Sheath motion in a capacitively coupled radio frequency discharge

The sheath motion in a capacitively coupled RF discharge is highly nonlinear. The voltage waveform on a cylindrical probe placed in the sheath region is measured as a function of position and time. A circuit model of the probe-discharge system relates the observed probe voltage to the sheath motion. The equations derived from this circuit model are solved numerically with varying nonlinear sheath motions; the resulting waveforms are compared with the experimental observations to determine the actual sheath motion. The time-varying plasma potential is also determined, indirectly, from the comparison. The authors also report observation of oscillations related to the plasma frequency, whose peak harmonic component can be calculated from a single plasma model. These oscillations can be a useful plasma diagnostic for determining plasma density. The presence of these high-frequency oscillations may significantly enhance the rate of stochastic heating of electrons     

Frequency effects in capacitively coupled radio-frequency glowdischarges: a comparison between a 2-D fluid model and experiments

The results of a 2-D fluid model for argon radiofrequency (RF) discharges in a closed cylindrical vacuum chamber are compared with experimental data from an amorphous silicon deposition reactor operated in argon. Good agreement is obtained for the relation between the DC autobias voltage and the dissipated power in the frequency range 40-100 MHz at pressures between 10 and 60 Pa. Scaling laws are presented for the dissipated power and for the ion fluxes toward the electrodes. These quantities are expressed in the DC bias voltage, the RF excitation frequency and the background pressure. Also the uniformity of the ion fluxes is studied. The model yields a linear relation between the applied RF voltage and the DC bias voltage. This relation depends only on the geometry of the discharge chamber and shows an offset     

Influence of a centered dielectric tube on inductively coupled plasma source: Chamber structures and plasma characteristics

A high-density RF ion source is an essential part of a neutral beam injector. In this study, the authors attempt to retrofit an original regular RF ion source reactor by inserting a thin dielectric tube through the symmetric axis of the discharge chamber. With the aid of this inner tube, the reactor is capable of generating a radial magnetic field instead of the original transverse magnetic field, which solves the E × B drift problem in the current RF ion source structure. To study the disturbance of the dielectric tube, a fluid model is introduced to study the plasma parameters with or without the internal dielectric tube, based on the inductively coupled plasma(ICP) reactor. The simulation results show that while introducing the internal dielectric tube into the ICP reactor, both the plasma density and plasma potential have minor influence during the discharge process, and there is good uniformity at the extraction region. The influence of the control parameters reveals that the plasma densities at the extraction region decrease first and subsequently slow down while enhancing the diffusion region.     

液相放电等离子体特性与激波特性的数值分析

以能量平衡方程为基础,采用不同的电导率唯象模型描述了液相放电等离子体圆柱形通道特性,得到了通道内半径、温度、电阻、电流和耗散能量随时间的变化关系,还给出了距离放电间隙中心一定距离处的冲击波压力变化,并与前人利用等离子体通道球状模型计算得到的结果进行了比较。结果表明:把等离子体通道看成球状和看成圆柱状在描述通道压力和通道半径时差异显著,而在描述其他物理特性时差别不大;三种电导率模型在描述等离子体通道物理特性时,变化趋势大体相同,而在描述激波特性时,电导率模型σ₂更符合实际;通过对比电学参数与压力参数的变化,就可以在实验中根据实验数据以及具体的研究问题进行模型的适用性选择。     

An experimental study of the plasma parameters of a hybrid low-pressure RF discharge

This paper presents an experimental study of the plasma parameters of a hybrid radio-frequency (RF) discharge, which is a new modification of an RF discharge. The discharge is maintained by both vortex and potential RF electrical fields. To generate a hybrid RF discharge, an RF power input unit is used in the form of a parallel-connected inductor and capacitor coatings. A blocking capacitor is included in the capacitive channel of the discharge. The paper presents data from the study of the influence of power coupled to the plasma, argon pressure, and blocking capacitance on the plasma parameters, i.e., the electron energy distribution, temperature, and density and plasma potential. The role of the capacitive channel in the variations in properties and the characteristics of the discharge are considered.     

Experimental study of patterns of energy input into hybrid low pressure discharge for varying RF generator power and argon pressure

Patterns of energy release in the plasma of a new modification of RF discharge, viz., hybrid RF discharge, which are maintained by vortex and potential RF electric fields, are experimentally studied. RF power input unit in the form of inductor and capacitor plates connected in parallel are used for initiating this discharge. This paper presents data obtained during the investigation of the influence of power supplied to the plasma and argon pressure on the equivalent resistance of the hybrid discharge, which is the measure of plasma capability of absorbing RF power. The role of the capacitive channel in the balance of RF power supplied to the discharge is considered.     

