Pmut structure and method of making the same

Abstract

The application discloses a PMUT structure and a preparation method thereof. The PMUT structure comprises a substrate, a barrier layer, a support layer, a lower electrode, a piezoelectric layer and an upper electrode which are sequentially stacked, a cavity penetrating through the substrate is arranged on the substrate, and the barrier layer, the support layer, the lower electrode, the piezoelectric layer and the upper electrode jointly form a vibrating diaphragm suspended above the cavity. A plurality of grooves are arranged on the vibrating diaphragm and surround the outer periphery of the upper electrode, the grooves penetrate from the piezoelectric layer to the support layer, and a suspended beam part is formed between two adjacent grooves. The PMUT structure and the preparation method thereof provided by the application form a suspended beam structure through the grooves, release the residual stress in the manufacturing process, effectively reduce the rigidity and improve the application reliability compared with the existing support plate structure, and improve the vibrating area and the vibrating displacement compared with the existing fixed boundary structure.

Description

Technical Field

[0001] This invention belongs to the field of piezoelectric device technology, specifically relating to a PMUT structure and its preparation method. Background Technology

[0002] Ultrasonic transducers play a crucial role in the conversion between acoustic and electrical signals. This conversion technology has been widely applied in various functions such as acoustic transmission, ultrasonic ranging, flow measurement, and acoustic imaging, playing a particularly vital role in medical imaging, industrial inspection, and consumer electronics. With continuous technological advancements, especially driven by trends towards intelligent and miniaturized designs, the design and functionality of ultrasonic transducers face increasingly severe challenges.

[0003] In recent years, with the development of microelectromechanical systems (MEMS) technology, a new type of MEMS ultrasonic transducer—PMUT (Piezoelectric Micromachined Ultrasonic Transducers)—has attracted widespread attention from research and industry. PMUTs are considered an ideal replacement for traditional bulk piezoelectric transducers due to their small size, low power consumption, and high integration performance. Despite the advantages offered by PMUTs, traditional PMUT designs still have some limitations, particularly in terms of effective vibrating area and vibration displacement. These limitations directly affect the transducer's core performance indicators, such as transmitted sound pressure level and receiver sensitivity, which is particularly problematic for applications requiring high sensitivity and high resolution.

[0004] Therefore, it is necessary to provide a new solution to the above-mentioned technical problems. Summary of the Invention

[0005] The purpose of this invention is to provide a PMUT structure and its preparation method, which has a large vibration area and vibration displacement, and can improve the output sound pressure and receiving sensitivity.

[0006] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0007] In a first aspect, the present invention provides a PMUT structure, comprising a substrate, a barrier layer, a support layer, a lower electrode, a piezoelectric layer, and an upper electrode stacked sequentially. A cavity penetrating the substrate is provided on the substrate. The barrier layer, support layer, lower electrode, piezoelectric layer, and upper electrode together constitute a vibrating diaphragm suspended above the cavity. The vibrating diaphragm is provided with a plurality of grooves surrounding the outer periphery of the upper electrode. The grooves extend from the piezoelectric layer to the support layer, and a cantilever portion is formed between two adjacent grooves.

[0008] In one or more embodiments, the projections of the groove and the cavity in the direction of the cavity axis at least partially overlap.

[0009] In one or more embodiments, the groove is arc-shaped or polygonal.

[0010] In one or more embodiments, the depth of the trench is equal to the sum of the thicknesses of the support layer, the lower electrode, and the piezoelectric layer.

[0011] In one or more embodiments, a boundary portion is formed on the vibrating diaphragm surrounding the periphery of the plurality of grooves, the boundary portion being formed by the edges of the piezoelectric layer and the lower electrode.

[0012] In one or more embodiments, the upper electrode is circular, the cavity is cylindrical, and the radius of the upper electrode is 60 to 80% of the radius of the cavity.

[0013] In one or more embodiments, the barrier layer is made of silicon dioxide, silicon nitride, silicon, or aluminum nitride.

