A PMUT device containing a bipolar piezoelectric structure and its fabrication method

Abstract

This invention provides a method for fabricating a PMUT device containing a bipolar piezoelectric structure, comprising: 1) providing a first substrate and forming a bottom electrode layer on the substrate surface; 2) forming a piezoelectric layer, including a first polar piezoelectric layer and a second polar piezoelectric layer formed sequentially; 3) sequentially depositing a first passivation layer and a top electrode layer, and patterning the top electrode layer; 4) forming a first via and depositing a support layer; 5) etching the support layer to form a first opening and a second opening; 6) providing a second substrate and flipping the structure so that the second substrate is bonded to the support layer, the first opening forms a cavity, and the first substrate is removed. The fabrication method of this invention employs two electrical connection methods. In the fabricated PMUT device, the piezoelectric layer is a single-layer bipolar film with no transition region, which maximizes the effective working area. Furthermore, this fabrication process is simple, has a small number of openings, a small wiring area, and significantly increases the array density of the PMUT.

Description

Technical Field

[0001] This invention belongs to the field of PMUT device technology, and in particular relates to a PMUT device containing a bipolar piezoelectric structure and its fabrication method. Background Technology

[0002] Piezoelectric micromachined ultrasonic transducers (PMUTs) are developed based on traditional bulk piezoelectric ultrasonic transducers. Traditional bulk transducers typically employ a d33 thickness vibration mode, with the resonant frequency primarily dependent on the thickness of the piezoelectric material. PMUTs, however, utilize MEMS technology and typically rely on a vibrating thin film composed of piezoelectric and electrode materials to generate and propagate mechanical elastic waves in a d31 mode. The resonant frequency of this wave is related to the size and thickness of the thin film. Currently, PMUT structures often employ a single-layer piezoelectric structure, resulting in small vibration displacement and low electromechanical conversion efficiency, which limits their applications. Therefore, increasing the maximum vibration displacement of the piezoelectric vibrating thin film under unit electrical excitation is of great significance.

[0003] To enhance vibration displacement, a double-piezoelectric-layer diaphragm structure is typically employed, which, compared to a single piezoelectric layer, effectively adds a vibration driving source. To achieve the cumulative vibration amplitude effect, opposing electric fields must be applied to the two piezoelectric layers, and the neutral layer of the entire vibrating film structure must be positioned between the two layers to prevent the driving effects from canceling each other out. Applying a reverse electric field requires placing electrodes between the two piezoelectric layers and on the other two surfaces, forming a five-layer "sandwich" structure. This introduces several manufacturing challenges, such as increasing the number of vias, increasing the wiring area, and consequently raising the technological requirements.

[0004] Therefore, improvements to the bilayer driving method are needed. Theoretically, this effect can be achieved by processing a single-layer thin film with opposite polarities. Such a film with inherent opposite polarities can double the vibration bending moment even when only two electrodes are laid. However, current processes for preparing single-layer bipolar piezoelectric films struggle to suppress the generation of polarity transition regions, resulting in non-ideal working areas within the piezoelectric film and reducing the effective working volume of the bipolar film.

[0005] Therefore, developing a new PMUT device with a bipolar piezoelectric structure and its fabrication method is beneficial to rapidly advance the development of PMUT and accelerate technological innovation in the field of ultrasonic testing. Summary of the Invention

[0006] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a PMUT device containing a bipolar piezoelectric structure and its fabrication method, so as to solve the problems of complex process and insufficient efficiency in the prior art.

[0007] To achieve the above and other related objectives, the present invention provides a method for fabricating a PMUT device containing a bipolar piezoelectric structure, the fabrication method comprising at least:

[0008] 1) A first substrate is provided, and a bottom electrode layer is formed on the surface of the first substrate;

[0009] 2) A piezoelectric layer is formed on the surface of the bottom electrode layer, the piezoelectric layer comprising a first polar piezoelectric layer and a second polar piezoelectric layer sequentially formed on the surface of the bottom electrode layer;

[0010] 3) A first passivation layer and a top electrode layer are sequentially deposited on the surface of the second polar piezoelectric layer, and the top electrode layer is patterned;

[0011] 4) Etch the first passivation layer, the second polar piezoelectric layer and the first polar piezoelectric layer to form a first via that exposes the bottom electrode layer, and deposit a support layer on the surface of the first passivation layer and the top electrode layer and in the first via;

[0012] 5) Etch the support layer to form a first opening exposing the top electrode layer and a second opening exposing the first passivation layer;

[0013] 6) Provide a second substrate and flip the structure obtained in step 5) so that the second substrate is bonded to the support layer, the first opening forms a cavity, and the first substrate is removed.

[0014] Preferably, step 1) further includes forming a transition layer on the surface of the first substrate before forming the bottom electrode layer, wherein the material of the transition layer includes one or a combination of piezoelectric materials or silicon oxide.

