Patent Publication Number: US-2022219198-A1

Title: Micromachined ultrasonic transources with dual out-of-plane and in-plane actuation and displacement

Description:
BACKGROUND 
     The subject matter disclosed herein relates to transducers and, in particular, micromachined ultrasonic transducers with dual out-of-plane and in-plane actuation and displacement. 
     An ultrasound device may be used for imaging targets such as organs and soft tissues in a human body, as well non-human targets. For example, an ultrasound device may be used for applications such as ultrasound/acoustic sensing, non-destructive evaluation (NDE), ultrasound therapy (e.g., High Intensity Focused Ultrasound (HIFU)), etc., in addition to ultrasound imaging of humans, animals, etc. 
     Ultrasound devices may use real time, non-invasive high frequency sound waves to produce a series of two-dimensional (2D) and/or three-dimensional (3D) images. The sound waves may be transmitted by a transmit transducer, and the reflections of the transmitted sound waves may be received by a receive transducer. The received sound waves may then be processed to display an image of the target. A conventional capacitive micromachined ultrasound transducer (CMUT) that is used as a transmit transducer and/or a receive transducer may include a top electrode and a bottom electrode, where the top electrode may move due to electrical signals to generate sound waves, or move due to receiving sound waves to generate electrical signals that can be processed. The top electrode and the bottom electrode may be separated by a gap, where the gap may comprise some level of vacuum or the gap may be filled with, for example, air. However, conventional and traditional CMUTs may have certain limitations or disadvantages. 
     BRIEF DESCRIPTION 
     Certain embodiments commensurate in scope with the originally claimed subject matter are summarized below. These embodiments are not intended to limit the scope of the claimed subject matter, but rather these embodiments are intended only to provide a brief summary of possible forms of the subject matter. Indeed, the subject matter may encompass a variety of forms that may be similar to or different from the embodiments set forth below. 
     In accordance with a first embodiment, a capacitive transducer is provided. The capacitive transducer includes a plate including a protruding center mass and a substrate with a center depression configured to accept the center mass. The capacitive transducer includes a first electrode coupled to a non-horizontal edge surface of the center mass and a second electrode coupled to a non-horizontal edge surface of the center depression. The capacitive transducer further includes a third electrode coupled to a horizontal edge surface of the center mass and a fourth electrode coupled to a horizontal edge surface of the center depression. The plate is coupled to the substrate at least along an outer perimeter area of the plate and the substrate. 
     In accordance with a second embodiment, a system is provided. The system includes a capacitive transducer. The capacitive transducer includes a plate including a protruding center mass and a substrate with a center depression configured to accept the center mass. The capacitive transducer further includes a first pair of electrodes arranged on non-horizontal surfaces of the capacitive transducer and a second pair of electrodes arranged on horizontal surfaces of the capacitive transducer. The system also includes circuitry configured to actuate the capacitive transducer by applying a direct current (DC) signal to the first pair of electrodes and applying an alternative current (AC) signal to the second pair of electrodes. 
     In accordance with a third embodiment, a capacitive transducer is provided. The capacitive transducer includes a plate including a protruding center mass and a substrate with a center depression configured to accept the center mass. The capacitive transducer also includes a first electrode coupled to a non-horizontal edge surface of the center mass and a second electrode coupled to a non-horizontal edge surface of the center depression. The capacitive transducer further includes a third electrode coupled to a horizontal edge surface of the center mass and a fourth electrode coupled to a horizontal edge surface of the center depression. The capacitive transducer even further includes a first insulation layer disposed on a portion of the third electrode, a second insulation layer disposed on a portion of the fourth electrode, or both the first insulation layer disposed on the portion of the third electrode and the second insulation layer disposed on the portion of the fourth electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present subject matter will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a block diagram of an exemplary ultrasound system that may be used in ultrasound imaging, in accordance with various embodiments; 
         FIGS. 2A and 2B  illustrate cross-sections of configurations for example capacitive micromachined ultrasound transducers (CMUTs) with dual out-of-plane and in-plane actuation and displacement, in accordance with various embodiments; 
         FIG. 3  illustrates example applicable dimensions of the example CMUT of  FIG. 2 , in accordance with various embodiments; 
         FIG. 4  illustrates example applicable dimensions of another CMUT, in accordance with various embodiments; 
         FIG. 5  illustrates another configuration for an example CMUT with dual out-of-plane and in-plane actuation and displacement, in accordance with various embodiments; 
         FIG. 6  illustrates another configuration for an example CMUT with dual out-of-plane and in-plane actuation and displacement, in accordance with various embodiments; 
         FIG. 7  illustrates another configuration for an example CMUT with dual out-of-plane and in-plane actuation and displacement, in accordance with various embodiments; 
         FIG. 8  illustrates another configuration for an example CMUT with dual out-of-plane and in-plane actuation and displacement, in accordance with various embodiments; 
         FIG. 9  illustrates an example graph comparing bandwidth of the CMUT of  FIG. 4  during fringe mode activation and dual mode activation, in accordance with various embodiments; 
         FIG. 10  illustrates an example series of graphs comparing the effect of in-plane electrode coverage for an example CMUTs during dual mode activation on pressure and bandwidth, in accordance with various embodiments; 
         FIG. 11  illustrates an example graph comparing the effect of in-plane electrode coverage for example CMUTs during dual mode activation on pressure and bandwidth, in accordance with various embodiments; and 
         FIGS. 12 and 13  illustrate schematic diagrams illustrating manufacture of an example CMUT with dual out-of-plane and in-plane actuation and displacement, in accordance with various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present subject matter, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Furthermore, any numerical examples in the following discussion are intended to be non-limiting, and thus additional numerical values, ranges, and percentages are within the scope of the disclosed embodiments. 
