A PMUT device containing a bipolar piezoelectric structure and a preparation method thereof

Abstract

The present invention provides a PMUT device containing a bipolar piezoelectric structure and a preparation method thereof, the preparation method at least comprising: 1) providing a substrate with a cavity, and forming a bottom electrode layer on the surface of the substrate; 2) forming a piezoelectric layer on the surface of the bottom electrode layer, the piezoelectric layer comprising a first polarity piezoelectric layer and a second polarity piezoelectric layer sequentially formed on the surface of the bottom electrode layer; 3) sequentially depositing a passivation layer and a top electrode layer on the surface of the second polarity piezoelectric layer, and patterning the top electrode layer; 4) preparing electrode lead-out structures of the bottom electrode layer and the top electrode layer. In the PMUT device obtained by the preparation method of the present invention, the piezoelectric layer is a single-layer bipolar film, which has the characteristics of no transition zone, and can maximize the effective working area. In addition, the preparation process of the single-layer bipolar film is simple, the number of openings is small, and the wiring area is small, so the array density of the PMUT can be greatly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of PMUT devices, and in particular relates to a PMUT device containing a bipolar piezoelectric structure and a preparation method thereof. Background Art

[0002] Piezoelectric Micromachined Ultrasound Transducer (PMUT) is developed on the basis of traditional piezoelectric bulk ultrasonic transducer. Traditional bulk type usually adopts d33 thickness vibration mode, and the resonant frequency mainly depends on the thickness of piezoelectric material. PMUT is a MEMS process, which usually relies on the vibrating film composed of piezoelectric material and electrode material to perform mechanical vibration in d31 mode, thereby generating and propagating mechanical elastic waves. The resonant frequency is related to the size and thickness of the film. At present, PMUT structure is subject to certain limitations in application because the vibrating film usually adopts a single-layer piezoelectric layer structure, its vibration displacement is small, and the electromechanical conversion efficiency is low. Therefore, it is of great significance to improve the maximum vibration displacement of the piezoelectric vibrating film under unit electrical excitation.

[0003] In order to increase the vibration displacement, a dual piezoelectric layer diaphragm structure is usually used, which is equivalent to adding a vibration driving source compared to a single piezoelectric layer. In order for the dual piezoelectric layer to achieve the effect of cumulative vibration amplitude, opposite electric fields must be applied to the two piezoelectric layers, and the position of the neutral layer of the entire vibration film structure must be controlled between the two piezoelectric layers to prevent the driving effects from canceling each other out. When applying a reverse electric field, electrodes need to be laid between the two piezoelectric layers and on the other two surfaces to form a five-layer stacked "sandwich" structure. This will bring many process problems, such as increasing the number of lead holes, increasing the wiring area, and the process requirements will also increase accordingly.

[0004] Therefore, it is necessary to improve the double-layer drive form, and processing a single-layer film with opposite polarity can theoretically achieve this effect. This film with opposite polarity can produce the effect of doubling the vibration bending moment when only the upper and lower electrodes are laid. However, the current process method used to prepare a single-layer bipolar piezoelectric film is difficult to suppress the generation of the film polarity transition zone, resulting in a non-ideal working area inside the piezoelectric film, reducing the effective working volume of the bipolar film.

[0005] Therefore, developing a new PMUT device containing a bipolar piezoelectric structure and a preparation method thereof will be beneficial to rapidly promote the development of PMUT and accelerate technological innovation in the field of ultrasonic testing. Summary of the invention

[0006] In view of the above-mentioned shortcomings of the prior art, an object of the present invention is to provide a PMUT device containing a bipolar piezoelectric structure and a preparation method thereof, so as to solve the problems of complex process and insufficient efficiency in the prior art.

[0007] To achieve the above-mentioned object and other related objects, the present invention provides a method for preparing a PMUT device having a bipolar piezoelectric structure, the method comprising at least:

[0008] 1) providing a substrate having a cavity, and forming a bottom electrode layer on the surface of the substrate;

[0009] 2) forming a piezoelectric layer on the surface of the bottom electrode layer, wherein the piezoelectric layer comprises a first polarity piezoelectric layer and a second polarity piezoelectric layer sequentially formed on the surface of the bottom electrode layer;

[0010] 3) depositing a passivation layer and a top electrode layer on the surface of the second polarity piezoelectric layer in sequence, and patterning the top electrode layer;

[0011] 4) Preparing electrode lead structures of the bottom electrode layer and the top electrode layer.

