A cross-band multistage adjustable radar wave-absorbing structure, a design method and a preparation method

Abstract

A cross-band multistage adjustable radar wave-absorbing structure, a design method and a preparation method belong to the technical field of electromagnetic wave regulation and control. The cross-band multistage adjustable radar wave-absorbing structure is prepared by using a photoetching process and a magnetron sputtering process, can realize absorption of a specific wave band under S / C / X wave bands, and sequentially comprises a patterned microwave absorbing layer, a chalcogenide phase change material layer, a heat bearing layer, a metal electrode layer and a flexible dielectric layer. The metal-insulating state conversion of the chalcogenide phase change material is realized by controlling the conductivity of the chalcogenide phase change material, the Fermi level of the chalcogenide phase change material is adjusted by controlling the chalcogenide phase change material, multi-band absorption adjustment is realized, perfect absorption is realized at multiple specific frequency points, and a certain angle insensitivity characteristic is possessed. Meanwhile, the structure of the chalcogenide phase change material layer is simple, mass production and processing requirements are met, the application of the phase change material in the GHz field is expanded, and the design is flexible, electrically adjustable, and widely applied.

Description

Technical Field

[0001] This invention belongs to the field of electromagnetic wave control technology, and relates to a cross-band multi-level adjustable radar absorbing structure, its design method, and its fabrication method. Specifically, it relates to a cross-band multi-level adjustable flexible radar absorbing structure based on chalcogenide phase change materials, its design method, and its fabrication method. Background Technology

[0002] Radar-absorbing structures are a key technology for achieving weapon stealth, effectively reducing the probability of targets being detected and tracked by radar waves, preventing interception, or significantly shortening the detection range. The quality of radar-absorbing stealth technology directly determines the survivability and penetration capability of weapons on the battlefield, influencing the course of war. Metamaterials are composite materials with artificial structures. Due to their unique properties such as negative refractive index, zero refractive index, and superabsorption, they have significant application value in the field of high-efficiency radar absorption technology. Metamaterial radar-absorbing structures can achieve radar wave absorption within a certain frequency range by designing different types of structures. However, once the structure is fabricated, it cannot be changed and can only operate within a fixed frequency range.

[0003] To address the broadband radar stealth requirements of weaponry throughout the entire combat process, Chinese invention patent CN202310732302.7 discloses a broadband sandwich structure based on a dual-band metasurface, achieving dual-band broadband wave transmission and possessing large-angle stability characteristics. Chinese invention patent CN202311458848.4 discloses a metasurface absorber, its device, and its applications, exhibiting ultra-wideband absorption effects. However, both of these absorbers, due to their fixed topological shape, cannot be altered and are limited to operating within a fixed frequency range, severely restricting the practical application of the absorbing structure. Chinese invention patent CN201910887551.7 discloses a graphene-based microwave-band dynamically adjustable absorber and its fabrication method, which dynamically controls the sheet resistance of graphene using a DC voltage source to adjust the absorption wave frequency. Chinese invention patent CN202311215016.X discloses an amplitude-adjustable absorbing structure controlled by a varactor diode. By adjusting the capacitance of the varactor diode in the parallel resonant microstrip stub, the resistance of the load resistor in the main resonant structure can be adjusted, thereby effectively regulating the absorption amplitude. Both of these passive adjustable devices suffer from drawbacks such as complex soldering and numerous feed lines, which are detrimental to reconfigurable characteristics and common surface applications.

[0004] When applied to engineering projects, the aforementioned technologies offer limited stealth capabilities, insufficient frequency band adjustment, and are primarily composed of rigid materials, making integration with equipment surfaces challenging. Therefore, there is an urgent need to develop a cross-band tunable flexible radar absorbing structure. Dithium-based phase change materials (DCTs), due to their non-volatility (the maintenance of their crystalline phase state does not require external energy) and the significant differences in refractive index and conductivity between different crystalline phase states, have become crucial for dynamically controlling the electromagnetic properties of metamaterials. Therefore, this paper proposes combining DCTs with flexible absorbing junctions to form a cross-band tunable flexible radar absorbing structure based on DCTs. This structure enables low-power active control of the absorption frequency, providing a new approach to cross-band absorption performance control and effectively improving the full-band stealth capability of weapons and equipment. Summary of the Invention

[0005] The main technical problem addressed by this invention is to overcome the shortcomings of existing methods. Addressing the issues of traditional radar absorbing structures' inability to achieve flexible control, narrow and fixed bandwidth, and unsuitability for complex curved surfaces, this invention proposes a multi-band, multi-level adjustable radar absorbing structure, its design method, and its fabrication method. This invention utilizes the characteristic that chalcogenide phase change materials have different dielectric constants under different lattice structures, breaking through the limitations of traditional absorbing structures' fixed and uncontrollable bands. It achieves near 100% peak absorption rates across different frequency bands. Furthermore, the combination of chalcogenide phase change materials with a flexible substrate avoids the welding problems of passive control devices, reduces the use of feed lines, improves assembly flexibility, and enhances the applicability and reliability of the flexible structure radar absorbing system.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A multi-band, multi-level tunable radar absorbing structure is fabricated using photolithography and magnetron sputtering processes. It can achieve absorption in specific bands under S / C / X bands, meeting the performance requirements of flexibility and tunable absorption bands. It solves the problem of passive device control being incompatible with flexible substrates. The system design and fabrication process are stable and have good application prospects.

[0008] Furthermore, the specific structure of the cross-band multi-level adjustable flexible radar absorbing structure, from top to bottom, consists of a patterned microwave absorbing layer, a chalcogenide phase change material layer, a heat-bearing layer, a metal electrode layer, and a flexible dielectric layer. Specifically:

[0009] The patterned microwave absorbing layer has a thickness ranging from 18 to 36 micrometers and a side length ranging from 10 to 15 millimeters, and the material is selected from copper.

[0010] The thickness of the sulfur-based phase change material layer ranges from 20 to 50 nanometers; the material is selected from GeTe, SeTe, or GeSbSeTe.