[0015] Preferably, the first polar piezoelectric layer and the second polar piezoelectric layer have opposite polarities. If the first polar piezoelectric layer is oriented upward, then the second polar piezoelectric layer is oriented downward; if the first polar piezoelectric layer is oriented downward, then the second polar piezoelectric layer is oriented upward.

[0016] Preferably, the material of the first polar piezoelectric layer includes one of AlN, PZT, quartz, PVDF, and ZnO, and the material of the second polar piezoelectric layer includes one of AlN, PZT, quartz, PVDF, and ZnO.

[0017] Preferably, step 2) further includes forming a patterned insulating layer or a stack of an insulating layer and an intermediate electrode layer on the surface of the first polar piezoelectric layer after the first polar piezoelectric layer is prepared and before the second polar piezoelectric layer is prepared.

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

[0019] Preferably, step 4) further includes etching the first passivation layer and the second polar piezoelectric layer to form a second via exposing the intermediate electrode layer, wherein the support layer is deposited in the second via.

[0020] Preferably, the support layer is made of a conductive material, and the PMUT device forms a downward electrical connection through the support layer to bring out the electrical properties of the bottom electrode layer, the middle electrode layer and the top electrode layer respectively.

[0021] Preferably, the support layer is made of a non-conductive material, then after completing step 6), the method further includes:

[0022] The bottom electrode layer is patterned, and a second passivation layer is formed on the surfaces of the bottom electrode layer and the first polar piezoelectric layer;

[0023] Etching is performed to form vias that expose the bottom electrode layer, the middle electrode layer, and the top electrode layer, respectively.

[0024] Electrode material is deposited in the via, and the PMUT device forms an upward electrical connection through the electrode material to bring out the electrical properties of the bottom electrode layer, the middle electrode layer and the top electrode layer respectively.

[0025] Preferably, in step 6), before flipping the structure obtained in step 5), the method further includes depositing a sacrificial layer in the first opening and the second opening, and after the second substrate is bonded to the support and the sacrificial layer, the sacrificial layer is etched away.

[0026] The present invention also provides a PMUT device containing a bipolar piezoelectric structure, the device comprising at least:

[0027] First passivation layer;

[0028] A piezoelectric layer and a bottom electrode layer are sequentially located on the surface of the first passivation layer. The piezoelectric layer includes a second polar piezoelectric layer and a first polar piezoelectric layer sequentially located on the surface of the first passivation layer.

[0029] The first through-hole penetrates the first passivation layer, the second polar piezoelectric layer, and the first polar piezoelectric layer.

[0030] A support layer having a first opening and a second opening is located between the first passivation layer and the second substrate, and fills the first through hole, wherein the first opening forms a cavity;

[0031] A top electrode layer is formed on the surface of the first passivation layer away from the second polar piezoelectric layer, and the top electrode layer is located in a cavity.

[0032] Preferably, the device further includes a transition layer formed on the surface of the bottom electrode layer away from the first polar piezoelectric layer, the thickness of the transition layer being no more than 30 nm, and the material of the transition layer including one or a combination of piezoelectric materials or silicon oxide.

[0033] Preferably, the material of the bottom electrode layer includes one or more of Pt, Mo, W, Al and Ti, and the thickness is between 150 nm and 300 nm. The material of the top electrode layer includes one or more of Pt, Mo, W, Al and Ti, and the thickness is between 150 nm and 300 nm.

[0034] Preferably, the first polar piezoelectric layer and the second polar piezoelectric layer have opposite polarities. If the first polar piezoelectric layer is oriented upward, then the second polar piezoelectric layer is oriented downward; if the first polar piezoelectric layer is oriented downward, then the second polar piezoelectric layer is oriented upward.

[0035] Preferably, the thickness of the first polar piezoelectric layer is between 0.5 μm and 4 μm, and the thickness of the second polar piezoelectric layer is between 0.5 μm and 4 μm.

[0036] Preferably, the device further includes a patterned insulating layer or a stack of an insulating layer and an intermediate electrode layer, wherein the insulating layer or the stack of an insulating layer and an intermediate electrode layer is formed between the first polar piezoelectric layer and the second polar piezoelectric layer, 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.

[0037] Preferably, the device further includes a second through-hole, which penetrates the first passivation layer and the second polar piezoelectric layer, and the support layer is also filled in the second through-hole.

[0038] Preferably, the support layer is made of a conductive material, and the PMUT device forms a downward electrical connection through the support layer to bring out the electrical properties of the bottom electrode layer, the middle electrode layer and the top electrode layer respectively.

[0039] Preferably, the support layer is made of a non-conductive material, the bottom electrode layer is a patterned bottom electrode layer, and the device further includes:

[0040] A second passivation layer is formed on the surfaces of the bottom electrode layer and the first polar piezoelectric layer;

[0041] Through-holes extend to the surfaces of the bottom electrode layer, the middle electrode layer, and the top electrode layer, respectively.

