A longitudinal leaky surface acoustic wave filter

Abstract

The application provides a longitudinal leaky surface acoustic wave filter, and relates to the field of passive electrical devices, comprising a plurality of electrically cascaded resonators for exciting longitudinal leaky surface acoustic wave modes; each resonator comprises a support substrate, a piezoelectric thin film structure and an electrode array; the electrode array comprises an interdigital electrode array and a reflective grating array; the wavelength of a series resonator is less than the wavelength of a parallel resonator; the first thickness of a first piezoelectric thin film structure of the series resonator is less than the second thickness of a second piezoelectric thin film structure of the parallel resonator, so that the difference between the normalized film thickness of the series resonator and the normalized film thickness of the parallel resonator is within a preset range, thereby suppressing the out-of-band spurious modes of the longitudinal leaky surface acoustic wave filter, reducing the in-band loss and improving the performance of the filter.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of passive electrical devices, in particular to a longitudinal leaky surface acoustic wave filter. BACKGROUND

[0002] A conventional surface acoustic wave filter based on piezoelectric thin film has a uniform piezoelectric film thickness h. The resonator, a component unit of the filter, has different wavelengths λ, so that a surface acoustic wave filter has multiple different h / λ. The resonator characteristics (including coupling coefficient, cutoff frequency, Q value, acoustic velocity, etc.) corresponding to different h / λ are quite different. For a surface acoustic wave filter of horizontal shear mode, different h / λ generally does not have a negative impact; but for the current new longitudinal leaky mode surface acoustic wave filter, too large h / λ will cause the existence of spurious modes outside the passband of the surface acoustic wave filter, and too small h / λ will make the cutoff frequency too low, resulting in excessive filter loss. SUMMARY

[0003] To solve the technical problems of the prior art that the longitudinal leaky surface acoustic wave filter has out-of-band spurious modes and high filter loss due to different h / λ, the present application discloses a longitudinal leaky surface acoustic wave filter, comprising:

[0004] a plurality of electrically cascaded resonators for exciting a longitudinal leaky surface acoustic wave mode; each of the resonators comprises a support substrate, a piezoelectric thin film structure and an electrode array; the electrode array comprises a parallelly arranged interdigital electrode array and a reflection grid array arranged at both ends of the interdigital electrode array; the resonators comprise series resonators and parallel resonators, wherein,

[0005] the wavelength of the series resonator is smaller than the wavelength of the parallel resonator; the first piezoelectric thin film structure of the series resonator has a first thickness; the second piezoelectric thin film structure of the parallel resonator has a second thickness;

[0006] the first thickness of the first piezoelectric thin film structure is smaller than the second thickness of the second piezoelectric thin film structure, so that the difference between the normalized film thickness of the series resonator and the normalized film thickness of the parallel resonator is within a preset difference range; the normalized film thickness represents the ratio of the piezoelectric thin film structure thickness of the resonator to the wavelength of the resonator.

[0007] Optionally, the first piezoelectric thin film structure and the second piezoelectric thin film structure are piezoelectric thin films or composite thin film structures formed by piezoelectric thin films and dielectric layers; the longitudinal leaky surface acoustic wave filter comprises at least two kinds of piezoelectric thin film structure thicknesses.

[0008] Optionally, the first piezoelectric thin film structure is a piezoelectric thin film, and the second piezoelectric thin film structure is a composite thin film structure formed by the piezoelectric thin film and a dielectric layer.

[0009] The piezoelectric thin film thickness of the first piezoelectric thin film structure is consistent with the piezoelectric thin film thickness of the second piezoelectric thin film structure, and the dielectric layer thickness of the second piezoelectric thin film structure is less than 1 micrometer; the material of the dielectric layer is one or a combination of silicon oxide, aluminum oxide, silicon nitride, and hafnium dioxide.

[0010] Optionally, the first piezoelectric thin film structure and the second piezoelectric thin film structure are both piezoelectric thin films; the piezoelectric thin film thickness of the first piezoelectric thin film structure is less than the piezoelectric thin film thickness of the second piezoelectric thin film structure.

[0011] Optionally, a transition layer is arranged between the piezoelectric thin film structure and the support substrate; the material of the transition layer is one or a combination of silicon oxide, aluminum oxide, silicon nitride, and hafnium dioxide.

[0012] Optionally, the material of the piezoelectric thin film of the first piezoelectric thin film structure and the second piezoelectric thin film structure is one of lithium niobate or lithium tantalate.

