Bulk acoustic wave resonator with double-layer piezoelectric film structure, preparation method and filter

Abstract

The invention relates to the field of microelectronic devices, in particular to a bulk acoustic wave resonator with a double-layer piezoelectric film structure, a preparation method and a filter, the bulk acoustic wave resonator comprises a cap wafer, and a dry film, a passivation layer, a silicon oxide layer, a high-resistance silicon wafer and RDL metal are sequentially arranged on the cap wafer; the dry film and the cap wafer form a dry film cavity, the passivation layer located above the dry film cavity extends into the dry film cavity, and a top electrode, a first piezoelectric film, a middle electrode, a second piezoelectric film and a bottom electrode are sequentially arranged on the passivation layer; the top electrode and the bottom electrode are used as first ports of the resonator; and the middle electrode is a second port. According to the resonator, the excitation mode of the resonator is changed through cooperation of the two layers of piezoelectric films with the same polarization direction and the intermediate electrode, a high-order resonance mode is jointly excited, meanwhile, a high electromechanical coupling coefficient is kept, and a wide fractional bandwidth is achieved at high frequency through the relatively thick piezoelectric films; the problems that a high-frequency resonator is weak in power bearing capacity, narrow in bandwidth, low in quality factor and poor in stability are solved.

Description

Technical Field

[0001] This invention relates to the field of bulk acoustic wave resonators with a double-layer piezoelectric film structure, their fabrication method, and filters. Background Technology

[0002] Wireless communication technology, as the cornerstone of modern information society, has always been at the forefront of continuous evolution and innovation. From early analog communication to digital communication, from 2G and 3G to 4G, each technological iteration has profoundly changed people's lifestyles and social operating models. Currently, wireless communication technology is at a critical juncture, transitioning from fifth-generation (5G) to fifth-generation enhanced (5G-Advanced) and sixth-generation (6G). With its ultra-high speed, ultra-low latency, and massive connection capacity, 5G technology has ushered in a new era of the Internet of Everything, widely applied in fields such as the Industrial Internet, intelligent transportation, telemedicine, and virtual reality. 5G-Advanced, as a further evolution of 5G technology, aims to achieve higher network performance and wider application scenario coverage through technological optimization and innovation, laying a solid foundation for the research and development of 6G technology. 6G technology is highly anticipated, expected to bring even more revolutionary changes, such as holographic communication, intelligent reflective surfaces, and terahertz communication, further blurring the boundaries between the physical and digital worlds and driving society towards a deep transformation towards intelligence and digitalization.

[0003] As a crucial component of wireless communication systems, the radio frequency (RF) front-end (RF front-end) undertakes core functions such as signal transmission and reception, filtering, and amplification. Its performance directly determines the overall performance and reliability of the wireless communication system. Key components of the RF front-end, especially high-frequency broadband filters, have become one of the key factors restricting the further development of wireless communication technology. Among the many types of high-frequency broadband filters, thin-film bulk acoustic wave (FBAR) filters have been widely used in the RF front-end field due to their unique advantages. The operating frequency of FBAR filters is mainly related to the thickness of the functional layer and the velocity of sound; typically, to achieve high operating frequencies, the film thickness needs to be reduced. However, for single-crystal piezoelectric thin film materials, the growth process of single-crystal piezoelectric thin films is extremely sensitive to process conditions. Reducing the film thickness further restricts atomic migration and arrangement during growth, leading to poorer crystallinity and deteriorating piezoelectric performance. Furthermore, when resonators are cascaded to form filters, a smaller film thickness results in a smaller electrode area for impedance matching, leading to enhanced parasitic effects on the electrodes and a lower quality factor of the resonator. Furthermore, as the film thickness decreases, the mechanical strength and power handling capability of both cavity-type and back-etched bulk acoustic wave resonators decrease. When the thickness decreases to a certain extent, the effects of interface and roughness are amplified, further increasing the acoustic loss of the filter. To reduce the loss caused by thickness variations, higher-order modes of the resonator can be used. However, in higher-order modes, the effective electromechanical coupling coefficient of the resonator decreases with the square of the order. This decrease in the effective electromechanical coupling coefficient means a reduction in the efficiency of the resonator in converting electrical energy to mechanical energy, thus affecting the filtering performance of the FBAR. Therefore, while higher-order modes can extend the operating bandwidth of the FBAR to some extent, they also introduce problems such as increased insertion loss and weakened out-of-band rejection, making it difficult to meet the stringent requirements of 5G-Advanced and 6G technologies for high-frequency broadband filter performance. Therefore, designing a bulk acoustic wave filter with a moderate piezoelectric layer thickness, excellent effective electromechanical coupling coefficient, and high-frequency applicability is an urgent problem to be solved. Summary of the Invention

[0004] To address the problems of reduced quality factor, short lifespan, increased acoustic loss, and poor stability of resonators caused by reduced film thickness in existing technologies, this invention provides a bulk acoustic resonator with a double-layer piezoelectric film structure, its fabrication method, and a filter.

