Gas Discharge Plasmas and Their Applications

Spectrochimica Acta Part B 57(2002) 609–658

0584-8547/02/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0584-8547Ž01.00406-2

Review

Gas discharge plasmas and their applications

Annemie Bogaerts *, Erik Neyts , Renaat Gijbels , Joost van der Mullena, a a b

University of Antwerp, Department of Chemistry, Universiteitsplein 1, B-2610 Wilrijk-Antwerp, Belgiuma

Eindhoven University of Technology, Department of Applied Physics, Den Dolech 2, Postbus 513, 5600 MB Eindhoven,b

The Netherlands

Received 6 June 2001; accepted 12 December 2001

Abstract

This paper attempts to give an overview of gas discharge plasmas in a broad perspective. It is meant for plasmaspectroscopists who are familiar with analytical plasmas(glow discharges, ICPs and microwave discharges), but whoare not so well aware of other applications of these and related plasmas. In the first part, an overview will be givenof the various types of existing gas discharge plasmas, and their working principles will be briefly explained. In thesecond part, the most important applications will be outlined.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Plasma; Gas discharge; Direct current; Radio-frequency; Pulsed; Dielectric barrier discharge; Magnetron; Surfacemodification; Lamps; Lasers; Plasma display panels

1. Introduction

Plasmas are ionized gases. Hence, they consistof positive (and negative) ions and electrons, aswell as neutral species. The ionization degree canvary from 100%(fully ionized gases) to very lowvalues(e.g. 10 –10 ; partially ionized gases).y4 y6

The plasma state is often referred to as thefourth state of matter. Much of the visible matterin the universe is in the plasma state. This is truebecause stars, as well as all visible interstellarmatter, are in the plasma state. Besides the astro-plasmas, which are omnipresent in the universe,we can also distinguish two main groups of labo-ratory plasmas, i.e. the high-temperature or fusion

*Corresponding author.E-mail address: [email protected](A. Bogaerts).

plasmas, and the so-called low-temperature plas-mas or gas discharges. The latter group of plasmasis the subject of the present review paper.In general, a subdivision can be made between

plasmas which are in thermal equilibrium andthose which are not in thermal equilibrium. Ther-mal equilibrium implies that the temperature of allspecies(electrons, ions, neutral species) is thesame. This is, for example, true for stars, as wellas for fusion plasmas. High temperatures arerequired to form these equilibrium plasmas, typi-cally ranging from 4000 K(for easy-to-ionizeelements, such as cesium) to 20 000 K(for hard-to-ionize elements, like helium) w1x. Often, theterm ‘local thermal equilibrium’(LTE) is used,which implies that the temperatures of all plasmaspecies are the same in localized areas in theplasma. On the other hand, interstellar plasma

610 A. Bogaerts et al. / Spectrochimica Acta Part B 57 (2002) 609–658

matter is typically not in thermal equilibrium, alsocalled ‘non-LTE’. This means that the temperaturesof the different plasma species are not the same;more precisely, that the electrons are characterizedby much higher temperatures than the heavy par-ticles (ions, atoms, molecules).The gas discharge plasmas can also be classified

into LTE and non-LTE plasmas. This subdivisionis typically related to the pressure in the plasma.Indeed, a high gas pressure implies many collisionsin the plasma(i.e. a short collision mean free path,compared to the discharge length), leading to anefficient energy exchange between the plasmaspecies, and hence, equal temperatures. A low gaspressure, on the other hand, results in only a fewcollisions in the plasma(i.e. a long collision meanfree path compared to the discharge length), andconsequently, different temperatures of the plasmaspecies due to inefficient energy transfer. Ofcourse, there are some exceptions to this rule, e.g.dielectric barrier discharges or atmospheric pres-sure glow discharges, as will be shown later inthis paper. The reason is that, as suggested above,not only does the pressure play a role, but also thedischarge length or the distance between the elec-trodes(which is very small in the above-mentionedexceptions). In general, it is the product of both(typically denoted aspD) which classifies theplasmas into LTE and non-LTE.In recent years, the field of gas discharge plasma

applications has rapidly expanded. This is due,among other things, to the large chemical freedomoffered by the non-equilibrium aspects of theplasma. This wide variety of chemical non-equilib-rium conditions is possible, since(external con-trol) parameters can easily be modified, such as:

● the chemical input(working gas; this definesthe different species in the plasma: electrons,atoms, molecules, ions, radicals, clusters);

● the pressure(ranging from approx. 0.1 Pa toatmospheric pressure; as mentioned above, ahigher pressure typically reduces the collisionmean free path, enhances the confinement andpushes the plasma toward equilibrium);

● the electromagnetic field structure(typicallyexternally imposed, but it can also be modifiedby the plasma species; these electric andyor

magnetic fields are used to accelerate, heat,guide and compress the particles);

● the discharge configuration(e.g. with or withoutelectrodes; discharge volume: a small volumetypically means large gradients and thus depar-ture from equilibrium); andyor

● the temporal behavior(e.g. pulsing the plasma).

Because of this multi-dimensional parameterspace of the plasma conditions, there exists a largevariety of gas discharge plasmas, employed in alarge range of applications. Three types of plasmas,i.e. the glow discharge(GD), inductively coupledplasma(ICP) and the microwave-induced plasma(MIP), are commonly used in plasma spectro-chemistry, and are, therefore, familiar to mostspectrochemists. However, these plasmas, as wellas related gas discharges, are more widely used inother(mainly technological) application fields.

LTE discharges, which are characterized byrather high temperatures, are typically used forapplications where heat is required, such as forcutting, spraying, welding or, as in the analyticalICP, for the evaporation of an analyte material.Non-LTE plasmas, on the other hand, are typicallyused for applications where heat is not desirable,such as for etching or the deposition of thin layers.The heavy particle temperature is low(often notmuch higher than room temperature), but theelectrons are at much higher temperatures, becausethey are light and easily accelerated by the appliedelectromagnetic fields. The high electron temper-ature gives rise to inelastic electron collisions,which, on the one hand, sustain the plasma(e.g.electron impact ionization) and on the other hand,result in a ‘chemically-rich’ environment. Theelectrons are, therefore, considered to be the ‘pri-mary agents’ in the plasma. Most of the applica-tions, on the other hand, result from the heavyparticle kinetics(e.g. sputtering, deposition, ionimplantation). Sometimes the plasma creation zoneand the application zone coincide in the plasma,but in other cases(so-called ‘remote plasmas’)these zones are separated in space(see below).In the first part of this paper, an overview of

the various kinds of gas discharge plasmas ispresented, and their working principles areexplained. We will mainly concentrate on non-

611A. Bogaerts et al. / Spectrochimica Acta Part B 57 (2002) 609–658

Fig. 1. Schematic overview of the basic plasma processes in aglow discharge. When a potential difference is applied betweentwo electrodes, the gas(e.g. argon) will break down into elec-trons and positive ions. The latter can cause secondary electronemission at the cathode. The emitted electrons give rise to col-lisions in the plasma, e.g. excitation(which is often followedby de-excitation with emission of radiation; hence explainingthe name of the ‘glow’ discharge) and ionization(which cre-ates new electrons and ions, and therefore makes the glow dis-charge a self-sustaining plasma). Besides, the argon ions, aswell as fast argon atoms bombarding the cathode, can also giverise to sputtering, which is important for several applications(e.g. in analytical spectrochemistry, and for sputter-depositionof thin films).

LTE plasmas, although some LTE plasmas will bebriefly described, where we find it appropriate orimportant in the framework of this paper. In thesecond part of the paper, the most importantapplications of gas discharge plasmas will beoutlined. The review is especially meant for plasmaspectrochemists, who want to become more famil-iar with gas discharge plasmas in an applicationrange much broader than analytical chemistry.

2. Overview of gas discharge plasmas

2.1. Direct current (d.c.) glow discharges

When a sufficiently high potential difference isapplied between two electrodes placed in a gas,the latter will break down into positive ions andelectrons, giving rise to a gas discharge. Themechanism of the gas breakdown can be explainedas follows: a few electrons are emitted from theelectrodes due to the omnipresent cosmic radiation.Without applying a potential difference, the elec-trons emitted from the cathode are not able tosustain the discharge. However, when a potentialdifference is applied, the electrons are acceleratedby the electric field in front of the cathode andcollide with the gas atoms. The most importantcollisions are the inelastic collisions, leading toexcitation and ionization. The excitation collisions,followed by de-excitations with the emission ofradiation, are responsible for the characteristicname of the ‘glow’ discharge. The ionizationcollisions create new electrons and ions. The ionsare accelerated by the electric field toward thecathode, where they release new electrons by ion-induced secondary electron emission. The electronsgive rise to new ionization collisions, creating newions and electrons. These processes of electronemission at the cathode and ionization in theplasma make the glow discharge a self-sustainingplasma.Another important process in the glow discharge

is the phenomenon of sputtering, which occurs atsufficiently high voltages. When the ions and fastatoms from the plasma bombard the cathode, theynot only release secondary electrons, but alsoatoms of the cathode material, which is calledsputtering. This is the basis of the use of glow

discharges for analytical spectrochemistry. Indeed,the material to be analyzed is then used as thecathode of the glow discharge, which is beingsputtered by the plasma species. The sputteredatoms can become ionized andyor excited in theplasma. The ions can be detected with a massspectrometer, and the excited atoms or ions emitcharacteristic photons which can be measured withoptical emission spectrometry. Alternatively, thesputtered atoms can also diffuse through the plas-ma and they can be deposited on a substrate(oftenplaced on the anode); this technique is used inmaterials technology, e.g. for the deposition of thinfilms (see further). A schematic picture of theelementary glow discharge processes describedabove is presented in Fig. 1. When a constantpotential difference is applied between the cathodeand anode, a continuous current will flow throughthe discharge; giving rise to adirect current (d.c.)glow discharge. It should be mentioned that in ad.c. glow discharge the electrodes play an essentialrole for sustaining the plasma by secondary elec-tron emission. When a time-varying potential dif-


Fig. 2. Schematic diagram of the spatial regions present in d.c.glow discharges,(a) at short cathode–anode distance andyorlow pressure;(b) at longer interelectrode distance andyor high-er pressure(CDSscathode dark space; NGsnegative glow;FDSsFaraday dark space; PCspositive column; AZsanodezone). The cathode(left) has a negative potential, whereas theanode(right) is grounded here. The solid line(left axis) rep-resents the potential distribution, whereas the dashed line(rightaxis) denotes the electric field distribution.

ference is applied, as in a capacitively coupledradio-frequency(rf) discharge(see below), therole of the electrodes becomes less important,because the electrons can oscillate in the plasmabetween the two electrodes, by the time-varyingelectric field. Eventually, the role of the electrodesbecomes even negligible, giving rise to electrode-less discharges, as in the case of the ICP(seebelow).The potential difference applied between the

two electrodes is generally not equally distributedbetween cathode and anode, but it drops almostcompletely in the first millimeters in front of thecathode(see Fig. 2a,b). This region adjacent tothe cathode, which is hence characterized by astrong electric field, is called the ‘cathode darkspace’ (CDS) or ‘sheath’. In the largest part ofthe discharge, the so-called ‘negative glow’(NG),

the potential is nearly constant and slightly positive(which is called the ‘plasma potential’) and hence,the electric field is very small. When the distancebetween cathode and anode is relatively long(e.g.a few cm, at 100 Pa argon, 400 V and 0.87 mAw2x), two more regions can be present, i.e. the‘Faraday dark space’(FDS) and the ‘positivecolumn’ (PC) (see Fig. 2b). They are character-ized by a slightly negative electric field, to conductthe electrons toward the anode. These two regionsare often present in glow discharges used as lasers(‘positive column lasers’) and as fluorescencelamps (see further). However, for most of theother applications of d.c. glow discharges(sputter-ing, deposition, chemical etching, analytical chem-istry, etc.), the distance between cathode and anodeis generally short, so that normally(i.e. at typicaldischarge conditions) only a short anode zone(AZ) is present beside CDS and NG, where theslightly positive plasma potential returns back tozero at the anode(see Fig. 2a).A d.c. glow discharge can operate over a wide

range of discharge conditions. The pressure canvary from below 1 Pa to atmospheric pressure(seebelow). It should, however, be realized that theproduct of pressure and distance between theelectrodes(pD) is a better parameter to character-ize the discharge(as mentioned above). Forinstance, at lower pressure, the distance betweencathode and anode should be longer to create adischarge with properties comparable to those ofhigh pressure with small distance. The voltage ismostly in the range between 300 and 1500 V, butfor certain applications it can increase to severalkV. The current is generally in the mA range. Thedischarge can operate in a rare gas(most oftenargon or helium) or in a reactive gas(N , O , H ,2 2 2

CH , SiH , SiF , etc.), as well as in a mixture of4 4 4

these gases.

2.2. Capacitively coupled (cc) radio-frequency (rf)discharges

To sustain a d.c. glow discharge, the electrodeshave to be conducting. When one or both of theelectrodes are non-conductive, e.g. when the glowdischarge is used for the spectrochemical analysisof non-conducting materials or for the deposition


Fig. 3. Development of self-bias(d.c.-bias) in a cc rf discharge,in the simplified case of a rectangular pulse:(a) applied volt-age; and(b) resulting voltage over the discharge as a functionof time, and resulting d.c.-bias(dashed line).

of dielectric films, where the electrodes becomegradually covered with insulating material, theelectrodes will be charged up due to the accumu-lation of positive or negative charges, and the glowdischarge will extinguish. This problem is over-come by applying an alternating voltage betweenthe two electrodes, so that each electrode will actalternately as the cathode and anode, and thecharge accumulated during one half-cycle will beat least partially neutralized by the opposite chargeaccumulated during the next half-cycle.The frequencies generally used for these alter-

nating voltages are typically in the radiofrequency(rf) range(1 kHz–10 MHz; with a most common3

value of 13.56 MHz). Strictly speaking, cc dis-charges can also be generated by alternating volt-ages in another frequency range. Therefore, theterm ‘alternating current’(a.c.) discharges, asopposed to d.c. discharges, might be more appro-priate. On the other hand, the frequency should behigh enough so that half the period of the alter-nating voltage is less than the time during whichthe insulator would charge up. Otherwise, therewill be a series of short-lived discharges with theelectrodes successively taking opposite polarities,instead of a quasi-continuous discharge. It can becalculated that the discharge will continue whenthe applied frequency is above 100 kHzw3x. Inpractice, many rf GD processes operate at 13.56MHz, because this is a frequency allotted byinternational communications authorities at whichone can radiate a certain amount of energy withoutinterfering with communications.At this point, it is important to note that the

term ‘capacitively coupled’ refers to the way ofcoupling the input power into the discharge, i.e.by means of two electrodes and their sheathsforming a kind of capacitor. Later in this paper, itwill be shown that rf power can also be coupledin another way to the discharge.At the typical rf frequencies, the electrons and

ions have a totally different behavior, which canbe explained by their different masses. The lightelectrons can follow the instantaneous electricfields produced by the applied rf voltage. Indeed,the characteristic frequency of electrons, or the so-called electron plasma frequency, is given byw1,4x:

2n ee y Ž .v s ; f s9000 n Hz (1)pe pe eym ´e 0

wheren is expressed in cm . When the electrony3e

density varies from 10 to 10 cm , the plasma10 13 y3

frequency ranges from 9=10 to 3=10 Hz,8 10

which is much higher than the typical rf-frequencyof 13.56 MHz. The ions, on the other hand, canfollow the field in the sheath if the frequency isless than the ion plasma frequency. Because theion plasma frequency is inversely proportional withmasswEq. (1)x, light ions can see the field varia-tions at typical rf-frequencies, whereas heavy ionscan only follow time-averaged electric fields.Another important aspect of cc rf discharges,

which also results from the differences in massbetween electrons and ions, is the phenomenon ofself-bias. The self-bias or d.c.-bias is formed(i)when both electrodes differ in size, and(ii) whena coupling capacitor is present between the rfpower supply and the electrode, or when theelectrode is non-conductive(because it then actsas a capacitor). When a certain voltage(assumingfor simplicity a square wave; see Fig. 3) is appliedover the capacitor formed by the electrodes, thevoltage over the plasma will initially have the


Fig. 4. Typical sinusoidal voltage in a cc rf discharge in thecase of a large, negative d.c.-bias. The solid line represents thevoltage between both electrodes, and the dashed line shows theresulting d.c.-bias at the rf-powered electrode.

