Large-aperture high-aspect-ratio convex blazed grating and preparation method thereof

Abstract

This invention discloses a method for fabricating a large-aperture convex grating with a high-depth-width blazed groove shape. The method includes: confirming the grating line density using dispersion theory; determining the grating's operating temperature under corresponding wavelength radiation backgrounds; determining the grating substrate material; performing a rough calculation of the blaze angle using scalar diffraction theory; performing a precise calculation of the blaze angle using the finite-difference time-domain method; determining the film material and thickness; designing the consistency of TE and TM diffraction efficiencies; analyzing the grating groove shape difference; confirming the grating groove shape parameters; electron beam exposure and development of the blazed groove shape; designing, exposing, and developing the exposure dose compensation; stitching together the large-aperture grating exposure areas; ion beam etching of the grating; and coating the grating with a metal reflective film. This fabrication method ensures the stability of the diffraction efficiency of the grating substrate and film materials under varying temperatures from room temperature to deep cryogenics, as well as the consistency of TE and TM diffraction efficiencies, thus guaranteeing the imaging quality and response stability of spectroscopic instruments.

Description

Technical Field

[0001] The invention relates to the field of convex grating technology, and in particular to a large-aperture, high-depth-wide convex blazed grating and its fabrication method. Background Technology

[0002] Spectroscopy is a crucial physical quantity for characterizing and detecting the microscopic structure of matter; it is the "color" and "fingerprint" of matter. Hyperspectral remote sensing technology can detect and identify the composition of materials on the Earth's surface over large areas, either in the field or on space. It is a milestone in the development of remote sensing technology and has been listed by the European Commission as one of the "100 Disruptive Technological Innovations for the Future." Depending on the dispersive method, hyperspectral imaging spectrometers can be classified into dispersive, interferometric, and filter types. The dispersive method of the spectral system directly affects the structural complexity, performance, size, and weight of the entire imaging spectrometer system. Dispersive imaging spectrometers have a simple structure, small size, high signal-to-noise ratio, and simple image data processing, making them suitable for the complex aerospace environment requiring high precision and lightweight design. The dispersive element is the core component of the imaging spectrometer, determining the system's key performance characteristics such as spectral resolution, dispersive efficiency, and accuracy.

[0003] A grating is a diffraction element consisting of equally spaced slits or a periodically distributed structure. Suitable for various spectral bands, it offers advantages such as uniform dispersion, a wide applicable spectral range, and high diffraction efficiency. The grating utilizes a triangular groove design to enhance the energy concentration (blaze) of the beam, broaden the coverage of high diffraction efficiency bands, and increase peak and overall energy. A specific grating line distribution achieves "dispersion" of wide-band polychromatic light, resulting in fine beam splitting. A convex grating beam splitting system based on an Offner structure achieves fine beam splitting with small spectral line bending, large flat field with low distortion, and low multispectral aliasing through a concentric structure. Furthermore, the system uses a reflective optical surface. With appropriate selection of optical and structural materials, the system can operate stably in cryogenic environments, ensuring that fluctuations or changes in external conditions do not affect the stability of weak signal spectral detection. This makes it the preferred choice for ultra-long-wavelength (12μm-16μm) hyperspectral detection of weak signals.

[0004] The design requirements for convex reflective gratings lie in the selection of grating groove shape and parameter design, as well as the selection of reflective film materials. Especially in cryogenic environments, the grating substrate material and reflective film material have a significant impact on the stability of spectrometer imaging intensity and imaging quality. Furthermore, when the grating operates over a wide spectral band, the polarization state of the incident light undergoes unpredictable changes after passing through the pre-telescope system. Ensuring consistent grating diffraction efficiency under both TE and TM extreme polarization conditions is crucial for maintaining stable grating diffraction energy. When spectral resolution is refined to the nanometer scale, the fabrication difficulty of convex gratings increases dramatically, requiring the creation of tens of thousands of triangular grooves with a width of tens of thousands of micrometers and a depth of tens of nanometers.

[0005] The core evaluation metric for convex gratings is diffraction efficiency. When the diffraction efficiency reaches its maximum, the grating performance is considered optimal. The diffraction efficiency is determined by the precision of the grating groove shape. As the spectrometer's wavelength range expands to the very low infrared (VLI) range, the corresponding convex grating line density decreases, the grating aperture increases, and the depth and width of the grating grooves increase significantly. This necessitates achieving highly consistent and deep / wide convex grating grooves within an aperture of nearly 100 millimeters. To meet the grating line and groove shape requirements of VLI spectrometer systems, using electron beam processing is the best method to ensure the performance of finely tuned spectrometers.

[0006] However, current grating groove design methods do not analyze the impact of temperature on the stability of the grating substrate and film materials, leading to risks in the application of gratings in deep cryogenic environments. In terms of grating fabrication, the very long-wave infrared band is not addressed, and wider and deeper (depth greater than 5μm, width greater than 50μm) triangular grooves are not achieved on large apertures (aperture greater than 100mm). This makes the grating grooves prone to deformation, resulting in protrusions or depressions, and controlling the consistency between the grating groove shape in the central region and the edge region of a large-area substrate is more difficult. These challenges require high-precision grating groove control technology and large-area groove splicing technology to overcome. Summary of the Invention

[0007] In view of this, the purpose of this invention is to provide a large-aperture, high-depth, and wide-convex blazed grating and its fabrication method, thereby solving one or more of the above-mentioned problems in the prior art.

