Nanostencil lithography for high-throughput ... - Semantic Scholar

Nanostencil lithography for high-throughput fabrication of infrared plasmonic sensors Serap Aksu 1,2, Ahmet A. Yanik 2,3 , Ronen Adato 2,3, Alp Artar 2,3, Min Huang 2,3, Hatice Altug 1,2,3* 1 Materials Science and Engineering, 2Photonics Center, 3Electrical and Computer Engineering, Boston University, Boston, MA, 02215, E-mail: [email protected] ABSTRACT We demonstrate a novel fabrication approach for high-throughput fabrication of engineered infrared plasmonic nanorod antenna arrays with Nanostencil Lithography (NSL). NSL technique, relying on deposition of materials through a shadow mask, offers the flexibility and the resolution to fabricate radiatively engineer nanoantenna arrays for excitation of collective plasmonic resonances. Overlapping these collective plasmonic resonances with molecular specific absorption bands can enable ultrasensitive vibrational spectroscopy. First, nanorod antenna arrays fabricated using NSL are investigated using SEM and optical spectroscopy, and compared against the nanorods with the same dimensions fabricated using EBL. No irregularities on the periodicity or the physical dimensions are detected for NSL fabricated nanorods. We also confirmed that the antenna arrays fabricated by NSL shows high optical quality similar to EBL fabricated ones. Furthermore, we show nanostencils can be reused multiple times to fabricate selfsame structures with identical optical responses repeatedly and reliably. This capability is particularly useful when high-throughput replication of the optimized nanoparticle arrays is desired. In addition to its high-throughput capability, NSL permits fabrication of plasmonic devices on surfaces that are difficult to work with electron/ion beam techniques. Nanostencil lithography is a resist free process thus allows the transfer of the nanopatterns to any planar substrate whether it is conductive, insulating or magnetic. As proof of the versatility of the NSL technique, we show fabrication of plasmonic structures in variety of geometries. We also demonstrate that nanostencil lithography can be used to achieve functional plasmonic devices in a single fabrication step, on variety of substrates. We introduced NSL for fabrication of nanoplasmonic structures including antenna arrays on rigid surfaces such as silicon, CaF2 and glass. In conclusion, Nanostencil Lithography enables plasmonic substrates supporting spectrally narrow far-field resonances with enhanced near-field intensities which are very useful for vibrational spectroscopy. We believe this nanofabrication scheme, enabling the reusability of stencil and offering flexibility on the substrate choice and nano-pattern design could significantly enhance wide-use of plasmonics in sensing technologies.

KEYWORDS: Shadow mask, nanostencil lithography, nanoplasmonics, optical nanoantenna, surface plasmons, nearfield effects, infrared spectroscopy.

1. INTRODUCTION Nanophotonics, optical studies of tailored dielectric and metallic nanostructures and their applications, has emerged as an exciting and rapidly growing field. In particular, plasmonics, optical study of tiny metallic structures, has taken significant attention. A new generation of plasmonic antennas operating at the optical and infrared frequencies is opening up a myriad of exciting possibilities by focusing light beyond the diffraction limit [1]. Enabling sub-wavelength light localization and dramatically strong local fields, plasmonic nanoantenna arrays have been used to enhance linear and non-linear optical phenomena including fluorescence, surface enhanced spectroscopy [2] and high-order harmonic generation. However, advances in nanoplasmonics are critically dependent on our ability to structure metals in a controllable way at high resolution. Although existing nanolithography techniques offer tremendous flexibility in creating large variety of nanostructure geometries and patterns at high resolution, their major drawback is the lowMicro- and Nanotechnology Sensors, Systems, and Applications III, edited by Thomas George, M. Saif Islam, Achyut K. Dutta, Proc. of SPIE Vol. 8031, 80312U © 2011 SPIE · CCC code: 0277-786X/11/$18 · doi: 10.1117/12.884105 Proc. of SPIE Vol. 8031 80312U-1 Downloaded from SPIE Digital Library on 14 Sep 2011 to 128.197.27.9. Terms of Use: http://spiedl.org/terms

