TWI726800B - Piezoelectric micromachined ultrasonic transducer and method of fabricating the same - Google Patents

Abstract

A PMUT includes a substrate, a membrane, and a sacrificial layer. The substrate has a cavity penetrating the substrate. The membrane is disposed over the cavity and includes a first piezoelectric layer, a bottom electrode, a top electrode, and a second piezoelectric layer. The first piezoelectric layer is disposed over the cavity and includes an anchor portion, where the anchor portion of the first piezoelectric layer is in direct contact with the substrate. The top and bottom electrodes are disposed over the first piezoelectric layer. The second piezoelectric layer is disposed between the bottom electrode and the top electrode. The sacrificial layer is disposed between the substrate and the first piezoelectric layer, and a vertical projection of the sacrificial layer does not overlap a vertical projection of portions of the membrane disposed over the cavity.

Description

Piezoelectric micromechanical ultrasonic transducer and manufacturing method thereof

This disclosure relates to the technical field of a Micro Electro Mechanical System (MEMS), and particularly relates to a piezoelectric micro-mechanical ultrasonic transducer (PMUT) and a manufacturing method thereof.

In the past few decades, Micro Machined Transducers (MUTs) have been extensively studied and have become an important component of various consumer electronics products, such as fingerprint sensors and proximity (proximity) sensing. Component parts of the device and gesture sensor. Generally speaking, MUTs can be divided into two categories, such as capacitive micromachined ultrasonic transducers (CMUTs) and piezoelectric micromachined ultrasonic transducers (PMUTs). For a typical piezoelectric micromechanical ultrasonic transducer, the piezoelectric micromechanical ultrasonic transducer includes a film layer composed of elastic materials, electrodes, and piezoelectric materials. This film layer is set on the acoustic wave resonator. On the cavity to improve the acoustic performance of the piezoelectric micromachined ultrasonic transducer. During the operation of the piezoelectric micro-machined ultrasonic transducer, the ultrasonic waves generated by the vibration of the film will be transmitted from the piezoelectric micro-machined ultrasonic transducer to the target, and then the piezoelectric micro-machined ultrasonic transducer can Detect the reflected sound waves generated by ultrasonic waves hitting the target.

Generally, the piezoelectric micromachined ultrasonic transducer will operate at the bending resonance frequency of the film. The bending resonance frequency can be determined by selecting the correct material, the size and thickness of the film. therefore, A good match of the resonance frequency of a single piezoelectric micromachined ultrasonic transducer is a necessary condition for normal operation. For the conventional PMUT, in order to control the bending resonance frequency of the film layer, the film layer disposed on the cavity usually includes an elastic layer with required elasticity, and the elastic layer is disposed at the bottom of the film layer, and the film layer The electrodes and piezoelectric layer are usually placed on the elastic layer. However, since the quality of the piezoelectric layer in the film layer is very sensitive to the surface texture of the underlying layer (such as the elastic layer), there are many restrictions on the material of the elastic layer and its manufacturing process. Therefore, even if the film layer can be made into a structure including an elastic layer, it is still difficult to freely control the resonance frequency of the PMUT.

Therefore, it is necessary to provide an improved PMUT and a manufacturing method thereof to solve the problems faced in the conventional PMUT.

In view of this, it is necessary to provide an improved PMUT and a manufacturing method thereof, which can arbitrarily regulate the elasticity of the film layer of the PMUT.

According to an embodiment of the present disclosure, a PMUT includes a substrate, a film layer, and a sacrificial layer. The substrate has a cavity penetrating the substrate. The film layer is disposed above the cavity and includes a first piezoelectric layer, a bottom electrode, a top electrode, and a second piezoelectric layer. The first piezoelectric layer is disposed above the cavity and includes an anchor portion, wherein the anchor portion of the first piezoelectric layer is in direct contact with the substrate. The bottom electrode is arranged above the first piezoelectric layer. The top electrode is arranged above the bottom electrode. The second piezoelectric layer is provided between the bottom electrode and the top electrode. The sacrificial layer is arranged between the substrate and the first piezoelectric layer, wherein the vertical projection of the sacrificial layer does not overlap with the vertical projection of the film layer directly above the cavity.

