CN112871612A

CN112871612A - Piezoelectric micromachined ultrasonic transducer with multiple piezoelectric layers

Info

Publication number: CN112871612A
Application number: CN202011512503.9A
Authority: CN
Inventors: 任俊彦; 何勒铭; 王言
Original assignee: Fudan University
Current assignee: Fudan University
Priority date: 2020-12-19
Filing date: 2020-12-19
Publication date: 2021-06-01

Abstract

The invention belongs to the technical field of micro-electro-mechanical systems, and particularly relates to a piezoelectric micro-mechanical ultrasonic transducer with multiple piezoelectric layers. The piezoelectric micromechanical ultrasonic transducer unit comprises a composite film, a supporting structure and a substrate; the supporting structure is a hollow thin layer, and the supporting structure and the substrate form bonding corresponding to the free boundary of the composite film adhered above the supporting structure; the composite film comprises a neutral layer, at least one piezoelectric layer and a plurality of metal layers; and the piezoelectric layer and the metal layer in the composite film drive the film structure to vibrate up and down together through the arrangement of the polarization direction and the electric connection of the piezoelectric layer. The invention can effectively accumulate the bending moment of the cross section of the component by utilizing the piezoelectric effect and improve the sensitivity of the sensor. The composite film can be released and supported by forming an etching cavity below the composite film, manufacturing a through hole by a deep etching method, or forming a supporting point by welding with a chip pin, so that sound waves are transmitted and received through a load medium (air, water, oil and the like).

Description

Piezoelectric micromachined ultrasonic transducer with multiple piezoelectric layers

Technical Field

The invention belongs to the technical field of micro-electro-mechanical systems, and particularly relates to a piezoelectric micro-mechanical ultrasonic transducer.

Background

Ultrasonic imaging is the most popular technology applied to medical imaging and industrial nondestructive testing by virtue of the advantages of no ionization side effect, high sensitivity, real-time imaging, no damage to tissues, low cost and the like. The important core component of the system is the ultrasonic transducer which is used for realizing the conversion between the electric energy of the imaging signal and the mechanical energy of the ultrasonic wave. The conventional transducer probe is formed by cutting and connecting units by piezoelectric ceramics or single crystals in a mechanical processing mode and the like, but the processing difficulty is high, the transducer probe is incompatible with an integrated circuit process, large-scale preparation and multi-chip packaging cannot be carried out, and the innovation and the application of high-frequency phased array imaging, microminiaturization application and a portable system are limited.

Piezoelectric micromachined ultrasonic transducers (pmuts) fill the gap of conventional piezoelectric transducers in portable devices, small volume catheters, high frequency, i.e., high resolution, ultrasound imaging. The piezoelectric film is stretched by applying an alternating current signal with a certain frequency through the electrodes based on the (inverse) piezoelectric effect of the piezoelectric film, and the neutral layer is bent due to the deformation of the piezoelectric film so as to push the medium to vibrate and radiate ultrasonic waves. On the contrary, when the ultrasonic wave with a certain frequency reaches the surface of the pMUT and presses the piezoelectric film, the piezoelectric material shows voltage response and changes along with the size of the sound pressure.

The piezoelectric micro-mechanical ultrasonic transducer has simple realization process and good stability, can be prepared on a large scale, has the potential of carrying out multi-chip packaging with an integrated circuit chip, reduces the problems of parasitism, channel imbalance, impedance mismatch and the like caused by interconnection, has attracted wide attention, and promotes the innovation and realization of new application in the medical and industrial fields.

Disclosure of Invention

The invention aims to provide a high-sensitivity piezoelectric micro-mechanical ultrasonic transducer (pMUT) which can be used in the fields of high-frequency phased array imaging, intravascular ultrasound or ultrasonic microscopes and the like.

