A laterally excited film bulk acoustic resonator and filter

Abstract

The present application relates to the technical field of resonator, and particularly relates to a transversely excited film bulk acoustic resonator and filter. The transversely excited film bulk acoustic resonator comprises a substrate, a bonding layer, a piezoelectric film layer and an excitation electrode. The substrate is used for supporting the piezoelectric film layer. The piezoelectric film layer comprises a first surface and a second surface opposite to each other. The bonding layer connects the second surface and the substrate together. The excitation electrode comprises a plurality of sub-electrodes. The sub-electrodes are arranged in the piezoelectric film layer. One end of the sub-electrodes is exposed to the first surface. The structure of the transversely excited film bulk acoustic resonator is that the electrode for exciting transverse electric field is completely arranged in the piezoelectric film, so that the formation of longitudinal electric field component is avoided, the uniformity of the transverse electric field is improved, the emergence of stray acoustic wave mode is reduced, and the impedance response curve is smoother.

Description

Technical Field

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

[0002] With the development of multifunctional wireless terminal technology, there are increasing demands for miniaturization, low power consumption, low cost, and high performance in frequency devices. Compared to traditional filters, film bulk acoustic resonators (FBARs) have become a focal point of attention in 5G technology due to their advantages in high-frequency applications. FBARs have advantages such as low insertion loss and high rectangularity factor, and are therefore widely used in modern wireless communication systems. FBAR resonators also exhibit frequency sensitivity to external environmental factors, responding to changes in external conditions based on frequency variations, making them suitable for sensor applications in biochemistry, medical diagnostics, and environmental monitoring.

[0003] However, as the required frequency and operating bandwidth of filters gradually increase, the design of AlN-based FBAR resonators becomes increasingly challenging. Only perfectly monocrystalline AlN or AlN with high Sc doping content can potentially meet the requirements of filters with bandwidths of hundreds of megahertz, and many technical problems remain to be solved. Currently, research on transverse excitation volume acoustic resonators (XBARs), spearheaded by the American company Resonant, is booming. XBAR is also a technology that utilizes longitudinal volume acoustic waves for resonance. It converts electrical energy into longitudinal acoustic waves through the inverse piezoelectric effect of a piezoelectric thin film. Since the resonator can only allow waves of specific frequencies to pass through, XBAR can meet the requirements of high-frequency and large-bandwidth filters.

[0004] However, in the XBAR design, the electrodes generating the transverse excitation electric field are located on the upper surface of the piezoelectric film. This results in transverse and longitudinal components in the electric field inside the piezoelectric film, reducing the uniformity of the electric field and making it prone to stray resonances in acoustic waves. Furthermore, when these stray resonance modes are cascaded to form a filter, they cause ripple jitter in the frequency response curve, degrading the filter's performance. Summary of the Invention

[0005] The technical problem to be solved by the present invention is that the electric field inside the piezoelectric film of the existing transversely excited thin film bulk acoustic resonator has transverse and longitudinal components, the electric field uniformity is reduced, and stray resonance of acoustic waves is easily generated.

[0006] To solve the above-mentioned technical problems, in a first aspect, embodiments of this application disclose a transversely excited thin-film bulk acoustic resonator, comprising: a substrate, a bonding layer, a piezoelectric thin film layer, and an excitation electrode;

[0007] The substrate is used to support the piezoelectric thin film layer;

[0008] The piezoelectric thin film layer includes opposing first and second surfaces, and the bonding layer connects the second surface to the substrate;

[0009] The excitation electrode includes a plurality of sub-electrodes, which are disposed in the piezoelectric thin film layer;

[0010] One end of the sub-electrode is exposed on the first surface.

[0011] Furthermore, the piezoelectric thin film layer has an effective resonant region, and the sub-electrode is disposed within the effective resonant region; an acoustic reflector is disposed between the substrate and the piezoelectric thin film layer, and the acoustic reflector includes at least an air and a Bragg reflector.

[0012] Furthermore, the sub-electrodes are periodically distributed within the effective resonant region.

[0013] Furthermore, the spacing between adjacent sub-electrodes is set according to a first preset function.

[0014] Furthermore, the sub-electrode forms a first electrode layer and a second electrode layer in the piezoelectric thin film layer, with the first electrode layer disposed on the first surface and the second electrode layer disposed on the second surface;

[0015] The first electrode layer includes a plurality of sub-electrodes connected to form a plurality of parallel first electrode bridges;

[0016] The second electrode layer includes multiple sub-electrodes connected to form multiple parallel second electrode bridges;

[0017] The projections of the first electrode bridge and the second electrode bridge onto the horizontal plane are perpendicular to each other.

[0018] Furthermore, the height of the sub-electrode is equal to the thickness of the piezoelectric thin film layer.

