Bulk acoustic wave resonator and method of manufacturing the same

Abstract

The application provides a bulk acoustic wave resonator and a preparation method thereof. The bulk acoustic wave resonator comprises a resonant unit, a first cavity is arranged below the resonant unit, the resonant unit comprises a bottom electrode, a piezoelectric layer and a top electrode from bottom to top, a dielectric layer is arranged on the bottom electrode and in the same layer as the piezoelectric layer, the dielectric layer is arranged around the piezoelectric layer and simultaneously forms an acoustic impedance mutation region and a temperature compensation region. By arranging the dielectric structure in the same layer around the piezoelectric layer, the integrated design of the lateral acoustic energy constraint and the temperature drift compensation is realized, and by matching the bottom electrode partition structure and the top electrode extension structure, the acoustic boundary characteristics and the electric field distribution of the device are improved. The bulk acoustic wave resonator provided by the application improves the resonant performance, temperature stability and preparation process reliability of the device while ensuring the compactness of the structure, effectively reduces the lateral acoustic energy leakage and improves the overall working stability of the device.

Description

Technical Field

[0001] This application relates to the field of acoustic resonator technology, and in particular to a bulk acoustic resonator and its fabrication method. Background Technology

[0002] Bulk Acoustic Wave (BAW) resonators are a type of radio frequency (RF) device that utilizes piezoelectric materials to generate bulk acoustic wave resonances along their thickness direction to achieve filtering functions. They offer advantages such as high operating frequency, high quality factor, small size, and ease of integration, and are widely used in mobile communications, wireless LANs, satellite communications, and RF front-end modules. Existing BAW resonators typically include a substrate, cavity, bottom electrode, piezoelectric layer, and top electrode. By forming a cavity in the substrate, the piezoelectric resonant structure is suspended above the cavity, reducing acoustic energy leakage towards the substrate and improving the device's resonant performance. To further improve performance, some designs incorporate air bridges, isolation grooves, or other boundary structures around the resonant region to reduce lateral acoustic wave leakage; others add a temperature compensation layer to mitigate frequency drift caused by temperature variations. However, these designs typically address lateral acoustic energy constraint and temperature compensation separately, requiring different functional structures. This not only increases the complexity of the device structure and manufacturing process but also may lead to insufficient process compatibility, increased manufacturing costs, and reduced device integration between different functional structures. Furthermore, some existing boundary structures are susceptible to the influence of etching media during device release processes, thereby affecting device consistency and manufacturing yield. Therefore, how to ensure device resonant performance while simultaneously considering lateral acoustic field constraint, temperature stability, and process reliability has become a pressing technical problem for those skilled in the art. Summary of the Invention

[0003] In view of this, this application provides a bulk acoustic resonator and its fabrication method to solve the technical problem that it is difficult to balance the acoustic performance and temperature performance of bulk acoustic resonators in the prior art.

[0004] Specifically, this application is implemented through the following technical solution:

[0005] The first aspect of this application provides a bulk acoustic resonator, comprising:

[0006] A first cavity is provided below the resonant unit;

[0007] The resonant unit is provided with at least a bottom electrode, a piezoelectric layer and a top electrode from bottom to top, and there is no cavity in the resonant unit;

[0008] Above the bottom electrode, a dielectric layer is provided in the same layer as the piezoelectric layer. The dielectric layer surrounds the piezoelectric layer. The top electrode covers the dielectric layer. The dielectric layer serves as both a region of abrupt change in acoustic impedance and a region of temperature compensation.

[0009] A second aspect of this application provides a method for fabricating a bulk acoustic resonator, the method comprising:

[0010] A substrate is provided, and a first cavity is formed in the substrate;

[0011] A bottom electrode is formed on the substrate, such that the bottom electrode is located in the region corresponding to the first cavity;

[0012] A piezoelectric layer is formed on the bottom electrode;

[0013] A dielectric layer is formed, which is disposed in the same layer as the piezoelectric layer and surrounds the piezoelectric layer in a plane. The dielectric layer simultaneously forms an acoustic impedance abrupt change region and a temperature compensation region in space.

[0014] A top electrode is formed above the piezoelectric layer, and the top electrode covers the dielectric layer.

[0015] The bulk acoustic wave resonator and its fabrication method provided in this application differ from existing bulk acoustic wave resonators that typically employ independent air bridge structures or independent temperature compensation layers to achieve lateral acoustic confinement and temperature stability adjustment respectively. This application utilizes a surrounding dielectric layer on the same layer as the piezoelectric layer, forming an integrated adjacent structure with the piezoelectric layer in the planar direction. Only one dielectric layer is added at the same location as the piezoelectric layer, without increasing the overall resonator thickness. Simultaneously, a single dielectric layer can achieve both acoustic impedance abrupt changes and temperature compensation, thus effectively suppressing lateral acoustic energy leakage and synergistically optimizing device temperature drift without introducing additional complex structures. Furthermore, by introducing a dielectric layer with a partitioned structure above the bottom electrode, forming a continuous lateral surrounding structure around the effective resonance region, laterally propagating sound waves undergo acoustic impedance discontinuity reflection at the interface between the dielectric layer, the piezoelectric layer, and the electrode material. This enhances the acoustic energy confinement capability in non-effective resonance regions, improving the energy concentration and resonance stability of the device. Furthermore, by introducing a material region with a positive temperature coefficient into the dielectric layer and forming a thermal coupling relationship with the piezoelectric layer and electrode structure, the frequency drift generated by the device during temperature changes is partially offset, thereby improving the device's temperature stability. Simultaneously, the synergistic arrangement of the dielectric layer and the outer protective region of the bottom electrode also improves the structural stability and fabrication reliability of the device during etching and release processes. Through the synergistic effect of the above structures, this application achieves integrated acoustic confinement and temperature compensation while maintaining device structural compactness and fabrication feasibility, thus enhancing the overall performance of the bulk acoustic resonator. Attached Figure Description

