A theory of the positive column in electronegative gases based on fluid-type momentum equations to describe charged particle motion is presented. It is assumed that quasi-neutrality conditions prevail and the ion inertial terms are neglected. The positive ions are assumed to be created by electron collisions with neutral molecules and the negative ions to be formed by dissociative electron attachment and destroyed by detachment in reactions with neutral species. The mathematical formulation consists of a two-point boundary value problem involving two independent parameters, functions of collisional and transport data, and two eigenvalues. One of these is the central ratio of the negative ion density to the electron density, while the other is related to the ionisation-loss balance and embodies a discharge characteristic for the maintenance field. These eigenvalues and the radial density distributions of the charged species were calculated for a wide range of variation of the independent parameters. An application of the theory to a positive column in oxygen is given as an illustrative example.

This paper reviews the formulation and updates some numerical procedures usually adopted in two-dimensional, time-dependent fluid models to study the transport of charged particles in radio-frequency capacitively coupled discharges. The description of charged particle transport is made by solving the continuity and momentum transfer equations for electrons and ions, coupled with Poisson's equation and the electron mean energy transport equations. Inertia terms are considered in the ion momentum transfer equations, by generalizing the earlier definition of effective electric field. The electron mean energy equations are written using specific energy transport parameters, deduced from integration over the electron energy distribution function (EEDF). The model adopts the local mean energy approximation, i.e. it computes the electron transport parameters as a function of the electron mean energy, using either a homogeneous, two-term Boltzmann equation solver or a Maxwellian EEDF. More appropriate boundary conditions for the electron and ion fluxes are used successfully. The model is solved for a GEC Cell reactor type (with 6.4 cm radius and 3.2 cm interelectrode distance) operating at frequency 13.56 MHz, pressures between 10 mTorr and 10 Torr and applied voltages from 100 to 500 V, in electropositive (helium) and electronegative (silane-hydrogen) gases or gas mixtures. The ion kinetics in silane and hydrogen is updated with respect to previous works, by further considering SiH + 2 , H + and H + 3 ions. In general, simulation results for some typical electrical parameters are closer to experimental measurements available than calculations reported in previous works.

A kinetic model for the low-pressure oxygen positive column is presented and discussed. The model is based on the electron Boltzmann equation and the rate balance equations for the dominant heavy-particle species, which are solved simultaneously in order to take into account the coupling between the electron and the heavy-particle kinetics. The effects of vibrationally excited molecules, dissociated atoms and metastable states on the electron kinetics are analysed in detail. The predicted populations of O2(X3 Sigma ), O2(a1 Delta ), O(3P), and O- are shown to agree satisfactorily with previously reported measurements. A combination of this kinetic model with the continuity and transport equations for the charged species e, O-, and O2+ is shown to provide characteristics for the maintenance field that agree reasonably well with experiment.

A two-dimensional numerical code, including three fluid modules to account for the description of electrical, thermal and chemical phenomena, has been developed for the modelling of hydrogenated amorphous silicon deposition from SiH 4 -H 2 radio-frequency glow discharges in a cylindrical PECVD reactor. The results of the model are compared to experimental data, obtained by different diagnostic techniques. The calculated radical densities are compared to those measured by threshold ionization mass spectrometry, at the centre of the substrate; the calculated SiH density profile between the electrodes is compared to those measured by laser-induced fluorescence and the radial distribution of the deposition rate on the substrate is compared to profilometry measurements. Globally, the model correctly predicts the main discharge characteristics for experimental conditions normally used for amorphous silicon deposition in the dust-free regime. The moderate agreement between model and experiment occurring for the hydrogen-dominated condition can be attributed to the simplified surface kinetics adopted in the model.