Introduction

 

Professor Din Ping Tsai from City University of Hong Kong and Professor Shumin Xiao from Harbin Institute of Technology (Shenzhen), jointly authored a review article titled "Advanced manufacturing of dielectric meta-devices". The article was published in the second issue of Photonics Insights in 2024. (Wenhong Yang, Junxiao Zhou, Din Ping Tsai, Shumin Xiao, "Advanced manufacturing of dielectric meta-devices," Photon. Insights 3, R04 (2024)).

 

In the cutting-edge field of nanophotonics, metasurfaces have demonstrated exceptional control over wavefront shaping. Specifically, dielectric metasurfaces composed of high-refractive-index, low-loss, and cost-effective materials, such as titanium dioxide (TiO₂), silicon (Si), and gallium nitride (GaN), have been widely applied in high-performance optical devices. Notably, the rapid advancement of micro-nano fabrication technologies has made it possible to explore new methods for fabricating metasurfaces, and the development of wafer-scale nanofabrication techniques has significantly accelerated the commercialization of dielectric metasurfaces. This paper reviews the latest advances in nanofabrication techniques for dielectric metasurfaces (Figure 1), including standard nanolithography (such as electron-beam lithography and focused ion beam lithography), advanced nanolithography (such as grayscale lithography and scanning probe lithography), large-scale nanolithography (such as nanoimprint lithography and deep ultraviolet lithography), and the use of extreme nanofabrication techniques to produce high aspect ratio, flexible, multilayer, and slanted novel high-performance dielectric metasurfaces. Ultimately, the paper concludes with a discussion on the future development and application prospects of nanofabrication technologies for dielectric metasurfaces.

 

Figure 1 The schematic diagram illustrates typical fabrication techniques for all-dielectric metasurfaces, including standard nanolithography, advanced nanolithography, large-scale manufacturing, and extreme nanofabrication techniques.

 

1. Development and High-Performance Applications of Dielectric Materials

 

Dielectric metasurfaces are composed of high-refractive-index and low-loss dielectric nanostructures, and based on Mie resonance theory, they can support both electric and magnetic multipole modes. By adjusting the geometric parameters of these nanostructures, their optical responses can be effectively controlled, enabling the development of efficient photonic devices. In nanofabrication, controlling precision and roughness is crucial for the performance of dielectric optical devices, especially for achieving ultra-narrow linewidth resonances, such as bound states in the continuum (BICs).

 

To precisely fabricate dielectric metasurfaces, various mature nanofabrication techniques are widely utilized, including electron-beam lithography (EBL) and focused ion beam (FIB) lithography. Additionally, rapid, wafer-scale manufacturing technologies have made significant strides. For instance, methods like nanoimprint lithography (NIL) and deep ultraviolet (DUV) lithography have achieved high throughput and low-cost fabrication of dielectric metasurfaces. Currently, a rich library of dielectric materials covers a broad range of wavelengths from ultraviolet (UV) to infrared (IR), as shown in Figure 2. Among these, Si has become a widely used dielectric material in the visible to near-infrared spectrum. This is due not only to its extensive application in the semiconductor industry but also to its mature manufacturing process and compatibility with modern CMOS technology, which meets the requirements for large-scale production. TiO₂ is another promising dielectric material with high refractive index and negligible optical loss across the visible to near-infrared range, and it is widely used in high-efficiency metasurface designs. Other materials, such as gallium nitride (GaN), silicon nitride (Si₃N₄), silicon dioxide (SiO₂), and gallium arsenide (GaAs), are also extensively used in designs from the visible to near-infrared spectrum. For mid-infrared applications, materials like tellurium (Te), antimony gallium (GaSb), and silicon carbide (SiC) have been employed in meta-devices. In the ultraviolet range, metasurfaces based on hafnium dioxide (HfO₂), zirconium dioxide (ZrO₂), and niobium pentoxide (Nb₂O₅) have achieved high-efficiency applications with 60%-80% efficiency. Selecting dielectric materials with suitable optical parameters and manufacturability is crucial. Through continuous innovation and development in nanofabrication technologies and material libraries, dielectric metasurfaces are set to play an increasingly important role in the realization of efficient photonic devices.

 

Figure 2 Comparison of the optical properties of different dielectric materials and their high-efficiency applications across the UV to IR spectrum.

