Patent Publication Number: US-11392249-B2

Title: Broadband ultrasonic sensor

Description:
TECHNICAL FIELD 
     This disclosure relates to ultrasonic transducer imaging array and, more particularly to techniques for providing a broadband ultrasonic imaging array. 
     DESCRIPTION OF THE RELATED TECHNOLOGY 
     Ultrasonic sensor systems may use a transmitter to generate and send an ultrasonic wave through a transmissive medium and towards an object to be detected and/or imaged. The ultrasonic transmitter may be operatively coupled with an ultrasonic sensor array configured to detect portions of the ultrasonic wave that are reflected from the object. At each material interface encountered buy the ultrasonic pulse, a portion of the ultrasonic pulse may be reflected. In some implementations, an ultrasonic pulse may be produced by starting and stopping the transmitter during a short interval of time (e.g. less than 1 microsecond). An ultrasonic sensor system may include biometric sensors, such as fingerprint or handprint sensors, and/or other ultrasonic imaging applications. 
     Piezoelectric ultrasonic transducers are attractive candidates for such applications and may be configured as a multilayer stack that includes a piezoelectric layer. The piezoelectric layer may convert vibrations caused by ultrasonic reflections into electrical output signals. In some implementations, the ultrasonic sensor system further includes a thin-film transistor (TFT) layer that may include an array of sensor pixel circuits that may, for example, amplify electrical output signals generated by the piezoelectric layer. The piezeoelectric layer may include one or more of lead zirconate titanate (PZT), single crystal lead magnesium niobate-lead titanate (PMN-PT), a PZT ceramic, polyvinylidene fluoride (PVDF), poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) and/or another PVDF copolymer. 
     In some applications, a two-dimensional array of a large number of transducer elements may be integrated with and disposed behind or “under” a platen (a “cover plate” or “cover glass”) configured as a display screen with which the user interacts. The display screen, for example, may provide a user touch interface and/or be incorporated in a personal electronic device such as a mobile phone or tablet and may include multi-layer stacks of glass, plastic and/or adhesive layers. 
     The piezoelectric ultrasonic transducer may be required to accommodate a variety of multi-layer stack configurations thickness and material properties, some of which may change during the life of the personal electronic device as a result, for example, of installation, removal or replacement of a screen protector. As a result, a piezoelectric ultrasonic transducer configuration exhibiting good performance over a broader range of frequencies is desirable. 
     SUMMARY 
     The systems, methods and devices of this disclosure each have several innovative aspects, no single one of which is solely responsible for the desirable attributes disclosed herein. 
     One innovative aspect of the subject matter described in this disclosure relates to an ultrasonic sensor including a substrate, a platen, and an acoustic stack disposed between the substrate and the platen, including at least one piezoelectric layer. The ultrasonic sensor is configured to exhibit a signal-to-noise ratio (SNR) of at least 4 over a frequency range of at least 9 to 16 MHz. 
     In some examples, the ultrasonic sensor may further include an OLED stack, the OLED stack and the platen each exhibiting an approximately similar acoustic impedance. In some examples, the OLED stack may include a multilayer structure including at least two of a polarizer, an OLED layer, and a touchscreen layer. 
     In some examples, the platen may include a polycarbonate (PC) layer. In some examples, the platen may include a multilayer structure including the PC layer and a poly methacrylate layer. 
     In some examples, the sensor may be configured to operate with ultrasonic waves having a characteristic wavelength and the platen may have a thickness less than ¼ of the characteristic wavelength. In some examples, the thickness may be less than 1/10 of the characteristic wavelength. 
     In some examples, the piezoelectric layer may include one or more of lead zirconate titanate (PZT), single crystal lead magnesium niobate-lead titanate (PMN-PT), a PZT ceramic, polyvinylidene fluoride (PVDF), poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) and/or another PVDF copolymer. 
     In some examples, the piezoelectric layer may include a thin-film transistor (TFT) layer. In some examples, the TFT layer may have a thickness less than 300 In some examples, the TFT layer may be configured to resonate at one or more frequencies in the range of 5-20 MHz. In some examples, the sensor may be configured to operate with ultrasonic waves having a characteristic wavelength and the acoustic stack may have a total thickness less than ½ the characteristic wavelength. In some examples, the acoustic stack may have a total thickness in a range of 90-200 In some examples, the ultrasonic sensor may further include a spacer disposed between the acoustic stack and the platen. In some examples, the sensor may be configured to operate with ultrasonic waves having a characteristic wavelength and a combined thickness of the acoustic stack and the spacer may be less than ½ the characteristic wavelength. In some examples, the combined thickness is in a range of 90-200 μm. 
     In some examples, the platen includes a cover glass layer and the acoustic stack comprises a TFT layer. In some examples, the ultrasonic sensor further includes a spacer and an adhesive layer disposed between the cover glass layer and the TFT layer, or behind the TFT layer. In some examples, the adhesive layer may be a pressure sensitive adhesive (PSA) or optical clear adhesive (OCA). 
     In some examples, the sensor may be configured to exhibit SNR of at least 4 over a frequency range of at least 5-20 MHz. 
     According to some implementations, an ultrasonic sensor includes a substrate, a platen, and an acoustic stack disposed between the substrate and the platen, including at least one piezoelectric layer. The piezoelectric layer includes a thin-film transistor (TFT) layer, the sensor is configured to operate with ultrasonic waves having a characteristic wavelength, and the acoustic stack has a total thickness less than ½ the characteristic wavelength. 
