A resonant tunable MEMS ultrasonic transducer and method of fabrication thereof

Abstract

This invention discloses a resonant tunable MEMS ultrasonic transducer, comprising a substrate, a fifth groove recessed downwards in the center of the top side of the substrate, a bias electrode flatly laid at the bottom of the fifth groove, a sealing cavity provided on the top side of the bias electrode, the top surface of the sealing cavity being sealed by an insulating layer, the bottom surface of the sealing cavity being sealed by the bias electrode, and the periphery of the sealing cavity being sealed by a structural layer; an insulating layer, a first electrode layer, a piezoelectric layer, a second electrode layer, and a structural layer are sequentially laid on the top side of the substrate; four release holes are symmetrically recessed in the center of the top side of the structural layer; the array element is a resonant structure composed of the sealing cavity, and a portion perpendicular to the sealing cavity and coinciding with the orthographic projection of the sealing cavity, the insulating layer, the first electrode layer, the piezoelectric layer, the second electrode layer, and the structural layer; a first lead electrode is connected to the first electrode layer and separate from the second electrode layer; a second lead electrode is connected to the bias electrode; and a third lead electrode is connected to the second electrode layer.

Description

Technical Field

[0001] This invention belongs to the field of microelectromechanical systems (MEMS) technology, and in particular relates to a resonant tunable MEMS ultrasonic transducer and its fabrication method. Background Technology

[0002] MEMS stands for Micro Electro Mechanical System. Microelectromechanical systems are high-tech devices with dimensions ranging from millimeters to smaller, and their internal structures are typically on the micrometer or even nanometer scale, forming independent intelligent systems. Microelectromechanical systems mainly consist of three parts: sensors, actuators, and micro-energy sources. They involve multiple disciplines and engineering technologies, including physics, semiconductors, optics, electronic engineering, chemistry, materials engineering, mechanical engineering, medicine, information engineering, and bioengineering, opening up broad applications in fields such as intelligent systems, consumer electronics, wearable devices, smart homes, systems biology, synthetic biology, and microfluidics. Common products include MEMS accelerometers, MEMS microphones, micromotors, micropumps, microoscillators, MEMS ultrasonic transducers, MEMS gyroscopes, MEMS humidity sensors, and their integrated products. An ultrasonic transducer is a device that converts electromagnetic energy into mechanical energy (sound energy), typically made of piezoelectric ceramics or other magnetostrictive materials.

[0003] During the fabrication of MEMS ultrasonic transducers, the cavity structures at different locations on the wafer exhibit dimensional deviations due to manufacturing processes. These dimensional deviations are strongly correlated with the resonant frequency. When these dimensional deviations exceed the allowable range, the ultrasonic transducer's frequency will exceed the permissible deviation for the product's application frequency, resulting in severe signal attenuation and defective products. Summary of the Invention

[0004] Objective of the Invention: To address the shortcomings of the prior art, this invention provides a resonant tunable MEMS ultrasonic transducer and its fabrication method. The resonant tunable MEMS ultrasonic transducer provided by this invention adjusts the resonant frequency of the ultrasonic transducer by placing a bias electrode at a certain distance from the lower end of the transducer and changing the bias voltage of the bias electrode to adjust the equivalent stiffness of the diaphragm, thereby ensuring a consistent resonant frequency across the product. This further unification of the resonant frequency improves the consistency of input / output sensitivity, achieving a preliminary sensitivity calibration effect.

[0005] Technical solution: A resonant tunable MEMS ultrasonic transducer, comprising a substrate, a bias electrode, a sealed cavity, an insulating layer, a first electrode layer, a piezoelectric layer, a second electrode layer, a structural layer, a first lead electrode, a second lead electrode, and a third lead electrode, wherein: The substrate has a fifth groove recessed downward in the middle of the top side. A bias electrode is laid flat at the bottom of the fifth groove. A sealing cavity is provided on the top side of the bias electrode. The top surface of the sealing cavity is sealed by an insulating layer. The bottom surface of the sealing cavity is sealed by the bias electrode. The periphery of the sealing cavity is sealed by a structural layer. An insulating layer is deposited on the top side of the substrate; A first electrode layer is laid on the top side of the insulating layer; A piezoelectric layer is laid on the top side of the first electrode layer; A second electrode layer is laid on the top side of the piezoelectric layer; A structural layer is laid on the top side of the second electrode layer; The top side of the structural layer has four symmetrically recessed release holes in the middle; The array element is a resonant structure composed of a sealed cavity, a bias electrode perpendicular to the sealed cavity and overlapping with its orthographic projection, an insulating layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a first electrode layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a piezoelectric layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a second electrode layer perpendicular to the sealed cavity and overlapping with its orthographic projection, and a structural layer perpendicular to the sealed cavity and overlapping with its orthographic projection. The first lead electrode is connected to the first electrode layer and is separate from the second electrode layer; The second lead electrode is connected to the bias electrode; The third lead electrode is connected to the second electrode layer.

[0006] When the signal source is connected to the third lead electrode, the piezoelectric layer vibrates due to the positive piezoelectric effect, causing the array elements to vibrate and emit sound signals. When an external sound signal is transmitted to the piezoelectric layer, the third lead electrode is connected to the output terminal, and the piezoelectric layer vibrates due to the inverse piezoelectric effect, generating an electrical signal and outputting a response from the output terminal.

[0007] When the frequency of the product does not conform to the design, a certain bias voltage is applied to the bias electrode through the second lead electrode to change the equivalent stiffness of the array element structure, thereby adjusting the resonant frequency and sensitivity of the array element.