     The presently contemplated embodiments provide micromachined ultrasonic transducers (e.g. CMUTs) with dual out-of-plane and in-plane actuation and displacement. In particular, a direct current (DC) signal or bias may be applied to a pair of vertical electrodes (e.g., for out-of-plane actuation) and an alternative current signal applied to a pair of horizontal electrodes (e.g., for in plane actuation). Actuation with the DC signal is orthogonal to a direction of displacement of a center mass of a plate toward a depression of a substrate, while actuation with the AC signal is parallel to the direction of displacement. The in-plane actuation with the AC signal increases the electromechanical coupling factor. This enables the CMUTs to have a higher transmit bandwidth when the DC signal is applied to the pair of vertical electrodes and the AC signal is applied to the pair of horizontal or in-plane electrodes as opposed to when both the AC and DC signals are applied to vertical pair of electrodes. The disclosed CMUTS can operate in both a conventional mode of operation and a mechanical collapse mode of operation (e.g., when the center mass of the plate contacts the substrate). Also, the disclosed CMUTs can operate in different drive configurations. For example, during operation in a transmit mode, only an AC signal may be applied to the pair of horizontal electrodes without a DC bias or an AC current signal being applied to the pair of vertical electrodes. Also, during operation in transmit and receive mode, the DC signal may be applied to either the pair of vertical electrodes or the pair of horizontal electrodes. For example, during the transmit portion of the transmit and receive mode, the DC signal may be applied to the pair of vertical electrodes and the AC signal applied to the pair of horizontal electrodes, while, during the receive portion of the transmit and receive mode, both the AC and DC signals may be applied to the pair of horizontal electrodes. 
     While a CMUT can be used for medical imaging, the CMUT may also be used for various other purposes such as, for example, ultrasound/acoustic sensing, non-destructive evaluation (NDE), ultrasound therapy (e.g., High Intensity Focused Ultrasound (HIFU)), etc., in addition to ultrasound imaging of humans or animals. 
     As used herein, the term “image” broadly refers to both viewable images and data representing a viewable image. However, many embodiments generate (or are configured to generate) at least one viewable image. In addition, as used herein, the phrase “image” is used to refer to an ultrasound mode such as B-mode (2D mode), M-mode, three-dimensional (3D) mode, CF-mode, PW Doppler, CW Doppler, MGD, and/or sub-modes of B-mode and/or CF such as Shear Wave Elasticity Imaging (SWEI), TVI, Angio, B-flow, BMI, BMI Angio, and in some cases also MM, CM, TVD where the “image” and/or “plane” includes a single beam or multiple beams. 
     Furthermore, the term processor or processing unit, as used herein, refers to any type of processing unit that can carry out the required calculations needed for the various embodiments, such as single or multi-core: CPU, Accelerated Processing Unit (APU), Graphics Board, DSP, FPGA, ASIC or a combination thereof. 
       FIG. 1  is a block diagram of an exemplary ultrasound system that may be used in ultrasound imaging, in accordance with various embodiments. Referring to  FIG. 1 , there is shown a block diagram of an exemplary ultrasound system  100 . The ultrasound system  100  comprises a transmitter  102 , an ultrasound probe  104 , a transmit beamformer  110 , a receiver  118 , a receive beamformer  120 , A/D converters  122 , an RF processor  124 , an RF/IQ buffer  126 , a user input device  130 , a signal processor  132 , an image buffer  136 , a display system  134 , and an archive  138 . The circuit  111  is a typical example of bias of a cMUT but many options are described in public literature. 
     The transmitter  102  may comprise suitable logic, circuitry, interfaces and/or code that may be operable to drive the ultrasound probe  104 . The ultrasound probe  104  may comprise, for example, a single element CMUT, a 1D array of CMUTs, 2D array of CMUTs, an annular (ring) array of CMUTs, etc. Accordingly, the ultrasound probe  104  may comprise a group of transducer elements  106  that may be, for example, CMUTs. In certain embodiments, the ultrasound probe  104  may be operable to acquire ultrasound image data covering, for example, at least a substantial portion of an anatomy, such as the heart, a blood vessel, or any suitable anatomical structure. Each of the transducer elements  106  may be referred to as a channel. 
