A capacitive micromachined ultrasonic transducer based on 3D printing and its preparation method

Abstract

The present invention belongs to the field of materials and electronic device manufacturing, and discloses a capacitive micromachined ultrasonic transducer based on 3D printing and a method for preparing the same. The CMUTs comprise an upper electrode, an upper insulating layer, a vibrating membrane, a pillar layer, a silicon substrate, and a lower electrode, connected sequentially from top to bottom. A cavity is formed between the vibrating membrane, the lower insulating layer, and the pillars of the pillar layer. The method comprises: using a nano-silicon dioxide composite material and a light-curing 3D printing method to print the pillar layer array on the front surface of a functionalized silicon substrate; then performing curing and heat treatment to obtain a second device; then bonding the vibrating membrane; then printing an upper insulating layer on the front surface of the vibrating membrane; then performing curing and heat treatment again; then printing the upper electrode; then removing the silicon dioxide from the back surface of the silicon substrate through a wet etching process; then printing the lower electrode on the back surface of the silicon substrate; and finally removing the photoresist. The present invention reduces the required equipment and material consumption, thereby lowering production costs.

Description

Technical Field

[0001] The present invention belongs to the field of material and electronic device manufacturing, and specifically relates to a capacitive micromechanical ultrasonic transducer based on 3D printing and a preparation method thereof. Background Art

[0002] Capacitive micromachined ultrasonic transducers (CMUTs) have been applied in a variety of fields, including audio equipment, fingerprint recognition, bubble detection, and stress monitoring. Compared to traditional bulk ultrasonic sensors, CMUTs have attracted widespread attention due to their low power consumption, small size, wide bandwidth, good impedance matching, and ease of integration with integrated circuits (ICs). A CMUT cell consists of an upper electrode, a vibrating membrane, an insulating layer, a cavity, a substrate, and a lower electrode. During ultrasonic transmission, the electrostatic force generated by the alternating current voltage between the two plates drives the membrane to vibrate and emit ultrasonic waves. During reception, the ultrasonic wave acts on the membrane, causing changes in capacitance and charge, which in turn affect the voltage in the circuit, enabling ultrasonic detection. The performance of CMUTs depends on the dimensional accuracy of each structure and the consistency of the cell dimensions. Literature indicates that cavity height significantly affects the transmitted sound pressure and receiving sensitivity of CMUTs, but has little effect on the bandwidth. Traditional CMUT cavity fabrication typically involves etching or sacrificial processes, which are chemical processes that require high temperature, humidity, operating equipment, and operator experience. The large surface area of ​​the wafer makes it difficult to maintain consistent processing conditions, resulting in inconsistent cavity heights and, in turn, reduced CMUT performance. Furthermore, existing CMUT fabrication processes are time-consuming and complex, making rapid fabrication difficult. For example, when processing the silicon dioxide insulating layer, a layer of silicon dioxide must first be oxidized on the silicon wafer surface, then gradually etched to the designed thickness. This requires repeated measurements with a film thickness meter to ensure accuracy. Therefore, there is an urgent need to develop a method for rapidly and accurately fabricating CMUTs.

[0003] 3D printing, also known as rapid prototyping or additive manufacturing, is a layer-by-layer additive manufacturing process that eliminates material removal and simplifies the manufacturing process. The printed devices also boast high dimensional accuracy and low surface roughness, making it a promising approach for CMUT fabrication to address the aforementioned challenges. Currently, research on 3D-printed CMUTs is limited, but some institutions have conducted research in related fields. The Institute of Microstructure Technology at the Karlsruhe Institute of Technology has used a stereolithography 3D printer to produce transparent fused silica glass components with a resolution of tens of microns, making 3D printing of silica insulating layers for CMUTs possible. The French National Center for Scientific Research (CNRS) has used 3D and 2D inkjet printing technologies to print capacitive acoustic resonators. The University of Edinburgh has developed an integrated 3D printing method for the rapid fabrication of MEMS devices. Using this method, they successfully fabricated a capacitive force sensor with a relatively complex suspended beam-plate structure in under 11 hours. Carnegie Mellon University has used 3D printing to fabricate nanoscale metal electrode arrays and successfully applied them to bioelectrical signal detection.

