A microstructure array-based cut-off filter and a method of manufacturing the same

Abstract

The application discloses a cut-off filter based on a microstructure array, which comprises a substrate, a dielectric film and a two-dimensional grating array structure arranged on the substrate in sequence; the substrate material is a high-refractive-index material; the dielectric film material is a low-refractive-index material; and the two-dimensional grating material is a high-refractive-index material. The application further provides a preparation method of the cut-off filter based on the microstructure array. The preparation method of the cut-off filter based on the microstructure array is simple in preparation, low in cost and convenient for large-scale and batch production. Therefore, the application can be widely applied in the fields of surveying and mapping remote sensing, environmental monitoring, sensing detection and clinical image analysis.

Description

Technical Field

[0001] This invention relates to an optical element, specifically to a cutoff filter based on a microstructure array and its fabrication method, which can be applied to fields such as surveying and remote sensing, environmental monitoring, sensing and detection, and clinical image analysis. Background Technology

[0002] As the most commonly used optical components, filters are facing the demand for miniaturization and integration. People are gradually beginning to try to integrate multiple filters together, making their size comparable to traditional filters, and allowing different regions of the filter to modulate different spectral effects.

[0003] The optical filtering techniques used in imaging spectrometers mainly include tunable filters, graded-pass filters, and filter arrays. Tunable filters are filters whose parameters, such as the passband center wavelength and transmission bandwidth, are adjustable. There are many types of these filters, commonly including Fabry-Perot tunable filters, acousto-optic tunable filters, liquid crystal tunable filters, birefringent tunable filters, and electro-optic tunable filters. These filters modulate the incident light using either acousto-optic diffraction, electro-optic effects, or liquid crystal light valves. Graded-pass filters are a type of graded-pass filter. Linearly graded-pass filters are filters whose optical properties change in one dimension. They can replace traditional gratings and prism dispersive elements to form compact, lightweight multicolor imagers, which are particularly useful in optical guidance and space exploration. Unlike tunable filters, graded filters do not require a driving circuit, making them particularly suitable for pushbroom remote sensing imaging. A multichannel filter is a filter whose optical properties are distributed along a specific direction in a two-dimensional plane. These filters have applications covering a wide wavelength range from ultraviolet to infrared. Compared to traditional bandpass filters, multichannel filters have more than one passband, thus carrying more information. Therefore, fewer filters are needed to carry the same amount of information, saving space and facilitating the miniaturization of spectrometers.

[0004] Because multi-channel filters can simultaneously acquire spatial image information of a target at different wavelengths, this technology greatly improves the sensitivity and accuracy of detection and imaging, leading to a surge of research on multi-channel array filters both domestically and internationally. As research into multi-channel array filters deepens, two main fabrication processes have emerged: splicing and masking. The splicing method involves pre-fabricating bandpass filters with different optical properties, then cutting and bonding them together to create a single filter. Compared to the metal masking method, this method involves complex and cumbersome cutting and bonding processes, making it difficult to avoid crosstalk between channels. Furthermore, individual channels cannot be made to the micrometer size, failing to meet the current demands for filter integration. The masking method is divided into metal masking and photoresist masking. The metal masking method uses a metal mask to block areas that do not need coating during deposition, followed by repeated coating. This method results in a shadowing effect at the boundaries between different blocks. Photoresist masks, on the other hand, use photoresist patterns to select coating areas, enabling the creation of micrometer-sized filter arrays—something that cannot be achieved using stitching or metal mask methods. However, array filters fabricated using photoresist masks are still limited by shadowing effects. Generally, the photoresist thickness should be at least three times the coating thickness. This can easily prevent vapor molecules incident at large angles from entering the photoresist pores, resulting in uneven film thickness at the edges and center after photoresist stripping, affecting spectral imaging and detection performance. Summary of the Invention

[0005] To address the aforementioned issues of channel crosstalk and coating shadows, this invention proposes a cutoff filter based on a microstructure array and its fabrication method. Micro / nanostructure-based filters possess excellent optical properties, good process compatibility, and outstanding integrability. These novel characteristics offer enormous potential for a wide range of research fields, such as mapping and remote sensing, environmental monitoring, sensor detection, and clinical image analysis.

[0006] This invention provides a cutoff filter based on a microstructure array. The filter has a simple structure, adopts a two-dimensional grating array structure, has a simple fabrication process, and exhibits stable performance.

[0007] The present invention also provides a method for fabricating a cutoff filter based on a microstructure array, the method involving photolithography, deposition pattern transfer, etching and other techniques to fabricate the microstructure array filter.

