Bulk acoustic wave resonator, method of manufacturing bulk acoustic wave resonator, and electronic device

Abstract

The present disclosure provides a bulk acoustic wave resonator and a preparation method thereof, and an electronic device, and belongs to the technical field of communication. The bulk acoustic wave resonator comprises a substrate substrate (10), a first electrode (11), a piezoelectric layer (12) and a second electrode (13). The bulk acoustic wave filter further comprises a first biasing resistance layer (17) arranged on the side of the first electrode (11) close to the substrate substrate (10), and a first electrical isolation layer (18) arranged between the first biasing resistance layer (17) and the first electrode (11). The material of the first biasing resistance layer (17) is a high resistivity material. And / or, a second biasing resistance layer (110) arranged on the side of the second electrode (12) away from the substrate substrate (10), and a second electrical isolation layer (19) arranged between the second biasing resistance layer (110) and the second electrode (13). The material of the second biasing resistance layer (19) is a high resistivity material.

Description

Technical Field

[0001] This disclosure belongs to the field of communication technology, specifically relating to a bulk acoustic resonator and its fabrication method, as well as electronic equipment. Background Technology

[0002] In the field of mobile communications, due to the narrow total available frequency range and the large number of frequency bands used for mobile communications, the spacing between adjacent frequency bands is very narrow (approximately a few megahertz to tens of megahertz), and the bandwidth of a single frequency band is very narrow (tens of megahertz). Therefore, filters used in mobile phones must possess performance characteristics such as low in-band ripple, high out-of-band rejection, and good rectangularity. Conventional microstrip filters are too large, have insufficient out-of-band rejection, and poor rectangularity, making them unsuitable. Cavity filters are also too large, making them unsuitable. Dielectric filters have high in-band insertion loss and poor rectangularity, making them unsuitable. IPD filters have large in-band ripple and poor rectangularity, making them unsuitable.

[0003] As the basic structural unit of a bulk acoustic wave (BAW) filter, the existing BAW resonator uses a silicon wafer as the substrate material, with a sandwich structure consisting of a first electrode, a piezoelectric layer, and a second electrode from bottom to top. The first and second electrodes are made of metal, and the piezoelectric layer is made of piezoelectric material.

[0004] The working principle of a bulk acoustic 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, the piezoelectric layer, and the second electrode. 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 at the interface between the metal electrode and the piezoelectric material through the piezoelectric effect. The resonator has a fixed resonant frequency. When the frequency of the RF signal equals the resonant frequency, the conversion efficiency from RF signal to acoustic signal to 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. To reduce insertion loss during the filtering process, the acoustic signal needs to be confined inside the piezoelectric material as much as possible to prevent it from spreading outward. Therefore, acoustic reflectors are usually built on the upper and lower surfaces of the resonator. Summary of the Invention

[0005] The present invention aims to solve at least one of the technical problems existing in the prior art, and to provide a bulk acoustic resonator and its preparation method, as well as an electronic device.

[0006] This disclosure provides a bulk acoustic wave resonator, comprising: a substrate, a first electrode, a piezoelectric layer, and a second electrode; the first electrode is disposed on the substrate, the second electrode is disposed on the side of the first electrode facing away from the substrate, the piezoelectric layer is disposed between the first electrode and the second electrode, and the orthographic projections of any two of the first electrode, the piezoelectric layer, and the second electrode onto the substrate at least partially overlap; wherein, the bulk acoustic wave filter further comprises:

[0007] A first bias resistor layer is disposed on the side of the first electrode near the substrate, and a first electrical isolation layer is disposed between the first bias resistor layer and the first electrode; the material of the first bias resistor layer is a high resistivity material; and / or,

[0008] A second bias resistor layer is disposed on the side of the second electrode away from the substrate, and a second electrical isolation layer is disposed between the second bias resistor layer and the second electrode; the material of the second bias resistor layer is a high resistivity material.

[0009] The substrate has a first cavity extending through it along its thickness direction.

[0010] It also includes at least one reflective mirror structure disposed on the side of the substrate near the first electrode; when the first bias resistor layer is disposed on the substrate, the reflective mirror structure is disposed on the side of the first bias resistor layer near the substrate.

[0011] The mirror structure includes a first substructure layer and a second substructure layer arranged sequentially along the direction away from the substrate, and the acoustic impedance of the material of the first substructure layer is greater than the acoustic impedance of the material of the second substructure layer.

[0012] Wherein, when the bulk acoustic resonator includes the first bias resistor layer, the material of the first bias resistor layer includes any one of the following materials: ITO, IZO, ZnO, IGO, IGZO, W, Mn, Cr, Ti, Ni, constantan alloy, manganese copper alloy, nickel-chromium alloy, and iron-chromium-aluminum alloy, in a single-layer structure or in a stacked structure of multiple materials.

[0013] Wherein, when the bulk acoustic resonator includes the second bias resistor layer, the second bias resistor layer includes a single-layer structure or a stacked structure of multiple materials selected from ITO, IZO, ZnO, IGO, IGZO, W, Mn, Cr, Ti, Ni, constantan alloy, manganese copper alloy, nickel-chromium alloy, and iron-chromium-aluminum alloy.

[0014] Wherein, when the bulk acoustic resonator includes the first electrical isolation layer, the material of the first electrical isolation layer includes a single-layer structure of any one of Si3N4, SiO2, Al2O3, AlN, and BN, or a stacked structure of multiple materials.

[0015] When the bulk acoustic resonator includes the second electrical isolation layer, the material of the second electrical isolation layer includes a single-layer structure of any one of Si3N4, SiO2, Al2O3, AlN, and BN, or a stacked structure of multiple materials.

[0016] The substrate includes any one of glass, Si, sapphire, SiC, GaAs, GaN, InP, BN, ZnO, and GaO.

[0017] The piezoelectric materials include AlN, doped AlN, 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; wherein the doped AlN includes 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 Any one of N.

[0018] The first electrode and the second electrode each include any one of Mo, Al, Cu, Co, Ag, Ti, Pt, Ru, W, and Au.

[0019] This disclosure provides a method for fabricating a bulk acoustic wave resonator, comprising: sequentially forming a first electrode, a piezoelectric layer, and a second electrode on a first substrate, wherein the orthographic projections of any two of the first electrode, the piezoelectric layer, and the second electrode on the first substrate at least partially overlap; wherein...

[0020] The fabrication method further includes: forming a first bias resistor layer on the side of the first electrode near the substrate, and forming a first electrical isolation layer between the first bias resistor layer and the first electrode; the first bias resistor layer is made of a high resistivity material; and / or,

[0021] A second bias resistor layer is formed on the side of the second electrode away from the substrate, and a second electrical isolation layer is formed between the second bias resistor layer and the second electrode; the material of the second bias resistor layer is a high resistivity material.

[0022] The method for fabricating the bulk acoustic resonator further includes:

[0023] The substrate is processed to form a first cavity that extends through the thickness direction of the substrate.

[0024] Wherein, at least one reflective mirror structure is formed on the side of the substrate near the first electrode; when the first bias resistor layer is formed on the substrate, the reflective mirror structure is formed on the side of the first bias resistor layer near the substrate.

[0025] The formation of the reflector structure includes sequentially forming a first substructure layer and a second substructure layer along a direction away from the substrate, wherein the acoustic impedance of the material of the first substructure layer is greater than the acoustic impedance of the material of the second substructure layer.

[0026] Wherein, when the bulk acoustic resonator includes the first bias resistor layer, the material of the first bias resistor layer includes any one of the following materials: ITO, IZO, ZnO, IGO, IGZO, W, Mn, Cr, Ti, Ni, constantan alloy, manganese copper alloy, nickel-chromium alloy, and iron-chromium-aluminum alloy, in a single-layer structure or in a stacked structure of multiple materials.

[0027] Wherein, when the bulk acoustic resonator includes the second bias resistor layer, the second bias resistor layer includes a single-layer structure or a stacked structure of multiple materials selected from ITO, IZO, ZnO, IGO, IGZO, W, Mn, Cr, Ti, Ni, constantan alloy, manganese copper alloy, nickel-chromium alloy, and iron-chromium-aluminum alloy.

