Patent Description:
This application is a continuation of <CIT>, having Attorney Docket No. IVS-<NUM>, and assigned to the assignee of the present application.

Fingerprint sensors have become ubiquitous in mobile devices as well as other applications for authenticating a user's identity. They provide a fast and convenient way for the user to unlock a device, provide authentication for payments, etc. Current fingerprint sensors are typically area sensors that obtain a two-dimensional image of the user's finger area presented to the sensor. Different technologies can be used to image the finger such as capacitive, ultrasound, and optical sensing. Once an image is obtained, that image is processed by a matcher to extract features and to compare against stored images to authenticate the user. As such, accuracy of captured images is essential to the performance of image matching for user authentication.

<CIT> describes, in accordance with its abstract, a system including a fingerprint sensor system and a control system. The system may be configured to transmit an ultrasonic wave including a first frequency. The control system may be configured to obtain dermis layer image data from a target object based on reflected portions of the ultrasonic waves received by the fingerprint sensor system. The dermis layer image data may correspond to ultrasonic waves received from the target object within a time interval corresponding with the dermis layer. The reflected portions of the ultrasonic wave corresponding to the dermis layer image data may include ultrasonic waves at a second frequency that is an integer multiple of the first frequency. The control system may be configured to determine whether a magnitude of the ultrasonic waves at the second frequency exceeds a harmonic threshold and, if the magnitude exceeds the harmonic threshold, the control system may perform an authentication process.

<CIT> describes, in accordance with its abstract, an ultrasonic button and methods for using the ultrasonic button. An ultrasonic button may include an ultrasonic transmitter configured to transmit an ultrasonic wave, a piezoelectric receiver layer configured to receive a reflected wave of the ultrasonic wave, a platen layer configured to protect the ultrasonic transmitter and the piezoelectric receiver layer, a first matching layer configured to match an acoustic impedance of the platen layer with an acoustic impedance of ridges of a finger, and an ultrasonic sensor array configured to detect the finger using the reflected wave.

<CIT> describes, in accordance with its abstract, a sensor device comprising a two-dimensional array of ultrasonic transducers, wherein the two-dimensional array of ultrasonic transducers is substantially flat, a non-uniform contact layer overlying the two-dimensional array of ultrasonic transducers, and a sensor processor. The sensor device is configured to: transmit ultrasonic signals using the two-dimensional array of ultrasonic transducers for reflection from an object in contact with the non-uniform contact layer, wherein the ultrasonic signals traverse the non-uniform contact layer, receive reflected ultrasonic signals at the two-dimensional array of ultrasonic transducers, obtain non-uniformity data characterizing the non-uniform contact layer, control operating parameters of the sensor device based on the non-uniformity data, and generate an image of the object in contact with the non-uniform contact layer based on the reflected ultrasonic signals, wherein the image is corrected for non-uniformity of the non-uniform contact layer.

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.

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. The presented embodiments are intended to cover alternatives and modifications, which may be included within the scope of the various embodiments without departing from the scope of 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. 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.

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.

Unless specifically stated otherwise as apparent from the following discussions, it is appreciated that throughout the description of embodiments, discussions utilizing terms such as "performing," "controlling," "capturing," "activating," "generating," "combining," "transmitting," "receiving," "activating," "combining," "determining," 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 processorexecutable instructions residing on some form of non-transitory processor-readable medium, such as program modules, executed by one or more computers or other devices. 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. 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 referto 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. 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.

Discussion begins with a description of an example piezoelectric micromachined ultrasonic transducer (PMUT), in accordance with various embodiments. Example arrays including PMUT devices are then described. Examples of the ultrasonic fingerprint sensors having a matching layer with a thickness substantially equal to a quarter wavelength of the first transmission frequency are then described. Examples of operations for operating an ultrasonic fingerprint sensor are then described.

Fingerprint sensors, in accordance with the described embodiments, are used for capturing fingerprint images that are used for performing fingerprint authentication. As the use of fingerprint sensors proliferates, there is a desire to include fingerprint sensors in devices having varying form factors. In many potential use cases, an ultrasonic fingerprint sensor is overlaid with a contact layer that is not flat (e.g., has a curved profile, a sloped profile, rounded edges, etc.) In other situations, the contact layer can be comprised of materials having varying acoustic impedances, where the acoustic impedance of a material may impact the amount of acoustic signal that can be transmitted through the contact layer. In such use cases, a matching layer between the contact layer and the fingerprint sensor is used to provide a transmission medium for ultrasonic signals between the contact layer and the fingerprint sensor. For example, the matching layer may be an adhesive layer (e.g., epoxy) for bonding the contact layer to the fingerprint sensor. Where the contact layer is not flat, the variation in material thickness (contact layer and matching layer combined) may result in a reduction of signal strength. Moreover, a matching layer having a non-uniform thickness will have inconsistent signal strength associated with a constant transmission frequency. Embodiments described herein account for the non-uniform thickness of the matching layer.

Embodiments described herein provide a sensor device including a two-dimensional array of ultrasonic transducers, a contact layer, a matching layer between the two-dimensional array and the contact layer, where the matching layer has a non-uniform thickness, and an array controller configured to control activation of ultrasonic transducers during an imaging operation for imaging a plurality of pixels within the two-dimensional array of ultrasonic transducers. In some embodiments, the matching layer is an adhesive layer for bonding the two-dimensional array of ultrasonic transducers to the contact layer.

In some embodiments, the matching layer has an acoustic impedance between an acoustic impedance of the two-dimensional array of ultrasonic transducers and an acoustic impedance of the contact layer. In some embodiments, the ultrasonic sensor further includes a packaging epoxy overlying the two-dimensional array of ultrasonic transducers, such that the matching layer is connected to the packaging epoxy and the contact layer. In some embodiments, a first thickness of the matching layer at a first region is substantially equal to a quarter wavelength of the first transmission frequency plus or minus a one-eighth wavelength of the first transmission frequency and a second thickness of the matching layer at a second region is substantially equal to a quarter wavelength of the second transmission frequency plus or minus a one-eighth wavelength of the second transmission frequency.

During the imaging operation, the array controller is configured to activate different subsets of ultrasonic transducers associated with different regions of the two-dimensional array of ultrasonic transducers at different transmission frequencies, where the different frequencies are determined such that a thickness of the matching layer at a region is substantially equal to a quarter wavelength of the first transmission frequency for the region. The array controller is also configured to combine the plurality of pixels into a compound fingerprint image that compensates for the non-uniform thickness of the matching layer. In some embodiments, a first region and a second region are associated with different blocks of ultrasonic transducers of the two-dimensional array of ultrasonic transducers. In some embodiments, a first region and a second region are associated with different columns of ultrasonic transducers of the two-dimensional array of ultrasonic transducers. In some embodiments, a first region and a second region are associated with different rows of ultrasonic transducers of the two-dimensional array of ultrasonic transducers.

Embodiments described herein and not forming part of the invention to which this European patent relates also provide a method for calibrating an ultrasonic sensor including a two-dimensional array of ultrasonic transducers, a contact layer overlying the two-dimensional array of ultrasonic transducers, and a matching layer between the two-dimensional array of ultrasonic transducers and the contact layer, where the matching layer has a non-uniform thickness overlying the two-dimensional array of ultrasonic transducers.

