Lithium niobate single crystal piezoelectric composite material, high-frequency ultrasonic transducer and preparation method thereof

Abstract

This application discloses a lithium niobate single-crystal piezoelectric composite material, a high-frequency ultrasonic transducer, and its fabrication method, belonging to the technical field of piezoelectric ultrasonic transducers. The composite material uses lithium niobate single crystals as the functional phase and a polymer as the matrix phase, composited through a 1-3 or 2-2 type spatial structure, with a thickness of 5-300 μm and a lithium niobate volume fraction of 40%-70%, effectively reducing the material's acoustic impedance. The high-frequency ultrasonic transducer uses the above-mentioned composite material as the piezoelectric layer, combined with a matching layer and a backing layer, significantly improving ultrasonic transmission efficiency and imaging resolution. This application also provides a corresponding fabrication method, achieving mass production of high-performance transducers through femtosecond laser etching, bonding thinning, and other processes.

Description

Technical Field

[0001] This application relates to a lithium niobate single-crystal piezoelectric composite material, a high-frequency ultrasonic transducer and its preparation method, belonging to the technical field of piezoelectric ultrasonic transducers. Background Technology

[0002] Acoustic impedance mismatch in piezoelectric ultrasonic transducers is a serious problem in practical applications, mainly caused by an excessive difference in acoustic impedance between the piezoelectric layer material and the load under test. Acoustic impedance mismatch causes most of the acoustic wave energy to be reflected at the interface between the piezoelectric layer and the tissue, resulting in reduced transmission efficiency and decreased receiving sensitivity. Furthermore, the acoustic waves reflected back from the interface resonate within the piezoelectric layer, leading to pulse prolongation and decreased resolution in the ultrasonic transducer.

[0003] The core idea behind solving the acoustic impedance mismatch problem lies in reducing the impedance gap between the high-impedance piezoelectric layer and the low acoustic impedance load, allowing the acoustic impedance gradient to change smoothly and transition gradually, thereby reducing ultrasonic wave reflection and improving transmission efficiency. The main technical methods for acoustic impedance matching include adding an acoustic impedance matching layer, an acoustic absorption backing layer, and optimizing the piezoelectric layer material. The matching layer consists of one or more layers of material with specific acoustic impedance and thickness loaded at the front end of the piezoelectric element. Its acoustic impedance is generally between that of the piezoelectric element and the load under test, acting as a transition medium for acoustic impedance. The backing layer is a layer of material with an acoustic impedance close to that of the piezoelectric layer loaded at the rear end of the piezoelectric element. Its main function is to absorb the acoustic energy transmitted backward by the piezoelectric element and suppress unnecessary vibrations within the piezoelectric element, thereby generating short pulses, improving resolution, and increasing bandwidth. Optimizing the piezoelectric layer material is a revolutionary approach that starts with the material itself. It generally involves combining piezoelectric materials with polymer materials, and then combining and interweaving them according to a specific spatial geometry to form a piezoelectric composite material. Since conventional polymer materials have low acoustic impedance, combining them with piezoelectric materials reduces the overall acoustic impedance of the material while also preserving the advantages of the piezoelectric material's electrical properties to a certain extent.

[0004] Lithium niobate (LiNbO3, LN) single crystals are an important piezoelectric material with a piezoelectric coefficient (up to 70 pC / N) far exceeding that of PZT ceramics, and extremely low dielectric constant (approximately 50) and dielectric loss (below 0.02%), theoretically making them highly suitable for high-performance ultrasonic transducers. However, the acoustic impedance of lithium niobate single crystals is approximately 34.5 MRayls, which differs significantly from the acoustic impedance of the load under test. Furthermore, the material is brittle and its coefficient of thermal expansion differs significantly from that of the polymer matrix. This presents significant challenges in precision machining, interfacial bonding with polymer composites, and large-scale fabrication. In particular, it is difficult to obtain large-area piezoelectric composite materials with high aspect ratios, intact structures, and excellent performance, which severely restricts its application in high-frequency ultrasonic transducers. Summary of the Invention