[0013] Optionally, the thickness of the piezoelectric thin film of the first piezoelectric thin film structure and the second piezoelectric thin film structure is less than 1 micrometer; the crystal cut type of the piezoelectric thin film is X-cut or Z-cut.

[0014] Optionally, the material of the support substrate is one of silicon carbide or diamond.

[0015] Optionally, the interdigital electrode array and the reflective grating electrode array of the electrode array have an inclination angle with the normal direction of the electrode array; the inclination angle is in the range of [-20°, +20°].

[0016] Optionally, an inductive element is further included, which is connected in series or bridged with the parallel resonator.

[0017] By adopting the technical scheme, the present application has the following beneficial effects:

[0018] The present application proposes a filter structure with different thin film thicknesses for a longitudinal leaky surface acoustic wave filter, reduces the thickness of the first piezoelectric thin film structure of the series resonator, or increases the thickness of the second piezoelectric thin film structure of the parallel resonator, so that the thickness of the first piezoelectric thin film structure of the series resonator is less than the thickness of the second piezoelectric thin film structure of the parallel resonator, the h / λ of all resonators in the longitudinal leaky surface acoustic wave filter is in an advantageous range, and the h / λ difference between the resonators is small, so as to suppress the out-of-band spurious mode of the longitudinal leaky surface acoustic wave filter, reduce the in-band loss, and improve the filter performance. Attached Figure Description

[0019] 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.

[0020] Figure 1 This is a schematic diagram of the cross-sectional structure of a resonator unit of a longitudinally leaking surface acoustic wave filter according to an embodiment of the present invention;

[0021] Figure 2 This is a schematic diagram of the cross-sectional structure of a resonator unit of another longitudinally leaking surface acoustic wave filter according to an embodiment of the present invention;

[0022] Figure 3 This is a schematic diagram of the cross-sectional structure of a resonator unit of another longitudinally leaking surface acoustic wave filter according to an embodiment of the present invention;

[0023] Figure 4 This is a top view schematic diagram of the resonator unit structure of a longitudinally leaking surface acoustic wave filter according to an embodiment of the present invention;

[0024] Figure 5 This is a schematic diagram of the topology of a longitudinally leaking surface acoustic wave filter according to an embodiment of the present invention;

[0025] Figure 6 This is a schematic diagram of the admittance of the resonator unit and the insertion loss of the filter in an embodiment of the longitudinal leakage surface acoustic wave filter of the present invention.

[0026] Figure 7 To Figure 6 A schematic diagram of the admittance of the resonator unit and the insertion loss of the filter in an optimized and adjusted longitudinal leakage surface acoustic wave filter.

[0027] Figure 8 This is a schematic diagram of the admittance and conductance of a resonator unit of another longitudinally leaking surface acoustic wave filter according to an embodiment of the present invention;

[0028] Figure 9 To Figure 8 A schematic diagram of the admittance and conductance of the resonator unit of another optimized and adjusted longitudinal leakage surface acoustic wave filter;

[0029] Figure 10 This is a schematic diagram of the admittance and conductance of a resonator unit in a horizontal shear surface acoustic wave filter in the prior art.

[0030] The following is supplementary explanation of the attached figures:

[0031] 1 - series resonator; 11 - first piezoelectric thin film structure; 2 - parallel resonator; 21 - second piezoelectric thin film structure; 3 - electrode array; 31 - interdigital electrode array; 32 - reflective grating array; 4 - support substrate; 5 - dielectric layer; 6 - transition layer. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0033] Herein, the term "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one implementation of the present application. In the description of the application, it should be understood that the terms "upper", "lower", "top", "bottom", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first", "second" are only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features referred to. Therefore, the features defined with "first", "second" can be explicitly or implicitly included one or more of the features. Moreover, the terms "first", "second", etc. are used to distinguish similar objects, and do not necessarily describe a particular order or sequence. It should be understood that the data thus used can be interchanged under appropriate circumstances, so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein.

[0034] For the purpose of the following detailed description, it is to be understood that the application can assume various alternative variations and step sequences, except where expressly specified to the contrary. Moreover, other than in any operating examples, or where otherwise indicated, all numbers expressing, for example, quantities of ingredients contained in the compositions and processes disclosed herein are to be understood as being modified in all instances by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by the present application. At the very least, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0035] Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the application are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. When numerical ranges are disclosed herein, these ranges are to be construed as continuous along their limits, and include every value within that range. Further, these ranges can be combined with other ranges disclosed herein. In other words, unless expressly stated to the contrary, any range of values disclosed herein is intended to include all values within that range. For example, a range of "1 to 10" is intended to include any and all sub-ranges between and including the minimum value of 1 and the maximum value of 10; that is, all sub-ranges beginning with a minimum value of 1 or more and ending with a maximum value of 10 or less, as well as every number from 1 to 10. Exemplary sub-ranges include, but are not limited to, 1 to 6.1, 3.5 to 7.8, 5.5 to 10, and the like.

