Large-bandwidth piezoelectric micromechanical ultrasonic transducer array and preparation method

Abstract

In conclusion, the embodiment of the invention provides a large-bandwidth piezoelectric micro-mechanical ultrasonic transducer array and a preparation method, and the large-bandwidth piezoelectric micro-mechanical ultrasonic transducer array comprises a substrate, a plurality of piezoelectric micro-mechanical ultrasonic transducers, a plurality of piezoelectric micro-mechanical ultrasonic transducers, a plurality of piezoelectric micro-mechanical ultrasonic transducers and a plurality of piezoelectric micro-mechanical ultrasonic transducers, the plurality of piezoelectric resonance units are positioned on the substrate; the plurality of piezoelectric resonance units form an array structure; each channel in the array structure comprises a plurality of vibrating diaphragm elements, the piezoelectric resonators in the array structure are divided into at least two groups, and the piezoelectric resonators in different groups have different structure parameters; and the piezoelectric layer of the array structure is polarized by a direct-current electric field with preset polarity. Bandwidth expansion is realized through frequency response superposition of different resonators, and the limitation that a traditional single-frequency PMUT is small in bandwidth and a multi-frequency PMUT easily sacrifices sensitivity is broken through. And meanwhile, the piezoelectric layer is subjected to direct-current electric field polarization treatment with preset polarity, so that the internal characteristics of the piezoelectric layer are optimized to improve the piezoelectric response.

Description

Technical Field

[0001] This invention relates to the field of microelectromechanical systems (MEMS) technology, specifically to a large-bandwidth piezoelectric micromechanical ultrasonic transducer array and its fabrication method. Background Technology

[0002] High-frequency piezoelectric micromechanical ultrasonic transducers (PMUTs), as key components in the field of microelectromechanical systems (MEMS), play an irreplaceable role in medical imaging, industrial sensing, and other fields. Currently, ultrasonic transducer-related technologies mainly fall into three categories: first, traditional bulk piezoelectric devices (such as lead zirconate titanate (PZT)-based bulk transducers), which are characterized by robust structures and stable sound pressure output; second, capacitive micromechanical ultrasonic transducers (CMUTs), which can achieve a wide operating bandwidth using MEMS processes; and third, piezoelectric micromechanical ultrasonic transducers (PMUTs), which integrate miniaturized design, good acoustic impedance matching with biological tissues / liquid media, and compatibility with CMOS processes, making them a research hotspot in recent years.

[0003] The existing technology development for PMUTs covers both single-frequency structure design and multi-frequency array design. Multi-frequency array design further attempts to extend the operating bandwidth through mutual acoustic coupling and gradual geometry (such as changes in resonator radius). Some solutions also employ positive polarization or non-polarization treatment on the core PZT piezoelectric film to optimize performance. However, existing technologies still have many limitations that urgently need to be addressed, making it difficult to meet the application requirements of high frequency, wide bandwidth, and high sensitivity. Summary of the Invention

[0004] The main objective of this invention is to provide a high-bandwidth piezoelectric micromechanical ultrasonic transducer array and its fabrication method, addressing the problems of existing PMUTs (Particle-to-Ultra-Ultrasound Transducers) such as difficulty in balancing bandwidth and sensitivity, and limited piezoelectric response performance. Specifically, by dividing the piezoelectric resonators in the array structure into at least two groups with different structural parameters, bandwidth expansion can be achieved by superimposing the frequency responses of different resonators, overcoming the limitation of small bandwidth in traditional single-frequency PMUTs. Simultaneously, by performing DC electric field polarization treatment on the piezoelectric layer with a preset polarity, the internal characteristics of the piezoelectric layer are optimized to improve the piezoelectric response, overcoming the limitation of traditional multi-frequency PMUTs that easily sacrifice sensitivity. Combined with the basic structural design of the substrate and piezoelectric resonator units, a high-frequency PMUT array with both high bandwidth and high sensitivity is finally realized, meeting the needs of high-resolution detection in fields such as medical imaging and industrial sensing, while ensuring that the device has advantages in miniaturization, structural stability, and process compatibility.

[0005] To achieve the above objectives, the embodiments of this application provide the following technical solutions: According to a first aspect of the embodiments of this application, a large-bandwidth piezoelectric micromechanical ultrasonic transducer array is provided, comprising: Substrate; and a plurality of piezoelectric resonant units located on the substrate; The plurality of piezoelectric resonant units constitute an array structure; each channel in the array structure includes a number of diaphragm elements, the piezoelectric resonators in the array structure are divided into at least two groups, and the piezoelectric resonators in different groups have different structural parameters; the piezoelectric layer of the array structure is polarized by a DC electric field of preset polarity.

[0006] Optionally, the array structure is divided into an outer region and a central region; the piezoelectric resonator in the outer region adopts a gradually changing structural parameter design, while the piezoelectric resonator in the central region adopts a fixed structural parameter design.

[0007] Optionally, the structural parameters include the planar dimensions of the piezoelectric resonators; the piezoelectric resonators with different planar dimensions are spatially distributed in the array structure, the piezoelectric resonators in the outer region of the array structure have a gradient change in size, and the piezoelectric resonators in the central region of the array structure have a consistent size.

[0008] Optionally, the piezoelectric resonator is a circular diaphragm, the structural parameter of which is the radius, and each channel of the array structure includes multiple circular diaphragms with different radii, the radius of which is in the range of 18μm to 22μm.

[0009] Optionally, the piezoelectric resonator includes a piezoelectric layer, which is made of a thin film material with ferroelectric properties; the piezoelectric layer is a lead zirconate titanate (PZT) thin film or a sodium potassium niobate (KNN) thin film.

[0010] Optionally, the preset polarity is negative, and the corresponding DC electric field polarization process is negative polarization; the negative polarization process is to apply a reverse DC voltage to the piezoelectric layer and maintain it for a set duration; the negative polarization process is to apply a DC voltage of a set magnitude to the piezoelectric layer and maintain it for a predetermined time.

[0011] Optionally, the substrate is a silicon-on-insulator (SOI) substrate with a cavity structure; the stacked structure of the array structure includes a bottom electrode layer, the piezoelectric layer, and a top electrode layer formed sequentially on the cavity SOI substrate.