[0019] Furthermore, the thickness of the piezoelectric thin film layer is 100nm-1000nm.

[0020] Furthermore, a limiting frame is provided on the first surface. The limiting frame is a closed ring structure and is disposed outside the effective resonant region.

[0021] Furthermore, the height of the limiting frame is 0.1μm-1μm; the cross-sectional dimensions of the limiting frame are 0.1μm-1μm.

[0022] Furthermore, the region between adjacent sub-electrodes is an inter-electrode region, and an acoustic microstructure is provided in the inter-electrode region, which is disposed on the first surface.

[0023] Furthermore, the height of the acoustic microstructure is 0.1μm-0.5μm; the cross-sectional size of the acoustic microstructure is 0.1μm-0.3μm.

[0024] Furthermore, the ratio of the size between adjacent acoustic microstructures to the size of the inter-electrode region is the duty cycle, which is set according to a second preset function.

[0025] Furthermore, the acoustic microstructure is made of a dielectric material.

[0026] Secondly, embodiments of this application disclose a filter, the filter including a parallel resonator and a series resonator, wherein the parallel resonator and the series resonator are cascaded to form the filter;

[0027] The parallel resonator and the series resonator are transversely excited thin-film bulk acoustic resonators as described above.

[0028] By adopting the above technical solution, the transversely excited thin-film bulk acoustic resonator and filter described in the embodiments of this application have the following beneficial effects:

[0029] The structure of this transversely excited thin-film bulk resonator places the electrodes that excite the transverse electric field entirely inside the piezoelectric thin film, avoiding the formation of longitudinal electric field components and improving the uniformity of the transverse electric field, reducing the occurrence of stray acoustic wave modes, and making its impedance response curve smoother. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 A schematic diagram of a transversely excited thin-film bulk acoustic resonator provided in an embodiment of this application;

[0032] Figure 2 A schematic diagram of a resonator with a gradually distributed sub-electrode, provided for an embodiment of this application;

[0033] Figure 3 A comparison diagram of the resonator effects of a prior art and an embodiment of this application is provided for the purposes of this application.

[0034] Figure 4 An effect diagram of a resonator with linearly varying sub-electrode spacing provided in an embodiment of this application;

[0035] Figure 5 A top view of a two-dimensional embedded sub-electrode arrangement structure provided in an embodiment of this application;

[0036] Figure 6 A cross-sectional view of a two-dimensional embedded sub-electrode arrangement structure provided in an embodiment of this application;

[0037] Figure 7 A schematic diagram of a resonator structure with a limiting frame on a piezoelectric thin film layer provided in this application embodiment;

[0038] Figure 8 This is a partial structural diagram of a resonator with an acoustic microstructure disposed on a piezoelectric thin film layer, provided in an embodiment of this application.

[0039] Figure 9 A schematic diagram of the distribution structure of an acoustic microstructure provided in an embodiment of this application;

[0040] Figure 10 A comparison diagram of resonator Q-value calculation between a prior art method and an embodiment of this application is provided for the purposes of this application.

[0041] Figure 11 A schematic diagram of a cascaded ladder topology of a filter provided in an embodiment of this application;

[0042] The following is supplementary explanation of the attached figures:

[0043] 1-Substrate; 2-Piezoelectric thin film layer; 3-Excitation electrode; 4-Bonding layer; 5-Acoustic microstructure; 6-Confinement frame; 7-First electrode bridge; 8-Circular electrode. Detailed Implementation

[0044] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0045] The term "an embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of this application. In the description of this application, it should be understood that the terms "upper," "lower," "top," "bottom," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. Moreover, the terms "first," "second," etc., are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of this application described herein can be implemented in orders other than those illustrated or described herein.

[0046] Because the speed of sound waves is about five orders of magnitude slower than that of electromagnetic waves, the size of an XBAR operating at the same frequency is much smaller than that of a dielectric ceramic device, and its operating frequency is determined by the thickness of the effective longitudinal region of its resonator.

[0047]

[0048] Among them, v l d is the longitudinal acoustic wave velocity propagating in the resonator, and d is the thickness of the thin film in the resonant region.

[0049] Thin-film bulk acoustic resonators are important components that determine the quality of radio frequency signals entering and exiting the filter. The various parameters of the resonator determine the performance of the filter: the electromechanical coupling coefficient determines the bandwidth of the filter, the resonant frequency determines the attenuation point of the filter, and the quality factor Q is one of the factors that determine the insertion loss of the filter.

