A low-absorption long-wave infrared polarizer and a method of manufacturing the same

Abstract

The application provides a low-absorption long-wave infrared polarizer and a manufacturing method thereof, the polarizer comprising a zinc sulfide and ytterbium fluoride film, a high-resistance silicon substrate, a silicon grating, and a double-layer gold film, the double-layer gold film being respectively covered on the bottom of the dielectric grating groove and the top of the grating line, since the refractive index of the zinc sulfide is lower than that of silicon and the zinc sulfide has no absorption, the filling layer can improve the transmittance of TM polarized light, meanwhile, since the zinc sulfide film belongs to a hard film, the gold film can be greatly protected, and the robustness of the polarizer is improved, ytterbium fluoride and zinc sulfide films are plated on the back side of the packaged grating, according to the optical absorption characteristics of the plating film material, the antireflection film design is optimized, and the transmittance can be further improved, the application solves the inherent absorption problem of 9um of the polarizer with a silicon substrate at home and abroad, further improves the absorption of the polarizer after 10um, and improves the stability of the polarizer while ensuring high extinction ratio and high transmittance.

Description

Technical Field

[0001] This invention belongs to the field of micro-nano fabrication technology of gratings, specifically relating to a low-absorption long-wave infrared polarizer and its manufacturing method. Background Technology

[0002] Traditional long-wave infrared polarizers suffer from problems such as low transmittance of TM polarized light, low extinction ratio, strong absorption in the long-wavelength range (greater than 9µm), and a sharp decline in performance. The 7-14µm range, as one of the three important atmospheric windows, is crucial for reducing polarizer absorption, increasing transmittance, and improving polarizer stability, making it significant in fields such as medicine and military applications.

[0003] One of the key issues to be addressed in the development of long-wave infrared polarizers is how to reduce absorption and improve the stability of the polarizer. Reducing absorption involves both decreasing the roughness of the grid sidewalls and the roughness of the metal film. The metal film needs to be as dense as possible. Furthermore, the anti-reflection design needs to be optimized, as conventional infrared anti-reflection films exhibit significant absorption in the long-wave infrared region. Currently, foreign long-wave infrared polarizers mainly utilize metal nanowire structures fabricated on silicon substrates. Commonly used optical materials in the long-wave infrared region include ZnSe, Ge, ZnS, and Si. ZnSe is soft, cannot withstand rain erosion, is toxic, and lacks sufficient processing strength. Ge has a small bandgap and its refractive index varies greatly with temperature, making it unsuitable for high-temperature environments. In contrast, Si is low-cost, has good processing performance, and mature processing technology. However, due to silicon's high refractive index, reflection needs to be reduced. Methods to reduce reflectivity include depositing anti-reflection films or creating anti-reflection structures. Materials suitable for long-wave infrared thin films mainly include zinc sulfide (ZnS) and ytterbium fluoride (YbF3). The main processes for fabricating long-wave infrared polarizers include: holographic ion beam etching, electron beam lithography, nanoimprinting, ICP etching, and electron beam evaporation.