Impedance model of helical resonator discharge

Modeling and measurement results of the RF input impedance of a helical resonator discharge are given. A simple microstrip transmission line model is used to extract an equivalent circuit of a helical resonator discharge. The transmission line model consists of a grounded cylinder for RF shielding, a helical copper coil for RF heating, and a high density plasma. Transmission line parameters for an experimental helical resonator discharge chamber are extracted, and the results are compared with the measured results. The measured transmission line parameters for the experimental reactor are the characteristic impedance of 200 Ω, the attenuation constant of 6×10^-3 Np/m, and the wave number of 0.36 rad/m, which have good agreement with the calculated values     

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     

One atmosphere glow discharge structure revealed by computermodeling

A one-dimensional and self-consistent model of an atmospheric pressure, dielectric controlled, parallel-plate RF discharge is presented. The computer modeling provided information about the unexpected uniform glow structure of this discharge. The calculated parameters of the plasma, including the electric field, the potential, and the space charge density are analogous to those observed in a normal DC glow discharge. Images of plasma properties are presented and discussed     

Effect of finite charging time on particulates in RF discharges

The effect of the finite charging time on spherical and cylindrical particulates in an RF discharge is considered. Using probe theory, analytical expressions for the charging rate under various conditions are derived and compared with numerical solutions. Scaling of the charging time with discharge parameters, in particular the electron temperature, is demonstrated. Using a one-dimensional fluid model for an rf discharge, the equilibrium electric and ion drag forces are compared for spherical and cylindrical particulates. The effect of the finite charging time on the dynamics of particulates of various sizes in the model discharge are discussed. Overall, a long cylindrical particle with the same mass as a spherical particle charges up more slowly to a larger net (negative) charge and has a significantly larger polarization. As a consequence, small cylindrical particles introduced near the electrodes are more likely to escape from the sheath region before becoming fully charged     

柱面天线射频感性耦合等离子体放电模式特性的实验研究

利用Z-scan、电流、电压探头,通过测量等离子体吸收功率、天线电流、电压、等离子体直流悬浮电位等多种参数,研究了匹配网络、天线耦合强度、导电地面积、气压等多种因素对E,H放电模式特性及模式转化行为的影响.基于Γ型阻抗匹配网络中串联电容对射频电源输出功率的影响,提出了E—H放电模式转化的正负反馈区概念.研究发现：在相同的其他放电条件下,处于正反馈区时等离子体放电易于产生跳变型模式转化,而且模式跳变的临界天线电流、回滞宽度、跳变临界功率、跳变功率差等参数均随阻抗匹配网络参数产生明显变化;在负反馈区内,模式转化过程趋于连续.由于阻抗匹配网络的影响,E—H模式的跳变电流并不是总大于H—E模式的跳变电流.在不同导电地面积、阻抗匹配网络、气压下,模式转化过程中等离子体直流悬浮电位的变化呈现多样性. 关键词： 射频等离子体 感性耦合 容性耦合 模式转化     

Improvement of the efficiency of a glow discharge-based ion emitter with oscillating electrons

Ion emission from the plasma of a low-pressure (≈5×10⁻² Pa) glow discharge with electrons oscillating in a weak (≈1 mT) magnetic field is studied in relation to the cold hollow cathode geometry. A hollow conic cathode used in the electrode system of a cylindrical inverted magnetron not only improves the extraction of plasma ions to ≈20% of the discharge current but also provides the near-uniform spatial distribution of the ion emission current density. The reason is the specific oscillations of electrons accelerated in the cathode sheath. They drift in the azimuth direction along a closed orbit and simultaneously move along the magnetic field toward the emitting surface of the plasma. A plasma emitter with a current density of ≈1 mA/cm² over an area of ≈100 cm² designed for an ion source with an operating voltage of several tens of kilovolts is described.     

A self-consistent model for the production and growth of nanoparticles in low-temperature plasmas

A theoretical global model is presented for describing the kinetics of generation and growth of clusters and nanoparticles in low-pressure plasmas, where important processes for clusters and grains are collisions with monomers, electrons, and ions and particle coagulation and loss because of diffusion and gas flow drag. Simple equations are given for calculations of monomer density, particle-size distribution function, critical cluster size, the rate of particle production and particle density and mean size, and plasma characteristics (the densities and average energies of “cold” and “hot” electrons and the density of positively charged ions). The model is self-consistent; that is, the above-mentioned properties of clusters, nanoparticles, electrons, and ions are calculated jointly from coupled equations as functions of a small number of radio frequency (RF) discharge parameters (discharge geometry; absorbed electric power; voltage across the RF sheath; gas pressure; composition; and flow rate). Comparisons are made with the experimental data on SiH₄-Ar mixtures. Published in Russian in Khimicheskaya Fizika, 2008, Vol. 27, No. 4, pp. 79–93. The article was translated by the authors.     

