Multi-section acoustic packaging structure of PMUT ultrasonic transducer and optimization method thereof

Abstract

This invention belongs to the field of ultrasonic sensing and microelectromechanical systems (MEMS) packaging technology, specifically relating to a multi-segment acoustic packaging structure and optimization method for a PMUT ultrasonic transducer. The multi-segment acoustic packaging structure for a PMUT ultrasonic transducer includes a base, a PMUT chip, and an acoustic cavity. The acoustic cavity includes a front cavity section, a tapered transition section, a conduit section, and a waterproof and acoustically permeable membrane. A first minimum gap is formed between the radiating surface of the PMUT chip and the bottom surface of the front cavity section; a second minimum gap is formed between the waterproof and acoustically permeable membrane and the outlet end of the conduit section. Furthermore, the acoustic cavity serves as either a transmitter or a receiver. When used as a transmitter, the depth of the front cavity section is greater than that of the front cavity section when used as a receiver, and the inner diameter of the conduit section is smaller than that of the conduit section when used as a receiver. This packaging structure, through optimization of geometric parameters and acoustic characteristics, achieves differentiated performance improvements for the transmitter and receiver, increasing the transmitted sound pressure and directivity, and expanding the bandwidth and receiving angle of the receiver.

Description

Technical Field

[0001] This invention belongs to the field of ultrasonic sensing and microelectromechanical systems (MEMS) packaging technology, specifically relating to a multi-segment acoustic packaging structure for a PMUT ultrasonic transducer and its optimization method. Background Technology

[0002] In existing ultrasonic sensing technologies, ultrasonic transducers operating in air or gas environments traditionally employ piezoelectric ceramic structures. However, these structures have significant limitations, such as large size leading to low integration, limited bandwidth making it difficult to meet high-precision measurement requirements, and complex manufacturing resulting in high costs. With the rapid development of microelectromechanical systems (MEMS) technology, piezoelectric MEMS ultrasonic transducers (PMUTs) are gradually showing broad application prospects in ultrasonic ranging and flow measurement due to their advantages such as small size, low power consumption, and ease of integration.

[0003] Despite its numerous advantages, the PMUT (Pressure Probe Unlocked) has a relatively small effective radiating area and a significant difference in acoustic impedance compared to air or fuel gas, leading to a series of acoustic problems. Specifically, the PMUT exhibits low sound pressure levels and unstable directionality during direct radiation, resulting in rapid sound field divergence and boundary reflection interference that affects measurement accuracy. Furthermore, the PMUT suffers from a long resonance tail, reducing the accuracy of time-of-flight (TOF) measurements. In waterproof applications, the waterproof membrane easily adheres to the duct opening, causing sound attenuation or even failure. More importantly, using the same packaging structure for both the transmitter and receiver makes it difficult to simultaneously meet the dual requirements of high sound pressure output and wide bandwidth reception.

[0004] Meanwhile, coaxial errors during cavity manufacturing can cause beam skew, further reducing the stability of through-beam TOF. Summary of the Invention

[0005] To address the aforementioned shortcomings of existing technologies, this invention provides a multi-segment acoustic packaging structure for PMUT ultrasonic transducers and its optimization method. This solves the problem that existing gas medium PMUT ultrasonic transducers, due to their symmetrical packaging structure, cannot simultaneously meet the requirements of high sound pressure and strong directivity at the transmitting end and wide bandwidth and large receiving angle at the receiving end. It also overcomes the limitation that conventional two-segment matching layer structures are not suitable for low-energy, multi-modal PMUTs.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A multi-segment acoustic packaging structure for a PMUT ultrasonic transducer is provided, including a base, a PMUT chip disposed inside the base, the PMUT chip being disposed on a PMUT chip main control board, an acoustic cavity disposed in the middle of the base, the acoustic cavity being disposed above the PMUT chip, and the acoustic cavity comprising, from bottom to top, a front cavity segment disposed opposite to the PMUT chip, a conical transition segment communicating with the front cavity segment, and a conduit segment communicating with the conical transition segment, and a waterproof and sound-permeable membrane disposed on the outer side of the outlet end of the conduit segment; A first minimum gap is formed between the radiating surface of the PMUT chip and the bottom surface of the front cavity segment; a second minimum gap is formed between the waterproof and acoustic membrane and the outlet end of the conduit segment. The acoustic cavity serves as the transmitter or receiver of the PMUT ultrasonic transducer. The depth of the front cavity section of the acoustic cavity serving as the transmitter is greater than the depth of the front cavity section of the acoustic cavity serving as the receiver, and the inner diameter of the conduit section of the acoustic cavity serving as the transmitter is smaller than the inner diameter of the conduit section of the acoustic cavity serving as the receiver.

[0007] Furthermore, the front cavity segment, tapered transition segment, and conduit segment in the multi-segment acoustic encapsulation structure differ in at least one aspect: effective acoustic impedance, cross-sectional size or rate of change of cross-section, equivalent acoustic path length, inner wall roughness, or sound absorption characteristics, and the front cavity segment, tapered transition segment, and conduit segment cannot be combined into an equivalent structure by a single equivalent acoustic parameter.