[0050] The Q value is an important parameter for evaluating thin-film bulk acoustic resonators. It is defined as the ratio of the total energy stored in the system to the energy lost by the resonator through various paths per cycle. The calculation method is as follows:

[0051]

[0052] Where ω is the angular frequency, E totalΔE represents the total energy stored in the system, while ΔE represents the energy lost by the resonator system through various pathways per cycle. A resonator with a high Q value indicates low energy loss. Filters composed of high-Q resonators have low insertion loss, steep stopband attenuation, good filter performance, and high rectangularity coefficient. Sensors composed of high-Q resonators have higher detection accuracy.

[0053] Secondly, in RF filter design, the impedance response curve of a single resonator unit has a significant impact on filter performance. Therefore, during resonator design, it is necessary to minimize the jitter of the impedance response curve and suppress the formation of spurious modes in the acoustic waves, which is beneficial for obtaining a filter with better performance.

[0054] Figure 1 A schematic diagram of a laterally excited thin-film bulk acoustic resonator provided in this application embodiment is shown below. Figure 1 As shown, the laterally excited thin-film bulk acoustic resonator includes: a substrate 1, a bonding layer 4, a piezoelectric thin film layer 2, and an excitation electrode 3. The substrate 1 supports the piezoelectric thin film layer 2. The piezoelectric thin film layer 2 includes opposing first and second surfaces, and the bonding layer 4 connects the second surface to the substrate 1. The excitation electrode 3 includes multiple sub-electrodes disposed within the piezoelectric thin film layer 2. One end of each sub-electrode exposes the first surface.

[0055] The transversely excited thin-film bulk acoustic resonator described in this application completely places the electrodes that excite the transverse electric field inside the piezoelectric thin film, avoiding the formation of longitudinal electric field components and improving the uniformity of the transverse electric field, reducing the occurrence of stray acoustic modes, and making its impedance response curve smoother. Based on this transversely excited thin-film bulk acoustic resonator structure, it has certain design guidance value in the field of radio frequency filters and has great practical potential in the industrial production of micro-electro-mechanical systems (MEMS).

[0056] In this embodiment, the structure of the laterally excited thin-film bulk acoustic resonator includes a high-resistivity silicon substrate 1, a piezoelectric thin film layer 2, and an excitation electrode 3 embedded in the piezoelectric thin film layer 2. The excitation electrode 3 is an interdigital transducer (IDT). Multiple sub-electrodes can be configured as interdigitals according to actual needs. A bonding layer 4 is also provided between the substrate 1 and the piezoelectric thin film layer 2, bonding the substrate 1 and the piezoelectric thin film layer 2 together. In some embodiments, an etching stop layer (or support layer) is also provided on the lower surface, i.e., the second surface, of the piezoelectric thin film layer 2 to support the piezoelectric thin film layer 2. In other embodiments, the upper surface of the IDT electrode can also be covered with a passivation layer film to prevent electrode oxidation.

[0057] In this embodiment, the laterally excited thin-film bulk acoustic resonator can achieve total reflection of sound waves at the interface using two structures: air-gap type and solid-state assembly type. The air-gap type forms a cavity between the substrate 1 and the resonator stack as an acoustic mirror. There are two structural types of air-gap type: one is a suspended film structure supported by the edge of the substrate 1. This is fabricated by first depositing a piezoelectric thin film layer 2 on the supporting substrate 1, and then etching away part of the substrate 1 to form the suspended film structure. The other type forms an air gap below the resonator. This is fabricated by etching a cavity on the upper surface of the substrate 1 using semiconductor technology, filling it with a sacrificial layer material, then fabricating a piezoelectric film on top to form the resonator, and finally removing the sacrificial layer using etching technology to obtain the air gap. The solid-state assembly type uses a Bragg reflector as the sound wave reflector, allowing the sound wave to be confined within the resonant cavity to prevent energy loss. This is fabricated by stacking thin films with different acoustic impedances, with the film thickness precisely controlled to 1 / 4 wavelength. In some embodiments, the acoustic mirror can also be other acoustic impedance structures that satisfy sound wave reflection. Figure 1 As shown, Figure 1 The diagram illustrates an air-gap type thin-film bulk acoustic resonator structure. In this structure, the longitudinal overlap region of the IDT electrode structure, the piezoelectric thin film layer 2, and the air cavity constitutes the effective resonant region. The lateral width of the cavity is L, and the spacing between the IDT electrode fingers is P. By applying a voltage to the busbars, adjacent fingers are sequentially connected to the signal and ground wires, respectively, forming a lateral electric field that excites the longitudinally propagating acoustic wave. By selecting a material with a large lateral electromechanical coupling coefficient as the substrate 1, a resonator with a large bandwidth can be obtained. By controlling the thickness of the piezoelectric thin film layer 2, a resonator with a higher operating frequency can be obtained.