[0028] Wherein, when the bulk acoustic resonator includes the first electrical isolation layer, the material of the first electrical isolation layer includes a single-layer structure of any one of Si3N4, SiO2, Al2O3, AlN, and BN, or a stacked structure of multiple materials.

[0029] When the bulk acoustic resonator includes the second electrical isolation layer, the material of the second electrical isolation layer includes a single-layer structure of any one of Si3N4, SiO2, Al2O3, AlN, and BN, or a stacked structure of multiple materials.

[0030] The substrate includes any one of glass, Si, sapphire, SiC, GaAs, GaN, InP, BN, ZnO, and GaO.

[0031] The piezoelectric materials include AlN, doped AlN, 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; wherein the doped AlN includes 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 Any one of N.

[0032] The first electrode and the second electrode each include any one of Mo, Al, Cu, Co, Ag, Ti, Pt, Ru, W, and Au.

[0033] This disclosure provides an electronic device that includes any of the bulk acoustic wave resonators described above. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of a back-etched bulk acoustic resonator.

[0035] Figure 2 This is a schematic diagram of a thin-film bulk acoustic resonator.

[0036] Figure 3 This is a schematic diagram of another type of thin-film bulk acoustic resonator.

[0037] Figure 4 This is a schematic diagram of a solid-state assembled bulk acoustic resonator.

[0038] Figure 5 This is a schematic diagram of the structure of a bulk acoustic resonator, which is a first example of an implementation of this disclosure.

[0039] Figure 6 for Figure 5 The flowchart shown is a fabrication process diagram of a bulk acoustic resonator.

[0040] Figure 7 This is a schematic diagram of the structure of a bulk acoustic resonator, which is a second example of an implementation of this disclosure.

[0041] Figure 8 for Figure 7 The flowchart shown is a fabrication process diagram of a bulk acoustic resonator.

[0042] Figure 9 This is a schematic diagram of the structure of a bulk acoustic resonator, which is a third example of an embodiment of this disclosure.

[0043] Figure 10 for Figure 9 The flowchart shown is a fabrication process diagram of a bulk acoustic resonator.

[0044] Figure 11 This is a schematic diagram of the structure of a bulk acoustic resonator, which is a fourth example of an embodiment of this disclosure.

[0045] Figure 12 for Figure 11 The flowchart shown is a fabrication process diagram of a bulk acoustic resonator.

[0046] Figure 13 This is a schematic diagram of the structure of a bulk acoustic resonator, which is a fifth example of an embodiment of this disclosure.

[0047] Figure 14 for Figure 13 The flowchart shown is a fabrication process diagram of a bulk acoustic resonator.

[0048] Figure 15 This is a schematic diagram of the structure of a bulk acoustic resonator, which is a sixth example of an embodiment of this disclosure.

[0049] Figure 16 for Figure 15 The flowchart shown is a fabrication process diagram of a bulk acoustic resonator. Detailed Implementation

[0050] 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.

[0051] Unless otherwise defined, the technical or scientific terms used in this disclosure shall have the ordinary meaning understood by one of ordinary skill in the art to which this disclosure pertains. The terms “first,” “second,” and similar terms used in this disclosure do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, the terms “an,” “a,” or “the,” and similar terms do not indicate a quantity limitation, but rather indicate the presence of at least one. The terms “including,” “comprising,” or “containing,” and similar terms mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. The terms “connected,” “linked,” or similar terms are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. The terms “upper,” “lower,” “left,” and “right,” etc., are used only to indicate relative positional relationships, and these relative positional relationships may change accordingly when the absolute position of the described objects changes.

[0052] like Figure 1-4 As shown, to reduce insertion loss during the filtering process, the bulk acoustic wave resonator needs to confine the acoustic signal as much as possible within the piezoelectric layer 12 between the first and second electrodes 13 to prevent the acoustic signal from spreading outward. Therefore, acoustic wave reflectors are usually constructed on the upper and lower surfaces of the resonator. The upper surface generally uses air as the reflector with low acoustic impedance. Depending on the construction of the acoustic wave reflector on the lower surface, bulk acoustic wave resonators are divided into three main categories, such as back-etched bulk acoustic wave resonators, etc. Figure 1 As shown; film bulk acoustic resonator (FBAR), a thin-film bulk acoustic resonator, such as... Figure 2 and 3 As shown; solid mounted resonator (SMR), such as... Figure 4 As shown. In this FBAR, a first groove 102 is etched and formed on the substrate 10 below the first electrode to serve as an air gap, and then the first electrode is supported by an isolation layer 14, as shown. Figure 2 As shown. Alternatively, a first groove 102 can be formed through the isolation layer 14 as an air gap, such as... Figure 3 As shown; SMR is an acoustic mirror structure 15 formed by alternating and repeating layers of high acoustic impedance layer 151 and low acoustic impedance material layer 152 below the first electrode; back etching type is a first cavity 101 formed on the substrate 10 as an air layer by deeply etching a cavity on the back side of the silicon substrate to form a cavity below the first electrode.

[0053] The inventors discovered that the piezoelectric materials in the piezoelectric layer of current bulk acoustic wave (BAW) filters are C-axis oriented AlN and Sc-doped AlN. With increasing temperature, the in-plane lattice constant increases while the out-of-plane lattice constant decreases, leading to a reduction in the sound wave propagation speed. This is determined by the qualitative formula: f = v / (2h), where f is the resonant frequency, v is the sound wave propagation speed along the thickness direction in the piezoelectric material, and h is the thickness of the piezoelectric material. This decrease in resonant frequency results in a temperature drift phenomenon (a negative temperature coefficient, typically -30ppm), which is detrimental to the performance of communication systems. Currently, to address this problem, thin films with a positive temperature coefficient are typically added near the piezoelectric layer. The main materials are SiO2 and F- and P-doped SiO2. However, because these films are insulating and non-conductive, they hinder the piezoelectric and inverse piezoelectric effects near the piezoelectric layer, reducing the effective electromechanical coupling coefficient of the resonator and leading to a decrease in the device's quality factor and performance degradation.

[0054] To address the aforementioned technical problems, this disclosure provides a bulk acoustic wave resonator. By applying a DC electric field to the piezoelectric layer, the propagation speed of sound waves in the piezoelectric layer material is modulated, thereby changing the resonant frequency of the bulk acoustic wave resonator. When the resonant frequency of the bulk acoustic wave resonator drifts towards lower frequencies due to temperature rise (i.e., temperature drift occurs), applying a suitable DC bias voltage to both sides of the piezoelectric layer generates a stable DC electric field penetrating the piezoelectric layer, causing the resonant frequency of the bulk acoustic wave resonator to drift towards higher frequencies (i.e., electric field drift occurs), ultimately keeping the frequency of the bulk acoustic wave resonator constant. Conversely, when the temperature decreases, the DC bias voltage is adjusted appropriately in the opposite direction to keep the frequency of the bulk acoustic wave resonator constant.

[0055] The following detailed examples illustrate specific examples of the bulk acoustic resonator and its fabrication method according to embodiments of this disclosure.

[0056] First example: Figure 5 This is a schematic diagram of the structure of a bulk acoustic resonator according to a first example of an embodiment of this disclosure; as shown... Figure 5As shown, the bulk acoustic wave resonator includes a substrate 10, and a first electrode 11, a piezoelectric layer 12, and a second electrode 13 sequentially disposed on the substrate 10. The orthogonal projections of any two of the first electrode 11, the piezoelectric layer 12, and the second electrode 13 onto the substrate 10 at least partially overlap. The substrate 10 has a first cavity 101 extending through its thickness direction. In this configuration, after a radio frequency (RF) signal is input into the bulk acoustic wave resonator, it is converted into an acoustic signal at the interface between the second electrode 13 and the piezoelectric layer 12 through the inverse piezoelectric effect. This acoustic signal propagates longitudinally within the piezoelectric layer 12, and upon reaching the interface between the first electrode 11 and the piezoelectric layer 12, it is converted back into an RF signal through the piezoelectric effect, finally exiting the resonator. The first cavity 101 below the bulk acoustic wave resonator and the air layer above it act as acoustic reflectors, confining the acoustic signal within the resonator structure rather than dissipating it, thus reducing the loss of the bulk acoustic wave resonator.