     In some examples, the ultrasonic sensor may be configured to exhibit a signal-to-noise ratio (SNR) of at least 4 over a frequency range of at least 9 to 16 MHz. In some examples, the ultrasonic sensor may further include an OLED stack, the OLED stack and the platen each exhibiting an approximately similar acoustic impedance. In some examples, the OLED stack may be a multilayer structure including at least two of a polarizer, an OLED layer, a touchscreen layer. In some examples, the platen may have a thickness less than ¼ of the characteristic wavelength. In some examples, the piezoelectric layer may include lead zirconate titanate (PZT), single crystal lead magnesium niobate-lead titanate (PMN-PT), a PZT ceramic, polyvinylidene fluoride (PVDF), poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) and/or another PVDF copolymer. In some examples, the TFT layer may have a thickness less than 300 μm. In some examples, the TFT layer may be configured to resonate at one or more frequencies in the range of 5-20 MHz. In some examples, the ultrasonic sensor may further include a spacer disposed between the acoustic stack and the platen. In some examples, a combined thickness of the acoustic stack and the spacer may be less than ½ the characteristic wavelength. In some examples, the combined thickness may be in a range of 90-200 In some examples, the platen may include a cover glass layer. In some examples, the ultrasonic sensor may further include a spacer and an adhesive layer disposed between the cover glass layer and the TFT layer. In some examples, the adhesive layer may be a pressure sensitive adhesive (PSA) or optical clear adhesive (OCA). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Details of one or more implementations of the subject matter described in this specification are set forth in this disclosure and the accompanying drawings. Other features, aspects, and advantages will become apparent from a review of the disclosure. Note that the relative dimensions of the drawings and other diagrams of this disclosure may not be drawn to scale. The sizes, thicknesses, arrangements, materials, etc., shown and described in this disclosure are made only by way of example and should not be construed as limiting. Like reference numbers and designations in the various drawings indicate like elements. 
         FIG. 1  shows a front view of a diagrammatic representation of an example of an electronic device that includes an ultrasonic sensing system according to some implementations. 
         FIG. 2A  shows a block diagram representation of components of an example of an ultrasonic sensing system, according to some implementations. 
         FIG. 2B  shows a block diagram representation of components of an example of an electronic device, according to some implementations. 
         FIGS. 3A-3C  show cross-sectional views of examples of an ultrasonic sensing system, according to some implementations. 
         FIG. 4  illustrates an example of range gate delay (RGD) and range gate window (RGW). 
         FIG. 5A  illustrates plots of signal-to-noise ratio as a function of range gate delay (RGD), according to the prior art. 
         FIG. 5B  illustrates plots of signal-to-noise ratio as a function of range gate delay (RGD), according to an implementation. 
         FIG. 6  illustrates an example of adjusting the RGD and performing a background calibration, according to some implementations. 
         FIG. 7  illustrates a test target template and corresponding received image associated with determination of sensor performance. 
         FIG. 8  illustrates examples of broadband ultrasonic sensors, according to some implementations. 
         FIG. 9  illustrates an example of a broadband ultrasonic sensor, according to some implementations. 
         FIG. 10  presents example performance results for a number of candidate implementations. 
         FIGS. 11A-11C  present example performance results for three additional candidate implementations. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is directed to certain implementations for the purposes of describing the innovative aspects of this disclosure. However, a person having ordinary skill in the art will readily recognize that the teachings herein may be applied in a multitude of different ways. The described implementations may be implemented in any device, apparatus, or system that includes a sensor system. In addition, it is contemplated that the described implementations may be included in or associated with a variety of electronic devices such as, but not limited to: mobile telephones, multimedia Internet enabled cellular telephones, mobile television receivers, wireless devices, smartphones, smart cards, wearable devices such as bracelets, armbands, wristbands, rings, headbands and patches, etc., Bluetooth® devices, personal data assistants (PDAs), wireless electronic mail receivers, hand-held or portable computers, netbooks, notebooks, smartbooks, tablets, printers, copiers, scanners, facsimile devices, global positioning system (GPS) receivers/navigators, cameras, digital media players (such as MP3 players), camcorders, game consoles, wrist watches, clocks, calculators, television monitors, flat panel displays, electronic reading devices (e.g., e-readers), mobile health devices, computer monitors, auto displays (including odometer and speedometer displays, etc.), cockpit controls and/or displays, steering wheels, camera view displays (such as the display of a rear view camera in a vehicle), electronic photographs, electronic billboards or signs, projectors, architectural structures, microwaves, refrigerators, stereo systems, cassette recorders or players, DVD players, CD players, VCRs, radios, portable memory chips, washers, dryers, washer/dryers, automated teller machines (ATMs), parking meters, packaging (such as in electromechanical systems (EMS) applications including microelectromechanical systems (MEMS) applications, as well as non-EMS applications), aesthetic structures (such as display of images on a piece of jewelry or clothing) and a variety of EMS devices. The teachings herein also may be used in applications such as, but not limited to, electronic switching devices, radio frequency filters, sensors, accelerometers, gyroscopes, motion-sensing devices, magnetometers, inertial components for consumer electronics, parts of consumer electronics products, varactors, liquid crystal devices, electrophoretic devices, drive schemes, manufacturing processes and electronic test equipment. Thus, the teachings are not intended to be limited to the implementations depicted solely in the Figures, but instead have wide applicability as will be readily apparent to one having ordinary skill in the art. 
