Stacked piezoelectric functional layer, method for manufacturing the same, and bulk acoustic wave resonator

Abstract

The application belongs to the technical field of microelectronics and MEMS manufacturing, and particularly relates to a laminated piezoelectric functional layer, a preparation method thereof and a bulk acoustic resonator. The preparation method comprises the following steps: under vacuum conditions, performing activation treatment on a first wafer by different plasmas respectively; taking the piezoelectric film layers of the first wafer and a second wafer as bonding surfaces, directly bonding the first wafer and the second wafer after the activation treatment, forming an amorphous transition layer at the bonding interface, and obtaining the laminated piezoelectric functional layer. By integrating the piezoelectric films of the first wafer and the second wafer through wafer-level hetero-bonding, the problem of quality deterioration of the piezoelectric films layer by layer in the traditional sequential deposition process is avoided, and it is ensured that each layer of the piezoelectric films in the laminated structure has high crystalline quality and low defect density, so that the effective electromechanical coupling coefficient and the quality factor of the bulk acoustic resonator can be significantly improved, and the laminated piezoelectric functional layer is suitable for high-frequency and large-bandwidth radio frequency front-end filter applications.

Description

Technical Field

[0001] This invention belongs to the field of microelectronics and MEMS manufacturing technology, specifically relating to a stacked piezoelectric functional layer, its preparation method, and a bulk acoustic resonator. Background Technology

[0002] Bulk acoustic wave (BAW) resonators are key components in the core architecture of wireless communication RF front-end filters, duplexers, and other similar devices. As mobile communication technology advances towards higher frequencies, wider bandwidths, and greater integration, higher demands are placed on the operating frequency, bandwidth, insertion loss, and power capacity of BAW resonators. The bandwidth of a BAW resonator is primarily determined by the effective electromechanical coupling coefficient, while insertion loss, frequency selectivity, and power capacity are closely related to the quality factor. Therefore, achieving both a high effective electromechanical coupling coefficient and a high quality factor simultaneously in high-frequency applications is crucial for improving the performance of BAW resonators.

[0003] Currently, mainstream bulk acoustic wave resonators typically employ a sandwich structure with a single piezoelectric layer sandwiched between upper and lower electrodes. The operating frequency of this structure is primarily influenced by the thickness of the piezoelectric layer. As the operating frequency increases, the piezoelectric layer thickness needs to be further reduced, making the effects of thin-film defects, interface damage, residual stress, and thickness fluctuations on device performance more significant. To improve the electromechanical coupling performance of single-layer piezoelectric structures, material modification methods such as elemental doping are commonly used. However, these methods suffer from complex processes, difficulty in controlling composition, and may introduce lattice defects and microstructure inhomogeneities, leading to increased acoustic wave scattering and energy loss, thus affecting the device's quality factor. Therefore, single-layer piezoelectric structures struggle to simultaneously meet the requirements of wide bandwidth, low insertion loss, and high power capacity in high-frequency applications, becoming a significant bottleneck limiting further performance improvements in bulk acoustic wave resonators.

[0004] To overcome the aforementioned technical bottlenecks, multilayer piezoelectric structures have gradually become an important development direction. This structure, by stacking multiple piezoelectric layers with electrode layers, utilizes the coupling and superposition effects of acoustic waves in multilayer media to potentially improve the effective electromechanical coupling coefficient and power capacity of bulk acoustic wave resonators. However, the performance of multilayer piezoelectric structures heavily depends on the crystal quality of each piezoelectric film. Existing multilayer piezoelectric film fabrication processes mostly employ top-down deposition processes such as physical vapor deposition (PVD), where a lower electrode, a first piezoelectric layer, a second piezoelectric layer, and a top electrode are deposited sequentially on a substrate to fabricate the multilayer piezoelectric structure. This makes the crystal quality of the upper piezoelectric film highly dependent on the crystal orientation and surface morphology of the lower piezoelectric film. This dependence results in an extremely narrow process window, making it difficult to ensure high orientation, low internal stress, and uniform thickness of each piezoelectric film while simultaneously achieving low-defect interlayer interfaces. Therefore, high-quality fabrication of multilayer piezoelectric films and low-damage interlayer integration have become the core obstacles currently limiting the high-performance applications of multilayer bulk acoustic wave resonators. Summary of the Invention

[0005] To address the technical problems inherent in existing top-down deposition processes for fabricating multilayer piezoelectric structures, such as layer-by-layer deterioration of piezoelectric film quality, numerous interlayer interface defects, and uncontrollable integration quality, this invention provides a multilayer piezoelectric functional layer, its fabrication method, and a bulk acoustic resonator.

