Inexpensive and fast wafer-scale fabrication of nanohole arrays in thin gold films for plasmonics This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2010 Nanotechnology 21 205301 (http://iopscience.iop.org/0957-4484/21/20/205301) Download details: IP Address: 131.170.6.51 The article was downloaded on 08/03/2013 at 14:25 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
Transcript
Inexpensive and fast wafer-scale fabrication of nanohole arrays in thin gold films for
plasmonics
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
Inexpensive and fast wafer-scalefabrication of nanohole arrays in thin goldfilms for plasmonicsMona J K Klein1,2, Mickael Guillaumee1, Bernard Wenger1,L Andrea Dunbar1, Jurgen Brugger2, Harry Heinzelmann1 andRaphael Pugin1
1 Centre Suisse d’Electronique et de Microtechnique S.A. (CSEM), Neuchatel, Switzerland2 Ecole Polytechnique Federale de Lausanne (EPFL), STI IMT LMIS1, Lausanne, Switzerland
Received 2 December 2009, in final form 1 April 2010Published 23 April 2010Online at stacks.iop.org/Nano/21/205301
AbstractIn this paper, a fast and inexpensive wafer-scale process for the fabrication of arrays ofnanoscale holes in thin gold films for plasmonics is shown. The process combines nanospherelithography using spin-coated polystyrene beads with a sputter-etching process. This allows thebatch fabrication of several 1000 μm2 large hole arrays in 200 nm thick gold films without theuse of an adhesion layer for the gold film. The hole size and lattice period can be tunedindependently with this method. This allows tuning of the optical properties of the hole arraysfor the desired application. An example application, refractive index sensing, is demonstrated.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
In this paper, we describe fast and inexpensive wafer-scalefabrication of nanoscale hole arrays in thin gold films. Holearrays in thin metal films are interesting for a number ofapplications related to plasmonics. For example, they can beused as surface plasmon resonance (SPR) sensors, transparentelectrodes for light-emitting diodes (LED), to enhance theefficiency of solar cells, or act as optical filters [1–8]. Previousmethods for the large-scale fabrication of nanoscale holearrays have been shown. These works include nanospherelithography (NSL), interference lithography (IL), nanoimprintlithography (NIL), phase-shifting lithography, or combinationsthereof [9–15]. However, most of these methods havedrawbacks which limit the fabrication parameters of the holearrays, and thus the design of their optical properties. Forexample, the gold film thickness is typically limited to lessthan 100 nm, and an adhesion layer for the gold such aschromium or titanium is required. Such an adhesion layeris disadvantageous for plasmonics in the visible wavelengthregime. Surface plasmons (SPs), which are surface-boundelectromagnetic modes, may propagate along a metal surfaceif the losses in the metal are not too high [16]. Thus, in thevisible wavelength regime, SPs can propagate along the surface
of e.g. gold or silver, but are not supported at the surface ofe.g. chromium or titanium.
The wavelength at which SPs can be excited by lightincident on hole arrays not only depends on the material’sdielectric properties, but also the hole array geometry.Therefore, to obtain the desired optical properties, it isimportant to control the hole spacing, hole size, hole shape,and hole depth [1, 17–19]. One example which solves theproblem of limited gold film thickness and use of an adhesionlayer is shown in [14]. Yet the drawbacks of this methodare the limited hole dimension control due to clogging in anevaporation process, and the wet-etch removal of the initiallypresent adhesion layer. The latter requires that a floating goldmembrane be transferred to a support substrate, which maybe a cumbersome and non-reliable process step, especially forlarge size gold films.
In this paper, we will present a process which combinesNSL [20, 21] with sputter-etching. This process allows thewafer-scale fabrication of hole arrays in 200 nm thick goldfilms and avoids the use of an adhesion layer for the gold. Thefirst part of the paper gives the fabrication parameters of thethree main fabrication steps. The second part shows the resultsobtained for each process step and explains the influence ofthe different process parameters. The emphasis is on how
Figure 1. Schematic of the fabrication process. Step 1: a 200 nmthick Au film is sputtered onto a glass wafer. Step 2: NSL is used tofabricate an aluminum hole mask on top of the gold layer. Step 3: thealuminum hole mask is transferred into the gold layer bysputter-etching. The size of the beads used in this process is419 nm ± 10 nm (wafer type A) and 517 nm ± 18 nm (wafer type B).
the hole dimensions can be tuned by varying the sputter-etchconditions. Finally, as a possible application, we show the useof the fabricated hole arrays for refractive index sensing.