[0008] The fabrication method of the resonant tunable MEMS ultrasonic transducer described in any one of the above methods comprises the following steps: (1) Preparation of sacrificial layer and bias electrode: Si wafer is selected as substrate, hard mask is prepared on the front side of Si wafer, and then a deep silicon etching machine is used to etch the middle of the front side of Si wafer to form a fifth groove with an etching depth of 0.5-5 μm. First, a metal thin film is prepared by physical vapor deposition, and then the metal thin film is patterned by photolithography and then etching. A bias electrode is formed at the bottom of the fifth groove. Then, phosphosilicate glass (PSG) or undoped silicate glass (USG) is prepared on the top surface of the bias electrode by chemical vapor deposition. The deposition thickness is 0.3-1 μm greater than the silicon etching thickness. Then, it is polished by chemical mechanical polishing to form a flat surface, which is the sacrificial layer. The thickness of the sacrificial layer after polishing is 0.5-3 μm. (2) An insulating layer, a first electrode layer, a piezoelectric layer, and a second electrode layer are sequentially fabricated on the top surface of the Si wafer processed in step (1), wherein: The insulating layer is prepared by chemical vapor deposition or physical vapor deposition. The first electrode layer was prepared by physical vapor deposition. The piezoelectric layer was prepared by physical vapor deposition. The second electrode layer is prepared by physical vapor deposition, and four symmetrically arranged first grooves and one second groove are prepared on the second electrode layer by photolithography followed by etching. The position of the first groove corresponds to the position of the prism-like tail of the fifth groove. (3) Using reactive ion etching, the second electrode layer, piezoelectric layer, first electrode layer and insulating layer of the Si wafer treated in step (2) are etched in sequence to form four symmetrically arranged release holes and four third grooves located outside the release holes. The third grooves are located inside the first grooves. The etching stops on the sacrificial layer. Then, the sacrificial layer is removed using a sacrificial layer release solution (NPW solution or VHF solution). After completion, a sealed cavity is formed. (4) A structural layer is prepared on the top side of the Si wafer treated in step (3) by chemical vapor deposition or physical vapor deposition. At this time: The structural layer is partially filled with release holes to form blind holes, and the structural layer is partially filled with a third groove to form a fourth groove. An array element is formed by a sealed cavity, a bias electrode perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, an insulating layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a first electrode layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a piezoelectric layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a second electrode layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, and a structural layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity. This array element is used to determine the resonant frequency of the ultrasonic transducer. (5) Using reactive ion etching, the top surface of the Si wafer processed in step (4) is etched to obtain the first blind hole exposing the first electrode layer, the four second blind holes exposing the bias electrode, and the third blind hole exposing the second electrode layer. Au metal first lead electrode, Au metal second lead electrode and Au metal third lead electrode are prepared by sacrificial layer process, or Al metal first lead electrode, Al metal second lead electrode and Al metal third lead electrode are prepared by physical vapor deposition and etching. The first lead electrode is disposed in the first blind hole, the first lead electrode is connected to the first electrode layer, and the first lead electrode is separate from the second electrode layer; Four second lead electrodes are arranged in four second blind holes. The second lead electrodes are connected to the bias electrodes and are separated from both the first electrode layer and the second electrode layer. The third lead electrode is connected to the second electrode layer.

[0009] This invention proposes a method for fabricating a resonant tunable MEMS ultrasonic transducer. A cavity is formed through surface etching, filling, chemical mechanical polishing, and release. Then, a structural layer for the array elements is fabricated using a low-stress deposition process. The release holes can be selectively filled through the structural layer to form a sealed cavity. The bias electrode is fabricated using physical vapor deposition followed by chemical mechanical polishing, which is compatible with the cavity fabrication process and does not require excessive additional processing costs. This improves product yield and generates better economic benefits.

[0010] The bias electrode and the first electrode layer constitute an electrostatic structure; by adjusting the voltage applied to the bias electrode, the equivalent stiffness of the diaphragm is changed, achieving an adjustable resonant frequency. Simultaneously, fine-tuning of the sensitivity can also be performed.

[0011] Beneficial Effects: The resonant tunable MEMS ultrasonic transducer and its fabrication method disclosed in this invention have the following beneficial effects: 1. By setting a bias electrode at a certain distance from the lower end of the ultrasonic transducer, the equivalent stiffness of the diaphragm can be adjusted by changing the bias voltage of the bias electrode, thereby adjusting the resonant frequency of the ultrasonic transducer, making the product resonant frequency consistent, and improving the product yield.

[0012] 2. By adjusting the applied bias voltage, the response efficiency of the resonant adjustable MEMS ultrasonic transducer is adjusted, thereby achieving the purpose of correcting its sensitivity and solving the technical problem of inconsistent sensitivity response at different locations. Attached Figure Description

[0013] Figures 1-5 This is a fluid diagram illustrating a method for fabricating a resonant tunable MEMS ultrasonic transducer disclosed in this invention. Figures 1-5 In the diagram, b represents a top view during the fabrication of a resonant tunable MEMS ultrasonic transducer. Figures 1-5 In the diagram, 'a' is the sectional view of A1A2 in the corresponding diagram 'b'.

[0014] Figure 6 This is a schematic diagram of the fifth groove, where a, b, and c correspond to different structures of the fifth groove.

[0015] Figure 7 The simulated vibration cloud diagram of the resonant tunable MEMS ultrasonic transducer prepared in specific embodiment 1.

[0016] Figure 8 This is a schematic diagram illustrating the effect of the sealed cavity size on the resonant frequency of the resonant tunable MEMS ultrasonic transducer prepared in Specific Embodiment 1.

[0017] Figure 9 This is a schematic diagram showing the effect of the bias voltage on the resonant frequency of the resonant adjustable MEMS ultrasonic transducer prepared in Specific Embodiment 1.

[0018] Figure label: Detailed Implementation

[0019] The specific embodiments of the present invention are described in detail below.

[0020] The "range" disclosed in this invention is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if a range of 10–50 is listed for a specific parameter, it is also expected that ranges of 10–40 and 20–50 are also included. Furthermore, if the minimum range values ​​are 1 and 2, and the maximum range values ​​are 3, 4, and 5, then the following ranges are all expected: 1–3, 1–4, 1–5, 2–3, 2–4, and 2–5. In this application, unless otherwise stated, the numerical range "a–b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0–5" means that all real numbers between "0–5" have been listed herein; "0–5" is merely a shortened representation of these numerical combinations.

[0021] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions.

[0022] Unless otherwise specified, all technical features and optional technical features of this application may be combined to form new technical solutions.

[0023] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0024] Unless otherwise specified, the terms "comprising" and "including" as used in this application can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0025] Unless otherwise specified, the reaction will proceed under normal temperature and pressure conditions.

[0026] Unless otherwise specified, all parts or percentages are by weight or by weight percentage.

[0027] In this invention, all the substances used are known substances that can be purchased or synthesized by known methods.

[0028] In this invention, all the devices or equipment used are conventional devices or equipment known in the art and are readily available.