     In some implementations, ultrasonic sensor systems include piezoelectric material for the transmission and receiving of ultrasonic waves. 
     For example, a voltage applied across piezoelectric material corresponding to a transmitter may result in the piezoelectric material stretching or contracting, e.g., being deformed such that the material is strained in response to the applied voltage, resulting in the generation of the ultrasonic wave, as previously discussed. The reflected signals (e.g., the reflected portions of the ultrasonic wave, as previously discussed) may result in the stretching or contracting of piezoelectric material corresponding to a receiver. This results in the generation of a surface charge, and therefore, a voltage across the piezoelectric material that may be used as an electrical output signal representing a portion of raw image data that represents fingerprint image data. 
     Some implementations of the subject matter described in this disclosure may be practiced to realize a broadband ultrasonic sensor offering one or more of the following potential advantages: a high signal-to-noise ratio over an expanded frequency range obviates or reduces the need for sensor tuning and accommodates a variety of user installed screen protectors and may be configured to obtain sub-dermis imaging as well as fingerprint imaging. 
       FIG. 1  shows a front view of a diagrammatic representation of an example of an electronic device  100  that includes an ultrasonic sensing system according to some implementations. The electronic device  100  may be representative of, for example, various portable computing devices such as cellular phones, smartphones, multimedia devices, personal gaming devices, tablet computers and laptop computers, among other types of portable computing devices. However, various implementations described herein are not limited in application to portable computing devices. Indeed, various techniques and principles disclosed herein may be applied in traditionally non-portable devices and systems, such as in computer monitors, television displays, kiosks, vehicle navigation devices and audio systems, among other applications. 
     In the illustrated implementation, the electronic device  100  includes a housing (or “case”)  102  within which various circuits, sensors and other electrical components may be disposed. In the illustrated implementation, the electronic device  100  also includes a display (that may be referred to herein as a “touchscreen display” or a “touch-sensitive display”)  104 . The display  104  may generally be representative of any of a variety of suitable display types that employ any of a variety of suitable display technologies. For example, the display  104  may be a digital micro-shutter (DMS)-based display, a light-emitting diode (LED) display, an organic LED (OLED) display, a liquid crystal display (LCD), an LCD display that uses LEDs as backlights, a plasma display, an interferometric modulator (IMOD)-based display, or another type of display suitable for use in conjunction with touch-sensitive user interface (UI) systems. 
     The electronic device  100  may include various other devices or components for interacting with, or otherwise communicating information to or receiving information from, a user. For example, the electronic device  100  may include one or more microphones  106 , one or more speakers  108 , and in some cases one or more at least partially mechanical buttons  110 . The electronic device  100  may include various other components enabling additional features such as, for example, one or more video or still-image cameras  112 , one or more wireless network interfaces  114  (for example, Bluetooth, WiFi or cellular) and one or more non-wireless interfaces  116  (for example, a universal serial bus (USB) interface or an HDMI interface). 
     The electronic device  100  may include an ultrasonic sensing system  118  capable of imaging an object signature, such as a fingerprint, palm print or handprint. In some implementations, the ultrasonic sensing system  118  may function as a touch-sensitive control button. In some implementations, a touch-sensitive control button may be implemented with a mechanical or electrical pressure-sensitive system that is positioned under or otherwise integrated with the ultrasonic sensing system  118 . In other words, in some implementations, a region occupied by the ultrasonic sensing system  118  may function both as a user input button to control the electronic device  100  as well as a sensor to enable security features such as user authentication based on, for example, a fingerprint, palm print or handprint. 
       FIG. 2A  shows a block diagram representation of components of an example of an ultrasonic sensing system, according to some implementations. In the illustrated implementation, an ultrasonic sensing system  200  includes a sensor system  202  and a control system  204  electrically coupled with the sensor system  202 . The sensor system  202  may be capable of scanning a target object and providing raw measured image data usable to obtain an object signature of, for example, a human appendage, such as one or more fingers or toes, a palm, hand or foot. The control system  204  may be capable of controlling the sensor system  202  and processing the raw measured image data received from the sensor system  202 . In some implementations, the ultrasonic sensing system  200  may include an interface system  206  capable of transmitting or receiving data, such as raw or processed measured image data, to or from various components within or integrated with the ultrasonic sensing system  200  or, in some implementations, to or from various components, devices or other systems external to the ultrasonic sensing system  200 . 
