Under-display ultrasonic fingerprint sensors for foldable displays

Abstract

An apparatus may include an ultrasonic sensor stack, a foldable display stack, and a transmission enhancement layer. The foldable display stack may include a display reinforcement and a display stack layer. The display stack layer may form one or more display stack resonators configured to enhance ultrasonic waves transmitted by the ultrasonic sensor stack within a first ultrasonic frequency range. In some embodiments, the transmission enhancement resonator may include the display reinforcement and the transmission enhancement layer. In some examples, the transmission enhancement resonator may include at least a portion of the ultrasonic sensor stack. The transmission enhancement resonator may be configured to enhance ultrasonic waves transmitted by the ultrasonic sensor stack within a first ultrasonic frequency range.

Description

Technical Field

[0001] Priority Statement

[0002] This application claims priority to U.S. Patent Application No. 17 / 446,436, filed August 30, 2021, entitled “UNDER-DISPLAY ULTRASONIC FINGERPRINT SENSORS FOR FOLDABLE DISPLAYS”, which is incorporated herein by reference for all purposes.

[0003] This disclosure generally relates to sensing devices and related methods, including but not limited to ultrasonic sensing systems and methods of using such systems. Prior Technology

[0004] Biometric authentication can refer to important characteristics used to control access to devices. Many existing products include some type of biometric authentication. While some existing biometric authentication technologies offer satisfactory performance, there is still a need for improved methods and devices. Summary of the Invention

[0005] The systems, methods, and apparatuses disclosed herein each possess several innovative forms, none of which are solely responsible for the desired properties disclosed herein.

[0006] One innovative embodiment of the subject matter described in this disclosure can be implemented in a device. The device may include an ultrasonic sensor stack, a foldable display stack, and a transmission enhancement layer. In some examples, the foldable display stack may include a display reinforcement and a display stack layer configured to cause ultrasonic waves transmitted by the ultrasonic sensor stack to have one or more display stack-induced local amplitude maxima within a first ultrasonic frequency range. In some examples, the display reinforcement and the transmission enhancement layer may form at least a portion of a transmission enhancement resonator. In some specific embodiments, the transmission enhancement resonator may be configured to cause ultrasonic waves transmitted by the ultrasonic sensor stack to have transmission enhancement resonator-induced local amplitude maxima within a first ultrasonic frequency range.

[0007] Depending on some specific implementations, the device can be a mobile device. The mobile device may include a stack of ultrasonic sensors, a stack of foldable displays, and a transmission enhancement layer.

[0008] In some examples, the maximum local amplitude induced by the transmission enhancement resonator can correspond to frequencies in the range of 5 MHz to 15 MHz. In other examples, the maximum local amplitude induced by the transmission enhancement resonator can correspond to frequencies in the range of 1 MHz to 20 MHz. In some examples, the thickness of the transmission enhancement resonator can correspond to a multiple of half the wavelength of the transverse or longitudinal wave within the first ultrasonic frequency range.

[0009] According to some examples, at least some of the display stack layers may include (and / or may form) one or more display stack resonators. In some such examples, the one or more display stack resonators may be configured to cause a local amplitude maxima induced by the one or more display stacks. In some examples, the one or more display stack resonators may include a first resonator defined by a glass layer of display reinforcement and a foldable display stack. According to some such examples, the first resonator may include multiple layers of an organic light-emitting diode display. In some examples, the one or more display stack resonators may include a second resonator defined by a glass layer and an outer surface of the foldable display stack.

[0010] In some examples, the transmission enhancement layer may be located between the ultrasonic sensor stack and the display reinforcement. According to some examples, the thickness of the transmission enhancement layer may be less than a quarter wavelength of a transverse or longitudinal wave in a first ultrasonic frequency range. In some specific embodiments, the transmission enhancement layer may be or may include one or more of the following: an aluminum layer with a thickness in the range of 50 to 100 micrometers, a copper layer with a thickness in the range of 25 to 50 micrometers, or a stainless steel layer with a thickness in the range of 25 to 50 micrometers. According to some examples, the transmission enhancement layer may include at least a portion of the ultrasonic sensor stack.

[0011] According to some embodiments, the device may include a first adhesive layer located between the transmission enhancement layer and the display reinforcement. In some embodiments, the device may include a second adhesive layer located between the transmission enhancement layer and the ultrasonic sensor stack. According to some examples, the thickness of the first adhesive layer and / or the second adhesive layer may be in the range of 3 micrometers to 10 micrometers.

[0012] In some examples, the ultrasonic sensor stack may include a thin-film transistor (TFT) substrate. According to some such examples, the transmission enhancement resonator may include a TFT substrate. In some cases, the thickness of the TFT substrate may be less than a quarter wavelength of the transverse or longitudinal wave in the first ultrasonic frequency range. In some examples, the thickness of the TFT substrate may range from 50 micrometers to 200 micrometers. According to some examples, the TFT substrate may be made of glass, or may include glass.

[0013] According to some specific implementations, the TFT substrate may have a first acoustic impedance value. In some such examples, the display reinforcement may have a second acoustic impedance value greater than the first acoustic impedance value. In some examples, the transmission enhancement layer may have a third acoustic impedance value greater than the first acoustic impedance value.

[0014] According to some examples, the display reinforcement may be or may include one or more of a metal or non-metal layer with an acoustic impedance of 10 MRayls or greater. In some examples, the thickness of the display reinforcement may range from 30 micrometers to 300 micrometers. According to some examples, the thickness of the display reinforcement may correspond to a multiple of half the wavelength of a transverse or longitudinal wave in a second ultrasonic frequency range higher than the first ultrasonic frequency range.

[0015] In some implementations, the device may include a control system. This control system may include one or more general-purpose single-chip or multi-chip processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs) or other programmable logic elements, discrete gate or transistor logic, discrete hardware components, or combinations thereof. According to some examples, the control system may be configured to cause an ultrasonic sensor stack to transmit a first ultrasonic wave within a first ultrasonic frequency range and to perform an authentication process based at least in part on ultrasonic sensor signals corresponding to the reflection of the first ultrasonic wave.

[0016] Other innovative variations of the subject matter described in this disclosure can be implemented in the method. In some examples, the method may involve controlling an ultrasonic transceiver layer of an ultrasonic sensor system via a control system to transmit ultrasonic waves through at least a first resonator and a second resonator, the first resonator being configured to cause a first local maximum in the ultrasonic wave transmission within a first ultrasonic wave frequency range, and the second resonator being configured to cause a second local maximum in the ultrasonic wave transmission within the first ultrasonic wave frequency range. In some examples, the first resonator may include a display reinforcement layer and a transmission enhancement layer. In some cases, the second resonator may include one or more display stack layers.

[0017] In some examples, the method may involve a control system receiving, and from an ultrasonic sensor system, an ultrasonic sensor signal corresponding to the reflection of transmitted ultrasonic waves from a portion of a target object located on the outer surface of a device including the ultrasonic sensor system. According to some examples, the method may involve a control system performing an authentication process based at least in part on the ultrasonic sensor signal.

[0018] In some specific implementations, the local maximum value of ultrasonic transmission may correspond to a frequency in the range of 5 MHz to 15 MHz. However, in other specific implementations, the local maximum value of ultrasonic transmission may correspond to a frequency in the range of 1 MHz to 20 MHz.

[0019] In some examples, the authentication process may involve extracting target object features from ultrasonic sensor signals. These target object features may, for example, include fingerprint features and / or subcutaneous features. In some examples, the method may involve controlling access to the device based at least in part on the authentication process.

[0020] Some or all of the operations, functions, and / or methods described herein can be executed by one or more devices according to instructions (e.g., software) stored on one or more non-transitory media. Such non-transitory media may include memory devices such as those described herein, including but not limited to random access memory (RAM) devices, read-only memory (ROM) devices, etc. Therefore, some innovative forms of the subject matter described herein can be implemented in one or more non-transitory media on which software is stored.

[0021] For example, the software may include instructions for controlling one or more devices to perform a method. According to some examples, the method may involve controlling an ultrasonic transceiver layer of an ultrasonic sensor system via a control system to transmit ultrasonic waves through at least a first resonator and a second resonator, the first resonator being configured to cause a first local maximum in a first ultrasonic frequency range, and the second resonator being configured to cause a second local maximum in the ultrasonic transmission within the first ultrasonic frequency range. In some examples, the first resonator may include a display reinforcement layer and a transmission enhancement layer. In some cases, the second resonator may include one or more display stack layers.

[0022] In some examples, the method may involve a control system receiving, and from an ultrasonic sensor system, an ultrasonic sensor signal corresponding to the reflection of transmitted ultrasonic waves from a portion of a target object located on the outer surface of a device including the ultrasonic sensor system. According to some examples, the method may involve a control system performing an authentication process based at least in part on the ultrasonic sensor signal.

[0023] In some specific implementations, the local maximum value of ultrasonic transmission may correspond to a frequency in the range of 5 MHz to 15 MHz. However, in other specific implementations, the local maximum value of ultrasonic transmission may correspond to a frequency in the range of 1 MHz to 20 MHz.

[0024] In some examples, the authentication process may involve extracting target object features from ultrasonic sensor signals. These target object features may, for example, include fingerprint features and / or subcutaneous features. In some examples, the method may involve controlling access to the device based at least in part on the authentication process. Simple Explanation of the Diagram

[0025] Details of one or more specific embodiments of the subject matter described in this specification are set forth in the drawings and the following description. Other features, appearances, and advantages will become apparent from the description, drawings, and requests. Note that the relative dimensions in the following drawings may not be drawn to scale. Similar element symbols and markings in the various drawings indicate the same elements.

[0026] Figure 1 is a block diagram illustrating exemplary components of an apparatus according to some disclosed specific implementations.

[0027] Figure 2 shows an example of a foldable display stack and an ultrasonic sensor stack.

[0028] Figure 3 shows an example of an ultrasonic transmission versus ultrasonic frequency curve corresponding to the example of the resonator shown in Figure 2.

[0029] Figure 4 shows an example of a foldable display stack and an ultrasonic sensor stack according to some disclosed specific implementations.