[0016] Figure 1 A cross-sectional schematic diagram of a bulk acoustic resonator structure embodiment provided for an exemplary embodiment of this application;

[0017] Figure 2 A top view schematic diagram of the positional relationship between the dielectric layer and the piezoelectric layer in a bulk acoustic resonator provided as an exemplary embodiment of this application;

[0018] Figure 3 A cross-sectional schematic diagram of a second embodiment of a bulk acoustic resonator structure provided for an exemplary embodiment of this application;

[0019] Figure 4 A cross-sectional schematic diagram of a third embodiment of a bulk acoustic resonator structure provided for an exemplary embodiment of this application;

[0020] Figure 5 A cross-sectional schematic diagram of a fourth embodiment of a bulk acoustic resonator structure provided for an exemplary embodiment of this application;

[0021] Figure 6A flowchart of an embodiment of a method for fabricating a bulk acoustic resonator provided in an exemplary embodiment of this application.

[0022] In the figure: 101, substrate; 102, cavity; 103, bottom electrode; 1031, first bottom electrode; 1032, second bottom electrode; 104, piezoelectric layer; 105, dielectric layer; 106, top electrode. Detailed Implementation

[0023] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with this application.

[0024] The terminology used in this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The singular forms “a,” “the,” and “the” used herein are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any and all possible combinations of one or more of the associated listed items.

[0025] It should be understood that although the terms first, second, third, etc., may be used in this application to describe various information, such information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, without departing from the scope of this application, first information may also be referred to as second information, and similarly, second information may also be referred to as first information. Depending on the context, the word "if" as used herein may be interpreted as "when," "when," or "in response to determination."

[0026] The following specific embodiments are given to illustrate the technical solution of this application in detail.

[0027] This application provides a bulk acoustic resonator, such as... Figure 1 As shown, the bulk acoustic resonator includes a resonant unit and also includes:

[0028] A first cavity 102 is provided below the resonant unit;

[0029] The resonant unit is provided with at least a bottom electrode 103, a piezoelectric layer 104 and a top electrode 106 from bottom to top, and there is no cavity in the resonant unit;

[0030] Above the bottom electrode 103, a dielectric layer 105 is provided in the same layer as the piezoelectric layer 104. The dielectric layer 105 surrounds the piezoelectric layer 104. The top electrode 106 covers the dielectric layer 105. The dielectric layer 105 serves as both a region of abrupt change in acoustic impedance and a temperature compensation region.

[0031] Figure 1 This is a cross-sectional schematic diagram of an embodiment of a bulk acoustic resonator structure provided for an exemplary embodiment of this application. (See attached diagram.) Figure 1 As shown, this embodiment provides a bulk acoustic wave resonator, which includes a substrate 101, a first cavity 102 formed in the substrate 101, and a resonant unit disposed on the substrate 101. The resonant unit is disposed on the substrate 101 and spans above the first cavity 102 to form a suspended resonant structure, thereby reducing the conduction loss of acoustic energy in the direction of the substrate 101. In this embodiment, the first cavity 102, the bottom electrode 103, the piezoelectric layer 104, and the top electrode 106 are all coaxially arranged about the same central axis in the planar projection direction, forming a coaxially aligned stacked structure. The planar projection range of the bottom electrode 103 is larger than the planar projection range of the first cavity 102, and the planar projection of the first cavity 102 at least covers the effective resonant region, so that the effective resonant region is located above the first cavity 102 to form a suspended structure. Furthermore, the planar projection range of the top electrode 106 is larger than the planar projection range of the bottom electrode 103 and the piezoelectric layer 104, and extends outward to the periphery of the effective resonant region to cover the temperature compensation layer subsequently formed on top of the piezoelectric layer, and form an outer electrode structure.

[0032] In this embodiment, the common projection area of ​​the first cavity 102, bottom electrode 103, piezoelectric layer 104, dielectric layer 105 and top electrode 106 in the vertical direction constitutes an effective resonant region. The effective resonant region is used to realize the electromechanical coupling and resonant response of the bulk acoustic wave, and its peripheral region is a non-effective region.

[0033] The resonant unit is the core functional structure of the bulk acoustic resonator for realizing bulk acoustic excitation and resonant response, and it includes at least a bottom electrode 103, a piezoelectric layer 104 and a top electrode 106 in sequence along the thickness direction.

[0034] The bottom electrode 103 is disposed on the side close to the first cavity 102, and is used to receive external driving signals and serve as the lower potential application end of the piezoelectric layer 104; the top electrode 106 is disposed on the side of the piezoelectric layer 104 away from the bottom electrode 103, and is used as the upper potential application end of the piezoelectric layer 104, and together with the bottom electrode 103, establishes an alternating electric field in the thickness direction of the piezoelectric layer 104.