In some embodiments, a thickness of the matching layer at each region is substantially equal to a quarter wavelength of a transmission frequency for the region. In some embodiments, the thickness of the matching layer at each region is substantially equal to a quarter wavelength of the transmission frequency plus or minus a one-eighth wavelength of the transmission frequency at the region. In some embodiments, the matching layer is an adhesive layer for bonding the two-dimensional array of ultrasonic transducers to the contact layer. In some embodiments, the matching layer has an acoustic impedance between an acoustic impedance of the two-dimensional array of ultrasonic transducers and an acoustic impedance of the contact layer.

Ultrasonic transducers of the two-dimensional array of ultrasonic transducers associated with a plurality of regions of the two-dimensional array of ultrasonic transducers are activated at a plurality of transmission frequencies, where the plurality of regions are non-overlapping. A signal for each of the plurality of transmission frequencies at the plurality of regions is monitored. For the plurality of regions, a transmission frequency of the plurality of transmission frequencies having a highest signal for a region is set as the transmission frequency for the region.

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. 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.

Embodiments described herein provide ultrasonic fingerprint sensors having non-uniform contact layers. It should be appreciated that different types of ultrasonic fingerprint sensors having different architectures may be utilized herein. For instance, some architectures include an array of ultrasonic transducers (e.g., PMUTs), embodiments of which are described herein. In some embodiments, the ultrasonic transducers may be bulk piezo actuator elements, e.g., lead zirconate titanate (PZT). Other architectures may utilize a film-based design. Although embodiments are described herein with respect to an array of ultrasonic transducers, the methods and techniques may be applied to other ultrasonic sensing architectures where the control of the operating parameters of different segments of the sensors can be adjusted separately to correct for the non-uniform contact surface. The embodiments described herein are with respect to sensors with a non-uniform contact layer, but the techniques and principles discussed may in some situations also be used to improve performance of sensors with uniform contact surfaces.

<FIG> is a diagram illustrating a PMUT device <NUM> having a center pinned membrane, according to some embodiments. PMUT device <NUM> includes an interior pinned membrane <NUM> positioned over a substrate <NUM> to define a cavity <NUM>. In one embodiment, membrane <NUM> is attached both to a surrounding edge support <NUM> and interior support <NUM>. In one embodiment, edge support <NUM> is connected to an electric potential. Edge support <NUM> and interior support <NUM> may be made of electrically conducting materials, such as and without limitation, aluminum, molybdenum, or titanium. Edge support <NUM> and interior support <NUM> 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 <NUM> or interior support <NUM>, electrically coupling lower electrode <NUM> to electrical wiring in substrate <NUM>.

In one embodiment, both edge support <NUM> and interior support <NUM> are attached to a substrate <NUM>. In various embodiments, substrate <NUM> may include at least one of, and without limitation, silicon or silicon nitride. It should be appreciated that substrate <NUM> may include electrical wirings and connection, such as aluminum or copper. In one embodiment, substrate <NUM> includes a CMOS logic wafer bonded to edge support <NUM> and interior support <NUM>. In one embodiment, the membrane <NUM> comprises multiple layers. In an example embodiment, the membrane <NUM> includes lower electrode <NUM>, piezoelectric layer <NUM>, and upper electrode <NUM>, where lower electrode <NUM> and upper electrode <NUM> are coupled to opposing sides of piezoelectric layer <NUM>. As shown, lower electrode <NUM> is coupled to a lower surface of piezoelectric layer <NUM> and upper electrode <NUM> is coupled to an upper surface of piezoelectric layer <NUM>. It should be appreciated that, in various embodiments, PMUT device <NUM> is a microelectromechanical (MEMS) device.

In one embodiment, membrane <NUM> also includes a mechanical support layer <NUM> (e.g., stiffening layer) to mechanically stiffen the layers. In various embodiments, mechanical support layer <NUM> may include at least one of, and without limitation, silicon, silicon oxide, silicon nitride, aluminum, molybdenum, titanium, etc. In one embodiment, PMUT device <NUM> also includes an acoustic coupling layer <NUM> above membrane <NUM> 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 <NUM> also includes platen layer <NUM> above acoustic coupling layer <NUM> for containing acoustic coupling layer <NUM> and providing a contact surface for a finger or other sensed object with PMUT device <NUM>. It should be appreciated that, in various embodiments, acoustic coupling layer <NUM> provides a contact surface, such that platen layer <NUM> is optional. Moreover, it should be appreciated that acoustic coupling layer <NUM> and/or platen layer <NUM> 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 <NUM> and/or platen layer <NUM>. It should be appreciated that platen layer <NUM> can include one or more materials, including without limitation: glass, plastic, metal (e.g., aluminum, titanium, stainless steel), etc. In some embodiments, platen <NUM> is a display device (e.g., an organic light emitting diode (OLED) display device) overlying one or more PMUT devices. In such an embodiment, platen <NUM> includes multiple layers of the display device.

<FIG> is identical to <FIG> in every way, except that the PMUT device <NUM>' of <FIG> omits the interior support <NUM> and thus membrane <NUM> is not pinned (e.g., is "unpinned"). There may be instances in which an unpinned membrane <NUM> is desired. However, in other instances, a pinned membrane <NUM> may be employed.

<FIG> is a diagram illustrating an example of membrane movement during activation of PMUT device <NUM>, according to some embodiments. As illustrated with respect to <FIG>, in operation, responsive to an object proximate platen layer <NUM>, the electrodes <NUM> and <NUM> deliver a high frequency electric charge to the piezoelectric layer <NUM>, causing those portions of the membrane <NUM> not pinned to the surrounding edge support <NUM> or interior support <NUM> to be displaced upward into the acoustic coupling layer <NUM>. 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 <NUM> 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 <NUM> 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 <NUM> 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 <NUM> 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 <NUM>, or a number of different or heterogonous device structures.

In various embodiments, the PMUT device <NUM> employs a piezoelectric layer <NUM>, comprised of materials such as, but not limited to, aluminum nitride (AlN), scandium doped aluminum nitride (ScAlN), lead zirconate titanate (PZT), quartz, polyvinylidene fluoride (PVDF), and/or zinc oxide, to facilitate both acoustic signal production and sensing. The piezoelectric layer <NUM> 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 <NUM> can sense mechanical vibrations caused by an ultrasonic beam and produce an electrical charge at the frequency (e.g., ultrasonic frequency) of the vibrations. Additionally, the piezoelectric layer <NUM> 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 <NUM>. It should be appreciated that the piezoelectric layer <NUM> can include almost any material (or combinationof 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 electricfields of opposite polarizations.

Further, the PMUT device <NUM> comprises electrodes <NUM> and <NUM> that supply and/or collect the electrical charge to/from the piezoelectric layer <NUM>. It should be appreciated that electrodes <NUM> and <NUM> can be continuous and/or patterned electrodes (e.g., in a continuous layer and/or a patterned layer). For example, as illustrated, electrode <NUM> is a patterned electrode and electrode <NUM> is a continuous electrode. As an example, electrodes <NUM> and <NUM> 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 <NUM>. In one embodiment, PMUT device also includes a third electrode, as illustrated in <FIG> and described below.

