Plasma Physics 1 APPH E6101x Columbia University Fall, 2015

Lecture22: Atmospheric Plasma Plasma Physics 1 APPH E6101x Columbia University Fall, 2015

1 http://www.plasmatreat.com/company/about-us.html

2 http://www.tantec.com

3 http://www.tantec.com/atmospheric-plasma-improved-features.html

4 5 6 PHYSICS OF PLASMAS 22, 121901 (2015)

Preface to Special Topic: Plasmas for Medical Applications Michael Keidar1,a) and Eric Robert2 1Mechanical and Aerospace Engineering, Department of Neurological Surgery, The George Washington University, Washington, DC 20052, USA 2GREMI, CNRS/Universite d’Orleans, 45067 Orleans Cedex 2, France (Received 30 June 2015; accepted 2 July 2015; published online 28 October 2015) Intense research effort over last few decades in low-temperature (or cold) atmospheric plasma application in bioengineering led to the foundation of a new scientiﬁc ﬁeld, plasma medicine. Cold atmospheric plasmas (CAP) produce various chemically reactive species including reactive oxygen species (ROS) and reactive nitrogen species (RNS). It has been found that these reactive species play an important role in the interaction of CAP with prokaryotic and eukaryotic cells triggering various signaling pathways in cells. VC 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4933406]

There is convincing evidence that cold atmospheric topic section, there are several papers dedicated to plasma plasmas (CAP) interaction with tissue allows targeted cell re- diagnostics. moval without necrosis, i.e., cell disruption. In fact, it was Shashurin and Keidar presented a mini review of diag- determined that CAP affects cells via a programmable pro- nostic approaches for the low-frequency atmospheric plasma cess called apoptosis.1–3 Apoptosis is a multi-step process jets. Robert et al. described analysis of atmospheric pressure leading to cell death, and this process is the one inherit to plasma propagation inside the long dielectric tubes through cells in the human body. non-intrusive and non-perturbative time resolving bi- Although CAP has been shown to be lethal to bacteria, directional electric field (EF) measurements. In particular, it appears to cause little to no damage to mammalian cells. their study reveals that plasma propagation occurs in a region Having different structure and morphology than that of pro- where longitudinal EF exists ahead of the ionization front. karyotes, mammalian cells exhibit different responses to EF components have a few kV/cm in amplitude in the case physical and chemical stresses induced by CAP. The ability of helium or neon plasmas and almost constant along a few of CAP to kill bacteria and to accelerate the proliferation of7 tens of cm long capillary. These measurements are in excel- specific tissue cells opened up the possibility to use plasma lent agreement with the previous calculations. in various medical applications. CAP is already proven to be Evaluation of efficacy of the plasma-activated media on effective in wound healing, skin diseases, hospital hygiene, cancer cells was described by Mohades et al. It was observed sterilization, antifungal treatments, dental care, and cosmet- that the chemistry induced by plasma in the aqueous state ics targeted cell/tissue removal.4–6 More recently, potential becomes crucial and usually determines the biological of CAP application in cancer therapy has been explored.7–10 response. They report on the measurements of concentrations It was demonstrated that CAP treatment leads to selective of the hydrogen peroxide, a species known to have strong bi- eradication of cancer cells in vitro and reduction of tumor ological effects. Yang et al. presented a study of atmospheric size in vivo. Recently, few successful clinical trials have pressure cold plasma effect on chemotherapeutic drugs- been performed. resistant cells. Incubation with plasma-treated medium for Further progress in this highly interdisciplinary field can 20 s induced more than 85% death rate in Bel7402 cells, be expected as a result of strong collaborative research while the same death ratio was achieved when Bel7402/5FU between plasma physicists, chemists, microbiologists, and cells were treated for as long as 300 s. They established that medical doctors. the hydrogen peroxide in the medium played a leading role It is due to such increased interest in plasma medicine to in the cytotoxicity effects. Miller et al. addressed an impor- the plasma physics community that Physics of Plasmas dedi- tant problem of atmospheric plasma effect on angiogenesis, cates this Special Topic Section on Plasma Medicine. The which is the process of formation of new blood vessels from proposed Special Topic Section will address the following pre-existing vessels. Stimulation of angiogenesis is a promis- aspects of plasma medicine: ing and an important step in treatment of peripheral artery disease. They demonstrated that atmospheric plasma treat- • plasma devices and plasma diagnostics for medical ment stimulates the production of VEGF, MMP-9, and applications CXCL 1 that, in turn, induces angiogenesis in mouse aortic • biological aspects of plasma interaction with cells and rela- rings. Ishaq et al. described biological mechanisms that tionship between plasma parameters and cell responses underpin plasma-induced death in cancer cells. In particular, Recently, various unique plasma diagnostic tools were they focused on effect of the plasma treatment dose on the developed to probe low-temperature plasmas. In this special expression of tumour suppressor protein TP73. They found that the plasma treatment induces dose-dependent up-regula- a)[email protected] tion of TP73 gene expression, resulting in significantly

1070-664X/2015/22(12)/121901/2/$30.00 22, 121901-1 VC 2015 AIP Publishing LLC

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.59.222.12 On: Wed, 09 Dec 2015 02:37:14 From: Physics of Plasmas [email protected] Subject: Now Online: Special Topic – Plasmas for Medical Applications Date: November 12, 2015 at 9:01 AM To: [email protected]

The #1 Journal in Google Scholar Metrics for the Plasma and Fusion Category View Online Or Forward to a friend.

PHYSICS OF PLASMAS 22, 121901 (2015)

Preface to Special Topic: Plasmas for Medical Applications 1,a) 2 Now Online: Special Topic – Upcoming Events Michael Keidar and Eric Robert DPP 57th Annual Meeting 1 Mechanical and Aerospace Engineering, Department of Neurological Surgery, The George Washington Plasmas for Medical November 16-18, 2015 University, Washington, DC 20052, USA Savannah, GA 2GREMI, CNRS/Universite d’Orleans, 45067 Orleans Cedex 2, France Applications These articles have been made free to download for a limited (Received 30 June 2015; accepted 2 July 2015; published online 28 October 2015) time. Intense research effort over last few decades in low-temperature (or cold) atmospheric plasma application in bioengineering led to the foundation of a new scientiﬁc ﬁeld, plasma medicine. Cold Preface to Special Topic: Plasmas for Medical Applications atmospheric plasmas (CAP) produce various chemically reactive species including reactive oxygen Michael Keidar, Eric Robert species (ROS) and reactive nitrogen species (RNS). It has been found that these reactive species Phys. Plasmas 22, 121901 (2015) play an important role in the interaction of CAP with prokaryotic and eukaryotic cells triggering Evaluation of the effects of a plasma activated medium on various signaling pathways in cells. VC 2015 AIP Publishing LLC. cancer cells [http://dx.doi.org/10.1063/1.4933406] S. Mohades, M. Laroussi, J. Sears, N. Barekzi, H. Razavi Phys. Plasmas 22, 122001 (2015)

There is convincing evidence that cold atmospheric topic section, there are several papers dedicatedExperimental to plasma approaches for studying non-equilibrium plasmas (CAP) interaction with tissue allows targeted cell re- diagnostics. atmospheric plasma jets A. Shashurin, M. Keidar moval without necrosis, i.e., cell disruption. In fact, it was Shashurin and Keidar presented a mini reviewPhys. ofPlasmas diag- 22, 122002 (2015) determined that CAP affects cells via a programmable pro- nostic approaches for the low-frequency atmospheric plasma cess called apoptosis.1–3 Apoptosis is a multi-step process jets. Robert et al. described analysis of atmosphericIntracellular pressure effects of atmospheric-pressure plasmas on plasma propagation inside the long dielectric tubesmelanoma through cancer cells leading to cell death, and this process is the one inherit to M. Ishaq, K. Bazaka, K. Ostrikov cells in the human body. non-intrusive and non-perturbative time resolvingPhys. Plasmas bi- 22, 122003 (2015) Although CAP has been shown to be lethal to bacteria, directional electric field (EF) measurements. In particular, it appears to cause little to no damage to mammalian cells. their study reveals that plasma propagation occursCancer in a region therapy using non-thermal atmospheric pressure plasma with ultra-high electron density Having different structure and morphology than that of pro- where longitudinal EF exists ahead of the ionizationHiromasa front. Tanaka, Masaaki Mizuno, Shinya Toyokuni, Shoichi karyotes, mammalian cells exhibit different responses to EF components have a few kV/cm in amplitudeMaruyama, in the case Yasuhiro Kodera, Hiroko Terasaki, Tetsuo Adachi, physical and chemical stresses induced by CAP. The ability of helium or neon plasmas and almost constant alongMasashi aKato, few Fumitaka Kikkawa, Masaru Hori of CAP to kill bacteria and to accelerate the proliferation of tens of cm long capillary. These measurements arePhys. in Plasmas excel- 22, 122004 (2015) specific tissue cells opened up the possibility to use plasma lent agreement with the previous calculations. Microsecond-pulsed dielectric barrier discharge plasma in various medical applications. CAP is already proven to be Evaluation of efficacy of the plasma-activatedstimulation media on of tissue macrophages for treatment of effective in wound healing, skin diseases, hospital hygiene, cancer cells was described by Mohades et al. It wasperipheral observed vascular disease sterilization, antifungal treatments, dental care, and cosmet- that the chemistry induced by plasma in the aqueousV. Miller, A. state Lin, F. Kako, K. Gabunia, S. Kelemen, J. ics targeted cell/tissue removal.4–6 More recently, potential becomes crucial and usually determines theBrettschneider, biological G. Fridman, A. Fridman, M. Autieri Phys. Plasmas 22, 122005 (2015) of CAP application in cancer therapy has been explored.7–10 response. They report on the measurements of concentrations It was demonstrated that CAP treatment leads to selective of the hydrogen peroxide, a species known to haveEffects strong of atmospheric bi- pressure cold plasma on human eradication of cancer cells in vitro and reduction of tumor ological effects. Yang et al. presented a study of atmospherichepatocarcinoma cell and its 5-fluorouracil resistant cell line size in vivo. Recently, few successful clinical trials have pressure cold plasma effect on chemotherapeuticH. Yang, drugs- R. Lu, Y. Xian, L. Gan, X. Lu, X. Yang been performed. resistant cells. Incubation with plasma-treated mediumPhys. Plasmas for 22, 122006 (2015) Further progress in this highly interdisciplinary field can 20 s induced more than 85% death rate in Bel7402 cells, be expected as a result of strong collaborative research while the same death ratio was achieved when Bel7402/5FUNew insights on the propagation of pulsed atmospheric plasma streams: From single jet to multi jet arrays between plasma physicists, chemists, microbiologists, and cells were treated for as long as 300 s. They establishedE. Robert, thatT. Darny, S. Dozias, S. Iseni, J. M. Pouvesle medical doctors. the hydrogen peroxide in the medium played a leadingPhys. Plasmas role 22, 122007 (2015) It is due to such increased interest in plasma medicine to in the cytotoxicity effects. Miller et al. addressed an important problem of atmospheric plasma effect on angiogenesis,Impact of plasma jet vacuum ultraviolet radiation on the plasma physics community that Physics of Plasmas dedi- reactive oxygen species generation in bio-relevant liquids cates this Special Topic Section on Plasma Medicine. The which is the process of formation of new blood vesselsH. Jablonowski, from R. Bussiahn, M. U. Hammer, K.-D. Weltmann, Th. proposed Special Topic Section will address the following pre-existing vessels. Stimulation of angiogenesisvon is a Woedtke, promis- S. Reuter aspects of plasma medicine: ing and an important step in treatment of peripheralPhys. Plasmas artery 22, 122008 (2015) disease. They demonstrated that atmospheric plasma treat- • plasma devices and plasma diagnostics for medical 8 Unsubscribe | Forward to a friend | Update Preferences ment stimulates the production of VEGF, MMP-9, and applications Copyright © 2015 AIP Publishing. All rights reserved. CXCL 1 that, in turn, induces angiogenesis in mouse1305 Walt aortic Whitman Rd., Melville, NY 11747 • biological aspects of plasma interaction with cells and rela- You are receiving this email because you are subscribed to alerts on our website or have submitted an article for rings. Ishaq et al. described biological mechanisms that tionship between plasma parameters and cell responses publication at some time. To guarantee delivery of this email please add [email protected] to your address book underpin plasma-induced death in cancer cells. In particular,and safe senders list. Recently, various unique plasma diagnostic tools were they focused on effect of the plasma treatment dose on the developed to probe low-temperature plasmas. In this special expression of tumour suppressor protein TP73. They found that the plasma treatment induces dose-dependent up-regula- a)[email protected] tion of TP73 gene expression, resulting in significantly

1070-664X/2015/22(12)/121901/2/$30.00 22, 121901-1 VC 2015 AIP Publishing LLC

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 128.59.222.12 On: Wed, 09 Dec 2015 02:37:14 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998 1685 The Atmospheric-Pressure Plasma Jet: A Review and Comparison to Other Plasma Sources Andreas Schutze,¨ James Y. Jeong, Steven E. Babayan, Jaeyoung Park, Gary S. Selwyn, and Robert F. Hicks

(Invited Paper)

Abstract—Atmospheric-pressure plasmas are used in a variety that materials processing proceeds at the same rate over large of materials processes. Traditional sources include transferred substrate areas. On the other hand, operating the plasma at arcs, plasma torches, corona discharges, and dielectric barrier reduced pressure has several drawbacks. Vacuum systems are discharges. In arcs and torches, the electron and neutral temperatures exceed 3000 C and the densities of charge species expensive and require maintenance. Load locks and robotic range from 10 –10 cm . Due to the high gas temperature, assemblies must be used to shuttle materials in and out of these plasmas are used primarily in metallurgy. Corona and di- vacuum. Also, the size of the object that can be treated is electric barrier discharges produce nonequilibrium plasmas with limited by the size of the vacuum chamber. gas temperatures between 50–400 C and densities of charged Atmospheric-pressure plasmas overcome the disadvantages species typical of weakly ionized gases. However, since these discharges are nonuniform, their use in materials processing is of vacuum operation. However, the difficulty of sustaining a limited. Recently, an atmospheric-pressure plasma jet has been glow discharge under these conditions leads to a new set of developed, which exhibits many characteristics of a conventional, challenges.Dr. Gary S. HigherSelwyn, Ph.D voltages founded are APJeT, required Inc., in for 2002 gas and breakdown serves as its low-pressure glow discharge. In the jet, the gas temperature atChief 760 Technology torr, and Officer. often arcingDr. Selwyn occurs is the principal between founder the electrodes.of APJeT, Inc. ranges from 25–200 C, charged-particle densities are 10 –10 Inand some was applications, successful in obtaining such as APJeT's plasma exclusive torches, license arcing for the is patent incm , and reactive species are present in high concentrations, tentionallyportfolio from sought LANL [3],and the [4]. necessary However, investment to prevent capital from arcing corporate and i.e., 10–100 ppm. Since this source may be scaled to treat large sponsors. He served as Chairman of the Board at APJeT, Inc., and serves areas, it could be used in applications which have been restricted loweras its the Director. gas temperature, When he is not several inventing, schemes Dr. Selwyn have enjoys been scuba devised, diving, to vacuum. In this paper, the physics and chemistry of the plasma suchskiing, as the sailing, use fishing, of pointed fitness electrodes activities, hiking, in corona camping, discharges museums, jet and other atmospheric-pressure sources are reviewed. [5]classical and insulating music, and insertsopera. Dr. in Selwyn dielectric has been barrier a pioneer discharges in the field [6]. of Index Terms—Atmospheric pressure, corona discharge, dielec- Aplasma drawback chemistry of theseand plasma sources processing is that of materials the plasmas for 25 years. are He not is tric barrier discharge, plasma jet, plasma torch, thermal and uniformwidely throughout known for his the discovery volume. of Recently, plasma-generated a plasma particulate jet has contamination in chip manufacturing processes nonthermal plasmas, transferred arc. been developed which uses flowing helium and a special electrode design to prevent arcing [7]. This source can etch I. INTRODUCTION 9 and deposit materials at low temperatures and may be suitable for a wide range of applications. OW-PRESSURE plasmas have found wide applications A number of review articles have been published on Lin materials processing and play a key role in manu- atmospheric-pressure plasmas [3], [4], [6], [8], [9]. However, facturing semiconductor devices [1], [2]. The advantages of these articles are devoted to either thermal, or nonthermal plasmas are well known. They generate high concentrations sources, and do not include recent advances in the field. In of reactive species that can etch and deposit thin films at rates this paper, we give an overview of these plasma sources and up to 10 m/min. The temperature of the gas is usually below highlight the recent development of the atmospheric-pressure 150 C, so that thermally sensitive substrates are not damaged. plasma jet. First, we consider the effect of pressure on the The ions produced in the plasma can be accelerated toward a physical properties of plasmas. Then in the following sections, substrate to cause directional etching of submicron features. each atmospheric-pressure source is examined. Topics of In addition, a uniform glow discharge can be generated, so interest include the current–voltage characteristics, electron and neutral temperatures, densities of charged particles, and Manuscript received March 30, 1998. This work was supported in part gas compositions for oxygen-based systems. Finally, in the last by the U.S. Department of Energy, the University of California, the Basic Energy Sciences, Environmental Management Sciences Program, and the section, a quantitative comparison is made between traditional Office of Science and Risk Policy under Award DE-F5607-96ER45621. The sources and the atmospheric-pressure plasma jet. work of A. Schutze¨ was supported by a fellowship from the Deutsche Forschungsgemeinschaft (DFG). The work of J. Y. Jeong and S. E. Babayan and was supported by fellowships from the University of California Center II. PROPERTIES OF PLASMAS for Environmental Risk Reduction and the Toxic Substances Research and Training Program. To ignite a plasma, the breakdown-voltage for the gas A. Schütze, J. Y. Jeong, S. E. Babayan, and R. F. Hicks are with the must be exceeded. This voltage depends on the electrode Department of Chemical Engineering, University of California, Los Angeles, spacing and the pressure as follows [2], [8], [10]: CA 90095-1592 USA (e-mail: [email protected]). J. Park and G. S. Selwyn are with Los Alamos National Laboratory, Los Alamos, NM 87545 USA. (1) Publisher Item Identifier S 0093-3813(98)09643-X.

0093–3813/98$10.00  1998 IEEE 11.1 DC-Discharges 333

Combining (11.11) and (11.12), we recover the Bessel-type equation (4.41), which has the solution ne(r) ne(0)J0(2.405r/a) shown in Fig. 4.14. This example shows how a short segment= of the positive column maintains its existence. Further, we can conclude that the positive column can be extended to an arbitrary length,2 provided that the discharge current is kept constant. In other words, the positive column requires a constant voltage drop per unit length (= axial electric ﬁeld), as can be seen in Fig. 11.2b.