[0005] To achieve the above objectives, the present invention employs the following technical solution: This invention provides a bulk acoustic resonator with a double-layer piezoelectric film structure, comprising a cap wafer, wherein a dry film, a passivation layer, a silicon oxide layer, a high-resistivity silicon wafer, and a first RDL metal are sequentially disposed on the cap wafer; The dry film and the capping wafer form a dry film cavity. A passivation layer located above the dry film cavity extends into the dry film cavity, and a top electrode, a first piezoelectric film, an intermediate electrode, a second piezoelectric film, and a bottom electrode are sequentially disposed on the passivation layer extending into the dry film cavity. The top electrode is connected to the bottom electrode and the first RDL metal in sequence through the second RDL metal, serving as the first port of the resonator. The intermediate electrode is connected to the first RDL metal and the second RDL metal, serving as the second port of the resonator.

[0006] Optionally, the first piezoelectric film and the second piezoelectric film have the same polarization direction and a thickness of 300-1000 nm.

[0007] Optionally, the first and second piezoelectric films are AlN piezoelectric films, Al 1-x Sc x N piezoelectric thin film, general formula is y Pb(Mg 1 / 3 Nb 2 / 3 O3 (1 y PbTiO3 lead magnesium niobate-lead titanate piezoelectric thin films, with the general formula: z Pb(In 1 / 2Nb 1 / 2 O3 a Pb(Mg 1 / 3 Nb 2 / 3 O3 (1 z a PbTiO3 piezoelectric thin films of lead indium niobate-lead magnesium niobate-lead titanate, single-crystal LiTaO3 piezoelectric thin films, and single-crystal LiNbO3 piezoelectric thin films, with the general formula Pb(Zr) 1-b Ti b One or more combinations of lead zirconate titanate piezoelectric films and barium titanate piezoelectric films of O3; wherein, 0.4 ≥ x> 0, 0.1≤ y ≤0.7, 0.15≤ z ≤0.3, 0.4≤ a ≤0.55, 0.4≤ b ≤0.6.

[0008] Optionally, the top electrode, intermediate electrode, and bottom electrode are made of one or more of the following materials: gold, molybdenum, aluminum, copper, tungsten, and platinum.

[0009] Optionally, the intermediate electrode is made of molybdenum with a thickness of 50-200 nm, and the silicon oxide layer has a thickness of 2-4 μm.

[0010] The present invention also provides a method for fabricating a bulk acoustic resonator with a double-layer piezoelectric film structure as described above, comprising: An AlN seed layer, a top electrode, a first piezoelectric film, and an intermediate electrode are sequentially grown on one surface of a double-sided thermally oxidized silicon wafer. After the intermediate electrode is patterned by photolithography, a second piezoelectric film, a bottom electrode, and a silicon oxide layer are sequentially grown on the patterned intermediate electrode. The silicon oxide layer is then etched to form a cavity and expose the bottom electrode, thus obtaining a wafer. The silicon oxide layer of the wafer is bonded to the high-resistivity silicon wafer to obtain the first assembly; Remove the double-sided thermally oxidized silicon wafer and AlN seed layer from the first assembly to expose the top electrode. Then, etch the top electrode and the first piezoelectric film in sequence to expose the middle electrode. Continue etching the second piezoelectric film to expose the bottom electrode. After depositing and patterning the second RDL metal to form an electrical connection, deposit a passivation layer to obtain the second assembly. The second assembly is bonded to a cap wafer covered with a dry film and having dry film cavities etched on the dry film to obtain the third assembly. The high-resistivity silicon wafer on the third assembly is thinned, and the high-resistivity silicon wafer and silicon oxide layer are etched to form a through hole, exposing the bottom electrode. The second piezoelectric film is etched downwards to expose the middle electrode. The first RDL metal is deposited and patterned. The top electrode, bottom electrode and middle electrode are led out and solder balls are attached to obtain a bulk acoustic resonator with a double-layer piezoelectric film structure.

[0011] Optionally, the top electrode, intermediate electrode, and bottom electrode are grown by magnetron sputtering or vacuum evaporation, and the top electrode, intermediate electrode, and bottom electrode are patterned by laser direct writing or inductively coupled plasma etching.

[0012] Optionally, the first and second piezoelectric films are grown by magnetron sputtering, pulsed laser deposition, or metal-organic chemical vapor deposition (MOCVD).

[0013] Optionally, the dry film cavity can be etched on the dry film using photolithography.

[0014] A bulk acoustic wave filter includes at least one of the above-mentioned bulk acoustic wave resonators having a double-layer piezoelectric film structure.

