Ultrasound sensor package with decoupled acoustic mode

Abstract

A piezoelectric micromechanical ultrasonic transducer (PMUT) sensor is implemented using a microelectromechanical sensor (MEMS) die that includes a membrane of the PMUT sensor that transmits and receives acoustic signals. A back volume within the MEMS sensor package has an acoustic resonant mode within an operating frequency range of the MEMS sensor. The MEMS die is located within the MEMS sensor package such that an acoustic pressure applied to the membrane is balanced across the membrane such that the back volume acoustic resonance mode is decoupled from the membrane operating mode.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 587,740, filed October 4, 2023, entitled “PMUT Package with Decoupled Acoustic Modes,” and U.S. Patent Application No. 18 / 902,498, filed September 30, 2024, entitled “Ultrasonic Sensor Package with Decoupled Acoustic Modes,” the entire contents of each of which are incorporated herein by reference for all purposes. Background Technology

[0003] Ultrasonic sensors (such as piezoelectric micromechanical ultrasonic transducers (“PMUT” sensors) transmit ultrasonic signals or waves to the environment of interest and measure the reflected signals received over time, where the timing and amplitude of the reflected or echo signals correspond to the distance to the object of interest and the characteristics of the object causing the reflection. Therefore, PMUT sensors are used in a variety of applications such as fingerprint sensors and object detection systems, and are subsequently integrated into numerous end-use devices such as security systems, door locks, computers, smartphones, tablets, vehicles, and the like.

[0004] PMUT sensors are implemented as microelectromechanical systems (MEMS) devices comprising thin films, which generate acoustic output signals based on, for example, an electrical transmission signal applied across a piezoelectric material layer by electrodes located on opposite sides of the piezoelectric material layer. Similarly, acoustic signals received at the piezoelectric material layer of the thin film are converted by the piezoelectric material layer into electrically reflected signals received and processed via adjacent electrode layers and additional processing circuitry. PMUT sensors are typically housed within a PMUT sensor package, where one side of the thin film (e.g., via a port and an internal back cavity volume encapsulated within the package of the PMUT sensor package on the opposite side of the thin film) is exposed to the external environment. The back cavity volume can also be acoustically excited by movement of the thin film, where a particularly large signal is generated when the thin film is excited at a frequency corresponding to the acoustic resonant mode of the back cavity. In these examples, a significant portion of the transmit and receive power of the PMUT sensor is consumed during the excitation of the back cavity rather than during the transmission and reception (e.g., pulse / echo or transmit / capture) of the signal of interest. Summary of the Invention

[0005] In embodiments of this disclosure, an ultrasonic sensor includes a sensor package comprising a rear cavity having an acoustic resonant mode at a first frequency and a port exposed to the external environment. The ultrasonic sensor also includes a MEMS die exposed in the rear cavity and the port. The MEMS die includes a thin film of the ultrasonic sensor having an operating frequency range encompassing the first frequency, wherein, based on the position of the MEMS die within the rear cavity, a sound pressure applied to the thin film corresponding to the acoustic resonant mode is balanced on the thin film.

[0006] In embodiments of this disclosure, a PMUT sensor includes a sensor package comprising a rear cavity having an acoustic resonance mode at a first frequency and a port exposed to the external environment. The PMUT sensor also includes a PMUT film located within the sensor package and having an operating frequency range encompassing the first frequency, wherein, based on the position of the PMUT film within the sensor package, sound pressure applied to the film corresponding to the acoustic resonance mode is balanced on the film.

[0007] In embodiments of this disclosure, a PMUT sensor includes a cover and a substrate, wherein the cover and substrate define a rear volume. The PMUT sensor also includes a processing circuitry system located within the rear volume and a PMUT film located within the rear volume, wherein, based on the position of the PMUT film within the rear volume, sound pressure applied to the film corresponding to the acoustic resonance mode of the rear volume is balanced on the film. Attached Figure Description

[0008] The above and other features, nature and various advantages of this disclosure will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, wherein:

[0009] Figure 1 A graphical PMUT system according to embodiments of the present disclosure is shown;

[0010] Figure 2 An exemplary PMUT sensor depicting an embodiment of the present disclosure having an unbalanced sound pressure distribution applied to a thin film of the PMUT due to a back cavity acoustic resonance mode;

[0011] Figure 3 An exemplary PMUT sensor depicting a balanced sound pressure distribution applied to a thin film of a PMUT, attributed to a back cavity acoustic resonance mode, according to an embodiment of the present disclosure;

[0012] Figure 4 An exemplary PMUT sensor according to an embodiment of the present disclosure is depicted, having multiple sound pressure distributions corresponding to different acoustic resonance modes of the rear cavity of the PMUT sensor;

[0013] Figure 5A graph depicting the net sound pressure applied to the PMUT film as the design parameters of the PMUT sensor vary according to embodiments of the present disclosure;

[0014] Figure 6 An exemplary top-port PMUT sensor according to an embodiment of the present disclosure is depicted;

[0015] Figure 7 The following describes different acoustic resonance modes corresponding to the rear cavity of the PMUT sensor according to embodiments of the present disclosure. Figure 6 Multiple sound pressure distributions in the circular version of the PMUT sensor;

[0016] Figure 8 The following describes different acoustic resonance modes corresponding to the rear cavity of the PMUT sensor according to embodiments of the present disclosure. Figure 6 Multiple sound pressure distributions in the rectangular version of the PMUT sensor; and

[0017] Figure 9 The exemplary steps of a PMUT sensor with a balanced thin film having an acoustic resonance mode of the PMUT sensor back cavity are described according to embodiments of the present disclosure. Detailed Implementation

[0018] Ultrasonic sensors (such as PMUT sensors) include PMUTs fabricated using MEMS fabrication techniques for specific purposes, such as transmitting acoustic signals toward an external volume of interest (e.g., based on an electrically transmitted signal provided by a processing circuitry system to excite a thin film of the PMUT), receiving reflections of the transmitted signal, and generating electrically reflected signals based on the received reflections. This disclosure is described in the context of ultrasonic sensors as PMUT sensors having a PMUT fabricated with a thin film-containing MEMS. The PMUT is implemented by a MEMS die, which serves as a component of the PMUT sensor package, and subsequently partially encloses the PMUT and provides signal paths (e.g., via ports) for transmitting signals to and receiving signals from the external volume. In some instances, the PMUT thin film transmits and receives acoustic signals via the front cavity of the MEMS die and the ports of the PMUT sensor package exposed to the external volume. The PMUT sensor package (e.g., comprising layers providing encapsulation for the PMUT sensor, such as a cap and a substrate) defines the enclosed back cavity of the PMUT sensor. In some examples, other components (such as an ASIC) are coupled to one of the PMUT sensor package layers within the back cavity. The air enclosed in the back cavity can be excited by the vibration of the thin film during the transmission and reception of acoustic signals, with the amplitude associated with the excitation being most intense at the frequency of the acoustic resonant mode of the back cavity. This problem can be particularly severe in MEMS ultrasonic sensors, where the size of the sensor package is comparable to the ultrasonic wavelength. Therefore, when the acoustic resonant mode of the back cavity is within the operating frequency range of the PMUT, energy that would normally be used to transmit acoustic signals or to convert reflected acoustic signals into electrically reflected signals is instead used to excite the back cavity.

