17726 DOI: 10.1021/la1028433 Langmuir 2010, 26(22), 17726–17732Published on Web 10/01/2010

pubs.acs.org/Langmuir

© 2010 American Chemical Society

Photo-Pens: A Simple and Versatile Tool for Maskless Photolithography

Chuanhong Zhou, Pradeep Ramiah Rajasekaran, Justin Wolff, Xuelian Li, and Punit Kohli*

Department of Chemistry & Biochemistry, Southern Illinois University, Carbondale, Illinois 62901

Received July 16, 2010

We demonstrate conical pores etched in tracked glass chips for fabricating patterns at the micrometer scale. Highlyfluorescent patterns based on photopolymerization of diacetylene films were formed by irradiating UV light throughconical pores called “photo-pens”. The properties of photopens were investigated through experiments, finite-difference-time-domain (FDTD) simulations and numerical calculations based on Fresnel equations. We show that the patterndimensions are easily controlled by adjusting the exposure time. Thus, patterns with a range of dimensions can be fabri-cated without any need of changes in the pore diameter. Parallel patterning was also demonstrated by simultaneouslyexposing the films to photons through multiple pores in the chip. Our method provides an inexpensive, versatile, andefficient way for patterning without the use of sophisticated masks.

Introduction

Photolithography is amicrofabrication technologywidely usedto selectively remove parts of a thin film or a bulk substrate.1-4 Ituses light to transfer a geometric shape present in a mask onto anoptically sensitive resist on the substrate. The pattern on the photo-resist is then generated on the substrate with a series of chemical orphysical development processes.3 To date, photolithography is themost commonly employed form of lithography.4 Therefore, it hasbeen extensively used in the fabrication of electronic integratedcircuits (IC),1,2 optical circuits,5 microelectromechanical systems(MEMS),6 microfluidic devices,7 and biochips.8 Compared withother lithographic methods such as dip-pen lithography,9 nano-pipette-based10 and nanofountain lithography,11 and electrifiedjet printing,12 photolithography is a massively parallel, cost-effective, and high output process. Its resolution is, however,limited due to diffraction limit. Unless special and expensive

methods are employed, photolithography renders a poorerresolution than probe-based lithography techniques. This is be-cause diffraction imposes physical limitations on the photonsirradiating the photoresist through themasks where the resolutionis limited to∼λ/2 (λ is thewavelength of the irradiation light).1 Animportant consideration in photolithography is the cost of thephotomask fabrication with features <2 μm which is usually acostly and time-consuming process, and often it relies on expen-sive instrumentation.7,8 Furthermore, the features in the maskscan not be altered once they are engraved on the mask, makingthis process less versatile and efficient. Therefore, there is a greatdemand for inexpensive and fast “maskless lithography” wherepatterning can be performed without the use of masks.

Here, we investigate pattern fabrication using conical poresfabricated in glass. Our finite-difference-time-domain (FDTD)simulation results showed that the light transmission was muchhigher for a conical pore than for a cylindrical pore whose porediameter was equal to the smaller pore diameter of the conicalpore. We fabricated the conical pores in tracked glass by using amature chemical etching technique13which is routinely performedunder ordinary laboratory conditions and does not require cleanroom facilities. The instrumentation involved in our fabricationmethod is inexpensive, and all elements of the fabrication processcan be performed in a wet laboratory. In principle, any given pat-tern in photoactive films on a variety of surfaces can be preparedusing this method with pattern resolution of ∼400 nm.

For the proof-of-concept experiments, we deposited electro-statically negatively charged self-assembled diacetylene liposomesas our photoresist on positively charged glass substrates. Follow-ing exposure to UV photons at 254 nm through pores and wash-ing and heating of the resist, we obtained stable and highly fluo-rescent patternswhichwere easily observable under amicroscope.A low power UV source of intensity 15 μW/cm2 was sufficient toperform the desired lithographywith an exposure time of<1min.

*To whom correspondence should be addressed. E-mail: pkohli@chem.siu.edu.(1) Madou, M. J. Fundamentals of microfabrication: the science of miniaturiza-

tion, 2nd ed.; CRC Press: Boca Raton, 2002.(2) Sheats, J. R.; Smith, B. W. Microlithography: science and technology; CRC

Press: Boca Raton, 1998.(3) Van Zant, P. Microchip Fabrication, 5th ed.; McGraw-Hill Professional:

New York, 2004.(4) Razeghi, M. Fundamentals of solid state engineering; Springer, 2002.(5) Li, Y. P.; Henry, C. H. IEE Proc. Optoelectron. 1996, 143, 263.(6) Miyajima, H.; Mehregany, M. J. Microelectromech. Syst. 1995, 4, 220.(7) Anderson, J. R.; Chiu, D. T.; Jackman, R. J.; Cherniavskaya, O.;

