Laser assisted particle removal from Silicon wafers

Mosbacheret al.,preprint,ParticlesonSurfaces7,K.L.Mittal(Ed.),2001 1

Laser assisted particle removal from Silicon wafers

M.MOSBACHER, 1;4;

H.-J.M



UNZER, 1

M.BERTSCH, 1

,V.DOBLER, 1

N. CHAOUI, 2

J.SIEGEL, 2

R.OLTRA, 3

D.B



AUERLE, 4

J.BONEBERG 1

andP.LEIDERER 1

1

UniversityofKonstanz,DepartmentofPhysics,FachM676,D-78457Konstanz,Germany

2

InstitutodeOptica,CSIC,Serrano121,28006Madrid,Spain

3

LaboratoiredeRecherchessurlaReactivitedesSolides,UMR5613CNRSLaboratoirede

PhysiqueUPRES-A5027,UniversitedeBourgogne,BP47870,21078DijonCedex,France

4

Johannes-Kepler-UniversitatLinz,InstitutfurAngewandtePhysik,A-4040Linz,Austria

Abstract{Wehavestudiedtheremovalofsubmicrometerparticlesfromsiliconwafersby

the steamlasercleaning (SLC)and drylasercleaning(DLC)processes. Theseprocesses

are currentlybeing investigated as newpromising cleaning technologies for complemen-

ting traditional methods inindustrial applications. For SLC a thinliquid layer(e.g. a

water-alcohol mixture) is condensed onto the substrate, and is subsequentlyevaporated

byirradiatingthesurfacewithashortlaserpulse. TheDLC process,ontheotherhand,

reliesonlyonthelaserpulse,withoutapplicationofavapor jet. Usingwell-characterized

monodispersepolystyreneandsilicaparticlesaswellasirregularlyshapedaluminaparticles

withdiametersdownto60nmwehavesystematicallyinvestigatedtheeÆciencyofthetwo

processes. Theinuenceof laserpulsedurationfrom thenanosecond tothe femtosecond

range was studied. Forthe DLCwe wereable to measure the acceleration ofthe silicon

surface due to thermal expansionfor DLC. Our results demonstrate that for the gentle

cleaning of silicon wafersthe SLC is a veryeÆcient methodand for particlessmaller in

diameter than 400nmit is superiorto DLC. This is dueto lower cleaning thresholdsin

laseruenceforSLCcomparedtoDLCfortheremovalofsmallparticles. DLCmaycause

serious surfacedamage by eld enhancement underthe contaminants,aneect thathas

onlyrarelybeentakenintoaccountinlasercleaningstudiessofarandisalsodiscussedhere.

Keywords: Particleremoval;lasercleaning;eldenhancement;cleaningmechanisms

1 Introduction

In the rapidly growing eld of nanotechnology the production of extremely clean

surfaces is one of the fundamental requirements that haveto be fullled in order

to increasetheintegrationdensitiesofdevices. Contaminations causedbyparticles

are responsibleforproductionlossesormalfunctionsin thiseld [1]. Thecleaning

of silicon wafers, the key material for micro- and nanoelectronics, therefore, is of

considerableinterestanddierentcleaningstrategiesarecurrentlybeinginvestigated

fortheircapabilitytoremovesubmicrometerparticles[2].

Thereareseveralrequirementsthat haveto befullled bysuch acleaningstra-

tegy: with the linewidth of ICs reaching 130nm by the end of the year 2001 [3]

particles with diameters larger than 60nm have to be removed. Any damage of

the substrate surfaceeither in topography or in theelectronic structurehas to be

avoidedstrictly. Theprocessshould be environmentallyfriendly,cost-eectiveand

should allowahighthroughput.

0

Towhomcorrespondenceshouldbeaddressedbyemail(mario.mosbacher@uni-konstanz.de),

telephone(+49-7531-882627)orfax(+49-7531-883127)

Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2007/2967/

URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-29675

fulll theserequirements. Recentlyitwasshownthatparticles(Al

2 O

3 ,SiO

2 ,MgO,

SiC, CeO

2

, BC,polystyrene)withdiameters assmallas100nmcanberemovedby

thismethodeÆciently[4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22].

Twogeneralconceptsforlasercleaningofparticleshavebeendevelopedduring the

last years. The rstone iscalled "Drylaser cleaning"(DLC) [4, 5, 6, 7, 8, 9, 10,

11, 12]. Here thesurfaceto becleaned isirradiatedbyashortlaserpulse anditis

assumedthatthermalexpansionofthesubstratesurfaceoroftheparticleduetothe

absorptionofthelaserenergyplaysthemajorroleinthecleaningmechanism. This

thermalexpansionisthoughttoacceleratetheparticlesandtoleadtoinertiaforces

strongenoughtoovercometheadhesionforcesactingontheparticles. Inthesecond

concept,"Steamlasercleaning"(SLC)[11,12,13,14,15,16,17,18,19,20,21,22],

a liquidis condensedonto thesurfacejust before the laser pulse. This liquid acts

asan energytransfer medium. As in DLC theenergy absorption in thesubstrate

leadstoarapidtemperatureincrease,butnowalsotheliquidisheated. Inthiscase

particle removal isgovernedby bubble nucleationat the solid-liquid interfaceand

thesubsequentexplosiveevaporationoftheliquidlmandnotbythermalsubstrate

expansionasinDLC.

Inthis work we will provide asummary of our recentstudies concerning both

techniques. Thesummary willbeinitiatedwith adescriptionof themostrelevant

resultsobtainedconcerningthephysicsunderlyingtheparticleremovalprocess. We

will rst present experimental resultsof surfaceaccelerationmeasurementsduring

theirradiationofsubstrateswithnslaserpulses. Thiswillbefollowedbyadiscussion

oftheeectsofeldenhancementunderneaththeparticles. Theseeectshaverarely

beenconsidered and, aswewill demonstrate,playan essentialrolein determining

the safe limits in terms of pulse duration, particle size and laser uence for the

applicationof laserinducedparticleremovalin "realworld"applications. Thiswill

also allow us to show that sub-picosecond laser pulses are of little use for both

DLCand SLC as in this casethe dominantmechanismfor particleremovalis the

localablationofthesubstrateunderneaththeparticlecausedbyeldenhancement

eects. Thesecondpartoftheresultssectionwill bedevotedtothecomparisonof

thecleaningperformanceofbothDLCandSLC usingnsandps pulses.

