Plasma Chemistry and Plasma Processing (v.30, #5)

Electrical, Thermal and Optical Diagnostics of an Atmospheric Plasma Jet System by C. E. Nwankire; V. J. Law; A. Nindrayog; B. Twomey; K. Niemi; V. Milosavljević; W. G. Graham; D. P. Dowling (537-552).
Plasma diagnostics of atmospheric plasmas is a key tool in helping to understand processing performance issues. This paper presents an electrical, optical and thermographic imaging study of the PlasmaStream atmospheric plasma jet system. The system was found to exhibit three operating modes; one constricted/localized plasma and two extended volume plasmas. At low power and helium flows the plasma is localized at the electrodes and has the electrical properties of a corona/filamentary discharge with electrical chaotic temporal structure. With increasing discharge power and helium flow the plasma expands into the volume of the tube, becoming regular and homogeneous in appearance. Emission spectra show evidence of atomic oxygen, nitric oxide and the hydroxyl radical production. Plasma activated gas temperature deduced from the rotational temperature of nitrogen molecules was found to be of order of 400 K: whereas thermographic imaging of the quartz tube yielded surface temperatures between 319 and 347 K.
Keywords: Plasma jet; Diagnostics; Plasma diagnostics; Optical emission; Infrared thermography; Corona

Effects of Oxygen Addition and Treating Distance on Surface Cleaning of ITO Glass by a Non-Equilibrium Nitrogen Atmospheric-Pressure Plasma Jet by M.-H. Chiang; K.-C. Liao; I.-M. Lin; C.-C. Lu; H.-Y. Huang; C.-L. Kuo; J.-S. Wu; C.-C. Hsu; S.-H. Chen (553-563).
Effects of oxygen addition and treating distance on cleaning organic contaminants on stationary and non-stationary (1–9 cm/s) ITO glass surfaces by a parallel-plate nitrogen-based dielectric barrier discharge (DBD) are investigated experimentally; the DBD is driven by a 60 kHz bipolar quasi-pulsed power source. The results show that two regimes of favorable operating condition for improving the hydrophilic property of the surface (reducing the contact angle from 84° to 25–30°) are found. The measured spatial distribution of NO-γ UV emission, O3 concentration and OES spectra are shown to strongly correlate with the measured hydrophilic property. At the near jet downstream locations (z < 10 mm), the metastable $$ N_{2} (A^{3} sum {_{u}^{ + } )} $$ and photo-induced dissociation of ozone play dominant roles in cleaning the ITO glass surface; while at the far jet downstream locations (z > 10 mm), where the ratio of oxygen to nitrogen is lower, only the long-lived metastable $$ N_{2} (A^{3} sum {_{u}^{ + } )} $$ plays a major role in cleaning the ITO glass surface.
Keywords: Atmospheric-pressure plasma jet; DBD; ITO; Quasi-pulsed; Surface cleaning

Packed Bed DBD Discharge Experiments in Admixtures of N2 and CH4 by G. Horvath; N. J. Mason; L. Polachova; M. Zahoran; L. Moravsky; S. Matejcik (565-577).
The physical, chemical and electrical properties of a coaxial Dielectric Barrier Discharge (DBD) fed by various mixtures of N2 and CH4 at atmospheric pressure and ambient temperature has been studied. FTIR spectral measurements of the products formed in the discharge have shown that the main chemical products are C2H2, HCN and C2H6 produced by dissociation of CH4, with small but significant traces of NH3.such a chemical transformation may provide some insight into the chemical processes prevalent in the atmosphere of Titan. During operation of the discharge a solid brown-yellowish deposit was formed on the central rod electrode and the dielectric pellets. Such a deposit is similar to that observed in other N2:CH4 discharges and is believed to be an analogue of the aerosol and dust observed in Titan’s atmosphere being composed of chemical species commonly known as ‘tholins’.
Keywords: DBD discharge; Infrared spectroscopy