[0012] Preferably, in step 1), the method for preparing the substrate with the cavity comprises one of a bonding method, a sacrificial layer method and DIRE.

[0013] Preferably, in the step 1), before forming the bottom electrode layer, a step of forming a transition layer on the surface of the substrate is also included, and the material of the transition layer includes piezoelectric material or silicon oxide or a combination of the two.

[0014] Preferably, the polarities of the first polarity piezoelectric layer and the second polarity piezoelectric layer are opposite, that is, when the first polarity piezoelectric layer is oriented upward, the polarity of the second polarity piezoelectric layer is oriented downward; when the first polarity piezoelectric layer is oriented downward, the polarity of the second polarity piezoelectric layer is oriented upward.

[0015] Preferably, the material of the first polarity piezoelectric layer includes one of AlN, PZT, quartz, PVDF, and ZnO, and the material of the second polarity piezoelectric layer includes one of AlN, PZT, quartz, PVDF, and ZnO.

[0016] Preferably, in the step 2), after preparing the first polarity piezoelectric layer and before preparing the second polarity piezoelectric layer, the step of forming a patterned insulating layer or a stack of an insulating layer and an intermediate electrode layer on the surface of the first polarity piezoelectric layer is also included.

[0017] Preferably, the step 4) further comprises the step of preparing an electrode lead-out structure of the intermediate electrode layer.

[0018] Preferably, in the step 2), the second polarity piezoelectric layer is formed on the surface of the first polarity piezoelectric layer by bonding, epitaxy or sputtering.

[0019] The present invention also provides a PMUT device containing a bipolar piezoelectric structure, the device at least comprising:

[0020] a substrate having a cavity;

[0021] A bottom electrode layer is formed on the surface of the substrate;

[0022] A piezoelectric layer is formed on the surface of the bottom electrode layer, wherein the piezoelectric layer comprises a first polarity piezoelectric layer and a second polarity piezoelectric layer sequentially formed on the surface of the bottom electrode layer;

[0023] A passivation layer and a top electrode layer are sequentially formed on the surface of the second polarity piezoelectric layer;

[0024] The electrode lead-out structure leads out the bottom electrode layer and the top electrode layer respectively.

[0025] Preferably, the substrate has a closed cavity, the substrate above the closed cavity serves as a support layer, and the thickness of the support layer is not greater than 10 um.

[0026] Preferably, the device further comprises a transition layer, wherein the transition layer is formed between the bottom electrode layer and the substrate, the thickness of the transition layer is not greater than 30 nm, and the material of the transition layer comprises piezoelectric material or silicon oxide, or a combination of both.

[0027] Preferably, the material of the bottom electrode layer includes a combination of one or more of Pt, Mo, W, Al and Ti, with a thickness between 150nm and 300nm, and the material of the top electrode layer includes a combination of one or more of Pt, Mo, W, Al and Ti, with a thickness between 150nm and 300nm.

[0028] Preferably, the polarities of the first polarity piezoelectric layer and the second polarity piezoelectric layer are opposite, that is, when the first polarity piezoelectric layer is oriented upward, the polarity of the second polarity piezoelectric layer is oriented downward; when the first polarity piezoelectric layer is oriented downward, the polarity of the second polarity piezoelectric layer is oriented upward.

[0029] Preferably, the thickness of the first polarity piezoelectric layer is between 0.5um and 4um, and the thickness of the second polarity piezoelectric layer is between 0.5um and 4um.

[0030] Preferably, the device also includes a patterned insulating layer or a stack of an insulating layer and an intermediate electrode layer, and the insulating layer or the stack of an insulating layer and an intermediate electrode layer is formed between the first polarity piezoelectric layer and the second polarity piezoelectric layer, the thickness of the insulating layer is not greater than 30nm, and the thickness of the intermediate electrode layer is between 30nm and 100nm.

