Bulk acoustic wave resonator and filter

Abstract

The application discloses a bulk acoustic wave resonator, a reflection function layer is located on one side surface of a substrate, a bottom electrode is located on the side surface of the reflection function layer away from the substrate, a piezoelectric thin film is located on the side surface of the bottom electrode away from the substrate, and a top electrode is located on the side surface of the piezoelectric thin film away from the substrate; the reflection function layer is provided with a cavity and a DBR reflection area; in the area where the top electrode and the bottom electrode overlap in the thickness direction, the area overlapping with the cavity is a working area, and the area not overlapping with the cavity is an invalid capacitance area; the DBR reflection area covers the invalid capacitance area, and a closed loop frame-shaped protrusion is arranged on the side surface of the top electrode away from the substrate and located in the working area. The DBR reflection area can inhibit the leakage of acoustic wave energy from the invalid capacitance area to the reflection function layer, the loop frame-shaped protrusion can inhibit the generation of a transverse derivative mode, and thus the quality factor of the bulk acoustic wave resonator as a whole is effectively increased. The application further provides a filter, which also has the beneficial effects.

Description

Technical Field

[0001] This invention relates to the field of acoustic resonator technology, and in particular to a bulk acoustic resonator and a filter. Background Technology

[0002] Bulk acoustic wave resonators (BAWs) typically refer to devices fabricated using silicon substrates with the aid of MEMS (Micro-Electro-Mechanical Systems) and thin-film technologies. They are used in wireless transceivers to achieve functions such as image cancellation, parasitic filtering, and channel selection, exhibiting high Q values ​​and ease of miniaturization. Thin-film BAWs, fabricated using the longitudinal resonance of piezoelectric thin films in the thickness direction, have become a viable alternative to surface acoustic wave (SAW) devices and quartz crystal resonators in mobile communications and high-speed serial data applications. RF front-end bulk wave filters / duplexers offer superior filtering characteristics, such as low insertion loss, a steep transition band, high power capacity, and strong electrostatic discharge (ESD) immunity. High-frequency thin-film BAW resonators with ultra-low frequency temperature drift exhibit low phase noise, low power consumption, and a wide bandwidth modulation range.

[0003] A bulk acoustic wave resonator comprises a thin film of piezoelectric material between two conductive electrodes. The piezoelectric layer of the bulk acoustic wave resonator is suspended in air by a supporting structure around it, and the air on both sides of the stack, i.e., the resonator interface, can capture energy generated during operation. The bulk acoustic wave resonator excites only thickness-extended (TE) modes, which are longitudinal mechanical waves with a propagation vector in the propagation direction. The TE modes travel in the thickness direction of the piezoelectric layer, for example, the z-direction.

[0004] However, in addition to the desired TE mode, there are also transverse modes generated in the acoustic stack. Transverse modes are mechanical waves with a k-vector perpendicular to the TE mode direction. These transverse modes travel along the area dimensions of the piezoelectric material, such as the x and y directions. Among other adverse effects, transverse modes detrimentally affect the quality factor Q of the BAW device. Specifically, the energy of the transverse modes is lost at the edges of the BAW device. This energy loss is the energy loss of the desired longitudinal mode and ultimately degrades the quality factor Q. Meanwhile, the acoustic waves transmitted in the piezoelectric film are specifically generated by the electrical signal applied between the top and bottom electrodes, and the energy generated by these acoustic waves easily leaks into the substrate, leading to a decrease in the Q value of the bulk acoustic resonator. Therefore, how to effectively improve the quality factor of the bulk acoustic resonator is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] The purpose of this invention is to provide a bulk acoustic resonator with a high quality factor; another purpose of this invention is to provide a filter with high performance.

[0006] To solve the above-mentioned technical problems, the present invention provides a bulk acoustic resonator, comprising a substrate, a reflective functional layer, a bottom electrode, a piezoelectric thin film, and a top electrode;

[0007] The reflective functional layer is located on one side surface of the substrate, the bottom electrode is located on the side surface of the reflective functional layer opposite to the substrate, the piezoelectric thin film is located on the side surface of the bottom electrode opposite to the substrate, and the top electrode is located on the side surface of the piezoelectric thin film opposite to the substrate.

[0008] The reflective functional layer is provided with a cavity and a DBR reflective area; the DBR reflective area is provided with alternating low acoustic impedance layers and high acoustic impedance layers along the thickness direction; the bottom electrode shields the cavity.

[0009] In the region where the top electrode and the bottom electrode overlap along the thickness direction, the region that overlaps with the cavity is the working region, and the region that does not overlap with the cavity is the ineffective capacitance region; the DBR reflection region covers the ineffective capacitance region, and a closed-loop frame-shaped protrusion is provided in the working region on the surface of the top electrode facing away from the substrate to adjust the distribution of the cutoff frequency in the working region.

[0010] Optionally, the cutoff frequency of the region where the closed-loop frame protrusion is located is less than the cutoff frequency of the central region, so that the strongest mode excited in the working area is the piston mode.

[0011] Optionally, the outer edge of the closed-loop frame protrusion is positioned along the inner sidewall of the cavity.