[0011] The thickness of the heat-bearing layer ranges from 40 to 70 nanometers; the material is selected from tungsten, gold, or copper.

[0012] The thickness of the metal electrode layer ranges from 70 to 100 nanometers; the material is selected from gold or copper.

[0013] The thickness of the flexible dielectric layer ranges from 30 to 800 micrometers; the material is selected from polyimide or flexible glass.

[0014] A design method for a cross-band multi-level tunable radar absorbing structure, which is a design method for a cross-band multi-level tunable flexible radar absorbing structure based on chalcogenide phase change materials, includes the following steps:

[0015] Step 1: Establish a mathematical model for the dielectric constant of chalcogenide phase change materials; specifically:

[0016] Based on the fundamentals of phase transition kinetics, the amorphous-crystalline phase transition in chalcogenide phase change materials is a temperature-activated process. Considering thermal effects, the amount of heat received by a chalcogenide phase change material is proportional to the square of its electrical resistance and voltage. The mathematical model is as follows:

[0017] (1)

[0018] Where v is the crystallization rate, v0 is the pre-exponential factor, and E a It is the activation energy of crystallization, k is the Boltzmann constant, T is the absolute temperature, and R is the activation energy of crystallization. m V is the resistance of the sulfide phase change material, V is the equivalent voltage, and T0 is the initial temperature of the sulfide phase change material.

[0019] Assuming a critical crystallization time, integrating the crystallization rate over time yields the control parameter t that regulates the crystallization transition of chalcogenide phase change materials:

[0020] (2)

[0021] Where a is a constant representing time; b is a correction factor representing the equivalent voltage.

[0022] At room temperature (25°C), for the sulfide-based phase change material used in this invention, by consulting the lattice parameters of the sulfide-based phase change material, the constant in formula (2) is obtained as follows: , .

[0023] Step 2: Based on the mathematical model of the dielectric constant of chalcogenide phase change materials, construct a flexible radar absorbing structure based on chalcogenide phase change materials, and optimize the structural parameters according to the absorption range and absorption efficiency within the working bandwidth; specifically:

[0024] Step 2.1: Based on the control parameters obtained in Step 1, determine the flexible radar absorbing structure. The flexible radar absorbing structure, from top to bottom, consists of a patterned microwave absorbing layer, a chalcogenide phase change material layer, a heat-bearing layer, a metal electrode layer, and a flexible dielectric layer. The minimum operating frequency of the flexible radar absorbing structure is then determined. Determine the theoretical minimum thickness h of the flexible radar absorbing structure, h= ,in It is the wavelength corresponding to the minimum operating frequency, and thus the thickness of each layer in the flexible radar absorbing structure, including the patterned microwave absorbing layer, the chalcogenide phase change material layer, the heat-bearing layer, the metal electrode layer, and the flexible dielectric layer, is optimized.

[0025] Step 2.2: Establish a flexible radar absorbing structure model. From top to bottom, the flexible radar absorbing structure model consists of a patterned microwave absorbing layer, a chalcogenide phase change material layer, a heat-bearing layer, a metal electrode layer, and a flexible dielectric layer. An equivalent circuit model is constructed through electromagnetic simulation. Based on this model, the analytical relationship between the reflection coefficient of the flexible radar absorbing structure and the geometric dimensions of the patterned microwave absorbing layer is derived. The specific steps are as follows:

[0026] Step 2.2.1: Calculate the value of the equivalent total resistance R of the equivalent circuit model;

[0027] (3)

[0028] Where R is the equivalent total resistance; S is the area of ​​the patterned microwave absorbing layer; A is the area through which current flows in the patterned microwave absorbing layer; R 21 It is the sheet resistance of the absorbing structure that constitutes the resonant part.

[0029] Step 2.2.2: Calculate the equivalent lumped inductance L and equivalent lumped capacitance C of the equivalent circuit model;

[0030] (4)

[0031] Where L0 and C0 are the equivalent inductance and capacitance of the absorbing layer in the air; ε r It is the equivalent dielectric constant of the entire flexible radar absorbing structure.

[0032] Then, based on the equivalent lumped inductance L and equivalent lumped capacitance C calculated in step 2.2.2, the equivalent impedance Z of the patterned microwave absorbing layer is calculated. r Establish a series model of R, C, and L, as shown in formula (5).

[0033] (5)

[0034] Among them, Z rR is the equivalent impedance of the patterned microwave absorbing layer; ω is the total equivalent resistance; L is the angular frequency; and C is the capacitance. It is represented as an imaginary unit.

[0035] Step 2.2.3: Calculate the input impedance Z of the entire flexible radar absorbing structure. in and reflection coefficient ;

[0036] The input impedance Z in Equivalent to the impedance Z of the absorbing layer r Equivalent impedance Z of base plate d The parallel connection is shown in formula (6):

[0037] (6)

[0038] Among them: Z d It can be obtained through formula (7);

[0039] (7)

[0040] Where Z0 is the transmission characteristic impedance; It is the free space wavenumber; It is the physical thickness of the dielectric layer; It is the relative permittivity of the base plate; It is the relative permittivity of air;

[0041] The reflection coefficient of the entire absorbing structure for:

[0042] (8)

[0043] in, Z0 is the reflection coefficient, and Z0 is the transmission characteristic impedance.

[0044] Step 2.3: Based on the analytical calculation formulas of the flexible radar absorbing structure shown in formulas (3) and (8), with the optimization objective of maximizing the minimum absorption rate within a certain frequency band, and with the thickness, length and width geometric dimensions of the patterned microwave absorbing layer, as well as the thickness of the chalcogenide phase change material layer, heat-bearing layer, metal electrode layer, and flexible dielectric layer in the flexible radar absorbing structure model as independent variables, the finite-difference time-domain method is used for simulation verification to optimize the entire flexible radar absorbing structure model, and finally obtain the geometric parameters of each layer of the patterned microwave absorbing layer, chalcogenide phase change material layer, heat-bearing layer, metal electrode layer, and flexible dielectric layer in the flexible radar absorbing structure.

