Plasma Chemistry and Plasma Processing (v.31, #2)

Thin Film Growth of Germanium Selenides from PECVD of GeCl4 and Dimethyl Selenide by Patrick J. Whitham; Dennis P. Strommen; Lisa D. Lau; René G. Rodriguez (251-256).
Plasma enhanced chemical vapor deposition (PECVD) of germanium selenide thin films from germanium tetrachloride and dimethyl selenide was studied to determine the viability of these reagents for thin film deposition. Germanium tetrachloride and alkylselenides were selected as candidates for these reactions due to their lower toxicities and higher availabilities compared to the more typical substitutes: germane and hydrogen selenide in the formation of germanium selenides. Dimethyl selenide was used successfully for the deposition of germanium selenides. Variation in film stoichiometry was observed by the modification of reactant gas flow ratios. Relative mass flow rates were varied in order to determine their effect on germanium chalcogenide deposition, and the effect of these flow rate modifications on the film thickness, structural properties, and composition are reported.
Keywords: Germanium selenide PECVD; Raman spectroscopy; Dimethyl selenide

Erratum to: Thin Film Growth of Germanium Selenides from PECVD of GeCl4 and Dimethyl Selenide by Patrick J. Whitham; Dennis P. Strommen; Lisa D. Lau; René G. Rodriguez (257-257).

A Comparative Study of HBr-Ar and HBr-Cl2 Plasma Chemistries for Dry Etch Applications by Alexander Efremov; Youngkeun Kim; Hyun-Woo Lee; Kwang-Ho Kwon (259-271).
The effects of HBr/Ar and HBr/Cl2 mixing ratios in the ranges of 0–100% Ar or Cl2 on plasma parameters, densities of active species influencing the dry etch mechanisms were analyzed at fixed total gas flow rate of 40 sccm, total gas pressure of 6 mTorr, input power of 700 W and bias power of 300 W. The investigation combined plasma diagnostics by Langmuir probes and the 0-dimensional plasma modeling. It was found that the dilution of HBr by Ar results in maximum effect on the ion energy flux with expected impact on the etch rate in the ion-flux-limited etch regime, while the addition of Cl2 influences mainly the relative fluxes of Br and Cl atoms on the etched surface with expected impact on the etch rate in the reaction-rate-limited etch regime.
Keywords: HBr; Cl2 ; Plasma; Dissociation; Ionization; Etch mechanism

Ethylene Epoxidation over Alumina- and Silica-Supported Silver Catalysts in Low-Temperature AC Dielectric Barrier Discharge by Thitiporn Suttikul; Thammanoon Sreethawong; Hidetoshi Sekiguchi; Sumaeth Chavadej (273-290).
In this work, ethylene epoxidation reaction for ethylene oxide production over silver catalysts loaded on two different supports (silica and alumina particles) in a low-temperature AC dielectric barrier discharge (DBD) reactor was investigated. The DBD plasma system was operated under the following base conditions: an O2/C2H4 feed molar ratio of 1/4, a total feed flow rate of 50 cm3/min, an electrode gap distance of 0.7 cm, an input frequency of 500 Hz, and an applied voltage of 19 kV. From the results, the presence of silver catalysts improved the ethylene oxide production performance. The silica support interestingly provided a higher ethylene oxide selectivity than the alumina support. The optimum Ag loading on the silica support was found to be 20 wt%, exhibiting the highest ethylene oxide selectivity of 30.6%.
Keywords: Epoxidation; Ethylene oxide; Dielectric barrier discharge; Alumina; Silica; Silver

