1、DOI/10/1007/s00339-007-3930-zAppl/Phys/A87/691695(2007)MaterialsScience&ProcessingAppliedPhysicsAy/hayasakia117d/kawamuraHigh-densitybumpformationonaglasssurfaceusingfemtosecondlaserprocessinginwaterDepartmentofOpticalScienceandTechnology/FacultyofEngineering/TheUniversityofTokushima/2-1Minamijosanj
2、ima-cho/Tokushima770-8506/JapanReceived/13November2006/Accepted/29January2007Publishedonline/29March2007Springer-Verlag2007ABSTRACTMicrometer-sizedbumpswereformedonaglasssurfaceusingafocusedfemtosecondlaserprocessinginwater/Thebumpswereformedoverawiderangesofpulseirradiationparameters/includingirrad
3、iationenergyandfocusposition/Thebumpsexhibitedawidevarietyofmorphologiesandsizesde-pendingontheparameters/Theuseofaliquid/namelyheavywater/whichreturnsafterbreakdownandcavitationbubblefor-mation/enabledustofabricatebumpswithhighspatialdensity/whichisnotpossibleusingasolidcoatingthatisablated/Ade-sir
4、edarrangementofbumpsonaglasssurfacewasfabricatedbytuningtheprocessingtimeintervaltobemorethanthedisap-pearancetimeofabubble/generatedbyfocusingafemtosecondlaserpulsenearthewater/glassinterface/PACS42/62/Cf；42/70/Ce；52/38/Mf；78/47/+p；79/20/Ds1IntroductionFemtosecondlasersarepowerfultoolsformicro-andn
5、ano-structuringoftransparentmaterialsbecausetheycanprocesswithhighspatialresolutionresultingfrommul-tiplephotonabsorption/andreducedthermaldamageduetotheultra-shortinteractiontimebetweenthelaserpulseandthematerial/aswellasvariousphysicalphenomenacausedbytheultra-highintensityofthelaserpulse111/Fem-t
6、osecondlaserprocessingisbeingincreasinglyappliedtothedevelopmentofthree-dimensionalopticalandfluidicde-vices7/8/1014/Asthemorphologyoftheprocessedtrans-parentmaterialisrelatedtothethermaleffectsofvaporizationanddissolutionduetothermaldiffusion/interactionwiththehotvaporplume/andalow-energy-densityre
7、gioninthelaserpulse/itishighlysensitivetonotonlythephysicalproper-tiesofthematerial/butalsotothelaserirradiationparameters/suchasthewavelength/pulseduration/pulseenergy/numer-icalapertureofthefocusedbeam/andthefocusposition/Inparticular/whenafemtosecondlaserpulseisfocusednearthesurfaceofatransparent
8、material/adifferenceinthefocuspos-itiongivesrisetoalargedifferenceinthesurfacemorphology/a117Fax/+81-88-656-9435/E-mail/hayasakiopt/tokushima-u/ac/jpThetypicalsurfacemorphologyofglassprocessedbyatightly-focusedfemtosecondlaserpulse/changesfromacavitytoabumpwhenthefocuspositionchangesfromtheoutsideto
9、theinsideoftheglass/Thecavityissurroundedbyaring-shapedprotrusionandscattereddebris/Theirsize/andtheamountofdebrisstronglydependsonthefocuspos-itionalso/Abumpwithadiameterfromseveralhundrednanometerstoseveralmicrometersisformedbymeltingtheglasssurfacewiththemeltedglassbeingpushedupbyami-croexplosion
10、insidetheglass1520/Duetotherangesoffocalpositionandirradiationpulseenergy/thesurfacemelt-ingandtheinternalmicroexplosionoccursimultaneouslyandthebumpsformedareverynarrow/Bumpstypicallyexhibitssmallvariationinsizeandstructure/Inapreviousstudy/wefoundthatatransparentcoatingontheglassfordecreasingtheam
11、ountofdebrisattachedtotheglasssurfaceallowsbumpformationoveraslightlywiderrangeoffocalpositionscomparedtobareglass/whenthecoatingthicknessissufficientlylargerthanthelengthofthefocalvolume19/21/Furthermore/wefoundthatwhenthecoatingthicknessisshorterthanthelengthofthefo-calvolume/thatis/whenthecoating
