Guidelines for establishing an etching procedure for dislocation density measurements on multicrystalline silicon samples

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Method Article

Guidelines for establishing an etching procedure for dislocation density measurements on

multicrystalline silicon samples

Krzysztof Adamczyk

^a,

*, Gaute Stokkan

^b

, Marisa Di Sabatino

^a

aDepartmentofMaterialsScienceandEngineering,NTNU,NO-7491Trondheim,Norway

bSintefMaterialsandChemistry,NO-7465Trondheim,Norway

ABSTRACT

Withmulticrystallinesiliconbecomingthemainmaterialusedforphotovoltaicapplicationsanddislocations beingoneofthemainmateriallimitationstobettersolarcellefﬁciency,etchpitdensitymeasurementsare gainingmoreimportance.Traditionally,etchpitdensitymeasurementsarebasedonselectiveetchingofsilicon samples.Themajorityoftheetchantshavebeendevelopedformonocrystallinesampleswithknownorientation, while those developed for multicrystalline samples have been less investigated and might need some optimization.Inthisstudy,weuseandcomparethePVScantool,whichprovidesaquickwaytoassessdislocation densityonselectivelyetchedsamples,andmicroscopeimageanalysis.Weshowhowtheetchingmethodsused fordislocationdensitymeasurementscanaffecttheresults,andwesuggesthowtooptimizetheSoporietching procedureformulticrystallinesiliconsampleswithhighdislocationdensities.WealsoshowhowtheSopori etchantcanbeusedtosubstituteSeccowhilemaintainingahighprecisionofdislocationdensitymeasurements, butwithoutthetoxichexavalentchromiumcompounds.

creativecommons.org/licenses/by/4.0/).

ARTICLE INFO

Methodname:Soporietchingforetchpitdensitymeasurements,CombinedwithPVScanandmicroscopeimageanalysis Keywords:Selectiveetching,Sopori,Secco,Etchpitdensity,EPD,Dislocations,Silicon,Photovoltaic

Articlehistory:Received18October2017;Accepted26September2018;Availableonline28September2018

*Correspondingauthor.

E-mailaddress:[email protected](K.Adamczyk).

https://doi.org/10.1016/j.mex.2018.09.013

creativecommons.org/licenses/by/4.0/).

ContentslistsavailableatScienceDirect

MethodsX

journalhomepage: www.elsevier.com/locate/mex

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SpeciﬁcationsTable

Subjectarea MaterialsScience

Morespeciﬁcsubjectarea Analysisofmethodsfordislocationdensitymeasurementsinsiliconmaterialforphotovoltaic applications

Methodname Soporietchingforetchpitdensitymeasurements,combinedwithPVScanandmicroscopeimage analysis

Nameandreferenceof originalmethod

Soporietching[1]

PVScan[2]

ImageJ[3]

Resourceavailability GlasswareandbenchsuitableforhandlingreagentswithHF Grindingmachine,grindingdiscwithgrainsize500and1200 Polishingmachine,diamondpastewithgrainsize9,3,1m^m Soporietchant–HF:CH3COOH:HNO3witharatioof36:15:2 Seccoetchant–HF:015molK2Cr2O7witharatio2:1 PVScan6000measurementsystem

Metallographicmicroscope

GIMP–GNUImageManipulationProgram ImageJ–ImageProcessingandAnalysisinJava Matlab–usedforPVScandataimaging MSExcel–usedforresultanalysisandplotting

Backgroundinformation

TheEtchPitDensity(EPD)measuredbythePVScantoolonanetchedmulticrystallinesiliconwafer istypicallyintherange10⁴and310⁶ cm ².Nolowerandhighervaluesaremeasured.

For smallerdislocationdensityareas,thereason fornodetectionmightbethat withthelow densityofetchingpitsachangeinthereﬂectedlasersignalistoosmalltobemeasured.Forlarger dislocationdensityareas,likeindislocationclusters,correctdislocationdensitycannotbemeasured becausetheetchpitsoverlap.

