Patent Publication Number: US-11651611-B2

Title: Device mountable packaging of ultrasonic transducers

Description:
RELATED APPLICATIONS 
     This application claims priority to, is a continuation of, and claims the benefit of co-pending U.S. non-Provisional patent application Ser. No. 15/415,716, filed on Jan. 25, 2017, entitled “DEVICE MOUNTABLE PACKAGING OF ULTRASONIC TRANSDUCERS,” by Julius Ming-Lin Tsai, and assigned to the assignee of the present application, which is herein incorporated by reference in its entirety. 
     U.S. non-Provisional patent application Ser. No. 15/415,716 claims priority to and the benefit of U.S. Patent Provisional Patent Application 62/331,919, filed on May 4, 2016, entitled “PINNED ULTRASONIC TRANSDUCERS,” by Ng et al., and assigned to the assignee of the present application, which is incorporated herein by reference in its entirety. 
     U.S. non-Provisional patent application Ser. No. 15/415,716 also claims priority to and the benefit of U.S. Provisional Patent Application 62/334,404, filed on May 10, 2016, entitled “EDGE MOUNTABLE PACKAGING OF ULTRASONIC TRANSDUCERS,” by Tsai, and assigned to the assignee of the present application, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Conventional fingerprint sensing solutions are available and deployed in consumer products, such as smartphones and other type of mobile devices. Common fingerprint sensor technologies generally rely on (1) a sensor and (2) a processing element. When the sensor is turned on, the sensor can take or can direct the device to take an image, which is digitized (e.g., level of brightness is encoded into a digital format), and send the image to the processing element. However, finger print sensors typically consume substantial amount of power (e.g., hundreds of μ Watts to several m Watts) and, therefore, may present a considerable drain on power resources of the mobile device by rapidly draining the battery of the mobile device. 
    
    
     
       BRIEF DESCRIPTION OF THE 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. 
         FIG.  1    is a diagram illustrating a PMUT device having a center pinned membrane, according to some embodiments. 
         FIG.  2    is a diagram illustrating an example of membrane movement during activation of a PMUT device, according to some embodiments. 
         FIG.  3    is a top view of the PMUT device of  FIG.  1   , according to some embodiments. 
         FIG.  4    is a simulated map illustrating maximum vertical displacement of the membrane of the PMUT device shown in  FIGS.  1 - 3   , according to some embodiments. 
         FIG.  5    is a top view of an example PMUT device having a circular shape, according to some embodiments. 
         FIG.  6    is a top view of an example PMUT device having a hexagonal shape, according to some embodiments. 
         FIG.  7    illustrates an example array of circular-shaped PMUT devices, according to some embodiments. 
         FIG.  8    illustrates an example array of square-shaped PMUT devices, according to some embodiments. 
         FIG.  9    illustrates an example array of hexagonal-shaped PMUT devices, according to some embodiments. 
         FIG.  10    illustrates an example pair of PMUT devices in a PMUT array, with each PMUT having differing electrode patterning, according to some embodiments. 
         FIGS.  11 A,  11 B,  11 C, and  11 D  illustrate alternative examples of interior support structures, according to various embodiments. 
         FIG.  12    illustrates a PMUT array used in an ultrasonic fingerprint sensing system, according to some embodiments. 
         FIG.  13    illustrates an integrated fingerprint sensor formed by wafer bonding a CMOS logic wafer and a microelectromechanical (MEMS) wafer defining PMUT devices, according to some embodiments. 
         FIG.  14 A  illustrates an example of an operational environment for sensing of human touch, according to some embodiments. 
         FIG.  14 B  illustrates an example sensing device, in accordance with various embodiments. 
         FIG.  15    illustrates an example array of ultrasonic transducers packaged in a chamber having a cover, in accordance with an embodiment. 
         FIG.  16    illustrates an example array of ultrasonic transducers packaged in a chamber having a cover and including refractive acoustic elements, in accordance with an embodiment. 
         FIG.  17    illustrates an example array of ultrasonic transducers packaged in a chamber having a curved cover, in accordance with an embodiment. 
         FIGS.  18 A-C  illustrate example packaging configurations including relief mechanisms, in accordance with various embodiments. 
         FIGS.  19 A-D  illustrate example mobile devices including an array of ultrasonic transducers in various locations, in accordance with various embodiments. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following Description of Embodiments is merely provided by way of example and not of limitation. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding background or in the following 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 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. 
     Notation and Nomenclature 
     Some portions of the detailed descriptions which follow are presented in terms of procedures, logic blocks, processing and other symbolic representations of operations on data within an electrical device. 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, 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 acoustic (e.g., ultrasonic) signals capable of being transmitted and received by an electronic device and/or electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated in an electrical device. 
     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 “adjusting” “determining,” “controlling,” “activating,” “detecting,” “interacting,” “capturing,” “sensing,” “generating,” “imaging,” “performing,” “comparing,” “updating,” “transmitting,” “entering,” or the like, refer to the actions and processes of an electronic device such as an electrical device. 
     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, executed by one or more computers 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, logic, 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 fingerprint sensing system and/or mobile electronic device described herein may include components other than those shown, including well-known components. 
     Various techniques described herein may be implemented in hardware, software, firmware, or any combination thereof, 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 processor-readable storage medium comprising instructions that, when executed, 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 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. 
     Various embodiments described herein may be executed by one or more processors, such as one or more motion processing units (MPUs), sensor processing units (SPUs), 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), a programmable logic controller (PLC), a complex programmable logic device (CPLD), a discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein, or other equivalent integrated or discrete logic circuitry. The term “processor,” as used herein may refer to any of the foregoing structures or any other structure suitable for implementation of the techniques described herein. As it employed in the subject specification, the term “processor” can refer to substantially any computing processing unit or device comprising, but not limited to comprising, single-core processors; single-processors with software multithread execution capability; multi-core processors; multi-core processors with software multithread execution capability; multi-core processors with hardware multithread technology; parallel platforms; and parallel platforms with distributed shared memory. Moreover, processors can exploit nano-scale architectures such as, but not limited to, molecular and quantum-dot based transistors, switches and gates, in order to optimize space usage or enhance performance of user equipment. A processor may also be implemented as a combination of computing processing units. 
     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 combination of an SPU/MPU and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with an SPU core, MPU core, or any other such configuration. 
     Overview of Discussion 
     Discussion begins with a description of an example piezoelectric micromachined ultrasonic transducer (PMUT), in accordance with various embodiments. Example arrays including ultrasonic transducers are then described. 
     In accordance with various embodiments, an electronic device includes an array of ultrasonic transducers for generating and receiving ultrasonic signals, and an acoustic coupling layer overlying the array of ultrasonic transducers, wherein the ultrasonic signals are propagated through the acoustic coupling layer. In one embodiment, the ultrasonic transducers include PMUT devices. In one embodiment, the ultrasonic transducers include Capacitive Micromachined Ultrasonic Transducer (CMUT) devices. In one embodiment, the electronic device further includes sidewalls bounding the array of ultrasonic transducers and the acoustic coupling layer. 
     In one embodiment, the electronic device further includes a cover overlying the acoustic coupling layer. In one embodiment, the cover is curved such that the acoustic coupling layer is thicker at a midpoint of the array of ultrasonic transducers than towards an edge of the array of ultrasonic transducers. In one embodiment, the cover has varying thickness such that the cover is thinner at a midpoint of the cover and thicker towards an edge of the cover. 
