A flexible piezoelectric micromachined ultrasonic transducer and its preparation method and application

Abstract

The present invention discloses a flexible piezoelectric micromechanical ultrasonic transducer and its preparation method and application. The flexible piezoelectric micromechanical ultrasonic transducer of the present invention includes a laminated structure and a flexible coating layer that encapsulates the laminated structure. The laminated structure includes a flexible substrate, an adhesive layer, a first supporting layer, a second supporting layer, a bottom electrode layer, a piezoelectric layer and a top electrode layer that are stacked in sequence. The first supporting layer is provided with a through-cavity structure. The first supporting layer and the second supporting layer are both made of epoxy-based negative photoresist. The composition of the piezoelectric layer includes polyvinylidene fluoride-trifluoroethylene and nano potassium sodium niobate ceramic particles. The flexible piezoelectric micromechanical ultrasonic transducer of the present invention has the advantages of high sensitivity, good tensile ductility and bending properties, flexible adjustment of resonant frequency, and easy arraying. In addition, its preparation method is simple, the production cost is low, and it is easy to mass produce, and it is suitable for large-scale industrial production and application.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor technology, and in particular to a flexible piezoelectric micromechanical ultrasonic transducer and a preparation method and application thereof. Background Art

[0002] A piezoelectric micromachined ultrasonic transducer is a microelectromechanical system (MEMS) device used to transmit or receive ultrasonic signals. It has the advantages of small size, low power consumption, low cost, no need for DC bias, and easy arraying. It has been widely used in many portable devices.

[0003] Traditional rigid silicon-based piezoelectric micromachined ultrasonic transducers have difficulty adhering closely to surfaces that are not flat and smooth, easily generating acoustic impedance mismatches that affect the accuracy of measurement data and significantly limit their application. Flexible ultrasonic devices, on the other hand, offer excellent flexibility and can adapt to varying surface shapes and curvatures, thereby improving data accuracy and stability when applied to non-planar surfaces or complex detection environments. (For example, flexible ultrasonic arrays based on row-column addressing can achieve a relatively better detection field of view and image resolution under specific curvatures.) Currently, flexible piezoelectric ultrasonic transducers based on thickness vibration are relatively mature, but such transducers often require mechanical cutting, laser cutting, and other methods to fabricate composite piezoelectric materials, limiting the minimum device spacing and size, hindering mass production. Furthermore, the operating frequency of such devices depends solely on the thickness of the piezoelectric material, hindering the flexible control of the device's resonant frequency. Therefore, flexible piezoelectric micromachined ultrasonic transducers that can flexibly control the resonant frequency based on the cavity diameter and diaphragm thickness have promising development prospects.

[0004] At present, the design schemes of flexible piezoelectric micromechanical ultrasonic transducers mainly include the following two categories: 1) Island-bridge design: a deep silicon etching process is used to form a silicon spring structure, so that the device has the ability to stretch and bend, or a piezoelectric micromechanical ultrasonic transducer based on a rigid substrate (for example, a silicon substrate) is bonded to a flexible substrate by gluing, embossing, etc., so that the device has the ability to bend. In order to ensure the transceiver sensitivity of the device, this type of design usually uses piezoelectric ceramics as the piezoelectric layer, but both piezoelectric ceramics and silicon have the disadvantages of low toughness and lack of ductility. When subjected to stress, the device tends to break by crack expansion, which cannot meet the application scenario requirements of a small curvature radius; 2) Fully flexible micromechanical ultrasonic transducers prepared based on micro-nano processing technology: This type of device usually has good bending ability, but the tensile performance is very limited. At the same time, the piezoelectric layer uses a single organic piezoelectric material, which is difficult to obtain high sensitivity. It can be seen that the existing flexible piezoelectric micromechanical ultrasonic transducers have obvious defects and are difficult to fully meet the requirements of practical applications.

[0005] Therefore, it is of great significance to develop a flexible piezoelectric micromechanical ultrasonic transducer with high sensitivity, good tensile ductility and bending properties, flexible adjustment of resonant frequency, easy arraying and easy mass production. Summary of the Invention

[0006] The purpose of the present invention is to provide a flexible piezoelectric micromechanical ultrasonic transducer and a preparation method and application thereof.

