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Plasmonic Nanolithography: A ReviewZhihua Xie&Weixing Yu&Taisheng Wang&Hongxin Zhang&Y ongqi Fu&Hua Liu&Fengyou Li&Zhenwu Lu&Qiang SunReceived: 9 January 2011 /Accepted: 23 May 2011 /Published online: 31 May 2011#Springer Science+Business Media, LLC 2011Abstract Surface plasmon polaritons (SPPs) has attractedgreat attention in the last decade and recently it has beensuccessfully applied to nanolithography due to its ability ofbeyond diffraction limit. This article reviews the recentdevelopment in plasmonic nanolithography, which isconsidered as one of the most remarkable technology fornext-generation nanolithography. Nanolithography experi-ments were highlighted on the basis of SPPs effect. Threetypes of plasmonic nanolithography methods: contactnanolithography, planar lens imaging nanolithography, anddirect writing nanolithography were reviewed in detail, andtheir advantages and shortages are analyzed and compared,respectively. Finally, the development trend of plasmonicnanolithography is suggested.Keywords Plasmonic nanolithography.Contactnanolithography.Planar lens imaging nanolithographyIntroductionLithography technique has always been considered as amainstream of fabrication methods in semiconductorindustry for its high throughput and cost effective incomparison to electron beam lithography 1 and focusedion beam writing over the past several decades. Higherthroughput, lower cost, higher resolution, and simplifica-tion of system configuration are the targets we alwayspursue. Various types of nanolithography techniques havebeen explored before, such as electron beam lithography,nanoimprint lithography 2, dip-pen lithography 3, 4, andothers. For electron beam lithography, the smallest resolu-tion of less than 10 nm has been demonstrated, but thethroughput of this technique is rather low so that it ismainly used for the fabrication of masks rather than massproduction. Nanoimprint lithography, with the resolution ofless than 10 nm and a high throughput, can be used in massproduction. However, as a replication method, there are stillsome issues for nanoimprint lithography to address. One ofthe issues is that the residual resist layer after imprintingcan arise which may limit its application 5. Dip-penlithography has the same disadvantage of low throughput aselectron beam lithography. Apart from the aforementionedtechniques, photolithography is another important tech-nique for nanolithography. The conventional photolithog-raphy techniques for nanolithography include opticalprojection lithography (193 immersion lithography) 68,X-ray lithography 9, extreme ultraviolet lithography 10,zone-plate-array lithography 1116, and so on. Theoptical projection lithography is mostly used in industryZ. Xie:T. Wang:H. Zhang:H. Liu:F. Li:Z. Lu:Q. SunOpto-electronic Technology Center,Changchun Institute of Optics, Fine Mechanics and Physics,Chinese Academy of Sciences,Changchun, Jilin 130033, ChinaZ. Xie:T. WangGraduate School of the Chinese Academy of Sciences,Beijing 100039, ChinaW. Yu (*)State Key Laboratory of Applied Optics,Changchun Institute of Optics, Fine Mechanics and Physics,Chinese Academy of Sciences,Changchun, Jilin 130033, Chinae-mail: Y. Fu (*)School of Physical Electronics,University of Electronic Science and Technology of China,Chengdu 610054, Sichuan Province, Peoples Republic of Chinae-mail: Plasmonics (2011) 6:565580DOI 10.1007/s11468-011-9237-0due to its high throughput. But with the requirement forsmaller feature sizes, conventional optical projection lithog-raphy technique cannot meet the resolution requirementanymore because it is diffraction limited. Conventionally,the resolution ofoptical projection lithographyisenhanced bymeans of reducing the illumination wavelength or increasingthe numerical aperture (NA), which brings too muchcomplexities and escalating cost. X-ray lithography can havea high throughput and has demonstrated a 50 nm resolution,but the X-ray lithography system is rather expensive, whichurges ustocastabout for a lithographysystemwithlower costto substitute it 17. Extreme ultraviolet lithography can alsoobtain a high resolution, but the high cost of light source andcomplexity of the optical exposure system limit its applica-tion in industry for mass production use. As to the zone-plate-array lithography, it is a novel schematic method to beable to expose arbitrary patterns with relative fast speed.However, its resolution is still diffraction limited.Near-field optical lithography provides a new route tobeat the diffraction limit and could achieve a resolutionwithout the limitation in theory. Recently, many sorts ofnear-field lithography systems have been reported 1822.Conventional near-field lithography has achieved sub-50 nm resolution using special masks such as light-coupling mask or phase-shift mask 23. But one of themain shortcomings of near-field lithography is that thetransmittance of the light is extremely low. For theapertures on the mask smaller than illumination wave-length, the amount of the light reaching resist is rather lowbecause most of the light has been diffracted and scatteredaway. This leads to a considerably long exposure time andlow image contrast.More recently, near-field lithography developed on thebasis of surface plasmon polaritons (SPPs) theory has beenproposed for the purpose of further improving the resolu-tion of near-field lithography. Surface plasmons (SPs) arecollective electrons existing at the interface of the metal andthe dielectric 24. SPs have its unique dispersion relation,which attributes to the resolution beyond the diffractionlimit. The dispersion relation is shown by Eq. 1:ksp2pl0?ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi(d(m(d(mr1where,l0is the light wavelength in vacuum,(dand(marethe permittivities of the dielectric and metal layers,respectively. From the dispersion equation, we can get thatthe wavelength of the SPs is shorter than the lightwavelength in vacuum (l0) and the shorter wavelength isresponsible for the resolution beyond diffraction limit.Surface plasmons exist in two forms, propagating andlocalized. On the smooth film, the SPPs wave is propagat-ing as an evanescent electromagnetic wave at the metaldielectric interface, and is the results of collective oscil-lations of the conduction free electrons in the metal surface.Generally, SPPs cannot easily be excited for the momentummismatch between light waves and the waves of the SP.The metal mask with perforations array and proper periodscan compensate the momentum mismatch and excite theSPPs. Localized surface plasmon resonance is not propa-gating at horizontal plane but localized electromagneticfields