飞秒激光器的应用.docx
- 文档编号:23885506
- 上传时间:2023-05-21
- 格式:DOCX
- 页数:17
- 大小:318.54KB
飞秒激光器的应用.docx
《飞秒激光器的应用.docx》由会员分享,可在线阅读,更多相关《飞秒激光器的应用.docx(17页珍藏版)》请在冰豆网上搜索。
飞秒激光器的应用
Industrialfemtosecondlasersandmaterialprocessing
01/22/2019
NORMANHODGSON,MICHAELLAHA,TONYS.LEE,ALBRECHTSTEINKOPFF, and SEBASTIANHEMING
Overthelastfiveyears,materialprocessingwithfemtosecondpulsesintherangeof300to900fshasgainedinpopularityduetothesmallheat-affectedzone(HAZ)andincreasedenergypenetrationdepthresultingfromthehighlaserpulseintensity.Industrialultrashort-pulse(USP)diode-pumpedsolid-stateandfiberlasersarenowbeingusedtocutfoilsforflatpaneldisplays,tocutstents,andtodrillfuelinjectornozzles,aswellasforwaferscribingandsurfacemicrostructuring.
Thefirstindustrialuseoffemtosecondlaserpulsesformicroprocessingdatesbacktothelate1990s,wheretitaniumsapphire(Ti:
sapphire)amplifierswereusedtorepairlithographymasksinintegratedcircuitfabrication.Atthattime,theonlychoiceincommerciallasersourceswere Q-switched,neodymium(Nd)-dopedsolid-statelasersdeliveringpulsedurationoftensofnanoseconds,andultrafastTi:
sapphireamplifiersthatexhibitedpulsedurationsof100fsandprovidedoutputpoweratthe1Wlevelat1kHz(FIGURE1).Thesmallfeaturesizeofthechromiumlayerontopofthefusedsilicasubstrateprohibitedtheuseofnanosecondpulsesduetothermaldamageofthechromium,andeventhesubstrate.ThenecessityforasmallHAZoutweighedthehighcostofaTi:
sapphireamplifiersystem,whichwasabout$300,000.
FIGURE1. Lithographymaskrepairusinga100fsTi:
sapphirelaserisshown,whereachromiumlayeronafusedsilicasubstrateisablated;widthoftheablatedlinesis750nm.1
Today,manydifferentfemtosecondlasersareavailable,providingoutputpulseenergiesofupto200µJ,withaverageoutputpowersinthekilowattrange.Especiallyoverthelastdecade,alargevarietyoffemtosecondsolid-statelaserandfiberlaserarchitectureshaveenteredthematerialprocessingfield,allofthembasedonytterbium(Yb)-dopedgainmaterials.Averageoutputpowersofupto100Warecurrentlyemployedinindustrialapplications,withpulsedurationsbetween300and900fsandrepetitionratesofupto2MHz.ComparedtotheoriginalTi:
sapphireamplifiersystems,theoutputpowershaveincreasedbytwoordersofmagnitude,whilesystemcosthasbeenconsiderablydecreasedatthesametime.
Amajorityofthelow-powerlasershavebeendeployedinophthalmicapplications.Inthe2000s,Nd:
glassregenerativeamplifierswereusedtocutthecornealflapinLASIKsurgerybygeneratingabubbleplaneinsidethecornea.TheseNd:
glasssystemswerelaterreplacedbyYb:
fibermaster-oscillatorpower-amplifiers(MOPAs),whichallowedhigherrepetitionratesatalowercost(FIGURE2).Typicalpulseenergiesrequiredforflapcuttingare2–4µJatrepetitionratesof50to200kHzandpulsedurationsofaround300fs.Asecond,morerecentlyemergingophthalmicapplicationislensdissectionaspartofcataractsurgery.Inthiscase,thepulseenergiesemployerareintherangeof20to40µJatrepetitionratesof50to100kHz,andpulsedurationsarepreferablybelow800fs.
FIGURE2. Theevolutionofthedeploymentoffemtosecondlasersinmicroprocessingapplicationsisshown;Ti:
sapphireregenerativeamplifiersformaskrepairhavebeenreplacedbypicosecondNd:
YVO4 lasersandNd:
glassregenerativeamplifiersbyfemtosecondYbfiberMOPAs.
AccordingtoaforecastbyStrategiesUnlimitedin2016,thetotalmarketin2019forfemtosecondandpicosecondlasersusedformaterialprocessing(includingophthalmicapplications)isprojectedtobe$460million(TABLE1).2 Halfofthisrevenueisgeneratedbypicosecondlasers,whichhavebeenwidelyusedinmicroelectronicmanufacturing.Theotherhalfissplitbetweenophthalmicfemtosecondlasers($136million)andfemtosecondlasersfornon-biologicalmaterialprocessing($98million).
TABLE1. The2016RevenueForecastoffemtosecondandpicosecondlasersinmaterialprocessingisshown,wherevaluesfor2014and2015areactualrevenues;femtosecondlasersincludeophthalmiclasers.2
Mechanismandbenefitsoffemtosecondlaserprocessing
Theinteractionoffemtosecondandpicosecondpulseswithmatterisgovernedbytheabsorptionofthelightbytheelectronsandsubsequentenergytransfertothelattice.Inthecaseofmetals,thephotonsareabsorbedbytheelectrongas,whichincreasesitstemperaturestovaluesofseveral10,000°C.Theelectronswilltransfertheirenergytothelatticewithintheelectron-phononrelaxationtime,whichformostmaterialsisintherangeof100fsto1psatroomtemperature.Thelatticehasabout100Xhigherheatcapacitycomparedtotheelectrons.Thisleadstoasubstantialdelaybetweentheincidenceofthelaserpulseandthetimewhenthelatticehasreachedmeltingtemperature(FIGURE3).Forhighlaserfluences,theablationoftheheatedmaterialoccursseveraltensofpicosecondsafterthelaserpulseisabsorbed.