     In one embodiment, the electronic device further includes a plurality of refractive acoustic elements. In one embodiment, refractive acoustic elements are disposed within the acoustic coupling layer such that a refractive acoustic element of the plurality of refractive acoustic elements is associated with an ultrasonic transducer of the array of ultrasonic transducers. In one embodiment, refractive acoustic elements are disposed adjacent to the array of ultrasonic transducers. In one embodiment, refractive acoustic elements are disposed within the cover such that a refractive acoustic element of the plurality of refractive acoustic elements is associated with an ultrasonic transducer of the array of ultrasonic transducers. It should be appreciated that refractive acoustic elements may be located in various locations. For instance, refractive acoustic elements may be located in the cover, suspended in the acoustic coupling material, and/or adjacent to the array of ultrasonic transducers in any combination. 
     In accordance with various embodiments, an array of ultrasonic transducers for generating and receiving ultrasonic signals includes sidewalls bounding the array of ultrasonic transducers, an acoustic coupling layer overlying the array of ultrasonic transducers and bounded by the sidewalls, wherein the ultrasonic signals are propagated through the acoustic coupling layer, the acoustic coupling layer comprises an acoustic material supporting transmission of the ultrasonic signals, and a cover overlying the sidewalls and the acoustic coupling layer. 
     In one embodiment, the sidewalls include a relief channel for allowing expulsion of excess acoustic material of the acoustic coupling layer from a cavity defined by the array of ultrasonic transducers, the sidewalls and the cover. In one embodiment, the relief channel is a groove situated at a top edge of the sidewalls. In one embodiment, the relief channel is an opening situated within the sidewalls. In one embodiment, the cover includes a relief channel for allowing expulsion of excess acoustic material of the acoustic coupling layer from a cavity defined by the array of ultrasonic transducers, the sidewalls and the cover. 
     In accordance with various embodiments, a mobile device includes a processor, a memory unit, a display device disposed on a first surface of the mobile device, and a fingerprint sensor disposed on a second surface of the mobile device. The fingerprint sensor includes an array of ultrasonic transducers for generating and receiving ultrasonic signals, and an acoustic coupling layer overlying the array of ultrasonic transducers, wherein the ultrasonic signals are propagated through the acoustic coupling layer. In one embodiment, the ultrasonic transducers include PMUT devices. In one embodiment, the ultrasonic transducers include CMUT devices. In one embodiment, the fingerprint sensor includes a button overlying the acoustic coupling layer such that the button is disposed on the second surface. 
     In one embodiment, the second surface is perpendicular to the first surface. In one embodiment, the second surface is a curved surface having a curvature, such that the fingerprint sensor is curved to match the curvature of the curved surface. In one embodiment, the acoustic coupling layer of the fingerprint sensor is curved to match the curvature of the curved surface. In another embodiment, the first surface is a top surface of the mobile device and the second surface is a bottom surface of the mobile device. 
     In one embodiment, the fingerprint sensor further includes a cover overlying the acoustic coupling layer, wherein the cover is curved to match the curvature of the curved surface. In one embodiment, the cover has varying thickness such that the cover is thinner at a midpoint of the cover and thicker towards an edge of the cover. 
     Piezoelectric Micromachined Ultrasonic Transducer (PMUT) 
     Systems and methods disclosed herein, in one or more aspects provide efficient structures for an acoustic transducer (e.g., a piezoelectric micromachined actuated transducer or PMUT). One or more embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the various embodiments. It may be evident, however, that the various embodiments can be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing the embodiments in additional detail. 
     As used in this application, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or”. That is, unless specified otherwise, or clear from context, “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, if X employs A; X employs B; or X employs both A and B, then “X employs A or B” is satisfied under any of the foregoing instances. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form. In addition, the word “coupled” is used herein to mean direct or indirect electrical or mechanical coupling. In addition, the word “example” is used herein to mean serving as an example, instance, or illustration. 
       FIG.  1    is a diagram illustrating a PMUT device  100  having a center pinned membrane, according to some embodiments. PMUT device  100  includes an interior pinned membrane  120  positioned over a substrate  140  to define a cavity  130 . In one embodiment, membrane  120  is attached both to a surrounding edge support  102  and interior support  104 . In one embodiment, edge support  102  is connected to an electric potential. Edge support  102  and interior support  104  may be made of electrically conducting materials, such as and without limitation, aluminum, molybdenum, or titanium. Edge support  102  and interior support  104  may also be made of dielectric materials, such as silicon dioxide, silicon nitride or aluminum oxide that have electrical connections on the sides or in vias through edge support  102  or interior support  104 , electrically coupling lower electrode  106  to electrical wiring in substrate  140 . 
     In one embodiment, both edge support  102  and interior support  104  are attached to a substrate  140 . In various embodiments, substrate  140  may include at least one of, and without limitation, silicon or silicon nitride. It should be appreciated that substrate  140  may include electrical wirings and connection, such as aluminum or copper. In one embodiment, substrate  140  includes a CMOS logic wafer bonded to edge support  102  and interior support  104 . In one embodiment, the membrane  120  comprises multiple layers. In an example embodiment, the membrane  120  includes lower electrode  106 , piezoelectric layer  110 , and upper electrode  108 , where lower electrode  106  and upper electrode  108  are coupled to opposing sides of piezoelectric layer  110 . As shown, lower electrode  106  is coupled to a lower surface of piezoelectric layer  110  and upper electrode  108  is coupled to an upper surface of piezoelectric layer  110 . It should be appreciated that, in various embodiments, PMUT device  100  is a microelectromechanical (MEMS) device. 
     In one embodiment, membrane  120  also includes a mechanical support layer  112  (e.g., stiffening layer) to mechanically stiffen the layers. In various embodiments, mechanical support layer  112  may include at least one of, and without limitation, silicon, silicon oxide, silicon nitride, aluminum, molybdenum, titanium, etc. In one embodiment, PMUT device  100  also includes an acoustic coupling layer  114  above membrane  120  for supporting transmission of acoustic signals. It should be appreciated that acoustic coupling layer can include air, liquid, gel-like materials, epoxy, or other materials for supporting transmission of acoustic signals. In one embodiment, PMUT device  100  also includes platen layer  116  above acoustic coupling layer  114  for containing acoustic coupling layer  114  and providing a contact surface for a finger or other sensed object with PMUT device  100 . It should be appreciated that, in various embodiments, acoustic coupling layer  114  provides a contact surface, such that platen layer  116  is optional. Moreover, it should be appreciated that acoustic coupling layer  114  and/or platen layer  116  may be included with or used in conjunction with multiple PMUT devices. For example, an array of PMUT devices may be coupled with a single acoustic coupling layer  114  and/or platen layer  116 . 
       FIG.  2    is a diagram illustrating an example of membrane movement during activation of PMUT device  100 , according to some embodiments. As illustrated with respect to  FIG.  2   , in operation, responsive to an object proximate platen layer  116 , the electrodes  106  and  108  deliver a high frequency electric charge to the piezoelectric layer  110 , causing those portions of the membrane  120  not pinned to the surrounding edge support  102  or interior support  104  to be displaced upward into the acoustic coupling layer  114 . This generates a pressure wave that can be used for signal probing of the object. Return echoes can be detected as pressure waves causing movement of the membrane, with compression of the piezoelectric material in the membrane causing an electrical signal proportional to amplitude of the pressure wave. 
