Piezoelectric flexible MEMS ultrasonic transducer and fabrication process

Abstract

The application discloses a piezoelectric flexible MEMS ultrasonic transducer and a preparation process, and all process steps are manufactured by using standard MEMS process. The transducer is wrapped by flexible material, and electrode material is also flexible. The application uses the manufacturing according to the MEMS process to ensure high efficiency and mass production in manufacturing, uses the wrapping of flexible material to ensure the flexibility of the whole device, uses the flexible electrode and the snake-shaped circuit to ensure the flexible functions such as bending, twisting and stretching, and uses specific manufacturing standards to ensure high consistency in manufacturing.

Description

Technical Field

[0001] This invention pertains to MEMS and ultrasonic transducer technology, and specifically relates to a piezoelectric flexible MEMS ultrasonic transducer and its fabrication process. Background Technology

[0002] Ultrasonic technology, due to its non-invasive, penetrating, and biocompatible characteristics, is widely used in fields such as medical and health care, industrial non-destructive testing, chemical production, and military sonar. Ultrasonic transducers, used for transmitting and receiving ultrasonic waves, are fundamental components for ultrasonic technology and play a decisive role in the performance of the entire ultrasonic system. There are many types of ultrasonic transducers, the most common being piezoelectric ultrasonic transducers. These transducers use the piezoelectric effect to convert ultrasonic signals into electrical signals, enabling the transmission and reception of ultrasonic waves. Piezoelectric ultrasonic transducers are also a research direction in MEMS ultrasonic transducers. Compared to traditional ultrasonic transducers, MEMS ultrasonic transducers are smaller, lighter, lower in cost, lower in power consumption, and easier to integrate with circuits. Currently, piezoelectric ultrasonic transducers are quite mature in terms of design theory, fabrication, and performance. With the continuous advancement of wafer-level material growth technology and the continuous improvement of MEMS processes and production lines, the potential of piezoelectric ultrasonic transducers is being further explored, ushering in a new era of technological upgrading in more and more fields.

[0003] However, current rigid sensors cannot achieve good coupling with irregular skin surfaces, leading to unreliable test results. Compared to rigid probes, wearable ultrasound transducers based on flexible materials offer advantages such as good impedance matching with the skin and seamless integration. Current research on flexible ultrasound probes can be divided into three main categories: using organic piezoelectric films as transducers, embedding piezoelectric ceramics into polymer substrates, and inserting a flexible layer between the rigid probe and the skin. Organic piezoelectric films, such as polyvinylidene fluoride (PVDF), possess good flexibility. However, their low electromechanical coupling coefficient and high dielectric loss result in low receiving and transmitting sensitivity, significantly reducing their measurement accuracy. Piezoelectric ceramics have good mechanical and electrical properties, and polymer substrates can conform to the skin, making devices embedding piezoelectric materials into polymer substrates suitable for BP waveform monitoring. Among these, some researchers use polyimide or flexible PCB circuits as substrates to give the devices flexible properties. However, due to the high elastic modulus of these polymer materials, the devices lack ductility and are difficult to adapt to the movement of human skin. Therefore, some ultrasonic devices use PDMS (polydimethylsiloxane) or Eco-flex materials with low Young's modulus to encapsulate polyimide-based serpentine electrodes, which are then bonded to tiny piezoelectric blocks (0.9 mm) using conductive silver binder. This gives the device high acoustic sensitivity and good flexibility, allowing for conformal contact with the body surface. However, despite the existence of tiny piezoelectric blocks and electrodes, the fabrication techniques used to manufacture these devices are not standardized and are incompatible with MEMS processes. In the fabrication techniques, manually applying conductive silver binder and bonding piezoelectric block units onto the tiny electrodes is extremely difficult and time-consuming, resulting in low yield, poor performance consistency, and difficulties in mass production. It is evident that although existing flexible ultrasonic transducers can conform to the skin, the incompatibility of processing techniques with MEMS leads to difficulties in device fabrication and poor device consistency, limiting their widespread application in many fields. This highlights the urgent need for the development of flexible ultrasonic transducers.

