In Situ Planarization of Huygens Metasurfaces by Nanoscale LocalOxidation of SiliconJonathan Bar-David, Noa Mazurski, and Uriel Levy*

Department of Applied Physics, the Benin School of Engineering and Computer Science, the Center for NanoscienceandNanotechnology, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel

*S Supporting Information

ABSTRACT: Metasurfaces are becoming a flourishing field ofresearch, with diverse applications, such as planar opticalcomponents and structural colors. While metallic metasurfacesare typically few tens of nanometers in their thickness, theirdielectric counterparts typically span few hundreds of nanometersin thickness variations. This makes the stacking of multilayers a bitchallenging. To mitigate this challenge, we have developed a newapproach for the realization of dielectric metasurfaces. Ourapproach is based on the nanoscale local oxidation of silicon(LOCOS), allowing to achieve planar metasurface structures. Wehave utilized this approach for the design, fabrication andcharacterization of amorphous silicon based all-dielectric Huygensmetasurfaces. These metasurfaces show clear electric andmagnetic resonances, which can be structurally tuned. Theobtained results are in good agreement with numerical simulations taking into account the unique shape of the nanoantennas.Relying on a robust approach for their realization, and combined with the important feature of in situ planarization, we believethat such planarized metasurfaces will become a viable technology for future applications.

KEYWORDS: Huygens metasurface, dielectric metasurface, LOCOS

Optical metasurfaces are nanopatterned surfaces exhibitingdesired optical characteristics depending on the nano

patterns and the structures which are defined on thesurface.1−21 This is in contrast to conventional optical elementsthat rely on the materials’ bulk properties. One may distinguishbetween two different types of metasurfaces, being resonant ornonresonant. While the latter relies on concepts such as phaseaccumulation, effective index and geometrical phase to controllight,2,3,5−7,9,10,22−27 the optical properties of resonant meta-surfaces originate at the optical resonant behavior of theindividual scattering elements, which are sometimes defined asoptical nanoantennas.1,11,13−18,28,29

The scattering, transmission, and absorption of electro-magnetic waves by small particles is an old topic firstconsidered by Mie,30−32 who calculated the inner and outerfields for scattering of plane waves by spheres, concluding theyare the sum of the basic multipole fields.31,32 The multipolefields are therefore the electromagnetic eigenmodes of the fieldsin and around small particles.31,32 These modes and theirresonant frequencies are determined mainly by particle size,geometry, aspect ratio, refractive index, and the index contrastbetween the particle and surrounding medium. Following therapid progress in nanofabrication techniques, these observa-tions pioneered by Mie are now of great significance to thephotonic research, as nowadays researchers are attempting toharness the optical response of small particles and cavities such

as nanoantennas for a large variety of photonic applications. byproperly designing nanoantennas, it is now possible to controlthe amplitude, phase, and polarization of the transmitted andreflected fields and by fabricating arrays of resonant nano-antennas one may fabricate resonant metasurfaces with tailoredoptical properties for different applications including, forexample, color engineering,11,18,19,21,28,33,34 nonlinear op-tics,35−38 reflectionless surfaces and perfect absorbers,1,8,39−41

metalenses and other optical elements,10,24,42−44 to name a few.The common to all these demonstrations is the ability tocontrol the polarization, phase, amplitude and frequency oftransmitted and reflected light, as well as the near-field of themetasurface, to design arbitrary optical properties. Manymetasurfaces use metals and plasmonic phenomena to achievesuch effects.17,18,28,29,45,45−51 Yet, metallic metasurfaces suffersignificant losses. In contrast, dielectric metasurfaces show onlylittle absorption in most cases and therefore offer hightransmission and reflection efficiency reaching the limit ofalmost no absorption losses and pure control of phase andpolarization.1,3,6−9,11−14,39,40,43,52 On the other hand, whilemetallic metasurface are nearly flat, a typical dielectricmetasurface consists of nanoantenna arrays which are situatedon the surface of a flat substrate, making the structure

Received: June 28, 2017Published: August 29, 2017

Article

pubs.acs.org/journal/apchd5

© 2017 American Chemical Society 2359 DOI: 10.1021/acsphotonics.7b00688ACS Photonics 2017, 4, 2359−2366

nonplanar. As a result, special efforts needs to be carried out inorder to planarize the surface if the stacking of additional layersis needed. Furthermore, the need for etching of the dielectricmetasurface structure leads to unavoidable surface roughnessand consequently scattering loss. In this work, we demonstratea novel approach which allows to fabricate smooth and planardielectric metasurfaces with optical resonances at the telecomregime. By doing so, we address some of the major issues ofcurrent metasurface technology and provide a path for evenbrighter future for metasurfaces.