same value as the applied voltage. When theapplied voltage is initially positive, as in Fig. 3,the electrons will be accelerated toward the elec-trode. Hence, the capacitor will be rapidly chargedup by the electron current, and the voltage overthe plasma will drop. When the applied potentialchanges polarity after one half-cycle, the voltageover the plasma changes with the same amount(i.e. twice the amplitude of the applied voltage).The capacitor will now be charged up by the ioncurrent, and the voltage over the plasma will,therefore, drop as well, but this second drop is lesspronounced, because of the much lower mobility(due to the higher mass) of the ions, and hence,the lower ion flux. At the next half-cycle, theapplied potential, and hence also the voltage overthe plasma, again changes polarity. The voltageover the plasma drops again more rapidly, becausethe capacitor is again charged up by the electronflux. This process repeats itself, until the capacitoris finally sufficiently negatively charged so thatthe ion and electron fluxes, integrated over one rf-cycle, are equal to each other. This results in atime-averaged negative d.c. bias at the rf-poweredelectrode(see dashed line in Fig. 3). It should bementioned that the same phenomenon happens atthe grounded electrode, but the effect is muchsmaller. Fig. 4 shows a typical potential as afunction of time in the rf cycle, as well as thecorresponding d.c. bias, in the case of a sinusoidalwave.Because of the negative d.c. bias, the ions

continue to be accelerated toward the rf-poweredelectrode, and they can, therefore, cause sputteringof the rf-electrode material. In fact, the cc rf

discharge often resembles a d.c. glow discharge,with a similar subdivision in different regions,similar operating conditions, and with similar proc-esses occurring in the plasma. This is especiallytrue if the cc rf discharge operates in the so-called‘g-regime’, where secondary electron emission andthe ionization due to accelerated electrons in therf-sheath, are the primary sustaining mechanismsin the discharge. Thisg-regime occurs typically inanalytical cc rf discharges, which operate at ratherhigh pressure(several hundred Pa) and voltage(rfamplitude voltage;1 kV), and where the rf-powered electrode(or the sample) is generallymuch smaller than the grounded electrode(or thecell walls), which results in a large self-bias(typically only approx. 80 V lower in absolutevalue than the rf amplitude voltagew5x). The otherimportant regime in which cc rf-discharges canoperate, is the ‘wave-riding’ or ‘a-regime’. In thiscase, the primary sustaining mechanism is ioniza-tion due to electrons in the bulk plasma(so-called‘volume ionization’). The electrons may havegained energy from the oscillating rf-electric fields(i.e. the rf sheath expansion and contraction), i.e.by so-called Ohmic and stochastic heatingw6x.This regime is typical at lower pressures andvoltages. Additionally, heating of the electrons canalso be realized in the bulk plasma, when the bulkelectric field is significant(so-called bulk Ohmicheating). This happens in the case of electronega-tive gases, or in rather long and narrow dischargetubes, where radial losses due to ambipolar diffu-sion to the walls are important. The mechanismhere is then similar to that in d.c. glow dischargeswith a positive column.The transition between thea andg regime has

been the subject of many theoretical, modelingand experimental investigations(e.g.w7–12x). Thedenotationsa and g mode are adopted from theTownsend’s first ionization coefficient(a) for theavalanche of charge carriers in the volume, andthe g coefficient for the ion-induced secondaryelectron emission from a target, respectivelyw13x.For plasma processing applications, cc rf dis-

charges, also called ‘rf-diodes’, consist, in thesimplest case, of a vacuum chamber containingtwo planar electrodes separated by a distance ofseveral cm. The substrate is normally placed on


one electrode. The typical driving voltage is 100–1000 V. The pressure is in the range of 1–100 Pa,and the electron density(or so-called plasmadensity) is of the order of 10 –10 cm w1x.9 11 y3

Hence, both the pressure and plasma density aresomewhat lower than in most analytical rf dis-charges(pressure of a few hundred Pa, and plasmadensity of the order of 10 –10 cm w5x).12 13 y3

2.3. Pulsed glow discharges

Besides applying an rf voltage to a glow dis-charge, the voltage can also be applied in the formof discrete pulses, typically with lengths in theorder of milli- to microseconds. Because a pulseddischarge can operate at much higher peak voltagesand peak currents for the same average power asin a d.c. glow discharge, higher instantaneoussputtering, ionization and excitation can be expect-ed, and hence better efficiencies(e.g. better sen-sitivities for analytical spectrochemistry). This isbecause the basic plasma phenomena, such asexcitation and ionization, are highly non-linearlydependent on field strength. Whereas the earlyanalytical investigations dealt primarily with mil-lisecond pulsed glow discharges, more recent workhas focused mainly on microsecond discharges,where even higher peak voltages and currents, andhence better sensitivities, can be obtainedw14x.

Typical analytical microsecond pulsed condi-tions are a voltage of 2 kV, which is applied duringa pulse of 10ms, with a pulse repetition frequencyof 200 Hz, giving rise to peak currents and powersof approximately 1 A and 2 kW, respectivelyw14x.Hence, the typical duty cycle is very short, i.e. theratio of ‘pulse-on’-period compared to ‘pulse-off’-period is very small. This means that the averageelectrical power is rather low, so that the samplewill not excessively be heated. Moreover, theoverall rate of sputtering will be low, so that thinfilms can be analyzedw15x.For the same reason, also in the semiconductor

industry, pulsed power operation has emerged as apromising technique for reducing charge-induceddamage and etch profile distortion, which is asso-ciated with continuous discharges. Another advan-tage of pulsed d.c. technology glow dischargescompared to rf technology is the simpler method

of up-scaling due to reduced impedance matchingnetwork and electromagnetic interference prob-lems, and the lower price of power supplies forlarger reactors. Typical operating conditions forpulsed technology plasmas involve discharge pulsedurations in the order of 100ms. The peak voltageis in the order of 500 V and the pressure isapproximately 100 Pa. It should also be mentionedthat the reactors are typically much larger, in theorder of several m . Well-known applications3

include the plasma nitriding of steelw16x and thedeposition of hard coatingsw17x.

As far as basic plasma processes are concerned,a pulsed glow discharge is very similar to a d.c.glow discharge, i.e. it can be considered as a shortd.c. glow discharge, followed by a generally longerafterglow, in which the discharge burns out beforethe next pulse starts. It should be mentioned thatnon-LTE is facilitated in pulsed discharges,because there is no excessive heating so that thegas temperature is lower than the electron temper-ature. Moreover, there is also non-chemical equi-librium because ionization(fragmentation) occurson a different time-scale compared to recombina-tion (on vs. off period).

2.4. Atmospheric pressure glow discharges(APGDs)

As mentioned above, a glow discharge canoperate over a wide pressure regime. The typicalpressure range is approximately 100 Pa. Operationat higher(even atmospheric) pressure is, however,possible, but it leads easily to gas and cathodeheating and arcing. According to the similaritylaws of classical theory, it is possible to increasethe gas pressure(p) if the linear dimension of thedevice(D) is decreased, the productpD being oneof the parameters kept constant during downscal-ing w18,19x. Miniaturized discharge devices are,therefore, expected to be able to generate glowdischarges at atmospheric(or even higher) pres-sure. In work by Schoenbach et al.w20x and Starkand Schoenbachw21,22x, an atmospheric pressuremicro-discharge in hollow-cathode geometry wasdeveloped. The hollow cathode diameter was typ-ically 100–200mm. In Czerfalvi et al.w23x andMezei et al. w24x, a small atmospheric pressure


Fig. 5. Schematic representation of an atmospheric pressure glow discharge(APGD), typically used for plasma polymerization. Theelectrodes are at a distance of a few mm, and they are both covered with a dielectric barrier. A high voltage(20 kV) is appliedbetween both electrodes, with a repetition frequency of 1–30 kHz. A flow of monomers in a carrier gas(e.g. helium) is brought inthe discharge. Due to chemical reactions in the plasma, a polymer film can be grown at the electrodes.

discharge was operated in open air with an elec-trolyte solution as the cathode, to be used foranalysis of water and waste water solutions. InEijkel et al. w25,26x, the use of an atmosphericpressure helium d.c. glow discharge on a micro-chip, as a molecular optical emission detector forgas chromatography(e.g. for methane, hexane,etc.), was reported. The typical dimensions were1–2 mm length, and a few hundredmm in widthand depth, leading to a typical plasma chambervolume of 50–180 nlw25,26x. Also, capacitivelycoupled rf discharges at atmospheric pressure havebeen used for quite a while by Blades et al.w27xand by Sturgeon and co-workersw28x for analyticalapplications, and they have also recently beenreported by a group in Romaniaw29x.Besides reducing the characteristic length of the

discharge chamber, stable atmospheric pressureglow discharges(APGDs) used for technologicalapplications have also been operated when otherconditions are fulfilled with respect to the structureof the electrodes, the carrier gas and the frequencyof the applied voltagew30,31x. Typically, inAPGDs, at least one of the electrodes is coveredwith a dielectric, and the discharge operates atalternating voltages. Furthermore, the kind of dis-charge gas determines the stability of the glowdischarge. For example, helium gives rise to astable homogeneous glow discharge, whereas

nitrogen, oxygen and argon easily cause the tran-sition into a filamentary glow discharge(seebelow). However, by changing the electrode con-figuration, it is still possible to let them operate ina homogenous glow discharge regimew30,31x.

Fig. 5 shows a schematic picture of a typicalAPGD used for plasma polymerization(see below)w32x. The glow discharge is generated betweentwo parallel electrodes, which are covered by adielectric layer(e.g. alumina). A gas flow, con-sisting in this case of plasma polymerization ofspecific monomers and helium as the carrier gas,is led through the discharge. An alternating voltageof 20 kV is applied with a frequency between 1and 30 kHz. The distance between the electrodesis typically a few millimetersw33x.

The main advantage of APGDs is the absenceof vacuum conditions, which greatly reduces thecost and complexity of the glow discharge opera-tion. Moreover, materials with a high vapor pres-sure, such as rubber, textiles and biomaterialsw34,35x can more easily be treated.Typical reported applications(see also in Sec-

tion 3) include the surface modification of mate-rials (e.g. enhancing the wettability of polymersused for paints and glues) w36x, the sterilization ofsurfaces(e.g. sterilization of micro-organisms onsurfaces in the healthcare industry) w37,38x, plasma


Fig. 6. Schematic representation of a dielectric barrier dis-charge(DBD) in 2 different configurations.(a) The volumedischarge(VD) consists of two parallel plates; the microdis-charge takes place in thin channels, which are generally ran-domly distributed over the electrode surface.(b) The surfacedischarge(SD) consists of a number of surface electrodes ona dielectric layer, and a counter-electrode on its reverse side.

polymerization w39,40x, the production of ozonew41x, etc.

2.5. Dielectric barrier discharges (DBDs)

Strongly related to the APGDs are the dielectricbarrier discharges(DBDs), historically also called‘silent discharges’w42–46x. They also operate atapproximately atmospheric pressure(typically0.1–1 atmw13x). An a.c. voltage with an amplitudeof 1–100 kV and a frequency of a few Hz to MHzis applied to the discharge, and a dielectric layer(made of glass, quartz, ceramic material or poly-mers) is again placed between the electrodes. Theinter-electrode distance varies from 0.1 mm(inplasma displays), over approximately 1 mm(inozone generators) to several cm(in CO lasers)2

w42x.The basic difference with APGDs is that the

latter are generally homogeneous across the elec-trodes and are characterized by only one currentpulse per half cycle, whereas the DBDs typicallyconsist of microdischarge filaments of nanosecondduration(hence, with many current pulses per halfcycle) w42x. In fact, this subdivision is ratherartificial, because the same electrode configurationcan give rise to an APGD or a DBD, dependingon the discharge conditions and the discharge gas(see also above). There are even indications thatthe homogeneous APGD is not really homogene-ous, but simply has a homogeneous or diffuselight emission. Therefore, an APGD can also beconsidered as a diffuse DBD.Two basic configurations of DBDs can be dis-

tinguishedw43x. The volume discharge (VD) con-sists of two parallel plates(see Fig. 6a). Themicrodischarge takes place in thin channels whichcross the discharge gap and are generally randomlydistributed over the electrode surface. The numberof microdischarges per period is proportional tothe amplitude of the voltage. Thesurface discharge(SD) consists of a number of surface electrodeson a dielectric layer and a counter electrode on itsreverse side(see Fig. 6b). There is no clearlydefined discharge gap. The so-called micro-dis-charges are, in this case, rather individual dischargesteps that take place in a thin layer on the dielectricsurface and can be considered homogeneous over

a certain distance. An increase in the voltage nowleads to an enlargement of the discharge area onthe dielectric. There also exist combinations ofthese two basic configurations, e.g. a co-planararrangement, such as used in plasma display panels(see Section 3), or a packed bed reactor, such asthose used in some plasma chemical reactorsw43x.

The nanosecond duration of DBDs is caused bya charge build-up at the dielectric surface, withina few ns after breakdown. Indeed, this reduces theelectric field at the location of the microdischargeto such an extent that the charge current at thisposition is interrupted. Because of the short dura-tion and the limited charge transport and energydissipation, this normally results in little gas heat-ing. Hence, in a microdischarge, a large fractionof the electron energy can be utilized for excitingatoms or molecules in the background gas, thusinitiating chemical reactions andyor emission ofradiation. This explains the great interest in DBDsfor many applications.In 1857, Siemensw47x already used this type of

discharge for the generation of ozone from air oroxygen. Today, these silent discharge ozonizers areeffective tools and a large number of ozone instal-lations are being used worldwide for water treat-


ment w48,49x. Other applications are the pumpingof CO lasers, the generation of excimer radiation2

in the UV and VUV spectral regions, the produc-tion of methanol from methaneyoxygen, variousthin-film deposition processesw40x, the remedia-tion of exhaust gases and for plasma display panelsw13,42x (see also Section 3). Recently, the use ofa DBD in analytical spectrometry has been report-ed w50x, i.e. as a microchip plasma for diode laseratomic absorption spectrometry of excited chlorineand fluorine in noble gases and in airynoble gasmixtures. It has been demonstrated that the DBDhas excellent dissociation capability for molecularspecies, such as CCl F , CClF and CHClFw50x.2 2 3 2

2.6. Corona discharges

Beside a glow discharge with two electrodes,there also exists another type of pulsed d.c. dis-charge, with the cathode in the form of a wirew51,52x. A high negative voltage(in the case of anegative corona discharge) is applied to the wire-cathode, and the discharge operates at atmosphericpressure. The name ‘corona discharge’ arises fromthe fact that the discharge appears as a lightingcrown around the wire.The mechanism of the negative corona discharge

is similar to that of a d.c. glow discharge. Thepositive ions are accelerated towards the wire, andcause secondary electron emission. The electronsare accelerated into the plasma. This is called astreamerw53x, i.e. a moving front of high-energyelectrons(with average energy approx. 10 eV),followed by a tail of lower energy electrons(orderof 1 eV). The high-energy electrons give rise toinelastic collisions with the heavy particles, e.g.ionization, excitation, dissociation. Hence, radicalscan be formed, which can crack larger moleculesin collisions. Therefore, there is a clear distinctionbetween the electron kinetics(to create the plas-ma) and the heavy particle kinetics of interest forthe applications(e.g. flue gas cleaning). The sep-aration between both is not accomplished here inspace(see Section 1), but in time. The coronadischarge is also strongly in non-equilibrium, bothwith respect to the temperatures and the chemistry.The main reason is the short time-scale of thepulses. If the source was not pulsed, there would

be a build-up of heat, giving rise to thermalemission and to a transition into an arc dischargeclose to equilibrium.It should be mentioned that beside the negative

corona discharge, there also exists a positive coro-na discharge, where the wire has a positive voltage,hence acting as anodew54x.

Applications of the corona discharge includeflue gas cleaning, the destruction of volatile com-pounds that escape from paints, water purification,etc. Dust particles are removed from the gas orliquid by the attachment of electrons from thedischarge to the dust particles. The latter becomesnegatively charged and will be drawn toward thewalls.

2.7. Magnetron discharges

In addition to applying a d.c. or rf potentialdifference(or electric field) to a glow discharge,a magnetic field can also be applied. The mostwell-known type of discharge characterized bycrossed magnetic and electric fields is the magne-tron dischargew1,4x. Three different types of mag-netron configurations can be distinguished, i.e.cylindrical, circular and planar magnetronsw3x.In a so-called ‘balanced planar magnetron’, an

axisymmetric magnetic field is applied with apermanent magnet behind the cathode, in such away that the magnetic field lines start and returnat the magnet(see Fig. 7). Hence, a kind of‘magnetic ring’ is formed at the cathode surface,with average radiusR and widthw, which ‘traps’the electrons that are accelerated away from thecathode by the electric field. The electrons willmove in helices around the magnetic field lines,and they will travel a much longer path-length inthe plasma than in conventional glow discharges,giving rise to more ionization collisions, and con-sequently, higher ion fluxes. Because the ions, dueto their higher mass, are much less influenced bythe magnetic field lines, they bombard the cathode,where they cause more secondary electron emis-sion because of their higher flux compared toconventional glow discharges. Hence, due to theenhanced ionization and secondary electron emis-sion, the magnetron generally operates at highercurrents (typically 1 A w55x), lower voltages


Fig. 7. Schematic representation of a planar magnetron dis-charge, indicating the magnetic field lines and the trapping ofelectrons in a magnetic ring with average radiusR and widthW. The magnet is placed behind the cathode. The cathode isused as sputtering target, whereas the substrate(for film dep-osition) is typically positioned at the anode.

(;500 V) and pressures(typically approx. 1 Pa)w56x, compared to a conventional glow discharge.The magnetic field strength lies in the range of0.01–0.1 Teslaw57x.

