Patent Publication Number: US-2021177378-A1

Title: Split electrode design for a transducer

Description:
CROSS-REFERENCE TO RELATED APPLICATION—PROVISIONAL 
     This application claims priority to and benefit of co-pending U.S. Provisional Patent Application No. 62/947,558 filed on Dec. 13, 2019 entitled “SPLIT ELECTRODE DESIGN FOR A TRANSDUCER” by Fabian T. Goericke et al., having Attorney Docket No. IVS-943-PR, and assigned to the assignee of the present application, the disclosure of which is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     A variety of devices exist which utilize sonic sensors (e.g., sonic emitters and receivers, or sonic transducers). By way of example, and not of limitation, a device may utilize one or more sonic sensors to track the location of the device in space, to detect the presence of objects in the environment of the device, and/or to avoid objects in the environment of the device. Such sonic sensors include transmitters which transmit sonic signals, receivers which receive sonic signals, and transducers which both transmit sonic signals and receive sonic signals. Piezoelectric Micromachined Ultrasonic Transducers (PMUTs), which may be air-coupled, are one type of sonic transducer, which operates in the ultrasonic range, and can be used for a large variety of sensing applications such as, but not limited to: virtual reality controller tracking, presence detection, and object avoidance for drones or other machines, etc. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of the Description of Embodiments, illustrate various embodiments of the subject matter and, together with the Description of Embodiments, serve to explain principles of the subject matter discussed below. Unless specifically noted, the drawings referred to in this Brief Description of Drawings should be understood as not being drawn to scale. Herein, like items are labeled with like item numbers. 
         FIGS. 1A and 1B  show example block diagrams of some aspects of a device, in accordance with various embodiments. 
         FIG. 2A  shows a top plan view of a split-electrode transducer, in accordance with various embodiments. 
         FIG. 2B  shows a sectional side elevational view of the split-electrode transducer of  FIG. 2A , in accordance with various embodiments. 
         FIG. 2C  shows a top plan view of a split-electrode transducer, in accordance with various embodiments. 
         FIG. 3  illustrates operation of the split-electrode piezoelectric transducer of  FIGS. 2A and 2B  in a transmit mode, in accordance with various embodiments. 
         FIG. 4A  illustrates operation of the split-electrode piezoelectric transducer of  FIGS. 2A and 2B  in a receive mode, in accordance with various embodiments. 
         FIG. 4B  illustrates operation of the split-electrode piezoelectric transducer of  FIGS. 2A and 2B  in a receive mode, in accordance with various embodiments. 
         FIG. 5A  illustrates an ultrasonic transducer device, in accordance with various embodiments. 
         FIG. 5B  illustrates the ultrasonic transducer device of  FIG. 5A  in a transmit configuration, in accordance with various embodiments. 
         FIG. 5C  illustrates the ultrasonic transducer device of  FIG. 5A  in a receive configuration, in accordance with various embodiments. 
         FIG. 5D  illustrates the ultrasonic transducer device of  FIG. 5A  in a receive configuration, in accordance with various embodiments. 
         FIG. 6  illustrates a method of manufacture of a piezoelectric micromachined transducer, in accordance with various embodiments. 
         FIG. 7A  shows a top plan view of a split-electrode transducer, in accordance with various embodiments. 
         FIG. 7B  shows a sectional side elevational view B-B of the split-electrode transducer of  FIG. 7A , in accordance with various embodiments. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Reference will now be made in detail to various embodiments of the subject matter, examples of which are illustrated in the accompanying drawings. While various embodiments are discussed herein, it will be understood that they are not intended to limit to these embodiments. On the contrary, the presented embodiments are intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the various embodiments as defined by the appended claims. Furthermore, in this Description of Embodiments, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present subject matter. However, embodiments may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the described embodiments. 
     Overview of Discussion 
     Air-coupled Piezoelectric Micromachined Ultrasonic Transducers (PMUTs), transmitters, and receivers can be used for a large variety of sensing applications. Conventionally, however, the application field for such PMUT sensing devices is limited by the maximum operating range, which in turn is limited by the strength of the transmitted signal (Tx) and the ability to resolve the received signal (Rx). Any significant improvement to either Tx or Rx could enable longer sensing ranges and thus new applications. The technology described herein presents improvements to both the Tx and the Rx functions and can be used in transmitting devices, receiving devices, and transducers. Conventional air-coupled PMUTs accomplish Tx and Rx with a single electrode in the center of a circular membrane. The signal-to-noise ratio (SNR) of conventional air-coupled PMUTs is limited due to their small size and the presence of interfering sonic signals from the environment. 
     The new technology described herein utilizes both a center electrode and a ring/outer electrode and splits them both into multiple sections. This new technology may be referred to more specifically as a split electrode design and provides for differential Tx which is stronger than conventional Tx and/or stacked-differential Rx which is more sensitive than conventional Rx. Stronger Tx and more sensitive Rx each improve the signal-to-noise ratio (SNR) and enable longer sensing ranges. Utilizing the split electrode design allows for stronger transmission from the PMUT and/or more sensitive receiving by the PMUT. 
     Discussion begins with a description of notation and nomenclature. Discussion then shifts to description of some block diagrams of example components of some example devices which may utilize a PMUT of the type described herein. The device may be any type of device which utilizes sonic sensing, for example any device which uses conventional PMUTs could utilize the new PMUTs described herein. Moreover, because of the improved Tx and Rx, many devices which cannot utilize conventional PMUTs due to their limitations may utilize the PMUTs described herein. Some example depictions of a PMUT are described. Utilization of an example PMUT for transmitting signals and for receiving signals is described. Discussion then moves to description of an example ultrasonic sensing device which includes a PMUT of the type described herein. Operation of the example ultrasonic sensing device for transmitting signals and receiving signals is then described. Finally, some example methods of manufacture of a PMUT, of the type described herein, are described. 
     Notation and Nomenclature 
     Some portions of the detailed descriptions which follow are presented in terms of procedures, logic blocks, processes, modules and other symbolic representations of operations on data bits within a computer memory. These descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. In the present application, a procedure, logic block, process, module, or the like, is conceived to be one or more self-consistent procedures or instructions leading to a desired result. The procedures are those requiring physical manipulations of physical quantities. Usually, although not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated in an electronic device/component. 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the description of embodiments, discussions utilizing terms such as “electrically coupling,” “generating,” “processing,” “decoupling,” “coupling,” “switching” or the like, may refer to the actions and processes of an electronic device or component such as: a host processor, a sensor processing unit, a sensor processor, a controller or other processor, a memory, some combination thereof, or the like; and/or a component such as a switch or an emitter, receiver, or transducer operating under control of a host processor, a sensor processing unit, a sensor processor, a controller or other processor, or the like. The electronic device/component manipulates and transforms data represented as physical (electronic and/or magnetic) quantities within the registers and memories into other data similarly represented as physical quantities within memories or registers or other such information storage, transmission, processing, or display components. 
