WO2025188908A1

WO2025188908A1 - Micro electrical mechanical system ultrasound transducer and methods of fabricating the same

Info

Publication number: WO2025188908A1
Application number: PCT/US2025/018592
Authority: WO
Inventors: Tyler RALSTON
Original assignee: Shifamed Holdings LLC
Current assignee: Shifamed Holdings LLC
Priority date: 2024-03-06
Filing date: 2025-03-05
Publication date: 2025-09-12
Anticipated expiration: 2026-09-06
Also published as: WO2025188908A8; TW202539583A

Abstract

A method of fabricating a micro electrical mechanical system (MEMS) ultrasound transducer is described. The method may include providing a structure with an application specific integrated circuit (ASIC). The method may include forming a patterned electrode on the structure, the patterned electrode being coupled to an ASIC on a first side of the patterned electrode. A patterned electrode can include at least a sensing electrode electrically coupled to an ASIC and has at least a portion removed according to a pattern. The method may include forming a cavity in an oxide layer positioned on a second side of the sensing electrode, opposite the first side of the patterned electrode. The method may include bonding a MEMS membrane to the structure at a first side of the MEMS membrane using low temperature fusion boding, a portion of the first side of the MEMS membrane proximate to the cavity.

Description

MICRO ELECTRICAL MECHANICAL SYSTEM ULTRASOUND TRANSDUCER AND METHODS OF FABRICATING THE SAME CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims priority to U.S. Prov. Patent App No. 63/562198 titled “MICRO ELECTRICAL MECHANICAL SYSTEM ULTRASOUND TRANSDUCER FABRICATION” and filed on March 6, 2024. The disclosure of the above-recited provisional application is hereby incorporated herein by reference in its entirety. TECHNICAL FIELD [0002] The present disclosure relates to micro electrical mechanical system (MEMS) devices. Specifically, the present disclosure relates to MEMS based ultrasound transducers used in ultrasound imaging devices. BACKGROUND [0003] Ultrasound imaging devices can be helpful in diagnosing condition in and operating on patients in clinical settings. In some instances, ultrasound imaging devices are inserted into a patient. For example, in a transesophageal echocardiography (TEE) a probe of an ultrasound imaging device is inserted into a patient’s esophagus and used to image the patient’s heart. As another example, in intracardiac echocardiography (ICE) a probe of an ultrasound imaging device is inserted into a patient (e.g., through the patient’s venous system) and into the patient’s heart. [0004] Reducing the size of an ultrasound imaging device that is inserted into a patient may be beneficial. For example, doing so may reduce discomfort experienced by the patient, increase maneuverability of the ultrasound imaging device within the patient, increase the applicability of the ultrasound imaging device for various procedures, and/or provide other benefits. However, many clinical applications may require a standard of fidelity, specifications for the transducer capabilities (e.g., capable frequencies, power usage, array sizes, and/or the like), and/or other have requirements for aspects of the ultrasound imaging device. SUMMARY [0005] In one aspect, the techniques described herein relate to a method of fabricating a micro electrical mechanical system (MEMS) ultrasound transducer, the method including: providing a structure with an application specific integrated circuit (ASIC); forming a patterned electrode on the structure, the patterned electrode being coupled to the ASIC on a first side of the patterned electrode, wherein the patterned electrode includes at least a sensing electrode electrically coupled to the ASIC and has at least a portion removed according to a pattern; forming a cavity in an oxide layer positioned on a second side of the sensing electrode, opposite the first side of the patterned electrode; and bonding a MEMS membrane to the structure at a first side of the MEMS membrane using low temperature fusion boding, wherein bonding occurs at a perimeter region of the structure, leaving a central portion of the MEMS membrane overlying the cavity free to deflect. [0006] In some embodiments, the method can include depositing a metal layer on a second oxide layer formed on a second side of the MEMS membrane, opposite the first side of the MEMS membrane; removing a portion of the metal layer to form a patterned metal layer; and applying a passivation layer to a surface of the structure. In some embodiments, the ASIC is a bipolar-complementary metal oxide semiconductor-double-diffused metal-oxide- semiconductor (BCD). In some embodiments, forming the patterned electrode includes depositing a conductive layer; and removing at least the portion of the conductive layer according to the pattern. In some embodiments, bonding the MEMS membrane to the structure includes: providing a wafer including the MEMS membrane and a handling wafer coupled to the MEMS membrane at a second side of the MEMS membrane, opposite the first side of the MEMS membrane; bonding the wafer to the structure at a first side of the MEMS membrane; and removing the handling wafer. In some embodiments, the patterned electrode further includes a second sensing electrode. In some embodiments, the patterned electrode includes at least four sub-sensing electrodes, each sub-sensing electrode individually electrically coupled to the ASIC and configured to receive separated voltages. In some embodiments, the patterned electrode includes a pull in electrode positioned centrally on the patterned electrode and electrically coupled to a separate voltage source, the pull in electrode configured to collapse a portion of the MEMS membrane by applying a threshold voltage on the MEMS membrane. In some embodiments, the separate voltage source is a power rail embedded in the structure. In some embodiments, the sensing electrode is configured to sense and/or induce movement in one or more moveable portions of the MEMS membrane, different than the collapsed portion. In some embodiments, forming the cavity in the oxide layer positioned on the second side of the sensing electrode includes: forming patterned passivation features on a top portion of a barrier layer, wherein the barrier layer is a portion of the oxide layer that remains between the cavity and the second side of the sensing electrode, and wherein the top portion of the barrier layer is adjacent to the cavity and a bottom portion, opposite the top portion is adjacent to the second side of the sensing electrode. [0007] In another aspect, the techniques described herein relate to a micro electrical mechanical system (MEMS) ultrasound sensor including: a transducer array including a plurality of capacitive micromachined ultrasonic transducers (CMUTs), wherein each of the plurality of CMUTs includes: a MEMS membrane, one or more cavities proximate to at least a portion of the MEMS membrane at a first side of the one or more cavities, and a patterned electrode positioned proximate to the one or more cavities at a second side of the one or more cavities, opposite the first side, the patterned electrode including at least a sensing electrode electrically coupled to an application specific integrated circuit (ASIC) and has at least a portion removed according to a pattern; a portion on a periphery of the transducer array including a plurality of capacitors, each capacitor electrically coupled to at least one CMUT of the plurality of CMUTs; and the ASIC electrically coupled to the transducer array and the plurality of capacitors, wherein, based on an electrical signal from the ASIC, the plurality of capacitors are configured to store charge and modulate the electrical signal delivered to the plurality of CMUTs and cause the plurality of CMUTs to transmit one or more ultrasonic or ultrasound waves. [0008] In some embodiments, the transducer array further includes a plurality of inter- element bypass capacitors, each inter-element bypass capacitor positioned between two or more CMUTs of the plurality of CMUTs and electrically coupled to one or more CMUTs of the plurality of CMUTs. In some embodiments, the plurality of inter-element bypass capacitors are configured to store charge and modulate the electrical signal delivered to the plurality of CMUTs and cause the plurality of CMUTs to transmit one or more ultrasonic waves based on the electrical signal from the ASIC. In some embodiments, the plurality of capacitors includes one or more CMUT structures storing energy using capacitances of the CMUT structures. In some embodiments, each of the plurality of CMUTs and the one or more CMUT structures include a same CMUT structure. [0009] In another aspect, the techniques described herein relate to a capacitive micromachined ultrasonic transducers (CMUT) including: a MEMS membrane, one or more cavities proximate to at least a portion of the MEMS membrane at a first side of the one or more cavities, and a patterned electrode positioned proximate to the one or more cavities at a second side of the one or more cavities, opposite the first side, the patterned electrode including at least a sensing electrode electrically coupled to the ASIC and has at least a portion removed according to a pattern. [0010] In some embodiments, at least the sensing electrode is formed by depositing a conductive layer and removing at least the portion according to the pattern. In some embodiments, the patterned electrode further includes a pull in electrode positioned centrally on the patterned electrode and electrically isolated from the sensing electrode, the pull in electrode configured to collapse a portion of the MEMS membrane by applying a threshold voltage on the MEMS membrane. In some embodiments, at least the sensing electrodes is configured to sense and/or induce movement in one or more moveable portions of the MEMS membrane, different than the collapsed portion. In some embodiments, wherein the patterned electrode includes a plurality of sub-sensing electrodes, each of the plurality of sub-sensing electrodes configured to sense movement in different portions of the MEMS membrane. [0011] In another aspects the techniques described herein relate to a patterned electrode of a capacitive micromachined ultrasonic transducers (CMUT), the patterned electrode including: a bypass electrode positioned on a periphery of the patterned electrode; one or more sensing electrodes, each electrically coupled to different voltage sources; and removed portions electrically isolating the bypass electrode and each of the one or more sensing electrodes. [0012] In some embodiments, the bypass electrode and the one or more sensing electrodes are formed by depositing a conductive layer and removing the removed portions. In some embodiments, the patterned electrode further includes a pull in electrode positioned centrally on the patterned electrode and electrically isolated from the one or more sensing electrodes and the bypass electrode, the pull in electrode configured to collapse a portion of a MEMS membrane by applying a threshold voltage on the MEMS membrane. In some embodiments, the one or more sensing electrodes are configured to sense and/or induce movement in one or more moveable portions of the MEMS membrane, different than the collapsed portion. In some embodiments, the one or more sensing electrodes include a plurality of sub-sensing electrodes, each of the plurality of sub-sensing electrodes configured to sense and induce movement in different portions of a MEMS membrane. In some embodiments, the removed portions are defined using a lithographic patterning and selective etching process, such that the remaining electrode regions are electrically isolated. BRIEF DESCRIPTION OF THE DRAWINGS [0013] Figure 1A is a schematic diagram illustrating an example ultrasound sensor. [0014] Figure 1B is a schematic diagram illustrating another example ultrasound sensor. [0015] Figure 2 illustrates an example portion of a transducer array. [0016] Figure 3 illustrates an example portion of a bond pad arrangement. [0017] Figures 4A-4L are schematic illustrations of cross sections