[0008] To achieve the above objectives, the present invention provides a method for fabricating a large-aperture, high-depth, and wide-width blazed convex grating, comprising the following steps:

[0009] S1. The density of grating lines was confirmed using dispersion theory;

[0010] S2. Determine the operating temperature of the grating under the corresponding wavelength radiation background;

[0011] S3. Determine the grating substrate material based on the influence of the grating substrate material on wavefront aberration at different operating temperatures;

[0012] S4. The blaze angle is roughly calculated using scalar diffraction theory.

[0013] S5. Based on the rough calculation results of step S4, set the blaze angle calculation range and use the finite difference time-domain method to calculate the blaze angle accurately.

[0014] S6. Determine the grating film material and film thickness based on the adaptability of the grating film material to different operating temperatures;

[0015] S7. Fine-tune the blaze angle or anti-blaze angle to ensure that the diffraction efficiency of TE and TM is consistent within the error range, and output the ideal grating slot design result.

[0016] S8. Calculate the grating groove roughness error function and parallelism error function;

[0017] S9. Based on the output of S7, calculate the grating resist groove parameters in combination with the ion beam etching selectivity.

[0018] S10. After electron beam exposure and development of the resist, the resist grating groove shape is obtained, and the resist grating groove shape distribution curve is obtained by testing.

[0019] S11. Compare the obtained resist grating groove shape distribution curve with the ideal resist groove shape curve, calculate the electron beam exposure compensation amount and compensation amount coefficient, expose and develop again, and determine whether the groove shape is qualified according to the grating groove shape difference calculated in step S8. If it is not qualified, repeat step S11. If it is qualified, proceed to step S12.

[0020] S12. Calculate the size of the single exposure area and the total number of areas based on the ideal grating groove width and the ideal grating aperture, perform large-aperture grating exposure area splicing, and then expose and develop again after splicing.

[0021] S13. Use the oscillating etching method to transfer the grating structure from the resist onto the grating substrate material;

[0022] S14. A layer of metal reflective film is deposited on the grating surface using the oscillating coating method to complete the fabrication of a large-aperture, high-depth, and wide convex blazed grating.

[0023] Furthermore, in step S2, the operating temperature of the grating under the corresponding band radiation background is as follows: less than 265k for visible light, near infrared, and short-wave infrared bands; less than 144k for mid-wave infrared bands; less than 132k for long-wave 1 infrared bands; less than 121k for long-wave 2 infrared bands; and less than 100k for very long-wave infrared bands.

[0024] Furthermore, the visible light band is 0.38–0.74 μm, the near-infrared band is 0.74–1.44 μm, the short-wave infrared band is 1.44–2.5 μm, the mid-wave infrared band is 2.5–5.15 μm, the long-wave 1 infrared band is 5.15–8.15 μm, the long-wave 2 infrared band is 8.15–12 μm, and the very long-wave infrared band is 12–16 μm.

[0025] Furthermore, the range of the coarsely calculated blaze angle θ is: 0 < θ < 90°.

[0026] Furthermore, in step S5, the precise blaze angle θ1 is 0 < θ-5° < θ1 < θ+5°, the angle scanning accuracy range is 0.1°-0.5°, and the simulation environment temperature is 300K.

[0027] Furthermore, the substrate material in step S3 is selected from any one of quartz JGS1, quartz JGS2, quartz JGS3, ULE, gold, and aluminum alloy.

[0028] Furthermore, in step S6, the film material is selected from any one of gold, silver, aluminum, and aluminum alloy, and the film thickness is 200-500 nm.

[0029] Furthermore, the different operating temperature ranges in steps S3 and S6 are 100–300K.

[0030] Furthermore, in step S7, the consistency of TE and TM satisfies the following: the polarization consistency error of the visible short-wavelength grating diffraction efficiency is less than 2%, the polarization consistency error of the mid-to-long-wavelength infrared grating diffraction efficiency is less than 5%, and the final output blaze angle is θ2.

[0031] Furthermore, the grating groove roughness error function in step S8 is:

[0032]

[0033] Where RMS represents the grating groove roughness error function, Δh(x n ) represents the difference between the ideal groove and the groove shape, x n This represents the coordinate position of the nth groove.

[0034] Furthermore, the groove roughness error is less than one-fortieth of the center wavelength.

[0035] Furthermore, the grating slot parallelism error function in step S8 is:

[0036]

[0037] Where P represents the grating slot parallelism error function, Δh(x n ) represents the difference between the ideal groove and the groove shape, x n This represents the nth coordinate position of the groove.

[0038] Furthermore, the parallelism error of the grating groove shape is less than 5%.

[0039] Furthermore, in step S9, the etching ratio ranges from 1.1:1 to 1.2:1, and the etching gas is CHF3.

[0040] Furthermore, in step S10, the diameter of the electron beam exposure focusing spot ranges from 5nm to 15nm, the intensity ranges from 1nA to 10nA, the single-point exposure dwell time is 0μs to 50μs, and the developer concentration is MIBK:IPA = 1:2 to 1:4.

[0041] Furthermore, step S11 specifically includes: calculating the difference Δh between the measured groove depth distribution curve and the ideal groove shape curve after grating development; calculating the exposure compensation amount based on the theoretical groove shape curve groove depth + k*Δh, where k is the compensation coefficient; keeping the exposure intensity constant, performing exposure compensation and development by dwell time; if the groove shape error after a single compensation does not meet the grating groove shape error requirement described in step S8, then the compensation coefficient k is corrected until the grating groove shape error is met.