Themainmotivationbehindthisworkwastoaddresstheseissuesbymodifyingtheetchingtimeto obtaindifferentsizeofdislocationetchpitsandallowamoreprecisemeasurementwiththefast,large- areaPVScantechnique.Itwashypothesizedthattomeasureintra-graindislocations,whichareinthe rangebelow10⁴cm ²,apossiblewaycouldbetoetchthesamplelongerandobtainbiggeretchpits.For dislocationclusters,decreasingtheetchingtimeshouldleadtosmalleretchpitsandlessetchpitoverlap.

Inthisworktheinﬂuenceofetchingtimeonetchpitsizeandetchpitdensitymeasurementswas evaluated.ThetestspresentedinthisworkwereperformedwiththeSoporietchantrecipe.Several studies important for the PV industry have alsobeen performed withthe Secco etchant recipe containingacarcinogenichexavalentchromiumcompound,potassiumdichromate,K₂Cr₂O₇[4,5].In additiontoitstoxicity,theCrpresentintheSeccoetchantcancontaminatethesampleandinﬂuence subsequentmeasurementsofchemicalcompositionanalysis.Becauseofthesefactors,partofthis workwas alsoaimedatcomparingtheSeccoandSoporirecipesfortheiruseinetchpitdensity measurements to establish if the Sopori etchant could be used instead of Secco for precise measurements.

PVScanandmicroscopeimageanalysisforEPDmeasurements

Allthemeasurementswereperformedonone55cmslabcomingfroma highperformance multicrystallinesilicon(HPMC-Si)ingot,seededwithSifeedstockfromtheﬂuidizedbedreactor(FBR) processandsolidiﬁedatNTNU/SINTEFlab [6].Topreparethesampleforetching,itssurfacewas groundonsandpaperandpolishedusingadiamondsuspensiondownto1

m

^m.

ThestandardrecipeusedatNTNUforselectiveetchingconsistsofthestepspresentedbelowand performedinoneetchingsession(ratios,ifnotstatedotherwise,aregiveninunitsofvolume):

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1 RCA1cleaning/10min 2 Dipindeionizedwater

3 5%hydroﬂuoricacid-HF/3min 4 Sopori/25s

5 HF:HNO₃(1:9)/5s 6 Dipindeionizedwater 7 Flushwithethanol

TheRCA1cleaningmixtureusedinstep1wasdevelopedintheRadioCorporationofAmericafor cleaningofsiliconwafers{Kern,1990#8}[7].It consistsof 5partsof deionizedwater,1 partof ammoniawater(29%HNO3)and1partofaqueousH2O2(30%H2O2).Itisappliedbydippingthesilicon waferinthemixtureatatemperatureof80Cwithagitation.Theremainingetchantsandmixturesare appliedbydippingthesiliconwaferintheetchantsatroomtemperatureandwithoutagitation,only withaveryslowmixingintroducedbymovingthesampleholderinthebath.Thedippingtimeisgiven foreachstep.Dipsindeionizedwaterdon’trequiretimecontrol.

TheSoporietchantusedinstep4isamixtureofHF:CH3COOH:HNO3witharatioof36:15:2.Silicon etchingwithsuchanetchantisamulti-reactionprocessinwhichalocalconcentrationofreaction productscouldleadtoincreasedetchinglocally.Theetchantmixingbymovingthesampleholderin thebathisintroducedtoavoidsuchchangesintheetchingrateandallowforamoreevendistribution ofsubstratesandproducts.

EvaluationofdifferentetchingtimewasperformedbychangingthetimeusedforSoporistepinthe standardNTNUrecipeandleavingtheremainingstepsunchanged.Intheﬁrstthreeetchingsessions thesamplewasetchedafterwardswithoutrepolishing,withSoporietchingstepslasting5,10and10s –upto25sintotal,asinthestandardprocedure.Thiswasdonetoevaluateifitispossibletoﬁrst obtaina measurementonasurface withsmalletchpitsandsubsequentlyetchthesamesample further,toobtainmeasurementscomparablewiththestandardprocedure.