[0029] A resonant tunable MEMS ultrasonic transducer includes a substrate, a bias electrode, a sealed cavity, an insulating layer, a first electrode layer, a piezoelectric layer, a second electrode layer, a structural layer, a first lead electrode, a second lead electrode, and a third lead electrode, wherein: The substrate has a fifth groove recessed downward in the middle of the top side. A bias electrode is laid flat at the bottom of the fifth groove. A sealing cavity is provided on the top side of the bias electrode. The top surface of the sealing cavity is sealed by an insulating layer. The bottom surface of the sealing cavity is sealed by the bias electrode. The periphery of the sealing cavity is sealed by a structural layer. An insulating layer is deposited on the top side of the substrate; A first electrode layer is laid on the top side of the insulating layer; A piezoelectric layer is laid on the top side of the first electrode layer; A second electrode layer is laid on the top side of the piezoelectric layer; A structural layer is laid on the top side of the second electrode layer; The top side of the structural layer has four symmetrically recessed release holes in the middle; The array element is a resonant structure composed of a sealed cavity, a bias electrode perpendicular to the sealed cavity and overlapping with its orthographic projection, an insulating layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a first electrode layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a piezoelectric layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a second electrode layer perpendicular to the sealed cavity and overlapping with its orthographic projection, and a structural layer perpendicular to the sealed cavity and overlapping with its orthographic projection. The first lead electrode is connected to the first electrode layer and is separate from the second electrode layer; The second lead electrode is connected to the bias electrode; The third lead electrode is connected to the second electrode layer.

[0030] Furthermore, the fifth groove includes four connecting bodies, one end of each of the four connecting bodies is connected to the intermediate body, and the other end of each of the four connecting bodies is connected to a type of quadrangular prism. The intermediate body is one of a type of cylinder, a type of regular tetrahedron, and a type of regular octahedron; The connector is a combination of a type of cylinder and a type of tetrahedron.

[0031] Furthermore, the orthographic projections of the four release holes coincide with the orthographic projection of the connector in the fifth groove; The orthographic projection of the second lead electrode coincides with the orthographic projection of the prism-like structure at the tail of the fifth groove.

[0032] Furthermore, the bias electrode is a layered electrode, which is one of Al layer, Au layer, Mo layer, Pt layer and Cu layer, and the thickness of the bias electrode is 50 to 500 nm.

[0033] Furthermore, the insulating layer is SiN. x The insulating layer is one of an Al2O3 layer and an AlN layer, wherein the thickness of the insulating layer is 50 to 500 nm.

[0034] Furthermore, the first electrode layer is one of a Pt layer, an Au layer, and a Mo layer, and the thickness of the first electrode layer is 50–300 nm.

[0035] Furthermore, the piezoelectric layer is one of AlN layer, AlScN layer and PZT layer, and the thickness of the piezoelectric layer is 0.5~2um.

[0036] Furthermore, the second electrode layer is one of a Pt layer, an Au layer, and a Mo layer, and the thickness of the second electrode layer is 50–300 nm.

[0037] Furthermore, the structural layer is SiN. x The structure is one of the following: an Al2O3 layer, an AlN layer, and a SiO2 layer, wherein the thickness of the structural layer is 1–6 μm.

[0038] In use, the first lead electrode is connected to the first electrode layer and to the external ground terminal, the second lead electrode is connected to the bias electrode and to the external bias voltage, and the third lead electrode is connected to the second electrode layer and to the external signal source or output terminal.

[0039] When the signal source is connected to the third lead electrode, the piezoelectric layer vibrates due to the positive piezoelectric effect, causing the array elements to vibrate and emit sound signals. When an external sound signal is transmitted to the piezoelectric layer, the third lead electrode is connected to the output terminal, and the piezoelectric layer vibrates due to the inverse piezoelectric effect, generating an electrical signal and outputting a response from the output terminal.

[0040] When the frequency of the product does not conform to the design, a certain bias voltage is applied to the bias electrode through the second lead electrode to change the equivalent stiffness of the array element structure, thereby adjusting the resonant frequency and sensitivity of the array element.

[0041] The fabrication method of the resonant tunable MEMS ultrasonic transducer described in any one of the above methods comprises the following steps: (1) Preparation of sacrificial layer and bias electrode: Si wafer is selected as substrate, hard mask is prepared on the front side of Si wafer, and then a deep silicon etching machine is used to etch the middle of the front side of Si wafer to form a fifth groove with an etching depth of 0.5-5 μm. First, a metal thin film is prepared by physical vapor deposition, and then the metal thin film is patterned by photolithography and then etching. A bias electrode is formed at the bottom of the fifth groove. Then, phosphosilicate glass (PSG) or undoped silicate glass (USG) is prepared on the top surface of the bias electrode by chemical vapor deposition. The deposition thickness is 0.3-1 μm greater than the silicon etching thickness. Then, it is polished by chemical mechanical polishing to form a flat surface, which is the sacrificial layer. The thickness of the sacrificial layer after polishing is 0.5-3 μm. (2) An insulating layer, a first electrode layer, a piezoelectric layer, and a second electrode layer are sequentially fabricated on the top surface of the Si wafer processed in step (1), wherein: The insulating layer is prepared by chemical vapor deposition or physical vapor deposition. The first electrode layer was prepared by physical vapor deposition. The piezoelectric layer was prepared by physical vapor deposition. The second electrode layer is prepared by physical vapor deposition, and four symmetrically arranged first grooves and one second groove are prepared on the second electrode layer by photolithography followed by etching. The position of the first groove corresponds to the position of the prism-like tail of the fifth groove. (3) Using reactive ion etching, the second electrode layer, piezoelectric layer, first electrode layer and insulating layer of the Si wafer treated in step (2) are etched in sequence to form four symmetrically arranged release holes and four third grooves located outside the release holes. The third grooves are located inside the first grooves. The etching stops on the sacrificial layer. Then, the sacrificial layer is removed using a sacrificial layer release solution (NPW solution or VHF solution). After completion, a sealed cavity is formed. (4) A structural layer is prepared on the top side of the Si wafer treated in step (3) by chemical vapor deposition or physical vapor deposition. At this time: The structural layer is partially filled with release holes to form blind holes, and the structural layer is partially filled with a third groove to form a fourth groove. An array element is formed by a sealed cavity, a bias electrode perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, an insulating layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a first electrode layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a piezoelectric layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a second electrode layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, and a structural layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity. This array element is used to determine the resonant frequency of the ultrasonic transducer. (5) Using reactive ion etching, the top surface of the Si wafer processed in step (4) is etched to obtain the first blind hole exposing the first electrode layer, the four second blind holes exposing the bias electrode, and the third blind hole exposing the second electrode layer. Au metal first lead electrode, Au metal second lead electrode and Au metal third lead electrode are prepared by sacrificial layer process, or Al metal first lead electrode, Al metal second lead electrode and Al metal third lead electrode are prepared by physical vapor deposition and etching. The first lead electrode is disposed in the first blind hole, the first lead electrode is connected to the first electrode layer, and the first lead electrode is separate from the second electrode layer; Four second lead electrodes are arranged in four second blind holes. The second lead electrodes are connected to the bias electrodes and are separated from both the first electrode layer and the second electrode layer. The third lead electrode is connected to the second electrode layer.

