Piezoelectric device and method of forming the same

Abstract

Various embodiments may relate to a piezoelectric device. The piezoelectric device may include a first electrode arrangement. The piezoelectric device may also include a second electrode arrangement. The piezoelectric device may further include one or more piezoelectric layers between the first electrode arrangement and the second electrode arrangement. The piezoelectric device may additionally include one or more structural layers in contact with the first electrode arrangement or the second electrode arrangement. The one or more piezoelectric layers may include a plurality of domains. The first electrode arrangement, the second electrode arrangement and the one or more piezoelectric layers may form a plurality of capacitors connected in series. Successive capacitors of the plurality of capacitors may be coupled by an electrode of the first electrode arrangement or the second electrode arrangement. The successive capacitors of the plurality of capacitors may have domains having or of opposite polarization directions.

Description

PIEZOELECTRIC DEVICE AND METHOD OF FORMING THE SAMECROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore application No.1020250081 OR filed March 27, 2025, the contents of it being hereby incorporated by reference in its entirety for all purposes.TECHNICAL FIELD

[0002] Various embodiments of this disclosure may relate to a piezoelectric device. Various embodiments of this disclosure may relate to a method of forming a piezoelectric device.BACKGROUND

[0003] Piezoelectric micromachined ultrasound transducers (pMUTs) are miniaturized devices that can generate and detect ultrasound waves by exploiting the piezoelectric effect. They have various applications in biomedical imaging, non-destructive testing, and wireless communication. One of the key performance parameters of pMUTs is the receiving sensitivity, which measures the ability of the device to convert the acoustic signal into an electrical signal. The higher the receiving sensitivity, the better the image quality and the signal-to-noise ratio. Therefore, improving the receiving sensitivity of pMUTs may be crucial for enhancing their functionality and efficiency, especially for the following applications:

[0004] (A) Biomedical imaging: pMUTs can be used to perform high-resolution ultrasound imaging of various organs and tissues, such as the heart, blood vessels, brain, and skin. The receiving sensitivity determines the quality and clarity of the images, as well as the depth of penetration and the contrast between different structures.

[0005] (B) Non-destructive testing: pMUTs can be used to inspect the integrity and defects of materials and structures, such as pipes, bridges, aircraft, and buildings. The receiving sensitivity affects the accuracy and reliability of the detection, as well as the resolution and range of the inspection.

[0006] (C) Wireless communication: pMUTs can be used to transmit and receive data using ultrasound waves, which can offer advantages over radio frequency waves, such as lower power consumption, higher security, and less interference. The receiving sensitivity influences the data rate and the error rate of the communication, as well as the distance and the bandwidth of the transmission.SUMMARY

[0007] Various embodiments may relate to a piezoelectric device. The piezoelectric device may include a first electrode arrangement. The piezoelectric device may also include a second electrode arrangement. The piezoelectric device may further include one or more piezoelectric layers between the first electrode arrangement and the second electrode arrangement. The piezoelectric device may additionally include one or more structural layers in contact with the first electrode arrangement or the second electrode arrangement. The one or more piezoelectric layers may include a plurality of domains. The first electrode arrangement, the second electrode arrangement and the one or more piezoelectric layers may form a plurality of capacitors connected in series. Successive capacitors of the plurality of capacitors may be coupled by an electrode of the first electrode arrangement or the second electrode arrangement. The successive capacitors of the plurality of capacitors may have domains (of the plurality of domains) having or of opposite polarization directions.