[0057] During real-time frequency compensation, a DC bias voltage is applied between the first electrode 11 and the second electrode 13, generating a DC electric field in the piezoelectric layer 12. When the direction of the DC electric field is from the first electrode 11 to the second electrode 13, the electric field strength increases from 0 to +100V / um as the DC bias voltage increases, and the resonant frequency of the bulk acoustic wave resonator shifts to a lower frequency. When the direction of the DC electric field is from the second electrode 13 to the first electrode 11, the electric field strength increases from 0 to -100V / um as the DC bias voltage increases, and the resonant frequency of the bulk acoustic wave resonator shifts to a higher frequency. Under DC electric field modulation (to prevent DC signals from mixing into the RF signal and breaking down the RF circuit, a DC blocker needs to be added in the RF circuit outside the bulk acoustic wave resonator), the resonant frequency of the bulk acoustic wave resonator shifts linearly in a controlled manner. The shift range is 28–58 ppm·f0 / (V / um). For AlN-doped piezoelectric materials, the shift range is greater than that of undoped AlN piezoelectric layer 12, where f0 is the initial resonant frequency without DC bias, and V / um is the electric field strength applied to piezoelectric layer 12. In a specific application scenario, when the ambient temperature changes, the resonant frequency of the bulk acoustic wave resonator will experience temperature drift (shifting to lower frequencies when heated and to higher frequencies when cooled), which in turn causes temperature drift in the filter curve. At this time, by adjusting the DC bias applied to piezoelectric layer 12, the resonant frequency of the bulk acoustic wave resonator is shifted in the opposite direction to the temperature drift, achieving the final effect of constant resonant frequency and ensuring that the filter curve remains unchanged when the ambient temperature changes.

[0058] In some examples, an encapsulation layer 16 is also provided on the side of the second electrode 13 facing away from the substrate 10 to isolate moisture and oxygen and prevent device damage.

[0059] Furthermore, the material of the encapsulation layer 16 is preferably an organic compound that can isolate moisture and oxygen, such as polyimide or epoxy resin, or an inorganic material such as SiN.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.

[0060] In some examples, the substrate 10 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.

[0061] In some examples, the material of the first electrode 11 is preferably molybdenum (Mo), 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 of these metals. The thickness of the first electrode 11 ranges from 1 nm to 10 μm.

[0062] 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 Hf0.25 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.

[0063] In some examples, the material of the second electrode 13 is preferably molybdenum (Mo), 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 these metals. The thickness of the second electrode 13 ranges from 1 nm to 10 μm.

[0064] Next, the fabrication method of the bulk acoustic resonator of the first example will be described. For example... Figure 6 As shown, the preparation method specifically includes the following steps.

[0065] S11, Provide a substrate 10.

[0066] Taking a monocrystalline silicon substrate as an example, step S11 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.

[0067] S12. A first electrode 11 is formed on the substrate 10.

[0068] When the first electrode 11 is made of a metal material, step S12 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 can 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 the first electrode 11. The etching process is preferably wet etching, but dry etching can also be used. Finally, a resist removal process is performed to complete the fabrication of the first electrode.

[0069] S13. A piezoelectric layer 12 is formed on the side of the first electrode 11 away from the substrate 10.

[0070] Taking the piezoelectric layer 12 as an example of an AlN monolayer structure, step S13 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.

[0071] S14. A second electrode 13 is formed on the side of the piezoelectric layer 12 opposite to the first electrode 11.

[0072] When the second electrode 13 is made of a metallic material, step S14 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.

[0073] S15. An encapsulation layer 16 is formed on the side of the second electrode 13 away from the substrate 10.

[0074] Taking the encapsulation layer 16 as an example of using an organic compound material, step S15 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.

[0075] S16. The substrate 10 with the above structure is flipped and etched to form a first cavity 101 that penetrates along the thickness direction of the substrate 10.

[0076] In some examples, step S16 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 101, and finally, a resist removal process is performed.

[0077] Second example: Figure 7 This is a schematic diagram of the structure of a bulk acoustic resonator according to a second example of an embodiment of this disclosure; as shown Figure 7 As shown, the bulk acoustic wave resonator includes a substrate 10, and a first bias resistor layer 17, a first electrical isolation layer 18, a first electrode 11, a piezoelectric layer 12, and a second electrode 13 sequentially disposed on the substrate 10. The orthogonal projections of any two of the first electrode 11, the piezoelectric layer 12, and the second electrode 13 onto the substrate 10 at least partially overlap. The substrate 10 has a first cavity 101 extending through its thickness direction. In this configuration, after a radio frequency (RF) signal is input into the bulk acoustic wave resonator, it is converted into an acoustic signal at the interface between the second electrode 13 and the piezoelectric layer 12 through the inverse piezoelectric effect. This acoustic signal propagates longitudinally within the piezoelectric layer 12, and upon reaching the interface between the first electrode 11 and the piezoelectric layer 12, it is converted back into an RF signal through the piezoelectric effect, finally exiting the resonator. The first cavity 101 below the bulk acoustic wave resonator and the air layer above it act as acoustic reflectors, confining the acoustic signal within the resonator structure rather than dissipating it, thus reducing the loss of the bulk acoustic wave resonator.

[0078] During real-time frequency compensation, a DC bias voltage is applied between the first bias resistor layer 17 and the second electrode 13, generating a DC electric field in the piezoelectric layer 12. When the direction of the DC electric field is from the first bias resistor layer 17 to the second electrode 13, the electric field strength increases from 0 to +100 V / µm with the increase of the DC bias voltage, and the resonant frequency of the bulk acoustic wave resonator shifts to a lower frequency. When the direction of the DC electric field is from the second electrode 13 to the first bias resistor layer 17, the electric field strength increases from 0 to -100 V / µm with the increase of the DC bias voltage, and the resonant frequency of the bulk acoustic wave resonator shifts to a higher frequency. Under the modulation of the DC electric field (to prevent DC signals from mixing into the RF signals and breaking down the RF circuit, a DC blocker needs to be added in the RF circuit outside the bulk acoustic wave resonator chip), the resonant frequency of the bulk acoustic wave resonator shifts linearly in a controlled manner, with a shift range of 28 to 58 ppm·f0 / (V / µm). For AlN piezoelectric materials, the shift range is greater than that for undoped AlN piezoelectric materials. Where f0 is the initial resonant frequency without DC bias, and V / um is the electric field strength applied to the piezoelectric layer 12. In a specific application scenario, when the ambient temperature changes, the resonant frequency of the bulk acoustic wave resonator experiences temperature drift (shifting to lower frequencies when heated and to higher frequencies when cooled), which in turn causes temperature drift in the filter's filtering curve. By adjusting the DC bias applied to the piezoelectric layer 12, the resonant frequency of the bulk acoustic wave resonator is shifted in the opposite direction to the temperature drift, achieving the final effect of a constant resonant frequency and ensuring that the filter's filtering curve remains unchanged regardless of ambient temperature variations.

[0079] In the second example, compared to the first example, a first bias resistor layer 17 and a first electrical isolation layer 18 are added between the first electrode 11 and the substrate 10 to isolate the DC signal and the radio frequency signal of the lower half of the bulk acoustic resonator.

[0080] In some examples, an encapsulation layer 16 is also provided on the side of the second electrode 13 facing away from the substrate 10 to isolate moisture and oxygen and prevent device damage.

[0081] Furthermore, the material of the encapsulation layer 16 is preferably an organic compound that can isolate moisture and oxygen, such as polyimide or epoxy resin, or an inorganic material such as SiN. 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.

[0082] In some examples, the substrate 10 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.

[0083] In some examples, the first bias resistor layer 17 is made of a conductive material with high resistivity, preferably ITO, but other materials such as IZO, ZnO, IGO, IGZO, W, Mn, Cr, Ti, Ni, constantan alloy, manganese copper alloy, nickel-chromium alloy, and iron-chromium-aluminum alloy can also be used. It can be a single material or a stack of various materials. The thickness of the first bias resistor layer 17 ranges from 1 nm to 10 μm.