     The transmit beamformer  110  may comprise suitable logic, circuitry, interfaces and/or code that may be operable to control the transmitter  102  that drives the group of transducer elements  106  to emit ultrasonic transmit signals into a region of interest (e.g., human, animal, underground cavity, physical structure and the like). The transmitted ultrasonic signals may be back-scattered from structures in the object of interest, like blood cells or tissue, to produce echoes. The echoes can then be received by the transducer elements  106 . For example, one or more drive circuits  111  may be coupled to and drive or control the electrodes of each transducer element  106 . For example, the one or more drive circuits may be coupled to separate AC and DC voltage sources. 
     The group of transducer elements  106  in the ultrasound probe  104  may be operable to convert the received echoes into analog signals and communicated to a receiver  118 . The receiver  118  may comprise suitable logic, circuitry, interfaces and/or code that may be operable to receive the signals from the ultrasound probe  104 . The analog signals may be communicated to one or more of the plurality of A/D converters  122 . 
     Accordingly, the ultrasound system  100  may multiplex such that ultrasonic transmit signals are transmitted during certain time periods and echoes of those ultrasonic signals are received during other time periods. Although not shown explicitly, various embodiments of the disclosure may allow simultaneous transmission of ultrasonic signals and reception of echoes from those signals. In such cases, the probe may comprise transmit transducer elements and receive transducer elements. 
     The plurality of A/D converters  122  may comprise suitable logic, circuitry, interfaces and/or code that may be operable to convert the analog signals from the receiver  118  to corresponding digital signals. The plurality of A/D converters  122  are disposed between the receiver  118  and the RF processor  124 . Notwithstanding, the disclosure is not limited in this regard. Accordingly, in some embodiments, the plurality of A/D converters  122  may be integrated within the receiver  118 . 
     The RF processor  124  may comprise suitable logic, circuitry, interfaces and/or code that may be operable to demodulate the digital signals output by the plurality of A/D converters  122 . In accordance with an embodiment, the RF processor  124  may comprise a complex demodulator (not shown) that is operable to demodulate the digital signals to form I/Q data pairs that are representative of the corresponding echo signals. The RF data, which may be, for example, I/Q signal data, real valued RF data, etc., may then be communicated to an RF/IQ buffer  126 . The RF/IQ buffer  126  may comprise suitable logic, circuitry, interfaces and/or code that may be operable to provide temporary storage of the RF or I/Q signal data, which is generated by the RF processor  124 . 
     Accordingly, various embodiments may have, for example, the RF processor  124  process real valued RF data, or any other equivalent representation of the data, with an appropriate RF buffer  126 . 
     The receive beamformer  120  may comprise suitable logic, circuitry, interfaces and/or code that may be operable to perform digital beamforming processing to sum, for example, delayed, phase shifted, and/or weighted channel signals received from the RF processor  124  via the RF/IQ buffer  126  and output a beam summed signal. The delayed, phase shifted, and/or weighted channel data may be summed to form a scan line output from the receive beamformer  120 , where the scan line may be, for example, complex valued or non-complex valued. The specific delay for a channel may be provided, for example, by the RF processor  124  or any other processor configured to perform the task. The delayed, phase shifted, and/or weighted channel data may be referred to as delay aligned channel data. 
     The resulting processed information may be the beam summed signal that is output from the receive beamformer  120  and communicated to the signal processor  132 . In accordance with some embodiments, the receiver  118 , the plurality of A/D converters  122 , the RF processor  124 , and the beamformer  120  may be integrated into a single beamformer, which may be digital. In various embodiments, the ultrasound system  100  may comprise a plurality of receive beamformers  120 . 
     The user input device  130  may be utilized to input patient data, scan parameters, settings, select protocols and/or templates, and the like. In an exemplary embodiment, the user input device  130  may be operable to configure, manage, and/or control operation of one or more components and/or modules in the ultrasound system  100 . In this regard, the user input device  130  may be operable to configure, manage and/or control operation of the transmitter  102 , the ultrasound probe  104 , the transmit beamformer  110 , the receiver  118 , the receive beamformer  120 , the RF processor  124 , the RF/IQ buffer  126 , the user input device  130 , the signal processor  132 , the image buffer  136 , the display system  134 , and/or the archive  138 . The user input device  130  may include switch(es), button(s), rotary encoder(s), a touchscreen, motion tracking, voice recognition, a mouse device, keyboard, camera, and/or any other device capable of receiving a user directive. In certain embodiments, one or more of the user input devices  130  may be integrated into other components, such as the display system  134  or the ultrasound probe  104 , for example. As an example, user input device  130  may comprise a touchscreen display. 
     The signal processor  132  may comprise suitable logic, circuitry, interfaces and/or code that may be operable to process ultrasound scan data (i.e., summed IQ signal) for generating ultrasound images for presentation on a display system  134 . The signal processor  132  is operable to perform one or more processing operations according to a plurality of selectable ultrasound modalities on the acquired ultrasound scan data. In an exemplary embodiment, the signal processor  132  may be operable to perform display processing and/or control processing, among other things. Acquired ultrasound scan data may be processed in real-time during a scanning session as the echo signals are received. Additionally or alternatively, the ultrasound scan data may be stored temporarily in the RF/IQ buffer  126  during a scanning session and processed in a live or off-line operation. In various embodiments, the processed image data can be presented at the display system  134  and/or stored at the archive  138 . The archive  138  may be a local archive, a Picture Archiving and Communication System (PACS), or any suitable device for storing images and related information. 