[0006] This invention provides a method for fabricating a multilayer piezoelectric functional layer, comprising the following steps: Under vacuum conditions, a first plasma activation treatment is performed on a first piezoelectric thin film layer of a first wafer to obtain a first activated piezoelectric thin film layer. A second plasma activation treatment is performed on a second piezoelectric thin film layer of a second wafer to obtain a second activated piezoelectric thin film layer. The first plasma and the second plasma have different ion types. The first activated piezoelectric thin film layer and the second activated piezoelectric thin film layer are bonded together under a pressure of 20KN to 40KN to form an amorphous transition layer at the bonding interface between the first activated piezoelectric thin film layer and the second activated piezoelectric thin film layer, thus obtaining a stacked piezoelectric functional layer formed by bonding the first activated piezoelectric thin film layer and the second activated piezoelectric thin film layer.

[0007] It should be noted that the piezoelectric thin film surfaces of the first wafer and the second wafer are activated by plasma with different ion types to construct asymmetric surface states in the same bonding system. The piezoelectric thin film surface of the first wafer is physically bombarded by a first ion beam to achieve atomic-level cleanliness and form high-density dangling bonds. The piezoelectric thin film surface of the second wafer is reactively activated by a second ion beam to introduce functional groups with chemical bonding activity to the dangling bonds. Thus, during bonding, atomic-level interfacial connections are formed through the directional reaction between the dangling bonds and the functional groups.

[0008] Preferably, the first plasma is Ar plasma and the second plasma is CF4 plasma.

[0009] Preferably, the conditions for the first plasma activation treatment and the second plasma activation treatment are the same, and the conditions for both the first plasma activation treatment and the second plasma activation treatment are: power of 50W, gas flow rate of 20sccm, and time of 180s.

[0010] In the bonding process of this invention, the piezoelectric thin film layers of the activated first wafer and the second wafer are used as bonding surfaces. The activated first wafer is flipped and then bonded to the second wafer. As a result, in the stacked piezoelectric functional layer formed after bonding, the first activated piezoelectric thin film layer and the second activated piezoelectric thin film layer have opposite polarization directions along the thickness direction.

[0011] Preferably, both the first wafer and the second wafer are POI wafers; the POI wafer is formed from a silicon substrate and a transition layer and a piezoelectric thin film layer sequentially stacked on the silicon substrate.

[0012] Preferably, the transition layer is a SiO2 layer or a metal layer; the thickness of the transition layer is 50 nm to 500 nm.

[0013] Preferably, the material of the piezoelectric thin film layer is lithium niobate, lithium tantalate, lead magnesium niobate-lead titanate, or lead zirconate titanate; the thickness of the piezoelectric thin film layer is 100 nm to 1000 nm.

[0014] Preferably, the thickness of the silicon substrate is 300 μm to 700 μm. Preferably, the metal layer material is selected from at least one of Mo, Al, Pt, Au and Ti.

[0015] The present invention provides a stacked piezoelectric functional layer, which is prepared by the above-mentioned method for preparing a stacked piezoelectric functional layer; the stacked piezoelectric functional layer is formed by bonding a first activated piezoelectric thin film layer and a second activated piezoelectric thin film layer.

[0016] This invention provides a bulk acoustic wave resonator, comprising: a package cap structure; a passivation layer bonded to the package cap structure to form a sealed cavity; a multilayer piezoelectric transducer structure disposed within the sealed cavity, comprising a frame structure electrode, a stacked piezoelectric functional layer, and a lower electrode stacked sequentially from top to bottom; an electrode connecting post vertically penetrating the multilayer piezoelectric transducer structure and electrically connected to the lower electrode, constituting the first driving electrode port of the bulk acoustic wave resonator; a support structure disposed on the multilayer piezoelectric transducer structure, comprising a supporting silicon layer and a patterned SiO2 layer located below it; and an outgoing electrode structure vertically penetrating the support structure and electrically connected to the frame structure electrode, constituting the second driving electrode port of the bulk acoustic wave resonator.

[0017] Compared with the prior art, the present invention has the following technical effects: This invention primarily involves plasma activation treatment of piezoelectric thin film layers on a first and second wafer using different ion types. The first activated piezoelectric thin film layer is then bonded to the second activated piezoelectric thin film layer using a direct bonding process to form a stacked piezoelectric functional layer with opposite polarization directions. This ensures that each piezoelectric thin film layer possesses independent high crystallinity and a low-defect interlayer interface. This stacked piezoelectric functional layer avoids the dependence of the second piezoelectric thin film layer on the crystal orientation and surface morphology of the first piezoelectric thin film layer in top-down deposition processes, ensuring that each piezoelectric thin film layer in the stacked functional layer has independent high crystallinity, uniform thickness, and excellent process consistency.