[0004] Companies like THORLABS and MOXTEX manufacture long-wave infrared polarizers, but their polarizers operating in the long-wave infrared (7-14µm) region suffer from strong absorption around 9µm, leading to performance degradation. This strong absorption near 9µm is caused by resonant absorption of silicon-oxygen bonds. Most low-refractive-index films operating in the long-wave infrared region exhibit strong absorption; for example, ytterbium fluoride and yttrium fluoride show significant absorption after 10µm. This severely limits the performance of long-wave infrared polarizers. Therefore, it is necessary to limit the thickness of ytterbium fluoride films as much as possible, optimize the film system, and even use antireflection structures for optimization. For the design of long-wave infrared antireflection films, considering the characteristics of the wide wavelength range and the stress matching problem of the film, the thickness of each film layer is on the order of micrometers, so too many layers cannot be designed. At the same time, the outermost layer should be a hard film if possible. Commonly used ytterbium fluoride films are porous, absorbent, and soft, making them unsuitable for extreme environments. Enhancing the stability of the antireflection film and reducing its absorption can greatly improve the stability and transmittance of long-wave infrared polarizers. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a low-absorption long-wave infrared polarizer and its manufacturing method, further improving the absorption issues of existing long-wave infrared polarizers. The design includes a high-resistivity silicon substrate, a silicon grating, a double-layer gold film, a zinc sulfide film, and a ytterbium fluoride film. Starting from the incident light direction, the polarizer sequentially passes through the zinc sulfide film, ytterbium fluoride film, zinc sulfide film, ytterbium fluoride film, substrate material, and another zinc sulfide film. A subwavelength grating is fabricated within the substrate material. A gold film is deposited at the bottom and top of the subwavelength grating grooves, respectively. The subwavelength grating grooves are filled with zinc sulfide material to form a filling layer. Zinc sulfide and ytterbium fluoride films are deposited on the back of the filled grating substrate as antireflection coatings. The high-resistivity silicon material exhibits good optical properties in the long-wave infrared band. The grating period is 300 nm; the grating groove depth is 300-600 nm; and the thickness of each layer of the double-layer gold film is 110 nm.

[0006] The present invention also provides a method for manufacturing a low-absorption long-wave infrared polarizer, comprising the following steps:

[0007] (1) Etching rectangular trenches into high-resistivity silicon;

[0008] (2) Electron beam evaporation deposits a gold film on the bottom and top of the grating groove, respectively;

[0009] (3) Fill the grating groove with zinc sulfide material;

[0010] (4) Deposit an antireflection coating on the planar side (back side of the grating) of the filled substrate.

[0011] The deposition of antireflective coatings on the planar side needs to be optimized based on actual deposition conditions to minimize absorption. The focus of this invention is on optimizing the grating structure and coating encapsulation to improve the stability and transmittance of the polarizer.

[0012] Beneficial effects:

[0013] This invention selects high-resistivity silicon as the substrate to reduce optical absorption, especially the strong absorption near 9µm. A subwavelength grating structure is etched onto this substrate, followed by the deposition of a gold film and then the encapsulation of the grating with zinc sulfide. Through optimization algorithms, the optimal grating parameters and the thickness of the zinc sulfide filling can be determined. This protects the gold film and improves the stability of the polarizer; the hardness of zinc sulfide also enhances the mechanical strength of the polarizer. Furthermore, the use of zinc sulfide filling, with controlled thickness, acts as an anti-reflection film for long-wave infrared radiation, reducing reflectivity and significantly improving transmittance. Compared to traditional long-wave infrared metal wire gratings, this design optimizes parameters, and the zinc sulfide filling improves the robustness of the polarizer and mitigates its absorption problems in long-wave infrared polarizers. In addition, a four-layer long-wave infrared anti-reflection film is designed, resulting in a smaller overall film thickness compared to a two-layer film. This reduces the thickness of the ytterbium fluoride film, further weakening the strong absorption caused by the ytterbium fluoride film. The outermost layer is a hard zinc sulfide film, further enhancing the stability of the polarizer. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the structure of a long-wave infrared polarizer in an embodiment of the present invention, where h1 is the depth of the grating groove, h2 represents the thickness of the gold film at the bottom and top of the grating groove, and h3 is the thickness of the zinc sulfide film.

[0015] Figure 2 This is a graph showing the change in transmittance of TM polarized light of the long-wave infrared polarizer in Embodiment 1 of the present invention as the incident light wavelength varies with the grating groove depth.

[0016] Figure 3 This is a graph showing the change in the extinction ratio of the long-wave infrared polarizer in Embodiment 1 of the present invention as the incident light wavelength varies with the grating groove depth.