[0045] A method for fabricating a multi-band, multi-level tunable radar absorbing structure, employing magnetron sputtering for thin film preparation, includes the following steps:

[0046] The first step is to pre-treat the flexible dielectric layer as a substrate; specifically:

[0047] Step 1.1: Using the flexible dielectric layer as a substrate, bake the flexible dielectric layer at a high temperature of 400℃-500℃ in a low-oxygen environment to eliminate most of the thermal stress inside the high-temperature durable substrate. This allows the atoms inside the substrate to obtain sufficient energy to rearrange themselves and also effectively reduces the occurrence of oxidation reactions, keeping the surface of the substrate clean and active.

[0048] Step 1.2: Immerse the flexible dielectric layer in isopropanol solution, place it in an ultrasonic cleaner to perform ultrasonic cleaning on the substrate surface, and wipe the substrate surface with deionized water after cleaning.

[0049] The second step involves processing the heat-bearing layer and the metal electrode layer, specifically:

[0050] Step 2.1: Spin-coat photoresist onto the top of the flexible dielectric layer. By controlling the spin coater's rotation time and speed, the photoresist thickness is controlled to be between 500-650 nm. After spin coating, the flexible dielectric layer coated with photoresist is heat-treated.

[0051] Furthermore, the heat treatment temperature is 100-150℃, and the time is 2-3 minutes to harden the photoresist, making the photoresist adhere more tightly to the substrate. The photoresist is LOR3A.

[0052] Step 2.2: Place the patterned photomask of the heat-bearing layer and metal electrode layer into the photolithography machine, expose the photoresist on the flexible dielectric layer, and then immerse the exposed flexible dielectric layer in the developer AZ400K to obtain the photolithographic pattern of the heat-bearing layer and metal electrode layer.

[0053] Furthermore, the exposure time is 5-8 seconds, and the soaking time is 5-10 seconds.

[0054] Step 2.3: Using tungsten metal and alumina as targets, respectively, a magnetron sputtering device is used to precisely deposit multilayer thin films.

[0055] Furthermore, the deposition parameters are: chamber background vacuum ≤ 5 × 10⁻⁶ -4 Pa, working gas flow rate of 4.5-5.5 mtorr, sputtering power of 70-90W, sputtering temperature of 80-150℃, distance between flexible dielectric layer and target material of 80-120mm, rotation speed of flexible dielectric layer of 15-25 rpm, magnetic field strength of 0.09-0.11 T.

[0056] Step 2.4: By using a stepwise deposition strategy, the deposition time is precisely controlled to adjust the thickness of each layer, thereby achieving the sequential deposition of metal electrode layers and heat-bearing layers of different thicknesses.

[0057] The third step, based on the processing results of the second step, involves processing a sulfur-based phase change material layer, specifically:

[0058] Step 3.1: Spin-coat photoresist onto the surface of the overall structure after the second step, including the unpatterned areas of the flexible dielectric layer, with a photoresist thickness of 100-200 nm. After spin-coating, heat-treat the flexible dielectric layer coated with photoresist.

[0059] Furthermore, by controlling the rotation time and speed of the spin coater, the photoresist thickness is controlled to be between 100-200 nm. After spin coating, the flexible dielectric layer coated with photoresist is placed on a heating stage, heated to 100-150°C for 2-3 minutes to harden the photoresist and make it adhere more tightly to the substrate. The photoresist used is LOR3A.

[0060] Step 3.2: Place the patterned photomask of the sulfide phase change material layer into a photolithography machine, expose the photoresist on the flexible dielectric layer, and then immerse the exposed flexible dielectric layer in the developer AZ400K to obtain the photolithographic pattern of the sulfide phase change material layer.

[0061] Furthermore, the exposure time is 5-8 seconds, and the soaking time is 5-10 seconds.

[0062] Step 3.3: Using sulfur-based phase change material as the target, a sulfur-based phase change material layer is deposited using magnetron sputtering equipment. The thickness of the sulfur-based phase change material layer is controlled by precisely controlling the deposition time.

[0063] Furthermore, the deposition parameters are: controlling the chamber background vacuum ≤ 5 × 10⁻⁶. -4 Pa, working gas flow rate of 4.5-5.5 mtorr, sputtering power of 100-130W, flexible dielectric layer temperature of 80-150℃, distance between flexible dielectric layer and target of 80-120 mm, rotation speed of flexible dielectric layer of 15-25 rpm, magnetic field strength of 0.09-0.11 T.

[0064] The fourth step, based on the processing results of the third step, involves fabricating a patterned microwave absorbing layer using magnetron sputtering. Specifically:

[0065] Step 4.1: Spin-coat photoresist onto the surface of the overall structure after step 3, including the unpatterned areas of the flexible dielectric layer. After spin-coating, heat the flexible dielectric layer coated with photoresist.

[0066] Furthermore, by controlling the rotation time and speed of the spin coater, the photoresist thickness is controlled to be between 18-30 μm. After spin coating, the flexible dielectric layer coated with photoresist is placed on a heating stage, heated to 100-150℃ for 2-3 minutes to harden the photoresist and make it adhere more tightly to the substrate. The photoresist used is LOR3A.

[0067] Step 4.2: Place the photomask of the patterned microwave absorption layer into the photolithography machine, expose the photoresist on the flexible dielectric layer for 5-8 seconds, and then immerse the exposed flexible dielectric layer in the developer AZ400K to obtain the photolithographic pattern of the patterned microwave absorption layer.

[0068] Furthermore, the exposure time is 5-8 seconds, and the soaking time is 20-40 seconds.

[0069] Step 4.3: Using copper (Cu) as the target material, a patterned microwave absorbing layer is precisely deposited by magnetron sputtering. The thickness of the patterned microwave absorbing layer is controlled by precisely controlling the deposition time.