McDonald, J. C.; Wu, H.; Whitesides, S. H.; Whitesides, G. M. Anal. Chem.2000, 72, 3158–3164.(8) Blawas, A. S.; Reichert, W. M. Biomaterials 1998, 19, 595.(9) (a) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283,

661–663. (b) Li, Y.; Maynor, B. W.; Liu, J. J. Am. Chem. Soc. 2001, 123, 2105–2106.(c) Lee, K. B.; Park, S. J.; Mirkin, C. A.; Smith, J. C.; Mrksich, M. Science 2002, 295,1702–1705.(10) (a) Rodolfa, K. T.; Bruckbauer, A.; Zhou, D.; Korchev, Y. E.; Klenerman,

D. Angew. Chem., Int. Ed. 2005, 44, 6854–6859. (b) Bruckbauer, A.; Zhou, D.; Ying,L.; Abell, C.; Klenerman, D.Nano Lett. 2004, 4, 1859–1862. (c) Bruckbauer, A.; Zhou,D.; Ying, L.; Korchev, Y. E.; Abell, C.; Klenerman, D. J. Am. Chem. Soc. 2003, 125,9834–9839.(11) (a) Loh, O. Y.; Ho, A.M.; Rim, J. E.; Kohli, P.; Patankar, N. A.; Espinosa,

H. D. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 16443. (b) Kim, K. H.; Moldovan, N.;Espinosa, H. D. Small 2005, 1(6), 632–635. (c) Wu, B.; Ho, A.; Moldova, N.; Espinosa,H. D. Langmuir 2007, 23, 9120–9123.(12) (a) Park, J.-U.;Hardy,M.;Kang, S. J.; Barton,K.; Adair,K.;Mukhopadhyay,

D. K.; Lee, C. Y.; Strano, M. S.; Georgiadis, J. G.; Ferreira, P. M.; Rogers, J. A.Nat. Mater. 2007, 6, 782–789. (b) Park, J.-U.; Lee, S.; Unarunotai, S.; Sun, Y.;Dunham, S.; Song, T.; Ferreira, P. M.; Alleyene, A. G.; Paik, U.; Rogers, J. A. NanoLett. 2010, 10, 584–591.

(13) (a) Siwy, Z. S.; Fulinski, A. Phys. Rev. Lett. 2002, 89, 198103/1–198103/4.(b) Siwy, Z. Adv. Funct. Mater. 2006, 16, 735–746. (c) Howorka, S.; Siwy, Z. Chem.Soc. Rev. 2009, 38, 2360–2384. (d) Mukaibo, H.; Horne, L. P.; Park, D.; Martin, C. R.Small 2009, 5, 2474–2479. (e) Siwy, Z.; Trofin, L.; Kohli, P.; Baker, L. A.; Trautmann,C.; Martin, C. R. J. Am. Chem. Soc. 2005, 127, 5000–5001. (f ) Sexton, L. T.; Horne,L. P.; Sherrill, S. S.; Bishop, G.W.; Baker, L. A.; Martin, C. R. J. Am. Chem. Soc. 2007,129, 13144–13152. (g) Heins, E. S.; Siwy, Z. S.; Baker, L. A.; Martin, C. R.Nano Lett.2005, 5, 1924–1829. (h) Fu, Y.; Tokuhisa, H.; Baker, L. A. Chem Commun. 2009,4877–4879.

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We also investigated experimentally and through simulations andnumerical methods the dependence of the pattern size on the porediameter and the exposure time. We found that the patterndimensions can be easily modulated through the use of exposure

time without any need to change pore size. Computer-controlledparallel patterning was also accomplished by exposing the resistthrough multiple conical pores fabricated in a chip without anydown time in the lithographic process. Since controlling the exposure

Figure 1. (A) SEM images of a cross section (left) and top view (right) of typical pores synthesized in a glass chip. (B) AFM image of thefluorescence diacetylene (DA) liposome monomers on a silanized glass slide. (C) Scheme showing 1,4 addition polymerization of self-assembled diacetylenic acid in liposomes. Blue square represents color of the film after polymerizationwhose quantumyield (Qy) is extremelysmall, whereas red color denotes “red”-PDA form whoseQy increased many orders of magnitude and was easily detected with fluorescencemicrocopy. The effective conjugation length is believed to be lower in the red-PDA form than that in the blue-PDA form. (D) Schematicdiagramof theUVphotolithographic process using conical pores. (E) Schematic of the exposure apparatusweused,whereA,B, andCare theX,Y, andZ piezoelectric stages, respectively;D is the holdermounted on theZ-stage, supporting theUV lamp systemand the glass chipmasksystem;E andFare theUV lampand the glass lampshade, respectively;GandHare the cushion sponge and glassmask; I is the substrate slideon which the patterning is recorded.

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time is relatively straightforward and inexpensive to accomplish, ourmethodwould provide an efficient, inexpensive, and versatile wayof photopatterning andwill find potential applications inmaterialsand life sciences.

