Piezoelectric micromechanical ultrasonic transducer and piezoelectric micromechanical ultrasonic transducer array

Abstract

The application discloses a piezoelectric micromachined ultrasonic transducer and a piezoelectric micromachined ultrasonic transducer array. The piezoelectric micromachined ultrasonic transducer comprises a substrate structure, a multilayer thin film resonance structure and an acoustic barrier structure, the substrate structure has a resonance cavity, the multilayer thin film resonance structure and the acoustic barrier structure are arranged on the substrate structure in a spaced manner, the multilayer thin film resonance structure is arranged above the resonance cavity along the z-axis direction of a three-dimensional coordinate system and completely covers the resonance cavity, and the acoustic barrier structure comprises a plurality of phononic crystal units, the plurality of phononic crystal units are arranged around the multilayer thin film resonance structure in a square structure. The application combines the phononic crystal unit and the PMUT, the phononic crystal unit has an anechoic effect on sound waves of a certain frequency, can block the sound signal in the acoustic barrier structure formed by the phononic crystal unit, converts the sound signal into internal energy through friction heat, reduces the transmittance of the transverse ultrasonic signal, and thus weakens the crosstalk between two PMUT units.

Description

TECHNICAL FIELD

[0001] The present application relates to a piezoelectric micromachined ultrasonic transducer and a piezoelectric micromachined ultrasonic transducer array, and belongs to the technical field of micro-electro-mechanical systems. BACKGROUND

[0002] For a PMUT (Piezoelectric Micromachined Ultrasonic Transducer) array with a transmitting function, due to process reasons, the resonant frequency of each PMUT unit cannot be completely consistent, and for a PMUT array device that needs to control the phase of transmitted ultrasonic waves to achieve more functions, it is necessary to reduce signal crosstalk between PMUT units by isolation. To solve this problem, two methods of designing isolation grooves and isolation columns are currently used.

[0003] The structure of a PMUT device with an isolation groove in the prior art is shown in Figure 1 The PMUT device includes, 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 includes an upper piezoelectric layer, a lower piezoelectric layer, an upper electrode layer, a middle electrode layer, and a lower electrode layer. The multilayer composite film has a concentric annular groove and covers the entire cavity area. This PMUT device effectively reduces crosstalk between PMUT units, reduces film stress, and significantly increases the piezoelectric driving bending moment in the film, thereby improving ultrasonic transmission and reception performance to meet the urgent need for low-power, high-performance ultrasonic transducers for small pipe flow detection applications. However, using isolation grooves or isolation columns to reduce crosstalk between device units, ultrasonic signals will be reflected multiple times between units or isolation columns, reducing signal propagation to adjacent units to weaken the impact of crosstalk. However, at the microscale, the transmittance of ultrasonic waves through the isolation groove or isolation column is still high, and the desired effect has not been achieved. SUMMARY

[0004] The main purpose of the present application is to provide a piezoelectric micromachined ultrasonic transducer and a piezoelectric micromachined ultrasonic transducer array to overcome the shortcomings of the prior art.

[0005] To achieve the above-mentioned application purposes, the technical solutions adopted by the present application include:

[0006] The first aspect of the present invention provides a piezoelectric micromechanical ultrasonic transducer, comprising: a substrate structure, a multilayer thin-film resonant structure, and a sound barrier structure. The substrate structure has a resonant cavity. The multilayer thin-film resonant structure and the sound barrier structure are spaced apart on the substrate structure. The multilayer thin-film resonant structure is positioned above the resonant cavity along the z-axis of a three-dimensional coordinate system and completely covers the resonant cavity. The sound barrier structure includes a plurality of phonon crystal units. The plurality of phonon crystal units are arranged in a square structure around the multilayer thin-film resonant structure. The linear distance between the central axes of two adjacent phonon crystal units is equal to the lattice size a of the phonon crystal unit. The multilayer thin-film resonant structure is located in the central region of the square structure.