[0042] Furthermore, the compensation coefficient k is 0.5-2.

[0043] Furthermore, step S12 specifically includes: stitching together the exposure areas with convex curvature distribution and drawing a virtual mask with angle changes; dividing the virtual mask into individual write fields of a specific size according to an integer multiple of the groove width; further dividing each write field into sub-write fields, the area of ​​which is the square of the width of a single groove; exposure and development.

[0044] Furthermore, the writing field size is 0.001mm*0.001mm-1mm*1mm, and the splicing accuracy is ≤30nm.

[0045] Furthermore, in step S13, the oscillation curvature accuracy of the oscillation etching is ±0.01mm, the oscillation angle range is -30° to 30°, the oscillation speed accuracy is 0.001rad / s, the etching accuracy is 20nm / min-45nm / min@ULE, and the effective etching area is ≥200mm.

[0046] Furthermore, in step S14, the oscillation curvature accuracy of the oscillation coating is ±0.01mm, the oscillation angle range is -30° to 30°, the oscillation speed accuracy is 0.001rad / s, the coating speed is 10nm / min-18nm / min, and the coating material is selected from any one of gold, silver, aluminum, and aluminum alloy.

[0047] Furthermore, the grating substrate is convex spherical in shape, with an effective wavelength range of ≥12μm-16μm, a groove width of ≥50μm, a groove depth of ≥5μm, an area of ​​≥100mm, a blaze angle of ≥5°, and an anti-blaze angle of ≥70°. The individual grooves of the grating are oriented along the vertical arc direction of the convex substrate, while the angles of adjacent grooves are continuously changing along the horizontal arc direction of the convex substrate.

[0048] Compared with existing technologies, the advantages of this invention are as follows: This invention proposes a method for fabricating a large-aperture convex grating with a high depth and width blazed groove shape. It analyzes the influence of temperature on the stability of the grating material, reducing the application risks of the grating in deep cryogenic environments; it analyzes the polarization characteristics of the grating, controlling the grating structure to improve diffraction efficiency and polarization consistency; it uses electron beam exposure and other processes for grating fabrication, ensuring the groove shape accuracy of the high depth and width blazed groove grating through exposure dose compensation; and it ensures the groove shape consistency of the large-aperture grating through large-area splicing. This fills the technological gap in high depth and width blazed groove large-aperture convex gratings for very long-wave infrared applications. Attached Figure Description

[0049] Figure 1 This is a flowchart illustrating a method for fabricating a large-aperture, high-depth, and wide-convex blazed grating according to an embodiment of the present invention;

[0050] Figure 2 This is a schematic diagram of a high-depth, wide-angle blazed groove-shaped large-diameter convex surface simulation model grating in one embodiment of the present invention.

[0051] Figure 3 This is a natural light diffraction efficiency curve of a grating in one embodiment of the present invention.

[0052] Figure 4 This is a grating polarization diffraction efficiency data curve in one embodiment of the present invention.

[0053] Figure 5 This is a schematic diagram of the grating groove distribution in one embodiment of the present invention.

[0054] Figure 6 This is a comparison curve of exposure dose compensation in one embodiment of the present invention.

[0055] Figure 7 This is a schematic diagram of the grating groove shape after exposure compensation in one embodiment of the present invention.

[0056] Figure 8 This is a schematic diagram illustrating the relationship between a single write field and the slot shape in one embodiment of the present invention.

[0057] Figure 9 This is a schematic diagram illustrating the relationship between the writing field and the entire grating in one embodiment of the present invention. Detailed Implementation

[0058] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it. Furthermore, it should be noted that, for ease of description, only the parts relevant to the invention are shown in the accompanying drawings, and not all of them.

[0059] To achieve the above objectives, this invention provides a design and analysis of the slot shape for a convex blazed grating, such as... Figure 1As shown, it includes the following steps:

[0060] S1. The density of grating lines was confirmed using dispersion theory;

[0061] Specifically, the grating line density f satisfies the formula: Where m is the diffraction order of the grating. Where m is the wavelength; typically, in the design of spectrometer imaging systems, m=1. Generally, the larger the wavelength, the lower the theoretical grating line density. That is, the grating line density gradually decreases from the ultraviolet to the very long-wave infrared band. The spectral resolution is: Where N is the total number of grating marks. When the detector and telescope parameters are the same, the same resolution requirement must be maintained. The larger the wavelength, the larger the grating area.

[0062] S2. Determine the operating temperature of the grating under the corresponding wavelength radiation background;

[0063] Specifically, based on the optimal operating temperature of the grating under background radiation analysis in the corresponding wavelength bands, an optomechanical-thermal integrated analysis is required. A thermal analysis model is established using SINDA, including the location of structural nodes, connection relationships, and thermal load, to calculate the structural temperature gradient distribution. The structural analysis model needs to project the temperature gradient from the thermal analysis onto a more detailed finite element analysis model. Given that the temperature gradient of structural components with the same material and regular shape exhibits a linear variation, a nonlinear transient thermal analysis is performed using the nearest neighbor interpolation method. The analysis yields the following operating temperatures for the grating under the corresponding wavelength radiation backgrounds: less than 265 K in the visible, near-infrared, and short-wave infrared bands; less than 144 K in the mid-wave infrared band; less than 132 K in the long-wave 1 infrared band; less than 121 K in the long-wave 2 infrared band; and less than 100 K in the very long-wave infrared band. More specifically, the visible light band is 0.38–0.74 μm, the near-infrared band is 0.74–1.44 μm, the short-wave infrared band is 1.44–2.5 μm, the mid-wave infrared band is 2.5–5.15 μm, the long-wave 1 infrared band is 5.15–8.15 μm, the long-wave 2 infrared band is 8.15–12 μm, and the very long-wave infrared band is 12–16 μm.