       FIG. 2B  shows a block diagram representation of components of an example of an electronic device, according to some implementations. In the illustrated example, an electronic device  210  includes the ultrasonic sensing system  200  of  FIG. 2A . For example, the electronic device  210  may be a block diagram representation of the electronic device  100  shown in and described with reference to  FIG. 1  above. The sensor system  202  of the ultrasonic sensing system  200  of the electronic device  210  may be implemented with an ultrasonic sensor array  212 . The control system  204  of the ultrasonic sensing system  200  may be implemented with a controller  214  that is electrically coupled with the ultrasonic sensor array  212 . While the controller  214  is shown and described as a single component, in some implementations, the controller  214  may collectively refer to two or more distinct control units or processing units in electrical communication with one another. In some implementations, the controller  214  may include one or more of a general purpose single- or multi-chip processor, a central processing unit (CPU), a digital signal processor (DSP), an applications processor, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device (PLD), discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions and operations described herein. 
     The ultrasonic sensing system  200  of  FIG. 2B  may include an image processing module  218 . In some implementations, raw measured image data provided by the ultrasonic sensor array  212  may be sent, transmitted, communicated or otherwise provided to the image processing module  218 . The image processing module  218  may include any suitable combination of hardware, firmware and software configured, adapted or otherwise operable to process the image data provided by the ultrasonic sensor array  212 . In some implementations, the image processing module  218  may include signal or image processing circuits or circuit components including, for example, amplifiers (such as instrumentation amplifiers or buffer amplifiers), analog or digital mixers or multipliers, switches, analog-to-digital converters (ADCs), passive or active analog filters, among others. In some implementations, one or more of such circuits or circuit components may be integrated within the controller  214 , for example, where the controller  214  is implemented as a system-on-chip (SoC) or system-in-package (SIP). In some implementations, one or more of such circuits or circuit components may be integrated within a DSP included within or coupled with the controller  214 . In some implementations, the image processing module  218  may be implemented at least partially via software. For example, one or more functions of, or operations performed by, one or more of the circuits or circuit components just described may instead be performed by one or more software modules executing, for example, in a processing unit of the controller  214  (such as in a general purpose processor or a DSP). 
     In some implementations, in addition to the ultrasonic sensing system  200 , the electronic device  210  may include a separate processor  220 , a memory  222 , an interface  216  and a power supply  224 . In some implementations, the controller  214  of the ultrasonic sensing system  200  may control the ultrasonic sensor array  212  and the image processing module  218 , and the processor  220  of the electronic device  210  may control other components of the electronic device  210 . In some implementations, the processor  220  communicates data to the controller  214  including, for example, instructions or commands. In some such implementations, the controller  214  may communicate data to the processor  220  including, for example, raw or processed image data. It should also be understood that, in some other implementations, the functionality of the controller  214  may be implemented entirely, or at least partially, by the processor  220 . In some such implementations, a separate controller  214  for the ultrasonic sensing system  200  may not be required because the functions of the controller  214  may be performed by the processor  220  of the electronic device  210 . 
     Depending on the implementation, one or both of the controller  214  and processor  220  may store data in the memory  222 . For example, the data stored in the memory  222  may include raw measured image data, filtered or otherwise processed image data, estimated PSF or estimated image data, and final refined PSF or final refined image data. The memory  222  may store processor-executable code or other executable computer-readable instructions capable of execution by one or both of the controller  214  and the processor  220  to perform various operations (or to cause other components such as the ultrasonic sensor array  212 , the image processing module  218 , or other modules to perform operations), including any of the calculations, computations, estimations or other determinations described herein (including those presented in any of the equations below). It should also be understood that the memory  222  may collectively refer to one or more memory devices (or “components”). For example, depending on the implementation, the controller  214  may have access to and store data in a different memory device than the processor  220 . In some implementations, one or more of the memory components may be implemented as a NOR- or NAND-based Flash memory array. In some other implementations, one or more of the memory components may be implemented as a different type of non-volatile memory. Additionally, in some implementations, one or more of the memory components may include a volatile memory array such as, for example, a type of RAM. 
     In some implementations, the controller  214  or the processor  220  may communicate data stored in the memory  222  or data received directly from the image processing module  218  through an interface  216 . For example, such communicated data can include image data or data derived or otherwise determined from image data. The interface  216  may collectively refer to one or more interfaces of one or more various types. In some implementations, the interface  216  may include a memory interface for receiving data from or storing data to an external memory such as a removable memory device. Additionally or alternatively, the interface  216  may include one or more wireless network interfaces or one or more wired network interfaces enabling the transfer of raw or processed data to, as well as the reception of data from, an external computing device, system or server. 