11.1.8 Similarity Laws

Two gas discharges in cylindrical tubes have the same plasma parameters ne and Te, when the following similarity conditions are fulfilled. The mean energy gain of an electron in the dc electric field is proportional to the product of the electric field and the mean free path, Eλmfp.Sincethemeanfreepath is inversely proportional to the gas density, the energetics in the different regions 11.1of DC-Discharges a glow discharge is determined by the quantity E/p, where the gas pressure p 325 represents gas density. Therefore, the electron temperature in the positive column is Fig. 11.2onlyAnatomy a function of of a glowE/p and the kindAston- of gas used. darkspace dischargeIn in particular, a long Townsend’s electron(a) cathode- multiplication Faraday- coefficient can be writtenanode- as cylindricalα pf tube(E with/p).Itisthenstraightforwardtoconcludethatthethicknessofthenormal plane cathodecathode= (C) and fall anode (11.8) (A). must be a fixed number of mean free paths for ionization, hence C A (a)Variousdarkspacesanddn 1/p.Likewise,thevoltagedropUn of the normal cathode fall is a fixed luminousmultiple∝ regions of are the energy gain per mean free path and therefore independent of gas indicated.pressure. (b)Sketchofthe This allows the following conclusions about the similarity parameter for electricthe potential dischargeΦ and current. axial The total discharge current is the same at each axial position electricof field the dischargeE in the normal tube. Hence, we cancathode calculatenegative it at the cathode.positive There, we haveanode a glow discharge layer glow column glow mobility-limited ion current, ji eniµi E0,withtheelectricfieldatthecathode (b)= E0 2Un/dn.Theiondensityisobtainedfromtheionspacechargeinthecathode = region nie ε0 E0/dn.Then,with je γ ji,thetotalcurrentbecomesΦ = = 2 E0 2 j ε0µi (1 γE) p . (11.13) = dn + ∝

2 1 1 The proportionality to p results from E0 p, dn p− ,andµi p− .Hence, positivej/p2 columnis a similarity becomes parameter. longer and fills∝ the additional∝ volume.∝ It is found that when the discharge gap is made shorter, the discharge is maintained even as the positive column and the Faraday dark space have been “consumed”, leaving only 2 Prof. Sanborn C. Brown told the anecdote of the gas-discharge pioneer Wilhelm Hittorf, who the negativeattempted glowto find the and maximum the dark length space of the adjacent positive column to the [353]. electrodes. “Week after week his discharge Thistube behavior grew as he added shows meter that afterthe meter glow [. . . ] His discharge tube went all can the way be across split the into room, two turned functional and units:came the back, cathodic turned again part, until which his laboratory is responsible seemed full for of thin liberating glass tubing. a sufficient It was summer number [. . . ] of and he opened the window to make it bearable. Suddenly from outside came the howl of a pack of electronsdogs in from full pursuit the cold and flying cathode through and the window ends in came the a terrified negative cat to glow, land [. and. . ] in the middle anodic of part consistingthe weeks of and positive weeks of column labor. ‘Until and an a unfortunate transition accident layer terminatedto the anode. my experiment’, Hittorf Thewrote, positive ‘the positive column column is appeared the active to extend part without in limit.’fluorescent ” tubes, in which a small admixture of mercury to a low-pressure10 argon discharge generates UV light that is converted to “white” visible light in a fluorescent coating of the inner tube wall. The typical red colour of the neon spectrum is found in neon displays. The potential distribution in a glow discharge and the corresponding axial electric field are sketched in Fig. 11.2b. The largest voltage drop occurs in the cathode region. The negative glow is mostly field-free, E 0. In the positive column, the axial electric field is constant. There is a small voltage≈ drop across the anode layer.

11.1.3 Processes in the Cathode Region

The key to understanding electrical breakdown and the steady-state of the cathode region is the interplay of electron multiplication in the gas and release of electrons from the cathode by ion bombardment.

11.1.3.1 Gas Breakdown When a primary electron is released from the cathode and is accelerated in the electric ﬁeld, which we assume to be homogeneous before breakdown, it can ionize gas atoms after a typical mean free path for ionization, λi.Inthisionizationprocess an electron-ion pair is created. Now two electrons are accelerated that generate 11.1 DC-Discharges 325

Fig. 11.2 Anatomy of a glow Aston- darkspace discharge in a long (a) cathode- Faraday- anode- cylindrical tube with plane cathode (C) and anode (A). (a)Variousdarkspacesand C A luminous regions are indicated. (b)Sketchofthe electric potential Φ and axial electric ﬁeld E in the normal cathode negative positive anode glow discharge layer glow column glow (b) Φ

334E 11 Plasma Generation

The influence of the tube radius a on the discharge is given by the production-loss 2 1 positivebalance column (4.43), becomesνion D longera (2.405 and/a) .Noticing,that fills the additionalνion volume.p and ItD isa foundp− , that whenwe find the dischargethat the product gap= is of made gas pressure shorter, and the tube discharge radius, pa is∝ maintained, must be a similarity even∝ as the positiveparameter. column Similar and scalings the Faraday with pd darkwere space discussed have forbeen the “consumed”, breakdown voltage, leaving the only thethickness negative of glow the normal and the glow, dark and space the adjacent dimensions to theof the electrodes. hollow cathode. ThisLast behavior not least, shows the pd thatscaling the glow explains, discharge why the can diameter be split of into fluorescent two functional tubes units:could the be cathodic reduced from part, 1 which.5′′ to 1′′ is,orevento1 responsible/4 for′′ in liberating compact fluorescent a sufficient tubes, number with of electronsacorrespondingincreaseingaspressure,afterphosphorcoatingsweredeveloped from the cold cathode and ends in the negative glow, and the anodic part consistingthat could of withstand positive the column increased and heata transition flux. It is layer also not to the surprising, anode. that modern gas discharges at atmospheric pressure are tiny objects of sub-millimeter dimensions. The positive column is the active part in fluorescent tubes, in which a small admixture of mercury to a low-pressure11 argon discharge generates UV light that is converted to “white” visible light in a fluorescent coating of the inner tube wall. The11.1.9 typical Discharge red colour of Modes the neon of Thermionic spectrum is found Discharges in neon displays. The potential distribution in a glow discharge and the corresponding axial elec- Low-pressure discharges with thermionic emitters are influenced by the formation tricof field electron are space-chargesketched in Fig. in the 11.2b. cathode The region. largest This voltage space-charge drop occurs cloud can in the limit cathode the region. The negative glow is mostly field-free, E 0. In the positive column, the discharge current by forming a virtual cathode (see≈ Sect. 9.2.2), or by a sufficient axialamount electric of ion field current, is constant. can vanish There and is leave a small a temperature-limited voltage drop across emission the anode current. layer. Four distinct discharge modes could be identified [354]:

11.1.31. the Processes anode-glow mode in the Cathode Region 2. the ball-of-fire mode 3. the Langmuir mode The key to understanding electrical breakdown and the steady-state of the cathode 4. the temperature-limited mode region is the interplay of electron multiplication in the gas and release of electrons fromFigure the cathode 11.10 showsby ion a bombardment. typical volt-ampere characteristic of a thermionic discharge [355] and identifies the topology of the potential distribution between cathode and anode. The anode glow mode is found in the regime A–B′ on the hysteresis curve. 11.1.3.1The corresponding Gas Breakdown potential distribution shows the formation of a potential barrier When(virtual a primary cathode) electron before the is released cathode. from In a layer the cathode before the and anode, is accelerated the potential in the rises electric field, which we assume to be homogeneous before breakdown, it can ionize gas atoms after a typical mean free path for ionization,D λi.Inthisionizationprocess C an electron-ion pair is created. Now two electrons are acceleratedUa that generate 800

TLM E 0 cathode anode

I (mA) 400 AGM Ua

B A F B' 0 0 0 10 20 30 40 U (V) Fig. 11.10 Hysteresis curve for a thermionic discharge (from [355]). The potential distribution for the anode glow mode and the temperature limit mode are sketched on the right IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998 1685 The Atmospheric-Pressure Plasma Jet: A Review and Comparison to Other Plasma Sources Andreas Schutze,¨ James Y. Jeong, Steven E. Babayan, Jaeyoung Park, Gary S. Selwyn, and Robert F. Hicks

(Invited Paper)

Abstract—Atmospheric-pressure plasmas are used in a variety that materials processing proceeds at the same rate over large of materials processes. Traditional sources include transferred substrate areas. On the other hand, operating the plasma at arcs, plasma torches, corona discharges, and dielectric barrier reduced pressure has several drawbacks. Vacuum systems are discharges. In arcs and torches, the electron and neutral temperatures exceed 3000 C and the densities of charge species expensive and require maintenance. Load locks and robotic range from 10 –10 cm . Due to the high gas temperature, assemblies must be used to shuttle materials in and out of these plasmas are used primarily in metallurgy. Corona and di- vacuum. Also, the size of the object that can be treated is electric barrier discharges produce nonequilibrium plasmas with limited by the size of the vacuum chamber. gas temperatures between 50–400 C and densities of charged Atmospheric-pressure plasmas overcome the disadvantages species typical of weakly ionized gases. However, since these discharges are nonuniform, their use in materials processing is of vacuum operation. However, the difficulty of sustaining a limited. Recently, an atmospheric-pressure plasma jet has been glow discharge under these conditions leads to a new set of developed, which exhibits many characteristics of a conventional, challenges. Higher voltages are required for gas breakdown low-pressure glow discharge. In the jet, the gas temperature at 760 torr, and often arcing occurs between the electrodes. ranges from 25–200 C, charged-particle densities are 10 –10 In some applications, such as plasma torches, arcing is incm , and reactive species are present in high concentrations, i.e., 10–100 ppm. Since this source may be scaled to treat large tentionally sought [3], [4]. However, to prevent arcing and areas, it could be used in applications which have been restricted lower the gas temperature, several schemes have been devised, to vacuum. In this paper, the physics and chemistry of the plasma such as the use of pointed electrodes in corona discharges jet and other atmospheric-pressure sources are reviewed. [5] and insulating inserts in dielectric barrier discharges [6]. Index Terms—Atmospheric pressure, corona discharge, dielec- A drawback of these sources is that the plasmas are not tric barrier discharge, plasma jet, plasma torch, thermal and uniform throughout the volume. Recently, a plasma jet has 12 nonthermal plasmas, transferred arc. been developed which uses flowing helium and a special electrode design to prevent arcing [7]. This source can etch I. INTRODUCTION and deposit materials at low temperatures and may be suitable for a wide range of applications. OW-PRESSURE plasmas have found wide applications A number of review articles have been published on Lin materials processing and play a key role in manu- atmospheric-pressure plasmas [3], [4], [6], [8], [9]. However, facturing semiconductor devices [1], [2]. The advantages of these articles are devoted to either thermal, or nonthermal plasmas are well known. They generate high concentrations sources, and do not include recent advances in the field. In of reactive species that can etch and deposit thin films at rates this paper, we give an overview of these plasma sources and up to 10 m/min. The temperature of the gas is usually below highlight the recent development of the atmospheric-pressure 150 C, so that thermally sensitive substrates are not damaged. plasma jet. First, we consider the effect of pressure on the The ions produced in the plasma can be accelerated toward a physical properties of plasmas. Then in the following sections, substrate to cause directional etching of submicron features. each atmospheric-pressure source is examined. Topics of In addition, a uniform glow discharge can be generated, so interest include the current–voltage characteristics, electron and neutral temperatures, densities of charged particles, and Manuscript received March 30, 1998. This work was supported in part gas compositions for oxygen-based systems. Finally, in the last by the U.S. Department of Energy, the University of California, the Basic Energy Sciences, Environmental Management Sciences Program, and the section, a quantitative comparison is made between traditional Office of Science and Risk Policy under Award DE-F5607-96ER45621. The sources and the atmospheric-pressure plasma jet. work of A. Schutze¨ was supported by a fellowship from the Deutsche Forschungsgemeinschaft (DFG). The work of J. Y. Jeong and S. E. Babayan and was supported by fellowships from the University of California Center II. PROPERTIES OF PLASMAS for Environmental Risk Reduction and the Toxic Substances Research and Training Program. To ignite a plasma, the breakdown-voltage for the gas A. Schütze, J. Y. Jeong, S. E. Babayan, and R. F. Hicks are with the must be exceeded. This voltage depends on the electrode Department of Chemical Engineering, University of California, Los Angeles, spacing and the pressure as follows [2], [8], [10]: CA 90095-1592 USA (e-mail: [email protected]). J. Park and G. S. Selwyn are with Los Alamos National Laboratory, Los Alamos, NM 87545 USA. (1) Publisher Item Identifier S 0093-3813(98)09643-X.

0093–3813/98$10.00  1998 IEEE 1686 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998

Fig. 3. Schematic of the electron and gas temperature as a function of Fig. 1. Breakdown potential in various gases as a function of the pressure pressure in a plasma discharge at constant current [14]. and gap distance for plane-parallel electrodes [8].

13 2). Regions 2) and 3) tend to shrink with increasing pressure, so that for many gases, spark ignition proceeds directly to arcing at 760 torr. In a weakly ionized gas at low pressure, the electron density ranges between 10 –10 cm [2]. Under these conditions, the collision rate between electrons and neutral molecules is insufﬁcient to bring about thermal equilibrium. Consequently, the electron temperature can be one to two orders of magnitude higher than the neutral and ion temperatures and . However, as the pressure increases, the collision rate will rise to a point where effective energy exchange occurs between the electrons and neutral molecules, so that . In this case, the electron density usually ranges from 10 –10 cm [2]. The trend in electron and neutral temperatures with pressure is illustrated in Fig. 3 for a plasma discharge with a mercury Fig. 2. Current–voltage characteristics of a low-pressure DC glow discharge and rare gas mixture [14], [15]. At 1 mtorr, the gas temperature at 1 torr [13]. is 300K, while the electron temperature is 10 000K (1 eV = 11 600K). These two temperatures merge together above 5 torr Here, and are constants found experimentally, and to an average value of about 5000K. Since the probability for is the secondary electron emission coefﬁcient of the cathode. energy exchange upon collision of an electron and molecule Paschen curves, showing the dependence of the breakdown depends on the nature of the molecule, the pressure at which voltage on electrode spacing and pressure, are presented in approaches should be a sensitive function of the gas Fig. 1, [8], [11], and [12]. Above , increases composition. For example, there may be compositions where rapidly with pressure at a constant electrode spacing. For and merge together above 760 torr. example, the breakdown voltage for argon is estimated to be Shibata et al. [16] have simulated the distribution of reac- 2500 V at 760 torr and with a 5 mm gap distance. A narrow tive species within a parallel-plate oxygen discharge using a gap is necessary to achieve a reasonable breakdown voltage relaxation-continuum model. Shown in Fig. 4 are the results at atmospheric pressure. obtained during half of the RF cycle with the cathode at . Insight into the operation of a plasma can be obtained The conditions are 0.15 torr O pressure, 200 power, from the dependence of the applied voltage on current. This and a 20 mm gap spacing. The simulation shows that the relationship is illustrated in Fig. 2 for a low-pressure DC density of charged species varies over a broad range, from glow discharge [13]. The discharge can be divided into four 10 –10 cm , depending on position. On the other hand, regions: 1) the “dark” or “Townsend discharge” prior to spark the concentrations of oxygen atoms and metastable oxygen ignition; 2) “normal glow” where the voltage is constant or molecules are much higher, equal to about 0.2 and 2.0 slightly decreasing with current; 3) “abnormal glow” where 10 cm , respectively. The concentration of metastable the voltage increases with current; and 4) “arc discharge” oxygen is constant across the gap, whereas the concentration of where the plasma becomes highly conductive. Low-pressure O atoms falls to zero at the walls due to surface recombination plasmas used in materials processing are operated in region [16]. This simulation is in good agreement with another 1686 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998