 

2. Standard nanolithography techniques

 

Standard semiconductor industry nanolithography processes include EBL and FIB, which are widely used in the fabrication of dielectric materials. EBL has become a dominant technique in dielectric material manufacturing over the past decades due to its excellent features—high resolution, design flexibility, and reliable repeatability. As shown in Figure 2(a)-(b), researchers have used Cr as a hard mask to achieve the fabrication of Si and GaN metasurfaces. FIB, on the other hand, can pattern directly on the target film without the need for a mask (Figure 2(c)). By selectively removing or depositing the target material, FIB can precisely create planar micro-nano or three-dimensional structures using either bottom-up or top-down approaches. In addition to nanofabrication, FIB integrates morphology characterization capabilities, allowing for simultaneous processing and surface observation in the working area. Moreover, laser nanolithography technology, another common nanolithography technique, is also widely used in the fabrication of dielectric metasurfaces due to its multifunctionality and mask-free manufacturing capabilities. Laser-based lithography techniques come in various forms, including Laser Interference Lithography (LIL) and Laser Direct Writing (LDW). Laser Direct Writing encompasses methods such as laser-induced melting or ablation and two-photon LDW (Figure 2(e)-(f)).

 

Figure 3 Applications of standard nanolithography methods in the fabrication of dielectric metasurfaces, including electron beam lithography, focused ion beam lithography, and laser-based nanomanufacturing.

 

3. Advanced Nanolithography Techniques

 

In addition to the standard nanolithography techniques previously mentioned, several innovative lithography methods have been widely applied in the fabrication of metasurfaces in recent years. These advanced nanolithography techniques include grayscale lithography, multi-step lithography, and atomic force microscope-based scanning probe lithography (SPL), with grayscale lithography and multi-step lithography being combinations of EBL, FIB, and LDW. These methods offer pathways to design and fabricate more complex dielectric metasurfaces. Unlike standard nanolithography techniques that can only produce nanostructures of the same height, grayscale lithography introduces variable light intensities during exposure, allowing for the creation of nanostructures with different heights or depths. This fine control enables simultaneous structure control in both the Z-direction and X-Y plane, significantly enhancing the spatial manipulation capabilities of metasurfaces. For example, Figure 4(a) shows the fabrication of TiO₂ nanopillars of varying heights using grayscale lithography, which allows precise control of nanopillar height on a single metasurface and opens up new possibilities for structural color applications. When single-step lithography techniques are insufficient to meet design requirements, more advanced nanofabrication techniques such as multi-step lithography have emerged. Multi-step lithography enables the fabrication of multi-layered and hybrid (different patterns, different materials) metasurfaces. This technique involves multiple patterning steps, such as EBL+EBL or EBL+LIL, each contributing to the final design, as demonstrated in Figure 4(b), where multi-step lithography was used to create novel metasurfaces combining silicon and gold. Another direct-patterning nanolithography technique is SPL, which uses a probe or tip close to the sample surface to directly pattern at the nanoscale. The interactions between the probe and the surface are crucial during the patterning process, including thermal, electrical, and mechanical interactions. In thermal SPL, the heat provided by a heated probe induces local material changes, as shown in Figure 4(c). These innovative lithography techniques broaden the scope of nanostructure design and fabrication, allowing researchers to select the most suitable method based on specific application needs and providing flexibility in creating complex nanostructures.

 

Figure 4 Applications of advanced nanolithography techniques in the fabrication of dielectric metasurfaces, including grayscale lithography, multi-step lithography, and scanning probe lithography.

 

4. Large-scale Nanolithography Techniques

 

Despite the rapid development of nanofabrication technologies, most remain at the experimental stage and face significant challenges in terms of practical application and commercialization. This is primarily due to the long processing times and lack of large-scale production capabilities, which are crucial for semiconductor devices and integrated circuits. Industrial applications demand processes that allow for large-scale, continuous production with high repeatability and stability, accommodating wafer sizes ranging from 2 inches to several tens of inches. Consequently, developing large-area manufacturing techniques with higher flexibility, precision, and uniformity is particularly important. To enhance the productivity of large-scale metasurface fabrication, significant advancements in techniques such as NIL, DUV Lithography, and molecular self-assembly have substantially reduced production costs and propelled the commercialization of dielectric metasurfaces. Among these, wafer-scale UV lithography uses specific UV light wavelengths to pattern the wafer during manufacturing. Currently, three UV lithography techniques are used in metasurface fabrication: i-line (λ = 365 nm) stepper lithography, argon fluoride (ArF) DUV (λ = 193 nm) immersion scanner lithography, and krypton fluoride (KrF) stepper DUV (λ = 248 nm) lithography. For example, using i-line step-and-repeat lithography, researchers successfully fabricated a single 2-cm-diameter silicon-based NIR metalens, showcasing high quality imaging and diffraction-limited focusing (see Figure 5(a)). However, the large wavelength of i-line stepper lithography prevents the creation of nanostructures with smaller feature sizes suitable for working at visible wavelengths. Current state-of-the-art photolithography tools employ DUV light with wavelengths of 248 nm (KrF) and 193 nm (ArF). Researchers have used KrF DUV lithography to fabricate a 4-inch wafer Ge dielectric resonator, enabling applications in infrared photonics and biosensing, as depicted in Figure 5(b).