     Embodiments described herein may be discussed in the general context of processor-executable instructions residing on some form of non-transitory processor-readable medium, such as program modules or logic, executed by one or more computers, processors, or other devices. Generally, program modules include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. The functionality of the program modules may be combined or distributed as desired in various embodiments. 
     In the figures, a single block may be described as performing a function or functions; however, in actual practice, the function or functions performed by that block may be performed in a single component or across multiple components, and/or may be performed using hardware, using software, or using a combination of hardware and software. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present disclosure. Also, the example electronic device(s) described herein may include components other than those shown, including well-known components. 
     The techniques described herein may be implemented in hardware, or a combination of hardware with firmware and/or software, unless specifically described as being implemented in a specific manner. Any features described as modules or components may also be implemented together in an integrated logic device or separately as discrete but interoperable logic devices. If implemented in software, the techniques may be realized at least in part by a non-transitory computer/processor-readable storage medium comprising computer/processor-readable instructions that, when executed, cause a processor and/or other components of a computer or electronic device to perform one or more of the methods described herein. The non-transitory processor-readable data storage medium may form part of a computer program product, which may include packaging materials. 
     The non-transitory processor-readable storage medium (also referred to as a non-transitory computer-readable storage medium) may comprise random access memory (RAM) such as synchronous dynamic random access memory (SDRAM), read only memory (ROM), non-volatile random access memory (NVRAM), electrically erasable programmable read-only memory (EEPROM), FLASH memory, other known storage media, and the like. The techniques additionally, or alternatively, may be realized at least in part by a processor-readable communication medium that carries or communicates code in the form of instructions or data structures and that can be accessed, read, and/or executed by a computer or other processor. 
     The various illustrative logical blocks, modules, circuits and instructions described in connection with the embodiments disclosed herein may be executed by one or more processors, such as host processor(s) or core(s) thereof, digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), application specific instruction set processors (ASIPs), field programmable gate arrays (FPGAs), sensor processors, microcontrollers, or other equivalent integrated or discrete logic circuitry. The term “processor” or the term “controller” as used herein may refer to any of the foregoing structures, any other structure suitable for implementation of the techniques described herein, or a combination of such structures. In addition, in some aspects, the functionality described herein may be provided within dedicated software modules or hardware modules configured as described herein. Also, the techniques could be fully implemented in one or more circuits or logic elements. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a plurality of microprocessors, one or more microprocessors in conjunction with an ASIC or DSP, or any other such configuration or suitable combination of processors. 
     In various example embodiments discussed herein, a chip is defined to include at least one substrate typically formed from a semiconductor material. A single chip may for example be formed from multiple substrates, where the substrates are mechanically bonded to preserve the functionality. Multiple chip (or multi-chip) includes at least two substrates, wherein the two substrates are electrically connected, but do not require mechanical bonding. 
     A package provides electrical connection between the bond pads on the chip (or for example a multi-chip module) to a metal lead that can be soldered to a printed circuit board (or PCB). A package typically comprises a substrate and a cover. An Integrated Circuit (IC) substrate may refer to a silicon substrate with electrical circuits, typically CMOS circuits but others are possible and anticipated. A MEMS substrate provides mechanical support for the MEMS structure(s). The MEMS structural layer is attached to the MEMS substrate. The MEMS substrate is also referred to as handle substrate or handle wafer. In some embodiments, the handle substrate serves as a cap to the MEMS structure. 
     Some embodiments may, for example, comprise an ultrsonic transducer device. This ultrasonic transducer device may operate in any suitable ultrasonic range. In some embodiments, the ultrasonic transducer device may be or include a split-electrode ultrasonic transducer which may be an air coupled PMUT. In some embodiments, the ultrasonic transducer device may include a digital signal processor (DSP) or other controller or processor which may be disposed as a part of an ASIC which may be integrated into the same package as the split-electrode ultrasonic transducer. 
     Example Device 
       FIGS. 1A and 1B  show some example components of a device  100  which utilizes an ultrasonic transducer device  150 , according to various embodiments. Some examples of a device  100  may include, but are not limited to: remote controlled vehicles, virtual reality remotes, a telepresence robot, an electric scooter, an electric wheelchair, a wheeled delivery robot, a flying drone operating near a surface or about to land on or take off from a surface, a wheeled delivery vehicle, an automobile, an autonomous mobile device, a floor vacuum, a smart phone, a tablet computer, and a robotic cleaning appliance. By way of example, and not of limitation, the device  100  may utilize one or more ultrasonic transducer devices  150  to track the location of the device  100  in space, to detect the presence of objects in the environment of the device  100 , to sense the absences of objects in the environment of device  100 , to characterize objects sensed in the environment of device  100 , and/or to avoid objects in the environment of the device  100 . 
       FIG. 1A  shows a block diagram of components of an example device  100 A, in accordance with various aspects of the present disclosure. As shown, example device  100 A comprises a communications interface  105 , a host processor  110 , host memory  111 , and at least one ultrasonic transducer device  150 . In some embodiments, device  100  may additionally include one or more of a transceiver  113 , and one or more motion sensors or other types of sensors. Some embodiments may include sensors used to detect motion, position, or environmental context; some examples of these sensors may include, but are not limited to, infrared sensors, cameras, microphones, and global navigation satellite system sensors (i.e., a global positioning system receiver). As depicted in  FIG. 1A , included components are communicatively coupled with one another, such as, via communications interface  105 . 
     The host processor  110  may, for example, be configured to perform the various computations and operations involved with the general function of device  100 . Host processor  110  can be one or more microprocessors, central processing units (CPUs), DSPs, general purpose microprocessors, ASICs, ASIPs, FPGAs or other processors which run software programs or applications, which may be stored in host memory  111 , associated with the general and conventional functions and capabilities of device  100 . 
     Communications interface  105  may be any suitable bus or interface, such as a peripheral component interconnect express (PCIe) bus, a universal serial bus (USB), a universal asynchronous receiver/transmitter (UART) serial bus, a suitable advanced microcontroller bus architecture (AMBA) interface, an Inter-Integrated Circuit (I2C) bus, a serial digital input output (SDIO) bus, or other equivalent and may include a plurality of communications interfaces. Communications interface  105  may facilitate communication between SPU  120  and one or more of host processor  110 , host memory  111 , transceiver  113 , ultrasonic transducer device  150 , and/or other included components. 
     Host memory  111  may comprise programs, modules, applications, or other data for use by host processor  110 . In some embodiments, host memory  111  may also hold information that that is received from or provided to sensor processing unit  120  (see e.g.,  FIG. 1B ). Host memory  111  can be any suitable type of memory, including but not limited to electronic memory (e.g., read only memory (ROM), random access memory (RAM), or other electronic memory). 
     Transceiver  113 , when included, may be one or more of a wired or wireless transceiver which facilitates receipt of data at device  100  from an external transmission source and transmission of data from device  100  to an external recipient. By way of example, and not of limitation, in various embodiments, transceiver  113  comprises one or more of: a cellular transceiver, a wireless local area network transceiver (e.g., a transceiver compliant with one or more Institute of Electrical and Electronics Engineers (IEEE) 802.11 specifications for wireless local area network communication), a wireless personal area network transceiver (e.g., a transceiver compliant with one or more IEEE 802.15 specifications (or the like) for wireless personal area network communication), and a wired a serial transceiver (e.g., a universal serial bus for wired communication). 