[0012] Optionally, the array structure is manufactured using a mask process; the patterning of the piezoelectric layer is performed using a wet etching method.

[0013] Optionally, the bottom electrode layer is made of platinum; the top electrode layer includes a chromium (Cr) layer and a gold (Au) layer deposited sequentially; both the bottom electrode layer and the top electrode layer are deposited by magnetron sputtering and patterned.

[0014] According to a second aspect of the embodiments of this application, a method for fabricating the large bandwidth piezoelectric micromechanical ultrasonic transducer array described in the first aspect is provided, the method comprising: A silicon-on-insulator (SOI) substrate with a cavity structure is selected as the substrate, and the surface of the cavity SOI substrate is cleaned to remove surface impurities and oxide layers. A platinum layer is deposited on the cleaned cavity SOI substrate using magnetron sputtering, and the thickness of the platinum layer is controlled to form a bottom electrode layer. The bottom electrode layer is then patterned using a masking process and etching to obtain a bottom electrode with a preset pattern. A buffer layer is deposited on the surface of the patterned bottom electrode layer, and then a lead zirconate titanate (PZT) film or a potassium sodium niobate (KNN) film is deposited on the surface of the buffer layer by magnetron sputtering to form the piezoelectric layer. The piezoelectric layer is patterned using a masking process combined with a wet etching method, and the bottom electrode of the preset area is exposed after etching. A chromium layer and a gold layer are sequentially deposited on the patterned piezoelectric layer using a magnetron sputtering process to form a top electrode layer; the top electrode layer is then patterned using a lift-off process to ensure precise matching between the top electrode layer and the piezoelectric layer and the bottom electrode layer. A reverse DC voltage of a set magnitude is applied to the piezoelectric layer and maintained for a set duration to complete the negative polarization process, thereby obtaining a large bandwidth piezoelectric micromechanical ultrasonic transducer array.

[0015] In summary, this application provides a large-bandwidth piezoelectric micromechanical ultrasonic transducer array and its fabrication method. The large-bandwidth piezoelectric micromechanical ultrasonic transducer array includes: a substrate; and multiple piezoelectric resonant units located on the substrate; the multiple piezoelectric resonant units constitute an array structure; each channel in the array structure includes a number of diaphragm elements; the piezoelectric resonators in the array structure are divided into at least two groups, and the piezoelectric resonators in different groups have different structural parameters; the piezoelectric layer of the array structure is polarized by a DC electric field of preset polarity. This solves the problems of existing PMUTs, such as the difficulty in balancing bandwidth and sensitivity, and the limited piezoelectric response performance. Specifically, by dividing the piezoelectric resonators in the array structure into at least two groups and designing different structural parameters, the bandwidth can be expanded by superimposing the frequency responses of different resonators, overcoming the limitations of traditional single-frequency PMUTs with small bandwidth and multi-frequency PMUTs that are prone to sacrificing sensitivity. At the same time, by performing DC electric field polarization treatment on the piezoelectric layer with preset polarity, the internal characteristics of the piezoelectric layer are optimized to improve the piezoelectric response. Combined with the basic structural design of the substrate and piezoelectric resonator unit, a high-frequency PMUT array with both large bandwidth and high sensitivity is finally realized, meeting the needs of high-resolution detection in fields such as medical imaging and industrial sensing, while ensuring that the device has advantages in miniaturization, structural stability and process compatibility. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0017] The structures, proportions, sizes, etc. illustrated in this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed herein, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0018] Figure 1 A schematic diagram of a high-bandwidth piezoelectric micromechanical ultrasonic transducer array provided in this application embodiment; Figure 2 This is a schematic diagram of the planar structure of a high-bandwidth, high-frequency PMUT array provided in an embodiment of this application; Figure 3 A schematic diagram of a PMUT array stacked structure based on a PZT piezoelectric thin film and a cavity SOI substrate provided in the embodiments of this application; Figure 4 A schematic diagram of the vibration displacement frequency response curves of PMUT units with different cavity radii provided in the embodiments of this application; Figure 5 A schematic diagram of the transmitted sound pressure frequency response curve of the multi-frequency PMUT array provided in the embodiments of this application; Figure 6 A schematic diagram of the ferromagnetic hysteresis loop of a 1μm thick PZT piezoelectric film provided in the embodiments of this application; Figure 7 A schematic diagram of the emission sound pressure frequency response curves of two PMUT structures with varying cavity size and the same cavity size, provided for embodiments of this application; Figure 8 A schematic diagram of the fabrication process of the large bandwidth piezoelectric micromechanical ultrasonic transducer array provided in the embodiments of this application; The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] It should be noted that all directional indications (such as up, down, left, right, front, back, etc.) in the embodiments of the present invention are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indication will also change accordingly.

[0021] Furthermore, in this invention, descriptions involving "first," "second," etc., are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of those features. In the description of this invention, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0022] In this invention, unless otherwise explicitly specified and limited, the terms "connection," "fixation," etc., should be interpreted broadly. For example, "fixation" can refer to a fixed connection, a detachable connection, or an integral part; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0023] Furthermore, the technical solutions of the various embodiments of the present invention can be combined with each other, but only if they are based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0024] Current technologies still have many limitations that need to be addressed, making it difficult to meet the application requirements of high frequency, wide bandwidth, and high sensitivity. First, while traditional bulk piezoelectric devices are durable, their narrow operating bandwidth is due to a severe mismatch between the acoustic impedance of the medium in applications such as biological tissues and liquids, making it impossible to support the imaging or detection requirements of high axial resolution. Although CMUTs can achieve wide bandwidth, they require a high DC bias voltage of 50-100V, which not only significantly increases system power consumption and complicates the circuit structure but also poses potential safety hazards. Second, the technical bottlenecks in the PMUT field are even more prominent: single-frequency PMUTs have a small inherent bandwidth, making it difficult to cover high-frequency application scenarios; while multi-frequency array designs expand the bandwidth through geometric optimization, they are accompanied by a significant decrease in sensitivity, making it difficult to maintain core performance indicators; and existing multi-frequency PMUTs have significant defects in the processing of PZT piezoelectric films: most schemes use positive polarization or non-polarization optimization, completely ignoring the coercive field shift characteristics of the PZT material itself, resulting in uneven domain arrangement inside the film and limited piezoelectric response performance. In summary, existing technologies have generally failed to overcome the inherent trade-off between bandwidth and sensitivity, especially lacking a negative polarization enhancement mechanism for PZT thin films, which makes it impossible to achieve efficient electromechanical coupling in high-frequency scenarios above 15MHz, thus restricting the performance improvement and scenario expansion of PMUT in high-resolution and high-frequency applications.