Experimental Section

Fabrication of Pores in the Glass Chips. We adopted achemical etching method to prepare the pores in the glass chips.The glass chips were tracked with an ion flux of 102-105 ions/cm2

at GSI (Darmstadt, Germany). The tracked glass chip wasadhered to amicroscope glass slide with a poly(dimethylsiloxane)(PDMS) film (∼0.2-0.5 mm in thickness). PDMS also acted as astress absorber cushion during mechanical handling and relatedprocesses such as etching and patterning. Before adhering the twoglasses, a hole (5 ( 0.5 mm in diameter) was drilled through themicroscope glass slide. The uncured PDMS was applied to thesurface of the altered microscope glass slide. During the curingprocess, the tracked glass was then adhered to the microscopeslide which left naked an area through which the chemical etchingwas performed.

Briefly, the fabrication of conical pores is as follows. We usedanisotropic etching to create conical pores in the glass chips. Ouretching apparatus consists of two half-U tubes in between thetracked glass chips. The conical pores were formed due toanisotropic etching of the chip of which one side was exposed toHF (24.5%) solution in NaCl (0.5 M, etching solution) while theother side of the glass chip was in contact with a saturatedCa(NO3)2 (blocking solution). NaCl in HF solution was used toenhance the ionic conductivity of HF solution. The etchingprocess was monitored by measuring ionic current; a spike inthe ionic current across the glass chip indicated through-poreformation in the glass. At this time, the etching was stopped. Theetching time for our chips was∼700-900 s. This corresponded toan etching rate of∼75-100 nm/s along the tracks, which is muchfaster than etching along the lateral direction.14 The characteriza-tion of the pores in glass chips was carried out by using FEIQuanta 600 FEG (equipped with a Bruker EDX detector) andHitachi 570 scanning electron microscopes (SEMs) (Figure 1A)and an optical microscope (Lieca DMIRB) equipped with aQImage (Cooled Mono 12-bit) CCD camera. The topology ofliposomes on surfaces was measured using an atomic forcemicroscope (AFM) (Topometrix TMX1010).

Synthesis and Characterization of Liposomes.15,16 Theprocedure to make the liposome solution is described as thefollowing: 10,12-Pentacosadiynoic acid (from GFS Chemicals)was dissolved in chloroform. The large aggregates from thesolution were filtered through nylon membranes with 450 nmpores. The solvent was then evaporated to form a thin dry film onthe surface of a round-bottom flask. Then 50 mL of deionizedwater was added into the flask coated with the dry film tomake amonomer concentration of 1 mM. The solution was sonicated(Branson 2510) until the film was suspended. The suspension wasfurther sonicated using a tip probe sonicator (model CV33, SonicsVibracell) for 30 min to reduce the size of the liposomes (50-400nm).15,16 To promote the self-assembling of monomers, the solu-tion was kept at 4 �C overnight.

Deposition of Unpolymerized Self-Assembled Diacety-

lene Liposomes on Aminosilane Functionalized Glass. Theresist for patterning was composed of self-assembled diacetyleneliposomes deposited on 3-aminopropyltriethoxysilane (APTS)treated glass slides. Figure 1C shows the 1,4 addition poly-merization reaction of self-assembled diacetylene molecules. Thephotopolymerization of diacetylene is well-known, and manyexcellent reviews and books are available on this topic.17,18 Prior

to APTS reaction, the glass slides were washed thoroughly withsoap and rinsedwith ethanol and deionized (DI) water. Theywerethen soaked in fresh piranha solution for 30 min followed bythorough rinsing in copious amounts of ethanol and DI water.Caution! Piranha solution is a strong oxidizer, and all the safetyprecautions must be taken when working with this. The functiona-lization of glass with aminosilane was performed by immersingthe glass slides in 10% v/v APTS in 95% ethanol containing solu-tion for 1 h. The slides were rinsed sequentially with ethanol andDI water and were cured at 90 �C for 5 h. The amino-functiona-lized slides were kept at 4 �C until further use.

The negatively charged liposomes were electrostatically ad-hered to positively charged glass substrates by immersing thesilanized glass slides in the self-assembled unpolymerized diace-tylene liposome solution at 25 �C for 4 h resulting in a single layerof liposomes on the surface. Figure 1B shows an AFM image ofdiacetylene liposome deposited from a diluted solution on a glassslide. The self-assembled liposomes had 3-D structureswith dimen-sions of 100-300 nm which matches with our previous studies.15