 

Additionally, the rapid development of nanoimprint lithography (NIL) technology has established it as a pivotal tool in nanomanufacturing. Unlike optical lithography, NIL achieves patterning by replicating patterns from a mold onto a substrate coated with a resist, typically using polymer materials through physical deformation or curing processes. NIL primarily includes two types based on curing mechanisms: thermal NIL (T-NIL) and ultraviolet NIL (UV-NIL). Compared to T-NIL, UV-NIL is widely used for metasurface fabrication due to its higher resolution and faster processing speed. For instance, as shown in Figure 5(c), researchers have developed UV-curable resins embedded with TiO₂ nanoparticles, enabling direct fabrication of dielectric metasurface lenses through single-step UV-NIL. In addition to these common large-area micro-nanofabrication techniques, Self-Assembled Monolayers (SAMs) technology has shown immense potential for manufacturing structures at the nanoscale. This method utilizes organic molecules that spontaneously organize into ordered lattices on a substrate, forming stable aggregates that can then be subjected to etching or deposition processes. SAMs are widely used in the fabrication of various dielectric metasurfaces, as illustrated in Figure 5(d).

 

Figure 5 Applications of large-scale manufacturing techniques in the fabrication of dielectric metasurfaces, including nanoimprint lithography, deep ultraviolet lithography, and self-assembled nanofabrication techniques.

 

5. Extreme Fabrication and Application

 

Extreme nanofabrication techniques encompass the fabrication of multilayer, flexible, inclined, and high aspect-ratio dielectric nanostructures. These innovative methods introduce additional degrees of freedom in the design of metasurface functionalities. Extreme fabrication not only advances the technology for metasurface nanofabrication but also enables structures with unprecedented properties and functionalities.

 

Multilayer dielectric metasurfaces involve creating metasurfaces with multiple layers of different structures or materials. The fabrication of multilayer structures typically requires multiple lithography and processing steps. Current advanced nanotechnology allows for simple yet versatile alignment schemes and procedures for manual or automated alignment of lithography equipment. Moreover, it is crucial to consider the impact of misalignment on optical responses to ensure optimal performance. Figure 6(a) illustrates the fabrication process of a bilayer silicon metasurface.

 

Flexible dielectric metasurfaces are designed to create optical devices that can adapt to curved or non-planar surfaces. The ability to bend and stretch on flexible substrates introduces a new dimension for electromagnetic wave manipulation. Figure 6(b) presents a method for fabricating a full-spectrum response mechanically tunable dielectric metasurface, consisting of TiO₂ nanoparticle arrays embedded in a PDMS substrate. These methods for manufacturing flexible metasurfaces offer new possibilities for designing and implementing optical devices, particularly in applications requiring bending and deformation, such as wearable devices and optical systems on complex surfaces.

 

Slanted dielectric metasurfaces involve the fabrication of nanostructures at non-perpendicular angles to the substrate, as shown in Figure 6(c). This metasurface consists of a TiO₂ slanted trapezoidal nanohole array on a glass substrate. The designed inclined geometry disrupts the symmetry both in-plane and out-of-plane, leading to the first observation of intrinsic chiral bound states in the continuum (BICs).

 

High aspect-ratio dielectric metasurface nanofabrication aims to achieve structures with high aspect ratios to enhance optical performance. Figure 6(d) shows the creation of high-aspect-ratio TiO₂ nanopillars using a top-down etching technique, achieving approximately 90° vertical sidewalls. This technique successfully realized a high-efficiency broadband achromatic metasurface lens suitable for near-infrared biological imaging.

 

Figure 6 Applications of extreme nanofabrication techniques in the fabrication of dielectric metasurfaces, including multilayer, flexible, inclined, and high aspect-ratio nanostructures.

 

6. Perspective and Outlook

 

The team led by Professor Din Ping Tsai from City University of Hong Kong and Professor Shumin Xiao from Harbin Institute of Technology (Shenzhen) has reviewed advanced manufacturing technologies for dielectric meta-devices, covering standard nanolithography, advanced nanolithography, large-scale nanolithography, and extreme fabrication methods. Each technique offers unique advantages suited for specific nanostructure fabrication requirements. The review highlights that transitioning from chip demonstrations to wafer-level production, or from wafer-level production to large-scale and small-batch chip applications, will be the mainstream trend in the future of metasurface manufacturing. Additionally, optimizing existing nanofabrication methods to enhance production efficiency, material utilization, and reduce fabrication costs remains the gold standard for metasurface manufacturing. High-performance dielectric meta-devices are expected to play a crucial role in building nanophotonics platforms and will drive advancements across various fields, including display technologies, optical sensors, and aerospace applications (see Figure 7).

 

Figure 7 Prospects for the bright future of all-dielectric meta-devices/optics with wafer-scale mass fabrication.

 

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