[0025] Figure 1 This application illustrates a wide-bandwidth piezoelectric micromechanical ultrasonic transducer array, comprising: a substrate; and multiple piezoelectric resonant units located on the substrate; the multiple piezoelectric resonant units constitute an array structure; each channel in the array structure includes a number of diaphragm elements, the piezoelectric resonators in the array structure are divided into at least two groups, and the piezoelectric resonators in different groups have different structural parameters; the piezoelectric layer of the array structure is polarized by a DC electric field of preset polarity.

[0026] This application provides a high-bandwidth piezoelectric micromechanical ultrasonic transducer array, specifically addressing the two core problems of limited piezoelectric response due to the inherent trade-off between bandwidth and sensitivity in existing PMUT technology. Existing single-frequency PMUTs have relatively small bandwidths, and while multi-frequency arrays can extend bandwidth, they often sacrifice sensitivity, and insufficient processing of the piezoelectric thin film leads to poor piezoelectric response. Therefore, this application addresses this issue by dividing the piezoelectric resonators in the array structure into at least two groups with different structural parameters. This utilizes the superposition effect of the frequency responses of resonators with different structural parameters to broaden the bandwidth. Simultaneously, the collaborative design of multiple resonators reduces sensitivity loss, overcoming the performance trade-off limitations of traditional designs. Furthermore, by subjecting the piezoelectric layer to DC electric field polarization with a preset polarity, combined with the characteristic of PZT thin film negatively polarized to optimize domain arrangement and overcome positive coercive field shift, the internal microstructure of the piezoelectric layer is improved, enhancing piezoelectric conversion efficiency and sound pressure output capability. Furthermore, the basic design of the array structure consisting of a substrate and multiple piezoelectric resonant units can also continue the inherent advantages of PMUT in miniaturization and acoustic impedance matching, adapting to the requirements of high resolution and high frequency performance in applications such as medical imaging and industrial sensing, and ultimately achieving performance improvement with large bandwidth, high sensitivity and high voltage response.

[0027] In one possible implementation, the array structure is divided into an outer region and a central region; the piezoelectric resonator in the outer region adopts a gradually changing structural parameter design, while the piezoelectric resonator in the central region adopts a fixed structural parameter design.

[0028] In one possible implementation, the structural parameters include the planar dimensions of the piezoelectric resonators; the piezoelectric resonators with different planar dimensions are spatially distributed in the array structure, the piezoelectric resonators in the outer region of the array structure have a gradient in size, and the piezoelectric resonators in the central region of the array structure have a consistent size.

[0029] In one possible implementation, the piezoelectric resonator is a circular diaphragm, the structural parameter of which is the radius, and each channel of the array structure includes multiple circular diaphragms with different radii, the radius of which ranges from 18 μm to 22 μm.

[0030] In the technical solution of the large bandwidth high-frequency PMUT array based on the negative polarization enhancement principle in this application, the design of the array structure and piezoelectric resonators is optimized by partitioning and adjusting the parameter gradient to further solve the problem of difficulty in balancing bandwidth expansion and sensitivity loss in existing PMUT multi-frequency arrays. Specifically, the array structure is divided into an outer region and a central region. The piezoelectric resonators in the outer region adopt a gradient structural parameter design, while the central region adopts a fixed structural parameter design. The gradient structural parameters (such as planar dimensions) in the outer region allow different resonators to cover different frequency ranges, achieving bandwidth expansion through frequency response superposition. For example, a diaphragm with a radius of 18-22μm corresponds to a parameter matching relationship of 17-24MHz. The fixed structural parameter design in the central region provides an anchoring effect for the entire array at the target operating frequency, avoiding performance fluctuations caused by the gradient of parameters across the entire array, ensuring that sensitivity is not excessively lost, and compensating for the shortcomings of existing multi-frequency arrays that rely solely on gradient geometry to broaden bandwidth while neglecting sensitivity preservation.

[0031] Furthermore, when the structural parameters are specifically the planar dimensions of the piezoelectric resonator, and the piezoelectric resonator is designed as a circular diaphragm with a radius limited to 18μm to 22μm, the quantitative implementation of bandwidth expansion is achieved. The resonant frequency of the circular diaphragm is directly related to its radius; a diaphragm with a radius of 18μm corresponds to a higher resonant frequency, and a diaphragm with a radius of 22μm corresponds to a lower resonant frequency. By integrating circular diaphragms of different radii into each channel in a gradient distribution on the outer side and a fixed center, the high-frequency range of 17-24MHz can be covered through the radius difference, extending the -6dB bandwidth to 15.5MHz (3 times that of traditional single-frequency designs). At the same time, relying on the stable response of the centrally fixed radius diaphragm, the sensitivity retention rate can be maintained at 84%. With the subsequent negative polarization treatment of the piezoelectric layer, the sound pressure output can be further increased to 6.25kPa / V, ultimately achieving the technical goal of large bandwidth, high sensitivity, and high-frequency output, which is fully adapted to the high-resolution detection requirements of medical imaging, industrial sensing, and other scenarios.

[0032] In one possible implementation, the piezoelectric resonator includes a piezoelectric layer made of a thin film material with ferroelectric properties; the piezoelectric layer is a lead zirconate titanate (PZT) thin film or a potassium sodium niobate (KNN) thin film.

[0033] In one possible implementation, the preset polarity is negative, and the corresponding DC electric field polarization process is negative polarization; the negative polarization process is to apply a reverse DC voltage to the piezoelectric layer and maintain it for a set duration; the negative polarization process is to apply a DC voltage of a set magnitude to the piezoelectric layer and maintain it for a predetermined time.