[0035] The piezoelectric layer 104 is disposed between the bottom electrode 103 and the top electrode 106. When an alternating electrical signal is applied between the bottom electrode 103 and the top electrode 106, the bottom electrode 103 and the top electrode 106 serve as the upper and lower potential application terminals, respectively, forming an alternating electric field distribution in the thickness direction of the piezoelectric layer 104. Under the action of this alternating electric field, the piezoelectric layer 104 generates a periodic mechanical strain response based on the piezoelectric effect, causing the piezoelectric layer 104 to undergo mechanical vibration in the thickness direction corresponding to the frequency of the electrical signal, thereby converting electrical energy into mechanical vibration energy in the thickness direction and forming a bulk acoustic wave within the piezoelectric layer 104. When the frequency of the alternating electrical signal matches the natural resonant frequency in the thickness direction of the piezoelectric layer 104, a stable standing wave in the thickness direction is formed in the effective resonant region, thereby realizing bulk acoustic wave resonance.

[0036] It should be noted that in existing bulk acoustic resonators, the piezoelectric layer 104 is typically formed continuously across the entire electrode coverage area. During resonance, laterally propagating acoustic waves easily diffuse from the effective resonant region to the periphery, leading to partial acoustic energy leakage and thus reducing the device's quality factor. Simultaneously, piezoelectric materials and electrode materials generally exhibit temperature-dependent frequency drift characteristics, causing the resonant frequency to shift with changes in operating temperature, affecting the device's stability. Therefore, it is necessary to optimize the lateral acoustic energy confinement capability and frequency-temperature stability without altering the main stacked structure of the resonant unit.

[0037] In one embodiment, a dielectric layer 105 is further disposed above the bottom electrode 103. The dielectric layer 105 is disposed in the same layer as the piezoelectric layer 104. The dielectric layer 105 fills the groove formed around the piezoelectric layer 104 and forms an acoustic impedance abrupt change region around the effective resonant region, and a temperature compensation region above the effective resonant region, thereby achieving integrated acoustic boundary construction and temperature compensation functions. Through the above structural configuration, the effective electromechanical coupling region of the piezoelectric layer 104 is concentrated in the effective resonant region. At the same time, since the continuous thin film structure of the piezoelectric layer 104 in the peripheral edge region is removed, the residual edge stress generated during the fabrication process of the piezoelectric layer 104 is released, and the risk of structural deformation or diaphragm cracking caused by stress concentration during cavity release is reduced, thereby improving the structural stability and fabrication yield of the resonant unit.

[0038] Furthermore, the top electrode 106 covers the dielectric layer 105. Specifically, the top electrode 106 is formed above the piezoelectric layer 104 and the dielectric layer 105, and covers at least the area where the dielectric layer 105 is located in the planar projection direction, so that the dielectric layer 105 is located between the top electrode 106 and the bottom electrode 103. Through the above structural arrangement, the dielectric layer 105 forms a confined space structure inside the resonant unit. During the subsequent release process of the first cavity 102, the top electrode 106 can provide structural protection for the dielectric layer 105, reducing the risk of the dielectric layer 105 being damaged or degraded due to direct contact between the etching medium and the dielectric layer 105 during the release process, thereby ensuring that the dielectric layer 105 can be stably retained in the resonant unit structure. At the same time, since the top electrode 106 covers the dielectric layer 105, the dielectric layer 105 can continuously form the temperature compensation region corresponding to the peripheral acoustic impedance modulation region and the effective resonance region, thereby ensuring the acoustic confinement performance and frequency temperature stability of the device.

[0039] Optionally, in one possible implementation, the dielectric layer 105 includes a first portion and a second portion, the first portion being used to form a region of abrupt change in acoustic impedance, and the second portion being used to compensate for the frequency temperature coefficient between the piezoelectric layer and the top electrode.

[0040] As an optional embodiment, such as Figure 1 As shown, the first part is disposed in the groove formed around the piezoelectric layer 104 and distributed along the lateral boundary region of the piezoelectric layer 104 to fill the structural region after the periphery of the piezoelectric layer 104 is removed, thereby forming a region of abrupt change in acoustic impedance around the piezoelectric layer 104; the second part is disposed in the region directly above the piezoelectric layer 104 and at least partially covers the upper surface region of the piezoelectric layer 104.

[0041] Figure 2 This is a top view schematic diagram illustrating the positional relationship between the dielectric layer and the piezoelectric layer in a bulk acoustic resonator provided as an exemplary embodiment of this application. Figure 2 As shown, to facilitate explanation of the planar positional relationship between the dielectric layer 105 and the piezoelectric layer 104, a top view schematic diagram of the positional relationship between the dielectric layer and the piezoelectric layer in a bulk acoustic resonator is presented. Figure 2It is understood that the piezoelectric layer 104 is located in the central region, and the dielectric layer 105 is disposed along the peripheral region of the piezoelectric layer 104 and distributed around the piezoelectric layer 104 in the planar projection direction. The dielectric layer 105 located on the periphery of the piezoelectric layer 104 forms a first portion, which is used to form an acoustic impedance abrupt change interface in the edge region of the piezoelectric layer 104 to limit the outward diffusion of laterally propagating sound waves. Further, the dielectric layer 105 also includes a second portion located above the piezoelectric layer 104, the second portion being connected to the first portion to form a continuous dielectric structure, used to achieve temperature compensation within the effective resonant region.

