Plasma Chemistry and Plasma Processing (v.33, #2)

Homoepitaxial Si thick films have been deposited by mesoplasma chemical vapor deposition (CVD) with SiHCl3 (TCS)–H2–Ar gas mixtures. The addition of a small amount of H2 has been found to not only modify the film structure from polycrystalline to epitaxial but also effectively improve the deposition efficiency and film purity by removing Cl in the form of HCl. However, an excess introduction of H2 decreases the deposition efficiency owing to the shrinkage of the plasma flame. On the other hand, an increase in TCS flow rate increases the epitaxial deposition rate despite exhibiting a saturating tendency, while the material yield tends to decrease gradually due possibly to an increase in the Cl atoms. Also, we observed a critical limit in the TCS flow rate for epitaxial growth, beyond which a polycrystalline film resulted. However, when RF input power was increased, not only the upper limit of TCS flow rate for epitaxy was extended but also the deposition yield was improved so that the deposition rate reached ~700 nm/s with the material yield of >50 % at 30 kW input power with an H2/TCS ratio of 1.5. Additionally, high input power is found to be beneficial to decrease Cl atom incorporation into the film and improve the Hall mobility of the films. An epitaxial film with a Cl atom concentration of less than 3 × 1016 cm−3 and a Hall mobility as high as 250 cm2/(V·s) was obtained at 30 kW input power.
Keywords: High deposition rate; High material yield; Epitaxial silicon film; Mesoplasma CVD

Dry Reforming of Methane by DC Spark Discharge with a Rotating Electrode by Mohammad Mahdi Moshrefi; Fariborz Rashidi; Hamid Reza Bozorgzadeh; Mohammad Ehtemam Haghighi (453-466).
A novel type of plasma reactor having a rotating electrode is proposed for CO2 reforming of methane without catalyst at room temperature and atmospheric pressure. Results indicated that employing rotating ground electrode leads to a stable discharge for any period of time. Effects of feed composition, feed flow rate, applied power and electrodes separation on the carbon dioxide and methane conversions as well as the products selectivity were investigated. Increasing CO2/CH4 molar ratio in the feed favors the reagents conversion and consequently promotes the formation of hydrogen and carbon monoxide. If the target product is hydrogen, it is proposed to operate the reactor at CO2/CH4 = 1 molar ratio and if the target product is carbon monoxide then CO2/CH4 = 3 molar ratio is the preferred option for feed composition. This reactor system has advantages of stable operation and high conversion ability. Also, the obtained syngas with flexible molar ratio of H2 to CO is suitable for vast industrial applications.
Keywords: Dry reforming methane; Spark discharge; Rotating electrode

Pulsed Discharge Regeneration of Diesel Particulate Filters by K. Graupner; J. Binner; N. Fox; C. P. Garner; J. E. Harry; D. Hoare; K. S. Ladha; A. Mason; A. M. Williams (467-477).
A novel method for the removal of soot from a diesel particulate filter using pulsed electric discharges is presented. High voltage pulses of between 18 and 25 kV of nano to microsecond duration and with pulse energies of typically 100–200 mJ were applied to the filter via a series spark gap. Initial slow erosion of the soot layer proceeds via the formation of microdischarges. Subsequent spark discharges removed the accumulated soot more effectively from a larger filter volume. Average soot removal rates of ∼0.1–0.2 g/min were achieved at 50 Hz breakdown frequency by optimizing both electrode geometry and breakdown voltage. On-engine long term testing of the technology showed soot removal by pulsed discharge to be reliable, efficient and uniform; a total of 100 g of soot was deposited and removed over 18 filter regeneration cycles.
Keywords: Microdischarge; Spark; Environmental applications; Plasma surface modification

Designing Atmospheric-Pressure Plasma Sources for Surface Engineering of Nanomaterials by Wei Yan; Zhao Jun Han; Wen Zheng Liu; Xin Pei Lu; B. Toan Phung; Kostya Ostrikov (479-490).
Atmospheric-pressure plasma processing techniques emerge as efficient and convenient tools to engineer a variety of nanomaterials for advanced applications in nanoscience and nanotechnology. This work presents different methods, including using a quasi-sinusoidal high-voltage generator, a radio-frequency power supply, and a uni-polar pulse generator, to generate atmospheric-pressure plasmas in the jet or dielectric barrier discharge configurations. The applicability of the atmospheric-pressure plasma is exemplified by the surface modification of nanoparticles for polymeric nanocomposites. Dielectric measurements reveal that representative nanocomposites with plasma modified nanoparticles exhibit notably higher dielectric breakdown strength and a significantly extended lifetime.
Keywords: Atmospheric-pressure plasma; Dielectric barrier discharge; Plasma jet; Polymeric nanocomposites; Dielectric property