at various stages during fabrication of a micro-electromechanical systems (MEMS) ultrasound sensor. [0018] Figures 5A-5D illustrate electrodes according to various embodiments of the disclosure. [0019] Figure 6 is a flowchart of an example process for fabricating an ultrasound sensor, according to various embodiments of this disclosure. [0020] Embodiments of the present disclosure and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures, wherein showings therein are for purposes of illustrating embodiments of the present disclosure and not for purposes of limiting the same. DETAILED DESCRIPTION [0021] Described herein is a device, such as an ultrasound sensor, manufactured using micro-electromechanical system (MEMS) and a complementary metal oxide semiconductor (CMOS) process(es). The devices and processes described herein may be used in ultrasound imaging devices, providing several advantages. For example, the ultrasound sensor described herein may provide improved performance and efficiency, such as providing improved operating voltages, utilizing patterned electrodes, and so on. As another example, the ultrasound sensor described herein may advantageously provide multiple operating modes, such as high frequency and low frequency modes. As another example, the ultrasound devices described herein may provide increased device reliability, such as lowering the dielectric breakdown and/or increasing resilience against dielectric breakdown. As another example, the ultrasound devices described herein may reduce stiction via a passivation approach. The examples above are not limiting, and other advantages, including other advantages described below, may result from the devices and techniques described herein. [0022] According to various embodiments of this disclosure, an ultrasound sensor can include a MEMS based ultrasound transducer (referred to herein as a “MEMS ultrasound transducer”). The MEMS ultrasound transducers described herein may be beneficial and provide the above described advantages, and other advantages apparent in this disclosure. The MEMS ultrasound transducers may provide compact form factors. The MEMS ultrasound transducer may include integration with associated complementary metal oxide semiconductor (CMOS) drivers to form the ultrasound sensor. MEMS ultrasound transducers may further provide a wide range of ultrasound frequencies. As such, MEMS ultrasound transducers may advantageously reduce the size of the ultrasound imaging devices they are implemented in while maintaining the standards needed for many clinical applications. This reduction in size may improve upon prior medical techniques associated with TEE, ICE, and/or other ultrasound procedures. [0023] The MEMS ultrasound transducer described herein can be implemented for, or otherwise included in, different ultrasound devices, including, but not limited to, TEE devices, ICE devices, intravascular ultrasound (IVUS) devices, or any other suitable ultrasound imaging device. Further, while aspects of this disclosure will be described as implementing capacitive micromachined ultrasonic transducers (CMUTs) will be described, other MEMS transducer may be used without departing from the disclosure. For example, in some implementation piezoelectric micromachined ultrasonic transducers (PMUTs) may be used. [0024] The disclosed technology will now be described in more detail. Example Ultrasound Sensor [0025] Figure 1A is a schematic diagram illustrating an example ultrasound sensor 100. The ultrasound sensor 100 may implemented in an ultrasound device, such as a TEE device, an ICE device, or another suitable ultrasound device. In some implementations, the ultrasound sensor 100 may be formed on one or more dies. The ultrasound sensor 100 may be formed on, or bonded to, an application specific integrated circuit (ASIC), such as a bipolar-CMOS- DMOS (abbreviated as “BCD”, where DMOS is an abbreviations of a double-diffused metal- oxide-semiconductor), a silicon-on-insulator (SOI) CMOS, a fully depleted SOI (FD-SOI), though other suitable ASICs may be used. In the illustrated example, the ultrasound sensor 100 includes a transducer array 102, bond pad arrangements 104, a process-control-monitor structures (PCM) reserved space 106, and a transducer array peripheral space 108. [0026] In the illustrated example, the ultrasound sensor 100 includes various distances, X1, X2, X3, X4, Y1, Y2, Y3, and Y4. These distances are provided for illustration only and provide an example of how the various components may be positioned on the ultrasound sensor 100. However, other distances and/or component positions may be implemented for the ultrasound sensor 100 without departing from this disclosure. In the illustrated example, X1 is a width of the transducer array 102, X2 is a width of the PCM reserved space 106, X3 is a distance from the edge of the transducer array peripheral space 108 to a far edge of a bond pad arrangement 104 (and/or a distance from a peripheral edge of a last bond pad arrangements 104 and a peripheral edge of the ultrasound sensor 100), and X4 is a distance from the edge of the transducer array 102 to the edge of the transducer array peripheral space 108) in a first dimensional axis. Further, in the illustrated embodiment Y1 is a height of the transducer array 102, Y2 is a height of the PCM reserved space 106, Y3 is a distance from the transducer array peripheral space 108 to a far edge of a bond pad arrangement 104, and Y4 is a distance from the edge of the transducer array 102 to the edge of the transducer array peripheral space 108 in a second dimensional axis. The various distances, X1, X2, X3, X4, Y1, Y2, Y3, and Y4 may depend on the particular application of the ultrasound sensor 100. For example, Y1 may be increased or reduced depending on application (e.g., Y1 may be smaller for implementations where a smaller probe is needed). Additionally, the application may increase and/or decrease component sizes on the ultrasound sensor 100. For example, lower frequency applications may use larger transducer elements and increase the overall size of the transducer array 102. In one example embodiment, X1 is 11520 m, X2 is 12920 m, X3 is 100 m, X4 is 300 m, Y1 is 8640 m, Y2 is 1500 m, Y3100 m, and Y4 is 750 m. However, X1 can have a value that is smaller or larger than 11520 m (e.g., 12000-15000 m, or larger or 9000-12000 m or smaller), X2 can have a value that is smaller or larger than 12920 m (e.g., 13000-15000 m, or larger or 9000-13000 m or smaller), X3 and Y3 can have values that are smaller or larger than 100 m (e.g., 50-100 m or smaller or 100-200 m or larger), X4 can have values that are larger or smaller than 300 m (e.g., 100-300 m or smaller or 300-600 m or larger), Y1 can have a value that is smaller or larger than 8640 m (e.g., 5000-9000 m or smaller or 9000- 11000 m or larger), Y2 can have a value that is smaller or larger than 1500 m (e.g., 1000- 1500 m or smaller or 1500-3000 m or larger, and Y4 can have a value that is smaller or larger than 750 m (e.g., 500-750 m or smaller or 750-1000 m or larger). [0027] Figure 1B is a schematic diagram illustrating another example ultrasound sensor _{150. The ultrasound sensor 150 can include any of the features and/or components described} above with respect to ultrasound sensor 100. In some embodiments, the ultrasound sensor 150 may represent a reduced size ultrasound sensor. As such, ultrasound sensor 150 may be suitable in applications where the ultrasound sensor benefits from a reduced size, does not need as many access points, and/or in other suitable applications. In one example implementation, the ultrasound sensor 150 may be included in a TEE or ICE probe. In such implementation, a group of bond pad arrangement 104 may be positioned together on a side of the ultrasound sensor 150 connected to catheter components (e.g., wire bundles) which may extend in direction D. [0028] The distances X5, X6, X7, X8, Y5, Y6, Y7, and Y8 may have similar values and constraints as discussed above with respect to X1, X2, X3, X4, Y1, Y2, Y3, and Y4 (e.g., X5 may be similar to X1, X7 may be similar to X3, X8 may be similar to X4, Y5 may be similar to Y1, Y6 may be similar to Y2, Y7 may be similar to Y3 and Y8 may be similar to Y4). However, some values may reflect the reduced size (e.g., X6 may be reduced compared to X2 due to fewer and/or smaller components). [0029] According to various embodiments, flex bonding packaging, integrated fan-out (InFO) packaging, and/or any other suitable packaging may be used with implementation of the ultrasound sensor 100 and/or the ultrasound sensor 150. Transducer Array 102 [0030] According to various implementations, the transducer array 102 can be an array of transducing elements, such as the transducer element 202 discussed in Figure 2. Each transducer element of the transducer array 102 may individually (and/or in conjunction with other transducer elements of the transducer array 102) convert electrical energy into mechanical vibrations, emitting ultrasonic waves (referred to herein as a “transmit mode” of the transducer element). Each transducer element of the transducer array 102 may individually (and/or in conjunction with other transducer elements of the transducer array 102) detect sonic waves (e.g., ultrasonic waves) and convert the sonic wave into an electrical signal (referred to herein as a “receive mode” of the transducer element). Each transducer element of the transducer array 102 may individually (and/or in conjunction with other transducer elements of the transducer array 102) be configured to operate at different frequencies (e.g., based on an electrical control signal sent to the transducer elements), thereby allowing the transducer array 102 to operate at multiple frequencies simultaneously. [0031] The timing and phase of transmitted waves from the transducer elements and the timing and/or phase of generated electrical signals from received waves by the transducer elements can be individually controlled. This may enable beam steering, focusing, spatial filtering and other beamforming functions for the transducer array 102. While the transducer array 102 is illustrated as a rectangular array, nonrectangular array shapes may be used without departing from this disclosure. [0032] The example transducer array 102 is illustrated as having dimensions X1, Y1. The dimensions X1, Y1 may depend on the number of transducer elements in the transducer array 102, the size and/or shape of the transducer elements in the transducer array 102, and/or operating features of the transducer array 102 (e.g., based on the specific application of the transducer array 102 and specifications needed for that specific application). In one example implementation, X1 is 11520 m and Y1 is 8640 m, though, as described above, other values of X1, Y1 may be implemented without departing from this disclosure. The values of X1 and/or Y1 may be constrained be application. For example, some ultrasound applications (e.g., TEE) may constrain the probe size, and by extension the transducer array 102. Similarly, the values of X1 and Y1 can be constrained to values of the dimensions of one transducer element. [0033] When transmitting, the transducer array 102 may be configured to receive electrical energy from an ASIC device (e.g., ASIC 401 illustrated in Figures 4A-4L) and/or receive electrical energy based on electrical signals from the ASIC device. For example, electrical energy may be delivered to electrodes of the transducer elements of the transducer array 102. As described below, in some implementations all, or a portion, of the electrical energy may be delivered from capacitors that are located in the transducer array 102, elsewhere on the ultrasound sensor 100, and/or on the ASIC. When receiving, the transducer array 102 may be configured to transmit electrical signals generated by the transducer elements from received waves to the ASIC. [0034] Figure 2 illustrates an example portion 200 of a transducer array, such as transducer array 102. In the illustrated example the portion 200 includes multiple transducer elements _{202. Each transducer element 202 has either an active CMUTs 204 or a bypass capacitor 206.