2. Fabrication
2.1. Preparation of gold substrates
Borosilicate glass wafers of 50 mm in diameter and 150 μmin thickness were obtained from Karl Hecht Assistent GmbH(Germany). The wafers were cleaned in organic solventsassisted by sonication and blown dry in a stream of nitrogen.Prior to the gold deposition, the wafer surface was exposedto an argon plasma for 5 min. Then, a 200 nm thickgold film was DC sputtered onto the wafers in a BalzersBAS450 at a rate of 10 A s−1 at room temperature (step 1 infigure 1). Under these deposition conditions, the adhesion ofthe gold film to the glass substrate is good; no delaminationhas been observed after sonication in organic solvents formore than 30 min. We attribute this to a mechanicaladhesion mechanism [22] as atomic force microscope (AFM)characterization showed that the glass wafers are decoratedwith small indentations of ca. 2 nm in depth and width, witha density of ca. 90 indentations μm−2, despite the overallsmoothness of the glass substrates (Rrms = 0.5 nm). Forcomparison, gold films deposited under the same conditionsonto glass substrates of similar Rrms, but without the smallindentations, delaminated immediately when sonicated inorganic solvents.
2.2. Nanosphere lithography
The NSL process step was used to fabricate a holey aluminumfilm on top of the gold substrates (step 2.1 through 2.4 infigure 1). Two different sizes of PS beads were used with anoriginal diameter Dbead,orig of 419 ± 10 nm and 517 ± 18 nmobtained from microparticles GmbH (Germany) as 10% (w/v)aqueous suspensions. In the remainder of this paper, we referto wafers A and B when 419 nm size and 517 nm size beads
Table 1. PS bead spin-coating parameters.
Bead size (nm) Speed (rpm) Acceleration (rpm s−1) Time (s)
419 500 300 2100 100 1
1500 500 60
517 500 300 5100 300 1
1500 500 60
were used in the NSL process step, respectively. This namingrule also holds for the figures shown in this paper.
For spin-coating of the beads, the suspensions werediluted with ethanol just before use (1.5% (v/v) for 419 nm and2.0% (v/v) for 517 nm size beads). The dilution with ethanolimproves the wetting of the wafer surface and thus only 100 μlof bead suspension was necessary for the coating of one wafer.The spin-coating was done at room temperature and the relativehumidity is controlled to between 30 and 40%. To obtain adense monolayer of beads, the spin-coating parameters (speed,acceleration, dwell time) were determined experimentally andare shown in table 1. The quality of the bead layers with respectto the arrangement of the beads (e.g. hexagonally close-packed(hcp) or non-close packed (ncp)) as well as the overall coverageof the wafer surface was analyzed by optical microscopy.
After deposition of the beads, the size of the beads wasreduced to Dbead,red in an oxygen plasma in a parallel platereactor (Oxford Plasmalab 80plus). The incident RF powerwas set to 100 W. At a pressure of 200 mTorr, this resulted in aself-bias voltage of −260 V. Thus, the etching of the beadsis slightly anisotropic with a faster etch rate in the verticalthan in the lateral direction. The beads on type A and Bwafers were etched in the oxygen plasma for 2 min 40 s and3 min 35 s, respectively, resulting in a Dbead,red of 195 ± 5 nmand 225 ± 5 nm, respectively.
In the next step, the size-reduced PS beads were usedas a lift-off template to fabricate a holey aluminum mask ontop of the gold layer. A 40 nm thick aluminum film wasevaporated at a rate of 4 A s−1 in a Leybold-Optics LAB600Hevaporator with a source–substrate distance of approximately1 m. Lift-off of the PS beads was done by overnight swellingin tetrahydrofuran (THF), followed by sonication in THF andrinsing in organic solvents. Gentle manual polishing duringthe rinsing process was applied on type A wafers to increasethe lift-off yield to almost 100%. The size of the holes in thealuminum layer, Dhole,Al, is identical to Dbead,red.