[0018] This invention utilizes plasma activation treatment with different ion types on the piezoelectric thin film layers of a first wafer and a second wafer to construct asymmetric surface states in the same bonding system. This results in high-density dangling bonds forming on one side of the surface and the introduction of functional groups with chemical bonding activity on the other side. During bonding, atomic-level interfacial connections are achieved, thereby improving the bonding strength and interfacial continuity between the piezoelectric thin films, resulting in a first activated piezoelectric thin film layer and a second activated piezoelectric thin film layer.

[0019] In the stacked piezoelectric functional layer prepared by this invention, the first and second piezoelectric layers have opposite polarization directions, enabling the stacked piezoelectric functional layer to generate a piezoelectric response with a specific phase relationship under the action of an external electric field. For non-target low-order modes or stray modes, the responses of the two piezoelectric films can partially cancel each other out, thereby achieving a suppression effect. For target high-order resonant modes where the strain direction is reversed in the thickness direction, the piezoelectric responses of the two piezoelectric layers can be superimposed in phase, thereby enhancing the electroacoustic conversion efficiency of the target mode and improving mode selectivity and spectral purity.

[0020] This invention enables higher frequency resonance without significantly reducing the total thickness of the piezoelectric film, avoiding the problems of difficult fabrication, reduced crystal quality, increased electrode loss, and reduced power handling capacity inherent in traditional ultrathin piezoelectric film solutions. It also reduces reliance on elemental doping to improve the electromechanical coupling coefficient, and lowers acoustic losses caused by doping defects and lattice distortion. This results in a balance of high effective electromechanical coupling coefficient, high quality factor, low insertion loss, and good power capacity, making it suitable for the mass production of high-frequency, high-performance RF filters, duplexers, and multi-band acoustic devices. Attached Figure Description

[0021] Figure 1 A process flow diagram for fabricating a bulk acoustic resonator in Application Example 1.

[0022] Figure 2 The diagram shows the structure of the bulk acoustic resonator prepared in Example 1.

[0023] Reference numerals: 101-First silicon substrate; 102-First SiO2 transition layer; 103-First PMN-PT piezoelectric thin film layer; 201-Second silicon substrate; 202-Second SiO2 transition layer; 203-Second PMN-PT piezoelectric thin film layer; 111-Lower electrode; 121-Electrode connection post; 131-Passivation layer; 141-Encapsulation cap structure; 211-Frame structure electrode; 221-Patternized SiO2 layer; 231-Supporting silicon; 241-Lead electrode post; 251-Pat; 261-Metal solder ball.

[0024] Figure 3The figure shows the COMSOL simulation results of the single-layer PMN-PT bulk acoustic resonator and the bulk acoustic resonator in Application Example 1. Detailed Implementation

[0025] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below in conjunction with specific embodiments and accompanying drawings.

[0026] Unless otherwise specified, all reagents used in this invention are commercially available, and all methods used are conventional techniques in the art.

[0027] Figure 2 This is a structural diagram of the bulk acoustic wave resonator prepared in Example 1. The bulk acoustic wave resonator of the present invention is a top-cap cavity device realized by wafer-level heterogeneous integration and three-dimensional packaging technology. Its overall structure is divided into a transducer structure, a support structure, and an interconnect structure from top to bottom.

[0028] Specifically, the transducer structure includes an encapsulation cap structure 141, a passivation layer 131, a multilayer piezoelectric transducer structure, and an electrode connection post 121. The multilayer piezoelectric transducer structure is disposed within a sealed cavity enclosed by the encapsulation cap structure 141 and the passivation layer 131. The multilayer piezoelectric transducer structure is formed by a lower electrode 111, a stacked piezoelectric functional layer, and a frame structure electrode 211 stacked sequentially from bottom to top. The stacked piezoelectric functional layer is formed by direct bonding of a first piezoelectric layer 103 and a second piezoelectric layer 203. The passivation layer 131 covers the surface of the multilayer piezoelectric transducer structure to protect its interior. The electrode connection post 121 vertically penetrates the stacked piezoelectric functional layer and is electrically connected to the lower electrode 111, forming the first driving electrode port of the bulk acoustic resonator.

[0029] The support structure is disposed above the multilayer piezoelectric transducer structure and is used to provide mechanical support and electrical connection. The support lead-out structure includes a patterned SiO2 layer 221 and a supporting silicon 231 arranged sequentially above the frame structure electrode 211.

