Nanopatterning Au chips for SPR refractometer by using interference lithography and chalcogenide photoresist

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1 PACS Mf, n, nd, Hp Nnoptterning Au chips for SPR refrctometer y using interference lithogrphy nd chlcogenide photoresist V.A. Dn ko, G.V. Dorozinsky, I.Z. Indutnyi, V.I. Myn ko, Yu.V. Ushenin, P.E. Shepelivyi, M.V. Lukniuk, A.A. Korchovyi, R.V. Khrystosenko V. Lshkryov Institute of Semiconductor Physics, NAS of Ukrine 41, prospect Nuky, Kyiv, Ukrine E-mil: indutnyy@isp.kiev.u Astrct. This study reports on development of the interference lithogrphy (IL) technique pplying the resist sed on chlcogenide glss films for friction of gold chips in the form of periodic surfce nnostructures for surfce plsmon resonnce (SPR) refrctometers. The IL technique ws optimized for ptterning the Au lyers nd formtion of one-dimensionl (grting) structures with the sptil frequency close to 3300 mm 1. The sptil frequency nd depth of grting grooves were selected with ccount of the condition for Brgg reflection of plsmons t the opertion wvelength of SPR refrctometer nd given environment (which is condition for enhncing iosensor sensitivity s compred to tht of flt Au chip surfce). It hs experimentlly een demonstrted tht the use of diffrction ptterns in SPR sensor increses its response y 17%. Keywords: interference lithogrphy, surfce plsmon resonnce, chlcogenide glss films, photoresist. Mnuscript received ; revised version received ; ccepted for puliction ; pulished online Introduction Surfce plsmon resonnce (SPR) devices hve found wide use in recent yers in iosensing pplictions due to their dvntges of high sensitivity, lel-free, reltime nd rpid detection. During the lst two decdes, it hs een mde gret progress in the development of instrumenttion nd prcticl pplictions [1, 2]. But conventionl reflection-type SPR iosensors employing prism in the ttenuted totl reflection configurtion (Kretschmnn configurtion) [3] present severl drwcks nd usully lck the sensitivity to detect interction of proteins with smll lignds [4]. To improve the performnce of SPR sensors, sensitivity enhncement hs een of tremendous interest in the development of these sensors. In recent yers, modified SPR sensing systems, such s gold nnoprticles immoilized on gold film [5], gold nnoprticleemedded dielectric lyer on gold film [6], nd metllic nnowires regulrly ptterned on metl film [7], hve een proposed theoreticlly nd experimentlly. Enhnced sensing chrcteristics re minly ssocited with shift or ending in dispersion reltion of propgting surfce plsmon, induced y its strong coupling with loclized plsmon resonnce modes in metllic nnostructures [8]. The recent theoreticl study [9] hs shown tht y formtion sinusoidl profile grting t the surfce of the metl lyer of the SPR sensor used in the Kretschmnn configurtion, the iosensor sensitivity 438

2 my e enhnced in comprison with tht of uncorrugted metl surfce. Surfce plsmons propgting in the direction perpendiculr to one-dimensionl grting (i.e. long the grting vector) will experience ck reflections. If the period of the grting corresponds to the Brgg condition, then ndgp will form where no surfce plsmons re permitted to propgte. The control over the position of the ndgp with respect to the wvelength nd ngle cn e ccomplished through the mnipultion of the period nd/or grting height. By designing the operting point of the sensor to e ner the ndgp, the sensitivity cn e improved s compred to the sensitivity with flt metllic lyer. But these results hve not yet een tested experimentlly. In order to get good plsmon resonnce nd enhnced sensor performnce, high-qulity metllic grtings or nother periodic sumicrometer structures over lrge res re needed. With n dvnce of nnofriction technology, periodic metllic nnostructures such s nnodot, nnohole nd nnogrting hve een relized in order to cquire designle nd predictle plsmonic properties. Depending on the period nd totl surfce re of these structures nd the mterils to e used, numer of different friction techniques cn e considered. Lst decde, interference (interferometric) lithogrphy (IL) is widely used for friction of defect-free periodic micrond nnostructures. IL is lrge re friction technique tht uses lser interference ptterns for rpid formtion of periodic structures such s grtings, nd igrtings (rrys). The totl processed re depends on the em intensity nd coherence length of the lser nd cn e up to dozens, or even hundreds of squre centimeters. The technology is much cheper nd simpler thn the electron em lithogrphy nd cn e used for mnufcturing of SPR sensor structures [10, 11]. In previous studies, the uthors hve shown tht IL with the use of chlcogenide photoresist is promising technology to form one- nd two-dimensionl sumicron periodic structures on the surfce of semiconductors nd dielectrics [12]. Chlcogenide photoresists sed on therml evported morphous films of chlcogenide glsses re chrcterized y high resolution, opticl uniformity, wide spectrl rnge of photosensitivity. In ddition, these photoresists possess high refrctive index rnging from 2.0 up to 3.0 nd re very perspective for immersion IL. In this pper, we report the IL technique with the use of resist sed on the chlcogenide glss (ChG) films for friction of gold chips with periodic surfce nnostructures for SPR refrctometers. 2. Experimentl The smples for our experiments were prepred y successive therml vcuum deposition of 1 3 nm thick (effective thickness) Cr dhesive lyer, lyer of metl (Au) with the thickness of 40 to 50 nm nd photoresist lyer (As 2 S 3 or GeSe 3 ) with the thickness from the rnge 50 to 300 nm onto polished glss sustrtes (F-1 glss, mm) t the residul pressure of P. The deposition rte nd film thickness were monitored in situ with KIT-1 qurtz thickness meter. After deposition, the film thickness ws controlled using MII-4 microinterferometer. To increse stility of the lyer, the gold chips were nneled t Т 120 С efore chlcogenide deposition [13]. Recording the periodic structures on ChG films ws crried out using the interference pttern formed with helium-cdmium lser (wvelength λ = 440 nm). The exposition vlue for the grtings recording (1D structures) ws J/cm 2. After exposure, the smples were chemiclly treted in non-wter lkline orgnic solutions to form resistive msk in the photoresist lyer, through which the metl film ws etched. After removing the photoresist residues in lkline solution, wshing nd drying the metl periodic structure ws otined. The surfce ptterns of otined structures were exmined with Dimension 3000 Scnning Proe tomic force microscope (Digitl Instruments Inc., Tonwnd, NY, USA) in the AFM tpping mode. Mesurements of ngulr dependences for light reflection (reflection curve) from the interfce nlyt/gold film on glss sustrte were performed using the two SPR refrctometers Plsmon-6 nd Plsmon-71 (V. Lshkryov Institute of Semiconductor Physics, NAS of Ukrine, Kyiv) with the opertion wvelengths 650 nd 850 nm, respectively. These devices llow mesuring the solute vlue of reflectivity s well s solute vlue of the ngle in the Kretschmnn configurtion. The wide rnge of ngles for scnning (up to 17 degree in ir) llows performing quntittive mesurements of full reflection curve for liquids with vrious refrction indexes from 1.33 (wter) up to 1.41 (hemogloin) for Plsmon-6 nd from 1.33 (wter) up to 1.47 (motor oil) for Plsmon-71 [14, 15]. 3. Results nd discussion Theoreticl modeling in [9] ws crried out for sinusoidl surfce relief grting with smll depth of the relief. For these grtings, we cn pproximtely estimte the vlue of the Brgg period (Λ B ) using the following simple formul: Λ B = 0.5λ 0 [(ε mr + ε D )/ε mr ε D ] 1/2, where λ 0 is the free-spce wvelength, ε D dielectric constnt of the dielectric nd ε mr rel prt of the dielectric constnt inherent to the metl. It ws otined tht, for our smples (Au lyer on F-1 glss sustrtes) t the wvelength 650 mn, the conditions of Brgg reflection of plsmons (which is the condition for enhncing the iosensor sensitivity s compred to tht for flt surfce) re stisfied t sptil frequencies of grtings ner 3000 mm 1 for the interfce Au-ir. For the interfce Au-wter, this condition is stisfied for the sme sptil frequency t the opertion wvelength 850 nm.