[0030] Figure 5 shows an example of an ultrasonic transmission versus ultrasonic frequency curve corresponding to the example of the resonator shown in Figure 4.

[0031] Figure 6 shows an example of a foldable display stack and an ultrasonic sensor stack.

[0032] Figure 7 shows exemplary components of an apparatus according to some disclosed specific implementations.

[0033] Figure 8 is a flowchart providing an example of operation based on some of the disclosed methods.

[0034] Figure 9 typically depicts various configurations of a 4×4 pixel array of sensor pixels used in an ultrasonic sensor system. Implementation

[0035] To describe the innovative features of this disclosure, the following description pertains to certain specific embodiments. However, those skilled in the art will readily recognize that the teachings herein can be applied in a variety of different ways. The described embodiments can be implemented in any device, apparatus, or system, including biometric identification systems, as disclosed herein. Furthermore, it is envisioned that the described embodiments can be included in or associated with a variety of electronic devices, such as, but not limited to: mobile phones, cellular phones supporting multimedia internet, mobile TV receivers, wireless devices, smartphones, smart cards, wearable devices (such as wristbands, armbands, wrist straps, rings, headbands, patches, etc.), Bluetooth® devices, personal data assistants (PDAs), wireless email receivers, handheld or portable computers, lightweight laptops, notebook computers, smartphones, tablets, printers, copiers, scanners, fax machines, global positioning systems (GPS), etc. GPS receivers / navigators, cameras, digital media players (such as MP3 players), camcorders, game consoles, watches, clocks, calculators, television monitors, flat panel displays, electronic reading devices (such as e-readers), mobile health devices, computer monitors, automotive displays (including odometer and speedometer displays), cockpit control units and / or displays, camera field-of-view displays (such as rearview camera displays in vehicles), electronic photographs, electronic billboards or signs, projectors, building structures, microwave ovens, refrigerators, stereo systems, cassette recorders or players, DVD players, CD players, video cassette recorders (VCRs), radios, portable storage chips, washing machines, dryers, washer / dryer units, parking timers, packaging (such as in electromechanical systems (EMS) applications, including microelectromechanical systems (MEMS) applications and non-EMS applications), aesthetic structures (such as displaying images on a piece of jewelry or clothing), and various EMS devices. The teachings herein can also be applied to 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 for consumer electronics products, steering wheels or other automotive parts, varactor diodes, liquid crystal elements, electrophoretic elements, drive systems, manufacturing processes, and electronic testing equipment. Therefore, these teachings are not intended to be limited to the specific implementations described only in the figures, but have broad applicability, as will be apparent to those skilled in the art.

[0036] Designing an under-display ultrasonic sensor system that provides acceptable performance is challenging. However, the assignee has successfully designed an under-display ultrasonic sensor system that is widely used in cellular phones and other display devices.

[0037] Designing an under-display ultrasonic sensor system for foldable display devices presents additional challenges. Foldable displays include a display reinforcement that helps maintain the physical integrity of the display stack when the foldable display is folded and unfolded. Display reinforcements are typically made of metal with a thickness ranging from approximately 30 micrometers to 300 micrometers, but they can also be formed of other materials with a relatively high modulus of elasticity and, in some cases, have other thicknesses. The inventors have discovered that display reinforcements significantly attenuate ultrasonic waves within a frequency range suitable for ultrasonic fingerprint sensors.

[0038] Some disclosed foldable display devices include an ultrasonic sensor stack, a transmission enhancement layer, and a foldable display stack with a display reinforcement. In some examples, the display reinforcement, the transmission enhancement layer, and at least a portion of the ultrasonic sensor stack form a transmission enhancement resonator configured to enhance the ultrasonic waves transmitted by the ultrasonic sensor stack within an ultrasonic frequency range suitable for an ultrasonic fingerprint sensor.

[0039] Specific embodiments of the subject matter described in this disclosure can be implemented to achieve one or more of the following potential advantages. Compared to foldable display devices lacking certain versions of the disclosed transmission enhancement layer, some disclosed foldable display devices enhance ultrasonic transmission within an ultrasonic frequency range suitable for ultrasonic fingerprint sensors. Specific embodiments in which the transmission enhancement layer is located between the ultrasonic sensor stack and the display reinforcement can be manufactured without modifying the existing foldable display stack, thereby avoiding additional manufacturing costs and time.

[0040] Figure 1 is a block diagram illustrating exemplary components of a device according to some disclosed embodiments. As with other disclosed embodiments, the number, type, and arrangement of elements shown in Figure 1 are presented by way of example only. Although not shown in Figure 1, device 100 may include other components such as a cover (which may be a cover glass or may include a cover glass), one or more adhesive layers, one or more electrode layers, etc. Some examples are described below. In some embodiments, device 100 may be a mobile device including the elements shown in Figure 1.

[0041] According to this example, device 100 includes an ultrasonic sensor stack 105. In some examples, the ultrasonic sensor stack 105 includes an ultrasonic transceiver layer 101 and a thin-film transistor (TFT) layer 102. In some such examples, the ultrasonic transceiver layer 101 may be configured to act as both an ultrasonic transmitter and an ultrasonic receiver. According to some specific embodiments, the ultrasonic transceiver layer 101 may be a single piezoelectric layer, while in other specific embodiments, the ultrasonic transceiver layer 101 may be a multilayer piezoelectric structure or an array of such structures.

[0042] For example, in some embodiments, the ultrasonic transceiver layer 101 may include a piezoelectric layer, such as a PVDF polymer layer or a PVDF-TrFE copolymer layer. In some embodiments, other piezoelectric materials, such as aluminum nitride (AlN) or lead zirconate titanate (PZT), may be used in the ultrasonic transceiver layer 101. Some alternative embodiments may include separate ultrasonic transmitter and ultrasonic receiver layers.

[0043] In some alternative examples, the ultrasonic transceiver layer 101 may include an array of ultrasonic transducer elements, such as an array of piezoelectric micromachined ultrasonic transducers (PMUTs), an array of capacitive micromachined ultrasonic transducers (CMUTs), etc. In some such examples, PMUT elements in a piezoelectric receiver layer, a single-layer array of PMUTs, or a single-layer array of CMUTs may be used as both an ultrasonic transmitter and an ultrasonic receiver.

[0044] The TFT layer 102 can be a metal-oxide-semiconductor field-effect transistor (MOSFET), fabricated by depositing a thin film of an active semiconductor layer, a dielectric layer, and metal contacts on a TFT substrate. In some examples, the TFT substrate can be a non-conductive material, such as glass. Depending on some specific embodiments, the TFT layer 102 can have a thickness ranging from 50 micrometers to 400 micrometers.

[0045] In this specific embodiment, the device includes a foldable display stack 111. According to this example, the foldable display stack 111 includes a display reinforcement 113 and a display stack layer 115. In some examples, the display stack layer 115 may include a light-emitting diode (LED) display layer, such as an organic light-emitting diode (OLED) display. Some examples of the display stack layer 115 are provided in this disclosure.

[0046] In this example, the display stack 115 forms one or more display stack resonators. In some examples, the display stack resonators may be configured to amplify ultrasonic waves transmitted within a first ultrasonic frequency range by the ultrasonic sensors stacked. In some examples, the one or more display stack resonators may include a first resonator defined by the display reinforcement 113 and the glass layer of the display stack 115. In some such examples, the first resonator may include multiple layers of an organic light-emitting diode display. In some examples, the one or more display stack resonators may include a second resonator defined by the glass layer and the outer surface of the foldable display stack.

[0047] In some examples, the display reinforcement 113 may have a relatively high acoustic impedance, such as 10 MRayls or higher. In some embodiments, the display reinforcement 113 may be or may include a metallic layer (e.g., a stainless steel layer with an acoustic impedance of approximately 47 MRayls). However, in other embodiments, the display reinforcement 113 may be or may include one or more other metals or non-metallic materials with a relatively high modulus of elasticity. According to some examples, the display reinforcement 113 may have a thickness ranging from 30 micrometers to 300 micrometers. According to some examples, the thickness of the display reinforcement 113 may correspond to a multiple of half the wavelength of a transverse or longitudinal wave in a second ultrasonic frequency range above the first ultrasonic frequency range. However, in some cases, the display reinforcement 113 may not be or may not include a material with high acoustic impedance. For example, in some cases, the display reinforcement 113 may be or may include a plastic layer, such as a polycarbonate layer. In some such cases, the disclosed transmission enhancement layer 103 may not be advantageous.

[0048] According to this example, device 100 includes a transmission enhancement layer 103. In some examples, the transmission enhancement layer 103 may be or may include the following: an aluminum layer with a thickness in the range of 50 to 100 micrometers, a copper layer with a thickness in the range of 25 to 50 micrometers, or a stainless steel layer with a thickness in the range of 25 to 50 micrometers. According to some examples, the thickness of the transmission enhancement layer 103 may be less than a quarter wavelength of a transverse or longitudinal wave in a first ultrasonic frequency range. In some examples, the transmission enhancement layer 103 may be located between the ultrasonic sensor stack 105 and the foldable display stack 111. In some such examples, the transmission enhancement layer 103 may be located between the ultrasonic sensor stack 105 and the display reinforcement 113. According to some such examples, the display reinforcement 113, the transmission enhancement layer 103, and at least a portion of the ultrasonic sensor stack 105 (e.g., the TFT substrate of the TFT layer 102) form a transmission enhancement resonator configured to enhance ultrasonic waves transmitted by the ultrasonic sensor stack in an ultrasonic frequency range suitable for an ultrasonic fingerprint sensor. Some examples are described below.

[0049] In some examples, device 100 may include control system 106. Control system 106 (where present) may include one or more general-purpose single-chip or multi-chip processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs) or other programmable logic elements, discrete gate or transistor logic, discrete hardware components, or combinations thereof. Control system 106 may also include (and / or be configured to communicate with) one or more memory devices, such as one or more random access memory (RAM) devices, read-only memory (ROM) devices, etc. Therefore, device 100 may have a memory system including one or more memory devices, although the memory system is not shown in FIG1. ​​Control system 106 may be able to receive and process data from ultrasonic transceiver layer 101 and / or from sensor pixel arrays, for example, as described below. In some specific implementations, the functionality of control system 106 may be divided among one or more controllers or processors, such as a dedicated sensor controller and an application processor for the mobile device.

