A pmut with isolated groove low stress dual piezoelectric layer structure and its working and preparation method

Abstract

The application discloses a PMUT with an isolated groove low-stress double piezoelectric layer structure and a working and preparation method thereof, and the PMUT comprises, from top to bottom, a circular multilayer composite film, an insulating layer, an upper substrate, a bonding layer and a lower substrate. The multilayer composite film comprises an upper piezoelectric layer, a lower piezoelectric layer, an upper electrode layer, a middle electrode layer and a lower electrode layer, and the multilayer composite film is provided with a concentric annular groove and covers the whole cavity area. The application can effectively reduce the crosstalk between PMUT units, reduce the film stress, greatly increase the piezoelectric driving bending moment in the film, and further improve the ultrasonic emission and receiving performance, so as to meet the urgent needs of low-power consumption and high-performance ultrasonic transducers for small pipe flow detection applications.

Description

TECHNICAL FIELD

[0001] The present application relates to MEMS and ultrasonic transducer technology, in particular to a PMUT with isolated groove low stress double piezoelectric layer structure for flow detection application and its working and preparation method. BACKGROUND

[0002] Ultrasound is a mechanical wave with a frequency above 20 kHz, which has the characteristics of strong directivity, good penetration, energy concentration and no noise pollution. With the development of modern science and technology, ultrasonic equipment is widely used in medical diagnosis, disease treatment, non-destructive testing, distance measurement, attitude recognition, fingerprint recognition and many other fields, and ultrasonic transducer is the sensor for transmitting and receiving ultrasonic waves. Although the design and preparation of traditional bulk ultrasonic sensors based on thickness vibration mode have become mature and have achieved wide application, the complex preparation process limits the further reduction of cost, the miniaturization of equipment and the design for advanced 2D array imaging system. With the development of microelectromechanical systems (MEMS), piezoelectric micromachined ultrasonic transducers (PMUT) based on MEMS technology gradually emerged in the industry, overcoming the shortcomings of traditional bulk piezoelectric ultrasonic transducers. PMUT has the characteristics of good impedance matching, no need for high-voltage pulse polarization, no need for high DC bias voltage, no strict restriction on the gap size between the vibration film and the substrate, etc., making its design more flexible and application more extensive.

[0003] In recent years, flow measurement is closely related to people's daily life, and is widely used in many fields such as petroleum, chemical industry, metallurgy, nuclear power, water conservancy, aviation and so on. Through detecting the change of flow, fault prediction and health management are realized. The ultrasonic flowmeter is a non-invasive measurement, which will not destroy the liquid flow field, has no pressure loss, almost no requirement for medium, non-fixed measurement, easy to install and remove, and has many advantages. Therefore, it has been widely used in the field of flow testing. At present, the ultrasonic flowmeter on the market is based on the traditional bulk PZT piezoelectric transducer. However, most of these traditional ultrasonic flowmeters are not feasible for flow measurement of small diameter pipes (such as diameter ≤ 15mm) for in-situ or real-time flow monitoring, because their shortcomings are large size, high power consumption and difficult to integrate with IC. With the development of MEMS technology, PMUT has become a promising alternative to traditional ultrasonic transducers because of its small size, no need for acoustic impedance matching layer, low power consumption, low manufacturing cost and easy integration with IC. These advantages enable MUT to achieve compact integration with measurement targets that have small pipes and high requirements for weight, size and power consumption, such as oil pipe systems in airplanes and aerospace vehicles or blood flow monitoring in medical diagnosis, to realize in-situ, long-term and real-time flow monitoring. Since the ultrasonic wave will quickly attenuate when propagating in the steel pipe, and the inconsistency of the acoustic impedance of the steel pipe and the PMUT impedance will also cause large reflection loss, therefore, the PMUT is required to have high strength ultrasonic emission capability and high receiving sensitivity. However, the conventional PMUT structure is composed of an upper electrode, a piezoelectric layer, a lower electrode, an insulation layer, a structure layer and a substrate, and mainly through designing a novel thin film structure to increase the average displacement of the thin film, thereby improving the output sound pressure and sensitivity. This method leads to complex thin film structure design, increases the process difficulty, and suppresses the consistency of the unit structure in the array. Therefore, the preparation process of the conventional structure PMUT is simple, but the output sound pressure, receiving performance and sensitivity are difficult to meet the performance requirements for flow detection of small pipes made of metal materials. SUMMARY

[0004] To solve the above technical problems, the present application provides a PMUT with a low-stress double piezoelectric layer structure with an isolation groove and its working and preparation method. The present application effectively reduces the crosstalk between PMUT units, reduces the stress of the thin film, and greatly increases the bending moment of the piezoelectric drive in the thin film, thereby improving the ultrasonic emission and receiving performance to meet the urgent needs of low-power and high-performance ultrasonic transducers for small pipe flow detection applications.