[0014] Secondly, the present invention provides a method for preparing the PMUT structure as described above, comprising:

[0015] An SOI silicon wafer is provided, with the bottom silicon layer of the SOI silicon wafer as the substrate, the buried oxide layer as the barrier layer, and the top silicon layer as the support layer; a lower electrode and a piezoelectric layer are sequentially formed on the top silicon layer of the SOI silicon wafer; the piezoelectric layer, the lower electrode, and the top silicon layer of the SOI silicon wafer are etched to form multiple trenches, and the unetched portions between the trenches form cantilever beams; an upper electrode located between the multiple trenches is formed on the piezoelectric layer; the bottom silicon layer of the SOI silicon wafer is etched to form a cavity penetrating the bottom silicon layer.

[0016] In one or more embodiments, the piezoelectric layer and the lower electrode are etched by dry etching, and the top silicon and bottom silicon of the SOI wafer are etched by deep reactive ion etching.

[0017] In one or more embodiments, the lower electrode, piezoelectric layer, and upper electrode are formed by magnetron sputtering.

[0018] Compared with the prior art, the PMUT structure and its preparation method provided by the present invention have the following advantages:

[0019] (1) By forming a cantilever structure through grooves, the residual stress in the manufacturing process is released. Compared with the existing support plate structure, the stiffness is effectively reduced while the application reliability is improved. Compared with the existing fixed boundary structure, the vibration area and vibration displacement are increased.

[0020] (2) The suspended membrane structure changes the vibration mode of PMUT. Compared with the traditional structure of the same volume, the vibration area is larger, the vibration displacement is higher, the mechanical properties are improved, and the receiving sensitivity is higher.

[0021] (3) The trench and back cavity of the present invention are not through structures. A barrier layer is provided between them. It can be used as an etching stop layer to control the deep silicon etching time, or as an air barrier layer to block the air. Compared with through-hole structures, it not only retains the superior mechanical vibration characteristics, but also avoids sound leakage, reduces sound loss, and improves acoustic performance.

[0022] (4) It does not require changing the device volume. The device resonant frequency can be controlled by adjusting the number and size of the grooves, thus overcoming the problems of high frequency, low output sound pressure and insufficient sensitivity caused by small-sized vibrating films in the prior art. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a perspective view of the PMUT structure in one embodiment of the present invention;

[0025] Figure 2 This is a cross-sectional view of the PMUT structure in one embodiment of the present invention;

[0026] Figure 3 This is a cross-sectional view of the PMUT structure in one embodiment of the present invention;

[0027] Figure 4 This is a flowchart of a method for preparing a PMUT structure according to an embodiment of the present invention;

[0028] Figure 5 This is a comparison chart of the output performance test of the PMUT structure in one embodiment of the present invention;

[0029] Figure 6 This is a comparison chart of the receiving performance of the PMUT structure in one embodiment of the present invention.

[0030] Explanation of key figure labels:

[0031] 1-Substrate, 11-Cavity, 2-Vibrating membrane, 21-Barrier layer, 22-Support layer, 23-Lower electrode, 24-Piezoelectric layer, 25-Upper electrode, 26-Trench, 27-Cantilever section, 28-Boundary section. Detailed Implementation

[0032] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0033] Unless otherwise expressly stated, throughout the specification and claims, the term "comprising" or its variations such as "including" or "comprises" shall be understood to include the stated elements or components without excluding other elements or other components.

[0034] Against the backdrop of modern technological development, the application of ultrasonic technology is becoming increasingly widespread, especially in fields such as high-resolution imaging and precise distance measurement, which places increasingly higher demands on the performance of ultrasonic transducers. Existing piezoelectric microelectromechanical systems (PMUTs) have made significant progress in terms of size, power consumption, and integration. However, based on in-depth analysis of existing technologies, the inventors discovered that traditional PMUTs have limitations in effective vibration area and vibration displacement, leading to deficiencies in sound pressure output and receiving sensitivity.