[0031] Preferably, the device further comprises an electrode lead-out structure of the intermediate electrode layer.

[0032] As described above, the present invention provides a PMUT device containing a bipolar piezoelectric structure and a preparation method thereof, the preparation method at least comprising: 1) providing a substrate having a cavity, and forming a bottom electrode layer on the surface of the substrate; 2) forming a piezoelectric layer on the surface of the bottom electrode layer, the piezoelectric layer comprising a first polarity piezoelectric layer and a second polarity piezoelectric layer sequentially formed on the surface of the bottom electrode layer; 3) sequentially depositing a passivation layer and a top electrode layer on the surface of the second polarity piezoelectric layer, and patterning the top electrode layer; 4) preparing electrode lead-out structures of the bottom electrode layer and the top electrode layer. In the PMUT device obtained by the preparation method of the present invention, the piezoelectric layer is a single-layer bipolar film, which has the characteristics of no transition zone, maximizing the effective working area, and the preparation process of the single-layer bipolar film is simple, the number of openings is small, and the wiring area is small, so the array density of the PMUT is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1~ Figure 5b The structure diagram of each step of the method for preparing a PMUT device having a bipolar piezoelectric structure of the present invention is presented. Figure 5a and Figure 5b It is a schematic structural diagram of a PMUT device containing a bipolar piezoelectric structure according to the present invention.

[0034] Component number description

[0035] 1 Substrate

[0036] 11 Cavity

[0037] 2 Bottom electrode layer

[0038] 3 Transition layer

[0039] 4 Piezoelectric layer

[0040] 41 First polarity piezoelectric layer

[0041] 42 Second polarity piezoelectric layer

[0042] 51 Insulation layer

[0043] 52 Intermediate electrode layer

[0044] 6 Passivation layer

[0045] 7 Top electrode layer

[0046] 81, 82 through hole DETAILED DESCRIPTION

[0047] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0048] Please refer to the attached drawings. It should be noted that the illustrations provided in this embodiment are only used to illustrate the basic concept of the present invention in a schematic manner, and the drawings only show the components related to the present invention rather than the number, shape and size of the components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed arbitrarily, and the component layout may also be more complicated.

[0049] This embodiment provides a method for preparing a PMUT device having a bipolar piezoelectric structure, the method comprising the following steps:

[0050] First, execute step 1), such as Figure 1a or Figure 1b and Figure 2 As shown, a substrate 1 having a cavity 11 is provided, and a bottom electrode layer 2 is formed on the surface of the substrate 1 .

[0051] As an example, a method for preparing the substrate 1 having the cavity 11 includes one of a bonding method, a sacrificial layer method and DIRE.

[0052] like Figure 1a The figure shows a process of preparing a substrate 1 with a closed cavity 11 by using a bonding method, specifically: preparing a substrate A made of silicon or silicon oxide or other suitable substrate materials, and another substrate B (also including but not limited to silicon and silicon oxide) with a cavity pattern after photolithography, bonding the two substrates, and thinning the substrate A above the cavity to a required thickness, thereby obtaining a substrate 1 with a closed cavity 11.

[0053] like Figure 1bThe process of preparing a substrate 1 with a closed cavity 11 by using a sacrificial layer method is shown, which can be specifically divided into two methods. The first method is: first, a sacrificial oxide layer material (including but not limited to polysilicon, silicon oxide or photoresist, etc.) is deposited in the cavity of the substrate, and a support layer is deposited on the surface of the obtained structure after surface flattening, and an etching hole is reserved, and the sacrificial layer material in the cavity is removed by etching through the etching hole to form a substrate 1 with a closed cavity 11. The second method is: a sacrificial layer material is deposited on the substrate, and the sacrificial layer material is patterned, and a flattening treatment is performed after the support layer is deposited, and an etching hole is reserved, and the sacrificial layer material is removed by etching through the etching hole to form a substrate 1 with a closed cavity 11.

