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

Abstract

This disclosure relates to porous matrices for acoustic impedance matching and improved touch sensing and fingerprint imaging. The invention discloses the use of acoustic impedance matching to improve the accuracy of ultrasonic touch sensing and fingerprint imaging. Acoustic impedance mismatch between the ultrasonic transducer array and the sensing plate can be reduced to maximize energy transfer and minimize parasitic reflections. Reduction of acoustic impedance mismatch can be achieved using: (i) a composite epoxy resin having a higher acoustic impedance than epoxy resin alone; (ii) one or more matching layers having an acoustic impedance approximately equal to the geometric mean of the acoustic impedance of the sensing plate and the acoustic impedance of the transducer array; (iii) holes or perforations in the sensing plate; or (iv) a geometry formed in the sensing plate. Furthermore, an absorbent layer can be used to suppress parasitic reflections.

Description

Technical Field

[0001] This disclosure relates generally 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

[0002] Currently, many types of electronic devices are available capable of receiving touch input to initiate operations. Examples of such devices include desktop computers, laptop and tablet computers, smartphones, media players, wearable devices such as watches and health monitoring devices, smart home control and entertainment devices, headphones and earphones, 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 buttons or keys, mice, trackballs, joysticks, touch panels, touchscreens, etc. Specifically, wearable devices provide users with readily accessible mechanisms to perform various functions and, in some cases, allow quick access to sensitive user information, either alone 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 control.

[0003] To provide secure authentication, fingerprint detection can be implemented on surfaces such as watch buttons. Because the surface can be made of metal and / or have a certain thickness (e.g., greater than 300 micrometers), which makes other sensing technologies impractical, ultrasonic touch sensing can be used to perform fingerprint imaging. In some examples, piezoelectric micromachining ultrasonic transducer (PMUT) arrays can be used to propagate ultrasonic waves through touch surfaces of various materials and thicknesses, where the presence or absence of fingerprint ridges can alter the amount of energy reflected back to the PMUT array. These variations in reflected energy can be used to determine an image of the user's fingerprint. However, when the PMUT array and associated electronics are attached to the back of the touch surface, the resulting stacking of materials (e.g., the PMUT array, adhesive material, sensing plate, etc.) can create an acoustic impedance mismatch, which may reduce the transmission of ultrasonic energy to / from the PMUT array and the touch surface, and may also produce parasitic reflections, both of which can adversely affect the PMUT array's ability to accurately perform fingerprint imaging. Summary of the Invention

[0004] The examples of this disclosure generally relate to providing ultrasonic touch sensing and fingerprint imaging capabilities, and more specifically to acoustic impedance matching for improved ultrasonic touch sensing and fingerprint imaging. In some examples of this disclosure, an epoxy resin with relatively low acoustic impedance may be combined with a filler material to increase the total acoustic impedance of the resulting composite epoxy resin. The composite epoxy resin may be used to adhere an ultrasonic transducer array to a top layer (i.e., a sensing plate or capping layer) configured to receive touch (e.g., from a finger). The composite epoxy resin (with the filler material) may have an acoustic impedance higher than that of the epoxy resin alone, and may more tightly match the acoustic impedance of the transducer array. The acoustic impedance of the composite epoxy resin reduces impedance mismatch between the transducer array and the sensing plate, which can result in reduced ultrasonic wave attenuation, less reflection, and more accurate touch sensing and fingerprint imaging.

[0005] In some examples of this disclosure, one or more matching layers may be formed on or attached to a sensing plate, the total acoustic impedance of which is approximately equal to the geometric mean of the acoustic impedance of the sensing plate and the acoustic impedance of the transducer array. In some examples, each of the one or more matching layers may be formed of one or more metals. The one or more matching layers (and the attached sensing plate) can then be adhered to the transducer array using the epoxy resin or composite epoxy resin described above. Each of the one or more matching layers may have a thickness approximately equal to a quarter wavelength of the ultrasonic wave propagating through the matching layer. The one or more matching layers can 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 can result in reduced ultrasonic wave attenuation, less reflection, and more accurate touch sensing and fingerprint imaging.

[0006] In some examples of this disclosure, the sensing plate may be fabricated with non-uniformly distributed holes (e.g., holes) or perforations along one or more surfaces of the sensing plate to generate one or more acoustic impedance gradients within the sensing plate. One or more acoustic impedance gradients can result in reduced ultrasonic wave attenuation, less reflection, a higher signal-to-noise ratio (SNR) touch signal (resulting in higher contrast touch and fingerprint images), and ultimately more accurate touch sensing and fingerprint imaging.

[0007] In some examples of this disclosure, geometries (e.g., cones) may be formed on the back side of the sensing plate. The geometries may have a peak-to-peak spacing much smaller than the wavelength of the ultrasound waves propagating through the sensing plate, and may have a peak-to-valley height at least three times the wavelength of the ultrasound waves. A lower acoustic impedance material may fill the gaps between the geometries to create an acoustic impedance gradient at the locations of the geometries and the filler material. This acoustic impedance gradient can result in reduced ultrasound attenuation, less reflection, and more accurate touch sensing and fingerprint imaging.

[0008] In some examples of this disclosure, because the transducer array can also generate unwanted ultrasound waves in a direction away from the sensing plate and through any underlying electronics, an absorbent layer can be formed on the back side of the ultrasonic sensing electronics to absorb the unwanted ultrasound waves and reduce the reflection of those waves. The absorbent layer can be a composite epoxy resin formed from tungsten filler and epoxy resin. This absorbent layer can have a higher acoustic impedance than conventional epoxy resins, resulting in high ultrasonic absorption. The absorbent layer reduces the energy reflected from unwanted ultrasound waves, which can lead to attenuation of unwanted / parasitic ultrasound waves, less reflection, and more accurate touch sensing and fingerprint imaging. Attached Figure Description

[0009] Figures 1A to 1G An example of an ultrasonic touch sensing and fingerprint imaging system with acoustic impedance matching is shown according to the present disclosure.

[0010] Figure 2 A block diagram of an electronic device including ultrasonic touch sensing with acoustic impedance matching and fingerprint imaging, according to an example of this disclosure, is shown.

[0011] Figure 3 An example of the process for ultrasonic touch sensing of an object (e.g., a fingerprint ridge) in contact with a touch-sensitive surface is shown according to the present disclosure.

[0012] Figure 4A A touch sensing stack using a PMUT array is shown as an example according to this disclosure.

[0013] Figure 4B The illustration shows parasitic reflections from acoustic impedance mismatch and finger signatures reflected from the touch surface of the sensing plate when using an epoxy resin with low acoustic impedance, according to an example of this disclosure.

[0014] Figure 5A An example of a composite epoxy resin according to the present disclosure is shown, which may include epoxy resin and filler particles suspended in epoxy resin.

[0015] Figure 5B The illustration shows parasitic reflections from acoustic impedance mismatch and finger signatures reflected from the touch surface of the sensing plate when using a composite epoxy resin, according to an example of this disclosure.

[0016] Figure 6A An example of a mating layer fixed to a sensing plate when separated from the epoxy resin and PMUT array is shown according to this disclosure.

[0017] Figure 6B A matching layer sandwiched between the sensing plate and the PMUT array according to an example of this disclosure is shown (the epoxy layer is omitted for clarity).

[0018] Figure 6C The example shown illustrates the transmission of ultrasonic signal energy through a single matched layer of a specific material within a range of frequencies and layer thicknesses, according to the present disclosure.

[0019] Figure 7A Several materials and techniques, according to examples of this disclosure, can be used to form a matching layer on the back side of a sensing plate.

[0020] Figure 7B An example of using plating to form a matching layer according to this disclosure is shown.

[0021] Figure 7C A plating process flow for forming and plating a stainless steel (SUS) sensing plate with a geometric structure, according to an example of this disclosure, is shown.

[0022] Figure 7D An example of forming a matching layer using a package layer according to this disclosure is shown.

[0023] Figure 7E The example shown is the formation of a matching layer using physical vapor deposition (PVD) according to this disclosure.

[0024] Figure 8A An example of a sensing plate with an internal acoustic impedance gradient generated by an aperture is shown according to the present disclosure.

[0025] Figure 8B A sensing plate with non-uniformly distributed holes is shown as an example according to this disclosure.

[0026] Figure 8C An example of a perforated sensing plate according to this disclosure is shown.

[0027] Figure 8D A sensing plate with density variation according to an example of this disclosure is shown.

[0028] Figure 8E A sensing plate with perforations and density variations, according to an example of this disclosure, is shown.

[0029] Figure 9A A sensing plate having a geometry for generating an acoustic impedance gradient is shown as an example according to this disclosure.

[0030] Figure 9B This is a perspective view of a sensing plate with a geometric structure according to an example of this disclosure.

[0031] Figure 9C An example of a stainless steel (SUS) sensing plate according to this disclosure is shown, having a geometry filled with a tin (Sn) matching layer, such as... Figure 7CThe process is completed as shown, and then epoxy resin is used to adhere it to the PMUT array.

[0032] Figure 10A The present disclosure illustrates an example of a touch sensing and fingerprint imaging stack using a PMUT array backed with an absorbent.