     The signal processor  132  may comprise one or more central processing units, microprocessors, microcontrollers, and/or the like. The signal processor  132  may be an integrated component, or may be distributed across various locations, for example. In an exemplary embodiment, the signal processor  132  may be capable of receiving input information from the user input device  130  and/or the archive  138 , generating an output displayable by the display system  134 , and manipulating the output in response to input information from the user input device  130 , among other things. The signal processor  132  may be capable of executing any of the method(s) and/or set(s) of instructions discussed herein in accordance with the various embodiments, for example. 
     The ultrasound system  100  may be operable to continuously acquire ultrasound scan data at a frame rate that is suitable for the imaging situation in question. Typical frame rates may range from 20-120 but may be lower or higher. The acquired ultrasound scan data may be displayed on the display system  134  at a display-rate that can be the same as the frame rate, or slower or faster. An image buffer  136  is included for storing processed frames of acquired ultrasound scan data that are not scheduled to be displayed immediately. Preferably, the image buffer  136  is of sufficient capacity to store at least several minutes worth of frames of ultrasound scan data. The frames of ultrasound scan data are stored in a manner to facilitate retrieval thereof according to its order or time of acquisition. The image buffer  136  may be embodied as any known data storage medium. 
     The display system  134  may be any device capable of communicating visual information to a user. For example, a display system  134  may include a liquid crystal display, a light emitting diode display, and/or any suitable display or displays. The display system  134  can be operable to present ultrasound images and/or any suitable information. 
     The archive  138  may be one or more computer-readable memories integrated with the ultrasound system  100  and/or communicatively coupled (e.g., over a network) to the ultrasound system  100 , such as a Picture Archiving and Communication System (PACS), a server, a hard disk, floppy disk, CD, CD-ROM, DVD, compact storage, flash memory, random access memory, read-only memory, electrically erasable and programmable read-only memory and/or any suitable memory. The archive  138  may include databases, libraries, sets of information, or other storage accessed by and/or incorporated with the signal processor  132 , for example. The archive  138  may be able to store data temporarily or permanently, for example. The archive  138  may be capable of storing medical image data, data generated by the signal processor  132 , and/or instructions readable by the signal processor  132 , among other things. 
     Components of the ultrasound system  100  may be implemented in software, hardware, firmware, and/or the like. The various components of the ultrasound system  100  may be communicatively linked. Components of the ultrasound system  100  may be implemented separately and/or integrated in various forms. For example, the display system  134  and the user input device  130  may be integrated as a touchscreen display. Additionally, while the ultrasound system  100  was described to comprise a receive beamformer  120 , an RF processor  124 , and a signal processor  132 , various embodiments of the disclosure may use various number of processors. For example, various devices that execute code may be referred to generally as processors. Various embodiments may refer to each of these devices, including each of the RF processor  124  and the signal processor  132 , as a processor. Furthermore, there may be other processors to additionally perform the tasks described as being performed by these devices, including the receive beamformer  120 , the RF processor  124 , and the signal processor  132 , and all of these processors may be referred to as a “processor” for ease of description. 
     Conventional CMUTs include two plates separated by either a vacuum or fluid gap. The plates are biased by a DC voltage and then superimposed with the AC signal of chosen frequency and amplitude. The working principle of CMUTs is based on Coulomb&#39;s laws of attraction. During the DC bias the electrostatic force and the mechanical restorative balance each other which keeps the membrane at the targeted displaced location. However, at a certain DC bias voltage, the electrostatic forces surpass the restorative force and the membrane touches the bottom electrode. For perfectly clamped CMUT plates, this physical phenomenon occurs at substantially ⅓rd of the effective gap height. The distance is called pull-in or collapse distance and voltage at which the phenomenon happens is called collapse or pull-in voltage. Operating the device in collapse mode provides higher levels of acoustic power and wider bandwidth during operation. One or more insulation layers may be sandwiched between the active membrane (top electrode), gap (vacuum or fluid) and the back-support structure (with bottom electrode) such that no short-circuit occurs during such collapse phenomenon. 
     The equation for collapsed voltage V col  is shown below in Equations 1 and 2 (example of flat rigid condenser model): 
     
       
         
           
             
               
                 
                   
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                   ( 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
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                     1 
                   
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     where, K is membrane stiffness, ε 0  is permittivity of free space, and A is the device area. The effective gap height is given by: 
     
       
         
           
             
               
                 
                   
                     g 
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     where g 0  is the vacuum/air gap, t i  is the insulation layer thickness, and ε i  is the permittivity of the insulation layer material. 
     In the following figures, it should be noted that the terms vertical and horizontal are defined relative to a longitudinal axis or length of the CMUT devices (e.g., extending in the X-direction). Thus, a vertical direction would be orthogonal to a longitudinal axis or length of a CMUT device and a horizontal direction would be parallel to a longitudinal axis or length of a CMUT device. 