[0030] The interconnect structure includes lead-out electrode posts 241, pads 251, and solder balls 261. The lead-out electrode posts 241 vertically penetrate the support structure and are formed by filling conductive material into TSV vias formed in the support structure. The lead-out electrode posts 241 form electrical contacts with the frame structure electrodes 211, constituting the second drive electrode port of the bulk acoustic wave resonator. The pads 251 are located at the top of the lead-out electrode posts 241, and the solder balls 261 are disposed on the upper end of the pads 251 for external electrical connection.

[0031] Thus, the multilayer piezoelectric transducer structure of the present invention achieves electrical lead-out through the frame structure electrode 211 leading out electrode posts 241, pads 251, and metal solder balls 261, forming the second driving electrode port of the bulk acoustic wave resonator; the lower electrode 111 of the multilayer piezoelectric transducer structure achieves electrical lead-out through electrode connection posts 121. This three-dimensional interconnect architecture, with the upper and lower electrodes respectively leading out from the top and bottom of the device, minimizes the electrical interconnect path and reduces parasitic effects.

[0032] The raw materials used in the following examples are from the following sources: POI wafer, Xi'an Jiaotong University (Model: 4-inch POI substrate, structure PMN-PT / SiO2 / Si), where PMN-PT is a piezoelectric thin film layer, Chinese name lead magnesium niobate-lead titanate, with a thickness of 150nm; SiO2 is a transition layer, with a thickness of 100nm; Si is a silicon substrate with a resistivity greater than 10kΩ·cm, and crystal orientation is... <100> Acetone, purchased from Suzhou Jingrui Chemical Co., Ltd. (SCCC) or Fujifilm Wako Pure Chemical Co., Ltd., with a purity of UP-SS or SC (electronic grade) and a metal impurity content of less than 1 ppb. Isopropanol, purchased from Suzhou Jingrui Chemical Co., Ltd. (SCCC) or Fujifilm Wako Pure Chemical Co., Ltd., with a purity of UP-SS or SC (electronic grade) and a metal impurity content of less than 1 ppb. Deionized water, with a resistivity greater than 18.2 MΩ·cm (at 25°C), prepared using a Milli-Q ultrapure water system. Nitrogen, high-purity nitrogen with a purity of 99.999% (5N grade).

[0033] Example 1 A method for fabricating a multilayer piezoelectric functional layer includes the following steps: Two sets of POI wafers with identical polarization and orientation were prepared. Each POI wafer was formed from a silicon substrate and a SiO2 transition layer and a PMN-PT piezoelectric thin film layer sequentially stacked on the silicon substrate. The silicon substrate thickness was 500 μm, the SiO2 transition layer thickness was 100 nm, and the PMN-PT piezoelectric thin film layer thickness was 150 nm. The first and second POI wafers were ultrasonically cleaned sequentially with acetone, isopropanol, and deionized water, with each cleaning step lasting 5 minutes. They were then dried with nitrogen and baked at 80°C for 15 minutes to remove surface-adsorbed water and organic residues. The cleaned first and second POI wafers were then loaded into the vacuum chamber of a surface activated bonding (SAB) apparatus and evacuated to a vacuum level of 5 × 10⁻⁶. -6To reduce the impact of residual gas and impurities in the cavity on the bonding interface, the surface of the second POI wafer is shielded, and the PMN-PT piezoelectric thin film layer of the first POI wafer is activated by Ar plasma at a power of 50 W, an Ar gas flow rate of 20 sccm, and a treatment time of 180 s, resulting in a first activated piezoelectric thin film layer. Then, the surface of the first POI wafer is shielded, and the PMN-PT piezoelectric thin film layer of the second POI wafer is activated by O2 plasma at a power of 50 W, an O2 gas flow rate of 20 sccm, and a treatment time of 180 s, resulting in a second activated piezoelectric thin film layer.

[0034] After in-situ surface activation, the first and second POI wafers are aligned in the same vacuum chamber, so that the first and second activated piezoelectric thin film layers are positioned opposite each other and in contact. A bonding pressure of 20 kN is applied under vacuum and maintained for 20 minutes to complete direct bonding. A uniform amorphous transition layer is formed at the bonding interface, which significantly improves the interfacial bonding strength.