     The described PMUT device  100  can be used with almost any electrical device that converts a pressure wave into mechanical vibrations and/or electrical signals. In one aspect, the PMUT device  100  can comprise an acoustic sensing element (e.g., a piezoelectric element) that generates and senses ultrasonic sound waves. An object in a path of the generated sound waves can create a disturbance (e.g., changes in frequency or phase, reflection signal, echoes, etc.) that can then be sensed. The interference can be analyzed to determine physical parameters such as (but not limited to) distance, density and/or speed of the object. As an example, the PMUT device  100  can be utilized in various applications, such as, but not limited to, fingerprint or physiologic sensors suitable for wireless devices, industrial systems, automotive systems, robotics, telecommunications, security, medical devices, etc. For example, the PMUT device  100  can be part of a sensor array comprising a plurality of ultrasonic transducers deposited on a wafer, along with various logic, control and communication electronics. A sensor array may comprise homogenous or identical PMUT devices  100 , or a number of different or heterogonous device structures. 
     In various embodiments, the PMUT device  100  employs a piezoelectric layer  110 , comprised of materials such as, but not limited to, Aluminum nitride (AlN), lead zirconate titanate (PZT), quartz, polyvinylidene fluoride (PVDF), and/or zinc oxide, to facilitate both acoustic signal production and sensing. The piezoelectric layer  110  can generate electric charges under mechanical stress and conversely experience a mechanical strain in the presence of an electric field. For example, the piezoelectric layer  110  can sense mechanical vibrations caused by an ultrasonic signal and produce an electrical charge at the frequency (e.g., ultrasonic frequency) of the vibrations. Additionally, the piezoelectric layer  110  can generate an ultrasonic wave by vibrating in an oscillatory fashion that might be at the same frequency (e.g., ultrasonic frequency) as an input current generated by an alternating current (AC) voltage applied across the piezoelectric layer  110 . It should be appreciated that the piezoelectric layer  110  can include almost any material (or combination of materials) that exhibits piezoelectric properties, such that the structure of the material does not have a center of symmetry and a tensile or compressive stress applied to the material alters the separation between positive and negative charge sites in a cell causing a polarization at the surface of the material. The polarization is directly proportional to the applied stress and is direction dependent so that compressive and tensile stresses results in electric fields of opposite polarizations. 
     Further, the PMUT device  100  comprises electrodes  106  and  108  that supply and/or collect the electrical charge to/from the piezoelectric layer  110 . It should be appreciated that electrodes  106  and  108  can be continuous and/or patterned electrodes (e.g., in a continuous layer and/or a patterned layer). For example, as illustrated, electrode  106  is a patterned electrode and electrode  108  is a continuous electrode. As an example, electrodes  106  and  108  can be comprised of almost any metal layers, such as, but not limited to, Aluminum (Al)/Titanium (Ti), Molybdenum (Mo), etc., which are coupled with and on opposing sides of the piezoelectric layer  110 . In one embodiment, PMUT device also includes a third electrode, as illustrated in  FIG.  10    and described below. 
     According to an embodiment, the acoustic impedance of acoustic coupling layer  114  is selected to be similar to the acoustic impedance of the platen layer  116 , such that the acoustic wave is efficiently propagated to/from the membrane  120  through acoustic coupling layer  114  and platen layer  116 . As an example, the platen layer  116  can comprise various materials having an acoustic impedance in the range between 0.8 to 4 MRayl, such as, but not limited to, plastic, resin, rubber, Teflon, epoxy, etc. In another example, the platen layer  116  can comprise various materials having a high acoustic impedance (e.g., an acoustic impendence greater than 10 MRayl), such as, but not limited to, glass, aluminum-based alloys, sapphire, etc. Typically, the platen layer  116  can be selected based on an application of the sensor. For instance, in fingerprinting applications, platen layer  116  can have an acoustic impedance that matches (e.g., exactly or approximately) the acoustic impedance of human skin (e.g., 1.6×10 6  Rayl). Further, in one aspect, the platen layer  116  can further include a thin layer of anti-scratch material. In various embodiments, the anti-scratch layer of the platen layer  116  is less than the wavelength of the acoustic wave that is to be generated and/or sensed to provide minimum interference during propagation of the acoustic wave. As an example, the anti-scratch layer can comprise various hard and scratch-resistant materials (e.g., having a Mohs hardness of over 7 on the Mohs scale), such as, but not limited to sapphire, glass, MN, Titanium nitride (TiN), Silicon carbide (SiC), diamond, etc. As an example, PMUT device  100  can operate at 20 MHz and accordingly, the wavelength of the acoustic wave propagating through the acoustic coupling layer  114  and platen layer  116  can be 70-150 microns. In this example scenario, insertion loss can be reduced and acoustic wave propagation efficiency can be improved by utilizing an anti-scratch layer having a thickness of 1 micron and the platen layer  116  as a whole having a thickness of 1-2 millimeters. It is noted that the term “anti-scratch material” as used herein relates to a material that is resistant to scratches and/or scratch-proof and provides substantial protection against scratch marks. 
     In accordance with various embodiments, the PMUT device  100  can include metal layers (e.g., Aluminum (Al)/Titanium (Ti), Molybdenum (Mo), etc.) patterned to form electrode  106  in particular shapes (e.g., ring, circle, square, octagon, hexagon, etc.) that are defined in-plane with the membrane  120 . Electrodes can be placed at a maximum strain area of the membrane  120  or placed at close to either or both the surrounding edge support  102  and interior support  104 . Furthermore, in one example, electrode  108  can be formed as a continuous layer providing a ground plane in contact with mechanical support layer  112 , which can be formed from silicon or other suitable mechanical stiffening material. In still other embodiments, the electrode  106  can be routed along the interior support  104 , advantageously reducing parasitic capacitance as compared to routing along the edge support  102 . 
     For example, when actuation voltage is applied to the electrodes, the membrane  120  will deform and move out of plane. The motion then pushes the acoustic coupling layer  114  it is in contact with and an acoustic (ultrasonic) wave is generated. Oftentimes, vacuum is present inside the cavity  130  and therefore damping contributed from the media within the cavity  130  can be ignored. However, the acoustic coupling layer  114  on the other side of the membrane  120  can substantially change the damping of the PMUT device  100 . For example, a quality factor greater than 20 can be observed when the PMUT device  100  is operating in air with atmosphere pressure (e.g., acoustic coupling layer  114  is air) and can decrease lower than 2 if the PMUT device  100  is operating in water (e.g., acoustic coupling layer  114  is water). 
       FIG.  3    is a top view of the PMUT device  100  of  FIG.  1    having a substantially square shape, which corresponds in part to a cross section along dotted line  101  in  FIG.  3   . Layout of surrounding edge support  102 , interior support  104 , and lower electrode  106  are illustrated, with other continuous layers not shown. It should be appreciated that the term “substantially” in “substantially square shape” is intended to convey that a PMUT device  100  is generally square-shaped, with allowances for variations due to manufacturing processes and tolerances, and that slight deviation from a square shape (e.g., rounded corners, slightly wavering lines, deviations from perfectly orthogonal corners or intersections, etc.) may be present in a manufactured device. While a generally square arrangement PMUT device is shown, alternative embodiments including rectangular, hexagon, octagonal, circular, or elliptical are contemplated. In other embodiments, more complex electrode or PMUT device shapes can be used, including irregular and non-symmetric layouts such as chevrons or pentagons for edge support and electrodes. 
       FIG.  4    is a simulated topographic map  400  illustrating maximum vertical displacement of the membrane  120  of the PMUT device  100  shown in  FIGS.  1 - 3   . As indicated, maximum displacement generally occurs along a center axis of the lower electrode, with corner regions having the greatest displacement. As with the other figures,  FIG.  4    is not drawn to scale with the vertical displacement exaggerated for illustrative purposes, and the maximum vertical displacement is a fraction of the horizontal surface area comprising the PMUT device  100 . In an example PMUT device  100 , maximum vertical displacement may be measured in nanometers, while surface area of an individual PMUT device  100  may be measured in square microns. 