FIGURE3. Interactionofultrafastpulseswithametalisshown.Theelectrongasabsorbsthelaserlight,leadingtoahot,thermalizedelectrondistributionwithin100fs;thelatticewillheatupwithadelayof4to30ps.
Thelight-matterinteractionforultrashortlaserpulsescanbemathematicallydescribedbytheTwo-TemperatureModel,whichprovidesthetemporalandspatialevolutionofthetemperatureofelectrongasandlatticebyincorporatingthecouplingbetweenbothsystemsviatheelectron–phononrelaxationtime.3 Thismodelhasbeenusedverysuccessfullyoverthelastdecadestocalculatedamagethresholdfluence,ablationrates,andHAZforultrashortpulseprocessing(FIGURE4).4-8 Themainresultsarethatforpulsesthatareshorterthan10ps,thedamagethresholdfluenceremainsconstant,whileforlargerpulsedurations,thethresholdfluenceincreasesinproportiontothesquare-rootofthepulseduration,independentofincidentlaserpulsewavelength.
FIGURE4. Calculatedtemporaltemperaturedistributionofelectrongas(left)andlatticeforcopperafterirradiationwitha100fspulseatanaveragefluenceof0.14J/cm2 andawavelengthof800nm.7
Similarly,theHAZremainsconstantforpulsedurationsbelow10ps,againindependentofthewavelengthofthelaserlight.Thebasicreasonforthisbehavioristhedelayinthetemperatureincreaseofthelatticeandofheatconductionintothematerial.Thisregimeofpulsedlaser-matterinteractionisthereforereferredtoascoldablation,sincethelatticestayscoldduringirradiationbythelaserpulse.Thisnameisabitmisleading,though,sincethematerialwillhavetoreachmeltingtemperaturetoinduceablation.
Themostinterestingeffectofultrashortpulseinteraction,however,istheincreaseintheenergypenetrationdepthandablationdepthwithdecreasingpulseduration.Decreasingthepulsedurationatagivenenergyfluenceleadstoanincreaseinthetemperatureoftheelectrongasandasimultaneousincreaseoftheelectron-phononrelaxationtime.
Inamoremechanicalmodel,thiscanbeeasilyunderstood:
thevelocityoftheelectronstravelingthroughthelatticecanbecomeashighas100,000m/sforfemtosecondpulsesduetotheveryhighintensity,andthishighspeedresultsindeeperpenetrationoftheelectronintothelatticewithouttransferringenergytothelattice.8
Thematerialprocessingefficiencyandqualitydependsonthedurationofthelaserpulses.Forpulsesinthenanosecondregime,theabsorptionofthelaserpulseisdeterminedbythelinearopticalabsorptiondepthofthelaserlightandtheenergydissipationisaresultofheatconductionintothematerial(FIGURE5).Forpulsesthatareshorterthan10ps,theinitialenergypenetrationdependsstronglyonthelightintensityandleadstodeeperpenetrationdepthforfemtosecondpulses.Inaddition,thelackofthermalconductionduringthepulseandthetimeoflatticeheatingresultsinaverylowHAZ.Formetals,HAZoflessthan5µmcanbeachieved,whileforplasticmaterials,theHAZistypicallyintherangeof30to50µm.
FIGURE5. Absorptionofalaserpulseinamediumisshown,whereenergypenetrationisrepresentedinblueandvolumeheatedviaheatconductioninred.
Theincreaseofthepenetrationdepthforshorterpulsesleadstoanincreaseinthemaximumablationrateswhenthepulsesbecomeshorterthanabout20ps(asshowninFIGURE6foraluminum).Aswillbediscussedsubsequently,atanypulsedurationthemaximumablationrateisachievedatapulsefluenceofabout7.5Xtheablationthresholdfluence.Comparedto Q-switchedlaserpulseswithpulsedurationsoftensofnanoseconds,theincreasedelectronvelocityinthematerialleadstoablationratesforsub-picosecondpulsesthatareonlyafactorofthreelower.
FIGURE6. MaximumablationratesofaluminumwithCoherent'smeasuredvalues(reddots)andusingvaluestakenfromdatapublishedbyBreitlingetal.(bluedots).9
Forpulsedlasers,ablationbecomesmostefficientatapulseenergyfluenceequaltoe2 timesthethresholdfluence.Thisisaresultofthesaturationoftheablatedvolumewithincreasingpulsefluence.Atafixedoutputpower,morevolumecanthereforebeablatediftheenergyfluenceislowered,whilesimultaneouslyincreasingpulserepetitionrateandthereforethroughput(FIGURE7).Themaximumvalueisachievedattheoptimumfluence.Theablationrate,C(inmm3/W/min),isgivenby:
where F isthepeakfluenceinJ/mm2, Fth isthepeakthresholdfluence,and d istheenergypenetrationdepthperpulseinmillimeters.10 Typicalvaluesfortheenergypenetrationdepthperpulseareintherangeof 20to100nmformetals,semiconductors,andplastics,and>500nmforglassesandtransparentcrystals.
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- 激光器 应用