Porous matrix for acoustic impedance matching and improved touch sensing and fingerprint imaging

Abstract

An improvement in the accuracy of ultrasonic touch detection and fingerprint imaging using acoustic impedance matching is disclosed. Acoustic impedance mismatch between an ultrasonic transducer array and a sensing plate can be reduced to maximize energy transfer and minimize parasitic reflections. Reduction of acoustic impedance mismatch can be achieved by using (i) a composite epoxy having a higher acoustic impedance than the epoxy alone, (ii) one or more matching layers having an acoustic impedance that is the approximate geometric mean of the acoustic impedance of the sensing plate and the acoustic impedance of the transducer array, (iii) pores or perforations within the sensing plate, or (iv) geometric structures formed on the sensing plate. Additionally, parasitic reflections can be suppressed by using an absorbent layer.

Description

Technology Field

[0001] Cross-reference regarding related applications

[0002] This application claims the benefit of U.S. Provisional Application No. 63 / 188,114 filed May 13, 2021 and U.S. Patent Application No. XX / XXX,XXX filed April XX, 2022, the contents of which are incorporated herein by reference in their entirety for all purposes.

[0003] Technology field

[0004] The present application generally relates to ultrasonic touch sensing and fingerprint imaging systems, and more specifically, to acoustic impedance matching for improved ultrasonic touch sensing and fingerprint imaging. Background Technology

[0005] Many types of electronic devices capable of accommodating touch input to initiate actions are currently available. Examples of these devices include desktop, laptop, and tablet computing devices; smartphones; media players; wearables such as watches and health monitoring devices; smart home control and entertainment devices; headphones and earbuds; and devices for computer-generated environments such as augmented reality, mixed reality, or virtual reality environments. Many of these devices can receive input via physical touch, such as buttons or keys, mice, trackballs, joysticks, touch panels, and touchscreens. In particular, wearable devices can provide users with readily accessible mechanisms to perform various functions, and in some cases, provide rapid access to sensitive user information, either individually or through paired devices. However, the possibility of easy access to device functions and / or sensitive information creates a need for secure authentication and access.

[0006] To provide security authentication, fingerprint detection can be implemented on a surface such as a watch button. Since the surface may be made of metal and / or may have a certain thickness (e.g., greater than 300 micrometers) that would render other sensing technologies impractical, ultrasonic touch sensing may be employed to perform fingerprint imaging. In some examples, an array of piezoelectric micro-machined ultrasonic transducers (PMUTs) may be used to propagate ultrasound through touch surfaces of various materials and thicknesses, where the presence or absence of fingerprint ridges can change the amount of energy reflected back to the PMUT array. This change in reflected energy can be used to determine an image of the user's fingerprint. However, when the PMUT array and related electronics are attached to the back of a touch surface, the resulting stack-up of materials (e.g., PMUT array, bonding material, sensing plate, etc.) can create an acoustic impedance mismatch, which can reduce the transmission of ultrasonic energy to and from the PMUT array and the touch surface and also generate parasitic reflections, both of which can negatively affect the PMUT array's ability to accurately perform fingerprint imaging.

[0007] Examples of the present disclosure generally relate to providing ultrasonic touch detection and fingerprint imaging capabilities, and more specifically, to acoustic impedance matching for improved ultrasonic touch detection and fingerprint imaging. In some examples of the present disclosure, an epoxy having a relatively low acoustic impedance may be combined with a filler material to increase the overall acoustic impedance of the resulting composite epoxy. The composite epoxy may be used to attach an ultrasonic transducer array to an upper layer (i.e., a sensing plate or cap layer) configured to receive a touch (e.g., from a finger). The composite epoxy (with the filler material) may have an acoustic impedance higher than that of the epoxy alone and may be more closely matched to the acoustic impedance of the transducer array. The acoustic impedance of the composite epoxy may reduce the impedance mismatch between the transducer array and the sensing plate, which may result in reduced attenuation of ultrasound, less reflection, and more accurate touch detection and fingerprint imaging.

[0008] In some examples of the present disclosure, one or more matching layers having a total acoustic impedance approximately equal to the geometric mean of the acoustic impedance of the sensing plate and the acoustic impedance of the transducer array may be formed on or attached thereto. In some examples, each of the one or more matching layers may be formed from one or more metals. Subsequently, one or more matching layers (and the attached sensing plate) may be bonded to the transducer array using the aforementioned epoxy or composite epoxy. Each of the one or more matching layers may have a thickness approximately equal to one-quarter wavelength of the ultrasound propagating through the matching layer. The one or more matching layers may create an acoustic impedance gradient between the transducer array and the sensing plate to reduce the acoustic impedance mismatch between the transducer array and the sensing plate, which may result in reduced attenuation of ultrasound, less reflection, and more accurate touch detection and fingerprint imaging.

[0009] In some examples of the present disclosure, the sensing plate may be manufactured with an uneven distribution of pores (e.g., holes) or perforations along one or more surfaces of the sensing plate to create one or more acoustic impedance gradients within the sensing plate. One or more acoustic impedance gradients can result in reduced attenuation of ultrasound, less reflection, higher signal-to-noise ratio (SNR) touch signals (which leads to higher contrast touch and fingerprint images), and ultimately more accurate touch detection and fingerprint imaging.

[0010] In some examples of the present disclosure, geometric structures (e.g., cones) may be formed on the back surface of a sensing plate. The geometric structures may have a peak-to-peak spacing much smaller than the wavelength of the ultrasound propagating through the sensing plate and a peak-to-trough height at least three times the wavelength of the ultrasound. To create an acoustic impedance gradient at the locations of the geometric structures and the filler material, a lower acoustic impedance material may fill the gaps between the geometric structures. This acoustic impedance gradient can result in reduced attenuation of the ultrasound, less reflection, and more accurate touch detection and fingerprint imaging.

[0011] In some examples of the present disclosure, since the transducer array may also generate undesirable ultrasound in a direction away from the sensing plate and through any underlying electronics, an absorber layer may be formed on the back surface of the ultrasonic sensing electronics to absorb the undesirable ultrasound and reduce the reflection of these waves. The absorber layer may be a composite epoxy formed from tungsten fillers and epoxy. This absorber layer may have a higher acoustic impedance than conventional epoxys, along with high ultrasound absorption. The absorber layer can reduce the reflected energy from the undesirable ultrasound, which can result in attenuation of undesirable / parasitic ultrasound, less reflection, and more accurate touch detection and fingerprint imaging. Brief explanation of the drawing

[0012] FIGS. 1a through 1g illustrate systems capable of employing ultrasonic touch sensing and fingerprint imaging having acoustic impedance matching according to examples of the present disclosure. FIG. 2 illustrates a block diagram of an electronic device comprising ultrasonic touch sensing and fingerprint imaging having acoustic impedance matching according to examples of the present disclosure. FIG. 3 illustrates a process for ultrasonic touch detection of an object (e.g., fingerprint ridges) in contact with a touch-sensitive surface according to examples of the present disclosure. FIG. 4a illustrates a touch sensing stackup using an array of PMUTs according to examples of the present disclosure. FIG. 4b illustrates a finger signature reflected back from the touch surface of a sensing plate and a parasitic reflection from acoustic impedance mismatch when an epoxy having low acoustic impedance is used, according to examples of the present disclosure. FIG. 5a illustrates a composite epoxy that may include epoxy and filler particles suspended in the epoxy, according to examples of the present disclosure. FIG. 5b illustrates a finger signature reflected back from the touch surface of a sensing plate and a parasitic reflection from an acoustic impedance mismatch when a composite epoxy is used according to examples of the present disclosure. FIG. 6a illustrates a matching layer attached to a sensing plate while being separated from the epoxy and PMUT array, according to examples of the present disclosure. FIG. 6b illustrates a matching layer sandwiched between a sensing plate and a PMUT array according to examples of the present disclosure (with the epoxy layer omitted for clarity). FIG. 6c illustrates the energy transmittance of an ultrasonic signal through a single matching layer of a specific material over a range of frequencies and layer thicknesses according to examples of the present disclosure. FIG. 7a illustrates various materials and techniques that can be used to form a matching layer on the back surface of a sensing plate according to examples of the present disclosure. FIG. 7b illustrates the formation of a matching layer using plating according to examples of the present disclosure. FIG. 7c illustrates a plating process flow for forming and plating a stainless steel (SUS) sensing plate having geometric structures according to examples of the present disclosure. FIG. 7d illustrates the formation of a matching layer using cladding according to examples of the present disclosure. FIG. 7e illustrates the formation of a matching layer using physical vapor deposition (PVD) according to examples of the present disclosure. FIG. 8a illustrates a sensing plate having an internal acoustic impedance gradient created by pores according to examples of the present disclosure. FIG. 8b illustrates a sensing plate having a non-uniform distribution of pores according to examples of the present disclosure. FIG. 8c illustrates a sensing plate having perforations according to examples of the present disclosure. FIG. 8d illustrates a sensing plate having density variations according to examples of the present disclosure. FIG. 8e illustrates a sensing plate having perforations and density variations according to examples of the present disclosure. FIG. 9a illustrates a sensing plate having geometric structures for generating an acoustic impedance gradient according to examples of the present disclosure. FIG. 9b is a perspective view of a sensing plate having geometric structures according to examples of the present disclosure. FIG. 9c illustrates a stainless steel (SUS) sensing plate having geometric structures filled with a finished tin (Sn) matching layer as shown in FIG. 7c, according to examples of the present disclosure, and subsequently bonded to a PMUT array using epoxy. FIG. 10a illustrates a touch sensing and fingerprint imaging stack-up using a PMUT array backed with an absorbent, according to examples of the present disclosure. FIG. 10b illustrates a finger signature reflected back from the touch surface of a sensing plate and from an acoustic impedance mismatch (e.g., combined effect of reflections) when a composite epoxy and an absorbent are used according to examples of the present disclosure. FIG. 11 illustrates a flowchart for acoustic impedance matching and improved touch sensing and fingerprint imaging according to examples of the present disclosure. Specific details for implementing the invention

[0013] In the following description of various examples, the attached drawings forming part of this specification are referenced, and specific examples that may be practiced are illustrated within the drawings. It should be understood that other examples may be used and structural modifications may be made without departing from the scope of the various examples.