       FIG.  5    is a top view of another example of the PMUT device  100  of  FIG.  1    having a substantially circular shape, which corresponds in part to a cross section along dotted line  101  in  FIG.  5   . Layout of surrounding edge support  102 , interior support  104 , and lower electrode  106  are illustrated, with other continuous layers not shown. It should be appreciated that the term “substantially” in “substantially circular shape” is intended to convey that a PMUT device  100  is generally circle-shaped, with allowances for variations due to manufacturing processes and tolerances, and that slight deviation from a circle shape (e.g., slight deviations on radial distance from center, etc.) may be present in a manufactured device. 
       FIG.  6    is a top view of another example of the PMUT device  100  of  FIG.  1    having a substantially hexagonal shape, which corresponds in part to a cross section along dotted line  101  in  FIG.  6   . Layout of surrounding edge support  102 , interior support  104 , and lower electrode  106  are illustrated, with other continuous layers not shown. It should be appreciated that the term “substantially” in “substantially hexagonal shape” is intended to convey that a PMUT device  100  is generally hexagon-shaped, with allowances for variations due to manufacturing processes and tolerances, and that slight deviation from a hexagon shape (e.g., rounded corners, slightly wavering lines, deviations from perfectly orthogonal corners or intersections, etc.) may be present in a manufactured device. 
       FIG.  7    illustrates an example two-dimensional array  700  of circular-shaped PMUT devices  701  formed from PMUT devices having a substantially circular shape similar to that discussed in conjunction with  FIGS.  1 ,  2  and  5   . Layout of circular surrounding edge support  702 , interior support  704 , and annular or ring shaped lower electrode  706  surrounding the interior support  704  are illustrated, while other continuous layers are not shown for clarity. As illustrated, array  700  includes columns of circular-shaped PMUT devices  701  that are offset. It should be appreciated that the circular-shaped PMUT devices  701  may be closer together, such that edges of the columns of circular-shaped PMUT devices  701  overlap. Moreover, it should be appreciated that circular-shaped PMUT devices  701  may contact each other. In various embodiments, adjacent circular-shaped PMUT devices  701  are electrically isolated. In other embodiments, groups of adjacent circular-shaped PMUT devices  701  are electrically connected, where the groups of adjacent circular-shaped PMUT devices  701  are electrically isolated. 
       FIG.  8    illustrates an example two-dimensional array  800  of square-shaped PMUT devices  801  formed from PMUT devices having a substantially square shape similar to that discussed in conjunction with  FIGS.  1 ,  2  and  3   . Layout of square surrounding edge support  802 , interior support  804 , and square-shaped lower electrode  806  surrounding the interior support  804  are illustrated, while other continuous layers are not shown for clarity. As illustrated, array  800  includes columns of square-shaped PMUT devices  801  that are in rows and columns. It should be appreciated that rows or columns of the square-shaped PMUT devices  801  may be offset. Moreover, it should be appreciated that square-shaped PMUT devices  801  may contact each other or be spaced apart. In various embodiments, adjacent square-shaped PMUT devices  801  are electrically isolated. In other embodiments, groups of adjacent square-shaped PMUT devices  801  are electrically connected, where the groups of adjacent square-shaped PMUT devices  801  are electrically isolated. 
       FIG.  9    illustrates an example two-dimensional array  900  of hexagon-shaped PMUT devices  901  formed from PMUT devices having a substantially hexagon shape similar to that discussed in conjunction with  FIGS.  1 ,  2  and  6   . Layout of hexagon-shaped surrounding edge support  902 , interior support  904 , and hexagon-shaped lower electrode  906  surrounding the interior support  904  are illustrated, while other continuous layers are not shown for clarity. It should be appreciated that rows or columns of the hexagon-shaped PMUT devices  901  may be offset. Moreover, it should be appreciated that hexagon-shaped PMUT devices  901  may contact each other or be spaced apart. In various embodiments, adjacent hexagon-shaped PMUT devices  901  are electrically isolated. In other embodiments, groups of adjacent hexagon-shaped PMUT devices  901  are electrically connected, where the groups of adjacent hexagon-shaped PMUT devices  901  are electrically isolated. While  FIGS.  7 ,  8  and  9    illustrate example layouts of PMUT devices having different shapes, it should be appreciated that many different layouts are available. Moreover, in accordance with various embodiments, arrays of PMUT devices are included within a MEMS layer. 
     In operation, during transmission, selected sets of PMUT devices in the two-dimensional array can transmit an acoustic signal (e.g., a short ultrasonic pulse) and during sensing, the set of active PMUT devices in the two-dimensional array can detect an interference of the acoustic signal with an object (in the path of the acoustic wave). The received interference signal (e.g., generated based on reflections, echoes, etc. of the acoustic signal from the object) can then be analyzed. As an example, an image of the object, a distance of the object from the sensing component, a density of the object, a motion of the object, etc., can all be determined based on comparing a frequency and/or phase of the interference signal with a frequency and/or phase of the acoustic signal. Moreover, results generated can be further analyzed or presented to a user via a display device (not shown). 
       FIG.  10    illustrates a pair of example PMUT devices  1000  in a PMUT array, with each PMUT sharing at least one common edge support  1002 . As illustrated, the PMUT devices have two sets of independent lower electrode labeled as  1006  and  1026 . These differing electrode patterns enable antiphase operation of the PMUT devices  1000 , and increase flexibility of device operation. In one embodiment, the pair of PMUTs may be identical, but the two electrodes could drive different parts of the same PMUT antiphase (one contracting, and one extending), such that the PMUT displacement becomes larger. While other continuous layers are not shown for clarity, each PMUT also includes an upper electrode (e.g., upper electrode  108  of  FIG.  1   ). Accordingly, in various embodiments, a PMUT device may include at least three electrodes. 
       FIGS.  11 A,  11 B,  11 C, and  11 D  illustrate alternative examples of interior support structures, in accordance with various embodiments. Interior supports structures may also be referred to as “pinning structures,” as they operate to pin the membrane to the substrate. It should be appreciated that interior support structures may be positioned anywhere within a cavity of a PMUT device, and may have any type of shape (or variety of shapes), and that there may be more than one interior support structure within a PMUT device. While  FIGS.  11 A,  11 B,  11 C , and  11 D illustrate alternative examples of interior support structures, it should be appreciated that these examples are for illustrative purposes, and are not intended to limit the number, position, or type of interior support structures of PMUT devices. 
     For example, interior supports structures do not have to be centrally located with a PMUT device area, but can be non-centrally positioned within the cavity. As illustrated in  FIG.  11 A , interior support  1104   a  is positioned in a non-central, off-axis position with respect to edge support  1102 . In other embodiments such as seen in  FIG.  11 B , multiple interior supports  1104   b  can be used. In this embodiment, one interior support is centrally located with respect to edge support  1102 , while the multiple, differently shaped and sized interior supports surround the centrally located support. In still other embodiments, such as seen with respect to  FIGS.  11 C and  11 D , the interior supports (respectively  1104   c  and  1104   d ) can contact a common edge support  1102 . In the embodiment illustrated in  FIG.  11 D , the interior supports  1104   d  can effectively divide the PMUT device into subpixels. This would allow, for example, activation of smaller areas to generate high frequency ultrasonic waves, and sensing a returning ultrasonic echo with larger areas of the PMUT device. It will be appreciated that the individual pinning structures can be combined into arrays. 