[0007] A second aspect of the present invention provides a piezoelectric micromechanical ultrasonic transducer array, comprising a plurality of piezoelectric micromechanical ultrasonic transducer units arranged in an array in the xy plane of the three-dimensional coordinate system, wherein each piezoelectric micromechanical ultrasonic transducer unit comprises a piezoelectric micromechanical ultrasonic transducer, and the first substrate of the plurality of piezoelectric micromechanical ultrasonic transducers is integral.

[0008] Compared with the prior art, the advantages of the present invention include: the present invention combines a phononic crystal unit and a PMUT. The phononic crystal unit has a noise reduction effect on sound waves of a certain frequency and can block the sound signal within the sound barrier structure formed by the phononic crystal unit. It converts the heat generated by friction into internal energy, reduces the transmittance of transverse ultrasonic signals, and thus weakens the crosstalk between the two PMUT units. Attached Figure Description

[0009] Figure 1 This is a schematic diagram of the structure of a piezoelectric micromechanical ultrasonic transducer in the prior art;

[0010] Figure 2 This is a schematic diagram of the structure of a piezoelectric micromechanical ultrasonic transducer provided in a typical embodiment of the present invention;

[0011] Figure 3 This is a schematic diagram of the structure of a piezoelectric micromechanical ultrasonic transducer provided in a typical embodiment of the present invention;

[0012] Figure 4 This is a cross-sectional structural schematic diagram of a piezoelectric micromechanical ultrasonic transducer provided in a typical embodiment of the present invention;

[0013] Figure 5 This is a cross-sectional structural schematic diagram of a piezoelectric micromechanical ultrasonic transducer provided in a typical embodiment of the present invention;

[0014] Figure 6 , Figure 7 , Figure 8These are all schematic diagrams of the orthographic projection of a phonon crystal unit in a piezoelectric micromechanical ultrasonic transducer in the xy plane, provided in a typical embodiment of the present invention.

[0015] Figure 9 This is a schematic diagram of a piezoelectric micromechanical ultrasonic transducer array with a 2*2 cycle provided in a typical embodiment of the present invention;

[0016] Figure 10 This is a typical embodiment of the present invention, which provides a phonon crystal unit band diagram in a piezoelectric micromechanical ultrasonic transducer.

[0017] Figure 11 This is a schematic diagram of the first Brillouin zone of the phononic crystal unit in this invention;

[0018] Figure 12 The sound pressure level transmission loss (1, 0) of a piezoelectric micromechanical ultrasonic transducer with two periodic phonon crystal units is provided in a typical embodiment of the present invention.

[0019] Figure 13 The sound pressure level transmission loss (1, 1) of a piezoelectric micromechanical ultrasonic transducer with two periodic phonon crystal units is provided in a typical embodiment of the present invention.

[0020] Figure 14 This is a typical embodiment of the present invention, which provides a cross-sectional sound pressure level distribution of a piezoelectric micromechanical ultrasonic transducer with a diameter of Z=40μm. Detailed Implementation

[0021] In view of the shortcomings of the prior art, the inventors of this invention, through long-term research and extensive practice, have proposed the technical solution of this invention. The following will further explain and illustrate this technical solution, its implementation process, and its principles.

[0022] The first aspect of the present invention provides a piezoelectric micromechanical ultrasonic transducer, comprising: a substrate structure, a multilayer thin-film resonant structure, and a sound barrier structure. The substrate structure has a resonant cavity. The multilayer thin-film resonant structure and the sound barrier structure are spaced apart on the substrate structure. The multilayer thin-film resonant structure is positioned above the resonant cavity along the z-axis of a three-dimensional coordinate system and completely covers the resonant cavity. The sound barrier structure includes a plurality of phonon crystal units. The plurality of phonon crystal units are arranged in a square structure around the multilayer thin-film resonant structure. The linear distance between the central axes of two adjacent phonon crystal units is equal to the lattice size a of the phonon crystal unit. The multilayer thin-film resonant structure is located in the central region of the square structure.

[0023] Furthermore, the multiple phonon crystal units contained in the sound barrier structure are configured as square lattices and surround the multilayer thin-film resonant structure with the multilayer thin-film resonant structure as the geometric center.