     Ultrasonic transducer device  150  includes a split-electrode ultrasonic transducer of the type described herein and is configured to emit and receive ultrasonic signals. In some embodiments, ultrasonic transducer device  150  may include a controller  151  for controlling the operation of the split-electrode ultrasonic transducer and/or other components of ultrasonic transducer device  150 . The controller  151  may be any suitable controller, many types of which have been described here. For example, controller  151  may turn amplifiers on or off, turn transmitters on or off, and/or operate selectable switches to electrically couple certain segments of electrodes during transmitting or during receiving and/or couple segments of electrodes to a drive transmitter, to ground, and/or to a front-end amplifier. Controller  151  may enable different modes of operation (e.g., transmitting, receiving, or continuous operation) and may enable variations within a mode (e.g., transmitting with some electrodes, but not others; and/or receiving with some electrodes, but not others). Additionally, or alternatively, in some embodiments, one or more aspects of the operation of ultrasonic transducer device  150  or components thereof may be controlled by an external component such as sensor processor  130  and/or host processor  110 . 
       FIG. 1B  shows a block diagram of components of an example device  100 B, in accordance with various aspects of the present disclosure. Device  100 B is similar to device  100 A except that it includes a sensor processing unit (SPU)  120  in which ultrasonic transducer device  150  is disposed. SPU  120 , when included, comprises: a sensor processor  130 ; an internal memory  140 ; and at least one ultrasonic transducer device  150 . In some embodiments, SPU  120  may additionally include one or more motion sensors and/or one or more other sensors such a light sensor, infrared sensor, GNSS sensor, microphone, etc. In various embodiments, SPU  120  or a portion thereof, such as sensor processor  130 , is communicatively coupled with host processor  110 , host memory  111 , and other components of device  100  through communications interface  105  or other well-known means. SPU  120  may also comprise one or more communications interfaces (not shown) similar to communications interface  105  and used for communications among one or more components within SPU  120 . 
     Sensor processor  130  can be one or more microprocessors, CPUs, DSPs, general purpose microprocessors, ASICs, ASIPs, FPGAs or other processors that run software programs, which may be stored in memory such as internal memory  140  (or elsewhere), associated with the functions of SPU  120 . In some embodiments, one or more of the functions described as being performed by sensor processor  130  may be shared with or performed in whole or in part by another processor of a device  100 , such as host processor  110 . 
     Internal memory  140  can be any suitable type of memory, including but not limited to electronic memory (e.g., read only memory (ROM), random access memory (RAM), or other electronic memory). Internal memory  140  may store algorithms, routines, or other instructions for instructing sensor processor  130  on the processing of data output by one or more of ultrasonic transducer device  150  and/or other sensors. In some embodiments, internal memory  140  may store one or more modules which may be algorithms that execute on sensor processor  130  to perform a specific function. Some examples of modules may include, but are not limited to: statistical processing modules, motion processing modules, object detection modules, and/or decision-making modules. 
     Ultrasonic transducer device  150 , as previously described, includes a split-electrode ultrasonic transducer of the type described herein and is configured to emit and receive ultrasonic signals. In some embodiments, ultrasonic transducer device  150  may include a controller  151  for controlling the operation of the split-electrode ultrasonic transducer and/or other components of ultrasonic transducer device  150 . The controller  151  may be any suitable controller, many types of which have been described here. For example, controller  151  may operate selectable switches to electrically couple certain segments of electrodes during transmitting or during receiving and or couple segments of electrodes to a drive transmitter, to ground, and/or to a front-end amplifier. Additionally, or alternatively, in some embodiments, one or more aspects of the operation of electrode ultrasonic transducer device  150  or components thereof may be controlled by an external component such as sensor processor  130  and/or host processor  110 . Ultrasonic transducer device  150  is communicatively coupled with sensor processor  130  by a communications interface, bus, or other well-known communication means. 
     Example Split-Electrode Piezoelectric Transducer 
       FIG. 2A  shows a top plan view of a split-electrode transducer  200 A, in accordance with various embodiments. In some embodiments, split-electrode piezoelectric transducer  200 A is an ultrasonic transducer and operates in the ultrasonic range. In some embodiments, split-electrode transducer  200 A is a Piezoelectric Micromachined Ultrasonic Transducer (PMUT), which may be an air-coupled PMUT. In some air coupled ultrasonic transducer embodiments, for example, split-electrode piezoelectric transducer  200 A operates in the 60 to 200 kHz range. In some air coupled ultrasonic transducer embodiments, for example, split-electrode piezoelectric transducer  200 A operates in the 40 to 400 kHz range; where higher frequencies may be used for sensing objects that are very near to a transducer. In other embodiments of an ultrasonic transducer which is not air coupled (i.e., the transducer is coupled to other media such as liquids, human flesh, or solids), different operating frequency ranges are possible. In a first example, in some medical device embodiments such as for ultrasound probes, an ultrasonic transducer as described herein may operate in the 1-10 MHz range. In a second example, in some fingerprint sensing embodiments, an ultrasonic transducer as described herein may operate in the 10-60 MHz range. Section line A-A shows the position and direction of a side sectional view illustrated in  FIG. 2B . 
     With reference to  FIG. 2A , the top view illustrates that transducer  200 A is formed in a circular shape, however other shapes may be utilized. Some non-limiting examples of other shapes include: square, rectangular, hexagonal, and ellipse. 
       FIG. 2B  shows a sectional side elevational view A-A of the split-electrode transducer  200 A of  FIG. 2A , in accordance with various embodiments. 
     With reference to  FIGS. 2A and 2B , split-electrode piezoelectric transducer  200 A includes: a top electrode layer, TE; a bottom electrode layer, BE; a membrane layer  204 , and a piezoelectric layer  203 . As will be described, the depicted order of the layers is just one example of their ordering; other orders of these layers may be utilized in some embodiments so long as the piezoelectric material is disposed between the TE layer and the BE layer. For example, an alternative ordering of the layers is illustrated in  FIGS. 7A and 7B . In some embodiments, other layers such as protective layers, filler layers, and/or electrically insulating layers may be included. These other layers have not been depicted in order to improve clarity. It should be appreciated that membrane  204  moves up and down (relative to  FIG. 2B ) at a desired frequency to produce sound through the displacement of membrane  204 , and that in  FIG. 2B  membrane  204  is depicted in a “displaced up” position of the transducer. 
     With continued reference to  FIG. 2A , the BE layer comprises conductive material disposed above and coupled with the membrane layer  204  and is split into at least two portions depicted as a first bottom electrode BE 1 , and a second bottom electrode BE 2 . In some embodiments, the first bottom electrode BE 1  and the second bottom electrode BE 2  are substantially equal in surface area. 