[0036] The acoustic resonator is the basic unit of the SAW filter, and the performance of the resonator will directly affect the performance of the filter. The SAW filter can be formed by cascading a plurality of series resonators and parallel resonators, and the resonator structure is usually formed by stacking a support substrate, a piezoelectric film and an electrode array. In the prior art, the piezoelectric film thickness h of each resonator of a conventional SAW filter based on a piezoelectric film is uniform, and the series resonator and the parallel resonator have different wavelengths λ, because according to the working frequency f=v / λ (where λ is the period of the interdigital electrode, and v is the acoustic velocity of the SAW), since the frequency of the series resonator is higher than that of the parallel resonator, under the premise that the acoustic velocity v is the same, the wavelength λ1 of the series resonator is smaller than the wavelength λ2 of the parallel resonator, so there are multiple different h / λ in a SAW filter, where h represents the thickness of the resonator piezoelectric film structure. Since the acoustic wave has a dispersion characteristic in a heterogeneous substrate, the resonator characteristics (including: coupling coefficient, cutoff frequency, Q value, acoustic velocity, etc.) corresponding to different h / λ are quite different. For a filter exciting a horizontal shear horizontal SAW mode (SH-SAW), different h / λ has little effect; for a filter exciting a longitudinal leaky SAW mode (LL-SAW), if h / λ is too large, it will cause the filter to have a spurious mode outside the passband, and if h / λ is too small, it will cause the cutoff frequency to be too low, resulting in excessive loss in the passband of the filter.

[0037] Therefore, for the filter of the longitudinal leaky surface acoustic wave mode (LL-SAW), in order to solve the problem of device performance deterioration caused by too large difference of h / λ of different resonators, make h / λ of each resonator unit in the filter be in an advantageous interval and the difference of h / λ between each resonator be small, the present application considers changing the thickness of the piezoelectric film of the resonator to change the size of h / λ. Since the wavelength of the series resonator is small and the wavelength of the parallel resonator is large, the thickness of the piezoelectric film of the series resonator can be reduced or the thickness of the piezoelectric film of the parallel resonator can be increased, that is, h1 is less than h2, finally h1 / λ1 is close to h2 / λ2, the series / parallel resonators with uniform performance are obtained, and finally a high-performance filter is formed.

[0038] The longitudinal leaky surface acoustic wave filter provided by the present application comprises:

[0039] A plurality of electrically cascaded resonators for exciting longitudinal leaky surface acoustic wave mode; each of the resonators comprises a support substrate 4, a piezoelectric film structure and an electrode array 3; the electrode array 3 comprises a parallelly arranged interdigital electrode array 31 and a reflection grid array 32 arranged at both ends of the interdigital electrode array 31; the resonators comprise series resonators 1 and parallel resonators 2, wherein,

[0040] The wavelength of the series resonator 1 is less than the wavelength of the parallel resonator 2; the first piezoelectric film structure 11 of the series resonator 1 has a first thickness; the second piezoelectric film structure 21 of the parallel resonator 2 has a second thickness;

[0041] The first thickness of the first piezoelectric film structure 11 is less than the second thickness of the second piezoelectric film structure 21, so that the difference between the normalized film thickness of the series resonator and the normalized film thickness of the parallel resonator is within a preset difference range; the normalized film thickness represents the ratio of the thickness of the piezoelectric film structure of the resonator to the wavelength of the resonator.

[0042] Specifically, the surface acoustic wave filter of the present application can be composed of one series resonator 1 and one parallel resonator 2, or one series resonator 1 and a plurality of parallel resonators 2, or a plurality of series resonators 1 and one parallel resonator 2, or a plurality of series resonators 1 and a plurality of parallel resonators 2, in the embodiments of the present application, the series resonators are generally used to collectively refer to the plurality of series resonators 1 in the filter, that is, in fact, the series resonator 1 can include a plurality of series resonators 1, and the parallel resonator 2 is used to collectively refer to the plurality of parallel resonators 2 in the filter, that is, the series resonator 1 and the parallel resonator 2 in the examples below are not limited to one.