[0070] Furthermore, the deposition parameters are: controlling the chamber background vacuum ≤ 5 × 10⁻⁶. -4 Pa, working gas flow rate of 4.5-5.5 mtorr, sputtering power of 50-100W, sputtering temperature of 80-150℃, distance between flexible dielectric layer and target material of 80-120 mm, rotation speed of flexible dielectric layer of 15-25 rpm, magnetic field strength of 0.09-0.11 T.

[0071] The beneficial effects of this invention are:

[0072] (1) Through the design of the stacked structure, the present invention achieves cross-band operation and multi-level adjustable absorption performance in the S / C / X bands. The reflection loss at specific absorption frequency points is less than -10dB. It also has the advantages of flexibility, light weight and thin thickness, overcoming the limitations of traditional absorbing materials such as fixed function, limited bandwidth and difficulty in tuning.

[0073] (2) At the design method level, this invention introduces chalcogenide phase change materials as functional media and, combined with their reversible phase change characteristics and the adjustable electromagnetic parameters, establishes a cross-band, multi-level adjustable radar absorption structure design model. Through the equivalent circuit model, the unit structure, material distribution, and interlayer matching are flexibly designed to achieve the control of absorption performance in multiple microwave bands, significantly improving the intelligence and adaptability of the radar absorption structure.

[0074] (3) The present invention adopts magnetron sputtering process, through substrate pretreatment and multi-parameter collaborative deposition (precise control of background vacuum, power, temperature and rotation speed), to ensure the bonding strength of heterogeneous material interface and the uniformity and stability of sulfide phase change material on flexible substrate. Combined with step-by-step deposition and precise thickness control, reliable preparation of sulfide phase change material patterning and multilayer flexible structure is realized.

[0075] In summary, this invention integrates chalcogenide phase change materials onto a flexible radar absorbing structure, significantly improving radar absorption performance without compromising efficiency. Compared to existing radar absorbing structures, it achieves continuous adjustment of the absorption frequency within the operating frequency band, enabling perfect absorption at the operating frequency. The system features low power consumption, fast response, and no driving structure. Furthermore, the fabrication process is simple and easy to implement, with strong compatibility and broad application prospects. This invention is of great significance for the development of radar absorbing structures in terms of frequency band coverage, control convenience, and applicability. Attached Figure Description

[0076] Figure 1 This is a schematic cross-sectional view of a preferred structure for a multi-band, multi-level adjustable radar absorbing structure.

[0077] Figure 2 This is a top view schematic diagram of the control structure region of a preferred sulfur-based phase change material for a multi-band, multi-level adjustable radar absorbing structure.

[0078] Figure 3 This is a schematic diagram of an embodiment of a multi-band, multi-level adjustable radar absorbing structure.

[0079] Figure 4 Absorption curves are shown for an embodiment of a multi-band, multi-level adjustable radar absorbing structure.

[0080] In the figure: 1 Patterned microwave absorbing layer; 2 Sulfur-based phase change material layer; 3 Heat-bearing layer; 4 Metal electrode layer; 5 Flexible dielectric layer; 6 Sulfur-based phase change material control structure region. Detailed Implementation

[0081] To better illustrate the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0082] This embodiment provides a design method for a multi-band, multi-level adjustable radar absorbing structure, including the following steps:

[0083] Step 1: Establish a mathematical model for the dielectric constant of chalcogenide phase change materials; specifically:

[0084] Based on the fundamentals of phase transition kinetics, the amorphous-crystalline phase transition in chalcogenide phase change materials is a temperature-activated process. Considering thermal effects, the amount of heat received by a chalcogenide phase change material is proportional to the square of its electrical resistance and voltage. The mathematical model is as follows:

[0085] (1)

[0086] Where v is the crystallization rate, v0 is the pre-exponential factor, and E a It is the activation energy of crystallization, k is the Boltzmann constant, T is the absolute temperature, and R is the activation energy of crystallization. m V is the resistance of the sulfide phase change material, V is the equivalent voltage, and T0 is the initial temperature of the sulfide phase change material.

[0087] Assuming a critical crystallization time, integrating the crystallization rate over time yields the control parameter t that regulates the crystallization transition of chalcogenide phase change materials:

[0088] (2)

[0089] Where a is a constant representing time; b is a correction factor representing the equivalent voltage.

[0090] At room temperature (25°C), for the sulfide-based phase change material used in this invention, by consulting the lattice parameters of the sulfide-based phase change material, the constant in formula (2) is obtained as follows: , The voltage parameters that guide the phase transition are calculated according to step 1 and formula (2), and the electrical control pulse parameters are as follows: the voltage parameter amplitude for positive phase transition is 90V, the pulse width is 500ns, and the rising and falling edges are both 100ns; the voltage parameter amplitude for reverse phase transition is 35V, and the pulse width is 300ms.

[0091] Step 2: Based on the control parameter t for the crystallization transformation of the chalcogenide phase change material obtained in Step 1, design a flexible radar absorbing structure based on the chalcogenide phase change material; specifically:

[0092] Step 2.1: Based on the control parameters obtained in Step 1, determine the flexible radar absorbing structure. The flexible radar absorbing structure, from top to bottom, consists of a patterned microwave absorbing layer 1, a chalcogenide phase change material layer 2, a heat-bearing layer 3, a metal electrode layer 4, and a flexible dielectric layer 5. The minimum operating frequency of the flexible radar absorbing structure is then determined. Determine the theoretical minimum thickness h of the flexible radar absorbing structure, h= ,in This refers to the wavelength corresponding to the minimum operating frequency. In this embodiment, the incident electromagnetic wave wavelength range is 2.8-12.4 GHz, and the incident angle is perpendicular. Based on this, the thickness of each dielectric layer is calculated optimally. Figure 1As shown, the patterned microwave absorbing layer 1 has a thickness of 18 μm; the sulfide phase change material layer 2 has a thickness of 30 nm; the heat-bearing layer 3 has a thickness of 50 nm; the metal electrode layer 4 has a thickness of 80 nm; and the flexible dielectric layer 5 has a thickness of 500 μm. A top view of the sulfide phase change material control structure region 6 is shown below. Figure 2 As shown. Step 2.2: Establish a flexible radar absorbing structure model. The flexible radar absorbing structure model, from top to bottom, consists of a patterned microwave absorbing layer 1, a chalcogenide phase change material layer 2, a heat-bearing layer 3, a metal electrode layer 4, and a flexible dielectric layer 5. Through electromagnetic simulation, an equivalent circuit model is constructed. Based on the equivalent circuit model of the flexible radar absorbing structure, the analytical relationship between the reflection coefficient of the flexible radar absorbing structure and the geometric dimensions of the patterned microwave absorbing layer 1 is derived. The specific steps are as follows:

