Piezoelectric structure with flexible substrate and preparation method and preparation device thereof

Abstract

The invention provides a piezoelectric structure with a flexible substrate, and a preparation method and a preparation device thereof. The preparation method comprises the following steps: firstly, carrying out high-temperature recrystallization annealing and cold rolling treatment on a flexible substrate base material; the two-phase gradient flexible substrate is composed of an upper surface layer ultra-fine grain layer, a lower surface layer ultra-fine grain layer and a toughened martensite layer located between the upper surface layer ultra-fine grain layer and the lower surface layer ultra-fine grain layer, and then an upper piezoelectric film and a lower piezoelectric film which are of the same structure are synchronously formed on the surfaces of the upper surface layer ultra-fine grain layer and the lower surface layer ultra-fine grain layer respectively. According to the invention, thermal mismatch stress caused by high-temperature recrystallization annealing in the process of forming the upper piezoelectric film and the lower piezoelectric film can be effectively relaxed. In addition, the piezoelectric films are formed on the two sides of the biphase gradient flexible substrate, buckling deformation of the flexible substrate is effectively restrained, and the output voltage / electric charge quantity is doubled.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of piezoelectric film preparation, and particularly relates to a piezoelectric structure with a flexible substrate and a preparation method and a preparation device thereof. BACKGROUND

[0002] The piezoelectric film on the flexible substrate solves the contradiction between high performance and strong robustness of the flexible piezoelectric device through material-structure-process synergy, provides bottom support for high-reliability flexible electronic systems, and has wide application prospects in wearable medical sensors (such as electrocardiogram monitoring), flexible robot tactile feedback, self-powered Internet of Things devices (microwatt energy collection), and implantable biological devices.

[0003] With the development of flexible electronic devices towards high performance and high reliability, higher requirements are put forward for the mechanical stability and functional integration of the piezoelectric film. For polymer-based piezoelectric films (such as PVDF-TrFE), their high-temperature thermal stability is poor, and the high-boiling-point solvents (such as DMF and NMP) used are highly toxic and carcinogenic, which does not conform to the green manufacturing trend. On the other hand, for perovskite oxide-based piezoelectric films (such as PZT), high-temperature (500-700℃) crystallization is required to obtain excellent piezoelectricity, and there is a significant difference in the thermal expansion coefficient (thermal expansion coefficient ≈ 30-40 ppm / K) between the stainless steel substrate and mica and the PZT (thermal expansion coefficient ≈ 4-8 ppm / K), which produces thermal mismatch stress, and then leads to: (1) microcracks or macrocracks in the piezoelectric film; (2) delamination of the film-substrate interface, thereby reducing the adhesion of the piezoelectric film; (3) residual stress weakens the piezoelectric performance (d 33 value drops by 20-40%) of the piezoelectric film; (4) substrate warping deformation, uneven shrinkage during cooling after high-temperature process, which is easy to cause the substrate to curl or permanently deform, especially for thin substrates with a thickness < 300 μm, which is more prone to curling or permanent deformation, affecting the flatness of the device and subsequent integration process, and seriously restricting the service life and performance of the device.

[0004] Therefore, it has become one of the focuses of the present research to provide a piezoelectric structure with the characteristics of resisting microcracks and cracking of the piezoelectric film, resisting delamination of the film-substrate interface, high piezoelectric performance, and resisting warping deformation of the substrate. SUMMARY

[0005] The technical problem to be solved by the present application is to provide a piezoelectric structure with a flexible substrate and a preparation method and a preparation device thereof, which can have the characteristics of resisting microcracks and cracking of the piezoelectric film, resisting delamination of the film-substrate interface, high piezoelectric performance, and resisting warping deformation of the substrate.

[0006] To solve the above problems, the present application provides a preparation method of a piezoelectric structure with a flexible substrate, comprising: providing a flexible substrate base material, the flexible substrate base material comprising an upper surface and a lower surface arranged oppositely; heating the flexible substrate base material at a set temperature for a certain time, and performing multi-pass cold rolling on the heated flexible substrate base material to a set thickness to form a dual-phase gradient flexible substrate, the dual-phase gradient flexible substrate having an upper surface layer ultra-fine crystal layer on the upper surface, a lower surface layer ultra-fine crystal layer on the lower surface, and a toughened martensite layer between the upper surface layer ultra-fine crystal layer and the lower surface layer ultra-fine crystal layer, the set temperature being higher than the recrystallization temperature of the flexible substrate base material; synchronously forming an upper piezoelectric thin film and a lower piezoelectric thin film with the same structure on the surface of the upper surface layer ultra-fine crystal layer and the surface of the lower surface layer ultra-fine crystal layer of the dual-phase gradient flexible substrate.

[0007] In an embodiment, the flexible substrate base material is one of stainless steel, titanium and titanium alloy, nickel-based alloy, copper and copper alloy.

[0008] In an embodiment, the step of heating the flexible substrate base material at a set temperature for a certain time further comprises that the set temperature is 700-900 degrees Celsius.

[0009] In an embodiment, the step of heating the flexible substrate base material at a set temperature for a certain time further comprises that the heating time is greater than 10 minutes.

[0010] In an embodiment, the flexible substrate base material is heated to the set temperature at a set heating rate, and the set heating rate ranges from 5 to 20 degrees Celsius per minute.

[0011] In an embodiment, the step of heating the flexible substrate base material at a set temperature for a certain time and performing multi-pass cold rolling on the heated flexible substrate base material to a set thickness further comprises that the heating and cold rolling of the flexible substrate base material are performed in the same chamber.

[0012] In an embodiment, the temperature of cold rolling is lower than the set temperature.

[0013] In an embodiment, the step of performing multi-pass cold rolling on the heated flexible substrate base material to a set thickness further comprises that the heated flexible substrate base material is cold rolled to a set thickness for at least three passes.

[0014] In an embodiment, the bi-phase gradient flexible substrate comprises a plating area and a non-plating area connected to the plating area, and the step of synchronously forming the upper piezoelectric thin film and the lower piezoelectric thin film with the same structure on the surface of the upper surface layer ultra-fine crystal layer and the surface of the lower surface layer ultra-fine crystal layer of the bi-phase gradient flexible substrate further comprises the step of synchronously forming the upper piezoelectric thin film and the lower piezoelectric thin film with the same structure only on the surface of the upper surface layer ultra-fine crystal layer and the surface of the lower surface layer ultra-fine crystal layer of the plating area of the bi-phase gradient flexible substrate.

[0015] In an embodiment, the non-plating area is arranged at two opposite ends of the plating area.

[0016] In an embodiment, the step of synchronously forming the upper piezoelectric thin film and the lower piezoelectric thin film with the same structure on the surface of the upper surface layer ultra-fine crystal layer and the surface of the lower surface layer ultra-fine crystal layer of the bi-phase gradient flexible substrate further comprises the steps of: synchronously forming an upper bottom electrode and a lower bottom electrode on the surface of the upper surface layer ultra-fine crystal layer and the surface of the lower surface layer ultra-fine crystal layer of the bi-phase gradient flexible substrate; synchronously forming an upper buffer layer and a lower buffer layer on the surface of the upper bottom electrode and the lower bottom electrode; synchronously forming an upper piezoelectric material layer and a lower piezoelectric material layer on the surface of the upper buffer layer and the lower buffer layer; performing heat annealing on the upper piezoelectric material layer and the lower piezoelectric material layer to crystallize the upper piezoelectric material layer and the lower piezoelectric material layer, thereby forming an upper piezoelectric layer and a lower piezoelectric layer; and synchronously forming an upper top electrode and a lower top electrode on the surface of the upper piezoelectric layer and the lower piezoelectric layer.

[0017] In an embodiment, the step of performing heat annealing on the upper piezoelectric material layer and the lower piezoelectric material layer further comprises the step of: the heat annealing temperature is less than the heat annealing temperature of the piezoelectric material layer formed directly on the flexible substrate base material.

[0018] The present application also provides a piezoelectric structure with a flexible substrate, comprising: a bi-phase gradient flexible substrate, the bi-phase gradient flexible substrate comprising an upper surface layer ultra-fine crystal layer and a lower surface layer ultra-fine crystal layer arranged oppositely, and a toughened martensite layer between the upper surface layer ultra-fine crystal layer and the lower surface layer ultra-fine crystal layer; an upper piezoelectric thin film arranged on the surface of the upper surface layer ultra-fine crystal layer; and a lower piezoelectric thin film arranged on the surface of the lower surface layer ultra-fine crystal layer, the lower piezoelectric thin film having the same structure as the upper piezoelectric thin film and being synchronously manufactured.

[0019] In an embodiment, the upper surface layer ultra-fine crystal layer and the lower surface layer ultra-fine crystal layer have the same structure and the same thickness.

[0020] In an embodiment, the upper surface layer ultra-fine crystal layer and the lower surface layer ultra-fine crystal layer have the same structure and the thickness range of 100-300 nm.

[0021] In an embodiment, the bi-phase gradient flexible substrate comprises a plated area and a non-plated area connected to the plated area, and the upper piezoelectric thin film and the lower piezoelectric thin film are only located in the plated area.