[0042] Furthermore, the hard mask layered structure in step (1) is a SiO2 layer or a SiN layer. x The layer or Al2O3 layer has a thickness of 50–500 nm.

[0043] Furthermore, in step (3), the diameter of the release hole is 5-30 μm, and the orthographic projection of the four release holes coincides with the orthographic projection of the connector of the fifth groove.

[0044] In one embodiment: A resonant tunable MEMS ultrasonic transducer includes a substrate, a bias electrode, a sealed cavity, an insulating layer, a first electrode layer, a piezoelectric layer, a second electrode layer, a structural layer, a first lead electrode, a second lead electrode, and a third lead electrode, wherein: The substrate has a fifth groove recessed downward in the middle of the top side. A bias electrode is laid flat at the bottom of the fifth groove. A sealing cavity is provided on the top side of the bias electrode. The top surface of the sealing cavity is sealed by an insulating layer. The bottom surface of the sealing cavity is sealed by the bias electrode. The periphery of the sealing cavity is sealed by a structural layer. An insulating layer is deposited on the top side of the substrate; A first electrode layer is laid on the top side of the insulating layer; A piezoelectric layer is laid on the top side of the first electrode layer; A second electrode layer is laid on the top side of the piezoelectric layer; A structural layer is laid on the top side of the second electrode layer; The top side of the structural layer has four symmetrically recessed release holes in the middle; The array element is a resonant structure composed of a sealed cavity, a bias electrode perpendicular to the sealed cavity and overlapping with its orthographic projection, an insulating layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a first electrode layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a piezoelectric layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a second electrode layer perpendicular to the sealed cavity and overlapping with its orthographic projection, and a structural layer perpendicular to the sealed cavity and overlapping with its orthographic projection. The first lead electrode is connected to the first electrode layer and is separate from the second electrode layer; The second lead electrode is connected to the bias electrode; The third lead electrode is connected to the second electrode layer.

[0045] Furthermore, the fifth groove includes four connecting bodies, one end of each of the four connecting bodies is connected to the intermediate body, and the other end of each of the four connecting bodies is connected to a type of quadrangular prism. The intermediate is a type of cylinder (such as...) Figure 6 (as shown in a). The connector is a combination of a type of cylinder and a type of tetrahedron.

[0046] Furthermore, the orthographic projections of the four release holes coincide with the orthographic projection of the connector in the fifth groove; The orthographic projection of the second lead electrode coincides with the orthographic projection of the prism-like structure at the tail of the fifth groove.

[0047] Furthermore, the bias electrode is a layered electrode, the bias electrode is an Al layer, and the thickness of the bias electrode is 50 nm.

[0048] Furthermore, the insulating layer is SiN. x The insulating layer has a thickness of 50 nm.

[0049] Furthermore, the first electrode layer is a Pt layer, and the thickness of the first electrode layer is 50.

[0050] Furthermore, the piezoelectric layer is an AlN layer, and the thickness of the piezoelectric layer is 0.5 μm.

[0051] Furthermore, the second electrode layer is a Pt layer, and the thickness of the second electrode layer is 50 nm.

[0052] Furthermore, the structural layer is SiN. x The layer has a thickness of 1 μm.

[0053] The fabrication method of the resonant tunable MEMS ultrasonic transducer described in any one of the above methods comprises the following steps: (1) Preparation of sacrificial layer and bias electrode: Si wafer is selected as substrate, hard mask is prepared on the front side of Si wafer, and then a fifth groove is formed by etching the middle of the front side of Si wafer using a deep silicon etching machine with an etching depth of 0.5um. First, a metal thin film is prepared by physical vapor deposition, and then the metal thin film is patterned by photolithography and then etching. A bias electrode is formed at the bottom of the fifth groove. Then, phosphosilicate glass (PSG) is prepared on the top surface of the bias electrode by chemical vapor deposition with a deposition thickness 0.3um greater than the silicon etching thickness. Then, it is polished by chemical mechanical polishing to form a flat surface, which is the sacrificial layer. The thickness of the sacrificial layer after polishing is 0.5um. (2) An insulating layer, a first electrode layer, a piezoelectric layer, and a second electrode layer are sequentially fabricated on the top surface of the Si wafer processed in step (1), wherein: The insulating layer is prepared by chemical vapor deposition. The first electrode layer was prepared by physical vapor deposition. The piezoelectric layer was prepared by physical vapor deposition. The second electrode layer is prepared by physical vapor deposition, and four symmetrically arranged first grooves and one second groove are prepared on the second electrode layer by photolithography followed by etching. The position of the first groove corresponds to the position of the prism-like tail of the fifth groove. (3) Using reactive ion etching, the second electrode layer, piezoelectric layer, first electrode layer and insulating layer of the Si wafer treated in step (2) are etched in sequence to form four symmetrically arranged release holes and four third grooves located outside the release holes. The third grooves are located inside the first grooves. The etching stops on the sacrificial layer. Then the sacrificial layer is removed using a sacrificial layer release solution (NPW solution). After completion, a sealed cavity is formed. (4) A structural layer is prepared on the top side of the Si wafer treated in step (3) by chemical vapor deposition. At this time: The structural layer is partially filled with release holes to form blind holes, and the structural layer is partially filled with a third groove to form a fourth groove. An array element is formed by a sealed cavity, a bias electrode perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, an insulating layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a first electrode layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a piezoelectric layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a second electrode layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, and a structural layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity. This array element is used to determine the resonant frequency of the ultrasonic transducer. (5) Using reactive ion etching, the top surface of the Si wafer processed in step (4) is etched to obtain the first blind hole exposing the first electrode layer, the four second blind holes exposing the bias electrode, and the third blind hole exposing the second electrode layer. Au metal first lead electrode, Au metal second lead electrode, and Au metal second lead electrode were fabricated using a sacrificial layer process. The first lead electrode is disposed in the first blind hole, the first lead electrode is connected to the first electrode layer, and the first lead electrode is separate from the second electrode layer; Four second lead electrodes are arranged in four second blind holes. The second lead electrodes are connected to the bias electrodes and are separated from both the first electrode layer and the second electrode layer. The third lead electrode is connected to the second electrode layer.