[0008] This invention provides the following technical solution:

[0009] A cutoff filter based on a microstructure array includes a substrate on which a dielectric thin film and a two-dimensional grating array structure are sequentially disposed; the substrate material is a high refractive index material; the dielectric thin film material is a low refractive index material; and the two-dimensional grating material is a high refractive index material.

[0010] Preferably, the two-dimensional grating array structure is composed of two or more filter blocks with different structures arranged alternately; wherein each filter block is composed of grating structure units arranged in an array.

[0011] When using multiple filter blocks, the overall size of the filter blocks can be equal or unequal, but equal sizes are generally chosen for ease of manufacturing and design. The size of a single filter block ranges from 5 to 500 micrometers, with 10 to 30 micrometers being preferred. Within each filter block, the grating structure unit structure is completely identical. Different filter blocks have different grating structure unit sizes to meet different filtering requirements.

[0012] Preferably, the high refractive index material has a refractive index greater than or equal to 1.9; the low refractive index material has a refractive index less than or equal to 1.65.

[0013] The substrate material is a high-refractive-index material, selected from single-element semiconductor materials such as silicon and germanium, oxides such as titanium dioxide, hafnium dioxide, and tantalum pentoxide, group II-VI semiconductor materials such as zinc sulfide and zinc selenide, and group II-VI compound semiconductor solid solutions such as mercury cadmium telluride. Silicon is preferably the substrate material.

[0014] The dielectric thin film material is a low-refractive-index material, selected from oxides such as silicon dioxide and aluminum oxide, fluorides such as magnesium fluoride, yttrium fluoride, and ytterbium fluoride, and low-refractive-index organic materials. The two-dimensional grating material is a high-refractive-index material, selected from single-element semiconductor materials such as silicon and germanium, oxides such as titanium dioxide, hafnium dioxide, and tantalum pentoxide, group II-VI semiconductor materials such as zinc sulfide and zinc selenide, and group II-VI compound semiconductor solid solutions such as mercury cadmium telluride. The dielectric thin film material is preferably ytterbium fluoride, and the grating material is preferably germanium.

[0015] Preferably, the filter blocks are arranged in a periodic array structure, and the structural parameters of the grating structure units differ within different types of filter blocks; the size of a single filter block is determined by actual requirements. Preferably, the grating structure units are arranged in a hexagonal or square pattern, and preferably, the gratings are arranged in a square pattern. The shape of the two-dimensional grating is selected from cylinders, cones, frustums, prisms, pyramids, frustums, etc. The preferred shape is a regular square prism.

[0016] As a further preferred embodiment, the grating structure unit is a regular square prism; the substrate material is silicon; the dielectric thin film material is ytterbium fluoride; and the grating material is germanium.

[0017] Preferably, the thickness of the dielectric film layer is 0.05 to 5 micrometers; further, the thickness of the dielectric film layer is 0.1 to 1 micrometer, and even further, the thickness of the dielectric film layer is 0.2 to 0.7 micrometers.

[0018] The filter of the present invention can be designed as a cutoff filter for different wavelength bands according to the required center wavelength. Preferably, the grating is arranged perpendicular to the dielectric film; the size (length, width, etc.) of the grating structure unit is 100-3000nm, preferably 200-1000nm; the height of the grating structure unit is 100-3000nm, preferably 200-1000nm; and the period of the grating structure unit is 200-4000nm, preferably 300-1500nm.

[0019] As one specific implementation scheme, there are two types of filter blocks, arranged alternately; one type of filter block consists of 9*9 array grating structure units, and the other type consists of 9*8 array grating structure units. The array block is a square with a side length of 15 micrometers. The grating structure unit is a regular square prism with dimensions of 0.5–1 micrometer side length and 0.5–1.5 micrometer height.

[0020] A cutoff filter based on a microstructure array is provided, wherein the array arrangement requires gratings with the same structural parameters to form a filter unit, corresponding to a cutoff filter block; the different cutoff filter blocks are arranged alternately to form an array filter.

[0021] During the design process, the number of cutoff filter blocks is determined by the actual detection requirements and is used as a known value in the optimization process. Similarly, the size of the filter blocks can also be determined manually, based on the size and period of the optimized grating structure units, to determine the number of grating structure units within each filter block.