[0014] Ultrasonic touch detection and fingerprint imaging systems can be negatively affected by acoustic impedance mismatch when ultrasound travels to a touch surface, such as a detection plate. This mismatch can not only cause undesirable reflection of ultrasound but also ultimately cause a reduction in ultrasonic energy that reaches the detection plate and is reflected back with different amplitudes depending on whether an object (e.g., fingerprint ridges) is touching the detection plate.

[0015] Accordingly, the examples of the present disclosure generally relate to providing ultrasonic touch detection and fingerprint imaging capabilities, and more specifically, to acoustic impedance matching for improved ultrasonic touch detection and fingerprint imaging. In some examples of the present disclosure, an epoxy having a relatively low acoustic impedance may be combined with a filler material to increase the overall acoustic impedance of the resulting composite epoxy. The composite epoxy may be used to attach an ultrasonic transducer array to an upper layer (i.e., a sensing plate or cap layer) configured to receive a touch (e.g., from a finger). The composite epoxy (containing the filler material) may have an acoustic impedance higher than that of the epoxy alone and may be more closely matched to the acoustic impedance of the transducer array. The acoustic impedance of the composite epoxy may reduce the impedance mismatch between the transducer array and the sensing plate, which may result in reduced attenuation of ultrasound, less reflection, and more accurate touch detection and fingerprint imaging.

[0016] In some examples of the present disclosure, one or more matching layers having a total acoustic impedance approximately equal to the geometric mean of the acoustic impedance of the sensing plate and the acoustic impedance of the transducer array may be formed on or attached thereto. In some examples, each of the one or more matching layers may be formed from one or more metals. Subsequently, one or more matching layers (and the attached sensing plate) may be bonded to the transducer array using the aforementioned epoxy or composite epoxy. Each of the one or more matching layers may have a thickness approximately equal to one-quarter wavelength of the ultrasound propagating through the matching layer. The one or more matching layers may create an acoustic impedance gradient between the transducer array and the sensing plate to reduce the acoustic impedance mismatch between the transducer array and the sensing plate, which may result in reduced attenuation of ultrasound, less reflection, and more accurate touch detection and fingerprint imaging.

[0017] In some examples of the present disclosure, the sensing plate may be manufactured with an uneven distribution of pores (e.g., holes) or perforations along one or more surfaces of the sensing plate to create one or more acoustic impedance gradients within the sensing plate. One or more acoustic impedance gradients can result in reduced attenuation of ultrasound, less reflection, higher signal-to-noise ratio (SNR) touch signals (which leads to higher contrast touch and fingerprint images), and ultimately more accurate touch detection and fingerprint imaging.

[0018] In some examples of the present disclosure, geometric structures (e.g., cones) may be formed on the back surface of a sensing plate. The geometric structures may have a peak-to-peak spacing much smaller than the wavelength of the ultrasound propagating through the sensing plate and a peak-to-trough height at least three times the wavelength of the ultrasound. To create an acoustic impedance gradient at the locations of the geometric structures and the filler material, a lower acoustic impedance material may fill the gaps between the geometric structures. This acoustic impedance gradient can result in reduced attenuation of the ultrasound, less reflection, and more accurate touch detection and fingerprint imaging.

[0019] In some examples of the present disclosure, since the transducer array may also generate undesirable ultrasound in a direction away from the sensing plate and through any underlying electronics, an absorber layer may be formed on the back surface of the ultrasonic sensing electronics to absorb the undesirable ultrasound and reduce the reflection of these waves. The absorber layer may be a composite epoxy formed from tungsten fillers and epoxy. This absorber layer may have a higher acoustic impedance than conventional epoxys, along with high ultrasound absorption. The absorber layer can reduce the reflected energy from the undesirable ultrasound, which can result in attenuation of undesirable / parasitic ultrasound, less reflection, and more accurate touch detection and fingerprint imaging.

[0020] FIGS. 1a through 1g illustrate systems capable of employing ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching according to examples of the present disclosure. FIG. 1a illustrates an exemplary mobile phone (136) capable of employing ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching according to examples of the present disclosure. FIG. 1b illustrates an exemplary digital media player (140) capable of employing ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching according to examples of the present disclosure. FIG. 1c illustrates an exemplary personal computer (144) capable of employing ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching according to examples of the present disclosure. FIG. 1d illustrates an exemplary tablet computing device (148) capable of employing ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching according to examples of the present disclosure. FIG. 1e illustrates an exemplary wearable device (150) (e.g., a watch) capable of employing ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching according to examples of the present disclosure. FIG. 1f illustrates another exemplary wearable device, an over-ear headphone (160), capable of employing ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching according to examples of the present disclosure. FIG. 1g illustrates another exemplary wearable device, an in-ear headphone or earbud (170), capable of employing ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching according to examples of the present disclosure. The exemplary devices illustrated in FIG. 1a through 1g are provided as examples, and it should be understood that other types of devices may employ ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching.

[0021] Ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching can be integrated into the aforementioned systems to improve the touch sensing and fingerprint imaging capabilities of the system. In some examples, a touch screen (e.g., capacitive, resistive, etc.) can be augmented with ultrasonic touch sensing and acoustic impedance matching to provide enhanced sensing capabilities (e.g., fingerprint imaging in addition to touch sensing). In some examples, an alternatively non-touch-sensitive display can be augmented with ultrasonic touch sensing and acoustic impedance matching to provide touch sensing capabilities (e.g., fingerprint imaging). In such examples, the display can be implemented without the stack-up required for a capacitive touch screen. In some examples, ultrasonic touch sensing with acoustic impedance matching can be used to provide touch sensing capabilities (e.g., fingerprint imaging) on ​​a non-display surface. For example, ultrasonic touch sensing with acoustic impedance matching can be used to provide fingerprint imaging capabilities for a button of a watch, an earbud, a trackpad (e.g., a trackpad (146) of a personal computer (144)), a scroll wheel, part or all of a housing, or any other surface of a device (e.g., front, back, or sides).

[0022] As illustrated by the examples of FIGS. 1e through 1g, some of the embodiments of the present disclosure relate particularly to wearable devices capable of providing users with readily accessible input mechanisms for initiating or performing various functions, and secure authentication and access to the wearable device and / or paired devices through fingerprint imaging. In some embodiments, ultrasonic fingerprint imaging capabilities may be integrated within existing wearable device structures. In these embodiments, a user may touch a touch-sensitive area on the wearable device structure with one or more fingers, and ultrasonic transducers with acoustic impedance matching may detect these touches and capture a fingerprint image to initiate or perform functions such as user verification. For example, ultrasonic touch detection and fingerprint imaging with acoustic impedance matching may be integrated into a button of a watch (150), or into accessible areas of over-ear headphones (160) or in-ear headphones (170).

[0023] FIG. 2 illustrates a block diagram of an electronic device comprising ultrasonic touch sensing and fingerprint imaging having acoustic impedance matching according to examples of the present disclosure. In some examples, the housing (202) of the device (200) (e.g., corresponding to the above devices (136, 140, 144, 148, 150, 160, 170)) may be coupled (e.g., mechanically) with one or more ultrasonic transducers (204). In some examples, the transducers (204) may be an array of piezoelectric transducers that can be manufactured to vibrate upon the application of electrical signals when acting as a transmitter and to generate electrical signals based on detected vibrations when acting as a receiver. In some examples, the transducers (204) may be formed from a PMUT array or a piezoelectric ceramic material (e.g., PZT or KNN) or a piezoelectric plastic material (e.g., PVDF or PLLA). In various examples, the transducers (204) may be bonded to the housing (202) by an adhesive (e.g., composite epoxy), deposited on one or more surfaces through processes such as deposition, lithography, etc., or formed integrally within the housing. When electrical energy is applied to the transducers (204) and causes them to vibrate, one or more surfaces in contact with the transducers may also vibrate, and the vibration of molecules of the surface material may propagate as ultrasound through one or more surfaces / materials. In some examples, the vibration of the transducers (204) may be used to generate ultrasound at a selected frequency within a medium on the surface of the electronic device.

[0024] In some examples, transducers (204) may be partially or completely disposed (or coupled thereto) on a portion of the display (208), which may be integrated with an additional (non-ultrasonic) touch circuit (212) in some examples to form a touch screen, but it should be understood that some exemplary devices do not include the display (208) nor the additional touch circuit (212) (the optional characteristics of which are indicated by dashed lines). The device (200) may further include an ultrasonic touch sensing circuit (206), which can perform touch sensing and fingerprint imaging and may include a circuit (e.g., a transmitting circuit) for driving electrical signals to stimulate vibrations of the transducers (204), as well as a circuit (e.g., a receiving circuit) for detecting electrical signals output by the transducers (204) when the transducers are stimulated by received ultrasonic energy. In some examples, timing operations for the ultrasonic touch sensing circuit (206) may be provided by a separate ultrasonic touch sensing controller (210) capable of controlling the timing of operations by the ultrasonic touch sensing circuit (206), including, optionally, touch and fingerprint detection and imaging. In some examples, the ultrasonic touch sensing controller (210) may be coupled between the ultrasonic touch sensing circuit (206) and the host processor (214). In some examples, controller functions may be integrated with the ultrasonic touch sensing circuit (206) (e.g., on a single integrated circuit). Output data from the ultrasonic touch sensing circuit (206) may be output to the host processor (214) for further processing to determine the location of an object in contact with the device (e.g., the location of fingerprint ridges). In some examples, processing for determining the position of the contacting object may be performed by an ultrasonic touch detection circuit (206), an ultrasonic touch detection controller (210), or a separate sub-processor (not shown) of the device (200).