[0034] In the technical solution of a high-bandwidth, high-frequency PMUT array based on the principle of negative polarization enhancement in this application, the piezoelectric layer of the piezoelectric resonator is selected from lead zirconate titanate (PZT) thin film with ferroelectric properties. PZT thin film has excellent piezoelectric performance and is a key material for realizing high-frequency ultrasonic signal conversion. However, its performance is highly dependent on the material processing method. In the prior art, positive polarization or non-polarization processing is mostly used, which fails to fully activate its piezoelectric potential. However, the embodiment of this application explicitly selects PZT thin film, which lays the material foundation for subsequent performance optimization. At the same time, it continues the advantages of PMUT miniaturization and compatibility with CMOS process, and adapts to the device integration requirements of scenarios such as medical imaging and industrial sensing.

[0035] For the piezoelectric layer treatment, a DC electric field polarization process with a preset negative polarity (i.e., negative polarization) is employed. This is achieved by applying a reverse DC voltage to the PZT film and maintaining it for a set duration. The coercive electric field of PZT material exhibits a characteristic of shifting towards the positive voltage direction. Its ferromagnetic hysteresis curve shows that the coercive field in the negative polarization state (-27 kV / cm) is significantly lower than that in the positive polarization state (74 kV / cm). Based on this, negative polarization can more easily overcome the coercive field under the same bias voltage, optimize the uniformity of domain arrangement within the PZT film, and solve the problems of uneven domain arrangement and limited piezoelectric response in existing positive polarization processes. Furthermore, by combining specific parameters, applying a -10V DC voltage for a predetermined time can significantly improve the piezoelectric conversion efficiency, increasing the acoustic pressure output of the PMUT array to 6.25 kPa / V. This provides a guarantee for efficient electromechanical coupling in high-frequency (>15 MHz) scenarios, ultimately contributing to achieving the technical goals of large bandwidth and high sensitivity.

[0036] In one possible implementation, the substrate is a silicon-on-insulator (SOI) substrate with a cavity structure; the stacked structure of the array structure includes a bottom electrode layer, the piezoelectric layer, and a top electrode layer formed sequentially on the cavity SOI substrate.

[0037] In one possible implementation, the array structure is manufactured using a masking process; the patterning of the piezoelectric layer is performed using a wet etching method.

[0038] In this application's technical solution for a high-bandwidth, high-frequency PMUT array based on the principle of negative polarization enhancement, the substrate is selected as a silicon-on-insulator (SOI) substrate with a cavity structure, and the array structure adopts a sequential stacking design of cavity SOI substrate, bottom electrode layer, piezoelectric layer, and top electrode layer. This solution directly addresses the problems of low cavity size control accuracy and poor compatibility of stacked structures in existing PMUT manufacturing. Traditional DRIE-based PMUT processes are difficult to precisely control cavity size, while the cavity SOI substrate selected in this application has a pre-defined cavity structure, which can significantly improve the control capability of cavity size, perfectly adapt to the manufacturing requirements of multiple sets of piezoelectric resonators with different structural parameters (such as circular diaphragms with different radii), ensure that each resonator can stably achieve the corresponding frequency response, and provide structural accuracy guarantee for bandwidth expansion; at the same time, the stacking order of bottom electrode layer-piezoelectric layer-top electrode layer not only conforms to the basic principle of piezoelectric conversion (driving the piezoelectric layer to vibrate by applying an electric field through electrodes), but also reduces signal loss and improves electromechanical coupling efficiency through the tight combination of each layer, continuing the advantages of PMUT miniaturization and CMOS process compatibility.

[0039] In terms of manufacturing process, the array structure adopts a mask process, and the patterning of the piezoelectric layer uses a wet etching method. The mask process has the advantages of simplified process and precise geometric control. A three-mask process can complete the deposition and patterning of each functional layer of the array, which can ensure the consistent manufacturing of multi-channel, multi-resonator arrays. As for the wet etching of the piezoelectric layer, unlike traditional acid etching (rate <5nm / min), the embodiments of this application can further combine an alkaline etching system containing a chelating agent (such as EDTA), which avoids the use of high-concentration hydrofluoric acid (HF) to improve safety, and can increase the etching rate to 20nm / min, significantly shortening the processing time. This process combination not only solves the problems of low precision, poor efficiency and insufficient safety in the existing PMUT manufacturing, but also adapts to the manufacturing requirements of cavity SOI substrates and stacked structures, ensuring that the patterned piezoelectric layer is precisely matched with the electrode layer, providing reliable process support for the realization of the array's large bandwidth and high sensitivity performance.

[0040] In one possible implementation, the bottom electrode layer is made of platinum; the top electrode layer comprises a chromium (Cr) layer and a gold (Au) layer deposited sequentially; both the bottom electrode layer and the top electrode layer are deposited by magnetron sputtering and patterned.

[0041] In the technical solution of a high-bandwidth high-frequency PMUT array based on the principle of negative polarization enhancement in this application, the bottom electrode layer is made of platinum (Pt), and the top electrode layer is designed as a chromium (Cr) layer and a gold (Au) layer deposited sequentially. Both types of electrode layers are deposited and patterned by magnetron sputtering. This design focuses on ensuring the compatibility between the electrodes and the piezoelectric layer, improving the efficiency of electrical signal transmission, and ensuring the stability of the array's high-frequency performance.

[0042] In terms of material selection, the core functional layer of the PMUT array is a PZT piezoelectric film. The bottom electrode needs to form a stable contact with the PZT film to achieve uniform application of the electric field. Platinum (Pt) has excellent chemical stability and conductivity, which not only avoids chemical reactions with the PZT film, but also ensures that the electric field is efficiently transmitted to the piezoelectric layer, providing a stable electrical basis for piezoelectric conversion. The top electrode adopts a composite structure of Cr layer + Au layer. This is because chromium (Cr) can enhance the adhesion between the electrode and the PZT film and prevent the electrode from falling off, while gold (Au) has extremely low resistivity, which can reduce the loss in the high-frequency signal transmission process. The combination of the two solves the problem of insufficient electrode adhesion or high signal loss of single material, ensuring the electrical signal transmission efficiency of the array in high-frequency scenarios above 15MHz.