1686 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998

2). Regions 2) and 3) tend to shrink with increasing pressure, so that for many gases, spark ignition proceeds directly to arcing at 760 torr. In a weakly ionized gas at low pressure, theFig. electron 3. density Schematic of the electron and gas temperature as a function of Fig. 1. Breakdown potential in various gases as a functionranges between of the 10 –10 pressurecm [2]. Under these conditions, the collision rate between electrons and neutralpressure molecules is in a plasma discharge at constant current [14]. and gap distance for plane-parallel electrodes [8]. insufficient to bring about thermal equilibrium. Consequently, the electron temperature can be one to two orders of magnitude higher than the neutral and ion temperatures and . However, as the pressure increases, the collision rate will rise to a point where effective energy exchange occurs between the electrons and neutral molecules, so that2). Regions. In 2) and 3) tend to shrink with increasing pressure, this case, the electron density usually ranges from 10 –10 cm [2]. so that for many gases, spark ignition proceeds directly to The trend in electron and neutral temperatures with pressure is illustrated in Fig. 3 for a plasma discharge with a mercury Fig. 2. Current–voltage characteristics of a low-pressure DC glow discharge and rare gas mixture [14], [15]. At 1 mtorr, thearcing gas temperature at 760 torr. at 1 torr [13]. is 300K, while the electron temperature is 10 000K (1 eV = 11 600K). These two temperatures merge together aboveIn 5a torr weakly ionized gas at low pressure, the electron density Here, and are constants found experimentally, and to an average value of about 5000K. Since the probability for is the secondary electron emission coefficient of the cathode. energy exchange upon collision of an electronranges and molecule between 10 –10 cm [2]. Under these conditions, Paschen curves, showing the dependence of the breakdown depends on the nature of the molecule, the pressure at which voltage on electrode spacing and pressure, are presented in approaches should be a sensitive function of the gas Fig. 1, [8], [11], and [12]. Above , increases composition. For example, there may be compositionsthe collision where rate between electrons and neutral molecules is rapidly with pressure at a constant electrode spacing. For and merge together above 760 torr. example, the breakdown voltage for argon is estimated to be Shibata et al. [16] have simulated the distributioninsufficient of reac- to bring about thermal equilibrium. Consequently, 2500 V at 760 torr and with a 5 mm gap distance. A narrow tive species within a parallel-plate oxygen discharge using a gap is necessary to achieve a reasonable breakdown voltage relaxation-continuum model. Shown in Fig. 4 are the results at atmospheric pressure. obtained during half of the RF cycle with thethe cathode at electron. temperature can be one to two orders of Insight into the operation of a plasma can be obtained The conditions are 0.15 torr O pressure, 200 power, from the dependence of the applied voltage on current. This and a 20 mm gap spacing. The simulationmagnitude shows that the higher than the neutral and ion temperatures and relationship is illustrated in Fig. 2 for a low-pressure DC density of charged species varies over a broad range, from glow discharge [13]. The discharge can be divided into four 10 –10 cm , depending on position. On the. other However, hand, as the pressure increases, the collision rate will regions: 1) the “dark” or “Townsend discharge” prior to spark the concentrations of oxygen atoms and metastable oxygen ignition; 2) “normal glow” where the voltage is constant or molecules are much higher, equal to about 0.2 and 2.0 slightly decreasing with current; 3) “abnormal glow” where 10 cm , respectively. The concentrationrise of metastable to a point where effective energy exchange occurs between the voltage increases with current; and 4) “arc discharge” oxygen is constant across the gap, whereas the concentration of where the plasma becomes highly conductive. Low-pressure O atoms falls to zero at the walls due to surfacethe recombination electrons and neutral molecules, so that . In plasmas used in materials processing are operated in region [16]. This simulation is in good agreementthis with another case, the electron density usually ranges from 10 –10 cm [2]. The trend in electron and neutral temperatures with pressure is illustrated in Fig. 3 for a plasma discharge with a mercury Fig. 2. Current–voltage characteristics of a low-pressure DC glow discharge and rare gas mixture [14], [15]. At 1 mtorr, the gas temperature at 1 torr [13]. is 300K, while the electron temperature is 10 000K (1 eV = 14 11 600K). These two temperatures merge together above 5 torr Here, and are constants found experimentally, and to an average value of about 5000K. Since the probability for is the secondary electron emission coefficient of the cathode. energy exchange upon collision of an electron and molecule Paschen curves, showing the dependence of the breakdown depends on the nature of the molecule, the pressure at which voltage on electrode spacing and pressure, are presented in approaches should be a sensitive function of the gas Fig. 1, [8], [11], and [12]. Above , increases composition. For example, there may be compositions where rapidly with pressure at a constant electrode spacing. For and merge together above 760 torr. example, the breakdown voltage for argon is estimated to be Shibata et al. [16] have simulated the distribution of reac- 2500 V at 760 torr and with a 5 mm gap distance. A narrow tive species within a parallel-plate oxygen discharge using a gap is necessary to achieve a reasonable breakdown voltage relaxation-continuum model. Shown in Fig. 4 are the results at atmospheric pressure. obtained during half of the RF cycle with the cathode at . Insight into the operation of a plasma can be obtained The conditions are 0.15 torr O pressure, 200 power, from the dependence of the applied voltage on current. This and a 20 mm gap spacing. The simulation shows that the relationship is illustrated in Fig. 2 for a low-pressure DC density of charged species varies over a broad range, from glow discharge [13]. The discharge can be divided into four 10 –10 cm , depending on position. On the other hand, regions: 1) the “dark” or “Townsend discharge” prior to spark the concentrations of oxygen atoms and metastable oxygen ignition; 2) “normal glow” where the voltage is constant or molecules are much higher, equal to about 0.2 and 2.0 slightly decreasing with current; 3) “abnormal glow” where 10 cm , respectively. The concentration of metastable the voltage increases with current; and 4) “arc discharge” oxygen is constant across the gap, whereas the concentration of where the plasma becomes highly conductive. Low-pressure O atoms falls to zero at the walls due to surface recombination plasmas used in materials processing are operated in region [16]. This simulation is in good agreement with another 1686 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998

2). Regions 2) and 3) tend to shrink15 with increasing pressure, so that for many gases, spark ignition proceeds directly to arcing at 760 torr. In a weakly ionized gas at low pressure, the electron density ranges between 10 –10 cm [2]. Under these conditions, the collision rate between electrons and neutral molecules is insufﬁcient to bring about thermal equilibrium. Consequently, the electron temperature can be one to two orders of magnitude higher than the neutral and ion temperatures and . However, as the pressure increases, the collision rate will rise to a point where effective energy exchange occurs between the electrons and neutral molecules, so that . In this case, the electron density usually ranges from 10 –10 cm [2]. The trend in electron and neutral temperatures with pressure is illustrated in Fig. 3 for a plasma discharge with a mercury Fig. 2. Current–voltage characteristics of a low-pressure DC glow discharge and rare gas mixture [14], [15]. At 1 mtorr, the gas temperature at 1 torr [13]. is 300K, while the electron temperature is 10 000K (1 eV = 11 600K). These two temperatures merge together above 5 torr Here, and are constants found experimentally, and to an average value of about 5000K. Since the probability for is the secondary electron emission coefﬁcient of the cathode. energy exchange upon collision of an electron and molecule Paschen curves, showing the dependence of the breakdown depends on the nature of the molecule, the pressure at which voltage on electrode spacing and pressure, are presented in approaches should be a sensitive function of the gas Fig. 1, [8], [11], and [12]. Above , increases composition. For example, there may be compositions where rapidly with pressure at a constant electrode spacing. For and merge together above 760 torr. example, the breakdown voltage for argon is estimated to be Shibata et al. [16] have simulated the distribution of reac- 2500 V at 760 torr and with a 5 mm gap distance. A narrow tive species within a parallel-plate oxygen discharge using a gap is necessary to achieve a reasonable breakdown voltage relaxation-continuum model. Shown in Fig. 4 are the results at atmospheric pressure. obtained during half of the RF cycle with the cathode at . Insight into the operation of a plasma can be obtained The conditions are 0.15 torr O pressure, 200 power, from the dependence of the applied voltage on current. This and a 20 mm gap spacing. The simulation shows that the relationship is illustrated in Fig. 2 for a low-pressure DC density of charged species varies over a broad range, from glow discharge [13]. The discharge can be divided into four 10 –10 cm , depending on position. On the other hand, regions: 1) the “dark” or “Townsend discharge” prior to spark the concentrations of oxygen atoms and metastable oxygen ignition; 2) “normal glow” where the voltage is constant or molecules are much higher, equal to about 0.2 and 2.0 slightly decreasing with current; 3) “abnormal glow” where 10 cm , respectively. The concentration of metastable the voltage increases with current; and 4) “arc discharge” oxygen is constant across the gap, whereas the concentration of where the plasma becomes highly conductive. Low-pressure O atoms falls to zero at the walls due to surface recombination plasmas used in materials processing are operated in region [16]. This simulation is in good agreement with another SCHUTZE¨ er al.: ATMOSPHERIC-PRESSURE PLASMA JET 1687

Fig. 6. Schematic of a transferred arc apparatus.

the other hand, the concentrations of O metastables and O atoms increases with pressure, because these species are created by the excitement and dissociation of O , which requires electron energies of only 1–5 eV. The trends shown in Fig. 5 suggest that at atmospheric pressure, ions will be relatively insigniﬁcant, so that the chemistry will be dominated Fig. 4. Spatial particle distributions in a parallel plate O RF discharge at by reactive neutral species. In the case of oxygen plasmas, ¨ for V [16]. SCHUTZE er al.: ATMOSPHERIC-PRESSURE PLASMA JET 1687 these species are O atoms, metastable O , and O .

III. TRANSFERRED ARCS AND PLASMA TORCHES Transferred arcs are used to cut [3], [4], [19], and [20], melt [4] and [21], and weld condensed materials. A schematic of such a device is shown in Fig. 6. It consists of a cylindrical shaped cathode, an outer grounded and water-cooled shield, and a workpiece as the anode. By feeding argon and hydrogen, oxygen, or air between the cathode and shield, and by applying DC power of up to 200 kW, an arc between the electrodes may be ignited and sustained. Typical operation conditions and properties are 1–15 l/s gas flow, 50–600 A, 10 MW/m , gas temperatures between 3000 and 20 000K, and a nozzle-to- Fig. 6. Schematic of a transferred arc apparatus. sample distance of 5–10 mm [19]–[22]. These parameters may vary, depending on the nozzle diameter and materials to be cut. Transferred arcs can slice steel plates up to 150 mm thick. For the other hand, the concentrations of O metastables and example, a plate, 40 mm thick, was cut at 0.7 m/min with a O atoms increases with pressure, because these species are 600 A arc [23]. created by the excitement and dissociation of O , which Several design variations on the transferred arc have been requires electron energies of only 1–5 eV. The trends shown developed. A plasma arc heater uses the shield as the anode in Fig. 5 suggest that at atmospheric pressure, ions will be instead of the workpiece. In addition, RF induction coils and relatively insignificant, so that the chemistry will be dominated Fig. 4. Spatial particle distributions in a parallel plate O RF discharge at Fig. 5. Time-averaged number densities of each particle in the center of the advanced gas injection schemes are sometimes employed to for V [16]. bydischarge reactive as a neutralfunction of species. pressure inIn the the range case between of oxygen 0.15 and plasmas, 1 torr [16]. enhance the plasma density and to restrict the cutting area [9]. these species are O atoms, metastable O , and O . Plasma torches are the same as plasma arc heaters, except that they contain a port for injecting precursor compounds modeling study by Klopovskii et al. [17]. By comparison, that are used in thin film deposition [3], [24]–[26]. In the SelwynIII. [18] TRANSFERRED measured theA ORCS atom AND concentrationPLASMA TORCHES in an oxygen literature, this technique is also referred to as “injection plasma andTransferred argon RF arcs discharge are used using to cut two-photon [3], [4], [19], laser and excitation. [20], melt He processing” [25]. The precursors, in the form of solid pellets, [4] and [21], and weld condensed materials. A schematic of detected O atom concentrations of about 5 10 cm at 0.1 powder, or volatile molecules, are introduced just downstream such a device is shown in Fig. 6. It consists of a cylindrical torr O , having a gap distance between two plane electrodes of the arc, where they are vaporized and/or dissociated into shaped cathode, an outer grounded and water-cooled shield, 16 of 55 mm and a self bias of 300 V. reactive species. The resultant mixture is sprayed onto a and a workpiece as the anode. By feeding argon and hydrogen, Shown in Fig. 5 is the effect of pressure on the plasma substrate, thereby coating it with a film at rates up to 10 oxygen, or air between the cathode and shield, and by applying composition as predicted by Shibata’s model. As the pressure m/min. Films which have been deposited with plasma torches DC power of up to 200 kW, an arc between the electrodes increases above 0.3 torr, the density of ions diminishes due include SiC [26], SiN [27], TiO [28], Y–Ba–Cu–O [24], [29], may be ignited and sustained. Typical operation conditions to the lower electron energies at higher pressure (cf. Fig. 3), Al 0 [30], and diamond [31]–[36]. The main purpose of these and properties are 1–15 l/s gas flow, 50–600 A, 10 MW/m , and thus, a lower production rate of ion-electron pairs. On coatings is to provide materials with added resistance to wear, gas temperatures between 3000 and 20 000K, and a nozzle-to- sample distance of 5–10 mm [19]–[22]. These parameters may vary, depending on the nozzle diameter and materials to be cut. Transferred arcs can slice steel plates up to 150 mm thick. For example, a plate, 40 mm thick, was cut at 0.7 m/min with a 600 A arc [23]. Several design variations on the transferred arc have been developed. A plasma arc heater uses the shield as the anode instead of the workpiece. In addition, RF induction coils and Fig. 5. Time-averaged number densities of each particle in the center of the advanced gas injection schemes are sometimes employed to discharge as a function of pressure in the range between 0.15 and 1 torr [16]. enhance the plasma density and to restrict the cutting area [9]. Plasma torches are the same as plasma arc heaters, except that they contain a port for injecting precursor compounds modeling study by Klopovskii et al. [17]. By comparison, that are used in thin film deposition [3], [24]–[26]. In the Selwyn [18] measured the O atom concentration in an oxygen literature, this technique is also referred to as “injection plasma and argon RF discharge using two-photon laser excitation. He processing” [25]. The precursors, in the form of solid pellets, detected O atom concentrations of about 5 10 cm at 0.1 powder, or volatile molecules, are introduced just downstream torr O , having a gap distance between two plane electrodes of the arc, where they are vaporized and/or dissociated into of 55 mm and a self bias of 300 V. reactive species. The resultant mixture is sprayed onto a Shown in Fig. 5 is the effect of pressure on the plasma substrate, thereby coating it with a film at rates up to 10 composition as predicted by Shibata’s model. As the pressure m/min. Films which have been deposited with plasma torches increases above 0.3 torr, the density of ions diminishes due include SiC [26], SiN [27], TiO [28], Y–Ba–Cu–O [24], [29], to the lower electron energies at higher pressure (cf. Fig. 3), Al 0 [30], and diamond [31]–[36]. The main purpose of these and thus, a lower production rate of ion-electron pairs. On coatings is to provide materials with added resistance to wear, SCHUTZE¨ er al.: ATMOSPHERIC-PRESSURE PLASMA JET 1687

Fig. 6. Schematic of a transferred arc apparatus.

the other hand, the concentrations of O metastables and O atoms increases with pressure, because these species are created by the excitement and dissociation of O , which requires electron energies of only 1–5 eV. The trends shown in Fig. 5 suggest that at atmospheric pressure, ions will be relatively insigniﬁcant, so that the chemistry will be dominated Fig. 4. Spatial particle distributions in a parallel plate O RF discharge at for V [16]. by reactive neutral species. In the case of oxygen plasmas, these species are O atoms, metastable O , and O .

SCHUTZE¨ er al.: ATMOSPHERIC-PRESSURE PLASMA JET 1687 III. TRANSFERRED ARCS AND PLASMA TORCHES Transferred arcs are used to cut [3], [4], [19], and [20], melt [4] and [21], and weld condensed materials. A schematic of such a device is shown in Fig. 6. It consists of a cylindrical shaped cathode, an outer grounded and water-cooled shield, and a workpiece as the anode. By feeding argon and hydrogen, oxygen, or air between the cathode and shield, and by applying DC power of up to 200 kW, an arc between the electrodes may be ignited and sustained. Typical operation conditions and properties are 1–15 l/s gas flow, 50–600 A, 10 MW/m , gas temperatures between 3000 and 20 000K, and a nozzle-to- sample distance of 5–10 mm [19]–[22]. These parameters may vary, depending on the nozzle diameter and materials to be cut. Transferred arcs can slice steel plates up to 150 mm thick. For Fig. 6. Schematic of a transferred arc apparatus. example, a plate, 40 mm thick, was cut at 0.7 m/min with a 600 A arc [23]. the other hand, the concentrations of O metastables andSeveral design variations on the transferred arc have been O atoms increases with pressure, because these species aredeveloped. A plasma arc heater uses the shield as the anode created by the excitement and dissociation of O , whichinstead of the workpiece. In addition, RF induction coils and Fig. 5.requires Time-averaged electron number energies densities of of only each particle 1–5 eV. in the The center trends of the shownadvanced gas injection schemes are sometimes employed to dischargein as Fig. a function 5 suggest of pressure that in at the atmospheric range between pressure, 0.15 and 1 torr ions [16]. willenhance be the plasma density and to restrict the cutting area [9]. relatively insignificant, so that the chemistry will be dominatedPlasma torches are the same as plasma arc heaters, except Fig. 4. Spatial particle distributions in a parallel plate O RF discharge at by reactive neutral species. In the case of oxygen plasmas,that they contain a port for injecting precursor compounds for V [16]. modeling study by Klopovskii et al. [17]. By comparison, these species are O atoms, metastable O , and O . that are used in thin film deposition [3], [24]–[26]. In the Selwyn [18] measured the O atom concentration in an oxygen literature, this technique is also referred to as “injection plasma and argon RF discharge using two-photon laser excitation. He III. TRANSFERRED ARCS AND PLASMA TORCHES processing” [25]. The precursors, in the form of solid pellets, detected O atom concentrations of about 5 10 cm at 0.1 powder, or volatile molecules, are introduced just downstream Transferred arcs are used to cut [3], [4], [19], and [20],17 melt torr O , having a gap distance between two plane electrodes of the arc, where they are vaporized and/or dissociated into [4] and [21], and weld condensed materials. A schematic of of 55 mm and a self bias of 300 V. reactive species. The resultant mixture is sprayed onto a such a device is shown in Fig. 6. It consists of a cylindrical Shown in Fig. 5 is the effect of pressure on the plasma substrate, thereby coating it with a film at rates up to 10 shaped cathode, an outer grounded and water-cooled shield, composition as predicted by Shibata’s model. As the pressure m/min. Films which have been deposited with plasma torches and a workpiece as the anode. By feeding argon and hydrogen, increases above 0.3 torr, the density of ions diminishes due include SiC [26], SiN [27], TiO [28], Y–Ba–Cu–O [24], [29], oxygen, or air between the cathode and shield, and by applying to the lower electron energies at higher pressure (cf. Fig. 3), Al 0 [30], and diamond [31]–[36]. The main purpose of these DC power of up to 200 kW, an arc between the electrodes and thus, a lower production rate of ion-electron pairs. On coatings is to provide materials with added resistance to wear, may be ignited and sustained. Typical operation conditions and properties are 1–15 l/s gas flow, 50–600 A, 10 MW/m , gas temperatures between 3000 and 20 000K, and a nozzle-to- sample distance of 5–10 mm [19]–[22]. These parameters may vary, depending on the nozzle diameter and materials to be cut. Transferred arcs can slice steel plates up to 150 mm thick. For example, a plate, 40 mm thick, was cut at 0.7 m/min with a 600 A arc [23]. Several design variations on the transferred arc have been developed. A plasma arc heater uses the shield as the anode instead of the workpiece. In addition, RF induction coils and Fig. 5. Time-averaged number densities of each particle in the center of the advanced gas injection schemes are sometimes employed to discharge as a function of pressure in the range between 0.15 and 1 torr [16]. enhance the plasma density and to restrict the cutting area [9]. Plasma torches are the same as plasma arc heaters, except that they contain a port for injecting precursor compounds modeling study by Klopovskii et al. [17]. By comparison, that are used in thin film deposition [3], [24]–[26]. In the Selwyn [18] measured the O atom concentration in an oxygen literature, this technique is also referred to as “injection plasma and argon RF discharge using two-photon laser excitation. He processing” [25]. The precursors, in the form of solid pellets, detected O atom concentrations of about 5 10 cm at 0.1 powder, or volatile molecules, are introduced just downstream torr O , having a gap distance between two plane electrodes of the arc, where they are vaporized and/or dissociated into of 55 mm and a self bias of 300 V. reactive species. The resultant mixture is sprayed onto a Shown in Fig. 5 is the effect of pressure on the plasma substrate, thereby coating it with a film at rates up to 10 composition as predicted by Shibata’s model. As the pressure m/min. Films which have been deposited with plasma torches increases above 0.3 torr, the density of ions diminishes due include SiC [26], SiN [27], TiO [28], Y–Ba–Cu–O [24], [29], to the lower electron energies at higher pressure (cf. Fig. 3), Al 0 [30], and diamond [31]–[36]. The main purpose of these and thus, a lower production rate of ion-electron pairs. On coatings is to provide materials with added resistance to wear, 1688 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998

