model are compared to results from a discharge model that fully resolves the sheaths, with generally good to excellent agreement     

Parameter space for collisionless RF sheaths

As plasma processing reactors approach higher density, the sheath models which neglect the radio frequency (RF) response of the ions become invalid. This work show that the nature of the collisionless RE ion sheath can be described in a number of different regimes of parameter space. These regimes can all be visualized on a single two-dimensional (2-D) plot where the horizontal axis is the ion plasma frequency divided by the frequency, and the vertical axis is the electron oscillating velocity divided by the ion sound speed     

Effects of gas pressure on plasma characteristics in dual frequency argon capacitive glow discharges at low pressure by a self-consistent fluid model

A self-consistent fluid model for dual radio frequency argon capacitive glow discharges at low pressure is established.Numerical results are obtained by using a finite difference method to solve the model numerically, and the results are analyzed to study the effect of gas pressure on the plasma characteristics. It shows that when the gas pressure increases from 0.3 Torr(1 Torr = 1.33322×10~2 Pa) to 1.5 Torr, the cycle-averaged plasma density and the ionization rate increase;the cycle-averaged ion current densities and ion energy densities on the electrodes electrode increase; the cycle-averaged electron temperature decreases. Also, the instantaneous electron density in the powered sheath region is presented and discussed. The cycle-averaged electric field has a complex behavior with the increasing of gas pressure, and its changes take place mainly in the two sheath regions. The cycle-averaged electron pressure heating, electron ohmic heating, electron heating, and electron energy loss are all influenced by the gas pressure. Two peaks of the electron heating appear in the sheath regions and the two peaks become larger and move to electrodes as the gas pressure increases.