Piezoelectric device and method of forming the same

Abstract

Various embodiments may relate to a piezoelectric device including a substrate at least partially defining a cavity, and a multi-layer stack over the cavity, the multi-layer stack including a piezoelectric stack. The piezoelectric stack may include a piezoelectric layer having a lateral dimension greater than a lateral dimension of the cavity, a first electrode in contact with a first surface of the piezoelectric layer and having a lateral dimension smaller than the lateral dimension of the cavity, and a second electrode in contact with a second surface of the piezoelectric layer and having a lateral dimension greater than the lateral dimension of the cavity. The multi-layer stack may also include an elastic layer in contact with the piezoelectric stack, the first electrode between the piezoelectric layer and the elastic layer. The piezoelectric device may be configured such that the first electrode extends along a neutral axis of the multi- layer stack.

Description

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

[0001] This application claims the benefit of priority of Singapore application No. 10202402469S filed August 15, 2024, 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 micro diaphragms have been widely employed as actuator-like piezoelectric micromachined ultrasonic transducers (pMUTs), as receiver-like microphones, and as transceiver-like imagers or rangefinders for different applications.SUMMARY

[0004] Various embodiments may relate to a piezoelectric device. The piezoelectric device may include a substrate at least partially defining a cavity. The piezoelectric device may also include a multi-layer stack over the cavity. The multi-layer stack may include a piezoelectric stack. The piezoelectric stack may include a piezoelectric layer having a first surface and a second surface opposite the first surface, the piezoelectric layer having a lateral dimension greater than a lateral dimension of the cavity The piezoelectric stack may also include a first electrode in contact with the first surface of the piezoelectric layer, the first electrode having a lateral dimension smaller than the lateral dimension of the cavity. The piezoelectric stack mayfurther include a second electrode in contact with the second surface of the piezoelectric layer, the second electrode having a lateral dimension greater than the lateral dimension of the cavity. The multi-layer stack may also include an elastic layer in contact with the piezoelectric stack such that the first electrode is between the piezoelectric layer and the elastic layer. The piezoelectric device may be configured such that the first electrode extends along a neutral axis of the multi-layer stack.

[0005] Various embodiments may relate to a method of forming a piezoelectric device. The method may include forming a multi-layer stack over a cavity at least partially defined by the substrate. The multi-layer stack may include a piezoelectric stack. The piezoelectric stack may include a piezoelectric layer having a first surface and a second surface opposite the first surface, the piezoelectric layer having a lateral dimension greater than a lateral dimension of the cavity. The piezoelectric stack may also include a first electrode in contact with the first surface of the piezoelectric layer, the first electrode having a lateral dimension smaller than the lateral dimension of the cavity. The piezoelectric stack may further include a second electrode in contact with the second surface of the piezoelectric layer, the second electrode having a lateral dimension greater than the lateral dimension of the cavity. The multi-layer stack may also include an elastic layer in contact with the piezoelectric stack such that the first electrode is between the piezoelectric layer and the elastic layer The piezoelectric device may be configured such that the first electrode extends along a neutral axis of the multi-layer stack.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] 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 followingdescription, various embodiments of the invention are described with reference to the following drawings.FIG. 1 shows (a) an optical microscopy image of the dual -el ectrode physical vapor deposition (PVD) lead zirconate titanate (PZT) piezoelectric micromachined ultrasonic transducers (pMUTs) in an array; and (b) a cross-sectional schematic of the fabricated physical vapor deposition lead zirconate titanate piezoelectric micromachined ultrasonic transducer.FIG. 2 shows schematics of the eigen frequencies of the micro diaphragm in (a) the flexible mode and (b) a higher mode.FIG. 3 shows (a) a schematic of a pressure test set up for the micro diaphragm; and (b) a schematic of impedance (in kilo-ohms or kQ) as a function of frequency (in kilo-hertz or kHz) illustrating the impedance spectrum with pressure response of the micro diaphragm.FIG. 4 shows (a) a plot of z-axis displacement (z-displacement, in micrometers or pm) as a function of distance (in micrometers or pm) illustrating the optical profile of the micro diaphragm under different residual stresses (SI and S2); and (b) a plot of frequency (in kiloHertz or kHz) as a function of pressure (in bars) comparing the frequency -pressure response curves of micro diaphragms with different residual stresses (SI and S2).FIG. 5A shows a general illustration of a piezoelectric device according to various embodiments.FIG. 5B shows a general illustration of a bimorph piezoelectric device according to various embodiments.FIG. 6 shows a general illustration of a method of forming a piezoelectric device according to various embodiments.FIG. 7 shows a cross-sectional view of a piezoelectric device according to various embodiments.FIG. 8 shows an illustration comparing the working concept of a conventional micro diaphragm and the micro diaphragm according to various embodiments.FIG. 9 A shows a schematic comparing (a) a first conventional micro diaphragm sensor, (b) a second conventional micro diaphragm sensor, and (c) a piezoelectric micro diaphragm sensor according to various embodiments.FIG. 9B shows a plot of surface z position (in micrometers or pm) as a function of arc location (in micrometers or pm) illustrating diaphragm deformation with response stress sweep in sensing layer of the first conventional micro diaphragm sensor.FIG. 9C shows a plot of surface z position (in micrometers or pm) as a function of arc location (in micrometers or pm) illustrating diaphragm deformation with response stress sweep in sensing layer of the second conventional micro diaphragm sensor.FIG. 9D shows a plot of surface z position (in micrometers or pm) as a function of arc location (in micrometers or pm) illustrating diaphragm deformation with response stress sweep in sensing layer of the piezoelectric micro diaphragm sensor according to various embodiments.FIG. 9E shows a plot of frequency (in kilo-hertz or kHz) as a function of response stress in sensing layer (in mega-Pascals or MPa) illustrating the frequency response sensitivity characterization of the first conventional micro diaphragm sensor, the second conventional micro diaphragm sensor and the piezoelectric micro diaphragm sensor according to various embodiments.FIG. 9F shows a schematic illustrating a piezoelectric device in which the neutral axis of the multi-layer stack is aligned with the patterned first electrode according to various embodiments, as well as in which the neutral axis is misaligned by +20% or -20% to the patterned first electrode.FIG. 9G shows a plot of surface z position (in micrometers or pm) as a function of arc location (in micrometers or pm) illustrating diaphragm deformation with response stress sweep insensing layer of a piezoelectric micro diaphragm sensor in which the neutral axis is misaligned by -20% to the patterned electrode.FIG. 9H shows a plot of surface z position (in micrometers or pm) as a function of arc location (in micrometers or pm) illustrating diaphragm deformation with response stress sweep in sensing layer of a piezoelectric micro diaphragm sensor in which the neutral axis is aligned by to the patterned electrode according to various embodiments.FIG. 91 shows a plot of surface z position (in micrometers or pm) as a function of arc location (in micrometers or pm) illustrating diaphragm deformation with response stress sweep in sensing layer of a piezoelectric micro diaphragm sensor in which the neutral axis is misaligned by +20% to the patterned electrode.FIG. 9J shows a plot of frequency (in kilo-hertz or kHz) as a function of response stress in sensing layer (in mega-Pascals or MPa) illustrating the frequency response sensitivity characterization of a piezoelectric device in which the neutral axis of the multi-layer stack is aligned with the patterned first electrode according to various embodiments, as well as in which the neutral axis is misaligned by +20% or -20% to the patterned first electrode.FIG. 10A shows a piezoelectric device according to various embodiments.FIG. 10B shows another piezoelectric device according to various embodimentsFIG. 10C shows yet another piezoelectric device according to various embodimentsFIG. 10D shows yet another piezoelectric device according to various embodiments.FIG. 11 shows schematics of top view of piezoelectric devices showing the cavity boundaries and the sole patterned first electrode boundaries according to various embodiments: (a) circular patterned first electrode in cavity with circular circumference; (b) square / quadrilateral patterned first electrode in cavity with square / quadrilateral perimeter; (c) circular patterned first electrode in cavity with square / quadrilateral perimeter; (d) hexagonal patterned first electrode in cavity with hexagonal perimeter; (e) octagonal patterned first electrode in cavity with octagonalperimeter; and (f) square / quadrilateral patterned first electrode in cavity with octagonal perimeter.FIG. 12 shows a schematic of (a) a piezoelectric device with a backport opening according to various embodiments; and (b) a piezoelectric device with a backside cavity according to various embodiments.DESCRIPTION