SCHUTZE¨ er al.: ATMOSPHERIC-PRESSURE PLASMA JET 1687

Fig. 6. Schematic of a transferred arc apparatus.

the other hand, the concentrations of O metastables and O atoms increases with pressure, because these speciesFig. 7. are Current–voltage characteristics of an atmospheric-pressure arc. created by the excitement and dissociation of O , which requires electron energies of only 1–5 eV. The trends shown in Fig. 5 suggest that at atmospheric pressure, ionscorrosion, will be and oxidation and may also have applications in relatively insignificant, so that the chemistry will be dominated Fig. 4. Spatial particle distributions in a parallel plate O RF discharge at electrical and electronic fields [37]. for V [16]. by reactive neutral species. In the case of oxygen plasmas,Fig. 7 shows the current–voltage characteristics of a plasma these species are O atoms, metastable O , and O . Fig. 8. Composition of an oxygen plasma arc at 0.95 atm as a function of torch. After exceeding the breakdown voltage (the maximum the temperature [42]. in the curve), the gas becomes highly ionized and conductive. III. TRANSFERRED ARCS AND PLASMA TORCHES This produces a rapid drop in voltage with increasing current. Transferred arcs are used to cut [3], [4], [19], and [20],Notice18 melt that 300 kV is required to achieve breakdown, whereas [4] and [21], and weld condensed materials. A schematicat normal of operating currents of 200–600 A, only about 100 V such a device is shown in Fig. 6. It consists of a cylindrical shaped cathode, an outer grounded and water-cooledis shield, needed to sustain the arc [19], [38]. and a workpiece as the anode. By feeding argon and hydrogen,Although plasma torches have been considered to be in oxygen, or air between the cathode and shield, and byequilibrium, applying Kruger et al. [39], [40] have shown that this is DC power of up to 200 kW, an arc between the electrodesnot normally the case. With regard to thermal equilibrium, may be ignited and sustained. Typical operation conditionsKim et al. [41] measured the electron and ion temperatures in and properties are 1–15 l/s gas flow, 50–600 A, 10 aMW/m plasma, torch as a function of the radial and axial positions. gas temperatures between 3000 and 20 000K, and a nozzle-to-They found that varied from 7.0–9.0 eV, while was sample distance of 5–10 mm [19]–[22]. These parameters may vary, depending on the nozzle diameter and materialsan to be order cut. of magnitude lower at 0.3–0.9 eV. In addition, they Transferred arcs can slice steel plates up to 150 mm thick.observed For electron densities near 3 10 cm . Fig. 9. Schematic of a corona discharge. example, a plate, 40 mm thick, was cut at 0.7 m/minIn with transferred a arcs and plasma torches, extremely high tem- 600 A arc [23]. peratures promote the complete dissociation of the feed gas. plasma usually exists in a region of the gas extending about 0.5 Several design variations on the transferred arc haveFor been example, Fig. 8 shows the effect of the gas temperature mm out from the metal point. In the drift region outside this developed. A plasma arc heater uses the shield as theon anode the distribution of oxygen species and electrons in an volume, charged species diffuse toward the planar electrode instead of the workpiece. In addition, RF inductionarc coils at and 0.95 atm of oxygen [42]. The oxygen molecules are advanced gas injection schemes are sometimes employed to and are collected. Fig. 5. Time-averaged number densities of each particle in the center of the approximately 99% dissociated with predominantly all of these discharge as a function of pressure in the range between 0.15 and 1 torr [16]. enhance the plasma density and to restrict the cutting area [9]. The restricted area of the corona discharge has limited molecules being converted into O atoms. By contrast, the Plasma torches are the same as plasma arc heaters, except its applications in materials processing. In an attempt to that they contain a port for injecting precursor compoundsconcentrations of O ,O, and O are several orders of modeling study by Klopovskii et al. [17]. By comparison, overcome this problem, two-dimensional arrays of electrodes that are used in thin film deposition [3], [24]–[26].magnitude In the lower. These results are consistent with the energies Selwyn [18] measured the O atom concentration in an oxygen have been developed. Some applications of coronas include the literature, this technique is also referred to as “injectionrequired plasma to dissociate and ionize oxygen: (O ) 5.11 eV, and argon RF discharge using two-photon laser excitation. He activation of polymer surfaces [48], [49], and the enhancement processing” [25]. The precursors, in the form of solid pellets,(O ) 6.48 eV, and (O) 13.62 eV [43]. detected O atom concentrations of about 5 10 cm at 0.1 powder, or volatile molecules, are introduced just downstream of SiO growth during the thermal oxidation of silicon wafers torr O , having a gap distance between two plane electrodes of the arc, where they are vaporized and/or dissociated into [50], [51]. of 55 mm and a self bias of 300 V. reactive species. The resultant mixture is sprayed onto a IV. CORONA DISCHARGE Shown in Fig. 10 is the dependence of the voltage on the Shown in Fig. 5 is the effect of pressure on the plasma substrate, thereby coating it with a film at rates upA to corona 10 discharge appears as a luminous glow localized current for a positive point-to-plane corona operating in air composition as predicted by Shibata’s model. As the pressure m/min. Films which have been deposited with plasmain torches space around a point tip in a highly nonuniform electric at 760 torr [5], [52]. The plasma ignites at a voltage of 2–5 increases above 0.3 torr, the density of ions diminishes due include SiC [26], SiN [27], TiO [28], Y–Ba–Cu–O [24], [29], kV and produces an extremely small current of 10 –10 A. to the lower electron energies at higher pressure (cf. Fig. 3), Al 0 [30], and diamond [31]–[36]. The main purposefield. of these The physics of this source is well understood [5], [8], and thus, a lower production rate of ion-electron pairs. On coatings is to provide materials with added resistance[12], to wear, [44]–[47]. The corona may be considered a Townsend Above 10 A, the voltage rapidly increases with current. This discharge or a negative glow discharge depending upon the coincides with the generation of micro-arcs, or “streamers,” field and potential distribution [45]. Fig. 9 shows a schematic that extend between the electrodes. A maximum voltage is of a point-to-plane corona. The apparatus consists of a metal recorded at about 5 10 A, where the device begins tip, with a radius of about 3 m, and a planar electrode to arc. Coronas are operated at currents below the onset separated from the tip by a distance of 4–16 mm [47]. The of arcing. 1688 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998

SCHUTZE¨ er al.: ATMOSPHERIC-PRESSURE PLASMA JET 1687

Fig. 6. Schematic of a transferred arc apparatus. Fig. 7. Current–voltage characteristics of an atmospheric-pressure arc.

the other hand, the concentrations of O metastables and corrosion, and oxidationO atoms increases and may with also pressure, have applications because these in species are electrical and electroniccreated by ﬁelds the [37]. excitement and dissociation of O , which Fig. 7 shows therequires current–voltage electron energies characteristics of only 1–5 of eV. a plasma The trends shown Fig. 8. Composition of an oxygen plasma arc at 0.95 atm as a function of torch. After exceedingin Fig. 5 the suggest breakdown that at voltage atmospheric (the maximum pressure, ionsthe will temperature be [42]. relatively insigniﬁcant, so that the chemistry will be dominated Fig. 4. Spatial particle distributions in a parallel plate O inRF the discharge curve), at the gas becomes highly ionized and conductive. for V [16]. This produces aby rapid reactive drop neutralin voltage species. with In increasing the case current. of oxygen plasmas, Notice that 300 kVthese is species required are to O achieve atoms, breakdown, metastable O whereas, and O . at normal operating currents of 200–600 A, only about 100 V is needed to sustain theIII. arc TRANSFERRED [19], [38]. ARCS AND PLASMA TORCHES

Although plasmaTransferred torches arcshave are been used considered to cut [3], [4], to [19], be in and [20],19 melt equilibrium, Kruger[4] andet al.[21],[39], and [40] weld have condensed shown materials. that this A is schematic of not normally thesuch case. a device With is regard shown to in thermal Fig. 6. It equilibrium, consists of a cylindrical Kim et al. [41] measuredshaped cathode, the electron an outer and grounded ion temperatures and water-cooled in shield, a plasma torch asand a a function workpiece of as the the radial anode. and By axial feeding positions. argon and hydrogen, They found thatoxygen,varied or air from between 7.0–9.0 the cathode eV, while and shield,was and by applying an order of magnitudeDC power lower of upat 0.3–0.9 to 200 kW, eV. anIn addition,arc between they the electrodes observed electronmay densities be ignited near and 3 sustained.10 cm Typical. operation conditionsFig. 9. Schematic of a corona discharge. In transferredand arcs properties and plasma are torches, 1–15 l/s gasextremely flow, 50–600 high tem- A, 10 MW/m , peratures promotegas the temperatures complete between dissociation 3000 andof the 20 000K, feed gas. and a nozzle-to- sample distance of 5–10 mm [19]–[22]. These parametersplasma may usually exists in a region of the gas extending about 0.5 For example, Fig. 8 shows the effect of the gas temperature vary, depending on the nozzle diameter and materialsmm to be out cut. from the metal point. In the drift region outside this on the distribution of oxygen species and electrons in an Transferred arcs can slice steel plates up to 150 mm thick.volume, For charged species diffuse toward the planar electrode arc at 0.95 atm of oxygen [42]. The oxygen molecules are example, a plate, 40 mm thick, was cut at 0.7 m/minand with are a collected. approximately 99% dissociated with predominantly all of these 600 A arc [23]. The restricted area of the corona discharge has limited molecules being converted into O atoms. By contrast, the Several design variations on the transferred arc haveits applications been in materials processing. In an attempt to concentrations of O ,O, and O are several orders of developed. A plasma arc heater uses the shield as theovercome anode this problem, two-dimensional arrays of electrodes magnitude lower. These results are consistent with the energies instead of the workpiece. In addition, RF inductionhave coils been and developed. Some applications of coronas include the required to dissociateadvanced and gas ionize injection oxygen: schemes(O are) sometimes5.11 eV, employed to Fig. 5. Time-averaged number densities of each particle in the center of the activation of polymer surfaces [48], [49], and the enhancement discharge as a function of pressure in the range between 0.15 and(O 1 torr) [16].6.48enhance eV, and the plasma(O) density13.62 and eV to [43]. restrict the cutting area [9]. of SiO growth during the thermal oxidation of silicon wafers Plasma torches are the same as plasma arc heaters, except that they contain a port for injecting precursor compounds[50], [51]. modeling study by Klopovskii et al. [17]. By comparison, IV. CORONA DISCHARGE that are used in thin film deposition [3], [24]–[26].Shown In the in Fig. 10 is the dependence of the voltage on the Selwyn [18] measured the O atom concentration in an oxygen A corona dischargeliterature, appears this technique as a luminous is also referred glow to localized as “injectioncurrent plasma for a positive point-to-plane corona operating in air and argon RF discharge using two-photon laser excitation. He in space aroundprocessing” a point tip [25]. in a The highly precursors, nonuniform in the form electric of solidat pellets, 760 torr [5], [52]. The plasma ignites at a voltage of 2–5 detected O atom concentrations of about 5 10field.cm Theat 0.1 physicspowder, of this or volatilesource ismolecules, well understood are introduced [5], [8],just downstreamkV and produces an extremely small current of 10 –10 A. torr O , having a gap distance between two plane[12], electrodes [44]–[47].of The the corona arc, where may they be considered are vaporized a Townsend and/or dissociatedAbove into 10 A, the voltage rapidly increases with current. This of 55 mm and a self bias of 300 V. discharge or a negativereactive species. glow discharge The resultant depending mixture upon is sprayedthe coincides onto a with the generation of micro-arcs, or “streamers,” Shown in Fig. 5 is the effect of pressure onfield the and plasma potentialsubstrate, distribution thereby [45]. coating Fig. 9 it shows with a a film schematic at ratesthat up to extend 10 between the electrodes. A maximum voltage is composition as predicted by Shibata’s model. Asof the a point-to-plane pressure m/min. corona. Films The which apparatus have been consists deposited of a with metal plasmarecorded torches at about 5 10 A, where the device begins increases above 0.3 torr, the density of ions diminishestip, with due a radiusinclude of SiCabout [26], 3 SiNm, [27], and TiO a planar[28], Y–Ba–Cu–O electrode [24],to arc. [29], Coronas are operated at currents below the onset to the lower electron energies at higher pressureseparated (cf. Fig. from 3), Al the0 tip[30], by and a distance diamond of [31]–[36]. 4–16 mm The [47]. main The purposeof of arcing. these and thus, a lower production rate of ion-electron pairs. On coatings is to provide materials with added resistance to wear, 1688 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998

Fig. 7. Current–voltage characteristics of an atmospheric-pressure arc.

1688 corrosion, and oxidationIEEE TRANSACTIONS and may also ON have PLASMA applications SCIENCE, in VOL. 26, NO. 6, DECEMBER 1998 electrical and electronic fields [37]. Fig. 7 shows the current–voltage characteristics of a plasma Fig. 8. Composition of an oxygen plasma arc at 0.95 atm as a function of torch. After exceeding the breakdown voltage (the maximum the temperature [42]. in the curve), the gas becomes highly ionized and conductive. This produces a rapid drop in voltage with increasing current. Notice that 300 kV is required to achieve breakdown, whereas at normal operating currents of 200–600 A, only about 100 V is needed to sustain the arc [19], [38]. Although plasma torches have been considered to be in equilibrium, Kruger et al. [39], [40] have shown that this is not normally the case. With regard to thermal equilibrium, Kim et al. [41] measured the electron and ion temperatures in a plasma torch as a function of the radial and axial positions. They found that varied from 7.0–9.0 eV, while was an order of magnitude lower at 0.3–0.9 eV. In addition, they observed electron densities near 3 10 cm . Fig. 9. Schematic of a corona discharge. In transferred arcs and plasma torches, extremely high temperatures promote the complete dissociation of the feed gas. plasma usually exists in a region of the gas extending about 0.5 For example, Fig. 8 shows the effect of the gas temperature mm out from the metal point. In the drift region outside this on the distribution of oxygen species and electrons in an volume, charged species diffuse toward the planar electrode arc at 0.95 atm of oxygen [42]. The oxygen molecules are and are collected. approximately 99% dissociated with predominantly all of these The restricted area of the corona discharge has limited molecules being converted into O atoms. By contrast, the its applications in materials processing. In an attempt to concentrations of O ,O, and O are several orders of overcome this problem, two-dimensional arrays of electrodes Fig. 7. Current–voltage characteristics of an atmospheric-pressuremagnitude arc. lower. These results are consistent with the energies have been developed. Some applications of coronas include the required to dissociate and ionize oxygen: (O ) 5.11 eV, activation of polymer surfaces [48], [49], and the enhancement (O ) 6.48 eV, and (O) 13.62 eV [43]. corrosion, and oxidation and may also have applications in of SiO growth during the thermal oxidation of silicon wafers [50], [51]. electrical and electronic fields [37]. IV. CORONA DISCHARGE Shown in Fig. 10 is the dependence of the voltage on the Fig. 7 shows the current–voltage characteristics ofA a corona plasma discharge appears as a luminous glow localized current for a positive point-to-plane corona operating in air Fig. 8. Composition of an oxygen plasma arc at 0.95 atm as a function of torch. After exceeding the breakdown voltage (thein maximum space around a point tip in a highly nonuniform electric at 760 torr [5], [52]. The plasma ignites at a voltage of 2–5 field. The physicsthe of this temperature source is [42]. well understood [5], [8], kV and produces an extremely small current of 10 –10 A. in the curve), the gas becomes highly ionized and conductive.[12], [44]–[47]. The corona may be considered a Townsend Above 10 A, the voltage rapidly increases with current. This This produces a rapid drop in voltage with increasingdischarge current. or a negative glow discharge depending upon the coincides with the generation of micro-arcs, or “streamers,” Notice that 300 kV is required to achieve breakdown,field whereas and potential distribution [45]. Fig. 9 shows a schematic that extend between the electrodes. A maximum voltage is of a point-to-plane corona. The apparatus consists of a metal recorded at about 5 10 A, where the device begins at normal operating currents of 200–600 A, only abouttip, with 100 a V radius of about 3 m, and a planar electrode to arc. Coronas are operated at currents below the onset is needed to sustain the arc [19], [38]. separated from the tip by a distance of 4–16 mm [47]. The of arcing. Although plasma torches have been considered to be in equilibrium, Kruger et al. [39], [40] have shown that this is not normally the case. With regard to thermal equilibrium, Kim et al. [41] measured the electron and ion temperatures in a plasma torch as a function of the radial and axial positions. They found that varied from 7.0–9.0 eV, while was an order of magnitude lower at 0.3–0.9 eV. In addition, they observed electron densities near 3 10 cm . Fig. 9. Schematic of a corona discharge. In transferred arcs and plasma torches, extremely high temperatures promote the complete dissociation of the feed gas. 20 plasma usually exists in a region of the gas extending about 0.5 For example, Fig. 8 shows the effect of the gas temperature mm out from the metal point. In the drift region outside this on the distribution of oxygen species and electrons in an volume, charged species diffuse toward the planar electrode arc at 0.95 atm of oxygen [42]. The oxygen molecules are and are collected. approximately 99% dissociated with predominantly all of these The restricted area of the corona discharge has limited molecules being converted into O atoms. By contrast, the its applications in materials processing. In an attempt to concentrations of O ,O, and O are several orders of overcome this problem, two-dimensional arrays of electrodes magnitude lower. These results are consistent with the energies have been developed. Some applications of coronas include the required to dissociate and ionize oxygen: (O ) 5.11 eV, activation of polymer surfaces [48], [49], and the enhancement (O ) 6.48 eV, and (O) 13.62 eV [43]. of SiO growth during the thermal oxidation of silicon wafers [50], [51]. IV. CORONA DISCHARGE Shown in Fig. 10 is the dependence of the voltage on the A corona discharge appears as a luminous glow localized current for a positive point-to-plane corona operating in air in space around a point tip in a highly nonuniform electric at 760 torr [5], [52]. The plasma ignites at a voltage of 2–5 field. The physics of this source is well understood [5], [8], kV and produces an extremely small current of 10 –10 A. [12], [44]–[47]. The corona may be considered a Townsend Above 10 A, the voltage rapidly increases with current. This discharge or a negative glow discharge depending upon the coincides with the generation of micro-arcs, or “streamers,” field and potential distribution [45]. Fig. 9 shows a schematic that extend between the electrodes. A maximum voltage is of a point-to-plane corona. The apparatus consists of a metal recorded at about 5 10 A, where the device begins tip, with a radius of about 3 m, and a planar electrode to arc. Coronas are operated at currents below the onset separated from the tip by a distance of 4–16 mm [47]. The of arcing. 1688 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998