[0035] The design concept of this invention is to change the structural design of the traditional PMUT diaphragm, thereby overcoming the aforementioned limitations. By designing a novel structure on the diaphragm, the stress state and vibration modes of the diaphragm can be altered, which directly affects the performance of the diaphragm. The technical implementation of this invention focuses on adjusting the mechanical structure of the diaphragm and optimizing its mode shapes to achieve a higher effective vibration area and vibration displacement, thereby enhancing the output sound pressure and improving the receiving sensitivity.

[0036] Please refer to Figure 1 , Figure 2 and Figure 3 As shown, the PMUT structure in one embodiment of the present invention includes a substrate 1, a barrier layer 21, a support layer 22, a lower electrode 23, a piezoelectric layer 24, and an upper electrode 25 stacked sequentially. A cavity 11 penetrating the substrate 1 is provided on the substrate 1. The barrier layer 21, support layer 22, lower electrode 23, piezoelectric layer 24, and upper electrode 25 together constitute a vibrating diaphragm 2 suspended above the cavity 11. The vibrating diaphragm 2 has multiple grooves 26 surrounding the outer periphery of the upper electrode 25. The grooves 26 extend from the piezoelectric layer 24 to the support layer 22, and a cantilever portion 27 is formed between adjacent grooves 26.

[0037] The design of the grooves 26 and the cantilever portion 27 on the diaphragm 2 is the core of this invention. The presence of the cantilever portion 27, by providing additional mechanical flexibility, can release residual stress generated during the manufacturing process. The presence of the grooves 26 reduces the overall stiffness of the diaphragm 2, allowing the diaphragm 2 to produce a larger displacement under the same excitation. This design increases the effective vibration area of ​​the diaphragm 2, thereby improving the output sound pressure level. At the same time, the arrangement of the cantilever portion 27 changes the stress condition of the diaphragm 2, allowing it to move more freely during vibration, further enhancing the vibration displacement.

[0038] During the fabrication of PMUTs, differences in the coefficients of thermal expansion of materials can lead to the generation of residual stress. This invention effectively releases these residual stresses by etching to form the cantilever portion 27, reducing material fatigue or damage caused by stress concentration and improving the reliability and service life of the PMUT.

[0039] Compared to traditional through-hole PMUT structures, the PMUT structure provided by this invention features a barrier layer 21, which prevents sound leakage and reduces ultrasonic energy loss, thus resulting in better acoustic performance. In this invention, the barrier layer 21 is not etched through, meaning it effectively prevents coupling between air on both sides. This design reduces energy loss during sound wave transmission, improves acoustic performance, and provides superior mechanical vibration characteristics, leading to higher output sound pressure and receiving sensitivity. Simultaneously, the barrier layer 21 also serves as an etching stop layer, providing more precise control over the fabrication process.

[0040] Furthermore, the PMUT structure proposed in this invention allows for control of the device's resonant frequency by adjusting the number and size of the grooves 26. The resonant frequency of existing PMUTs is significantly affected by the radius of the cavity 11; a larger radius results in a lower frequency. However, for the PMUT structure of this invention, the resonant frequency can be lowered by increasing the area of ​​the grooves 26 for low-frequency applications. This results in a smaller device size compared to traditional structures at the same frequency, and also provides a higher output sensitivity. When used in arrays, the fill factor can be increased. For high-frequency applications, the area of ​​the grooves 26 can be reduced. This structure overcomes the problems of high frequency, low output sound pressure, and insufficient sensitivity caused by small-sized vibrating films in existing technologies.

[0041] The upper electrode 25 and lower electrode 23 in the PMUT structure can be connected to a circuit, which can either emit pulse waves to excite the vibrating diaphragm 2 to vibrate and emit ultrasonic waves through the cavity 11, or receive ultrasonic echo signals through the cavity 11 and convert them into voltage signals through the vibration of the vibrating diaphragm 2.

[0042] In one exemplary embodiment, the projections of the groove 26 and the cavity 11 of the PMUT structure onto the axis of the cavity 11 at least partially overlap. The diaphragm 2 is a key component of the PMUT used to convert electrical signals and acoustic signals. The design of the groove 26 allows the diaphragm 2 to generate a larger displacement when excited by an electrical signal, thereby emitting a stronger sound wave. When the projections of the groove 26 and the cavity 11 partially overlap, the portion of the diaphragm 2 above the cavity 11 can vibrate more freely because this portion of the diaphragm 2 is not directly supported by the substrate 1, thereby increasing the vibration displacement and output sound pressure.

