Medium-infrared superstring-shaped holographic grating film coating and shape restoring regulation and control method and holographic grating

Abstract

The invention relates to an intermediate infrared superstring-shaped holographic grating film coating and shape restoring regulation and control method, which comprises the following steps of: preparing a photoresist grating mask on a substrate by adopting a holographic exposure method, depositing a film material on the photoresist grating mask as a matching layer by utilizing a film coating technology to form a superstring-shaped mask structure, and then depositing a functional layer to prepare a superstring-shaped holographic grating. According to the method, fine regulation and control of the grating mask are realized by utilizing a coating process, the limitation that the morphology regulation and control of the grating mask only depends on photoresist under a wet development process is broken through, the problem that the preparation of the holographic grating mask in the middle-infrared band is difficult is solved, and a new scheme is provided for the preparation of the holographic grating in the middle-infrared band.

Description

Technical Field

[0001] The present invention relates to the field of grating preparation, in particular to a mid-infrared super string-shaped holographic grating coating complex control method and a holographic grating. Technical Background

[0002] Mid-infrared gratings are core optical components in spectral analysis instruments, pulse compression devices, and spectral beam combiners. They are in high demand in fields such as high-field physics, optoelectronic countermeasures, laser processing, medical treatment, and environmental monitoring. Holographic gratings fabricated using holographic interferometry offer advantages such as high linear density accuracy, high resolution, short fabrication cycles, and simple process flow, making them an ideal solution for fabricating mid- and long-wave infrared gratings.

[0003] Currently, reflective holographic gratings primarily consist of a grating structure and a highly reflective layer. The grating structure diffracts incident light by phase-modulating it, while the highly reflective layer provides high reflectivity. Existing fabrication methods are primarily divided into pre-coating and post-coating techniques [Prior Art 1: BW Shore, et al., J. Opt. Soc. Am. A 14, 1124-1136 (1997)]. Post-coating technology is simple and widely used in grating fabrication. The main steps are: a photoresist grating mask is prepared on a grating substrate using a holographic interference exposure method, followed by coating a highly reflective film layer onto the mask. The grating structure produced using this method typically remains consistent with the mask structure. Therefore, holographic gratings fabricated using post-coating processes typically rely on controlling the photoresist grating mask and ignore subtle changes to the grating structure caused by the coating process. In the visible to near-infrared wavelength range, the groove depth of holographic gratings is typically less than 500 nanometers, and the grating period is less than 2000 nanometers. At this structural scale, string-shaped gratings can be easily fabricated using holographic exposure and wet etching processes [Prior Art 2: CN111580205B]. However, in the mid-infrared wavelength range, the grating groove depth design ranges from 500 to 2000 nanometers, and the grating period exceeds 2000 nanometers. At this scale, the structure of the photoresist grating mask deviates from the string shape due to the solubility characteristics of the photoresist [Prior Art 3: CN117406328A]. Conventional methods for controlling the morphology of photoresist grating masks rely solely on changes to the structure of the photoresist grating mask [Prior Art 5: Han Jian, et al. Acta Physica Sinica, 2012, 32(03):9-15.] Furthermore, the formation of the mask structure is affected by a combination of factors, including the type of photoresist, exposure dose, light field distribution, and development process. Precisely controlling the structure is extremely difficult and has low controllability. Furthermore, due to the solubility characteristics of the photoresist, the degree of freedom in controlling the mask structure is very limited. However, by depositing a thin film on the photoresist grating mask layer through a coating process, and by varying process parameters such as coating time, deposition rate, and working gas profile, and utilizing the stacking effect of the thin film material, the mask morphology can be further adjusted, enabling the fabrication of a string-shaped grating mask. To our knowledge, no research has reported on methods for controlling grating complexes using coating technology in the field of holographic grating fabrication. Summary of the Invention

[0004] To address the aforementioned difficulties in fabricating mid-infrared holographic gratings, the present invention provides a method for controlling the coating replica of a mid-infrared superstring holographic grating. This method, based on post-coating technology, utilizes a coating process to further deposit a matching film layer on the surface of a photoresist grating mask to achieve replica control of the mask structure, thereby precisely evolving and fabricating the superstring holographic grating structure. The string holographic grating fabricated using this method is suitable for the 2-12 micron mid-infrared band and can be used in various spectral analysis, laser pulse compression, or spectral beam combining devices.