Lithography Using Conical Pores in Glass Chips.Weusedahome-assembledxyzpiezoelectric stage to accurately control theexposure of the pattern positions. A schematic of the lithographicapparatus is shown in Figure 1E. A, B, and C are the X, Y, andZ piezoelectric stages respectively. X, Y, and Z stages can bemoved independent of one another with a step size of 50 nm. D isan “L” shaped holdermounted onCwhich supports theUV lampsystem (E and F) and the photomask system (G and H). E andF are the UV lamp and UV-light blocker, respectively. The UV-light blocker protects the operator from harmful UV exposure. Gis a stress absorbing cushion made of rubber or sponge for thepurpose of mechanical passive compliance and for protecting theglass chip H from excessive stress. I is a microscope slide forproviding mechanical support for the pore-containing chip. Weadopted the contact mode for themonomer coatingUV exposureportion of the experiment. The exposure position was accuratelymanipulated by the piezoelectric stages driven by controller soft-ware. The liposome coated glass slide was placed on the X andY piezoelectric stages. The stages were then moved in a desiredpattern via a computer program. TheUV exposure along the pro-grammed path produced the desired pattern after washing off theunpolymerized liposomes from the surface.

As-photopolymerized diacetylene (PDA) films were in “blue:form (Figure 1C). Because the quantum yield (Qy) of blue-PDA isextremely small (<10-4),15-18 the emission of patterns were notobservable. Heating the patterns to 70-90 �C or exposure toethanol (in vapor or liquid form) for a few minutes (1-5 min)converted them into “red” form.18 Qy of red-PDA is known toincrease many orders of magnitude15-18 and was easily observedas an intense red emission when excited with a 41004 Texas Redfilter using a CCD camera (exciting and emitting band widths ofthe filter used were 527-567 nm and 605-682 nm, respectively).

Results and Discussion

Using conical pores, we investigate and demonstrate an inex-pensive, fast, and easy to implement lithographic technique. Itsversatility is further demonstrated by the fact that we were able tofabricate patterns with feature sizes ranging from 1.5 to 50 μmunder wet laboratory conditionswithout any clean room facilities.Conical Pore Characterization. Figure 1A shows SEM

images of a cross section (left) and top view (right) of typicalpores etched in a glass chip. The pore has a conical structure with

(14) Rajasekaran, P. R.; Wolff, J.; Zhou, C.; Kinsel, M.; Trautmann, C.;Aouadi, S.; Kohli, P. J. Mater. Chem. 2009, 19, 8142–8149.(15) Li, X.; McCarroll, M.; Kohli, P. Langmuir 2006, 22, 8615–8617.(16) Li, X.; Matthews, S.; Kohli, P. J. Phys. Chem. B 2008, 112, 13263.

(17) Olmsted, J., III; Strand, M. J. Phys. Chem. 1983, 87, 4790.(18) (a) Okada, S.; Peng, S.; Spevak, W.; Charych, D. Acc. Chem. Res. 1998, 31,

229–239. (b) Charych, D. H.; Nagy, J. O.; Spevak, W.; Bednarski, M. D. Science 1993,261, 585–588. (c) Polydiacetylenes: Synthesis, Structure and Electronic Properties;Bloor, D., Chance, R. R., Eds.; NATO Science Series E; Brussels, 1985. (d) Wegner, G.Z. Naturforsch., B: Chem. Sci. 1969, 24, 824. (e)Wegner, G.Makromol. Chem. 1972,154, 35. (f ) Tieke, B.; Wegner, G.; Naegele, D.; Ringsdorf, H. Angew. Chem., Int. Ed.Engl. 1976, 15, 764.

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smaller (ds) and larger pore (dl) diameters of ∼10 and 63 μm,respectively, and the length of the pores is ∼70 μm. The porediameter and number of pores in a given chip were easily con-trolled by choosing the appropriate etching conditions such asconcentration, temperature of etching solution, etching time, andpore-tracking density for a desired result. In our experiments,we have prepared chips with smaller (ds) pore diameters rangingfrom1 to 20μm, and larger (dl) pore diameters from15 to 100μm.14

A typical optical micrograph of a conical pore fabricated in glasschip is shown in Figure 2A. The dark rings with the bright centerindicate the positions of pores, the dark rings corresponding to theconical sidewalls of the pores. They lookeddarkbecause lightwasrefracted away from these regions, and the transmission of thelight to the detector was drastically reduced. The bright centers,on the other hand, corresponded to the small openings of poresthrough which the transmitted light reached the detector.14 Theabsorption spectrumof the glass in the 200-800 nm region showsthat the photons with wavelength <300 nm were almost com-pletely absorbed by the glass (Figure 2B). For a wavelength of254 nmwhere liposome polymerization was performed, the trans-mission efficiency through glass chip was <1%. This corre-sponded to a transmission intensity of <15 nW/cm2 that passedthrough the glass matrix and was insufficient for photopolymer-ization of self-assembled diacetylene monomers for shorter ex-posure times of<5 s (see below). Our glass chips are, thus, an ex-cellent choice for pattern fabrication on photoresists that utilizeUV photons for development.FDTD Simulations. We used FDTD simulations to gain a

better understanding of light propagation through the pores.FDTD simulations were based on solving Maxwell’s electrody-namic equations (eqs 1 and 2).19 A home-written code was usedfor all the simulations.