[0084] In some embodiments, the first electrical isolation layer 18 is selected from insulating materials, preferably Si3N4, but can also be selected from materials such as SiO2, Al2O3, AlN, BN, etc., and can be a single material or a stack of various materials. The thickness of the electrical isolation layer ranges from 1 nm to 10 μm.

[0085] 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 of these metals. The thickness of the first electrode 11 ranges from 1 nm to 10 μm.

[0086] 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 0.25Ti 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.

[0087] 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 these metals. The thickness of the second electrode 13 ranges from 1 nm to 10 μm.

[0088] Next, the fabrication method of the bulk acoustic resonator in the second example will be described. For example... Figure 8 As shown, the preparation method specifically includes the following steps.

[0089] S21. Provide a substrate 10.

[0090] Taking a monocrystalline silicon substrate as an example, step S21 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.

[0091] S22. A first bias resistor layer 17 is formed on the substrate 10.

[0092] Specifically, step S22 can first involve depositing a thin film of a high-resistivity conductive material. The preferred deposition method is radio frequency magnetron sputtering (DC magnetron sputtering is also acceptable), but pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), thermal evaporation, electron beam evaporation, or attaching a metal foil or alloy foil can also be used. The conductive material thin film undergoes photolithography, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Finally, etching is performed, preferably using a wet etching process, but a dry etching process can also be chosen to complete the fabrication of the first bias resistor layer 17.

[0093] S23. A first electrical isolation layer 18 is formed on the side of the first bias resistor layer 17 away from the substrate 10.

[0094] Specifically, step S23 may include first depositing an electrically insulating material, the deposition method of which may include radio frequency control sputtering, pulsed laser sputtering (PLD), atomic layer deposition (ALD), or plasma chemical vapor deposition (PECVD). The electrically insulating material undergoes a photolithography process, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Finally, etching is performed, preferably using a wet etching process, but a dry etching process can also be selected to complete the fabrication of the first electrically insulating layer 18.

[0095] S24. A first electrode 11 is formed on the side of the substrate 10 away from the first electrical isolation layer 18.

[0096] When the first electrode 11 is made of a metal material, step S24 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 the first electrode 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 the first electrode.

[0097] S25. A piezoelectric layer 12 is formed on the side of the first electrode 11 away from the substrate 10.

[0098] Taking the piezoelectric layer 12 as an example of an AlN monolayer structure, step S25 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.

[0099] S26. A second electrode 13 is formed on the side of the piezoelectric layer 12 opposite to the first electrode 11.

[0100] When the second electrode 13 is made of a metallic material, step S26 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.

[0101] S27. An encapsulation layer 16 is formed on the side of the second electrode 13 away from the substrate 10.

[0102] Taking the encapsulation layer 16 as an example of using an organic compound material, step S27 may specifically include: firstly, coating with an organic liquid material, which may be done by spin coating, spraying, inkjet printing, transfer, etc., and then heating and curing to form the pattern of the encapsulation layer 16.

[0103] S28. The substrate 10 with the above structure is flipped and etched to form a first cavity 101 that penetrates along the thickness direction of the substrate 10.

[0104] In some examples, step S28 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 101, and finally, a resist removal process is performed.

[0105] The third example: Figure 9 This is a schematic diagram of the structure of a bulk acoustic resonator according to a third example of an embodiment of this disclosure; as shown Figure 9 As shown, the bulk acoustic wave resonator includes a substrate 10, and a first bias resistor layer 17, a first electrical isolation layer 18, a first electrode 11, a piezoelectric layer 12, a second electrode 13, a second electrical isolation layer 19, and a second bias resistor layer 110 sequentially disposed on the substrate 10. The orthographic projections of any two of the first electrode 11, the piezoelectric layer 12, and the second electrode 13 onto the substrate 10 at least partially overlap. The substrate 10 has a first cavity 101 extending through its thickness direction. In this configuration, after a radio frequency (RF) signal is input into the bulk acoustic wave resonator, it is converted into an acoustic wave signal at the interface between the second electrode 13 and the piezoelectric layer 12 through the inverse piezoelectric effect. This acoustic wave propagates longitudinally within the piezoelectric layer 12, and upon reaching the interface between the first electrode 11 and the piezoelectric layer 12, it is converted back into an RF signal through the piezoelectric effect, finally exiting the resonator. The first cavity 101 below the bulk acoustic resonator and the air layer above it act as acoustic reflectors. Their function is to confine the acoustic signal within the resonator structure rather than dissipate it, thereby reducing the loss of the bulk acoustic resonator.

[0106] During real-time frequency compensation, a DC bias voltage is applied between the first bias resistor layer 17 and the second bias resistor layer 110, generating a DC electric field in the piezoelectric layer 12. When the direction of the DC electric field is from the first bias resistor layer 17 to the second bias resistor layer 110, the electric field strength increases from 0 to +100 V / µm with increasing DC bias voltage, and the resonant frequency of the bulk acoustic wave resonator shifts to lower frequencies. When the direction of the DC electric field is from the second bias resistor layer 110 to the first bias resistor layer 17, the electric field strength increases from 0 to -100 V / µm with increasing DC bias voltage, and the resonant frequency of the bulk acoustic wave resonator shifts to higher frequencies. Under the modulation of the DC electric field, the resonant frequency of the bulk acoustic wave resonator shifts linearly in a controlled manner, with a shift range of 28–58 ppm·f0 / (V / µm). For AlN-doped piezoelectric materials, the shift range is greater than that for undoped AlN piezoelectric materials. Here, f0 is the initial resonant frequency without DC bias voltage, and V / µm is the electric field strength applied to the piezoelectric layer 12. In a specific application scenario, when the ambient temperature changes, the resonant frequency of the bulk acoustic wave resonator will experience temperature drift (shifting to lower frequencies when the temperature rises and shifting to higher frequencies when the temperature falls), which in turn causes the filter curve of the filter to drift due to temperature. At this time, by adjusting the DC bias voltage applied to the piezoelectric layer 12, the resonant frequency of the bulk acoustic wave resonator is moved in the opposite direction to the temperature drift, so as to achieve the final effect of keeping the resonant frequency unchanged and ensuring that the filter curve of the filter remains unchanged when the ambient temperature changes.

[0107] The third example, compared to the second example, adds an upper second electrical isolation layer 19 and a second bias resistor layer 110 on the second electrode 13, which completely isolates the DC signal and the radio frequency signal on both sides of the bulk acoustic resonator, so that no DC blocker is needed outside the piezoelectric filter.

[0108] In some examples, an encapsulation layer 16 is also provided on the side of the second electrode 13 facing away from the substrate 10 to isolate moisture and oxygen and prevent device damage.

[0109] Furthermore, the material of the encapsulation layer 16 is preferably an organic compound that can isolate moisture and oxygen, such as polyimide or epoxy resin, or an inorganic material such as SiN. 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.

[0110] In some examples, the substrate 10 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.

[0111] In some examples, both the first bias resistor layer 17 and the second bias resistor layer 110 are made of conductive materials with high resistivity, preferably ITO, but IZO, ZnO, IGO, IGZO, W, Mn, Cr, Ti, Ni, constantan alloy, manganese copper alloy, nickel-chromium alloy, iron-chromium-aluminum alloy, etc., can also be selected. They can be a single material or a stack of various materials. The thickness of the first bias resistor layer 17 ranges from 1 nm to 10 μm.

[0112] In some embodiments, both the first electrical isolation layer 18 and the second electrical isolation layer 19 can be made of insulating materials, preferably Si3N4, but also SiO2, Al2O3, AlN, BN, etc., and can be a single material or a stack of various materials. The thickness of the electrical isolation layer ranges from 1 nm to 10 μm.

[0113] 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 of these metals. The thickness of the first electrode 11 ranges from 1 nm to 10 μm.

[0114] 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.

[0115] 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 these metals. The thickness of the second electrode 13 ranges from 1 nm to 10 μm.