[0058] In this embodiment, the resistivity of the high-resistivity silicon substrate 1 is 1-100 kΩ·cm. Besides silicon, the substrate 1 can also be made of other semiconductor materials, such as sapphire, quartz, indium phosphide, gallium arsenide, etc. The piezoelectric thin film layer 2 can be made of aluminum nitride, zinc oxide, lead zirconate titanate, single-crystal lithium niobate, and single-crystal lithium tantalate, etc. The excitation electrode 3 can be made of metals such as molybdenum, aluminum, tungsten, ruthenium, platinum, and their alloys.

[0059] In this embodiment, in the laterally excited thin-film bulk acoustic resonator, the sub-electrode is completely embedded inside the piezoelectric thin film layer 2. The height of the sub-electrode can be less than, equal to, or slightly greater than the thickness of the piezoelectric thin film layer 2. The optimal height of the sub-electrode is equal to the thickness of the piezoelectric thin film layer 2. This arrangement of the excitation electrode 3 can obtain a highly uniform lateral electric field and avoid stray modes caused by the presence of longitudinal electric field components.

[0060] In this embodiment, in a laterally excited thin-film bulk acoustic resonator, when the spacing between the IDT electrode fingers is large, the resonant frequency of the resonator is mainly determined by the thickness h of the piezoelectric thin film layer 2. Resonance occurs when the thickness h is equal to half the longitudinal wave wavelength. Longitudinal wave wavelength λ l It can be calculated from the equivalent film thickness of the resonator, using the following formula:

[0061]

[0062] Among them, v l The longitudinal wave velocity t in piezoelectric thin film layer 2 n and v n These represent the thickness of each film layer and the longitudinal wave velocity, respectively.

[0063] This structure can utilize a laterally excited electric field to excite longitudinally propagating acoustic wave modes. Due to the large electromechanical coupling coefficient of the piezoelectric thin film layer 2 in this direction, the resonator achieves a high resonant frequency and ultra-large bandwidth while controlling the thickness of the piezoelectric thin film layer 2. Optionally, the thickness h of the piezoelectric thin film layer 2 can be varied in the range of 100nm-1000nm. As an example, using a high-resistivity silicon substrate 1, a single-crystal lithium niobate piezoelectric thin film layer 2, and silicon dioxide bonding layer 4, the resonator's resonant frequency is approximately 5GHz and the bandwidth exceeds 1GHz when h = 400nm. Compared to AlN-based FBAR resonators, this bandwidth is significantly larger for the same piezoelectric thin film layer 2 thickness.

[0064] In this embodiment, the edges of the sub-electrodes arranged in the piezoelectric thin film layer 2 can extend beyond the cavity or be located within the corresponding region inside the cavity. Preferably, the sub-electrode region is entirely located inside the cavity to form an effective resonant region, preventing acoustic wave energy from leaking directly from the silicon substrate 1 and degrading the resonator's performance. Optionally, such as Figure 1 As shown, the sub-electrodes in the piezoelectric thin film layer 2 are arranged in an equally spaced manner. The distance between the sub-electrodes can be designed according to the suppression of stray sound wave modes or the wavelength of the transversely propagating sound waves. Optionally, the sub-electrodes in the piezoelectric thin film layer 2 can be arranged in a gradually varying manner. The minimum period of the sub-electrode arrangement is determined according to the suppression of stray sound waves, with the period increasing sequentially from small to large. By optimizing the arrangement of the excitation electrodes 3 of this resonator and then combining it with their transverse dimensions to determine the number of pairs of excitation electrodes 3, stray sound wave modes caused by poor electric field uniformity in the prior art are avoided, thus preventing a reduction in the filter performance of the application.

[0065] As an optional implementation method, Figure 2 This application provides a schematic diagram of the structure of a resonator with a gradually distributed sub-electrode, as shown in the embodiment of the present application. Figure 2As shown, the spacing between adjacent sub-electrodes gradually increases from both sides of the cavity edge towards the center. The minimum spacing between adjacent sub-electrodes needs to be determined by the impedance response curve of a single resonator. The larger the spacing between adjacent sub-electrodes, the higher the uniformity of the transverse electric field and the flatter the impedance response curve. Correspondingly, the smaller the spacing, the easier it is to generate stray acoustic wave resonance. As an example, the gradual change in the spacing between adjacent sub-electrodes can follow the functional relationship y = Δ5 × N, where N is a positive integer. That is, the minimum spacing on the outer side of the sub-electrode can be 5 μm, and the spacing between subsequent adjacent sub-electrodes can be designed as 10 μm, 15 μm, and 20 μm respectively. At this time, the impedance response curve of the resonator is relatively flat, and the stray mode gradually weakens with the increase of the period.