[0042] Electrode material is deposited in the via, and the PMUT device forms an upward electrical connection through the electrode material to bring out the electrical properties of the bottom electrode layer, the middle electrode layer and the top electrode layer respectively.

[0043] Preferably, the thickness of the support layer is between 2µm and 10µm.

[0044] As described above, the present invention provides a method for fabricating a PMUT device containing a bipolar piezoelectric structure. The fabrication method includes at least: 1) providing a first substrate and forming a bottom electrode layer on the substrate surface; 2) forming a piezoelectric layer, the piezoelectric layer comprising a first polar piezoelectric layer and a second polar piezoelectric layer formed sequentially; 3) sequentially depositing a first passivation layer and a top electrode layer, and patterning the top electrode layer; 4) forming a first via and depositing a support layer; 5) etching the support layer to form a first opening and a second opening; 6) providing a second substrate and flipping the structure so that the second substrate is bonded to the support layer, the first opening forms a cavity, and the first substrate is removed. The fabrication method of the present invention employs two electrical connection methods. In the fabricated PMUT device, the piezoelectric layer is a single-layer bipolar film with no transition region, which maximizes the effective working area. Furthermore, the fabrication process is simple, with few openings and a small wiring area, significantly increasing the array density of the PMUT. Attached Figure Description

[0045] Figures 1-12 This is a schematic diagram illustrating the structural steps of the fabrication method of the PMUT device containing a bipolar piezoelectric structure according to the present invention. Figure 9a , Figure 9b and Figure 12 This is a structural diagram of the PMUT device containing a bipolar piezoelectric structure according to the present invention.

[0046] Component designation explanation

[0047] 1 First substrate

[0048] 2 Bottom electrode layer

[0049] 3. Transition Layer

[0050] 4 Piezoelectric layer

[0051] 41 First polar piezoelectric layer

[0052] 42 Second polar piezoelectric layer

[0053] 51 Insulation layer

[0054] 52 Intermediate Electrode Layer

[0055] 6 First passivation layer

[0056] 7 Top Electrode Layer

[0057] 81 First Through Hole

[0058] 82 Second Through Hole

[0059] 83 Through Hole

[0060] 9 Support layer

[0061] 91 First Opening

[0062] 92 Second opening

[0063] 10 Second substrate

[0064] 11 Second passivation layer

[0065] 12 Sacrificial Layer Detailed Implementation

[0066] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0067] Please refer to the accompanying drawings. It should be noted that the illustrations provided in this embodiment are only schematic representations of the basic concept of the present invention. Therefore, the drawings only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0068] Example 1

[0069] This embodiment provides a method for fabricating a PMUT device containing a bipolar piezoelectric structure, the method comprising the following steps:

[0070] First, perform step 1), such as Figure 1 As shown, a first substrate 1 is provided, and a bottom electrode layer 2 is formed on the surface of the first substrate.

[0071] As an example, the material of the first substrate 1 is generally a silicon-based substrate, a silicon oxide substrate, or a CMOS substrate.

[0072] As an example, the material of the bottom electrode layer 2 can be metals such as Pt, Mo, W, Al, and Ti, 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.

[0073] Preferably, such as Figure 1 As shown, before forming the bottom electrode layer 2, a step of forming a transition layer 3 on the surface of the first substrate 1 may be included. That is, the transition layer 3 is formed between the bottom electrode layer 2 and the first substrate 1. The material of the transition layer 3 may be a piezoelectric material or silicon oxide, etc., and the thickness is about 0 to 30 nm (for example, it may be 10 nm, 15 nm, 20 nm, 25 nm, etc.). This layer can serve as a seed layer to protect the growth of the upper piezoelectric material, and can also serve as a protective layer for the subsequent bottom electrode layer 2.

[0074] Then proceed to step 2), such as Figure 2 As shown, a piezoelectric layer 4 is formed on the surface of the bottom electrode layer 2. The piezoelectric layer 4 includes a first polar piezoelectric layer 41 and a second polar piezoelectric layer 42 sequentially formed on the surface of the bottom electrode layer 2.

[0075] As an example, the first polar piezoelectric layer 41 is made of materials such as AlN (aluminum nitride), PZT (lead zirconatetitanate), quartz, PVDF (polyvinylidene fluoride), and ZnO (zinc oxide), and the growth thickness is generally between 0.5 and 4 μm. The polar orientation of the first polar piezoelectric layer 41 is upward or downward.

[0076] As an example, the material of the second polar piezoelectric layer 42 can be AlN (aluminum nitride), PZT (lead zirconate titanate), quartz, PVDF (polyvinylidene fluoride), ZnO (zinc oxide), etc. If a piezoelectric single crystal material is used, the c-axis of the crystal structure must be ensured to be downward or upward. For piezoelectric polycrystalline materials such as PZT, its polarization direction must be ensured to be downward or opposite to the polarity of the lower piezoelectric material. The thickness is usually 0.5–4 μm.