[0064] S3. Determine the grating substrate material based on the influence of the grating substrate material on wavefront aberration at different operating temperatures;

[0065] Specifically, SigFit software was used for temperature change analysis. Under normal operating conditions, the substrate PV = 120 nm and RMS = 12 nm were set. The ideal operating temperature was set as the target temperature. The thermal expansion coefficient of the material was set, and simulation was used to obtain the rigid body deformation displacement data after the change. The PV and RMS values ​​at the ideal operating temperature were calculated. The substrate material with the smallest change in PV and RMS values ​​was selected as the most suitable substrate material. Preferably, the substrate material was selected from any one of quartz JGS1, quartz JGS2, quartz JGS3, ULE, gold, and aluminum alloy. The operating temperature range was 100K-300K.

[0066] S4. The blaze angle is roughly calculated using scalar diffraction theory.

[0067] Specifically, the rough formula for calculating the blaze angle in step S4 is derived from the grating equation, which is mλ = d(sinα ± sinβ), where m is the grating order, λ is the diffraction wavelength, α is the incident angle, β is the diffraction angle, and d is the grating period. This indicates that the principal poles of the same order appear in different orientations at different wavelengths, with longer wavelengths having larger diffraction angles and shorter wavelengths having smaller diffraction angles. m can take values ​​of 0, ±1, ±2, etc., and the corresponding spectra are called the zero-order spectrum, first-order spectrum, second-order spectrum, etc. The + and - signs indicate that the incident angle and diffraction angle are on the same or opposite sides of the normal, respectively. (dsinα + dsinβ) is the net optical path difference between the incident ray and the diffracted ray, which is equal to an integer multiple of the wavelength, mλ. Based on the relationship between the incident angle, blaze angle, and diffraction angle... The rough formula for calculating the blaze angle is derived as follows: Where m is the grating order. For the blaze wavelength, To roughly calculate the gleam angle, Let be the incident angle, and d be the grating period. The relationship between the period d and the grating line density f is: d = 1 / f. Substituting the grating usage conditions and requirements, the blaze angle calculation results are obtained. Calculation results without imaginary parts are selected, and the blaze angle selection criteria are: .

[0068] S5. Based on the rough calculation results of step S4, set the blaze angle calculation range and use the finite difference time-domain method to calculate the blaze angle accurately.

[0069] Specifically, the finite-difference time-domain method is used to construct the grating groove model, and preferably, the precise blaze angle is defined. for The angle range can be adjusted based on the coarsely calculated blaze angle. The angle scanning accuracy range is 0.1°-0.5°. The number of calculations is the scan range / number of scans + 1. The simulation environment temperature is 300K. By comparing multiple calculation results, the blaze angle corresponding to the optimal diffraction efficiency is selected as the finely calculated optimal blaze angle value. The anti-blaze value is δ = 90° - θ1, and the evaluation criterion is the diffraction efficiency value. Diffraction efficiency is an important parameter of grating performance, reflecting the signal-to-noise ratio of the spectrometer and the utilization rate of light energy. The structural parameters of the grating determine the diffraction efficiency of the grating. The criteria for judging the accurate blaze angle are: ensuring that the diffraction efficiency value corresponding to the blaze wavelength is the maximum, and the diffraction efficiency at both ends is relatively high.

[0070] S6. Determine the grating film material and film thickness based on the adaptability of the grating film material to different operating temperatures;

[0071] Specifically, such as Figure 1 As shown, the temperature range of the simulation environment was changed to 100K-300K. The grating diffraction efficiency curves under different materials were compared, and the smoothest curve was selected to improve efficiency stability. Preferably, the film material was selected from any one of gold, silver, aluminum, or aluminum alloy, and the film thickness ranged from 200nm to 500nm. To ensure that the diffraction efficiency fitting curve accurately reflects the actual simulation value, the wavelength discontinuity step size generally did not exceed 10nm. During the process, there may be local wavelength points with high or low efficiency, resulting in "spicules" on the efficiency curve, which cannot be eliminated by changing all film materials. This phenomenon is a "resonance" phenomenon between the scribe line density and the wavelength band, which is normal and not used as a basis for judging the smoothness of the curve.

[0072] S7. Fine-tune the blaze angle or anti-blaze angle to ensure that the diffraction efficiency of TE and TM is consistent within the error range, and output the ideal grating slot design result.

[0073] Specifically, fine-tuning the blaze angle or anti-blaze angle improves the consistency of TE and TM diffraction efficiencies. Adjusting the blaze angle changes the position of the center wavelength, with an adjustment range of ±0.1°. Adjusting the anti-blaze angle changes the diffraction efficiency values ​​at both ends, with an adjustment range of ±20°. This achieves a polarization consistency error of less than 2% for visible short-wavelength grating diffraction efficiency and less than 5% for mid- and long-wavelength infrared grating diffraction efficiency. More specifically, the formula for calculating polarization consistency error is:

[0074] q= 100%;

[0075] Where q is the polarization consistency error. This represents the diffraction efficiency value under TE polarization. This represents the diffraction efficiency value under TM polarization.

[0076] S8. Calculate the grating groove roughness error function and parallelism error function, that is, perform groove manufacturing tolerance analysis on the grating designed in steps S1-S7.