[0043] The presence of cavity 11 reduces acoustic wave coupling between diaphragm 2 and substrate 1, thereby reducing energy loss during sound wave transmission and improving transmission efficiency. The overlap between the groove 26 and the projected portion of cavity 11 further optimizes this coupling effect, making the sound field above diaphragm 2 more uniform, thus improving the transmission and reception efficiency of sound waves.

[0044] Specifically, the groove 26 is arc-shaped or polygonal (such as rectangular, square, parallelogram, etc.). The arc-shaped or polygonal groove 26 can effectively change the vibration mode of the PMUT diaphragm 2. Different shapes will affect the stress distribution and the position of the vibration nodes on the diaphragm 2, so the vibration mode can be designed in a targeted manner to optimize specific performance, such as improving sensitivity or adjusting the frequency response.

[0045] The arc-shaped groove 26 helps to create a more uniform energy distribution on the diaphragm 2 because the arc shape reduces stress concentration at sharp edges and avoids energy concentration in unnecessary areas. The polygonal design can concentrate energy in certain directions, improving acoustic wave output in specific directions. At the microscale, the smooth edges of the arc shape can reduce stress concentration caused by material fatigue or processing defects, contributing to improved device reliability and durability.

[0046] In one exemplary embodiment, the depth of the trench 26 is equal to the sum of the thicknesses of the support layer 22, the lower electrode 23, and the piezoelectric layer 24. That is, the trench 26 completely penetrates the support layer 22 from the piezoelectric layer 24, resulting in a larger trench 26 depth. The larger trench 26 depth allows the vibrating diaphragm 2 to have a larger vibration space, thereby achieving a larger vibration displacement without increasing the driving voltage.

[0047] In one exemplary embodiment, a boundary portion 28 is formed on the diaphragm 2 surrounding the periphery of the plurality of grooves 26, the boundary portion 28 being formed by the edges of the piezoelectric layer 24 and the lower electrode 23. The presence of the boundary portion 28 helps to localize vibrational energy on the diaphragm 2. This design, by defining the vibration region through the boundary portion 28 formed by the edges of the piezoelectric layer 24 and the lower electrode 23, helps to concentrate energy in the desired vibration region, thereby improving the efficiency of the transducer.

[0048] The boundary portion 28 provides structural integrity and mechanical support for the diaphragm 2. The structure formed by the edges of the piezoelectric material and the lower electrode 23 provides sufficient strength and stability to protect the diaphragm 2 from excessive mechanical stress, especially at high power output.

[0049] The boundary portion 28 also serves to guide sound waves, directing acoustic energy towards the center of the vibrating diaphragm 2. This structural design facilitates sound wave focusing, increasing the output intensity of ultrasound in a specific direction, making it suitable for applications requiring directional sound wave emission. When an electrical signal is applied to the piezoelectric layer 24, the vibration energy is confined to a specific area due to the presence of the boundary portion 28, which increases the amplitude of the vibration displacement, thereby increasing the generated sound pressure.

[0050] In one exemplary embodiment, the upper electrode 25 is circular, and the cavity 11 is cylindrical, with the radius of the upper electrode 25 being 60-80% of the radius of the cavity 11. The design of the circular upper electrode 25 and the cylindrical cavity 11 simplifies the analysis and control of vibration modes. The circular structure readily generates axisymmetric vibration modes, which is beneficial for predicting and adjusting resonant frequencies. Selecting the radius of the upper electrode 25 to be 60% to 80% of the radius of the cavity 11 can optimize the vibration characteristics of the diaphragm 2, such as improving the vibration efficiency of specific modes or adjusting the modes to meet specific application requirements.