[0093] Step 2.2.1: Calculate the value of the equivalent total resistance R of the equivalent circuit model;

[0094] (3)

[0095] Where R is the equivalent total resistance; S is the area of ​​the patterned microwave absorbing layer; A is the area through which current flows in the patterned microwave absorbing layer 1; R 21 It is the sheet resistance of the absorbing structure that constitutes the resonant part.

[0096] Step 2.2.2: Calculate the equivalent lumped inductance L and equivalent lumped capacitance C of the equivalent circuit model;

[0097] (4)

[0098] Where L0 and C0 are the equivalent inductance and capacitance of the absorbing layer in the air; ε r It is the equivalent dielectric constant of the entire flexible radar absorbing structure.

[0099] Based on the equivalent lumped inductance L and equivalent lumped capacitance C calculated in step 2.2.2, the equivalent impedance Z of the patterned microwave absorbing layer 1 is determined. r Establish a series model of R, C, and L, as shown in formula (5).

[0100] (5)

[0101] Among them, Z r R is the equivalent impedance of the patterned microwave absorbing layer 1; ω is the total equivalent resistance; L is the angular frequency; C is the inductance; and j is the imaginary unit.

[0102] Step 2.2.3: Calculate the input impedance Z of the entire flexible radar absorbing structure. in and reflection coefficient ;

[0103] The input impedance Z in Equivalent to the impedance Z of the absorbing layer r Equivalent impedance Z of base plate d The parallel connection is shown in formula (6):

[0104] (6)

[0105] Among them: Z d It can be obtained through formula (7);

[0106] (7)

[0107] Where Z0 is the transmission characteristic impedance; It is the free space wavenumber; It is the physical thickness of the dielectric layer; It is the relative permittivity of the base plate; Given the relative permittivity of air, the reflection coefficient of the entire absorbing structure is... for:

[0108] (8)

[0109] in, Z0 is the reflection coefficient, and Z0 is the transmission characteristic impedance.

[0110] Step 2.3: Based on the analytical calculation formulas for the flexible radar absorbing structure shown in formulas (3) and (8), with the optimization objective of maximizing the minimum absorption rate within a certain frequency band, and using the thickness, length and width geometric dimensions of the patterned microwave absorbing layer 1, the thickness of the chalcogenide phase change material layer 2, the heat-bearing layer 3, the metal electrode layer 4, and the flexible dielectric layer 5 in the flexible radar absorbing structure model as independent variables, the finite-difference time-domain method is used for simulation verification to optimize the entire flexible radar absorbing structure model, and finally obtains the flexible radar absorbing structure as shown in the figure. Figure 3 As shown, the patterned microwave absorbing layer 1 preferably has a thickness of 18 μm, and its side dimensions are a = 10 mm, b = 0.5 mm, c = 0.2 mm, d = 4.8 mm, e = 0.2 mm, and f = 0.5 mm. The sulfur-based phase change material layer 2 preferably has a thickness of 30 nm. The heat-bearing layer 3 preferably has a thickness of 50 nm. The metal electrode layer 4 preferably has a thickness of 80 nm. The flexible dielectric layer 5 has a thickness of 500 μm.

[0111] A method for fabricating a multi-band, multi-level tunable radar absorbing structure includes the following steps:

[0112] The first step is to pre-treat the flexible dielectric layer 5; specifically:

[0113] Step 1.1: A flexible glass with a size of 50mm×50mm×500μm is used as the flexible dielectric layer 5 of the adjustable flexible radar absorbing structure. The flexible dielectric layer 5 is baked at 500℃ in a low oxygen environment to eliminate most of the thermal stress inside the high-temperature durable substrate. This allows the atoms inside the substrate to obtain sufficient energy to rearrange themselves and also effectively reduces the occurrence of oxidation reactions, keeping the surface of the substrate clean and active.

[0114] Step 1.2: Immerse the flexible dielectric layer 5 in isopropanol solution, place it in an ultrasonic cleaner to perform ultrasonic cleaning on the substrate surface, and wipe the substrate surface with deionized water after cleaning.

[0115] The second step involves processing the heat-bearing layer 3 and the metal electrode layer 4, specifically:

[0116] Step 2.1: Spin-coat photoresist onto the top of the flexible dielectric layer 5. The spin coater is set to a rotation time of 150 seconds and a rotation speed of 3000 rpm to control the photoresist thickness to 550 nm. After spin-coating, the flexible dielectric layer 5 coated with photoresist is placed on a heating stage and baked at 110°C for 2 minutes to harden the photoresist and improve adhesion between the photoresist and the substrate. The photoresist used is LOR3A.

[0117] Step 2.2: Place the patterned photomask of the heat-bearing layer 3 and the metal electrode layer 4 into the photolithography machine, expose the photoresist on the flexible dielectric layer 5 for 6 seconds using the photolithography machine, and then immerse the exposed flexible dielectric layer 5 in the developer solution AZ400K for 8 seconds to obtain the photolithographic pattern of the heat-bearing layer 3 and the metal electrode layer 4.

