High-Precision Solvent Vapor Annealing for Block Copolymer Thin Films

21 Pages·2017·6.39 MB·English

micromachines
Article
High-Precision Solvent Vapor Annealing for Block
Copolymer Thin Films
GunnarNelson,ChloeS.Drapes,MeaganA.Grant,RyanGnabasik,JeffreyWong
andAndrewBaruth* ID
DepartmentofPhysics,CollegeofArtsandSciences,CreightonUniversity,2500CaliforniaPlaza,
Omaha,NE68178,USA;[email protected](G.N.);[email protected](C.S.D.);
[email protected](M.A.G.);[email protected](R.G.);[email protected](J.W.)
* Correspondence:[email protected];Tel.:[phone]
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:19April2018;Accepted:25May2018;Published:29May2018
Abstract: Despiteitsefficacyinproducingwell-ordered,periodicnanostructures,theintricaterole
multipleparametersplayinsolventvaporannealinghasnotbeenfullyestablished. Insolventvapor
annealingathinpolymerfilmisexposedtoavaporofsolvent(s)thusformingaswollenandmobile
layertodirecttheself-assemblyprocessatthenanoscale. Recentdevelopmentsinboththeoryand
experiments have directly identified critical parameters that govern this process, but controlling
them in any systematic way has proven non-trivial. These identified parameters include vapor
pressure,solventconcentrationinthefilm,andthesolventevaporationrate. Toexploretheirrole,
a purpose-built solvent vapor annealing chamber was designed and constructed. The all-metal
chamberisdesignedtobeinerttosolventexposure. Computer-controlled,pneumaticallyactuated
valvesallowforprecisiontimingintheintroductionandwithdrawalofsolventvaporfromthefilm.
Themassflowcontroller-regulatedinlet,chamberpressuregauges,insituspectralreflectance-based
thicknessmonitoring,andlowflowmicrometerreliefvalvegivereal-timemonitoringandcontrol
duringtheannealingandevaporationphaseswithunprecedentedprecisionandaccuracy.Thereliable
andrepeatablealignmentofpolylactidecylindersformedfrompolystyrene-b-polylactide, where
cylinders stand perpendicular to the substrate and span the thickness of the film, provides one
illustrativeexample.
Keywords: blockpolymers;self-assembly;thinfilms;solventvaporannealing;nanolithography
1. Introduction
Techniquestoachieveperiodicnanostructuresviatraditional“topdown”methods,including
photolithography, have become increasingly challenging within the semiconductor industry.
Ultra-small feature production is approaching fundamental resolution limits (193 nm ultraviolet
(UV)lithography,forexample,recentlyreachingsub-30nmfeatures)[1–11]. Onepromisingstrategy
is investigating “bottom up” approaches that rely on nanoscale self-assembly. In 2007, directed
self-assembly was first considered as a potential scaling solution, according to the International
TechnologyRoadmapforSemiconductors(ITRS)[11]. Intheir2013report,thedirectedself-assembly
ofcomplexstructureswithlowannealtime,lowdefectdensity,andhighreproducibilitywasidentified
asoneofthe“GrandChallenges”toextendMoore’slaw[11]. Thedirectedself-assemblyofblock
polymer(BP)thinfilmshasbecomeaparticularlystrongcandidatetoachievesub-20nmdimensions,
where the size and morphology is controlled by varying the molecular weight of the constituent
polymerblocks.
Duetothoroughinvestigationsoverdecades,theBPresearchcommunitygenericallyconsiders
the bulk behavior of many BPs well known [12,13]. Bulk morphologies are characterized by the
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Micromachines2018,9,271 2of21
Flory–Hugginsinteractionparameter(χ),thedegreeofpolymerization(N),thevolumefractionsof
theconstituentblocks(f),andtheparticulararchitecture[13]. Furthermore,muchisknownabout
bulkbehaviorunderavarietyofstimuli,whetherthermal,solvent,ormechanical[12]. Inparticular,
measuredorknownquantitiescanwellpredictthealignmentofstructuresandmorphologieswithin
thebulk.Onthecontrary,theconfinementofathinfilmandtheassociatedsurfaceenergycontributions
introducesadditionalconfounders[14,15]. Inparticular,thecomplexityofsurfaceinteractions(both
with the substrate and free surface) impose new, often asymmetric, boundary conditions [16,17].
Regardless,techniquesdesignedtopromoteorderingofBPthinfilmshaveprogressedrapidly,driven
byapplicationsrequiringlong-rangelateralordering,uniformityinfeaturesizeandhighplacement
precision. MostofthehistoricalapproachestoBPorderinghavebeenlargelyunpredictableandare
oftensloworenergy-intensive: includingthermalannealing[18,19],electricfieldalignment[20,21],
andincorporationoflowsurfaceenergymidblocks[22,23]. Morerecently,theuseofpre-patterned
substrates (chemical or topographical) to act as a guide for BP assembly has been successfully
incorporatedbutcanbetime-intensiveandinvolvesmultiplelithographicapproaches[24]. Therefore,
itisnotnecessarilycost-effectiveforhigh-throughputapplications. Asaresult,amaturingtechnique
issolventvaporannealing(SVA)[25].
SVA was originally introduced as an alternative to thermal annealing for BPs exhibiting
thermaldegradation,problematicthermally-driventransitions,orslowdynamicsduetohighmolar
mass[26–28]. TheinterestinSVAofBPshasgrownwellbeyondthisinrecentyears. Ithasbeenshown
tooptimizeorganizationquicklyduetotheincreasedchainmobility,apossiblydecreasedχ(dependent
onsolventpolarity), andtunablesurfaceenergies[25]. Inthisprocess, aBPfilmisexposedtothe
vaporsofoneormoreorganicsolvents,offeringdirectcontroloverlyotropictransitions(cylindersto
spheres,forexample)whileinthesolvatedstateaswellasduringevaporation[29,30]. Thistechnique
alsohastheabilitytoreducedefectdensitydramatically[31],whileimprovinglateralordering,both
at the free surface and into the bulk of a film. This ordering is achieved more quickly (by several
ordersofmagnitude)andcompletelythanpreviousmethods[32]. Althoughmuchisknownaboutthe
interactionsofBPswithsolventinbulk[33],theeffectsinthinfilmsexhibitdifferentbehaviorsdueto
thepresenceofconfiningsurfacesandthedynamicexchangewiththesolventvaporatmosphere[17].
Thus,thistechniquecontinuestosufferfromreliabilityproblemsandnostandardizedmethodshave
becomeapparent. Itisbecomingincreasinglyclearthatacontinuedunderstandingofthespecific
orderingmechanismsofaBPsystemisparamount,wherethequestforgenericunderstandingofany
BPthinfilmsystemremainselusive.
OfcriticalimportancetomanytechnologicalapplicationsistheabilitytodirectBPthinfilms
toformcylindersthatstandperpendiculartothesurface,traversethroughthethicknessofthefilm,
andlaterallypackwithhexagonalorder. Arecentlydemonstratedapproachtonanolithographyofa
magneticthinfilm,forexample,utilizedsuchBP-basedlithographymasksusingaDamascene-like
approach. This approach was able to synthesize hexagonally-packed magnetic nanodots with a
diameter of 19 nm with high fidelity and retention of robust ferromagnetism [34]. Furthermore,
this approach achieved diameter control, down to 14 nm, and the potential for high-temperature
processingwithanadditionalatomiclayerdepositionstepofZnO[35]. Thesetechniquesrelyonthe
verticalalignmentofcylinders. Onthecontrary,manycurrentadvancesinBPalignment,including
importantworkinsolventconcentrationgradients[36],havefocusedsolelyonthefreesurfaceofthe
film. Thisisinsufficienttobeadirectsubstitutionformosttraditionallithographicapproaches[37].