[0019] The pressure distribution generated by excitation of the back cavity at frequencies associated with the acoustic resonant modes can be varied based on characteristics of the PMUT sensor design, such as the size of the back cavity and the relative position of the PMUT film of the MEMS die relative to the back volume. The relative dimensions of the back cavity volume, and thus the pressure distribution of the acoustic resonant modes, can be optimized, for example, by changing the shape and total volume of the back cavity (e.g., based on the size of the cap or other encapsulation layer), the relative positions of the components within the back cavity, and by modifying the component positions and (e.g., the size of the ASIC in the back cavity). The position of the MEMS die within the back cavity is positioned such that the net pressure experienced by the PMUT film of the MEMS die attributable to the relevant acoustic resonant modes of the back cavity (e.g., acoustic resonant modes within the operating frequency range of the PMUT) is balanced (e.g., within 1%, 3%, or 5% of zero net pressure or other pre-selected thresholds). In this way, the acoustic resonant modes of the back cavity are decoupled from the operating modes of the PMUT sensor.

[0020] Figure 1 A graphical PMUT system 100 according to an embodiment of the present disclosure is shown. Although in Figure 1 While specific components are depicted, it should be understood that other suitable combinations of PMUT, other sensors (e.g., MEMS or pressure sensors), processing components, memory, and other circuitry can be utilized depending on the needs of different applications and systems. According to this disclosure, a PMUT system may include a PMUT sensor 102 and additional sensors 108. Although this disclosure is described in the context of signals received from certain PMUT sensor designs and configurations, it should be understood that the decoupling of the acoustic resonance modes of the back cavity within the operating frequency range of the PMUT sensor according to this disclosure from the transmission and reception of acoustic signals can be utilized with various PMUT designs, including various thin film and encapsulation materials, thin film and encapsulation layers, thin film and encapsulation shapes, manufacturing technologies, and combinations thereof.

[0021] The processing circuitry system 104 may include one or more components that provide processing based on the requirements of the PMUT system 100. In some embodiments, the processing circuitry system 104 may include hardware control logic that may be integrated into the chip of a sensor (e.g., a die belonging to the PMUT sensor 102 or other sensor 108 or on a portion of the chip adjacent to the PMUT sensor 102 or other sensor 108) to control the operation of the PMUT sensor 102 or other sensor 108 and perform aspects of the processing of the PMUT sensor 102 or other sensor 108. In some embodiments, the PMUT sensor 102 and other sensors 108 may include one or more registers that allow modification of aspects of the operation of the hardware control logic (e.g., by modifying register values). In some embodiments, the processing circuitry system 104 may also include a processor, such as a microprocessor, that executes, for example, software instructions stored in memory 106. The microprocessor may control the operation of the PMUT sensor 102 by interacting with the hardware control logic and processing signals received from the PMUT sensor 102. The microprocessor may interact with other sensors 108 in a similar manner. In some embodiments, some or all of the functions of the processing circuit system 104, and in some embodiments, some or all of the functions of the memory 106 may be implemented on an application-specific integrated circuit (“ASIC”) and / or a field-programmable gate array (“FPGA”), for example, within the rear cavity of a PMUT sensor.

[0022] Although in some embodiments ( Figure 1In embodiments not depicted, PMUT sensor 102 or other sensors 108 may communicate directly with external circuitry (e.g., via a serial bus or a direct connection to sensor outputs and control inputs). However, in this embodiment, processing circuitry 104 may process data received from PMUT sensor 102 and other sensors 108 and communicate with external components via communication interface 110 (e.g., a Serial Peripheral Interface (SPI) or I2C bus, a Controller Area Network (CAN) or Local Area Interconnect Network (LIN) bus in automotive applications, or other suitable wired or wireless communication interfaces known in the art for other applications). Processing circuitry 104 may convert signals received from PMUT sensor 102 and other sensors 108 into appropriate units of measurement (e.g., based on settings provided by other computing units communicating via communication interface 110).

[0023] The PMUT sensor 102 can be implemented in a PMUT sensor package having back-cavity acoustic resonant modes decoupled from the acoustic transmission and reception of the PMUT sensor 102. Although these back-cavity acoustic resonant modes can occur within the operating frequency range of the PMUT sensor, a thin film located on a MEMS die situated within and partially defining the back volume of the PMUT sensor package can be positioned within the back cavity such that any acoustic pressure applied to the back cavity during the acoustic resonant mode is balanced on the thin film. In this way, a negligible portion of the total signal power used for transmitting and receiving acoustic signals via the thin film is consumed by exciting the acoustic resonant modes. Other acoustic resonant modes may have resonant frequencies outside the operating frequency range of the PMUT sensor 102.

[0024] Figure 2 An exemplary PMUT sensor is depicted according to an embodiment of the present disclosure, which has an unbalanced sound pressure distribution on a thin film of the PMUT due to a back cavity acoustic resonance mode. Figure 2The PMUT sensor includes a PMUT sensor package 200, which depicts a horizontal cross-sectional view of the sensor package, while a top cross-sectional view 250 shows the sound pressure distribution of certain components within the PMUT sensor package 200 based on the top view from the cross-section line 250. It should be understood that the PMUT sensor package 200 may have several different configurations and components. In an exemplary embodiment, the PMUT sensor 200 is constructed from (such as) MEMS, semiconductors, and laminated components and includes a substrate 202, a cover 204, a port 206, a PMUT implemented as including a MEMS die 210 containing a thin film 214, and a processing circuitry system such as an ASIC 212. The package of the PMUT sensor package 200 includes a substrate layer 202 and a cover 204, wherein the port 206 is located within and through the substrate 202. MEMS die 210 is fixed to substrate 202 and includes a thin film 214 of PMUT, wherein a volume is in fluid communication with port 206 and is enclosed by MEMS die 210 to define a front cavity 216. Processing circuitry (such as ASIC die 212) is located within the enclosed rear cavity volume of PMUT sensor package 200, for example, bonded to the upper surface of substrate 202. Figure 2 In this embodiment, the enclosed internal volume of the PMUT sensor package is a rear cavity 218 defined by the geometric configuration of the inner surfaces of the substrate 202, the cover 204, and the internal surfaces of the internal components (such as the MEMS die 210 and the ASIC die 212). The rear cavity enclosure may contain a volume of gas such as atmospheric pressure air, or in other embodiments, a volume containing other gases and pressures such as vacuum. In the exemplary embodiment, the components are shown in specific locations with specific dimensions, but these can be modified to accommodate different PMUT configurations and designs. Furthermore, additional components may be added and / or removed, and the relative positions of the components may be changed (e.g., moving the ASIC die outside the sensor package or adding filler material within the rear cavity).