throughput. In addition, the choice of substrates is also limiting factor for some widely used lithography techniques. These limitations have fueled our research on flexible nanofabrication methods that can be utilized for high-throughput nanopatterning. The most common top-down nanopatterning techniques with high resolution are electron beam and focused ion beam lithography. Electron beam lithography (EBL) is mostly used for on-chip plasmonic nanoparticle array fabrication, while focused ion beam (FIB) tools are reserved primarily to fabricate nanoapertures in metallic films. Both EBL and FIB lithography offer tremendous flexibility in creating large variety of nanostructure geometries and patterns at high resolution. However, their major drawback is the low-throughput. Due to the serial nature of these lithographic processes, each nanostructure has to be created one at a time, which is both slow and costly. For EBL, the choice of substrates is also limited due to the dependence of the e-beam exposure on the substrate conductivity. For example, patterning on glass based substrates require addition of a conductive layer (such as ITO), which may interfere with the optical responses of the fabricated nanostructures. EBL for plasmonic nanoparticle and nanowire fabrication often involves a lift-off process, which can be restrictive in creating nanostructures with high aspect ratios. Although, multilayer lithographic processes are offered to be solutions for this limitation, these processes are cumbersome due to the involvement of multiple fabrication steps. All these difficulties have fueled the research on flexible nanofabrication methods that can be utilized for high-throughput nanopatterning. One innovative approach is nano-stencil lithography (NSL). NSL [3, 4] is a shadow-mask patterning technique that can allow fabrication of structures down to sub-100 nanometers. The method relies on direct deposition of materials through a pre-patterned mask. The deposited material could be metal, dielectric and even organic (such as cells). Here, we demonstrate high-throughput fabrication of infrared plasmonic gold nanorod antenna arrays using nanostencil lithography. The collective exitations on antenna arrays can be utilized to create spectrally narrower far-field extinction resonances and far stronger near-field enhancements than what is achievable with that of the individual constituent nanoparticles [2]. We successfully confirm that stencil lithography can be effectively used to fabricate nanorod arrays supporting sharp collective plasmonic resonances with narrow linewidths at mid-infrared wavelengths. The reflection spectra of these antenna arrays have identical optical quality compared to EBL fabricated ones. More importantly, we show that nanostencil masks can be reused multiple times to create same nanoantenna arrays with identical optical quality. Our observation is confirmed in optical measurements as well as scanning electron microscopy images. This high-throughput nanofabrication approach, by enabling the re-usability of the stencil mask multiple times and offering flexibility on the substrate choice and the nano-pattern design, can significantly advance nanoplasmonics field and speed up the its transition into the applications such as ultrasensitive vibrational spectroscopy.

2. METHODOLOGY Stencil technique is actually well known in macro-world for arts and crafts, and decorative painting. Stencils are made to transfer a single pattern, like alphabet stencils. In positive stencils, first apertures complementary to the shape of the desired pattern are cut out and then paint, ink, or another agent is applied through the apertures. Here, the stencil behaves like a mask and shadows the applied agent around the pattern, at the end giving the shape complementary to the apertures. Nanostencil lithography is basically exploiting the same notion at nano-scales. To create gold nanorod antenna arrays, we fabricate a nanostencil with rectangular shaped nanoaperture arrays, place on a substrate and then deposit gold through the nanoapertures. When the nanostencil is removed, we get gold nanorod particles on our substrate with shapes complementary to the nanoapertures used in the stencil. Distinct feature of this technique is that any pattern will be transferred positively to any surface in a single-step process. Existing lithography uses resist base patterning, which means extra chemical lift-off process is needed. More importantly, like stencils in macro-world, nanostencils could be reused many times to repeatedly fabricate the same nanorod arrays. This capability is particularly useful when high-throughput replication of the optimized nanoparticle arrays is desired. As we show in our paper [3], series of plasmonic nanoantenna arrays created from a single nanostencil gives nearly identical optical responses. In the following, we will explain the fabrication procedure in detail. Nanostencil technique that we employed for the parallel fabrication of plasmonic nanostructures is summarized in Figure 1&2. This fabrication technique consists of three steps: (i) fabrication of the free standing membrane, (ii) patterning of the mask and (iii) direct deposition of