       FIG.  12    illustrates an embodiment of a PMUT array used in an ultrasonic fingerprint sensing system  1250 . The fingerprint sensing system  1250  can include a platen  1216  onto which a human finger  1252  may make contact. Ultrasonic signals are generated and received by a PMUT device array  1200 , and travel back and forth through acoustic coupling layer  1214  and platen  1216 . Signal analysis is conducted using processing logic module  1240  (e.g., control logic) directly attached (via wafer bonding or other suitable techniques) to the PMUT device array  1200 . It will be appreciated that the size of platen  1216  and the other elements illustrated in  FIG.  12    may be much larger (e.g., the size of a handprint) or much smaller (e.g., just a fingertip) than as shown in the illustration, depending on the particular application. 
     In this example for fingerprinting applications, the human finger  1252  and the processing logic module  1240  can determine, based on a difference in interference of the acoustic signal with valleys and/or ridges of the skin on the finger, an image depicting epi-dermis and/or dermis layers of the finger. Further, the processing logic module  1240  can compare the image with a set of known fingerprint images to facilitate identification and/or authentication. Moreover, in one example, if a match (or substantial match) is found, the identity of user can be verified. In another example, if a match (or substantial match) is found, a command/operation can be performed based on an authorization rights assigned to the identified user. In yet another example, the identified user can be granted access to a physical location and/or network/computer resources (e.g., documents, files, applications, etc.) 
     In another example, for finger-based applications, the movement of the finger can be used for cursor tracking/movement applications. In such embodiments, a pointer or cursor on a display screen can be moved in response to finger movement. It is noted that processing logic module  1240  can include or be connected to one or more processors configured to confer at least in part the functionality of system  1250 . To that end, the one or more processors can execute code instructions stored in memory, for example, volatile memory and/or nonvolatile memory. 
       FIG.  13    illustrates an integrated fingerprint sensor  1300  formed by wafer bonding a CMOS logic wafer and a MEMS wafer defining PMUT devices, according to some embodiments.  FIG.  13    illustrates in partial cross section one embodiment of an integrated fingerprint sensor formed by wafer bonding a substrate  1340  CMOS logic wafer and a MEMS wafer defining PMUT devices having a common edge support  1302  and separate interior support  1304 . For example, the MEMS wafer may be bonded to the CMOS logic wafer using aluminum and germanium eutectic alloys, as described in U.S. Pat. No. 7,442,570. PMUT device  1300  has an interior pinned membrane  1320  formed over a cavity  1330 . The membrane  1320  is attached both to a surrounding edge support  1302  and interior support  1304 . The membrane  1320  is formed from multiple layers. In one embodiment, integrated fingerprint sensor  1300  includes sensor  1350  in the CMOS logic wafer (e.g., a temperature sensor). In one embodiment, the CMOS logic wafer includes at least one drive circuit for driving transmission of ultrasonic signals from ultrasonic transducers of the array of ultrasonic transducers and at least one receive circuit for receiving reflected ultrasonic signals from ultrasonic transducers of the array of ultrasonic transducers. 
     Example Arrays Including Ultrasonic Transducers 
     Devices described herein, in accordance with various embodiments, provide packaging of a two-dimensional array of ultrasonic transducers (e.g., an array of PMUTs). One or more embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the various embodiments. It may be evident, however, that the various embodiments can be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing the embodiments in additional detail. 
     Piezoelectric materials facilitate conversion between mechanical energy and electrical energy. Moreover, a piezoelectric material can generate an electrical signal when subjected to mechanical stress, and can vibrate when subjected to an electrical voltage. Piezoelectric materials may be utilized in piezoelectric ultrasonic transducers to generate acoustic waves based on an actuation voltage applied to electrodes of the piezoelectric ultrasonic transducer. 
     A piezoelectric ultrasonic transducer able to generate and detect pressure waves can include a membrane with the piezoelectric material, a supporting layer, and electrodes combined with a cavity beneath the electrodes. Miniaturized versions known as PMUTs have been developed for a variety of medical and other ultrasound application. 
     PMUTs can be arranged into physical blocks with separately timed ultrasonic wave emission and detection. This advantageously reduces crosstalk issues and simplifies signal processing. In addition, because the fabrication of a PMUT through the Micro Electro Mechanical Systems (MEMS) process allows for repeatable variations of the hardware circuitry, supplemental modes may be supported through hardware. Supplemental modes may be supported through operation of the hardware in different modes through computer program code. 
     The systems described herein, in one or more embodiments thereof, relate to a PMUT array for ultrasonic wave generation and sensing. The PMUT devices include edge support structures and a membrane attached to the edge support structure to allow movement at ultrasonic frequencies. The membrane includes a piezoelectric layer, first and second electrodes at least partially attached to opposing sides of the piezoelectric layer. The membrane may optionally include an interior support structure connected to the piezoelectric layer within an area defined by the edge support structures. The PMUT devices can be arranged to form various virtual transmit and receive blocks on the two dimensional array. However, it should be appreciated that the described embodiments are not limited to PMUT devices, and that other types of ultrasonic transducers, such as CMUT devices, may be used. 
     In one embodiment, the PMUT array may form a fingerprint sensor system comprising a substrate having sidewalls to form a chamber fillable with an acoustic coupling material, also referred to herein as an ultrasound propagation material, such as an epoxy, polydimethylsiloxane (PDMS), perfluorotripentylamine (e.g., Fluorine™ FC-70), glass, metal, oil, gel, or various cyclic or linear siloxanes. In some embodiments, the acoustic coupling material has acoustic properties that act to minimize unwanted ultrasonic reflections at an interface between the acoustic coupling material and the cover. The PMUT array that transmits ultrasonic beams and receives ultrasonic signals is positioned in the chamber and a cover is positioned over the PMUT array to contain the ultrasound propagation material. 
     In some embodiments the acoustic coupling material is a gel material such as PDMS. The cover can be a platen directly touchable by a finger, allowing use under mobile device touchscreens, home buttons in mobile devices, or dedicated fingerprint sensor regions. Because ultrasonic systems can be tuned to penetrate intermediate layers, it is also possible to locate the PMUT in non-traditional locations for the mobile device. In certain embodiments, the cover can be a platen mounted on an edge of a mobile device and touchable by a finger, the cover can be on a curved area or region of a device, or the cover can be made of glass, metal, plastic, or composites among other materials and without limitation. 
     In various embodiments, the cover can include refractive acoustic elements, including ultrasonic lensing elements, acoustic wedges, focusing metamaterials, or microlenses. Similarly, PMUT devices (transducers) of the PMUT array can have affixed refractive acoustic elements, including ultrasonic lensing elements, acoustic wedges, focusing metamaterials, or microlenses. The refractive acoustic elements are used for refractive beamforming, e.g., to focus a transmit beam to a desired point. For instance, one or more refractive acoustic elements may be used to focus an ultrasonic signal to a point at or near the top of a platen (e.g., the contact point between a finger and a fingerprint sensing system. However, it should be appreciated that the refractive acoustic elements form a focal point at other depths (e.g., at a point above the platen for sensing into a finger rather than just the surface of the finger). 
     In various embodiments, a fingerprint sensor system includes a substrate having sidewalls to form a chamber fillable with an acoustic coupling material, and a PMUT array for transmitting ultrasonic beams and receiving ultrasonic signals positioned in the chamber. A cover is positioned over the PMUT array to contain the ultrasound propagation material. The cover or sidewalls of the PMUT array packaging include a relief mechanism (e.g. perforations or relief channels) for releasing acoustic coupling material when the cover is installed or mounted. The use of a relief mechanism simplifies use of the PMUT array as an under glass or under metal sensor system for mobile devices or other security applications. 