[0066] It should be noted that, Figure 2 The distribution of sub-electrodes shown is only an illustrative explanation of one case. That is, the relationship between the spacing between adjacent sub-electrodes is not limited to a linear function, but can also be a quadratic function or an exponential function with a positive correlation.

[0067] Figure 3 A comparison diagram of the resonator effects of a prior art technology and an embodiment of this application is provided for the purposes of this application. Figure 3 As shown, Figure (3a) illustrates the effect of a resonator in the prior art, and Figure (3b) illustrates the effect of a resonator with equally spaced sub-electrodes provided in an embodiment of this application. Figure 3 As shown, the resonant frequency of the resonator hardly changes. Because the IDT electrode is placed inside the piezoelectric thin film layer 2, the generation of the longitudinal component of the electric field, which is present in the prior art, is avoided, the uniformity of the transverse electric field is improved, stray sound wave resonance is reduced, and its impedance response curve is flatter.

[0068] Figure 4 An effect diagram of a resonator with linearly varying sub-electrode spacing provided in an embodiment of this application is shown below. Figure 4 As shown, Figure 4 It shows Figure 2 The optimization effect of the sub-electrode arrangement shown. Figure 4 The Δ5, Δ15, and Δ25 shown are the periodically varying step sizes. As the periodic step size increases, the stray acoustic wave modes in the impedance response curve gradually weaken.

[0069] As another alternative implementation, the shape of the excitation electrode 3 disposed in the piezoelectric thin film layer 2 is not limited to a finger-like shape, but can also be a two-dimensional structure. Figure 5 This is a top view of a two-dimensional embedded sub-electrode arrangement structure provided in an embodiment of this application. Figure 6 A cross-sectional view of a two-dimensional embedded sub-electrode arrangement structure provided in an embodiment of this application, as shown below. Figure 5 and Figure 6As shown, sub-electrodes form a first electrode layer and a second electrode layer distributed vertically within the piezoelectric thin film layer 2. The first electrode layer is disposed on the upper surface (first surface) of the piezoelectric thin film layer 2, and the second electrode layer is disposed on the second surface. The first electrode layer and multiple sub-electrodes are connected to form multiple parallel first electrode bridges. The second electrode layer includes multiple sub-electrodes connected to form multiple parallel second electrode bridges. Figure 5 As shown, the electrode bridge 7 is an electrical bridge structure that ensures electrical interconnection between electrodes, and each electrode bridge 7 includes multiple circular electrodes 8. In some embodiments, the shape of the electrodes can also be a regular shape such as rectangular or rhomboid, or other irregular shapes. Figure 5 and Figure 6 As shown, the projections of the first electrode bridge and the second electrode bridge onto the horizontal plane are perpendicular to each other, and the projections of the sub-electrodes in the first electrode bridge and the sub-electrodes in the second electrode bridge onto the horizontal plane coincide. That is, two-dimensional embedded electrodes are set in both the upper and lower halves of the piezoelectric thin film layer 2, the orientations of the upper and lower halves of the electrode bridges differ by 90°, and the positive projection of each positive electrode onto the opposite surface must completely cover a negative electrode, and the positive projection of each negative electrode onto the opposite surface must completely cover a positive electrode, thereby forming a positive and negative overlapping signal excitation acoustic wave mode.

[0070] Figure 7 This is a schematic diagram of a resonator structure with a limiting frame 6 on a piezoelectric thin film layer 2, as provided in an embodiment of this application. Figure 7 As shown, a limiting frame 6 is provided on the first surface. The limiting frame 6 is a closed ring structure and is located outside the effective resonance region.

[0071] In this embodiment, a raised limiting frame 6 can be provided in the invalid resonant region located outside the cavity. This limiting frame 6 is disposed on the upper surface of the piezoelectric thin film layer 2. Figure 7 As shown, the confinement frame 6 forms a closed shape around the entire ineffective region of the resonator. The lateral width of the confinement frame 6 is defined as d, and the longitudinal thickness is defined as t. The setting of the confinement frame 6 can improve the acoustic impedance mismatch between the outer ineffective region and the inner effective region of the resonator, enhance the ability to limit the transversely transmitted leakage sound waves, and improve the quality factor Q value of the resonator.

[0072] The height of the confinement frame 6 is 0.1μm-1μm; the cross-sectional dimensions of the confinement frame 6 are 0.1μm-1μm.

