Filter circuit and electronic equipment

Abstract

The invention provides a filter circuit, which comprises a plurality of bulk acoustic wave resonators which are electrically connected, and the resonant frequency of each bulk acoustic wave resonator is within a preset frequency band. Wherein the resonant frequencies of at least part of the bulk acoustic wave resonators are different, so that the number of transmission zero points of the S21 curve of the filter circuit is not less than three. The filter circuit disclosed by the invention is high in out-of-band rejection capability and good in transmission performance.

Description

Technical Field

[0001] This disclosure belongs to the field of electronic technology, specifically relating to a filter circuit and an electronic device. Background Technology

[0002] With the rapid development of mobile communication technology, the application of many radio frequency devices has increased significantly, among which the filter market is poised for explosive growth. Currently, the filtering devices used in personal mobile terminals (such as mobile phones) are mainly piezoelectric acoustic wave (BAW) filters. The resonators that make up these filters are mainly: FBAR (Bulk Acoustic Resonator), SMR (Solid State Mesh Resonator), and SAW (Surface Acoustic Wave Resonator), with FBAR and SMR collectively referred to as BAW. Compared with SAW, BAW has advantages such as lower insertion loss, higher Q value, steeper roll-off characteristics, and larger power capacity, but SAW is cheaper and has the advantage of impedance transformation. Summary of the Invention

[0003] The present invention aims to at least solve one of the technical problems existing in the prior art, and provides a filter circuit including a plurality of electrically connected bulk acoustic wave resonators, wherein the resonant frequency of each bulk acoustic wave resonator is within a preset frequency band; wherein at least some of the bulk acoustic wave resonators have different resonant frequencies, so that the number of transmission zeros of the S21 curve of the filter circuit is not less than three.

[0004] In some embodiments, the filter circuit includes N electrically connected filter sub-circuits; the filter sub-circuit includes two series-connected bulk acoustic wave resonators, and the connection node between the two is a first node, and the two series-connected bulk acoustic wave resonators are a first bulk acoustic wave resonator and a second bulk acoustic wave resonator, respectively.

[0005] For the i-th filter sub-circuit, the two ends of the first bulk acoustic resonator are respectively connected to the first node of the (i-1)-th filter sub-circuit and the first node of the (i+1)-th filter sub-circuit; i is an integer from 2 to N-1, where N is an integer greater than or equal to 3;

[0006] For the filter circuit, at least one of the first body acoustic resonators has a different resonant frequency than the other first body acoustic resonators, and / or at least one of the second body acoustic resonators has a different resonant frequency than the other second body acoustic resonators.

[0007] In some embodiments, the bulk acoustic resonator includes: a substrate, and a first electrode, a piezoelectric layer, and a second electrode sequentially stacked on the substrate; the orthographic projections of any two of the first electrode, the piezoelectric layer, and the second electrode on the substrate at least partially overlap.

[0008] The thickness of the second electrode of at least one of the first bulk acoustic wave resonators is different from the thickness of the second electrode of the other first bulk acoustic wave resonators, and / or the thickness of the second electrode of at least one of the second bulk acoustic wave resonators is different from the thickness of the second electrode of the other second bulk acoustic wave resonators.

[0009] In some embodiments, the thickness of the second electrode of the first bulk acoustic resonator is 0.05-0.15 μm.

[0010] In some embodiments, the thickness of the second electrode of the second bulk acoustic resonator is 0.15-0.25 μm.

[0011] In some embodiments, the thickness of the first electrode is 0.1-0.2 μm.

[0012] In some embodiments, the bulk acoustic resonator further includes a first groove disposed on the side of the substrate near the first electrode, wherein the orthographic projection of the piezoelectric layer on the substrate covers the orthographic projection of the first groove on the substrate.

[0013] In some embodiments, the bulk acoustic resonator further includes multiple layers of first impedance layer and second impedance layer disposed between the substrate and the first electrode; the first impedance layer and the second impedance layer are alternately disposed, and the acoustic impedance of the first impedance layer is greater than the acoustic impedance of the second impedance layer.

[0014] In some embodiments, the substrate comprises any one of glass, Si, sapphire, SiC, GaAs, GaN, InP, BN, ZnO, and GaO.

[0015] In some embodiments, both the first electrode and the second electrode comprise any one of Mo, Al, Cu, Co, Ag, Ti, Pt, Ru, W, and Au.

[0016] In some embodiments, the piezoelectric layer includes ZnO, PZT, GaN, InN, CdS, CdSe, ZnS, CdTe, ZnTe, GaAs, GaSb, InAs, InSb, GaSe, GaP, AlP, quartz crystal, LiTaO3, LiNbO3, La3Ga5SiO 14 Any one of BaTiO3, PbNb2O6, PBLN, LiGaO3, LiGeO3, TiGeO3, PbTiO3, PbZrO3, and PVDF.

[0017] Based on the same inventive concept, this disclosure also provides an electronic device including the above-described filtering circuit. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of a back-etched bulk acoustic resonator.

[0019] Figure 2 This is a schematic diagram of a thin-film bulk acoustic resonator.

[0020] Figure 3 This is a schematic diagram of a solid-state assembled bulk acoustic resonator.

[0021] Figure 4a The impedance curves of the second electrode of the first-body acoustic resonator at different thicknesses are shown.

[0022] Figure 4b The impedance curves of the second electrode of the second-body acoustic resonator at different thicknesses are shown.

[0023] Figure 5 This is a circuit diagram of a filter circuit.

[0024] Figure 6 for Figure 5 The simulation curve of the S21 of the filter circuit.

[0025] Figure 7 The image shows the simulation curve of the filter circuit in Example 1.

[0026] Figure 8 The image shows the simulation curve of the filter circuit in Example 2.

[0027] Figure 9 The image shows the simulation curve of the filter circuit in Example 3.

[0028] Figure 10 The image shows the simulation curve of the filter circuit in Example 4.

[0029] Figure 11 The image shows the simulation curve of the filter circuit in Example 5.

[0030] Figure 12 The image shows the simulation curve of the filter circuit in Example 6.

[0031] Figure 13 This is a flowchart illustrating the fabrication process of a back-etched bulk acoustic resonator in a filter circuit.

[0032] Figure 14 This is a flowchart illustrating the fabrication process of a thin-film bulk acoustic resonator in a filter circuit.