[0015] Compared with the prior art, the present invention has the following beneficial effects: This invention provides a bulk acoustic resonator with a double-layer piezoelectric film structure, comprising a cap wafer on which a dry film, a passivation layer, a silicon oxide layer, a high-resistivity silicon wafer, and a first RDL metal are sequentially disposed. The dry film and the cap wafer form a dry film cavity. A passivation layer located above the dry film cavity extends into the dry film cavity, and a top electrode, a first piezoelectric film, an intermediate electrode, a second piezoelectric film, and a bottom electrode are sequentially disposed on the passivation layer extending into the dry film cavity. The resonator has two piezoelectric layers. By adding an intermediate electrode between the two piezoelectric films to change the signal terminal of the resonator, the two piezoelectric layers can be excited simultaneously to work together and jointly excite high-frequency acoustic waves. This not only achieves high-frequency resonance without excessively reducing the thickness of a single piezoelectric film, but also maintains an electromechanical coupling coefficient comparable to that of a single piezoelectric film of the same thickness, achieving efficient energy conversion. The dry film cavity formed by the dry film and the capping wafer, and the cavity in the silicon oxide layer, provide a relatively independent and stable space for acoustic wave propagation in the resonator. The difference in acoustic impedance between the cavity and the piezoelectric stack confines the acoustic waves within the piezoelectric stack, helping to reduce interference from the external environment and improve the resonator's quality factor, thereby enhancing its frequency selectivity and stability. The passivation layer extends into the dry film cavity, upon which electrodes and piezoelectric films are sequentially disposed. This not only enhances the overall stability and reliability of the structure but also effectively protects the internal electrodes and piezoelectric films from external environmental erosion and mechanical damage, solving the problems of reduced resonator quality factor, short lifespan, increased acoustic loss, and poor stability caused by reduced film thickness in existing technologies. This resonator meets the high-frequency performance requirements of high-frequency communication systems, providing stable and efficient frequency selection and signal processing functions for high-frequency communication equipment. In high-frequency communication terminals such as wireless communication base stations, smartphones, and IoT devices, this resonator can be applied to key components such as filters and oscillators, helping to improve the performance and reliability of communication equipment and promoting the further development of high-frequency communication technology.

[0016] The present invention also provides a method for fabricating a bulk acoustic wave resonator with a double-layer piezoelectric film structure as described above. The method involves sequentially growing an AlN seed layer, a top electrode, a first piezoelectric film, an intermediate electrode, a second piezoelectric film, a bottom electrode, and a silicon oxide layer on the surface of a double-sided thermally oxidized silicon wafer. The silicon oxide layer is then etched to expose the bottom electrode, resulting in a wafer. The oxide layer of the wafer is bonded to a high-resistivity silicon wafer, and the double-sided thermally oxidized silicon wafer and the AlN seed layer are sequentially removed to expose the top electrode. The top electrode, the first piezoelectric film, the intermediate electrode, and the second piezoelectric film are then sequentially etched to expose the intermediate electrode and the bottom electrode, respectively. A second RDL metal and a passivation layer are deposited to obtain a second assembly. The second assembly is bonded to a cap wafer covered with a dry film and having dry film cavities etched on the dry film. Through-holes are etched to expose the bottom electrode, and etching continues downwards to expose the intermediate electrode. A first RDL metal is deposited, and the top electrode, bottom electrode, and intermediate electrode are led out and solder balls are implanted to obtain a bulk acoustic wave resonator with a double-layer piezoelectric film structure. This method is simple and enables stable integration of multilayer bulk acoustic wave resonators, ensuring tight bonding between layers. It features high efficiency, stability, and low cost. The bulk acoustic wave resonators fabricated using this method can achieve high-frequency resonance while maintaining the electromechanical coupling coefficient. The filters formed by cascading these resonators have a larger electrode area, solving the parasitic effects caused by using smaller electrode areas for impedance matching due to the thinner piezoelectric material film. This further reduces the acoustic loss of the filter and improves the energy conversion efficiency, providing a reliable design solution for RF front-end filters applied in S-band, up to 6GHz, and even higher frequency bands.

[0017] A bulk acoustic wave filter includes at least one bulk acoustic wave resonator with a double-layer piezoelectric film structure as described above. Because this resonator can maintain a relatively high electromechanical coupling coefficient in higher-order resonant modes, the filter constructed from it possesses the core advantages of ultra-low loss, ultra-wide bandwidth, and high power capacity, providing a cost-effective filtering solution for 5G or 6G communication and the Internet of Things. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of a bulk acoustic resonator with a double-layer piezoelectric film structure according to the present invention.

[0019] Figure 2 This is a flowchart illustrating the fabrication method of a bulk acoustic resonator with a double-layer piezoelectric film structure according to the present invention.

[0020] Figure 3 This is a process state diagram of a method for fabricating a bulk acoustic resonator with a double-layer piezoelectric film structure according to the present invention.

[0021] Figure 4 The impedance spectrum of the thin-film bulk acoustic resonator with a double-layer piezoelectric film structure prepared in Example 1 of the present invention.

[0022] Figure 5 The impedance spectrum of a single-layer AlN thin-film bulk acoustic resonator.

[0023] Figure 6 The impedance spectrum of the thin-film bulk acoustic resonator with a double-layer piezoelectric film structure prepared in Example 2 of the present invention.

[0024] Figure 7 Single layer Al 0.7 Sc 0.3 Impedance spectrum of N-film bulk acoustic resonator.

[0025] Among them, 1-cap wafer, 2-dry film, 3-passivation layer, 4-silicon oxide layer, 5-high resistivity silicon wafer, 6-first RDL metal, 7-top electrode, 8-intermediate electrode, 9-first piezoelectric film, 10-second piezoelectric film, 11-bottom electrode, 12-second RDL metal. Detailed Implementation

[0026] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0027] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0028] The present invention will be further described in detail below with reference to specific embodiments. These descriptions are for explanation purposes only and are not intended to limit the scope of the invention.