[0025] The substrate 202 provides a support material layer (e.g., a package laminate or other suitable material) for other components of the PMUT sensor package 200 and, in some instances, provides an interface for electrical and / or mechanical connections to other components of the end-use device (e.g., providing bonding pads for transmitting electrical signals back and forth to the ASIC 212 and physical connections or other connections to other components to provide, for example, providing port 206 to access the area of ​​interest for the transmission and reflection of acoustic signals). Figure 2In this embodiment, the substrate 202 may provide physical attachment surfaces for bonding other functional components, such as MEMS die 210, ASIC die 212, cover 204, or other processing circuitry or sensors. The substrate 202 may also provide circuit paths for routing electrical signals via electrical signal paths (not depicted) within the substrate layer 202. The substrate 202, together with the cover 204, provides structural support for the entire package. Figure 2 In the embodiments, each of the substrate 202 and cover 204 forming the external package is generally rectangular, but it should be understood that the substrate 202, cover 204 and other components can be configured in various shapes, such as rectangular, circular, elliptical, polygonal, irregular, and combinations thereof.

[0026] Port 206 may be fabricated through substrate 202 to allow a fluid path (e.g., via front cavity 216) between the internal thin film 214 of the MEMS die 210 and the external volume of interest. Port 206 is generally an aperture within substrate 202 that allows direct access to the PMUT thin film 214. The exposed volume between the thin film 214 and port 206, enclosed by the MEMS die, is called front cavity 216. The acoustic characteristics of front cavity 216 can be modified by varying the geometry (e.g., circular, rectangular, elliptical) and overall size of port 206. Therefore, port 206 may have various shapes and configurations to suitably access the front cavity of the thin film 214 exposed to the PMUT, such as rectangular, circular, elliptical, polygonal, or irregular shapes. Furthermore, port 206 may include features for mitigating undesirable interference to signals transmitted and received via port 206 and front cavity 216 (such as for blocking light or particles from entering port 206 and front cavity 216).

[0027] Although port 206 is Figure 2 Port 206 is depicted extending through substrate 202, but it should be understood that port 206 can be located on any suitable surface to provide a path for transmitting and receiving acoustic signals, such as a semiconductor sidewall (not depicted) or cap 204, wherein the positions of MEMS die 210 and the front and rear cavities are appropriately modified. In an additional embodiment (not depicted), port 206 may extend through cap, such that the so-called rear cavity 218 in this embodiment can act as the front cavity by facilitating signal transmission, while the so-called front cavity 216 in this embodiment is an enclosed volume optimized for decoupling the rear volume acoustic resonant modes described herein (e.g., without a port fluidly connected to an external volume).

[0028] Cover 204 can be manufactured using various technologies and materials (e.g., semiconductors, metals, polymers, composites) and provides structural support and physical attachment surfaces for bonding other functional components. Furthermore, cover 204 provides a physical barrier and thus protection for the entire PMUT sensor package 200 containing the MEMS die 210 and ASIC die 212. The acoustic resonance mode of the rear cavity 218 depends in part on various geometries of the cover and its spatial position relative to the components within the rear cavity. Variations in the volume enclosed by cover 204 can subsequently alter the acoustic resonance mode (e.g., frequency, pressure distribution, amplitude, etc.) by changing the relative dimensions (e.g., height to width to depth) and relative size and position of the components housed within the cover.

[0029] MEMS die 210 contacts both the front cavity 216 and the rear cavity 218 and is depicted herein in a simplified form. The PMUT can be fabricated as MEMS die 210 using MEMS semiconductor manufacturing processes, wherein MEMS die 210 also includes structural features such as those for bonding and attachment to the substrate 202. It should be understood that although this embodiment is described in the context of a MEMS die 210 comprising a thin film 214 of the PMUT, in other embodiments, PMUTs fabricated from different processes and materials may have undesired acoustic resonance modes decoupled from PMUT operation using the structures and processes described herein. Figure 2 In one embodiment, the MEMS die 210 has a rectangular shape and includes a circular thin film 214 in contact with both the front cavity 216 and the rear cavity 218. However, each of the MEMS die 210 and the thin film 214 may have different shapes, such as rectangular, circular, elliptical, polygonal, irregular, or combinations thereof. The thin film 214 (e.g., from a processing circuit system such as ASIC 212) receives an electrical transmission signal, which is converted by the thin film 214 into an acoustic transmission signal and transmitted to the external environment via port 206. The signal is then reflected back to the thin film 214 by objects in the external environment via port 206 as an acoustic reflection signal and converted by the thin film 214 into an electrical reflection signal for further processing by a processing circuit system such as ASIC 212.

[0030] Modifications to the geometry, size, and spatial position of the MEMS die 210 affect not only the acoustic properties of the front cavity 216 but also the acoustic resonant modes of the rear cavity 218. The PMUT film 214 is an assembly of the MEMS die 210 exposed to both the front cavity 216 and the rear cavity 218, with a variable thickness (typically in the low micrometer range). Furthermore, the size and shape of the MEMS die 210 (e.g., the thickness and lateral dimensions defining the front and rear cavities) can substantially influence the acoustic resonant modes of the rear cavity 218. The PMUT has an operating frequency range designed based on the PMUT and film 214. As described herein, certain acoustic resonant modes of the rear cavity 218 can be used within the operating frequency range of the PMUT to "power" these acoustic resonant modes with power originally intended for transmitting and receiving acoustic signals. As described herein, interaction with the acoustic resonant modes within the rear cavity reduces the sensitivity and accuracy of the PMUT when the acoustic resonant modes are not decoupled from the PMUT film 214.

[0031] The processing circuitry (e.g., ASIC die 212) for interfacing with other circuitry systems and external circuitry systems of the thin film 214 and MEMS 210 chips is located within the rear volume 218 of the PMUT sensor package 200. Although in Figure 2 The ASIC chip 212 is depicted herein, but the processing circuitry may include other circuitry and / or additional processing circuitry, which may also be housed together with the ASIC 212 within the rear cavity 218. The ASIC chip 212 generates electrical signals that are converted into acoustic transmission signals via the PMUT film 214 and also processes electrical signals generated by sound waves reflected back to the PMUT film 214. Modifications to the geometry, size, and spatial location of other processing circuitry such as the ASIC chip 212 can affect the acoustic resonance modes of the rear cavity 218. Encapsulation materials, epoxy resins, and films (not depicted) that may cover the ASIC chip 212 can also alter the acoustic resonance modes of the rear cavity 218, such as shape and total encapsulation volume, and any reflection, attenuation, or amplification of sound waves generated externally or internally. Therefore, in addition to the decoupling described herein, PMUT sensor designs may also include modifications to the size, shape, location, and other properties of processing circuitry (e.g., the ASIC chip 212) to modify the characteristics (e.g., frequency, amplitude, position, etc.) of specific acoustic resonance modes.