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metallic plasmonic devices. Processing steps for free standing membrane fabrication is illustrated in Figure 1. An important consideration here is the mechanical strength of the membranes. Mechanically highly robust Low Pressure Chemical Vapor Deposition (LPCVD) SiNx films are excellent choice [3]. We start with 550 µm thick silicon wafer coated with 400 nm thick LPCVD SiNx on double sides. After cleaning with organic solvents, 2 µm thick MICROPOSIT™ S1818™ positive photoresist is spin coated. 800 µm x 800 µm apertures on the bottom SiNx layer are defined by photolithography using UV exposure with SUSS MicroTec MA/BA6 Mask Aligner and reactive ion etching (RIE) with Plasma Therm 790 RIE/PECVD System. Then, the chips are immersed in KOH solution to selectively etch Si layer from the backside. Finally, ~200 µm x 200 µm and 400 nm thick free standing SiNx membranes are obtained once the etching stops at the top SiNx layer of the wafer .

Figure 1. Nanostencil lithography, a shadow-mask patterning technique, is used for high-throughput fabrication of plasmonic antenna arrays. Fabrication of free standing membrane and nanostencil is illustrated from 1 to 4. Dry and wet etching processes and EBL are used for achieving precisely defined pattern of nanoapertures/slits on the membrane.

Si is etched with 54.7º angle side-wall profile as shown at Fig.1, step 2. Prior to patterning, the membranes are thinned down to ~100 nm with RIE. Nanoaperture patterning is performed using electron-beam lithography. This process is only needed once for the creation of the mask, and the mask can be used multiple times. The process involves spinning of positive e-beam resist poly (methylmethacrylate) (PMMA) followed by e-beam exposure using Zeiss Supra 40VP with GEMINI electron-optics column. After development of PMMA resist, patterns are transferred to the membrane by RIE dry etching of SiNx with mixture of SF6 and Ar gases. The resulting structure acts as stencil. Final part of our nano-stencil method involves direct deposition of the plasmonic structures to the desired surface. For highest quality of the structures, the gap between substrate and mask must be minimized. The stencil is directly put on the substrate and secured tightly with clips so that the silicon side is facing up, while the patterned SiN layer is nominally in contact with the substrate. Here, the toughness of the LPCVD SiNx layer plays important role for the durability of the mask. Directional e-beam gold deposition at 3x 10-6 Torr is performed in CHA-600S e-beam evaporator for 100 nm gold film without depositing any prior gold adhesion layer (Fig 2). Since NSL does not require metal lift-off processes, an adhesion layer is not necessary. When the mask is removed from the substrate, it leaves plasmonic nanostructures on the substrate with the shapes complimentary to the nanoapertures.

Figure 2. On the left, SEM images of the nanostencil and fabricated nanoantennas are shown. On the right, gold deposition scheme with reusable mask is illustrated.


3.

RESULTS

Nanorod arrays with 1100 nm length, 230 nm width and 100 nm height fabricated by NSL technique on silicon are investigated using SEM and compared against the nanorods with the same dimensions fabricated using EBL. No irregularities on the periodicity or the physical dimensions are detected for nanorods in the array fabricated using NSL. For optical characterization, reflection spectra are obtained from the NSL fabricated arrays and the results are compared with the spectra of the structures fabricated using EBL. The spectral linewidth of the resonances of the antenna arrays fabricated by NSL is comparable to that of the arrays fabricated using EBL. This observation indicates high optical quality of nanorod arrays fabricated using NSL. In this work, we tune the plasmonic resonances to the amide-I and amide-II vibrational bands of the protein backbones, which are around 1550-1650 cm-1. In a recent paper, we have shown that nanorod antenna arrays fabricated using EBL could enable detection of vibrational signatures of proteins at zepto-mole sensitivity levels [2]. We also achieved detection of proteins with nanorod antennas fabricated using NSL. Further studies on these NSL fabricated antennas for spectroscopic application are currently underway in our research group.

