Non-equal-diameter cavity membrane structure back cavity emission type piezoelectric miniature ultrasonic transducer and manufacturing method thereof

Abstract

The invention discloses a non-equal-diameter cavity membrane structure back cavity emission type piezoelectric miniature ultrasonic transducer and a manufacturing method, and belongs to the field of miniature ultrasonic transducers, the non-equal-diameter cavity membrane structure back cavity emission type piezoelectric miniature ultrasonic transducer comprises a substrate, the substrate is provided with a back cavity and a vibrating diaphragm with a single end shielding the back cavity, an air cavity is arranged between the substrate and the vibrating diaphragm, and the air cavity is communicated with the back cavity; the radial profile of the air cavity is larger than that of the back cavity, and the back cavity is within the axial projection range of the air cavity. According to the invention, better comprehensive balance can be obtained between sensitivity and sound pressure, the edge displacement of a high-order mode is suppressed, the vibration boundary stability is improved, the sound energy coupling efficiency of a main mode is improved, the unimodal characteristic and the signal purity of a transmitted waveform are improved, the effective sound radiation area of a back cavity and the rigidity of the air spring can be flexibly adjusted, and the reliability is high. And the far-field sound pressure can be improved, the ultrasonic radiation capability of the transducer can be enhanced, the realizable working bandwidth can be expanded, the process tolerance can be improved, and the device is insensitive to the deviation of the back etching diameter, so that the consistency and reliability of the device are improved.

Description

Technical Field

[0001] This invention relates to a non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer and its manufacturing method, belonging to the field of micro ultrasonic transducers. Background Technology

[0002] Ultrasonic transducers are one of the core components in the field of ultrasonic sensing and detection, widely used in smartphones, smart homes, automotive driver assistance systems, drones, industrial inspection, and other fields, playing important roles in distance measurement, obstacle detection, spatial modeling, and acoustic imaging. With the rapid development of smart terminals and automated equipment, higher demands are being placed on ultrasonic sensors: achieving longer detection distances and higher measurement accuracy in a smaller size and with lower power consumption.

[0003] Traditional ultrasonic transducers mostly employ a thin-film forward-emitting structure, where sound waves primarily propagate from the front of the diaphragm. To achieve higher sound pressure output under the same driving conditions, improve directivity by ensuring sound waves primarily propagate along the cavity opening, and reduce module size, a more novel cavity-backed emission design is adopted. This cavity-backed emission design typically consists of a diaphragm and a cavity. Currently, this design still suffers from limited acoustic coupling efficiency, excessively high quality factor, narrow bandwidth, weak resistance to manufacturing deviations, and limited sound pressure, necessitating optimization of its structural design. Summary of the Invention

[0004] To overcome the shortcomings of the prior art, the present invention provides a non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer and its manufacturing method, so as to improve the problems of narrow bandwidth, insufficient sound pressure, sensitivity to process deviation and severe high-order mode interference in the prior art.

[0005] The technical solution adopted by this invention to solve its technical problem is: In a first aspect, this application provides a non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer, including a substrate, a back cavity and a diaphragm that blocks the back cavity at one end are disposed on the substrate, an air cavity is disposed between the substrate and the diaphragm, the air cavity is connected to the back cavity, the radial profile of the air cavity is larger than the radial profile of the back cavity, and the back cavity is within the axial projection range of the air cavity.

[0006] The non-uniform diameter cavity-structured back-cavity emitting piezoelectric micro-ultrasonic transducer provided in this application features a diaphragm suspension area larger than the radial cross-sectional area of ​​the back cavity. By setting a solid support region at the edge of the diaphragm (where the substrate and diaphragm are directly connected), a differential coupling region is formed between the back cavity and the diaphragm, effectively controlling the acoustic energy distribution and air spring effect. This achieves broadband characteristics, increases the emitted sound pressure, and suppresses higher-order modes. Simultaneously, the diaphragm size and the back cavity size can be set separately according to different performance requirements, allowing the mechanical vibration characteristics of the transducer and the acoustic response of the back cavity to be controlled independently without mutual restriction, effectively improving overall acoustic performance and design freedom. It is suitable for various scenarios such as distance measurement, robot perception, gesture recognition, and gas detection.

[0007] Furthermore, a piezoelectric oscillator is provided on the side of the diaphragm facing away from the back cavity, and the area of ​​the piezoelectric oscillator is smaller than that of the diaphragm.