[0029] This invention discloses a bulk acoustic resonator with a double-layer piezoelectric film structure, with reference to... Figure 1 ,include See Figure 1The present invention provides a bulk acoustic resonator with a double-layer piezoelectric film structure, including a cap wafer 1, on which a dry film 2, a passivation layer 3, a silicon oxide layer 4, a high-resistivity silicon wafer 5 and a first RDL metal 6 are sequentially disposed; The dry film 2 and the capping wafer 1 form a dry film cavity. The passivation layer 3 located above the dry film cavity extends into the dry film cavity, and a top electrode 7, a first piezoelectric film 9, an intermediate electrode 8, a second piezoelectric film 10, and a bottom electrode 11 are sequentially disposed on the passivation layer 3 extending into the dry film cavity. The top electrode 7 is connected to the bottom electrode 11 and the RDL metal 6 in sequence through the second RDL metal 12, serving as the first port of the resonator. The intermediate electrode 8 is connected to the first RDL metal 6 and the second RDL metal 12, serving as the second port of the resonator.

[0030] Preferably, the thickness of both the first piezoelectric film 9 and the second piezoelectric film 10 is 300-1000 nm. Preferably, the first piezoelectric film 9 and the second piezoelectric film 10 are AlN piezoelectric films, Al... 1-x Sc x N piezoelectric thin film, general formula is y Pb(Mg 1 / 3 Nb 2 / 3 O3 (1 y PbTiO3 lead magnesium niobate-lead titanate piezoelectric thin films, with the general formula: z Pb(In 1 / 2 Nb 1 / 2 O3 a Pb(Mg 1 / 3 Nb 2 / 3 O3 (1 z a PbTiO3 piezoelectric thin films of lead indium niobate-lead magnesium niobate-lead titanate, single-crystal LiTaO3 piezoelectric thin films, and single-crystal LiNbO3 piezoelectric thin films, with the general formula Pb(Zr) 1-b Ti b One or more combinations of lead zirconate titanate piezoelectric films and barium titanate piezoelectric films of O3; wherein, 0.4 ≥ x> 0, 0.1≤ y ≤0.7, 0.15≤ z ≤0.3, 0.4≤ a ≤0.55, 0.4≤ b≤0.6. Preferably, the top electrode 7, the middle electrode 8, and the bottom electrode 11 are made of one or more of gold, molybdenum, aluminum, copper, tungsten, and platinum; more preferably, the electrode material is molybdenum. The thickness of the silicon oxide layer 4 is 2-4 μm.

[0031] The piezoelectric layer of this resonator is a two-layer composite thin film. By using two piezoelectric thin films with the same polarization direction in conjunction with the intermediate electrode to change the excitation mode of the resonator, high-frequency resonance can be achieved, while maintaining an electromechanical coupling coefficient comparable to that of a single-layer piezoelectric film of the same thickness. This bulk acoustic wave resonator with a double-layer piezoelectric film structure can solve the problems of poor film crystallinity, poor film quality, low mechanical strength, and low quality factor caused by the small film thickness in high-frequency bulk acoustic wave resonators. In addition, the filter formed by cascading multilayer film bulk acoustic wave resonators provided by this invention has a larger electrode area, which solves the parasitic effect caused by using a small electrode area for impedance matching due to the small film thickness of the piezoelectric material. This further reduces the acoustic loss of the filter and improves the energy conversion efficiency, providing a reliable design solution for RF front-end filters applied in S-band, up 6GHz, and even higher frequency bands.

[0032] See Figure 2 and Figure 3 The present invention also provides a method for fabricating a bulk acoustic resonator with a double-layer piezoelectric film structure as described above, comprising: S1: An AlN seed layer 13, a top electrode 7, a first piezoelectric film 9, and an intermediate electrode 8 are sequentially grown on one surface of a double-sided thermally oxidized silicon wafer 12. After the intermediate electrode 8 is patterned by photolithography, a second piezoelectric film 10, a bottom electrode 11, and a silicon oxide layer 4 are sequentially grown on the patterned intermediate electrode 8. The silicon oxide layer 4 is then etched to form a cavity and expose the bottom electrode 11, thus obtaining a wafer. Preferably, the AlN seed layer 13 has a thickness of 30-120 nm and is grown by magnetron sputtering or pulsed laser deposition; the top electrode 7 has a thickness of 100-300 nm and is made of one or more of gold, molybdenum, aluminum, copper, tungsten, and platinum, preferably molybdenum, and is grown by magnetron sputtering or vacuum evaporation; the first piezoelectric film 9 has a thickness of 300-1000 nm; and the intermediate electrode 8 has a thickness of 50-200 nm and is grown using a lift-off process or by deposition followed by etching. The intermediate electrode 8 is patterned by etching to achieve interconnection of the resonators. The intermediate electrode 8 is grown by magnetron sputtering or vacuum evaporation, and the electrode material is one or more of gold, molybdenum, aluminum, copper, tungsten, and platinum, with molybdenum being preferred as the material for the intermediate electrode. The thickness of the second piezoelectric thin film 10 is 300-1000 nm. Both the first piezoelectric thin film 9 and the second piezoelectric thin film 10 are grown by magnetron sputtering, MOCVD, or pulsed laser deposition. The first piezoelectric thin film 9 and the second piezoelectric thin film 10 are AlN piezoelectric thin films, Al 1-x Sc x N piezoelectric thin film, general formula is y Pb(Mg 1 / 3 Nb 2 / 3 O3 (1 y PbTiO3 lead magnesium niobate-lead titanate piezoelectric thin films, with the general formula: z Pb(In 1 / 2 Nb 1 / 2 O3 a Pb(Mg 1 / 3 Nb 2 / 3 O3 (1 z a PbTiO3 piezoelectric thin films of lead indium niobate-lead magnesium niobate-lead titanate, single-crystal LiTaO3 piezoelectric thin films, and single-crystal LiNbO3 piezoelectric thin films, with the general formula Pb(Zr) 1-b Ti b One or more combinations of lead zirconate titanate piezoelectric films and barium titanate piezoelectric films of O3, wherein 0.4 ≥ x> 0, 0.1≤ y ≤0.7, 0.15≤ z ≤0.3, 0.4≤ a ≤0.55, 0.4≤ b≤0.6; The thickness of the bottom electrode 11 is 100-300nm, and the electrode material is one or more of gold, molybdenum, aluminum, copper, tungsten and platinum, with molybdenum being preferred as the material of the bottom electrode 11. The protruding electrode boundary and the recessed electrode structure are etched in two steps using inductively coupled plasma etching technology and frequency correction technology, and then the shape of the bottom electrode 11 of the resonator is etched; The silicon oxide layer 4 is grown by plasma-enhanced chemical vapor deposition, and the silicon oxide layer 4 is polished using chemical mechanical polishing technology. The thickness of the silicon oxide layer 4 is 2-4μm; The silicon oxide layer 4 is etched using inductively coupled plasma etching technology to expose the bottom electrode 11, and a wafer is obtained.