[0033] Figure 15 This is a flowchart illustrating the fabrication process of a solid-state assembled bulk acoustic resonator in a filter circuit. Detailed Implementation

[0034] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0035] Unless otherwise defined, the technical or scientific terms used in this application shall have the ordinary meaning understood by one of ordinary skill in the art to which this application pertains. The terms “a,” “an,” “an,” “the,” and similar words used in this application do not indicate quantity limitation and may indicate singular or plural. The terms “comprising,” “including,” “having,” and any variations thereof used in this application are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or device that includes a series of steps or modules (units) is not limited to the listed steps or units, but may also include steps or units not listed, or may include other steps or units inherent to these processes, methods, products, or devices. The terms “connected,” “linked,” “coupled,” and similar words used in this application are not limited to physical or mechanical connections, but may include electrical connections, whether direct or indirect. “Multiple” used in this application refers to two or more. “And / or” describes the relationship between related objects, indicating that three relationships may exist; for example, “A and / or B” can represent: A alone, A and B simultaneously, and B alone. The character " / " generally indicates that the preceding and following objects are in an "or" relationship. The terms "first," "second," and "third" used in this application are merely to distinguish similar objects and do not represent a specific ordering of objects. "Above," "below," "left," and "right" are only used to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0036] Bulk acoustic wave (BAW) resonators, as the basic structural unit of bulk acoustic wave filters, currently use silicon wafers as the substrate material. A sandwich structure is employed on the BAW, consisting of a first electrode, a piezoelectric layer, and a second electrode from bottom to top. The first and second electrodes are metal electrodes, while the piezoelectric layer is made of piezoelectric material. For example... Figure 1-3 As shown, to reduce insertion loss during the filtering process, bulk acoustic wave (BAW) resonators need to confine the acoustic signal as much as possible within the piezoelectric layer between the first and second electrodes to prevent outward diffusion. Therefore, acoustic wave reflectors are usually constructed on the upper and lower surfaces of the resonator. The upper surface typically uses air as the reflector, which has low acoustic impedance. Based on the different construction methods of the acoustic wave reflector on the lower surface, BAW resonators are divided into three main categories, such as back-etched BAW resonators. Figure 1As shown; filmbulk acoustic resonator (abbreviated as FBAR), a thin-film bulk acoustic resonator, such as... Figure 2 As shown; solid-mounted resonator (SMR), a solid-state assembled bulk acoustic resonator, such as... Figure 3 As shown. In the back-etching type, a cavity is formed by deep etching on the back side of the silicon substrate to create a cavity below the first electrode, thus constructing a first cavity 101 on the substrate 10 as an air layer, as shown. Figure 1 As shown. FBAR involves creating a first trench 102 etched onto the substrate 10 below the first electrode as an air gap, and then supporting the first electrode through an insulating layer 14, as shown. Figure 2 As shown, the SMR constructs an acoustic mirror structure 15 below the first electrode. The acoustic mirror structure 15 is formed by alternating and repeating layers of a first impedance layer 151 and a second impedance material layer 152, as shown. Figure 3 As shown, the acoustic impedance of the first impedance layer 151 is greater than that of the second impedance layer 152.

[0037] The working principle of a bulk acoustic wave (BAW) resonator is as follows: A radio frequency (RF) signal is input from one electrode of the resonator, and then converted into a mechanically vibrating acoustic signal at the interface between the piezoelectric material and the metal electrode through the inverse piezoelectric effect. This acoustic signal forms a resonant standing wave with a certain frequency in the sandwich structure of the first electrode 11, the piezoelectric layer 12, and the second electrode 13. The frequency of the RF signal is equal to the resonant frequency of the resonator. The acoustic signal is transmitted to the other electrode of the resonator, where it is converted back into an RF signal through the piezoelectric effect at the interface between the metal electrode and the piezoelectric material. The resonator has a fixed resonant frequency. When the frequency of the RF signal equals the resonant frequency, the conversion efficiency of RF signal → acoustic signal → RF signal is high. When the frequency of the RF signal is not equal to the resonant frequency, the conversion efficiency is very low, and most of the RF signal cannot be transmitted through the resonator. In other words, the resonator functions as a filter, filtering the RF signal. The resonant frequency of the resonator is determined by the thickness of the piezoelectric layer 12 and the electrode layer, i.e., fp = v / 2t, where t is the equivalent thickness of the bulk acoustic wave determined by the piezoelectric layer 12 and the two electrodes, and v is the speed of sound. Therefore, the resonant frequency of the bulk acoustic wave resonator can be changed by altering the thickness of the piezoelectric layer 12 and the two electrodes.

[0038] For bulk acoustic wave (BAW) filters, key performance indicators include insertion loss, out-of-band rejection, roll-off, and transmission zeros. Insertion loss is often represented by the parameter IL (Insert Loss). Because the signal cannot completely reach the output, energy loss is inevitable when passing through the filter. Insertion loss is defined as the ratio of input power Pin to output power PL, i.e., IL(dB) = 10lg(Pin / PL) = -S²1, where S²1 is the transmission coefficient from the input port to the output port, which can be measured by a vector network analyzer. Out-of-band rejection is the attenuation outside the filter's passband, representing its ability to suppress unwanted frequency signals. The roll-off, also known as the rectangular coefficient, describes the steepness of the filter's transition band; a steeper roll-off indicates better frequency selectivity. It can be represented by the ratio of 60dB bandwidth to 3dB bandwidth. Transmission zeros are specific frequency points where the filter's transfer function equals zero. Energy cannot pass through these points, thus achieving complete isolation. Transmission zeros can help improve out-of-band rejection. In practical design, by introducing a finite number of transmission zeros, complete signal isolation can be achieved at specific frequencies, thereby improving filter performance. Existing technologies typically increase out-of-band rejection and the number of transmission zeros by introducing inductors, but this increases layout area and losses, as well as circuit complexity.

[0039] Based on this, the present disclosure provides a filter circuit, which includes multiple electrically connected bulk acoustic wave resonators, each of which has a resonant frequency within a preset frequency band. At least some of the bulk acoustic wave resonators have different resonant frequencies, thereby ensuring that the number of transmission zeros in the S21 curve of the filter circuit is not less than three, while increasing the out-of-band rejection of the filter circuit, without the need to add additional inductor components.

[0040] In some examples, the preset frequency band can range from 2.15 to 2.45 GHz.

[0041] In some examples, the filter circuit includes N electrically connected filter sub-circuits, where N is an integer greater than or equal to 3. Each filter sub-circuit includes two bulk acoustic wave resonators connected in series, with the connection node being the first node. One of the two bulk acoustic wave resonators in each filter sub-circuit is a first bulk acoustic wave resonator, and the other is a second bulk acoustic wave resonator. Both have the same structure and can both be back-etched type bulk acoustic wave resonators, FBAR resonators, or SMR resonators.