[0008] Various embodiments may relate to a method of forming a piezoelectric device. The method may include forming a first electrode arrangement. The method may also includeforming a second electrode arrangement. The method may further include forming one or more piezoelectric layers between the first electrode arrangement and the second electrode arrangement. The method may additionally include forming one or more structural layers in contact with the first electrode arrangement or the second electrode arrangement. The one or more piezoelectric layers may include a plurality of domains. The first electrode arrangement, the second electrode arrangement and the one or more piezoelectric layers may form a plurality of capacitors connected in series. Successive capacitors of the plurality of capacitors may be coupled by an electrode of the first electrode arrangement or the second electrode arrangement. The successive capacitors of the plurality of capacitors may have domains (of the plurality of domains) of opposite polarization directions.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.FIG. 1 shows a general illustration of a piezoelectric device according to various embodiments. FIG. 2 shows a general illustration of a method of forming a piezoelectric device according to various embodiments.FIG. 3 A shows a cross-sectional schematic of a piezoelectric device according to various embodiments.FIG. 3B shows a schematic illustrating the equivalent circuit of the piezoelectric device as shown in FIG. 3A according to various embodiments.FTG. 3C shows a possible top planar view of the piezoelectric device corresponding to the cross-sectional schematic illustrated in FIG. 3A according to various embodiments.FIG. 3D shows (i) a vibration mode of a membrane of the piezoelectric device according to various embodiments under operation; (ii) one example of the piezoelectric device in which all the capacitors are at a tensile stress region of the piezoelectric device according to various embodiments; and (iii) another example of the piezoelectric device in which all the capacitors are at a compressive stress region of the piezoelectric device according to various embodiments. FIG. 4A shows a conventional piezoelectric device and its equivalent circuit.FIG. 4B shows a piezoelectric device and its equivalent circuit according to various embodiments.FIG. 4C shows a top planar view of the piezoelectric device shown in FIG 4B according to various embodiments.FIG. 4D shows a top planar view of a conventional piezoelectric device.FIG. 5A shows a plot of impedance (in arbitrary units or a.u.) as a function of frequency (in kilo-Hertz or kHz) illustrating simulated comparison between the impedance spectrum of the conventional piezoelectric device and the impedance spectrum of the piezoelectric device according to various embodiments.FIG. 5B shows a plot of sensing voltage (in arbitrary units or a.u.) as a function of frequency (in kilo-Hertz or kHz) illustrating simulated comparison between the sensing voltage spectrum of the conventional piezoelectric device and the sensing voltage spectrum of the piezoelectric device according to various embodiments.FIG. 6 shows a cross-sectional schematic of a piezoelectric device according to various embodiments.FIG. 7 shows a cross-sectional schematic of a piezoelectric device according to various embodiments.FTG. 8 shows a cross-sectional schematic of a piezoelectric device according to various embodiments.FIG. 9 shows a schematic illustrating the mode shape and stress distribution of a membrane of a piezoelectric device according to various embodiments under resonance.FIG. 10A shows a conventional piezoelectric device, in which the top electrode and the bottom electrode each covers an entire surface of the piezoelectric device.FIG. 10B shows another conventional piezoelectric device with a top electrode arrangement including multiple electrodes and a single bottom electrode.FIG. IOC shows yet another conventional piezoelectric device with a top electrode arrangement including multiple electrodes and a single bottom electrode.DESCRIPTION

[0010] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0011] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and / or combinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0012] Tn the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0013] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance, e g. within 10% of the specified value.

[0014] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items. In other words, “A and / or B” may include or refer to A, B, or both A and B.

[0015] By “comprising” it is meant including, but not limited to, whatever follows the word “comprising”. Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0016] By “consisting of’ is meant including, and limited to, whatever follows the phrase “consisting of’. Thus, the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0017] Embodiments described in the context of one of the piezoelectric devices are analogously valid for another of the piezoelectric devices, embodiments described in the context of a method are analogously valid for a piezoelectric device, and vice versa.

[0018] Various embodiments may relate to a piezoelectric device having improved receiving sensitivity.

[0019] FIG. 1 shows a general illustration of a piezoelectric device according to various embodiments The piezoelectric device may include a first electrode arrangement 102. The piezoelectric device may also include a second electrode arrangement 104. The piezoelectric device may further include one or more piezoelectric layers 106 between the first electrode arrangement 102 and the second electrode arrangement 104. The piezoelectric device may additionally include one or more structural layers 108 in (physical) contact with the firstelectrode arrangement 102 or the second electrode arrangement 104. The one or more piezoelectric layers may include a plurality of domains 106a, 106b. The first electrode arrangement 102, the second electrode arrangement 104 and the one or more piezoelectric layers 106 may form a plurality of capacitors (electrically) connected in series. Successive capacitors of the plurality of capacitors may be coupled (i.e., electrically connected) by an electrode of the first electrode arrangement 102 or the second electrode arrangement 104. The successive capacitors of the plurality of capacitors may have domains (of the plurality of domains 106a, 106b) having or of opposite polarization directions.

[0020] In other words, various embodiments may relate to a piezoelectric device including a piezoelectric layer (or layers) 106 between a first electrode arrangement 102 and a second electrode arrangement 104. The piezoelectric device may further include a structural layer (or layers) 108 in contact with either the first electrode arrangement 102 or the second electrode arrangement 104. The piezoelectric layer (or layers) 106 may include multiple domains 106a, 106b. The electrode arrangements 102, 104 and the piezoelectric layer(s) 106 may form multiple capacitors electrically connected in series by electrode(s) from the first electrode arrangement 102 and / or the second electrode arrangement 104, with neighboring capacitors having domains 106a, 106b (i.e., regions of the piezoelectric layer 106) in opposite polarization directions, i.e., with domains having polarization direction away from the structural layer(s) 108 and polarization direction towards the structural layer(s) 108.