Recentresultsrevealthatoptimizedorderingofhexagonally-packedcylinderscanpotentiallyoccurin
secondsandextendthroughthefilmusingSVA,butthisapproachcanbesomewhatunreliable[38],
wherenear100%successofformingthedesiredmorphologyhasyettobeachieved. Advancesin
computersimulationaswellasinsituX-rayandneutronscatteringcontinuetobothfurtherunderstand
thisprocessandimproveuponit[39–41]. Whilesuchanunderstandingoftheself-assemblybehavior
iscriticaltotheadvancementofthisfield,utilizingastatisticalapproachtoquantifyingSVAwithlarge
samplesets,withagoalofovercomingreliabilitybarriers,requirestheabilitytoidentify,measure
Micromachines2018,9,271 3of21
anddevelopcontrolsforallofthepertinentvariableswithreliableprecision. Thisincludeschamber
pressure,solventexposuretime,solventconcentrationinthefilm,solventevaporationrate,solvent
purityorcombinations,ambienttemperature,sampleandsolventtemperature,humidity[42],andfilm
thicknesstonameafew. Theseareaddressedinthepresentmanuscript,withaprimaryfocusonthe
roleofcontrollingchamberpressureandsolventevaporationratesatfasttimescalesof~10ms.Wenote
that this chamber does not actively control sample [41] or solvent temperature [43–45], or include
multiplesolvents[46],whichhavebeenshownimportanttocontrollingannealingkinetics. So,these
results work to keep these parameters as consistent as possible. To that end, we present here our
strictannealingprotocolsandourclimate-controlledSVAchamberwithcomputer-controlledsolvent
vaporflowandpressuremanagementandinsituspectralreflectance-basedsolventconcentration
(φ)measurements. Thischamberallowsustofixpotentialvariableswhileinvestigatingonlyone.
ThisleadstounprecedentedcontrolovertheSVAprocesswithagoalofsystematicstudieswithhigh
reliabilityandrepeatabilitythatmayhaveadvantagesasBPSVAalignmentmovesfromitscurrent
researchphase(relyingheavilyoninsituX-rayandneutronscattering)intoscaled-upprocessing. One
criticalandunprecedentedadvancementistheabilitytostabilizesolventconcentrationwithinaBP
thinfilmforanarbitrarilylongtimeatauser-definedchamberpressure,anecessaryprerequisitefor
anytemporalstudiesofcrystallization.
Todate,therearethreeprimarymethodsofcontrolledSVAintheliteraturewithdifferinglevelsof
complexity,asillustratedinFigure1. TheseprocesseswererecentlyreviewedbyPosseltetal. andGu
etal.[32,47],whereeachmethodtypicallyincorporatesanopticalinterferometertomonitorthickness
(i.e.,swellingduetosolventuptake)inrealtimethatisplacedabovethechamberandshinesdownon
thesamplesurface. Briefly,in“jarannealing,”depictedinFigure1a,asolventreservoirisplacedina
sealedvesselwiththeBPfilm.Thesolventvaporpressure,andthusthefilmswelling,isparameterized
by the ratio of liquid solvent surface area to the chamber volume [48]. The solvent evaporation is
eitherdoneviaopeningthelid,whichisdifficulttoquantify,orintroducingaleakuntilallsolvent
hasevaporated[48,49]. AnaturalextensionofthisSVAmethodistheinclusionofinlet/outletflow
lines,asseeninFigure1b,wheretheincorporationofaninertgasflowcanservetomodifythevapor
pressureandBPfilmswellingmoredirectly[50]. However,thereisstillonlylimitedcontroloverthe
solventevaporationrate. Thenextextensionisdisplacingthesolventreservoirfrominsidethesealed
chamberintoaseparatesealedreservoir,asshowninFigure1c. InthisSVAmethod,solventiscarried
inthevaporphasebyacarriergasintothechamber. Boththeabsolutepressureandvaporpressure
insidethechamberarecontrolledviatheflowratesofthevaporlineandasecondinletforaninertgas
stream. ThisgivessignificantdynamiccontrolofBPfilmswelling,althoughthehighestachievable
vaporpressureisreducedcomparedtoFigure1a,bduetoadilutionofvaporfromthecarriergas.
ThesolventevaporationrateiscontrolledinasimilarfashiontothemethoddepictedinFigure1b,
butwithoutthepresenceofthesolventreservoir,thusofferingmorecontrol. Similarmethodshave
proliferated[30,51–53],butonlyfewutilizereal-timecomputercontrol[41,45]andthereiscurrently
aninabilitytomaintainafixedsolventconcentrationforanarbitrarilylongtimescale. Thepresent
manuscriptdescribesanewevolutionintheSVAmethodthatbuildsontheseexistingmodels,adding
additionalcontroloversolventintroductionintoandevaporationoutofaBPfilmwithahighdegree
ofreliabilityandreproducibilitybycontrollingthevaporpressurethroughinlet/outletflows.
Several recent in situ studies utilizing Grazing Incidence Small Angle X-ray and Neutron
Scattering(GISAXSandGISANS)servetomotivatetheabilitytostabilizeaBPfilmwithaspecific
solvent concentration [41,54]. These studies have shown to be critical in identifying the role of
the order-to-disorder transition in directed self-assembly, as well as the role of solvent polarity
and temperature. Their results could be further extended with an increased level of control over
solvent concentration during the anneal and during the solvent evaporation. For example, Gu et
al. show a high degree of ordering in poly(styrene)-b-poly(2-vinyl pyridine) using GISAXS with
an increasing solvent concentration [30]. Upon rapid evaporation, scanning electron micrographs
(SEM) of the resultant films exhibit long correlation lengths of in-plane cylinders. Furthermore,
Micromachines 2018, 9, x 3 of 20
solvent evaporation rate, solvent purity or combinations, ambient temperature, sample and solvent
temperature, humidity [42], and film thickness to name a few. These are addressed in the present
manuscript, with a primary focus on the role of controlling chamber pressure and solvent
evaporation rates at fast time scales of ~10 ms. We note that this chamber does not actively control
sample [41] or solvent temperature [43–45], or include multiple solvents [46], which have been
shown important to controlling annealing kinetics. So, these results work to keep these parameters
as consistent as possible. To that end, we present here our strict annealing protocols and our
climate-controlled SVA chamber with computer-controlled solvent vapor flow and pressure
management and in situ spectral reflectance-based solvent concentration (φ) measurements. This
chamber allows us to fix potential variables while investigating only one. This leads to
unprecedented control over the SVA process with a goal of systematic studies with high reliability
and repeatability that may have advantages as BP SVA alignment moves from its current research
phase (relying heavily on in situ X-ray and neutron scattering) into scaled-up processing. One
critical and unprecedented advancement is the ability to stabilize solvent concentration within a BP
thin film for an arbitrarily long time at a user-defined chamber pressure, a necessary prerequisite for
any temporal studies of crystallization.
To date, there are three primary methods of controlled SVA in the literature with differing
levels of complexity, as illustrated in Figure 1. These processes were recently reviewed by Posselt et
al. and Gu et al. [32,47], where each method typically incorporates an optical interferometer to
monitor thickness (i.e., swelling due to solvent uptake) in real time that is placed above the chamber
and shines down on the sample surface. Briefly, in “jar annealing,” depicted in Figure 1a, a solvent
reservoir is placed in a sealed vessel with the BP film. The solvent vapor pressure, and thus the film
swelling, is parameterized by the ratio of liquid solvent surface area to the chamber volume [48]. The
solvent evaporation is either done via opening the lid, which is difficult to quantify, or introducing a
leak untiMl iacrlolm saochlinvees2n0t1 8h,9a,s27 e1vaporated [48,49]. A natural extension of this SVA method is t4hoef 2i1nclusion
of inlet/outlet flow lines, as seen in Figure 1b, where the incorporation of an inert gas flow can serve
to modifrye ctehnet rvesaupltosrin pdriceastseuthraet aapnpdr oBaPch ifnilgma ssowlveelnltincogn cmenotrraet idonirceocntslyis te[5n0tw]. iHthothweeovrdeerr,- ttoh-deriseo ridse rstill only
limited ctornantrsoitli oonvleera dthseto stohlevelanrgt eesvtacpororrealattiioonn lreantget.h Tsh[3e2 n].eTxht iesxstoelvnesniotnco insc denistrpaltaiocninisg atlhsoe csoonlvsiestnetn rteservoir
with enhanced defect removal [31]. Annealing at solvent concentrations above the order-disorder
from inside the sealed chamber into a separate sealed reservoir, as shown in Figure 1c. In this SVA
transitionwill,instead,resultinapotentiallydisorderedfilm,aspreviouslyshown[34]. Inaddition,
method, solvent is carried in the vapor phase by a carrier gas into the chamber. Both the absolute
Sintureletal.revealedsimilarorderinginasolvatedpoly(styrene)-b-poly(lactide)film,whereGISAXS
pressure and vapor pressure inside the chamber are controlled via the flowrates of the vapor line
wasmonitoredwithincreasingsolventconcentration[29]. Thedatashowsthatthecorrelationlength
and a secoofnpder pinenledti cfuolra ralyn ailnigenretd gcaysl isntdreerasmin.q Tuhiciksl ygidvrieesd sfiilgmnsifisiclaarngte dstyfonraamhiicg hcosonlvtreonlt ocofn BcePn tfrialmtio nswelling,
although( btehloew hitghehoersdt earc-tho-ideivsaorbdleer vtraapnsoirti opnr)e.sFsinuarlely i,sB erreedzukicneedt aclo.mrecpeanrtleyds htoow Feidguhorew 1mai,xbe ddusoel vteon ats dilution
andtheuseofelevatedtemperatureinacontrolledSVAchamber(similartoFigure1c)furtherenhance
of vapor from the carrier gas. The solvent evaporation rate is controlled in a similar fashion to the
controloverfinalmorphologies. TheseGISAXSstudies,whileextremelyinsightful,relyonsomewhat
method depicted in Figure 1b, but without the presence of the solvent reservoir, thus offering more
uncontrolled solvent exposure methods, as presented in Figure 1. In each case, no static solvent
control. Similar methods have proliferated [30,51–53], but only few utilize real-time computer
concentration(oftendescribedasswellingratio)wasachievedoveranyappreciableamountoftime.
