Vladimir D. Stojanovic

The role of heavy particles in kinetics of low current discharges in argon at high electric field to gas number density ratio

We have developed a coupled system of Monte Carlo simulation codes for modeling of electron, ion, and fast neutral kinetics in low pressure, low current discharges. The procedure was tested for ion drift tube data by using different cross-section sets including the recent anisotropic set of Phelps [A. V. Phelps, J. Appl. Phys. 76, 747 (1994)]. The code was applied to modeling absolute emission profiles obtained at very high electric field to gas number density ratio, where heavy particle excitation dominates electron excitation by more than two orders of magnitude, and also to an inhomogeneous field experiment of Scott and Phelps [D. A. Scott and A. V. Phelps, Phys. Rev. A 43, 3043 (1991)]. The qualitative agreement between the Monte Carlo and experimental results is good, giving confidence in the available models and data for ion and fast neutral transport in gas discharges, and for their interaction with surfaces.

Comparison of the results of Monte Carlo simulations with experimental data for electron swarms in from moderate to very high electric field to gas density ratios

A code for Monte Carlo simulation (MCS) of electron motion adjusted to include boundary and non-equilibrium effects is developed, tested and applied to model low-pressure, low-current, moderate to very high experiments in nitrogen. It has the flexibility to include effects of anisotropy, non-conservative collisions, steady-state Townsend (SST) conditions and electron reflection and backscattering at the anode. The results of the simulation are compared with available experimental data such as spatial distribution of emission of first negative band in nitrogen, ionization coefficients and charge multiplication. Good agreement exists between the results of simulations and the experimental data indicating that the basic understanding of non-equilibrium development is available, while some differences point to the need to adjust the corresponding cross sections or models of scattering.

Excitation by electrons and fast neutrals in nitrogen discharges at very high electric field to gas number density ratios

A Monte Carlo code for simulation of electron, ion and fast neutral transport was developed to model the spatial distribution of excitation under nonequilibrium conditions at very high electric field to gas number density ratios (E/N) and low currents. The code includes the most detailed representation of electron scattering and transport including the reflection and multiplication at the anode and simulation of ion and fast neutral transport for realistic geometry of the experiments of B. M. Jelenkovic and A. V. Phelps [Phys. Rev. A 36, 5310 (1987)] and V. T. Gylys, B. M. Jelenkovic, and A. V. Phelps [J. Appl. Phys. 65, 3369 (1989)]. A good agreement between the simulations and experimental data was achieved by using reasonable data for scattering cross sections from the literature. Simulations confirm the model proposed by A. V. Phelps, B. M. Jelenkovic and L. C. Pitchford [Phys. Rev. A 36, 5327 (1987)] of transport and excitation kinetics at very high E/N where fast neutral excitation is the dominant p...

Resonant vibrational excitation/de-excitation of N2 (v) by electrons

In this paper we have applied a semi-empirical theory of Mihajlov and Pivovar to calculate the cross sections and rate coefficients for excitation from one vibrationally excited level (v) of nitrogen to another (k). Calculations were performed for 0 v, k8, which is the limit set by the applicability of the model. The semi-empirical model assumes that the process of excitation to a higher vibrationally excited state may be split into two separate parts, which is justified if the lifetime of the negative ion compound state is much longer than the flight time of the projectile electron across the dimension of the molecule. The semi-empirical model calculations were based on the best available experimental data for elastic scattering of electrons and vibrational excitation from the ground state. Results for all combinations of the vibrational quantum numbers were obtained. Both Maxwellian and non-equilibrium rate coefficients were determined and it was found that there is a great difference between the two sets of coefficients due to a very specific shape of the electron energy distribution function in nitrogen at moderate E/N (where E is electric field and N is gas number density).

Electron transport coefficients in mixtures of CF4 and CF2 radicals

Electron kinetics determines the rate of production of chemically active species in processing plasmas. Precise transport coefficients are needed to describe conditions such as those found in plasma assisted technologies for semiconductor production, but these are affected by the density of free radicals, which in themselves depend on the chemical kinetics. We present transport coefficients for electrons in mixtures of CF4 with CF2 (and we also show similar results for other radicals) for ratios of the electric field to the gas number density E/N from 1 to 1000 Td (1 Td = 10−21 V m2). Our analysis of non-conservative collisions revealed a range of E/N where electron attachment to radicals significantly changes the electron kinetics compared with pure CF4 gas. The results are obtained using simple solutions for Boltzmanns equation and exact Monte Carlo simulations.