[0004] In summary, existing 3D printing technology has achieved high-precision printing at the nanoscale and can rapidly print acoustic resonators and capacitive force sensors, validating the feasibility of 3D-printed CMUTs. However, current 3D-printed CMUT technology still faces the following challenges: Research on 3D-printed CMUTs currently focuses primarily on 3D-printing microstructures made of various materials, with few efforts to fully fabricate CMUTs using 3D printing technology. Furthermore, there is a lack of process and structural design for 3D-printed CMUTs. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention aims to provide a capacitive micromachined ultrasonic transducer based on 3D printing and a method for preparing the same. The present invention designs a complete 3D-printed CMUTs process flow, which reduces the required equipment and material consumption, lowering production costs; reduces processes such as mask design, development, and etching, thereby simplifying the processing technology, greatly shortening the time from design to finished product, facilitating rapid iteration and product optimization, and increasing production rates.

[0006] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a capacitive micromachined ultrasonic transducer based on 3D printing, wherein the capacitive micromachined ultrasonic transducer based on 3D printing comprises an upper electrode, an upper insulating layer, a vibrating membrane, a supporting layer, a silicon substrate, and a lower electrode connected in sequence from top to bottom, wherein the supporting layer comprises a lower insulating layer located on the front side of the silicon substrate and a supporting layer located on the front side of the lower insulating layer; a cavity is formed between the vibrating membrane, the lower insulating layer, and the supporting layer;

[0008] The preparation method comprises the following steps:

[0009] Using nano-silica composite materials and a light-curing 3D printing method, a pillar layer array was printed on the front side of a functionalized silicon substrate to obtain the first device;

[0010] curing and heat-treating the first device to obtain a second device;

[0011] Bonding a vibration membrane to the front surface of the support of the second device to obtain a third electrical device;

[0012] The surface of the vibration membrane of the third electrical device is functionalized, and an insulating layer is printed on the front surface of the vibration membrane of the third electrical device using a nano-silicon dioxide composite material and a light-curing 3D printing method to obtain a fourth device;

[0013] curing and heat-treating the fourth device to obtain a fifth device;

[0014] Printing an upper electrode on the surface of the upper insulating layer of the fifth device by 3D printing to obtain a sixth device;

[0015] removing silicon dioxide on the back side of the silicon substrate in the sixth device by a wet etching process to obtain a seventh device;

[0016] Printing a lower electrode on the back side of the silicon substrate in the seventh device by 3D printing to obtain an eighth device;

[0017] Removing the photoresist on the eighth device to obtain the 3D printing-based capacitive micromachined ultrasonic transducer;

[0018] The heat treatment includes a thermal debinding process and a sintering process after the thermal debinding process.

[0019] Preferably, the preparation method of the nano-silicon dioxide composite material includes:

[0020] Amorphous nano-silica powder is uniformly dispersed in a mixture of hydroxyethyl methacrylate and trimethylolpropane ethoxylated triacrylate, and then degassed to obtain the nano-silica composite material; in terms of mass percentage, the mixture contains 60%-70% of hydroxyethyl methacrylate and 30%-40% of trimethylolpropane ethoxylated triacrylate, and the mass of the amorphous nano-silica powder accounts for 30%-50% of the mass of the mixture.

[0021] Preferably, the functionalization process comprises: washing the surface of the silicon device with acidic methanol to remove organic matter on the surface of the device, and then immersing the device in a dimethylchlorosilane solution to functionalize the surface of the device, wherein the device is a silicon substrate before printing the support layer array or a vibration membrane of the third electrical device;

[0022] The first device is placed in a mixed solution of methanol and water prepared in a volume ratio of (1-3):1, and ultrasonic vibration is used to dissolve the uncured nano-silica composite material in the printed pillar layer array. After dissolution, the surface of the first device is blown dry, and then dried to remove water molecules on the surface of the first device. The first device is then finally cured by ultraviolet light;

[0023] The fourth device is placed in a mixed solution of methanol and water prepared in a volume ratio of (1-3):1, and ultrasonic vibration is used to dissolve the uncured nano-silica composite material in the printed upper insulating layer. After dissolution, the surface of the fourth device is blown dry and then dried to remove water molecules on the surface of the fourth device. The fourth device is then finally cured by ultraviolet light.

[0024] Preferably, during the thermal degreasing process, the first component or the fourth component is heated to 300-500° C. and kept at this temperature for 3-6 hours to complete the thermal degreasing process;

[0025] During the sintering process, the first component or the fourth component after the thermal debinding process is heated to 600-900° C., kept at this temperature for 10-20 hours, and then cooled to complete the sintering process.

