High temperature stable spectrally selective infrared film and method of making same

Abstract

The application discloses a high-temperature stable spectrum-selective infrared film and a preparation method thereof, and belongs to the technical field of infrared functional materials. The film adopts a Fabry-Perot cavity multilayer structure, and from bottom to top, the film comprises a bottom infrared reflection layer prepared from a high-infrared reflectivity metal, an intermediate medium resonance layer prepared from an infrared transparent semiconductor material, an upper partial absorption layer which is a metal film, and a top encapsulation protective layer prepared from an infrared transparent dielectric material. The intermediate medium resonance layer is a crystal structure formed through heat pretreatment, and the crystal structure fundamentally inhibits the mutual diffusion of elements between film layers and the structural phase change at high temperatures. The design makes the film have low emissivity in the 3-5 mu m and 8-14 mu m atmospheric window wave bands, high emissivity in the 5-8 mu m non-detection wave band, and can be used at a temperature of 600 DEG C for a long time.

Description

Technical Field

[0001] This invention relates to the fields of advanced functional materials and infrared technology, specifically to a high-temperature stable spectrally selective infrared thin film and its preparation method. Background Technology

[0002] With the rapid development of infrared detection technology, extremely high requirements have been placed on the infrared stealth capabilities of military equipment and key facilities. An ideal infrared stealth material should have low emissivity in the atmospheric window bands of 3-5μm and 8-14μm, which are sensitive to infrared detectors, to evade detection, while maintaining high emissivity in the non-sensitive band of 5-8μm to achieve effective radiative heat dissipation and maintain its own thermal balance.

[0003] In existing technologies, multilayer film structures based on Fabry-Perot resonators are one of the effective ways to achieve spectrally selective radiation. However, these structures generally face two major technical bottlenecks: First, the material system is limited to specific noble metals, such as gold and platinum, and semiconductors, such as germanium, which restricts their optimization space in terms of cost and adaptability to specific environments; second, and more critically, there is insufficient high-temperature stability. When the operating temperature rises, interdiffusion of elements and phase transformation between film layers can destroy the delicate Fabry-Perot resonator structure, causing spectrally selective failure and making it difficult to apply to high-temperature components such as engines and high-speed aircraft.

[0004] Therefore, there is an urgent need in this field for an infrared radiation thin film with a wide range of material choices, capable of long-term stable operation at high temperatures, and exhibiting spectral selectivity in different wavelength bands. Summary of the Invention

[0005] The purpose of this invention is to provide a high-temperature stable spectrally selective infrared thin film and its preparation method, so as to solve the problems of single material system and insufficient high-temperature stability in the existing technology.

[0006] This invention provides the following technical solution: a high-temperature stable spectrally selective infrared thin film, the thin film having a Fabry-Perot cavity structure, comprising, from bottom to top: a bottom infrared reflective layer made of a high infrared reflectivity metal; an intermediate dielectric resonant layer made of an infrared transparent semiconductor material; an upper partial absorption layer, which is a metal thin film; and a top encapsulation protective layer made of an infrared transparent dielectric material; wherein the intermediate dielectric resonant layer is a crystalline structure formed through thermal pretreatment.

[0007] This invention also provides a method for preparing a high-temperature stable spectrally selective infrared thin film, comprising: determining the optimal thickness range of each layer and preparing the high-temperature stable spectrally selective infrared thin film, wherein determining the optimal thickness range of each layer includes:

[0008] S1: Obtain the physicochemical parameters of each layer of material, and calculate the thickness range of each layer based on the infrared spectral characteristics of the thin film;

[0009] S2: Optimize the thickness range of each layer based on the target spectral emissivity;

[0010] S3: Based on the preset optimization algorithm, the thickness of each layer is used as the optimization variable. An evaluation function is constructed based on the deviation between the actual spectral emissivity and the target spectral emissivity of the thin film. The value of the evaluation function is minimized through iterative calculation, and the optimal thickness range of each layer is obtained.

