Time-resolved imaging of flyer dynamics for femtosecond laser-induced backward transfer of solid polymer thin films

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Full

Length

Article

Time-resolved

imaging

of

ﬂyer

dynamics

for

femtosecond

laser-induced

backward

transfer

of

solid

polymer

thin

ﬁlms

M. Feinaeugle

a,∗

_,

_P.

_{Gregorˇciˇc}

b

_,

_D.J.

_Heath

a

_,

_B.

_Mills

a

_,

_R.W.

_Eason

a a_{Optoelectronics}_Research_Centre,_University_of_Southampton,_Southampton,_SO17_1BJ,_UK

b_Faculty_of_Mechanical_Engineering,_University_of_Ljubljana,_{Aˇskerˇceva}_6,_1000,_Ljubljana,_Slovenia

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received15July2016

Receivedinrevisedform27October2016 Accepted15November2016

Availableonline19November2016 Keywords:

Laser-inducedbackwardtransfer Time-resolvedshadowgraphy Femtosecondlaser-induced micro-processing Polymerthinﬁlms Additivemanufacturing SU-8

a

b

s

t

r

a

c

t

Wehavestudiedthetransferregimesanddynamicsofpolymerflyersfromlaser-inducedbackward transfer(LIBT)viatime-resolvedshadowgraphy.ImagingoftheflyerejectionphaseofLIBTof3.8␮m and6.4␮mthickSU-8polymerfilmsongermaniumandsiliconcarriersubstrateswasperformedover atimedelayrangeof1.4–16.4␮safterarrivalofthelaserpulse.Theexperimentswerecarriedoutwith 150fs,800nmpulsesspatiallyshapedusingadigitalmicromirrordevice,andlaserfluencesofupto 3.5J/cm2_while_images_were_recorded_via_a_CCD_camera_and_a_spark_discharge_lamp._Velocities_of_flyers

foundintherangeof6–20m/s,andtheintactandfragmentedejectionregimes,wereafunctionofdonor thickness,carrierandlaserfluence.Thecraterprofileofthedonoraftertransferandtheresultingflyer profileindicateddifferentflyerejectionmodesforSicarriersandhighfluences.Theresultscontributeto betterunderstandingoftheLIBTprocess,andhelptodetermineexperimentalparametersforsuccessful LIBTofintactdeposits.

1. Introduction

Additive methods for the microfabrication of devices have

recentlygainedinterestoverconventionaltechniquesduetotheir

versatility,simplicityandresultinghighspeedoffabrication[1–3].

Amongthese,laser-basedtechniquesareapromisingwaytoenable

deviceprintinginacontactlessfashionwithdemonstrated

micron-scaleresolution.Auniqueadvantageisthatthesemethodsallow

thedepositionofmaterialsthatnotonlyhaveaspeciﬁcstructural

role,butalsohaveelectronic,photonicorevenbiomedical

func-tionality.

Inparticular,laser-inducedforwardtransfer(LIFT)hasproven

itscapabilitytoallowmanufacturingofawiderangeof

materi-als,suchasmetals[4],ceramics,semiconductors,superconductors

[5],2D materialsand structures for e.g.MEMS [6],waveguides

[7],biomedicalsensors[8]orthermoelectricgenerators[9].More

recently,thetransferofsilverpastes[10,11],3-dimensional

micro-objects[12,13]andmetalvias[14]hasshownthepotentialofLIFT

formicrofabrication.DuringLIFT(e.g.withatransparentdonor),

∗ Correspondingauthor.Currentaddress:ChairofAppliedLaserTechnology, LaboratoryofMechanicalAutomationandMechatronics,FacultyofEngineering Technology,UniversityofTwente,Enschede,TheNetherlands.

E-mailaddresses:m.feinaeugle@utwente.nl(M.Feinaeugle), bm602@orc.soton.ac.uk(B.Mills).

shownschematicallyinFig.1a,apulsedlaserbeamisfocussedor

imagedattheinterfacebetweenatransparentcarriersubstrateand

asandwichofthinﬁlms,consistingofanabsorbingmaterialand

thedonor.Asaconsequenceoftheabsorbedlaserenergy,asmall

volumeofthedonorisejectedandtransferredontoareceiver

sub-stratewhichislocatedparalleltothedonorsurface.Insomecases,

thedonoritselfactsasanabsorberandnoadditionalinterfacial

layerisrequired.Thespacingbetweenthedonorandreceiveris

typicallyinthefewtotensofmicrometresrange.

The minimum featuresizesof structures fabricatedvia LIFT

ismainlylimitedtoopticalresolutionsforcongruenttransferof

devices[15,16].However,formoltentransfer,structuresthatare

smallerthanthediffraction-limitedsizeoftheincidentlaserpulse

havebeendemonstrated[17,18].Speciﬁcallyforthefabricationof

thosestructures,laser-inducedbackwardtransfer(LIBT)[19]has

producedsubmicron-structureswithhighrepeatabilitywhichmay

provetobeanadvantageousalternativetoLIFTforspeciﬁc

appli-cations[20,21].DuringLIBT,shownschematicallyinFig.1b,the

receiverwhoseabsorptionislowincomparisonwiththecarrier

issituatedinthepathofthelaser,whilethedonoriscoatedon

a(bulk)carriersubstrate.Theincidentlaserpulseenergythatis

eitherabsorbedinthedonoror-forpartiallytransparent

donors-thecarrier leadsto thetransfer ofa volume of thedonor in a

directionoppositetothatofthelaserbeampath,hencetheterm

‘backward’.

http://dx.doi.org/10.1016/j.apsusc.2016.11.120

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Fig.1.Schematicside-viewof(a)laser-inducedforwardtransfer(LIFT)and(b) laser-inducedbackwardtransfer(LIBT)foratransparentdonor.

IncomparisonwithLIFT,LIBThasdifferentrequirementsand

restrictionsconcerningthetransparencyofthereceiverandthe

donor,butthepossibilitytouseabulkcarriersubstratemightprove

advantageousforcertainapplications.Theseadvantagescanalso

beusedfortransferofothermaterialsasdemonstratedinprevious

work,wheremetals[22–25],oxides[25,26],CrSi2[27],TiN[24]

andcorrodedsurfaces[28]havebeenthesubjectofstudiesofLIBT

methods.

Recently,we havedemonstrated thattheuseofa bulk

sub-strate facilitatesthe imprint-based laser-induced fabrication of

sub-micron-sizestructuresviaLIBTofsolidpolymers[29].

WhileLIFThasbeentheobjectofmanystudiestodate,much

lessefforthasbeenputintofullyunderstandingandexploitingthe

processofLIBT.Tohelpinpredictingtheoutcomeofanexperiment

viaLIBTe.g.withanewmaterial,imaging[24]andsimulation[30]

oftheprocesscanbeusefultools.Also,asLIBTiscloselyrelatedto

theprocessesoflaserlift-off[31],lasercleaning,laserscribing,or

evenablation[32],studyingLIBTcouldalsoaidinunderstanding

theseprocesses.Themaindifferencewithrespecttopreviouswork

isthatforourexperiments,weareinterestedintheejectedmaterial

beinginanintactstate,andthatitsshapeisgeometricallysimilar

totheincomingspatialshapeofthelaserpulse.

