Structure and mid-infrared optical properties of spin-coated polyethylene films developed for integrated photonics applications

Resea ch A icle Vol. 12, No. 6 / 1 Jun 2022 / Op ical Ma e ials Exp ess 2168 S uc u e and mid-in a ed op ical p ope ies o spin-coa ed polye hylene ilms de eloped o in eg a ed pho onics applica ions MAURO DAVID,1DAVIDE DISNAN,2ANNA LARDSCHNEIDER,1 DOMINIK WACHT,3HANH T. HOANG,1GEORG RAMER,3HERMANN DETZ,1,4 BERNHARD LENDL,3ULRICH SCHMID,2GOTTFRIED STRASSER,1AND BORISLAV HINKOV1,* 1Ins i u e o Solid S a e Elec onics, TU Wien, Vienna, Aus ia 2Ins i u e o Senso and Ac ua o Sys ems, TU Wien, Vienna, Aus ia 3Ins i u e o Chemical Technologies and Analy ics, TU Wien, Vienna, Aus ia 4Cen al Eu opean Ins i u e o Technology, B no Uni e si y o Technology, B no, Czech Republic *bo isla [email p o ec ed] Abs ac : Polye hylene is a p omising polyme o mid-in a ed in eg a ed op ics due o i s b oad anspa ency and op imal e ac i e index. Howe e , simple ab ica ion p o ocols ha p ese e i s op ical cha ac e is ics a e needed o os e a wide ange o applica ions and unlock i s ull po en ial. This wo k p esen s in es iga ions o he op ical and s uc u al p ope ies o spin-coa ed linea low-densi y polye hylene ilms ab ica ed unde humidi y-con olled condi ions. The ilm hickness on polyme concen a ion dependence shows a non-linea beha io , in ag eemen wi h p e iously epo ed heo e ical models and allowing p edic i e concen a ion-dependen hickness deposi ion wi h high epea abili y. The su ace oughness is on he nanome e -scale o all in es iga ed concen a ions be ween 1% and 10%. The c ys allini y o he ilms was s udied wi h he Raman spec oscopy echnique. Mid-in a ed ellipsome y measu emen s show a b oad anspa ency ange as expec ed o bulk ma e ial. Laye exposu e o sol en s e ealed good s abili y o he ilms, indica ing ha he ab ica ed laye s can ou las u he ab ica ion s eps. These in es iga ions con i m he excellen p ope ies o spin-coa ed hin ilms ab ica ed wi h ou no el me hod, c ea ing new oppo uni ies o he use in pho onic in eg a ed ci cui s Published by Op ica Publishing G oup unde he e ms o he C ea i e Commons A ibu ion 4.0 License. Fu he dis ibu ion o his wo k mus main ain a ibu ion o he au ho (s) and he published a icle’s i le, jou nal ci a ion, and DOI. 1. In oduc ion The mid-in a ed (mid-IR) spec al ange ( ∼ 2–20 µ m) is a pa o he elec omagne ic spec um o pa icula scien i ic and echnological in e es o sensing applica ions and op ical ee-space communica ion. [1–3]. While lase sou ces ope a ing a hese wa eleng hs ha e de eloped in o comme cially a ailable ins umen s o e he las yea s, he ad ancemen o in eg a ed ci cui s has aced conside able challenges. A lo o wo k was dedica ed o op imizing op oelec onic de ices [4–7], including no el ma e ials [8,9] and al e na i e a chi ec u es o mid-IR emission [10–12]. Howe e , nex -gene a ion in eg a ion s a egies [13,14], including implemen a ion o no el anspa en ma e ials, we e ela i ely sca ce [15,16]. This is pa icula ly ue o he long-wa e in a ed (LWIR) spec al ange, whe e an impo an a mosphe ic window is loca ed (8–14 µ m). The e, he ma e ial selec ion o highly anspa en ma e ials wi h sui able cha ac e is ics and exis ing p ocessing p o ocols is e y limi ed [17,18]. This is pa icula ly he case conce ning he in eg a ion o hese de ices on o Indium Phosphide (InP) subs a es o he ealiza ion o mid-IR pho onic in eg a ed ci cui