[0004] In summary, while current research has made significant progress in piezoelectric flexible ultrasonic transducers, its compatibility with MEMS processes is not ideal. This limits the large-area manufacturing of ultrasonic transducers and affects their processing efficiency. Furthermore, compared to all-MEMS processes, current piezoelectric flexible ultrasonic transducer manufacturing processes exhibit significant inconsistencies in terms of consistency. Therefore, the manufacturing process for piezoelectric flexible ultrasonic transducers needs to be compatible with MEMS processes to achieve mass production and high efficiency.

[0005] Currently, the manufacturing process for piezoelectric flexible ultrasonic transducers, which embed piezoelectric ceramics into polymers, utilizes polymer materials with low Young's modulus to achieve high acoustic sensitivity and good ductility. However, the manufacturing process is not fully compatible with MEMS technology, inevitably leading to inconsistencies during production and limiting its mass production and large-area manufacturing. This restricts its widespread application in many fields. Therefore, it is necessary to innovate the manufacturing process to meet the requirements of compatibility with MEMS technology, in order to achieve mass production, high consistency, and large-area manufacturing of such piezoelectric flexible ultrasonic transducers. Summary of the Invention

[0006] To address the problems existing in the prior art, the present invention aims to provide a piezoelectric flexible MEMS ultrasonic transducer and its fabrication process. The present invention enables the mass production of flexible ultrasonic transducers with high consistency and large area. Specifically for piezoelectric flexible ultrasonic transducers that embed piezoelectric ceramics in polymers, the invention effectively improves the manufacturing efficiency of such flexible ultrasonic transducers, enables large-area manufacturing, reduces the inconsistencies of manual manufacturing, and achieves good acoustic impedance matching characteristics while adhering well to human skin.

[0007] The technical solution adopted in this invention is as follows:

[0008] A piezoelectric flexible MEMS ultrasonic transducer includes a top flexible layer, a flexible upper electrode layer, a piezoelectric layer, a flexible lower electrode layer, and a flexible substrate. The flexible upper electrode layer, piezoelectric layer, flexible lower electrode layer, and flexible substrate are arranged sequentially from top to bottom. The piezoelectric layer includes multiple piezoelectric transducer units. The upper ends of the multiple piezoelectric transducer units are connected by connecting lines in the flexible upper electrode layer, and the lower ends of the multiple piezoelectric transducer units are connected by connecting lines in the flexible lower electrode layer.

[0009] The space between each piezoelectric transducer unit in the piezoelectric layer and the space between the connecting lines in the flexible lower electrode layer and the upper surface of the flexible substrate are filled with a central flexible layer material, which serves as the central flexible layer.

[0010] The area between the connecting lines in the flexible upper electrode layer is filled with a top flexible layer material, and a top flexible layer material is provided on the upper surface of the flexible upper electrode layer. The top flexible layer material filling the area between the connecting lines in the flexible upper electrode layer and the top flexible layer material provided on the upper surface of the flexible upper electrode layer constitute the top flexible layer.

[0011] Preferably, the surface of the flexible upper electrode layer is provided with a support structure at a position directly opposite to each piezoelectric transducer unit, and the top flexible layer is disposed on the surface of the support structure and fills the area between each support structure.

[0012] The support structure includes a support layer and a cavity sealing layer. The support layer is disposed on the surface of the flexible upper electrode layer. A cavity is formed on the surface of the support layer away from the flexible upper electrode layer. The cavity sealing layer is provided on this surface of the support layer to seal the cavity opening.

[0013] Preferably, the flexible lower electrode layer includes a conductive adhesive layer, a flexible lower electrode, and a lower electrode-substrate connection layer. The lower electrode-substrate connection layer is disposed on the surface of the flexible substrate, the flexible lower electrode is disposed on the surface of the lower electrode-substrate connection layer, and the conductive adhesive layer is disposed between the flexible lower electrode and the piezoelectric layer.

[0014] Preferably, the conductive adhesive layer and the piezoelectric layer have the same lateral shape and size. The area on the flexible lower electrode that is directly opposite the piezoelectric layer and the area that is directly opposite the connecting line in the flexible lower electrode layer are set as blank areas. The flexible lower electrode and the lower electrode-substrate connection layer have the same shape.

[0015] Preferably, the material of the lower electrode-substrate connection layer is polyimide or polyethylene terephthalate, and the material of the conductive adhesive layer is conductive silver paste.

