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The fabrication of meta-devices is a COMS-compatible progress. To the authors’ knowledge, a majority of nanofabrication techniques used in semiconductor industries can be applied to the manufacturing of metasurfaces. These technologies are divided into three categories: maskless lithography, masked lithography, as well as other technologies that have gained attention with the emergence of metasurfaces. The comparison between different fabrication technologies is summarized in Table 1 to facilitate the reader’s quick review. The categories are derived based on the way the subwavelength-scale patterns are first created. Fabrication steps that help pattern transfer, for example, etching procedures, are included in the discussion of fabrication process flow and only a few outstanding etching works are highlighted here.
Strength Weakness Maskless lithography E-beam lithography High resolution Low throughput
High costFocused ion beam lithography High resolution
One-step fabrication
Freeform patterningLow throughput
High cost
Ion contaminationLaser direct write lithography Low cost
Large areaNone batch production Laser interference lithography Large area
Low costPeriodic patterns only Masked lithography Photolithography Large area
High throughputHigh cost Nanoimprint lithography High throughput
Low cost
Large areaHigh resolution master mold
Residual imprint layerSelf-assembly lithography Large area
Low cost
SimplePeriodic patterns only
Mask defectsOther techniques Two-photon polymerization Lithography 3D structure patterning Low throughput Laser ablation Low cost
High throughputUniformity Table 1. Summary of metasurface fabrication methods
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Electron-beam lithography (EBL) is one of the most common fabrication techniques used in the manufacture of metasurface. A focused electron beam is directly adopted to define the nanostructure patterns on the electron beam-sensitive resist. EBL provides the desired patterns with ultra-high resolution in a mask-free exposure process. Depending on the metasurface design, further fabrication steps (e.g., etching) are generally required to transfer the EBL-defined pattern to the target working layer.
Wang et al. proposed a broadband achromatic meta-lens working in the visible region54. EBL is adopted to fabricate the proposed meta-lens, as depicted in Fig. 1a. In the first step, a layer of GaN, which acts as the desired meta-atom layer, is grown on the sapphire substrate by metalorganic chemical vapor deposition (MOCVD). Then, a SiO2 layer, which serves as a hard mask for etching high aspect ratio GaN nanopillar, is deposited by plasma enhanced chemical vapor deposition (PECVD). In the third step, a layer of resist is spin-coated on the SiO2 layer. After exposure under EBL, followed by the development process, a layer of chromium (Cr) is deposited using e-beam evaporation. The pattern is transferred to the Cr layer after the lift-off process. The SiO2 layer is then dry etched using reactive ion etching (RIE) with Cr as the etching mask. Using etched SiO2 layer as a hard mask, the GaN is etched by inductively coupled plasma reactive ion etching (ICP-RIE). After removing the remaining SiO2 layer using buffered oxide etch (BOE), the desired nanostructures that provide the required optical response are finally defined. Fig. 1b shows the scanning electron microscope (SEM) images of the fabricated achromatic meta-lens with a numerical aperture (NA) of 0.15. The GaN nanopillars and the inverse nanostructures are seen from the SEM images. The minimum feature size of the design is 45 nm, proving that EBL can offer high-resolution fabrication.
Fig. 1 Electron-beam lithography. a The process flow for the fabrication of GaN achromatic meta-lens working in the visible. b The corresponding SEM images of GaN nanopillars (top) and the inverse nanostructures (bottom)54. c The tilted view SEM images of the TiO2 nanostructures, including circular pillars, crosses, and rectangle pillars9.
Titanium dioxide (TiO2) is currently one of the most commonly used materials for metasurfaces in the visible and near-infrared bands, but most of the preparation of TiO2 metasurfaces still used the bottom-up atomic layer deposition (ALD) on EBL patterned resist, followed by dry etching the top TiO2 layer and removing the residual resist55,56. The height and aspect ratio of nanopillars prepared by this method are limited to 600 nm and 15, respectively. Despite the seemingly high aspect ratio given by this method, the allowed fabrication limit is only suitable for Huygens metasurface57,58 and propagation-based metasurface. A much higher aspect ratio is preferred for applications such as chromatic aberration elimination. Xiao et al. have redeveloped a more efficient top-down dry etching method for TiO29. Similar to the preparation process in Fig. 1a, the metasurface pattern is applied to the resist using EBL, and then the pattern is transferred to the Cr layer through the lift-off process. The dry etching method by RIE is used to obtain the TiO2 nanostructures array. As shown in Fig. 1c, the structure has almost vertical sidewalls with record-high aspect ratios of 37.5. This etching technology provides strong support for the large-scale and efficient production of TiO2-based meta-devices.