[0022] In an embodiment, the non-plated area is arranged at both ends of the plated area.

[0023] The present application also provides a preparation device for a piezoelectric structure with a flexible substrate as described above, comprising: a high-temperature recrystallization and cold rolling combined chamber for heating a flexible substrate base material at a set temperature for a certain time, and performing multi-pass cold rolling on the heated flexible substrate base material to a set thickness to form a bi-phase gradient flexible substrate, the bi-phase gradient flexible substrate has an upper surface layer ultra-fine crystal layer on the upper surface, a lower surface layer ultra-fine crystal layer on the lower surface, and a toughened martensite layer between the upper surface layer ultra-fine crystal layer and the lower surface layer ultra-fine crystal layer; a piezoelectric thin film preparation chamber comprising a plurality of sub-chambers for synchronously forming an upper piezoelectric thin film and a lower piezoelectric thin film with the same structure on the upper surface layer ultra-fine crystal layer surface and the lower surface layer ultra-fine crystal layer surface of the bi-phase gradient flexible substrate; a conveying device sequentially passing through each sub-chamber of the high-temperature recrystallization and cold rolling combined chamber and the piezoelectric thin film preparation chamber for conveying the flexible substrate base material and the bi-phase gradient flexible substrate.

[0024] The technical scheme is characterized in that: the flexible substrate base material is subjected to high-temperature recrystallization annealing and cold rolling treatment, to form a dual-phase gradient flexible substrate composed of an upper surface ultrafine crystal layer, a lower surface ultrafine crystal layer and a toughened martensite layer between the upper surface ultrafine crystal layer and the lower surface ultrafine crystal layer, and then upper piezoelectric thin films and lower piezoelectric thin films with the same structure are synchronously formed on the surfaces of the upper surface ultrafine crystal layer and the lower surface ultrafine crystal layer. The presence of the upper surface ultrafine crystal layer and the lower surface ultrafine crystal layer enables the thermal mismatch stress between the upper piezoelectric thin films and the lower piezoelectric thin films and the dual-phase gradient flexible substrate to drive the viscous slip of the grain boundaries (the interface between the upper piezoelectric thin films and the upper surface ultrafine crystal layer and the interface between the lower piezoelectric thin films and the lower surface ultrafine crystal layer) in the cooling step of the subsequent annealing process for forming the upper piezoelectric thin films and the lower piezoelectric thin films, so that the strain energy is released, the thermal mismatch stress caused by the high-temperature recrystallization annealing in the process of forming the upper piezoelectric thin films and the lower piezoelectric thin films can be effectively relaxed, and the micro-cracks, cracks and film-substrate interface delamination caused by the large deformation of the upper piezoelectric thin films and the lower piezoelectric thin films are avoided. Meanwhile, the toughened martensite layer between the upper surface ultrafine crystal layer and the lower surface ultrafine crystal layer can effectively absorb the bending stress and improve the bending fatigue life of the piezoelectric thin films. In addition, the preparation method of the piezoelectric structure with the flexible substrate provided by the embodiment of the present application forms piezoelectric thin films on both sides of the dual-phase gradient flexible substrate, effectively suppresses the warping deformation of the flexible substrate, thereby maintaining the geometric stability, and is helpful to promote the subsequent device processing and integration process; and the piezoelectric output performance of the piezoelectric structure is improved, the polarization directions of the double-sided piezoelectric thin films are opposite, the electric charges generated during deformation are superimposed in the same direction, the output voltage / charge amount is doubled, and the sensitivity of the piezoelectric response of the flexible piezoelectric structure is improved. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0026] Figure 1 is a step schematic diagram of the preparation method of the piezoelectric structure with the flexible substrate provided by the embodiment of the present application; Figure 2 is a schematic diagram of the flexible substrate base material in the preparation method of the piezoelectric structure with the flexible substrate provided by the embodiment of the present application; Figure 3 is a schematic diagram of the dual-phase gradient flexible substrate in the preparation method of the piezoelectric structure with the flexible substrate provided by the embodiment of the present application; Figure 4is a schematic diagram of a piezoelectric structure formed by a preparation method of a piezoelectric structure with a flexible substrate provided by an embodiment of the present application; Figure 5 is a top view of a piezoelectric structure prepared by a preparation method provided by an embodiment of the present application; Figure 6 is a TEM and SEM characterization schematic diagram of a two-phase gradient flexible substrate 300 in a preparation method of a piezoelectric structure with a flexible substrate provided by an embodiment of the present application; Figure 7 is a SEM cross-section characterization schematic diagram of a piezoelectric structure formed by a preparation method of a piezoelectric structure with a flexible substrate provided by an embodiment of the present application; Figure 8 is a PZT piezoelectric coefficient and bending cycle number relationship graph of a piezoelectric structure with a flexible substrate provided by an embodiment of the present application; Figure 9 is a PZT leakage current density and bending cycle number relationship graph of a piezoelectric structure with a flexible substrate provided by an embodiment of the present application; Figure 10 is a schematic diagram of a preparation device of a piezoelectric structure with a flexible substrate provided by an embodiment of the present application.

[0027] Explanation of reference signs: 200, flexible substrate base material; 300, two-phase gradient flexible substrate; 300A, film-coated area; 300B, non-film-coated area; 310, upper surface layer ultrafine crystal layer; 320, lower surface layer ultrafine crystal layer; 330, toughened martensite layer; 410, upper piezoelectric thin film; 411, upper bottom electrode; 412, upper buffer layer; 413, upper piezoelectric layer; 414, upper top electrode; 415, upper adhesion layer; 420, lower piezoelectric thin film; 421, lower bottom electrode; 422, lower buffer layer; 423, lower piezoelectric layer; 424, lower top electrode; 425, lower adhesion layer; 430, piezoelectric structure; 500, recrystallization and cold rolling combined chamber; 501, heater; 502, double roll cold rolling mill; 510, piezoelectric thin film preparation chamber; 511, adhesion layer deposition sub-chamber; 512, bottom electrode deposition sub-chamber; 513, buffer layer deposition sub-chamber; 514, piezoelectric material layer deposition sub-chamber; 515, high temperature annealing sub-chamber; 516, top electrode deposition chamber; 520, conveying device; 530, cathode; 540, heating element; 550, baffle; D1, first direction; D2, second direction. DETAILED DESCRIPTION

[0028] The specific embodiments of the piezoelectric structure with a flexible substrate and the preparation method and device thereof provided by the present application will be described in detail below with reference to the accompanying drawings.

[0029] Figure 1 is a schematic diagram of the steps of the preparation method of the piezoelectric structure with a flexible substrate provided by the specific embodiments of the present application, please refer to Figure 1 , the preparation method comprises the following steps: step S10, providing a flexible substrate base material, the flexible substrate base material comprises an upper surface and a lower surface arranged oppositely; step S11, heating the flexible substrate base material at a set temperature for a certain time, and cold rolling the heated flexible substrate base material in multiple passes to a set thickness to form a dual-phase gradient flexible substrate, the dual-phase gradient flexible substrate has an upper surface layer ultra-fine crystal layer on the upper surface and a lower surface layer ultra-fine crystal layer on the lower surface, and a toughened martensite layer between the upper surface layer ultra-fine crystal layer and the lower surface layer ultra-fine crystal layer, the set temperature is higher than the recrystallization temperature of the flexible substrate base material; step S12, synchronously forming upper and lower piezoelectric thin films with the same structure on the surfaces of the upper surface layer ultra-fine crystal layer and the lower surface layer ultra-fine crystal layer of the dual-phase gradient flexible substrate.

[0030] The preparation method of the piezoelectric structure with a flexible substrate provided by the specific embodiments of the present application first forms a dual-phase gradient flexible substrate composed of an upper surface layer ultra-fine crystal layer, a lower surface layer ultra-fine crystal layer and a toughened martensite layer between the upper surface layer ultra-fine crystal layer and the lower surface layer ultra-fine crystal layer, and then synchronously forms upper and lower piezoelectric thin films with the same structure on the surfaces of the upper surface layer ultra-fine crystal layer and the lower surface layer ultra-fine crystal layer, respectively.

[0031] The existence of the upper surface layer and the lower surface layer can effectively relax the thermal mismatch stress caused by high-temperature recrystallization annealing in the process of forming the upper piezoelectric film and the lower piezoelectric film, and avoid micro-cracks, cracking and delamination of the film-substrate interface caused by large deformation of the upper piezoelectric film and the lower piezoelectric film. At the same time, the toughened martensite layer can effectively absorb bending stress and improve the bending fatigue life of the piezoelectric film.

