A method for fabricating a large-aperture superlens and the superlens

Abstract

This invention relates to the field of micro-nano optics technology and discloses a method for fabricating a large-aperture superlens and the superlens itself. The method includes: spin-coating a first substrate to form a photosensitive layer on its surface; using laser interference to form periodically distributed nano-stripes on the photosensitive layer to obtain a processed first substrate; selectively exposing, developing, and etching the processed first substrate according to the phase distribution map of the target superlens to obtain an imprint master with a nanostructure array; using the imprint master as a template, fabricating a metal hard mask corresponding to the nanostructure array using imprinting and electroplating processes; using the metal hard mask as an etching barrier layer to etch the target substrate, forming a nanostructure array on the target substrate, and obtaining the target superlens. This invention simultaneously solves the problem of simultaneously achieving large-aperture, complex nanostructure arrays and low cost and high efficiency in superlens manufacturing.

Description

Technical Field

[0001] This invention relates to the field of micro-nano optics technology, specifically to a method for fabricating a large-aperture superlens and the superlens itself. Background Technology

[0002] The fabrication of existing large-aperture superlenses is highly dependent on electron beam lithography or deep ultraviolet stepping lithography. When fabricating large-sized superlenses, electron beam lithography has low fabrication efficiency due to the point-by-point scanning characteristic of the electron beam. Deep ultraviolet stepping lithography has a limited exposure area per exposure, and the substrate needs to be moved during the fabrication process to perform multiple "stepping" or "scanning" exposures. However, the high cost of deep ultraviolet stepping lithography equipment leads to high fabrication costs for superlenses. Summary of the Invention

[0003] In view of the above problems, embodiments of the present invention provide a method for fabricating a large-aperture superlens and a superlens, so as to solve the problem that efficiency and cost cannot be balanced in the existing superlens fabrication technology.

[0004] According to one aspect of the present invention, a method for fabricating a large-aperture superlens is provided, the method comprising: A photosensitive layer is formed on the surface of the first substrate by spin coating, and a periodically distributed nano-stripes are formed on the photosensitive layer by laser interference to obtain the processed first substrate. Based on the phase distribution map of the target superlens, the processed first substrate is subjected to selective grayscale exposure; then, through development and etching, the etching depth is extended into the interior of the first substrate to obtain an imprint master with a nanostructure array. Using the embossing master as a template, a metal hard mask corresponding to the nanostructure array is prepared by embossing and electroplating processes. Using the metal hard mask as an etching barrier layer, the target substrate is etched to form a nanostructure array on the target substrate, thereby obtaining the target superlens.

[0005] In one alternative approach, the selective processing of the processed first substrate based on the phase distribution map of the target superlens to obtain an imprint master with a nanostructure array includes: Based on the optical parameters of the target superlens, the corresponding phase distribution map is calculated and generated; Based on the phase distribution map, the target phase values ​​at different locations are converted into corresponding exposure doses to generate a grayscale exposure map; Based on the grayscale exposure image, the processed first substrate is subjected to selective exposure processing, followed by etching processing, so as to selectively control the local feature size of the nano-stripes by superimposed exposure doses. The first substrate, after selective exposure processing, is developed to form the nanostructure array with spatially varying dimensions on the first substrate, thereby obtaining the imprint master.

[0006] In one alternative approach, the step of using the imprinting master as a template and employing imprinting and electroplating processes to fabricate a metal hard mask corresponding to the nanostructure array includes: A second substrate is obtained, the second substrate comprising an intermediate substrate, a metal seed layer and a polymer layer stacked sequentially; Using the embossing master as a template, the nanostructure array is copied onto the polymer layer of the second substrate using an embossing process to form an embossed pattern; The second substrate having the imprinted pattern is processed to expose the metal seed layer. Using the metal seed layer as a cathode, metal is deposited in the trenches of the imprinted pattern using an electroplating process until the trenches are filled and a continuous metal layer is formed on the surface of the imprinted pattern. A temporary bonding layer is formed on the surface of the metal layer using a coating process; The temporary bonding layer is peeled off to separate the metal layer from the second substrate, thereby obtaining the metal hard mask with the temporary bonding layer.

[0007] In one alternative approach, the etching of the target substrate using the metal hard mask as an etching barrier layer to form a nanostructure array on the target substrate and obtain the target superlens includes: The metal hard mask is tightly bonded to the surface of the target substrate by heating and pressurizing to obtain the substrate to be etched. Using the metal hard mask as the etching barrier layer, the substrate to be etched is deeply etched to obtain the target superlens.

