Preparation method of three-dimensional curved surface microstructure based on shape memory alloy phase change driving

Abstract

The invention belongs to the technical field of micro-nano machining and advanced manufacturing, and particularly belongs to a preparation method of a three-dimensional curved surface microstructure based on shape memory alloy phase change driving. Carrying out thermomechanical treatment on the substrate, and endowing the three-dimensional curved surface with a memory state to obtain a trained SMA substrate; the trained SMA substrate is cooled to the temperature below the martensite phase transformation completion temperature, external mechanical force is applied, the three-dimensional curved surface shape in the memory state is stretched and flattened into a temporary two-dimensional plane state, then the SMA substrate is fixed to a rigid carrier plate, and the SMA substrate in the temporary plane state is obtained; and processing on the SMA substrate in the temporary plane state by using a plane micro-nano processing technology, then recovering to a three-dimensional curved surface shape in a memory state, and removing redundant structures to obtain the three-dimensional curved surface microstructure. Through phase change driving of the shape memory alloy, two-dimensional plane machining is converted into three-dimensional curved surface result forming.

Description

Technical Field

[0001] This invention belongs to the field of micro-nano fabrication and advanced manufacturing technology, and in particular, to a method for preparing three-dimensional curved surface microstructures based on shape memory alloy phase transformation driven by phase transformation. Background Technology

[0002] With the increasing integration and functional demands of microsystems, the performance bottlenecks of traditional planar structures are becoming increasingly apparent. In cutting-edge fields such as beam shaping and spectral modulation, optical elements with three-dimensional curved surface features exhibit irreplaceable advantages. Typical three-dimensional micro-optical devices include curved microlens arrays, three-dimensional waveguide devices, and three-dimensional diffraction elements. These structures directly control the light field distribution through physical morphology, playing a crucial role in simplifying optical architecture, improving energy conversion efficiency, and correcting aberrations. Beyond micro-optics, the applications of three-dimensional micro / nano structures are very extensive, mainly including the following aspects: first, biochips, such as organ-on-a-chip, biomimetic devices, microfluidic sensors, biological detection, and analytical instruments; second, nanophotonics, such as sensors, lasers, and optical micromotors; and third, mechanical MEMS devices, such as pressure sensors and three-dimensional micro-assemblies.

[0003] While three-dimensional micro / nano structures possess significant performance advantages and broad application prospects, their actual development is limited by complex fabrication processes. Traditionally, micro / nano fabrication systems centered on photolithography have only been designed for manufacturing planar two-dimensional structures. To construct three-dimensional micro / nano structures, multi-layer photolithography is typically employed, involving the successive stacking of different two-dimensional patterns in conjunction with etching processes to form three-dimensional structures with height differences. However, this method has significant limitations: not only is the processing efficiency low, but it also requires extremely high interlayer alignment precision. More importantly, it can only generate step-like three-dimensional micro / nano structures, making it difficult to achieve complex three-dimensional features such as continuous transitions between curved surfaces, which greatly limits the expansion of device functionality. To address these limitations, researchers have developed several groundbreaking three-dimensional microfabrication technologies, including: direct-write photolithography, grayscale mask photolithography, moving mask photolithography, holographic photolithography, and digital maskless photolithography, as well as a template-assisted forming technology.

[0004] Direct-write lithography is a fabrication method that selectively exposes photoresist surfaces point-by-point by precisely controlling the scanning path of a focused light beam. Its main technical branches include laser direct-write, electron beam direct-write, ion beam direct-write, and two-photon direct-write. Among these, two-photon laser direct-write lithography can overcome the dimensional limitations of traditional lithography, achieving sub-micron precision fabrication of arbitrarily complex curved three-dimensional micro / nano structures. However, limited by the point-by-point scanning mechanism, its processing speed is difficult to meet the demands of industrial mass production. Therefore, this technology currently focuses mainly on the prototype development and customized fabrication of small-size, highly complex three-dimensional devices and is not suitable for large-scale or large-size three-dimensional micro / nano structure fabrication.

[0005] Grayscale lithography achieves different exposure doses at different locations on the same photoresist film by modulating the transmittance of a grayscale mask or controlling the electron beam / laser direct writing energy. After exposure at different doses, the resulting photoresist structure, after complete or partial development in a developer, exhibits a 3D contour structure. However, grayscale lithography can only fabricate continuous tilted or stepped structures, making it difficult to achieve complex free-form surfaces. Furthermore, the process control is complex, and it has poor compatibility with standard CMOS processes.

[0006] Moving mask lithography is a technique for fabricating three-dimensional structures by dynamically adjusting the relative position of a mask and a substrate. Its core principle is to precisely control the mask's displacement trajectory during exposure, allowing different areas of the photoresist to accumulate varying exposure doses, thus forming a highly gradient three-dimensional structure after development. Compared to grayscale mask lithography, its advantage lies in requiring only a two-dimensional periodic pattern design for the mask, significantly reducing processing complexity and manufacturing costs. However, this technology is limited by the exposure mechanism of mask movement and is mainly suitable for fabricating three-dimensional micro / nano structures with rotational symmetry and periodic arrangement. It is difficult to fabricate non-periodic distributions or complex curved surface structures, thus limiting its application in the fabrication of multifunctional integrated devices.

[0007] Holographic lithography is based on the reflective interference effect of light to fabricate micro and nanostructures. It can achieve higher resolution and imaging contrast than traditional lithography techniques, and has greater flexibility and designability. However, there is an inherent contradiction between high resolution and large-size processing. To obtain fine structures, large-angle interference is usually required, but this reduces the processing area; moreover, the optical system is complex and has stringent stability requirements.