[0054] Furthermore, in step (1), the hard mask layered structure is a SiO2 layer with a thickness of 50 nm.

[0055] Furthermore, in step (3), the diameter of the release hole is 5 μm, and the orthographic projection of the four release holes coincides with the orthographic projection of the connector of the fifth groove.

[0056] In another embodiment: A resonant tunable MEMS ultrasonic transducer includes a substrate, a bias electrode, a sealed cavity, an insulating layer, a first electrode layer, a piezoelectric layer, a second electrode layer, a structural layer, a first lead electrode, a second lead electrode, and a third lead electrode, wherein: The substrate has a fifth groove recessed downward in the middle of the top side. A bias electrode is laid flat at the bottom of the fifth groove. A sealing cavity is provided on the top side of the bias electrode. The top surface of the sealing cavity is sealed by an insulating layer. The bottom surface of the sealing cavity is sealed by the bias electrode. The periphery of the sealing cavity is sealed by a structural layer. An insulating layer is deposited on the top side of the substrate; A first electrode layer is laid on the top side of the insulating layer; A piezoelectric layer is laid on the top side of the first electrode layer; A second electrode layer is laid on the top side of the piezoelectric layer; A structural layer is laid on the top side of the second electrode layer; The top side of the structural layer has four symmetrically recessed release holes in the middle; The array element is a resonant structure composed of a sealed cavity, a bias electrode perpendicular to the sealed cavity and overlapping with its orthographic projection, an insulating layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a first electrode layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a piezoelectric layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a second electrode layer perpendicular to the sealed cavity and overlapping with its orthographic projection, and a structural layer perpendicular to the sealed cavity and overlapping with its orthographic projection. The first lead electrode is connected to the first electrode layer and is separate from the second electrode layer; The second lead electrode is connected to the bias electrode; The third lead electrode is connected to the second electrode layer.

[0057] Furthermore, the fifth groove includes four connecting bodies, one end of each of the four connecting bodies is connected to the intermediate body, and the other end of each of the four connecting bodies is connected to a type of quadrangular prism. The intermediate is a type of regular tetrahedron (such as...) Figure 6 (as shown in b). The connector is a combination of a type of cylinder and a type of tetrahedron.

[0058] Furthermore, the orthographic projections of the four release holes coincide with the orthographic projection of the connector in the fifth groove; The orthographic projection of the second lead electrode coincides with the orthographic projection of the prism-like structure at the tail of the fifth groove.

[0059] Furthermore, the bias electrode is a layered electrode, the bias electrode is an Au layer, and the thickness of the bias electrode is 500 nm.

[0060] Furthermore, the insulating layer is an Al2O3 layer, and the thickness of the insulating layer is 500 nm.

[0061] Furthermore, the first electrode layer is an Au layer, and the thickness of the first electrode layer is 300 nm.

[0062] Furthermore, the piezoelectric layer is an AlScN layer, and the thickness of the piezoelectric layer is 2 μm.

[0063] Furthermore, the second electrode layer is an Au layer, and the thickness of the second electrode layer is 300 nm.

[0064] Furthermore, the structural layer is an Al2O3 layer with a thickness of 6 μm.

[0065] The fabrication method of the resonant tunable MEMS ultrasonic transducer described in any one of the above methods comprises the following steps: (1) Preparation of sacrificial layer and bias electrode: Si wafer is selected as substrate, hard mask is prepared on the front side of Si wafer, and then a deep silicon etching machine is used to etch the middle of the front side of Si wafer to form a fifth groove with an etching depth of 5um. First, a metal thin film is prepared by physical vapor deposition, and then the metal thin film is patterned by photolithography and then etching. A bias electrode is formed at the bottom of the fifth groove. Then, undoped silicate glass (USG) is prepared on the top surface of the bias electrode by chemical vapor deposition with a deposition thickness 1um greater than the silicon etching thickness. Then, it is polished by chemical mechanical polishing to form a flat surface, which is the sacrificial layer. The thickness of the sacrificial layer after polishing is 3um. (2) An insulating layer, a first electrode layer, a piezoelectric layer, and a second electrode layer are sequentially fabricated on the top surface of the Si wafer processed in step (1), wherein: The insulating layer was prepared by physical vapor deposition. The first electrode layer was prepared by physical vapor deposition. The piezoelectric layer was prepared by physical vapor deposition. The second electrode layer is prepared by physical vapor deposition, and four symmetrically arranged first grooves and one second groove are prepared on the second electrode layer by photolithography followed by etching. The position of the first groove corresponds to the position of the prism-like tail of the fifth groove. (3) Using reactive ion etching, the second electrode layer, piezoelectric layer, first electrode layer and insulating layer of the Si wafer treated in step (2) are etched in sequence to form four symmetrically arranged release holes and four third grooves located outside the release holes. The third grooves are located inside the first grooves. The etching stops on the sacrificial layer. Then the sacrificial layer is removed using a sacrificial layer release solution (VHF solution). After completion, a sealed cavity is formed. (4) A structural layer is prepared on the top side of the Si wafer treated in step (3) by physical vapor deposition. At this time: The structural layer is partially filled with release holes to form blind holes, and the structural layer is partially filled with a third groove to form a fourth groove. An array element is formed by a sealed cavity, a bias electrode perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, an insulating layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a first electrode layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a piezoelectric layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a second electrode layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, and a structural layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity. This array element is used to determine the resonant frequency of the ultrasonic transducer. (5) Using reactive ion etching, the top surface of the Si wafer processed in step (4) is etched to obtain the first blind hole exposing the first electrode layer, the four second blind holes exposing the bias electrode, and the third blind hole exposing the second electrode layer. Al metal first lead electrode, Al metal second lead electrode, and Al metal third electrode were fabricated using physical vapor deposition and etching. The first lead electrode is disposed in the first blind hole, the first lead electrode is connected to the first electrode layer, and the first lead electrode is separate from the second electrode layer; Four second lead electrodes are arranged in four second blind holes. The second lead electrodes are connected to the bias electrodes and are separated from both the first electrode layer and the second electrode layer. The third lead electrode is connected to the second electrode layer.

[0066] Furthermore, the hard mask layered structure in step (1) is SiN. x The layer has a thickness of 500 nm.