[0024] Furthermore, the phonon crystal unit has a mirror-symmetric structure, the axis of the phonon crystal unit is parallel to the z-axis of the three-dimensional coordinate system, and the orthographic projection pattern formed by the phonon crystal unit along its own axis has two perpendicular axes of symmetry. These two axes of symmetry are parallel to the two perpendicular sides of the square structure, and are also parallel to the x-axis and y-axis of the three-dimensional coordinate system.

[0025] Furthermore, the phonon crystal unit is a columnar structure with multiple approximately Helmholtz resonant cavities.

[0026] Furthermore, the phonon crystal unit is a columnar structure with four approximately Helmholtz resonant cavities.

[0027] Furthermore, the phonon crystal unit has at least one complete bandgap, and the complete bandgap covers the optimal operating frequency of the piezoelectric micromechanical ultrasonic transducer, which is located at a position where the sound pressure level loss is greater than 40dB.

[0028] Furthermore, along the axial direction of the phonon crystal unit, the top surface of the phonon crystal unit is positioned higher than the top surface of the multilayer thin-film resonant structure.

[0029] Furthermore, along the axial direction of the phonon crystal unit, the top surface of the phonon crystal unit is located higher than the highest point of deformation of the multilayer thin-film resonant structure during operation.

[0030] Furthermore, in the axial direction of the phonon crystal unit, the vertical distance between the top surface of the phonon crystal unit and the top surface of the multilayer thin film resonant structure is greater than 10 μm and less than the radius of the resonant cavity. This not only maintains the sound insulation performance of the sound barrier structure, but also reduces the impact on the PMUT radiation angle.

[0031] In a more specific embodiment, the substrate structure includes a first substrate and a second substrate. The second substrate is disposed on the first substrate along the z-axis direction of the three-dimensional coordinate system. The resonant cavity penetrates the first substrate and the second substrate along a selected direction. The multilayer thin film resonant structure is disposed on the second substrate. The sound barrier structure is disposed on the first substrate. The bottom end face of the phonon crystal unit is in direct contact with the first substrate. The sound barrier structure is not in direct contact with the second substrate or the multilayer thin film resonant structure.

[0032] Furthermore, the multilayer thin-film resonant structure is positioned directly above the resonant cavity along the z-axis direction of the three-dimensional coordinate system.

[0033] Furthermore, the resonant cavity has a cylindrical structure.

[0034] Furthermore, the multilayer thin-film resonant structure is coaxially arranged with the resonant cavity.

[0035] Furthermore, the multilayer thin-film resonant structure, the second substrate, and the resonant cavity are coaxially arranged.

[0036] Furthermore, the thickness of the second substrate is greater than 10 μm to improve the shielding effect of the phononic crystal unit and block the propagation of crosstalk in the form of surface acoustic waves.

[0037] Furthermore, the multilayer thin-film resonant structure includes an elastic layer, a bottom electrode, a piezoelectric layer, and a top electrode sequentially stacked on the substrate structure.

[0038] A second aspect of the present invention provides a piezoelectric micromechanical ultrasonic transducer array, comprising a plurality of piezoelectric micromechanical ultrasonic transducer units arranged in an array in the xy plane of the three-dimensional coordinate system, wherein each piezoelectric micromechanical ultrasonic transducer unit comprises a piezoelectric micromechanical ultrasonic transducer, and the first substrate of the plurality of piezoelectric micromechanical ultrasonic transducers is integral.

[0039] Furthermore, the piezoelectric micromechanical ultrasonic transducer array has a mirror-symmetric structure. The orthographic projection of the piezoelectric micromechanical ultrasonic transducer array in the xy plane of the three-dimensional coordinate system has two perpendicular axes of symmetry, which are parallel to the x-axis and y-axis of the three-dimensional coordinate system, respectively.

[0040] The following will provide a further explanation of the technical solution, its implementation process, and its principles, in conjunction with the accompanying drawings and specific implementation examples.