control [S4u1c,4h5c]o natnrodl itshaeprere riesq cuuisritreefnotrlyan aynte imnpaobriallitsytu tdoy mofaoirndteariinng ak ifniexteicds asnodlvfuerntht ecroonpctiemnitzraattioionn for an
arbitrariloyf alnonangn teiamlineg spcraolteo.c oTl.hMe opvrinegsefonrtw maradn,eusspcerciipaltl ydwehscenricboenss iad enrienwg secavlionlguutipotnh eisne ttehceh nSiqVuAes method
towardsmassproduction,creatingahighlycontrolledSVAchambercouldassistinavoidingtheuse
that builds on these existing models, adding additional control over solvent introduction into and
ofcontinuedinsituX-rayorneutronscatteringtechniquesthatarenoteasilyextensibletoscalingup.
evaporation out of a BP film with a high degree of reliability and reproducibility by controlling the
Instead,ahighlycontrolledSVAchambercouldestablishreliabilityandrepeatabilitythatincorporates
vapor pressure through inlet/outlet flows.
thecriticalparametersthathavebeenestablishedfromthesescatteringtechniques.

Figure1.Primarymethodsofsolventvaporannealing;(a)“jarannealing,”whereasealedchamber
Figure 1. Primary methods of solvent vapor annealing; (a) “jar annealing,” where a sealed chamber
containsthesampleandasolventreservoir;(b)“jarannealing”withinlet/outletlinesforinertgasflow;
contains the sample and a solvent reservoir; (b) “jar annealing” with inlet/outlet lines for inert gas
(c)solventvaporflowviaacarriergasthroughinlet/outletlines.Blockpolymerfilmsaredepictedin
flow; (c) bsluoel,vweintht lviqaupidorso flvloenwt dvepiaic tae dcianrgrrieeern .gas through inlet/outlet lines. Block polymer films are
depicted in blue, with liquid solvent depicted in green.
Inadditiontoacontrolledstaticsolventconcentration,theevaporationrateservesasanother

complicating factor to be addressed. Solvent evaporation rate has been shown to strongly affect
the surface morphology of BP films [52], where the evaporation has been shown to produce an
orderingfrontthatpropagatesthroughthefilmfromthefreesurfacetothesubstrate[55]. Thetiming
of solvent removal dominates this effect. Enviable results were reported in 2004 from Kim et al.
revealingdefect-freeorderingofcylindersatthefreesurfaceoverlargelaterallengthscales,where
GISAXS data aimed to expose the role of this ordering front [55]. The ordering depends strongly
on the exact trajectory through the BP phase diagram as solvent is removed [56], with changes in
polymerdynamicsandorder–ordertransitionsservingaspotentialcomplicatingfactors[57]. Assuch,
conflicting results dictate that either slow [51,52] or fast [58] evaporation times may lead to ideal
morphologies. Recently,studiesofsolventconcentrationprofiles[58]havesuggestedthatthecylinder
growthrateduringevaporationisaproductofthepolymerchainmobilityandthedrivingforceto
phaseseparate. DynamicalfieldtheorysimulationsfromParadisoetal. furtherexposetheroleof
rapidsolventevaporationandsegregationstrength(χ)oncylinderorientation[57]. Itisincreasingly
clearthattheabilitytosystematicallyremovesolventfromthefilm,includingatrapidtimescales,is
importanttounderstandingfinalfilmmorphologies. Inparticular,revealinghowagivenmorphology
propagates into the film during solvent evaporation, which is a necessary requirement for many
nanolithographicapplications. Itisimportanttonotethattheroleofsolventremovalonrapidtime
scales(lessthan100ms)hasnotbeensuccessfullymonitoredusingGISAXS,whereintegrationtimes
Micromachines2018,9,271 5of21
canbesomewhatlong(1–10s)[32]; however, recentuseofcharge-coupleddevice(CCD)cameras
havebeenshowntobeeffectiveforstudyingfastkinetics[47]. Thisopensthedoorforquantifying
the effects of solvent removal in real time. Presently, in many of these studies, combining known
structuralpropertiesfromGISAXSinthesolvatedstatewithfinalmorphologiesimagedwithatomic
forcemicroscopy(AFM)orSEMfollowingrapidevaporation,asaddressedabove,havebeenusedto
revealimportantrelationshipsbetweenevaporationtrajectoriesandfinalmorphologies. Thepresent
manuscript describes a systematically improved SVA method that offers a potential to elucidate
multiplekeyvariablesintheSVAprocessandoffersalevelofcontrol,reliabilityandreproducibility
toenhancetheunderstandingoforderingkineticsbothduringannealingand,critically,evaporation.
Specifically,thisimprovedSVAmethodmonitorsandcontrolschamberpressure,solventexposure
time,solventconcentrationinthefilm,solventpurityandsolventevaporationrate,whileconcurrently
monitoringambienttemperatureandhumidity.
2. MaterialsandMethods
2.1. SolventVaporAnnealing
Optically polished substrates were single-crystalline silicon <111> wafers (n-type, 5 Ω·cm)
with a native oxide layer (University Wafer, South Boston, MA, USA). No attempt was taken
to remove the native oxide. Toluene (ACS Certified), tetrahydrofuran (THF) (Certified, contains
0.025% butylated hydroxytoluene as a preservative), acetone (99.5%), methanol (ACS Certified),
and sodium hydroxide granules were all purchased from Fisher Chemical, Pittsburgh, PA, USA.
1,1,1,3,3,3-Hexamethyldisilazane(HMDS)(98%)and3Å,4to8mesh,3333werepurchasedfromAcros
Organics,Belgium,WI,USA.
Construction of the solvent vapor-annealing chamber (SAC) required chemically inert
stainless-steel tubing and valves with Swagelok tube fittings. Pneumatically actuated stainless
steelballvalves(1/4”and3/4”)utilizingDowCorningM111(heavy-consistencydimethylsilicone
compound)lubricantandModifiedPTFEpacking(SS-T12-S-065-20andSS-45S8-33C),stainlesssteel
quarter-turnplugvalves(1/4”)withKalrezO-rings(SS-4P4T),astainlesssteellow-flowmetering
valve(1/4”)utilizingaKalrezO-ring(SS-SS4-KZ-VH),astainlesssteellow-pressure(5psig/34.5kPa)
unfilledpressuregauge(LP1-SS-254-5PSI),astainlesssteelunfilledpressuregauge(60psig/413.7kPa,
PGI-63C-PG60-LAOX),and1/4”and3/4”stainlesssteeltubing(SS-T4-S-035-20andSS-T12-S-065-20,
respectively) were purchased from the Swagelok Company, Saarland, OH, USA. The body and
bubblerportionsoftheSACutilizedvacuum-gradeConflatflangeswithoxygen-freecoppergaskets.