[0054] In addition, the cavity 11 may also be formed by back etching such as DIRE and other methods, which will not be described in detail here.

[0055] As an example, the material of the bottom electrode layer 2 can be selected from metals such as Pt, Mo, W, Al, Ti, etc., and the thickness is generally between 150 and 300 nm, for example, it can be 150 nm, 180 nm, 200 nm, 220 nm, 250 nm, 280 nm, etc.

[0056] Preferably, if Figure 2 As shown, before forming the bottom electrode layer 2, a step of forming a transition layer 3 on the surface of the substrate 1 can also be included, that is, the transition layer 3 is formed between the bottom electrode layer 2 and the substrate 1, and the material of the transition layer 3 can be a piezoelectric material or silicon oxide, etc., with a thickness of about 0 to 30nm (for example, it can be 10nm, 15nm, 20nm, 25nm, etc.), and this layer can serve as a seed layer to protect the growth of the upper piezoelectric material.

[0057] Then execute step 2), such as Figure 3a As shown, a piezoelectric layer 4 is formed on the surface of the bottom electrode layer 2 , and the piezoelectric layer 4 includes a first polarity piezoelectric layer 41 and a second polarity piezoelectric layer 42 which are sequentially formed on the surface of the bottom electrode layer 4 .

[0058] As an example, the first polarity piezoelectric layer 41 is made of materials such as AlN (aluminum nitride), PZT (lead zirconatetitanate), quartz, PVDF (polyvinylidene fluoride), and ZnO (zinc oxide). The growth thickness is generally between 0.5 and 4 um, and the polarity orientation of the first polarity piezoelectric layer 41 is upward or downward.

[0059] As an example, the material of the second polarity piezoelectric layer 42 may be AlN (aluminum nitride), PZT (lead zirconate titanate), quartz, PVDF (polyvinylidene fluoride), ZnO (zincoxide), etc. If a piezoelectric single crystal material is used, it is necessary to ensure that the c-axis of the crystal structure is downward or upward, and the piezoelectric polycrystalline such as PZT must ensure that its polarization direction is downward or opposite to the polarity of the lower piezoelectric material, and the thickness is usually 0.5 to 4 um.

[0060] In short, the polarities of the first polarity piezoelectric layer 41 and the second polarity piezoelectric layer 42 must be opposite. If the first polarity piezoelectric layer 41 is oriented upward, the polarity of the second polarity piezoelectric layer 42 is oriented downward; if the first polarity piezoelectric layer 41 is oriented downward, the polarity of the second polarity piezoelectric layer 42 is oriented upward.

[0061] As a preferred solution, Figure 3b As shown, in this step, after preparing the first polarity piezoelectric layer 41 and before preparing the second polarity piezoelectric layer 42, a step of forming a patterned insulating layer 51 or a stack of the insulating layer 51 and the intermediate electrode layer 52 on the surface of the first polarity piezoelectric layer 41 is also included. That is, the insulating layer 51 or the stack of the insulating layer 51 and the intermediate electrode layer 52 is formed between the first polarity piezoelectric layer 41 and the second polarity piezoelectric layer 42.

[0062] The material of the insulating layer 51 can be silicon oxide, silicon nitride, etc., which is used to protect the surface of the piezoelectric layer. This layer is also used to adjust the position of the neutral layer of the vibrating membrane structure to avoid the neutral layer of the vibrating film being located in the first polarity piezoelectric layer or the second polarity piezoelectric layer, resulting in the mutual cancellation of polarities under opposite excitations. Its thickness can be 0 to 30 nm (for example, it can be 10 nm, 15 nm, 20 nm, 25 nm, etc.). A metal conductive material can also be deposited on the insulating layer 51 as an intermediate electrode layer 52 to add an output signal and improve the PMUT signal reception sensitivity by connecting a correction circuit. The thickness is generally 30 to 100 nm. After growth, the insulating layer 51 and the intermediate electrode layer 52 are patterned to avoid electrical short circuits.