[0050] Some specific implementations of device 100 may include interface system 107. In some examples, the interface system may include a wireless interface system. In some specific implementations, the interface system may include a user interface system, one or more network interfaces, one or more interfaces between control system 106 and memory system, and / or one or more interfaces (e.g., ports or application processors) between control system 106 and one or more external device interfaces.

[0051] Interface system 107 can be configured to provide communication between components of device 100 (which may include wired or wireless communication, such as electrical communication, radio communication, etc.). In some such examples, interface system 107 can be configured to provide communication between control system 106 and ultrasonic receiver layer 101, communication between control system 106 and one or more display stack layers 115, and / or communication between control system 106 and sensor pixel array. According to some such examples, a portion of interface system 107 may, for example, couple at least a portion of control system 106 to ultrasonic receiver layer 101 and / or sensor pixel array via a conductive material.

[0052] According to some examples, interface system 107 can be configured to provide communication between device 100 and other devices and / or humans. In some such examples, interface system 107 may include one or more user interfaces. In some examples, interface system 107 may include one or more network interfaces and / or one or more external device interfaces (such as one or more Universal Serial Bus (USB) interfaces). In some specific implementations, device 100 may include a memory system. In some examples, interface system 107 may include at least one interface between control system 106 and memory system.

[0053] Device 100 can be used in a variety of different environments, and many examples of these different environments are disclosed herein. For example, in some embodiments, mobile devices (such as cellular phones, smartphones, tablets, laptops (e.g., laptop touchpads), etc.) may include at least a portion of device 100. In some embodiments, wearable devices may include at least a portion of device 100. For example, a wearable device may be a watch, bracelet, armband, wristband, ring, headband, or patch. In some embodiments, control system 106 may reside in more than one device. For example, a portion of control system 106 may reside in a wearable device, and another portion of control system 106 may reside in another device, such as a mobile device (e.g., a smartphone or tablet) and / or a server. In some such examples, interface system 107 may also reside in more than one device.

[0054] Figure 2 shows an example of a stack of foldable displays and a stack of ultrasonic sensors. The types, quantities, and arrangements of the components shown in Figure 2 are merely examples. Other examples may include components of different types, quantities, and / or arrangements. Furthermore, the components shown in Figure 2 are not drawn to scale.

[0055] Device 200 is similar to device 100 shown in FIG. 1, but does not include the transmission enhancement layer 103. Device 200 includes an example of the ultrasonic sensor stack 105 and foldable display stack 111 of FIG. 1. Here, the ultrasonic sensor stack 105 is attached to the foldable display stack 111 via an adhesive layer 202a. The adhesive layer 202a may be or may include a thin pressure-sensitive adhesive (PSA). In some cases, the adhesive layer 202a may be in the range of 2 micrometers to 10 micrometers.

[0056] The ultrasonic sensor stack 105 includes a TFT layer 102, an ultrasonic transceiver layer 101, and an electrode layer 210. The TFT layer 102 is located between the ultrasonic transceiver layer 101 and the foldable display stack 111, and an adhesive layer 202a connects the TFT layer 102 to the foldable display stack 111. In some alternative examples, the ultrasonic transceiver layer 101 may be located between the TFT layer 102 and the foldable display stack 111. The ultrasonic transceiver layer 101 may be or may include one or more piezoelectric materials, such as piezoelectric polymers and / or piezoelectric copolymers. The electrode layer 210 may be or may include conductive ink (e.g., silver ink). In this case, the ultrasonic sensor stack 105 includes a passivation layer 212. The passivation layer 212 may be or may include an epoxy resin film.

[0057] The foldable display stack 111 includes a display reinforcement 113 located between the TFT layer 102 and the other layers of the foldable display stack 111. The display reinforcement 113 provides structural support for the other layers of the foldable display stack 111. In some examples, the display reinforcement 113 may be or may include a high-impedance material (in other words, a material with high acoustic impedance), such as a metal.

[0058] The foldable display stack 111 includes one or more screen protective layers 211, which in some cases may include a polyethylene terephthalate (PET) layer. The foldable display stack 111 includes a glass layer 203, a polarizer layer 222, an OLED panel 214, and one or more protective films 215. Here, an optically clear adhesive (OCA) layer 204a connects one or more screen protective layers 211 to the glass layer 203, and an OCA layer 204b connects the glass layer 203 to the polarizer layer 222. A polarizer pressure-sensitive adhesive 213a connects the polarizer 222 to the OLED panel 214, and a pressure-sensitive adhesive 213b connects the OLED panel 214 to one or more protective films 215. The polarizer pressure-sensitive adhesive 213a may be, for example, an optically clear adhesive (OCA).

[0059] In this example, the adhesive layer 202b and the layers of the foldable display stack 111 form a resonator 250a, which is defined by a glass layer 203 and a display reinforcement 113. According to this example, the resonator 250a includes an OCA layer 204b, a polarizer layer 222, a polarizer pressure-sensitive adhesive 213a, an OLED panel 214, pressure-sensitive adhesive 213b, one or more protective films 215, and the adhesive layer 202b. In some examples, the thickness of the resonator 250a may correspond to a multiple N of half the wavelength at the peak frequency of the ultrasonic frequency range transmitted by the ultrasonic sensor stack 105, where N is an integer greater than or equal to 1. In some such implementations, the resonator 250a may result in local maxima within the ultrasonic frequency range. According to some examples, local maxima may correspond to frequencies in the range of 5 MHz to 15 MHz or 1 MHz to 20 MHz. According to some of these specific implementations, the frequency range of the local ultrasonic transmission maximum value caused by resonator 250a can correspond to the frequency range of the local ultrasonic transmission maximum value caused by resonator 250c.

[0060] According to this example, the foldable display stack 111 also includes a resonator 250c formed by one or more screen protector layers 211 and OCA 204a, and defined by a glass layer 203: here, the glass layer 203 has a higher acoustic impedance than the one or more screen protector layers 211 or OCA 204a. In some examples, the thickness of the resonator 250c may correspond to a multiple N of a quarter wavelength at the peak frequency of the ultrasonic frequency range transmitted by the ultrasonic sensor stack 105, where N is an integer greater than or equal to 1. In some such embodiments, the resonator 250c may cause local maxima within the ultrasonic frequency range. According to some examples, the local maxima may correspond to frequencies in the range of 5 MHz to 15 MHz or 1 MHz to 20 MHz. According to some such embodiments, the frequency range including the local ultrasonic transmission maximum value caused by the resonator 250a may correspond to the frequency range including the local ultrasonic transmission maximum value caused by the resonator 250c.

[0061] In this specific implementation, glass layer 203 forms resonator 250b. According to some examples, the maximum transmission frequency of ultrasound waves through resonator 250b can be outside (e.g., below) the frequency range that includes the local maxima caused by resonators 250a and 250c. In some examples, the thickness of resonator 250b can be less than a multiple N of the quarter-wavelength at the peak frequency of the ultrasound waves transmitted by the ultrasound sensor stack 105, where N is an integer greater than or equal to 1.

[0062] In this example, the display reinforcement 113 forms a resonator 250d. According to this example, the resonator 250d results in very low ultrasonic wave transmission in the frequency range of 5 MHz to 15 MHz. The low ultrasonic wave transmission caused by the display reinforcement 113 poses a significant challenge for the design of under-display ultrasonic sensors.

[0063] Figure 3 shows an example of ultrasonic transmission versus ultrasonic frequency curves corresponding to the example resonator shown in Figure 2. In these examples, curve 305a indicates the ultrasonic transmission amplitude of resonator 250a at various frequencies, curve 305b indicates the ultrasonic transmission amplitude of resonator 250b at various frequencies, curve 305c indicates the ultrasonic transmission amplitude of resonator 250c at various frequencies, and curve 305d indicates the ultrasonic transmission amplitude of resonator 250d at various frequencies. However, these ultrasonic transmission amplitudes are only specific examples based on the thickness and materials used in the particular foldable display stack 111.

[0064] In these examples, frequency band 301 between frequencies F1 and F2 is the desired frequency band for ultrasound imaging related to authentication. In some examples, frequency band 301 may be suitable for fingerprint imaging, while in others, it may be suitable for subcutaneous imaging. In some cases, frequency band 301 may include frequencies in the range of 5 MHz to 15 MHz or in the range of 1 MHz to 20 MHz. In one example, frequency band 301 is in the range of 6 MHz to 8 MHz. In another example, frequency band 301 is in the range of 11 MHz to 14 MHz.

[0065] According to the example shown in Figure 3, resonators 250a and 250c cause local ultrasonic transmission amplitude maxima within ultrasonic band 301. These local amplitude maxima are examples of what may be referred to herein as "display stack-induced local amplitude maxima". In these examples, the peak frequency of the local ultrasonic transmission maxima caused by resonator 250a is lower than the peak frequency of the local ultrasonic transmission maxima caused by resonator 250c. In this case, graph 305b indicates that resonator 250b does not cause a local ultrasonic transmission maxima within ultrasonic band 301. However, the ultrasonic transmission level caused by resonator 250b is greater for frequencies within ultrasonic band 301 than for higher frequencies. The terms "band" and "frequency range" are used as synonyms in this disclosure.

[0066] In this example, resonator 250d—corresponding to display reinforcement 113 shown in Figure 2—results in a local maximum ultrasonic transmission at peak frequency F3, which falls within a second ultrasonic band 307 that spans a higher frequency range than ultrasonic band 301 (in this example, from frequency F4 to frequency F5). However, in this example, resonator 250d results in a very low ultrasonic transmission level within band 301. Therefore, graph 305d illustrates an example of a low ultrasonic transmission level within the band suitable for ultrasonic fingerprint sensor imaging caused by display reinforcement 113.