According to an embodiment, the acoustic impedance of acoustic coupling layer <NUM> is selected to be similar to the acoustic impedance of the platen layer <NUM>, such that the acoustic wave is efficiently propagated to/from the membrane <NUM> through acoustic coupling layer <NUM> and platen layer <NUM>. As an example, the platen layer <NUM> can comprise various materials having an acoustic impedance in the range between <NUM> to <NUM> MRayl, such as, but not limited to, plastic, resin, rubber, Teflon, epoxy, etc. In another example, the platen layer <NUM> can comprise various materials having a high acoustic impedance (e.g., an acoustic impendence greater than <NUM> MRayl), such as, but not limited to, glass, aluminum-based alloys, sapphire, etc. Typically, the platen layer <NUM> can be selected based on an application of the sensor. For instance, in fingerprinting applications, platen layer <NUM> can have an acoustic impedance that matches (e.g., exactly or approximately) the acoustic impedance of human skin (e.g., <NUM>×<NUM><NUM> Rayl). Further, in one aspect, the platen layer <NUM> can further include a thin layer of anti-scratch material. In various embodiments, the anti-scratch layer of the platen layer <NUM> 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 <NUM> 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 <NUM> can operate at <NUM> and accordingly, the wavelength of the acoustic wave propagating through the acoustic coupling layer <NUM> and platen layer <NUM> can be <NUM>-<NUM> 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 <NUM> micron and the platen layer <NUM> as a whole having a thickness of <NUM>-<NUM> 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 <NUM> can include metal layers (e.g., Aluminum (Al)/Titanium (Ti), Molybdenum (Mo), etc.) patterned to form electrode <NUM> in particular shapes (e.g., ring, circle, square, octagon, hexagon, etc.) that are defined inplane with the membrane <NUM>. Electrodes can be placed at a maximum strain area of the membrane <NUM> or placed at close to either or both the surrounding edge support <NUM> and interior support <NUM>. Furthermore, in one example, electrode <NUM> can be formed as a continuous layer providing a ground plane in contact with mechanical support layer <NUM>, which can be formed from silicon or other suitable mechanical stiffening material. In still other embodiments, the electrode <NUM> can be routed along the interior support <NUM>, advantageously reducing parasitic capacitance as compared to routing along the edge support <NUM>.

For example, when actuation voltage is applied to the electrodes, the membrane <NUM> will deform and move out of plane. The motion then pushes the acoustic coupling layer <NUM> it is in contact with and an acoustic (ultrasonic) wave is generated. Oftentimes, vacuum is present inside the cavity <NUM> and therefore damping contributed from the media within the cavity <NUM> can be ignored. However, the acoustic coupling layer <NUM> on the other side of the membrane <NUM> can substantially change the damping of the PMUT device <NUM>. For example, a quality factor greater than <NUM> can be observed when the PMUT device <NUM> is operating in air with atmosphere pressure (e.g., acoustic coupling layer <NUM> is air) and can decrease lower than <NUM> if the PMUT device <NUM> is operating in water (e.g., acoustic coupling layer <NUM> is water).

<FIG> illustrates an example two-dimensional array <NUM> of square-shaped PMUT devices <NUM> formed from PMUT devices having a substantially square shape similar to that discussed in conjunction with <FIG>, <FIG>, and <FIG>. Layout of square surrounding edge support <NUM>, interior support <NUM>, and square-shaped lower electrode <NUM> surrounding the interior support <NUM> are illustrated, while other continuous layers are not shown for clarity. As illustrated, array <NUM> includes columns of square-shaped PMUT devices <NUM> that are in rows and columns. It should be appreciated that rows or columns of the square-shaped PMUT devices <NUM> may be offset. Moreover, it should be appreciated that square-shaped PMUT devices <NUM> may contact each other or be spaced apart. In various embodiments, adjacent square-shaped PMUT devices <NUM> are electrically isolated. In other embodiments, groups of adjacent square-shaped PMUT devices <NUM> are electrically connected, where the groups of adjacent square-shaped PMUT devices <NUM> are electrically isolated.

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> illustrates an example fingerprint sensor <NUM>, in accordance with various embodiments. In one embodiment, fingerprint sensor <NUM> includes an array <NUM> of ultrasonic transducers (e.g., PMUT devices), a processor460, and a memory <NUM>. It should be appreciated that some or all operations can be performed a processor external to fingerprint sensor <NUM> (e.g., a host processor or an application processor) In various embodiments, processor <NUM> performs certain operations in accordance with instructions stored within memory <NUM>. It should be appreciated that components of fingerprint sensor <NUM> are examples, and that certain components, such as processor460 and/or memory <NUM> may not be located within fingerprint sensor415.

in one embodiment, fingerprint sensor <NUM> includes processor <NUM> for performing the pixel capture, where pixel capture is performed using subsets of ultrasonic transducers (e.g., PMUTs) of fingerprint sensor <NUM>. In some embodiments, pixel capture for groups of pixels is handled the same, such that the groups of pixels utilize same imaging operations (e.g., utilize a same relative activation of the first subset of ultrasonic transducers and the second subset of ultrasonic transducers according to a same local angle). In other embodiments, processor <NUM> can perform at least some signal analysis, e.g., thresholding, to determine whether an object has interacted with fingerprint sensor <NUM>. In other embodiments, processor <NUM> can analyze captured pixels and determine whether the object has characteristics of finger, e.g., a pattern resembling the ridge/valley pattern of a fingerprint. In other embodiments, processor460 can capture an image of the fingerprint and forward it to a processor of system circuitry for further analysis.

In accordance with embodiments, processor <NUM> is configured to control the activation of the subsets of ultrasonic transducers to generate an ultrasonic signal, also referred to herein as an ultrasonic beam. Using multiple ultrasonic transducers, some of which are time delayed with respect to other ultrasonic transducers, embodiments described herein provide for focusing a transmit beam (e.g., forming a beam) of an ultrasonic signal to a desired point, allowing for high resolution sensing of a fingerprint, or other object. For instance, transmitting an ultrasonic signal from multiple PMUTs, where some PMUTs transmit at a time delay relative to other PMUTs, provides for focusing the ultrasonic beam to a contact point of a fingerprint sensing system (e.g., a top of a contact layer) for sensing a high resolution image of a pixel associated with the transmitting PMUTs. In some embodiments, the activation includes transmitting ultrasonic signals from a first group of ultrasonic transducers of the plurality of ultrasonic transducers, wherein at least some ultrasonic transducers of the first group of ultrasonic transducers are phase delayed with respect to other ultrasonic transducers of the first group of ultrasonic transducers, the first group of ultrasonic transducers for forming a focused ultrasonic beam. The activation also includes receiving reflected ultrasonic signals at a second group of ultrasonic transducers of the plurality of ultrasonic transducers.

While the embodiment of <FIG> includes processor <NUM> and memory <NUM>, as described above, it should be appreciated that various functions of processor <NUM> and memory <NUM> may reside in other components of an electronic device. Moreover, it should be appreciated that processor <NUM> may be any type of processor for performing any portion of the described functionality (e.g., custom digital logic).