[0021] For avoidance of doubt, FIG. 1 seeks to provide a general illustration of features of a piezoelectric device according to various embodiments, and is not intended to limit, for instance, the number, shape, dimensions, arrangement etc. of the various features. For instance, while FIG. 1 shows a single piezoelectric layer 106 with two domains 106a, 106b, various embodiments may include a piezoelectric layer 106 with more than two domains. The piezoelectric device may be a single transducer, or may be a transducer array including aplurality of transducers. Accordingly, in embodiments in which the piezoelectric device includes a transducer array, the piezoelectric device may include multiple piezoelectric layers 106 (e.g., from different transducers forming the piezoelectric device, the multiple piezoelectric layers 106 arranged laterally to one another). Further, while FIG. 1 shows a single structural layer 108, various embodiments may include multiple structural layers 108 (e.g., from different transducers forming the piezoelectric device, the multiple structural layers 108 arranged laterally to one another). Also, while FIG. 1 shows that the second electrode arrangement 104 has a single electrode and the first electrode arrangement 102 has two electrodes, various embodiments may relate to a piezoelectric device having a first electrode arrangement 102 having any suitable number of electrodes, and a second electrode arrangement 104 having any suitable number of electrodes. While FIG. 1 shows the structural layer 108 in contact with the second electrode arrangement 104, it may be envisioned that in various embodiments, the structural layer 108 may instead be in contact with the first electrode arrangement 102.

[0022] In various embodiments, the piezoelectric device may further include one or more substrates. Each of the one or more structural layers 108 may be part of or in contact with a respective substrate of the one or more substrates. Each of the one or more piezoelectric layers 106 and each of the one or more structural layers 108 may be suspended over a cavity at least partially defined by the respective substrate.

[0023] As mentioned above, in various embodiments, the piezoelectric device may be a single transducer. The piezoelectric device may include a substrate, with a structural layer 108 being part of or in contact with the substrate. The piezoelectric device may include a stack including a piezoelectric layer 106, the first electrode arrangement 102, and the second electrode arrangement 104 over the structural layer 108. In various other embodiments, the piezoelectric device may be a transducer array including a plurality of transducers.

[0024] Tn various embodiments, the one or more piezoelectric layers 106 may be ferroelectric. In various embodiments, the one or more piezoelectric layers 106 may include any suitable piezoelectric or ferroelectric material(s), e.g., aluminum nitride (AIN), scandium aluminum nitride (ScAlN), lead zirconate titanate (PZT), lead titanate (PbTiO j, hafnium zirconium oxide (HfxZri.xC>2, where x is of any suitable value) or barium titanate (BaTiCh).

[0025] Tn various embodiments, the structural layer(s) may include any suitable material(s). e.g., epitaxial silicon, silicon oxide, silicon oxynitride, or silicon nitride, scandium aluminum nitride (ScAlN), aluminum nitride (AIN), lead zirconate titanate (PZT), polyvinylidene fluoride (PVDF) or polymer. The structural layer(s) may serve to shift a neutral axis of the piezoelectric device, such that the entire piezoelectric layer(s) 106 may be on one side of the neutral axis. The neutral axis may be a line within a cross-section of a structural member (i.e , the stacked arrangement including the first electrode arrangement 102, the second electrode arrangement 104, the piezoelectric layer 106 and the structural layer 108) undergoing pure bending where both the longitudinal normal stress and the longitudinal strain are both zero.

[0026] In various embodiments, the first electrode arrangement 102 and / or the second electrode arrangement 104 may include electrode(s) including any suitable electrically conductive material(s), e.g., molybdenum (Mo), platinum (Pt), aluminum (Al), gold (Au), silver (Ag) or titanium (Ti)

[0027] Tn various embodiments, the piezoelectric device may include one or more buffer layers in contact with portions of the one or more piezoelectric layers 106, the one or more buffer layers for setting the polarization directions of the plurality of domains 106a, 106b. The buffer layer(s) may help to set or determine the initial polarity (polarization direction) of certain domain(s) of the piezoelectric layer 106.

[0028] In various embodiments, a first electrode of the first electrode arrangement 102 may be connected to a reference port, the reference port for coupling to ground or a referencevoltage. A second electrode of the first electrode arrangement 102 may be connected to a sensing port, the sensing port for coupling to a voltage or current sensing circuit. The piezoelectric device may be configured to generate an electrical signal at the sensing port in response to an acoustic wave incident onto the piezoelectric device.

[0029] In various embodiments, the piezoelectric device may further include one or more polarization configuration ports connected to one or more floating electrodes of the first electrode arrangement 102 and / or the second electrode arrangement 104, the one or more polarization configuration ports for setting the polarization directions of the plurality of domains 106a, 106b.

[0030] In various embodiments, the plurality of capacitors may be all at a tensile region of the piezoelectric device or all at a compressive region of the piezoelectric device.

[0031] FIG. 2 shows a general illustration of a method of forming a piezoelectric device according to various embodiments. The method may include, in 202, forming a first electrode arrangement. The method may also include, in 204, forming a second electrode arrangement. The method may further include, in 206, forming one or more piezoelectric layers between the first electrode arrangement and the second electrode arrangement The method may additionally include, in 208, forming one or more structural layers in contact with the first electrode arrangement or the second electrode arrangement. The one or more piezoelectric layers may include a plurality of domains. The first electrode arrangement, the second electrode arrangement and the one or more piezoelectric layers may form a plurality of capacitors connected in series. Successive capacitors of the plurality of capacitors may be coupled by an electrode of the first electrode arrangement or the second electrode arrangement. The successive capacitors of the plurality of capacitors may have domains (of the plurality of domains )of opposite polarization directions.