[0067] Although graphs 305a-305d are merely examples corresponding to specific foldable display stack configurations, the inventors have found that the display reinforcement of a foldable display stack generally results in low ultrasonic transmission within frequency bands suitable for fingerprint imaging (e.g., in the range of 10 MHz to 20 MHz) and / or within frequency bands suitable for subepidermal imaging (e.g., in the range of 1 MHz to 10 MHz). In response to this design challenge, the inventors have developed methods and apparatus for improving ultrasonic transmission within one or more frequency bands suitable for fingerprint imaging and / or subepidermal imaging. In some disclosed specific embodiments, the display reinforcement, the transmission enhancement layer, and at least a portion of the ultrasonic sensor stack form a transmission enhancement resonator configured to enhance the ultrasonic waves transmitted by the ultrasonic sensor stack within the ultrasonic frequency range suitable for ultrasonic fingerprint sensors.

[0068] Figure 4 illustrates an example of a stack of foldable displays and a stack of ultrasonic sensors according to some disclosed embodiments. As with other disclosed embodiments, the types, quantities, and arrangements of elements shown in Figure 4 are merely examples. Other embodiments may include elements of different types, quantities, and / or arrangements. Furthermore, the elements shown in Figure 4 are not drawn to scale.

[0069] In this example, device 100a is similar to device 200 shown in FIG. 2. However, in this specific embodiment, device 100a includes an example of the transmission enhancement layer 103 described above with reference to FIG. 1. According to this example, device 100a includes an example of the ultrasonic sensor stack 105 and the foldable display stack 111 of FIG. 1. In this example, the transmission enhancement layer 103 is attached to the foldable display stack 111 via adhesive layer 202a, and the ultrasonic sensor stack 105a is attached to the transmission enhancement layer 103 via adhesive layer 202c. In some examples, adhesive layers 202a and / or 202c may be or may include a thin pressure-sensitive adhesive (PSA). In some cases, adhesive layers 202a and / or 202c may be in the range of 1 micrometer to 10 micrometers.

[0070] According to this example, the ultrasonic sensor stack 105a includes a TFT layer 102, an ultrasonic transceiver layer 101, and an electrode layer 210. According to some embodiments, the thickness of the TFT layer 102 can range from 50 micrometers to 150 micrometers, for example, 50 micrometers, 55 micrometers, 60 micrometers, 65 micrometers, 70 micrometers, 75 micrometers, 80 micrometers, 85 micrometers, 90 micrometers, 95 micrometers, 100 micrometers, 105 micrometers, 110 micrometers, 115 micrometers, 120 micrometers, 125 micrometers, 130 micrometers, 135 micrometers, 140 micrometers, 145 micrometers, or 150 micrometers. In this example, the TFT layer 102 is located between the ultrasonic transceiver layer 101 and the foldable display stack 111, and an adhesive layer 202a connects the TFT layer 102 to the foldable display stack 111. In some alternative embodiments, the ultrasonic transceiver layer 101 may be located between the TFT layer 102 and the foldable display stack 111.

[0071] According to some examples, the ultrasonic transceiver layer 101 may be or may include one or more piezoelectric materials, such as piezoelectric polymers and / or piezoelectric copolymers. In some examples, the thickness of the ultrasonic transceiver layer 101 may be in the range of 5 micrometers to 20 micrometers, for example, 5 micrometers, 6 micrometers, 7 micrometers, 8 micrometers, 9 micrometers, 10 micrometers, 11 micrometers, 12 micrometers, 13 micrometers, 14 micrometers, 15 micrometers, 16 micrometers, 17 micrometers, 18 micrometers, 19 micrometers, or 20 micrometers.

[0072] In some embodiments, electrode layer 210 may be or may include conductive ink (e.g., silver ink). However, in other embodiments, electrode layer 210 may be or may include other types of conductors, such as copper, gold, etc. In some examples, the thickness of electrode layer 210 may range from 10 micrometers to 30 micrometers, for example, 10 micrometers, 11 micrometers, 12 micrometers, 13 micrometers, 14 micrometers, 15 micrometers, 16 micrometers, 17 micrometers, 18 micrometers, 19 micrometers, 20 micrometers, 21 micrometers, 22 micrometers, 23 micrometers, 24 micrometers, 25 micrometers, 26 micrometers, 27 micrometers, 28 micrometers, 29 micrometers, or 30 micrometers.

[0073] In this configuration, the ultrasonic sensor stack 105a includes a passivation layer 212. According to some examples, the passivation layer 212 may be or may include an epoxy resin film. In some examples, the passivation layer 212 may be in the range of 10 micrometers to 30 micrometers, for example, 10 micrometers, 11 micrometers, 12 micrometers, 13 micrometers, 14 micrometers, 15 micrometers, 16 micrometers, 17 micrometers, 18 micrometers, 19 micrometers, 20 micrometers, 21 micrometers, 22 micrometers, 23 micrometers, 24 micrometers, 25 micrometers, 26 micrometers, 27 micrometers, 28 micrometers, 29 micrometers, or 30 micrometers.

[0074] In this example, the foldable display stack 111 includes the same layers described above with reference to FIG. 2. In this case, the foldable display stack 111 includes a display reinforcement 113 located between the TFT layer 102 and the other layers of the foldable display stack 111. The display reinforcement 113 provides structural support for the other layers of the foldable display stack 111. In some examples, the display reinforcement 113 may be or may include a high-resistivity material, such as a metal. According to some such embodiments, the display reinforcement 113 may include stainless steel. However, in other embodiments, the display reinforcement 113 may be or may include one or more other metals or non-metallic materials having a relatively high modulus of elasticity. In some examples, the thickness of the display reinforcement 113 can range from 30 micrometers to 300 micrometers, for example, 30 micrometers, 35 micrometers, 40 micrometers, 45 micrometers, 50 micrometers, 55 micrometers, 60 micrometers, 65 micrometers, 70 micrometers, 75 micrometers, 80 micrometers, 85 micrometers, 90 micrometers, 95 micrometers, 100 micrometers, 105 micrometers, 110 micrometers, 115 micrometers, 120 micrometers, 125 micrometers, 130 micrometers, 135 micrometers, 140 micrometers, 145 micrometers, 150 micrometers, 15 5 micrometers, 160 micrometers, 165 micrometers, 170 micrometers, 175 micrometers, 180 micrometers, 185 micrometers, 190 micrometers, 195 micrometers, 200 micrometers, 205 micrometers, 210 micrometers, 215 micrometers, 220 micrometers, 225 micrometers, 230 micrometers, 235 micrometers, 240 micrometers, 245 micrometers, 250 micrometers, 255 micrometers, 260 micrometers, 265 micrometers, 270 micrometers, 275 micrometers, 280 micrometers, 285 micrometers, 290 micrometers, 295 micrometers, or 300 micrometers.

[0075] In this example, the adhesive layer 202b and the layers of the foldable display stack 111 form a resonator 250a, which is defined by the glass layer 203 and the display reinforcement 113. In some examples, the thickness of the resonator 250a may correspond to a multiple N of half the wavelength at the peak frequency of the ultrasonic frequency range transmitted by the ultrasonic sensor stack 105a, where N is an integer greater than or equal to 1. In some such embodiments, the resonator 250a may cause a local maximum within the ultrasonic frequency range. According to some examples, the local maximum may correspond to frequencies in the range of 5 MHz to 15 MHz or 1 MHz to 20 MHz. According to some such embodiments, the frequency range including the local ultrasonic transmission maximum caused by the resonator 250a may correspond to the frequency range including the local ultrasonic transmission maximum caused by the resonator 250c.

[0076] According to this example, the foldable display stack 111 also includes a resonator 250c formed by one or more screen protector layers 211 and OCA 204a, and defined by a glass layer 203: here, the glass layer 203 has a higher acoustic impedance than the one or more screen protector layers 211 or OCA 204a. In some examples, the thickness of the resonator 250c may correspond to a multiple N of a quarter wavelength at the peak frequency of the ultrasonic frequency range transmitted by the ultrasonic sensor stack 105a, where N is an integer greater than or equal to 1. In some such embodiments, the resonator 250c may cause a local maximum within the ultrasonic frequency range. According to some examples, the local maximum may correspond to frequencies in the range of 5 MHz to 15 MHz or 1 MHz to 20 MHz. According to some such embodiments, the frequency range including the local ultrasonic transmission maximum caused by the resonator 250a may correspond to the frequency range including the local ultrasonic transmission maximum caused by the resonator 250c.

[0077] In this specific implementation, glass layer 203 forms resonator 250b. According to some examples, the maximum transmission frequency of ultrasound waves through resonator 250b can be outside the frequency range that includes the local maximum value caused by resonator 250a (e.g., below that frequency range).

[0078] In this example, resonator 250e includes display reinforcement 113, transmission enhancement layer 103, and TFT layer 102, as well as adhesive layers 202a and 202c. Although resonator 250d formed by display reinforcement 113 alone results in very low ultrasonic transmission in the frequency range of 5 MHz to 15 MHz, in this example, resonator 250e results in at least one local ultrasonic transmission maximum in the frequency range of 5 MHz to 15 MHz or 1 MHz to 20 MHz.

[0079] Resonator 250e is an example herein referred to as a “transmission enhancement resonator”, configured to enhance ultrasonic waves transmitted by the ultrasonic sensor stack within at least a first ultrasonic frequency range (e.g., band 301 of FIG. 3 or band 501 of FIG. 5). According to the example shown in FIG. 4, the transmission enhancement layer 103, the display reinforcement 113, and at least a portion of the ultrasonic sensor stack 105a (e.g., TFT layer 102) form the transmission enhancement resonator 250e. In some cases, the transmission enhancement resonator 250e can be configured to cause the ultrasonic waves transmitted by the ultrasonic sensor stack 105a to have a local amplitude maximum within the first ultrasonic frequency range (which may be referred to herein as a “transmission enhancement resonator-induced local maximum”). Such a local maximum is an example herein of “enhancing” the ultrasonic waves transmitted by the ultrasonic sensor stack within the first ultrasonic frequency range. In some examples, the transmission enhancement resonator 250e can "enhance" ultrasound in the ultrasonic frequency range by allowing ultrasound in that frequency range to pass through at a relatively high amplitude, compared to the amplitude of ultrasound in that frequency range in a device without the transmission enhancement resonator 250e. In some such specific implementations, the transmission enhancement resonator 250e can "enhance" ultrasound in the ultrasonic frequency range by acting as a bandpass filter for ultrasound in the ultrasonic frequency range. According to some examples, the transmission enhancement resonator 250e can "enhance" ultrasound in the ultrasonic frequency range by reducing the acoustic impedance of the display reinforcement 113 alone (in other words, the display reinforcement 113 in a device without the transmission enhancement resonator 250e, such as that shown in FIG. 2) to ultrasound in that ultrasonic frequency range. The combination of the transmission enhancement layer 103 and the display reinforcement 113 (and in some examples, at least a portion of the ultrasonic sensor stack 105a) can allow ultrasonic waves in this frequency range to pass through with a relatively high amplitude (compared to the amplitude of ultrasonic waves in this frequency range passed by a device excluding the transmission enhancement resonator 250e), and / or can result in local amplitude maxima in this frequency range.