[0043] In terms of fabrication process, magnetron sputtering is selected to deposit the electrode layer. Magnetron sputtering can achieve precise control of the electrode layer thickness (e.g., the bottom electrode layer is 100nm thick, the Cr layer is 20nm thick, and the Au layer is 200nm thick), ensuring consistent electrode performance for different channels and resonators, and avoiding differences in electric field distribution caused by uneven thickness. Subsequent patterning processing allows the electrode layer to be precisely matched with the piezoelectric layer and resonator structure, further reducing electrical interference. Combined with the multi-frequency design of the array and the negative polarization treatment of the piezoelectric layer, it supports the high bandwidth and high sensitivity performance target of the PMUT array, meeting the precise conversion requirements of high-frequency ultrasound signals in fields such as medical imaging and industrial sensing.

[0044] In one possible implementation, the high-bandwidth high-frequency PMUT array includes an array structure composed of multiple resonators, with 3×40 elements arranged in each channel. The resonators adopt a circular diaphragm structure with a radius varying from 18μm to 22μm. By combining resonators of different radii according to their frequency response characteristics, the overall bandwidth of the array is broadened by utilizing the frequency superposition effect. The array is divided into an outer region and a central region. The resonators in the outer region adopt a radius gradient design to cover a wider frequency range, while the resonators in the central region maintain a constant radius, providing an anchoring effect for the array at the target operating frequency and ensuring the stability of the array performance.

[0045] In one possible implementation, the core functional layer of the PMUT array includes a piezoelectric layer, which is made of lead zirconate titanate (PZT) film. The PZT film is subjected to negative polarization treatment to optimize its internal domain arrangement. The specific conditions for negative polarization treatment are: applying a DC voltage of -10V to the piezoelectric layer for 5 minutes. After this treatment, the sound pressure output of the array is increased to 6.25kPa / V. When operating in FC-3283 medium, the center frequency reaches 18.1MHz, and the -6dB bandwidth is extended to 15.5MHz, meeting the requirements of high-frequency and broadband applications.

[0046] In one possible implementation, the PMUT array uses a silicon-on-insulator (SOI) substrate with a cavity structure (i.e., a cavity SOI substrate) as the substrate. Compared with the PMUT process based on DRIE, the cavity SOI substrate has higher control precision for cavity size, which can accurately match the manufacturing requirements of resonators with different radii. It is especially suitable for the processing of multi-cavity radius arrays, ensuring the consistency of the performance of each resonator in the array.

[0047] In one possible implementation, the patterning of the piezoelectric layer during the fabrication of the PMUT array is performed using a wet etching method. The etching system used in the wet etching process consists of hydrogen peroxide, ammonium hydroxide, and ethylenediaminetetraacetic acid (EDTA). EDTA acts as a chelating agent, which can stabilize the etching reaction and accelerate the complexation and dissolution process of metal ions. This etching system can enable the initial etching rate of the KNN film to reach 20 nm / min, which is much higher than the rate of less than 5 nm / min of traditional acidic wet etching, thus significantly shortening the processing time and improving production efficiency.

[0048] In one possible implementation, the wet etching process uses common silicon oxide (SiO2) as a hard mask, eliminating the need for precious metal materials such as platinum. This avoids the procurement costs of expensive metal masks and additional process steps, significantly reducing the overall manufacturing cost of the array while ensuring etching accuracy. It also simplifies the process flow and is more suitable for large-scale mass production scenarios.

[0049] In one possible implementation, the etching system used in the wet etching process does not contain high concentrations of hydrofluoric acid (HF). The etching function is achieved through the synergistic effect of hydrogen peroxide, ammonium hydroxide, and EDTA. This system effectively reduces the health risks to operators, while also reducing the corrosive hazards to production equipment and improving the safety of the process operation.

[0050] In one possible implementation, the PMUT array has a -6dB bandwidth of 15.5MHz, which is three times that of a traditional single-frequency PMUT array. While significantly expanding the bandwidth, the array sensitivity is retained at 84%. Compared with the common problem of bandwidth expansion accompanied by a significant decrease in sensitivity in existing multi-frequency PMUT arrays, this array achieves an efficient balance between bandwidth and sensitivity, which can meet the demanding performance requirements of scenarios such as high-resolution imaging and industrial sensing.

[0051] In one possible implementation, when the PMUT array is tested in FC-3283 medium, its center frequency is stable at 18.1MHz, its -6dB bandwidth is 15.5MHz, and its sound pressure output reaches 6.25kPa / V. These performance parameters enable the array to operate stably in the high-frequency range and have strong acoustic signal output capabilities, which can meet the requirements of high axial resolution and high signal strength in medical ultrasound imaging, as well as the requirements of accurate detection of microstructural defects in industrial sensing.

[0052] To clearly present the technical details of the large bandwidth high frequency PMUT array based on the negative polarization enhancement principle of this application, and to intuitively demonstrate the array structure design, performance characteristics and physical form corresponding to key processes, the following is a detailed explanation of the technology and experimental verification results of this application in conjunction with the accompanying drawings.

[0053] Figure 2 This is a schematic diagram of the planar structure of a high-bandwidth, high-frequency PMUT array, visually illustrating the design logic of the multi-frequency PMUT array in this application. As shown, the array achieves broadband characteristics by superimposing the frequency responses of multiple circular resonators with different radii and corresponding resonant frequencies. Each channel contains 3×40 diaphragms, and the array is divided into an outer region and a central region. The diaphragm radii in the outer region exhibit gradient changes (e.g., 36μm, 38μm, 40μm, 42μm, 44μm, etc.) to cover a wider frequency range; the diaphragms in the central region maintain a constant radius, providing an anchoring effect for the array at the target frequency and maintaining stable array performance. The spatial variation of the diaphragm radius broadens the overall frequency response, enabling the array to effectively cover a wider frequency range, providing a structural basis for achieving a -6dB bandwidth of 15.5MHz (center frequency 18.1MHz). Furthermore, combining the first-order bending mode equation of the circular parallel plate, it can be seen that:

[0054] Where a is the radius, ρ is the surface density, E(z) is the Young's modulus, and v(z) is the Poisson's ratio of the material at a distance z from the neutral axis. When the PMUT array operates in a liquid medium, the resonant frequency changes further due to increased damping. Compared to air, the center frequency of the PMUT decreases in the fluid. This adjustment is described by the following equation:

[0055] Where ρ fluid It is the fluid density. The resonant frequency of each diaphragm is determined by its radius and thickness. When the PMUT array is running in a liquid medium (such as FC-3283), the fluid density will further adjust the resonant frequency. This also explains the performance of the center frequency and bandwidth of the array in this application when tested in a liquid medium, further verifying the rationality and practicality of the array design.