       FIG. 2A  illustrates a cross-section of a configuration for an example CMUT with dual out-of-plane and in-plane actuation and displacement, in accordance with various embodiments. Referring to  FIG. 2 , there is shown a CMUT  200  comprising a plate  202  and a substrate  204 . The plate  202  may comprise a center mass  203 . The center mass  203  protrudes down into a depression  205  in a corresponding area of the substrate  204 . The substantially vertical edges (or non-horizontal edges) of the center mass  203  include electrode(s)  210  and the substantially vertical edges of the depression  205  include electrode(s)  211 . A bottom surface  212  of the center mass  203  faces a top surface  214  of the depression  205 . The substantially horizontal edge of the bottom surface  212  includes electrode  216  and the substantially horizontal edge of the top surface  214  includes electrode  218 . While not shown, when viewed from the top at the X-Y plane, the electrodes  216 ,  218  may have a circular shape (as shown with electrodes  1301  and  1401  in  FIGS. 7 and 8 , respectively), a rectangular shape, or any other shape. The electrodes  210 ,  211  and the electrodes  216 ,  218  may be provided with electrical signals (DC bias and AC signal) used to move the plate  202  in the Z direction to generate sonic waves. The CMUT  200  may operate in transmit mode (Tx), receive (Rx) mode, and/or transmit and receive (Tx-Rx) mode. In addition, the CMUT  200  may operate in a conventional mode or mechanical collapse mode (e.g., when the bottom surface  212  contacts the top surface  214  of the depression  205 ). Different drive configurations may be utilized for the CMUT  200 . For example, DC bias may applied to either the electrodes  210 ,  211  (fringe electrodes) or the electrodes  216 ,  218  (in-plane electrodes or central electrodes). For example, during operation of the CMUT  200  in the Tx-Rx mode, a DC bias signal is applied to the electrodes  210  and  211  and an AC signal applied only to the electrodes  216  and  218  during the transmit portion, while during the receive portion both the DC bias signal and the AC signal are applied to the electrodes  210  and  211 . In such a configuration, different DC voltage levels may be applied to the respective electrode pairs (electrodes  210 ,  211  and electrodes  216 ,  218 ) during transmitting, receiving, or both transmitting and receiving. During operation of the CMUT  200  in the Tx mode, only the AC signal may be applied to the electrodes  216 ,  218  without the DC signal applied to either of the electrodes pairs (electrodes  210 ,  211  and electrodes  216 ,  218 ). In this scenario, the CMUT  200  may be driven at half the targeted frequency to receive the harmonic at the output. In the Rx mode, both the DC bias signal and the AC signal are applied to the electrodes  210  and  211 . 
     One or more insulation layers or bumps or high contact resistance layers or bumps may be sandwiched between the electrodes  216 ,  218  in case of mechanical collapse or accidental contact to avoid a short circuit. As depicted, an insulation layer or bump  220  (or high contact resistance layer or bump) is disposed on a portion of the electrode  216  and an insulation layer or bump  222  (or high contact resistance layer or bump) is disposed on a portion of the electrode  218 . In certain embodiments, only the insulation layer or bump  220  (or high contact resistance layer or bump) is disposed on the electrode  216 . In other embodiments, only the insulation layer or bump  222  (or high contact resistance layer or bump) is disposed on the electrode  218 . 
     The plate  202  may be coupled to the substrate  204  at the outer perimeter area  202 A of the plate  202 . The coupling may be via any appropriate methods, including processes known in MEMS fabrication, such as, for example, wafer bonding. 
     It may be seen that the CMUT  200  has an upper vertical gap  206 A and a lower vertical gap  206 B between the plate  202  and the substrate  204 . There is also a horizontal gap  208  between the electrodes  210  and  211 . Further, there is a vertical gap  224  between the electrodes  216  and  218  (and respective insulation layers  220  and  222  or high contact resistance layers). The horizontal gap  208  and the vertical gap  224  may be referred to as an electrode gap  208  and an electrode gap  224 . When the DC bias is applied to the electrodes  210 ,  211  and the AC signal is applied to the electrodes  216 ,  218  (i.e., dual mode actuation), the vertical gaps  206 A and  206 B are the actuation boundaries making the AC signal parallel to the device displacement direction along the Z axis and the DC bias orthogonal to the device displacement direction along the Z-axis. When both the DC bias and the AC signal are applied to the electrodes (i.e., fringe mode activation), the vertical gaps  206 A and  206 B are the actuation boundaries making the DC bias and the AC signal orthogonal to the device displacement direction along the Z axis. Operation in dual mode actuation improves electromechanical coupling as opposed to fringe mode operation. While not shown, when viewed from the top at the X-Y plane, the CMUT  200  may have a circular shape, a rectangular shape, or any other shape. 
     Since the CMUT plate  202  has a mechanical constraint in the X direction due to clamping at the edges (outer perimeter area  202 A of the plate  202 ), the displacement degree of freedom may be predominantly in the Z direction. Accordingly, even though the actuation is in the X-direction (e.g., during fringe mode actuation and by DC bias in the dual mode actuation), the X-direction displacement may be much less than the Z-direction displacement due to the mechanical constraint. Various embodiments of the disclosure may have, for example, a displacement ratio of 10 or more for Z-direction displacement versus X-direction displacement. It may be noted that the displacement ratio may be determined for specific usage for the CMUT  200 .  FIG. 2B  illustrates an alternative embodiment for the CMUT having a larger outer perimeter area  202 A where the plate  202  is coupled to the substrate  204 . 