Energy distributions of electrons in a low-current self-sustained nitrogen discharge

Electron energy distribution functions (EEDF) in N2 Townsend discharges for E/N between 1 and 30 kTd (1 Td=10−21 V m2) were measured using a multigridded energy analyzer behind a small (0.1 mm) aperture in the graphite anode. Experimental results are further analyzed by applying Monte Carlo simulations in order to describe nonequilibrium transport in steady state Townsend discharges and to determine the origin of the observed EEDF features. Boundary effects at electrodes are described by allowing exact representation of absorption, reflection, and secondary electron production at the anode. It was found that it is necessary to include electron reflection and secondary electron production in order to model the low energy part of the observed EEDF.

Modelling of anomalous Doppler broadened lines, thermalization of electrons and the role of radicals in discharges at high E/N

In this paper we show how Monte Carlo technique developed for swarm studies may be applied to model some real discharges or to provide data for realistic plasma models. In all cases discussed here the Monte Carlo technique without self consistent field is driven to a much greater level of complexity than usually found in swarm studies. We discuss three conceptually quite different situations that require exact modelling by Monte Carlo Simulations (MCS). The first is the case of transport in gases where degree of dissociation is large and thereby radicals formed by the discharge may affect the transport coefficients. Based on recent calculations for electron CFX radical cross sections we calculated how transport coefficients would be affected by the presence of radicals in pure CF4 and CF4/Ar mixtures covering range of abundances often found in plasma etching devices. The second case studied here is modelling of thermalization of high energy electrons in the atmospheric gases as an attempt to provide the basis for detection of very high energy cosmic particles. In that case pressure dependent emission efficiency is studied as a function of electron energy and the system is developed to an arbitrary degree of secondary electron production. Finally we give some recent advances and current status of modelling of high E/N discharges operating in Townsends regime in hydrogen where anomalous broadening was observed and explained by fast neutral excitation in both directions along and against the acceleration due to electric field.

Electron, Ion and Atom Collisions Leading to Anomalous Doppler Broadening in Hydrogen

DC high E/N swarm experiment (self sustained discharge operating in the Townsend regime) may be modeled directly and exactly as it does not require self consistent calculation of the electric field and is thus open to a simple Monte Carlo simulation that may include complexity at the level of representation of collisional events. Thus such an experiment operating under swarm conditions is the best way to test the mechanism of generation of anomalously large Doppler broadening that was often observed in low pressure discharges.In this paper we show revised results of modeling of Doppler profiles by using a well tested Monte Carlo procedure and revised models of heavy particle collisions. In particular we study the importance of fast H2 and fast H particles, the effect of different models of angular distribution of particles scattered of the surface and of the molecules and we study the role of the energy losses due to vibrational excitation.

Modeling elastic momentum transfer cross-sections from mobility data

In this letter we present a new method to simply obtain the elastic momentum transfer cross-section which predicts a maximum of reduced mobility and its sensitivity to the temperature variation at low energies. We first determined the transport cross-section which resembles mobility data for similar closed-shell systems by using the Monte Carlo method. Second, we selected the most probable reactive processes and compiled cross-sections from experimental and theoretical data. At the end, an elastic momentum transfer cross-section is obtained by subtracting the compiled cross-sections from the momentum transfer cross-section, taking into account the effects of the angular scattering distributions. Finally, the cross-section set determined in such a way is used as an input in a final Monte Carlo code run, to calculate the flux and bulk reduced mobility for Ne+ + CF4 which were discussed as functions of the reduced electric field E/N (N is the gas density) for the temperature T = 300 K.

Effect Of Anode Surface on Doppler Profile in Townsend Discharge in Pure Hydrogen

In this work we show results of Monte Carlo modeling of electrons and heavy particles induced spatially resolved emission intensity or the Doppler profile in pure H2 discharge focusing on anode boundary. In particular we present study of fast H2 and fast H particles, scattered both at the surface and of the molecules. Since the anode boundary is characterized by a large electron flux, role of electron reflection and production of secondary electrons at the anode surface is part of the complete picture of spatial emission profile. In this work we verify our models of electron backscattering from the anode including both the reflection and the secondary electron production by making comparisons with the drift tube experiments. In order to achieve consistency with our previous results we select conditions of simulation appropriate for very high E/N (E‐electric field, N‐gas density) that are selected from experimental Townsend discharges in pure H2.