Monolithic Integration of Continuously Tunable Plasmonic

LETTER pubs.acs.org/NanoLett

Monolithic Integration of Continuously Tunable Plasmonic Nanostructures Nathan C. Lindquist,† Timothy W. Johnson,† David J. Norris,‡ and Sang-Hyun Oh*,† † ‡

Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States Department of Mechanical and Process Engineering, ETH Z€urich, Z€urich, Switzerland

bS Supporting Information ABSTRACT: We demonstrate precise three-dimensional integration of smooth bumps, grooves, and apertures in optically thick metal films using template stripping. Patterned silicon wafers are used as high-quality, reusable templates. The heights or depths of the metallic features are controlled to within 2 nm, giving continuously tunable optical properties with sharp and intense plasmonic resonances. Furthermore, we demonstrate a pick-and-place template stripping method in situ, enabling versatile three-dimensional micromanipulation, imaging, and characterization of nanoscale devices. KEYWORDS: Template stripping, plasmonics, focused ion beam (FIB), grating, trapped rainbow

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he ability to engineer precise metallic nanostructures has many important applications in plasmonics1 4 such as enhanced transmission,5,6 biosensing,7 11 data storage,12,13 waveguiding,14 16 nanofocusing of light,17 19 surface-enhanced spectroscopy,20 23 optical antennas,24 26 solar cells,27,28 and photon sorting.29 To generate and manipulate surface plasmons,30 typical surface patterning consists of subwavelength bumps, grooves, and apertures. The excitation of surface plasmons by these nanostructures largely determines their optical behavior, with the electromagnetic fields tightly confined to the surface. Because of this, the smoothness and purity of the patterned metal nanostructures is critical.31 Even nanometric surface roughness is enough to severely degrade the performance of a plasmonic device.18 Due to this extreme sensitivity to conformation and local geometry, practical implementation has been challenging. Until recently, the metals that are required for plasmonics have not been amenable to large-area patterning at the required nanometric resolution because as-deposited metal films are inherently rough and no simple, high-throughput method existed to pattern these metal films over a large area. Standard fabrication techniques such as direct-write electron-beam or focused ion beam32 (FIB) lithography cannot be scaled to low-cost, large-area patterning. Furthermore, direct FIB milling, while also implanting ion impurities, worsens the roughness of metals due to grainorientation-dependent sputtering rates. Although techniques such as nanoimprinting,33 nanotransfering,34 and nanomolding35 can easily pattern metals over large areas, these techniques have not been suitable due to low throughput, if the molds are etched away, or due to surface roughness from poor wetting of metals on polymers. To address these problems, we have shown that r 2011 American Chemical Society

template stripping36 is able to pattern ultrasmooth metals over large areas.31 However, high-throughput fabrication and monolithic integration of complex three-dimensional shapes with precisely configured geometry both above and below or even through an optically thick metallic film remains a significant challenge. In this paper, we demonstrate precision three-dimensional patterning of bumps, grooves, and apertures in optically thick metallic films obtained via template stripping.31,36 Patterned silicon wafers are used as master templates (Figure 1a). With mature silicon fabrication technology, a variety of templates can be fabricated. With subsequent metal deposition on the template (Figure 1b), a smooth surface at the metal silicon interface is formed, which can be peeled off using a backing layer such as epoxy or electrodeposited metal foils (Figure 1c,d). The smoothness of the metal nears that of the silicon mold. Importantly, the mold can then be reused for mass production. With this process, high-fidelity replication and monolithic integration of bumps, grooves, and apertures is possible. We also show that precise tuning of the height or depth of a metallic bump or groove, even on 1 2 nm length scales, significantly changes the optical properties of the device. Such continuous and fine control over the local geometry is essential for the rational design of practical plasmonic devices. To characterize or engineer these devices, it is important to have free access to both sides of the metallic film. Figure 2 demonstrates our template-stripping method in situ by using a sharp tungsten needle micromanipulator. This technique offers an arbitrary series of copy, cut, move, rotate, and paste functions. Within a FIB chamber, the tungsten needle was welded to a Received: February 17, 2011 Published: August 11, 2011 3526

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Nano Letters 300 nm thick silver film deposited on a silicon template. With FIB-induced deposition of platinum,37 the weld is strong enough to peel out a 30 μm long strip (Figure 2a,b) which can be rotated and imaged in situ. This silver foil can then be placed elsewhere in the chamber (Supplemental Video S1, Supporting Information), similar to the preparation of transmission electron microscope (TEM) samples. Here, however, removing the sample does not require extensive preparation such as deep FIB milling, polishing,

Figure 1. Processing schematic. (a) A silicon mold is first prepared for template stripping bumps, grooves, and apertures. (b) Subsequent metal deposition forms a smooth interface with the silicon, though the backside of the as-deposited metal is rough. (c) By attaching a backing layer such as epoxy, (d) the entire metal film, transferred with the patterning, is stripped. Forming apertures requires that the deposited film is discontinuous.