s (PICs). Semiconduc o - o Dielec ic Loaded Su ace Plasmon Pola i on Wa eguides (S/D-LSPPW) ha e been ound o be a p omising solu ion o hese issues #458667 h ps://doi.o g/10.1364/OME.458667 Jou nal © 2022 Recei ed 15 Ma 2022; e ised 15 Ap 2022; accep ed 21 Ap 2022; published 12 May 2022 Resea ch A icle Vol. 12, No. 6 / 1 Jun 2022 / Op ical Ma e ials Exp ess 2169 [14,15]. Howe e , only a limi ed numbe o ma e ials can simul aneously o e high anspa ency and low e ac i e index ( o educe in e ace e lec ions) o ma ch hei coun e pa ’s pe o mance a elecom equencies (1.55 µ m). Polyme s ha e a ac ed emendous in e es o in eg a ed op ical applica ions o many yea s [19] bu so a lea ing ou he LWIR spec al ange. Thei a o able op ical cha ac e is ics, such as high anspa ency and low e ac i e index, a e decisi e ac o s o hei use in op oelec onic de ices and applica ions. PMMA and SU-8 a e o en used dielec ic ma e ials o plasmonic de ices in he nea -IR [20–22]. Howe e , nei he o bo h ma e ials can o e high anspa ency in he mid-IR spec al ange [23,24] due o nume ous abso p ion lines especially loca ed in he LWIR (see Fig. 1). Fig. 1. Imagina y pa o he complex e ac i e index a a wa eleng h o λ= 9.12 µ m o some o he mos anspa en polyme s in he LWIR. Da a aken om [36,37] In con as , Polye hylene (PE) shows b oad anspa ency o a wide spec al ange [25]. I was so a exploi ed as b oadband in e connec o [26], lexible an enna [27], and senso pla o m [28] in he THz spec al egion ( > 20 µ m). Theo e ical in es iga ions ha e shown i s g ea po en ial as a co e ma e ial o DLSPPW a elecom and THz equencies [29] and as a cladding laye o hyb id plasmonic THz wa eguides [30,31]. Howe e , o unlock i s ull po en ial in he mid-IR wi h i s much smalle geome ical dimensions, i is necessa y o de elop ad an ageous ab ica ion echniques ha o e epea able hin ilm deposi ion wi h excellen laye quali y. The ilms a e expec ed o ha e a composi ion simila o he bulk ma e ial, wi h low su ace oughness and con olled hickness, while main aining cons an e ac i e index and high anspa ency. Mo eo e , sui able ilms mus supply he necessa y chemical and mechanical s abili y o ou las ambien exposu e and u he p ocessing s eps. Se e al ypes o PE ha e been shown so a , and a e ypically classi ied acco ding o hei densi y and b anching. The wo basic ypes a e High-Densi y PE (HDPE) and Low-Densi y PE (LDPE). HDPE is cha ac e ized by igh ly packed molecules ha o m c ys alline pa ches, while LDPE ypically has a highe deg ee o amo phous ac ions. In gene al, anspa ency ends o dec ease wi h a highe deg ee o c ys alliza ion, as he ligh de lec s along c ys alline bounda ies [32]. The e o e, he choice among he di e en PE ypes na u ally alls owa ds LDPE, mo i a ed by i s highe ansmi ance compa ed o HDPE [33]. Nowadays, LDPE has almos en i ely been eplaced by linea LDPE (LLDPE), which has a highe mel ing empe a u e and c ack s eng h [34,35]. Spin coa ing is a s anda d echnique o p oduce high-quali y and uni o m polyme ilms o adjus able hickness on a plane subs a e [38]. Howe e , he s a e-o - he-a echnology o Resea ch A icle Vol. 12, No. 6 / 1 Jun 2022 / Op ical Ma e ials Exp ess 2170 p oducing spin-coa ed PE ilms is cha ac e ized by complex and ime-consuming p ocesses, including p ehea ed equipmen and employing oxic sol en s (e.g., Decalin) used a hei boiling empe a u es, ollowed by se e al hou s o annealing [39–41]. Fu he mo e, only e y