[0035] The silicon substrate of the second POI wafer was thinned to 10 μm using a chemical mechanical polishing process. Subsequently, the thinned second POI wafer was placed in a 25 wt.% tetramethylammonium hydroxide aqueous solution and etched at 80°C until the silicon substrate was completely removed. The silicon substrate-removed second POI wafer was then placed in a 5 wt.% HF aqueous solution to remove the SiO2 transition layer, thereby exposing the PMN-PT piezoelectric thin film layer of the second POI wafer.

[0036] The silicon substrate of the first POI wafer was thinned to 10 μm using a chemical mechanical polishing process. The thinned first POI wafer was then placed in a 20 wt.% tetramethylammonium hydroxide aqueous solution and etched at 80°C until the silicon substrate was completely removed. Subsequently, the first POI wafer with the silicon substrate removed was placed in a 5 wt.% HF aqueous solution to remove the SiO2 transition layer, thereby exposing the PMN-PT piezoelectric thin film layer of the first POI wafer, resulting in a stacked piezoelectric wafer composed of a second piezoelectric layer 203 (the PMN-PT piezoelectric thin film layer of the second POI wafer is referred to as the second piezoelectric layer) and a first piezoelectric layer 103 (the PMN-PT piezoelectric thin film layer of the first POI wafer is referred to as the first piezoelectric layer).

[0037] Application Example 1 like Figure 2As shown, the bulk acoustic wave resonator includes an encapsulation cap structure 141; a passivation layer 131 bonded to the encapsulation cap structure 141, forming a sealed cavity; a multilayer piezoelectric transducer structure disposed within the sealed cavity, including a frame structure electrode 211, a stacked piezoelectric functional layer, and a lower electrode 111 stacked sequentially from top to bottom; the stacked piezoelectric functional layer is formed by bonding a first activated piezoelectric thin film layer and a second activated piezoelectric thin film layer; an electrode connecting post 121 vertically penetrating the multilayer piezoelectric transducer structure and electrically connected to the lower electrode 111, constituting the first driving electrode port of the bulk acoustic wave resonator; a support structure disposed on the multilayer piezoelectric transducer structure, including a supporting silicon layer 231 and a patterned SiO2 layer 221 located below it; and an outgoing electrode structure 241 vertically penetrating the support structure and electrically connected to the frame structure electrode 211, constituting the second driving electrode port of the bulk acoustic wave resonator.

[0038] The fabrication method of a bulk acoustic resonator includes the following steps: Step 1: Prepare two sets of POI wafers with identical polarization and orientation. The POI wafer is formed from a silicon substrate and a SiO2 transition layer and a PMN-PT piezoelectric thin film layer sequentially stacked on the silicon substrate. The silicon substrate thickness is 500 μm, the SiO2 transition layer thickness is 100 nm, and the PMN-PT piezoelectric thin film layer thickness is 150 nm. Figure 1 As shown in Figure a, the first POI wafer is formed from a first silicon substrate 101 and a first SiO2 transition layer 102 and a first PMN-PT piezoelectric thin film layer 103 sequentially stacked on the first silicon substrate. The second POI wafer is formed from a second silicon substrate 201 and a second SiO2 transition layer 202 and a second PMN-PT piezoelectric thin film layer 203 sequentially stacked on the second silicon substrate 201.

[0039] The first and second POI wafers were ultrasonically cleaned sequentially with acetone, isopropanol, and deionized water, with each cleaning step lasting 5 minutes. They were then dried with nitrogen and baked at 80°C for 15 minutes to remove surface-adsorbed water and organic residues. The cleaned first and second POI wafers were then loaded into the vacuum chamber of a surface activation bonding apparatus, and a vacuum of 5 × 10⁻⁶ was applied. -6Pa is used to reduce the impact of residual gas and impurities in the cavity on the bonding interface. The surface of the second POI wafer is shielded, and the first PMN-PT piezoelectric thin film layer 103 of the first POI wafer is subjected to Ar plasma activation treatment with a power of 50 W, an Ar gas flow rate of 20 sccm, and a treatment time of 180 s. Then, the surface of the first POI wafer is shielded, and the second PMN-PT piezoelectric thin film layer 203 of the second POI wafer is subjected to O2 plasma activation treatment with a power of 50 W, an O2 gas flow rate of 20 sccm, and a treatment time of 180 s.

[0040] After completing the in-situ surface activation treatment, the first PMN-PT piezoelectric thin film layer 103 and the second PMN-PT piezoelectric thin film layer 203 are used as the bonding surfaces, such as Figure 1 As shown in b, the first POI wafer and the second POI wafer are aligned in the same vacuum chamber, so that the first PMN-PT piezoelectric thin film layer 103 of the first POI wafer and the second PMN-PT piezoelectric thin film layer 203 of the second POI wafer are positioned opposite each other and in contact. A bonding pressure of 20 kN is applied in a vacuum environment and maintained for 20 minutes to complete direct bonding. A uniform amorphous transition layer is formed at the bonding interface, which can significantly improve the interfacial bonding strength.