[0005] The technical scheme adopted by the present application is as follows:

[0006] A PMUT with a low-stress double piezoelectric layer structure with an isolation groove, comprising a first transducer unit and a second transducer unit, the first transducer unit and the second transducer unit are the same in structure;

[0007] The first transducer unit and the second transducer unit each comprise a multilayer composite film, an insulating layer, an upper substrate and a lower substrate, the insulating layer, the upper substrate and the lower substrate are sequentially stacked, a circular cavity is formed in the upper substrate, and the multilayer composite film is arranged on the insulating layer and has a circular shape at a portion above the circular cavity;

[0008] The first transducer unit and the second transducer unit share the insulating layer, the upper substrate and the lower substrate, and the multilayer composite film of the first transducer unit and the second transducer unit is provided with an annular groove at an outer periphery of the portion above the circular cavity, and the annular groove extends from a surface of the multilayer composite film to a surface of the insulating layer;

[0009] The portion of the multilayer composite film above the circular cavity comprises an upper piezoelectric layer, a lower piezoelectric layer, a top electrode, a middle electrode and a bottom electrode, the bottom electrode is arranged on a surface of the insulating layer, the lower piezoelectric layer is arranged on a surface of the bottom electrode, the middle electrode is arranged on a surface of the lower piezoelectric layer, the upper piezoelectric layer is arranged on a surface of the middle electrode, the middle electrode comprises a circular middle inner electrode and an annular middle outer electrode located at an outer periphery of the middle inner electrode, a gap is formed between an outer edge of the middle inner electrode and an inner edge of the middle outer electrode, and the top electrode is arranged on a surface of the upper piezoelectric layer.

[0010] Preferably, the upper substrate and the lower substrate are bonded by a bonding layer.

[0011] Preferably, the upper piezoelectric layer and the lower piezoelectric layer are made of aluminum nitride piezoelectric material, the insulating layer and the bonding layer are made of silicon dioxide material, and the upper substrate and the lower substrate are made of silicon material.

[0012] Preferably, the portion of the multilayer composite film above the circular cavity is coaxial with the circular cavity.

[0013] Preferably, a transverse dimension of the portion of the multilayer composite film above the circular cavity is greater than a diameter of the circular cavity.

[0014] Preferably, the top electrodes of the first transducer unit and the second transducer unit are connected by a lead wire, the middle outer electrodes of the first transducer unit and the second transducer unit are connected by a lead wire, the middle inner electrodes of the first transducer unit and the second transducer unit are connected by a lead wire, the first transducer unit and the second transducer unit are connected by a lead wire, and the bottom electrodes of the first transducer unit and the second transducer unit are connected by a lead wire.

[0015] The working method of the PMUT with the isolation groove low-stress double piezoelectric layer structure as described above comprises the following processes:

[0016] A driving voltage of 0V is loaded on the top electrode and the bottom electrode, driving voltages with equal amplitude and opposite phase are loaded on the middle outer electrode and the middle inner electrode, in-surface stresses with equal size and opposite direction are generated in the upper piezoelectric layer and the lower piezoelectric layer, and the upper piezoelectric layer and the lower piezoelectric layer are warped to generate bending vibration.

[0017] Preferably, the driving voltage V1 loaded on the middle inner electrode and the driving voltage V2 loaded on the middle outer electrode are both sine signals, and the amplitudes of the driving voltage V1 and the driving voltage V2 are equal and the phases are different by π.