[0116] Next, the fabrication method of the third example of a bulk acoustic resonator will be described. Figure 10 As shown, the preparation method specifically includes the following steps.

[0117] S31, Provide a substrate 10.

[0118] Taking a monocrystalline silicon substrate as an example, step S31 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.

[0119] S32. A first bias resistor layer 17 is formed on the substrate 10.

[0120] Specifically, step S32 can first involve depositing a thin film of a high-resistivity conductive material. The preferred deposition method is radio frequency magnetron sputtering (DC magnetron sputtering is also acceptable), but pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), thermal evaporation, electron beam evaporation, or attaching a metal foil or alloy foil can also be used. The conductive material thin film undergoes photolithography, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Finally, etching is performed, preferably using a wet etching process, but a dry etching process can also be chosen to complete the fabrication of the first bias resistor layer 17.

[0121] S33. A first electrical isolation layer 18 is formed on the side of the first bias resistor layer 17 away from the substrate 10.

[0122] Specifically, step S33 may include first depositing an electrically insulating material, the deposition method of which may include radio frequency control sputtering, pulsed laser sputtering (PLD), atomic layer deposition (ALD), or plasma chemical vapor deposition (PECVD). The electrically insulating material undergoes a photolithography process, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Finally, etching is performed, preferably using a wet etching process, but a dry etching process can also be selected to complete the fabrication of the first electrically insulating layer 18.

[0123] S34. A first electrode 11 is formed on the side of the substrate 10 away from the first electrical isolation layer 18.

[0124] When the first electrode 11 is made of a metal material, step S34 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 the first electrode 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 the first electrode.

[0125] S35. A piezoelectric layer 12 is formed on the side of the first electrode 11 away from the substrate 10.

[0126] Taking the piezoelectric layer 12 as an example of an AlN monolayer structure, step S35 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.

[0127] S36. A second electrode 13 is formed on the side of the piezoelectric layer 12 opposite to the first electrode 11.

[0128] When the second electrode 13 is made of a metallic material, step S36 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.

[0129] S37. A second electrical isolation layer 19 on the side of the second electrode 13 facing away from the substrate 10.

[0130] The second electrical isolation layer 19 is formed using the same process as the first electrical isolation layer 18, so it will not be described in detail here.

[0131] S38. A second bias resistor layer 110 is formed in the first step where the second electrical isolation layer 19 is away from the substrate 10.

[0132] The second bias resistor layer 110 is formed using the same process as the first bias resistor layer 17, so it will not be described in detail here.

[0133] S39. An encapsulation layer 16 is formed on the side of the second electrode 13 away from the substrate 10.

[0134] Taking the encapsulation layer 16 as an example of using an organic compound material, step S39 may specifically include first coating the organic material liquid, which may be done by spin coating, spraying, inkjet printing, transfer printing, etc., and then heating and curing to form the pattern of the encapsulation layer 16.

[0135] S310, the substrate 10 with the above structure is flipped and etched to form a first cavity 101 that penetrates along the thickness direction of the substrate 10.

[0136] In some examples, step S310 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 101, and finally, a resist removal process is performed.

[0137] Fourth example: Figure 11 This is a schematic diagram of the structure of a bulk acoustic resonator according to a fourth example of an embodiment of this disclosure; as shown Figure 11 As shown, the bulk acoustic wave resonator includes a substrate 10, and at least one acoustic mirror structure 15, a first electrode 11, a piezoelectric layer 12, and a second electrode 13 sequentially disposed on the substrate 10. The orthogonal projections of any two of the first electrode 11, the piezoelectric layer 12, and the second electrode 13 onto the substrate 10 at least partially overlap. The mirror structure 15 includes a first substructure and a second substructure sequentially disposed along a direction away from the first substrate 10, and the acoustic impedance of the material of the first substructure is greater than that of the material of the second substructure. For ease of description and understanding, the first substructure is referred to as the high acoustic impedance layer 151, and the second substructure as the low acoustic impedance layer 152. In this case, after the radio frequency signal is input into the bulk acoustic wave resonator, it is converted into an acoustic wave signal at the interface between the second electrode 13 and the piezoelectric layer 12 through the inverse piezoelectric effect and propagates longitudinally in the piezoelectric layer 12. When it reaches the interface between the first electrode 11 and the piezoelectric layer 12, it is converted back into a radio frequency signal through the piezoelectric effect and finally emitted from the resonator. The first cavity 101 below the bulk acoustic resonator and the air layer above it act as acoustic reflectors. Their function is to confine the acoustic signal within the resonator structure rather than dissipate it, thereby reducing the loss of the bulk acoustic resonator.

[0138] During real-time frequency compensation, a DC bias voltage is applied between the first electrode 11 and the second electrode 13, generating a DC electric field in the piezoelectric layer 12. When the direction of the DC electric field is from the first electrode 11 to the second electrode 13, the electric field strength increases from 0 to +100V / um as the DC bias voltage increases, and the resonant frequency of the bulk acoustic wave resonator shifts to a lower frequency. When the direction of the DC electric field is from the second electrode 13 to the first electrode 11, the electric field strength increases from 0 to -100V / um as the DC bias voltage increases, and the resonant frequency of the bulk acoustic wave resonator shifts to a higher frequency. Under DC electric field modulation (to prevent DC signals from mixing into the RF signal and breaking down the RF circuit, a DC blocker needs to be added in the RF circuit outside the bulk acoustic wave resonator), the resonant frequency of the bulk acoustic wave resonator shifts linearly in a controlled manner. The shift range is 28–58 ppm·f0 / (V / um). For AlN-doped piezoelectric materials, the shift range is greater than that of undoped AlN piezoelectric layer 12, where f0 is the initial resonant frequency without DC bias, and V / um is the electric field strength applied to piezoelectric layer 12. In a specific application scenario, when the ambient temperature changes, the resonant frequency of the bulk acoustic wave resonator will experience temperature drift (shifting to lower frequencies when heated and to higher frequencies when cooled), which in turn causes temperature drift in the filter curve. At this time, by adjusting the DC bias applied to piezoelectric layer 12, the resonant frequency of the bulk acoustic wave resonator is shifted in the opposite direction to the temperature drift, achieving the final effect of constant resonant frequency and ensuring that the filter curve remains unchanged when the ambient temperature changes.

[0139] In some examples, an encapsulation layer 16 is also provided on the side of the second electrode 13 facing away from the substrate 10 to isolate moisture and oxygen and prevent device damage.

[0140] Furthermore, the material of the encapsulation layer 16 is preferably an organic compound that can isolate moisture and oxygen, such as polyimide or epoxy resin, or an inorganic material such as SiN. 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.

[0141] In some examples, the substrate 10 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.

[0142] 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.

[0143] 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 of these metals. The thickness of the first electrode 11 ranges from 1 nm to 10 μm.

[0144] 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) Tax 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.

[0145] 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 these metals. The thickness of the second electrode 13 ranges from 1 nm to 10 μm.

[0146] Next, the fabrication method of the fourth example of a bulk acoustic resonator will be explained. For example... Figure 12 As shown, the preparation method specifically includes the following steps.

[0147] S41, Provide a substrate 10.

[0148] Taking a monocrystalline silicon substrate as an example, step S41 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.

[0149] S42. A mirror structure 15 is formed on the substrate 10.

[0150] Step S42 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.

[0151] S43. A first electrode 11 is formed on the substrate 10.

[0152] When the first electrode 11 is made of a metal material, step S43 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 the first electrode 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 the first electrode.

[0153] S44. A piezoelectric layer 12 is formed on the side of the first electrode 11 away from the substrate 10.

[0154] Taking the piezoelectric layer 12 as an example of an AlN monolayer structure, step S44 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.

[0155] S45. A second electrode 13 is formed on the side of the piezoelectric layer 12 opposite to the first electrode 11.

[0156] When the second electrode 13 is made of a metallic material, step S45 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.

[0157] S46. An encapsulation layer 16 is formed on the side of the second electrode 13 away from the substrate 10.

[0158] Taking the encapsulation layer 16 as an example of using an organic compound material, step S46 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.