ToimprovetheeffectivenessofLIBT,factorssuchaslowﬂyer

velocityandreducedshockgenerationplayamajorrolein

trans-ferringaﬂyerinanintactstate[33].Anexperimentaltime-resolved

imagingstudycouldthereforesupportfutureeffortstomodelthe

LIBTprocesstooptimiseexperimentalconditions,andto

under-standtheadvantagesandlimitationsofthistechnique.Previously,

thefemtosecondlaserablationofsilicagrownontopofsiliconwas

investigatedintheﬁrst∼10nsafterthearrivalofthelaserpulse

[34].Inadifferentstudy,asimplemodelofsilicaonaAgsubstrate

wassimulatedonapicosecondtimescale[30].

Whilethefocusofthosestudieswasontheobservationofshock

andﬁlmdynamicsintheﬁrstnanosecondsafterthearrivalofthe

laserpulse,here,wehaveexaminedthedynamicsoftheemerging

ﬂyerandfragmentsonamicrosecondscaletoobtainexperimental

parametersforLIBTofintactdeposits.

Inthisstudy,wehaveimagedtheLIBTprocessfroman

epoxy-basedSU-8polymerdonorﬁlmfromplanarsiliconandgermanium

carriersviaafemtosecondpulsedlasersource.SU-8isanexample

ofatransparentdonormaterialthatcanbeusedfore.g.photonicor

microﬂuidicdevices.Thispolymer,oncedeveloped,hasarelatively

highchemicalresistance,andhasbeenpreviouslyusedin

micro-opto-electro-mechanicalsystems(MOEMS).Itisroutinelyusedfor

lithographicpatterningonthemicro- andnanoscale,andhence

beneﬁcialforthecreationofsmallstructures.Siliconand

Germa-niumcarrierswereusedasreadilyavailablebulksubstratesthatare

widelyusedinmicrofabricationofelectronicandphotonicdevices,

thusforwhichalargenumberofmicrofabricationprocessesare

known.

Whenusingpolymersasdonormaterial,photophysicaleffects

asdamagemechanismsneedtobeconsideredduringtransfer,and

thesemechanismsincludephotochemicaldecomposition,thermal

Fig.2. Setupfortime-resolvedimagingofLIBT.ThelaserpulseswithGaussianbeam proﬁlearehomogenisedtoatophatproﬁleviaarefractivebeamshaper(BS).Laser triggering,time-resolvedimaging,DMDmaskdisplayandbeamattenuationare controlledbyacomputer.

ablation,spallationandphotopolymerisationofmonomerchains

[35,36].Theuseofshortpulsesandinfraredwavelengthdecrease

thelikelihoodofdamageviathermaleffectsordirectruptureof

polymericbonds respectively, while multiphoton effects would

onlybeexpectedforthehighestﬂuencesused.Atthesametime,

thermaleffectstothesemiconductorcarriersareexpectedtobe

reducedwithshortlaserpulseswhencomparedtolongerpulsed

lasersources,andthishasfurthermotivatedourchoiceoflaser

sourcefortheseexperiments[37].

Withthehelpofatime-resolvedshadowgraphysetup,wehave

recordedthepositionoftheemergingﬂyerasafunctionofpulse

energy,donor thickness,carrier material,delay time afterlaser

pulsearrival,andbeamintensitydistribution.Shadowgraphycan

beusedtodeterminetheexistenceandpositionofparticlesand

ﬂyerejectedfromthedonorsurfaceandisalsosensitivetochanges

intherefractiveindexofthesurroundingatmosphere,e.g.through

gradientsinpressureorgaseouselements.Generally,

shadowgra-phycanbemostreadilyperformedwiththepresenceofareceiver

tostudyimpactandlandingoftheﬂyer,andthereceiver’s

inter-actionwithpressurewaves.Insteadwehavechosentostudythe

dynamicsoftheﬂyerejectionwithoutreceiverwhichisacase

rel-evanttotheLIBTprocessasthevelocity,integrityandorientation

oftheﬂyer,andthepossiblecreationofdebrisorshockcanbe

observedoveralargerrangethanpossiblewithareceiverinplace.

Inthefollowingwereferto‘transfer’forthedynamicsofﬂyer

ejec-tionandpropagationfortargetsin aLIBTconﬁgurationasused

here.Toallowamore directcomparisonwiththeLIBTprocess,

wehavebrieﬂycontrastedtheresultsfromshadowgraphywith

thoseofstandardtransferexperiments,wherewehavemeasured

theratioofintactﬂyersfoundonareceivertoﬂyersimagedinan

intactstate.

Wewillﬁrstintroducetheexperimentaldetailsandmethods

ofthetime-resolvedstudiesofLIBT.Thenwepresent

experimen-talresultsfromvaryingtimedelay,laserﬂuence,donorthickness

andcarrier.Further,wewilldiscussthedifferentobserved

trans-ferregimesandtheeffectsofexperimentalparametersonLIBTof

SU-8.

2. Experimental

The imaging of the LIBT process was carried out using the

setupshowninFig.2.Itconsistedofthreedifferentopticalbeam

lines,onefor liveimaging ofthesample surface,onefor

laser-inducedtransferandthethirdonefortime-resolvedshadowgraph

imaging.TransferwasinducedviapulsesfromaTi:sapphirelaser

oscillator-ampliﬁersystem(Mira/Legend,Coherent)withacentral

wavelengthof800nm,andpulselengthsof150fs.Themaximum

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neutraldensityﬁlter.TheGaussianintensityproﬁlefromthelaser

wastransformedintoa top-hatintensityproﬁleviaarefractive

beamshaper(Pi-Shaper,AdlOptica).Theselaserpulseswith

top-hatproﬁlethenilluminated thesurfaceof a608×684element

digitalmicromirrordevice(DLP3000, TexasInstruments)whose

mirrorswereactuatedtoformadynamicintensitymask.Todo

so,mirrorsinthe‘on’positiondirectedlightintothebeampath

showninFig.2,while mirrorsin‘off’positionsteeredthelaser

pulsesintoa beamstop(notshown).Thesurfaceof thedigital

micromirrordevice(DMD)displayingauser-speciﬁedmaskwas

imagedandde-magniﬁedattheinterfacebetweenthedonorand

carrierintheLIBTtargetwitha50×de-magniﬁcationmicroscope

objective(Mitutoyo).

Forsamplepositioningandfocussing,thesample,illuminated

withawhitelightsource,isimagedcontinuouslyonaCMOS

cam-era,whoseimagepathiscollineartothelaserbeampath.More

detailsonthesetupandontheconﬁgurationoftheDMDforimage

projectioncanbefoundinpreviouswork[38].

