Olivera Šašić

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.

Measurements and analysis of electron transport coefficients obtained by a pulsed Townsend technique

A review of a wide range of electron swarm studies in several pure gases and gas mixtures is given. These studies include the determination of the cross section set for electrons in C2H2F2 (R134a) based on recent measurements of transport data, the re-analysis of the cross sections for electrons in N2O and its mixtures with N2 and SF6 and, finally, the analysis of electron transport in N2–Ar and Xe–He mixtures. It was found that in the case of R134a further studies of the characteristic energy are needed for its mixtures with argon in pure gases in order to obtain a reliable set of cross sections. For N2O, a set has been developed that fits a wide range of data. However, some verification of significant changes in the shape of the attachment cross section should still be done. In two different sets of data for the mixtures of Xe and He and of Ar and N2, the existing cross sections do a very good job throughout most of the energy range, although some small adjustments may be sought at the higher end of the relevant energy range for xenon. In this paper we summarize the work already described in separate papers for each of the He–Xe and Ar–N2 mixtures, and we present here a number of transport coefficients and analyses that were not included in the original papers.

Transport coefficients for electrons in mixtures of Ar and HBr

We present calculations of swarm data and rate coefficients for electrons in mixtures of Ar and HBr. The transport data were calculated using a Monte Carlo simulation over a broad range of E/N (electric field E to the gas number density N ratio) and with the idea to provide a basis for models of plasma etching involving HBr. The total cross section has an almost a constant collision frequency, which leads to rather uneventful E/N dependences of the transport data in pure HBr, but the mixtures with Ar involve more complex kinetic phenomena.

Excitation coefficients and cross-sections for electron swarms in methane

We have measured electron excitation coefficients for production of excited fragments H(n=3), H(n=4), CH(A2Δ) and CH(B2Σ−) from the ground-state methane molecules by an electron swarm in a drift tube type experiment (parallel plate Townsend discharge at very low currents). Hα and Hβ lines as well as CH(A2Δ–X2Π) and CH(B2Σ−–X2Π) bands were used. Cross-sections from binary collision experiments were renormalized by fitting the measured data with the results of Boltzmann equation and Monte Carlo calculations for electron transport.

Scattering cross sections for electrons in C2 H2F4 and its mixtures with Ar from measured transport coefficients

Previous measurements of the drift velocities, W, and the density-normalized effective ionization (multiplication) coefficients (ionization minus attachment), (?????)/N, measured in C2H2F4 (1,1,1,2 tetrafluoroethane) and in C2H2F4?Ar mixtures have been analysed with a standard swarm procedure. As a result of this analysis a set of electron collision cross sections for the C2H2F4 molecule has been obtained. This set has been further used to calculate other transport parameters such as the characteristic energies and rate coefficients for individual processes.

Transport coefficients and cross sections for electrons in N2O and N2O/N2 mixtures

A standard swarm analysis of electron scattering cross sections in nitrous oxide (N2O) is presented. The experimental results for drift velocities and effective ionization coefficients (differences between the ionization and attachment coefficients), obtained over an extended range of E/N (electric field normalized to the gas number density) by the pulsed-Townsend technique, are compared with the numerical solution of the Boltzmann equation. Our analysis shows that commonly used sets of cross sections have to be modified in order to fit the new experimental data, in particular the dissociative cross sections for attachment and electronic excitation (with a threshold energy of around 4.0 eV). Using a single set of cross sections it was possible to fit both the data for pure N2O and those for the N2O/N2 mixtures with 20%, 40%, 60% and 80% N2O. S Online supplementary data available from stacks.iop.org/PSST/19/025005/mmedia (Some figures in this article are in colour only in the electronic version)

Electron impact ionization and transport in nitrogen?argon mixtures

The electron drift velocities and ionization coefficients in the nitrogen–argon mixtures have been measured and calculated over a wide range of E/N and mixture concentrations. It was found that standard cross section sets for low and moderate energies fit the transport data very well. Derived quantities such as the mean energy for electrons and the ratio between the transverse diffusion coefficients and the electron mobility have also been calculated.

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.

Data Bases for Modeling Plasma Devices for Processing of Integrated Circuits

In this paper we compile the data on electron methane scattering cross-sections and transport coefficients that provide a data base for plasma models of etching, deposition and other technologies involving CH4. Cross section sets were compiled and tested against the swarm data and transport coefficients were calculated and measured for DC and RF fields. We also indicate the conditions where kinetic effects in the RF field require an extension of the present day models of plasma etching deposition and cleaning.

Scattering cross section set for electrons in CH 3 OCH 3

Summary form only given. Scattering cross section set for electrons in CH3OCH3 (dimethyl ether, DME) is developed by using the standard swarm procedure. Our work was motivated by the interest for this molecule as it is one of the largest organic molecule in the interstellar space, with the wide industrial use and application in particle detectors, especially in micro-strip gas chambers [1]. Therefore, there was a need for accurate and reliable collision and transport data that can be used in plasma models.The first objective of our analysis was to examine how well the available and the most commonly used cross sections [2] reproduced experimental transport data [3]. In order to achieve that goal we started from that set and we calculated electron drift velocity (W) and the density normalized ionization coefficient (α/N) over a wide range of reduced electric fields (E/N), for pure gas and its mixtures with Ar and Ne. Calculations were made by two term approximation of the Boltzmann equation, and also by using our Monte Carlo simulation code. Comparison of calculated and experimental data showed that some modifications in the starting cross section set need to be made to fit the available experimental data [4-6]. In particular the elastic momentum transfer and electronic excitation energy dependences (with the threshold energy of εth=7.7 eV) were modified both in their magnitude and shape as well as the vibrational excitation (εth=0.349 eV). The process of cross section modifications was performed in iterations, for pure gas and mixtures simultaneously until a consistent set was developed. The alterations in calculated W appear to be small and only for the low electron energies. At the same time, the α/N is now much more accurate in comparison to the experimental data, over the entire investigated electron energy range. That fact justifies our revision of the cross section set, especially having in mind the significance of the ionization coefficient from the application point of view.