2 Experimental

2.1 Sample preparation

In our quantitative studies on the cleaning eÆciency we did not use irregularly

shapedparticlecontaminantscommonlyusedinmanylasercleaningstudies(Al

2 O

3 ,

Si

3 N

4

,...),butsphericalcolloidalpolystyrene(PS;InterfacialDynamicsCorporation,

Portland,OR97224USA)andSiO

2

(BangsLaboratoriesInc.,Fishers,IL,USAand

DukeScienticCorp.,2463FaberPlace,PaloAlto,CA94303,USA)particles. These

particles are advantageous for investigations of the underlying physical processes

of laser cleaning due to their narrow size distribution ( 5% for PS, 20% for

SiO

2

)comparedtoirregularparticles. ThisenablesstudiesofremovaleÆcienciesfor

various,well-denedsizes. Theirsphericalshapewillmakeafuturecomparisonwith

theoretical models easier, as adhesionforces of particles are mostly calculated for

thegeometryofasphereonaatsubstrate. Someexperimentswerealsoperformed

usingirregularlyshapedAl

2 O

3

particles(SUMMITCHEMICALSEUROPEGMBH,

Dusseldorf,Germany)ascontaminants.

Figure 1: Typical sample as

used inthelaser cleaningex-

periments. The displayed

areais4.3mx5mand the

particlesizeis480nm.

Germany)that werecleaned in isopropanol(IPA)in anultrasonic bathbefore ap-

plyingthecontaminants.

Theparticlesweredepositedonthesiliconsamplebyaspincoatingprocess. Af-

terdilutionwithIPA(1:2500)theparticlesuspensionwasspunontothesiliconwafer

(2000-3000Rpm). Byadjustingtherotationspeed,theconcentrationofparticlesin

thesuspensionandthetotalamountofsuspensionappliedontoeachsamplewewere

abletopreparesampleswithmorethan95%ofisolatedspheresatparticledensities

1000 percm 2

[21]. A typical examplecanbe seenin Fig. 1where 480nm sized

PS particlesweredeposited onto aSi wafer. The preventionof particleagglomer-

atesonthesamplesisimportantforquantitativeexperiments,assuchagglomerates

complicatetheinterpretationofcleaning results[8]andexhibit adierentcleaning

behaviourcomparedtosingleparticles[19,23].

2.2 Laser Sources

Variouslasersourceswereemployedinthedierentexperiments. Femtosecondlaser

pulses (150fs,800nm)were generatedusingamode-lockedTi:Al

2 O

3

oscillatorcou-

pled to a regenerative amplier. Picosecond pulses (30ps, 583nm) were obtained

using asynchronously-pumped,mode-locked dye laserseedingapulseddyeampli-

er. ThelattersystemwhenseededwithaCWtunabledyelaserallowstoproduce

also2.5nslaserpulsesatthesamewavelength. Othernslaserpulsesourcesweresev-

eralfrequency-doubledNd:YAGlasers(532nm,2.5,6.5,7and8ns),aKrFexcimer

laser (30ns, 248nm) and, nally, an optical parametric oscillator (OPO) pumped

withthethirdharmonicofaNd:YAGlaser. TheOPOwastunedat800nminorder

to allow adirect comparison of the results obtained under ns laser pulses (6.5ns)

andfs pulsesatthesamewavelength.

2.3 Evaluation of the cleaning eÆciency

Theparticleconcentrationonthesamplewasdeterminedbymeasuringthescattered

lightofa5mWHeNelaser(=633nm)illuminatingaspotwithadiameterof<0.5

mm by aphotomultiplier. The cleaning eÆciency wasobtainedby comparing the

DM ⁰⁰

00 00 00 00 11 11 11 11 11

Nd:YAG BS

L Si

PD

P P

IF

A KDP

P

Figure 2: Interferometric measurementof the laser-induced surface displacement.

Thefrequencydoubled Nd-YAGpulseisattenuatedbyglassplates (A)andguided

tothesiliconsubstratebyseveralprisms(P).Aheterodyneinterferometer(IF,B.M.

industries,SH-130,bandwidth200kHz-45MHz)measuresthesurfacedisplacement

of the sample. Thepulse shape is captured by aphotodiode (PD). Displacement

and pulse shapeare recordedon adigitalstorageoscilloscope. A lenswasused to

increasethelaser uenceat thesubstrate.

imaging of the cleaned areas with a microscope and counting theparticles before

and after cleaning weveried that thescattering measurements provided accurate

quantitativedata[21].

2.4 Determination of laser uence

Thedeterminationoflaseruencesforthenanosecondpulsesisdescribedindetailin

ref. [21]. Briey,thelaseruencewasdeterminedrelativetothewellknownmelting

thresholduenceofSimakinguseofthehigherreectivityofthemoltenwithrespect

to thesolidphase. Thiswasdonebysimultaneoustime-resolvedmonitoringof the

reectedlight(probingmelting)andthescatteredlight(eÆciency)oftheHeNelaser

(=633nm)during theexperiment.

Forthepspulses,theuencevaluesweredeterminedbyamethodsuggestedby

Liu[24]whichisbasedonthedetectionofphasetransformationsofsuitablematerials

irradiatedbythelaserpulse. Inourcaseweused[20]athin(approx. 50nm)GeSb

lm on aglass substrate asdescribed in detailin [25, 26]. The melting threshold

of the substrate was determined by measuring its optical reectivity in real-time.

A streakcamera provided picosecond resolution [27]. From these experimentswe

found avaluefor theonset of melting foroperatingourcleaning laser (=583nm,

FWHM=30ps)atapproximately220mJ/cm 2

.

A third method was used in the caseof femtosecond pulses: the beam prole

wasimagedon aCCD cameraandthe totalpulseenergy wasdetermined foreach

pulse. A combination ofthe resultsprovided aspatially resolvedmappingoflaser

uencesin theirradiatedarea. With anoptical microscopewedirectly imagedthe

area where particles were removed and correlatedthis image with themap of the

2.5 Surface accelerations measurement setup

Wedeterminedthesurfaceaccelerationanddecelerationbyderivingnumericallythe

measuredsurfacedisplacementduringirradiation ofasiliconsubstratebyapulsed

laser. Theexperimentalsetupisshowningure2. 600pulseswereaveragedtogive

smooth displacements and the spot monitored by the interferometer was at least

tentimessmallerthantheareairradiatedbytheNd:YAGlaser(=532nm,FWHM

variedfrom10-20ns).