Stainless steel tees (275-150-CFT), 6-way cube (275-CUBE-OS), and all Conflat blanks (275-000-T)
andSwagelokadaptorswerepurchasedfromLDSVacuumProducts,Inc.,Longwood,FL,USA,or
modifiedin-house. ConflatflangedzerolengthdeepUVquartz(CorningHPFS7980FusedSilica)
viewportwaspurchasedfromtheKurtJ.LeskerCompany,JeffersonHills,PA,USA(VPZL-275Q).Dry
N inletwasachievedwithaDrieritegaspurifier,andwascontrolledviaanApex500SCCMmass
2
flowcontrollerwithRS-232digitalcontrol(SchoonoverInc.,Canton,GA,USA).DryN pressurefrom
2
thegastankweremonitoredandcontrolledwithadual-stagegasregulator(0–344.7kPa)andflowrates
weremonitoredandcontrolledwithaPanel-MountFlowmeter(OMEGAEngineering,Stanford,CT,
USA)forair,withabrassvalve,withtwodifferentflowranges(3–30SCFHand30–300SCFH)from
McMaster-Carr, Elmhurst, IL, USA. Pneumatic valve control was achieved with 3-way 1/4” NPT,
normallyclosed,120V,100psisolenoids,variousOne-touchfittings(1/4”)and1/4”nylontubing
purchased from the Swagelok Company, Saarland, OH, USA. Electric control of pneumatics was
achievedwithaNationalInstrumentsUSB,8input,12-Bit,10kS/s,MultifunctionDAQ;25A,250V
solidstaterelays;andLabVIEW2016software(NationalInstruments,Austin,TX,USA).Finally,film
thicknesswasdeterminedinsituwithspectralreflectanceviaaFilmetricsF20-UV,SanDiego,CA,
USA,general-purposefilmthicknessmeasurementsystemwithbothhalogenanddeuteriumsources.
Micromachines2018,9,271 6of21
2.2. SynthesisofPoly(Styrene)-Block-Poly(Lactide)
Thesynthesisofpoly(styrene)-block-poly(lactide)(PS-b-PLA)isdescribedfullyelsewhere[34].
Succinctly,hydroxyl-terminatedPS(Mn=42.5kDa)wassynthesizedvialivinganionicpolymerization.
ThesubsequentPLAwassynthesizedviaringopeningtransesterificationpolymerization(ROTEP)of
D,L-lactideindichloromethaneatroomtemperatureusing1,8-diazabicyclo[5.4.0]undec-7-ene(DBU)as
acatalystfor1h. PS-b-PLAwasobtainedbyprecipitatinginmethanolafterterminationwithbenzoic
acid. ThefinalPS-b-PLAhadatotalMn=63kDa,withaPLAvolumefractionof0.28(byvolume)
yieldingacylindricalmorphologywithapolydispersityindexof1.08,asdeterminedwith1HNuclear
MagneticResonance(NMR)andSize-ExclusionChromatography(SEC)[34].
2.3. ThinFilmPreparation
Typicalsolutionsof1.5%(w/v)PS-b-PLAintoluene(non-selectivesolvent)werespincoated
ontoHMDStreated,natively-oxidizedsiliconwafers(20mm×20mm). HMDStreatmentoftheSi
waferswascarriedoutbyultrasonicallycleaningsubstratesinorganicsolvents(acetonefollowedby
methanol),treatingthemina1:5(v/v)HMDS:toluenesolutionfor16h,thenrinsingintolueneto
removeanyexcessHMDSthathadnotgraftedfullytothesubstrate,andblowingdrywithN gas.
2
Treatedwaferswerethenplacedina75◦Coventoremoveanyresidualsolvent. Thefilmswerespin
coatedat1000–3000rpmfor30s,dicedinto12–16pieces(~5mm×5mm),andimmediatelyplaced
ina75◦C(belowtheglasstransitiontemperatureofeitherblock[34])ovenfordrying. Thisprocess
yielded~60nmthickfilms,dependentonexactspinspeedandsolutionconcentration. Sampleswere
driedforaminimumofoneday. Periodicatomicforcemicrographsindicatenoapparentagingissues
orannealingeffectswhilestoringfilmsatthistemperature.
Forthepresentexperiment,thinfilmswerehot-loadedintoaN purged,over-pressurizedsolvent
2
annealingchamberandimmediatelysealedtoavoidwatercontamination. Thesampleswereallowed
tocooltoroomtemperatureandnofurthertemperaturecontrolwasimplemented. Thisisdescribed
infulldetailbelow. THFliquidwasstoredinasealedErlenmeyerflaskoveractivated3Åmolecular
sievesforseveraldaysbeforeintroducingittotheSAC,resultinginwaterlevelsof4.1ppmorless[59].
AdditionalactivatedmolecularsievesresideinthesolventreservoiroftheSACtocontinueactive
dryingduringsubsequentsolventvaporanneals. Followingsolventvaporexposureandcomplete
solvent evaporation from the film, the samples were immediately transferred to a 0.05 M NaOH
solution(H O:CH OH=6:4byvolume)forPLAminoritydomaindegradationandlefttosoakfor
2 3
45min,wherethedegradationrateissensitivetomolarity[60]. Afterremoval,filmswerewashedwith
deionizedwater/methanolfor5min. Byremovingtheminoritycomponentimmediately,thisserves
toensurethemorphologyofthefilmisimmobilizedforsubsequentimaging. Furthermore,toremove
anyadditionalsurfacecontamination,a150W,50KHzO reactiveionetch(PE-50,PlasmaEtch,Inc.,
2
Carson,NV,USA)isemployedfor10sat100mTorr. Thisprocessremoves~2–3nmoforganicmaterial.
Samplesatthisstagewereimmediatelyimagedwithatomicforcemicroscopy(AFM),withoutany
furthermodification.
2.4. MeasuringFilmThickness
FilmthicknesswasdeterminedwithaFilmetricsF20-UV(SanDiego,CA,USA)general-purpose
filmthicknessmeasurementsystemwithbothhalogenanddeuteriumsources. Spectralreflectance
datawastakenatdifferingtimeintervals(asdiscussedbelow),between10ms–3switha10–250ms
integrationtime.Experimentaldatawasmodeledoveraspectralrangeof270–900nmwithathree-layer
model(Si+PS-b-PLA+air).Wedevelopedanticipatedrefractiveindexprofilesbasedonknownvalues
(e.g.,PS,n=1.59;PLA,n=1.482;THF,n=1.407). Therefore,weexpectedanindexofrefractionof
1.55fortheneatfilm,droppingto1.45withincreasingsolvent(THF)concentrationuptoφ=0.55[34].
Samplesnotfollowingthistrendinrefractiveindexwithincreasingsolventwereabortedanddisposed.
Micromachines2018,9,271 7of21
2.5. AtomicForceMicroscopy(AFM)
TappingmodeAFMwasperformedonanAgilent5420microscope(SantaClara,CA,USA)under
ambientconditionsusingengagementsetpointsbetween0.9–0.95ofthefreeamplitudeoscillation.
Thetappingmodecantilevers(BudgetSensors,Sofia,BulgariaandBruker,Billerica,MA,USA)had
a resonant frequency of 300 kHz and a force constant of 40 N/m. For imaging the film/substrate
interface,thePLA-removedthinfilmswereplacedupsidedownondouble-sidedtransparenttape
(ScotchBrand,St. Paul,MN,USA)andplacedinliquidN for30s. FollowingliquidN exposure,
2 2
theSiwaferwaspeeledawayfromthefilmprovidingaccesstotheunderside. Thesefilmswereagain
exposedtoa10–20sO reactiveionetch(150W50KHzin100mTorr)ontheundersidetoremoveany
2
HMDS,athinPSwettinglayeroradhesiveresidue[34,61].