[0042] For the entire filtering circuit, one end of the first bulk acoustic wave resonator in the first filtering sub-circuit is multiplexed as the signal input terminal of the filtering circuit, and the other end is connected to the corresponding second bulk acoustic wave resonator. The connection node between the two is the first node of the first filtering sub-circuit, and the other end of the second bulk acoustic wave resonator is connected to the reference ground. One end of the first bulk acoustic wave resonator in the i-th filtering sub-circuit is connected to the first node of the (i-1)-th filtering sub-circuit, and the other end is connected to the corresponding second bulk acoustic wave resonator. The connection node between the two is the first node of the i-th filtering sub-circuit, and the other end of the second bulk acoustic wave resonator is connected to the reference ground. i takes an integer from 2 to N-1. One end of the first bulk acoustic wave resonator in the N-th filtering sub-circuit is connected to the first node of the N-1 filtering sub-circuit, and the other end is connected to the corresponding second bulk acoustic wave resonator. The connection node between the two is the first node of the N-th filtering sub-circuit, and the other end of the second bulk acoustic wave resonator is connected to the reference ground. It should be noted that each first bulk acoustic wave resonator and each second bulk acoustic wave resonator can share a single substrate 10, and the orthographic projections of each bulk acoustic wave resonator on the substrate 10 do not overlap to prevent interference. It should also be noted that the connecting line between two resonators can be disposed in the same layer as the corresponding electrode or in a different layer. For example, for the first node in the filter sub-circuit, it is constituted by the connecting line connecting the first electrode 11 of the first bulk acoustic wave resonator and the first electrode 11 of the second bulk acoustic wave resonator. Therefore, this connecting line can be disposed in the same layer as the first electrode 11 of the first bulk acoustic wave resonator and the first electrode 11 of the second bulk acoustic wave resonator, and completed through a one-time patterning process to simplify the manufacturing process. As another example, for the connecting line connecting the first electrode 11 of one resonator and the second electrode 13 of another resonator, the connection can be achieved by drilling holes in the intermediate film layer (e.g., the insulating layer or the encapsulation layer 16).

[0043] By setting the resonant frequencies of at least some bulk acoustic wave resonators in the filter circuit to be different, the number of transmission zeros in the S21 curve of the filter circuit can be changed, thereby improving out-of-band suppression. Specifically, at least one first bulk acoustic wave resonator can be set to have a different resonant frequency than other first bulk acoustic wave resonators, or at least one second bulk acoustic wave resonator can be set to have a different resonant frequency than other second bulk acoustic wave resonators, or at least one first bulk acoustic wave resonator can be set to have a different resonant frequency than other first bulk acoustic wave resonators, and at least one second bulk acoustic wave resonator can also have a different resonant frequency than other second bulk acoustic wave resonators.

[0044] Furthermore, the above analysis shows that the resonant frequency of the bulk acoustic resonator is determined by the thickness of the piezoelectric layer 12 and the electrode layer, i.e., fp = v / 2t, where t is the equivalent thickness of the bulk acoustic wave determined by the piezoelectric layer 12 and the two electrodes, and v is the speed of sound. It should be noted that in this disclosure, the resonant frequency of the resonator is changed by altering the thickness of the second electrode in the bulk acoustic resonator, as described above. Figures 4a-4b , Figure 4a The red curve in the figure represents the impedance curve of the resonator when the thickness of the second electrode in the first bulk acoustic resonator is set to 0.1 micrometers, and the black curve represents the impedance curve of the resonator when the thickness of the second electrode in the first bulk acoustic resonator is set to 0.15 micrometers. The resonant frequency of the former is 2.2 GHz, and the resonant frequency of the latter is 2.22 GHz. Figure 4b The red curve in the figure shows the impedance curve of the bulk acoustic wave resonator when the thickness of the second electrode is set to 0.2 micrometers, and the black curve shows the impedance curve of the resonator when the thickness of the second electrode is set to 0.15 micrometers. The resonant frequency of the former is 2.4 GHz, and the resonant frequency of the latter is 2.36 GHz. Therefore, the resonant frequency of the bulk acoustic wave resonator can be changed by changing the thickness of the second electrode.

[0045] For example, this disclosure provides S21 simulation curves of the filter circuit with different parameters for the thickness of the second electrode 13 in the bulk acoustic wave resonator. The simulation results show that the filter circuit of this disclosure has at least three transmission zeros and exhibits good out-of-band suppression and high selectivity, making it applicable to various scenarios. Specifically, the thickness of the second electrode 13 of the first bulk acoustic wave resonator is 0.05-0.15 μm, the thickness of the second electrode 13 of the second bulk acoustic wave resonator is 0.15-0.25 μm, and the thickness of the first electrode 11 of both the first and second bulk acoustic wave resonators is 0.1-0.2 μm.

[0046] Before describing the specific embodiments, it should be noted that in the following embodiments, only N=4 is used as an example to illustrate the filter circuit of this disclosure, but this does not constitute a limitation of this disclosure, and other similar ladder circuits are also within the protection scope of this disclosure.

[0047] Specifically, refer to Figure 5 The filtering circuit includes four electrically connected filter sub-circuits: a first filter sub-circuit 51, a second filter sub-circuit 52, a third filter sub-circuit 53, and a fourth filter sub-circuit 54. Each filter sub-circuit includes a first bulk acoustic wave filter S and a second bulk acoustic wave filter P connected in series, and the connection node between them is the first node Q1 of the filter sub-circuit. (Refer to...) Figure 5In the first filter sub-circuit 51, one end (i.e., the first electrode 11) of the first bulk acoustic wave resonator S1 is multiplexed as the signal input terminal of the filter circuit, and the other end (i.e., the second electrode 13) is connected to one end (i.e., the first electrode 11) of the second bulk acoustic wave resonator P1. The other end (i.e., the second electrode 13) of the second bulk acoustic wave resonator P1 is connected to the reference ground. In the second filter sub-circuit 52, one end (i.e., the first electrode 11) of the first bulk acoustic wave resonator S2 is connected to the first node Q11 in the first filter sub-circuit 51 (to reduce jumpers, it can be directly connected to the first electrode 11 of the second bulk acoustic wave resonator P1 in the first filter sub-circuit 51), and the second electrode 13 is connected to one end (i.e., the first electrode 11) of the second bulk acoustic wave resonator P2. The other end (i.e., the second electrode 13) of the second bulk acoustic wave resonator P2 is connected to the reference ground. One end (i.e., the first electrode 11) of the first bulk acoustic wave resonator S3 in the third filter sub-circuit 53 is connected to the first node Q12 in the second filter sub-circuit 52 (to reduce jumpers, it can be directly connected to the first electrode 11 of the second bulk acoustic wave resonator P2 in the second filter sub-circuit 52). The second electrode 13 is connected to one end (i.e., the first electrode 11) of the second bulk acoustic wave resonator P3, and the other end (i.e., the second electrode 13) of the second bulk acoustic wave resonator P3 is connected to the reference ground. One end (i.e., the first electrode 11) of the first bulk acoustic wave resonator S4 in the fourth filter sub-circuit 54 is connected to the first node Q13 in the third filter sub-circuit 53 (to reduce jumpers, it can be directly connected to the first electrode 11 of the second bulk acoustic wave resonator P3 in the third filter sub-circuit 53). The second electrode 13 is connected to one end (i.e., the first electrode 11) of the second bulk acoustic wave resonator P4, and the other end (i.e., the second electrode 13) of the second bulk acoustic wave resonator P4 is connected to the reference ground. It should be noted that the thickness of the second electrode 13 of each bulk acoustic resonator in the above-mentioned filter circuit is the same, ranging from 0.1 to 0.2 micrometers.