12、surfaceisablatedbyasinglelaserpulsefocusedattheboundarybetweenthetransparentcoatingandtheglass/bumpswereproducedoverafairlywiderangeoffocuspositionscomparedtousingathickcoating20/Fromthoseinvestigations/webelievethattheamountofcoatingmaterialablatedinthefocalvol-ume/whichdependsonthecoatingthickness
13、/affectsthestrengthofashieldingeffectoftheplasmageneratedwhenablatingthecoating/Asaresult/thesizeandstructureoftheformedbumpcanbechanged/Thetransparentcoatingmethodhasthedisadvantagethatthespatialdensityofthebumpsislimitedtoseveralmicrometersbecauseofablationofthetransparentcoating/Inordertoachievec
14、ontrollablefabri-cationofbumpswithahighdensity/itispossibletouseliquidonthetransparentmaterialinplaceofthetransparentcoatingduringfemtosecondlaserprocessing/becausetheliquidnatu-rallyreturnsafterbreakdownandbubbleformation/Fabrica-tionofcomplexstructuresonasiliconsurfacebyfemtosecondlaserprocessingi
15、nwaterhasbeendemonstrated2224/Inthispaper/wedemonstrateformationofhigh-densitymicrometer-sizedbumpsbyfemtosecondlaserprocessinginwater/InSect/2/wedescribetheexperimentalsetupand692AppliedPhysicsAMaterialsScience&Processingprocedure/InSect/3/wedescribetheexperimentalresults/Weinvestigatedtheeffectsof
16、irradiationparameters/includ-ingenergyandfocalposition/onthemorphologyandsizeofthebumps/Wedemonstratedthat/bytuningtheprocess-ingtimeintervaltobemorethanthedisappearancetimeofabubble2528generatedbyafemtosecondlaserpulsefo-cusednearthewater/glassinterface/wecouldfabricateade-siredstructureontheglasss
17、urface/composedofhigh-densitybumps/InSect/4/weconcludeourstudy/2ExperimentalsetupandprocedureTheexperimentalsetupconsistedofanamplifiedfemtosecondlaserandanopticalmicroscopeandisshowninFig/1/Itwasthesameasthesetupusedinourpre-viouswork19/20/Theamplifiedfemtosecondlaserpro-ducedpulseswithapeakwavelen
18、gthof800nm/adurationof150fs/andamaximumrepetitionrateof1kHz/TheirradiationpulseenergyEatthesamplewascontrolledbyneutraldensityfilters/andisgivenbytheproductoftheenergymeasuredbeforeintroducingthelaserpulseintotheopticalmicroscope(Olympus/IX70)andthetransmittanceoftheop-ticalmicroscope/includinga40ob
19、jectivelens(numericalaperture/NA=0/65)/Thetransmittanceofthemicroscopewas0/69/Theprocessedareawasobservedundertransmittedilluminationbyausualcharge-coupleddevice(CCD)imagesensorwiththeframerateof30frames/s/ThefocuspositionZofthelaserpulsewasdefinedasthedistancemovedalongtheopticalaxisbythemicroscope
20、stage/Thezeroposition(Z=0)wasdefinedasthepositionwhereastructurewasformedontheglasssurfacebyirradiationofalaserpulsewithnearablationthresholdenergy/ThestructureofthesampleisalsoshowninFig/1/Thesamplewaspreparedasfollows/Fourordinarymicroscopecoverslips(Matsunami)whichweresubjectedtoultrasoniccleanin
21、ginethanolandpurewaterwereprepared/TheywereFIGURE1Experimentalsetupandthestructureofthesample/Thespacerglasseswereremovedwhentheprocessingwasperformedatargetglass/awindowglassforsandwichingwater/andtwospacerglasseswithathicknessof130m/Poly-methylmethacrylate(PMMA)withtoluenesolventwasusedtoformwalls
22、onthewindowglass/Aftersufficientlyevaporatingthesolventthespacerglasseswereremoved/andasmallcham-berwithasidelengthof1015mmcomposedofthePMMAwallsontheglasswasformed/WaterwasdroppedinthesmallchamberandthetargetglasswasfixedonthechamberwithasmallamountofthePMMAthatwasusedasaglue/Inthisexperiment/deute
23、riumoxide(heavywater/hereafterreferredtosimplyas“water”)wasusedbecauseofitslowlinearabsorptionaroundthewavelengthof800nm/Afterpro-cessing/thetargetglasswasremovedfromthechamberandsubjectedtoultrasoniccleaninginpurewaterandethanol/Thesurfacestructureoftheprocessedareawasobservedwithanatomicforcemicro