Room-Temperature Silicon Nitrides Prepared with Very High Rates (>50 nm/s) in Atmospheric-Pressure Very High-Frequency Plasma by Hiroaki Kakiuchi; Hiromasa Ohmi; Kei Nakamura; Yoshihito Yamaguchi; Kiyoshi Yasutake (579-590).
We have investigated the structure and stability of SiN x films deposited with very high rates (>50 nm/s) in atmospheric-pressure (AP) He-based plasma excited by a 150 MHz very high-frequency (VHF) power using a cylindrical rotary electrode at room temperature. The SiN x films are prepared on Si(001) substrates with varying VHF power density (P VHF), H2 concentration and source ratio (NH3/SiH4). Infrared absorption spectroscopy is used to analyze the bonding configurations in the films. The results show that increasing H2 concentration under the supply of a moderately large P VHF, together with the adjustment of NH3/SiH4 ratio, enables us to prepare SiN x showing reasonable stability against a buffered hydrofluoric acid solution in spite of the very high deposition rate of 130 nm/s. The achievement of such a high-rate deposition at room temperature is primarily due to the significant enhancement of both gas-phase and surface-phase reactions in AP-VHF plasma.
Keywords: Atmospheric-pressure plasma; Silicon nitride; High-rate deposition; Room temperature

Hydrogen production from methane reforming using the partial oxidation reaction is carried out using a bipolar pulse-driven plasma reactor operating at atmospheric pressure. The influence of axial and radial injections of CH4 and O2 on the reforming performance is investigated in conjunction with the spatial distribution of their spectral emissions by varying the O2/C ratio at a fixed discharge power. With increasing the O2/C ratio, the axial injection achieves the better conversion and efficiency. The measurement of spectral emissions reveals that the higher efficiency of axial injection is caused by its efficient use of oxidation reaction heat.
Keywords: Methane reforming; Hydrogen production; Partial oxidation reaction; Bipolar pulse-driven plasma; Spectral emission distribution

The measurements of electro-optical discharge characteristics and concentration of produced ozone were performed to evaluate the efficiency of ozone production in an AC surface dielectric barrier discharge (SDBD) in pure oxygen at atmospheric pressure. The discharge was driven in an amplitude-modulated regime with a driving AC frequency of 1 kHz, variable discharge duty cycle of 0.01–0.8 and oxygen flow rate of 2.5–10 slm. We observed asymmetric SDBD behaviour as evidenced by the variation in the ratio of the OI/O2 + emission intensities registered during the positive/negative half-periods and complemented by the transferred charge measurements through the Lissajous figures. We also found a strong dependence of O3 concentration on the discharge duty cycle. The highest calculated ozone production yield reached 170 g/kWh with a corresponding energy cost of about 10 eV/molecule when combining the lowest inspected duty cycle with the lowest AC high voltage amplitude.
Keywords: Ozone; Surface DBD; Oxygen; Production efficiency

Time-resolved Optical Emission Spectroscopy in Water Electrical Discharges by C. Miron; M. A. Bratescu; N. Saito; O. Takai (619-631).
The characteristics of the plasma initiated in ultrapure water between pairs of tungsten and tantalum electrodes were investigated by time-resolved optical emission spectroscopy. The deexcitation processes of the reactive species formed in the water plasma depended on the electrode material, but had been independent on the polarity of the applied voltage pulses. All the reactive species presented the same evolution with time and have been identified with high concentration in the emission spectra between the pulses. The current–voltage characteristics showed the features of a spark discharge for the both types of electrodes used in the process. When tantalum electrodes were used to generate the discharge, a broad emission continuum (350–940 nm) dominated the spectrum due to a transition to arc discharge.
Keywords: Water electrical discharges; Reactive species; Time-resolved optical emission spectroscopy

Highly Selective and Low Damage Etching of GaAs/AlGaAs Heterostructure using Cl2/O2 Neutral Beam by B. J. Park; J. K. Yeon; W. S. Lim; S. K. Kang; J. W. Bae; G. Y. Yeom; M. S. Jhon; S. H. Shin; K. S. Chang; J. I. Song; Y. T. Lee; J. H. Jang (633-640).
Highly selective and low damage etching of the GaAs cap layer on AlGaAs is essential in fabricating GaAs/AlGaAs high electron mobility transistors. The GaAs on AlGaAs was etched using a low energy Cl2/O2 neutral beam and the Schottky device characteristics fabricated on the exposed AlGaAs were compared with those fabricated after the etching using wet etching and a Cl2/O2 ion beam. Using a low energy Cl2/O2 ion beam or a Cl2/O2 neutral beam, highly selective etching of the GaAs cap layer to AlGaAs similar to wet etching could be achieved through the formation of Al2O3 on the exposed AlGaAs during the etching. When the electrical characteristics of the Schottky devices were compared, the devices fabricated after the etching using the neutral beam showed the best electrical characteristics such as electrical stability, low leakage current, higher barrier height, etc. by showing low damage to the exposed AlGaAs surface.
Keywords: Neutral beam; Etch selectivity; GaAs; AlGaAs; HEMT