[0011] The innovation and superiority of this invention lie in:

[0012] (1) This invention innovatively proposes a strategy of active thermal pretreatment and in-situ crystallization locking. During the thin film preparation stage, an intermediate dielectric resonant layer is actively deposited under a controlled high-temperature environment, allowing it to directly form a stable crystalline endpoint structure. This advances the unstable phase transition process from the usage stage to the manufacturing stage. During use, the intermediate dielectric resonant layer is already in a thermodynamically stable state and no longer undergoes changes, fundamentally suppressing structural failures caused by crystallization or interdiffusion during subsequent use, and increasing the long-term stable operating temperature of the thin film from the traditional <400℃ to 600℃.

[0013] (2) The present invention uses the thickness of the intermediate dielectric resonant layer, the thickness and sheet resistance of the upper absorption layer, and the thickness of the top encapsulation protective layer to coordinate the design so that the final prepared film exhibits the reflection state of Fabry-Perot cavity in the 3-5μm and 8-14μm bands, and the strong absorption resonant state of Fabry-Perot cavity in the 5-8μm band.

[0014] (3) The design goal of this invention is not simply to match a certain emissivity curve, but to construct a high-quality Fabry-Perot absorption resonant mode in the 5-8μm band by precisely controlling the reflection phase of the bottom metal mirror, the optical length of the crystalline semiconductor cavity, and the impedance of the top ultrathin metal mirror, while simultaneously mismatching the resonance in the two side bands, so that the system is in a reflection-dominated state. This dual-state design of one cavity is an innovative extension of the function of traditional FP cavities.

[0015] (3) Based on this material system, the present invention can quickly optimize the optimal thickness parameters for different material combinations using optical simulation software, such as the simplex method, to achieve the target spectrum of low emission in the atmospheric window and high emission in the non-window. It has a large degree of design freedom and wide adaptability. The optimized film has an emissivity of less than 0.2 in the 3-5μm and 8-14μm bands and an emissivity of more than 0.8 in the 5-8μm band.

[0016] (4) The preparation method of the present invention is highly compatible with the traditional magnetron sputtering process, with a clear process and strong controllability, and is suitable for large-area, uniform production. The total thickness of the entire film is less than 1 μm, with a simple structure of only four layers, which is very suitable for large-area, uniform preparation using mature magnetron sputtering technology. It has good compatibility with existing semiconductor and optical coating processes and has industrialization prospects. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of the high-temperature stable spectrally selective infrared thin film of the present invention.

[0018] Figure 2 The image shows the spectral curves of the optimized Pt / Ge / Pt / HfO2 multilayer thin film structure obtained using simulation software in Example 1.

[0019] Figure 3 The average emissivity of the Pt / Ge / Pt / HfO2 multilayer thin film in Example 1 at 0-600℃ in different wavelength bands. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, preferred embodiments are described below to further illustrate the invention in detail. However, it should be noted that many details listed in the specification are merely to provide the reader with a thorough understanding of one or more aspects of the invention, and these aspects can be achieved even without these specific details.

[0021] Unless otherwise stated, all raw materials, reagents, instruments and equipment used in this application are available on the market or can be prepared by existing methods.

[0022] It should be noted that, in order to efficiently suppress infrared radiation characteristics from both the perspectives of reducing emissivity and reducing temperature, the material needs to maintain low emissivity in the atmospheric window bands (3-5 μm and 8-14 μm) to avoid infrared-guided attacks or infrared detection; and maintain high emissivity in the non-infrared detection band (5-8 μm) to achieve radiative cooling. Based on the inherent infrared properties of the material, it is difficult to achieve precise selective control of the infrared spectrum using a single material. Therefore, it is necessary to optimize the structure based on multiple materials, i.e., design spectrally selective infrared radiation materials. By combining the tunneling effect of ultrathin metals with the multi-beam interference effect of FP cavity interferometers, asymmetric FP structure infrared spectrally selective emission materials with simple structure, easy fabrication, and excellent spectral performance can be designed.

[0023] According to a first aspect of this application, the present invention provides a high-temperature stable spectrally selective infrared thin film, the thin film having a Fabry-Perot cavity structure, comprising, from bottom to top:

[0024] The bottom infrared reflective layer is made of a metal with high infrared reflectivity and has a thickness of 60nm to 150nm.

[0025] The intermediate dielectric resonant layer is made of infrared transparent semiconductor material with an extinction coefficient k of less than 0.1 in the 3-14μm band, and its thickness is 200nm to 500nm.