[0043] Reference Figure 1Fig. 1 is a schematic diagram of a resonator unit cross-sectional structure of a longitudinal leaky surface acoustic wave filter according to an embodiment of the present application, from bottom to top, there are a support substrate 4, a piezoelectric thin film structure and an electrode array 3. It should be noted that the resonator unit includes each series resonator 1 and parallel resonator 2, so each series resonator 1 is composed of the support substrate 4, the first piezoelectric thin film structure 11 and the electrode array 3, and each parallel resonator 2 is composed of the support substrate 4, the second piezoelectric thin film structure 21 and the electrode array 3. Figure 1 It can be seen that the thickness h1 of the first piezoelectric thin film structure 11 of the series resonator 1 is less than the thickness h2 of the second piezoelectric thin film structure 21 of the parallel resonator 2, so that the difference between the normalized film thickness of the series resonator 1 and the normalized film thickness of the parallel resonator 2 is within a preset difference range, in the embodiment of the present application, the preset difference range is (β, β+1.5%), i.e. β < h1 / λ1, h2 / λ2,.... < (β+1.5%), wherein β satisfies 16% < β < 30.5%, the value of β is able to ensure that the surface acoustic wave filter is as little as possible to be affected by high-frequency stray modes when in the longitudinal leaky surface acoustic wave mode (LL-SAW), for example, in an embodiment, the preset difference range is (24%, 25.5%), i.e. 24% < h1 / λ1, h2 / λ2,.... < 25.5% of the normalized film thickness of the series resonator 1 and the normalized film thickness of the parallel resonator 2. In the prior art, the h / λ of each resonator of the longitudinal leaky surface acoustic wave (LL-SAW) filter is quite different, and cannot be guaranteed to be in the same advantage interval, the h / λ of each resonator of the longitudinal leaky surface acoustic wave (LL-SAW) filter according to the embodiment of the present application is less different, and is in the same advantage interval.

[0044] In addition to h1 / λ1 and h2 / λ2, there can be other h / λ, which can be understood as, in a possible implementation, the first piezoelectric thin film structure 11 and the second piezoelectric thin film structure 21 are piezoelectric thin films, or composite thin film structures formed by piezoelectric thin films and dielectric layers 5; the longitudinal leaky surface acoustic wave filter includes at least two kinds of piezoelectric thin film structure thickness. Specifically, the first piezoelectric thin film structure 11 and the second piezoelectric thin film structure 21 can be single-layer or multi-layer piezoelectric thin films composed of a single piezoelectric material, i.e. only including piezoelectric thin films; or the first piezoelectric thin film structure 11 and the second piezoelectric thin film structure 21 can be composite thin film structures composed of piezoelectric materials and other materials, i.e. including piezoelectric thin films and dielectric layers 5. Moreover, the first piezoelectric thin film structure 11 of multiple series resonators 1 can have different thicknesses, and the second piezoelectric thin film structure 21 of multiple parallel resonators 2 can also have different thicknesses, which need to meet the condition that the thickness of the first piezoelectric thin film structure 11 is less than the thickness of the second piezoelectric thin film structure 21, and the specific thickness can be set and adjusted according to the actual situation and performance parameters of the surface acoustic wave filter.

[0045] In a possible implementation, the material of the piezoelectric film of the first piezoelectric film structure 11 and the second piezoelectric film structure 21 is one of lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). The thickness of the piezoelectric film of the first piezoelectric film structure 11 and the second piezoelectric film structure 21 is less than 1 micrometer; the crystal cut type of the piezoelectric film is X-cut or Z-cut.

[0046] In a possible implementation, referring to Figure 2 , the first piezoelectric film structure 11 is a piezoelectric film, and the second piezoelectric film structure 21 is a composite film structure formed by a piezoelectric film and a dielectric layer 5; the thickness of the piezoelectric film of the first piezoelectric film structure 11 is consistent with the thickness of the piezoelectric film of the second piezoelectric film structure 21, and the thickness of the dielectric layer 5 of the second piezoelectric film structure 21 is less than 1 micrometer; the material of the dielectric layer 5 is one or a combination of silicon oxide, aluminum oxide, silicon nitride, or hafnium dioxide.