[0159] Fifth example: Figure 13 This is a schematic diagram of the structure of a bulk acoustic resonator according to a fifth example of an embodiment of this disclosure; as shown Figure 13 As shown, the bulk acoustic wave resonator includes a substrate 10, and at least one acoustic mirror structure 15, a first bias resistor layer 17, a first electrical isolation layer 18, a first electrode 11, a piezoelectric layer 12, and a second electrode 13 sequentially disposed on the substrate 10. The orthogonal projections of any two of the first electrode 11, the piezoelectric layer 12, and the second electrode 13 onto the substrate 10 at least partially overlap. The mirror structure 15 includes a first substructure and a second substructure sequentially disposed along a direction away from the first substrate 10, and the acoustic impedance of the material of the first substructure is greater than that of the material of the second substructure. For ease of description and understanding, the first substructure is referred to as the high acoustic impedance layer 151, and the second substructure as the low acoustic impedance layer 152. In this case, after the radio frequency signal is input into the bulk acoustic wave resonator, it is converted into an acoustic wave signal at the interface between the second electrode 13 and the piezoelectric layer 12 through the inverse piezoelectric effect and propagates longitudinally in the piezoelectric layer 12. When it reaches the interface between the first electrode 11 and the piezoelectric layer 12, it is converted back into a radio frequency signal through the piezoelectric effect and finally emitted from the resonator. The first cavity 101 below the bulk acoustic resonator and the air layer above it act as acoustic reflectors. Their function is to confine the acoustic signal within the resonator structure rather than dissipate it, thereby reducing the loss of the bulk acoustic resonator.

[0160] During real-time frequency compensation, a DC bias voltage is applied between the first bias resistor layer 17 and the second electrode 13, generating a DC electric field in the piezoelectric layer 12. When the direction of the DC electric field is from the first bias resistor layer 17 to the second electrode 13, the electric field strength increases from 0 to +100 V / µm with the increase of the DC bias voltage, and the resonant frequency of the bulk acoustic wave resonator shifts to a lower frequency. When the direction of the DC electric field is from the second electrode 13 to the first bias resistor layer 17, the electric field strength increases from 0 to -100 V / µm with the increase of the DC bias voltage, and the resonant frequency of the bulk acoustic wave resonator shifts to a higher frequency. Under the modulation of the DC electric field (to prevent DC signals from mixing into the RF signals and breaking down the RF circuit, a DC blocker needs to be added in the RF circuit outside the bulk acoustic wave resonator chip), the resonant frequency of the bulk acoustic wave resonator shifts linearly in a controlled manner, with a shift range of 28 to 58 ppm·f0 / (V / µm). For AlN piezoelectric materials, the shift range is greater than that for undoped AlN piezoelectric materials. Where f0 is the initial resonant frequency without DC bias, and V / um is the electric field strength applied to the piezoelectric layer 12. In a specific application scenario, when the ambient temperature changes, the resonant frequency of the bulk acoustic wave resonator experiences temperature drift (shifting to lower frequencies when heated and to higher frequencies when cooled), which in turn causes temperature drift in the filter's filtering curve. By adjusting the DC bias applied to the piezoelectric layer 12, the resonant frequency of the bulk acoustic wave resonator is shifted in the opposite direction to the temperature drift, achieving the final effect of a constant resonant frequency and ensuring that the filter's filtering curve remains unchanged regardless of ambient temperature variations.

[0161] In the fifth example, compared to the fourth example, a first bias resistor layer 17 and a first electrical isolation layer 18 are added between the first electrode 11 and the acoustic mirror structure 15 to isolate the DC signal and the radio frequency signal of the lower half of the bulk acoustic resonator.

[0162] In some examples, an encapsulation layer 16 is also provided on the side of the second electrode 13 facing away from the substrate 10 to isolate moisture and oxygen and prevent device damage.

[0163] Furthermore, the material of the encapsulation layer 16 is preferably an organic compound that can isolate moisture and oxygen, such as polyimide or epoxy resin, or an inorganic material such as SiN. 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.

[0164] In some examples, the substrate 10 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.

[0165] 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.

[0166] In some examples, the first bias resistor layer 17 is made of a conductive material with high resistivity, preferably ITO, but other materials such as IZO, ZnO, IGO, IGZO, W, Mn, Cr, Ti, Ni, constantan alloy, manganese copper alloy, nickel-chromium alloy, and iron-chromium-aluminum alloy can also be used. It can be a single material or a stack of various materials. The thickness of the first bias resistor layer 17 ranges from 1 nm to 10 μm.

[0167] In some embodiments, the first electrical isolation layer 18 is selected from insulating materials, preferably Si3N4, but can also be selected from materials such as SiO2, Al2O3, AlN, BN, etc., and can be a single material or a stack of various materials. The thickness of the electrical isolation layer ranges from 1 nm to 10 μm.

[0168] 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 of these metals. The thickness of the first electrode 11 ranges from 1 nm to 10 μm.

[0169] 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.

[0170] 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 these metals. The thickness of the second electrode 13 ranges from 1 nm to 10 μm.

[0171] Next, the fabrication method of the fifth example of a bulk acoustic resonator will be described. Figure 14 As shown, the preparation method specifically includes the following steps.

[0172] S51, Provide substrate 10.

[0173] Taking a monocrystalline silicon substrate as an example, step S51 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.

[0174] S52. A mirror structure 15 is formed on the substrate 10.

[0175] Step S52 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, a photoresist is coated (or sprayed) onto the high acoustic impedance layer 151 thin film, followed by pre-baking, exposure, development, post-baking, and etching 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, a photoresist is coated (or sprayed) onto the low acoustic impedance layer 152 thin film, followed by pre-baking, exposure, development, post-baking, and etching 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.

[0176] S53. A first bias resistor layer 17 is formed on the side of the reflector structure 15 away from the substrate 10.

[0177] Specifically, step S53 can first involve depositing a high-resistivity conductive material thin film. The preferred deposition method is radio frequency magnetron sputtering (DC magnetron sputtering is also acceptable), but pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), thermal evaporation, electron beam evaporation, or attaching metal or alloy foils can also be used. The conductive material thin film undergoes photolithography, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Finally, etching is performed, preferably using a wet etching process, but a dry etching process can also be chosen to complete the fabrication of the first bias resistor layer 17.

[0178] S54. A first electrical isolation layer 18 is formed on the side of the first bias resistor layer 17 away from the substrate 10.

[0179] Specifically, step S54 may include first depositing an electrically insulating material, the deposition method of which can be selected from radio frequency control sputtering, pulsed laser sputtering (PLD), atomic layer deposition (ALD), or plasma chemical vapor deposition (PECVD). The electrically insulating material undergoes a photolithography process, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Finally, etching is performed, preferably using a wet etching process, but a dry etching process can also be selected to complete the fabrication of the first electrically insulating layer 18.

[0180] S55, A first electrode 11 is formed on the first electrical isolation layer 18 away from the substrate 10.

[0181] When the first electrode 11 is made of a metal material, step S55 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 the first electrode 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 the first electrode.

[0182] S56. A piezoelectric layer 12 is formed on the side of the first electrode 11 away from the substrate 10.

[0183] Taking the piezoelectric layer 12 as an example of an AlN monolayer structure, step S56 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.

[0184] S57. A second electrode 13 is formed on the side of the piezoelectric layer 12 opposite to the first electrode 11.

[0185] When the second electrode 13 is made of a metallic material, step S57 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.

[0186] S58. An encapsulation layer 16 is formed on the side of the second electrode 13 away from the substrate 10.

[0187] Taking the encapsulation layer 16 as an example of using an organic compound material, step S58 may specifically include first coating the organic material liquid, which may be done by spin coating, spraying, inkjet printing, transfer, etc., and then heating and curing to form the pattern of the encapsulation layer 16.

