H Tagashira

Electron swarm development in SF6. I. Boltzmann equation analysis

The electron swarm behaviour in SF6 gas is studied for E/N values from 141 to 707 Td by a three-term Boltzmann equation method, in which the effect of generation and loss of electrons due to ionisation and attachment is considered properly. A consistent set of electron collision cross sections, which gives the swarm parameter values in agreement with previous measurements, is determined considering the latest cross section data. The calculation is performed mainly for the steady-state Townsend condition. The validity of the results obtained has been confirmed by a Monte Carlo simulation carried out parallel to the analysis. The present results are also compared with those of the usual two-term Boltzmann analysis. It is found that the two-term approximation is fully valid for deduction of the swarm parameters for E/N values as considered despite the fact that SF6 is a strongly electronegative gas.

Electron transport coefficients in SF6

The electron swarm behaviour in SF6 gas is re-analysed over the E/N range 141-7000 Td by a six-term Boltzmann equation method and by a Monte Carlo simulation considering the latest cross section data, in particular, that of the elastic momentum transfer cross section. The Boltzmann equation analysis shows that the present set of cross sections gives the values of swarm parameters such as ionization and electron attachment coefficients, drift velocity, longitudinal and transverse diffusion coefficients in excellent agreement with the respective measurements for a wide range of E/N. The swarm parameters calculated by the six-term approximation analysis agree well with those by Monte Carlo calculation. Furthermore, the Monte Carlo simulation confirms the results of a previous computer simulation study for correspondence between experimental and theoretical electron drift velocities; that is, the drift velocity deduced from Schlumbohms experiment (1965) assumes a value close to but slightly larger than Wm and the drift velocity deduced from Frommholds experiment (1959) assumes a value represented by (Wr+Wm)/2, where Wm and Wr are the mean arrival time and the centre-of-mass electron drift velocities, respectively.

Boltzmann equation analysis of electron swarm behaviour in nitrogen

The electron swarm behaviour in nitrogen is studied for E/ rho 0 from 20 to 200 V cm-1 Torr-1 by a Boltzmann equation method. A set of electron collision cross sections is determined using newly published data. The modification of these cross sections when necessary is kept to within experimental error. Moreover, the validity of the vibrational excitation cross sections obtained theoretically by Hazi and co-workers (1981) is examined. The results show that the calculated swarm parameters are in close agreement with those obtained by a photon flux experiment of Wedding and co-workers (1985). This suggests that the set of electron collision cross sections determined in the present work is an appropriate one as far as the swarm parameter analysis is concerned. The electron energy distribution and electronic excitation coefficients and frequencies to various excited states are also calculated and discussed.

Electron transport coefficients in SF6 and c-C4F8 mixtures

The ionization alpha , electron attachment eta and other electron transport coefficients for SF6 and perfluorocyclobutane (c-C4F8) gas mixtures have been calculated by a three-term Boltzmann equation method over the E/N range 283-707 Td. A set of electron collision cross-sections that gives transport parameters in agreement with measurements for the pure gases is used. The calculation is performed for important experimental conditions, i.e. the steady-state Townsend (SST), pulsed Townsend (PT) and time-of-flight (TOF) methods. It is found that the greater the reduction in E/N the more pronounced is the peak of eta values of mixtures, e.g. at E/N=283 Td the peak appears at the SF6 partial pressure k around 20%, though alpha values show monotonical variation with k, at a fixed E/N for the E/N range studied here. As a result, the E/N value at which alpha = eta , i.e. the limiting E/N of 20% SF6 and 80% c-C4F8, can exceed the values for the respective pure gases. This fact suggests that this combination may be one desirable synergistic mixture since the limiting E/N is an important parameter for predicting the dielectric characteristics in electronegative gases.

Electron energy distribution and transport coefficients of electron swarms in SF6 and nitrogen mixtures

Electron swarm behaviour in SF6 and nitrogen mixtures is analysed over the E/N range from 141 to 707 Td by a three-term Boltzmann equation method. A set of electron collision cross sections, which was determined consistently with measurements for the respective pure gases by the authors ((SF6, Itoh et al., 1988); N2(Ohmori et al., 1988)), is used. The calculation is carried out for the steady-state Townsend (SST), pulsed Townsend (PT) and time-of-flight (TOF) experiments. In particular the TOF parameters of these mixtures are deduced for the first time as far as the authors are aware. It is found that the calculated values of the effective ionisation coefficient alpha shows a monotonic variation with percentage of SF6 at a fixed E/N, the same as previous measurements. The E/N value at which alpha =0, i.e. the limiting E/N, is also deduced and in close agreement with measurements. The variations of the electron energy distribution and other transport coefficients with the partial SF6 pressure are calculated and discussed in detail.

Development of electron swarms in SF6

The electron swarm behaviour in SF6 is calculated and analysed for a wide range of E/N values from 71 to 5656 Td by both a Boltzmann equation method and a Monte Carlo simulation, using a set of electron collision cross sections determined by the authors. The Monte Carlo result shows clearly that the difficulty in deducing the electron energy distribution increases as the E/N values decrease due to a large electron attachment probability, while the result of the Boltzmann equation analysis shows that the set of cross sections determined by the authors is again fully valid for E/N>707 Td, since the effective ionisation coefficient agrees with measurements at very high E/N. The swarm parameters for important experimental conditions are deduced and the validity of the usual two-term method is discussed in detail.

Measurement of electron transport coefficients in tetramethylsilane vapour

This paper reports four electron transport coefficients in tetramethylsilane (TMS, Si(CH3)4) vapour which is utilized in plasma-enhanced chemical vapour deposition (PECVD) of thin films such as SiC and SiN. The coefficients, namely the density-normalized effective ionization coefficient (αeff/N), the mean-arrival-time drift velocity (Wm) and the product of the longitudinal electron diffusion coefficient and the gas density (NDL) have been experimentally determined using a double-shutter drift tube method based on an arrival-time spectra theory over a reduced electric field (E/N) range of 30–5000 Td (1 Td = 10−21 V m2). The result shows that the coefficient αeff/N increases rapidly at an E/N value of about 200 Td and then reaches a value of 9.84 × 10−16 cm2 at an E/N value of 5000 Td. It is revealed that the values of the coefficient αeff/N are practically equal to those in SiH4 gas which is a representative gas used in PECVD. The reliability of the αeff/N values is checked by a steady-state Townsend experiment. The drift velocity (Wm) monotonically increases with E/N except at 50 Td < E/N < 200 Td, where a slight negative differential conductivity is observed. The longitudinal diffusion coefficient (NDL) has a minimum value at an E/N value of about 100 Td in the present measurement. Its variation profile against E/N is similar to that in SiH4 although its profile shifts to higher E/N. The ratio of the longitudinal diffusion coefficient to the mobility (DL/μ) is deduced from the Wm and the NDL values, which monotonically increases from 0.1 to about 10 eV with E/N in the present E/N range.

Electron kinetics in proportional counters

Abstract A Boltzmann equation description of the electron behavior under the non-uniform field geometry is presented for modeling the gas amplification process of the proportional counters. Specifically, we apply the arrival-time spectra (ATS) method to deduce the equation leading to the counter gas gain. In addition, the behavior of the electron multiplication process in spatially distorted field conditions is demonstrated using a Monte Carlo simulation to investigate the physical aspects of the ionization coefficient.

Computer simulation study of correspondence between experimental and theoretical electron drift velocities. II. Constant total collision frequency model gases

Electron drift velocities WSC and WFD obtained respectively from Schlumbohms and Frommholds experiments are calculated by a Monte Carlo simulation for five cases of model gases. The obtained drift velocities are compared with the theoretical drift velocities defined as the average velocity Vd of the steady state Townsend condition and the average velocity Wv, the centre of mass drift velocity Wr and the mean arrival time drift velocity Wm of an isolated electron swarm, which are calculated by a separate Monte Carlo simulation in a free space for the respective model gases. It is found that WSC is almost equal to Wm, and that WFD is almost equal to drift velocity Ww represented as 1/2 (Wr+Wm), for the E/N values studied here. It is also found that the difference between electron drift velocities depends not only on the E/N value but also on the value of the effective ionization frequency.

Properties of TiN Films on Heated Substrate Below 550°C by 50 Hz Plasma-Enhanced Chemical Vapor Deposition

Titanium nitride (TiN) films have been deposited on silicon substrates at low substrate temperature (400–550°C) by 50 Hz plasma-enhanced chemical vapor deposition (CVD) using a TiCl4+N2+H2 gas mixture. A substrate bias circuit with two diodes was used to enhance the deposition through ion processes. The Vickers hardness values of the deposited TiN films at Tsub ranging from 450 to 550°C were above 2200 Hv, and resistivity was below 100 µΩcm at Tsub500°C. The composition of the deposited films was investigated by the Rutherford backscattering spectroscopy (RBS) method to determine that the at.% value of the Cl content [=Cl/(Ti+N+O+C+Cl)] in the TiN films is about 1% at Tsub=550°C. X-ray diffraction (XRD) spectra revealed that the film has the preferred orientation of δ-TiN with (200) and (220) under the deposition condition of Tsub500°C.