The laser-induced events above the sample surface were

recordedviailluminationfromawhitelightsparkdischargeﬂash

lamp (Nanolite KL-K, HSPS) witha pulse duration of 8ns [39].

TheﬂashlampwasplacedatthefocusofmicroscopeobjectiveL1

(10×magniﬁcation,LeitzWetzlar)usedasacollimator.Asecond

microscopeobjectiveL2(50×magniﬁcation,Nikon)wasusedto

concentratetheilluminatinglightattheinteractionarea.The

inter-actionoftheilluminatingbeamsandthelaser-inducedobjectsor

differencesinrefractiveindiceswerethenobservedasintensity

gradients[40]onaCCDcamera(scA1400–17fm,1.4Mpx,Basler

AG)equippedwithamicroscopeobjectiveL3.Dependingonthe

resolutionrequiredandtheﬁeldofview,weusedoneoftwo

dif-ferent(20×and100×magniﬁcation,Nikon)microscopeobjectives

L3.Oursetupthereforeprovidedthefollowingtheoretical

reso-lutions:90nm/pixeland420nm/pixel,whiletheﬁeldofviewin

thedirectionofﬂyermovementequalledapproximately120␮m

and400_␮mrespectively.The excitationlaser,ﬂash lamp,

shut-terandtheCCDcameraweresynchronisedbyasignalgenerator

(Tektronix,AFG3102)whilethedisplayoftheDMDimagemask,

lasertriggeringandattenuationlevelwerecontrolledbya

com-puter.

Followinglasertriggering,thesignalgeneratorcausedtheCCD

cameratobeactivefor∼20msandatthesametimeactuatedthe

ﬂashlampatachosendelaytimewithaminimumvalueof1.4␮s.A

snapshotwasthereforetakenafterthechosendelaywithan

expo-suretimeof∼8ns.Wevariedthisdelayduringexperimentsover

therange between1.4␮sand16.4␮swithanestimated

uncer-taintyof100ns.Foreachdelayanewareaofthedonorwasselected,

sothedatapresentedbelowconsistsofsequentialﬂyersimagedat

differentdelaysbutotherwisesimilarconditions.Toreduceerrors

duetonaturalﬂuctuationsinﬂyerbehaviour,eachsetofsimilar

conditionswasrepeatedatleastﬁvetimes.Thedelaywaschosen

toshowﬂyerstravellingthefullextentoftheimageframevisible

tothecamera.

Fig.3ashowsaschematicoftheimagingsetupandtheimage

showninFig.3bisatypicalshadowgramrecordedwiththeCCD

Fig.3.(a)Schematicside-viewofshadowgraphyimagingsetupand(b)imageframe asrecordedbyCCDcameraanda100×microscopeobjective.

camerawhere ﬂyerand donor surfaceorientationappear atan

anglerelativetoeachotherasaresultofcameraperspective.The

contrast,brightnessandgammavaluesofthecapturedimageswere

modiﬁedtooptimisevisibilityofthelaser-inducedevents.

TheLIBTtargets(i.e.thedonor-coatedcarrier)werefabricated

viaspin-coatingofSU-8photoresist(Microchem)ontosiliconand

germaniumcarriersubstrates(300–600␮mthick).Thegermanium

carriersconsistedofa3␮mthicklayerofGegrownonaSisubstrate.

Before spin-coating, the carriers were ultrasonically cleaned in

sequentialbathsofacetone,isopropanolandwaterfor30minand

subsequentlydriedbypressurisednitrogen.Aftercoating,theLIBT

targetswerebakedonahotplatefor3minrampingfrom60to90◦C,

andthesamplewasheldat90◦Cforafurtherminutetosolidifythe

polymerandtoremoveanyresidualgamma-butyrolactonesolvent

[41].Finaldonorthicknesseswere3.8␮m(referredtolateras‘thin’)

and6.4␮m(referredtoas‘thick’),measuredwithamechanical

proﬁler.Thevariationofdonorthicknessmeasurementwasinthe

rangeof±100nmwhilethemeasurementerrorbythe

mechani-calproﬁlerusedwasestimatedtobesmallerthanthisvariation.

Theﬁlmswerecoatedatspinspeedsof2000rpmand4000rpmfor

maximumaccelerationsof300rpm/sandaspindurationof30s.

Theﬁlmthicknessofafewmicronswasatypicalthicknessusedin

SU-8basedMEOMSandaroundthestandardthicknessforSU-85

usedinlithography.

Thetargetswerethen cleavedinthecentreforbetter

imag-ingofthecentralpartofthedonortoavoidshadingbythethick

donorbeadfoundattheperimeterofaspin-coatedsampleand

experimentswereperformedatleast100␮mawayfromthe

sam-ple(donor)edgestoavoidvariationofmaterialproperties,suchas

areduceddonor-carrieradhesion.

3. Resultsanddiscussion

3.1. Velocityoftheﬂyer

Intheﬁrstexperimentswevariedthedelaybetweenvaluesof

1.4–10␮sbetweenincidenceofthelaserpulseandrecordingof

thepositionoftheﬂyerabovethesubstrate.Asequenceofimages

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Fig.5.Distancetravelledbyﬂyersforvaryingdelaytimesfrom(a)Siand(b)Ge carrierswiththickandthinSU-8donors.Theerrorbaristhestandarddeviationat acertaindelaytime.

takenfordifferentdelaysataﬂuenceof1.39J/cm2_for_ﬂyers_from

athickSU-8onsilicontargetareshowninFig.4.

Aﬂyeremergesfromthesurfacenotasacylindricaldiscwhich

couldbethecircularshapewithtop-hatspatialintensityprojected

attheinterface,butisslightlytaperedandadditionallyfeaturesa

thinperipheralrimrippedoutofthedonortoresultina

‘saucer-like’structure.Mostoftheshadowgraphimagesshowtwobright

areas,oneinthecentreoftheimageandasecondoneinthecrater

onthedonor.Theﬁrstspotoriginatesfromdirectimagingofthe

sparkgapillumination,andwhilethepresenceofthisbrightspot

intheimagewasundesired,thisgeometrywasusedtomaximise

ﬂyerillumination.Thesecondspotisaconsequenceofscattering

ofthelaserpulsevisibleduetothelongexposuretimeoftheCCD

camera.

Theimagealsorevealsthatthebottomsideofthetiltedﬂyer

carriessomedamageappearinginthecentralregionoftheﬂyer.

Thisdamage,seenasdarkspotswithinaﬂyer,occurredtosomeof

theﬂyersandwasmostlikelycausedbyimperfectionsofthedonor

orﬂuctuationsofthelaserpulseintensityduetoimperfectoptics

orlaseroutput.Asﬂyerswerenotcollected,furtherevaluationof

thisdamagewasnotpossible.TheparticlesshowninFig.4b–dis

debriswhichlikelyhasitsorigininthecentraldamagedspotofthe

ﬂyer.