[0067] Furthermore, in step (3), the diameter of the release hole is 30 μm, and the orthographic projection of the four release holes coincides with the orthographic projection of the connector of the fifth groove.

[0068] In yet another embodiment: A resonant tunable MEMS ultrasonic transducer includes a substrate, a bias electrode, a sealed cavity, an insulating layer, a first electrode layer, a piezoelectric layer, a second electrode layer, a structural layer, a first lead electrode, a second lead electrode, and a third lead electrode, wherein: The substrate has a fifth groove recessed downward in the middle of the top side. A bias electrode is laid flat at the bottom of the fifth groove. A sealing cavity is provided on the top side of the bias electrode. The top surface of the sealing cavity is sealed by an insulating layer. The bottom surface of the sealing cavity is sealed by the bias electrode. The periphery of the sealing cavity is sealed by a structural layer. An insulating layer is deposited on the top side of the substrate; A first electrode layer is laid on the top side of the insulating layer; A piezoelectric layer is laid on the top side of the first electrode layer; A second electrode layer is laid on the top side of the piezoelectric layer; A structural layer is laid on the top side of the second electrode layer; The top side of the structural layer has four symmetrically recessed release holes in the middle; The array element is a resonant structure composed of a sealed cavity, a bias electrode perpendicular to the sealed cavity and overlapping with its orthographic projection, an insulating layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a first electrode layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a piezoelectric layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a second electrode layer perpendicular to the sealed cavity and overlapping with its orthographic projection, and a structural layer perpendicular to the sealed cavity and overlapping with its orthographic projection. The first lead electrode is connected to the first electrode layer and is separate from the second electrode layer; The second lead electrode is connected to the bias electrode; The third lead electrode is connected to the second electrode layer.

[0069] Furthermore, the fifth groove includes four connecting bodies, one end of each of the four connecting bodies is connected to the intermediate body, and the other end of each of the four connecting bodies is connected to a type of quadrangular prism. The intermediate is a type of regular octahedron (such as...) Figure 6 (as shown in c). The connector is a combination of a type of cylinder and a type of tetrahedron.

[0070] Furthermore, the orthographic projections of the four release holes coincide with the orthographic projection of the connector in the fifth groove; The orthographic projection of the second lead electrode coincides with the orthographic projection of the prism-like structure at the tail of the fifth groove.

[0071] Further, the bias electrode is a layered electrode, the bias electrode is a Mo layer, and the thickness of the bias electrode is 300 nm. In another embodiment, the bias electrode is a layered electrode, the bias electrode is a Pt layer, and the thickness of the bias electrode is 75 nm. In yet another embodiment, the bias electrode is a layered electrode, the bias electrode is a Cu layer, and the thickness of the bias electrode is 480 nm.

[0072] Furthermore, the insulating layer is an AlN layer, and the thickness of the insulating layer is 200 nm.

[0073] Furthermore, the first electrode layer is a Mo layer, and the thickness of the first electrode layer is 200 nm.

[0074] Furthermore, the piezoelectric layer is a PZT layer, and the thickness of the piezoelectric layer is 1 μm.

[0075] Furthermore, the second electrode layer is a Mo layer, and the thickness of the second electrode layer is 100 nm.

[0076] Furthermore, the structural layer is an AlN layer with a thickness of 3 μm. In another embodiment, the structural layer is a SiO2 layer with a thickness of 2 μm.

[0077] The fabrication method of the resonant tunable MEMS ultrasonic transducer described in any one of the above methods comprises the following steps: (1) Preparation of sacrificial layer and bias electrode: Si wafer is selected as substrate, hard mask is prepared on the front side of Si wafer, and then a deep silicon etching machine is used to etch the middle of the front side of Si wafer to form a fifth groove with an etching depth of 2um. First, a metal thin film is prepared by physical vapor deposition, and then the metal thin film is patterned by photolithography and then etching. A bias electrode is formed at the bottom of the fifth groove. Then, phosphosilicate glass (PSG) is prepared on the top surface of the bias electrode by chemical vapor deposition. The deposition thickness is 0.5um greater than the silicon etching thickness. Then, it is polished by chemical mechanical polishing to form a flat surface, which is the sacrificial layer. The thickness of the sacrificial layer after polishing is 2.2um. (2) An insulating layer, a first electrode layer, a piezoelectric layer, and a second electrode layer are sequentially fabricated on the top surface of the Si wafer processed in step (1), wherein: The insulating layer was prepared by physical vapor deposition. The first electrode layer was prepared by physical vapor deposition. The piezoelectric layer was prepared by physical vapor deposition. The second electrode layer is prepared by physical vapor deposition, and four symmetrically arranged first grooves and one second groove are prepared on the second electrode layer by photolithography followed by etching. The position of the first groove corresponds to the position of the prism-like tail of the fifth groove. (3) Using reactive ion etching, the second electrode layer, piezoelectric layer, first electrode layer and insulating layer of the Si wafer treated in step (2) are etched in sequence to form four symmetrically arranged release holes and four third grooves located outside the release holes. The third grooves are located inside the first grooves. The etching stops on the sacrificial layer. Then the sacrificial layer is removed using a sacrificial layer release solution (NPW solution). After completion, a sealed cavity is formed. (4) A structural layer is prepared on the top side of the Si wafer treated in step (3) by physical vapor deposition. At this time: The structural layer is partially filled with release holes to form blind holes, and the structural layer is partially filled with a third groove to form a fourth groove. An array element is formed by a sealed cavity, a bias electrode perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, an insulating layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a first electrode layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a piezoelectric layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a second electrode layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, and a structural layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity. This array element is used to determine the resonant frequency of the ultrasonic transducer. (5) Using reactive ion etching, the top surface of the Si wafer processed in step (4) is etched to obtain the first blind hole exposing the first electrode layer, the four second blind holes exposing the bias electrode, and the third blind hole exposing the second electrode layer. Au metal first lead electrode, Au metal second lead electrode, and Au metal second lead electrode were fabricated using a sacrificial layer process. The first lead electrode is disposed in the first blind hole, the first lead electrode is connected to the first electrode layer, and the first lead electrode is separate from the second electrode layer; Four second lead electrodes are arranged in four second blind holes. The second lead electrodes are connected to the bias electrodes and are separated from both the first electrode layer and the second electrode layer. The third lead electrode is connected to the second electrode layer.

[0078] Furthermore, in step (1), the hard mask layered structure is an Al2O3 layer with a thickness of 200 nm.