[0012] Optionally, the length of the DBR reflective region is greater than the length of the ineffective capacitor region.

[0013] Optionally, the film layer in the DBR reflection region that is in contact with the bottom electrode is a low acoustic impedance layer.

[0014] Optionally, the depth of the cavity is greater than the thickness of the low acoustic impedance layer that is in direct contact with the bottom electrode.

[0015] Optionally, the reflective functional layer is provided with a plurality of cavities, and each cavity having a bottom electrode, a piezoelectric film, and a top electrode on the side facing away from the substrate.

[0016] Optionally, the DBR reflection zone is provided between adjacent cavities.

[0017] Optionally, an air gap is provided in the DBR reflective area located between adjacent cavities.

[0018] The present invention also provides a filter comprising a bulk acoustic resonator as described in any of the preceding claims.

[0019] The present invention provides a bulk acoustic wave resonator, comprising a substrate, a reflective functional layer, a bottom electrode, a piezoelectric thin film, and a top electrode; the reflective functional layer is located on one side surface of the substrate, the bottom electrode is located on the side surface of the reflective functional layer facing away from the substrate, the piezoelectric thin film is located on the side surface of the bottom electrode facing away from the substrate, and the top electrode is located on the side surface of the piezoelectric thin film facing away from the substrate; the reflective functional layer is provided with a cavity and a DBR reflective region; the DBR reflective region is provided with alternating low acoustic impedance layers and high acoustic impedance layers along the thickness direction; the bottom electrode shields the cavity; in the region where the top electrode and the bottom electrode overlap along the thickness direction, the region overlapping with the cavity is the working region, and the region not overlapping with the cavity is the ineffective capacitance region; the DBR reflective region covers the ineffective capacitance region; a closed-loop frame-shaped protrusion is provided in the working region on the side surface of the top electrode facing away from the substrate to adjust the distribution of the cutoff frequency in the working region.

[0020] By setting a DBR reflection zone covering the invalid capacitance area in the functional reflection layer, the leakage of acoustic energy from the invalid capacitance area to the reflection functional layer can be suppressed, which can effectively reduce the longitudinal energy leakage; and by setting a closed-loop frame-shaped protrusion to adjust the distribution of the cutoff frequency in the working area, the generation of transverse derived modes can be effectively suppressed, thereby effectively increasing the overall quality factor of the bulk acoustic resonator.

[0021] The present invention also provides a filter that has the same beneficial effects as described above, which will not be described in detail here. Attached Figure Description

[0022] To more clearly illustrate the technical solutions of the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a schematic diagram of the structure of a bulk acoustic resonator provided in an embodiment of the present invention;

[0024] Figure 2 for Figure 1 A top-view structural diagram;

[0025] Figure 3 This is a schematic diagram of a specific bulk acoustic resonator provided in an embodiment of the present invention;

[0026] Figure 4 Smith chart for setting up a closed-loop frame-shaped protrusion front and rear body acoustic resonator.

[0027] In the figure: 1. Substrate, 2. Reflective functional layer, 21. Cavity, 22. Low acoustic impedance layer, 23. High acoustic impedance layer, 24. Air gap, 3. Bottom electrode, 4. Piezoelectric film, 5. Top electrode, 51. Closed-loop frame protrusion. Detailed Implementation

[0028] The core of this invention is to provide a bulk acoustic wave resonator. In the prior art, the ineffective capacitance region where the top and bottom electrodes overlap but do not overlap with the cavity leaks energy to the substrate, resulting in energy leakage. This leaked energy generates derived modes; simultaneously, transverse modes also exist within the acoustic stack. These transverse modes are mechanical waves with a k-vector perpendicular to the TE mode direction. These transverse modes travel along the area dimensions of the piezoelectric material, such as the x and y directions. Among other adverse effects, transverse modes also detrimentally affect the quality factor of the BAW device.

[0029] The bulk acoustic wave resonator provided by this invention includes a substrate, a reflective functional layer, a bottom electrode, a piezoelectric thin film, and a top electrode. The reflective functional layer is located on one side of the substrate, the bottom electrode is located on the side of the reflective functional layer facing away from the substrate, the piezoelectric thin film is located on the side of the bottom electrode facing away from the substrate, and the top electrode is located on the side of the piezoelectric thin film facing away from the substrate. The reflective functional layer is provided with a cavity and a DBR reflective region. The DBR reflective region is provided with alternating low acoustic impedance layers and high acoustic impedance layers along the thickness direction. The bottom electrode shields the cavity. In the region where the top electrode and the bottom electrode overlap along the thickness direction, the region overlapping with the cavity is the working region, and the region not overlapping with the cavity is the ineffective capacitance region. The DBR reflective region covers the ineffective capacitance region. A closed-loop frame-shaped protrusion is provided in the working region on the side of the top electrode facing away from the substrate to adjust the distribution of the cutoff frequency in the working region. This can typically flatten the waveform generated when the central region inside the closed-loop frame-shaped protrusion is working, thereby improving the quality factor of the bulk acoustic wave resonator.