[0032] The rear cavity 218 is an encapsulated volume defined by several characteristics, such as the geometry, thickness, and spatial location of the substrate 202, cover 204, MEMS die 210, and ASIC die 212. The presence of other sensors, components, and encapsulation materials also affects the rear cavity volume. All these characteristics and configurations will alter the number and specific characteristics of the acoustic resonant modes present within the rear cavity 218. Acoustic vibrations generated by the PMUT film 214 can couple with these acoustic resonant modes, leading to signal interference and attenuation, which will reduce the overall efficiency of the PMUT sensor package 200 (i.e., the energy transmitted by the film 214 to generate acoustic waves in the front cavity 216 and exit through port 206 will be used to excite the volume within the rear cavity 218, thus reducing the amplitude and fidelity of the acoustic waves exiting through port 206).

[0033] Pressure distribution 250 illustrates an exemplary acoustic pressure distribution associated with a specific resonant mode of the PMUT sensor package 200, based on a top view of the self-section line 250. Negative and positive pressure regions are represented in dark gray, while locations with zero net pressure are represented in light gray. Positive ("+") and negative ("-") phase pressure regions are depicted by markers within continuous gray areas. In the acoustic mode present within pressure distribution 250, a first pressure node line 252a is vertically generated across the ASIC die 212, and a second pressure node line 252b is vertically generated across the MEMS die 210 and the thin film 214. The second pressure node line 252b is not aligned with the lateral spatial symmetry lines 254a or 254b present across the PMUT thin film 214, thus creating an unbalanced pressure distribution across the entire thin film 214, leading to coupling between the PMUT and the acoustic resonant mode. In this embodiment, a specific sound pressure distribution with only two pressure node lines exists at a set frequency. However, in other PMUT configurations and designs (e.g., containing other components, sensors, processing circuitry, dimensions, etc.), there may be more or fewer nodes and different sound pressure distributions because both the frequency and operating band of the PMUT sensor package 200 can be varied. For example, in some embodiments, multiple acoustic resonance modes may occur at close frequencies, allowing them to couple to PMUT operation at the operating frequency.

[0034] The acoustic excitation of air in the rear cavity 218 generates a sound pressure distribution across the PMUT sensor package 200. In addition to the presence of other objects (e.g., encapsulation material, epoxy resin, etc.), the geometry, position, size, and shape of the components within the PMUT sensor package 200 (e.g., ASIC die 212) also affect the sound pressure distribution and the position of the nodal lines. In the exemplary embodiment of FIG. 250, a first pressure nodal line 252a spans the ASIC die 212, and a second pressure nodal line 252b spans the MEMS die 210 and the thin film 214. Depending on the application frequency and operating band of the PMUT sensor package 200, the sound pressure distribution across all components can vary (e.g., the number, position, and shape of pressure nodules). The alignment of the pressure nodal lines with the vertical 254a and horizontal 254b spatial lateral symmetry lines of the thin film 214 determines the net pressure distribution on the thin film 214. In this embodiment, in a specific acoustic resonance mode, the configuration causes the pressure node line to misalign with the spatial lateral symmetry line, thus resulting in a non-zero net pressure (i.e., a positive pressure region) across the thin film 214. This leads to a reduction in acoustic efficiency and negatively impacts the performance of the PMUT sensor package 200 due to coupling between the PMUT and the acoustic resonance mode. Consequently, the pressure distribution of the acoustic resonance mode is unbalanced on the thin film 214.

[0035] Figure 3 An exemplary PMUT sensor is depicted according to an embodiment of the present disclosure, having a balanced sound pressure distribution applied to a thin film of the PMUT due to a back cavity acoustic resonance mode. Figure 3 The PMUT sensor comprises a PMUT sensor package 300 fabricated as a MEMS package, depicting a horizontal cross-sectional view of the device, while pressure distribution 350 shows the acoustic pressure distribution of the PMUT sensor package 300 based on a top view of the cross-section line 350. It should be understood that the PMUT sensor package 300 may have several different configurations and components. The PMUT sensor package 300 and its numbered components are similar to and operate in a manner similar to that of the PMUT sensor package 200 (and its respective components). However, the thin film 314 of the MEMS die 310 and the port 306 are positioned such that the acoustic pressure distribution across the thin film 314 is balanced on the thin film 314, where the thin film 314 experiences zero or near-zero net pressure. In the context of this disclosure, the thin film with a balanced acoustic pressure distribution utilizes negligible energy of the volume after excitation in its acoustic resonant mode, for example, less than 5%, 3%, or 1% of the total transmitted power applied to the thin film in the acoustic resonant mode.

[0036] Pressure distribution 350 illustrates an exemplary acoustic resonance mode pressure distribution of the PMUT sensor package 300 based on a top view of the self-section line 350. Negative and positive pressures across locations are represented in dark gray, while neutral pressure is represented in light gray. In the acoustic resonance mode corresponding to pressure distribution 350, a first pressure node line 352a occurs perpendicularly across the ASIC die 312, and a second pressure node line 352b occurs perpendicularly across the MEMS die 310 and the thin film 314. The second pressure node line 352b is aligned with a vertical lateral symmetry line 354a across the PMUT thin film 314, thus creating a balanced pressure distribution across the entire thin film 314. In this embodiment, a specific acoustic pressure distribution with only two pressure node lines exists at a specific frequency associated with the acoustic resonance mode, but in other PMUT configurations and designs (e.g., containing other components, sensors, configurations, etc.), there may be more or fewer nodes and different acoustic pressure distributions, as both the frequency and operating band of the PMUT sensor package 300 can be varied. As described in this disclosure, the PMUT sensor package 300 is designed to be configured such that pressure is balanced across the PMUT film for all acoustic resonant modes corresponding to the operating frequency range of the PMUT sensor package.

[0037] Figure 4 An exemplary PMUT sensor according to an embodiment of the present disclosure is depicted, having multiple sound pressure distributions corresponding to different rear cavity acoustic resonance modes of the PMUT sensor's rear cavity. Negative and positive pressures across positions are represented in dark gray, while neutral pressure is represented in light gray. The respective phases of the time harmonic pressures of the acoustic resonance modes are indicated by "+" or "-" signs. It should be understood that different PMUT designs and configurations will have different numbers and types of acoustic resonance modes within the operating frequency range, and can provide similarities to these designs. Figure 4 The description. In Figure 4 In exemplary embodiments, such as Figure 3 The six acoustic resonance modes of the PMUT design are described in the paper.