[0025] The host processor (214) receives ultrasonic and optionally other touch sensor outputs (e.g., capacitive) and non-touch sensor outputs, and can initiate or perform actions based on these sensor outputs. The host processor (214) can also be connected to a program storage (216) and optionally a display (208). The host processor (214) can, for example, communicate with the display (208) to generate an image such as an image of a user interface (UI) on the display, and can detect a touch, for example, a touch input and / or force input on or near the display (208), using the ultrasonic touch detection circuit (206) (and, in some examples, their respective controllers), and in some examples, the touch detection circuit (212). Touch input may be used by computer programs stored in the program store (216) to perform actions such as security authentication and access, moving objects such as a cursor or pointer, scrolling or panning, adjusting control settings, opening files or documents, viewing menus, making selections, executing commands, operating peripheral devices connected to a host device, receiving phone calls, making phone calls, ending phone calls, changing volume or audio settings, storing information related to telephone communication such as addresses, frequently dialed numbers, received calls, missed calls, logging into a computer or computer network, allowing authorized personal access to restricted areas of a computer or computer network, loading a user profile associated with user preference arrangements on a computer desktop, allowing access to web content, launching specific programs, encrypting or decoding messages, etc., but not limited to these.The host processor (214) can also perform additional functions that may not be related to touch processing.

[0026] Note that one or more of the functions described herein may be performed by firmware stored in memory and executed by the ultrasonic touch sensing circuit (206) (or their respective controllers) and, in some embodiments, by the touch circuit (212), or by firmware stored in the program storage (216) and executed by the host processor (214). The firmware may also be stored and / or transmitted in any non-transient computer-readable storage medium for use by or in connection with an instruction execution system, device, or device, such as a computer-based system, a processor-containing system, or other system capable of fetching instructions from and executing instructions from an instruction execution system, device, or device. In the context of this specification, "non-transient computer-readable storage medium" may be any medium (excluding signals) capable of containing or storing a program for use by or in connection with an instruction execution system, device, or device. Non-transient computer-readable media storage may include, but is not limited to, electronic, magnetic, optical, electromagnetic, infrared, or semiconductor systems, devices or devices, portable computer diskettes (magnetic), random access memory (RAM) (magnetic), read-only memory (ROM) (magnetic), erasable programmable read-only memory (EPROM) (magnetic), portable optical discs such as CD, CD-R, CD-RW, DVD, DVD-R, or DVD-RW, or flash memory such as Compact Flash cards, secure digital cards, USB memory devices, and memory sticks.

[0027] Firmware may also be propagated within any transmission medium for use by or in connection with an instruction execution system, device, or device, such as a computer-based system, a processor-containing system, or other system capable of fetching instructions from or executing instructions from an instruction execution system, device, or device. In the context of this specification, "transmission medium" may be any medium capable of delivering, propagating, or transmitting a program for use by or in connection with an instruction execution system, device, or device. A transmission readable medium may include, but is not limited to, electronic, magnetic, optical, electromagnetic, or infrared wired or wireless radio media.

[0028] It should be understood that the device (200) is not limited to the components and configurations of FIG. 2 and may include other or additional components in a number of configurations according to various examples. Additionally, the components of the device (200) may be included within a single device or distributed among a number of devices. Additionally, it should be understood that the connections between the components are exemplary and, regardless of the arrows shown in the configuration of FIG. 2, different unidirectional or bidirectional connections between the components may be included according to the embodiment.

[0029] FIG. 3 illustrates an exemplary process (300) for ultrasonic touch detection of an object (e.g., fingerprint ridges) in contact with a touch-sensitive surface according to examples of the present disclosure. In 302, ultrasonic energy may be transmitted through the thickness of a material in the form of ultrasound (e.g., by an array of transducers). In some examples, the waves may propagate as bulk compression waves or bulk shear waves. Other modes of propagation for the transmitted ultrasonic energy may also exist based on the properties of the surface material, the geometry, and the manner of energy transmission from the transducers to the surface of the device. In some examples, wave propagation discontinuities may occur at material layer boundaries and in the surface material (e.g., when ultrasound propagates to a touch surface opposite the transducer). The transmitted energy may propagate through the thickness until the wave propagation discontinuity at the material layer boundary or until it reaches the surface, which may cause a portion of the energy to be reflected. When transmitted energy reaches one of the aforementioned wave propagation discontinuities, some of the energy may be reflected, and some of the reflected energy may be directed toward one or more transducers. Objects such as fingers or fingerprint ridges in contact with the surface may affect the amount of energy reflected at surface propagation discontinuities.

[0030] In 304, returning ultrasonic energy may be received, and the ultrasonic energy may be converted into an electrical signal by one or more transducers. In 306, the ultrasonic sensing system may determine whether one or more objects are in contact with the surface of the device and may further detect the location of one or more objects based on the received ultrasonic energy. In some examples, baseline reflected energy from one or more intentionally included wave propagation discontinuities (e.g., edges) may be compared with the measured value of the reflected energy corresponding to one or more wave propagation discontinuities. The baseline reflected energy may be determined during a measurement when no object (e.g., fingerprint ridges) is in contact with the surface. Deviations of the reflected energy from the baseline may be correlated with the presence of an object touching the surface.

[0031] As described above, the process (300) generally refers to a reflected wave received by the same transducer(s) that transmitted the wave, but in some examples, the transmitter and receiver functions may be separated so that the transmission of ultrasonic energy at 302 and the reception of the ultrasound at 304 may occur at different transducers located together (e.g., one transducer is the transmitting configuration and one transducer is the receiving configuration). In some examples, the ultrasonic energy may be transmitted along and / or through the surface by one or more transducers and received at different locations along the surface by one or more additional transducers (not shown). The attenuation of the received ultrasonic energy may be used to detect the presence of one or more objects on the surface and / or to identify their locations. In some examples, the transmitted ultrasonic energy may be received at the transmitting transducer and may also be received at one or more other non-transmitting transducers located at different locations (e.g., different locations along the surface). Energy can be reflected from one or more objects at multiple angles, and the energy received at all receiving transducers can be used to determine the positions of one or more objects.

[0032] FIG. 4a illustrates a touch sensing stackup (400) using an array of PMUTs (402) according to examples of the present disclosure. In the example of FIG. 4a, the PMUT array (402) may be used to provide ultrasonic waves (416) to a touch surface through a sensing plate (404), wherein the presence or absence of fingerprint ridges (406) may change the amount of energy reflected back to the PMUT array as a finger signature (418). This change in reflected energy may be used to determine an image of the user's fingerprint. However, when the PMUT array (402) and the associated electronic device (408) are attached to the back of the sensing plate (404) using an adhesive such as epoxy (410), the resulting stackup of materials (e.g., PMUT array, epoxy, and sensing plate) may create an acoustic impedance mismatch at the material layer boundaries (412). In particular, the epoxy (410) may have a relatively low acoustic impedance (e.g., about 3 MRayl), whereas the sensing plate (404) may have a much higher acoustic impedance of about 46 MRayl. This acoustic impedance mismatch can reduce the transmission of ultrasonic energy to / from the PMUT array (402) and generate parasitic reflections (414), both of which can adversely affect the PMUT array's ability to perform touch detection or fingerprint imaging.

[0033] The sensing plate (404) can be manufactured from a wide range of materials such as aluminum (Al), stainless steel (SUS), ceramics, such as zirconia (ZrO2), titanium (Ti), etc. Although FIG. 4a illustrates a PMUT array (402), in other examples, different types, numbers, and arrangements of ultrasonic transducers may also be used.

[0034] FIG. 4b illustrates a parasitic reflection (414) from an acoustic impedance mismatch and a finger signature (418) reflected back from the touch surface of the sensing plate (404) when an epoxy (410) having a low acoustic impedance is used according to examples of the present disclosure. In the example of FIG. 4b, an acoustic impedance mismatch may be formed between the epoxy (410) and the sensing plate (404). Due to this mismatch, acoustic signals generated by the PMUT array (402) may be reflected back from the boundary (412) between the epoxy (410) and the sensing plate (404) (regardless of whether a finger is present on the touch surface of the sensing plate) and may be received as a parasitic reflection (414) in the PMUT array. Note that because the parasitic reflection (414) travels a shorter distance than the finger signature (418), the parasitic reflection may arrive back at the PMUT array (402) before the finger signature. Therefore, during the first few cycles of the parasitic reflection (414) illustrated in FIG. 4b, the finger signature (418) is not present. As illustrated by the example in FIG. 4b, since a significant amount of ultrasonic energy can be reflected in the form of the parasitic reflection (414), the amplitude (energy) of the parasitic reflection can be large, while the amplitude of the finger signature (418) can be adversely reduced. Generally, ultrasonic fingerprint imaging systems are often constrained by a number of parameters, so the signal-to-noise ratio (SNR) of the system may be undesirably low when high acoustic impedance sensing plates are used. The examples of the present disclosure reduce acoustic impedance mismatch to improve the SNR.

[0035] Examples of the present disclosure relate to improving the accuracy of ultrasonic touch detection and fingerprint imaging by employing acoustic impedance matching to reduce parasitic reflections and increase finger signature energy in devices such as desktop, laptop, and tablet computing devices; wearables such as smartphones, media players, watches, and health monitoring devices; smart home control and entertainment devices; headphones and earbuds; and devices for computer-generated environments such as augmented reality, mixed reality, or virtual reality environments. Some examples of the present disclosure relate particularly to wearable devices capable of providing users with immediately accessible input mechanisms for initiating or performing various functions, and secure authentication and access to wearable devices and / or paired devices via fingerprint imaging.