} Inter-element bypass capacitors 208 can be positioned (e.g., interspaced, interspersed, interwoven, and so on) between the transducer elements 202 (e.g., located at shared corners of the transducer elements 202). In some embodiments, the bypass capacitors, such as capacitors 208, may be connected to the ASIC described herein via, at least in part, a bypass electrode for individual CMUTs. An example bypass electrode is described in more detail below with respect to Figures 4A-5D. [0035] The active CMUTs 204 can be CMUTs used for transmitting and receiving signals (e.g., ultrasonic waves). As described above, when the transducer element 202 is in a transmit mode, the active CMUT 204 receives electrical energy and converts the electrical energy into a signal (e.g., ultrasonic waves) that is transmitted from the transducer element 202. When the transducer element 202 is in a receive mode, the active CMUT 204 converts a received signal (e.g., ultrasonic, and/or other sound waves) into an electrical signal. [0036] The bypass capacitor 206 may be used to provide the electrical energy (or a portion of the electrical energy) used by the active CMUTs 204 when generating the transmitted signals. For instance, in some applications, the active CMUTs 204 may use relatively high energy when transmitting signals. The bypass capacitor 206 may help the active CMUTs 204 reach the needed energy levels. For example, the bypass capacitors 206 may each discharge electrical energy based on a signal from an ASIC (e.g., from a pulser on the ASIC) into one or more of the active CMUTs 204. The bypass capacitor 206 may be connected to a power supply or power rail (e.g., a pulser or other analog or digital supply) which can be used to charge and/or discharge the bypass capacitor 206. According to various implementations the bypass capacitors 206 may use a CMUT structure (e.g., the same CMUT structure as the active CMUTs 204) and store the electrical energy using the capacitance of the CMUT structure. For example, the CMUT structure of a bypass capacitor 206 may store electrical energy rather than converting the electrical energy into transmitted signals. Thus, the capacitors may be configured to store charge and modulate electrical signals delivered to the CMUTs, for example to control ultrasonic wave transmission. [0037] According to some implementations, the active CMUTs 204 form the transducer array 102 illustrated in Figures 1A and 1B and the bypass capacitors 206 form the transducer array peripheral space 108 on the periphery of the transducer array 102. In some implementations, the bypass capacitors 206 may be positioned (e.g., interspaced, interspersed, interwoven, and so on) between in the transducer array 102. [0038] In some implementations, a particular CMUT structure may operate as both an active CMUT 204 and a bypass capacitor 206, depending on the context. For example, in some uses, a transducer array may use fewer (or more) transducer elements 202. As such, in these examples, the particular CMUT structure may operate as an active CMUT 204 when more transducer element 202 are used and a bypass capacitor 206 when the particular CMUT structure is not used from transmission and reception. In other implementations, the CMUT structure of the transducer array may perform fixed functionality (e.g., the CMUT structures may operate, such as may only operate, as an active CMUT 204 or a bypass capacitor 206 regardless of context). [0039] While the active CMUTs 204 and bypass capacitor 206 structures are illustrated as circular, in some implementations, the active CMUTs 204 and bypass capacitor 206 structures may be non-circular. For example, in some implementations the active CMUTs 204 and bypass capacitor 206 structures may be elliptical, non-uniform, and/or otherwise shaped. The sizes of the CMUTs 204 and bypass capacitor 206 may depend on the application of the ultrasound sensor 100. For example, in some applications X9 and Y9 might have values of approximately 180 m. However, other applications may require a different frequency range (e.g., need to transmit and/or receive at lower or higher frequencies) and the values of X9 and Y9 may be larger than or smaller than 180 m. Further, X9 and Y9 may not be equal (e.g., when the CMUTs 204 and/or bypass capacitor 206 is elliptical). [0040] The inter-element bypass capacitors 208 can also provide electrical energy to the active CMUTs 204. The inter-element bypass capacitors 208 may be formed using CMUT structures (e.g., using electrodes and membranes) and/or using other structure types (e.g., using metal-oxide-semiconductor (MOS) capacitor structures, metal-insulator-metal (MIM) capacitor structures, metal-interlayer-metal (MiMCap) capacitor structures, metal-oxide-metal (MOM) capacitor structures, and/or other suitable capacitor structures). The inter-element bypass capacitors 208 and/or the bypass capacitor 206 may perform other functions for the transducer array such as noise reduction, impedance matching, electrostatic discharge protection, signal isolation, and/or any other suitable function of capacitors in MEMS ultrasound transducers. Bond Pad Arrangements 104 [0041] The bond pad arrangements 104 may each include a plurality of contact pads, such as the contact pads 302 illustrated in Figure 3. The contact pads may provide input/output (I/O) and/or other interfacing for the ultrasound sensor 100. For example, some of the contact pads of the bond pad arrangements 104 may be connected to signal cables (e.g., coaxial cables) that extend from the ultrasound sensor 100 and up a catheter and/or connected to other components of an ultrasound probe. Some of the contact pads of the bond pad arrangements 104 may provide electrical connection to the ASIC that the ultrasound sensor 100 is bonded to (or formed on). Some of the contact pads of the bond pad arrangements 104 may be otherwise routed. For example, some of the contact pads of the bond pad arrangements 104 may rout directly to one or more transducer elements of the transducer array 102 to structures in the PCM reserved space 106, to capacitors of the transducer array 102 (or elsewhere on the ultrasound sensor 100), and/or otherwise routed. [0042] The contact pads of the bond pad arrangements 104 may be formed on the surface of the ultrasound sensor 100 using a conductive and/or semiconductive material. For example, the contact pads of the bond pad arrangements 104 may be formed of a metal (e.g., gold, platinum, aluminum, copper, etc.) deposited onto the ultrasound sensor 100 and etched into the contact pads). [0043] Figure 3 illustrates an example portion 300 of a bond pad arrangement, such as a bond pad arrangement 104. In the illustrated example, the portion 300 includes contact pads 302, each positioned within a unit cell 304. As described above, the contact pads 302 can provide I/O and/or other interfacing for an ultrasound sensor. Each contact pads 302 may be formed of a metal and connected to an electrical routing path to the desired component. For example, a contact pad 302 may be connected to an electrical routing path leading to the ASIC, thereby providing an I/O interface to the ASIC. [0044] The unit cells 304 may provide footprints for the total area associated with each contact pads 302 (e.g., the space of the contact pad 302 plus a pitch distance). For example, the unit cells unit cells 304 may have dimensions D1xD1 where D1 is the side length D2 of the unit cell 304 plus a pitch distance. While the unit cells 304 and contact pads 302 are illustrated as having equal dimensions along a first and second axis (e.g., they are approximately square), in some instances one or both of the unit cells 304 and contact pads 302 may have other proportions (e.g., they may be rectangular or otherwise shaped). PCM Reserved Space 106 [0045] The PCM reserved space 106 can include structures and/or components used in the manufacture, testing, and/or monitoring of the ultrasound sensor 100 and/or the ASIC the ultrasound sensor 100 is bonded (or formed on). Transducer Array Peripheral Space 108 [0046] The transducer array peripheral space 108 can provide capacitor structures used by the ultrasound sensor 100. For example, in some implementations, the transducer array peripheral space 108 may include bypass capacitors 206 illustrated in Figure 2. As described above, the capacitors may provide electrical energy (e.g., current) used in ultrasound transmission by the transducer array 102. The capacitors may also provide other functionality, such as noise reduction, impedance matching, electrostatic discharge protection, signal isolation, and/or any other suitable function of capacitors in MEMS ultrasound transducers. Example Fabrication a MEMS Ultrasound Sensor [0047] Figures 4A-4L are schematic illustrations of cross sections at various stages during fabrication of the disclosed MEMS ultrasound sensor. The various components illustrated in Figures 4A-4L are not necessarily shown to scale. Further, fabrication steps other than those described herein may be used without departing from this disclosure. In some implementations low temperature fusion bonding may be used for one or more of the fabrication steps described below. For example, low temperature fusion bonding may be used to bond an ASIC 401 to the structure forming MEMS ultrasound sensor, bond a wafer (e.g., a wafer with a handler wafer 417 and MEMS membrane 416) to the structure forming the MEMS ultrasound sensor. [0048] The low temperature fusion bonding may allow the combining of components at relatively low temperature (e.g., below 500 degrees C, below 450 degrees C, below 400 degrees C, below 300 degrees C, or lower) and can include plasma-activated direct bonding, anodic bonding, adhesive bonding, solder bonding, thermocompression, and/or other suitable low temperature bonding techniques. In some implementation eutectic bonding may be used for one or more of the fabrication steps described below. In some embodiments, gold, gold alloys, tin, and/or any other suitable material may be used in the bonding (e.g., the low temperature fusion bonding and/or the eutectic bonding). [0049] Cross section 400 of Figure 4A illustrates the fabrication process at a first instance. The cross section 400 includes an ASIC 401, ASIC top metal 402, a redistribution layer (RDL) 403, and oxide 404. While the illustrated example shows the ASIC 401 as a BCD, other suitable ASICs may be used. The ASIC 401 includes ASIC top metal 402 that provides an interface to the ASIC to send and receive electrical signals. The RDL 403 can include metal or other conductive traces and help facilitate alignment between the ASIC 401 and components of the MEMS ultrasound sensor (e.g., the sensing metal 411 described below). The oxide 404 can be formed of any suitable insulating material (e.g., silicon dioxide) formed from (e.g., from oxidization processes) and/or deposited on or bonded to the MEMS ultrasound sensor. [0050] Cross section 405 of Figure 4B illustrates the fabrication process at an instance following cross section 400. In cross section 405 the oxide 404 is extended above the top of the RDL. Vias 406 have been formed in the oxide 404 providing an electrical path to the RDL. The vias 406 can be formed of any suitable conductive material. A sensing layer 407 can be formed (e.g., deposited on and/or bonded to) the oxide 404. The vias 406 can provide electrical paths to and from the sensing layer 407 and the RDL. The sensing layer 407 may provide the base for electrodes used for the MEMS transducer (e.g., in the CMUT structure). The sensing layer 407 can any suitable material used in CMUT electrodes (e.g., metals, doped silicon, or other suitable materials). In some embodiments, the sensing layer 407 may be overlayed and/or treated for processing into one or more patterned electrodes (e.g., the electrodes 411 described below). For example, a patterned mask, a lithographic pattern, and/or another suitable technique may be used on the sensing layer 407 in preparation for process(es) of removing a portion of the sensing layer 407 (e.g., dry etching, wet etching, a milling process, and/or another suitable removal process). [0051] Cross section 