Experiments were carried out just above ﬂyer removal

ﬂu-encethresholdsforthe3.8␮mand6.4␮mthickﬁlms.Fordonors

onsiliconthesethresholdﬂuenceswerebetween0.80J/cm2 _and

1.35J/cm2 _as_a_function_of_donor_thickness._Germanium_carriers

hadthresholdﬂuencesbetween0.55J/cm2_and_0.60_J/cm2_._The

dif-ferencesinthresholdﬂuencesobservedbetweenexperimentswith

thedifferentcarriersubstratescouldbereducedtoboththe

differ-entphysicalpropertiesofthecarrierortothedifferentadhesionof

donoroncarrier,andfurthermeasurementsofadhesionwouldbe

requiredtodeterminetheeffectoftheseproperties.

Thedistanced travelledbytheresulting intactﬂyersfor an

imagetakenafteraspeciﬁcdelayisshowninFig.5.

ExperimentsforthinSU-8onsiliconwereperformedatﬂuences

justaboveandat20%higherthanthresholdforcomparisontosee

ifthereisameasurableinﬂuenceofﬂuenceonvelocity.Thehigher

valuewaschosentobebetweenthethresholdandtheﬂuenceat

whichthelikelihoodofbreakingupwouldincreasedramatically.

Thevelocityv(t)ofaﬂyerasafunctionofdelaytasshowninTable1

andFig.6wasdeﬁnedas:

v

(t) =d (t) −d (t1)

t−t1

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witht1theminimumdelay(1.4␮s).Frompreviousexperiments,it

isexpectedthatﬂyerejection(occurringattimet0)isinitiatedon

thetimescaleofhundredsofnanosecondsafterlaserpulsearrival

andisafunctionofﬂuence[42,43].Generally,usingshortpulses

Table1

Intactflyervelocitiesfordifferentcarrier/donorcombinationsextractedfrom dis-tancevs.delaydata.ThefluencevaluesforthesamplesareshowninFig.5.The temporalmeanofthevelocityisshownfordifferentfluencesandcombinationsof donorandcarrier.

Carrier/Donor(Fluence) Meanvelocity[m/s]

Si/ThinSU-8(0.84J/cm2₎ _18.1_±_2.1

Si/ThinSU-8(1J/cm2₎ _19.8_±_2.3

Si/ThickSU-8 9.3±2.8

Ge/ThinSU-8 13.5±1.5

Ge/ThickSU-8 9.1±5.4

Fig.6.Velocityasafunctionofdelaytimefor(a)siliconand(b)germaniumcarriers withthin(3.8␮m)andthick(6.4␮m)SU-8donors.

and higher ﬂuencescan decrease t0. However,theinﬂuence of

dynamicreleaselayers(DRL)suchasAu[44]isinconclusive,while

foraTriazeneDRL[45],athickerﬁlmcandecreaset0signiﬁcantlyas

comparedtoathinDRL[43].Whilethepressureofthe

surround-ingatmospheredoeshavealargeeffectonpropagationvelocity,

nomajorinﬂuenceonejectiontimecouldbeobservedinliterature

[33].

Resulting mean velocities for ﬂyers were in the range of

9–20m/s,andthelowestvelocityrecordedwas6m/s.Forboth

car-riers,thethickerﬂyershadalowervelocitythanthethinﬂyers.The

comparisonofathinﬂyerfromaSicarrierinTable1showsslightly

highervelocitiesforaﬂyerejectedat20%higherﬂuencebut

oth-erwisesimilarconditionsforthemeanvelocity.Fig.5alsoshows

thatﬂyersejectedathigherﬂuencehavealwaystravelledfurther

atcomparabledelaytimes,hintingatasmallerﬂyerreleasetimet0

forhigherﬂuences.Thevelocitiesasafunctionofdelayareplotted

inFig.6.

Noindicationofanydeceleration,e.g.bydrag,couldbeseen

inthevelocityoverthetimedelaysstudied.Theﬂyerpropagation

wasassumedtobeinﬂuencedbythedecelerationthroughthe

sur-roundingatmosphereaswellasbytheﬂyerrotationinducedduring

ﬂyerejection.Gravityactingintheoppositedirectionoftravelwas

neglectedbeingseveralordersofmagnitudesmallerthandragfrom

thesurroundingair atatmospheric pressure.Thedifferentﬂyer

velocitiesobservedarecrucialtoestimatetheoutcomeofaLIBT

experimentasahighervelocityincreasestheimpactwhenlanding

onareceiver.

3.2. Influenceoflaserfluenceonflyerpropagation

AsobservedinFig.5forthethindonorﬁlms,distancetravelled

inaspeciﬁctimeperiodincreasedforhigherlaserpulseenergies

incidentonthetargetsasseenearlier[46].Thisrelationwas

inves-tigatedinmoredetailbyrecordingﬂyerpropagationfora ﬁxed

delaytimebutvaryinglaserﬂuenceforGeandSicarriers.Fig.7

showstheresultingbehaviourforﬂyersfromathickSU-8donor

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Fig.7. Distancetravelledbyflyerfortwodelaytimeswhenvaryingincidentpulse fluenceforaGe carrierandathickSU-8donor film.Thefirst datapointsat ∼400mJ/cm2_show_that_no_flyer_had_emerged_from_the_donor.

Fig.8.Distancetravelledasafunctionofincidentlaserﬂuenceforaﬂyerfroma thickSU-8donoronSicarrier.

Duringthisexperimentonlyﬂyersuptoaﬂuenceof0.9J/cm2

and2.3J/cm2_for_Ge_and_Si_carriers_respectively_were_observed_to_be

inanintactstate.Forhigherﬂuences,morethan90%ofﬂyerswere

foundtohavefragmented.Thedistancesplottedatlargerﬂuences

showthedistancetravelledbythemainfragmentsejectedfrom

thedonorsurface.Beforebreakup,themeasuredaverage

veloci-tiesofthefastestﬂyerswere12.2±7.7m/s.Theerrorfoundfor

thesefastestﬂyerswasrelativelylargeandweassumedthatthe

tiltofsomeoftherecordedﬂyersmayhavecontributedtotheir

deceleration.Foragermaniumcarrier,allﬂyersejectedatahigh

ﬂuencetravelfurther,thusatahigheraveragevelocity,thanﬂyers

ejectedatalowerﬂuenceclosetothreshold.Theresultinglinear

increaseofvelocityasafunctionofﬂuencewasintherangeof

0.023±0.01m/smJ−1cm2_for_ﬂyers_in_an_intact_state.

Asimilarplotofdistanceoverdelaytimefromanexperiment

withsiliconcarriersisshowninFig.8.Here,thedistancecurves

aresplitataﬂuencevalueof∼1.8J/cm2_into_a_{‘sawtooth’}_function

withpositivegradients.Theﬁrstpartoftheﬁttedsawtooth

func-tion(1.25–1.8J/cm2_),_showing_the_distances_of_intact_ﬂyers,_has_a

gradientofvelocityof0.014±0.01m/smJ−1cm2_._The_velocity_of

thefastestﬂyersherewas16.8±4.6m/s.