[0017] Figure 4 This is a graph showing the variation of the transmittance of TM polarized light of the long-wave infrared polarizer in Embodiment 2 of the present invention with the incident light wavelength at different gold film thicknesses.

[0018] Figure 5 This is a graph showing the extinction ratio of the long-wave infrared polarizer in Embodiment 2 of the present invention as a function of incident light wavelength for different gold film thicknesses.

[0019] Figure 6 This is a graph showing the transmittance of TM polarized light from a long-wave infrared polarizer in Embodiment 3 of the present invention as a function of incident light wavelength for different zinc sulfide thicknesses.

[0020] Figure 7 This is a graph showing the extinction ratio of the long-wave infrared polarizer in Embodiment 3 of the present invention as a function of incident light wavelength for different zinc sulfide thicknesses.

[0021] Figure 8This is a graph showing the transmittance and extinction ratio of TM polarized light as a function of incident light wavelength, obtained after optimizing the parameters of the long-wave infrared polarizer in Embodiment 4 of the present invention.

[0022] Figure 9 This is a graph showing the transmittance and extinction ratio of TM polarized light with and without gaps in the long-wave infrared polarizer of the present invention, as a function of incident light wavelength.

[0023] Figure 10 This is a schematic diagram of the structure of a long-wave infrared polarizer after an antireflection film is deposited on the planar side in Embodiment Six of the present invention, where h4 and h6 are the thicknesses of two ytterbium fluoride films, and h5 and h7 are the thicknesses of two zinc sulfide films.

[0024] Figure 11 This is a graph showing the changes in transmittance and extinction ratio of TM polarized light as a function of incident light wavelength after an antireflection coating is deposited on the planar side of a long-wave infrared polarizer in Embodiment Six of the present invention. Detailed Implementation

[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 embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0026] like Figure 1 As shown, the long-wavelength infrared polarizer of the present invention includes a high-resistivity silicon substrate 1 and a gold film 2 with a thickness of h2. A subwavelength rectangular groove grating is formed on the high-resistivity silicon substrate 1 by holographic ion beam etching and ICP etching. A gold film 2 is deposited at the bottom and top of the subwavelength grating groove by electron beam evaporation, respectively. Then, a zinc sulfide film 4 is deposited to fill the grating groove, and the grating groove depth is h1. Due to the characteristics of electron beam evaporation deposition, gaps 3 appear in the grating groove after filling with zinc sulfide, and arc-shaped undulations appear at the top of the grating.

[0027] like Figure 1 As shown, the zinc sulfide-plated grating groove is filled. According to the equivalent medium theory, the filled zinc sulfide, gold film, and grating structure can be equivalent to a multilayer thin film. It has different optical effects for TM polarized light and TE polarized light, which is equivalent to an artificial birefringent material. Optimizing the parameters can reduce the interface reflection of the high-resistivity silicon substrate 1, while protecting the gold film and grating groove. Since zinc sulfide has high hardness, it can improve the mechanical strength of the polarizer.

[0028] The long-wave infrared polarizer of the present invention consists of, starting with incident light 5, a high-resistivity silicon substrate 1, a grating structure, and a zinc sulfide thin film 4.

[0029] The grating period is 300nm; the grating groove depth is 300-600nm; the thickness of the gold film 2 is 80-120nm; the thickness of the zinc sulfide film is greater than the grating groove depth; and the grating aspect ratio is 0.5.

[0030] The manufacturing method of the long-wave infrared polarizer includes three steps: holographic ion beam etching, ICP etching, and electron beam evaporation deposition. A silicon oxide grating mask is fabricated on a high-resistivity silicon substrate 1 by holographic ion beam etching, and a high aspect ratio grating is etched by ICP etching. Then, a gold film 2 is deposited by electron beam evaporation, and finally, zinc sulfide is deposited by electron beam evaporation to fill the grating grooves.