[0077] Specifically, the roughness error function of the grating groove is:

[0078] ;

[0079] Where RMS represents the grating groove roughness error function, Δh(x n ) represents the difference between the ideal groove and the groove shape, x n This indicates the nth coordinate position of the groove. Preferably, the groove roughness error is less than one-fortieth of the center wavelength.

[0080] Specifically, the grating slot parallelism error function is:

[0081] P=Σ / n*100%;

[0082] Where P represents the grating slot parallelism error function, Δh(x n ) represents the difference between the ideal groove and the groove shape, x n This indicates the coordinate position of the nth groove. Preferably, the parallelism error of the grating groove shape is less than 5%.

[0083] Based on the grating groove roughness error function and the grating groove parallelism error value requirement, calculate the grating groove roughness error Δh(x) n ).

[0084] S9. Based on the output of S7, calculate the grating resist groove parameters in combination with the ion beam etching selectivity.

[0085] Specifically, the etching ratio r is in the range of 1.1:1-1.2:1, the etching gas is CHF3, and preferably, the electron beam resist is 950PMMA series, and the substrate is ULE substrate. Let the ideal trench depth be h(x). n If the electron beam resist groove distribution curve is h1(x), then the electron beam resist groove distribution curve is h1(x). n )=h(x n )*r, where h(x) n )for:

[0086]

[0087] S10. After electron beam exposure and development of the resist, the resist grating groove shape is obtained, and the resist grating groove shape distribution curve is obtained by testing.

[0088] Specifically, the grating groove is triangular, and the grating blaze surface is approximated by several steps. The electron beam scans and exposes the grating with a step width as the interval. An appropriate exposure dose is selected based on the height of each step. After development, a stepped groove shape is obtained. The finer the steps, the closer it is to the ideal sawtooth shape. Each step occupies one pixel, so each grating groove consists of multiple pixels. Different step heights require different exposure doses. Using the same accelerating voltage during exposure results in different electron beam dwell times at each step height, thus achieving different exposure doses. Preferably, the electron beam focusing spot diameter ranges from 5nm to 15nm, the intensity ranges from 1nA to 10nA, the voltage is 1-10kV, the single-point exposure dwell time is 0μs-50μs, the developer concentration is MIBK:IPA = 1:2-1:4, and the measured groove depth distribution curve after exposure and development is h2(x n ).

[0089] S11. Compare the obtained resist grating groove shape distribution curve with the ideal resist groove shape curve, calculate the electron beam exposure compensation amount and compensation amount coefficient, expose and develop again, and determine whether the groove shape is qualified according to the grating groove shape difference calculated in step S8. If it is not qualified, repeat step S11. If it is qualified, proceed to step S12.

[0090] Specifically, an error segmentation compensation and correction method is used to compensate for the electron beam exposure dose. Generally, no exposure dose compensation is needed for grooves with a width of 0-10 μm; exposure compensation is required for grooves with a width >10 μm. More specifically, the compensation design and operation are completed as follows: The measured groove depth distribution curve h2(x) after grating development is calculated. n ) and the ideal trough curve h1(x n The difference in trench depth Δh = h2(x) n )-h1(x n According to the theoretical groove shape curve, the groove depth h1(x) n )+k*Δh to calculate the exposure compensation amount, where k is the compensation coefficient, preferably k is 0.5-2; keep the exposure intensity constant, and perform exposure compensation and development by dwell time; if the groove description error after a single compensation does not meet the grating groove description error requirement described in step S8, then the compensation coefficient k is corrected until the grating groove description error is met.

[0091] S12. Calculate the size of the single exposure area and the total number of areas based on the ideal grating groove width and the ideal grating aperture, perform large-aperture grating exposure area splicing, and then expose and develop again after splicing.

[0092] Specifically, the smaller the write field area of ​​electron beam exposure, the higher the achievable pattern resolution. Exposure of large-area patterns is achieved through electron beam deflection and sample stage movement. When the area of ​​the exposed pattern is larger than the deflection field range, multiple write fields need to be stitched together. More specifically, this involves stitching together exposure areas with convex curvature distribution and drawing virtual masks with varying angles; dividing the virtual mask into individual write fields of specific sizes according to integer multiples of the groove width; further dividing each write field into sub-write fields, with the sub-write field area being the square of the individual groove width; and then exposure and development. Preferably, the write field size is 0.001mm*0.001mm-1mm*1mm, and the stitching accuracy is ≤30nm. If the size of the scanning field is distorted, phenomena such as repeated exposure, underexposure, or misalignment will occur at the pattern stitching points. To reduce these issues, the moving stage and field distortion need to be corrected before large-size stitching.

[0093] S13. Use the oscillating etching method to transfer the grating structure from the resist onto the grating substrate material;

[0094] Specifically, the transfer of the grating groove shape is achieved using a wobbling etching method. The electron beam resist grating mask obtained by electron beam exposure can directly form the desired pattern on the convex surface. Ion beam etching simply transfers the pattern directly to the substrate without the need for etching to form the blaze angle. During etching, the sample stage is rotated so that the ion beam is perpendicular to the convex surface and the local etching area is etched. Using wobbling etching ensures consistent etching depth across all positions of the large-area grating. Preferably, the wobbling curvature accuracy is ±0.01 mm, the wobbling angle range is -30° to 30°, the wobbling speed accuracy is 0.001 rad / s, the etching accuracy is 20 nm / min to 45 nm / min @ ULE, and the effective etching area is ≥200 mm.

