PMUT with partially inactive piezoelectricity

Abstract

A piezoelectric micromachined ultrasonic transducer (PMUT) device has different transduction efficiencies at different portions of the PMUT device depending on design characteristics such as material, material thickness, and shape. The metal layer of the PMUT device is patterned such that some portions of the piezoelectric layer of the PMUT device are inactive. Only the active portion of the piezoelectric layer is used for transmission and reception of ultrasonic signals, while the total PMUT device capacitance is reduced due to the lack of active capacitors in the inactive region (s) of the PMUT device, resulting in a PMUT design with increased sensitivity. For the differential PMUT device, patterning may be performed to match a capacitance associated with the differential piezoelectric region.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 530,560, filed August 3, 2023, entitled “PMUT With Partially Inactive Piezoelectric,” and U.S. Patent Application No. 18 / 403,964, filed January 4, 2024, both of which are incorporated herein by reference in their entirety. Background Technology

[0003] Devices such as piezoelectric micromechanical ultrasonic transducers (PMUTs) can be used in a variety of applications, including fingerprint sensing, time-of-flight sensing, and medical imaging. A PMUT device may include a piezoelectric material that provides an electromechanical response based on a given input signal, which is provided via a conductive (e.g., metallic) layer positioned adjacent to the piezoelectric material. When an electrical signal is applied to the piezoelectric material via the conductive layer, the piezoelectric material may exhibit a mechanical response (e.g., generating an ultrasonic output signal) depending on the characteristics of the electrical signal. In response to the received mechanical (e.g., ultrasonic) signal, the piezoelectric material may generate a corresponding electrical response that can be sensed via the metallic layer.

[0004] Piezoelectric and metal layers can be stacked on top of PMUT film layers, such that the piezoelectric layer defines the dielectric volume between the metal layers, thereby creating a capacitor with characteristics based on the metal layer material, metal layer shape, piezoelectric layer material, and piezoelectric material thickness. A large capacitance in such a capacitor can lead to a mismatch with the associated PMUT circuitry (such as transmitting and receiving circuits), resulting in reduced signal-to-noise ratio and stability issues in the PMUT device due to excessive or mismatched capacitance. Modifications to the PMUT design to reduce this capacitance also affect the PMUT's mechanical response, for example, by reducing the sensitivity of the PMUT design. Summary of the Invention

[0005] In embodiments of this disclosure, a piezoelectric micromechanical ultrasonic transducer (PMUT) includes a PMUT film, a first metal layer, a second metal layer, and a piezoelectric layer, the piezoelectric layer being patterned at least as a first piezoelectric layer portion over only a portion of the PMUT film. The first piezoelectric layer portion is at least partially located between the first metal layer and the second metal layer, wherein the first metal layer is patterned to define an active region and an inactive region of the first piezoelectric layer portion.

[0006] In embodiments of the present disclosure, a method for selecting patterning of a metal layer of a piezoelectric micromechanical ultrasonic transducer (PMUT) includes providing an initial configuration of a piezoelectric layer between a first metal layer and a second metal layer over a portion of a PMUT membrane layer. The method can further include determining a vibration mode shape of the PMUT and identifying one or more portions of the piezoelectric layer having a higher transduction efficiency compared to other portions of the piezoelectric layer based on modal shape properties in the piezoelectric layer. The method can further include patterning the first metal layer such that the one or more portions of the piezoelectric layer are active regions of the piezoelectric layer and the other portions of the piezoelectric layer are inactive regions of the piezoelectric layer.

[0007] In embodiments of the present disclosure, a process for designing a piezoelectric micromechanical ultrasonic transducer (PMUT) can include providing an initial configuration of a piezoelectric layer between a first metal layer and a second metal layer over a portion of a PMUT membrane layer, and patterning one of the first metal layer or the second metal layer to modify a capacitance parameter of the PMUT without substantially affecting structural mechanics of the PMUT. BRIEF DESCRIPTION OF DRAWINGS

[0008] The above-described and other features, nature, and various advantages of the present disclosure will be more clearly understood from consideration of the following detailed description in conjunction with the accompanying drawings, in which:

[0009] Figure 1 An illustrative MEMS system having a PMUT device is shown in accordance with embodiments of the present disclosure;

[0010] Figure 2 An example conventional PMUT device is shown;

[0011] Figure 3 An example PMUT device including inactive piezoelectric regions is shown in accordance with embodiments of the present disclosure;

[0012] Figure 4 An example conventional differential PMUT device is shown;

[0013] Figure 5A An example differential PMUT device including a patterned metal layer defining inactive piezoelectric regions is shown in accordance with embodiments of the present disclosure;

[0014] Figure 5B An example differential PMUT device including a patterned metal layer defining inactive piezoelectric regions in accordance with an associated transduction efficiency pattern is shown in accordance with embodiments of the present disclosure;

[0015] Figure 6An exemplary PMUT device including multiple inactive piezoelectric regions and a partially removed metal layer is shown in accordance with embodiments of the present disclosure;

[0016] Figure 7 An exemplary PMUT device including selected inactive piezoelectric regions is shown in accordance with embodiments of the present disclosure;

[0017] Figure 8 An exemplary PMUT device including a distributed patterned metal layer defining active piezoelectric regions is shown in accordance with embodiments of the present disclosure;

[0018] Figure 9 Improvements in PMUT device sensitivity based on modification of inactive piezoelectric regions are shown in accordance with embodiments of the present disclosure; and

[0019] Figure 10 Exemplary steps for optimizing PMUT designs with inactive piezoelectric regions are shown in accordance with embodiments of the present disclosure. DETAILED DESCRIPTION

[0020] MEMS devices such as piezoelectric micromechanical ultrasonic transducer (PMUT) devices can be used for a variety of applications such as fingerprint sensing, time-of-flight imaging, and medical imaging. The general principle of a PMUT device is that a piezoelectric material produces an ultrasonic wave based on an electrical input signal, which is in turn transmitted in the direction of the environment of the PMUT device. In many applications, the ultrasonic wave reflects back in the direction of the PMUT device, where the piezoelectric material of the PMUT device (e.g., a transmitter device or a dedicated receiver device) receives the reflected ultrasonic signal and generates a corresponding electrical output signal. In this way, a PMUT device or an array of PMUT devices can be operated at suitable power and frequency to generate ultrasonic signals suitable for providing ultrasonic signals to a region of interest, receiving reflections of those signals, and generating a composite image or other complex output based on the reflected signals.