3. Results
3.1. DesignofaPurpose-BuiltSolventVaporAnnealingChamber
Thereproducibilityofafinalmorphologyanditspropagationintosolventvapor-annealedBP
filmsremainssomewhatelusive,despitenumerousadvancesinthefield. Todirectlyaddressthisissue,
wehavedesigned,constructedandtestedapurpose-builtsolventvaporannealingchamber(SAC)
thatprovidesunparalleledcontroloverintroducingandmaintainingprecisesolventconcentrations
within the film during the annealing process as well as during the evaporation of solvent from
thefilm. Concurrently,wemaintaincontroloverseveraladditionalnecessaryparameters(Table1),
including an exceptionally low dew point in the sample cell with active dry N purging, solvent
2
vaporflowrate,filmthicknessasaproxyforsolventconcentrationinthefilm[34],chamberpressure,
solventselectivity,andsubstratesurfacepreparation. Thepresentinvestigationuses~60nmthick
films of a prototypical BP, PS-b-PLA, that adopts a cylindrical morphology in the bulk. We use at
roomtemperaturearelativelyneutralsolvent,THF,fortheanneal[29,34,62];aPS-selectivesubstrate,
HMDS-functionalized Si; and maintain a low dew point (−100 ◦C) for the N carrier gas, due to
2
thehygroscopicnatureofTHFandPLA.ThefollowingSACdescriptionisasignificantevolution
overinitialworkindevelopingtheseprotocols, includingtheadditionofpneumatically-actuated,
computercontrolledsolventflowratesandvalveactuation[34,38]. Inourmethod,acopper-gasket
sealed,all-metalchambercontrolstheSVAclimate(i.e.,humidity,chamberpressure,andevaporation
times). Theall-metalconstruction,exceptforchemicallyinertmodifiedpolytetrafluoroethylenesealed
ballvalvesandaperfluoroelastomersealedlow-flowmeteringvalve,ensuresnegligibleinteraction
withthesolventduringtheannealingprocess. Inparticular,THFcanbeparticularlyaggressiveon
traditional organic gaskets and lubricants. Following hundreds of hours of exposure to THF, no
degradationtoanyvalvesinthechamberisevident.
Thechamber,showninFigure2(additionalimagesareavailableinAppendixA),isconnectedto
anactivelydriedN2line(dewpointguaranteedto−100◦C)(cid:13)1 ,whichsplitsintotwo1/4”stainless
steeltubingpaths.Path1(inred)purgesthesamplespacebeforeannealingandduringsampleloading;
thisensuresalowdewpointduringtheSVA.Additionally,BPfilmsarehot-loadedfroma75◦Coven
intothepurged,over-pressurizedchamberviaaConflat-flangeddoorandimmediatelysealedtoavoid
watercontaminationandallowedtocooltoroomtemperature. Path1isalsovitaltothecontrolled
evacuationofsolventvaporfromtheSACduringtheevaporationphase. N flowispassedthroughan
2
acrylic,block-styleflowmeter(3–30SCFHor30–300SCFH,dependentonflowratechosen)tocontrol
andmeasureflowratesduringsolventevaporation.
Micromachines2018,9,271 8of21
Table1. Acompilationofvariablesconsideredinthedesignofsolventvaporannealingprotocols,
alongwiththeirimportanceandthestepstakeninthedesigntoaddressthem.
Variable Importance ProtocolsinDesign
Samplesarestoredin75◦Coven
Samplesarehotloadedintochamber
Waterisapolarsolvent,which Vacuum-grade,coppergasketsand
willmodifythesolubility. Swageloksealsthroughoutchamber
Humidity
Forexample,wateris Molecularsieve-driedsolvent
PLAselective (tetrahydrofuran(THF))
Activelypurged(dryN )samplecell
2
DrieriteGasPurifier(−100◦Cdewpoint)
Flowrateisproportionalto Computer-controlledmassflowcontroller
SolventVaporFlowRate
solventuptakeinfilm Low-flowmeteringoutletvalve
Solventconcentrationinfilm Insituopticaldetectionofsolvent
SolventConcentration duringsolventvaporannealing concentration,assumingproportionalto
(SVA)modifiesmobility filmthickness,with10–20msresolution
Evaporationrateinlinkedto Computer-controlledpneumaticvalves
SolventEvaporationRate
morphologyalignment VariableflowN purgeline
2
Filmsareallspuncastataconstantspin
Roleofcommensuratethickness
InitialThickness speedfromthesamesolution
onmorphology
concentration
VaporpressureduringSVA Highandlowpressuregauges
VaporPressure modifiessolventuptakeand Low-flowmeteringoutletvalvecanfinely
evaporation adjustvaporpressure
Solventselectivitymodifies
THFisarelativelyneutralsolventforPS
SolventSelectivity surfaceenergyand
andPLA.IthasslightPSselectivity
polymer-polymerinteractions
Substratepreparationmodifies HMDS-functionalizedSisubstratesurface
SurfaceSelectivity
surfaceenergy promotesPS(majorityblock)adhesion
Path2(ingreen)flowisgovernedbyamassflowcontroller(cid:13)2 connectedtoaLabVIEW-enabled
computer(NationalInstruments,Austin,TX,USA).Flowratesrangefrom0–500SCCM,dependenton
intendedannealconditions. Theflowofthemetered,activelydriedN gasiscomputercontrolledvia
2
solenoidactuatedpneumaticvalves(cid:13)3 thatcontrol80psi(551kPa)airflowtoeachoffourpneumatic
processvalves(cid:13)4 . FollowingtheflowcontMroicrlolmearch,intehs e201N8, 92, xg asispassedthroughaprimarysafetyvalve 9 of 20
(cid:13)4 (this protects the mass flow controller from liquid solvent exposure). The controlled gas flow
continuesonitswaytoasealed,moleculacro-msipeuvteer- cdonritreodllesdo vliva esonletnroeids earctvuoatierd(cid:13) p5n,euamsattaici nvalelvsess ③ste tehlatC coonntrflola 8t0 psi (551 kPa) air flow
to each of four pneumatic process valves ④. Following the flow controller, the N2 gas is passed
teewithviewwindow(FigureA5). Thisrtehsroeurgvho iar pcroimnatrayi nsasfeatyn vaadlvde it④io n(tahils inprloettecttus bthee wmiatshs falonwo cromntarollllyer from liquid solvent
closedplugvalvethatisusedforliquidsolevxepnostulroe)a. dTihne gcowntirtohlloedu gtabs rfeloawk icnongtiannueys Cono intsfl waatys teoa als seFailgedu, rmeoAlec6u)l.ar-sieve-dried solvent
The solvent reservoir is backed by a saferteyserrveosier r⑤vo, air st(cid:13)a6in,leass sstteaeiln Cloensfslats tteeee wliCtho vnieflwa wtitnedeoww (Fitighurve iAew5). iTnhgis reservoir contains an
additional inlet tube with a normally closed plug valve that is used for liquid solvent loading
window. Thesafetyreservoirwillcollectfluidsolventifbackpressureispresent,avoidingexposure
without breaking any Conflat seals Figure A6). The solvent reservoir is backed by a safety reservoir
tothemassflowcontroller. Thesafetyres⑥er,v ao sitarincloesns tsateienl sCoannflaat dtede iwtiitohn vaielwtiungb weiwndiotwh. Tahne osarfmetya rlelsyercvoloirs weidll collect fluid solvent if
backpressure is present, avoiding exposure to the mass flow controller. The safety reservoir contains
plugvalvetoremoveanyliquidsolventwithoutopeninganyConflatseals. Ultimately,thedryN is
2
an additional tube with a normally closed plug valve to remove any liquid solvent without opening
bubbledthroughthesolventreservoir(cid:13)5 ,wanhy iCchonsfluatb sseeaqlsu. eUnlttimlyatcealyr, rtihees dsoryl vNe2n its ibnubthbleedv athproourgph hthaes esoilnvetnot reservoir ⑤, which
thesamplespace(cid:13)7 ,a6-wayConflatcubseub(7se0qumenmtly ×car7ri0es msolmven×t in7 t0hem vmapo),r pvhiaasea inctoom thpe suatmeprl-ec sopnacter o⑦ll,e ad 6,-way Conflat cube (70
pneumatically-actuatedchambervalve(cid:13)4 .mAml l×fl 7o0 wmmin ×t o70t hmems),a vmia pa lceomsppautceer-ceoxnittrsoltlehdr,o pungeuhm3a/tic4a”llys-taactiunalteesds chamber valve ④. All
flow into the sample space exits through 3/4” stainless steel tubing via a pneumatically-actuated ball
steeltubingviaapneumatically-actuatedbvaalllvve a⑧lv eo(cid:13)r8 ao rloawl-oflwow-fl omwetemrinegt evrailnvge v⑨al vteo (cid:13)9a tfuomae fuhmooed. hTohoed .computer controlled,
Thecomputercontrolled,pneumatically-apcnteuumataetidcalvlya-alvcteuaste(cid:13)d4 v,awlvietsh ④c,o wmithp croemspseredsseadi raiirn inlelett ⑩ aanndd 12102 V0 soVlenoid valves ③, allow
solenoidvalves(cid:13)3 ,allowustoquicklyinituias ttoe qaunicdklyte irnmitiaitne aatned ttheremSinVatAe twhe iStVhAa wsipthe ca isfipeecdifileedv leevleol foft tiimmiinngg. .Of critical importance,
initial testing indicates a controlled SVA evaporation time down to 15 ms or any time longer with
Ofcriticalimportance,initialtestingindicatesacontrolledSVAevaporationtimedownto15msor
10–20 ms temporal resolution. To our knowledge, this is the fastest recorded SVA evaporation time
anytimelongerwith10–20mstemporalrefsoor lau BtPio tnhi.nT fiolmo. urknowledge,thisisthefastestrecordedSVA
The BP sample resides in the sample space ⑦ on a custom-built mount (Figure A7). The sample
evaporationtimeforaBPthinfilm.