[0008] Furthermore, the diaphragm is provided with a central hole that connects to the back cavity, and the central hole penetrates the diaphragm and the piezoelectric oscillator.

[0009] Furthermore, the piezoelectric oscillator stack includes a bottom electrode, a piezoelectric thin film layer, and a top electrode in sequence from near the back cavity to away from the back cavity, and the overlapping area of ​​the bottom electrode, the piezoelectric thin film layer, and the top electrode is located within the axial projection range of the back cavity.

[0010] Furthermore, the substrate has a pad area on the side near the piezoelectric oscillator, the pad area being outside the axial projection of the air cavity, the piezoelectric oscillator having an extension connected to the pad area, the pad area having a first pad and a second pad that do not overlap, the piezoelectric thin film layer and the top electrode being connected to the first pad via the extension, and the bottom electrode being connected to the second pad via the extension.

[0011] Furthermore, the material of the piezoelectric thin film layer is selected from one of the following: single-crystal aluminum nitride, polycrystalline aluminum nitride, zinc oxide, PZT ceramic, lithium niobate, lithium tantalate, and polyvinylidene fluoride.

[0012] Furthermore, the materials of the bottom electrode and the top electrode are each independently selected from one of molybdenum, tungsten, platinum, titanium, gold, and ruthenium.

[0013] Furthermore, a first protective layer is provided on the side of the piezoelectric oscillator away from the back cavity, and a second protective layer is provided on the side of the substrate close to the piezoelectric oscillator. The materials of the first protective layer and the second protective layer are each independently selected from one of aluminum nitride, silicon oxide, silicon carbide, and silicon nitride.

[0014] Furthermore, the radial cross-section of the back cavity is circular, and the radial cross-section of the air cavity is circular or rectangular.

[0015] Furthermore, in the direction from near the diaphragm to away from the diaphragm, except for the area that is in the same plane as the air cavity, the radial cross section of the back cavity increases from small to large.

[0016] Furthermore, the inner wall of the back cavity is divided into multiple steps.

[0017] Furthermore, the back cavity is frustum-shaped.

[0018] Furthermore, the thickness of the air cavity is 0.2μm~1.5μm.

[0019] Furthermore, the substrate material is selected from one of the following: non-metallic elements, silicon oxide, silicon carbide, sapphire, and lithium gallium oxide.

[0020] Secondly, this application provides a method for fabricating a non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer, including the following steps: A sacrificial layer is constructed on the substrate; Annular grooves are etched into the sacrificial layer; A diaphragm is deposited on the sacrificial layer, and the annular groove is deposited into an annular stop gate. The substrate is etched from the side without the diaphragm, down to the sacrificial layer, with the etching range extending within the axial projection of the cut-off gate, to obtain the back cavity; The sacrificial layer within the area surrounding the cutoff gate is removed through the back cavity to obtain an air cavity.

[0021] The beneficial effects of this invention are: it achieves a better overall balance between sensitivity and sound pressure level. The non-uniform diameter structure allows the diaphragm edge to fall on the solid support area, thereby suppressing edge displacement of higher-order modes and improving vibration boundary stability, effectively enhancing the acoustic energy coupling efficiency of the main mode, and improving the single-peak characteristics and signal purity of the emitted waveform. Simultaneously, because the effective acoustic radiation area of ​​the back cavity and the stiffness of the air spring can be flexibly adjusted, this structure can significantly improve far-field sound pressure level, enhance the ultrasonic radiation capability of the transducer, and expand the achievable operating bandwidth. Furthermore, the independent adjustability of the back cavity and diaphragm dimensions improves process tolerance, making the device insensitive to back etching diameter deviations, thereby improving the consistency and reliability of the device.

[0022] Other features and advantages of this application will be set forth in the following description and will be apparent in part from the description or may be learned by practicing the application. The objectives and other advantages of this application may be realized and obtained by means of the structures particularly pointed out in the written description and the accompanying drawings. Attached Figure Description

[0023] Figure 1This is one of the cross-sectional views of a non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer provided in the embodiments of this application.

[0024] Figure 2 This is a schematic diagram of the structure of a non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer after partial cross-section, provided in an embodiment of this application.

[0025] Figure 3 This is a schematic diagram illustrating a method for fabricating a non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer provided in an embodiment of this application.

[0026] Figure 4 This is the second cross-sectional view of a non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer provided in the embodiments of this application.

[0027] Figure 5 This is the third cross-sectional view of a non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer provided in the embodiments of this application.