Oxidative Conversion of Hexane to Olefins-Influence of Plasma and Catalyst on Reaction Pathways by C. Boyadjian; A. Ağıral; J. G. E. Gardeniers; L. Lefferts; K. Seshan (291-306).
An integrated plasma-Li/MgO system is efficient for the oxidative conversion of hexane. In comparison to the Li/MgO catalytic system, it brings considerable improvements in the yields of light olefins (C 2 = –C 5 = ) at relatively low temperatures indicating synergy from combination of plasma and catalyst. The study on the influence of temperature on the performance of the integrated plasma-Li/MgO system shows dominancy of plasma chemistry at the lower temperature (500°C), while contribution from the catalyst both in hexane activation and in enhancing olefin formation becomes significant at the higher temperature (600°C). At 500°C significant amount of acetylene formation is observed. This is minimized at 600°C at oxygen depleting conditions.
Keywords: Barrier discharge; Plasma; Hexane; Oxygen; Oxidative conversion; Olefins; Oxicracking; Li/MgO

Principal Limitations in Homogeneous Gas Phase Chemistry in Non-Thermal Plasmas by F. Holzer; R. Köhler; U. Roland; E. Stelter; F.-D. Kopinke (307-314).
This study highlights the oxidation of H2, CH4, and HCl present in the range of some volume percent in a homogeneous O2 or air phase in a flow through glass barrier discharge reactor. The oxidation of all three compounds is highly exothermic and exergonic at ambient temperature and proceeds at sufficiently high temperatures as radical chain reaction. The conversion of each compound was below 10% in a non-thermal oxygen plasma under various reaction conditions. Increasing concentrations of H2 and CH4 above the lower explosion limit did not lead to higher conversion degrees. It is assumed that only initial radical formation by electron impact dissociation and exothermic steps within the chain process run in a sufficiently fast manner at ambient temperature. For endothermic steps within the radical chain, the necessary activation energy is not available and the chain reaction aborts, most likely, after formation of peroxyl (hydro- or methyl-peroxyl) radicals.
Keywords: Non-thermal plasma; Gas phase oxidation; Radical chain reaction; Hydrogen; Methane

Carbon Dioxide Reforming of Methane Using a Dielectric Barrier Discharge Reactor: Effect of Helium Dilution and Kinetic Model by Valentin Goujard; Jean-Michel Tatibouët; Catherine Batiot-Dupeyrat (315-325).
The carbon dioxide reforming of methane to synthesis gas was investigated in a dielectric barrier discharge reactor at room temperature. The influence of dilution of reactants by helium was studied. We showed that, at a fixed contact time, the conversions of CH4 and CO2 increase when the amount of helium in the gas mixture increases. This result is attributed to the “penning ionization” phenomenon, which corresponds to an energy transfer from excited He to molecules in ground state (CH4, CO2). The selectivity to products is affected by the dilution factor. As soon as helium is present in a large amount the formation of products resulting from recombination of methyl radicals (such as C2, C3 and C4) is less favourable due to the lowest probability of collisions to proceed. A kinetic model is proposed based on the assumption that the reactant molecules CH4 or CO2 are attacked by active species produced by the plasma discharges, and the production of this active species are function of the plasma power. This model which takes into account the dilution by helium fits particularly well the experimental data we obtained.
Keywords: Plasma; Dielectric barrier discharge; Carbon dioxide reforming of methane; Kinetic model

Methane Decomposition Leading to Deposit Formation in a DC Positive CH4–N2 Corona Discharge by G. Horvath; M. Zahoran; N. J. Mason; S. Matejcik (327-335).
We have explored the formation of chemical species produced in a positive coaxial corona discharge fed by a mixture of N2 and CH4 at atmospheric pressure and ambient temperature. Gaseous products were detected by IR spectroscopy whilst solid products, deposited on the electrodes, were examined using Scanning Electron Microscopy and Energy Dispersive X-ray Analysis. The temporal evolution of gaseous products C2H2, HCN and C2H6 are also reported. These results may assist in the interpretation of results from the recent Cassini Huygens space mission as they may provide a simulation of the chemical processes occuring in Titan’s atmosphere.
Keywords: CH4–N2 corona discharge; FTIR spectroscopy; SEM–EDS spectroscopy; Deposit

