Method for preparing special functionalized lithium niobate superstructure surface based on focused ion beam

Abstract

The application belongs to the technical field of micro-nano integrated devices. A method for preparing a special functional lithium niobate superstructure surface based on a focused ion beam is provided. The cut thin film lithium niobate is cleaned, a hard mask is prepared on the substrate surface by magnetron sputtering, and annealing modification is performed; the thin film lithium niobate with the mask is etched by a gallium-based focused ion beam, the beam current and the bombardment angle are matched as needed, the processed superstructure unit array is used as a reference to realize regional splicing and expansion, the hard mask is stripped after processing, and secondary cleaning is performed to obtain a functional lithium niobate superstructure surface with a large area or a tilted configuration. The application can expand the processing area without additional alignment marks, and can prepare an adjustable tilted chiral structure in a single etching process, solving the problems of limited processing area and difficulty in forming special configurations in traditional processes, and being suitable for chiral polarization control and nonlinear optical device preparation.

Description

Technical Field

[0001] This invention relates to the field of micro-nano integrated device technology, and specifically to a method for preparing a specially functionalized lithium niobate metasurface based on focused ion beam. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Lithium niobate, with its excellent electro-optic properties and wide transmission spectrum, has become a core functional material in the field of integrated photonic chips. Bonded thin-film lithium niobate substrates further enhance the light field confinement capability, meeting the R&D needs of various micro / nano metasurface devices. Metasurfaces achieve special optical manipulation effects through the arrangement of micro / nano units, showing broad application prospects in areas such as chiral polarization control and nonlinear optical frequency conversion. Focused ion beam (FIP) technology, relying on physical sputtering to achieve maskless direct writing, can fabricate three-dimensional micro / nano structures that are difficult to form using conventional etching processes. It is gradually becoming the mainstream processing method for the refined fabrication of hard dielectric crystal metasurfaces, continuously driving the process iteration and product commercialization of lithium niobate-based metasurface devices.

[0004] Current focused ion beam (FIB) fabrication processes for lithium niobate metasurfaces suffer from a critical shortcoming: they cannot simultaneously achieve large-area processing and integrated molding of controllable tilted structures. Conventional processing methods can only complete etching of small, single-area regions, and cannot leverage pre-formed microstructures for autonomous alignment and splicing to expand the processing area. Furthermore, it is difficult to prepare oriented, tilted chiral nanostructures by controlling the tilt angle in a single etching process. This limits the mass production of large-size, specially configured functionalized lithium niobate metasurfaces and hinders the industrialization of related polarization-controlled metasurface devices. Summary of the Invention

[0005] To address the problem that traditional processes cannot simultaneously achieve the integrated fabrication of large-area splicing and controllable tilted configurations, this invention provides a method for fabricating specially functionalized lithium niobate metasurfaces based on focused ion beams. The fabricated spliced ​​metasurfaces can effectively expand the processing area while maintaining the period and feature size unchanged. This makes the impact of excessive etching at the seams due to dose accumulation on the overall device performance less than the performance gain brought about by the area expansion.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing specially functionalized lithium niobate metasurfaces based on focused ion beams includes the following steps: Clean lithium niobate film is obtained by cleaning the cut and shaped thin film lithium niobate; A hard mask layer was prepared on the clean surface of thin-film lithium niobate by magnetron sputtering and then annealed to obtain a masked thin-film lithium niobate. The gallium-based focused ion beam is used to etch and shape the masked thin film of lithium niobate. The ion beam parameters and ion beam bombardment angle are selected according to the optical function of the device. As needed, the multi-region splicing and expansion is completed by using the positioning reference of the pre-formed metacell array. After all structural processing is completed, the hard mask layer is peeled off and cleaned again to obtain a functionalized lithium niobate metasurface with a large area or tilted configuration.

[0007] In one implementation of the present invention, the thin-film lithium niobate is configured with a Z-axis or X-axis cross-section, and the thickness of the thin-film lithium niobate ranges from 300 nanometers to 900 nanometers.

[0008] In one implementation of the present invention, the cleaning process sequentially employs acetone ultrasonic cleaning, isopropanol rinsing, deionized water rinsing, and nitrogen purging and drying, with the ultrasonic cleaning time for acetone controlled between 1 and 3 minutes.

[0009] In one implementation of the present invention, the hard mask layer is made of metallic chromium, the deposition environment of metallic chromium magnetron sputtering is an argon atmosphere with a pressure between 2 Pa and 3 Pa, and the thickness of the hard mask layer is set to 25 nanometers to 35 nanometers.

[0010] In one implementation of the present invention, the annealing modification treatment is carried out under a nitrogen protective atmosphere, the annealing ambient temperature is 180 degrees Celsius to 220 degrees Celsius, and the annealing holding time is 8.5 hours to 9.5 hours.

[0011] In one implementation of the present invention, the accelerating voltage of the gallium-based focused ion beam is fixed at 30 kV, and the ion residence time parameter is controlled between 8 microseconds and 12 microseconds.

[0012] In one implementation of the present invention, the ion beam current is adjustable in the range of 0.02 nanoamperes to 0.3 nanoamperes, and the ion beam bombardment tilt angle is adjustable in the range of 65 degrees to 90 degrees.

[0013] In one implementation of the present invention, the multi-region splicing expansion uses the edge rows and columns of the pre-processed meta-unit array as positioning references, and achieves precise alignment and splicing of adjacent processed regions based on sample stage displacement adjustment and ion beam scanning deflection correction.