2.3. Sputter-etching of holes
The holey aluminum film was used as a hard mask to transferthe hole pattern into the gold substrate by sputter-etching inan argon plasma (step 3 in figure 1). The etching was donein an Alcatel SCM600 sputtering machine operated in reversemode. The pressure was set to 5 mTorr and the plasma issustained under an incident RF power of 400 W, resulting ina self-bias voltage of −190 V. Wafers of type A and B weresputter-etched in a continuous (Cont.) and intermittent (Int.)etching procedure (see table 2). The continuous etching was
2
Nanotechnology 21 (2010) 205301 M J K Klein et al
Figure 2. (a) Photograph of an A wafer after spin-coating,(b) optical microscope image of a bead monolayer on a B wafer.Hexagonally close-packed (hcp) and non-close packed (ncp) regionscan be distinguished as indicated by the arrows.
Table 2. Etch protocol for the continuous and intermittent etching oftype A (bead size 419 nm) and B (bead size 517 nm) wafers.
Wafer type Procedure Cont./Int. Etch time (min)
A Cont. 30
B Cont. 30
A Int. 20+10 = 30+10 = 40+10 = 50
B Int. 20+10 = 30+10 = 40+10 = 50
done for 30 min; the intermittent etching was done in foursteps from 20 to 50 min of cumulative etch time. The resultinghole geometry was analyzed after each etch run by scanningelectron microscope (SEM) and AFM imaging.
3. Results
3.1. NSL
3.1.1. Spin-coating of PS beads. Both A and B type waferscould be homogeneously coated with a monolayer of PS beadsby spin-coating. A photograph of a wafer after spin-coating isshown in figure 2(a) (wafer type A). Regions of hcp and ncpbead monolayers can be found on the wafer surface. Thoseregions can be distinguished as shown in figure 2(b) (wafertype B). The typical domain size of hcp regions is on the orderof several 1000 μm2. Low magnification optical microscopeimages were taken at several locations across the wafer surface.They reveal long-range order defects in the bead monolayersuch as areas not covered by beads or bead multilayers. Basedupon an image analysis3, it was found that the overall defectarea occupies less than 10% of the wafer surface. This showsthat NSL provides a method for wafer-scale nanopatterning.
3.1.2. Size reduction of PS beads. The etch behavior of PSbeads in the oxygen plasma has been investigated for bothbead sizes. Figure 3 shows a typical example of 517 nm sizebeads on gold films before and after being etched in an oxygen
3 Image analysis was performed with the free software tool ImageJ v1.42. Theimage threshold settings were set manually so as to select defect areas in thebead monolayer. Thus, the percentage of defect bead layer could be extracted.
Figure 3. SEM images of 419 nm size beads on a gold surfacebefore (a) and after (b) size reduction by O2-RIE. The shrinkage ofthe beads is slightly anisotropic as illustrated in (c).
Figure 4. SEM images of the aluminum hole mask on type A (a)and type B (b) wafers after lift-off. The PS bead template could beremoved successfully and the holes in the aluminum mask preciselyreproduce the top-view shading cross-section of the size-reduced PSbeads.
plasma. The evolution of the bead geometry is illustratedby the schematic insets which are based on AFM and SEMmeasurements. With increasing etch time, the lateral etch rateat which the diameter of the beads shrinks increases. Thiscan be explained by the slightly anisotropic etch behavior inthe parallel plate reactor oxygen plasma, together with thespherical shape of the PS beads [23]. Another parameter whichleads to an increase of both lateral and vertical etch rate is theheating of the substrate. As a consequence, the etch time needsto be calibrated for each desired Dbead,red.
3.1.3. Aluminum hole mask. Figure 4 shows SEM imagesof both wafer types after the lift-off procedure. The holesin the 40 nm thick aluminum layer precisely reproduce thetop-view shading cross-section of the size-reduced PS beads.The size of the aluminum mask holes is 195 nm on typeA and 225 nm on type B wafers. As the bead surface isroughened during the size reduction, this roughening translatesinto small notches in the overall circular holes in the aluminummask.
3
Nanotechnology 21 (2010) 205301 M J K Klein et al
Figure 5. Holes etched into a 200 nm thick gold film on a type Bwafer after continuous sputter-etching for 30 min. (a) shows a lowmagnification image with hcp and ncp regions as indicated by theannotation. Figures (b) and (c) are a zoom into the hcp and ncpregions with a FFT image as the inset.
On the type B wafers, the lift-off yield was 100% afterswelling and sonication in THF. In the case of the A wafers, theyield had to be increased by applying a gentle manual polishingprocedure. This difference in lift-off behavior can be explainedby the fact that the rim height (see sketch in figure 3(c)) of the419 nm size beads after size reduction, hrim, is only 50 nm, ascompared to 70 nm in the case of the 517 nm size beads.