[0031] The polarizer of this invention achieves excellent transmittance in the 7-14µm range, without considering backside reflection from the substrate. With a grating period of 300nm, a grating groove depth of 355nm, a zinc sulfide film thickness of 858nm, a gold film thickness of 110nm, and an aspect ratio of 0.5, the average transmittance of the TM polarizer in the 7-14µm range is greater than 89%, reaching a maximum of 98%, and the extinction ratio is higher than 53dB. Combined with the characteristics of holographic lithography, large-area wire gratings can be fabricated. Compared to wire grating polarizers on silicon substrates both domestically and internationally, it exhibits lower absorption at 9µm and achieves better stability.

[0032] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0033] Example 1:

[0034] See Figure 1 This is a schematic diagram of a long-wave infrared grating polarizer (the back side of the grating is not coated with an anti-reflection film).

[0035] The substrate material is a high-resistivity silicon substrate 1, the grating period p = 300nm, the aspect ratio is 0.5, the incident light 5 is TM or TE linearly polarized light, incident normally from the silicon substrate, with a wavelength of 7-14um, the gold film 2 has a thickness h2 = 110nm, the zinc sulfide has a thickness h3 = 858nm, the grating groove depth h1 varies, and the gap 3 is made of air.

[0036] Extinction ratio is defined as:

[0037]

[0038] T TM It is the transmittance of TM polarized light, T TE The transmittance is denoted by TE, where TM polarization represents the electric field vector of linearly polarized light perpendicular to the grating lines, and TE polarization represents the electric field vector of linearly polarized light parallel to the grating lines. The extinction ratio ER is expressed in dB. As the groove depth increases, the peak transmittance redshifts. At 400 nm, the average transmittance across the entire wavelength is 89.7%, and the extinction ratio is greater than 53 dB. Figure 2This is a graph showing the change in transmittance of TM polarized light of the long-wave infrared polarizer in Embodiment 1 of the present invention as the incident light wavelength varies with the grating groove depth. Figure 3 This is a graph showing the change in the extinction ratio of the long-wave infrared polarizer in Embodiment 1 of the present invention as the incident light wavelength varies with the grating groove depth.

[0039] Example 2:

[0040] See Figure 1 The grating period p = 300 nm, the aspect ratio is 0.5, the incident light 5 is TM or TE linearly polarized light, incident orthogonally from the silicon substrate, with a wavelength of 7-14 μm, the gold film 2 thickness h2 varies, the zinc sulfide thickness h3 = 858 nm, the grating groove depth h1 = 355 nm, and the gap 3 material is air. The relationship between the TM polarized light transmittance and extinction ratio and wavelength under different gold film thicknesses is as follows: Figure 4 , Figure 5 As shown, a gold film thickness of 110-120 nm can achieve high transmittance and high extinction ratio.

[0041] Example 3:

[0042] See Figure 1 The grating period p = 300 nm, the aspect ratio is 0.5, the incident light 5 is TM or TE linearly polarized light, incident orthogonally from the silicon substrate, with a wavelength of 7-14 μm, the gold film 2 thickness h2 = 110 nm, the zinc sulfide thickness h3 varies, the grating groove depth h1 = 355 nm, and the gap 3 material is air. The relationship between the TM polarized light transmittance and extinction ratio and wavelength for zinc sulfide films of different thicknesses is as follows: Figure 6 , Figure 7 As shown, with the increase of zinc sulfide film thickness, the transmittance peak redshifts, and high transmittance and high extinction ratio can be achieved in the 800nm-1000nm range.