[0014] In one implementation of the present invention, the stripping operation of the hard mask layer uses a mixed etchant of nitric acid and cerium ammonium nitrate, and the soaking time at room temperature is controlled to be 2 to 4 minutes. After the stripping is completed, the sample is rinsed with deionized water.

[0015] In one implementation of the present invention, a chiral nanopillar array metasurface with a continuously adjustable tilt angle of 5 to 20 degrees is prepared on the surface of a thin film lithium niobate by adjusting the ion beam bombardment tilt angle and the sample stage spatial tilt angle in a single etching process.

[0016] Compared with the prior art, the beneficial effects of the present invention are: This invention specifically addresses the shortcomings of traditional processes that cannot simultaneously achieve the integrated fabrication of large-area splicing and controllable tilted configurations. First, the thin-film lithium niobate is cleaned, and then a hard chromium mask is prepared by magnetron sputtering and annealed for modification. The mask not only conducts the surface charge accumulation on the insulating substrate and avoids processing deviations caused by ion beam skew, but also improves the regularity of the etched sidewalls, ensuring the accuracy of subsequent focused ion beam forming. Using a gallium-based focused ion beam for etching allows for flexible selection of beam current and bombardment tilt angle. Leveraging the equipment's five-axis angle adjustment capability, tilted chiral nanostructures with continuously adjustable tilt angles can be fabricated in a single etching operation, eliminating multiple sequential processing steps. This approach uses an existing meta-cell array as an in-situ alignment reference for splicing and expansion, eliminating the need for additional alignment marks and avoiding alignment errors caused by mark processing. It effectively expands the overall processing area of ​​the meta-surface without changing the cell period and size. After processing, the hard mask is removed and a second cleaning is performed, ultimately producing a functionalized lithium niobate meta-surface with both a large surface area and a special tilted configuration. The nonlinear optical response of the product is significantly improved, effectively expanding the practical application range of lithium niobate meta-surfaces in chiral polarization control and nonlinear optical devices.

[0017] This invention utilizes the high-resolution imaging capability of a FIB / SEM dual-beam system, using the edge of the metasurface unit array in the already processed area as a natural alignment reference. No additional positioning markers are required. By adjusting the sample stage position and ion beam scanning deflection, precise alignment and continuous processing of adjacent processing areas are achieved. A key feature of this technology is that the in-situ observation capability of the FIB system allows the already processed structure itself to be used directly as an alignment reference, avoiding the additional errors introduced by marker preparation in traditional electron beam lithography splicing processes. The spliced ​​metasurfaces prepared using this technology can achieve effective expansion of the processing area while maintaining the period and feature size. The excessive etching at the seams due to dose accumulation has a lower impact on the overall device performance than the performance gain from the increased area.

[0018] This invention utilizes the five-axis degree-of-freedom design of a focused ion beam stage to provide a single-step fabrication method for tilted nanostructures. By adjusting the stage tilt angle and the ion beam incident direction, an array of nanopillars with an overall tilted orientation can be directly fabricated on a single-layer thin film, with the tilt angle continuously adjustable within the range of 5° to 20°. Compared to traditional inductively coupled plasma etching, which can only achieve vertical incidence and is difficult to fabricate controllable tilted structures in a single process, this method has irreplaceable technological advantages in the fabrication of tilted structures. The geometry of this tilted nanopillar array itself possesses chiral characteristics, enabling it to produce differential optical responses to left-handed and right-handed circularly polarized light. This makes it suitable for chiral metasurfaces, asymmetric light transmission, and beam manipulation. This invention is also applicable to next-generation semiconductor materials with high hardness and low conductivity, and is expected to realize application value in the integrated circuit industry.

[0019] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0020] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0021] Figure 1 This is a schematic diagram illustrating the processing flow and characterization of a spliced ​​metasurface, provided as an exemplary embodiment of the present invention. Figure 1 (a) is a three-dimensional process principle diagram of 2×2 partition in-situ splicing to prepare lithium niobate metasurface. It shows the complete processing flow from blank LNOI substrate, sequentially FIB etching of each region, and four splicing to form a whole large-area metasurface array. Figure 1 (b) is a high-magnification SEM micro-morphology image of the splicing seam with a scale bar of 1 μm. It clearly shows the arrangement details of the nano-cylinder array on both sides of the splicing boundary line. The nano-cylinder period at the seam position is continuous without obvious misalignment. Figure 1 (c) is a SEM image of the spliced ​​meta-array under medium field of view, with a scale bar of 2μm, showing the overall arrangement of the local array and the macroscopic morphology of the splicing seam. The blank area in the lower right corner is the unetched original lithium niobate substrate. Figure 1 (d) is a panoramic SEM image of the entire large-area metasurface after complete 2×2 splicing, with a scale bar of 5μm, which intuitively presents the complete array outline and overall splicing boundary formed by splicing four sub-regions. Figure 1 (e) in the figure is a comparative spectrum curve of second harmonic (SHG) performance; the horizontal axis is the second harmonic wavelength (360~440nm), and the vertical axis is the relative intensity of the harmonic. The blue curve is the test result of the small-sized metastructure without splicing, and the red curve is the test result of the spliced ​​large-area metastructure of the present invention, which shows that the nonlinear frequency doubling intensity of the spliced ​​device is greatly improved.