[0063] If the insulating layer 51 or the stack of the insulating layer 51 and the intermediate electrode layer 52 is not made on the surface of the first polarity piezoelectric layer 41, the second polarity piezoelectric layer 42 can be formed on the surface of the first polarity piezoelectric layer 41 by using the bonding method. Specifically, the second polarity piezoelectric layer 42 is first grown on a temporary substrate, and the temporary substrate on which the second polarity piezoelectric layer 42 is grown is flipped 180 degrees so that the second polarity piezoelectric layer 42 is at the bottom, so that the second polarity piezoelectric layer 42 is bonded to the first polarity piezoelectric layer 41. The polarity of the flipped second polarity piezoelectric layer 42 needs to be opposite to the polarity of the first polarity piezoelectric layer 41. After bonding, mechanical thinning and chemical etching are performed to remove the temporary substrate. The final structure is as shown in FIG. Figure 3a shown.

[0064] If the insulating layer 51 or the stacked layer of the insulating layer 51 and the intermediate electrode layer 52 is formed on the surface of the first polarity piezoelectric layer 41, the second polarity piezoelectric layer 42 can be formed on the surface of the first polarity piezoelectric layer 41 by epitaxy or sputtering. Figure 3b The figure shows a schematic diagram of a stack of an insulating layer 51 and an intermediate electrode layer 52 made on the surface of a first polarity piezoelectric layer 41. Specifically, a second polarity piezoelectric layer 42 covering the first polarity piezoelectric layer 41 and the intermediate electrode layer 52 is deposited on the surface of the first polarity piezoelectric layer 41 and the intermediate electrode layer 52 through process control, such as controlling gas flow (oxygen, nitrogen, inert gas, etc.).

[0065] It should be noted that after the second polarity piezoelectric layer 42 is formed, in fact, the interface between the first polarity piezoelectric layer 41 and the second polarity piezoelectric layer 42 is integrated, but the polarities of the upper and lower parts of the piezoelectric layer 4 as a whole are opposite. Therefore, the piezoelectric layer 4 produced by the present invention can be called a single-layer bipolar piezoelectric film, and there is no transition zone inside the piezoelectric film.

[0066] Then execute step 3), such as Figure 4a Or as shown in FIG4 b , a passivation layer 6 and a top electrode layer 7 are sequentially deposited on the surface of the second polarity piezoelectric layer 42 , and the top electrode layer 7 is patterned.

[0067] As an example, the passivation layer 6 is usually made of silicon oxide or other materials, but not limited thereto, with a thickness of about 0 to 30 nm. The top electrode layer 7 can be made of metal materials such as Pt, Mo, W, Al, Ti, etc., with a thickness of generally 150 to 300 nm.

[0068] It should also be noted that the thickness of the transition layer, the bottom electrode layer, the first polarity piezoelectric layer, the second polarity piezoelectric layer, the insulating layer, the intermediate electrode layer, the passivation layer and the top electrode layer can be adjusted within a certain range, but ultimately the position of the neutral layer during operation must be at the interface between the first polarity piezoelectric layer and the second polarity piezoelectric layer to prevent the polarities from canceling each other out under opposite excitations.

[0069] Finally, execute step 4), such as Figure 5a or Figure 5b As shown, the electrode lead-out structure of the bottom electrode layer 2 and the top electrode layer 7 is prepared.

[0070] Through the electrode lead-out structure, the electrical properties of the bottom electrode layer 2 and the top electrode layer 7 can be led out respectively. Figure 5a As shown, the passivation layer 6, the second polarity piezoelectric layer 42 and the first polarity piezoelectric layer 41 are etched to form a through hole 81 exposing the bottom electrode layer 2, and then the electrode lead material is deposited in the through hole 81 to lead out the electrical property of the bottom electrode layer 2, and the electrode lead material in contact with the top electrode layer 7 is directly deposited to lead out the electrical property of the top electrode layer 7. For the convenience of illustration, Figure 5a The through hole 81 structure is shown in the figure, and the electrode lead material is not drawn.