Intheﬁrstpartofthecurve,distanceincreasesmonotonically.

However,around2.0–2.5J/cm2_,_propagation_is_much_lower_than

expected.Onlyforvaluesof3.5J/cm2_is_propagation_again_larger

Fig.9. CraterdiameterforvaryingﬂuencemeasuredontwoSiandGedonorsafter LIBTexperimentsforathickSU-8donor.Theinsetsshowsmicroscopeimagesof cratersinthedonoraftertransfer(forSicarrier).Thescalebaroftheinsetsis25␮m.

thanforﬂyersejected at1.8J/cm2_._Such_behaviour_would_seem

toindicateadecelerationorachangeoftransferregimeforthose

higherﬂuences.Tofurtherinvestigatethisbehaviour,wehave

mea-suredthediameteroftheablationcratersinthedonorﬁlmsleft

behindbytheejectedﬂyerasshowninFig.9.

Foraﬁxedimagemaskasusedthroughoutthisexperiment,it

wasexpectedthatcratersizewouldremainconstantorincrease

onlyslightlyforincreasingﬂuenceduetoareasintheperimeter

oftheimagedmaskfeature,whereintensity,decayinginan

expo-nentialfashionduetoimperfectimaging,exceededthetransfer

threshold,resultinginalargerﬂyerbeingejected.However,Fig.9

showsalocalminimumincratersizeforSU-8/Sitargetsataround

2.5J/cm2_conﬁrming_that_craters,_and_as_a_consequence_ﬂyers,_do

nothaveaconstantorlinearincreasingdiameter.

Thebeamdiameteratthedonor-carrierinterfacewas∼20␮m,

estimatedfrommicroscopeimagesofthedonordamageatlow

ﬂu-ences.FromFig.9andpreviousexperimentswecanseethatthe

flyershapeismuchlarger.Duringflyerrelease,theflyershearsoff

additionalneighbouringdonorareasandthusresultsinanincrease

inﬂyerdiameter.Althoughanexactcauseoftheobservedvariation

inflyerdiameterisdifficulttoconfirm,weassumedthatdifferent

factorscouldcontributetotheobservedﬂyersshapedistribution.

Ithadbeenobservedearlierinpolymersthathigherimpact

veloci-tiesonpolymercanleadtodifferentfailuremodes[47].Slowdonor

loadingwouldinducebrittlefracture(tensilefailuremode)while

fastloadswouldinduceatransitiontoaductile(failure)regime

whereshearcrackgrowthispreferred.Thusafast‘push’couldlead

toadifferenttransferregimepreferringstraightedgesincraterand

resultingﬂyer.Thisdifferentfailuremechanismwouldthencausea

differentamountofkineticenergytobedeliveredtotheﬂyerduring

ﬂyerejectionorontheotherhandthedonoracceleratedat

dif-ferentrateswouldsufferfromthesedifferentfailureregimes.The

effectofvaryingcratersizeandvelocitywasnotseeninthe

exper-imentswithGecarrierandhencecouldbeaconsequenceofthe

relativelyhighﬂuencesrequiredfortheSicarriers.Ingeneral,the

failuremodedeterminestheresultingﬂyeredgequalityandshape.

Additionally,theincreasedﬂuencecouldincreasethelikelihood

ofnon-linearmultiphotonabsorption,shock-inducedchanges[48]

orevenheatingoftheﬂyerandhenceitschangeinglobalorlocal

mechanicalpropertiesexplainingtheobservedchangesinﬂyerand

cratershape.

Fromour experimentshere,we canalso estimatethe

inﬂu-enceofthereceiver bydeterminingtheﬂuencewindowFW,in

(6)

Fig.10.ShadowgraphimageofflyersejectedfromathickSU-8filmonasiliconcarrierejectedat(a)1.39J/cm2_,_(b)_1.69_J/cm2_,_and_(c)_2.05_J/cm2_{respectively.}_Delay_times were3.4␮s.Imagesweretakenviaa100×objective.Thescalebaris20␮minallfigures.

Fig.11.ShadowgraphimagesfromLIBTeventsofathickSU-8donoronsiliconcarrier.Fluenceswere(a)1.39J/cm2,(b)2.05J/cm2,and(c)3.46J/cm2.Imagesweretaken withaCCDmounted20xobjectiveanddelaywas2.4␮s.Thescalebaris50␮minallﬁgures.

orintactforLIBTexperimentsprintingontoareceiver.Thisresults

inatransferwindowdeﬁnedas:

FW= Fu−Fth

Fth

(2)

withFu and Fththerespectivemaximumandminimumﬂuence

forwhich ﬂyersare ejectedor depositedinintactstate.FW for

thickﬂyersonGeand Siwasapproximately60%. For

compari-son,inanexperimentusingapolydimethylsiloxane-coatedglass

receiverandathinSU-8donorfromaSicarrier(ﬂyersintransfer

werenotimaged),FWwas∼16%,comparedto∼38%for

shadowg-raphyexperiments,forﬂyersejectedfromthedonor.Thisindicates

thattheinﬂuenceofthereceivercontributestoareductionofthis

transferwindowbyapproximatelyafactoroftwo(≈38%/16%).As

shownpreviouslyforLIFT,thereductionoftransferwindowcanbe

explainedbyshockwavesreﬂectingoffthesurfaceofthereceiver

[33]ordestructionduetoimpactonthereceiver[49].Further,it

mayaswellbepossiblethatthereceivermightcauseaberrations

leadingtoimperfectimagingoftheobjectattheimageplanewhich

inturnwouldincreasethelikelihoodoffracturedﬂyerejection.

3.3. Transferregimesofﬂyers

Theinﬂuenceoflaserpulseenergy,andthusﬂuencedeliveredto

theLIBTtarget,onthevelocityandshapeofaﬂyer,isfurthershown

inimagesof ﬂyerejectionevents. Fig.10 shows shadowgraphs

takenwitha100×objectivefromathinSU-8onsilicontargetat

delaysof3.4␮s.Fortheselectedﬂuencesof1.39J/cm2_,_1.69_J/cm2

and2.05J/cm2_propagation_distance_is_described_{approximately}_by

thedatapresentedinFig.8.

Theﬂyerswiththesameconditionsasfortheoneshownin

Fig.10chaveadifferentproﬁlecomparedtotheotherﬂyers

con-ﬁrmingthedataofcraterdiametershowninFig.9.Theyhavea

smallerdiameterandappeartomissthethinrimseenintheother

ﬂyersofFig.10.However,theyonlypropagatetoadistancesimilar

toaﬂyerejectedataﬂuenceof1.69J/cm2_,_hence_not_all_the_excess

energydepositedintothetargetisusedforaccelerationoftheﬂyer

inadirectionawayfromthedonor.