[0080] In some implementations, a specific frequency range (and / or the frequency corresponding to the local maximum ultrasonic transmission value) can be selected to match the high transmission frequency range of one or more resonators of the foldable display stack 111. In some such examples, the thickness of resonator 250e may correspond to a multiple N of half the wavelength at the peak frequency of the ultrasonic frequency range transmitted by the ultrasonic sensor stack 105a, where N is an integer greater than or equal to 1. According to some such implementations, the frequency range including the local maximum ultrasonic transmission value caused by resonator 250e may correspond to the frequency range including the local maximum ultrasonic transmission value caused by resonator 250a and / or the frequency range including the local maximum ultrasonic transmission value caused by resonator 250c.

[0081] Figure 5 shows an example of ultrasonic transmission versus ultrasonic frequency curves corresponding to the example resonator shown in Figure 4. In these examples, curve 505a indicates the ultrasonic transmission amplitude of resonator 250a at various frequencies, curve 505b indicates the ultrasonic transmission amplitude of resonator 250b at various frequencies, curve 505c indicates the ultrasonic transmission amplitude of resonator 250c at various frequencies, and curve 505e indicates the ultrasonic transmission amplitude of resonator 250e at various frequencies. However, these ultrasonic transmission amplitudes are only specific examples based on the thickness and materials used in the particular foldable display stack 111.

[0082] In these examples, frequency band 501 between frequencies F1 and F2 is the desired frequency band for ultrasound imaging related to authentication. In some examples, frequency band 501 may be suitable for fingerprint imaging. In some specific implementations, frequency band 501 may be suitable for subcutaneous imaging. In some cases, frequency band 501 may include frequencies in the range of 5 MHz to 15 MHz or 1 MHz to 20 MHz. In one example, frequency band 501 may be in the range of 6 MHz to 8 MHz. In one example, frequency band 501 may be in the range of 11 MHz to 14 MHz.

[0083] According to the example shown in Figure 5, resonators 250a and 250c cause local ultrasonic transmission amplitude maxima within frequency band 501. In these examples, the peak frequency of the local ultrasonic transmission amplitude maxima caused by resonator 250a is lower than the peak frequency of the local ultrasonic transmission amplitude maxima caused by resonator 250c. In this case, graph 505b indicates that resonator 250b does not cause local ultrasonic transmission amplitude maxima within frequency band 501. However, the ultrasonic transmission level caused by resonator 250b is greater for frequencies within frequency band 501 than for frequencies above frequency band 501.

[0084] Unlike the resonator 250d described above—corresponding solely to the display reinforcement 113 shown in Figure 2—resonator 250e causes a local peak in ultrasonic transmission amplitude within frequency band 501. As described above, resonator 250e is an example herein referred to as a “transmission-enhancing resonator” configured to amplify ultrasonic waves transmitted by a stack of ultrasonic sensors within at least a first ultrasonic frequency range (in this example, frequency band 501). The local peak in ultrasonic transmission amplitude within frequency band 501 is an example herein referred to as “enhancing” ultrasonic waves transmitted by a stack of ultrasonic sensors within a first ultrasonic frequency range. In some examples, the transmission-enhancing resonator 250e can “enhance” ultrasonic waves within an ultrasonic frequency range by allowing ultrasonic waves in that frequency range to pass through at a relatively high amplitude, compared to the amplitude of ultrasonic waves in that frequency range in a device without the transmission-enhancing resonator 250e. In some such specific implementations, the transmission-enhancing resonator 250e can “enhance” ultrasonic waves within an ultrasonic frequency range by acting as a bandpass filter for ultrasonic waves within that frequency range.

[0085] Figure 6 illustrates an example of a stack of foldable displays and a stack of ultrasonic sensors. As with other disclosed embodiments, the types, quantities, and arrangements of elements shown in Figure 6 are merely examples. Other embodiments may include elements of different types, quantities, and / or arrangements. Furthermore, the elements shown in Figure 6 are not drawn to scale.

[0086] In this example, device 100b is similar to device 100a shown in FIG. 4 because, in this specific implementation, device 100b includes an example of the aforementioned transmission enhancement layer 103. Similar to the example of device 100a described above with reference to FIG. 4, device 100b also includes an example of the ultrasonic sensor stack 105 and the foldable display stack 111 of FIG. 1. As in the example described above with reference to FIG. 4, in this example, the transmission enhancement layer 103 is attached to the foldable display stack 111 via adhesive layer 202a, and the ultrasonic sensor stack 105a is attached to the transmission enhancement layer 103 via adhesive layer 202c. In some examples, adhesive layers 202a and / or 202c may be or may include a thin pressure-sensitive adhesive (PSA). In some cases, adhesive layers 202a and / or 202c may be in the range of 1 micrometer to 10 micrometers.

[0087] According to this example, the ultrasonic sensor stack 105a includes a TFT layer 102, an ultrasonic transceiver layer 101, an electrode layer 210, and a passivation layer 212. However, unlike the specific embodiment described above with reference to FIG4, in this example, the ultrasonic transceiver layer 101, the electrode layer 210, and the passivation layer 212 are located between the TFT layer 102 and the transmission enhancement layer 103.

[0088] In this example, the foldable display stack 111 includes the same layers described above with reference to Figures 2 and 4. Other embodiments may provide other foldable display stack layers, foldable display stack layers with different materials and / or thicknesses.

[0089] In this example, the adhesive layer 202b and the layers of the foldable display stack 111 form a resonator 250a, which is defined by the glass layer 203 and the display reinforcement 113. In some examples, the thickness of the resonator 250a may correspond to a multiple N of half the wavelength at the peak frequency of the ultrasonic frequency range transmitted by the ultrasonic sensor stack 105b, where N is an integer greater than or equal to 1. In some such embodiments, the resonator 250a may cause a local maximum within the ultrasonic frequency range. According to some examples, the local maximum may correspond to frequencies in the range of 5 MHz to 15 MHz or 1 MHz to 20 MHz. According to some such embodiments, the frequency range including the local ultrasonic transmission maximum caused by the resonator 250a may correspond to the frequency range including the local ultrasonic transmission maximum caused by the resonator 250c.

[0090] According to this example, the foldable display stack 111 also includes a resonator 250c formed by one or more screen protector layers 211 and OCA 204a, and defined by a glass layer 203: here, the glass layer 203 has a higher acoustic impedance than the one or more screen protector layers 211 or OCA 204a. In some examples, the thickness of the resonator 250c may correspond to a multiple N of a quarter wavelength at the peak frequency of the ultrasonic frequency range transmitted by the ultrasonic sensor stack 105b, where N is an integer greater than or equal to 1. In some such embodiments, the resonator 250c may cause local maxima within the ultrasonic frequency range. According to some examples, the local maxima may correspond to frequencies in the range of 5 MHz to 15 MHz or 1 MHz to 20 MHz. According to some such embodiments, the frequency range including the local ultrasonic transmission maximum value caused by the resonator 250a may correspond to the frequency range including the local ultrasonic transmission maximum value caused by the resonator 250c.

[0091] In this specific implementation, glass layer 203 forms resonator 250b. According to some examples, the maximum transmission frequency of ultrasound waves through resonator 250b can be outside the frequency range that includes the local maximum value caused by resonator 250a (e.g., below that frequency range).

[0092] Resonator 250f is another example, which may be referred to herein as a “transmission-enhancing resonator,” and is configured to enhance ultrasonic waves transmitted by an ultrasonic sensor stack within at least a first ultrasonic frequency range (e.g., band 301 of FIG. 3 or band 501 of FIG. 5). In this example, resonator 250f includes a display reinforcement 113, a transmission-enhancing layer 103, and an adhesive layer 202a.

[0093] According to this specific embodiment, and unlike the specific embodiment described above with reference to Figure 4, in this example, the transmission enhancement resonator does not include the TFT layer 102. This is because, in this example, the ultrasonic transceiver layer 101, electrode layer 210, and passivation layer 212 (all of which typically have a lower acoustic impedance than the display reinforcement 113 and the transmission enhancement layer 103) are located between the TFT layer 102 and the transmission enhancement layer 103. In some such examples, the thickness of the resonator 250f may correspond to a multiple N of half the wavelength at the peak frequency of the ultrasonic frequency range transmitted by the ultrasonic sensor stack 105b, where N is an integer greater than or equal to 1. In some examples, the thickness of the resonator 250f may be in the range of 50-150 micrometers. Although the resonator 250d formed by the display reinforcement 113 results in very low ultrasonic transmission in the frequency range of 5 MHz to 15 MHz, in this example, the resonator 250f results in at least one local ultrasonic transmission maximum in the frequency range of 5 MHz to 15 MHz or 1 MHz to 20 MHz.

[0094] In some embodiments, a specific frequency range (and / or the frequency corresponding to the local ultrasonic transmission maximum value) can be selected as the high transmission frequency range matching one or more resonators of the foldable display stack 111. According to some such embodiments, the frequency range including the local ultrasonic transmission maximum value caused by resonator 250f can correspond to the frequency range including the local ultrasonic transmission maximum value caused by resonator 250a and / or the frequency range including the local ultrasonic transmission maximum value caused by resonator 250c.