[0005] To address the problem in existing piezoelectric ultrasonic transducer technologies where the significant acoustic impedance difference between lithium niobate single crystals and biological tissues (approximately 34.5 MRayls for lithium niobate and approximately 1.5 MRayls for biological tissues) leads to substantial ultrasonic wave reflection at the interface and reduced transmission efficiency, this application provides a lithium niobate single crystal piezoelectric composite material technology and its application in high-frequency ultrasonic transducers. By combining lithium niobate piezoelectric single crystals with organic substrate materials, piezoelectric composite materials with different spatial interconnected structures can be formed, effectively reducing the acoustic impedance of the piezoelectric layer in the ultrasonic transducer, improving ultrasonic wave transmission efficiency, and significantly increasing the electromechanical coupling coefficient of the piezoelectric layer. This application utilizes a unique bonding and thinning process to achieve the mass production of large-size, structurally complex lithium niobate single crystal composite materials with controllable thickness, greatly reducing process costs. This technology can meet the different frequency requirements of ultrasonic transducers in the field of medical ultrasound imaging, thus broadening the application prospects of high-frequency ultrasonic transducers based on lithium niobate piezoelectric single crystals in medical imaging.

[0006] The technical solution adopted in this application is as follows: According to a first aspect of this application, a lithium niobate single-crystal piezoelectric composite material is provided, comprising a functional phase and a matrix phase; The functional phase and the matrix phase are interwoven in a pre-defined spatial interconnection structure to form a piezoelectric composite material; The functional phase is lithium niobate single crystal; The matrix phase is a polymer material.

[0007] In one possible design, the spatial connectivity structure is either type 1-3 or type 2-2.

[0008] In one possible design, the thickness of the lithium niobate single-crystal piezoelectric composite material is 5~300 μm.

[0009] In one possible design, the volume fraction of lithium niobate single crystals in the lithium niobate single crystal piezoelectric composite material is 40% to 70%.

[0010] According to a second aspect of this application, a method for preparing the lithium niobate single-crystal piezoelectric composite material is provided, comprising: Provide a lithium niobate single crystal wafer, and thin the lithium niobate single crystal wafer to a target thickness; A pre-defined spatial interconnected groove was formed on the surface of the thinned lithium niobate single crystal wafer using femtosecond laser lithography. Fill and cure the groove with polymer material; Excess polymer material is removed from the surface of the lithium niobate single crystal wafer, and the side of the lithium niobate single crystal wafer away from the groove is thinned to expose the polymer material, thereby forming a lithium niobate single crystal wafer with several lithium niobate single crystal piezoelectric composite materials. The lithium niobate single crystal wafer is cut to form an independent lithium niobate single crystal piezoelectric composite material.

[0011] In one possible design, the lithium niobate single-crystal wafer is cut to form individual lithium niobate single-crystal piezoelectric composite materials, including: passivating the lithium niobate single-crystal wafer to form a polymer film on its surface, and etching to cut the lithium niobate single-crystal wafer into several lithium niobate single-crystal piezoelectric composite materials.

[0012] According to a third aspect of this application, a high-frequency ultrasonic transducer is provided, comprising a matching layer, an upper electrode layer, a piezoelectric layer, a lower electrode layer, a backing layer, an adhesive layer, and an encapsulation substrate stacked sequentially. The matching layer, the piezoelectric layer, and the backing layer are sequentially bonded together to form an acoustic laminated structure; The packaging substrate is provided with electrode pathways, and the upper electrode layer and the lower electrode layer are led out and connected to the electrode pathways through wires; The backing layer is connected to the encapsulation substrate via the adhesive layer; The piezoelectric layer is selected from at least one of the aforementioned lithium niobate single-crystal piezoelectric composite materials or lithium niobate single-crystal piezoelectric composite materials obtained according to the aforementioned preparation method.

[0013] In one possible design, the thicknesses of the upper electrode layer and the lower electrode layer are independently 0.01~2μm.

[0014] In one possible design, the thickness of the backing layer is 0.5 to 5 mm.

