A method for preparing a three-dimensional micro-nano structure based on MEMS

Abstract

The application provides a three-dimensional micro-nano structure preparation method based on MEMS. The preparation method comprises the following steps: providing a substrate layer, depositing and patterning a sacrificial layer and a stress matching layer, arranging a functional layer on the stress matching layer, spin-coating a polyimide precursor, and then performing first heat treatment to form a non-fully cured polyimide layer and patterning, removing the sacrificial layer to make the composite film layer self-curl, and then performing second high-temperature heat treatment to fine-tune the curvature to a preset value. The three-dimensional micro-nano structure comprises a stress matching layer for providing compressive stress, a functional layer for realizing specific functions, and a polyimide layer for regulating curling. The application has the advantages of accurate curvature regulation, compatibility with MEMS and CMOS processes, batch manufacturing, adaptation to multiple scenes, improvement of product yield, solution to the pain points of traditional technologies, and wide application in the fields of on-chip energy storage, radio frequency communication, biological medicine, optical sensing, micro-fluid control and the like.

Description

Technical Field

[0001] This invention relates to the field of three-dimensional micro-nano device technology, and more specifically, to a method for fabricating three-dimensional micro-nano structures based on MEMS. Background Technology

[0002] Three-dimensional micro- and nanostructures are core elements driving technological innovation in microelectronics, optoelectronics, biomedicine, energy storage, and other fields. Their fabrication precision and application flexibility directly impact the performance improvement of related devices. Microelectromechanical systems (MEMS) technology, with its advantages of miniaturization, low power consumption, and mass production capabilities, has become the mainstream fabrication method for three-dimensional micro- and nanostructures. However, mainstream MEMS technology is still primarily limited to two-dimensional planar fabrication, creating performance bottlenecks in several cutting-edge fields such as on-chip energy storage, radio frequency communication, biomedicine, and optical sensing.

[0003] In existing 3D forming technologies, 3D printing precision is difficult to achieve at the nanometer level and is incompatible with complementary metal-oxide-semiconductor (CMOS) processes. Self-rolling technology achieves autonomous transformation from two-dimensional planes to three-dimensional structures by constructing thin-film structures with interlayer stress mismatch. However, existing self-rolling technologies still have many limitations in practical applications. First, their curvature control precision is insufficient. Traditional technologies mostly rely on differences in film thickness or inherent material stress to achieve rolling, resulting in a fixed structural shape after forming. The lack of effective fine-tuning methods leads to uneven rolling density and large gaps, making it difficult to meet the requirements of high-precision applications. Second, their application adaptability is limited. Traditional technologies are mostly designed for specific scenarios, requiring different process platforms for different functional needs, making it difficult to achieve universal fabrication for multiple scenarios.

[0004] Therefore, there is an urgent need for a MEMS-based three-dimensional micro / nano structure and its fabrication method that is process-compatible, has precise and controllable curvature, is highly efficient in fabrication, and is suitable for multiple application scenarios, in order to solve the problems of uncorrectable curvature, poor process compatibility, and limited applicability in existing technologies. Summary of the Invention

[0005] Based on existing technologies, the objective of this invention is to provide a method for fabricating three-dimensional micro / nano structures using MEMS, which can solve problems such as uncorrectable curvature, poor process compatibility, and limited applicability of three-dimensional micro / nano structures in existing technologies, and promote the large-scale application of three-dimensional micro / nano structures in fields such as high-precision sensing, on-chip energy storage, and radio frequency communication.

[0006] According to the present invention, the above-mentioned task is solved by a MEMS-based method for fabricating three-dimensional micro / nano structures.

[0007] In a first aspect, the present invention provides a method for fabricating three-dimensional micro / nano structures based on MEMS, comprising: A sacrificial layer is deposited on the substrate layer, and the sacrificial layer is patterned. A stress-matching layer is deposited on the patterned sacrificial layer, and the stress-matching layer is patterned. The functional layers are arranged on the graphical stress matching layer; After applying a polyimide precursor to the functional layer and the stress matching layer, a first heat treatment is performed to allow the polyimide to partially cure and shrink to form a polyimide layer. The polyimide layer is then patterned. The sacrificial layer is removed, allowing the stress-matching layer, functional layer, and polyimide layer to automatically curl up due to stress mismatch to form a three-dimensional micro / nano structure. The polyimide layer undergoes a second heat treatment, driving it to further shrink until fully cured, thereby correcting the radius of curvature of the three-dimensional micro / nano structure to a preset value, wherein: The temperature of the second heat treatment is higher than the temperature of the first heat treatment.

[0008] Furthermore, the method also includes: A polyimide film is spin-coated and cured onto the substrate layer as a flexible substrate layer.

[0009] Furthermore, the graphical processing includes: A pattern layer is formed on the surface of the layer to be treated using photoresist; The layer to be processed, which is not protected by the pattern layer, is removed by etching; and The patterned layer is removed by ultrasonic removal using a removal solution; The layer to be processed is the sacrificial layer, stress-matching layer, or polyimide layer.

[0010] Furthermore, the photoresist is a UV-sensitive positive photoresist.