[0188] Sixth example: Figure 15 This is a schematic diagram of the structure of a bulk acoustic resonator according to a sixth example of an embodiment of this disclosure; as shown Figure 15 As shown, the bulk acoustic wave resonator includes a substrate 10, and at least one acoustic mirror structure 15, a first bias resistor layer 17, a first electrical isolation layer 18, a first electrode 11, a piezoelectric layer 12, a second electrode 13, a second electrical isolation layer 19, and a second bias resistor layer 110 sequentially disposed on the substrate 10. The orthogonal projections of any two of the first electrode 11, the piezoelectric layer 12, and the second electrode 13 onto the substrate 10 at least partially overlap. The mirror structure 15 includes a first substructure and a second substructure sequentially disposed along a direction away from the first substrate 10, and the acoustic impedance of the material of the first substructure is greater than the acoustic impedance of the material of the second substructure. For ease of description and understanding, the first substructure is referred to as the high acoustic impedance layer 151, and the second substructure is referred to as the low acoustic impedance layer 152. In this configuration, after the radio frequency signal is input into the bulk acoustic resonator, it is converted into an acoustic signal through the inverse piezoelectric effect at the interface between the second electrode 13 and the piezoelectric layer 12. This acoustic signal propagates longitudinally within the piezoelectric layer 12, and upon reaching the interface between the first electrode 11 and the piezoelectric layer 12, it is converted back into a radio frequency signal through the piezoelectric effect, finally exiting the resonator. The first cavity 101 below the bulk acoustic resonator and the air layer above it act as acoustic reflectors, confining the acoustic signal within the resonator structure rather than dissipating it, thus reducing the loss of the bulk acoustic resonator.

[0189] During real-time frequency compensation, a DC bias voltage is applied between the first bias resistor layer 17 and the second bias resistor layer 110, generating a DC electric field in the piezoelectric layer 12. When the direction of the DC electric field is from the first bias resistor layer 17 to the second bias resistor layer 110, the electric field strength increases from 0 to +100 V / µm with increasing DC bias voltage, and the resonant frequency of the bulk acoustic wave resonator shifts to lower frequencies. When the direction of the DC electric field is from the second bias resistor layer 110 to the first bias resistor layer 17, the electric field strength increases from 0 to -100 V / µm with increasing DC bias voltage, and the resonant frequency of the bulk acoustic wave resonator shifts to higher frequencies. Under the modulation of the DC electric field, the resonant frequency of the bulk acoustic wave resonator shifts linearly in a controlled manner, with a shift range of 28–58 ppm·f0 / (V / µm). For AlN-doped piezoelectric materials, the shift range is greater than that for undoped AlN piezoelectric materials. Here, f0 is the initial resonant frequency without DC bias voltage, and V / µm is the electric field strength applied to the piezoelectric layer 12. In a specific application scenario, when the ambient temperature changes, the resonant frequency of the bulk acoustic wave resonator will experience temperature drift (shifting to lower frequencies when the temperature rises and shifting to higher frequencies when the temperature falls), which in turn causes the filter curve of the filter to drift due to temperature. At this time, by adjusting the DC bias voltage applied to the piezoelectric layer 12, the resonant frequency of the bulk acoustic wave resonator is moved in the opposite direction to the temperature drift, so as to achieve the final effect of keeping the resonant frequency unchanged and ensuring that the filter curve of the filter remains unchanged when the ambient temperature changes.

[0190] The sixth example, compared to the fifth example, adds an upper second electrical isolation layer 19 and a second bias resistor layer 110 on the second electrode 13, which completely isolates the DC signal and the radio frequency signal on both sides of the bulk acoustic resonator, so that no DC blocker is required outside the piezoelectric filter.

[0191] In some examples, an encapsulation layer 16 is also provided on the side of the second electrode 13 facing away from the substrate 10 to isolate moisture and oxygen and prevent device damage.

[0192] Furthermore, the material of the encapsulation layer 16 is preferably an organic compound that can isolate moisture and oxygen, such as polyimide or epoxy resin, or an inorganic material such as SiN. 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.

[0193] In some examples, the substrate 10 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.

[0194] 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.

[0195] In some examples, the materials of the first bias resistor layer 17 and the second bias resistor layer 110 can both be selected from conductive materials with high resistivity, preferably ITO, but IZO, ZnO, IGO, IGZO, W, Mn, Cr, Ti, Ni, constantan alloy, manganese copper alloy, nickel-chromium alloy, iron-chromium-aluminum alloy, etc. can also be selected. It can be a single material or a stack of various materials. The thickness of the first bias resistor layer 17 ranges from 1 nm to 10 μm.

[0196] In some embodiments, the materials of the first electrical isolation layer 18 and the second electrical isolation layer 19 can both be insulating materials, preferably Si3N4, but also SiO2, Al2O3, AlN, BN, etc., and can be a single material or a stack of various materials. The thickness of the electrical isolation layer ranges from 1 nm to 10 μm.

[0197] 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 of these metals. The thickness of the first electrode 11 ranges from 1 nm to 10 μm.

[0198] 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 14Materials 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.

[0199] 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 these metals. The thickness of the second electrode 13 ranges from 1 nm to 10 μm.

[0200] Next, the fabrication method of the sixth example of the bulk acoustic resonator will be explained. Figure 16 As shown, the preparation method specifically includes the following steps.

[0201] S61, Provide a substrate 10.

[0202] Taking a monocrystalline silicon substrate as an example, step S61 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.

[0203] S62. A mirror structure 15 is formed on the substrate 10.

[0204] Step S62 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.

[0205] S63. A first bias resistor layer 17 is formed on the side of the reflector structure 15 away from the substrate 10.

[0206] Specifically, step S63 can first involve depositing a thin film of a high-resistivity conductive material. The preferred deposition method is radio frequency magnetron sputtering (DC magnetron sputtering is also acceptable), but pulsed laser sputtering (PLD), molecular beam epitaxy (MBE), thermal evaporation, electron beam evaporation, or attaching a metal foil or alloy foil can also be used. The conductive material thin film undergoes photolithography, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Finally, etching is performed, preferably using a wet etching process, but a dry etching process can also be chosen to complete the fabrication of the first bias resistor layer 17.

[0207] S64. A first electrical isolation layer 18 is formed on the side of the first bias resistor layer 17 away from the substrate 10.

[0208] Specifically, step S64 may include first depositing an electrically insulating material, the deposition method of which may include radio frequency control sputtering, pulsed laser sputtering (PLD), atomic layer deposition (ALD), or plasma chemical vapor deposition (PECVD). The electrically insulating material undergoes a photolithography process, including resist coating (or spraying), pre-baking, exposure, development, and post-baking. Finally, etching is performed, preferably using a wet etching process, but a dry etching process can also be selected to complete the fabrication of the first electrically insulating layer 18.

[0209] S65. A first electrode 11 is formed on the first electrical isolation layer 18 away from the substrate 10.

[0210] When the first electrode 11 is made of a metal material, step S65 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 the first electrode 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 the first electrode.

[0211] S66. A piezoelectric layer 12 is formed on the side of the first electrode 11 away from the substrate 10.

[0212] Taking the piezoelectric layer 12 as an example of an AlN monolayer structure, step S56 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.

[0213] S67. A second electrode 13 is formed on the side of the piezoelectric layer 12 opposite to the first electrode 11.

[0214] When the second electrode 13 is made of a metallic material, step S67 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.

[0215] S68. A second electrical isolation layer 19 on the side of the second electrode 13 facing away from the substrate 10.

[0216] The second electrical isolation layer 19 is formed using the same process as the first electrical isolation layer 18, so it will not be described in detail here.

[0217] S69. A second bias resistor layer 110 is formed in the first step where the second electrical isolation layer 19 is away from the substrate 10.

[0218] The second bias resistor layer 110 is formed using the same process as the first bias resistor layer 17, so it will not be described in detail here.

[0219] S610, an encapsulation layer 16 is formed on the side of the second electrode 13 away from the substrate 10.

[0220] Taking the encapsulation layer 16 as an example of using an organic compound material, step S610 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.

[0221] It should be noted that in the above example of the bulk acoustic wave resonator, only the formation of the first electrical isolation layer 18 and the first bias resistor layer 17 on the side of the first electrode 11 close to the substrate 10, and the formation of the first electrical isolation layer 18 and the first bias resistor layer 17 on the side of the first electrode 11 close to the substrate 10, while the formation of the second electrical isolation layer 19 and the second bias resistor layer 110 on the side of the second electrode 13 away from the substrate 10, should be understood. It is also feasible to form the second electrical isolation layer 19 and the second bias resistor layer 110 only on the side of the second electrode 13 away from the substrate 10.