[0077] In summary, the polarities of the first polar piezoelectric layer 41 and the second polar piezoelectric layer 42 must be opposite. If the orientation of the first polar piezoelectric layer 41 is upward, then the polar orientation of the second polar piezoelectric layer 42 is downward; if the orientation of the first polar piezoelectric layer 41 is downward, then the polar orientation of the second polar piezoelectric layer 42 is upward.

[0078] As a preferred solution, such as Figure 2 As shown, this step, after preparing the first polar piezoelectric layer 41 and before preparing the second polar piezoelectric layer 42, also includes forming a patterned insulating layer 51 or a stack of insulating layer 51 and intermediate electrode layer 52 on the surface of the first polar piezoelectric layer 41. That is, the insulating layer 51 or the stack of insulating layer 51 and intermediate electrode layer 52 is formed between the first polar piezoelectric layer 41 and the second polar piezoelectric layer 42.

[0079] The insulating layer 51 can be made of silicon oxide, silicon nitride, etc., and is used to protect the surface of the piezoelectric layer. This layer also helps to adjust the position of the neutral layer in the vibrating film structure, so as to prevent the neutral layer of the vibrating film from being located in the first or second polarity piezoelectric layer, which would cause the polarities to cancel each other out under opposite excitation. Its thickness can be 0-30 nm (e.g., 10 nm, 15 nm, 20 nm, 25 nm, etc.). A conductive metal material can also be deposited on top of the insulating layer 51 as an intermediate electrode layer 52 to add an output signal. By connecting a correction circuit, the signal receiving sensitivity of the PMUT can be improved. The thickness is generally 30-100 nm. After growth, the insulating layer 51 and the intermediate electrode layer 52 are patterned to avoid electrical short circuits.

[0080] If an insulating layer 51 or a stack of insulating layer 51 and intermediate electrode layer 52 is not formed on the surface of the first polar piezoelectric layer 41, the second polar piezoelectric layer 42 can be formed on the surface of the first polar piezoelectric layer 41 using a bonding method. Specifically: First, the second polar piezoelectric layer 42 is grown on a temporary substrate. The temporary substrate on which the second polar piezoelectric layer 42 is grown is flipped 180 degrees so that the second polar piezoelectric layer 42 is on the bottom, so that the second polar piezoelectric layer 42 can bond with the first polar piezoelectric layer 41. The polarity of the flipped second polar piezoelectric layer 42 needs to be opposite to that of the first polar piezoelectric layer 41. After bonding, the temporary substrate is removed by mechanical thinning and chemical etching. In the final structure, the first polar piezoelectric layer 41 is directly bonded.

[0081] If an insulating layer 51 or a stack of insulating layer 51 and intermediate electrode layer 52 is formed on the surface of the first polar piezoelectric layer 41, the second polar piezoelectric layer 42 can be formed on the surface of the first polar piezoelectric layer 41 using epitaxy or sputtering. Figure 2 The diagram shows a stack of insulating layer 51 and intermediate electrode layer 52 fabricated on the surface of the first polar piezoelectric layer 41. Specifically, a second polar piezoelectric layer 42 is deposited on the surface of the first polar piezoelectric layer 41 and intermediate electrode layer 52 by process control, such as controlling the gas flow rate (oxygen, nitrogen, inert gas, etc.).

[0082] It should be noted that after the second polar piezoelectric layer 42 is formed, the interface between the first polar piezoelectric layer 41 and the second polar piezoelectric layer 42 is actually integrated. It is just that the polarities of the upper and lower parts of the piezoelectric layer 4 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 region inside the piezoelectric film.

[0083] Next, proceed to step 3), such as Figure 3a As shown, a first passivation layer 6 and a top electrode layer 7 are sequentially deposited on the surface of the second polar piezoelectric layer 42, and the top electrode layer 7 is patterned.

[0084] As an example, the first passivation layer 6 is typically made of materials such as silicon oxide, but is not limited to this, 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, and Ti, with a thickness of generally 150 to 300 nm.

[0085] It should be noted that, Figure 3b This is also the structure obtained in this step, namely, Figure 3a and Figure 3b This is a longitudinal cross-sectional view of the same structure at different locations. The formed top electrode layer 7 includes a middle electrode layer (which can be circular) and an edge electrode layer (which can be a strip structure). The edge electrode layer facilitates the extraction of the electrical properties of the middle electrode layer.

[0086] Next, proceed to step 4), such as... Figure 4a As shown, the first passivation layer 6, the second polar piezoelectric layer 42 and the first polar piezoelectric layer 41 are etched to form a first via 81 that exposes the bottom electrode layer 2, and a support layer 9 is deposited on the surface of the first passivation layer 6 and the top electrode layer 7 and in the first via 81.