=� E ¼ -DB=Dt ð1Þ

=�H ¼ ε DE=Dt ð2Þwhere =� = (i∂/∂x þ j∂/∂y þ k∂/∂z)� is curl operator, i, j, andk are orientation unit vectors, E and H are electric and magneticfield of the light, respectively, and B = μH. μ and ε are the

permeability and permittivity, respectively, of the media (air inour case). A two-dimensional FDTD method was adopted tosimulate the propagation of light. We assume that transversemagnetic (TM) polarized light (λ=254 nm) is normally incidenton the larger opening side of the glass chip, and a CCD detectorwas located at the smaller side of the chip. The refractive indexand absorption index coefficient of the glass chip were 1.54 and65.8 mm-1, respectively. The transmission through the conicalpore can be classified into three groups (Figure 3): the first portionpropagated directly out of the smaller pore of the cone (labeled asA0); the secondpart impingedon the sideof pore andwas reflectedand then transmitted out of the pore (labeled as B0); and the thirdpart impinged on the side of pore, was refracted into the glass chipafter a significant attenuation, and then penetrated into free space(denoted as C0). The transmission through the cylindrical poreis highly nonuniform because few light modes are coupled out.Thesemodes interfere with one another to generate a nonuniformintensity distribution.

The ratio of the integrated intensity for conical and cylindricalpores was obtained with FDTD simulations, 1.47. Higher trans-mission output for conical pores can be attributed to a largerphoton coupling efficiency for conical structures that significantlyenhances the transmission output. Therefore, the use of conicalpores over cylindrical pores is preferred for photolithographicapplications. Furthermore, the measured photon transmissionout of the smaller pore diameter of conical pores (ds∼ 12 μm anddl ∼ 42 μm) was found to be ∼20% higher than that of incidentlight intensity. The reason why the light transmission for conicalpores was higher than the incident intensity can be explained bythe intensityoverlapat the smaller openingof thepore (transmissionA0 and reflection B0 overlap at the small opening of the conicalpore in Figure 3A).14

Fluorescent Patterning. A bright fluorescent 6� 5 dot arrayof PDA films fabricated on a glass substrate is shown in Figure 4A.For each dot, UV exposure time through a conical pore (dl =21 μm and ds = 2.8 μm) was 15 s. An SEM image of the smallopening of the pore used for patterning is shown in the inset ofFigure 4A. The diameter of the dot patterns was 3.85 ( 0.4 μm.The pattern dimensions and emission intensities between differentspots were ∼8.4% and ∼9%. These variations are attributedprimarily to small differences in the liposomal coating thicknessand photon exposure intensity over the pattern areas. Althoughthe pore was in contact with the PDA coated substrate, thepatternswere slightly larger than the pore size. This is discussed in

Figure 2. (A) Optical micrograph of conical pores in the glass. Bright dots at the centers of the circles represent small pores, whereas darkconcentric circles represent walls of the conical pores. (B) Typical UV-vis absorption spectrum of an unetched glass chip.

(19) Taflove, A.; Hagness, S. C. Computational electrodynamics: The finite-difference time-domain method; Artech House: Norwood, 2005.

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detail in the following paragraphs. To evaluate the patternquality, we adopted the Michelson contrast which is defined as(Imax - Imin)/(Imax þ Imin), where Imax and Imin represent themaximum and minimum emission intensity of the patterning andthe background, respectively.20 For Figure 4A, a contrast valuebetween dots and background was ∼0.81 (maximum contrastvalue is 1), suggesting an insignificant background emission afterwashing off the unpolymerized liposomes at nonirradiated areas.

After polymerization of the liposomes, the patterns werechemically stable for a long duration (>7 days). We immersedthe patterns into methanol for 2 h but did not find any obviouschanges in the emission or dimensions. We also investigated thecontrast values for the patterns after 1 month and found insig-nificant changes in the contrast values (<5%). These studiesclearly suggested that the adhesion of the polymerized patterns onamine-functionalized glass was strong which contributed to theirresistance to washing with organic solvents whereas the nonpo-lymerized liposomal coating adhered poorly onpositively chargedglass substrates. Although it is not fully clear why the adhesion ofpolymerized liposomes was much stronger than that of unpoly-merized liposomes, we believe that this could be attributed to acooperative effect of multidentate electrostatic interactions be-tweenmultiple negatively charged groups present on polymerized

liposomes with abundant positively charged amine groups on thesubstrate surface. On the other hand, for unpolymerized surfacecases, the interaction between positively charged amine andnegatively charged diacetylene carboxylic acid is probably muchweaker, and the self-assembly was easily destroyed by washing inethanol or water solutions.Exposure Time-Dependent Patterning.Agiven patternwas