[0047] Specifically, the first piezoelectric thin film structure 11 of the series resonator 1 is a single-layer or multi-layer piezoelectric thin film layer composed of piezoelectric material, and the second piezoelectric thin film structure 21 of the parallel resonator 2 is a piezoelectric thin film layer + dielectric layer 5 composite thin film structure composed of piezoelectric material and other material. In this embodiment, the piezoelectric thin film layer composed of piezoelectric material in the first piezoelectric thin film structure 11 and the second piezoelectric thin film structure 21 has the same thickness h1. To achieve the condition that the thickness of the first piezoelectric thin film structure 11 is less than the thickness of the second piezoelectric thin film structure 21, a dielectric layer 5 is arranged in the second piezoelectric thin film structure 21. In one embodiment, the dielectric layer 5 can be arranged on the upper layer of the piezoelectric material in the second piezoelectric thin film structure 21. In another embodiment, the dielectric layer 5 can also be arranged on the lower layer of the piezoelectric material in the second piezoelectric thin film structure 21. The dielectric layer 5 is a low acoustic velocity dielectric layer, and the material of the low acoustic velocity dielectric layer can be one or a combination of silicon oxide, aluminum oxide, silicon nitride, or hafnium dioxide. The silicon oxide includes silicon oxide (SiO), silicon dioxide (SiO2), etc., and the silicon nitride includes silicon nitride (Si3N4), etc. The thickness of the dielectric layer 5 satisfies (h2-h1), and the thickness of the dielectric layer 5 is less than 1 micrometer. For example, in one embodiment, the thickness of the piezoelectric thin film layer composed of piezoelectric material in the first piezoelectric thin film structure 11 and the second piezoelectric thin film structure 21 is 400 nanometers, and the thickness of the dielectric layer 5 in the second piezoelectric thin film structure 21 is 55 nanometers. Through the above embodiment, the thickness h1 of the first piezoelectric thin film structure 11 of the series resonator 1 is less than the thickness h2 of the second piezoelectric thin film structure 21 of the parallel resonator 2, and then the difference between the thicknesses of the resonators of the longitudinal leaky surface acoustic wave (LL-SAW) filter is small, and all the resonators are in the same advantageous interval, which eliminates the existence of stray modes outside the passband of the filter and improves the performance of the filter.

[0048] It should be noted that the "low acoustic velocity" of the low acoustic velocity dielectric layer in the embodiment of the present application is the acoustic velocity of the dielectric layer 5 relative to the acoustic velocity of the longitudinal leaky surface acoustic wave (LL-SAW) mode (>6000 m / s). For example, aluminum oxide (Al2O3) is a low acoustic velocity dielectric material relative to the longitudinal leaky surface acoustic wave (LL-SAW) mode, but is a high acoustic velocity dielectric material relative to the horizontal shear surface acoustic wave (SH-SAW) mode.

[0049] In one possible embodiment, the first piezoelectric thin film structure 11 and the second piezoelectric thin film structure 21 are both piezoelectric thin films; the piezoelectric thin film thickness of the first piezoelectric thin film structure 11 is less than the piezoelectric thin film thickness of the second piezoelectric thin film structure 21.

[0050] Specifically, the first piezoelectric film structure 11 and the second piezoelectric film structure 21 are not provided with the medium layer 5, and the first piezoelectric film structure 11 and the second piezoelectric film structure 21 are both composed of a single layer or multiple layers of piezoelectric film layer of piezoelectric material, but the piezoelectric film thickness h1 of the first piezoelectric film structure 11 is less than the piezoelectric film thickness h2 of the second piezoelectric film structure 21. This embodiment can be understood as reducing the thickness of the first piezoelectric film structure 11 of the series resonator 1, or increasing the thickness of the second piezoelectric film structure 21 of the parallel resonator 2, or reducing the thickness of the first piezoelectric film structure 11 of the series resonator 1 and increasing the thickness of the second piezoelectric film structure 21 of the parallel resonator 2 at the same time, which can be realized in different embodiments, so that the longitudinal leaky surface acoustic wave (LL-SAW) filter resonators have a small h / λ difference, and are in the same advantageous interval, so that the spurious mode is effectively suppressed, and the performance of the filter is improved.

[0051] In a possible implementation mode, referring to Figure 3 , a transition layer 6 is arranged between the piezoelectric film structure and the support substrate 4, and the material of the transition layer 6 is one or a combination of silicon oxide, aluminum oxide, silicon nitride, and hafnium dioxide. The thickness of the transition layer 6 is less than 1 micrometer; the material of the transition layer 6 is one or a combination of silicon oxide, aluminum oxide, silicon nitride, and hafnium dioxide, wherein the silicon oxide includes silicon oxide (SiO), silicon dioxide (SiO2), etc., and the silicon nitride includes silicon nitride (Si3N4) etc. Without the transition layer 6, the piezoelectric film is directly covered on the support substrate 4 during the filter manufacturing process, which may cause low product yield. The transition layer 6 can play a buffering role to improve the product yield of the filter. Further, the transition layer 6 can also improve the device performance such as Q value and electromechanical coupling coefficient, and improve the temperature stability of the filter.