[0041] Please see Figure 2 and Figure 3 A piezoelectric micromechanical ultrasonic transducer includes a substrate structure, a multilayer thin-film resonant structure, and a sound barrier structure. The substrate structure has a resonant cavity that runs through the substrate structure along the z-axis of a three-dimensional coordinate system. The multilayer thin-film resonant structure and the sound barrier structure are disposed on the substrate structure along the z-axis of the three-dimensional coordinate system. The z-axis of the multilayer thin-film resonant structure is positioned directly above the resonant cavity and completely covers the resonant cavity. The sound barrier structure includes multiple phonon crystal units. The multiple phonon crystal units are arranged in a square structure around the multilayer thin-film resonant structure with the multilayer thin-film resonant structure as the geometric center. The phonon crystal units have no direct contact with the multilayer thin-film resonant structure. The straight-line distance between the central axes of two adjacent phonon crystal units is equal to the lattice size a of the phonon crystal unit.

[0042] Understandably, the surface of the substrate structure is parallel to the xy plane of the three-dimensional coordinate system. Multiple phonon crystal units are arranged sequentially and spaced apart around the multilayer thin film resonant structure along the x and y axes, maintaining the square lattice characteristics of the phonon crystal units, thus forming a sound barrier structure with a square outline. Furthermore, there is a gap between the phonon crystal units and the multilayer thin film resonant structure in the xy plane, that is, the phonon crystal units and the multilayer thin film resonant structure are in a non-contact form to prevent surface acoustic waves from being transmitted through the phonon crystal units.

[0043] Please see Figure 4 and Figure 5 The substrate structure includes a first substrate and a second substrate arranged sequentially from bottom to top along the z-axis. The resonant cavity passes through the first substrate and the second substrate along the z-axis. The multilayer thin film resonant structure is disposed on the second substrate, and the sound barrier structure is disposed on the first substrate. In the z-axis direction, the bottom end face of the phonon crystal unit is in direct contact with the first substrate, and the top end face of the phonon crystal unit is higher than the top end face of the multilayer thin film resonant structure. Furthermore, the top end face of the phonon crystal unit is higher than the highest point of deformation of the multilayer thin film resonant structure during operation. In the xy plane direction, the phonon crystal unit has no direct contact with the second substrate or the multilayer thin film resonant structure.

[0044] Specifically, the first substrate serves as the main supporting structure of the entire structure, and the area of ​​the second substrate is smaller than that of the first substrate (this area is the projected area of ​​both in the xy plane). The circumferential sides of the first and second substrates form a stepped structure, which can disrupt the planar propagation structure of surface acoustic waves and reduce crosstalk in the form of surface acoustic waves. Specifically, the thickness of the second substrate (the thickness in this invention refers to the dimension along the z-axis) needs to be ≥10μm, and in the z-axis direction, the height difference between the top surface of the phononic crystal unit and the top surface of the multilayer thin film resonant structure is greater than 10μm and less than the radius of the resonant cavity.

[0045] As a preferred embodiment, the first substrate has a square structure, with its sides parallel to the x-axis and y-axis, respectively. The second substrate and the multilayer thin-film resonant structure are both circular structures, and the resonant cavity is cylindrical. Furthermore, the resonant cavity, the second substrate, and the multilayer thin-film resonant structure are coaxially arranged. More specifically, the geometric centers of the orthographic projections of the resonant cavity, the second substrate, the multilayer thin-film resonant structure, the sound barrier structure, and the first substrate in the xy-plane coincide to maintain the symmetry of the sound barrier structure in terms of sound shielding effect, for example, maintaining the same degree of attenuation in the +x-axis direction and the -x-axis direction.

[0046] Please refer to further information. Figure 4The multilayer thin-film resonant structure comprises four layers stacked sequentially from bottom to top along the z-axis on a second substrate: an elastic layer, a bottom electrode, a piezoelectric layer, and a top electrode. It should be noted that the materials and other parameters of the elastic layer, bottom electrode, piezoelectric layer, and top electrode are known to those skilled in the art and can be selected according to specific needs; no specific limitations are made here. For example, the elastic layer can be made of silicon, the bottom and top electrodes can be made of metals, and the piezoelectric layer can be made of Al. 0.8 Sc 0.2 N and other piezoelectric materials.