[0018] The preparation method of the PMUT with the isolation groove low-stress double piezoelectric layer structure as described above comprises the following steps:

[0019] Step 1: remove the top layer of silicon dioxide on the surface of the low-resistance double-side polished SOI wafer by dry etching, and then remove the top layer of silicon of the low-resistance double-side polished SOI wafer by wet etching;

[0020] Step 2: photoetch the bottom layer of silicon dioxide of the low-resistance double-side polished SOI wafer, and etch the corresponding area of the circular cavity on the silicon substrate of the low-resistance double-side polished SOI wafer by dry etching, and stop etching on the bottom surface of the middle silicon dioxide of the low-resistance double-side polished SOI wafer, at this time, the circular cavity is formed, the thickness of the middle silicon dioxide of the low-resistance double-side polished SOI wafer is thinned, an insulating layer is formed, the insulating layer serves as a vibrating diaphragm, and a first structure is obtained;

[0021] Step 3: take another single-side polished and thinned single crystal silicon wafer as a lower substrate, bond the single crystal silicon wafer and the insulating layer of the first structure by diffusion, at this time, the vacuum degree in the circular cavity is controlled to reach the design required vacuum suction force, and finally the sealing of the circular cavity is completed;

[0022] Step 4: magnetron sputter a first metal layer on the surface of the insulating layer, then photoetch and patternize, and finally form a bottom electrode by dry etching;

[0023] Step 5: magnetron sputter a first piezoelectric layer on the surface of the bottom electrode, then photoetch and patternize, and finally form a lower piezoelectric layer by combining dry etching and wet etching;

[0024] Step 6: magnetron sputter a second metal layer on the surface of the lower piezoelectric layer, then photoetch and patternize, form the area of the middle inner electrode and the middle outer electrode, and then form a middle electrode by dry etching;

[0025] Step 7: magnetron sputter a second piezoelectric layer on the surface of the middle electrode, then photoetch and patternize, and finally form an upper piezoelectric layer by combining dry etching and wet etching;

[0026] Step 8: magnetron sputter a third metal layer on the surface of the upper piezoelectric layer, then photoetch and patternize, and finally form a top electrode by dry etching, and the preparation is completed.

[0027] Preferably, in step 3, when the single crystal silicon wafer is bonded with the insulating layer of the first structure by diffusion, the balance between the downward vacuum suction in the circular cavity and the upward processing stress of the insulating layer of the first structure is achieved by adjusting the vacuum degree of the circular cavity between the single crystal silicon wafer and the insulating layer of the first structure.

[0028] Compared with the prior art, the present application has the following beneficial effects:

[0029] In the PMUT, the annular grooves are arranged on the outer periphery of the part of the multilayer composite film above the circular cavity of the first transducer unit and the second transducer unit, the annular grooves extend from the surface of the multilayer composite film to the surface of the insulating layer, the annular isolation grooves between the first transducer unit and the second transducer unit can inhibit the propagation of ultrasonic transverse waves along the structure layer, reduce the cross talk between units, and realize the improvement of acoustic power; the part of the multilayer composite film above the circular cavity adopts a double piezoelectric layer, the double piezoelectric layer can be driven differentially, the differential driving double piezoelectric layer structure can increase the transverse stress in the film, thereby greatly improving the piezoelectric bending moment, and the excitation mode of the reverse electric field of the double piezoelectric layer realizes the concentration of the bending moment in the film at the vibration center and the reduction at the edge, further improves the average deformation in the film, and further improves the output sound pressure; in the present application, the balance between the downward vacuum suction and the upward film processing stress is achieved by adjusting the vacuum degree in the circular cavity, thereby reducing the stress in the vibrating film, thereby effectively increasing the sensitivity. In summary, the PMUT of the present application effectively reduces the cross talk of the PMUT while greatly improving the ultrasonic emission and reception performance, and can meet the urgent needs of small pipe flow detection applications for low-power and high-performance ultrasonic transducers. BRIEF DESCRIPTION OF DRAWINGS

[0030] Fig. 1(a) is a top view of the PMUT with an isolation groove low-stress double piezoelectric layer structure of the present application;

[0031] Fig. 1(b) is a sectional view of A-A in Fig. 1(a).

[0032] Figure 2 Fig. 1(c) is a schematic diagram of the driving signal loading and working vibration shape of the PMUT with an isolation groove low-stress double piezoelectric layer structure of the present application (A-A section in Fig. 1(a)).

[0033] Figure 3 Fig. 1(d) is a schematic diagram of the preparation method steps of the PMUT with an isolation groove low-stress double piezoelectric layer structure of the present application.