near the surface of the isolated nanoparticles 25. Fora single subwavelength aperture, the transmission can beenhanced due to the existence of localized surface plasmon(LSP). As for the periodic aperture array, the transmissionenhancement can be explained as the integrated effect ofLSP and SPPs 26. The lithography technique based onsurface plasmon theory is called plasmonic photolithogra-phy. For plasmonic photolithography, resolution and con-trast can be improved significantly due to the extraordinaryenhancement of transmittance. Recently, many SPP-basedlithography experiments have been conducted. Computa-tional numerical simulations show that the resolution ashigh as 20 nm can be achieved using the illumination lightwith 365 nm wavelength 24. And it seems that plasmonicphotolithography has the potential to satisfy the require-ments of high resolution, low cost, and high throughputsimultaneously. In this article, we will review the typicalplasmonic photolithography experiments firstly. Then acomparison of these experiments is concluded. Finally, thedevelopment trend of the plasmonic nano-photolithographyis put forth.Plasmonic NanolithographyGenerally, plasmonic nano-photolithography can be catego-rized into three types in terms of the exposure methods:namely contact nanolithography, planar lens imaging nano-lithography, and direct writing nanolithography, respectively.In the following section, we will try to review the plasmonicnanolithography experiments as many as possible by dividingall these experiments into the three types techniques men-tioned above.Plasmonic Contact LithographyPlasmonic contact lithography is a modified form ofevanescent near-field optical lithography designed toimprove subwavelength image quality 27. In this method,the photoresist is exposed by SPPs which is originated fromthe metal mask. As the SPPs can only propagate tens ofnanometers beneath the metal film, the intimate contactbetween the mask and photoresist film is necessary. Manyefforts have been devoted to the exploitation of thistechnique.566Plasmonics (2011) 6:565580In 2005, Srituravanich et al. experimentally demonstratednanolithography with a half-pitch resolution down to 60 nmusing a 2D holes array perforated through Ag mask 24. Afiltered mercury lamp with a radiation peak at 365 nm wasemployed as the illumination source. The schematicdiagram is illustrated in Fig. 1a. The mask is composed ofan Ag layer perforated with a 2D periodic holes arraypatterned and sandwiched between quartz and OmniCoatlayers (refractive index is 1.48 and 1.57, respectively). TheAg film has a thickness of 40 nm, and the holes array has aperiodicity of 120 nm and each hole has a diameter of60 nm. It is noted that an adhesion layer of 3 nm thicktitanium film was added between the Ag and quartzsubstrate so as to increase adhesion of the Ag film andreduce the surface roughness. With this configuration, smallfeature sizes are fabricated in Ag film with significantly lowsurface roughness as illustrated in Fig. 1b. A 15 nm thickspacer layer of OmniCoat is spin coated on the patternedAg film. A negative photoresist (SU-8) is directly spincoated on top of the spacer layer and polymerized on themask to eliminate the gap variation between the mask andphotoresist in the photolithography process. 2D holes arraywith the feature size as small as 60 nm (equivalent tol0/6)has been obtained with the exposure does of 80 mJ/cm2andillustrated in Fig. 2. Feature size obtained on photoresist isthe same size with the patterns on the mask. Therefore, theresolution is mainly limited by the resolution on the mask.The pattern fidelity is significantly high due to theparticularly short propagation length of SPPs in this case(20 nm). It is noted that Al film can also be employed asthe mask since it can excite the SPPs in the ultraviolet (UV)range and a sub-100 nm dot array pattern with a 170-nmperiod has been successfully obtained using a 365-nmwavelength light source by Srituravanich et al. 28. Butsimulation results calculated by use of commercial finite-difference time-domain software (Microsoft Studio) showedthat under the plane wave illumination (l=365 nm), theelectric field transmitted through the Ag holes array has asignificant enhancement on the field strength and tightconfinement of the field distribution compared with that ofthe Al holes array 29. Therefore, it can be concluded thatfor the 365 nm wavelength irradiation, Ag film can realize abetter pattern than that of Al film.Shao et al. exploited a surface plasmon-assisted nano-lithography system similar to the one discussed above 30,whereas a UV lamp is employed to provide light irradia-tion. The photomask is made from a 70-nm thick titaniumfilm with the patterns of grating structures and ringapertures. The mask is intimately in contact with thephotoresist without the spacing layer. An 80-nm thicktitanium shield is added in between the photoresist and thesubstrate to focus the light intensity in the photoresist so asto achieve nanoscale patterning with high density andabsorb the light as it reaches the substrate. In thisexperiment, both the number of apertures and the period-icity are the critical factors affecting the lithography results.The grating with a period of 400 nm was transferredfairly well and the pattern obtained in the resist has aheight of 35 nm. However, the pattern that has a singleaperture was not transferred to the resist, which indicates thatthe performance of the mask mainly depends on design of theaperture shape and aperture size in the mask.Zayats and Smolyaninov proposed a new method toachieve strongly enhanced transmittance of an individualsubwavelength aperture 31. They demonstrated that theoptical transmission of an individual subwavelength aper-ture in a multi-layered metal film was shown to be stronglyenhanced compared with that of a homogeneous single-layered metal film due to the light coupling to surfaceFig. 