According to another embodiment of the present disclosure, a method of manufacturing PMUT is disclosed, including the following steps. First, a substrate is provided, and a sacrificial layer is formed on the substrate, wherein the sacrificial layer includes at least one hole exposing the substrate. Then, a piezoelectric layer is formed in the at least one hole and on the sacrificial layer. After that, a cavity penetrating the substrate is formed to expose a portion of the sacrificial layer. After that, the piezoelectric layer is used as an etching stop structure, To remove the exposed part of the sacrificial layer from the cavity.

According to the embodiment of the present disclosure, the elastic layer will not be arranged between the substrate and the film layer, but will be arranged on the upper part of the film layer. Therefore, the crystallinity of the piezoelectric layer in the film layer will no longer be affected by the elastic layer, and thus the elasticity of the film layer in the PMUT can be freely controlled.

100: Piezoelectric micromachined ultrasonic transducer (PMUT)

102: substrate

102A: Top surface

102B: bottom surface

106: Membrane

114: first contact pad

116: second contact pad

120: Cavity

120e: Edge

124: Sacrifice Layer

126: Hole

126S: side wall

132: The first piezoelectric layer

134: bottom electrode

136: second piezoelectric layer

138: Top electrode

140: passivation layer

140A: Top surface

140S: side wall

142: Elastic layer

200: method

202: Step

204: Step

206: Step

208: Step

210: Step

D: distance

O: opening

θ 1: Angle

θ 2: Angle

In order to make the following easier to understand, the drawings and detailed text descriptions can be referred to when reading this disclosure. Through the specific embodiments in this text and with reference to the corresponding drawings, the specific embodiments of the present disclosure are explained in detail, and the principle of the specific embodiments of the present disclosure is explained. In addition, for the sake of clarity, the features in the drawings may not be drawn according to the actual scale, so the size of some features in some drawings may be deliberately enlarged or reduced.

FIG. 1 is a schematic top view of a piezoelectric micromachined ultrasonic transducer (PMUT) according to an embodiment of the disclosure.

Fig. 2 is a schematic cross-sectional view along the tangent line A-A' of Fig. 1 according to an embodiment of the present disclosure.

FIG. 3 is a schematic cross-sectional view after a sacrificial layer is formed on the substrate according to an embodiment of the disclosure.

Figure 4 is a schematic cross-sectional view after a film layer is formed on the substrate.

Figure 5 is a schematic cross-sectional view after the elastic layer is formed on the film layer.

FIG. 6 is a schematic cross-sectional view after a cavity penetrating the substrate is formed according to an embodiment of the disclosure.

FIG. 7 is a flowchart of a method of making PMUT according to an embodiment of the disclosure.

The present disclosure provides several different embodiments, which can be used to implement different features of the present disclosure. To simplify the description, this disclosure also describes examples of specific components and settings. The purpose of providing these examples is only for illustration, and not for any limitation. For example, the following description of "the first feature is formed on or above the second feature" can mean "the first feature is in direct contact with the second feature", or it can mean "the first feature is in direct contact with the second feature". There are other features among the features", so that the first feature and the second feature are not in direct contact. In addition, various embodiments in the present disclosure may use repeated reference symbols and/or text annotations. These repeated reference symbols and notes are used to make the description more concise and clear, rather than to indicate the relevance between different embodiments and/or configurations.

In addition, regarding the space-related narrative vocabulary mentioned in this disclosure, for example: "below", "low", "below", "above", "above", "below", "top" "," "bottom" and similar words, for ease of description, their usage is to describe the relative relationship between one element or feature and another (or more) elements or features in the drawing. In addition to the swing outward shown in the diagram, these spatially related words are also used to describe the possible swing directions of the semiconductor device during use and operation. As the swing direction of the semiconductor device is different (rotated by 90 degrees or other orientations), the space-related narrative used to describe its swing direction should also be interpreted in a similar way.

Although this disclosure uses terms such as first, second, and third to describe various elements, components, regions, layers, and/or sections, it should be understood that these elements, components, regions, layers, And/or blocks should not be restricted by these terms. These terms are only used to distinguish an element, component, region, layer, and/or block from another element, component, region, layer, and/or block, and they do not mean or represent the element. Any previous ordinal number does not represent the arrangement order of a component and another component, or the order in the manufacturing method. Therefore, without departing from the scope of the specific embodiments of the present disclosure, the first element, component, region, layer, or block discussed below can also be referred to as the second element, component, region, layer, or block It.