also easily programmed by controlling the piezoelectric stagemovements using a commercial software. The logos of the “NSF”and “NIH” were fabricated by controlling the stage movementswhile simultaneously irradiating the liposomal films with 254 nmphotons (Figure 4B). These logos were constructed by exposingthe film with different exposure times. For example, letters “N”,“S and I”, and “F andH”were exposed for 10, 20, and 30 s, respec-tively, through a conical pore with ds = 11.2 μm and dl =47.3 μm.Surprisingly, the diameter of “N”, “S and I”, and “F and H”patterns were (13.5 ( 0.4) μm, (17.5 ( 0.6) μm, and (19.5 ( 0.5)μm, respectively. The resolution of our instrument is ∼0.33 μm,and the dimensions of patterns were significantly larger than theinstrumental resolution. Thus, the measured dimensions of pat-terns represented the true pattern dimensions. These experimentsclearly indicated that pattern sizewas dependent on exposure timeand that it can be easily modulated without changing the poresize. This will be extremely useful in photolithography and cansignificantly reduce the cost and down time for photopatterning.

To further investigate the exposure time-pattern size depen-dence, we synthesized a dot array with different exposure times (t)(inset in Figure 5A). The dots in the same column had the sameexposure duration, and each subsequent column had an exposuretime incremented by 5 s. Initially, the intensity and the dimensionsof the patterns increasedwith exposure time until a saturationwasreached (Figure 5A and 5B, respectively). The x-axis in Figure 5Bis natural logarithmic.

Now the question is why does pattern size increase withexposure time?More insight about this was obtained by perform-ing FDTD simulations that indicated two distinct photon trans-mission intensity levels for a conical pore (with ds and dl of 11.8and 47 μm, respectively, Figure 5C): (1) the central high intensityregion with width din of ∼9 μm and (2) the exponential decayedside lobe with full-width-at-half-maxima (fwhm) of dmax of∼25 μm(Figure 5C). The appearance of the fast decaying portion of thelight transmission out of the smaller pore is due to leakage of thelight out of the small pore. Because of the conical shape of thepore, the glass matrix is very thin near the smaller pore diameterthat resulted in incomplete absorption of light and so a smallfraction of light leaked out of it. The dimensions of patternsformed at short (∼5 s) and very long exposure (180 s) times were

Figure 4. (A) A 6 � 5 dot array PDA fluorescence patternfabricated using a pore (ds=21 μm and dl=2.8 μm) in a trackedglass chip.Eachdothas the sameexposure timeof20s.TheMichelsoncontrast for the patterns was 0.81. The inset in Figure 4A shows anSEM image of the small diameter of a pore used for lithography,ds ∼3.85 μm. (B) Fluorescent patterns of “NSF” and “NIH”logos. The exposure times for “N”; “S and I”; and “F and H” are10, 20, and 30 s, respectively. The corresponding averaged con-trast value was 0.85.

Figure 3. FTDT simulations of the light passing through a conical and a cylindrical pore. The intensity distributions of the UV light passingthrough (A) a conical pore and (B) a cylinder pore in a glass chip. The integrated photon intensity for conical pores (ds = 12 μm and dl =47 μm) was ∼47% larger than that of cylindrical pores (diameter of the pore =12 μm).

(20) Valberg, A. Light vision color; John Wiley and Sons: Chichester, 2005.

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in agreement with the values of din and dmax, respectively. Forexample, the feature sizes of the fluorescent patterns for short andlong exposures were (9.2 ( 1) μm and (25 ( 2) μm, respectively,whichmatchedwell with our simulation data.At shorter exposuretimes, the large intensity of the central transmission region (din)was large enough to photopolymerize self-assembled diacetylenemolecules. This produced pattern dimensions corresponding todin (Figure 5B). On the other hand, the number of excitationphotons in the exponent decayed curves is ∼40-60% less thanthat of the central intense region. Therefore, a longer time wasneeded for photopolymerized patterns to occur with leaked pho-tons having dimensions corresponding to dmax. Our simulationsalso predicted that the transmission intensity in the fast decayedtransmission profile is an exponential function of distance fromthe center (Figure 5C). This also explains an exponential increase(which is a mirror image of the exponential decayed profile inthe Figure 5C) in the pattern dimensions with time (Figure 5B).The limiting pattern dimensions (dmax) will also depend upon thecone angle of the pore. For example, the larger cone angles willprovide larger limiting pattern dimensions (dmax). Thus, the shapeand size of the conical pores provides unique opportunities forphotolithography where a range of patterns can be synthesizedwithout changing pore size in the chips.

