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An analysis of plasma density distributions at arbitrary ion–atom collisionality for one-dimensional axially symmetrical cylindrical and annular plasmas is presented. Perturbations of plasma densities caused by a cylindrical probe are studied for arbitrary ion–atom collisionality. Analytical expressions for the plasma characteristics near the probe for low collisionality have been obtained. The plasma was modeled by the hydrodynamic neutral plasma equations, taking into account ionization, ion inertia, and a non-linear ion frictional force, which dominates the plasma transport at low gas pressures. Significant plasma density depletion around the probe has been observed for a wide range of ion–atom collisionality. The presented results predict underestimation of plasma density obtained from the classical Langmuir probe procedure and should provide a better understanding of electrostatic, magnetic, and microwave probes inserted into plasmas at low gas pressure. [ABSTRACT FROM AUTHOR]

In this paper, we discuss a number of problems found in the literature related to experimental measurements of rf discharge electrical and plasma parameters with different electromagnetic probes. Incorrect evaluations of discharge power and the inaccurate measurement of basic plasma parameters with electrical (Langmuir), magnetic (B-dot), and microwave probes are among the troubling issues found in many recent publications on rf plasma. The purpose of this review is to show the origination of errors and ways to their mitigation based on the three-decade development of contemporary rf discharge diagnostics. [ABSTRACT FROM AUTHOR]

The feasibility of helicon plasma thrusters, commonly considered the most promising for deep space propulsion, is evaluated in terms of their plasma production efficiency, longevity, operation stability, and need for massive magnets with large power consumption. It is shown that helicon thrusters should be inferior to those based on inductively coupled plasmas. [ABSTRACT FROM AUTHOR]

The plasma-sheath model and the Bohm criterion introduced by Bohm are among the earliest attempts to separately model the plasma and the sheath and to find a way to join the plasma and the sheath solutions. Although there is hardly a paper on plasma-sheath modeling that does not quote the Bohm criterion, Bohm's paper and his results are widely misunderstood. The reason for this is that, in his paper, Bohm himself misinterpreted his result by concluding that the sheath edge coincides with the reference point of his plasma-sheath model. As a result, the criterion for the reference point obtained by Bohm to ensure monotonicity of his sheath solution (i.e., the Bohm criterion) was erroneously applied to the sheath edge and was used in literature as a criterion for sheath formation. In this paper, we show that the Bohm criterion when applied to the sheath edge contradicts Bohm's own definition of the sheath and cannot be obtained from Bohm's plasma-sheath model. Index Terms--Bohm criterion, plasma-sheath model.

Nonequilibrium effects associated with spatial and temporal nonlocality between electron energy distribution and electromagnetic field in gas discharge plasmas at low gas pressures are reviewed in this paper. Formation of nonequilibrium EEDF is discussed for capacitive and inductive radio-frequency discharges. The possibility of electron temperature control is considered for gas discharge plasmas at nonequilibrium condition. Index Terms--Electron energy distribution function (EEDF), electron kinetics, gas discharge, nonequilibrium.

In our recent paper [Sternberg and Godyak (2004)], we have solved the plasma-sheath problem using asymptotic matching techniques. We have shown that the position of the sheath edge obtained by asymptotic matching yields the position of the ion sheath edge determined by Godyak's condition (E = k[T.sub.e]/(e[[lambda].sub.D1])) [Godyak (1982)]. In addition, we were able to show how the sheath width depends on the voltage applied to the wall. Our results are theoretical, grounded in mathematics. We used numerical computations mainly to illustrate our point. We have written our paper with great care, so that any scientist, even one not familiar with the intricacies of asymptotic matching, could understand our exposition and be convinced in the correctness of our results. Kaganovich (2004) reproduced our theoretical results numerically up to E = [10.sup.-11]. Franklin (2004) published a review adapting our results and interpretations. However, we were not able to convince Riemann, although he reproduced some of our results numerically [Riemann (2004)]. This paper is a response to Riemann's comments [Riemann (2004)].