[0043] Example 4:

[0044] See Figure 1 Based on the Monte Carlo optimization algorithm, three variables were set for the model: grating aspect ratio, grating groove depth, and zinc sulfide filling thickness. Global optimization was performed with the goal of maximizing transmittance, resulting in the final parameters. The grating period p = 300 nm, the aspect ratio is 0.5, and the incident light 5 is TM or TE linearly polarized light, incident normally from the silicon substrate, with a wavelength of 7-14 μm. The gold film 2 has a thickness h2 = 110 nm, the zinc sulfide thickness h3 = 858 nm, the grating groove depth h1 = 355 nm, and the gap 3 is made of air. The relationship between TM polarized light transmittance and extinction ratio as a function of wavelength is shown below. Figure 8 As shown, the average transmittance in the long-wave infrared is 89.2%, and the extinction ratio is greater than 53dB. In actual processing, deep etching of silicon should be avoided as much as possible. Once the etching depth is reached, factors such as groove deformation and a reduced aspect ratio will degrade the polarizer's performance.

[0045] Example 5:

[0046] See Figure 1 The grating period p = 300 nm, the aspect ratio is 0.5, the incident light 5 is TM or TE linearly polarized light, incident orthogonally from the silicon substrate, with a wavelength of 7-14 μm, the gold film 2 thickness h2 = 110 nm, the zinc sulfide thickness h3 = 858 nm, and the grating groove depth h1 = 355 nm. The relationships between the transmittance and extinction ratio of TM polarized light with wavelength are as follows: when gap 3 exists (i.e., the material is air) and when gap 3 does not exist (i.e., the material is zinc sulfide). Figure 9 As shown in the figure. The results indicate that the impact of zinc sulfide filling gaps on polarizer performance is within acceptable limits, significantly improving process tolerance.

[0047] Example 6:

[0048] See Figure 1 The grating period p = 300 nm, the aspect ratio is 0.5, the gold film 2 thickness h2 = 110 nm, the zinc sulfide thickness h3 = 858 nm, and the grating groove depth h1 = 355 nm. The gap 3 is made of air. The first antireflective coating 6 is made of ytterbium fluoride, with thicknesses h4 = 100 nm and h6 = 600 nm. The second antireflective coating 7 is made of zinc sulfide, with thicknesses h5 = 600 nm and h7 = 145 nm. Figure 11 This is a graph showing the transmittance and extinction ratio of TM-polarized light after an antireflection coating is deposited on the planar side of the long-wave infrared polarizer in Embodiment Six of this invention, as a function of incident light wavelength. The results show that the absorption peak near 9 μm is suppressed, solving the problem of strong absorption near 9 μm in polarizers made with silicon substrates currently used both domestically and internationally.

[0049] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-absorption long-wave infrared polarizer, characterized in that, The device includes a substrate on which a subwavelength rectangular groove grating is etched. A gold film is deposited at the bottom and top of the subwavelength rectangular groove grating, and then a zinc sulfide film is deposited to fill the rectangular grating groove. After filling with zinc sulfide, gaps appear in the grating groove and arc-shaped undulations appear at the top of the grating. An antireflection film is deposited on the plane of the side of the substrate opposite to the grating side. The thickness of the zinc sulfide film is greater than the depth of the grating groove. The grating period is 300 nm. The grating groove depth is 300-600 nm. The gold film thickness is 80-120 nm. The substrate material is infrared high-resistivity silicon. The grating aspect ratio is 0.

5.

2. The polarizer according to claim 1, characterized in that: Antireflective coatings include ytterbium fluoride coatings and zinc sulfide coatings, with the zinc sulfide coating located on the outermost side of the antireflective coating, away from the substrate.

3. A method for manufacturing a low-absorption long-wave infrared polarizer according to any one of claims 1-2, characterized in that, The manufacturing method includes the following steps: (1) The substrate material is high-resistivity silicon; (2) Subwavelength rectangular groove gratings were formed on a high-resistivity silicon substrate by holographic ion beam etching and ICP etching; (3) Electron beam evaporation deposits a gold film at the bottom and top of the grating groove, respectively; (4) Fill the grating groove with zinc sulfide material; gaps appear in the grating groove after filling with zinc sulfide; (5) An antireflection film is deposited on the plane of the filled substrate on the side opposite to the grating side, wherein the outermost part of the antireflection film is a zinc sulfide film.

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