[0015] According to a fourth aspect of this application, a method for manufacturing the aforementioned high-frequency ultrasonic transducer is provided, comprising: Provide lithium niobate single-crystal piezoelectric composite materials; The upper electrode layer and the lower electrode layer are respectively prepared on the upper and lower opposite sides of the lithium niobate single crystal piezoelectric composite material; A backing layer is prepared on the side of the lower electrode layer away from the lithium niobate single-crystal piezoelectric composite material; The side of the backing layer away from the lower electrode layer is connected to the packaging substrate via an adhesive layer; Electrode pathways are fabricated on the packaging substrate, wires are led out from the upper electrode and the lower electrode and connected to the electrode pathways, and the electrode pathways are connected to a coaxial line; A matching layer is prepared on the surface of the upper electrode layer away from the lithium niobate single-crystal piezoelectric composite material.

[0016] The beneficial effects of this application include: This application provides a lithium niobate single-crystal piezoelectric composite material and a high-frequency ultrasonic transducer. Utilizing the lithium niobate single-crystal piezoelectric composite material as the piezoelectric layer effectively solves the problems of low emission efficiency and poor sensitivity in ultrasonic transducers caused by the excessively high acoustic impedance of piezoelectric materials. Furthermore, this application employs bonding and thinning processes to achieve precision machining of the lithium niobate single-crystal composite material, avoiding processing problems such as wafer warping and cracking, and holds promise for the mass production and fabrication of high-frequency ultrasonic transducers. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the lithium niobate composite material structure prepared in batches according to this application; Figure 2 This is a schematic diagram of the high-frequency lithium niobate ultrasonic transducer of this application.

[0018] Attached Figure Labels 1. Lithium niobate single crystal wafer; 2. Piezoelectric composite material; 3. Lithium niobate piezoelectric single crystal pillar; 4. Polymer matrix; 5. Matching layer; 6. Upper electrode layer; 7. Lower electrode layer; 8. Backing layer; 9. Adhesive layer; 10. Encapsulation substrate; 11. Electrode path. Detailed Implementation

[0019] The present application is described in detail below with reference to the embodiments, but the present application is not limited to these embodiments.

[0020] Unless otherwise specified, all raw materials used in the embodiments of this application were purchased through commercial channels.

[0021] Unless otherwise specified, all test methods are standard and all instrument settings are those recommended by the manufacturer.

[0022] Acoustic impedance reflects the influence of the damping characteristics at any point in a medium on the vibration of particles caused by disturbances. The propagation medium of sound waves is defined by its acoustic impedance. If the acoustic impedance difference between two media is too large, most of the sound wave energy will be reflected. Currently, piezoelectric materials in commercially available ultrasonic transducers have high acoustic impedance (e.g., the common PZT piezoelectric ceramic has an acoustic impedance of approximately 35 MRayls), while the acoustic impedance of commonly used loads such as water is approximately 1.5 MRayls, resulting in a significant acoustic impedance difference. This acoustic impedance mismatch causes a large number of ultrasonic waves emitted by the transducer to be reflected at the interface with the load under test. Only a small portion of the ultrasonic waves can enter the load under test for information transmission, and the reflected ultrasonic waves continue to oscillate within the ultrasonic transducer, greatly reducing the transducer's transmission efficiency and receiving sensitivity. Therefore, reducing the acoustic impedance of the material itself is the most effective way to solve the acoustic impedance mismatch problem.

[0023] The efficient synthesis and large-scale production of piezoelectric single-crystal composites is currently the biggest obstacle to their widespread application. The main reasons include: piezoelectric single-crystal materials are brittle and prone to microcracks during precision machining; the significant difference in thermal expansion coefficients between piezoelectric single-crystal materials and polymer materials makes the polymer materials susceptible to internal stress during curing or transducer operation, affecting interfacial bonding and causing material damage; and the piezoelectric composites require piezoelectric single-crystal materials with a large aspect ratio, but the fabrication of small-sized piezoelectric single-crystal micropillar arrays is complex and difficult. Therefore, it is necessary to achieve a perfect combination of piezoelectric single-crystals with a polymer matrix through sophisticated microstructure design and controllable fabrication processes without compromising their excellent intrinsic properties, and to realize the mass production of composite materials based on large-sized piezoelectric single-crystal wafers.