[0008] The beneficial effects of adopting the above technical solution are as follows: This multi-segment acoustic encapsulation structure integrates a front cavity segment, a conical transition segment, a conduit segment, and a waterproof and acoustically permeable membrane. By differentially optimizing the geometric parameters of each segment to meet the different acoustic requirements of transmission and reception, it achieves differentiated performance enhancements at both the transmitting and receiving ends. This not only significantly improves the transmitted sound pressure and directivity, ensuring stable propagation of sound waves in air or gas media, but also expands the bandwidth and receiving angle of the receiving end, enhancing the adaptability and stability of signal reception, thereby improving the overall performance of the PMUT ultrasonic transducer in gaseous media. Specifically, by constructing at least three differentiated acoustic functional zones and coordinating their design based on the vibration characteristics of the PMUT, the acoustic impedance gradient and sound field distribution can be precisely controlled. This effectively suppresses non-axial acoustic energy leakage caused by the low energy and multimodal characteristics of the PMUT, guiding energy more concentratedly to axial output, thus solving the performance bottleneck of conventional two-segment structures in PMUT applications. The front cavity section, with its larger inner diameter and appropriate height, effectively expands the effective radiation area of ​​the PMUT and enhances near-field sound pressure. When the acoustic cavity serves as the transmitting end of the PMUT ultrasonic transducer, the deeper front cavity section further increases the equivalent acoustic impedance within the cavity, contributing to the enhancement of sound pressure. Conversely, when used as the receiving end, the shallower front cavity section reduces cavity resonance, widens the frequency response range, and reduces signal distortion. The tapered transition section, with its continuously decreasing inner diameter along the height direction, forms a gradually changing acoustic impedance structure, effectively reducing sound wave reflection and scattering during propagation. When the acoustic cavity serves as the transmitting end of the PMUT ultrasonic transducer, the tapered transition section acts as an acoustic energy compression section, concentrating sound wave energy into the conduit section, improving sound pressure and directivity. In the receiving end, the tapered transition section acts as a smooth transition section, reducing step reflections and cavity standing waves, maintaining signal purity and stability. By using a specific length-to-diameter ratio, the conduit segment can create a quasi-plane sound velocity in the air medium, significantly improving the directionality of sound waves and the stability of TOF measurements. When the acoustic cavity can be used as the transmitting end of a PMUT ultrasonic transducer, the conduit segment, with its smaller inner diameter and longer length, can suppress higher-order propagation modes and enhance the collimation of the sound beam. When used as the receiving end, the larger inner diameter and shorter length reduce propagation loss and high-frequency attenuation, expand the receiving angle, and improve the sensitivity of signal reception.The waterproof and acoustically permeable membrane is placed on the outside of the outlet end of the conduit section, and a second minimum gap is formed between the waterproof and acoustically permeable membrane and the outlet end of the conduit section. This effectively prevents the adsorption of the waterproof membrane under the action of gas pressure difference, which not only ensures the unobstructed propagation of sound waves, but also avoids sound attenuation and signal distortion caused by membrane adsorption, thus improving the reliability and stability of the transducer in complex environments. The first minimum gap formed between the radiation surface of the PMUT chip and the bottom surface of the front cavity section ensures the free vibration conditions of the diaphragm during operation, avoiding the resonant frequency shift and sensitivity reduction caused by the solid wall loading effect and cavity compression effect. This ensures the stable operation of the PMUT chip in air or gas medium, and improves the sound radiation efficiency and signal quality.

[0009] Furthermore, the height of the anterior lumen segment is 0.3 to 1.5 times the height of the catheter segment.

[0010] Furthermore, the height of the anterior lumen segment is 0.5 to 1.2 times the height of the catheter segment.

[0011] Furthermore, the height of the first minimum gap According to the center operating frequency of the PMUT chip The calculation relationship is as follows:

[0012] in, The height of the first minimum gap is expressed in millimeters (mm). This is the center operating frequency of the PMUT chip, measured in kilohertz (kHz). These are dimensionless coefficients related to the acoustic boundary conditions; The speed of sound in air is measured in meters per second (m / s).

[0013] The beneficial effects of adopting the above technical solution are: by utilizing the center operating frequency of the PMUT chip... f Speed ​​of sound in air c Combined with the dimensionless coefficient K, which is closely related to the acoustic boundary conditions, it can be dynamically adjusted. The value of is adjusted to ensure that the diaphragm is in a near-free boundary condition. This not only avoids the solid-wall loading effect and resonant frequency shift caused by too small a gap, but also prevents the formation of standing wave cavity and phase distortion caused by too large a gap, thereby improving the sound radiation efficiency and enhancing the stability of the resonant frequency.