[0032] In addition, in some existing piezoelectric structure preparation methods, a piezoelectric film is prepared on one side of a flexible substrate by a single-sided deposition process, and a continuous and dense piezoelectric layer is formed by high-temperature annealing crystallization. The piezoelectric function is realized by using the interface bonding between the piezoelectric film and the flexible substrate to guide the charge under mechanical deformation. However, high-performance piezoelectric structures need to meet strict conditions such as thermal mismatch stress balance, crack resistance and interface stability. If the thermal mismatch stress is unbalanced (for example, when the piezoelectric film is deposited on one side of the flexible substrate, the asymmetric stress after high-temperature annealing is greater than 500 MPa during the cooling process), the following problems will occur: (1) the flexible substrate is significantly warped and the piezoelectric coefficient is attenuated; (2) the dynamic bending stress tends to concentrate on the edge of the piezoelectric film, and the micro-crack initiation threshold is reduced; (3) the piezoelectric film leakage current increases by 2 orders of magnitude, increasing the power consumption of the device.

[0033] The preparation method of the piezoelectric structure with a flexible substrate provided in the embodiment of the present application synchronously forms piezoelectric films on both sides of the two-phase gradient flexible substrate to form a symmetrical structure, so that the thermal mismatch stress on both sides of the two-phase gradient flexible substrate after high-temperature annealing and cooling process satisfies the moment balance equation: σ top * t top * h top = σ bottom * t bottom * h bottom , wherein, σ top represents the thermal mismatch stress of the upper piezoelectric film, t top represents the thickness of the upper piezoelectric film, h top represents the distance from the upper piezoelectric film to the two-phase gradient flexible substrate, σ bottom represents the thermal mismatch stress of the upper piezoelectric film, t bottom represents the thickness of the upper piezoelectric film, h bottomThe distance between the upper piezoelectric film and the two-phase gradient flexible substrate is represented. It can be seen that the net bending moment of the piezoelectric structure is 0, which effectively suppresses the warping deformation of the two-phase gradient flexible substrate, thereby maintaining its geometric stability and facilitating subsequent device processing and integration process.

[0034] In addition, the polarization direction of the upper piezoelectric film is opposite to that of the lower piezoelectric film, and the charges generated during deformation are superimposed in the same direction: Q total = ∣Q top ∣ + ∣Q bottom ∣ =2* d 33 * F , wherein, Q total Q is the total electric quantity of the piezoelectric structure, Q top Q1 is the electric quantity of the upper piezoelectric film, Q bottom Q2 is the electric quantity of the lower piezoelectric film, d 33 d33 is the piezoelectric strain constant, F F is the mechanical force applied to the piezoelectric film. It can be seen that the piezoelectric films are formed on both sides of the two-phase gradient flexible substrate, so that the output voltage / charge quantity of the piezoelectric structure is doubled, thereby improving the sensitivity of the piezoelectric response of the flexible device and improving the piezoelectric output performance of the piezoelectric structure.

[0035] The preparation method of the piezoelectric structure with a flexible substrate provided by the embodiment of the present application can maintain the geometric stability, mechanical integrity and higher electromechanical function of the flexible piezoelectric structure through the cooperative control of the two-phase gradient flexible substrate and the piezoelectric film formed on both sides.

[0036] Figures 2-4 is the process flow chart of the preparation method of the piezoelectric structure with a flexible substrate provided by the embodiment of the present application, and the following will be described in detail. Figures 1-4 The specific embodiments of the present application will be described in detail.

[0037] Please refer to Figure 1 and Figure 2 , wherein, Figure 2 is the schematic diagram of the flexible substrate base material in the preparation method of the piezoelectric structure with a flexible substrate provided by the embodiment of the present application, and step S10 provides a flexible substrate base material 200, which includes an upper surface and a lower surface arranged oppositely.

[0038] In an embodiment, the flexible substrate 200 is a stainless steel substrate, including but not limited to 316L stainless steel, 304L stainless steel, 310S stainless steel, etc. Stainless steel has significant advantages as a flexible substrate material in flexible electronics: excellent high-temperature resistance, stainless steel substrates can withstand high-temperature processes (such as thin film deposition temperatures up to 400°C or more), much higher than polymer substrates (such as PET temperature resistance only about 150°C), suitable for device preparation requiring high-temperature processing; high strength and resistance to mechanical stress, high mechanical strength of stainless steel (tensile strength can reach more than 500 MPa), can withstand tensile stress in the continuous production process of roll-to-roll, avoid deformation or breakage, while supporting ultra-thin design (such as thickness can be as low as 0.25 mm); good thermal expansion matching, the coefficient of thermal expansion (CTE) of stainless steel is close to that of many optoelectronic materials, which can reduce the interfacial stress under temperature changes and reduce the risk of device delamination or cracking; corrosion resistance and environmental stability, the chromium oxide passivation film formed on the surface of stainless steel can resist moisture, salt spray and chemical corrosion, suitable for outdoor or harsh environments (such as marine climate, chemical industry scene), prolonging the service life of the device; electrical conductivity and process compatibility, stainless steel itself has electrical conductivity and can be used as a back electrode, simplifying the device structure and reducing costs. In addition, its surface has high smoothness, which is suitable for high-precision thin film deposition processes; flexibility and lightweight, although stainless steel has high hardness, its flexibility can be adjusted through rolling and annealing processes to achieve a certain degree of bending; economic and potential for large-scale production, stainless steel is less expensive than special alloys (such as titanium), and can be mass-produced through mature processes such as rolling and stamping, suitable for low-cost roll-to-roll continuous production, promoting the industrialization of flexible electronics.

[0039] In an embodiment, the flexible substrate 200 is a titanium and titanium alloy substrate (such as pure titanium TA1, titanium alloy TC4). Titanium and titanium alloy have excellent plastic deformation ability after heating, and can form ultra-fine crystal layers on the upper and lower surfaces of the flexible substrate 200 through multi-pass cold rolling, with β-martensite toughening phase remaining in the core.

[0040] In an embodiment, the flexible substrate 200 is a nickel-based alloy substrate (such as pure nickel, nickel-copper alloy Monel400). The nickel-based alloy substrate can be ultra-finely crystallized (grain size can be controlled in the range of 100-300 nm) on the surface and form a solid solution phase with good toughness in the core after heating through multi-pass cold rolling. Nickel-based alloys have excellent corrosion resistance (especially in seawater, acid and alkali environments), which can expand the application of the patent piezoelectric structure in "marine environment flexible sensors, self-powered equipment in the chemical industry".

[0041] In one embodiment, the flexible substrate base material 200 is a copper and copper alloy substrate (e.g. oxygen-free copper TU1, brass H62), which has excellent plasticity after heating, and can be efficiently used for surface layer ultra-fine crystallization and formation of a center part with excellent toughness and face-centered cubic structure through multi-pass cold rolling. 8 Ωm, and the electrical conductivity of stainless steel is about 70 x 10⁻ 8 Ωm), the "bottom electrode deposition" step in the preparation method can be omitted, and the flexible substrate can be directly used as an electrode, thereby simplifying the process and reducing the cost, and being suitable for "self-powered Internet of Things devices with high electrical conductivity requirements".

[0042] Please refer to Figure 1 and Figure 3 wherein, Figure 3 is a schematic diagram of a two-phase gradient flexible substrate 300 in a preparation method of a piezoelectric structure with a flexible substrate provided in one embodiment of the present application, step S11, the flexible substrate base material 200 is heated at a set temperature for a certain time, and the flexible substrate base material 200 after heating is subjected to multi-pass cold rolling to a set thickness to form a two-phase gradient flexible substrate 300, the two-phase gradient flexible substrate 300 has an upper surface layer ultra-fine crystalline layer 310 on the upper surface, and a lower surface layer ultra-fine crystalline layer 320 on the lower surface, the upper surface layer ultra-fine crystalline layer 310 and the lower surface layer ultra-fine crystalline layer 320 are a toughened martensite layer 330, and the set temperature is higher than the recrystallization temperature of the flexible substrate base material 200. Figure 6 is a TEM and SEM characterization schematic diagram of a two-phase gradient flexible substrate 300 in a preparation method of a piezoelectric structure with a flexible substrate provided in one embodiment of the present application, wherein (a) is a general cross-sectional view, (b) is a surface layer fine-grained region, and (c) is a center part needle-shaped toughened martensite region.