3 Surface acceleration during DLC of silicon

Particleremovalin the DLCprocessis explainedbymost authorsin thefollowing

way: during the laser pulse its energy is absorbed in the substrate. Due to the

subsequentthermalexpansionthe surfaceand theadheringparticle isaccelerated,

theparticle gains kineticenergy (someenergy isalso stored in elasticdeformation

of bothparticle andsurface). At theendof thepulsethe expansionof thesurface

stops andtheparticleleavesthesurfacedueto itsinertia.

Fromamodelingpointofviewthisparticledetachmentcanbedescribedintwo

ways-intermsofforcesorenergies. Inthecaseofforcesonecomparestheadhesion

force to the cleaning force that arises from the deceleration via F = ma. If the

deceleration of thesurface is tooslow (i.e. theadhesion force exceeds the inertia

force)thekineticenergyoftheparticlewillbetransferredtothesubstrateandthe

particle willnot bedetachedevenifits maximumkineticenergyin thelaboratory

frameexceededitsbondingenergy. Amorethoroughwaytomodelthedetachmentis

tocomputethepotentialactingontheparticle. Thispotentialwellcanbeovercome

bythekineticenergyoftheparticleintheframeofthemoving substratesurface.

Fromtheseconsiderationsitisclearthatameasurementofthedynamicsofthe

substrate expansionupon illuminationwith alaser pulse isof vitalimportance for

the understanding oflaser cleaning by thermalsubstrate expansion, and therefore

wemeasured[38]itbyaninterferometer (Fig.2).

Figure 3(a) shows a typical displacement and the corresponding acceleration.

Thesurfaceexpandsduring thepulse andreachesaplateau d

max

(here3.5nm).

ThemaximumdecelerationimportantforthemodelingofDLCisfoundtobea

max

310 6

g.

From a theoretical point of view the surface displacement can be modeled by

calculatingthe1Dthermalexpansionofthesubstrate. LetRdenotethereectivity,

the linearthermalexpansioncoeÆcient,C

P

thespecic heat of thesubstrate,

itsdensity,f(t)thetimenormalizedintensityofthelaserandFitsuencethen(1

R )F R

t

1 f(t

0

)dt 0

istheabsorbedenergyperunitarea. Thisleadstoadisplacement

d(t)=

C

P

(1 R )F Z

t

1 f(t

0

)d t 0

(1)

astheGruneisen parameter=(C

P

)isalmosttemperatureindependent. This

canbederivedfor aknownpulse shapef(t). Thermoelasticeects which account

for thegenerated stress in thematerial [39, 40]modifythis formula by afactorof

2(1+)with thePoissonratio.

Equation(1)foragaussianpulsewithFWHM yieldsamaximumdeceleration

of

a

max

= C10 6

g ns

2

2 F

2

(2)

−0.5 0 0.5 1 1.5 2 2.5 3 3.5 4

0 20 40 60 80 100

−3

−2

−1 0 1 2 3

d [nm] a [10 6 g]

time [ns]

d(t) a(t)

(a)Surfacedisplacementandacceleration

0.5 0.6 0.7 0.8 0.9 1 1.1 1.2

8 10 12 14 16 18 20

− a max / d max [10 6 g / nm]

FWHM τ [ns]

(b) a

max

=d

max

Figure 3: Experimentaldata ofthedisplacementmeasurements. Fig.3(a)displays

thesurfacedisplacementd(t)asmeasuredbytheinterferometerandtheacceleration

a(t) obtainedby numericallyderiving thed(t) signaltwice. Thepeak of thelaser

intensity is located at the center of the graph. The ratio a

max

=d

max

plotted in

Fig. 3(b) can be used to illustrate the inuence of the pulse duration . Shorter

pulsesyieldhigheracceleration.

with astrong1=

2

dependence onthe pulselength andaconstantC with avalue

of4.10including,or1.71neglectingthermoelasticeects.

Asexpectedfromtheory,themeasuredmaximaldisplacementandthemaximal

decelerationarelinearfunctionsoftheincidentlaseruenceF (forapulsewidthof

10ns):

d

max 11:1

pm

mJ/cm 2

F; a

max

10:210 3

g F

mJ/cm 2

(3)

The ratio a

max

=d

max

is, therefore, independent of the energy density F and

canbeused to illustrate theinuence of thetemporal pulse shape. Thisis shown

in gure 3(b) for pulse widths varying between 10 and 18ns using dierent ash

lamp energies. Longerpulses yieldlowerdecelerationsat thesameuenceF. The

dependence on is not as strong as expected for a somewhat idealized gaussian

temporal pulse prole. This can be explained by the non-gaussian shape of the

longerpulsesastheNd:YAGlaser isoperatedat itslimit.

Experimentallywefound a

max

1:2810 6

gat 10ns and100mJ/cm 2

whichis

ingoodagreementwiththevalueobtainedfromeqn.2whenavalueofC=1:71is

chosenandthusthermoelasticeectsareneglected. Thisndingcanbeunderstood

ifonetakesinto accountthat theareawhere thedisplacement isdetectedis much

smallerthentheilluminatedspotand locatedin itscenter. Thusatthisspotthere

arein arstapproximationnolateralthermalgradients.

As shorter pulse widths are expected to yield higher decelarations, and thus

higher cleaning eÆciencies, westudied laser cleaning using ps pulses (see Sec. 6).

Forfs pulsesitisshowninSec.4.2.1thatparticleremovalisduetolocalsubstrate

Figure 4: Calculated

local light intensity

distributions for PS

spheres (n=1.6) with

diameters of 1700nm

(a, d, g), 800nm (b,

e, h) and 320nm (c,

f, i) irradiated at

=800nm. A cross

section seen from un-

derneath the particles

(a-c) and from the

side (d-i) is plotted.

Theambientina)-f)is

vacuum,ing)-i)water.

Thewhitecirclesshow

the transverse section

ofthespheres.

4 Optical eld enhancement in laser cleaning

With respect to afuture industrial application of laser cleaning one fundamental

requirementistoavoidanysurfacedamage,i.e. structuralandtopologicalchanges,

ofthecleanedwafer.Asupperboundfortheapplicablelaseruence,onlythresholds

referringtobaresubstrates(meltingthreshold,changesin wetting)weretakeninto

accountso far,butnotdamagesinducedbythepresenceoftheadheringparticles.