[0071] If a stack of an insulating layer 51 and an intermediate electrode layer 52 is formed between the first polarity piezoelectric layer 41 and the second polarity piezoelectric layer 42, an electrode lead-out structure of the intermediate electrode layer 52 needs to be additionally formed, such as Figure 5b As shown, the passivation layer 6 and the second polarity piezoelectric layer 42 are etched to form a through hole 82 exposing the intermediate electrode layer 52 , and then an electrode lead-out material is deposited in the through hole 82 to lead out the electrical property of the intermediate electrode layer 52 .

[0072] like Figure 5a As shown, this embodiment also provides a PMUT device containing a bipolar piezoelectric structure, including but not limited to being prepared by the above method, and the device at least includes:

[0073] A substrate 1 having a cavity 11;

[0074] A bottom electrode layer 2 is formed on the surface of the substrate 1;

[0075] A piezoelectric layer 4 is formed on the surface of the bottom electrode layer 2, and the piezoelectric layer 4 includes a first polarity piezoelectric layer 41 and a second polarity piezoelectric layer 42 which are sequentially formed on the surface of the bottom electrode layer 2;

[0076] A passivation layer 6 and a top electrode layer 7 are sequentially formed on the surface of the second polarity piezoelectric layer 42;

[0077] The electrode lead-out structure leads out the bottom electrode layer 2 and the top electrode layer 7 respectively.

[0078] As an example, the substrate 1 has a closed cavity 11 or an open cavity 11, and the substrate above the closed cavity 11 serves as a support layer, and the thickness of the support layer is not greater than 10 um.

[0079] As an example, Figure 5b As shown, the device further includes a transition layer 3, which is formed between the bottom electrode layer 2 and the substrate 1, and has a thickness of no more than 30 nm. The material of the transition layer 3 includes piezoelectric material or silicon oxide, or a combination of both.

[0080] As an example, the material of the bottom electrode layer 2 includes a combination of one or more of Pt, Mo, W, Al and Ti, with a thickness between 150nm and 300nm, and the material of the top electrode layer 7 includes a combination of one or more of Pt, Mo, W, Al and Ti, with a thickness between 150nm and 300nm.

[0081] As an example, the polarities of the first polarity piezoelectric layer 41 and the second polarity piezoelectric layer 42 are opposite. If the first polarity piezoelectric layer 41 is oriented upward, the polarity of the second polarity piezoelectric layer 42 is oriented downward; if the first polarity piezoelectric layer 41 is oriented downward, the polarity of the second polarity piezoelectric layer 42 is oriented upward.

[0082] As an example, the thickness of the first polarity piezoelectric layer 41 is between 0.5 um and 4 um, and the thickness of the second polarity piezoelectric layer 42 is between 0.5 um and 4 um.

[0083] As an example, Figure 5b As shown, the device also includes a patterned insulating layer 51 or a stack of an insulating layer 51 and an intermediate electrode layer 52, and the insulating layer 51 or the stack of the insulating layer 51 and the intermediate electrode layer 52 is formed between the first polarity piezoelectric layer 41 and the second polarity piezoelectric layer 42, the thickness of the insulating layer 51 is not more than 30nm, and the thickness of the intermediate electrode layer 52 is between 30nm and 100nm.

[0084] As an example, the device further includes an electrode lead-out structure of the intermediate electrode layer 52 .

[0085] For further introduction to the PMUT device containing a bipolar piezoelectric structure, please refer to the description of the above-mentioned preparation method, which will not be repeated for the purpose of brevity.

[0086] In summary, the present invention provides a PMUT device containing a bipolar piezoelectric structure and a preparation method thereof, the preparation method at least comprising: 1) providing a substrate 1 having a cavity 11, and forming a bottom electrode layer 2 on the surface of the substrate 1; 2) forming a piezoelectric layer 4 on the surface of the bottom electrode layer 2, the piezoelectric layer 4 comprising a first polarity piezoelectric layer 41 and a second polarity piezoelectric layer 42 sequentially formed on the surface of the bottom electrode layer 2; 3) depositing a passivation layer 6 and a top electrode layer 7 on the surface of the second polarity piezoelectric layer 42 in sequence, and patterning the top electrode layer 7; 4) preparing electrode lead-out structures of the bottom electrode layer 2 and the top electrode layer 7. In the PMUT device obtained by the preparation method of the present invention, the piezoelectric layer 4 is a single-layer bipolar film, which has the characteristics of no transition zone, and can maximize the effective working area. In addition, the preparation process of the single-layer bipolar film is simple, the number of openings is small, and the wiring area is small, so the array density of the PMUT can be greatly improved.