[0028] Figure 6 This is one of the bottom views of a non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer provided in the embodiments of this application.

[0029] Figure 7 This is a second bottom view of a non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer provided in the embodiments of this application.

[0030] Figure 8 The graph shows the output voltage sensitivity test results for each embodiment.

[0031] Reference numerals: 100, substrate; 101, diaphragm; 102, air cavity; 103, back cavity; 106, second pad; 107, extension segment; 108, first pad; 109, center hole; 300, bottom electrode; 400, piezoelectric thin film layer; 500, top electrode; 600, first protective layer; 601, second protective layer; 110, sacrificial layer; 111, annular groove; 112, stop gate. Detailed Implementation

[0032] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0033] The following disclosure provides many different embodiments or examples for implementing different structures of the invention. To simplify the disclosure, specific examples of components and arrangements are described below. Of course, these are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed.

[0034] It should be understood that, without conflict, any and all embodiments of the present invention can be combined with technical features of any other embodiment or multiple other embodiments to obtain other embodiments. The present invention includes such combinations to obtain other embodiments.

[0035] Unless otherwise specified, all technical and scientific terms used herein have the standard meaning in the field to which the claimed subject matter pertains. Where multiple definitions exist for a term, the definition herein shall prevail.

[0036] A typical design for the back cavity 103 emission structure is that the diameter of the diaphragm 101 is the same as the diameter of the back cavity 103. This structure is easy to manufacture, but it has the following problems: Limited acoustic coupling efficiency: The maximum stress point at the edge of the diaphragm 101 coincides with the edge of the back cavity 103, resulting in excessive coupling of high-order mode energy and insufficient acoustic energy dissipation efficiency.

[0037] High quality factor and narrow bandwidth: When the diaphragm 101 and the back cavity 103 are perfectly matched in size, the acoustic damping is small and the Q value of the PMUT (piezoelectric micromechanical ultrasonic transducer) is too high. Under air coupling conditions, this results in a narrow bandwidth, which is not suitable for applications such as broadband ultrasonic ranging, frequency modulation excitation, and coded excitation.

[0038] Weak resistance to process deviations: The diameter of the back cavity 103 formed by back etching usually has a dimensional deviation of ±1–3 μm. When the diameter of the back cavity 103 is the same as that of the diaphragm 101, the deviation will significantly affect the performance stability.

[0039] Sound pressure limitation: Due to the limitations of the target resonant frequency and sensitivity of the diaphragm 101, the size of the back cavity 103 cannot be designed and determined independently according to acoustic principles.

[0040] In summary, existing medium-diameter cavity-membrane structures cannot simultaneously meet the comprehensive requirements of high sound pressure, wide bandwidth, clean modes, and strong resistance to deviations, and a better structural design method is urgently needed.

[0041] To meet the demands of most practical applications for ultrasonic transducers that typically require wide bandwidth, high sound pressure level output, high signal purity, and strong structural reliability, we need a structural design that can optimize acoustic load, quality factor, and modal distribution by adjusting the geometric relationship between the diaphragm and the back cavity. (Refer to...) Figure 1 and Figure 2 This application provides a non-uniform diameter cavity-film structure back-cavity emission type piezoelectric micro ultrasonic transducer, including a substrate 100, a back cavity 103 and a diaphragm 101 that blocks the back cavity 103 at one end disposed on the substrate 100, and an air cavity 102 disposed between the substrate 100 and the diaphragm 101. The air cavity 102 communicates with the back cavity 103, and the radial profile of the air cavity 102 is larger than the radial profile of the back cavity 103. The back cavity 103 is within the axial projection range of the air cavity 102. Radially, both the back cavity 103 and the air cavity 102 are within the surrounding range of the substrate 100. Here, the axial direction refers to the direction perpendicular to the diaphragm 101, and correspondingly, the radial direction is the direction perpendicular to the axial direction.

[0042] Reference Figure 1 In this embodiment, the diameter d of the diaphragm 101 and the diameter D of the back cavity 103 are designed to be different sizes to form a differentially coupled acoustic structure, thereby achieving a high sound pressure, wide bandwidth, and strong stability ultrasonic transduction effect. The diameter of the diaphragm 101 is larger than the diameter of the back cavity 103, and a solid support area is retained between the edge of the diaphragm 101 and the back cavity 103.

