Abstract-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 C and the densities of charge species range from 10 16 -10 19 cm 03 . 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 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 C, charged-particle densities are 10 11 -10 12 cm 03 , 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.Index Terms-Atmospheric pressure, corona discharge, dielectric barrier discharge, plasma jet, plasma torch, thermal and nonthermal plasmas, transferred arc.

The atmospheric pressure plasma jet ͑APPJ͒ ͓A. Schütze et al., IEEE Trans. Plasma Sci. 26, 1685 ͑1998͔͒ is a nonthermal, high pressure, uniform glow plasma discharge that produces a high velocity effluent stream of highly reactive chemical species. The discharge operates on a feedstock gas ͑e.g., He/O 2 /H 2 O͒, which flows between an outer, grounded, cylindrical electrode and an inner, coaxial electrode powered at 13.56 MHz rf. While passing through the plasma, the feedgas becomes excited, dissociated or ionized by electron impact. Once the gas exits the discharge volume, ions and electrons are rapidly lost by recombination, but the fast-flowing effluent still contains neutral metastable species ͑e.g., O 2 * , He*͒ and radicals ͑e.g., O, OH͒. This reactive effluent has been shown to be an effective neutralizer of surrogates for anthrax spores and mustard blister agent. Unlike conventional wet decontamination methods, the plasma effluent does not cause corrosion and it does not destroy wiring, electronics, or most plastics, making it highly suitable for decontamination of sensitive equipment and interior spaces. Furthermore, the reactive species in the effluent rapidly degrade into harmless products leaving no lingering residue or harmful by-products.

A plasma jet has been developed for etching materials at atmospheric pressure and between 100 and 275 • C. Gas mixtures containing helium, oxygen and carbon tetrafluoride were passed between an outer, grounded electrode and a centre electrode, which was driven by 13.56 MHz radio frequency power at 50 to 500 W. At a flow rate of 51 l min −1 , a stable, arc-free discharge was produced. This discharge extended out through a nozzle at the end of the electrodes, forming a plasma jet. Materials placed 0.5 cm downstream from the nozzle were etched at the following maximum rates: 8.0 µm min −1 for Kapton (O 2 and He only), 1.5 µm min −1 for silicon dioxide, 2.0 µm min −1 for tantalum and 1.0 µm min −1 for tungsten. Optical emission spectroscopy was used to identify the electronically excited species inside the plasma and outside in the jet effluent.

An atmospheric pressure plasma source operated by radio frequency power has been developed. This source produces a unique discharge that is volumetric and homogeneous at atmospheric pressure with a gas temperature below 300°C. It also produces a large quantity of oxygen atoms, ϳ5ϫ10 15 cm Ϫ3 , which has important value for materials applications. A theoretical model shows electron densities of 0.2-2ϫ10 11 cm Ϫ3 and characteristic electron energies of 2-4 eV for helium discharges at a power level of 3-30 W cm Ϫ3 .

The reaction chemistry in the afterglow of a non-equilibrium, capacitive discharge, operated at 600 Torr total pressure with (0.5 to 5.0) × 10 17 cm -3 of oxygen in helium, has been examined by ultraviolet absorption spectroscopy, optical emission spectroscopy, and numerical modeling. The densities of the active species,and O 3 , have been determined as a function of the operating conditions. At RF power densities between 6.1 and 30.5 W/cm 3 and a neutral temperature of 100 ( 40°C, the plasma generated (0.2 to 1.0) × 10 16 cm -3 of O( 3 P) and O 2 ( 1 ∆ g ), (0.2 to 2.0) × 10 15 cm -3 of O 2 ( 1 Σ g + ), and (0.1 to 4.0) × 10 15 cm -3 of O 3 . After the power was turned off, the singlet-sigma and singlet-delta states decayed within 0.1 and 30.0 ms, respectively. The concentration of oxygen atoms remained constant for about 0.5 ms, then fell rapidly due to recombination with O 2 to form O 3 . It was found that the etching rate of polyimide correlated with the concentration of oxygen atoms in the afterglow, indicating that the O atoms were the active species involved in this process.