[0041] Step 2: The second silicon substrate 201 of the second POI wafer is thinned to 10 μm using a chemical mechanical polishing process. Subsequently, the thinned second POI wafer is placed in a 25 wt.% tetramethylammonium hydroxide aqueous solution and etched at 80°C until the silicon substrate is completely removed. The second POI wafer with the silicon substrate removed is then placed in a 5 wt.% HF aqueous solution to remove the second SiO2 transition layer 202, thereby exposing the second PMN-PT piezoelectric thin film layer 203, simply referred to as the second piezoelectric layer 203.

[0042] Step 3: Deposit the first Mo metal on the surface of the second piezoelectric layer 203 using DC magnetron sputtering. Before deposition, evacuate the sputtering chamber to a base vacuum level not exceeding 8 × 10⁻⁶. -5 Pa, and Ar plasma was used to perform in-situ cleaning of the surface of the second piezoelectric layer 203, with a cleaning power of 50W and a cleaning time of 60s. Then, Ar gas was introduced at a flow rate of 30sccm and a working pressure of 0.5Pa. DC magnetron sputtering was performed using a Mo target with a purity of 99.99%, a sputtering power of 200W, a substrate temperature of room temperature, and a deposition thickness of 100nm. Subsequently, the first Mo metal layer was patterned using photolithography and dry etching processes to form the frame structure electrode 211 of the bulk acoustic wave resonator, as shown below. Figure 1As shown in d, the geometry and boundary conditions of the frame structure electrode 211 are used to confine acoustic wave energy, suppress stray modes, and improve the electromechanical conversion efficiency and spectral purity of the resonator.

[0043] Step 4: Using plasma-enhanced chemical vapor deposition (PECVD), a SiO2 layer is deposited on the framework structure electrode 211 in three stages, with each deposition layer being 1 μm thick, resulting in a total SiO2 layer thickness of 3 μm. Subsequently, the surface of the SiO2 layer is planarized using chemical mechanical polishing (CMP). Then, a patterned SiO2 layer 221 is formed using photolithography and dry etching processes, creating windows on the framework structure electrode 211 to expose a portion of the electrode for subsequent electrical lead-out, such as... Figure 1 As shown in e.

[0044] Step 5, as follows Figure 1 As shown in f, a silicon substrate is used as the supporting silicon 231, and the supporting silicon 231 is bonded to the patterned SiO2 layer 221 to form a support structure composed of the supporting silicon 231 and the patterned SiO2 layer 221.

[0045] Step 6, as follows Figure 1 As shown in g, the first silicon substrate of the first POI wafer is thinned to 10 μm using a chemical mechanical polishing process. The thinned first POI wafer is then placed in a 20 wt.% tetramethylammonium hydroxide aqueous solution and etched at 80°C until the silicon substrate is completely removed. Subsequently, the first POI wafer with the silicon substrate removed is placed in a 5 wt.% HF aqueous solution to remove the first SiO2 transition layer 102, thereby exposing the first PMN-PT piezoelectric thin film layer 103, simply referred to as the first piezoelectric layer 103.

[0046] Step 7, as follows Figure 1 As shown in h, a second Mo metal layer is deposited on the surface of the first piezoelectric layer 103 using a DC magnetron sputtering process and then patterned to serve as the lower electrode 111. Before deposition, the sputtering chamber is evacuated to a base vacuum level not exceeding 8 × 10⁻⁻⁻⁶. 5The surface of the first piezoelectric layer 103 was cleaned in situ using Ar plasma at a power of 50 W for 60 seconds. Then, Ar gas was introduced at a flow rate of 30 sccm and a working pressure of 0.5 Pa. A Mo target with a purity of at least 99.99% was used for DC magnetron sputtering at a power of 200 W, resulting in a deposition thickness of 100 nm. Subsequently, the second Mo metal layer was patterned using photolithography and dry etching to form a piezoelectric etching window as the lower electrode 111, exposing a portion of the first piezoelectric layer 103. The lower electrode 111 was etched using SF6 / Ar plasma at a power of 300 W and a chamber pressure of 5 mTorr, resulting in a multilayer piezoelectric transducer structure consisting of a frame structure electrode 211, the second piezoelectric layer 203, the first piezoelectric layer 103, and the lower electrode 111.