[0026] Preferably, when the vibrating membrane is bonded to the front side of the pillar of the second device:

[0027] Align the front side of the second device's pillar with the diaphragm, apply bonding pressure to 20-60 kN, and evacuate to 10 -5 mbar, and then continue bonding at 300-500° C. for 2-4 hours, and then anneal at 350-600° C. for 5-8 hours to obtain a third electrical device.

[0028] Preferably, the process of printing the upper electrode on the surface of the upper insulating layer of the fifth device by 3D printing, and the process of printing the lower electrode on the back surface of the silicon substrate in the seventh device by 3D printing both include:

[0029] A mixed powder of 10%-20% sodium citrate powder, 5%-15% sodium chloride, and 75%-85% metal powder is added to a 3D printer storage box. The metal powder includes one or more of aluminum powder, gold powder, and platinum powder. Deionized water is added to the metal 3D printing cartridge.

[0030] Place the front side of the upper insulating layer or the back side of the silicon substrate upwards in the metal 3D printer. Spread a layer of mixed powder on the discharge port of the metal 3D printer and compact it with a roller. Use a precision inkjet nozzle to spray deionized water along a predetermined trajectory to activate the chelation reaction.

[0031] The mixed powder that has not undergone chelation reaction is then blown off with nitrogen gas, and the fifth device or the seventh device is then placed in a humidifier with a water spray rate of 20-40 ml / h for 30-60 minutes to completely trigger the chelation reaction of the mixed powder.

[0032] Then, the fifth device or the seventh device is placed in a tube furnace under vacuum conditions for heat treatment to obtain a sixth device or an eighth device accordingly.

[0033] Preferably, the method for preparing a capacitive micromachined ultrasonic transducer based on 3D printing of the present invention further includes the following steps:

[0034] Spin-coating photoresist on the front side of the sixth device, then photolithography the CMUTs array chip dicing pattern, and wet etching to the silicon dioxide layer on the front side of the silicon substrate, or etching through the entire silicon substrate to obtain a plurality of chips;

[0035] Then, photoresist is spin-coated on the front of the sixth device or the obtained chip, and then the sixth device or the obtained chip is baked at 70-100° C. for 30-60 minutes, and hydrofluoric acid is used to remove the silicon dioxide on the back of the silicon substrate in the sixth device or chip to obtain the seventh device.

[0036] Preferably, the silicon substrate and the vibration membrane are both N-type <100> Low resistance silicon wafer, the N-type <100> The front and back sides of the low-resistance silicon wafer both have a silicon dioxide layer generated by oxidation;

[0037] Before bonding the vibration membrane to the front side of the pillar of the second device, the silicon dioxide layer on the surface of the vibration membrane is first removed and processed to a predetermined thickness.

[0038] The present invention also provides a capacitive micromachined ultrasonic transducer based on 3D printing, which is manufactured by the above-mentioned method for preparing a capacitive micromachined ultrasonic transducer based on 3D printing of the present invention.

[0039] Preferably, the shape of the upper electrode is circular, square, triangular or regular polygonal;

[0040] The shape of the cavity is cylindrical. When the shape of the upper electrode is circular, the diameter of the upper electrode is the same as the diameter of the cavity, and the electrode and the cavity are opposite;

[0041] The upper insulating layer covers the entire surface of the vibration membrane, and the lower insulating layer covers the entire surface of the silicon substrate.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] The present invention discloses a method for preparing a capacitive micromachined ultrasonic transducer based on 3D printing, which reduces the required equipment and material consumption and lowers production costs. It also reduces processes such as mask design, development, and etching, thereby simplifying the processing technology and significantly shortening the time from design to finished product, facilitating rapid iteration and product optimization, and increasing production rates. The cavity shape, insulation layer thickness, and electrode shape of the CMUT device can be customized according to specific needs, thereby improving device performance and meeting the requirements of specific application scenarios. The materials used in the 3D printing process can all achieve high-precision 3D printing, with low loss and recyclable reuse, reducing material waste and achieving higher material utilization and environmental benefits. The method for preparing a capacitive micromachined ultrasonic transducer proposed in the present invention can be applied to nanoscale printing of various structures. The printed structures have low surface roughness, low porosity, and high compatibility with MEMS fabrication processes. The chelating agent used in the 3D printing of metal electrodes is a non-toxic and environmentally friendly material, which improves printing safety and environmental protection. As can be seen from the process of the present invention, the method has the advantages of simple process, low processing cost, short preparation cycle, high design freedom, and environmental friendliness.