mount features two recessed ports, which speeds sample loading and helps avoid direct flow from
Paths 1 and 2 that can shift sample position. One recessed port holds the BP film, while the other
holds a blank Si wafer for use as an optical standard. Recess ports are sized to hold up to an 8 mm × 8
mm BP film and a blank Si wafer. The chamber offers direct optical access to the sample space
through a fused silica viewport ⑪, enabling continuous monitoring of the sample chamber with
high precision pressure gauges ⑫ to directly measure chamber pressure and in situ spectral
reflectance-based measurements of film thickness with 0.1 nm thickness and 10–20 ms temporal
resolution. These thickness measurements are directly related to solvent volume concentration (φ)
within the film by:
𝑉 −𝑉 𝑡 −𝑡 𝑡
ϕ= 𝑠𝑜𝑙𝑣𝑒𝑛𝑡+𝑓𝑖𝑙𝑚 𝑓𝑖𝑙𝑚= 𝑠𝑜𝑙𝑣𝑒𝑛𝑡+𝑓𝑖𝑙𝑚 𝑓𝑖𝑙𝑚=1− 𝑓𝑖𝑙𝑚 (1)
𝑉 𝑡 𝑡
𝑠𝑜𝑙𝑣𝑒𝑛𝑡+𝑓𝑖𝑙𝑚 𝑠𝑜𝑙𝑣𝑒𝑛𝑡+𝑓𝑖𝑙𝑚 𝑠𝑜𝑙𝑣𝑒𝑛𝑡+𝑓𝑖𝑙𝑚
where φ is solvent concentration, V denotes volume and t is film thickness. As supported by direct
observation, the areas for the two film states (i.e., swollen versus dry) are taken to be nominally
identical and thus only a thickness measurement is required to obtain a real-time in situ probe of φ
during SVA.
3.2. Theory of Operation
Through the active control of solvent vapor inlet and outlet flows, the solvent concentration
within the BP film is controlled and monitored as a function of time. As shown in Figure 3, using an
in situ measurement of φ, we divide the SVA process of a 60 nm PS-b-PLA film into four distinct
time regimes. During the first three time regimes, thickness data was taken every 3 s with a 249 ms
integration time. The initial regime (blue) includes the opening of the solvent reservoir to the sample
chamber and an initial, constant N2 inlet flow of 30–100 SCCM (dependent on desired chamber
pressure, 50 SCCM in the present study), resulting in an exponential increase in ϕ. The thickness
data is well modeled assuming a copolymer refractive index of 1.55, consistent with the neat
PS-b-PLA film (Appendix B). This exponential region typically lasts ~60 s where the time
dependence is well modeled with a single rate constant (0.03–0.1 s−1) with 0.088 s−1 for Figure 3 (see
Figure A8 for the exponential fit), which is dependent on exact inlet and outlet flow rates. Outlet
flow rates are governed by a low flow micrometer relief valve, which is set to a flow coefficient, Cv, of
0.0002–0.002 (1–6 turns), with 0.0004 being used in Figure 3. This, along with inlet flow, dictates the
Micromachines 2018, 9, x 8 of 20
Vapor pressure during SVA High and low pressure gauges
Vapor
modifies solvent uptake and Low-flow metering outlet valve can finely
Pressure
evaporation adjust vapor pressure
Solvent selectivity modifies surface
Solvent THF is a relatively neutral solvent for PS
energy and polymer-polymer
Selectivity and PLA. It has slight PS selectivity
interactions
Surface Substrate preparation modifies HMDS-functionalized Si substrate surface
Selectivity surface energy promotes PS (majority block) adhesion
The chamber, shown in Figure 2 (additional images are available in Appendix A), is connected
to an actively dried N2 line (dew point guaranteed to −100 °C) ①, which splits into two 1/4” stainless
steel tubing paths. Path 1 (in red) purges the sample space before annealing and during sample
loading; this ensures a low dew point during the SVA. Additionally, BP films are hot-loaded from a
75 °C oven into the purged, over-pressurized chamber via a Conflat-flanged door and immediately
sealed to avoid water contamination and allowed to cool to room temperature. Path 1 is also vital to
the controlled evacuation of solvent vapor from the SAC during the evaporation phase. N2 flow is
passed through an acrylic, block-style flowmeter (3–30 SCFH or 30–300 SCFH, dependent on flow
Micromachines2018,9,271 9of21
rate chosen) to control and measure flow rates during solvent evaporation.
Micromachines 2018, 9, x 9 of 20
Micromachines 2018, 9, x 9 of 20
computer controlled via solenoid actuated pneumatic valves ③ that control 80 psi (551 kPa) air flow
computer controlled via solenoid actuated pneumatic valves ③ that control 80 psi (551 kPa) air flow
to each of tfoo uera cphn eouf mfoautirc ppnreoucmesas tivca lpvreosc e④ss . vFaolvlleosw ④ing. Fthoell oflwowin gc othnetr ofllolewr, ctohne trNo2l legra,s thise pNas2 sgeads is passed
through a primary safety valve ④ (this protects the mass flow controller from liquid solvent
through a primary safety valve ④ (this protects the mass flow controller from liquid solvent
exposure). The controlled gas flow continues on its way to a sealed, molecular-sieve-dried solvent
exposure). The controlled gas flow continues on its way to a sealed, molecular-sieve-dried solvent
reservoir ⑤, a stainless steel Conflat tee with view window (Figure A5). This reservoir contains an
reservoir ⑤, a stainless steel Conflat tee with view window (Figure A5). This reservoir contains an
additional inlet tube with a normally closed plug valve that is used for liquid solvent loading
additional inlet tube with a normally closed plug valve that is used for liquid solvent loading
without breaking any Conflat seals Figure A6). The solvent reservoir is backed by a safety reservoir
without breaking any Conflat seals Figure A6). The solvent reservoir is backed by a safety reservoir
⑥, a stainless steel Conflat tee with viewing window. The safety reservoir will collect fluid solvent if
⑥, a stainless steel Conflat tee with viewing window. The safety reservoir will collect fluid solvent if
backpressure is present, avoiding exposure to the mass flow controller. The safety reservoir contains
backpressure is present, avoiding exposure to the mass flow controller. The safety reservoir contains
an additional tube with a normally closed plug valve to remove any liquid solvent without opening
an additional tube with a normally closed plug valve to remove any liquid solvent without opening
any Conflaatn yse aClos.n fUlaltt imseaatlesl.y ,U tlhtiem adtreyly N, t2h ies dbruyb bNle2d i st hbruobubglhe dt hteh rsooulgvhen tth ree sseorlvvoeinrt ⑤res, ewrvhoiicrh ⑤, which
subsequently carries solvent in the vapor phase into the sample space ⑦, a 6-way Conflat cube (70
subsequently carries solvent in the vapor phase into the sample space ⑦, a 6-way Conflat cube (70
mm × 70 mmmm × × 7 07 0m mmm), ×v i7a0 a m Fcomimg)u,p vruietae ra-2 cc.o onm(tarpo)u llteeCdr-o,c pmonnpetruuomltleeardti-,ca apildnlyee-udamc taudtaiecteasdlilg ycn-ha acmt(uCbaetAerd Dv ac)hl vaeam n④bde. r Av(blall )v er ④ea.l Ailml age of a computer-controlled,
flow into the sampleF sipgaucere ex2i.ts( ath)rCouogmh p3/u4t”e srt-aainidleesds sdteeesli tgunbi(nCgA viDa )a apnndeu(mba)triceaalllyi-macatugaeteodf baacllo mputer-controlled,pneumatically
flow into the sample space exitsp tnherouumgha t3i/c4a”l slyta iancletussa stteedel stuoblvinegn vti av aa pponre uamnanteicaallilyn-ga cctuhaatmedb bearl.l More images can be found in Appendix A.