[0024] To address at least one of the aforementioned problems, this application provides a technical solution for a lithium niobate single-crystal piezoelectric composite material and a high-frequency ultrasonic transducer. The high-frequency ultrasonic transducer includes a matching layer, a piezoelectric layer, an electrode layer, a backing layer, an adhesive layer, and a packaging substrate. The piezoelectric layer is a lithium niobate piezoelectric single-crystal composite material. By combining lithium niobate piezoelectric single crystals with organic substrate materials, piezoelectric composite materials with different spatial interconnected structures can be formed, effectively reducing the acoustic impedance of the piezoelectric layer in the ultrasonic transducer, improving ultrasonic wave transmission efficiency, and significantly increasing the electromechanical coupling coefficient of the piezoelectric layer. This application utilizes a unique bonding and thinning process to achieve the mass production of large-size, structurally complex lithium niobate single-crystal composite materials with controllable thickness, greatly reducing process costs. This can meet the different frequency requirements of ultrasonic transducers in the field of medical ultrasound imaging, broadening the application prospects of high-frequency ultrasonic transducers based on lithium niobate piezoelectric single crystals in medical imaging.

[0025] According to one embodiment of this application, a lithium niobate single-crystal piezoelectric composite material includes a functional phase and a matrix phase; The functional phase and the matrix phase are interwoven in a pre-defined spatial interconnection structure to form a piezoelectric composite material; The functional phase is lithium niobate single crystal; The matrix phase is a polymer material.

[0026] In one embodiment, the spatial connectivity structure is either type 1-3 or type 2-2. The spatial structure of piezoelectric composites is typically represented in the industry using two numbers: the former represents the connectivity dimension of the piezoelectric phase structure, and the latter represents the connectivity dimension of the polymer phase structure. For example, type 1-3 indicates that the lithium niobate single crystal is one-dimensionally connected, and the polymer substrate is three-dimensionally connected. Type 1-3 (piezoelectric pillar array embedded in polymer) and type 2-2 (alternating layers) are proven optimal macroscopic structures that significantly reduce acoustic impedance while maintaining piezoelectric activity. The type 1-3 structure better utilizes the piezoelectric properties of the single crystal, while the type 2-2 structure is easier to fabricate large-area uniform composite materials.

[0027] In one embodiment, the thickness of the lithium niobate single-crystal piezoelectric composite material is 5–300 μm. This thickness range clearly defines the suitability of the material for high-frequency ultrasonic transducers.

[0028] In one embodiment, the volume fraction of lithium niobate single crystals in the lithium niobate single crystal piezoelectric composite material is 40% to 70%. This volume fraction range aims to balance the seemingly contradictory relationship between "low acoustic impedance" and "high piezoelectric performance." If the volume fraction is too low, the piezoelectric performance is insufficient; if it is too high, the acoustic impedance reduction effect is not significant. Within this range, the composite material can significantly reduce the acoustic impedance to a level that matches the load while ensuring a sufficient electromechanical coupling coefficient, thereby guaranteeing that the transducer has high transmission efficiency and receiving sensitivity.

[0029] In one embodiment, the polymer material is selected from at least one of epoxy resin, polyvinylidene fluoride, polydimethylsiloxane, polyimide, and polyurethane. This type of insulating material not only has low acoustic impedance but also exhibits good adhesion to lithium niobate single crystals. Its mechanical properties and curing process are suitable for microfabrication, ensuring the structural integrity and long-term reliability of the composite material and preventing performance degradation due to internal stress or interfacial delamination. This is crucial for achieving mass production.

[0030] According to one embodiment of this application, a method for preparing a lithium niobate single-crystal piezoelectric composite material includes: A lithium niobate single-crystal wafer is provided, and the lithium niobate single-crystal wafer is thinned to a target thickness; the target thickness is greater than the thickness of the lithium niobate single-crystal piezoelectric composite material; A pre-defined spatial interconnected groove was formed on the surface of the thinned lithium niobate single crystal wafer using femtosecond laser lithography. Fill and cure the groove with polymer material; Excess polymer material is removed from the surface of the lithium niobate single crystal wafer, and the side of the lithium niobate single crystal wafer away from the groove is thinned to expose the polymer material, thereby forming a lithium niobate single crystal wafer with several lithium niobate single crystal piezoelectric composite materials. The lithium niobate single crystal wafer is cut to form an independent lithium niobate single crystal piezoelectric composite material.