     In other embodiments, a fingerprint sensor system includes a cover having a varying thickness and including acoustic coupling material positioned over the PMUT array to contain the ultrasound propagation material. The cover can be curved to allow mounting as a platen mounted on an edge of a mobile device. In some embodiments, acoustic microlenses of varying sizes and arrangements can be subtractively or additively formed on the cover. 
     Other techniques known in ultrasound can be used to enhance propagation, or to deter losses of reflections, through any aberrating layers that are part of the device and through which the PMUT communicates with a finger. Beamforming, beam steering, and similar techniques for control of the PMUT may also be used to enhance further the capability of the PMUT with regard to a known device design and related packaging constraints. 
     Systems described herein can be used, for example, for analysis of acoustically sensed data in various applications, such as, but not limited to, medical applications, security systems, biometric systems (e.g., fingerprint sensors and/or motion/gesture recognition sensors), mobile communication systems, industrial automation systems, consumer electronic devices, robotics, etc. In one embodiment, the system can include PMUT devices that can facilitate ultrasonic signal generation and sensing (e.g., transducers). Moreover, the sensing component can include a silicon wafer having a two-dimensional (or one-dimensional) array of ultrasonic transducers. 
     In one embodiment, the PMUT array may be used in a substantially passive listening mode. In a typical active operational mode for fingerprint recognition, the PMUT elements are activated during a transmit phase in which an acoustic signal is generated. The reflected signal or echo is then received during a receive phase. The timing of the received reflection is used to construct a fingerprint image. Alternatively, in the presence of ambient external energy of suitable frequency, the PMUT array may be placed in a low-power listen mode that is substantially passive. Ambient signals are then received during a receive phase without the need to generate an acoustic signal during a transmit phase. This allows the device to consume substantially less power and to provide sensing capabilities other than ultrasonic fingerprint recognition. 
     In various embodiments, the ultrasonic transducers comprise PMUT devices. In various embodiments, the ultrasonic transducers comprise CMUT devices. In various embodiments, the control module comprises a processor. 
     With reference to the drawings,  FIG.  14 A  illustrates an example of an operational environment  1400  for sensing of human touch in accordance with one or more embodiments of the disclosure. As illustrated, a device  1410  includes a fingerprint sensor  1430  or other type of surface sensitive to touch. In one embodiment, fingerprint sensor  1430  is disposed beneath a touch-screen display device  1415  of device  1410 . In another embodiment, fingerprint sensor  1430  is disposed in an edge of device  1410 . In another embodiment, fingerprint sensor  1430  is disposed adjacent or close to a touch-screen display device  1415  of device  1410 . In another embodiment, fingerprint sensor  1430  is comprised within a touch-screen display device  1415  of device  1410 . It should be appreciated that device  1410  includes a fingerprint sensor  1430  for sensing a fingerprint of a finger interacting with device  1410 . 
     In one embodiment, a human finger (represented by a hand  1420 ), can touch or interact with a specific area of device  1410  proximate fingerprint sensor  1430 . In various embodiments, fingerprint sensor  1430  can be hard and need not include movable parts, such as a sensor button configured to detect human touch or otherwise cause the device  1410  to respond to human touch. The device  1410  can include circuitry that can operate in response to touch (human or otherwise) of the touch-screen display device and/or fingerprint sensor  1430  (or, in some embodiments, another type of touch sensitive surface). 
     In accordance with the described embodiments, device  1410  includes sensing device  1430  and system circuitry  1440 . It should be appreciated that components of sensing device  1430  and system circuitry  1440  might be disposed within the same componentry, and are conceptually distinguished herein such that fingerprint sensor  1430  includes components that are always-on, or mostly always-on, and system circuitry  1440  includes components that are powered off until they are powered on, for example, in response to an activation signal received from sensing device  1430 . For example, such circuitry can be operatively coupled (e.g., electrically coupled, communicative coupled, etc.) via a bus architecture  1435  (or bus  1435 ) or conductive conduits configured to permit the exchange of signals between the sensing device  1430  and the system circuitry  1440 . In some embodiments, a printed circuit board (PCB) placed behind a touch-screen display device can include the sensing device  1430 , the system circuitry  1440 , and the bus  1435 . In one embodiment, the sensing device  1430  and the system circuitry  1440  can be configured or otherwise arranged in a single semiconductor die. In another embodiment, the sensing device  1430  can be configured or otherwise arranged in a first semiconductor die and the system circuitry  1440  can be configured or otherwise arranged in a second semiconductor die. In addition, in some embodiments, the bus  1435  can be embodied in or can include a dedicated conducting wire or a dedicated data line that connects the sensing device  1430  and the system circuitry  1440 . 
     The sensing device  1430  can operate as sensor for human touch and the system circuitry  1440 , or a portion thereof, can permit or otherwise facilitate analysis of the human touch. As described herein, sensing device  1430  includes fingerprint sensor  1430 . For example, responsive to capturing an image of a fingerprint, fingerprint sensor  1430  can transmit the captured image to system circuitry for analysis. 
     The analysis can include fingerprint recognition or other types of biometric evaluations. The sensing device  1430  can be energized or otherwise power-on continuously or nearly continuously and can be configured to monitor touch of sensing device  1430 . In addition, in response to human touch (e.g., touch by a human finger or other human body part), the sensing device  1430  can be further configured to trigger detection and/or another type of analysis of elements of the human touch or a human body associated therewith. 
       FIG.  14 B  illustrates an example sensing device  1430 , in accordance with various embodiments. In one embodiment, sensing device  1430  includes an array  1450  of ultrasonic transducers (e.g., PMUT devices), a control module  1460 , a memory  1470 , and an external interface  1485 . In various embodiments, control module  1460  performs certain operations in accordance with instructions stored within memory  1470 . It should be appreciated that components of sensing device  1430  are examples, and that certain components, such as control module  1460  and/or memory  1470  may not be located within sensing device  1430  (e.g., control module  1460  and/or memory  1470  may reside within system circuitry  1440 ). For example, sensing device  1430  or system circuitry  1440  may include a processor and/or memory for performing certain operations. 
     In one embodiment, sensing device  1430  includes control module  1460  for performing the pixel capture. In other embodiments, control module  1460  can perform thresholding to determine whether an object has interacted with fingerprint sensor  1430 . In other embodiments, control module  1460  can analyze captured pixels and determine whether the object is a finger. In other embodiments, control module  1460  can capture an image of the fingerprint and forward it to a processor of system circuitry  1440  for further analysis. 
     While the embodiment of  FIG.  14 B  includes control module  1460  and memory  1470 , as described above, it should be appreciated that various functions of control module  1460  and memory  1470  may reside in other components of device  1410  (e.g., within sensing device  1430  or system circuitry  1440 ). Moreover, it should be appreciated that control module  1460  may be any type of processor for performing any portion of the described functionality (e.g., custom digital logic). 
     In various embodiments, fingerprint sensor  1430  can include ultrasonic transducers (e.g., PMUTs or CMUTs) able to generate and detect pressure waves. Examples of PMUT devices and arrays of PMUT devices are described in accordance with  FIGS.  1 - 13    above. In embodiments, a device  1410  includes sensing device  1430  comprised of an array of PMUT devices that can facilitate ultrasonic signal generation and sensing (transducer). For example, sensing device  1430  can include a silicon wafer having a two-dimensional (or one-dimensional) array of ultrasonic transducers. 