[0014] Furthermore, the dimensionless coefficients related to acoustic boundary conditions The value range is 0.05≤ ≤0.30.

[0015] Furthermore, the dimensionless coefficients related to the acoustic boundary conditions The value range is 0.10≤ ≤0.20.

[0016] Furthermore, the inner diameter of the tapered transition section continuously narrows along its axial direction from the end connected to the anterior lumen section to the end connected to the catheter section. The end of the tapered transition section near the anterior lumen section is the larger end, and the end of the tapered transition section near the catheter section is the smaller end.

[0017] The beneficial effects of adopting the above technical solution are as follows: the inner diameter of the tapered transition section continuously narrows along its axial direction from the large end connected to the front cavity section to the small end connected to the conduit section, realizing a gradual transition of acoustic impedance and effectively reducing the reflection and scattering losses of sound waves during propagation. Furthermore, when the acoustic cavity can be used as the transmitting end of the PMUT ultrasonic transducer, the sound wave energy is gradually compressed and concentrated to the conduit section through the tapered transition section, enhancing sound pressure and directivity. When the acoustic cavity can be used as the receiving end of the PMUT ultrasonic transducer, the tapered transition section reduces step reflection and cavity standing waves through a smooth transition, maintaining signal purity and stability, thus simultaneously meeting the requirements of the transmitting end for high sound pressure and strong directivity, and the receiving end for wide bandwidth and large receiving angle.

[0018] Furthermore, the inner diameter of the anterior cavity segment is larger than the characteristic diameter of the effective radiation surface of the PMUT chip, while the inner diameter of the catheter segment is smaller than the inner diameter of the anterior cavity segment.

[0019] The beneficial effects of adopting the above technical solution are as follows: when the inner diameter of the front cavity section is larger than the characteristic diameter of the effective radiation surface of the PMUT chip, the acoustic radiation area can be effectively expanded and the acoustic impedance change can be reduced, thereby enhancing the near-field sound pressure and reducing the initial reflection; while when the inner diameter of the conduit section is smaller than that of the front cavity section, the acoustic impedance gradient is formed, which further concentrates the sound wave in the small cross-section area, thereby improving the sound pressure intensity and directionality of the transmitting end, and optimizing the signal purity and bandwidth of the receiving end by suppressing the propagation of higher-order modes.

[0020] Furthermore, the proportional relationship between the length of the catheter segment and its inner diameter is as follows:

[0021] in, The length of the catheter segment; This refers to the inner diameter of the catheter segment.

[0022] The beneficial effects of adopting the above technical solution are as follows: the ratio of the length of the duct segment to its inner diameter is kept between 1.0 and 4.0, which ensures that the duct segment forms a stable quasi-planar sound beam in the air medium. It suppresses the divergence of higher-order sound modes and enhances the directionality through sufficient length, while avoiding high-frequency attenuation and propagation loss caused by excessive length, thereby effectively balancing the directionality and energy loss of sound wave propagation.

[0023] Furthermore, the proportional relationship between the length of the catheter segment and its inner diameter is as follows:

[0024] in, The length of the catheter segment; This refers to the inner diameter of the catheter segment.

[0025] Furthermore, the surface roughness of the inner wall of the acoustic cavity ≤3.2μm.

[0026] Furthermore, the surface roughness of the inner wall of the acoustic cavity ≤1.6μm.

[0027] Furthermore, the surface roughness of the inner wall of the acoustic cavity ≤0.8μm.

[0028] Based on the above-mentioned multi-segment packaging structure of PMUT ultrasonic transducers, this invention provides an optimization method for the multi-segment acoustic packaging structure of PMUT ultrasonic transducers, specifically including the following steps: S1. Determine the target acoustic operating frequency band: Based on the center frequency and bandwidth parameters of the PMUT chip, determine the target acoustic operating frequency band; S2. Divide the acoustic functional areas: Divide the acoustic cavity into at least three segments along the direction of sound propagation; S3. Set initial parameters: Set the initial acoustic impedance, geometric parameters and material parameters for each acoustic functional zone; S4. Co-simulation: Based on the acoustic transmission matrix model or finite element simulation, perform co-simulation of multiple structural segments; S5. Iterative optimization: Using echo amplitude, signal-to-noise ratio, or resonance peak stability as the objective function, iteratively optimize the parameters of each segment. S6. Output optimization parameters: Output a combination of multi-segment acoustic encapsulation structure parameters that meet preset thresholds.