[0016] Preferably, the piezoelectric layer is made of PZT-5H rigid piezoelectric material or type 1-3 piezoelectric composite material.

[0017] Preferably, the top flexible layer, the middle flexible layer, and the flexible substrate are made of PDMS flexible material.

[0018] Preferably, both the flexible upper electrode layer and the flexible lower electrode layer adopt a serpentine circuit.

[0019] Preferably, the surface of the flexible substrate has a bottom gel layer.

[0020] The fabrication process of the piezoelectric flexible MEMS ultrasonic transducer of the present invention, as described above, includes the following steps:

[0021] A flexible material is spin-coated onto the surface of a glass slide, and the flexible material is then treated with oxygen plasma to form a flexible substrate.

[0022] A flexible lower electrode layer is fabricated on a flexible substrate;

[0023] A piezoelectric layer is attached to the flexible lower electrode layer;

[0024] The piezoelectric layer was etched into multiple piezoelectric transducer units using laser ablation.

[0025] Patterning of the flexible lower electrode layer was performed using laser ablation.

[0026] After the flexible lower electrode layer is patterned, a central flexible layer material is spin-coated onto the surface of the piezoelectric layer, so that the central flexible layer material fills the space between each piezoelectric transducer unit in the piezoelectric layer, as well as the space between the connecting lines in the flexible lower electrode layer and the upper surface of the flexible substrate.

[0027] After the flexible layer material in the middle is spin-coated, a metal layer is formed on the surface of the piezoelectric layer 4 using a magnetron sputtering method;

[0028] The metal layer is patterned using a laser ablation method, and an upper electrode layer is formed by etching.

[0029] The piezoelectric flexible MEMS ultrasonic transducer is obtained by spin-coating a top flexible layer material onto the surface of the flexible top electrode layer.

[0030] Preferably, the flexible lower electrode layer includes a conductive adhesive layer, a flexible lower electrode, and a lower electrode-substrate connection layer. The lower electrode-substrate connection layer is disposed on the surface of the flexible substrate, the flexible lower electrode is disposed on the surface of the lower electrode-substrate connection layer, and the conductive adhesive layer is disposed between the flexible lower electrode and the piezoelectric layer.

[0031] When fabricating a flexible lower electrode layer on a flexible substrate, the lower electrode-substrate connection layer and the flexible lower electrode layer are pressed onto the flexible substrate; then a conductive adhesive layer is spin-coated onto the surface of the flexible lower electrode, the piezoelectric layer is placed on the conductive adhesive layer, and then baked to bond the flexible lower electrode and the piezoelectric layer through the conductive adhesive layer.

[0032] When the piezoelectric layer is etched into multiple piezoelectric transducer units by laser ablation, the conductive adhesive layer is etched together with the piezoelectric layer, and the etching stops at the surface of the flexible lower electrode.

[0033] When patterning the flexible lower electrode layer using laser ablation, the flexible lower electrode and the lower electrode-substrate connection layer are etched together, and the etching stops at the surface of the flexible substrate.

[0034] The present invention has the following beneficial effects:

[0035] Compared to existing technologies, this invention features a MEMS-compatible process. The piezoelectric flexible MEMS ultrasonic transducer of this invention can be manufactured entirely using standard MEMS processes, achieving high consistency and enabling large-area manufacturing, thereby improving the manufacturing efficiency of the piezoelectric flexible ultrasonic transducer. Furthermore, this invention employs a top flexible layer, a flexible upper electrode layer, a flexible lower electrode layer, and a flexible substrate, and the piezoelectric layer includes multiple independent piezoelectric transducer units. Therefore, the piezoelectric flexible MEMS ultrasonic transducer of this invention possesses excellent flexibility, enabling bending and torsion. In summary, the piezoelectric flexible MEMS ultrasonic transducer of this invention achieves compatibility with MEMS processes, enabling large-scale, efficient manufacturing, avoiding inconsistencies inherent in manual manufacturing, and offering advantages such as long service life.

[0036] Furthermore, the present invention features a piezoelectric unit wrapped in flexible PDMS material, and the connection between each piezoelectric unit adopts a serpentine circuit after patterning flexible electrodes. In terms of device flexibility, it can achieve a large bending and twisting, so it can fit the skin surface of different shapes.