The invention provides a piezoelectric micro-mechanical ultrasonic transducer (pMUT), which is provided with a plurality of piezoelectric layers, namely a piezoelectric layer structure comprising a patterned etched piezoelectric layer, a laminated layer and a double-side distribution. Specifically, the piezoelectric micromachined ultrasonic transducer (pMUT) unit includes a composite thin film, a support structure, and a substrate, which are sequentially disposed from top to bottom; wherein:

the supporting structure is a hollow thin layer for supporting, namely, the middle part of the supporting structure is provided with a hole corresponding to the free boundary of the composite film adhered to the upper part of the supporting structure; and the support structure forms a bond with the substrate through at least one thin layer of silicon dioxide;

the composite film comprises a neutral layer, at least one piezoelectric layer and a plurality of metal layers; and the piezoelectric layer and the metal layer in the composite film drive the film structure to vibrate up and down together through the arrangement of the polarization direction and the electric connection of the piezoelectric layer.

In the invention, the composite film is based on a neutral layer, and metal layers and piezoelectric layers are deposited on the upper surface and the lower surface of the composite film, specifically:

for a pMUT unit with a single piezoelectric layer, a bottom metal layer (M), a piezoelectric layer (P) and a top metal layer (M) are manufactured on the upper surface of a neutral layer, namely an MPM sandwich piezoelectric layer structure, and routing and electrodes are led out in a proper mode;

for a pMUT unit with a double-layer piezoelectric layer, MPM sandwich piezoelectric layer structures are respectively manufactured on the upper surface and the lower surface of a neutral layer, or MPM double-piezoelectric layer structures are manufactured on the upper surface of the neutral layer, so that the aim of improving the theoretical sensitivity by one time is fulfilled;

for the pMUT unit with the multilayer piezoelectric layers, a multi-piezoelectric-layer structure of MPM (and/or PM) · is manufactured on the upper side, the lower side or one side of a neutral layer, the sensitivity of the pMUT unit is further improved, finally, the routing and the electrodes are led out in a proper mode, and the two-port units are connected in series or in parallel in a proper mode.

In the present invention, the composite film is substantially circular or square.

The piezoelectric layer of the conventional pMUT unit is not etched or is slightly etched, and in the present invention, the piezoelectric layer is etched into a specific shape, that is, a circular or square shape with a fixed ratio which is similar to the pattern of the free boundary (circular or square) of the bottom surface of the composite membrane, so as to improve the rigidity and vibration mode of the membrane, thereby improving the sensitivity and bandwidth of the pMUT unit.

When the piezoelectric layer is circular, the ratio of the radius of the piezoelectric layer to the radius of the circular free boundary of the lower surface of the composite film is 0.6-0.9; when the piezoelectric layer is square, the ratio of the side length of the piezoelectric layer to the side length of the square free boundary of the lower surface of the composite film is 0.6-0.9.

In order to prepare the piezoelectric layer MPM (and/or PM) · structure on both sides of the neutral layer, firstly, a single-layer or multi-layer piezoelectric layer structure with patterns and leads is prepared on the upper side of the neutral layer through multiple deposition and photoetching steps, then the unfinished composite film is turned over and bonded with another substrate with a prepared support structure in a mode of transferring the film, and then the single-layer or multi-layer piezoelectric layer MPM (or PM) · structure is prepared on the other side.

In order to realize the bonding of the pMUT unit on the chip substrate, the film is turned over and bonded to the bonding column in the manner described in the above paragraph, so as to transfer the film to a specified substrate; after the film is transferred, the original substrate can be reserved, and through holes are manufactured in deep etching and other modes, so that the transducer transmits sound waves outwards through the through holes, and indexes such as the vibration mode and far-field directivity of the pMUT unit are improved.

In the basic structure of the pMUT unit, a polymer material with the thickness of 10-50 microns can be added on the upper surface of the pMUT unit, so that the acoustic matching between the composite film structure for bending vibration and a sound field medium is realized, and the acoustic emission efficiency is improved. For the pMUT unit with the through hole etched on the substrate, polymer can be filled in the through hole to serve as waveguide material, and then the acoustic matching purpose is achieved, and meanwhile indexes such as far field directivity of a sound field and the like which are crucial to phased array imaging are improved.

In the present invention, the substrate is composed of an integral part of a silicon wafer, a bulk silicon part of a silicon-on-insulator (SOI) element, or a chip substrate bonded after thin film transfer.

In the invention, the neutral layer of the composite film is a mechanical vibration layer without piezoelectric property, and the material is silicon, silicon oxide or silicon nitride.