[0029] The beneficial effects of adopting the above technical solution are as follows: This optimization method, through systematic acoustic parameter design and differentiated configuration, improves the performance of PMUT ultrasonic transducers in gaseous media. It not only resolves the contradiction between the high sound pressure and strong directivity of the transmitter and the wide bandwidth and large receiving angle of the receiver, which are inherently difficult to achieve with traditional symmetrical encapsulation structures, but also significantly improves sound pressure output, directivity stability, and signal reception quality through precise control of multi-segment acoustic cavities, while enhancing the anti-adsorption capacity of the waterproof membrane. This method, by first determining the target frequency band and dividing it into at least three functional zones, and then employing a joint simulation and iterative optimization strategy, can collaboratively optimize the multi-segment acoustic structure for the low-energy, multi-modal characteristics of PMUT. This effectively suppresses non-dominant mode leakage and maximizes axial performance, overcoming the shortcomings of traditional two-segment matching layer optimization methods. Specifically, S1 determines the target operating frequency band based on the PMUT center frequency, providing a clear optimization direction for subsequent multi-segment structure design. S2, by dividing the acoustic functional zones into at least three segments, provides a structural foundation for constructing complex acoustic impedance gradients and modal control. S3 sets initial parameters for each functional area, providing a starting point for refined optimization. S4 uses acoustic transfer matrix or finite element simulation for joint simulation, which can accurately predict the overall acoustic performance of the multi-segment structure in the target frequency band. S5 performs iterative optimization with key performance indicators as targets, ensuring the optimal comprehensive performance of the final structural parameter combination. The parameter combination output by S6 can be directly used to guide the manufacturing of the packaging structure, ensuring the effective achievement of the design goals.

[0030] Furthermore, in S5, during the iterative optimization process, the height of the second minimum gap is determined. At least the following should be met:

[0031] in, This represents the maximum pressure differential under the expected operating conditions. The height of the second minimum gap; The safety factor has a value range of 1.1 ≤ ≤2.

[0032] Furthermore, in S5, it is necessary to ensure a second minimum gap is formed between the waterproof and acoustically permeable membrane and the outlet end of the duct segment. To meet the anti-adsorption requirements of the waterproof membrane, the calculation method is as follows: Based on the material properties, geometric dimensions, and maximum working pressure difference of the waterproof and sound-permeable membrane, calculate the maximum deflection of the membrane. :

[0033] in, The effective radius of the waterproof membrane; For the bending stiffness of the membrane material, , For elastic modulus, For film thickness, Poisson's ratio; This represents the maximum pressure differential under the expected operating conditions.

[0034] Furthermore, in S6, for the transmitting end, the depth of its front cavity segment is set to be within a first depth range and the inner diameter of the conduit segment is set to be within a first inner diameter range; for the receiving end, the depth of its front cavity segment is set to be within a second depth range and the inner diameter of the conduit segment is set to be within a second inner diameter range; wherein, the first depth range is greater than the second depth range, and the first inner diameter range is less than the second inner diameter range.

[0035] In summary, the multi-segment acoustic encapsulation structure and its optimization method for a PMUT ultrasonic transducer provided by this invention have the following beneficial effects: (1) This multi-segment acoustic encapsulation structure integrates a front cavity segment, a tapered transition segment, a conduit segment, and a waterproof and acoustically permeable diaphragm, achieving high sound pressure output, low resonance tail, good directivity, and excellent anti-interference capability. Specifically, the front cavity segment adopts an inner diameter structure larger than the characteristic diameter of the effective radiation surface of the PMUT chip, effectively expanding the sound radiation area, enhancing the near-field sound pressure, and providing a smooth transition region for sound waves. At the same time, a first minimum gap is set between the radiation surface of the PMUT chip and the bottom surface of the front cavity segment, and the height of the first minimum gap is determined according to the center operating frequency of the PMUT chip. This protects the mechanical safety of the diaphragm and places it under approximately free boundary conditions, thereby ensuring the stability of the resonant frequency and radiation efficiency. The tapered transition segment realizes a continuous gradual change in acoustic impedance, reducing reflection and scattering caused by abrupt changes in cross-section, allowing sound energy to be effectively concentrated and guided to the conduit segment. The duct section, with its specific length-to-inner-diameter ratio, forms a collimated sound beam in the air, significantly improving the directionality of the sound waves and reducing propagation loss. This enhances the stability and repeatability of time-of-flight measurements. The second minimum gap between the duct section outlet and the waterproof acoustic membrane effectively prevents the membrane from adhering to the outlet under pressure changes, ensuring the stability and reliability of sound transmission. Specifically, by constructing at least three differentiated acoustic functional zones and designing them collaboratively, this structure can precisely match the vibration modes of the PMUT, effectively suppressing non-dominant mode energy loss and more efficiently converting limited vibration energy into axial acoustic energy output. This solves the compatibility problem of conventional matching layer structures in PMUT applications.