3.2. Sputter-etching
3.2.1. Uniform etching of large area hole arrays. Theadvantage of sputter-etching is that it is a parallel processallowing hole fabrication on a full wafer in a single etchingstep. Figure 5 shows SEM images of a type B waferafter continuous etching for 30 min. The uniformity ofthe etching procedure and of the distribution of holes isshown in figure 5(a). Two distinct areas can be identified infigure 5(a)—regions of hexagonally close-packed (hcp) andnon-close packed (ncp) holes. Domains of good short-rangeorder in the hcp region appear as a Moire pattern.
Figures 5(b) and (c) show a zoom into the areas markedas hcp and ncp in figure 5(a). The insets show a fast-Fourier-transform (FFT) image of domains in figures 5(b) and (c). TheFFT of the hcp image shows the peaks of a hexagonal crystalstructure, while the FFT of the ncp region shows a ring pattern.The radius of the inner ring corresponds to the characteristicdistance between the holes in the absence of array order.
3.2.2. Etch rate selectivity and widening of the maskholes. Sputter-etching is a purely physical etching process.Therefore, the etch rate mainly depends on the materialshardness. While the bulk hardnesses of gold and aluminumare similar, aluminum oxide is much harder than both gold or
Figure 6. (a) The conical hole cross-section is illustrated by theAFM images of type A and B wafers after 20 min of etching.(b) The hole sidewall slope measured after different cumulative etchtimes when applying the intermittent etching procedure.
aluminum [24]. The etch rate selectivity Au:Al for plain 20 nmthick films was measured to be �10. This may be explainedby the presence of a thin aluminum oxide layer on top of thealuminum film. Oxidation of the aluminum mask also has asignificant influence on the etch results obtained by either thecontinuous or the intermittent etching procedure (see below formore details).
For dense patterns with sub-μm features, faceting [25],which erodes the masking layer at the pattern edges, may leadto an early mask failure. In the present case, the masking layerconsists of closely spaced holes in a thin aluminum film. Thus,faceting leads to a widening of the mask holes until they startto merge, eventually resulting in a complete removal of themasking layer. Thus, in order to obtain a certain hole size fora given hole spacing, it is important to take into account thecontinuous widening of the mask holes.
When etching the holes into the gold film, the hole willeventually reach the glass substrate and the glass substrate willstart to be etched. The etch rate selectivity of Au:SiO2 wasmeasured to be �2 for plain substrates. Therefore, althoughthe glass substrate is etched, it is at a much lower etch rate thanthe gold.
3.2.3. Conical hole cross-section. Figure 6 illustratesthe cross-sectional hole profile obtained by sputter-etching.Figure 6(a) shows AFM images of both wafer types after20 min of etching. Figure 6(b) shows the measured sidewallslope for different cumulative etch times when the intermittentetch procedure was used. The data shown in figure 6(b) wereobtained by combining AFM height with SEM topographymeasurements. An image analysis software was used to extractthe hole diameters Dhole,AuGlass and Dhole,AuAir from the SEMimages (software: analySIS by OLYMPUS).
4
Nanotechnology 21 (2010) 205301 M J K Klein et al
Figure 7. SEM images obtained on type A and type B wafers afterthe continuous (1) and intermittent (2) etch procedure. In the case ofthe intermittent etch procedure, the same sample was analyzed aftereach additional etch step.
For varying cumulative etch times, the sidewall slope isconstant around 58◦±3◦. This value is in good agreement withtheory, according to which the highest etch rate is obtained forions incident under an angle of 60◦–70◦ [26]. Mask faceting,the ion angular distribution and redeposition of non-volatileetch products may explain the slightly lower value of 58◦.
3.2.4. Continuous versus intermittent etching procedure.Figure 7 shows SEM images of holes on both wafer types after30 min of continuous etching and after the intermittent etchingprocedure for cumulative etch times from 20 to 50 min.
Figure 8. The hole diameter at the gold–glass interface as a functionof increasing cumulative etch time.
After 30 min of continuous etching, the hole diameter atthe gold–glass interface, Dhole,AuGlass, is 80 nm and 170 nmon type A and type B wafers, respectively. Considering thehole size at the gold–glass interface, this corresponds to a holefilling factor ff of 3% (A) and 10% (B), where the ff is the holearea at the gold–glass interface divided by the area of a hcpunit cell. The diameter at the gold–air interface, Dhole,AuAir, is280 nm (A) and 320 nm (B). Originally, the diameter of thealuminum mask holes Dhole,Al was 195 nm (A) and 225 nm(B). Thus, the widening of the mask hole’s diameter during theetching is about 90 nm on both types of wafers. This may beexplained by faceting at the mask edges (also see above).