[0022] Figure 2 A schematic diagram and a morphological illustration of a tilted metasurface provided as an exemplary embodiment of the present invention, wherein, Figure 2 (a) is a schematic diagram of the cross-sectional principle of preparing a metasurface of tilted chiral nanopillars of lithium tantalate by tilted incident focused ion beam (FIB) etching. The tilt angle of the ion beam incident is marked θ, which intuitively shows the formation mechanism of the trapezoidal nanopillar array with tilted sidewalls formed by physical sputtering when the ion beam bombards the LiTaO3 substrate at an angle of θ. Figure 2(b) shows the top-view SEM morphology of the finished nanopillar arrays with four different tilt angles. From left to right, they correspond to ion tilt angles θ=5°, θ=10°, θ=15°, and θ=20°, respectively. The scale of each single icon is 1μm, which clearly and intuitively shows the molding effect that the tilt degree of the nanopillars increases synchronously with the gradual increase of the incident tilt angle. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0024] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0025] Lithium niobate (LiNbO3, LN) has become an important material for high-performance photonic integrated circuits due to its wide transparency window, high electro-optic coefficient, and ultra-low optical loss. Recent breakthroughs in thin-film technology have enabled the integration of submicron-thick single-crystal LN films with low-refractive-index substrates (such as SiO2) via ion slicing and wafer bonding. The resulting LN-on-insulator (LNOI) platform provides significantly improved refractive index contrast and optical field confinement, thus laying an ideal material foundation for the integration of micro / nano photonic devices. However, the inherent high hardness and chemical stability of LN pose significant challenges to nanoscale patterning: ICP etching, commonly used in semiconductor processes, introduces LiF impurities; the material's hardness and brittleness lead to microcracks and amorphization; and lattice anisotropy results in different etching rates for different orientations. To address these challenges, researchers have explored processes including ion implantation-assisted etching, hard mask-assisted dry etching, and heterogeneous integration. However, these approaches are inherently limited by complex processes and byproduct contamination. Developing a high-precision, low-damage direct fabrication method is of significant research importance.

[0026] Focused ion beam (FIB) is a high-precision micro / nano fabrication technique. Its core principle is to focus ions emitted from a liquid metal ion source into a beam and bombard the sample surface, removing material from specific locations through physical sputtering. Applying FIB to the micro / nano fabrication of hard materials such as LN has several inherent advantages: (1) FIB is a purely physical sputtering process, independent of the chemical properties of the material, and does not require chemical etchants, thus avoiding common semiconductor process problems such as byproduct generation; (2) This technology does not require a mask and removes material from specific areas through direct writing, significantly shortening the process cycle; (3) Through precise beam control and scanning, FIB can fabricate special three-dimensional nanostructures, which is difficult to achieve with traditional photolithography and etching methods; (4) In addition to sputtering removal, FIB can also utilize ion beam-induced deposition, implantation, and stress engineering effects to further enrich morphology control methods.

[0027] However, for dielectric crystals such as LN, FIB fabrication faces additional challenges due to the low conductivity of the material. Insulating samples accumulate surface charge under ion beam bombardment, and the resulting electrostatic field deflects the incident ion beam, leading to a decrease in beam focus quality and introducing significant positioning errors and image distortion. Using a metal mask layer not only improves the steepness of the sidewalls but also provides an effective charge dissipation path, and has been widely used in FIB fabrication of LN devices. However, FIB-based fabrication is still limited to small-scale, simple structural devices. Based on this, this invention further optimizes the FIB process and systematically studies the influence of key parameters on the morphology of LN nanostructures.

[0028] The method for preparing specially functionalized lithium niobate metasurfaces based on focused ion beams proposed in this invention includes the following steps: Step S101: Clean the cut thin film lithium niobate (TFLN); specifically, the thin film lithium niobate is z-cut or x-cut, and the thickness is 300-900nm.

[0029] Lithium niobate single crystals belong to the trigonal crystal system. Different crystal orientation slices have significant differences in optical refractive index, electro-optic coefficient, and ion sputtering etching rate: Z-axis slices (Z-cut LN) have lower optical field transmission loss and excellent electro-optic modulation coefficient, and are mostly used for the preparation of polarization-controlled chiral metasurfaces; X-axis slices (X-cut LN) have higher nonlinear second-order polarizability and outstanding second harmonic conversion efficiency, and are preferred for the processing of nonlinear frequency conversion and frequency doubling metasurfaces.

[0030] Optionally, the selection of film thickness (300-900 nm) is determined by the height of the target meta-nanopillar: 300-500 nm thin LN films are suitable for short nanopillar metastructures with a height of less than 200 nm, used for visible light polarization devices; 600-900 nm thick films are suitable for 300-700 nm high aspect ratio nanopillar arrays, meeting the light field confinement requirements of near-infrared nonlinear metasurfaces. A standard 2-5 μm silica buffer layer is installed under the film. Silica has a much lower refractive index than lithium niobate, relying on the refractive index difference to achieve total internal reflection of the light field, confining it within the lithium niobate film. The bottom 250-300 μm lithium tantalate / silicon substrate provides mechanical support. The thermal expansion coefficient of the lithium tantalate substrate is close to that of lithium niobate, making the film less prone to warping and cracking during high and low temperature processes. The silicon substrate is also cheaper, suitable for low-cost commercial device mass production.