[0095] Figure 7 illustrates exemplary components of an apparatus according to some disclosed embodiments. As with other disclosed embodiments, the types, numbers, and arrangements of the elements, as well as the dimensions of the elements, are merely examples. According to this example, apparatus 100c is configured to perform at least some of the methods disclosed herein. According to this embodiment, the ultrasonic sensor stack 105a includes an ultrasonic transceiver layer 101, an electrode layer 210 on one side of the ultrasonic transceiver layer 101, and a sensor pixel array 706 on a second opposite side of the ultrasonic transceiver layer 101. In this embodiment, the ultrasonic transceiver layer 101 comprises one or more piezoelectric polymers. In other embodiments, the ultrasonic transceiver layer 101 may comprise other types of piezoelectric materials.

[0096] According to this example, electrode layer 210 is located between passivation layer 212 and ultrasonic transceiver layer 101. In some examples, passivation layer 212 may include an adhesive, such as an epoxy resin film, a polymer layer (such as a polyethylene terephthalate (PET) layer), etc.

[0097] In this example, TFT layer 102 includes a TFT substrate and circuitry for the sensor pixel array 706. TFT layer 102 can be a metal-oxide-semiconductor field-effect transistor (MOSFET), fabricated by depositing a thin film of an active semiconductor layer, a dielectric layer, and metal contacts on the TFT substrate. In some examples, the TFT substrate can be a non-conductive material, such as glass.

[0098] In this example, device 100c includes a foldable display stack 111, which in this example includes a display reinforcement 113 and a display stack layer 115. According to this example, the display stack layer 115 forms one or more display stack resonators. In some cases, the one or more display stack resonators may be or may include the resonators 250a, 250b and / or 250c described above.

[0099] According to some examples, a transmission enhancement resonator, including display reinforcement 113 and transmission enhancement layer 103, can be configured to enhance ultrasonic waves transmitted by ultrasonic sensor stack 105a within at least one ultrasonic frequency range. In some specific embodiments, the transmission enhancement resonator may also include TFT layer 102. In some examples, the transmission enhancement resonator can cause a local maximum in ultrasonic wave transmission within at least one ultrasonic frequency range. In some cases, the at least one ultrasonic frequency range can be the frequency range within which one or more display stack resonators cause a local maximum in ultrasonic wave transmission.

[0100] According to this specific implementation, the TFT layer 102, the sensor pixel array 706, and the electrodes are electrically coupled to at least a portion of the control system 106 and one side of the ultrasonic transceiver layer 101 via a portion of the interface system 107, which in this case includes conductive material and flexible printed circuit (FPC).

[0101] In this example, device 100c is configured to perform at least some of the methods disclosed herein. In this example, control system 106 is configured to control the ultrasonic sensor system to transmit one or more ultrasonic waves 713. According to this example, the ultrasonic waves 713 are transmitted through TFT layer 102, transmission enhancement layer 103, display reinforcement 113, and display stack layer 115. According to this example, the reflection 714 of the ultrasonic waves 713 is caused by the acoustic impedance contrast at (or near) interface 715, which is located between the outer surface of cover 718 and anything in contact with that outer surface (which can be air or the surface of a target object, such as the ridges and grooves of a fingerprint). (As used herein, the term "finger" can refer to any finger, including the thumb. Therefore, a thumbprint will be considered a type of "fingerprint.")

[0102] According to some examples, the reflection 714 of the ultrasonic wave 713 can be detected by the sensor pixel array 706. The corresponding ultrasonic signal can be provided to the control system 106. In some such embodiments, the ultrasonic signal used by the control system 106 for fingerprint-based authentication can be based on the reflection 714 from the cover / finger interface detected by the sensor pixel array 706. In some embodiments, the reflection 714 corresponding to the cover / air interface can be detected by the electrode layer 210, and the corresponding background ultrasonic signal can be provided to the control system 106.

[0103] Figure 8 is a flowchart providing operational examples according to some of the disclosed methods. For example, the blocks in Figure 8 can be executed by the device 100 of Figure 1 (e.g., by the control system 106 and the ultrasonic sensor stack 105) or by a similar device including frequency micro-layers. As with other methods disclosed herein, the methods outlined in Figure 8 may include more or fewer blocks than shown. Furthermore, the blocks in the methods disclosed herein do not necessarily have to be executed in the indicated order. In some cases, one or more blocks may be executed simultaneously.

[0104] In this example, block 805 relates to controlling an ultrasonic transceiver layer (e.g., ultrasonic transceiver layer 101) of an ultrasonic sensor system via a control system (e.g., via control system 106) to transmit ultrasonic waves (e.g., ultrasonic wave 713 shown in FIG. 7) through at least a first resonator and a second resonator, the first resonator being configured to cause a first local maximum of ultrasonic wave transmission within a first ultrasonic wave frequency range, and the second resonator being configured to cause a second local maximum of ultrasonic wave transmission within the first ultrasonic wave frequency range. According to this example, the first resonator includes a display reinforcement layer (such as display reinforcement 113) and a transmission enhancement layer (such as transmission enhancement layer 103). In some examples, the first resonator may include a TFT layer.

[0105] The first resonator is an example that may be referred to herein as a transmission-enhancing resonator. In some examples, the thickness of the first resonator may correspond to a multiple of half the wavelength of a transverse or longitudinal wave within the frequency range of the first ultrasonic wave. In some cases, the first ultrasonic frequency range may be from 5 MHz to 15 MHz. In some examples, the first ultrasonic frequency range may be from 1 MHz to 20 MHz.

[0106] In this specific implementation, the second resonator includes one or more display stack layers. In some cases, the second resonator may be the resonator 250a or resonator 250c described above.

[0107] According to this specific embodiment, block 810 relates to a control system receiving, and from an ultrasonic sensor system, an ultrasonic sensor signal corresponding to the reflection of transmitted ultrasonic waves from a portion of a target object located on the outer surface of a device including the ultrasonic sensor system. According to some examples, the ultrasonic sensor signal may correspond to reflection from the interior of that portion of the target object. If the target object is a finger, the first signal may correspond to the reflection of a first ultrasonic wave from a lower layer of the finger, such as from one or more subepidermal features. Alternatively or additionally, the ultrasonic sensor signal may correspond to the reflection of transmitted ultrasonic waves from the surface of that portion of the target object. If the target object is a finger, the ultrasonic sensor signal may correspond to the reflection of a second ultrasonic wave from the surface of the finger, such as from the ridges and grooves of a fingerprint.

[0108] According to this specific implementation, block 815 relates to performing an authentication process by a control system based at least in part on ultrasonic sensor signals. In some specific implementations, method 800 may relate to controlling access to the device or other devices based at least in part on the authentication process.

[0109] According to some specific implementations, block 815 may involve obtaining fingerprint data based on a portion of an ultrasonic sensor signal received within a time interval corresponding to the fingerprint. This time interval may be measured, for example, relative to the time it takes for the ultrasonic waves to travel. For example, obtaining fingerprint data may involve extracting target object features from the ultrasonic sensor signal. For example, target object features may include fingerprint features. According to some examples, fingerprint features may include fingerprint details, key points, and / or sweat pores. In some examples, fingerprint features may include ridge end information, ridge bifurcation information, short ridge information, ridge flow information, island information, branch information, increment information, core information, etc.

[0110] In some examples, block 815 may involve comparing fingerprint features with those of an authorized user. For instance, the authorized user's fingerprint features may have already been received during a previous registration process.

[0111] In some implementations, block 815 may involve extracting subcutaneous features from ultrasound sensor signals. For example, the subcutaneous features of an authorized user may have already been received during a previous registration process. According to some implementations, the authentication process may involve comparing the subcutaneous features extracted from the ultrasound sensor signals with the subcutaneous features of the authorized user.

[0112] In some such implementations, subepidermal features may include subepidermal information corresponding to the reflection of received ultrasound waves from that portion of the target object within a time interval corresponding to the subepidermal region. For example, subepidermal features may include dermal information corresponding to the reflection of a second received ultrasound wave from that portion of the target object. This dermal information may be obtained within a time interval corresponding to the dermis. The authentication process may be based at least in part on the dermal information. Alternatively or additionally, subepidermal features may include information about other subepidermal layers (such as the papillary layer, reticular layer, subcutaneous tissue, etc.), any blood vessels, lymphatic vessels, sweat glands, hair follicles, dermal papillae, lipid lobules, etc. (which may be present within such tissue layers, muscle tissue, bone material, etc.).

[0113] Figure 9 typically depicts various configurations of a 4×4 pixel array for an ultrasonic sensor system. Each pixel 934 may be associated, for example, with a local region of piezoelectric sensor material (PSM), a peak detection diode (D1), and a readout transistor (M3); many or all of these elements may be formed on or in a substrate to form pixel circuitry 936. In practice, the local region of the piezoelectric sensor material of each pixel 934 can convert the received ultrasonic energy into electrical charge. The peak detection diode D1 can record the maximum amount of charge detected by the local region of the piezoelectric sensor material PSM. Each column of the pixel array 935 can then be scanned, for example, by a column selection mechanism, a gate driver, or a shift register, and the readout transistor M3 of each row can be triggered to allow the magnitude of the peak charge of each pixel 934 to be read by additional circuitry (e.g., a multiplexer and an A / D converter). Pixel circuitry 936 may include one or more TFTs to allow gating, addressing, and resetting of the pixels 934.

[0114] Each pixel circuit 936 can provide information about a small portion of the object detected by the ultrasonic sensor system. Although the example shown in Figure 9 has a relatively coarse resolution for illustrative purposes, ultrasonic sensors with a resolution of approximately 500 pixels per inch or higher can be configured with appropriately sized structures. The detection area of ​​the ultrasonic sensor system can be selected depending on the object to be detected. For example, the detection area can range from approximately 5mm × 5mm for a single finger to approximately 3 inches × 3 inches for four fingers. Smaller and larger areas, including square, rectangular, and non-rectangular geometries, can be used depending on the needs of the target object.