[0011] Furthermore, the arrangement of the functional layer on the stress-matching layer includes: Depending on the target application scenario, a functional layer is formed by at least one of the following processes: photolithography-evaporation-stripping, sputtering-etching, wet transfer, and solution coating. The functional layer is used to realize at least one of the following: biosensing, magnetic sensing, optical sensing, on-chip energy storage, radio frequency communication, and microfluidics.

[0012] Furthermore: The temperature of the first heat treatment is set to 120°C to 300°C, and the heating time is set to 1 hour to 3 hours. The temperature of the second heat treatment is set to be higher than the temperature of the first heat treatment but not higher than 300°C, and the heating time of the second heat treatment is controlled until the radius of curvature of the three-dimensional micro / nano structure reaches a preset value.

[0013] Furthermore, controlling the heating time of the second heat treatment includes: During the heat treatment process, the radius of curvature of the three-dimensional micro / nano structure is monitored in real time, and the heating time of the second heat treatment is controlled based on the obtained radius of curvature.

[0014] The present invention also proposes a three-dimensional micro / nano structure based on MEMS, fabricated according to the method proposed in the first aspect of the present invention, wherein the three-dimensional micro / nano structure comprises: A stress-matching layer, configured to provide basic compressive stress, forms a stress mismatch with the polyimide layer, providing power for the self-curling of the three-dimensional micro / nanostructure; A functional layer, disposed on the stress-matching layer, is configured to implement corresponding functions according to the target application scenario, the functions including at least one of biosensing, magnetic sensing, optical sensing, on-chip energy storage, radio frequency communication, and microfluidics; and A polyimide layer is disposed on the functional layer and the stress matching layer, and cooperating with the stress matching layer to encapsulate the functional layer. The polyimide layer is configured to drive the three-dimensional micro / nanostructure to self-roll and adjust the radius of curvature of the three-dimensional micro / nanostructure.

[0015] Furthermore, the stress matching layer includes a silicon oxide layer with a thickness of 500 nm to 3 μm.

[0016] Furthermore, the functional layer includes at least one of the following: an electrode structure, a membrane stack structure, a graphene layer, an energy storage stack structure, a metal wire structure, and a resistance wire structure.

[0017] The present invention proposes a method for fabricating three-dimensional micro / nano structures based on MEMS, which has at least the following beneficial effects: (1) The method described in this invention utilizes the chemical curing kinetics of polyimide materials. With the film thickness remaining constant, the material modulus and stress can be adjusted over a wide range by simply changing the heat treatment temperature, thereby accurately setting the curl radius.

[0018] (2) The method described in this invention achieves precise control of the curvature radius of the three-dimensional micro-nano structure through two heat treatments of the polyimide layer. After the three-dimensional micro-nano structure is formed by the first heat treatment, it can be finely adjusted by the second heating, which effectively improves the structural size accuracy and yield, and solves the problem that the shape cannot be corrected after the traditional self-curling technology is formed.

[0019] (3) The entire process of the method described in this invention is fully compatible with standard microelectromechanical systems (MEMS) processes and complementary metal-oxide-semiconductor (CMOS) processes, and has the potential for large-scale wafer-level manufacturing, reducing production costs and integration difficulties. At the same time, the process platform has good versatility, and can be adapted to different application requirements by replacing the functionally sensitive layer, without the need to design special processes for specific scenarios.

[0020] (4) The three-dimensional micro-nano structure described in this invention has achieved breakthroughs in various high-performance devices: In the field of on-chip energy storage, a multi-layer concentric cylindrical structure in the form of a "Swiss roll" can be used to achieve an energy density several times that of a planar battery in a small footprint; In the field of radio frequency, a three-dimensional air-core solenoid inductor can be fabricated to reduce substrate loss and significantly improve the quality factor (Q value); In the field of biomedicine, flexible electrodes can be used to adaptively wrap around micro nerves or organoids, solving the problem of poor contact of planar electrodes.

[0021] In summary, the MEMS-based three-dimensional micro / nanostructure fabrication method proposed in this invention solves the problems of uncorrectable curvature, poor process compatibility, and limited applicability in existing technologies. It can be widely applied in on-chip energy storage to manufacture miniature lithium batteries or supercapacitors; in radio frequency communication to fabricate hollow inductors or tubular antennas; in biomedicine to construct flexible neural cuff electrodes or organoid encapsulation structures; in optical sensing to form high-quality whispering-gallery mode optical resonators; and in microfluidics to realize micropump or microvalve functions, providing strong support for performance improvement and integrated applications of high-end devices in multiple fields. Attached Figure Description

[0022] To further illustrate the advantages and other features of the various embodiments of the present invention, a more specific description of the embodiments of the present invention will be presented with reference to the accompanying drawings. It is understood that these drawings depict only typical embodiments of the invention and are therefore not intended to limit its scope. In the drawings, identical or corresponding parts will be indicated by the same or similar reference numerals for clarity.

[0023] Figure 1 A flowchart of the method described in this invention is shown.

[0024] Figure 2 A process flow diagram of the method described in this invention is shown.

[0025] Figure 3 A schematic diagram illustrating the principle of polyimide-controlled curvature of three-dimensional micro / nano structures is shown.