The term "about" or "substantially" mentioned in this disclosure usually means within 20% of a given value or range, preferably within 10%, and more preferably within 5%, or Within 3%, or within 2%, or within 1%, or within 0.5%. It should be noted that the numerical ranges, quantities, values, and percentages provided in the description are approximate quantities, that is, without a specific description of "about" or "substantially", the "about" or "about" can still be implied. The meaning of "substantially".

The specific steps or block levels in the steps/processes described below are examples. According to design preferences, specific steps or block levels in the steps/processes described below can be rearranged. Furthermore, some blocks can be merged or deleted. In addition, the following method request items list the corresponding elements of the above different blocks in a simple order, and this method request item should not be limited to the specific steps or block levels described above.

Although specific embodiments are used to describe the invention of the present disclosure below, the principles of the invention of the present disclosure are defined by the deep application patent scope of this case, and therefore can also be applied to embodiments that are not specifically described in the specification. In addition, in order not to obscure the spirit of the present invention, specific details will not be described in the specification, and the undescribed details belong to the knowledge range of those with ordinary knowledge in the technical field.

FIG. 1 is a schematic top view of a piezoelectric micromachined ultrasonic transducer (PMUT) according to an embodiment of the disclosure. Referring to Figure 1, a piezoelectric micromachined ultrasonic transducer (PMUT) 100 includes at least a substrate 102, a cavity 120 formed through the substrate 102, and an etching stop structure (not shown) provided along the opening of the cavity 120 , A film layer (not shown) (for example, a multilayer structure) disposed on the cavity 120, and an elastic layer 142 disposed on the film layer and separated from the cavity 120. According to an embodiment of the present disclosure, the etch stop structure provided along the opening of the cavity 120 may be a ring structure protruding from the bottom surface of the film layer, so that the film layer can be attached to the substrate 102. The shape of the etch stop structure is not limited to this, and the etch stop structure may also be a polygonal shape or an arc shape arranged along the opening of the cavity 120. The first contact pad 114 and the second contact pad 116 disposed on opposite sides may be electrically coupled to electrodes of the film layer, respectively. In addition, in order to avoid the first contact pad 114 Unnecessary parasitic capacitance is generated between the first contact pad 114 and the second contact pad 116, and the size of the first contact pad 114 and the second contact pad 116 can be reduced as much as possible, but is not limited to this. According to an embodiment of the present disclosure, the first contact pad 114 and the second contact pad 116 can be arranged on the same side or any position of the film layer 106, as long as the first contact pad 114 and the second contact pad 116 can be electrically coupled to the film layer 106. Layer of electrodes. Additional conductive traces (not shown) may be provided on the substrate 102 and electrically coupled to the first contact pad 114 and the second contact pad 116 in order to transmit electrical signals to or from the film layer . During the operation of the PMUT 100, when sound waves apply sound pressure to the film or electrical signals are applied to the film, the film may vibrate. By using the etch stop structure, the size and position of the film layer above the cavity 120 can be accurately defined, regardless of the size and position of the cavity 120 below the film layer. Therefore, the uniformity of the resonance frequency of each PUMT 100 can be effectively increased.