[0031] In one embodiment, the lithium niobate single crystal wafer is thinned to a target thickness, including: first coarse thinning to a set thickness, and then fine thinning to the target thickness; the thinning rate of the coarse thinning is 10-50 μm / min, and the thinning rate of the fine thinning is 1-3 μm / min; the polishing process is chemical mechanical polishing.

[0032] In one embodiment, the lithium niobate single-crystal wafer is diced to form individual lithium niobate single-crystal piezoelectric composite materials. This includes: passivating the lithium niobate single-crystal wafer to form a polymer film on its surface, and etching to dice the lithium niobate single-crystal wafer into several lithium niobate single-crystal piezoelectric composite materials. This method of alternating passivation and etching enables high aspect ratio, vertical sidewall dicing, and ensures the independence and performance consistency of each transducer element. It is a core process step for achieving transducer arraying and integration.

[0033] According to one embodiment of this application, a high-frequency ultrasonic transducer includes a matching layer, an upper electrode layer, a piezoelectric layer, a lower electrode layer, a backing layer, an adhesive layer, and an encapsulation substrate, which are stacked sequentially. The matching layer, the piezoelectric layer, and the backing layer are sequentially bonded together to form an acoustic laminated structure; The packaging substrate is provided with electrode pathways, and the upper electrode layer and the lower electrode layer are led out and connected to the electrode pathways through wires; The piezoelectric layer is selected from at least one of the aforementioned lithium niobate single-crystal piezoelectric composite materials or lithium niobate single-crystal piezoelectric composite materials obtained according to the aforementioned preparation method.

[0034] The material of the matching layer is selected from at least one of epoxy resin, polyimide, silicone rubber, polyurethane, and parylene; the thickness of the matching layer is calculated based on the operating frequency of the high-frequency ultrasonic transducer. Preferably, the materials of the upper electrode layer and the lower electrode layer are independently selected from at least one of gold, chromium, molybdenum, platinum, and aluminum; In one embodiment, the thicknesses of the upper and lower electrode layers are independently 0.01–2 μm. This thickness range is designed to balance conductivity with the impact on vibration. If the electrodes are too thin, the resistance and losses are high; if they are too thick, the piezoelectric layer becomes rigid, suppressing its vibration and reducing sensitivity. This ensures good conductivity while minimizing the negative impact of the mass load and stiffness on the vibration modes of the piezoelectric layer, thereby guaranteeing efficient energy conversion of the transducer.

[0035] Preferably, the backing material is selected from at least one of epoxy resin, rubber and its composites, and conductive adhesive; In one embodiment, the thickness of the backing layer is 0.5–5 mm; this thickness range is designed to ensure that the backing layer can adequately absorb rearward-propagating acoustic energy and suppress residual vibrations within the piezoelectric layer. A sufficiently thick backing layer can generate short-pulse, wide-bandwidth acoustic signals, which directly determines the axial resolution of ultrasound imaging. Insufficient thickness results in poor absorption, pulse tailing, and blurred images. The adhesive layer is made of at least one material selected from epoxy resin, polyimide, and silicone adhesive. The adhesive layer serves to bond the main structure of the ultrasonic transducer to the encapsulation substrate.

[0036] According to one embodiment of this application, the method for manufacturing the aforementioned high-frequency ultrasonic transducer includes: Provide lithium niobate single-crystal piezoelectric composite materials; An upper electrode layer and a lower electrode layer are respectively prepared on the upper and lower opposite sides of the lithium niobate single crystal piezoelectric composite material; A backing layer is prepared on the side of the lower electrode layer away from the lithium niobate single-crystal piezoelectric composite material; The side of the backing layer away from the lower electrode layer is connected to the packaging substrate via an adhesive layer; Electrode pathways are fabricated on the packaging substrate, wires are led out from the upper electrode and the lower electrode and connected to the electrode pathways, and the electrode pathways are connected to a coaxial line; A matching layer is prepared on the surface of the upper electrode layer away from the lithium niobate single-crystal piezoelectric composite material.