[0007] 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.

[0008] 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.

[0009] 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.

[0010] In the context of various embodiments, the terms “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.

[0011] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0012] 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.

[0013] 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.

[0014] Piezoelectric micro diaphragm structure usually includes two main parts as shown in FIG. 1. FIG. 1 shows (a) an optical microscopy image of the dual-electrode PVD PZT pMUTs in an array; and (b) a cross-sectional schematic of the fabricated PVD PZT pMUT One is the top piezoelectric stack, and the other is a below attached elastic layer. The piezoelectric stack is a sandwiched structure involving a top electrode layer, a piezoelectric layer, and a bottom electrode layer. The below elastic layer is a mechanical structure with several micrometers thickness to provide structural support for the whole diaphragm.

[0015] In most practical applications, the micro diaphragm works in the flexible vibration mode under its structural eigen frequency, and only occasionally works in higher mode(s). FIG. 2 shows schematics of the eigen frequencies of the micro diaphragm in (a) the flexible mode and (b) a higher mode. The top piezoelectric stack may be responsible for energy conversion from electrical energy to mechanical kinetic energy when the diaphragm is used as actuators, or from mechanical kinetic energy to electrical energy when it is used as receivers. The eigen frequency or resonant frequency can be dependent on the structure of the micro diaphragm, e g., overall diaphragm stiffness, which in turn can be impacted by the membrane thickness, membrane dimension(s), and / or the young’s modulus properties of the material. Additionally,the eigen frequency or resonant frequency may also be dependent on the cavity defined anchor boundary.

[0016] Given the flexibility of thin elastic membrane structures, micro diaphragms may potentially be suitable for highly sensitive stress sensors. The scandium aluminum nitride (ScAlN) micro diaphragm has previously been demonstrated for ambient gas pressure monitoring, and the optimal device shows high-sensitivity (sensing frequency (fsen) = 112kHz / bar, figure of merit (FOM) = fsen / initial frequency (fo) =9 8-- 104ppm / bar) and wide range (0 ~ 3 bars) when sensing pressure, as shown in FIG. 3, well-suited for tire pressure monitoring in vehicles. FIG. 3 shows (a) a schematic of a pressure test set up for the micro diaphragm; and (b) a schematic of impedance (in kilo-ohms or kQ) as a function of frequency (in kilo-hertz or kHz) illustrating the impedance spectrum with pressure response of the micro diaphragm.

[0017] It is found that the resonance characteristic of the micro diaphragm is severely affected by the initial condition and residual stress of the membrane. Devices from different parts of the wafer may be divided into 2 types based on different fabrication residual stresses - SI : large tensile stress (-2.7 pm deformation); and S2: relatively neutral stress (- 0.5pm deformation). FIG. 4 shows (a) a plot of z-axis displacement (z-displacement, in micrometers or pm) as a function of distance (in micrometers or pm) illustrating the optical profile of the micro diaphragm under different residual stresses (SI and S2); and (b) a plot of frequency (in kilo-Hertz or kHz) as a function of pressure (in bars) comparing the frequency-pressure response curves of micro diaphragms with different residual stresses (SI and S2) FIG. 4(b) shows that S2 with a near neutral stress exhibits a sensitivity of 112kHz / bar in range of 0 - 3 bars, which is about 1 5 times higher than SI (76 kHz / bar). Notably, the presence of larger tensile stress makes SI an initial over-concave condition leading to the singularity issue when pressure changes from 0 to 0.5 bar. This initial deformation (caused by residual stress) resultsin the sensor having the same output signal at 2 different pressure points (under 0.16 bar and 0.33 bar), leading to a “blind area” of measurement range from 0 to 0.5 bar.