[0043] Reference Figure 3 The fabrication method of this non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer is as follows, including the following steps: S1: Construct a sacrificial layer 110 on substrate 100. When the substrate is silicon, the substrate surface can be thermally oxidized to use silicon oxide as the sacrificial layer. When the substrate is made of other materials, the sacrificial layer can be formed by deposition (PECVD or ALD). Figure 3 middle The substrate material in the example is monocrystalline silicon, which becomes silicon dioxide after thermal oxidation. Since the substrate is thermally oxidized as a whole, sacrificial layers are formed on both sides, with each sacrificial layer being about 3 μm thick.

[0044] S2: An annular groove 111 is etched into the sacrificial layer 110. For example... Figure 3 middle The annular groove 111 extends to the unoxidized portion of the substrate 100. For safety, Figure 3 In the corresponding embodiment, two coaxial annular grooves 111 are provided.

[0045] S3: A diaphragm 101 is deposited on the sacrificial layer 110, and simultaneously, the annular groove 111 is deposited into an annular stop gate 112, as shown. Figure 3 middle The deposited material is selected from one of polycrystalline silicon, SiC, SiN, and AlN. Figure 3 As can be seen in the corresponding embodiments, the cutoff gate and the diaphragm are made of the same material. Figure 3 middle This indicates that the diaphragm is thinned by CMP polishing. Figure 3 middle This indicates that the back side of the substrate was thinned (while removing the excess sacrificial layer on the other side) to approximately 400 μm.

[0046] S4: Etch the substrate 100 from the side without the diaphragm 101, down to the sacrificial layer 110, within the axial projection range of the stop gate 112, to obtain the back cavity 103, as shown. Figure 3 middle . Figure 3 middle to The preparation of piezoelectric oscillators will be explained in detail later.

[0047] S5: The sacrificial layer 110 within the surrounding area of ​​the stop gate 112 is removed through the back cavity 103 to obtain the air cavity 102, as follows: Figure 3 middle .

[0048] In some embodiments, refer to Figure 6 The radial cross-section of the back cavity 103 is circular, and the radial cross-section of the air cavity 102 is circular. Figure 6 and Figure 7 In the diagram, unfilled areas represent portions containing only substrate material, while gray-filled areas represent portions containing other materials (e.g., a protective layer). The inner dashed line represents the area of ​​the back cavity 103, and the outer dashed line represents the area of ​​the air cavity 102.

[0049] Based on acoustic theory and engineering experience, at the first-order modal frequency of the circular diaphragm 101, the diaphragm diameter d is related to the resonant frequency f, and the equivalent diaphragm thickness h is related to the diaphragm diameter d, the resonant frequency f, and the equivalent diaphragm thickness h. eff Receiver sensitivity S rec Residual stress σ tension The design relationships are as follows: ; The design parameters related to the cavity diameter D, far-field sound pressure level (SPL), quality factor (Q), bandwidth (BW), and ring-down region are expressed as follows: ; It is evident that the diameter of the diaphragm 101 needs to be designed around the resonant frequency and sensitivity, while the diameter of the back cavity 103 needs to be designed around acoustic loads such as sound pressure level, bandwidth, and residual vibration time. The design goals of the two are not entirely consistent, and even partially contradictory, therefore it is impossible to simultaneously meet all performance requirements using a constant-diameter structure. This invention employs a non-constant-diameter cavity-diaphragm structure, allowing the diaphragm 101 and back cavity 103 to be optimized separately, thereby simultaneously meeting multiple performance requirements such as resonant frequency, sensitivity, sound pressure level, bandwidth, and residual vibration. At the same time, this design improves process tolerance and has resistance to back erosion deviation. Back erosion diameter deviations of ±1–3 μm no longer affect the effective vibration zone of the diaphragm 101, reducing performance fluctuations to less than 20% of those of traditional structures.

[0050] In some embodiments, refer to Figure 7 The radial cross-section of the back cavity 103 is circular, while the radial cross-section of the air cavity 102 is square. In conventional constant-diameter structures, the shape of the diaphragm 101 needs to be the same as the radial cross-sectional shape of the back cavity 103. A rectangular diaphragm 101 is more advantageous in certain applications requiring specific directionality or better matching with rectangular chip layouts. The embodiments of this application provide flexibility in shape, allowing designers to choose according to their final acoustic performance goals.