[0118] Step 2.3: Precision deposition of multilayer thin films is performed using magnetron sputtering equipment, with tungsten metal (W) and alumina as targets, respectively. The chamber background vacuum is controlled to be ≤5×10⁻⁶. -4 Pa, working gas flow rate is 5 mtorr, sputtering power is 80 W, sputtering temperature is 90℃, the distance between the flexible dielectric layer 5 and the target is 90 mm, the rotation speed of the flexible dielectric layer 5 is 20 rpm, and the magnetic field strength is 0.1 T.

[0119] Step 2.4: Through a stepwise deposition strategy, metal electrode layer 4 and heat-bearing layer 3 are formed sequentially. By precisely controlling the deposition time of metal electrode layer 4 to 40 minutes and the deposition time of heat-bearing layer 3 to 25 minutes, the thickness of metal electrode layer 4 to 80 nm and the thickness of heat-bearing layer 3 to 50 nm are respectively deposited.

[0120] The third step, based on the processing results of the second step, involves processing a sulfur-based phase change material layer, specifically:

[0121] Step 3.1: Spin-coat photoresist onto the surface of the overall structure after the second step, including the unpatterned area of ​​the flexible dielectric layer 5. Control the spin coater rotation time to 40 seconds and the rotation speed to 3600 rpm to maintain the photoresist thickness at 150 nm. After spin coating, place the photoresist-coated flexible dielectric layer 5 on a heating stage and bake at 100-150°C for 3 minutes to harden the photoresist and improve adhesion between the photoresist and the substrate. The photoresist used is LOR3A.

[0122] Step 3.2: Place the patterned photomask of the sulfur-based phase change material layer 2 into the photolithography machine, expose the photoresist on the flexible dielectric layer 5 for 6 seconds using the photolithography machine, and then immerse the exposed flexible dielectric layer 5 in the developer AZ400K for 8 seconds to obtain the photolithographic pattern of the sulfur-based phase change material layer 2.

[0123] Step 3.3: Precise deposition of the sulfide phase change material layer 2 thin film is carried out using a magnetron sputtering system, with the sulfide phase change material as the target material. The background vacuum of the chamber is controlled to be ≤5×10⁻⁶. -4 The working gas flow rate is 5 mtorr, the sputtering power is 110 W, the sputtering temperature is 90 °C, the distance between the flexible dielectric layer 5 and the target is 80 mm, the rotation speed of the flexible dielectric layer 5 is 20 rpm, the magnetic field strength is 0.1 T, and by precisely controlling the deposition time of the sulfur-based phase change material layer 2 to 15 minutes, the thickness of the sulfur-based phase change material layer 2 is 30 nm.

[0124] The fourth step, based on the processing results of the third step, involves fabricating a patterned microwave absorbing layer using magnetron sputtering. Specifically...

[0125] Step 4.1: Spin-coat photoresist onto the surface of the overall structure after the third step, including the unpatterned area of ​​the flexible dielectric layer 5. Control the spin coater rotation time to 30 seconds and the rotation speed to 300 rpm to maintain a photoresist thickness of 30 μm. After spin coating, place the photoresist-coated flexible dielectric layer 5 on a heating stage and bake at 100°C for 2.5 minutes to harden the photoresist and improve adhesion between the photoresist and the substrate. The photoresist used is LOR3A.

[0126] Step 4.2: Place the photomask of the patterned microwave absorption layer 1 into the photolithography machine, expose the photoresist on the flexible dielectric layer 5 for 8 seconds using the photolithography machine, and then immerse the exposed flexible dielectric layer 5 in the developer AZ400K for 25 seconds to obtain the photolithographic pattern of the patterned microwave absorption layer 1.

[0127] Step 4.3: A patterned microwave absorption layer 1 thin film is precisely deposited using magnetron sputtering equipment, with copper (Cu) as the target material. The chamber background vacuum is controlled to be ≤5×10⁻⁶. -4The working gas flow rate is 4.5 mtorr, the sputtering power is 50 W, the sputtering temperature is 80 °C, the distance between the flexible dielectric layer 5 and the target is 80 mm, the rotation speed of the flexible dielectric layer 5 is 15 rpm, the magnetic field strength is 0.09 T, and by precisely controlling the deposition time to 4 hours, the thickness of the patterned microwave absorbing layer 1 is 18 μm.

[0128] The absorption rate of the sample in the embodiment was analyzed and tested. Absorption experiments were conducted on the chalcogenide phase change material in the tunable flexible radar absorbing structure in crystalline (metallic), intermediate (semi-conductive), and amorphous (insulating) states. The final test results are as follows: Figure 4 As shown, specifically:

[0129] When the sulfur-based phase change material is in the amorphous state, it achieves a microwave absorption effect of -25dB at 4.5GHz; when the sulfur-based phase change material is in the intermediate state, it achieves a microwave absorption effect of -24dB at 8.1GHz; and when the sulfur-based phase change material is in the amorphous state, it achieves a microwave absorption effect of -23dB at 11.4GHz, thus achieving the goal of perfect microwave absorption.

[0130] A flexible radar absorbing structure with S / C / X band tunability based on chalcogenide phase change material is fabricated using the above-mentioned method. It is prepared by photolithography and magnetron sputtering processes and can achieve absorption in specific bands under S / C / X bands, meeting the performance requirements of flexibility and tunable absorption bands. It solves the problem of passive device control being incompatible with flexible substrates. The system design and fabrication process are stable and have good application prospects.