[0021] An exemplary PMUT device includes multiple layers, including a piezoelectric layer of material that generates a mechanical response to an applied electrical signal (e.g., to generate a desired ultrasonic output signal) and an electrical signal in response to an applied mechanical force (e.g., from a received reflected ultrasonic signal). The PMUT device can also include processing circuitry (e.g., an integrated processing layer) including signal paths and signal processing circuitry (e.g., amplifiers, filters, etc.) for providing electrical signals to and receiving electrical signals from the piezoelectric layer. The electrical signals can be applied to and received from the piezoelectric layer by electrodes formed from metal layers on opposite sides of the piezoelectric layer.

[0022] Processing circuitry (e.g., transmit and / or receive circuitry) utilized with PMUTs can typically only accommodate PMUTs with a capacitance below a certain value, where the capacitance limit varies depending on the PMUT and circuit design. Additionally, increased capacitance can result in increased noise, even if the PMUT is otherwise operating well. This linkage between PMUT mechanical design and electrical characteristics limits the PMUT designs that can be used, e.g., PMUT size, piezoelectric material, and / or piezoelectric layer thickness. PMUT electrodes are formed from metal layers and are typically either as close to the center of the PMUT membrane as possible or as close to the edge of the PMUT membrane as possible, as these configurations are generally considered to maximize sensitivity. Capacitance is typically reduced by reducing electrode size, increasing piezoelectric layer thickness, and / or changing piezoelectric material. These traditional methods of reducing PMUT capacitance also affect the mechanical behavior of the PMUT device, such as resonance frequency, modal shape, and static deflection.

[0023] According to the present disclosure, the electrical and mechanical performance of the sensor are substantially decoupled, allowing for advantageous mechanical characteristics to be realized without losing / altering those benefits in order to reduce capacitance, and as a result, a significant improvement in sensor sensitivity (e.g., signal-to-noise ratio). In designs utilizing differential PMUTs and circuitry, the electrical performance is improved when the two electrodes are capacitively matched. By matching the capacitance in a differential design without requiring additional modifications to the mechanical design, the overall sensitivity of the differential sensor is greatly improved.

[0024] PMUT capacitance is reduced and adjusted by patterning the metal (electrode) layer(s) to define certain regions of the piezoelectric layer (e.g., portions of the piezoelectric layer between the patterned metal layer and another metal layer on the opposite side of the piezoelectric layer) as non-active and other portions as active. Patterning can be performed by removing portions of the metal layer (e.g., the top metal layer) to electrically isolate portions of the metal layer. Only some of the isolated portions are then connected to the associated processing circuitry or a signal is selectively applied by the processing circuitry, such that the portions of the piezoelectric layer are non-active and do not contribute to the transmission or reception of ultrasonic signals. The corresponding non-active portions of the metal layer also do not contribute to the capacitance of the PMUT device.

[0025] Selection of active and inactive portions of the metal layer can be based on electromechanical transduction efficiency of the PMUT design. For example, active metal portions and associated active piezoelectric portions can be selected based in part on portions of the piezoelectric layer having the highest transduction efficiency. This will vary with PMUT designs based on materials such as piezoelectric materials, layer thicknesses, other device component sizes and compositions (e.g., of the PMUT membrane), plate and component stresses, and other design parameters. Active metal layer sizes can be further patterned to achieve desired capacitance reduction and / or capacitance matching (e.g., for differential designs), which can scale in a predictable manner (e.g., where PMUT capacitance scales linearly with electrode area and inversely with piezoelectric film thickness).

[0026] Figure 1 An illustrative PMUT sensor system 100 is shown in accordance with embodiments of the present disclosure. While specific components are shown in Figure 1 particular, it will be understood that other suitable combinations of MEMS, processing components, memory, and other circuitry can be utilized in accordance with the needs of different applications and systems. In embodiments as described herein, a PMUT sensor system can include at least a PMUT sensor 102 and support circuitry such as processing circuitry 104 and memory 106. In some embodiments, one or more additional sensors 108 (e.g., additional PMUT devices, MEMS gyroscopes, MEMS accelerometers, MEMS pressure sensors, compasses, etc.) can be included within the PMUT sensor system 100. While the present disclosure will be described in the context of specific configurations and designs of PMUT sensors, it will be understood that the selective utilization of inactive piezoelectric regions as described can be utilized with various suitable PMUT types, configurations, shapes, and designs.

[0027] The processing circuitry 104 can include one or more components that provide processing based on the requirements of the PMUT sensor system 100. In an exemplary PMUT system, this processing circuitry includes typical PMUT components, such as transmit and receive circuitry for transmitting and processing ultrasound signals (e.g., charge pumps, receivers, analog front-ends, etc.). In some embodiments, the processing circuitry 104 can include hardware control logic that can be integrated within the sensor’s chip (e.g., on a base substrate of the PMUT sensor 102 or other sensors 108, or on a portion of the chip that is proximate to the PMUT sensor 102 or other sensors 108) to control the operation of the PMUT sensor 102 or other sensors 108 and perform aspects of processing for the PMUT sensor 102 or other sensors 108. In some embodiments, the PMUT sensor 102 and other sensors 108 can include one or more registers that allow aspects of the operation of the hardware control logic to be modified (e.g., by modifying the values of the registers). In some embodiments, the processing circuitry 104 can also include a processor, such as a microprocessor, that executes software instructions (e.g., software instructions stored in the memory 106). The microprocessor can 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 can interact with the other sensors 108 in a similar manner. In some embodiments, some or all of the functionality of the processing circuitry 104, and in some embodiments some or all of the functionality of the memory 106, can be implemented on an application-specific integrated circuit (“ASIC”) and / or a field-programmable gate array (“FPGA”).