[0018] The undesired "blind area" resulting from initial deformation may pose a significant challenge for resonance pressure sensors. Similarly, it can be detrimental for crucial micro particles detection like gases, chemical molecules, or biomolecules. It can seriously influence the minimum detection limit and even bring an unpredictable frequency response shift due to the residual stress variation. Therefore, addressing the initial deformation induced by fabrication residual stress may be imperative for piezoelectric micro diaphragm sensors.

[0019] Various embodiments may relate to a piezoelectric device that precisely adjusts the positioning of the mid-patterned electrode relative to the neutral axis of the entire diaphragm stack (i.e , multi-layer stack). This may enable the creation of a structure resistant to initial deformation, even when confronted with varying residual stresses across different layers. By accommodating chemical or physical sensing reactions, various embodiments may increase or maximize the sensitivity of the frequency response. The frequency response output may only be dictated by the stress variations within the sensing layer without any stack deformation issues. Various embodiments may effectively resolve the "blind area" issue, which makes the piezoelectric micro diaphragm a promising alternative to traditional quartz crystal microbalance (QCM) sensor platform.

[0020] FIG. 5A shows a general illustration of a piezoelectric device according to various embodiments. The piezoelectric device may be or may include a piezoelectric micro diaphragm (alternatively referred to as “piezo micro diaphragm”) or a piezoelectric micro diaphragm sensor (alternatively referred to as “piezo micro diaphragm sensor”). The piezoelectric micro diaphragm may be a pMUT The piezoelectric device may include a substrate 502 at least partially defining a cavity (indicated by dashed lines). The piezoelectric device may also include a multi-layer stack 504 over the cavity. The multi-layer stack 504 may include a piezoelectricstack 506. The piezoelectric stack 506 may include a piezoelectric layer 506a having a first surface and a second surface opposite the first surface, the piezoelectric layer 506a having a lateral dimension greater than a lateral dimension of the cavity. The piezoelectric stack 506 may also include a first electrode 506b in contact with the first surface of the piezoelectric layer 506a, the first electrode 506b having a lateral dimension smaller than the lateral dimension of the cavity. The piezoelectric stack 506 may further include a second electrode 506c in contact with the second surface of the piezoelectric layer 506a, the second electrode 506c having a lateral dimension greater than the lateral dimension of the cavity. The multi-layer stack 504 may also include an elastic layer 508 in contact with the piezoelectric stack 506 such that the first electrode 506b is between the piezoelectric layer 506a and the elastic layer 508. The piezoelectric device may be configured such that the first electrode 506b extends along a neutral axis of the multi-layer stack 504.

[0021] In other words, various embodiments may relate to a piezoelectric device including a piezoelectric stack 506 and an elastic layer 508 over a substrate 502 with a cavity. The piezoelectric stack 506 may have a first electrode 506b with a width smaller than a width of the cavity. The piezoelectric stack 506 may also include a piezoelectric layer 506a and a second electrode 506c with widths greater than the width of the cavity. The first electrode 506b may be arranged along a neutral axis of the multi-layer stack 504.

[0022] For avoidance of doubt, FIG. 5A is intended to illustrate features of a piezoelectric device according to various embodiments, and is not intended to limit for instance, the dimensions, arrangement, orientation etc. of the various features For instance, while FIG. 5 A shows the piezoelectric stack 506 over the elastic layer 508, it may be envisioned that in some embodiments, the elastic layer 508 may be over the piezoelectric stack 506.

[0023] The neutral axis may be a line within a cross-section of a structural member (e g., a beam or in this case, the multi-layer stack 504) undergoing pure bending where both thelongitudinal normal stress and the longitudinal strain are both zero. A thickness of each layer of the multi-layer stack 504 may be configured so that the first electrode 506a extends along the neutral axis of the multi-layer stack and a target resonant frequency is achieved by the multilayer stack 504. In other words, the thickness of the layers of the multi-layer stacked may be tuned or controlled such that the first electrode 506a lies along the neutral axis and piezoelectric device is of the target resonant frequency. The first electrode 506b extending or lying along the neutral axis of the multi-layer stack 504 may mean or refer to the neutral axis of the multi-layer stack 504 aligned within a thickness of the first electrode 506b. The target resonant frequency may also be dependent on dimensions of the cavity and elastic property of each layer (of the multi-layer stack 504). By aligning the first electrode 506b and the neutral axis of the multilayer stack 504, the piezoelectric device may be a deformation-proof structure. A response stress variation on the piezoelectric device may translate only to stiffness change (instead of deformation and stiffness change in a conventional device, which may lead to frequency shift in opposite directions), thereby increasing the ultimate frequency shift sensitivity.

[0024] In various embodiments, the muti-layer stack may further include a sensing layer (alternatively referred to as “sensing function layer”) over the substrate 502. In various embodiments, the piezoelectric device may be a micro diagram resonance sensor. In various embodiments, the piezoelectric stack 506 may be over the elastic layer 508, and the sensing layer may be over the piezoelectric stack 506. In various other embodiments, the elastic layer 508 may be over the piezoelectric stack 506, and the sensing layer may be over the elastic layer 508. In yet various embodiments, the elastic layer 508 may be over the sensing layer, and the piezoelectric stack 506 may be over the elastic layer 508. In yet various other embodiments, the piezoelectric stack 506 may be over the sensing layer, and the elastic layer 508 may be over the piezoelectric stack 506.