       FIG.  15    illustrates an example ultrasonic transducer array system  1500  including an array of ultrasonic transducers packaged in a chamber (also referred to herein as a cavity or a container) having a cover, in accordance with an embodiment. System  1500  includes sidewalls  1502  and a cover  1504  that, together with substrate mounted ultrasonic transducer array  1508 , define a chamber. In one embodiment, array  1508  includes PMUTs. In another embodiment, array  1508 , includes CMUTs. 
     The chamber is filled with an acoustic coupling material  1506 , such as such as an epoxy, PDMS, perfluorotripentylamine (e.g., Fluorinert™ FC-70), glass, metal, oil, gel, or various cyclic or linear siloxanes. In various embodiments, the acoustic coupling material is selected to have acoustic matching properties with the cover  1504 . In some embodiments, the acoustic coupling material  1506  has acoustic properties that act to minimize unwanted ultrasonic reflections at an interface between the acoustic coupling material  1506  and the cover. In some embodiments the cover  1504  can be directly used as a platen (e.g., platen  1216  of  FIG.  12   ) for an ultrasonic fingerprint sensor, while in other embodiments the cover can be attached to a separate platen (not shown) that includes one or more additional material layers. 
       FIG.  16    illustrates an example ultrasonic transducer array system  1600  including an array of ultrasonic transducers packaged in a chamber having a cover and including refractive acoustic elements, in accordance with an embodiment. System  1600  includes sidewalls  1602  and a cover  1604  that, together with substrate mounted ultrasonic transducer array  1608 , define a chamber. In one embodiment, array  1608  includes PMUTs. In another embodiment, array  1608  includes CMUTs. 
     The chamber is filled with an acoustic coupling material  1606 , such as such as an epoxy, PDMS, perfluorotripentylamine (e.g., Fluorinert™ FC-70), glass, metal, oil, gel, or various cyclic or linear siloxanes. In various embodiments, the acoustic coupling material  1606  is selected to have acoustic matching properties with the cover  1604 . In some embodiments, the acoustic coupling material  1606  has acoustic properties that act to minimize unwanted ultrasonic reflections at an interface between the acoustic coupling material and the cover. In some embodiments the cover  1604  can be directly used as a platen (e.g., platen  1216  of  FIG.  12   ) for an ultrasonic fingerprint sensor, while in other embodiments the cover can be attached to a separate platen (not shown) that includes one or more additional material layers. 
     System  1600  also includes refractive acoustic elements. In some embodiments, system  1600  includes refractive acoustic elements  1605  that are attached to or disposed within cover  1604 . In some embodiments, system  1600  includes refractive acoustic elements  1610  that are disposed within (e.g., suspended) acoustic coupling material  1606 . In some embodiments, system  1600  includes refractive acoustic elements  1612  that are attached to or disposed adjacent to ultrasonic transducers of array  1608 . 
     In accordance with various embodiments, the acoustic refractive elements  1605 ,  1610  and  1612  can be additively or subtractively formed, can be of various sizes, and can completely or partially match or be associated with ultrasonic transducers in the array  1608 . For example, curved elements, lens elements, wedge elements, waveguides, or other geometric acoustic refractive elements can be used. Proper selection of material properties such as density, heterogeneous acoustic wave propagation characteristics, or anomalous acoustic wave propagation using metamaterials or the like can also be used to redirect ultrasonic signals. In certain embodiments, selected ultrasonic transducers in the array  1608  may only be used either for transmission or reception of ultrasonic signals, with type, size, and position of the acoustic refractive elements being selected to enhance efficiency of the system  1600 . It should be appreciated that acoustic refractive elements  1605 ,  1610  and  1612  can be comprised of any material having different acoustic transmission properties than acoustic coupling material  1606 . 
     The refractive acoustic elements are used for refractive beamforming, e.g., to focus a transmit beam to a desired point. For instance, one or more refractive acoustic elements may be used to focus an ultrasonic signal to a point at or near the top of a platen (e.g., the contact point between a finger and a fingerprint sensing system. However, it should be appreciated that the refractive acoustic elements form a focal point at other depths (e.g., at a point above the platen for sensing into a finger rather than just the surface of the finger). It should be appreciated that refractive acoustic elements may be located in various locations. For instance, refractive acoustic elements may be located in the cover, suspended in the acoustic coupling material, and/or adjacent to the array of ultrasonic transducers in any combination. 
       FIG.  17    illustrates an example ultrasonic transducer array system  1700  including an array of ultrasonic transducers packaged in a chamber having a curved cover  1704 , in accordance with an embodiment. Array system  1700  may be well suited, for example, for mounting on an edge of a device. System  1700  includes sidewalls  1702  and a cover  1704  that, together with substrate mounted ultrasonic transducer array  1708 , define a chamber. In one embodiment, array  1708  includes PMUTs. In another embodiment, array  1708 , includes CMUTs. 
     The chamber is filled with an acoustic coupling material  1706 , such as such as an epoxy, PDMS, perfluorotripentylamine (e.g., Fluorinert™ FC-70), glass, metal, oil, gel, or various cyclic or linear siloxanes. In various embodiments, the acoustic coupling material is selected to have acoustic matching properties with the cover  1704 . In some embodiments, the acoustic coupling material  1706  has acoustic properties that act to minimize unwanted ultrasonic reflections at an interface between the acoustic coupling material  1706  and the cover. In some embodiments the cover  1704  can be directly used as a platen (e.g., platen  1216  of  FIG.  12   ) for an ultrasonic fingerprint sensor, while in other embodiments the cover can be attached to a separate platen (not shown) that includes one or more additional material layers. 
     Cover  1704  is curved such that acoustic coupling material  1706  is thicker at a midpoint of the array  1708  of ultrasonic transducers than towards an edge (e.g., towards a sidewall  1702 ) of the array  1708  of ultrasonic transducers. In one embodiment, cover  1704  has a varying thickness, with a greater thickness near the sidewalls  1702 . This can allow for improvement in ultrasonic transducer efficiency for those ultrasonic transducers situated near the edge of array  1708 . In some embodiments, an asymmetric phase delay pattern for edge situated ultrasonic transducers can be used, causing formation of an ultrasonic beam focused at a point above and outside the boundaries of the array  1708 . In another embodiment, cover  1704  has a varying thickness, with a greater thickness near the center of array  1708 , such that cover  1704  is thinner at a midpoint of the cover and thicker towards the sidewalls  1702 . The acoustic refractive characteristics of cover  1704  can aid in detection of reflected ultrasonic signals from that focal point outside the boundaries of the array. 
       FIGS.  18 A-C  illustrate example packaging configurations including relief mechanisms, in accordance with various embodiments. As illustrated in  FIG.  18 A , a package  1800  for ultrasonic transducer array system  1805  includes a cover  1802  having a relief channel  1804  to allow expulsion of excess acoustic coupling material contained in a cavity. In accordance with various embodiments, the relief channel  1804  can be a groove defined or etched in either or both the cover and top of the sidewalls  1806 . In the illustrated embodiment, four relief channels situated at corners of the sidewalls are shown. However, it should be appreciated that the relief channels may be located in different positions or greater or lesser numbers of relief channels can be used. 
     In various embodiments, during manufacture, the cavity is first filled with the acoustic coupling material. Cover  1802  is then placed atop sidewalls  1806 . Excess acoustic coupling material may be expelled via relief channels  1804 , for example, as a result of cover  1802  being placed or during curing of the acoustic coupling material that might result in expansion of the acoustic coupling material. It should be appreciated that in accordance with various embodiments, after fixing the cover  1802  in place, or upon completion or partial completion of curing of the acoustic coupling material, relief channels  1804  can be sealed with a suitable epoxy or similar adhesive. 