[0036] (2) This multi-segment acoustic encapsulation structure can be used as a transmitter or receiver depending on the application requirements. When used as a transmitter, the acoustic cavity adopts a deeper front cavity segment and a smaller inner diameter conduit segment. The deeper front cavity segment increases the equivalent acoustic impedance of the cavity, thereby enhancing the transmitted sound pressure. The smaller inner diameter conduit segment strengthens the collimation effect of the sound beam, making the sound wave energy more concentrated and the directionality stronger, which is conducive to efficiently transmitting sound energy to the far field and is suitable for transmission tasks that require high sound pressure and precise pointing. When used as a receiver, the acoustic cavity adopts a shallower front cavity segment and a larger inner diameter conduit segment. The shallower front cavity segment reduces cavity resonance and suppresses the generation of standing waves, thereby widening the frequency response bandwidth. The larger inner diameter conduit segment expands the receiving angle, making the transducer less sensitive to the deviation of the incident sound wave, improving the signal acquisition capability and adaptability. At the same time, the shorter conduit segment reduces propagation loss and high frequency attenuation, which is conducive to the rapid response and clarity of the received signal. Through these two asymmetric optimization designs for transmission and reception tasks, energy output efficiency can be improved at the transmitting end and signal reception quality can be improved at the receiving end, thereby achieving synergistic optimization of high sound pressure transmission and wide bandwidth reception, which significantly improves the overall measurement accuracy, stability and signal-to-noise ratio. Attached Figure Description

[0037] Figure 1 This is a cross-sectional view of the overall encapsulation structure in this invention; Figure 2 This is a schematic diagram of the acoustic cavity structure in this invention; Figure 3 This is a schematic diagram of the PMUT chip mounting and the first minimum gap in this invention; Figure 4 This is a schematic diagram of the second minimum gap between the conduit segment and the waterproof membrane in this invention; Figure 5 This is a schematic diagram of the structure of the present invention as the transmitting end; Figure 6 This is a schematic diagram of the structure of the present invention as the receiving end; Among them, 1. base; 2. PMUT main control board; 3. waterproof and sound-permeable membrane; 4. PMUT chip; 5. conduit section; 6. conical transition section; 7. front cavity section; 8. waterproof membrane fixing ring; 9. waterproof membrane gap section; 10. first minimum gap; 11. second minimum gap. Detailed Implementation

[0038] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0039] like Figures 1-6 As shown, the multi-segment acoustic packaging structure of the PMUT ultrasonic transducer provided by the present invention includes a base 1, a PMUT chip 4 disposed inside the base 1, the PMUT chip 4 being fixed on the PMUT main control board 2, an acoustic cavity disposed in the middle of the base 1, the acoustic cavity being disposed above the PMUT chip 4, an acoustic functional area disposed within the acoustic cavity, the acoustic functional area including at least, from bottom to top, a front cavity segment 7 disposed opposite to the PMUT chip 4, a conical transition segment 6 communicating with the front cavity segment 7, and a conduit segment 5 communicating with the conical transition segment 6, a waterproof and acoustically permeable membrane 3 disposed on the outer side of the outlet end of the conduit segment 5, the waterproof and acoustically permeable membrane 3 being tensioned and fixed by a waterproof fixing ring, and a waterproof membrane gap segment 9 being formed between the waterproof and acoustically permeable membrane 3 and the conduit segment 5.

[0040] This multi-segment acoustic encapsulation structure integrates a front cavity segment 7, a tapered transition segment 6, a conduit segment 5, and a waterproof acoustic membrane 3. By optimizing the geometric parameters and acoustic characteristics of each segment, it achieves differentiated performance enhancements for both the transmitter and receiver. This not only significantly improves the transmitted sound pressure and directivity, ensuring stable sound wave propagation in air or gas media, but also expands the bandwidth and receiving angle of the receiver, enhancing the adaptability and stability of signal reception, thereby improving the overall performance of the PMUT ultrasonic transducer in gaseous media. Specifically, the front cavity segment 7 is positioned opposite the PMUT chip 4, providing excellent acoustic matching for the transmitter and facilitating high sound pressure output. The tapered transition segment 6 acts as an acoustic focuser, enhancing the directivity of the transmitter. The conduit segment 5 optimizes the sound wave propagation path, and together with the waterproof acoustic membrane 3 tensioned and fixed by a waterproof retaining ring on the outer side of the outlet, it creates a superior acoustic environment for the receiver while ensuring waterproof performance. This effectively broadens the receiving bandwidth and increases the receiving angle, thus simultaneously meeting the requirements of high sound pressure and strong directivity at the transmitter and wide bandwidth and large receiving angle at the receiver.

[0041] In this invention, such as Figure 3 As shown, a first minimum gap 10 is formed between the radiating surface of the PMUT chip 4 and the bottom surface of the front cavity section 7. The first minimum gap 10 ensures the free vibration conditions of the diaphragm during operation, avoids the resonant frequency shift and sensitivity decrease caused by the solid wall loading effect and cavity compression effect, thereby ensuring the stable operation of the PMUT chip 4 in air or gas medium and improving the sound radiation efficiency and signal quality.

[0042] like Figure 4As shown, a second minimum gap 11 is formed between the waterproof and sound-permeable membrane 3 and the outlet end of the conduit section 5. The second minimum gap 11 effectively prevents the adsorption phenomenon of the waterproof membrane under the action of gas pressure difference, which not only ensures the unobstructed propagation of sound waves, but also avoids sound attenuation and signal distortion caused by membrane adsorption, thereby improving the reliability and stability of the transducer in complex environments.