[0037] Furthermore, the present invention has a bottom gel layer on the surface of the flexible substrate. The self-adhesive hydrogel film enables the transducer of the present invention to have good adhesion to the skin, improves the coupling between the device and the skin, and has good acoustic impedance matching characteristics.

[0038] The piezoelectric flexible MEMS ultrasonic transducer of this invention is fabricated using standard MEMS processes, which helps to enhance the overall robustness of the device, prevents the splitting of structural layers during use, and contributes to improving the overall performance and lifespan of the device. Attached Figure Description

[0039] Figure 1 This is a schematic diagram (longitudinal cross-sectional view) of a piezoelectric flexible MEMS ultrasonic transducer in one embodiment of the present invention;

[0040] Figure 2 This is a schematic diagram (longitudinal cross-sectional view) of a piezoelectric flexible MEMS ultrasonic transducer in another embodiment of the present invention;

[0041] Figure 3 This is a schematic diagram showing the piezoelectric transducer units in the piezoelectric layer of the present invention connected by connecting lines;

[0042] Figure 4 For the present invention Figure 1 The fabrication process flow diagram of the piezoelectric flexible MEMS ultrasonic transducer of the embodiment shown is illustrated.

[0043] In the figure: 1-Top flexible layer, 2-Flexible upper electrode, 3-Middle flexible layer, 4-Piezoelectric layer, 4-1-Piezoelectric transducer unit, 5-Conductive adhesive layer, 6-Flexible lower electrode, 7-Lower electrode-substrate connection layer, 8-Flexible substrate, 9-Bottom gel layer, 10-Slide, 11-PI layer, 12-Copper foil, 13-PZT-5H piezoelectric film, 14-Conductive silver paste, 15-Top Au / Ti metal layer, 16-Si layer, 17-Cavity, 18-SiO2 layer, 19-Connecting line. Detailed Implementation

[0044] The present invention will now be described in detail with reference to the accompanying drawings and embodiments:

[0045] like Figure 1 As shown, the piezoelectric flexible MEMS ultrasonic transducer in this embodiment includes a top flexible layer 1, a flexible upper electrode 2, a middle flexible layer 3, a piezoelectric layer 4, a conductive adhesive layer 5, a flexible lower electrode 6, a lower electrode-substrate connection layer 7, a flexible substrate 8, and a bottom gel layer 9. The top flexible layer 1, middle flexible layer 3, and flexible substrate 8 are sequentially connected and are made of the same material. The top flexible layer 1, middle flexible layer 3, and flexible substrate 8 completely enclose the piezoelectric transducer unit and its serpentine circuit. The flexible layers (i.e., top flexible layer 1, middle flexible layer 3, and flexible substrate 8) are interconnected as a whole, with each piezoelectric transducer unit individually enclosed within them, and the upper and lower electrodes connected to the transducer units respectively. The flexible upper electrode 2, piezoelectric layer 4, and flexible lower electrode 6 are stacked sequentially from top to bottom. The piezoelectric layer 4 and flexible lower electrode 6 are connected by the conductive adhesive layer 5, and the connection between the flexible lower electrode 6 and the flexible substrate 8 is achieved through the lower electrode-substrate connection layer 7. Specifically, the top flexible layer 1 covers the flexible upper electrode 2, the flexible substrate 7 covers the flexible lower electrode 6, and the middle flexible layer 3 contacts the top flexible layer 1 and the flexible substrate 7, isolating the flexible upper electrode 2 and the flexible lower electrode 6 and filling the space between the piezoelectric layer 4. The flexible upper electrode 2, the piezoelectric layer (4), the conductive adhesive layer 5 and the flexible lower electrode 6 have the same shape and the same lateral dimensions.

[0046] Among them, the top flexible layer 1, the middle flexible layer 3, and the flexible substrate 8 are made of PDMS flexible material. The purpose of using flexible material is to ensure the overall flexibility of the device. PDMS flexible material can ensure the overall flexibility of the device, realize the flexibility requirements such as stretching, torsion, and bending, and can fit complex curved surfaces well.