[0131] In summary, the above is merely a preferred example of the present invention exhibiting absorption peaks at 3-5 GHz before modulation, 7-9 GHz in the intermediate state after modulation, and 10-12 GHz after modulation. It is not intended to limit the present invention in any way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent examples. However, any simple modifications, equivalent changes, and alterations made to the above examples based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A multi-band, multi-level adjustable radar absorbing structure, characterized in that, The cross-band multi-level adjustable radar absorbing structure is fabricated using photolithography and magnetron sputtering processes. It can achieve absorption of a preset band in the S / C / X bands. From top to bottom, it consists of a patterned microwave absorbing layer, a chalcogenide phase change material layer, a heat-bearing layer, a metal electrode layer, and a flexible dielectric layer. The patterned microwave absorbing layer has a thickness ranging from 18 to 36 micrometers and a side length ranging from 10 to 15 millimeters, and the material is selected from copper; The thickness of the sulfur-based phase change material layer ranges from 20 to 50 nanometers; the material is selected from GeTe, SeTe, or GeSbSeTe. The thickness of the heat-bearing layer ranges from 40 to 70 nanometers; the material is selected from tungsten, gold, or copper. The thickness of the metal electrode layer ranges from 70 to 100 nanometers; the material is selected from gold or copper; The thickness of the flexible dielectric layer ranges from 30 to 800 micrometers; the material is selected from polyimide or flexible glass.

2. A design method for a cross-band multi-level adjustable radar absorbing structure as described in claim 1, characterized in that, The design method Includes the following steps: Step 1: Establish a mathematical model for the dielectric constant of chalcogenide phase change materials; specifically: When considering thermal effects, the heat received by a sulfide phase change material is proportional to the square of the material's resistance and voltage. The mathematical model is as follows: （1） Where v is the crystallization rate, v0 is the pre-exponential factor, and E a It is the activation energy of crystallization, k is the Boltzmann constant, T is the absolute temperature, and R is the activation energy of crystallization. m is the resistance of the sulfide phase change material, V is the equivalent voltage, and T0 is the initial temperature of the sulfide phase change material; Assuming a critical crystallization time, the crystallization rate is integrated over time to obtain the control parameter t that controls the crystallization transformation of sulfide phase change materials. Step 2: Based on the mathematical model of the dielectric constant of chalcogenide phase change materials, construct a flexible radar absorbing structure based on chalcogenide phase change materials. Optimize the structural parameters based on the absorption range and absorption efficiency within the operating bandwidth. Specifically: Step 2.1: Based on the control parameters obtained in Step 1, determine the flexible radar absorbing structure, and determine the minimum operating frequency of the flexible radar absorbing structure. Determine the theoretical minimum thickness h of the flexible radar absorbing structure, and then optimize the calculation of the thickness of each layer in the flexible radar absorbing structure. Step 2.2: Establish a flexible radar absorbing structure model, which consists of a patterned microwave absorbing layer, a chalcogenide phase change material layer, a heat-bearing layer, a metal electrode layer, and a flexible dielectric layer from top to bottom. Construct an equivalent circuit model through electromagnetic simulation. Based on the equivalent circuit model of the flexible radar absorbing structure, obtain the analytical relationship between the reflection coefficient of the flexible radar absorbing structure model and the geometric dimensions of the patterned microwave absorbing layer. Step 2.3: Based on the analytical relationship obtained in Step 2.2, with the optimization objective of maximizing the minimum absorption rate within a certain frequency band, and using the thickness, length and width of the patterned microwave absorbing layer, as well as the thicknesses of the chalcogenide phase change material layer, heat-bearing layer, metal electrode layer, and flexible dielectric layer in the flexible radar absorbing structure model as independent variables, the finite-difference time-domain method is used for simulation verification to optimize the entire flexible radar absorbing structure model. Finally, the geometric parameters of each layer in the flexible radar absorbing structure, including the patterned microwave absorbing layer, chalcogenide phase change material layer, heat-bearing layer, metal electrode layer, and flexible dielectric layer, are obtained.

3. The design method of a cross-band multi-level adjustable radar absorbing structure according to claim 2, characterized in that, In the first step, the control parameter t is obtained using formula (2): （2） Where a is a constant representing time; b is a correction factor representing the equivalent voltage.

4. The design method of a multi-band, multi-level adjustable radar absorbing structure according to claim 3, characterized in that, In the second step: In step 2.1, the theoretical minimum thickness h of the flexible radar absorbing structure is h = ,in It is the wavelength corresponding to the minimum operating frequency; Step 2.2 is as follows: Step 2.2.1: Calculate the value of the equivalent total resistance R of the equivalent circuit model; （3） Where R is the equivalent total resistance; S is the area of ​​the patterned microwave absorbing layer; A is the area through which current flows in the patterned microwave absorbing layer; R 21 It is the sheet resistance of the absorbing structure that constitutes the resonant part; Step 2.2.2: Calculate the equivalent lumped inductance L and equivalent lumped capacitance C of the equivalent circuit model; （4） Where L0 and C0 are the equivalent inductance and capacitance of the absorbing layer in the air; ε r It is the equivalent dielectric constant of the entire flexible radar absorbing structure; Based on the equivalent lumped inductance L and equivalent lumped capacitance C, the equivalent impedance Z of the patterned microwave absorbing layer is... r Establish a series model of R, C, and L, as shown in formula (5); （5） Among them, Z r R is the equivalent impedance of the patterned microwave absorbing layer; ω is the total equivalent resistance; L is the angular frequency; and C is the capacitance. Represented as the imaginary unit; Step 2.2.3: Calculate the input impedance Z of the entire flexible radar absorbing structure. in and reflection coefficient The reflection coefficient of the entire absorbing structure is... for: （8） in, Z0 is the reflection coefficient, and Z0 is the transmission characteristic impedance.

5. The design method of a cross-band multi-level adjustable radar absorbing structure according to claim 4, characterized in that, In step 2.2.3, the input impedance Z in and reflection coefficient The calculation formula is as follows: The input impedance Z in Equivalent to the impedance Z of the absorbing layer r Equivalent impedance Z of base plate d The parallel connection is shown in formula (6): （6） Among them: Z d It can be obtained through formula (7); （7） Where Z0 is the transmission characteristic impedance; It is the free space wavenumber; It is the physical thickness of the dielectric layer; It is the relative permittivity of the base plate; It is the relative permittivity of air.