limi ed da a al eady exis s, epo ing he mid-IR s uc u al and op ical p ope ies o PE, and none in he case o laye s p oduced by spin-coa ing [42]. Finally, add essing mid-IR applica ions, he hickness needs o be p edic ed p ecisely in he 100 nm – 6 µ m ange, which has no ye been ealized in li e a u e. In his con ex , we p esen an op imized ab ica ion echnique pe o med by spin-coa ing o LLDPE wi h he scope o p ese ing i s ad an ageous op ical p ope ies and ob aining high epea able esul s in e ms o laye hickness h ough con olling ambien condi ions. The excellen op ical p ope ies o he ilms, and hei s abili y owa ds sol en exposu e, con i m ha hin polye hylene laye s ob ained by ou spin-coa ing echnique can be used o ealize he in eg a ion o c ucial building elemen s o PIC de ices. 2. Ma e ials and me hods 2.1. Ma e ials Double side polished 2-inch Si (100) wa e s wi h 275 µ m hickness we e used as subs a e ma e ial. They we e clea ed in o 11 × 11 mm 2 pieces, cleaned in an ul asonic ba h wi h ace one, insed in isop opanol, and d ied wi h ni ogen. Subsequen ly, 8 nm o Ti and 100 nm o Au we e spu e ed (25 W, p wo k = 8e-3 mba ) in mul iple cycles (30 s pe cycle), al e na ing wi h cool-down pe iods o 60 s, un il eaching he desi ed ilm hickness. LLDPE powde (ρ=0.906 g/mL, a e age Mw∼35000 and Mn∼7700, used wi hou u he pu i ica ion) was pu chased om Sigma-Ald ich and dispe sed in oluene (Sigma-Ald ich, anhyd ous, pu i y 99.8%). 2.2. Polyme ilm ab ica ion The ab ica ion o he ilms was conduc ed in a humidi y-con olled glo e box (see Fig. 2). A low cha o he ab ica ion p ocess is shown in Fig. 3. Fig. 2. Side (a) and op- iew (b) pic u es o he ab ica ion se up in he glo e box. Tempe a u e and humidi y du ing ab ica ion we e moni o ed wi h a digi al hyg ome e . The glo e box s abilizes he humidi y du ing he ab ica ion p ocess by a cons an supply o d y ai lux and by he usage o silica gel g ains. The ela i e humidi y was kep a 15% ± 0.9% du ing he p ocess. Resea ch A icle Vol. 12, No. 6 / 1 Jun 2022 / Op ical Ma e ials Exp ess 2171 Fig. 3. Flow cha o he ilm ab ica ion p ocess. The LLDPE powde was dissol ed in oluene wi h di e en weigh a ios, anging om 1% o 10%, and s i ed a 100 ° C un il comple e dissolu ion. The empe a u e o he solu ion was acked wi h a p obe he mome e . Be o e deposi ing he polyme solu ion, he subs a es we e p ehea ed on a ho pla e se o 120 ° C. The ilm was p oduced om he hea ed solu ion by using he dynamic dispense me hod (i.e., deposi ing he polyme solu ion on he subs a e wa e du ing spinning) o a ixed amoun o solu ion (100 µ L). The spin-coa ing was ca ied ou a 1000 pm o 35 s wi h 5 seconds o accele a ion o all coa ings, ollowed by a pos -baking a 120 ° C o 15 min o e apo a e any sol en esidual. 2.3. Film cha ac e iza ion The a e age hickness o he ab ica ed ilms was measu ed a h ee di e en spo s on each sample wi h a Vis-Nea -IR Ellipsome e (Woollam Alpha-SE, assuming n LLDPE ∼ 1.54 and measu ed n Au ). The measu emen s o he a e age ilm hickness o some o he hicke samples we e also con i med wi h a su ace p o ilome e (B uke Dek ak XT, ip o ce 1 mg). The mo phology analysis o he su ace o he LLDPE laye s was conduc ed wi h he p o ilome e i s and hen on he nanome e scale using A omic Fo ce Mic oscope (AFM). The measu emen s o he analysis wi h he su ace p o ilome e we e aken by scanning along he diagonal di ec ion o he sample. Each scan was 8 mm long o ensu e a