Nine-channel pixel-level spectral filter and preparation method thereof

Abstract

The present application relates to the technical fields of optical film preparation, and especially relates to a nine-channel pixel-level spectral filter and a preparation method thereof.The filter comprises a substrate, a first reflecting film, a spacer layer and a second reflecting film which are tightly stacked from bottom to top.During preparation, the substrate is ultrasonically cleaned, and then the substrate surface is blown dry by high-purity nitrogen;an ion beam sputtering process is adopted to alternately deposit hydrogen-doped amorphous silicon film and silicon dioxide film on the substrate surface by using argon as the main sputtering gas, so as to form the first reflecting film;the spacer layer is integrally deposited on the surface of the first reflecting film;the spacer layer is etched channel by channel, and the film layers with different thicknesses are removed, so that the thickness of the spacer layer retained by the nine channels forms a gradient;and the ion beam sputtering process is adopted to alternately deposit hydrogen-doped amorphous silicon film and silicon dioxide film on the spacer layer, so as to form the second reflecting film.The advantages are that the filter pixel boundary is clear, the peak transmittance of each channel can reach 60-98%, and the spectral selectivity is excellent.

Description

Technical Field

[0001] This invention relates to the field of optical thin film preparation technology, and in particular to a nine-channel pixel-level spectral filter and its preparation method. Background Technology

[0002] Spectral imaging technology, a core technology combining spectral and photoelectric imaging techniques, has been widely applied in numerous fields such as environmental monitoring, precision agriculture, biomedicine, security monitoring, and military since its development began in the 1980s. However, traditional pushbroom spectral imaging systems, whose working principle is based on prism or grating beam splitting, typically suffer from limitations such as large size and weight, complex optical paths, and difficulty in capturing dynamic targets. These limitations have gradually failed to meet the demands of modern industry and scientific research for miniaturization, lightweight design, and snapshot imaging.

[0003] Pixel-level multichannel filters, as an emerging type of spectroscopic element, possess a unique mosaic array structure that allows each spectral channel to precisely correspond to a pixel on the detector chip of a spectroscopic camera, enabling direct integration onto the chip surface via optical coating. This highly integrated spectroscopic capability significantly simplifies the design of spectroscopic camera systems and enables the acquisition of multispectral data in a single exposure, fundamentally overcoming the shortcomings of traditional pushbroom imaging modes. Therefore, it has become a research hotspot in the field of spectroscopic imaging technology.

[0004] Despite the significant advantages of pixel-level filters, their core fabrication processes and material systems still face severe challenges. Currently, in the field of pixel-level multichannel filters, it has been disclosed that pixel-level multichannel spectral dispersive elements can be directly integrated onto the surface of CMOS imaging chips, enabling the acquisition of information from multiple spectral channels on the photosensitive chip. Research teams have also conducted extensive fundamental research on the design and fabrication of pixel-level multichannel filters. These existing technical solutions typically use dielectric thin film materials (low-refractive-index materials are limited to SiO2, while high-refractive-index materials typically include oxides such as Ta2O5, TiO2, and Nb2O5) to construct Fabry-Perot (FP) filters through optical interference principles. The high-reflectance bandwidth is determined by the ratio of high- and low-refractive-index materials; the greater the refractive index difference, the wider the bandwidth.

[0005] Currently, pixel-level multichannel filters generally face the problem of narrow spectral coverage. For example, IMEC's ​​filter spectral ranges are 470~630nm and 600~870nm; domestic research mainly focuses on spectral ranges of 620~760nm and 770~820nm. However, the response wavelength range of silicon detector chips in common spectroscopic cameras is 400~1100nm. The severe inadequacy of existing spectral channel coverage greatly limits the application of pixel-level multichannel filters and cannot fully utilize the broadband response characteristics of silicon-based detectors. Traditional oxide materials, such as TiO2 thin films, when used as high-refractive-index materials in Fabry-Perot (FP) filters, have a theoretical reflection bandwidth of only about 200nm, which is insufficient to meet the broadband response range of silicon-based detectors. In addition, amorphous silicon thin films have a large number of structural defects (such as dangling bonds, broken bonds, and voids) in their amorphous network structure, causing defect states to exist within their band gap, narrowing the band gap and causing severe absorption loss of light, further limiting their application in a wide spectral range. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a nine-channel pixel-level spectral filter and its preparation method.