[0095] S14. A layer of metal reflective film is deposited on the grating surface using the oscillating coating method to complete the fabrication of a large-aperture, high-depth, and wide convex blazed grating.

[0096] Specifically, a magnetron sputtering oscillating deposition method is employed, in which high-speed positive ions bombard the surface of the film material. Through momentum transfer, the molecules or atoms gain sufficient kinetic energy to escape from the target surface and condense into a film on the deposited surface. Its theoretical basis is the glow discharge of gas: at a gas pressure of 1–10 Pa, the gas between high-voltage electrodes ionizes to form a low-pressure, high-current conductor, accompanied by a glow discharge phenomenon. Preferably, the oscillation curvature accuracy of the oscillation deposition is ±0.01 mm, the oscillation angle range is -30° to 30°, the oscillation speed accuracy is 0.001 rad / s, the deposition speed is 10 nm / min–18 nm / min, the deposition material is selected from any one of gold, silver, aluminum, or aluminum alloys, and the deposition thickness is 150–200 nm.

[0097] The fabricated grating is a large-aperture, high-depth, convex, wide-angle blazed grating. The grating substrate is a convex spherical surface, with an effective wavelength range ≥12μm-16μm, groove width ≥50μm, groove depth ≥5μm, area ≥100mm², blaze angle ≥5°, and anti-blaze angle ≥70°. Individual grooves of the grating are etched along the vertical arc direction of the convex substrate, while the angle between adjacent grooves changes continuously along the horizontal arc direction of the convex substrate. For example... Figure 5 As shown, 1 represents the grating groove shape, 2 represents the grating substrate, and the grating groove shape always remains perpendicular to the tangent.

[0098] Example 1

[0099] This embodiment provides a method for fabricating a large-aperture convex grating with a high depth and wide blazed groove shape, including the following steps:

[0100] S1. Rough calculation of the blaze angle using scalar diffraction theory; specifically, based on grating line density analysis theory and optical design methods, the very long-wave infrared grating line density is obtained as 17.2 lp / mm, and the incident angle of the principal ray in the central region is... =25.8°, with a wavelength range of 12-16μm, and the desired blaze wavelength is λ. d =14μm, ideal order is m=1 order.

[0101] S2. Determine the operating temperature of the grating under the corresponding wavelength radiation background;

[0102] Specifically, based on the optimal operating temperature of the background radiation analysis of the band in which the grating is used, an optomechanical-thermal integrated analysis is required. A thermal analysis model is established using SINDA, including the location of structural nodes, connection relationships and thermal load, and the temperature gradient distribution of the structure is calculated. The analysis shows that the operating temperature of the very long-wave infrared band is less than 100K.

[0103] S3. Determine the grating substrate material based on the influence of the grating substrate material on wavefront aberration at different operating temperatures;

[0104] Specifically, SigFit software was used to analyze temperature changes. Under normal operating conditions, the surface area PV = 120nm and RMS = 12nm were set, and the ideal operating temperature was set to 100K. When the substrate material was ULE, the surface area change was minimal, with PV = 250nm and RMS = 21nm.

[0105] S4. The grating period d = 1 / f = 58.139 μm was calculated. The scalar diffraction theory was used to roughly calculate the grating blaze angle, and the blaze angles were obtained as -172.78°-15i, -76.41°, 103.58°-16i, and 8.21°. According to the blaze angle selection criteria, the final result of the rough calculation of the blaze angle is θ = 8.21°.

[0106] S5. The blaze angle is precisely calculated using the finite-difference time-domain method. Based on the rough calculation results from scalar diffraction theory, the blaze angle calculation range is set, and the optimal blaze angle for a normal temperature environment is output. The blaze angle calculation range is 7° < θ1 < 9°, the angle scanning accuracy is 0.1°, the reflective film material is selected as silver film, the ambient temperature is 300K, and the calculated ideal blaze angle is 7.8°, and the anti-blaze angle is 82.2°. Figure 3 The figure shows the natural light diffraction efficiency curves of the grating. In the figure, 1 represents the diffraction efficiency curve with a blaze angle of 7.3° and an anti-blaze angle of 82.7°, 2 represents the diffraction efficiency curve with a blaze angle of 7.8° and an anti-blaze angle of 82.2°, and 3 represents the diffraction efficiency curve with a blaze angle of 8.3° and an anti-blaze angle of 81.7°.

[0107] S6. Change the simulation environment temperature range to 100K, change the substrate material, and compare the smoothness and stability of the diffraction efficiency curves of different materials. When the substrate material is ULE, the film material is gold, and the film thickness is greater than 250nm, a better diffraction efficiency is obtained.

[0108] S7. Fine-tuning the blaze angle or anti-blaze angle improves the consistency of TE and TM diffraction efficiency. Adjusting the blaze angle changes the center wavelength position, and adjusting the anti-blaze angle changes the diffraction efficiency values ​​at both ends. For example... Figure 2 The figure shows the grating polarization diffraction efficiency analysis model, where 1 represents the effective calculation area, 2 represents the polarization state of the incident light, 3 represents the incident light, 4 represents the grating groove shape, and 5 represents the grating substrate. This patent uses a long-wave infrared grating, requiring a diffraction efficiency polarization consistency error of less than 5%. The actual optimized blaze angle is 7.7°, and the anti-blaze angle is 79.5°. Figure 4 The figure shows the grating polarization diffraction efficiency data curves, where 1 represents the diffraction efficiency under TE polarization, 2 represents the diffraction efficiency under natural light, and 3 represents the diffraction efficiency under TM polarization. The polarization consistency error of the diffraction efficiency is 1.95%, which is less than 2%.