[0028] While in some embodiments (e.g., in a PMUT sensor system 100 that is a component of a mobile device), Figure 1In some embodiments, the PMUT sensors 102 and other sensors 108 can be directly connected to the processing circuitry 104, and the processing circuitry 104 can be directly connected to the communication interface 110. In other embodiments, the PMUT sensors 102 and other sensors 108 can be directly in communication with external circuitry (e.g., via a serial bus or direct connection to sensor outputs and control inputs) in the vehicle, but in embodiments the processing circuitry 104 can process data received from the PMUT sensors 102 and other sensors 108 and communicate with external components via the communication interface 110 (e.g., a serial peripheral interface (SPI) or I2C bus, a controller area network (CAN) or local interconnect network (LIN) bus in automotive applications, or a suitable wired or wireless communication interface as known in the art in other applications). The processing circuitry 104 can convert signals received from the PMUT sensors 102 and other sensors 108 into appropriate measurement units (e.g., based on settings provided by other computing units communicating over the communication interface 110) and perform more complex processing to identify information of interest, such as to model a 3-dimensional area of interest (such as a fingerprint, a medical area of interest, a room, or an area proximate to an exterior surface of a vehicle) appropriately by combining multiple PMUT sensor outputs (e.g., based on time of flight of returned transmitted signals received at the PMUT sensors).

[0029] In embodiments of the present disclosure, the PMUT device includes a piezoelectric layer between two metal layers and over a PMUT membrane layer, but it will be understood that PMUTs as described herein can have other configurations, e.g., including a membrane layer overlying the metal and piezoelectric layers. Additionally, in some embodiments, the piezoelectric layer can overlie other components, such as a surrounding die area of the PMUT device. The piezoelectric layer has different transduction efficiencies at different areas of the piezoelectric layer based on the particular mechanical design and configuration. The configuration of the piezoelectric layer and metal layers can be optimized such that active piezoelectric areas (e.g., portions of the piezoelectric layer between electrically active portions of the metal layers) correspond to those portions of the mechanical design that have higher transduction efficiencies, e.g., by removing portions of the piezoelectric layer and / or metal layers, and selectively connecting portions of the metal layers to active electrical signal paths for transmitting and receiving ultrasound signals. In this way, the mechanical design (e.g., resulting in a particular transduction efficiency profile of the piezoelectric layer) is decoupled from the electrical design (e.g., particular configuration of active portions of the metal layers) in a way that allows efficient transmission and reception of ultrasound signals while limiting the overall capacitance between the metal layers, and thus, substantially reduces noise associated with PMUT capacitance and improves the signal-to-noise ratio of the PMUT sensor.

[0030] Figure 2 An example conventional PMUT device is shown. In Figure 2In the exemplary PMUT devices shown in FIGS. 2A and 2B, two different PMUT devices 200 and 250 are shown from a top view and a side view, where the top view corresponds to a cross-sectional view taken from below the top metal layer (e.g., below the top metal layers 208 and 258). The particular shapes, relative sizes of components and layers, etc. are provided for illustration only, and it will be understood that similar configurations can be achieved using different shapes and relative sizes of components and layers. As shown in FIGS. 2A and 2B, the PMUT devices 200 and 250 are configured to provide a single active piezoelectric region in the center of the piezoelectric layer. Figure 2 As can be seen in FIGS. 2A and 2B, each of the PMUT device 200 and the PMUT device 250 includes a single active piezoelectric region over the entirety of the center portion of the piezoelectric layer.

[0031] The PMUT device 200 includes a PMUT membrane layer 202, a bottom metal layer 204, a piezoelectric layer 206, a top metal layer 208, and processing circuitry 210. The piezoelectric layer 206 is located between the top metal layer 208 and the bottom metal layer 204. The processing circuitry is connected to the top metal layer 208 and the bottom metal layer 204 to provide for transmission of ultrasound signals (e.g., by actively applying a signal to the top metal layer 208 and / or the bottom metal layer 204 to initiate a mechanical response of the piezoelectric layer 206 located between the top metal layer 208 and the bottom metal layer 204) and reception of ultrasound signals (e.g., by receiving an electrical signal generated by the piezoelectric layer 206 located between the top metal layer 208 and the bottom metal layer 204 in response to a received signal, such as an acoustic signal). In the PMUT device 200, the entirety of the piezoelectric layer 206 is active, resulting in a relatively large capacitor formed between the top metal layer 208 and the bottom metal layer 204. The configuration of the PMUT device 200 results in vibration mode shapes based on the mechanical configuration of its components (e.g., the PMUT membrane layer 202, the bottom metal layer 204, the piezoelectric layer 206, and the top metal layer 208) having different transduction efficiencies at different portions of the piezoelectric layer 206, but does not distinguish between the active regions of the piezoelectric layer 206 to optimize the average transduction efficiency of the active regions, such that the metal layer electrode coverage matches the mechanical performance of the PMUT device 200.

[0032] PMUT device 250 is similar to PMUT device 200, with PMUT membrane layer 252 corresponding to PMUT membrane layer 202, bottom metal layer 254 corresponding to bottom metal layer 204, top metal layer 258 corresponding to top metal layer 208, and processing circuitry 260 corresponding to processing circuitry 210. PMUT device 250 includes a single piezoelectric layer 256 that includes an active piezoelectric layer region 256a located between top metal layer 258 and bottom metal layer 254, and an inactive piezoelectric layer region 256b surrounding active piezoelectric layer region 256a and covering the entirety of PMUT membrane layer 252. Because inactive piezoelectric layer region 256b is not located between respective top and bottom metal layers, it does not contribute to the overall capacitance of PMUT device 250, but it does contribute to the overall mechanical response and modal shape of PMUT device 250. Due to the lack of patterning on the piezoelectric layer, the piezoelectric layer cannot have a selectively patterned piezoelectric response, preventing full and complete optimization of a PMUT design that balances both mechanical response and capacitance. The configuration of PMUT device 250 results in modal shapes based on the mechanical configuration of its components (e.g., PMUT membrane layer 252, bottom metal layer 254, piezoelectric layer portions 256a and 256b, and top metal layer 258) having different transduction efficiencies at different portions of piezoelectric layer 256a-b, but only selecting the central portion of the piezoelectric layer as the active piezoelectric layer portion 256a, potentially failing to optimize the transduction efficiency (e.g., if portion 256b of the piezoelectric layer has a higher transduction efficiency) while still maintaining a corresponding reduction in capacitance and signal-to-noise ratio.

[0033] Figure 3 An example PMUT device including an inactive piezoelectric region is shown in accordance with embodiments of the present disclosure. While Figure 3 will be described in the context of particular applications and system components, it will be understood that the present disclosure can be utilized with various PMUT designs, applications, configurations, shapes, etc. While Figure 3 particular components are shown and described in Figure 3 the example shown in Figure 3 the example shown inFigure 3 and other figures shown and described herein (e.g., Figures 5A-8 ) in a particular order and arrangement, but it will be understood that in some embodiments one or more intermediate layers (e.g., a substrate, a bonding layer, etc.) not shown can be included.