[0037] In one embodiment, the upper electrode layer and the lower electrode layer are prepared on opposite upper and lower surfaces of the lithium niobate single crystal piezoelectric composite material, respectively, including: preparing metal electrodes on the surface of the lithium niobate piezoelectric single crystal wafer by deposition methods such as magnetron sputtering or thermal evaporation to form the upper electrode layer and the lower electrode layer.

[0038] Example 1 This embodiment illustrates a lithium niobate single-crystal piezoelectric composite material, obtained based on a lithium niobate single-crystal wafer, such as... Figure 1As shown, the main structure consists of a lithium niobate single-crystal wafer (1), a piezoelectric composite material (2), a lithium niobate piezoelectric single-crystal pillar (3), and a polymer matrix (4). The piezoelectric composite material is a type 1-3 piezoelectric composite material, the piezoelectric single-crystal pillar has a square cross-section, and the polymer matrix is ​​made of epoxy resin. The designed high-frequency ultrasonic transducer operates at a frequency of 50 MHz. The lithium niobate single crystal has a thickness of 70 μm; the element size of a single composite material is 0.55 × 0.45 mm; the square piezoelectric single-crystal pillar in the composite material has a side length of 42 μm and a kerf width of 10 μm, i.e., a pillar spacing of 52 μm, and the composite material volume fraction is 65%.

[0039] Based on the above-mentioned lithium niobate single-crystal piezoelectric composite material, a high-frequency ultrasonic transducer using this lithium niobate single-crystal piezoelectric composite material as the piezoelectric layer is further exemplified. The structure is described in the following example. Figure 2 As shown, the structure mainly includes a matching layer 5, an upper electrode layer 6, a piezoelectric layer 2, a lower electrode layer 7, a backing layer 8, an adhesive layer 9, a packaging substrate 10, and an electrode pathway 11. The matching layer is made of parylene C with a thickness of 12 μm; the upper and lower electrode layers are made of Cr / Au alloy with a thickness of 0.15 μm; the backing layer is made of high-performance, silver-filled conductive adhesive with a thickness of 3 mm; the adhesive layer is made of epoxy resin, which fills the space between the material and the metal shell to prevent the upper and lower electrodes of the ultrasonic transducer from conducting; the packaging substrate is made of titanium alloy or silicon, which has good biocompatibility.

[0040] The steps for preparing the above-mentioned lithium niobate single-crystal piezoelectric composite material and further preparing it into a high-frequency ultrasonic transducer with a piezoelectric layer include: (1) Select a 6-inch lithium niobate piezoelectric single crystal wafer with a thickness of 500 μm and use acetone, ethanol and deionized water to perform ultrasonic cleaning on the wafer.

[0041] (2) The surface of the 500 μm lithium niobate wafer is coarsely thinned to a thickness of 80 μm. Further, fine thinning is performed to a target thickness of 75 μm. Further, the thinned LN wafer surface is polished using chemical mechanical planarization (CMP), and the total thickness variation (TTV) of the lithium niobate wafer is finally controlled within 2 μm.

[0042] (3) Etching of lithium niobate piezoelectric single crystals was performed using femtosecond laser lithography, according to... Figure 1The arrangement of the composite materials is used to etch grooves into the device within a predetermined size range of 0.45 × 0.55 mm. The parameters are as follows: femtosecond laser lithography with a laser wavelength of 355 nm, a laser modulation frequency of 5 kHz, a laser power of 0.2 W, a laser spot size of 4 μm, a laser travel speed of 50 mm / s, 100 laser processing cycles, and a laser processing depth of 75 μm.

[0043] (4) The grooves formed by femtosecond laser lithography are filled with epoxy resin to form a type 1-3 lithium niobate piezoelectric single crystal composite material. After the epoxy resin is cured, the excess epoxy resin on the surface is removed by grinding, and the lithium niobate wafer is thinned on the reverse side to make the overall thickness of the wafer 70 μm.

[0044] (5) Cr / Au electrodes were fabricated on the upper and lower surfaces of the wafer using magnetron sputtering. The thickness of Cr was 20 nm and the thickness of Au was 100 nm, thus completing the fabrication of the upper and lower electrode layers. The vacuum level required for magnetron sputtering of the Cr / Au electrodes was 3 × 10⁻⁶. -3 The sputtering pressure was MPa, the required sputtering time was 45 min, and the final sputtered metal electrode thickness was 0.15 μm.