[0030] By setting a DBR reflection zone covering the invalid capacitance area in the functional reflection layer, the leakage of acoustic energy from the invalid capacitance area to the reflection functional layer can be suppressed, which can effectively reduce the longitudinal energy leakage; and by setting a closed-loop frame-shaped protrusion to adjust the distribution of the cutoff frequency in the working area, the generation of transverse derived modes can be effectively suppressed, thereby effectively increasing the overall quality factor of the bulk acoustic resonator.

[0031] To enable those skilled in the art to better understand the present invention, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Please refer to Figure 1as well as Figure 2 , Figure 1 This is a schematic diagram of the structure of a bulk acoustic resonator provided in an embodiment of the present invention; Figure 2 for Figure 1 A top-view structural diagram.

[0033] See Figure 1 In this embodiment of the invention, the bulk acoustic wave resonator includes a substrate 1, a reflective functional layer 2, a bottom electrode 3, a piezoelectric thin film 4, and a top electrode 5; the reflective functional layer 2 is located on one side surface of the substrate 1, the bottom electrode 3 is located on the side surface of the reflective functional layer 2 opposite to the substrate 1, the piezoelectric thin film 4 is located on the side surface of the bottom electrode 3 opposite to the substrate 1, and the top electrode 5 is located on the side surface of the piezoelectric thin film 4 opposite to the substrate 1; the reflective functional layer 2 is provided with a cavity 21 and a DBR reflective region; the DBR reflective region... Alternating low acoustic impedance layers 22 and high acoustic impedance layers 23 are disposed along the thickness direction; the bottom electrode 3 shields the cavity 21; in the region where the top electrode 5 overlaps with the bottom electrode 3 along the thickness direction, the region overlapping with the cavity 21 is the working region, and the region not overlapping with the cavity 21 is the ineffective capacitance region; the DBR reflection region covers the ineffective capacitance region, and a closed-loop frame-shaped protrusion 51 is disposed on the surface of the top electrode 5 facing away from the substrate 1 within the working region to adjust the distribution of the cutoff frequency within the working region.

[0034] The aforementioned substrate 1 is the base of the bulk acoustic wave resonator. Substrate 1 primarily serves a load-bearing function, and the various functional structures within the bulk acoustic wave resonator are typically sequentially arranged on the surface of substrate 1. The specific thickness and material of substrate 1 can be determined according to actual conditions; details can be found in existing technologies and will not be elaborated upon here. Typically, in this embodiment of the invention, a silicon substrate 1 can be used as the substrate 1 of the bulk acoustic wave resonator.

[0035] The aforementioned reflective functional layer 2 is mainly used to limit the leakage of acoustic wave energy from the piezoelectric thin film 4 towards the substrate 1. Since the resonator provided in this embodiment is specifically a bulk acoustic wave resonator, and the direction of acoustic wave energy transmission in the bulk acoustic wave resonator is specifically along the thickness direction of the piezoelectric thin film 4, this energy is obviously prone to leakage towards the substrate 1. In this embodiment, to reduce the leakage of the aforementioned acoustic wave energy, a reflective functional layer 2 is provided on one side of the substrate 1 surface. This reflective functional layer 2 is used to reflect the acoustic wave energy leaking towards the substrate 1 back to the piezoelectric thin film 4.

[0036] Specifically, in this embodiment of the invention, the reflective functional layer 2 is provided with a cavity 21 and a DBR (distributed Bragg reflection) reflective layer. The area where the DBR reflective layer is provided in the horizontal direction is the DBR reflective region. In the DBR reflective region, alternating low acoustic impedance layers 22 and high acoustic impedance layers 23 are provided along the thickness direction of the functional reflective layer. The alternating low acoustic impedance layers 22 and high acoustic impedance layers 23 constitute the DBR reflective layer to reflect the acoustic wave energy leaking towards the substrate 1.

[0037] The bottom electrode 3 is located on the surface of the reflective functional layer 2 facing away from the substrate 1, and it shields the cavity 21. Specifically, the cavity 21 is typically located on the surface of the reflective functional layer 2 facing away from the substrate 1, and the bottom electrode 3 must at least span the cavity 21 to shield it. "Spanning" means that the bottom electrode 3 must at least be positioned above the cavity 21, but it does not necessarily require the bottom electrode 3 to completely cover the cavity 21. It should be noted that when the bottom electrode 3 shields the cavity 21, it necessarily has an area above the cavity 21 and an area in contact with the reflective functional layer 2 surrounding the cavity 21. The area where the bottom electrode 3 contacts the reflective functional layer 2 surrounding the cavity 21 can be considered as a support structure to support the bottom electrode 3, as well as the subsequent piezoelectric film 4 and top electrode 5, above the cavity 21.

[0038] The piezoelectric thin film 4 is located on the surface of the bottom electrode 3 facing away from the substrate 1, while the top electrode 5 is located on the surface of the piezoelectric thin film 4 facing away from the substrate 1. The bottom electrode 3, piezoelectric thin film 4, and top electrode 5, which form a sandwich-like stacked structure, constitute the main structure of the resonant cavity in the bulk acoustic resonator provided in this embodiment of the invention. Under the excitation of an electrical signal, the acoustic wave energy vibrates along the thickness direction of the piezoelectric thin film 4. The specific materials, thicknesses, and other related parameters of the bottom electrode 3, piezoelectric thin film 4, and top electrode 5 can be set according to actual conditions. The specific details can be found in the prior art and will not be elaborated here.