[0056] Figure 3 This is a schematic diagram of the PMUT array stack structure based on a PZT piezoelectric thin film and a cavity SOI substrate, clearly showing the interlayer composition and process design logic of the PMUT array in this application. As can be seen from the figure, the stack structure of the array, from bottom to top, consists of a Si substrate, a SiO2 layer, a SiO2 / TixO buffer layer, a Pt bottom electrode layer, a PZT piezoelectric layer, and a top electrode layer composed of a Cr layer and an Au layer.

[0057] First, a custom cavity SOI (CSOI) wafer was selected as the substrate, with a device layer thickness of 3µm. Then, a 100nm Pt layer was deposited on the CSOI wafer as the bottom electrode, followed by a 100nm SiO2 / TixO layer as a buffer layer to improve the interface between the bottom electrode and the piezoelectric layer. Next, a 1µm PZT layer was deposited by magnetron sputtering as the core piezoelectric layer, and the PZT layer was patterned using wet etching to expose the bottom electrode. Finally, a 20nm Cr layer and a 200nm Au layer were deposited by magnetron sputtering, and patterned after a lift-off process to form the top electrode. This stacked structure design and process combination ensures precise stacking and geometric control of each functional layer, while optimizing the piezoelectric conversion efficiency through the material selection of the buffer layer and electrode layer. Ultimately, this achieves a large bandwidth and high sensitivity performance for the array, while simultaneously meeting the requirements for simplified process and consistent manufacturing.

[0058] Figure 4 The vibration displacement frequency response curves of PMUT units with different cavity radii present the vibration characteristics of the circular diaphragm in the multi-frequency PMUT array of this application. As shown in the figure, the curves correspond to circular diaphragms with radii of 18μm, 19μm, 20μm, 21μm, and 22μm, respectively, and their resonant frequencies are distributed in the range of 17MHz to 24MHz, which is closely consistent with the theoretical prediction results. Tests using a laser Doppler vibrometer show that the maximum displacement sensitivity of all tested radii is inversely proportional to the resonant frequency. For example, the diaphragm with a radius of 22μm has a resonant frequency of approximately 17MHz and a relatively high peak vibration displacement; the diaphragm with a radius of 18μm has a resonant frequency of approximately 24MHz and a relatively low peak vibration displacement. This attenuation trend mainly stems from two mechanisms: first, the effective stiffness of the diaphragm increases with increasing frequency, suppressing the vibration displacement amplitude; second, the mechanical damping of the material increases with increasing frequency, further reducing the displacement sensitivity. The test results validate the design logic of this application, which broadens the bandwidth by superimposing the frequencies of diaphragms with different radii. The resonant frequency ranges of diaphragms with different radii overlap with each other, providing performance support for achieving a -6dB bandwidth of 15.5MHz. It also explains the displacement response characteristics of the array at high frequencies, demonstrating the rationality of the design and the predictability of the performance.

[0059] Figure 5The figure shows the frequency response curves of the emitted sound pressure of a multi-frequency PMUT array, illustrating the differences in emitted sound pressure performance under three conditions: no poling, positive poling, and negative poling. As can be seen from the figure, the emitted sound pressure sensitivity of the negatively polarized PMUT array is significantly higher than that under positive and no-polarization conditions. Its sound pressure output gain advantage is evident in the 5-25MHz frequency range, especially in the high-frequency range (e.g., 15-20MHz). This result verifies the technical solution of this application, which optimizes PZT domain alignment through negative polarization to overcome positive coercive field shift and improve piezoelectric response. Negative polarization enhances piezoelectric conversion efficiency by adjusting the domain alignment within the PZT thin film, thereby increasing the array's sound pressure output to 6.25 kPa / V, directly demonstrating the significant advantage of negative polarization in improving the piezoelectric response of the PMUT.

[0060] Figure 6 The ferromagnetic hysteresis loop of a 1 μm thick PZT piezoelectric film reveals the shift characteristics of the coercive electric field of the PZT material. As shown in the figure, the hysteresis loop exhibits significant asymmetry, with coercive fields of -27 kV / cm and 74 kV / cm corresponding to negative and positive polarization states, respectively. This shift of the coercive electric field towards the positive voltage direction results in a significantly lower coercive electric field strength to be overcome in the negative polarization direction. Based on this, under the same bias voltage, negative polarization treatment more easily overcomes the coercive field, thereby achieving a greater polarization intensity. This characteristic is the basis for optimizing PZT domain arrangement through negative polarization treatment in this application. Utilizing the lower coercive field of PZT film in negative polarization, negative polarization treatment by applying a reverse DC voltage can efficiently optimize the domain arrangement inside the PZT film, significantly improve piezoelectric conversion efficiency, and ultimately achieve a substantial increase in array acoustic pressure output (e.g., 6.25 kPa / V), providing a crucial material-level guarantee for the high performance of large-bandwidth high-frequency PMUT arrays.

[0061] Figure 7 The emission sound pressure level frequency response curves for two PMUT structures with varying and identical cavity sizes illustrate the difference in emission performance between multi-frequency and single-frequency PMUT arrays. The first curve represents a multi-frequency PMUT array using diaphragms of different radii (VariousRadii), while the second curve represents a single-frequency PMUT array using diaphragms of the same radius (SameRadius). As shown in the figure, the multi-frequency PMUT array achieves a -6dB bandwidth of 15.5MHz, a center frequency of 18.1MHz, and a sound pressure level sensitivity of 6.2kPa / V; while the single-frequency PMUT array has a bandwidth of only 4.9MHz, a center frequency of 14.4MHz, and a sound pressure level sensitivity of 7.4kPa / V.