■ FABRICATION

Our approach relies on the local oxidation of silicon (LOCOS)fabrication technique. LOCOS is a fabrication methodoriginally used for the isolation of adjacent MOS transistorsfrom each other. In short, it is based on the oxidation of silicon

in an oxygen reach furnace, with the end result of having silicondioxide replacing the silicon in desired location across thedevice. More recently, this approach has been used for thepurpose of defining smooth, low-loss optical waveguides onsilicon substrate.53−56 In this technique, a protective layer isdeposited above a silicon (Si) substrate. Next, photo- orelectron beam-lithography patterns are transferred to theprotective layer by etching. Finally, these patterns aretransferred to the silicon substrate by oxidation, rather thanby further etch steps. The oxidation process allows for smoothstructures to be formed, significantly reducing roughness andconsequently undesired scattering of light. The originalapproach results in a highly nonplanar structure due to thehigher volume of the silicon dioxide layer that is formed insteadof the silicon. However, Recent publications also show that bypartial etching of the Si substrate prior to oxidation, one may

Figure 1. LOCOS fabrication scheme. (A) a-Si, Si3N4 and e-beam resist layers are deposited on a Quartz substrate. (B) ZEP etch mask is defined byelectron beam lithography. (C) RIE etch is performed through the Si3N4 and some of the a-Si layer, leaving a thin a-Si layer close to substrate. (D)Silicon oxidation followed by removal of Si3N4 layer. The end result is a quasi-planar metasurface.

Figure 2. SEM micrographs of aSi LOCOS nanoantennas. (Left) cylindrical nanoantennas with radius of ∼130 nm and gap of ∼150 nm. (Right)rectangular nanoantennas with length of ∼420 nm, width of ∼160 nm, and gaps of ∼220 nm. In both antenna types the a-Si thickness is ∼270 nm.The a-Si antennas are seen as dark gray areas, and the lighter “frame” around them is the oxide layer.

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obtain quasi-planar structures without jeopardizing the smooth-ness of the obtained structures.55 An illustration of ourfabrication process is shown in Figure 1. Unlike previousdemonstrations, in this work, we chose to utilize the LOCOStechnique to fabricate quasi-planar nanoantenna arrays in anamorphous silicon (a-Si) layer.The fabrication process begins by depositing a 275 nm thick

hydrogenated a-Si layer on a quartz substrate by PECVD(Oxford). Atop this layer, we deposited the oxidation-shieldlayer, a 160 nm thick silicon nitride (Si3N4) by PECVD. Samplewas then spin coated with electron beam (e-beam) resist (ZEP520), and nanoantenna patterns were created by e-beamlithography (Raith eline). Note that ZEP is a positive resist,hence, e-beam patterns are the interantenna gaps. After e-beamlithography, sample was developed and the pattern wastransferred to the silicon nitride layer by a reactive ion etch(RIE, Corial), where the ZEP e-beam resist is used as the etchmask. In this step we etched the full height of the silicon nitridelayer and exposed the a-Si layer in the bottom of the createdtrenches.Next, a second etch step was performed, partiallyetching the a-Si layer, to compensate for oxidation swelling.Finally, the e-beam resist was removed and the sample wasoxidized by heating the sample to ∼1000 °C in an H2O richatmosphere. During oxidation, the silicon nitride layer acts asan oxidation shield defining the oxidized regions. Finally, thesilicon nitride layer is removed, and the planarized structure ofamorphous-silicon nanoantennas, embedded in SiO2 is formed.A scanning electron microscope (SEM) image showing the topview of some of the fabricated samples is shown in Figure 2.Atomic force microscope (AFM) scans of the fabricatedsamples are shown in Figure 3. An SEM cross section revealingthe detailed subterranean structure of the nanoantennas isshown in Figure 4. These scans clearly show the oxidized ringsurrounding each of the Si nanoantennas. The surface heightvariation is of the order of ±25 nm. The relation betweenantenna layer thickness and final surface variation was notinvestigated directly. However, based on current results weassume that the antenna thickness does not have a significanteffect on the final flatness, which is governed mostly by theaccuracy of the etch and the oxidation steps. We have chosen tofabricate arrays of identical antennas with radii and periodvarying between arrays in order to better understand andcontrol the fabrication process and the optical properties of theantenna structures.

The cross section shown in Figure 4 further reveals theantenna sidewall geometry which is curved inward. Thisgeometry is an outcome of the oxidation process, the curvedstructure is a result of oxygen diffusion into the silicon and thein- and outward swelling of the formed silicon dioxide. Thisunique structure was considered in the numerical simulations,as will be discussed later on.