[0036] FIG. 5a illustrates a composite epoxy (520) that may comprise an epoxy (522) and filler particles (524) suspended in the epoxy, according to examples of the present disclosure. In some examples, the materials and sizes of the components of the composite epoxy (520) may be selected to reduce the acoustic impedance mismatch between the PMUT array and the sensing plate. Because the acoustic impedance of the sensing plate may be very high compared to the much lower acoustic impedance of the epoxy (522) (e.g., 3 MRayl) (e.g., 46 MRayl in the case of SUS), in some examples, filler particles (524) may be added to the epoxy to produce a composite epoxy (520) having an increased acoustic impedance compared to the epoxy. In optimal examples, the acoustic impedance of the composite epoxy (520) may be increased to match the acoustic impedance of the underlying PMUT array (e.g., 11 MRayl) or electronic device (e.g., 22 MRayl). However, in other examples that may be easier to achieve, the composite epoxy (520) may have an increased acoustic impedance of 7 MRayl (increased relative to the epoxy itself). The increased acoustic impedance may reduce the acoustic impedance mismatch between the PMUT array and the sensing plate, which may increase the transmission of ultrasonic energy to / from the PMUT array and reduce parasitic reflections, both of which may improve the ability of the PMUT array to perform fingerprint imaging.

[0037] In some examples, the composite epoxy (520) may be formulated to be temperature-stable (i.e., low thermal drift) so that its properties do not change with temperature. Thus, the epoxy (522) may be selected to have a "glass transition" (the temperature at which the epoxy begins to transition from a hard or brittle state to a softer, more viscous state) well above the expected operating temperature of the device (e.g., up to 60°C). In some examples, the epoxy (522) may be selected to have a glass transition of 80 to 100°C so that the properties of the epoxy remain stable over the expected operating temperature range. Above this temperature range, the epoxy may become too soft, and the modulus may drop, which may cause an undesirable shift in acoustic impedance.

[0038] In some examples, the diameter of the filler particles (524) d ) can be selected to be much smaller than the wavelength of the ultrasound propagating through the composite epoxy (520) so that ultrasonic energy reflection / scattering is minimized. Diameter ( d If the ) is too large, the filler particles (524) may reflect / scatter the waves. In some examples, since the average wavelength of the ultrasound propagating through the composite epoxy (520) may be about 100 to 150 micrometers, the particle size may be selected to be 5 to 15 micrometers, and in some cases less than 5 micrometers.

[0039] The volume fraction of particles (524) within the epoxy (522) can be selected to achieve a desired acoustic impedance. Ideally, the acoustic impedance of the composite epoxy (520) should match the acoustic impedance of the PMUT array (e.g., having an acoustic impedance of about 11 MRayl), but this may not be achievable in practice. Therefore, in some examples, the volume fraction of particles (524) can be selected to be about 60 to 70%. A higher volume fraction may result in a higher acoustic impedance, but at a volume fraction greater than about 70%, the integrity of the composite epoxy (520) may be compromised. For example, the adhesive ability may be reduced, and the viscosity may be too high, making it difficult to distribute and apply the composite epoxy to a desired thickness (e.g., 15 to 45 micrometers).

[0040] In the above view, in some examples of the present disclosure, an 0-3 composite epoxy comprising an alumina filler (40% by volume, particle size < 5 µm) and an epoxy (e.g., Epo-tek® 301 epoxy having an acoustic impedance of about 3 MRayl) may be used. The alumina filler may increase the acoustic impedance of the composite epoxy to about 7 MRayl, and in some cases, to 8 to 11 MRayl. Although the sensing plate may have a much higher acoustic impedance (e.g., about 46 MRayl for stainless steel (SUS), 30 MRayl for titanium, and 27 MRayl for aluminum), nevertheless, the increased acoustic impedance of the composite epoxy (compared to the lower acoustic impedance of 3 MRayl for epoxy) reduces the impedance mismatch between the PMUT array and the sensing plate, which can produce reduced attenuation of ultrasound, increased finger signature energy, and improved ultrasonic touch detection and fingerprint imaging.

[0041] FIG. 5b illustrates a parasitic reflection (514) from an acoustic impedance mismatch and a finger signature (518) reflected back from the touch surface of the sensing plate (504) when a composite epoxy (520) is used according to examples of the present disclosure. Due to the reduced acoustic impedance mismatch provided by the composite epoxy (520), less energy from the ultrasonic signals generated by the PMUT array can be reflected (regardless of whether a finger is present on the touch surface of the sensing plate) and can be received back from the PMUT array as a parasitic reflection. Note that because the parasitic reflection (514) can travel a shorter distance than the finger signature (518), the parasitic reflection can arrive back from the PMUT array before the finger signature. Therefore, during the first few cycles of the parasitic reflection (514) shown in FIG. 5b, the finger signature (518) is not present. As illustrated by the example in FIG. 5b, since less ultrasonic energy can be reflected in the form of parasitic reflection (514), the amplitude (energy) of the parasitic reflection can be reduced, while the amplitude of the finger signature (518) can be advantageously increased (compared to FIG. 4b).

[0042] As mentioned above, the materials and sizes of the components of the composite epoxy can be selected to reduce acoustic impedance mismatch between the PMUT array and the sensing plate. To further improve acoustic impedance matching, in other examples of the present disclosure, a matching layer may be formed on the back surface of the sensing plate to create an acoustic impedance gradient between the lower acoustic impedance PMUT array and the high acoustic impedance sensing plate.

[0043] FIG. 6a illustrates a matching layer (626) attached to a sensing plate (604) while being separated from the epoxy (610) and PMUT array (602) according to examples of the present disclosure. The matching layer (626) can create a more gradual acoustic impedance transition (i.e., a bridge or gradient) between the PMUT array (602) and the sensing plate (604). This acoustic impedance gradient can reduce the amount of ultrasonic reflection generated at the interface between the PMUT array (602) and the sensing plate (604) and allow more energy to pass through the surface of the sensing plate for touch detection and fingerprint imaging. By selecting the thickness and composition of the matching layer (626) according to the characteristics of the sensing plate (604), different matching layers can be developed for use with different sensing plate materials.

[0044] FIG. 6b illustrates a matching layer (626) sandwiched between a sensing plate (604) and a PMUT array (602) according to examples of the present disclosure (an epoxy layer is omitted for clarity). In some examples, the matching layer (626) may have an acoustic impedance (Zm) between the acoustic impedance (Zt) of the PMUT array (602) and the acoustic impedance (Zp) of the sensing plate (604). In one example, Zm may be the geometric mean of Zp (46 MRayl for SUS) and Zt (11 MRayl), or 22.5 MRayl. Selecting the matching layer (626) to have an acoustic impedance approximately equal to the geometric mean of two adjacent materials can improve the energy transfer of ultrasound when ultrasound propagates through the matching layer. In some examples, the thickness of the matching layer (626) ( d ) can be 1 / 4 of the wavelength of the ultrasound propagating through the matching layer. Maximum energy transfer of the signal through the matching layer (626) can occur at this 1 / 4 wavelength thickness.

[0045] The table below provides exemplary matching layer acoustic impedances for four different sensing plate materials and one PMUT array according to some examples of the present disclosure.

[0046]

[0047] Since aluminum, tin, and titanium have acoustic impedances in the range of 13.7 to 23.2 MRayl, these materials can be used as matching layers. Using a stainless steel sensing plate as an example, an aluminum matching layer with a thickness of 30 micrometers can have an acoustic impedance of about 17 MRayl and a maximum transmittance of 94%, a titanium matching layer with a thickness of 30 micrometers can have an acoustic impedance of about 27 MRayl and a maximum transmittance of 95%, and a tin matching layer with a thickness of 17 micrometers can have an acoustic impedance of about 24 MRayl and a maximum transmittance of 99%. For broadband pulses, transmittance decreases when the frequency is far from the optimal frequency.

[0048] In other examples, the matching layer (626) may be formed from a plurality of matching sub-layers, and each sub-layer in the stack of sub-layers has an ultrasonic impedance that increases from the bottom sub-layer to the top sub-layer adjacent to the sensing plate. In some examples, all matching sub-layers may have a total thickness approximately equal to 1 / 4 wavelength of the frequency propagating through the matching sub-layers. In other examples, each matching sub-layer may have a thickness approximately equal to 1 / 4 wavelength of the frequency propagating through the matching sub-layer. Referring to FIG. 6b, the matching layer (626) may be replaced by a plurality of matching sub-layers formed on the back surface of the sensing plate (604) and later bonded to the lower PMUT layer (602). In one example, an aluminum matching sub-layer may be formed on the sensing plate, and then a magnesium matching sub-layer (having a lower acoustic impedance) may be formed on the aluminum matching sub-layer. In some examples, each matching sub-layer can be tuned to have an acoustic impedance that is the approximate geometric mean of the acoustic impedances of the materials on both sides of the matching sub-layer.

[0049] FIG. 6c illustrates the energy transmittance of an ultrasonic signal through a single matching layer of a specific material over a range of frequencies and layer thicknesses according to examples of the present disclosure. In the example of FIG. 6c, the center frequency of the ultrasonic signal ( f c In ), a thickness equal to 1 / 4 wavelength of the ultrasonic signal ( d Energy transfer through a matching layer having ) is maximized, as indicated by a point (644) located in a high-energy transmission band (630). Meanwhile, frequency components of the ultrasonic signal far from the center frequency ( f h ) can have much less energy transfer, as indicated by the point (646) located in the lower energy transmission band (634).

[0050] However, as described above, when multiple matching sub-layers are employed, acoustic energy transfer can be improved over a wider frequency band. For example, the transmittance graph of multiple matching layers may have a wider high-energy transmission band (630) than shown in the example of FIG. 6c. This wider high-energy transmission band (630) can enable more energy from the ultrasound to propagate to the touch surface of the sensing plate with less reflection.