[0062] Experimental results show that the multi-frequency PMUT array proposed in this application achieves a three-fold increase in bandwidth while reducing sensitivity by 16%. It maintains high sensitivity while significantly expanding bandwidth, which can improve sound pressure output and axial resolution. It is suitable for high-resolution imaging and microstructure defect detection, and fully verifies the technical advantages of multi-cavity radius array design in overcoming the bandwidth-sensitivity trade-off problem.

[0063] In summary, this application provides a large-bandwidth piezoelectric micromechanical ultrasonic transducer array, comprising: a substrate; and multiple piezoelectric resonant units located on the substrate; the multiple piezoelectric resonant units constitute an array structure; each channel in the array structure includes a number of diaphragm elements, the piezoelectric resonators in the array structure are divided into at least two groups, and the piezoelectric resonators in different groups have different structural parameters; the piezoelectric layer of the array structure is polarized by a DC electric field of preset polarity. This solves the problems of existing PMUTs, such as the difficulty in balancing bandwidth and sensitivity, and the limited piezoelectric response performance. Specifically, by dividing the piezoelectric resonators in the array structure into at least two groups and designing different structural parameters, the bandwidth can be expanded by superimposing the frequency responses of different resonators, overcoming the limitations of traditional single-frequency PMUTs with small bandwidth and multi-frequency PMUTs that are prone to sacrificing sensitivity. At the same time, by performing DC electric field polarization treatment on the piezoelectric layer with preset polarity, the internal characteristics of the piezoelectric layer are optimized to improve the piezoelectric response. Combined with the basic structural design of the substrate and piezoelectric resonator unit, a high-frequency PMUT array with both large bandwidth and high sensitivity is finally realized, meeting the needs of high-resolution detection in fields such as medical imaging and industrial sensing, while ensuring that the device has advantages in miniaturization, structural stability and process compatibility.

[0064] Based on the same technical concept, this application also provides a method for fabricating a large-bandwidth piezoelectric micromechanical ultrasonic transducer array, such as... Figure 8 As shown, the method includes: Step 801: Select a silicon-on-insulator (SOI) substrate with a cavity structure as the substrate, and clean the surface of the cavity SOI substrate to remove surface impurities and oxide layers; Step 802: A platinum layer is deposited on the cleaned cavity SOI substrate using magnetron sputtering, and the thickness of the platinum layer is controlled to form a bottom electrode layer; the bottom electrode layer is patterned by masking and etching to obtain a bottom electrode with a preset pattern. Step 803: Deposit a buffer layer on the surface of the patterned bottom electrode layer, and then deposit a lead zirconate titanate (PZT) film or a potassium sodium niobate (KNN) film on the surface of the buffer layer using a magnetron sputtering process to form the piezoelectric layer; Step 804: The piezoelectric layer is patterned using a masking process combined with a wet etching method, and the bottom electrode of the preset area is exposed after etching; Step 805: A chromium layer and a gold layer are sequentially deposited on the patterned piezoelectric layer surface using a magnetron sputtering process to form a top electrode layer; the top electrode layer is patterned using a stripping process to ensure precise matching between the top electrode layer and the piezoelectric layer and the bottom electrode layer. Step 806: Apply a reverse DC voltage of a set magnitude to the piezoelectric layer and maintain it for a set duration to complete the negative polarization process, thereby obtaining a large bandwidth piezoelectric micromechanical ultrasonic transducer array.

[0065] The first step is the selection and cleaning of the cavity SOI substrate. Based on the process advantages of the cavity SOI substrate in terms of better control of cavity size, an SOI substrate with its own cavity structure can be selected. This avoids the problem of insufficient cavity size accuracy in the traditional DRIE process and lays the foundation for the subsequent manufacturing of piezoelectric resonators with different structural parameters (such as diaphragms with different radii). The surface cleaning process removes impurities and oxide layers to ensure the bonding stability between the bottom electrode layer and the substrate.

[0066] The second step involves the fabrication and patterning of the bottom electrode layer. A platinum (Pt) layer is deposited by magnetron sputtering and patterned using a masking process. A design can be adopted where a 100nm Pt layer is deposited on a CSOI wafer as the bottom electrode. Platinum has excellent conductivity and chemical stability, which can form a stable electrical contact with the subsequent PZT piezoelectric layer, avoiding the impact of interface reactions on piezoelectric conversion. The magnetron sputtering process can precisely control the thickness of the platinum layer (e.g., 100nm), and the patterning process ensures that the bottom electrode matches the preset resonator layout, providing a guarantee for the uniform application of the electric field to the piezoelectric layer.

[0067] The third step involves the fabrication of a buffer layer and a piezoelectric layer. A buffer layer (such as a SiO2 / TixO layer) is deposited on the bottom electrode, followed by magnetron sputtering of a PZT thin film. A design with the PZT thin film as the core piezoelectric layer can be adopted: the buffer layer can reduce the lattice mismatch between the bottom electrode and the piezoelectric layer, improving the crystal quality of the PZT thin film; magnetron sputtering of the PZT thin film can precisely control the thickness (such as 1 μm), ensuring that the piezoelectric layer has stable ferroelectric properties, providing a material basis for subsequent negative polarization treatment and piezoelectric response activation.

[0068] The fourth step is wet etching patterning of the piezoelectric layer. This involves combining PZT patterning with wet etching and a process optimization using an alkaline etching system containing chelating agents. Wet etching can achieve fine patterning of the PZT film, exposing the bottom electrode in a predetermined area to build an electrical pathway. Subsequently, a hydrogen peroxide + ammonium hydroxide + EDTA etching system can be further used, which avoids the safety hazards of high-concentration HF and can improve the etching rate (up to 20nm / min), balancing process safety and efficiency.

[0069] The fifth step involves the fabrication and patterning of the top electrode layer. A Cr / Au composite layer is deposited by magnetron sputtering and patterned using a lift-off process. A 20nm Cr layer and a 200nm Au layer can be deposited by magnetron sputtering as the top electrode design: the Cr layer enhances the adhesion between the electrode and the PZT layer, while the Au layer reduces high-frequency signal transmission loss. The combination of the two solves the performance defects of electrodes made of single materials. The lift-off process ensures precise alignment of the top electrode with the piezoelectric layer and the bottom electrode, reducing electrical interference.