comple e o e iew o he su ace inhomogenei ies and oughness dis ibu ion in he inne pa o he sample. The s anda d p ac ice o eliably ex ac ing op ical and elec ical pa ame e s o o ganic hin ilms is by spec ally esol ed ellipsome y in he spec al ange o in e es [43]. The e o e, he op ical p ope ies o he ab ica ed laye s we e in es iga ed using a mid-IR ellipsome e (IR-VASE, J.A Woollam) in he ange o 2–12 µm. A e ilm ab ica ion, selec ed samples we e subme ged in deionized wa e and ace one o se e al minu es. This s ep was pe o med o con i m he obus ness o he ab ica ed coa ings o u he p ocessing s eps wi h espec o adhesion and delamina ion o he ilms. Film c ys allini y was de e mined using Raman spec oscopy by i ing he CH 2 wis ing band a ound 1300 cm −1 , as ou lined by Lin e al. [44]. Raman spec a we e collec ed using Resea ch A icle Vol. 12, No. 6 / 1 Jun 2022 / Op ical Ma e ials Exp ess 2172 a con ocal Raman spec ome e using a 532 nm lase sou ce a 50 mW and a 100x (Zeiss EC Epiplan-Neo lua , NA 0.9) objec i e, using 10 s in eg a ion ime and 10 a e ages. The spec um was i wi h wo Gauss-Lo en z bands, one ixed a 1303 cm −1 and he second loa ing be ween 1290 cm −1 and 1295 cm −1 . The amo phous con en was calcula ed as he a io o he a ea o he 1303 cm−1di ided by he sum o bo h bands. 3. Resul s and discussion 3.1. Fab ica ed samples Syn hesis o PE-based hin ilms is ypically pe o med a empe a u es abo e 100 ° C, wi h p ehea ed equipmen ollowed by long annealing imes [45,41,39,46,40]. S a e-o - he-a echniques o spin coa ing LLDPE ypically equi e he p ehea ing o all he ins umen a ion (subs a e, solu ion and spin coa e chuck) o e y high empe a u es ( > 150 ° C). Wang e al. [39] in es iga ed he mo phology o ul a- hin LLDPE ilms coa ed on HF passi a ed silicon wa e s. To p e en he ilms om c ys allizing, hey dissol ed polye hylene in boiling oluene and spun-cas he h solu ion on hea ed subs a es (150 ° C). The ilms’ hickness was con olled wi h he concen a ion (0.05–5%) and spin-coa ed wi h 2500 pm, ob aining ilms wi h hickness be ween 10 and 460 nm. Two c ys alliza ion condi ions ha e been s udied o imp o e he su ace quali y: non-iso he mal and iso he mal. The i s consis ed o hea ing he ab ica ed ilms in an oil apped acuum o en a 140 ° C o 30 min o mel he ilms. The empe a u e was hen dec eased o oom empe a u e o 12 h be o e exposing he sample o he a mosphe e. The iso he mal p ocess was pe o med by annealing o e en highe empe a u es o 30 min and hen quickly mo ing o ano he acuum o en se a 60 °C, whe e he samples we e u he annealed o 12 h be o e exposing hem o oom empe a u e. Unde non-iso he mal c ys alliza ion condi ions, hey ob ained la su aces wi h ypical sphe uli es. Howe e , a discussion on he su ace oughness o he ab ica ed ilms is missing. They epo ed a high di e ence o 10–15 nm be ween he lamellae idges and he la lamellae egion and a deg ee o he amo phous ac ion a ound 60%. Jeon and K ishnamoo i [40] in es iga ed he indi idual e ec o con inemen and he mal his o y on he supe molecula and lamellae mo phology a ia ion by pe o ming iso he mal c ys alliza ion wi h di e en empe a u es. Fo his pu pose, hey i s dissol ed LLDPE in o boiling oluene and pou ed he solu ion in o me hanol a oom empe a u e in o de o p ecipi a e he polyme a e il a ion, ollowed by o e nigh d ying in a acuum o en a 110 ° C. Subsequen ly, he cleaned LLDPE sample was hea ed o 160 ° C o 5 min and cooled a a