[0031] Optionally, before formally entering the S101 cleanroom process, the thin-film lithium niobate substrate must first be cut. Commercial finished LNOI wafers are mostly 4-inch or 6-inch large circular wafers, which cannot be directly placed into the magnetron sputtering chamber and FIB equipment sample stage. They need to be cut according to the device design size using a diamond wafer dicing machine. The preferred cutting sizes for conventional experimental samples are 10mm×10mm and 15mm×15mm. During the cutting process, the high-speed grinding of the diamond wheel will leave three major types of contaminants on the sample surface: diamond powder debris, coolant organic oil stains, and wafer protective wax. At the same time, dust particles in the ambient air and human fingerprint oil will also adhere to the surface of the thin-film lithium niobate during wafer transportation and storage. If the above impurities are not thoroughly removed, they will cause pinholes and peeling defects in the subsequent chromium mask sputtering coating, directly damaging the accuracy of the subsequent FIB etching pattern.

[0032] Optionally, the graded chemical cleaning scheme in this step is designed to target the removal of different types of contaminants: The first step is acetone ultrasonic cleaning. Acetone is a highly polar organic solvent that can fully dissolve residual protective paraffin, organic cutting fluid, and fingerprint-like organic contaminants. Short ultrasonic cleaning (1-3 minutes) ensures thorough dissolution of organic matter while avoiding delamination and cracking at the interface between the thin film lithium niobate and the underlying silica insulating layer caused by prolonged ultrasonic cleaning. The second step is isopropanol rinsing. Isopropanol and acetone are miscible in any proportion and are used to replace residual acetone in the sample crevices, preventing the precipitation of solutes and the formation of new organic residues during rapid acetone evaporation. The third step is high-pressure rinsing with deionized water to remove inorganic dust and solid particulate impurities from the sample surface. A water with a resistivity of 18.2 MΩ is selected. cm ultrapure water is used to prevent the precipitation and scaling of calcium and magnesium ions from tap water; the fourth step is high-purity nitrogen purging and drying, using industrial high-purity nitrogen with a purity of 99.999%, and blowing the sample at a uniform speed with the nitrogen gun outlet at a 45° angle, using high-speed airflow to remove the surface moisture and avoid water stains left by the evaporation of water solvent during natural air drying.

[0033] Step S102: A hard mask is deposited on TFLN by magnetron sputtering, followed by annealing. Specifically, annealing is performed for 8.5 to 9.5 hours at 180 to 220°C under N2 atmosphere. The hard mask used in this invention is chromium (Cr-), which is deposited by magnetron sputtering in an Ar gas environment with a pressure of 2 to 3 Pa. The thickness of the hard mask is 25 to 35 nm.

[0034] Optionally, the magnetron sputtering equipment used in this step is a high-vacuum DC magnetron sputtering coating system. Before starting the coating process, the sputtering chamber needs to be pre-treated: first, the mechanical pump pre-evacuates the chamber to below 5 Pa, then the molecular pump is turned on to evacuate the chamber to a high vacuum, and the base vacuum is evacuated to 5 × 10⁻⁶ Pa. -4 After Pa, the evacuation valve is closed, and high-purity argon gas is introduced as a sputtering protection and ionization gas. The chamber pressure is precisely controlled and stabilized in the range of 2~3 Pa. Argon gas, as an inert gas, is ionized by high voltage between the cathode chromium target and the anode sample base to form argon plasma. High-energy argon ions in the plasma bombard the high-purity metallic chromium target (chromium target purity ≥99.99%, circular planar target specification Φ50mm) under the acceleration of the electric field. After the chromium atoms on the target surface are sputtered off, they are directionally deposited on the surface of the thin film lithium niobate sample below. The chromium film thickness is precisely controlled and stabilized at 25~35nm by adjusting the sputtering power and sputtering time.

[0035] Optionally, after chromium film deposition, the film is immediately transferred to a tubular annealing furnace for annealing modification. Before annealing, high-purity nitrogen is continuously purged for 30 minutes to replace the air inside the furnace tube, thoroughly removing oxygen and moisture to prevent the chromium film from oxidizing at high temperatures and forming a chromium oxide insulating layer that would damage its conductivity. Nitrogen is circulated at a constant rate throughout the process to maintain an inert protective atmosphere. The furnace heating rate is controlled at 2℃ / min, slowly increasing to 180~220℃ and setting a holding temperature to avoid cracking caused by rapid heating due to thermal expansion and contraction of the chromium film. After holding at this temperature for 8.5~9.5 hours, the film is naturally cooled to room temperature at a rate of 1℃ / min to prevent a sudden increase in internal stress caused by rapid cooling. After annealing modification, the chromium hard mask exhibits a conductivity uniformity improvement of over 40% and significantly enhanced resistance to ion bombardment sputtering.

[0036] Step S103: Etch the metasurface on the TFLN using a focused ion beam (FIB). The focused ion beam is Ga. + The ion beam has an accelerating voltage of 30 kV and a dwell time of 8–12 μs.

[0037] Optionally, a fixed acceleration voltage of 30kV for the gallium-based focused ion beam is selected as the optimal parameter after hundreds of comparative experiments: when the acceleration voltage is below 25kV, the kinetic energy of Ga+ ions is insufficient, resulting in low lithium niobate sputtering yield and a significant decrease in etching efficiency; when the acceleration voltage is above 35kV, high-energy ions are prone to ion implantation, and a large number of gallium ions implanting into the surface lattice of lithium niobate causes amorphization of the material and deteriorates the optical properties of the metasurface. Therefore, 30kV is locked as the unified acceleration voltage. The ion dwell time of 8~12μs represents the duration of bombardment of a single beam of ions at a single pixel: a dwell time of <8μs results in insufficient material removal by single-point sputtering, and the etching depth of the nanopillars does not meet the design specifications; a dwell time of >12μs results in excessive ion dose at a single point, leading to local over-etching and sidewall chipping problems. 10μs is a compromise and preferred intermediate value.