[0051] Reference Figure 1 , Figure 4 and Figure 5 A piezoelectric oscillator is disposed on the side of the diaphragm 101 facing away from the back cavity 103, and the area of ​​the piezoelectric oscillator is smaller than that of the diaphragm 101. A central hole 109 is disposed on the diaphragm, connecting the back cavity 103 and penetrating the diaphragm 101 and the piezoelectric oscillator. The piezoelectric oscillator includes a bottom electrode 300, a piezoelectric thin film layer 400, and a top electrode 500 in sequence from the direction near the back cavity 103 to the direction away from the back cavity 103. The overlapping area of ​​the bottom electrode 300, the piezoelectric thin film layer 400, and the top electrode 500 is located within the axial projection range of the back cavity 103.

[0052] Reference Figure 2 The substrate 100 has a pad area on the side near the piezoelectric oscillator, which extends beyond the axial projection of the air cavity 102. The piezoelectric oscillator has an extension 107 connecting to the pad area. The pad area has a first pad 108 and a second pad 106 that do not overlap. The piezoelectric thin film layer 400 and the top electrode 500 are connected to the first pad 108 via the extension 107, and the bottom electrode 300 is connected to the second pad 106 via the extension 107. It should be noted that... Figure 6 and Figure 7 The focus is on showing the shape; only the extension segment is drawn, not the pad area, which does not represent... Figure 6 and Figure 7This embodiment does not require a pad area. A first protective layer 600 is provided on the side of the piezoelectric oscillator away from the back cavity 103, and a second protective layer 601 is provided on the side of the substrate 100 near the piezoelectric oscillator. The materials of the first protective layer 600 and the second protective layer 601 are each independently selected from one of AlN, SiO2, SiC, and Si3N4. When the first protective layer 600 and the second protective layer 601 are made of the same material, the first protective layer 600 and the second protective layer 601 can be connected into one piece, i.e., as shown... Figure 6 , Figure 7 Like the filled part.

[0053] The material of the piezoelectric thin film layer 400 is selected from one of the following: single crystal AlN, polycrystalline AlN, ZnO, PZT, LiNbO3, LiTaO3, and PVDF.

[0054] The material of the bottom electrode 300 is selected from one of Mo, W, Pt, Ti, Au, and Ru.

[0055] The material of the top electrode 500 is selected from one of Mo, W, Pt, Ti, Au, and Ru.

[0056] The substrate 100 is made of a material selected from non-metallic elements (such as Si), SiO2, SiC, sapphire, and LiGaO2.

[0057] Reference Figure 3 The detailed steps for the fabrication of a piezoelectric oscillator are as follows; to A cutoff gate is etched onto a hot-oxidized sheet.

[0058] Polycrystalline layers are deposited via LPCVD or plasma-enhanced chemical vapor deposition (PECVD). Figure 3 Example: polycrystalline silicon (formed as diaphragm and cut-off gate). Other materials may be selected from non-metallic elements (such as Si), SiO2, SiC, sapphire, and LiGaO2.

[0059] to First, a bottom electrode 300 is deposited, with the material selected from Mo, W, Pt, Ti, Au, and Ru. The piezoelectric thin film layer 400 is deposited, with the material selected from single-crystal AlN, polycrystalline AlN, ZnO, PZT, LiNbO3, LiTaO3, and PVDF, and is prepared using a sol-gel method, magnetron sputtering, or thin film deposition method. Specifically, a 200 nm Mo bottom electrode can be grown first, followed by a 20 nm aluminum nitride seed layer on the bottom electrode, and then a 1 μm aluminum nitride piezoelectric layer on the aluminum nitride seed layer.

[0060] The top electrode is deposited 500 nm thick using sputtering or evaporation processes, with the same material as the bottom electrode. Specifically, a Mo top electrode with a thickness of 200 nm is used.

[0061] The top electrode is etched using either dry or wet etching methods.

[0062] to The piezoelectric layer and bottom electrode are etched using either dry or wet etching methods. to The etching is for the formation of piezoelectric oscillators, such as Figure 2 A ring structure.

[0063] The growth protective layer (the first and second protective layers are generated simultaneously) is made of one of the inorganic materials such as AlN, SiO2, SiC, and Si3N4. The etched protective layer is specifically removed from the following areas: the solder pad area and the area on the diaphragm without piezoelectric resonant stacks.

[0064] Etching of the polycrystalline layer (thinning, etching 200nm top silicon) using either dry or wet etching methods.

[0065] The metal pads are grown using materials selected from Mo, W, Pt, Ti, Au, and Ru.