[0008] Digital maskless lithography uses digital micromirror devices or spatial light modulators as dynamic "digital masks." Computer-controlled patterning projects ultraviolet light onto photoresist for exposure, and three-dimensional structures are constructed by accumulating exposure amounts or grayscale patterns. This technology eliminates the mask manufacturing step, reducing process costs and increasing the flexibility of pattern design. However, it is limited by the size of the micromirror units, resulting in lower pattern resolution than traditional physical masks. Furthermore, when constructing tilted or curved surfaces, the well-known "staircase effect" occurs, leading to uneven surfaces and stepped sidewalls. While this can be mitigated by reducing layer thickness, it significantly sacrifices processing efficiency.

[0009] Template-assisted molding technology involves transferring or depositing material onto a pre-fabricated 3D mold. However, manufacturing the mold itself is challenging; demolding is difficult, it can easily damage delicate structures, and it is difficult to integrate multi-layered heterogeneous materials.

[0010] In summary, existing technologies generally suffer from the dilemma of being unable to simultaneously achieve "complex surface forming," "large-area fabrication," "material multifunctionality," and "shape programmability." The industry needs a new manufacturing method that can extend mature planar micro / nano fabrication technology to three-dimensional curved surfaces, with a controllable and predictable forming process. Summary of the Invention

[0011] To overcome the shortcomings of the prior art, the present invention provides a method for preparing three-dimensional curved surface microstructures based on shape memory alloy phase transformation driven by the prior art.

[0012] The technical solution of the present invention includes the following: A method for fabricating three-dimensional curved surface microstructures driven by phase transformation of shape memory alloys includes the following steps: The substrate is subjected to thermomechanical treatment to impart a three-dimensional surface memory state, resulting in a trained SMA substrate. The trained SMA substrate is cooled below its martensitic phase transformation completion temperature, and external mechanical force is applied to stretch and flatten the three-dimensional curved shape from the memory state into a temporary two-dimensional plane state. Then it is fixed on a rigid plate to obtain an SMA substrate in a temporary plane state. On an SMA substrate in a temporary planar state, planar micro-nano fabrication technology is used to process the substrate, and then restore it to a three-dimensional curved surface shape in a memory state. Excess structures are removed to obtain a three-dimensional curved surface microstructure. The planar micro-nano fabrication technology includes one or more of the following: interface cleaning and activation, deposition of barrier layers and stress buffer layers, deposition of adhesive layers, and microstructure preparation.

[0013] Furthermore, the substrate includes shape memory metal sheets or thin films, and the shape memory metal includes nickel-titanium alloy; the barrier layer is made of Si3N4, the stress buffer layer is made of one or more of DLC, PI, and stress buffer layer solutions, and the adhesive layer is made of Cr and / or Ti.

[0014] Furthermore, thermomechanical treatment includes high-temperature annealing followed by quenching under fixture constraints.

[0015] Furthermore, the memory state is the austenitic parent phase state, and the temperature of the entire processing of the planar micro-nano fabrication process is lower than the austenitic phase transformation initiation temperature of the SMA substrate.

[0016] Furthermore, interface cleaning and activation includes the step of in-situ plasma cleaning of the SMA substrate.

[0017] Furthermore, the microstructure fabrication includes the steps of: fabricating the desired planar micro / nanostructure by photolithography and / or sacrificial layer and / or etching techniques, the planar micro / nanostructure including metal tip structures and / or metal microbridge structures.

[0018] Furthermore, restoring the three-dimensional curved surface shape to the memory state includes the following steps: removing the rigid carrier plate, and then heating the entire structure to ≥60°C using an external heat source. The SMA substrate undergoes a phase transformation from martensite to austenite, restoring it to the originally assigned three-dimensional curved surface memory state. The structure on the SMA substrate is then loaded onto the three-dimensional curved surface to form the final three-dimensional curved surface microstructure.

[0019] Furthermore, removing excess structures includes leaving only the adhesive layer and microstructure.

[0020] Preferably, the method for preparing the stress buffer layer solution includes the following steps: Microcapsules and epoxy resin were ultrasonically dispersed in a PI solution, and then alumina particles and nano-silica were added and mechanically stirred to obtain a stress-relieving layer solution.

[0021] It should be noted that a combination of microcapsules, polyimide (PI), epoxy resin, alumina particles, and nano-silica was used in the preparation of the stress-relieving layer solution. PI provides the matrix material for the stress-relieving layer; PI possesses excellent electrical insulation, thermal stability, and mechanical strength, making it a good coating material. Epoxy resin increases the mechanical strength of the stress-relieving layer and helps improve the crosslinking density of the coating, enhancing the durability of the structure. Microcapsules provide self-healing capabilities. When stress or microcracks form, the microcapsules rupture and release a repair agent, achieving self-healing. Alumina particles provide additional mechanical strength, enhancing the material's fatigue resistance and wear resistance. Nano-silica enhances the coating's hardness and corrosion resistance, and improves the overall performance of the coating.

[0022] The combination of microcapsules with a PI matrix, epoxy resin, and inorganic fillers enables the stress-relieving layer to not only provide excellent mechanical properties but also possess self-healing capabilities. This self-healing ability is achieved by the release of a repair agent (epoxy resin) within the microcapsules upon the occurrence of stress or damage. Compared to traditional stress-relieving layers, this design improves the material's service life and reliability.

[0023] Preferably, the method for preparing microcapsules includes the following steps: IPDI and polyether polyol were stirred and reacted under dry nitrogen protection to obtain polyurethane prepolymer; The polyurethane prepolymer and epoxy resin are dissolved in acetone to obtain a polyurethane solution. Add emulsifier and polyvinyl alcohol to deionized water, stir well, then gradually add polyurethane solution dropwise while stirring continuously to obtain an emulsion; DETA was added to the emulsion, the mixture was stirred and centrifuged, and the precipitate was retained. The precipitate was a microcapsule.