[0038] Step S104: Select appropriate ion beam current and bombardment angle according to the specific requirements of the device; specifically, the ion beam current adjustment range is 0.02~0.3nA, depending on the size of the target device, to achieve a balance between processing quality and efficiency. The ion beam bombardment angle adjustment range is 65-90°, depending on the functional requirements of the device design.

[0039] Optionally, the ion beam current of 0.02~0.3nA allows for bidirectional control of processing efficiency and precision: a small beam current of 0.02~0.1nA results in a smaller beam spot size, suitable for fine etching of high-precision meta-units in short cycles; a medium to large beam current of 0.1~0.3nA provides higher ion flux, removing more material per unit time through sputtering, suitable for accelerated processing of large-area spliced ​​meta-surfaces. The native bombardment angle of the ion beam, 65°~90°, is the angle between the ion beam and the sample normal. Combined with the pitch rotation of the five-axis sample stage, this effectively changes the actual incident angle of the ions, ultimately achieving the overall tilting of nanopillars from 5° to 20°. A 90° vertical incident angle corresponds to conventional upright nanopillar meta-structures, while the 65° tilt native beam angle, combined with sample stage tilt adjustment, covers the entire target tilt range.

[0040] Step S105: Based on the required fabrication area, select whether a splicing operation is needed, and the number of splices; the specific operation of the splicing process is as follows: the edge row of the metasurface unit array already fabricated in the previous region is used as a natural reference. By adjusting the sample stage position and ion beam scanning deflection, the pattern layout of the current region is precisely aligned with the previous region, thereby enabling the continuous fabrication of adjacent regions while maintaining a consistent period and feature size.

[0041] Optionally, the splicing process of this invention abandons the traditional photolithographic alignment mark preparation process. Instead, it relies on in-situ imaging using a FIB-SEM dual-beam system to achieve markless self-sponsoring. A single large-area metastructure to be processed is pre-divided into N equal sub-regions in the CAD layout. The sub-regions have uniform specifications, and the size of each sub-region is controlled within the FIB single-processing stability limit (50μm×50μm). The processing sequence follows a left-to-right, top-to-bottom, block-by-block processing method. After etching the first reference sub-region, the SEM high-resolution imaging mode is activated to magnify and observe the morphology of the outermost row and column of metastructure nanopillar array in the reference region. The outer contour edge of the nanopillars serves as the natural physical alignment reference. Two corrections are performed through the FIB equipment software control system: first, coarse adjustment of the sample stage's X / Y axis displacement at the micrometer level to achieve overall coarse alignment of the next sub-region to be processed; second, fine adjustment of the ion beam scanning deflection signal software to perform nanometer-level precise offset compensation for the ion beam scanning range, eliminating residual errors in the sample stage's mechanical displacement and ensuring seamless connection of adjacent sub-region metastructure units and continuous and uniform array periods.

[0042] In practical engineering applications, various splicing array specifications such as 2×2, 3×3, and 4×4 can be flexibly set according to the target total area of ​​the device. In response to the problem of excessive etching due to the inherent dose accumulation at the splicing seam, this invention slightly reduces the etching dose of the adjacent column of nanopillars at the seam by 5% to 8% in advance during the layout design stage to compensate for the ion dose superposition effect and further reduce the optical loss caused by excessive etching at the seam. Experimental data proves that this fine-tuning scheme can reduce the second harmonic loss of the splicing device by more than 12%.

[0043] Step S106: Remove the hard mask and clean the sample to complete the preparation of the special functionalized metasurface. Specifically, the method for removing the hard mask and cleaning is as follows: immerse the processed sample in a nitric acid-cerium ammonium nitrate mixed solution for 2-4 minutes at room temperature and rinse with deionized water.

[0044] Optionally, the etching solution for chromium mask stripping is a deionized aqueous solution of nitric acid and cerium ammonium nitrate, with a preferred mass ratio of 6% nitric acid, 12% cerium ammonium nitrate, and 82% ultrapure water. Nitric acid provides an acidic etching environment, while cerium ammonium nitrate, as a strong oxidizing ligand, undergoes a coordination oxidation reaction with solid metallic chromium under room temperature and atmospheric pressure. The elemental chromium is oxidized to form soluble chromium coordination salts that dissolve in the aqueous solution and will not corrode the underlying lithium niobate crystals. Strict control is maintained for immersion at room temperature for 2-4 minutes: immersion for less than 2 minutes will prevent the chromium film from being fully stripped, leaving residual dot-like chromium film that obscures the metastructure units; immersion for more than 4 minutes will allow the etching solution to seep into the gaps on the sidewalls of the nanopillars in small amounts, corroding the surface of the lithium niobate and causing structural dimensional loss.

[0045] After the mask is peeled off, the process involves three stages of deionized water cleaning: the first stage is a dynamic water rinse for 1 minute to remove large pieces of dissolved residue from the surface; the second stage is a static soaking for 3 minutes to dissolve residual corrosion liquid in the crevices; and the third stage is a high-purity nitrogen blow-drying and sealing for storage. The entire cleaning process is carried out at room temperature, and high-temperature drying is prohibited to avoid thermal deformation of the nanostructure.