The higher ion fluxes bombarding the cathodealso give rise to more sputtering. Moreover, thesputtered atoms are less subject to scattering col-lisions in the plasma, due to the reduced pressure,and they can be more directed to the substrate fordeposition (located at the grounded electrode).The sputtering of cathode material, and the sub-sequent deposition of thin films on a substrate, isthe most important application of magnetrondischarges.Balanced magnetrons are particularly useful in

cases where low charged particle fluxes are desir-able, such as for the deposition of plastic coatings.There is, however, another class of industrial appli-cations where there is a need for combined sputtersources, i.e. for the deposition of thin films withion beam assistancew56x. It is generally recognizedthat ion bombardment during the growth of thinfilms produces changes in the nucleation charac-teristics, in morphology, composition, crystallinity,and in the film stress. The changes are usuallyattributed to resputtering of the condensing species,leading to a modified microstructure of the films.Some examples where ion-beam assistance hasproved to be valuable include the deposition of

thin films for hard protective coatings, for opticalfibers and for hard and wear resistant metallurgicalcoatingsw56x. Such ion-beam assisted depositioncan, in principle, be realized with a balancedmagnetron and an additional ion source, but thisleads to complicated experimental set-ups, espe-cially because additional ion sources normallyoperate at lower pressures than the magnetrons.This problem can be overcome in so-called ‘unbal-anced magnetron sources’, where the magneticfield lines are not closed at the cathode surface(as in Fig. 7), but lead the electrons, and byambipolar diffusion, ions toward the substrate,where the latter can influence the film properties.Hence, by altering the magnetic field configura-tion, thin film growth can be achieved with eitherlow ion and electron bombardment, or bombard-ment by ions and electrons at a flux greater than,or equal to, that of the depositing neutral species.A characteristic feature of conventional planar

magnetrons is the presence of the race-track at thesputtering target, as a consequence of the chargedparticle confinement in the magnetic field. Thisrace-track generally limits the complete target util-ization, resulting in higher working costs. Thisproblem can be overcome by a so-called ‘rotatingcylindrical magnetron’ w58,59x. In this concept, acylindrical target tube is supported and poweredby two end blocks: one delivers target cooling andelectrical power, while the other sustains the rota-tion. Within the rotating tube, a stationary magnetconfiguration is positioned, pointing in the direc-tion of the substrate. Although a stationary plasmatrack is present during the sputtering process, norace-track groove, corresponding to the magnetconfiguration, is formed in the rotating target, andvery high material utilization may be achieved.A variation also exists to the rotating cylindrical

magnetron, in which a planar magnetron is mount-ed on the end block bases of the rotating magne-trons, allowing an easy and fast exchange betweenplanar and cylindrical magnetronsw59x. This hassome advantages, because some ceramic materialsmay be produced much more easily in planar formand in particular applications, sputtering from aplanar target may be beneficial for certain coatingpropertiesw60x. Compared to flat cathodes, wherethe target consumption is typically approximately


30%, the consumption for rotating cathodesamounts up to 90%, which gives an importantdecrease of cost-of-ownership of a sputter-coatingline w60x.Finally, at this point it is interesting to note that,

besides the above described d.c. magnetron dis-charges, there exist some other technologicallyinteresting discharges where crossed magnetic andelectric fields are applied to enhance the plasmadensities. Some examples include the electroncyclotron resonance plasma(ECR; see below;Section 2.8.1), and the magnetically enhancedreactive ion etcher(MERIE, also called rf mag-netron). The latter has been developed to improveperformance of the rf reactor. A d.c. magnetic fieldof 0.005–0.03 Tesla is applied parallel to thepowered electrode, on which the wafer is placed.The magnetic field increases the efficiency ofpower transfer from the source to the plasma andalso enhances plasma confinement. This results ina reduced sheath voltage and an increased plasmadensity. However, MERIE systems do not havegood uniformity, which may limit their applicabil-ity to next-generation, submicrometer device fab-rication w1x.

2.8. Low-pressure, high-density plasmas

In recent years, a number of low-pressure, high-density plasma discharges have been developed,mainly as alternatives to capacitive rf discharges(rf diodes; see above) and their magneticallyenhanced variants, for etching and depositionapplicationsw1,61,62x. Indeed, one of the disad-vantages of rf-diodes is that voltage and currentcannot be independently controlled, and hence, theion-bombardment flux and bombarding energycannot be varied independently of each other,except when applying different frequencies, whichis not always very practical. Hence, for a reason-able ion flux, sheath voltages at the driven elec-trode must be high. For wafers placed on thedriven electrode, this can result in undesirabledamage, due to the high bombarding energies.Furthermore, the combination of low ion flux andhigh ion energy leads to a relatively narrow processwindow for many applicationsw1x. The low proc-ess rates resulting from the limited ion flux in rf

diodes often mandates multi-wafer or batch proc-essing, with the consequent loss of wafer-to-waferreproducibility. To overcome these problems, themean ion bombarding energy should be controlla-ble independently of the ion and neutral fluxes.Some control over ion-bombarding energy can beachieved by placing the wafer on the undrivenelectrode, and independently biasing this electrodewith a second rf source. Although these so-calledrf-triode systems are in use, processing rates arestill low at low pressures and sputtering contami-nation is an issuew1x. Various magneticallyenhanced rf diodes and triodes have also beendeveloped to increase the plasma density, andhence the ion fluxes, of which MERIE is the mostwell-known. However, as mentioned above, thelatter may not have good uniformity, limiting theirsuitability for plasma processing applicationsw1x.The new generation of low-pressure, high-den-

sity plasma sources are characterized, as predictedby the name, by low pressure(typically 0.1–10Pa) and by higher plasma densities(typically10 –10 cm at these low pressure conditions11 13 y3

w1x), and consequently by higher ion fluxes thancc rf discharges of similar pressures. In addition,a common feature is that the rf or microwavepower is coupled to the plasma across a dielectricwindow, rather than by direct connection to anelectrode in the plasma, as for an rf diode. This isthe key to achieving low voltages across all plasmasheaths at electrode and wall surface. D.c. voltages,and hence also ion acceleration energies, are typi-cally only 20–30 V at all surfaces. To control theion energy, the electrode on which the wafer isplaced can be independently driven by a capaci-tively coupled rf source. Hence, independent con-trol of the ionyradical fluxes(through the sourcepower) and the ion-bombarding energy(throughthe wafer electrode power) is possiblew62x.Although the need for low pressures, high fluxes

and controllable ion energies has motivated high-density source development in recent years, thereare still many issues that need to be resolved. Acritical issue is achieving the required processuniformity over large wafer surfaces(e.g. 20–30cm diameter). In contrast to the nearly one-dimen-sional geometry of typical rf diodes(i.e. twoclosely spaced parallel electrodes), high-density


Fig. 8. Schematic representation of an electron cyclotron res-onance(ECR) discharge source, illustrating the spatial sepa-ration into resonance region(electron kinetics) and application(process) region, typical for a remote plasma.

Fig. 10. Schematic representation of a helical resonator source,illustrating the spatial separation into resonance region(elec-tron kinetics) and application(process) region, typical for aremote plasma.

Fig. 9. Schematic representation of a helicon discharge source,illustrating the spatial separation into resonance region(elec-tron kinetics) and application(process) region, typical for aremote plasma.

Fig. 11. Schematic representation of an inductively coupledplasma(ICP) source, for plasma processing applications,(a)in cylindrical geometry, with a helical coil wound around thedischarge, and(b) in planar geometry, with a flat helix or spiralwound from near the axis to near the outer radius of the dis-charge chamber.

cylindrical sources are essentially two-dimensional(e.g. length equal or larger than diameter). Hence,plasma formation and transport are inherently radi-ally non-uniform. Another critical issue is efficientpower transfer(coupling) across the dielectricwindows over a wide range of plasma operatingconditions. Degradation of and deposition on thewindow can also lead to irreproducible sourcebehavior and the need for frequent, costly cleaningcycles. Finally, the low-pressure operation leads tosevere pumping requirements for high deposition

or etching rates, and hence the need for large,expensive vacuum pumpsw1x.

Figs. 8–11 present the four most important high-density plasma sourcesw1x. The common featuresof power transfer across dielectric windows andseparate bias supply at the wafer electrode are


clearly illustrated. Moreover, it shows the commonidea of separate plasma creation chamber andprocess chamber, where the substrate is located(so-called ‘remote plasma sources’). However, thesources differ significantly in the means by whichpower is coupled to the plasma. In principle, someof these discharges could also be classified in othersections, e.g. the ECR discharge(see below) wasalready mentioned in Section 2.7 because it usesa magnetic field, and it could also be treated inSection 2.9, as microwave-induced plasma). How-ever, we have chosen the present classification,because of their common growing importance inplasma processing applications.

2.8.1. Electron cyclotron resonance sources(ECRs)Electron cyclotron resonance(ECR) plasmas

(see Fig. 8) are generated from the interactionbetween an electric field at microwave frequencyand a superimposed magnetic field, in such a waythat the electrons are in resonance with the micro-wave field w63–65x. As follows from Fig. 8, theECR reactor consists essentially of two parts: aresonance region and a process region(with thesurface to be treated). The plasma flows along themagnetic field lines from the resonance region intothe process region, where energetic ions and freeradicals from the plasma can bombard the surface.When a magnetic fieldB is applied, the charged

particles will be characterized by a gyration aroundthe magnetic field lines, with a frequencyv sceBym. This frequency is called the angular cyclo-tron frequency, and is independent of the velocityof the particle. Therefore, the cyclotron frequencyof the electrons is a characteristic parameter of thesystem, e.g. at a magnetic field of 875 Gauss, theelectron cyclotron frequency, and hence the micro-wave frequency needed for resonance is 2.45 GHz.The d.c. magnetic field in the ECR is generated

by one or more electromagnetic windings andvaries in the axial direction. Moreover, a linearlypolarized microwave field is axially appliedthrough a dielectric wall into the plasma. Thisfield can be decomposed in two circularly polar-ized waves, rotating in opposite directions: a right-handed circularly polarized wave(RHP) and aleft-handed circularly polarized wave(LHP). The

electric field vector of the RHP wave rotates inright-handed way around the magnetic field witha frequencyv, while the electrons also gyrate inright-handed way around a magnetic field line, butwith a frequencyv . This means that when thec

magnetic field is such thatv sv, the forceyeEc

accelerates the electrons on their circular path,resulting in a continuous transverse energy gain,as if the electrons ‘feel’ a d.c. electric field. Hence,resonance occurs between the microwaves and theelectrons, and the power absorption is at its max-imum. The LHP field, on the other hand, producesan oscillating force, whose time-averaged value isequal to zero, and does not give rise to an energygain. However, the energy gain from the RHPfield is sufficient for the electrons to cause ioni-zation collisions. Because the heating of the elec-trons occurs essentially by absorption of the RHPwave energy, this type of discharge is also some-times called ‘wave-heated discharge’w1x.

2.8.2. Helicon sourcesHelicon discharges are also a form of wave-

heated dischargew1x. A dielectric cylinder thatforms the source chamber is surrounded by amagnet coil(see Fig. 9). An rf-powered antennalaunches rf waves that propagate along the plasmatube. The plasma electrons are heated by absorbingthe energy from the wave. Hence, the energytransfer occurs by wave–particle interactions. Sim-ilar to ECRs, the plasma flows again from thesource chamber to the process chamber, where thesubstrate is located(see Fig. 9). In comparisonwith ECRs, a much smaller magnetic field(typi-cally 20–200 G for processing discharges) isrequired for wave propagation and absorption, andan rf source (e.g. 13.56 MHz), instead of amicrowave source, is used, which makes the heli-con much cheaper. However, the resonant couplingof the helicon mode at the antenna can lead to anon-uniform variation of the plasma density withvarying source parametersw1x.

2.8.3. Helical resonator sourcesIn the helical resonator source, shown in Fig.

10, the dielectric discharge chamber is surroundedby an external helix and a grounded conductingcylinder, which form a resonant structure in the


MHz range w1x. This can be used for efficientplasma generation at low pressures. Helical reso-nator plasmas operate conveniently at radio fre-quencies(3–30 MHz) with simple hardware, andthey do not require a d.c. magnetic field, as doECRs and helicons(see above) w1x.

2.8.4. Inductively coupled plasmas (ICPs)In the inductively coupled source, as shown in

Fig. 11, the plasma chamber is mostly also sur-rounded by a coil. Simply speaking, the rf currentsin the coil (inductive element) generate an rfmagnetic flux, which penetrates the plasma region.Following Faraday’s law:

≠B=xEsy (2)

≠t

the time-varying magnetic flux density induces asolenoidal rf electric field, which accelerates thefree electrons and sustains the dischargew66x.Inductive sources have the same advantages as

the helical resonator sources over the high-densitywave-heated sources(i.e. ECRs and helicons; seeabove), including the simplicity of concept, norequirement for d.c. magnetic fields, and rf ratherthan microwave source power. In contrast to ECRsand helicons, which can be configured to achieveplasma densitiesn G10 cm , inductive dis-13 y3

0

charges may have natural density limits,n F10130

cm , for efficient power transfer to the plasma.y3

However, the density regime 10Fn F1011 120

cm for efficient inductive discharge operation,y3

is still typically 10 times higher than for capacitiverf discharges in the same pressure range(;1 Pa)w1x.Basically, two different coil configurations can

be distinguished in inductive discharges for proc-essing applications, i.e. cylindrical and planar(seeFig. 11a,b). In the first configuration, a coil iswound around the discharge chamber, as a helix.In the second configuration, which is more com-monly used for materials processing, a flat helixor spiral is wound from near the axis to near theouter radius of the discharge chamber, separatedfrom the discharge region by a dielectric. Advan-tages of the latter are reduced plasma loss andbetter ion generation efficiency; a disadvantage isthe higher sputter-contamination, UV-damage and

heating of neutrals at the substrate. Multipolepermanent magnets can be used around the processchamber circumference, as shown in Fig. 11b, toincrease radial plasma uniformity. The planar coilcan also be moved close to the wafer surface,resulting in a near-planar source geometry, havinggood uniformity properties, even in the absence ofmultipole confinementw1x.

It should be mentioned that the coupling in ICPsis generally not purely inductive, but has a capac-itive component as well, through the wall of thereactor. Indeed, when an inductive coupling isused, deposition on the walls is often observed tofollow a pattern matching the shape of the coil.This is an indication of localized stronger electricfields on the walls, showing that the coupling isat least partly capacitive through the walls of thereactorw4x. In fact, two distinct power modes existin inductive discharges for processing applications:a dominant capacitively coupled discharge(so-called E-mode) at low power, and a dominantinductively coupled discharge(so-called H-mode)at high power. The latter mode is characterized bya much higher luminosity and densityw66x. Gen-erally, the capacitive coupling can be reduced usingshielding structures.At this point, it is worthwhile to mention that

inductively coupled plasmas are not only used asmaterials processing discharges, but they are alsoapplied in other fields, albeit in totally differentoperating regimes. Indeed, ICPs are the mostpopular plasma sources in plasma spectrochemis-try. A typical analytical ICP is illustrated in Fig.12 w67x. They operate typically at atmosphericpressure, although low power(12–15 W) reducedpressure(order of 100 Pa) ICPs have also beenreported for mass spectrometric detectionw68x.Although the principle of inductive coupling is thesame, the atmospheric pressure ICPs are clearlydifferent from the above-described low-pressureinductive discharges used for processing applica-tions. Indeed, at low pressure, the electrons willnot undergo many collisions with the gas atoms,and they will, therefore, be characterized by ratherhigh energy, whereas the gas temperature remainslow. Hence, the low-pressure ICPs are far fromLTE. In the analytical, atmospheric pressure ICP,on the other hand, the electrons undergo many


Fig. 12. Schematic representation of an analytical ICP torch.

collisions with the gas atoms, and they are char-acterized by only moderate energy(typical elec-tron temperature of 8000–10 000 K), whereas thegas temperature is almost as high(typically 4500–8000 K) w67x. Hence, the analytical ICP is nearLTE w67x. Because most spectrochemists are famil-iar with this type of plasma, we will not go intomore detail about analytical ICPs.Finally, ICPs at intermediate pressure(typically

a few tens of Pa) are also used for lightingapplications, in the so-called electrodeless dis-charge lamps(see below, Section 3) w69,70x. Incontrast to the other types of ICPs described above,here the induction coil is typically placed insidethe plasma(i.e. at the center axis), although ICPlamps with external coils do exist as well(seeSection 3).

2.9. Microwave induced plasmas

All plasmas that are created by the injection ofmicrowave power, i.e. electromagnetic radiation inthe frequency range of 300 MHz to 10 GHz, canin principle be called ‘microwave induced plas-mas’ (MIPs) w71,72x. This is, however, a generalterm which comprises several different plasmatypes, e.g. cavity induced plasmas, free expandingatmospheric plasma torches, ECRs, surface wavedischarges(SWD), etc. These different plasmatypes operate over a wide range of conditions, i.e.

a pressure ranging from less than 0.1 Pa to a fewatmospheres, a power between a few W andseveral hundreds of kW, sustained in both noblegases and molecular gasesw71x. Some differenttypes of plasmas generated by microwave powerwill be described below in more detail, except forthe ECR discharge, which has already beendescribed above.

2.9.1. Resonant cavity plasmasResonator cavities are the oldest systems to

generate microwave plasmas. A standing wave isgenerated and energy is coupled into the discharge.They can usually be operated both at atmosphericand reduced pressure. The most well-known reso-nant cavity is the Beenakker cavityw73,74x. Theresonance frequency in a cylindrical resonatordepends, among others, on the cavity radius. Thismeans that a specific cavity is only resonant in alimited frequency range. This results, on one hand,in very efficient coupling of the power, but on theother hand, also in severe distortions of the powerabsorption with small frequency changes.

2.9.2. Free expanding atmospheric plasma torchesThe free expanding atmospheric plasma torches

operate in the open air, hence at atmosphericpressure, and they can be considered as alternativesto the atmospheric ICPs in analytical chemistry.Compared to the ICP, they benefit from a compactset-up and low gas consumption. Two examplesare the TIA (Torche a Injection Axiale) w75–77x`and the MPT(Microwave Plasma Torch) w78,79x.The TIA creates a freely expanding plasma con-sisting of a thin, almost needle-like convergingfilamentary cone with a tail flame on top of it.The MPT creates a plasma with a similar structure,although the plasma cone is usually hollow with alarger diameter and a shorter length. The plasmacones of TIA and MPT are, in any case, smallerthan the voluminous plasma of the analytical ICP.The TIA is, as the name suggests, a torch with

axial gas injection, which is driven with typically1–2 kW microwave power. The gas temperatureis typically approximately 3000 K, whereas theelectron temperature is in the order of 20 000 Kw80x. The MPT is operated at a microwave powerof the order of 100 W. Electron number densities


and temperatures are in the range of 10 –1014 15

cm and 16 000–18 000 K, respectivelyw79x.y3

Both TIA and MPT plasmas are far from LTE.Whereas the electron energy distribution functionin the TIA is more or less Maxwellian, this is notthe case in the MPT. The excitation and ionizationpower of the TIA appears to be stronger than thatof the MPT w76x.