[0043] In this step, the flexible substrate base material 200 is heated at a set temperature for a certain time, and the heated flexible substrate base material 200 is cold-rolled in multiple passes to form a two-phase gradient flexible substrate 300. Specifically, in this step, the flexible substrate base material 200 is first heated at a set temperature for a certain time to reach the recrystallization temperature, so that the original coarse / deformed grains are converted into stress-free equiaxed grains, the internal stress of the substrate processing is eliminated, the substrate has a plastic state that can be further refined by cold rolling, and the foundation for the subsequent cold rolling to form a gradient structure is laid. The heated flexible substrate base material 200 is then cold-rolled in multiple passes. Through the cumulative deformation of multiple passes of cold rolling, an upper surface layer of ultra-fine grains 310 is formed on the upper surface of the flexible substrate base material 200, a lower surface layer of ultra-fine grains 320 is formed on the lower surface of the flexible substrate base material 200, and a central toughened martensite layer 330 is reserved. Finally, a two-phase gradient structure is formed. On the one hand, the heated flexible substrate base material 200 has good plasticity, and the upper and lower surface layers of the flexible substrate base material 200 are repeatedly broken and refined through the continuous extrusion and shearing effect of multiple passes of cold rolling, and finally an upper surface layer of ultra-fine grains 310 and a lower surface layer of ultra-fine grains 320 with a thickness of 100-300 nm are formed. On the other hand, the surface layer of the flexible substrate base material 200 is directly in contact with the roller, and the degree of deformation is large (the grains are broken to ultra-fine grains); while the central part has a smaller degree of deformation, and the equiaxed grains after recrystallization are converted into a toughened martensite layer 330 under the plastic stress of cold rolling. The role of the toughened martensite layer 330 is to absorb bending stress and improve the fatigue resistance of the substrate. In a specific embodiment, the step of heating the flexible substrate base material 200 at a set temperature for a certain time further comprises: the set temperature is 300-900 degrees Celsius. The set temperature corresponding to the flexible substrate base material 200 of different materials is different. Specifically, for a stainless steel substrate, the set temperature is 700-900 degrees Celsius. For titanium and titanium alloy substrates, the set temperature is 600-800 degrees Celsius. The recrystallization temperature of pure titanium TA1 is about 550-600 degrees Celsius, and the recrystallization temperature of TC4 titanium alloy is about 700-750 degrees Celsius. The set temperature needs to be higher than this interval to ensure complete recrystallization, while avoiding excessive temperature leading to oxidation of titanium alloy (titanium oxidation in air environment below 800 degrees Celsius is controllable). For nickel-based alloy substrates, the set temperature is 450-750 degrees Celsius. The recrystallization temperature of pure nickel is about 370-450 degrees Celsius, and the recrystallization temperature of Monel400 (nickel-copper alloy) is about 650-700 degrees Celsius. The set temperature is higher than this interval, which can realize the recrystallization of nickel-based alloy, and 450-750 degrees Celsius will not cause the volatilization of elements in nickel-based alloy (such as the volatilization temperature of copper is much higher than 750 degrees Celsius).For copper and copper alloy substrates, the set temperature is 300-600°C, the recrystallization temperature of oxygen-free copper TU1 is about 200-300°C, and the recrystallization temperature of brass H62 is about 400-500°C. The set temperature higher than the interval can ensure recrystallization, and below 600°C can avoid copper oxidation (copper oxidation film is thin below 600°C, which does not affect the subsequent film bonding).

[0044] In an embodiment, the step of heating the flexible substrate base material 200 at a set temperature for a period of time further comprises: controlling the temperature rising rate during the temperature rising of the flexible substrate base material 200 to the set temperature, and rising the flexible substrate base material to the set temperature at a set temperature rising rate, so as to form the uniform upper surface ultrafine crystal layer 310 and the lower surface ultrafine crystal layer 320 and avoid cracking of the upper surface ultrafine crystal layer 310 and the lower surface ultrafine crystal layer 320.

[0045] In an embodiment, the temperature rising rate is controlled to be 5-20°C / min. At the temperature rising rate, the internal temperature gradient of the flexible substrate base material 200 is small, atoms have sufficient time to diffuse and arrange according to the principle of minimum energy, uniform size, stress-free equiaxed grains can be formed, and the internal stress remaining in the flexible substrate base material 200 during rolling can be fully eliminated; and "abnormal grain growth" caused by local overheating can also be avoided (such as stainless steel, if local overheating, coarse grains are easily formed, which is difficult to break and refine during subsequent cold rolling, affecting the uniformity of the surface ultrafine crystal layer). If the temperature rising rate is too large, the temperature rises too fast, the temperature difference between the inside and outside of the flexible substrate base material 200 is large, thermal stress is easily generated (which may cause micro-deformation of the flexible substrate base material 200), and the atomic diffusion time is insufficient, the recrystallized grain size is uneven (part of the area is still residual original deformed structure, and the surface ultrafine crystal layer is easily cracked locally during subsequent cold rolling), which destroys the integrity of the dual-phase gradient structure.

[0046] In an embodiment, the step of heating the flexible substrate base material 200 at a set temperature for a period of time further comprises: the heating time is greater than 10 minutes, and the balance between recrystallization sufficiency and grain size control is achieved by controlling the heating time, which ensures that the flexible substrate base material 200 completes recrystallization and avoids excessive grain growth to form a uniform ultrafine crystal layer during subsequent cold rolling.

[0047] In an embodiment, the heating and cold rolling of the flexible substrate 200 are performed in the same chamber, which can reduce the exposure of the substrate to contamination and oxidation, and ensure the bonding strength of the film-substrate interface. In addition, the process continuity and stability can be improved, which is suitable for large-scale production. In an embodiment, a protective gas is used during the heating and cold rolling of the flexible substrate 200. The protective gas can be nitrogen or inert gas. In an embodiment, the temperature of the cold rolling is lower than the set temperature. In addition, the temperature of the cold rolling is 300-500°C.

[0048] The heated flexible substrate 200 is subjected to multi-pass cold rolling to a set thickness. For example, in an embodiment, the heated flexible substrate 200 is subjected to three-pass cold rolling to a set thickness. For example, for a flexible substrate 200 with a thickness of 500 μm, the first-pass cold rolling reduction is 18% (thickness to 410 μm), the second-pass cold rolling reduction is 20% (thickness to 328 μm), and the third-pass cold rolling reduction is 24% (thickness to 250 μm), and the total reduction is 50%. In other embodiments, more than three-pass cold rolling can be performed to form a dual-phase gradient flexible substrate 300 with a set thickness.

[0049] In an embodiment, the upper surface layer ultrafine crystal layer 310 and the lower surface layer ultrafine crystal layer 320 are formed synchronously, and the thicknesses of the two layers are the same, which can further form a symmetrical structure on both sides of the dual-phase gradient flexible substrate 300, maintain the geometric stability of the piezoelectric structure, and improve the piezoelectric performance of the piezoelectric structure. In addition, the thicknesses of the upper surface layer ultrafine crystal layer 310 and the lower surface layer ultrafine crystal layer 320 are both in the range of 100-300 nm.

[0050] In this step, the dual-phase gradient flexible substrate 300 is formed by high-temperature recrystallization and cold rolling, and a new system with continuous change of grain size and phase composition in the material is achieved. In the dual-phase gradient flexible substrate 300, the surface layer is treated as an ultrafine crystal region with a controllable thickness, and the core is converted into a martensite phase with high toughness. The two are naturally connected through a transition zone with a gradual change in composition and structure, forming a gradient distribution of mechanical properties.

[0051] Please refer to Figure 1 and Figure 4 wherein, Figure 4 is a schematic diagram of the piezoelectric structure 430 formed by the preparation method of the piezoelectric structure with a flexible substrate provided in an embodiment of the present application. In step S12, the upper piezoelectric thin film 410 and the lower piezoelectric thin film 420 with the same structure are synchronously formed on the surface of the upper surface layer ultrafine crystal layer 310 and the surface of the lower surface layer ultrafine crystal layer 320 of the dual-phase gradient flexible substrate 300. Figure 7is a schematic diagram of SEM cross-section characterization of a piezoelectric structure formed by a method for manufacturing a piezoelectric structure with a flexible substrate according to an embodiment of the present application.

[0052] In this step, when forming the upper piezoelectric thin film 410 and the lower piezoelectric thin film 420, an annealing process is used to make the piezoelectric material high-temperature recrystallize to form a piezoelectric layer. In the cooling step of the annealing process, the thermal mismatch stress between the upper piezoelectric thin film 410 and the lower piezoelectric thin film 420 and the bi-phase gradient flexible substrate 300 drives the grain boundary (the interface between the upper piezoelectric thin film 410 and the upper surface layer ultra-fine crystal layer 310 and the interface between the lower piezoelectric thin film 420 and the lower surface layer ultra-fine crystal layer 320) to viscous slip, thereby releasing its strain energy and effectively relaxing the thermal mismatch stress caused by annealing, avoiding micro-cracks, cracking and film-substrate interface delamination caused by large deformation of the upper piezoelectric thin film 410 and the lower piezoelectric thin film 420.

[0053] In this step, the upper piezoelectric thin film 410 and the lower piezoelectric thin film 420 with the same structure are formed on the surface of the upper surface layer ultra-fine crystal layer 310 and the surface of the lower surface layer ultra-fine crystal layer 320, respectively, thereby forming a symmetrical structure, effectively suppressing the warping deformation of the bi-phase gradient flexible substrate 300, thereby maintaining its geometric stability, and doubling the output voltage / charge of the piezoelectric structure, thereby improving the sensitivity of the piezoelectric response of the flexible device and enhancing the piezoelectric output performance of the piezoelectric structure.