[0046] Optionally, the thin-film lithium niobate used in this invention is a commercially available product with a Z-shaped or X-shaped tangent and a thickness of 300-900 nm. The underlying layer of the thin film has a 2-5 μm thick SiO2 layer, and the next layer is a 250-300 μm thick lithium tantalate or silicon substrate. Any combination of the parameters regarding the tangent, padding, and substrate is applicable to the processing method proposed in this invention. For example, four types of commercially available substrate combinations can be selected to complete the control processing test: Combination 1: Z-shaped + 500 nm N + 3 μm SiO2 + 280 μm lithium tantalate substrate; Combination 2: X-shaped + 700 nm N + 2 μm SiO2 + 260 μm lithium tantalate substrate; Combination 3: Z-shaped + 400 nm N + 4 μm SiO2 + 300 μm silicon substrate; Combination 4: X-shaped + 800 nm N + 5 μm SiO2 + 250 μm silicon substrate.

[0047] The magnetron sputtering instrument used in this invention is a commercially available high-vacuum three-chamber magnetron sputtering thin film deposition system, and the focused ion beam instrument used is a commercially available focused ion beam instrument.

[0048] The following specific examples illustrate the positive effects of the present invention: Example 1: A method for preparing a large-scale lithium niobate metasurface based on focused ion beam splicing technology, comprising the following steps: (1) First, ultrasonically clean the cut TFLN with acetone solution for 2 minutes; then rinse with isopropanol solution and deionized water in sequence, and finally dry with nitrogen gun.

[0049] Specifically, the test samples were commercially available Z-cut thin-film lithium niobate substrates. The single-layer thickness of the thin-film lithium niobate was 500nm, the thickness of the intermediate silicon dioxide isolation layer was 3μm, and the bottom support substrate was a 280μm lithium tantalate wafer. The samples were cut into 10mm×10mm square samples by a diamond cutter. After cutting, the samples were placed in a dust-free sealed box and stored at room temperature away from light for 72 hours to remove cutting stress. Then the cleaning process was started. The ultrasonic cleaning was precisely controlled for 2 minutes with acetone. The ultrasonic cleaner power was set to 80W and the water bath temperature was kept constant at 25℃ to avoid the difference in cleaning effect caused by water temperature fluctuations.

[0050] (2) A Cr mask with a thickness of 30 nm was deposited on TFLN by magnetron sputtering and then annealed at 200 °C for 9 h in N2 atmosphere; wherein the deposition power of Cr was 16 W and the deposition time was 400 s.

[0051] The base vacuum of the magnetron sputtering chamber was evacuated to 4×10⁻⁶. -4 After introducing high-purity argon gas, the pressure was stabilized in a closed loop to 2.5 Pa (the middle value of the preferred range of 2~3 Pa in this invention). The DC sputtering power was stabilized at 16W for 400s. Three parallel samples were randomly selected using a step tester to detect the thickness of the chromium film. The measured thickness was 29.2~30.7nm, which is in line with the 25~35nm process range. Nitrogen gas was introduced into the annealing furnace at a flow rate of 50sccm, and the temperature was increased to 200℃ at a rate of 2℃ / min. The temperature was then precisely held for 9h, and the furnace was cooled at a rate of 1℃ / min. After cooling, the surface resistivity of the chromium film was detected using a four-probe tester. The surface resistivity of the chromium film after annealing was stable at 35~40Ω / □, indicating excellent conductivity.

[0052] (3) The layout was drawn using AutoCAD software. Based on the layout, a metasurface with a period of 500 nm, a diameter of 300 nm, and a height of 200 nm was etched on the annealed TFLN using FIB. The focused ion beam was Ga... + The accelerating voltage was 30 kV, the ion beam current was 0.23 nA, and the dwell time was 10 μs.

[0053] The CAD layout shows a single basic sub-region with dimensions of 40μm × 40μm. The meta-nanopillar array within the sub-region has a period of 500nm and a cylinder diameter of 300nm, with a target etching depth of 200nm. The FIB equipment uses a serpentine reciprocating scanning path, which can effectively balance the ion dose distribution compared to unidirectional scanning and reduce the problem of over-etching one sidewall caused by unidirectional scanning. The 0.23nA beam current and 10μs residence time are the preferred intermediate parameters of this invention, balancing etching efficiency and dimensional accuracy. The single etching depth fluctuation of parallel samples is controlled within ±5nm.

[0054] (4) After etching, the edge row of the metasurface unit array already fabricated in the previous region is used as a natural reference. By adjusting the sample stage position and ion beam scanning deflection, the pattern layout of the current region is precisely aligned with the previous region, thus enabling the continuous fabrication of adjacent regions while maintaining a consistent period and feature size. This process is repeated 4 times to obtain a large-scale metasurface composed of 2×2 local metasurfaces. A schematic diagram of this process is shown below. Figure 1 As shown in (a).

[0055] Processing sequence: Step 1: Process the upper left reference sub-region; Step 2: Align with the upper left array edge and process the upper right sub-region; Step 3: Align with the left array and process the lower left sub-region; Step 4: Complete the stitching of the lower right sub-region. After four stitching steps, the overall metastructure size is 80μm×80μm. SEM full-field scanning observation of the four sub-region stitching seams showed no misalignment of the nanopillars at the seams. Only a slight over-etching was observed in the 50nm narrow band at the center of the seam. The over-etching depth was <15nm, which is much lower than the total height of the metastructure of 200nm and has a negligible impact on the overall optical performance.

[0056] (5) Immerse the processed sample in a nitric acid-cerium ammonium nitrate mixed solution for 2-4 minutes at room temperature, and rinse with deionized water to complete the preparation of the spliced ​​metasurface.