410 of Figure 4C illustrates the fabrication process at an instance following cross section 405. In cross section 410, portions of the sensing metal 407 have been removed (e.g., through removal processes, such as the etching processes described above) to form one or more electrodes 411. The oxide 404 is extended above the top of the electrodes _{411. Cavities 413 and cavities 412 have been formed in the oxide 404(e.g., by application of} a lithographic patterned and an etching process). The cavities 413 may help provide an electrical pathway from the BCD and RDL to components added later in fabrication. The cavities 412 may form a basis for a gap between the electrodes 411 and a membrane added at a later step. Some oxide 404 may remain between the cavities 412 and the electrodes 411, forming a barrier layer 414 between the cavities 412 and the electrodes 411. [0052] As illustrated in Figure 4C, an element of the MEMS ultrasound sensor (e.g., an individual CMUT) may include multiple cavities 412 and multiple electrodes 411. In some embodiments, an individual CMUT may include an individual cavity such that the illustrated multiple cavities 412 may correspond to multiple CMUTs. The cavities 413 and cavities 412 may be formed using any suitable technique for removing an oxide. For example, in some implementations, the cavities 413 and cavities 412 are formed using dry etching. [0053] According to various embodiments, the electrodes 411 may be implemented using one or more of the electrodes discussed in Figures 5A-5D (e.g., electrode 500, patterned electrode 520, patterned electrode 550, and/or patterned electrode 570), which will be described in more detail with respect to Figures 5A-5D. Patterning may include, for example lithographic patterning and/or selective etching processes. As will be described, a patterned electrode may have at least a portion of sensing metal (e.g., sensing electrode) removed. The patterned electrode may have, in some embodiments, two or more regions which are electrically isolated. [0054] The fabrication process illustrated in Figures 4A-4C may include forming electrical paths (e.g., traces) to and from individual portions of a patterned electrode 411 and/or from each electrode 411 and the ASIC (e.g., electrically coupling the electrode 411 to the ASIC). For example, the ASIC top metal 402, RDL 403, and vias 406 may form multiple traces from the ASIC 401 to individual portions of the electrodes 411 (e.g., a trace for each of the sub- sensing electrodes 522 illustrated in Figure 5B), electrically coupling the individual portions to the ASIC. Electrically coupling, as used herein, describes an electrical connection between components, which may include one or more intervening components, such that an electrical signal can travel to and/or from the components. [0055] Cross section 415 of Figure 4D illustrates the fabrication process at an instance following cross section 410. In cross section 415 a MEMS membrane 416 has been bonded to over an oxide 404 that extends above the cavities 413 and cavities 412. The bonding can include low temperature fusion bonding as described above and/or another suitable bonding technique. The bonding between the MEMS membrane 416 and the oxide 404 can occur at a perimeter region of the MEMS ultrasound sensor (e.g., at the perimeter of the structure shown in cross section 415) leaving a central portion of the MEMS membrane overlying the cavities 412 and free to deflect. A handler wafer 417 may be used to help with the boding process (e.g., to enable the handling and boding of the MEMS membrane 416 without damaging the MEMS membrane 416). The MEMS membrane 416 may be a flexible structure that can move in response to stimuli (e.g., from the electrodes 411 or from incoming ultrasonic waves). The MEMS membrane 416 can be formed of any suitable material, including silicon nitride, doped or undoped silicon (e.g., polysilicon), or other suitable materials. [0056] Cross section 420 of Figure 4E illustrates the fabrication process at an instance following cross section 415. In cross section 420, the handler wafer 417 has been removed (e.g., etched away). After the handler wafer 417 has been removed, the MEMS membrane 416 and an oxide layer 404 remain. [0057] Cross section 425 of Figure 4F illustrates the fabrication process at an instance following cross section 420. In cross section 425, cavities 427 are formed in the oxide 404 (e.g., by etching away portions of the oxide). The cavities 426 are formed through, e.g., by etching away, portions of the oxide 404, the MEMS membrane, and more oxide 404, exposing some of the RDL. [0058] Cross section 430 of Figure 4G illustrates the fabrication process at an instance following cross section 425. In cross section 430, a MEMS top metal layer 431 has been deposited. The MEMS top metal layer 431 can be formed of a conductive material (e.g., a metal). The MEMS top metal layer 431 can contact the MEMS membrane in one or more locations. For example, the MEMS top metal layer 431 can contact at portion 433, located above one or more electrodes, at portion 432, located in the etched-out cavities 426, and at portion 434 located on a side of the cavities 426 opposite the electrodes. [0059] Cross section 435 of Figure 4H illustrates the fabrication process at an instance following cross section 430. In cross section 435, portions of the MEMS top metal have been removed (e.g., by etching) into a MEMS top metal pattern. The MEMS top metal pattern can have a portion 437 contacting the MEMS membrane, portion 436 in one cavity contacting the MEMS membrane, and portion 434 contacting the MEMS membrane and forming an electrical path to the RDL. [0060] Cross section 440 of Figure 4I illustrates the fabrication process at an instance following cross section 435. In cross section 440, trench 441 has been formed in the MEMS membrane. Trench 441 may isolate one portion of the MEMS membrane from another. For example, the MEMS membrane on the left of the trench 441 may form a part of one CMUT while the MEMS membrane on the right of the trench 441 may form a part of another CMUT. Trenches may be used, for example, for membrane isolation such for electrical isolation, stiction reduction, and so on. [0061] Cross section 445 of Figure 4J illustrates the fabrication process at an instance following cross section 440. In cross section 445, oxide 404 is extended above the MEMS top metal pattern. A passivation layer 446 is deposited on a top surface. The passivation layer 446 may be formed material that can provide electrical insulation and help prevent chemical and mechanical breakdown of the MEMS ultrasound sensor. The passivation layer 446 may be formed of silicon nitride, parylene, silicon dioxide, and/or another suitable material. [0062] Cross section 450 of Figure 4K illustrates the fabrication process at an instance following cross section 445. In cross section 450, a portion of the passivation layer is removed (e.g., etched away), exposing a contact portion 451 of the MEMS top metal. [0063] Cross section 455 of Figure 4L illustrates the fabrication process at an instance following cross section 450. In cross section 455, a conductive bump 456 is attached to (or deposited on) the exposed MEMS top metal. The conductive bump 456 may be formed of a conductive material such as plated copper, gold, aluminum, and/or any other suitable electrically conductive material. Cross section 455 illustrates a finished MEMS ultrasound sensor. In the finished MEMS ultrasound sensor, an electrical pathway is formed from the conductive bump 456, through the MEMS metal, through the RDL, and to the ASIC. [0064] In the completed MEMS ultrasound sensor, the CMUT structure can transmit a signal (e.g., an ultrasonic wave) based on an electrical signal received from the ASIC. The electrical signal can cause the electrode to exert electrostatic forces on the MEMS membrane. The electrostatic forces can cause the MEMS membrane to emit an ultrasonic wave with known characteristics (frequency, phase, amplitude, etc.). The CMUT structure can receive a signal (e.g., an ultrasonic wave) which can cause the MEMS membrane to vibrate and induce an electrical signal by the electrode, which is transmitted to the ASIC, where it can be processed. The received signal may be reflected from a portion of the patient (e.g., heart tissue). The resulting electrical signal may be used to display a result in an ultrasound image (e.g., by aggregating multiple electrical signals from various CMUT structures, using the known relationship between ultrasonic waves and anatomic structures, the known characteristics of the transmitted signal, and/or using other factors). [0065] According to various embodiments, the oxide 404 may include patterned oxides (e.g., the barrier layer 414 may be, or include, patterned oxides). The patterned oxides (which can also be referred to as “patterned passivation features”) may include variations on one or more surfaces of the oxide. For example, a top portion of the barrier layer (e.g., the portion of the barrier layer 414 facing the cavities 412) may have posts of material extending further into the cavity 412 and/or recessed portions recessing further into the barrier layer 414, though other suitable patterns may be used. Suitable patterns for the patterned oxides can include, but are not limited to, grid patterns, sets of points that have pairwise ratios different than one another, and/or structured patterns. The patterns may represent a deviation from a mean surface level of the barrier layer. Such deviations may range from a few Angstroms to a few nanometers, in some embodiments. In some embodiments the deviations can be less than a few angstroms or greater than a few nanometers. [0066] Patterned oxides may provide one or more benefits to the structure and/or the MEMS ultrasound sensor. For example, the patterned oxides may optimize electromechanical coupling in the MEMS ultrasound sensor (e.g., by modifying and/or focusing electric field profiles to effective portions, by optimizing mechanical properties such as membrane displacement and resonance characteristics, or other optimizations). As another example, patterned oxides may provide increased dielectric insulation and reliability (e.g., increase breakdown protection and/or breakdown voltages and/or reduce leakage current). As another example, patterned oxides may improve charge trapping and stability (e.g., help control charge trapping) and/or reduce charge drift and unwanted charge polarization effects. As another example, patterned oxides may provide electromechanical performance optimization (e.g., provide improved control of electric field distribution on the MEMS membrane, improve uniformity in MEMS membrane deflection, and/or enhancing sensitivity and bandwidth of the membrane). As another example, patterned oxides may help reduce parasitic capacitance between separate electrodes 411, which can help signal-to-noise ration and impedance matching with other components. Example Electrodes [0067] Figures 5A-5D illustrate electrodes according to various embodiments of the disclosure. In CMUT devices the electrodes described herein (also referred to herein as “sensing metal”) may described the bottom electrode of an electrode pair, with the top electrode being a MEMS membrane. The electrodes illustrated in Figures 5A-5D may be used as the electrodes 411 of the MEMS ultrasound sensor described in Figures 4C-4L. [0068] According to various implementations, when the CMUT is in transmit mode, voltage is applied to the electrode and electrostatic attraction causes the MEMS membrane to deflect towards the electrode. The deflection can cause a wave (e.g., an ultrasonic wave) to be generated and propagated from the CMUT. When the CMUT is in receive mode, an ultrasonic wave interacts with (e.g., strikes) the MEMS membrane, which can displace the MEMS membrane and cause a variation in capacitance between the MEMS membrane and the electrodes. The variation in capacitance can be converted into an electrical signal. [0069] When the MEMS membrane deflects too far (in either transmit mode from a biasing voltage or in receive mode from an acoustic