■ NUMERICAL SIMULATIONSThe optical transmission spectrum of the fabricated structurewas simulated by 3D FDTD software (Lumerical FDTDsolutions). The transmission of nanoantenna arrays wasconsidered by applying periodic boundary conditions andplane-wave illumination. Due to the resonant nature of thesenanoantennas, even a slight change of few nanometers in thesimulated antenna geometry, for example, its height, radius, orsidewall profile, results in a noticeable variation in opticalperformance and shift in the resonance frequency. Figure 5presents the results of such simulations, calculating thetransmitted amplitude for cylindrical nanoantennas with heightof 275 nm and varying radii of 60 to 400 nm, for a broad SWIRspectrum with wavelength ranging from 600 to 1600 nm. Thegap was kept constant at 200 nm, that is, the period is

Figure 3. AFM measurement. (Left) AFM scan of nanoantenna array (same as in Figure 2, left). Blue line marks the cross section. (Right) AFMcross-section reveals a ±25 nm height variation due to oxidation swelling.

Figure 4. Scanning electron microscope (SEM) micrograph image(colored for clarity) showing a cross section in fabricated nanoantenna.The nanoantennas are seen as rounded-side trapezoids in middle partof the image. Due to the resonant nature of these nanoantennas, theexact fabricated shape has a major influence on the transmissionspectrum of these structures. Interface between the amorphous siliconlayer and quartz substrate is clearly seen.

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increasing with the radius. As may be seen in Figure 5A, thetransmission properties of these nanoantennas change with theantenna radius. The change is mostly observable as awavelength shift and separation of the two distinct transmissionminima. These two resonances can also be observed in Figure5b, which is a line scan of Figure 5a at a constant cylinder radiusof 329 nm. Indeed, the two distinct minima, labeled as “electricdipole” (ED) and “magnetic dipole” (MD) resonances are seen,as also reported in, for example, refs 8, 31, 40, and 57.These resonances are actually the two first Mie resonances,

which “switch order”, that is, the ED mode is found at a longerwavelength (or lower frequency) than the MD mode, due tothe small aspect ratio of the nanoantennas, with a height of 275nm and a radii of 330 nm. Figure 6 shows the calculated electricand magnetic fields for the two modes indicated in Figure 5B. Itmay be seen that under illumination with a x-polarized planewave, the MD modal field at a wavelength of 1306 nm has theexpected strong magnetic field in the y direction and the

circular vortex-shaped electric field in the xz plane, as depictedin Figure 6 (top row), while the ED modal fields at thewavelength of 1446 nm have a strong electric field at the x direction and a vortex-shaped magnetic field loop at the yzplane, as depicted in Figure 6 (bottom row). It may also benoticed that the ED modal fields are actually concentrated closeto the nanoantenna boundary due to the asymmetric antennastructure and (relatively) low index contrast between theantenna and the surrounding medium (here, the substrate),which increase the effective volume of the nanoantenna.

■ EXPERIMENTAL CHARACTERIZATIONFollowing the design and fabrication of the LOCOS basedmetasurface, we have experimentally characterized its opticalresponse. Transmission measurements were done by illuminat-ing the sample with a white light source (tungsten-halogenlamp) through a microscope condenser lens. The transmittedlight is collected by an objective lens (Nikon, 50×, NA 0.45)

Figure 5. FDTD simulation of antenna transmission. (A) Transmission spectrum of an antenna array for 600−1600 nm. The MD-ED mode splittingis clearly seen at a wavelength of ∼1200 nm for antenna radii of 250−300 nm. Diffraction effects are seen in the upper left corner, for shortwavelengths and (relatively) large antennas, and the Kerker condition,58 a decrease and narrowing of the resonant reflection, is met by antennas ofradius ∼150 nm at a wavelength of about 900 nm. The dashed line marks the antenna radius for which we present the spectra in subfigure (B). (B)Transmission spectrum of a single antenna array with antenna radius of 330 nm. MD, “magnetic dipole” mode; ED, “electric dipole” mode. The EDmode is at longer wavelength due to antenna shape and aspect ratio. The curved sidewall has been approximated by a parabolic shape with acurvature of about 30 nm inward.