[0079] Furthermore, in step (3), the diameter of the release hole is 20 μm, and the orthographic projection of the four release holes 301 coincides with the orthographic projection of the connector of the fifth groove. Specific Implementation Example 1 like Figures 1-5 As shown, a method for fabricating a resonant tunable MEMS ultrasonic transducer includes the following steps: (1) Preparation of sacrificial layer 101 and bias electrode 102: Si wafer is selected as substrate 103. A hard mask is prepared on the front side of the Si wafer. Then, a deep silicon etching machine is used to etch the middle of the front side of the Si wafer to form a fifth groove with an etching depth of 2um. First, a metal Mo film is prepared by physical vapor deposition with a metal film thickness of 100nm. Then, the metal film is patterned by photolithography followed by etching. A bias electrode 102 is formed at the bottom of the fifth groove. Then, a phosphosilicate glass (PSG) is prepared on the top surface of the bias electrode 102 by chemical vapor deposition with a deposition thickness 0.5um greater than the silicon etching thickness. Then, it is polished by chemical mechanical polishing to form a flat surface, which is the sacrificial layer 101. The thickness of the sacrificial layer 101 after polishing is 1.8um. (2) An insulating layer 201, a first electrode layer 202, a piezoelectric layer 203, and a second electrode layer 204 are sequentially fabricated on the top surface of the Si wafer processed in step (1), wherein: The insulating layer 201 is prepared by physical vapor deposition and is an AlN thin film with a thickness of 100 nm. The first electrode layer 202 is prepared by physical vapor deposition and is a Mo thin film with a thickness of 200 nm. The piezoelectric layer 203 is prepared by physical vapor deposition and is an AlScN thin film with a thickness of 1000 nm. The second electrode layer 204 is prepared by physical vapor deposition, and four symmetrically arranged first grooves and one second groove are prepared on the second electrode layer 204 by photolithography followed by etching. The position of the first groove corresponds to the position of the prism-like tail of the fifth groove. It is a Mo thin film with a thickness of 200 nm. (3) Using reactive ion etching, the second electrode layer 204, piezoelectric layer 203, first electrode layer 202, and insulating layer 201 of the Si wafer treated in step (2) are etched sequentially to form four symmetrically arranged release holes 301 and four third grooves 603 located outside the release holes 301. The third grooves 603 are located inside the first grooves 601. The etching stops on the sacrificial layer 101. Then, the sacrificial layer is removed using a sacrificial layer release solution (NPW solution). After completion, a sealed cavity 302 is formed. (4) The top side of the Si wafer treated in step (3) is prepared by chemical vapor deposition to form the structural layer 401. At this time: The structural layer 401 partially fills the release hole 301 to form a blind hole, and the structural layer 401 partially fills the third groove 603 to form the fourth groove 604. The sealed cavity 302, and the bias electrode 102, which is perpendicular to the sealed cavity 302 and coincides with the orthographic projection of the sealed cavity 302, the insulating layer 201, which is perpendicular to the sealed cavity 302 and coincides with the orthographic projection of the sealed cavity 302, the first electrode layer 202, which is perpendicular to the sealed cavity 302 and coincides with the orthographic projection of the sealed cavity 302, the piezoelectric layer 203, which is perpendicular to the sealed cavity 302 and coincides with the orthographic projection of the sealed cavity 302, the second electrode layer 204, which is perpendicular to the sealed cavity 302 and coincides with the orthographic projection of the sealed cavity 302, and the structural layer 401, which is perpendicular to the sealed cavity 302 and coincides with the orthographic projection of the sealed cavity 302, constitute an array element 402, which is used to determine the resonant frequency of the ultrasonic transducer. (5) Using reactive ion etching, the top surface of the Si wafer processed in step (4) is etched to obtain the first blind hole exposing the first electrode layer 202, the four second blind holes exposing the bias electrode 102, and the third blind hole exposing the second electrode layer 204. Al metal first lead electrode, Al metal second lead electrode, and Al metal third electrode were fabricated using physical vapor deposition and etching. The first lead electrode 501 is disposed in the first blind hole, the first lead electrode 501 is connected to the first electrode layer 202, and the first lead electrode 501 is separated from the second electrode layer 204. Four second lead electrodes 502 are arranged in four second blind holes. The second lead electrodes 502 are connected to the bias electrode 102, and the second lead electrodes 502 are separated from the first electrode layer 202 and the second electrode layer 204. The third lead electrode 503 is connected to the second electrode layer 204.

[0081] Furthermore, in step (1), the hard mask is a layered structure, which is a SiO2 layer with a thickness of 200 nm.

[0082] Furthermore, in step (3), the diameter of the release hole 301 is 20 μm, and the orthographic projection of the four release holes 301 coincides with the orthographic projection of the connector of the fifth groove.

[0083] Taking the resonant tunable MEMS ultrasonic transducer prepared in Specific Embodiment 1 as an example, a simulation experiment of the bias voltage was conducted. The experimental results are as follows: Figure 7-9As shown, for an 80kHz resonant adjustable MEMS ultrasonic transducer, a 20µm size deviation in the sealed cavity results in a 6.94% resonant frequency deviation. The 0-25V bias voltage adjusts the resonant frequency amplitude to 8.46%. Currently, FAB production lines can typically control the size deviation within 15µm. That is, the technical solution of this invention can make the resonant frequencies of 6-inch / 8-inch / 12-inch wafers consistent by setting the bias voltage, thereby achieving the goal of improving product yield.

[0084] The embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above embodiments, and various changes can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention.

Claims

1. A resonant tunable MEMS ultrasonic transducer, characterized in that, It includes a substrate, a bias electrode, a sealing cavity, an insulating layer, a first electrode layer, a piezoelectric layer, a second electrode layer, a structural layer, a first lead electrode, a second lead electrode, and a third lead electrode, wherein: The substrate has a fifth groove recessed downward in the middle of the top side. A bias electrode is laid flat at the bottom of the fifth groove. A sealing cavity is provided on the top side of the bias electrode. The top surface of the sealing cavity is sealed by an insulating layer. The bottom surface of the sealing cavity is sealed by the bias electrode. The periphery of the sealing cavity is sealed by a structural layer. An insulating layer is deposited on the top side of the substrate; A first electrode layer is laid on the top side of the insulating layer; A piezoelectric layer is laid on the top side of the first electrode layer; A second electrode layer is laid on the top side of the piezoelectric layer; A structural layer is laid on the top side of the second electrode layer; The top side of the structural layer has four symmetrically recessed release holes in the middle; The array element is a resonant structure composed of a sealed cavity, a bias electrode perpendicular to the sealed cavity and overlapping with its orthographic projection, an insulating layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a first electrode layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a piezoelectric layer perpendicular to the sealed cavity and overlapping with its orthographic projection, a second electrode layer perpendicular to the sealed cavity and overlapping with its orthographic projection, and a structural layer perpendicular to the sealed cavity and overlapping with its orthographic projection. The first lead electrode is connected to the first electrode layer and is separate from the second electrode layer; The second lead electrode is connected to the bias electrode; The third lead electrode is connected to the second electrode layer.