[0066] Dry or wet etching of the center through hole.

[0067] The back cavity is etched using processes such as anisotropic wet etching or deep reactive ion etching (DRIE).

[0068] Release the heat-oxygen layer. In some embodiments, the radial cross section of the back cavity 103 increases from near the diaphragm 101 to away from the diaphragm 101, except for the area that is in the same plane as the air cavity 102 (the plane is parallel to the diaphragm 101).

[0069] For example Figure 4 The diaphragm 101 adopts a circular structure, and the back cavity 103 adopts a frustum geometry, that is, the opening is smaller on the side closer to the diaphragm 101 and larger on the side farther away from the diaphragm 101. The diameter of the top opening of the back cavity 103 is significantly smaller than the effective diameter of the diaphragm 101, and the bottom opening can be equal to or larger than the diameter of the diaphragm 101.

[0070] For example, the inner wall of the dorsal cavity 103 is divided into multiple steps. Specifically, as shown... Figure 5 The back cavity 103 adopts a two-stage or three-stage ( Figure 5 It is a structure composed of three stepped cavities of different diameters and depths, with each step transitioning through a step difference. The opening diameter of the uppermost layer is smaller than the diameter of the diaphragm 101.

[0071] Preferably, the thickness of the air cavity 102 is 0.2 μm to 1.5 μm. The thickness of the air cavity 102 refers to its dimension in the direction perpendicular to the diaphragm, and can also be considered as the depth of the countersunk hole. The thickness of the air cavity 102 is related to the required cutoff layer thickness in the etching process of the back cavity 103, and also to the downward limit vibration displacement of the diaphragm 101 during operation. Based on this, the thickness of the air cavity 102 is minimized as much as possible to make the pmut back cavity 103 conform to acoustic design. Calculations show that a suitable thickness of 1 μm for the air cavity 102 at commonly used pmut frequencies is 1 μm.

[0072] Example 1 according to Figure 6 The non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer is fabricated, that is, the back cavity 103 and the diaphragm 101 are uniformly circular, the diameter of the back cavity 103 is 860μm, and the diameter of the air cavity 102 is 900μm. The diameter difference "40" represents Example 1.

[0073] Example 2 according to Figure 6 The non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer is fabricated, that is, the back cavity 103 and the diaphragm 101 are uniformly circular, the diameter of the back cavity 103 is 860μm, and the diameter of the air cavity 102 is 920μm. The diameter difference "60" represents Example 2.

[0074] Example 3 according to Figure 6 The non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer is fabricated, that is, the back cavity 103 and the diaphragm 101 are uniformly circular, the diameter of the back cavity 103 is 860μm, and the diameter of the air cavity 102 is 940μm. The diameter difference "80" represents Example 3.

[0075] Example 4 according to Figure 6 The non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer is fabricated, that is, the back cavity 103 and the diaphragm 101 are uniformly circular, the diameter of the back cavity 103 is 860μm, and the diameter of the air cavity 102 is 960μm. The diameter difference "100" represents Example 4.

[0076] Example 5 according to Figure 6The non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer is fabricated, that is, the back cavity 103 and the diaphragm 101 are uniformly circular, the diameter of the back cavity 103 is 860μm, and the diameter of the air cavity 102 is 980μm. The diameter difference "120" represents Example 5.

[0077] Output voltage sensitivity experiments were conducted on Examples 1, 2, 3, 4, and 5. The receiver output voltage sensitivity was measured. Specifically, a calibrated sound source was used to apply a sinusoidal sound pressure excitation of known amplitude to the PMUT, and the open-circuit output voltage across the device was measured in receiving mode. The output voltage as a function of frequency was obtained by frequency sweeping, and then divided by the incident sound pressure to obtain the receiver output voltage sensitivity (V / Pa) response curve as a function of frequency. During the test, the PMUT was in open-circuit condition without DC bias to characterize the intrinsic acoustic-to-electrical conversion characteristics of the device. The results are as follows: Figure 8 As shown. Figure 8 In the graph, the horizontal axis represents frequency (kHz), and the vertical axis represents voltage sensitivity (V / Pa).

[0078] It can be seen that within this diameter difference range, the larger the diameter difference, the lower the resonant frequency and the higher the output voltage sensitivity.