[0034] The piezoelectric layer can be a single layer that includes two separate shaded portions (e.g., shown as continuous in Figure 3 ), namely, an active piezoelectric layer portion 306a and an inactive piezoelectric layer portion 306b. The nomenclature applied to the portions 306a and 306b of the piezoelectric layer is based on the respective patterning and connection of the metal layers 306a and 306b. In conjunction with the other components of the PMUT device 300, a certain modal shape is associated with the physical configuration of the piezoelectric layer and the other components such that the desired transduction efficiency characteristics exist at a certain portion of the piezoelectric layer (e.g., the piezoelectric layer portion 306a). In the embodiment shown in Figure 3 , the bottom metal layer 304 is a continuous layer 304 and is connected to the processing circuitry 310 to facilitate the emission and reception of ultrasonic signals such as acoustic waves. The top metal layer is patterned into two separate metal layer portions including an active top metal layer portion 308a and an inactive top metal layer portion 308b. The top metal layer portions are physically separated from each other such that there is no direct electrical connection between the layers, and only one of the top metal layer portions (i.e., the active top metal layer portion 308a) is connected to the processing circuitry 310 to facilitate the emission and reception of ultrasonic signals such as acoustic waves.

[0035] The non-active piezoelectric layer portion 306b is non-active because the non-active top metal layer portion 308b is disconnected from the processing circuitry 310, or in some embodiments, the non-active top metal layer portion 308b is connected to a signal that makes the top metal layer portion 308b effectively non-active (e.g., a floating potential, a high impedance ground path, or the same potential or same signal as the bottom metal layer 304). In this way, the size and shape of the active portion of the piezoelectric layer can be selected by patterning of the top metal layer, for example, in a way that optimizes transduction efficiency while minimizing the capacitance between the active top metal layer portion 308a and the bottom metal layer 304. The mechanical performance of the PMUT device remains substantially the same, while the total capacitance is greatly reduced, resulting in an overall improvement in the signal-to-noise ratio of the sensor. In examples where the transduction efficiency is higher near the outer portion of the piezoelectric layer or at multiple locations of the piezoelectric layer, the location of the active electrode can be modified (e.g., based on the selection of the active portion of the metal layer), for example, by connecting the top metal electrode layer portion 308b instead of the top metal electrode layer portion 308a to the processing circuitry. In such embodiments, the piezoelectric layer portion 306b would be an active piezoelectric layer portion, while the non-active piezoelectric portion 306a would not have an associated capacitance that contributes to PMUT noise.

[0036] Figure 4 An example conventional differential PMUT device is shown. In Figure 4 Two different PMUT devices 400 and 450 are shown in top view and side view, where the top view corresponds to a cross-sectional view taken from below the top metal layer (e.g., below the top metal layers 408 and 458). The particular shapes, relative dimensions of components and layers, etc. are provided for illustration only, and it will be understood that similar configurations can be implemented using different shapes and relative dimensions of components and layers. As can be seen in Figure 4 As can be seen in, each of the PMUT device 400 and the PMUT device 450 is a differential PMUT device having multiple active piezoelectric regions.

[0037] PMUT device 400 includes a PMUT film layer 402, a bottom metal layer 404, a central piezoelectric layer portion 406a, an outer piezoelectric layer portion 406c, a central top metal layer portion 408a, an outer top metal layer portion 408c, and a processing circuit system 410. The central piezoelectric layer portion 406a is located between the central top metal layer portion 408a and the bottom metal layer 404, and each of the central top metal layer portion 408a and the bottom metal layer 404 is connected to the processing circuit system, making the central piezoelectric portion 406a active. The outer piezoelectric layer portion 406c is located between the outer top metal layer portion 408c and the bottom metal layer 404, and each of the outer top metal layer portion 408c and the bottom metal layer 404 is connected to the processing circuit system, making the outer piezoelectric portion 406b active. Signals provided by the processing circuit system 410 to the respective central top metal layer portion 408a and outer top metal layer portion 408c are provided differentially to facilitate differential transmission and reception of ultrasonic signals. In PMUT device 400, the entire piezoelectric layer is active, resulting in a relatively large capacitor formed between the top metal layer 408a / c and the bottom metal layer 404. The configuration of PMUT device 400 results in modal shapes based on the mechanical configuration of its components (e.g., PMUT film layer 402, bottom metal layer 404, piezoelectric layer 406a / c, and top metal layer 408c / b) with different transducer efficiencies at different portions of the piezoelectric layer, but without differentiation between active regions of the piezoelectric layer to optimize the average transducer efficiency of the active regions, thus inherently linking the metal layer electrode coverage to the mechanical properties of the PMUT device.

[0038] PMUT device 450 is similar to PMUT device 400, wherein PMUT film layer 452 corresponds to PMUT film layer 402, bottom metal layer 454 corresponds to bottom metal layer 404, top metal layer portions 458a and 458c correspond to top metal layer portions 408a and 408c, and processing circuit system 460 corresponds to processing circuit system 410. PMUT device 450 includes a single piezoelectric layer 456, which includes active piezoelectric layer regions 456a and 456c located between the top metal layer portions 458a / 458c and the bottom metal layer 454, and inactive piezoelectric layer region 456b located between the active piezoelectric layer portions 456a and 456c. In this way, the inactive piezoelectric layer portion 456b provides a buffer between the active piezoelectric layer portions 456a and 456c involved in differential transmission and reception. Because the inactive piezoelectric layer region 456b is not located between the corresponding top and bottom metal layers, it does not contribute to the total capacitance of the PMUT device 450, but it does contribute to the total mechanical response and modal shape of the PMUT device 450. The configuration of the PMUT device 450 results in modal shapes with different transduction efficiencies at different portions of the piezoelectric layers 456a-c based on the mechanical configuration of its components (e.g., PMUT film layer 452, bottom metal layer 454, piezoelectric layer portions 456a-c, and top metal layer portions 458a / c). However, by selecting only the central and outer portions of the piezoelectric layers as the active piezoelectric layer portions 456a and 456c, it may be impossible to optimize the transduction efficiency while still maintaining a considerable reduction in capacitance and signal-to-noise ratio.