Forthenextsessions,thesamplewasrepolishedbeforeeachetching.SessionswiththeSoporistep lasting25,75and150swereperformedtoobtainstandardandlargeretchpits.Finally,thesamplewas repolishedandetchedfor5swiththeSoporimixturediluteddowntoHF:CH3COOH:HNO3ratioof 36:20:1toobtainetchpitssmallerthanintheﬁrstthreesessions.

TheSeccoetchingprocedureusedinthisworkwasasfollows:

1 Dipinacetone 2 Dipinethanol

3 Dipindeionizedwater 4 Secco/60s

TheSeccoetchantisamixtureofonepartof0.15MsolutionofK2Cr2O7inH2Oandtwopartsof49%

HF.

EtchpitdensitymeasurementswithPVScanandmicroscopeanalysiswasperformedaftereach etchingsession.

PVScan6000isasurfacescanningtoolwhichallowsforrelativelyfastevaluationofetchpitdensity byintegratingthediffusedlightreﬂectedbytheetchedsurface.Thebasicprincipleofoperationofthis instrumentispresentedinFig.1.Alaserbeamisusedtoilluminatethesurface.Surfacefreeofany defectslikedislocationclustersorgrainboundarieswillreﬂectthelightdirectlyandthesignalcoming fromthediffused light detectoris weak. Whenthelaser illuminatesdefects,thesignalis varied dependingonthedensityofthedefects.Asmentionedearlier,thistechniquefailswhentheetchpits arenotdenseenough,orwhentheirdensityistoohighandleadstooverlapping.Microscopeimage analysisallowstomeasureetchpitdensitymoreprecisely,butrequiresmoretimeandusuallyallows analysisofmuchsmallerareas.

CalculatingtheetchpitdensitybymicroscopeimageanalysiscanbeexplainedwithFig.2showing thesubsequentstepsintheprocess.AllthepresentedstepswereperformedinImageJsoftware.The imagewasﬁrstconvertedfromthecolorimageformatintoablackandwhiteimage.Thisallowedfor thresholding,whichisadivisionofthepixelsonimageintotwogroupsdependingontheirintensity

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values,higherandlowerthanthethreshold,andassigningthemtoonlytwovalues,blackandwhite.

Tocompensate foretchpitoverlap,the‘watershed’operationwas performedontheimage.This operationsearchesforcasesofoverlappingparticlesontheimage,basingontheirshape,anddivides themaccordingly.Whileitmayleadtodividingsingleetchpitsintoseveralareasontheimageiftheir shape is farfromcircular,themajorityof theoverlappingetch pitsare separated.Theetch pits fulﬁllingtheanalysisconditionsarethencountedbythesoftware.

Tocomparebothmethodsdifferentareaswereselectedonthesamples,onewithlargedislocation densitiesforanalysisofshorterSoporietchingtimes,onewithsmallerdislocationdensitiesforlonger etchingtimes.TheselectionwasmadeonPVScandislocationdensitymapswhich,whilelessprecise, areeasierandfastertoobtain.

ThedislocationdensitymapsfromPVScanarediscrete,thatiseachvaluewasobtainedbystepping overthescannedareawithadiscretestepsize.Inthecaseofreportedmeasurementsthisstepsize equaled100

m

^m.^Becauseôf^this^the^microscopeîmages,^coveringâ^limitedârea,^werefirstmanually stitchedinGIMPsoftwareintolargerimagescoveringareaswhichcouldbemoreeasilycompared withPVScanmaps,andthendividedintoareascorrespondingtoPVScanpixels.Thiscanbemore easilyunderstoodwhenlookingatFig.3.Theresulting‘pixel’microscopeimageswereautomatically

Fig.1.SchematicpresentingthebasicprincipleofPVScanoperation[2].

Fig.2.EtchpitdensitymeasurementbymicroscopeimageanalysiswithImageJsoftware.Theimageshownherewasslightly trimmeddownfrom100100m^m^forpresentation.Itsanalysisresultedinacountof58etchpits,leadingtoadensityof 5.810⁵dislocations/cm².Eachframeshowsaneffectofaprocessingstep.

a)Microscopeimageisconvertedto8-bitblackandwhiteimage.

b)Imageisthresholdedinto2values–whiteforetchpit,blackforbackgroundc)Watershedoperationperformedinorderto divideoverlappingetchpitsd)EtchpitsfulﬁllingtheanalysisconditionsarecountedbyParticleAnalysistool.