[0022] Considering the one-to-one correspondence between detectors and filter arrays, to improve the resolution of the spectrometer, we need a sufficient number of detectors per unit volume, and correspondingly, a sufficient number of filters per unit volume. However, if the array block is too small, the number of structural units will be too small, resulting in ineffective filtering. As a preferred option, we set the array side length to 15 micrometers.

[0023] Photoresist masking is a method for fabricating array filters. It involves using a photoresist mask pattern to block areas that don't need coating during the deposition process, and then repeating the deposition process until the desired filter array is created. However, this method has some problems. In the post-deposition photoresist removal stage, to ensure successful removal, the photoresist thickness often needs to be three times the required coating thickness. Therefore, the photoresist pattern thickness depends on the filter's film thickness. This leads to a problem: at the boundaries between adjacent blocks, the presence of photoresist creates coating "shadows." These shadows not only significantly affect the filter's beam splitting performance but also occupy space, which doesn't meet our miniaturization requirements.

[0024] To facilitate processing and eliminate the shadowing effect during the fabrication process, as a preferred approach, during the structural design optimization stage, we kept the dielectric film thickness and the two-dimensional grating thickness consistent in the two cutoff filter arrays (i.e., the dielectric film thickness and grating thickness of two or more blocks were consistent). During the sample preparation stage, we adopted an etching technique (the selected etching technique is a dry etching technique with strong anisotropy and high selectivity, which uses molecular gas plasma in a vacuum system for etching, and can utilize ion-induced chemical reactions to achieve anisotropic etching) instead of the traditional dissolution and stripping method, in order to eliminate the shadowing effect generated during the coating stage of the fabrication process.

[0025] The present invention also provides a method for preparing the above-mentioned cutoff filter based on a microstructure array, comprising the following steps:

[0026] (1) Based on the center wavelength of the filter to be made, optimize the dielectric film layer material and thickness, grating material and thickness, grating size and grating arrangement period; the optimization process can be carried out using existing methods; during the optimization process, ensure that the dielectric film layer thickness and two-dimensional grating thickness are consistent in different cutoff filter blocks.

[0027] (2) Deposit a dielectric film on the substrate with the same thickness as the dielectric film layer designed in (1);

[0028] (3) Deposit another grating layer on the dielectric film, with the same thickness as the grating thickness designed in (1);

[0029] (4) Spin-coat a layer of photoresist, electronic resist, or imprinting adhesive onto the substrate after coating.

[0030] (5) The photoresist spin-coated in (4) is patterned; the photolithography process involves exposing the photoresist to light and then washing away the unwanted photoresist through a development process. Then, through fixing and drying processes, the desired photoresist pattern is formed on the substrate.

[0031] (6) After the patterning is completed, the sample is sent to the etching machine for etching;

[0032] (7) Remove the residual adhesive to obtain the cutoff filter based on the microstructure array; that is, after etching, the sample is placed in acetone solution for cleaning to remove the residual adhesive.

[0033] As a preferred option, step (1) uses the particle swarm optimization algorithm to optimize the parameters to be optimized. During the optimization process, the finite difference time method is used to simulate and calculate the filtering performance. The optimization is determined based on the calculation results. When the requirements are met, the optimization is stopped. The obtained parameters are the optimal parameters for subsequent processing steps. At the same time, the thickness of the dielectric thin film layer and the thickness of the grating structure unit are consistent in different cutoff filter blocks.

[0034] The cutoff filter based on microstructure array of the present invention differs from traditional array filters. It adopts a two-dimensional grating structure combined with a low refractive index dielectric thin film layer, which can achieve the cutoff effect of the corresponding wavelength band.

[0035] The beneficial effects of this invention are reflected in:

[0036] The cutoff filter based on a microstructure array prepared in this invention utilizes a particle swarm optimization algorithm in the early design and optimization stage, and employs the finite-difference time-domain method for simulation. During the process, different blocks are simultaneously simulated and optimized to minimize crosstalk between channels caused by optimizing individual blocks. In the sample preparation process, etching technology is used instead of the traditional dissolution and stripping method, eliminating coating shadows between structural units and between arrays. This improves the space utilization of the optical device and the accuracy of detection and imaging, making it more conducive to the fabrication of highly integrated and high-precision filters, and thus offering broader application prospects.

[0037] This invention presents a method for fabricating a cutoff filter based on a microstructure array. The method is simple, low-cost, and suitable for large-scale, mass production. Therefore, this invention is expected to have wide applications in fields such as surveying and remote sensing, environmental monitoring, sensor detection, and clinical image analysis. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the structure of a single cutoff filter of the present invention.