[0033] Figure 10B The illustration shows, according to an example of this disclosure, parasitic reflections from acoustic impedance mismatch (e.g., combined effects of reflections) and finger signatures reflected from the touch surface of the sensing plate when using a composite epoxy resin and an absorbent.

[0034] Figure 11 A flowchart illustrating an example of touch sensing and fingerprint imaging for acoustic impedance matching and improvement according to this disclosure is shown. Detailed Implementation

[0035] Cross-reference to related applications

[0036] This application claims the benefit of U.S. Provisional Application No. 63 / 188114, filed May 13, 2021, and U.S. Patent Application No. 17 / 660308, filed April 22, 2022, the contents of which are incorporated herein by reference in their entirety for all purposes.

[0037] In the following description of various examples, reference will be made to the accompanying drawings, which form part of the following description, and specific examples that can be implemented are illustrated in the drawings by way of illustration. It should be understood that other examples may be used and structural changes may be made without departing from the scope of the various examples.

[0038] When ultrasonic waves propagate to a touch surface (such as a sensor plate), ultrasonic touch sensing and fingerprint imaging systems can be adversely affected by acoustic impedance mismatch. In addition to the reduction in the energy of the ultrasonic waves that eventually reach the sensor plate and are reflected back at different amplitudes depending on whether the object (e.g., a fingerprint ridge) is touching the sensor plate, these mismatches can lead to undesirable reflections of the ultrasonic waves.

[0039] Therefore, examples of this disclosure generally relate to providing ultrasonic touch sensing and fingerprint imaging capabilities, and specifically to acoustic impedance matching for improved ultrasonic touch sensing and fingerprint imaging. In some examples of this disclosure, an epoxy resin with relatively low acoustic impedance may be combined with a filler material to increase the total acoustic impedance of the resulting composite epoxy resin. The composite epoxy resin may be used to adhere an ultrasonic transducer array to a top layer (i.e., a sensing plate or capping layer) configured to receive touch (e.g., from a finger). The composite epoxy resin (with the filler material) may have an acoustic impedance higher than that of the epoxy resin alone, and may more closely match the acoustic impedance of the transducer array. The acoustic impedance of the composite epoxy resin reduces impedance mismatch between the transducer array and the sensing plate, which can result in reduced ultrasonic wave attenuation, less reflection, and more accurate touch sensing and fingerprint imaging.

[0040] In some examples of this disclosure, one or more matching layers may be formed on or attached to a sensing plate, the total acoustic impedance of which is approximately equal to the geometric mean of the acoustic impedance of the sensing plate and the acoustic impedance of the transducer array. In some examples, each of the one or more matching layers may be formed of one or more metals. The one or more matching layers (and the attached sensing plate) can then be adhered to the transducer array using the epoxy resin or composite epoxy resin described above. Each of the one or more matching layers may have a thickness approximately equal to a quarter wavelength of the ultrasonic wave propagating through the matching layer. The one or more matching layers can 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 can result in reduced ultrasonic wave attenuation, less reflection, and more accurate touch sensing and fingerprint imaging.

[0041] In some examples of this disclosure, the sensing plate may be fabricated with non-uniformly distributed holes (e.g., holes) or perforations along one or more surfaces of the sensing plate to generate one or more acoustic impedance gradients within the sensing plate. One or more acoustic impedance gradients can result in reduced ultrasonic wave attenuation, less reflection, a higher signal-to-noise ratio (SNR) touch signal (resulting in higher contrast touch and fingerprint images), and ultimately more accurate touch sensing and fingerprint imaging.

[0042] In some examples of this disclosure, geometries (e.g., cones) may be formed on the back side of the sensing plate. The geometries may have a peak-to-peak spacing much smaller than the wavelength of the ultrasound waves propagating through the sensing plate, and may have a peak-to-valley height at least three times the wavelength of the ultrasound waves. A lower acoustic impedance material may fill the gaps between the geometries to create an acoustic impedance gradient at the locations of the geometries and the filler material. This acoustic impedance gradient can result in reduced ultrasound attenuation, less reflection, and more accurate touch sensing and fingerprint imaging.

[0043] In some examples of this disclosure, because the transducer array can also generate unwanted ultrasound waves in a direction away from the sensing plate and through any underlying electronics, an absorbent layer can be formed on the back side of the ultrasonic sensing electronics to absorb the unwanted ultrasound waves and reduce the reflection of those waves. The absorbent layer can be a composite epoxy resin formed from tungsten filler and epoxy resin. This absorbent layer can have a higher acoustic impedance than conventional epoxy resins, resulting in high ultrasonic absorption. The absorbent layer reduces the energy reflected from unwanted ultrasound waves, which can lead to attenuation of unwanted / parasitic ultrasound waves, less reflection, and more accurate touch sensing and fingerprint imaging.

[0044] Figures 1A to 1G An example of an ultrasonic touch sensing and fingerprint imaging system with acoustic impedance matching is shown according to the present disclosure. Figure 1A An exemplary mobile phone 136, which may employ ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching, is shown as an example according to this disclosure. Figure 1B An exemplary digital media player 140, which may employ ultrasonic touch sensing with acoustic impedance matching and fingerprint imaging, is shown as an example according to this disclosure. Figure 1C An exemplary personal computer 144, which may employ ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching, is shown as an example according to the present disclosure. Figure 1D An exemplary tablet computing device 148, which may employ ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching, is shown according to an example of this disclosure. Figure 1E An exemplary wearable device 150 (e.g., a watch) with ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching is shown according to an example of this disclosure. Figure 1F Another exemplary wearable device (over-ear headphones 160) with acoustic impedance matching and ultrasonic touch sensing and fingerprint imaging is shown as an example according to this disclosure. Figure 1G Another exemplary wearable device (in-ear headphones or earbuds 170) with ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching is shown according to an example of this disclosure. It should be understood that... Figures 1A to 1G The exemplary device shown is provided as an example, and other types of devices may employ ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching.

[0045] Acoustic impedance matching ultrasonic touch sensing and fingerprint imaging can be incorporated into the aforementioned systems to improve the system's touch sensing and fingerprint imaging capabilities. In some examples, touchscreens (e.g., capacitive, resistive, etc.) can be enhanced 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, additional non-touch-sensitive displays can be enhanced 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 structure required for capacitive touchscreens. In some examples, acoustic impedance matching ultrasonic touch sensing can be used to provide touch sensing capabilities (e.g., fingerprint imaging) for non-display surfaces. For example, acoustic impedance matching ultrasonic touch sensing can be used to provide fingerprint imaging capabilities on watch buttons, earpieces, touchpads (e.g., touchpad 146 of personal computer 144), scroll wheels, part or all of the casing, or any other surface of the device (e.g., on the front, back, or sides).

[0046] like Figures 1E to 1G As illustrated in the examples, some examples of this disclosure specifically relate to wearable devices that provide users with readily accessible input mechanisms for initiating or performing various functions, as well as secure authentication and access to the wearable device and / or paired devices via fingerprint imaging. In some examples, ultrasonic fingerprint imaging capabilities can be integrated within the existing structure of the wearable device. In these examples, a user can touch a touch-sensitive area on the structure of the wearable device with one or more fingers, and an acoustically impedance-matched ultrasonic transducer can detect these touches and capture fingerprint images to initiate or perform functions such as user authentication. For example, acoustically impedance-matched ultrasonic touch sensing and fingerprint imaging can be integrated into the buttons of a watch 150, or into an accessible area of ​​an over-ear headphone 160 or an in-ear headphone 170.

[0047] Figure 2A block diagram of an electronic device including ultrasonic touch sensing and fingerprint imaging with acoustic impedance matching, according to examples of this disclosure, is shown. In some examples, the housing 202 of device 200 (e.g., corresponding to devices 136, 140, 144, 148, 150, 160, and 170 described above) may be coupled (e.g., mechanically) to one or more ultrasonic transducers 204. In some examples, the transducers 204 may be an array of piezoelectric transducers that can vibrate upon application of an electrical signal when acting as a transmitter and generate an electrical signal based on detected vibrations when acting as a receiver. In some examples, the transducers 204 may be formed of 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., a composite epoxy resin), deposited on one or more surfaces by processes such as deposition, photolithography, etc., or integrally formed within the housing. When electrical energy is applied to transducers 204 and they vibrate, one or more surfaces in contact with the transducers can also vibrate, and the vibration of molecules in the surface material can propagate as ultrasonic waves through the one or more surfaces / materials. In some examples, the vibration of transducers 204 can be used to generate ultrasonic waves at a selected frequency in the medium on the surface of an electronic device.