[0048] Figure 6 The simulation diagram of the S21 curve of the above-mentioned filter circuit shows that the filter circuit has two transmission zeros, corresponding to resonant frequencies of 2.26 GHz and 2.44 GHz, respectively. Based on the above circuit structure, a specific embodiment is given below.

[0049] Example 1

[0050] In this embodiment, the thickness of the second electrode 13 of the first bulk acoustic wave resonator S1 in the first filter sub-circuit 51 is set to 0.1 micrometers, the thickness of the second electrode 13 of the other seven bulk acoustic wave resonators S2 / S3 / S4 / P1 / P2 / P3 / P4 in the filter circuit is set to 0.15 micrometers; and the thickness of the first electrode 11 of the eight bulk acoustic wave resonators S1~S4 / P1~P4 in the filter circuit is 0.15 micrometers.

[0051] Figure 7The simulation curve of the above filter circuit is shown in the figure. It can be seen from the figure that the curve has three transmission zeros, and the corresponding resonant frequencies are 2.26GHz, 2.44GHz and 2.48GHz, respectively.

[0052] Example 2

[0053] In this embodiment, the thickness of the second electrode 13 of the second bulk acoustic wave resonator P1 in the first filter sub-circuit 51 is set to 0.2 micrometers, the thickness of the second electrode 13 of the other seven bulk acoustic wave resonators S1 / S2 / S3 / S4 / P2 / P3 / P4 in the filter circuit is set to 0.15 micrometers; and the thickness of the first electrode 11 of the eight bulk acoustic wave resonators S1~S4 / P1~P4 in the filter circuit is 0.15 micrometers.

[0054] Figure 8 The simulation curve of the above filter circuit is shown in the figure. It can be seen from the figure that the curve has three transmission zeros, and the corresponding resonant frequencies are 2.22GHz, 2.26GHz and 2.44GHz, respectively.

[0055] Example 3

[0056] In this embodiment, the thickness of the second electrode 13 of the first bulk acoustic wave resonator S1 in the first filter sub-circuit 51 is set to 0.1 micrometers, the thickness of the second electrode 13 of the second bulk acoustic wave resonator P1 in the first filter sub-circuit 51 is set to 0.2 micrometers, the thickness of the second electrode 13 of the other six bulk acoustic wave resonators S2 / S3 / S4 / P2 / P3 / P4 in the filter circuit is 0.15 micrometers; and the thickness of the first electrode 11 of the eight bulk acoustic wave resonators S1~S4 / P1~P4 in the filter circuit is 0.15 micrometers.

[0057] Figure 9 The simulation curve of the above filter circuit is shown in the figure. It can be seen from the figure that the curve has four transmission zeros, and the corresponding resonant frequencies are 2.22GHz, 2.26GHz, 2.44GHz and 2.48GHz.

[0058] Example 4

[0059] In this embodiment, the thickness of the second electrode 13 of the first bulk acoustic wave resonator S1 in the first filter sub-circuit 51 is set to 0.1 micrometers, the thickness of the second electrode 13 of the first bulk acoustic wave resonator S2 in the second filter sub-circuit 52 is set to 0.05 micrometers, the thickness of the second electrode 13 of the other six bulk acoustic wave resonators S3 / S4 / P1 / P2 / P3 / P4 in the filter circuit is 0.15 micrometers; and the thickness of the first electrode 11 of the eight bulk acoustic wave resonators S1~S4 / P1~P4 in the filter circuit is 0.15 micrometers.

[0060] Figure 10 The simulation curve of the above filter circuit is shown in the figure. It can be seen from the figure that the curve has four transmission zeros, and the corresponding resonant frequencies are 2.26GHz, 2.44GHz, 2.46GHz and 2.49GHz.

[0061] Example 5

[0062] In this embodiment, the thickness of the second electrode 13 of the second bulk acoustic wave resonator P1 in the first filter sub-circuit 51 is set to 0.2 micrometers, the thickness of the second electrode 13 of the second bulk acoustic wave resonator P2 in the second filter sub-circuit 52 is set to 0.25 micrometers, the thickness of the second electrode 13 of the other six bulk acoustic wave resonators S1 / S2 / S3 / S4 / P3 / P4 in the filter circuit is 0.15 micrometers; and the thickness of the first electrode 11 of the eight bulk acoustic wave resonators S1~S4 / P1~P4 in the filter circuit is 0.15 micrometers.

[0063] Figure 11 The simulation curve of the above filter circuit is shown in the figure. It can be seen from the figure that the curve has four transmission zeros, and the corresponding resonant frequencies are 2.2GHz, 2.22GHz, 2.26GHz and 2.44GHz.

[0064] Example 6

[0065] In this embodiment, the thickness of the second electrode 13 of the first bulk acoustic wave resonator S1 in the first filter sub-circuit 51 is set to 0.1 micrometers, the thickness of the second electrode 13 of the second bulk acoustic wave resonator P1 in the first filter sub-circuit 51 is set to 0.2 micrometers, the thickness of the second electrode 13 of the first bulk acoustic wave resonator S2 in the second filter sub-circuit 52 is set to 0.05 micrometers, the thickness of the second electrode 13 of the second bulk acoustic wave resonator P2 in the second filter sub-circuit 52 is set to 0.25 micrometers, the thickness of the second electrode 13 of the other four bulk acoustic wave resonators S3 / S4 / P3 / P4 in the filter circuit is set to 0.15 micrometers; and the thickness of the first electrode 11 of the eight bulk acoustic wave resonators S1~S4 / P1~P4 in the filter sub-circuit is 0.15 micrometers.