[0007] The technical solution adopted by the present invention is:

[0008] A flexible piezoelectric micromechanical ultrasonic transducer comprises a stacked structure and a flexible covering layer encapsulating the stacked structure; the stacked structure comprises a flexible substrate, an adhesive layer, a first supporting layer, a second supporting layer, a bottom electrode layer, a piezoelectric layer, and a top electrode layer stacked in sequence; the first supporting layer is provided with a through-cavity structure; the first supporting layer and the second supporting layer are both made of epoxy-based negative photoresist; and the piezoelectric layer comprises polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) and nano-potassium sodium niobate ceramic particles.

[0009] Preferably, the flexible coating layer is made of one of silicone rubber and polydimethylsiloxane potting compound.

[0010] Preferably, the silicone rubber is Ecoflex silicone rubber from Smooth-On.

[0011] Preferably, the polydimethylsiloxane potting adhesive is SYLGARD 184 from Dow Corning.

[0012] Preferably, the flexible substrate is made of epoxy-based negative photoresist.

[0013] Preferably, the thickness of the flexible substrate is 30 μm to 60 μm.

[0014] Preferably, the adhesive layer is made of photosensitive low-temperature curing flexible epoxy resin adhesive.

[0015] Preferably, the thickness of the adhesive layer is 4 μm to 7 μm.

[0016] Preferably, the photosensitive low-temperature curing flexible epoxy resin adhesive is Kayaku's photosensitive low-temperature curing epoxy resin adhesive PermiNex 1005.

[0017] Preferably, the thickness of the first supporting layer is 50 μm to 80 μm.

[0018] Preferably, the second supporting layer has a thickness of 5 μm to 10 μm.

[0019] Preferably, the surfaces of the second supporting layer and the first supporting layer corresponding to the through-cavity structure are further provided with protruding structures.

[0020] Preferably, the protruding structure is cylindrical, with a diameter of 20 μm to 80 μm and a height of 30 μm to 40 μm.

[0021] Preferably, the number of the protrusion structures is one or more.

[0022] Preferably, the bottom electrode layer is composed of at least one of Au, Cr, Ag, and Ti.

[0023] Preferably, the bottom electrode layer has a thickness of 200 nm to 500 nm.

[0024] Preferably, the area of the bottom electrode layer is not less than the opening area of the through-cavity structure of the first supporting layer.

[0025] Preferably, the top electrode layer is composed of at least one of Au, Cr, Ag, and Ti.

[0026] Preferably, the thickness of the top electrode layer is 200 nm to 500 nm.

[0027] Preferably, the top electrode layer comprises a plurality of contacts arranged in an array, and adjacent contacts are connected via a double-path serpentine wire.

[0028] Preferably, the area of the top electrode layer is 60% to 80% of the opening area of the first support layer penetrating the cavity structure.

[0029] Preferably, the through cavity structure is cylindrical, with a diameter of 90 μm to 110 μm.

[0030] Preferably, the epoxy-based negative photoresist is one of epoxy-based negative photoresist SU-8 2005 and epoxy-based negative photoresist SU-8 2025 produced by Suzhou Zhongxin Qiheng Scientific Instrument Co., Ltd.

[0031] Preferably, the mass ratio of the polyvinylidene fluoride-trifluoroethylene to the nano potassium sodium niobate ceramic particles is 1.5 to 3.0:1.

[0032] Preferably, the polyvinylidene fluoride-trifluoroethylene is from Arkema FC 20.

[0033] Preferably, the particle size of the nano potassium sodium niobate ceramic particles is 300 nm to 500 nm.

[0034] Preferably, the piezoelectric layer has a thickness of 1 μm to 4 μm.

[0035] Preferably, the piezoelectric layer is provided with a through hole for leading a wire from the bottom electrode layer to the same side of the top electrode layer.

[0036] A method for preparing the flexible piezoelectric micromechanical ultrasonic transducer as described above comprises the following steps:

[0037] 1) depositing a sacrificial layer material on a single surface of a rigid substrate by magnetron sputtering to form a first sacrificial layer;

[0038] 2) depositing an electrode material on a portion of the surface of the first sacrificial layer using an evaporation process to form a top electrode layer;

[0039] 3) coating the surface of the first sacrificial layer and the top electrode layer with a piezoelectric material by a spin coating method to form a piezoelectric layer;

[0040] 4) depositing an electrode material on a portion of the surface of the piezoelectric layer using an evaporation process to form a bottom electrode layer;

[0041] 5) annealing the device obtained in step 4);

