A novel lithography technique for formation of large areas of uniform nanostructures

Transcription

1 A novel lihography echnique for formaion of large areas of uniform nanosrucures Wei Wu, Dibyendu Dey, Omer G. Memis, Alex Kasnelson, Hooman Mohseni a a Dep. of EECS, Norhwesern Univ., 2145 Sheridan Rd, Evanson, IL 60208, USA ABSTRACT Wih nanoechnology becoming widely used, many applicaions such as plasmonics, sensors, sorage devices, solar cells, nano-filraion and arificial kidneys require he srucures wih large areas of uniform periodic nanopaerns. Mos of he curren nano-manufacuring echniques, including phoolihography, elecron-beam lihography, and focal ion beam milling, are eiher slow or expensive o be applied ino he areas. Here, we demonsrae an alernaive and novel lihography echnique Nanosphere Phoolihography (NSP) ha generaes a large area of highly uniform periodic nanoholes or nanoposs by uilizing he monolayer of hexagonally close packed (HCP) silica microspheres as superlenses on op of phooresis. The size of he nanopaerns generaed is almos independen of he sphere sizes and hence exremely uniform paerns can be obained. We demonsrae ha he mehod can produce hexagonally packed arrays of hole of sub-250 nm size in posiive phooresis using a convenional exposure sysem wih a broadband UV source cenered a 400 nm. We also show a large area of highly uniform gold nanoholes (~180 nm) and nanoposs (~300nm) array wih he period of 1 µm fabricaed by he combinaion of lif-off and NSP. The process is no limied o gold. Similar srucures have been shown wih aluminum and silicon dioxide layer. The period and size of he srucures can also be uned by changing proper parameers. The echnique applying self-assembled and focusing properies of micro- /nano-spheres ino phoolihography esablishes a new paradigm for mask-less phoolihography echnique, allowing rapid and economical creaion of large areas of periodic nanosrucures wih a high hroughpu. Keywords: micro-spheres, super-lenses, phooresis, lif-off, and nanosrucures 1. Inroducion As nanoechnology being developed widely, here is an increasing demand for rapid and massively parallel fabricaion sraegies for large areas of uniform periodic subwavelengh srucures. Arrays of he nanosrucures have been poenially found o have many imporan applicaions such as surface plasmonics [1], daa sorage [2], opoelecronic devices [3], and nano-filraion [4]. To fabricae he nanosrucures, sandard lihography echniques, such as x-ray lihography [5], elecron-beam lihography [6], and focal ion beam milling [7] mehods, have high enough resoluions o be applied. However, x-ray lihography is oo expensive because of he expensive insrumens used and he e-beam lihography and focal ion beam milling mehods are oo slow because of heir inheren serial naure. Low-cos and high hroughpu mehods such as self-assembly are anoher class of fabricaion echniques. Generally, he self-assembly mehods can be defined as he sponaneous formaion processes which lead o complex hierarchical srucures from predesigned building blocks. The self-assembly mehods, such as Copolymer lihography [8] and Nanosphere lihography (NSL) [9], have a lo of advanages for cerain applicaions, bu hey are no as fully developed as he sandard lihography mehods. The developmen of alernaive parallel lihography mehods is sill necessary for differen applicaions. Here we will presen an alernaive mehod, Micro-/nano-sphere Phoolihography (NSP), for fabricaion of large areas of uniform periodic nanopaerns using a monolayer of silica micro-spheres o focus UV ligh. Recen heoreical and experimenal daa show ha spheres wih cerain refracive index can produce phoonic-jes, over a wide a hmohseni@ece.norhwesern.edu; Fax: (847) ; Phone: (847) ; bisol.norhwesern.edu Nanoengineering: Fabricaion, Properies, Opics, and Devices V, edied by Elizabeh A. Dobisz, Louay A. Eldada, Proc. of SPIE Vol. 7039, 70390P, (2008) X/08/$18 doi: / Proc. of SPIE Vol P-1