Fingerprint sensors are used in electronic devices for user authentication, such as mobile electronic devices, building locks, automobile locks, etc. In many situations, the surface area of the fingerprint sensor needs to be as flat and as uniform as possible in order to obtain a good fingerprint. However, in some situation it may be desirable to have a non-flat or non-uniform fingerprint surface. For example, when a fingerprint sensor is mounted on the back of a mobile phone, a concave form or structured surface may help guide the user's finger to the correct position on the FP sensor. In other situations, the design employs a non-flat form factor, such as a convex button, a rounded doorknob or automobile handle. The design may also employ cover materials of different hardness, such as stainless steel, plastic, ceramic, aluminum, etc. Some designs may require a smooth surface, while other designs may require a rough or textured surface. In further situations, the fingerprint sensor may be integrated in a wearable device, e.g., a smart watch, either on the face of the watch (e.g., behind the display), or on the side of the body. While in the former placement the surface can be relatively flat, in the latter placement the sensor surface may have a small radius of curvature. In general, the smaller the device, or section of the device, where the sensor is integrated, the smaller the radius of curvature, i.e., the more curved the surface is.

Embodiments described herein provide a sensor device including a two-dimensional array of ultrasonic transducers, a contact layer, a matching layer between the two-dimensional array and the contact layer, where the matching layer has a uniform or non-uniform thickness, and an array controller configured to control activation of ultrasonic transducers during an imaging operation for imaging a plurality of pixels within the two-dimensional array of ultrasonic transducers. In some embodiments, the matching layer is an adhesive layer for bonding the two-dimensional array of ultrasonic transducers to the contact layer. During the imaging operation, the array controller is configured to activate different subsets of ultrasonic transducers associated with different regions of the two-dimensional array of ultrasonic transducers at different transmission frequencies, where the different frequencies are determined such that a thickness of the matching layer at a region is substantially equal to a quarter wavelength of the first transmission frequency for the region. The array controller is also configured to combine the plurality of pixels into a compound fingerprint image that compensates for the non-uniform thickness of the matching layer.

Although embodiments are described with respect to an array of ultrasonic transducers, the methods and techniques may be applied to other ultrasound sensing architectures where the control of the operating parameters of different segments of the sensors can be adjusted separately to correct for the non-uniform thickness of the matching layer. Moreover, the methods and techniques may be applied to ultrasonic sensors having matching layers of uniform thickness.

<FIG> and <FIG> illustrate cross section views of ultrasonic fingerprint sensors having matching layers of uniform thickness, according to embodiments. <FIG> illustrates a cross section view of an ultrasonic fingerprint sensor <NUM>, according to an embodiment. As illustrated, ultrasonic fingerprint sensor <NUM> includes several different layers. Ultrasonic fingerprint sensor <NUM> includes an active sensing layer <NUM> on top of a substrate <NUM>. The active sensing layer <NUM> may use various techniques to sense or detect the fingerprint, e.g., acoustic or ultrasonic techniques. It should be appreciated that active sensing layer <NUM> may not extend to the edges of contact layer <NUM>. For an ultrasonic fingerprint sensor, the sensing layer may comprise an array of ultrasonic transducers (e.g., PMUTs <NUM> of <FIG>, PMUTs <NUM>' of <FIG>), or bulk piezo actuator elements, or piezoelectric film (e.g., PVDF) that may be used emit and detect ultrasonic waves. In some embodiments, sensing layer <NUM> includes a transmission layer for transmitting ultrasonic signals and a detection layer for receiving reflected ultrasonic signals. Matching layer <NUM> overlays sensing layer <NUM>. Contact layer <NUM> overlays matching layer <NUM>, where the outer surface of contact layer <NUM> is contact surface <NUM>. In some embodiments, matching layer <NUM> is an adhesive layer (e.g., an epoxy) for bonding contact layer <NUM> to sensing layer <NUM>. Contact surface <NUM> of contact layer <NUM> is substantially flat and parallel to sensing layer <NUM>. For example, a user using ultrasonic fingerprint sensor <NUM> places his or her finger in contact with contact surface <NUM>.

In some embodiments, other layers such as display devices, (not shown) may be used between sensing layer <NUM> and contact layer <NUM>, above or below matching layer <NUM>. In some embodiments, matching layer <NUM> is an acoustic coupling layer. It should be appreciated that in some embodiments, contact layer <NUM> and the acoustic coupling layer are a single layer. It should be further appreciated that ultrasonic fingerprint sensor <NUM> may include other layers, such as other bonding layers and laminate layers, and is not intended to be limited to the specific layers shown. In other embodiments, an ultrasonic fingerprint sensor including separate signal transmission and signal detection layers may be used (e.g., within sensing layer <NUM>).

In an ultrasonic fingerprint sensor <NUM>, the acoustic waves travel from the sensing layer <NUM> through matching layer <NUM> and contact layer <NUM> to contact surface <NUM>, interact with the object on the surface (e.g., a finger), and may then be reflected back to the sensing layer <NUM>, thereby again traversing matching layer <NUM> and contact layer <NUM>. In some embodiments, matching layer <NUM> has an acoustic impedance between an acoustic impedance of sensing layer <NUM> and an acoustic impedance of contact layer <NUM>. In some embodiments, matching layer <NUM> has a hardness between a hardness of sensing layer <NUM> and a hardness of contact layer <NUM>. In some embodiments, a thickness of matching layer <NUM> is substantially equal to a quarter wavelength (or N wavelengths plus a quarter wavelength, where N is an integer) of the transmission frequency of sensing layer <NUM>. Matching layer <NUM> operates as an acoustic coupling layer between sensing layer <NUM> and contact layer <NUM>. Embodiments herein aim to optimize the transmission signal and reflection signal to provide a high signal through contact layer <NUM>. In some embodiments, a thickness of matching layer <NUM> is substantially equal to a quarter wavelength of the transmission frequency of sensing layer <NUM> plus or minus a one-eighth wavelength of the transmission frequency.

<FIG> illustrates a cross section view of an ultrasonic fingerprint sensor <NUM>, according to another embodiment. Ultrasonic fingerprint sensor <NUM> operates in a similar manner to, and includes the components of, ultrasonic fingerprint sensor <NUM> of <FIG>, with the exception of the composition of sensing layer <NUM>. As illustrated, sensing layer <NUM> includes piezoelectric layer <NUM> and packaging layer <NUM> overlaying piezoelectric layer <NUM>. In some embodiments, sensing layer <NUM> is a packaged sensing device such that packaging layer <NUM> is a packaging epoxy overlaying piezoelectric layer <NUM>. In some embodiments, piezoelectric layer <NUM> includes a two-dimensional array of ultrasonic transducers (e.g., PMUTs <NUM> of <FIG>, PMUTs <NUM>' of <FIG>, or bulk piezo actuator elements). In some embodiments, packaging layer <NUM> is an epoxy, also referred to as a packaging epoxy. Packaging layer <NUM> overlaying piezoelectric layer <NUM> is an example of a packaged ultrasonic sensor, where the piezoelectric layer <NUM> (e.g., a two-dimensional array of ultrasonic transducers) is packaged into a sensor unit by sealing piezoelectric layer <NUM> with packaging layer <NUM>. It should be appreciated that a sensing layer including a packaging layer overlaying a piezoelectric layer can be utilized in the other embodiments of ultrasonic sensors described herein.