[0008] In one alternative approach, the step of using the metal hard mask as the etching barrier layer to deeply etch the substrate to be etched and obtain the target superlens includes: The substrate to be etched is placed in the reaction chamber, and deep reactive ion etching based on Bosch technology is performed alternately in the following steps: A passivation gas is introduced into the reaction chamber to deposit a passivation protective layer on the sidewall surface of the substrate to be etched. Fluorine-containing etching gas is introduced into the reaction chamber to perform anisotropic etching on the exposed area of ​​the target substrate in the substrate to be etched. Repeat the above steps until the nanostructure array of a predetermined depth is formed in the target substrate; After etching is completed, the metal hard mask is removed to obtain the target superlens.

[0009] In one alternative embodiment, the fluorine-containing etching gas is SF6 gas, and the passivation gas is C4F8 gas.

[0010] In one alternative approach, the step of using heat and pressure to tightly bond the metal hard mask to the surface of the target substrate to obtain the substrate to be etched further includes: removing the temporary bonding layer in the substrate to be etched.

[0011] In one alternative approach, the temporary bonding layer is a water-soluble polymer.

[0012] In one alternative embodiment, the second substrate comprises a silica glass substrate, an indium tin oxide conductive layer, and an ultraviolet-curable imprinting adhesive layer stacked sequentially.

[0013] According to another aspect of the present invention, a superlens is provided, which is prepared by any one of the large-aperture superlens preparation methods described above.

[0014] This invention combines laser interference with selective exposure to achieve efficient fabrication of imprint master plates with large areas and complex nanostructure arrays. It also uses nanoimprinting and electroplating to prepare a metal hard mask as an etching barrier layer for the target substrate for high-fidelity deep etching. This simultaneously solves the problem of simultaneously achieving large aperture, complex nanostructure arrays, low cost, and high efficiency in the fabrication of superlenses.

[0015] The hard metal mask obtained in this embodiment can meet the high strength requirements of deep silicon etching in superlens fabrication. Furthermore, the fabrication method provided by this invention enables the repeated acquisition of the hard metal mask, solving the problem of needing to re-fabricate the patterned mask each time a superlens is fabricated in existing processes, thus further improving the fabrication efficiency of superlenses and reducing manufacturing costs.

[0016] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, and in order to make the above and other objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention are described below. Attached Figure Description

[0017] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Furthermore, the same reference numerals denote the same parts throughout the drawings. In the drawings: Figure 1 A schematic flowchart of the large-aperture superlens fabrication method provided in an embodiment of the present invention is shown. Detailed Implementation

[0018] The embodiments of the technical solution of the present invention will now be described in detail with reference to the accompanying drawings. These embodiments are merely illustrative of the technical solution of the present invention and are therefore intended to limit the scope of protection of the present invention. Example

[0019] Please see Figure 1 , Figure 1 A schematic flowchart of the large-aperture superlens fabrication method provided in an embodiment of the present invention is shown, as follows: Figure 1 As shown, this invention provides a highly efficient, low-cost manufacturing method suitable for fabricating large-aperture superlenses. By combining laser interference lithography, grayscale exposure, nanoimprinting, and electroplating transfer processes, it achieves large-scale replication of high-precision, high-aspect-ratio nanostructure arrays, obtaining target superlenses with the target nanostructure array. The fabrication method includes: S1: Spin-coating the first substrate to form a photosensitive layer on the surface of the first substrate, and using laser interference exposure to form periodically distributed nano-stripes on the photosensitive layer to obtain the processed first substrate.

[0020] The first substrate is preferably a silicon (Si) wafer or a quartz wafer with good thermal and chemical stability. The first substrate is treated to ensure a smooth and clean surface. A layer of photosensitive material is uniformly coated onto the surface of the first substrate using a spin-coating process to form a photosensitive layer with the required thickness and uniformity. This photosensitive material is typically a positive photoresist.

[0021] This embodiment employs laser interference lithography to expose the photosensitive layer: two coherent laser beams interfere on the surface of the photosensitive layer, forming alternating bright and dark, periodically distributed nanostripes, resulting in a first substrate with preliminary exposure. Through a single exposure process, perfectly periodic sinusoidal or near-sinusoidal intensity distribution nanostripes can be formed over an area of ​​several square centimeters or even larger. This allows the first substrate to possess a large-area, high-precision, and highly uniform periodic nanostripe pattern in a low-cost manner, significantly reducing manufacturing costs.

[0022] S2: Selective grayscale exposure is performed on the processed first substrate according to the phase distribution map of the target superlens, and then development and etching are performed to extend the etching depth into the interior of the first substrate to obtain an imprint master with a nanostructure array.