       FIG. 3  illustrates example applicable dimensions of the example CMUT of  FIG. 2 , in accordance with various embodiments. Referring to  FIG. 3 , there is shown a partial view of a CMUT  300  that may be similar to the CMUT  200 . The dimensions may include, for example, a plate radius (P r )  302 , a mass radius (M r )  304 , a horizontal gap (G h )  306 , vertical gaps (G v )  307  and  309 , a mass thickness (M t )  305 , a plate thickness (P t )  303 , an in-plane electrode radius (Eip R ) or electrode coverage area (EC)  310 . The horizontal gap  306  and the vertical gap  309  may be referred to as the electrode gap  306  and the electrode gap  309 , respectively. The vertical gap  309  is defined as the gap between the any insulation layers (e.g., insulation layers  220  and/or  222 ) disposed on and between the in-plane electrodes (e.g., electrodes  216  and  218  in  FIG. 2 ). 
     The vertical gaps (G v )  307  and  309  may be equal to each other. The vertical gap (G v )  307  and/or  309  may be equal to the horizontal gap (G h )  306 . The vertical gap (G v )  307  and/or  309  may be greater or smaller than the horizontal gap (G h )  306 . The various gaps may be measured in any appropriate units such as, for example, microns, nanometers, etc. 
     A term “E PI ” may be a voltage for electrical pull-in, or the DC bias needed to make the side electrodes collapse to each other in the X-direction. A term “E MC ” may be used for voltage needed for mechanical collapse. Mechanical collapse is defined as the phenomenon when at a certain DC bias the center mass  203  touches the bottom of the depression  205  of the substrate  204 . 
     The E MC  among various embodiments may vary due to differences in the above parameters. For example, the CMUT  300  may have the following dimensions: P r  of 50 μm, M r  of P r /3, P t  of 300 nm, M t  of 2*G v , G v  of 100 nm, G h  of 0.2*G v . These dimensions may result in an E MC  of 34 volts (V). Increasing the ratio of the plate radius, P r , to the mass radius, M r , may result (e.g., to P r /M r  of 2) in increasing the E MC  (e.g., to 36 V). Decreasing the ratio of the plate radius, P r , to the mass radius, M r , may result (e.g., to P r /M r  of  4 ) in decreasing the E MC  (e.g., to 30 V). Eliminating the vertical gap  307  (e.g., between the plate  202  and the substrate  204 , as depicted in the CMUT  350  in  FIG. 4 , may significantly increase the E MC  (e.g., to 182 V). The E MC  may also vary depending on the materials used for the different layers. 
       FIG. 5  illustrates another configuration for an example CMUT with dual out-of-plane and in-plane actuation and displacement, in accordance with various embodiments. Referring to  FIG. 5 , there is shown a CMUT  500  that is similar to the CMUT  200 , except that the edges of the center mass  503  of the plate  502  and the edges of the depression  505  of the substrate  504  are diagonal. Accordingly, orientation of the electrodes  506  and  507  are also diagonal. The diagonal edges may also be referred to as non-horizontal edges where the angle of inclination (θ) is greater than 0 degrees (e.g., conventional device) and less than 90 degrees (e.g., fringe device). The orthogonal distance between the electrodes  506  and  507  may be referred to as the electrode gap  508 . Similar to CMUT  200 , the CMUT  500  includes horizontal or in-plane electrodes  510  and  512  and respective insulation layers  514  and  516 . 
       FIG. 6  illustrates another configuration for an example CMUT with dual out-of-plane and in-plane actuation and displacement, in accordance with various embodiments. Referring to  FIG. 6 , there is shown a CMUT  600  that is similar to the CMUT  200 , except that the edges of the center mass  603  and the edges of the depression  605  are corrugated. Accordingly, the electrodes  606  and  607  are offset horizontally from the electrodes  608  and  609 . There is a horizontal gap  610  between the electrodes  606  and  607  and between the electrodes  608  and  609 . The corrugated edges may also be referred to as non-horizontal edges. The horizontal gap  610  may be referred to as the electrode gap  610 . Similar to CMUT  200 , the CMUT  600  includes horizontal or in-plane electrodes  612  and  614  and respective insulation layers  616  and  618 . 
       FIG. 7  illustrates another configuration for an example CMUT with dual out-of-plane and in-plane actuation and displacement, in accordance with various embodiments. Referring to  FIG. 7 , there is shown a top cross-sectional view (for example, X-Y plane) of a CMUT  1300  including an in-plane actuation electrode  1301  that shows a pattern of a horizontal gap  1310 . There are also shown side cross-sectional views (for example, X-Z plane)  1302  and  1304  that show the horizontal gap  1312  between the electrodes (e.g., vertical or fringe electrodes), as well as the upper vertical gap  1314  and the lower vertical gap  1316  (e.g., between the in-plane or horizontal electrodes). The horizontal gap  1312  may be similar to the horizontal gap  306  in  FIG. 3 . The upper vertical gap  1314  may be similar to the upper vertical gap  307  in  FIG. 3  and the lower vertical gap  1316  may be similar to the lower vertical gap  309  in  FIG. 3 . As can be seen, the cross-sectional view  1302  is for an outer portion of the horizontal gap  1310 , and the cross-sectional view  1304  is for an inner portion of the horizontal gap  1310 . 