[0052] In one possible implementation, the material of the support substrate 4 is one of silicon carbide or diamond. Specifically, in the embodiment of the present application, the support substrate 4 needs to use one of the extremely high acoustic velocity crystal materials: silicon carbide or diamond, because the acoustic velocity of the longitudinal leaky surface acoustic wave (LL-SAW) mode is high, and the acoustic velocity of materials such as silicon and quartz is relatively low, which cannot effectively excite and constrain the energy of the longitudinal leaky surface acoustic wave (LL-SAW) mode; and the use of silicon carbide or diamond can effectively excite and constrain the longitudinal leaky surface acoustic wave (LL-SAW) mode. In other embodiments, the support substrate 4 can also be a composite substrate structure composed of a Bragg multilayer film structure and a support substrate, wherein the Bragg multilayer film structure is composed of high and low acoustic impedance layers stacked alternately with a total number of layers greater than 2, and the thickness of each layer is less than the device wavelength, and the high acoustic impedance layer material can be aluminum nitride, tungsten, platinum, hafnium dioxide, etc., and the low acoustic impedance layer material can be silicon dioxide.

[0053] Reference Figure 4 , which shows a top view structural schematic diagram of a resonator unit of a longitudinal leaky surface acoustic wave filter according to an embodiment of the present application. As can be seen, the electrode array 3 of each resonator includes a parallelly arranged interdigital electrode array 31 and a reflection grid array 32 arranged at both ends of the interdigital electrode array 31, and the normal direction of the electrode array 3 is the propagation direction of the acoustic wave. In one possible implementation, the interdigital electrode array 31 and the reflection grid electrode array 32 of the electrode array 3 have an inclination angle with the normal direction of the electrode array 3; the inclination angle is in the range of [-20°, +20°]. For example, in one embodiment, the inclination angle is set to 10°. By arranging the interdigital electrode array 31 and the reflection grid array 32 at an inclination angle on the piezoelectric film, the Q value of the resonator can be improved, and the spurious wave mode of the parallel resonator 2 can be suppressed.

[0054] In one possible implementation, the longitudinal leaky surface acoustic wave filter of the present application further includes an inductive element, and the inductive element is connected in series with the parallel resonator 2. By arranging an external inductive element on the parallel branch, the electrical performance of the filter is further optimized, and the flexibility of circuit design is improved. Specifically, by connecting an inductive element in series with the parallel arm resonator, the resonant frequency of the parallel resonator 2 will move to the low frequency side, i.e., the electromechanical coupling coefficient of the resonator is improved, and the corresponding filter bandwidth is improved. The number and value of the inductive element are determined by the specific design requirements. In one embodiment, referring to Figure 5 , the longitudinal leaky surface acoustic wave filter is electrically cascaded by three series resonators 1 and two parallel resonators 2, and one of the parallel resonators 2 is connected in series with an inductive element.

[0055] In order to further understand the technical solutions of the present application, the following will be described in detail through specific embodiments:

[0056] Embodiment 1

[0057] The longitudinal leaky surface acoustic wave filter, the first piezoelectric thin film structure 11 and the second piezoelectric thin film structure 21 of the series resonator 1 and the parallel resonator 2 are both piezoelectric thin film layers composed of piezoelectric materials, the thickness of the piezoelectric thin film layer is based on single h = 400 nm X-cut lithium niobate (LiNbO3), the thickness of the electrode array 3 is 100 nm, the support substrate 4 is silicon carbide, the wavelength λ1 of the series resonator 1 is 1.47 microns, h1 / λ1 is 27.2%; the wavelength λ2 of the parallel resonator 2 is 1.65 microns, h2 / λ2 is 24.2%. From Figure 6 It can be seen that, due to the larger h1 / λ1 of the series resonator 1, the cutoff frequency of the series resonator 1 is relatively large with respect to the anti-resonance frequency, so that the frequency of the high-frequency side of the spurious mode (response near 5.25 GHz) is lower than the cutoff frequency point 1, resulting in no leakage, and finally causing the problem of deterioration of the out-of-band suppression in the stopband of the filter; while the h2 / λ2 of the parallel resonator 2 is smaller, it can be found that the frequency of the cutoff frequency point 2 is lower than the frequency of the high-frequency side of the spurious mode, so that the spurious mode is effectively suppressed.