2.9.3. Capacitive microwave plasmas (CMPs)In the capacitive microwave plasma(CMP)

w81–84x the microwaves generated by a magnetronare not coupled with an external resonant cavityor an antenna, but they are conducted through aco-axial waveguide to the tip of a central singleelectrode, where the plasma is formedw81x. It is abrush-formed microwave discharge at the tip of ametal electrode. It also operates at atmosphericpressure, but at much lower gas consumption(typically 2–3 lymin) than the ICP. The inputpower is typically 600 Ww82x. The advantage ofthe CMP compared to other MIPs is that CMPsare stable over a wide range of power levels, theyare more tolerant to the introduction of molecularspecies, and can be sustained with a wide varietyof gasesw81x.

2.9.4. Microstrip plasmas (MSPs)Following trends in analytical chemistry to min-

iaturize parts of or entire chemical analysis sys-tems, a miniaturized MIP using microstriptechnology has been developed by Broekaert andco-workersw85,86x. This microstrip plasma source(MSP) consists of a single 1.5-mm sapphire waferwith a straight grown-in gas channel of 0.9 mm indiameter and 30 mm length. The electrodelessplasma is operated at atmospheric pressure, with amicrowave input power of 5–30 W. It has thepotential to become very useful in analytical chem-istry for the emission detection of non-metals likehalogens and chalcogensw85x.

2.9.5. Surface wave dischargesFinally, surface wave discharges(SWDs) are

also classified here, because they are usually sus-tained by microwave power, with typical frequencybetween 1 and 10 GHz, especially when high-density plasmas have to be created. Nevertheless,

it should be mentioned that rf-powering is alsopossiblew87x. SWDs are sustained by(running orstanding) waves that are conducted parallel to thesurface of the dielectric walls surrounding theplasmaw1x. The most common SWD configurationis the surfatron. Other types include the guide-surfatron, the surfacan, the surfaguide and the rho-box w71x.Similar to the resonant cavity plasmas, SWDs

can be created both at reduced and at atmosphericpressure. In the high-pressure regime(typicallyabove 0.1 atm), the SWD plasmas also approacha state of LTEw88x. In contrast to the resonantcavities, where the radius of the cavity is a criticaldimension, the surfatron is less dependent ongeometrical dimensions. Indeed, the length of thesurfatron is the result of the electromagnetic energyabsorption by the plasma, and it can vary depend-ing on frequency, pressure and kind of gas.The wave is absorbed by electrons near the

interface, where the electric field component ofthe wave is strong. Hence, the electrons are beingheated here, and transfer the energy into the plasmabulk by diffusion. Because the power transfer isin the axial direction and the electrons absorb moreenergy near the position where the wave islaunched, the wave amplitude, and hence thepower transfer, decay as a function of axial direc-tion; hence, the plasma density is non-uniform inthis direction. One of the possibilities to enhancethe uniformity is to make use of standing insteadof running waves. Standing waves can be obtainedby placing a reflecting plate at each end of thecylinder and generating the wave from the middleof the tube, travelling in both directions.There are generally two types of SWDs, with

cylindrical and flat configurations, but the operat-ing mechanism is the same. Cylindrical configu-rations, typically with diameters of 3–10 cm, aremostly usedw89x, but planar configurations aredeveloped to treat larger surfaces(in plasma dep-osition or etching) w90x.Applications of SWDs include, among others,

particle sources(e.g. the creation of ion beams)w91x, light sourcesw92x, lasersw93x, detoxificationof gasesw94x, deposition of thin filmsw95x, etchingw96x, as well as in analytical chemistryw97x. It hasbeen demonstratedw97x that the surfatron possesses


Fig. 13. Schematic representation of the cascaded arc, and theexpanding plasma(not to scale), typically used for depositionpurposes. The cascaded arc consists of an anode plate, a stackof electrically isolated copper plates(i.e. the cascade plates)and three cathode pins positioned in a cathode housing, placedat an angle of 458 with respect to the cascade plates. The arcplasma expands supersonically to a region of low pressure.

a number of advantages over other kinds of micro-wave-induced plasma devices for elemental anal-ysis, such as flexibility to optimize the dischargeparameters for the best analytical performance, andthe possibility of adjusting the length of the plasmacolumn, and hence the residence time of samples.

2.10. The expanding plasma jet

The expanding plasma jet is based on a pressuregradient, inducing bulk transport of the plasmaspecies by supersonic expansion. The major advan-tage is that the plasma creation region and theapplication region(for materials processing) areseparated, giving rise to remote plasma treatment.An example is the expanding plasma jet generatedfrom a thermal, high-density cascaded arc. Thecascaded arc is essentially different from most ofthe plasma sources described above, because it isa thermal(LTE) plasma, characterized by equalgas and electron temperatures of approximately10 K (1 eV). However, because of its great4

importance for deposition purposes(see below),the operating principles of this plasma are alsodescribed here.The cascaded arc is a so-called wall-stabilized,

high-intensity arcw88x. It is a thermal(pulsed)d.c. plasma, operating over a wide range of pres-sures (0.1–100 atm) and currents(5–2000 A).The electron densities are very high(10 –1015 18

cm ) w98x.y3

A typical example of a cascaded arc is illustratedin Fig. 13 (upper part). It consists of an anodeplate, a stack of electrically isolated copper plates(i.e. the cascade plates) and three cathode pinspositioned in a cathode housing, which is placedat an angle of 458 with respect to the cascadeplatesw98x.

The so-called ‘wall-stabilization’ of the arc isachieved because the walls of the cylinder keepthe arc centered along the axis. Indeed, a shift inthe position of the arc toward the walls of the tubewill be compensated by higher heat transfer to thewalls, which reduces the temperature, and there-fore, the electrical conductivity at this location.Therefore, the arc will be forced to return back toits equilibrium positionw88x.

The discharge gas is usually argon, or a reactivegas for deposition purposes. The arc plasmaexpands to a region of much lower pressure(typ-ically a few hundred Pa). This background pressuredetermines the radial expansion of the plasmacolumn. The important characteristics of the plas-ma are a strong supersonic expansion, in whichthe neutral particle and ion densities drop threeorders of magnitude, followed by a stationaryshock front. After the shock front, the plasmaexpands further subsonicallyw99x. This ‘down-stream’ plasma is not in LTE. It is a recombiningplasma with gas and electron temperature both inthe order of 1000–2500 K. Fig. 13 presents notonly the cascaded arc, but the entire setup, alsoincluding the expanding plasma.Cascaded arcs are, among others, used for dep-

osition purposes. Some examples include the dep-


Fig. 14. Schematic representation of the presence of dust par-ticles in a plasma, in the case of(a) large particles, drawn tothe lower electrode as a result of the gravitation force,(b)smaller particles, drawn toward both electrodes, and(c) verysmall particles, uniformly distributed in the plasma and mainlyaffected by the electric force.

osition of hydrogenated amorphous carbonw100xor silicon layersw101x, and compounds consistingof C, N and Hw102x. With this set-up, the growthrate of thin film coatings can be as high as 70nmys w101x, which is much higher(typically afactor of 100) than for other deposition techniques.Moreover, it has been demonstrated that the qual-ity, in terms of hardness and infrared refractiveindex, increases with increasing growth rate with-out the need for an additional ion bombardmentw101x. Another advantage, as mentioned above, isthat, due to the supersonic expansion, the plasmaproduction(arc) and the downstream or depositionregion are decoupled, i.e. the downstream plasmais remote. This means that plasma settings andsubstrate conditions can be optimized separately,in contrast to most non-remote plasmas, whereboth parameters are mutually connectedw102x.

2.11. Dusty plasmas

The term ‘dusty plasmas’ does not actuallyindicate a type of discharge, but rather a propertyof many gas discharges, i.e. the presence of littledust, particles or clusters, in the plasma. This is atypical illustration that the chemical compositionis an important ingredient for the plasma compo-sition. Indeed, the dust particles not only changethe chemistry, but also the electric field. Thesedust particles are normally considered to be con-taminants, which reduce the performance of theplasma processes and the quality of the final result.Indeed, with the decreasing feature sizes on inte-grated circuits, the tolerance for dust particlesbecomes smaller. Now that feature sizes are below0.25mm, very small dust particles with a diameterof only a few tenths of amm, can kill the devicew103x. However, in particular cases, the presenceof dust particles now appears to be an advantageinstead of a disadvantage(see below). Becausedusty plasmas are a ‘hot-topic’ in plasma process-ing applications, especially in IC-technology andfor amorphous silicon deposition, we will brieflydescribe their main features.The properties of dusty plasmas will be deter-

mined by how and where the particles move inthe plasma. This is controlled by the various forcesacting on the particlesw103,104x:

● The electric force: since every free object in aplasma is exposed to ion and electron fluxes,and since the electrons have a much highermobility than the ions, the objects will benegatively charged. This applies to the walls,the electrodes, and also to dust particles whichmight be present in the plasma. Hence, the dustparticles will be negatively charged, and theywill be trapped in the negative glow plasma bythe sheath fields.

● The gravitation force pulls the particles down,but it is only important for particles)10 mmdiameter.

● The gas flow through the discharge cell carriesthe particles with it, as a result of friction.

● The so-called ion wind: when the ions move ina certain direction, such as at the sheath–bulkinterface, collisions between the ions and thedust particles can carry the particles with theion flux.

● Thermophoresis: when the gas temperatureshows a gradient, the particles will move in thedirection of the lower temperature.

The combination of these forces determines thebehavior of the dust particles. Generally, threecases can be distinguishedw103x (see Fig. 14):

1. The largest particles(severalmm in diameter)will be drawn to the lower electrode, as a resultof the gravitation force.

2. The smaller particles feel mainly the ion wind,and are drawn toward both electrodes, formingtwo ‘clouds’ of dust in front of the electrodes.

3. The smallest particles(-0.1mm diameter) aremainly affected by the electric force, and they


are generally uniformly distributed in theplasma.

When the particle density is relatively high, orwhen the particle size is large, it is possible toobserve the locations of particles with the nakedeye, by observing the scattering from a HeNe laserw104x.The reason that the dust particles are not nec-

essarily considered as being a disadvantage in theplasma is due to the fact that they can ‘grow’ inthe plasma(‘coagulation’) via negative ion chem-istry. When the particles are large enough, theirelectronegativity rises considerably. Because thecoagulated particles attract the electrons, theyresult in a drop in electron density and a rise inelectron temperature, which affects, e.g. the etch-ing process. If it was possible to control theproperties of the plasma and the dust particles,certain plasma processes might be improved. Forexample, it has been proven that the smallest dustparticles in SiH plasmas are crystalline, so that4

microcrystalline dust particles, with controllablegrain sizes, can be madew103x. An importantapplication is the production of stable solar cellsbased on an a-Si:H film in which nanocrystalliteshave been incorporated. Other applications, suchas the production of photoluminescent and electro-luminescent materials, submicron catalysts, fuller-enes, nanotubes and other clusters have also beendeveloped with the use of dusty plasmasw103x.

2.12. Electron-beam-produced plasmas

Finally, although this paper deals mainly withelectrical discharge plasmas(i.e. created by elec-trical power), we briefly mention here plasmadischarges produced by the interaction of an elec-tron beam with a gaseous medium, because oftheir importance for large area plasma processingapplications w13x. An example is the large-areaplasma processing system(LAPPS) developed atthe Naval Research Laboratoryw105x. The plasmais generated by a sheet electron beam with voltagesand current densities of the order of kVs and tensof mAycm . The plasma dimensions are 1 m by2 2

a few cm thick. The beam is guided by a magneticfield of 0.005–0.03 Tesla. Since an electron beam

of this type efficiently ionizes any gas, highelectron densities of 10 –10 cm are easily12 13 y3

generated at 4–10 Pa pressure. In addition to thelarge area and the high electron density, the LAPPShas some other advantages for plasma processing,including independent control of ion and freeradical fluxes to the surface, very high uniformity,very low electron temperature, and a geometrywell suited for many applications(etching, depo-sition and surface treatment). The major disadvan-tages comprise the trade-off between efficiencyand uniformity, the need for a beam source andthe need for a magnetic field to confine the beamw105x.

3. Applications of gas discharge plasmas

As mentioned in the introduction, one applica-tion field of plasmas is in analytical spectrochem-istry, for the trace analysis of solids, liquids andgases. The ICP(mostly at atmospheric pressure),the microwave induced plasma and the glow dis-charge(in d.c., rf, or pulsed mode) are the mostwell-known analytical plasmas, but magnetron dis-charges, d.c. plasma jets and SWDs have also beenused for analytical purposes(see above). Sincethe present review is aimed at making analyticalplasma spectrochemists more familiar with gasdischarge plasmas in a wider application range,we will not go into any detail here about theanalytical applications, which have been thorough-ly reviewed in several good booksw67,106,107x,but we will focus instead on the other applicationfields.Plasmas find well-established use in industrial

applications(e.g. for surface modification, lasers,lighting, etc.), but they are also gaining moreinterest in the field of life sciences, related toenvironmental issues and biomedical applications.From a scientific point of view, the plasma

yields a transformation of either(i) particles,(ii)momentum or(iii ) energy. Indeed, either particles,momentum or energy can be considered as inputin the plasma, whereas the output is again eitherparticles (with changed chemical composition),momentum(e.g. acceleration, beaming) or energy(e.g. heat, light). Keeping this in mind, the follow-ing subdivision of applications could be made.


1. Transformation of particles, i.e. plasma chem-istry, either at the surface(surface modification,such as etching, deposition, etc.) or in theplasma itself (e.g. powder formation, ozonegeneration, environmental applications);

2. Transformation of momentum, i.e. plasmabeaming, such as for lasers, plasma thrusters,rocket propulsion;

3. Transformation of energy, e.g. creation of light,such as in lamps, plasma displays or lasers.

In the following, we will describe some of themost widespread applications in more detail. Wewill confine ourselves to various kinds of surfacemodification, to lamps, plasma displays, lasers,ozone generation, environmental and biomedicalapplications, and particle sources.

3.1. Surface modifications

Surface modification by gas discharge plasmasplays a crucial role in the microelectronics industryw1,4,62,108x. For the microfabrication of an inte-grated circuit(IC), one-third of the hundreds offabrication steps are typically plasma based. A fewexamples arew62x:

● argon or oxygen discharges to sputter-depositaluminum, tungsten or high temperature super-conducting films;

● oxygen discharges to grow SiO films on2

silicon;● SiH Cl yNH and Si(OC H ) yO discharges2 2 3 2 5 4 2

for the plasma-enhanced chemical vapor depo-sition of Si N and SiO films, respectively;3 4 2

● BF discharges to implant dopant(B) atoms3

into silicon;● CF yCl yO discharges to selectively remove4 2 2

silicon films;● O discharges to remove photoresist or polymer2

films.

These types of steps(i.e. deposit or grow, dopeor modify, etch or remove) are repeated again andagain in the manufacture of a modern integratedcircuit. They are the equivalent, on amm-scalesize, of cm-sized manufacturing using metal andcomponents, bolts and solder, and drill press andlathe w62x.

Fig. 15 w62x shows a typical set of steps tocreate a metal film patterned with sub-micronfeatures on a large area wafer substrate.

1. The film is deposited.2. A photoresist layer is deposited over the film.3. The resist is selectively exposed to light

through a pattern.4. The resist is developed, removing the exposed

resist regions and leaving behind a patternedresist mask.

5. The pattern is transferred into the film by anetch process, i.e. the mask protects the under-lying film from being etched.

6. Finally, the remaining resist mask is removed.

Of these six steps, plasma processing is gener-ally used for film deposition(a) and etching(e),and may also be used for resist development(d)and removal(f) w62x.Another important application field of plasma

surface modification is in materials technology.With plasma processing, materials and surfacestructures can be fabricated that are not attainableby any other commercial method, and the surfaceproperties of materials can be modified in uniqueways.The different kinds of plasma processes that

play a role in surface modification will be outlinedbelow w4x. We limit ourselves here to applicationsof non-LTE plasmas, but it is interesting to notethat other surface modification processes also exist,based on LTE plasmas, i.e. where heat is required,such as for welding, cutting, spraying, etc.w88x,which will not be further discussed here.