1688 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998

Fig. 7. Current–voltage characteristics of an atmospheric-pressure arc.

corrosion, and oxidation and may also have applications in electrical and electronic fields [37]. Fig. 7 shows the current–voltage characteristics of a plasma Fig. 8. Composition of an oxygen plasma arc at 0.95 atm as a function of torch. After exceeding the breakdown voltage (the maximum the temperature [42]. in the curve), the gas becomes highly ionized and conductive. This produces a rapid drop in voltage with increasing current. Notice that 300 kV is required to achieve breakdown, whereas at normal operating currents of 200–600 A, only about 100 V is needed to sustain the arc [19], [38]. Although plasma torches have been considered to be in equilibrium, Kruger et al. [39], [40] have shown that this is not normally the case. With regard to thermal equilibrium, Kim et al. [41] measured the electron and ion temperatures in a plasma torch as a function of the radial and axial positions. They found that varied from 7.0–9.0 eV, while was an order of magnitude lower at 0.3–0.9 eV. In addition, they observed electron densities near 3 10 cm . Fig. 9. Schematic of a corona discharge. In transferred arcs and plasma torches, extremely high temperatures promote the complete dissociation of the feed gas. plasma usually exists in a region of the gas extending about 0.5 For example, Fig. 8 shows the effect of the gas temperature mm out from the metal point. In the drift region outside this on the distribution of oxygen species and electrons in an volume, charged species diffuse toward the planar electrode arc at 0.95 atm of oxygen [42]. The oxygen molecules are and are collected. approximately 99% dissociated with predominantly all of these The restricted area of the corona discharge has limited molecules being converted into O atoms. By contrast, the its applications in materials processing. In an attempt to Fig. 7. Current–voltage characteristics of an atmospheric-pressureconcentrations arc. of O ,O, and O are several orders of overcome this problem, two-dimensional arrays of electrodes magnitude lower. These results are consistent with the energies have been developed. Some applications of coronas include the required to dissociate and ionize oxygen: (O ) 5.11 eV, activation of polymer surfaces [48], [49], and the enhancement SCHUTZE¨ (Oer) al.: ATMOSPHERIC-PRESSURE6.48 eV, and (O) PLASMA13.62 eV JET [43]. 1689 of SiO growth during the thermal oxidation of silicon wafers corrosion, and oxidation and may also have applications in [50], [51]. IV. CORONA DISCHARGE Shown in Fig. 10 is the dependence of the voltage on the electrical and electronic fields [37]. A corona discharge appears as a luminous glow localized current for a positive point-to-plane corona operating in air in space around a point tip in a highly nonuniform electric at 760 torr [5], [52]. The plasma ignites at a voltage of 2–5 Fig. 7 shows the current–voltage characteristics offield. a plasma The physics of this source is well understood [5], [8], kV and produces an extremely small current of 10 –10 A. [12], [44]–[47]. The coronaFig. may 8. be considered Composition a Townsend ofAbove an oxygen 10 A, the plasma voltage rapidly arc increases at 0.95 with atm current. as This a function of torch. After exceeding the breakdown voltage (the maximumdischarge or a negativethe glow dischargetemperature depending [42]. upon the coincides with the generation of micro-arcs, or “streamers,” field and potential distribution [45]. Fig. 9 shows a schematic that extend between the electrodes. A maximum voltage is in the curve), the gas becomes highly ionized and conductive.of a point-to-plane corona. The apparatus consists of a metal recordedFig. 12. at Schematic about 5 of a10 silent discharge;A, where (1) the metallic device electrodes begins and (2) tip, with a radius of about 3 m, and a planar electrode todielectric arc. Coronas barrier are coating. operated at currents below the onset This produces a rapid drop in voltage with increasingseparated current. from the tip by a distance of 4–16 mm [47]. The of arcing. species are still lower at an average density of about 10 Notice that 300 kV is required to achieve breakdown, whereas cm . This distribution of reaction products differs greatly at normal operating currents of 200–600 A, only about 100 V from that seen in a low-pressure glow discharge (cf. Figs. 4 and 5). is needed to sustain the arc [19], [38]. V. DIELECTRIC BARRIER DISCHARGE Although plasma torches have been considered to be in Dielectric barrier discharges are also called “silent” and Fig. 10. Current–voltage characteristics for a positive point-to-plane corona discharge with a gap of 13 mm in 1 atm of air [5]. “atmospheric-pressure-glow” discharges [6], [54]–[56]. A equilibrium, Kruger et al. [39], [40] have shown that this is schematic of this source is shown in Fig. 12. It consists not normally the case. With regard to thermal equilibrium, of two metal electrodes, in which at least one is coated with a dielectric layer. The gap is on the order of several mm, and Kim et al. [41] measured the electron and ion temperatures in the applied voltage is about 20 kV. The plasma is generated through a succession of micro arcs, lasting for 10–100 ns, and a plasma torch as a function of the radial and axial positions. randomly distributed in space and time. These streamers are believed to be 100 m in diameter and are separated from They found that varied from 7.0–9.0 eV, while was each other by as much as 2 cm [6], [56]. Dielectric barrier discharges are sometimes confused with coronas, because the an order of magnitude lower at 0.3–0.9 eV. In addition, they latter sources may also exhibit microarcing. observed electron densities near 3 10 cm . Fig. 9. Schematic of a coronaDielectric discharge. barrier discharges have been examined for several material processes, including the cleaning of metal surfaces In transferred arcs and plasma torches, extremely high tem- [57] and the plasma-assisted chemical vapor deposition of polymers [56] and glass films [58]. However, since the plasma peratures promote the complete dissociation of the feed gas. 21 is not uniform, its use in etching and deposition is limited to plasma usually exists in a region of the gas extending about 0.5 For example, Fig. 8 shows the effect of the gas temperature cases where the surface need not be smooth. For example, in mm out from the metalthe study point. of SiO Indeposition, the drift it was region found that the outside surface this on the distribution of oxygen species and electrons in an roughness exceeded 10% of the film thickness [58]. volume, charged speciesEliasson diffuse and coworkers toward [6], [54] the have modeled planar silent electrode dis- arc at 0.95 atm of oxygen [42]. The oxygen molecules are charges and have concluded that the electron temperature and are collected. ranges from 1–10 eV. They have also simulated air and oxygen approximately 99% dissociated with predominantly all of these discharges, and their results for pure oxygen at 760 torr are Fig. 11. Concentrations of species inThe a corona discharge restricted as a function of area the shown of in the Fig. 13. corona The axis is dischargetime following the ignition has of limited radial distance between the electrodes for a potential difference of molecules being converted into O atoms. By contrast, the a single micro arc, and the axis is the concentration relative kV [53]. its applications in materials processing. In an attempt to concentrations of O ,O, and O are several orders of to the initial amount of oxygen 2 10 cm . Eliasson’s overcome this problem,model two-dimensional predicts that the electrons and ionsarrays exist for of a very electrodes short magnitude lower. These results are consistent with theIn energies the plasma near the tip, the density of charged species time, from 2–100 ns. These charged species produce O atoms, rapidly decreases with distancehave from been about 10 developed.–10 cm [5], Somewhich in turn applications react with oxygen molecules of coronas to produce include ozone. the required to dissociate and ionize oxygen: (O ) [8]. The5.11 electron eV, temperature within the plasma averages about Beyond about 20 ms, the discharge achieves a steady-state 5 eV. In the drift regionactivation outside the discharge, of polymer the electron production surfaces of ozone [48], of 0.1% [49], of the and initial the oxygen enhancement density. (O ) 6.48 eV, and (O) 13.62 eV [43].density is much lower, near 10 cm . Silent discharges are efficient ozone generators, and this has Pontiga et al. [53] haveof modeled SiO the distributiongrowth of duringspecies proven the to thermal be their principal oxidation industrial application of silicon [6]. wafers within a negative corona discharge, operating with pure oxygen at 760 torr and an applied[50], voltage [51]. of 4.25 kV. His model VI. PLASMA JET IV. CORONA DISCHARGE is based on a discharge betweenShown two coaxial in electrodes, Fig. a wire10 isShown the in dependence Fig. 14 is a schematic of of an the atmospheric-pressure voltage on the cathode, and an outer cylinder. Fig. 11 shows the concentration plasma jet [7], [59]. This new source consists of two concentric A corona discharge appears as a luminous glowof these localized species as a functioncurrent of the radial for distance. a positive Ozone electrodes point-to-plane through which a mixture corona of helium, operating oxygen, and in air is the dominant product at a concentration of 5 10 other gases flow. By applying 13.56 MHz RF power to the in space around a point tip in a highly nonuniformcm . electric Metastable oxygenat molecules 760 and torr O atoms [5], are [52]. five to inner The electrode plasma at a voltage ignites between at 100–250 a voltage V, the gas of 2–5 field. The physics of this source is well understoodsix orders [5], of [8], magnitudekV lower andin concentration. produces The ionized an extremelydischarge is ignited. small current of 10 –10 A. [12], [44]–[47]. The corona may be considered a Townsend Above 10 A, the voltage rapidly increases with current. This discharge or a negative glow discharge depending upon the coincides with the generation of micro-arcs, or “streamers,” field and potential distribution [45]. Fig. 9 shows a schematic that extend between the electrodes. A maximum voltage is of a point-to-plane corona. The apparatus consists of a metal recorded at about 5 10 A, where the device begins tip, with a radius of about 3 m, and a planar electrode to arc. Coronas are operated at currents below the onset separated from the tip by a distance of 4–16 mm [47]. The of arcing. SCHUTZE¨ er al.: ATMOSPHERIC-PRESSURE PLASMA JET 1689 SCHUTZE¨ er al.: ATMOSPHERIC-PRESSURE PLASMA JET 1689

Fig. 12. Schematic of a silent discharge; (1) metallic electrodes and (2) dielectric barrier coating.

species are still lower at an average density of about 10 cm . This distribution of reaction products differs greatly from that seen in a low-pressure glow discharge (cf. Figs. 4 Fig. 12. Schematicand 5). of a silent discharge; (1) metallic electrodes and (2) dielectric barrier coating. V. DIELECTRIC BARRIER DISCHARGE Dielectric barrier discharges are also called “silent” and Fig. 10. Current–voltage characteristics for a positive point-to-plane corona discharge with a gap of 13 mm in 1 atm of air [5]. species are“atmospheric-pressure-glow” still lower at an averagedischarges [6], density [54]–[56]. of A about 10 schematic of this source is shown in Fig. 12. It consists cm . Thisof two distribution metal electrodes, of in reaction which at least products one is coated differs with greatly from that seena dielectric in layer. a low-pressure The gap is on the order glow of several discharge mm, and (cf. Figs. 4 the applied voltage is about 20 kV. The plasma is generated and 5). through a succession of micro arcs, lasting for 10–100 ns, and randomly distributed in space and time. These streamers are believed to be 100 m in diameter and are separated from eachV. other DIELECTRIC by as much asB 2 cmARRIER [6], [56].D DielectricISCHARGE barrier Dielectricdischarges barrier are sometimes discharges confused are with also coronas, called because the “silent” and Fig. 10. Current–voltage characteristics for a positive point-to-plane corona latter sources may also exhibit microarcing. Dielectric barrier discharges have been examined for several discharge with a gap of 13 mm in 1 atm of air [5]. “atmospheric-pressure-glow”22 discharges [6], [54]–[56]. A material processes, including the cleaning of metal surfaces schematic[57] of and this the source plasma-assisted is shown chemical vapor in Fig.deposition 12. of It consists of two metalpolymers electrodes, [56] and glass in films which [58]. However, at least since one the plasma is coated with is not uniform, its use in etching and deposition is limited to a dielectriccases layer. where The the surface gap need is on not bethe smooth. order For of example, several in mm, and the appliedthe voltage study of SiO is aboutdeposition, 20 it kV. was found The that plasma the surface is generated roughness exceeded 10% of the film thickness [58]. through a successionEliasson and coworkers of micro [6], arcs, [54] have lasting modeled for silent 10–100 dis- ns, and randomly distributedcharges and have in concluded space that and the time. electron These temperature streamers are ranges from 1–10 eV. They have also simulated air and oxygen believed todischarges, be 100 and theirm results in diameter for pure oxygen and at 760 are torr separated are from Fig. 11. Concentrations of species in a corona discharge as a function of the shown in Fig. 13. The axis is time following the ignition of radial distance between the electrodes for a potential differenceeach of other by as much as 2 cm [6], [56]. Dielectric barrier kV [53]. a single micro arc, and the axis is the concentration relative dischargesto are the initial sometimes amount of confusedoxygen 2 with10 cm coronas,. Eliasson’s because the model predicts that the electrons and ions exist for a very short In the plasma near the tip, the density of chargedlatter species sourcestime, frommay 2–100 also ns. exhibit These charged microarcing. species produce O atoms, rapidly decreases with distance from about 10 –10 cmDielectric[5], which barrier in turn reactdischarges with oxygen have molecules been to produce examined ozone. for several [8]. The electron temperature within the plasma averages about Beyond about 20 ms, the discharge achieves a steady-state 5 eV. In the drift region outside the discharge,material the electron processes,production of includingozone of 0.1% the of the cleaning initial oxygen of density. metal surfaces density is much lower, near 10 cm . [57] and theSilent plasma-assisted discharges are efficient ozone chemical generators, vapor and this deposition has of Pontiga et al. [53] have modeled the distribution of species proven to be their principal industrial application [6]. within a negative corona discharge, operating withpolymers pure oxy- [56] and glass films [58]. However, since the plasma gen at 760 torr and an applied voltage of 4.25 kV.is His not model uniform, its use inVI. etching PLASMA andJET deposition is limited to is based on a discharge between two coaxial electrodes,cases a wire whereShown the surface in Fig. 14 is need a schematic not of be an atmospheric-pressure smooth. For example, in cathode, and an outer cylinder. Fig. 11 shows the concentration plasma jet [7], [59]. This new source consists of two concentric of these species as a function of the radial distance.the study Ozone ofelectrodes SiO throughdeposition, which a mixture it was of helium, found oxygen, that and the surface is the dominant product at a concentration ofroughness 5 10 other exceeded gases flow. 10% By applying of the 13.56 film MHz thickness RF power to [58]. the cm . Metastable oxygen molecules and O atoms are five to inner electrode at a voltage between 100–250 V, the gas six orders of magnitude lower in concentration. TheEliasson ionized discharge and coworkers is ignited. [6], [54] have modeled silent discharges and have concluded that the electron temperature ranges from 1–10 eV. They have also simulated air and oxygen discharges, and their results for pure oxygen at 760 torr are Fig. 11. Concentrations of species in a corona discharge as a function of the shown in Fig. 13. The axis is time following the ignition of radial distance between the electrodes for a potential difference of kV [53]. a single micro arc, and the axis is the concentration relative to the initial amount of oxygen 2 10 cm . Eliasson’s model predicts that the electrons and ions exist for a very short In the plasma near the tip, the density of charged species time, from 2–100 ns. These charged species produce O atoms, rapidly decreases with distance from about 10 –10 cm [5], which in turn react with oxygen molecules to produce ozone. [8]. The electron temperature within the plasma averages about Beyond about 20 ms, the discharge achieves a steady-state 5 eV. In the drift region outside the discharge, the electron production of ozone of 0.1% of the initial oxygen density. density is much lower, near 10 cm . Silent discharges are efficient ozone generators, and this has Pontiga et al. [53] have modeled the distribution of species proven to be their principal industrial application [6]. within a negative corona discharge, operating with pure oxygen at 760 torr and an applied voltage of 4.25 kV. His model VI. PLASMA JET is based on a discharge between two coaxial electrodes, a wire Shown in Fig. 14 is a schematic of an atmospheric-pressure cathode, and an outer cylinder. Fig. 11 shows the concentration plasma jet [7], [59]. This new source consists of two concentric of these species as a function of the radial distance. Ozone electrodes through which a mixture of helium, oxygen, and is the dominant product at a concentration of 5 10 other gases flow. By applying 13.56 MHz RF power to the cm . Metastable oxygen molecules and O atoms are five to inner electrode at a voltage between 100–250 V, the gas six orders of magnitude lower in concentration. The ionized discharge is ignited. 23 SCHUTZE¨ er al.: ATMOSPHERIC-PRESSURE PLASMA JET 1691

TABLE II DENSITIES OF CHARGE SPECIES IN THE PLASMA DISCHARGES

SCHUTZE¨ er al.: ATMOSPHERIC-PRESSURE PLASMA JET 1691

TABLE II DENSITIES OF CHARGE SPECIES IN THE PLASMA DISCHARGES

Fig. 17. Schematic of a cold plasma torch (m.b. matchbox) [62].

TABLE I BREAKDOWN VOLTAGES OF THE PLASMA DISCHARGES

Fig. 17. Schematic of a cold plasma torch (m.b. matchbox) [62].