2 wavelengh range, o generae a beam wais much smaller han he wavelengh of ligh [10][11][12]. Using his effec, we inroduced a new subwavelengh nano-paerning process ha is based on he convenional UV ligh sources widely used in phoolihography indusry, such as g-line (405 nm), i-line (365 nm), or oher deep-uv ligh. Since he FWHM of he focused ligh is a very weak funcion of he diameers of he micro-spheres, a very uniform array of nanopaerns can be generaed by his mehod. Compared wih oher nanolihography echniques, NSP is a maskless echnique. The microspheres can be assembled as monolayer on op of phooresis by many mehods, such as drop-coaing [13], spincoaing [14] and convecive assembly [15]. NSP has he advanages of being fas, high hroughpu and low-cos. I uilizes boh he self-assembled propery of micro-spheres and parallel advanage of phoolihography. I can also be widely applied ino many areas, since i generaes nanopaerns in phooresis, which have been widely sudied and used. I is also a unable process. The sizes of he nanopaerns generaed can be changed by differen exposure dose and developmen of phooresis, while he period of he array can be uned by he diameers of micro-spheres. The combinaion of NSP echnique wih oher sandard processes such as lif-off and eching grealy broadens NSP ino he fabricaion of nanopillars and nanoholes in oher maerials. Applying he lif-off wih he nanoholes in posiive phooresis or nanopillars of negaive phooresis, we produced a large area of periodic uniform nanoposs of gold or nanoholes perforaed in gold film. Differen meals, dielecrics or muli-sacks wih he nanoholes perforaed can also be generaed. Using a broadband wavelengh ligh source cenered around 400 nm we have produced he nanoholes of he diameer of abou 250 nm in posiive phooresis and he nanopillars of he diameer of abou 200 nm wih negaive phooresis. The nanopillars generaed have a high aspec raio and under-cu shape for lif-off. As small as 180 nm nanohole arrays perforaed in ianium/gold films were produced. The nanohole arrays perforaed in aluminum and silicon dioxide films were also shown. 2. Simulaions The UV ligh propagaion hrough he silica micro-sphere has been sudied and simulaed by 3D-FDTD modeling mehods. The basic model for he simulaions is he silica micro-sphere sanding on op of he phooresis wih he subsrae below, as shown in figure 1(a). The variables in he simulaions are he diameers of he micro-spheres, he indices of he spheres (differen maerials), and differen inpu wavelenghs of UV ligh. During he simulaions, we changed one variable while keeping oher variables consan. Figure 1(b) shows one example of he focus process by he silica micro-sphere. We are using he plane wave from he ligh source. Wih he diameer of micro-sphere of abou 1 µm and he UV ligh s wavelengh of 365 nm, he FWHM value of he focused ligh is abou 150 nm and he inensiy is abou 30 imes sronger han he inpu one. Proc. of SPIE Vol P-2

3 Figure 1 (a) he basic simulaion model, (b) one example of he focus process by 1 µm silica micro-sphere Changing he diameers of he spheres from abou 0.5 µm o abou 5 µm, we found ha he FWHM values of he focused ligh were almos unchanged. Figure 2 is he normalized inensiy of he focused ligh versus he posiion in he plane of he ligh s propagaion wih differen diameers of micro-spheres. The variaion of he FWHM values of he focused ligh versus he spheres diameer is less han 1% considering a linear relaionship. The FWHM value can be approximaely used as he sizes of nano-feaures generaed in phooresis since i is he hreshold value beween he exposed and unexposed area. In hence, he nanopaerns array generaed would be very uniform. Wavekngli 4OO'un 0-3 -&1 91 Pa.Hlon JiTII Figure 2 The normalized inensiy of he focused ligh versus he posiion wih differen diameers of micro-spheres Proc. of SPIE Vol P-3