[0066] Figure 12 The simulation curve of the above filter circuit for S21 shows that the curve has six zeros, corresponding to resonant frequencies of 2.2GHz, 2.22GHz, 2.26GHz, 2.44GHz, 2.46GHz and 2.49GHz.

[0067] Based on the same inventive concept, this disclosure also provides a method for fabricating the above-mentioned filter circuit, referring to... Figure 13-15 It should be noted that, for ease of description, Figure 13-15The diagram only shows the fabrication process of one bulk acoustic wave resonator in the filter circuit. It can be understood that the fabrication processes for all bulk acoustic wave filters are roughly the same and share a common substrate, differing only in the thickness of the second electrode. The method for fabricating a bulk acoustic wave resonator includes the following steps:

[0068] S1. Provide a substrate 10.

[0069] Taking a monocrystalline silicon substrate as an example, step S1 may specifically include: first, ultrasonically cleaning the monocrystalline silicon substrate with deionized water; then, immersing it in a mixed solution of H2SO4:H2O = 3:1 and heating it to 250°C for 15 minutes; then immersing it in deionized water for ultrasonic cleaning; next, immersing it in a mixed solution of NH4OH:H2O = 1:6 and heating it to 80°C for 15 minutes; then rinsing it in deionized water; next, immersing it in a mixed solution of HCl:H2O2:H2O = 1:1:5 and heating it to 85°C for 15 minutes; then rinsing it in a dilute hydrofluoric acid solution of HF:H2O = 1:20 for 10 seconds to remove the oxide layer on the surface; finally, immersing it in deionized water for ultrasonic cleaning for 20 minutes and drying it with an air knife to complete the entire cleaning process of the substrate 10.

[0070] In some examples, the substrate 1010 is preferably made of glass, but materials such as Si, sapphire, SiC, GaAs, GaN, InP, BN, ZnO, and GaO can also be selected. The thickness of the substrate 10 ranges from 0.1 μm to 10 mm.

[0071] S2. A bulk acoustic resonator is formed on the substrate 10.

[0072] In some examples, the bulk acoustic wave resonator is a back-etched type bulk acoustic wave resonator. Reference is then made. Figure 13 Step S2 includes:

[0073] S211. A first electrode 11 of a bulk acoustic resonator is formed on a substrate 10.

[0074] When the first electrode 11 is made of a metal material, step S211 may include depositing a first metal thin film on the substrate 10. The deposition method is preferably DC magnetron sputtering (RF magnetron sputtering is also acceptable), but pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), thermal evaporation, electron beam evaporation, or copper foil deposition may also be used. Next, a photolithography process is performed on the first metal thin film, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Etching is then performed to form a pattern including multiple first electrodes 11. The etching process is preferably wet etching, but dry etching may also be used. Finally, a resist removal process is performed to complete the fabrication of each first electrode 11. The thickness of the first electrode 11 is preferably set to 0.15 micrometers.

[0075] In some examples, the material of the first electrode 11 is preferably molybdenum, because its lattice size is very close to that of the piezoelectric layer 12 material AlN and the doped AlN lattice structure and size. The material of the first electrode 11 can also be Al, Cu, Co, Ag, Ti, Pt, Ru, W, Au, or a multilayer or alloy material formed from the above metals.

[0076] S212. A piezoelectric layer 12 is formed on the side of the first electrode 11 away from the substrate 10.

[0077] Taking the piezoelectric layer 12 as an example of an AlN monolayer structure, step S212 may include forming a piezoelectric material layer on the side of the first electrode 11 away from the substrate 10, and performing oriented growth of the piezoelectric material layer. The preferred method for forming the piezoelectric material layer is radio frequency magnetron sputtering (DC-controlled sputtering is also acceptable). For AlN piezoelectric material, Al is selected as the target. By controlling the Ar and N2 gas pressure and temperature during the deposition process, as well as the post-annealing time and temperature, an AlN C-axis oriented piezoelectric material layer is formed. The preferred growth orientation is (001). Other methods for depositing the piezoelectric material layer include pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), and plasma-enhanced chemical vapor deposition (PECVD). Next, a photolithography process is performed on the piezoelectric layer 12, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Then, etching is performed to form a pattern including the piezoelectric layer 12. A wet etching process is preferred, but a dry etching process can also be used. Finally, a desizing process is performed to complete the preparation of the piezoelectric layer 12.

[0078] In some examples, the material of the piezoelectric layer 12 is preferably AlN or doped AlN, such as Al... (1-x) Sc x N, Al (1-x) Cr x N, Al (1-x) Y x N, Al (1-x) Ti x N, Al (1-x) Zr x N, Al (1-x) Hf x N, Al (1-x) Yb x N, Al (1-x) Ta x N, Mg 0.5x Nb 0.5x Al (1-x) N, Mg 0.5x Ti 0.5x Al (1-x) N, Mg0.5x Zr 0.5x Al (1-x) N, Mg 0.5x Hf 0.5x Al (1-x) N, Mg 0.5x Si 0.5x Al (1-x) N, Zn 0.25 Ti 0.25 Al 0.5 N, Zn 0.25 Zr 0.25 Al 0.5 N, Zn 0.25 Hf 0.25 Al 0.5 N. The material of the piezoelectric layer 12 can also be selected from ZnO, PZT, GaN, InN, CdS, CdSe, ZnS, CdTe, ZnTe, GaAs, GaSb, InAs, InSb, GaSe, GaP, AlP, quartz crystal, LiTaO3, LiNbO3, La3Ga5SiO 14 Materials such as BaTiO3, PbNb2O6, PBLN, LiGaO3, LiGeO3, TiGeO3, PbTiO3, PbZrO3, and PVDF can be used. The piezoelectric layer 12 can be a single piezoelectric material or a stack of the above piezoelectric materials. The thickness of the piezoelectric layer 12 ranges from 10 nm to 100 μm.

[0079] S213. A second electrode 13 is formed on the side of the piezoelectric layer 12 opposite to the first electrode 11.