[0058] Therefore, in this embodiment, the corresponding device parameters of the parallel resonator 2 are kept unchanged (h2 / λ2 = 24.2%), and the thickness of the piezoelectric thin film layer of the series resonator 1 is adjusted to adjust h1 / λ1, the thickness of the piezoelectric thin film layer of the series resonator 1 is based on single h1 = 364 nm X-cut lithium niobate (LiNbO3), the wavelength λ1 is 1.5 microns, and the corresponding h1 / λ1 is 24.3%. It can be seen that the h1 / λ1 of the series resonator 1 is close to the h2 / λ2 of the parallel resonator 2, and the difference between them is 0.1%, which does not exceed 1.5%, and both are in the range of β < h1 / λ1, h2 / λ2,.... < (β + 1.5%), where β satisfies 16% < β < 30.5%. Referring to Figure 7 The operating frequency, electromechanical coupling coefficient and admittance ratio of the series resonator 1 are almost unchanged before and after adjustment, but the high-frequency spurious mode is effectively suppressed, the performance of the series / parallel resonator 2 is close, and the stopband response of the longitudinal leaky surface acoustic wave filter constructed is greatly optimized.

[0059] Embodiment 2

[0060] The longitudinal leaky surface acoustic wave filter, the first piezoelectric thin film structure 11 and the second piezoelectric thin film structure 21 of the series resonator 1 and the parallel resonator 2 are both piezoelectric thin film layers composed of piezoelectric materials, the thickness of the piezoelectric thin film layer is based on single h = 400 nm X-cut lithium niobate (LiNbO3), the thickness of the electrode array 3 is 100 nm, the support substrate 4 is silicon carbide, the wavelength λ1 of the series resonator 1 is 1.7 microns, h1 / λ1 is 23.5%; the wavelength λ2 of the parallel resonator 2 is 2.1 microns, h2 / λ2 is 19.0%. FromFigure 8 It can be seen that the series resonator 1 and the parallel resonator 2 do not have high-frequency spurious modes after the anti-resonance point, but it can be seen from the admittance and conductance curves that the cutoff frequency of the parallel resonator 2 with λ = 2.1 microns is too close to the anti-resonance point, and the "height difference" of the admittance curve and the conductance is small, indicating that the Q value of the parallel resonator 2 is low and is difficult to apply to the filter. The series resonator 1 does not have the above problems due to the large h / λ.

[0061] Therefore, in the embodiment, the device parameters corresponding to the series resonator 1 are kept unchanged (h1 / λ1 is 23.5%), the thickness of the piezoelectric film layer of the parallel resonator 2 is adjusted to adjust h2 / λ2, and the thickness of the piezoelectric film layer of the parallel resonator 2 is based on a single h2 = 455 nanometer X-cut lithium niobate (LiNbO3), the wavelength λ2 is 2.03 microns, and the corresponding h2 / λ2 is 22.4%, which is close to the series resonator 1. From the admittance and conductance curves of the parallel resonator 2 shown in FIG. 4, it can be seen that the cutoff frequency point 2 is higher than the cutoff frequency point 1, and the conductance curve is partially reduced, indicating that the Q value of the resonator is improved to a certain extent under the premise of unchanged frequency and bandwidth. Figure 9 It can be seen that, compared with the comparative example, the frequency of the cutoff frequency point 2 is greatly improved, and the conductance curve is partially reduced, indicating that the Q value of the resonator is improved to a certain extent under the premise of unchanged frequency and bandwidth.

[0062] The embodiments of the present application are all for longitudinal leaky surface acoustic wave filters, that is, the resonator units all excite longitudinal leaky surface acoustic wave (LL-SAW) modes. Figure 10 Three resonators exciting horizontal shear surface acoustic wave (SH-SAW) modes with different wavelengths are shown, the piezoelectric film structure is a 500 nanometer thick lithium tantalate (LiTaO3) film, the support substrate 4 is a 500 nanometer thick silicon dioxide-silicon material, and the corresponding h / λ is 20.8%, 22.7%, and 25.0%, respectively. It can be seen that the high-frequency spurious modes of the resonators are not affected, that is, the cutoff frequency point is always higher than the spurious mode, and the Q value (admittance ratio or conductance) of the horizontal shear surface acoustic wave (SH-SAW) mode response is not greatly affected. Therefore, for the horizontal shear surface acoustic wave (SH-SAW) mode, different h / λ has little effect, and there is no need to adjust the h / λ of the series / parallel resonator 2. Therefore, the present application is specifically to solve the specific engineering problems encountered by the longitudinal leaky surface acoustic wave filter.