[0024] Preferably, the alumina particles include α-phase alumina particles; the polyether polyol includes PEG; and the emulsifier includes Tween80.

[0025] It should be noted that during the microcapsule preparation process: IPDI is a polyurethane prepolymer that provides a robust shell structure. Polyether polyols react with IPDI to form a flexible portion of the polyurethane matrix, controlling the elasticity and stability of the microcapsule shell.

[0026] Emulsifiers in the aqueous phase help emulsify the oil phase (polyurethane solution) to form a stable emulsion structure.

[0027] Epoxy resin, encapsulated inside microcapsules as a repair agent, has strong adhesive properties and can provide repair function when the microcapsules rupture.

[0028] DETA acts as a crosslinking agent, promoting the curing of epoxy resin and enhancing the stability and robustness of the microcapsule shell.

[0029] According to the inventive concept of the present invention, the core technical problem to be solved by the present invention is: how to break through the existing three-dimensional micro-manufacturing technology and provide a method for preparing large-area three-dimensional microstructures that can take into account high precision, complex curved surface forming, multi-functional integration and dynamic adjustability.

[0030] Existing 3D microstructure manufacturing technologies face several inherent bottlenecks: ① The contradiction between manufacturing precision and structural complexity: Traditional planar micromachining techniques (such as photolithography) are difficult to implement on curved surfaces; while 3D manufacturing techniques (such as two-photon polymerization and holographic lithography) can directly form shapes, they are either limited in the design of complex free-form surfaces or suffer from inefficiency and stair-step effects, making it difficult to simultaneously achieve high precision and high complexity. ② The disconnect between functional integration and 3D forming: Most existing 3D forming technologies (such as template methods) are mainly carried out on passive materials (such as photoresists and polymers), making it impossible to efficiently and non-destructively transfer multi-material, multi-functional devices (such as IC and MEMS sensors) that have been maturely fabricated on planar surfaces to 3D curved surfaces, thus limiting the functional diversity of 3D microsystems. ③ Lack of dynamic adjustability: 3D structures manufactured by existing methods are usually static and fixed, unable to achieve active and reversible shape transformations, limiting their application in cutting-edge fields such as reconfigurable optics and adaptive systems.

[0031] Therefore, this invention provides a method for preparing three-dimensional curved surface microstructures based on phase transformation driven by shape memory alloys. This invention relates to the field of micro-nano fabrication and advanced manufacturing, and particularly to a method for preparing three-dimensional curved surface microstructures using the intelligent deformation characteristics of functional materials. Specifically, it is a technology that achieves the transformation from two-dimensional planar processing to three-dimensional curved surface formation through phase transformation driven by shape memory alloys.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Introducing a "2D Machining, 3D Forming" Manufacturing Strategy: This invention utilizes the unique shape memory effect (SMA) of shape memory metals to combine the most mature and precise planar micro-nano machining with the intelligent deformation capabilities of shape memory metals, perfectly separating the two processes of "microstructure manufacturing" and "3D forming." First, complex microstructures are manufactured on a planar surface using high-precision processes, and then 3D forming is triggered through the shape memory effect, solving the problem of directly performing micromachining on curved surfaces.

[0033] 2. Programmable design and precise control of complex surfaces are achieved: the final three-dimensional surface shape is determined by the initial shape of the shape memory metal, which can be achieved through thermomechanical training. This method realizes a deterministic mapping from two-dimensional patterns to three-dimensional morphologies.

[0034] 3. A dynamic / reconfigurable 3D microsystem platform is provided: Since the phase transition of shape memory alloys is a reversible process, the 3D structure can be reversibly switched between a "temporary planar shape" and a "memory curved surface shape" by controlling the temperature. This provides an unprecedented platform for the development of tunable optical devices, reconfigurable antennas, and dynamic biomimetic structures.

[0035] 4. Good process compatibility and high functional integration: In the planar processing stage, multiple functional materials (such as semiconductors, metals, and dielectrics) can be integrated to achieve heterogeneous integration on three-dimensional curved surfaces, which greatly expands the functional range of three-dimensional microstructures.

[0036] 5. This invention improves the material of the stress buffer layer, which can use polyimide (PI). PI is a widely used material with good thermal stability, mechanical strength, and electrical insulation, but its performance in stress dispersion and self-healing ability is relatively limited. Typically, PI materials cannot self-repair when cracks or micro-damage occur, which limits their long-term performance under high stress environments. PI stress relief layers mainly rely on the mechanical properties of the material itself to disperse stress, but they are prone to cracking or material fatigue under high stress conditions, leading to performance loss.

[0037] This invention prepares a stress-relieving layer solution. By embedding microcapsules and repair agents (such as epoxy resin), the repair agent can be automatically released to repair cracks or damage, thereby improving the fatigue resistance and long-term stability of the material.