[0115] Specific implementation examples are described in the following numbered clauses:

[0116] 1. An apparatus comprising: an ultrasonic sensor stack; a foldable display stack including a display reinforcement and a display stack layer configured such that ultrasonic waves transmitted by the ultrasonic sensor stack have one or more display stack-induced local amplitude maxima within a first ultrasonic frequency range; and a transmission enhancement layer, wherein the display reinforcement and the transmission enhancement layer include a transmission enhancement resonator configured such that the ultrasonic waves transmitted by the ultrasonic sensor stack have transmission enhancement resonator-induced local amplitude maxima within the first ultrasonic frequency range.

[0117] 2. The apparatus according to Clause 1, wherein the thickness of the transmission enhancement resonator corresponds to a multiple of half the wavelength of a transverse or longitudinal wave with a frequency in the first ultrasonic frequency range.

[0118] 3. The apparatus according to Clause 1 or Clause 2, wherein the display stack layer includes one or more display stack resonators, and wherein the one or more display stack resonators are configured to cause a local amplitude maximum induced by the one or more display stacks.

[0119] 4. The apparatus according to Clause 3, wherein the one or more display stack resonators include a first resonator defined by the display reinforcement and the glass layers of the foldable display stack.

[0120] 5. The apparatus according to Clause 4, wherein the first resonator comprises a plurality of layers of an organic light-emitting diode display.

[0121] 6. The apparatus according to Clause 4 or Clause 5, wherein the one or more display stack resonators include a second resonator defined by the outer surface of the glass layer and the foldable display stack.

[0122] 7. The apparatus according to any one of clauses 1 to 6, wherein the maximum local amplitude induced by the transmission enhancement resonator corresponds to a frequency in the range of 5 MHz to 15 MHz.

[0123] 8. The apparatus according to any one of claims 1 to 7, further comprising a control system configured to cause the ultrasonic sensor stack to transmit a first ultrasonic wave within the first ultrasonic wave frequency range and to perform an authentication process based at least in part on an ultrasonic sensor signal corresponding to a reflection of the first ultrasonic wave.

[0124] 9. The apparatus according to any one of clauses 1 to 8, wherein the transmission enhancement layer is located between the ultrasonic sensor stack and the display reinforcement.

[0125] 10. The apparatus according to any one of clauses 1 to 9, wherein the thickness of the transmission enhancement layer is less than one-quarter wavelength of a transverse or longitudinal wave with a frequency in the first ultrasonic frequency range.

[0126] 11. The apparatus according to any one of clauses 1 to 10, wherein the transmission enhancement layer comprises: an aluminum layer with an aluminum layer thickness in the range of 50 micrometers to 100 micrometers, a copper layer with a copper layer thickness in the range of 25 micrometers to 50 micrometers, or a stainless steel layer with a stainless steel layer thickness in the range of 25 micrometers to 50 micrometers.

[0127] 12. The apparatus according to any one of clauses 1 to 11, the apparatus further comprising a first adhesive layer between the transmission enhancement layer and the display reinforcement and a second adhesive layer between the transmission enhancement layer and the ultrasonic sensor stack.

[0128] 13. The apparatus according to Clause 12, wherein at least one of the first adhesive layer or the second adhesive layer has a thickness in the range of 3 micrometers to 10 micrometers.

[0129] 14. The apparatus according to any one of clauses 1 to 13, wherein the ultrasonic sensor stack includes a thin-film transistor (TFT) substrate, and wherein the transmission enhancement resonator includes the TFT substrate.

[0130] 15. The apparatus according to Clause 14, wherein the thickness of the TFT substrate is less than one-quarter wavelength of a transverse or longitudinal wave with a frequency in the first ultrasonic frequency range.

[0131] 16. The apparatus according to Clause 14 or Clause 15, wherein the TFT substrate has a thickness in the range of 50 micrometers to 200 micrometers.

[0132] 17. The apparatus according to any one of clauses 14 to 16, wherein the TFT substrate comprises glass.

[0133] 18. The apparatus according to any one of clauses 14 to 17, wherein: the TFT substrate has a first acoustic impedance value; the display reinforcement has a second acoustic impedance value greater than the first acoustic impedance value; and the transmission enhancement layer has a third acoustic impedance value greater than the first acoustic impedance value.

[0134] 19. The apparatus according to any one of clauses 1 to 18, wherein the display reinforcement comprises one or more of a metal layer or a non-metal layer having an acoustic impedance of 10 MTayls or greater.

[0135] 20. The apparatus according to any one of clauses 1 to 19, wherein the display reinforcement has a thickness in the range of 30 micrometers to 300 micrometers.

[0136] 21. The apparatus according to any one of clauses 1 to 20, wherein the thickness of the display reinforcement corresponds to a multiple of half the wavelength of a transverse or longitudinal wave in a second ultrasonic frequency range higher than the first ultrasonic frequency range.

[0137] 22. The apparatus according to any one of clauses 1 to 21, wherein the apparatus is a mobile device, the mobile device comprising the ultrasonic sensor stack, the foldable display stack, and the transmission enhancement layer.

[0138] 23. The apparatus according to any one of clauses 1 to 22, wherein the transmission enhancement layer comprises at least a portion of the ultrasonic sensor stack.

[0139] Other specific implementation examples are described in the following numbered clauses:

[0140] 24. An apparatus comprising: an ultrasonic sensor component; a foldable display stack including a display reinforcement and one or more display stack resonators configured to amplify ultrasonic waves transmitted by the ultrasonic sensor component within a first ultrasonic frequency range; and a transmission enhancement layer comprising at least a portion of the display reinforcement, the transmission enhancement layer, and the ultrasonic sensor component forming a transmission enhancement resonator configured to amplify the ultrasonic waves transmitted by the ultrasonic sensor component within the first ultrasonic frequency range.

[0141] 25. The apparatus according to Clause 24, wherein the thickness of the transmission enhancement resonator corresponds to a multiple of half the wavelength of a transverse or longitudinal wave with a frequency in the first ultrasonic frequency range.

[0142] 26. The apparatus according to Clause 24, wherein enhancing the ultrasonic waves in the first ultrasonic frequency range includes causing a local maximum value of the ultrasonic waves in the first ultrasonic frequency range.

[0143] Other specific implementation examples are described in the following numbered clauses:

[0144] 27. A method comprising: controlling an ultrasonic transceiver layer of an ultrasonic sensor system via a control system to transmit ultrasonic waves through at least a first resonator and a second resonator, the first resonator being configured to cause a first local maximum in ultrasonic wave transmission within a first ultrasonic wave frequency range, the second resonator being configured to cause a second local maximum in ultrasonic wave transmission within the first ultrasonic wave frequency range, the first resonator including a display reinforcement layer and a transmission enhancement layer, the second resonator including one or more display stack layers; receiving, by the control system and from the ultrasonic sensor system, an ultrasonic sensor signal corresponding to a reflection of transmitted ultrasonic waves from a portion of a target object located on an outer surface of a device including the ultrasonic sensor system; and performing an authentication process by the control system at least in part based on the ultrasonic sensor signal.

[0145] 28. The method according to Clause 27, wherein the local maximum value of ultrasonic transmission corresponds to a frequency in the range of 5 MHz to 15 MHz.

[0146] 29. The method according to Clause 27, wherein the authentication process involves extracting target object features from the ultrasonic sensor signal.

[0147] 30. The method according to Clause 29, wherein the target object features include at least one of fingerprint features or subepidermal features.

[0148] 31. The method according to Clause 27, the method further comprising controlling access to the device based at least in part on the authentication process.

[0149] As used in this article, the phrase “at least one of” in a list of items refers to any combination of these items, including a single member. As an example, “at least one of a, b, or c” is intended to cover: a, b, c, ab, ac, bc, and abc.

[0150] The various illustrative logics, logic blocks, modules, circuits, and algorithmic processes described in conjunction with the specific implementations disclosed herein can be implemented as electronic hardware, computer software, or a combination of both. The interchangeability of hardware and software has been generally described 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 on the specific application and the design constraints imposed on the overall system.

[0151] Hardware and data processing apparatuses for implementing the various illustrative logics, logic blocks, modules, and circuits described in conjunction with the various forms disclosed herein can be implemented or performed by: general-purpose single-chip or multi-chip processors, digital signal processors (DSPs), application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs) or other programmable logic elements, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor can be a microprocessor, or any conventional processor, controller, microcontroller, or state machine. Processors can also be implemented as combinations of computing devices, such as a combination of a DSP and a microprocessor, multiple microprocessors, one or more microprocessors combined with a DSP core, or any other such configuration. In some specific implementations, particular processes and methods can be performed by circuitry specific to a given function.

[0152] In one or more embodiments, the described functions can be implemented in hardware, digital electronic circuitry, computer software, firmware (including the structures disclosed herein and their structural equivalents), or any combination thereof. Specific implementations of the subject matter described herein can also be implemented as one or more computer programs, i.e., one or more modules of computer program instructions encoded on a computer storage medium for execution by a data processing device or for controlling the operation of a data processing device.

[0153] If implemented in software, these functions can be stored as one or more instructions or code on or transmitted over a computer-readable medium, such as a non-transitory medium. The flow of the methods or algorithms disclosed herein can be implemented as a processor-executable software module, which can reside on a computer-readable medium. Computer-readable media includes both computer storage media and communication media, encompassing any media that allows a computer program to be transferred from one place to another. Storage media can be any available media accessible to a computer. By way of example and not limitation, non-transitory media can include RAM, ROM, EEPROM, CD-ROM or other optical disc storage devices, magnetic disk storage devices or other magnetic storage devices, or any other media that can be used to store desired program code in the form of instructions or data structures and is accessible to a computer. Furthermore, any connection can be appropriately referred to as computer-readable media. As used herein, magnetic disks and optical disks include compact discs (CDs), laser discs, optical discs, digital versatile discs (DVDs), floppy disks, and Blu-ray discs, where magnetic disks typically reproduce data magnetically, while optical disks reproduce data optically using lasers. Combinations of the above should also be included within the scope of computer-readable media. Furthermore, the operation of a method or algorithm may be located as one or any combination of code and instruction sets on machine-readable and computer-readable media, which may be incorporated into computer program products.