[0038] exist Figure 4In the embodiments described, the PMUT operates at a frequency range of 50 kHz to 100 kHz, and acoustic resonance modes 2 to 5 are within the operating frequency range of the PMUT, while acoustic resonance modes 1 and 6 are outside the operating frequency range of the PMUT. For example, acoustic resonance mode 1 may occur at a frequency of 35 kHz, acoustic resonance mode 2 may occur at a frequency of 55 kHz, acoustic resonance mode 3 may occur at a frequency of 65 kHz, acoustic resonance mode 4 may occur at a frequency of 85 kHz, acoustic resonance mode 5 may occur at a frequency of 95 kHz, and acoustic resonance mode 6 may occur at a frequency of 105 kHz. It should be understood that different PMUT sensor designs and configurations will have different numbers of associated back-cavity acoustic resonance modes occurring at different frequencies, and this disclosure can be similarly applied to other such designs and configurations by simulating the back-cavity acoustic resonance modes by the physical position of the film relative to the PMUT sensor package, by modifying the film position and / or acoustic resonance modes using test equipment, and / or by testing various prototypes with predetermined modifications to the film position and / or PMUT sensor package configuration.

[0039] exist Figure 4 In the embodiments, neither acoustic resonant mode 1 nor acoustic resonant mode 6 corresponds to the operating frequency range of the PMUT and therefore is not excited during proper operation of the PMUT. However, each of acoustic resonant modes 2 to 5 occurs within the operating frequency range and should be considered as optimizing the performance of the PMUT. For each of acoustic resonant modes 2 to 5, the acoustic resonant mode is decoupled from the operating mode of the PMUT by balancing the total pressure applied to the PMUT film attributable to the acoustic resonant mode. For each of acoustic resonant modes 2 to 5, the PMUT is designed such that the applied pressure is distributed on the PMUT film oriented around one or more transverse lines of symmetry around the PMUT film (e.g., for a circular film, the diameter lines intersect at right angles at the center point of the circular film).

[0040] For example, at the frequency associated with acoustic resonant mode 2, a balanced sound pressure distribution is achieved across the PMUT film based on the neutral pressure occurring at the vertical transverse symmetry line of the film and equal and opposite positive and negative pressures (e.g., with opposite phases) on both sides of the vertical transverse symmetry line. At the frequency associated with acoustic resonant mode 3, a balanced sound pressure distribution is achieved across the PMUT film based on the neutral pressure occurring at the horizontal transverse symmetry line of the film and equal and opposite positive and negative pressures (e.g., with opposite phases) on both sides of the horizontal transverse symmetry line. At the frequency associated with acoustic resonant mode 4, a balanced sound pressure distribution is achieved across the PMUT film based on the neutral pressure occurring at the horizontal transverse symmetry line of the film and equal and opposite positive and negative pressure patterns (e.g., with opposite phases) on both sides of the vertical transverse symmetry line. At the frequency associated with acoustic resonant mode 5, a balanced sound pressure distribution is achieved across the PMUT film based on the neutral pressure occurring at the horizontal and vertical transverse symmetry lines of the film and equal and opposite positive and negative pressures (e.g., with opposite phases) in the phase boundaries defined by the horizontal and vertical transverse symmetry lines.

[0041] Figure 5 A graph depicting the net sound pressure applied to the PMUT film as a function of varying PMUT sensor design parameters according to embodiments of the present disclosure. Figure 5 The embodiments described correspond to (for example) Figure 4 The acoustic resonance mode 2 is depicted in the disclosure. According to this disclosure, the frequency at which the acoustic resonance mode occurs and the pressure distribution of the acoustic resonance mode (e.g., including the location of pressure transitions) can be modified by modifying components that jointly define the cavity volume (e.g., substrate, cover, ASIC / processing circuit system, MEMS die, etc.). Furthermore, the positions of the MEMS die and the corresponding PMUT film relative to this pressure distribution can also be modified to (e.g.) correspond to positions where the pressure distribution applied to the PMUT film is balanced based on the acoustic resonance mode. As an example, Figure 5 This describes the changes in net pressure experienced by the PMUT film of the MEMS die, ASIC thickness, and cap position in response to modifications. It should be understood that... Figure 5 The instance corresponds to a specific PMUT design (such as Figures 2 to 4 The PMUT design described and depicted herein can be used to perform similar analysis on other designs, and other parameters (such as y-axis MEMS die position, cover height, total back cavity volume, etc.) can be considered when designing and implementing PMUT sensor packages with the decoupled acoustic resonance modes and operating modes described herein. Figure 5 In the graph, the "zero" value on the x-axis corresponds to the parameter value where the PMUT membrane pressure balance is optimized.

[0042] Figure 502 depicts the effect of variations in the x-axis position of the MEMS die on the net pressure distribution experienced by the PMUT film. The horizontal axis of Figure 502 corresponds to the x-axis position of the MEMS die within the back volume in micrometers (μm), while the vertical axis corresponds to the normalized net pressure on the PMUT film ranging from +1 to -1. As depicted in Figure 502, for modes corresponding to pressure distributions of 250 and 350, and... Figure 4 Acoustic mode 2, the negative x-axis shift of the MEMS die corresponds to an increase in net pressure on the PMUT film, wherein the depicted acoustic resonance mode has a positive net pressure in the negative x-direction from the neutral pressure line / position on the MEMS die. Similarly, corresponding to pressure distributions 250 and 350 and Figure 4 Acoustic mode 2, the positive x-axis shift of the MEMS die corresponds to a negative increase in the net pressure on the PMUT film, wherein the depicted acoustic resonance mode has a negative net pressure in the positive x-direction from the neutral pressure line / position on the MEMS die. The "zero" position on the x-axis of curve 502 corresponds to zero net pressure, for example, as... Figure 3 As depicted in the text.

[0043] Curve 504 depicts the attribution to Figure 5 The effect of variations in the thickness (e.g., height) of the ASIC die within the back cavity for a specific acoustic mode depicted in Figure 504 on the total net pressure experienced by the PMUT film. The horizontal axis of Figure 504 corresponds to the z-axis thickness of the ASIC within the back volume in micrometers (μm), while the vertical axis of Figure 504 corresponds to the normalized net pressure on the PMUT film in the range of +1 to -1. As depicted in Figure 504, the ASIC height alters the characteristics of the acoustic resonance mode (e.g., incorporating a neutral pressure location). The “zero” value of the ASIC thickness in Figure 504 corresponds to the ASIC height where the net pressure on the PMUT film is zero, while changing the height results in a change in the net pressure on the PMUT film.

[0044] Curve 506 depicts the attribution to Figure 5The effect of changing the x-axis position of the cover of a PMUT sensor package for a specific acoustic mode on the total net pressure experienced by the PMUT film is illustrated in Figure 506. The horizontal axis of Figure 506 corresponds to the x-axis position of the cover (e.g., assuming the same overall size of the cover, moving the cover laterally along the x-axis to different positions relative to the MEMS and ASIC chips on the substrate), in micrometers (μm), while the vertical axis of Figure 506 corresponds to the normalized net pressure on the PMUT film in the range of +1 to -1. As depicted in Figure 506, the x-axis position of the cover alters the characteristics of the acoustic resonance mode (e.g., including neutral pressure positions). The "zero" value of the cover position in Figure 504 corresponds to the cover position where the net pressure on the PMUT film is zero, while changing the x-axis position results in a change in the net pressure on the PMUT film.