[0222] This disclosure also provides an electronic device that may include any of the above-described bulk acoustic resonators.

[0223] 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 bulk acoustic resonator, comprising: Substrate, first electrode, piezoelectric layer, and second electrode; The first electrode is disposed on the substrate, the second electrode is disposed on the side of the first electrode facing away from the substrate, the piezoelectric layer is disposed between the first electrode and the second electrode, and the orthographic projections of any two of the first electrode, the piezoelectric layer, and the second electrode on the substrate at least partially overlap; wherein, the bulk acoustic resonator further includes: A first bias resistor layer is disposed on the side of the first electrode near the substrate, and a first electrical isolation layer is disposed between the first bias resistor layer and the first electrode; the first bias resistor layer is made of a high resistivity material, so that when a DC voltage is applied between the first bias resistor layer and the second electrode, the change in the resonant frequency of the bulk acoustic wave resonator is opposite to the temperature drift of the resonant frequency of the bulk acoustic wave resonator; and / or, A second bias resistor layer is disposed on the side of the second electrode away from the substrate, and a second electrical isolation layer is disposed between the second bias resistor layer and the second electrode; the material of the second bias resistor layer is a high resistivity material, so that when a DC voltage is applied between the first bias resistor layer and the second bias resistor layer, the change in the resonant frequency of the bulk acoustic wave resonator is opposite to the temperature drift of the resonant frequency of the bulk acoustic wave resonator.

2. The bulk acoustic resonator according to claim 1, wherein, The substrate has a first cavity extending through it along its thickness direction.

3. The bulk acoustic resonator according to claim 1, wherein, It also includes at least one reflective mirror structure disposed on the side of the substrate near the first electrode; when the first bias resistor layer is disposed on the substrate, the reflective mirror structure is disposed on the side of the first bias resistor layer near the substrate. The mirror structure includes a first substructure layer and a second substructure layer arranged sequentially along the direction away from the substrate, and the acoustic impedance of the material of the first substructure layer is greater than the acoustic impedance of the material of the second substructure layer.

4. The bulk acoustic resonator according to claim 1, wherein, When the bulk acoustic resonator includes the first bias resistor layer, the material of the first bias resistor layer includes any one of the following materials: ITO, IZO, ZnO, IGO, IGZO, W, Mn, Cr, Ti, Ni, constantan alloy, manganese copper alloy, nickel-chromium alloy, and iron-chromium-aluminum alloy, in a single-layer structure or a multi-layer structure.

5. The bulk acoustic resonator according to claim 1, wherein, When the bulk acoustic resonator includes the second bias resistor layer, the second bias resistor layer includes a single-layer structure or a stacked structure of multiple materials selected from ITO, IZO, ZnO, IGO, IGZO, W, Mn, Cr, Ti, Ni, constantan alloy, manganese copper alloy, nickel-chromium alloy, and iron-chromium-aluminum alloy.

6. The bulk acoustic resonator according to claim 1, wherein, When the bulk acoustic resonator includes the first electrical isolation layer, the material of the first electrical isolation layer includes a single-layer structure of any one of Si3N4, SiO2, Al2O3, AlN, and BN, or a stacked structure of multiple materials.

7. The bulk acoustic resonator according to claim 1, wherein, When the bulk acoustic resonator includes the second electrical isolation layer, the material of the second electrical isolation layer includes a single-layer structure of any one of Si3N4, SiO2, Al2O3, AlN, and BN, or a stacked structure of multiple materials.

8. The bulk acoustic resonator according to claim 1, wherein, The substrate includes any one of glass, Si, sapphire, SiC, GaAs, GaN, InP, BN, ZnO, and GaO.

9. The bulk acoustic resonator according to claim 1, wherein, The piezoelectric material of the piezoelectric layer includes AlN, doped AlN, 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; wherein the doped AlN includes 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 Any one of N.

10. The bulk acoustic resonator according to claim 1, 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. A method for fabricating a bulk acoustic resonator, comprising: The step of sequentially forming a first electrode, a piezoelectric layer, and a second electrode on a first substrate, wherein the orthographic projections of any two of the first electrode, the piezoelectric layer, and the second electrode on the first substrate at least partially overlap; wherein... The fabrication method further includes: forming a first bias resistor layer on the side of the first electrode near the substrate, and forming a first electrical isolation layer between the first bias resistor layer and the first electrode; the first bias resistor layer is made of a high resistivity material, so that when a DC voltage is applied between the first bias resistor layer and the second electrode, the change in the resonant frequency of the bulk acoustic wave resonator is opposite to the temperature drift of the resonant frequency of the bulk acoustic wave resonator; and / or, A second bias resistor layer is formed on the side of the second electrode away from the substrate, and a second electrical isolation layer is formed between the second bias resistor layer and the second electrode; the material of the second bias resistor layer is a high resistivity material, so that when a DC voltage is applied between the first bias resistor layer and the second bias resistor layer, the change in the resonant frequency of the bulk acoustic wave resonator is opposite to the temperature drift of the resonant frequency of the bulk acoustic wave resonator.

12. The method for fabricating a bulk acoustic resonator according to claim 11, wherein, Also includes: The substrate is processed to form a first cavity that extends through the thickness direction of the substrate.

13. The method for fabricating a bulk acoustic resonator according to claim 11, wherein, At least one reflective mirror structure is formed on the side of the substrate near the first electrode; when the first bias resistor layer is formed on the substrate, the reflective mirror structure is formed on the side of the first bias resistor layer near the substrate. The formation of the reflector structure includes sequentially forming a first substructure layer and a second substructure layer along a direction away from the substrate, wherein the acoustic impedance of the material of the first substructure layer is greater than the acoustic impedance of the material of the second substructure layer.

14. The method for fabricating a bulk acoustic resonator according to claim 11, wherein, When the bulk acoustic resonator includes the first bias resistor layer, the material of the first bias resistor layer includes any one of the following materials: ITO, IZO, ZnO, IGO, IGZO, W, Mn, Cr, Ti, Ni, constantan alloy, manganese copper alloy, nickel-chromium alloy, and iron-chromium-aluminum alloy, in a single-layer structure or a multi-layer structure.

15. The method for fabricating a bulk acoustic resonator according to claim 11, wherein, When the bulk acoustic resonator includes the second bias resistor layer, the second bias resistor layer includes a single-layer structure or a stacked structure of multiple materials selected from ITO, IZO, ZnO, IGO, IGZO, W, Mn, Cr, Ti, Ni, constantan alloy, manganese copper alloy, nickel-chromium alloy, and iron-chromium-aluminum alloy.

16. The method for fabricating a bulk acoustic resonator according to claim 11, wherein, When the bulk acoustic resonator includes the first electrical isolation layer, the material of the first electrical isolation layer includes a single-layer structure of any one of Si3N4, SiO2, Al2O3, AlN, and BN, or a stacked structure of multiple materials.

17. The method for fabricating a bulk acoustic resonator according to claim 11, wherein, When the bulk acoustic resonator includes the second electrical isolation layer, the material of the second electrical isolation layer includes a single-layer structure of any one of Si3N4, SiO2, Al2O3, AlN, and BN, or a stacked structure of multiple materials.

18. The method for fabricating a bulk acoustic resonator according to claim 11, wherein, The substrate includes any one of glass, Si, sapphire, SiC, GaAs, GaN, InP, BN, ZnO, and GaO.

19. The method for fabricating a bulk acoustic resonator according to claim 11, wherein, The piezoelectric material of the piezoelectric layer includes AlN, doped AlN, 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; wherein the doped AlN includes 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 Any one of N.

20. The method for fabricating a bulk acoustic resonator according to claim 11, wherein, Both the first electrode and the second electrode include any one of Mo, Al, Cu, Co, Ag, Ti, Pt, Ru, W, and Au.

21. An electronic device comprising the bulk acoustic resonator according to any one of claims 1-10.

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