[0087] Therefore, the present invention effectively overcomes various shortcomings in the prior art and has high industrial utilization value.

[0088] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A method for preparing a PMUT device containing a bipolar piezoelectric structure, characterized in that: The preparation method at least comprises: 1) providing a substrate having a cavity, and forming a bottom electrode layer on the surface of the substrate; 2) forming a piezoelectric layer on the surface of the bottom electrode layer: forming a first polarity piezoelectric layer on the surface of the bottom electrode layer; growing a second polarity piezoelectric layer on a temporary substrate, flipping the temporary substrate on which the second polarity piezoelectric layer is grown, bonding the second polarity piezoelectric layer to the first polarity piezoelectric layer, so that the interface between the first polarity piezoelectric layer and the second polarity piezoelectric layer is integrated, and there is no transition zone inside the piezoelectric layer; the first polarity piezoelectric layer and the second polarity piezoelectric layer have opposite polarities; 3) depositing a passivation layer and a top electrode layer on the surface of the second polarity piezoelectric layer in sequence, and patterning the top electrode layer; 4) Preparing electrode lead structures of the bottom electrode layer and the top electrode layer.

2. The method for preparing a PMUT device having a bipolar piezoelectric structure according to claim 1, characterized in that: In the step 1), the method for preparing the substrate with the cavity includes one of a bonding method, a sacrificial layer method and DIRE.

3. The method for preparing a PMUT device having a bipolar piezoelectric structure according to claim 1, characterized in that: In the step 1), before forming the bottom electrode layer, a transition layer is also included on the surface of the substrate, and the material of the transition layer includes piezoelectric material or silicon oxide, or a combination of the two.

4. The method for preparing a PMUT device having a bipolar piezoelectric structure according to claim 1, characterized in that: If the first polarity piezoelectric layer is oriented upward, the polarity of the second polarity piezoelectric layer is oriented downward; or if the first polarity piezoelectric layer is oriented downward, the polarity of the second polarity piezoelectric layer is oriented upward.

5. The method for preparing a PMUT device having a bipolar piezoelectric structure according to claim 1, characterized in that: The material of the first polarity piezoelectric layer includes one of AlN, PZT, quartz, PVDF, and ZnO, and the material of the second polarity piezoelectric layer includes one of AlN, PZT, quartz, PVDF, and ZnO.

6. A method for preparing a PMUT device containing a bipolar piezoelectric structure, characterized in that: The preparation method at least comprises: 1) providing a substrate having a cavity, and forming a bottom electrode layer on the surface of the substrate; 2) forming a piezoelectric layer on the surface of the bottom electrode layer: forming a first polarity piezoelectric layer on the surface of the bottom electrode layer; forming a patterned insulating layer on the surface of the first polarity piezoelectric layer, or forming a stack of a patterned insulating layer and an intermediate electrode layer on the surface of the first polarity piezoelectric layer; forming a second polarity piezoelectric layer on the surface of the first polarity piezoelectric layer by epitaxy or sputtering; the first polarity piezoelectric layer and the second polarity piezoelectric layer are integrated at the interface, and there is no transition zone inside the piezoelectric layer; the first polarity piezoelectric layer and the second polarity piezoelectric layer have opposite polarities; 3) depositing a passivation layer and a top electrode layer on the surface of the second polarity piezoelectric layer in sequence, and patterning the top electrode layer; 4) Preparing electrode lead structures of the bottom electrode layer and the top electrode layer.

7. The method for preparing a PMUT device having a bipolar piezoelectric structure according to claim 6, characterized in that: The step 4) also includes the step of preparing the electrode lead-out structure of the intermediate electrode layer.