4 By changing differen wavelenghs of he UV ligh while keeping he diameer of he microspheres as 1 µm, we found ha he FWHM of he focused ligh would ge smaller as he wavelenghs ge smaller. In figure 3, i shows he normalized inensiy disribuion of he focused ligh wih differen wavelenghs. The FWHM values of he ligh change from abou 203 nm o 121 nm when he wavelenghs are from 500 nm o 300 nm. So, NSP is a scalable process. Wih a shorer wavelengh, smaller nano-feaures can be generaed. I PosIion (pm) Figure 3 The normalized inensiy of he focused ligh versus he posiion wih differen wavelenghs of UV ligh 3. Experimen The basic process flow is shown in shown in figure 4. Phooresis was spun on he subsrae. Eiher posiive or negaive phooresis can be applied. A monolayer of micro-spheres was formed on op of he phooresis. Silica microspheres were bough from Bangs Lab. The sample wih he monolayer of micro-spheres on he phooresis was exposed wih a convenional Quinel exposure insrumen wih broadband wavelengh cenered abou 400 nm. The micro-spheres were removed by ulrasonicaion mehod in D.I. waer afer exposure. An array of periodic uniform nanoholes was formed afer he developmen of posiive phooresis. The nanoposs made of Ti/Au layer were formed by elecron-beam evaporaion of Ti/Au and lif-off. Similar processes were done wih aluminum layer. For negaive phooresis, an array of periodic uniform nanopillars was formed. The nanoholes perforaed in Ti/Au films were formed by lif-off. The whole process of NSP was quie similar as he sandard phoolihography echniques. Bu since i is a maskless process, i avoids using he expensive masks in he phoolihography mehod. I also uses a longer wavelengh o fabricae he nanopaerns wih he same resoluions compared wih he sandard phoolihography. Proc. of SPIE Vol P-4

5 (e) oqj awl lqs aiansqns 1 earns An a amso aiaiold 151$ r (a) (a) a)iulsqns &IkflSl5 ll f) s)socl-ouu (i) selmimluw lu p1o sills aq a) Figure 4 The schemaic process of NSP for fabricaion of nanoposs in Ti/Au layers and nanoholes perforaed in he films, from (b) o (c), i swiches o wo differen processes wih wo differen kinds of phooresis used () 4. Resuls Figure 5(a) is a Scanning Elecron Microscope (SEM) image showing ha a HCP monolayer of silica microspheres was formed on op of he phooresis. Figure 5(b) is he enlarged view of he micro-spheres on op of he phooresis. These micro-spheres are close packed. :... :... H,, - Figure 5 (a) SEM image of HCP monolayer of silica micro-spheres, (b) Enlarged cross-secion view of micro-spheres on op of he phooresis Using he UV ligh wih λ cenered abou 400 nm o expose he posiive phooresis wih silica micro-spheres, we produced a periodic uniform nanohole array wih he diameer of abou 250 nm shown in figure 6(a). The period of he array is abou 1 µm, which is he same as he diameer of he microspheres used. Figure 6(b) is he cross secion view of he nanohole. I shows abou 1:2 aspec raio and almos verical sidewall. I can be furher used for lif-off and eching process. As you can see, a he boom of he nanohole he diameer is even smaller and i can be poenially used for generaing smaller feaures by eching-ransfer he nanopaerns. To show he unable abiliy of NSP, we also used Proc. of SPIE Vol P-5

6 differen diameers of silica micro-spheres o produce he nanohole arrays. As expeced, we have generaed he nanoholes of almos he same sizes bu wih differen periods. Figure 7(a) and (b) show he nanohole arrays wih he periods of abou 2 and 4 µm and he nanohole sizes are around 300 nm. Using he negaive phooresis in NSP, we produced a periodic uniform array of nanopillars. The nanopillars can also be uned wih differen sizes and sidewalls in he cross secion. Figure 8(a) and (b) show an array of periodic uniform nanopillars of phooresis wih differen sizes and cross secion shapes. Wih conrolled parameers, he size of he nanopillars in figure 8(a) is abou 250 nm and he sidewall is almos verical; he nanopillars in figure 8 (b) have he size of abou 200 nm and i shows a very srong undercu shape. As you can see, he nanopillar fallen down shows a clear undercu cross-secion shape of he pillar. (b) * 500 urn Figure 6 (a) A uniform nanohole array in phooresis wih he period of abou 1 um, (b) he cross secion view of a single nanohole V Figure 7 (a) A uniform nanohole array in phooresis wih he period of abou 2 um, (b) a uniform nanohole array in phooresis wih he period of abou 4 um Proc. of SPIE Vol P-6

