Electrical characteristics of parallel-plate RF discharges in argon

Electrical characteristics were measured in a parallel-plate, capacitively coupled (E-type), low-pressure, symmetrical RF discharge driven at 13.56 MHz. The discharge voltage, current, and phase shift between them were measured over a very wide range of discharge parameters (gas pressures between 3 mtorr and 3 torr with discharge power between 20 mW and 100 W). From these measurements the discharge impedance components, the power dissipated in the plasma and in the sheaths, the sheath width, and the ion current to the RF electrodes were found over a wide range of discharge conditions. Some of the general relationships between the various measured and determined parameters are discussed. The experimental results can be used as a database for straightforward comparison with existing RF discharge models and numerical simulations     

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     

Reply to comments on "On asymptotic matching and the sheath Edge"

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=kT/sub e//(e/spl 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 /spl epsi/=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)].     

A comparison of RF electrode sheath models

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     

Good news: you can patch active plasma and collisionless sheath

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.     

Nonlocal electron kinetics in collisional gas discharge plasmas

Nonlocal phenomena in electron kinetics of collisional gas discharge plasmas, their kinetic treatment by a nonlocal approach, and relevant experimental results are reviewed in this paper. Using the traditional two-term approximation for the electron distribution function, a general method to analyze electron kinetics in nonuniform plasmas in DC and RF fields for atomic gases is presented for the nonlocal case, when the electron energy relaxation length exceeds the characteristic spatial scale of bounded plasmas. The nonlocal method, which is based on the great difference between the electron mean free path for the momentum transfer and the electron energy relaxation length, considerably simplifies the solution of the kinetic equation and, in a number of cases, allows one to obtain analytical and semi-analytical solutions. The main simplification is achieved for trapped electrons by averaging the Boltzmann equation over space and fast electron motion. Numerous examples of spatial nonlocality are considered in the positive column and near the electrodes of DC glow discharges, in spatial relaxation of the electron distribution and in striations, and in capacitively and inductively coupled low-pressure RF discharges. The modeling of fast beam-like electrons is based on a continuous-energy-loss approximation with the assumption of forward scattering. Simple analytic expressions for the fast electron spectrum are obtained in cathode regions of DC discharges with planar and hollow cathodes     

Smooth plasma-sheath transition in a hydrodynamic model

In the hydrodynamic plasma and sheath models, when the two models are treated separately, the use of Bohm's (1949) criterion to obtain boundary conditions at the plasma-sheath interface leads to singularities and discontinuities in some physical quantities. This problem can be overcome by using an appropriate boundary condition; namely, by specifying a particular finite value of the electric field at the plasma-sheath interface. Using such a boundary condition and arbitrary collision parameters, sheath characteristics are then calculated continuously from the collisionless to the collisional sheath. When applied to the plasma model, this new boundary condition takes into account the space-charge effect on the behavior of the plasma at its boundary