[0057] Optionally, the etching solution ratio is: 6wt% nitric acid + 12wt% cerium ammonium nitrate + 82wt% deionized water. Soak at room temperature (23℃) for 3 minutes (midpoint of the range). After soaking, rinse with three-stage deionized water, blow dry with high-purity nitrogen, and then store in a nitrogen-protected moisture-proof sample box in a sealed, light-proof container for subsequent nonlinear optical performance characterization tests.

[0058] Example 2: A method for preparing tilted thin-film lithium niobate metasurfaces based on focused ion beams, comprising the following steps: (1) First, ultrasonically clean the cut TFLN with acetone solution for 2 minutes; then rinse with isopropanol solution and deionized water in sequence, and finally dry with nitrogen gun.

[0059] Optionally, an X-cut thin-film lithium niobate substrate is selected, with an LN film thickness of 600nm, a SiO2 isolation layer of 4μm, and a silicon substrate of 270μm. The X-cut lithium niobate has a higher nonlinear coefficient and is more suitable for the circular polarization differential response test of tilted chiral metastructures. The cutting size is also 10mm×10mm, and the cleaning process parameters, ultrasonic power, and water bath temperature are completely consistent with those in Example 1 to achieve a single variable (crystal orientation, tilt angle) control test.

[0060] (2) A Cr mask with a thickness of 30 nm was deposited on TFLN by magnetron sputtering and then annealed at 200 °C for 9 h in N2 atmosphere; wherein the deposition power of Cr was 16 W and the deposition time was 400 s.

[0061] Optionally, the magnetron sputtering and annealing processes of Example 1 are fully replicated, with the actual thickness of the chromium film being 29.5~30.4 nm. The conductivity after annealing is consistent with that of the Sample 1, eliminating the interference of mask process differences on the formation of tilted nanopillars and ensuring that the only experimental variable is the ion incident tilt angle.

[0062] (3) The layout was drawn using AutoCAD software. Based on the layout, a metasurface with a period of 500 nm, a side length of 300 nm, and a height of 300 nm was etched on the annealed TFLN using FIB. The focused ion beam was Ga... + The accelerating voltage was 30 kV, the ion beam current was 0.23 nA, and the dwell time was 10 μs. The ion beam bombarded the sample surface at tilted angles of 5°, 10°, 15°, and 20°. A schematic diagram of the tilting process is shown below. Figure 2 (a) and Figure 2 As shown in (b) of the diagram.

[0063] Optionally, the same 10mm×10mm sample is divided into four independent 40μm×40μm partitions on the CAD layout. These four partitions correspond to four tilted square column configurations of 5°, 10°, 15°, and 20°, respectively. The square column cross-section has a side length of 300nm, an array period of 500nm, and an etching height of 300nm. By rotating the five-axis sample stage at the corresponding angles, combined with the ion beam's native 65° fixed bombardment angle, the overall directional tilt of the nanopillars is effectively achieved. The tilted arrangement of the square cross-sections naturally forms a geometric chiral structure, which can produce differentiated absorption and transmission of left- and right-hand circularly polarized light, making it the core structure of chiral polarization devices. After fabrication, the actual tilt angle is tested partition by partition using SEM. The measured tilt angle error is all <0.8°, demonstrating excellent tilt angle controllability.

[0064] Comparative Example: A method for preparing thin-film lithium niobate metasurfaces using focused ion beam, with specific steps as in Example 1, except that step (4) splicing process is omitted; the substrate, cleaning, coating, annealing, and FIB etching parameters of the comparative example sample are all the same as those in Example 1, except that the splicing process is omitted, and a single FIB processing only prepares a single 40μm×40μm small-area metasurface as a performance blank control group; the 80μm×80μm large-area sample after splicing in Example 1 and the single sample of the comparative example are tested for second harmonic spectroscopy on the same optical testing platform, with the same incident laser power and the same ambient temperature, thus avoiding data errors caused by the testing environment.

[0065] Experimental Example: Due to dose accumulation at the edges of adjacent regions and submicron positioning errors related to stage movement, the stitching operation inevitably introduces a certain degree of over-etching or overlapping etching at the seams between adjacent sub-regions, such as... Figure 1 As shown in (b), (c), and (d) of the paper. However, experimental results of this invention show that the optical resonance enhancement caused by the increased supersurface area far outweighs the optical loss caused by boundary damage. Therefore, from an engineering perspective, this splicing method is both acceptable and efficient. Using this method, this invention successfully fabricated an 80×80μm supersurface containing 150×150 unit cells. 2The TFLN metasurface, compared to the unspliced ​​small-area metasurface (Comparative Example 1), exhibits approximately twice the second harmonic intensity when spliced, such as... Figure 1 As shown in (e) in the diagram.

[0066] Optionally, in the experimental example, a 1550nm femtosecond pulsed fiber laser was used as the pump source for optical performance testing. The pulse width was 100fs and the repetition frequency was 80MHz. The focusing objective perpendicularly incident the pump laser onto the lithium niobate metasurface under test. The second harmonic signal in the visible light band generated by the metasurface was dispersed by a monochromator and the spectral data was collected by a photomultiplier tube. The wavelength range of the spectral test was 360~440nm, corresponding to the wavelength range on the horizontal axis of the test curve in the attached figure. The test data showed that the peak second harmonic signal intensity of the comparative single 40μm×40μm sample was 1.0, while the peak intensity of the 80μm×80μm large-area sample spliced ​​in Example 1 increased to 1.98~2.03 times, which is highly consistent with the experimental conclusion of a two-fold enhancement. The optical loss caused by local over-etching at the seam only caused an overall signal decrease of less than 3%, and the nonlinear gain brought by doubling the area completely covered the seam loss.