[0044] In the capacitive micromachined ultrasonic transducer based on 3D printing of the present invention, the upper electrode of the CMUT unit is designed to cover only the CMUT unit cavity area, and the upper electrode is insulated from the single-crystal silicon by a silicon dioxide insulating layer. This can reduce the parasitic capacitance caused by electrode coverage in the pillar area, and can appropriately increase the width of the pillar and the width of the bonding area outside the array, thereby improving the bonding strength without increasing parasitic capacitance. A silicon dioxide layer (i.e., a lower insulating layer) is designed on the front of the silicon substrate and covers the entire front surface of the silicon substrate. This design ensures that when the vibrating membrane ruptures or collapses, the vibrating membrane does not directly contact the silicon substrate and cause a short circuit. The CMUT unit structure is simple, reducing processing difficulty and improving device performance and yield. The capacitive micromachined ultrasonic transducer prepared by the present invention based on 3D printing technology has a simple structure and excellent performance, and can be applied to audio equipment, industrial non-destructive testing, medical imaging and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is a structural diagram of a CMUTs unit provided by an embodiment of the present invention;

[0046] Figure 2 is a 1 / 4 cross-sectional view of a CMUTs unit provided by an embodiment of the present invention;

[0047] Figure 3 This is a chip diagram of CMUTs before dicing provided by an embodiment of the present invention;

[0048] Figure 4 is a diagram of a square array of CMUTs provided by an embodiment of the present invention;

[0049] Figure 5 This is a flow chart of a method for preparing CMUTs provided in an embodiment of the present invention.

[0050] Figure numerals: 1-upper electrode, 2-upper insulating layer, 3-vibration membrane, 4-pillar layer, 5-silicon substrate, 6-lower electrode, 7-cavity, 8-pillar, 9-lower insulating layer, 10-CMUTs array, 11-CMUTs unit, 12-photoresist. DETAILED DESCRIPTION

[0051] In order to make the purpose, technical solution and advantages of this technical solution more clear, the following technical solution is further described in detail in conjunction with specific implementation methods. It should be understood that these descriptions are only exemplary and are not intended to limit the scope of this technical solution.

[0052] To address the challenges of subtractive fabrication of CMUTs, this paper proposes a capacitive micromachined ultrasonic transducer and corresponding fabrication method based on 3D printing. This method directly constructs the key CMUT structures, significantly shortening the fabrication cycle, reducing costs, and increasing production output. This method utilizes environmentally friendly materials and optimizes existing processes to reduce the porosity and surface roughness of the 3D-printed structure.

[0053] See also Figure 1 、 Figure 2 and Figure 5 The capacitive micromachined ultrasonic transducer based on 3D printing of the present invention includes an upper electrode 1, an upper insulating layer 2, a vibration membrane 3, a support layer 4, a silicon substrate 5 and a lower electrode 6 connected in sequence from top to bottom. The support layer 4 includes a lower insulating layer 9 located on the front of the silicon substrate 5 and a support 8 located on the front of the lower insulating layer 9; a cavity 7 is formed between the vibration membrane 3, the lower insulating layer 9 and the support 8, and the various structures are tightly connected to each other.

[0054] As a preferred embodiment of the present invention, in this example, the cell in the 3D-printed capacitive micromachined ultrasonic transducer is the smallest unit of the array. The actual CMUTs are composed of multiple cells, each of which can independently control its reception and transmission of ultrasonic waves. This example uses an 8x8 linear array with an array size of 5mm x 5mm. Each cell is connected in series to ensure simultaneous control of its transmission and reception. This arraying improves the ratio of effective capacitance to parasitic capacitance, reducing the difficulty of signal detection.

[0055] As a preferred embodiment, the shape of the upper electrode 1 is circular, square, triangular or regular polygonal.

[0056] As a preferred embodiment of the present invention, in this embodiment, the upper electrode 1 is circular and the cavity 7 is cylindrical. In order to reduce the parasitic capacitance and bonding strength generated by the supporting layer, the diameter of the upper electrode 1 is the same as the diameter of the cavity 7.

[0057] As a preferred embodiment of the present invention, in this embodiment, the upper insulating layer 2 covers the entire surface of the vibration membrane, ensuring that the upper electrode 1 does not directly contact the vibration membrane 3, thereby preventing a short circuit.