[0025] In various embodiments, the sensing layer may include palladium (Pd), polyimide (PI), zinc oxide (ZnO), Molecularly Imprinted Polymers (MIPs), carbon nanotubes, graphene or quantum dots In various other embodiments, the sensing layer may include any other suitable material. The material chosen for the sensing layer (e.g., Pd, PI etc.) may be based on the target molecule (e g., hydrogen (H2) or water vapor (HcO) etc.) to be detected.

[0026] In various embodiments, a lateral boundary of the first electrode 506b and a lateral boundary of the cavity may form a concentric arrangement. The lateral boundary of the cavity may have a shape selected from a group consisting of a circle, a square, a hexagon and an octagon.

[0027] In various embodiments, a surface area of a planar surface of the first electrode 506b may be any percentage selected from a range from 5% to 95% of a traverse cross-section of the cavity.

[0028] In various embodiments, the second electrode 506c may extend over an entire second surface of the piezoelectric layer 506a. In various other embodiments, the second electrode 506c may not extend over the entire second surface of the piezoelectric layer 506a.

[0029] In various embodiments, the piezoelectric layer 506a may include scandium aluminum nitride (ScAlN), aluminum nitride (AIN), lead zirconate titanate (PZT), quartz or poly vinylidene fluoride (PVDF). In various other embodiments, the piezoelectric layer 506a may include any other suitable material.

[0030] In various embodiments, the first electrode 506b and / or the second electrode 506c may include molybdenum (Mo), platinum (Pt), aluminum (Al), gold (Au), silver (Ag) or titanium (Ti). In various other embodiments, the first electrode 506b and / or the second electrode 506c may include any other suitable material.

[0031] In various embodiments, the elastic layer 508 may include epitaxial silicon, silicon oxide, silicon oxynitride, or silicon nitride, scandium aluminum nitride (ScAlN), aluminumnitride (AIN), lead zirconate titanate (PZT), polyvinylidene fluoride (PVDF) or polymer. In various other embodiments, the elastic layer 508 may include any other suitable material.

[0032] In various embodiments, the cavity may extend from a first surface of the substrate 502 to a second surface of the substrate 502 to form a backport opening, as shown in FIG. 5A. In various other embodiments, the cavity may be an enclosed cavity defined by the substrate 502 and the multi-layer stack 504.

[0033] In various embodiments, the piezoelectric device may be an unimorph pMUT, such as that illustrated in FIG. 5A. In various other embodiments, the piezoelectric device may be a bimorph structure including two piezoelectric stacks and two elastic layers, in which each of the two piezoelectric stacks includes a first electrode between a respective piezoelectric layer and a corresponding elastic layer

[0034] FIG. 5B shows a general illustration of a bimorph piezoelectric device according to various embodiments. The piezoelectric device may include a substrate 502 at least partially defining a cavity (indicated by dashed lines). The piezoelectric device may also include a multilayer stack 504 over the cavity. The multi-layer stack 504 may include a piezoelectric stack 506. The piezoelectric stack 506 may include a piezoelectric layer 506a having a first surface and a second surface opposite the first surface, the piezoelectric layer 506a having a lateral dimension greater than a lateral dimension of the cavity The piezoelectric stack 506 may also include a first electrode 506b in contact with the first surface of the piezoelectric layer 506a, the first electrode 506b having a lateral dimension smaller than the lateral dimension of the cavity. The piezoelectric stack 506 may further include a second electrode 506c in contact with the second surface of the piezoelectric layer 506a, the second electrode 506c having a lateral dimension greater than the lateral dimension of the cavity. The piezoelectric stack 506 may also include a further piezoelectric layer 506a’ having a first surface and a second surface opposite the first surface, the further piezoelectric layer 506a’ having a lateral dimension greater than alateral dimension of the cavity. The first electrode 506b may be in contact with the second surface of the further piezoelectric layer 506a’. The piezoelectric stack 506 may additionally include a further second electrode 506c’ in contact with the first surface of the further piezoelectric layer 506a’, the further second electrode 506c’ having a lateral dimension greater than the lateral dimension of the cavity. The multi-layer stack 504 may also include an optional elastic layer 508 in contact with the piezoelectric stack 506 such that the first electrode 506b, the further piezoelectric layer 506a’ and the further second electrode 506c’ are between the piezoelectric layer 506a and the elastic layer 508. The piezoelectric device may be configured such that the first electrode 506b extends along a neutral axis of the multi-layer stack 504. In various embodiments, the piezoelectric stack 506 may be symmetrical (about a length of the first electrode 506) such that the first electrode 506b extends along the neutral axis of the multilayer stack 504.

[0035] In other words, various embodiments may relate to a bimorph structure including two piezoelectric layers 506a, 506a’. The patterned first electrode 506b may be between the two piezoelectric layers 506a, 506a’.

[0036] For avoidance of doubt, FIG. 5B is intended to illustrate features of a piezoelectric device according to various embodiments, and is not intended to limit for instance, the dimensions, arrangement, orientation etc of the various features.

[0037] Various embodiments may be used as an acoustic transducer, while various other embodiments may be used as a sensor, a physical sensor, a chemical sensor, a biomedical sensor, a gas molecule sensor, or a liquid molecule sensor.

[0038] FIG. 6 shows a general illustration of a method of forming a piezoelectric device according to various embodiments The method may include, in 602, forming a multi-layer stack over a cavity at least partially defined by the substrate. The multi-layer stack may include a piezoelectric stack. The piezoelectric stack may include a piezoelectric layer having a firstsurface and a second surface opposite the first surface, the piezoelectric layer having a lateral dimension greater than a lateral dimension of the cavity. The piezoelectric stack may also include a first electrode in contact with the first surface of the piezoelectric layer, the first electrode having a lateral dimension smaller than the lateral dimension of the cavity. The piezoelectric stack may further include a second electrode in contact with the second surface of the piezoelectric layer, the second electrode having a lateral dimension greater than the lateral dimension of the cavity. The multi-layer stack may also include an elastic layer in contact with the piezoelectric stack such that the first electrode is between the piezoelectric layer and the elastic layer. The piezoelectric device may be configured such that the first electrode extends along a neutral axis of the multi-layer stack.