The aluminum masking layer is used up after 30 minof continuous etching. The absence of a residual aluminummask was confirmed by energy dispersive x-ray spectroscopymeasurements taken before and after the etching procedure andcompared to bare 200 nm thick gold films on glass substrates.This shows how an in situ removal of the masking layer ispossible, eliminating the need for a post-treatment to removethe masking layer.
On the other hand, further etching would result in athinning of the gold layer and in a continuous wideningof Dhole,AuGlass. This limits the attainable Dhole,AuGlass, andthus the ff, to rather small values. A possible approach toincrease Dhole,AuGlass and reduce the effect of the mask holewidening could be to use a thicker aluminum mask. A thickeraluminum mask, however, may only be realized at the trade-off of accepting a larger Dhole,Al (see section 3.1.3). Using alarger Dhole,Al, however, faceting would pose a problem as theholes in the aluminum mask will merge even sooner. Despitea thicker masking layer, early mask failure could be the case.Thus, a more practical solution could be to either have a moreresistant etch mask, or to modify the etching process.
Figure 7(2) shows SEM images of holes on type A andB wafers after each step of the intermittent etching procedure.The hole size increases with increasing cumulative etch timedue to faceting of the mask holes. The relationship betweenetch time and hole diameter is shown in figure 8. With thecumulative etch time increasing from 20 to 50 min, Dhole,AuGlass
increases from 65 nm to 195 nm and from 105 nm to 250 nmfor type A and type B wafers, respectively. The correspondinghole filling factor can thus be tuned from 2% to 19% (A) and4% to 20% (B). This shows how the continuous widening of
5
Nanotechnology 21 (2010) 205301 M J K Klein et al
the holes in the aluminum mask provides a convenient wayto obtain different hole diameters for originally identical maskdimensions.
Another observation from figure 7 is that the aluminummask is still valid even after 50 min of cumulative etchtime, while it was completely removed after only 30 min ofcontinuous etching. Two factors contribute to the prolongedmask lifetime: (a) a re-oxidation of the aluminum maskinglayer in between the consecutive etch runs and (b) reducedheating during the etching process. The prolonged masklifetime allows us to use a thinner masking layer and stillachieve a large hole size without reducing the gold filmthickness. A thinner masking layer also allows us to realizesmaller mask hole diameters which are limited by the height ofthe PS bead lift-off template in the NSL process step.
In summary, the combination of NSL with sputter-etchingprovides a flexible process for the fabrication of wafer-scalehole arrays of various hole sizes. Tuning parameters includethe size of the beads used, the size reduction of the PS beads,the thickness of the aluminum layer and the parameters of thefinal etching procedure.
4. Refractive index sensing
Hole arrays fabricated by the continuous etch procedure(see figure 7(1)) were used for refractive index (RI) sensingexperiments. Optical transmission measurements wereperformed using an inverted optical microscope with a lownumerical aperture (NA) (<0.1) white light illuminationsource. The light is collected through a 40× objective (NA =0.6) and analyzed using a spectrometer coupled with a liquidnitrogen cooled CCD camera. The light intensity I transmittedthrough the hole arrays was normalized to the incident fluxthrough a glass slide, I0.
Figure 9 shows the transmission spectra obtained for typeB hole arrays in air and when immersing the arrays in differentconcentrations of glycerine in water (�RI = 0.0568). Inair, up to 15% of the light incident through the glass slideis transmitted, while the hole filling factor ff is only 10%(Dhole,AuGlass = 170 nm). This means that more light istransmitted through the holes as compared to the light incidenton the area occupied by holes. Such an enhanced lighttransmission is known as extraordinary optical transmission(EOT) [1, 27]. However, taking the conical shape of the holesand the gold skin depth at optical frequencies into account,the effective hole diameter becomes larger than 170 nm; thus,also the ff is larger than 10%. Therefore, quantification whichjustifies to claim EOT is difficult in this case.