[0023] On the photoresist on the surface of the first substrate after processing in S1, selective exposure processing is performed based on the phase distribution map using techniques such as high-resolution projection lithography and grayscale direct-write lithography. Corresponding to different positions on the first substrate, the exposure dose of each pixel at different positions is controlled, and the crosslinking rate of the photoresist is locally changed based on the existing nanostripes. Then, the first substrate is developed and etched to extend the etching depth into the interior of the first substrate, thereby forming a nanostructure array with depth or shape varying with position on the first substrate, thus obtaining an imprint master.

[0024] This embodiment achieves large-area periodic nanostripes through initial exposure, and combines it with selective secondary exposure to enable independent depth / morphology control at different locations on the first substrate based on the corresponding phase distribution. This allows for high-precision achievement of almost arbitrary continuous or discrete phase distributions, meeting the requirements of superlenses for complex wavefront manipulation (such as phase amplitude polarization). Simultaneously, combined with projection high-resolution laser direct writing, it achieves feature size control and phase control precision at the tens of nanometer level, ensuring that the diffraction efficiency of the fabricated target superlens approaches the theoretical limit. Furthermore, selective exposure allows for local modification of the pre-fabricated highly uniform periodic nanostripes, preserving the large-area uniformity advantage of the periodic nanostripes while introducing the required non-periodic phase changes, making the process of preparing the imprint master both efficient and precise.

[0025] Furthermore, S2 includes: S21: Calculate and generate the corresponding phase distribution map based on the optical parameters of the target superlens; S22: Based on the phase distribution map, the target phase values ​​at different locations are converted into corresponding exposure doses to generate a grayscale exposure map; S23: Based on the grayscale exposure image, selective exposure processing is performed on the processed first substrate to selectively control the local feature size of the nano-stripes by superimposed exposure doses; S24: The first substrate, after selective exposure processing, is developed and then etched to form the nanostructure array with spatially varying dimensions on the first substrate, thereby obtaining the imprint master.

[0026] The phase distribution function of the target superlens is usually optimized by electromagnetic simulation software (such as FDTD, RCWA). A two-dimensional phase distribution map is obtained based on this phase distribution function. This two-dimensional phase distribution map is then converted into a grayscale image, where the grayscale values ​​(0-255) linearly correspond to the required nanostructure depth or shape changes. At the same time, this grayscale data needs to be overlaid and aligned with the grating period obtained in step S1.

[0027] The first substrate with nano-stripes obtained in S1 is placed in a grayscale direct-write lithography machine (such as a laser direct-write machine or a projection lithography system). The existing nano-stripes (grating structure) on the first substrate are used as alignment marks for precise positioning. The system controls the pixel modulation of the focused laser beam according to the input phase grayscale image to perform "pixel-level" variable dose exposure on the photoresist on the first substrate. Different exposure energies are applied to the positions where different phase delays are required.

[0028] The first substrate, after undergoing secondary exposure, is then developed. Due to spatial variations in the exposure dose (different spatial distributions), the width / duty cycle of the dissolved region of the photoresist differs at different locations, resulting in a nano-striped pattern with a constant period and spatially varying linewidth / duty cycle on the first substrate after development. Depth (or sidewall profile) modulation is generated on top of the original periodic stripes. For example, at locations requiring π phase delay, the grating ridges may be etched deeper or wider. Finally, the developed first substrate is etched to extend the etching depth into its interior, forming a complex array (nanostructure array) on the first substrate where the geometric parameters (height, diameter, shape) of nanostructure units (such as nanopillars and nanopores) vary with position, thus creating an imprint master. The structure on this imprint master directly reflects the target phase distribution.

[0029] S3: Using the embossing master as a template, a metal hard mask corresponding to the nanostructure array is prepared by embossing and electroplating processes.

[0030] Using the imprint master obtained in S2 as a template, the nanostructure array on the imprint master is copied onto the imprinting adhesive of the second substrate using nanoimprint lithography (NIL). After curing the imprinting adhesive, the imprinting adhesive is separated from the imprint master, resulting in an imprinted second substrate with a structure complementary to (or identical to) that of the imprint master. The imprinted image on the imprinted second substrate has an imprinted pattern corresponding to the nanostructure array on the imprint master.

[0031] A second substrate with an imprinted pattern is processed to expose the metal structure on the second substrate. The processed second substrate is then used as a cathode for electroplating (e.g., nickel electroplating, with metallic nickel as the anode). Metal ions (e.g., nickel ions) in the electrolyte are reduced and deposited on the cathode surface under the action of an electric field, gradually filling and covering the imprinted pattern to form a metal layer. Once the electroplated metal layer reaches the required thickness, it is separated from the second substrate to obtain a hard metal (e.g., nickel) template. The surface of this hard metal template has a precise relief pattern corresponding to the nanostructure array on the target superlens.