valve ⑧ voarl vea ⑧low -oflro wa lmoawect-etfrulionawgte dvmaselovteelrv ine⑨gn t vvtaoal vpeao r⑨faunm neto e ahlaoi ondfgu. mcTheh aemh obcooedmr. .pMTuhtoeer r eccoiommntparuogtleelesrd c, caonntbroellfeodu, ndinAppendixA.Numbersand
pneumatically-actuaNteud mvablveerss ④ an, dw ictho lcoormepdr easrsreodw aisr ianrleet d⑩es acnrdib 1e2d0 Vin s othleen oteidx vt.a lves ③, allow
pneumatically-actuated valves c④ol,o wreitdh caormropwresssaerde adire isnclerti b⑩ed anidn 1t2h0e Vt esxolte.noid valves ③, allow
us to quickly initiate and terminate the SVA with a specified level of timing. Of critical importance,
us to quickly initiate and terminate the SVA with a specified level of timing. Of critical importance,
initial testiningi tiinald ticeasttiensg a incodnictraoPtelalse tdah cS oVn2Atr oelv(lieandp oSrVagtArioe neev tnaimp) oer afdtloioowwnn t itmoi es1 5d omgwson vo rteo ar n1n5ye mtdims eob rl oyann gyea rt i mwmeit hlao nsgse r fwloitwh controller ② connected to a
10–20 ms te1m0–p2o0r mal sr etesmolLuptoairobanlV. rTTeIoshEo oeWluuBtri- oPkennn.so aTwaobml eloedupdgrle ek,c ntorhoeimwss liipesdd tueghtesee, fitranh si(tseNt hsista e rtthesiceoao fnrmadsaetpled sl ItSen rVessActpor earudvcmaeepdoe (cid:13)Sr7nVattAioso ,nne vA taaimpucoesur taistnitoo,n mT tiXm-b,e u UilStAm).o Fulnotw(F riagtuesre raAn7g).e Tfrhoemsa 0m–5p0le0
for a BP thin film.
for a BP thin film. mountfeaturestworecessedports,whichspeedssampleloadingandhelpsavoiddirectflowfrom
The BP samTphlee BrePs isdaSemCs piCnle tM rhees, isddaemesp pinlee t nshpeda secaenm ⑦tp ol eon nsp aian ccuete s⑦tnomd oen-b dau iclaut nsmtnoomeuan-btl u (cFiloitg mnurdoeui Atnit7o )(.Fn Tisgh.ue rT sea hAme7p ).fl elT ohwe s aomfp tlhe e metered, actively dried N2 gas is
mount featmuroeus nttw foe arteucreeP sssa etwtdh ops or1ertcsea,s nwsedhdi c2pho tsrhptsae, etwdchsa iscnahm ssphpleieef tdlossa asdamimngpp allene dlop ahodeslipnitgsi aoavnnod.i dhO edlniprese cartv efolcoiedws d sfireroedmct p floowrt fhroomld stheBPfilm,whiletheotherholds
Paths 1 andP a2t htsh a1t acnadn 2sah tihbfta ltsa acnmaknp lsSeh iipfwto ssaiatfmioenpr.l feOo pnroeus irtseiecoenas.s sOedan nep oroerctp ehtsioscledadsl ptshoterat nBhPdo lafdirlsmd t,h. weR BheiPlce ef itslhmse, p owtohhreitrlse tahree osthizere dtoholduptoan8mm×8mmBP
holds a blank Si wafer for use as an optical standard. Recess ports are sized to hold up to an 8 mm × 8
holds a blank Si wafer fofir ulmse aasn adn oaptbiclaal nstkanSdiarwd.a Rfeecre.ssT phoertsc harae msizbeedr too hfofeldr supd tior eanc t8 ompmti ×c a8 laccesstothesamplespacethroughafused
mm BP film and a blank Si wafer. The chamber offers direct optical access to the sample space
mm BP film and a blank Si wafer. The chamber offers direct optical access to the sample space
through a fusseidli csialicva iveiwewppoorrtt ⑪,, eennaabblinlign cgonctoinnutoiunsu mouonsitmorionng iotof trhine gsamofptleh cehsaammbepr lweitchh amberwithhighprecisionpressure
through a fused silica viewport ⑪, enabling continuous monitoring of the sample chamber with
high precision pressure gauges ⑫ to directly measure chamber pressure and in situ spectral
high precision pressureg gaauuggeess ⑫ ttoo ddiirreecctltyl ymmeaesuarseu crheacmhbaerm pbreesrsuprree asnsdu rien asnitud sipnecstirtaul spectralreflectance-basedmeasurementsof
reflectance-based measurements of film thickness with 0.1 nm thickness and 10–20 ms temporal
reflectance-based measurements of film thickness with 0.1 nm thickness and 10–20 ms temporal
filmthicknesswith0.1nmthicknessand10–20mstemporalresolution. Thesethicknessmeasurements
resolution. These thickness measurements are directly related to solvent volume concentration (φ)
resolution. These thickness measurements are directly related to solvent volume concentration (φ)
within the filma rbey:d irectlyrelatedtosolventvolumeconcentration(φ)withinthefilmby:
within the film by:
𝑉 −𝑉 𝑡 −𝑡 𝑡
ϕ=𝑉𝑠𝑜𝑙𝑣𝑉𝑒𝑠𝑛𝑜𝑡ϕ𝑙+𝑣𝑒𝑓=𝑛𝑖𝑙𝑡𝑚+𝑓𝑠−𝑜𝑖𝑙𝑙𝑚𝑣𝑉𝑉𝑒𝑓𝑠𝑛𝑖𝑜𝑙𝑡𝑙𝑚+𝑣𝑒𝑓𝑛=𝑖𝑙𝑡𝑚+𝑡𝑓𝑠𝑖𝑜𝑙φ𝑚𝑙𝑣𝑡𝑒𝑓𝑠𝑛𝑖𝑜=𝑙𝑡𝑙𝑚+𝑣𝑒𝑓𝑛=𝑖V𝑙𝑡𝑚+s𝑓𝑠−o𝑖𝑜𝑙l𝑙𝑚𝑣v𝑡𝑡𝑒𝑓e𝑠𝑛𝑖𝑜n𝑙𝑡𝑙𝑚t+𝑣+𝑒𝑓𝑛=𝑖𝑙𝑡f𝑚+i1l𝑓m𝑖−𝑙𝑚−𝑡𝑓𝑠𝑖𝑜𝑙𝑚𝑙V𝑣𝑡𝑒𝑓=𝑛𝑖f𝑙𝑡i𝑚+1lm𝑓−𝑖𝑙𝑚𝑡= 𝑠 𝑜𝑙𝑣t𝑒𝑓𝑛s𝑖𝑙o𝑡𝑚+lv𝑓𝑖e𝑙n𝑚t +f(il1m) −tfil(m1) =1− tfilm (1)
where φ isw sohlevreen φt cios nscoelnvternatt icoonn, cVen dtreantoiotens, vVo dluemnoet easn dvo tl uiVsm sfoiell mvane ntdht i+tc kifsni lfemislms. Athsi cskunpepsos.r tAedst ssbouylp vdpeinortre+tcetd fi lbmy direct tsolvent+film
observation, the areas for the two film states (i.e., swollen versus dry) are taken to be nominally
observation, the areas for the two film states (i.e., swollen versus dry) are taken to be nominally
identical anidde tnhtuicsa ol nanlyd a wt hthuhisce kornneelysφs a m ithseiacsskounrleevsmse enmntet aicsso urrneeqcmueiernnettd ri stao rt eoiqobutnairi,neVd a trode aoelbn-ttaiomitnee a si nrve saiolt-ultu ipmmreo beine asoinft uφd ptroibse fiofl mφ thickness. Assupportedbydirect
during SVA.
