Hirotake Sugawara

One-dimensional modelling of low-frequency and high-pressure Xe barrier discharges for the design of excimer lamps

A self-consistent one-dimensional modelling of a Xe gas discharge between electrodes covered with dielectric barrier is presented for power frequencies from 50 kHz to 1 MHz and gas pressures from 10 to 400 Torr. Spatiotemporal profiles of the concentration of electrons, ions, excited atoms and excimers are obtained. Excimers are mainly produced in the sheath regions. The efficiencies of spontaneous emission from excimers and resonance-state atoms increase with an increase in the input powers for gas pressures higher than 50 Torr. A characteristic period during which the barrier wall charge significantly influences the electric field in bulk region was found, and discharge properties in the period are discussed.

Predicting the amount of carbon in carbon nanotubes grown by CH4 rf plasmas

Carbon nanotubes (CNTs) were grown on Si substrates by rf CH4 plasma-enhanced chemical vapor deposition in a pressure range of 1–10Torr, and then characterized by scanning electron microscopy. At 1Torr, the CNTs continued growing up to 60min, while their height at 4Torr had leveled off at 20min. CNTs hardly grew at 10Torr and amorphous carbon was deposited instead. CH4 plasma was simulated using a one-dimensional fluid model to evaluate the production and transport of radicals, ions, and nonradical neutrals. The amount of simulated carbon supplied to the electrode surface via the flux of radicals and ions such as CH3, C2H5, and C2H5+ was consistent with estimations from experimental results.

Analysis of Oxidation State of Multilayered Catalyst Thin Films for Carbon Nanotube Growth Using Plasma-Enhanced Chemical Vapor Deposition

We synthesized vertically aligned carbon nanotubes (CNTs) using multilayered catalyst thin films (Fe/Al2O3 and Al2O3/Fe/Al2O3) by RF (13.56 MHz) CH4/H2/Ar plasma-enhanced chemical vapor deposition. Pretreatment of the catalyst is crucial for CNT growth. In this paper, we analyzed the effect of catalyst reduction on CNT growth. Catalyst thin films on substrates were reduced by H2 plasma pretreatment at 550 °C to form nanometer-sized catalyst particles. The multilayered thin films were analyzed; the chemical composition and oxidation state by X-ray photoelectron spectroscopy (XPS) and the surface morphology by scanning electron microscopy (SEM). The Fe 2p peak of the XPS spectra showed that FexOy in the as-deposited catalyst was effectively reduced to Fe by a pretreatment of duration 4 min. Using this catalyst, we obtained CNTs with an average diameter of 10.7 nm and an average length of 5.3 µm. However, pretreatment longer than 4 min resulted in shorter CNTs and the Fe peak was shifted from Fe to Fe3O4. These transitions (Fe2O3→Fe3O4→Fe→Fe3O4) can be explained by the enthalpy of the oxides. This result indicates the presence of an optimum ratio between Fe and FexOy to maximize the CNT lengths.

Amorphous fluorocarbon polymer (a-C:F) films obtained by plasma enhanced chemical vapor deposition from perfluoro-octane (C8F18) vapor. II. Dielectric and insulating properties

Amorphous fluorocarbon polymer films (a-C:F) have been grown by plasma enhanced chemical vapor deposition from a new precursor, namely perfluoro-octane (C8F18) vapor. The dielectric and insulating properties of the films have been assessed by means of capacitance–voltage and current–voltage characteristics, breakdown voltage measurements, scanning electron microscopy, and ellipsometric analyses. In the investigated frequency range, 120 Hz–1 MHz, the films have a low dielectric constant (≈2.4) and a low dielectric loss (⩽3×10−2). The electrical conduction mechanism is ohmic in the weak electric field regime, with a bulk resistivity of 4×1015 Ω cm, and a trap modulated space charge limited conduction in the strong electric field regime. For a film thickness between 1 and 12 μm, the dielectric strength varies with film thickness according to EB∼h−0.53. For a film thickness of 1 μm the film dielectric strength was 2.7 MV/cm while for 12 μm it decreased to 0.9 MV/cm. Measurements of the breakdown voltage in ni...