Recently some authors [8, 28] reported a decrease of ablation thresholds for

glass coveredwith irregularlyshapedAl

2 O

3

particles. Forsphericalparticleswith

diameters d larger than the wavelength d > it is known that they may act as

spherical lenses [29]. This was discussed [9, 30] as apossible explanation for the

discrepancy betweentheoretically predictedandexperimentallymeasuredcleaning

thresholds. But also enhancement of the incident optical eld at particles with

d<is expected fromMie-theory,asexperimentallyshownbyKawataet al. [31]

recently. This group used a CW laser for the illumination of colloidal particles

on apolymeric substrate and modied locally the chemical substrate composition

by photochemistry. As these particles are the most interesting in laser cleaning

applications and submicrometer spherical particles are frequently used as model

contaminantsin laser cleaningstudies [7,9, 10, 18, 20,21, 22] weinvestigated the

consequencesoflocaleldenhancementonDLCsystematically[23,32].

4.1 Mie-calculations of eld intensities

Inordertoobtainarstestimationoftheenhancedlaserintensitiesattheirradiated

particlesonasiliconwafer,weperformedcalculationsbasedontheMie-theory[33]

using amodied versionof aprogram describedin [34]. Asa rstapproachthese

calculations havebeen carried outfor dielectric PS spheres(n=1.6) in freespace,

neglectingtheinuenceofthesubstratesurface. Morerecentcalculations[30]take

Figure 5: Holes created byfs pulses in aSilicon substrate at the former positions

of1700nmPSspheres(a),400nmarbitrarilyshapedAl

2 O

3

particles(b)and320nm

PSspheres. In(a)theformerparticlepositionscanbedetectedfromthefaintrings

resultingfrom themarkingbyanadditionallayerofSiO

2

thatwasevaporatedonto

thesamplebeforecleaning. Thedistancein betweentheholesin (b)is400nmand

(c)is about2.75mx2.75min size.

by afactorof about1.5 higher thanthose calculatedfor freespace, therefore, our

resultsprovidealowerbound.

InFig. 4 theresults of these computations are displayed for particles 1700nm

(a, d, g), 800nm(b, e,h)and 320nm(c,f,i)in diameter. Thelocal lightintensity

distributions have been plotted bothin a cross section oriented along the surface

planeofanimaginaryplanelocatedundertheparticle(a-c)aswellasparalleltothe

incoming laserbeam(d-i). Forcomputation wechose alaser wavelengthwechose

accordingtoourexperiments=800nmandthelaserentersfromthetop(d-i). This

resultsin intensity enhancementat thelightaverted sidesof the particlesranging

from2(320nm,d==1.26)to30(1700nm,d==6.68). Ingeneralwithincreasing

d= theintensityenhancementalso increases,however,pronouncedresonancescan

befoundas discussedin [30]. Themaximumintensityislocalized toanareasome

hundreds of nanometers in diameter. One candistinguish two separated maxima

as theyare well-knownfor aHertziandipole. Forparticleswith diameters d,

the resultsfrom Mie calculationscanbeunderstood in a moredescriptive way by

geometricaloptics. However,forparticlescomparabletoorevensmallerindiameter

than the wavelength this approach fails, whereas the Mie-theory is valid for all

particlesizes.

AccordingtotheMie-theorytheenhancementoftheincidentintensitydepends

ontheratiooftherefractiveindexesofparticleandambientmediumand,therefore,

it should be dierent in air/vacuum (a-f, corresponding to DLC) and water (g-i,

correspondingtoSLC).Andindeed, in awaterambienttheintensityenhancement

isonlyabout1/3oftheenhancementinair.

4.2 Surface damage induced by local optical eld enhance-

ment

From the results of the computations described above it is clear that there is a

great riskofdamaging thesurfaceunderneath aparticle dueto theenhancedlaser

intensity, evenifthenominal laseruenceis wellbelowthedamagethreshold ofa

baresubstrate. Therefore,itisofgreatinteresttodeterminethethresholdsforlocal

4.2.1 Irradiation with femtosecond pulses

In a rst set of experiments [32, 35] we irradiated our samples with fs pulses

(=800nm, FWHM=150fs). Heat diusion in the substrate during the pulse was

minimizedduetotheshortpulselength(thermaldiusionlengthsometensofnm)

thusincreasingthelocalpeakintensitiesachievedin thematerial. Toclearlyrelate

anydamagetotheparticles,wepreparedsampleswithhighparticledensity(about

3-4timescomparedtothoseforcleaningexperiments). Theparticles'locationsbe-

forethelasercleaningprocedure weremarkedbyevaporatingaSiO

2

layerof10nm

thicknessonto thesample. Asit is transparentto the laser lightand the waferis

already coveredwith anative oxide layerthis oxidelayershould not inuence the

cleaning processorthe opticalpropertiesof theparticles. Afterparticle removalit

waspossibletodeterminetheirsizeandpositionfromlocalcontrastchangesin the

SEM due to thedierentthicknesses ofthe oxidelayersatand outsidethe former

particleposition.

AtypicalexampleofsuchanirradiatedsamplecanbeseeninFig. 5(a). Under

eachof thePSspheres1700nmin diameter aholewascreatedinthe centerof the

markedareaexactlyat thepointwhere thehighestintensitywaspredictedby the

numericalcalculations. Suchholeswithtypicaldiametersof200-400nmanddepths

ofseveraltensof nanometershavebeenfoundforPSsphereswith diametersdown

to 320nm (see Fig. 5(c)), the smallestsize investigated until now. As the320nm

particles are smaller in size than =2, it is evidentthat geometricaloptics cannot

describetheprocessproperly,andfocusingintheneareldoftheparticlehastobe

takenintoaccount. Andindeeditiseasytoidentifyinthepicturethetwomaxima

in lightintensitypredictedbythecalculationsabove.

Inarealcleaningsituation,however,theparticlesonthesurfacesarenotspher-

icalingeometry. AssomewhatmorerealisticcontaminantswestudiedAl

2 O

3 parti-

clesthatareirregularinshapeandusede.g. asslurryparticlesinpolishingprocesses

ofthewafer. Theseparticlesweredepositedbasicallyin thesamewayasdescribed

inSec. 2. Inadditiontotheproceduredescribedabovetheprocessincludedseveral

stepsofsonicationandsedimentationoftheparticle/IPAsuspensionwhichincreased

thenumberofisolatedparticlesamongthedeposited onestoabout50%. Asitcan

beseenfromFig. 5(b),holeswerealsocreated.