[0043] Based on the multi-segment acoustic encapsulation structure of the PMUT ultrasonic transducer described above, the acoustic cavity in this multi-segment acoustic encapsulation structure can serve as either the transmitter (TX) or receiver (RX) of the PMUT ultrasonic transducer. Furthermore, the transmitter and receiver employ different acoustic cavity sizes and structural parameters to optimize their acoustic performance respectively. The specific optimization method for the multi-segment acoustic encapsulation structure of the PMUT ultrasonic transducer includes the following steps: S1. Determine the target acoustic operating frequency band: Based on the center frequency and bandwidth parameters of the PMUT chip, determine the target acoustic operating frequency band; S2. Divide the acoustic functional areas: Divide the acoustic cavity into at least three segments along the direction of sound propagation; S3. Set initial parameters: Set the initial acoustic impedance, geometric parameters and material parameters for each acoustic functional zone; S4. Co-simulation: Based on the acoustic transmission matrix model or finite element simulation, perform co-simulation of multiple structural segments; S5. Iterative optimization: Using echo amplitude, signal-to-noise ratio, or resonance peak stability as the objective function, iteratively optimize the parameters of each segment. S6. Output optimization parameters: Output a combination of multi-segment acoustic encapsulation structure parameters that meet preset thresholds.

[0044] This optimization method, through systematic acoustic parameter design and differentiated configuration, improves the performance of PMUT ultrasonic transducers in gaseous media. It not only resolves the contradiction between the high sound pressure and strong directionality of the transmitter and the wide bandwidth and large receiving angle of the receiver, which are inherent limitations of traditional symmetrical encapsulation structures, but also significantly enhances sound pressure output, directional stability, and signal reception quality through precise control of multi-segment acoustic cavities, while simultaneously strengthening the anti-adsorption capability of the waterproof membrane. By dividing multiple functional areas and conducting joint simulation optimization, this method enables refined and collaborative design of the acoustic encapsulation structure, specifically addressing the low-energy characteristics and complex modes of PMUT, effectively improving axial acoustic energy output efficiency. During the optimization process, it is crucial to ensure that the height of the second minimum gap 11 formed between the waterproof acoustic membrane 3 and the outlet end of the conduit segment 5 meets the anti-adsorption requirements. Furthermore, the material properties of the waterproof acoustic membrane 3 (elastic modulus) are also considered. film thickness Poisson's ratio Effective radius ), calculate its bending stiffness , Subsequently, based on the expected maximum pressure difference Calculate the maximum deflection of the membrane. ,in, The effective radius of the waterproof membrane; For the bending stiffness of the membrane material, , For elastic modulus, For film thickness, Poisson's ratio; The maximum pressure differential under the expected operating conditions; and the height of the second minimum clearance 11. At least should meet ,in, This represents the maximum pressure differential under the expected operating conditions. The height of the second minimum gap 11; The safety factor has a value range of 1.1 ≤ To ensure complete transmission of sound waves, the specific structures of the transmitter and receiver are described below using two examples.

[0045] Example 1 The acoustic cavity structure described in this embodiment is used for the transmitting end of a PMUT ultrasonic transducer and is suitable for ultrasonic transmission scenarios requiring high sound pressure and strong directivity, such as the signal transmitting unit in a gas flow meter.

[0046] In this embodiment, such as Figure 5 As shown, the inner diameter of the front cavity section 7 is larger than the diameter corresponding to the effective radiation surface area of ​​the PMUT chip 4, which can effectively expand the near-field acoustic radiation area and enhance the initial sound pressure. The height of the front cavity section 7 and the height of the conduit section 5 satisfy a specific proportional relationship. The height of the front cavity section 7 is 0.3 to 1.5 times the height of the conduit section 5, more preferably 0.5 to 1.2 times. Increasing the depth of the front cavity can improve the equivalent acoustic impedance of the cavity, thereby further enhancing the sound pressure coupled from the PMUT chip 4. Simultaneously, the large end of the tapered transition section 6 connects to the front cavity section 7, and the small end of the tapered transition section 6 connects to the conduit section 5. Its inner diameter continuously decreases along the height direction, forming a smoothly contracting acoustic energy compression section. This achieves a geometrical gradual change in acoustic impedance, which can smoothly and continuously compress and guide the acoustic wave energy converged by the front cavity section 7 to the conduit section 5 with a smaller cross-sectional area. This significantly reduces acoustic wave reflection caused by abrupt changes in cross-section and concentrates the energy, laying the foundation for forming a collimated sound beam. The inner diameter of the conduit section 5 is smaller than the inner diameter of the cavity section, and the length of the conduit section 5... Its inner diameter Key proportional relationships must be met. More preferably is ,in, The length of catheter segment 5; This refers to the inner diameter of catheter segment 5. When the catheter length... Smaller than its inner diameter At this time, it is difficult for the sound field to form a stable quasi-plane wave; while when If the duration is too long, it will cause unnecessary high-frequency attenuation and propagation loss. By controlling the signal within this preferred range, higher-order propagation modes can be effectively suppressed in the air medium, creating a highly collimated planar sound beam, thereby significantly improving the directionality of sound wave propagation and the stability of time-of-flight measurements.