[0034] The reference numerals in the drawings are shown in the following table:

[0035] DETAILED DESCRIPTION

[0036] The application will be described in detail below in conjunction with the drawings and examples:

[0037] The application provides a PMUT with an isolated groove low-stress double piezoelectric layer structure, which realizes a substantial increase in the piezoelectric driving bending moment in the thin film while reducing the film stress and the cross talk between PMUT units, and further realizes the improvement of ultrasonic emission and receiving performance to meet the demand for low-power high-performance ultrasonic transducers in small pipe flow detection applications.

[0038] As shown in FIG. 1(a) and FIG. 1(b), the PMUT with an isolated groove low-stress double piezoelectric layer structure of the application includes a multilayer composite film 1, an insulating layer 3, an upper substrate 4, a bonding layer 6 and a lower substrate 7 from top to bottom;

[0039] As shown in FIG. 1(b), the multilayer composite film 1 includes a piezoelectric driving layer 1-2, a top electrode 1-1, a middle electrode 1-3 and a bottom electrode 1-4; wherein the piezoelectric driving layer 1-2 includes an upper electrode layer 1-2-1 and a lower electrode layer 1-2-2 from top to bottom in the thickness direction; the middle electrode 1-3 is arranged between the upper electrode layer 1-2-1 and the lower electrode layer 1-2-2, and the middle electrode 1-3 includes a middle outer electrode 1-3-2 and a middle inner electrode 1-3-1 from the boundary to the center along the radial direction, as shown in FIG. 1(a), the middle inner electrode 1-3-1 is a circular structure, the middle outer electrode 1-3-2 is an annular structure and is sleeved on the outer circle of the middle inner electrode 1-3-1, and there is a gap between the outer edge of the middle inner electrode 1-3-1 and the inner edge of the middle outer electrode 1-3-2.

[0040] The multilayer composite film 1 can be set as a whole during processing, and then an annular groove 2 is etched on the outer edge of the part above the circular cavity 5 in the upper substrate 4 of the multilayer composite film 1, the part in the annular groove 2 is a circular structure and is located above the circular cavity 5, and the annular groove 2 and the circular cavity 5 are in concentric position; the insulating layer 3, the upper substrate 4, the bonding layer 6 and the lower substrate 7 seal the circular cavity 5 from top to bottom.

[0041] The top electrodes 1-1 of the adjacent first transducer unit and the second transducer unit are interconnected by a top electrode lead 8, the middle outer electrodes 1-3-2 of the adjacent first transducer unit and the second transducer unit are interconnected by a middle outer electrode lead 10, the middle inner electrodes 1-3-1 of the adjacent first transducer unit and the second transducer unit are interconnected by a middle inner electrode lead 9, and the bottom electrodes 1-4 of the adjacent first transducer unit and the second transducer unit are interconnected by a bottom electrode lead 11.

[0042] The top electrode 1-1, the piezoelectric driving layer 1-2, the middle outer electrode 1-3-2 and the bottom electrode 1-4 have the same lateral size; the distance between the inner wall of the middle outer electrode 1-3-2 and the outer wall of the middle inner electrode 1-3-1 is 2-5 μm; the lateral size of the top electrode 1-1, the piezoelectric driving layer 1-2, the middle outer electrode 1-3-2 and the bottom electrode 1-4 is larger than the lateral size of the cavity 5;

[0043] In the PMUT with the isolated groove low stress double piezoelectric layer structure, the piezoelectric driving layer 1-2 is made of aluminum nitride piezoelectric material, the insulation layer 3 and the bonding layer 6 are made of silicon dioxide material, and the upper substrate 4 and the lower substrate 7 are made of silicon material. The piezoelectric driving layer 1-2 is not only a driving layer for providing a piezoelectric driving bending moment, but also a structure layer for realizing self vibration, replacing a traditional vibration structure layer, and reducing the thickness of the unit as a whole.

[0044] In the PMUT with the isolated groove low stress double piezoelectric layer structure, the driving voltage of the top electrode 1-1 and the bottom electrode 1-4 is 0 V, the driving voltage of the middle outer electrode 1-3-2 and the middle inner electrode 1-3-1 has the same amplitude but opposite phase, in-plane stress with the same size and opposite direction is generated in the upper piezoelectric layer 1-2-1 and the lower piezoelectric layer 1-2-2 in the middle electrode 1-3 covering area, and then the upper piezoelectric layer 1-2-1 and the lower piezoelectric layer 1-2-2 are warped to generate bending vibration, see Figure 2 The driving signal V1 and the driving signal V2 loaded on the middle inner electrode 1-3-1 and the middle outer electrode 1-3-2 are both sine signals, and the amplitude of the driving signal V1 is equal to the amplitude of the driving signal V2, and the phase difference is π.