[0039] Specifically, the piezoelectric film 4 preferably extends beyond all edges of the bottom electrode 3. Since the piezoelectric material used in the piezoelectric film 4, such as AlN, is a columnar material, it tends to descend easily at the edges of the bottom electrode 3. This characteristic of the piezoelectric material leads to the generation of derived modes due to material discontinuities when the edge of the bottom electrode 3 terminates within the cavity 21 region, resulting in a decrease in the performance of the bulk acoustic wave resonator. Simultaneously, when the bottom electrode 3 completely covers the cavity 21, i.e., when the bottom electrode 3 overlaps all sides of the cavity 21, the quality factor Q of the bulk acoustic wave resonator increases.

[0040] Typically, a pad is also required in the bulk acoustic resonator to be electrically connected to the bottom electrode 3 and the top electrode 5 to facilitate the transmission of electrical signals. The specific structure and location of the pad can be referred to the existing technology, and no specific limitation is made again.

[0041] In this embodiment of the invention, since the bottom electrode 3 needs to be in direct contact with the reflective functional layer 2 to achieve a cross-positioning across the cavity 21, the area where the top electrode 5, the bottom electrode 3, and the cavity 21 overlap along the thickness direction is specifically referred to as the working area, while the area where the top electrode 5 overlaps with the bottom electrode 3 but does not overlap with the cavity 21 is the ineffective capacitance area. It should be noted that by moving the stepped edges of the bottom electrode 3 and the piezoelectric film 4 away from the cavity 21, i.e., increasing the width and area of ​​the aforementioned ineffective capacitance area, the bulk acoustic resonator can be made more robust and able to withstand a wider range of pressures, including forces generated during compression and tension. However, since the aforementioned ineffective capacitance area contains a portion of the electrode and the top electrode 5, acoustic energy transmitted in the piezoelectric film 4 will obviously also be generated in the aforementioned ineffective capacitance area. However, since this ineffective capacitance area is in direct contact with the reflective functional layer 2, the acoustic energy generated in the ineffective capacitance area will directly leak to the reflective functional layer 2, reducing the quality factor Q of the bulk acoustic resonator.

[0042] In this embodiment of the invention, the aforementioned DBR reflective region overlaps with at least the ineffective capacitance region. This allows the DBR reflective region to limit the acoustic wave energy leaking from the ineffective capacitance region toward the reflective functional layer 2. When the DBR reflective region at least covers the ineffective capacitance region, it can at least prevent some of the acoustic wave energy leaking from the ineffective capacitance region from propagating toward the substrate 1, thereby effectively reducing the generation of derived modes and effectively increasing the overall Q value of the bulk acoustic resonator. It should be noted that the low acoustic impedance layer 22 and high acoustic impedance layer 23, which are alternately arranged along the thickness direction in the aforementioned DBR reflective region, are specifically arranged on the side of the reflective functional layer 2 facing away from the substrate 1. That is, the aforementioned lower electrode is usually in direct contact with the uppermost low acoustic impedance layer 22 or high acoustic impedance layer 23 in the DBR reflective region to improve the reflection effect of the DBR reflective region on the acoustic wave energy leaking toward the substrate 1.

[0043] Furthermore, since the acoustic energy generated between the bottom electrode 3 and the top electrode 5 leaks not only from the ineffective capacitance region but also from the ineffective capacitance region along the horizontal direction away from the cavity 21, in this embodiment of the invention, in order to limit the acoustic energy leaking towards the substrate 1 as much as possible, the preferred DBR reflection region not only needs to cover the ineffective capacitance region, but the length of the DBR reflection region also needs to be greater than the ineffective capacitance region, so as to limit the acoustic energy that wants to be transmitted in the lower direction as much as possible through the DBR reflection layer. It should be noted that in the DBR reflection region, the acoustic isolation of the excitation characteristic mode provided by the DBR layer, that is, the distributed multilayer film reflection structure, also depends on the contrast between the acoustic impedances of adjacent acoustic impedance layers in the DBR layer. The greater the contrast, the better the acoustic reflection of the vertical component of the characteristic mode. Therefore, in this embodiment of the invention, the specific materials of the low acoustic impedance layer 22 and the high acoustic impedance layer 23 can be specifically adjusted to improve the reflection effect of the DBR reflection region.

[0044] See Figure 2 In this embodiment of the invention, a closed-loop frame-shaped protrusion 51 is provided on the surface of the top electrode 5 facing away from the substrate 1 within the working area to adjust the distribution of the cutoff frequency within the working area. The material of the protrusion is usually the same as that of the top electrode 5, meaning the protrusion is typically integral with the top electrode 5. Of course, other materials can be used for the protrusion, as long as they can change the thickness distribution of the top electrode 5 within the working area; no specific limitation is made here.

