Steven E. Babayan

The atmospheric-pressure plasma jet: a review and comparison to other plasma sources

Atmospheric-pressure plasmas are used in a variety of materials processes. Traditional sources include transferred arcs, plasma torches, corona discharges, and dielectric barrier discharges. In arcs and torches, the electron and neutral temperatures exceed 3000/spl deg/C and the densities of charge species range from 10/sup 16/-10/sup 19/ cm/sup -3/. Due to the high gas temperature, these plasmas are used primarily in metallurgy. Corona and dielectric barrier discharges produce nonequilibrium plasmas with gas temperatures between 50-400/spl deg/C and densities of charged species typical of weakly ionized gases. However, since these discharges are nonuniform, their use in materials processing is limited. Recently, an atmospheric-pressure plasma jet has been developed, which exhibits many characteristics of a conventional, low-pressure glow discharge. In the jet, the gas temperature ranges from 25-200/spl deg/C, charged-particle densities are 10/sup 11/-10/sup 12/ cm/sup -3/, and reactive species are present in high concentrations, i.e., 10-100 ppm. Since this source may be scaled to treat large areas, it could be used in applications which have been restricted to vacuum. In this paper, the physics and chemistry of the plasma jet and other atmospheric-pressure sources are reviewed.

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 0.4u2009vol% 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 2083u2009Wu2009cm−3. Optical emission confirmed that sheath breakdown occurred at the transition point. The maximum light intensity shifted from a position 0.25u2009mm 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.

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 2u2009vol% oxygen. The precursors examined were hexamethyldisilazane, hexamethyldisiloxane, tetramethyldisiloxane, tetramethylcyclotetrasiloxane and tetraethoxysilane. With hexamethyldisilazane, glass films were deposited at rates of up to 0.25u2009µmu2009min−1 and contained as little as 13.0u2009mol% hydroxyl groups. These films exhibited low porosity and superior hardness and abrasion resistance. With tetramethyldisiloxane, glass films were deposited at rates up to 0.91u2009µmu2009min−1. However, these coatings contained significant amounts of carbon and hydroxyl impurities (~20u2009mol% 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.

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 0.5–8.0u2009Torr 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 4.0u2009Torr CF4 and SF6 were calculated to be 7.0± 2.0 × 1011u2009cm−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 ~1013u2009cm−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.

Materials processing with atmospheric-pressure plasma jets

Summary form only given. Atmospheric-pressure plasma jets can be used for a wide range of materials processing applications, including surface cleaning and modification, selective etching, and thin-film deposition. The plasma source consists of two closely spaced electrodes through which helium and other gases flow (O/sub 2/, CF/sub 4/, etc.). A variety of electrode configurations can be used, and the source is suitable for continuous and large-area processing of materials. Another advantage of the plasma jet is that it achieves etching and deposition rates that are higher than those obtained in low-pressure plasmas. At the meeting, the plasma source will be described in detail, and results for several materials processing applications will be presented.

Properties of Reactive Argon Plasmas at Atmospheric Pressure

Summary form only given. A new atmospheric pressure plasma source has been developed that shows exceedingly high processing rates. For example, kapton films have been etched at 5.0 mum/s using an argon and oxygen discharge with 6.0 vol.% O2 and a temperature of 280degC. The plasma source consisted of a small quartz tube that was capacitively coupled to radio frequency power at 13.56 MHz. The input plasma power could be increased up to 150 W/cm3 without arcing, or forming a streamer like discharge. At this power density, the gas temperature was determined by spectroscopic methods to be 300plusmn30degC. The O atom concentration was measured in the plasma afterglow by nitric oxide titration, and was found to be 1.2plusmn0.6times1017 cm-3 at 150 W/cm3 and 6.0 vol.% O2 in Ar. The concentration of ozone in the downstream region equaled 4.3plusmn0.5times1014 cm -3, as determined by UV absorption spectroscopy. These results were found to be in good agreement with a numerical model of the plasma and afterglow that included the reaction mechanism and the plasma electron density and temperature as calculated from current-voltage measurements