[0032] In this embodiment, the nanoimprint lithography process can almost perfectly replicate the subwavelength features on the master lithography template, and a large-area structure can be transferred in a single imprint, resulting in extremely high efficiency. Combined with electroplating, the soft polymer structure is transformed into a hard metal layer, perfectly solving the problem of the original template being easily damaged and unable to be directly used for multiple etching processes. Furthermore, the second substrate with the imprinted pattern obtained in this embodiment can be repeatedly used to obtain multiple hard metal templates from a single imprinted patterned second substrate, enabling the fabrication of multiple target superlenses with the same nanostructure array, further reducing the manufacturing cost of the superlenses.

[0033] S4: Using the metal hard mask as an etching barrier layer, the target substrate is etched to form a nanostructure array on the target substrate, thereby obtaining the target superlens.

[0034] Metal hard masks are characterized by high mechanical strength and good wear resistance. The metal hard mask is tightly bonded and fixed to the surface of the target substrate. The metal hard mask acts as an etching barrier layer, protecting the areas covered by it. Through etching processes, the desired nanostructure array is fabricated on the areas of the target substrate not covered by the metal hard mask, resulting in a large-aperture superlens.

[0035] The large-aperture superlens fabrication method provided in this invention involves forming large-area, periodically varying nanostripes on a first substrate using laser interference. Secondary exposure is used to locally control and modify the first substrate with the nanostripes. After development, an imprint master is obtained, significantly reducing the area and data volume required for direct writing to the first substrate. This further improves the fabrication efficiency of the imprint master without sacrificing design freedom. Using the imprint master as a template, a metal hard mask corresponding to the nanostructure array is fabricated using imprinting and electroplating processes. The metal hard mask serves as an etching barrier layer to etch the target substrate, resulting in the superlens. The imprint master in this embodiment can be reused to fabricate the metal hard mask, reducing the manufacturing cost of the superlens. The metal hard mask, acting as an etching barrier layer, allows for the processing of any suitable etchable solid material (such as TiO2, Si3N4, Si, GaN, etc.) as the target substrate. This enables the free selection of high-refractive-index, low-loss intrinsic optical materials based on the operating wavelength (visible light, infrared, terahertz) during superlens fabrication.

[0036] Furthermore, S3 includes: S31: Obtain a second substrate, the second substrate comprising an intermediate substrate, a metal seed layer and a polymer layer stacked sequentially; S32: Using the imprinting master as a template, the nanostructure array is copied onto the polymer layer of the second substrate using an imprinting process to form an imprinted pattern; S33: Process the second substrate having the imprinted pattern to expose the metal seed layer, and use the metal seed layer as a cathode to deposit metal in the trenches of the imprinted pattern using an electroplating process until the trenches are filled and a continuous metal layer is formed on the surface of the imprinted pattern. S34: A temporary bonding layer is formed on the surface of the metal layer using a coating process; S35: Peel off the temporary bonding layer to separate the metal layer from the second substrate and obtain the metal hard mask with the temporary bonding layer.

[0037] The intermediate substrate is usually made of low-cost glass with a flat surface. The metal seed layer is a metal material covering the surface of the intermediate substrate and is used as a cathode during electroplating. The polymer layer is a layer of polymer material (imprinting adhesive) coated on the metal seed layer that can be nanoimprinted. It is usually a UV-curable resin or thermoplastic polymer. The thickness of the polymer layer is required to be slightly thicker than the maximum depth of the nanostructure on the imprinting master.

[0038] The pattern face of the embossing master is aligned and pressed into the polymer layer of the second substrate. Under appropriate pressure, the polymer fills all the nanoscale grooves of the embossing master. Subsequently, the polymer is cured by ultraviolet light irradiation (for UV adhesives) or heating (for hot melt adhesives). After separating the second substrate and the embossing master, an embossed pattern with the opposite texture to the embossing master is formed in the polymer layer of the second substrate. The embossed pattern consists of a series of grooves (corresponding to the master's protrusions) and protrusions (corresponding to the master's grooves).

[0039] Residual polymer layer material remains at the bottom of the trench formed on the second substrate, causing the metal seed layer to be completely covered. This residual material is removed by etching or other processes to expose the underlying metal seed layer, ensuring that the second substrate can be used as a cathode in the future.