[0045] The adjacent first transducer unit and the second transducer unit are connected by the isolated groove 2 to reduce the ultrasonic crosstalk. At the same time, the multi-layer composite film 1 of the unit is connected by the isolated groove 2 to reduce the boundary stress and improve the vibration displacement.

[0046] By adjusting the vacuum degree of the circular cavity 5 between the insulation layer 3 and the bonding layer 6, the downward vacuum suction force and the upward processing stress of the film are balanced.

[0047] When used as an ultrasonic wave emission mode, the circular middle inner electrode and the ring-shaped middle outer electrode are excited with a certain phase difference to realize differential driving of the double piezoelectric layers, increase the piezoelectric bending moment in the film, the ring-shaped isolated groove has the functions of suppressing the crosstalk between the units and reducing the boundary stress to improve the vibration displacement, the downward vacuum suction force generated by the vacuum cavity is balanced with the internal stress of the film, and the ultrasonic emission performance of the unit is greatly improved; when used as an ultrasonic wave receiving mode, the incident ultrasonic wave makes the multi-layer composite film vibrate to generate a detectable electric signal, realizing the reception of the ultrasonic wave.

[0048] Referring toFigure 3 The method for manufacturing the PMUT with the isolation groove low-stress double piezoelectric layer structure shown in FIG. 1 comprises the following steps:

[0049] Step (1): Take a low-resistance double-side polished SOI wafer (which can be referred to as SOI wafer) as an upper substrate structure, remove the top layer of silicon dioxide 13 on the upper surface of the SOI wafer by dry etching, and then remove the top layer of silicon 14 by wet etching, leaving a middle layer of silicon dioxide, which is subsequently processed into an insulating layer 3.

[0050] Step (2): Photoetch the bottom layer of silicon dioxide 12 of the SOI wafer, etch away the silicon substrate (which is subsequently processed into an upper substrate 4) in the back cavity pattern (i.e. the area corresponding to the circular cavity 5) by dry etching, and stop etching on the bottom surface of the middle layer of silicon dioxide. The thickness of the silicon substrate is thinned by mechanical grinding to the required height of the circular cavity 5. At this time, the circular cavity 5 of the vibrating diaphragm (the area corresponding to the circular cavity 5 on the insulating layer 3) is formed. Select another single crystal silicon wafer which is single-side polished and thinned as a combination of a lower substrate 7 and a bonding layer 6. The lower substrate 7 is bonded to the silicon substrate of the SOI wafer with completed back cavity etching by diffusion bonding. At this time, the vacuum degree needs to be controlled to ensure the design required vacuum suction. Finally, the sealing of the circular cavity 5 is completed.

[0051] Step (3): Sputter a first metal layer by magnetron sputtering, then photoetch and patternize, remove the corresponding first metal layer at the isolation groove 2, and finally form the bottom electrode 1-4 by dry etching.

[0052] Step (4): Sputter a first aluminum nitride layer by magnetron sputtering, then photoetch and patternize, remove the corresponding first aluminum nitride layer at the isolation groove 2, and finally form the lower piezoelectric layer 1-2-2 by a combination of dry etching and wet etching.

[0053] Step (5): Sputter a second metal layer by magnetron sputtering, then photoetch and patternize, remove the corresponding second metal layer at the isolation groove 2, and remove the second metal layer between the outer edge of the middle electrode 1-3-1 and the inner edge of the middle outer electrode 1-3-2, and then form the middle electrode 1-3 by dry etching.

[0054] Step (6): Sputter a second aluminum nitride layer by magnetron sputtering, then photoetch and patternize, remove the corresponding second aluminum nitride layer at the isolation groove 2, and finally form the upper piezoelectric layer 1-2-1 by a combination of dry etching and wet etching.

[0055] Step (7): Sputter a third metal layer by magnetron sputtering, then photoetch and patternize, remove the corresponding third metal layer at the isolation groove 2, and finally form the top electrode 1-1 by dry etching. Thus, the PMUT with the isolation groove low-stress double piezoelectric layer structure is manufactured.