Low frequency (100 kHz) discharge in Ar-H2 and CH3SiCl3-Ar-H2 mixtures was studied to obtain information on the processes involved in plasma deposition of SixCy:H films from CH3SiCl3-Ar-H2 plasma once the properties of Ar-H2 plasma are known. The plasmas were studied using optical emission spectroscopy. The addition of small amounts of nitrogen to the plasma mixtures also permitted the use of an actinometry technique. First, plasma parameters (electron density and temperature) and actinometric concentrations of atomic hydrogen in an argon–hydrogen plasma were investigated as a function of the hydrogen content in the feed. Second, the emission intensities of Si, Si+, CH, H, Ar and Ar+ species produced in the CH3SiCl3-Ar-H2 discharge were analysed as a function of time following the introduction of CH3SiCl3 (methyltrichlorosilane, MTCS) to the argon–hydrogen plasmas with various proportions of the feed gasses. The results reveal a rapid decay of the Si-excited state number density versus time, while those of Si+ and CH fell off more slowly. The emission of atomic silicon was believed to be a result of electron impact dissociative and excitation processes occurring in the bulk of the discharge, whereas the Si+ and CH seemed to originate mainly from products of sputtering of the growing film surface. The fragmentation of the MTCS associated with HCl formation and enhanced atomic hydrogen production as a result of HCl dissociation are proposed. Variations in the radical densities of H and CH3 were determined using an actinometry technique. The results indicate a significant role for H2 in gas-phase reactions occurring in the CH3SiCl3-Ar-H2 plasma, as well as in gas-surface interactions, leading to competition between deposition and chemical sputtering of already deposited material.
Keywords: Methyltrichlorosilane; Emission spectroscopy; Gas phase reaction; Plasma CVD

Radiofrequency argon plasma has been used for cobalt–boron surface modification. The argon plasma treatments enhance the cobalt–boron reactivity in hydrolysis of NaBH4 to release H2. The improved activity for cobalt–boron after plasma treatment is attributed to the enhanced synergistic effect between cobalt and boron and the large specific surface area resulting from the mesopores formed on the surface during the treatment. The synergistic effect, pore size and pore volume of cobalt–boron can be adjusted by changing either the plasma power and time or the argon pressure. High plasma power was found to cause the decomposition of cobalt–boron. Argon plasma treatment also improved the cycle performance of Co–B alloy in the hydrolysis of NaBH4 to release H2.
Keywords: Plasma treatment; Co–B amorphous alloy; NaBH4 ; H2

Novel Surface Modification of Sulfur by Plasma Polymerization and its Application in Dissimilar Rubber-Rubber Blends by Rui Guo; Auke G. Talma; Rabin N. Datta; Wilma K. Dierkes; Jacques W. M. Noordermeer (679-695).
In this study, surface modification of elemental sulfur by plasma polymerization with acetylene, perfluorohexane and acrylic acid is described, with the aim of changing the surface properties of sulfur without losing the bulk properties and reactivities in the vulcanization process. Significant improvements are obtained in dissimilar elastomer blends using the encapsulated sulfur powders. The conditions for the plasma polymerization were varied in order to obtain the optimal performance of the modified sulfur. The imperfections in the shell structure, obtained with plasma polymers, act as gateways to release sulfur for the vulcanization reaction.
Keywords: Plasma polymerization; Sulfur; Rubber blends; Vulcanization; Surface polarity

Surface inactivation is a phenomenon that causes poor adhesion. A wood surface exposed to contaminants such as dust or atmospheric grime can experience surface inactivation. Inactivation mechanisms can reduce the attractive forces on the wood surface and lead to a decrease in wettability. Plasma treatment has been applied to recover inactivated wood surfaces for better adhesion and bonding. Plasma treatment technology is very simple and the cost is rather low. In addition, this treatment produces no environmental pollution. In this study, low pressure plasma treatment was applied to reactivate the surfaces of spruce wood for glue bonding and to increase wettability after a 9-year period of natural surface inactivation. Changes in contact angles, surface energy, surface colour and bonding strength of inactivated and oxygen plasma treated wood surfaces were studied. Wettability, bonding and other mechanical strength properties of plywood panels increased with the oxygen plasma treatment.
Keywords: Wood; Plasma treatment; Natural surface inactivation; Wettability; Bonding strength