[0033] S2: Bond the silicon oxide layer 4 of the wafer to the high-resistivity silicon wafer 5 to obtain the first assembly; Preferably, the silicon oxide layer 4 of the wafer is bonded to the high-resistivity silicon wafer 5 using surface activation bonding technology or hydrophilic bonding technology, and the resistivity of the high-resistivity silicon wafer 5 is >5000Ω·cm.

[0034] S3: Remove the double-sided thermally oxidized silicon wafer 12 and AlN seed layer 13 from the first assembly to expose the top electrode 7, and then etch the top electrode 7 and the first piezoelectric film 9 in sequence to expose the middle electrode 8. Continue to etch the second piezoelectric film 10 to expose the bottom electrode 11, deposit and pattern the second RDL metal 12 to form an electrical connection, and deposit the passivation layer 3 to obtain the second assembly. Preferably, the silicon substrate in the first assembly is thinned by chemical mechanical polishing, then completely removed by wet etching with potassium hydroxide solution or TMAH solution, and silicon oxide and seed layer 13 are removed by HF wet etching to expose the top electrode 7, which facilitates the patterning of the top electrode 7. The top electrode 7 is patterned by laser direct writing technology or inductively coupled plasma etching technology, while leaving a window for etching the piezoelectric layer. The first piezoelectric film 9 is first etched by inductively coupled plasma etching technology to expose the intermediate electrode 8, and then the first piezoelectric film 9 and the second piezoelectric film 10 are etched to expose the bottom electrode 11. Al / Cu is electroplated to form two RDL metals 12 and pads. The Si3N4 passivation layer 3 is grown by plasma-enhanced chemical vapor deposition technology to obtain the second assembly.

[0035] S4: Bond the second assembly to the cap wafer 1 covered with dry film 2 and with dry film cavities etched on dry film 2 to obtain the third assembly; Preferably, the method for preparing the cap wafer 1 covered with dry film 2 and having dry film cavities etched on dry film 2 is as follows: dry film 2 is coated on cap wafer 1, and dry film 2 is patterned by photolithography to form dry film cavities. The second assembly was bonded to the cap wafer 1 by hot pressing to obtain the third assembly, and the high-resistivity silicon wafer 5 in the third assembly was thinned to 50-100 μm by chemical mechanical polishing.

[0036] S5: Thin the high-resistivity silicon wafer 5 on the third assembly and then etch the high-resistivity silicon wafer 5 and silicon oxide layer 4 to form a through hole, exposing the bottom electrode 11. Continue photolithography to etch the second piezoelectric film 10 downwards to expose the middle electrode 8. Deposit and pattern the first RDL metal 6. Lead out the top electrode 7, bottom electrode 11 and middle electrode 8 respectively and perform solder ball placement to obtain a bulk acoustic resonator with a double-layer piezoelectric film structure. Preferably, an inductively coupled plasma etching technique is first used to etch a first via on the high-resistivity silicon wafer 5 to expose the bottom electrode 11. Then, an inductively coupled plasma etching technique is used to etch a second piezoelectric thin film 10 to form a second via, exposing the intermediate electrode 8. Finally, a first RDL metal 6 is deposited, and the top electrode 7, bottom electrode 11 and intermediate electrode 8 are led out to the surface of the high-resistivity silicon wafer 5 and metal balls are implanted. Preferably, the RDL metal is Ti / Cu.

[0037] Example 1 An AlN seed layer 13 with a thickness of 30-120 nm is grown on the surface of a double-sided thermally oxidized silicon wafer 12 by magnetron sputtering. A 100-300 nm thick molybdenum metal layer is then grown on this AlN seed layer 13 to serve as the top electrode 7 of the resonator. The top electrode 7 is also grown by magnetron sputtering.