[0038] 6. Regarding the stress buffer layer solution, the combination of microcapsules with the PI matrix, epoxy resin, and inorganic fillers enables the stress relief layer to not only provide good mechanical properties but also possess self-healing capabilities. This self-healing ability is achieved by the release of the repair agent (epoxy resin) within the microcapsules when stress or damage occurs. Compared to traditional stress relief layers, this design improves the service life and reliability of the material. Specifically, (1) the stress buffer layer solution includes microcapsules, the outer shell of which is formed by the combined action of PU and epoxy resin. The polyurethane outer shell provides mechanical strength and flexibility, while the cross-linking effect of the epoxy resin ensures the stability of the outer shell. The polyurethane outer shell formed by the reaction of polyether polyol and IPDI not only ensures the structural strength of the microcapsules but also provides adjustable elasticity, which helps to play a repair role after crack formation. The epoxy resin is used as a repair agent within the microcapsules and is released after the microcapsules rupture to perform repair. DETA promotes the curing of the epoxy resin, ensuring the durability and effectiveness of the repair agent. (2) Other components: Polyimide (PI) provides high-temperature resistance and electrical insulation, while epoxy resin enhances the coating's hardness and adhesion. Together, they provide comprehensive mechanical and chemical properties for the stress-relieving layer. The microcapsule shell material (polyurethane) and epoxy resin jointly construct the coating. The microcapsules enhance the coating's crack resistance by providing repair functions, while the epoxy resin enhances the overall structural strength of the coating. Alumina particles and nano-silica, two inorganic fillers, enhance the mechanical properties of the stress-relieving layer, improving its fatigue strength and crack resistance. Alumina improves the coating's toughness, while nano-silica enhances its hardness and corrosion resistance. Attached Figure Description

[0039] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 This is a schematic diagram of the SMA substrate with a memory surface after thermomechanical training according to the present invention. Figure 2 This is a schematic diagram of the structural process of the SMA substrate (temporary planar state) being flattened and fixed on a temporary carrier plate at low temperature according to the present invention; Figure 3 This is a schematic diagram of the planar structure of the metal needle tip of the present invention; Figure 4 This is a schematic diagram illustrating the structural process of how the SMA substrate carrying microstructures recovers to a preset three-dimensional curved surface shape after being unfixed and heated according to the present invention. Figure 5This is a schematic diagram of the structure of the curved metal needle tip structure obtained by separating the SMA substrate from the three-dimensional structure in this invention; Figure 6 This is a schematic diagram of the planar structure of the metal microbridge of the present invention; Figure 7 This is a schematic diagram illustrating the structural process of how the SMA substrate carrying microstructures recovers to a preset three-dimensional curved surface shape after being unfixed and heated according to the present invention. Figure 8 A schematic diagram of the structural process for separating the SMA substrate from the three-dimensional structure and releasing the sacrificial layer to obtain the curved metal microbridge structure of the present invention; Figure 9 Schematic diagram of different phases of SMA; Figure 10 This is a schematic diagram of the martensitic phase transformation temperature of SMA. Figure 11 Schematic diagram of shape memory effect The following are labeled in the figure: 1. SMA substrate; 2. Rigid carrier plate; 3. Temporary bonding adhesive; 4. Barrier layer; 5. Stress buffer layer; 6. Adhesive layer; 7. Metal needle tip structure; 8. Sacrificial layer; 9. Metal microbridge structure. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described below in conjunction with specific embodiments. Unless otherwise specified, the methods described are conventional methods, and the raw materials described are all available from publicly available commercial sources.

[0042] Any embodiment described herein as “exemplary” is not necessarily superior to other embodiments. Although various aspects of the embodiments are shown in the accompanying drawings, the drawings are not necessarily drawn to scale unless specifically indicated otherwise.

[0043] Method 1 A method for fabricating three-dimensional curved surface microstructures driven by phase transformation of shape memory alloys includes the following steps: S1: Thermomechanical training and heat treatment of shape memory alloy (SMA) substrates First, based on the target 3D curved surface microstructure and application scenario, a suitable shape memory metal sheet or film is selected as the smart substrate (such as NiTi alloy, whose phase transformation temperature can be precisely controlled between -50℃ and 200℃ through composition ratio). Next, through thermomechanical treatment (high-temperature annealing followed by quenching under specific fixture constraints), a predefined and precise 3D curved surface shape is given as its "memory" state (i.e., the austenitic parent phase state), resulting in the trained SMA substrate 1.

[0044] S2: Martensitic phase transformation and shape fixation The trained SMA substrate 1 was cooled to its martensitic phase transformation completion temperature (M). f Below this point, in this low-temperature martensitic phase, the material softens, and by applying external mechanical force, it can be stretched and flattened from its original three-dimensional curved shape into a temporary two-dimensional planar state. Using temporary bonding adhesive 3 or mechanical jigs, it is fixed to a rigid carrier plate 2 (such as a silicon wafer) in this planar state, resulting in an SMA substrate in a temporary planar state.

[0045] S3: Planar Micro / Nano Fabrication On the SMA substrate 1 in a temporary planar state, using a planar micro / nano fabrication process based on mechanical energy standards, the temperature throughout the entire fabrication process must be below the austenitic phase transformation initiation temperature (A) of the SMA. s This is to ensure that it remains planar. This includes, but is not limited to: Interface cleaning and activation: The SMA substrate 1 was subjected to rigorous in-situ plasma cleaning (such as Ar / O2) to thoroughly remove organic matter and weak boundary layers, and to activate the surface; Deposition of barrier layer 4 and stress buffer layer 5: On a fixed planar SMA substrate 1, a barrier layer 4 (such as Si3N4) is first deposited (PECVD can be used). The main function of this layer is to prevent atoms from diffusing into the SMA substrate 1 during subsequent processes, and at the same time to prevent elements such as Ni and Ti in the SMA substrate 1 from diffusing outward and contaminating the functional layer. For electronic devices, this provides electrical insulation. Next, a stress buffer layer 5 (such as diamond-like carbon DLC, polyimide PI, etc.) is deposited. This layer has a certain degree of flexibility and high fracture toughness, and can absorb and dissipate most of the strain energy generated during the SMA recovery process through its own micro-elastic deformation. Adhesive layer 6, deposited as a functional structure: This adhesive layer 6 (such as Cr, Ti, etc.) is directly in contact with the subsequent microstructure above. Metals such as Ti / Cr can form a strong bond with the upper metal layer, while also having good adhesion to the stress buffer layer 5 or dielectric layer below; Microstructure fabrication: The required planar micro and nanostructures are fabricated through photolithography, etching, and sacrificial layer techniques.