[0051] FIG. 7a illustrates various materials and techniques that may be used to form a matching layer (726) on the back surface of a sensing plate (704) according to examples of the present disclosure. In the left stack-up of FIG. 7a, a thin layer of nickel (Ni) having a thickness of less than 5 micrometers may be deposited using physical vapor deposition (PVD) on the back surface of a sensing plate (704) formed from SUS or zirconia (ZrO2)—both having an acoustic impedance of about 50 MRayl—and is followed by a layer of tin (Sn) having a thickness of about 17 micrometers. The Ni layer is required as a seed layer when ZrO2 is used as the sensing plate (704), because ZrO2 is a non-conductive ceramic. After the Ni layer is formed, a Sn layer may be plated on the Ni layer. Because the nickel is too thin, the tin layer primarily provides matching layer characteristics. In the center stack-up of FIG. 7a, aluminum (Al) with a thickness of about 30 micrometers can be clad (diffusion bonded) with a sensing plate (704) formed from SUS or titanium (Ti) to form a matching layer (726). In the right stack-up of FIG. 7a, PVD can be used to deposit both Al and Ti materials on the back surface of a sensing plate (704) formed from SUS, ZrO2, or Ti to form a matching layer (726). Any of these three methods can be used to form the matching layer (726), but in practice, the three methods can result in different types of non-uniformities that can affect the acoustic impedance.

[0052] FIG. 7b illustrates the formation of a matching layer using plating according to examples of the present disclosure. In the example of FIG. 7b, a thin film of nickel (e.g., less than 2 micrometers) may first be deposited using PVD onto a SUS (450 micrometers) or ZrO2 (800 micrometers) sensing plate (704) (note that FIG. 7b shows the sensing plate at the bottom). The Sn layer can protect delicate structures and improve reliability. Since the nickel layer is thin, it is negligible in terms of the desired quarter wavelength thickness. The Sn layer may have a desired quarter wavelength thickness of about 17 micrometers and may be a major contributor to acoustic impedance matching. Note that the matching layer (726) does not need to have an exact 1 / 4 wavelength thickness, because the acoustic impedances of the PMUT array, matching layer, sensing plate, etc., are highly interdependent, so the actual preferred matching layer thickness can be determined by simulations of the entire stackup. The two stackups on the left in FIG. 7b illustrate Ni and Sn plated on flat SUS and ZrO2 sensing plates (704). The two stackups on the right in FIG. 7b illustrate Ni and Sn plated on SUS and ZrO2 sensing plates (704) formed with geometric structures (e.g., cones) for improved acoustic impedance matching, as described below.

[0053] FIG. 7c illustrates a plating process flow for forming and plating a SUS sensing plate having geometric structures according to examples of the present disclosure. In the example of FIG. 7c (from left to right), raw SUS material can be laser-textured to form a sensing plate (704) having geometric structures (734) (note that FIG. 7c shows the sensing plate from the bottom). Subsequently, the sensing plate (704) can be deposited with a thin layer of Ni (< 5 micrometers) using PVD, followed by a thicker layer of Sn, which can primarily act as a matching layer. Nickel plating can serve as a seed layer so that the Sn layer can have material to be plated in the plating bath. Since Sn is plated over an irregular surface, the plated Sn may also have an irregular surface. Therefore, the final step of FIG. 7c is a desired 1 / 4 wavelength thickness ( d The Sn layer can be flattened to have a thickness of (e.g., 30 micrometers) and to provide a smooth surface for epoxy bonding.

[0054] FIG. 7d illustrates the formation of a matching layer using cladding according to examples of the present disclosure. In the three stack-ups on the left in FIG. 7d, Al and / or magnesium (Mg), each having a thickness of about 30 micrometers, can be clad with a SUS sensing plate (704) having a thickness of about 450 micrometers. Cladding involves fusing rolls of dissimilar metals together using high-pressure rollers and high heat. At the material boundaries, some atoms of one material diffuse into the other material, causing the two materials to stick together. When clad, the Al or Mg material can serve as a matching layer (726). The leftmost stack-up in FIG. 7d illustrates a cladding of aluminum (also having a thickness of about 30 micrometers) and Mg having a thickness of about 30 micrometers to form a plurality of matching layers as described above. In the far right stack-up of FIG. 7d, Al having a thickness of about 30 micrometers can be clad with a Ti sensing plate (704) having a thickness of about 450 micrometers. When clad, the Al layer can serve as a matching layer (726). The second stack-up from the right in FIG. 7d illustrates a cladding of Al having a thickness of about 390 micrometers with a much thinner SUS layer having a thickness of 60 micrometers. In this stack-up, the SUS layer may be a decorative layer having a thickness equal to about half the wavelength of the acoustic signal propagating through the SUS layer to create a magnetic-resonant layer.

[0055] FIG. 7e illustrates the formation of a matching layer using PVD according to examples of the present disclosure. In the left stack-up of FIG. 7e, Al having a thickness of about 30 micrometers can be progressively built up on a ZrO2 sensing plate (704) using PVD to form a matching layer (726). In the middle stack-up of FIG. 7e, the matching layer (726) having a thickness of about 30 micrometers can be progressively built up from a plurality of layers of Ti, Al, or combined Ti / Al (e.g., in ratios of 75% / 25% or 50% / 50%) each having a thickness of about 6 micrometers, and applied on a ZrO2 sensing plate (704) using PVD. In the right stack-up of FIG. 7e, a matching layer (726) having a thickness of about 30 micrometers is progressively built up from a plurality of layers of Ti, Al, or combined Ti / Al (e.g., in ratios of 75% / 25% or 50% / 50%) each having a thickness of about 6 micrometers, and can be applied onto a SUS sensing plate (704) using PVD. In the middle and right stack-ups of FIG. 7e, the material layers forming the matching layer (726) can be selected with different acoustic impedances to create an acoustic impedance gradient across the matching layer, which can cause less parasitic reflection.

[0056] The acoustic impedance gradient that can be achieved by the multiple matching layer examples of FIG. 7d and the multiple material matching layer examples of FIG. 7e can also be achieved in other ways. For example, introducing micro-air bubbles or perforations into the sensing plate can reduce the effective acoustic impedance of the sensing plate. The terms "pore" and the property of "porousness" may be used herein to collectively refer to closed voids (i.e., pockets or bubbles) of air, gas, or other low acoustic impedance materials (compared to the acoustic impedance of the sensing plate), or open-ended perforations (i.e., notches or cavities) filled with air or other low acoustic impedance materials. Due to the low acoustic impedance of air, selected gases, and other low acoustic impedance materials, the presence of pores can lead to a reduction in the average acoustic impedance of the sensing plate at the locations of the pores. The formation of pores in a non-uniform (variable density) distribution with respect to the sensing plate can create an acoustic impedance gradient through the sensing plate, which can lead to less waveform reflection, increased wave transmission energy, and more accurate touch detection and fingerprint imaging.

[0057] FIG. 8a illustrates a sensing plate (804) having an internal acoustic impedance gradient created by pores according to examples of the present disclosure. In the example of FIG. 8a, a PMT array (802) is attached to the sensing plate (804) (an epoxy layer is omitted for clarity), and ultrasound is transmitted to / from a touch surface through the sensing plate. In some examples, the sensing plate (804) may include a plurality of closed pores (e.g., air gaps, voids, or bubbles) with a non-uniform distribution within the sensing plate (symbolically illustrated in FIG. 8a), wherein the bottom portion of the sensing plate has the highest porosity (highest pore concentration) to lower the acoustic impedance (Z1) of that portion of the sensing plate and provide a closer match with the acoustic impedance of the PMUT array (802). The porosity of the sensing plate (804) can be gradually reduced at increasing elevations within the sensing plate (in the view of FIG. 8a), so that the upper part of the sensing plate has the lowest porosity (lowest pore concentration) to increase the acoustic impedance (Z2) of that part of the sensing plate and provide acoustic impedance matching closer to, for example, a touching finger. In other words, when viewed from the bottom to the top, the effective acoustic impedance of the sensing plate (804) can be gradually increased as the volume fraction ratio of the higher impedance to the lower impedance material becomes dominant.

[0058] FIG. 8b illustrates a sensing plate (804) having pores (828) of non-uniform distribution according to examples of the present disclosure. As illustrated in the example of FIG. 8b, the closed pores (828) may be filled with air, gas, or other low acoustic impedance material and have a diameter selected such that it is smaller than the wavelength of the acoustic signal propagating through the sensing plate (804) to minimize ultrasonic reflection (in some cases, much smaller than the wavelength, e.g., less than 1 / 10 of the wavelength). dIt can have ). In some examples, d The size may be approximately 3 to 10 micrometers. In some examples, pores (828) may be added to the sensing plate (804) using a metal injection molding process, wherein a bonding material having a volume fraction that varies over time is added to the metal powder as the sensing plate is gradually formed. Once the sensing plate (804) is fully built up, de-binding and sintering may be performed on the bonding layer to remove the bonding material and create porosity throughout the bonding layer. Since the density of the pores (828) is greatest at the bottom of the sensing plate (804) and lowest at the top, the acoustic impedance may gradually increase from Z1 at the bottom to Z2 at the top throughout the sensing plate.