[0042] 6) coating the surface of the piezoelectric layer and the bottom electrode layer with a support layer material by spin coating to form a second support layer;

[0043] 7) coating a support layer material on a portion of the surface of the second support layer by spin coating to form a first support layer;

[0044] 8) coating a sacrificial layer material on one side of another rigid substrate by spin coating to form a second sacrificial layer;

[0045] 9) coating the surface of the second sacrificial layer with a flexible substrate material by spin coating to form a flexible substrate;

[0046] 10) bonding the first supporting layer and the flexible substrate with an adhesive to form an adhesive layer;

[0047] 11) A solvent is used to dissolve the second sacrificial layer, and the corresponding rigid substrate is removed. The device is then fixed in a mold with the flexible substrate facing downward and maintaining a certain distance from the bottom surface of the mold. The coating material is then injected until the portion below the first sacrificial layer of the device is immersed, and then solidified. The first sacrificial layer is then dissolved by a solvent, and the corresponding rigid substrate is removed. The coating material is then sprayed on the surface of the piezoelectric layer and the top electrode layer, and then solidified to form a flexible coating layer, thereby obtaining a flexible piezoelectric micromechanical ultrasonic transducer.

[0048] Preferably, the rigid substrate in step 1) is one of a glass substrate, a silicon substrate, and a quartz substrate.

[0049] Preferably, the sacrificial layer material in step 1) is aluminum.

[0050] Preferably, the rigid substrate in step 8) is one of a glass substrate, a sapphire substrate and a quartz substrate.

[0051] Preferably, the sacrificial layer material in step 8) is a negative photoresist.

[0052] An ultrasonic device comprises the flexible piezoelectric micromechanical ultrasonic transducer.

[0053] Preferably, the ultrasonic device comprises multiple flexible piezoelectric micromachined ultrasonic transducers, electrically connected to each other via parallel top and bottom electrode layers, with the connecting conductors each utilizing a dual-path serpentine-shaped wire. This redundant design of serpentine-shaped wires achieves interconnection, maintaining electrical connectivity while forming an island-bridge structure between the transducer units. This increases the device's stretchability and flexibility, reduces localized stress concentration when subjected to external forces, and improves the device's overall durability.

[0054] Preferably, the ultrasonic device is one of a portable ultrasonic imaging device, an active tactile generating device, and a wearable vital sign monitoring device.

[0055] The working principle of the flexible piezoelectric micromechanical ultrasonic transducer of the present invention is as follows: when the transducer works in the transmitting mode, a pulse excitation voltage signal is applied to the top electrode layer, the bottom electrode layer is grounded, the piezoelectric layer vibrates with the frequency of the pulse excitation signal, and the electrical energy is converted into mechanical energy based on the inverse piezoelectric effect to realize the transmission of the ultrasonic signal; when the transducer works in the receiving mode, the incident ultrasonic signal causes the piezoelectric layer to vibrate, and the mechanical energy is converted into electrical energy based on the positive piezoelectric effect, and the signal is extracted and amplified through signal conditioning circuits such as charge amplifiers, voltage amplifiers, and transimpedance amplifiers to realize the reception and preprocessing of the ultrasonic signal.

[0056] The beneficial effects of the present invention are: the flexible piezoelectric micromechanical ultrasonic transducer of the present invention has the advantages of high sensitivity, good tensile ductility and bending, flexible adjustment of resonant frequency, and easy arraying, and its preparation method is simple, the production cost is low, and it is easy to mass produce, and is suitable for large-scale industrial production and application.

[0057] Specifically:

[0058] 1) The flexible piezoelectric micromechanical ultrasonic transducer of the present invention uses a flexible coating layer, which gives the transducer greater tensile ductility and bendability, and can effectively relieve and disperse the stress applied to the transducer structural unit. The first and second supporting layers are made of an epoxy-based negative photoresist with high mechanical strength and chemical corrosion resistance, which gives the first and second supporting layers a certain bending ability, reducing the risk of fracture. When the transducer is subjected to large compressive / tensile stress, the rigid support structure (first and second supporting layers) will not produce excessive deformation, thereby avoiding the resonant frequency and transceiver sensitivity from being significantly offset due to external forces, which may affect the normal performance of the device.