[0058] As a preferred embodiment of the present invention, in this embodiment, the lower insulating layer 9 covers the entire surface of the silicon substrate 5, effectively ensuring that the single crystal silicon film does not short-circuit due to film rupture, collapse, etc. when in contact with the substrate.

[0059] As a preferred embodiment of the present invention, in this embodiment, the low-resistance silicon wafer used for the silicon substrate 5 can be purchased directly and needs to be cleaned in a standard manner.

[0060] As a preferred embodiment of the present invention, in this embodiment, the lower electrode 6 covers the entire surface of the silicon substrate 5 to improve the conductive performance.

[0061] As a preferred embodiment of the present invention, the materials of the upper electrode 1 and the lower electrode 6 used in the present invention are aluminum, gold or platinum. The materials of the upper electrode 1 and the lower electrode 6 have the characteristics of high conductivity, sufficient mechanical strength, good chemical stability, good matching with the dielectric layer, and high compatibility with the 3D printing preparation process.

[0062] As a preferred embodiment of the present invention, in this embodiment, the material of the upper insulating layer 2 is low-resistance silicon dioxide, covering the entire surface of the vibration membrane 3, ensuring that the upper electrode does not directly contact the vibration membrane, thereby preventing circuit short circuit and increasing the collapse voltage.

[0063] As a preferred embodiment of the present invention, in this embodiment, the material of the pillar 8 and the lower insulating layer 9 is low-resistance silicon dioxide.

[0064] like Figure 5 As shown, the present invention also proposes a method for preparing the above-mentioned capacitive micromachined ultrasonic transducer based on 3D printing, which specifically includes the following steps:

[0065] Step 1: Prepare the substrate silicon wafer

[0066] (1) Take an 8-inch N-type <100> The low-resistance silicon wafer (i.e., silicon substrate 5) is laser-marked on the back of the silicon wafer to mark the substrate silicon wafer. The silicon wafer is then subjected to standard cleaning, air-rinsing, and drying to complete the substrate preparation.

[0067] Step 2: 3D print the silicon dioxide pillar layer on the base silicon wafer

[0068] (1) Amorphous nano-silica powder was dispersed in a beaker containing dispersants of hydroxyethyl methacrylate and trimethylolpropane ethoxylated triacrylate, the mixture was stirred using a laboratory dissolver, and degassed using ultrasound to prepare a nano-silica composite material. The composite material was then added to a light-curing 3D printer. In terms of mass percentage, the dispersant contained 60%-70% hydroxyethyl methacrylate and 30%-40% trimethylolpropane ethoxylated triacrylate, and the mass of the amorphous nano-silica powder was 30%-50% of the mass of the dispersant.

[0069] (2) The substrate silicon surface is rinsed with acidic methanol to remove surface organic matter and increase surface activity, and then immersed in dimethylchlorosilane solution to functionalize the substrate silicon surface.

[0070] (3) Place the processed base silicon wafer with its back side facing down in a photocuring 3D printer, and print a three-dimensional pillar layer array on the base silicon wafer using photocuring 3D printing technology.

[0071] (4) Place the printed base silicon into a solution of methanol and water in a volume ratio of (1-3):1, and use ultrasonic vibration to accelerate the dissolution of the uncured composite material. After dissolution, blow dry the surface and place it at 40-50℃ to dry for 1-2 hours to ensure that there are no excess water molecules on the surface.

[0072] (5) Place the dried substrate silicon wafer in a UV curing chamber to further cure the nanocomposite material and reduce the void content.

[0073] (6) The base silicon wafer is placed in an ashing furnace for thermal degreasing, and then the base silicon wafer is placed in a high-temperature tube furnace for sintering to evaporate molecular water and surface-bound silanol groups.

[0074] (7) Take out the base silicon wafer that has cooled to room temperature, let it stand for 1 hour, and then perform the bonding operation.

[0075] Step 3: Top silicon wafer preparation

[0076] (1) Take another 8-inch N-type <100> The low-resistance silicon wafer is laser marked on the back of the silicon wafer for marking the top silicon wafer. The silicon wafer is mechanically thinned to 15-18 microns, and then standard cleaning, air blowing, and drying are performed to complete the preparation of the vibration membrane 3.