[0080] When the second electrode 13 is made of a metallic material, step S213 may specifically include: firstly, depositing a second metal thin film on the side of the piezoelectric layer 12 opposite to the first electrode 11. The deposition method is preferably DC magnetron sputtering (RF magnetron sputtering is also acceptable), but pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), thermal evaporation, electron beam evaporation, etc., can also be selected. Next, a photolithography process is performed on the second metal thin film, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Then, etching is performed to form a pattern including the second electrode 13. The etching process is preferably wet etching, but dry etching can also be selected. Finally, a resist removal process is performed to complete the fabrication of the second electrode 13.

[0081] In some examples, the material of the second electrode 13 is preferably molybdenum, because its lattice size is very close to that of the piezoelectric layer 12 material AlN and the doped AlN lattice structure and size. The material of the second electrode 13 can also be Al, Cu, Co, Ag, Ti, Pt, Ru, W, Au, or a multilayer or alloy of the above metals.

[0082] S214. An encapsulation layer 16 is formed on the side of the second electrode 13 away from the substrate 10.

[0083] Taking the encapsulation layer 16 as an example of using an organic compound material, step S214 may specifically include first coating the organic material liquid, which can be done by spin coating, spraying, inkjet printing, transfer, etc., and then heating and curing to form the pattern of the encapsulation layer 16.

[0084] In some examples, the material of the encapsulation layer 1616 is preferably an organic compound that can isolate moisture and oxygen, such as polyimide, epoxy resin, etc., but inorganic materials such as SiN can also be selected. x Al2O3, etc. The 1616 encapsulation layer can be a single layer of a single material or a stack of multiple materials.

[0085] S215. The substrate 10 with the above structure is flipped and etched to form a first cavity that penetrates along the thickness direction of the substrate 10.

[0086] In some examples, step S215 may include flipping the substrate 10 forming the above structure, preparing a mask pattern on the back side of the substrate 10, performing a photolithography process, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Then, HF acid wet etching is performed to form the first cavity, and finally, a resist removal process is performed.

[0087] In some examples, the bulk acoustic resonator is an FBAR resonator. In this case, refer to... Figure 14 Step S2 includes:

[0088] S221. A first groove 102 is formed on the substrate 10.

[0089] In some examples, step S221 may include preparing a mask pattern on the substrate 10, performing a photolithography process including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Then, HF acid etching is performed to form the first trench 102, and finally, a resist removal process is performed.

[0090] S222. A corresponding first electrode 11 is formed on the side of the first trench 102 away from the substrate 10.

[0091] When the first electrode 11 is made of a metal material, step S222 may include depositing a first metal thin film on the substrate 10. The deposition method is preferably DC magnetron sputtering (RF magnetron sputtering is also acceptable), but pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), thermal evaporation, electron beam evaporation, or copper foil deposition may also be used. Next, a photolithography process is performed on the first metal thin film, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Etching is then performed to form a pattern including multiple first electrodes 11. The etching process is preferably wet etching, but dry etching may also be used. Finally, a resist removal process is performed to complete the fabrication of each first electrode 11. The thickness of the first electrode 11 is preferably set to 0.15 micrometers.

[0092] In some examples, the material of the first electrode 11 is preferably molybdenum, because its lattice size is very close to that of the piezoelectric layer 12 material AlN and the doped AlN lattice structure and size. The material of the first electrode 11 can also be Al, Cu, Co, Ag, Ti, Pt, Ru, W, Au, or a multilayer or alloy material formed from the above metals.

[0093] S223. A piezoelectric layer 12 is formed on the side of the first electrode 11 away from the substrate 10.

[0094] Taking the piezoelectric layer 12 as an example of an AlN monolayer structure, step S223 may include forming a piezoelectric material layer on the side of the first electrode 11 away from the substrate 10, and performing oriented growth of the piezoelectric material layer. The preferred method for forming the piezoelectric material layer is radio frequency magnetron sputtering (DC-controlled sputtering is also acceptable). For AlN piezoelectric material, Al is selected as the target. By controlling the Ar and N2 gas pressure and temperature during the deposition process, as well as the post-annealing time and temperature, an AlN C-axis oriented piezoelectric material layer is formed. The preferred growth orientation is (001). Other methods for depositing the piezoelectric material layer include pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), and plasma-enhanced chemical vapor deposition (PECVD). Next, a photolithography process is performed on the piezoelectric layer 12, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Then, etching is performed to form a pattern including the piezoelectric layer 12. A wet etching process is preferred, but a dry etching process can also be used. Finally, a desizing process is performed to complete the preparation of the piezoelectric layer 12.

[0095] In some examples, the material of the piezoelectric layer 12 is preferably AlN or doped AlN, such as Al... (1-x) Sc x N, Al (1-x) Cr x N, Al (1-x) Y x N, Al(1-x) Ti x N, Al (1-x) Zr x N, Al (1-x) Hf x N, Al (1-x) Yb x N, Al (1-x) Ta x N, Mg 0.5x Nb 0.5x Al (1-x) N, Mg 0.5x Ti 0.5x Al (1-x) N, Mg 0.5x Zr 0.5x Al (1-x) N, Mg 0.5x Hf 0.5x Al (1-x) N, Mg 0.5x Si 0.5x Al (1-x) N, Zn 0.25 Ti 0.25 Al 0.5 N, Zn 0.25 Zr 0.25 Al 0.5 N, Zn 0.25 Hf 0.25 Al 0.5 N. The material of the piezoelectric layer 12 can also be selected from ZnO, PZT, GaN, InN, CdS, CdSe, ZnS, CdTe, ZnTe, GaAs, GaSb, InAs, InSb, GaSe, GaP, AlP, quartz crystal, LiTaO3, LiNbO3, La3Ga5SiO 14 Materials such as BaTiO3, PbNb2O6, PBLN, LiGaO3, LiGeO3, TiGeO3, PbTiO3, PbZrO3, and PVDF can be used. The piezoelectric layer 12 can be a single piezoelectric material or a stack of the above piezoelectric materials. The thickness of the piezoelectric layer 12 ranges from 10 nm to 100 μm.

[0096] S224. A second electrode 13 is formed on the side of the piezoelectric layer 12 opposite to the first electrode 11.

[0097] When the second electrode 13 is made of a metallic material, step S224 may specifically include: firstly, depositing a second metal thin film on the side of the piezoelectric layer 12 opposite to the first electrode 11. The deposition method is preferably DC magnetron sputtering (RF magnetron sputtering is also acceptable), but pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), thermal evaporation, electron beam evaporation, etc., can also be selected. Next, a photolithography process is performed on the second metal thin film, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Then, etching is performed to form a pattern including the second electrode 13. The etching process is preferably wet etching, but dry etching can also be selected. Finally, a resist removal process is performed to complete the fabrication of the second electrode 13.