a e o 100 ° C o ec ys alliza ion and dissol ed in o boiling decalin (200 ° C) o 1 h. The solu ion was hen cooled and main ained a a empe a u e o 160 ° C and spin coa ed a 1800 pm wi h he s a ic dispense me hod. To p e en ea ly c ys alliza ion p io he spinning p ocess, he spinne chuck was also p ehea ed o 100–140 ° C. They obse ed ha he quali y o he ilms s ongly depended on he ime o he accele a ion pe iod. A longe accele a ion imes he ilms we e no mac oscopically uni o m. The concen a ion anged om 0.15 o 3.5%, and hey ob ained hickness anges be ween 20 o 800 nm. Fo iso he mal c ys alliza ion s udies, he spin-coa ed ilms we e mel ed o 2 h a 160 ° C in a acuum o en, hen quickly mo ed o a second p ehea ed acuum o en whe e hey we e iso he mally c ys allized o e a empe a u e ange o 90–119 ° C o 2 h, and quickly quenched in o liquid ni ogen. As he ocus o he s udy was he mo phological ansi ions upon c ys alliza ion p ocedu e, da a on he oughness and op ical p ope ies o he ilms a e missing. Fo many op oelec onic applica ions, oughness, anspa ency, and epea abili y o a ab ica ion me hod a e essen ial cha ac e is ics in o de o ob ain laye s wi h desi ed hickness and su icien (su ace) quali y. Ambien condi ions may ha e a signi ican e ec on spin-coa ed ilm p ope ies. Typically, spin-coa ing p ocesses o polyme s a e pe o med in esea ch lab ume hoods, cha ac e ized by luc ua ing ela i e humidi y and empe a u e alues, bo h also depending on ambien (wea he ) condi ions. In con as , glo e boxes limi empe a u e and humidi y luc ua ions o a minimum. In gene al, he impac o he humidi y on he ilm hickness is a he Resea ch A icle Vol. 12, No. 6 / 1 Jun 2022 / Op ical Ma e ials Exp ess 2173 known [47,48], and i can c ea e undesi ed hickness a ia ion and lowe laye quali y. The a ia ion in hickness ∆ D o a spin-coa ed laye is ela ed o he Rela i e Humidi y (RH) h ough ∆ D = (1-RH) β , whe e β is an empi ically de e mined cons an . Indeed, om some p elimina y expe imen s pe o med in a s anda d lab ume hood, we ob ained PE laye s wi h a a he b oad hickness ange (be ween 200–500nm o w =2%). By pe o ming he ab ica ion in he glo e box, humidi y luc ua ions ha a ec he hickness epea abili y a e a oided. As discussed p e iously, s a e-o - he-a p ocesses in ol e dissolu ion a sol en boiling empe a u es and p ehea ed equipmen . To a oid excess apo iza ion bu ensu e comple e dissolu ion, we limi ed he empe a u e o he solu ion o 100 ° C (i.e., en deg ees below he boiling poin o he sol en ). Howe e , when using he s a ic dispense me hod, we ob ained inhomogeneous laye s wi h low op ical cla i y, in ag eemen wi h he indings in li e a u e [40]. We ound ha his issue is ci cum en ed by using he dynamic dispense me hod. In his way, epea able homogenous laye s can be ob ained. Films ab ica ed wi h his me hod show no isible inhomogenei ies o he naked eye (Fig. 4). Mo eo e , a e pos -baking, he syn hesized ilms we e soaked in ace one and deionized wa e , p o ing s ong adhesion o he unde nea h subs a e. These a e common s eps pe o med in clean ooms du ing pho oli hog aphy-based p ocesses o he ab ica ion o pho onic de ices, making he coa ings’ s abili y o hese sol en s essen ial o u he de elopmen s. In con as , he laye s ab ica ed ou side he glo e box o en expe ienced delamina ion and showed adhesion p oblems a e he pos -bake s ep. Fig. 4. Mac oscopic pic u es o ab ica ed samples in he glo e box wi h deposi ion pe o med be o e spinning (a), (b), and du ing spinning (c),(d). 