[0042] Optionally, in one possible implementation, the first portion forms an acoustic impedance discontinuity interface with the piezoelectric layer 104 and the electrode material to generate acoustic wave reflection in the lateral propagation direction, thereby suppressing acoustic energy leakage to the overlapping region.

[0043] Specifically, the first part fills the groove formed around the piezoelectric layer. Since there is a material acoustic impedance difference between the first part and the piezoelectric layer 104, the bottom electrode 103, and the top electrode 106, an acoustic impedance mismatch interface is formed in the lateral propagation direction to reflect the lateral propagating sound waves and suppress the diffusion of sound energy to the non-effective resonance region.

[0044] Optionally, in one possible implementation, the second part includes a dielectric material with a positive temperature coefficient to counteract the resonant frequency drift caused by the negative temperature coefficients of the piezoelectric layer and the electrode material.

[0045] Specifically, the second part is made of a dielectric material with a positive temperature coefficient and forms a thermal coupling structure with the piezoelectric layer 104, the bottom electrode 103, and the top electrode 106. This structure is used to compensate for the resonant frequency drift caused by the change in the elastic constant, density, and sound propagation speed of the piezoelectric layer 104 and the electrode material with temperature, thereby improving the frequency and temperature stability of the device.

[0046] In this embodiment, the first part and the second part are preferably integrally formed using the same dielectric material and a continuous dielectric layer is formed through a single deposition process. The only difference lies in their positions within the resonant unit, which respectively form a peripheral acoustic impedance abrupt change region and a temperature compensation region on the effective resonant region. The material type, thickness, and coverage of the dielectric layer can be optimized according to performance indicators such as the device's target frequency, quality factor, and frequency temperature coefficient. Through this structural arrangement, the dielectric layer 105 achieves integrated acoustic constraint and temperature compensation functions without altering the main stacked structure of the resonant unit.

[0047] Optionally, in one possible implementation, the dielectric layer includes a second portion disposed directly above the piezoelectric layer and a first portion disposed horizontally on the second portion and the piezoelectric layer, the first portion being in communication with the second portion.

[0048] Specifically, the first part and the second part are spatially interconnected, so that the dielectric layer 105 forms an integrated continuous structural region. Without increasing the number of layers stacked in the thickness direction of the resonant unit, a temperature compensation region is simultaneously formed on the outer acoustic impedance abrupt change region and the effective resonant region.

[0049] Optionally, in one possible implementation, the material of the dielectric layer includes at least one of silicon dioxide, fluorine-doped silicon dioxide, silicon nitride, aluminum nitride, or polyimide.

[0050] Specifically, the dielectric layer 105 can be composed of one or more materials selected from silicon dioxide, fluorine-doped silicon dioxide (SiOF), silicon nitride, aluminum nitride, or polyimide. Different materials can be used as acoustic impedance modulation media or temperature compensation media according to the functional requirements of the device, to meet the requirements for acoustic performance and frequency temperature stability under different process conditions.

[0051] like Figure 1 As shown, this embodiment provides a bulk acoustic wave resonator, which includes a substrate 101 and a resonant unit disposed on the substrate 101. The resonant unit is the core structure for realizing electromechanical conversion and resonant response, and is composed of a bottom electrode 103, a piezoelectric layer 104 and a top electrode 106 arranged sequentially along the thickness direction. The substrate 101 is used to support the resonant unit and a first cavity 102 is formed inside it.

[0052] Furthermore, structural support regions are formed on the substrate 101 on both sides of the first cavity 102 to support the resonant unit, and together with the top electrode 106 and the bottom electrode 103, they form a protective structure for the dielectric layer 105.

[0053] The structural support region is formed by retaining the substrate 101 or by patterning the substrate 101. The structural support region is located on both sides of the first cavity 102, allowing the resonant unit to span the first cavity 102 in the thickness direction. The structural support region can be formed according to device structure and fabrication process requirements, for example, by patterning the substrate, piezoelectric layer, or other structural layers, or by retaining a portion of the structural region. Specifically, during the release process of the first cavity 102, the release etching medium needs to enter the first cavity 102 to remove the sacrificial layer. Since the dielectric layer 105 is disposed in the peripheral region of the piezoelectric layer 104, and is protected by the top electrode 106 above it, the bottom electrode 103 below it, and the surrounding structural support region, an isolation is formed between the dielectric layer 105 and the release etching medium, thereby reducing the risk of corrosion or damage to the dielectric layer 105 during cavity release and improving device structural stability.

[0054] The first cavity 102 is formed inside the substrate 101 and located in the region below the bottom electrode 103, and is used to isolate the solid-state acoustic propagation path between the resonant unit and the substrate 101 in the thickness direction.

[0055] The first cavity 102 is positioned at least corresponding to the effective resonant region in the planar projection direction, so that the effective resonant region is located above the first cavity 102 to form a floating structure, thereby reducing the conduction loss of acoustic energy to the substrate 101 during resonance.