[0059] 2) The piezoelectric layer in the flexible piezoelectric micromechanical ultrasonic transducer of the present invention uses a 0-3 type piezoelectric composite material, which can improve the piezoelectric performance of the piezoelectric layer. The nano-potassium sodium niobate ceramic particles therein can improve the electromechanical coupling coefficient of the piezoelectric layer, which not only effectively improves the sensitivity of the piezoelectric layer, but also ensures the flexibility and bending ability of the piezoelectric layer.

[0060] 3) The present invention adopts a reverse micro-nano processing technology to prepare a flexible piezoelectric micromechanical ultrasonic transducer, which is compatible with the preparation process of piezoelectric materials requiring high-temperature annealing, avoiding the impact of high temperature on other structures. In addition, the epoxy-based negative photoresist used can form a high-aspect ratio structural pattern by spin coating lithography. Therefore, the structural height can be adjusted by controlling its viscosity and spin coating speed to achieve flexible control of the resonant frequency, and reduce production costs, facilitating large-scale production and preparation. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 Schematic diagram of the structure of the flexible piezoelectric micromechanical ultrasonic transducer of the present invention.

[0062] Figure 2 This is a structural schematic diagram (top view) of the lateral stretching process of the top electrode layer in the flexible piezoelectric micromechanical ultrasonic transducer of the present invention.

[0063] Explanation of the accompanying symbols: 10, flexible covering layer; 20, flexible substrate; 30, adhesive layer; 40, first supporting layer; 50, second supporting layer; 60, bottom electrode layer; 70, piezoelectric layer; 701, polyvinylidene fluoride-trifluoroethylene; 702, nano potassium sodium niobate ceramic particles; 80, top electrode layer; 801, contact; 802, serpentine winding wire.

[0064] Figure 3 This is the normalized sound pressure-frequency curve of the flexible piezoelectric micromechanical ultrasonic transducer of Example 1.

[0065] Figure 4 This is the normalized sound pressure-frequency curve of the flexible piezoelectric micromechanical ultrasonic transducer of Example 2.

[0066] Figure 5 This is the sound pressure-frequency curve of the flexible piezoelectric micromechanical ultrasonic transducer of Example 3 at 1 mm in the axial sound field. DETAILED DESCRIPTION

[0067] The present invention will be further explained and illustrated below with reference to specific embodiments.

[0068] Example 1:

[0069] A flexible piezoelectric micromechanical ultrasonic transducer (structural diagram as shown in Figure 1 As shown, it consists of a stacked structure and a flexible covering layer 10 that encapsulates the stacked structure; the flexible covering layer 10 is made of Ecoflex silicone rubber from Smooth-On; the stacked structure consists of a flexible substrate 20, an adhesive layer 30, a first supporting layer 40, a second supporting layer 50, a bottom electrode layer 60, a piezoelectric layer 70, and a top electrode layer 80 stacked in sequence from bottom to top;

[0070] The flexible substrate 20 is made of epoxy negative photoresist SU-8 2025 from Suzhou Zhongxin Qiheng Scientific Instrument Co., Ltd., with a thickness of 50 μm;

[0071] The adhesive layer 30 is made of Kayaku's photosensitive low-temperature curing epoxy resin adhesive PermiNex 1005, with a thickness of 5 μm;

[0072] The first supporting layer 40 is provided with a through cavity structure, which is cylindrical and has a diameter of 110 μm / 106 μm / 102 μm / 98 μm / 94 μm / 90 μm;

[0073] The first supporting layer 40 is made of epoxy negative photoresist SU-82025 from Suzhou Zhongxin Qiheng Scientific Instrument Co., Ltd., with a thickness of 50 μm;

[0074] The second supporting layer 50 is made of epoxy negative photoresist SU-82005 from Suzhou Zhongxin Qiheng Scientific Instrument Co., Ltd., with a thickness of 5 μm;

[0075] The bottom electrode layer 60 is made of Au-Cr alloy and has a thickness of 400 nm. The area of the bottom electrode layer 60 is equal to the opening area of the first support layer 40 that penetrates the cavity structure.

[0076] The piezoelectric layer 70 is made of polyvinylidene fluoride-trifluoroethylene 701 (Arkema FC 20, composed of a molar ratio of 4:1 between vinylidene fluoride units and trifluoroethylene units) and nano-potassium sodium niobate ceramic particles 702 (particle size 300nm-500nm) in a mass ratio of 3:1, with a thickness of 2μm;

[0077] The top electrode layer 80 includes a plurality of contacts 801 arranged in an array, and adjacent contacts 801 are connected by a double-path serpentine wire 802 (the schematic diagram of the structure of the top electrode layer during the lateral stretching process is shown in FIG. Figure 2 As shown; top view); the top electrode layer 80 is made of Au-Cr alloy and has a thickness of 400 nm; the area of the top electrode layer 80 is 70% of the opening area of the first support layer 40 that penetrates the cavity structure.