[0047] Regarding the height of the first minimum gap 10 The design has a minimum clearance of 10 units in height. It needs to be based on the center operating frequency of PMUT chip 4 Based on the formula The calculation results show that, The height of the first minimum gap is 10; This is the center operating frequency of PMUT chip 4; These are dimensionless coefficients related to the acoustic boundary conditions; The dimensionless coefficients related to the acoustic boundary conditions are: (1) the speed of sound in air; (2) to ensure the PMUT diaphragm operates under approximately free boundary conditions, avoiding rigid loading effects due to excessively small gaps or standing wave cavities caused by excessively large gaps; and (3) the speed of sound in air. The preferred range is set at 0.05 ≤ ≤0.30, more preferably 0.10≤ ≤0.20, to achieve stable acoustic output.

[0048] Regarding the height of the second minimum gap 11 The design, the height of the second minimum gap 11 It is necessary to ensure that the waterproof and acoustically permeable membrane 3 does not adhere to the duct outlet under the maximum working pressure differential. Firstly, this should be determined based on the material properties of the waterproof membrane (elastic modulus). Poisson's ratio Film thickness ) and effective radius Calculate its bending stiffness , Subsequently, based on the system's expected maximum pressure difference... Calculate the maximum deflection of the membrane. , To avoid adsorption, the second minimum gap is 11. At least should meet ,in, This represents the maximum pressure differential under the expected operating conditions. The height of the second minimum gap 11; The safety factor has a value range of 1.1 ≤ ≤2, to ensure complete transmission of sound waves.

[0049] Example 2 The acoustic cavity structure described in this embodiment is used for the receiving end of a PMUT ultrasonic transducer and is suitable for ultrasonic signal receiving scenarios that require wide bandwidth response and a large receiving range, such as the signal receiving unit in a gas flow meter.

[0050] In this embodiment, such as Figure 6 As shown, the inner diameter of the front cavity section 7 is larger than the effective radiating surface diameter of the PMUT chip 4 to maintain an appropriate acoustic receiving area. However, unlike the transmitter, the ratio of the height of the front cavity section 7 to the height of the conduit section 5 is chosen to be relatively small. The shallower front cavity depth helps reduce the resonance peak of the cavity itself, reduces signal tailing, and thus effectively broadens the frequency response bandwidth of the receiver, making it less sensitive to frequency changes of the incident sound wave. The inner diameter of the tapered transition section 6 of the receiver also decreases continuously along the height direction, but its contraction is more gradual than that of the transmitter. The tapered transition section 6 of the receiver mainly serves as a smooth transition section, aiming to achieve a continuous change in acoustic impedance to reduce the possible step reflection when the sound wave enters the front cavity section 7 from the conduit section 5 and suppress the generation of standing waves in the cavity, thereby maintaining the purity and stability of the received signal, which is beneficial for the accurate extraction of the time-of-flight signal. The inner diameter of the conduit section 5 of the receiver is also smaller than the inner diameter of its front cavity section 7, but its inner diameter value is larger than that of the conduit of the transmitter. At the same time, the ratio of the length of the conduit section 5 to its inner diameter tends to be chosen to be... A smaller value is used to form a larger throat and a shorter length. A larger throat inner diameter can increase the opening angle of the receiving sound wave, making the receiving end less sensitive to the incident angle deviation caused by assembly errors or sound path offset, thus improving the signal acquisition capability. A shorter duct length is beneficial to reduce the propagation loss of sound waves in the duct, especially the attenuation of high frequency components, and can further shorten the overall resonance time of sound waves in the structure.

[0051] In the receiver structure, the design principles, calculation methods, and functions of the first minimum gap 10 and the second minimum gap 11 are exactly the same as those of the transmitter in Embodiment 1.

[0052] In summary, through Embodiments 1 and 2, this invention provides an asymmetric, separately optimizable PMUT acoustic packaging structure. The transmitting end, by employing a deeper front cavity, a significantly tapered compression section, a smaller throat diameter, and a longer conduit, achieves effective focusing and high collimation of acoustic energy, achieving the transmission target of high sound pressure and strong directionality. The receiving end, by employing a shallower front cavity, a gently tapered transition section 6, a larger throat diameter, and a shorter conduit, achieves wide-angle, low-loss, and fast reception of broadband sound waves. This allows the same PMUT technology platform to simultaneously meet the dual requirements of high sound pressure transmission and high-quality reception, thereby significantly improving the signal-to-noise ratio, measurement accuracy, and stability.