In the invention, the supporting structure corresponds to the composite film, and the basic shape of the supporting structure is a round or square hollow thin layer; when the support structure is used for the interconnection bonding of the thin film and the chip, the support structure is a plurality of micro-columns for bonding.

In the invention, when the substrate and the supporting structure are prepared by deeply etching the back surface of the substrate to form the through hole and form the free boundary of the lower surface of the composite film, the substrate and the supporting structure are the same part and are not obviously distinguished. In the invention, the upper surface and the lower surface of the piezoelectric layer are in contact with the metal layer, wherein one layer is a grounding electrode, and the other layer is a signal electrode.

In the composite film, the piezoelectric layer is required to be etched to form a round/square film with smaller size, and the ratio of the radius/side length of the round/square film to the radius/side length of the free boundary on the lower surface of the composite film is 0.6-0.9.

In the composite film of the present invention, when a plurality of piezoelectric layers are provided, the piezoelectric layers and the metal layers in contact therewith are distributed on one side or both sides of the neutral layer, and the polarization direction of the piezoelectric material can be configured to be upward or downward.

In the composite film, when a plurality of piezoelectric layers are arranged, each piezoelectric layer and the metal layer contacted with the piezoelectric layer are combined into a dual-port unit, and at least two electrodes can be finally led out between the units in a parallel or serial mode to be used as the input of excitation and grounding signals.

When the piezoelectric micromachined ultrasonic transducer (pMUT) works in a transmitting mode, the main mechanism is that metal layers in contact with the upper side and the lower side of a piezoelectric layer are respectively used as a top electrode and a bottom electrode (or the bottom electrode and the top electrode), an electric field is generated in the longitudinal direction under the excitation of voltage signals with any amplitude and waveform, the piezoelectric layer responds to stretching or shrinking strain in the horizontal direction based on the inverse piezoelectric effect, and then a neutral layer is driven to drive the composite film to be integrally bent into a convex or concave shape. The flexural vibration compression of the membrane causes mechanical vibration of the acoustic medium (air, water, oil, elastic material, etc.) loaded above, generating longitudinal (or longitudinal and transverse) ultrasonic waves, known as the emission of ultrasonic waves.

When the pMUT provided by the present invention operates in a receiving mode, the main mechanism is that when the ultrasonic wave in the sound-guiding medium is conducted to the surface of the composite membrane of the pMUT unit, the whole composite membrane is driven to perform bending vibration, so that the piezoelectric layer therein generates tensile or contraction strain in the horizontal direction, and based on the piezoelectric effect, the piezoelectric layer responds to charge and voltage signals in the longitudinal direction and is received by a connected receiving circuit, which is called as receiving of the ultrasonic wave.

According to the pMUT provided by the invention, as the piezoelectric layer is subjected to the graphical etching of the specified shape and size, the rigidity of the boundary of the composite film is reduced, the integral vibration mode of the composite film is improved, and further, the larger receiving and transmitting sensitivity and bandwidth can be obtained; under the excitation of voltage signals with the same amplitude, the accumulated horizontal direction stretching and shrinking stress of the composite film is doubled compared with that of the pMUT unit with a single piezoelectric layer, namely, the emission sensitivity is doubled; the pMUT unit with the multiple piezoelectric layers distributed on two sides of the neutral layer has the advantages that the structure is symmetrical, residual stress of the composite film can be remarkably reduced, the position of a reference plane of bending moment in the structure is kept unchanged, and the driving efficiency of the film under the multiple piezoelectric layers can be improved.

Drawings

Fig. 1 is a schematic diagram of a first version of a pMUT cell with a dual layer piezoelectric layer.

Fig. 2 is a schematic diagram of a second version of a pMUT cell with a dual layer piezoelectric layer.

Fig. 3 is a schematic diagram of a first version of a pMUT cell having multiple piezoelectric layers.

Fig. 4 is a schematic diagram of a second version of a pMUT cell having multiple piezoelectric layers.

Fig. 5 is a schematic diagram of a second approach to implementing the transducer support structure.

Fig. 6 is a schematic diagram of a third scheme for implementing the transducer support structure.