[0056] By setting the first cavity 102, the resonant unit forms a suspended resonant structure during operation. The sound field in the thickness direction is mainly confined inside the resonant unit, reducing the propagation of sound energy to the substrate 101, thereby reducing sound energy loss and improving the resonant performance of the device.

[0057] Optionally, in one possible implementation, the top electrode 106 includes a main body portion located above the piezoelectric layer 104, and an extensional portion extending to both sides from the main body portion.

[0058] The epitaxial portion covers the dielectric layer 105 in the planar projection direction and extends to the structural support regions on both sides of the substrate 101.

[0059] Specifically, the main body portion is disposed on the upper surface region of the piezoelectric layer 104, and is used to form a major electric field interaction region in the thickness direction with the bottom electrode 103 to realize the electromechanical conversion between electrical signals and acoustic signals. The epitaxial portion is integrally formed with the main body portion or electrically connected to it, and extends to both sides of the main body portion in the planar direction.

[0060] The epitaxial portion covers the area above the dielectric layer 105 in the planar projection direction, such that the top electrode 106 forms an extension structure across the boundary of the effective resonant region in the lateral direction. Furthermore, the epitaxial portion extends to the structural support regions on both sides of the substrate 101, so that the top electrode 106 forms a structurally extended coverage at the device edge region.

[0061] Through the thickness-direction overlap structure between the main body of the top electrode 106 and the bottom electrode 103, a primary electric field excitation region can be formed in the piezoelectric layer 104 for electromechanical energy conversion. The epitaxial portion covers the area above the dielectric layer 105 in the planar projection direction and extends to the structural support region, allowing the top electrode 106 to cover the temperature compensation region and form a stable connection with the structural support region. This structural arrangement enables the top electrode 106 to form an effective electromechanical coupling structure with the bottom electrode 103 while also covering the dielectric layer 105 and the structural support region, thus meeting the requirements for device electrical connection and structural layout.

[0062] Optionally, in one possible implementation, the bottom electrode includes a first bottom electrode 1031 and a second bottom electrode 1032 stacked along the thickness direction, with the first bottom electrode 1031 disposed on the side near the first cavity 102 and the second bottom electrode 1032 disposed on the side near the piezoelectric layer 104.

[0063] The planar projection of the second bottom electrode 1032 is located within the planar projection range of the first bottom electrode 1031, and the planar projection of the first cavity 102 is located within the planar projection range of the first bottom electrode 1031.

[0064] Specifically, the planar projection of the second bottom electrode 1032 is located within the planar projection range of the first bottom electrode 1031, thereby covering the second bottom electrode 1032 with the first bottom electrode 1031 in the lateral direction. At the same time, the planar projection of the first cavity 102 is located within the planar projection range of the first bottom electrode 1031, enabling the bottom electrode 1031 to form a continuous support structure in the region above the cavity.

[0065] Specifically, the first bottom electrode 1031 and the second bottom electrode 1032 can be electrically connected to form a continuous bottom electrode structure, while the acoustic characteristics are partitioned in space through different material regions.

[0066] Optionally, in one possible implementation, the first bottom electrode 1031 is divided into a first region and a second region in the horizontal direction. The first region is in contact with the second bottom electrode 1032, and the second region is in contact with the dielectric layer 105. The second region is located outside the first region in the horizontal direction. The first region includes tungsten or a tungsten-based high acoustic impedance material, and the second region includes a molybdenum material to act as an etching barrier layer to protect the dielectric layer 105.

[0067] The first region is located on the side close to the second bottom electrode 1032 and is in direct contact with the second bottom electrode 1032 to form a bottom electrode structure within the effective resonant region. The second region is located horizontally outside the first region and is in contact with the dielectric layer 105.

[0068] Specifically, the second region is in structural contact with the dielectric layer 105 and is used as an etch barrier layer in subsequent fabrication processes to protect the dielectric layer 105 from erosion by the etchant during release or etching processes, thereby improving the structural stability of the device.

[0069] The first region includes tungsten or tungsten-based high acoustic impedance material to improve the overall acoustic impedance characteristics of the bottom electrode and reduce acoustic energy loss; the second region includes molybdenum material to form a process-compatible epitaxial protection structure.

[0070] It should be noted that, because the first region and the second region are made of different materials, the first bottom electrode 1031 forms an acoustic impedance difference structure in the horizontal direction. This acoustic impedance difference creates an acoustic impedance gradient interface in the edge region of the bottom electrode, thereby producing a certain reflection or attenuation effect on the sound wave in the lateral propagation direction, enhancing the boundary constraint capability of the sound energy, and reducing the diffusion of sound energy into the non-effective resonant region.

[0071] In summary, as Figure 1 In the illustrated embodiment, the combination of the first cavity, the bottom electrode, and the dielectric layer allows bulk acoustic waves to propagate primarily along the thickness direction. Simultaneously, the acoustic impedance boundary formed by the dielectric layer and the acoustic impedance gradient structure formed at the edge of the bottom electrode constrain the laterally propagating acoustic waves, thereby increasing the concentration of acoustic energy. Furthermore, the second portion of the dielectric layer, positioned above the piezoelectric layer, forms a thermally coupled structure with the piezoelectric layer and the electrode, compensating for resonant frequency drift caused by temperature changes and improving the device's temperature stability. Moreover, the interaction between the epitaxial portion of the top electrode and the dielectric layer and structural support region optimizes the electric field distribution at the edge of the effective resonant region and, to a certain extent, suppresses edge parasitic modes, improving the device's resonant performance and operational stability.