[0048] In some examples, transducer 204 may be partially or completely disposed on (or coupled to) a portion of display 208, which in some examples may be integrated with additional (non-ultrasonic) touch circuitry 212 to form a touchscreen; however, it should be understood that some exemplary devices do not include display 208 or additional touch circuitry 212 (the optional nature of which is indicated by dashed lines). Device 200 may also include ultrasonic touch sensing circuitry 206, which performs touch sensing and fingerprint imaging, and may include circuitry (transmitting circuitry) for driving electrical signals to stimulate vibrations of transducer 204, and circuitry (e.g., receiving circuitry) for sensing electrical signals output by transducer 204 when the transducer is stimulated by received ultrasonic energy. In some examples, the timing operation of ultrasonic touch sensing circuitry 206 may optionally be provided by a separate ultrasonic touch sensing controller 210, which controls the timing of operation of ultrasonic touch sensing circuitry 206, including touch and fingerprint sensing 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, the controller functionality 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 touching the device (e.g., the location of a fingerprint ridge). In some examples, the processing for determining the location of the object touching may be performed by the ultrasonic touch sensing circuit 206, the ultrasonic touch sensing controller 210, or a separate subprocessor (not shown) of the device 200.

[0049] The host processor 214 can receive ultrasonic outputs and optionally other touch sensor outputs (e.g., capacitive) and non-touch sensor outputs, and initiate or perform actions based on these sensor outputs. The host processor 214 can also be connected to a program storage device 216 and optionally to a display 208. The host processor 214 can, for example, communicate with the display 208 to generate images on the display, such as images of a user interface (UI), and can use ultrasonic touch sensing circuitry 206 (and in some examples, its corresponding controller), and in some examples, touch sensing circuitry 212 to detect touches on or near the display 208, such as touch inputs and / or force inputs at the displayed UI. Touch input can be used by a computer program stored in program storage device 216 to perform actions, including but not limited to: security authentication and access, moving objects such as cursors or pointers, scrolling or panning, adjusting control settings, opening files or documents, viewing menus, making selections, executing commands, operating peripherals connected to the host device, answering telephone calls, making telephone calls, terminating telephone calls, changing volume or audio settings, storing information related to telephone communication (such as addresses, frequently dialed numbers, received calls, missed calls), logging onto a computer or computer network, allowing authorized individuals to access restricted areas of a computer or computer network, loading user profiles associated with the user's preferred computer desktop layout, allowing access to web page content, launching specific programs, encrypting or decrypting messages, etc. The host processor 214 may also perform additional functions that may not be related to touch processing.

[0050] It should be noted that one or more of the functions described herein may be executed by firmware stored in memory and performed by ultrasonic sensing touch sensing circuitry 206 (or its corresponding controller) and, in some examples, by touch circuitry 212, or stored in program storage device 216 and executed by host processor 214. This firmware may also be stored and / or delivered in any non-transitory computer-readable storage medium for use or in combination with an instruction execution system, apparatus, or device, such as a computer-based system, a processor-integrated system, or other system that can retrieve and execute instructions from and with an instruction execution system, apparatus, or device. In the context of this document, "non-transitory computer-readable storage medium" can be any medium (excluding signals) that can contain or store programs for use or in combination with an instruction execution system, apparatus, and device. Non-transitory computer-readable media storage may include, but is not limited to, electronic, magnetic, optical, electromagnetic, infrared or semiconductor systems, devices or equipment, portable computer disks (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 memory cards, security digital cards), USB storage devices, memory sticks, etc.

[0051] This firmware can also be propagated in any transmission medium for use or in combination with an instruction execution system, apparatus, or device, such as a computer-based system, a processor-based system, or other system capable of retrieving and executing instructions from and with an instruction execution system, apparatus, or device. In the context of this document, "transmission medium" can be any medium that can transmit, propagate, or transfer a program for use or in combination with an instruction execution system, apparatus, or device. Transmission readable media can include, but are not limited to, electronic, magnetic, optical, electromagnetic, or infrared wired or wireless transmission media.

[0052] It should be understood that equipment 200 is not limited to Figure 2 The components and configurations of device 200 are as follows, but other components or additional components may be included in multiple configurations according to various examples. Furthermore, the components of device 200 may be included within a single device or distributed among multiple devices. Moreover, it should be understood that the connections between components are exemplary, and depending on the implementation, different unidirectional or bidirectional connections may be included between components, regardless of... Figure 2 How do the arrows in the configuration look?

[0053] Figure 3An exemplary process 300 for ultrasonic touch sensing of an object (e.g., a fingerprint ridge) in contact with a touch-sensitive surface, according to an example of this disclosure, is shown. At 302, ultrasonic energy may be transmitted through the thickness of a material in the form of ultrasonic waves (e.g., by an array of transducers). In some examples, the wave may propagate as a volume compression wave or a volume shear wave. Other propagation modes for the transmitted ultrasonic energy may also exist based on the properties, geometry, and manner of energy transmission from the transducer to the device surface. In some examples, wave propagation interruptions may occur at material layer boundaries and at the surface material (e.g., when ultrasonic waves propagate to a touch surface opposite the transducer). The transmitted energy may propagate through the thickness until wave propagation is interrupted at a material layer boundary or when it reaches the surface, which may result in a portion of the energy being reflected. When the transmitted energy reaches one of the aforementioned wave propagation interruptions, some of the energy may be reflected, and a portion of the reflected energy may be directed to one or more transducers. The object (such as a finger or fingerprint ridge in contact with the surface) may have an influence on the amount of energy reflected at the surface propagation interruption.

[0054] At 304, returned ultrasonic energy can be received and converted into an electrical signal by one or more transducers. At 306, the ultrasonic sensing system can determine whether one or more objects are in contact with the surface of the device, and can further detect the position 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 interruptions (e.g., edges) can be compared with a measurement of reflected energy corresponding to one or more wave propagation interruptions. When an object (e.g., a fingerprint ridge) is not in contact with the surface, the baseline reflected energy can be determined during measurement. The deviation of the reflected energy from the baseline can be correlated with the presence of an object touching the surface.

[0055] Although, as described above, process 300 generally refers to the reflected wave received by the same transducer of the transmitted wave, in some examples, the transmitter and receiver functions can be separated, such that the transmission of ultrasonic energy at 302 and the reception of ultrasonic energy at 304 can occur at different co-located transducers (e.g., one transducer in a transmission configuration and one transducer in a reception configuration). In some examples, ultrasonic energy can be transmitted along and / or through a 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 can be used to detect the presence of one or more objects on the surface and / or identify the location of one or more objects on the surface. In some examples, the transmitted ultrasonic energy can be received at a transmitting transducer and also at one or more other non-transmitting transducers located at different locations (e.g., at 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 location of one or more objects.

[0056] Figure 4A A touch sensing stack 400 using an array of PMUT 402 is shown as an example according to this disclosure. Figure 4A In the example, the PMUT array 402 can be used to deliver ultrasonic waves 416 to a touch surface via a sensing plate 404, where the presence or absence of fingerprint ridges 406 can alter the amount of energy reflected back to the PMUT array as a finger signature 418. These variations in reflected energy can be used to determine an image of a user's fingerprint. However, when the PMUT array 402 and associated electronics 408 are attached to the back of the sensing plate 404 using an adhesive such as epoxy resin 410, the resulting stack of materials (e.g., the PMUT array, epoxy resin, and sensing plate) can create an acoustic impedance mismatch at the material layer boundaries 412. In particular, the epoxy resin 410 may have a relatively low acoustic impedance (e.g., about 3 MNayl), while the sensing plate 404 may have a much higher acoustic impedance of about 46 MNayl. These acoustic impedance mismatches can reduce the transmission of ultrasonic energy to / from the PMUT array 402 and can produce parasitic reflections 414, both of which can adversely affect the PMUT array's ability to perform touch sensing or fingerprint imaging.

[0057] The sensing plate 404 can be made of a variety of materials, such as aluminum (Al), stainless steel (SUS), ceramics such as zirconium oxide (ZrO2), titanium (Ti), etc. Although Figure 4A The PMUT array 402 is shown, but in other examples, ultrasonic transducers of different types, numbers and arrangements may also be used.

[0058] Figure 4BThe illustration shows, according to an example of this disclosure, parasitic reflections 414 from acoustic impedance mismatch and finger signatures 418 reflected back from the touch surface of the sensing plate 404 when using an epoxy resin 410 with low acoustic impedance. Figure 4B In the example, an acoustic impedance mismatch can form between the epoxy resin 410 and the sensing plate 404. Due to this mismatch, the acoustic signal generated by the PMUT array 402 can be reflected back at the boundary 412 between the epoxy resin 410 and the sensing plate 404 (regardless of whether a finger is present on the touch surface of the sensing plate) and received at the PMUT array as a parasitic reflection 414. It should be noted that because the parasitic reflection 414 travels a shorter distance than the finger signature 418, the parasitic reflection can return to the PMUT array 402 before the finger signature. Therefore, for Figure 4B In the first few cycles of the parasitic reflection 414 shown, there is no finger signature 418. Figure 4B As illustrated in the example, since significant ultrasonic energy can be reflected as parasitic reflection 414, the amplitude (energy) of the parasitic reflection can be large, while the amplitude of the fingerprint signature 418 can be adversely reduced. Typically, ultrasonic fingerprint imaging systems are often limited by several parameters, which can result in an undesirably low signal-to-noise ratio (SNR) when using a high acoustic impedance sensing plate. The examples in this disclosure reduce acoustic impedance mismatch to improve SNR.