[0026] The upper absorption layer is composed of an ultrathin metal film with a thickness of 2nm to 10nm.

[0027] The top encapsulation protective layer is made of infrared transparent dielectric material with a thickness of 150nm to 250nm.

[0028] The intermediate dielectric resonant layer is a crystalline structure formed through thermal pretreatment.

[0029] In some embodiments of this application, the high infrared reflectivity metal is preferably at least one of platinum, gold, silver, aluminum, and iridium; the infrared transparent semiconductor material is preferably at least one of germanium, silicon, zinc sulfide, and zinc selenide; the material of the ultrathin metal film is preferably at least one of platinum, nickel, chromium, titanium, and nickel-chromium alloy, and the thickness is 2-10 nm; the infrared transparent dielectric material is preferably at least one of hafnium oxide, silicon dioxide, aluminum oxide, silicon nitride, and yttrium oxide.

[0030] In some embodiments of this application, the thickness of the crystalline semiconductor layer, the thickness and sheet resistance of the ultrathin metal layer, and the thickness of the protective layer are synergistically designed so that the final thin film exhibits a Fabry-Perot cavity reflection state in the 3-5μm and 8-14μm bands, and a strong absorption resonance state of the Fabry-Perot cavity in the 5-8μm band.

[0031] According to the second aspect of this application, for the infrared radiation thin film provided by the present invention, after selecting specific materials, it is necessary to use optical thin film design software to optimize the structure of the constructed multilayer thin film in order to determine the optimal thickness of each functional layer. This mainly involves the following steps:

[0032] S1: Import spectral data and add boundary conditions.

[0033] Optical constants of the metal or nonmetal in the 3-14 μm band, such as refractive index n and extinction coefficient k, obtained using elliptic polarization, are imported into the software. The infrared spectral characteristics of the initial structure are optimized, primarily focusing on the thickness of each layer. To ensure the feasibility of multilayer thin film fabrication and structural stability, further restrictions are needed on the thickness range of each layer. After determining the structural constraints, target spectral emissivity is set: emissivity ≤ 0.2 in the 3-5 μm band, ≥ 0.8 in the 5-8 μm band, and ≤ 0.2 in the 8-14 μm band.

[0034] S2: Determine the algorithm and calculate

[0035] The initial structure selection algorithm was optimized using the simplex method. Based on the physical principles of optical thin film optimization problems, the simplex method continuously optimizes the film thickness according to the objective function, ensuring the thin film structure meets design requirements. An evaluation function is automatically established during the calculation process. The algorithm iteratively calculates to minimize the value of the evaluation function F, thus obtaining the structure that satisfies the boundary conditions and best matches the preset objective's spectral characteristics. The corresponding thin film structure is the optimal solution. The optimized structure exhibits spectral selectivity characteristics, and the total film thickness does not exceed 1 μm.

[0036] According to a third aspect of this application, after obtaining optimized thin film data, the thin film is prepared using the above-described method for preparing infrared radiation thin films provided by this invention, comprising the following steps:

[0037] S1: Deposit the underlying infrared reflective layer on the substrate;

[0038] S2: The substrate with the underlying infrared reflective layer deposited is heated to a preset crystallization temperature T and kept at that temperature. The temperature T is higher than the threshold of harmful metal-induced crystallization that may occur in the subsequent working environment of the thin film. An intermediate dielectric resonant layer is deposited at this temperature T, so that the layer forms a stable crystalline structure in situ during the deposition process.

[0039] S3: After heating is stopped and cooling is completed, the upper absorption layer is deposited on the crystalline dielectric resonant layer;

[0040] S4: Deposit the top encapsulation protective layer on the upper absorption layer;

[0041] The crystallization temperature T is between 300℃ and 450℃, which is 60% to 90% of the maximum operating temperature of the thin film. The thickness of each layer is designed in a synergistic optical manner, so that the thin film forms a Fabry-Perot strong absorption resonance in the 5-8μm band.

[0042] In some embodiments of this application, the crystallization temperature T in step S2 is 400℃±20℃. Steps S1, S2, S3, and S4 are all performed using magnetron sputtering.