[0078] Depend on Figure 2 It can be seen that when the top electrode layer is stretched and deformed, the serpentine winding wire can adapt to the deformation well, effectively disperse the force, reduce the concentration of local stress, and prevent the transducer cavity structure from producing excessive deformation within a certain stretching range, thereby improving the flexibility and durability of the entire device.

[0079] The preparation method of the flexible piezoelectric micromechanical ultrasonic transducer is as follows:

[0080] 1) A glass substrate was ultrasonically cleaned with acetone, ethanol, and deionized water in sequence, and then dried. A 1 μm thick aluminum film was then deposited on one side of the glass substrate using magnetron sputtering to form a first sacrificial layer.

[0081] 2) Spin-coating a stripped photoresist on the surface of the first sacrificial layer, then performing thermal baking and exposure and development to form a top electrode layer preparation area, then rinsing with deionized water and drying with a nitrogen gun, then depositing a 400 nm thick Au-Cr alloy thin film on the top electrode layer preparation area by an evaporation process, and then immersing in acetone to remove the photoresist to form a top electrode layer;

[0082] 3) 3-aminopropyltriethoxysilane is coated on the surface of the first sacrificial layer and the top electrode layer (to improve the adhesion of the piezoelectric layer to the electrode), and then a piezoelectric material (a dispersion of 15% polyvinylidene fluoride-trifluoroethylene and 5% nano-potassium sodium niobate ceramic particles in N,N-dimethylformamide as the solvent) is coated on the surface of the first sacrificial layer and the top electrode layer by spin coating. The layers are then placed on a hot plate and baked at 70°C for 15 minutes. The photoresist is then spin-coated and stripped, followed by heat baking, exposure, and development. The layers are then rinsed with deionized water and blown dry with a nitrogen gun. The layers are then patterned by RIE etching (to reduce crosstalk between transducer units and to reserve a through hole for leading the wires from the bottom electrode layer to the same side of the top electrode layer). The etching gases are O2, C4F8, and Ar, and the etching power is 100 W, to form a piezoelectric layer with a thickness of 2 μm.

[0083] 4) Spin-coating the stripped photoresist on the surface of the piezoelectric layer, then performing heat baking and exposure and development to form a bottom electrode layer preparation area, then rinsing with deionized water and drying with a nitrogen gun, and then depositing a 400 nm thick Au-Cr alloy thin film on the bottom electrode layer preparation area by an evaporation process, and then immersing in acetone to remove the photoresist to form a bottom electrode layer;

[0084] 5) annealing the device obtained in step 4) at 130° C. in a vacuum environment for 60 min (to increase the crystallinity of the piezoelectric composite material, induce phase transition, increase the β-phase content of the piezoelectric film in the organic polymer, and enhance the interfacial bonding strength between the top / bottom electrode layer and the piezoelectric layer), then naturally cooling to room temperature, and polarizing the piezoelectric layer by corona poling at 80° C. (to preferentially orient the dipoles and further enhance the piezoelectric performance) at a polarization voltage of 2 kV for 20 min;

[0085] 6) Spin coating the surfaces of the piezoelectric layer and the bottom electrode layer with epoxy negative photoresist SU-8 2005 from Suzhou Zhongxin Qiheng Scientific Instrument Co., Ltd., followed by heat baking, exposure, development, and hardening, followed by rinsing with deionized water and drying with a nitrogen gun to form a second support layer with a thickness of 5 μm;

[0086] 7) Spin coating a portion of the surface of the second support layer with epoxy negative photoresist SU-8 2025 manufactured by Suzhou Zhongxin Qiheng Scientific Instrument Co., Ltd., followed by heat baking, exposure, development, and hardening, followed by rinsing with deionized water and drying with a nitrogen gun to form a first support layer with a thickness of 50 μm;

[0087] 8) A glass substrate was ultrasonically cleaned with acetone, ethanol, and deionized water in sequence, dried, and then a negative photoresist with a thickness of 3 μm was spin-coated on one side of the glass substrate to form a second sacrificial layer;