[0026] Figure 4 The results of the nanoindentation test are shown.

[0027] Figure 5The curve showing the relationship between curing temperature and Young's modulus of polyimide is presented.

[0028] Figure 6 The infrared spectra of polyimide after heat treatment and curing at different temperatures are shown.

[0029] Figure 7 Optical photographs of the three-dimensional electrode manufactured by the method described in this invention before and after the second heat treatment are shown.

[0030] Figure 8 An optical photograph of a multilayer Swiss roll three-dimensional micro / nano structure fabricated by the method described in this invention is shown.

[0031] List of reference numerals 100 Three-dimensional micro / nano structures 101 Basal layer 102 Sacrificial Layer 103 Stress Matching Layer 104 Functional Layer 105 Polyimide layer Detailed Implementation It should be noted that the components in the various figures may be shown exaggeratedly for illustrative purposes and are not necessarily to scale. In each figure, the same reference numerals are used for components that are identical or have the same function.

[0032] In this invention, the various embodiments are merely intended to illustrate the solutions of the invention and should not be construed as limiting.

[0033] In this invention, unless otherwise specified, the quantifiers “a” and “one” do not exclude scenarios involving multiple elements.

[0034] It should also be noted that, in the embodiments of the present invention, only a portion of the components or parts may be shown for clarity and simplicity. However, those skilled in the art will understand that, under the teachings of the present invention, necessary components or parts can be added as needed for specific scenarios. Furthermore, unless otherwise stated, features in different embodiments of the present invention can be combined with each other. For example, a feature in the second embodiment can replace a corresponding or functionally identical or similar feature in the first embodiment, and the resulting embodiment will also fall within the scope of disclosure or description of this application.

[0035] In this invention, the term "PECVD process" refers to plasma-enhanced chemical vapor deposition, a commonly used thin film deposition technology in the semiconductor industry. This technology combines the basic principles of chemical vapor deposition with plasma technology, enabling the production of high-quality thin films with precise control over their properties.

[0036] Furthermore, the steps of the methods of the present invention are not limited in terms of the execution order of the method steps. Unless otherwise specified, the method steps may be executed in different orders.

[0037] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0038] Figure 1 A flowchart of the method described in this invention is shown. Figure 2 A process flow diagram of the method described in this invention is shown.

[0039] This invention proposes a method for fabricating three-dimensional micro / nano structures based on MEMS. In one embodiment of this invention, such as... Figure 1 and Figure 2 As shown, the method includes the following steps: Step S100, Substrate Treatment: as follows Figure 2 As shown in (a), the substrate material is cleaned to remove surface contaminants, serving as the substrate layer 101 of the three-dimensional micro / nano structure 100. The substrate material includes a silicon wafer or a glass substrate. In embodiments of the present invention, depending on the actual application scenario, a polyimide (PI) film is spin-coated and cured onto the substrate layer 101 as a flexible substrate if necessary.

[0040] Step S200, Sacrificial layer preparation: as follows Figure 2 As shown in (b), a sacrificial layer 102 is deposited on the substrate layer 101, and the sacrificial layer 102 is patterned. In a specific embodiment of the present invention, a copper (Cu) sacrificial layer 102 with a thickness of 100 nm to 500 nm is deposited on the substrate layer 101 using magnetron sputtering. A pattern layer with a thickness of 3 μm is patterned on the surface of the sacrificial layer 102 using ultraviolet photolithography with positive photoresist. Subsequently, the sacrificial layer 102 not protected by photoresist is removed by dry etching with an ion beam, and the photoresist is removed by ultrasonic removal with acetone to complete the patterning process of the sacrificial layer 102.

[0041] Step S300, stress matching layer preparation: as follows Figure 2 (c) and Figure 2 As shown in (d), a stress-matching layer 103 with compressive stress is deposited on the patterned sacrificial layer 102, and the stress-matching layer 103 is patterned. In a specific embodiment of the present invention, a silicon oxide (SiO2) layer with compressive stress is deposited as the stress-matching layer 103 using a PECVD process, and its thickness is set to 500 nm to 3 μm. Subsequently, the stress-matching layer 103 not protected by photoresist is removed by plasma etching, and the photoresist is removed by ultrasonic removal with acetone to complete the patterning of the stress-matching layer 103.

[0042] Step S400, Functional layer processing: such as Figure 2As shown in (e), the functional layer 104 is arranged on the patterned stress-matching layer 103. In embodiments of the present invention, different fabrication processes can be used to process the functional layer 104 depending on the application scenario.

[0043] In one specific embodiment of the present invention, at the biosensing application level, a positive UV photoresist is spin-coated onto the stress matching layer 103 and pre-baked. A mask with a pre-defined pattern is used for UV exposure, followed by treatment with a developing solution to form a patterned protective layer of photoresist. A Ti transition layer and an Au functional layer are sequentially deposited using a vacuum evaporation process, wherein the Ti layer is used to enhance the adhesion between the Au layer and the stress matching layer 103. After evaporation, a stripping solution is used to remove the photoresist and its surface-attached metal layers, retaining the Au / Ti structure in the photoresist-protected area, forming the microelectrodes and leads required for biosensing.