4.2.2 Irradiation with nanosecond pulses

In laser cleaning applications thetypicalpulse length chosen issome nanoseconds

ratherthanfemtoseconds. Withregardtosubstratedamagingbyintensityenhance-

mentat particlesthese longerpulses maylowertherisk ofsurfacedamageasthey

allowheatdiusionin thesubstrateduringthelaserpulse.

Ourexperiments using aNd:YAG laser (=532nm, FWHM=7ns) showed that

holecreationundertheparticlesispossibleevenatnominallaseruencesbelowthe

melting thresholdofthe baresubstrate. Evenforthesmallestparticles that could

beremovedsurfacedamagewasobserved. Thiswill bediscussedin moredetailin

Sec. 5. Preliminary experiments using even longer pulses of aKrF-Excimer laser

(=248nm,FWHM=30ns)forcleaningofSiO

2

spheres(transparentat248nm[36])

have shown asomewhat dierentpicture. Inthis casedepending on the particles

sizetheremayexistintervalsinlaseruencewhereparticlescanberemovedwithout

formation of a hole in the DLC process. Future experiments will provide more

Figure 6: Holes created at the edge of the cleaned region for DLC of 800nm PS

spheres. Theholeformationthreshold forbothfs (a)and ns (b)pulses equalsthe

cleaning threshold. The sizes of the displayed areasare 50mx 40m in (a) and

9mx11min (b)

4.3 Consequences for the laser cleaning process

Therearethreemajorconsequencesfromtheeldenhancementatparticles. First,as

onedealswithenhancedlaserintensitiesunderneaththeparticles,thisenhancement

has to be incorporated into any model that describes the laser cleaning process.

Modelsforthecomputationof thecleaningthresholdsin bothDLCandSLC were

proposedinseveralpublications[4,7,9,22]. Noneofthesemodelstakesintoaccount

theenhancedintensitiesunderneaththeparticles.

The second consequence is a new upperbound for laser uences that may be

appliedin theprocess. Thisboundshould bedictatedbythethresholduencesfor

holecreation,denotedasholethreshold,ratherthanbythethresholdfortheonsetof

meltingofabaresubstrate. Wedeterminedtheseholethresholdsinourexperiments.

Forthe fs DLC weobtained values of 11, 25 and 80mJ/cm 2

for PS sphereswith

diameters of 1700, 800 and 320nm, respectively. Values for the nanosecond case

are discussedin Sec. 5. Theseholethresholds perfectly coincide withthecleaning

thresholdsintheexperimentascanbeseenfromFig. 6. Inthisguretheedgeofthe

cleaned regionapplyinga150fspulse(=800nm,(a))and a7nspulse (=532nm,

(b))intheDLCof800nmPSspheresisdisplayed. Theedgeof thecleaned region

marksthecleaningthresholduence. Itisclearlyvisiblethatinbothcasesholeswere

createdeven atthis edge. Thiscould beseen forfemtosecondpulses and particles

down to 320nm in diameter as wellas for nanosecondpulses at particles down to

800nmindiameter(smalleronestobeinvestigated). Bytakingaseriesofimagesat

thecleaningedge weveriedthatwheneveraparticlewithasizementionedabove

was removed, a hole was created at its former position. After dissolving the PS

spheresin toluene and imagingthecleaning edge again wefound that outside the

cleanedareanoholeswereformed.

Fromthis coincidenceof holeformationwith cleaningthresholdfollowsathird

consequenceofeldenhancementwithregardstoparticleremovalmechanism. Par-

ticle removalin DLCis attributed to the thermalexpansionof the substrate ona

however,suggestsanothermechanism: particleremovalbylocalablationof thesil-

iconsubstrate. A further illustration forthis is givenby acomparison ofthe hole

formationthresholdof25mJ/cm 2

for800nmPSspheresirradiatedwithfemtosecond

pulses (800nm, 150fs) with the ablation threshold of abare surfaceof about 200-

250mJ/cm 2

[37]. Althoughthenominalintensityisfarbelowtheablationthreshold,

theintensityunderneaththeparticleisenhancedbyafactorofabout10(ascanbe

seeninFig. 4)whichisin therangeofablation, henceholesarecreated.

Thislocalablationmechanismisclearlypredominantforthefemtosecondpulses.

Inthenanosecondcaseforthe30nspulsesusingtheexcimerlasertherewereregimes

ofparticleremovalgovernedbythermalexpansion(noholescreated). For7nspulses

andatleastforparticleswithdiameterslargerthan800nmlocalablationplayedan

important role again. To quantify these regimes will be the goal of future work.

Inaddition itshouldbenotedthat forSLCthesituation mightbedierentas the

computations reveal values for the eld enhancement that are signicantly lower

thanin air(seeSec. 4.1).

5 Laser cleaning with nanosecond pulses

Totheauthors'knowledgealllasercleaningstudiespublishedsofarhaveemployed

nanosecond pulsed lasers such as excimer- (ArF/KrF, =193/248nm), Nd:YAG-

(=1064nm,532nm,355nm)andCO

2

-(=10.6m)lasersforparticleremoval. The

choiceofaspeciclaserisgovernedbythedesiredenergyabsorptionmechanismand

thusbytheopticalpropertiesofsubstrate,particleandenergytransfermedium. In

ourstudiesusingPS,SiO

2 andAl

2 O

3

particlesweusedafrequencydoubledNd:YAG

laser(=532nm,FWHM7ns)toinvestigatethecleaningbehaviourintheregimeof

transparentparticlesonanabsorbingsubstrate.

ThegoaloftheexperimentswastodeterminecleaningeÆcienciesbothforDLC

andSLCbyusingthecharacterizedcolloidalparticlesandtocomparethetwopro-

cesseswithrespecttotheireÆciency. Asprocessparameterswevariedthenumber

ofcleaningstepsandtheappliedlaseruence.

5.1 Steam laser cleaning

For SLC we used [21] the same experimental setup as described above for DLC

but supplementeditwith asteam providingunit [14]: acontrolled owof ltered,

pressurized air was directed through a reservoir with a water/IPA mixture (90%

water) heated to 330K. Then the steam/air mixture was directed to the sample

via anozzle at adistance of 1.5 cm from thecleaned area. TheIPAsrole wasto

improvethe wetting of the steam condensed onto the silicon wafer and to enable

theformation ofaliquid lm. Weestimated thelm thicknessusing ellipsometric

measurementstobeabout200-400nm.