[0087] The growth thickness of the support layer 9 determines the cavity depth, and the thickness is approximately 2 to 10 μm, for example, it can be 3 μm, 5 μm, 8 μm, 10 μm, etc.

[0088] In this embodiment, a conductive material (such as Pt, Mo, W, Al, Ti, etc.) is selected as the material of the support layer 9. The conductive support layer 9 can provide an electrical connection with the subsequent second substrate 10.

[0089] Specifically, the process of forming the support 9 includes: first, growing a relatively thick support layer 9, and then planarizing the support layer 9 material. Planarization includes mechanical thinning and chemical etching. CMP processing is usually used for planarization.

[0090] As an example, this step may further include etching the first passivation layer 6 and the second polar piezoelectric layer 42 to form a second via 82 exposing the intermediate electrode layer 52, and the support layer 9 is further deposited and filled into the second via 82. The support layer 9 filled in the second via 82 serves as the electrical lead-out of the subsequent intermediate electrode layer 52, while the support layer 9 in the first via 81 serves as the electrical lead-out of the subsequent bottom electrode layer 2.

[0091] Similarly, Figure 4b This is also the structure obtained in this step, namely, Figure 4a and Figure 4b These are longitudinal sectional views of the same structure at different locations.

[0092] Then perform step 5), such as Figure 5a As shown, the support layer 9 is etched to form a first opening 91 that exposes the top electrode layer 7 and a second opening 92 that exposes the first passivation layer 6.

[0093] Similarly, Figure 5b This is also the structure obtained in this step, namely, Figure 5a and Figure 5b These are longitudinal sectional views of the same structure at different locations; therefore... Figure 5b Due to the location of the cross-sectional view, the second opening 92 cannot be shown, but its existence should be known.

[0094] The first opening 91 can form a cavity later, and the second opening 92 can separate the support layer 9 for leading out the electrical properties of the bottom electrode layer 2 and the support layer 9 for leading out the electrical properties of the top electrode layer 7, thus avoiding short circuits.

[0095] Finally, execute step 6), as follows: Figure 9a As shown, a second substrate 10 is provided, and the structure obtained in step 5) is flipped so that the second substrate 10 is bonded to the support layer 9, the first opening 91 forms a cavity, and the first substrate 10 is removed.

[0096] As an example, the material of the second substrate 10 is generally silicon-based material, aluminum oxide, etc.

[0097] As an example, the first substrate 10 can be removed by mechanical thinning and its surface can be planarized by chemical etching to reduce the surface roughness. Preferably, after removing the first substrate 10, a transition layer 3 is exposed. The top transition layer 3 can be used to protect the bottom electrode layer 2, and the transition layer 3 acts as a passivation layer.

[0098] As a preferred option, in this step, such as Figure 6aAs shown, before flipping the structure obtained in step 5), the process further includes depositing a sacrificial layer 12 in the first opening 91 and the second opening 92, and then please refer to... Figure 7a The second substrate 10 is bonded to the support layer 9 and the sacrificial layer 12. Finally, as shown in the figure... Figure 8a As shown, the sacrificial layer 12 is removed by introducing an etchant through holes (not shown) in the second substrate 10. By depositing the sacrificial layer 12 in the first opening 91 and the second opening 92, the bonding between the second substrate 10 and the support layer 9 can be made more stable.

[0099] in, Figure 6b and Figure 6a These are cross-sectional views of the same structure at different locations. Figure 7b and Figure 7a These are cross-sectional views of the same structure at different locations. Figure 8b and Figure 8a These are cross-sectional views of the same structure at different locations. Figure 9b and Figure 9a These are cross-sectional views of the same structure at different locations.

[0100] It should also be noted that the thicknesses of the transition layer, the bottom electrode layer, the first polar piezoelectric layer, the second polar piezoelectric layer, the insulating layer, the intermediate electrode layer, the first passivation layer, and the top electrode layer can be adjusted within a certain range. However, the neutral layer must ultimately be positioned at the interface between the first and second polar piezoelectric layers during operation to prevent the polarities from canceling each other out under opposite excitations.

[0101] Since this embodiment uses a conductive material as the material for the support layer 9, the PMUT device obtained after completing step 6) can directly form a downward electrical connection through the support layer 9 to bring out the electrical properties of the bottom electrode layer 2, the middle electrode layer 52 and the top electrode layer 7 respectively.

[0102] Example 2

[0103] This embodiment provides a method for fabricating a PMUT device containing a bipolar piezoelectric structure. The difference between this method and the method in Embodiment 1 is that the support layer 9 in this embodiment uses a non-conductive material, and after completing step 6), the upper surface needs to be treated to form an upward electrical connection. For clarity, [details omitted]. Figure 9b The structure shown is used as a basis for demonstrating subsequent processes. The preparation method includes the following steps:

[0104] First, steps 1) to 6) in Embodiment 1 are performed, wherein the support layer 9 is made of a non-conductive material.