Figure 3. a) SEM images of nanorods with 1100 nm length, 230 nm width and 100 nm height fabricated using NSL (left) and EBL (right) are shown. b) Nanorods fabricated using two different techniques give identical reflection spectrum with resonances at ~1700 cm-1.

The unique advantage of the nanostencil lithography is that stencils can be reused for multiple times by simply removing the metal residue with a mild metal etchant. This capability is particularly useful when high-throughput replication of the optimized nanoparticle arrays is desired. Our SEM images reveal that there is no sign of degradation and deformation on stencil after its fourth usage. We also checked the reflection spectrum of the four different series of nanorod arrays fabricated on different silicon chips by using the same stencil. The resulting spectra for all the structures have very similar spectral profiles. They show strong resonances around 1700 cm-1 with deviations in the spectral peak position less than 3.5%. Our observations clearly indicate that with a single stencil, optimized designs can be replicated many times with a high degree of plasmonic antenna uniformity and similar optical response. In addition to its highthroughput capability, NSL lithography enables fabrication of plasmonic devices on surfaces otherwise difficult to work with electron/ion beam techniques. We demonstrated [3] high optical quality responses of plasmonic nanorod antennas on glass and CaF2 due to high transmission properties of these substrates in visible and mid-IR, respectively.


Figure 4. Mask can be reused multiple times. a) SEM images of the same mask are shown before its first usage (left) and after the fourth usage (right). Apertures have dimensions of 1050 nm length, 200 nm width and 100 nm height. No sign of degradation and deformation is observed on the mask after fourth usage. b) Reflectance spectra are shown for different nanorod arrays obtained from four consecutive depositions using the same mask. The resulting spectra show negligible deviations on resonances (3.5 %) around 1700 cm-1.

4. CONCLUSION In conclusion, plasmonic devices fabricated using NSL technique are shown to have comparable optical characteristics with respect to the arrays fabricated by EBL. Moreover, we show the stencil can be reused for high-throughput fabrication of antenna arrays. We also show that NSL offers flexibility and resolution for fabrication of radiatively engineer nanoantenna arrays for excitation of collective plasmonic resonances. These excitations, by enabling spectrally narrow far-field resonances and enhanced near-field intensities, are highly suitable for ultrasensitive vibrational nanospectroscopy [2]. We believe this nanofabrication scheme, enabling the reusability of stencil and offering flexibility on the substrate choice and nano-pattern design could significantly enhance wide-use of plasmonics in sensing technologies.

5. REFERENCES [1] Ozbay, E. “Plasmonics: Merging Photonics and Electronics at Nanoscale Dimensions” Science, 311, 189-193 (2006). [2] Adato, R.,Yanik, A., Amsden, J.J., Kaplan, D.L., Omenetto, M.K.H., Erramili, S., Altug, H., "Ultra-sensitive vibrational spectroscopy of protein monolayers with plasmonic nanoantenna arrays", Proceedings of National Academy of Science (PNAS) 106, 19227-18232 (2009). [3] Aksu, S., Yanik, A., Adato, R., Artar, A., Huang, M., Altug, H., "High-throughput Nanofabrication of Plasmonic Infrared NanoAntenna Arrays for Vibrational Nanospectroscopy", Nano Lett., 10 (7), pp 2511–2518 (2010). >@9D]TXH]0HQD29LOODQXHYD*6DYX96LGOHU.YDQGHQ%RRJDDUW0$)DQG%UXJJHU-´0HWDOOLF1DQRZLUHV E\)XOO:DIHU6WHQFLO/LWKRJUDSK\´1DQR/HWWHUV SS