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processedwithImageJsoftware,asdescribed above.Theetchpitdensity resultswerecompared betweeneachPVScanpixelandacorresponding‘pixel’fromImageJmicroscopeimageanalysis.

Forthelowdislocationdensityareas,thelowestdensitymeasured–thatisif1etchpitwasvisible on100100

m

^m^square–wasequalto10⁴dislocations/cm².Sincelowervalueswereexpectedinlow dislocationdensityareas,theyweredividedintoagridof200x200

m

^m^instead^of¹⁰⁰100

m

^m.^Such

larger‘pixels’werecomparedwithPVScanmaps,whichwererecalculatedtothesamepixelsize–one newpixelwithadensityvaluecalculatedasanaverageoffouroriginalPVScanpixels.Suchbinning allowedcomparisonwithmicroscopeimageanalysisofareaswithetchpitdensitiesdownto2.510³ dislocations/cm².ItneedstobenotedthatthePVScanlaserbeamhasadiameterofabout800

m

^m,^but

reanalysingthemicroscopedatasothateach pixelresultwasaweightedaverageofitselfandits surroundingpixelswithweightsaccountingforthePVScanbeamintensitydidnotleadtodifferent conclusionsthantheoneswithoutsuchrecalculation,meaningthattheintensityofthelaserbeamis notuniformandmajorityofPVScansignalcomesfromscatteringmostlyitshigh-intensitycenterpart.

Forsimplicitythemicroscopeanalysisresultspresentedbelowarebasedonsinglepixels,withoutthe weightedaveragerecalculation.

EtchpitsizewasmeasuredwithImageJsoftwareonthemicroscopeimagesobtainedafteretching.

Fig.3. Imagesofsameareaofthesiliconsample,obtainedwithdifferenttechniques:ontheleftisaPVScanetchpitdensitymap andontherightisamicroscopeimage,stitchedanddividedintoareascorrespondingtoPVScanpixels.Theorientationaxis systemonthePVScanmapreferstopixelpositions.Eachsuchpixelcorrespondsto100100mmsquareonthemicroscope image.

Fig.4. Etchpitsizeforeachoftheetchingprocedurestestedintheexperiment.150randometchpitdiametersaveragedfor eachetchingtime.

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EtchpitsizecomparisonispresentedinFig.4.AsexpectedforSopori,theaverageetchpitdiameter waslargestforthelongestetchingtime,andsmallestforshortesttimeofetchinginadiluteetchant.

TheetchpitsizeobtainedaccordingtotheSeccoetchingprocedurewassmallerthanthesizeobtained fromashortetchindilutedSopori.

TheresultsshowingacomparisonbetweenPVScanmeasurementsandmicroscopeimageanalysis withImageJarepresentedinFigs.5and6.Thedataforanalysisofhighdislocationdensitypresented inFig.5forbothchartscomesfromthesameareaaspresentedinFig.4,whiledataforlowdislocation densityareaswasobtainedfromadifferentsamplearea,freeoflargerdislocationclusters.

Theblacklinedividingthechartareaisalinearrepresentationofthecasewheretheresultsfrom PVScanareequaltotheresultsfrommicroscopeimageanalysis.Ifthepointisabovethisline,PVScan measuredavaluehigherthanobtainedbymicroscopeanalysisonthesamearea.Ifthepointisbelow theline,thePVScanvaluewaslower.Adistinctionismadewithinthedatabetweenpixelscoveringan areawithgrainboundariesvisibleinadditiontoetchpitsandpixelscoveringareaswithetchpits,but withoutgrainboundaries.

Ageneral conclusionfromtheabovecomparison isthat ifagrainboundaryispresent inthe scannedarea,PVScanreturnsahigherdislocationdensityvaluethancanbeobtainedwithmicroscope analysis.Theetchedgrainboundaryalsoscattersthelaser beamandmoresignalismeasuredby PVScan.