[0098] In some examples, the material of the second electrode 13 is preferably molybdenum, because its lattice size is very close to that of the piezoelectric layer 12 material AlN and the doped AlN lattice structure and size. The material of the second electrode 13 can also be Al, Cu, Co, Ag, Ti, Pt, Ru, W, Au, or a multilayer or alloy of the above metals.

[0099] S225. An encapsulation layer 16 is formed on the side of the second electrode 13 away from the substrate 10.

[0100] Taking the encapsulation layer 16 as an example of using an organic compound material, step S225 may specifically include first coating the organic material liquid, which can be done by spin coating, spraying, inkjet printing, transfer, etc., and then heating and curing to form the pattern of the encapsulation layer 16.

[0101] In some examples, the material of the encapsulation layer 16 is preferably an organic compound that can isolate moisture and oxygen, such as polyimide, epoxy resin, etc., but inorganic materials such as SiN can also be selected. x Al2O3, etc. The encapsulation layer 16 can be a single layer of a single material, or it can be a stack of multiple materials.

[0102] In some examples, the bulk acoustic resonator is an SMR resonator. In this case, refer to... Figure 15 Step S2 includes:

[0103] S231. A mirror structure 15 is formed on the substrate 10.

[0104] Step S231 may specifically include: (a) depositing a high acoustic impedance layer 151 thin film material, preferably by DC magnetron sputtering (RF magnetron sputtering is also acceptable), or by pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), thermal evaporation, electron beam evaporation, etc. Then, coating (or spraying) a resist onto the high acoustic impedance layer 151 thin film, pre-baking, exposure, development, post-baking, and etching are performed to form the high acoustic impedance layer 151. The etching process is preferably wet etching, but dry etching is also acceptable. (b) depositing a low acoustic impedance layer 152 thin film material, preferably by DC magnetron sputtering (RF magnetron sputtering is also acceptable), or by pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), thermal evaporation, electron beam evaporation, etc. Then, coating (or spraying) a resist onto the low acoustic impedance layer 152 thin film, pre-baking, exposure, development, post-baking, and etching are performed to form the low acoustic impedance layer 152. The etching process preferably uses a wet etching process, but a dry etching process can also be selected. Then, steps (a) and (b) are repeated until an acoustic reflector structure 15 with the required number of layers is obtained.

[0105] In some examples, the acoustic reflector structure 15 consists of alternating high acoustic impedance layers 151 and low acoustic impedance layers 152. The acoustic impedance of a material is equal to the speed of sound propagation in the material multiplied by the density of the material. Theoretically, when the thickness of the high acoustic impedance layer 151 is equal to one-quarter of the wavelength of the sound wave at the resonant frequency of the bulk acoustic resonator propagating in the high acoustic impedance layer 151, and the thickness of the low acoustic impedance layer 152 is equal to one-quarter of the wavelength of the sound wave at the resonant frequency of the bulk acoustic resonator propagating in the low acoustic impedance layer 152, the alternating arrangement of high and low acoustic impedance layers 152 (high / low / high / low... or low / high / low / high...) acts as an acoustic reflector, reflecting the sound wave signal leaking from above back. A reflector structure 15 consisting of high acoustic impedance layers 151 and low acoustic impedance layers 152 generally requires 3 to 4 sets to achieve a good acoustic reflection effect; of course, the more sets, the better, but the cost will increase. The number of layers is not limited, and the range of selectable reflector structures is 1 to 100 layers. There is also no restriction on whether the layer is equal to one-quarter of the wavelength; any thickness is acceptable. Materials for the high acoustic impedance layer 151 can include W, Ir, Pt, Ru, Au, Mo, Ta, Ti, Cu, Ni, Zn, Al, Al2O3, Ag, etc., while commonly used low acoustic impedance materials include SiO2, Si3N4, Mg, rubber, nylon, polyimide, polyethylene, polystyrene, Teflon, etc. Depending on the resonant frequency and the sound velocity of different materials, the thickness range of a single high acoustic impedance layer 151 and a single low acoustic impedance layer 152 is 1 nm to 10 μm.

[0106] S232. A plurality of first electrodes 11 are formed on the side of the mirror structure 15 away from the substrate 10.

[0107] When the first electrode 11 is made of a metal material, step S232 may include depositing a first metal thin film on the substrate 10. The deposition method is preferably DC magnetron sputtering (RF magnetron sputtering is also acceptable), but pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), thermal evaporation, electron beam evaporation, or copper foil deposition may also be used. Next, a photolithography process is performed on the first metal thin film, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Etching is then performed to form a pattern including multiple first electrodes 11. The etching process is preferably wet etching, but dry etching may also be used. Finally, a resist removal process is performed to complete the fabrication of each first electrode 11. The thickness of the first electrode 11 is preferably set to 0.15 micrometers.

[0108] In some examples, the material of the first electrode 11 is preferably molybdenum, because its lattice size is very close to that of the piezoelectric layer 12 material AlN and the doped AlN lattice structure and size. The material of the first electrode 11 can also be Al, Cu, Co, Ag, Ti, Pt, Ru, W, Au, or a multilayer or alloy material formed from the above metals.

[0109] S233. A piezoelectric layer 12 is formed on the side of each first electrode 11 facing away from the substrate 10.

[0110] Taking the piezoelectric layer 12 as an example of an AlN monolayer structure, step S233 may include forming a piezoelectric material layer on the side of the first electrode 11 away from the substrate 10, and performing oriented growth of the piezoelectric material layer. The preferred method for forming the piezoelectric material layer is radio frequency magnetron sputtering (DC-controlled sputtering is also acceptable). For AlN piezoelectric material, Al is selected as the target. By controlling the Ar and N2 gas pressure and temperature during the deposition process, as well as the post-annealing time and temperature, an AlN C-axis oriented piezoelectric material layer is formed. The preferred growth orientation is (001). Other methods for depositing the piezoelectric material layer include pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), metal-organic chemical vapor deposition (MOCVD), and plasma-enhanced chemical vapor deposition (PECVD). Next, a photolithography process is performed on the piezoelectric layer 12, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Then, etching is performed to form a pattern including the piezoelectric layer 12. A wet etching process is preferred, but a dry etching process can also be used. Finally, a desizing process is performed to complete the preparation of the piezoelectric layer 12.