1 a Schematic of lithography setup designed by Srituravanich etal. b The silver mask with hole array used in this experiment.Reprinted with permission from 24Plasmonics (2011) 6:565580567plasmon excited by a periodically structured film. Thismethod enhances optical transmission in up to two ordersof magnitude in comparison to an aperture with the samesize perforated in a homogeneous film. Moreover, themulti-layered metallic film can be conventionally preparedwith well-developed thin-film deposition techniques. Suchstructures are robust and are easy to fabricate due to theabsence of the fabrication of complex periodic structurerequired by conventional single-metal-film plasmonicmask. This proposal can be used in lithography and itmay deliver a better lithography result. The application inlithography is going to be demonstrated experimentally.Similarly, a photolithography scheme using a designedmetal-dielectric multilayer was proposed to generate sub-wavelength feature size 32. Generally speaking, thepatterns fabricated in photoresist are identical to thepatterns in mask for the scheme. However, numericalexperiments show that a much smaller feature size couldbe obtained such as for a 400-nm period grating structure ina mask, a 67-nm period grating structure can be fabricatedin photoresist.The utilization of an array of holes in noble metalsshows transmission enhancement with the aid of theresonant excitation of surface plasmon, while the spatialresolution is somehow sacrificed by the period of the holesarray 33. Recent research showed that sharp-ridgedapertures in metal, such as bowtie apertures or antennas,C-, H-shaped apertures and others, may realize a betterresult. The bowtie antenna was an important aperture typeand was proposed firstly by Grober et al. and used as anear-field optical probe with high transmission efficiency atmicrowave frequency 34. Subsequently, a lot of effortshave been devoted to the research of the bowtie structure.And the applied frequency was extended to visible andeven UV range. Highly confined hot spots with enhancedintensity were observed in the near-field of bowtiestructures due to the localized surface plasmon resonance.It has been applied in many domains due to the enhancedtransmittance of bowtie structures, such as high efficiencyexcitation of plasmonic waveguides 35, high-harmonicgeneration, and so on 36. Recently, it has been success-fully applied in nanolithography as a novel method toimprove the resolution.A bowtie aperture is the counterpart of a bowtie antennaas shown in Fig. 3. Both of two structures consist of twoarms and a small gap formed by two sharp tips pointingtoward each other. The simulation results indicate that theresonance at 505 nm, the corresponding field intensity atthe bowtie apex is more than 15,000 times of that ofillumination field, which is comparable to the bowtieantenna 37. But the actual performance would be greatlyaffected by the aperture size, metal material, wavelength,polarization, and some other factors. A mask with bowtieapertures can be used in contact lithography and a fineresult has been demonstrated. Xu et al. firstly applied thebowtie aperture in plasmonic contact lithography experi-mentally and 2D holes with feature size of sub-50 nm wereresolved using the bowtie apertures 38. The bowtieapertures with a 30-nm gap size fabricated in 150-nm thickAl film coated on quartz substrates were employed as themask. Al was selected as the mask material because of itssmall skin depth and high reflectivity. The mask wasilluminated by a diode-pumped solid-state 355 nm laserbeam polarized in the direction across the gap. Theexperimental setup is illustrated in Fig. 4. The experimentwas performed in a class-10 clean room rating glove box tominimize the contamination and to prevent the environ-mental light from exposing the photoresist. With a 3UVobjective lens, the laser beam is focused into a 110 mdiameter spot on the mask. The polarization of the laserbeam is directed across the gap of the bowtie aperture. Apositive photoresist (Shipley S1805) was used in thisexperiment. As the depth and dimension of holes weredirectly affected by the exposure time, therefore, precisecontrol of exposure time is required. The exposure time iscontrolled using an electric shutter with millisecond timingFig. 2 AFM image of the exposure pattern. Reprinted withpermission from 24Fig. 3 Schematic of bowtie aperture (left) and antenna (right).Reprinted with permission from 38568Plasmonics (2011) 6:565580precision. As a result, subdiffraction-limit lithography holesas small as 4050 nm were obtained in the positivephotoresist with a 1.3-s exposure time.Recently, Ueno et al. explored a new plasmonic lithogra-phy method using the localized field of nanogaps 39. Themask consisting of pairs of rectangular gold nanoblockswhich are separated by nanogaps fabricated on glasssubstrate. In this experiment, a femtosecond laser (Tsunami,Spectra Physics) with a central wavelength oflL=800 nm, apulse length ofCP=100 fs, and a repetition rate of f=82 MHzwere employed as the illumination source. The beam isnormally incident with respect to the substrate (xy plane)and is linearly polarized along the diagonal of the blocks.The schematic diagram of gold nanoblock pairs wasillustrated in Fig. 5. The nanoblocks are rectangular goldcuboids with a size of 808035 nm arranged into pairsoriented along a common diagonal parallel to the x-axiswith the gap size of 4 nm. The mask consists of largeperiodic arrays of nanoblock pairs arranged on a squarelattice with a period of 400 nm.An experimental setup with 70-nm thick positive TSMRV-90 photoresist film is illustrated in Fig. 6a. The resist wasspin coated on a glass substrate. The substrate is broughtinto direct contact with the top surface of the nanoblocks.The exposure is expected to occur in regions of photoresistadjacent to the nanogaps. And the experimental result isillustrated in Fig. 6b. For a lower irradiance of 0.6 W/cm2,only the regions adjacent to the nanogaps are exposed andit was reported that single pit with a typical lateral size ofabout 5 nm has been obtained (the inset image of Fig. 6b),which is about 0.6% of the laser wavelength. The mainimage of Fig. 6b is the pattern obtained with an exposure of60 W/cm2. This method can be conducted with othergeometries of metallic nanostructures. The research grouphas also demonstrated a mask with structures consisting ofpairs of gold strips with a dimension of 1,00010036 nmseparated by 20-nm wide nanogaps and arranged with aperiod of 600 nm on the substrate. Series of lines with anaverage width of 4050 nm extending along the entirelength of the stripes was recorded. In this configuration,laser beam is incident on the photoresist from the backsideof the substrate that was coated by the same resist. Incomparison to the nanoblocks mask, this schematic has alower enhancement factor and a higher contrast.From above, we can conclude that the bowtie structure-based lithography and the nanogap-assisted lithographytechnique have a better resolution compared to the maskwith holes array. Therefore, these two types of techniquesmay have a brighter future.In addition, it is worth to mention that some researchershave tried to use spherical or nonspherical particles asmasks for nanopatterning as well 24, 40. In this case, thehole size and lattice period could be tuned independently.Although the transmittance of plasmonic nanostructurehas been greatly enhanced, plasmon