24、scope(AFM；DigitalInstruments/Di-mension3000)/3ExperimentalresultsFigure2showsstructuresprocessedinwateroverarangeofZfrom4/0to12/0mwhentheenergyEwas2/1J/Figure2aandbshowanAFMimageanditscorres-pondingprofile/whoseverticalrangeis500nm/Figure2canddshowtopandsideviewsoftheprocessedareaobservedwiththetran
25、smissionopticalmicroscope/Figure2eshowsthediameterandheightofthebumps/whichwereobtainedfromtheAFMobservation/andthelengthofavoid/whichFIGURE2(a)AFMimagesoftheprocessedareaand(b)theirprofiles/Theirradiationenergywas2/1J/Theverticalrangeis500nm/(c)Topand(d)sideviewsobservedwithatransmissionopticalmicr
26、oscope/(e)Diameterandheightofbumpsversusfocusposition/andthelengthofvoidsformedintheglassversusfocuspositionHAYASAKIetal/High-densitybumpformationonaglasssurfaceusingfemtosecondlaserprocessinginwater693wasobtainedfromasideviewobservation/ThebumpswereformedontheglasssurfaceoverawiderangeofZ/from4/0to
27、8/0m/AsZincreased/theheightanddiameterofthebumpsincreased/WhenZwas6/0m/thebumphadamax-imumheightof400nmandadiameterof3/6m/WhenZwas8/0m/alowbumpwithaheightof50nmwasformed/WhenZwasgreaterthan8/0m/voidswereformedinsidetheglassandnostructurewasformedontheglasssurface/Thelengthofthevoidunderthebumpalsoin
28、creasedasZincreased/ThevoidsformedwhenZwas4to12mwerenearlyequalinlength/UndermoredetailedobservationinthesideviewshowninFig/2d/wefoundthatthevoidshaddif-ferentgraylevelswhenZwasbetween6/0and8/0m/ThedarkhueofthevoidsunderthehighbumpsatZ=3/0mandZ=6/0mwasdarkerthanthoseofthevoidsformedcom-pletelyinside
29、theglass/Weexpectedthevoidinthehighbumptohavelowerdensitythantheothers/becauseaninternalmi-croexplosiondisplacedtheglassmaterialfromthefocalpointandformedthehighbump/thuscausingadecreaseindensity/Thisbumpformationphenomenonisthesameasthatob-servedinourpreviousstudyinwhichglasshavingatrans-parentpoly
30、mercoatingwasprocessed/Theprincipleofbumpformationinthatstudywasbasedonthesuppressionofthematerialemissionfromtheglasssurfacebyashieldingeffectofplasmageneratedbyablationofthepolymerandbyphys-icalblockingofthepolymer/Onedifferenceinthepresentstudyisthatthebumpformationintheglassprocessedinwa-teroccu
31、rsoverawiderrangeofZ/asshowninFig/3/Theirradiationbeamparameterswerealmostthesameasourpre-viousexperiments(showninFig/3in19)/TheirradiationenergywasE=0/69J/Whenprocessingglasswithapoly-mercoating/bumpformationwasobservedwhenZwas1/0to4/0m20whereaswhenprocessinginwater/bumpfor-mationwasobservedwhenZwa
32、s4/0to7/0m/Themainreasonforthedifferenceisthatthephysicalblockingofwa-terisweakerthanthatofthepolymercoating/Thisisfurthersupportedbytheresultsforstructuresprocessedwithhighpulseenergies/aboveseveralmicrojoules/discussedinthenextparagraph/Figure4showsAFMimagesoftheprocessedstructuresforvariousenergi
33、esEwhenZ=0/BumpswereformedwhenEwas0/17to6/9J/andtheirstructuresdrasticallychangeddependingonE/Thediameterandheightofthebumpin-creasedasEincreasedto4/1J/WhenEwas4/1J/thediameterwas5/1mandtheheightwas1/57m/Withfur-therincreaseofE/bothdimensionsdecreased/WhenE2/1J/therewaslittledebrisaroundtheperiphery
34、ofthebump/Although/whenE2/1J/debriswasdistributedaroundtheperiphery/andtheamountofdebrisincreasedasEincreased/ThescatteredregionofthedebrisisindicatedbythesquaresonthesolidlinesinFig/4/Processinginwa-terproducedmorescattereddebrisaroundthebumpthanprocessingwithanappliedpolymercoating/Thisfurthersup-