More complicated geometries can be realized by combining other techniques with the EBL process. Chen et al. developed a kind of metasurface based on TiO2 with inclined nanopillar distribution by combining EBL and RIE dry etching techniques59. The process flow is shown in Fig. 2a. EBL is still used to pattern the trapezoidal arrays in the resist, and the pattern is then transferred to the Cr layer through dry etch by ICP-RIE. In the RIE dry etching step, the etched sample is tilted and covered with Faraday cages to orient the plasma distribution on the sample surface. Fig. 2c shows the side-view and cross-sectional SEM images of the inclined metasurface. The use of adjustable tilt etching technology breaks the traditional mirror symmetry of metasurfaces, unleashing the flexible characteristics of metasurfaces, and providing innovative ideas for the development and application of meta-devices. 3D nanostructures are also possible with advanced dry etching technologies. An uniaxial isotropic metamaterial composed of 3D metallic split-ring resonators (SRRs) is experimentally demonstrated by EBL process, as given in Fig. 2d60. A layer of positive e-beam resist, PMMA, is spin-coated on the silicon substrate, followed by EBL process to define the ring pattern. Au/Ni double metal layers are deposited on the patterned sample. Additional Au/Ni are removed by the lift-off process. After dry etching the silicon substrate using CF4, the planar metal nanostructures are spontaneously folded by the higher tensile stress in the top Au layer than in the bottom Ni layer. The fabricated fourfold-symmetric SRRs are exhibited in Fig. 2e. The proposed metamaterial shows great potential in generating a bi-anisotropic response. These fabrication techniques also apply to other lithography methods for complex pattern creation.
Fig. 2 Electron-beam lithography. a The process flow for the fabrication of inclined metasurface. b Schematic illustration of the slanted RIE system. c Side-view (left) and cross-sectional (right) scanning electron microscope images of inclined metasurface. Scale bar, 300 nm59. d The fabrication process for 3D SRR using EBL. e The SEM images of 3D SRRs with four-fold symmetry60.
EBL also acts as a one-step fabrication technique without further pattern transfer steps. Huang et al. experimentally demonstrated an all-dielectric metasurface supporting ultrahigh Q resonance61. In their design, a patterned resist, ZEP520, is directly utilized as the top perturbation structure, which avoids the etching steps and ensures a higher fabrication accuracy. However, unlike conventional photolithography, EBL is a pixel-by-pixel technology, which means EBL only writes a small pixel at a time. The high cost, long processing time, and highly stable requirements prevent EBL from large area and high-volume manufacturing. Owing to the electron scattering in the resist and substrate, the resolution of EBL is mainly determined by electron scattering instead of beam size of incident electrons62.
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Instead of using electrons to expose the surface resist, focused ion beam (FIB) lithography applies heavy ion beams to bombard the sample surface, resulting in the direct removal of neutral and ionized atoms from the surface63. FIB provides a maskless one-step milling process with nanometer-scale precision. Through the combination of FIB with SEM to form the dual-beam system, direct imaging and analysis of the target sample can be realized.
Tseng et al. experimentally realized a 3D chiral metasurface by FIB adopting gallium ions64. As illustrated in Fig. 3a, the metasurface consists of an array of Archimedean spiral lines made of Si3N4/Au bilayer film. The Archimedean spiral patterns are obtained by one-step milling across both Si3N4 and Au thin film using FIB. Due to the stress and defects introduced in the milling process, the planar spiral lines stretch towards the out-of-plane direction and form the 3D ones. The top and tilted SEM images of fabricated spirals are shown in Fig. 3b. The linewidth of the spiral is 80 nm. Materials, film thickness, quality of the ion beams, and milling directions determine the final geometry of the 3D nanostructures. The experimental results show that the spirals have strong chiral dissymmetry over a broad infrared region and can be applied to chiral photomechanical sensors. The shape of 3D nanostructures can further be precisely manipulated by controlling the ion beam irradiation65.
Fig. 3 Focused ion beam lithography. a Schematic of Archimedean spiral metasurface fabrication process by FIB. b The corresponding top (left panel) and tilted (right panel) SEM images of the fabricated metasurface64. c SEM images of LN maetasurface for second harmonic enhancement70. d Schematic configuration of the metasurface unit cell and the corresponding SEM images71.
FIB shows its great potential in etching difficult-to-etch materials. For example, lithium niobate (LN) is hard to etch and faces challenges in realizing vertical sidewalls and large etching depth66-69. Fig. 3c depicts the SEM images of an LN metasurface, which boosts the second harmonic in the visible region70. The LN thin film is milled directly by FIB. The side walls of LN nanopillars have an angle of 83.6°.
FIB is suitable for freeform surface patterning. Gorkunov et al. show a chiral metasurface on crystal silicon film fabricated by digitally controlled FIB71, as given in Fig. 3d. The SEM images show that, instead of flat patterns with vertical side walls, the meta-atoms have geometry with smoothly varying height and four-fold rotational symmetry. Gholipour et al. also adopted FIB in the fabrication of a perovskite metasurface72. FIB is a more flexible patterning method than EBL. However, it suffers from high costs and low throughput and cannot be applied to massive manufacturing. Ion doping, sample damage, and sample displacement during milling will degrade the performance of the fabricated samples.