[0047] Step 8, as follows Figure 1 As shown in Figure i, the second piezoelectric layer 203 and the first piezoelectric layer 103 are patterned and etched using photolithography and ICP dry etching processes to expose the pad area for fabricating the bottom electrode connection. In this pad connection area, an AlCu alloy layer with a Cu content of 1 wt.% and a deposition thickness of approximately 2 μm is deposited using DC magnetron sputtering. Before deposition, the cavity is evacuated to 8 × 10⁻⁻⁻⁸. 5 The pressure was below Pa, and in-situ cleaning was performed using Ar plasma for 60 seconds. During deposition, the Ar flow rate was 30 sccm, the working pressure was 0.5 Pa, and the sputtering power was 300 W. Subsequently, the AlCu alloy layer was patterned by photolithography and Cl2 / BCl3 / Ar dry etching process to form electrode connection pillars 121 electrically connected to the lower electrode 111.

[0048] Step 9, as follows Figure 1 As shown in j, a silicon nitride passivation layer 131 is formed on the bottom, sidewalls and surface of electrode connection post 121 of the lower electrode 111 by chemical vapor deposition process. This layer is used to protect the internal structure from damage caused by subsequent processes and environmental factors, and to ensure the long-term reliability of the device.

[0049] Step 10, as follows Figure 1 As shown in k, a protective cavity structure with a thickness of 15 μm, namely the encapsulation cap structure 141, is formed on a 750 μm thick silicon wafer using a dry film patterning process. Then, the encapsulation cap structure 141 is bonded to the surface of the passivation layer 131 to form a sealed cavity, thereby sealing the multilayer piezoelectric transducer structure within it. The inner wall of the cavity of the encapsulation cap structure 141 avoids the edges of the electrode pattern to ensure reliable bonding.

[0050] Step 11, as follows Figure 1As shown in Figure 1, the supporting silicon 231 was thinned to approximately 100 μm by chemical mechanical polishing. Subsequently, a photoresist with a thickness of approximately 15 μm was spin-coated onto the surface of the supporting silicon 231, and the TSV pattern was defined by photolithography. TSV etching was then performed using Bosch deep silicon etching technology. The SF6 flow rate was 120 sccm, the C4F8 flow rate was 80 sccm, the etching pressure was 20 mTorr, the ICP power was 1200 W, and the bias power was 30 W, forming TSV vias with a diameter of approximately 40 μm until the underlying framework structure electrode 211 was exposed.

[0051] Step 12, as follows Figure 1 As shown in m, a 50nm Ti barrier layer and a 200nm Cu seed layer are sequentially deposited on the TSV via structure using magnetron sputtering. The TSV vias are then filled with copper using an acidic copper sulfate electroplating process to form lead-out electrode posts 241. After electroplating, excess metal layers on the wafer surface are removed by chemical mechanical polishing, making the copper-filled structure flush with the wafer surface. Subsequently, a Cu / Ni / Au pad structure 251 is formed, where the Ni layer is approximately 5μm thick and the Au layer is approximately 0.1μm thick. Finally, SnAgCu solder balls 261 with a diameter of approximately 100μm are implanted at the pad locations, and three-dimensional interconnect packaging is completed by reflow soldering at 250℃ to obtain a bulk acoustic wave resonator.

[0052] Application Example 2 A method for fabricating a bulk acoustic resonator includes the following steps: The difference from Application Example 1 lies in the POI wafer structure used. The POI wafer is formed from a silicon substrate and a metal transition layer and a PMN-PT piezoelectric thin film layer sequentially stacked on the silicon substrate. The metal layer material is selected from Mo, and the thickness of the Mo metal transition layer is 100 nm. The Mo metal transition layer can be directly used as an electrode for the bulk acoustic wave resonator during subsequent device fabrication. By pre-introducing the Mo metal layer, the number of subsequent electrode fabrication steps can be reduced, and the interface bonding between the electrode and the piezoelectric thin film, as well as the acoustic performance, can be improved.

[0053] Therefore, in this embodiment, steps 3 and 7 of Embodiment 1, which involve metal deposition to form the upper and lower electrodes, are no longer performed. Instead, after removing the silicon substrate, the Mo metal transition layer is directly patterned to form the bottom electrode structure of the bulk acoustic wave resonator. The remaining bonding, thinning, etching, packaging, and interconnection processes are basically the same as in Embodiment 1.