3.2. Film hickness and concen a ion dependence F om a de ice design pe spec i e, i is desi able o be able o p edic he hickness o he ab ica ed laye s wi h ela i ely high p ecision om simple, measu able quan i ies. Typically, his is done by changing he concen a ion o he solu ion and/o he o a ional speed o he spin coa ing p ocess. In i s app oxima ion, he hickness depends mo e s ongly on he concen a ion, and he e o e we decided o ix he o a ional speed o 1000 pm (a a he low alue as we aimed o ela i ely hick ilms). Wi h ou me hod, i was possible o epea edly ex ac he non-linea ilm hickness dependence as a unc ion o he LLDPE concen a ion om he ilms ab ica ed. To desc ibe his beha io , we used he ela ion p oposed by Chen [47] ha models he hickness-dependence o a spin-coa ed laye om sol en ype, solu ion, and spin coa ing pa ame e s: D=K0µ0.36ω−0.50 (︃Eλ Cµ)︃0.6 (1) Resea ch A icle Vol. 12, No. 6 / 1 Jun 2022 / Op ical Ma e ials Exp ess 2174 whe e Dis he ilm hickness, K0 is a cons an o ola ile o ganic sol en s, µ is he iscosi y o he coa ing solu ion, ω is he o a ional speed, Eis he sol en -e apo a ion a e, λ is he la en hea o e apo a ion and Cµ is he sol en hea capaci y. E en hough he ela ionship be ween iscosi y and polyme concen a ion can be a he complex [45,49,50], i is possible o app oxima e i by a powe -law unc ion [45,51,52]. The e o e, he da a we e i ed o: logD =C+0.36α(w )(2) whe e Cis a pa ame e ha includes all he cons an s om Eq. (1), w is he concen a ion, and α ep esen s he coe icien desc ibing he phenomenological powe law ha cha ac e izes concen a ion and iscosi y. In gene al, he iscous beha io o a solu ion can be es ima ed wi h good p ecision o highe concen a ions, by expanding he iscosi y-concen a ion equa ion in o a Taylo se ies [50]: µ=µ0(1+[µ]w +kH[µ]2w 2+· · ·) (3) whe e [µ] is he in insic iscosi y and kH is he Huggins cons an . Figu e 5shows he ob ained esul s when i ing he hickness o he model. F om ou measu emen s, we could ex ac wo dis inc alues o α ha di ide he solu ion in o wo egimes. We ob ained a alue o α= 2.8 o concen a ions anging be ween 1 and 4%, indica ing ha he solu ion in his concen a ion ange can be conside ed iscoelas ic [52]. Fo concen a ions abo e 5% we ex ac ed a alue o α= 7.72. This ela i ely high alue indica es ha he solu ion is in a highly concen a ed egime, ypically cha ac e ized by en angled mac omolecules [49,50,52]. w (%) D (nm)   1 138 ± 1 2.83 2 271 ± 4 2.83 4 560 ± 11 2.83 6 1246 ± 13 7.72 8 2670 ± 2 7.72 10 5186 ± 17 7.72 (a) (b) Fig. 5. (a) Film hickness dependence on LLDPE concen a ion including expe imen al da apoin s ( ed c osses) and i s o he model in bo h ob ained egimes (solid line). All ilms we e ab ica ed in he glo e box. (b) Table epo ing he p ecise hickness alues measu ed wi h he ellipsome e . To es he epea abili y o ou ab ica ion me hod, we ab ica ed 10 coa ings wi h mass po ion o 10%. The a e age measu ed hickness was 5159 ± 87 nm, showing he excellen epea abili y o ou ab ica ion p ocess. 