[0105] Then, as Figure 10 As shown, the bottom electrode layer 2 is graphically represented, as follows: Figure 11 As shown, a second passivation layer 11 is formed on the surface of the bottom electrode layer 2 and the first polar piezoelectric layer 41.

[0106] As a preferred embodiment, the surface of the bottom electrode layer 2 is covered with a transition layer 3. While patterning the bottom electrode layer 2, the transition layer 3 is also patterned. Please refer to [reference needed]. Figure 10 .

[0107] As an example, the second passivation layer 11 is typically made of materials such as silicon oxide, but is not limited to this, and its thickness is generally no more than 30 nm.

[0108] Next, as Figure 12 As shown, etching is performed to form through-holes 83 that expose the bottom electrode layer 2, the intermediate electrode layer 52 and the top electrode layer 7, respectively.

[0109] Finally, electrode material is deposited in the via 83, and the PMUT device forms an upward electrical connection through the electrode material to bring out the electrical properties of the bottom electrode layer 2, the intermediate electrode layer 52 and the top electrode layer 7 respectively.

[0110] For ease of illustration, the electrode material is not shown.

[0111] Example 3

[0112] This embodiment provides a PMUT device containing a bipolar piezoelectric structure, such as Figure 9a As shown, the device includes, but is not limited to, those prepared using the preparation method in Example 1, and the device includes the following:

[0113] First passivation layer 6;

[0114] The piezoelectric layer 4 and the bottom electrode layer 2 are sequentially located on the surface of the first passivation layer 6. The piezoelectric layer 4 includes a second polar piezoelectric layer 42 and a first polar piezoelectric layer 41 sequentially located on the surface of the first passivation layer 6.

[0115] The first through hole 81 penetrates the first passivation layer 6, the second polar piezoelectric layer 42 and the first polar piezoelectric layer 41;

[0116] A support layer 9 having a first opening 91 and a second opening 92 is located between the first passivation layer 6 and the second substrate 10 and fills the first through hole 81, wherein the first opening 91 forms a cavity.

[0117] A top electrode layer 7 is formed on the surface of the first passivation layer 6 away from the second polar piezoelectric layer 42 and the top electrode layer 7 is located in the cavity.

[0118] As an example, the device further includes a transition layer 3 formed on the surface of the bottom electrode layer 2 away from the first polar piezoelectric layer 41, the thickness of the transition layer 3 being no more than 30 nm, and the material of the transition layer 3 including one or a combination of piezoelectric materials or silicon oxide.

[0119] As an example, the material of the bottom electrode layer 2 includes one or more combinations of Pt, Mo, W, Al and Ti, and the thickness is between 150 nm and 300 nm. The material of the top electrode layer 2 includes one or more combinations of Pt, Mo, W, Al and Ti, and the thickness is between 150 nm and 300 nm.

[0120] As an example, the first polar piezoelectric layer 41 and the second polar piezoelectric layer 42 have opposite polarities. If the first polar piezoelectric layer 41 is oriented upward, then the second polar piezoelectric layer 42 is oriented downward; if the first polar piezoelectric layer 41 is oriented downward, then the second polar piezoelectric layer 42 is oriented upward.

[0121] As an example, the thickness of the first polar piezoelectric layer 41 is between 0.5 μm and 4 μm, and the thickness of the second polar piezoelectric layer 42 is between 0.5 μm and 4 μm.

[0122] As an example, the device further includes a patterned insulating layer 51 or a stack of insulating layer 51 and intermediate electrode layer 52, wherein the insulating layer 51 or the stack of insulating layer 51 and intermediate electrode layer 52 is formed between the first polar piezoelectric layer 41 and the second polar piezoelectric layer 42, the thickness of the insulating layer 51 is no greater than 30 nm, and the thickness of the intermediate electrode layer 52 is between 30 nm and 100 nm.

[0123] As an example, the device further includes a second via 82 that penetrates the first passivation layer 6 and the second polar piezoelectric layer 42, and the support layer 9 is also filled in the second via 82.

[0124] As an example, the thickness of the support layer 9 is between 2µm and 10µm.

[0125] In this embodiment, the support layer 9 is made of conductive material, and the PMUT device forms a downward electrical connection through the conductive support layer 9 to bring out the electrical properties of the bottom electrode layer 2, the intermediate electrode layer 52 and the top electrode layer 7 respectively.

[0126] This embodiment can also be referred to the appendix. Figure 9b , Figure 9a and Figure 9b These are cross-sectional views of the same structure at different locations.

[0127] For further details on PMUT devices containing bipolar piezoelectric structures, please refer to the description of the fabrication method in Example 1, which will not be repeated here for the sake of brevity.