     It should be appreciated that electrical traces are required to be coupled to the electrodes in order to route various signals and/or provide various couplings (such as to another electrode, to ground, etc.), however in the interest of clarity these traces are not illustrated. Any suitable routing may be used for such these traces. 
     With continued reference to  FIGS. 2A and 2B , a piezoelectric layer  203  is disposed above and coupled with the bottom electrode layer (i.e., bottom electrodes BE 1  and BE 2 ). In some embodiments, the piezoelectric layer  203  may comprise a first piezoelectric portion  203 - 1  disposed above the first bottom electrode BE 1  and a second piezoelectric portion  203 - 2  disposed above the second bottom electrode BE 2 . In the embodiment of  FIG. 2B , the gap represented by dashed lines  206  and  207  in  FIG. 2A  is etched or otherwise created to form two segments of the piezoelectric layer  203 . In some embodiments, the surface area (i.e., surface area from a top plan view measurement of surface area) of first piezoelectric portion  203 - 1  and second piezoelectric portion  203 - 2  may be equal or substantially equal (e.g., within manufacturing tolerances of e.g., a few percent) to one another. In other embodiments, where the top electrode layer TE and the bottom electrode layer BE may be etched while the piezoelectric layer  203  is not. In such an embodiment, piezoelectric layer  203  may be a single, unetched layer that is not divided into multiple parts and the gap in piezoelectric layer  203  represented by dashed lines  206  and  207  in  FIG. 2A  would not be present. 
     With continued reference to  FIGS. 2A and 2B , a top electrode layer TE comprised of conductive material is disposed above and coupled with the piezoelectric layer  203 . Top electrode layer TE comprises a segmented center electrode with segments TE 1  and TE 2  that are disposed above a center portion of the membrane layer  204 . Gap  201 , illustrated in  FIG. 2A , shows the location of an electrical disconnect/gap between center electrode segments TE 1  and TE 2 . The top electrode layer TE also comprises a segmented outer electrode with segments TE 3  and TE 4  that are spaced apart, outward, from the segmented center electrode segments TE 1  and TE 2 . In a circular embodiment, as depicted in  FIG. 2A , the outer electrode segments TE 3  and TE 4  are spaced radially outward, apart from the center electrode segments TE 1  and TE 2 . Gaps  202  and  205 , illustrated in  FIG. 2A , show the locations of electrical disconnects/gaps between outer electrode segments TE 3  and TE 4 . The segmented outer electrode (e.g., segments TE 3  and TE 4 ) is disposed such that it is spaced apart, away from the center of the membrane layer and around (i.e., surrounding except for the gaps) the segmented center electrode (e.g., around segments TE 1  and TE 2 ). In  FIG. 2A , outer electrode segments TE 3  and TE 4  form a segmented circular ring around segments TE 1  and TE 2  of a circular center. However, in other transducer shapes (e.g., square, hexagonal, rectangular, oval) the segmented outer electrode as well as the center electrodes may have other shapes (e.g., square, hexagonal, rectangular, oval) and the segmented outer electrode forms a perimeter or periphery which is spaced apart and outward from the segmented center electrodes. Although the center electrode and outer electrode are each divided into two segments, following the same principles each may be divided into a larger number of segments. For example, each of the center electrode and the outer electrode may be divided into three segments, four segments, five segments six segments, etc. In some embodiments, each of the segments of the center electrode is equal or substantially equal (e.g., within manufacturing tolerances of a few percent) in surface area to one another. In some embodiments, each of the segments of the outer electrode is equal or substantially equal (e.g., within manufacturing tolerances of a few percent) in surface area to one another. In some embodiments, each of the individual segments of the segmented center electrode and of the segmented outer electrode is equal or substantially equal (e.g., within manufacturing tolerances of a few percent) in surface area to one another. That is, in some embodiments the plan view surface area of TE 1 =the surface area of TE 2 =the surface area of TE 3 =the surface area TE 4 . It should be appreciated that the gaps  202  and  205  between the outer electrode segments TE 3  and TE 4  are not required to be aligned with the gap  201  between the center electrode segments TE 1  and TE 2 . They may be offset by any suitable angle, such as 7 degrees, 20 degrees, 37 degrees, 45 degrees, 90 degrees, etc. 
     With reference to  FIG. 2C , a top view is illustrated of an example split-electrode transducer  200 B, in accordance with various embodiments.  FIG. 2C  shows a circular embodiment of a split-electrode transducer where the gaps ( 202 ,  205 ) between the outer electrode segments TE 3  and TE 4  are not aligned with the gap  201  between the center electrode segments TE 1  and TE 2 . In some such embodiments, the lack of alignment between the gap  201  and gaps  202  and  205  reduces physical weakness of transducer  200 B, and in particular the top electrode layer TE, by positioning gaps along different axes instead of all being positioned along the same axis. As described above in conjunction with  FIG. 2A , in other transducer shapes (e.g., square, hexagonal, rectangular, oval) the segmented outer electrode as well as the center electrodes may have other shapes (e.g., square, hexagonal, rectangular, oval). Electrical operation of the split-electrode transducer  200 B is not altered, with respect to split electrode transducer  200 A, when gap  201  is not aligned with gaps  202  and  205 . 
     With continued reference to  FIGS. 2A and 2B , a first segment TE 4  of the segmented outer electrode and a first segment TE 2  of the segmented center electrode are disposed above the first bottom electrode BEL while a second segment TE 3  of the segmented outer electrode and a second segment TE 1  of the segmented center electrode are disposed above the second bottom electrode B 2 . In some embodiments the segments TE 1  and TE 2  of the segmented center electrode and segments TE 3  and TE 4  of the segmented outer electrode are positioned on the piezoelectric layer  203  based on a curvature of the piezoelectric layer  203  when it is displaced up or down (shown displaced up in  FIG. 2B ). That is, they are arranged such that the curvature of the center electrode segments TE 1  and TE 2  is opposite of the curvature of the outer electrode segment TE 3  and TE 4  when transducer  200 A is fully displaced up or fully displaced down. That is, one of the split center electrode and the split outer electrode inside the deflection point of the displaced piezoelectric layer  203  while the other is outside of a deflection point of the displaced piezoelectric layer  203 . 
     With continued reference to  FIGS. 2A and 2B , one way to mathematically describe the shapes of the top electrode layer (TE 1 , TE 2 , TE 3 , TE 4 ) with respect to the piezoelectric layer  203  is that the top electrode layer is disposed above and coupled with the piezoelectric layer  203  and comprises a segmented first electrode (segmented center electrode segments TE 1  and TE 2 ) disposed above a section of the membrane layer  204 , in which the Laplacian of the out-of-plane displacement in the piezoelectric layer  203  has a positive sign in a given displaced shape, and a segmented second electrode (segmented outer electrode segments TE 3  and TE 4 ) spaced radially apart from the segmented first electrode (TE 1 , TE 2 ), in which the Laplacian of the out-of-plane displacement in the piezoelectric layer  203  has a negative sign in the same given displaced shape. Another way to mathematically describe the shapes of the top electrode layer (TE 1 , TE 2 , TE 3 , TE 4 ) with respect to the piezoelectric layer  203  is that the top electrode layer is disposed above and coupled with the piezoelectric layer  203  and comprises a segmented first electrode (segmented center electrode segments TE 1  and TE 2 ) disposed above a section of the membrane layer  204 , in which the sum of the normal components of the in-plane strain in the piezoelectric layer  203  has a positive sign in a given displaced shape, and a segmented second electrode (segmented outer electrode segments TE 3  and TE 4 ) spaced apart (radially apart in the depicted embodiment) from the segmented first electrode (TE 1 , TE 2 ), in which the sum of the normal components of the in-plane strain in the piezoelectric layer  203  has a negative sign in the same given displaced shape. 