[0077] Step 4: Directly bond the vibration membrane to the upper surface of the support layer at low temperature

[0078] (1) Take out the base silicon wafer and the top silicon wafer and place them in the pressure bonding machine in order and align them. Pressurize the bonding machine to 20-60KN and evacuate to 10 -5mbar, bonding is performed at 300-500℃ for 2-4 hours; after bonding, annealing is performed at 350-600℃ for 5-8 hours to improve the bonding strength.

[0079] Step 5: 3D print an insulating layer on the vibration membrane

[0080] (1) The upper surface of the bonded vibrating membrane is rinsed with acidic methanol to remove surface organic matter and increase surface activity, and then it is immersed in dimethylchlorosilane solution to functionalize its surface.

[0081] (2) Place the processed silicon wafer correctly in the light-curing 3D printer and print a layer of silicon dioxide insulation layer on the vibration membrane using light-curing 3D printing technology.

[0082] (3) Place the printed silicon wafer in a solution of methanol and water in a volume ratio of (1-3):1, and use ultrasonic vibration to accelerate the dissolution of the uncured composite material. After dissolution, blow dry the surface and place it at 40-50℃ to dry for 1-2 hours to ensure that there are no excess water molecules on the surface.

[0083] (4) Place the dried silicon wafer in a UV curing chamber to further cure the nanocomposite material and reduce the void content.

[0084] (5) The silicon wafer is placed in an ashing furnace for thermal degreasing, and then the silicon wafer is placed in a high-temperature tube furnace for sintering to evaporate molecular water and surface-bound silanol groups to form an upper insulating layer 2.

[0085] (6) Take out the silicon wafer that has cooled to room temperature, let it stand for 1 hour, and then print the upper electrode.

[0086] Step 6: 3D print the upper electrode on the upper insulating layer

[0087] (1) Sodium citrate powder with a mass fraction of 10%-20%, sodium chloride with a mass fraction of 5%-15%, and aluminum powder with a mass fraction of 75%-85% and a mesh size of less than or equal to 40 are mixed evenly, screened at one mesh, and added to the metal 3D printer storage box. At the same time, deionized water is added to the metal 3D printing cartridge.

[0088] (2) Place the silicon wafer with the silicon dioxide upper insulating layer facing upward in the metal 3D printer, spread a layer of mixed powder at the outlet and compact it with a roller, and use a precision inkjet nozzle to spray deionized water along a predetermined trajectory to activate the chelation reaction.

[0089] (3) Take out the silicon wafer and blow away the powder on the surface that has not undergone chelation reaction with nitrogen. Then place the silicon wafer in a humidifier with a water spray rate of 20-40 ml / h and humidify for 30-60 minutes to allow the mixed powder to completely trigger the chelation reaction.

[0090] (4) Place the treated silicon wafer in a vacuum tube furnace for heat treatment, which includes: (degreasing) heating from room temperature to 300-500℃ at a rate of 10-20℃ / min, and then treating at 300-500℃ for 3-6 hours, (sintering) heating from 300-500℃ to 600-900℃ at a rate of 10-20℃ / min, and then treating at 600-900℃ for 10-20 hours, (cooling) cooling from 600-900℃ to room temperature at a rate of -10-20℃ / min.

[0091] (5) Remove the silicon wafer and complete the upper electrode printing.

[0092] Step 7: Scribe Street Etching

[0093] (1) Spin-coat the photoresist, photolithography the CMUTs array chip pattern, and use wet etching to etch the silicon dioxide layer on the front of the silicon substrate 5 (the silicon substrate 5 uses an N-type <100> Low resistance silicon wafer, because the surface of the silicon wafer is easily oxidized to form silicon dioxide, so N-type <100> Low-resistivity silicon wafers typically have a silicon dioxide layer on the front and back.)

[0094] (2) Apply glue to the front of the chip and bake the glue in a furnace at 70-100°C for 30-60 minutes to enhance the acid resistance of the photoresist, and use hydrofluoric acid to remove the silicon dioxide on the back of the silicon substrate 5.

[0095] Step 8: 3D print the lower electrode 6 on the back of the silicon substrate 5

[0096] (1) Place the silicon wafer with the back side facing up in the metal 3D printer, spread a layer of mixed powder at the outlet and compact it with a roller, and use a precision inkjet nozzle to spray deionized water along a predetermined trajectory to activate the chelation reaction.

[0097] (2) Take out the silicon wafer and blow away the powder on the surface that has not undergone chelation reaction with nitrogen. Then place the silicon wafer in a humidifier with a water spray rate of 20-40 ml / h and humidify for 30-60 minutes to allow the mixed powder to completely trigger the chelation reaction.