[0040] The second substrate with the exposed metal seed layer is immersed in the electroplating solution. The exposed metal seed layer at the bottom of the trench serves as the cathode. Electroplating is performed by connecting the power supply. Metal ions (such as Ni²⁺) gain electrons on the cathode surface (i.e., the exposed metal seed layer and the surface of the deposited metal), are reduced to metal atoms and deposited until a complete and robust metal layer is formed. This ensures that the metal layer has sufficient mechanical strength and durability as an etching mask.

[0041] A temporary bonding material is uniformly coated onto the surface of the metal layer using a spin-coating process to form a temporary bonding layer. When separating the second substrate and the metal hard mask, the metal hard mask, which is firmly adhered to the surface of the temporary bonding layer, is peeled off from the second substrate as a whole through this temporary bonding layer, resulting in a metal hard mask with the temporary bonding layer. Under specific conditions (such as heating, ultraviolet irradiation, or dissolution in a specific solvent), the adhesion of the material in this temporary bonding layer can be significantly reduced or eliminated, facilitating peeling.

[0042] After separating the metal hard mask with a temporary bonding layer from the second substrate, the second substrate is used as a repeatable electroplating template to continue preparing the metal hard mask through an electroplating process, which further improves the preparation efficiency of the superlens and reduces the preparation cost, so that the superlens preparation process can be both efficient and cost-effective.

[0043] Furthermore, S4 includes: S41: Heat and pressure are applied to make the metal hard mask fit tightly against the surface of the target substrate to obtain the substrate to be etched; S42: Using the metal hard mask as the etching barrier layer, perform deep etching on the substrate to be etched to obtain the target superlens.

[0044] Specifically, the material of the target substrate can be selected according to the working wavelength of the superlens. For example, for the visible light band, a transparent dielectric film with high refractive index and low loss is preferred, such as titanium dioxide (TiO2) or silicon nitride (Si3N4) films grown on transparent substrates (such as quartz or glass) by atomic layer deposition (ALD) or magnetron sputtering.

[0045] A hard metal mask is aligned with the surface of a target substrate. Uniform pressure (e.g., 0.1 to 1 MPa) and an appropriate temperature (e.g., 100°C to 200°C) are applied to the aligned hard metal mask and target substrate. Under the combined action of heat and pressure, the hard surface of the hard metal mask and the surface of the target substrate achieve close contact, minimizing air gaps between the interfaces and obtaining the substrate to be etched. Using dry etching technology, with the tightly fitted hard metal mask as protection, the material of the unprotected target substrate is selectively and anisotropically (vertically) removed, thereby transforming the two-dimensional planar pattern on the hard metal mask into a three-dimensional nanostructure on the target substrate.

[0046] Furthermore, S42 includes: S421: Place the substrate to be etched into the reaction chamber, and perform the following steps alternately using deep reactive ion etching based on Bosch technology: S4211: Passivating gas is introduced into the reaction chamber to deposit a passivation protective layer on the sidewall surface of the substrate to be etched. S4212: Fluorine-containing etching gas is introduced into the reaction chamber to perform anisotropic etching on the exposed area of ​​the target substrate in the substrate to be etched; S4213: Repeat steps S4211 and S4212 above until the nanostructure array of a predetermined depth is formed in the target substrate; S422: After etching is completed, the metal hard mask is removed to obtain the target superlens.

[0047] During deep etching, the metal hard mask, due to its extremely high chemical stability and physical hardness, is almost completely inert to etching gases and ion bombardment, perfectly serving as an etching barrier layer. The active groups and ions in the plasma can only etch the target substrate areas not protected by the metal mask. During etching, by precisely controlling parameters such as etching time, gas flow rate, pressure, and power, highly anisotropic etching can be achieved, forming deep trenches or pillars with vertical sidewalls and flat bottoms. The etching depth is achieved by controlling the process time to reach a preset value.

[0048] After etching reaches the target depth, the etching gas supply is stopped, the chamber is evacuated, and inert gas is introduced. The etched metal hard mask-target substrate is removed, and the metal hard mask and target substrate are separated. The separated target substrate is cleaned to obtain the target superlens.

[0049] This embodiment utilizes highly anisotropic dry etching to replicate patterns from a metal hard mask into the target material almost without damage or distortion. This is crucial for achieving subwavelength, high aspect ratio (5:1) nanostructures and forms the technological basis for obtaining broadband, high-efficiency superlenses. The metal hard mask (especially nickel) has an extremely high etching selectivity relative to the target substrate material. During the etching process, the wear of the metal hard mask itself is negligible, ensuring the uniformity and consistency of the etching depth on the target substrate. In addition, the metal hard mask is tightly bonded to the target substrate through a heat-pressurized bonding method, which helps to compensate for the microscopic warpage or unevenness of the large-size target substrate and the metal hard mask itself, ensuring contact uniformity across the entire large-aperture range (e.g., 200 mm diameter). This enables high uniformity of nanostructure depth and morphology across the entire superlens, ensuring consistent optical performance of the large-aperture superlens.