[0039] The different layers may be formed by any suitable fabrication method(s) / techniques(s). For instance, the first electrode, the second electrode and / or the piezoelectric layer may be formed by any suitable deposition process, e.g., physical vapor deposition. The first electrode may be patterned by depositing metal to form a metal layer, depositing a photoresist on the metal layer, using photolithography to pattern the photoresist, removing parts of the photoresist exposed to photolithography, and using a suitable etching method to remove portions of the metal layer exposed by the photoresist. The elastic layer may be formed by any suitable deposition process, e.g., epitaxy, for epitaxial silicon. The sensing layer may be formed by any suitable deposition process, e.g., by physical metal deposition for palladium (Pd).

[0040] Various embodiments may relate to a micro diaphragm resonance sensor including a piezoelectric micro diaphragm combined with a sensing layer. The piezoelectric micro diaphragm may be configured to provide electrical frequency signal generation, while the sensing layer may include variant functionalized films tailored for specific target molecule reactions exposed to ambient. The sensing layer may be optional, and can be removed when thesensor is just used as a pressure sensor, a flow meter, a mass detector, or for measuring other physical properties.

[0041] The piezoelectric micro diaphragm may be an initial deformation-proof structure (resistant to the impact of residual stress) including a piezoelectric stack, an elastic layer, and an anchor (substrate) with cavity. The piezoelectric stack may include a piezoelectric layer and two electrodes, i.e., a first electrode and a second electrode. One of the two electrodes may be patterned (e.g., 5% - 95%) smaller than a lateral dimension of the cavity, while the other electrode and the piezoelectric layer may remain intact (i.e., fully covering the cavity).

[0042] The piezoelectric stack may be on the elastic layer, or alternatively below the elastic layer. The patterned electrode may be sandwiched between the electric layer and the piezoelectric layer of the piezoelectric stack.

[0043] The thickness of each layer may be meticulously tailored in accordance with the elastic properties of materials of the layer, the dimensions of the cavity, and the targeted resonant frequency, ensuring precise alignment (i.e., within 50% error as tolerance) of the above patterned electrode with the neutral axis of the entire multi-layer stack. The multi-layer stack may be mounted in a substrate acting as an anchor, the substrate having the cavity.

[0044] The sensing layer may be a separate layer having a material different from the material of the electrodes, or may be the second electrode. The sensing layer may cover the top or the bottom of the entire multi-layer stack.

[0045] FIG. 7 shows a cross-sectional view of a piezoelectric device according to various embodiments The piezoelectric device may be a hydrogen micro diaphragm sensor. The piezoelectric device may include a substrate 702 including a cavity, an elastic layer 708, a piezoelectric layer 706a, a patterned first electrode 706b, a second electrode 706c and a sensing layer 710. As shown in FIG. 7, the first electrode 706b, sandwiched between the elastic layer 708 and the piezoelectric layer 706a, may be 50% partially patterned, while the remaining layers708, 706a, 706c, 710 may remain intact. The material composition of each layer 708, 706a, 706b, 706c, 710 can be customized to fulfil the specific functional roles within the piezoelectric device, provided they effectively serve the intended purpose of the respective layer. The conductive material of the electrodes 706b, 706c may be molybdenum (Mo), the material of the piezoelectric layer 706a may be scandium aluminum nitride (ScAlN), the material of the mechanical elastic layer 708 may be epitaxial silicon (Epi Si), the substrate may include silicon (Si), and the material of the sensing layer 710 may be palladium (Pd).

[0046] The position of the neutral axis Zstack of the entire multi-layer stack above the cavity may be provided by Zstackwhere Z(- is the center position of the layer z, £) is theYoung’s modulus of the layer z, and;is the cross-sectional area of the layer z.

[0047] The resonance frequency of the micro diaphragm may be provided by f0«where a is the first mode circular constant, r is the radius of the cavity, D is the flexural rigidity, p is the average membrane density and t is the thickness of the micro diaphragm. The flexuralEt rigidity31) may be provided by D —127T(1_v2y where v is the effective Poisson’s ratio.

[0048] The thickness of each layer 708, 706a, 706b, 706c, 710 may be defined by the above equations to ensure the ZBOI Eie being well aligned with the neutral axis Zstack of the whole diaphragm stack. Firstly, the f0equation and the targeting frequency may be referred to determine the cavity radius and estimate a thickness range of the entire micro diaphragm. Thereafter, correlation between the Zstack equation and Z(may be used to precisely tailor the thickness (7,) of each layer. Another feasible way is to use Finite Element Modeling (FEM) simulation. For instance, the targeting frequency may be 120kHz and the radius of cavity may be 300 pm. The thickness of each layer (7]) may be defined as Tsm~ Pd 50 nm, TropEie ~ Mo 100 nm, Tpte ~ ScAlN 1 pm, TBotEie ~ Mo 100 nm, TEIO ~ Epi Si 1.6 pm, where the Zstack ~ 1.70 pm is almost the same as the ZBotEie ~ 1.76 pm verified in FEM simulation. In other words,according to some embodiments, the piezoelectric device may include a 1 pm thick piezoelectric layer including ScAlN, a 100 nm thick first electrode including Mo, a 100 nm thick second electrode including Mo, a 1 6 pm thick elastic layer including epitaxial Si, and a 50 nm thick sensing layer including Pd. The neutral axis of the multi-layer stack Zstack ~ 1.70 pm may be aligned within a thickness of the first electrode (i.e., i.e., ±0.1 pm of ZuotEie ~ 1.76 pm).