Whenusinganobjectivewith20×magniﬁcationwithathick

SU-8/Sitarget,debrisdistributionandfastparticlescanbebetter

detectedduetothelargerﬁeldofview.Fig.11showstransferevents

for low (1.39J/cm2_), _medium_(2.05_J/cm2₎ _and _high_(3.46_J/cm2₎

fluences.Notethatforthesemediumfluences,intactflyersonly

occurredin∼50%oftransfereventsandwereneverseenforhigh

(7)

Fig.12.DistanceversusdelayforthickSU-8/Sianddifferenttransferregimes.Note thatforthetwohigherfluences,distancevaluesareshownforthemainfragments, whileforthelowestfluence,onlyintactflyersareshown.

Forlowﬂuencesinmorethan90%ofthecases,theﬂyerwas

intact.AsshowninFig.11,smallamountsofdebrispresumably

fromtheperimeter oftheshearedﬂyerarevisible.Astheﬂyer

appearstobeslightlysmallerthanshowninFig.9,weassumed

thatoneofthesourcesofsuchdebrisistheperimeteroftheﬂyer.

FormediumﬂuencesasfortheﬂyerinFig.11b,disintegrationof

themainﬂyerisapparentatadistanceofaround50␮m,the

num-berofsmallparticlesordebrishasincreased,andatthecentreof

theimage,anadditionalcompactandrelativelylargefeaturecanbe

seenwhichisconnectedwithalongstringofmaterialtothelower,

mainﬂyer.Suchastringorjetisanindicatorofmoltenmaterial

andcouldexplainthesaturationofthemainﬂyervelocitycaused

byachangeindonormaterialphaseandhencedifferent

mechan-icalproperties[50].Also,meltingofthecarrierattheserelatively

largeﬂuencesislikelytooccur.

Forhighﬂuences, themoltenfeatures arestillvisible inthe

upperpartoftheimagetogetherwithalargequantityofsmall

par-ticles.Themainﬂyerisseentohavebrokenupinseveralpiecesina

typicalfashionfornon-intacttransfer.Forthetwohigherﬂuences,

seennearthecrater,residuallightemissionisvisible,originating

fromtheincidentlaserpulseastherelativelylongcameraexposure

timeincludesbothlaserpulseandﬂashlamppulseevents.

Fig.12 emphasises furtherthe differenttransfer regimesby

showingpropagationdistancesofintactﬂyersandlargefragments

ofthemainﬂyer.Thelargestextractedvelocitiesforfragmentsseen

athighﬂuenceswas∼40m/s.

4. Conclusions

Wehavedemonstratedthetime-resolvedshadowgraph

imag-ing of thin transparent epoxy-based polymer SU-8 ﬁlms via

femtosecond laser-induced backward transfer. Flyer velocity,

transfer regimes and intact transfer window were determined

for different thicknesses (3.4␮m and 6.8␮m), delay times

(1.4–16.4␮s)andcarriersubstratesofsiliconandgermaniumfor

ﬂuencesof 0.5–3.5J/cm2_._Observed_velocities _were_in_the_range

of 6–20m/s for different donor thicknesses, carriers and laser

ﬂuences.We haveseen thatﬂyer velocityis afunctionof laser

ﬂuencewitha gradientof 0.023±0.01m/smJ−1cm2 _for_Ge _and

0.014±0.01m/smJ−1cm2_for_Si_carriers_in_intact_state_and_ﬂyers

removedaroundthresholdﬂuence.However,forSicarriers and

largeﬂuences,thecraterfoundinthedonor,theﬂyershapeand

reducedﬂyerpropagationvelocityindicateadifferentﬂyerfailure

regimethanforlowﬂuences.Also,wehavenotdetectedanyshock

waveswhichareknowntocompromiseﬂyerintegrityduringLIFT

experiments.Thereceiverhasshowntoberesponsiblefora

reduc-tionoftheﬂuencetransferwindowbyapproximatelyafactorof

two.Amongthetestedcarriers,duetotheirrelativelylow

ejec-tionthresholdagermaniumcarrieris preferredover thesilicon

one.TheseﬁndingsarehelpfulforbetterunderstandingoftheLIBT

process,fore.g.futuremodelling,andtodetermineexperimental

parametersforLIBTprintingintactdeposits.

Acknowledgements

ThisworkwasfundedundertheUKEngineeringandPhysical

SciencesResearchCouncil(EPSRC)GrantsNos.EP/L022230/1and

EP/J008052/1.Theauthorsalsowouldliketoacknowledgeﬁnancial

supportfromthestatebudgetbytheSlovenianResearchAgency

[ProgrammeNo.P2-0392].GoranMashanovichiskindly

acknowl-edgedforprovidinggermaniumsamples.Thedataforthiswork

areaccessiblethroughtheUniversityofSouthamptonInstitutional

ResearchRepository(DOI:10.5258/SOTON/398008).

References

[1]M.Vaezi,H.Seitz,S.Yang,Areviewon3Dmicro-additivemanufacturing technologies,Int.J.Adv.Manuf.Technol.67(2013)1721–1754,http://dx.doi. org/10.1007/s00170-012-4605-2.

[2]Y.Zhang,C.Liu,D.Whalley,Direct-writetechniquesformasklessproduction ofmicroelectronics:Areviewofcurrentstate-of-the-arttechnologies,in: 2009Int.Conf.Electron.Packag.Technol.HighDensityPackag.,IEEE,2009,pp. 497–503,http://dx.doi.org/10.1109/ICEPT.2009.5270702.

[3]A.delCampo,E.Arzt,Fabricationapproachesforgeneratingcomplex micro-andnanopatternsonpolymericsurfaces,Chem.Rev.108(2008)911–945, http://dx.doi.org/10.1021/cr050018y.

[4]J.A.Grant-Jacob,B.Mills,M.Feinaeugle,C.L.Sones,G.Oosterhuis,M.B. Hoppenbrouwers,R.W.Eason,Micron-scalecopperwiresprintedusing femtosecondlaser-inducedforwardtransferwithautomateddonor replenishment,Opt.Mater.Express3(2013)747,http://dx.doi.org/10.1364/ OME.3.000747.

[5]C.B.Arnold,P.Serra,A.Piqué,Laserdirect-writetechniquesforprintingof complexmaterials,MRSBull.32(2007)23–32,http://dx.doi.org/10.1557/ mrs2007.11.

[6]A.J.Birnbaum,H.Kim,N.A.Charipar,A.Pique,Laserprintingofmulti-layered polymer/metalheterostructuresforelectronicandMEMSdevices,Appl.Phys. A:Mater.Sci.Process.99(2010)711–716, http://dx.doi.org/10.1007/s00339-010-5743-8.

[7]C.L.Sones,K.S.Kaur,P.Ganguly,D.P.Banks,Y.J.Ying,R.W.Eason,S.Mailis, Laser-induced-forward-transfer:arapidprototypingtoolforfabricationof photonicdevices,Appl.Phys.A101(2010)333–338,http://dx.doi.org/10. 1007/s00339-010-5827-5.