[0046] S4: Shape Restoration and 3D Shaping After completing the planar micro / nano fabrication, the fixation between the SMA substrate 1 and the temporary carrier plate is first released; then, the entire structure is heated to its austenite phase transformation completion temperature (A) using an external heat source (such as a hot stage, infrared radiation, or electric current Joule heating). f The above applies. When the SMA substrate 1 undergoes a martensitic-to-austenitic phase transformation, it generates enormous recovery stress, driving itself to precisely recover to the memory surface shape predefined in step S1. Simultaneously, the planar microstructures integrated on it are also loaded onto this three-dimensional surface, forming the final three-dimensional surface microstructure.

[0047] S5: Structural Release and Post-processing The PI layer is removed using an oxygen plasma dry method. If a sacrificial layer 8 is prepared, it also needs to be removed to achieve clean, controllable, and non-destructive separation of the three-dimensional curved surface microstructure from the SMA substrate 1.

[0048] After step S5, if necessary, the obtained device wafer can be subjected to performance testing and characterization.

[0049] Based on Method 1, specific implementations are carried out, and various different planar micro / nano structures are presented, as follows: It should be noted that specific parameters are provided for the purpose of comparing technical effects later. In actual use, appropriate parameters can be selected according to the needs.

[0050] Example 1 In this embodiment, a metal needle tip structure 9 is constructed.

[0051] A method for fabricating three-dimensional curved surface microstructures driven by phase transformation of shape memory alloys includes the following steps: S1: Thermomechanical training and heat treatment of shape memory alloy (SMA) substrates Firstly, a NiTi alloy with a phase transformation temperature control range of -50℃ to 200℃ was selected as the smart substrate, with a thickness of 50μm. Specifically, the phase transformation temperature range of the NiTi alloy was designed as A. s =40℃, A f =55℃, M s =30℃, M f =15℃.

[0052] Next, through thermomechanical treatment, specifically, a high-precision three-dimensional metal mold was used to constrain its curved surface. The mold material was selected as the high-temperature resistant Inconel 718 alloy. The mold was placed in a box furnace and heated to 500°C under the protection of inert argon gas, holding for 15 minutes. It was then rapidly quenched in room temperature water to complete the thermomechanical training. Finally, it was tempered at 300°C for 5 minutes to stabilize the crystal structure. After natural cooling to room temperature, an SMA substrate with shape memory was formed.

[0053] By applying a predefined, precise three-dimensional surface shape through thermomechanical processing as its memory state (i.e., the austenite parent phase state), a trained SMA substrate 1 is obtained. The results are referenced... Figure 1 .

[0054] S2: Martensitic phase transformation and shape fixation Cool the trained SMA substrate 1 to below 10°C and maintain for 10 minutes.

[0055] At low temperatures, the material softens, and by applying planar tensile stress, it can be stretched from its original three-dimensional curved shape into a temporary two-dimensional planar state. Temporary bonding adhesive 3 is used to fix it onto a rigid carrier 2 in this planar state. A silicon wafer is selected as the temporary carrier 2, resulting in an SMA substrate 1 in a temporary planar state. The results are referenced... Figure 2 .

[0056] S3: Planar Micro / Nano Fabrication On the SMA substrate 1, which is in a temporary planar state, a planar micro-nano fabrication process based on mechanical energy standards is used. The temperature of the entire fabrication process is controlled at <30℃ to ensure that it remains planar.

[0057] 1. Interface cleaning and activation: Using Ar / O2 plasma with a ratio of 3:1, a power of 100W, and a time of 60 seconds, the SMA substrate 1 was subjected to rigorous in-situ plasma cleaning to thoroughly remove organic matter and weak boundary layers and activate the surface.

[0058] 2. Deposition of Barrier Layer and Stress Buffer Layer: On a fixed planar SMA substrate 1, a barrier layer 4 of approximately 100 nm is first deposited using PECVD (Plasma Enhanced Chemical Vapor Deposition). The barrier layer 4 is made of Si3N4. The main function of this layer is to prevent atoms from diffusing into the SMA substrate 1 during subsequent processes, and to prevent elements such as Ni and Ti in the SMA substrate 1 from diffusing outwards and contaminating the functional layer. For electronic devices, this provides electrical insulation.

[0059] Next, a stress buffer layer 5 is coated, which is made of polyimide (PI). This layer has a certain degree of flexibility and high fracture toughness, and can absorb and dissipate most of the strain energy generated during the SMA recovery process through its own micro-elastic deformation.

[0060] 3. Deposition of the adhesion layer 6 as a functional structure: A 10nm layer of Ti (titanium) is deposited using PECVD as the adhesion layer 6. The adhesion layer 6 is a functional structure that directly contacts the subsequent microstructures above it. The Ti metal can form a strong bond with the upper metal layer, and at the same time has good adhesion to the stress buffer layer 5 below. Microstructure fabrication: A metal layer (Au, Pt, or Ni) with a thickness of approximately 100 nm is deposited on the adhesion layer. Next, photoresist is coated onto the surface, and exposure is performed using a grayscale mask. After development, a microneedle array pattern is formed (the opposite of the metal microneedle structure). Then, using the photoresist as a masking layer, the metal layer is etched. Finally, the photoresist is removed, forming a conical metal needle tip structure 7. In this embodiment, the planar micro / nano structure is the metal needle tip structure 7, and the results are referenced. Figure 3 .