[0072] Figure 3A cross-sectional schematic diagram of a second embodiment of a bulk acoustic resonator structure provided for an exemplary embodiment of this application, as shown below. Figure 3 As shown, this embodiment simplifies the structure based on embodiment one. The basic structure of its overall bulk acoustic resonator is the same as that of embodiment one, including substrate 101, first cavity 102, piezoelectric layer 104, dielectric layer 105, and top electrode 106. The same structure will not be described again here.

[0073] The main difference between Embodiment 2 and Embodiment 1 lies in the arrangement of the bottom electrode 103 structure. Specifically, in this embodiment, the bottom electrode 103 is a single-layer structure, which is disposed below the piezoelectric layer 104 and above the first cavity 102. It is used together with the top electrode 106 to form an electric field excitation condition in the thickness direction within the piezoelectric layer 104, thereby realizing electromechanical energy conversion and bulk acoustic wave resonance excitation.

[0074] The single-layer bottom electrode 103 at least partially overlaps with the piezoelectric layer 104 and the top electrode 106 in the planar projection direction to define an effective resonant region and provide continuous lower electrode boundary conditions for the piezoelectric layer 104, enabling the piezoelectric layer 104 to form a vibration mode in the thickness direction under the action of alternating electrical signals. Compared with the multi-layer bottom electrode structure using a first bottom electrode 1031 and a second bottom electrode 1032 in Embodiment 1, this embodiment uses a single-layer bottom electrode 103, eliminating the need for a double-layer electrode structure, thereby reducing the interlayer interface of the bottom electrode and simplifying the device fabrication process.

[0075] Compared to the double-layer bottom electrode 103 structure, the single-layer bottom electrode 103 structure reduces the process steps such as bottom electrode deposition, patterning and interlayer alignment, which helps to reduce the complexity of device fabrication and the number of interlayer interfaces, thereby improving the continuity of electrode conductivity and the consistency of device fabrication to a certain extent.

[0076] Optionally, the edge region of the bottom electrode 103 is configured with a slope transition structure, so that the thickness of the bottom electrode 103 gradually decreases along the edge direction. By setting the slope transition structure, the coverage quality of subsequent thin films such as the piezoelectric layer 104 in the edge region of the bottom electrode can be improved, and phenomena such as cracks, gaps or discontinuities in the film at the step edge can be reduced, thereby improving the subsequent film deposition quality and device fabrication reliability.

[0077] It should be noted that the single-layer bottom electrode 103 in Embodiment 2 can be formed of metal materials such as tungsten, molybdenum, gold, silver, copper, aluminum, titanium or their alloys to meet the requirements of conductivity and device process.

[0078] With the above structural setup, Embodiment 2 adopts a single-layer bottom electrode structure while maintaining the basic resonance mechanism of the bulk acoustic wave resonator, thereby reducing the complexity of device fabrication and improving process consistency. At the same time, the edge slope transition structure improves the coverage quality of subsequent thin films and enhances the reliability of device fabrication.

[0079] Figure 4 A cross-sectional schematic diagram of a third embodiment of a bulk acoustic resonator structure provided for an exemplary embodiment of this application, as shown below. Figure 4 As shown, this embodiment is based on the first embodiment with structural adjustments. The basic structure of its overall bulk acoustic resonator is the same as that of the first embodiment, including a substrate 101, a first cavity 102, a bottom electrode 103, a piezoelectric layer 104, and a top electrode 106. The same structure will not be described again here.

[0080] The main difference between Embodiment 3 and Embodiment 1 lies in the structural configuration of the dielectric layer 105. Specifically, in Embodiment 3, the upper surface of the piezoelectric layer 104 is in direct contact with the top electrode 106, and the dielectric layer 105 structure located above the piezoelectric layer 104 is no longer provided. The outer dielectric layer 105 is used to form a region of abrupt change in acoustic impedance to constrain transversely propagating sound waves, and no longer serves the function of temperature compensation in the effective resonant region.

[0081] In this structure, the bottom electrode 103, the piezoelectric layer 104, and the top electrode 106 still constitute the main electromechanical coupling structure in the thickness direction, and form an effective resonant region in the planar projection direction to realize bulk acoustic wave resonant excitation.

[0082] Figure 5 A cross-sectional schematic diagram of a fourth embodiment of a bulk acoustic resonator structure provided for an exemplary embodiment of this application, as shown below. Figure 5 As shown, Embodiment 4 is based on Embodiment 1 with structural adjustments. The basic structure of its bulk acoustic resonator is the same as that of Embodiment 1, including substrate 101, first cavity 102, bottom electrode 103, piezoelectric layer 104 and top electrode 106. The same structure will not be described again here.

[0083] The main difference between Example 4 and Example 1 lies in the structure of the resonant unit and the material of the dielectric layer.

