Xiawan Yang

Physics of high-pressure helium and argon radio-frequency plasmas

The physics of helium and argon rf discharges have been investigated in the pressure range from 50 to 760Torr. The plasma source consists of metal electrodes that are perforated to allow the gas to flow through them. Current and voltage plots were obtained at different purity levels and it was found that trace impurities do not affect the shape of the curves. The electron temperature was calculated using an energy balance on the unbound electrons. It increased with decreasing pressure from 1.1 to 2.4eV for helium and from 1.1 to 2.0 for argon. The plasma density calculated at a constant current density of 138mA∕cm2 ranged from 1.7×1011 to 9.3×1011cm−3 for helium and from 2.5×1011 to 2.4×1012cm−3 for argon, increasing with the pressure. At atmospheric pressure, the electron density of the argon plasma is 2.5 times that of the helium plasma.

Properties of an atmospheric pressure radio-frequency argon and nitrogen plasma

An atmospheric pressure capacitive discharge source has been developed that operates at power densities over 100 W cm−3. The ground state nitrogen atom concentration was measured at the exit of the source by titration with NO, and it was found to reach a maximum of 3.0 ± 0.8 × 1017 cm−3 at 6.0 vol% N2 in argon, 250 °C and 150 W cm−3. This is equivalent to 2.3 vol% of N atoms in the afterglow. At these conditions, the electron density and temperature are estimated to be 3.1 × 1012 cm−3 and 1.2 eV. A plug-flow model of the plasma and afterglow was developed, and it was determined that the maximum N atom concentration achievable is limited by three body recombination.

Comparison of an atmospheric pressure, radio-frequency discharge operating in the α and γ modes

The α and γ modes of an atmospheric pressure, radio-frequency plasma have been investigated. The plasma source consisted of two parallel electrodes that were fed with helium and 0.4 vol% nitrogen. The transition from α to γ was accompanied by a 40% drop in voltage, a 12% decrease in current and a surge in power density from 25 to 2083 W cm−3. Optical emission confirmed that sheath breakdown occurred at the transition point. The maximum light intensity shifted from a position 0.25 mm above the electrodes to right against the metal surfaces. The average density of ground-state nitrogen atoms produced in the atmospheric plasma was determined from the temporal decay of N2(B) emission in the afterglow. It was found that 5.2% and 15.2% of the N2 fed were dissociated into atoms when the plasma was operated in the α and γ modes, respectively. The lower efficiency of the γ discharge may be attributed to the non-uniform distribution of the discharge between the electrodes.

A radio-frequency nonequilibrium atmospheric pressure plasma operating with argon and oxygen

A capacitively coupled, atmospheric pressure plasma has been developed that produces a high concentration of reactive species at a gas temperature below 300°C. The concentration of ground-state oxygen atoms produced by the discharge was measured by NO titration, and found to equal 1.2vol%, or 1.2±0.4×1017cm−3, using 6.0vol% O2 in argon at 150W∕cm3. The ozone concentration determined at the same conditions was 4.3±0.5×1014cm−3. A model of the gas phase reactions was developed and yielded O atom and O3 concentrations in agreement with experiment. This plasma source etched Kapton® at 5.0μm∕s at 280°C and an electrode-to-sample spacing of 1.5cm. This fast etch rate is attributed to the high O atom flux generated by the plasma source.

Chamberless plasma deposition of glass coatings on plastic

A chamberless, remote plasma deposition process has been used to coat silicon and plastic substrates with glass at ambient conditions. The films were deposited by introducing an organosilane precursor into the afterglow of an atmospheric plasma fed with helium and 2 vol% oxygen. The precursors examined were hexamethyldisilazane, hexamethyldisiloxane, tetramethyldisiloxane, tetramethylcyclotetrasiloxane and tetraethoxysilane. With hexamethyldisilazane, glass films were deposited at rates of up to 0.25 µm min−1 and contained as little as 13.0 mol% hydroxyl groups. These films exhibited low porosity and superior hardness and abrasion resistance. With tetramethyldisiloxane, glass films were deposited at rates up to 0.91 µm min−1. However, these coatings contained significant amounts of carbon and hydroxyl impurities (~20 mol% OH), yielding a higher density of voids and poor abrasion resistance. In summary, the properties of glass films produced by remote atmospheric plasma deposition strongly depend on the organosilane precursor selected.