[0061] S4: Shape Restoration and 3D Shaping After completing the planar micro-nano fabrication, first release the fixation between the SMA substrate 1 and the temporary carrier plate; the temporary carrier plate along with the structure can be placed on a hot stage for heating; soften the temporary bonding adhesive 3, so that the temporary bonding adhesive 3 and the rigid carrier plate 2 attached to it can be removed from the SMA substrate 1. After removal, the surface of the SMA substrate 1 can be wiped with IPA (isopropyl alcohol) to remove the residue of the temporary bonding adhesive 3. Next, the entire structure is heated to above 60°C using an external heat source (such as a hot stage, infrared radiation, or Joule heating). When the SMA substrate 1 undergoes a martensitic-to-austenitic phase transformation, it generates enormous recovery stress, driving it to precisely recover to the memory surface shape predefined in step S1. Simultaneously, the planar micro / nano structures integrated on it are also loaded onto this three-dimensional surface, forming the final three-dimensional surface microstructure. The result is referenced... Figure 4 .

[0062] S5: Structural Release and Post-processing The stress buffer layer 5 was removed using an oxygen plasma dry method, achieving clean, controllable, and non-destructive separation of the three-dimensional curved surface microstructure from the SMA substrate 1. The results are referenced... Figure 5 .

[0063] Example 2 In this embodiment, a metal microbridge structure is constructed.

[0064] A method for fabricating three-dimensional curved surface microstructures driven by phase transformation of shape memory alloys includes the following steps: S1: Thermomechanical training and heat treatment of shape memory alloy (SMA) substrates Firstly, a NiTi alloy with a phase transformation temperature control range of -50℃ to 200℃ was selected as the smart substrate, with a thickness of 50μm. Specifically, the phase transformation temperature range of the NiTi alloy was designed as A. s =40℃, A f =55℃, M s =30℃, M f =15℃.

[0065] Next, through thermomechanical treatment, specifically, a high-precision three-dimensional metal mold was used to constrain its curved surface. The mold material was selected as the high-temperature resistant Inconel 718 alloy. The mold was placed in a box furnace and heated to 500°C under the protection of inert argon gas, holding for 15 minutes. It was then rapidly quenched in room temperature water to complete the thermomechanical training. Finally, it was tempered at 300°C for 5 minutes to stabilize the crystal structure. After natural cooling to room temperature, an SMA substrate with shape memory was formed.

[0066] By applying a predefined, precise three-dimensional surface shape through thermomechanical processing as its memory state (i.e., the austenite parent phase state), a trained SMA substrate 1 is obtained. The results are referenced... Figure 1 .

[0067] S2: Martensitic phase transformation and shape fixation Cool the trained SMA substrate to below 10°C and maintain for 10 minutes.

[0068] At low temperatures, the material softens, and by applying planar tensile stress, it can be stretched from its original three-dimensional curved shape into a temporary two-dimensional planar state. Temporary bonding adhesive 3 is used to fix it onto a rigid carrier 2 in this planar state. A silicon wafer is selected as the temporary carrier 2, resulting in an SMA substrate 1 in a temporary planar state. The results are referenced... Figure 2 .

[0069] S3: Planar Micro / Nano Fabrication On the SMA substrate 1, which is in a temporary planar state, a planar micro-nano fabrication process based on mechanical energy standards is used. The temperature of the entire fabrication process is controlled at <30℃ to ensure that it remains planar.

[0070] 1. Interface cleaning and activation: Using Ar / O2 plasma with a ratio of 3:1, a power of 100W, and a time of 60 seconds, the SMA substrate 1 was subjected to rigorous in-situ plasma cleaning to thoroughly remove organic matter and weak boundary layers and activate the surface.

[0071] 2. Deposition of Barrier Layer 4 and Stress Buffer Layer 5: On a fixed planar SMA substrate 1, a barrier layer 4 of approximately 100 nm is first deposited using PECVD (Plasma Enhanced Chemical Vapor Deposition). The barrier layer 4 is made of Si3N4. The main function of this layer is to prevent atoms from diffusing into the SMA substrate 1 during subsequent processes, and to prevent elements such as Ni and Ti in the SMA substrate 1 from diffusing outwards and contaminating the functional layer. For electronic devices, this provides electrical insulation.

[0072] Next, a stress buffer layer 5 is coated, which is made of polyimide (PI). This layer has a certain degree of flexibility and high fracture toughness, and can absorb and dissipate most of the strain energy generated during the SMA recovery process through its own micro-elastic deformation.

[0073] 3. Deposition of the adhesion layer 6 as a functional structure: A 10nm layer of Ti (titanium) is deposited using PECVD as the adhesion layer 6. The adhesion layer 6 is a functional structure that directly contacts the subsequent microstructures above it. The Ti metal can form a strong bond with the upper metal layer, and at the same time has good adhesion to the stress buffer layer 5 below. Microstructure fabrication: A sacrificial layer 8 is fabricated on the adhesive layer 6. The sacrificial layer 8 is a strip structure and its width is smaller than that of the adhesive layer 6. A metal microbridge structure 9 is fabricated on the sacrificial layer 8.

[0074] Specifically, photoresist is coated onto the adhesion layer 6, and then exposed and developed to form the sacrificial layer 8. A metal, approximately 100 nm thick, is deposited on the sacrificial layer 8 using physical vapor deposition (PVD). The metal can be Ti / Au or Cr / Pt. The deposited metal covers the surface and both sides of the sacrificial layer 8, forming a microbridge structure 9. In this embodiment, the planar micro / nano structure is a metal microbridge structure 9, and the results are referenced. Figure 6 .