[0032] n other words, various embodiments may relate to a method of forming a piezoelectric device as described herein.

[0033] For avoidance of doubt, FIG. 2 is intended to provide a general illustration of steps of forming a piezoelectric device, and is not intended to limit the sequence of the various steps. The steps may be carried out in any appropriate sequence. In various embodiments, step 208 (forming the one or more structural layers) may be carried out first, followed by step 204 (forming the second electrode arrangement), step 206 (forming the one or more piezoelectric layers) and step 202 (forming the first electrode arrangement).

[0034] In various embodiments, the method may include forming one or more buffer layers in contact with portions of the one or more piezoelectric layers, the one or more buffer layers for setting the polarization directions of the plurality of domains.

[0035] In various embodiments, the method may include connecting one or more polarization configuration ports to one or more floating electrodes of the first electrode arrangement and / or the second electrode arrangement, the one or more polarization configuration ports for setting the polarization directions of the plurality of domains.

[0036] In various embodiments, the plurality of capacitors may be all at a tensile region of the piezoelectric device or may be all at a compressive region of the piezoelectric device.

[0037] In various embodiments, a first electrode of the first electrode arrangement may be connected to a reference port, the reference port for coupling to ground or a reference voltage. A second electrode of the first electrode arrangement may be connected to a sensing port, the sensing port for coupling to a voltage or current sensing circuit.

[0038] In various embodiments, the piezoelectric device may be configured to generate an electrical signal at the sensing port in response to an acoustic wave incident onto the piezoelectric device.

[0039] Tn various embodiments, each of the one or more structural layers may be part of or in contact with a respective substrate of one or more substrates.

[0040] In various embodiments, each of the one or more piezoelectric layers and each of the one or more structural layers may be suspended over a cavity at least partially defined by the respective substrate.

[0041] In various embodiments, the one or more piezoelectric layers may be ferroelectric.

[0042] In various embodiments, the piezoelectric device may include one transducer or a transducer array including a plurality of transducers.

[0043] FIG. 3A shows a cross-sectional schematic of a piezoelectric device according to various embodiments. The piezoelectric device may include a first electrode arrangement 302 including electrodes 302a, 302b, 302c. The piezoelectric device may also include a second electrode arrangement 304 including electrodes 304a, 304b. The piezoelectric device may further include a piezoelectric layer 306 between the first electrode arrangement 302 and the second electrode arrangement 304, the piezoelectric layer 306 including a plurality of domains 306a, 306b, 306c, 306d. The piezoelectric device may additionally include a structural layer 308 in contact with the second electrode arrangement 304. The piezoelectric device may be a micromachined ultrasonic transducer (pMUT) with enhanced receiving sensitivity.

[0044] As shown in FIG 3A, the stacked arrangement or membrane may be anchored to the substrate 310. As also shown in FIG. 3A, the cavity may extend through an entire thickness of the substrate 310. However, it may also be envisioned that the cavity may not extend through an entire thickness of the substrate 310, but may instead be an enclosed cavity defined by the structural layer 308 and the substrate 310.

[0045] A first electrode 302a of the first electrode arrangement 302 may be connected to a reference voltage, i.e. ground (G), while a second electrode 302b of the first electrode arrangement 302 may be connected to a voltage sensing port (S). The first electrodearrangement 302, the second electrode arrangement 304, the piezoelectric layer 306 may form a plurality of capacitors (i.e., four as shown in FIG. 3A, Ci - C4) connected in series. The capacitors may alternatively be referred to as sub-capacitors. In more detail, the first electrode 302a of the first electrode arrangement 302, a first electrode 304a of the second electrode arrangement 304 and a first domain 306a of the piezoelectric layer 306 may form a first capacitor (Ci), a middle electrode 302c of the first electrode arrangement 302, the first electrode 304a of the second electrode arrangement 304 and a second domain 306b of the piezoelectric layer 306 may form a second capacitor (C2), the middle electrode 302c of the first electrode arrangement 302, a second electrode 304b of the second electrode arrangement 304 and a third domain 306c of the piezoelectric layer 306 may form a third capacitor (C3), and the second electrode 302b of the first electrode arrangement 302, the second electrode 304b of the second electrode arrangement 304 and a fourth domain 306d of the piezoelectric layer 306 may form a fourth capacitor (C4). FIG. 3B shows a schematic illustrating the equivalent circuit of the piezoelectric device as shown in FIG. 3A according to various embodiments. The labels of the electrical nodes ‘G’, ‘S’, ‘A’, ‘B’ and C’ in FIG. 3B correspond to the same labels in FIG. 3A. In the equivalent circuit, the adjacent capacitors (‘adjacent’ here refers to the equivalent circuit, and may not mean physically adjacent) may be connected by electrodes 302a-c of the first electrode arrangement 302, or electrodes 304a-b of the second electrode arrangement 304 without crossing the piezoelectric layer 306. As shown in FIG. 3A, domains of neighboring or successive capacitors may be in opposite polarization directions. For instance, the polarization direction of domains 306a (Ci) and 306c (C3) may be away from the structural layer 308, while the polarization direction of domains 306b (C2) and 306d (C4) may be towards the structural layer 308.