具有空心阴极放电特征的射频放电的两电子组模型

 运用两电子组模型，考虑了射频放电中的α过程和γ过程两种电离机制，并结合流体模型，研究了中等气压下窄电极间隙容性耦合射频放电在运行模式转变区的等离子体密度以及电离速率分布等特性。理论研究表明，γ电离过程在高电流模式运行中起主要作用，并证实了此类放电中存在显著的电子摆钟效应，具有类似于空心阴极放电的特征。     

The Radio Frequency Hollow Cathode Discharge Induced by the RF Discharge in the Plasma‐Jet Chemical Reactor

In the recent decade an RF driven, low‐pressure plasma reactor with supersonic plasma jet was developed (RPJ). This reactor was successfully used for deposition of thin films of various materials. The deposition of thin films indicates that the properties of the deposited films are dependent on the sputtering or reactive sputtering processes appearing inside the nozzle (hollow athode). The nozzle (hollow athode) fabricated of different kinds of materials and alloys works both as a cathode of the radio frequency (RF) hollow cathode discharge and as a nozzle for plasma jet channel generation as well. The RF hollow cathode discharge is a secondary discharge, which is induced by the primary RF plasma generated in the reactor chamber. The present paper deals with the experimental study of this RF hollow cathode discharge. The stress is laid on the investigation of the axial distribution of discharge parameters and sputtering processes inside the nozzle. On the base of experiments, the simple model of the axial distribution of the investigated RF hollow cathode discharge has been developed.     

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为探究射频离子源驱动器线圈电气参数对射频放电的影响,主要进行了射频离子源等离子体激发的物理分析,并计算了射频电源的频率选择与线圈放电电流、线圈匝间电压以及放电气压之间的关系,设计了射频离子源驱动器的主要参数。研制的射频离子源驱动器装置,成功获得氢等离子体射频放电。实验结果和理论计算结果符合很好。     

射频放电阻抗测量用于等离子体诊断研究

利用自行研制的传感器和测量装置,通过对射频放电电压、电流以及其相位角的精确测定,算出放电管的总阻抗,结合放电管的等效电路模型与Godyak等建立的射频放电模型,对射频激励铜离子激光管在氦气中的放电特性进行了研究,得出射频激励铜离子激光器不同气压及电流密度下的等离子体电阻、容抗、鞘层厚度及电子密度 关键词： 射频放电 阻抗测量 等离子体诊断     

大面积NBI 射频离子源电气参数设计分析

为探究射频离子源驱动器线圈电气参数对射频放电的影响,主要进行了射频离子源等离子体激发的物理分析,并计算了射频电源的频率选择与线圈放电电流、线圈匝间电压以及放电气压之间的关系,设计了射频离子源驱动器的主要参数。研制的射频离子源驱动器装置,成功获得氢等离子体射频放电。实验结果和理论计算结果符合很好。     

Electrodynamic Model of the Gas Discharge Sustained by Azimuthal Surface Waves

Results of studying an electrodynamical model of the plasma producer based on azimuthal surface waves (ASW) propagation both in a cylindrical metal waveguide filled by magnetoactive plasma and along metal cylindrical antenna placed into the plasma are presented. Parameters of these gas discharges are studied in the two frequency ranges where propagation of the ASW is possible and also are compared. The value of plasma density that can be achieved by this producer is calculated. The spatial distribution of the wave electromagnetic fields, dependencies of their penetration depth into plasma and angular discharge length on the discharge parameters are studied analytically and numerically. Advantages of the operation in the both frequency ranges are indicated. Possibility of resonant absorption of the ASW energy due to their conversion into the bulk upper hybrid mode is examined as well.     

Intense emission from a grid-stabilized plasma cathode

Intense emission from a grid-stabilized plasma cathode based on a glow discharge with an expanded anode area is studied. In the electrode system of the ion source, the potential difference between a large-mesh grid electrode (a hole diameter of 4–6 mm) and cathode and anode plasma reaches 200 V and the glow discharge current is up to 1 A. The current distribution over the electrodes of the plasma cathode is taken, and the dependences of the electron extraction efficiency and electron-emitting-plasma potential on the gas pressure and discharge parameters are obtained. A relationship between the geometric parameters of the grid, cathode plasma potential, and efficiency of electron extraction from the plasma is derived. It is shown that stable intense emission from the plasma cathode can be provided in wide ranges of gas pressure and discharge current by varying the geometry and mesh size of the plasma cathode grid. Discharge contraction in the grid plane at elevated gas pressures is explained. It is assumed that the emitting plasma becomes inhomogeneous due to variation in the thickness of near-electrode layers in the holes of the grid, which makes the distribution of the emission current from the plasma more nonuniform.