[0084] Specifically, in Embodiment 4, the bottom electrode 103 adopts a single-layer structure, that is, it only includes a single electrode layer structure, and the double-layer bottom electrode structure of Embodiment 1 is no longer used. For related details, please refer to the description of Embodiment 2, which will not be repeated here.

[0085] Specifically, in Embodiment 4, the piezoelectric layer 104 is in direct contact with the top electrode 106, and the dielectric layer 105 structure located above the piezoelectric layer 104 is no longer provided. For related details, please refer to the description of Embodiment 3, which will not be repeated here.

[0086] Furthermore, in this embodiment, the dielectric layer 105 is made of polyimide (PI) material. The PI material is disposed in the lateral region of the piezoelectric layer 104 and surrounds the piezoelectric layer 104 in the planar direction, thereby forming an acoustic impedance abrupt change structure region around the piezoelectric layer 104, thereby replacing the traditional air bridge structure and realizing lateral acoustic wave confinement.

[0087] Specifically, the PI material does not react with the etching medium used for cavity release, so that the dielectric layer 105 does not require additional protection during cavity release and can be stably retained in the resonant unit structure.

[0088] In this structure, the bottom electrode 103, the piezoelectric layer 104, and the top electrode 106 still constitute the main electromechanical coupling structure in the thickness direction and are used to realize the bulk acoustic wave resonant excitation and resonant response.

[0089] The bulk acoustic wave resonator provided in this application has a first cavity below the resonant unit, making the resonant unit a suspended resonant structure and reducing the conduction of acoustic energy towards the substrate. A dielectric layer, co-layered with and surrounding the piezoelectric layer, is disposed above the bottom electrode, allowing the dielectric layer to simultaneously form a region of abrupt acoustic impedance change and a temperature compensation region within the same continuous structure. The outer region of the dielectric layer forms a lateral acoustic boundary, constraining laterally propagating sound waves and reducing acoustic energy leakage to the effective resonant region. The region of the dielectric layer above the piezoelectric layer is used to compensate for the frequency-temperature characteristics of the device, reducing the drift of the resonant frequency due to temperature changes. Therefore, this application achieves integrated acoustic boundary construction and temperature compensation functions through a single dielectric layer, improving the device's lateral acoustic energy constraint capability, enhancing its frequency-temperature stability, and facilitating miniaturization and fabrication.

[0090] In addition to providing a bulk acoustic wave resonator, this application also provides a method for fabricating a bulk acoustic wave resonator. The method provided in this application is described below.

[0091] Figure 6 For a flowchart of the fabrication method of a bulk acoustic resonator provided in an exemplary embodiment of this application, please refer to Embodiment 1. Figure 6 The method for fabricating a bulk acoustic resonator provided in this embodiment includes:

[0092] S601. Provide a substrate and form a first cavity 102 in the substrate 101.

[0093] Specifically, a silicon, silicon carbide, sapphire, or spinel substrate 101 is provided. A first cavity 102 is formed through photolithography, etching, and resist removal processes, and a sacrificial layer is formed within the first cavity. The sacrificial layer may be formed using phosphorus-doped silicon dioxide (PSG) material. Subsequently, a CMP process is used to planarize the sacrificial layer to obtain a flat surface for subsequent thin film deposition.

[0094] S602, A bottom electrode 103 is formed on the substrate 101, such that the bottom electrode 103 is located in the corresponding region of the first cavity 102.

[0095] Specifically, a bottom electrode 103 is deposited on the upper surface of the substrate 101, and a corresponding structure is formed through a patterning process, such that the bottom electrode 103 is located above the corresponding region of the first cavity 102. In some possible implementations, the bottom electrode 103 may include a first bottom electrode 1031 and a second bottom electrode 1032 to form a double-layer bottom electrode structure. The first bottom electrode 1031 has high acoustic impedance and Young's modulus, and high tolerance to the etch medium released from the cavity; the second bottom electrode 1032 is disposed above the first bottom electrode 1031 to form the main electrode structure. The thickness of the first bottom electrode 1031 is less than half the thickness of the second bottom electrode 1032. By combining different materials and thicknesses, the acoustic performance of the effective resonant region is improved, while the parasitic effects in the peripheral region are reduced.

[0096] S603, A piezoelectric layer 104 is formed on the bottom electrode 103.

[0097] Specifically, a piezoelectric layer 104 is formed on the bottom electrode 103, and the piezoelectric layer 104 is patterned to form a groove structure around the effective resonant region. The inner side of the groove corresponds to the region of the first cavity 102, and the outer side corresponds to the peripheral region of the first cavity 102. The groove serves two purposes: firstly, to release the internal residual stress generated during the fabrication of the piezoelectric layer, reducing the risk of diaphragm cracking during subsequent cavity release; and secondly, to accommodate the subsequently formed dielectric layer.

[0098] S604. Form a dielectric layer 105, such that the dielectric layer 105 is disposed in the same layer as the piezoelectric layer 104 and surrounds the piezoelectric layer 104 in a plane, wherein the dielectric layer 105 simultaneously forms an acoustic impedance abrupt change region and a temperature compensation region in space.