Referring back nowto <FIG>, <FIG>, <FIG>, and <FIG>, and matching layer <NUM> (e.g., acoustic coupling layer <NUM>). Acoustic coupling layer <NUM> and matching layer <NUM> may have different thicknesses depending on the application, and the material may be selected to help transmit acoustic energy from the sensing layer <NUM> to contact layer <NUM>. In some embodiments, the thickness of matching layer <NUM> may be selected to increase the coupling of acoustic energy into contact layer <NUM>. For example, matching layer <NUM> may have a thickness of approximately a quarter (¼) wavelength, also referred to as a quarter lambda layer or quarterwavelength layer. Although the term quarter wavelength layer is used, the layer may also be thicker as long as it is an nth order quarterwavelength, meaning any integer value + quarter lambda. Internal reflection and constructive interference effects may then increase the transmitted acoustic energy into the contact layer.

in embodiments where a quarter wavelength layer is used, the thickness of the layer may have to be tuned depending on the frequency of the acoustic waves and the speed of sound of the material. For example, take matching layer <NUM> made of epoxy with a speed of sound of <NUM>/s. At an acoustic frequency of <NUM>, the matching layer <NUM> should have a thickness of approximately <NUM>. Care should be taken that the thickness variation of the acoustic coupling layer is not too large to have an opposite effect of destructive interference. Destructive interference occurs when the layer thickness is close the half (½) wavelength. This means that the tolerance of the quarter wavelength layer thickness is one-eight (<NUM>/<NUM>) wavelength. If the difference in thickness is larger, negative destructive interference effects start to occur. In the above example, the one eighth wavelength is approximately <NUM>, which means that the thickness variation in the <NUM> layer should not be more than plus or minus <NUM>. In situations and devices where the thickness of the matching layer <NUM> cannot be controlled to within the desired specification, the wavelength can be adapted through frequency tuning to obtain the constructive interference effects of the quarter wavelength layer.

In some embodiments, the frequency of the acoustic waves is adapted to the local thickness of the matching layer <NUM>, or any other layer used for creating constructive interference. Frequency grouping maps similar to those described in relation to <FIG> below may be used for this purpose. Thefrequency for each transducer or group of transducers may be determined in a calibration phase, where the frequency is varied and the effect on the signal strength is observed. As the frequency is varied, the signal will go through maxima and minima, corresponding to quarter wavelength and half wavelength effects, respectively. A frequency corresponding to a maximum may then be selected for optimum quarter wavelength effect. The use of the quarter wavelength layer may be used with flat surface or surfaces of non-uniform thickness. The use of the quarter wavelength layer may be especially important in combination with materials with a large acoustic impedance mismatch, e.g., stainless steel.

<FIG> illustrates a cross section view of an ultrasonic fingerprint sensor <NUM> having a matching layer <NUM> of uniform thickness, according to an embodiment. Ultrasonic fingerprint sensor <NUM> includes cover layer630 over piezoelectric layer <NUM>, where matching laying <NUM> is between cover layer <NUM> over piezoelectric layer610. In some embodiments, matching layer <NUM> is an adhesive layer (e.g., an epoxy) for bonding cover layer630 to piezoelectric layer610. As illustrated, matching layer <NUM> has a thickness that is substantially equal to a quarter wavelength of the transmission frequency of piezoelectric layer610.

<FIG> illustrates a cross section view of an ultrasonic fingerprint sensor650 having a matching layer <NUM> of non-uniform thickness, according to an embodiment. Ultrasonic fingerprint sensor650 includes cover layer680 over piezoelectric layer <NUM>, where matching laying <NUM> is between cover layer <NUM> over piezoelectric layer <NUM>. in some embodiments, matching layer <NUM> is an adhesive layer (e.g., an epoxy) for bonding cover layer <NUM> to piezoelectric layer <NUM>.

As illustrated, matching layer <NUM> has a non-uniform thickness. In accordance with various embodiments, piezoelectric layer660 is operable to transmit at different frequencies associated with different regions of thickness of matching layer670, where a thickness at each region is substantially equal to a quarter wavelength of the transmission frequency of piezoelectric layer <NUM>. In some embodiments, the thickness at each region is substantially equal to a quarter wavelength of the transmission frequency of piezoelectric layer <NUM> plus or minus a one-eighth wavelength (or the required accuracy) of the transmission frequency at the region. It should be appreciated that ultrasonic fingerprint sensor <NUM> can have any number of transmission regions of piezoelectric layer660, ranging from individual transducers to large regions of transducers. While matching layer <NUM> is shown as having various thickness at the two-dimensional cross section view, it should be appreciated that matching layer670 can have varying frequency over the entire three-dimensional volume of matching layer <NUM>.

<FIG>, <FIG>, and <FIG> illustrate cross section views of ultrasonic fingerprint sensors having matching layers of non-uniform thickness, according to embodiments. <FIG> illustrates a cross section view of ultrasonic fingerprint sensor <NUM> having a matching layer <NUM> of linear non-uniform thickness, according to an embodiment. As illustrated, ultrasonic fingerprint sensor <NUM> includes an active sensing layer <NUM> on top of a substrate <NUM>, matching layer <NUM> on top of sensing layer <NUM>, and contact layer <NUM> on top of matching layer <NUM>. In some embodiments, sensing layer <NUM> may comprise an array of ultrasonic transducers (e.g., PMUTs <NUM> of <FIG>, PMUTs <NUM>' of <FIG>, or bulk piezo actuator elements) that may be used emit and detect ultrasonic waves. In some embodiments, matching layer <NUM> is an adhesive layer (e.g., an epoxy) for bonding contact layer <NUM> to sensing layer <NUM>. Contact layer <NUM> has contact surface <NUM>. For example, a user using ultrasonic fingerprint sensor <NUM> places his or her finger in contact with contact surface <NUM>.

As illustrated, matching layer <NUM> includes two regions 714a and 714b, where the thickness of each region 714a and 714b is within a tolerance range of a quarter wavelength, or substantially a quarter wavelength, of the transmission frequency of each corresponding region 712a and 712b of sensing layer <NUM>. In some embodiments, the thickness at each corresponding region 714a and 714b is substantially equal to a quarter wavelength of the transmission frequency of the corresponding region 712a and 712b of sensing layer <NUM> plus or minus a one-eighth wavelength of the transmission frequency at the region (e.g., a range of a quarter wavelength plus or minus a one-eighth wavelength of the transmission frequency). In some embodiments, the transmission frequency is tuned such that a quarter wavelength of the transmission frequency is substantially equal to a thickness of regions 714a and 714b. The thickness of region 714a of matching layer <NUM> is substantially equal to a quarter wavelength of the transmission frequency of corresponding region 712a of sensing layer712 and the thickness of region 714b of matching layer <NUM> is substantially equal to a quarter wavelength of the transmission frequency of corresponding region 712b of sensing layer <NUM>, such that the thickness of region 714a is different than regions 714b.

It should be appreciated that the slope of matching layer <NUM> may dictate or influence the number of different regions of matching layer <NUM>, and that the two regions 714a and 714b of matching layer <NUM> are examples. The number of different regions may vary based on the tolerance for accuracy, the slope of the linear non-uniform thickness, the number of available transmission frequencies, the control of the available transmission frequencies, the applications utilizing the fingerprint sensor, etc. For example, where a system has a certain number of adjustable or selectable transmission frequencies, embodiments herein are configured to group ultrasonic transducers to best fit the available transmission frequencies, within an acceptable tolerance, into regions of sensing layer <NUM>, corresponding to the regions of matching layer <NUM>.