3D Unsteady State MHD Modeling of a 3-Phase AC Hot Graphite Electrodes Plasma Torch by Christophe Rehmet; Vandad Rohani; François Cauneau; Laurent Fulcheri (491-515).
We present, in this paper, the magnetohydrodynamic (MHD) modeling of a three-phase plasma torch. The MHD equations are solved using CFD software Code Saturne ®, a computational fluid dynamics software which is based on colocated finite volume. The model developed is 3-D, time dependent, and assumes Local Thermodynamic Equilibrium (LTE). Regarding numerical issues, the modeling of the three-phase AC discharge is particularly tricky since the arcs ignition, by the rotating electrical potential, is relative to the electron density of the electrode gap middle. However, despite these challenging difficulties, the numerical model has been successfully implemented by a LTE assumption. After a detailed description of the model, the results are presented, analyzed, and discussed. The influence of current and nitrogen flow rate over the arc characteristics are studied in terms of temperature, arc behavior (position and motion), velocity and electrical potential. The model gave significant information on parameters that could hardly be obtained experimentally. This study has shown the strong influence of the electrode jets on the overall arc and flow behavior. This work is likely to open the way toward a better understanding of three-phase discharges, which technologies are currently encountering an important development in many application fields.
Keywords: Plasma; MHD modelling; Alternative current; 3-Phase; Electric arc

Erosion of a hafnium cathode in Plasma Arc Cutting torch using oxygen as plasma gas is considered. It is shown that approximately 0.001 fraction of the evaporated particles participate in a net erosion, the rest of the evaporated particles return back to the cathode after spending some time in a near-cathode plasma. Along with erosion rate, the suggested equations allow one to the calculate current density at the cathode, the cathode temperature inside the arc attachment and the electron temperature at the cathode-plasma boundary. Comparison of the obtained values with the available information on these parameters shows a reasonable agreement.
Keywords: Plasma arc cutting; Electrode; Hafnium; Erosion rate

Etching Characteristics and Mechanisms of Mo and Al2O3 Thin Films in O2/Cl2/Ar Inductively Coupled Plasmas: Effect of Gas Mixing Ratios by Sungchil Kang; Alexander Efremov; Sun Jin Yun; Jinyoung Son; Kwang-Ho Kwon (527-538).
An investigation of etching behaviors for Mo and Al2O3 thin films in O2/Cl2/Ar inductively coupled plasmas at constant gas pressure (6 mTorr), input power (700 W) and bias power (200 W) was carried out. It was found that an increase in Ar mixing ratio for Cl2/Ar plasma results in non-monotonic etching rates with the maximums of 160 nm/min at 60 % Ar for Mo and 27 nm/min at 20 % Ar for Al2O3. The addition of O2 in the Cl2/Ar plasma causes the non-monotonic Mo etching rate (max. 320 nm/min at 40–45 % O2) while the Al2O3 etching rate decreases monotonically. The model-based analysis of etching kinetics allows one to relate the non-monotonic etching rates in Cl2/Ar plasma to the change in the etching regime from the ion-flux-limited mode (at low Ar mixing ratios) to the neutral-flux-limited mode (for high Ar mixing ratios). In the Cl2/O2/Ar plasma, the non-monotonic Mo etching rate is probably due to the change in reaction probability.
Keywords: Mo; Al2O3 ; O2/Cl2/Ar plasma; Plasma diagnostics; Modeling; Etching mechanism

1,3-Butadiene as an Adhesion Promoter Between Composite Resin and Dental Ceramic in a Dielectric Barrier Discharge Jet by Geum-Jun Han; Sung-No Chung; Bae-Hyeock Chun; Chang-Keun Kim; Kyu Hwan Oh; Byeong-Hoon Cho (539-551).
A pencil-type floating electrode dielectric barrier discharge (FE-DBD) jet was designed to improve adhesion of composite resin to dental ceramic by plasma deposition. Among various monomers used for plasma deposition, 1,3-butadiene (BD) merged as a promising monomer. Shear bond strength (SBS) and fracture modes were evaluated with specimens prepared at various flow rates of BD. The SBS values of the experimental groups were significantly higher than that of the negative control group and approached that of the positive control group when flow rate was higher than or equal to 2 sccm. Surface characterizations of plasma polymer-deposited ceramic surfaces were performed with FTIR-ATR and XPS. The deposited polymer on the ceramic surface contained methyl and methylene groups, ether and ester groups, and carbon–carbon double bonds. Formation of plasma deposited layer from BD was verified with TEM and EDS from specimens prepared using a focused ion beam technique. Adhesion between ceramic and composite resin was enhanced with BD plasma deposition using the FE-DBD jet. The adhesion effect was stemmed from chemical reactions between C=C double bonds remaining in the plasma deposited polymer and those in the adhesive monomers as well as increased wettability due to the ester and ether groups involved in deposited polymer.
Keywords: Dielectric barrier discharge (DBD) jet; Precursor monomer; Plasma enhanced chemical vapor deposition; Dental ceramic adhesion; Surface characterization