Comparingcleaning eÆciencies after 1, 2, 5, 10 and 20 cleaning steps (steam

condensationandsubsequentlaserpulse)previousexperiments[23]haveshownthat

the cleaning process is statistical in a way that the number N

r

of the remaining

particlesafterncleaningstepsisgivenbyN

r

=N

0

=(1 p) n

asfunctionofthesingle

shotcleaning eÆciencypand theoriginal particlenumberN

0

. Therefore,ourway

of samplepreparation allowed quantitativeexperimentsand for thesakeof clarity

in Fig. 7weplotted onlythecleaningeÆcienciesafter 20steps.

Thisgureshowsthedependenceof thecleaningeÆciency ontheappliedlaser

uence for particles of dierent sizes (60nm-800nm), materials (PS, SiO

2 , Al

2 O

3 )

andgeometries(sphericalPS,SiO andarbitrarilyshapedAl O ). Someimportant

Figure 7: Cleaning

eÆciency of various

colloidal particles for

SLC with =532nm,

FWHM=7ns. The

cleaning threshold is

found to be the same

forallparticles.

resultscanbeobtainedfromthisgraph. Apredominantfeatureistheexistenceofa

universal cleaningthresholdforallparticlesinvestigatedatalaseruenceofabout

110mJ/cm 2

. Forslightlyhigherlaser uencesweobservedaverysteep increasein

thecleaningeÆciencyandvalues90%arereachedwellbelowthemeltingthreshold

ofabaresiliconsubstrate,evenforparticlesassmallas60nm.

5.2 Dry laser cleaning

Thedrylasercleaningexperimentswerecarriedoutinambientconditions(relative

humidity 30-40%). After contamination of the samples with PS colloidal spheres

havingdiameters from140nm-2000nm,theywereirradiatedbyasinglelaserpulse.

A owof pressurizedlteredairwasused toblowawaytheremovedparticlesand

to prevent redeposition. In contrast to the SLC experiments described above for

allthedierentparticlesizesdierentvaluesofthecleaningthresholduencewere

measured. Againstthebackgroundoftheeld enhancementexperimentsdescribed

abovewedeterminedthethresholdforsurfacemodication(melting,localsubstrate

ablation)in additiontothecleaningthreshold.

5.3 Comparison of DLC and SLC results

TheresultsobtainedbothfromtheDLCandSLCexperimentsareplottedinFig.8.

WhencomparingthecleaningeÆciencyofbothmethodsthemoststrikingdierence

is the dierent dependence of the cleaning eÆciency on the particle diameter. In

DLCthecleaningthresholdsvarystronglywithparticlediameter. Thiswasalready

describedbyother authorsandisascribedto thestronglydierentadhesionforces

acting on the dierent particles [41, 42]. None of these previous experiments in-

vestigatedsomanydierentparticlesizesandgainedsuchcompleteinformationon

the dependence of the DLCcleaning thresholdin ambientairon theapplied laser

uence. It canbe seenclearly that smaller particles are morediÆcult to remove.

Thethresholduencefollowsan1=r k

tendency asdescribed in[43],where ris the

particle radius and k 1. However, a quantitative comparison with theoretical

predictions is notpossible, as the experimentswere carried outin airand asit is

Figure 8: Cleaning

thresholds for PS

spheres for DLC

in air. A Nd:YAG

Laser (=532nm,

FWHM=7ns) was

used.

The SLC universal cleaning threshold is indicated by a horizontal line. With

regardtotheDLCexperimentstheuniversalityofthisthresholdisquitesurprising.

Our results are also in contrast to theoreticalpredictions [44] and to experiments

usingarbitrarilyshapedAl

2 O

3

particles[19].

NoneoftheproposedmodelsforSLC[9,22,6,44]predictsauniversalcleaning

threshold. This may be an indication that the assumptions made to simplify the

complexprocess(particleadhesion,geometryofliquidlayer,dynamicsofevaporation

of the liquid layer, eld enhancement at the particles) are either notvalid or are

oversimplied.

Althoughthemodelingof SLC fails, yet previousexperimentsonlaser induced

bubblenucleationin liquidsmayprovideaqualitativeinsightintothenatureofthe

universal threshold. Asthecleaning thresholdisthe samefor particlesdieringin

size aswellas in material andgeometry propertiesweconcludethat thethreshold

is determined by the nucleation of bubbles capable of removing the particles and

thereforeby thedynamics oftheheating ofthe liquidlayerandbubble formation,

rather than by particle properties. For this laser inducedbubble nucleation there

alsoexists asharpthresholdin laseruence[45]whichmightexplaintheexistence

ofacleaningthreshold . Thedependencyofbubblepressureonappliedlaseruence

shows a very steep increase above the bubble nucleation threshold which should

result in high cleaning forces exerted on the particles at laser uences very close

to thethresholduence. Thismayexplain thesteep increasein cleaningeÆciency.

The highcleaning forces provided bythegrowingbubblesforces couldexplain the

universality ofthecleaningthreshold: althoughtheadhesionforcesforthedierent

investigatedparticlesvarybymorethanoneorderofmagnitude,thecleaningforces

maybyfarexceedtheadhesionforalltheparticlesinvestigatedleadingtothesame

cleaningthresholdforallsortsofparticles.

AcomparisonoftheeÆciencyofbothprocessesshowsthatforPSparticleslarger

than about 400nm in diameter the threshold uences in DLC are lower, whereas

for smaller particles (and thus those particles that are of highest interest for an

industrial cleaning application) SLC is far moreeÆcient. As theupper bound for

thelaseruencethatmaybeappliedforcleaningisgivenbythemeltingthreshold

ofthebaresiliconsubstrateonecanalsoobtainfromthegraphtheinformationthat

couldberemovedwithaneÆciency>90%. SiO

2 andAl

2 O

3

particlesareevenmore

diÆcultto removebyDLC[22], whichmakesthis methodevenmoreunfavourable

forapplication.

Whencomparing thetwoprocesseswith regardtotheireÆciencyonemustnot

onlyfocusontheremovalofparticles,butshouldalsopayattentiononpossiblesur-

facedamage. ThedashedlineinFig.8indicatesthethresholdinlaseruencewhere

we found a modication of the substrate at the former positions of the particles.

This damage is caused by local eld enhancement asdescribed above. As clearly

canbeseenfromthegraph,thismodicationthresholdisidentical withthecleaning

thresholdforall particlesinDLC.Removingaparticlealwaysmeantdamagingthe

substrate-damage freeDLCisnotpossibleforthelaser parametersweused. The

coincidenceofcleaningand damagethresholdmayalsoindicatethat localablation

mayplayaroleinDLCascleaningmechanisminadditiontothethermalexpansion

ofthesubstrate.