[0051] The sound waves generated by the circular upper electrode 25 when excited can propagate more uniformly outward due to the presence of the cylindrical cavity 11. This design helps to improve the transmission efficiency and reception sensitivity of the sound waves. The ratio between the radius of the upper electrode 25 and the radius of the cavity 11 determines the energy distribution of the sound waves within the cavity 11, thereby affecting the sound pressure level and sensitivity of the transducer.

[0052] In other embodiments, the upper electrode 25 and the cavity 11 can also be designed in other shapes. For example, the upper electrode 25 can be designed as a square, a rectangle, etc., and the cavity 11 can be designed as a horn shape.

[0053] In one exemplary embodiment, the barrier layer 21 is made of silicon dioxide, silicon nitride, silicon, or aluminum nitride. These materials exhibit good thermal stability over a wide temperature range. During PMUT operation, especially at high power output, heat may be generated. The material of the barrier layer 21 needs to withstand these temperature variations, maintain its structural and functional stability, and not negatively impact PMUT performance.

[0054] The material of the barrier layer 21 needs to have good mechanical strength and toughness to withstand the stress that may occur during vibration. Silica, silicon nitride, silicon, and aluminum nitride all have high hardness and tensile strength, which can protect the film from damage and extend the service life of the equipment.

[0055] Specifically, the upper electrode 25 can be made of gold, the piezoelectric layer 24 can be made of scandium-doped aluminum nitride, the lower electrode 23 can be made of molybdenum, and the support layer 22 and the substrate 1 can be made of silicon.

[0056] Please refer to Figure 4 As shown, the present invention also provides a method for preparing the aforementioned PMUT structure, which specifically includes the following steps:

[0057] S401: Provides SOI silicon wafers, with the bottom silicon layer as the substrate, the buried oxide layer as the barrier layer, and the top silicon layer as the support layer.

[0058] It should be noted that SOI (Silicon-On-Insulator) wafers are three-layer silicon wafers, consisting of a top silicon layer, a buried oxide layer, and a bottom silicon layer. These wafers are widely used in microelectronics and MEMS manufacturing because they offer excellent electrical insulation and mechanical stability.

[0059] In this invention, the bottom silicon layer serves as a substrate to provide structural support, the buried oxide layer acts as an electrically insulating barrier layer, and the top silicon layer serves as a support layer for the vibrating diaphragm. This structural design facilitates subsequent processing steps and ensures the basic functional requirements of the PMUT.

[0060] S402: A lower electrode and a piezoelectric layer are sequentially formed on the top silicon layer of an SOI silicon wafer.

[0061] The bottom electrode is typically made of a metallic material, such as aluminum or molybdenum, to provide good electrical conductivity; the piezoelectric layer is made of a piezoelectric material, such as PZT (lead zirconium titanate) or aluminum nitride (AlN), to convert electrical energy into mechanical energy. The bottom electrode and the piezoelectric layer can be formed using magnetron sputtering.

[0062] S403: Etching the piezoelectric layer, the lower electrode, and the top silicon layer of the SOI silicon wafer to form multiple trenches. The unetched portions between the trenches form cantilever beams.

[0063] Multiple trenches are formed on the piezoelectric layer, the lower electrode, and the top silicon layer using etching techniques. The unetched portions between the trenches naturally form cantilever beams. The piezoelectric layer and the lower electrode can be etched using dry etching, while the top silicon layer can be etched using deep reactive ion etching (DRIE).

[0064] S404: An upper electrode is formed on the piezoelectric layer between multiple trenches.

[0065] The upper and lower electrodes together hold the piezoelectric layer, forming an electric field to drive or induce the vibration of the piezoelectric layer.

[0066] S405: Etches the bottom silicon layer of the SOI silicon wafer to form a cavity that penetrates the bottom silicon layer.

[0067] Please refer to Figure 5 The figure shown is a comparison of the output performance of a PMUT structure (dual-stiffness PMUT with SiO2 film) prepared in one embodiment of the present invention, a PMUT structure without a barrier layer (dual-stiffness PMUT without SiO2 film), and a traditional PMUT structure.