Characterization of the Active Species in the Afterglow of a Nitrogen and Helium Atmospheric-Pressure Plasma

The concentrations of the neutral active species in the afterglow of a nitrogen and helium atmospheric-pressure plasma have been determined by optical emission and absorption spectroscopy and by numerical modeling. For operation with 10 Torr N2 and 750 Torr He, at 15.5 W/cm3 rf power, 30.4 L/min flow rate, and a neutral temperature of 50°C, the plasma produced 4.8×1015 cm−3 of ground state nitrogen atoms, N(4S), 2.1×1013 cm−3 of N2(A3Σu), 1.2×1012 cm−3 of N2(B3Πg), and 3.2×109 cm−3 of N2(C3Πu). The concentration of nitrogen atoms and metastable state nitrogen molecules, N2(A), increased gradually with the rf power and the nitrogen partial pressure. Both the model and experiments indicate that ground-state nitrogen atoms are the dominant active species in the afterglow beyond 2.0 ms.

High stability of atmospheric pressure plasmas containing carbon tetrafluoride and sulfur hexafluoride

The properties of electronegative discharges operating at atmospheric pressure have been investigated. The plasma source consisted of two parallel metal electrodes into which helium was fed with 0.5–8.0 Torr carbon tetrafluoride or sulfur hexafluoride. It was found that the ionization mechanism changed from the α- to the γ-mode at a critical RF power level. In pure helium, this resulted in an abrupt drop in the voltage with increasing current, whereas for the fluorine-containing plasmas, a smooth, continuous transition was observed along the I–V curve. The electron densities in the plasmas fed with 4.0 Torr CF4 and SF6 were calculated to be 7.0± 2.0 × 1011 cm−3, or about 30 times lower than that estimated for the pure helium case. Free electrons were consumed by electron attachment to the gas molecules, which boosted the density of negative ions to ~1013 cm−3. Compared with pure He, the lower electron density in the fluorine-containing plasmas removed the ionization instabilities encountered in the γ -mode and made it possible to sustain these discharges over substantially larger areas.

Measurement of the fluorine atom concentration in a carbon tetrafluoride and helium atmospheric-pressure plasma

The fluorine atom concentration has been measured in the downstream region of a low-temperature, atmospheric-pressure plasma fed with 739.0 Torr helium and 12.6 Torr carbon tetrafluoride (3.1×1017 cm−3). The fluorine atoms were titrated with H2 molecules, and the HF reaction product was detected by infrared spectroscopy. The radio-frequency gas discharge produced 1.2×1015 cm−3 of F atoms, which was about two orders of magnitude higher than that found in low-pressure plasmas. The average electron density and temperature in the plasma was estimated to be 6.1×1011 cm−3 and 2.5 eV, respectively. A numerical model of the plasma indicated that most of the fluorine atoms were generated by the reaction of CF4 with metastable helium atoms.

Operating modes of an atmospheric pressure radio frequency plasma

The physics of an atmospheric pressure, radio frequency plasma has been investigated. The discharge is generated with helium and 0.4 vol.% nitrogen passing between two metal electrodes with a 3.0-mm gap. It was discovered that at a critical power density of 2.1 kW/cm/sup 3/, the plasma undergoes sheath breakdown and transitions from the /spl alpha/- to the /spl gamma/-mode. Photographs are presented showing the unique distribution of excited-state species under these conditions.

The reactions of silane in the afterglow of a helium-nitrogen plasma

Optical emission and infrared spectroscopy have been used to study the reactions of nitrogen atoms with silane introduced into the afterglow of an atmospheric-pressure helium–nitrogen plasma. Our experimental observations show that the direct reaction between silane and ground-state nitrogen atoms is slow, with an estimated rate constant no greater than 4 × 10−16 cm3 s−1, which is 2 × 102–3 × 105 times lower than the values previously reported. Using numerical modelling based on the spectroscopic measurements, we propose that silane dissociation occurs via a two-step process in which energy is first transferred to the molecule from vibrationally excited N2 and then the activated species collides with N atoms to form SiH3 and NH. A kinetic model has been developed that allows one to predict the distribution of gas-phase intermediates generated in a remote plasma-enhanced chemical vapour deposition process for silicon nitride.