[0109] S8. Calculate the grating groove roughness error function and parallelism error function to obtain the groove roughness error Δh. Δh is a numerical array with an array size of 1*N.

[0110] S9. The electron beam resist is 950PMMA A11, and the etching ratio with the ULE substrate is r = 1.2:1. The ideal groove width of the grating is x(N) = 58.139μm, and the groove depth is h(x(N)) = 6.2μm. Therefore, the groove width of the resist grating is x(N) = 58.139μm, and the groove depth is h1(x(N)) = h(x(N))*r = 7.44μm.

[0111] S10, the electron beam exposure method has a focused beam diameter of 10nm, an intensity range of 7nA, a single-point exposure dwell time of 0μs-22.32μs, and a developer concentration of MIBK:IPA = 1:4.

[0112] S11. Perform exposure compensation with a groove width ranging from 10 to 58.139 μm and a compensation coefficient of 0.9. After exposure and development, the measured groove depth distribution is h2(x(n)). The difference in groove depth between the measured and ideal groove shape curves is Δh(x) = h1(d1(x)) - h1(d2(x)). The exposure compensation is adjusted by changing the dwell time, which follows the trend of tan(θ)*d1(x) + k*Δh(x). Figure 6 As shown, this is a comparison curve of exposure dose compensation. Here, 2 represents the theoretical groove distribution formed by the uncompensated theoretical exposure dose, 1 represents the actual groove distribution formed by the uncompensated theoretical exposure dose, and 3 represents the theoretical groove distribution after adding exposure compensation. The grating groove preparation result after adding exposure compensation is shown below. Figure 7 As shown, the tolerance Δh is satisfied, where 1 represents the theoretically required groove shape distribution and 2 represents the actual groove shape distribution after adding exposure compensation.

[0113] S12, Large-aperture grating exposure stitching: The size of a single area ranges from 0.905mm*0.905mm, with an aperture of 101mm. The stitching accuracy is 45nm, and the stitching count is 9780 times. Exposure areas are stitched according to the convex curvature distribution. For example... Figure 8 The diagram shows the relationship between a single write field and the groove shape, where 1 represents a single write field and 2 represents the distribution of the grating grooves within that single write field. Figure 9 The diagram shows the relationship between the write field and the entire grating, where 1 represents the write field and 2 represents the entire grating. All write fields ensure that the entire grating is covered.

[0114] S13. The oscillating etching method is adopted, with an oscillation curvature accuracy of ±0.01mm, a motion speed accuracy of 0.001rad / s, an etching accuracy of 31nm / min@ULE, a total etching time of 199min, and an effective area of ​​≥200mm.

[0115] S14. A magnetron sputtering coating method is used, employing a oscillating coating technique with an oscillation curvature accuracy of ±0.01mm and a motion speed accuracy of 0.001rad / s. The coating material can be gold, and the coating thickness is 310nm, resulting in a convex grating.

[0116] The fabricated grating is a large-aperture, high-depth, convex, wide-angle blazed grating. The grating substrate is a convex spherical surface with an effective wavelength range of ≥12μm-16μm. The groove width is 58.139μm, the groove depth is 7.19μm, the area is 104mm, the blaze angle is 7.7°, and the anti-blaze angle is 81°. The individual grooves of the grating are etched along the vertical arc direction of the convex substrate, while the angles of adjacent grooves are continuously changed along the horizontal arc direction of the convex substrate.

Claims

1. A method for fabricating a large-aperture, high-depth, and wide-convex blazed grating, characterized in that, Includes the following steps: S1. The density of grating lines was confirmed using dispersion theory; S2. Determine the operating temperature of the grating under the corresponding wavelength radiation background; S3. Determine the grating substrate material based on the influence of the grating substrate material on wavefront aberration at different operating temperatures; S4. The blaze angle is roughly calculated using scalar diffraction theory. S5. Based on the rough calculation results of step S4, set the blaze angle calculation range and use the finite difference time-domain method to calculate the blaze angle accurately. S6. Determine the grating film material and film thickness based on the adaptability of the grating film material to different operating temperatures; S7. Fine-tune the blaze angle or anti-blaze angle to ensure that the diffraction efficiency of TE and TM is consistent within the error range, and output the ideal grating slot design result. S8. Calculate the grating groove roughness error function and parallelism error function; S9. Based on the output of S7, calculate the grating resist groove parameters in combination with the ion beam etching selectivity. S10. After electron beam exposure and development of the resist, the resist grating groove shape is obtained, and the resist grating groove shape distribution curve is obtained by testing. S11. Compare the obtained resist grating groove shape distribution curve with the ideal resist groove shape curve, calculate the electron beam exposure compensation amount and compensation amount coefficient, expose and develop again, and determine whether the groove shape is qualified according to the grating groove shape difference calculated in step S8. If it is not qualified, repeat step S11. If it is qualified, proceed to step S12. S12. Calculate the size of the single exposure area and the total number of areas based on the ideal grating groove width and the ideal grating aperture, perform large-aperture grating exposure area splicing, and then expose and develop again after splicing. S13. Use the oscillating etching method to transfer the grating structure from the resist onto the grating substrate material; S14. A layer of metal reflective film is deposited on the grating surface using the oscillating coating method to complete the fabrication of a large-aperture, high-depth, and wide convex blazed grating.