damping originatedfrom intrinsic metal absorption limits the achievable aspectratio of the fabricated structures. To obtain higher fielddepth, K. Sathiyamoorthy proposed a novel concept ofemploying the dye medium to enhance plasmon propaga-tion by compensating intrinsic loss associated with metal41. In this proposal, polymethylmethacrylate (PMMA)doped with the dye material was employed as the masksubstrate. Simulation results show that a 14.5-fold of fieldenhancement in the PMMA/dye medium can be obtainedcompared to that of the bare PMMA. In addition, Kid et al.exploited metal nanoparticle arrays for near-field opticallithography and spot sizes ranging from 30 to 80 nm withexposure depth ranging from 12 to 45 nm have beenachieved using broad beam illumination with visible lightand standard resist 42.Plasmonic contact lithography has the advantages ofhigh resolution and high throughput. But the mask used inthis technique is rather expensive and the fabricationprocess is very complex. In addition, the intimate contactbetween the mask and substrate will result in the contam-ination of the mask and thus reduce lifetime of the mask.These intrinsic shortcomings will constrain its applicationsin mass production of industry.Planar Lens Imaging NanolithographyPlanar lens imaging nanolithography is the method to usethe so-called “superlens” positioned underneath the mask toproject the nanopatterns through the mask onto the imageFig. 5 Schematic of gold nanoblock pairs. Reprinted with permissionfrom 39Fig. 4 Schematic diagram of the experimental lithography systemdesigned by Xu et al. Reprinted with permission from 38Plasmonics (2011) 6:565580569plane (normally in photoresist) which is about tens ofnanometers under the superlens to make patterns inphotoresist. The concept of “superlens” was theoreticallyproposed by Pendry first 43. He predicted that a slab ofnegative refractive index material (NIM) has the power toproject the near-field image with subdiffraction limit ontothe image plane because the negative index medium canbend light into a negative angle with respect to the surfacenormal. The property of NIM can make a slab of NIM a“superlens”. The principle of superlens is illustrated inFig. 7. Light formerly diverging from a point source is setin reverse and converges back to a point when transmittingthrough the superlens as illustrated in Fig. 7a. The near-field evanescent waves can be strongly enhanced across thelens as illustrated in Fig. 7b 44. This enhancement isassisted by the excitation of surface plasmons on the metalfilm. The materials with negative refractive index do notexist in nature. Although many approaches have emerged toobtain artificial NIMs, such as photonic crystals 45, 46and metamaterial nanostructures 4752, there are stillchallenges for the NIM to be applied for photolithographyso far. Professor Pendry proposed that the ultrathin Ag filmwhose permittivity has a negative real part and smallpositive imaginary part can be employed as the superlens.The physical mechanism of this type of superlens is that theimaginary part of the metal film was ignored because theabsolute value of real part is far larger than that of theimaginary part 53. The small imaginary part means thatthe amount of the absorbed light is too less to be consideredin practice. The superlens is capable of focusing all Fouriercomponents from the objects onto a 2D image with aresolution far beyond the diffraction limit 54. Mostsuperlenses were put forth based on the condition that thesuperlens and the surrounded dielectric have a matchedpermittivity (Re(m)=d) at the working wavelength,where the mand ddenotes the permittivity of metal anddielectric, respectively 55. For different superlens materi-als, different wavelengths are chosen so that the superlens isindex matched to dielectric medium. Therefore, the super-lenses based on bulk metal can only operate at a singlefrequency for the given surrounded dielectric. However,Cai et al. proposed a new superlens which composes ofmetal and dielectric and the superlens can operate atpractically any desired wavelength in the visible and near-infrared ranges by tuning the metal-filling factor 56. Thisproposal has been verified by the simulations but not beendemonstrated in experiment yet. Ag is an ideal metalmaterial in UV range which can be used as the superlens forits low absorption. Experiments have shown that evanes-cent waves can be enhanced significantly in Ag slabs 57.It is predicted that the image with a resolution of aboutl/20 can be realized in principle with the proper superlenses58. But the metal absorption in the lenses and thedispersive nature of the lenses limited the spatial resolution.The spatial dispersion will remain and limit the resolutioneven if the absorption is reduced or compensated by theFig. 6 a The setup designed byUeno et al. b SEM images ofperiodical pits in the photoresist.Reprinted with permissionfrom 39Fig. 7 a An NIM flat lensbrings all the diverging raysfrom an object into a focusedimage. b The NIM can enhancethe evanescent waves across thelens. Reprinted with permissionfrom 44570Plasmonics (2011) 6:565580optical gain 55. Therefore, it is difficult to achieve thetheoretical resolution in practice.Many experiments have been implemented to confirmthis prediction. Melville demonstrated optical imagingthrough a thin planar Ag layer with a thickness of 120 nmusing a light source with a wavelength of 341 nm in 200459. Feature size as small as 350 nm (with 700 nm period)was well resolved. Subwavelength imaging was notdemonstrated because the Ag lens is not thin enough. In2005, a modified system with a thinner lens with athickness of 50 nm was demonstrated to be able to resolvea subwavelength image 60. Where, the superlens structureconsists of 25 nm PMMA/50 nm Ag/10 nm SiO2.A 350 Wmercury lamp with a 365 nm wavelength, which gives anintensity of 6.7 mW/cm2was employed as the light source.The substrate is a 1-in. diameter silicon wafer. The image ofa grating with a period of down to 145 nm has beensuccessfully resolved. Numerical simulation shows that thesmallest resolution that can be achieved using this methodis 40 nm. A comparison between 50 nm single- and double-layer lens with two 30-nm layers was made by Melville in2005 61. Gratings with 170 nm period have been resolvedfor the double-layer lens. The results demonstrated thatdouble-layer lens has a shorter exposure time. Theenhancement of the transmission has been achieved throughthe double-layer stack despite the increase in total thick-ness. The resolution limit for the double layer is no lessthan a single layer with the same total Ag thickness.However, double-layer lens does not always lead