[0045] Figure 6 A horizontal cross-sectional view of an exemplary top-port PMUT sensor according to an embodiment of the present disclosure is depicted. It should be understood that the PMUT sensor may have several different configurations and components. In this exemplary embodiment, the device includes a MEMS package comprising a substrate layer 602, a sidewall layer 604, and a top cover substrate layer 606, with a port 608 through the top cover substrate layer 606 opening to the external environment. A PMUT MEMS die 610 is attached to the top cover substrate layer 606, wherein a volume defined by the MEMS die 610, in fluid communication with the port 606, and in contact with a thin film 614 defines a front cavity 616. Additionally, a processing circuitry system, such as an ASIC die 612, is located within the enclosed volume of the PMUT sensor package, for example, bonded to the substrate layer 602. Figure 6 In this embodiment, the enclosed internal volume of the PMUT sensor package is a rear cavity 618 defined by the geometric configuration of the inner surfaces of the substrate layer 602, sidewall layer 604, top cover substrate layer 606, and internal components (such as MEMS die 610 and ASIC die 612). Cross-sectional line 650 is used as... Figure 7 (For example, a PMUT sensor with a cylindrical rear cavity) and Figure 8 (For example, a PMUT sensor with a rectangular back cavity) This is a reference. Furthermore, the PMUT MEMS grains 610 and the thin film 614 can have various shapes (e.g., circular, rectangular, elliptical, irregular, etc.), such as... Figure 7 and Figure 8 The rectangular MEMS die 610 and circular thin film 614 are depicted in the illustration. In the exemplary embodiment, the components are shown in a specific location with specific dimensions, but they can be modified to accommodate different PMUT configurations and designs. Furthermore, additional components can be added and / or removed, and the relative positions of the components can be changed (e.g., moving the ASIC die outside the cover or adding filler material within the rear cavity).

[0046] Figure 7 Depicting different acoustic resonance modes corresponding to the rear cavity in the PMUT sensor package according to embodiments of the present disclosure. Figure 6 The circular version of the PMUT sensor is shown with multiple acoustic pressure distributions based on a top view of section line 650. The respective phases of the pressure distributions are represented in dark gray, while neutral pressure is represented in light gray. The respective phases of the time harmonic pressures of the acoustic resonant modes are indicated by "+" or "-" signs. It should be understood that different PMUT designs and configurations will have different numbers and types of acoustic resonant modes within the operating frequency band, and can provide similar... Figure 7 The description. In Figure 7 In exemplary embodiments, such as Figure 6 The four acoustic resonance modes of the circular version of the PMUT design are depicted in the illustration.

[0047] exist Figure 7 In the embodiments, acoustic resonance mode 1 may occur at a frequency of 45 kHz, acoustic resonance mode 2 may occur at a frequency of 60 kHz, acoustic resonance mode 3 may occur at a frequency of 85 kHz, and acoustic resonance mode 4 may occur at a frequency of 105 kHz. It should be understood that different PMUT sensor designs and configurations will have different numbers of associated back-cavity acoustic resonance modes occurring at different frequencies, and this disclosure can be similarly applied to other such designs and configurations by simulating the back-cavity acoustic resonance modes through the physical position of the film relative to the PMUT sensor package, modifying the film position and / or acoustic resonance modes using test equipment, and / or testing various prototypes with predetermined modifications to the film position and / or PMUT sensor configuration. Figure 7 In one embodiment, the acoustic resonance mode of the rear cavity can be analyzed to determine the usable operating frequency range of the designed PMUT sensor.

[0048] For example, regarding Figure 7 The acoustic resonance mode 1 (e.g., corresponding to 45 kHz) is described, where pressure is balanced on the PMUT film, with equal and opposite positive and negative phase pressure regions on both sides of the film's horizontal transverse symmetry line. For Figure 7 The acoustic resonance mode 2 (e.g., corresponding to 60 kHz) has pressure balanced on the PMUT film, with equal and opposite positive and negative phase pressure regions on both sides of the film's vertical transverse symmetry line. For Figure 7 The acoustic resonance mode 3 (e.g., corresponding to 85 kHz) has pressure balanced on the PMUT film, with equal and opposite positive and negative phase pressure regions in each of the four quadrants around the center point of the film and at 45° relative to the horizontal and vertical transverse symmetry lines of the film. For Figure 7The acoustic resonant mode 4 (e.g., corresponding to 105 kHz) has pressure balanced on the PMUT film, with equal and opposite positive and negative phase pressure regions in each of the four quadrants around the center point of the film, aligned with the horizontal and vertical transverse symmetry lines of the film. Therefore, all four acoustic resonant modes between 45 kHz and 105 kHz have their balanced pressure distribution on the film of the PMUT sensor package and are thus decoupled from PMUT operation within these frequency ranges. Based on the above, Figure 7 The operating frequency range of the PMUT sensor can be at least from 45 kHz to 105 kHz (for example, where the possible additional range below 45 kHz and above 105 kHz is based on the known frequencies of the subsequent acoustic resonance modes).

[0049] Figure 8 Depicting different acoustic resonance modes corresponding to the rear cavity in the PMUT sensor according to embodiments of the present disclosure. Figure 6 The rectangular version of the PMUT sensor is shown in a top view based on a 650° section line, depicting multiple acoustic pressure distributions. Negative and positive pressures across locations are represented in dark gray, while neutral pressure is represented in light gray. The respective phases of the time harmonic pressures of the acoustic resonant modes are indicated by "+" or "-" signs. It should be understood that different PMUT designs and configurations will have different numbers and types of acoustic resonant modes, and can provide similar [specific characteristics] for such designs. Figure 8 The description. In Figure 8 In exemplary embodiments, such as Figure 6 The rectangular version of the PMUT design depicted in the paper has four acoustic resonance modes.

[0050] exist Figure 8 In the embodiments, acoustic resonance mode 1 may occur at a frequency of 50 kHz, acoustic resonance mode 2 may occur at a frequency of 65 kHz, acoustic resonance mode 3 may occur at a frequency of 80 kHz, and acoustic resonance mode 4 may occur at a frequency of 105 kHz. It should be understood that different PMUT sensor designs and configurations will have different numbers of associated back-cavity acoustic resonance modes occurring at different frequencies, and this disclosure can be similarly applied to other such designs and configurations by simulating the back-cavity acoustic resonance modes by the physical position of the thin film relative to the PMUT sensor package, by modifying the thin film position and / or acoustic resonance modes using test equipment, and by testing various prototypes with predetermined modifications to the thin film position and / or PMUT sensor configuration. Figure 8 In one embodiment, the acoustic resonance mode of the rear cavity can be analyzed to determine the usable operating frequency range of the designed PMUT sensor.