[0043] In some embodiments of this application, the temperature T in step S2 needs to be higher than the threshold temperature at which the semiconductor material may undergo metal-induced crystallization in subsequent practical use, but lower than the temperature at which it undergoes severe interdiffusion with the underlying metal. The advantage is that by depositing at this temperature T, the semiconductor atoms gain sufficient kinetic energy upon reaching the substrate and directly arrange themselves into a lower-energy lattice structure, i.e., a crystalline structure. This process preemptively completes the harmful phase transition process that would normally occur during device use, thereby removing the unstable amorphous-to-crystalline phase transition point from the device's operating temperature range and fundamentally eliminating the risk of high-temperature failure.

[0044] It is understandable that, in addition to magnetron sputtering, physical vapor deposition methods such as electron beam evaporation and pulsed laser deposition, or chemical vapor deposition methods, can also be used to prepare the specific functional layer of this invention. However, magnetron sputtering has the best overall performance in terms of film quality, thickness control, and process integration.

[0045] The present invention will be further described in detail below through Examples 1-5, such as... Figure 2 and 3 As shown, Figure 2 The image shows the spectral curves of the optimized Pt / Ge / Pt / HfO2 multilayer thin film structure obtained using simulation software in Example 1. Figure 3 The average emissivity of the Pt / Ge / Pt / HfO2 multilayer thin film in Example 1 at 0-600℃ in different wavelength bands.

[0046] It should be noted that the scope of protection of this invention is not limited thereto. These embodiments demonstrate different material choices, structural designs, and process parameters to verify the universality and excellent effects of this invention.

[0047] The preparation schemes and process parameters of Examples 1-5 are shown in Table 1.

[0048] Table 1

[0049]

[0050] The properties of the films in Examples 1-5 are shown in Table 2.

[0051] Table 2

[0052]

[0053] According to the performance data, the infrared radiation films of Examples 1-5 have an average emissivity of less than 0.2 in the 3-5 μm and 8-14 μm bands, an average emissivity of greater than 0.8 in the 5-8 μm band, and an emissivity variation of less than 0.1 in each band at a temperature of 0-600℃. They can also be used for more than 400 hours at a minimum temperature of 600℃.

[0054] To further demonstrate the technical advantages of this application, comparative examples 1-5 are provided for illustration.

[0055] Comparative Examples 1-5 used the exact same material system and film thickness design as Examples 1-5. The only difference between them and the Examples is that the substrate was kept at room temperature (approximately 25°C) during the deposition of the intermediate dielectric resonant layer, without any preheating treatment. Therefore, the deposited intermediate layers were all amorphous structures.

[0056] The preparation schemes and process parameters of comparative examples 1-5 are shown in Table 3.

[0057] Table 3

[0058]

[0059] The properties of the films in Comparative Examples 1-5 are shown in Table 4.

[0060] Table 4

[0061]

[0062] It can be observed that after heating the infrared radiation films of Comparative Examples 1-5 at 600℃ for less than 20 hours, the characteristic emission peak in the 5-8 μm band showed significant attenuation, while the emissivity in the 3-5 μm and 8-14 μm bands increased significantly. Upon further heating, the spectral selectivity was completely lost, exhibiting low emissivity across the entire spectral band. After cooling to room temperature, the performance was irrecoverable, with average emissivity greater than 0.5 in the 3-5 μm and 8-14 μm bands and less than 0.7 in the 5-8 μm band.

[0063] The above comparison fully demonstrates that the thermal pretreatment crystallization process adopted in this invention is not a conventional selection of process parameters, but rather a key and innovative step in solving the high-temperature failure problem of this type of thin film. It raises the upper limit of the stable operating temperature of the thin film from less than 400°C in traditional methods to 600°C, and increases the working life at high temperatures by an order of magnitude, achieving unexpected technical results.

[0064] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements, modifications and substitutions can be made without departing from the principle of the present invention, and these improvements, modifications and substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A high-temperature stable spectrally selective infrared thin film, characterized in that, The thin film has a Fabry-Perot cavity structure, which includes, from bottom to top: The bottom infrared reflective layer is made of a metal with high infrared reflectivity. The intermediate dielectric resonant layer is fabricated using infrared transparent semiconductor material; The upper absorption layer is a thin metal film; The top encapsulation protective layer is made of infrared transparent dielectric material; The intermediate dielectric resonant layer is a crystalline structure formed through thermal pretreatment.