Upon the immersion of the arrays in liquid, thetransmission spectra undergo a marked change as compared tothe spectra in air. The long wavelength transmission reaches30% due to the increase of the refractive index of the dielectricinside the holes. When the RI of the liquid is further changedfrom 1.333 (0 vol% glycerine in water) to 1.3898 (50 vol%glycerine in water), the transmission maxima undergo a red-shift. This shift can be correlated to the RI of the liquid.The inset in figure 9 shows the shift of the long wavelengthtransmission peak as a function of the RI. It shifts by 230 nm/RI
Figure 9. Transmission spectra obtained for type B wafers(continuous etch procedure) measured in air and after immersion inwater–glycerine mixtures. The glycerine concentrations wereincreased from 0 to 50 vol%, corresponding to a refractive index (RI)change from 1.333 (pure water) to 1.3898 (50 vol% glycerine inwater), that is �RI = 0.0568. Shown in the inset is the longwavelength transmission peak’s shift as a function of the RI.
unit (RIU). RI sensitivities when using hole arrays for RIsensing can be as high as 500 nm/RIU (e.g. [3]). Thus, in orderto obtain a higher RI sensitivity, the hole array geometry needsto be optimized.
Despite the rather low RI sensitivity observed in this case,an important advantage thanks to the large size of the holearrays is the large amount of transmitted light. This allows fastmeasurements with still a high signal-to-noise ratio, which isan important factor for high through-put analysis of differentanalytes or for online monitoring of chemical reactions orsurface binding events. In a primary experiment, the non-specific binding of bovine serum albumin could be monitoredsuccessfully [28].
5. Conclusions
By combining nanosphere lithography (NSL) using spin-coated polystyrene (PS) beads with a sputter-etching process,nanoscale hole arrays in 200 nm thick gold films werefabricated on 50 mm in diameter glass wafers. All processsteps are wafer-based and thus allow a low-cost, high through-put fabrication.
The hole array periodicity is determined by the originalsize of the PS beads (419 and 517 nm in this paper). The orderof the arrays is dependent on the spin-coating parameters andcan be optimized. Alternatively, a different bead depositionmethod might be considered [10].
By size reduction of the deposited PS beads in an oxygenplasma, the hole size in the aluminum etch mask can be tuned(nanosphere lithography process step). Sputter-etching is thenused to transfer the aluminum hole mask into the gold layer.The resulting holes have a conical cross-section with a sidewallslope of ca. 58◦. By adapting the sputter-etch parameters,different hole sizes in the gold film can be realized withoutthe need to modify the aluminum mask. For example, thehole size in 200 nm thick gold films could be tuned from 65to 195 nm for a mask hole diameter of 195 nm, or from 105to 250 nm for a mask hole diameter of 225 nm. Refractive
6
Nanotechnology 21 (2010) 205301 M J K Klein et al
index (RI) sensing was demonstrated in optical transmissionmeasurements by immersing the hole arrays in liquids ofvarying RI. The sensitivity, measured as the peak transmissionwavelength shift, is around 230 nm/RI unit. To obtain a highersensitivity, the hole array geometry can be optimized.
Further interesting applications of hole arrays are relatedto photovoltaic or light-emitting devices. The continuousgold layer decorated with hole arrays may serve as a highlytransparent, yet low resistivity electrode. At the same time,the confinement of the electromagnetic field in the vicinityof the holes and at the gold–dielectric interface may serve toincrease the efficiency of optical devices through increasedlight absorption in the active device region.
Acknowledgments
We would like to thank Vladislav Spassov for the fruitfuldiscussions and experimental support concerning sputter-etching techniques. CSEM thanks the Swiss Federal Office forEduction and Science (OFES) for funding.