[0045] Generally, bulk acoustic resonators (BARs) comprise different peripheral regions that may be affected by different types of resonance or resonant modes. The transverse modes generated during BAR operation extensively influence both the resonant and peripheral regions. The resonant region is roughly defined by the overlap between the two plate electrodes and the piezoelectric material, while the peripheral region is defined as the area outside the resonant region. Specifically, there are two peripheral regions: one located between the edge of the resonant region and the edge of the air cavity, and the other where at least one plate electrode and piezoelectric material overlap with the substrate—the aforementioned ineffective capacitance region. The resonant region is affected by electro-excitation modes generated by the electric field between the two plate electrodes. Both the resonant and peripheral regions are influenced by certain derivative modes generated by energy scattering from these electro-excitation modes. Electro-excitation modes include piston modes formed by longitudinal acoustic waves, with their boundaries at the edge of the resonant region.

[0046] Derivative modes typically include transverse modes excited by transverse acoustic waves at the edges of the resonant region and surrounding regions. Transverse modes promote the continuity of appropriate mechanical particle velocities and stresses between the resonant region and surrounding regions. They can either propagate freely (i.e., propagation modes) or decay exponentially from the excitation point (i.e., wave energy gradually dissipates) and recombination modes. These derivative modes can be excited by transverse structural discontinuities, including interfaces between regions of different thicknesses in the resonant region, the edges of the top or bottom electrodes 3, or electric field discontinuities, such as the edge of the top electrode 5, where the electric field abruptly terminates. Transverse modes generally have a detrimental effect on the performance of bulk acoustic resonators.

[0047] For the region in the aforementioned edge region where the plate electrode and piezoelectric material overlap with the substrate, i.e., the ineffective capacitance region, the aforementioned DBR reflection region in this embodiment of the invention reduces the generation of its derived modes. For the region in the aforementioned edge region located between the edge of the resonant region and the edge of the air cavity, the aforementioned protrusion is specifically provided in this embodiment of the invention. The aforementioned protrusion needs to form a closed-loop frame, creating a closed-loop frame protrusion 51. This closed-loop frame protrusion 51 is located in the working region, thereby further enclosing a central region in the working region, i.e., the aforementioned resonant region, which is the region enclosed by the closed-loop frame protrusion 51. Typically, the outer edge of the closed-loop frame protrusion 51 is positioned along the inner wall of the cavity 21 to ensure that the central region has sufficient area. The aforementioned closed-loop frame protrusion 51 effectively increases the thickness of the top electrode 5 in its region, thereby causing a difference between the cutoff frequency at the location of the closed-loop frame protrusion 51 and the cutoff frequency of the central region. Typically, the cutoff frequency at the location of the closed-loop frame protrusion 51 will be lower than the cutoff frequency of the central region, because the thicker the electrode, the lower its cutoff frequency.

[0048] In this embodiment of the invention, by setting the aforementioned closed-loop frame-shaped protrusion 51 and appropriately selecting the acoustic characteristics and width of the frame-shaped boundary region, the characteristics of the resonant mode that can be excited piezoelectrically in the central region can be adjusted. Specifically, in this embodiment of the invention, the cutoff frequency of the region where the closed-loop frame-shaped protrusion 51 is located can be made lower than the cutoff frequency of the central region, so that the strongest mode excited in the working area is a piston mode, thus enabling the acoustic resonator to operate in piston mode. A piston mode indicates that its displacement is uniform within a certain region of the resonator. In this embodiment of the invention, by setting the closed-loop frame-shaped protrusion 51 that meets the necessary boundary conditions, including setting the closed-loop frame-shaped protrusion 51 with appropriate width and thickness, the displacement in the central region can be made uniform, forming a piston mode. Therefore, setting the closed-loop frame-shaped protrusion 51 can effectively suppress the generation of transverse derived modes.

[0049] In this embodiment of the invention, by adjusting the acoustic characteristics and width of the closed-loop frame protrusion 51, the displacement of the strongest piezoelectrically excited wave mode is substantially uniform in the central region of the resonator when the resonator is excited piezoelectrically. Considering a piezoelectric film 4 with a certain thickness in the vertical direction and electrodes on its horizontal surface, when a thickness-expanding wave is excited piezoelectrically, the particles of the piezoelectric film 4 will displace in the vertical direction; in other words, they will displace in the direction of the applied electric field. When a shear wave is excited piezoelectrically, the particles of the piezoelectric material will displace in the horizontal direction; in other words, they will displace in the direction perpendicular to the applied electric field. When the resonant structure in this embodiment of the invention is excited piezoelectrically, the displacement in the central region of the resonator is substantially uniform. When the piezoelectrically excited wave is a thickness-expanding wave, it means that the thickness of the central region changes over time, making the thickness of the central region the same at every moment at substantially every point within this region. Similarly, when the piezoelectrically excited wave is a shear wave, the displacement of the particles in the horizontal direction is uniform. Since the piston mode is defined as having a uniform displacement within a certain region of the resonator, in this embodiment of the invention, the uniform displacement related to the piston mode specifically occurs in the central region of the resonator. This central region is precisely where the piston mode operates.