[0075] S4: Shape Restoration and 3D Shaping After completing the planar micro-nano fabrication, first release the fixation between the SMA substrate 1 and the temporary carrier plate; the temporary carrier plate along with the structure can be placed on a hot stage for heating; soften the temporary bonding adhesive 3, so that the temporary bonding adhesive 3 and the rigid carrier plate 2 attached to it can be removed from the SMA substrate 1. After removal, the surface of the SMA substrate can be wiped with IPA (isopropyl alcohol) to remove the residue of the temporary bonding adhesive 3. Next, the entire structure is heated to above 60°C using an external heat source (such as a hot stage, infrared radiation, or Joule heating). When the SMA substrate 1 undergoes a martensitic-to-austenitic phase transformation, it generates enormous recovery stress, driving it to precisely recover to the memory surface shape predefined in step S1. Simultaneously, the planar micro / nano structures integrated on it are also loaded onto this three-dimensional surface, forming the final three-dimensional surface microstructure. The result is referenced... Figure 7 .

[0076] S5: Structural Release and Post-processing The stress buffer layer 5 was removed using an oxygen plasma dry method to release the sacrificial layer 8. The structure was then immersed in a photoresist stripping solution to remove the sacrificial layer 8. This method achieves clean, controllable, and non-destructive separation of the three-dimensional curved surface microstructure from the SMA substrate 1. The results are referenced. Figure 8 .

[0077] Example 3 The specific steps for preparing microcapsules are as follows: Raw materials used: Isophorone diisocyanate: IPDI, available at BASF, Desmodur® I Polyether polyol: PEG-2000 Emulsifier: Tween 80 Diethylenetriamine: DETA Epoxy Resin: Purchase EPON TM Resin 828 Preparation using the above-mentioned raw materials: Under dry nitrogen protection, 4.4 g IPDI and 2.0 g polyether polyol were added to a reaction flask and stirred at 80 °C for 4 hours to obtain polyurethane prepolymer.

[0078] A polyurethane solution was obtained by dissolving polyurethane prepolymer and 0.5g epoxy resin in 131.1g acetone.

[0079] Add 1.0g of emulsifier and 0.2g of polyvinyl alcohol to 100ml of deionized water, stir well, and then gradually add polyurethane solution dropwise. Stir with a high-shear mixer at 8000rpm for 10 minutes to obtain the emulsion.

[0080] Add 1.0g DETA to the emulsion, stir and react at room temperature for 1 hour, react at 60℃ for 2 hours, then centrifuge at 4000rpm for 10 minutes, retain the precipitate, the precipitate is microcapsules, wash and store.

[0081] It should be noted that this embodiment uses interfacial polymerization to prepare microcapsules with a polyurethane-epoxy resin interpenetrating polymer network composite shell. An oil phase containing dissolved isocyanate-terminated polyurethane prepolymer and epoxy resin is emulsified under high shear in an aqueous phase containing an emulsifier to form uniform oil droplets. Subsequently, a water-soluble polyamine is added, which reacts rapidly with the isocyanate groups of the prepolymer at the oil-water interface to form a primary shell of polyurethane and polyurea. Simultaneously, the polyamine diffuses into the interior of the oil droplets, initiating cross-linking and curing of the epoxy resin. Ultimately, a thermodynamically stable interpenetrating polymer network is constructed within the microcapsule shell, which enhances the mechanical strength, thermal stability, and interfacial bonding force with the polymer matrix of the microcapsules.

[0082] Example 4 The specific steps for preparing the stress-relieving layer solution are as follows: Raw materials used: Polyimide (PI) N-Methylpyrrolidone: NMP Microcapsules: Polyurethane (PU) microcapsules, using the microcapsules prepared in Example 3; Epoxy Resin: Purchase EPON TM Resin 828 Alumina particles: α phase, 5μm, AKP-30 Sumitomo Chemical Nano silica: Purchase CAB-O-SIL M-5 Preparation using the above-mentioned raw materials: Dissolve 10g of PI in 90ml of NMP solvent and stir until homogeneous to obtain a PI solution.

[0083] 1.5g of microcapsules and 0.5g of epoxy resin were ultrasonically dispersed in a PI solution for 30 minutes at 300W. Then, 0.8g of alumina particles and 0.2g of nano-silica were added and mechanically stirred at 500rpm for 30 minutes to obtain a stress-relieving layer solution.

[0084] Example 5 The steps that differ from those in Example 1 are as follows: Next, a stress buffer layer 5 is coated. The material of the stress buffer layer 5 is the stress relief layer solution of Example 4, and it is heat-cured at 80°C for 1-2 hours. This layer has a certain degree of flexibility and high fracture toughness, and can absorb and dissipate most of the strain energy generated during the SMA recovery process through its own micro-elastic deformation.

[0085] The remaining steps are the same as in Example 1.

[0086] Example 6 The steps that differ from those in Example 2 are as follows: Next, a stress buffer layer 5 is coated. The material of the stress buffer layer 5 is the stress relief layer solution of Example 4, and it is heat-cured at 80°C for 1-2 hours. This layer has a certain degree of flexibility and high fracture toughness, and can absorb and dissipate most of the strain energy generated during the SMA recovery process through its own micro-elastic deformation.

[0087] The remaining steps are the same as in Example 2.

[0088] Analysis: In the embodiment, the materials of the stress buffer layer 5 are divided into PI and stress relief layer solution.