[0088] 9) Spin coating the surface of the second sacrificial layer with epoxy negative photoresist SU-8 2025 from Suzhou Zhongxin Qiheng Scientific Instrument Co., Ltd., followed by heat baking, exposure, development, and hardening, followed by rinsing with deionized water and drying with a nitrogen gun to form a flexible substrate with a thickness of 50 μm;

[0089] 10) Adhere the first support layer and the flexible substrate using Kayaku's photosensitive low-temperature curing epoxy resin adhesive PermiNex 1005, then perform heat baking, exposure and development, and heat curing to form an adhesive layer with a thickness of 5 μm;

[0090] 11) Acetone was used to dissolve the second sacrificial layer, and the corresponding glass substrate was removed. The device was then rinsed with deionized water and blown dry with a nitrogen gun. The device was then bombarded with oxygen plasma for 120 seconds (to make the sample more hydrophilic). The device was then fixed in a mold with the flexible substrate facing down and maintained at a certain distance from the bottom of the mold. Ecoflex silicone rubber was then injected until the area below the first sacrificial layer of the device was submerged. The device was then allowed to stand and cure at room temperature. The first sacrificial layer was then dissolved with an 85% by mass phosphoric acid solution, and the corresponding glass substrate was removed. Ecoflex silicone rubber with a thickness of 10 μm was then sprayed on the surface of the piezoelectric layer and the top electrode layer. The device was then allowed to stand and cure at room temperature to form a flexible coating layer, thereby obtaining a flexible piezoelectric micromechanical ultrasonic transducer.

[0091] The normalized sound pressure-frequency curves of the flexible piezoelectric micromechanical ultrasonic transducers of this embodiment (six types of transducers, with the diameters R1 of the cavity structure penetrating therethrough being denoted as a to f from small to large) are shown in FIG. Figure 3 shown.

[0092] Depend on Figure 3 It can be seen that as the diameter of the through-cavity structure changes, the resonant frequency of the flexible piezoelectric micromechanical ultrasonic transducer with the maximum transmitted sound pressure changes significantly.

[0093] Example 2:

[0094] A flexible piezoelectric micromechanical ultrasonic transducer, wherein the thickness of the second supporting layer is 5μm / 6μm / 7μm / 8μm / 9μm / 10μm, the diameter R1 of the through-cavity structure is 110μm, and the rest is exactly the same as the flexible piezoelectric micromechanical ultrasonic transducer of Example 1.

[0095] The normalized sound pressure-frequency curves of the flexible piezoelectric micromechanical ultrasonic transducers of this embodiment (six types of transducers, the thickness R2 of the second supporting layer is denoted as a to f from small to large) are as follows: Figure 4 shown.

[0096] Depend on Figure 4 It can be seen that as the thickness of the second supporting layer changes, the resonant frequency of the flexible piezoelectric micromechanical ultrasonic transducer with the maximum transmitted sound pressure changes significantly.

[0097] Example 3:

[0098] A flexible piezoelectric micromechanical ultrasonic transducer, wherein a cylindrical protrusion is provided at the center of the surface of the second supporting layer (the surface corresponding to the through-cavity structure of the first supporting layer) (the size specifications of the cylindrical protrusion are diameter 50μm×height 40μm / diameter 60μm×height 30μm / diameter 80μm×height 40μm; the composition of the cylindrical protrusion is the same as that of the second supporting layer), the diameter R1 of the through-cavity structure is 110μm, and the rest is exactly the same as the flexible piezoelectric micromechanical ultrasonic transducer of Example 1.

[0099] The sound pressure-frequency curves of the flexible piezoelectric micromechanical ultrasonic transducer of this embodiment (three types of transducers, with cylindrical protrusions denoted as a to c in descending order of volume) at 1 mm in the axial sound field are shown in FIG. Figure 5 (The flexible piezoelectric micromechanical ultrasonic transducer of Example 1 with a through-cavity structure diameter R1 of 110 μm and no additional protrusion structure is used as a control; R3 represents the diameter of the cylindrical protrusion, and H represents the height of the cylindrical protrusion).

[0100] Depend on Figure 5 It can be seen that by selecting the appropriate size of the cylindrical protrusion and making the device perform piston-like motion, the sensitivity of the device can be further improved without reducing the resonant frequency.