[0099] Specifically, a dielectric layer 105 is formed on the piezoelectric layer 104 and planarized using a CMP process. The dielectric layer 105 is then patterned so that it covers the grooves in the piezoelectric layer 104 and is positioned above the effective resonant region. The dielectric layer 105 positioned above the effective resonant region forms a temperature compensation region; the dielectric layer 105 located in the grooves of the piezoelectric layer 104 forms a region of abrupt changes in acoustic impedance.

[0100] S605. A top electrode 106 is formed above the piezoelectric layer 104, and the top electrode 106 covers the dielectric layer 105.

[0101] Specifically, a top electrode 106 is formed on the piezoelectric layer 104 and the dielectric layer 105, and a corresponding structure is formed through a patterning process, so that the top electrode 106 covers the area where the dielectric layer 105 is located. In some embodiments, the top electrode 106 includes a main body portion and an epitaxial portion, the epitaxial portion covering the dielectric layer 105 and extending to the structural support region to form a complete resonant unit structure.

[0102] Optionally, after forming the top electrode 106, a mass loading layer or a protrusion structure may be formed on the top electrode 106 to improve the device quality factor. Then, the sacrificial layer in the first cavity 102 is removed using a cavity release process to form a suspension structure corresponding to the first cavity 102, thereby creating an acoustic isolation space between the resonant unit and the substrate, completing the fabrication of the bulk acoustic wave resonator.

[0103] Through the above steps, the bulk acoustic resonator in the aforementioned embodiment can be prepared. Since the dielectric layer 105 adopts an integrated continuous structure, simultaneously forming a region of abrupt change in acoustic impedance and a temperature compensation region, the synergistic integration of lateral acoustic constraint and temperature compensation functions is achieved without adding additional structural layers, which is beneficial for improving the device's resonant performance and temperature stability.

[0104] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A bulk acoustic resonator, comprising a resonant unit, characterized in that: A first cavity is provided below the resonant unit; The resonant unit is provided with at least a bottom electrode, a piezoelectric layer and a top electrode from bottom to top, and there is no cavity in the resonant unit; Above the bottom electrode, a dielectric layer is provided in the same layer as the piezoelectric layer. The dielectric layer surrounds the piezoelectric layer. The top electrode covers the dielectric layer. The dielectric layer serves as both a region of abrupt change in acoustic impedance and a region of temperature compensation.

2. The bulk acoustic resonator according to claim 1, characterized in that: The dielectric layer includes a first part and a second part. The first part is used to form a region of abrupt change in acoustic impedance, and the second part is used to compensate for the frequency temperature coefficient between the piezoelectric layer and the top electrode.

3. The bulk acoustic resonator according to claim 2, characterized in that: The first part forms an acoustic impedance discontinuity interface with the piezoelectric layer and electrode material to generate sound wave reflection in the lateral propagation direction, thereby suppressing sound energy leakage to the overlapping area.

4. The bulk acoustic resonator according to claim 2, characterized in that: The second part includes a dielectric material with a positive temperature coefficient to counteract the resonant frequency drift caused by the negative temperature coefficient of the piezoelectric layer and electrode material.

5. The bulk acoustic resonator according to claim 1, characterized in that: The dielectric layer includes a second portion disposed directly above the piezoelectric layer and a first portion disposed on the horizontal side of the second portion and the piezoelectric layer, wherein the first portion is connected to the second portion.

6. The bulk acoustic resonator according to claim 1, characterized in that: The bottom electrode includes a first bottom electrode and a second bottom electrode stacked along the thickness direction. The first bottom electrode is disposed on the side close to the first cavity, and the second bottom electrode is disposed on the side close to the piezoelectric layer. The planar projection of the second bottom electrode is located within the planar projection range of the first bottom electrode, and the planar projection of the first cavity is located within the planar projection range of the first bottom electrode.

7. The bulk acoustic resonator according to claim 1, characterized in that: The top electrode includes a main body portion located above the piezoelectric layer, and an extensional portion extending to both sides from the main body portion; The epitaxial portion covers the dielectric layer in the planar projection direction and extends to the structural support regions on both sides of the substrate.

8. The bulk acoustic resonator according to claim 6, characterized in that: The first bottom electrode has a first region and a second region in the horizontal direction. The first region is in contact with the second bottom electrode, and the second region is in contact with the dielectric layer. The second region is located outside the first region in the horizontal direction. The first region includes tungsten or tungsten-based high acoustic impedance material, and the second region includes molybdenum material, so as to act as an etching barrier layer to protect the dielectric layer.

9. The bulk acoustic resonator according to claim 1, characterized in that: The material of the dielectric layer includes at least one of silicon dioxide, fluorine-doped silicon dioxide, silicon nitride, aluminum nitride, or polyimide.

10. A method for fabricating a bulk acoustic resonator, characterized in that, The preparation method is used to prepare a bulk acoustic resonator as described in any one of claims 1-9, comprising: A substrate is provided, and a first cavity is formed in the substrate; A bottom electrode is formed on the substrate, such that the bottom electrode is located in the region corresponding to the first cavity; A piezoelectric layer is formed on the bottom electrode; A dielectric layer is formed, which is disposed in the same layer as the piezoelectric layer and surrounds the piezoelectric layer in a plane. The dielectric layer simultaneously forms an acoustic impedance abrupt change region and a temperature compensation region in space. A top electrode is formed above the piezoelectric layer, and the top electrode covers the dielectric layer.

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