Fig.5.ChartspresentingthecomparisonbetweendislocationdensityvaluesmeasuredbyPVScanandbymicroscopeimage analysis.Thedifferentetchingtimesweretestedonthesamearea,repolishedaftereachetchingsession.

a)StandardNTNUSoporietchant,usedfor25sasinstandardprocedure.

b)DilutedSoporietchantusedonlyfor5s,toobtainlowersizeofetchpitsforanalysisofhighdislocationdensityareas.

Fig.6.ComparisonbetweendislocationdensityvaluesmeasuredbyPVScanandbymicroscopeimageanalysisonasample etchedwiththestandardSoporietchantusedfor150stoobtainlargersizeofetchpitsforanalysisoflowdislocationdensity areas.

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AnotherfeaturesimilarforallthedatasetsisthatthePVScanshowedaverylowsignaltonoise ratiofordislocationdensitiesbelow10⁵dislocations/cm².Thisisacutoffvalueforthedatapointson chartsa)andb).Evenforpixelswhereetchpitswerevisiblewithmicroscopeindensitiescloserto10⁴ cm ²,PVScanassignedvaluesabove10⁵cm ².Itwasexpectedtohavelowsignaltonoiseratiocoming fromareaswithlowdensityofsmalletchpits,whichisvisibleonthechartb)inFig.5,butincreasing theetchpitsizebylongeretchingdidnotresultinbettersensitivityofthePVScansystem.Thechartin Fig.6showsthatthereseemstobenocorrelationbetweendislocationdensitymeasuredbyPVScan andwithmicroscopeimageanalysisinthelowdensityrange.Thismightberelatedtotheetching conditions,wherethelongetchingtimenotonlyledtoanincreaseinetchpitsize,butalsoresultedin introducingsigniﬁcantartifactsonthesamplesurface.SomeoftheseartifactsarevisibleinFig.7.As wasseeninFig.4,theetchpitsafter150sofSoporietchingarenotuniform,theirsizediffersacrossa widerangeofvalues.Thereasonforsuchartifactscanbethatforsuchlongetchingtimesthemixing introducedbymovingthesampleinthebathisnotefﬁcientenough.Differenttechniquesshouldbe usedtoincreasethemixing.Theartifactscanaffecttheresultsfrombothtechniques,withPVScan beingprobablymorepronetoerrorbecauseofit.EachoftheseartifactsscatterslightofthePVScan laserbeam,resultinginincreasedsignalforthistechnique.Majorityoftheartifactsisnotcountedas etchpitsbythemicroscopeimageanalysisalgorithmsduetotheircapabilitytodistinguishdefectsby theirsize.Etchingthesamplewiththreesubsequentstepswithoutrepolishing,with5,10and10s steps,alsoledtohavingsimilarartifactsonthesurface.

In therangeofhighdislocationdensities,thedatacomingfroma sampleetchedina diluted etchant gave a smaller spread of values comingfrom PVScan.The PVScan settings are a linear calibrationofthemeasuredsignal:

r

⁼^C*S,^where^thedislocationdensity

r

^is^calculated^from^the

signalSbasedonacalibrationconstantC[8].Thecalibrationconstantcanbeadjustedforthediluted etchantshowingapossibilityofcloserﬁtbetweenPVScanandmicroscopeimageanalysisintherange above10⁵cm ².Modifyingthecalibrationequationtocontainaconstantfactoraccountingforthe noiseinlowdislocationdensityareascanbealsoconsidered.Thiswouldincreasetheprecisionofthis fastmeasurementtechniqueandmakeitmorecomparablewiththemoretedious,butalsomore precisemicroscopeanalysis.