[0044] In one specific embodiment of the present invention, at the magnetic sensing application level, a multilayer film with giant magnetoresistive (GMR) film is deposited on the surface of stress-matching layer 103 using magnetron sputtering. After the film deposition is completed, a positive photoresist is spin-coated using ultraviolet light, and after exposure and development, a photoresist protective layer corresponding to the Wheatstone bridge structure is formed. The GMR multilayer film not protected by photoresist is selectively removed using an ion beam etching process. After etching, the photoresist is removed by ultrasonic cleaning with an organic solvent to obtain the Wheatstone bridge structure for the magnetic sensor.

[0045] In one specific embodiment of the present invention, at the photosensitive application level, graphene grown by chemical vapor deposition (CVD) is applied to the surface of the stress-matching layer 103 using a wet transfer process to ensure that the graphene and the stress-matching layer are tightly bonded and wrinkle-free. Subsequently, a positive photoresist is spin-coated using ultraviolet light and patterned. Using the photoresist as a protective layer, an oxygen plasma etching process is used to remove the unprotected graphene areas. After etching, the photoresist is removed to obtain the patterned graphene layer required for photosensitive applications.

[0046] In one specific embodiment of the present invention, at the on-chip energy storage application level, a metal current collector layer is deposited on the surface of the stress matching layer 103 using a magnetron sputtering process, followed by the deposition of a positive electrode material layer. After the positive electrode material layer is prepared, a solid electrolyte layer is laid down using a solution coating process and then cured. Finally, a negative electrode material layer is deposited by magnetron sputtering, forming an energy storage functional stacked structure in which the current collector, positive electrode material, solid electrolyte, and negative electrode material are sequentially stacked.

[0047] In one specific embodiment of the present invention, at the radio frequency communication application level, a positive photoresist is spin-coated onto the surface of the stress matching layer 103 using ultraviolet light and patterned to form a protective layer corresponding to the metal wire pattern on the surface of the stress matching layer. A Cu or Au metal layer is deposited using a magnetron sputtering process, with the metal layer thickness set according to the radio frequency transmission requirements. After deposition, the sample is subjected to ultrasonic lift-off to remove the photoresist and excess metal from the surface, retaining the patterned parallel metal wires as the core functional layer of the radio frequency device.

[0048] In one specific embodiment of the present invention, at the microfluidic application level, a photoresist pattern corresponding to the heating resistance wire is formed on the surface of the stress matching layer 103 using a photolithography process. A metal resistive material layer is deposited using a magnetron sputtering process, and then annealed to stabilize the resistive performance. Subsequently, the metal layer not protected by the photoresist is removed by wet etching, retaining the metal structure in the photoresist-covered area, forming the functional layer of the heating resistance wire required for microfluidics. The diameter of this resistance wire can be adjusted by temperature changes.

[0049] Step S500, polyimide layer spin coating and gradient curing: as follows Figure 2 As shown in (f), a photosensitive or non-photosensitive polyimide (PI) precursor is spin-coated onto the surfaces of the functional layer 104 and the stress-matching layer 103. The entire sample is then subjected to a first heat treatment (pre-curing) to bring the PI to a preset incompletely cured state, resulting in a polyimide layer 105. The temperature of the first heat treatment ranges from 120°C to 300°C, and the treatment time ranges from 1 hour to 3 hours. Subsequently, a 3μm patterned layer is patterned using UV lithography with positive photoresist. After selectively removing the unprotected polyimide layer 105 by plasma etching, the photoresist is removed using acetone ultrasonication.

[0050] Step S600, Sacrificial layer release: as follows Figure 2 (g) and Figure 2 As shown in (h), the sacrificial layer 102 is removed, allowing the stress-matching layer 103, functional layer 104, and polyimide layer 105 to automatically curl up due to stress mismatch, forming a three-dimensional micro / nano structure 100. In a specific embodiment of the present invention, a copper sacrificial layer 102 is removed using a copper etching solution.

[0051] Step S700, Secondary Kinetic Fine-tuning: A second heat treatment is performed on the incompletely cured three-dimensional micro / nano structure 100 at a temperature higher than the first heat treatment but not exceeding 300°C. By controlling the heating time, the polyimide layer 105 is driven to further shrink, correcting the radius of curvature of the three-dimensional micro / nano structure 100 to a preset value. In a specific embodiment of the present invention, the heating time is controlled in 1-minute increments during the second heat treatment until the radius of curvature of the three-dimensional micro / nano structure 100 reaches the preset value. The radius of curvature is observed and measured in real-time using an optical microscope.

[0052] Figure 3 A schematic diagram showing the curvature of a three-dimensional micro / nano structure controlled by polyimide is shown.

[0053] like Figure 3 As shown, as the heat treatment temperature increases, the polyimide layer 105 solidifies and shrinks, resulting in an increase in the curvature of the three-dimensional micro / nano structure 100. The principle behind this is further explained below.