[0054] As an example, the step of forming the upper piezoelectric thin film 410 and the lower piezoelectric thin film 420 with the same structure on the surface of the upper surface layer ultra-fine crystal layer 310 and the surface of the lower surface layer ultra-fine crystal layer 320 of the bi-phase gradient flexible substrate 300 further comprises: The upper bottom electrode 411 and the lower bottom electrode 421 are formed on the surface of the upper surface layer ultra-fine crystal layer 310 and the surface of the lower surface layer ultra-fine crystal layer 320 of the bi-phase gradient flexible substrate 300, respectively. The upper bottom electrode 411 and the lower bottom electrode 421 can be one or more of platinum, gold, silver, copper, etc. As an example, the bi-phase gradient flexible substrate 300 is conveyed to a bottom electrode deposition chamber by a roll-to-roll motor, and a Pt target is used to deposit the upper bottom electrode 411 and the lower bottom electrode 421 on both surfaces of the bi-phase gradient flexible substrate 300 by direct current magnetron sputtering at room temperature. The upper bottom electrode 411 and the lower bottom electrode 421 are both Pt bottom electrode layers, and the thickness of the Pt bottom electrode layer is controlled to be about 150 nm.

[0055] In one embodiment, before depositing the upper bottom electrode 411 and the lower bottom electrode 421, the preparation method further comprises depositing an upper adhesion layer 415 and a lower adhesion layer 425, the upper bottom electrode 411 is deposited on the upper adhesion layer 415 to increase the bonding force between the upper bottom electrode 411 and the dual-phase gradient flexible substrate 300, and the lower bottom electrode 421 is deposited on the lower adhesion layer 425 to increase the bonding force between the lower bottom electrode 421 and the dual-phase gradient flexible substrate 300. The upper adhesion layer 415 and the lower adhesion layer 425 can both be metal adhesion layers, and the metal adhesion layers include one or more of titanium, zirconium, tantalum, and chromium. As an example, the dual-phase gradient flexible substrate 300 after cold rolling combined with high-temperature recrystallization treatment is conveyed to an adhesion layer deposition chamber by a roll-to-roll motor, and a metal Ti target is used for room temperature deposition on both surfaces of the dual-phase gradient flexible substrate 300 by direct current magnetron sputtering to deposit the upper adhesion layer 415 and the lower adhesion layer 425. The upper adhesion layer 415 and the lower adhesion layer 425 are both Ti metal adhesion layers, and the thickness of the Ti adhesion layer is controlled to be about 50 nm.

[0056] After the step of forming the upper bottom electrode 411 and the lower bottom electrode 421, an upper buffer layer 412 and a lower buffer layer 422 are simultaneously formed on the surfaces of the upper bottom electrode 411 and the lower bottom electrode 421. The upper buffer layer 412 is used to adjust the lattice mismatch between the upper bottom electrode 411 and the upper piezoelectric layer 413, promote the preferred orientation growth of the piezoelectric material, thereby improving the piezoelectric coefficient and the electromechanical coupling efficiency, and can relieve the interface stress caused by the difference in thermal expansion coefficient to prevent film cracking or peeling. The upper buffer layer 412 and the lower buffer layer 422 can be an oxide layer, a nitride layer, or a composite buffer layer. The upper buffer layer 412 and the lower buffer layer 422 can both be an oxide buffer layer, and the oxide buffer layer includes one or more of lanthanum nickel oxide (LaNiO3), magnesium oxide (MgO), and strontium ruthenate (SrRuO3). As an example, the dual-phase gradient flexible substrate 300 after depositing the Pt bottom electrode layer is conveyed to a buffer layer deposition chamber by a roll-to-roll motor, and a conductive oxide lanthanum nickel oxide (LNO) target is used for room temperature deposition on both surfaces of the dual-phase gradient flexible substrate 300 by direct current magnetron sputtering to deposit the upper buffer layer 412 and the lower buffer layer 422. The upper buffer layer 412 and the lower buffer layer 422 are both LNO buffer layers, and the thickness of the LNO buffer layer is controlled to be about 150 nm. 1+x O3), magnesium oxide (MgO), and strontium ruthenate (SrRuO3). As an example, the dual-phase gradient flexible substrate 300 after depositing the Pt bottom electrode layer is conveyed to a buffer layer deposition chamber by a roll-to-roll motor, and a conductive oxide lanthanum nickel oxide (LNO) target is used for room temperature deposition on both surfaces of the dual-phase gradient flexible substrate 300 by direct current magnetron sputtering to deposit the upper buffer layer 412 and the lower buffer layer 422. The upper buffer layer 412 and the lower buffer layer 422 are both LNO buffer layers, and the thickness of the LNO buffer layer is controlled to be about 150 nm.

[0057] After the step of forming the upper buffer layer 412 and the lower buffer layer 422, the upper piezoelectric material layer and the lower piezoelectric material layer are formed on the surfaces of the upper buffer layer 412 and the lower buffer layer 422. The upper piezoelectric material layer and the lower piezoelectric material layer can be one or more of potassium sodium niobate (KNN), lead zirconate titanate (PZT), and aluminum nitride (AIN). As an example, the bi- gradient flexible substrate 300 with the upper buffer layer 412 and the lower buffer layer 422 deposited thereon is transferred to a piezoelectric layer deposition chamber by a roll-to-roll motor, and a non-conductive lead zirconate titanate (PZT) target is used for room temperature deposition of the upper piezoelectric material layer and the lower piezoelectric material layer on both surfaces of the bi- gradient flexible substrate 300 by radio frequency magnetron sputtering, the upper piezoelectric material layer and the lower piezoelectric material layer being PZT piezoelectric layers, and the thickness of the PZT piezoelectric layers being controlled to be about 2 μm.

[0058] After the step of forming the upper piezoelectric material layer and the lower piezoelectric material layer, the upper piezoelectric material layer and the lower piezoelectric material layer are subjected to thermal annealing to crystallize the upper piezoelectric material layer and the lower piezoelectric material layer, thereby forming the upper piezoelectric layer 413 and the lower piezoelectric layer 423.

[0059] Further, the thermal annealing temperature is lower than the thermal annealing temperature of the piezoelectric material layer formed directly on the flexible substrate base material, for example, the thermal annealing temperature in this step is 500-600 °C. This has the following advantages: (1) significantly reducing thermal mismatch stress and avoiding cracking of the piezoelectric film and film-substrate delamination: the formula for calculating the thermal mismatch stress is σ = E • α • ΔT (E is the elastic modulus, α is the difference in thermal expansion coefficient, and ΔT is the process temperature difference), if the temperature is controlled to be 500-600 °C, the ΔT of "heating-cooling" in the process is significantly reduced, which directly leads to a reduction in thermal mismatch stress; (2) protecting the core microstructure of the bi- gradient flexible substrate and maintaining its mechanical properties: the temperature controlled to be 500-600 °C can precisely protect the structure. On the one hand, the stability of the toughened martensite layer is improved: the martensite tempering phase transition temperature of stainless steel is usually higher than 600 °C, and below this interval, the martensite can avoid "tempering sorbite" (the toughness decreases by more than 30% after phase transition), ensuring that it remains highly tough and effectively absorbs dynamic bending stress (such as Figure 8The piezoelectric coefficient remains stable after 10^6 bending cycles, and the low-temperature process is an important guarantee for this performance. On the other hand, the integrity of the surface ultra-fine crystal layer is maintained: the role of the ultra-fine crystal layer is to release thermal stress through grain boundary sliding. If the temperature is too high, it is easy to cause the "grain growth" of the ultra-fine crystal (the number of grain boundaries decreases, and the stress relaxation ability decreases), and controlling the temperature at 500-600°C can inhibit grain coarsening and maintain the integrity of the ultra-fine crystal structure (100-300nm), ensuring that its stress relaxation function does not fail. On the other hand, the microstructure of the piezoelectric material is optimized, taking into account the crystallinity and piezoelectric performance. High temperature (>600°C) can easily lead to PZT grain growth to more than 100nm, and excessive grain size can increase grain boundary defects (such as vacancy aggregation), leading to a decrease in the piezoelectric coefficient d33 (as mentioned in the background art, "high temperature reduces d33 by 20-40%"). Controlling the grain size below this range to 50-80nm can meet the needs of perovskite crystallization (XRD characteristic peak half-width narrow, crystallinity >90%), and also enhance the piezoelectric response sensitivity through fine grain strengthening (d33>300pC / N in the present invention).

[0060] As an example, the bi-phase gradient flexible substrate 300 with the upper piezoelectric material layer and the lower piezoelectric material layer deposited thereon is conveyed to a high-temperature thermal annealing chamber by a roll-to-roll motor, and the bi-phase gradient flexible substrate 300 with the upper piezoelectric material layer and the lower piezoelectric material layer deposited thereon is heated on both sides at a high temperature (the heating temperature is controlled at 500-600°C) to promote the crystallization of the piezoelectric material and form a tetragonal perovskite structure, and finally to form a flexible substrate with an anti-curl deformation piezoelectric film. In this step, the thermal mismatch stress between the upper piezoelectric film 410 and the lower piezoelectric film 420 and the bi-phase gradient flexible substrate 300 drives the grain boundary (the interface between the upper piezoelectric film 410 and the upper surface ultra-fine crystal layer 310 and the interface between the lower piezoelectric film 420 and the lower surface ultra-fine crystal layer 320) to occur viscous sliding, thereby releasing its strain energy, which can effectively relax the thermal mismatch stress caused by thermal annealing in the process of forming the upper piezoelectric film 410 and the lower piezoelectric film 420, and avoid micro-cracks, cracking and film-substrate interface delamination caused by large deformation of the upper piezoelectric film 410 and the lower piezoelectric film 420.