We have also developed a quantitative model that explainsthe increase in the dimension size of the patterns (d) withexposure time. This model is based on the ray diagram shownin Figure 5D. Since the photopolymerization rate of the centralregion is much faster than that corresponding to the side-lobedarea, we assume the pattern size (d) consists of a constantcentral region specified with radius r1 and time-dependent sideregions specified with r2, that is, d = 2(r1 þ r2). In the sideregions, the emission intensity Ie can be written as (see the

Supporting Information for the derivation of eq 3):

IeðrÞ ¼ Ies 1- e- aIpðrÞt� �

ð3Þ

where Ies is the saturated emission intensity for exposure time(t) >120 s, a is a polymerization coefficient that depends uponthe interaction of photons with diacetylene monomers, andIp(r) is the exposure intensity as a function of the position (r) atthe smaller pore diameter to which the photoresist was exposedto. We defined r2 as the distance whose intensity decayed tohalf of the saturated intensity (Figure 5D), that is, Ie(r2) =0.5Ies, and solving eq 3 provided

Ipðr2Þ ¼ lnð2Þat

ð4Þ

The exposure intensity (Ip(r)) at the smaller porewas calculatedas follows: The intensity of the incident lightwas Iinc (15 μW/cm2),and the refractive index of glass was n (=1.54) (Figure 5D). Thelight is refracted by the side wall of the pore and penetrates intothe glass (2). After a significant absorption across the glass, thelight impinges on the bottom of the glass and transmits into freespace (3). The intensity transmittivity from (1) to (2) and from (2)and (3) are T12 and T23, respectively, and the absorption coeffi-cient is ε. The refractive intensity Ip can be expressed by21

IpðrÞ ¼ T23T12Iinc eð- εLÞ ð5Þ

with

L ¼ sinðθ1Þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1-

sinðθ1Þn

� �2s r2 ð6Þ

and Tij (i=1,2; j=2,3) satisfies Fresnel’s transmission formula:

Tij ¼ sinð2θiÞ sinð2θjÞsin2ðθi þ θjÞ cos2ðθi - θjÞ

ð7Þ

whereθi andθj are incident and refractive angles, respectively, andsatisfy Snell’s law21

ni sinðθiÞ ¼ nj sinðθjÞ ð8Þni and nj are the refractive indices of the ith and the jth spaces,respectively. The refractive indices of air and our chip were 1 and1.54, respectively; the measured absorption coefficient (ε) of theglass at 254 nm was 65.8 mm-1 and thickness w = 70 μm. Thepore etched in the glass chip has a large opening dl = 47 μmand asmall opening ds = 12 μm, so the incident angle θ1 = 76�. Bycombining eqs 5-8, we obtained Ip(r):

Ipðr2Þ ¼ 0:86Iinc eð- 0:3569r2Þ ð9Þ

where Iinc is the intensity of the incident light at the larger pore.Combining eqs 4 and 9 and solving for r2, we obtained

r2 ¼ 2:78 lnð1:24aIinctÞ ð10Þ

Figure 5. (A) Fluorescence intensity versus exposure time for anarray of dots fabricated using a pore with ds and dl of 11.8 and47 μm, respectively. The red line is a fit for the experimental dotarray which is shown in the inset of the figure. The exposure timesfor the leftmost and rightmost columns are 5-45 s with a 5 s incre-ment for successive columns. (B) Feature size of the patterns versuslog (exposure time) for anarrayof dots fabricatedusing a porewithds and dl of 11.8 and 47 μm, respectively. The red line is a fit for theexperimental data. (C) The calculated FDTD transmission inten-sity profile at the small opening of a conical pore (ds=11.8μmanddl = 47 μm), where din and dmax depict the sizes of the patterns thatcan be fabricated at short and long exposure times, respectively,using this pore. (D) Light ray tracing diagram showing the leakageof the light that led to increased pattern size with longer exposuretime.

(21) Born, M.; Wolf, E.; Bhatia, A. B. Principles of optics: Electromagnetictheory of propagation, interference and diffraction of light; Cambridge UniversityPress: Cambridge, 1999.

17732 DOI: 10.1021/la1028433 Langmuir 2010, 26(22), 17726–17732

Article Zhou et al.

Using aIinc= 0.09 s-1 (obtained from fitting of the experimentaldata in Figure 5A), the diameter of the patterns (d) is

d ¼ 2r1 þ 2r2 ∼ 5:6 lnðtÞ ð11ÞThe experimental data in Figure 5B fit well with the theoreticalmodel (eq 11). Excellent fit between experimental and theoreticaldatawill be amajor advantage of thismethod over existingphoto-lithographic techniquesbecause in the present case thedimensionsof the patterns can be empirically predicted before hand usinga simple equation. This will save time and will reduce cost ofpatterning.