wave), such as greater than a threshold, the MEMS membrane can contact a barrier layer (e.g., the barrier layer 414 in Figure 4C) position between the MEMS membrane and the electrode, which can be referred to as being in collapse. Stiction occurs, as an example, when the MEMS membrane becomes stuck (e.g., permanently or semi permanently) to the barrier layer (or the bottom electrode) which can impair or destroy the operation of the CMUT. Further, repeated collisions between the MEMS membrane and the barrier layer can cause the barrier layer to become damage, potentially causing a short between the MEMS membrane and the electrode. In various implementations, during operation a CMUT may be collapsed (referred to herein as operating in a “collapsed mode”). For example, in some uses a collapsed mode may provide increased sensitivity, increased bandwidth, an increased ability to sense and/or reduce nonlinearities, and/or provide other performance benefits in the use. [0070] Figure 5A illustrates example electrode 500 which may be implemented in the MEMS ultrasound sensor in various embodiments. In the illustrated example, the electrode 500 includes a bypass electrode 502, a sensing electrode 504, and removed portions 506. The removed portions 506 can be portions where the metal of the electrode 500 has been removed, which can electrically isolate the bypass electrode 502 and the sensing electrode 504. The relative size of the removed portions 506 can influence the operation of the electrode 500. For example, when removed portions 506 is smaller, the sensing electrode 504 becomes closer to the bypass electrode 502 which can result in added dead capacitance (e.g., capacitance that is not indicative and/or the product of received and transmitted waves). When removed portions 506 become larger the sensing electrode 504 can decrease in size which can reduce the effectiveness of the sensing electrode 504 (e.g., the sensing electrode 504 can have a reduced force exerted on the membrane). [0071] The sensing electrode 504 can be used to apply a desired voltage to transmit a signal, when used for transmit mode, and sense changes in capacitance, when used for receive mode. The bypass electrode 502 can provide electrical connection to one or more bypass capacitors and/or one or more inter-element bypass capacitors (such as the bypass capacitors 206 and inter-element bypass capacitors 208 illustrated in Figure 2). In some embodiments, the bypass electrode 502 may act as a secondary electrode which can have a separate voltage (e.g., is grounded, biased to a reference voltage, etc.). The bypass electrode 502 help evenly distribute electrostatic forces, filter out voltage spikes in received signals, and/or perform other suitable functionality. The sensing electrode 504 and/or the bypass electrode 502 may be connected to a pulser (and/or charged capacitors) and receive a relatively high voltage pulse, which can then exert a proportional electrostatic force on the membrane and create an ultrasonic wave. [0072] Figure 5B illustrates an example patterned electrode 520, which may be implemented in the MEMS ultrasound sensor in various embodiments. The patterned electrode 520 includes a bypass electrode 502, removed portions 506, and sub-sensing electrodes 522. The removed portions 506 includes removed metal from the center of the patterned electrode 520 and between four distinct sub-sensing electrodes 522. The removed center portion may help manipulate the e-field distribution in the CMUT for better performance, help eliminate undesired parasitic capacitance (which can often occur from at the cavity center), reduce stiction, and/or provide other benefits. [0073] The sub-sensing electrodes 522 can be used to apply a desired voltage to transmit a signal when used for transmit mode and sense changes in capacitance when used for receive mode. While four sub-sensing electrodes 522 are illustrated in Figure 5B, more or fewer sub- sensing electrodes 522 may be used in other implementations. Each of the sub-sensing electrodes 522 may be able to operate individual (e.g., having separated voltages in transmission and/or reception, sensing separate capacitances in reception, and being routed separately to the ASIC), which can provide an ability to transmit or receive on sub-apertures (e.g., using sub-portions of the same MEMS membrane), increase performance in high- frequency and/or high-resolution imaging, increase beamforming capabilities of the transducer array, and/or provide other benefits. Separate traces may be used to connect each of the sub- sensing electrodes 522 to the ASIC which may be fabricated as shown in Figures 4A-4L. [0074] In some embodiments, the sub-sensing electrodes 522 may operate separately in reception and operate together in transmission. For example, when receiving ultrasonic signals, each of the sub-sensing electrodes 522 may sense changes in capacitance separately, which are received by the ASIC using separate traces. In the example, when transmitting the sub-sensing electrodes 522 may be configured to receive the same pulses from the pulser (not shown). Components of the ASIC (or elsewhere) may be used to configure the sub-sensing electrodes 522 to operate together when transmitting and configure the sub-sensing electrodes 522 to operate separately when receiving. For example, a transmit/receive switch (T/R switch) may enable the change in operation when configuring the transducer to a transmit mode or a receive mode. [0075] Figure 5C illustrates an example patterned electrode 550, which may be implemented in the MEMS ultrasound sensor in various embodiments. The patterned electrode 550 includes a bypass electrode 502, removed portions 506, a sensing electrode 552, and a pull in electrode 554. When a CMUT is operating in collapse mode the center of the MEMS membrane contacts the barrier above the electrode which can cause the center to remain relatively stationary during transmission and reception. This can cause a dead capacitance at the center of the sensing electrode which can interfere with the transmission and reception when some electrodes, such as the sensing electrode 504 are used. The patterned electrode 550 can have increased performance in collapsed mode in certain respects (e.g., increased sensitivity, increased bandwidth, increased ability to sense and/or reduce nonlinearities, and/or provide other benefits to performance). [0076] The pull in electrode 554 can apply a static voltage to the MEMS membrane. The voltage can be at a voltage threshold high enough to cause the CMUT go into collapse mode or be near or in partial collapse (e.g., more than 5V, more than 10V, more than 20V, more than 40V, or more). The sensing electrode 552 may then be free to send transmission voltage and sense capacitance changes on the portions of the MEMS membrane that are not in contact with the barrier layer (and therefore, not stationary or fixed). The pull in electrode 554 can be connected to a separate voltage source than the sensing electrode 552, be tied to ground, and/or otherwise have a separate bias voltage applied from the sensing electrode 552. In some embodiments the pull in electrode 554 may be connected to a power rail configured to provide the needed voltage. The power rail may be connected to the pull in electrodes 554 of more than one CMUT. In some embodiments, the voltage may be about 20 voltages. In some embodiments, the voltage may be between 20-40 volts, 40-60, 80-100, and so on. In some embodiments, the pull in electrode 554 may be formed of different materials than the sensing electrode 552 (e.g., using separate fabrication processes) and/or formed from the same materials. [0077] In some implementations, combinations of the previous embodiments of electrodes may be used. For example, Figure 5D illustrates an example patterned electrode 570 that includes both a pull in electrode 554 and sensing electrodes 552, which can both operate similarly to the pull in electrode 554 and sensing electrodes 552 described above. In some embodiments, a sensing electrode similar to sensing electrode 552 (e.g., a circular electrode with a central removed portion) may be implemented without a pull in electrode 554, leaving a removed portion 506 in the center of the sensing electrode 552. Example Processes of Fabrication [0078] Figure 6 is a flowchart of an example process 600 for fabricating an ultrasound sensor, according to various embodiments of this disclosure. The process 600 describes aspects of the above-described Figures 1A-5D and disclosure included in Figures 1A-5D may be included below and disclosure included below may be relevant to Figures 1A-5D. [0079] At block 602 a structure with an ASIC is provided. The structure can for example, include all, or a portion, of the cross section 400, including the ASIC 401. The structure can also include the ASIC top metal 402, RDL 403, and oxide 404. If the structure does not include one or more of the ASIC top metal 402, RDL 403, and oxide 404 than the ASIC top metal 402, RDL 403, or oxide 404 can be formed on the structure using suitable techniques. [0080] At block 604, a sensing electrode is formed on the structure. The sensing electrode can, for example, be the electrodes 411 illustrated in the cross section 410 of Figure 4C. As shown in Figures 4B and 4C the sensing electrode can be formed by depositing a conductive layer (e.g., sensing layer 407) and removing portions to form the sensing electrode (e.g., according to a pattern). For example, the conductive layer may be etched away. Forming the sensing electrode can include forming traces to individual sub-sensing electrodes. In some embodiments, a pull in electrode may also be formed on the structure (e.g., by removing portions of the conductive layer in a similar manner as the sensing electrode or by using a separate process). Each trace may be tied to a separate source such that the sub-sensing electrodes can receive and/or transmit separately (e.g., as discussed with respect to Figures 5A- 5D. [0081] At block 606, a cavity (or cavities) is formed in an oxide layer on a side of the sensing electrode. The cavity can include, for example, one or the cavities 412 illustrated in cross section 410 of Figure 4C. As shown in Figure 4C, the cavity can be formed in an oxide layer (e.g., oxide 404) formed above the sensing electrode (as oriented in cross section 410). For example, a portion of the oxide may be etched away. [0082] At block 608, a membrane is bonded to the structure with a portion of the membrane proximate to the cavities. The membrane can be, for example, the MEMS membrane 416 illustrated in cross section 415. Bonding the membrane to the structure can include bonding a handling wafer (e.g., using low temperature fusion bonding) that includes the membrane to the structure and etching away the handling wafer, leaving the MEMS membrane (and potentially an oxide layer on a side of the membrane opposite side as the cavities as shown in Figures 4D and 4E). The bonding can include low temperature fusion bonding as previously described and/or another suitable bonding technique. The bonding between the MEMS membrane and the structure can occur at a perimeter region of the MEMS ultrasound sensor (e.g., at the perimeter of the structure shown in cross section 415) leaving a central portion of the MEMS membrane overlying the cavities and free to deflect. [0083] At block 610, a metal layer is deposited on an oxide layer oxide layer on a side of the membrane opposite side as the cavities. The metal layer can, for example, be the MEMS top metal layer 431 illustrated in cross section 430 of Figure 4G. The metal layer can be deposited using suitable techniques. [0084] At block 612, a patterned metal layer is formed. The patterned metal layer can, for example, be the portion 438, portion 436, and portions 437 illustrated in cross section 435 of Figure 4H. The patterned layer can, for example, be etched from the deposited metal layer of block 610. In some implementations, after the patterned metal layer is formed, a portion of the membrane may be removed (e.g., etched away) separating the membrane into two or more portions. Each portion may form a different