     With reference to  FIG. 2B , split-electrode transducer  200 A is shown with a curvature which occurs as the transducer  200 A moves during operation. As depicted, in some instances during upward deflection of the piezoelectric layer  203 , when segments TE 1  and TE 2  of the segmented center electrode (TE 1  and TE 2  together) present a concave surface disposed toward membrane layer  204 ; at the same time segments TE 3  and TE 4  of the segmented outer electrode (TE 3  and TE 4  together) present a convex surface oriented toward membrane layer  204 . Similarly, in other instances during downward deflection of the piezoelectric layer  203  (not depicted), when segments TE 1  and TE 2  of the segmented center electrode (TE 1  and TE 2  together) present a convex surface disposed toward membrane layer  204 ; at the same time segments TE 3  and TE 4  of the segmented outer electrode (TE 3  and TE 4  together) present a concave surface oriented toward membrane layer  204 . These concave and convex curvatures and orientations are due to the shape of the deflected piezoelectric layer  203  as it moves in response to an applied signal. 
     With reference to  FIGS. 2A, 2B, and 2C , the depicted gaps may be air gaps or may be filled, such as with an insulative material. The width of the gaps is very small and may only be a few microns, in some embodiments. Generally, a gap is made as narrow as is feasible, as making it wider limits surface area of the split electrodes and reduces performance of the transducer. The lower limit of the narrowness of the gap widths is typically governed either by the limits of the lithography or other techniques used to deposit materials or etch the gaps or by the avoidance of fringe capacitive coupling between the electrodes on each side of the gap which may occur if the gap is too narrow. In some embodiments, conductive routing traces may be disposed in/routed through a gap. For example, the split center electrodes TE 1  and TE 2  may have one or more conductive routing traces which are disposed in gap  201 . Accordingly, in such an embodiment, gap  201  may be wider than the minimum narrowness possible to facilitate presence of the conductive trace(s). 
     With reference to  FIG. 2B , in some embodiments, an additional electrode (not depicted) can be added below membrane  204 . In such embodiments, the additional electrode can be grounded and/or electrically isolated from bottom electrode layer BE and used as a shield to reduce interference. 
     Although described herein as an ultrasonic transducer, the principles of the split-electrode piezoelectric transducer  200  illustrated in  FIGS. 2A, 2B, and 2C  may be utilized with transducers operating in other frequency ranges (e.g., human audible or infrasound). Further, the principles may be applied to sonic transmitters or sonic receivers, not just to sonic transducers. For example, the principles described herein may be utilized to improve the receive function of a microphone. 
     Operation of the Example Split Electrode PMUT 
       FIG. 3  illustrates operation of the split-electrode PMUT  200 A of  FIGS. 2A and 2B  in a transmit (Tx) mode, in accordance with various embodiments. In  FIG. 3 , a circuit  300  is depicted which includes split-electrode PMUT  200 A. As illustrated, a signal/pulse generator  330  is coupled with the input of a drive transmitter  310  generates and provides a repeating pulse or repeating waveform on the input. In some embodiments, a charge pump  320  may also be coupled with drive transmitter  310 . Charge pump  320 , when included, supplies additional charge for drive transmitter  310  to amplify the input to drive transmitter  310 . In some embodiments, for example, a charge pump may be included when aluminum nitride (AlN) is used in the piezoelectric layer  203  as certain configurations of such a split-electrode transducer may require additional supplied charge (voltage), over the voltage natively provided by drive transmitter  310 , to transmit. 
     In the Tx mode illustrated in  FIG. 3 , in some embodiments differential drive embodiments, center electrode segment TE 1  and center electrode segment TE 2  are connected together (as a unified center electrode TE 1 ,TE 2 ), while outer electrode segment TE 3  and outer electrode segment TE 4  are connected together (as a unified outer electrode TE 3 ,TE 4 ). A differential drive signal from the non-inverted output of drive transmitter  310  is applied on center electrode TE 1 ,TE 2 ; and from the inverted output of drive transmitter  310  the 180 degree out of phase version of the drive signal is applied on outer electrode TE 34 . BE 1  and BE 2  are both grounded. By driving differential signals simultaneously on both the center electrode TE 1 , TE 2  and the outer electrode TE 3 , TE 4 , the achieved displacement of the split electrode PMUT  200 A is increased compared to driving only on the either the center electrode TE 1 , TE 2  or on outer electrode TE 3 , TE 4 . The increased displacement results in increased transmission range of the transmitted ultrasonic signal over conventional approaches. 
     Transmit modes that do not utilize differential drive may be employed, in some embodiments. For example, in a continuous mode of operation (rather than where driving is pulsed on/off), transmitting may be accomplished by driving only the segmented center electrodes TE 1 , TE 2  (at the same time with the same signal) but not driving the segmented outer electrodes TE 3 , TE 4 ; or transmitting may be accomplished by driving only the segmented outer electrodes TE 3 , TE 4  (at the same time with the same signal) but not driving the segmented center electrodes TE 1 , TE 2 . In a continuous mode of operation, some TE electrodes may be driven while other TE electrodes are used to receive. For example, while a signal is driven on one or both segmented center electrodes TE 1  and TE 2 , returned signals may be received on one or both of segmented outer electrodes TE 3  and TE 4 . Controller  151  may configure components of an ultrasonic transducer device  150  to operate in a continuous mode or a differential mode. 
     To transmit with some modicum of amplitude control, instead of driving on all four of the TE electrodes, only one, two or three of the TE electrodes may be driven. To support a continuous mode of operation, undriven TE electrodes may be used to simultaneously receive while other TE electrodes are being driven. Controller  151  may configure components of an ultrasonic transducer device  150  to drive with selected electrodes and/or to receive with selected electrodes. 