[0049] FIG. 8 shows an illustration comparing the working concept of a conventional micro diaphragm and the micro diaphragm according to various embodiments. For the conventional micro diaphragm (C-Micro diaphragm), the response stress variation within sensing layer may deform the whole diaphragm stack and change the stack stiffness at the same time. Both the diaphragm deformation and the stiffness change may result in frequency shift, but in opposite directions. The ultimate response signal output may be totally determined by the above frequency shift.

[0050] To enhance the frequency shift sensitivity, the proposed P-Mirco diaphragm according to various embodiments may employ a deformation-proof solution, effectively converting response stress variations into changes in stack stiffness. Various embodiments may lead to a significant improvement in the frequency shift sensitivity, thereby leading to a substantial increase in the overall response output.

[0051] FIG. 9A shows a schematic comparing (a) a first conventional micro diaphragm sensor, (b) a second conventional micro diaphragm sensor, and (c) a piezoelectric micro diaphragm sensor according to various embodiments. In the first conventional micro diaphragm sensor, the piezoelectric stack may include a bottom electrode and a top electrode along with a piezoelectric layer, all patterned to dimensions smaller than the cavity size (50% covering). These layers are on or over the top surface of the elastic layer. In the second conventional micro diaphragm sensor, only the top electrode is patterned, covering 50% of the cavity diameter,while the piezoelectric layer and bottom electrode remain fully intact, covering the entire top surface of the elastic layer.

[0052] Different to the above conventional micro diaphragm sensors, the piezoelectric micro diaphragm sensor may feature a patterned bottom electrode (i.e., first electrode), while the top electrode (i.e., second electrode) and the piezoelectric layer may be kept fully intact. The sole patterned 1 ay er / el ectrode, i.e., the bottom electrode, may be sandwiched between the elastic layer and the piezoelectric layer. Significantly, the thickness of each layer has been meticulously calculated according to the equations provided above to ensure precise alignment between neutral axis Zstack and the sole patterned layer ZBotEie ~ 1.7pm to form the deformation resistant structure.

[0053] The 3 devices as shown in FIG 9A are tested and verified through COMSOL simulation. With a consideration of average fabrication residual stress existing in the piezoelectric stack (i.e. stress within layers such as electrodes piezoelectric layers etc.) — 300MPa, the initial status of each device is illustrated in FIG. 9B. FIG. 9B shows a plot of surface z position (in micrometers or pm) as a function of arc location (in micrometers or pm) illustrating diaphragm deformation with response stress sweep in sensing layer of the first conventional micro diaphragm sensor. FIG. 9C shows a plot of surface z position (in micrometers or pm) as a function of arc location (in micrometers or pm) illustrating diaphragm deformation with response stress sweep in sensing layer of the second conventional micro diaphragm sensor. FIG. 9D shows a plot of surface z position (in micrometers or pm) as a function of arc location (in micrometers or pm) illustrating diaphragm deformation with response stress sweep in sensing layer of the piezoelectric micro diaphragm sensor according to various embodiments. The first conventional micro diaphragm sensor and the second conventional micro diaphragm sensor are susceptible with a 0.5 ~ 0.6 pm initial convex deformation caused by the same residual stress. In contrast, the piezoelectric micro diaphragmsensor according to various embodiments exhibits 0 gm initial deformation, thereby demonstrating excellent resistance to structural deformation induced by fabrication residual stress.

[0054] Subsequently, a response stress sweep within the palladium (Pd) sensing layer was conducted to simulate the interaction between the sensing layer and varying concentrations of hydrogen (H2 gas) Tn various embodiments, the sensing layer may include alternative sensing material(s), based on the specific target molecules to be detected, as different sensing materials react with different specific target molecules. For instance, a polymer or zinc oxide (ZnO) sensing layer may be used for humidity / water molecules sensing. During the response stress sweep ranging from -800 MPa to + 800 MPa (simulating stress induced in the sensing layer after physical or chemical reaction), both the first conventional micro diaphragm sensor and the second conventional micro diaphragm sensor exhibit a serious response diaphragm deformation of -1.3 pm. In contrast, the piezoelectric micro diaphragm sensor according to various embodiments demonstrates a remarkable deformation-resistant capability, maintaining a negligible response deformation of approximately 0 pm. The ultimate response frequency sensitivity curves are depicted in FIG. 9E, showcasing how the initial diaphragm deformation resulting from fabrication residual stress, as well as the response deformation caused by response stress, may significantly degrade the sensitivity of the first conventional micro diaphragm sensor and the second conventional micro diaphragm sensor. FIG. 9E shows a plot of frequency (in kilo-hertz or kHz) as a function of response stress in sensing layer (in megaPascals or MPa) illustrating the frequency response sensitivity characterization of the first conventional micro diaphragm sensor, the second conventional micro diaphragm sensor and the piezoelectric micro diaphragm sensor according to various embodiments.

[0055] The deformation proof structure of the proposed piezoelectric micro diaphragm sensor according to various embodiments may maximize the frequency shift with a sensitivityvalue of 62 Hz / MPa. In contrast, the sensitivity value of the first conventional micro diaphragm sensor is 16 Hz / MPa, and the sensitivity value of the second conventional micro diaphragm sensor is 26 Hz / MPa. In addition, it may be worth noting that the second conventional micro diaphragm sensor experiences singularity point issues stemming from both the impact of initial deformation status and the response deformation change. Thus, benefiting from the stress caused deformation-resistant structure, the proposed piezoelectric micro diaphragm sensor according to various embodiments may effectively avoid the singularity point issues and enhance the resonance frequency sensitivity. Various embodiments may avoid deformation deterioration on frequency response sensitivity caused by residual stress.