2. The preparation method according to claim 1, characterized in that, In step S2, the operating temperature of the grating under the corresponding band radiation background is as follows: less than 265k for visible light, near infrared, and short-wave infrared bands; less than 144k for mid-wave infrared bands; less than 132k for long-wave 1 infrared bands; less than 121k for long-wave 2 infrared bands; and less than 100k for very long-wave infrared bands.

3. The preparation method according to claim 2, characterized in that, The visible light band is 0.38–0.74 μm, the near-infrared band is 0.74–1.44 μm, the short-wave infrared band is 1.44–2.5 μm, the mid-wave infrared band is 2.5–5.15 μm, the long-wave 1 infrared band is 5.15–8.15 μm, the long-wave 2 infrared band is 8.15–12 μm, and the very long-wave infrared band is 12–16 μm.

4. The preparation method according to claim 1, characterized in that, The range of the coarsely calculated blaze angle θ is: 0 < θ < 90°.

5. The preparation method according to claim 4, characterized in that, In step S5, the precise blaze angle θ1 is 0 < θ-5° < θ1 < θ+5°, the angle scanning accuracy range is 0.1°-0.5°, and the simulation environment temperature is 300K.

6. The preparation method according to claim 1, characterized in that, The substrate material in step S3 is selected from any one of quartz JGS1, quartz JGS2, quartz JGS3, ULE, gold, and aluminum alloy.

7. The preparation method according to claim 1, characterized in that, In step S6, the film material is selected from any one of gold, silver, aluminum, and aluminum alloy, and the film thickness is 200-500 nm.

8. The preparation method according to claim 1, characterized in that, The different operating temperature ranges in steps S3 and S6 are 100 to 300 K.

9. The preparation method according to claim 1, characterized in that, In step S7, the consistency of TE and TM is satisfied as follows: the polarization consistency error of the visible short-wave grating diffraction efficiency is less than 2%, the polarization consistency error of the mid- and long-wave infrared grating diffraction efficiency is less than 5%, and the final output blaze angle is θ2.

10. The preparation method according to claim 1, characterized in that, The grating groove roughness error function in step S8 is: Where RMS represents the grating groove roughness error function, Δh(xn) represents the ideal groove groove roughness error, and xn represents the nth coordinate position of the groove.

11. The preparation method according to claim 10, characterized in that, The groove roughness error is less than one-fortieth of the center wavelength.

12. The preparation method according to claim 1, characterized in that, The grating slot parallelism error function in step S8 is: Where P represents the parallelism error function of the grating groove, Δh(xn) represents the ideal groove shape error, and xn represents the nth coordinate position of the groove.

13. The preparation method according to claim 12, characterized in that, The parallelism error of the grating groove shape is less than 5%.

14. The preparation method according to claim 1, characterized in that, In step S9, the etching ratio ranges from 1.1:1 to 1.2:1, and the etching gas is CHF3.

15. The preparation method according to claim 1, characterized in that, In step S10, the electron beam exposure focusing spot diameter ranges from 5nm to 15nm, the intensity ranges from 1nA to 10nA, and the single-point exposure dwell time is from 0μs to 50μs; the developer concentration is MIBK:IPA = 1:2 to 1:

4.

16. The preparation method according to claim 1, characterized in that, Step S11 specifically includes: calculating the difference Δh between the measured groove depth distribution curve and the ideal groove shape curve after grating development; calculating the exposure compensation amount based on the theoretical groove shape curve groove depth + k*Δh, where k is the compensation coefficient; keeping the exposure intensity constant, performing exposure compensation and development by dwell time; if the groove shape error after a single compensation does not meet the grating groove shape error requirement described in step S8, then the compensation coefficient k is corrected until the grating groove shape error is met.

17. The preparation method according to claim 16, characterized in that, The compensation coefficient k is 0.5-2.

18. The preparation method according to claim 1, characterized in that, Step S12 specifically includes: stitching together the exposure areas with convex curvature distribution and drawing a virtual mask with angle changes; dividing the virtual mask into individual write fields of a specific size according to an integer multiple of the groove width; further dividing each write field into sub-write fields, the area of ​​which is the square of the width of a single groove; exposure and development.

19. The preparation method according to claim 18, characterized in that, The writing field size is 0.001mm*0.001mm-1mm*1mm, and the splicing accuracy is ≤30nm.

20. The preparation method according to claim 1, characterized in that, In step S13, the oscillation curvature accuracy of the oscillation etching is ±0.01mm, the oscillation angle range is -30° to 30°, the oscillation speed accuracy is 0.001rad / s, the etching accuracy is 20nm / min-45nm / min@ULE, and the effective etching area is ≥200mm.

21. The preparation method according to claim 1, characterized in that, In step S14, the oscillation curvature accuracy of the oscillation coating is ±0.01mm, the oscillation angle range is -30° to 30°, the oscillation speed accuracy is 0.001rad / s, the coating speed is 10nm / min-18nm / min, and the coating material is selected from any one of gold, silver, aluminum, and aluminum alloy.

22. The preparation method according to claim 1, characterized in that, The large-aperture, high-depth, and wide-convex blazed grating obtained in step S14 has a substrate shape of a convex spherical surface, an effective wavelength range of ≥12μm-16μm, a groove width of ≥50μm, a groove depth of ≥5μm, an area of ​​≥100mm, a blaze angle of ≥5°, and an anti-blaze angle of ≥70°. The individual grooves of the grating are along the vertical arc direction of the convex substrate, and the angles of adjacent grooves are continuously changing along the horizontal arc direction of the convex substrate.

Images