[0054] The initial state of the polyimide precursor is a solvent-containing polymer system with randomly coiled molecular chains, weak intermolecular forces, and a large amount of free volume. During the first heat treatment, the solvent in the precursor gradually evaporates, and the active groups on the molecular chains undergo preliminary cross-linking reactions, transforming the molecular chains from a loose and disordered state to a partially ordered cross-linked structure. During this process, the distance between molecular chains shortens, and the free volume decreases, causing the polyimide to undergo initial volume shrinkage; simultaneously, the formation of the cross-linked structure increases the rigidity of the molecular chains. In the second heat treatment stage, the higher temperature provides sufficient energy for the molecular chain movement, prompting the unreacted active groups to further cross-link, forming a denser three-dimensional network structure with a higher cross-linking density. As the cross-linking reaction deepens, the molecular chains become more regularly and tightly arranged, the free volume is further compressed, and the polyimide layer undergoes secondary shrinkage; the perfection of the three-dimensional network structure makes relative slippage of the molecular chains difficult, marking the complete curing of the polyimide layer.

[0055] Figure 4 The results of the nanoindentation test are shown in the figure.

[0056] like Figure 4 As shown, the nanoindentation test accurately measures the mechanical interaction between the indenter and polyimide treated at different curing temperatures, reflecting the load-displacement curves of polyimide treated at different curing temperatures, and calculates Young's modulus by combining the contact mechanics model.

[0057] Figure 5 The curve showing the relationship between curing temperature and Young's modulus of polyimide is presented.

[0058] The trend of Young's modulus test results is highly synchronized with the shrinkage process of the polyimide layer: as the heat treatment (curing) temperature increases, the Young's modulus increases from low to high, corresponding to an increase in the crosslinking density of the polyimide molecular chains. This increase in crosslinking density directly drives a continuous reduction in the free volume between molecular chains, ultimately resulting in the gradual shrinkage of the polyimide. This shrinkage is controllable; by adjusting the heat treatment temperature, the final value of the Young's modulus can be precisely controlled. In the three-dimensional micro / nano structure 100, the Young's modulus determines the stress transfer efficiency and deformation resistance of the polyimide layer 105. Furthermore, by controlling the interlayer stress balance, the curl curvature of the composite film layer can be regulated, thereby controlling the degree of shrinkage of the polyimide layer 105 and providing a precise basis for controlling the curl curvature of the three-dimensional micro / nano structure 100.

[0059] The following further explains the principle of how the stress-matching layer 103, the functional layer 104, and the polyimide layer 105 automatically curl up to form a three-dimensional micro / nano structure 100 through stress mismatch, and how the radius of curvature of the three-dimensional micro / nano structure 100 is precisely controlled through a second heat treatment.

[0060] In one embodiment of the present invention, the core principle of forming the three-dimensional micro / nano structure 100 is the release of mechanical equilibrium caused by interlayer stress mismatch, as detailed below: When the silica stress-matching layer 103 is deposited using the PECVD process, inherent compressive stress is generated during its atomic stacking process. This stress causes the stress-matching layer 103 to have a natural tendency to shrink inward. After the first heat treatment (pre-curing), the polyimide layer 105 undergoes partial cross-linking of its molecular chains, accompanied by volume shrinkage, which in turn generates tensile stress that stretches outward. Since the stress-matching layer 103 and the polyimide layer 105 are firmly bonded as a whole through the previous process, and the stress directions of the two layers are opposite and their magnitudes are unequal, a stable stress constraint state is formed. That is, the shrinkage tendency of the stress-matching layer 103 is limited by the stretching effect of the polyimide layer 105, and the stretching tendency of the polyimide layer 105 is also constrained by the shrinkage effect of the stress-matching layer 103. At this time, the overall structure remains planar under the support of the sacrificial layer 102. After the sacrificial layer 102 is removed, the supporting effect of the base layer 101 on the composite film composed of the stress-matching layer 103, the functional layer 104, and the polyimide layer 105 disappears. The originally constrained interlayer stress imbalance loses its external support and constraint. To achieve a new mechanical equilibrium, the composite film must release its internal stress through morphological deformation. Since the two layers are tightly bonded and cannot be separated, the stress release process can only be achieved through bending and curling: the compressive stress of the stress-matching layer 103 drives the film to shrink inward, while the tensile stress of the polyimide layer 105 drives the film to extend outward. The combined force of the two causes the composite film to bend towards the stress-dominant side, ultimately forming a stable three-dimensional curled structure. Throughout the process, the degree of stress mismatch determines the initial state of the curl curvature. The subsequent second heat treatment further solidifies and shrinks the polyimide layer 105, adjusting the stress magnitude of the polyimide layer 105 and further precisely correcting the curled shape, so that the three-dimensional micro / nano structure 100 achieves the designed three-dimensional configuration.

[0061] Figure 6 The infrared spectra of polyimide after heat treatment and curing at different temperatures are shown.

