Ž. Nikitović

Kinetic phenomena in charged particle transport in gases, swarm parameters and cross section data*

In this review we discuss the current status of the physics of charged particle swarms, mainly electrons. The whole field is analysed mainly through its relationship to plasma modelling and illustrated by some recent examples developed mainly by our group. The measurements of the swarm coefficients and the availability of the data are briefly discussed. More time is devoted to the development of complete electron?molecule cross section sets along with recent examples such as NO, CF4 and HBr. We extend the discussion to the availability of ion and fast neutral data and how swarm experiments may serve to provide new data. As a point where new insight into the kinetics of charge particle transport is provided, the role of kinetic phenomena is discussed and recent examples are listed. We focus here on giving two examples on how non-conservative processes make dramatic effects in transport, the negative absolute mobility and the negative differential conductivity for positrons in argon. Finally we discuss the applicability of swarm data in plasma modelling and the relationship to other fields where swarm experiments and analysis make significant contributions.

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.

Transport parameters of F− ions in Ar/BF3 mixtures

In this paper we show predictions for the low-energy cross-sections and transport properties for the F− in Ar/BF3 mixtures which does not exist in the literature. These data are needed for modelling in numerous applications of technologically important Ar/BF3 discharges. Results for transport coefficients as a function of E/N (E is the electric field, N the gas density) were obtained by using the Monte Carlo technique. The Monte Carlo method is applied to obtain swarm parameters at the temperature .

Measurements and modeling of electron energy distributions in the afterglow of a pulsed discharge in BF3

In this paper we use experimental data (Radovanov S. and Godet L., J. Phys.: Conf. Ser., 71 (2007) 012014) for time-resolved electron energy distribution function in boron trifluoride (BF3) discharges together with cross-sections for electron excitation processes and attachment in order to explain electron dynamics in the pulsed plasma doping system. A Monte Carlo simulation (MCS) was used to perform calculations of the electron energy probability function (EEPF) in pulsed DC electric fields as found in practical implantation devices. It was found that in the afterglow, electric field in the plasma is not zero but still has a significant reduced electric field (E/N) albeit below the breakdown condition. Our analysis assuming free diffusion conditions in the afterglow led to the calculation of EEPF for a range of E/N corresponding to different afterglow times of a pulsed DC discharge. Calculated and experimental EEPF agree fairly well for a given set of cross-sections (see paper by Radovanov and Godet quoted above) and assumed initial distributions. In addition we have modeled the kinetics of production of negative ions in the afterglow as observed by experiment and found an increase in the production of negative ions in the early afterglow. Electron attachment in BF3 with 0.1% of F2 is a possible explanation for the observed rate of negative-ion production as predicted by our Monte Carlo simulation. However, the most likely cause for the increase in detected number density of ions is the collapse of the field-controlling electrons.

Effect of exothermic reactions on the mobility of Ar+ in CF4

In this letter we present a cross-section set and transport properties for Ar+ scattering on CF4 for relative energies up to 1000 eV. Monte Carlo simulation method is applied to accurately calculate transport parameters in hydrodynamic regime. We present new data for Ar+ ions in CF4 as a function of reduced electric fields E/N (N-gas density) where the gas temperature is used as a parameter. Values of the reduced mobility are discussed. Discrepancy of reduced mobility for and from the polarization limit was previously theoretically considered but rarely measured especially in the presence of exothermic reactions. We find that internally resonant exothermic dissociative charge transfer cross-section for production significantly increases zero-field ion mobility with respect to the polarization limit.

Transport of F− ions in F2

Transport properties of F− ions in F2 in DC fields were calculated using the Monte Carlo simulation technique. In the absence of a more reliable theory or experiments we have employed a simple technique to predict the cross-sections and to separate elastic from reactive collisions. We present reaction rate coefficients, characteristic energies, mean energy and drift velocities for the conditions of low and moderate reduced electric fields E/N (E is the electric field, N the gas density) accounting also for the non-conservative collisions.

Transport coefficients for electron scattering in CF4/Ar/O2 mixtures with a significant presence of Fx or CFx radicals

We present transport coefficients for electrons in mixtures of CF4 with Ar and O2 which are used in plasma etching, for ratios of the electric field to the gas number density E/N from 1 Td to 1000 Td (1Td=10−21 Vm2). We then add a certain percentage of radicals produced by dissociation of CF4. Our analysis of non-conservative collisions revealed a range of E/N where electron attachment introduced by radicals significantly changes the electron kinetics obtained for mixtures without dissociation of the CF4 gas. Results are obtained by using a simple, two-term solution for Boltzmanns equation and by Monte Carlo simulations.

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.

Comparison between transport parameters for K+ and Li+ in 1, 2-dimethoxy ethane (DXE) gas

In this paper, a theoretical study of 1, 2-dimethoxy ethane (DXE) and

Transport of electrons in Ar/H2 mixtures

\text{K}^+/\text{Li}^+