[0089] Polyimide (PI) is a widely used material with good thermal stability, mechanical strength, and electrical insulation, but its performance in stress dispersion and self-healing ability is relatively limited. Typically, PI materials cannot self-repair when cracks or micro-damage occur, which limits their long-term performance under high-stress environments.

[0090] PI stress relief layers mainly rely on the mechanical properties of the material itself to disperse stress, but they are prone to cracking or material fatigue under high stress conditions, which can lead to performance loss.

[0091] Stress-relieving layer solutions, by embedding microcapsules and repair agents (such as epoxy resin), can automatically release repair agents to repair cracks or damage when they occur, thereby improving the fatigue resistance and long-term stability of materials.

[0092] The following experiment will verify this: PI (Kapton® PI) and the stress relief layer solution prepared in Example 4 were coated on Si3N4 substrates respectively, and cured into coatings. The coating thickness was controlled to be 50 μm, and two sets of samples were made, with the sample size being 50 mm × 10 mm.

[0093] Two groups of samples were exposed to a high temperature and high humidity environment (70℃, 95% humidity) for 1000 hours to test the changes in the mechanical properties of the materials.

[0094] Tensile strength was tested using a universal testing machine with a tensile rate of 5 mm / min, in accordance with GB / T 1040.1-2018.

[0095] The coating hardness was tested using a Vickers hardness tester. The load was set to 100g, and the load was maintained for 10-15 seconds before being released, in accordance with GB / T 4340.1-2017.

[0096] The result is: PI sample: tensile strength is 140MPa, hardness is 330HV.

[0097] Stress relief layer solution sample: tensile strength 150MPa, hardness 350HV.

[0098] In comparison, the stress-relieving layer solution sample performed better.

[0099] In summary, the specific embodiments and comparative examples described above are merely for clearly illustrating the present invention and should not be construed as limiting the present invention. Those skilled in the art should understand that various equivalent substitutions, modifications, changes, or improvements can be made to the technical solutions and implementation methods of the present invention without departing from the spirit and scope of the invention, and all such changes or improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A method for preparing a three-dimensional curved microstructure based on shape memory alloy phase transition driving, characterized in that, The method comprises the following steps: a hot mechanical treatment is performed on the substrate to impart a three-dimensional curved surface memory state, and a trained SMA substrate is obtained; the trained SMA substrate is cooled to below the temperature at which the martensite phase transformation is completed, an external mechanical force is applied to stretch and flatten the three-dimensional curved surface shape from the memory state into a temporary two-dimensional planar state, and then the SMA substrate in the temporary planar state is fixed on a rigid carrier plate to obtain the SMA substrate in the temporary planar state; a planar micro-nano processing technology is used to process the SMA substrate in the temporary planar state, and then the SMA substrate is restored to the three-dimensional curved surface shape of the memory state, and the excess structure is removed to obtain a three-dimensional curved surface microstructure; the planar micro-nano processing technology comprises one or more of interface cleaning and activation, deposition of a barrier layer and a stress buffer layer, deposition of an adhesion layer, and microstructure preparation.

2. The method according to claim 1, wherein the method is characterized by: The substrate comprises a shape memory metal sheet or a shape memory metal film, and the shape memory metal comprises Nitinol alloy; the material of the barrier layer comprises Si3N4; the material of the stress buffer layer comprises one or more of DLC, PI, and a stress buffer layer solution; and the material of the adhesion layer comprises Cr and / or Ti.

3. The method according to claim 1, wherein the method is characterized by: The hot mechanical treatment comprises high-temperature annealing followed by quenching under the constraint of a clamp.

4. The method according to claim 1, wherein, The memory state is an austenite parent phase state, and the temperature of the entire processing process of the planar micro-nano processing technology is lower than the austenite phase transformation start temperature of the SMA substrate.

5. The method according to claim 1, wherein the method is characterized by: The interface cleaning and activation comprises the step of in-situ plasma cleaning of the SMA substrate.

6. The method according to claim 1, wherein, The microstructure preparation comprises the step of preparing a required planar micro-nano structure by means of photolithography and / or a sacrificial layer and / or etching technology, and the planar micro-nano structure comprises a metal needle tip structure and / or a metal micro-bridge structure.

7. The method according to claim 1, wherein the method is characterized by: The restoration to the three-dimensional curved surface shape of the memory state comprises the steps of removing the rigid carrier plate, heating the entire structure to ≥60℃ by means of an external heat source, causing the SMA substrate to undergo a phase transformation from martensite to austenite, restoring the SMA substrate to the originally imparted three-dimensional curved surface memory state, and loading the structure on the SMA substrate onto the three-dimensional curved surface to form a final three-dimensional curved surface microstructure.

8. The method according to claim 1, wherein, The removal of the excess structure comprises leaving only the adhesion layer and the microstructure.

9. The method according to claim 2, wherein the method is characterized by: The preparation method of the stress buffer layer solution comprises the steps of: The microcapsules and epoxy resin are ultrasonically dispersed in a PI solution, and then alumina particles and nanosilica are added and mixed by mechanical stirring to obtain a stress buffer layer solution.

10. The method according to claim 9, wherein the method is characterized by: The preparation method of the microcapsules comprises the steps of: IPDI and polyether polyol are stirred and reacted under dry nitrogen protection to obtain a polyurethane prepolymer; The polyurethane prepolymer and epoxy resin are dissolved in acetone to obtain a polyurethane solution; An emulsifier and polyvinyl alcohol are added to deionized water and stirred uniformly, and then the polyurethane solution is gradually added dropwise, and stirring is continued to obtain an emulsion; DETA is added to the emulsion, and after stirring and reaction, the precipitate is retained by centrifugation, and the precipitate is the microcapsules.

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