[0050] Furthermore, selective wet etching is employed to remove the metal mask, achieving separation between the hard metal mask and the target substrate. This avoids the risks associated with simple physical peeling, which can lead to the breakage, collapse, or detachment of fragile, high aspect ratio nanostructures (such as silicon nanopillars) from the target substrate under peeling forces, resulting in device damage. For example, aqua regia and other oxidizing acids can efficiently dissolve metals (hard metal masks) such as nickel (Ni) and gold (Au). However, aqua regia has a slow etching rate for silicon (Si) and many dielectric materials (such as silicon dioxide). By precisely controlling the etching time, the hard metal mask can be cleanly and thoroughly dissolved without damaging the underlying etched silicon nanopillar structure, ensuring the integrity, purity, and sidewall smoothness of the nanostructure on the superlens.

[0051] In one implementation of this embodiment, in step S1, the period of the nanostripes can be precisely controlled by adjusting the wavelength, incident angle, and optical path of the laser. By utilizing the wave nature of light, a large-area, periodically uniform nanostripe pattern can be generated in a single exposure across the entire illuminated area of ​​the first substrate, resulting in high efficiency and eliminating the need for complex masks. In step S2, different exposure doses are set at different locations using a phase distribution map to selectively expose the first substrate, followed by development to obtain an imprint master.

[0052] Specifically, the laser wavelength used in the interference exposure is 405nm, and the exposure dose is 4.0-5.0mJ / cm²; the secondary exposure dose of the direct laser writing system is 30-40mJ / cm², the scanning step size is 3600nm, the spot size is 4800nm, and the photoresist is preferably positive photoresist AZ701.

[0053] Furthermore, in step S3, the metal electroplated is preferably nickel or a nickel alloy.

[0054] In one implementation of this embodiment, sulfur hexafluoride (SF6) is used as the fluorine-containing etching gas, and octafluorocyclobutane (C4F8) is used as the passivation gas. An infrared superlens with single-crystal silicon (Si) as the target substrate is fabricated using the large-aperture superlens fabrication method. A nickel mask is used as the metal hard mask. The nickel mask-silicon substrate (the substrate to be etched) is processed using a Bosch process to obtain a high aspect ratio, high-fidelity silicon nanostructure array on the silicon substrate. This includes: placing the substrate to be etched into the reaction chamber of an inductively coupled plasma etching machine, maintaining the pressure inside the reaction chamber at 20 mTorr, and setting the radio frequency (ICP) power to 500 W, and alternately performing the following steps: a) Introduce C4F8 (5 sccm) into the reaction chamber and perform a 10-second sidewall passivation treatment to deposit a passivation protective layer on the sidewall surface of the substrate to be etched. b) Introduce SF6 (50 sccm) into the reaction chamber and perform isotropic etching on the exposed area of ​​the target substrate in the substrate to be etched for 8 seconds.

[0055] Repeat steps a) and b) above approximately 35 times, with a total etching time of approximately 0.5 hours (the specific number of cycles and total etching time can be set according to requirements). During the etching process, the silicon beneath the nickel mask is selectively removed, while the silicon not protected by the nickel mask is gradually etched, ultimately forming a silicon nanopillar array with a height of approximately 7 μm and a period of 3.2 μm. The diameter of the silicon nanopillars continuously varies within the range of 0.9–2.6 μm according to phase requirements.

[0056] The nickel mask is consumed in very small amounts during the entire etching process. After etching is completed, the nickel mask-silicon substrate is soaked in aqua regia (a mixture of hydrochloric acid and nitric acid) to completely remove the residual nickel mask and obtain a pure infrared superlens composed of high aspect ratio silicon nanopillars.

[0057] Different target substrate materials require different etching gases. For example, for TiO2 substrates, a mixed gas of Cl2 / BCl3 / Ar is commonly used: Cl2 and BCl3 provide chlorine radicals that react with titanium to generate volatile TiCl4, while Ar ions assist the reaction through physical bombardment and enhance anisotropy. For Si3N4 substrates, a mixed gas of CF4 / CHF3 / O2 is commonly used: fluorocarbon groups react with silicon to generate volatile SiF4, CHF3 provides carbon to form protective polymer sidewalls, and O2 regulates plasma chemistry by generating oxygen radicals, which oxidize and consume excess fluorocarbon polymers, preventing excessive polymer buildup in the etched area (especially the bottom) that could lead to "black silicon" or etching cessation.