[0128] Example 4

[0129] This embodiment provides a PMUT device containing a bipolar piezoelectric structure, such as Figure 12 As shown, the device includes, but is not limited to, the preparation method described in Example 2. The difference between this example and Example 3 is that the support layer 9 in this example is made of a non-conductive material. Based on the structure of Example 3, it also includes a structural layer formed by treating the upper surface, creating an upward electrical connection. For clarity, [details omitted]. Figure 9b The upper surface structure layer is then displayed based on the existing structure.

[0130] Please refer to the appendix. Figure 12 The bottom electrode layer 2 is a patterned bottom electrode layer 2. Of course, as a preferred embodiment, the surface of the bottom electrode layer 2 has a transition layer 3, and the transition layer 3 is also a patterned transition layer 3.

[0131] The device also includes:

[0132] The second passivation layer 11 is formed on the surface of the bottom electrode layer 2 and the first polar piezoelectric layer 41;

[0133] Through-hole 83 extends through and stops to the surfaces of the bottom electrode layer 2, the middle electrode layer 52 and the top electrode layer 7 respectively;

[0134] Electrode material is deposited in the via 83, and the PMUT device forms an upward electrical connection through the electrode material to bring out the electrical properties of the bottom electrode layer 2, the intermediate electrode layer 52 and the top electrode layer 7 respectively.

[0135] For ease of illustration, the electrode material in the through hole is not shown.

[0136] In summary, this invention provides a method for fabricating a PMUT device containing a bipolar piezoelectric structure. The method includes at least: 1) providing a first substrate and forming a bottom electrode layer on the substrate surface; 2) forming a piezoelectric layer, comprising a first polar piezoelectric layer and a second polar piezoelectric layer formed sequentially; 3) sequentially depositing a first passivation layer and a top electrode layer, and patterning the top electrode layer; 4) forming a first via and depositing a support layer; 5) etching the support layer to form a first opening and a second opening; 6) providing a second substrate and flipping the structure so that the second substrate is bonded to the support layer, the first opening forms a cavity, and the first substrate is removed. This method employs two electrical connection methods, resulting in a PMUT device with a single-layer bipolar piezoelectric layer that has no transition region, maximizing the effective working area. Furthermore, the fabrication process is simple, with few openings and a small wiring area, significantly increasing the array density of the PMUT.

[0137] Therefore, this invention effectively overcomes the various shortcomings of the prior art and has high industrial application value.

[0138] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can 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 those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for fabricating a PMUT device containing a bipolar piezoelectric structure, characterized in that, The preparation method includes at least: 1) A first substrate is provided, and a bottom electrode layer is formed on the surface of the first substrate; 2) A piezoelectric layer is formed on the surface of the bottom electrode layer, the piezoelectric layer comprising a first polar piezoelectric layer and a second polar piezoelectric layer sequentially formed on the surface of the bottom electrode layer; 3) A first passivation layer and a top electrode layer are sequentially deposited on the surface of the second polar piezoelectric layer, and the top electrode layer is patterned; 4) Etch the first passivation layer, the second polar piezoelectric layer and the first polar piezoelectric layer to form a first via that exposes the bottom electrode layer, and deposit a support layer on the surface of the first passivation layer and the top electrode layer and in the first via; 5) Etch the support layer to form a first opening exposing the top electrode layer and a second opening exposing the first passivation layer; 6) Provide a second substrate and flip the structure obtained in step 5) so that the second substrate is bonded to the support layer, the first opening forms a cavity, and the first substrate is removed.

2. The method for fabricating a PMUT device containing a bipolar piezoelectric structure according to claim 1, characterized in that: In step 1), before forming the bottom electrode layer, a step of forming a transition layer on the surface of the first substrate is further included. The material of the transition layer includes one or a combination of piezoelectric materials or silicon oxide.

3. The method for fabricating a PMUT device containing a bipolar piezoelectric structure according to claim 1, characterized in that: The first polar piezoelectric layer and the second polar piezoelectric layer have opposite polarities. The first polar piezoelectric layer is oriented upwards, while the second polar piezoelectric layer is oriented downwards. If the first polar piezoelectric layer is oriented downwards, then the second polar piezoelectric layer is oriented upwards.

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

5. The method for fabricating a PMUT device containing a bipolar piezoelectric structure according to claim 1, characterized in that: In step 2), after preparing the first polar piezoelectric layer and before preparing the second polar piezoelectric layer, the method further includes forming a patterned insulating layer or a stack of an insulating layer and an intermediate electrode layer on the surface of the first polar piezoelectric layer.

6. The method for fabricating a PMUT device containing a bipolar piezoelectric structure according to claim 1, characterized in that: In step 2), the second polar piezoelectric layer is formed on the surface of the first polar piezoelectric layer by bonding, epitaxy, or sputtering.

7. The method for fabricating a PMUT device containing a bipolar piezoelectric structure according to claim 5, characterized in that: Step 4) further includes etching the first passivation layer and the second polar piezoelectric layer to form a second via exposing the intermediate electrode layer, wherein the support layer is deposited in the second via.