1692 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 26, NO. 6, DECEMBER 1998 TABLE I BREAKDOWN VOLTAGES OF THE PLASMA DISCHARGES TABLE III [16] M. Shibata, N. Nakano, and T. Makabe, “Effect of O on plasma ¨ SCHUTZE er al.: ATMOSPHERIC-PRESSURE PLASMA JET 1691 DENSITIES OF OXYGEN SPECIES IN THE PLASMA DISCHARGES structures in oxygen radio frequency discharges,” J. Appl. Phys., vol. 80, no. 11, pp. 6142–6147, 1996. [17] K. S. Klopovskii, A. M. Popov, A. T. Rakhimov, T. V. Rakhimov, and V. A. Feoktistov, “Self-consistent model of an discharge at low TABLE II pressure in an oxygen plasma,” Plasma Phys. Rep., vol. 19, no. 7, pp. DENSITIES OF CHARGE SPECIES IN THE PLASMA DISCHARGES 473–477, 1993. [18] G. S. Selwyn, “Spatially resolved detection of O atoms in etching Fig. 17. The powered electrode consists of a metal needle with plasmas by two-photon laser-induced fluorescence,” J. Appl. Phys., vol. a thickness of 1 mm. This needle is inserted into a grounded 60, no. 8, pp. 2771–2774, 1986. [19] S. Ramakrishnan and M. W. Rogozinski, “Properties of electric arc metal cylinder. In addition, a quartz tube is placed between plasma for metal cutting,” J. Appl. Phys. D, vol. 30, pp. 636–644, 1997. Fig. 18. Comparison of the gas and electron temperatures for different[20] V. A. Nemchinsky, “Dross formation and heat transfer during plasma the cathode and anode, which makes this device resemble a atmospheric-pressure plasmas versus low-pressure plasmas. arc cutting,” J. Appl. Phys. D, vol. 30, pp. 2566–2572, 1997. Fig. 17. The powered electrode consists of a metal needle with obtain time-averaged values for the reactive species. However, [21] D. R. Mac Rae, “Plasma arc process systems, reactors, and applications,” dielectric1692 barrier discharge. Mixtures of rare gases, He and Ar, IEEE TRANSACTIONSperusal of the ON literature PLASMA suggests SCIENCE, that the VOL. time averaged 26, NO. 6, con- DECEMBERPlasma 1998 Chem. Plasma Process., vol. 9, no. 1, pp. 85S–118S, 1989. a thickness of 1 mm. This needle is inserted into a grounded [22] W. H. Gauvin, “Some characteristics of transferred-arc plasmas,” and other species are fed between the metal needle and quartz centrations should be similar to those found in a corona. Also, Plasma Chem. Plasma Process., vol. 9, no. 1, pp. 65S–84S, 1989. metal cylinder. In addition, a quartz tube is placed between Fig. 18.pressure Comparison plasmasthe of values the gas have shown and breakdown for electron the plasma temperatures jet voltages correspond for above different to the gas 10 in kV.[23] A G. Laroche and M. Orfeuil, Plasma in Industry. Avon, France: Doppée tube at flow velocities of about 5 m/s at 200–400 C. The gasesatmospheric-pressure plasmasbetween versusthe electrodes. low-pressure In the plasmas. downstream jet, the distribution Collection, 1989, p. 699. the cathode and anode, which makesTABLE this III device resemble a comparison[16] M. Shibata, of the N.– Nakano,curves and T. reveals Makabe, that “Effect the of O corona[24] andon J.plasma Heberlein and E. Pfender, “Thermal plasma chemical vapor deposi- are ionized and exit the source as a small jet. Koinuma and of species changes as indicated in Fig. 16. dielectric barrierDENSITIES discharge. OF OXYGEN MixturesSPECIES of INrare THE gases,PLASMA HeD andISCHARGES Ar, the plasmastructures jet exhibit in oxygen normal radio and frequency abnormal discharges,” glow regionsJ. Appl. that Phys.,tion,” invol.Materials Science Forum, J. J. Pouch and S. A. Alterovitz, Ed. coworkers have employed the cold plasma torch in a number of In the corona and dielectric barrier discharge, ozone is the Aedermannsdorf, Switzerland: Trans. Tech. Publications Ltd., 1993, vol. and other species are fed between the metal needle and quartz 80,main no. reaction 11, pp. product, 6142–6147, whereas 1996. in the other plasmas, oxygen 140–142, pp. 477–496. pressureare plasmas characteristic have breakdown of a weakly voltages ionized above plasma. 10 kV. However, A [25] the P. Fauchais, M. Vardelle, A. Vardelle, and L. Bianchi, “Plasma spray: tubematerials at flow processes, velocities of including about 5 m/s silicon at 200–400 etchingC. [62], The photoresist gases [17] K.atoms S. Klopovskii, represent a large A. M. fraction Popov, of the A. reactive T. Rakhimov, species. In T. a V. Rakhimov, comparisoncurrent of in theand a V. corona– A. Feoktistov,curves is limited reveals “Self-consistent to that below the modelcorona 10 of and anA, whichdischarge isStudy at lowof the coating generation,” Ceram. Int., vol. 22, no. 4, pp. areashing ionized [63], and deposition exit the source of SiO as a,small and TiO jet. Koinumafilms [64]–[67], and low-pressure glow discharge the concentrations of ions and 295–303, 1996. the plasmainsufficient jet exhibitpressureatoms to ionize normal are in an lower a oxygen and reasonable than abnormal plasma,” in an atmospheric-pressure volume glowPlasma regions Phys.of the Rep., that gas. plasma.vol. 19,[26] no. T. Yoshida, 7, pp. “The future of thermal plasma processing,” Materials Trans., JIM, vol. 31, no. 1, pp. 1–11, 1990. coworkerstreatment have of vulcanized employed the rubber cold plasma [68], torch and inthe a number production of of 473–477,However, 1993. the impingement rate of these species on a substrate Fig. 17. Schematic of a cold plasma torch (m.b. matchbox) [62]. are characteristicPresented of in a Tableweakly II ionized are the plasma. densities However, of charged the species[27] Y. Chang, R. M. Young, and E. Pfender, “Silicon nitride synthesis in an materials processes, including silicon etching [62], photoresist [18] G. S. Selwyn, “Spatially resolved detection of O atoms inatmospheric etching pressure convection-stabilized arc,” Plasma Chem. Plasma fullerenes [69]. current in a coronamay is be limited about the to same below in both 10 cases,A, since which the isflux to ashing [63], deposition of SiO , and TiO films [64]–[67], in the differentplasmasthe surface plasma by two-photon increases discharges. with laser-induced decreasing Except pressure. fluorescence,” for Taking the transferredJ. into Appl. Phys.,Process.,vol.vol. 9, no. 2, pp. 277–289, 1989. Koinuma and coworkers [69] measured the electron temper-insufficient to ionize a reasonable volume of the gas. [28] N. Kubota, M. Ayabe, T. Fukuda, and M. Akashi, “Characterization of TABLE I treatment of vulcanized rubber [68], and the production of arc and plasma60,account no. torch, 8, all pp. the all2771–2774, properties the plasmas of 1986. the plasmas, exhibit it appears electron that densitiesthe TiO thick films photoelectrodes prepared by the plasma-spray coating,” BREAKDOWN VOLTAGES OF THE PLASMA DISCHARGES ature in the plasma effluent with a Langmuir probe and foundPresented[19] in TableS.atmospheric-pressure Ramakrishnan II are the and densities plasma M. W.jet exhibitsof Rogozinski, charged the greatest species “Properties similarity of electricMater. Res. arc Soc., vol. 458, pp. 449–452, Dec. 1997. in the same range as a low-pressure discharge. However,[29] in A. G. Cunha, M. T. D. Orlando, F. G. Emmerich, C. Larica, and E. M. fullerenes [69]. to a low-pressure glow discharge. Consequently, this device it to be between 1–2 eV depending on the gas composition.in the different plasma for discharges. metal cutting,” ExceptJ. Appl. for Phys. the transferred D, vol. 30, pp. 636–644,Baggio-Saitovitch, 1997. “Optimization of Yba Cu O superconducting thick Koinuma and coworkers [69] measured the electron temper- the corona[20] V.shows and A. Nemchinsky, promise the dielectric for being“Dross used barrier formation in a discharge, number and heat of materials transfer the plasma duringlayers plasma produced by plasma spray technique,” Physica C, vol. 282–287, arc and plasma torch,applications all the that plasmas are now exhibit limited to electron vacuum. densities pp. 489–490, 1997. ature in the plasma effluent with a Langmuir probe and found is restrictedarc cutting,”to a smallJ. Appl. region Phys. of D, spacevol. 30, and pp. 2566–2572, is not available 1997.[30] J. R. Fincke, W. D. Swank, and D. C. Haggard, “Plasma spray obtainVIII. time-averaged COMPARISON values OF forPLASMA the reactiveSOURCES species. However,in the same[21] range D. R. as Mac a low-pressure Rae, “Plasma arc discharge. process systems, However, reactors, in and applications,”of alumina: Plasma and particle flow fields,” Plasma Chem. Plasma it to be between 1–2 eV depending on the gas composition. 24 for uniformly treating largeREFERENCES substrate areas. Recent results perusal of the literature suggests that the time averagedthe con- corona andPlasma the dielectric Chem. Plasma barrier Process., discharge,vol. 9, the no. plasma 1, pp. 85S–118S, 1989.Process., vol. 13, no. 4, pp. 579–600, 1993. Low-pressure plasma discharges are widely used in ma- obtained in our laboratory indicate that the plasma jet can[31] be Z. P. Lu, L. Stachowicz, P. Kong, J. Heberlein, and E. Pfender, is restricted[22] to a W. small[1] H. H. Schmid,Gauvin, region B. of Kegel,“Some space W. characteristics Petasch, and and is notG. Liebel, available of “Low transferred-arc pressure plasmas,”“Diamond synthesis by thermal plasma CVD at 1 atm,” Plasma Chem. centrations should be similar to those found in a corona. Also, plasma processing in microelectronics,” in Proc. Joint 24th Int. Conf. terials processing,VIII. COMPARISON because OF theyPLASMA haveS aOURCES number of distinct scaled upPlasma to dimensions Chem. Plasma required Process., invol. industrial 9, no. 1, processes pp. 65S–84S, [70]. 1989.Plasma Process., vol. 11, no. 3, pp. 387–394, 1991. for uniformly treatingMicroelectronics large substrate (MIEL) and areas.32nd Symp. Recent Devices Materials results SD’96, [32] Z. P. Lu, J. Heberlein, and E. Pfender, “Process study of thermal plasma the values shown for the plasma jet correspond to the gas in [23] G. LarocheNova Gorcia, and M. Slovenia, Orfeuil, 1996,Plasma pp. 17–35. in Industry. Avon, France:chemical Doppée vapor deposition of diamond—Part I: Substrate material, tem- advantages:Low-pressure 1) low plasma breakdown discharges voltages; are widely 2) a used stable in operating ma- obtainedShown in our laboratory in[2] Fig. M. A. Lieberman 18 indicate are and the A. that J. Lichtenberg, ranges the plasmaPrinciples of neutral jet of Plasma can Dischargesand be electron between the electrodes. In the downstream jet, the distribution Collection, 1989, p. 699. perature, and methane concentration,” Plasma Chem. Plasma Process., terialswindow processing, between because spark ignition they have and a arcing;number 3)of distinct an electronscaledtemperatures up to[24] dimensions J. Heberlein thatand Materials requiredapply and Processing. E.to in Pfender, each industrialNew plasma “Thermal York: Wiley,processes discharge. 1994. plasma [70]. chemical The plasma vaporvol. deposi- 12, no. 1, pp. 35–53, 1992. of species changes as indicated in Fig. 16. [3] P. Fauchais and A. Vardelle, “Thermal plasmas,” IEEE Trans. Plasma [33] , “Process study of thermal plasma chemical vapor deposition Fig. 17. The powered electrode consists of a metal needle with jet and coronation,”Sci., inclearlyMaterialsvol. 25, fall pp. 1258–1280, Science within Forum, Dec. the 1997. rangeJ. J. Pouch commonly and S. A. utilized Alterovitz,of diamond—Part Ed. II: Pressure dependence and effect of substrate advantages:temperature 1) capable low breakdown of dissociating voltages; molecules 2) a stable (1–5 operating eV), but aShown in Fig. 18[4] R. are W.the Smith, ranges D. Wei, and of D. neutral Apelian, “Thermal and electron plasma materials pretreatment,” Plasma Chem. Plasma Process., vol. 12, no. 1, pp. 55–69, a thickness of 1 mm. This needle is inserted into a grounded In the corona and dielectric barrier discharge, ozone is the Aedermannsdorf,processing—Applications Switzerland: and opportunities,” Trans. Tech.Plasma Publications Chem. Plasma Ltd., 1993, vol. windowlow neutral between temperature; spark ignition 4) relatively and arcing; high 3) concentrations an electron temperatures of in low-pressure that apply plasma to each discharges. plasma discharge. The electron The plasma temperature of1992. Process., vol. 9, no. 1, pp. 135S–165S, 1989. 140–142, pp. 477–496. [34] J. M. Olson and M. J. Dawes, “Examination of the material properties metal cylinder. In addition, a quartz tube is placed betweentemperatureFig.main 18. reaction Comparison capable product, of of dissociating the gas whereas and moleculeselectron in the temperatures (1–5 other eV), plasmas, for but different a oxygenjet and corona clearly[5] M. fall Goldman within and R. the S. Sigmond, range “Corona commonly and insulation,” utilizedIEEE Trans. ions and radicals to drive etching and deposition reactions; and the plasma[25] P. jet Fauchais, is estimated M. Vardelle, to be A. Vardelle,between and 1–2 L. eV. Bianchi, However, “Plasmaand spray: performance of thin-diamond film cutting tool inserts produced by the cathode and anode, which makes this device resemble a atmospheric-pressure plasmas versus low-pressure plasmas. Elect. Insulation, vol. EI-17, no. 2, pp. 90–105, 1982. low5) aatoms neutral uniform represent temperature; glow over a large 4) a large relatively fraction gas volume. high ofthe concentrations It reactive is instructive species. of in to low-pressure Inthere a is a plasmaStudy sufficient[6] B. of Eliassondischarges. the population and coating U. Kogelschatz, The generation,” electron of “Nonequilibrium electrons temperatureCeram. volume at Int., plasma energies ofvol. chem- 22, high no.arc-jet 4, and pp. hot filament chemical vapor deposition,” J. Mater. Res., vol. dielectric barrier discharge. Mixtures of rare gases, He and Ar, ical processing,” IEEE Trans. Plasma Sci., vol. 19, pp. 1063–1077, Dec. 11, no. 7, pp. 1765–1775, 1996. ions and radicals to drive etching and deposition reactions; and the plasma jet is295–303, estimated 1996. to be between 1–2 eV. However, [35] S. K. Baldwin, Jr., T. G. Owano, and C. H. Kruger, “Growth rate comparelow-pressure atmospheric-pressure glow discharge plasmas the concentrations against these criteria, of ions andenough to dissociate1991. many molecules, including O and N and other species are fed between the metal needle and quartz5) a uniform glow over a large gas volume. It is instructive to there is a sufficient[26] T. Yoshida,[7] population J. Y. Jeong, “The S. E.future of Babayan, electrons of thermal V. J. Tu, at J. plasma Park, energies R. processing,” F. Hicks, high and G.Materials S. studies Trans., of CVD diamond in an RF plasma torch,” Plasma Chem. Plasma topressure seeatoms if any areplasmas of lower these have sourcesthan breakdown in can an matchvoltages atmospheric-pressure the above performance 10 kV. A plasma. of [70]. Selwyn, “Etching materials with an atmospheric-pressure plasma jet,” Process., vol. 14, no. 4, pp. 383–406, 1994. tube at flow velocities of about 5 m/s at 200–400 C. The gases JIM, vol. 31, no. 1, pp. 1–11, 1990. [36] S. K. Baldwin, Jr., T. G. Owano, M. Zhao, and C. H. Kruger, “Enhanced comparecomparisonHowever, atmospheric-pressure theof the impingement– curves plasmas rate reveals of against these that these species the corona criteria, on a and substrateenough to dissociatePlasma many Source molecules, Sci. Technol., vol. including 7, no. 3, pp. O 282–285,and 1998. N are ionized and exit the source as a small jet. Koinuma andlow-pressure discharges. Listed[27] in Y.[8] Table Chang, Y. P. III R.Raizer, M. areGas Young, the Discharge average and Physics. E. Pfender, densitiesNew York: “Silicon Springer-Verlag, of oxygen nitride synthesis ions,deposition in an rate of diamond in atmospheric pressure plasma CVD: Effects to seethemay plasmaif any be of aboutjet these exhibit the sources normal same can and in match abnormal both the cases, performance glow since regions theof that flux[70]. to atmospheric1991. pressure convection-stabilized arc,” Plasma Chem.of Plasma a secondary discharge,” Diam. Relat. Mater., vol. 6, no. 2–4, pp. coworkers have employed the cold plasma torch in a number of Shown in Table I are breakdown voltages for the different oxygen atoms,[9] M. and I. Boulos, ozone “Thermal in the plasma different processing,” IEEE atmospheric-pressure Trans. Plasma Sci., 202–206, 1997. low-pressureare characteristic discharges. of a weakly ionized plasma. However, the Listed in TableProcess., III arevol. 19,vol. the pp. average 9, 1078–1089, no. 2,densities Dec. pp. 1991. 277–289, of oxygen 1989. ions, [37] L. Pawlowski and P. Fauchais, “Thermally sprayed coatings for electri- materials processes, including silicon etching [62], photoresistplasmathe surface sources. increases The plasma with jet decreasing has a pressure.below thatTaking of intoplasma discharges. Since the dielectric barrier discharge op-cal and electronic applications,” in Materials Science Forum, J. J. Pouch Showncurrent in in Table a corona I are is breakdown limited to voltages below for 10 theA, different which isoxygen atoms,[28] and N.[10] Kubota,ozone E. E. Kunhardt,in M. the Ayabe, different “Electrical T. Fukuda, breakdown atmospheric-pressure and of gases: M. TheAkashi, prebreakdown “Characterization of ashing [63], deposition of SiO , and TiO films [64]–[67], account all the properties of the plasmas, it appears that the TiO thickstage,” IEEE films Trans. photoelectrodes Plasma Sci., vol. PS-8,prepared pp. 130–138, by the June plasma-spray 1980. coating,”and S. A. Alterovitz, Eds. Aedermannsdorf, Switzerland: Trans. Tech. plasmaa low-pressureinsufficient sources. to ionize The discharge, plasma a reasonable whereas jet has volume a the ofotherbelow the gas. atmospheric- that of plasmaerates discharges. as a[11] seriesSince R. Woo, the “RF of voltage dielectric transient breakdown barrier microarcs, and the Paschen discharge curve,” it isProc. op- difficult IEEE, toPublications Ltd., 1993, vol. 140–142, pp. 497–520. treatment of vulcanized rubber [68], and the production of atmospheric-pressure plasma jet exhibits the greatest similarity Mater.vol. Res. 62, no. Soc., 4, p.vol. 521, 458,1974. pp. 449–452, Dec. 1997. [38] S. Ramakrishnan, M. Gershenzon, F. Polivka, T. N. Kearney, and M. W. a low-pressurePresented in discharge, Table II are whereas the densities the other of chargedatmospheric- specieserates as a[29] series A.[12] G. of J. Cunha, M. transient MeekM. and J. T. D. microarcs, D. Craggs, Orlando,Electrical it F. Breakdown is G. Emmerich,difficult of Gases. toLondon, C. Larica, andRogozinski, E. M. “Plasma generation for the plasma cutting process,” IEEE fullerenes [69]. to a low-pressure glow discharge. Consequently, this device U.K.: Oxford Univ. Press, 1953. Trans. Plasma Sci., vol. 25, pp. 937–946, Oct. 1997. in the different plasma discharges. Except for the transferred Baggio-Saitovitch,[13] G. Francis, “The glow “Optimization discharge at low of pressure,” Yba Cu in EncyclopediaO superconducting of [39] C. H. thick Kruger, “Nonequilibrium effects in thermal plasma chemistry,” Koinuma and coworkers [69] measured the electron temper- Physics, Gas Discharges II, vol. XXII, S. Flugge,¨ Ed. Berlin, Germany: Plasma Chem. Plasma Process., vol. 9, no. 4, pp. 435–443, 1989. arcshows and plasma promise torch, for all being the plasmas used exhibit in a electron number densities of materials layers produced by plasma spray technique,” Physica C, vol. 282–287, ature in the plasma effluent with a Langmuir probe and found Springer-Verlag, 1956, pp. 53–208. [40] C. H. Kruger, T. G. Owano, and C. O. Laux, “Experimental investigation inapplications the same range that as are a low-pressurenow limited discharge. to vacuum. However, in pp.[14] 489–490, W. Elenbaas, 1997.The High Pressure Mercury Vapor Discharge. Amster- of atmospheric pressure nonequilibrium plasma chemistry,” IEEE Trans. it to be between 1–2 eV depending on the gas composition. [30] J. R.dam, Fincke, The Netherlands: W. D. North-Holland, Swank, and 1951. D. C. Haggard, “PlasmaPlasma spray Sci., vol. 25, pp. 1042–1051, Oct. 1997. the corona and the dielectric barrier discharge, the plasma [15] E. Pfender, “Electric arcs and arc heaters,” in Gaseous Electronics, vol. [41] H. Kim and S. H. Hong, “Comparative measurements on thermal plasma of alumina:1, M. N. Hirsh Plasma and H. and J. Oskam, particle Eds. flowNew York: fields,” Academic,Plasma 1978. Chem.jet Plasma characteristics in atmospheric and low pressure plasma sprayings,” is restricted to a small regionREFERENCES of space and is not available Process., vol. 13, no. 4, pp. 579–600, 1993. VIII. COMPARISON OF PLASMA SOURCES for uniformly treating large substrate areas. Recent results [31] Z. P. Lu, L. Stachowicz, P. Kong, J. Heberlein, and E. Pfender, [1] H. Schmid, B. Kegel, W. Petasch, and G. Liebel, “Low pressure Low-pressure plasma discharges are widely used in ma- obtained in our laboratory indicate that the plasma jet can be “Diamond synthesis by thermal plasma CVD at 1 atm,” Plasma Chem. plasma processing in microelectronics,” in Proc. Joint 24th Int. Conf. Plasma Process., vol. 11, no. 3, pp. 387–394, 1991. terials processing, because they have a number of distinct scaledMicroelectronics up to dimensions (MIEL) requiredand in32nd industrial Symp. Devices processes Materials [70]. SD’96, [32] Z. P. Lu, J. Heberlein, and E. Pfender, “Process study of thermal plasma advantages: 1) low breakdown voltages; 2) a stable operating ShownNova in Gorcia, Fig. 18 Slovenia, are the 1996, ranges pp. 17–35. of neutral and electron chemical vapor deposition of diamond—Part I: Substrate material, tem- window between spark ignition and arcing; 3) an electron temperatures[2] M. A. Lieberman that apply and to A. each J. Lichtenberg, plasma discharge.Principles The of Plasma plasma Discharges perature, and methane concentration,” Plasma Chem. Plasma Process., and Materials Processing. New York: Wiley, 1994. vol. 12, no. 1, pp. 35–53, 1992. temperature capable of dissociating molecules (1–5 eV), but a jet[3] and P. corona Fauchais clearly and A. fall Vardelle, within “Thermal the range plasmas,” commonlyIEEE utilized Trans. Plasma low neutral temperature; 4) relatively high concentrations of in low-pressure plasma discharges. The electron temperature of [33] , “Process study of thermal plasma chemical vapor deposition Sci., vol. 25, pp. 1258–1280, Dec. 1997. of diamond—Part II: Pressure dependence and effect of substrate [4] R. W. Smith, D. Wei, and D. Apelian, “Thermal plasma materials ions and radicals to drive etching and deposition reactions; and the plasma jet is estimated to be between 1–2 eV. However, pretreatment,” Plasma Chem. Plasma Process., vol. 12, no. 1, pp. 55–69, processing—Applications and opportunities,” Plasma Chem. Plasma 5) a uniform glow over a large gas volume. It is instructive to there is a sufficient population of electrons at energies high 1992. Process., vol. 9, no. 1, pp. 135S–165S, 1989. [34] J. M. Olson and M. J. Dawes, “Examination of the material properties compare atmospheric-pressure plasmas against these criteria, enough[5] M. to Goldman dissociate and R.many S. Sigmond, molecules, “Corona including and insulation,” O andIEEE N Trans. and performance of thin-diamond film cutting tool inserts produced by to see if any of these sources can match the performance of [70]. Elect. Insulation, vol. EI-17, no. 2, pp. 90–105, 1982. low-pressure discharges. [6]Listed B. Eliasson in Table and III U. are Kogelschatz, the average “Nonequilibrium densities of oxygen volume ions, plasma chem- arc-jet and hot filament chemical vapor deposition,” J. Mater. Res., vol. ical processing,” IEEE Trans. Plasma Sci., vol. 19, pp. 1063–1077, Dec. 11, no. 7, pp. 1765–1775, 1996. Shown in Table I are breakdown voltages for the different oxygen atoms, and ozone in the different atmospheric-pressure [35] S. K. Baldwin, Jr., T. G. Owano, and C. H. Kruger, “Growth rate plasma sources. The plasma jet has a below that of plasma1991. discharges. Since the dielectric barrier discharge op- [7] J. Y. Jeong, S. E. Babayan, V. J. Tu, J. Park, R. F. Hicks, and G. S. studies of CVD diamond in an RF plasma torch,” Plasma Chem. Plasma a low-pressure discharge, whereas the other atmospheric- eratesSelwyn, as a series “Etching of materials transient with microarcs, an atmospheric-pressure it is difficult plasma to jet,” Process., vol. 14, no. 4, pp. 383–406, 1994. Plasma Source Sci. Technol., vol. 7, no. 3, pp. 282–285, 1998. [36] S. K. Baldwin, Jr., T. G. Owano, M. Zhao, and C. H. Kruger, “Enhanced [8] Y. P. Raizer, Gas Discharge Physics. New York: Springer-Verlag, deposition rate of diamond in atmospheric pressure plasma CVD: Effects 1991. of a secondary discharge,” Diam. Relat. Mater., vol. 6, no. 2–4, pp. [9] M. I. Boulos, “Thermal plasma processing,” IEEE Trans. Plasma Sci., 202–206, 1997. vol. 19, pp. 1078–1089, Dec. 1991. [37] L. Pawlowski and P. Fauchais, “Thermally sprayed coatings for electri- [10] E. E. Kunhardt, “Electrical breakdown of gases: The prebreakdown cal and electronic applications,” in Materials Science Forum, J. J. Pouch stage,” IEEE Trans. Plasma Sci., vol. PS-8, pp. 130–138, June 1980. and S. A. Alterovitz, Eds. Aedermannsdorf, Switzerland: Trans. Tech. [11] R. Woo, “RF voltage breakdown and the Paschen curve,” Proc. IEEE, Publications Ltd., 1993, vol. 140–142, pp. 497–520. vol. 62, no. 4, p. 521, 1974. [38] S. Ramakrishnan, M. Gershenzon, F. Polivka, T. N. Kearney, and M. W. [12] J. M. Meek and J. D. Craggs, Electrical Breakdown of Gases. London, Rogozinski, “Plasma generation for the plasma cutting process,” IEEE U.K.: Oxford Univ. Press, 1953. Trans. Plasma Sci., vol. 25, pp. 937–946, Oct. 1997. [13] G. Francis, “The glow discharge at low pressure,” in Encyclopedia of [39] C. H. Kruger, “Nonequilibrium effects in thermal plasma chemistry,” Physics, Gas Discharges II, vol. XXII, S. Flugge,¨ Ed. Berlin, Germany: Plasma Chem. Plasma Process., vol. 9, no. 4, pp. 435–443, 1989. Springer-Verlag, 1956, pp. 53–208. [40] C. H. Kruger, T. G. Owano, and C. O. Laux, “Experimental investigation [14] W. Elenbaas, The High Pressure Mercury Vapor Discharge. Amster- of atmospheric pressure nonequilibrium plasma chemistry,” IEEE Trans. dam, The Netherlands: North-Holland, 1951. Plasma Sci., vol. 25, pp. 1042–1051, Oct. 1997. [15] E. Pfender, “Electric arcs and arc heaters,” in Gaseous Electronics, vol. [41] H. Kim and S. H. Hong, “Comparative measurements on thermal plasma 1, M. N. Hirsh and H. J. Oskam, Eds. New York: Academic, 1978. jet characteristics in atmospheric and low pressure plasma sprayings,” APPLIED PHYSICS LETTERS 89, 221504 ͑2006͒