to betterimaging performance because of two reasons. One is thatthe lens has a specific resonance and the feature of theimage will be distorted at spatial frequencies out ofresonance. The other is that the double-layer lens has anincreasing attenuation of the DC component of transmittedimages, which reduces the image fidelity, particularly fordark-line features 62.In 2005, Zhang et al. demonstrated subdiffraction-limited imaging with a 60 nm half-pitch resolution 63.The setup of their experiment is shown in Fig. 8. As can beseen in Fig. 8, a set of embedded objects are inscribed intothe 50-nm thick chrome (Cr) film. Left side is an array of60-nm wide slots with a 120-nm pitch, and right side is anarbitrary object “NANO”. The line width of the “NANO”object (illustrated in Fig. 9a) is 40 nm. The Ag film isseparated from the objects by a 40-nm thick PMMA layer.Following the PMMA layer is photoresist. A UV light witha wavelength of 365 nm is employed as the light source. Acontrol experiment in which the Ag film is replaced byPMMA was conducted as well for comparison. A 120-nmthick negative photoresist NFR 105 G, Japan SyntheticRubber Microelectronics (JSR Micro) is coated on Ag filmto record the near-field image. The substrate is thenexposed under an I-line filtered mercury lamp with flux of8 mW/cm2and an optimal exposure time of 60 s. Using thismethod, the arbitrary pattern of “NANO” (illustrated inFig. 9b) and 1D lines with width of 60 nm (aboutl/6;illustrated in Fig. 10a), with typical average height of 510 nm, was fabricated in photoresist. The control experi-mental results are shown in Figs. 9c and 10b. To make acomparison, the cross-sectional line width of letter “A”patterned in both experiments was measured. In theexperiment with the superlens, the line width was about89 nm while in the control experiment; the full width athalf-maximum line width is 32110 nm. Apparently, theline width obtained with the superlens beats diffractionlimit but is still larger than the width of the objects in themasks. The experimental results clearly demonstrated theimaging improvement using the superlens. The same setupwas utilized to realize the imaging of a 50 nm half-pitchobject atl0/7 resolution 64.Currently, the best resolution achieved by the planarlens lithography is about 30 nm. And such a resolutionhas been demonstrated by Du et al. and Chaturvedi,respectively, using different methods. They employed thelocalized surface plasmon mask composed with poly-dimethylsiloxane (PDMS) soft mold and thin Ag filmserving as the “superlens” to realize a 30-nm resolution23. The PDMS was selected as the mask material due toits two unique advantages: the transparent property fortransporting light with a high transmittance and the softproperty which assures the intimate contact between themold and the metal film. TM polarized UV light withwavelength of 365 nm was used to expose the resist. AnAg layer with a thickness of 18 nm served as the superlensand resist (AR-3170) with a thickness of 100 nm wasused. After 15-s exposure and 20-s development, the finegroove patterns with a dimension of 30 nm (about 1/12 ofthe exposing wavelength) and a depth about 20 nm wereFig. 8 Optical superlens designed by Zhangs group. Reprinted withpermission from 63Plasmonics (2011) 6:565580571obtained in photoresist. The patterns with a 30-nm widthare illustrated in Fig. 11.Another method using smooth and low loss Ag opticalsuperlens capable of resolving features at 1/12 of theillumination wavelength with high fidelity has beendemonstrated by Chaturvedi et al. 65. The setup used bythem is illustrated in Fig. 12a, where an array of chromegratings with 40 nm thick and 30 nm half-pitch serve as theobject. The Cr grating was patterned using nanoimprintlithograph method. Then, a 6-nm thick spacer layer wasdeposited on top of the object as the planarization layer toreduce the surface modulation below 1.3 nm. To enhancethe contrast, a 35 nm thick Cr window layer is patterned ontop of the spacer layer using photolithography. Then 1 nmthick Ge and 15 nm thick Ag are evaporated over theFig. 9 a FIB image of “NANO” object. b The developed image withsilver superlens. c The image in the control experiment. Reprintedwith permission from 63Fig. 10 a 1-D image patternedwith the superlens. b 1-D imagepatterned without the superlens.c The averaged cross-sectionalprofile of Fig. 10a. d Theaveraged cross-sectional profileof Fig. 10b. Reprinted withpermission from 63Fig. 11 SEM photography for a lithographic result fabricated byusing the LSP lithographic method with the PDMS soft mold in 2 and0.8 m line width. The inset is a magnified image highlighting theachieved minimum feature size of 30 nm. Reprinted with permissionfrom 23572Plasmonics (2011) 6:565580window layer followed by coating with a thick layer ofphotoresist (NOA-73). The Ge layer is introduced toimprove the Ag film surface morphology and make thesurface roughness less than 0.8 nm. UV-LED light with awavelength of 380 nm and a power of 80 mW wasemployed as the light source. In their experiments, 30 nmhalf-pitch patterns were obtained in photoresist with anexposure time of 120 s. The experiment result is illustratedin Fig. 12b.Lately, Zhong Shi proposed a method using a 193 nm Alfilm-based superlens with index-matching layer. Theirsimulation indicated that 20-nm resolution can be resolvedby the introduction of an “index matching layer” betweenmetal layer and the dielectric layer 66, 67. The proposed193-nm superlens may provide an alternative way to reachthe 22-nm lithography node. In addition, Xu et al. proposeda metal-cladding superlens to effectively localize thesurface plasmons for projecting deep subwavelength pat-terns 68. In this configuration, an additional Ag layer wasinserted between the photoresist and the substrate. With thisspecial configuration, both the intensity contrast of theinterference pattern and the image can be remarkablyimproved.For planar lens nanolithogrpahy, the light source is animportant factor affecting the experimental results. Acomparison about the illumination light with broadbandand narrowband has been made experimentally byBlaikie. The results show that broadband light sourceoffers the advantage of shorter exposure time and higherthroughput while the depth of the pattern in the resistand the degree of line edge roughness are almost thesame as that obtained by narrowband source. However,the resolution of the former is not good as that of thelatter due to the dispersive nature of the broadband lightsource which will result in the aberrations of the planarlens 61. Therefore, the choice of light source is crucialfor planar lens lithography experiment.The near-field planar