[0050] The bulk acoustic wave resonator provided in this embodiment of the invention effectively reduces longitudinal energy leakage by setting a DBR reflection region covering the invalid capacitance region in the functional reflection layer to suppress the leakage of acoustic wave energy from the invalid capacitance region to the reflection functional layer 2; and by setting a closed-loop frame-shaped protrusion 51 to adjust the distribution of the cutoff frequency in the working area, the generation of transverse derived modes can be effectively suppressed. Through the setting of the closed-loop frame-shaped protrusion 51 and the DBR reflection region, the influence of derived modes can be reduced in all directions, thereby effectively increasing the overall quality factor of the bulk acoustic wave resonator.

[0051] The specific details of the bulk acoustic resonator provided by this invention will be described in detail in the following embodiments.

[0052] Unlike the embodiments described above, the embodiments of the present invention further define the structure of the bulk acoustic resonator based on the embodiments described above. The remaining details have been described in detail in the embodiments described above and will not be repeated here.

[0053] In this embodiment of the invention, the DBR reflection area covers the cavity 21. Since the area occupied by the cavity 21 in the horizontal direction is typically the working area, energy leakage towards the substrate 1 is also easily generated from the periphery of the working area. In this embodiment of the invention, to further limit the energy leakage towards the substrate 1 within the entire bulk acoustic wave resonator, the DBR reflection area can cover the cavity 21 to minimize the energy leakage towards the substrate 1 within the entire bulk acoustic wave resonator.

[0054] Furthermore, the aforementioned DBR reflection area can cover the entire reflection functional layer 2, that is, the reflection functional layer 2 can be entirely composed of the DBR reflection layer, and the aforementioned cavity 21 can specifically be a cavity 21 etched in the DBR reflection layer, thereby limiting as much energy as possible that leaks from the entire bulk acoustic resonator toward the substrate 1.

[0055] Specifically, in this embodiment of the invention, the low acoustic impedance layer 22 and the high acoustic impedance layer 23 are alternately distributed along the thickness direction from the surface of the reflective functional layer 2 facing away from the substrate 1. That is, the aforementioned DBR layer can be specifically disposed on the surface of the reflective functional layer 2 facing away from the substrate 1, so that the lower electrode and other structures are in direct contact with the low acoustic impedance layer 22 or the high acoustic impedance layer 23, so as to maximize the limiting effect of the DBR reflective layer on the acoustic wave energy generated between the bottom electrode 3 and the top electrode 5.

[0056] Typically, in this embodiment of the invention, the film layer in the DBR reflective region that contacts the bottom electrode 3 is a low acoustic impedance layer 22. That is, the uppermost film layer in the DBR reflective layer is the low acoustic impedance layer 22. In practice, the low acoustic impedance layer 22 is softer than the high acoustic impedance layer 23. Therefore, in this embodiment of the invention, making the film layer in the DBR reflective region that contacts the bottom electrode 3 a low acoustic impedance layer 22 can effectively reduce dislocations and defects between the low acoustic impedance layer 22 and the bottom electrode 3, or between the low acoustic impedance layer 22 and the piezoelectric thin film 4, ensuring that the final bulk acoustic resonator has high performance.

[0057] Specifically, in this embodiment of the invention, the operating region of the bulk acoustic resonator is defined by the presence of air at the top and bottom boundaries, i.e., the cavity 21. Therefore, the vertical stress component at the boundary of the operating region is zero. Similarly, by appropriately adjusting the materials of the low acoustic impedance layer 22 and the high acoustic impedance layer 23, specifically adjusting the material of the low acoustic impedance layer 22 that is in direct contact with the bottom electrode 3, the low acoustic impedance layer 22 may have a very low acoustic impedance compared to the bottom electrode 3, which can also reduce the vertical stress at the boundary between the bottom electrode 3 and the low acoustic impedance layer 22.

[0058] Specifically, in this embodiment of the invention, the depth of the cavity 21 is typically greater than the thickness of the low acoustic impedance layer 22 that directly contacts the bottom electrode 3. Because in this embodiment, the acoustic energy leaking to the reflective functional layer 2 propagates not only along the thickness direction towards the substrate 1 but also laterally. When the depth of the cavity 21 is less than or equal to the thickness of the low acoustic impedance layer 22 that directly contacts the bottom electrode 3, the low acoustic impedance layer 22 cannot effectively block the lateral propagation of acoustic energy relative to the cavity 21. Therefore, in this embodiment of the invention, preferably, the depth of the cavity 21 needs to be greater than the thickness of the low acoustic impedance layer 22 that directly contacts the bottom electrode 3 to ensure that the DBR layer can limit the lateral propagation of acoustic energy. It should be noted that the correspondence between the depth of the cavity 21 and the overall thickness of the DBR layer is not specifically limited in this embodiment of the invention. The depth of the cavity 21 can be greater than, equal to, or less than the overall thickness of the DBR layer, depending on the specific circumstances, and is not specifically limited here.

[0059] It should be noted that, in this embodiment of the invention, the specific materials, thicknesses, number of cycles, and other specific parameters of the low acoustic impedance layer 22 and the high acoustic impedance layer 23 within the DBR reflection zone can be set according to actual conditions and are not specifically limited here. It should also be noted that, in this embodiment of the invention, an additional acoustic impedance layer can be provided between the high acoustic impedance layer 23 and the low acoustic impedance layer 22 within the DBR reflection zone; this is not specifically limited in this embodiment of the invention.