     A power supply  224  may provide power to some or all of the components in the electronic device  210 . The power supply  224  may include one or more of a variety of energy storage devices. For example, the power supply  224  may include a rechargeable battery, such as a nickel-cadmium battery or a lithium-ion battery. Additionally or alternatively, the power supply  224  may include one or more supercapacitors. In some implementations, the power supply  224  may be chargeable (or “rechargeable”) using power accessed from, for example, a wall socket (or “outlet”) or a photovoltaic device (or “solar cell” or “solar cell array”) integrated with the electronic device  210 . Additionally or alternatively, the power supply  224  may be wirelessly chargeable. 
     As used hereinafter, the term “processing unit” refers to any combination of one or more of a controller of an ultrasonic system (for example, the controller  214 ), an image processing module (for example, the image processing module  218 ), or a separate processor of a device that includes the ultrasonic system (for example, the processor  220 ). In other words, operations that are described below as being performed by or using a processing unit may be performed by one or more of a controller of the ultrasonic system, an image processing module, or a separate processor of a device that includes the ultrasonic sensing system. 
       FIG. 3A  shows a cross-sectional of an example of an ultrasonic sensing system according to some implementations.  FIG. 3B  shows an enlarged cross-sectional side view of the ultrasonic sensing system of  FIG. 3A  according to some implementations. In the illustrated example, the ultrasonic sensing system  300  may implement the ultrasonic sensing system  118  described with reference to  FIG. 1  or the ultrasonic sensing system  200  shown and described with reference to  FIGS. 2A and 2B . The ultrasonic sensing system  300  may include an ultrasonic transducer  302  that overlies a substrate  304  and that underlies a platen (a “cover plate” or “cover glass”)  306 . The ultrasonic transducer  302  may include both an ultrasonic transmitter  308  and an ultrasonic receiver  310 . 
     The ultrasonic transmitter  308  may be configured to generate ultrasonic waves towards the platen  306 , and a target object  312  positioned on the upper surface of the platen  306 . In the illustrated implementation the object  312  is depicted as finger, but any appendage or body part may be contemplated by the present techniques, as well as any other natural or artificial object. In some implementations, the ultrasonic transmitter  308  may more specifically be configured to generate ultrasonic plane waves towards the platen  306 . In some implementations, the ultrasonic transmitter  308  includes a layer of piezoelectric material such as, for example, polyvinylidene fluoride (PVDF) or a PVDF copolymer such as poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE). For example, the piezoelectric material of the ultrasonic transmitter  308  may be configured to convert electrical signals provided by the controller of the ultrasonic sensing system into a continuous or pulsed sequence of ultrasonic plane waves at a scanning frequency. In some implementations, the ultrasonic transmitter  308  may additionally or alternatively include capacitive ultrasonic devices. 
     The ultrasonic receiver  310  may be configured to detect ultrasonic reflections  314  resulting from interactions of the ultrasonic waves transmitted by the ultrasonic transmitter  308  with ridges  316  and valleys  318  defining surface texture of the target object  312  being scanned. In some implementations, the ultrasonic transmitter  308  overlies the ultrasonic receiver  310  as, for example, illustrated in  FIGS. 3A and 3B . In some other implementations, the ultrasonic receiver  310  may overlie the ultrasonic transmitter  308 . The ultrasonic receiver  310  may be configured to generate and output electrical output signals corresponding to the detected ultrasonic reflections. In some implementations, the ultrasonic receiver  310  may include a second piezoelectric layer different than the piezoelectric layer of the ultrasonic transmitter  308 . For example, the piezoelectric material of the ultrasonic receiver  310  may be any suitable piezoelectric material such as, for example, a layer of PVDF or a PVDF copolymer. The piezoelectric layer of the ultrasonic receiver  310  may convert vibrations caused by the ultrasonic reflections into electrical output signals. In some implementations, the ultrasonic receiver  310  further includes a thin-film transistor (TFT) layer. In some such implementations, the TFT layer may include an array of sensor pixel circuits configured to amplify the electrical output signals generated by the piezoelectric layer of the ultrasonic receiver  310 . The amplified electrical signals provided by the array of sensor pixel circuits may then be provided as raw measured image data to the processing unit for use in processing the image data, identifying a fingerprint associated with the image data, and in some applications, authenticating a user associated with the fingerprint. In some implementations, a single piezoelectric layer may serve as the ultrasonic transmitter  308  and the ultrasonic receiver  310 . In some implementations, the substrate  304  may be a glass, plastic or silicon substrate upon which electronic circuitry may be fabricated. In some implementations, an array of sensor pixel circuits and associated interface circuitry of the ultrasonic receiver  310  may be configured from CMOS circuitry formed in or on the substrate  304 . In some implementations, the substrate  304  may be positioned between the platen  306  and the ultrasonic transmitter  308  and/or the ultrasonic receiver  310 . In some implementations, the substrate  304  may serve as the platen  306 . One or more protective layers, acoustic matching layers, anti-smudge layers, adhesive layers, decorative layers, conductive layers or other coating layers (not shown) may be included on one or more sides of the substrate  304  and the platen  306 . 
     The platen  306  may be formed of any suitable material that may be acoustically coupled with the ultrasonic transmitter  308 . For example, the platen  306  may be formed of one or more of glass, plastic, ceramic, sapphire, metal or metal alloy. In some implementations, the platen  306  may be a cover plate such as, for example, a cover glass or a lens glass of an underlying display. In some implementations, the platen  306  may include one or more polymers, such as one or more types of parylene, and may be substantially thinner. In some implementations, the platen  306  may have a thickness in the range of about 10 microns (μm) to about 1000 μm or more. In some implementations, the platen  306  may be configured as a multilayer stack of multi-layer stacks of glass, plastic and/or adhesive layers. 