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and thinning. Often, TEM sample preparation can take several hours, whereas with our in situ template-stripping method, only a single FIB cut is required for sample removal. Furthermore, the underlying smooth interface is protected against oxidation or contamination until it is stripped out, keeping the surface pristine until used.38 In situ stripping of patterned metal films is also possible. Before metal deposition, the mold was patterned via FIB milling. Due to its crystallinity, FIB pattering in silicon is extremely smooth and precise. Furthermore, any ion contamination is implanted only into the mold, giving pure metal samples after template stripping. Figure 2c shows a FIB image of an Ag grating peeling from the mold. After stripping, the grating was rotated and placed elsewhere for imaging in a high-resolution scanning electron microscope. The contrast between the rough (Figure 2d) and smooth (Figure 2e) sides of the metal foil is then clearly seen. Interestingly, both sides of the metal film are patterned and perfectly aligned. This may be useful when surface plasmons are exploited to transmitted light through a patterned film.6 In situ template stripping offers tremendous flexibility in sample preparation, manipulation, and characterization, allowing close inspection from arbitrary angles and precise three-dimensional placement of smooth patterned metals. On another silicon mold, by using either FIB milling or the aforementioned ion-beam-induced platinum deposition, both concave or convex structures can be created. Our previous work demonstrated convex structures obtained by patterning into the silicon.31 When features are fabricated above the silicon surface, the resulting metal patterns will be concave. Figure 3a shows a mold with features both milled below and deposited above the silicon surface. An 11 nm thick overlayer of silica was deposited in an atomic layer deposition chamber to facilitate template stripping from the platinum stripes. The resulting template-stripped

Figure 2. In situ template stripping. (a) A long strip of 300 nm thick silver deposited onto a silicon wafer is cut. (b) The strip can be lifted out with a sharp tungsten probe. The probe is welded to the metal film via focused ion beam (FIB) deposition of platinum. (c) FIB image showing that metallic films can also be stripped from prepatterned silicon templates. The film is then rotated and placed elsewhere on the sample for (d) imaging in a scanning electron microscope (SEM) chamber. The contrast between the rough side and (e) smooth side is clearly evident. Interestingly, both sides of the metallic film are patterned with perfect alignment. 3527

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Nano Letters silver film contains both bumps and grooves, shown in Figure 3b. These plasmonic elements can be situated side by side, offering extra degrees of freedom both above and below the smooth surrounding surface. Panels c and d of Figure 3 show the smooth and well-defined patterned surfaces.

Figure 3. Template-stripped bumps and grooves. (a) SEM image of a silicon template prepared by either milling into or depositing platinum onto the silicon surface via FIB. (b) Cross-sectional SEM of the template-stripped silver film showing both bumps and grooves above and below the flat metallic surface. (c) Zoomed images of the bumps and (d) grooves replicated in silver show the smooth, well-defined features.

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The geometry of these features can be controlled on nanometric scales, shown in Figure 4a. Here, the bump height increases linearly from 10 to 120 nm, with a periodicity of 520 nm. The bump width also increases from 210 to 270 nm. Ramped groove gratings (Supplemental Figure S1, Supporting Information) were also made, with the groove depth increasing from 20 to 100 nm, with the width increasing from 180 to 290 nm, and with a periodicity of 530 nm. From atomic force microscope scans of the template (Supplemental Figure S2, Supporting Information), the heights or depths of these bumps or grooves are seen to be controlled to within 1 2 nm from line to line. Ramped plasmonic gratings offer straightforward optimization of the surface plasmon generation efficiency. By using an imaging spectrometer (Supplementary Figure S3, Supporting Information) and an inverted microscope (5, NA = 0.15), we measured the reflectivity of the gratings as a function of bump height (Figure 4b) or groove depth (Figure 4c). The vertical axis represents space, or position along the grating, whereas the horizontal axis represents wavelength. The data are normalized to the reflection of light polarized parallel to the grating lines, where surface plasmons are not excited. The sharp dip in reflectivity indicates the generation of surface plasmons and is a strong function of bump height or groove depth. In our samples, the minimum reflectivity occurred for a bump height of ∼60 nm and a groove depth of ∼60 nm. The width of the reflectivity dip for the bumps is ∼10 nm. In general, the reflectivity dip for grooves shifts to the red for increasing depth. In our samples, since the groove width is also increasing, shifting the resonance to the blue, we observe minimal overall spectral shift (Supplemental Figure S4, Supporting Information). Cross sections through the minima

Figure 4. Continuously tunable plasmonic gratings. (a) SEM of a ramped plasmonic grating. Within the grating, the bump height is finely tuned for maximizing the surface plasmon generation efficiency. (b) Reflection map of the grating. The vertical axis represents space, or position along the grating, and is labeled with the corresponding bump height. The horizontal axis represents wavelength, and the data are normalized to the reflection of light polarized parallel to the grating lines, where surface plasmons are not excited. The central dark region indicates the generation of surface plasmons. (c) Similar results were obtained for a ramped grating of grooves. The horizontal dashed lines show (d) cross sections through the reflectivity minima. The sharp reflectivity dips indicate a well-tuned grating, corroborated with finite-difference time-domain (FDTD) computer simulations. (e) Visualizing the time-averaged electric field in the bump grating verifies that surface plasmons are efficiently excited only for a certain range of bump heights. 3528