This paper is concerned with patching the plasma and the sheath solutions for collisionless ions, in order to approximate the full solution of the plasma-wall problem from the center to the wall. A method to find an adequate patching point is developed, and analytic expressions for the plasma characteristics at the patching point are given. The patched solution gives a good approximation of the full solution and can replace the full solution for all practical purposes. The method presented in this paper does not depend on any parameter and can, therefore, be used even when the solution of the plasma-wall problem cannot be computed numerically. Index Terms--Patching, plasma, plasma-wall problem, sheath.

The position of the sheath edge and the value of the electric field at the sheath edge are found using asymptotic matching techniques for collisionless plasma-wall systems. It is demonstrated that specifying the position of the sheath edge by the Bohm criterion or by Riemann's weak electric field is inconsistent with the asymptotic matching theory and contradicts the commonly accepted definition of the sheath as a region where ionization can be neglected. It is shown that the sheath found using the asymptotic matching theory is the ion sheath, characterized by negligible electron density, and that the value of the electric field at the sheath edge practically coincides with the value of Godyak's electric field. In addition, analytical expressions are given for the position of the wall, the sheath width, and sheath characteristics for arbitrary potentials at the wall. Index Terms--Asymptotic matching, patching, plasma, plasma-wall problem, sheath, sheath edge.

The present paper is concerned with the effects of boundary conditions on the electrical sheath characteristics in the dynamic rf sheath model. The dynamic r sheath model, which is described by a moving boundary problem, is solved analytically and numerically in the collisionless case for arbitrary values of the electric field at the plasma-sheath interface. The results that are obtaine show that even for relatively high rf sheath voltages, the rf sheath capacitanc and the dc sheath voltage are influenced in a significant way by the choice of the boundary condition for the electric field.

Analytic expressions have been found from different time-averaged and dynamic models for the electrode sheath width in a capacitively driven RF discharge. The sheath widths predicted from all the models under consideration are shown to practically coincide if they are expressed in terms of dc sheath voltage and ion current to the RF electrode. However, these models have fundamental differences in their RF discharge scaling laws that result in different theoretical predictions for RF discharge electrical characteristics. Analytic expressions for the sheath width with arbitrary collisionality at moderate RF voltages are found to be in good agreement with experimental measurements made over a wide range of discharge voltage and gas pressure.

A material etching system was developed by combining beam electron injection from a direct current hollow cathode (HC) electron source with the downstream reactive environment of a remote CF4/O2 low temperature plasma. The energy of the injected beam electrons is controlled using an acceleration electrode biased positively relative to the HC argon discharge. For an acceleration voltage greater than the ionization potential of Ar, the extracted primary electrons can produce a secondary plasma in the process chamber. The authors characterized the properties of the secondary plasma by performing Langmuir probe measurements of the electron energy probability function (EEPF) 2.5 cm below the extraction ring. The data indicate the existence of two major groups of electrons, including electrons with a primary beam electron energy that varies as the acceleration voltage is varied along with low energy electrons produced by ionization of the Ar gas atoms in the process chamber by the injected beam electrons. When combining the HC Ar beam electron with a remote CF4/O2 electron cyclotron wave resonance plasma, the EEPF of both the low energy plasma electron and beam electron components decreases. Additionally, the authors studied surface etching of Si3N4 and polycrystalline Si (poly-Si) thin films as a function of process parameters, including the acceleration voltage (0-70 V), discharge current of the HC discharge (1-2 A), pressure (2-100 mTorr), source to substrate distance (2.5-5 cm), and feed gas composition (with or without CF4/O2). The direction of the incident beam electrons was perpendicular to the surface. Si3N4 and polycrystalline silicon etching are seen and indicate an electron-neutral synergy effect. Little to no remote plasma spontaneous etching was observed for the conditions used in this study, and the etching is confined to the substrate area irradiated by the injected beam electrons. The electron etched Si3N4 surface etching rate profile distribution is confined within a ~30 mm diameter circle, which is slightly broader than the area for which poly-Si etching is seen, and coincides closely with the spatial profile of beam electrons as determined by the Langmuir probe measurements. The magnitude of the poly-Si etching rate is by a factor of two times smaller than the Si3N4 etching rate. The authors discuss possible explanations of the data and the role of surface charging. [ABSTRACT FROM AUTHOR]

It is shown that the proof of physical inconsistency of the discrete plasma-sheath model with a continuous electric field is based on mathematical error in solving the basic sheath equation. Index Terms--Plasma sheaths.