[0039] Figure 5A An exemplary differential PMUT device comprising a patterned metal layer defining inactive piezoelectric regions is shown according to an embodiment of the present disclosure. Although Figure 5A This will be described in the context of specific applications and system components, but it will be understood that this disclosure can be utilized with a wide variety of PMUT designs, configurations, shapes, and materials. Although in Figure 5A Specific components are shown and described herein, but it will be understood that components may be added, removed, replaced, or modified in accordance with this disclosure. For example, in some embodiments, metal layer patterning may be performed on the bottom metal layer instead of performing metal layer patterning on the top metal layer or as a supplement to the top metal layer. Figure 5AIn the example shown, the differential PMUT device 500 includes a PMUT film layer 502, a bottom metal layer 504, a central active piezoelectric layer portion 506a, a central inactive piezoelectric layer portion 506b, an outer inactive piezoelectric layer portion 506c, an outer active piezoelectric layer portion 506d, a central active top metal layer portion 508a, a central inactive top metal layer portion 508b, an outer inactive top metal layer portion 508c, an outer active top metal layer portion 508d, and a processing circuit system 510. The PMUT device 500 is shown from a top view and a side view, wherein the top view corresponds to a cross-sectional view taken from below the top metal layer (e.g., below the top metal layer portions 508a-508d) towards the other layers.

[0040] The piezoelectric layer includes a central portion (e.g., including a central active piezoelectric portion 506a and a central inactive piezoelectric portion 506b, shown as continuous in the embodiment of FIG. 5) and an outer portion (e.g., including an outer active piezoelectric portion 506d and an outer inactive piezoelectric portion 506c, shown as continuous in the embodiment of FIG. 5) to facilitate differential operation. The naming of the corresponding active portions 506a and 506d applied to the piezoelectric layer is based on the corresponding patterning and connection of the top metal layer portions 508a, 508b, 508c, and 508d. In conjunction with other components of the PMUT device 500, a certain modal shape is associated with the physical configuration of the piezoelectric layer and other components such that the desired transduction efficiency characteristics exist at a certain portion of the piezoelectric layer (e.g., the inner portion 506a of the central piezoelectric layer 506a / 506b and the outer portion 506d of the outer piezoelectric layer 506c / 506d). In the embodiment shown in Figure 5, the bottom metal layer 504 is a continuous layer 504 and is connected to the processing circuitry system 510 to facilitate the transmission and reception of ultrasonic signals such as sound waves. The top metal layer is patterned as two separate metal layer portions for each region (center and outer) of the differential PMUT, including an active center top metal layer portion 508a associated with an active center piezoelectric layer portion 506a, an inactive center top metal layer portion 508b associated with an inactive center piezoelectric layer portion 506b, an inactive outer top metal layer portion 508c associated with an inactive outer piezoelectric layer portion 506c, and an active outer top metal layer portion 508d associated with an active outer piezoelectric layer portion 506d. The top metal layer portions are physically separated from each other such that there is no direct electrical connection between the layer portions, and only one of the top metal layer portions (i.e., the active top metal layer portions 508a and 508d) for each differential region of the PMUT device is connected to the processing circuitry system 510 to facilitate the transmission and reception of ultrasonic signals such as sound waves.

[0041] The inactive central piezoelectric layer portion 506b is inactive because the inactive central top metal layer portion 508b is disconnected from the processing circuitry 510, or in some embodiments, the inactive central top metal layer portion 508b is connected to a signal that effectively inactivates the top metal layer portion 508a (e.g., a floating potential, a high-impedance ground path, or the same potential or the same signal as the bottom metal layer 504). Similarly, the inactive outer piezoelectric layer portion 506c is inactive because the inactive central top metal layer portion 508c is disconnected from the processing circuitry 510. In this way, the size and shape of the active piezoelectric layer portions can be selected by patterning the top metal layer, for example, to optimize transduction efficiency while minimizing the capacitance between the active top metal layer and the bottom metal layer. Therefore, the mechanical properties of the PMUT device remain substantially the same, while the total capacitance is significantly reduced, resulting in an overall improvement in the signal-to-noise ratio of the sensor. In an example where the transducer efficiency is higher near the outer portion of the central piezoelectric layer, the processing circuitry can be actively connected to the outer top metal layer 508b, making the outer piezoelectric region 506b active. Similarly, in an example where the transducer efficiency is higher near the central portion of the outer piezoelectric layer, the processing circuitry can be actively connected to the central top metal layer 508c, making the central piezoelectric region 506c active. Furthermore, the size, position, and configuration of the active top metal layer and the piezoelectric layer can be selected to match the transmit and receive responses of the piezoelectric layer for differential sensing, and to match the capacitance of the active portion to achieve improved differential operation.

[0042] Figure 5B An exemplary differential PMUT device according to an embodiment of the present disclosure is shown, the exemplary differential PMUT device including a patterned metal layer defining an inactive piezoelectric region according to an associated transduction efficiency mode. Although Figure 5B This will be described in the context of specific applications and system components, but it will be understood that this disclosure can be utilized with a wide variety of PMUT designs, configurations, shapes, and materials. Although in Figure 5B Specific components are shown and described herein, but it will be understood that components may be added, removed, replaced, or modified in accordance with this disclosure. Figure 5B In the example shown, except that the active components (e.g., the piezoelectric layer portion and the top metal layer portion) are selected according to the transduction efficiency mode 520, the components of the PMUT device 500 are... Figure 5A same.

[0043] The transducer efficiency of a given piezoelectric region is determined by the local curvature (Laplace operator) of the resonant vibrational mode shape. The vibrational mode shape can be influenced by factors such as the piezoelectric film coverage, device composition, size, film properties, and stress. Accordingly, the choice of piezoelectric layer design and metal layer patterning can be determined based on the specific transducer efficiency modes (e.g., mode shapes) for a particular device, whether experimentally or through modeling. Figure 5B In the example transducer efficiency pattern 520 shown, the horizontal axis corresponds to the radial dimension of the PMUT device 500, while the vertical axis corresponds to the transducer efficiency (e.g., piezoelectric sensitivity) at the corresponding radial location. The transducer efficiency is shown only for the portion of the PMUT device 500 that includes the piezoelectric layer.