       FIG. 4A  illustrates operation of the split-electrode PMUT  200 A of  FIGS. 2A and 2B  in a receive mode, in accordance with various embodiments. In  FIG. 4A , a circuit  400 A is depicted which includes split-electrode PMUT  200 A. In the Rx mode, BE 1  and BE 2  are floating and TE 1  and TE 4  are grounded. Center electrode segment TE 2  and outer electrode segment TE 3  are connected, respectively, to the two differential inputs of front-end amplifier  410  of the receive circuitry. As depicted, for example, split electrode segment TE 2  is coupled with the non-inverting input, while outer electrode segment TE 3  is coupled to the inverting input. In this setup, the path from ground through TE 1  to BE 2  to TE 3  to the inverting input of front-end amplifier  410  represents a set of two stacked (i.e., series) capacitors, such that the overall capacitance in this path is halved, and the voltage is consequently doubled while doubling the source impedance. More particularly, the first capacitor has plates of TE 1  and BE 2  separated by a piezoelectric layer  203 - 2  as a dielectric, while the second capacitor has plates of BE 2  and TE 3  separated by piezoelectric layer  203 - 2  as a dielectric. Because BE 2  is a common plate/node in this set of capacitors, it provides a series electrical coupling between these two capacitors. The same is true of the path from ground through TE 4  to BE 1  to TE 2  to the non-inverting input of front-end amplifier  410 , which represents a second set of two stacked (i.e., series) capacitors. More particularly, the first capacitor has plates of TE 4  and BE 1  separated by a piezoelectric layer  203 - 1  as a dielectric, while the second capacitor has plates of BE 2  and TE 2  separated by piezoelectric layer  203 - 1  as a dielectric. Because BE 1  is a common plate/node in this second set of capacitors, it provides a series electrical coupling between these two capacitors. By using both TE 2  and TE 3  as differential input to the front end, the receive voltage is doubled again for an overall four times gain in comparison to a transducer which only utilizes a center electrode design. This four times gain increases the Rx sensitivity over conventional approaches. 
       FIG. 4B  illustrates operation of the split-electrode piezoelectric transducer  200 A of  FIGS. 2A and 2B  in a receive mode, in accordance with various embodiments. In  FIG. 4B , a circuit  400 B is depicted which includes split-electrode PMUT  200 A. When the surface areas of TE 1 , TE 2 , TE 3 , and TE 4 , are equal or substantially equal, the effective capacitances of C 1  and C 2  are equal, and any interference signal (V INT ) becomes common mode and is reduced at the output of amplifier  410  by the common mode rejection ratio of amplifier  410 . 
       FIG. 5A  illustrates an ultrasonic transducer device  150 , in accordance with various embodiments.  FIG. 5A  represents a combination of the circuits and as components illustrated in  FIGS. 4A and 4B  in order to form an ultrasonic transducer device  150  which is configured to both transmit and receive. Selectable switches SW 1 , SW 2 , SW 3 , SW 4 , SW 5 , SW 6 , SW 7 , SW 8 , SW 9 , and SW 10  have also been added to select various modes of operation. These selectable switches are all shown in an open position. In some embodiments, controller  151  (not depicted) or another processor or logic operates the selectable switches to select different modes of operation. In some embodiments, SW 1 , SW 2 , SW 3 , and SW 4  are high voltage switches and operate to pass high voltages during transmitting. In some embodiments, switches SW 5  and SW 6  are low voltage switches. In some embodiments, SW 7 , SW 8 , SW 9 , and SW 10  are high voltage switches which provide a low Equivalent Series Resistance (ESR) and also block high voltages from front-end amplifier  410  when transmitting is occurring. In some embodiments, high voltage is in the range of 4V to ˜120V or more. In some embodiments, high voltage is in the range of ˜10V to ˜40V. In some embodiments, low voltage is in the range of 1V to ˜3.5V or slightly more (e.g., 5V). In some embodiments, low voltage is in the range of 1.2V to 2.2V or slightly more. 
       FIG. 5B  illustrates the ultrasonic transducer device  150  of  FIG. 5A  in a transmit configuration, in accordance with various embodiments. In such embodiments, controller  151  or another processor or logic, has closed selectable switches SW 1 , SW 2 , SW 3 , SW 4 , SW 5  and SW 6  and opened selectable switches SW 7 , SW 8 , SW 9 , and SW 10  in response to an instruction to place PMUT  200 A in a transmit mode. This creates the same conditions previously discussed in conjunction with  FIG. 3 , where: BE 1  and BE 2  are coupled to ground; the non-inverting output of drive transmitter  310  is coupled with center electrode segments TE 1  and TE 2 ; and the inverting output of drive transmitter  310  is coupled with outer electrode segments TE 3  and TE 4 . 
     It should be appreciated that switches S 1 , S 2 , S 3 , and S 4  may not be present or used in some embodiments. That is, the modes described herein, may be implemented without these switches. For example, in such embodiments, two transmitter  310  may instead be used (rather than the single depicted transmitter  310 ). The positive output of the first transmitter  310  is connected to TE 2  and the negative output of the first transmitter  310  is connected to TE 4 . The positive output of the second transmitter  310  is connected to TE 1  and the negative output of the second transmitter  310  is connected to TE 3 . Each of the transmitters  310  is designed to have a high-impedance state wherein the internal switches in a transmitter  310  may be turned off by controller  151  when it is not active and especially when the circuit is configured in receive mode. 
       FIG. 5C  illustrates the ultrasonic transducer device  150  of  FIG. 5A  in a receive configuration, in accordance with various embodiments. In such embodiments, controller  151  or another processor or logic, has opened selectable switches SW 1 , SW 2 , SW 3 , SW 4 , SW 5  and SW 6  and closed selectable switches SW 7 , SW 8 , SW 9 , and SW 10  in response to an instruction to place PMUT  200 A in a receive mode or an instruction to switch from a transmit mode to a receive mode. This creates the same conditions previously discussed in conjunction with  FIG. 4A , where: BE 1  and BE 2  are floating and TE 1  and TE 4  are grounded. Center electrode segment TE 2  and outer electrode segment TE 3  are connected, respectively, to the two differential front-end inputs of the receive circuitry. In some embodiments, instead of immediately opening switches SW 5  and SW 6  when PMUT is switched from transmitting to receiving, controller  151  may leave them closed for a short period of time (such as the ringdown period after differential transmitting with transducer  200 A, or slightly longer) in order to reduce the amplitude of membrane vibration caused by ringdown and thus control the gain (such as by preventing front-end amplifier  410  from being driven to saturation by a ringdown signal when it is set to amplify received signals that do not include much or any of the ringdown signal). 
     SW 5  and SW 6 , when closed, reduce the signal received from the transducer  200 A by shorting out the signal from TE 1  and TE 4 . These switches can be opened and/or closed during a receive cycle. This can be used to reduce the overall dynamic range requirement of the front end and can be used to increase the gain (by opening these switches) during the receive cycle after a certain time has elapsed since the transmitting phase. In this manner front-end may be set at a higher gain, without clipping/saturation being caused by ringdown signals. In some embodiments, the ringdown period is at least as long as a transmit period immediately preceding the PMUT being switched from a transmit mode to a receive mode. Reducing the received signal by closing SW 5  and SW 6  may be utilized to create a “close object detection mode,” which reduces the gain while sensing for nearby objects which would generate a strong return. In this manner, gain of the front-end amplifier  410  does not have to be adjusted to prevent saturation which might be caused by sensing nearby objects. Opening switches SW 5  and SW 6  allows switching from the close object detection mode to a “far object detection mode” due to increasing the gain. For example, if it is determined that there is no close object detected, switches SW 6  and SW 6  can be opened to detect for objects farther away. 