The emission by electronically excited CN molecules is a prominent feature in the spectrum of active nitrogen containing traces of carbonaceous species. A large amount of experimental work has been devoted to an investigation of CN emission. However, up to now the plasma chemistry of CN radicals and processes leading to the excitation of CN electronic states are still poorly understood. The results of experimental measurements and numerical simulations are compared in order to establish the role of various channels of CN creation in the ground and excited states.
Keywords: Elementary processes in plasma; Elementary kinetics; CN creation and emission; CN radicals

Atmospheric Pressure Plasma Discharge for Polysiloxane Thin Films Deposition and Comparison with Low Pressure Process by Riccardo A. Siliprandi; Stefano Zanini; Elisa Grimoldi; Francesco S. Fumagalli; Ruggero Barni; Claudia Riccardi (353-372).
An atmospheric pressure dielectric barrier plasma discharge has been used to study a thin film deposition process. The DBD device is enclosed in a vacuum chamber and one of the electrodes is a rotating cylinder. Thus, this device is able to simulate continuous processing in arbitrary deposition condition of pressure and atmosphere composition. A deposition process of thin organosilicon films has been studied reproducing a nitrogen atmosphere with small admixtures of hexamethyldisiloxane (HMDSO) vapours. The plasma discharge has been characterized with optical emission spectroscopy and voltage-current measurements. Thin films chemical composition and morphology have been characterized with FTIR spectroscopy, atomic force microscopy (AFM) and contact angle measurements. A strong dependency of deposit character from the HMDSO concentration has been found and then compared with the same dependency of a typical low pressure plasma enhanced chemical vapour deposition process.
Keywords: Plasma-enhanced chemical vapour deposition; Hexamethyldisiloxane; Polysiloxane; Dielectric barrier discharge; Low pressure glow discharge

Modeling study is performed to reveal the momentum and heat/mass transfer characteristics of a turbulent or laminar plasma reactor consisting of an argon plasma jet issuing into ambient air and interacting with a co-axially counter-injected argon jet. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of argon in the argon–air mixture for the laminar and the turbulent regimes, respectively. Modeling results presented include the streamline, isotherm and argon mass fraction distributions for the cases with different jet-inlet parameters and different distances between the counter-injected jet exit and the plasma torch exit. It is shown that there exists a quench layer with steep temperature gradients inside the reactor; a great amount of ambient air is always entrained into the plasma reactor; and the flow direction of the entrained air, the location and shape of the quench layer are dependent on the momentum flux ratio of the plasma jet to the counter-injected cold gas. Two quite different flow patterns are obtained at higher and lower momentum flux ratios, and thus there exists a critical momentum flux ratio to separate the different flow patterns and to obtain the widest quench layer. There exists a high argon concentration or even ‘air-free’ channel along the reactor axis. No appreciable difference is found between the turbulent and laminar plasma reactors in their overall plasma parameter distributions and the quench layer locations, but the values of the critical momentum flux ratio are somewhat different.
Keywords: Thermal plasma; Turbulent and laminar jet; Counter-flow; Modeling

Carbon Nano-Flakes Produced by an Inductively Coupled Thermal Plasma System for Catalyst Applications by Ramona Pristavita; Jean-Luc Meunier; Dimitrios Berk (393-403).
Carbon material was produced using an inductively coupled thermal plasma torch system of 35 kW and a conical shape reactor. The carbon nanopowders were obtained by plasma decomposition of methane at various flow rates and show a uniform microstructure throughout the reactor. The product has a crystalline graphitic structure, with a stacking of between 6 and 16 planes and a nano-flake morphology with particles dimensions of approximately 100 nm long, 50 nm wide and 5 nm thick. Nitrogen was also introduced in some synthesis experiments along with the methane precursor using flow rates of 0.1 and 0.2 slpm. The resulting product has the same structural properties and the nitrogen is incorporated into the graphitic structure through pyridinic type bonds.
Keywords: Carbon nano-flakes; Thermal plasma; Nitrogen functionalization; Graphitic structure