[0068] Please refer to Figure 6 The figure shown is a comparison of the receiving performance of a PMUT structure (dual-stiffness PMUT with SiO2 film) prepared in one embodiment of the present invention, a PMUT structure without a barrier layer (dual-stiffness PMUT without SiO2 film), and a traditional PMUT structure.

[0069] In summary, the PMUT structure and its preparation method provided by this invention have the following advantages:

[0070] (1) By forming a cantilever structure through grooves, the residual stress in the manufacturing process is released. Compared with the existing support plate structure, the stiffness is effectively reduced while the application reliability is improved. Compared with the existing fixed boundary structure, the vibration area and vibration displacement are increased.

[0071] (2) The suspended membrane structure changes the vibration mode of PMUT. Compared with the traditional structure of the same volume, the vibration area is larger, the vibration displacement is higher, the mechanical properties are improved, and the receiving sensitivity is higher.

[0072] (3) The trench and back cavity of the present invention are not through structures. A barrier layer is provided between them. It can be used as an etching stop layer to control the deep silicon etching time, or as an air barrier layer to block the air. Compared with through-hole structures, it not only retains the superior mechanical vibration characteristics, but also avoids sound leakage, reduces sound loss, and improves acoustic performance.

[0073] (4) It does not require changing the device volume. The device resonant frequency can be controlled by adjusting the number and size of the grooves, thus overcoming the problems of high frequency, low output sound pressure and insufficient sensitivity caused by small-sized vibrating films in the prior art.

[0074] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0075] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A PMUT structure, characterized in that, The device includes a substrate, a barrier layer, a support layer, a lower electrode, a piezoelectric layer, and an upper electrode stacked sequentially. A cavity is provided on the substrate, and the barrier layer, support layer, lower electrode, piezoelectric layer, and upper electrode together constitute a vibrating diaphragm suspended above the cavity. The vibrating diaphragm has multiple grooves surrounding the outer periphery of the upper electrode. The projections of the grooves and the cavity in the axial direction of the cavity at least partially overlap. The grooves extend from the piezoelectric layer to the support layer, and the depth of the grooves is equal to the sum of the thicknesses of the support layer, the lower electrode, and the piezoelectric layer. A cantilever section is formed between two adjacent grooves. The barrier layer is provided between the groove and the cavity. The barrier layer is not penetrated by the groove, so that the groove and the cavity are not a transparent structure, and the barrier layer blocks the air.

2. The PMUT structure according to claim 1, characterized in that, The groove is arc-shaped or polygonal.

3. The PMUT structure according to claim 1, characterized in that, The vibrating diaphragm has a boundary portion formed around the periphery of the plurality of grooves, the boundary portion being formed by the edge of the piezoelectric layer and the lower electrode.

4. The PMUT structure according to claim 1, characterized in that, The upper electrode is circular, the cavity is cylindrical, and the radius of the upper electrode is 60-80% of the radius of the cavity.

5. The PMUT structure according to claim 1, characterized in that, The barrier layer is made of materials including silicon dioxide, silicon nitride, silicon, or aluminum nitride.

6. A method for preparing a PMUT structure as described in any one of claims 1 to 5, characterized in that, include: SOI silicon wafers are provided, with the bottom silicon layer as the substrate, the buried oxide layer as the barrier layer, and the top silicon layer as the support layer. A lower electrode and a piezoelectric layer are sequentially formed on the top silicon layer of an SOI silicon wafer; The piezoelectric layer, the lower electrode, and the top silicon of the SOI silicon wafer are etched to form multiple trenches, and the unetched portions between the trenches form cantilever beams. An upper electrode is formed on the piezoelectric layer between multiple trenches; The bottom silicon layer of the SOI silicon wafer is etched to form a cavity that penetrates the bottom silicon layer.

7. The method for preparing the PMUT structure according to claim 6, characterized in that, The piezoelectric layer and the lower electrode are etched using dry etching, while the top and bottom silicon layers of the SOI silicon wafer are etched using deep reactive ion etching.

8. The method for preparing the PMUT structure according to claim 6, characterized in that, The lower electrode, piezoelectric layer, and upper electrode are formed using magnetron sputtering.

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