[0051] For example, regarding Figure 8The acoustic resonance mode 1 (e.g., corresponding to 50 kHz) is described, where pressure is balanced on the PMUT film, with equal and opposite positive and negative phase pressure regions on both sides of the film's vertical transverse symmetry line. For Figure 8 The acoustic resonance mode 2 (e.g., corresponding to 65 kHz) has pressure balanced on the PMUT film, with equal and opposite positive and negative phase pressure regions on both sides of the film's horizontal transverse symmetry line. For Figure 8 The acoustic resonance mode 3 (e.g., corresponding to 80 kHz) has pressure balanced on the PMUT film, with equal and opposite positive and negative phase pressure regions in each of the four quadrants around the center point of the film, aligned with the horizontal and vertical transverse symmetry lines of the film. For Figure 8 In acoustic resonant mode 4 (e.g., corresponding to 105 kHz), the positive phase pressure occurs across almost the entire thin film of the PMUT sensor package, while the opposite negative phase pressure is mostly present adjacent to the film. Therefore, a large net positive pressure is applied to the film in acoustic resonant mode 4, causing the acoustic resonant mode 4 of the back cavity to be excited during PMUT sensor operation, resulting in a substantial reduction in transmitted and received power and a decrease in signal-to-noise ratio. Based on the above results, Figure 8 The operating frequency range of the PMUT sensor can be at least from 50 kHz to 85 kHz (for example, where the possible additional range of known frequencies based on the next acoustic resonance mode is below 50 kHz or above 85 kHz and the guard band range is such as 5% to 10%).

[0052] Figure 9 Illustrative steps are described in the design of a PMUT sensor with a balanced thin film and a decoupled post-cavity acoustic resonance mode according to embodiments of the present disclosure. Although Figure 9 A particular sequence of steps is described, but steps may be removed, modified, or replaced. Furthermore, additional steps (and their order) may be added in some embodiments.

[0053] Processing begins at step 902, where the configuration of the PMUT sensor is determined. The PMUT sensor includes characteristics such as the device design (e.g., top port design) and the overall package shape (e.g., elliptical, rectangular, polygonal, irregular, etc.). The materials and thicknesses of the substrate and cover (if applicable) can be selected. These properties will affect the overall footprint of the device, define the containment constraints of any components (e.g., the MEMS die of the PMUT), affect the volume of the back cavity, and influence the acoustic resonance modes present within the back cavity. Once the package configuration is set, processing can proceed to step 904.

[0054] In step 904, the configuration of the MEMS chips and their initial spatial position within the device can be determined. General parameters such as shape, size, material, film thickness, spatial position within the PMUT sensor package, and the inclusion of any chip encapsulation material (e.g., epoxy resin, film) can be determined. The volume of the back cavity and its acoustic resonance mode will be affected by the configuration of the MEMS chips. Once the MEMS chip configuration and spatial position are set, the process can continue to step 906.

[0055] In step 906, it can be determined whether the ASIC and / or other processing circuitry is contained within the rear cavity of the PMUT sensor package. When co-located with the MEMS die within the rear cavity, the size, shape, position, and material can affect the acoustic resonance mode of the rear cavity. If the ASIC die is contained within the rear cavity, the process can continue to step 908. If the ASIC die is not contained within the package, the process can continue to step 910.

[0056] In step 908, the configuration of the processing circuitry system (e.g., an ASIC) within the rear cavity and its initial spatial position within the rear cavity are determined. General parameters such as shape, size, material, position within the PMUT sensor package (e.g., outside the package), and the inclusion of any die encapsulation material (e.g., epoxy resin, film) can be determined. The volume of the rear cavity and its acoustic resonance mode can be affected by the configuration of the processing circuitry system. Once the configuration and position of the processing circuitry system are set, processing can continue to step 910.

[0057] In step 910, the inclusion of any other modifications within the rear cavity may be evaluated. In some cases, this may include the addition of other components (e.g., overmolding material). If other encapsulation modifications are included within the rear cavity, the process may proceed to step 912. If other encapsulation modifications are not included within the rear cavity, the process may proceed to step 914.

[0058] In step 912, the configuration of the package modification can be determined. General parameters such as size, material, thickness, and position within the PMUT sensor package can be determined. The volume of the rear cavity and its acoustic resonance mode can be affected by the package modification configuration. Once the package modification configuration is set, the process can continue to step 914.

[0059] In step 914, tests and / or simulations may be performed to determine the acoustic resonance mode of the rear cavity. The rear cavity is defined by several characteristics, such as the geometry, thickness, and spatial location of the substrate, cover, MEMS die, and processing circuitry (e.g., ASIC die). The presence of other sensors, components, and encapsulation materials will also directly affect the encapsulation volume, and this volume and associated component dimensions will directly affect the acoustic resonance mode frequency and pressure distribution. All these characteristics and configurations will alter the number and specific characteristics (e.g., pressure distribution, amplitude, etc.) of the acoustic resonance modes present in the rear cavity. Acoustic vibrations generated by the PMUT film can couple with these acoustic resonance modes, leading to signal interference and attenuation, which will reduce the overall efficiency of the PMUT sensor (i.e., the energy transmitted by the film to generate sound waves in the front cavity and exiting through the port will be used to excite the volume in the rear cavity, thus reducing the total amplitude and number of sound waves exiting through the port, resulting in similar signal loss when receiving reflected sound signals). Once the acoustic resonance mode of the rear cavity is determined, the process can continue to step 916.

[0060] In step 916, the acoustic pressure distribution across the PMUT film for the acoustic resonance mode (determined in step 914) can be determined. The number and spatial location of pressure nodes (where pressure is balanced) within the acoustic pressure distribution can be noted. Once the acoustic pressure distribution across the film is determined, the process can continue to step 918.

[0061] In step 918, it can be determined whether the acoustic pressure distribution across the PMUT film is balanced within the operating range of the sensor. During this step, the alignment of the spatial lateral symmetry line of the PMUT film can be compared with the pressure distribution applied to the film by the acoustic resonance mode. If the net pressure on the PMUT film is balanced within the relevant operating frequency range of the PMUT sensor in the acoustic resonance mode (e.g., within thresholds of 1%, 3%, 5%, or other similar thresholds), then the acoustic resonance mode is decoupled from the PMUT sensor operation and the process is complete. If a balanced net pressure is not obtained at the PMUT film within the operating frequency range, the process continues to step 920.

[0062] In step 920, it can be determined whether the unbalanced sound pressure distribution across the PMUT film can be balanced by changing the size or position of the PMUT sensor components. The position and / or size of the ASIC, MEMS die, port holes, cap, and substrate within the package can affect the sound pressure distribution across the entire device. Therefore, shifting the position and / or modifying the size of one or more of these components of the PMUT sensor can allow their spatial lateral symmetry lines to align with the pressure node lines to produce a balanced sound pressure distribution across the film. If a balanced sound pressure distribution across the film can be achieved by changing the position and / or size of the PMUT sensor, the process can continue to step 922. If changing the position and / or size of the PMUT sensor does not produce a balanced sound pressure distribution across the film, the process continues to step 902 to consider more significant modifications to the overall configuration of the PMUT sensor.