[0046] As mentioned earlier, to sense the acoustic waves, the first electrode 302a of the first electrode arrangement 302 may be connected to the reference voltage, i.e. ground (G), whilethe second electrode 302b of the first electrode arrangement 302 may be connected to the voltage sensing port (S). The remaining electrodes (i.e., middle electrode 302c of the first electrode arrangement 302 and electrodes 304a-b of the second electrode arrangement 304) may be left floating during the sensing operation. FIG. 3C shows a possible top planar view of the piezoelectric device corresponding to the cross-sectional schematic illustrated in FIG. 3A according to various embodiments The dashed line in FIG. 3C may correspond to the cross-sectional schematic illustrated in FIG. 3A according to various embodiments.

[0047] To achieve the highest receiving sensitivity, the series-connected capacitors may be located in the regions with the same stress direction (compressive or tensile) under the designed vibration mode. In other words, the plurality of capacitors may all at a tensile (stress) region of the piezoelectric device or all at a compressive (stress) region of the piezoelectric device, as illustrated in FIG. 3D. FIG. 3D shows (i) a vibration mode of a membrane of the piezoelectric device according to various embodiments under operation; (ii) one example of the piezoelectric device in which all the capacitors are at a tensile stress region of the piezoelectric device according to various embodiments; and (iii) another example of the piezoelectric device in which all the capacitors are at a compressive stress region of the piezoelectric device according to various embodiments.

[0048] By dividing a stack including the first electrode arrangement 302, the second electrode arrangement 304 and the piezoelectric layer 306 into several capacitors or subcapacitors with opposite polarization directions and electrically connecting them in series, various embodiments may achieve higher receiving sensitivity than conventional pMUT devices with the same stack configuration and similar process complexity. Various embodiments may also increase the bandwidth and reduce the form factor of the pMUT device, making it suitable for various applications such as ultrasonic imaging, finger-print identification and photo-acoustic imaging.

[0049] FIG. 4A shows a conventional piezoelectric device and its equivalent circuit. FIG.4B shows a piezoelectric device and its equivalent circuit according to various embodiments. The conventional piezoelectric device includes a piezoelectric layer sandwiched between two electrodes, i.e. a top electrode and a bottom electrode. The piezoelectric device shown in FIG.4B may include a first electrode arrangement 402 including a first electrode 402a and a second electrode 402b, a second electrode arrangement 404 including a single electrode 404a, and a piezoelectric layer 406 between the first electrode arrangement 402 and the second electrode arrangement 404, the piezoelectric layer 406 including domains 406a, 406b. The electrode arrangements 402, 404 and the piezoelectric layer 406 may therefore form two capacitors in series connection. The electrode arrangements 402, 404 and the piezoelectric layer 406 may be formed over structural layer 408, and may be suspended over a cavity of a substrate 410.

[0050] With reference to the conventional piezoelectric device shown in FIG. 4A, for sensing the ultrasound acoustic wave, the membrane will resonate in response to the ultrasound acoustic wave with a specific frequency, which is determined by the dimension of the membrane. At this resonant frequency, the membrane vibrates with one particular flexural deformation pattern, in which the stress beneath the top electrode region is transduced to charge by the piezoelectric material. The sensing signal is the induced voltage (V) across the top electrode and the bottom electrode, which may be written as V =in which Q relates to the flexural mode shape, C is the capacitance between the sensing port (top electrode) and reference port, i.e., ground (bottom electrode). From the equation, one can deduce that with an identical induced charge (Q), a smaller capacitance will result in a higher sensing voltage.

[0051] With reference to the piezoelectric device according to various embodiments as shown in FIG. 4B, the first electrode arrangement 402 (i.e. top electrode arrangement) may be divided into two parts (i.e., 402a, 402b), with both the sensing port and reference port (ground) connected to the first electrode arrangement 402, i.e. electrode 402b and electrode 402arespectively. Accordingly, the equivalent circuit between the sensing port and ground may be a circuit of two capacitors connected in series via the electrode of the bottom electrode arrangement 404, resulting in a decrease in effective capacitance as compared to the conventional piezoelectric device shown in FIG.4A. The charge polarity in both capacitors may need to match the circuit route (i.e., sensing port — > bottom electrode arrangement — > ground), or the induced voltages would negate each other. The charge polarity depends on both stress direction and piezoelectric material polarization (i.e., reversing the polarization direction but not the stress direction can invert the charge polarity). As shown in FIG. 4B, with both capacitors under similar stress (i.e., in same direction away from the structural layer 408) but opposite polarization directions (i.e., polarization direction of domain 406a away from the structural layer 408 and polarization direction of domain 406b towards the structural layer 408), the charged polarities in neighboring or successive domains 406a, 406b may be reversed (i.e., opposite to each other, with charge direction of domain 406a away from the structural layer 408 and charge direction of domain 406b towards the structural layer 408), aligning with the circuit route as described above to boost the sensing voltage. As such, opposite polarization directions in neighboring domains 406a, 406b may be important for charge direction alignment.