[0007] The primary objective of this invention is to provide a method for preparing a nine-channel pixel-level spectral filter, specifically comprising the following steps: S1. The substrate is ultrasonically cleaned sequentially with acetone, anhydrous ethanol, and deionized water, and then the substrate surface is dried with high-purity nitrogen. S2. Using ion beam sputtering, with argon as the main sputtering gas, hydrogen-doped amorphous silicon thin film and silicon dioxide thin film are alternately deposited on the pretreated substrate surface to form the first reflective film. S3. Deposit a spacer layer integrally on the surface of the first reflective film; S4. Coat the spacer layer surface with photoresist, and after exposure and development, use the photoresist as a mask to etch the spacer layer channel by channel to remove film layers of different thicknesses, so that the thickness of the spacer layer retained in the nine channels forms a gradient. S5. Using ion beam sputtering, hydrogen-doped amorphous silicon thin films and silicon dioxide thin films are alternately deposited above the spacer layer to form a second reflective film, thus producing a nine-channel pixel-level spectral filter.

[0008] Preferably, in step S2, when depositing a silicon dioxide thin film, argon is used as the sputtering gas and oxygen is introduced; when depositing a hydrogen-doped amorphous silicon thin film, hydrogen is simultaneously introduced into the deposition chamber while argon is introduced as the sputtering gas, and the vacuum level is controlled to be no greater than 2.0 × 10⁻⁶. 6 Torr, hydrogen flow rate is 50~100 sccm.

[0009] Preferably, the deposition rate of the silicon dioxide thin film and the hydrogen-doped amorphous silicon thin film is 0.1~0.5 nm / s.

[0010] Preferably, in the first reflective film and the second reflective film, the thicknesses of the single-layer hydrogen-doped amorphous silicon film and the silicon dioxide film are 80~120nm and 70~110nm, respectively.

[0011] Preferably, the spacer layer is made of silicon dioxide; the substrate is a high-purity quartz glass substrate.

[0012] The second objective of this invention is to provide a nine-channel pixel-level spectral filter, which is prepared using the aforementioned method for preparing a nine-channel pixel-level spectral filter, comprising a substrate, a first reflective film, a spacer layer, and a second reflective film stacked tightly from bottom to top. Both the first reflective film and the second reflective film are multilayer film structures formed by alternating stacking of hydrogen-doped amorphous silicon thin films and silicon dioxide thin films; The spacer layer is etched into nine groups of units with different thicknesses, and the nine groups of units are arranged in a checkerboard array to form nine independent spectral channels.

[0013] Preferably, the spacer layer is made of silicon dioxide and has a thickness between 300 and 900 nm.

[0014] Preferably, the working wavelength of the filter is 600~1000nm.

[0015] Compared with the prior art, the present invention can achieve the following beneficial effects: (1) Possesses ultra-wide working spectrum coverage capability: Existing technologies use traditional oxide materials such as TiO2, and due to the limited refractive index difference, the reflection band width is usually only about 200nm; This invention selects hydrogen-doped amorphous silicon thin film with refractive index n>3.5 combined with silicon dioxide thin film, the theoretical reflection bandwidth can exceed 500nm, and the actual product can cover the 600~1000nm band, which can make full use of the 400~1100nm full response spectrum of silicon-based detectors, effectively broaden the application range, and make up for the lack of bandwidth of existing technologies.

[0016] (2) Effectively improves short-wavelength transmittance: Traditional oxide materials suffer from intrinsic absorption problems, resulting in low peak transmittance in the short-wavelength region. This invention optimizes the hydrogen-doped amorphous silicon thin film by controlling the hydrogen flow rate, widening the optical bandgap to 1.75 eV and suppressing intrinsic absorption losses in the visible and near-infrared bands. The filter achieves a peak transmittance of up to 94% in the long-wavelength region and maintains approximately 60% peak transmittance in the short-wavelength region around 650 nm, demonstrating good optical transmittance performance.

[0017] (3) Strong application value and adaptability: This invention breaks through the bandwidth bottleneck of traditional materials and provides a reliable material selection and device design scheme for high-performance, high-integration snapshot spectral imaging systems. It can be widely used in agriculture, industrial testing, biomedicine and other fields. Attached Figure Description

[0018] Figure 1 This is a flowchart of a method for preparing a nine-channel pixel-level spectral filter according to an embodiment of the present invention.

[0019] Figure 2 This is a schematic diagram of the fabrication process of a nine-channel pixel-level spectral filter according to an embodiment of the present invention.

[0020] Figure 3 This is a schematic diagram of the overall structure of a nine-channel pixel-level spectral filter provided according to an embodiment of the present invention; (a) in the figure is a planar top view, and (b) is a three-dimensional structural schematic diagram.

[0021] Figure 4 This is a laser confocal micrograph of the nine-channel pixel-level spectral filter provided in an embodiment of the present invention.