[0059] FIG. 8c illustrates a sensing plate (804) having pores in the form of perforations, notches, or voids according to examples of the present disclosure. In the exemplary left sensing plate (804) of FIG. 8c, laser micro-perforations (830) having a depth of 25 to 30 micrometers and a pitch of 25 micrometers may be created on the upper surface (touch surface) and lower surface (surface facing the PMUT array) of the sensing plate, which may be made of SUS or ZrO2. Since the perforations (830) do not extend throughout the entire sensing plate (804), an uneven distribution of pores is created. In some examples, the open-end notches or voids created by the laser micro-perforations may be filled with a low acoustic impedance material. Laser micro-perforations (830) on the upper surface of the sensing plate (804) may have a width and depth selected to lower the acoustic impedance at the top of the sensing plate to match, for example, the acoustic impedance of a touching finger, whereas laser micro-perforations on the bottom surface may have a different width and depth selected to lower the acoustic impedance at the bottom of the sensing plate to match, for example, the acoustic impedance of the underlying epoxy layer or PMUT array. In the exemplary right-side sensing plate (804) of FIG. 8c, deep laser micro-perforations (832) having a depth of about 120 micrometers and a pitch of 50 micrometers may be created on the bottom surface of the sensing plate (the surface facing the PMUT array), which may be made of SUS or ZrO2. Since the perforations (832) do not extend throughout the entire sensing plate (804), an uneven distribution of pores is created. Deep laser micro-perforations (832) on the bottom surface may have a selected width and depth to lower the acoustic impedance at the bottom of the sensing plate so as to match, for example, the acoustic impedance of the underlying epoxy layer or PMUT array more closely.In other examples, the perforations (830, 832) of FIG. 8c can be formed by building up the material rather than removing the material using a laser.

[0060] FIG. 8d illustrates a sensing plate (804) having density variations according to examples of the present disclosure. In the exemplary left sensing plate (804) of FIG. 8d, additive manufacturing techniques (i.e., 3D printing) may be employed to form a non-uniform material density gradient across the entire sensing plate, which may be manufactured from SUS or other materials. The material density at the top surface and bottom surface may be selected to be lower (i.e., more closed pores or voids) to create a lower acoustic impedance (Z1) that is more closely matched to the acoustic impedance of a touching finger at the top surface, and to be more closely matched to the acoustic impedance of an epoxy or PMUT array below the bottom surface. The material density toward the middle of the sensing plate (804) may be selected to be higher (i.e., fewer closed pores or voids) to create a higher acoustic impedance (Z2) in that region. In some examples, the material density may vary from 50 to 96%. In the exemplary right-side sensing plate (804) of FIG. 8d, additive manufacturing techniques (i.e., 3D printing) may be employed to form a non-uniform material density gradient across the entire sensing plate. The material density at the bottom surface may be selected to be lower (i.e., more closed pores or voids) to produce a lower acoustic impedance (Z1) that matches more closely with the acoustic impedance of the epoxy or PMUT array below the bottom surface. The material density at the top surface of the sensing plate (804) may be selected to be higher (i.e., fewer closed pores or voids) to produce a higher acoustic impedance (Z2) in that area.

[0061] FIG. 8e illustrates a sensing plate (804) having perforations and density variations according to examples of the present disclosure. An exemplary left sensing plate (804) of FIG. 8e may be a combination of the right sensing plates of FIG. 8c and FIG. 8d, wherein deep micro-perforations (created by removing or building up material) are present on the bottom surface and material density variations change from low on the bottom surface to high on the top surface. An exemplary right sensing plate (804) of FIG. 8e may be a combination of the left sensing plates of FIG. 8c and FIG. 8d, wherein micro-perforations (created by removing or building up material) are present on the top and bottom surfaces and material density variations change from low on the top and bottom surfaces to high toward the middle.

[0062] While prior examples utilized pores and / or perforations to generate one or more acoustic impedance transitions (e.g., gradients) with a sensing plate, in other examples of the present disclosure, geometric structures may be used alternatively or additionally to generate such acoustic impedance transitions.

[0063] FIG. 9a illustrates a sensing plate (904) having geometric structures (934) for generating an acoustic impedance gradient according to examples of the present disclosure. In the example of FIG. 9a, the sensing plate (904) may have an upper surface for receiving one or more touches and a lower portion formed of geometric structures (934) (e.g., conical structures) to generate a gradual acoustic impedance transition. In some examples, the geometric structures (934) may be formed on sensing plates made of SUS or ZrO2. The geometric structures may have distal endpoints (964) and valleys (966). The geometric structures (934) may be generated by laser ablation, microindentation, molding, additive manufacturing, etc., and may be aligned in an orthogonal xy array, offset in a diagonal array, or arranged in other regular or random arrays. Shapes other than cones, such as pyramids or generally conical structures having non-linear surfaces, may also be used, but in some examples, conical shapes may be easier to manufacture using available micro-manufacturing methods. Subsequently, the spaces between the geometric structures (934) may be filled with a lower acoustic impedance material (936). The material (936) may be an adhesive layer such as the aforementioned epoxy or composite epoxy (where the geometric structures are in direct contact with the epoxy layer), a matching layer as described above (where the matching layer is sandwiched between the geometric structures and the epoxy layer), or other materials. Essentially, the lower acoustic impedance filler material (936) is "geometrically mixed" with the higher acoustic impedance geometric structures (934) within the sensing plate (904) to create a gradual acoustic impedance transition at frequencies of interest.

[0064] Width of each geometric structure (934) d)(i.e., the distance between two peaks, or between two troughs; pitch) may be selected to be smaller than the average wavelength of the ultrasound propagating through the material (936) and the sensing plate (904) in order to reduce scattering and reflection of the waves (and in some cases, to be much smaller, e.g., less than 1 / 3 of the wavelength). In some examples, d The height of the geometric structure (934) from the valleys (966) to the distal end point (964) can be about 30 to 60 micrometers. h ) can be selected to be large enough to provide a smooth and gradual acoustic impedance transition. In some examples, h can be three times the wavelength of the ultrasound. In some examples, the height can be about 100 to 300 micrometers. In other examples, in the case of SUS sensing plates, the height ( h ) can be about 120 micrometers, and in the case of ZrO2, the height( h ) can be about 160 micrometers.

[0065] width( d When the wavelength of the ultrasound is chosen to be much smaller than the wavelength of the ultrasound, the acoustic impedance at the boundary of the sensing plate (904) (i.e., at the location of the geometric structures (934)) can be approximated by the average properties of the sensing plate and the filler material (936). The average acoustic impedance is approximately proportional to the area fraction of the two materials at any point along the cross-section of such materials in a theoretical plane parallel to the upper surface of the sensing plate. Thus, the acoustic impedance can transition from the acoustic impedance of the material (936) to the acoustic impedance of the sensing plate (904) as the theoretical plane moves from the distal ends of the geometric structures (934) to the valleys. The geometric structure of the geometric structures (934) (e.g., slope or taper) can determine the rate of change of the acoustic impedance.

[0066] FIG. 9b is a perspective view of a sensing plate (904) having geometric structures (934) according to examples of the present disclosure.

[0067] As discussed above, in some examples of the present disclosure, the material (936) may be a matching layer. FIG. 7c discussed above illustrates an exemplary process for creating a SUS sensing plate having geometric structures and an associated tin matching layer.

[0068] FIG. 9c illustrates a SUS sensing plate having geometric structures (934) filled with a finished Sn matching layer (926) as shown in FIG. 7c, according to examples of the present disclosure, and subsequently bonded to a PMUT array (902) using epoxy (910).

[0069] Referring again to FIG. 4a, the PMUT array (402) can generate parasitic ultrasound (448), which propagates through the electronic device (408), is reflected from the components and rear surface of the electronic device, and returns to the PMUT array as parasitic reflection (438). Like parasitic reflection (414), parasitic reflection (438) can negatively affect the ability of the PMUT array (402) to perform touch detection and fingerprint imaging. Accordingly, in some examples of the present disclosure, an absorbent may be formed on the rear surface of the electronic device (408) to absorb ultrasound (448) and reduce parasitic reflection (438).

[0070] FIG. 10a illustrates a touch sensing and fingerprint imaging stackup (1000) using a PMUT array (1002) backed with an absorbent (1040) according to examples of the present disclosure. FIG. 10a is similar to FIG. 4a except for the addition of an absorbent (1040) formed on the back surface of an electronic device (1008). Since the PMUT array (1002) will also generate undesirable (parasitic) ultrasound (1048) in the opposite direction from the sensing plate (1004) and through any underlying electronic device (1008), reflections (1038) from these waves may interfere with the proper detection of a touch finger or fingerprint ridge. In some examples, the electronic device (1008) may be about 190 micrometers thick, most of which are silicon wafers. Therefore, the acoustic impedance of the electronic device (1008) is uniform and is basically the same as the acoustic impedance of a single-crystal SiO2 wafer, which is about 22 MRayl.

[0071] In the example of FIG. 10a, the absorber (1040) formed on the back surface of the electronic device (1008) can absorb undesirable ultrasound (1048) and reduce the reflection (1038) of these waves. In optimal examples, the absorber (1040) can be formulated to have an acoustic impedance that matches the acoustic impedance of the electronic device (1008) (e.g., 22 MRayl). The absorber (1040) can be formed from a 0-3 epoxy composite made of a tungsten filler (particle size > 5 to 15 μm) and an epoxy (e.g., Epo-tek® 301 epoxy having an acoustic impedance of about 3 MRayl). The base epoxy can be selected to have high attenuation. As with the aforementioned composite epoxy, the absorber (1040) can have an acoustic impedance higher than that of the epoxy alone, along with high absorption. Tungsten can be used as a filler because it has a high acoustic impedance mismatch with epoxy—which can cause high scattering and increased energy absorption—but other filler materials with a high acoustic impedance mismatch with epoxy in other examples may also be used. Additionally, the high acoustic impedance of tungsten can cause a high overall acoustic impedance for the epoxy composite, which can result in better acoustic impedance matching with the electronic device (1008). The diameter of the filler particles may be 1 / 4 wavelength of the ultrasound propagating through the absorber (1040) for maximized scattering.

[0072] FIG. 10b illustrates a parasitic reflection (1042) from acoustic impedance mismatch (e.g., a combined effect of reflections (1014, 1038)) and a finger signature (1018) reflected back from the touch surface of a sensing plate (1004) when a composite epoxy (1020) and an absorbent (1040) are used according to examples of the present disclosure. In the example of FIG. 10b, the composite epoxy (1020) can reduce the acoustic impedance mismatch between the epoxy and the sensing plate (1004), and the absorbent (1040) can reduce the reflection received back from the electronic device (1008). Since less energy from the parasitic reflection is received back from the PMUT array (1002), the amplitude (energy) of the parasitic reflection (1042) can be reduced, while the amplitude of the finger signature (1018) can be advantageously increased.