[0056] FIG. 9F shows a schematic illustrating a piezoelectric device in which the neutral axis of the multi-layer stack is aligned with the patterned first electrode according to various embodiments, as well as the piezoelectric device in which the neutral axis is misaligned by +20% or -20% to the patterned first electrode. FIG. 9G shows a plot of surface z position (in micrometers or pm) as a function of arc location (in micrometers or pm) illustrating diaphragm deformation with response stress sweep in sensing layer of a piezoelectric micro diaphragm sensor in which the neutral axis is misaligned by -20% to the patterned electrode. FIG 9H shows a plot of surface z position (in micrometers or pm) as a function of arc location (in micrometers or pm) illustrating diaphragm deformation with response stress sweep in sensing layer of a piezoelectric micro diaphragm sensor in which the neutral axis is aligned by to the patterned electrode according to various embodiments. FIG. 91 shows a plot of surface z position (in micrometers or pm) as a function of arc location (in micrometers or pm) illustrating diaphragm deformation with response stress sweep in sensing layer of a piezoelectric micro diaphragm sensor in which the neutral axis is misaligned by +20% to the patterned electrode. FIG. 9J shows a plot of frequency (in kilo-hertz or kHz) as a function of response stress in sensing layer (in mega-Pascals or MPa) illustrating the frequency response sensitivitycharacterization of a piezoelectric device in which the neutral axis of the multi-layer stack is aligned with the patterned first electrode according to various embodiments, as well as in which the neutral axis is misaligned by +20% or -20% to the patterned first electrode. When the neutral axis is misaligned by -20% to the patterned first electrode, half of the range may be invalid due to non-linearity. When the neutral axis is misaligned by +20% to the patterned first electrode, the sensitivity value is 38 Hz / MPa, and when the neutral axis is aligned to the patterned first electrode, the sensitivity value is 62 Hz / MPa. Various embodiments in which the neutral axis is aligned to the patterned first electrode may avoid deformation negative impact on the sensor frequency response.

[0057] FIG. 10A shows a piezoelectric device according to various embodiments. The device shown in FIG. 10A may be similar to the device shown in FIG. 7 The piezoelectric device may include a multi-layer stack 1004 on the substrate 1002. The multi-layer stack 1004 may include an elastic layer 1008, a piezoelectric stack 1006 on the elastic layer 1008, and a sensing layer 1010 on the piezoelectric stack 1006. The piezoelectric stack 1006 may include a patterned first electrode 1006b on the elastic layer 1008, a piezoelectric layer 1006a on the first electrode 1006b, and a second electrode 1006c on the piezoelectric layer 1006a. In other words, the piezoelectric stack 1006 and the sensing layer 1010 may be over the elastic layer 1008, and the sole patterned first electrode 1006b may be a bottom electrode between the piezoelectric layer 1006a of the piezoelectric stack 1006 and the elastic layer 1008.

[0058] FIG. 10B shows another piezoelectric device according to various embodiments. The piezoelectric device may include a multi-layer stack 1004 on the substrate 1002. The multilayer stack 1004 may include a piezoelectric stack 1006, an elastic layer 1008 on the piezoelectric stack 1006, and a sensing layer 1010 on the elastic layer 1008. The piezoelectric stack 1006 may include a second electrode 1006c on the substrate 1002, a piezoelectric layer 1006a on the second electrode 1006c and a patterned first electrode 1006b on the piezoelectric 1layer 1006a. In other words, the piezoelectric stack 1006 may be below the elastic layer 1008, and the sensing layer 1010 may be on the elastic layer 1008. The sole patterned first electrode 1006b may be a top electrode between the piezoelectric layer 1006a of the piezoelectric stack 1006 and the elastic layer 1008.

[0059] The devices shown in FIGS. 10A - B may need to be mounted with the top sensing layer 1010 exposed to the test environment.

[0060] FIG. 10C shows yet another piezoelectric device according to various embodiments. The piezoelectric device may include a multi-layer stack 1004 on the substrate 1002. The multilayer stack 1004 may include a sensing layer 1010 on the substrate 1002, an elastic layer 1008 on the sensing layer 1010, and a piezoelectric stack 1006 on the elastic layer 1008. The piezoelectric stack 1006 may include a patterned first electrode 1006b on the elastic layer 1008, a piezoelectric layer 1006a on the first electrode 1006b, and a second electrode 1006c on the piezoelectric layer 1006a. In other words, the piezoelectric stack 1006 may be on the elastic layer 1008, and the sensing layer 1010 may be below the elastic layer 1008. The sole patterned first electrode 1006b may be a bottom electrode between the piezoelectric layer 1006a of the piezoelectric stack 1006 and the elastic layer 1008.

[0061] FIG. 10D shows yet another piezoelectric device according to various embodiments. The piezoelectric device may include a multi-layer stack 1004 on the substrate 1002. The multilayer stack 1004 may include a sensing layer 1010 on the substrate 1002, a piezoelectric stack 1006 on the sensing layer 1010, and an elastic layer 1008 on the piezoelectric stack 1006. The piezoelectric stack 1006 may include a second electrode 1006c on the sensing layer 1010, a piezoelectric layer 1006a on the second electrode 1006c, and a patterned first electrode 1006b on the piezoelectric layer 1006a. In other words, the piezoelectric stack 1006 may be below the elastic layer 1008, and the sensing layer 1010 may be below the piezoelectric stack 1006. Thesole patterned first electrode 1006b may be a top electrode between the piezoelectric layer 1006a of the piezoelectric stack 1006 and the elastic layer 1008.

[0062] The devices shown in FIGS. 10C - D may be required to be mounted with back port exposed to the test environment.

[0063] All the devices shown in FIGS. 10A - D may have the sole patterned first electrode 1006b aligned to the neutral axis of the whole multi-layer stack 1004 (i.e., diaphragm stack).