[0044] As illustrated by the transduction efficiency pattern 520, for the central piezoelectric layer region (e.g., corresponding to piezoelectric layer portions 506a and 506b), the transduction efficiency is relatively low in a first transduction efficiency region 522a closer to the center of this region, and relatively high in a second transduction efficiency region 522b closer to the periphery of this region. Similarly, for the outer piezoelectric layer region (e.g., corresponding to piezoelectric layer portions 506c and 506d), the transduction efficiency is relatively low in a third transduction efficiency region 522c closer to the center of this region, and relatively high in a fourth transduction efficiency region 522d closer to the periphery of this outer region. For example, between the higher transduction efficiency regions 522b and 522d, the fourth transduction efficiency region 522d can be more efficient than the third transduction efficiency region 522b. The size and selection of the active patterned top electrode layer portions (e.g., the outer active top metal layer 508d and the central active top metal layer 508b) can be determined based on both the transduction efficiency pattern and the capacitance to match both ultrasonic transmission and reception sensitivities. For example, such as Figure 5B As shown, the relatively reduced width of the outer top metal layer 508d (and thus the associated outer active piezoelectric layer portion 506d) compared to the active center top metal layer 508b (and thus the associated central active piezoelectric layer 506b) is appropriate because the fourth transducer efficiency region 522d corresponds to a higher efficiency than the second transducer efficiency region 522b. Furthermore, since the annular region defined by the outer active top metal layer 508d surrounds the annular region defined by the central active top metal layer 508b, and would otherwise have a larger capacitance surface area than the annular region defined by the central active top metal layer 508b, the reduced width of the outer active top metal layer 508d better matches the capacitance between the two portions / regions.

[0045] Regardless of whether the design has a single or multiple (e.g., differential) active emitter and receiver regions, determining or estimating the resonant vibrational mode shape provides a means to optimize PMUT device operation via patterning of one or more metal layers. By selecting portions of the piezoelectric layer as inactive (typically those with lower transduction efficiency) through patterning of one or more metal layers, the overall mode shape of the PMUT remains undisturbed or modified, while the overall capacitance of the PMUT device is significantly reduced. Therefore, the mechanical (e.g., mode shape) properties of the PMUT device are decoupled from their electrical (e.g., capacitance and emitter sensitivity) properties and can be modified individually for overall signal-to-noise ratio optimization. This optimization can be performed for any suitable piezoelectric material, material thickness, PMUT film type and material, shape, and other PMUT parameters, for example, by first identifying the desired design and / or mode shape and then optimizing the electrical characteristics associated with that design and / or mode shape.

[0046] Figure 6 An exemplary PMUT device comprising a plurality of inactive piezoelectric regions and a partially removed metal layer is shown according to an embodiment of the present disclosure. Although Figure 6 This will be described in the context of specific applications and system components, but it will be understood that this disclosure can be utilized with a wide variety of PMUT designs, configurations, shapes, and materials. Although in Figure 6 Specific components are shown and described herein, but it will be understood that components may be added, removed, replaced, or modified in accordance with this disclosure. For example, in some embodiments, metal layer patterning may be performed on the bottom metal layer instead of performing metal layer patterning on the top metal layer or as a supplement to the top metal layer. Figure 6 In the example shown, the differential PMUT device 600 includes a PMUT film layer 602, a bottom metal layer 604, a central inactive piezoelectric layer portion 606a, a central active piezoelectric layer portion 606b, an outer inactive piezoelectric layer portion 606c, an outer active piezoelectric layer portion 606d, a central active top metal layer portion 606b, an outer active top metal layer portion 608d, and a processing circuit system 610. The PMUT device 600 is shown from top and side views, where the top view corresponds to a cross-sectional view taken from below the top metal layer (e.g., below the top metal layer portions 608b and 608d) towards the other layers.

[0047] exist Figure 6In the embodiment shown, the vibration mode shape may be as previously analyzed to determine that the transduction efficiency of the PMUT device 600 is maximized at the piezoelectric portions corresponding to the outer active piezoelectric layer portion 606d (which may be continuous with the outer inactive piezoelectric layer portion 606c) and the central active piezoelectric layer portion 606b (which may be continuous with the central inactive piezoelectric layer portion 606a). Accordingly, the outer active metal layer portion 608d and the central active metal layer portion 608b, located above the active piezoelectric layer portions, are each connected to the processing circuitry system 610 for the transmission and reception of ultrasonic signals. Figure 6 In the embodiment shown, the inactive piezoelectric portions (e.g., the central inactive piezoelectric layer portion 606a and the outer inactive piezoelectric layer portion 606c) are not covered by the top metal layer. Depending on the manufacturing technology employed, excluding the inactive metal layer portions or removing the inactive metal portions entirely can provide advantages in terms of manufacturing speed and / or yield. Material costs can be reduced if the inactive top metal portions are not initially deposited. While the inactive metal layer portions generally do not significantly contribute to the total capacitance of the PMUT sensor, excluding the inactive metal layer can prevent coupling to RF signals, etc., thereby further reducing the signal-to-noise ratio.

[0048] Figure 7 An exemplary PMUT device including selected inactive piezoelectric regions is shown according to an embodiment of this disclosure. Although Figure 7 This will be described in the context of specific applications and system components, but it will be understood that this disclosure can be utilized with a wide variety of PMUT designs, configurations, shapes, and materials. Although in Figure 7 Specific components are shown and described herein, but it will be understood that components may be added, removed, replaced, or modified in accordance with this disclosure. For example, in some embodiments, metal layer patterning may be performed on the bottom metal layer instead of performing metal layer patterning on the top metal layer or as a supplement to the top metal layer. Figure 7 In the example shown, the differential PMUT device 700 includes a PMUT film layer 702, a bottom metal layer 704, a central piezoelectric layer 706a, an outer inactive piezoelectric layer portion 706b, an outer active piezoelectric layer portion 706c, a central top metal layer 706a, an outer inactive top metal layer portion 708b, an outer active top metal layer portion 708c, and a processing circuit system 710. The PMUT device 700 is shown from a top view and a side view, wherein the top view corresponds to a cross-sectional view taken from below the top metal layer (e.g., below the top metal layer portions 708a-708c) towards the other layers.