[0047] The piezoelectric layer 4 uses a rigid piezoelectric material such as PZT-5H. Using a rigid piezoelectric material can ensure the integrity of the device's working performance. The piezoelectric layer 4 can also use a type 1-3 composite piezoelectric material. Compared with traditional piezoelectric ceramic materials, type 1-3 composite piezoelectric materials have the advantages of low acoustic impedance, good flexibility, low density, and higher electromechanical properties.

[0048] The flexible upper electrode 2 mainly uses flexible metal materials such as Au and Ti, while the flexible lower electrode 6 mainly uses flexible metal materials such as Cu. It also employs a patterned upper electrode design. (See [link to relevant documentation]). Figure 3 The flexible upper electrode 2 and flexible lower electrode 6 are made of flexible materials, which makes the serpentine circuit very flexible and helps to improve the overall flexibility of the device.

[0049] Furthermore, adding silver nanowires to the flexible upper electrode (including flexible upper electrode 2 and flexible lower electrode 6) can further enhance the bonding strength between the flexible upper electrode and the piezoelectric layer and improve the flexibility of the flexible upper electrode itself.

[0050] The lower electrode-substrate connection layer 7 is made of flexible materials such as PI (Polyimide) and PET (polyethylene terephthalate), and adopts the same patterned design as the flexible lower electrode 6.

[0051] The top flexible layer 1, the middle flexible layer 3, and the flexible substrate 8 are all formed by spin coating, which is compatible with MEMS processes. The resulting flexible layers are more uniform, have better adhesion, and more tightly wrap the piezoelectric transducer unit, which is beneficial to improving the flexibility and service life of the device.

[0052] The conductive adhesive layer 5 uses conductive silver paste as an adhesive, which is uniformly spin-coated using MEMS technology and then cured at high temperature to firmly connect the piezoelectric layer and the conductive layer. This achieves compatibility with MEMS technology during manufacturing and has better adhesion strength than manual spin-coating, preventing the piezoelectric layer from falling off the conductive layer and causing the acoustic and electrical signals to fail to convert.

[0053] The bonding method between the flexible lower electrode 6 and the flexible substrate 8 adopts a MEMS-compatible lamination method and is subjected to plasma oxidation treatment to improve the bonding strength between the two, thereby preventing the flexible layer from falling off during use and improving the service life of the device under long-term bending and torsion.

[0054] The flexible lower electrode 6 and the flexible upper electrode 2 were formed using sputtering and etching processes in standard MEMS technology. The piezoelectric layer 4 was also formed using etching processes in standard MEMS technology to improve the consistency of the device manufacturing process.

[0055] The bottom gel layer 9 is a self-adhesive hydrogel film. After the transducer of the present invention is encapsulated and cured, it is formed on the flexible substrate 8 using a processing method compatible with MEMS, so as to achieve good and close adhesion between the flexible device and the skin for a longer period of time.

[0056] The manufacturing process of the piezoelectric flexible ultrasonic transducer device adopts MEMS-compatible process steps, and the compatible process can be applied to other piezoelectric flexible ultrasonic transducers with similar structures using the same materials.

[0057] Reference Figure 4 The fabrication method of the piezoelectric flexible MEMS ultrasonic transducer according to the above embodiments of the present invention specifically includes the following steps:

[0058] (1) Take a glass slide 10 and clean it. The glass slide 10 is a low-resistivity double-sided polished single crystal silicon wafer.

[0059] (2) A layer of PDMS substrate is spin-coated on the surface of glass slide 10 and cured in a vacuum oven at 80-100°C for 1-2 hours. The PDMS substrate is then subjected to oxygen plasma treatment to form a flexible substrate 8. Plasma oxidation treatment is used to enhance the surface adhesion of the flexible substrate 8.

[0060] (3) The PI layer 12 (25μm) and the copper foil 11 (17μm) are laminated and bonded on the flexible substrate 8 under a preset pressure, wherein the PI layer 12 serves as the lower electrode-substrate connection layer 7 and the copper foil 11 serves as the flexible lower electrode 6.

[0061] (4) Spin coat conductive silver paste adhesive 14 onto the laminated copper foil 11, bond PZT-5H piezoelectric film 13 to the copper foil 11 through conductive silver paste adhesive 14, and bake at 150-200℃ for 30-50 minutes. Conductive silver paste adhesive 14 forms conductive adhesive layer 5, and PZT-5H piezoelectric film 13 serves as piezoelectric layer 4.