Fig. 7 is a schematic diagram of an acoustic matching layer scheme for a pMUT cell with a dual piezoelectric layer.

Detailed Description

The invention will be described in more detail below with reference to the accompanying drawings. Like elements are represented by like numbers in the various figures. For purposes of clarity, the various features in the drawings are not necessarily drawn to scale. Moreover, some well-known elements may not be present.

Numerous specific details of the invention, such as device structures, material sizing processes, and techniques, are set forth in the following description in order to provide a more thorough understanding of the invention. However, as will be understood by those skilled in the art, the present invention may be practiced without these specific details.

Fig. 1 shows a schematic diagram of a first scheme of a pMUT cell of the present invention having a dual-layer piezoelectric layer.

As shown in fig. 1, a metal layer (M) -a piezoelectric layer (P) -a metal layer (M), that is, a sandwich piezoelectric layer structure of MPM, is respectively fabricated on two sides of a neutral layer 140 of a pMUT unit 100 based on a double-layer piezoelectric film. The MPM structure formed by the metal layer 110, the piezoelectric layer 120 and the metal layer 130 is substantially mirror-symmetrical to the MPM structure formed by the metal layer 111, the piezoelectric layer 121 and the metal layer 131, wherein the metal layer 131 may further include a thin silicon dioxide or silicon nitride layer added on the surface for bonding. The cavity structure 155 is formed on the substrate 150 by etching or additionally depositing a silicon-based material, and is generally circular or square, and is spatially complementary to the support structure 151; the support structure 151 is process controlled and may be made of the same material as the substrate, mainly silicon, or silicon dioxide, silicon nitride, etc.

In order to improve the vibration mode of the composite membrane of the transducer, the radial

piezoelectric layers

120 and 130 are etched to be round or square with a smaller size similar to the cavity structure 155, and when the radial

piezoelectric layers

120 and 130 are round, the ratio of the radius of the

piezoelectric layers

120 and 130 to the radius of the cavity structure 155 is 0.6-0.9; when the square shape is adopted, the ratio of the side length of the

piezoelectric layers

120 and 130 to the side length of the cavity structure 155 is 0.6-0.9; the transducer units in the figures all adopt the scheme to improve the vibration mode of the composite film and improve the overall sensitivity and bandwidth of the transducer.

In an embodiment, MPM piezoelectric layer structures on the upper side and the lower side of the neutral layer 140 respectively constitute a two-port device, wherein the polarization direction of the piezoelectric layer 120 is the same as or opposite to the polarization direction of the piezoelectric layer 121. When the polarization directions of

piezoelectric layers

120 and 121 are the same (both downward or upward), two effective electrical connections that can be implemented but are not limited to this are: the

metal layers

130 and 131 are grounded together, the

metal layers

110 and 111 are connected with the signal line together, and at the moment, the two MPM dual-port units are in a parallel connection state; the metal layer 111 is grounded, the

metal layers

130 and 131 are connected in a short circuit, and the metal layer 110 is connected to a signal line, so that the two MPM dual-port units are connected in series. When the polarization directions of

piezoelectric layers

120 and 121 are opposite, one effective electrical connection that can be implemented, but is not limited to, is: the metal layers 130 and 131 are grounded together, the metal layers 110 and 111 are connected to two ends of the differential signal, and the two MPM dual-port units form a three-port input unit of the differential input.

Fig. 2 shows a schematic diagram of a second version of a pMUT cell of the invention having a dual-layer piezoelectric layer.

As shown in fig. 2, the neutral layer 240 of the pMUT unit 200 based on the dual-layer piezoelectric film has a piezoelectric layer structure formed only on the upper side, and the metal layer 230, the piezoelectric layer 220, the metal layer 210, the piezoelectric layer 221, and the metal layer 231 form an mpmpmpm piezoelectric layer structure. Since the back side does not require the fabrication of a piezoelectric structure, the thin layer 245 is primarily a silicon dioxide or silicon nitride layer that bonds the neutral layer 240 to the substrate 250 with support structure, and the thin layer 245 may be distributed on the surface of the substrate with support structure in addition to being distributed on the bottom side of the neutral layer, at the same point that the support point and the neutral layer 240 may be in effective contact, so that the composite membrane may vibrate efficiently.