[0056] From the above scheme, the present application first reduces the acoustic crosstalk between adjacent units by the design of isolation grooves, and further reduces the constraint of the thin film deformation region on the thin film deformation; secondly, the design of differential driving of double piezoelectric layers increases the bending moment of the thin film; finally, the design of the vacuum cavity realizes the balance with the processing stress of the thin film. These designs greatly increase the deformation displacement of the thin film under the condition of ensuring the thickness of the unit, and further improve the output sound pressure, the electromechanical coupling coefficient, the emission and receiving sensitivity and other performances. In addition, the preparation process of the present application is simple, and the prepared PMUT structure and performance consistency are good. The PMUT of the present application effectively reduces the PMUT crosstalk while greatly improving the ultrasonic emission and receiving performance, and can meet the urgent needs of small pipe flow detection applications for low-power and high-performance ultrasonic transducers.

[0057] The above only describes several embodiments of the present application, which are not all or the only embodiments. Any equivalent transformation of the technical solutions of the present application adopted by a person of ordinary skill in the art by reading the specification of the present application is covered by the claims of the present application.

Claims

1. A PMUT with a low-stress double piezoelectric layer structure having an isolation groove, characterized in that, It includes a first transducer unit and a second transducer unit, and the first transducer unit and the second transducer unit have the same structure; The first transducer unit and the second transducer unit both include a multilayer composite film (1), an insulating layer (3), an upper substrate (4) and a lower substrate (7). The insulating layer (3), the upper substrate (4) and the lower substrate (7) are stacked in sequence. A circular cavity (5) is opened in the upper substrate (4). The multilayer composite film (1) is disposed on the insulating layer (3) and the part located above the circular cavity (5) is circular. The first transducer unit and the second transducer unit share an insulating layer (3), an upper substrate (4) and a lower substrate (7). The outer periphery of the multilayer composite film (1) of the first transducer unit and the second transducer unit located above the circular cavity (5) is provided with an annular groove (2). The annular groove (2) extends from the surface of the multilayer composite film (1) to the surface of the insulating layer (3). The portion of the multilayer composite film (1) above the circular cavity (5) includes an upper piezoelectric layer (1-2-1), a lower piezoelectric layer (1-2-2), a top electrode (1-1), a middle electrode (1-3), and a bottom electrode (1-4). The bottom electrode (1-4) is disposed on the surface of the insulating layer (3), the lower piezoelectric layer (1-2-2) is disposed on the surface of the bottom electrode (1-4), the middle electrode (1-3) is disposed on the surface of the lower piezoelectric layer (1-2-2), and the upper piezoelectric layer (1-2-1) is disposed on the surface of the middle electrode (1-3). The middle electrode (1-3) includes a circular inner electrode (1-3-1) and an annular outer electrode (1-3-2) located around the inner electrode (1-3-1). There is a gap between the outer edge of the inner electrode (1-3-1) and the inner edge of the outer electrode (1-3-2). The top electrode (1-1) is disposed on the surface of the upper piezoelectric layer (1-2-1). By adjusting the vacuum level inside the circular cavity (5), a downward vacuum suction force is formed, which balances the upward thin film processing stress. The upper piezoelectric layer (1-2-1) and the lower piezoelectric layer (1-2-2) are made of aluminum nitride piezoelectric material, the insulating layer (3) and the bonding layer (6) are made of silicon dioxide material, and the upper substrate (4) and the lower substrate (7) are made of silicon material.

2. The PMUT with a low-stress double piezoelectric layer structure having an isolation groove according to claim 1, characterized in that, The upper substrate (4) and the lower substrate (7) are bonded together by a bonding layer (6).

3. The PMUT with a low-stress double piezoelectric layer structure having an isolation groove according to claim 1, characterized in that, The portion of the multilayer composite film (1) located above the circular cavity (5) is coaxial with the circular cavity (5).

4. A PMUT with a low-stress double piezoelectric layer structure having an isolation groove according to claim 1, characterized in that, The lateral dimension of the portion of the multilayer composite film (1) located above the circular cavity (5) is larger than the diameter of the circular cavity (5).