[0038] AlN is grown on the top electrode 7 as a first piezoelectric thin film 9 with a thickness of 300-1000 nm using magnetron sputtering. Molybdenum is grown on the first piezoelectric thin film 9 as an intermediate electrode 8 with a thickness of 50-200 nm. The intermediate electrode 8 is patterned using a lift-off process or a deposition-then-etch method to achieve interconnection of the resonators. The intermediate electrode 8 is grown by magnetron sputtering.

[0039] AlN is grown on the intermediate electrode 8 as a second piezoelectric thin film 10 with a thickness of 300-1000 nm, and the growth method is magnetron sputtering.

[0040] Molybdenum metal, with a thickness of 100-300 nm, is grown on the second piezoelectric thin film 10 by magnetron sputtering. The raised electrode boundary and the recessed electrode structure are etched in two steps using inductively coupled plasma etching (ICP-C) to form the shape of the bottom electrode 11 of the resonator.

[0041] Silicon oxide 4 is deposited over the patterned bottom electrode 11 using plasma-enhanced chemical vapor deposition. The silicon oxide 4 layer is then smoothed using chemical mechanical polishing (CMP), and this process is repeated 3-5 times until the silicon oxide layer thickness is 2-3 μm and the surface is flat. Subsequently, inductively coupled plasma etching (ICP-C) is used to etch the silicon oxide to form grooves on the bottom electrode, resulting in a wafer.

[0042] A high-resistivity silicon wafer 5 is bonded to a first silicon wafer using surface activation bonding or hydrophilic bonding techniques to form a cavity below the bottom electrode 11, resulting in a first assembly. The silicon substrate of the double-sided thermally oxidized silicon wafer 12 in the first assembly is first thinned by chemical mechanical polishing, and then completely removed by wet etching with potassium hydroxide solution or TMAH solution. Finally, HF wet etching is used to remove the silicon oxide and seed layer 3, exposing the top electrode 7 for patterning.

[0043] The top electrode 7 is patterned using inductively coupled plasma etching (ICP-E) technology, while leaving a window for etching the piezoelectric layer. First, the first piezoelectric film 9 is etched using ICP-E to expose the intermediate electrode 8. Then, the first piezoelectric film 9 and the second piezoelectric film 10 are etched to expose the bottom electrode 11. Al / Cu is electroplated to form the second RDL metal 12 and pads. A Si3N4 passivation layer 3 is grown using plasma-enhanced chemical vapor deposition (PECVD) technology to obtain the second assembly.

[0044] Dry film 2 is coated onto capped silicon wafer 1, and patterned using photolithography to form capped wafer 1 with dry film cavities. The second assembly is bonded to capped wafer 1 using thermoforming to obtain the third assembly. The high-resistivity silicon wafer 5 in the third assembly is thinned to 50-100 μm using chemical mechanical polishing.

[0045] First, an inductively coupled plasma etching (ICP-C) technique is used to etch a first via on the high-resistivity silicon wafer 5, exposing the bottom electrode 11. Then, ICP-C is used again to etch a second piezoelectric thin film 10, forming a second via and exposing the intermediate electrode 8. Finally, a first RDL metal 6 is deposited, and the top electrode 7, bottom electrode 11, and intermediate electrode 8 are led out to the silicon surface, followed by metal balling. Preferably, the RDL metal is Ti / Cu.

[0046] The bulk acoustic resonator with a double-layer piezoelectric film structure obtained using the above preparation method can achieve resonance at 6.47 GHz with a total thickness of 900 nm for the double film, and the effective electromechanical coupling coefficient is maintained at 6.5%. (See [link to documentation]). Figure 4 The resonant frequency is higher than that of a single-layer AlN thin-film bulk acoustic resonator of the same thickness, while maintaining an effective electromechanical coupling coefficient comparable to that of a single-layer AlN thin-film bulk acoustic resonator. See [link to relevant documentation]. Figure 5Meanwhile, this ensures the excellent and stable piezoelectric performance of the piezoelectric film, increasing the mechanical strength of the thin-film bulk acoustic wave resonator. Filters formed by cascading these bulk acoustic wave resonators with a double-layer piezoelectric film structure offer advantages such as large bandwidth and low loss, and can be stably applied in S-band, up to 6GHz, and even higher frequency RF front-end scenarios.

[0047] Example 2 An AlN seed layer 13 with a thickness of 30-120 nm is grown on the surface of a double-sided thermally oxidized silicon wafer 12 by magnetron sputtering. A 100-300 nm thick molybdenum metal layer is then grown on this AlN seed layer 13 to serve as the top electrode 7 of the resonator. The top electrode 7 is also grown by magnetron sputtering.

[0048] Al was grown on the top electrode 7 by magnetron sputtering. 0.7 Sc 0.3 N serves as the first piezoelectric thin film 9, with a thickness of 300-1000 nm. Molybdenum metal is grown on the first piezoelectric thin film 9 as an intermediate electrode 8, with a thickness of 50-200 nm. The intermediate electrode 8 is patterned using a lift-off process or a deposition-then-etching method to achieve interconnection of the resonators. The intermediate electrode 8 is grown by magnetron sputtering.

[0049] Al is grown on intermediate electrode 8 0.7 Sc 0.3 N, as the second piezoelectric thin film 10, has a thickness of 300-1000 nm and is grown by magnetron sputtering.