In an embodiment, the metal layer 230, the piezoelectric layer 220, and the metal layer 210 are an MPM dual port piezoelectric layer structure unit, and the MPM dual port piezoelectric layer structure unit, which is the metal layer 210, the piezoelectric layer 221, and the metal layer 231, are commonly connected through the metal layer 210, wherein the polarization directions of the

piezoelectric layers

220 and 221 are the same or opposite. When the polarization directions of

piezoelectric layers

120 and 121 are opposite, one effective electrical connection that can be implemented, but is not limited to, is: metal layers 230,231 are commonly grounded and metal layer 210 is connected to a signal line, with the two piezoelectric structures being combined together to form a two-port cell. When the polarization directions of

piezoelectric layers

120 and 121 are the same, one effective electrical connection that can be implemented, but is not limited to, is: the metal layer 210 is grounded, and the metal layers 230 and 231 are respectively connected to two ends of the differential signal, and at this time, the two piezoelectric structures are combined together to form a transducer unit with three-terminal differential input.

Fig. 3 shows a schematic diagram of a first version of a pMUT cell of the invention having multiple piezoelectric layers; obviously, mpmpmpm multilayer piezoelectric layer structures are respectively fabricated on the upper and lower sides of the neutral layer 340 of the pMUT cell 300 based on a multilayer piezoelectric film. The support structure substrate 350 and the cavity 355 are formed in the same manner as the base structure of the

support structures

150, 151. In general, the metal layer 330, the piezoelectric layer 320, the metal layer 310, the piezoelectric layer 321, and the metal layer 331 constitute an mpmpmpm three-port input multi-piezoelectric layer structure, which is mirror-symmetrical to the mpm three-port input multi-piezoelectric layer structure constituted by the metal layer 332, the piezoelectric layer 323, the metal layer 311, the piezoelectric layer 324, and the metal layer 333.

In an embodiment, the pMUT cell described in fig. 1 and 2 may already provide sufficient reference to drive the 300 cell. The piezoelectric layers in the pair of three-terminal input units in 300 may have different or the same polarization directions, and are coupled with a suitable electrical connection scheme, so that the positive (inverse) piezoelectric effect of the piezoelectric film enhances the sensitivity of the transducer unit 300 rather than canceling each other in the process of exciting or receiving the acoustic wave. In addition, the reasonable common ground and polarization direction arrangement can simplify the number of leads to two simple leads, thereby reducing the complexity of layout manufacture and subsequent work.

Fig. 4 shows a schematic diagram of a second version of a pMUT cell of the invention having multiple piezoelectric layers; the neutral layer 440 of the pMUT unit 400 based on multi-layer piezoelectric film is fabricated with a piezoelectric layer structure only on the upper side, but the number of piezoelectric layers is more than that of the

units

100 and 200, and the pmutm is a three-piezoelectric layer structure. Metal layer 430, piezoelectric layer 420, metal layer 410, piezoelectric layer 421, metal layer 431, piezoelectric layer 422, and metal layer 411 constitute a four-port input piezoelectric layer structure for MPMPMPM. Based on the evolution of the

cells

200, 400, transducer cells with more laminated electrical layer mpm (pm) structures can be prepared to further increase the sensitivity of the transducer. In addition, based on the evolution of the

units

200 and 300, the unit 400 can also fabricate more piezoelectric layer structures on the other side of the neutral layer 440 according to the evolution route, so as to balance the force neutral plane of the composite membrane and improve the sensitivity of the transducer unit. In the neutral layer 440, the substrate 450 with support structures and the cavities 455 are formed in the same way as the base structure of the

support structures

150, 151.

In an embodiment, the pMUT cell described in fig. 1 and 2 may already provide sufficient reference to drive the 400 cell. The

piezoelectric layers

420, 421 and 422 can have different polarization directions to match with the appropriate electrical connection method to effectively drive the vibration of the composite membrane. One effective configuration, which may be implemented but is not limited to, is that the

piezoelectric layers

420 and 422 are polarized in the same direction, and the metal layers 430 and 431 are commonly grounded, and the metal layers 410 and 411 are commonly connected to signal lines, so that the mpmpmpmpm triple piezoelectric layer structure of the cell 400 forms a dual port cell that can effectively improve the sensitivity of the transducer.