35、portstheassertionthatwaterhadweakerphysicalblockingthanthepolymercoating/Mostofthedebriswasnotremovedbyultrasoniccleaninginwater/Therefore/theglassmaterialscatteredintheliquidstateattheglass/waterinterfacead-heredtotheglasssurfaceandsolidified/Figure5showbumpsarrangedinastraightlinewithhighdensity/T
36、helinearly-arrangedbumpswereprocessedbyir-FIGURE3Diameterandheightofbumpsversusfocusposition/Ewas0/69JFIGURE4AFMimagesofthestructuresprocessedwith(a)E=0/69J/(b)E=2/8J/(c)E=4/1J/(d)E=4/8J/(e)E=5/5Jand(f)E=6/9J/(g)Diameterandheightofbumpanddebrisdiameterversusirradiationenergyradiatingthelaserpulsesat
37、aspatialintervalshorterthanthediameterofasinglebump/Inthiscase/thespatialintervalDwassetto2/0m/undertheconditionthatasinglebumpwithadiameterof3/6mandaheightof56nmwasformedwhenEwas3/5JandZwas6/0m/Thestructurewasprocessedbyscanningthemicroscopestagesothatasinglepulsewasirradiatedateachlocation/repeate
38、datarepetitionrateRof1Hz/Theshapeofthelinearly-arrangedbumpswascontrolledbychangingD/asshowninFig/6aandb/WhenDwas0/8m/thebumpsweresmoothlyconnected/toformalineofbumps/WhenDwas5/0m/thatis/whenDwassuffi-cientlylargerthanthebumpdiameter/thebumpshadisolatedpeaks/694AppliedPhysicsAMaterialsScience&Proces
39、singFIGURE5AFMobservationoflinearly-arrangedbumpsformedunderE=3/5J/Z=6/0m/R=1Hz/andD=2/0m/(a)and(b)arethepro-filesacrossandalongthelinearly-arrangedbumps/Theverticalrangeoftheprofilesis250nmanditshorizontallengthis60mFIGURE6Surfacestructuresformedundervariousconditions/ThesameirradiationenergyofE=2/
40、1Jwasused/In(a)and(b)/Z=6/0mandR=1Hz/andthepulseirradiationspatialintervalsof(a)D=0/8mand(b)D=5/0mweredifferent/In(c)and(d)/R=1HzandD=0/5m/andthefocuspositionsof(c)Z=6/0mand(d)Z=3/0mweredifferent/In(e)and(f)/Z=6/0mandD=0/5m/andtherepetitionratesof(e)R=2Hzand(f)R=5Hzweredifferent/TheAFMimagesare88m2T
41、ofabricatebumpswithhighdensity/ZandRwerecarefullychosen/inadditiontoEandD/Withtheirradi-ationconditionsZ=6/0m/E=2/1J/D=0/5m/andR=1Hz/asmoothlineofbumpswithauniformheightwasFIGURE7Bubblesgeneratedonthewater/glassinterfaceobservedwithaCCDimagesensor/whentheelapsedtime(a)t=2/30/(b)8/30/(c)12/30/and(d)1
42、3/30s/(e)Thedisappearancetimeofbubblesforthepulseenergy/Threemeasurementsateachpulseenergyareindicatedasthecenterfilledcircleandthebarsformed/asshowninFig/6c/Thewidthandheightofthelineofthebumpswereabout4/2mand60nm/respectively/WiththeirradiationconditionsZ=3/0m/E=2/1J/D=0/5m/andR=1Hz/manysub-microm
43、etersizedspikeswereformed/asshowninFig/6d/Theirregularlyshapedstructureswereformedasaresultofasinglebumpformedbythepreviouslaserpulsebeingdestroyedbythenextlaserpulse/becausetheenergydensityattheglasssurfaceenabledablationoftheformedbumpwhenthefocuspositionwasneartheglasssurface/Selectionoftherepeti
44、tionrateRwasalsoimportantinforminghigh-densitybumps/Figure6eandfshowAFMim-agesofastructureprocessedwithR=2and5Hz/respec-tively/Theotherconditions(Z=6/0m/E=2/1J/andD=0/5m)werethesameasthoseintheexperimentshowninFig/6c/ThisdifferencedependingonlyonRwasstronglyrelatedtothedisappearancetimeofthecavitati
45、onbubblegen-eratedbyplasmaformationatthewater/glassinterface/Figure7adshowthebubblegeneratedatthewater/glassinterfaceobservedwiththeCCDimagesensorwhenE=4/8JandZ=0/0m/Astheexpansionofabubbleislessthan10s28/itcannotbecapturedwithanordinaryCCDimagesensor/Onlythecontractionofabubblewasobserved/asshowninFig/7ac/InFig/7d/thecircularpatternwasthelaser-processedstructure/becauseitdidntchangetempo-rally/Theelapsedtimet=0wasdefinedasthetimewhenthebubblewasobserved/ThedisappearancetimeofthebubbleTd
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