3 The IL technique ws optimized (thickness of photoresist, exposure time, selectivity of etchnt nd time of the post-exposure etching) for ptterning the Au lyer nd formtion of one-dimensionl structures with the sptil frequency up to 3300 mm 1. IL technology ws pplied in the mode of little over-exposure of photoresist to provide cycloid form of groove profile of periodic chlcogenide msk. By chnging the time of selective etching of photoresist, it is possile to chnge the width of the elements of lithogrphic msks nd, ccordingly, the width of opened intervls etween the elements of the msk, through which further etching the metl lyer is fulfilled. Etching the photoresist ws monitored in situ y registrtion of non-photoctive longwve light diffrcted from relief structure tht is formed in the photoresist lyer. For given photoresistive msk, form of profile nd duty cycle (the rtio of width to period) of the elements in the periodic structure, otined s result of wet etching the metl lyer through the chlcogenide msk, were lso determined y the rte nd time of metl etching. Fig. 1 shows the AFM imge of the gold grting formed using IL on the gold lyer with thickness ner 40 nm y wet etching through ChG resistive msk. The period of this grting is 330 nm (sptil frequency 3000 mm 1 ). On this smple, the photoresist msk ws formed with the rtio of the strip width to intervl width close to unity. The depth of the relief reches 36 nm, nd the groove sectionl profile is trpezoid, even close to rectngulr with flt tops (Fig. 1). It mens tht the gold lyer is etched throughout the whole thickness to the glss sustrte. This grting ws nneled in vcuum t 400 C for 15 min. Fig. 2 shows the imge nd groove profile of the sme grting fter nneling. One cn see significnt chnges in the groove profile s result of nneling. Trpezoid flt-topped groves were trnsformed to convex grooves with smller hlf-width nd greter height. Investigtion of groove heights histogrm hs shown tht the initil height of the grooves of 36±2 nm hs incresed up to 49±5 nm s result of the nneling. Tht is the flt gold nno-strips were trnsformed during the nneling into semi-cylindricl nno-wires y the forces of surfce tension. So, using the dditionl nneling we cn chnge the shpe of the grting grooves, nd to some extent vry condition (resonnce ngle, wvelength) of plsmon resonnce. Enhncement of the iosensor sensitivity ws theoreticlly studied [9] for grting on the Ag surfce with the depths of the relief 20 nd 40 nm (respective totl thicknesses of the metl lyers were 70 nd 90 nm). The optiml thickness of gold films for SPR refrctometer chips is nm. To investigte opticl chrcteristics nd sensitivity of the sensors with diffrction grting, we produced severl grtings with smll depth of relief nd the sptil frequency close to 3000 mm 1 on the gold film with the thickness 50 nm. Fig. 1. AFM imge nd cross-sectionl profile of the Au grting with the period 330 nm. Fig. 2. The sme grting s in Fig. 1 fter nneling for 15 min t 400 C. Fig. 3 shows the AFM imge nd cross-sectionl profile of the Au grting with the period 330 nm nd relief depth close to 20±4 nm. 440

4 sensor response (S) with diffrction grting s compred to the sensor without it. S ws clculted s the rtio Δθ dsp / Δθ sp, (where Δθ dsp is the shift of ngulr position for SPR minimum for the sensor with grting t the trnsfer from wter to ethnol, Δθ sp the sme vlue for the stndrd sensor) nd ws Fig. 3. AFM imge nd profile of the Au grting with the period 330 nm nd depth of the relief 20±4 nm (totl thickness of Au lyer is 50 nm). Fig. 4. SPR curves plotted for sensing element with diffrction grting (white line) nd without it (lck line) in mient ir. SPR curves were mesured using the device Plsmon-6. Formtion of diffrction grting on the surfce of the gold film chnges the shpe of the ngulr dependence for reflection from the opertion surfce of the sensor ner resonnce. Fig. 4 shows the curves of surfce plsmon resonnce for stndrd sensor with the flt surfce of gold film (lck line) nd sensor with diffrction grting (white line) with the sme prmeters (sptil frequency, depth relief) s for the grting in Fig. 3. The externl environment in these mesurements ws ir. The dependence of the reflectnce on the ngle of incidence (SPR curves) were mesured using the device Plsmon-6 (opertion wvelength 650 nm). From the figure, one cn see tht SPR curve for the sensor with grting is significntly expnded nd shifted to greter ngles. In ddition, two mxim re seen, indicting proximity to the Brgg resonnce. For comprison, study of sensor sensitivity ws crried out for oth sensors in liquid environment. SPR curves were mesured using the device Plsmon-71 (opertion wvelength 850 nm). Fig. 5 shows the SPR curves for the sensor with diffrction grting possessing the sme prmeters s in Fig. 3, nd Fig. 6 shows the SPR curves for stndrd sensor. The externl environments in these mesurements were distilled wter (white lines on oth grphs) nd 96% queous solutions of ethnol (lck lines). Their refrction indexes were mesured independently using the Aé refrctometer IRF-22 nd were for wter nd for ethnol solutions. The ngulr position of SPR minimum ws clculted for two mient liquids. Also, we clculted enhncement of the Fig. 5. SPR curves for distilled wter (white line) nd ethnol solution (lck line) for sensing element with diffrction grting. SPR curves were mesured using the device Plsmon-71. Fig. 6. SPR curves for distilled wter (white line) nd ethnol solution (lck line) for stndrd sensor. SPR curves were mesured using the device Plsmon