       FIG. 8  illustrates another configuration for an example CMUT with dual out-of-plane and in-plane actuation and displacement, in accordance with various embodiments. Referring to  FIG. 8 , there is shown a top cross-sectional view (for example, X-Y plane) of a CMUT  1400  including an in-plane actuation electrode  1401  that shows a pattern of a horizontal gap  1410 . There are also shown side cross-sectional views (for example, X-Z plane)  1402  and  1404  that show the horizontal gap  1412  between the electrodes (e.g., vertical or fringe electrodes), as well as the lower vertical gap  1416  (e.g., between the in-plane or horizontal electrodes). The horizontal gap  1412  may be similar to the horizontal gap  306  in  FIG. 3 . The lower vertical gap  1416  may be similar to the lower vertical gap  309  in  FIG. 3 . As can be seen, the cross-sectional view  1402  is for an outer portion of the horizontal gap  1410 , and the cross-sectional view  1404  is for an inner portion of the horizontal gap  1410 . 
     While two example configurations are shown for increasing the total surface area of electrodes that can be used for the horizontal gaps  1310  and  1410  of the CMUTs  1300  and  1400 , respectively, a horizontal gap when seen from the top (for example, X-Y plane) may be any of various shapes such as, for example, a circle, an oval, a regular or irregular polygon, etc. The horizontal gap may be, for example, continuous as shown in  FIGS. 7 and 8 , one or more discrete pieces that together do not go all the way around a CMUT, or one or more discrete pieces that together go around a CMUT. Accordingly, when viewed from above (for example, the X-Y plane), the horizontal gap of a CMUT may comprise one or more gaps, where each gap may be any geometric shape with any pattern. 
     Additionally, any CMUT may have any geometric shape when viewed from the top (for example, the X-Y plane). For example, while the CMUTs  1300  and  1400  are shown to be circular, a CMUT may be elliptical, oval, a polygon, etc. Additionally, while several configurations were shown, various embodiments of the disclosure need not be so limited. For example, the CMUT  200  may have multiple electrodes  210  and  211  similar to the CMUT  600 . That is, while the edges may be planar, there may be multiple electrodes may be multiple electrodes  210  and corresponding multiple electrodes  211 . Or there may be a different number of electrodes  210  than electrodes  211 , where, for example, multiple electrodes  210  may be used for a single electrode  211  or vice versa. 
     Additionally, the center mass  203 ,  503 ,  603 , etc., may be different shapes than the examples disclosed. For example, the center mass  503  may have rounded (convex) edges and the depression  505  of the substrate  504  may have rounded (concave) edges so that the depression  505  may accept the center mass  503 . Accordingly, various embodiments of the disclosure may have appropriately rounded electrodes  506  and  507 . 
     However, the shape of a center mass and/or a depression of a substrate need not be limited to just what is mentioned in the disclosure. Rather, any appropriate shape may be used. Furthermore, the electrodes placed on the edge surfaces of a center mass and/or a depression may have conforming shapes to the edge surfaces or shapes that are different than the edge surfaces. 
     Additionally, while various descriptions were made of edges, surfaces, electrodes, the edge, surface, or electrode may be a single, continuous edge/surface/electrode. For example, when the center mass  203  is cylindrical, the center mass  203  may comprise a single vertical surface. Accordingly, there may be a single electrode  210  and a single electrode  211  for the CMUT  200 . However, even when there is a single surface, there may be multiple electrodes  210  and multiple electrodes  211  placed at regular intervals along the single surface of the center mass  203  of the plate  202  and/or the single surface of the depression  205  of the substrate  204 . 
     Furthermore, the gaps described in the various figures may be filled with fluid, such as, for example, air, or may comprise some level of vacuum. Accordingly, in various embodiments of the disclosure, the capacitive transducers may be configured such that the gaps are air-tight. 
       FIG. 9  illustrates an example graph comparing bandwidth of the CMUT  350  of  FIG. 4  during fringe mode activation and dual mode activation, in accordance with various embodiments. Referring to  FIG. 9 , there is shown a graph  1500  with frequency in megahertz (MHz) along the X-axis, surface pressure in decibels (dB) along the Y-axis on the left side of the graph  1500 , and surface pressure in dB along the Y-axis on the right side of the graph  1500 . Plot  1502  (e.g., shown as a solid plot) shows the transmit bandwidth of the CMUT  350  of  FIG. 4  in fringe mode activation (e.g., both the DC bias and the AC signal are only applied to the vertical or fringe electrodes) relative the surface pressure indicated on the right of the graph  1500 . Plot  1504  (e.g., shown as a dashed plot) shows the transmit bandwidth of the CMUT  350  of  FIG. 4  in dual mode activation (e.g., DC bias applied to the vertical or fringe electrodes and the AC signal applied to the horizontal or in-plane electrodes). As depicted in  FIG. 9 , the transmit bandwidth of the CMUT  350  of  FIG. 4  is considerably higher during dual mode activation due to the added electromechanical coupling with the AC actuation on the in-plane electrodes. 