[0059] Examples of this disclosure relate to improving the accuracy of ultrasonic touch sensing and fingerprint imaging by employing acoustic impedance matching to reduce parasitic reflections and increasing finger signature energy in devices such as desktop computing devices, laptop and tablet computing devices, smartphones, media players, wearable devices such as watches and health monitoring devices, smart home control and entertainment devices, headphones and earphones, and devices for computer-generated environments such as augmented reality, mixed reality, or virtual reality environments. Some examples of this disclosure particularly relate to wearable devices that provide users with an immediately accessible input mechanism for initiating or performing various functions, as well as secure authentication and access to the wearable device and / or paired devices via fingerprint imaging.

[0060] Figure 5AA composite epoxy resin 520 according to an example of this disclosure is shown, which may include epoxy resin 522 and filler particles 524 suspended in the epoxy resin. In some examples, the materials and dimensions of the components of the composite epoxy resin 520 may be selected to reduce acoustic impedance mismatch between the PMUT array and the sensing plate. Because the acoustic impedance of the sensing plate can be very high (e.g., 46 MNayl for SUS) compared to the much lower acoustic impedance (e.g., 3 MNayl) of the epoxy resin 522, in some examples, filler particles 524 may be added to the epoxy resin to produce a composite epoxy resin 520 with an increased acoustic impedance compared to the epoxy resin. In a preferred example, the acoustic impedance of the composite epoxy resin 520 may be increased to match the acoustic impedance of the PMUT array below (e.g., 11 MNayl) or the acoustic impedance of the electronics (e.g., 22 MNayl). However, in other examples that may be easier to implement, the composite epoxy resin 520 may have an increased acoustic impedance of 7 MNayl (an increase compared to the epoxy resin itself). The increased acoustic impedance reduces the acoustic impedance mismatch between the PMUT array and the sensing plate, which increases the transmission of ultrasonic energy to / from the PMUT array and reduces parasitic reflections. Both of these improve the PMUT array's ability to perform fingerprint imaging.

[0061] In some examples, the composite epoxy resin 520 can be formulated to be temperature stable (i.e., with low thermal drift), so that its properties do not change with temperature. Therefore, epoxy resin 522 can be selected to have a "glass transition" (the temperature at which the epoxy resin begins to transition from a hard or brittle state to a softer, more viscous state) much higher than the expected operating temperature of the equipment (e.g., up to 60 degrees Celsius). In some examples, epoxy resin 522 can be selected to have a glass transition of 80-100 degrees Celsius, such that the properties of the epoxy resin are stable within the expected operating temperature range. Above this temperature range, the epoxy resin can become too soft, and the modulus can decrease, which can lead to an undesirable shift in acoustic impedance.

[0062] In some examples, the diameter d of the filler particles 524 can be selected to be much smaller than the wavelength of the ultrasound waves propagating through the composite epoxy resin 520, thereby minimizing the reflection / scattering of ultrasonic energy. If the diameter d is too large, the filler particles 524 may reflect / scatter the waves. In some examples, the average wavelength of the ultrasound waves propagating through the composite epoxy resin 520 can be approximately 100-150 micrometers, therefore the particle size can be selected between 5-15 micrometers, and in some cases less than 5 micrometers.

[0063] The volume fraction of particles 524 in epoxy resin 522 can be selected to achieve the desired acoustic impedance. Although the acoustic impedance of the composite epoxy resin 520 should ideally match the acoustic impedance of the PMUT array (e.g., with an acoustic impedance of about 11 MRayl), this may not be practically achievable. Therefore, in some examples, the volume fraction of particles 524 can be selected to be about 60%–70%. A higher volume fraction can result in a higher acoustic impedance, but at a volume fraction greater than about 70%, the integrity of the composite epoxy resin 520 may be compromised. For example, adhesion may be reduced, and viscosity may become so high that the composite epoxy resin may be difficult to dispense and apply at the desired thickness (e.g., 15–45 micrometers).

[0064] In view of the above, in some examples of this disclosure, a 0-3 composite epoxy resin can be used, comprising alumina filler (40% volume fraction, particle size <5µm) and epoxy resin (e.g., having an acoustic impedance of about 3MRayl). (301 epoxy resin). Alumina filler can increase the acoustic impedance of the composite epoxy resin to about 7 MRayl, and in some cases, between 8 MRayl and 11 MRayl. Although the sensing plate can have much higher acoustic impedance (e.g., about 46 MRayl for stainless steel (SUS), 30 MRayl for titanium, and 27 MRayl for aluminum), the increased acoustic impedance of the composite epoxy resin (compared to the lower acoustic impedance of 3 MRayl for epoxy resin) reduces the impedance mismatch between the PMUT array and the sensing plate, resulting in reduced ultrasonic attenuation, increased finger signature energy, and improved ultrasonic touch sensing and fingerprint imaging.

[0065] Figure 5B The illustration shows, according to an example of this disclosure, a parasitic reflection 514 from acoustic impedance mismatch and a finger signature 518 reflected back from the touch surface of the sensing plate 504 when using a composite epoxy resin 520. Due to the reduced acoustic impedance mismatch provided by the composite epoxy resin 520, less energy from the ultrasonic signal generated by the PMUT array can be reflected (regardless of whether a finger is present at the touch surface of the sensing plate) and received back at the PMUT array as a parasitic reflection. It should be noted that because the parasitic reflection 514 can travel a shorter distance than the finger signature 518, the parasitic reflection can return to the PMUT array before the finger signature. Therefore, for Figure 5B In the first few cycles of the parasitic reflection 514 shown, there is no finger signature 518. (As shown...) Figure 5B As the example shows, 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...). Figure 4B compared to).

[0066] As described above, the materials and dimensions of the components of the composite epoxy resin 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 this disclosure, a matching layer may be formed on the back side of the sensing plate to create an acoustic impedance gradient between the lower acoustic impedance PMUT array and the higher acoustic impedance sensing plate.

[0067] Figure 6A A matching layer 626, according to an example of this disclosure, is fixed to a sensing plate 604 when separated from epoxy resin 610 and PMUT array 602. Matching layer 626 can create a more gradual acoustic impedance transition (i.e., bridging or gradient) between PMUT array 602 and sensing plate 604. This acoustic impedance gradient can reduce the amount of ultrasonic wave reflection generated at the interface between PMUT array 602 and sensing plate 604, and allow more energy to reach the surface of the sensing plate for touch detection and fingerprint imaging. By selecting the thickness and composition of matching layer 626 according to the characteristics of sensing plate 604, different matching layers can be developed for use with different sensing plate materials.

[0068] Figure 6B A matching layer 626 (epoxy layer omitted for clarity) sandwiched between a sensing plate 604 and a PMUT array 602 according to an example of this disclosure is shown. 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. Choosing the matching layer 626 to have an acoustic impedance approximately equal to the geometric mean of the two adjacent materials improves the energy transfer of ultrasound as it propagates through the matching layer. In some examples, the thickness d of the matching layer 626 may be one-quarter of the wavelength of the ultrasound propagating through the matching layer. The maximum energy transfer of the signal through the matching layer 626 may occur at this one-quarter wavelength thickness.

[0069] The table below provides exemplary matching layer acoustic impedances for four different sensing plate materials and PMUT arrays, according to some examples of this disclosure.

[0070]

[0071]

[0072] Aluminum, tin, and titanium have acoustic impedances ranging from 13.7 MRayl to 23.2 MRayl, making them suitable for use as matching layers. Using a stainless steel sensing plate as an example, a 30-micron-thick aluminum matching layer achieves approximately 17 MRayl of acoustic impedance and 94% of maximum transmission; a 30-micron-thick titanium matching layer achieves approximately 27 MRayl of acoustic impedance and 95% of maximum transmission; and a 17-micron-thick tin matching layer achieves approximately 24 MRayl of acoustic impedance and 99% of maximum transmission. For broadband pulses, transmission decreases as the frequency deviates from the optimal frequency.

[0073] In other examples, the matching layer 626 may be formed of multiple matching sublayers, each sublayer in the stack having an increasing ultrasonic impedance from the bottom sublayer to the top sublayer adjacent to the sensing plate. In some examples, all matching sublayers may have a total thickness approximately equal to a quarter wavelength of the frequency propagating through the matching sublayer. In other examples, each matching sublayer may have a thickness approximately equal to a quarter wavelength of the frequency propagating through that matching sublayer. Reference Figure 6B The matching layer 626 can be replaced by a plurality of matching sublayers formed on the back side of the sensing plate 604 and subsequently adhered to the lower PMUT layer 602. In one example, an aluminum matching sublayer can be formed on the sensing plate, and a magnesium matching sublayer (with lower acoustic impedance) can then be formed on the aluminum matching sublayer. In some examples, each matching sublayer can be tuned to have an acoustic impedance that approximates the geometric mean of the acoustic impedance of the material on either side of that matching sublayer.

[0074] Figure 6C This illustrates, according to an example of the present disclosure, the transmission of ultrasonic signal energy through a single matched layer of a specific material within a range of frequencies and layer thicknesses. Figure 6C In the example, at the center frequency f of the ultrasound signal c Below this, energy transfer through a matching layer with a thickness d equal to a quarter wavelength of the ultrasonic signal is maximized, as indicated by point 644 in the high-energy transmission band 630. On the other hand, the frequency component f of the ultrasonic signal far from the center frequency... h It can have much less energy transfer, as indicated by point 646 in the lower energy transfer band 634.