FIG. 2 is a schematic cross-sectional view taken along the tangent line AA′ of FIG. 1 according to an embodiment of the disclosure. Referring to FIG. 2, the etch stop structure may be a part of the first piezoelectric layer 132 that is anchored to the substrate 102 and is in direct contact with the cavity 120, which may be referred to as the “anchor portion” of the first piezoelectric layer 132. The opening O of the cavity 120 on the top surface 102A of the substrate 102 can be regarded as being sealed by the first piezoelectric layer 132. The substrate 102 may be a semiconductor substrate, such as a bulk silicon substrate, but is not limited thereto. The substrate 102 may be monocrystalline silicon, polycrystalline silicon, amorphous silicon, glass, ceramic materials or other suitable materials. According to an embodiment of the present disclosure, the substrate 102 may be an SOI substrate. The sacrificial layer 124 may be disposed between the substrate 102 and the first piezoelectric layer 132, and the composition of the sacrificial layer 124 may be different from the composition of the substrate 102 and the first piezoelectric layer 132. According to an embodiment of the present disclosure, when the composition of the substrate 102 includes a semiconductor material, such as silicon, the sacrificial layer 124 may be, for example, a dielectric layer _{of silicon oxide (SiO x} ) or _{silicon dioxide (SiO 2 ).} In addition, according to another embodiment of the present disclosure, most of the sacrificial layer 124 can be omitted to increase the contact area between the first piezoelectric layer 132 and the substrate 102. The film layer 106, for example, a multilayer structure including a bottom electrode 134, a second piezoelectric layer 136, and a top electrode 138, may be disposed on the first piezoelectric layer 132. The part of the film layer 106 may be disposed above the cavity 120, and the cavity 120 includes an edge 120 e close to the film layer 106. According to an embodiment of the present disclosure, the vertical projection of the sacrificial layer 124 may not overlap the vertical projection of the film layer 106 disposed directly above the cavity 120. The bottom electrode 134 and the top electrode 138 of the PMUT 100 may be electrically coupled to the first contact pad 114 and the second contact pad 116, respectively. The selective passivation layer 140 may be further disposed on the film layer 106, and its composition may be a dielectric layer. An elastic layer with desired elasticity may be disposed on the selective passivation layer 140 so that when sound waves or electrical signals are applied to the PMUT 100, the film layer 106 can vibrate at a specific frequency. It should be noted that since the elasticity of the elastic layer 142 is higher than the elasticity of other layers under the elastic layer 142, the mechanical behavior of the film layer 106 is mainly dominated by the elastic layer 142.

In order to enable those with ordinary knowledge in the technical field to implement the disclosed invention, the method of manufacturing a piezoelectric micromechanical ultrasonic transducer is further described below. In addition, because piezoelectric micromachined ultrasonic transducers can be manufactured through standard CMOS processes, related electronic components, such as field-effect devices, can also be manufactured through the same CMOS process on the same substrate of piezoelectric micromachined ultrasonic transducers. Crystals, amplifiers and integrated circuits.

FIG. 3 is a schematic cross-sectional view after forming a sacrificial layer on a substrate according to an embodiment of the disclosure. FIG. 7 is a flowchart of a method for manufacturing a piezoelectric micromachined ultrasonic transducer according to an embodiment of the disclosure. Referring to FIG. 3, in step 202 of the method 200, a substrate 102 is provided. According to different requirements, the substrate 102 can be selected from a semiconductor substrate or an insulating substrate. According to an embodiment of the present disclosure, the substrate 102 may be a single crystal silicon substrate. Then, in step 204, a sacrificial layer 124 is deposited on the top surface 102A of the substrate 102. There may be at least two holes 126 in the sacrificial layer 124 so that a part of the substrate 102 may be exposed from the bottom of the hole 126. Because the position of the hole 126 can be precisely defined through the photolithography process, the distance between the two holes 126 can be accurately controlled. It should be noted that in the subsequent process, some layers will be deposited on the sacrificial layer 124, and in order to increase the electrical performance or crystallinity of these layers, the sidewall 126S of the sacrificial layer 124 and the top surface 102A of the substrate 102 The angle θ 1 should be set in the range of 10°-40°, such as 10°, 20°, 30°, or 40°, but is not limited thereto.

Figure 4 is a schematic cross-sectional view after a film layer is formed on the substrate. In step 206, the first piezoelectric layer 132 may be deposited on the substrate 102 and filled into the holes 126 of the sacrificial layer 124. The first piezoelectric layer 132 may be made of an insulating material, such as aluminum nitride (AlN), scandium-doped aluminum nitride (ScAlN), lead zirconate titanate (PZT), zinc oxide (ZnO), polyvinylidene fluoride ( polyvinylidene fluoride, PVDF), lead mangnesium niobate-lead titanate (PMN-PT), but not limited to this. According to an embodiment of the present disclosure, the first piezoelectric layer 132 may also be used as a seed layer of a certain layer subsequently deposited on the first piezoelectric layer 132. In addition, the surface texture of the first piezoelectric layer 132 may affect the crystallinity of some layers deposited thereon. After that, the bottom electrode 134, the second piezoelectric layer 136, the top electrode 138, and the passivation layer 140 may be sequentially deposited on the first piezoelectric layer 132. The bottom electrode 134 and the top electrode 138 may be the same or different materials composed of molybdenum (Mo), titanium (Ti), aluminum (Al), or platinum (Pt), but are not limited thereto. The second piezoelectric layer 136 may be made of aluminum nitride (AlN), scandium-doped aluminum nitride (ScAlN), lead zirconate titanate (PZT), zinc oxide (ZnO), polyvinylidene fluoride (PVDF), niobium The composition of lead acid-lead titanate (PMN-PT), but not limited to this. The passivation layer 140 may be a selective layer made of an insulating material, such as SiO ₂ , SiON, or AlN, but is not limited thereto. In addition, the material of the second piezoelectric layer 136 may be the same as the material of the first piezoelectric layer 132. In addition, a plurality of grooves may be formed on the surface of the passivation layer 140, and each groove may have an angle θ 2 of 5°-35° between the sidewall 140S of the passivation layer 140 and the top surface 140A of the passivation layer 140. Holes may be formed in the film layer 106 to expose the bottom electrode 134 and the top electrode 138 respectively, and then the contact pads, that is, the first contact pad 114 and the second contact pad 116 may be filled into the holes. As such, the first contact pad 114 may be electrically coupled to the bottom electrode 134, and the second contact pad 116 may be electrically coupled to the top electrode 138.