5 Thus, the use of diffrction ptterns in SPR sensor increses sensor response y 17%. This vlue is much higher thn the possile experimentl error (experimentl ccurcy for determintion of the ngulr position of SPR minimum for Plsmon-71 is degrees, nd in our experiment Δθ sp = nd Δθ dsp = ), ut is smller thn the theoreticlly possile [9] mximum vlue (600%). It should e noted tht enhncement of sensitivity, ccording to the simultion results [9], strongly depends on the refrctive index of the environment on the externl surfce of the gold film. Significnt enhncement should e oserved in nrrow rnge of vrition of the refrctive index (Δn 0.01) for given grting prmeters. A lrge intervl of Δn (s in our cse Δn ~ 0.03) gives the verge vlue of enhncement, which should e much less thn the theoreticlly possile mximum. In ddition, theoreticl modeling were performed for the idel structures without surfce roughness nd other possile defects. Such inhomogeneities cn cuse scttering of plsmons, rodening of the resonnce curve nd decrese the sensor sensitivity. To otin higher sensitivity, further optimiztion in friction procedure nd prmeters of grtings is currently underwy. 4. Conclusion Being sed on the presented results, we cn conclude tht IL method with dditionl nneling enles successful friction of ptterned SPR chips with desired chrcteristics (sptil frequency, depth of relief, form of profile) on the re lrger thn 5 cm 2. The results of mesuring the sensor sensitivity in liquid environment confirm theoreticl prediction out enhncement of SPR iosensor sensitivity y formtion of grting with the pproprite period nd depth on the rer fce of SPR chip. The uthors grtefully cknowledge tht this work hve een prtly funded y the Swiss Ntionl Science Foundtion (SNSF, Bern) under grnt no. IZ73Z0_ (SCOPES). References 1. J. Homol, Surfce plsmon resonnce sensors for detection of chemicl nd iologicl species // Chem. Rev. 108(2), p (2008). 2. B.A. Snopok, E.V. Kostyukevich, S.I. Lysenko et l., Opticl iosensors sed on the surfce plsmon resonnce phenomenon: optimiztion of the metl lyer prmeters // Semiconductor Physics, Quntum Electronics nd Optoelectronics, 4(1), p (2001). 3. E. Kretschmnn, H. Rether, Rditive decy of non-rditive surfce plsmons excited y light // Z. Nturforsch. A, 23, p (1968). 4. P. Engleienne, A. Vn Hooncker, M. Verhs, Surfce plsmon resonnce: principles, methods nd pplictions in iomedicl sciences // Spectroscopy, 17(2, 3), p (2003). 5. R.S. Moirngthem, Y.-C. Chng, nd P.-K. Wei, Ellipsometry study on gold-nnoprticle-coted gold thin film for iosensing ppliction // Biomed. Opt. Exp. 2(9), p (2011). 6. W.P. Hu, S.-J. Chen, K.-T. Hung, J.H. Hsu, W.Y. Chen, G.L. Chng, nd K.-A. Li, A novel ultrhigh resolution surfce plsmon resonnce iosensor with n Au nnocluster-emedded dielectric film // Biosens. Bioelectron. 19(11), p (2004). 7. K.M. Byun, S.J. Yoon, D. Kim, nd S.J. Kim // Experimentl study of sensitivity enhncement in surfce plsmon resonnce iosensors y use of periodic metllic nnowires // Opt. Lett. 32(13), (2007). 8. Nk-hyeon Kim, Munsik Choi, Jung Woo Leem et l., Improved iomoleculr detection sed on plsmonic nnoporous gold film fricted y olique ngle deposition // Opt. Exp. 23(14), p (2015). 9. C.J. Alleyne, A.G. Kirk, R.C. McPhedrn, N-A.P. Nicorovici nd D. Mystre // Enhnced SPR sensitivity using periodic metllic structures // Opt. Exp. 15, p (2007). 10. A. Arriol, A. Rodriguez, N. Perez et l., Friction of high qulity su-micron Au grtings over lrge res with pulsed lser interference lithogrphy for SPR sensors // Opt. Mter. Exp. 2(11), p (2012). 11. M. Vl nd J. Homol, Flexile method sed on four-em interference lithogrphy for friction of lrge res of perfectly periodic plsmonic rrys // Opt. Exp. 22(15), p (2014). 12. V. Dn ko, I. Indutnyi, M. Min ko, P. Shepelyvyi, Interference photolithogrphy with the use of resists on the sis of chlcogenide glssy semiconductors // Optoelectronics, Instrumenttion nd Dt Processing, 46(5), p (2010). 13. B.A. Snopok, E.V. Kostyukevich, S.I. Lysenko et l., Opticl iosensors sed on the surfce plsmon resonnce phenomenon: optimiztion of the metl lyer prmeters // Semiconductor Physics, Quntum Electronics nd Optoelectronics, 4(1), p (2001). 14. N. Gridin, G. Dorozinsky, R. Khristosenko, V. Mslov, A. Smoylov, Yu. Ushenin, Yu. Shirshov, Surfce plsmon resonnce iosensor // Sensors & Trnsducers J. 149(2), p (2013). 15. G.V. Dorozinsky, A.I. Liptug, V.I. Gordienko, V.P. Mslov, V.V. Pidgornyi, Dignostics of motor oil qulity y using the device sed on surfce plsmon resonnce phenomenon // Scholrs J. Eng. nd Technol. (SJET), 3, p (2015). 442