element of the ultrasound sensor and may be accompanied by a different cavity and sensing electrode discussed above. [0085] At block 614, the structure is passivated. For example, by forming a passivation layer (e.g., by depositing or bonding the passivation layer to the structure. The passivation layer can, for example, be the passivation layer 446 illustrated in cross section 445 of Figure 4J. Portions of the passivation layer may be removed (e.g., etched away) to expose portions of the structure, such as the patterned metal layer (e.g., as shown in cross section 450 of Figure 4K). The exposed portions may provide electrical access to portions of the structure, such as the ASIC. Additional Example 1_{. A micro electrical mechanical system (MEMS) ultrasound transducer comprising:} first passivation layer disposed above a buried oxide (BOX) layer; a top layer beneath the BOX layer; a bottom layer beneath the top layer; a cavity formed in the bottom layer; a sensing metal formed in the bottom layer and position below the cavity; a second passivation layer beneath the bottom layer; a bipolar-CMOS-DMOS ("BCD") beneath the second passivation layer; a first plurality of metal vias, each of the first plurality of metal vias extending through the BOX layer, the top layer, and into the bottom layer; a first metal layer formed on a portion of the BOX layer and positioned in a gap of the first passivation layer, the first metal layer connected to at least a first metal via of the first plurality of metal vias; and a second plurality of metal vias, each of the second plurality of metal vias extending from the bottom layer, through the second passivation layer, and connected to a plurality of metal connections of the BCD, wherein a portion of the second plurality of metal vias connect the first plurality of metal vias to the BCD and, wherein a second metal via of the second plurality of metal vias connects the sensing metal to the BCD. 2_{. The ultrasound transducer of Embodiment 1, wherein the ultrasound transducer is} a capacitive micromachined ultrasonic transducer (CMUT). 3_{. A process of fabrication the ultrasound transducer of Embodiment 1, as described} in the description above. 4_{. The ultrasound transducer of any of the preceding Embodiments according to any} of the aspects of Additional Examples II, III, IV, V, VI, and/or VII. Additional Example II 1_{. A method of fabricating a micro electrical mechanical system (MEMS) ultrasound} transducer, the method comprising: providing a structure with an application specific integrated circuit (ASIC); forming a patterned electrode on the structure, the patterned electrode being coupled to the ASIC on a first side of the patterned electrode, wherein the patterned electrode comprises at least a sensing electrode electrically coupled to the ASIC and has at least a portion removed according to a pattern; forming a cavity in an oxide layer positioned on a second side of the sensing electrode, opposite the first side of the patterned electrode; and bonding a MEMS membrane to the structure at a first side of the MEMS membrane using low temperature fusion boding, a portion of the first side of the MEMS membrane proximate to the cavity. 2_{. A method of fabricating a micro electrical mechanical system (MEMS) ultrasound} transducer, the method comprising: providing a structure with an application specific integrated circuit (ASIC); forming a patterned electrode on the structure, the patterned electrode being coupled to the ASIC on a first side of the patterned electrode, wherein the patterned electrode comprises at least a sensing electrode electrically coupled to the ASIC and has at least a portion removed according to a pattern, wherein the portion removed according to the pattern comprises a portion removed centrally on the patterned electrode, the sensing electrode circumscribing the portion removed centrally; forming a cavity in an oxide layer positioned on a second side of the sensing electrode, opposite the first side of the patterned electrode; and bonding a MEMS membrane to the structure at a first side of the MEMS membrane using low temperature fusion boding, a portion of the first side of the MEMS membrane proximate to the cavity. 3_{. A method of fabricating a micro electrical mechanical system (MEMS) ultrasound} transducer, the method comprising: providing a structure with an application specific integrated circuit (ASIC); forming a patterned electrode on the structure, the patterned electrode being coupled to the ASIC on a first side of the patterned electrode, wherein the patterned electrode comprises two or more sensing electrodes electrically coupled to the ASIC and has at least a portion removed according to a pattern, wherein the portion removed according to the pattern comprises a portion separating each of the two or more sensing electrodes; forming a cavity in an oxide layer positioned on a second side of the sensing electrode, opposite the first side of the patterned electrode; and bonding a MEMS membrane to the structure at a first side of the MEMS membrane using low temperature fusion boding, a portion of the first side of the MEMS membrane proximate to the cavity. 4_{. The method of any of Embodiments 1-3, wherein the patterned electrode comprises} a bypass electrode positioned at the periphery of the patterned electrode and separated from the sensing electrode by the portion removed according to the pattern. 5_{. The method of any of Embodiments 1-3, further comprising:} depositing a metal layer on a second oxide layer formed on a second side of the MEMS membrane, opposite the first side of the MEMS membrane; removing a portion of the metal layer to form a patterned metal layer; and applying a passivation layer to a surface of the structure. 6_{. The method of any of Embodiments 1-3, wherein the ASIC is a bipolar-} complementary metal oxide semiconductor-double-diffused metal-oxide-semiconductor (BCD). _{7. The method of any of Embodiments 1-3, wherein forming the patterned electrode} comprises: depositing a conductive layer; and removing at least the portion of the conductive layer according to the pattern. 8_{. The method of any of Embodiments 1-3, wherein bonding the MEMS membrane to} the structure comprises: providing a wafer comprising the MEMS membrane and a handling wafer coupled to the MEMS membrane at a second side of the MEMS membrane, opposite the first side of the MEMS membrane; bonding the wafer to the structure at a first side of the MEMS membrane; and removing the handling wafer. 9_{. The method of any of Embodiments 1-3, wherein the patterned electrode further} comprises a second sensing electrode. 1_{0. The method of any of Embodiments 1-3, wherein the patterned electrode comprises} at least four sub-sensing electrodes, each sub-sensing electrode individually electrically coupled to the ASIC and configured to receive separated voltages. 1_{1. The method of any of Embodiments 1-3, wherein the patterned electrode comprises} a pull in electrode positioned centrally on the patterned electrode and electrically coupled to a separate voltage source, the pull in electrode configured to collapse a portion of the MEMS membrane by applying a threshold voltage on the MEMS membrane. 1_{2. The method of any of Embodiments 1-3, wherein the separate voltage source is a} power rail embedded in the structure. 1_{3. The method of Embodiment 12, wherein the sensing electrode is configured to sense} and/or induce movement in one or more moveable portions of the MEMS membrane, different than the collapsed portion. 1_{4. The method of any of Embodiments 1-3, wherein forming the cavity in the oxide} layer positioned on the second side of the sensing electrode comprises: forming patterned passivation features on a top portion of a barrier layer, wherein the barrier layer is a portion of the oxide layer that remains between the cavity and the second side of the sensing electrode, and wherein the top portion of the barrier layer is adjacent to the cavity and a bottom portion, opposite the top portion is adjacent to the second side of the sensing electrode. 1_{5. A method of fabricating a micro electrical mechanical system (MEMS) ultrasound} transducer, the method comprising: providing a structure with an application specific integrated circuit (ASIC); forming a patterned electrode on the structure, the patterned electrode being coupled to the ASIC on a first side of the patterned electrode, wherein the patterned electrode comprises at least a sensing electrode electrically coupled to the ASIC and has at least a portion removed according to a pattern; forming a cavity in an oxide layer positioned on a second side of the sensing electrode, opposite the first side of the patterned electrode; and bonding a MEMS membrane to the structure at a first side of the MEMS membrane using low-temperature fusion bonding, wherein the bonding occurs at a perimeter region, leaving a central portion of the MEMS membrane overlying the cavity free to deflect. 1_{6. The method of any of the preceding Embodiments according to any of the aspects} of Additional Examples I, III, IV, V, VI, and/or VII. Additional Example III 1_{. A micro electrical mechanical system (MEMS) ultrasound sensor comprising:} a transducer array comprising a plurality of capacitive micromachined ultrasonic transducers (CMUTs), wherein each of the plurality of CMUTs comprises: a MEMS membrane, one or more cavities proximate to at least a portion of the membrane at a first side of the one or more cavities, and a patterned electrode positioned proximate to the one or more cavities at a second side of the one or more cavities, opposite the first side, the patterned electrode comprising at least a sensing electrode electrically coupled to an application specific integrated circuit (ASIC) and has at least a portion removed according to a pattern; a portion on a periphery of the transducer array comprising a plurality of capacitors, each capacitor electrically coupled to at least one CMUT of the plurality of CMUTs; and the ASIC electrically coupled to the transducer array and the plurality of capacitors, wherein, based on an electrical signal from the ASIC, the plurality of capacitors are configured to store charge and modulate the electrical signal delivered to the plurality of CMUTs and cause the plurality of CMUTs to transmit one or more ultrasonic waves. 2_{. A micro electrical mechanical system (MEMS) ultrasound sensor comprising:} a transducer array comprising a plurality of capacitive micromachined ultrasonic transducers (CMUTs), wherein each of the plurality of CMUTs comprises: a MEMS membrane, one or more cavities proximate to at least a portion of the membrane at a first side of the one or more cavities, and a patterned electrode positioned proximate to the one or more cavities at a second side of the one or more cavities, opposite the first side, the patterned electrode comprising at least a sensing electrode electrically coupled to the ASIC and has at least a portion removed according to a pattern, wherein the portion removed according to the pattern comprises a portion removed centrally on the patterned electrode, the sensing electrode circumscribing the portion removed centrally; a portion on a periphery of the transducer array comprising a plurality of capacitors, each capacitor electrically coupled to at least one CMUT of the plurality of CMUTs; and the ASIC electrically coupled to the transducer array and the plurality of capacitors, wherein, based on an electrical signal from the ASIC, the plurality of capacitors are configured to store charge and modulate the electrical signal delivered to the plurality of CMUTs and cause the plurality of CMUTs to transmit one or more ultrasonic waves. 