Claims

1. A multi-segment acoustic packaging structure for a PMUT ultrasonic transducer, characterized in that: The device includes a base, inside which a PMUT chip is disposed, the PMUT chip being mounted on a PMUT chip main control board, an acoustic cavity being disposed in the middle of the base, the acoustic cavity being disposed above the PMUT chip, an acoustic functional area being disposed within the acoustic cavity, the acoustic functional area including at least a front cavity section disposed opposite to the PMUT chip from bottom to top, a conical transition section communicating with the front cavity section, and a conduit section communicating with the conical transition section, the outer side of the outlet end of the conduit section being provided with a waterproof and sound-permeable membrane; A first minimum gap is formed between the radiating surface of the PMUT chip and the bottom surface of the front cavity segment; a second minimum gap is formed between the waterproof and acoustically permeable membrane and the outlet end of the conduit segment. The acoustic cavity serves as either the transmitter or receiver of the PMUT ultrasonic transducer. The depth of the front cavity section of the acoustic cavity serving as the transmitter is greater than the depth of the front cavity section of the acoustic cavity serving as the receiver, and the inner diameter of the conduit section of the acoustic cavity serving as the transmitter is smaller than the inner diameter of the conduit section of the acoustic cavity serving as the receiver.

2. The multi-segment acoustic encapsulation structure of the PMUT ultrasonic transducer according to claim 1, characterized in that: Height of the first minimum gap According to the center operating frequency of the PMUT chip It is confirmed that the calculation relationship is as follows: in, The height of the first minimum gap is expressed in millimeters (mm). This is the center operating frequency of the PMUT chip, measured in kilohertz (kHz). These are dimensionless coefficients related to the acoustic boundary conditions; The speed of sound in air is measured in meters per second (m / s).

3. The multi-segment acoustic encapsulation structure of the PMUT ultrasonic transducer according to claim 2, characterized in that: The dimensionless coefficients related to the acoustic boundary conditions The value range is 0.05≤ ≤0.

30.

4. The multi-segment acoustic packaging structure of the PMUT ultrasonic transducer according to claim 1, characterized in that: The inner diameter of the tapered transition section continuously narrows along its axial direction from the end connected to the anterior cavity section to the end connected to the catheter section. The end of the tapered transition section near the anterior cavity section is the larger end, and the end of the tapered transition section near the catheter section is the smaller end.

5. The multi-segment acoustic packaging structure of the PMUT ultrasonic transducer according to claim 1, characterized in that: The inner diameter of the anterior cavity segment is larger than the characteristic diameter of the effective radiation surface of the PMUT chip, and the inner diameter of the catheter segment is smaller than the inner diameter of the anterior cavity segment.

6. The multi-segment acoustic packaging structure of the PMUT ultrasonic transducer according to claim 5, characterized in that: The ratio between the length of the catheter segment and its inner diameter is as follows: in, The length of the catheter segment; This refers to the inner diameter of the catheter segment.

7. The multi-segment acoustic encapsulation structure of the PMUT ultrasonic transducer according to claim 1, characterized in that: The surface roughness of the inner wall of the acoustic cavity ≤3.2μm.

8. An optimization method for a multi-segment acoustic encapsulation structure of a PMUT ultrasonic transducer, based on the multi-segment encapsulation structure of the PMUT ultrasonic transducer according to any one of claims 1 to 6, characterized in that, Specifically, the following steps are included: S1. Determine the target acoustic operating frequency band: Based on the center frequency and bandwidth parameters of the PMUT chip, determine the target acoustic operating frequency band; S2. Divide the acoustic functional areas: Divide the acoustic cavity into at least three segments along the direction of sound wave propagation; S3. Set initial parameters: Set the initial acoustic impedance, geometric parameters and material parameters for each acoustic functional zone; S4. Co-simulation: Based on the acoustic transmission matrix model or finite element simulation, perform co-simulation of multiple structural segments; S5. Iterative optimization: Using echo amplitude, signal-to-noise ratio, or resonance peak stability as the objective function, iteratively optimize the parameters of each segment. S6. Output optimization parameters: Output a combination of multi-segment acoustic encapsulation structure parameters that meet preset thresholds.

9. The optimization method for the multi-segment acoustic encapsulation structure of the PMUT ultrasonic transducer according to claim 8, characterized in that: In step S5, during the iterative optimization process, the height of the second minimum gap is determined. At least the following should be met: in, This represents the maximum pressure differential under the expected operating conditions. The height of the second minimum gap; The safety factor has a value range of 1.1 ≤ ≤2.

10. The optimization method for the multi-segment acoustic encapsulation structure of the PMUT ultrasonic transducer according to claim 8, characterized in that: In step S6, for the transmitting end, the depth of its front cavity segment is set to be within a first depth range and the inner diameter of the conduit segment is set to be within a first inner diameter range; for the receiving end, the depth of its front cavity segment is set to be within a second depth range and the inner diameter of the conduit segment is set to be within a second inner diameter range; wherein, the first depth range is greater than the second depth range and the first inner diameter range is less than the second inner diameter range.

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