[0061] The heating rate during thermal annealing can affect the piezoelectric layer formed. Specifically, slow heating (e.g., a heating rate of 10-20°C / min): the piezoelectric precursor (i.e., the piezoelectric material layer) can be gradually decomposed and complete the crystal phase transition, avoiding "incomplete decomposition of the precursor" (residual amorphous phase or impurity phase, such as pyrochlore phase) due to rapid temperature rise; at the same time, slow heating can reduce "grain boundary defects" (such as oxygen vacancies, vacancy aggregation), ensure uniform growth of perovskite grains, and improve film density (insufficient density will lead to increased leakage current, such as the high-temperature process defects mentioned in the background art easily make the leakage current rise by 2 orders of magnitude). Fast heating (e.g., a heating rate greater than 25°C / min) has the disadvantage that local temperature rise easily leads to "thermal mismatch stress instantaneous increase" between the piezoelectric film and the substrate (which can cause film cracking or film-substrate delamination), and rapid decomposition of the precursor easily produces "gas phase impurities" (such as PbO volatilization too fast, leading to deviation from the stoichiometric ratio of the components), forming a non-ferroelectric phase (such as pyrochlore phase), directly leading to a decrease in the piezoelectric coefficient d33 (such as d33 from 300 pC / N to below 200 pC / N), which does not meet the "high piezoelectric performance" target of the present application. In a specific embodiment, the preferred heating rate range is 10-20°C / min, which can meet the quality requirements of "substrate recrystallization uniformity" and "high crystallinity of the piezoelectric layer", and can avoid the low process efficiency caused by slow rate (such as <5°C / min will make the single process time length extended to several hours, which is not conducive to large-scale production), fully consistent with the technical positioning of "high performance and industrialization" of the present application.

[0062] After the steps of forming the upper piezoelectric layer 413 and the lower piezoelectric layer 423, the upper top electrode 414 and the lower top electrode 424 are simultaneously formed on the surfaces of the upper piezoelectric layer 413 and the lower piezoelectric layer 423. The upper top electrode 414 and the lower top electrode 424 together with the upper bottom electrode 411 and the lower bottom electrode 421 form the electric field application path, and the piezoelectric layer generates inverse piezoelectric effect (mechanical deformation) or collects positive piezoelectric effect (charge output) through external voltage driving. The upper top electrode 414 and the lower top electrode 424 can both be metal electrodes, for example, the upper top electrode and the lower top electrode 424 can both be one of platinum, gold, silver, copper, etc. As an example, the bi-phase gradient flexible substrate 300 with the deposited upper piezoelectric layer 413 and lower piezoelectric layer 423 is conveyed to a top electrode deposition chamber by a roll-to-roll motor, a specific mask is attached to test the electrical properties of the piezoelectric film 410 on the flexible substrate, and the Pt target is used for direct current magnetron sputtering to deposit the upper top electrode 414 and the lower top electrode 424 on both sides of the bi-phase gradient flexible substrate 300 at room temperature. The upper top electrode 414 and the lower top electrode 424 are both Pt top electrode layers, and the thickness of the Pt top electrode layer is controlled to be about 150 nm.

[0063] The method for preparing a piezoelectric structure with a flexible substrate provided in the specific embodiments of the present invention involves depositing a piezoelectric thin film on both sides of the flexible substrate at room temperature and then performing high-temperature recrystallization annealing to crystallize the piezoelectric thin film into a perovskite phase structure and impart piezoelectric properties to it.

[0064] In one specific implementation, such as Figure 5 The diagram shows a top view of a piezoelectric structure prepared using a specific embodiment of the present invention. The dual-phase gradient flexible substrate 300 includes a coated region 300A and a non-coated region 300B connected to the coated region 300A. The step of simultaneously forming an upper piezoelectric thin film 410 and a lower piezoelectric thin film 420 with the same structure on the surface of the upper ultrafine crystalline layer 310 and the lower ultrafine crystalline layer 320 of the dual-phase gradient flexible substrate 300 further includes: simultaneously forming an upper piezoelectric thin film 410 and a lower piezoelectric thin film 420 with the same structure only on the surface of the upper ultrafine crystalline layer 310 and the lower ultrafine crystalline layer 320 of the coated region 300A of the dual-phase gradient flexible substrate 300.

[0065] The upper piezoelectric film 410 and the lower piezoelectric film 420 are formed only in the coating area 300A. They are not formed in the non-coating area 300B. The non-coating area 300B acts as a flexible hinge, absorbing the strain energy caused by the thermal mismatch stress between the piezoelectric film and the flexible substrate in the coating area 300A through localized plastic deformation. Simultaneously, it prevents cracks from extending into the coating area 300A, thereby further improving the fatigue performance and device lifespan of the flexible piezoelectric film. In each sub-chamber of the piezoelectric film fabrication, a mask is used to shield the non-coating area 300B. Precise positioning by the roll-to-roll conveyor ensures that the mask and the biphase gradient flexible substrate 300 are not misaligned, thus ensuring that the upper piezoelectric film 410 and the lower piezoelectric film 420 are formed only in the coating area 300A.

[0066] Furthermore, the uncoated region 300B is disposed at opposite ends of the coated region 300A to enhance the performance of the uncoated region 300B as a flexible hinge. Specifically, as Figure 5 As shown, during the fabrication of the piezoelectric structure, the biphase gradient flexible substrate 300 moves along the first direction D1 and passes through each chamber in sequence. The non-coated region 300B is disposed at opposite ends of the coated region 300A in the second direction D2, which is perpendicular to the first direction D1.

[0067] Based on the same inventive concept, this invention also provides a piezoelectric structure with a flexible substrate prepared using the aforementioned method. Please refer to [link to specific embodiments]. Figure 4, the piezoelectric structure comprises a dual-phase gradient flexible substrate 300, an upper piezoelectric film 410 and a lower piezoelectric film 420.

[0068] The dual-phase gradient flexible substrate 300 comprises oppositely arranged upper surface layer ultrafine crystal layer 310 and lower surface layer ultrafine crystal layer 320, and toughened martensite layer 330 between the upper surface layer ultrafine crystal layer 310 and the lower surface layer ultrafine crystal layer 320. The thickness of the upper surface layer ultrafine crystal layer 310 and the lower surface layer ultrafine crystal layer 320 is less than the thickness of the toughened martensite layer 330.

[0069] In a specific embodiment, the upper surface layer ultrafine crystal layer 310 and the lower surface layer ultrafine crystal layer 320 have the same structure and the same thickness, further forming a symmetrical structure on both sides of the dual-phase gradient flexible substrate 300, maintaining the geometric stability of the piezoelectric structure, and improving the piezoelectric performance of the piezoelectric structure. Further, the thickness of the upper surface layer ultrafine crystal layer 310 and the lower surface layer ultrafine crystal layer 320 is in the range of 100-300 nm.

[0070] The upper piezoelectric film 410 is arranged on the surface of the upper surface layer ultrafine crystal layer 310. In a specific embodiment, the upper piezoelectric film 410 comprises upper adhesive layer 415, upper bottom electrode 411, upper buffer layer 412, upper piezoelectric layer 413 and upper top electrode 414 arranged in sequence.

[0071] The lower piezoelectric film 420 is arranged on the surface of the lower surface layer ultrafine crystal layer 320, and the lower piezoelectric film 420 has the same structure as the upper piezoelectric film 410 and is synchronously manufactured. In a specific embodiment, the lower piezoelectric film 420 comprises lower adhesive layer 425, lower bottom electrode 421, lower buffer layer 422, lower piezoelectric layer 423 and lower top electrode 424 arranged in sequence.

[0072] The upper adhesive layer 415 and the lower adhesive layer 425 have the same thickness and the same structure, the upper bottom electrode 411 and the lower bottom electrode 421 have the same thickness and the same structure, the upper buffer layer 412 and the lower buffer layer 422 have the same thickness and the same structure, the upper piezoelectric layer 413 and the lower piezoelectric layer 423 have the same thickness and the same structure, and the upper top electrode 414 and the lower top electrode 424 have the same thickness and the same structure, thereby forming a symmetrical structure on both sides of the dual-phase gradient flexible substrate 300, effectively inhibiting the warping deformation of the dual-phase gradient flexible substrate 300, thereby maintaining its geometric stability, and doubling the output voltage / charge of the piezoelectric structure, thereby improving the sensitivity of the piezoelectric response of the flexible device and improving the piezoelectric output performance of the piezoelectric structure.

[0073] In a specific embodiment, as shown in FIG. 1, the piezoelectric structure comprises a dual-phase gradient flexible substrate 300, an upper piezoelectric film 410 and a lower piezoelectric film 420. Figure 5As shown, the bi-phase gradient flexible substrate 300 includes a plated area 300A and a non-plated area 300B connected with the plated area 300A, and the upper piezoelectric film 410 and the lower piezoelectric film 420 are only located in the plated area 300A. The non-plated area 300B acts as a flexible hinge to absorb the strain energy of the plated area 300A caused by the thermal mismatch stress between the piezoelectric film and the flexible substrate, while being able to block the crack from extending to the plated area 300A, thereby further improving the fatigue performance of the flexible piezoelectric film and the service life of the device. Further, the non-plated area 300B is arranged at the opposite ends of the plated area 300A to increase the performance of the non-plated area 300B as a flexible hinge.