6. A method for fabricating a multi-band, multi-level adjustable radar absorbing structure as described in claim 1, characterized in that, The preparation method includes the following steps: The first step is to pretreat the flexible dielectric layer as a substrate; The second step involves processing the heat-bearing layer and the metal electrode layer, specifically: Step 2.1: Spin coat the photoresist onto the top of the flexible dielectric layer. By controlling the rotation time and speed of the spin coater, the thickness of the photoresist is controlled to be 500-650nm. After spin coating, the flexible dielectric layer coated with photoresist is heat-treated. Step 2.2: Place the patterned photomask of the heat-bearing layer and metal electrode layer into the photolithography machine, expose the photoresist on the flexible dielectric layer, and then immerse the exposed flexible dielectric layer in the developing solution to obtain the photolithographic pattern of the heat-bearing layer and metal electrode layer. Step 2.3: Using tungsten metal and alumina as targets, respectively, a magnetron sputtering device is used to precisely deposit multilayer thin films; Step 2.4: By using a stepwise deposition strategy, the deposition time is precisely controlled to adjust the thickness of each layer, thereby achieving the sequential deposition of metal electrode layers and heat-bearing layers of different thicknesses. The third step, based on the processing results of the second step, involves processing a sulfur-based phase change material layer, specifically: Step 3.1: Spin-coat photoresist onto the surface of the overall structure after the second step, including the unpatterned area of ​​the flexible dielectric layer, with a photoresist thickness of 100-200nm; After spin-coating, heat-treat the flexible dielectric layer coated with photoresist. Step 3.2: Place the patterned photomask of the sulfide phase change material layer into the photolithography machine, expose the photoresist on the flexible dielectric layer, and then immerse the exposed flexible dielectric layer in the developing solution to obtain the photolithographic pattern of the sulfide phase change material layer. Step 3.3: Using sulfur-based phase change material as the target, a sulfur-based phase change material layer is deposited using magnetron sputtering equipment. The thickness of the sulfur-based phase change material layer is controlled by precisely controlling the deposition time. The fourth step, based on the processing results of the third step, involves fabricating a patterned microwave absorbing layer using magnetron sputtering. Specifically: Step 4.1: Spin-coat photoresist onto the surface of the overall structure after the third step, including the unpatterned area of ​​the flexible dielectric layer; after spin-coating, heat the flexible dielectric layer coated with photoresist. Step 4.2: Place the photomask of the patterned microwave absorption layer into the photolithography machine, expose the photoresist on the flexible dielectric layer for 5-8 seconds, and then immerse the exposed flexible dielectric layer in the developing solution to obtain the photolithographic pattern of the patterned microwave absorption layer. Step 4.3: Using copper (Cu) as the target material, a patterned microwave absorbing layer is precisely deposited by magnetron sputtering. The thickness of the patterned microwave absorbing layer is controlled by precisely controlling the deposition time.

7. The method for fabricating a multi-band, multi-level adjustable radar absorbing structure according to claim 6, characterized in that, The first step is specifically as follows: Step 1.1: Using the flexible dielectric layer as a substrate, bake the flexible dielectric layer at a high temperature of 400℃-500℃ in a low-oxygen environment. Step 1.2: Immerse the flexible dielectric layer in isopropanol solution, place it in an ultrasonic cleaner to perform ultrasonic cleaning on the substrate surface, and wipe the substrate surface with deionized water after cleaning.

8. The method for fabricating a multi-band, multi-level tunable radar absorbing structure according to claim 7, characterized in that, In the second step: In step 2.1, the heat treatment temperature is 100-150℃ and the time is 2-3 minutes to harden the photoresist, making the photoresist adhere more tightly to the substrate; the photoresist is LOR3A. In step 2.2, the exposure time is 5-8 seconds and the soaking time is 5-10 seconds. In step 2.3, the deposition parameters are: chamber background vacuum ≤ 5 × 10⁻⁶. -4 Pa, working gas flow rate of 4.5-5.5 mtorr, sputtering power of 70-90 W, sputtering temperature of 80-150℃, distance between flexible dielectric layer and target material of 80-120 mm, rotation speed of flexible dielectric layer of 15-25 rpm, magnetic field strength of 0.09-0.11 T.

9. The method for fabricating a multi-band, multi-level tunable radar absorbing structure according to claim 8, characterized in that, In the third step: In step 3.1, the photoresist thickness is controlled to be 100-200 nm by controlling the rotation time and speed of the spin coater; after spin coating, the flexible dielectric layer coated with photoresist is placed on a heating stage, and the heating temperature is 100-150℃ for 2-3 minutes to harden the photoresist and make the photoresist adhere more tightly to the substrate; the photoresist is LOR3A. In step 3.2, the exposure time is 5-8 seconds and the soaking time is 5-10 seconds. In step 3.3, the deposition parameters are: controlling the chamber background vacuum ≤ 5 × 10⁻⁶. -4 Pa, working gas flow rate of 4.5-5.5 mtorr, sputtering power of 100-130W, flexible dielectric layer temperature of 80-150℃, distance between flexible dielectric layer and target of 80-120 mm, rotation speed of flexible dielectric layer of 15-25 rpm, magnetic field strength of 0.09-0.11 T.

10. The method for fabricating a multi-band, multi-level tunable radar absorbing structure according to claim 9, characterized in that, In the fourth step: In step 4.1, the photoresist thickness is controlled to be 18-30 μm by controlling the rotation time and speed of the spin coater; after spin coating, the flexible dielectric layer coated with photoresist is placed on a heating stage, and the heating temperature is 100-150℃ for 2-3 minutes to harden the photoresist and make the photoresist adhere more tightly to the substrate; the photoresist is LOR3A. In step 4.2, the exposure time is 5-8 seconds and the soaking time is 20-40 seconds; In step 4.3, the deposition parameters are: controlling the chamber background vacuum ≤ 5 × 10⁻⁶. -4 Pa, working gas flow rate is 4.5-5.5 mtorr, sputtering power is 50-100W, sputtering temperature is 80-150℃, distance between flexible dielectric layer and target is 80-120mm, rotation speed of flexible dielectric layer is 15-25 rpm, magnetic field strength is 0.09-0.11 T.

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