[0047] Please refer to the following for details. Figure 6 , Figure 7 and Figure 8 The phonon crystal unit is a columnar structure with multiple approximate Helmholtz resonant cavities, for example, a columnar structure with four approximate Helmholtz resonant cavities. The phonon crystal unit is a mirror-symmetric structure, and the axis of the phonon crystal unit is parallel to the z-axis of the three-dimensional coordinate system. The orthographic projection of the phonon crystal unit in the xy plane has two perpendicular axes of symmetry, which are parallel to the x-axis and y-axis of the three-dimensional coordinate system, respectively. These two axes of symmetry are parallel to the two perpendicular sides of the square sound barrier structure. It can be understood that the overall outline of the sound barrier structure is square, and the sides of the square are parallel to the x-axis and y-axis.

[0048] Specifically, the phonon crystal unit has at least one complete bandgap, and the complete bandgap covers the optimal operating frequency of the piezoelectric micromechanical ultrasonic transducer, which is located at a position where the sound pressure level loss is greater than 40dB.

[0049] Specifically, PMUT crosstalk only occurs in array-type devices. The piezoelectric micromechanical ultrasonic transducer array in this invention includes at least two piezoelectric micromechanical ultrasonic transducer units, which are arrayed in the xy plane of the three-dimensional coordinate system. The piezoelectric micromechanical ultrasonic transducer units include the aforementioned piezoelectric micromechanical ultrasonic transducers.

[0050] Please see Figure 9 The piezoelectric micromechanical ultrasonic transducer array of the present invention comprises multiple piezoelectric micromechanical ultrasonic transducer units arranged in a square pattern. That is, the multiple piezoelectric micromechanical ultrasonic transducer units are arranged sequentially along the x-axis and y-axis. Furthermore, the orthographic projection pattern of the piezoelectric micromechanical ultrasonic transducer array in the xy-plane is a mirror-symmetric structure, and its orthographic projection pattern also has two perpendicular axes of symmetry. These two axes of symmetry are parallel to the x-axis and y-axis, respectively. In other words, the two axes of symmetry of the orthographic projection pattern of the piezoelectric micromechanical ultrasonic transducer array correspond to and are parallel to the two axes of symmetry of the orthographic projection pattern of the phonon crystal unit.

[0051] It should be noted that the number of periods of the phonon crystal units arranged around each piezoelectric micromechanical ultrasonic transducer unit is ≥1, that is, the number of periods of the phonon crystal units between two piezoelectric micromechanical ultrasonic transducer units is ≥2. In the sound barrier structure formed by the phonon crystal units, the two nearest phonon crystal units need to be distributed on the same axis (i.e., the x-axis or y-axis of the three-dimensional coordinate system), and the distance between the central axes needs to be equal to the lattice size 'a' of the phonon crystal unit. Furthermore, the first substrate of the multiple piezoelectric micromechanical ultrasonic transducer units included in this piezoelectric micromechanical ultrasonic transducer array can be integral.

[0052] The working process of the piezoelectric micromechanical ultrasonic transducer array of the present invention specifically includes:

[0053] An AC voltage drive signal with the same resonant frequency as the PMUT (Piezoelectric Micromechanical Ultrasonic Transducer) is applied to the top electrode, and the bottom electrode is grounded. The piezoelectric layer deforms and vibrates, thereby generating an ultrasonic signal of that frequency. The ultrasonic signal can be divided into a beneficial signal in the axial (z-axis direction) direction of the PMUT unit and a transverse interference signal. The transverse interference signal propagates laterally to the phonon crystal unit. (Sound insulation principle) When the sound wave propagates in the porous sound-absorbing material or sound-absorbing structure, the resonant friction of the phonon crystal unit converts the sound energy into internal energy, thereby greatly weakening the crosstalk signal. Overall, it exhibits a low ultrasonic transmittance, making it difficult for it to propagate to other PMUT units around this PMUT unit.