[0005] The technical solutions of the present invention are as follows:

[0006] A method for controlling the complex shape of a mid-infrared superstring holographic grating coating is characterized in that the method comprises the following steps:

[0007] Step 1: coating a layer of photoresist on the surface of the grating substrate, and preparing a photoresist grating mask after double-beam interference exposure and development;

[0008] Step 2: depositing a matching film layer on the surface of the photoresist grating mask using a coating process, adjusting the deposition rate, deposition time and working gas ratio parameters of the coating machine to form a superstring-shaped matching layer grating mask having the same structure as the target grating;

[0009] Step 3: Using a coating process, a functional layer is further deposited on the surface of the matching layer grating mask to prepare a reflective string-shaped holographic grating.

[0010] Preferably, the coating process is a magnetron sputtering coating process or an electron beam evaporation coating process, and the controllable process parameters include deposition rate, deposition time and working gas ratio.

[0011] Preferably, the thickness of the matching film layer is 500-1000 nanometers, and the material can be any one or more thin film materials.

[0012] Preferably, the functional layer is various types of highly reflective metal films, dielectric films or mixed material films.

[0013] The superstring holographic grating structure prepared based on the above method can be represented by the slot function: H(x) = h×max{0,1-[cos(πxΛ) / sin[πf / 2]] 2σ}, where h is the groove depth, Λ is the grating period, f is the grating groove duty ratio, and σ is the grating shape factor. The groove depth h is 500 to 2500 nanometers, the grating period Λ is 2000 to 7000 nanometers, the duty ratio f is 0 to 1, and σ is any positive real number.

[0014] The technical effects of the present invention are as follows:

[0015] 1) The present invention combines the holographic interference exposure process with the coating process, which solves the difficulty of controlling the mask structure in the traditional holographic exposure process, expands the degree of freedom of mask structure control, and realizes the precise complex control of the superstring grating mask structure, providing a new idea for the preparation of mid-infrared holographic gratings.

[0016] 2) The superstring holographic grating prepared based on the method of the present invention has controllable diffraction efficiency and adjustable polarization characteristics, and is suitable for various spectral analysis, laser pulse compression or spectral beam combining devices in the 2 to 12 micron band.

[0017] 3) The mask control method of the present invention is universal and can be extended to the preparation of gratings in other bands and non-periodic microstructures. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram of the superstring diffraction grating structure provided by the present invention. In the figure, 1 is the grating substrate, 2 is the photoresist mask layer, 3 is the matching film layer, 4 is the functional layer, h is the grating groove depth, Λ is the grating period, and f is the duty ratio.

[0019] Figure 2 This is a spectrum diagram showing the variation of the TM polarization-1st order reflection diffraction efficiency of the superstring-shaped diffraction grating in the 6-9 μm long-wave infrared band at an incident angle of 40 degrees calculated in Example 2 with wavelength.

[0020] Figure 3 This is a spectrum diagram showing the TM polarization, TE polarization, and average -1 order reflection diffraction efficiency of the superstring diffraction grating in the 4.6-4.7 μm mid-infrared band at an incident angle of 51° calculated in Example 3 as a function of wavelength.

[0021] Figure 4 This is a schematic diagram of the superstring diffraction grating preparation process and the evolution process of the mask layer to the superstring grating structure provided by the present invention.

[0022] Figure 5 This is a cross-sectional view of the superstring diffraction grating prepared by SEM observation in Example 1. DETAILED DESCRIPTION

[0023] The specific implementation of the present invention will be further described below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention should not be limited thereto.

[0024] Example 1:

[0025] According to the present invention, Figure 1 The schematic diagram of the superstring grating structure shown is based on the superstring profile function H(x) to design a high-efficiency diffraction grating in the 4.6-4.7μm band, where 1 uses 50mm×50mm×1.5mm fused quartz as the grating substrate, 2 is the photoresist mask layer, 3 is the matching film layer, which is composed of a 500nm germanium (Ge) film, and 4 is the functional layer. The film system is L(HL)^4+4.88H, where L represents the low-refractive index material YF3 with a thickness of 730nm, and H represents the high-refractive index material Ge with a thickness of 282nm. The period of the superstring holographic grating structure is 3000nm, the groove depth is 3328nm, and the duty cycle is 0.24. The diffraction efficiency of the grating is calculated as follows Figure 2As shown, the grating has polarization-independent characteristics. At an incident angle of 51°, the TE polarization, TM polarization and average diffraction efficiency in the 4.6-4.7 μm band are all over 96%, and the average diffraction efficiency is over 97%.