[0070] The sixth step involves negative polarization of the piezoelectric layer. A reverse DC voltage is applied to the PZT layer and maintained for a set duration. Utilizing the positive shift of the coercive field of the PZT material (negative polarization coercive field -27kV / cm, much lower than the positive polarization 74kV / cm), the reverse DC voltage can more easily overcome the coercive field, achieving uniformity of the PZT thin film domain arrangement, improving piezoelectric response performance, and ultimately enabling the array acoustic pressure output to reach 6.25kPa / V, providing performance support for efficient electromechanical coupling and bandwidth expansion at high frequencies (>15MHz).

[0071] In summary, this fabrication method fully realizes the core technical solution of multi-frequency bandwidth design + negative polarization to enhance piezoelectric performance through stepwise precise manufacturing of substrate-electrode-piezoelectric layer and negative polarization treatment. This ensures that the final array has high bandwidth (e.g., 15.5MHz) and high sensitivity (84% retention), making it suitable for high-frequency and high-resolution applications such as medical imaging and industrial sensing.

[0072] It should be noted that the above embodiments are illustrative of this application and not limiting of it, and that those skilled in the art can devise alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses should not be construed as limiting the claims. The word "enclosed" does not exclude the presence of elements or steps not listed in the claims. The word "one" or "a" preceding an element does not exclude the presence of a plurality of such elements. This application can be implemented by means of hardware comprising several different elements and by means of a suitably programmed computer. In the unit claims enumerating several means, several of these means may be embodied by the same item of hardware. The use of the words "first," "second," and "third," etc., does not indicate any order. These words can be interpreted as names.

[0073] The above description is merely a preferred embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

[0074] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention specification and drawings under the concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A large-bandwidth piezoelectric micromechanical ultrasonic transducer array, characterized in that, include: Substrate; and a plurality of piezoelectric resonant units located on the substrate; The plurality of piezoelectric resonant units form an array structure; Each channel in the array structure includes a number of diaphragm elements. The piezoelectric resonators in the array structure are divided into at least two groups, and the piezoelectric resonators in different groups have different structural parameters. The piezoelectric layer of the array structure is polarized by a DC electric field of preset polarity.

2. The large bandwidth piezoelectric micromechanical ultrasonic transducer array as described in claim 1, characterized in that, The array structure is divided into an outer region and a central region; the piezoelectric resonator in the outer region adopts a gradually changing structural parameter design, while the piezoelectric resonator in the central region adopts a fixed structural parameter design.

3. The large bandwidth piezoelectric micromechanical ultrasonic transducer array as described in claim 2, characterized in that, The structural parameters include the planar dimensions of the piezoelectric resonators; the piezoelectric resonators with different planar dimensions are spatially distributed in the array structure, the piezoelectric resonators in the outer region of the array structure have a gradient change in size, and the piezoelectric resonators in the central region of the array structure have a consistent size.

4. The large bandwidth piezoelectric micromechanical ultrasonic transducer array as described in claim 3, characterized in that, The piezoelectric resonator is a circular diaphragm, and the structural parameter of the circular diaphragm is its radius. Each channel of the array structure includes multiple circular diaphragms with different radii, and the radius of the circular diaphragms ranges from 18μm to 22μm.

5. The large bandwidth piezoelectric micromechanical ultrasonic transducer array as described in claim 1, characterized in that, The piezoelectric resonator includes a piezoelectric layer, which is made of a thin film material with ferroelectric properties; the piezoelectric layer is a lead zirconate titanate (PZT) thin film or a sodium potassium niobate (KNN) thin film.

6. The large bandwidth piezoelectric micromechanical ultrasonic transducer array as described in claim 5, characterized in that, The preset polarity is negative, and the corresponding DC electric field polarization process is negative polarization; the negative polarization process is to apply a reverse DC voltage to the piezoelectric layer and maintain it for a set duration; the negative polarization process is to apply a DC voltage of a set magnitude to the piezoelectric layer and maintain it for a predetermined time.

7. The large bandwidth piezoelectric micromechanical ultrasonic transducer array as described in claim 1, characterized in that, The substrate is a silicon-on-insulator (SOI) substrate with a cavity structure; the array structure includes a bottom electrode layer, a piezoelectric layer, and a top electrode layer formed sequentially on the cavity SOI substrate.

8. The large bandwidth piezoelectric micromechanical ultrasonic transducer array as described in claim 7, characterized in that, The array structure is manufactured using a masking process; the patterning of the piezoelectric layer is performed using a wet etching method.

9. The large bandwidth piezoelectric micromechanical ultrasonic transducer array as described in claim 1, characterized in that, The bottom electrode layer is made of platinum; the top electrode layer includes a chromium (Cr) layer and a gold (Au) layer deposited sequentially; both the bottom electrode layer and the top electrode layer are deposited and shaped by magnetron sputtering and are patterned.

10. A method for fabricating a large-bandwidth piezoelectric micromechanical ultrasonic transducer array as described in any one of claims 1-9, characterized in that, The method includes: A silicon-on-insulator (SOI) substrate with a cavity structure is selected as the substrate, and the surface of the cavity SOI substrate is cleaned to remove surface impurities and oxide layers. A platinum layer is deposited on the cleaned cavity SOI substrate using magnetron sputtering, and the thickness of the platinum layer is controlled to form a bottom electrode layer. The bottom electrode layer is then patterned using a masking process and etching to obtain a bottom electrode with a preset pattern. A buffer layer is deposited on the surface of the patterned bottom electrode layer, and then a lead zirconate titanate (PZT) film or a potassium sodium niobate (KNN) film is deposited on the surface of the buffer layer by magnetron sputtering to form the piezoelectric layer. The piezoelectric layer is patterned using a masking process combined with a wet etching method, and the bottom electrode of the preset area is exposed after etching. A chromium layer and a gold layer are sequentially deposited on the patterned piezoelectric layer using a magnetron sputtering process to form a top electrode layer; the top electrode layer is then patterned using a lift-off process to ensure precise matching between the top electrode layer and the piezoelectric layer and the bottom electrode layer. A reverse DC voltage of a set magnitude is applied to the piezoelectric layer and maintained for a set duration to complete the negative polarization process, thereby obtaining a large bandwidth piezoelectric micromechanical ultrasonic transducer array.

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