[0075] However, if multiple matching sublayers are used as described above, acoustic energy transfer can be improved over a wider frequency band. For example, the transmission curves of multiple matching layers can have a higher efficiency than those of other matching sublayers. Figure 6C The example shows a wider high-energy transmission band 630. This wider high-energy transmission band 630 allows more energy from the ultrasound to propagate through the touch surface of the sensing plate with less reflection.

[0076] Figure 7ASeveral materials and techniques, according to examples of this disclosure, that can be used to form a matching layer 726 on the back side of a sensing plate 704. Figure 7A In the left-hand stack, a thin nickel (Ni) layer less than 5 micrometers thick is deposited using physical vapor deposition (PVD) onto the back side of the sensing plate 704, formed of SUS or zirconium oxide (ZrO2), both of which have an acoustic impedance of approximately 50 M Nayl, followed by a tin (Sn) layer approximately 17 micrometers thick. When ZrO2 is used as the sensing plate 704, the Ni layer is required as a seed layer because ZrO2 is a non-conductive ceramic. After forming the Ni layer, the Sn layer can be deposited on top of the Ni layer. Because the nickel is so thin, the tin layer primarily provides matching layer characteristics. Figure 7A In the central stack, an approximately 30-micrometer-thick layer of aluminum (Al) can be clad (diffusion bonded) with a sensing plate 704 formed of SUS or titanium (Ti) to form a matching layer 726. Figure 7A In the right-hand stack, PVD can be used to deposit Al and Ti materials onto the back side of the sensing plate 704 formed of 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, all three methods can lead to different types of non-uniformity that may affect the acoustic impedance.

[0077] Figure 7B An example of forming a matching layer using plating according to this disclosure is shown. Figure 7B In the example, a nickel thin film (e.g., less than 2 micrometers) can first be deposited onto a SUS (450 micrometers) or ZrO2 (800 micrometers) sensing plate 704 using PVD (note that...). Figure 7B (The sensing plate at the bottom is shown). The Sn layer protects the delicate structure and improves reliability. Because the nickel layer is thin, it is negligible in terms of the desired quarter-wavelength thickness. The Sn layer can have a desired quarter-wavelength thickness of about 17 micrometers and can be the major contributor to acoustic impedance matching. Note that the matching layer 726 does not need to have a precise quarter-wavelength thickness because the acoustic impedance of the PMUT array, matching layer, sensing plate, etc., is so interdependent that the actual preferred matching layer thickness can be determined by simulating the entire stack. Figure 7B The two layers on the left show Ni and Sn plated on a flat SUS and ZrO2 sensing plate 704. Figure 7B The two layers on the right show Ni and Sn plated on SUS and ZrO2 sensing plates 704, which are formed with geometry (e.g., cones) for improved acoustic impedance matching, as will be described below.

[0078] Figure 7C A plating process flow for forming and plating an SUS sensor plate with a geometric structure, according to an example of this disclosure, is shown. Figure 7CIn the example (from left to right), SUS raw material can be laser-textured to form sensor plate 704 using geometry 734 (note) Figure 7C The sensing plate at the bottom is shown. The sensing plate 704 can then be deposited using PVD with a thin Ni layer (<5 micrometers), followed by a thicker Sn layer (which can primarily act as a matching layer). Nickel plating can be used as a seed layer, allowing the Sn layer to have material plated onto it in the plating bath. Because the Sn is plated on an irregular surface, the plated Sn can also have an irregular surface. Therefore, Figure 7C The final step in the process can flatten the Sn layer to have a desired quarter-wavelength thickness d (e.g., 30 micrometers) and provide a smooth surface for epoxy bonding.

[0079] Figure 7D The example shown illustrates the use of a packet layer to form a matching layer according to this disclosure. Figure 7D In the three layers on the left, each of Al and / or magnesium (Mg) with a thickness of approximately 30 micrometers can be clad with an SUS sensing plate 704 with a thickness of approximately 450 micrometers. Cladding involves fusing the different metals together using high-pressure rollers and high heat. At the material boundaries, some atoms from one material diffuse into the other, causing the two materials to adhere together. When added, Al or Mg material can be used as a matching layer 726. Figure 7D The leftmost layer stack shows cladding of Mg, also approximately 30 micrometers thick, with aluminum (approximately 30 micrometers thick) to form multiple matching layers as described above. Figure 7D In the rightmost stack, an Al layer with a thickness of approximately 30 micrometers can be clad with a Ti sensing plate 704 with a thickness of approximately 450 micrometers. When added, the Al layer can be used as a matching layer 726. Figure 7D The second layer stack from the right shows an Al layer with a thickness of approximately 390 micrometers clad with a much thinner SUS layer of 60 micrometers. In this stack, the SUS layer can be a decorative layer with a thickness equal to approximately half the wavelength of the acoustic signal propagating through the SUS layer, to create a self-resonant layer.

[0080] Figure 7E The example shown illustrates the use of PVD to form a matching layer according to this disclosure. Figure 7E In the left-hand stack, Al with a thickness of approximately 30 micrometers is gradually constructed on the ZrO2 sensing plate 704 using PVD to form a matching layer 726. Figure 7E In the intermediate stack, a matching layer 726 with a thickness of approximately 30 micrometers can be gradually constructed from multiple layers of Ti, Al, or a combination of Ti / Al (e.g., in a ratio of 75% / 25% or 50% / 50%), each having a thickness of approximately 6 micrometers, and applied to the ZrO2 sensing plate 704 using PVD. Figure 7EIn the right-hand stack, a matching layer 726 with a thickness of approximately 30 micrometers can be progressively constructed from multiple layers of Ti, Al, or a combination of Ti / Al (e.g., in a ratio of 75% / 25% or 50% / 50%), each having a thickness of approximately 6 micrometers, and applied to the SUS sensing plate 704 using PVD. Figure 7E In the middle and right layers of the stack, the material layers forming the matching layer 726 can be selected to have different acoustic impedances to generate an acoustic impedance gradient on the matching layer, which may result in less parasitic reflection.

[0081] can be Figure 7D Multiple matching layer examples and Figure 7E The acoustic impedance gradient achieved by the multiple material-matching layers example can also be achieved in other ways. For example, introducing microbubbles or perforations into the sensing plate can reduce the effective acoustic impedance of the sensing plate. The characteristics of the terms "pore" and "porosity" can be used herein to refer together to closed voids (i.e., bags or bubbles) of air, gas, or other low acoustic impedance materials (compared to the acoustic impedance of the sensing plate), or open perforations (i.e., notches or cavities) filled with air or other low acoustic impedance materials. The presence of pores can result in a reduction in the average acoustic impedance of the sensing plate at the location of the pores due to the low acoustic impedance of air, the chosen gas, and other low acoustic impedance materials. Forming pores in the sensing plate with a non-uniform (variable density) distribution can generate an acoustic impedance gradient across the sensing plate, which can lead to less wave reflection, increased wave transfer energy, and more accurate touch detection and fingerprint imaging.

[0082] Figure 8A A sensing plate 804 with an internal acoustic impedance gradient generated by an aperture is shown as an example according to this disclosure. Figure 8A In one example, the PMT array 802 is fixed to the sensing plate 804 (the epoxy layer is omitted for clarity), and ultrasonic waves are transmitted through the sensing plate to / from the touch surface. In some examples, the sensing plate 804 may include a plurality of closed holes (e.g., air gaps, voids, or bubbles) unevenly distributed within the sensing plate. Figure 8A (Symbolically shown in the image), where the bottom of the sensing plate has the highest porosity (highest pore concentration) to reduce the acoustic impedance Z1 of that portion of the sensing plate and provide a closer match to the acoustic impedance of the PMUT array 802. For example, the porosity of the sensing plate 804 can be increased within the sensing plate by a height (from...). Figure 8A The porosity gradually decreases from bottom to top (from an angle of view), resulting in the lowest porosity (lowest pore concentration) at the top of the sensing plate. This increases the acoustic impedance Z2 of that portion of the sensing plate and provides an acoustic impedance match closer to that of a touching finger. In other words, as viewed from bottom to top, the effective acoustic impedance of the sensing plate 804 gradually increases as the volume fraction ratio of higher impedance material to lower impedance material dominates.

[0083] Figure 8B A sensing plate 804 with non-uniformly distributed holes 828 is shown as an example according to this disclosure. Figure 8B As illustrated in the examples, the closed aperture 828 may be filled with air, gas, or other low acoustic impedance material and may have a diameter d, which is chosen to be smaller than (and in some cases much smaller, such as less than one-tenth of the wavelength) the acoustic signal propagating through the sensing plate 804 to minimize ultrasonic wave reflection. In some examples, d may be approximately 3 micrometers to 10 micrometers. In some examples, the aperture 828 may be added to the sensing plate 804 using a metal injection molding process, wherein a binder material with a volume fraction that varies over time is added to the metal powder as the sensing plate is gradually formed. When the sensing plate 804 is fully constructed, debonding and sintering may be performed on the mating layer to remove the binder and create porosity throughout the mating layer. Because the density of the aperture 828 is greatest at the bottom and lowest at the top of the sensing plate 804, the acoustic impedance may gradually increase from Z1 at the bottom to Z2 at the top throughout the sensing plate.