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Figure 5. Template-stripped apertures and bumps. (a) SEM image of a bump and aperture pattern. By adjustment of the depth of the FIB-milled features, as shown in Figure 1, both bumps and apertures can be replicated in a 100 nm thick silver film. The checkerboard pattern shows (b) a similar transmission spectrum to an array of only apertures, with lower absolute transmission due to fewer apertures, and to an array of opposite polarity (bumps to apertures). This indicates that the bumps are able to efficiently generate the grating-coupled surface plasmons responsible for enhanced optical transmission. (c) Similarly, bumps can be arranged around a single aperture, (d) boosting the transmission spectrum compared to that of an isolated aperture. Interestingly, both sides of the silver film are patterned with perfect alignment, as shown in Figure 2, which means that surface plasmons can be manipulated on both the input and output sides of the film.

of reflectivity are shown in Figure 4d. With these ramped devices, it is seen that for similar periodicities, the reflection minimum for a metallic bump grating is to the blue of that for a metallic groove grating, or 537 nm versus 580 nm. This is corroborated with finite-difference time-domain (FDTD) computer simulations of the devices, also shown. Visualizing the electric field of the bump ramp grating (Figure 4e) shows an optimal height, or position along the ultrasmooth grating, for surface plasmon generation. With optical energy localized at specific points along the ramped grating, these structures may be useful for plasmonic applications such as photovoltaics,27 photon sorting,29 and trapped rainbows.39,40 Because the bumps protrude from the surface, these structures may also benefit plasmonic trapping.41 It is also possible to template strip apertures that are decorated with other plasmonic elements in an optically thick metal foil. Previously, we demonstrated apertures through a 30 nm thick Ag film.31 Here, the Ag film is 100 nm thick and is suitable for plasmon-enhanced transmission experiments. For replication of a clean aperture, it is critical that the silicon template sidewalls be nearly vertical, as shown in Figure 1a. This was accomplished by FIB milling into the silicon mold through a 25 nm thick titanium layer, later removed. Figure 5a shows a checkerboard device consisting of a small array of alternating bumps and apertures. On comparison of the optical transmission spectrum in Figure 5b to that of an array of opposite polarity (i.e., bumps to apertures) and to an array of only apertures, it is clear that the bumps are able to efficiently excite surface plasmons and contribute to an extraordinarily high optical transmission through the film.42 Decorating a single aperture with a series of concentric rings (Figure 5c) directs surface plasmons toward the aperture, boosting the optical transmission (Figure 5d). This bull’s eye device is patterned

on both sides of the film with perfect alignment, as in Figure 2, allowing both in-coupling and out-coupling of optical energy.6 By combining FIB milling and FIB-induced metal deposition with template stripping, we have shown the integrated fabrication of both convex and concave plasmonic nanostructures that are perfectly aligned with subwavelength apertures in a one-step, peel-off process. This offers three-dimensional control of plasmonic fields above, below, and through the ultrasmooth metallic film. Using a patterned silicon template, our process allows faithful replication of smooth bumps, grooves, and apertures with minimal sample-to-sample variations. The patterning process is performed only once to produce the reusable mold, allowing mass production with 1 2 nm precision. Furthermore, template stripping in situ enables an arbitrary series of copy, cut, move, rotate, and paste functions, providing powerful new methods for three-dimensional nanomachining, imaging, and device characterization. Template stripping can also easily integrate sharp metallic tips18 or multilayered metamaterials31 with integrated bumps, grooves, and apertures. Because these smooth patterned metal films can be transferred onto a hard backing layer, a stretchable, flexible substrate, or even used as a freestanding metal foil after electroplating, this new set of monolithically integrated nanostructures and in situ manipulation schemes will further enrich the toolbox and building blocks available for plasmonics and nanophotonics.

’ ASSOCIATED CONTENT

bS

Supporting Information. Details of experimental methods, Figures S1 through S4 showing template and device for grooves, AFM scans of the mold, schematic of the imaging

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Nano Letters spectrometer setup, FDTD calculations, and Video S1 showing the in situ template stripping process. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by grants to S.-H.O. from the Office of Naval Research (ONR) Young Investigator Program, the National Science Foundation (NSF CAREER Award and CBET #1067681), and the ACS PRF Doctoral New Investigator Award. We thank Luke Jordan for help with AFM measurements and Prashant Nagpal, Si Hoon Lee, and Hyungsoon Im for helpful discussions. N.C.L. was supported with a University of Minnesota Doctoral Dissertation Fellowship. We also utilized resources at the University of Minnesota, including the Nanofabrication Center, which receives partial support from NSF through the National Nanotechnology Infrastructure Network (NNIN), and the Characterization Facility, which has received capital equipment funding from NSF through the MRSEC program.

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