[0045] Specifically, an epoxy resin / tungsten powder layer is coated on the surface of the lower electrode layer of the composite material using a spin coating method as a backing layer. After mixing the two-component, silver-filled epoxy resin conductive adhesive, 400% by mass of 800-mesh tungsten powder is added before curing. The mixture is stirred thoroughly until the tungsten powder and epoxy resin are evenly mixed. The mixture is placed at 60°C for 8 hours to allow the film to fully cure, resulting in a backing layer with a thickness of 3 mm.

[0046] (6) The wafer is patterned using dry etching to form independent composite material structures. The wafer is passivated in a reaction chamber using a C4F8 / Ar mixed gas (conditions: passivation power 200 W, reaction pressure 20 mTorr), forming a polymer film through a chemical reaction. The advantage of this C4F8 / Ar passivation step is that Ar… + The synergistic effect of physical bombardment and chemical deposition of C4F8 achieves sidewall protection and selective opening of the bottom; SF6 gas is further introduced into the reaction chamber to perform physical and chemical etching on the wafer. In a single cycle (passivation + etching), the flow rates of C4F8 / Ar and SF6 are 80 accm and 10 accm, respectively; the pulse power and etching selectivity are 50 W and 1500, respectively; the step frequency is 10 steps / s; the etching rate is 5 μm / min; and the etching time is 60 min.

[0047] Specifically, the silicon substrate is bonded to the composite material backing layer using an adhesive method. The adhesive layer is Epo-tek 301-2 epoxy resin, and the encapsulation substrate is a silicon substrate. A high-precision chip mounter is used, with an optical recognition system, to precisely align the wafer and the silicon substrate. After applying slight pressure, thermal curing is performed.

[0048] (7) The silicon substrate is patterned using dry etching, and the electrode pathway is formed by repeated magnetron sputtering. The packaging substrate has grooves, each with a length and width of 0.9 × 1.1 mm and a depth of 3.9 mm. The final overall size of a single packaging substrate is 1.2 × 1.4 mm and a depth of 6 mm. The electrode pathways are located at the center of both sides of the packaging substrate, with a width of 0.15 mm, thus obtaining the main structure of the ultrasonic transducer.

[0049] (8) Using a micro-electro-mechanical systems (MEMS) wire bonding method, wires are led out from the upper and lower electrodes and connected to the electrode path on the silicon substrate. The electrodes under the silicon substrate are then connected to the coaxial line by welding. The specific steps are as follows: ① Perform plasma cleaning on the main structure of the ultrasonic transducer to remove organic matter and oxide layer from the electrode surface to ensure solderability. ② Feed a predetermined length of gold wire from the spool using the wire bonding machine's cutting tool. The electronic spark rod generates a high-voltage electric spark below the cutting tool, instantly melting the end of the extended gold wire into a perfect sphere, called a "free air ball". ③ The cutting tool holds the gold wire and lowers it with the gold ball, precisely aligning it with the surface electrode. Apply a certain pressure, and simultaneously turn on ultrasonic vibration and supplement with heating. Under the combined action of heat, pressure, and ultrasonic energy, the gold ball and the metal electrode form an intermetallic compound, completing a strong metallurgical bond and forming the first weld point. ④ Lead out the gold wire and connect it to the electrode path.

[0050] (9) A matching layer was prepared using chemical vapor deposition (CVD) with Parylene C as the material and a thickness of 12 μm. Parylene C was sublimated from solid to gaseous state. The gaseous Parylene C was then transported to a high-temperature pyrolysis chamber for decomposition at 650-700℃. The active monomer vapor was then introduced into a room-temperature deposition chamber to deposit Parylene C onto the wafer surface. A higher vacuum, typically ~0.1 Torr, was maintained in the deposition chamber. Finally, a high-frequency ultrasonic transducer based on lithium niobate single crystal was obtained.