<FIG> illustrates a cross section view of ultrasonic fingerprint sensor <NUM> having a matching layer <NUM> of convex non-uniform thickness, according to an embodiment. As illustrated, ultrasonic fingerprint sensor <NUM> includes an active sensing layer <NUM> on top of a substrate <NUM>, matching layer734 on top of sensing layer <NUM>, and contact layer <NUM> on top of matching layer <NUM>. In some embodiments, sensing layer <NUM> may comprise an array of ultrasonic transducers (e.g., PMUTs <NUM> of <FIG>, PMUTs <NUM>' of <FIG>, or bulk piezo actuator elements) that may be used emit and detect ultrasonic waves. In some embodiments, matching layer <NUM> is an adhesive layer (e.g., an epoxy) for bonding contact layer <NUM> to sensing layer <NUM>. Contact layer <NUM> has contact surface <NUM>. For example, a user using ultrasonic fingerprint sensor <NUM> places his or her finger in contact with contact surface <NUM>.

As illustrated, matching layer <NUM> includes five regions 734a through 734e, where the thickness of each region 734a through 734e is substantially equal to a quarterwavelength of the transmission frequency of each corresponding region 732a through 732e of sensing layer <NUM>. In some embodiments, the thickness at each corresponding region 734a through 734e is substantially equal to a quarter wavelength of the transmission frequency of the corresponding region 732a through 732e of sensing layer <NUM> plus or minus a one-eighth wavelength of the transmission frequency at the region (e.g., a range of a quarterwavelength plus or minus a one-eighth wavelength of the transmission frequency). As illustrated, the thickness of region 734a of matching layer <NUM> is substantially equal to a quarter wavelength of the transmission frequency of corresponding region 732a of sensing layer <NUM>, the thickness of region 734b of matching layer <NUM> is substantially equal to a quarter wavelength of the transmission frequency of corresponding region 732b of sensing layer <NUM>, the thickness of region 734c of matching layer <NUM> is substantially equal to a quarter wavelength of the transmission frequency of corresponding region 732c of sensing layer <NUM>, the thickness of region 734d of matching layer <NUM> is substantially equal to a quarter wavelength of the transmission frequency of corresponding region 732d of sensing layer <NUM>, and the thickness of region 734e of matching layer <NUM> is substantially equal to a quarter wavelength of the transmission frequency of corresponding region 732e of sensing layer <NUM>. It should be appreciated that the thickness of at least two regions 734a through 734e are different. As illustrated, the thickness of regions 734a and 734e are substantially equal and the thickness of regions 734b and 734d are substantially equal.

It should be appreciated that the slope or curve of matching layer <NUM> may dictate or influence the number of different regions of matching layer <NUM>, and that the five regions 734a through 734b of matching layer <NUM> are examples. The number of different regions may vary based on the tolerance for accuracy, the slope of the convex non-uniform thickness, the number of available transmission frequencies, the control of the available transmission frequencies, the applications utilizing the fingerprint sensor, etc. For example, where a system has a certain number of adjustable or selectable transmission frequencies, embodiments herein are configured to group ultrasonic transducers to best fit the available transmission frequencies, within an acceptable tolerance, into regions of sensing layer <NUM>, corresponding to the regions of matching layer <NUM>.

<FIG> illustrates a cross section view of ultrasonic fingerprint sensor <NUM> having a matching layer <NUM> of concave non-uniform thickness, according to an embodiment. As illustrated, ultrasonic fingerprint sensor <NUM> includes an active sensing layer <NUM> on top of a substrate <NUM>, matching layer754 on top of sensing layer752, and contact layer756 on top of matching layer <NUM>. In some embodiments, sensing layer <NUM> may comprise an array of ultrasonic transducers (e.g., PMUTs <NUM> of <FIG>, PMUTs <NUM> of <FIG>, or bulk piezo actuator elements) that may be used emit and detect ultrasonic waves. In some embodiments, matching layer <NUM> is an adhesive layer (e.g., an epoxy) for bonding contact layer <NUM> to sensing layer <NUM>. Contact layer <NUM> has contact surface <NUM>. For example, a user using ultrasonic fingerprint sensor <NUM> places his or her finger in contact with contact surface <NUM>.

As illustrated, matching layer <NUM> includes five regions 754a through 754e, where the thickness of each region 754a through 754e is substantially equal to a quarter wavelength of the transmission frequency of each corresponding region 752a through 752e of sensing layer <NUM>. In some embodiments, the thickness at each corresponding region 754a through 754e is substantially equal to a quarter wavelength of the transmission frequency of the corresponding region 752a through 752e of sensing layer <NUM> plus or minus a one-eighth wavelength of the transmission frequency at the region (e.g., a range of a quarterwavelength plus or minus a one-eighth wavelength of the transmission frequency). As illustrated, the thickness of region 754a of matching layer <NUM> is substantially equal to a quarter wavelength of the transmission frequency of corresponding region 752a of sensing layer <NUM>, the thickness of region 754b of matching layer <NUM> is substantially equal to a quarter wavelength of the transmission frequency of corresponding region 752b of sensing layer <NUM>, the thickness of region 754c of matching layer <NUM> is substantially equal to a quarter wavelength of the transmission frequency of corresponding region 752c of sensing layer <NUM>, the thickness of region 754d of matching layer <NUM> is substantially equal to a quarter wavelength of the transmission frequency of corresponding region 752d of sensing layer <NUM>, and the thickness of region 754e of matching layer <NUM> is substantially equal to a quarter wavelength of the transmission frequency of corresponding region 752e of sensing layer <NUM>. It should be appreciated that the thickness of at least two regions 754a through 754e are different. As illustrated, the thickness of regions 754a and 754e are substantially equal and the thickness of regions 754b and 754d are substantially equal.

It should be appreciated that the slope or curve of matching layer <NUM> may dictate or influence the number of different regions of matching layer <NUM>, and that the five regions 754a through 754b of matching layer <NUM> are examples. The number of different regions may vary based on the tolerance for accuracy, the slope of the concave non-uniform thickness, the number of available transmission frequencies, the control of the available transmission frequencies, the applications utilizing the fingerprint sensor, etc. For example, where a system has a certain number of adjustable or selectable transmission frequencies, embodiments herein are configured to group ultrasonic transducers to best fit the available transmission frequencies, within an acceptable tolerance, into regions of sensing layer <NUM>, corresponding to the regions of matching layer <NUM>.

In some embodiments, the frequency of the acoustic waves is adapted to the local thickness of the matching layer, or any other layer used for creating constructive interference. Frequency grouping maps similar to those described in relation to <FIG> may be used for this purpose. The frequency for each transducer or group of transducers may be determined in a calibration phase, where the frequency is varied and the effect on the signal strength is observed. As the frequency is varied, the signal will go through maxima and minima, corresponding to quarter wavelength and half wavelength effects, respectively. A frequency corresponding to a maximum may then be selected for optimum quarter wavelength effect, e.g., as illustrated in <FIG>. The use of the quarter wavelength layer may be used with flat surface or surfaces of non-uniform thickness. The use of the quarter wavelength layer may be especially important in combination with materials with a large acoustic impedance mismatch, such as e.g. stainless steel.