[0039] Figure 2 (a) and (b) are schematic diagrams of the array arrangement of the cutoff filter based on microstructure array of the present invention.

[0040] Figure 3 for Figure 2 (a) and (b) are schematic diagrams of the structural units in Block I and Block II.

[0041] Figure 4 This is a flowchart of the fabrication method of the cutoff filter based on microstructure array according to the present invention.

[0042] Figure 5 (a) and (b) are the transmission spectra of Block I (long-wavelength pass array) and Block II (short-wavelength pass array) in the embodiments of the present invention. Detailed Implementation

[0043] The present invention will now be described in further detail with reference to the accompanying drawings.

[0044] like Figure 1 As shown, a cutoff filter based on a microstructure array consists of a substrate 1, a low-refractive-index dielectric film 2 fixed on the substrate 1, and a two-dimensional grating array structure 3 vertically disposed on the dielectric film 2. Preferably, the grating material is germanium, the low-refractive-index film 2 material is ytterbium fluoride, and the substrate material is silicon.

[0045] like Figure 2 As shown in (a) and (b), the two-dimensional grating array structure is composed of cutoff filter block I and cutoff filter block II alternately laid on the substrate plane.

[0046] like Figure 3 As shown, corresponding to Figure 2 (a) and (b) show that cutoff filter block I consists of grating structure units arranged in a 9*9 array; cutoff filter block II consists of grating structure units arranged in a 9*8 array. The grating structure units within the same cutoff filter block have the same structural parameters and are arranged in a periodic array to form a filter unit; different cutoff filter blocks are arranged alternately to form an array filter.

[0047] The cutoff filter based on microstructure array of the present invention differs from traditional array filters. It adopts a two-dimensional grating structure combined with a low refractive index thin film layer to achieve the cutoff effect of the corresponding wavelength band.

[0048] like Figure 4 As shown: A method for fabricating a cutoff filter based on a microstructure array, comprising the following steps:

[0049] (1) Based on the center wavelength of the filter to be made, the substrate material, dielectric film layer thickness, dielectric film material, grating material, grating thickness, bottom edge length of the grating and grating period are optimized. The optimization process is carried out using the particle swarm optimization algorithm, and the simulation is performed using the finite difference time method. The simulation results are used to guide the optimization process. Different blocks are simulated and optimized simultaneously during the optimization process to eliminate the channel crosstalk problem caused by optimizing different blocks separately. During the optimization process, the dielectric film layer thickness and the height of the grating structure unit are consistent in different cutoff filter blocks.

[0050] The types of cutoff filter blocks, their overall size, and their arrangement can be determined based on experience and requirements. There are multiple possible arrangements for the cutoff filter blocks, and the optimal arrangement can be determined through optimization. Alternatively, a specific arrangement can be selected based on experience; for example, a specific arrangement could be used. Figure 2 The periodic alternation arrangement of (a) and (b) is then followed by the optimization process described above.

[0051] Step (1) can obtain parameter values ​​such as the material of the substrate, the thickness of the dielectric thin film layer, the material of the dielectric thin film, the material of the grating, the thickness of the grating, the bottom edge length of the grating, and the period of the grating.

[0052] (2) Deposit a dielectric film on the substrate.

[0053] (3) Then deposit a high-refractive-index grating layer on the dielectric film. Since multiple blocks use the same dielectric film layer thickness and grating height, it is not necessary to distinguish between multiple blocks; the entire process can be performed in one go.

[0054] (4) Spin-coat a layer of photoresist, electronic resist or imprinting adhesive onto the substrate after coating. The thickness of the adhesive needs to meet the minimum etching thickness requirement, i.e., meet the etching resistance requirement.

[0055] (4) The designed pattern is transferred onto the photoresist / electron resist or imprinting adhesive by exposure or imprinting, and then the unwanted resist is washed away by the development process. Then, the desired resist pattern is formed on the substrate by the fixing and drying processes.

[0056] (5) After the patterning is completed, the sample is sent to the etching machine for etching (reactive ion etching).

[0057] (6) After etching, the sample is placed in acetone solution to clean it and remove the residual adhesive.

[0058] Example: A cutoff filter with a center wavelength of λ = 4.25 μm and an array block side length of 15 μm is designed. Two arrays (two filter blocks) are designed. One array is required to achieve a short-wavelength pass effect with high transmission in the 3.7 μm to 4.0 μm range and transmission cutoff in the 4.5 μm to 4.8 μm range. The other array is required to achieve a long-wavelength pass effect with transmission cutoff in the 3.7 μm to 4.0 μm range and high transmission in the 4.5 μm to 4.8 μm range. The specific method is the same as the fabrication method of the cutoff filter based on the microstructure array in the specific implementation.