[0073] In the embodiments of this application, such as Figure 7The transversely excited thin-film bulk acoustic resonator shown requires the transverse width d of the confinement frame 6 to be determined based on one-quarter of the transverse acoustic wavelength at the resonant frequency, and can further be taken as odd multiples such as 1, 3, 5, and 7. As an example, the transverse acoustic wave velocity inside a single-crystal lithium niobate thin film is approximately in the range of 3300 m / s to 4000 m / s. At a resonant frequency of 5 GHz, with a piezoelectric film thickness h of 400 nm, the calculated wavelength is approximately 0.66 μm. According to the formula d = 0.25 × λ × N, where N is a positive integer, the width d can range from 0.165 μm to 9.075 μm, and the longitudinal thickness t can range from 0.1 μm to 1 μm. The width d and thickness t of the confinement frame 6 need to be reasonably set according to the suppression of stray acoustic waves in the impedance response curve to obtain the optimal performance of the transversely excited thin-film bulk acoustic resonator.

[0074] Figure 8 This application provides a partial structural diagram of a resonator with an acoustic microstructure 5 disposed on a piezoelectric thin film layer 2, as shown in the embodiment of the present application. Figure 8 As shown, the area between adjacent sub-electrodes is the inter-electrode region, and an acoustic microstructure 5 is provided in the inter-electrode region. The acoustic microstructure 5 is disposed on the first surface.

[0075] In the embodiments of this application, such as Figure 8 As shown, acoustic microstructures 5 are disposed between adjacent electrode pairs and located on the upper surface of the piezoelectric thin film layer 2. The acoustic microstructures 5 are arranged laterally and periodically between adjacent electrode pairs. Since the acoustic microstructures 5 are subwavelength, the periodically distributed uneven structure cannot be identified by longitudinally propagating sound waves, and is equivalent to a horizontal plane. The ratio of the size between adjacent acoustic microstructures 5 to the size of the inter-electrode region is the duty cycle. Different duty cycles result in different heights of the equivalent plane, thus forming resonators with different frequencies.

[0076] In this embodiment, the acoustic microstructure 5 does not require an electrical signal and can be made of a dielectric material. While a metallic material can also be used, a dielectric material is preferred. Using a dielectric material avoids the generation of floating voltages in the metal, which could affect the internal electric field of the piezoelectric thin film layer 2.

[0077] Frequency tuning of a transversely excited thin-film bulk acoustic resonator can be easily achieved through a single etching process. In existing technologies, frequency tuning of series and parallel resonators requires depositing a tuning layer on the electrode surface. These layers may have different thicknesses, necessitating thickness correction and making the process cumbersome. Furthermore, precise frequency matching requires optimization of the tuning layer thickness, which is challenging. In this embodiment, by incorporating acoustic microstructure 5 technology, a periodically varying or duty-cycle-varying acoustic microstructure 5 is formed on the surface of the piezoelectric thin film layer 2. This allows for frequency tuning of both series and parallel resonators through a single etching process, reducing the complexity of the fabrication process and production costs. The acoustic subwavelength can be determined based on the transverse width of the acoustic microstructure 5; that is, the transverse width of the acoustic microstructure 5 needs to be smaller than the longitudinal acoustic wave propagation wavelength λ. l Preferably, its width can be less than λ. l / 2. Wherein, the longitudinal wave wavelength λ l It can be calculated from the equivalent film thickness of the resonator and formula (3).

[0078] In this embodiment, the height of the acoustic microstructure 5 is 0.1 μm-0.5 μm, and the cross-sectional dimension of the acoustic microstructure 5 is 0.1 μm-0.3 μm. As an example, the transverse acoustic wave velocity within a single-crystal lithium niobate film is approximately in the range of 3300 m / s to 4000 m / s. With a resonant frequency of 5 GHz, a piezoelectric film thickness h of 400 nm, and a calculated wavelength of approximately 0.66 μm, the transverse width (i.e., the cross-sectional dimension of the acoustic microstructure 5) can range from 0.165 μm to 2.475 μm, and the longitudinal thickness (i.e., the height of the acoustic microstructure 5) can range from 0.1 μm to 0.5 μm. Placing the acoustic microstructure 5 between adjacent electrode pairs can improve the quality factor Q of the transversely excited thin-film bulk acoustic resonator while simultaneously achieving frequency tuning.

[0079] In this embodiment, the acoustic microstructure 5 can be arranged in an equally spaced manner between adjacent sub-electrodes, or the duty cycle can be set according to a preset function. Figure 9 This is a schematic diagram of the distribution structure of an acoustic microstructure 5 provided in an embodiment of this application, as shown below. Figure 9 As shown, with Figure 8 The acoustic microstructures 5 shown are arranged differently, Figure 9The duty cycle of the acoustic microstructure 5 on the upper surface of the medium piezoelectric thin film layer 2 is no longer uniform everywhere, but varies from the outer ends to the inner ends. Specifically, the period P2 between the outer IDT electrode pairs corresponds to the duty cycle η2, and the period P1 between the inner IDT electrode pairs corresponds to the duty cycle η1. Different duty cycles correspond to different equivalent thicknesses, thus forming a confinement region for acoustic wave energy between each IDT electrode pair, thereby further improving the Q value of the resonator. The lateral width of the duty cycle η2 of the outer microstructure is defined as m, and the specific value of m needs to be determined based on the quarter wavelength of the transversely transmitted acoustic wave in the lithium niobate film. Optionally, it can be designed as an odd multiple; as an optional implementation, the value of m is approximately 5.775 μm.