[0049] It will be understood that, depending on the PMUT design, shape, material type (e.g., piezoelectric material, metal layer material, etc.), material thickness, etc., various vibrational mode shapes and capacitance conditions will be available. This disclosure enables the optimization of PMUT devices by combining any piezoelectric and / or metal layer patterning described herein. As an example, in Figure 7 In the embodiments, the vibrational mode shape and capacitive response enable optimization of the signal-to-noise ratio by making the entire central piezoelectric layer 706a active, for example, by patterning any top metal layer not included on the central metal layer 708a. A portion of the outer piezoelectric layer (i.e., the inactive outer piezoelectric portion 706b) is rendered inactive by patterning the outer metal layer to electrically isolate the inactive outer metal layer portion 708b from the active outer metal layer portion 708c, while only connecting the active outer metal layer portion 708c to the processing circuitry system 710.

[0050] Figure 8 An exemplary PMUT device comprising a distributed patterned metal layer defining active piezoelectric regions is shown according to an embodiment of the present disclosure. Although Figure 8 This will be described in the context of specific applications and system components, but it will be understood that this disclosure can be utilized with a wide variety of PMUT designs, configurations, shapes, and materials. Although in Figure 8 Specific components are shown and described herein, but it will be understood that components may be added, removed, replaced, or modified in accordance with this disclosure. For example, in some embodiments, metal layer patterning may be performed on the bottom metal layer instead of performing metal layer patterning on the top metal layer or as a supplement to the top metal layer. Figure 8 In the example shown, the PMUT device 800 includes a PMUT film layer 802, a bottom metal layer 804, a piezoelectric layer 806, and patterned top metal layer portions 8081-808. n And processing circuit system 810. PMUT device 800 is shown from top and side views.

[0051] The embodiments described herein have shown circular PMUT devices having a circular pattern and surrounding annular regions arranged in a concentric pattern. It will be understood that this disclosure is similarly applicable to other PMUT device shapes and patterns, including but not limited to squares, rectangles, polygons, and other shapes. Furthermore, the active and inactive layers can be located in various suitable positions and do not need to be concentrically arranged, but can be formed in a concentric shape according to the expectations of a particular end-use application.

[0052] Patterning of metal layers, and especially of the top metal layer, can be relatively flexible compared to patterning of other layers such as piezoelectric layers. Accordingly, as Figure 8 As shown, the top metal layer may include patterned top metal layer portions 8081-808 distributed on the piezoelectric layer.n .exist Figure 8 In an exemplary embodiment, each patterned metal layer portion is shown as being individually connected to the processing circuitry system; however, in some embodiments, the patterned top metal layer portions 8081-808... n Some or all of them can be interconnected, such as via recessed traces. Exemplary top metal layer portions 8081-808 n The square metal layer portions are shown as uniformly distributed over the piezoelectric layer 806, but they can be any desired shape or distribution, for example, to utilize a specific portion of the piezoelectric layer 806 with higher transduction efficiency or to provide a specific desired transmit or receive mode. In the example of the differential PMUT device (not shown), complex patterning can be included on one or both of the piezoelectric layer regions.

[0053] In some embodiments, the active and inactive portions can be dynamically selected by the processing circuitry system, for example, by selectively connecting the metal layer portions to specific signals, such as a floating potential, ground, or other voltages that inactivate the metal layer portions and their corresponding piezoelectric layer portions. This selection can provide different transmit and receive characteristics from a single PMUT device while retaining the advantage of removing portions of the top metal layer from the total capacitance. For example, in Figure 3 and Figures 5A-7 In any of the embodiments, each top metal layer may be connected to the processing circuitry system and selectively made active or inactive. Figure 8 In an example where all top metal layer portions are individually connected to the processing circuit system, selective activation of the top metal layer portion and the piezoelectric layer portion can be achieved in the top metal layer portion 8081-808. n Perform on any desired pattern.

[0054] In the embodiments described herein, such as regarding Figure 3 , Figure 5A , Figure 5B , Figure 6 , Figure 7 and Figure 8 Patterning can also be performed on the piezoelectric layer itself, for example, by providing patterning on the piezoelectric layer directly below and / or directly above the patterned metal layer, which can provide cost reduction and / or yield improvement during manufacturing.

[0055] Figure 9 An improvement in the sensitivity of a PMUT device based on modifications to the active piezoelectric region according to embodiments of this disclosure is illustrated. Figure 9 In the example, the energy conversion efficiency mode can be compared with... Figure 5BThe transduction efficiency pattern 520 is shown in the diagram. For PMUT device 902, the active region is shown by dot patterning as the innermost circular portion of the central piezoelectric region and the outermost annular portion of the outer piezoelectric region. For PMUT device 904, the active region pattern is shown by dot patterning as the outermost annular portion of the central piezoelectric region and the outermost annular portion of the outer piezoelectric region. (Return to Reference) Figure 5B It will be noted that PMUT device 902 has an active center piezoelectric region corresponding to a first transducer efficiency region 522a with relatively low transducer efficiency, while PMUT device 904 has an active center piezoelectric region corresponding to a second transducer efficiency region 522b with relatively high transducer efficiency. As shown in the normalized PMUT speed diagram 906 driven by the applied voltage, a significant efficiency improvement is achieved solely by utilizing the second transducer efficiency region 522b within the central piezoelectric layer.

[0056] Figure 10 Exemplary steps for optimizing a PMUT design with inactive piezoelectric regions according to embodiments of this disclosure are shown. While for Figure 10 Specific steps are shown in a particular order, but in some embodiments, steps may be removed, modified, or replaced, additional steps may be added, and in some embodiments, the order of certain steps may be modified.

[0057] The process begins at step 1002, where an initial PMUT physical configuration is determined. The PMUT physical configuration can be determined based on the end-use application and can be based on various constraints, such as desired transmit power, receive sensitivity, frequency response, resonant frequency, component size, manufacturing process, and power consumption. The PMUT physical configuration involves the selection and size of materials (such as metal layers, PMUT films, and piezoelectric layers) and connecting components and materials (such as substrates). Once the PMUT physical configuration for the PMUT device is determined, the process can proceed to step 1004.

[0058] At step 1004, the PMUT device is analyzed to determine the vibrational mode shape. This analysis identifies portions of the PMUT device with different levels of transduction efficiency. Proceeding to step 1006, high transduction efficiency portions of the piezoelectric layer are identified, thus providing a range of options for making those portions of the piezoelectric layer inactive. The process can then proceed to step 1008.