[0052] A simple calculation can be conducted. Assuming the piezoelectric devices in FIGS.4A - B have identical geometries with the top electrode FIG. 4A being split into two smaller electrodes of the top electrode arrangement in FIG. 4B, as the membrane's size and material remain unchanged, both devices will exhibit similar resonant frequencies and deformation mode shapes. Therefore, the resulting Q distribution should match, barring orientation. If the charge and capacitance of one of the capacitors in FIG. 4B is denoted as Qoand Co, the conventional piezoelectric device in FIG. 4A can be considered as two capacitors (each capacitor with capacitance Co) in parallel. The sensing voltage ( Va) of the conventionalpiezoelectric device in FIG. 4A may be provided by Va= - = — = — For the piezoelectricdevice according to various embodiments as shown in FIG. 4B, as the two capacitors (each capacitor with capacitance Co) is series connected, the sensing voltage ( V],) may be provided

[0053] FIG. 4C shows a top planar view of the piezoelectric device shown in FIG. 4B according to various embodiments. The top electrode arrangement and bottom electrode arrangement are not shown in FIG. 4C. FIG. 4D shows a top planar view of a conventional piezoelectric device.

[0054] From the above, the sensing voltage of the piezoelectric device according to various embodiments may be twice that of the conventional piezoelectric device (assuming same acoustic load). This conclusion can be proved by the finite element method (FEM) simulation, as shown in FIGS. 5A - B. FIG. 5 A shows a plot of impedance (in arbitrary units or a.u.) as a function of frequency (in kilo-Hertz or kHz) illustrating simulated comparison between the impedance spectrum of the conventional piezoelectric device and the impedance spectrum of the piezoelectric device according to various embodiments. FIG. 5B shows a plot of sensing voltage (in arbitrary units or a.u.) as a function of frequency (in kilo-Hertz or kHz) illustrating simulated comparison between the sensing voltage spectrum of the conventional piezoelectric device and the sensing voltage spectrum of the piezoelectric device according to various embodiments.

[0055] The above described idea may be extended to more than 2 capacitors, e.g., the piezoelectric device with 4 capacitors as shown in FIG. 3A. To achieve a high sensitivity piezoelectric device, it may be important to properly pattern electrodes of the top electrode arrangement and electrodes of the bottom electrode arrangement , to form a series connection of all the capacitors. For the charge direction of the series-connected capacitors to be consistent, the polarization directions of the capacitors may need to be adjusted so that neighboring capacitors have opposite polarization directions Furthermore, to ensure charge directionmatches periodic polarization direction reversal, the capacitors may all be placed in the region where the stress direction is the same for a particular deformation pattern at the resonant frequency. The voltage across two ends of this series-connected capacitors may be captured as the sensing voltage.

[0056] FIG. 6 shows a cross-sectional schematic of a piezoelectric device according to various embodiments. The piezoelectric device may include a first (top) electrode arrangement 602 including electrodes 602a, 602b, 602c. The piezoelectric device may also include a second (bottom) electrode arrangement 604 including electrodes 604a, 604b. The piezoelectric device may further include a piezoelectric layer 606 between the first electrode arrangement 602 and the second electrode arrangement 604, the piezoelectric layer 606 including a plurality of domains 606a, 606b, 606c, 606d. The piezoelectric device may additionally include a structural layer 608 in contact with the second electrode arrangement 604. The stacked arrangement including the first electrode arrangement 602, the second electrode arrangement 604, the piezoelectric layer 606 and the structural layer 608 may be suspended over a cavity of a substrate 610.

[0057] The piezoelectric device may further include buffer layers 612a, 612b in contact with portions of the piezoelectric layer 606, the buffer layers 612a, 612b for setting the polarization directions of certain domains of the plurality of domains. The buffer layers 612a, 612b may be above the bottom electrode arrangement 604. In particular, with reference to the piezoelectric device shown in FIG. 6, the buffer layer 612a may have a bottom surface in contact with the first electrode 604a of the bottom electrode arrangement 604, and a top surface in contact with the second domain 606b of the piezoelectric layer 606, while the buffer layer 12b may have a bottom surface in contact with the second electrode 604b of the bottom electrode arrangement 604, and a top surface in contact with the fourth domain 606d of the piezoelectric layer 606. The buffer layers 612a, 612b may help to determine the initial polarity of certain domains ofthe piezoelectric layer 606 (i.e domains 606b, 606d) and to achieve or realize the desired polarization pattern. In other words, the buffer layers 612a, 612b may control the as-deposited polarization polarity of domains 606b, 606d. The buffer layers 612a, 612b may include a transition metal (e g., molybdenum (Mo), tungsten (W), titanium (Ti) or aluminum (Al)), a metal nitride (e.g., titanium nitride (TiN), tantalum nitride (TaN), zirconium nitride (ZrN) or gallium nitride (GaN)), or a substrate material (e g., silicon carbide (SiC) or sapphire).