[0059] Using the optimization method mentioned in step (1), the final parameters are as follows: silicon substrate; ytterbium fluoride dielectric film with a thickness of 0.44 μm; germanium grating material with a height of 1.15 μm; long-pass array grating spacing of 0.55 μm and grating period of 1.35 μm; short-pass array grating spacing of 0.3 μm and grating period of 1.4 μm. The spectrum of this structure is as follows: Figure 5 (a) Figure 5 As shown in (b), both arrays of this structure exhibit good cutoff performance. The long-pass array has an average transmittance of only 7.15% in the 3.7μm–4μm band and an average transmittance of 80.79% in the 4.5μm–4.8μm band; the short-pass array has an average transmittance of 64.90% in the 3.7μm–4μm band and an average transmittance of only 11.46% in the 4.5μm–4.8μm band.

Claims

1. A cutoff filter based on a microstructure array, characterized in that, The device includes a substrate on which a dielectric thin film and a two-dimensional grating array structure are sequentially disposed; the substrate material is a high refractive index material; the dielectric thin film material is a low refractive index material; and the two-dimensional grating material is a high refractive index material. The two-dimensional grating array structure is composed of two or more filter blocks with different structures arranged alternately; each filter block is composed of grating structure units arranged in an array. The filter blocks are periodically arranged array structures, and the structural parameters of the grating structure units are different in different types of filter blocks; The grating structure units within the same cutoff filter block have the same structural parameters and are arranged in a periodic array. The dielectric film layer thickness is consistent in different cutoff filter blocks, and the two-dimensional grating thickness is consistent in different cutoff filter blocks.

2. The cutoff filter based on a microstructure array according to claim 1, characterized in that, The high refractive index material has a refractive index greater than or equal to 1.9; the low refractive index material has a refractive index less than or equal to 1.

65.

3. The cutoff filter based on a microstructure array according to claim 1, characterized in that, The high refractive index material is selected from one or more of silicon, germanium, titanium dioxide, hafnium dioxide, tantalum pentoxide, zinc sulfide, zinc selenide, and mercury cadmium telluride; the low refractive index material is selected from one or more of silicon dioxide, aluminum oxide, magnesium fluoride, yttrium fluoride, ytterbium fluoride, and low refractive index organic materials.

4. The cutoff filter based on a microstructure array according to claim 1, characterized in that, The size of a single filter block is 5 to 500 micrometers.

5. The cutoff filter based on a microstructure array according to claim 1, characterized in that, The grating structure unit is a cylinder, cone, frustum, prism, pyramid, or frustum; the substrate material is silicon; the dielectric thin film material is ytterbium fluoride; and the grating material is germanium.

6. The cutoff filter based on a microstructure array according to claim 5, characterized in that, The grating structure unit is arranged perpendicular to the dielectric film; the size of the grating structure unit is 100~3000nm; the height of the grating structure unit is 100~3000nm; the period of the grating structure unit is 200~4000nm; and the thickness of the dielectric film layer is 0.05~5 micrometers.

7. A method for fabricating a cutoff filter based on a microstructure array as described in any one of claims 1 to 6, characterized in that, Includes the following steps: (1) Based on the center wavelength of the filter to be made, optimize the dielectric film layer material and thickness, grating material and thickness, grating size and grating arrangement period; (2) Deposit a dielectric film on the substrate with the same thickness as the dielectric film layer designed in (1); (3) Deposit another grating layer on the dielectric film, with the same thickness as the grating thickness designed in (1); (4) Spin-coat a layer of photoresist, electronic resist, or imprinting adhesive onto the substrate after coating; (5) The adhesive spin-coated in step (4) is patterned; (6) After the patterning is completed, the sample is sent to the etching machine for etching; (7) Remove the residual adhesive to obtain the cutoff filter based on the microstructure array.

8. The method for fabricating a cutoff filter based on a microstructure array according to claim 7, characterized in that, Step (1) The particle swarm optimization algorithm is used to optimize the parameters to be optimized. During the optimization process, the finite difference time method is used to simulate and calculate the filtering performance. At the same time, the thickness of the dielectric thin film layer of different cutoff filter blocks is consistent, and the thickness of the two-dimensional grating of different cutoff filter blocks is consistent.

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