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INSTITUTEOFPHYSICSPUBLISHINGJOURNALOFMICROMECHANICSANDMICROENGINEERINGJ.Micromech.Microeng.142004415–422PIIS096013170469783XImplementationandanalysisofpolymericmicrostructurereplicationbymicroinjectionmoldingYuChuanSu1,2,JatanShah3andLiweiLin1,21BerkeleySensorActuatorCenter,UniversityofCalifornia,Berkeley,CA94720,USA2DepartmentofMechanicalEngineering,UniversityofCalifornia,Berkeley,CA94720,USA3DepartmentofMechanicalEngineering,UniversityofMichigan,AnnArbor,MI48105,USAEmailyuchsume.berkeley.eduReceived30September2003Published17December2003Onlineatstacks.iop.org/JMM/14/415DOI10.1088/09601317/14/3/015AbstractThispaperpresentstheadaptationofaconventionalinjectionmoldingprocesstothemassreplicationofpolymericmicrostructureswithappropriatemolddesignandprocesscontrol.Usingwetetchedsiliconwaferswithmicrostructuresonthesurfacesasmoldinserts,wehavesuccessfullypredicted,improvedandoptimizedthereplicationresults.Theflowbehaviorsofpolymermeltsinmicromoldcavitiesarecharacterizedbybothsimulationandexperiments.Amongvariousprocessparameters,temperatureisidentifiedasthekeyfactorthatdecisivelydeterminesthequalityofinjectionmoldedmicrostructures.Basedonthecollectedexperimentalandsimulationresults,processoptimizationisperformedtoimprovereplicationqualityandtoestablishguidelinesforpotentialapplications.Becauseofitshighspeedandlowcost,theadaptationoftheinjectionmoldingprocesstomicrofabricationwillleadtoapromisingtechnologyforMEMSapplications.1.IntroductionBecauseoftheiruniqueproperties,polymershavebeenincreasinglyusedinawiderangeofapplicationsincludingbothmacroandmicrodevices.InordertoexpandthefieldofMEMStopolymerbaseddevices,itisimportanttointroduceeffectivetechniquesforthefabricationofpolymericmicrostructuresatalowcostandwithhighprecision.Inrecentyears,anumberoftechnologiesforpolymericmicrostructurereplicationhavebeenproposed,includingtheLIGAprocess1,2thatuseseitherhotembossing3orinjectionmolding4toduplicatepolymericmicrostructures.Usingmoldinsertsfabricatedbyxraylithography,theLIGAprocessprovidesthepossibilitytomanufacturemicrostructureswitharbitrarylateralgeometryandhighdepthforhighaspectratiodevicesfromavarietyofmaterialssuchasmetals,polymersandceramicsbyvariousmoldingprocesses.Amongdifferentmoldingtechniques,injectionmoldingisthemostprominentonewithadvantagesoflowcostandhighprecisionformassproduction.Successfulresultsforthereplicationofpolymericmicrostructureshavebeenachievedbyusingspecialinjectionmoldingprocesses5–12andconventionalCDinjectionmoldingtechniques13,14.However,theflowbehaviorsofpolymermeltsinmicromoldcavitiesarenotfullyunderstood.Itisbelievedthatduetothelargesurfacetovolumeratio,surfaceeffectswilldominatetheflowbehavioratthemicroscale15.Thispaperaimstoinvestigatetheflowbehaviorofpolymermeltinthemicromoldcavityanddeterminethenecessarystrategiestoadaptthetraditionalinjectionmoldingprocessforthereplicationofpolymericmicrostructures.First,thedirectapplicationoftheconventionalinjectionmoldingprocessinthereplicationofpolymericmicrostructuresisanalyzedusingasimulationsoftwareCMOLD16.Differentcombinationsofprocessparametersarethensimulatedtoinvestigatetheflowbehaviorofpolymermelt,therelationshipbetweenprocessparametersandthequalityofmoldedmicrostructures.Usingtheseresults,themostsignificantparameterscanbeidentifiedandpossibleprocessingstrategiescanbeproposedandsimulatedtotestthefeasibility.Finally,09601317/04/0304150830.00©2004IOPPublishingLtdPrintedintheUK415YCSuetal2bxyzyzVelocityprofilePolymermeltPressureandmaterialsupplyFigure1.Schematicofpolymermeltflowinginathincavity.thesestrategiesareappliedinmoldtrialstoevaluatetheirvalidity.2.TheoreticalmodelsBecausemostinjectionmoldedpolymericpartshavecomplicatedthreedimensional3DconfigurationsandtherheologicalresponseofpolymermeltisgenerallynonNewtonianandnonisothermal,itisextremelydifficulttoanalyzethefillingprocesswithoutsimplifications.ThegeneralizedHeleShawGHSflowmodelintroducedbyHieberandShen17isthemostcommonapproximationthatprovidessimplifiedgoverningequationsfornonisothermal,nonNewtonianandinelasticflowsinathincavity,asshowninfigure1.TheassumptionsoftheGHSflowmodelare1Thethicknessofthecavityismuchsmallerthantheotherdimensions.2Thevelocitycomponentinthedirectionofthicknessisneglected,andpressureisafunctionofxandyonly.3TheflowregionsareconsideredtobefullydevelopedHeleShawflowsinwhichinertiaandgravitationalforcesaremuchsmallerthanviscousforces.4Theflowkinematicsissheardominatedandtheshearviscosityistakentobebothtemperatureandshearratedependent.ThedetailedderivationshavebeendevelopedbyHieberandShen,andtheseassumptionsapplywellforthemicroinjectionmoldingprocess.Inviewoftheseassumptionsandneglectingcompressibilityduringthefillingstages,themomentumequationintheCartesiancoordinatesystemreducesto17∂∂zbracketleftbiggη∂vx∂zbracketrightbigg∂P∂x∂∂zbracketleftbiggη∂vy∂zbracketrightbigg∂P∂y1wherevxandvyarevelocitycomponentsinthexandydirections,respectivelyPx,yisthepressure,ηγprime,Tistheshearviscosity,γprimeistheshearrateandTistemperature.Underthepresentassumptions,γprimeisgivenbyγprimebraceleftBiggbracketleftbigg∂vx∂zbracketrightbigg2bracketleftbigg∂vy∂zbracketrightbigg2bracerightBigg1/2.2Applyingthelubricationapproximation,thethicknessaveragedcontinuityequationresultsin∂b¯vx∂x∂b¯vy∂y03where¯vxand¯vyareaveragedvelocitiesoverz,andbishalfofthethickness.Afterseveralderivativesteps,thegoverningequationfortheflowofthepolymermeltcanbereducedtothecelebratedReynoldsequation∂∂xbracketleftbiggS∂P∂xbracketrightbigg∂∂ybracketleftbiggS∂P∂ybracketrightbigg04whereSistheflowconductancewhichisdefinedasSintegraldisplayb0z2ηdz.5ThevelocitiesandshearratecanbeobtainedasvxLambda1xintegraldisplaybzz1ηdz1vyLambda1yintegraldisplaybzz1ηdz1γprimezLambda1η6whereLambda1x−∂P∂x,Lambda1y−∂P∂yandLambda1bracketleftbigLambda12xLambda12ybracketrightbig1/2.Becauseofthetemperaturedifferencebetweenmoldandpolymermeltandtheviscousheatinginsidetheflow,thefillingprocessshouldbetreatedasanonisothermalcase.Heatconductioninthedirectionofflowisneglectedbasedontheassumptionthatthethickness2bismuchsmallerthantheothertwodimensions.Theenergyequationinthemeltregionbecomesρcpbracketleftbigg∂T∂tvx∂T∂xvy∂T∂ybracketrightbiggk∂2T∂z2ηγprime27wheretheηγprime2istheviscousheatingterm,andρ,cpandkaredensity,specificheatandthermalconductivity,respectively.Forsimplicity,itisassumedthatthevelocityandtemperaturearesymmetricinthezdirection,thevelocitiesofpolymermeltonthemoldsurfacesarezeroandthetemperatureofmoldremainsatTwduringfilling.Theboundaryconditionsaregivenbyvxvy0atzb∂vx∂z∂vy∂z0atz0TTwatz±b∂T∂z0atz0.8Ascanbeseen,theequationsofthismodelarenonlinearandcoupled.Itisdifficulttosolvetheseequationsanalytically.Inthispaper,simulationsoftwareCMOLDthatemploysnumericalsolversbasedonahybridfiniteelement/finitedifferencemethodisusedtosolvethepressure,velocityandtemperaturefieldsoftheGHSmodel.Becauseoftheseapproximations,aGHSmodelcannotpredicttheexactflowfieldneartheadvancingflowfrontorattheedgesofthemold.Thismightcauseerrorsinpredictingtheflowbehaviornearmicroscalemoldcavities.3.DesignandfabricationofmoldingapparatusAnaluminummoldismanufacturedforthereplicationprocess.Theschematicdiagramandaphotographofthealuminummold,whichconsistsofcavityandcorehalves,areshowninfigure2.Thecavityhalfincorporatesthecavityinwhichamoldinsertiskept.A4inchsilicon416ImplementationandanalysisofpolymericmicrostructurereplicationbymicroinjectionmoldingMountingplateStripperplateMoldinsertSiliconwaferMountingplateCorehousingplateSprueCavityhousingplateInsulationlayerHeaterBaseplateFigure2.Injectionmoldsetup.Figure3.Microstructuresonasiliconmoldinsert.waferwithbulkmicromachinedmicrostructuresisusedasthemoldinsert.Figure3showsthesiliconmicromoldinsertthatisetchedtohaveacavitydepthof110µm.Thesquarecavitieshaveopeningsof320µm,160µm,80µmand40µmandareetchedbymeansofanisotropicsiliconetchinginTMAHtetramethylammoniumhydroxide.Aheaterisinstalledintheinjectionmoldtocontrolthetemperatureduringthemoldingprocess.Tohavebetterthermalconductivityandshortercoolingtime,weemployedanaluminummoldthatisalsoeasiertomanufactureandmodify.Inaddition,withappropriatethermalinsulationandacoolingsystem,theproblemofdimensionalvariationcausedbythermalexpansioncanbecontrolledandanaluminummoldcanbeusedasamoreeconomicaltoolforthereplicationprocess.Themoldedcomponentcanberemovedfromthemoldmanuallyorbyusingtheejectionsystem.Unliketheprocessesdescribedinthepreviousliterature,asiliconwaferthatservesasthemoldinsertisplacedinthemoldcavity.Usingsiliconwaferasmoldinserthastheadvantageofshortturnaroundtime.Inaddition,thewearofasiliconmoldinsertismuchsmallerascomparedtoatraditionalnickeltool18.However,asiliconmoldinsertismorebrittlethananickelone.Toavoidthebreakageofthewaferduringthemoldingprocess,theedgeofsiliconwafershouldmatchthecavityboundary.Agapbetweenthemoldinsertandcavitycanallowpolymermelttosolidifywithin,whichwouldeventuallyliftthewaferFigure4.ArburgAllrounder221M35075injectionmoldingmachine.fromthecavityduringmoldopeningandresultinthebreakageofthewafer.4.ExperimentsAnArburgAllrounder221M35075conventionalinjectionmoldingmachine,asshowninfigure4,withasinglecavity,coldrunnermoldisemployed.ThematerialusedformoldtrialsisBayerMakrolon2205polycarbonatePCthermoplasticresin.Becauseofitsexcellentoptical,chemicalandmechanicalproperties,PCcanbeusedinapplicationssuchasmedicalinstruments,biochemicalsensorsanddatastoragesystems.Thepolymerisinjectedintothemoldcavityatapressurerangingfrom40to50MPa.Themelttemperatureinthefeedingzoneismaintainedatabout300◦C.Themoldtemperatureiscontrolledbyaheaterandmaintainedatatemperaturelowerthan200◦C.Thecycletimeofthemoldingprocessis65s,andpolymermeltandmoldareallowedtocooldownfor30safterthefillingstage.Figure5showsthetypicalpressureversustimeandcorrespondingflowrateversustimerelationshipofthemoldingprocess.Forthemicromoldingprocess,injectionpressure,moldtemperatureand417YCSuetalHolding0t1t2t30t1t2t3InjectionPackingHoldingInjectionPackingTimeTimePressureFlowrateFigure5.Typicalpressureversustimeandcorrespondingflowrateversustimerelationshipsduringtheinjectionmoldingprocess.Figure6.SEMmicrographofmoldingresultsinjectionpressure45MPa,moldtemperature25◦C.injectionvelocityarerecognizedasthedrivingparameters.Thedepthtoopeningratiosofmoldedmicrostructuresareusedtomeasurethequalityofmoldingresults.Ahigherdepthtoopeningratiomeansbetterfillingstatusandmoldingquality.Thepresenceofvoidsplaysamajorroleinthemoldingprocess.Preheatingofthepolymerpriortothemoldingprocessreducesthechancesofentrapmentofvoids.Conventionalventingmethodsaredifficulttouseformicroinjectionmoldingduetothehighpossibilityofundesiredstructuralchangesinthemoldedcomponent.Hence,anevacuatedmoldisrecommendedtoobtainagoodreplicationprocess.Inthefirstmoldtrial,ordinaryinjectionmoldingparameterswereusedandnoadditionalcontrolunitwasactivated.Themoldingresultisshowninfigure6.Ascanbeseen,themoldingresultshaveasmalldepthtoopeningratiowhichmeanspolymermeltcannotfillthemicromoldcavity.Inthissituation,polymericmicrostructurescannotbesuccessfullyreplicated.Beforedoingmoremoldtrialstoimprovethemoldingresults,asimulationtoolwasusedtounderstandtheflowbehaviorofpolymermeltinthemicromoldcavityforfeasiblemodificationstoimprovethemoldingresults.5.SimulationItiswellknownthatcomputeraidedengineeringCAEcanimprovethetrialanderrortechniques,andcomputermodelscanbereliedupontopredictflowbehaviorandmoldresults.Ideally,CAEanalysisprovidesinsightthatisusefulindesigningparts,moldsandmoldingprocesses.ByusingCAEanalysistoiterateandevaluatealternativedesignsandmaterials,engineeringknowhowintheformofdesignguidelinescanbeestablishedrelativelyfastandcosteffectively.TheCAEsoftwareCMOLDdevelopedbyACTechnologyisemployedasthenumericalcomputationtool.ThemoldfillingprocessismodeledbytheGHSmodeldescribedintheprevioussection.Thenumericalsolutionsarebasedonahybridfiniteelement/finitedifferencemethodtosolveforthepressure,flowandtemperaturefieldsandacontrolvolumemethodtotrackmovingmeltfronts.Afiniteelementmeshisusedtoapproximatethecircularshapebaseplatewithconvexmicrostructuresononesurface,asshowninfigure7.Thisfiniteelementmodeliscomposedof6008nodes,2672twodimensional2Dtriangularelementsand4607onedimensional1Dpartrunnerelements.The2Dtriangularelements,whichdisregardtheshearandcoolingfromsidewalls,areusedtomodelthesubstrateplate.The1Dpartrunnerelements,whichconsidertheshearandcoolingfromallthecontactsurfaces,areusedtomodelthemicrostructuresonthesurface.ThefollowingconditionsareconsideredinthisworktocontrolandinvestigatetheinjectionmoldingprocessFillingtime/injectionpressure.Inordertogenerateuniformmolecularorientationthroughoutthepart,itisrecommendedtomaintainaconstantvelocityatthemeltfront.However,onlyadvancedinjectionmoldingmachineshavetheabilitytoexactlyachievethisrequiredvelocityprofile.InCMOLD,eitherfillingtimeorinjectionpressurecanbeusedtocontroltheprocesssequence.Moldtemperature.Itisbelievedthatsurfaceeffectswilldominatetheflowbehavioratthemicroscale,andmelttemperaturesarethekeythatdeterminesthefluidpropertysuchasviscosity,specificheatandthermalconductivity.However,hightemperaturemightcausethedegradationofpolycarbonate,soapredefinedmaximalallowablemelttemperatureisusedinthesimulationprocess19.Thicknessofthebaseplate.Thebaseplateisemployedtosupportmicrostructuresandthethicknessofthebaseplatewillaffectthebalanceofpolymermeltandthequalityofmoldedresults.Becausethethicknessofthebaseplateismuchlargerthantheindividualopeningof418
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