[0063] In step 922, the new positions and / or dimensions of the PMUT sensor components may be updated. The process then returns to step 916 to determine the membrane pressure distribution, and may continue to cycle through steps 916 to 922 to optimize the PMUT sensor design until the desired measurement of PMUT membrane equilibrium is achieved in step 918.

[0064] The above description includes exemplary embodiments based on this disclosure. These examples are for illustrative purposes only and are not intended to be limiting. It should be understood that this disclosure may be implemented in forms other than those expressly described and depicted herein, and various modifications, optimizations, and variations consistent with the following claims can be implemented by those skilled in the art.

Claims

1. An ultrasonic sensor, comprising: Sensor package, comprising: The rear cavity has an acoustic resonant mode at the first frequency; and Ports that are exposed to the external environment; and A microelectromechanical system (MEMS) die exposed in the rear cavity and the port, the MEMS die including a thin film of the ultrasonic sensor having an operating frequency range including the first frequency, wherein, based on the position of the MEMS die in the rear cavity, the sound pressure applied to the thin film corresponding to the acoustic resonance mode is balanced on the thin film.

2. The ultrasonic sensor of claim 1, wherein when the thin film receives an electrical transmission signal to convert it into a transmitted acoustic signal within the operating frequency range, the balance of the applied sound pressure in the acoustic resonance mode prevents the energy of the electrical transmission signal from being used to excite the acoustic resonance mode of the rear cavity.

3. The ultrasonic sensor of claim 1, wherein when the thin film receives an acoustic signal to convert it into an electrical signal within the operating frequency range, the balance of the applied sound pressure in the acoustic resonance mode prevents the energy of the received acoustic signal from being used to excite the acoustic resonance mode of the rear cavity.

4. The ultrasonic sensor of claim 1, wherein the acoustic resonance mode includes a first acoustic resonance mode having an applied first sound pressure, and the ultrasonic sensor further includes a second acoustic resonance mode at a second frequency within the operating frequency range of the thin film, wherein the second sound pressure applied to the thin film corresponding to the second acoustic resonance mode is balanced on the thin film based on the location of the MEMS die within the sensor package.

5. The ultrasonic sensor of claim 4, further comprising a third acoustic resonance mode at a third frequency within the operating frequency range of the thin film, wherein a third acoustic pressure applied to the thin film corresponding to the third acoustic resonance mode is balanced on the thin film based on the location of the MEMS die within the sensor package.

6. The ultrasonic sensor of claim 5, further comprising a fourth acoustic resonance mode at a fourth frequency within the operating frequency range of the thin film, wherein a fourth acoustic pressure applied to the thin film corresponding to the fourth acoustic resonance mode is balanced on the thin film based on the location of the MEMS die within the sensor package.

7. The ultrasonic sensor of claim 5, further comprising a fourth acoustic resonance mode at a fourth frequency outside the operating frequency range of the thin film, wherein, based on the location of the MEMS die within the sensor package, a fourth acoustic pressure applied to the thin film corresponding to the fourth acoustic resonance mode is unbalanced on the thin film.

8. The ultrasonic sensor of claim 1, further comprising a second acoustic resonance mode at a second frequency outside the operating frequency range of the thin film, wherein, based on the location of the MEMS die within the sensor package, a second acoustic pressure applied to the thin film corresponding to the second acoustic resonance mode is unbalanced on the thin film.

9. The ultrasonic sensor of claim 1, further comprising a processing circuit system located within the sensor package, wherein the processing circuit system is configured to provide an electrical transmission signal in the operating frequency range to the thin film to cause the thin film to transmit an acoustic signal in the operating frequency range via the port.

10. The ultrasonic sensor of claim 9, wherein the processing circuitry is further configured to receive an electrically reflected signal based on the reflection of the transmitted acoustic signal received by the thin film via the port.

11. The ultrasonic sensor of claim 9, wherein the location of the MEMS die within the sensor package is at least partially based on the location and size of the processing circuitry within the sensor package.

12. The ultrasonic sensor of claim 1, wherein the package includes a substrate and a cover, wherein the port is located in the substrate, wherein a first portion of the MEMS die surrounds the port such that the thin film faces the port, and wherein the rear cavity encloses an air volume defined by the cover, the inner surface of the substrate and a second portion of the MEMS die opposite the first portion.

13. The ultrasonic sensor of claim 12, wherein the air volume is further defined by a processing circuitry system located within the rear cavity.

14. The ultrasonic sensor of claim 1, wherein the sensor package includes at least three stacked layers, wherein the port is located in a first layer of the stacked layers, wherein a first portion of the MEMS die surrounds the port such that the thin film faces the port, wherein a second layer is positioned parallel to the first layer, and wherein a third layer couples the first layer to the second layer to define the rear cavity.

15. The ultrasonic sensor of claim 1, wherein the rear cavity has a rectangular shape, a circular shape, an elliptical shape, a polygonal shape, or an irregular shape.

16. The ultrasonic sensor of claim 1, wherein the thin film has one of a rectangular shape, a circular shape, an elliptical shape, a polygonal shape, or an irregular shape.

17. The ultrasonic sensor of claim 1, wherein the applied sound pressure includes applying a first net pressure to a first portion of the thin film on a first lateral side of the thin film, and applying a second net pressure equal to and opposite to the first pressure to a second portion of the thin film on a second lateral side opposite to the first side, in a second pressure region.

18. The ultrasonic sensor of claim 17, wherein the transverse symmetry line of the thin film separates the first side from the second side, and wherein the second portion of the thin film is located on the second side of the symmetry line opposite to the first portion of the thin film on the first side of the symmetry line.

19. The ultrasonic sensor of claim 17, wherein the lateral center of the thin film corresponds to the line of symmetry of the rear cavity.

20. The ultrasonic sensor of claim 1, wherein the position of the MEMS die within the sensor package is at least partially based on the position and size of the filler material within the sensor package.

21. The ultrasonic sensor of claim 1, wherein the volume of the rear cavity is filled with air at near atmospheric pressure.

22. A piezoelectric micromechanical ultrasonic transducer (PMUT) sensor, comprising: Sensor package, comprising: The rear cavity has an acoustic resonant mode at the first frequency; and Ports that are exposed to the external environment; and A PMUT film is located within the sensor package and has an operating frequency range that includes the first frequency, wherein, based on the position of the film within the sensor package, the sound pressure applied to the film corresponding to the acoustic resonance mode is balanced on the film.

23. A piezoelectric micromechanical ultrasonic transducer (PMUT) sensor, comprising: build; A substrate, wherein the cover and the substrate define the volume; The processing circuitry is located within this rear volume; and The PMUT sensor has a thin film located within the rear volume, wherein, based on the position of the thin film within the rear volume, the sound pressure applied to the thin film corresponding to the acoustic resonance mode of the rear volume is balanced on the thin film.

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