[0063] The application is directed to a longitudinal leaky surface acoustic wave filter, and proposes a filter structure with different film thicknesses, by reducing the thickness of the first piezoelectric film structure 11 of the series resonator 1, or increasing the thickness of the second piezoelectric film structure 21 of the parallel resonator 2, so that the thickness of the first piezoelectric film structure 11 of the series resonator 1 is less than the thickness of the second piezoelectric film structure 21 of the parallel resonator 2, the h / λ of all resonators in the longitudinal leaky surface acoustic wave filter is in an advantageous interval, and the h / λ difference between the resonators is small, so as to suppress the out-of-band spurious mode of the longitudinal leaky surface acoustic wave filter, reduce the in-band loss, and improve the filter performance.

[0064] The above description is merely preferred embodiments of the present application, and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A longitudinal leaky surface acoustic wave filter, characterized by, The application relates to a longitudinal leaky surface acoustic wave filter, comprising: a plurality of electrically cascaded resonators for exciting a longitudinal leaky surface acoustic wave mode; each of the resonators comprises a supporting substrate, a piezoelectric thin film structure and an electrode array; the electrode array comprises a parallelly arranged interdigital electrode array and a reflection grating array arranged at both ends of the interdigital electrode array; the resonators comprise series resonators and parallel resonators, wherein the wavelength of the series resonators is smaller than the wavelength of the parallel resonators; the first piezoelectric thin film structure of the series resonators has a first thickness; the second piezoelectric thin film structure of the parallel resonators has a second thickness; the first piezoelectric thin film structure and the second piezoelectric thin film structure are piezoelectric thin films or composite thin film structures formed by piezoelectric thin films and dielectric layers; the longitudinal leaky surface acoustic wave filter comprises at least two kinds of piezoelectric thin film structure thicknesses; the first thickness of the first piezoelectric thin film structure is smaller than the second thickness of the second piezoelectric thin film structure, so that the normalized film thickness of the series resonators and the normalized film thickness of the parallel resonators are both within a preset range; the normalized film thickness represents the ratio of the piezoelectric thin film structure thickness of the resonator to the wavelength of the resonator; when the first piezoelectric thin film structure is a piezoelectric thin film and the second piezoelectric thin film structure is a composite thin film structure formed by a piezoelectric thin film and a dielectric layer, the piezoelectric thin film thickness of the first piezoelectric thin film structure is consistent with the piezoelectric thin film thickness of the second piezoelectric thin film structure, and the dielectric layer thickness of the second piezoelectric thin film structure is less than 1 micron; the dielectric layer of the second piezoelectric thin film structure is arranged on the upper surface or the lower surface of the piezoelectric thin film of the second piezoelectric thin film structure; the preset range is (beta, beta+1.5%); wherein beta satisfies 16%<beta<30.5%.

2. The longitudinal leakage surface acoustic wave filter according to claim 1, characterized by, The material of the dielectric layer is one or a combination of silicon oxide, aluminum oxide, silicon nitride or hafnium dioxide.

3. The longitudinal leakage surface acoustic wave filter according to claim 1, wherein, The first piezoelectric thin film structure and the second piezoelectric thin film structure are both piezoelectric thin films; the piezoelectric thin film thickness of the first piezoelectric thin film structure is smaller than the piezoelectric thin film thickness of the second piezoelectric thin film structure.

4. The longitudinal leakage surface acoustic wave filter according to claim 1, wherein, A transition layer is arranged between the piezoelectric thin film structure and the supporting substrate; the material of the transition layer is one or a combination of silicon oxide, aluminum oxide, silicon nitride or hafnium dioxide.

5. The longitudinal leakage surface acoustic wave filter according to any one of claims 2 to 4, characterized in that, The material of the piezoelectric thin film of the first piezoelectric thin film structure and the second piezoelectric thin film structure is one of lithium niobate or lithium tantalate.

6. The longitudinal leakage surface acoustic wave filter according to any one of claims 2 to 4, characterized by, The thickness of the piezoelectric thin film of the first piezoelectric thin film structure and the second piezoelectric thin film structure is less than 1 micron; the crystal cut type of the piezoelectric thin film is X-cut or Z-cut.

7. The longitudinal leakage surface acoustic wave filter according to claim 1, wherein, The material of the supporting substrate is one of silicon carbide or diamond.

8. The longitudinal leakage surface acoustic wave filter of claim 1, wherein, The interdigital electrode array and the reflection grating electrode array of the electrode array have an inclination angle with the normal direction of the electrode array; the inclination angle is within the range of [-20 DEG, +20 DEG].

9. The longitudinal leakage surface acoustic wave filter of claim 1, wherein, An inductance element is further arranged in cascade or bridge connection with the parallel resonators.

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