3.3. Mo phology analysis o ilms LLDPE ypically p esen s a deg ee o c ys allini y anging om 20% o 50% [32,39,44] and, he e o e, na u ally ends o a ange in o a mo e amo phous phase. Since he scope o ou wo k is Resea ch A icle Vol. 12, No. 6 / 1 Jun 2022 / Op ical Ma e ials Exp ess 2175 o a ain homogenous and anspa en coa ings, we did no need o ocus on achie ing a maximal deg ee o c ys alliza ion, because i would ha e been coun e p oduc i e, as desc ibed in he in oduc ion. AFM su ace scans o he ab ica ed LLDPE ilms o di e en polyme concen a ions a e p esen ed in Fig. 6. Di e en a angemen s o chains along he su ace can be iden i ied om he heigh p o ile eco ded wi h he AFM. The b igh a eas a e connec ed o a highe polyme chains concen a ion. Howe e , since we did no pe o m any iso he mal o non-iso he mal c ys alliza ion p ocess, no highly s uc u ed and o de ed mo phology based on sphe uli es can be obse ed. F om Fig. 6, i can be seen ha inc easing he pe cen age o LLDPE dissol ed in he solu ion esul s in an enla gemen o he polyme mic os uc u e. The su ace ea u es o he LLDPE hin ilms become mo e p ominen as he polyme concen a ion inc eases, wi hou a ec ing he gene al cha ac e is ics o he chain a angemen . Fu he mo e, he su ace’s heigh eco ded by he AFM inc eases by he enla gemen o he mic os uc u e. Fig. 6. AFM images o LLDPE hin ilm su aces o di e en mass concen a ions. The change in o de ing o he s uc u es in he di e en egimes can be obse ed. In o de o e alua e he quali y o he laye s, we de e mined he mean oughness as a unc ion o he polyme concen a ion om he AFM measu emen s (Fig. 7). E en hough he su ace oughness inc eases wi h concen a ion, i emains below 15 nm o all concen a ions. The su ace o he ilms, analyzed by he p o ilome e , does no e eal inhomogenei ies on he mm-scale ac oss he samples. Such “wa y-like” luc ua ions o he hickness ha e been obse ed in p e ious s udies on PE coa ings [39] and a e associa ed wi h he Gibbs-Ma angoni e ec . 3.4. Film c ys allini y The amo phous con en o he ilm was ound o dec ease wi h inc easing LLDPE concen a ion in he coa ing solu ion (see Fig. 8) om a ound 80%(m/m) a 1% concen a ion down o a ound 65%(m/m) a 10% ( i ed spec um is p o ided in Supplemen 1). Simila ly, he a iance o amo phous con en was ound o dec ease wi h inc easing PE con en . Howe e , as Raman spec a ep esen he make up o an a ea smalle han 500 nm in diame e , he dec ease in s anda d de ia ion can be in e p e ed as being in pa dependen on he homogenei y o he ilm. A highe Resea ch A icle Vol. 12, No. 6 / 1 Jun 2022 / Op ical Ma e ials Exp ess 2176 Fig. 7. Mean oughness (S a ) e sus concen a ion o he di e en concen a ions o LLDPE. concen a ion o he s ock solu ion leads no only o a dec ease in amo phous ac ion o he ilm, bu also o mo e homogeneous ilms. Fig. 8. Mass ac ion o amo phous con en o he ilm, as de e mined om con ocal Raman spec oscopy. E o ba s ep esen he s anda d de ia ion o measu emen s aken in 5 di e en loca ions. 3.5. Mid-in a ed op ical cha ac e iza ion The wa eleng h-dependen mid-IR e ac i e index and pe mi i i y p o ile o a ypical LLDPE ilm ab ica ed in he glo e box is shown in Fig. 9( he able wi h he da a is p o ided in Supplemen 1). We hus expec ed hei p esence due o he in insic polyme in e band ansi ions [3]. The high anspa ency o he ab ica ed ilms is expe imen ally con i med, indica ing he excellen op ical p ope ies o he ab ica ed laye s a mid-IR o LWIR equencies. This da a p esen s he i s mid-IR ellipsome y measu emen o spin-coa ed LLDPE and con i ms he po en ial o ou ab ica ion echnique o p oducing obus , con ollable, and ime-e icien LLDPE coa ings. Due o he op imal e ac i e index cha ac e is ics, hey can be employed as passi a ion laye s, an i e lec ion-coa ing, and wa eguides (cladding o co e)