<FIG> illustrates an example graph <NUM> of transmission versus matching layer thickness of an ultrasonic fingerprint sensor for different contact layer materials, according to embodiments. Graph <NUM> illustrates that for various contact layer materials (e.g., aluminum, titanium, and stainless steel), the transmission frequency signal is highest at a matching layer having a thickness of a quarter wavelength of the transmission frequency. For example, an aluminum cover has an impedance ZL of approximately <NUM>. At a transmission frequency of <NUM>, as shown at line <NUM>, the thickness of a matching layer that provides the highest signal is approximately <NUM>, as indicated by arrow <NUM> (as well as approximately <NUM>, approximately <NUM>, etc.) This highest signal at approximately <NUM> corresponds to substantially a quarter wavelength of the transmission frequency of <NUM>. Moreover, as can be seen in <FIG>, the thickness of a matching layer that provides the highest signal is also approximately <NUM> for titanium and stainless steel covers. While the signal strength varies for all cover materials (e.g., based on material acoustic impedance), the thickness of the matching layer for a given transmission frequency is the same.

As can be seen in graph <NUM> at range <NUM>, the thickness of a matching layer that provides the highest signal is approximately <NUM>, the range <NUM> at which a signal strength is at or close to the highest is substantially a quarter wavelength plus or minus a one-eighth wavelength of the transmission frequency. Beyond range <NUM>, at the flexion point of the curve, the signal strength decreases. In some embodiment, a narrower range <NUM> is used (e.g., a quarter wavelength plus or minus a one-sixteenth wavelength of the transmission frequency) for providing an smaller tolerance of signal.

In some embodiments, the thickness of the matching layer is selected based on the transmission frequency of the underlying sensor and the cover material. In some embodiments, the transmission frequency is tuned according to the thickness of the matching layer that provides the highest signal. In some embodiments, a thickness of a matching layer is selected based on a desired transmission frequency, and the transmission frequency is then tuned or slightly adjusted to achieve a transmission frequency where the thickness is substantially equal to a quarter wavelength. This then compensates for the fact that the actual thickness may not be exactly equal to the desired thickness.

<FIG>, <FIG> illustrate examples of transmission frequency regions for use by ultrasonic transducers of an ultrasonic fingerprint sensor underlying a matching layer of non-uniform thickness, according to embodiments. As the thickness of the matching layer varies over the surface of the ultrasonic sensor layer, different regions of the matching layer correspond to different transmission frequencies for the underlying ultrasonic transducers for generating ultrasonic signals that have the highest signal strength. It should be appreciated that the regions can cover a range of signal strengths corresponding to a quarter wavelength plus or minus a one-eighth wavelength of the transmission frequency. The grouping of ultrasonic transducers into regions allows for the collective control of ultrasonic transducers, and is useful in selecting regions where the tolerance of the signal strength can be collectively controlled within the appropriate signal strength range.

<FIG> illustrates an example of block transmission frequency regions 910a through 910n (generically referred to as a region <NUM>) for use by an ultrasonic fingerprint sensor <NUM> underlying a matching layer of non-uniform thickness, according to an embodiment. As illustrated, ultrasonic fingerprint sensor900 has <NUM>×<NUM> regions having varying transmission frequencies. It should be appreciated that a region <NUM> can include one or more ultrasonic transducers. It should also be appreciated that some regions <NUM> can have the same operating transmission frequency of ultrasonic transducers. Although the regions are shown with substantially equivalent sizes, the size of the different regions may vary, depending on the thickness of the matching layer and/or selected frequencies.

<FIG> illustrates an example of ring-shaped transmission frequency regions 930a through 930n (generically referred to as a region <NUM>) for use by an ultrasonic fingerprint sensor <NUM> underlying a matching layer of non-uniform thickness, according to an embodiment. For example, ring-shaped transmission frequency regions <NUM> of <FIG> may be used in a dimple or curved (e.g., concave or convex) shaped matching layer. As illustrated, ultrasonic fingerprint sensor <NUM> four regions having varying transmission frequencies where region 930d is circular, regions 930b and 930c are ring-shaped, and region 930a is square shaped with a circular cutout. It should be appreciated that a region <NUM> can include one or more ultrasonic transducers. It should also be appreciated that some regions <NUM> can have the same operating transmission frequency of ultrasonic transducers.

<FIG> illustrates an example of column-shaped transmission frequency regions 950a through 950n (generically referred to as a region <NUM>) for use by an ultrasonic fingerprint sensor <NUM> underlying a matching layer of non-uniform thickness, according to an embodiment. For example, column-shaped transmission frequency regions <NUM> of <FIG> may be used in a matching layer having a thickness change in the horizontal direction. As illustrated, ultrasonic fingerprint sensor940 has five column-shaped regions having varying transmission frequencies. It should be appreciated that a region <NUM> can include one or more ultrasonic transducers. It should also be appreciated that some regions <NUM> can have the same operating transmission frequency of ultrasonic transducers.

<FIG> illustrates an example of row-shaped transmission frequency regions 970a through 970n (generically referred to as a region <NUM>) for use by an ultrasonic fingerprint sensor <NUM> underlying a matching layer of non-uniform thickness, according to an embodiment. For example, row-shaped transmission frequency regions <NUM> of <FIG> may be used in a matching layer having a thickness change in the vertical direction. As illustrated, ultrasonic fingerprint sensor960 has five row-shaped regions having varying transmission frequencies. It should be appreciated that a region <NUM> can include one or more ultrasonic transducers. It should also be appreciated that some regions <NUM> can have the same operating transmission frequency of ultrasonic transducers.

During imaging, different portions of an image are captured using the different transmission frequencies at the different regions. Pixels of the frequency regions may be captured based on the order of the frequency regions, for example with reference to <FIG>, starting with region 910a, then 910b, etc. Alternatively, pixels of the frequency regions may be captured in order of frequency, going from a low frequency to a high frequency, or vice-versa. It should be appreciated that portions can be individually captured or all portions using the same transmission frequency can be captured at the same imaging operation. In other embodiments, entire images may be captured using particular transmission frequencies, and the pertinent pixels may be extracted from these images for use in the final output image. The pixels corresponding to the frequency regions are then combined into a single output image.

It should be appreciated that each frequency region may operate to capture pixels during an imaging process in parallel, in series, or concurrently. Where the pixels are captured in series, the image capture may have latency, so the decision as to whether to capture pixels of different frequency regions in parallel, in series, or concurrently may be device or application specific. For example, different applications may have a requirement for higher resolution images (e.g., security applications). Accordingly, security applications may utilize in series pixel capture to capture a high resolution image, accepting the additional latency, while other applications may utilize different pixel capture ordering in parallel or concurrently to capture images with lower latency.

<FIG> and <FIG> illustrate flow diagrams of example methods for operating a fingerprint sensor comprised of ultrasonic transducers, according to various embodiments. Procedures of these methods will be described with reference to elements and/or components of various figures described herein. It is appreciated that in some embodiments, the procedures may be performed in a different order than described, that some of the described procedures may not be performed, and/orthat one or more additional procedures to those described may be performed. The flow diagrams include some procedures that, in various embodiments, are carried out by one or more processors (e.g., a host processor or a sensor processor) under the control of computer-readable and computer-executable instructions that are stored on non-transitory computer-readable storage media. It is further appreciated that one or more procedures described in the flow diagrams may be implemented in hardware, or a combination of hardware with firmware and/or software.