[0079] In this invention, the diaphragm diameter and the cavity diameter can be set independently according to different application requirements: the geometry of the diaphragm 101 is mainly used to determine the transducer's inherent resonant frequency and electromechanical sensitivity, while the diameter and depth of the cavity 103 are used to regulate the air spring effect, acoustic damping, acoustic radiation impedance, and far-field sound pressure output. Through this structural design that decouples the "mechanical vibration parameters" from the "acoustic load parameters," this invention can achieve a better overall balance between sensitivity and sound pressure.

[0080] Compared to existing common constant-diameter cavity-film PMUT structures, this invention achieves comprehensive optimization of adjustable resonant frequency, controllable sensitivity, enhanced bandwidth, and increased sound pressure level while maintaining MEMS (Micro-Electro-Mechanical Systems) process compatibility and structural compactness. This provides a novel structural solution for the miniaturization, broadbanding, and high sound pressure level of high-performance air-coupled ultrasonic transducers and their arrays. This ultrasonic transducer not only overcomes the shortcomings of traditional thin-film forward emission but also provides a new design concept and implementation approach for future high-sensitivity, long-distance, and low-power ultrasonic sensing applications.

[0081] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., indicate that a specific feature, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0082] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer, comprising a substrate (100), wherein a back cavity (103) is disposed on the substrate (100) and a diaphragm (101) that blocks the back cavity (103) at one end, characterized in that, An air cavity (102) is provided between the substrate (100) and the diaphragm (101). The air cavity (102) is connected to the back cavity (103). The radial profile of the air cavity (102) is larger than the radial profile of the back cavity (103). The back cavity (103) is within the axial projection range of the air cavity (102).

2. The non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer according to claim 1, characterized in that, The diaphragm (101) has a piezoelectric oscillator on the side facing away from the back cavity (103), and the area of ​​the piezoelectric oscillator is smaller than that of the diaphragm (101).

3. The non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer according to claim 2, characterized in that, The diaphragm (101) is provided with a central hole (109) that connects to the back cavity (103), and the central hole (109) penetrates the diaphragm (101) and the piezoelectric oscillator.

4. The non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer according to claim 2, characterized in that, The piezoelectric oscillator includes a bottom electrode (300), a piezoelectric thin film layer (400), and a top electrode (500) in sequence from the direction near the back cavity (103) to the direction away from the back cavity (103). The overlapping area of ​​the bottom electrode (300), the piezoelectric thin film layer (400), and the top electrode (500) is located within the axial projection range of the back cavity (103).

5. The non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer according to claim 4, characterized in that, The substrate (100) has a pad area on the side near the piezoelectric oscillator. The pad area is outside the axial projection of the air cavity (102). The piezoelectric oscillator has an extension (107) connected to the pad area. The pad area has a first pad (108) and a second pad (106) that do not overlap. The piezoelectric thin film layer (400) and the top electrode (500) are connected to the first pad (108) via the extension (107). The bottom electrode (300) is connected to the second pad (106) via the extension (107).

6. The non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer according to claim 2, characterized in that, A first protective layer (600) is provided on the side of the piezoelectric oscillator away from the back cavity (103), and a second protective layer (601) is provided on the side of the substrate (100) close to the piezoelectric oscillator. The materials of the first protective layer (600) and the second protective layer (601) are each independently selected from one of aluminum nitride, silicon oxide, silicon carbide and silicon nitride.

7. The non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer according to claim 1, characterized in that, The radial cross-section of the back cavity (103) is circular, and the radial cross-section of the air cavity (102) is circular or rectangular.

8. The non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer according to claim 1, characterized in that, In the direction from the diaphragm (101) away from the diaphragm (101), the radial cross section of the back cavity (103) increases from small to large, except for the area that is in the same plane as the air cavity (102).

9. The non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer according to claim 1, characterized in that, The thickness of the air cavity (102) is 0.2μm~1.5μm.

10. A method for fabricating a non-uniform diameter cavity membrane structure back cavity emission type piezoelectric micro ultrasonic transducer, characterized in that, Includes the following steps: A sacrificial layer (110) is constructed on the surface of the substrate (100); A ring groove (111) is etched on the sacrificial layer (110); A diaphragm (101) is deposited on the sacrificial layer (110), while the annular groove (111) is deposited into an annular stop gate (112). The substrate (100) is etched from the side without the diaphragm (101) to the sacrificial layer (110), with the etching range within the axial projection range of the stop gate (112), to obtain the back cavity (103). The sacrificial layer (110) within the area surrounding the cutoff gate (112) is removed through the back cavity (103) to obtain the air cavity (102).

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