[0062] In one embodiment of the present invention, fully cured polyimide and incompletely cured polyimide have different characteristic peaks, and the relative heights of the characteristic peaks change with the degree of curing. Therefore, the degree of curing of the polyimide layer 105 in the three-dimensional micro / nano structure 100 can be determined by analyzing the infrared spectral characteristic peaks of polyimide treated under different heat treatment conditions. Figure 6 As shown, the infrared characteristic peaks of polyimides cured by heat treatment at 150℃, 175℃, 200℃, and 300℃ exhibit significant differences, specifically in the following aspects: The imidization of polyimide exhibits two characteristic peaks, one at 1780 cm⁻¹. -1 The C=O symmetric and asymmetric stretching vibration peaks (Imide I peak) and 1380 cm⁻¹ -1The CNC axial stretching vibration peak (Imide II peak) is observed at the location. As the heat treatment temperature increases, the degree of curing of polyimide gradually improves, and the relative peak heights of the two imidization characteristic peaks mentioned above also continue to increase.

[0063] The method flow of the present invention will be further illustrated below through a specific embodiment.

[0064] In a specific embodiment of the present invention, the preparation of a flexible neural cuff electrode suitable for the biomedical field using the method described in the present invention includes the following steps: Substrate preparation: Silicon wafers were selected as the substrate, and conventional semiconductor cleaning processes were used to remove oil, oxide layers, and impurities from the substrate surface, ensuring a clean and smooth surface. Subsequently, a polyimide precursor was spin-coated onto the clean silicon substrate, which was then placed in a heating device and cured at 300°C for 2 hours to form a 20μm thick flexible polyimide substrate layer, providing stable support for subsequent functional layers.

[0065] Sacrificial layer preparation: A copper sacrificial layer was deposited on the surface of the flexible substrate using magnetron sputtering, with deposition parameters controlled to achieve a sacrificial layer thickness of 150 nm. Next, a positive UV photoresist AZ4330 was spin-coated, followed by pre-baking, exposure, and development to form a 3 μm thick patterned layer corresponding to the pre-defined retention area of ​​the sacrificial layer. Subsequently, the sample was etched using an ion beam dry etching process for 4 min to selectively remove the copper layer portions not protected by the photoresist. After etching, the sample was immersed in an acetone solution for ultrasonic cleaning for 10 min to remove residual photoresist, yielding the patterned copper sacrificial layer.

[0066] Compressive stress layer preparation: A silicon oxide layer was deposited on the patterned copper sacrificial layer using PECVD for 4 min 37 s, forming a 500 nm thick silicon oxide stress-matching layer with compressive stress. This layer serves both as insulation and stress-providing layer. Subsequently, a positive photoresist was spin-coated using UV lithography, and a 3 μm thick photoresist protective layer was formed after patterning. The unprotected silicon oxide layer was then selectively etched using plasma etching for 90 s. After etching, the sample was ultrasonically cleaned in acetone solution for 10 min to remove the photoresist, yielding the patterned silicon oxide stress-matching layer.

[0067] Electrode layer fabrication: A Ti layer (transition layer) and an Au layer (functional layer) were sequentially deposited on the surface of the silicon oxide stress-matching layer using magnetron sputtering. The Ti layer was 20 nm thick, and the Au layer was 200 nm thick. The Ti layer was used to enhance the bonding stability between the Au layer and the silicon oxide layer. Subsequently, a positive UV photoresist AZ4330 was spin-coated, and a 3 μm thick photoresist pattern layer was formed after patterning. This pattern layer corresponds to the preset shape of the electrodes and leads. Ion beam dry etching was used for 8 minutes to remove the Ti / Au layer portions not protected by the photoresist. After etching, the sample was ultrasonically cleaned in acetone solution for 10 minutes to remove the photoresist, resulting in the patterned Ti / Au microelectrode array and lead structure.

[0068] PI Layer Spin Coating and Gradient Curing: A non-photosensitive polyimide precursor was spin-coated onto the electrode layer and silicon oxide layer. The sample was then placed in a heating device for the first heat treatment, cured at 150°C for 2 hours to allow the polyimide to reach a semi-cured state, forming a polyimide layer. Subsequently, UV photoresist AZ4620 was spin-coated, followed by pre-baking, exposure, and development to form a 10 μm thick patterned layer. This patterned layer was used to protect the pre-reserved polyimide areas. Plasma etching was used to selectively remove the polyimide portions not protected by the photoresist, exposing the underlying copper sacrificial layer and electrode sites. After etching, the sample was immersed in acetone solution and ultrasonically cleaned for 20 minutes to thoroughly remove residual photoresist.

[0069] Sacrificial layer release: The sample treated in the above steps is immersed in a 10% FeCl3 copper etching solution to remove the copper sacrificial layer through chemical etching. As the sacrificial layer is gradually removed, the supporting effect of the substrate on the upper composite film disappears. A significant mismatch occurs between the compressive stress of the silicon oxide stress matching layer and the tensile stress of the semi-cured polyimide control layer. Under stress drive, the composite film automatically curls up to form a three-dimensional structure.

[0070] Secondary fine-tuning of dynamics: The rolled-up three-dimensional structure is placed in a heating device for a second heat treatment at 200℃. Based on the preset neural cuff diameter requirement, the heating time is precisely controlled in 1-minute increments, driving the semi-cured polyimide control layer to further cross-link and shrink. This allows the curvature of the three-dimensional structure to be precisely corrected to the design value, ultimately resulting in a dimensionally accurate, flexible neural cuff electrode that can tightly wrap around the nerve bundle.