[0154] Various modifications to the specific embodiments described in this disclosure will be readily apparent to those skilled in the art, and the general principles defined herein can be applied to other embodiments without departing from the spirit or scope of this disclosure. Therefore, this disclosure is not intended to be limited to the specific embodiments shown herein, but should be given the broadest possible scope consistent with the claims, the principles disclosed herein, and the novel features. Where applicable, the word “exemplary” is used exclusively herein to mean “as an example, instance, or illustration.” Any specific embodiment described herein as “exemplary” is not necessarily to be construed as preferentially superior to or better than other specific embodiments.

[0155] Some features described in this specification in the context of a single embodiment may also be implemented in combination in a single embodiment. Conversely, various features described in the context of a single embodiment may also be implemented individually or in any suitable sub-combination in multiple embodiments. Furthermore, although features may be described above as functioning in certain combinations, or even originally claimed in this way, in some cases, one or more features from the claimed combination may be separated from that combination, and the claimed combination may point to a sub-combination or a variation of the sub-combination.

[0156] Similarly, although operations are depicted in a specific order in the diagram, this should not be construed as requiring such operations to be performed in the specific order shown or in a sequential order, or requiring the execution of all operations in the diagram to achieve the desired result. In some cases, multitasking and parallel processing may be advantageous. Furthermore, the separation of various system components in the above embodiments should not be construed as requiring such separation in all embodiments, and it should be understood that the described program components and systems can generally be integrated together in a single software product or encapsulated in multiple software products. Additionally, other embodiments are also within the scope of the appended request. In some cases, the actions described in the request can be performed in a different order and still achieve the desired result.

[0157] It should be understood that unless any features in a particular embodiment described are explicitly identified as incompatible with each other or the surrounding context suggests that they are mutually exclusive and not easily combined in a complementary and / or supporting sense, the overall content of this disclosure conceives and envisions the selective combination of specific features of those complementary embodiments to provide one or more comprehensive but slightly different technical solutions. Therefore, it will be further understood that the above description is given only as an example and may be modified in detail within the scope of this disclosure.

[0158] 100: Device 105: Ultrasonic sensor stack 101: Ultrasonic transceiver layer 102: Thin Film Transistor Layer 103: Transmission Enhancement Layer 106: Control System 107: Interface System 111: Foldable Display Stack 113: Display reinforcement 115: Display Stacking Layer 200: Device 250c: Resonator 211: Screen Protector 204a: Optically transparent adhesive layer 250b: Resonator 203: Glass layer 250a: Resonator 204b: Optically transparent adhesive layer 222: Polarizer layer 213a: Polarizer pressure-sensitive adhesive 214: OLED panel 213b: Pressure-sensitive adhesive 215: Protective film 202b: Adhesive layer 250d: Resonator 202a: Adhesive layer 210: Electrode layer 212: Passivation layer 305c: Curve graph 305b: Curve Graph 305a: Curve Graph 305d: Curve Graph 301: Frequency Band F1: Frequency F2: Frequency 307: Second ultrasonic frequency band F3: Frequency F4: Frequency F5: Frequency 100a:Device 250e: Resonator 202c: Adhesive layer 105a: Ultrasonic sensor stack 505c: Curve Graph 505b: Curve Graph 505a: Curve Graph 505e: Curve Graph 501: Frequency Band 100b: Device 250f: Resonator 105b: Ultrasonic sensor stack 100c: Device 715: Interface 714: Reflection 713: Ultrasound 706: Sensor pixel array 800: Method 805: Square 810: Square 815: Square 934: pixels 935: Pixel Array 936: Pixel Circuit 937: D1: Peak detection diode PSM: Piezoelectric sensing material

Claims

1. An ultrasonic sensor device, the device comprising: Stacked ultrasonic sensors; A foldable display stack includes: a display reinforcement; and a display stack layer configured to enhance ultrasonic waves transmitted by the ultrasonic sensor stack; and a transmission enhancement layer, wherein the display reinforcement and the transmission enhancement layer include a transmission enhancement resonator configured to enhance the ultrasonic waves transmitted by the ultrasonic sensor stack, wherein the transmission enhancement resonator has a thickness corresponding to a multiple of half the wavelength of a shear wave or longitudinal wave within a first ultrasonic frequency range.

2. The apparatus as claimed in claim 1, wherein, The display stack includes one or more display stack resonators, and the one or more display stack resonators are configured to cause a local amplitude maximum induced by the one or more display stacks.

3. The apparatus as claimed in claim 2, wherein, The one or more display stack resonators include a first resonator defined by the display reinforcement and the glass layers stacked by the foldable display.

4. The apparatus as claimed in claim 3, wherein, The first resonator includes multiple layers of an organic light-emitting diode display.

5. The apparatus as claimed in claim 3, wherein, The one or more display stack resonators include a second resonator defined by the outer surface of the glass layer and the foldable display stack.

6. The apparatus as claimed in claim 1, wherein, The maximum local amplitude induced by the transmission enhancement resonator corresponds to a frequency in the range of 5MHz to 15MHz.

7. The apparatus of claim 1, further comprising a control system configured to: cause the ultrasonic sensor stack to transmit a first ultrasonic wave within the first ultrasonic wave frequency range, and to perform an authentication process based at least in part on ultrasonic sensor signals reflected from the first ultrasonic wave.

8. The apparatus as claimed in claim 1, wherein, The transmission enhancement layer is located between the ultrasonic sensor stack and the display reinforcement, or the transmission enhancement layer includes at least a portion of the ultrasonic sensor stack.

9. The apparatus as claimed in claim 1, wherein, The thickness of the transmission enhancement layer is less than one-quarter of the wavelength of a shear wave or longitudinal wave with a frequency in the first ultrasonic frequency range.

10. The apparatus as claimed in claim 1, wherein, The transmission enhancement layer includes: an aluminum layer with a thickness ranging from 50 micrometers to 100 micrometers, a copper layer with a thickness ranging from 25 micrometers to 50 micrometers, or a stainless steel layer with a thickness ranging from 25 micrometers to 50 micrometers.

11. The apparatus of claim 1, further comprising: A first adhesive layer located between the transmission enhancement layer and the display reinforcement, and a second adhesive layer located between the transmission enhancement layer and the ultrasonic sensor stack.

12. The apparatus as claimed in claim 11, wherein, At least one of the first adhesive layer or the second adhesive layer has a thickness in the range of 3 micrometers to 10 micrometers.

13. The apparatus as claimed in claim 1, wherein, The ultrasonic sensor stack includes a thin-film transistor (TFT) substrate, and the transmission enhancement resonator includes the TFT substrate.

14. The apparatus as claimed in claim 13, wherein, The thickness of the TFT substrate is less than one-quarter of the wavelength of a shear wave or longitudinal wave with a frequency within the first ultrasonic frequency range.

15. The apparatus as claimed in claim 13, wherein, The TFT substrate has a thickness ranging from 50 micrometers to 200 micrometers.

16. The apparatus as claimed in claim 13, wherein, The TFT substrate includes glass.

17. The apparatus as claimed in claim 13, wherein: The TFT substrate has a first acoustic impedance value; the display reinforcement has a second acoustic impedance value greater than the first acoustic impedance value; and the transmission enhancement layer has a third acoustic impedance value greater than the first acoustic impedance value.

18. The apparatus as claimed in claim 1, wherein, The display reinforcement includes one or more of a metal layer or a non-metal layer with an acoustic impedance of 10 MTayls or greater.

19. The apparatus as claimed in claim 1, wherein, The display reinforcement has a thickness ranging from 30 micrometers to 300 micrometers.

20. The apparatus as claimed in claim 1, wherein, The thickness of the display reinforcement corresponds to a multiple of half the wavelength of a shear wave or longitudinal wave in a second ultrasonic frequency range that is higher than the first ultrasonic frequency range.

21. The apparatus as claimed in claim 1, wherein, The device is a mobile device, which includes the ultrasonic sensor stack, the foldable display stack, and the transmission enhancement layer.

22. The apparatus as claimed in claim 1, wherein, The transmission enhancement layer includes at least a portion of the ultrasonic sensor stack.

23. An ultrasonic sensor device, the device comprising: Ultrasonic sensor components; A foldable display stack includes: a display reinforcement; and one or more display stack resonators configured to amplify ultrasonic waves transmitted by the ultrasonic sensor component; and a transmission enhancement layer, wherein the display reinforcement, the transmission enhancement layer, and at least a portion of the ultrasonic sensor component form a transmission enhancement resonator configured to amplify the ultrasonic waves transmitted by the ultrasonic sensor component, wherein the transmission enhancement resonator has a thickness corresponding to a multiple of half the wavelength of a shear wave or longitudinal wave within a first ultrasonic frequency range.

24. The apparatus as claimed in claim 23, wherein, Enhancing ultrasound within the first ultrasonic frequency range includes causing a local maximum value of the ultrasound within the first ultrasonic frequency range.

25. An ultrasonic sensing method, the method comprising: The system controls an ultrasonic transceiver layer of an ultrasonic sensor stack to transmit ultrasonic waves via at least a first resonator and a second resonator. The first resonator is configured to amplify the ultrasonic waves transmitted by the ultrasonic sensor stack, and the second resonator is configured to amplify the ultrasonic waves transmitted by the ultrasonic sensor stack. The first resonator includes a display reinforcement layer and a transmission enhancement layer, and the first resonator has a thickness corresponding to a multiple of half the wavelength of a shear wave or longitudinal wave with a frequency in a first ultrasonic frequency range. The second resonator includes one or more display stack layers. The system receives ultrasonic sensor signals corresponding to the reflection of transmitted ultrasonic waves from a portion of a target object located on the outer surface of a device including the ultrasonic sensor system. The system performs an authentication process based at least in part on the ultrasonic sensor signals.

26. The method as described in claim 25, wherein, Amplifying ultrasound within the first ultrasonic frequency range includes causing one or more local maxima within the first ultrasonic frequency range.

27. The method as described in claim 25, wherein, The authentication process involves extracting target object features from the ultrasonic sensor signals.

28. The method as described in claim 27, wherein, The target object features include at least one of fingerprint features or subepidermal features.

29. The method of claim 25, wherein the method optionally further includes controlling access to the device based at least in part on the authentication process.

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