<FIG> illustrates a flow diagram <NUM> of an example method for operating an ultrasonic sensor including a two-dimensional array of ultrasonic transducers, according to embodiments. The ultrasonic sensor includes a two-dimensional array of ultrasonictransducers, a contact layer overlying the two-dimensional array of ultrasonic transducers, and a matching layer between the two-dimensional array of ultrasonic transducers and the contact layer for bonding the two-dimensional array of ultrasonic transducers to the contact layer, where the matching layer has a non-uniform thickness overlying the two-dimensional array of ultrasonic transducers.

in some embodiments, the matching layer has an acoustic impedance between an acoustic impedance of the two-dimensional array of ultrasonic transducers and an acoustic impedance of the contact layer. In some embodiments, the ultrasonic sensor further includes a packaging epoxy overlying the two-dimensional array of ultrasonic transducers, such that the matching layer is connected to the packaging epoxy and the contact layer. In some embodiments, a first thickness of the matching layer at a first region is substantially equal to a quarter wavelength of the first transmission frequency plus or minus a one-eighth wavelength of the first transmission frequency and a second thickness of the matching layer at a second region is substantially equal to a quarter wavelength of the second transmission frequency plus or minus a one-eighth wavelength of the second transmission frequency.

At procedure <NUM> of flow diagram <NUM>, a first subset of ultrasonic transducers of the two-dimensional array of ultrasonic transducers associated with a first region of the two-dimensional array of ultrasonic transducers is activated at a first transmission frequency to image a first subset of pixels, wherein the first transmission frequency is determined such that a first thickness of the matching layer at the first region is substantially equal to a quarter wavelength of the first transmission frequency.

At procedure <NUM>, a second subset of ultrasonic transducers of the two-dimensional array of ultrasonic transducers associated with a second region of the two-dimensional array of ultrasonic transducers is activated at a second transmission frequency to image a second subset of pixels, where the second transmission frequency is determined such that a second thickness of the matching layer at the second region is substantially equal to a quarter wavelength of the second transmission frequency, wherein the first region and the second region are non-overlapping regions of the two-dimensional array of ultrasonic transducers and wherein the first thickness and the second thickness are different. It should be appreciated that procedure <NUM> can be performed sequential to, parallel to, or concurrent to procedure <NUM>.

In some embodiments, as shown at procedure <NUM>, a third subset of ultrasonic transducers of the two-dimensional array of ultrasonic transducers associated with a third region of the two-dimensional array of ultrasonic transducers is activated at a third transmission frequency to image a third subset of pixels, wherein a third thickness of the matching layer at the third region is substantially equal to a quarter wavelength of the third transmission frequency, wherein the first region, the second region, and the third region are non-overlapping regions of the two-dimensional array of ultrasonic transducers. It should be appreciated that procedure <NUM> can be performed sequential to, parallel to, or concurrent to procedures <NUM> and/or <NUM>.

At procedure <NUM>, the first subset of pixels and the second subset of pixels are combined into a compound fingerprint image, wherein the compound fingerprint image compensates for the non-uniform thickness of the matching layer. In one embodiment, the third subset is also combined into the compound fingerprint image.

<FIG> illustrates a flow diagram <NUM> of an example method for calibrating an ultrasonic sensor including a two-dimensional array of ultrasonic transducers, according to embodiments. The ultrasonic sensor includes a two-dimensional array of ultrasonic transducers, a contact layer overlying the two-dimensional array of ultrasonic transducers, and a matching layer between the two-dimensional array of ultrasonic transducers and the contact layer, where the matching layer has a non-uniform thickness overlying the two-dimensional array of ultrasonic transducers. In some embodiments, a thickness of the matching layer at each region is substantially equal to a quarter wavelength of a transmission frequency for the region. In some embodiments, the thickness of the matching layer at each region is substantially equal to a quarter wavelength of the transmission frequency plus or minus a one-eighth wavelength of the transmission frequency at the region. In some embodiments, the matching layer is an adhesive layer for bonding the two-dimensional array of ultrasonic transducers to the contact layer. In some embodiments, the matching layer has an acoustic impedance between an acoustic impedance of the two-dimensional array of ultrasonic transducers and an acoustic impedance of the contact layer.

At procedure <NUM> of flow diagram <NUM>, ultrasonic transducers of the two-dimensional array of ultrasonic transducers associated with a plurality of regions of the two-dimensional array of ultrasonic transducers are activated at a plurality of transmission frequencies, where the plurality of regions are non-overlapping. At procedure <NUM> of flow diagram <NUM>, a signal for each of the plurality of transmission frequencies at the plurality of regions is monitored.

in one embodiment, as shown at procedure <NUM>, it is determined how ultrasonic transducers are grouped with the available frequencies. In one embodiment, where there are a certain number of fixed transmission frequencies, it is determined which ultrasonic transducers are grouped with which transmission frequency, where the frequency is chosen such that tile matching layer thickness is closest to a quarter wavelength. In another embodiment, where there are a certain number of flexible transmission frequencies, it is determined which ultrasonic transducers are grouped with which transmission frequency. For example, a distribution of transmission frequencies and groups is determined to get the best result, e.g., optimizing a loss function for the total sensor, where a loss function depends on deviation from ideal quarter wavelength setting. In another embodiment, where there are unlimited or individually controllable transmission frequencies, each ultrasonic transducers is assigned a transmission frequency. At procedure <NUM>, for the plurality of regions, a transmission frequency of the plurality of transmission frequencies having a highest signal for a region is set as the transmission frequency for the region.

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 without departing from the scope of the appended claims. Accordingly, the claimed subject matter is intended to embrace all such alterations, modifications, and variations that fall within the scope of the appended claims.

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.

Claim 1:
An ultrasonic sensor comprising:
a two-dimensional array of ultrasonic transducers, wherein the two-dimensional array of ultrasonic transducers is substantially flat;
a contact layer overlying the two-dimensional array of ultrasonic transducers;
a matching layer between the two-dimensional array of ultrasonic transducers and the contact layer, wherein the matching layer has a non-uniform thickness overlying the two-dimensional array of ultrasonic transducers; and
an array controller configured to control activation of ultrasonic transducers during an imaging operation for imaging a plurality of pixels at a plurality of positions within the two-dimensional array of ultrasonic transducers, such that during the imaging operation, the array controller is configured to:
activate (<NUM>) a first subset of ultrasonic transducers of the two-dimensional array of ultrasonic transducers associated with a first region of the two-dimensional array of ultrasonic transducers at a first transmission frequency, wherein the first transmission frequency is determined such that a first thickness of the matching layer at the first region is substantially equal to a quarter wavelength of the first transmission frequency;
activate (<NUM>) a second subset of ultrasonic transducers of the two-dimensional array of ultrasonic transducers associated with a second region of the two-dimensional array of ultrasonic transducers at a second transmission frequency, wherein the second transmission frequency is determined such that a second thickness of the matching layer at the second region is substantially equal to a quarter wavelength of the second transmission frequency, wherein the first region and the second region are non-overlapping regions of the two-dimensional array of ultrasonic transducers and wherein the first thickness and the second thickness are different; and
combine (<NUM>) the plurality of pixels into a compound fingerprint image, wherein the compound fingerprint image compensates for the non-uniform thickness of the matching layer.