Figure 5 is a schematic cross-sectional view after the elastic layer is formed on the film layer. Referring to FIG. 5, a layer with desired elasticity may be deposited on the film layer 106 and then patterned to form an elastic layer 142 separated from the first contact pad 114 and the second contact pad 116. The elastic layer 142 may be composed of materials with suitable elasticity, such as crystalline silicon (c-Si), amorphous silicon (a-Si), silicon-rich nitride (SiN _x ), silicon carbide (SiC), molybdenum (Mo), Titanium (Ti), aluminum (Al), or platinum (Pt), but not limited thereto. Since the elastic layer 142 will not be disposed under the second piezoelectric layer 136, the crystallinity of the second piezoelectric layer 136 will no longer be affected by the surface texture of the elastic layer 142.

FIG. 6 is a schematic cross-sectional view after a cavity penetrating the substrate is formed according to an embodiment of the disclosure. Referring to FIG. 6, in step 208, the back surface of the substrate 102 is etched to form a cavity 120 penetrating the substrate 102. Therefore, part of the bottom surface of the sacrificial layer 124 may be exposed from the cavity 120. The cavity 120 may have an opening O on the front side of the substrate 102. The opening O is defined by the edge 120e of the cavity 120 adjacent to the film layer 106, and the length defined by the opening O may be smaller than that of the first piezoelectric layer. The distance D defined by the anchor portion of 132. Since the distance D used to define the position of the film layer in the PMUT is mainly determined by the anchor portion of the first piezoelectric layer 132, even if the position or size of the opening O is slightly shifted, the position of the film layer of the PMUT And the size will not change.

Then, in step 210, by using the first piezoelectric layer 132 as an etching stop structure, an etching process is performed to remove the sacrificial layer 124 exposed from the cavity 120. When the composition of the sacrificial layer 124 is silicon oxide, the etchant may be Vapor HF (VHF). In the process of removing the sacrificial layer 124 exposed from the cavity 120, since the etching selection ratio of the sacrificial layer 124 to the first piezoelectric layer 132 is greater than 10, only the sacrificial layer 124 directly in contact with the etchant may be removed. In addition, since the anchor portion of the first piezoelectric layer 132 can prevent the etchant from reaching the remaining portion of the sacrificial layer 124, the remaining portion of the sacrificial layer may be prevented from being removed during the etching process. As a result, the structure shown in Fig. 2 can be obtained.

According to the embodiment of the present disclosure, the elastic layer is not disposed between the substrate and the film layer, but is disposed on the top surface of the film layer. Therefore, the crystallinity of the piezoelectric layer in the film layer will no longer be affected by the surface texture of the elastic layer, so that the overall elasticity of the PMUT film layer can be freely controlled.

The foregoing descriptions are only preferred embodiments of the present invention, and all equivalent changes and modifications made in accordance with the scope of the patent application of the present invention should fall within the scope of the present invention.