     When the capacitance of PMUT  200 A (or  200 B when it is utilized) is much larger than the parasitic capacitance (of front-end amplifier  410 , wire bonds, etc.) any reduction in PMUT capacitance is beneficial. Thus, in this manner, the TE electrodes may be divided into more pieces than illustrated in order to increase the number of series capacitors and thus decrease capacitance. The gains realized from reducing the capacitance of the PMUT (via these further divisions of the top electrode (center and outer portions) and the bottom electrode) only diminishes when the PMUT capacitance becomes the same or less than the parasitic capacitance while in a receive mode. For example, the series stacking can be taken to the next level by splitting each top and bottom electrode up again. This would further increase the receive voltage. However, it would require connecting top electrode areas together without connecting the underlying bottom electrodes. In some embodiments, this connecting can be accomplished in a process where the bottom electrode is patterned before the deposition of the piezoelectric layer; or by connecting the top electrodes externally (e.g., with wire bonds or switches). The advantage of further stacking additional series capacitors disappears when the reduction in capacitance means that the PMUT capacitance becomes comparable to parasitic capacitance considering also that the latter might increase due to additional wire bonds, switches in the ASIC, etc. 
       FIG. 5D  illustrates the ultrasonic transducer device  150  of  FIG. 5A  in a receive configuration, in accordance with various embodiments.  FIG. 5D  is the same as  FIG. 5C  except that it illustrates an embodiments previously described in conjunction with  FIG. 4B  in which the surface areas of TE 1 , TE 2 , TE 3 , and TE 4 , are equal or substantially equal, causing the effective capacitances of C 1  and C 2  to be equal, and thus causing an interference signal (VINT) to become common mode and rejected to ground. 
     Example Method of Manufacture 
       FIG. 6  illustrates a method of manufacture of a piezoelectric micromachined transducer, in accordance with various embodiments. In some embodiments, the piezoelectric micromachined transducer may operate in the ultrasonic range and it may be referred to as a PMUT. In some embodiments the piezoelectric micromachined transducer is air coupled. Procedures of the method illustrated by flow diagram  600  of  FIG. 6  will be described with reference to elements and/or components of one or more of  FIGS. 2A, 2B, and 2C . It is appreciated that in some embodiments, the procedures may be performed in a different order than described in a flow diagram, that some of the described procedures may not be performed, and/or that one or more additional procedures to those described may be performed. 
     With reference to  FIG. 6 , at procedure  610  of flow diagram  600 , in various embodiments, a membrane layer is provided. In some embodiments, the membrane is provided or built-up through deposition, it can then be patterned as required to create gaps by using photolithographic patterning, etching, or lift-off. The membrane layer may be similar to membrane  204  of  FIG. 2B . 
     With continued reference to  FIG. 6 , at procedure  620  of flow diagram  600 , in various embodiments, a bottom electrode layer is deposited above the membrane layer. In some embodiments, the deposition is accomplished through deposition cycles. The bottom electrode layer may be similar to bottom electrodes BE 1  and BE 2  in  FIG. 2B . The bottom electrode layer may be deposited and the gap may be created later, such as through patterning, etching, etc. 
     With continued reference to  FIG. 6 , at procedure  630  of flow diagram  600 , in various embodiments, a piezoelectric layer is disposed above the bottom electrode layer. In some embodiments, the deposition is accomplished through deposition cycles. The piezoelectric layer may be similar to piezoelectric layer  203  of  FIG. 2B . The piezoelectric layer may be deposited with a gap such that it is divided into piezoelectric portions  203 - 1  and  203 - 2  or the gap may be created later (if desired), such as through etching. 
     With continued reference to  FIG. 6 , at procedure  640  of flow diagram  600 , in various embodiments, a top electrode layer is deposited above the piezoelectric layer. With reference to  FIGS. 2A, 2B, and 2C  the top electrode layer comprises a center electrode TE 1 , TE 2  disposed above a center of the membrane layer  204  and a outer electrode TE 3 , TE 4  spaced apart from the center electrode TE 1 , TE 2 . The segmented outer electrode (e.g., segments TE 3  and TE 4 ) is disposed such that it is spaced apart, away from the center of the membrane layer and around (i.e., surrounding except for the gaps) the segmented center electrode (e.g., around segments TE 1  and TE 2 ). In  FIG. 2A , outer electrode segments TE 3  and TE 4  form a segmented circular ring around segments TE 1  and TE 2  of a circular center. However, in other transducer shapes (e.g., square, hexagonal, rectangular, oval) the segmented outer electrode as well as the center electrodes may have other shapes (e.g., square, hexagonal, rectangular, oval) and the segmented outer electrode forms a perimeter or periphery which is spaced apart and outward from the segmented center electrodes. Although the center electrode and outer electrode are each divided into two segments, following the same principles each may be divided into a larger number of segments. In some embodiments, the deposition is accomplished through deposition cycles. In some embodiments, the outer electrode and the center electrode are deposited such that they are substantially equal in surface area. In some embodiments, the top electrode layer may be deposited with gaps as illustrated in  FIG. 2A  such that the center electrode is divided into two portions TE 1  and TE 2  and the outer electrode is divided into two portions TE 3  and TE 4 . In some embodiments, the gaps in the top electrode layer which are illustrated in  FIG. 2A  may be created later, after deposition, such as through etching. It should be appreciated that the gap between the segmented center electrodes TE 1  and TE 2  does not have to be aligned (i.e., on the same axis as the gaps between the outer electrode segments TE 3  and TE 4 ), and may be purposely misaligned in order to decrease the overall weakening of the transducer which may result by aligning all of the gaps on a single axis. 
     With continued reference to  FIG. 6 , at procedure  650  of flow diagram  600 , in various embodiments, the top electrode layer, the piezoelectric layer, and the bottom electrode layer are etched. The creation of gaps may be accomplished via photolithographic patterning, etching, or lift-off. With reference to  FIGS. 2A and 2B , the etching of the bottom electrode layer creates a first bottom electrode BE 1  and a second bottom electrode BE 2  from the bottom electrode layer. In some embodiments BE 1  and BE  2  are equal or substantially equal in surface area. With reference to  FIGS. 2A and 2B , the etching of the piezoelectric layer creates: a first piezoelectric portion  203 - 1  and a second piezoelectric portion  203 - 2  from the piezoelectric layer  203 , where the first piezoelectric portion  203 - 1  disposed above the first bottom electrode BE 1  and the second piezoelectric portion  203 - 2  disposed above the second bottom electrode BE 2 . With reference to  FIGS. 2A and 2B , the etching of the top electrode layer creates a first outer electrode segment TE 4  and a second outer electrode segment TE 3  from the outer electrode, where the first outer electrode segment TE 4  is disposed above the first bottom electrode BE 1  and the second outer electrode segment TE 3  is disposed above the second bottom electrode BE 2 . With reference to  FIGS. 2A and 2B , the etching of the top electrode layer also creates a first center electrode segment TE 1  and a second center electrode segment TE 2  from the center electrode, where the first center electrode segment TE 1  is disposed above the first bottom electrode BE 1  and the second center electrode segment TE 2  is disposed above the second bottom electrode BE 2 . In some embodiments the etching is accomplished to create the first center electrode segment TE 1  and the second center electrode segment TE 2  such that they are equal or substantially equal in surface area to the first outer electrode segment TE 4  and the second outer electrode segment TE 3 . Put differently, the surface areas of TE 1 , TE 2 , TE 3 , and TE 4  are equal or substantially equal. 