[0054] Figure 3The simulation results of the impedance frequency response of the single-layer PMN-PT bulk acoustic resonator and the stacked PMN-PT bulk acoustic resonator of this invention are shown. The dashed line represents the single-layer PMN-PT resonator, and the solid line represents the stacked PMN-PT resonator of this invention. As shown in the figure, the series resonant frequency of the single-layer PMN-PT resonator is... f s The parallel resonant frequency is 1.501 GHz. f p The effective electromechanical coupling coefficient is 1.835 GHz. k 2 eff It is 36.79%; the series resonant frequency of the stacked PMN-PT resonator f s The parallel resonant frequency is 3.070 GHz. f p The effective electromechanical coupling coefficient is 3.791 GHz. k 2 eff The efficiency was 38.00%. Compared to a single-layer PMN-PT structure, the present invention, using a stacked PMN-PT piezoelectric structure formed by direct bonding, increased the series resonant frequency of the device from 1.501 GHz to 3.070 GHz and the parallel resonant frequency from 1.835 GHz to 3.791 GHz, approximately twice that of the single-layer structure. Simultaneously, the effective electromechanical coupling coefficient was slightly improved, indicating that the stacked piezoelectric structure design with opposite polarization directions constructed in this invention is beneficial for enhancing the piezoelectric response of the target vibration mode, improving the effective electromechanical coupling performance of the device, and thus further enhancing the overall performance of the bulk acoustic resonator.

[0055] It should be noted that when numerical ranges are involved in this invention, it should be understood that the two endpoints of each numerical range, as well as any value between the two endpoints, can be selected. Since the steps and methods used are the same as in the embodiments, preferred embodiments are described in this invention to avoid redundancy. Although preferred embodiments of this invention have been described, those skilled in the art, once they understand the basic inventive concept, can make other changes and modifications to these embodiments, and all such changes and modifications fall within the scope of this invention.

[0056] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. If these modifications and variations fall within the scope of equivalents of this invention, then this invention also intends to include these modifications and variations.

Claims

1. A method for fabricating a multilayer piezoelectric functional layer, characterized in that, Includes the following steps: Under vacuum conditions, the first piezoelectric thin film layer of the first wafer is subjected to a first plasma activation treatment to obtain a first activated piezoelectric thin film layer, and the second piezoelectric thin film layer of the second wafer is subjected to a second plasma activation treatment to obtain a second activated piezoelectric thin film layer; wherein the first plasma and the second plasma have different ion types; The first activated piezoelectric thin film layer and the second activated piezoelectric thin film layer are bonded together under a pressure of 20KN to 40KN to form an amorphous transition layer at the bonding interface between the first activated piezoelectric thin film layer and the second activated piezoelectric thin film layer, thus obtaining a stacked piezoelectric functional layer formed by bonding the first activated piezoelectric thin film layer and the second activated piezoelectric thin film layer.

2. The method for preparing the multilayer piezoelectric functional layer according to claim 1, characterized in that, The first plasma is Ar plasma, and the second plasma is CF4 plasma.

3. The method for preparing the multilayer piezoelectric functional layer according to claim 2, characterized in that, The conditions for the first plasma activation treatment and the second plasma activation treatment are the same: power of 50W, gas flow rate of 20sccm, and time of 180s.

4. The method for preparing the multilayer piezoelectric functional layer according to claim 1, characterized in that, The first activated piezoelectric thin film layer and the second activated piezoelectric thin film layer in the stacked piezoelectric functional layer have opposite polarization directions along the thickness direction.

5. The method for preparing the multilayer piezoelectric functional layer according to claim 1, characterized in that, Both the first and second wafers are POI wafers; POI wafers are formed from a silicon substrate and a transition layer and a piezoelectric thin film layer that are sequentially stacked on the silicon substrate.

6. The method for preparing the multilayer piezoelectric functional layer according to claim 5, characterized in that, The materials for the piezoelectric thin film layer are LiNbO3, LiTaO3, lead magnesium niobate-lead titanate, or lead zirconate titanate; The thickness of the piezoelectric thin film layer is 100nm to 1000nm.

7. The method for preparing the multilayer piezoelectric functional layer according to claim 5, characterized in that, The transition layer is a SiO2 layer or a metal layer; the thickness of the transition layer is 50nm to 500nm.

8. A multilayer piezoelectric functional layer, characterized in that, The stacked piezoelectric functional layer is prepared by the method of any one of claims 1 to 7; the stacked piezoelectric functional layer is formed by bonding a first activated piezoelectric thin film layer and a second activated piezoelectric thin film layer.

9. A bulk acoustic wave resonator, said bulk acoustic wave resonator comprising a stacked piezoelectric functional layer, characterized in that, The stacked piezoelectric functional layer is the stacked piezoelectric functional layer as described in claim 8.

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