8. A PMUT device having a bipolar piezoelectric structure, characterized in that: The device at least comprises: a substrate having a cavity; A bottom electrode layer is formed on the surface of the substrate; A piezoelectric layer is formed on the surface of the bottom electrode layer, the piezoelectric layer includes a first polarity piezoelectric layer and a second polarity piezoelectric layer sequentially formed on the surface of the bottom electrode layer, the second polarity piezoelectric layer is bonded to the first polarity piezoelectric layer, and the interface between the first polarity piezoelectric layer and the second polarity piezoelectric layer is integrated, and there is no transition zone inside the piezoelectric layer; the first polarity piezoelectric layer and the second polarity piezoelectric layer have opposite polarities; A passivation layer and a top electrode layer are sequentially formed on the surface of the second polarity piezoelectric layer; The electrode lead-out structure leads out the bottom electrode layer and the top electrode layer respectively.

9. The PMUT device having a bipolar piezoelectric structure according to claim 8, characterized in that: The substrate has a closed cavity therein, and the substrate above the closed cavity serves as a support layer, and the thickness of the support layer is no more than 10 um.

10. The PMUT device having a bipolar piezoelectric structure according to claim 8, characterized in that: The device further comprises a transition layer, which is formed between the bottom electrode layer and the substrate, has a thickness of no more than 30 nm, and is made of a material comprising piezoelectric material or silicon oxide, or a combination of both.

11. The PMUT device having a bipolar piezoelectric structure according to claim 8, characterized in that: The material of the bottom electrode layer includes a combination of one or more of Pt, Mo, W, Al and Ti, with a thickness between 150nm and 300nm. The material of the top electrode layer includes a combination of one or more of Pt, Mo, W, Al and Ti, with a thickness between 150nm and 300nm.

12. The PMUT device having a bipolar piezoelectric structure according to claim 8, characterized in that: If the first polarity piezoelectric layer is oriented upward, the polarity of the second polarity piezoelectric layer is oriented downward; or if the first polarity piezoelectric layer is oriented downward, the polarity of the second polarity piezoelectric layer is oriented upward.

13. The PMUT device having a bipolar piezoelectric structure according to claim 8, characterized in that: The thickness of the first polarity piezoelectric layer is between 0.5um and 4um, and the thickness of the second polarity piezoelectric layer is between 0.5um and 4um.

14. A PMUT device having a bipolar piezoelectric structure, characterized in that: The device at least comprises: a substrate having a cavity; A bottom electrode layer is formed on the surface of the substrate; A piezoelectric layer is formed on the surface of the bottom electrode layer, the piezoelectric layer includes a first polarity piezoelectric layer and a second polarity piezoelectric layer sequentially formed on the surface of the bottom electrode layer, the interface between the first polarity piezoelectric layer and the second polarity piezoelectric layer is integrated, and there is no transition zone inside the piezoelectric layer; the first polarity piezoelectric layer and the second polarity piezoelectric layer have opposite polarities; the second polarity piezoelectric layer is formed on the surface of the first polarity piezoelectric layer by epitaxy or sputtering; a patterned insulating layer is further included between the first polarity piezoelectric layer and the second polarity piezoelectric layer, or a stack of a patterned insulating layer and an intermediate electrode layer is further included between the first polarity piezoelectric layer and the second polarity piezoelectric layer; A passivation layer and a top electrode layer are sequentially formed on the surface of the second polarity piezoelectric layer; The electrode lead-out structure leads out the bottom electrode layer and the top electrode layer respectively.

15. The PMUT device having a bipolar piezoelectric structure according to claim 14, characterized in that: The thickness of the insulating layer is not greater than 30 nm, and the thickness of the intermediate electrode layer is between 30 nm and 100 nm.

16. The PMUT device having a bipolar piezoelectric structure according to claim 14, characterized in that: The device further comprises an electrode lead-out structure of the intermediate electrode layer.

17. The PMUT device having a bipolar piezoelectric structure according to claim 14, characterized in that: If the first polarity piezoelectric layer is oriented upward, the polarity of the second polarity piezoelectric layer is oriented downward; or if the first polarity piezoelectric layer is oriented downward, the polarity of the second polarity piezoelectric layer is oriented upward.

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