[0080] In some embodiments, the duty cycle change of the acoustic microstructures 5 arranged from both sides towards the middle can be abrupt, a linear gradual change of a linear function (y = kx), or a nonlinear gradual change of a quadratic function (y = -ax). 2 +b), etc., the function coefficients need to be determined by the frequency change caused by the actual duty cycle change. Valid coefficients can be (k = 0.1, a = 0.04, b = 0.74), where x is defined as the position of the outermost microstructure and is a positive integer, and y is defined as the duty cycle size of the microstructure corresponding to position x. Alternatively, a step function with gradual changes can be used (y = step(Δx = N*c)), where N and c are positive integers. It should be understood that valid function expressions are not limited to the formulas given above and can also be other types of functions with different patterns.

[0081] In the embodiments of this application, Figure 9 The transversely excited thin-film bulk acoustic resonator shown can also be combined with variations of techniques such as microstructure duty cycle abrupt changes, gradual changes, and gradual function forms through the joint use of the upper and lower surfaces to achieve an optimal solution that gives the resonator a high Q value.

[0082] Figure 10 A comparison diagram of resonator Q-value calculation between the prior art and the embodiments of this application is provided for the purposes of this application, as shown in the figure. Figure 10 As shown, the solid line represents the above. Figure 9 The image shows the Q-value of a laterally excited thin-film bulk acoustic resonator; the dotted line represents the Q-value of a prior art resonator. From... Figure 10 As can be seen, when the acoustic microstructure 5 is applied to the membrane layer of the thin-film bulk acoustic resonator, transverse acoustic energy leakage can be effectively suppressed, resulting in a higher Q value for the thin-film bulk acoustic resonator compared to existing resonators. This application provides a highly feasible solution for filters used in high-frequency applications.

[0083] The key technology of the laterally excited thin-film bulk acoustic resonator structure described in this application embodiment lies in: setting electrodes for the laterally excited electric field inside the piezoelectric thin film layer 2, so that the electric field inside the piezoelectric layer is distributed laterally, avoiding the presence of a longitudinal component, thus improving the uniformity of the laterally excited electric field. The longitudinal thickness of the electrodes can be less than, equal to, or slightly larger than the piezoelectric thin film, with the optimal value being when both are equal. The arrangement of the electrodes can be equally spaced or periodically gradually varied, with a large periodic electrode design usually being optimal, which is beneficial for suppressing stray acoustic modes of the resonator. In addition, the laterally excited thin-film bulk acoustic resonator proposed in this application embodiment also incorporates an acoustic microstructure 5, which can achieve adjustment of the resonant frequency and further suppression of stray acoustic modes, improve the quality factor Q value, and enable filters using it to have superior performance, showing greater application potential in 5G and even higher frequency fields.

[0084] This application also discloses a filter, which includes a parallel resonator and a series resonator, cascaded together to form the filter. The parallel and series resonators are laterally excited thin-film bulk acoustic resonators as described above.

[0085] Figure 11 A schematic diagram of a cascaded ladder topology of a filter provided in an embodiment of this application is shown below. Figure 11 As shown, the cascaded filter topology includes one series branch and four parallel branches. The series branch contains five thin-film bulk acoustic resonators, such as... Figure 11 As shown, the first resonator S1, the second resonator S2, the third resonator S3, the fourth resonator S4, and the fifth resonator S5 have parallel resonators P1, P2, P3, and P4 connected in parallel between each pair of adjacent series resonators. Each parallel resonator has a grounding inductor between it and the grounding node, forming a transmission zero to adjust the out-of-band suppression strength of the filter. Each resonator is a transversely excited thin-film bulk acoustic resonator as described in this embodiment. It should be understood that the number of series and parallel resonators is not limited. Figure 11 As shown in the diagram, the number of filters can be increased or decreased according to the filter design specifications.

[0086] This application proposes a transversely excited thin-film bulk acoustic resonator structure. By completely placing the excitation electrode 3 inside the piezoelectric thin film layer 2, the longitudinal electric field component present inside the piezoelectric thin film layer 2 in the prior art is avoided, thereby achieving the beneficial technical effect of reducing stray acoustic wave modes of the resonator. At the same time, the RF filter using it has better performance.