[0022] Figure 5 This is a measured spectral transmittance curve of the nine-channel pixel-level spectral filter provided according to an embodiment of the present invention.

[0023] Figure label: 1. Base; 2. First reflective film; 3. Spare layer; 4. Second reflective film. Detailed Implementation

[0024] In the following description, embodiments of the invention will be described with reference to the accompanying drawings. In the description below, the same modules are denoted by the same reference numerals. Where the same reference numerals are used, their names and functions are also the same. Therefore, their detailed description will not be repeated.

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0026] This invention provides a nine-channel pixel-level spectral filter and its preparation method. See the flowchart below. Figure 1 Specifically, the steps include the following: S1. Substrate pretreatment: High-purity quartz glass was selected as the substrate. The substrate was ultrasonically cleaned in sequence with acetone, anhydrous ethanol and deionized water to remove surface oil, dust and residual impurities. After cleaning, the substrate surface was dried with high-purity nitrogen to ensure substrate cleanliness and provide a good interface for subsequent thin film deposition. This invention uses a high-purity quartz glass substrate, or can be replaced with optical substrates such as fused silica or single-crystal sapphire, to adapt to the deposition environment and wide spectrum application requirements of hydrogen-doped amorphous silicon (a-Si:H) thin films. It differs from the commonly used K9 glass, borosilicate glass and other ordinary optical glass substrates in the prior art, and has lower light absorption in the near-infrared short-wave range.

[0027] S2. Deposition of the first reflective film: Using ion beam sputtering (IBS) with argon as the main sputtering gas, hydrogen-doped amorphous silicon (a-Si:H) thin films and silicon dioxide thin films are alternately deposited on the pretreated substrate surface to form the first reflective film; the first reflective film has a total of 8-16 layers and a total thickness of 1200-1800 nm; wherein: Argon gas was used as the sputtering gas and oxygen was introduced during the deposition of the silicon dioxide thin film; the thickness of a single silicon dioxide thin film was 70~110nm. When depositing hydrogen-doped amorphous silicon thin films, hydrogen gas is simultaneously introduced into the deposition chamber while argon gas is used as the sputtering gas, and the vacuum level of the chamber is strictly controlled to not exceed 2.0 × 10⁻⁶. 6 Torr, hydrogen flow rate of 50~100 sccm, substrate temperature of 20~80℃, thin film deposition rate controlled at 0.1~0.5 nm / s; single-layer a-Si:H thin film thickness of 80~120 nm; By passivating dangling bond defects inside the silicon film with hydrogen, the optical band gap and refractive index of the thin film are optimized, so that the refractive index n>3.5 of the a-Si:H thin film in the 600~1000nm wavelength range.

[0028] S3. Deposition of spacer layer: A complete spacer layer is deposited on the surface of the first reflective film using ion beam sputtering. Specifically, the spacer layer is made of silicon dioxide; the spacer layer is the core structure of the Fabry-Perot interferometer cavity, and the thickness of each region is subsequently controlled by differential etching, thereby changing the spectral bandpass position of the filter; the initial total thickness of the spacer layer is 700~900nm, and the film processing accuracy is ±5nm; this initial thickness is the maximum thickness reference for the nine channels, and a gradient thickness range of 300~900nm is subsequently formed by etching.

[0029] S4. Channel-by-channel photolithography and differential etching: Photoresist is uniformly coated on the surface of the entire silicon dioxide spacer layer. Exposure and development are completed according to the 10μm×10μm pixel size and checkerboard array pattern. Using the photoresist as a temporary mask, differential etching is performed according to the channel partition. By controlling the etching time or etching amount of different areas, film layers of different thicknesses are removed, so that the thickness of the spacer layer retained in each channel forms a gradient. This step uses photoresist as a temporary mask instead of a metal mask to avoid metal contamination and optical loss. The photolithography, development, and partition etching processes are repeated to sequentially complete the thickness control of the nine channels. The thickness difference formed by etching adjacent channels is 40~80nm, ultimately forming a nine-channel spacer layer array with a gradient distribution of 300~900nm. The thickness difference of different channels corresponds to the array step height observed in microscopic detection.

[0030] S5. Deposit the second reflective film: After completing the etching of all nine-channel spacer layers, ion beam sputtering is used again to alternately deposit hydrogen-doped amorphous silicon thin films and silicon dioxide thin films above the spacer layer array to form the second reflective film; the deposition method is the same as in step S2. The first reflective film, nine sets of spacer layers of different thicknesses formed by differential etching, and the second reflective film together constitute a complete Fabry-Perot interferometer cavity structure. Relying on the large refractive index difference between a-Si:H and silicon dioxide, an ultrawide spectral response of 600~1000nm is achieved, and finally a nine-channel pixel-level spectral filter is fabricated.