References
[1] Genet C and Ebbesen T W 2007 Light in tiny holes Nature445 39–46
[2] Sinton D, Gordon R and Brolo A G 2008 Nanohole arrays inmetal films as optofluidic elements: progress and potentialMicrofluid Nanofluid 4 107–16
[3] Ferreira J, Santos M J L, Rahman M M, Brolo A G, Gordon R,Sinton D and Girotto E M 2009 Attomolar protein detectionusing in-hole surface plasmon resonance J. Am. Chem. Soc.131 436–7
[4] Lindquist N C, Lesuffleur A, Im H and Oh S-H 2009Sub-micron resolution surface plasmon resonance imagingenabled by nanohole arrays with surrounding Bragg mirrorsfor enhanced sensitivity and isolation Lab Chip 9 382–7
[5] Geyer U, Hetterich J, Diez C, Hu D Z, Schaadt D M andLemmer U 2008 Nano-structured metallic electrodes forplasmonic optimized light-emitting diodes Proc. SPIE7032 B320
[6] Liu C, Kamaev V and Vardeny Z V 2005 Efficiencyenhancement of an organic light-emitting diode with acathode forming two-dimensional periodic hole array Appl.Phys. Lett. 86 143501
[7] Rivas J G, Vecchi G and Giannini V 2008 Surface plasmonpolariton-mediated enhancement of the emission of dyemolecules on metallic gratings New J. Phys. 10 105007
[8] Dintinger J, Klein S and Ebbesen T W 2006 Molecule surfaceplasmon interactions in hole arrays: enhanced absorption,refractive index changes, and all-optical switching Adv.Mater. 18 1267–70
[9] Jiang P and McFarland M J 2005 Nanohole arrays templatedfrom two-dimensional nonclose-packed colloidal crystalsJ. Am. Chem. Soc. 127 3710–1
[10] Lee S H, Bantz K C, Lindquist N C, Oh S H and Haynes C L2009 Self-assembled plasmonic nanohole arrays Langmuir25 13685–93
[11] Kuiper S, van Wolferen H, van Rijn G, Nijdam W, Krijnen Gand Elwenspoek M 2009 Fabrication of microsieves withsub-micron pore size by laser interference lithography JMicromech. Microeng. 11 33–7
[12] Skinner J L, Hunter L L, Talin A A, Provine J and Horsley D A2008 Large-area subwavelength aperture arrays fabricatedusing nanoimprint lithography IEEE Trans. Nanotechnol. 7527–31
[13] Chen J, Shi J, Decanini D, Cambril E, Chen Y andHaghiri-Gosnet A-M 2009 Gold nanohole arrays forbiochemical sensing fabricated by soft UV nanoimprintlithography Microelectron. Eng. 86 632–5
[14] Kwak E-S, Henzie J, Chang S-H, Gray S K, Schatz G C andOdom T W 2005 Surface plasmon standing waves inlarge-area subwavelength hole arrays Nanot Lett. 5 1963–7
[15] Henzie J, Lee J, Lee M H, Hasan W and Odom T W 2009Nanofabrication of plasmonic structures Annu. Rev. Phys.Chem. 60 147–65
[16] Raether H 1988 Surface plasmons on Smooth and RoughSurfaces and on Gratings (Berlin: Springer)
[17] Coe J V, Heer J M, Teeters-Kennedy S, Tian H andRodriguez K R 2008 Extraordinary transmission of metalfilms with arrays of subwavelength holes Annu. Rev. Phys.Chem. 59 179–202
[18] Laux E, Genet C and Ebbesen T W 2009 Enhanced opticaltransmission at the cutoff transition Opt. Express17 6920–30
[19] Przybilla F, Degiron A, Laluet J Y, Genet C and Ebbesen T W2006 Optical transmission in perforated noble and transitionmetal films J. Opt. A: Pure Appl. Opt. 8 458–63
[20] Hulteen J C and Van Duyne P 1995 Nanosphere lithography: amaterials general fabrication process for periodic particlearray surfaces J. Vac. Sci. Technol. A 13 1553–8
[21] Fredriksson H, Alaverdyan Y, Dmitriev A, Langhammer C,Sutherland D S, Zaech M and Kasemo B 2007 Hole-maskcolloidal lithography Adv. Mater. 19 4297–302
[22] Ohring M 2002 Materials Science of Thin Films 2nd edn(Amsterdam: Elsevier) chapter 12.7.3, p 767
[23] Haginoya C, Ishibashi M and Koike K 1997 Nanostructurearray fabrication with a size-controllable natural lithographyAppl. Phys. Lett. 20 2934–6
[24] Lide D R 1992 CRC Handbook of Chemistry and Physics72nd edn (Boca Raton, FL: CRC Press)
[25] Madou J M 2002 Fundamentals of Microfabrication 2nd edn(Boca Raton, FL: CRC Press)
[26] Menz W, Mohr J and Paul O Mikrosystemtechnik furIngenieure 3rd edn (Weinheim: Wiley–VCH)
[27] Ebbesen T W, Lezec H J, Ghaemi H F, Thio T and Wolff P A1998 Extraordinary optical transmission throughsub-wavelength hole arrays Nature 391 667–9
[28] Results Presented at SPP4 Conf. (Amsterdam) 2009 data uponrequest