[0054] Example 1

[0055] One PC-PMUT structure in this embodiment is as follows: Figures 2-8 As shown, in this embodiment, the PMUT array unit design using phonon crystal units for isolation employs Al as the piezoelectric layer. 0.8 Sc 0.2 The piezoelectric material is N, the top and bottom electrodes are made of molybdenum, the elastic layer and substrate structure (first substrate and second substrate) are made of silicon, and the four-cavity phonon crystal unit is made of silicon. In this embodiment, the piezoelectric layer has a circular structure, and the phonon crystal units are arranged in a square around the multilayer thin-film resonant structure. The optimal operating frequency of its PC-PMUT is 1.806MHz. Specific related dimensional parameters are shown in Table 1.

[0056] Table 1. Relevant parameters of PC-PMUT in Example 1

[0057]

[0058] The central axis in Table 1 is the central axis of the resonant cavity, the multilayer resonant film, and the PMUT unit.

[0059] In this embodiment, the phonon crystal unit adopts a structure with four Helmholtz resonant cavities (e.g., Figure 6 (As shown) and has highly symmetrical features, its relevant dimensions are shown in Table 1, such as Figure 10 , Figure 11 As shown, the phononic crystal unit has multiple complete band gaps, with the third band gap having the largest width (1.2MHz-2.7MHz), covering the optimal operating frequency of the PMUT device at 1.802MHz. Figure 12 and Figure 13 The frequency-sound pressure level loss relationship of a PMUT device with two periodic phononic crystal units is shown respectively. Figure 12 To account for the transmission loss of ultrasonic waves in the (1,0) direction, Figure 13 The transmission loss of the ultrasonic wave in the (1,1) direction is given. At a frequency of 1.806 MHz, the transmission loss of the sound pressure is greater than 40 dB (equivalent to 99% transmission loss on the sound power, or less than 1% sound transmittance).

[0060] Figure 14 The image shows the sound pressure level distribution of the PC-PMUT in the xy plane at a height of 40 μm above the top plane of the first substrate in this embodiment. The sound pressure level at the center is the highest at 164 dB. After passing through the sound barrier formed by one cycle of phononic crystal units, the sound pressure level is reduced to a minimum of 102 dB, which is 62 dB away from the highest point. This shows an ultra-low sound wave transmittance (less than 0.1%) and a sound focusing effect can be seen in the Helmholtz resonant cavity of the phononic crystal unit, indicating that the sound wave is confined in the resonant cavity and converted into internal energy.

[0061] This invention combines a phononic crystal unit and a PMUT. The phononic crystal unit has a noise reduction effect on sound waves of a certain frequency and can block the sound signal within the sound barrier structure formed by the phononic crystal unit. It converts the sound signal into internal energy through frictional heating, thereby reducing the transverse ultrasonic signal transmittance and weakening the crosstalk between the two PMUT units. Compared with the existing solutions, it has a more obvious advantage in weakening effect.

[0062] It should be understood that the above embodiments are merely illustrative of the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A piezoelectric micromechanical ultrasonic transducer, characterized in that, include: The system comprises a substrate structure, a multilayer thin-film resonant structure, and a sound barrier structure. The substrate structure contains a resonant cavity. The multilayer thin-film resonant structure and the sound barrier structure are spaced apart on the substrate structure. The multilayer thin-film resonant structure is positioned above the resonant cavity along the z-axis of a three-dimensional coordinate system and completely covers the resonant cavity. The sound barrier structure includes multiple phonon crystal units, each of which is a columnar structure with four Helmholtz resonant cavities. The multiple phonon crystal units are arranged in a square structure around the multilayer thin-film resonant structure. The linear distance between the central axes of two adjacent phonon crystal units is equal to the lattice size 'a' of the phonon crystal unit. The multilayer thin-film resonant structure is located in the central region of the square structure.