[0051] The above description is merely a few embodiments of this application and is not intended to limit this application in any way. Although this application discloses preferred embodiments as described above, it is not intended to limit this application. Any changes or modifications made by those skilled in the art without departing from the scope of the technical solution of this application using the disclosed technical content are equivalent to equivalent implementation cases and fall within the scope of the technical solution.

Claims

1. A lithium niobate single-crystal piezoelectric composite material, characterized in that, Includes functional phase and matrix phase; The functional phase and the matrix phase are interwoven in a pre-defined spatial interconnection structure to form a piezoelectric composite material; The functional phase is lithium niobate single crystal; The matrix phase is a polymer material.

2. The lithium niobate single-crystal piezoelectric composite material according to claim 1, characterized in that, The spatial connectivity structure is of type 1-3 or type 2-2.

3. The lithium niobate single-crystal piezoelectric composite material according to claim 1, characterized in that, The thickness of the lithium niobate single-crystal piezoelectric composite material is 5~300 μm.

4. The lithium niobate single-crystal piezoelectric composite material according to claim 1, characterized in that, The volume fraction of lithium niobate single crystals in the lithium niobate single crystal piezoelectric composite material is 40%~70%.

5. A method for preparing a lithium niobate single-crystal piezoelectric composite material according to any one of claims 1 to 4, characterized in that, include: Provide a lithium niobate single crystal wafer, and thin the lithium niobate single crystal wafer to a target thickness; A pre-defined spatial interconnected groove was formed on the surface of the thinned lithium niobate single crystal wafer using femtosecond laser lithography. Fill and cure the groove with polymer material; Excess polymer material is removed from the surface of the lithium niobate single crystal wafer, and the side of the lithium niobate single crystal wafer away from the groove is thinned to expose the polymer material, thereby forming a lithium niobate single crystal wafer with several lithium niobate single crystal piezoelectric composite materials. The lithium niobate single crystal wafer is cut to form an independent lithium niobate single crystal piezoelectric composite material.

6. The preparation method according to claim 5, characterized in that, The step of cutting the lithium niobate single-crystal wafer to form independent lithium niobate single-crystal piezoelectric composite materials includes: The lithium niobate single crystal wafer is passivated to form a polymer film on its surface, and then etched to slit the lithium niobate single crystal wafer into several lithium niobate single crystal piezoelectric composite materials.

7. A high-frequency ultrasonic transducer, characterized in that, It includes a matching layer, an upper electrode layer, a piezoelectric layer, a lower electrode layer, a backing layer, an adhesive layer, and a packaging substrate, which are stacked in sequence. The matching layer, the piezoelectric layer, and the backing layer are sequentially bonded together to form an acoustic laminated structure; The packaging substrate is provided with electrode pathways, and the upper electrode layer and the lower electrode layer are led out and connected to the electrode pathways through wires; The backing layer is connected to the encapsulation substrate via the adhesive layer; The piezoelectric layer is selected from at least one of the lithium niobate single crystal piezoelectric composite materials according to any one of claims 1 to 4 or the lithium niobate single crystal piezoelectric composite materials obtained by the preparation method according to claim 5 or 6.

8. The high-frequency ultrasonic transducer according to claim 7, characterized in that, The thickness of the upper electrode layer and the lower electrode layer are independently 0.01~2μm.

9. The high-frequency ultrasonic transducer according to claim 7, characterized in that, The thickness of the backing layer is 0.5~5mm.

10. A method for preparing the high-frequency ultrasonic transducer according to any one of claims 7 to 9, characterized in that, include: Provide lithium niobate single-crystal piezoelectric composite materials; An upper electrode layer and a lower electrode layer are respectively prepared on the upper and lower opposite sides of the lithium niobate single crystal piezoelectric composite material; A backing layer is prepared on the side of the lower electrode layer away from the lithium niobate single-crystal piezoelectric composite material; The side of the backing layer away from the lower electrode layer is connected to the packaging substrate via an adhesive layer; Electrode pathways are fabricated on the packaging substrate, wires are led out from the upper electrode and the lower electrode and connected to the electrode pathways, and the electrode pathways are connected to a coaxial line; A matching layer is prepared on the surface of the upper electrode layer away from the lithium niobate single-crystal piezoelectric composite material.

Images