[0111] In some examples, the material of the piezoelectric layer 12 is preferably AlN or doped AlN, such as Al... (1-x) Sc x N, Al(1-x) Cr x N, Al (1-x) Y x N, Al (1-x) Ti x N, Al (1-x) Zr x N, Al (1-x) Hf x N, Al (1-x) Yb x N, Al (1-x) Ta x N, Mg 0.5x Nb 0.5x Al (1-x) N, Mg 0.5x Ti 0.5x Al (1-x) N, Mg 0.5x Zr 0.5x Al (1-x) N, Mg 0.5x Hf 0.5x Al (1-x) N, Mg 0.5x Si 0.5x Al (1-x) N, Zn 0.25 Ti 0.25 Al 0.5 N, Zn 0.25 Zr 0.25 Al 0.5 N, Zn 0.25 Hf 0.25 Al 0.5 N. The material of the piezoelectric layer 12 can also be selected from ZnO, PZT, GaN, InN, CdS, CdSe, ZnS, CdTe, ZnTe, GaAs, GaSb, InAs, InSb, GaSe, GaP, AlP, quartz crystal, LiTaO3, LiNbO3, La3Ga5SiO 14 Materials such as BaTiO3, PbNb2O6, PBLN, LiGaO3, LiGeO3, TiGeO3, PbTiO3, PbZrO3, and PVDF can be used. The piezoelectric layer 12 can be a single piezoelectric material or a stack of the above piezoelectric materials. The thickness of the piezoelectric layer 12 ranges from 10 nm to 100 μm.

[0112] S234. A second electrode 13 is formed on the side of the piezoelectric layer 12 opposite to the first electrode 11.

[0113] When the second electrode 13 is made of a metallic material, step S234 may specifically include: firstly, depositing a second metal thin film on the side of the piezoelectric layer 12 opposite to the first electrode 11. The deposition method is preferably DC magnetron sputtering (RF magnetron sputtering is also acceptable), but pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), thermal evaporation, electron beam evaporation, etc., can also be selected. Next, a photolithography process is performed on the second metal thin film, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Then, etching is performed to form a pattern including the second electrode 13. The etching process is preferably wet etching, but dry etching can also be selected. Finally, a resist removal process is performed to complete the fabrication of the second electrode 13.

[0114] In some examples, the material of the second electrode 13 is preferably molybdenum, because its lattice size is very close to that of the piezoelectric layer 12 material AlN and the doped AlN lattice structure and size. The material of the second electrode 13 can also be Al, Cu, Co, Ag, Ti, Pt, Ru, W, Au, or a multilayer or alloy of the above metals.

[0115] S235. An encapsulation layer 16 is formed on the side of the second electrode 13 away from the substrate 10.

[0116] Taking the encapsulation layer 16 as an example of using an organic compound material, step S235 may specifically include first coating the organic material liquid, the specific method of which may be spin coating, spraying, inkjet printing, transfer printing, etc., and then heating and curing to form the pattern of the encapsulation layer 16.

[0117] Based on the same inventive concept, this disclosure also provides an electronic device that includes the filtering circuit of any of the above embodiments.

[0118] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A filter circuit comprising a plurality of electrically connected bulk acoustic wave resonators, wherein the resonant frequency of each bulk acoustic wave resonator is within a preset frequency band; wherein, At least some of the bulk acoustic resonators have different resonant frequencies, so that the number of transmission zeros in the S21 curve of the filter circuit is not less than three.

2. The filter circuit according to claim 1, wherein, The filtering circuit includes N electrically connected filtering sub-circuits; the filtering sub-circuit includes two series-connected bulk acoustic wave resonators, and the connection node between the two is the first node, and the two series-connected bulk acoustic wave resonators are the first bulk acoustic wave resonator and the second bulk acoustic wave resonator, respectively. For the i-th filter sub-circuit, the two ends of the first bulk acoustic resonator are respectively connected to the first node of the (i-1)-th filter sub-circuit and the first node of the (i+1)-th filter sub-circuit; i is an integer from 2 to N-1, where N is an integer greater than or equal to 3; For the filter circuit, at least one of the first body acoustic resonators has a different resonant frequency than the other first body acoustic resonators, and / or at least one of the second body acoustic resonators has a different resonant frequency than the other second body acoustic resonators.

3. The filter circuit according to claim 2, wherein, The bulk acoustic wave resonator includes: a substrate, and a first electrode, a piezoelectric layer, and a second electrode sequentially stacked on the substrate; the orthographic projections of any two of the first electrode, the piezoelectric layer, and the second electrode on the substrate at least partially overlap. The thickness of the second electrode of at least one of the first bulk acoustic wave resonators is different from the thickness of the second electrode of the other first bulk acoustic wave resonators, and / or the thickness of the second electrode of at least one of the second bulk acoustic wave resonators is different from the thickness of the second electrode of the other second bulk acoustic wave resonators.

4. The filter circuit according to claim 3, wherein, The thickness of the second electrode of the first bulk acoustic resonator is 0.05-0.15 μm.

5. The filter circuit according to claim 3, wherein, The thickness of the second electrode of the second bulk acoustic resonator is 0.15-0.25 μm.

6. The filter circuit according to claim 3, wherein, The thickness of the first electrode is 0.1-0.2 μm.

7. The filter circuit according to claim 3, wherein, The bulk acoustic resonator further includes a first groove disposed on the side of the substrate near the first electrode, and the orthogonal projection of the piezoelectric layer on the substrate covers the orthogonal projection of the first groove on the substrate.

8. The filter circuit according to claim 3, wherein, The bulk acoustic resonator further includes multiple layers of first impedance layer and second impedance layer disposed between the substrate and the first electrode; the first impedance layer and the second impedance layer are alternately disposed, and the acoustic impedance of the first impedance layer is greater than the acoustic impedance of the second impedance layer.

9. The filter circuit according to claim 3, wherein, The substrate includes any one of glass, Si, sapphire, SiC, GaAs, GaN, InP, BN, ZnO, and GaO.

10. The filter circuit according to claim 3, wherein, Both the first electrode and the second electrode include any one of Mo, Al, Cu, Co, Ag, Ti, Pt, Ru, W, and Au.

11. The filter circuit according to claim 3, wherein, The piezoelectric layer includes ZnO, PZT, GaN, InN, CdS, CdSe, ZnS, CdTe, ZnTe, GaAs, GaSb, InAs, InSb, GaSe, GaP, AlP, quartz crystal, LiTaO3, LiNbO3, and La3Ga5SiO2. 14 Any one of BaTiO3, PbNb2O6, PBLN, LiGaO3, LiGeO3, TiGeO3, PbTiO3, PbZrO3, and PVDF.

12. An electronic device comprising a filter circuit as claimed in any one of claims 1-11.

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