[0058] The polarization directions may alternatively be adjusted by applying a direct current (DC) pulse of a defined polarity and strength across certain regions of the piezoelectric layer. FIG. 7 shows a cross-sectional schematic of a piezoelectric device according to various embodiments. The piezoelectric device may include a first (top) electrode arrangement 702 including electrodes 702a, 702b, 702c. The piezoelectric device may also include a second (bottom) electrode arrangement 704 including electrodes 704a, 704b. The piezoelectric device may further include a piezoelectric layer 706 between the first electrode arrangement 702 and the second electrode arrangement 704, the piezoelectric layer 706 including a plurality of domains 706a, 706b, 706c, 706d. The piezoelectric device may additionally include a structural layer 708 in contact with the second electrode arrangement 704. The stacked arrangement including the first electrode arrangement 702, the second electrode arrangement 704, the piezoelectric layer 706 and the structural layer 708 may be suspended over a cavity of a substrate 710.

[0059] The piezoelectric device may also include polarization configuration ports connected to floating electrode 702c of the first electrode arrangement 702, and floating electrodes 704a, 704b of the second electrode arrangement 704. Voltages may be applied to the polarization configuration ports for setting the polarization directions of certain domains of the plurality of domains. It should be noted that these intermediate electrodes (i.e., floating electrodes 702c,704a, 704b) may only be accessed during the polarization configuring stage, and may remain floating during sensing of acoustic waves.

[0060] FIG. 8 shows a cross-sectional schematic of a piezoelectric device according to various embodiments. The piezoelectric device shown in FIG. 8 may be implemented on a plurality of transducers, i.e., 4 transducers, instead of a single transducer. The piezoelectric device may include a first (top) electrode arrangement 802 including electrodes 802a, 802b, 802c. The piezoelectric device may also include a second (bottom) electrode arrangement 804 including electrodes 804a, 804b. The piezoelectric device may further include a plurality of piezoelectric layers 806a, 806b, 806c, 806d between the first electrode arrangement 802 and the second electrode arrangement 804, the plurality of piezoelectric layers 806a, 806b, 806c, 806d arranged laterally to one another. More specifically, the electrode 804a may be in contact with the piezoelectric layers 806a, 806b, and the electrode 804b may be in contact with the piezoelectric layers 806c, 806d. The piezoelectric device may additionally include a plurality of structural layers 808a, 808b, 808c, 8008d in contact with the second electrode arrangement 804. Each of the piezoelectric layers 806a - 806d shown in FIG. 8 may include a single domain. The first electrode arrangement 802, the second electrode arrangement 804 and the plurality of piezoelectric layers 806a-d may form a plurality of capacitors (electrically) connected in series. Successive capacitors of the plurality of capacitors may be coupled (i.e , electrically connected) by an electrode of the first electrode arrangement 802 or the second electrode arrangement 804. The first and second capacitors may be electrically connected by electrode 804a of the bottom electrode arrangement 804, the second and third capacitors may be electrically connected by electrode 802c of the top electrode arrangement 804, and the third and fourth capacitors may be electrically connected by electrode 804b of the bottom electrode arrangement 804. The successive capacitors of the plurality of capacitors may have domains of opposite polarization directions.

[0061] FIG. 9 shows a schematic illustrating the mode shape and stress distribution of a membrane of a piezoelectric device according to various embodiments under resonance. When the membrane is at its resonant frequency, the stress experienced in the middle part is the reverse of what occurs at the edges of the membrane.

[0062] FIG. 10A shows a conventional piezoelectric device, in which the top electrode and the bottom electrode each covers an entire surface of the piezoelectric device. The continuous top electrode and continuous bottom electrode cover areas of expansion and contraction, analogous to a parallel coupling of these areas. The charge alignment is achieved by setting the polarization directions of the piezoelectric film in the contraction zone in reverse to that in the expansion zone. As mentioned above, the top and bottom electrodes are continuous. Also, regions with different polarization directions are also under opposite stress directions.