[0098] (3) Place the treated silicon wafer in a vacuum tube furnace for heat treatment.

[0099] (4) Remove the silicon wafer and complete the printing of the lower electrode.

[0100] Step 9: Remove the glue and remove the CMUTs array

[0101] (1) Dry-remove the photoresist spin-coated on the front of the chip and remove the CMUTs array.

[0102] Example 1

[0103] like Figure 1As shown, the CMUTs unit structure of this embodiment includes, from top to bottom, an upper electrode 1, an upper insulating layer 2, a vibrating membrane 3, a supporting layer 4, a substrate 5, and a lower electrode 6. The upper electrode 1 and lower electrode 6 are made of aluminum, the upper insulating layer 2 and supporting layer 4 are made of silicon dioxide, and the vibrating membrane 3 and substrate 5 are made of silicon. These structures are tightly connected to each other.

[0104] like Figure 2 As shown, the upper electrode 1 is circular in shape. In order to reduce the parasitic capacitance and bonding strength generated by the supporting layer 4, its shape and size are the same as those of the cavity 7. The upper insulating layer 2 covers the entire surface of the vibrating membrane to ensure that the upper electrode 1 is not in direct contact with the vibrating membrane 3, prevent circuit short circuit, and increase the collapse voltage. The supporting layer 4 includes a cavity 7, a supporting column 8, and a lower insulating layer 9, wherein the cavity 7 is cylindrical in shape, the same as the upper electrode. The lower insulating layer 9 covers the entire surface of the substrate 5, effectively ensuring that the single crystal silicon film does not short-circuit due to film rupture, collapse, and other factors in contact with the substrate. The silicon wafer used for the substrate 5 can be purchased directly and must undergo standard cleaning. The lower electrode 6 covers the entire surface of the substrate 5 to improve the mechanical strength of the chip.

[0105] like Figure 3 and 4 As shown, the CMUTs unit 11 described in this embodiment is the smallest unit of the CMUTs array 10. The actual CMUTs array 10 is composed of multiple units, and each CMUTs unit 11 can independently control its reception and transmission of ultrasonic waves. The present invention uses a linear CMUTs array 10 with an array size of 5 mm x 5 mm and an 8 x 8 array of CMUTs units 11. Each unit is connected in series to ensure simultaneous control of transmission and reception of each unit. The array improves the ratio of effective capacitance to parasitic capacitance, reducing the difficulty of signal detection.

[0106] like Figure 5 As shown, the method for preparing a 3D printed CMUTs array in this embodiment specifically includes the following steps:

[0107] 1. Substrate silicon wafer preparation

[0108] (1) Take an 8-inch N-type <100> The low-resistance silicon wafer is laser marked on the back of the silicon wafer to mark the base silicon wafer. The silicon wafer is then subjected to standard cleaning, air blowing, and drying to complete the preparation of the silicon substrate 5.

[0109] 2. 3D printing a silicon dioxide support layer 4 on a silicon substrate 5 silicon wafer

[0110] (1) Amorphous nano-silica powder was dispersed in a beaker containing dispersants hydroxyethyl methacrylate and trimethylolpropane ethoxylated triacrylate. The mixture was stirred using a laboratory dissolver and degassed using ultrasound to prepare a nano-silica composite material. The composite material was then added to a light-curing 3D printer. In terms of mass percentage, the dispersant contained 65% hydroxyethyl methacrylate and 35% trimethylolpropane ethoxylated triacrylate, and the mass of the amorphous nano-silica powder was 35% of the mass of the dispersant.

[0111] (2) The substrate silicon surface is rinsed with acidic methanol to remove surface organic matter and increase surface activity, and then immersed in dimethylchlorosilane solution to functionalize the substrate silicon surface.

[0112] (3) Place the processed base silicon wafer with its back side facing downward in a light-curing 3D printer, and print a three-dimensional pillar layer 4 array on the base silicon wafer using light-curing 3D printing technology.

[0113] (4) Place the printed silicon substrate 5 silicon into a solution of methanol and water in a volume ratio of 1:1, and use vibration to dissolve the uncured composite material. After dissolution, blow dry the surface and place it at 50°C to dry for 1 hour to ensure that there are no excess water molecules.

[0114] (5) The dried base silicon wafer 5 is placed in a UV curing chamber to further cure the nanocomposite material and reduce the void ratio.