     As illustrated in  FIGS. 3A and 3B , the target object  312  is in direct contact with the platen  306 . However, in some implementations a screen protector may be disposed over the platen  306 .  FIG. 3C  illustrates an implementation in which a screen protector  3000  is disposed above the platen  306 . Such a screen protector may be installed (or removed) by a user or third party after factory calibration of the ultrasonic sensing system  300 . 
     In order to accommodate variations in thickness and material properties of the platen (plus, in some instances screen protector), the ultrasonic transducers, advantageously, may be operable at a variety of values of range gate delay. 
       FIG. 4  illustrates an example of range gate delay as the term is used herein. More particularly,  FIG. 4  graphically illustrates an example of transmitter excitation signals and receiver bias voltage levels as a function of time. The transmitter excitation signals (upper graph) may be provided to an ultrasonic transmitter, whereas the receiver bias voltage (lower graph) may be applied to an RBias electrode of an ultrasonic sensor element. One or more cycles of an ultrasonic transmitter excitation signal may be applied to the ultrasonic transmitter, as shown in the upper graph of  FIG. 4 . In some implementations, a single transmitter excitation cycle may be used. In some implementations, as illustrated, multiple excitation cycles may be used, such as two cycles, three cycles, four cycles, five cycles or more. The transmitter excitation signals in some implementations may be square waves, rectangular waves, partial waves, pulsed waves, multiple-frequency waves, chirped waves, low or high duty-cycle waves, variable-amplitude waves, variable-frequency waves, or other suitable waveform for driving an ultrasonic transmitter. During a first portion of time (“Tx Block”) when transmission of the outgoing ultrasonic wave is occurring, the bias voltage applied to the RBias electrode may correspond to a “block value” such that the receiver bias electrode prevents signals reflected from outgoing transmitted waves from being captured by a sensor pixel circuit. 
     During a subsequent portion of time (“Rx Sample”), the bias level of the control signal applied to the RBias electrode is set to a “sample value” and the reflected ultrasonic signals may be captured a sensor pixel. The Rx Sample period may start upon completion of the range gate delay (“RGD”) period. The RGD period may typically be in a range of 0.5-2 microseconds. The duration of the Rx sample period may be referred to as the range gate window (“RGW”) period. The RGW period may typically be less than one microsecond. In some implementations, the RGW period may be in the range of about 50 to 1000 nanoseconds. To prevent detection of unwanted internal reflections, the bias level applied to the receiver bias electrode may be brought back to the block value upon completion of the RGW period. The RGW period, in the illustrated implementation, may correspond to a time interval that is roughly similar to the period of a transmitter excitation cycle (“tone burst”). In other implementations, the RGW period may be shorter or longer than the period of the tone burst. During RGW period, the sensor pixel may be said to be in a “read mode” of operation. During or near the RGW period, the receiver may output signals, resulting from or corresponding to localized electrical charges generated by the piezoelectric receiver layer and collected by the pixel input electrodes. 
     In the absence of the presently disclosed techniques, an ultrasonic sensor may be operable (i.e., may exhibit a sufficiently high signal-to-noise ratio (SNR)) over a relatively narrow range of frequency versus RGD.  FIG. 5A  illustrates a contour plot of SNR as a function of frequency and RGD for an ultrasonic sensor configured according to the prior art. It may be observed that SNR is greater than four only in a region  503  that is generally within a frequency range of 10-12.5 MHz and a normalized RGD of 0.95 to 1.15. In a region  502 , SNR is in the range of 2-4, whereas, in a region  501 , SNR is less than 2. 
       FIG. 5B  illustrates a contour plot of SNR as a function of frequency and RGD, according to an implementation of the presently disclosed techniques. It may be observed that SNR is greater than four over a broad range of frequencies (at least, 9-16 MHz), over a normalized RGD of at least 0.8 to 1.4. In a region  602 , SNR is in the range of 2-4, whereas, in a region  601 , SNR is less than 2. Because the contour plot of  FIG. 6  demonstrates a desirably high value of SNR over a broad range of frequencies (at least 9-16 MHz), ultrasonic sensors exhibiting such performance are referred to herein as “broadband” sensors. Advantageously, such broadband sensors may substantially reduce a need for tuning of the sensor operating parameters such as frequency and RGD. In some implementations, the broadband sensor may operate satisfactorily without adjusting such parameters, whether or not a screen protector is installed removed or replaced. Advantageously the broadband sensor may be operable at a low frequency at which sub-dermis imaging may be performed. In some implementations, the broadband sensor may exhibit SNR&gt;4 over a range of frequencies of at least 5-20 MHz. 
     For the plots illustrated in  FIGS. 5A and 5B , and in the drawings, specification and claims, generally, SNR may be determined by using a test target template with light and dark regions similar to a pattern exhibited by a fingerprint and obtaining a measured test target image.  FIG. 6  illustrates an example of a test target template (Detail A) and an acquired test target image (Detail B). The template may be applied as a mask to the acquired image to extract pixel values within respective dark regions  641  and light regions  642 . SNR, as used herein, and in the claims, is the ratio of “signal” (defined as the difference in median pixel values of the light and dark regions) and “noise” (defined as the root-mean-square of the standard deviation, σ, of the light and dark regions); thus: 
     