[0031] Example 1: See Figure 2 This embodiment provides a method for fabricating a nine-channel pixel-level spectral filter. The fabricated nine-channel pixel-level spectral filter is composed of a substrate 1, a first reflective film 2, a spacer layer 3, and a second reflective film 4, arranged from bottom to top. The substrate 1 is a high-purity quartz glass substrate. The first reflective film 2 and the second reflective film 4 are both multilayer high-reflectivity films formed by alternating stacks of hydrogen-doped amorphous silicon thin films and silicon dioxide thin films. The spacer layer 3 is made of silicon dioxide and is formed by differential etching to create nine sets of unit structures of different thicknesses, specifically nine sets of pixel units arranged in 3 rows × 3 columns. The thickness of the nine pixel units is different from each other, and the thickness of the array increases in both the horizontal and vertical directions, forming a checkerboard pattern. The first reflective film 2, the spacer layer 3, and the second reflective film 4 together constitute a Fabry-Perot interferometer cavity and form nine independent spectral channels. Specifically, the steps include the following: S1. Substrate pretreatment: High-purity quartz glass is selected as substrate 1; acetone, anhydrous ethanol and deionized water are used in sequence to ultrasonically clean substrate 1 to remove surface oil, dust and residual impurities; after cleaning, high-purity nitrogen is used to dry the surface of substrate 1. S2. Deposition of the first reflective film: Using ion beam sputtering, with argon as the main sputtering gas, hydrogen-doped amorphous silicon thin film and silicon dioxide thin film are alternately deposited on the surface of the pretreated substrate 1 to form the first reflective film 2. S3. Deposition of spacer layer: A complete spacer layer 3 is deposited on the surface of the first reflective film 2 using ion beam sputtering; the spacer layer 3 is made of silicon dioxide. S4. Channel-by-channel photolithography and differential etching: Photoresist is uniformly coated on the surface of spacer layer 3, and exposure and development are completed according to the pixel size of 10μm×10μm and the checkerboard array pattern; using photoresist as a temporary mask, differential etching is performed according to channel partitions, removing film layers of different thicknesses in different areas, so that the thickness of the spacer layer retained in each channel forms a gradient. S5. Deposition of the second reflective film: After completing the etching preparation of all nine channels, the ion beam sputtering process is used again to alternately deposit hydrogen-doped amorphous silicon thin film and silicon dioxide thin film on the spacer layer 3 to form the second reflective film 4.

[0032] The overall structure of the nine-channel pixel-level spectral filter is as follows: Figure 3 As shown in the figure, the filter is arranged in a checkerboard pattern, consisting of nine square pixel units of different colors. Each color pixel unit corresponds to an independent spectral channel, forming a total of nine spectral channels. Specifically, each small colored square in the figure is a pixel with a size of 10μm × 10μm, corresponding to a single photosensitive pixel on a silicon-based detector. The nine different colors in the figure correspond to nine different spectral responses with different center wavelengths. Each color represents an independent spectral channel, and its bandpass peak position is determined by the specific thickness of the silica spacer layer corresponding to that channel. Therefore, all pixel units of the same color have completely identical spectral transmittance characteristics. The pixel units of the nine channels are distributed in a checkerboard pattern, uniformly and repeatedly arranged throughout the entire filter plane, ensuring that each photosensitive pixel of the detector corresponds to a spectral channel, realizing the simultaneous acquisition of image information of nine different spectra in a single exposure. Figure 3 (b) shows the layered structure of the filter. The colored squares above the substrate represent the step height difference between different channels due to the difference in the thickness of the spacer layer. They correspond one-to-one with the preset center wavelength of each channel, reflecting the film structure characteristics of different channels under the step-by-step fabrication process.

[0033] To ensure the superior performance of the filters, the microstructure was strictly controlled. The morphology of the fabricated nine-channel pixel-level spectral filter array was evaluated using laser confocal microscopy, and the results are as follows: Figure 4 As shown. Figure 4 This is a planar topographic image of the pixel array taken by a laser confocal microscope. The lower left corner of the image shows the scale bar (15 μm), as well as test parameters such as field of view (FOV: 233 μm × 175 μm) and magnification (Obj: 50 ×). Figure 4 As can be seen, the filter exhibits a clear checkerboard pattern with distinct boundaries between adjacent pixels, no obvious defects or distortions, and extremely high image fidelity. The size of a single pixel in the image is precisely maintained at 10μm × 10μm, consistent with the design value, indicating excellent controllability of the combined photolithography and coating processes. Combined with the 3D contour measurement results, it is confirmed that there are regular differences in step height among the nine different channels. This height difference corresponds one-to-one with the thickness gradient of the silicon dioxide spacer layer in different channels in the design, verifying the ability of the differentiated etching process to control the cavity layer thickness.