[0067] From an industrialization cost perspective: traditional segmented, unmarked splicing relying on coordinate positioning has a scrap rate exceeding 35%, while the in-situ self-aligned splicing process of this invention improves the yield to over 92%, reducing the overall manufacturing cost of single-chip large-area meta-devices by 28%, demonstrating significant economic advantages in the mass production of nonlinear photonic chips. Furthermore, Example 2, with four groups of chiral meta-circular polarization spectral tests at different tilt angles, shows that the larger the tilt angle (5°~20°), the higher the extinction ratio of left-hand and right-hand circular polarization. The extinction ratio of the 20° tilted sample can reach 18dB, meeting the requirements for on-chip polarization screening chips.

[0068] In summary, the process of this invention is not limited to a single material system of lithium niobate. After minor parameter adjustments, it can be extended to the preparation of wide-bandgap hard dielectric crystal metasurfaces such as lithium tantalate, aluminum nitride, and silicon carbide, expanding its applications in fields such as ultraviolet metadevices and high-frequency piezoelectric microelectromechanical systems. Further research and development can be iteratively optimized in three directions: first, optimizing the doping modification of the magnetron sputtering chromium film to further improve the mask's conductivity and sputtering resistance; second, developing a multi-batch fully automated program splicing algorithm to achieve automated continuous unattended processing of hundreds of areas; and third, combining it with ion beam vapor deposition to achieve in-situ modification of the metasurface sidewall functional coating, further expanding the cutting-edge applications of lithium niobate metasurfaces in quantum optics and super-resolution imaging.

[0069] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing specially functionalized lithium niobate metasurfaces based on focused ion beam, characterized in that, The process includes the following: Clean lithium niobate film is obtained by cleaning the cut and shaped thin film lithium niobate; A hard mask layer was prepared on the clean state thin film lithium niobate surface by magnetron sputtering and then annealed to obtain a masked thin film lithium niobate. The lithium niobate film covered by the gallium-based focused ion beam is etched and shaped. The ion beam parameters and ion beam bombardment angle are selected according to the optical function of the device. As needed, the multi-region splicing and expansion is completed based on the positioning reference of the already formed metacell array. After all structural processing is completed, the hard mask layer is peeled off and cleaned again to obtain a functionalized lithium niobate metasurface with a large area or tilted configuration.

2. The method for preparing specially functionalized lithium niobate metasurfaces based on focused ion beam as described in claim 1, characterized in that, The thin-film lithium niobate is configured with a Z-axis or X-axis cross-section, and the thickness of the thin-film lithium niobate ranges from 300 nanometers to 900 nanometers.

3. The method for preparing specially functionalized lithium niobate metasurfaces based on focused ion beam as described in claim 1, characterized in that, The cleaning process involves ultrasonic cleaning with acetone, rinsing with isopropanol, rinsing with deionized water, and drying with nitrogen purging. The ultrasonic cleaning time with acetone is controlled between 1 and 3 minutes.

4. The method for preparing specially functionalized lithium niobate metasurfaces based on focused ion beam as described in claim 1, characterized in that, The hard mask layer is made of metallic chromium, and the deposition environment for the metallic chromium magnetron sputtering is an argon atmosphere with a pressure between 2 and 3 Pa. The thickness of the hard mask layer is set to 25 nanometers to 35 nanometers.

5. The method for preparing specially functionalized lithium niobate metasurfaces based on focused ion beam as described in claim 1, characterized in that, The annealing modification treatment is carried out under a nitrogen protective atmosphere, with an annealing temperature of 180°C to 220°C and an annealing holding time of 8.5 hours to 9.5 hours.

6. The method for preparing specially functionalized lithium niobate metasurfaces based on focused ion beam as described in claim 1, characterized in that, The accelerating voltage of the gallium-based focused ion beam is fixed at 30 kV, and the ion residence time parameter is controlled between 8 microseconds and 12 microseconds.

7. The method for preparing specially functionalized lithium niobate metasurfaces based on focused ion beam as described in claim 1, characterized in that, The ion beam current can be adjusted from 0.02 nanoamperes to 0.3 nanoamperes, and the ion beam bombardment tilt angle can be adjusted from 65 degrees to 90 degrees.

8. The method for preparing specially functionalized lithium niobate metasurfaces based on focused ion beam as described in claim 1, characterized in that, The multi-region splicing expansion uses the edge rows and columns of the pre-processed meta-unit array as positioning references, and achieves precise alignment and splicing of adjacent processed regions based on sample stage displacement adjustment and ion beam scanning deflection correction.

9. The method for preparing specially functionalized lithium niobate metasurfaces based on focused ion beam as described in claim 1, characterized in that, The stripping operation of the hard mask layer uses a mixed etchant of nitric acid and cerium ammonium nitrate, with the soaking time at room temperature controlled between 2 and 4 minutes. After the stripping is completed, the sample is rinsed with deionized water.

10. The method for preparing specially functionalized lithium niobate metasurfaces based on focused ion beam as described in claim 1, characterized in that, By adjusting the ion beam bombardment tilt angle and the sample stage tilt angle, a chiral nanopillar array metasurface with a continuously adjustable tilt angle of 5 to 20 degrees is prepared by single etching on the surface of the thin film lithium niobate.

Images