Thecomparisonbetweenmeasurementsofhighdislocationdensityareasshowedanadditional issuewithprecisemeasurement.AscanbeseeninFig.5,intherangebelow10⁵cm ²thereisa differenceinwhatwasfoundbymicroscopeimageanalysisafter25sinthestandardetchantandafter 5sinadilutedetchant.Asanalysisoftheetchedsurfacesreveals,thestandardetchresultsinavariety ofetch pits, rangingfrom circular toelliptic, and theelliptic etchpits seem more shallow.The differencein etchpit shape can be explained by differing anglesbetween the surface and the dislocationcore.Theetchpitsonthesampleaftertheshorter,dilutedetcharemuchmoreuniform,as ifonlythedislocationsatacertainpreferentialangletothesurfacewereetched.Thesmalletchpitsize withfeweroverlappingetchpitsallowedforamoreprecisequantiﬁcationofhighdislocationdensity areas,butthefactthatpartofthedislocationsalignedinlesspreferentialorientationsdidn’tetch

Fig.7. Imageofsamplesurfaceafter150setchinginstandardSoporietchant.

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needstobeconsideredwhen applyingthis technique.Acomparisonofthesurfaceafterthetwo discussedetchingstepsisshowninFig.8.

Astheaboveresultsshow,dividingtheetchingprocedureintoshortetchingforhighdislocation densityanalysisandsubsequentlongetchingforlowdislocationdensityanalysisdoesnotyieldgood results,especiallyinthelowdensityrangeafterlongetching.Combininglongetchingwithshort etchingwithoutrepolishingsamplesbetweensuchsessionsmayintroduceetchingartifactsandmake furthermeasurementsunprecise. Shorteretchingtimescanbeapplicableifonlyhighdislocation densityareasneedtobecharacterizedwithmoreprecision.Properoptimizationoftheetchingtime canallowpreciseanalysisonthePVScaninstrument.

ComparisonbetweenSoporiandSeccoetching

ForacomparisonbetweentheSeccoetchantandSoporietchant,theetchpitdensitywasmeasured inthesameareasafteretchingfor5sinstandardSoporiandthenafterrepolishingandetchingin Seccofor60s.IntheoriginalpaperonSeccoitwassuggestedtoetchmonocrystallinesiliconsamples for5minwithultrasonicagitation.60swithoutagitationisenoughtoselectivelyetchmulticrystalline siliconsamples,thus60swasusedforthiscomparison.Theresultsofthemicroscopeimageanalysis Fig.8.Microscopeimagesofthesamesamplearea,a)etchedfor5sinadilutedSoporietchantandb)for25sinstandardSopori etchant.Thesamplewasrepolishedbetweeneachetchingsession.

Fig.9. EffectofetchingprocedureonEPDmeasurementsbymicroscopeimageanalysis.60sSeccoand5sstandardSoporiare compared.

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ofthesurfaceobtainedaftereachoftheseproceduresareshownin9.Microscopeimageanalysiswas chosenasthetechniqueforthiscomparison,becauseduetotheetchpitsizethePVScansignalwas veryweak,resultinginlowerdislocationdensities.Fig.9indicatesthateven5sistoomuchforthe Soporietchanttoobtainprecisedislocationdensitymeasurementsinthehighdensityrange.The60s Seccoetchgivesanetchpitsizeofabout0.5–1

m

^m,^while^the⁵^s^Sopori^etch^resultsⁱⁿ²

m

^m^etch^pits.

Themaincauseforthedifferenceinmeasurementbetweenthetwoetchingproceduresistheoverlap ofetchpitsintherangeabove10⁵cm ²forSopori.

TheconclusionfromthiscomparisonisthattheSeccoetchantcanbereplacedwithSoporietching, buttheSoporietchingprocedureneedstobefurtheroptimizedformeasurementsofhighdislocation densityareas.EtchingtimeswiththedilutedSoporietchantbelow5saresuggestedinsuchcase.

Replacing theSeccoetchant withSoporigivessigniﬁcantbeneﬁts sinceone canavoid toxicand carcinogenicCrcompounds.

Acknowledgements

TheauthorswouldliketothankTorildKrogstadforherhelpwiththeetchingperformedinthis study. The work reportedin this paper was performed in the projectImpurity Controlin High Performance Multicrystalline Silicon, 228930/E20, funded by the Norwegian Research Council’s ENERGIXprogrammeandindustrypartnersRECSolar,RECSilicon,SteulerSolar,andTheQuartzCorp.

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