[0062] (5) The PZT-5H film 13 and its adhesive layer 14 are etched into multiple small units (i.e., piezoelectric transducer units 4-1) using a laser ablation system.

[0063] (6) The PI layer 12 and the copper foil 11 are patterned by using a laser ablation system. The etching stops at the surface of the flexible substrate 8 to construct a bottom serpentine circuit pattern with a resolution of 0.15 mm, thereby obtaining the flexible lower electrode 6 and the lower electrode-substrate connection layer 7.

[0064] (7) PDMS is coated on the surface of the device obtained in step (6) by spin coating to form a central flexible layer 3. The central flexible layer 3 can isolate the bottom electrode and fill the gap between the piezoelectric units of the piezoelectric layer PZT-5H.

[0065] (8) A top Au / Ti metal layer 15 is formed on the surface of the piezoelectric layer 4 using a magnetron sputtering process. In the top Au / Ti metal layer 15, the thickness of the Ti layer is 30 nm and the thickness of the Au layer is 200 nm.

[0066] (9) The top Au / Ti metal layer 15 is patterned and etched using a laser ablation system to form a flexible upper electrode 2;

[0067] (10) Spin-coat a layer of PDMS onto the surface of the flexible top electrode 2 as the top flexible layer 1, and bake it at 80-100°C for 1-2 hours;

[0068] (11) The integral device obtained in step (10) is treated with benzophenone solution and then bonded together with the self-adhesive hydrogel prepolymer in a mold by UN irradiation to form the bottom gel layer 9.

[0069] Figure 2 The diagram shown illustrates the structure of another piezoelectric flexible MEMS ultrasonic transducer according to the present invention. Figure 1 A modified structure based on the structure shown. Figure 2 The changing structure shown is Figure 1 The difference in the structure shown is that, in the original structure, the thickness of the piezoelectric layer 4 is reduced, and a SiO2 layer 18 and a Si layer 16 with a cavity 17 are added to the top of the piezoelectric transducer unit to form a piezoelectric unit structure with a support structure. Specifically, the Si layer 16 serves as a support layer and is disposed on the surface of the flexible upper electrode layer. The top surface of the Si layer 16 has an inwardly formed cavity 17, and the SiO2 layer 18 serves as a cavity sealing layer disposed on the top surface of the Si layer 16 and seals the cavity 17. At this time, the top flexible layer 1 covers the surface of the SiO2 layer 18 and fills the area between each piezoelectric unit structure with a support structure. In this embodiment, a cavity-type structure similar to that of a traditional PMUT (piezoelectric micromechanical ultrasonic transducer) is adopted, and the thickness of the piezoelectric layer 4 is reduced, so that the piezoelectric layer 4 serves as a vibrating film. This changes the basic working vibration mode of the ultrasonic transducer in this embodiment, enabling it to reach a higher resonant frequency to adapt to more types of application scenarios.

[0070] The above description is only one embodiment of the present invention, and not all or the only embodiment. Any equivalent modifications made by those skilled in the art to the technical solution of the present invention by reading the present invention specification are covered by the claims of the present invention.

Claims

1. A fabrication process for a piezoelectric flexible MEMS ultrasonic transducer, characterized in that, The piezoelectric flexible MEMS ultrasonic transducer includes a top flexible layer, a flexible upper electrode layer, a piezoelectric layer, a flexible lower electrode layer, and a flexible substrate. The flexible upper electrode layer, piezoelectric layer, flexible lower electrode layer, and flexible substrate are arranged sequentially from bottom to top. The piezoelectric layer includes multiple piezoelectric transducer units. The upper ends of the multiple piezoelectric transducer units are connected by connecting lines in the flexible upper electrode layer, and the lower ends of the multiple piezoelectric transducer units are connected by connecting lines in the flexible lower electrode layer. The spaces between the piezoelectric transducer units in the piezoelectric layer and the spaces between the connecting lines in the flexible lower electrode layer and the upper surface of the flexible substrate are filled with a middle flexible layer material. The top flexible layer is disposed on the upper surface of the flexible upper electrode layer and fills the areas between the connecting lines in the flexible upper electrode layer. The preparation process includes the following steps: A flexible material is spin-coated onto the surface of a glass slide, and the flexible material is then treated with oxygen plasma to form a flexible substrate. A flexible lower electrode layer is fabricated on a flexible substrate; A piezoelectric layer is attached to the flexible lower electrode layer; The piezoelectric layer was etched into multiple piezoelectric transducer units using laser ablation. Patterning of the flexible lower electrode layer was performed using laser ablation. After the flexible lower electrode layer is patterned, a central flexible layer material is spin-coated onto the surface of the piezoelectric layer, so that the central flexible layer material fills the space between each piezoelectric transducer unit in the piezoelectric layer, as well as the space between the connecting lines in the flexible lower electrode layer and the upper surface of the flexible substrate. After the flexible layer material in the middle is spin-coated, a metal layer is formed on the surface of the piezoelectric layer using a magnetron sputtering method; The metal layer is patterned using a laser ablation method, and an upper electrode layer is formed by etching. The piezoelectric flexible MEMS ultrasonic transducer is obtained by spin-coating a top flexible layer material onto the surface of the flexible top electrode layer.