[0074] In the piezoelectric structure provided by the embodiment of the present application, the piezoelectric film is formed on the surface of the surface layer ultra-fine crystal layer (the upper piezoelectric film 410 is formed on the surface of the upper surface layer ultra-fine crystal layer 310, and the lower piezoelectric film 420 is formed on the surface of the lower surface layer ultra-fine crystal layer 320), and in the cooling step after annealing, the thermal mismatch stress between the upper piezoelectric film 410 and the lower piezoelectric film 420 and the bi-phase gradient flexible substrate 300 drives the viscous slip of the grain boundary (the interface between the upper piezoelectric film 410 and the upper surface layer ultra-fine crystal layer 310 and the interface between the lower piezoelectric film 420 and the lower surface layer ultra-fine crystal layer 320), thereby releasing the strain energy thereof, which can effectively relax the thermal mismatch stress caused by the high-temperature recrystallization annealing in the process of forming the upper piezoelectric film 410 and the lower piezoelectric film 420, and avoid the micro-cracks, cracking and film-substrate interface delamination caused by large deformation of the upper piezoelectric film 410 and the lower piezoelectric film 420. Moreover, the toughened martensite layer 330 between the upper surface layer ultra-fine crystal layer 310 and the lower surface layer ultra-fine crystal layer 320 can effectively absorb the bending stress, thereby improving the bending fatigue life of the piezoelectric film.

[0075] The piezoelectric structure provided by the embodiment of the present application has the characteristics of resisting micro-cracks and cracking of the piezoelectric film, resisting film-substrate interface delamination, high piezoelectric performance, and resisting substrate warping deformation. The piezoelectric structure provided by the embodiment of the present application has the following characteristics: piezoelectric coefficient (d 33,f ) > 300 pC / N, leakage current density (Leakage) < 10 7 A / cm 2 , breakdown voltage (E anti ) > 80 V / μm, bending fatigue resistance (F) > 10 6 cycle, and bow degree (Bow) < 10 nm / 100 mm.

[0076] Figure 8 is a PZT piezoelectric coefficient and bending cycle number relationship diagram of the piezoelectric structure with a flexible substrate provided by the embodiment of the present application, Figure 9This is a graph showing the relationship between the PZT leakage current density and the number of bending cycles of a piezoelectric structure with a flexible substrate provided in a specific embodiment of the present invention. It can be seen that as the number of bending cycles increases, the PZT piezoelectric coefficient and the PZT leakage current density remain stable. The piezoelectric structure provided by the present invention has good resistance to bending fatigue.

[0077] Based on the same inventive concept, this invention also provides a fabrication apparatus for the aforementioned piezoelectric structure with a flexible substrate. For example... Figure 10 The diagram shows a schematic of a fabrication apparatus for a piezoelectric structure 430 with a flexible substrate provided in a specific embodiment of the present invention. The fabrication apparatus includes a high-temperature recrystallization and cold rolling chamber 500, used to heat the flexible substrate 200 at a set temperature for a certain time, and then perform multiple passes of cold rolling on the heated flexible substrate 200 to a set thickness to form a biphase gradient flexible substrate 300. The biphase gradient flexible substrate 300 has an upper surface ultrafine crystalline layer 310 on its upper surface and a lower surface ultrafine crystalline layer 320 on its lower surface. The upper surface ultrafine crystalline layer 310 and... The lower surface ultrafine crystalline layer 320 is separated by a toughened martensite layer 330; the piezoelectric film preparation chamber 510 includes multiple sub-chambers for simultaneously forming an upper piezoelectric film 410 and a lower piezoelectric film 420 with the same structure on the surface of the upper surface ultrafine crystalline layer 310 and the surface of the lower surface ultrafine crystalline layer 320 of the biphase gradient flexible substrate 300; the conveying device 520 passes sequentially through the high-temperature recrystallization and cold rolling combined chamber 500 and each sub-chamber of the piezoelectric film preparation chamber 510 for conveying the flexible substrate 200 and the biphase gradient flexible substrate 300.

[0078] As an example, the high-temperature recrystallization and cold rolling combined chamber 500 includes a heater 501 and a twin-roll cold rolling mill 502. The heater 501 is used to heat the flexible substrate 200, and the twin-roll cold rolling mill 502 is used to cold roll the flexible substrate 200. The piezoelectric film preparation chamber 510 includes an adhesion layer deposition sub-chamber 511, a bottom electrode deposition sub-chamber 512, a buffer layer deposition sub-chamber 513, a piezoelectric material layer deposition sub-chamber 514, a high-temperature annealing sub-chamber 515, and a top electrode deposition chamber 516. The conveying device 520 is a roll-to-roll conveying device. The roll-to-roll transport device drives the flexible substrate 200 through the high-temperature recrystallization and cold rolling combined chamber 500 to form the biphase gradient flexible substrate 300. The biphase gradient flexible substrate 300 is then transported sequentially through the adhesion layer deposition sub-chamber 511, the bottom electrode deposition sub-chamber 512, the buffer layer deposition sub-chamber 513, the piezoelectric material layer deposition sub-chamber 514, the high-temperature annealing sub-chamber 515, and the top electrode deposition chamber 516 to form an upper piezoelectric film layer and a lower piezoelectric film layer.

[0079] A baffle 550 is arranged in each sub-chamber of the piezoelectric thin film preparation chamber 510 to isolate each sub-chamber, and a cathode 530 is arranged opposite to each sub-chamber (for example, the adhesion layer deposition sub-chamber 511, the bottom electrode deposition sub-chamber 512, the buffer layer deposition sub-chamber 513, the piezoelectric material layer deposition sub-chamber 514, and the top electrode deposition chamber 516) in which sputtering needs to be performed, and the double-sided gradient flexible substrate 300 passes between the two cathodes 530 to achieve double-sided sputtering. Two heating elements 540 are arranged opposite to each other in the high-temperature annealing sub-chamber 515, and the double-sided gradient flexible substrate 300 passes between the two heating elements 540 to achieve double-sided heat annealing of the piezoelectric material layer.

[0080] The present application also provides a preparation method of a piezoelectric structure with a flexible substrate.

[0081] Embodiment 1 (1) High-temperature recrystallization and cold rolling of the stainless steel substrate: The stainless steel (initial thickness 500 μm) is placed in a high-temperature recrystallization and cold rolling combined chamber, and first high-temperature recrystallization is performed at an argon flow rate of 80 sccm, a pressure of 0.5 Pa, a temperature increasing rate of 10°C / min, and a holding time of 15 min; then cold rolling is performed, and the stainless steel substrate is stepped into a double-roller cold rolling machine, the first pass reduction is 18% (thickness to 410 μm), the second pass reduction is 20% (thickness to 328 μm), the third pass reduction is 24% (thickness to 250 μm), and the total reduction is 50%; (2) Double-sided metal Ti adhesion layer plating of the stainless steel: The high-temperature recrystallization and cold rolling treated stainless steel substrate is stepped into a Ti chamber, argon flow rate is 80 sccm, pressure is controlled at 0.5 Pa, room temperature direct current sputtering power is 500 W, target-substrate distance is 20 mm, sputtering time is 10 s, and the thickness of the Ti adhesion layer is controlled at 50 nm; (3) Double-sided metal Pt bottom electrode layer plating of the stainless steel: The stainless steel substrate with double-sided Ti adhesion layer plating is stepped into a Pt chamber, and the stainless steel is double-sided plated with a Pt bottom electrode layer, argon flow rate is 80 sccm, pressure is controlled at 0.5 Pa, room temperature direct current sputtering power is 500 W, target-substrate distance is 20 mm, sputtering time is 60 s, and the thickness of the Pt bottom electrode layer is controlled at 150 nm; (4) Double-sided LNO oxide buffer layer plating of the stainless steel: The stainless steel substrate with double-sided Pt bottom electrode layer plating is stepped into an LNO chamber, and the stainless steel is double-sided plated with an LNO oxide buffer layer, argon flow rate is 80 sccm, oxygen flow rate is 20 sccm, pressure is controlled at 0.5 Pa, room temperature direct current sputtering power is 1000 W, target-substrate distance is 30 mm, sputtering time is 300 s, and the thickness of the LNO oxide buffer layer is controlled at 150 nm; (5) stainless steel double side plated oxide PZT piezoelectric layer: the stainless steel substrate double side plated LNO buffer layer is stepped into the PZT chamber, the stainless steel double side plated oxide PZT piezoelectric layer, the argon flow is 80 sccm, the oxygen flow is 20 sccm, the gas pressure control is 0.5 Pa, the room temperature radio frequency magnetron sputtering power is 1000 W, the target base distance is 30 mm, the sputtering time is 3600 s, and the LNO oxide buffer layer thickness is controlled to be 2000 nm; (6) PZT piezoelectric layer high temperature recrystallization: the stainless steel substrate double side plated PZT at room temperature is stepped into the high temperature heat annealing chamber, the PZT piezoelectric layer is subjected to high temperature heat annealing, the argon flow is 80 sccm, the oxygen flow is 20 sccm, the gas pressure control is 0.5 Pa, the heating rate is 20 ℃ / min (from room temperature to 550 ℃), the holding time is 900 s, and the cooling rate is 40 ℃ / min; (7) the stainless steel substrate double side high temperature heat annealed PZT piezoelectric film is stepped into the Pt chamber, and the top electrode is deposited in cooperation with a mask plate, and the specific process parameters and steps (3) are consistent; (8) the above step controls the tension of the roll-to-roll motor to be 5 N, and the stepping transmission rate is 0.1 m / min.