[0101] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A flexible piezoelectric micromechanical ultrasonic transducer, characterized in that: A flexible covering layer comprising a stacked structure and an encapsulated stacked structure; the stacked structure comprises a flexible substrate, an adhesive layer, a first supporting layer, a second supporting layer, a bottom electrode layer, a piezoelectric layer, and a top electrode layer stacked in sequence; the first supporting layer is provided with a through-cavity structure; the first supporting layer and the second supporting layer are both made of epoxy-based negative photoresist; the piezoelectric layer comprises polyvinylidene fluoride-trifluoroethylene and nano-potassium sodium niobate ceramic particles; the mass ratio of the polyvinylidene fluoride-trifluoroethylene to the nano-potassium sodium niobate ceramic particles is 1.5 to 3.0:1; and the particle size of the nano-potassium sodium niobate ceramic particles is 300 nm to 500 nm.

2. The flexible piezoelectric micromachined ultrasonic transducer according to claim 1, characterized in that: The flexible coating layer comprises one of silicone rubber and polydimethylsiloxane potting glue; the flexible substrate is made of epoxy-based negative photoresist; and the adhesive layer is made of photosensitive low-temperature curing flexible epoxy resin glue.

3. The flexible piezoelectric micromachined ultrasonic transducer according to claim 1, characterized in that: The through cavity structure is cylindrical, and has a diameter of 90 μm to 110 μm.

4. The flexible piezoelectric micromachined ultrasonic transducer according to claim 1, characterized in that: The thickness of the first supporting layer is 50 μm to 80 μm; the thickness of the second supporting layer is 5 μm to 10 μm.

5. The flexible piezoelectric micromachined ultrasonic transducer according to claim 1, characterized in that: The composition of the bottom electrode layer includes at least one of Au, Cr, Ag, and Ti; the thickness of the bottom electrode layer is 200nm to 500nm; the composition of the top electrode layer includes at least one of Au, Cr, Ag, and Ti; the thickness of the top electrode layer is 200nm to 500nm.

6. The flexible piezoelectric micromachined ultrasonic transducer according to claim 1, characterized in that: The thickness of the piezoelectric layer is 1 μm to 4 μm.

7. The flexible piezoelectric micromachined ultrasonic transducer according to any one of claims 1 to 6, characterized in that: The surfaces of the second supporting layer and the first supporting layer corresponding to the through-cavity structure are further provided with protruding structures.

8. A method for preparing a flexible piezoelectric micromachined ultrasonic transducer according to any one of claims 1 to 7, characterized in that: The following steps are involved: 1) depositing a sacrificial layer material on a single side of a rigid substrate using magnetron sputtering to form a first sacrificial layer; 2) depositing electrode material on a portion of the surface of the first sacrificial layer using an evaporation process to form a top electrode layer; 3) Spin coating the piezoelectric material on the surface of the first sacrificial layer and the top electrode layer to form a piezoelectric layer; 4) depositing electrode material on a portion of the surface of the piezoelectric layer using an evaporation process to form a bottom electrode layer; 5) Annealing the device obtained in step 4); 6) Spin coating the support layer material on the surface of the piezoelectric layer and the bottom electrode layer to form a second support layer; 7) coating a support layer material on a portion of the surface of the second support layer by spin coating to form a first support layer; 8) Spin coating a sacrificial layer material on one side of another rigid substrate to form a second sacrificial layer; 9) Spin coating the surface of the second sacrificial layer with a flexible substrate material to form a flexible substrate; 10) Adhesive bonding the first supporting layer and the flexible substrate to form an adhesive layer; 11) A solvent is used to dissolve the second sacrificial layer, and the corresponding rigid substrate is removed. The device is then fixed in a mold with the flexible substrate facing downward and maintaining a certain distance from the bottom surface of the mold. The coating material is then injected until the portion below the first sacrificial layer of the device is immersed, and then cured. The first sacrificial layer is then dissolved by a solvent, and the corresponding rigid substrate is removed. The coating material is then sprayed onto the surface of the piezoelectric layer and the top electrode layer, and then cured to form a flexible coating layer, thereby obtaining a flexible piezoelectric micromechanical ultrasonic transducer.

9. An ultrasonic device, characterized in that The flexible piezoelectric micromechanical ultrasonic transducer comprises the flexible piezoelectric micromechanical ultrasonic transducer according to any one of claims 1 to 7.

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