[0084] Figure 8C A sensing plate 804 having holes in the form of perforations, notches, or cavities is shown as an example according to this disclosure. Figure 8C In an exemplary left-side sensing plate 804, laser micro-perforations 830 with a depth of 25-30 micrometers and a spacing of 25 micrometers can be formed in the top surface (touch surface) and bottom surface (surface facing the PMUT array) of the sensing plate, which can be made of SUS or ZrO2. Because the perforations 830 do not extend through the entire sensing plate 804, a non-uniformly distributed distribution of holes is created. In some examples, the open notches or voids created by the laser micro-perforations can be filled with a low acoustic impedance material. For example, the laser micro-perforations 830 on the top surface of the sensing plate 804 can have a width and depth selected to reduce the acoustic impedance at the top of the sensing plate to more closely match the acoustic impedance of a touching finger, while the laser micro-perforations on the bottom surface can have different widths and depths selected to reduce the acoustic impedance at the bottom of the sensing plate to more closely match the acoustic impedance of the underlying epoxy layer or PMUT array. Figure 8C In the exemplary right-side sensing plate 804, deep laser-perforated holes 832 with a depth of approximately 120 micrometers and a spacing of 50 micrometers can be formed in the bottom surface of the sensing plate (the surface facing the PMUT array), which can be made of SUS or ZrO2. Because the perforations 832 do not extend through the entire sensing plate 804, a non-uniformly distributed distribution of holes is created. For example, the deep laser-perforated holes 832 on the bottom surface can have a width and depth selected to reduce the acoustic impedance at the bottom of the sensing plate to more closely match the acoustic impedance of the underlying epoxy layer or PMUT array. In other examples, Figure 8CThe perforations 830 and 832 in the design can be formed by building up the material rather than by removing the material using a laser.

[0085] Figure 8D A sensing plate 804 with density variation according to an example of this disclosure is shown. Figure 8D In the exemplary left-side sensing plate 804, additive manufacturing techniques (i.e., 3D printing) can be used to create a non-uniform material density gradient across the sensing plate, which can be made of SUS or other materials. The material density at the top and bottom surfaces can be selected to be lower (i.e., more closed holes or voids) to produce a lower acoustic impedance Z1 that more closely matches the acoustic impedance of a touching finger at the top surface and more closely matches the acoustic impedance of the epoxy or PMUT array beneath the bottom surface. The material density toward the center of the sensing plate 804 can be selected to be higher (i.e., fewer closed holes or voids) to produce a higher acoustic impedance Z2 in this region. In some examples, the material density can vary between 50% and 96%. Figure 8D In the exemplary right-side sensing plate 804, additive manufacturing techniques (i.e., 3D printing) can be used to create a non-uniform material density gradient across the entire sensing plate. The material density at the bottom surface can be selected to be lower (i.e., more closed holes or voids) to produce a lower acoustic impedance Z1, which more closely matches the acoustic impedance of the epoxy resin or PMUT array beneath the bottom surface. The material density at the top surface of the sensing plate 804 can be selected to be higher (i.e., fewer closed holes or voids) to produce a higher acoustic impedance Z2 in that region.

[0086] Figure 8E A sensing plate 804 with perforations and density variations, according to an example of this disclosure, is shown. Figure 8E An exemplary left-side sensing panel 804 may be Figure 8C and Figure 8D The combination of the right-side sensing plates, wherein deep micro-perforations (created by removing or building material) are on the bottom surface, and the material density changes from low at the bottom surface to high at the top surface. Figure 8E An exemplary right-side sensing panel 804 may be Figure 8C and Figure 8D The combination of the left-side sensing plates, wherein micro-perforations (created by removing or building material) are on the top and bottom surfaces, and the material density changes from low at the top and bottom surfaces to high towards the middle.

[0087] While the foregoing examples utilize holes and / or perforations to generate one or more acoustic impedance transitions (e.g., gradients) with respect to the sensing plate, in other examples of this disclosure, geometry may alternatively or otherwise be used to generate these acoustic impedance transitions.

[0088] Figure 9A A sensing plate 904 having geometry 934 for generating an acoustic impedance gradient is shown as an example according to this disclosure. Figure 9A In some examples, the sensing plate 904 may have a top surface for receiving one or more touches and a lower portion with a geometry 934 (e.g., a conical structure) formed to produce a gradual acoustic impedance transition. In some examples, the geometry 934 may be formed in a sensing plate made of SUS or ZrO2. The geometry may have a distal endpoint 964 and a valley 966. The geometry 934 may be produced by laser ablation, microindentation, molding, additive manufacturing, etc., and may be aligned in an orthogonal xy array, offset diagonally, or arranged in other regular or random arrangements. Shapes other than cones may also be used, such as pyramids or generally conical structures with nonlinear surfaces, but in some examples, the cone shape can be more easily fabricated using available microfabrication methods. The spaces between the geometries 934 may then be filled with a lower acoustic impedance material 936. Material 936 can be an adhesive layer, such as the epoxy resin or composite epoxy resin described above (if the geometry is in direct contact with the epoxy resin layer), a matching layer as described above (if the matching layer is sandwiched between the geometry and the epoxy resin layer), or other materials. Essentially, the lower acoustic impedance filler material 936 in the sensing plate 904 is "geometrically mixed" with the higher acoustic impedance geometry 934 to produce a gradual acoustic impedance transition at the frequency of interest.

[0089] The width d of each geometry 934 (i.e., the distance between two peaks, or between two valleys; the spacing) can be chosen to be smaller than the average wavelength of the ultrasound waves propagating through the material 936 and the sensing plate 904 (and in some cases much smaller, such as less than one-third of the wavelength) to reduce the scattering and reflection of said waves. In some examples, d can be about 30 micrometers to 60 micrometers. The height h of the geometry 934 from the valley 966 to the distal end 964 can be chosen to be large enough to provide a smooth and gradual transition in acoustic impedance. In some examples, h can be three times the wavelength of the ultrasound waves. In some examples, the height can be about 100 micrometers to 300 micrometers. In other examples, for the SUS sensing plate, the height h can be about 120 micrometers, and for ZrO2, the height h can be about 160 micrometers.

[0090] When the width d 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 geometry 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 these materials in a theoretical plane parallel to the top surface of the sensing plate. Therefore, as the theoretical plane moves from the distal endpoint to the valley of geometry 934, the acoustic impedance can be transformed from the acoustic impedance of material 936 to the acoustic impedance of sensing plate 904. The geometry of geometry 934 (e.g., slope or taper) determines the rate of change of acoustic impedance.

[0091] Figure 9B This is a perspective view of a sensing plate 904 having geometry 934 according to an example of this disclosure.

[0092] As described above, in some examples of this disclosure, material 936 can be a matching layer. The above discussion... Figure 7C An exemplary process for producing a SUS sensing board with a geometry and associated tin-matching layers is shown.

[0093] Figure 9C An example of an SUS sensing plate according to this disclosure is shown, having a geometry 934 filled with a Sn matching layer 926, such as... Figure 7C The process is completed as shown, and then epoxy resin 910 is used to adhere it to the PMUT array 902.

[0094] See you again Figure 4A The PMUT array 402 can generate parasitic ultrasonic waves 448 that can propagate through the electronics 408, reflect from the components and back surface of the electronics, and return to the PMUT array as parasitic reflections 438. Like parasitic reflections 414, parasitic reflections 438 can adversely affect the PMUT array 402's ability to perform touch sensing and fingerprint imaging. Therefore, in some examples of this disclosure, an absorbent can be formed on the back surface of the electronics 408 to absorb the ultrasonic waves 448 and reduce parasitic reflections 438.

[0095] Figure 10A An example of a touch sensing and fingerprint imaging stack 1000 using a PMUT array 1002 backed with absorbent 1040 is shown according to the present disclosure. Figure 10A Similar to Figure 4AIn addition to adding an absorbent 1040 formed on the back surface of the electronics 1008, the PMUT array 1002 will also generate unwanted (parasitic) ultrasonic waves 1048 in the opposite direction to the sensing plate 1004 and through any underlying electronics 1008. Reflections 1038 from these waves can interfere with the correct detection of a touching finger or fingerprint ridge. In some examples, the electronics 1008 can be approximately 190 micrometers thick, mostly made of silicon wafer. Therefore, the acoustic impedance of the electronics 1008 is uniform and substantially the same as that of a single-crystal SiO2 wafer, which has an acoustic impedance of approximately 22 MRayl.