[8]P.Serra,M.Colina,J.M.Fernandez-Pradas,L.Sevilla,J.L.Morenza,Preparation offunctionalDNAmicroarraysthroughlaser-inducedforwardtransfer,Appl. Phys.Lett.85(2004)1639–1641,http://dx.doi.org/10.1063/1.1787614. [9]M.Feinaeugle,C.L.Sones,E.Koukharenko,B.Gholipour,D.W.Hewak,R.W.

Eason,Laser-inducedforwardtransferofintactchalcogenidethinﬁlms: resultantmorphologyandthermoelectricproperties,Appl.Phys.A112(2013) 1073–1079,http://dx.doi.org/10.1007/s00339-012-7491-4.

[10]D.Munoz-Martin,C.F.Brasz,Y.Chen,M.Morales,C.B.Arnold,C.Molpeceres, Laser-inducedforwardtransferofhigh-viscositysilverpastes,Appl.Surf.Sci. 366(2016)389–396,http://dx.doi.org/10.1016/j.apsusc.2016.01.029. [11]E.Breckenfeld,H.Kim,R.C.Y.Auyeung,N.Charipar,P.Serra,A.Pique,

Laser-inducedforwardtransferofsilvernanopasteformicrowave

interconnects,Appl.Surf.Sci.331(2015)254–261,http://dx.doi.org/10.1016/ j.apsusc.2015.01.079.

[12]M.Zenou,Z.Kotler,Printingofmetallic3Dmicro-objectsbylaserinduced forwardtransfer,Opt.Express24(2016)1431,http://dx.doi.org/10.1364/OE. 24.001431.

[13]C.W.Visser,R.Pohl,C.Sun,G.-W.Römer,B.Huisin‘tVeld,D.Lohse,Toward 3Dprintingofpuremetalsbylaser-inducedforwardtransfer,Adv.Mater.27 (2015)4087–4092,http://dx.doi.org/10.1002/adma.201501058.

[14]BertHuisin’tVeld,L.Overmeyer,M.Schmidt,K.Wegener,A.Malshe,P. Bartolo,Microadditivemanufacturingusingultrashortlaserpulses,CIRP Ann.–Manuf.Technol.64(2015)701–724,http://dx.doi.org/10.1016/j.cirp. 2015.05.007.

[15]D.J.Heath,M.Feinaeugle,J.A.Grant-Jacob,B.Mills,R.W.Eason,Dynamic spatialpulseshapingviaadigitalmicromirrordeviceforpatterned laser-inducedforwardtransferofsolidpolymerﬁlms,Opt.Mater.Express5 (2015)1129,http://dx.doi.org/10.1364/OME.5.001129.

[16]J.Ihlemann,R.Weichenhain-Schriever,Patterneddepositionofthin SiOX-ﬁlmsbylaserinducedforwardtransfer,ThinSolidFilms550(2014) 521–524,http://dx.doi.org/10.1016/j.tsf.2013.10.128.

(8)

[17]A.I.Kuznetsov,A.B.Evlyukhin,C.Reinhardt,A.Seidel,R.Kiyan,W.Cheng,A. Ovsianikov,B.N.Chichkov,Laser-inducedtransferofmetallicnanodropletsfor plasmonicsandmetamaterialapplications,J.Opt.Soc.Am.B26(2009)B130, http://dx.doi.org/10.1364/josab.26.00b130.

[18]D.P.Banks,C.Grivas,J.D.Mills,R.W.Eason,I.Zergioti,Nanodropletsdeposited inmicroarraysbyfemtosecondTi:sapphirelaser-inducedforwardtransfer, ApplPhys.Lett.89(2006)193107,http://dx.doi.org/10.1063/1.2386921. [19]M.L.Levene,R.D.Scott,B.W.Siryj,Materialtransferrecording,Appl.Opt.9

(1970)2260,http://dx.doi.org/10.1364/AO.9.002260.

[20]U.Zywietz,A.B.Evlyukhin,C.Reinhardt,B.N.Chichkov,Laserprintingof siliconnanoparticleswithresonantopticalelectricandmagneticresponses, Nat.Commun.5(2014)3402,http://dx.doi.org/10.1038/ncomms4402. [21]A.I.Kuznetsov,J.Koch,B.N.Chichkov,Laser-inducedbackwardtransferof

goldnanodroplets,Opt.Express17(2009)18820,http://dx.doi.org/10.1364/ OE.17.018820.

[22]A.I.Kuznetsov,C.Unger,J.Koch,B.N.Chichkov,Laser-inducedjetformation anddropletejectionfromthinmetalﬁlms,Appl.Phys.A106(2012)479–487, http://dx.doi.org/10.1007/s00339-011-6747-8.

[23]B.Liu,Z.Hu,Y.Che,Ultrafastpulsedlasermicro-depositionprintingon transparentmedia,in:Micromach,MicromachiningandMicrofabrication ProcessTechnologyXV(2010)759002–759006,http://dx.doi.org/10.1117/12. 842806.

[24]S.G.Koulikov,D.D.Dlott,Ultrafastmicroscopyoflaserablationofrefractory materials:ultralowthresholdstress-inducedablation,J.Photochem. Photobiol.AChem.145(2001)183–194, http://dx.doi.org/10.1016/S1010-6030(01)00581-0.

[25]P.Papakonstantinou,N.A.Vainos,C.Fotakis,MicrofabricationbyUV femtosecondlaserablationofPt,Crandindiumoxidethinﬁlms,Appl.Surf. Sci.151(1999)159–170,http://dx.doi.org/10.1016/S0169-4332(99)00299-8. [26]H.Sakata,S.Chakraborty,M.Wakaki,PatterningofBi2O3ﬁlmsusing

laser-inducedforwardandbackwardtransfertechniques,Microelectron.Eng. 96(2012)56–60,http://dx.doi.org/10.1016/j.mee.2012.02.002.

[27]A.Luches,S.A.Mulenko,V.P.Veiko,A.P.Caricato,V.Chuiko,Y.V.Kudryavtsev, A.V.Lopato,A.A.Petrov,F.Romano,D.Valerini,Laser-assistedsynthesisof semiconductorchromiumdisilicideﬁlms,Appl.Surf.Sci.253(2007) 6512–6516,http://dx.doi.org/10.1016/j.apsusc.2007.01.023.

[28]V.P.Veiko,E.a.Shakhno,V.N.Smirnov,G.D.Nikishin,S.P.Rho,Laserablation andlocaldeposition:physicalmechanismsandapplicationfor

decontaminationofradioactivesurfaces,J.KoreanPhys.Soc.51(2007)345, http://dx.doi.org/10.3938/jkps.51.345.