[0060] The bulk acoustic wave resonator provided in this embodiment of the invention suppresses the leakage of acoustic wave energy from the invalid capacitance region to the reflective functional layer 2 by setting a DBR reflective region that overlaps with the invalid capacitance region in the functional reflective layer, thereby effectively reducing the generation of derived modes and thus effectively increasing the overall Q value of the bulk acoustic wave resonator.

[0061] The specific details of the bulk acoustic resonator provided by this invention will be described in detail in the following embodiments.

[0062] Please refer to Figure 3 as well as Figure 4 , Figure 3 This is a schematic diagram of a specific bulk acoustic resonator provided in an embodiment of the present invention; Figure 4 Smith chart for setting up a closed-loop frame protrusion 51 front and rear body acoustic resonator.

[0063] Unlike the embodiments described above, the embodiments of the present invention further define the structure of the bulk acoustic resonator based on the embodiments described above. The remaining details have been described in detail in the embodiments described above and will not be repeated here.

[0064] See Figure 3In this embodiment of the invention, the reflective functional layer 2 is provided with a plurality of cavities 21. Each cavity 21, on the side facing away from the substrate 1, is provided with a bottom electrode 3, a piezoelectric film 4, and a top electrode 5. That is, in this embodiment of the invention, a single bulk acoustic resonator can be provided with multiple resonant cavities, and correspondingly, the reflective functional layer 2 can be provided with a plurality of cavities 21. These cavities 21 are typically located on the surface of the reflective functional layer 2 facing away from the substrate 1. Accordingly, each cavity 21, on the side facing away from the substrate 1, is provided with a bottom electrode 3, a piezoelectric film 4, and a top electrode 5 stacked in a sandwich-like structure, thereby forming a resonant cavity. It should be noted that the corresponding bottom electrodes 3 of different cavities 21 can be interconnected, and the corresponding top electrodes 5 of different cavities 21 can also be interconnected. In this case, the resonant cavities are typically connected in series. Of course, the specific connection relationship between different resonant cavities can be determined according to the specific structure, and is not specifically limited here. It should be noted that in this embodiment of the invention, each cavity 21 is typically near a DBR reflective region corresponding to an ineffective capacitance region.

[0065] Furthermore, in this embodiment of the invention, a DBR reflection region is provided between adjacent cavities 21. To avoid unnecessary signal crosstalk between adjacent resonant cavities, the aforementioned DBR reflection region can be further provided between adjacent cavities 21 in this embodiment of the invention. The alternating high acoustic impedance layer 23 and low acoustic impedance layer 22 provided in the DBR reflection region block and cancel the signal leaking between adjacent resonant cavities in the vertical direction.

[0066] Furthermore, in this embodiment of the invention, an air gap 24 is provided in the DBR reflection region located between adjacent cavities 21. This air gap 24 can reduce conductor-related parasitic capacitance, thereby reducing energy loss. Specifically, in this embodiment, an air gap 24 can be further provided in the DBR reflection region located between adjacent cavities 21. This air gap 24 is typically provided along the thickness direction of the reflective functional layer 2, allowing it to block signals leaking between adjacent resonant cavities in the horizontal direction. The specific depth of the air gap 24 can be referenced to the depth of the cavity 21, and the width of the air gap 24 can be set according to actual conditions, without specific limitations. In practice, the depth of the cavity 21 needs to be greater than a preset value so that the cavity 21 can effectively reflect acoustic wave energy back to the piezoelectric film 4. However, in this embodiment of the invention, the depth of the cavity 21 is not related to the common thickness of the high acoustic impedance layer 23 and the low acoustic impedance layer 22 in the DBR reflection region; there is no corresponding relationship between them. Specifically, the depth of the cavity 21 usually needs to be no less than 1 μm to ensure that the cavity 21 can effectively reflect the sound waves back to the piezoelectric film 4.

[0067] It should be noted that the area of ​​the DBR reflective region affects the thin film stress within the device; a larger DBR reflective region results in increased accumulated thin film stress, making it more prone to cracking and affecting yield. Therefore, in practice, the DBR area should be minimized. It should also be noted that the total thickness of the high acoustic impedance layer 23 and the low acoustic impedance layer 22 within the DBR reflective region also affects the thin film stress. Consequently, in practice, DBR structures with too many layers should be avoided to reduce the risk of thin film cracking.

[0068] In this embodiment of the invention, by setting a DBR reflective region to acoustically isolate the connection edge between the top electrode 5 and the substrate, the influence caused by the ineffective capacitance region is essentially eliminated. The aforementioned DBR reflective region also prevents the gradually decaying and recombination modes of wave energy from areas other than the top electrode 5, such as the edge of the top electrode 5 and the edge of the substrate 1, from coupling to the substrate 1, because these modes decay exponentially from the excitation edge located at the edge of the top electrode 5. Simultaneously, the arrangement of the closed-loop frame-shaped protrusion 51 ensures that when the resonator is piezoelectrically excited, the displacement of the strongest piezoelectrically excited mode is substantially uniform within the central region of the resonator, allowing the bulk acoustic resonator to operate in piston mode.