3.1.1. Deposition of thin filmsPlasma deposition processesw109x can be sub-

divided in two groups: sputter-deposition and plas-ma-enhanced chemical vapor deposition.(1) Sputter-deposition comprises physical sput-

tering and reactive sputtering. Inphysical sputter-ing, ions (and atoms) from the plasma bombardthe target, and release atoms(or molecules) of thetarget material. This can be compared with ‘sand-blasting’ at the atomic level. The sputtered atomsdiffuse through the plasma and arrive at the sub-strate, where they can be deposited. Inreactivesputtering, use is made of a molecular gas. Beside


Fig. 15. Different steps in making an integrated circuit:(a) metal film deposition;(b) photoresist deposition;(c) optical exposurethrough a pattern;(d) photoresist development;(e) plasma etching; and(f) remaining photoresist removal.

the positive ions from the plasma that sputter-bombard the target, the dissociation products fromthe reactive gas will also react with the target.Hence, the film deposited at the substrate will bea combination of sputtered target material and thereactive gas.The mechanism of deposition is illustrated in

Fig. 16. When the sputtered atoms arrive at thesubstrate they can be(temporarily) adsorbed(a);they can, however, also migrate through the surfaceor become re-evaporated. When a second atomarrives at the substrate, it can form a doublet withthe first atom, which is more stable than a singleatom, and has more chance to remain ‘stuck’(b).New atoms arrive at the substrate and can formtriplets, etc. This initial stage is called ‘nucleation’(c). Little atomic islands are formed(d) thatcoalesce together(e), until a continuous film isformed(f) w3x.D.c. glow discharges are the simplest source

configurations for sputter-deposition. However,they are not suitable for the deposition of non-conductive materials because of charging-up of thetarget material. This problem can be overcome bycc rf discharges(see above). In particular, mag-netron discharges are also often used for sputter-

deposition applications(see above; Section 2.7).The introduction of the magnetic field has espe-cially resulted in much higher deposition rates andmore flexibility in substrate geometry and com-position in comparison to diode sputtering.(2) Another method of deposition is byplasma

enhanced chemical vapor deposition (PE-CVD).The discharge operates in a reactive gas. Bychemical reactions in the plasma(mainly electronimpact ionization and dissociation), different kindsof ions and radicals are formed which diffusetoward the substrate and are deposited by chemicalsurface reactions. The major advantage comparedto simple chemical vapor deposition(CVD) is thatPE-CVD can operate at much lower temperatures.Indeed, the electron temperature of 2–5 eV in PE-CVD is sufficient for dissociation, whereas in CVDthe gas and surface reactions occur by thermalactivation. Hence, some coatings, which are diffi-cult to form by CVD due to melting problems,can be deposited more easily with PE-CVD. Vari-ous kinds of plasma sources have been used forthis application, ranging from d.c., cc-rf and pulseddischarges, to microwave discharges, ECRs, ICPsand expanding plasma jets(see above; Section 2).


Fig. 16. Different steps in the deposition of a film on a substrate:(a) a single atom arrives and can migrate over the surface;(b)arrival of a second atom and combination into a doublet;(c) nucleation into ‘islands of atoms’;(d) island growing;(e) coalescenceof the islands; and(f) formation of a continuous film.

The two most well-known applications of PE-CVD are the deposition of amorphous hydrogen-ated silicon (a:Si-H) w110–112x and carbon(a:C-H) w113,114x layers. Amorphous hydrogen-ated silicon layers are often used for the fabricationof solar cellsw115x. An advantage of amorphoussilicon deposited by PE-CVD over crystalline sil-icon produced by a conventional technique is thelow cost of production. Amorphous silicon absorbsphotons much more efficiently so that the solarcell can be much thinner compared to crystallinesilicon. Another advantage of amorphous siliconproduced by PE-CVD is that it can be producedat lower temperature compared to that of thermallydriven CVD (see above) w112x.

Amorphous hydrogenated carbon layers, alsocalled ‘diamond-like carbon layers’(DLC) arecommonly used as protective hard coatings onvarious kinds of industrial components(metals,glass, ceramics and plastics). They have some veryinteresting characteristics, e.g. a high hardness(inthe range 1000–3000 kg.mmw4x), extremely low2

friction coefficients(between 0.01 and 0.28w4x),

a good adhesion on most materials, chemicalinertness against most solvents and acids, thermalstability (up to above 3008C), and interestingoptical properties(i.e. transparent in the VIS andIR ranges) w113x. This makes them very suitablefor a large number of applicationsw116–119x,especially to protect materials, e.g. for the depo-sition of magnetic hard disks, the protection of sunglasses, as anti-reflective coatings for IR opticalmaterials, etc. In general, it can be stated that DLClayers create a hard surface with self-lubricatingproperties. Therefore, they are very suitable toreduce wear and friction when two componentsare in contact with each other(e.g. protection ofmachine components).

3.1.2. EtchingPlasma etching is essentially used to remove

material from a surfacew120x. It can be conductedwith a variety of discharge sources, such as d.c.glow discharges, cc rf discharges, SWDs, ICPsand ECRs. The three most important parameters


Fig. 17. Schematic representation of(a) anisotropic etching, inwhich the material is removed in the vertical direction only,and(b) isotropic etching, which is characterized by equal ver-tical and horizontal etch rates, leading to ‘undercutting’.

Fig. 18. Schematic representation of the four basic etch mech-anisms: (a) sputter-etching;(b) chemical etching(i.e. bychemical reactions between plasma species and the target sur-face, to form projectiles in the gas phase); (c) ion-enhancedenergetic etching(i.e. chemical etching, enhanced by energeticion bombardment); and(d) ion-enhanced inhibitor etching(i.e.so-called inhibitor precursor molecules are deposited on thetarget material, to form a protective layer; the bombarding ionflux removes this layer, so that the target surface can be selec-tively exposed to the etching species).

for etching are etch rate uniformity, anisotropy andselectivity w4,121x.

The etch rate uniformity is a major concern insemiconductor processing, since it can have asignificant impact on manufacturing yield. Incapacitively coupled systems, the geometry of theelectrode assembly at the wafer perimeter is mostcritical for etch rate uniformity, especially in thewafer edge region. In inductively coupled plasmasystems, the etch rate uniformity across the entirewafer can be affected by spatially varying the rfcoupling to the backside of the substrate.

Anisotropic etching means that the material isremoved in the vertical direction only, whereas thehorizontal etch rate is zero(see Fig. 17a). Opposedto that is isotropic etching(see Fig. 17b), whichis characterized by equal vertical and horizontaletch rates. Many years ago, feature spacings(e.g.between trenches) were tens of microns, i.e. muchlarger than the required film thickness. So-called‘undercutting’ (shown in Fig. 17b) was thenacceptable. This is no longer true with sub-micronfeature spacings. The reduction in feature sizesand spacings makes anisotropic etch processesessential. Plasma processing is the only commer-cial technology capable of controlling the anisot-ropy. Hence, anisotropy has been a major drivingforce in the development of plasma processingtechnologyw62x.Another critical process parameter for IC man-

ufacture isselectivity (i.e. removing one type ofmaterial while leaving other materials unaffected).It appears that highly-selective plasma etch proc-esses are not easily designed. In fact, selectivityand anisotropy often compete in the design of aplasma etch processw62x.

There are basically four different low pressureplasma etch mechanisms, i.e.(1) sputter-etching,(2) chemical etching,(3) ion-enhanced energetic


etching and(4) ion-enhanced inhibitor etching(see Fig. 18). The degree of anisotropy and selec-tivity will depend on the etch mechanism used.

1. The mechanism ofsputter-etching is identicalto sputter-deposition, but the discharge para-meters are now optimized for the efficientremoval of material from the target instead ofthe deposition of the sputtered atoms on asubstrate. Sputtering is a more or less non-selective process. Indeed, the sputter yielddepends only on the energy of the bombardingspecies, the masses of projectile and targetspecies, and the surface binding energyw122x.Consequently, there is not a great differencebetween sputter yields for different materials.However, sputtering is an anisotropic processwhich depends greatly on the angle of inci-dence of the bombarding ions. Indeed, a max-imum in the sputtering yield is typicallyreached at incident angles of approximately60–808 relative to the surface normalw123x.At low incident angles with respect to thesurface normal, the sputtering yield increaseswith rising angle due to the increased probabil-ity of the collisional cascade to propagate backto the cathode surface and hence to result insputtering. At incident angles higher than 60–808 relative to the surface normal, the sputter-ing yield decreases, since the incomingparticles are more likely to reflect off thesurface without any penetration or momentumtransfer, so that sputtering becomes unlikely.

2. The second mechanism ispure chemical etch-ing. Atoms or radicals from the dischargeplasma can chemically react with the targetsurface to form projectiles in the gas phase,e.g.: Si(s)q4F ™SiF (g). Chemical etching0

4

is essentially isotropic, because the etchingatoms arrive at the target surface with a nearlyuniform angular distribution. Anisotropic etch-ing is only possible when the reaction takesplace between etching atoms and a crystal,where the etching rate depends on the crystal-lographic orientation. The etch rate is generallynot determined by the flux of etching atoms,but rather by the set of reactions at the surface.

3. The third mechanism ision-enhanced energeticetching w124x. The discharge provides etchingparticles and energetic ions for the etch process.It appears that the combination of etchingatoms and energetic ions is much more effec-tive than the separate mechanisms of sputter-etching and chemical etching. The etchingprocess itself is probably chemical, but with areaction rate determined by the energetic ion-bombardment. This etching method can also bevery anisotropic, but the selectivity is generallylower than with chemical etching.

4. The last etching mechanism ision-enhancedinhibitor etching. The discharge provides notonly etching atoms and energetic ions, but alsoso-called inhibitor precursor molecules. Thelatter are deposited on the target material, toform a protective layer. The etching atoms arechosen in order to obtain a high chemical etchrate in the absence of ion bombardment andthe inhibitor. The bombarding ion flux preventsthe formation of an inhibitor layer and itremoves this layer so that the target surfacecan be exposed to the etching atoms. Hence,the target surface will only be etched wherethere is no inhibitor layer, or in other words,where the ion flux reaches the target. In thisway, the ion-enhanced inhibitor etching is veryselective, as well as anisotropicw4x.

An example of ion-enhanced inhibitor etchingis the anisotropic etching of aluminum trencheswith CCl yCl or CHCl yCl discharges. Both Cl4 2 3 2

and Cl can efficiently chemically etch aluminum,2

but it results in an isotropic pattern. By addingcarbon to the mixture, a protective carbon–chlo-rine polymer film is formed at the surface. Ionbombardment removes the film at the bottom ofthe trench, enabling etching at the bottom. Becausethe protective film is also formed at the walls ofthe trench, whereas the ions bombard only thebottom, steep walls can be formed(see Fig. 18d).Possible problems of ion-enhanced inhibitor etch-ing are, however, the contamination of the surface,and the removal of the protective film after theetching step.


To reach a compromise between parameters suchas efficiency, precision, selectivity and anisotropy,the different etch mechanisms can be coupled toeach other(parallel or after each other).

3.1.3. Plasma-immersion ion implantationThe process of ion implantation causes the

injection of an energetic ion beam into a material,changing herewith the atomic composition andstructure, and hence the properties, of the materialsurface layer. Conventional ion implantation iscarried out in a vacuum chamber, where an ionsource is used to create an intense ion beam of thespecies to be implanted. The ion beam has to befocussed and accelerated by a potential differenceof several tens to hundreds of kV, which makesthis technique mechanically complex andexpensive.In plasma-immersion ion implantation(PIII)

w125–128x, also called plasma source ion implan-tation (PSII), a number of steps required in theconventional method can be removed, such asbeam extraction, focussing and scanning over thetarget material. Instead, the target material is‘immersed’ in a plasma, and the ions are directlyextracted from the plasma and accelerated towardthe target by a number of negative high voltagepulses.PIII has become a routine process in the semi-

conductor industry, mainly for doping of the tar-gets. In order to obtain high implantation fluxes atlow pressure, ECR sources are mostly used(typi-cally at 2.45 GHz and a pressure of 0.1 Pa orlower). Moreover, this technology is also gainingincreasing interest in the metallurgical industry, tomake new surface alloys with enhanced hardnessas well as corrosion and wear resistancew4x.

3.1.4. Surface activation and functionalization ofpolymersWhen a plasma is brought into contact with

polymers, this can give rise to chemical andphysical modifications at the surface, e.g. produc-ing more reactive sites, or changes in cross-linkingor molecular weightw129–135x. In this way, mate-rials with desired properties can be obtained, suchas wettability, adhesion, barrier protection, materialselectivity and even biocompatibilityw40,129x.

Plasma surface treatments allow the modificationof the surface characteristics of polymers to obtainimproved bonding, without affecting the bulkproperties.

Surface activation of polymers is carried out byexposure to a non-polymer-forming plasma, suchas O , N , NH and the inert gasesw130x. The2 2 3

bombardment by energetic particles breaks thecovalent bonds at the surface, leading to theformation of surface radicals. The latter can reactwith the active plasma species to form differentchemically active functional groups at the surface.Surface contaminants and weakly bound polymerlayers can dissociate in volatile side products,which can be pumped away. Oxygen and nitrogenplasma exposures for several minutes make thesurfaces of most polymers hydrophilicw129x. Theactive plasma species attack the polymer surfacesand cause the incorporation of hydrophilic groups,such as carbonyl, carboxyl, hydroxyl and aminogroups. It has been reported that radical species,rather than ions and electrons, play an importantrole in the hydrophilic modificationw129x.

Polymer surfaces can also be functionalized by‘plasma induced grafting’, which is a combinationof plasma activation and conventional chemistryw4x. An inert gas(typically argon or helium) isused to activate the polymer surface, by formingradicals, e.g.:

*Ar•CyCyCyCyC™CyCyCyCyC

(3)

substrate activated substrate

After this plasma activation, the polymer surfaceis exposed to unsaturated monomers, ‘grafting’ apolymer layer on the activated surface:

acrylic acid•CyCyCyCyC ™ CyCyCyCyC

± (4)

CyCOOH

3.1.5. Plasma polymerizationBesides the surface activation of polymers, thin

polymer films can also be deposited by so-called


‘plasma polymerization’ w40,129–135x. This isessentially a PECVD process(see above). It refersto the deposition of polymer films through plasmadissociation and to the excitation of an organicmonomer gas and subsequent deposition andpolymerization of the excited species on the sur-face of a substrate. Plasma polymerization is usedto deposit films with thicknesses from several tensto several thousands of angstroms. The depositedfilms are called plasma polymers, and are generallychemically and physically different from conven-tional polymersw4x. Whereas conventional polym-erization is based on molecular processes duringwhich rearrangements of the atoms within themonomer seldom occur, plasma polymerization isessentially an atomic process. Polymers formed byplasma polymerization are, in most cases, highlybranched and highly cross-linked(with approx.one cross-link per 6–10 chain carbon atoms) w4x.Plasma polymerization is characterized by severalfeaturesw130x:

1. Plasma polymers are not characterized byrepeating units, as is typical for conventionalpolymers.

2. The properties of the plasma polymer are notdetermined by the monomer being used butrather by the plasma parameters. For example,an ethylene plasma does not simply give riseto polyethylene, but to a variety of products(including unsaturated groups, aromatic groupsand side branches), depending on the plasmaconditions.

3. The monomer used for plasma polymerizationdoes not have to contain a functional group,such as a double bond.

Plasma polymerization takes place through sev-eral reaction stepsw133x. In the initiation stage,free radicals and atoms are produced by collisionsof electrons and ions with monomer molecules, orby dissociation of monomers adsorbed on thesurface of the sample. The next step, i.e. propa-gation of the reaction, is the actual formation ofthe polymeric chain. This can take place both inthe gas phase(by adding radical atoms to otherradicals or molecules) and on the deposited poly-mer film (by interactions of surface free radicalswith either gas phase or adsorbed monomers).

Finally, termination can also take place in the gasphase or at the polymer surface, by similar proc-esses as in the propagation step, but ending eitherwith the final product or with a closed polymerchain.Applications of plasma-polymerized films are

associated with biomedical uses(e.g. immobilizedenzymes, organelles and cells, sterilization andpasteurization, blood bag, blood vessels, artificialkidneys, etc.), the textile industry(e.g. anti-flam-mability, anti-electrostatic treatment, hydrophilicimprovement, water-repellence, dyeing affinity,etc.), electronics(e.g. amorphous semiconductors),electrics(insulators, thin film dielectrics, separa-tion membrane for batteries), optical applications(anti-reflecting coatings, anti-dimming coatings,improvement of transparency, optical fibers, con-tact lenses, etc.), chemical processing(reverseosmosis membrane, permselective membrane, etc.)and surface modification(adhesive improvement,protective coatings, etc.) w4x. The main advantageof plasma polymerization is that it can occur atmoderate temperatures compared to conventionalchemical reactions(because the cracking of mon-omers and the formation of radicals occurs byelectron impact reactions in the plasma).For plasma deposition and activation of polymer

films, plasma sources operating at atmosphericpressure are preferably used, because they can treatmaterials with high vapor pressure. In the past,d.c. corona discharges were typically used for thispurpose, but nowadays, DBDs and APGDs(seeabove) are more important plasma sources for thisapplication.

3.1.6. CleaningCleaning of surfaces means the removal of all

possible undesired residues, such as oxides, metal-lic and organic contaminants or photoresist in thesemiconductor industry. Plasma-assisted cleaningof surfaces offers some alternatives to conventionalwet (chemical) cleaningw4x.

Wet cleaning can cause problems by the intro-duction of the cleaning liquids in the narrowchannels, in the case of ICs. Moreover, for thisapplication, all traces of contaminants have to becompletely removed from the wafer.


Fig. 19. Schematic representation of ashing(i.e. removal of a photoresist, consisting mainly of carbon and hydrogen) with oxygen.The plasma gives rise to chemically active atomic oxygen, which reacts with the resist, to form volatile products, such as CO,CO and water vapor. The latter can then be pumped out of the reactor.2

In the case of organic contaminants, this prob-lem can be solved by volatilization, and subsequentremoval of the volatile components from the gasphase. A ‘cleaning plasma’ thus contains an oxi-dizing gas(e.g. O), which converts the surface2

contaminants into volatile oxides.The removal of metallic surface contaminants

from silicon wafers is, however, more complicated,because it concerns, typically, a range of differentmetals, with different reactivities, segregation coef-ficients and diffusion coefficients in silicon andsilicon oxide. The removal of the metals needs tobe carried out at low temperatures to avoid diffu-sion of the metal atoms into the silicon beforethey can be removed. However, metal atoms andtheir compounds are generally not volatile at lowtemperatures. Therefore, a two-step process needsto be applied. First, the metal needs to be convertedinto a complex, in order to capture the metal atomsor compounds and to avoid diffusion into thesilicon. Consequently, the metal complexes needto be volatilized by increasing the substrate tem-perature and reducing the pressure, to obtain anappreciable vapor pressure. Volatile compounds ofmetals can be produced with halogens. In particu-lar, chlorine reacts with nearly all contaminatingmetals and produces more volatile compounds thanthe other halogens. Hence, chlorine-based plasmasare often used to remove metal contaminants fromsurfaces.