8. The method for fabricating a PMUT device containing a bipolar piezoelectric structure according to claim 7, characterized in that: The support layer is made of conductive material, and the PMUT device forms a downward electrical connection through the support layer to bring out the electrical properties of the bottom electrode layer, the middle electrode layer and the top electrode layer respectively.

9. The method for fabricating a PMUT device containing a bipolar piezoelectric structure according to claim 7, characterized in that: If the support layer is made of a non-conductive material, then after completing step 6), the process further includes: The bottom electrode layer is patterned, and a second passivation layer is formed on the surfaces of the bottom electrode layer and the first polar piezoelectric layer; Etching is performed to form vias that expose the bottom electrode layer, the middle electrode layer, and the top electrode layer, respectively. Electrode material is deposited in the via, and the PMUT device forms an upward electrical connection through the electrode material to bring out the electrical properties of the bottom electrode layer, the middle electrode layer and the top electrode layer respectively.

10. The method for fabricating a PMUT device containing a bipolar piezoelectric structure according to claim 1, characterized in that: In step 6), before flipping the structure obtained in step 5), a step of depositing a sacrificial layer in the first opening and the second opening is included. After the second substrate is bonded to the support layer and the sacrificial layer, the sacrificial layer is etched away.

11. A PMUT device containing a bipolar piezoelectric structure, characterized in that, The device includes at least: First passivation layer; A piezoelectric layer and a bottom electrode layer are sequentially located on the surface of the first passivation layer. The piezoelectric layer includes a second polar piezoelectric layer and a first polar piezoelectric layer sequentially located on the surface of the first passivation layer. The first through hole penetrates the first passivation layer, the second polar piezoelectric layer, and the first polar piezoelectric layer. A support layer having a first opening and a second opening is located between the first passivation layer and the second substrate, and fills the first through hole, wherein the first opening forms a cavity; A top electrode layer is formed on the surface of the first passivation layer away from the second polar piezoelectric layer, and the top electrode layer is located in a cavity.

12. The PMUT device containing a bipolar piezoelectric structure according to claim 11, characterized in that: The device further includes a transition layer formed on the surface of the bottom electrode layer away from the first polar piezoelectric layer. The thickness of the transition layer is no greater than 30 nm. The material of the transition layer includes one or a combination of piezoelectric materials or silicon oxide.

13. The PMUT device containing a bipolar piezoelectric structure according to claim 11, characterized in that: The bottom electrode layer is made of one or more of Pt, Mo, W, Al and Ti, and has a thickness between 150 nm and 300 nm. The top electrode layer is made of one or more of Pt, Mo, W, Al and Ti, and has a thickness between 150 nm and 300 nm.

14. The PMUT device containing a bipolar piezoelectric structure according to claim 11, characterized in that: The first polar piezoelectric layer and the second polar piezoelectric layer have opposite polarities. The first polar piezoelectric layer is oriented upwards, while the second polar piezoelectric layer is oriented downwards. If the first polar piezoelectric layer is oriented downwards, then the second polar piezoelectric layer is oriented upwards.

15. The PMUT device containing a bipolar piezoelectric structure according to claim 11, characterized in that: The thickness of the first polar piezoelectric layer is between 0.5 μm and 4 μm, and the thickness of the second polar piezoelectric layer is between 0.5 μm and 4 μm.

16. The PMUT device containing a bipolar piezoelectric structure according to claim 11, characterized in that: The device further includes a patterned insulating layer or a stack of an insulating layer and an intermediate electrode layer, wherein the insulating layer or the stack of an insulating layer and an intermediate electrode layer is formed between the first polar piezoelectric layer and the second polar piezoelectric layer, 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.

17. The PMUT device containing a bipolar piezoelectric structure according to claim 16, characterized in that: The device further includes a second through-hole that penetrates the first passivation layer and the second polar piezoelectric layer, and the support layer is also filled in the second through-hole.

18. The PMUT device containing a bipolar piezoelectric structure according to claim 17, characterized in that: The support layer is made of conductive material, and the PMUT device forms a downward electrical connection through the support layer to bring out the electrical properties of the bottom electrode layer, the middle electrode layer and the top electrode layer respectively.

19. The PMUT device containing a bipolar piezoelectric structure according to claim 17, characterized in that: The support layer is made of a non-conductive material, the bottom electrode layer is a patterned bottom electrode layer, and the device further includes: A second passivation layer is formed on the surfaces of the bottom electrode layer and the first polar piezoelectric layer; Through-holes extend to the surfaces of the bottom electrode layer, the middle electrode layer, and the top electrode layer, respectively. Electrode material is deposited in the via, and the PMUT device forms an upward electrical connection through the electrode material to bring out the electrical properties of the bottom electrode layer, the middle electrode layer and the top electrode layer respectively.

20. The PMUT device containing a bipolar piezoelectric structure according to claim 11, characterized in that: The thickness of the support layer is between 2µm and 10µm.

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