Our calculations presented in Sec. 4.1 yielded, on the other hand, a signi-

cantlylowereld enhancementin water comparedto air. Andindeed, preliminary

SLC experimentswith PSspheres800nmin diameter showedthat in this casethe

holeformationthresholdwaswellabove thecleaning threshold,thus enablingsub-

stratedamage-freeremovalincontrasttoDLC.Thisreductionineldenhancement

combined with the uniform cleaning threshold seems to be the advantage of SLC

comparedto DLCwithregardtoapplications.

6 Laser cleaning with picosecond pulses

A reductionofthe pulselengthhasimportantconsequencesforboththeDLCand

SLCprocess. InDLCahigherdecelerationofthesubstratesurfaceisexpected(see

Sec.3);inSLCthemoreeÆcientheatingoftheliquid-solidinterfaceduetoalower

heat diusionintodeeperlayersofthesubstrateinuencesthedynamicsofbubble

growth.

As SLC is based on the nucleation and the rapid growth of gas bubbles the

inherent time scale of this method is not given by the heating of the liquid-solid

interfacebut bythetimeit takesto superheatasuÆcientlythickliquid layerand,

therefore,bytheheattransportwithintheliquid. Toourknowledge,thetimescale

of the heating of a liquid is not known so far. Therefore, it is not clear a priori

whethershorterpulse lengthswilllowerthecleaningthresholdinSLC.

Besidesthereductionofcleaningthresholdswhichisfavorableforthelaserclean-

ingamoreeÆcientheatingofthesubstratealsohastwounfavorableconsequences.

First, a more eÆcient heating of the substrate means a reduction in the melting

threshold for the bare substrate and second the possibility of surface damage in-

creaseswithdecreasingpulse length(see. Sec. 4.2). Theresultsofourexperiments

[20]forbothDLCandSLC employingpicosecondpulseswillbediscussed below.

6.1 DLC using picosecond pulses

In Fig. 9weplotted the cleaning eÆciency forbothDLC andSLC for 800nm PS

spheresafter20cleaningstepsasafunctionoftheapplied laseruence. Compared

to the nanosecond cleaning threshold in DLC of about 70mJ/cm 2

, in the case of

picosecond pulses this value reducedconsiderably to about10-15mJ/cm 2

. This is

evenmoreastonishingastherelativereductionin themeltingthresholdfromnano-

2

Figure 9: The clean-

ing eÆciency as a

function of the ap-

plied laser uence for

both DLC and SLC

with ps (=583nm,

FWHM=30ps)pulses.

removal for ps DLC to be due to substrate thermal expansion then this discrep-

ancyisquitesurprisingasthereductioninthemeltingthresholdisalsodetermined

thermally. And indeed, a subsequent investigation of the irradiated samples with

anSEMshowedholesalloverthecleanedarea-just asin thefemtosecondregime.

Thismightprovideanexplanationforsuchadrasticchangeinthecleaningthreshold

whenchangingthepulsedurationinDLC.

6.2 SLC using picosecond pulses

When compared to DLC the SLC cleaning threshold is slightly higher, about 20

mJ/cm 2

, but still astonishingly low compared to the universal threshold of 110

mJ/cm 2

determined forSLC underns pulses. Comparingthemeltingthresholdof

thebaresurfaceforpspulses(220mJ/cm 2

)tothecleaningthreshold(20mJ/cm 2

)

and the eld enhancement factor of about 3 expected in water for a particle size

of 800nm, it is apparentlythat ps pulses may provide a safewindow for a really

low threshold SLC without surface damage. However, whether this threshold is

determinedbyadierentheatingprocessoftheliquidcomparedtothenanosecond

caseorbylocalsubstrateablationwillbethesubjectoffutureexperiments.

7 Summary and Conclusions

Inourexperimentsweinvestigatedthelaserassistedparticleremovalbythesteam

(SLC) and dry laser cleaning (DLC) processes. For the latter the accelerationof

the substrate surface due to thermal expansion is an important parameter. We

determined this acceleration to be on the order of some 10 6

m/s 2

for applications

carried outwith typical laser parameters. This valueis in good agreementwith a

simplemodelforthermalexpansionneglectingthermoelasticeects.

An investigation of theintensityenhancementof theapplied laser pulse at the

contaminantparticlesshowedthatevennominallaseruencesfarbelowthemelting

thresholdforthebaresubstratecouldinducesurfacedamageinformofholesatthe

formerparticlepositions. Thecreationofholesleadstoaparticleremovalmechanism

of both processes were compared. In DLC we found dierent cleaning thresholds

for dierent particle sizes. In SLC there exists a size and material independent

cleaning threshold for all particles investigated. A comparison of both methods

showedthatSLCwasmoreeÆcientforparticlediameterssmallerthanabout400nm.

Highcleaning eÆcienciesofmorethan90%forparticlesassmallas60nmcouldbe

reached in SLC. InDLC the removalof PS particles always led to damage of the

surfaceinducedbyeldenhancementeects atthecontaminants.

ApplyingpspulsesbothtoDLC andSLCwefoundthatitispossibletoremove

particleswithhigheÆcienciesandremarkablylowlaseruences. Estimationsbased

ontheeldenhancementinwaterandthecorrespondingcleaningandmeltingthres-

hold uences suggest that sub-ns pulses may beextremely useful for SLC at very

lowlaser uences. However,the nature ofthe removalmechanism-local ablation

or bubble nucleation - is not completely clear so far for and will require further

experiments.

Against the background of these experiments we conclude that laser assisted

particle removal is a cleaning method capable of removing particles even smaller

than100nmin diameterfrom siliconwafers. CleaningeÆcienciesabove90%canbe

reachedinSLCwhichmakesthistechniqueverypromising. Ontheotherhand,one

has to be clearlyawareof the danger ofsurface damagedue to eld enhancement

eectsespeciallyinDLC,wherenodamagefreeparticleremovalwasobservedinour

experiments. ComparingtheDLCto SLC method, theSLC seemsto befavorable

due to its highereÆciency, theexistence of a universal cleaning threshold and its

lowereld enhancementattheparticles.