[29]M.Feinaeugle,D.J.Heath,B.Mills,J.A.Grant-Jacob,G.Z.Mashanovich,R.W. Eason,Laser-inducedbackwardtransferofnanoimprintedpolymerelements, Appl.Phys.A.122(2016)398,http://dx.doi.org/10.1007/s00339-016-9953-6. [30]E.T.Karim,M.Shugaev,C.Wu,Z.Lin,R.F.Hainsey,L.V.Zhigilei,Atomistic

simulationstudyofshortpulselaserinteractionswithametaltargetunder conditionsofspatialconﬁnementbyatransparentoverlayer,J.Appl.Phys. 115(2014)183501,http://dx.doi.org/10.1063/1.4872245.

[31]D.A.Zayarnyi,A.A.Ionin,S.I.Kudryashov,S.V.Makarov,A.A.Rudenko,E.A. Drozdova,S.B.Odinokov,Speciﬁcfeaturesofsingle-pulsefemtosecondlaser micronandsubmicronablationofathinsilverﬁlmcoatedwitha micron-thickphotoresistlayer,Quantum.Electron.45(2015)462–466, http://dx.doi.org/10.1070/QE2015v045n05ABEH015788.

[32]C.Zhang,J.Yao,S.Lan,V.A.Troﬁmov,T.M.Lysak,Effectsofplasma conﬁnementonthefemtosecondlaserablationofsilicon,Opt.Commun.308 (2013)54–63,http://dx.doi.org/10.1016/j.optcom.2013.06.052.

[33]R.Fardel,M.Nagel,F.Nuesch,T.Lippert,A.Wokaun,Laser-inducedforward transferoforganicLEDbuildingblocksstudiedbytime-resolved

shadowgraphy,J.Phys.Chem.C.114(2010)5617–5636,http://dx.doi.org/10. 1021/jp907387q.

[34]J.P.McDonald,J.A.Nees,S.M.Yalisove,Pump-probeimagingoffemtosecond pulsedlaserablationofsiliconwiththermallygrownoxideﬁlms,J.Appl. Phys.102(2007),http://dx.doi.org/10.1063/1.2778740.

[35]P.E.Dyer,Excimerlaserpolymerablation:twentyyearson,Appl.Phys.A: Mater.Sci.Process.77(2003)167–173, http://dx.doi.org/10.1007/s00339-003-2137-1.

[36]M.Malinauskas,M.Farsari,A.Piskarskas,S.Juodkazis,Ultrafastlaser nanostructuringofphotopolymers:adecadeofadvances,Phys.Rep.533 (2013)1–31,http://dx.doi.org/10.1016/j.physrep.2013.07.005. [37]B.N.Chichkov,C.Momma,S.Nolte,F.vonAlvensleben,A.Tunnermann,

Femtosecond,picosecondandnanosecondlaserablationofsolids,Appl.Phys. A:Materi.Sci.Process.63(1996)109–115.

[38]B.Mills,M.Feinaeugle,C.L.Sones,N.Rizvi,R.W.Eason,Sub-micron-scale femtosecondlaserablationusingadigitalmicromirrordevice,J.Micromech. Microeng.23(2013)35005,http://dx.doi.org/10.1088/0960-1317/23/3/ 035005.

[39]M.S.Rabasovi ć,D. ˇSevi ć,N.Luka ć,M.Jezerˇsek,J.Moˇzina,P.Gregorˇci ć, Evaluationoflaser-inducedthin-layerremovalbyusingshadowgraphyand laser-inducedbreakdownspectroscopy,Appl.Phys.A122(2016)186,http:// dx.doi.org/10.1007/s00339-016-9697-3.

[40]G.S.Settles,SchlierenandShadowgraphTechniques:VisualizingPhenomena inTransparentMedia,Springer-Verlag,Berlin,2006.

[41]Microchem,SU-8DataSheets,2016http://www.microchem.com/Prod-SU8. htm.

[42]M.Feinaeugle,A.P.Alloncle,P.Delaporte,C.L.Sones,R.W.Eason,

Time-resolvedshadowgraphimagingoffemtosecondlaser-inducedforward transferofsolidmaterials,Appl.Surf.Sci.258(2012)8475–8483,http://dx. doi.org/10.1016/j.apsusc.2012.04.101.

[43]K.S.Kaur,R.Fardel,T.C.May-Smith,M.Nagel,D.P.Banks,C.Grivas,T.Lippert, R.W.Eason,Shadowgraphicstudiesoftriazeneassistedlaser-inducedforward transferofceramicthinﬁlms,J.Appl.Phys.105(2009)113118–113119 http://link.aip.org/link/?JAP/105/113119/1.

[44]L.Rapp,C.Cibert,A.P.Alloncle,P.Delaporte,S.Nenon,C.Videlot-Ackermann, F.Fages,Comparativetimeresolvedshadowgraphicimagingstudiesof nanosecondandpicosecondlasertransferoforganicmaterials,XviiIntSymp. GasFlow,Chem.Lasers,High-PowerLasers7131(2009)71311L,http://dx.doi. org/10.1117/12.817481.

[45]R.Fardel,M.Nagel,F.Nuesch,T.Lippert,A.Wokaun,Shadowgraphy investigationoflaser-inducedforwardtransfer:frontsideandbackside ablationofthetriazenepolymersacriﬁciallayer,Appl.Surf.Sci.255(2009) 5430–5434http://www.sciencedirect.com/science/article/pii/

S0169433208018035.

[46]T.Mattle,J.Shaw-Stewart,A.Hintennach,C.W.Schneider,T.Lippert,A. Wokaun,Shadowgraphicinvestigationsintothelaser-inducedforward transferofdifferentSnO2precursorﬁlms,in:Appl.Surf.Sci.ElsevierB.V., 2013pp.77–8110.1016/j.apsusc.2012.11.146.

[47]K.Ravi-Chandar,J.Lu,B.Yang,Z.Zhu,Failuremodetransitionsinpolymers underhighstrainrateloading,Int.J.Fract.101(2000)33–72,http://dx.doi. org/10.1023/A:1007581101315.

[48]K.Brown,R.Conner,Y.Fu,H.Fujiwara,D.Dlott,Microscopicstatesofshocked polymers,AIPConfProc.(2012)1593–1596,http://dx.doi.org/10.1063/1. 3686589.

[49]M.Feinaeugle,P.Horak,C.L.Sones,T.Lippert,R.W.Eason,Polymer-coated compliantreceiversforintactlaser-inducedforwardtransferofthinﬁlms: experimentalresultsandmodelling,Appl.Phys.A116(2014)1939–1950, http://dx.doi.org/10.1007/s00339-014-8360-0.

[50]T.Kumada,H.Akagi,R.Itakura,T.Otobe,M.Nishikino,A.Yokoyama, Non-thermaleffectsonfemtosecondlaserablationofpolymersextracted fromtheoscillationoftime-resolvedreﬂectivity,ApplPhys.Lett.106(2015) 221605,http://dx.doi.org/10.1063/1.4921854.