We have also fabricated more complex patterns at higherthroughput rates byusingmultiple conical pores in the glass chips.Figure 6A shows the programmed parallel patterns (a 2 � 6 dotarray and the “SIU” logos) using a chip with two pores, 2.3 and4.5 μm, by controlling the movements of the x-y stages. Theexposure time for eachdotwas 10 s.A typical line patternwas alsofabricated by scanning a pore over a liposomal film (Figure 6B).Thus, with this method, any discrete, continuous, and parallelpatterns with the multiple pores can be fabricated with almostzero down time. Although the multiple pores in the chips greatlyimproved the throughput rates, each pattern still however has tobe generated by writing in a serial fashion.Emission Intensity-Exposure Time Dependence. Surpris-

ing, we also observed that the pattern emission intensity wasdependent upon exposure time. To further investigate the impactsof exposure time on the emission intensity of the patterns, wesynthesized a dot array pattern with different exposure times(inset in Figure 5A). The dots in the same column had the sameexposure time, and there was a 5 s increment in the exposure timefor dots in the subsequent column. Initially, the intensity of patternsincreased quickly with the exposure time, but it reached saturationat longer exposure times (Figure 3A). The emission intensity-exposure time curve resembles a Langmuir isotherm plot.

In general, for a radical photopolymerization process, thedegree of polymerization depends upon many factors includingthe presence of initiators, quenchers and photon intensity ofpolymerizingbeam. In the present case, the presenceof the oxygenmolecules in the films is believed to play a crucial role in thepattern emission intensity dependence on the exposure time.Oxygen is a known powerful free-radical quencher22,23 and canreact with the growing chains, terminating them through coupling

or disproportion reactions.23 Oxygen molecules are hydrophobicin nature and are known to present in hydrophobicmolecules andbilayers. Our bilayer liposomes contained both hydrophilic andhydrophobic parts. The hydrophobic part of the bilayer (theemissive functionality, the backbone of PDA is also hydrophobic)contained a higher oxygen concentration than that in/near thehydrophilic portion of the liposomes.24 The degree of polymer-ization was lower for shorter exposure time due to the presence ofoxygen which can readily quench growing PDA (both biradicaland bicarbene) chains. A lower polymerization degree of PDAresulted in a lower emission intensity. However, with longerirradiation time an higher PDA content in the patterns wasformed that provided a higher emission intensity (Figure 5A).Resolution of the Patterning. We now comment on achiev-

ing a lower resolution using this technique. In principle, ourpatterning method can be used for making nanoscale patternsthrough the use of nanoscale conical pores fabricated in the chips.The transmission intensity through nanoscale pores however willbe significantly attenuated. This can be compensated either bylonger exposure time or by use of very intense light sources.Another option is coupling of the photons with surface plasmonswhere the photon transmission output from nanoscale pores canbe significantly increased.25 Photon-surface plasmon couplingalong with exposure time and intensity can be used for patterndevelopment at the nanoscale. Furthermore, one can also im-prove the pattern resolution and intensity by optimizing the shapeand size of pores.

In summary, we demonstrate a “maskless” photolithographicmethod for patterning at the micrometer scale by using conicalpores etched in tracked glass chips. The functionality of the maskwas providedbymovable stages.We found that the dimensions ofthe patterns can be easily modulated by controlling the exposuretime without changing the size of pores in the chips which willsignificantly reduce the cost and downtime of patterning.We alsoshow that different patterns can be developed simultaneously byexposing photoactive polymer through multiple pores present inthe same chip. This method can potentially provide an inexpen-sive, versatile, and efficientway for patterningmolecules and filmsfor applications inmicroelectronics, photonics, materials, and lifesciences. Note: When this manuscript was in press, a paper fromChad Mirkin’s group was published in Nature Nanotechnologythat is based on a principle similar to the present studies.26

Acknowledgment. We acknowledge financial support fromthe National Science Foundation (CAREER award) and theNational Institutes of Health (GM 8071101). Dr. ChristinaTrautmann’s assistance for tracking the glass chips is greatlyacknowledged. We also acknowledge Rashid Zakeri’s (IndianaUniversity) help in the fabrication of conical pores. Dr. JohnBozzola, Dr. Steve Schmitt, and Ms. Hillary Gates (IMAGEcenter at SIUC) helped us in the electron microscopy analysis.

Supporting InformationAvailable: Derivation of eq 3. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

Figure 6. (A) Fluorescence micrograph showing the results ofparallel patterning (dot pattern and SIU logos) made using twopores in the chip. (B) Fabrication of line patterns by scanning thepores over photoresists. Exposure time was 25 s.

(22) Schweitzer, C.; Schmidt, R. Chem. Rev. 2003, 103, 1685–1757.(23) Odian, G. The Principles of Polymerization, 3rd ed.; John Wiley & Sons Inc.:

New York, 1991.

(24) Subczynski, W. K.; Wisniewska, A.; Yin, J.; Hyde, J. S.; Kusumi, A.Biochemistry 1994, 33, 7670–7681.

(25) Genet, C.; Ebbesen, T. W. Nature 2007, 445, 39–46.(26) Huo, F.; Zheng, G.; Liao, X.; Giam, L. R.; Chai, J.; Chen, X.; Shim, W.;

Mirkin, C. A. Nature Nanotechnology 2010, 5, 637–640.