lens lithography relaxes the rigidrequest for intimate contact between mask and substrate toa certain extent. Therefore, damage to the mask due to theintimate contact can be reduced. But the superiority of thetechnique is still restricted because the image plane islocated in the proximity of the superlens (normally theworking distance is a few tens of nanometers or even less).Hence, the far-field superlens which can form subdiffrac-tion limit image in far field of the superlens was proposedand experimentally demonstrated by Liu et al. 69. The far-field superlens consists of a conventional superlens and ananoscale coupler. The coupler can convert evanescentwave into propagating wave by shifting the incident fieldwave vector into various diffraction orders and selectivelyenhance the evanescent waves from the object 70. In theirexperiment, an optical microscope (Zeiss Axiovert mat 200,100 oil immersion objective, NA=1.4) was used toilluminate the object with a 377-nm wavelength lightsource, combining with the far-field superlens, 50-nmwidth lines separated by 70 nm has been recorded byCCD. The experiment has successfully demonstrated theimaging ability of the far-field superlens, showing thepotential to be applied to nanolithography.Planar lens lithography has the advantage over thecontact lithography at some aspects. However, it is stillmask-based nanolithographpy technology. Therefore, it hasall the disadvantages brought by the masks. In the nextsection, we will introduce the maskless nanolithographymethod based on plasmonics, i.e., plasmonic direct writingnanolithography method.Plasmonic Direct Writing NanolithographyThe techniques mentioned above are all mask-basedlithography. As well-known, the fabrication of the maskwith nano-feature size is complicated, time consuming, aswell as with very high cost. It was reported that the costwith a set of mask for developing a new product for 193-nm immersion lithography is normally more than 1 milliondollars 71. Moreover, mask-based lithography is notflexible because it is only able to fabricate the specificFig. 12 a Schematic drawingof a smooth silver superlenswith embedded 30 nm chromegrating designed by Chaturvediet al. b The image of the 30 nmhalf-pitch Cr grating arearecorded on the photoresist layerafter exposure and development.Reprinted with permissionfrom 65Plasmonics (2011) 6:565580573patterns defined by mask. Considering the aforementioneddisadvantages, more and more attention is paid to direct-writing lithography which is more flexible and can be usedto fabricate arbitrary patterns. Scanning probe lithography7277 is an important lithography technique belonging todirect writing lithography. The critical component ofscanning probe lithography is the scanning probe 78,which is used to transmit light toward the tip of a taperedplasmonic waveguide to expose the resist 79. Varioustypes of probes have been investigated. Solid immersionlens can also be used as the scanning probe 71, 80, 81.The scanning microscope probe is the most commonly usednear-field scanning probe 82, 83. Conventional scanningprobe lithography utilizes optical fiber with tapered tip asprobe. The scanning microscope probe, which is coatedwith a metal to prevent leakage of the electric field laterallythrough cone section of the probe, was employed as thescanning probe to expose the photoresist beneath the probetip 8486. Some experiments demonstrated that metal thinfilm under the probe can also be etched utilizing the photo-thermal effect 8789. The photo-thermal effect occursowing to absorption of the light with high-energy density90. Conventional scanning probe lithography, both usingsolid immersion lens and scanning microscope probe, hastheir intrinsic disadvantages. For the solid immersion lens,the resolution would be limited by the diffraction limit andfor the scanning microscope lithography, the transmittanceefficiency of the aperture is so low that the throughput ofpatterning is too low to be applied for industrial applica-tions 91. Plasmonic direct writing lithography, which is apromising technique due to its subdiffraction resolutionlimit, can be defined as an extension of the scanning probelithography. In this technique, resist is scanned and exposedby the superfocusing light spot focused by the plasmoniclenses. Many plasmonic direct writing systems are anintegration of conventional probe (solid immersion lens orscanning microscope probe) and plasmonic subwavelengthdevice. Zhangs group demonstrated a practical plasmonicnear-field scanning optical microscopy system (NSOM)experimentally for near-field lithography for the first time92. The conic plasmonic lens demonstrated by themconsists of a subwavelength aperture at the apex of the conesurrounded by concentric through rings in an Al thin filmdeposited on a tapered fiber tip, as shown in Fig. 13. Thecone angle was set to be 75 and simulation results showthat the optimal sizes for aperture diameter, ring periodicity,ring width and Al layer thickness are 100, 300, 50, and80 nm, respectively. The working wavelength was selectedas 365 nm. During the lithography, the laser beam wascoupled into the near-field scanning microscope tip toexpose the positive photoresist (IX965G, JSR Microelec-tronic Inc.) on a Si substrate. By using this method, tightfocus of approximately a 100-nm beam spot was obtainedat the center of the circular grating. Simulation results showthat light intensity at the focal point was enhanced 36 withrespect to the intensity through a single aperture due toconstructive interference of SPPs. The NSOM probe canalso be integrated with ridged apertures such as C-shape, H-shape, bowtie-shaped apertures, and so on. This type ofintegration requires a flat surface on top of the NSOMprobe. The NSOM probe with a bowtie aperture has beendemonstrated recently 93, 94. Experiments have shownthat bowtie apertures can both increase the transmissionefficiency and shrink the focused light spot size. Murphy-Dubay et al. reported their experimental demonstration offield enhancement of bowtie-shaped apertures integratedwith NSOM probe and consistent lines with width of assmall as 24 nm were written on resist using the NSOMprobes integrated with the bowtie apertures 95. In thisexperiment, 800 nm Ti: sapphire femtosecond laser wasemployed as the light source and thin Al film with athickness of 120 nm was selected as the bowtie aperturematerial due to its small skin depth and high reflectivity.But in the experimental process, the probe is required to bein intimate contact with the substrate, which is detrimentalto the probe because it will cause abrasion. Althoughutilization of the probe integrated with a metal film canprovide a satisfying result, it has inherent shortage ofthermal effect due to the thermal expansion caused by thepartial energy absorption by the metallic film, whichprevents it from some specific applications 96, 97.Kim et al. proposed a plasmonic lithography techniquewith bowtie aperture contact probes 98. The bottomsurface of the probe is covered with a 10-nm thick silicaglass film for the gap control. In this method, a relativehigh-speed patterning without external gap distance controlcan be realized. The experimental setup is illustrated inFig. 