2. The fabrication process of the piezoelectric flexible MEMS ultrasonic transducer according to claim 1, characterized in that, The flexible lower electrode layer includes a conductive adhesive layer, a flexible lower electrode, and a lower electrode-substrate connection layer. The lower electrode-substrate connection layer is disposed on the surface of the flexible substrate, the flexible lower electrode is disposed on the surface of the lower electrode-substrate connection layer, and the conductive adhesive layer is disposed between the flexible lower electrode and the piezoelectric layer. When fabricating a flexible lower electrode layer on a flexible substrate, the lower electrode-substrate connection layer and the flexible lower electrode layer are pressed onto the flexible substrate; Next, a conductive adhesive layer is spin-coated onto the surface of the flexible lower electrode, and the piezoelectric layer is placed on the conductive adhesive layer. Then, it is baked to bond the flexible lower electrode and the piezoelectric layer through the conductive adhesive layer. When the piezoelectric layer is etched into multiple piezoelectric transducer units by laser ablation, the conductive adhesive layer is etched together with the piezoelectric layer, and the etching stops at the surface of the flexible lower electrode. When patterning the flexible lower electrode layer using laser ablation, the flexible lower electrode and the lower electrode-substrate connection layer are etched together, and the etching stops at the surface of the flexible substrate.

3. The fabrication process of the piezoelectric flexible MEMS ultrasonic transducer according to claim 2, characterized in that, The conductive adhesive layer and the piezoelectric layer have the same lateral shape and size. The area on the flexible lower electrode that is directly opposite the piezoelectric layer and the area that is directly opposite the connecting line in the flexible lower electrode layer are set as blank areas. The flexible lower electrode and the lower electrode-substrate connection layer have the same shape.

4. The fabrication process of the piezoelectric flexible MEMS ultrasonic transducer according to claim 2, characterized in that, The material of the lower electrode-substrate connection layer is polyimide or polyethylene terephthalate, and the material of the conductive adhesive layer is conductive silver paste.

5. The fabrication process of the piezoelectric flexible MEMS ultrasonic transducer according to claim 1, characterized in that, The surface of the flexible upper electrode layer has a support structure at a position directly opposite each piezoelectric transducer unit, and the top flexible layer is disposed on the surface of the support structure and fills the area between each support structure. The support structure includes a support layer and a cavity sealing layer. The support layer is disposed on the surface of the flexible upper electrode layer. A cavity is formed on the surface of the support layer away from the flexible upper electrode layer. The cavity sealing layer is provided on this surface of the support layer to seal the cavity opening.

6. The fabrication process of the piezoelectric flexible MEMS ultrasonic transducer according to claim 1 or 5, characterized in that, The piezoelectric layer is made of PZT-5H rigid piezoelectric material or type 1-3 piezoelectric composite material.

7. The fabrication process of the piezoelectric flexible MEMS ultrasonic transducer according to claim 1 or 5, characterized in that, The top flexible layer, the middle flexible layer, and the flexible substrate are made of PDMS flexible material.

8. The fabrication process of the piezoelectric flexible MEMS ultrasonic transducer according to claim 1 or 5, characterized in that, The surface of the flexible substrate has a bottom gel layer.

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