       
         
           
             SNR 
             = 
             
               
                 
                   
                     ( 
                     
                       Median 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       dark 
                     
                     ) 
                   
                   - 
                   
                     ( 
                     
                       Median 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       light 
                     
                     ) 
                   
                 
                 
                   
                     
                       
                         σ 
                         Dark 
                         2 
                       
                       + 
                       
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               . 
             
           
         
       
     
       FIG. 7  illustrates a broadband ultrasonic sensor, according to some implementations. In the illustrated example, an ultrasonic sensor  700  includes a substrate  704 , a platen  706 , and an acoustic stack  720  disposed between the substrate  704  and the platen  706 . The acoustic stack  720  may include a piezoelectric layer and a thin film transistor (TFT) layer. The platen  706  may include one or more layers of plastic and/or glass and may, in some configurations, include a plastic organic light-emitting diode (OLED) layer. Optionally, a spacer (not illustrated) may be disposed between the platen  706  and the acoustic stack  720 . By appropriate selection of design guidelines and drive schemes, the ultrasonic sensor  700  may be configured to exhibit a signal-to-noise ratio of at least 4 over a frequency range of at least 9 to 16 MHz. For example, as described in more detail below, dimensions and/or material properties of components of the acoustic stack  720 , of the platen  706  and/or of the optional spacer disposed therebetween may be selected so as to obtain the desired broadband operating characteristics. 
       FIG. 8  illustrates examples of broadband ultrasonic sensors, according to some implementations. Referring first to Detail C, an ultrasonic sensor  800 A includes a substrate  804 A, a platen  806 A that includes an organic light emitting diode (OLED) layer. The OLED layer may be flexibly disposed within two or more layers of plastic, and be referred to, collectively, as a plastic OLED stack. The ultrasonic sensor  800 A includes an acoustic stack  820 A that includes a TFT layer disposed between the substrate  804 A and the platen  806 A. The TFT layer of acoustic stack  820 A may or may not be configured as a resonator (i.e., may be configured to resonate at one or more frequencies within the operating frequency band of interest, which may be in the range of 5-20 MHz, for example). Advantageously, the acoustic stack  820 A has a total thickness less than λ/2 where λ is a characteristic wavelength of ultrasonic waves passing through the TFT layer. For example, for a 12 MHz signal, λ may be approximately 500 In some implementations the thickness of the TFT layer may be in the range of 90-200 μm. 
     Referring next to Detail D, an ultrasonic sensor  800 B includes a substrate  804 B, a platen  806 B that includes a plastic OLED stack, an acoustic stack  820 B that is or includes a TFT layer disposed between the substrate  804 B and a spacer  830 B, and the spacer  830 B disposed between the acoustic stack  820 B and the platen  806 B. The TFT layer of acoustic stack  820 A may or may not be configured as a resonator. The spacer  830 B may be characterized as being “soft”, meaning it has no resonant frequency near an operating frequency of the ultrasonic sensor. In some implementations, the OLED stack may include a multilayer structure including at least two of a polarizer, an OLED layer, a touchscreen layer. Advantageously, the OLED stack and the platen exhibit little or no acoustic mismatch. More particularly, an acoustic impedance of the platen and the OLED stack may be approximately similar (i.e., identical within approximately 10%). Alternatively or in addition, in some implementations, a thickness of the platen  806 B is small relative to a characteristic wavelength of ultrasonic waves passing through the platen. For example the thickness may be less than ¼ of the characteristic wavelength. Advantageously, in some implementations, the thickness may be less than 1/10 of the characteristic wavelength. 
     Advantageously, the combined thickness of the acoustic stack  820 B and the spacer  830 B has a total thickness less than λ/2 where λ is a characteristic wavelength of ultrasonic waves passing through the TFT layer. For example, for a 12 MHz signal, λ, may be approximately 500 μm. In some implementations, the combined thickness may be in the range of 90-200 μm. 
     Referring next to Detail E, an ultrasonic sensor  800 C includes a substrate  804 C, a platen  806 C that includes a plastic OLED stack, an acoustic stack  820 C that includes a TFT layer disposed between the substrate  804 C and a spacer  840 C. The spacer  840 C is disposed between the acoustic stack  820 C and the platen  806 C. The spacer  830 B may be characterized as being “hard”, meaning it is configured as a resonator. In such a configuration, advantageously, the TFT layer of acoustic stack  820 C is not configured as a resonator. Advantageously, the combined thickness of the acoustic stack  820 C and the spacer  840 C has a total thickness less than λ/2 where λ is a characteristic wavelength of ultrasonic waves passing through the TFT layer. For example, for a 12 MHz signal, λ, may be approximately 500 μm. In some implementations, the combined thickness may be in the range of 90-200 μm. 
     Advantageously, any of the ultrasonic sensors  800 A,  800 B and  800 C may be driven by a broadband input, (i.e., over a frequency range of about 5-20 MHz). An appropriate drive schemes may include a signal pulse half pulse the tap a 30 V direct drive square signal, or a chirp signal. 
       FIG. 9  illustrates an example of a broadband ultrasonic sensor, according to some implementations. In the illustrated example, an ultrasonic sensor  900  includes a substrate  904 , a platen  906  that includes a cover glass layer, and an acoustic stack  920  that includes a TFT layer disposed between the substrate  904  and an adhesive layer  960 . The adhesive layer  960  is disposed between the TFT layer of acoustic stack  920  and a spacer  950 . The spacer  950  is disposed between the adhesive layer  960  and the cover glass  906 . The TFT layer of acoustic stack  820 A may or may not be configured as a resonator. The cover glass of platen  906  may be a multilayer structure including any number of transparent layers. For example, the cover glass may include a polarizer, an OLED layer, a touchscreen layer and/or a screen protector. The adhesive layer  960  may be or include a pressure sensitive adhesive (PSA). 
     The present inventors have simulated performance of the ultrasonic sensor  900 , using various assumed dimensional and material properties. For example, parametric studies of performance as a function of thickness of the thickness of the spacer  950 , thickness of the adhesive layer  960 , and thickness of the acoustic stack layer  920  were performed.  FIG. 10  presents example performance results for a number of candidate implementations. The results plot SNR as a function of frequency for 50 different combinations of spacer thickness, adhesive layer thickness and acoustic stack thickness. As indicated in the legend of  FIG. 10 , spacer thickness and acoustic stack thickness were each modeled as having a thickness of 50, 100, 150, 200 and 250 For each of the resulting 25 combinations of spacer thickness and acoustic stack thickness, a cell includes two plots: a first plot labeled ‘t’ represents modeled results for an adhesive layer thickness of 0.1 μm and a second plot labeled ‘T’ represents modeled results for an adhesive layer thickness of 10 μm. A dashed horizontal line in each of the 25 cells represents a desired SNR of about 4. 