[0073] FIG. 11 illustrates a flowchart for acoustic impedance matching and improved touch sensing and fingerprint imaging according to examples of the present disclosure. In the exemplary flowchart of FIG. 11, dashed blocks indicate that any one of these blocks is optional and that any one or more of these blocks may be performed individually or in combination. Although the blocks appear sequentially, it should be understood that their arrangement is merely for convenience of illustration and that any of these blocks may be implemented in any order. In block (1150), the acoustic impedance of the epoxy attaching the transducer array to the sensing plate may be increased to reduce the acoustic impedance mismatch between the transducer array and the sensing plate. In block (1152), a matching layer may be added between the transducer array and the sensing plate, wherein the acoustic impedance of the matching layer is the geometric mean of the acoustic impedances of the transducer array and the sensing plate. In block (1154), the thickness of the matching layer can be set to one-quarter the wavelength of the ultrasonic signal. In block (1156), non-uniform porosity can be added to the sensing plate to create an acoustic impedance gradient. In block (1158), the sensing plate and the matching layer can be "geometrically blended" by forming geometric structures on the sensing plate. In block (1160), ultrasound directed at the transducer electronics can be absorbed to reduce parasitic reflections.

[0074] Although various examples have been illustrated and described above primarily in separate drawings and paragraphs for clarity, it should be understood that various combinations of the described examples may be used together according to additional examples of the present disclosure. For example, any combination of two or more of a composite epoxy layer, a matching layer, porosity and / or perforations in a sensing plate, geometric structures on a sensing plate, and an absorbent layer may also be employed according to examples of the present disclosure.

[0075] Accordingly, according to the foregoing, some embodiments of the present disclosure relate to a touch sensing device, wherein the touch sensing device comprises a first surface, a second surface and a sensing plate having a first acoustic impedance—the first surface is configured to receive one or more touches and the second surface is formed of a plurality of geometric structures—and one or more ultrasonic transducers configured to propagate ultrasound to the first surface through the sensing plate, wherein the plurality of geometric structures taper from valleys toward distal ends oriented toward the one or more ultrasonic transducers, and the plurality of geometric structures are configured to reduce the acoustic impedance mismatch between the one or more ultrasonic transducers and the sensing plate. In addition to or alternatively to one or more of the embodiments disclosed above, in some embodiments, the pitch between adjacent distal ends of the plurality of geometric structures is smaller than the wavelength of the ultrasound expected to propagate through the geometric structures when generated by one or more ultrasonic transducers. In addition to or alternatively to one or more of the embodiments disclosed above, in some embodiments, the pitch between adjacent distal ends is about 30 to 60 micrometers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the height of a plurality of geometric structures from the bones to the distal endpoints is greater than the wavelength of the ultrasound expected to propagate through the geometric structures when generated by one or more ultrasonic transducers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the height of the plurality of geometric structures is about 100 to 300 micrometers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the touch sensing device further comprises a filler material disposed between the plurality of geometric structures and one or more ultrasonic transducers, wherein the filler material has a second acoustic impedance lower than a first acoustic impedance.In addition to or alternatively to one or more of the examples disclosed above, in some examples, a plurality of geometric structures and a filler material are configured to generate an average acoustic impedance that transitions from a second acoustic impedance of the filler material at the distal ends of the plurality of geometric structures to a first acoustic impedance of the sensing plate at the valleys of the plurality of geometric structures. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the filler material is an adhesive layer. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the filler material is a matching layer sandwiched between the geometric structures and the adhesive layer, and the second acoustic impedance of the matching layer is configured to be the approximate geometric mean of the first acoustic impedance of the sensing plate and the third acoustic impedance of the adhesive layer. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the adhesive layer is a composite adhesive layer formed from particles suspended in an epoxy having a fourth acoustic impedance, and the composite epoxy has a third acoustic impedance greater than the fourth acoustic impedance. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the filler material is a matching layer sandwiched between a plurality of geometric structures and one or more ultrasonic transducers, and the second acoustic impedance of the matching layer is configured to be the approximate geometric mean of the acoustic impedance of the sensing plate and the acoustic impedances of one or more ultrasonic transducers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, one or more of the plurality of geometric structures comprises a conical structure. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the taper of the conical structure is configured to produce a predetermined transition rate from the second acoustic impedance of the filler material to the first acoustic impedance of the sensing plate.

[0076] Some examples of the present disclosure relate to a method for reducing acoustic impedance mismatch between a sensing plate of a touch sensing device having a first acoustic impedance and one or more ultrasonic transducers, the method comprising: forming a plurality of geometric structures on a second surface of a sensing plate facing one or more ultrasonic transducers; tapering the plurality of geometric structures from valleys toward distal endpoints oriented toward one or more ultrasonic transducers; and transitioning the acoustic impedance experienced by ultrasound generated by one or more ultrasonic transducers from a second acoustic impedance smaller than the first acoustic impedance at the distal endpoints of the plurality of geometric structures to the first acoustic impedance at the valleys of the plurality of geometric structures. In addition to or alternative to one or more of the examples disclosed above, in some examples, the method further comprises forming a plurality of geometric structures having a pitch between adjacent distal endpoints smaller than the wavelength of ultrasound expected to propagate through the geometric structures when generated by one or more ultrasonic transducers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of forming a plurality of geometric structures having a height between valleys and distal endpoints greater than the wavelength of the ultrasound expected to propagate through the geometric structures when generated by one or more ultrasonic transducers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of placing a filler material between the plurality of geometric structures and one or more ultrasonic transducers, wherein the filler material has a second acoustic impedance lower than a first acoustic impedance.In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of generating an average acoustic impedance from distal endpoints of a plurality of geometric structures to valleys, transitioning from the acoustic impedance of the filler material to the acoustic impedance of the sensing plate. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of bonding the sensing plate to one or more ultrasonic transducers using the filler material as an adhesive. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of formulating the filler material as a matching layer having an acoustic impedance that is the approximate geometric mean of the first acoustic impedance of the sensing plate and the acoustic impedance of the adhesive layer.

[0077] Some examples of the present disclosure relate to a touch sensing device, wherein the touch sensing device comprises a first surface, a second surface and a sensing plate having a first acoustic impedance—the first surface being configured to receive one or more touches—one or more ultrasonic transducers configured to propagate ultrasound through the sensing plate to the first surface, and a non-uniform distribution of a plurality of pores within the sensing plate, wherein the plurality of pores are configured to create an acoustic impedance gradient within the sensing plate and to reduce an acoustic impedance mismatch between the one or more ultrasonic transducers and the sensing plate. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the plurality of pores comprises a plurality of sealed voids having a second acoustic impedance lower than the first acoustic impedance. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the diameter of the plurality of pores is smaller than the wavelength of the ultrasound expected to propagate through the sensing plate when generated by the one or more ultrasonic transducers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the diameter of the plurality of pores is about 3 to 10 micrometers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, a second surface of the sensing plate is closest to one or more ultrasonic transducers, and the sensing plate includes a larger distribution of pores near the second surface compared to the distribution of pores near the first surface to create an acoustic impedance gradient across the sensing plate. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the plurality of pores include a plurality of open-end perforations that partially extend into the sensing plate, and the plurality of open-end perforations cause a second acoustic impedance lower than the first acoustic impedance in the sensing plate at the location of the plurality of open-end perforations.In addition to or alternatively to one or more of the examples disclosed above, in some examples, a second surface of the sensing plate is closest to one or more ultrasonic transducers, and the second surface comprises a plurality of open-end type perforations. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the plurality of open-end type perforations on the second surface have a width and depth configured such that the second acoustic impedance of the sensing plate near the second surface is more closely matched with the third acoustic impedance of the layer beneath the sensing plate. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the first surface also comprises a plurality of open-end type perforations. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the plurality of open-end type perforations on the first surface have a width and depth configured such that the acoustic impedance of the sensing plate near the first surface is more closely matched with the acoustic impedance of the object in contact with the first surface. In addition to or alternatively to one or more of the examples disclosed above, in some examples, a second surface of the sensing plate is closest to one or more ultrasonic transducers, and the sensing plate includes a larger distribution of pores near the first surface and near the second surface compared to the distribution of pores near the center of the sensing plate to create an acoustic impedance gradient across the sensing plate. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the sensing plate is composed of a non-uniform material density gradient such that the highest material density is near the center of the sensing plate.

[0078] Some examples of the present disclosure relate to a method for reducing acoustic impedance mismatch between one or more ultrasonic transducers and a sensing plate within a touch sensing device, wherein the sensing plate has first and second surfaces and a first acoustic impedance, and the present method comprises the steps of forming a plurality of pores within the sensing plate and arranging the plurality of pores in a non-uniform distribution to create an acoustic impedance gradient within the sensing plate, wherein the acoustic impedance gradient creates a second acoustic impedance smaller than the first acoustic impedance near the second surface of the sensing plate closest to the one or more ultrasonic transducers. In addition to or alternative to one or more of the examples disclosed above, in some examples, the present method further comprises the step of forming a plurality of pores as a plurality of sealed voids having a third acoustic impedance lower than the first acoustic impedance. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of forming a plurality of pores having a diameter smaller than the wavelength of the ultrasound expected to propagate through the sensing plate when generated by one or more ultrasonic transducers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of arranging a plurality of pores in a non-uniform distribution having a larger distribution of pores near a second surface compared to a distribution of pores near a first surface to create an acoustic impedance gradient across the sensing plate. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of forming a plurality of pores as a plurality of open-end perforations that partially extend into the sensing plate. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of forming a plurality of open-end perforations on a second surface of the sensing plate.In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of forming a plurality of open-end type perforations on a first surface of a sensing plate. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of forming a larger distribution of pores near the first surface and near the second surface compared to the distribution of pores near the center of the sensing plate to create an acoustic impedance gradient across the sensing plate.