[0058] In one implementation, after step S41, the method further includes: removing the temporary bonding layer from the substrate to be etched; the temporary bonding layer is preferably a water-soluble polymer, such as polyvinyl alcohol (PVA), polyethylene oxide (PEO), or derivatives thereof. The temporary bonding layer is used to assist in the separation of the release metal layer from the second substrate. After the metal hard mask-target substrate, with the temporary bonding layer attached and fixed, is placed in deionized water or a specific mild water-based solution to dissolve the temporary bonding layer. After dissolution, the metal hard mask-target substrate is removed from the water and rinsed with a large amount of deionized water to ensure that all polymer residues are washed away. Subsequently, the metal hard mask-target substrate is dried with high-purity nitrogen (N2) or under low pressure to prevent moisture residue.

[0059] In one implementation of this embodiment, the second substrate includes a silicon dioxide glass substrate, an indium tin oxide conductive layer, and an ultraviolet-curable imprinting adhesive layer stacked sequentially.

[0060] When obtaining the second substrate, an indium tin oxide (ITO) conductive layer is first formed on a 300nm thick silicon dioxide glass substrate through an electroplating process, and then a UV-curable imprinting adhesive is formed on the surface of the indium tin oxide (ITO) conductive layer through a spin coating process.

[0061] Using the imprint master obtained in step S1 as a template, the nanostructure array is copied onto the UV-cured imprinting adhesive layer of the second substrate using UV nanoimprinting technology. By precisely controlling the imprinting pressure and time, it is ensured that the UV-cured imprinting adhesive layer is completely filled into the trenches of the nanostructure array and the residual layer thickness is uniform (about 50 nm), thus obtaining a second substrate with an imprinted pattern.

[0062] The second substrate with the imprinted pattern is processed (e.g., dry etching) to expose the indium tin oxide (ITO) conductive layer at the bottom of the trenches in the imprinted pattern. The exposed ITO conductive layer on the second substrate is then immersed in a nickel plating solution, using ITO as the cathode for electroplating. The plating current density is 10 mA / cm², and the plating time is determined based on the required nickel mask thickness (approximately 250 nm). After plating, the trenches of the imprinted pattern are filled with metallic nickel, forming a nickel metal layer.

[0063] A water-soluble polyvinyl alcohol (PVA) film of about 15 μm thickness was spin-coated onto the surface of the electroplated nickel metal layer and cured at 80°C. The cured PVA film, together with the nickel metal layer embedded in the PVA film, was peeled off from the glass / ITO substrate to obtain a nickel mask with a PVA film.

[0064] A nickel mask with a PVA film is attached to the surface of a silicon wafer (target substrate) and hot-pressed at 90°C and 0.5 MPa for 30 seconds to create a strong adhesion between the nickel mask and the silicon wafer, thus obtaining the substrate to be etched. The substrate to be etched is then immersed in deionized water and gently shaken until the PVA film is completely dissolved, leaving a nickel mask tightly adhered to the surface of the silicon wafer, serving as an etching barrier layer for the silicon wafer. Example

[0065] Based on the large-aperture superlens fabrication method provided in Example 1, Example 2 provides a superlens fabricated using the same method. This superlens has a nanostructure array occupying a large area on its surface. The superlens is made of functional optical materials with high refractive index and low optical loss, including but not limited to: amorphous silicon (a-Si), hydrogenated amorphous silicon (a-Si:H), silicon nitride (Si3N4), titanium dioxide (TiO2), tantalum oxide (Ta2O5), single-crystal silicon (c-Si), germanium (Ge), and gallium arsenide (GaAs).

[0066] The aperture of this superlens is no longer limited by the scanning field of the direct writing device, enabling large sizes at the centimeter or even wafer level. At the same time, because the superlens is based on high refractive index materials and high aspect ratio structures, it has excellent optical performance with high numerical aperture (NA), high diffraction efficiency (>80%) and wide working bandwidth.

[0067] It should be noted that, unless otherwise stated, the technical or scientific terms used in the embodiments of the present invention should have the ordinary meaning as understood by those skilled in the art to which the embodiments of the present invention pertain.

[0068] Furthermore, technical terms such as "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. In the description of embodiments of the present invention, "a plurality of" means two or more, unless otherwise explicitly defined.