[0082] It should be noted that the terms "include" and "have" and their variants involved in the file of the present application are intended to cover non-exclusive inclusion. The terms "first", "second" and the like are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence, unless the context clearly indicates otherwise, and it should be understood that the data thus used can be interchanged under appropriate circumstances. The term "one or more" can be used to describe a feature, structure or characteristic in a singular sense, or can be used to describe a combination of features, structures or characteristics in a plural sense, depending at least in part on the context. The term "based on" can be understood as not necessarily expressing a set of exclusive factors, but can instead, depending at least in part on the context, allow the presence of other factors not necessarily explicitly described. In addition, the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. Furthermore, in the above description, the description of well-known components and technologies is omitted to avoid unnecessary confusion of the concepts of the present application. In each of the above embodiments, each embodiment focuses on the difference from other embodiments, and the same / similar parts between the embodiments can be referred to each other.

[0083] The above only describes the preferred embodiments of the present application, and it should be noted that those skilled in the art can make several improvements and refinements without departing from the principles of the present application, and these improvements and refinements should also be considered within the protection scope of the present application.

Claims

1. A method for fabricating a piezoelectric structure with a flexible substrate, characterized in that, include: A flexible substrate is provided, the flexible substrate including an upper surface and a lower surface disposed opposite to each other; The flexible substrate is heated at a set temperature for a certain period of time, and the heated flexible substrate is cold rolled in multiple passes to a set thickness to form a biphase gradient flexible substrate. The biphase gradient flexible substrate has an upper surface ultrafine crystalline layer on the upper surface and a lower surface ultrafine crystalline layer on the lower surface. A toughened martensite layer is located between the upper surface ultrafine crystalline layer and the lower surface ultrafine crystalline layer. The set temperature is higher than the recrystallization temperature of the flexible substrate. An upper piezoelectric thin film and a lower piezoelectric thin film with the same structure are simultaneously formed on the surface of the upper and lower ultrafine crystalline layers of the dual-phase gradient flexible substrate.

2. The method for fabricating a piezoelectric structure with a flexible substrate according to claim 1, characterized in that, The flexible substrate is one of stainless steel, titanium and titanium alloys, nickel-based alloys, copper and copper alloys.

3. The method for fabricating a piezoelectric structure with a flexible substrate according to claim 1, characterized in that, The step of heating the flexible substrate at a set temperature for a certain period of time further includes: the set temperature is 300~900 degrees Celsius.

4. The method for fabricating a piezoelectric structure with a flexible substrate according to claim 3, characterized in that, The step of heating the flexible substrate at a set temperature for a certain period of time further includes: the heating time being greater than 10 minutes.

5. The method for fabricating a piezoelectric structure with a flexible substrate according to claim 1, characterized in that, The flexible substrate is heated to the set temperature at a set heating rate, the set heating rate being in the range of 5~20℃ / min.

6. The method for fabricating a piezoelectric structure with a flexible substrate according to claim 1, characterized in that, The step of heating the flexible substrate at a set temperature for a certain period of time and then performing multiple cold rolling passes on the heated flexible substrate to a set thickness further includes: heating and cold rolling of the flexible substrate are carried out in the same chamber.

7. The method for fabricating a piezoelectric structure with a flexible substrate according to claim 1, characterized in that, The cold rolling temperature is lower than the set temperature.

8. The method for fabricating a piezoelectric structure with a flexible substrate according to claim 1, characterized in that, The step of performing multiple cold rolling operations on the heated flexible substrate to a set thickness further includes: performing at least three cold rolling operations on the heated flexible substrate to a set thickness.

9. The method for fabricating a piezoelectric structure with a flexible substrate according to claim 1, characterized in that, The dual-phase gradient flexible substrate includes a coated region and a non-coated region connected to the coated region. The step of simultaneously forming an upper piezoelectric thin film and a lower piezoelectric thin film with the same structure on the surface of the upper ultrafine crystalline layer and the surface of the lower ultrafine crystalline layer of the dual-phase gradient flexible substrate further includes forming an upper piezoelectric thin film and a lower piezoelectric thin film with the same structure only on the surface of the upper ultrafine crystalline layer and the surface of the lower ultrafine crystalline layer of the coated region of the dual-phase gradient flexible substrate.

10. The method for fabricating a piezoelectric structure with a flexible substrate according to claim 9, characterized in that, The non-coated areas are located at opposite ends of the coated areas.

11. The method for fabricating a piezoelectric structure with a flexible substrate according to claim 1, characterized in that, The step of simultaneously forming an upper piezoelectric thin film and a lower piezoelectric thin film with the same structure on the surface of the upper and lower ultrafine crystalline layers of the dual-phase gradient flexible substrate further includes: An upper bottom electrode and a lower bottom electrode are simultaneously formed on the surface of the upper and lower ultrafine crystalline layers of the dual-phase gradient flexible substrate. An upper buffer layer and a lower buffer layer are simultaneously formed on the surfaces of the upper bottom electrode and the lower bottom electrode; An upper piezoelectric material layer and a lower piezoelectric material layer are simultaneously formed on the surfaces of the upper buffer layer and the lower buffer layer; The upper piezoelectric material layer and the lower piezoelectric material layer are thermally annealed to crystallize them, forming the upper piezoelectric layer and the lower piezoelectric layer. An upper top electrode and a lower top electrode are simultaneously formed on the surfaces of the upper piezoelectric layer and the lower piezoelectric layer.

12. The method for fabricating a piezoelectric structure with a flexible substrate according to claim 11, characterized in that, The step of thermally annealing the upper piezoelectric material layer and the lower piezoelectric material layer further includes: the thermal annealing temperature is lower than the thermal annealing temperature of the piezoelectric material layer formed directly on the flexible substrate.

13. A piezoelectric structure with a flexible substrate, characterized in that, include: A dual-phase gradient flexible substrate, comprising an upper surface ultrafine crystalline layer and a lower surface ultrafine crystalline layer disposed opposite to each other, and a toughened martensite layer located between the upper surface ultrafine crystalline layer and the lower surface ultrafine crystalline layer; An upper piezoelectric thin film is disposed on the surface of the upper ultrafine crystalline layer; A lower piezoelectric thin film is disposed on the surface of the lower ultrafine crystalline layer. The lower piezoelectric thin film has the same structure as the upper piezoelectric thin film and is fabricated simultaneously.

14. The piezoelectric structure with a flexible substrate according to claim 13, characterized in that, The upper surface ultrafine crystalline layer and the lower surface ultrafine crystalline layer have the same structure and the same thickness.

15. The piezoelectric structure with a flexible substrate according to claim 13, characterized in that, The thickness range of both the upper and lower surface ultrafine crystalline layers is 100~300nm.

16. The piezoelectric structure with a flexible substrate according to claim 13, characterized in that, The dual-phase gradient flexible substrate includes a coated region and a non-coated region connected to the coated region, and the upper piezoelectric thin film and the lower piezoelectric thin film are located only in the coated region.

17. The piezoelectric structure with a flexible substrate according to claim 16, characterized in that, The non-coated areas are located at both ends of the coated areas.

18. An apparatus for fabricating a piezoelectric structure with a flexible substrate as described in any one of claims 13-17, characterized in that, include: A high-temperature recrystallization and cold rolling combined chamber is used to heat a flexible substrate at a set temperature for a certain period of time, and then perform multiple cold rolling passes on the heated flexible substrate to a set thickness to form a biphase gradient flexible substrate. The biphase gradient flexible substrate has an upper surface ultrafine crystalline layer on the upper surface and a lower surface ultrafine crystalline layer on the lower surface. A toughened martensite layer is located between the upper surface ultrafine crystalline layer and the lower surface ultrafine crystalline layer. The piezoelectric thin film fabrication chamber includes multiple sub-chambers for simultaneously forming an upper piezoelectric thin film and a lower piezoelectric thin film with the same structure on the surface of the upper and lower ultrafine crystalline layers of the biphase gradient flexible substrate. The conveying device sequentially passes through each sub-chamber of the high-temperature recrystallization and cold rolling combined chamber and the piezoelectric thin film preparation chamber to convey the flexible substrate and the dual-phase gradient flexible substrate.

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