5. A PMUT with a low-stress double piezoelectric layer structure having an isolation groove according to claim 1, characterized in that, The top electrodes (1-1) of the first transducer unit and the second transducer unit are connected by leads. The middle and outer electrodes (1-3-2) of the first transducer unit and the second transducer unit are connected by leads. The middle and inner electrodes (1-3-1) of the first transducer unit and the second transducer unit are connected by leads. The bottom electrodes (1-4) of the first transducer unit and the second transducer unit are connected by leads.

6. The operating method of a PMUT with a low-stress double piezoelectric layer structure having an isolation groove as described in any one of claims 1-5, characterized in that, The process includes the following: A driving voltage of 0V is applied to the top electrode (1-1) and the bottom electrode (1-4). A driving voltage of equal amplitude and opposite phase is applied to the middle outer electrode (1-3-2) and the middle inner electrode (1-3-1). In-plane stress of equal magnitude and opposite direction is generated in the upper piezoelectric layer (1-2-1) and the lower piezoelectric layer (1-2-2), causing the upper piezoelectric layer (1-2-1) and the lower piezoelectric layer (1-2-2) to warp and produce bending vibration.

7. The operating method of a PMUT with a low-stress double piezoelectric layer structure having an isolation groove according to claim 6, characterized in that, The driving voltage V1 applied to the inner electrode (1-3-1) and the driving voltage V2 applied to the outer electrode (1-3-2) are both sinusoidal signals, and the amplitudes of the driving voltages V1 and V2 are equal and their phases differ by π.

8. A method for fabricating a PMUT with a low-stress double piezoelectric layer structure having an isolation groove, as described in any one of claims 1-5, characterized in that, Includes the following steps: Step 1: Remove the top layer silicon dioxide on the upper surface of the low-resistivity double-sided polished SOI wafer by dry etching (13), and then use wet etching to remove the top layer silicon of the low-resistivity double-sided polished SOI wafer (14). Step 2: Photolithography of the bottom silicon dioxide (12) of the low-resistivity double-sided polished SOI wafer, and dry etching of the corresponding area of ​​the circular cavity (5) on the silicon substrate of the low-resistivity double-sided polished SOI wafer. The etching stops at the bottom surface of the middle silicon dioxide of the low-resistivity double-sided polished SOI wafer, at which point the circular cavity (5) is formed. The thickness of the middle silicon dioxide of the low-resistivity double-sided polished SOI wafer is reduced to form an insulating layer (3). The insulating layer (3) serves as a vibration film to obtain the first structure. Step 3: Take another single-sided polished and thinned monocrystalline silicon wafer as the lower substrate, and bond the monocrystalline silicon wafer to the insulating layer (3) of the first structure through diffusion bonding. At this time, control the vacuum degree in the circular cavity (5) to reach the vacuum suction force required by the design, and finally complete the sealing of the circular cavity (5). Step 4: The first metal layer is magnetron sputtered on the surface of the insulating layer (3), then photolithographically patterned, and finally the bottom electrode (1-4) is formed by dry etching. Step 5: The first piezoelectric layer is magnetron sputtered on the surface of the bottom electrode (1-4), then photolithographically patterned, and finally the lower piezoelectric layer (1-2-2) is formed by a combination of dry and wet etching. Step 6: A second metal layer is magnetron sputtered on the surface of the lower piezoelectric layer (1-2-2), and then photolithographically patterned to form the regions of the inner electrode (1-3-1) and the outer electrode (1-3-2). The middle electrode (1-3) is then formed by dry etching. Step 7: A second piezoelectric layer is magnetron sputtered on the surface of the middle electrode (1-3), then photolithographically patterned, and finally the upper piezoelectric layer (1-2-1) is formed by a combination of dry and wet etching. Step 8: A third metal layer is magnetron sputtered onto the surface of the upper piezoelectric layer (1-2-1), then patterned by photolithography, and finally the top electrode (1-1) is formed by dry etching, thus completing the fabrication.

9. The method for fabricating a PMUT with a low-stress double piezoelectric layer structure having an isolation groove as described in claim 8, characterized in that, In step 3, when the single crystal silicon wafer is bonded to the insulating layer (3) of the first structure by diffusion bonding, the vacuum degree of the circular cavity (5) between the single crystal silicon wafer and the insulating layer (3) of the first structure is adjusted to achieve a balance between the downward vacuum suction force in the circular cavity (5) and the upward processing stress of the insulating layer (3) of the first structure.

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