3.1.7. AshingAshing is mainly used to remove organic frag-

ments from inorganic surfaces. Hence, the appli-

cation of ashing is closely related to cleaning.Plasma ashing is typically carried out in an O2

plasmaw4x, although for certain applications(e.g.proteins) H plasmas can also be used.2

One of the first real applications of cold plasmachemistry was the ashing of photoresists, alsocalled ‘resist stripping’ in the 1960sw4x. Theorganic resist consists mainly of carbon and hydro-gen. At room temperature, it does not react withmolecular oxygen. However, the chemically activeatomic oxygen created in a plasma does react moreeasily at room temperature with the resist, formingvolatile products such as CO, CO and water vapor2

(see Fig. 19), which are then pumped out of thereactor.

3.1.8. OxidationOxidation is also a form of surface modification

w4x. When a metal or semiconductor surface isimmersed in an oxygenyargon plasma, an oxidelayer can be formed on top of the surface. Whenthe surface is at ‘floating potential’, no currentflows toward the substrate during the oxide growth,and the process is called ‘plasma oxidation’. Theplasma species(neutrals, electrons, positive andnegative ions) can reach the substrate by diffusion,and the formed oxide layer is generally thin,typically less than 10 nm. When a positive bias isapplied to the surface, electrons and negative ionsare accelerated toward the substrate, and the oxidegrowth is stimulated. This process is then generallycalled ‘plasma anodization’, and the oxide layerscan reach a thickness of severalmm.


Fig. 20. Schematic representation of plasma nitriding, indicat-ing the different steps in formation of a nitride film. The target(cathode) is bombarded by positive ions(N ), causing sput-q

2

tering of the metallic atoms(e.g. Fe), which react in the plasmato form FeN. The nitrides are deposited at the target, wherethey react further with the target(Fe) atoms, into Fe N and2

Fe N, until finally the stable Fe N is formed. The N atoms3 4

released in these reactions, can diffuse into the target, to formthe final nitride layer.

In some cases, a negative bias is applied to thesubstrate. The thickness of the oxide layer is thencontrolled by diffusion, which can be enhanced bythe bombardment of positive ions. In this case, anequilibrium can be reached between the growthrate and the rate of sputter-removal. By varyingthe plasma parameters, such as electrical power,pressure and argonyoxygen ratio, the thickness ofthe oxide layer can be very accurately controlled.The advantage of plasma oxidation and anodi-

zation is that it can be performed at lower temper-atures than thermal oxidation. Typically, the plasmaoxidation of silicon occurs at a temperature of300–5008C. Another application of plasma oxi-dation is the fabrication of highT super conduc-c

tors by ECR reactorsw4x.

3.1.9. Surface hardeningSurface hardening comprises three related meth-

ods: nitriding, carburizing and boridingw4x. Thesetechniques are similar to PIII(see above), but themodification mechanism is different. The aim ofthese techniques is to harden the surface layer,without affecting the bulk properties of thematerial.

Nitriding is mainly applied to enhance wear andcorrosion resistance, to avoid fatigue of materials,and to increase the hardness of a surface. Thehardness achieved with nitriding is maintained athigh temperatures. Conventional thermal nitridingis typically carried out in partially dissociatedammonia at temperatures between 480 and 6508C. It takes a long time(up to 50 h) before thedesired hardness is reached.Plasma nitriding, alsocalled ‘ion nitriding’, is a surface modificationprocess similar to plasma oxidation. However,nitrogen does not form negative ions in the plasmaor near the surface. Hence, plasma nitriding iscarried out as a cathodic process, accompanied bythe bombardment of positive ions. The targettypically serves as the cathode of a d.c. glowdischarge, which is maintained at a pressure ofseveral hundred Pa, and a voltage between 1000and 1500 V. The target is bombarded by positiveions. This causes sputtering of the metallic atoms(e.g. Fe). The latter react in the plasma with thenitrogen atoms, to form a nitride(FeN). Thesenitrides are deposited at the target, but they are

unstable and react further with the target(Fe)atoms. Finally, they form the stable product Fe N.4

Hence, atomic nitrogen is released and can diffuseinto the target material, to form the final nitridelayer (see Fig. 20). Because the surface materialis saturated with nitrogen, plasma nitriding is muchfaster than conventional nitriding, so that the entireprocess takes less time. Moreover, the compositionof the nitride layer can be controlled by adaptingthe plasma parameters. This is very useful, sincethe final surface properties greatly depend on thecomposition of the nitride layer. Only a smallfraction of the nitriding processes occurs by directimplantation of nitrogen in the target material(i.e.PIII).

Carburizing occurs by increasing the amount ofcarbon in the surface layer of the material. In the


Fig. 21. Schematic representation of the working principles ofa fluorescent lamp. A potential difference is applied betweentwo electrodes, placed at the ends of the tube. Electrons areemitted from the cathode, and are accelerated in the plasma,giving rise to collisions, e.g. excitation of mercury atoms. Theexcited atoms decay with emission of UV photons(254 nm).The latter are absorbed by a fluorescence layer, applied to theinner walls of the tube, and are converted into visible photons.

conventional method, this has to be followed by aheat treatment to obtain the desired hardness.Plasma carburizing is similar to plasma nitriding.It occurs typically in a d.c. glow discharge at 100–3000 Pa. The plasma provides carbon atoms tomove towards the surface, and it promotes surfacereactions. An important difference with plasmanitriding is that the temperature is here very high.For example, plasma carburizing of steel occurs at1050 8C. This temperature is required to enablediffusion of carbon in the austenitic phase of thesteel material. Due to the high temperature, a post-treatment is generally required, which is not nec-essary for plasma nitriding. The advantages ofplasma carburizing, compared to conventional car-burizing, are the uniformity of the layer and thereduction of the process time. The uniformity isachieved because the plasma follows the profile ofthe target material(e.g. working tools).

A combination of plasma nitriding and carburiz-ing of steel, i.e. so-calledplasma carbonitriding,is another plasma hardening process which iscommercially used. It is carried out in a gasmixture of nitrogen and methane.Finally, boriding is a surface hardening method

which is mainly used to enhance the wear resis-tance of steel. As suggested by the name, harden-ing of the surface is achieved by the formation ofborides. Conventional boriding occurs at tempera-tures above 9008C. Plasma boriding is achievedin a mixture of diborane and argon, at a tempera-ture of 600 8C. Diborane is dissociated in theplasma and the boron is deposited on the materialsurface and diffuses into the steel to form FeB andFe B boride phases. An increase in surface hard-2

ness of 2.65–9.8 GPa after plasma boriding hasbeen reportedw4x.

3.2. Lamps

Several types of light sources are based on gasdischarge plasmas. In the following, we will makea subdivision into conventional ‘electroded lamps’and the more recent types of ‘electrodeless dis-charge lamps’. In both categories, low-pressurenon-LTE lamps (e.g. fluorescence lamps) andhigh-pressure, thermal LTE-lamps(e.g. high-inten-sity discharge(HID) lamps) exist.

3.2.1. Electroded lamps

3.2.1.1. Electroded low-pressure lamps. Fluores-cence lamps are electroded low-pressure non-LTElamps, which operate in the positive column ofd.c. glow discharges, in a mixture of a rare gaswith mercury; the latter is present both in liquidand gaseous formw136x. The electrodes, placed atthe ends of the tube, are covered with a mixtureof the oxides of barium, strontium and calcium.These materials are characterized by a high elec-tron emission coefficient, in order to supply thedischarge with sufficient electrons. The electricalenergy applied to the discharge in the form of apotential difference between the electrodes, is con-verted into kinetic energy of the electrons byacceleration in the electric field. The electronscollide with mercury atoms leading to excitationand subsequent radiative decay. The UV photonsemitted by this process are absorbed by a fluores-cence layer, which is applied to the inner walls ofthe discharge tube. This layer converts theabsorbed UV photons into visible photons. Thewhole process of generating visible radiation fromelectrical energy is schematically illustrated in Fig.21 w137x. Approximately 60–70% of electricalpower in these discharges is converted to UVradiation by mercury atoms. Taking into accountthe conversion of UV to visible light, the totalconversion efficiency of electrical power to visiblelight is approximately 25%w70x. It has been


estimated that 80% of the world’s artificial lightis fluorescentw136x.

The rare gas pressure in the discharge is typi-cally several hundred Pa whereas the mercurypartial pressure is in the order of 0.5 Pa. Neverthe-less, most of the electron energy is consumed forelectron impact excitation and ionization of themercury atoms. Indeed, the mean electron energyis typically only 1 eV, which is generally highenough to excite the mercury atoms, but notsufficient for excitation of the rare gas atoms,because the latter are characterized by much higherexcitation energies than the mercury atoms. How-ever, the rare gas plays an essential role in thedischarge:(i) to reduce the mean free path of thecharged plasma species;(ii) to reduce evaporationand sputtering of the electrode material; and(iii )to facilitate breakdown of the discharge.The fluorescence layer typically consists of three

phosphors, with their own emission spectrum. Bymixing them, the desired color can be obtained.However, once the fluorescence layer is depositedon the inner tube walls, the color cannot bechanged anymore. Nevertheless, in some cases, itwould be convenient to change the color, depend-ing on the application(e.g. museums need bettercolor rendering than industrial halls; butchers needmore red light to show their customers a ‘healthy’piece of red meat, etc).Changing the color of fluorescence lamps can

be done in several ways(i.e. so-called ‘variablecolor fluorescence lamps’) w137–139x.

1. A simple solution consists of using two lampswith different colors, and varying the relativeenergy input to the lamps. This principle canbe extended to one discharge tube with two‘discharge paths’ with different colors.

2. Another method is based on the different decaytimes of the phosphors.

3. The most common solution is to change theelectron energy distribution function(EEDF),by varying the applied voltage, because thedischarge emission, and hence, the lamp color,depends strongly on the EEDFw136x.

4. Changing colors can also be achieved in so-called ‘hybrid discharges’, where a seconddischarge, e.g. a cc rf discharge, is introduced

in the lamp, to change the EEDF locally.Typically, neon is then used as the dischargegas. In the region where the cc rf discharge isactive, the electrons can gain sufficient energyto excite and ionize the neon atoms. Thesubsequent radiative decay of the neon atomsgives rise to red light. By adjusting the powerdissipated in the cc rf discharge, the intensityof the red neon light can be changed, andhence, the lamp color can be variedw137x.

5. Finally, the emission spectrum of the lamp canalso be varied by reducing the density of themercury atoms, so that the discharge producesa significant amount of rare gas radiation. Thisis achieved in the so-called ‘depleted dis-charge’ w137x, either by controlling the tem-perature of the coldest region of the discharge(where the liquid mercury is found), or byintroducing radial cataphoresis, i.e. the partialsegregation of the gas components in the elec-tric dischargew140x.

3.2.1.2. Electroded high-pressure lamps. Electrod-ed high-pressure lamps, also called HID(highintensity discharge) lamps, operate in the regimeof an arc discharge. The pressure is typically afew atm. In contrast to low pressure light sources,which are far from LTE, HID lamps are close toLTE, with gas and electron temperatures typicallyapproximately 6000 K at the cell axis and approx-imately 1000 K near the wallsw70x.

The first HID lamp having technical importancewas the high pressure mercury(HPM) lamp w70x.It produces visible light, without the need for usingphosphors. Indeed, the UV radiation(254 nm),which is dominantly present under low pressureconditions, is reabsorbed under these high pressureconditions. That is why phosphors are not needed.The strongest mercury lines are green, but differentcolors can be obtained by adding metal halides tothe discharge, which will evaporate and dissociate,and subsequently emit lines at different colors.The radiation is mainly emitted from the hot regionnear the lamp axis(see temperatures above). Thishot region is surrounded by a cold region, whichserves as radiation absorber and to stabilize thearc. Another well-known type of HID lamp is the


Fig. 22. Schematic representation of the re-entrant cavity ICPlamp. A ferrite core, surrounded by an isolated induction coil,is placed in a cavity. The latter is surrounded by a bulb, whichis covered at the inside with a fluorescent layer, to convert UVphotons into visible photons.

high pressure sodium(HPS) lamp w141x, which ismostly employed for outdoor lighting(industriallighting, road lighting, security lighting, etc.).

3.2.2. Electrodeless lampsIn conventional electroded discharge lamps, the

energy is typically supplied as d.c. or low-frequen-cy a.c. current, so that electrodes inside the tubeare required to sustain the discharge. The presenceof electrodes causes serious limitations to the lampdesign. Moreover, the electrodes are responsiblefor the limited lifetime of such lamps. To overcomethis problem, electrodeless lamps have been devel-oped, which can, in principle, be excited by fourdischarge mechanismsw70x, i.e. inductive or H-discharges, capacitive or E-discharges, microwavedischarges and travelling wave discharges(e.g.SWDs). However, as will be shown below, onlytwo of them, namely the inductive and microwavedischarges, have been successfully exploited up tonow as commercial light sources. The other twoconfigurations still suffer from technical problems,complex operating conditions, too high costs, toolow efficiency, etc. to be used as commerciallamps. Again, a subdivision can be made into lowpressure, non-LTE(fluorescence) lamps and high-pressure LTE(HID) lamps.

3.2.2.1. Electrodeless low-pressure lamps. Themost well-known, and the only commercial typesof electrodeless low-pressure fluorescence lamps,are theelectrodeless ICP lamps, where the energyis inductively coupled to the discharge plasma.Three different designs can be distinguishedw70x.In ‘re-entrant cavity lamps’ w142x a ferrite core issurrounded by an isolated induction coil, placed ina cavity. The latter is surrounded by a ‘bulb’(seeFig. 22). The discharge is ignited by an rf current,at a frequency of 2.65 MHz, which is in the bandallowed for lighting. Commercial examples of thisconfiguration are the Philips ‘QL lamp’w143,144xand the GE Lighting ‘Genura lamp’w145x. Bothlamps operate at a pressure below 100 Pa, whichis much lower than in electroded fluorescencelamps. The discharge gas is argon for the QLlamp, and krypton for the Genura lamp. Besidesthe rare gas, mercury is also present, which isnecessary in all fluorescence lamps. The QL lamp

has an additional phosphor layer at the surface ofthe cavity to enhance the conversion efficiency ofthe UV to the visible light. The main feature ofthe QL lamp is its exceptional life(100 000 h),making it ideal for situations where maintenanceis difficult. The Genura lamp is mainly used toreplace incandescent reflector lamps. It has a lowerlifetime (typically 10 000 h, but this is not themajor issue for this application), but it offers afour-fold improvement in efficacy compared to theincandescent reflector lamp that it replacesw70x.

ICP lamps with external coils also exist. Theyrequire extra electromagnetic screening by meansof a Faraday cage. An example is the ‘Everlight’of Matsushitaw146x. This lamp operates at an rffrequency of 13.56 MHz and uses neon as a buffergas.The third ICP lamp configuration is thetoroidal

lamp. An example is the ‘Endura lamp’(or ‘Ice-tron’ in Northern America), from Osram Sylvaniaw70x. This lamp operates at the lowest frequencyof all commercially available ICP lamps(i.e. 250kHz). It operates in krypton as buffer gas, at apressure below 100 Pa. The Endura is a highpower, high brightness lamp, with a long lifetime,which makes it useful for inaccessible areas.


Beside the electrodeless ICP lamps, two othertypes of electrodeless low-pressure fluorescentlamps have been developed, namely theSWDlamps w147x and the cc discharge lamp w148x.However, because of some technical difficulties,no commercial lamps of these types have beendeveloped yetw70x.

3.2.2.2. Electrodeless high-pressure lamps. Theonly commercial electrodeless high-pressure, orHID lamp is a microwave HID resonant cavitylamp. The discharge tube is placed in a resonantcavity and microwave power is coupled to thedischarge through this cavity. There was only onecommercial lamp of this type used for generallighting purposes, i.e. the Fusion Lighting ‘Solar1000’ w145,149x, which is, however, no longercommercially available. The lamp operates at ahigh power of 1425 W. The main constituents aresulfur (at a typical pressure of 5–7 bar) and argon(typically at 1 bar). Sulfur molecules emit a widequasi-continuum in the visible region, whichexplains why this lamp emits very ‘white’ light.Because sulfur reacts chemically with most metals,it cannot be used in conventional lamps withelectrodes.Beside this resonance cavity microwave HID

lamp, research has also been conducted oncapac-itively coupled HID lamps andinductively coupledrf HID lamps, but without commercial exploitationw70x.

3.3. Plasma displays

Until now the television(TV) market has beendominated by bulky cathode ray tube(CRT) dis-plays. Yet, display researchers have always beenlooking for more elegant alternatives. Recently,two alternative display technologies have emergedthat offer the possibility of large, lightweight, flatTV monitors. Both are based on small gas dis-charges: microdischarges. The most well-known isthe plasma display panel(PDP) technology, usingmicrodischarges to generate the light of the display.The other is the plasma addressed liquid crystal(PALC) technology, where microdischarges serveas electrical switches.