[0050] Molybdenum metal, with a thickness of 100-300 nm, is grown on the second piezoelectric thin film 10 by magnetron sputtering. The raised electrode boundary and the recessed electrode structure are etched in two steps using inductively coupled plasma etching (ICP-C) to form the shape of the bottom electrode 11 of the resonator.

[0051] Silicon oxide 4 is deposited over the patterned bottom electrode 11 using plasma-enhanced chemical vapor deposition. The silicon oxide 4 layer is then smoothed using chemical mechanical polishing (CMP), and this process is repeated 3-5 times until the silicon oxide layer thickness is 2-3 μm and the surface is flat. Subsequently, inductively coupled plasma etching (ICP-C) is used to etch the silicon oxide to form grooves on the bottom electrode, resulting in a wafer.

[0052] A high-resistivity silicon wafer 5 is bonded to a first silicon wafer using surface activation bonding or hydrophilic bonding techniques to form a cavity below the bottom electrode 11, resulting in a first assembly. The silicon substrate of the double-sided thermally oxidized silicon wafer 12 in the first assembly is first thinned by chemical mechanical polishing, and then completely removed by wet etching with potassium hydroxide solution or TMAH solution. Finally, HF wet etching is used to remove the silicon oxide and seed layer 3, exposing the top electrode 7 for patterning.

[0053] The top electrode 7 is patterned using inductively coupled plasma etching (ICP-E) technology, while leaving a window for etching the piezoelectric layer. First, the first piezoelectric film 9 is etched using ICP-E to expose the intermediate electrode 8. Then, the first piezoelectric film 9 and the second piezoelectric film 10 are etched to expose the bottom electrode 11. Al / Cu is electroplated to form the second RDL metal 12 and pads. A Si3N4 passivation layer 3 is grown using plasma-enhanced chemical vapor deposition (PECVD) technology to obtain the second assembly.

[0054] Dry film 2 is coated onto capped silicon wafer 1, and patterned using photolithography to form capped wafer 1 with dry film cavities. The second assembly is bonded to capped wafer 1 using thermoforming to obtain the third assembly. The high-resistivity silicon wafer 5 in the third assembly is thinned to 50-100 μm using chemical mechanical polishing.

[0055] First, an inductively coupled plasma etching (ICP-C) technique is used to etch a first via on the high-resistivity silicon wafer 5, exposing the bottom electrode 11. Then, ICP-C is used again to etch a second piezoelectric thin film 10, forming a second via and exposing the intermediate electrode 8. Finally, a first RDL metal 6 is deposited, and the top electrode 7, bottom electrode 11, and intermediate electrode 8 are led out to the silicon surface, followed by metal balling. Preferably, the RDL metal is Ti / Cu.

[0056] The bulk acoustic resonator with a double-layer piezoelectric film structure obtained using the above preparation method can achieve resonance at 4.904 GHz with a total thickness of 900 nm for the double-layer film, and the effective electromechanical coupling coefficient is maintained at 16.84%. (See [link to documentation]). Figure 6 Higher than a single layer of Al of the same thickness 0.7 Sc 0.3 The resonant frequency of the N-film bulk acoustic resonator remains consistent with that of the monolayer Al. 0.7 Sc 0.3 The effective electromechanical coupling coefficient of the N-film bulk acoustic resonator is equivalent to that of the N-film bulk acoustic resonator. See [link / reference]. Figure 7 Meanwhile, this ensures the excellent and stable piezoelectric properties of the piezoelectric film, increasing the mechanical strength of the thin-film bulk acoustic resonator. Filters formed by cascading these bulk acoustic resonators with a double-layer piezoelectric film structure offer advantages such as high bandwidth and low loss, and can be stably applied in S-band and even higher frequency RF front-end scenarios.

[0057] It is evident that the bulk acoustic resonator prepared using this method can achieve high-frequency resonance while maintaining the electromechanical coupling coefficient. The filter formed by cascading this resonator has a larger electrode area, which solves the parasitic effects caused by using a smaller electrode area for impedance matching due to the thinner piezoelectric material film, further reducing the acoustic loss of the filter and improving the energy conversion efficiency.

[0058] The present invention also provides a filter, comprising at least one of the aforementioned bulk acoustic wave resonators with a double-layer piezoelectric film structure. Because this resonator can maintain a relatively high electromechanical coupling coefficient in higher-order resonant modes, the filter constructed from it possesses the core advantages of ultra-low loss, ultra-wide bandwidth, and high power capacity, providing a cost-effective filtering solution for 5G or 6G communication and the Internet of Things.

[0059] The above description is merely a preferred embodiment of the present invention and is not intended to limit the technical solution of the present invention in any way. Those skilled in the art should understand that, without departing from the spirit and principles of the present invention, the technical solution can be modified and replaced in several simple ways, and these modifications and replacements are all within the scope of protection covered by the claims.