Acknowledgements

This work was supported by the Konstanz Center of Modern Optics and the EU

(TMRERB-CT98-0188: Modelinganddiagnosticofpulsedlaser-solidinteractions:

applications to laser cleaning). Wacker Siltronic supplied the industrial silicon

wafers. The authors would like to thank Maria Erikssonfor her assistance in the

experimentsandCarmenN.AfonsoandJavierSolisfortheirsupervisionandmany

instructivediscussions.

References

[1] R. DeJule,SemiconductorIntl.8,65-68(1998).

[2] R. Kohli,this volume.

[3] InternationalTechnologyRoadmapforSemiconductors,SematechInc. (2000).

[4] J.D. KelleyandF.E.Hovis,MicroelectronicEngg.20,159(1993).

[5] R.N.Legge,D.J.Convey,J.D.Peterson,C.M.Freeman,D.L.Thompson,T.R.O'

Keee,A.C.Engelsberg,A.F.Cruz,M.E.KingandP.M.Randall,inParticles on

Surfaces5&6: Detection,AdhesionandRemoval,K.L.Mittal(Ed.),pp.267-287,

VSP,Utrecht(1999).

[6] Y.F. Lu,W.D. SongandT.S. Low,Mater.Chem. Phys.54,181-185(1998).

[7] G.Vereecke,E. RohrandM.M.Heyns,J.Appl.Phys.85,3837(1999).

[9] Y.F. Lu,Y.W.ZhengandW.D.Song,J.Appl.Phys.87,1534(2000).

[10] G.Vereecke,E.RohrandM.M.Heyns,Appl.Surf.Sci.157 ,67(2000).

[11] W.Zapka,W. ZiemlichandA.C.Tam, Appl.Phys.Lett.58, 2217(1991).

[12] A.C.Tam,W.P. Leung,W.ZapkaandW. Ziemlich,J.Appl.Phys.71, 3515

(1992).

[13] K.Imen,J.LeeandS.D.Allen,Appl.Phys.Lett.58,203-205(1991).

[14] H.K.Park,C.P.Grigoropoulos,W.P.LeungandA.C.Tam,IEEETrans.on

CPMTA, 17,631(1994).

[15] J.B. Heroux, S. Boughaba, I. Ressejac, E. Sacher and M. Meunier, J. Appl.

Phys.79,2857(1996).

[16] S.Boughaba,X.Wu, E.SacherandM.Meunier,J.Adhesion, 293-307(1997).

[17] A.C.Tam,H.K.ParkandC.P.Grigoropoulos,Appl.Surf.Sci. 127-129,721-

725(1998).

[18] P.Leiderer,J.Boneberg,M.Mosbacher,A.SchillingandO.Yavas,Proc.SPIE

3274,68(1998).

[19] M.She,D. KimandC.P.Grigoropoulos,J.Appl.Phys.86,6519(1999).

[20] M. Mosbacher, N. Chaoui, J. Siegel, V. Dobler, J. Solis, J. Boneberg, C.N.

AfonsoandP.Leiderer,Appl.Phys.A69[Supp.],331(1999).

[21] M.Mosbacher,V.Dobler,J.BonebergandP.Leiderer,Appl.Phys.A70,669

(2000).

[22] X.Wu, E.SacherandM.Meunier,J.Appl.Phys.87, 3618(2000).

[23] P.Leiderer,J.Boneberg,V. Dobler,M. Mosbacher,H.-J.Munzer,N. Chaoui,

J. Siegel, J. Solis, C.N. Afonso, T. Fourrier, G. Schrems and D. Bauerle, Proc.

SPIE4065(2000).

[24] J.M.Liu,OpticsLett.7,196(1982).

[25] J.Siegel,Structuraltransformation dynamics inGe lmsupon ultrashort laser

pulse irradiation, PhDThesisatUniversidadAutonomadeMadrid(1999).

[26] J.Solis,C.N.Afonso,S.C.W.Hyde,N.P.BarryandP.M.W.French,Phys.Rev.

Lett.76,2519(1999).

[27] J.Solis,J.Siegel,C.N.Afonso,Rev.Sci.Instr. 711595-1601(2000).

[28] D.M.KaneandD.R.Halfpenny,J.Appl.Phys.87,pp.4548-4552(2000).

[29] S.Hayashi,Y.Kamamoto,T.SutukiandT.Hirai,J.ColloidInterfaceSci.144,

538(1991).

[30] B.S.Lukyanchuk,Y.W.ZhengandY.F.Lu,Proc.SPIE4065(2000).

[31] Y.Kawata,C.Egami,O.Nakamura,O.Sugihara,N.Okamoto,M.Tsuchimori

Leiderer,Appl.Phys.A72,41(2001).

[33] G.Mie: Ann.Physik4,377-445(1908).

[34] P.W.BarberandS.C.Hill,LightScatteringbyParticles: ComputationalMeth-

ods,WorldScienticPublishing,Singapore(1989).

[35] H.-J.M"unzer,M.Mosbacher,M.Bertsch, J.Zimmermann,P.LeidererandJ.

Boneberg,J.Microscopy202,129(2001).

[36] D.Bauerle,LaserProcessingandChemistry ,2ndEd.,Springer(1996).

[37] J.Bonse, S. Baudach, J.Kruger,W. KautekandM. Lenzner,Appl. Phys.A,

submitted.

[38] V.Dobler,R.Oltra,J.P.Boquillon,M.Mosbacher,J.BonebergandP.Leiderer,

Appl.Phys.A69, 335(1999).

[39] S.V.Buntsents,S.G.DmitrievandO.G.Shagimuratov,Phys.SolidStat.,38,

552-557(1996).

[40] M. Vicanek, A. Rosch, F. Piron and G. Simon, Appl. Phys. A, 59 407-412

(1994).

[41] V.M. Muller, V.S. Yushenko, B.V. Derjaguin, J. ColloidInterface Sci. 92, 92

(1983).

[42] R.A.Bowling,in: Particleson Surfaces1: Detection,Adhesion andRemoval,

K.L.Mittal(Ed.), pp.129-142,Plenum,NewYork(1988).

[43] T. Fourrier,G. Schrems, T. Muhlberger, J.Heitz, N. Arnold,D. Bauerle, M.

Mosbacher,J.BonebergandP.Leiderer,Appl.Phys.A72, 1-6(2001).

[44] Y.F. Lu,Y. Zhang, Y.H.Wanand W.D.Song, Appl. Surf.Sci. 138-139,140

(1999).

[45] O.Yavas,A. Schilling,J.Bischof,J.BonebergandP.Leiderer,Appl.Phys.A

64,331(1997).