3.3.1. Plasma display panels (PDPs)A plasma display panel(PDP) w150–153x con-

sists of two glass plates placed at a distance of100–200mm from each other. The region betweenthe plates is filled with a gas at a pressure of 0.6atm. The plates are covered on the inner side witha large number of thin parallel electrodes, in sucha way that the electrodes of one plate are placedperpendicular to the electrodes of the other plate.Hence, they form the rows and columns of adisplay. At each intersection between a row and acolumn electrode, a discharge can be formed,independent of the other intersections, by applyingsuitable voltage pulses to the electrodes.The discharge gives rise to a plasma that emits

visible and UV light. In monochrome PDPs thevisible light can be used directly. In color PDPs,the UV light is used to excite a phosphor, whichthen emits either red or green or blue light.The discharge gas is typically a mixture of rare

gases(xenonyneonyhelium). Color PDPs alwayscontain a percentage of xenon(typically 5%),because it is the most efficient emitter of UVradiation. Nevertheless, the efficiency of the dis-charge to produce UV radiation is still quite low,typically 10%.The discharge can be operated in d.c. or a.c.

mode. In the first case, the electrodes are in directcontact with the discharge gasw152x, whereas inthe second case the PDPs are covered with adielectric layer(as in a dielectric barrier discharge)w42x.In principle, two types of a.c. PDPs can be

distinguished. In thecounter-electrode type, thedischarge is formed between row and columnelectrodes of the opposite plates. In thecoplanar-electrode type (see Fig. 23), the discharge issustained between two pairs of electrodes at theupper plate, but it is ignited by the electrodes atthe lower plate. The latter type is also calledsurface discharge PDP. The electrodes of the upperplate are then transparent and covered with a thinglass layer, which serves as the dielectric. A MgOthin film is deposited to protect against the bom-barding ion flux and to enhance secondary electronemission at the surface.To prevent electrical and optical interaction

between the columns, dielectric barriers are placed


Fig. 23. Schematic representation of a coplanar-electrode a.c. plasma display panel(PDP). The discharge is sustained between twopairs of electrodes at the upper plate(sustaining electrodes), whereas the electrodes at the lower plate(address electrodes) are usedto ignite the discharge. One picture element(i.e. ‘pixel’) is formed by the intersection of one pair of sustaining electrodes and oneinitiating electrode. The electrodes at the upper plate are transparent, and are covered with a thin glass dielectric. Furthermore, aMgO thin film is deposited, to enhance secondary electron emission at the surface, and to protect the electrodes against thebombarding ion flux. Dielectric barrier ribs are placed between the initiating electrodes at the lower plate, to prevent electrical andoptical interactions between the columns. The lower plate is also covered with phosphors.

between the initiating electrodes at the lower plate.This plate is also covered with phosphors(alter-nating in red, blue and green for consecutivecolumns). One picture element(called ‘pixel’) isformed by the intersection of one pair of sustainingelectrodes and one initiating electrode. A squarewave voltage with a frequency of 50–250 kHz iscontinuously applied between the sustaining elec-trodes. This voltage should be just below thebreakdown voltage, so that it cannot ignite thedischarge. To activate a certain pixel, an additionalvoltage pulse has to be applied between one of thesustaining electrodes and an initiating electrode.The resulting discharge will, however, quickly beextinguished as a result of charge accumulation atthe dielectric. However, during the next half cycle,the sustaining voltage changes polarity. The accu-mulated surface charge reinforces the effect of thesustaining voltage, so that a new discharge isignited, even when the sustaining voltage is lowerthan the breakdown voltage. Again there is surfacecharge accumulation, but now on the other elec-trode. The discharge is again extinguished, butwill be ignited again when the sustaining voltagechanges polarity. Hence, in this way a new tran-

sient discharge is formed during each half cycleof the sustaining voltage pulse. The pixel is turnedoff by applying a so-called ‘erase pulse’ betweenthe initiating and sustaining electrodes, whichdestroys or removes the surface dischargedistribution.When the pixel is on, pulses of UV photons

reach the phosphors with a frequency twice thesustaining frequency(see above), which results intime-averaged intensities of the emitted visiblelight. The intensity of the pixel can be determinedby controlling the on and off times of the pixel.Plasma display panels(PDPs) were invented in

1964 by Bitzer and Slottoww150x. In 1971, amonochrome display with 412=512 pixels(30 cmdiagonal) had been developed. In the late 1980s,a monochrome display with 640=480 pixels(25cm diagonal) was produced for the emerginglaptop computer market and 500 000 units per yearwere shipped. This later dropped to 250 000 unitsper year, due to the penetration of the laptopmarket by liquid crystal displays(LCDs). In the1990s, research and development has concentratedon color displays with 90–180 cm diagonal, forflat panel high definition television. There has


been an explosive growth of investments in thelast 10 years, mainly in Asia. Most of the world’smajor TV manufacturers have been involved inthe development of PDPsw62x. In 1998, alreadyapproximately 50 000 PDPs of 1-m diagonal weresold. The market volume is expected to increaseto 5 million sets in the year 2005w42x.There are several reasons why plasma displays

are attractive for flat panel TV-screensw150x. Dueto the V–I characteristic of the plasma dischargeat each pixel, there is a very strong non-linearity:for a pixel voltage less than some threshold volt-age, there is no light output. This implies that onecan matrix address the pixels in very large sizearrays, e.g. a screen with 2048=2048 pixels and105 cm diagonal. Moreover, a pixel, once turnedon, stays on until commanded to turn off, and viceversa. This leads to high brightness, flicker-freedisplays. A lifetime exceeding 10 000 h has beendemonstrated for color PDPs. Moreover, PDPshave wide viewing angles. A decisive advantageis also the fabrication technology for PDP displays:most of the structures and films required to fabri-cate a display are screen-printed on inexpensive(glass) substrates, rather than using(expensive)photolithography on expensive substrate materialsw62x.A disadvantage at this moment is that PDPs are

still characterized by very low luminous intensi-ties: ca. 1 lmyW, compared to ca. 4 lmyW ofconventional CRTsw152x. The overall efficiencyis only 0.5%, because it is the result of fourcombined efficiencies:(i) the discharge efficiencyfor UV photon production is approximately 10%;(ii) the efficiency of UV photon capture by thephosphors is approximately 40%;(iii ) the efficien-cy of UV to visible light conversion is approxi-mately 30%; and(iv) the efficiency of visiblephoton capture is approximately 40%w152x. Theseefficiencies will have to be increased considerably,if PDPs really want to compete in the future withconventional TVs. Another issue that still needsimprovement is the writing and erasing of thepixels: this should be reliable and — in view ofthe cost price — not require too high drivingvoltages w152x. Last but not least, large colordisplays need to be produced at high volume, inorder to reduce their price considerably and to

make them competitive with conventional TVsw62x.

3.3.2. Plasma addressed liquid crystal (PALC)technologyBesides PDPs, plasma addressed liquid crystals

(PALCs) are possible candidates as new types ofTVs and monitorsw152x. The PALC technology isa variation of the LCD(liquid crystal display)technology, which is nowadays widely used, e.g.in laptops.Similar to LCDs, PALC displays are essentially

‘passive’, i.e. they do not generate light, but theymodulate the light generated by a source behindthe display (‘backlight’) by making use of theunique optical properties of liquid crystals. Indeed,a liquid crystal is an electro-optical material: itchanges the polarization state of the light whichpasses through it, and the change in polarizationdepends on the electric field applied to the liquidcrystal. In LCDs, the light from a source firstpasses through a polarizing layer, then it passesthrough a liquid crystal layer, and finally througha second polarizing layer. The amount of light thatpasses through the second polarizing layer dependson the electric field in the liquid crystal. The latteris independently controlled by an electric switchfor each display element(pixel). In conventionalLCDs, these switches are typically thin film tran-sistors(TFTs). However, the TFTs limit the max-imum size of the LCDs, so that the latter will notbecome suitable for large TV screens.In PALC displays, the pixels are turned on and

off by plasma switches, which can be more easilyfabricated. This permits the production of largedisplays, which makes them possible candidatesfor large TV screens. Fig. 24 shows a typicalPALC panel. The LC and the protecting microlayerare placed between two glass plates. The backplate contains parallel channels filled with a dis-charge gas(mostly helium or a helium-basedmixture, at a pressure of 0.1–0.3 atm) w152x. Thechannels correspond to the picture rows of thedisplay. Two narrow parallel electrodes are placedat the bottom of every channel. The front glassplate consists of patterns formed by the so-calleddata electrodes, i.e. transparent, conducting strips


Fig. 24. Schematic representation of a plasma-activated liquid crystal(PALC). The liquid crystal(LC) is placed between two glassplates. The lower(back) plate contains parallel channels filled with a discharge, which correspond to the picture rows of the display.The upper(front) plate consists of patterns formed by the so-called data electrodes, which correspond to the picture columns of thedisplay.

Fig. 25. Schematic representation of the laser action in a He–Ne laser. In a first step, the He atoms are excited by electron impactexcitation. In a second step, the excited He* atoms exchange energy with ground state Ne atoms, thereby populating excited Ne*atoms in 2s or 3s levels, which are the upper levels for laser action. This clearly illustrates the combination of electron kinetics(asa precursor for the application, in step 1) and heavy particle kinetics(for the real application, in step 2).

of indium–tin-oxide(ITO), which correspond tothe picture columns of the display.A pixel is formed at the intersection of a channel

and a data electrode. The picture on a display is

switched on row by row, i.e. all pixels of one roware activated at the same time. To activate a certainrow, a discharge is created in the correspondingchannel by applying a short d.c. voltage pulse


between the channel electrodes. During the after-glow of the discharge, small voltages are appliedbetween channel and data electrodes, which rep-resent the data that have to be ‘written’ on thepixels of that row. The extinguishing plasma inthe channels screens itself from the resulting elec-tric fields by accumulating a surface charge on themicrolayer. The plasma keeps building up thesurface charge until the fields in the channel havedisappeared, and the data voltages are present overthe entire LC layer and microlayer. When theplasma is completely extinguished, the surfacecharge remains constant. The electric fields in theLC and the transmission through the second polar-izer will now remain constant, until the nextdischarge pulse is applied at the channel.The PALC technology was invented at Tektron-

ics in 1990 w154x. At present, several displaymanufacturers are involved in the development ofPALC displays. The major problem of the currentPALC technology is the addressing speed. ThePALC discharges should ignite quickly and have ashort decay time. Indeed, if the plasma is notentirely gone by the end of the addressing time,charging errors will occur. In addition, it is impor-tant that the charging of the microsheet is accurateand as uniform as possible: inhomogenities in thecharging translate directly into a loss of contrast.Another problem is the lifetime of the displays,which might be limited by ion-induced sputteringof channel electrode material. Moreover, the PALCdisplays should also not emit stray light. Finally,in view of the cost price of PALC displays, it isimportant that the required driving voltages arenot too high and that the microdischarge geometrycan be manufactured easilyw152x.

3.4. Lasers

Gas discharges are also used for laser applica-tions, more specifically as gas lasers. Several kindsof gas lasers exist(see below), but they have onecommon characteristic, i.e. the mechanism of pop-ulation inversion, necessary for laser action, alwaysoccurs via gas discharges. The gas, at reducedpressure, is contained within a glass discharge tubewith mirrors at the ends of the tube. Anode andcathode can be placed at both ends of the tube.

Alternatively, the cathode can have a hollow cath-ode geometry(see also below), with anode ringsat the ends. Generally, three classes of gas laserscan be distinguished, depending on whether thelasing transition occurs between energy levels ofatoms, ions or moleculesw155x.

3.4.1. Atomic lasers

3.4.1.1. He–Ne laser. The He–Ne laser is one ofthe most-used lasersw155x. As predicted by thename, the active medium is a mixture of heliumand neon in a typical ratio of 10:1. It operates ina glow discharge tube of a few mm diameter anda length of 0.1–1 m. The total pressure is approx-imately 1000 Pa. The lasing transitions take placebetween the energy levels of neon. Several differ-ent transitions are possible, all starting from the 3sand 2s levels. Excitation of these upper laser levelsoccurs in two steps(see Fig. 25):

1. electron impact excitation of helium: eqy

He™e qHe*; andy

2. followed by energy exchange between He* andNe: He*qNe™HeqNe* (in 2s or 3s levels).

This is a typical example of combined electronexcitation as creation process(step 1) and heavyparticle kinetics to realize the application(step 2)as discussed in the Introduction.The three main transitions are at 3.39mm, 1.15

mm and 632.8 nm. Because these transitions haveeither the same upper or lower laser levels, theyare competing with each other. Therefore, themirrors should be selective, by being highly reflec-tive only at the desired wavelength.A disadvantage of the He–Ne laser is that it

gives a low power output(typically 0.5–10 mW),because higher power gives saturation due topopulation in the glow discharge of the lower laserlevels. Advantages are the excellent beam quality,the very narrow laser linewidth and the relativelysmall sizew155x.The He–Ne laser is used for optical alignment,

by producing a visible line which can be used forpositioning an object, for guidance of equipmentin construction(aircrafts, ships), or for the align-ment of other (e.g. IR) lasers, as well as for


interferometry(e.g. in the Michelson interferome-ter). Other applications are in laser printers andoptical disksw155x.

3.4.1.2. Copper vapor laser. The copper vaporlaser(CVL) is based on a glow discharge at hightemperature(wall temperature typically 1400–1500 8C). The discharge tube is typically almost1 m long with a diameter of 2 cmw156x. It isfilled with neon at a pressure of 3000–7000 Pa.Metallic copper is present in a reservoir, and itevaporates due to the high temperature.Lasing occurs at two wavelengths, i.e. at 510.6

and 578.2 nm. The upper levels of both laser linesare the Cu 3d 4p levels, and the lower laser0 10

levels are the Cu 3d 4s levelsw156x. Because0 9 2

the latter are metastable levels with a relativelylong lifetime, the lasing will be short, until thepopulation inversion is destroyed. Therefore, thelaser is also called ‘self-terminating CVL’. Typi-cally, it takes 25ms to deactivate the lower laserlevels, and to produce new laser action. Hence,the CVL is operated in the pulsed mode, with apulse repetition frequency in the order of 1–100kHz. The average power output is 10–100 W. TheCVL has a high gain(1% per mm), and theoverall efficiency is rather high(up to 2%) w155x.

The CVL has found application in medicine(dermatology and oncologyw157x), as pumpingsources for dye and Tiysapphire lasers for tunableoutput w158x, and for industrial materials process-ing w159x.

3.4.2. Ion lasers

3.4.2.1. Argon ion laser. In conventional argon ionlasers, the active medium is the positive columnregion of a high current density argon glow dis-chargew160x. The mechanism for laser activationtypically occurs in two steps:(i) ionization ofargon, and(ii) excitation of Ar w155x. Becauseq

of the two-step process, and because of the highenergies required for ionization and excitation(i.e.15.76 eV for ionization, and typically 19.68 eVfor excitation of the ions to the upper laser level),the positive column argon ion laser has a lowpower efficiency(typically 0.05%) w160x. Hence,the glow discharge must be very intense, with an

electrical dissipated power in the kW rangew155x.This requires considerable engineering skills. Thelaser efficiency can, however, be increased byapplying a magnetic field along the tube axisw155x. Alternatively, an electron beam with energyclose to the peak of the direct excitation crosssection has been used to directly excite the argonion upper laser levelsw161x.Several laser lines are possible between 454 nm

and 529 nm, corresponding to 4p–4s transitions.Hence, the laser can be tuned to one specificwavelength, if needed. The two most intense linesare at 488 nm and at 514.5 nm.Argon ion lasers are used for laser printers,

optical disks and for Raman spectroscopy. More-over, they are also used in medicine, for thetreatment of detached retinas(ophtalmology). Theradiation is strongly absorbed by red blood cellsand the resulting thermal effects lead to a re-attachment of the retinaw155x.

A variation to the argon ion laser is the kryptonion laser, which has a lower gain and is lesspowerful, but on the other hand, it has an evenbroader wavelength range(between 337 and 800nm, with the most intense laser line being at 647nm), and it is often attractive for this reasonw155x.

3.4.2.2. Metal-vapor ion lasers. The metal-vaporion lasers(MVILs) are probably most similar toanalytical glow dischargesw162–165x. They oper-ate in a rare gas(mostly helium or neon), at apressure of 100–1000 Pa. The metal vapor istraditionally introduced by thermal evaporation.For some metals(e.g. Cu, Ag, Au, etc.) thetemperature has to be very high, which may leadto technical difficulties for the development ofMVILs. Therefore, the metal vapor is sometimesobtained from volatile compounds of the metal(e.g. CuBr, CuCl) which are dissociated in theplasma by electron impact, after which the metalatoms can be excited. In the case of Cu, thetemperature can in this way be reduced from 15008C to 400–6008C w163x. Another way to introducethe metal vapor into the discharge is by sputtering,as in analytical glow discharges. This typicallyoccurs in hollow cathode discharge lasersw166x.The main advantage is that lower temperatures canbe used, which simplifies the experimental design.


A disadvantage, however, is that the dischargecurrent and metal vapor pressure cannot be con-trolled independently of each other, because themetal vapor pressure is determined by the amountof sputtering, which is defined by the dischargecurrent. The metal vapor pressure varies typicallybetween 0.1 and 100 Pa.A large number of different elements have been

considered as metal vapor. Experimentally, approx-imately 300 ion laser lines have been found alreadyw163x, in more than 30 different elements, e.g. Be,B, C, Mg, Al, Si, P, S, Ca, Cr, Ni, Cu, Zn, Ge,As, Se, Br, Sr, Ag, Cd, In, Sn, Te, I, Ba, Eu, Yb,Au, Hg, Tl, Pb and Biw162x. MVILs can operatein the positive column of d.c. glow discharges, inhollow cathode discharges(HCDs) or in cc rfdischarges.Depending on the buffer gas–metal vapor com-

bination, and on the laser geometry, differentplasma processes can be responsible for the pop-ulation inversion(note the analogy with processesin analytical discharges).