[0071] It should be noted that the functional layer fabrication methods listed in the foregoing embodiments are merely exemplary solutions of the present invention and are not intended to limit the fabrication methods of the functional layers. The functional layer fabrication method of the three-dimensional micro / nano structure described in the present invention can be flexibly adjusted according to the functional requirements of actual application scenarios to adapt to the usage requirements of different fields.

[0072] The method for fabricating three-dimensional micro / nano structures based on MEMS proposed in this invention has the following advantages: (1) The method described in this invention utilizes the chemical curing kinetics of polyimide material to adjust the Young's modulus and stress of the three-dimensional micro-nano structure over a wide range by simply changing the temperature of the heat treatment without changing the thickness of the polyimide, thereby accurately setting its curling radius.

[0073] (2) The method described in this invention proposes a fault-tolerant process of "secondary heating and fine-tuning". In traditional self-curving processes, the shape of the device is fixed after the sacrificial layer is released and formed, lacking effective means of curvature correction. Once the curvature of the initial forming does not meet the design requirements, it is difficult to remedy, resulting in limited yield. The method described in this invention utilizes the activity of incompletely cured polyimide, which retains operational space for curvature adjustment after the device is automatically rolled and formed. The secondary heating process provides energy for further cross-linking of the polyimide molecular chains, driving the polyimide layer to undergo controllable secondary shrinkage, thereby precisely micro-shaping the curvature of the three-dimensional structure and effectively compensating for the small curvature deviations that may occur during the initial forming process. This fault-tolerant process greatly improves the flexibility and fault tolerance of the process, significantly increases the yield, and can fully meet the stringent requirements of high-precision application scenarios for structural dimensions, successfully solving the key pain point of the inability to correct the shape after forming in traditional self-curving technology.

[0074] Figure 7 Optical photographs of the three-dimensional electrode manufactured by the method described in this invention before and after the second heat treatment are shown. Figure 7 As shown, in a specific embodiment of the present invention, the three-dimensional electrode manufactured using the method described in the present invention undergoes a second heat treatment and heating to solidify, resulting in a further curling of its structure and a secondary control of its radius of curvature.

[0075] This invention also proposes a three-dimensional micro / nano structure based on MEMS, fabricated according to the method described in the foregoing embodiments of this invention. In one embodiment of this invention, the three-dimensional micro / nano structure 100 includes: The stress-matching layer 103, which is configured to provide basic compressive stress and form a stress mismatch with the polyimide layer 105, provides power for the self-curling of the three-dimensional micro / nanostructure 100; A functional layer 104, disposed on the stress-matching layer 103, is configured to implement corresponding functions according to the target application scenario. These functions include biosensing, magnetic sensing, optical sensing, on-chip energy storage, radio frequency communication, and / or microfluidics. A polyimide layer 105 is disposed on the functional layer 104 and the stress matching layer 103, and cooperates with the stress matching layer 103 to cover the functional layer 104. The polyimide layer 105 is configured to shrink after a first heat treatment when it is not fully cured, and to shrink further to full curing after a second high-temperature heat treatment, thereby driving the three-dimensional micro-nano structure 100 to self-roll and adjusting the radius of curvature of the three-dimensional micro-nano structure 100.

[0076] In one embodiment of the present invention, the stress matching layer 103 is a silicon oxide layer with a thickness of 500 nm to 3 μm.

[0077] In an embodiment of the present invention, functional layer 104 includes: Au / Ti microelectrode arrays and leads, which are rolled up to form C-shaped cuffs or cage-like structures, are used for biosensors to wrap nerve bundles or organoids. GMR multilayer film stacked Wheatstone bridge structure for magnetic sensors; Patterned CVD graphene layers or quantum dot materials are rolled up to form whispering-gallery mode (WGM) optical resonators for use in optical sensors. The current collector positive electrode material and solid electrolyte negative electrode material stacked structure, after being rolled up, form a "Swiss roll" style multi-layer concentric cylindrical structure, which is used for micro lithium batteries or supercapacitors. Parallel-arranged Cu / Au wires, coiled in three-dimensional space, form a solenoid coil, constituting a hollow inductor or tubular antenna for use in radio frequency devices; and Heating resistance wires and curling them to form hollow tubes serve as fluid channels. By heating and changing the tube diameter, micropump / microvalve functions are achieved, which is used for microfluidics.

[0078] Figure 8 An optical photograph of a multilayer Swiss roll three-dimensional micro / nano structure fabricated by the method described in this invention is shown.