     Similarly,  FIG.  18 B  illustrates a package  1810  for ultrasonic transducer array system  1815  that includes a cover  1812  having at least one relief channel  1814  to allow expulsion of excess acoustic coupling material contained in a cavity. In one embodiment, relief channels  1814  are vertically etched into cover  1812 . In accordance with various embodiments, the relief channel  1814  can be a hole or opening in cover  1812 . In the illustrated embodiment, four relief channels  1814  situated at corners of cover  1812 . However, it should be appreciated that the relief channels may be located in different positions or greater or lesser numbers of relief channels can be used. 
     In various embodiments, during manufacture, the cavity is first filled with the acoustic coupling material. Cover  1812  is then placed atop sidewalls  1816 . Excess acoustic coupling material may be expelled via relief channels  1814 , for example, as a result of cover  1812  being placed or during curing of the acoustic coupling material that might result in expansion of the acoustic coupling material. It should be appreciated that in accordance with various embodiments, after fixing the cover  1812  in place, or upon completion or partial completion of curing of the acoustic coupling material, relief channels  1814  can be sealed with a suitable epoxy or similar adhesive. 
     In yet another embodiment shown in  FIG.  18 C , package  1820  for ultrasonic transducer array system  1825  includes a cover  1822  and a relief channel  1824  situated in the sidewall  1826  of the package  1820  to allow expulsion of excess acoustic coupling material contained in a cavity. In accordance with various embodiments, the relief channel  1824  can be an opening in sidewall  1826 . In the illustrated embodiment, four relief channels  1824  situated at midpoints of sidewalls  1826  are shown. However, it should be appreciated that the relief channels may be located in different positions or greater or lesser numbers of relief channels can be used. 
     In various embodiments, during manufacture, the cavity is first filled with the acoustic coupling material. Cover  1822  is then placed atop sidewalls  1826 . Excess acoustic coupling material may be expelled via relief channels  1824 , for example, as a result of cover  1822  being placed or during curing of the acoustic coupling material that might result in expansion of the acoustic coupling material. It should also be appreciated that multiple packages  1820  may be arranged adjacently, such that acoustic coupling material flows through multiple packages  1820 , such that excess acoustic coupling material is expelled from all packages via relief channels  1824  of outer packages  1820 . It should be appreciated that in accordance with various embodiments, after fixing the cover  1822  in place, or upon completion or partial completion of curing of the acoustic coupling material, relief channels  1824  can be sealed with a suitable epoxy or similar adhesive. 
     In various embodiments, the use of relief channel(s) allows fingerprint sensors using ultrasonic transducer arrays to be filled with acoustic coupling material just prior to attachment to a fingerprint sensor platen. Alternatively, edge mounted sensors with curved glass or metal covers can be more easily assembled. In some embodiments, mobile computers or telecommunication devices that include touchscreen glass or plastic display device can have a fingerprint sensor assembled under the display device, with acoustic coupling material (e.g., gel) being injected into the cavity through the grooves, perforations, or channels after attachment. 
       FIGS.  19 A-D  illustrate example mobile devices including an array of ultrasonic transducers in various locations, in accordance with various embodiments. As illustrated in  FIGS.  19 A-D , example mobile devices that incorporate a ultrasonic transducer array in non-traditional locations are shown. Non-traditional placement is possible because ultrasonic transducers can be tuned to penetrate intermediate layers, which are layers that reside between the sensor (e.g., ultrasonic transducer array) and the element sensed (e.g., a finger). There are limitations with alternative fingerprint sensing technologies. Optical-based detection requires line-of-sight between the sensor and the sensed element. Any intermediate layer must be optically transparent, so most opaque materials are not a packaging option. Capacitive-based detection requires line-of-touch between the sensor and element sensed. Any intermediate layer is part of the electrical circuit and places limitations on the material choice of the packaging and the use of non-planar designs. 
     Accordingly, it is possible to locate the array of ultrasonic transducers in non-traditional locations for the mobile device.  FIG.  19 A  shows an example mobile device  1900  where the cover  1910  can be a platen mounted on an edge  1920  of mobile device  1900  and touchable by a finger  1930 . In one embodiment, edge  1920  is perpendicular to a top surface (e.g., the surface with the display device) of mobile device  1900 . In the present embodiment, ultrasonic transducer array system  1940  has a rectangular layout with substantially more columns than rows, so that mobile device  1900  can be as thin as possible while still providing sufficient area for the device to provide acceptable fingerprint recognition.  FIG.  19 B  shows a close-up of mobile device  1900  and ultrasonic transducer array system  1940 , where edge  1920  is planar, or substantially planar. 
       FIG.  19 C  shows a close-up of mobile device  1900  and ultrasonic transducer array system  1940  where edge  1920  is curved. As illustrated, edge  1920  is a curved surface having a curvature, such that a surface of ultrasonic transducer array system  1940  is curved to match the curvature of the curved surface. In one embodiment, the acoustic coupling layer of ultrasonic transducer array system  1940  is curved to match the curvature of the curved surface. In another embodiment, the cover and/or platen of ultrasonic transducer array system  1940  is curved to match the curvature of the curved surface. 
       FIG.  19 D  shows a close-up of mobile device  1900  and ultrasonic transducer array system  1940  where ultrasonic transducer array system  1940  is incorporated beneath a physical button  1980  that is disposed in edge  1920 . In one embodiment, physical button  1980  is proximate to cover  1910 . In another embodiment, physical button  1980  is cover  1910 . 
     It will be appreciated that the cover or platen can be made of glass, metal, plastic, or composites among other materials. Other techniques known in ultrasound can be used to enhance propagation, or to deter losses of reflections, through any such intermediate or aberrating layers that are part of the device and through which the ultrasonic transducers communicate with a finger. Beamforming, beam steering, and similar techniques for control of the ultrasonic transducers may also be used to enhance further the capability of the ultrasonic transducers with regard to a known device design and related packaging constraints. It will be appreciated that the mobile device may be any portable computing device such as a smart phone, tablet computer, laptop computer, or wearable device. It is also possible to apply the invention to other security applications—unlocking traditional doors or vehicle doors, lock box, ATM or other security access, etc. 
     What has been described above includes examples of the subject disclosure. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the subject matter, but it is to be appreciated that many further combinations and permutations of the subject disclosure are possible. Accordingly, the claimed subject matter is intended to embrace all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims. 
     In particular and in regard to the various functions performed by the above described components, devices, circuits, systems and the like, the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (e.g., a functional equivalent), even though not structurally equivalent to the disclosed structure, which performs the function in the herein illustrated exemplary aspects of the claimed subject matter. 
     The aforementioned systems and components have been described with respect to interaction between several components. It can be appreciated that such systems and components can include those components or specified sub-components, some of the specified components or sub-components, and/or additional components, and according to various permutations and combinations of the foregoing. Sub-components can also be implemented as components communicatively coupled to other components rather than included within parent components (hierarchical). Additionally, it should be noted that one or more components may be combined into a single component providing aggregate functionality or divided into several separate sub-components. Any components described herein may also interact with one or more other components not specifically described herein. 
     In addition, while a particular feature of the subject innovation may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “includes,” “including,” “has,” “contains,” variants thereof, and other similar words are used in either the detailed description or the claims, these terms are intended to be inclusive in a manner similar to the term “comprising” as an open transition word without precluding any additional or other elements. 
     Thus, the embodiments and examples set forth herein were presented in order to best explain various selected embodiments of the present invention and its particular application and to thereby enable those skilled in the art to make and use embodiments of the invention. 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 of the invention to the precise form disclosed.