Boltzmann equation analysis of electron swarm parameters and related properties of Xe/He and Xe/Ne mixtures used for plasma display panels

The electron swarm parameters of Xe/He and Xe/Ne mixtures are analysed with a Boltzmann equation method in which secondary generations of electrons through excited atoms are properly considered. The swarm parameters are examined for a wide range of the electron concentrations from the Townsend discharge region to the working condition of plasma display panels (PDPs). Using ionization coefficients obtained in this work and referred values of secondary electron emission coefficients from MgO film, the discharge onset voltage V b is discussed. The Paschen curves around (p 0 d )min at which V b has a minimum are compared for Xe/He and Xe/Ne mixtures. Under the working conditions for PDP discharges, the radiation rates of ultraviolet light ( = 130, 147 and 173 nm) from the Xe resonance levels and Xe excimer are calculated. A suitable discharge condition for designing PDP is discussed with respect to various values of the gas mixture ratio, gas pressure and reduced electric field.

Transient behaviour of CF4 rf plasmas after step changes of power source voltage

CF4 radio-frequency (rf) plasmas are often driven in time-modulation modes to obtain higher etching performance. For dynamic and stable control of CF4 plasmas, we investigated the transient behaviour of the plasmas after step up or down of the amplitude of the power source voltage Vs(t). Using a one-dimensional fluid model, we simulated a CF4 capacitively coupled plasma that transfers from a periodic steady state to another at 0.2 Torr in a 20 mm parallel-plate gap. In addition, we compared such plasma responses to the step down to 0 V at the phases at which Vs(t) takes zero and peak values. Fundamental properties of transient CF4 plasmas were discussed.

Timesaving techniques for decision of electron-molecule collisions in Monte Carlo simulation of electrical discharges

Techniques to reduce the computational load for determination of electron-molecule collisions in Monte Carlo simulations of electrical discharges have been presented. By enhancing the detection efficiency of the no-collision case in the decision scheme of the collisional events, we can decrease the frequency of access to time-consuming subroutines to calculate the electron collision cross sections of the gas molecules for obtaining the collision probability. A benchmark test and an estimation to evaluate the present techniques have shown a practical timesaving efficiency.

Fluorinated Carbon Films with Low Dielectric Constant Made from Novel Fluorocarbon Source Materials by RF Plasma Enhanced Chemical Vapor Deposition

An attempt to fabricate fluorinated carbon (fluoropolymer) films by rf (13.56 MHz) plasma enhanced chemical vapor deposition was made using novel fluorocarbon source materials of C7F16, (C3F7)3N/(C4F9)3N and C8F18/C8F16O. The deposited films were transparent and displayed excellent electrical properties as interlayer dielectrics for LSI using deep-submicron technology, namely, a dielectric constant as low as 2.0 and a dielectric strength higher than 2 MV/cm. The refractive index of these films was 1.38.

Analyses of electron swarms in gases under steady-state Townsend conditions

A new simulation technique for obtaining the equilibrium electron energy distributions of electron swarms in gases under steady-state Townsend conditions is proposed based on a propagator method. The electron energy distribution function could be obtained from calculations in only one slab in real space, assuming exponential spatial growth of an electron swarm and similarity of the electron energy distribution at any position. The electron energy distributions were analysed in argon at E/N=1414 Td for a case of a positive large value of the effective ionization coefficient alpha and in an argon ((90%)/fluorine (10%) mixture at E/N=56.6, 84.8, 113.1 and 141.1 Td for a case of values of alpha crossing zero. The results agreed excellently with those of a Monte Carlo simulation and a Boltzmann equation analysis. This propagator method was found to deduce stably proper solutions for both positive and negative values of alpha . A relation between the present method for a steady-state Townsend condition and a similar method for a pulsed Townsend condition is discussed.

Hydrogen-Sensing Response of Carbon-Nanotube Thin-Film Sensor with Pd Comb-Like Electrodes

Hydrogen gas sensors based on single-walled carbon nanotube (SWCNT) films were fabricated with Pd comb-like electrodes. The SWCNT films were deposited by Mo/Co catalyst-assisted chemical vapor deposition using alcohol. The film included a high-density network of SWCNTs. The SWCNT film sensor showed a large conductance change and a fast response (20 s) upon exposure to 1% H2 gas in air at room temperature, while the conductance change was negligible for devices using Au electrodes.