[0087] This application proposes a transversely excited thin-film bulk acoustic resonator structure, the minimum value of which needs to be determined according to the stray mode of the acoustic wave, thereby improving the design flexibility.

[0088] This application proposes a thin-film bulk acoustic resonator based on a subwavelength acoustic microstructure 5. It can achieve fine-tuning of the frequency difference between series and parallel resonators with only one etching technique, without the need for additional process steps, avoiding the need to adjust each resonator sequentially, and reducing the complexity of the process.

[0089] This application embodiment, through the rational design of the proposed subwavelength acoustic microstructure 5, achieves fine-tuning of the frequency difference between series and parallel resonators, while the width of the acoustic microstructure 5 can be designed according to the transverse propagation wavelength, so that the resonator has a high Q value, reduces transverse leakage of sound waves, and further reduces the passband insertion loss of the cascaded filter.

[0090] This application proposes a transversely excited thin-film bulk acoustic resonator structure. The manufacturing method of this resonator is entirely based on mature MEMS semiconductor process technology. It can be repeatedly mass-produced using existing equipment without the need to introduce other traditional processes, which greatly reduces costs.

[0091] The above description is only 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 protection scope of this application.

Claims

1. A transversely excited thin-film bulk acoustic resonator, characterized in that, include: The substrate (1), bonding layer (4), piezoelectric thin film layer (2) and excitation electrode (3) are included. The substrate (1) is used to support the piezoelectric thin film layer (2); The piezoelectric thin film layer (2) includes a first surface and a second surface opposite to each other, and the bonding layer (4) connects the second surface to the substrate (1); The excitation electrode (3) includes a plurality of sub-electrodes, which are completely embedded inside the piezoelectric thin film layer (2); One end of the sub-electrode is exposed on the first surface; the height of the sub-electrode is equal to the thickness of the piezoelectric thin film layer (2); the sub-electrodes are periodically distributed within the effective resonant region of the piezoelectric thin film layer (2); A limiting frame (6) is provided on the first surface. The limiting frame (6) protrudes relative to the first surface. The limiting frame (6) is a closed ring structure and is located outside the effective resonant region. The height of the limiting frame (6) is 0.1μm-1μm. The cross-sectional dimension of the limiting frame (6) is 0.1μm-1μm. The width of the limiting frame (6) ranges from 0.165μm to 9.075μm. The longitudinal thickness of the limiting frame (6) ranges from 0.1μm to 1μm.

2. The transversely excited thin-film bulk acoustic resonator according to claim 1, characterized in that, An acoustic reflector is provided between the substrate (1) and the piezoelectric thin film layer (2), and the acoustic reflector includes at least an air and a Bragg reflector.

3. The transversely excited thin-film bulk acoustic resonator according to claim 1, characterized in that, The spacing between adjacent sub-electrodes is set according to a first preset function.

4. The transversely excited thin-film bulk acoustic resonator according to claim 1, characterized in that, The sub-electrode forms a first electrode layer and a second electrode layer in the piezoelectric thin film layer (2), with the first electrode layer disposed on the first surface and the second electrode layer disposed on the second surface; The first electrode layer includes a plurality of sub-electrodes connected to form a plurality of parallel first electrode bridges; The second electrode layer includes multiple sub-electrodes connected to form multiple parallel second electrode bridges; The projections of the first electrode bridge and the second electrode bridge onto the horizontal plane are perpendicular to each other.

5. The transversely excited thin-film bulk acoustic resonator according to claim 1, characterized in that, The thickness of the piezoelectric thin film layer (2) is 100nm-1000nm.

6. The transversely excited thin-film bulk acoustic resonator according to claim 1, characterized in that, The region between adjacent sub-electrodes is called the inter-electrode region, and an acoustic microstructure (5) is provided in the inter-electrode region. The acoustic microstructure (5) is disposed on the first surface.

7. The transversely excited thin-film bulk acoustic resonator according to claim 6, characterized in that, The height of the acoustic microstructure (5) is 0.1μm-0.5μm; the cross-sectional size of the acoustic microstructure (5) is 0.1μm-0.3μm.

8. The transversely excited thin-film bulk acoustic resonator according to claim 7, characterized in that, The ratio of the size between adjacent acoustic microstructures (5) to the size of the inter-electrode region is the duty cycle, which is set according to a second preset function.

9. The transversely excited thin-film bulk acoustic resonator according to claim 8, characterized in that, The acoustic microstructure (5) is made of a dielectric material.

10. A filter, characterized in that, The filter includes a parallel resonator and a series resonator, and the parallel resonator and the series resonator are cascaded to form the filter; The parallel resonator and the series resonator are the transversely excited thin-film bulk acoustic resonators as described in any one of claims 1-9.

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