2. The resonant tunable MEMS ultrasonic transducer as described in claim 1, characterized in that, The fifth groove includes four connectors, one end of each connector is connected to the intermediate body, and the other end of each connector is connected to a type of quadrangular prism. The intermediate body is one of a type of cylinder, a type of regular tetrahedron, and a type of regular octahedron; The connector is a combination of a type of cylinder and a type of tetrahedron.

3. The resonant tunable MEMS ultrasonic transducer as described in claim 2, characterized in that, The orthographic projections of the four release holes coincide with the orthographic projection of the connector in the fifth groove; The orthographic projection of the second lead electrode coincides with the orthographic projection of the prism-like structure at the tail of the fifth groove.

4. The resonant tunable MEMS ultrasonic transducer as described in claim 1, characterized in that, The bias electrode is a layered electrode, which is one of Al layer, Au layer, Mo layer, Pt layer and Cu layer, and the thickness of the bias electrode is 50 to 500 nm.

5. The resonant tunable MEMS ultrasonic transducer as described in claim 1, characterized in that, The insulating layer is SiN x One of the following: an insulating layer, an Al2O3 layer, and an AlN layer, wherein the thickness of the insulating layer is 50–500 nm, and / or The first electrode layer is one of Pt layer, Au layer and Mo layer, and the thickness of the first electrode layer is 50-300 nm.

6. The resonant tunable MEMS ultrasonic transducer as described in claim 1, characterized in that, The piezoelectric layer is one of AlN, AlScN, and PZT layers, and the thickness of the piezoelectric layer is 0.5–2 μm, and / or The second electrode layer is one of Pt, Au, and Mo layers, and the thickness of the second electrode layer is 50–300 nm.

7. The resonant tunable MEMS ultrasonic transducer as described in claim 1, characterized in that, The structural layer is SiN. x The structure is one of the following: an Al2O3 layer, an AlN layer, and a SiO2 layer, wherein the thickness of the structural layer is 1–6 μm.

8. A method for fabricating a resonant tunable MEMS ultrasonic transducer as described in any one of claims 1-7, characterized in that, The steps are as follows: (1) Preparation of sacrificial layer and bias electrode: Si wafer is selected as substrate, hard mask is prepared on the front side of Si wafer, and then a deep silicon etching machine is used to etch the middle of the front side of Si wafer to form a fifth groove with an etching depth of 0.5-5 μm. First, a metal thin film is prepared by physical vapor deposition, and then the metal thin film is patterned by photolithography and then etching. A bias electrode is formed at the bottom of the fifth groove. Then, phosphosilicate glass or undoped silicate glass is prepared on the top surface of the bias electrode by chemical vapor deposition. The deposition thickness is 0.3-1 μm greater than the silicon etching thickness. Then, it is polished by chemical mechanical polishing to form a flat surface, which is the sacrificial layer. The thickness of the sacrificial layer after polishing is 0.5-3 μm. (2) An insulating layer, a first electrode layer, a piezoelectric layer, and a second electrode layer are sequentially fabricated on the top surface of the Si wafer processed in step (1), wherein: The insulating layer is prepared by chemical vapor deposition or physical vapor deposition. The first electrode layer was prepared by physical vapor deposition. The piezoelectric layer was prepared by physical vapor deposition. The second electrode layer is prepared by physical vapor deposition, and four symmetrically arranged first grooves and one second groove are prepared on the second electrode layer by photolithography followed by etching. The position of the first groove corresponds to the position of the prism-like tail of the fifth groove. (3) Using reactive ion etching, the second electrode layer, piezoelectric layer, first electrode layer and insulating layer of the Si wafer treated in step (2) are etched in sequence to form four symmetrically arranged release holes and four third grooves located outside the release holes. The third grooves are located inside the first grooves. The etching stops on the sacrificial layer. Then the sacrificial layer is removed using the sacrificial layer release solution. After completion, a sealed cavity is formed. (4) A structural layer is prepared on the top side of the Si wafer treated in step (3) by chemical vapor deposition or physical vapor deposition. At this time: The structural layer is partially filled with release holes to form blind holes, and the structural layer is partially filled with a third groove to form a fourth groove. An array element is formed by a sealed cavity, a bias electrode perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, an insulating layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a first electrode layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a piezoelectric layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, a second electrode layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity, and a structural layer perpendicular to the sealed cavity and coinciding with the orthographic projection of the sealed cavity. This array element is used to determine the resonant frequency of the ultrasonic transducer. (5) Using reactive ion etching, the top surface of the Si wafer processed in step (4) is etched to obtain the first blind hole exposing the first electrode layer, the four second blind holes exposing the bias electrode, and the third blind hole exposing the second electrode layer. Au metal first lead electrode, Au metal second lead electrode and Au metal third lead electrode are prepared by sacrificial layer process, or Al metal first lead electrode, Al metal second lead electrode and Al metal third lead electrode are prepared by physical vapor deposition and etching. The first lead electrode is disposed in the first blind hole, the first lead electrode is connected to the first electrode layer, and the first lead electrode is separate from the second electrode layer; Four second lead electrodes are arranged in four second blind holes. The second lead electrodes are connected to the bias electrodes and are separated from both the first electrode layer and the second electrode layer. The third lead electrode is connected to the second electrode layer.

9. The method for fabricating a resonant tunable MEMS ultrasonic transducer as described in claim 8, characterized in that, The hard mask in step (1) has a layered structure, which is either a SiO2 layer or a SiN layer. x The layer or Al2O3 layer has a thickness of 50–500 nm.

10. The method for fabricating a resonant tunable MEMS ultrasonic transducer as described in claim 8, characterized in that, In step (3), the diameter of the release hole is 5-30 μm, and the orthographic projection of the four release holes coincides with the orthographic projection of the connector of the fifth groove.

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