FIG. 5 is a schematic diagram of a second implementation of the transducer support structure of the present invention; previously fig. 1 described the transducer unit 100 in a way that a cavity is made, so that the complementary structure is called the support structure 151. As shown in fig. 5, the substrate 510 of the cell 500 is provided with a circular or square via pattern 515, which is formed by a deep etching process after the piezoelectric structure is fabricated. And a stopper layer 520 made of silicon oxide or silicon nitride is etched upward from the bottom surface of the substrate 510. In specific implementation, the through hole 515 may be left empty and not filled to directly transmit sound waves through load media such as air, water, oil, and the like, or may be filled with a polymer as a waveguide material to perform an acoustic impedance matching function between the composite film and the load media while conducting the sound waves, thereby improving the efficiency of sound emission and reception.

FIG. 6A shows a schematic diagram of a third implementation of the transducer support structure of the present invention; in order to co-package a large-scale pMUT array with a chip to reduce the influence of size and various parasitic effects and improve imaging quality, it is necessary to transfer a composite film that plays a core role onto a chip substrate 620. After the fabrication of the piezoelectric structure on one or both sides of the transducer neutral layer is completed, the composite film can be effectively bonded to the chip substrate 620 by fabricating suitable bonding micro-pillars 611 on the chip substrate 620, and finally the aforementioned support structure, cavity and silicon-based substrate are released. In this

cell

600a, 610 acts as a new support structure, 620 acts as a new substrate, and 625 acts as a cavity that is spatially complementary to 611, together with the composite membrane constituting an effective pMUT cell. As described above for the

cells

100, 200, 300, 400, 500, the driving method and the polarization method have been fully described.

Fig. 6B shows a transducer cell 600B designed to improve pMUT cell far-field directivity and acoustic emission efficiency in accordance with the present invention. For the transducer using the deep through hole method, after the bonding process of the composite film and the chip substrate 620 is completed, the original substrate, the supporting structure and the 630b formed by the through holes can be retained, and a suitable polymer is filled as a material of the guided wave and acoustic matching layer to effectively couple the acoustic wave into the load medium. For phased array applications and the like, excellent directivity is crucial for imaging, and imaging and detection quality can be improved by using the reserved structure 630b filling material.

Figure 7 shows a schematic diagram of another acoustic matching layer implementation of the present invention. After the transducer unit 710 is fabricated, an alternative way to protect the transducer is to deposit a few microns thin passivation layer over it to protect the unit from the high humidity environment. In the present invention, the unit 710 is further attached with a polymer material having a thickness of 10 to 50 μm as an acoustic matching layer, couples the bending vibration composite film with an acoustic medium (water, oil, etc.), and effectively absorbs and transmits ultrasonic waves through longitudinal and transverse wave resonance modes in the thickness direction inside the acoustic matching layer, thereby improving the sensitivity and bandwidth performance of the transducer.

Although the present invention and its advantages have been described in detail, it should be understood that the scope of the invention is not limited to the particular embodiments of the methods and steps described in the specification, and that various changes and modifications can be made by one skilled in the art without departing from the spirit and scope of the invention.

Claims

1. A piezoelectric micromechanical ultrasonic transducer with multiple piezoelectric layers is marked as pMUT, and is characterized in that a piezoelectric micromechanical ultrasonic transducer unit comprises a composite film, a support structure and a substrate which are sequentially arranged from top to bottom; wherein:

2. Piezoelectric micromachined ultrasonic transducer according to claim 1, wherein the composite membrane is based on a neutral layer, and metal layers and piezoelectric layers are deposited on the upper and lower surfaces thereof, in particular:

for a pMUT unit with a single piezoelectric layer, a bottom metal layer (M), a piezoelectric layer (P) and a top metal layer (M) are manufactured on the upper surface of a neutral layer, the structure is called an MPM sandwich piezoelectric layer structure, and routing and electrodes are led out in a proper mode;

3. Piezoelectric micromachined ultrasonic transducer according to claim 1 or 2, wherein the composite membrane is circular or square.