14. The bowtie aperture with the smallest gap of 20 nmwas employed to excite plasmon wave. In this experiment,a conical shape solid immersion lens with a flat surfacewith 30 m diameter on top was employed. A 120-nm Alfilm was coated on the flat surface and a thin Cr layer of2 nm thickness is added to ensure the adhesion between theAl film and the glass substrate. Bowtie nano-apertures withabout 140-nm outline dimension were formed on the metalfilm by use of focused ion beam (FIB) lithography. A 300-Fig. 13 Schematic drawingof the conic plasmonic lens.Reprinted with permissionfrom 92574Plasmonics (2011) 6:565580nm thick silica glass film was deposited on the Al film witha subsequent process of FIB milling to reduce the silicaglass film thickness down to 10 nm. The use of the silicaglass is not only for the control of the gap between theprobe and photoresist but also for the protection of aperturefrom contamination. A single atom self-assembled film wascoated on the surface of SiO2film to reduce the frictionbetween the probe and the photoresist during scanning. Inthis experiment, the probe is in intimate contact with resistby means of applying a certain pressure (132.5 N) whilethe substrate was fixed on nanometer-precision stage. Apositive photoresist (Shipley S1805) with 400-nm thicknesswas coated on the substrate. In this experiment, the solidimmersion lens is coupled with a high NA (0.8) objectivelens (Nikon CFI LU Plan Epi ELWD 100) to focus adiode laser beam of 405 nm wavelength onto the nano-aperture. The spot size focused by the solid immersion lensis estimated to be 340 nm. Then the nano-aperture excitessurface plasmons to expose the photoresist. The lasersource (CrystaLaser, BCL-025-405S) has a polarization-maintaining optical fiber to keep the degree of thepolarization at about 0.98. The nano-patterns can befabricated in resist by scanning the stage beneath theoptical probe. The smallest line width of 50 nm illustratedin Fig. 15a can be realized when the polarized laser lightwith 405 nm wavelength and a power of 0.5 mW wasemployed using this method. When the laser power is set as1.25 mW, the lines with a width of 150 nm and a pitch of1 m can be fabricated, as shown in Fig. 15b. The systemhas a highest throughput of 10 mm/s.Srituravanich et al. in the University of California,Berkeley reported another promising new method with ahigh throughput 99. The technique is to use a flyingplasmonic lens to focus the surface plasmon wave on asubstrate rotating with high speed. The principle of theirexperiment is illustrated in Fig. 16. In this experiment, aUV continuous-wave laser was focused down to a spot ofseveral micrometers onto a plasmonic lens, which is thenfurther focused into a sub-100 nm beam spot to expose thespinning disk for writing arbitrary patterns. An inorganicTeOx-based thermal photoresist deposited on a glass diskwas employed. A setup called “plasmonic flying head”, onthe back surface of which is the fabricated plasmonic lens,was employed as the direct writing probe. The plasmoniclens consists of a concentric ring grating with a throughhole in the center. The plasmonic lens was illustrated inFig. 17. As the subwavelength spots are only produced atthe near field of the plasmonic lens, the distance betweenthe plasmonic lens and resist has to be controlled at about20 nm. To meet this request, a method of a self-spacing airbearing that can fly the plasmonic lens at just 20 nm abovethe substrate was adopted. The substrate is spinning atFig. 14 a The structure of thedirect writing probe designed byKim et al. b A bowtie aperturedesigned with silica glass as themap material. Reprinted withpermission from 98Fig. 15 a A single line of50 nm width recorded at speedof 10 mm/s. b Multiple linepattern of 150 nm width with1m pitch. Reprinted withpermission from 98Plasmonics (2011) 6:565580575speeds of between 4 and 12 ms1. The experimentdemonstrated that the distance can be controlled within202 nm by using this method. Apart from the rigiddistance control, the parallelism of the plasmonic lensand the substrate is also needed to be controlled preciselyto ensure the exposure uniformity. A line width of 80 nmwith the writing speed of 10 m/s has been demonstrated.According to the authors, if 1,000 plasmonic lens wereadopted at the same time, a 12-in. wafer can beprocessed in just 2 min. Therefore, a high throughputof 30 wafer/h can be achieved.Recently, the same research group integrated the lithog-raphy system with another plasmonic lens for the purposeof photolithography 100. The plasmonic lens adopted isshown in Fig. 18. The metallic thin-film structure consist-ing of two concentric rings with an H-shaped aperture intheir center is used as the plasmonic lens. With the 355 nmlaser beam illumination, 50 nm lines width can be resolvedon the TeOxbased thermal resist with a speed of about10 m/s. It is said that the flying head in this work can carryup to 16,000 lenses within 2 nm gap tolerance. So thethroughput can be greatly enhanced.In addition, plasmonic interference lithography, which isan important part of plasmonic part, is another masklesslithography technique. Conventional plasmonic interferencelithography scheme created on the basis of attenuated totalreflection-coupling mode has three important components:a high refractive index prism, a metal layer, and thephotoresist coated on substrate. A high refractive index ispreferred because the higher refractive index of a prismleads to a smaller critical resonance angle. And it wasdemonstrated that the finer experimental results are easy toachieve at smaller incident angle 101. Using this type oflithography system, 1D patterns using two-beam interfer-ence and 2D patterns using four-beam interference havebeen demonstrated 101111. The interferential beams areFig. 