7 Figure 8 A uniform nanopillar array in negaive phooresis wih he period of abou 1 um (a) he nanohole size of abou 250 nm wih a verical sidewall, (b) he nanohole size of abou 200 nm wih a srong undercu Using he nanoholes and nanopillars produced by NSP, we successfully produced a uniform array of gold nanoposs and nanoholes. Figure 9(a) shows an array of hexagonally disribued nanoposs of Ti/Au and figure 9(b) shows he array of hexagonally disribued nanoholes perforaed in Ti/Au films. The diameer of nanoposs and nanoholes in figure 9(a) and (b) are abou 300 nm and 180 nm separaely, and he periods are boh abou 1 um. All hese parameers can be uned since NSP is a unable process. Similar resuls can also be obained wih he use of aluminum, as shown in figure 10 (a). The diameer of he nanoholes perforaed in he Al film is abou 250 nm. The edge of he nanoholes is a lile rough because aluminum deposied by e-beam evaporaion is no as uniform as oher meals. Besides lif-off process, using Reacive Ion Eching (RIE) and he meal layer wih he nanoholes as mask, we also produced a similar nanohole array in he dielecric of silicon dioxide (SiO 2 ) film in figure 10(b). I is a iled cross secion image. There are clearly wo layers of maerials on op of he subsrae, which are SiO 2 and Al hin films. The size of he nanohole is almos he same as he one in he meal films. - - ;r I, r c a a a e rs 4¼ as r r ee r Figure 9 (a) An array of hexagonally disribued uniform nanoposs of Ti/Au, (b) an array of hexagonally disribued uniform holes perforaed in Ti/Au films Proc. of SPIE Vol P-7

8 Figure 10 (a) An array of hexagonally disribued uniform nanoholes perforaed in aluminum layer, (b) using RIE o ransfer he nanoholes paerns in he SiO 2 film below 5. Conclusions We have demonsraed a maskless phoolihography mehod o generae subwavelengh srucures. The mehod uilizes he self-assembled monolayer of silica micro-spheres as super-lenses o focus UV ligh ino phooresis and generaed highly uniform arrays of nanoholes or nanopillars. We also presened he similar nanosrucures in differen meals and dielecrics using he mehod wih oher sandard processing mehods. The echnique is a conrollable process. We believe ha i can be used for many areas, such as fabricaion of elecronic and opoelecronic devices. Acknowledgemens This work is parially suppored by Naional Science Foundaion gran # ECCS and Defense Advanced Research Projec Agency gran # N References [1] Lezec, H. J., Degiron, A., Devaux, E., Linke, R. A., Marin-Moreno, A., Garcia-Vidal F. J. and Ebbesen T. W., Science, 297, (2002). [2] Weekes, S. M., Ogrin, F. Y. and Murray, W. A., Langmuir, 20, (2004) [3] Brolo, A. G., Arcander, E., Gordon, R., Leahem, B. and Kavanagh, K. L., Nano Le., 4, (2004). [4] Siwy, Z. and Fulinski, A., Am. J. Phys., 72, (2004) [5] Rease, R. F. W., J. Vac. Sci. Technol. B, 10, 278 (1992) [6] Haynes, C. L., Kall, M. e al, J. Phys. Chem. B, 107, 7337 (2003) [7] Tong, H. D., and Elwennspoek. M., e al, Nano Le., 4, 283 (2004) [8] Park, M., Harrison, C., Chaikin, P. M., Regiser, R. A. and Adamson D. H., Science, 276, (1997) [9] Huleen, J. C. and Van Duyne, R. P., J. Vac. Sci. Technol. A, 13, 1553 (1995) [10] Chen, Z., Taflove, A. and Backman V., Op. Express, 12, 1214 (2004) [11] Wu, W., Kanelson, A., Memis, G. and Mohseni, H., Nanoechnology, 18, (2007) [12] Kong, S., Sahakian, A. V., Heifez, A., Taflove, A. and Backman V., APL, 92, (2008) [13] Denkov, N. D., Nagayama, K. e al, Langmuir, 8, 3183 (1992) [14] Fishes, U. C. and Zingsheim, P., J. Vac. Sci. Technol. 19, 881 (1981) [15] Kim, M. H., Im, S. H. and Park, O. O., Adv. Func. Maer., 15, (2005) Proc. of SPIE Vol P-8