[0063] FIG. 10B shows another conventional piezoelectric device with a top electrode arrangement including multiple electrodes and a single bottom electrode. The regions of the piezoelectric layer under different electrodes of the top electrode arrangement are under different stress directions, while the polarization direction is the same throughout the entire piezoelectric layer The shared bottom electrode is grounded, while signals from different electrodes of the top electrode arrangement over regions of opposite stress directions are captured independently

[0064] FIG. 10C shows yet another conventional piezoelectric device with a top electrode arrangement including multiple electrodes and a single bottom electrode. The capacitors may be serially connected via the single bottom electrode, and such an arrangement may align the charge direction with the circuit route. However, the regions of the piezoelectric layer under different electrodes of the top electrode arrangement are under different stress directions, while the polarization direction is the same throughout the entire piezoelectric layer.

Claims

Claims1. A piezoelectric device comprising:a first electrode arrangement;a second electrode arrangement;one or more piezoelectric layers between the first electrode arrangement and the second electrode arrangement;one or more structural layers in contact with the first electrode arrangement or the second electrode arrangement;wherein the one or more piezoelectric layers comprise a plurality of domains; wherein the first electrode arrangement, the second electrode arrangement and the one or more piezoelectric layers form a plurality of capacitors connected in series;wherein successive capacitors of the plurality of capacitors are coupled by an electrode of the first electrode arrangement or the second electrode arrangement; andwherein the successive capacitors of the plurality of capacitors have domains of the plurality of domains of opposite polarization directions.

2. The piezoelectric device according to claim 1, further comprising:one or more buffer layers in contact with portions of the one or more piezoelectric layers, the one or more buffer layers for setting the polarization directions of the plurality of domains.

3. The piezoelectric device according to claim 1, further comprising:one or more polarization configuration ports connected to one or more floating electrodes of the first electrode arrangement or the second electrode arrangement, the one or more polarization configuration ports for setting the polarization directions of the plurality of domains.

4. The piezoelectric device according to claim 1,wherein the plurality of capacitors are all at a tensile region of the piezoelectric device or all at a compressive region of the piezoelectric device.

5. The piezoelectric device according to claim 1,wherein a first electrode of the first electrode arrangement is connected to a reference port, the reference port for coupling to ground or a reference voltage; andwherein a second electrode of the first electrode arrangement is connected to a sensing port, the sensing port for coupling to a voltage or current sensing circuit.

6. The piezoelectric device according to claim 5,wherein the piezoelectric device is configured to generate an electrical signal at the sensing port in response to an acoustic wave incident onto the piezoelectric device.

7. The piezoelectric device according to claim 1, further comprising:one or more substrates,wherein each of the one or more structural layers is part of or in contact with a respective substrate of the one or more substrates.

8. The piezoelectric device according to claim 7,wherein each of the one or more piezoelectric layers and each of the one or more structural layers is suspended over a cavity at least partially defined by the respective substrate.

9. The piezoelectric device according to claim 1,wherein the one or more piezoelectric layers are ferroelectric.

10. The piezoelectric device according to claim 1,wherein the piezoelectric device includes one transducer or a transducer array including a plurality of transducers.

11. A method of forming a piezoelectric device, the method comprising:forming a first electrode arrangement,forming a second electrode arrangement;forming one or more piezoelectric layers between the first electrode arrangement and the second electrode arrangement;forming one or more structural layers in contact with the first electrode arrangement or the second electrode arrangement,wherein the one or more piezoelectric layers comprise a plurality of domains; wherein the first electrode arrangement, the second electrode arrangement and the one or more piezoelectric layers form a plurality of capacitors connected in series;wherein successive capacitors of the plurality of capacitors are coupled by an electrode of the first electrode arrangement or the second electrode arrangement; andwherein the successive capacitors of the plurality of capacitors have domains of the plurality of domains of opposite polarization directions.

12. The method according to claim 11, further comprising:forming one or more buffer layers in contact with portions of the one or more piezoelectric layers, the one or more buffer layers for setting the polarization directions of the plurality of domains.

13. The method according to claim 11, further comprising:connecting one or more polarization configuration ports to one or more floating electrodes of the first electrode arrangement or the second electrode arrangement, the one or more polarization configuration ports for setting the polarization directions of the plurality of domains.

14. The method according to claim 11,wherein the plurality of capacitors are all at a tensile region of the piezoelectric device or all at a compressive region of the piezoelectric device.

15. The method according to claim 11,wherein a first electrode of the first electrode arrangement is connected to a reference port, the reference port for coupling to ground or a reference voltage; andwherein a second electrode of the first electrode arrangement is connected to a sensing port, the sensing port for coupling to a voltage or current sensing circuit.

16. The method according to claim 15,wherein the piezoelectric device is configured to generate an electrical signal at the sensing port in response to an acoustic wave incident onto the piezoelectric device.

17. The method according to claim 11,wherein each of the one or more structural layers is part of or in contact with a respective substrate of one or more substrates.

18. The method according to claim 17,wherein each of the one or more piezoelectric layers and each of the one or more structural layers is suspended over a cavity at least partially defined by the respective substrate.

19. The method according to claim 11,wherein the one or more piezoelectric layers are ferroelectric20. The method according to claim 11,wherein the piezoelectric device includes one transducer or a transducer array including a plurality of transducers.

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