2. The high-temperature stable spectrally selective infrared thin film according to claim 1, characterized in that, The thickness of the bottom infrared reflective layer is 60-150nm; the high infrared reflectivity metal includes at least one of platinum, gold, silver, aluminum, and iridium; The thickness of the intermediate dielectric resonant layer is 200-500 nm; the infrared transparent semiconductor material includes at least one of germanium, silicon, zinc sulfide, and zinc selenide; The thickness of the upper absorption layer is 2-10 nm; the material of the metal thin film includes at least one of platinum, nickel, chromium, titanium, and nickel-chromium alloy; The thickness of the top encapsulation protective layer is 150-250nm; the infrared transparent dielectric material includes at least one of hafnium oxide, silicon dioxide, aluminum oxide, silicon nitride, and yttrium oxide.

3. The high-temperature stable spectrally selective infrared thin film according to claim 1 or 2, characterized in that, The thickness of the high-temperature stable spectrally selective infrared thin film is <1 μm.

4. A method for preparing a high-temperature stable spectrally selective infrared thin film, used to prepare the high-temperature stable spectrally selective infrared thin film according to any one of claims 1-3, characterized in that, include: Determining the optimal thickness range for each layer and preparing high-temperature stable spectrally selective infrared thin films, wherein determining the optimal thickness range for each layer includes: S1: Obtain the physicochemical parameters of each layer of material, and calculate the thickness range of each layer based on the infrared spectral characteristics of the thin film; S2: Optimize the thickness range of each layer based on the target spectral emissivity; S3: Based on the preset optimization algorithm, the thickness of each layer is used as the optimization variable. An evaluation function is constructed based on the deviation between the actual spectral emissivity and the target spectral emissivity of the thin film. The value of the evaluation function is minimized through iterative calculation, and the optimal thickness range of each layer is obtained.

5. The method for preparing a high-temperature stable spectrally selective infrared thin film according to claim 4, characterized in that, The target spectral emissivity is as follows: the thin film exhibits a Fabry-Perot cavity reflection state in the 3-5 μm and 8-14 μm bands, while exhibiting a strong absorption resonance state of the Fabry-Perot cavity in the 5-8 μm band.

6. The method for preparing a high-temperature stable spectrally selective infrared thin film according to claim 5, characterized in that, The emissivity of the thin film is ≤0.2 in the 3-5μm band, ≥0.8 in the 5-8μm band, and ≤0.2 in the 8-14μm band. At a temperature of 600℃, the emissivity variation in each band is less than 0.

1.

7. The method for preparing a high-temperature stable spectrally selective infrared thin film according to claim 4, characterized in that, The preparation of high-temperature stable spectrally selective infrared thin films includes: S4: Deposit the underlying infrared reflective layer on the substrate; S5: Heat the substrate with the underlying infrared reflective layer deposited to a preset crystallization temperature and keep it at that temperature. The preset crystallization temperature is higher than the threshold for harmful metal-induced crystallization in the environment in which the prepared film is used. S6: Deposit an intermediate dielectric resonant layer at a preset crystallization temperature, and the intermediate dielectric resonant layer forms a crystalline structure; S7: After heating is stopped and cooling is completed, the upper absorption layer is deposited on the intermediate dielectric resonant layer; S8: Deposit a top encapsulation protective layer on the upper partial absorption layer.

8. The method for preparing a high-temperature stable spectrally selective infrared thin film according to claim 7, characterized in that, The preset crystallization temperature is 300℃ to 450℃, which is 60% to 90% of the target maximum operating temperature of the prepared thin film; the thickness of each layer of the thin film is sufficient to form a Fabry-Perot strong absorption resonance in the 5-8μm wavelength band.

9. The method for preparing a high-temperature stable spectrally selective infrared thin film according to claim 7, characterized in that, The preset crystallization temperature is 400℃±20℃.

10. The method for preparing a high-temperature stable spectrally selective infrared thin film according to claim 7, characterized in that, Each layer was deposited layer by layer using magnetron sputtering.

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