Microplasma jet at atmospheric pressure Yong Cheol Honga͒ and Han Sup Uhm Department of Molecular Science and Technology, Ajou University, San 5, Wonchon-Dong, Youngtong-Gu, Suwon 443-749, Korea ͑Received 26 August 2006; accepted 22 October 2006; published online 30 November 2006͒ A nitrogen microplasma jet operated at atmospheric pressure was developed for treating thermally sensitive materials. For example, the plasma sources in treatment of vulnerable biological materials must operate near the room temperature at the atmospheric pressure, without any risk of arcing or electrical shock. The microplasma jet device operated by an electrical power less than 10 W exhibited a long plasma jet of about 6.5 cm with temperature near 300 K, not causing any harm to human skin. Optical emission measured at the wide range of 280–800 nm indicated various reactive species produced by the plasma jet. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2400078͔

Nonthermal plasmas generated at reduced pressure have dielectric disk can be made of glass, quartz, Teflon, etc. The well established for broad applications in material science. assembled electrodes and dielectric disks are inserted in a Nonthermal atmospheric plasmas get much attention lately dielectric case of the same diameter as that of the dielectric because they can provide a cheaper and more convenient disk. To prevent an electrical shock and damage caused from alternative in comparison with low-pressure plasmas.1 Vari- electrode by accidental contact to human body, the front ous configurations and applications of nonthermal plasmas electrode is also covered with the cylindrical dielectric case, have been extensively investigated nowadays. Capacitively as shown in Fig. 1. The dielectric case has the same size of coupled radio-frequency ͑rf͒ discharges are studied in con- hole as that of the electrode. Once nitrogenAPPLIED is introduced PHYSICS LETTERS 89, 221504 ͑2006͒ nection with material surface processing2,3 and plasma dis- through the aligned holes of the electrodes and dielectric play panels,4 dc microhollow cathode discharges can serve as disks, and ac high voltage is applied, a discharge is fired in efficient sources of vacuum UV radiation,5,6 and dielectric the gap between the electrodes and a long plasma jet reach- barrier discharges have been turned out to be useful in sur-Microplasmaing lengths up to 6.5 jet cm is at ejected atmospheric to open air through the pressure 7,8 face modification and gas conversion. More recently, much front electrode, as shown ina the͒ inset of Fig. 1. Therefore, the effort is endowed in creating an appropriate plasma source deviceYong can Cheolbe handheld, Hong and theand long Han and narrow Sup plasma Uhm jet for biomedical applications.9–11 Particularly, Stoffels et al.12 of theDepartment device can of be Molecular directed towards Science a target and surface. Technology, Ajou University, San 5, Wonchon-Dong, Youngtong-Gu, developed a nondestructive atmospheric plasma source, so SuwonBecause 443-749, the nitrogen Korea microplasma jet in Fig. 1 remains called rf plasma needle, for the study of the plasma interac- near the room temperature, it can be touched by bare hands tions with living cells and tissues. Such a source has to meet or scanned͑Received on human 26 August skin without 2006; establishing accepted any 22 conduc- October 2006; published online 30 November 2006͒ many requirements to be suitable in applications for treating tive pathway. As one example for treating a vulnerable tar- thermally sensitive materials. For instance, the plasma source get,A Fig. nitrogen2 reveals microplasma a photograph of jet the operated microplasma at atmospheric jet in pressure was developed for treating thermally for biomedical applications must provide truly nonthermal contactsensitive with bare materials. hand. From For the example, measurement the of plasma gas tem- sources in treatment of vulnerable biological materials plasma working at atmospheric pressure and near the room peraturemust by operate a thermocouple, near the it was room shown temperature that the tempera- at the atmospheric pressure, without any risk of arcing or temperature without any electrical and chemical risks. The ture of the plasma jet at 2 cm from the dielectric case was microhollow discharge occurs by applying external dc or belowelectrical 300 K. In shock. Fig. 2, the The plasma microplasma operated at 6.3 jet lpm device͑liters operated by an electrical power less than 10 W time-varying voltage electrodes. Tens or hundreds of micro- perexhibited minute͒ nitrogen a long was plasma ejected jet at the of about speed of 6.5 approxi- cm with temperature near 300 K, not causing any harm to sized cathode cavity is then formed in reduced pressure, con- matelyhuman 535 m/s, skin. producing Optical afterglowemission at measured high pressure at the and wide range of 280–800 nm indicated various reactive fining the glow discharge in inert gases.5,6,13,14 Meanwhile, cooling down to the room temperature. The microplasma de- we present a microplasma jet device operated at the atmo- vicespecies can also produced produce a by short the plasma plasma less jet. than © 52006 mm in American Institute of Physics. spheric pressure, which can produce a long cold plasma jet ͓DOI: 10.1063/1.2400078͔ 221504-2 Y. C. Hong and H. S. Uhm Appl. Phys. Lett. 89,221504͑2006͒ of several centimeters in nitrogen gas and which might be useful in treating thermally sensitive materials. Figure 1 is a schematic presentation of the microplasma jet device at atmospheric pressure. The ac power supplier is Nonthermal plasmas generated at reduced pressure have dielectric disk can be made of glass, quartz, Teflon, etc. The a commercially available transformer for neon light operatedwell established for broad applications in material science. assembled electrodes and dielectric disks are inserted in a at 20 kHz. The applied voltage is connected to two elec-Nonthermal atmospheric plasmas get much attention lately dielectric case of the same diameter as that of the dielectric trodes with a hole of 500 ␮m diameter, through which nitro-because they can provide a cheaper and more convenient disk. To prevent an electrical shock and damage caused from gen gas is flowing. Each of the two electrodes is made of an 1 aluminum disk with 20 mm diameter and 3 mm thicknessalternative in comparison with low-pressure plasmas. Vari- electrode by accidental contact to human body, the front attached to the surface of a centrally perforated dielectricous configurations and applications of nonthermal plasmas electrode is also covered with the cylindrical dielectric case, disk with 1.5 mm thickness. The hole in the center ofhave the been extensively investigated nowadays. Capacitively as shown in Fig. 1. The dielectric case has the same size of dielectric disk has the same diameter with the electrodes. The coupled radio-frequency ͑rf͒ discharges are studied in con- hole as that of the electrode. Once nitrogen is introduced FIG. 1. ͑Color online͒ Schematic presentation of a simple2,3 nitrogen micro- FIG. 2. ͑Color online͒ Photograph of the N2 microplasma jet in contact with a͒ nection with material surface processing and plasma dis- through the aligned holes of the electrodes and dielectric Author to whom corresondence should be addressed; electronic mail: plasma jet4 device at atmospheric pressure. The inset is the photograph of the human skin. [email protected] play panels,microplasmadc jet microhollow at 6.3 lpm N2. cathode discharges can serve as disks, and ac high voltage is applied, a discharge is fired in 5,6 efficient sources of vacuum UV radiation, and dielectric the gap between the electrodes and a9,12 long plasma jet reach- 0003-6951/2006/89͑22͒/221504/3/$23.0089,221504-1 © 2006 American Institute of Physics length such as the device known as the plasma needle by Downloaded 06 May 2009 to 66.112.232.233. Redistribution subjectbarrier to AIP discharges license or copyright; have been see http://apl.aip.org/apl/copyright.jsp turned out to be useful25 in sur-controllinging gas lengths flow rate up and to applied 6.5 cm voltage. is ejected to open air through the 7,8 The measured voltage and the discharge current charac- FIG. 4. Gas temperature estimated from measured and simulated optical face modification and gas conversion. More recently, much front electrode, as shown in the inset of Fig. 1emissions. Therefore, of OH molecules the near 309 nm with a spectral resolution of teristic to the present microplasma jet device are shown in effort is endowed in creating an appropriate plasma source device can be handheld, and the long and narrow0.3 nm for plasma the N2 microplasma jet jet at atmospheric pressure. 9–11 Fig.12 3, when 3 lpm nitrogen was injected. The length of the for biomedical applications. Particularly, Stoffels et al.plasma jetof for the this device flow rate can was be about directed 3.3 cm. towards The black aand target surface. developed a nondestructive atmospheric plasma source, sogray lines representBecause the measured the nitrogen voltage microplasma and discharge cur- jet inelectric Fig. 1 fieldremainsE, the current density J, and the electron mo- called rf plasma needle, for the study of the plasma interac-rent, respectively.near the The room peak-to-peak temperature, voltage it and can current be weretouchedbility by␮ baree. The hands electron density is defined as ne =J/͑E␮ee͒. As tions with living cells and tissues. Such a source has to meet1.92 kVor͑0.28 scanned kV in rms on͒ humanand 1.02 skin A ͑0.038 without A in rms establishing͒, re- an example, any conduc- we consider nitrogen plasma at 1 atm. The respectively. The discharge pattern showed sharp current duced electric field ͑E/ P͒ is estimated to be many requirements to be suitable in applications for treating tive pathway. As one example for treating a vulnerable tar- pulses occurred mainly at a steep voltage gradient, though 16.84 ͑V/cm Torr͒, which gives a value of ␮ep for nitrogen thermally sensitive materials. For instance, the plasma sourcethe peakget, current Fig. value2 reveals varied. This a photograph is more apparent of from the a microplasmagas as 0.42ϫ jet106 incm2 Torr/V s.16 The current density was for biomedical applications must provide truly nonthermalclose-upcontact in the inset with of thebare part hand. marked From with dottedthe measurement line in obtained of gas from tem- Figs. 1 and 3 to be 19.38 A/cm2 by consider- plasma working at atmospheric pressure and near the roomFig. 3.perature This seems by to a be thermocouple, similar to that in it wasatmospheric- shown thating the effective tempera- discharge radius between the two electrodes. pressure glow discharge sustained between two parallel-plate Eventually, the electron density is estimated to be temperature without any electrical and chemical risks. Theelectrodes.ture15 The of typicalthe plasma operational jet powerat 2 cm of the from plasma the jet dielectric1.71ϫ10 case13/cm was3 at the midplane in the diode where the dis- microhollow discharge occurs by applying external dc oris aboutbelow 10 W. The 300 electron K. In Fig. temperature2, theT plasmae can be operated roughly atcharge 6.3 lpm occurs.͑liters time-varying voltage electrodes. Tens or hundreds of micro-calculatedper from minute swarm͒ parametersnitrogen of was electrons ejected in nitrogen at the as speedThe of gas approxi- temperature was estimated by making use of an sized cathode cavity is then formed in reduced pressure, con-well-knownmately Einstein’s 535 m/s,equation, producingkBTe /eϷDe afterglow/␮e, where ␮ ate, highoptical pressure spectroscopy, and as shown in Fig. 4. The rotational struc- 5,6,13,14 kB, and De are drift mobility, Boltzmann constant ture of diatomic gases provides information of the rotational fining the glow discharge in inert gases. Meanwhile,͑1.38ϫ10cooling−23 J/K͒ down, and the to diffusion the room constant, temperature. which is ex- The microplasmatemperature. Molecules de- in the rotational states and the neu- we present a microplasma jet device operated at the atmo-pressedvice as a function can also of E/N produce. From Fig. a3, short the electric plasma field E lesstral than gas molecules5 mm in are in equilibrium due to the low energies spheric pressure, which can produce a long cold plasma jetis estimated to be 12.8 kV/cm, which is consistent to needed for rotational excitation and the short transition 5.1ϫ10−16 V cm2 ͓Ϸ51 Td ͑tomosecond͔͒ in reduced elec- times. Therefore, the gas temperature can be obtained from of several centimeters in nitrogen gas and which might betric field E/N.16 According to Nakamura,17 the ratio of the the rotational temperature.3 The experimental spectrum in useful in treating thermally sensitive materials. diffusion coefficient to the electron mobility is given by Fig. 4 was obtained at the same experimental parameters as Figure 1 is a schematic presentation of the microplasmaDe /␮e =0.53 V for E/N=51 Td. As the results, the electron that of Fig. 3. By comparing measured and simulated optical temperature is predicted to be 0.56 eV from the Einstein emissions of OH radicals around 309 nm with a spectral jet device at atmospheric pressure. The ac power supplier is 18 equation. The averaged electron density ͑ne͒ can also be es- resolution of 0.3 nm, gas temperature was found to be ap- a commercially available transformer for neon light operatedtimated from the electrical parameters of the discharge, the proximately 290 K in a good agreement with measured data at 20 kHz. The applied voltage is connected to two elec- by a thermocouple, as mentioned earlier, revealing low- trodes with a hole of 500 ␮m diameter, through which nitro- temperature operation, the most important parameter in biomedical applications. This is similar to the observation from gen gas is flowing. Each of the two electrodes is made of an a pulsed cold atmospheric plasma jet.19 aluminum disk with 20 mm diameter and 3 mm thickness To identify various excited plasma species generated by attached to the surface of a centrally perforated dielectric the microplasma jet device, optical emission spectroscopy disk with 1.5 mm thickness. The hole in the center of the was applied in a wide range of 280–800 nm wavelengths. Figure 5 shows the optical emissions of the microplasma jet, dielectric disk has the same diameter with the electrodes. The when 3 lpm nitrogen gas was injected. The emission spectrum was mainly dominated by the presence of excited nitro- FIG. 1. ͑Color online͒ Schematic presentation of a simple nitrogen micro- a͒ gen species, containing N2 second and first positive Author to whom corresondence should be addressed; electronic mail: plasma jet device at atmospheric pressure. The inset is thesystems. photograph20 In of addition, the highly reactive radicals such as hy- [email protected] microplasma jet at 6.3 lpm N2. droxyl ͑OH͒ at 308.9 nm and atomic oxygen at 616 and 777.1 nm were detected due to the opening to the ambient 0003-6951/2006/89͑22͒/221504/3/$23.0089,221504-1 © 2006 American Instituteair. These of radicals Physics can play important roles in plasma-surface interactions in applications. Downloaded 06 May 2009 to 66.112.232.233. Redistribution subjectFIG. 3. to͑Color AIP online license͒ Measured or copyright; voltage and discharge see http://apl.aip.org/apl/copyright.jsp current of the N2 microplasma jet of Fig. 1 with applied voltage with a sine wave form. The In summary, the nitrogen microplasma jet device pow- inset is a magnified view of the part marked with dotted line. ered by a commercially available power supplier showed a Downloaded 06 May 2009 to 66.112.232.233. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp PHYSICS OF PLASMAS 22, 122005 (2015)