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Despite the high resolution of patterning with focus beams of particles, maskless fabrication techniques with flexibility, high throughput, and large-area fabrication are heavily desired. The direct application of laser into pattern writing provides a competitive solution.
Laser direct writing (LDW) lithography is a common technology adopted in mask writing in the semiconductor industry. As it is named, LDW applies laser to expose the photoresist directly. The exposure position is controlled by the moving stage. Zhang et al. demonstrated a plasmonic metasurface by LDW73. After the multiple depositions of gold, GST, and MgF2 films on the glass substrate, followed by spin-coating the photoresist, the laser beam writes on the top to define the geometry of the plasmonic nanoantenna. Fig. 4a shows the SEM image of the plasmonic metasurface. The metasurface achieves reconfigurable beam shaping within wavelengths ranging from 8.5 µm to 10.5 µm. Based on the LDW technique, an all-dielectric metasurface which produces polarization-sensitive optical vortices is also fabricated74. As given in Fig. 4b, Si nanorods with various geometric configurations are clearly observed. The expense for LDW technique is relatively low. Through the integration with other optical components, such as spatial light modulators (SLM), large-area manufacturing with high throughput is realized75,76. However, each sample must be prepared independently, making it difficult to carry out large-scale batch production.
Fig. 4 Laser lithography. a SEM image of plasmonic metasurface fabricated by LDW73. b SEM image of the LDW-fabricated dielectric metasurface for optical vortices generation74. c SEM image of metasurface beam splitter manufactured by LIL77. d Schematic setup for four beam interference lithography. e, f Schematic of dewetting process for Mie resonator formation e and the corresponding SEM images f. Scale bar: 1 μm78.
Laser interference lithography (LIL) applies two or more coherent laser beams to create interference patterns with bright and dark regions. Such configurations ensure large-area periodic nanostructure manufacturing without the requirement for masks. Fig. 4c illustrates the SEM image of a metasurface beam splitter77. The periodic grating nanostructures are fabricated by LIL. More complicated structures other than periodic line shapes can be realized via LIL. Berzinš et al. experimentally show metasurface supporting Mie resonance using four-beam LIL78. The fabrication setup is given in Fig. 4d. A diffractive optical component is applied to split the incident light into four beams, which reach the sample surface after passing through the lens system. The four beams interfere and form a square-shaped lattice pattern. Single pulse laser is adopted to provide high power for melting the silicon thin film. As shown in Fig. 4e, the melted silicon diffuses to the dark region in the interference and reshapes to a hemi-spherical configuration to minimize the surface energy. The SEM images of the intermediate and final pattern of Si nano-resonator are depicted in Fig. 4f. Despite the fact that more complex configurations can be fabricated by applying more interference beams or controlling the beam intensity distribution79, LIL still cannot draw arbitrary or non-periodic patterns.
Meta-device: advanced manufacturing
- Light: Advanced Manufacturing 5, Article number: (2024)
- Received: 30 September 2023
- Revised: 27 December 2023
- Accepted: 03 January 2024 Published online: 07 March 2024
doi: https://doi.org/10.37188/lam.2024.005
Abstract: Metasurfaces are one of the most promising devices to break through the limitations of bulky optical components. By offering a new method of light manipulation based on the light-matter interaction in subwavelength nanostructures, metasurfaces enable the efficient manipulation of the amplitude, phase, polarization, and frequency of light and derive a series of possibilities for important applications. However, one key challenge for the realization of applications for meta-devices is how to fabricate large-scale, uniform nanostructures with high resolution. In this review, we review the state-of-the-art nanofabrication techniques compatible with the manufacture of meta-devices. Maskless lithography, masked lithography, and other nanofabrication techniques are highlighted in detail. We also delve into the constraints and limitations of the current fabrication methods while providing some insights on solutions to overcome these challenges for advanced nanophotonic applications.
Research Summary
Advanced Manufacturing for metasurface realization
The advanced nanofabrication technologies ensure the realization of metasurfaces. This paper provides an overview of state-of-the-art metasurface fabrication technologies, including maskless lithography, masked lithography, and other nanofabrication techniques. Din Ping Tsai from City University of Hong Kong, China, Shumin Xiao from Harbin Institute of Technology, China, and their colleagues review the recent achievements of nanofabrication techniques that are compatible with the manufacture of metasurfaces. Lithography with or without the mask and other nanofabrication techniques for metasurface are discussed. This article reviews serval well-fabricated metasurface works and summarizes the strengths and limitations of the nanofabrication techniques.
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