[0026] Example 2:

[0027] According to the present invention, Figure 1 The schematic diagram of the superstring grating structure is shown in Figure 1. Based on the superstring profile function H(x), a high-efficiency diffraction grating in the 7-9 μm band is designed. 1 uses 50 mm × 50 mm × 1.5 mm fused silica as the grating substrate, 2 is a photoresist grating mask, 3 is a matching layer consisting of a 1000 nm thick Ge film, and 4 is a functional layer consisting of a 200 nm thick gold (Au) film. The period of the prepared superstring grating structure is 6667 nm, the duty ratio is 0.65, and the groove depth is 2250 nm. The diffraction efficiency of the grating is calculated as follows: Figure 5 As shown, the grating is TM polarized. At an incident angle of 40°, the -1 order diffraction efficiency in the 7-9 μm band exceeds 95%, the highest diffraction efficiency exceeds 98%, and the bandwidth with diffraction efficiency above 90% exceeds 3 μm.

[0028] Example 3:

[0029] According to the preparation method provided by the present invention, a superstring holographic grating with a period of 2500nm, a groove depth of 800nm, and a duty ratio of 0.75 is prepared:

[0030] Step 1: Use 50mm×50mm×1.5mm fused quartz as the grating substrate. After wiping it clean with acetone, spin-coat 800nm thick photoresist on the substrate surface, and use a holographic interference exposure system to prepare a photoresist mask layer at a wavelength of 325nm.

[0031] Step 2: Place the sample in the vacuum coating chamber at a background vacuum of 8×10 -4 Under Pa conditions, the argon flow rate was 40 sccm, the power was 300 W in AC mode, the working pressure was 3 Pa, the deposition time was 180 min, and a 500 nm thick Ge film was deposited.

[0032] Step 3: Place the sample in the vacuum coating machine chamber. Under the condition of background vacuum of 8×10-4Pa, introduce argon gas at a flow rate of 40sccm, power of 300W in DC mode, working pressure of 0.5Pa, deposition time of 71s, and deposit a 200nm thick Au film.

[0033] Step 4: Then use SEM to test the cross section of the sample. Figure 4 It clearly shows that after the matching layer is deposited on the photoresist grating mask, the mask structure is effectively controlled and the evolution to a superstring structure is achieved. The profile curve of the grating is tested using AFM. Figure 5 As shown, the measured grating period is 2498.5nm, the groove depth is 796nm, and the duty ratio is 0.76. It can be seen that the profile of the prepared grating sample is basically consistent with the designed superstring groove function curve.

Claims

1. A method for controlling the complex shape of a mid-infrared super string holographic grating coating, characterized in that: The following steps are involved: Step 1: coating a layer of photoresist on the surface of the grating substrate (1), and forming a photoresist grating mask (2) having a periodic structure through double-beam interference exposure and development processing; Step 2: depositing a matching film layer (3) on the surface of the photoresist grating mask (2) using a coating process, and adjusting the deposition rate, deposition time and working gas ratio parameters so that the matching film layer (3) forms a superchordal profile consistent with the target grating structure; Step 3: Using a coating process to continue depositing a functional layer (4) on the surface of the matching layer grating mask to prepare a reflective string-shaped holographic grating.

2. A mid-infrared super string holographic grating coating complex control method according to claim 1, characterized in that: The coating process is selected as a magnetron sputtering coating process or an electron beam evaporation coating process, and the controllable process parameters include deposition rate, deposition time and working gas ratio.

3. The method for controlling the complex shape of a mid-infrared super string holographic grating coating according to claim 1, characterized in that: The thickness of the matching film layer (3) is 500 to 1000 nanometers, and the material is any one or more thin film materials.

4. The method for controlling the complex shape of a mid-infrared super string holographic grating coating according to claim 1, wherein: The functional layer (4) can be various types of highly reflective metal films, dielectric films or mixed material films.

5. A mid-infrared superstring holographic plane grating prepared by the method according to any one of claims 1 to 4, characterized in that: include: grating substrate; The superstring grating structure formed on the surface of the grating substrate has a groove function of: H(x) = h×max{0,1-[cos(πxΛ) / sin[πf / 2]] 2σ }, where h is the groove depth, Λ is the grating period, f is the grating groove duty ratio, and σ is the grating shape factor; wherein the groove depth h is 500 to 2500 nanometers, the grating period Λ is 2000 to 7000 nanometers, the duty ratio f is 0 to 1, and σ is any positive real number; A functional reflective layer covers the surface of the grating structure.

6. The holographic grating according to claim 5, characterized in that The superstring holographic grating is suitable for use in various spectrum analyses, laser pulse compression or spectrum beam combining devices in the mid-infrared band of 2 to 12 microns.

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