3_{. A micro electrical mechanical system (MEMS) ultrasound sensor comprising:} a transducer array comprising a plurality of capacitive micromachined ultrasonic transducers (CMUTs), wherein each of the plurality of CMUTs comprises: a MEMS membrane, one or more cavities proximate to at least a portion of the membrane at a first side of the one or more cavities, and a patterned electrode positioned proximate to the one or more cavities at a second side of the one or more cavities, opposite the first side, the patterned electrode comprising two or more sensing electrodes electrically coupled to the ASIC and has at least a portion removed according to a pattern, wherein the portion removed according to the pattern comprises a portion separating each of the two or more sensing electrodes; a portion on a periphery of the transducer array comprising a plurality of capacitors, each capacitor electrically coupled to at least one CMUT of the plurality of CMUTs; and the ASIC electrically coupled to the transducer array and the plurality of capacitors, wherein, based on an electrical signal from the ASIC, the plurality of capacitors are configured to store charge and modulate the electrical signal delivered to the plurality of CMUTs and cause the plurality of CMUTs to transmit one or more ultrasonic waves. 4_{. The MEMS ultrasound sensor of any of Embodiments 1-3, wherein the transducer} array further comprises a plurality of inter-element bypass capacitors, each inter-element bypass capacitor positioned between two or more CMUTs of the plurality of CMUTs and electrically coupled to one or more CMUTs of the plurality of CMUTs. 5_{. The MEMS ultrasound sensor of Embodiment 4, wherein the plurality of inter-} element bypass capacitors are configured to store charge and modulate the electrical signal delivered to the plurality of CMUTs and cause the plurality of CMUTs to transmit one or more ultrasonic waves based on the electrical signal from the ASIC. 6_{. The MEMS ultrasound sensor of any of Embodiments 1-3, wherein the plurality of} capacitors comprises one or more CMUT structures storing energy using capacitances of the CMUT structures. 7_{. The MEMS ultrasound sensor of Embodiment 6, wherein each of the plurality of} CMUTs and the one or more CMUT structures comprise a same CMUT structure. 8_{. The method of any of the preceding Embodiments according to any of the aspects} of Additional Examples I, II, IV, V, VI, and/or VII. Additional Example IV 1_{. A capacitive micromachined ultrasonic transducers (CMUT) comprising:} a MEMS membrane, one or more cavities proximate to at least a portion of the MEMS membrane at a first side of the one or more cavities, and a patterned electrode positioned proximate to the one or more cavities at a second side of the one or more cavities, opposite the first side, the patterned electrode comprising: a bypass electrode positioned on a periphery of the patterned electrode and electrically coupled with a first voltage source; one or more sensing electrodes electrically coupled to at least one different voltage source, each sensing electrode of the one or more sensing electrodes configured to be biased at different voltages; and removed portions electrically isolating the bypass electrode and each of the one or more sensing electrodes. 2_{. The CMUT of Embodiment 1, wherein the bypass electrode and the one or more} sensing electrodes are formed by depositing a conductive layer and removing the removed portions. 3_{. The CMUT of Embodiment 1, wherein the patterned electrode further comprises a} pull in electrode positioned centrally on the patterned electrode and electrically isolated from the one or more sensing electrodes and the bypass electrode, the pull in electrode configured to collapse a portion of the MEMS membrane by applying a threshold voltage on the membrane. 4_{. The CMUT of Embodiment 3, wherein the one or more sensing electrodes are} configured to sense and/or induce movement in one or more moveable portions of the MEMS membrane, different than the collapsed portion. 5_{. The CMUT of Embodiment 1, wherein the one or more sensing electrodes comprise} a plurality of sub-sensing electrodes, each of the plurality of sub-sensing electrodes configured to sense and induce movement in different portions of the MEMS membrane. 6_{. The method of any of the preceding Embodiments according to any of the aspects} of Additional Examples I, II, III, V, VI, and/or VII. Additional Example V 1_{. A capacitive micromachined ultrasonic transducers (CMUT) comprising:} a MEMS membrane, one or more cavities proximate to at least a portion of the MEMS membrane at a first side of the one or more cavities, and a patterned electrode positioned proximate to the one or more cavities at a second side of the one or more cavities, opposite the first side, the patterned electrode comprising at least a sensing electrode electrically coupled to the ASIC and has at least a portion removed according to a pattern. 2_{. A capacitive micromachined ultrasonic transducers (CMUT) comprising:} a MEMS membrane, one or more cavities proximate to at least a portion of the MEMS membrane at a first side of the one or more cavities, and a patterned electrode positioned proximate to the one or more cavities at a second side of the one or more cavities, opposite the first side, the patterned electrode comprising at least a sensing electrode electrically coupled to the ASIC and has at least a portion removed according to a pattern, wherein the portion removed according to the pattern comprises a portion removed centrally on the patterned electrode, the sensing electrode circumscribing the portion removed centrally. 3_{. A capacitive micromachined ultrasonic transducers (CMUT) comprising:} a MEMS membrane, one or more cavities proximate to at least a portion of the MEMS membrane at a first side of the one or more cavities, and a patterned electrode positioned proximate to the one or more cavities at a second side of the one or more cavities, opposite the first side, the patterned electrode comprising two or more sensing electrodes electrically coupled to the ASIC and has at least a portion removed according to a pattern, wherein the portion removed according to the pattern comprises a portion separating each of the two or more sensing electrodes. 4_{. The method of any of the preceding Embodiments according to any of the aspects} of Additional Examples I, II, III, IV, VI, and/or VII. Additional Example VI 1_{. A patterned electrode of a capacitive micromachined ultrasonic transducers} (CMUT), the patterned electrode comprising: a bypass electrode positioned on a periphery of the patterned electrode and electrically coupled with a first voltage source; one or more sensing electrodes electrically coupled to at least one different voltage source, each sensing electrode of the one or more sensing electrodes configured to be biased at different voltages; and removed portions electrically isolating the bypass electrode and each of the one or more sensing electrodes. 2_{. The patterned electrode of Embodiment 1, wherein the bypass electrode and the one} or more sensing electrodes are formed by depositing a conductive layer and removing the removed portions. 3_{. The patterned electrode of Embodiment 2, further comprising a pull in electrode} positioned centrally on the patterned electrode and electrically isolated from the one or more sensing electrodes and the bypass electrode, the pull in electrode configured to collapse a portion of a MEMS membrane by applying a threshold voltage on the membrane. 4_{. The patterned electrode of Embodiment 3, wherein the one or more sensing} electrodes are configured to sense and/or induce movement in one or more moveable portions of the MEMS membrane, different than the collapsed portion . 5_{. The patterned electrode of Embodiment 21, wherein the one or more sensing} electrodes comprise a plurality of sub-sensing electrodes, each of the plurality of sub-sensing electrodes configured to sense and induce movement in different portions of a MEMS membrane. 6_{. The method of any of the preceding Embodiments according to any of the aspects} of Additional Examples I, II, III, IV, V, and/or VII. Additional Example VII 1_{. A patterned electrode of a capacitive micromachined ultrasonic transducers} (CMUT), the patterned electrode comprising: a bypass electrode positioned on a periphery of the patterned electrode; one or more sensing electrodes, each electrically coupled to different voltage sources; and removed portions electrically isolating the bypass electrode and each of the one or more sensing electrodes. 2_{. A patterned electrode of a capacitive micromachined ultrasonic transducers} (CMUT), the patterned electrode comprising: a bypass electrode positioned on a periphery of the patterned electrode; one or more sensing electrodes, each electrically coupled to different voltage sources; and removed portions electrically isolating the bypass electrode and each of the one or more sensing electrodes, wherein the removed portion comprises a portion removed centrally on the patterned electrode, the sensing electrode circumscribing the removed portion. 3_{. A patterned electrode of a capacitive micromachined ultrasonic transducers} (CMUT), the patterned electrode comprising: a bypass electrode positioned on a periphery of the patterned electrode; two or more sensing electrodes, each electrically coupled to different voltage sources; and removed portions electrically isolating the bypass electrode and each of the one or more sensing electrodes wherein the removed portion comprises a portion separating each of the two or more sensing electrodes. 4_{. The patterned electrode of any of Embodiments 1-3, wherein the removed portions} are defined using a lithographic patterning and selective etching process, such that the remaining electrode regions are electrically isolated. _{5. The method of any of the preceding Embodiments according to any of the aspects} of Additional Examples I, II, III, IV, V, and/or VI. Additional Aspects [0086] Conditional language such as, among others, “can,” “could,” “might” or “may,” unless specifically stated otherwise, are understood within the context as used in general to convey that certain embodiments include, while other embodiments do not include, certain features, elements, and/or steps. Thus, such conditional language is not generally intended to imply that features, elements and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements and/or steps are included or are to be performed in any particular embodiment. [0087] Disjunctive language such as the phrase “at least one of X, Y, or Z,” unless specifically stated otherwise, is understood with the context as used in general to present that an item, term, etc., may be either X, Y, or Z, or any combination thereof (for example, X, Y, and/or Z). Thus, such disjunctive language is not generally intended to, and should not, imply that certain embodiments require at least one of X, at least one of Y, or at least one of Z to each be present. [0088] Any process descriptions, elements or blocks in the flow diagrams described herein and/or depicted in the attached figures should be understood as potentially representing modules, segments, or portions of code which include one or more executable instructions for implementing specific logical functions or elements in the process. Alternate implementations are included within the scope of the embodiments described herein in which elements or functions may be deleted, executed out of order from that shown, or discussed, including substantially concurrently or in reverse order, depending on the functionality involved as would be understood by those skilled in the art. [0089] Unless otherwise explicitly stated, articles such as “a” or “an” should generally be interpreted to include one or more described items. Accordingly, phrases such as “a device configured to” are intended to include one or more recited devices. Such one or more recited devices can also be collectively configured to carry out the stated recitations. For example, “a processor configured to carry out recitations A, B and C” can include a first processor configured to carry out recitation A working in conjunction with a second processor configured to carry out recitations B and C. [0090] It should be emphasized that many variations and modifications may be made to the above-described embodiments, the elements of which are to be understood as being among other acceptable examples. All such modifications and variations are intended to be included herein within the scope of this disclosure.