● Electron impact excitation from the metal vaporion ground state to the resonant excited levels.The latter are the upper laser levels, whichdecay to lower, mostly metastable levels.Because the metastable levels cannot sponta-neously decay to the ground state, lasers basedon this mechanism are called ‘self-terminatinglasers’. They operate only in the pulsed mode,because the lower laser levels have to bequenched in between the pulses by collisions,before new laser radiation can be emitted dur-ing the following pulse. This type of popula-tion-inversion occurs mainly with pulsedpositive column lasers, e.g. He–Cd , Ne–Cuq q

w162,167x.● Penning ionization of the metal vapor by col-

lisions with the buffer gas in a metastable state.This process plays a role in the He–Cdq

cataphoretic laser(see below).● Asymmetric charge transfer between the rare

gas ions and the metal vapor atoms, yieldingmetal vapor ions in excited levels. This processrequires good energy overlap between the excit-ed levels of the metal ion and the rare gas ionground state. Hence, it is a very selective

process, which will give rise to different laserlines of the same metal vapor, depending onthe buffer gas. This process is probably themost important for population inversion inhollow cathode lasersw164x.

The combination of a high average output pow-er, an excellent beam quality and laser lines in theUV range make the MVILs promising for use inthe industry for precision-material treatments(e.g.in the semiconductor industry) w163x. SomeMVILs are already commercially successful, suchas thecataphoretic He–Cd ion laser in the d.c.q

positive column geometry. The most importantwavelength is at 441.6 nm, where output powersof 200 mW are commercially availablew163x. Therelatively short wavelength, low price, simple con-struction and relatively low operating temperatures(300–3508C), make this laser suitable for appli-cations such as fluorescence diagnostics, photo-lithography, photoresist technology, high-speedlaser printing and laser microscopyw162,163x.

Another commercial He–Cd ion laser is theq

so-called‘white-light’ laser w168x. It operates inthe hollow cathode geometry, and the principalCd laser lines, at 636.0 nm, 533.7–537.8 nmq

(doublet) and 441.6 nm, lie sufficiently close tothe ‘ideal’ primary colors(at 610, 540 and 450nm) to be used for ‘white-light’ applications, e.g.color holography, laser television, medical diag-nostics, color printingw162,163x. In spite of theadvantages of the hollow cathode geometry com-pared to the positive column geometry as a moreefficient excitation medium to produce white light,the technical difficulties involved in producinghollow cathode lasers with long lifetimes are stillquite large. Hence, there is only one producer ofwhite-light He–Cd lasers, with output powersq

reaching 30 mWw163x.Other hollow cathode MVILs (e.g. Ne–Cu ,q

He–Ag , He–Au ) are also promising for severalq q

applicationsw164x, mainly because of their far UVtransitions(200–300 nm), e.g. to save informationon photomaterial with high resolution and at highspeedw162x. An important characteristic of MVILsis their excellent beam quality. At the moment,excimer lasers(KrF, ArF), which also emit laserlines in the far UV range(;200 nm), are fre-


Fig. 26. Schematic representation of excimer energy levels:(a)unstable ground state dimer, and(b) stable excited dimer(exci-mer). The population of the excited dimer state does not occurvia the dimer ground state, but it occurs typically in a two-stepprocess(e.g. electron attachment followed by positive–nega-tive-ion recombination). Population inversion, necessary forlaser action, is easy to achieve because the dimer ground stateis unstable.

quently used in microlithography. However, thereare strong requirements for the beam quality, inorder to obtain the optimal spatial resolution.MVILs could serve as alternatives for the emissionof UV-radiation, with excellent beam quality.Because they have lower intensities than the exci-mer lasers, they could be amplified to the desiredpower by excimer amplifiers(so-called metal-vapor excimer hybrid lasers) w169x.

3.4.2.3. Recombination lasers. Finally, we wouldalso like to mention here recombination lasers, inwhich the lasing transition can occur betweenenergy levels of either atoms or ions. It is initiatedby electron-ion recombination, which populates ahighly excited atomic or ionic level. The latterdecays to a lower level, which corresponds to thelaser transition, typically at low wavelengths.

3.4.3. Molecular lasers

3.4.3.1. CO laser. The CO laser operates in a2 2

low-pressure d.c. discharge, in a mixture of He,CO and N w170,171x. The laser transition occurs2 2

between vibrational energy levels of CO . The2

excitation is a two-step process and occurs via N2

vibrational levels. The efficiency of this laser isvery high (typically 30%). The high efficiencyresults from the low energy necessary for excita-tion (only a few eV, compared to 20–30 eV forthe He–Ne and Ar ion laser). Moreover, the N2excited levels are metastable, which ensures effi-cient energy transfer to CO . The presence of2

helium as the buffer gas further enhances theefficiency, e.g. by deactivating the lower laserlevels, and hence improving the populationinversion.A large number of transitions occur between 9.2

and 10.8mm, with the strongest band at 10.6mm.This means that normal optical materials, such asglass or quartz, cannot be used for componentssuch as Brewster windows, because they exhibitvery high absorption at this wavelength. Hence,some alternatives have to be used, such as alkalihalides, gallium arsenide and germaniumw155x.The CO laser is a very versatile laser. It is2

available with a wide range of output powers(continuous power typically tens of kW) and at

reasonable costw155x. This makes it important fortechnological applications(e.g. welding and cut-ting of steel, pattern cutting, laser fusion, etc), aswell as for medical applications(e.g. as cuttingtool for surgery) w155x.

3.4.3.2. N laser. Unlike the CO laser, the lasing2 2

transitions in the N laser occur between electron2

energy levels of N , and consequently, they are in2

the UV range(at 337 nm). Because the lowerlaser level has a longer lifetime than the upperlaser level, only short laser pulses are possible,before self-termination of the laser occurs.

3.4.3.3. Excimer lasers. An ‘excimer’ is anexciteddimer. It is a diatomic molecule, which is stablein the excited level, but unstable in the groundstate(see typical potential energy curves in Fig.26). Hence, excitation does not occur via the dimerground state, but in a two-step process, e.g. forArF:

● electron attachment: eqF ™F qF; andy y2


● followed by positive–negative-ion recombina-tion: Ar qF ™(ArF)*.q y

Because the dimer ground state is unstable,population inversion is easy to achieve. Pumpingof excimer lasers occurs mostly in gas discharges,but it can also be done in an electron or a photonbeamw155x. Excimer formation is more effectiveat high pressure(i.e. high collision frequencies)and also when efficient excitation and ionizationof the precursor species takes place, hence at non-equilibrium high-pressure gas discharge conditionstypical of DBDs w42x.A large number of excimer lasers exist, with

output wavelengths between 120 and 500 nm. TheXeF and KrF lasers are very efficient(up to 10–15%), and high powers can be obtained. Thismakes them particularly useful for the pumping ofdye lasersw155x.

3.4.3.4. Chemical lasers. Chemical energy can alsobe used to create population inversion, i.e. whenthe products of a chemical reaction are producedin an excited state. In most chemical lasers, thereaction products are formed in highly excitedvibrational levels. An example is the continuoushydrogen fluoride(HF) chemical laser, which iscreated in a SWD, in a mixture of SF , He and6

O . The fluorine atoms resulting from the dissoci-2

ation of SF react with H molecules, which are6 2

directly injected into the cavity(FqH ™HFq2

H). HF molecules in vibrationally excited statesare formed, and population inversion may beobtained between these states and the ground state.Laser action has been observed in the range of2.5–3.4mm. The role of helium consists in pro-viding electrons with rather high energy, whichfacilitates the dissociation of SF by electron6

impact collisions.

3.5. Ozone generation

Ozone(O ) generation is also a typical appli-3

cation of DBDs or high pressure GDsw41,42,45,48,49x. Ozone can be generated fromoxygen, air or from other NyO mixtures. The2 2

first step towards ozone formation in gas discharg-es is the dissociation of O molecules by electron2

impact and by reactions with N atoms or excited

N molecules, if nitrogen is present. Ozone is then2

formed in a three-body reaction involving O andO .2In recent years, considerable progress was made

with respect to attainable ozone concentrations andenergy consumption. Ozone concentrations up to5 wt.% from air and up to 18 wt.% from technicaloxygen can now be reachedw42x. Large ozonegenerating facilities produce several hundred kgozone per hour at a power consumption of severalmegawatts. With modern technology, ozone canbe produced at a price less than 2 US$ykg w42x.The main applications are in water treatment andin pulp bleaching. Applications in organic synthe-sis include the ozonation of oleic acids and theproduction of hydroquinone, piperonal, certain hor-mones, antibiotics, vitamins, flavors and perfumesw42x.

3.6. Environmental applications

In the above application, DBDs serve as chem-ical reactors to produce ozone. A similar use, butwith different reactions, makes plasmas(both ther-mal and non-thermal) interesting for environmentalapplicationsw172,173x. The thermal plasmas most-ly used for this purpose include several kinds ofarcs, as well as ICPsw174,175x. Thermal plasmasoffer some unique advantages for the destructionof hazardous wastes compared to classical com-bustion. The high energy density and temperaturesassociated with thermal plasmas and the corre-sponding fast reaction times offer the potential oflarge throughputs in a small reactor. Moreover, theuse of electric energy reduces gas flow needs andoff-gas treatment requirements, and offers controlover the chemistry. Finally, thermal plasmas caneasily be integrated into a manufacturing processwhich generates hazardous wastes, thus permittingthe destruction of the wastes at the source. Thermalplasmas are widely applicable for the destructionof noxious compounds, either in the form ofcomplex mixtures like mixed liquid wastes, in theform of contaminated soil or sludge, or in the formof mixed liquid and solid waste. Organic com-pounds can be destroyed with high efficiency,metals can be recycled, and heavy metals and lowlevel radioactive materials can be vitrified in a


non-leachable slag. Because the present paperdeals mainly with non-thermal plasmas, we willnot go into more detail here, but more informationcan be found in refs.w172–175x.

The non-thermal plasmas used for environmen-tal applicationsw172,176x are mainly high-pressuredischarges, such as DBDsw42,177x, pulsed coronadischargesw178,179x and dielectric packed bedreactorsw180x, as well as electron beam sustainedplasmasw181x. An increasing number of investi-gations are devoted to the decomposition of nitro-gen and sulfur oxides in flue gasesw182x, and ofvolatile organic compounds(VOCs) emanatingfrom various industrial processesw183x. Manyhazardous organic compounds are readily attackedby exciting species, free radicals, electrons, ionsandyor UV photons generated in DBDsw42x.Moreover, investigations are going on to use DBDsfor the generation of H and elemental sulfur from2

H S and for the conversion of the greenhouse2

gases CO and CH to syngas or liquid fuelsw177x.2 4

The removal of hazardous organic pollutantsfrom waste water is also a growing issue inenvironmental research. In particular, pulsed coro-na discharges, DBDs and contact glow dischargeelectrolysis techniques are being studied for thepurpose of cleaning waterw184x. A well-estab-lished means of water purification is by ozonetreatment. The ozone synthesis typically takesplace in electrical discharges. However, electricaldischarges in aerated water are also possible, andthey produce some other strongly oxidizing agents,such as OH , H , O , O and H Ow184x. Therefore,• • •

3 2 2

instead of ‘ex situ’ electrical discharges for ozoneproduction, the ‘in situ’ electrical discharges inwater may provide a means to utilize most of thesechemically active species for water purificationw178,184x. Moreover, the strong electric fields ofelectric discharges are also lethal to several kindsof microorganisms in waterw185x. Furthermore,the electrical discharges in water may also produceUV radiation, which helps in the destruction ofpollutantsw186x.

Besides these high pressure discharges, reducedpressure discharges, such as SWDs, have alsoproven their suitability for environmental applica-tions, e.g. for the transformation of volatile organiccompounds(VOCs) detrimental to health, and

molecules utilized in the micro-electronics indus-try, such as SF and C F , which strongly contrib-6 2 6

ute to the green house effectw87x. Arno et al.w187x describes the detoxification of chlorinatedhydrocarbon pollutants(trichloroethylene) in aSWD in oxygen or airywater mixture. It was foundthat the trichloroethylene conversion was limitedto light gases, primarily CO , CO, HCl and Cl .2 2

This is believed to result from the relatively shortresidence times, which limit the number of slowmulti-step mechanisms essential in the formationof non-parent chlorohydrocarbon species. Thisdemonstrates the interest in using SWDs whereone can optimize the plasma column length, andthus, control the residence timew187x.

3.7. Biomedical applications

Another increasing application field of gas dis-charges, mostly at atmospheric pressure, is forbiomedical applications. Plasmas are used toimprove the biocompatibility of materialsw129x.Surfaces of biomaterial polymers employed inbiomedical products are typical materials modifiedby plasma treatmentw4,129x. Biomaterials arematerials used in contact with body fluids, butforeign to the body. They are, for example, usedfor medical implants(vascular prostheses, cathe-ters, etc.), for contact lenses and for cell culturingcontainersw4,129x.

Plasma sterilization of biological samples, main-ly with APGDs, is also gaining increasing interestin the healthcare industryw37,38,188–190x. Itfunctions near room temperature and is muchshorter than the 12-h period characteristic of con-ventional methods such as autoclaving and ethyl-ene oxide exposure. It has been demonstrated thata plasma exposure time of several minutes leadsto a reduction in the population of microorganisms(such asEscherichia coli bacteria andStaphylo-coccus aureus) by six orders of magnitudew188x.

3.8. Particle sources

Finally, because several kinds of plasma speciesare present in the plasma(electrons, ions, excitedatoms and ions, radicals, molecules, etc.), some of


the plasmas described above have also been usedas a source for specific particles.SWDs have been used to provide heavy(kryp-

ton) ions for a joint French–Soviet space missionw191,192x, as part of a remote chemical analysisexperiment at the surface of the Martian moonPhobos w193x, by long distance(50 m) SIMS.Other applications include the use of rare gasmetastable atoms present in gas discharge plasmas,for Penning ionization electron spectrometryw194x,and the remote plasma processing with e.g. N-atoms. The latter are created in the spatial after-glow of N discharges, where there is no electric2

field and little charged particlesw195,196x.Because N-atoms are long-lived, they can be trans-ported outside the plasma by the gas flow. Becausethey have a much higher reactivity than the groundstate molecules, they can induce chemical surfacereactions for thin film deposition and surfacemodifications.

4. Conclusion

As shown above, gas discharge plasmas exist ina large variety of excitation modes. The classicalconfiguration, a d.c. glow discharge operatingeither at reduced or atmospheric pressure, is stillwidely used. However, new excitation modes, suchas powering with an rf generator(either in thecapacitive or inductive configuration), applying amagnetic field or microwave power, etc, have beendeveloped to increase the plasma density and theefficiency of power absorption.These different types of plasmas are used in a

wide range of growing application fields. Surfacemodification (both in the semiconductor industryand for materials technology) is probably the mostimportant application field. Plasma processesappear to have some distinct advantages comparedto conventional(wet chemical) processes. A num-ber of different plasma technologies are essentialto different steps in the fabrication of ICs. The useof plasmas as lamps, more specifically fluorescentlamps, is probably the oldest application. Nowa-days, new types of so-called electrodeless lampsare being developed, their main advantage being alonger lifetime because damaging of the electrodes(e.g. by sputtering) is avoided. An application

which attracts a lot of interest from a broaderpublic is the use of low temperature plasmas fordisplays, to be developed as large and flat televi-sion screens, either directly as plasma displaypanels, or indirectly as plasma switches for liquidcrystal displays. However, in order to becomecompetitive with the conventional(cathode raytube) TV technology, the luminous efficiency ofplasma displays still has to be considerablyincreased, and their price has to be lowered.Another application is in laser technology. A largevariety of gas discharge lasers exist, based on lasertransitions between energy levels of atoms, ions ormolecules. Finally, we have also outlined someemerging applications of(mainly) high-pressuregas discharges(DBDs, APGDs) as ozone genera-tors and for environmental and biomedicalapplications.

Acknowledgments

A. Bogaerts is indebted to the Flemish Fund forScientific Research(FWO) for financial support.The authors also acknowledge financial supportfrom the Federal Services for Scientific, Technicaland Cultural Affairs(DWTCySSTC) of the PrimeMinister’s Office through IUAP-IV(Conv. P4/10).Finally, we would like to thank E. Dekempeneer,O. Goossens, C. Leys and M.C.M. van de Sandenfor supplying us with useful information.

Appendix A: List of acronyms:

ac: alternating currentAPGD: atmospheric pressure glow dischargeAZ: anode zonecc: capacitively coupledCDS: cathode dark spaceCMP: capacitive microwave plasmaCRT: cathode ray tubeCVD: chemical vapor depositionCVL: copper vapor laserDBD: dielectric barrier discharged.c.: direct currentDLC: diamond-like carbonECR: electron cyclotron resonanceEEDF: electron energy distribution functionFDS: Faraday dark space


GD: glow dischargeHCD: hollow cathode dischargeHID: high intensity dischargeHPM: high pressure mercuryHPS: high pressure sodiumIC: integrated circuitICP: inductively coupled plasmaIR: infra redLAPPS: large area plasma processing systemLCD: liquid crystal displayLHP: left-handed circularly polarizedLTE: local thermal equilibriumMERIE:magnetically enhanced reactive ion

etcherMIP: microwave induced plasmaMPT: microwave plasma torchMSP: microstrip plasmaMVIL: metal vapor ion laserPALC: plasma addressed liquid crystalPC: positive columnPDP: plasma display panelPE-CVD: plasma enhanced chemical vapor

depositionPIII: plasma-immersion ion implantationPSII: plasma source ion implantationrf: radio-frequencyRHP: right-handed circularly polarizedSD: surface dischargeSIMS: secondary ion mass spectrometrySWD: surface wave dischargeTIA: torche a injection axiale`TFT: thin film transistorTV: televisionUV: ultravioletVD: volume dischargeVIS: visibleVOC: volatile organic compoundsVUV: vacuum ultraviolet

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Gas Discharge Plasmas and Their Applications

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