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and directed into a spectrometer (Horiba, microHR\OceanOptics, Flame). All measurements are later normalized to thetransmission through the system. Figure 7 presents themeasured transmission of our fabricated cylindrical nano-antenna arrays. As may be seen, the main features predicted bysimulation are clearly seen in our measurements, namely thetwo resonant modes and their overlap for nanoantennas ofradius about 150 nm. To further compare our measured resultsto the simulated prediction, we present in Figure 8 acomparison of the numerical simulations (left) to themeasurements (right) of nanoantennas with gradually increas-

ing radius. This comparison reveals a good agreement betweenthe two. Furthermore, the gradual spectral separation betweenthe electric and magnetic modes can be clearly observed.In addition to the characterization of cylindrical nano-

antennas, we have also fabricated and characterized rectangularnanoantenna arrays (such as depicted in Figure 2). Thetransmission through such arrays was measured by the samemethod described above, with light polarized along thenanoantennas’ long axis. Due to the geometry of thesenanoantennas, with length of 300−400 nm and width of∼150 nm, the resonant frequencies of these antennas are closer

Figure 6. Electric and magnetic fields at resonance, for antenna with a 330 nm radius. Light propagates toward the antenna along the z axis. Theblack lines denote the antenna cross-section. As described above, sidewall curvature is approximated by a parabola. Black arrows are added as guidesto the eye to indicate the direction of electric and magnetic fields. (Top row) The magnetic dipole mode at λ = 1306 nm exhibits an “electric fieldloop” at xz plane and strong magnetic field at the y direction. (Bottom row) the electric dipole mode at λ = 1446 nm exhibits a “magnetic field loop”at the yz plane and a strong electric field at the x direction.

Figure 7. Measured transmission spectrum for antenna with radii of 70−300 nm. With the reduction of aspect ratio, the resonant modes begin toseparate, and for nanoantennas with radius >250 nm the modes are easy to distinguish. On the other hand, for smaller antenna radius of about 150nm, one may observe spectral overlap between the MD and the ED modes, followed by enhanced transmission around 800 nm wavelength. This isknown as the Kerker condition. The transmission is limited by the band to band absorption in the a-Si layer.

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Figure 8. Transmission spectrum of cylindrical nanoantennas with radii of ∼90 to ∼250 nm. (Left) full wave numerical simulation, (Right) -measured results. The shifting of modes toward longer wavelengths and the splitting of the MD and ED modes are clearly seen in both simulationand measured data. The Kerker condition, in which MD and ED resonances overlap is denoted by the red box. Full transmission is not obtained asthe wavelength is below the material band gap.

Figure 9. Transmission spectrum of rectangular nanoantennas, with light linearly polarized along the antennas’ long axis: (left) simulation, (right)measurement. Notice the gradual change in resonant frequencies as the antenna length is increased from 300 nm at the bottom row to 400 nm at thetop. Spectrum complexity arises primarily from the intrinsic absorption of a-Si for frequencies above the material band gap. The curved sidewall hasbeen approximated by a segment shape with a curvature of about 30 nm inward.

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to the visible spectrum (compared to the cylindrical nano-antennas shown previously). As such, the demonstration ofLOCOS based nanoantennas allows for an additional degree offreedom in mitigating specific applications toward the visibleband, while maintaining dimensions which are not too difficultto fabricate. Figure 9 shows the simulated (left) and themeasured (right) transmission of nanoantenna arrays withlength varying from 300 to 400 nm. Here, the transmissionspectra are more complex due to the intrinsic silicon absorptionat these frequencies resulting from interband transition.Nevertheless, we are able to identify the major trends in bothsimulation and measured data, namely, the general spectrallocation of the antenna resonances and their gradual shifttoward longer wavelengths as the antenna length increases.

■ SUMMARY

We have demonstrated the viability of the LOCOS techniquefor the fabrication of planar, optically resonant dielectricmetasurfaces in a-Si layers. Measurements of our fabricatedmetasurfaces show good agreement with simulation predic-tions, indicating that these metasurfaces can operate in thevisible and NIR spectral regimes, above and below the Si bandgap frequency. We have designed and fabricated metasurfacesbased on nanoantennas of different sizes and geometries and bythis we have shown the fabrication robustness provided by thisapproach, as well as it is compatibility with current CMOStechniques. Following the results reported here, one may usethis technique for the implementation of planar metasurfacedevices, for example, metalenses, metafilters, metasplitters,metaholograms, and more. Finally, the LOCOS technique andthe use of PECVD deposited materials on commerciallyavailable substrates pave the way for future implementation inoptical devices and in few-layer stack structures.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsphoto-nics.7b00688.

Figure S1 (PDF).

■ AUTHOR INFORMATION

Corresponding Author*E-mail: ulevy@mail.huji.ac.il.

ORCID

Jonathan Bar-David: 0000-0002-4464-636XUriel Levy: 0000-0002-5918-1876Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.

FundingThis research was supported by the ActiPlAnt program ofEinstein foundation, Berlin, Germany, and the three-dimen-sional printing project from the Israeli Ministry of Science.

NotesThe authors declare no competing financial interest.

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ACS Photonics Article

DOI: 10.1021/acsphotonics.7b00688ACS Photonics 2017, 4, 2359−2366

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