Claims

WHAT IS CLAIMED IS: 1_{. A method of fabricating a micro electrical mechanical system (MEMS) ultrasound} transducer, the method comprising: providing a structure with an application specific integrated circuit (ASIC); forming a patterned electrode on the structure, the patterned electrode being coupled to the ASIC on a first side of the patterned electrode, wherein the patterned electrode comprises at least a sensing electrode electrically coupled to the ASIC and has at least a portion removed according to a pattern; forming a cavity in an oxide layer positioned on a second side of the sensing electrode, opposite the first side of the patterned electrode; and bonding a MEMS membrane to the structure at a first side of the MEMS membrane using low temperature fusion boding, wherein bonding occurs at a perimeter region of the structure, leaving a central portion of the MEMS membrane overlying the cavity free to deflect. 2_{. The method of Claim 1, further comprising:} depositing a metal layer on a second oxide layer formed on a second side of the MEMS membrane, opposite the first side of the MEMS membrane; removing a portion of the metal layer to form a patterned metal layer; and applying a passivation layer to a surface of the structure. 3_{. The method of Claim 1, wherein the ASIC is a bipolar-complementary metal oxide} semiconductor-double-diffused metal-oxide-semiconductor (BCD). 4_{. The method of Claim 1, wherein forming the patterned electrode comprises:} depositing a conductive layer; and removing at least the portion of the conductive layer according to the pattern. 5_{. The method of Claim 1, wherein bonding the MEMS membrane to the structure} comprises: providing a wafer comprising the MEMS membrane and a handling wafer coupled to the MEMS membrane at a second side of the MEMS membrane, opposite the first side of the MEMS membrane; bonding the wafer to the structure at a first side of the MEMS membrane; and removing the handling wafer. 6_{. The method of Claim 1, wherein the patterned electrode further comprises a second} sensing electrode.

_{7. The method of Claim 1, wherein the patterned electrode comprises at least four sub-} sensing electrodes, each sub-sensing electrode individually electrically coupled to the ASIC and configured to receive separated voltages. 8_{. The method of Claim 1, wherein the patterned electrode comprises a pull in} electrode positioned centrally on the patterned electrode and electrically coupled to a separate voltage source, the pull in electrode configured with a biasing voltage to collapse a portion of the MEMS membrane by applying a threshold voltage on the MEMS membrane. 9_{. The method of Claim 8, wherein the separate voltage source is a power rail} embedded in the structure. 1_{0. The method of Claim 8, wherein the sensing electrode is configured to sense and/or} induce movement in one or more moveable portions of the MEMS membrane, different than the collapsed portion. 1_{1. The method of Claim 1, wherein forming the cavity in the oxide layer positioned} on the second side of the sensing electrode comprises: forming patterned passivation features on a top portion of a barrier layer, wherein the barrier layer is a portion of the oxide layer that remains between the cavity and the second side of the sensing electrode, and wherein the top portion of the barrier layer is adjacent to the cavity and a bottom portion, opposite the top portion is adjacent to the second side of the sensing electrode. 1_{2. A micro electrical mechanical system (MEMS) ultrasound sensor comprising:} a transducer array comprising a plurality of capacitive micromachined ultrasonic transducers (CMUTs), wherein each of the plurality of CMUTs comprises: a MEMS membrane, one or more cavities proximate to at least a portion of the MEMS membrane at a first side of the one or more cavities, and a patterned electrode positioned proximate to the one or more cavities at a second side of the one or more cavities, opposite the first side, the patterned electrode comprising at least a sensing electrode electrically coupled to an application specific integrated circuit (ASIC) and has at least a portion removed according to a pattern; a portion on a periphery of the transducer array comprising a plurality of capacitors, each capacitor electrically coupled to at least one CMUT of the plurality of CMUTs; and the ASIC electrically coupled to the transducer array and the plurality of capacitors, wherein, based on an electrical signal from the ASIC, the plurality of capacitors are configured to store charge and modulate the electrical signal delivered to the plurality of CMUTs and cause the plurality of CMUTs to transmit one or more ultrasonic waves. 1_{3. The MEMS ultrasound sensor of Claim 12, wherein the transducer array further} comprises a plurality of inter-element bypass capacitors, each inter-element bypass capacitor positioned between two or more CMUTs of the plurality of CMUTs and electrically coupled to one or more CMUTs of the plurality of CMUTs. 1_{4. The MEMS ultrasound sensor of Claim 13, wherein the plurality of inter-element} bypass capacitors are configured to store charge and modulate the electrical signal delivered to the plurality of CMUTs and cause the plurality of CMUTs to transmit one or more ultrasonic waves based on the electrical signal from the ASIC. 1_{5. The MEMS ultrasound sensor of Claim 12, wherein the plurality of capacitors} comprises one or more CMUT structures storing energy using capacitances of the CMUT structures. 1_{6. The MEMS ultrasound sensor of Claim 15, wherein each of the plurality of CMUTs} and the one or more CMUT structures comprise a same CMUT structure. 1_{7. A capacitive micromachined ultrasonic transducers (CMUT) comprising:} a MEMS membrane, one or more cavities proximate to at least a portion of the MEMS membrane at a first side of the one or more cavities, and a patterned electrode positioned proximate to the one or more cavities at a second side of the one or more cavities, opposite the first side, the patterned electrode comprising at least a sensing electrode electrically coupled to the ASIC and has at least a portion removed according to a pattern. 1_{8. The CMUT of Claim 17, wherein at least the sensing electrode is formed by} depositing a conductive layer and removing at least the portion according to the pattern. 1_{9. The CMUT of Claim 17, wherein the patterned electrode further comprises a pull} in electrode positioned centrally on the patterned electrode and electrically isolated from the sensing electrode, the pull in electrode configured to collapse a portion of the MEMS membrane by applying a threshold voltage on the MEMS membrane. 2_{0. The CMUT of Claim 19, wherein at least the sensing electrodes is configured to} sense and/or induce movement in one or more moveable portions of the MEMS membrane, different than the collapsed portion.

_{21. The CMUT of Claim 17, wherein the patterned electrode comprises a plurality of} sub-sensing electrodes, each of the plurality of sub-sensing electrodes configured to sense movement in different portions of the MEMS membrane. 2_{2. A patterned electrode of a capacitive micromachined ultrasonic transducers} (CMUT), the patterned electrode comprising: a bypass electrode positioned on a periphery of the patterned electrode; one or more sensing electrodes, each electrically coupled to different voltage sources; and removed portions electrically isolating the bypass electrode and each of the one or more sensing electrodes. 2_{3. The patterned electrode of Claim 22, wherein the bypass electrode and the one or} more sensing electrodes are formed by depositing a conductive layer and removing the removed portions. 2_{4. The patterned electrode of Claim 23, further comprising a pull in electrode} positioned centrally on the patterned electrode and electrically isolated from the one or more sensing electrodes and the bypass electrode, the pull in electrode configured to collapse a portion of a MEMS membrane by applying a threshold voltage on the MEMS membrane. 2_{5. The patterned electrode of Claim 24, wherein the one or more sensing electrodes} are configured to sense and/or induce movement in one or more moveable portions of the MEMS membrane, different than the collapsed portion . 2_{6. The patterned electrode of Claim 22, wherein the one or more sensing electrodes} comprise a plurality of sub-sensing electrodes, each of the plurality of sub-sensing electrodes configured to sense and induce movement in different portions of a MEMS membrane. 2_{7. The patterned electrode of Claim 22, wherein the removed portions are defined} using a lithographic patterning and selective etching process, such that the remaining electrode regions are electrically isolated.