100: Piezoelectric micromachined ultrasonic transducer (PMUT)

102: substrate

102A: Top surface

102B: bottom surface

106: Membrane

114: first contact pad

116: second contact pad

120: Cavity

120e: Edge

124: Sacrifice Layer

126: Hole

126S: side wall

132: The first piezoelectric layer

134: bottom electrode

136: second piezoelectric layer

138: Top electrode

140: passivation layer

142: Elastic layer

D: distance

O: opening

Claims (17)

A piezoelectric micromechanical ultrasonic transducer includes: a substrate including a cavity penetrating the substrate; a film layer disposed on the cavity, the film layer including: a first piezoelectric layer disposed on the On the cavity, the first piezoelectric layer includes an anchor portion, wherein the anchor portion of the first piezoelectric layer directly contacts the substrate; a bottom electrode disposed on the first piezoelectric layer; and a top electrode , Disposed on the bottom electrode; and a second piezoelectric layer, disposed between the bottom electrode and the top electrode; and a sacrificial layer, disposed between the substrate and the first piezoelectric layer, wherein the sacrificial layer The vertical projection of the layer does not overlap the vertical projection of the part of the film layer directly above the cavity. The piezoelectric micromachined ultrasonic transducer according to claim 1, wherein an opening of the cavity is adjacent to the film layer. The piezoelectric micromachined ultrasonic transducer according to claim 1, wherein the anchor portion of the first piezoelectric layer that directly contacts the substrate is disposed along an opening of the cavity, and the anchor portion Including ring, polygon or arc structure. The piezoelectric micromachined ultrasonic transducer according to claim 1, further comprising an interface between one end of the sacrificial layer and the first piezoelectric layer, wherein between the interface and the top surface of the substrate The angle is 10°-40°. The piezoelectric micromachined ultrasonic transducer according to claim 1, wherein when the gaseous hydrogen fluorine is used When acid is used as an etchant, the etching selection ratio of the sacrificial layer to the first piezoelectric layer is greater than 10. The piezoelectric micromachined ultrasonic transducer according to claim 5, wherein the material of the first piezoelectric layer is the same as the material of the second piezoelectric layer. The piezoelectric micromachined ultrasonic transducer according to claim 6, wherein the first piezoelectric layer directly contacts the bottom electrode. The piezoelectric micromachined ultrasonic transducer according to claim 5, further comprising an elastic layer provided on the film layer. The piezoelectric micromechanical ultrasonic transducer according to claim 8, further comprising at least one contact pad, and the at least one contact pad is disposed on the sacrificial layer and separated from the elastic layer. The piezoelectric micromachined ultrasonic transducer according to claim 8, further comprising a passivation layer disposed between the film layer and the elastic layer. A method for manufacturing a piezoelectric micromechanical ultrasonic transducer includes: providing a substrate; forming a sacrificial layer on the substrate, wherein the sacrificial layer includes at least one hole exposing the substrate; and forming a piezoelectric layer on the at least A hole in and on the sacrificial layer; forming a cavity penetrating the substrate to expose a part of the sacrificial layer; and using the piezoelectric layer as an etch stop structure to remove the sacrificial layer exposed from the cavity section, Wherein, before the step of forming the cavity penetrating the substrate, a plurality of layers are formed on the piezoelectric layer and the sacrificial layer, and the layers include: a bottom electrode; a top electrode disposed on the bottom electrode ; And another piezoelectric layer, disposed between the bottom electrode and the top electrode. The method for manufacturing a piezoelectric micromachined ultrasonic transducer according to claim 11, wherein the angle between the side wall of the at least one hole and the top surface of the substrate is 10°-40°. The method for manufacturing a piezoelectric micromachined ultrasonic transducer according to claim 11, wherein the piezoelectric layer includes an anchor portion directly contacting the substrate. The method of manufacturing a piezoelectric micromachined ultrasonic transducer as described in claim 11, further comprising forming an elastic layer on the layers. The method for manufacturing a piezoelectric micromachined ultrasonic transducer according to claim 14, further comprising forming at least one contact pad, the at least one contact pad is disposed on the layers and electrically coupled to the top electrode or the bottom electrode One of them, wherein the elastic layer is separated from the at least one contact pad. The method for manufacturing a piezoelectric micromachined ultrasonic transducer according to claim 11, wherein when the step of removing the part of the sacrificial layer exposed from the cavity is completed, the other part of the sacrificial layer remains in the On the substrate. The method for manufacturing a piezoelectric micromachined ultrasonic transducer as described in claim 11, wherein During the step of removing the portion of the sacrificial layer exposed from the cavity, the etching selectivity ratio of the sacrificial layer to the piezoelectric layer is greater than 10.

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