18 SEM picture of the plasmonic lens adopted by Pan et al.Reprinted with permission from 100Fig. 17 SEM image of an array of plasmonic lens. Reprinted withpermission from 99Fig. 16 a The schematic showing the plasmonic lens focusing thelight onto the rotating substrate. b The plasmonic head flying 20 nmabove the rotating substrate. c Schematic of process control system.Reprinted with permission from 99576Plasmonics (2011) 6:565580generally constituted from the first order diffraction lightgenerated from the grating diffraction. In the conventionalinterference lithography, the photoresist and metal layermust be in contact closely. Therefore, the surface of themetal film and resist film are easily damaged or polluted.Furthermore, oxidation of the metal film is another problemdue to exposure in air. To overcome these disadvantages,He et al. proposed and experimentally demonstrated a newapproach based on backside-exposure technique 112. Thestructure of this system composes a prism, matching fluidlayer and glass substrate, Ag film and resist layer. This newstructure not only prevents the damage and pollution butalso eliminates the high refractive index prism from thesystem. Using this method, interference fringes with featuresize below 65 nm were experimentally obtained. Thethroughput of interference lithography is quite high andthe image quality is acceptable. But it is applicable only toperiodic and quasiperiodic patterns, which limits itsapplications 111.Finally, we supplemented a quantum effect-based newoptical method which can realize the subwavelengthlithography 113. The method is similar to the traditionallithography but adding a critical step before dissociating thechemical bound of the photoresist. The subwavelengthpattern is achieved by inducing the multi-Rabi oscillationbetween the two atomic levels. The proposed method doesnot require multiphoton absorption and the entanglement ofphotons. Initially, the molecules are in the ground state.Then two laser pulses with different frequencies weresequentially turned on. The first laser pulse, whosefrequency is resonant with the energy difference betweenthe two ground states, will induce Rabi oscillations betweenthese two states. After that, the second laser pulse wasturned on, and it will only dissociate the molecules that arein the excited states but not those in the ground states. Themolecules that are dissociated will change their chemicalproperties, especially the solubility. The resulting patternsof the photoresist will thus depend on the spatial distribu-tion of the excited state induced by the first laser pulse.It is worthy to point out that as an alternative lithographytechnique, extreme-ultraviolet lithography at 13.5 nm isexpected to be possibly introduced in high-volume semi-conductor chip production over the next 3 years. Researchis now underway to investigate sub-10 nm light sources thatcould support lithography over the coming decades 114.An extension of this technology could be an alternativeoption for next-generation sub-13.5-nm lithography. Manyof the technical issues, such as target regeneration and high-repetition rate operation, are already solved. However,improving the low conversion efficiency (about 0.5% for2.48 nm) remains a critical point. Moreover, high expen-diture is a drawback issue in comparison to plasmonicnanolithography technique.Comparison Between Different Plasmonic LithographyMethodsThe three nanolithography methods have been reviewed indetails in above sections. Below gives a comparison ofthree different plasmonic lithography methods.Generally speaking, the plasmonic contact lithographycan get a higher resolution using masks with ridged shapeapertures. Although large-area lithography can be realizedthrough the step and repeat exposure and the production isrelatively high, it still cannot satisfy the industrial require-ment. In addition, the fabrication of conformable masks iscomplex and the cost is rather high. Intimate contact, whichis assured by the external force sometimes, could cause thesevere damage on the mask and substrate. The complexityin conjunction with the rigid exposure requirement ofintimate contact between mask and substrate limits itsfurther applications in many aspects. Researches on the newkind masks, such as elastic masks, can be done toconsummate this method. Furthermore, the environmentshould be maintained rigidly clean in the process oflithography since any particles may affect the experimentresult and even destroy the experiment.In comparison to the contact lithography, the planar lenslithography relaxes the complexity of process controlwithout the requirement of intimate contact between themask and substrate. Thus, the masks and substrates can beprevented from being destroyed. But the superiority isrestrained by the limited working distance (a few tens ofnanometers or even less). Therefore, the research on thesuperlens with larger working distance should be made infuture. In addition, this method is relatively underdevelopedcompared with the contact lithography. It is commonlyknown that the resolution of this method is inferior toplasmonic contact lithography. Researches should be madeto enhance the resolution.Plasmonic direct writing nanolithography is consideredas the most promising technique for its flexibility and lowcost. In addition, it is a maskless lithography method.Therefore, it avoids all the complexity aroused by themasks. Arbitrary patterns with the resolution as small as50 nm (l/8,l=405 nm) has been demonstrated by usingthis method. In addition, plasmonic direct writing lithogra-phy can be implemented in air and there is no such rigidenvironment requirement as the plasmonic contact lithog-raphy. Without the requirement of vacuum environment, theexperiments are relatively easy to implement and it is agreat superiority of this method. Forming arbitrary patternsand working in normal environment are the two superior-ities of this method over former two lithography techniques.One of issues of the direct writing method is the lowthroughput. However, to expose the spinning substrate withflying plasmonic lens has the potential to achieve aPlasmonics (2011) 6:565580577throughput that is two to five orders of magnitude higherthan other maskless lithography and thus to be able tosatisfy the industrial requirement. However, issues forflying plasmonic nanolithography like pattern data man-agement, lithography linewidth control, pattern overlay, andresist defect reduction are still need to be addressed to applythis technology for nanomanufacturing industry.SummaryIn this article, we reviewed a technology of plasmonicnanolithography in three catalogs: contact nanolithography,planar lens nanolithography, and direct writing nanolithog-raphy. Some experiments about the three kinds of lithog-raphy techniques are presented and the comparison betweenthem is also made. From the analysis presented above, wecan conclude that the higher resolution is the uniquesuperiority of plasmonic lithography. 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