Microsecond-pulsed dielectric barrier discharge plasma stimulation of tissue macrophages for treatment of peripheral vascular disease V. Miller,1,a) A. Lin,1 F. Kako,2 K. Gabunia,2 S. Kelemen,2 J. Brettschneider,1 G. Fridman,1 A. Fridman,1 and M. Autieri2 1AJ Drexel Plasma Institute, Drexel University, Camden, New Jersey 08103, USA 2Department of Physiology, Independence Blue Cross Cardiovascular Research Center, Temple University School of Medicine, Philadelphia, Pennsylvania 19140, USA (Received 13 May 2015; accepted 17 June 2015; published online 20 October 2015) Angiogenesis is the formation of new blood vessels from pre-existing vessels and normally occurs during the process of inﬂammatory reactions, wound healing, tissue repair, and restoration of blood ﬂow after injury or insult. Stimulation of angiogenesis is a promising and an important step in the treatment of peripheral artery disease. Reactive oxygen species have been shown to be involved in stimulation of this process. For this reason, we have developed and validated a non-equilibrium atmospheric temperature and pressure short-pulsed dielectric barrier discharge plasma system, which can non-destructively generate reactive oxygen species and other active species at the surface of the tissue being treated. We show that this plasma treatment stimulates the production of vascular endothelial growth factor, matrix metalloproteinase-9, and CXCL 1 that in turn induces angiogenesis in mouse aortic rings in vitro. This effect may be mediated by the direct effect of plasma generated reactive oxygen species on tissue. VC 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4933403]

I. INTRODUCTION Various growth factors essential for angiogenesis have been identified. Vascular Endothelial Growth Factor (VEGF-A) Peripheral Artery Disease (PAD) is characterized by a is one of the most important regulators of angiogenesis.4,5 decrease in blood flow to the extremities (ischemia) and is Its production by macrophages and stromal cells is stimu- associated with pain and often loss of limb function or even lated by Reactive Oxygen Species (ROS).6,7 Macrophages the limb itself. A major goal of therapy is to stimulate vascu- are also a potent source of Matrix metalloproteinase-9 logenesis in ischemic limbs, either through the growth of (MMP-9), also known as Collagenase, which breaks down new vessels or through collateral angiogenesis from existing26 1 the basal lamina in blood vessels and extracellular matrix to vessels. Angiogenesis, the growth of new blood vessels, 8 normally occurs during the process of inflammatory reac- make room for new vessels. Another important molecule, tions, wound healing, tissue repair, and restoration of blood CXCL 1, is a chemokine secreted by both macrophages and endothelial cells. It is important for migration and prolifera- flow after injury or insult. Physiologically, angiogenesis is 9,10 required to meet the increased nutritional and oxygen needs tion of endothelial cells. Angiogenesis is a complex pro- during embryogenesis and growth, but is a rare phenomenon cess and multiple other factors play a role, for example, in healthy adult tissues.2 Interleukin 8, Interleukin 19, Fibroblast Growth Factor, 11–13 Formation of new capillaries is a multi-step process. It Platelet Derived Endothelial Growth Factor, etc. The begins with the degradation of endothelial basement mem- key cells involved in the process of angiogenesis include en- 13 brane and the local extracellular matrix to make space for dothelial cells, stromal cells, and macrophages. Their acti- the growing vessels. This is followed by induction of vation, juxtacrine communication, and coordination are a migration and proliferation of endothelial cells. Finally, tightly regulated process. A number of factors required for factors that inhibit angiogenesis are produced to regulate angiogenesis are produced by endothelial cells and macro- the vascular network and allow for maturation of capilla- phages, mononuclear phagocytic cells that are ubiquitously 14,15 ries.3 Hence, there is a balance between growth promoting present. and growth inhibiting processes resulting in a tight control Available pro-angiogenic strategies for PAD include over angiogenesis. growth factor/cytokine therapy, gene therapy, and stem cell Any disturbance in the equilibrium results in either too therapy. While promising in the laboratory, growth factor much or too little capillary growth and is recognized as an based therapies have only been partially successful in clini- essential component of many serious diseases such as can- cal trials.11 Current research approaches are focused on mod- cers, cardiovascular disease, strokes, diabetic ulcers, and ulation of growth factor delivery to achieve sustained, macular degeneration.4 temporal release by employing newly engineered biomateri- als.16 Stem cell based therapies face other limitations and their clinical efficacy is not yet well proven.17 Genetic engi- a)Author to whom correspondence should be addressed. Electronic mail: neering approaches are being used to optimize the survival, 18 [email protected] homing, and growth factor production by stem cells.

1070-664X/2015/22(12)/122005/5/$30.00 22, 122005-1 VC 2015 AIP Publishing LLC

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 24.228.70.136 On: Wed, 09 Dec 2015 03:43:45 122005-2 Miller et al. Phys. Plasmas 22, 122005 (2015)

Combination therapies are also being tried. Based on limitations analysis program by circling the extent of microvessel out- of current, systemic therapies, what is needed is a non-invasive, growth at 7 days, and subtracting the area of the aortic ring. localized therapy to induce angiogenesis in the ischemic limb. All animal procedures were approved by the Institutional Dielectric barrier discharge (DBD) plasma is a promising, non- Animal Care and Use Committee of Temple University. At invasive approach for the treatment of PAD. the end of the experiment, RNA was extracted from repre- We have previously shown that DBD plasma can stimu- sentative aortic rings and analyzed by Reverse Transcription late the migration of cultured macrophages in an in vitro Polymerase Chain Reaction (qRT-PCR). The following ex- wound healing scratch assay.19 Short-pulsed non-equilibrium perimental groups were included in the study: atmospheric pressure plasmas have been shown to produce high concentrations of reactive oxygen and reactive nitrogen (a) Negative control: Aortic rings with no treatment species, charges, and ultraviolet radiation.20,21 Specifically, (b) Plasma experimental group: Aortic rings with msDBD the authors have developed a Floating Electrode-DBD (FE- plasma treatment DBD) system that allows safe application of Non-Thermal (c) Positive control: Aortic rings with VEGF treatment Plasma (NTP) to cells and living tissues without damage.22,23 (d) Synergy experimental group: Aortic rings with VEGF FE-DBD systems generate plasma in direct contact with tis- and msDBD plasma treatment sue by applying fast, high voltage pulses between a quartz dielectric-covered copper electrode and the biological target. B. Microsecond pulsed DBD treatment Active species known for their biochemical activity, gener- Treatment of the aortic rings was performed on glass ated by NTP, include H2O2, OH, N2þ,O2À,O2(1Dg), NO, 24 slides with microsecond pulsed DBD (msDBD) plasma ONOO•, and others. We hypothesize that since non-thermal plasma can stim- (Fig. 1). Plasma was produced by applying a voltage pulse of 19 approximately 20 kV between the high voltage electrode and ulate macrophage function, cells important for VEGF-A, 24,25 MMP-9, and CXCL 1 secretion, and since ROS are impor- a grounded metal plate underneath the slide. Aortic rings tant signaling molecules for angiogenesis via VEGF-A secre- were at floating potential. Treatment time and gap distance 122005-2 Miller et al. tion, DBD treatment of aorticPhys. rings Plasmas will induce22, 122005 angiogenesis. (2015) were fixed at 10 s and 2 mm, respectively, and the frequency In this manuscript, we use mouse aortic rings to determine if was internally controlled. Three frequencies, 50, 830, and Combination therapies are also being tried. Based on limitations analysisnon-thermal program DBD by circling plasma treatment the extent can of microvessel elicit angiogenesis. out- 1000Hz, were used for low, medium, and high plasma of current, systemic therapies, what is needed is a non-invasive, growthThis at assay 7 days, is anda commonly subtracting used, the area physiologically of the aortic relevant ring. treatment. The energy per pulse of a single msDBD plasma localized therapy to induce angiogenesis in the ischemic limb. Allmodel animal to proceduresidentify angiogenic were approved factors in by the the absence Institutional of ische- discharge was measured and calculated using the methods 26 Dielectric barrier discharge (DBD) plasma is a promising, non- Animalmia and Care inflammation. and Use Committee of Temple University. At stated in our previous work. This was found to be invasive approach for the treatment of PAD. the end of the experiment, RNA was extracted from repre- 0.56 mJ/pulse, and the total energy delivered to the cells We have previously shown that DBD plasma can stimu- sentativeII. MATERIALS aortic rings AND and METHODS analyzed by Reverse Transcription (dose) during treatment for the three frequencies are 0.3 mJ late the migration of cultured macrophages in an in vitro Polymerase Chain Reaction (qRT-PCR). The following ex- (50 Hz), 4.6 mJ (830 Hz), and 5.6 mJ (1000 Hz). Tissue was 19 A. Aortic ring assay wound healing scratch assay. Short-pulsed non-equilibrium perimental groups were included in the study: immediately transferred to a 24-well plate well containing atmospheric pressure plasmas have been shown to produce The aortic ring assay was performed as described by matrigel. (a) Negative control: Aortic rings with no treatment high concentrations of reactive oxygen and reactive nitrogen Jain et al.12 Aorta from C57B6 mice (8–10 weeks old) were 20,21 (b) Plasma experimental group: Aortic rings with msDBD species, charges, and ultraviolet radiation. Specifically, removed from arch to renal arteries under sterile conditions C. RT-PCR plasma treatment the authors have developed a Floating Electrode-DBD (FE- and placed in sterile phosphate buffered saline (PBS) con- (c) Positive control: Aortic rings with VEGF treatment Total RNA was isolated from control and stimulated DBD) system that allows safe application of Non-Thermal taining penicillin and streptomycin (pen-strep). Perivascular 22,23 (d) Synergy experimental group: Aortic rings with VEGF aortic rings. One microgram of total RNA was reverse tran- Plasma (NTP) to cells and living tissues without damage. 12 fat was removed and the aorta was cut into 3 mm rings. scribed by standard methods as described. MMP-9 and FE-DBD systems generate plasma in direct contact with tis- and msDBD plasma treatment Matrigel (BD 356231) was thawed overnight at 4 C and GAPDH mRNA were targeted using primer pairs from sue by applying fast, high voltage pulses between a quartz kept on ice throughout the procedure to prevent polymeriza- Integrated DNA Technologies, (Coralville, IA), SYBR green dielectric-covered copper electrode and the biological target. B.tion. Microsecond Fifty microliter pulsed of DBD matrigel treatment was added to each well of a Active species known for their biochemical activity, gener- 24 well plate. To identify the safe therapeutic dose range, Treatment of the aortic rings was performed on glass ated by NTP, include H2O2, OH, N2þ,O2À,O2(1Dg), NO, each aortic ring was plasma treated at a different energy 24 slides with microsecond pulsed DBD (msDBD) plasma ONOO•, and others. (0.3 J corresponding to 50 Hz, 4.6 J corresponding to 830 Hz, (Fig. 1). Plasma was produced by applying a voltage pulse of We hypothesize that since non-thermal plasma can stim- and 5.6 J corresponding to 1000 Hz) and then placed in 19 approximately 20 kV between the high voltage electrode and ulate macrophage function, cells important for VEGF-A, matrigel in the prepared plate. The plate was incubated for a grounded metal plate underneath the slide.24,25 Aortic rings MMP-9, and CXCL 1 secretion, and since ROS are impor- 30 min at 37 C. 100 ll of matrigel was added on top to were at floating potential. Treatment time and gap distance tant signaling molecules for angiogenesis via VEGF-A secre- cover each ring, and the plate was incubated for another 15 were fixed at 10 s and 2 mm, respectively, and the frequency tion, DBD treatment of aortic rings will induce angiogenesis. min at 37 C. Then 700 ll of MCDB media (Corning was internally controlled. Three frequencies, 50, 830, and In this manuscript, we use mouse aortic rings to determine if 15–100-CV) containing pen-strep, L-glutamine, and 10% non-thermal DBD plasma treatment can elicit angiogenesis. 1000Hz,Fetal Bovine were used Serum for (FBS) low, by medium, volume and was slowlyhigh plasma added to This assay is a commonly used, physiologically relevant treatment.each well. The Four energy hundred per pul ng/mlse of VEGF-A a single was msDBD added plasma to appro- model to identify angiogenic factors in the absence of ische- discharge was measured and calculated using the methods priate wells. Aortic rings were26 examined daily, and digital mia and inflammation. statedimages in were our previous taken at day work. 7 forThis quantitative was found analysis to of be the 0.56area mJ/pulse, of vessel and outgrowth the total by energy the SPOT delivered Advanced to the imaging cells (dose) during treatment for the three frequencies are 0.3 mJ II. MATERIALS AND METHODS program (Media Cybernetics, Sterling Heights, MI). FIG. 1. Photograph of the microsecond-pulsed dielectric barrier discharge (50Microvessel Hz), 4.6 mJ (830outgrowth Hz), and was 5.6 calculated mJ (1000 usingHz). Tissue the Image was J generated on top of a glass slide. A. Aortic ring assay immediately transferred to a 24-well plate well containing The aortic ring assay was performed as described by matrigel. 12 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: Jain et al. Aorta from C57B6 mice (8–10 weeks old) were 24.228.70.136 On: Wed, 09 Dec 2015 03:43:45 removed from arch to renal arteries under sterile conditions C. RT-PCR and placed in sterile phosphate buffered saline (PBS) con- Total RNA was isolated from control and stimulated taining penicillin and streptomycin (pen-strep). Perivascular aortic rings. One microgram of total RNA was reverse tran- fat was removed and the aorta was cut into 3 mm rings. scribed by standard methods as described.12 MMP-9 and Matrigel (BD 356231) was thawed overnight at 4 C and GAPDH mRNA were targeted using primer pairs from kept on ice throughout the procedure to prevent polymeriza- Integrated DNA Technologies, (Coralville, IA), SYBR green tion. Fifty microliter of matrigel was added to each well of a 27 24 well plate. To identify the safe therapeutic dose range, each aortic ring was plasma treated at a different energy (0.3 J corresponding to 50 Hz, 4.6 J corresponding to 830 Hz, and 5.6 J corresponding to 1000 Hz) and then placed in matrigel in the prepared plate. The plate was incubated for 30 min at 37 C. 100 ll of matrigel was added on top to cover each ring, and the plate was incubated for another 15 min at 37 C. Then 700 ll of MCDB media (Corning 15–100-CV) containing pen-strep, L-glutamine, and 10% Fetal Bovine Serum (FBS) by volume was slowly added to each well. Four hundred ng/ml VEGF-A was added to appropriate wells. Aortic rings were examined daily, and digital images were taken at day 7 for quantitative analysis of the area of vessel outgrowth by the SPOT Advanced imaging program (Media Cybernetics, Sterling Heights, MI). FIG. 1. Photograph of the microsecond-pulsed dielectric barrier discharge Microvessel outgrowth was calculated using the Image J generated on top of a glass slide.