2. The piezoelectric micromechanical ultrasonic transducer according to claim 1, characterized in that: The phonon crystal unit has a mirror-symmetric structure. The axis of the phonon crystal unit is parallel to the z-axis of the three-dimensional coordinate system. The orthographic projection pattern formed by the phonon crystal unit along its own axis has two perpendicular axes of symmetry. These two axes of symmetry are parallel to the two perpendicular sides of the square structure, and are also parallel to the x-axis and y-axis of the three-dimensional coordinate system.

3. The piezoelectric micromechanical ultrasonic transducer according to claim 1 or 2, characterized in that: The phonon crystal unit has at least one complete bandgap, and the complete bandgap covers the optimal operating frequency of the piezoelectric micromechanical ultrasonic transducer, which is located at a position where the sound pressure level loss is greater than 40 dB.

4. The piezoelectric micromechanical ultrasonic transducer according to claim 2, characterized in that: Along the axial direction of the phonon crystal unit, the top surface of the phonon crystal unit is positioned higher than the top surface of the multilayer thin-film resonant structure.

5. The piezoelectric micromechanical ultrasonic transducer according to claim 2, characterized in that: Along the axial direction of the phonon crystal unit, the top surface of the phonon crystal unit is located higher than the highest point of deformation of the multilayer thin-film resonant structure during operation.

6. The piezoelectric micromechanical ultrasonic transducer according to claim 4 or 5, characterized in that: In the axial direction of the phonon crystal unit, the vertical distance between the top surface of the phonon crystal unit and the top surface of the multilayer thin film resonant structure is greater than 10 μm and less than the radius of the resonant cavity.

7. The piezoelectric micromechanical ultrasonic transducer according to claim 1 or 2, characterized in that: The substrate structure includes a first substrate and a second substrate. The second substrate is disposed on the first substrate along the z-axis of the three-dimensional coordinate system. The resonant cavity penetrates the first substrate and the second substrate along a selected direction. The multilayer thin film resonant structure is disposed on the second substrate. The sound barrier structure is disposed on the first substrate. The bottom end face of the phonon crystal unit is in direct contact with the first substrate. The sound barrier structure is not in direct contact with the second substrate or the multilayer thin film resonant structure.

8. The piezoelectric micromechanical ultrasonic transducer according to claim 7, characterized in that: The multilayer thin-film resonant structure is positioned directly above the resonant cavity along the z-axis of the three-dimensional coordinate system.

9. The piezoelectric micromechanical ultrasonic transducer according to claim 8, characterized in that: The resonant cavity has a cylindrical structure.

10. The piezoelectric micromechanical ultrasonic transducer according to claim 8 or 9, characterized in that: The multilayer thin-film resonant structure is coaxially arranged with the resonant cavity.

11. The piezoelectric micromechanical ultrasonic transducer according to claim 10, characterized in that: The multilayer thin-film resonant structure, the second substrate, and the resonant cavity are arranged coaxially.

12. The piezoelectric micromechanical ultrasonic transducer according to claim 7, characterized in that: The thickness of the second substrate is greater than 10 μm.

13. The piezoelectric micromechanical ultrasonic transducer according to claim 1, characterized in that: The multilayer thin-film resonant structure includes an elastic layer, a bottom electrode, a piezoelectric layer, and a top electrode, which are sequentially stacked on the substrate structure.

14. A piezoelectric micromechanical ultrasonic transducer array, comprising a plurality of piezoelectric micromechanical ultrasonic transducer units arranged in an array in the xy plane of the three-dimensional coordinate system, characterized in that: The piezoelectric micromechanical ultrasonic transducer unit includes the piezoelectric micromechanical ultrasonic transducer according to any one of claims 1-13, wherein the first substrate of the plurality of piezoelectric micromechanical ultrasonic transducers is integral.

15. The piezoelectric micromechanical ultrasonic transducer array according to claim 14, characterized in that: The piezoelectric micromechanical ultrasonic transducer array has a mirror-symmetric structure. The orthographic projection of the piezoelectric micromechanical ultrasonic transducer array in the xy plane of the three-dimensional coordinate system has two perpendicular axes of symmetry, which are parallel to the x-axis and y-axis of the three-dimensional coordinate system, respectively.

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