Claims

1. A bulk acoustic wave resonator having a double-layer piezoelectric film structure, characterized by, The device includes a capped wafer (1), on which a dry film (2), a passivation layer (3), a silicon oxide layer (4), a high-resistivity silicon wafer (5), and a first RDL metal (6) are sequentially disposed. The dry film (2) and the cap wafer (1) form a dry film cavity. The passivation layer (3) located above the dry film cavity extends into the dry film cavity. The passivation layer (3) extending into the dry film cavity is provided with a top electrode (7), a first piezoelectric film (9), an intermediate electrode (8), a second piezoelectric film (10), and a bottom electrode (11) in sequence. The top electrode (7) is connected to the bottom electrode (11) and the first RDL metal (6) in sequence through the second RDL metal (12) to serve as the first port of the resonator. The intermediate electrode (8) is connected to the first RDL metal (6) and the second RDL metal (12) to serve as the second port of the resonator.

2. The bulk acoustic wave resonator having a double-layer piezoelectric film structure according to claim 1, characterized by, The first piezoelectric film (9) and the second piezoelectric film (10) have the same polarization direction and a thickness of 300-1000 nm.

3. The bulk acoustic resonator with a double-layer piezoelectric film structure according to claim 1, characterized in that, The first piezoelectric film (9) and the second piezoelectric film (10) are AlN piezoelectric films and Al 1-x Sc x N piezoelectric thin film, general formula is y Pb(Mg 1 / 3Nb 2 / 3 O3 (1 y PbTiO3 lead magnesium niobate-lead titanate piezoelectric thin films, with the general formula: z Pb(In 1 / 2 Nb 1 / 2 O3 a Pb(Mg 1 / 3Nb 2 / 3 O3 (1 z a PbTiO3 piezoelectric thin films of lead indium niobate-lead magnesium niobate-lead titanate, single-crystal LiTaO3 piezoelectric thin films, and single-crystal LiNbO3 piezoelectric thin films, with the general formula Pb(Zr) 1-b Ti b One or more combinations of lead zirconate titanate piezoelectric films and barium titanate piezoelectric films of O3; wherein, 0.4 ≥ x> 0, 0.1≤ y ≤0.7, 0.15≤ z ≤0.3, 0.4≤ a ≤0.55, 0.4≤ b ≤0.

6.

4. The bulk acoustic resonator with a double-layer piezoelectric film structure according to claim 1, characterized in that, The top electrode (7), the middle electrode (8), and the bottom electrode (11) are all made of one or more of the following materials: gold, molybdenum, aluminum, copper, tungsten, and platinum.

5. The bulk acoustic resonator with a double-layer piezoelectric film structure according to claim 1, characterized in that, The intermediate electrode (8) is made of molybdenum with a thickness of 50-200 nm, and the silicon oxide layer (4) has a thickness of 2-4 μm.

6. A method for fabricating a bulk acoustic resonator with a double-layer piezoelectric film structure as described in any one of claims 1-5, characterized in that, include: An AlN seed layer (13), a top electrode (7), a first piezoelectric film (9), and an intermediate electrode (8) are sequentially grown on one surface of a double-sided thermally oxidized silicon wafer (12). After the intermediate electrode (8) is patterned by photolithography, a second piezoelectric film (10), a bottom electrode (11), and a silicon oxide layer (4) are sequentially grown on the patterned intermediate electrode (8). The silicon oxide layer (4) is etched to form a cavity and expose the bottom electrode (11) to obtain a wafer. The silicon oxide layer (4) of the wafer is bonded to the high-resistivity silicon wafer (5) to obtain the first assembly; Remove the double-sided thermally oxidized silicon wafer (12) and AlN seed layer (13) from the first assembly to expose the top electrode (7), and then etch the top electrode (7) and the first piezoelectric film (9) in sequence to expose the middle electrode (8), continue to etch the second piezoelectric film (10) to expose the bottom electrode (11), deposit and pattern the second RDL metal (12) to form an electrical connection, and then deposit a passivation layer (3) to obtain the second assembly; The second assembly is bonded to a cap wafer (1) covered with a dry film (2) and having a dry film cavity etched on the dry film (2) to obtain the third assembly; Thin the high-resistivity silicon wafer (5) on the third assembly, and etch the high-resistivity silicon wafer (5) and silicon oxide layer (4) to form a through hole, exposing the bottom electrode (11). Continue to etch the second piezoelectric film (10) downwards, and after exposing the middle electrode (8), deposit and pattern the first RDL metal (6). Lead out the top electrode (7), bottom electrode (11) and middle electrode (8) respectively and perform solder ball placement to obtain a bulk acoustic resonator with a double-layer piezoelectric film structure.

7. The method for fabricating a bulk acoustic resonator with a double-layer piezoelectric film structure according to claim 6, characterized in that, The top electrode (7), intermediate electrode (8) and bottom electrode (11) are grown by magnetron sputtering or vacuum evaporation, and the top electrode (7), intermediate electrode (8) and bottom electrode (11) are patterned by laser direct writing or inductively coupled plasma etching.

8. The method for fabricating a bulk acoustic resonator with a double-layer piezoelectric film structure according to claim 6, characterized in that, The first piezoelectric thin film (9) and the second piezoelectric thin film (10) are grown by magnetron sputtering, pulsed laser deposition or MOCVD.

9. The method for fabricating a bulk acoustic resonator with a double-layer piezoelectric film structure according to claim 6, characterized in that, The dry film cavity was etched on the dry film (2) by photolithography.

10. A bulk acoustic wave filter, characterized in that, It includes at least one bulk acoustic resonator with a double-layer piezoelectric film structure as described in any one of claims 1-5.

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