[0069] Please refer to Figure 4 Since the fundamental mode of the bulk acoustic resonator is the longitudinally extended mode, this mode is excited by applying an alternating voltage to the two electrodes at the resonant frequency of the bulk acoustic resonator, where the piezoelectric film 4 converts electrical energy into mechanical energy. In an ideal bulk acoustic resonator with infinitely thin electrodes, resonance occurs when the applied frequency is equal to the speed of sound in the piezoelectric medium divided by twice the thickness of the piezoelectric medium.

[0070] f = vac / (2*T);

[0071] Where T is the thickness of the piezoelectric medium, and vac is the acoustic phase velocity. For a resonator with electrodes of finite thickness, this equation is modified by the weighted velocity and thickness of the electrodes.

[0072] By plotting the ratio (Γ) of reflected energy to applied energy as frequency changes on a Smith chart, a quantitative and qualitative understanding of the resonator's quality factor Q can be obtained. As the frequency of the applied energy increases, the amplitude / phase of the BAW resonator sweeps a circle on the Smith chart. This is called the Q-circle. The point where the Q-circle first intersects the real axis, i.e., the horizontal axis, corresponds to the frequency fs, with a true impedance (in ohms) of Rs. As the Q-circle continues to circle the perimeter of the Smith chart, it intersects the real axis again. The second point where the Q-circle intersects the real axis is labeled fp, the anti-resonant frequency of the bulk acoustic wave resonator. The residual real part of the impedance is labeled Rp. Figure 4It can be seen that this application can effectively improve the quality factor of the bulk acoustic resonator by setting the closed-loop frame protrusion 51.

[0073] The present invention also provides a filter, including the bulk acoustic resonator provided in any of the above embodiments. Further details regarding the filter's structure can be found in the prior art and will not be elaborated upon here.

[0074] Because the bulk acoustic wave resonator provided in the above embodiments of the invention has a high quality factor Q, the filter provided in the embodiments of the invention can also have high performance. Of course, in addition to being used as a structure in a filter, the above-mentioned bulk acoustic wave resonator can also be used as a structure in other devices, such as duplexers, multiplexers, etc., and no specific limitation is made here.

[0075] The various embodiments in this specification are described in a progressive manner. Each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0076] Finally, it should be noted that in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0077] The present invention has provided a detailed description of a bulk acoustic resonator and a filter. Specific examples have been used to illustrate the principles and implementation methods of the invention. The descriptions of these embodiments are merely illustrative and are intended to help understand the method and core ideas of the invention. It should be noted that those skilled in the art can make various improvements and modifications to the invention without departing from its principles, and these improvements and modifications also fall within the scope of protection of the claims.

Claims

1. A bulk acoustic resonator, characterized in that, It includes a substrate, a reflective functional layer, a bottom electrode, a piezoelectric thin film, and a top electrode; The reflective functional layer is located on one side surface of the substrate, the bottom electrode is located on the side surface of the reflective functional layer opposite to the substrate, the piezoelectric thin film is located on the side surface of the bottom electrode opposite to the substrate, and the top electrode is located on the side surface of the piezoelectric thin film opposite to the substrate. The reflective functional layer is provided with a cavity and a DBR reflective area; the DBR reflective area is provided with alternating low acoustic impedance layers and high acoustic impedance layers along the thickness direction; the bottom electrode shields the cavity. In the region where the top electrode and the bottom electrode overlap along the thickness direction, the region that overlaps with the cavity is the working region, and the region that does not overlap with the cavity is the ineffective capacitance region; the DBR reflection region covers the ineffective capacitance region, and a closed-loop frame-shaped protrusion is provided in the working region on the surface of the top electrode facing away from the substrate to adjust the distribution of the cutoff frequency in the working region; The cutoff frequency of the region where the closed-loop frame-shaped protrusion is located is less than the cutoff frequency of the central region, so that the strongest mode excited in the working area is the piston mode; wherein, the central region is the region in the working area framed by the closed-loop frame-shaped protrusion.

2. The bulk acoustic resonator according to claim 1, characterized in that, The outer edge of the closed-loop frame-shaped protrusion is positioned along the inner sidewall of the cavity.

3. The bulk acoustic resonator according to claim 1, characterized in that, The length of the DBR reflective region is greater than the length of the ineffective capacitor region.

4. The bulk acoustic resonator according to claim 1, characterized in that, The film layer in the DBR reflection zone that is in contact with the bottom electrode is a low acoustic impedance layer.

5. The bulk acoustic resonator according to claim 4, characterized in that, The depth of the cavity is greater than the thickness of the low acoustic impedance layer that is in direct contact with the bottom electrode.

6. The bulk acoustic resonator according to claim 1, characterized in that, The reflective functional layer is provided with a plurality of cavities, and each cavity has a bottom electrode, a piezoelectric film and a top electrode disposed on the side facing away from the substrate.

7. The bulk acoustic resonator according to claim 6, characterized in that, The DBR reflective area is provided between each adjacent cavity.

8. The bulk acoustic resonator according to claim 7, characterized in that, An air gap is provided in the DBR reflective area located between adjacent cavities.

9. A filter, characterized in that, Includes a bulk acoustic resonator as described in any one of claims 1 to 8 above.

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