INSTITUTE OF PHYSICS PUBLISHING PLASMA SOURCES SCIENCE AND TECHNOLOGYPlasma Sources Sci. Technol. 14 (2005) 314–320 doi:10.1088/0963-0252/14/2/013Comparison of an atmospheric pressure,radio-frequency discharge operating intheαand γ modesX Yang1, M Moravej1,GRNowling1, S E Babayan2, J Panelon2,J P Chang1and R F Hicks11Chemical Engineering Department, University of California, Los Angeles, CA 90095, USA2Surfx Technologies LLC, 3617 Hayden Ave., Culver City, CA 90232, USAE-mail: [email protected] 23 July 2004Published 22 March 2005Online atstacks.iop.org/PSST/14/314AbstractThe α and γ modes of an atmospheric pressure, radio-frequency plasmahave been investigated. The plasma source consisted of two parallelelectrodes that were fed with helium and 0.4 vol% nitrogen. The transitionfrom α to γ was accompanied by a 40% drop in voltage, a 12% decrease incurrent and a surge in power density from 25 to 2083 W cm−3. Opticalemission confirmed that sheath breakdown occurred at the transition point.The maximum light intensity shifted from a position 0.25 mm above theelectrodes to right against the metal surfaces. The average density ofground-state nitrogen atoms produced in the atmospheric plasma wasdetermined from the temporal decay of N2(B) emission in the afterglow.It was found that 5.2% and 15.2% of the N2fed were dissociated into atomswhen the plasma was operated in the α and γ modes, respectively. Thelower efficiency of the γ discharge may be attributed to the non-uniformdistribution of the discharge between the electrodes.(Some figures in this article are in colour only in the electronic version)1. IntroductionPlasma technologies play a vital role in materials processingfor the microelectronics, automotive, aerospace and bio-medical industries. Chemically reactive plasma dischargesare widely used for etching, thin film deposition, surfaceactivation and bio-sterilization [1, 2]. Recently, there hasbeen increased interest in using atmospheric pressure plasmasfor materials processing [3–15]. These plasmas do notrequire vacuum systems and are suitable for continuous in-lineprocessing. Thermal atmospheric plasmas, i.e. torches, maybe used when the substrates are not thermally sensitive,or when the contact time can be kept very short [4].Low-temperature atmospheric plasmas include coronas [10],dielectric barrier discharges [11, 12], microhollow cathodedischarges [13], the one-atmosphere-uniform-glow-dischargeplasma (OAUGDP) [14] and surface-wave discharges [15].Coronas have been well studied since they were introducedseveral decades ago. However, the other designs weredeveloped more recently and are increasingly used formaterials processing.We have shown that a stable, capacitive discharge maybe produced at atmospheric pressure by feeding heliumor argon between two metal electrodes driven with radio-frequency (RF) power at 13.56 MHz [3, 16–26]. This plasmaoperates at temperatures below 100˚C and is homogenousin space and time. A high density of reactive species isgenerated in the source, which is suitable for the downstreamprocessing of substrates, regardless of their size, thicknessor shape [17, 21, 24]. So far this plasma source has beenused for plasma-enhanced chemical vapour deposition ofsilicon oxide, silicon nitride and amorphous hydrogenatedsilicon [20, 22, 25]. In addition, the atmospheric plasma hasbeen applied to the etching of kapton, silicon dioxide, tantalumand uranium oxide [16, 18, 26].RF capacitive discharges at moderate pressure(10–200 Torr) have been studied for many years [28–33].Raizer et al [29–32] have shown that these plasmas may0963-0252/05/020314+07$30.00 © 2005 IOP Publishing Ltd Printed in the UK 314Investigation on α and γ modesexist in two operating modes, α and γ , depending on thedominant ionization process. In the α mode, the plasma issustained by bulk ionization, while in the γ mode, it is sus-tained by secondary electron emission from the electrode sur-face. Sheath breakdown causes the transition from the bulkto the surface, i.e. the γ discharge occurs when the oscilla-tion amplitude of the plasma boundary exceeds half the totalthickness of the α sheath.An atmospheric pressure, capacitive discharge plasma,fed with helium gas, has been examined by Park et al [19].They found that their parallel-plate source could be operatedin the α mode up to a current density of 38 mA cm−2, andthereafter it converted to a filamentary arc. Recently, Shiet al [8] reported that a new operating regime may be producedin atmospheric pressure helium plasmas. They denoted thisregime the ‘recovery mode’, and showed that it consumedmuch greater quantities of RF power. Their electrode designconsisted of a powered metal pin held a few millimetres abovea grounded metal plate. The transition to the ‘recovery’mode was most likely due to sheath breakdown as observedin capacitive discharges operated at moderate pressures.In this paper, we describe the properties of the α and γoperating modes of an atmospheric pressure, RF plasma, fedwith helium and nitrogen. This system has been studiedwith current, voltage and power measurements and opticalemission spectroscopy. Furthermore, we have determined theconcentration of ground-state nitrogen atoms produced in theplasma. Somewhat surprisingly, the reactive species density isonly three times higher in the γ mode compared to the α mode.2. Experimental methodsA schematic of the atmospheric pressure plasma used in theseexperiments is shown in figure 1. It consisted of two parallelelectrodes made of aluminium and separated by a gap of1.6 mm across. The upper electrode was 7.6 × 7.6mm2, andwas connected to an RF power supply (13.56 MHz). It was(a)(b)Grounded ElectrodeAluminium BlockCeramic BlockMatchingNetworkAEProbeTo Power SupplyPoweredElectrode(0.3x0.3 in2)SapphireWindowTeflonPlasmaAfterglowMonochromator(PMT)TeflonHigh-PassFilterFigure 1. Schematic of the experimental apparatus: (a) side view,(b) top view.embedded in a ceramic block of dimensions 10.2 × 10.2cm2.An aluminium block, 10.2×10.2cm2, was placed downstreamof this electrode. The lower electrode, 10.2 cm wide and20.4 cm long, was grounded and cooled with chilled water. Theupper electrode was positioned above the centre of the lowerelectrode. These parts were assembled together to provide auniform duct 1.6 mm in height throughout the length of thedevice. Teflon spacers were placed in the duct to force thegas to flow past the upper