[0079] In one specific embodiment of the present invention, such as Figure 8As shown, the functional layer 104 of the three-dimensional micro / nano structure 100 is a stacked structure of current collector, positive electrode material, solid electrolyte, and negative electrode material, which, after being rolled up, forms a "Swiss roll" type multi-layer concentric cylindrical structure. This "Swiss roll" type multi-layer concentric cylindrical structure has significant advantages and can effectively solve the core pain points of traditional planar micro batteries in the field of on-chip energy storage. Through the rolled arrangement of the multi-layer stacked structure, it significantly increases the effective contact area of ​​the positive electrode material, negative electrode material, and solid electrolyte within a limited space, while improving the unit volume load of the electrode material, thereby significantly improving the energy density of micro energy storage devices, which can better meet the endurance requirements of micro robots, implantable devices, etc. The compact structure of the multi-layer concentric cylinder makes the functional layers more tightly bonded, which not only enhances the mechanical stability of the device and avoids performance degradation caused by structural loosening during use, but also reduces the ion transport path length, lowers the interface contact resistance, and improves charge and discharge efficiency and cycle stability.

[0080] It should be noted that the functional layer structures listed in the foregoing embodiments are merely exemplary solutions of the present invention and are not intended to limit the type and form of the functional layers. The functional layers of the three-dimensional micro / nano structure described in this invention can be flexibly adjusted according to the functional requirements of actual application scenarios to adapt to the usage requirements of different fields.

[0081] Although various embodiments of the invention have been described above, it should be understood that they are presented by way of example only and not as limitations. It will be apparent to those skilled in the art that various combinations, modifications, and alterations can be made without departing from the spirit and scope of the invention. Therefore, the breadth and scope of the invention disclosed herein should not be limited by the exemplary embodiments disclosed above, but should be defined solely by the appended claims and their equivalents.

Claims

1. A method for fabricating three-dimensional micro / nano structures based on MEMS, characterized in that, include: A sacrificial layer is deposited on the substrate layer, and the sacrificial layer is patterned. A stress-matching layer is deposited on the patterned sacrificial layer, and the stress-matching layer is patterned. The functional layers are arranged on the graphical stress matching layer; After applying a polyimide precursor to the functional layer and the stress matching layer, a first heat treatment is performed to allow the polyimide to partially cure and shrink to form a polyimide layer. The polyimide layer is then patterned. The sacrificial layer is removed, allowing the stress-matching layer, functional layer, and polyimide layer to automatically curl up due to stress mismatch to form a three-dimensional micro / nano structure. The polyimide layer undergoes a second heat treatment, driving it to further shrink until fully cured, thereby correcting the radius of curvature of the three-dimensional micro / nano structure to a preset value, wherein: The temperature of the second heat treatment is higher than the temperature of the first heat treatment.

2. The method according to claim 1, characterized in that, The method further includes: A polyimide film is spin-coated and cured onto the substrate layer as a flexible substrate layer.

3. The method according to claim 1, characterized in that, The graphical processing includes: A pattern layer is formed on the surface of the layer to be treated using photoresist; The layer to be processed, which is not protected by the pattern layer, is removed by etching; and The patterned layer is removed by ultrasonic removal using a removal solution; The layer to be processed is the sacrificial layer, stress-matching layer, or polyimide layer.

4. The method according to claim 3, characterized in that, The photoresist is a UV-photolithography positive photoresist.

5. The method according to claim 1, characterized in that, The step of arranging the functional layer on the stress matching layer includes: Depending on the target application scenario, a functional layer is formed by at least one of the following processes: photolithography-evaporation-stripping, sputtering-etching, wet transfer, and solution coating. The functional layer is used to realize at least one of the following: biosensing, magnetic sensing, optical sensing, on-chip energy storage, radio frequency communication, and microfluidics.

6. The method according to claim 1, characterized in that: The temperature of the first heat treatment is set to 120°C to 300°C, and the heating time is set to 1 hour to 3 hours. The temperature of the second heat treatment is set to be higher than the temperature of the first heat treatment but not higher than 300°C, and the heating time of the second heat treatment is controlled until the radius of curvature of the three-dimensional micro / nano structure reaches a preset value.

7. The method according to claim 6, characterized in that, Controlling the heating time of the second heat treatment includes: During the heat treatment process, the radius of curvature of the three-dimensional micro / nano structure is monitored in real time, and the heating time of the second heat treatment is controlled based on the obtained radius of curvature.

8. A three-dimensional micro / nano structure based on MEMS, characterized in that, The three-dimensional micro / nano structure is prepared according to any one of claims 1-7, and comprises: A stress-matching layer, configured to provide basic compressive stress, forms a stress mismatch with the polyimide layer, providing power for the self-curling of the three-dimensional micro / nanostructure; A functional layer, disposed on the stress-matching layer, is configured to implement corresponding functions according to the target application scenario, the functions including at least one of biosensing, magnetic sensing, optical sensing, on-chip energy storage, radio frequency communication, and microfluidics; and A polyimide layer is disposed on the functional layer and the stress matching layer, and cooperating with the stress matching layer to encapsulate the functional layer. The polyimide layer is configured to drive the three-dimensional micro / nanostructure to self-roll and adjust the radius of curvature of the three-dimensional micro / nanostructure.

9. The three-dimensional micro / nano structure according to claim 8, characterized in that, The stress matching layer includes a silicon oxide layer with a thickness of 500 nm to 3 μm.

10. The three-dimensional micro / nano structure according to claim 8, characterized in that, The functional layer includes at least one of the following: electrode structure, membrane stack structure, graphene layer, energy storage stack structure, metal wire structure, and resistance wire structure.

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