[0064] FIG. 11 shows schematics of top view of piezoelectric devices showing the cavity boundaries and the sole patterned first electrode boundaries according to various embodiments: (a) circular patterned first electrode in cavity with circular circumference; (b) square / quadrilateral patterned first electrode in cavity with square / quadrilateral perimeter; (c) circular patterned first electrode in cavity with square / quadrilateral perimeter; (d) hexagonal patterned first electrode in cavity with hexagonal perimeter; (e) octagonal patterned first electrode in cavity with octagonal perimeter; and (f) square / quadrilateral patterned first electrode in cavity with octagonal perimeter. The shapes of the cavity and / or the sole patterned first electrode may for instance be circular, quadrilateral, hexagonal, and / or octagonal, as long as the location relationship remains concentric. Symmetrical distribution may be required to balance the moment of residual stress to form the initial deformation resistant structure. FIG. 12 shows a schematic of (a) a piezoelectric device with a backport opening according to various embodiments; and (b) a piezoelectric device with a backside cavity according to various embodiments. The backside processes may be varied to form the backport opening or the backside cavity.

Claims

Claims1. A piezoelectric device comprising: a substrate at least partially defining a cavity; a multi-layer stack over the cavity, the multi-layer comprising: a piezoelectric stack comprising: a piezoelectric layer having a first surface and a second surface opposite the first surface, the piezoelectric layer having a lateral dimension greater than a lateral dimension of the cavity; a first electrode in contact with the first surface of the piezoelectric layer, the first electrode having a lateral dimension smaller than the lateral dimension of the cavity; and a second electrode in contact with the second surface of the piezoelectric layer, the second electrode having a lateral dimension greater than the lateral dimension of the cavity; and an elastic layer in contact with the piezoelectric stack such that the first electrode is between the piezoelectric layer and the elastic layer; wherein the piezoelectric device is configured such that the first electrode extends along a neutral axis of the multi-layer stack.

2. The piezoelectric device according to claim 1, wherein a thickness of each layer of the multi-layer stack is configured so that the first electrode extends along the neutral axis of the multi-layer stack and a target resonant frequency is achieved by the multi-layer stack.

3. The piezoelectric device according to claim 2,wherein the target resonant frequency is also dependent on dimensions of the cavity and elastic property of each layer.

4. The piezoelectric device according to claim 1, wherein the muti-layer stack further includes a sensing layer over the substrate.

5. The piezoelectric device according to claim 4, wherein the piezoelectric stack is over the elastic layer; and wherein the sensing layer is over the piezoelectric stack.

6. The piezoelectric device according to claim 4, wherein the elastic layer is over the piezoelectric stack; and wherein the sensing layer is over the elastic layer.

7. The piezoelectric device according to claim 4, wherein the elastic layer is over the sensing layer; and wherein the piezoelectric stack is over the elastic layer.

8. The piezoelectric device according to claim 4, wherein the piezoelectric stack is over the sensing layer; and wherein the elastic layer is over the piezoelectric stack.

9. The piezoelectric device according to claim 4, wherein the sensing layer comprises palladium (Pd), polyimide (PI), zinc oxide (ZnO), Molecularly Imprinted Polymers (MIPs), carbon nanotubes, graphene or quantum dots.

10. The piezoelectric device according to claim 1, wherein a lateral boundary of the first electrode and a lateral boundary of the cavity form a concentric arrangement.

11. The piezoelectric device according to claim 10, wherein the lateral boundary of the first electrode has a shape selected from a group consisting of a circle, a square, a hexagon and an octagon.

12. The piezoelectric device according to claim 10, wherein the lateral boundary of the cavity has a shape selected from a group consisting of a circle, a square, a hexagon and an octagon.

13. The piezoelectric device according to claim 1, wherein a surface area of a planar surface of the first electrode is any percentage selected from a range from 5% to 95% of a traverse cross-section of the cavity.

14. The piezoelectric device according to claim 1, wherein the second electrode extends over an entire second surface of the piezoelectric layer.

15. The piezoelectric device according to claim 1, wherein the piezoelectric layer comprises scandium aluminum nitride (ScAlN), aluminum nitride (AIN), lead zirconate titanate (PZT), quartz or polyvinylidene fluoride (PVDF).

16. The piezoelectric device according to claim 1,wherein the first electrode or the second electrode includes molybdenum (Mo), platinum (Pt), aluminum (Al), gold (Au), silver (Ag) or titanium (Ti).

17. The piezoelectric device according to claim 1, wherein the elastic layer includes epitaxial silicon, silicon oxide, silicon oxynitride, silicon nitride, scandium aluminum nitride (ScAlN), aluminum nitride (AIN), lead zirconate titanate (PZT), polyvinylidene fluoride (PVDF) or polymer.

18. The piezoelectric device according to claim 1, wherein the cavity extends from a first surface of the substrate to a second surface of the substrate to form a backport opening.

19. The piezoelectric device according to claim 1, wherein the cavity is an enclosed cavity defined by the substrate and the multilayer stack.

20. A method of forming a piezoelectric device, the method comprising: forming a multi-layer stack over a cavity at least partially defined by the substrate, the multi-layer comprising: a piezoelectric stack comprising: a piezoelectric layer having a first surface and a second surface opposite the first surface, the piezoelectric layer having a lateral dimension greater than a lateral dimension of the cavity;a first electrode in contact with the first surface of the piezoelectric layer, the first electrode having a lateral dimension smaller than the lateral dimension of the cavity; and a second electrode in contact with the second surface of the piezoelectric layer, the second electrode having a lateral dimension greater than the lateral dimension of the cavity; and an elastic layer in contact with the piezoelectric stack such that the first electrode is between the piezoelectric layer and the elastic layer; wherein the piezoelectric device is configured such that the first electrode extends along a neutral axis of the multi-layer stack.

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