during SVA. observation, the areas for the two film states (i.e., swollen versus dry) are taken to be nominally
3.2. Theory of Oidpeernattioinc alandthusonlyathicknessmeasurementisrequiredtoobtainareal-timeinsituprobeofφ
3.2. Theory of Operation
Throughd thuer ianctgiveS VcoAnt.rol of solvent vapor inlet and outlet flows, the solvent concentration
Through the active control of solvent vapor inlet and outlet flows, the solvent concentration
within the BP film is controlled and monitored as a function of time. As shown in Figure 3, using an
within the BP film is controlled and monitored as a function of time. As shown in Figure 3, using an
in situ measurement of φ, we divide the SVA process of a 60 nm PS-b-PLA film into four distinct
in situ measurement of φ, we divide the SVA process of a 60 nm PS-b-PLA film into four distinct
time regimes. During the first three time regimes, thickness data was taken every 3 s with a 249 ms
time regimes. During the first three time regimes, thickness data was taken every 3 s with a 249 ms
integration time. The initial regime (blue) includes the opening of the solvent reservoir to the sample
integration time. The initial regime (blue) includes the opening of the solvent reservoir to the sample
chamber anchda manb einr iatinadl, caonn isntaitniat l,N c2o innslteatn ftl oNw2 ionfl e3t0 –fl1o0w0 SoCf C30M–1 (0d0e pSCenCdMen t( doenp ednedseirnetd o cnh admesbireerd chamber
pressure, 50 SCCM in the present study), resulting in an exponential increase in ϕ. The thickness
pressure, 50 SCCM in the present study), resulting in an exponential increase in ϕ. The thickness
data is well modeled assuming a copolymer refractive index of 1.55, consistent with the neat
data is well modeled assuming a copolymer refractive index of 1.55, consistent with the neat
PS-b-PLA film (Appendix B). This exponential region typically lasts ~60 s where the time
PS-b-PLA film (Appendix B). This exponential region typically lasts ~60 s where the time
dependence is well modeled with a single rate constant (0.03–0.1 s−1) with 0.088 s−1 for Figure 3 (see
dependence is well modeled with a single rate constant (0.03–0.1 s−1) with 0.088 s−1 for Figure 3 (see
Figure A8 for the exponential fit), which is dependent on exact inlet and outlet flow rates. Outlet
Figure A8 for the exponential fit), which is dependent on exact inlet and outlet flow rates. Outlet
flow rates afrloe wgo rvaetersn eadre b gyo av elornwe dfl obwy am liocwro fmloewte rm rieclrioefm veatlevre r, ewlihefi cvha livs es,e wt thoi ach f liosw se cto teof fai cfileonwt ,c Cove,f ofifc ient, Cv, of
0.0002–0.002 (1–6 turns), with 0.0004 being used in Figure 3. This, along with inlet flow, dictates the
0.0002–0.002 (1–6 turns), with 0.0004 being used in Figure 3. This, along with inlet flow, dictates the
Micromachines2018,9,271 10of21
3.2. TheoryofOperation
Through the active control of solvent vapor inlet and outlet flows, the solvent concentration
withintheBPfilmiscontrolledandmonitoredasafunctionoftime. AsshowninFigure3, using
aninsitumeasurementofφ,wedividetheSVAprocessofa60nmPS-b-PLAfilmintofourdistinct
timeregimes. Duringthefirstthreetimeregimes,thicknessdatawastakenevery3switha249ms
integrationtime. Theinitialregime(blue)includestheopeningofthesolventreservoirtothesample
chamber and an initial, constant N inlet flow of 30–100 SCCM (dependent on desired chamber
2
pressure,50SCCMinthepresentstudy),resultinginanexponentialincreaseinφ. Thethicknessdata
iswellmodeledassumingacopolymerrefractiveindexof1.55, consistentwiththeneatPS-b-PLA
film(AppendixB).Thisexponentialregiontypicallylasts~60swherethetimedependenceiswell
modeledwithasinglerateconstant(0.03–0.1s−1)with0.088s−1 forFigure3(seeFigureA8forthe
exponentialfit),whichisdependentonexactinletandoutletflowrates. Outletflowratesaregoverned
byalowflowmicrometerreliefvalve,whichissettoaflowcoefficient,C ,of0.0002–0.002(1–6turns),
v
with0.0004beingusedinFigure3. This,alongwithinletflow,dictatesthechamberpressure,which
wasmaintainedbelow3.5kPatoobtainFigure3. FollowingthedisplacementofresidualN inthe
2
chamberwithsolventvapor,thesolventuptakeentersasecondregime(red). Thisregimeinvolvesthe
metereduptakeofsolventintothefilm,indicatedbyacontrolled,linearincreaseinthicknessover
aperiodof2min(highlytunable,basedonrelativeflowrates),untilthefilmreachesthetargetedφ.
Theinletandoutletflowratesremainedthesameforthesecondregimeasgivenforthefirstregimein
Figure3,demonstratingthischangeinsolventuptake. Thethicknessdataiswellmodeledassuminga
copolymerrefractiveindexthatgraduallyapproaches1.45,consistentwithavolumetriccombination
of neat PS-b-PLA and THF (Appendix B). During this phase, two possible methods drive solvent
into the film and increase the thickness. If the inlet flow is higher than outlet flow, the increasing
pressure in the chamber will increasingly force solvent into the film (a relatively fast mechanism).
Iftheinletandoutletflowsarecomparable,theincreaseinrelativesolventconcentrationwithinthe
chamberleadstoanincreaseduptakeofsolventintothefilm(arelativelyslowmechanism). Therefore,
itispossibletoswellafilmwithachamberpressurethatisnominallyatmospheric. Dependenton
desiredchamberpressureandultimatethickness,theinletandoutletflowratesaretuned. Precise
controlisbestachievedbyadjustingthemassflowcontroller-regulatedinlet. Wehavenotseenany
impactwiththerateofswellinginthissecondregimeonthefinalmorphologyofthefilm,otherthan
a potential dependence on the pressure in the chamber (faster swelling is typically accomplished
withanincreaseinchamberpressure). Thethicknessdatainthisregimeiswellmodeledassuminga
copolymerrefractiveindexof1.45(FigureA10),consistentwithasolvatedPS-b-PLAfilmcontaining
φ=0.55.
Asthetargetφisapproached,decreasingtheinletfloworincreasingtheoutletflowcausesφto
leveloff. Thethirdregime(green)ischaracterizedbyanextremelyconstant(standarddeviationis
regularlylessthan∆φ=0.002)solventconcentration. Thisconstantconcentrationcanbemaintained
for nearly any specified anneal time (3 min is shown in Figure 3, but we have maintained similar
consistencyformorethananhour). Itismaintainedthroughslightmanualvariationsininlet/outlet
flowrates,withthefuturepotentialofcomputerfeedbackcontrol. Figure3wasachievedwithafixed
outletflow,consistentwithregimes1and2andbycontrollingtheinletflowbetween10–30SCCMto
achieveaconstantthickness(solventconcentration). Inthefinalsecondsofthisperiod,theintegration
time and thickness acquisition interval are switched to 10 ms and 0 s, respectively. This is the
fastestwecanacquirespectralreflectancedataandstillgetahigh-fidelitymodeltoextractthickness.
This rapid data acquisition allows for close examination of the solvent evaporation period. In the
fourth regime (magenta), evaporation of the solvent from the film occurs. Computer-controlled,
pneumatically-actuatedvalvesopenPath1andclosePath2. Throughthereleaseofpressureinthe
chamber,intandemwithN flow,thesolventvaporisreleasedfromthefilmandevacuatedfromthe
2
chamber. Detailsontimingcontrol,whicharedependentonPath1flowratesandchamberpressure,
are given below. The initial evaporation of solvent from the film is not complete, where φ ≈ 0.2