[0096] exist Figure 10A In one example, an absorber 1040 formed on the back side of the electronic device 1008 absorbs unwanted ultrasonic waves 1048 and reduces the reflection 1038 of these waves. In a preferred example, the absorber 1040 may be formulated to have an acoustic impedance (e.g., 22 MRayl) that matches the acoustic impedance of the electronic device 1008. The absorber 1040 may be formed from a 0-3 epoxy resin composite material composed of tungsten filler (particle size >5µm-15µm) and epoxy resin (e.g., having an acoustic impedance of approximately 3 MRayl). It is made of 301 epoxy resin. The base epoxy resin can be selected to be highly attenuating. Like the composite epoxy resin described above, the absorber 1040 can have a higher acoustic impedance than the epoxy resin alone, resulting in a high absorption rate. Tungsten can be used as a filler because it has a high acoustic impedance mismatch with the epoxy resin, which can lead to high scattering and increased energy absorption; however, different filler materials with high acoustic impedance mismatch with the epoxy resin can also be used in other examples. Furthermore, the high acoustic impedance of tungsten can result in a high total acoustic impedance of the epoxy resin composite, which can better match its acoustic impedance with that of the electronic device 1008. The diameter of the filler particles can be a quarter wavelength of the ultrasound waves propagating through the absorber 1040 to maximize scattering.

[0097] Figure 10B The illustration shows, according to an example of this disclosure, parasitic reflection 1042 from acoustic impedance mismatch (e.g., the combined effect of reflections 1014 and 1038) and finger signature 1018 reflected from the touch surface of the sensing plate 1004 when using composite epoxy resin 1020 and absorbent 1040. Figure 10B In the example, the composite epoxy resin 1020 reduces the acoustic impedance mismatch between the epoxy resin and the sensing plate 1004, and the absorbent 1040 reduces reflections received from the electronics 1008. Because less energy from parasitic reflections is received back at the PMUT array 1002, the amplitude (energy) of the parasitic reflection 1042 can be reduced, while the amplitude of the fingerprint signature 1018 can be advantageously increased.

[0098] Figure 11 A flowchart illustrating touch sensing and fingerprint imaging for acoustic impedance matching and improvement, according to an example of this disclosure, is shown. Figure 11 In the exemplary flowchart, dashed boxes indicate that any of these boxes is optional, and any one or more of these boxes can be performed individually or in combination. It should be understood that although the boxes appear in sequence, this arrangement is for illustrative purposes only, and any of these boxes can be implemented in any order. At box 1150, the acoustic impedance of the epoxy resin to which the transducer array is adhered to the sensing plate can be increased to reduce acoustic impedance mismatch between the transducer array and the sensing plate. At box 1152, a matching layer can 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. At box 1154, the thickness of the matching layer can be set to a quarter wavelength of the ultrasonic signal. At box 1156, non-uniform porosity can be added to the sensing plate to create an acoustic impedance gradient. At box 1158, the sensing plate and the matching layer can be “geometrically blended” by forming a geometry on the sensing plate. At box 1160, ultrasonic waves directed to the transducer electronics can be absorbed to reduce parasitic reflections.

[0099] While various examples have been illustrated and described above primarily in separate figures and paragraphs for clarity, it should be understood that various combinations of the described examples can be used together according to other examples of this disclosure. For example, according to examples of this disclosure, combinations of any two or more of the following can also be employed: a composite epoxy layer, a matching layer, porosity and / or perforations in the sensing plate, geometry on the sensing plate, and an absorbent layer.

[0100] Therefore, based on the foregoing, some examples of this disclosure relate to a touch sensing device comprising a 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 and the second surface having a plurality of geometries formed thereon; and one or more ultrasonic transducers configured to propagate ultrasonic waves through the sensing plate to the first surface, wherein the plurality of geometries taper from valleys to distal endpoints oriented toward the one or more ultrasonic transducers, and the plurality of geometries being configured to reduce 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 for the foregoing, in some examples, the spacing between adjacent distal endpoints of the plurality of geometries is smaller than the wavelength of the ultrasonic waves expected to propagate through the geometries when generated by the one or more ultrasonic transducers. In addition to or alternatively to one or more of the examples disclosed for the foregoing, in some examples, the spacing between adjacent distal endpoints is between about 3 micrometers and 10 micrometers. In addition to one or more of the examples disclosed above, or alternatively, in some examples, the height of the plurality of geometries from the valley to the distal endpoint is greater than the wavelength of the ultrasonic waves expected to propagate through the geometries when generated by one or more ultrasonic transducers. In addition to one or more of the examples disclosed above, or alternatively, in some examples, the height of the plurality of geometries is between about 100 micrometers and 300 micrometers.

[0101] In addition to or alternatively to one or more of the examples disclosed above, in some examples, the touch sensing device further includes a filler material disposed between a plurality of geometries and one or more ultrasonic transducers, the filler material having 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 plurality of geometries and the filler material are configured to generate an average acoustic impedance that transitions from the second acoustic impedance of the filler material at the distal endpoints of the plurality of geometries to the first acoustic impedance of the sensing plate at the valleys of the plurality of geometries. 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 geometries and the adhesive layer, the second acoustic impedance of the matching layer being configured to approximate the 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 of particles suspended in an epoxy resin having a fourth acoustic impedance, the composite epoxy resin having 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 geometries and one or more ultrasonic transducers, the second acoustic impedance of the matching layer being configured to approximate the geometric mean of the acoustic impedance of the sensing plate and the acoustic impedance of the 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 geometries includes a tapered structure. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the tapering of the tapered structure is configured to produce a predetermined rate of transition from the second acoustic impedance of the filler material to the first acoustic impedance of the sensing plate.

[0102] Some examples of this disclosure relate to a method for reducing acoustic impedance mismatch between one or more ultrasonic transducers and a sensing plate of a touch sensing device having a first acoustic impedance. The method includes forming a plurality of geometries on a second surface of the sensing plate facing the one or more ultrasonic transducers, tapering the plurality of geometries from valleys to distal endpoints oriented towards the one or more ultrasonic transducers, and transforming the acoustic impedance encountered by sound waves generated by the one or more ultrasonic transducers from a second acoustic impedance less than the first acoustic impedance at the distal endpoints of the plurality of geometries to a first acoustic impedance at the valleys of the plurality of geometries. In addition to one or more examples disclosed above, or alternatively, in some examples, the method further includes forming a plurality of geometries wherein the spacing between adjacent distal endpoints is less than the wavelength of ultrasonic waves expected to propagate through the geometries when generated by the one or more ultrasonic transducers. In addition to one or more examples disclosed above, or alternatively, in some examples, the method further includes forming a plurality of geometries wherein the height between the valleys and the distal endpoints is greater than the wavelength of ultrasonic waves expected to propagate through the geometries 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 method further includes distributing a filler material having a second acoustic impedance lower than the first acoustic impedance between a plurality of geometries and one or more ultrasonic transducers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the method further includes generating an average acoustic impedance from the distal endpoints of the plurality of geometries to a valley, the average acoustic impedance being converted 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 method further includes using the filler material as an adhesive to adhere the sensing plate to one or more ultrasonic transducers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the method further includes formulating the filler material as a matching layer having an acoustic impedance approximately equal to the geometric mean of the first acoustic impedance of the sensing plate and the acoustic impedance of the adhesive layer.

[0103] Some examples of this disclosure relate to a touch sensing device including a 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 configured to propagate ultrasonic waves through the sensing plate to the first surface; and a plurality of non-uniformly distributed holes within the sensing plate configured to generate an acoustic impedance gradient within the sensing plate and reduce 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 holes include a plurality of closed gaps 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 holes is smaller than the wavelength of the ultrasonic waves 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 holes is between about 30 micrometers and 60 micrometers. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the second surface of the sensing plate is closest to one or more ultrasonic transducers, and the sensing plate includes more holes distributed near the second surface than holes distributed near the first surface, to generate an acoustic impedance gradient on the sensing plate. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the plurality of holes includes a plurality of open perforations that partially extend into the sensing plate, generating a second acoustic impedance lower than the first acoustic impedance at the location of the plurality of open perforations on the sensing plate. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the second surface of the sensing plate is closest to one or more ultrasonic transducers, and the second surface includes a plurality of open perforations. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the plurality of open 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 more closely matches 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 includes a plurality of open perforations. In addition to one or more of the examples disclosed above, or alternatively, in some examples, a plurality of open perforations on the first surface have a width and depth configured such that the acoustic impedance of the sensing plate near the first surface more closely matches the acoustic impedance of an object in contact with the first surface.In addition to one or more of the examples disclosed above, or alternatively, in some examples, the second surface of the sensing plate is closest to one or more ultrasonic transducers, and the sensing plate includes more holes distributed near the first surface and the second surface than holes distributed near the center of the sensing plate, to generate an acoustic impedance gradient on the sensing plate. In addition to one or more of the examples disclosed above, or alternatively, in some examples, the sensing plate is configured with a non-uniform material density gradient such that the highest material density is near the center of the sensing plate.