     Example Alternative Split-Electrode Piezoelectric Transducer 
     A variety of alternative arrangements of the layer stack-up of transducer  200 A of  FIGS. 2A and 2B  are possible and anticipated. Some non-limiting examples of alternative designs include: 1) using the membrane layer itself as the bottom electrode BE, by doping portions of the membrane layer to make them conductive; 2) inverting the functionality of the bottom electrode and top electrode and with respect to  FIGS. 2A and 2B  and switching these layers in the stack-up (i.e., the top electrode would have two portions and the bottom electrode would have four portions); 3) inverting the order of the stack-up/layer ordering shown in  FIGS. 2A and 2B  (i.e., the top-to-bottom layer ordering becomes: membrane, bottom electrode layer, piezoelectric layer, and top electrode layer); and 4) placing the bottom electrode layer below the membrane layer. 
       FIG. 7A  shows a top plan view of a split-electrode transducer  700 , in accordance with various embodiments. This is a depiction of the second alternative example mentioned above. In some embodiments, split-electrode piezoelectric transducer  700  is an ultrasonic transducer and operates in the ultrasonic range. Except for the stack-up ordering which reverses the functionality of the top electrode layer and the bottom electrode layer, the operation and other characteristics are similar or the same as those described previously in conjunction with split-electrode transducer  200 A. Section line B-B shows the position and direction of a side sectional view illustrated in  FIG. 7B . 
       FIG. 7B  shows a sectional side elevational view B-B of the split-electrode transducer  700  of  FIG. 7A , in accordance with various embodiments. 
     With reference to  FIGS. 7A and 7B , split-electrode piezoelectric transducer  700  includes: a top electrode a top electrode layer, TE; a bottom electrode layer, BE; a piezoelectric layer  203 ; and a membrane layer  204 . In some embodiments, other layers such as protective layers, filler layers, and/or electrically insulating layers may be included. These other layers have not been depicted in order to improve clarity. It should be appreciated that membrane  204  moves up and down (relative to  FIG. 7B ) at a desired frequency to produce sound through the displacement of membrane  204 , and that in  FIG. 7B  membrane  204  is depicted in a “displaced up” position of the transducer. 
     With continued reference to  FIG. 7A , the BE layer comprises conductive material disposed above and coupled with the membrane layer  204  and it is split into at least four portions depicted as: a first bottom electrode BE 1 ′, a second bottom electrode BE 2 ′, a third bottom electrode BE 3 , and a fourth bottom electrode BE 4 . Bottom electrode layer BE comprises a segmented center electrode with segments BE 1 ′ and BE 2 ′ that are disposed above a center portion of the membrane layer  204  (in the same shape and fashion as TE 1  and TE 2  of  FIG. 2A ). Gap  702  shows the location of an electrical disconnect/gap between center electrode segments BE 1 ′ and BE 2 ′. The bottom electrode layer BE also comprises a segmented outer electrode with segments BE 3  and BE 4  that are spaced apart, outward, from the segmented center electrode segments BE 1 ′ and BE 2 ′ (in the same fashion as TE 3  and TE 4  of  FIG. 2A ). In a circular embodiment, as depicted in  FIGS. 7A and 7B , the outer electrode segments BE 3  and BE 4  are spaced radially outward, apart from the center electrode segments BE 1 ′ and BE 2 ′. The segmented outer electrode (e.g., segments BE 3  and BE 4 ) is disposed such that it is spaced apart, away from the center of the membrane layer and around (i.e., surrounding except for the gaps) the segmented center electrode (e.g., around segments BE 1 ′ and BE 2 ′). In  FIG. 7A , outer electrode segments BE 3  and BE 4  form a segmented circular ring around segments TE 1  and TE 2  of a circular center. However, in other transducer shapes (e.g., square, hexagonal, rectangular, oval) the segmented ring as well as the center electrodes may have other shapes (e.g., square, hexagonal, rectangular, oval) and the segmented outer electrode forms a perimeter or periphery which is spaced apart and outward from the segmented center electrodes. Although the center electrode and outer electrode are each divided into two segments, following the same principles each may be divided into a larger number of segments. For example, each of the center electrode and the outer electrode may be divided into three segments, four segments, five segments six segments, etc. In some embodiments, each of the segments of the center electrode is equal or substantially equal (e.g., within manufacturing tolerances of a few percent) in surface area to one another. In some embodiments, each of the segments of the outer electrode is equal or substantially equal (e.g., within manufacturing tolerances of a few percent) in surface area to one another. In some embodiments, each of the individual segments of the segmented center electrode and of the segmented outer electrode is equal or substantially equal (e.g., within manufacturing tolerances of a few percent) in surface area to one another. That is, in some embodiments the plan view surface area of BE 1 ′=the surface area of BE 2 ′=the surface area of BE 3 =the surface area BE 4 . 
     It should be appreciated that electrical traces are required to be coupled to the electrodes in order to route various signals and/or provide various couplings (such as to another electrode, to ground, etc.), however in the interest of clarity these traces are not illustrated. Any suitable routing may be used for such these traces. 
     With continued reference to  FIGS. 7A and 7B , a piezoelectric layer  203  is disposed above and coupled with the bottom electrode layer (i.e., bottom electrodes BE 1 ′, BE 2 ′, BE 3 , and BE 4 ). In some embodiments, the piezoelectric layer  203  may comprise a first piezoelectric portion  203 - 1 ′ disposed above BE 2 ′ and BE 4  a second piezoelectric portion  203 - 2 ′ disposed above BE 1 ′ and BE 3 . 
     With continued reference to  FIGS. 7A and 7B , a top electrode layer TE comprised of conductive material is disposed above and coupled with the piezoelectric layer  203 . The top electrode layer TE is split into at least two portions depicted as a first top electrode TE 1 ′, and a second top electrode TE 2 ′. In some embodiments, the first top electrode TE 1 ′ and the second top electrode TE 2 ′ are substantially equal in surface area from a top plan view. 
     CONCLUSION 
     The examples set forth herein were presented in order to best explain, to describe particular applications, and to thereby enable those skilled in the art to make and use embodiments of the described examples. However, those skilled in the art will recognize that the foregoing description and examples have been presented for the purposes of illustration and example only. The description as set forth is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims. 
     Reference throughout this document to “one embodiment,” “certain embodiments,” “an embodiment,” “various embodiments,” “some embodiments,” or similar term means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of such phrases in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics of any embodiment may be combined in any suitable manner with one or more other features, structures, or characteristics of one or more other embodiments without limitation.