       FIG. 10  illustrates an example series of graphs comparing the effect of in-plane electrode (e.g., horizontal electrode with AC actuation) coverage for an example CMUT during dual mode activation on pressure and bandwidth, in accordance with various embodiments. Referring to  FIG. 10 , there are shown graphs  1510 ,  1512 ,  1514 , and  1516  with frequency in MHz along the X-axis and normalized pressure (normalized to 1) along the Y-axis. The graphs  1510 ,  1512 ,  1514 , and  1516  are for electrode coverage percentages (e.g., relative to the mass radius) of 25 percent, 33 percent, 50 percent, and 80 percent respectively. Plots  1518 ,  1520 ,  1522 , and  1524  show the effect of in-plane electrode coverage on both pressure and bandwidth. For example, as electrode coverage increases the bandwidth decreases and the normalized pressure increases. 
     Similarly,  FIG. 11  illustrates an example graph comparing the effect of in-plane electrode (e.g., horizontal electrode with AC actuation) coverage for an example CMUT during dual mode activation on pressure and bandwidth, in accordance with various embodiments. Referring to  FIG. 11 , there is shown graph  1530  with frequency in MHz along the X-axis and normalized pressure (normalized to corresponding max) along the Y-axis. Plots  1532  (shown as solid plot),  1534  (shown as dashed plot), and  1536  (shown as dotted-dashed plot) are for electrode coverage percentages (e.g., relative to the mass radius) of 25 percent, 33 percent, and 50 percent, respectively. Plots  1532 ,  1534 , and  1536  show the effect of in-plane electrode coverage on both pressure and bandwidth. For example, as electrode coverage increases the bandwidth decreases. As depicted in  FIGS. 10 and 11  varying the in-plane electrode coverage areas enables optimization of the output power (e.g., acoustic power) and bandwidth. 
       FIGS. 12 and 13  illustrate schematic diagrams illustrating manufacture of an example CMUT with dual out-of-plane and in-plane actuation and displacement, in accordance with various embodiments. In  FIG. 12 , the manufacturing of the plate and substrate with the vertical and horizontal electrodes discussed above are shown on the left and right, respectively, corresponding to  FIG. 3 . The various manufacturing steps are similar but the plate and substrate have complementary orientations. It should be noted that only portions of the plate and the substrate are shown and the components are not to scale. The plate and substrate may be subjected to other manufacturing steps besides those described below. 
     As depicted in  FIG. 12 , a wafer (e.g., silicon wafer) is subjected to etching (e.g., deep reactive-ion etching (DRIE)) to form the mass in the plate and the depression in the substrate. Following etching, metal, via metal deposition is deposited on the plate and substrates on the surfaces (e.g., horizontal and vertical surfaces) that will face each other. The metal is then subjected to metal etching to form the respective horizontal and vertical electrodes on the plate and the substrate. Following metal etching, insulation layers or high contact resistance material may be disposed on and etched on the in-plane or horizontal electrodes of the plate and the substrate. In certain embodiments, respective insulation layers are disposed on both the plate and the substrate. In other embodiments, an insulation layer is disposed on the plate only. In other embodiments, an insulation layer is disposed on the substrate only. Upon manufacturing the plate and the substrate with the electrodes and insulation layers, as depicted in  FIG. 13 , the plate and substrate are bonded together forming the CMUT (as well as the enclosed gap between the plate and the substrate) (e.g., corresponding to  FIG. 3 ). 
     Technical effects of the disclosed subject matter include providing CMUTs with dual out-of-plane and in-plane actuation and displacement. In particular, a direct current (DC) signal may be applied to a pair of vertical electrodes (e.g., for out-of-plane actuation) and an alternative current signal applied to a pair of horizontal electrodes (e.g., for in plane actuation). Actuation with the DC signal is orthogonal to a direction of displacement of a center mass of a plate toward a depression of a substrate, while actuation with the AC signal is parallel to the direction of displacement. The in-plane actuation with the AC signal increases the electromechanical coupling factor in comparison to CMUT devices just having fringe electrodes. This enables the CMUTs to have a higher transmit bandwidth during dual mode activation when the DC signal is applied to the pair of vertical electrodes and the AC signal is applied to the pair of horizontal or in-plane electrodes as opposed to during fringe mode activation when both the AC and DC signals are applied to the vertical pair of electrodes. The disclosed CMUTs can operate in both a conventional mode of operation and a mechanical collapse mode of operation. In addition, the CMUTs can operate in either fringe mode activation or dual mode activation. 
     This written description uses examples to disclose the subject matter, including the best mode, and also to enable any person skilled in the art to practice the disclosed subject matter, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.