[0104] Some examples of this disclosure relate to a method for reducing acoustic impedance mismatch between one or more ultrasonic transducers and a sensing plate in a touch sensing device, the sensing plate having a first surface and a second surface and a first acoustic impedance. The method includes forming a plurality of holes within the sensing plate, and arranging the plurality of holes in a non-uniform distribution to generate an acoustic impedance gradient within the sensing plate, wherein the acoustic impedance gradient generates a second acoustic impedance less than the first acoustic impedance near the second surface of the sensing plate closest to the one or more ultrasonic transducers. In addition to one or more examples disclosed above, or alternatively, in some examples, the method further includes forming the plurality of holes as a plurality of closed voids having a third acoustic impedance less than the first acoustic impedance. In addition to one or more examples disclosed above, or alternatively, in some examples, the method further includes forming a plurality of holes with a diameter smaller than the wavelength of ultrasonic waves expected to propagate through the sensing plate when generated by the one or more ultrasonic transducers. In addition to one or more of the examples disclosed above, or alternatively, in some examples, the method further includes arranging a plurality of holes in a non-uniform distribution, wherein more holes are distributed near the second surface than holes distributed near the first surface to generate an acoustic impedance gradient on the sensing plate. In addition to one or more of the examples disclosed above, or alternatively, in some examples, the method further includes forming the plurality of holes as a plurality of open perforations partially extending into the sensing plate. In addition to one or more of the examples disclosed above, or alternatively, in some examples, the method further includes forming a plurality of open perforations on the second surface of the sensing plate. In addition to one or more of the examples disclosed above, or alternatively, in some examples, the method further includes forming a plurality of open perforations on the first surface of the sensing plate. In addition to one or more of the examples disclosed above, or alternatively, in some examples, the method further includes forming more holes near the first surface and near the second surface than holes distributed near the center of the sensing plate to generate an acoustic impedance gradient on the sensing plate.

[0105] Some examples of this disclosure relate to a touch sensing device comprising 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 ultrasonic waves 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 approximately equal to the geometric mean of the first and third acoustic impedances, for generating an acoustic impedance gradient between the one or more ultrasonic transducers and the sensing plate and reducing acoustic impedance mismatch between the one or more ultrasonic transducers and the sensing plate. In addition to one or more of the examples disclosed above, or alternatively, in some examples, the thickness of the matching layer is approximately one-quarter wavelength of the ultrasonic waves expected to propagate through the matching layer when generated by the one or more ultrasonic transducers. In addition to one or more of the examples disclosed above, or alternatively, 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 includes a nickel layer with a thickness of less than 5 micrometers 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 includes multiple matching sublayers formed of various materials, each sublayer having an increasing ultrasonic impedance from a bottom sublayer to a top sublayer 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 includes multiple matching sublayers formed of various materials, each sublayer having an acoustic impedance approximately equal to the geometric mean of the acoustic impedance of the materials on either side of the sublayer. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the matching layer includes multiple matching sublayers formed of various materials, the total thickness of all matching sublayers being approximately one-quarter wavelength of the ultrasonic wave expected to propagate through the matching sublayers 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 sublayer is formed of a combination of various materials in a specific ratio. In addition to one or more of the examples disclosed above, or alternatively, in some examples, the matching layer comprises multiple matching sublayers, each sublayer having a thickness of approximately one-quarter wavelength of the ultrasonic waves expected to propagate through that sublayer when generated by one or more ultrasonic transducers.

[0106] Some examples of this disclosure relate to a method for reducing acoustic impedance mismatch between one or more ultrasonic transducers having a third acoustic impedance and a metallic sensing plate having a first acoustic impedance in a touch sensing device. The method includes distributing a matching layer between the one or more ultrasonic transducers and the metallic sensing plate, and selecting the matching layer to have a second acoustic impedance approximately equal to the geometric mean of the first and third acoustic impedances to create an acoustic impedance gradient between the one or more ultrasonic transducers and the sensing plate and reduce the acoustic impedance mismatch between them. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the method further includes selecting the thickness of the matching layer to be approximately one-quarter wavelength of the ultrasonic waves 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 method further includes distributing a seed layer between the metallic sensing plate and the matching layer. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the method further includes generating a matching layer from a plurality of matching sublayers formed of a variety of materials, each sublayer having an increasing ultrasonic impedance from a bottom sublayer to a top sublayer adjacent to a metallic sensing plate. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the method further includes generating a matching layer from a plurality of matching sublayers formed of a variety of materials, each sublayer having an acoustic impedance approximately equal to the geometric mean of the acoustic impedance of the materials on either side of the sublayer. In addition to or alternatively to one or more of the examples disclosed above, in some examples, the method further includes generating a matching layer from a plurality of matching sublayers formed of a variety of materials, the total thickness of all matching sublayers being approximately one-quarter wavelength of the ultrasonic wave expected to propagate through the matching sublayers 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 method further includes generating at least one matching sublayer from a combination of a variety of materials in a specific ratio. In addition to one or more of the examples disclosed above, or alternatively, in some examples, the method further includes applying a combination of multiple materials at a specific ratio using physical vapor deposition. In addition to one or more of the examples disclosed above, or alternatively, in some examples, the method further includes generating a matching layer from multiple matching sublayers, each sublayer having a thickness approximately one-quarter wavelength of the ultrasonic wave expected to propagate through that sublayer when generated by one or more ultrasonic transducers. In addition to one or more of the examples disclosed above, or alternatively, in some examples, the method further includes forming the matching layer using one or more of plating, cladding, and physical vapor deposition (PVD).

[0107] While examples of this disclosure have been fully described with reference to the accompanying drawings, it should be noted that various changes and modifications will become apparent to those skilled in the art. It should be understood that such changes and modifications are considered to be included within the scope of the examples of this disclosure as defined by the appended claims.

Claims

1. A touch sensing device, comprising: A 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, the one or more ultrasonic transducers being configured to propagate ultrasonic waves through the sensing plate to the first surface; as well as A plurality of non-uniformly distributed holes within the sensing plate are configured to generate an acoustic impedance gradient within the sensing plate and reduce the 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. Compared with the holes distributed near the center of the sensing plate, the sensing plate includes more holes distributed near the first surface and near the second surface to generate an acoustic impedance gradient on the sensing plate.

2. The touch sensing device according to claim 1, wherein the plurality of holes comprises a plurality of closed gaps, the plurality of closed gaps having a second acoustic impedance lower than the first acoustic impedance.

3. The touch sensing device of claim 1, wherein the diameter of the plurality of holes is smaller than the wavelength of the ultrasonic waves expected to propagate through the sensing plate when generated by the one or more ultrasonic transducers.

4. The touch sensing device of claim 3, wherein the diameter of the plurality of holes is between 3 micrometers and 10 micrometers.

5. The touch sensing device of claim 1, wherein the sensing plate is configured with a non-uniform material density gradient such that the highest material density is near the center of the sensing plate.

6. A touch sensing device, comprising: A 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, the one or more ultrasonic transducers being configured to propagate ultrasonic waves through the sensing plate to the first surface; as well as A plurality of non-uniformly distributed holes within the sensing plate are configured to generate an acoustic impedance gradient within the sensing plate and reduce the acoustic impedance mismatch between the one or more ultrasonic transducers and the sensing plate, wherein: The plurality of holes includes a plurality of open perforations that partially extend into the sensing plate, the plurality of open perforations generating a second acoustic impedance lower than the first acoustic impedance at the location of the plurality of open perforations on the sensing plate.

7. The touch sensing device of 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 perforations.

8. The touch sensing device of claim 7, wherein the plurality of open perforations on the second surface have a width and depth, the width and depth being configured such that the second acoustic impedance of the sensing plate near the second surface more closely matches the third acoustic impedance of the layer beneath the sensing plate.

9. The touch sensing device according to claim 7, wherein the first surface further includes the plurality of open perforations.

10. The touch sensing device of claim 9, wherein the plurality of open perforations on the first surface have a width and depth, the width and depth being configured such that the first acoustic impedance of the sensing plate near the first surface more closely matches the acoustic impedance of an object in contact with the first surface.

11. A touch sensing device, comprising: A 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, wherein: The second surface of the sensing plate is closest to one or more ultrasonic transducers; and Compared to the holes distributed near the first surface, the sensing plate includes more holes distributed near the second surface to generate an acoustic impedance gradient on the sensing plate; The one or more ultrasonic transducers, configured to propagate ultrasonic waves through the sensing plate to the first surface; and The sensing plate contains a plurality of non-uniformly distributed holes configured to generate an acoustic impedance gradient within the sensing plate and reduce the acoustic impedance mismatch between the one or more ultrasonic transducers and the sensing plate.

12. The touch sensing device of claim 11, wherein the plurality of holes comprises a plurality of closed gaps, the plurality of closed gaps having a second acoustic impedance lower than the first acoustic impedance.

13. The touch sensing device of claim 11, wherein the diameter of the plurality of holes is smaller than the wavelength of the ultrasonic waves expected to propagate through the sensing plate when generated by the one or more ultrasonic transducers.

14. The touch sensing device of claim 13, wherein the diameter of the plurality of holes is between 3 micrometers and 10 micrometers.

15. The touch sensing device of claim 11, wherein the plurality of holes comprises a plurality of open perforations that partially extend into the sensing plate, the plurality of open perforations generating a second acoustic impedance lower than the first acoustic impedance at the location of the plurality of open perforations on the sensing plate.

16. The touch sensing device of claim 15, 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 perforations.

17. The touch sensing device of claim 16, wherein the plurality of open perforations on the second surface have a width and depth, the width and depth being configured such that the second acoustic impedance of the sensing plate near the second surface more closely matches the third acoustic impedance of the layer beneath the sensing plate.

18. The touch sensing device of claim 16, wherein the first surface further includes the plurality of open perforations.

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