[0079] Some examples of the present disclosure relate to a touch sensing device, wherein the touch sensing device comprises a metal sensing plate having a first surface, a second surface and a first acoustic impedance—the first surface being configured to receive one or more touches—one or more ultrasonic transducers having a third acoustic impedance and configured to propagate ultrasound through the sensing plate to the first surface, and a matching layer disposed between the one or more ultrasonic transducers and the sensing plate, wherein the matching layer is configured to have a second acoustic impedance which is the approximate geometric mean of the first acoustic impedance and the third acoustic impedance to create an acoustic impedance gradient between the one or more ultrasonic transducers and the sensing plate and to reduce the acoustic impedance mismatch between the one or more ultrasonic transducers and the sensing plate. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the thickness of the matching layer is about one-quarter wavelength of the ultrasound expected to propagate through the matching layer when generated by the one or more ultrasonic transducers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the metal sensing plate is made of stainless steel and the matching layer is made of tin. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the touch sensing device further comprises a nickel layer less than 5 micrometers thick formed between the metal sensing plate and the matching layer. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the matching layer comprises a plurality of matching sub-layers formed from a plurality of materials, each sub-layer having an ultrasonic impedance that increases from a bottom sub-layer to an upper sub-layer adjacent to the metal sensing plate.In addition to or alternatively to one or more of the examples disclosed above, in some examples, the matching layer comprises a plurality of matching sub-layers formed from a plurality of materials, each sub-layer having an acoustic impedance that is the approximate geometric mean of the acoustic impedances of the materials on both sides of the sub-layer. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the matching layer comprises a plurality of matching sub-layers formed from a plurality of materials, and all matching sub-layers have a total thickness that is about 1 / 4 wavelength of the ultrasound expected to propagate through the matching sub-layers when generated by one or more ultrasonic transducers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, at least one matching sub-layer is formed from a combination of a plurality of materials in a specific ratio. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the matching layer comprises a plurality of matching sub-layers, each sub-layer having a thickness of about 1 / 4 wavelength of the ultrasound expected to propagate through the sub-layer when generated by one or more ultrasound transducers.

[0080] Some embodiments of the present disclosure relate to a method for reducing acoustic impedance mismatch between one or more ultrasonic transducers having a third acoustic impedance and a metal sensing plate having a first acoustic impedance in a touch sensing device, the method comprising: placing a matching layer between one or more ultrasonic transducers and a metal sensing plate; and selecting the matching layer to have a second acoustic impedance which is the approximate geometric mean of the first acoustic impedance and the third acoustic impedance to create an acoustic impedance mismatch between one or more ultrasonic transducers and a sensing plate and to reduce the acoustic impedance mismatch between one or more ultrasonic transducers and a sensing plate. In addition to or alternative to one or more of the examples disclosed above, in some examples, the method further comprises the step of selecting the thickness of the matching layer such that it is about one-quarter wavelength of the ultrasound expected to propagate through the matching layer when generated by one or more ultrasonic transducers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of placing a seed layer between a metal sensing plate and a matching layer. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of forming a matching layer from a plurality of matching sub-layers formed from a plurality of materials, each sub-layer having an ultrasonic impedance that increases from a bottom sub-layer to an upper sub-layer adjacent to the metal sensing plate. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of forming a matching layer from a plurality of matching sub-layers formed from a plurality of materials, each sub-layer having an acoustic impedance that is the approximate geometric mean of the acoustic impedances of the materials on both sides of the sub-layer.In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of forming a matching layer from a plurality of matching sub-layers formed from a plurality of materials, wherein all matching sub-layers have a total thickness of about 1 / 4 wavelength of ultrasound expected to propagate through the matching sub-layers when generated by one or more ultrasonic transducers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of forming at least one matching sub-layer from a combination of a plurality of materials in a specific ratio. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of applying a combination of a plurality of materials in a specific ratio using physical vapor deposition. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of forming a matching layer from a plurality of matching sub-layers, wherein each sub-layer has a thickness of about 1 / 4 wavelength of ultrasound expected to propagate through the sub-layer when generated by one or more ultrasonic transducers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the present method further comprises the step of forming a matching layer using one or more of plating, cladding, and physical vapor deposition (PVD).

[0081] Although the examples of the present disclosure have been sufficiently described with reference to the accompanying drawings, it should be noted that various changes and modifications will be apparent to those skilled in the art. Such changes and modifications should be understood to be included within the scope of the examples of the present disclosure as defined by the appended claims.

Claims

Claim 1 A touch sensing device comprising: a sensing plate having a first surface, a second surface and a first acoustic impedance, wherein the first surface is configured to receive one or more touches; one or more ultrasonic transducers configured to propagate ultrasound to the first surface through the sensing plate; and a non-uniform distribution of a plurality of closed pores within the sensing plate, wherein the plurality of closed pores are configured to create an acoustic impedance gradient within the sensing plate and to reduce an acoustic impedance mismatch between the one or more ultrasonic transducers and the sensing plate, wherein the second surface of the sensing plate is closest to the one or more ultrasonic transducers, and the sensing plate comprises a larger distribution of closed pores near the first surface and near the second surface compared to a distribution of closed pores near the center of the sensing plate to create an acoustic impedance gradient across the sensing plate. Claim 2 A touch sensing device according to claim 1, wherein the plurality of closed pores comprise a plurality of sealed voids having a second acoustic impedance lower than the first acoustic impedance. Claim 3 A touch sensing device according to claim 1, wherein the diameter of the plurality of closed pores is smaller than the wavelength of the ultrasound expected to propagate through the sensing plate when generated by the one or more ultrasonic transducers. Claim 4 A touch sensing device according to paragraph 3, wherein the diameter of the plurality of closed pores is 3 to 10 micrometers. Claim 5 A touch sensing device according to paragraph 2, wherein the second surface of the sensing plate is closest to the one or more ultrasonic transducers, and the sensing plate comprises a larger distribution of closed pores near the second surface compared to a distribution of closed pores near the first surface to create an acoustic impedance gradient across the sensing plate. Claim 6 A touch sensing device comprising: a sensing plate having a first surface, a second surface and a first acoustic impedance, wherein the first surface is configured to receive one or more touches; one or more ultrasonic transducers configured to propagate ultrasound to the first surface through the sensing plate; and a non-uniform distribution of a plurality of pores within the sensing plate, wherein the plurality of pores are configured to create an acoustic impedance gradient within the sensing plate and reduce an acoustic impedance mismatch between the one or more ultrasonic transducers and the sensing plate, wherein the plurality of pores include a plurality of open-ended perforations that partially extend into the sensing plate, and wherein the plurality of open-ended perforations cause a second acoustic impedance lower than the first acoustic impedance in the sensing plate at the location of the plurality of open-ended perforations. Claim 7 A touch sensing device according to claim 6, wherein the second surface of the sensing plate is closest to the one or more ultrasonic transducers, and the second surface includes the plurality of open-end type perforations. Claim 8 A touch sensing device according to claim 7, wherein the plurality of open-end type perforations on the second surface have a width and depth configured such that the second acoustic impedance of the sensing plate near the second surface is more closely matched with the third acoustic impedance of the layer beneath the sensing plate. Claim 9 A touch sensing device according to claim 7, wherein the first surface also includes the plurality of open-end type perforations. Claim 10 A touch sensing device according to claim 9, wherein the plurality of open-end type perforations on the first surface have a width and depth configured such that the first acoustic impedance of the sensing plate near the first surface is more closely matched with the acoustic impedance of the object in contact with the first surface. Claim 11 A touch sensing device according to claim 1, wherein the sensing plate is composed of a non-uniform material density gradient such that the highest material density is near the center of the sensing plate. Claim 12 A touch sensing device comprising: a sensing plate having a first surface, a second surface and a first acoustic impedance; wherein the first surface is configured to receive one or more touches, the second surface of the sensing plate is closest to one or more ultrasonic transducers, and the sensing plate comprises a larger distribution of pores near the second surface compared to a distribution of pores near the first surface to create an acoustic impedance gradient across the sensing plate; one or more ultrasonic transducers configured to propagate ultrasound to the first surface through the sensing plate; and a non-uniform distribution of a plurality of pores within the sensing plate, wherein the plurality of pores are configured to create an acoustic impedance gradient within the sensing plate and reduce an acoustic impedance mismatch between the one or more ultrasonic transducers and the sensing plate. Claim 13 A touch sensing device according to claim 12, wherein the plurality of pores comprises a plurality of sealed voids having a second acoustic impedance lower than the first acoustic impedance. Claim 14 A touch sensing device according to claim 12, wherein the diameter of the plurality of pores is smaller than the wavelength of the ultrasound expected to propagate through the sensing plate when generated by the one or more ultrasonic transducers. Claim 15 A touch sensing device according to claim 14, wherein the diameter of the plurality of pores is 3 to 10 micrometers. Claim 16 A touch sensing device according to claim 12, wherein the plurality of pores include a plurality of open-end type perforations that partially extend into the sensing plate, and the plurality of open-end type perforations cause a second acoustic impedance lower than the first acoustic impedance in the sensing plate at the location of the plurality of open-end type perforations. Claim 17 A touch sensing device according to claim 16, wherein the second surface of the sensing plate is closest to the one or more ultrasonic transducers, and the second surface comprises the plurality of open-end type perforations. Claim 18 A touch sensing device according to claim 17, wherein the plurality of open-end type perforations on the second surface have a width and depth configured such that the second acoustic impedance of the sensing plate near the second surface is more closely matched with the third acoustic impedance of the layer beneath the sensing plate. Claim 19 A touch sensing device according to claim 17, wherein the first surface also comprises the plurality of open-end type perforations. Claim 20 delete

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