     The results demonstrate that, for instances where the adhesive layer is “thin” (0.1 μm) a combined thickness of the acoustic stack  920  and the spacer  950  may be as large as 250 μm while still maintaining SNR&gt;4 through at least most of the frequency range of 5 to 20 MHz. For instances where the adhesive layer is “thick” (10 μm), the spacer  950  is advantageously avoided or made very thin, and a thickness of the acoustic stack  920  may, advantageously, be less than 250 μm. 
     As a further example, parametric studies of performance as a function of range gate window (RGW, as described hereinabove) and for variously composed acoustic stacks were performed.  FIGS. 11A-11C  present example performance results for three additional candidate implementations. SNR as a function of frequency is depicted for each of the example implementations. The plot illustrated in  FIG. 11A , of an implementation including an acoustic stack including a zirconate titanate (PZT) piezoelectric and using a relatively narrow RGW value of 50 ns compares favorably to two alternative implementations illustrated in  FIGS. 11B and 11C .  FIG. 11B  models performance of an implementation using the same acoustic stack as  FIG. 11A , but operated with a relatively wide RGW value of 800 ns.  FIG. 11C  depicts performance of an implementation including an acoustic stack including polyvinylidene fluoride, instead of PZT, with an RGW value of 50 ns. 
     Thus, an improved ultrasonic sensor has been disclosed. It will be appreciated that a number of alternative configurations and operating techniques may be contemplated. 
     As used herein, a phrase referring to “at least one of” a list of items refers to any combination of those items, including single members. As an example, “at least one of: a, b, or c” is intended to cover: a, b, c, a-b, a-c, b-c, and a-b-c. 
     The various illustrative logics, logical blocks, modules, circuits and algorithm processes described in connection with the implementations disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. The interchangeability of hardware and software has been described generally, in terms of functionality, and illustrated in the various illustrative components, blocks, modules, circuits and processes described above. Whether such functionality is implemented in hardware or software depends upon the particular application and design constraints imposed on the overall system. 
     The hardware and data processing apparatus used to implement the various illustrative logics, logical blocks, modules and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose single- or multi-chip processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor or any conventional processor, controller, microcontroller, or state machine. A processor also may be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. In some implementations, particular processes and methods may be performed by circuitry that is specific to a given function. 
     In one or more aspects, the functions described may be implemented in hardware, digital electronic circuitry, computer software, firmware, including the structures disclosed in this specification and their structural equivalents thereof, or in any combination thereof. Implementations of the subject matter described in this specification also may be implemented as one or more computer programs, i.e., one or more modules of computer program instructions, encoded on a computer storage media for execution by or to control the operation of data processing apparatus. 
     If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium, such as a non-transitory medium. The processes of a method or algorithm disclosed herein may be implemented in a processor-executable software module which may reside on a computer-readable medium. Computer-readable media include both computer storage media and communication media including any medium that may be enabled to transfer a computer program from one place to another. Storage media may be any available media that may be accessed by a computer. By way of example, and not limitation, non-transitory media may include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, or any other medium that may be used to store desired program code in the form of instructions or data structures and that may be accessed by a computer. Also, any connection may be properly termed a computer-readable medium. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. Additionally, the operations of a method or algorithm may reside as one or any combination or set of codes and instructions on a machine readable medium and computer-readable medium, which may be incorporated into a computer program product. 
     Various modifications to the implementations described in this disclosure may be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other implementations without departing from the spirit or scope of this disclosure. Thus, the disclosure is not intended to be limited to the implementations shown herein, but is to be accorded the widest scope consistent with the claims, the principles and the novel features disclosed herein. The word “exemplary” is used exclusively herein, if at all, to mean “serving as an example, instance, or illustration.” Any implementation described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other implementations. 
     Certain features that are described in this specification in the context of separate implementations also may be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation also may be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination may in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed to achieve desirable results. Further, the drawings may schematically depict one more example processes in the form of a flow diagram. However, other operations that are not depicted may be incorporated in the example processes that are schematically illustrated. For example, one or more additional operations may be performed before, after, simultaneously, or between any of the illustrated operations. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems may generally be integrated together in a single software product or packaged into multiple software products. Additionally, other implementations are within the scope of the following claims. In some cases, the actions recited in the claims may be performed in a different order and still achieve desirable results. It will be understood that unless features in any of the particular described implementations are expressly identified as incompatible with one another or the surrounding context implies that they are mutually exclusive and not readily combinable in a complementary and/or supportive sense, the totality of this disclosure contemplates and envisions that specific features of those complementary implementations may be selectively combined to provide one or more comprehensive, but slightly different, technical solutions. It will therefore be further appreciated that the above description has been given by way of example only and that modifications in detail may be made within the scope of this disclosure.