[0069] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention. In particular, as long as there is no structural conflict, the various technical features mentioned in the embodiments can be combined in any way. The present invention is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A method for fabricating a large-aperture superlens, characterized in that, The preparation method includes: A photosensitive layer is formed on the surface of the first substrate by spin coating, and a periodically distributed nano-stripes are formed on the photosensitive layer by laser interference exposure to obtain the processed first substrate. Based on the phase distribution map of the target superlens, the processed first substrate is subjected to selective grayscale exposure, followed by development and etching to extend the etching depth into the interior of the first substrate, thereby obtaining an imprint master with a nanostructure array. Using the embossing master as a template, a metal hard mask corresponding to the nanostructure array is prepared by embossing and electroplating processes. Using the metal hard mask as an etching barrier layer, the target substrate is etched to form a nanostructure array on the target substrate, thereby obtaining the target superlens.

2. The method for preparing a large-aperture superlens according to claim 1, characterized in that, The process involves selective grayscale exposure and development of the processed first substrate based on the phase distribution map of the target superlens, followed by etching to extend the etching depth into the interior of the first substrate, resulting in an imprint master with a nanostructure array. This includes: Based on the optical parameters of the target superlens, the corresponding phase distribution map is calculated and generated; Based on the phase distribution map, the target phase values ​​at different locations are converted into corresponding exposure doses to generate a grayscale exposure map; Based on the grayscale exposure image, the processed first substrate is subjected to selective exposure processing to selectively control the local feature size of the nanostripes by superimposing exposure doses; The first substrate, after undergoing selective exposure processing, is developed and then etched to form the nanostructure array with spatially varying dimensions on the first substrate, thereby obtaining the imprint master.

3. The method for preparing a large-aperture superlens according to claim 2, characterized in that, The process of using the imprinting master as a template and employing imprinting and electroplating processes to fabricate a metal hard mask corresponding to the nanostructure array includes: A second substrate is obtained, the second substrate comprising an intermediate substrate, a metal seed layer and a polymer layer stacked sequentially; Using the embossing master as a template, the nanostructure array is copied onto the polymer layer of the second substrate using an embossing process to form an embossed pattern; The second substrate having the imprinted pattern is processed to expose the metal seed layer. Using the metal seed layer as a cathode, metal is deposited in the trenches of the imprinted pattern using an electroplating process until the trenches are filled and a continuous metal layer is formed on the surface of the imprinted pattern. A temporary bonding layer is formed on the surface of the metal layer using a coating process; The temporary bonding layer is peeled off to separate the metal layer from the second substrate, thereby obtaining the metal hard mask with the temporary bonding layer.

4. The method for preparing a large-aperture superlens according to claim 3, characterized in that, The process of etching the target substrate using the metal hard mask as an etching barrier layer to form a nanostructure array on the target substrate and obtaining the target superlens includes: The metal hard mask is tightly bonded to the surface of the target substrate by heating and pressurizing to obtain the substrate to be etched. Using the metal hard mask as the etching barrier layer, the substrate to be etched is deeply etched to obtain the target superlens.

5. The method for preparing a large-aperture superlens according to claim 4, characterized in that, The step of using the metal hard mask as the etching barrier layer to perform deep etching on the substrate to be etched to obtain the target superlens includes: The substrate to be etched is placed in the reaction chamber, and deep reactive ion etching based on Bosch technology is performed alternately in the following steps: A passivation gas is introduced into the reaction chamber to deposit a passivation protective layer on the sidewall surface of the substrate to be etched. Fluorine-containing etching gas is introduced into the reaction chamber to perform anisotropic etching on the exposed area of ​​the target substrate in the substrate to be etched. Repeat the above steps until the nanostructure array of a predetermined depth is formed in the target substrate; After etching is completed, the metal hard mask is removed to obtain the target superlens.

6. The method for preparing a large-aperture superlens according to claim 5, characterized in that, The fluorine-containing etching gas is SF6 gas, and the passivation gas is C4F8 gas.

7. The method for preparing a large-aperture superlens according to claim 4, characterized in that, The process involves heating and pressurizing the metal hard mask to tightly adhere to the surface of the target substrate to obtain the substrate to be etched. Afterward, the process further includes removing the temporary bonding layer from the substrate to be etched.

8. The method for preparing a large-aperture superlens according to claim 7, characterized in that, The temporary bonding layer is a water-soluble polymer.

9. The method for preparing a large-aperture superlens according to claim 3, characterized in that, The second substrate includes a silicon dioxide glass substrate, an indium tin oxide conductive layer, and an ultraviolet-curable imprinting adhesive layer stacked sequentially.

10. A superlens, characterized in that, The superlens is prepared by the method for preparing a large-aperture superlens as described in any one of claims 1-9.

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