[0059] At step 1008, it can be determined whether the PMUT device design is a differential sensor design. If the design is not differential, the process can proceed to step 1012. If the design is differential, the process can proceed to step 1010 to balance the inactive and active portions between the corresponding differential piezoelectric layers of the PMUT device. In an example with a central circular piezoelectric layer region and surrounding concentric ring piezoelectric layer regions, the coverage of the active metal layer in each region can be selected to be closely matched in total surface area (e.g., assuming the material and thickness are the same) and thus match the capacitance. The portion of the covered piezoelectric layer region can then be selected to match the overall sensitivity, which may include selecting only those regions with the highest transduction efficiency, or in some embodiments may involve selecting to match the transmit and receive sensitivities. The process can then proceed to step 1012.

[0060] At step 1012, inactive portions of the piezoelectric layer can be selected to optimize PMUT device sensitivity. For example, portions of the piezoelectric layer associated with the highest transduction efficiency (e.g., greater than a threshold for a particular application) can be selected such that lower transduction efficiency regions do not contribute to the total PMUT device capacitance. The process can then proceed to step 1014.

[0061] At step 1014, the metal layer is patterned to generate a desired distribution of active and inactive piezoelectric layer portions. Patterning can be performed in various suitable manner, such as removing material from the metal layer to create electrically isolated metal layer portions, where only the portions associated with the active piezoelectric layer portions are connected to the active electrical signals. Once the metal layer has been patterned, the process can end.

[0062] The foregoing description includes exemplary embodiments according to this disclosure. These examples are provided for illustrative purposes only and are not intended to be limiting. It will be understood that this disclosure may be implemented in forms different from those expressly described and shown herein, and various modifications, optimizations, and variations can be made by those skilled in the art in accordance with the appended claims.

Claims

1. A piezoelectric micromechanical ultrasonic transducer (PMUT), comprising: a PMUT membrane layer; a first metal layer; a second metal layer; and a piezoelectric layer, the piezoelectric layer being patterned at least as a first piezoelectric layer portion over only a portion of the PMUT membrane layer, wherein the first piezoelectric layer portion is at least partially located between the first metal layer and the second metal layer, and wherein the first metal layer is patterned to define an active region of the first piezoelectric layer portion and an inactive region of the first piezoelectric layer portion.

2. The PMUT of claim 1, wherein the first metal layer is located between the PMUT membrane layer and the piezoelectric layer.

3. The PMUT of claim 1, wherein the second metal layer and the piezoelectric layer are located between the PMUT membrane layer and the first metal layer.

4. The PMUT of claim 1, wherein the active region corresponds to a first patterned portion of the first metal layer, and wherein the first patterned portion of the first metal layer is connected to an active electronic signal path to transmit and receive signals using the active region of the first piezoelectric layer portion.

5. The PMUT of claim 4, wherein the inactive region corresponds to a second patterned portion of the first metal layer, and wherein the second patterned portion of the first metal layer is at a floating potential or connected to ground.

6. The PMUT of claim 4, wherein the inactive region corresponds to a portion of the first metal layer that has been removed, such that the inactive region is not coupled to the first metal layer.

7. The PMUT of claim 1, wherein the active region corresponds to a region of the piezoelectric layer that has a higher transduction efficiency than a region of the piezoelectric layer that corresponds to the inactive region.

8. The PMUT of claim 7, wherein the active region having a higher transduction efficiency is identified based on PMUT vibration modal shape properties in the piezoelectric layer.

9. The PMUT of claim 1, wherein the active region comprises a first active region and the inactive region comprises a first inactive region, wherein a second piezoelectric layer portion is at least partially located between the first metal layer and the second metal layer, and wherein the first metal layer is patterned to define a second active region of the second piezoelectric layer portion and a second inactive region of the second piezoelectric layer portion.

10. The PMUT of claim 9, wherein the first active region corresponds to a first capacitor formed between the first metal layer and the second metal layer and the second active region corresponds to a second capacitor formed between the first metal layer and the second metal layer, and wherein the first capacitor and the second capacitor have substantially matched capacitances.

11. The PMUT of claim 9, wherein the first active region and the second active region have substantially matched transduction efficiencies.

12. The PMUT of claim 9, wherein the first active region corresponds to a first capacitor formed between the first metal layer and the second metal layer and the second active region corresponds to a second capacitor formed between the first metal layer and the second metal layer, and wherein a received acoustic signal generates a first electrical signal output from the first capacitor, the first electrical signal being a differential signal compared to a second electrical signal output from the second capacitor. ​ 13. The PMUT of claim 12, wherein a first electrical signal input provided to the first capacitor to generate the first transmit acoustic signal output from the first capacitor is a differential signal compared to a second electrical signal input provided to the second capacitor to generate the second transmit acoustic signal.

14. The PMUT of claim 9, wherein the second active region surrounds the first active region.

15. The PMUT of claim 14, wherein the second active region and the first active region form a concentric shape.

16. The PMUT of claim 15, wherein the concentric shape is at least a portion of a circular shape for the first active region and a ring shape for the second active region.

17. The PMUT of claim 1, wherein the active region comprises a plurality of spaced apart active regions within the first piezoelectric layer portion.

18. The PMUT of claim 16, wherein the plurality of spaced apart active regions are distributed within the first piezoelectric layer portion.

19. The PMUT of claim 1, wherein the active region comprises a plurality of spaced apart inactive regions within the first piezoelectric layer portion.

20. A method for selecting patterning of a metal layer for a piezoelectric micromechanical ultrasonic transducer (PMUT), comprising: providing an initial configuration of a piezoelectric layer between a first metal layer and a second metal layer over a portion of a PMUT membrane layer; determining a vibrational modal shape of the piezoelectric layer; identifying one or more portions of the piezoelectric layer having a higher transduction efficiency compared to other portions of the piezoelectric layer based on the modal shape of the piezoelectric layer; and patterning the first metal layer such that the one or more portions of the piezoelectric layer are active regions of the piezoelectric layer and the other portions of the piezoelectric layer are inactive regions of the piezoelectric layer.

21. A method for designing a piezoelectric micromechanical ultrasonic transducer (PMUT), comprising: providing an initial configuration of a piezoelectric layer between a first metal layer and a second metal layer over a portion of a PMUT membrane layer; and patterning one of the first metal layer or the second metal layer to modify a capacitance parameter of the PMUT without substantially affecting a structural mechanics of the PMUT. ​ ​

Images