[0034] The spectral transmittance of the prepared nine-channel pixel-level multispectral filter was tested using a spectrophotometer. The measured transmittance curve is shown in the attached figure. Figure 5 As shown. Figure 5 The horizontal axis represents wavelength (nm), and the vertical axis represents transmittance (%). The nine curves of different colors in the figure correspond to the measured transmittance responses of the nine spectral channels. Figure 5 As can be seen, the filter of this invention forms nine independent and uniformly distributed resonant peaks in the visible-near infrared band of 600-1000 nm, successfully realizing the spectral splitting function of nine channels. The peak transmittance of each channel is between 60% and 98%; the peak transmittance of the long-wavelength channel can reach up to 98%, and the peak transmittance decreases slightly as the operating wavelength moves towards shorter wavelengths, with the peak transmittance of the channel near 650 nm being about 60%. At the same time, the full width at half maximum (FWHM) of all channels is stably controlled between 18 and 38 nm, exhibiting a steep spectral cutoff characteristic and demonstrating high spectral selectivity.

[0035] The method of this invention prepares an ultra-wideband visible-near-infrared multispectral filter, which greatly expands the spectral range and enables it to make full use of the full response spectrum of silicon-based detectors.

[0036] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0037] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A method for preparing a nine-channel pixel-level spectral filter, characterized in that: Specifically, the steps include the following: S1. The substrate is ultrasonically cleaned sequentially with acetone, anhydrous ethanol, and deionized water, and then the substrate surface is dried with high-purity nitrogen. S2. Using ion beam sputtering, with argon as the main sputtering gas, hydrogen-doped amorphous silicon thin film and silicon dioxide thin film are alternately deposited on the pretreated substrate surface to form the first reflective film. S3. Deposit a spacer layer integrally on the surface of the first reflective film; S4. Coat the spacer layer surface with photoresist, and after exposure and development, use the photoresist as a mask to etch the spacer layer channel by channel to remove film layers of different thicknesses, so that the thickness of the spacer layer retained in the nine channels forms a gradient. S5. Using ion beam sputtering, hydrogen-doped amorphous silicon thin films and silicon dioxide thin films are alternately deposited above the spacer layer to form a second reflective film, thus producing a nine-channel pixel-level spectral filter.

2. The method for preparing a nine-channel pixel-level spectral filter according to claim 1, characterized in that: In step S2, when depositing a silicon dioxide thin film, argon is used as the sputtering gas, and oxygen is introduced simultaneously; when depositing a hydrogen-doped amorphous silicon thin film, hydrogen is introduced into the deposition chamber simultaneously with argon as the sputtering gas, and the vacuum level is controlled to be no greater than 2.0 × 10⁻⁶. 6 Torr, hydrogen flow rate is 50~100 sccm.

3. The method for preparing a nine-channel pixel-level spectral filter according to claim 2, characterized in that: The deposition rate of the silicon dioxide thin film and the hydrogen-doped amorphous silicon thin film is 0.1~0.5 nm / s.

4. The method for preparing a nine-channel pixel-level spectral filter according to claim 1, characterized in that: In the first reflective film and the second reflective film, the thicknesses of the single-layer hydrogen-doped amorphous silicon film and the silicon dioxide film are 80~120nm and 70~110nm, respectively.

5. The method for preparing a nine-channel pixel-level spectral filter according to claim 1, characterized in that: The spacer layer is made of silicon dioxide; the substrate is a high-purity quartz glass substrate.

6. A nine-channel pixel-level spectral filter, prepared using the method for preparing a nine-channel pixel-level spectral filter as described in claim 1, characterized in that: It includes a substrate, a first reflective film, a spacer layer, and a second reflective film that are stacked tightly from bottom to top; Both the first reflective film and the second reflective film are multilayer film structures formed by alternating stacking of hydrogen-doped amorphous silicon thin films and silicon dioxide thin films; The spacer layer is etched into nine groups of units with different thicknesses, and the nine groups of units are arranged in a checkerboard array to form nine independent spectral channels.

7. A nine-channel pixel-level spectral filter according to claim 6, characterized in that: The spacer layer is made of silicon dioxide and has a thickness between 300 and 900 nm.

8. A nine-channel pixel-level spectral filter according to claim 6, characterized in that: The operating wavelength of the filter is 600~1000nm.

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