4. The piezoelectric micromachined ultrasonic transducer of claim 3, wherein the piezoelectric layer is etched to a specific shape that is an approximate, fixed ratio of circles or squares with respect to the composite membrane bottom surface free boundary circle or square pattern to improve the stiffness and vibration mode of the membrane, thereby increasing the sensitivity and bandwidth of the pMUT cell.

5. The piezoelectric micromachined ultrasonic transducer of claim 4, wherein when the piezoelectric layer is circular, the ratio of the radius of the piezoelectric layer to the radius of the circular free boundary of the lower surface of the composite membrane is 0.6-0.9; when the piezoelectric layer is square, the ratio of the side length of the piezoelectric layer to the side length of the square free boundary of the lower surface of the composite film is 0.6-0.9.

6. Piezoelectric micromachined ultrasonic transducer according to claim 5, wherein the piezoelectric layer MPM (and/or PM) · -structure is prepared on both sides of the neutral layer, resulting from: firstly, a single-layer or multi-layer piezoelectric layer structure with patterns and lead wires is manufactured on the upper side of a neutral layer through multiple deposition and photoetching steps; then, turning over the unfinished composite film and bonding the unfinished composite film with another substrate with a manufactured support structure in a film transferring mode; subsequently preparing a single-layer or multilayer piezoelectric layer MPM (and/or PM) · structure on the other side;

the supporting structure and the substrate form bonding, which is realized by turning the film and bonding the film and the bonding column, so that the film is transferred to the specified substrate; after the film is transferred, the original substrate is reserved, through holes are manufactured in deep etching and other modes, the transducer is enabled to transmit sound waves outwards through the through holes, and therefore vibration modes and far field directivity indexes of the pMUT unit are improved.

7. The piezoelectric micromachined ultrasonic transducer of claim 1, 2, 4, 5 or 6, wherein a layer of polymer material with a thickness of 10-50 μm is further added on the upper surface of the pMUT unit basic structure for acoustic matching between the composite membrane structure for bending vibration and the acoustic field medium, so as to improve acoustic emission efficiency; for the pMUT unit with the substrate etched with the through hole, the polymer is filled in the through hole to be used as a waveguide material, and then the acoustic matching purpose is achieved, and meanwhile, the directivity index of a far field of a sound field is improved.

8. A piezoelectric micromachined ultrasonic transducer according to claim 1, 2, 4, 5, or 6, wherein the substrate is comprised of an integral part of a silicon wafer, a bulk silicon part of a silicon-on-insulator cell, or a chip substrate bonded after thin film transfer;

the neutral layer of the composite film is a mechanical vibration layer without piezoelectric property, and the material is silicon, silicon oxide or silicon nitride.

9. Piezoelectric micromachined ultrasonic transducer according to claim 1, 2, 4, 5 or 6, wherein the support structure, corresponding to the composite membrane, is substantially shaped as a thin layer, which is hollow, circular or square; when the support structure is used for the interconnection bonding of the thin film and the chip, the support structure is a plurality of micro-columns for bonding;

according to the substrate and the supporting structure, when the preparation method is that through holes are formed through deep etching on the back surface of the substrate to form the free boundary of the lower surface of the composite film, the substrate and the supporting structure are the same part and are not distinguished obviously.

10. A piezoelectric micromachined ultrasonic transducer according to claim 1, 2, 4, 5, or 6, wherein the upper and lower surfaces of the piezoelectric layer are in contact with a metal layer, one of which is a ground electrode and the other of which is a signal electrode;

in the composite film: when a plurality of piezoelectric layers are provided, the piezoelectric layers and the metal layers in contact with the piezoelectric layers are distributed on one side or both sides of the neutral layer, and the polarization direction of the piezoelectric material is configured to be upward or downward; when multiple piezoelectric layers are provided, each piezoelectric layer and the metal layer contacted with the piezoelectric layer are combined into a dual-port unit, and at least two electrodes are finally led out from the units in a parallel or serial mode to be used as input of excitation and grounding signals.