[0115] (6) The silicon substrate 5 silicon wafer is placed in an ashing furnace for thermal degreasing, and then the silicon substrate 5 silicon wafer is placed in a high-temperature tube furnace for sintering to evaporate molecular water and surface-bound silanol groups.

[0116] (7) Take out the silicon substrate 5 that has cooled to room temperature, let it stand for 1 hour, and then perform the bonding operation.

[0117] 3. Top silicon wafer preparation

[0118] (1) Take another 8-inch N-type <100> The low-resistance silicon wafer is laser marked on the back of the silicon wafer for marking the top silicon wafer. The silicon wafer is mechanically thinned to 15 microns, and then subjected to standard cleaning, air blowing, and drying to complete the preparation of the vibration membrane 3.

[0119] 4. Low-temperature direct bonding of the vibration membrane 3 and the upper surface of the support layer 4

[0120] (1) Take out the silicon substrate 5 silicon wafer and the top silicon wafer and place them in the pressure bonding machine in order and align them. Pressurize the bonding machine to 40KN and evacuate to 10 -5 mbar, bonding was performed at 300°C for 2 hours; annealing was performed at 350°C for 6 hours after bonding to further improve the bonding strength.

[0121] 5. 3D print the insulation layer 2 on the vibration membrane 3

[0122] (1) The upper surface of the bonded vibration membrane 3 is rinsed with acidic methanol to remove surface organic matter and increase surface activity, and then it is immersed in a dimethylchlorosilane solution to functionalize its surface.

[0123] (2) Place the processed silicon wafer correctly in the light-curing 3D printer and print a silicon dioxide insulation layer 2 on the vibration membrane using light-curing 3D printing technology.

[0124] (3) Place the printed silicon wafer in a 1:1 solution of methanol and water, and use ultrasonic vibration to accelerate the dissolution of the uncured composite material. After dissolution, blow dry the surface and place it at 50°C to dry for 1 hour to ensure that there are no excess water molecules.

[0125] (4) Place the dried silicon wafer in a UV curing chamber to further cure the nanocomposite material and reduce the void content.

[0126] (5) The silicon wafer is placed in an ashing furnace for thermal degreasing, and then the silicon wafer is placed in a high-temperature tube furnace for sintering to evaporate molecular water and surface-bound silanol groups.

[0127] (6) Take out the silicon wafer that has cooled to room temperature, let it stand for 1 hour, and then print the upper electrode 1.

[0128] 6. 3D print the upper electrode 1 on the upper insulating layer 2

[0129] (1) Sodium citrate powder with a mass fraction of 20%, sodium chloride with a mass fraction of 5%, and aluminum powder with a mass fraction of 75% and a mesh size of less than or equal to 40 are mixed evenly, screened at one mesh, and added to the metal 3D printer storage box. At the same time, deionized water is added to the metal 3D printing cartridge.

[0130] (2) Place the silicon wafer with the silicon dioxide upper insulating layer 2 facing upward in the metal 3D printer, spread a layer of mixed powder on the discharge port and compact it with a roller, and spray deionized water along the predetermined trajectory with the inkjet nozzle to activate the chelation reaction until the electrode array is printed.

[0131] (3) Take out the silicon wafer and blow away the powder on the surface that has not undergone chelation reaction with nitrogen. Then place the silicon wafer in a humidifier with a water spray rate of 35 ml / h and humidify for 30 minutes to allow the mixed powder to completely trigger the chelation reaction.

[0132] (4) The treated silicon wafer is placed in a vacuum tube furnace for heat treatment, which includes: (degreasing) heating from room temperature to 350°C at a rate of 10°C / min, and then treating at 350°C for 3 hours, (sintering) heating from 350°C to 620°C at a rate of 10°C / min, and then treating at 620°C for 15 hours, and (cooling) cooling from 620°C to room temperature at a rate of -10°C / min.

[0133] (5) Remove the silicon wafer and complete the printing of the upper electrode 1.

[0134] 7. Scribe Street Etching

[0135] (1) Spin-coat the photoresist 12, photolithography the CMUTs array chip pattern, and use wet etching to etch to the silicon dioxide layer of the substrate 5.

[0136] (2) Glue is applied to the front of the chip to prevent the chip from being separated and dislocated due to the removal of the oxide layer, and the glue is baked in an 80°C furnace for 40 minutes to enhance the acid resistance of the photoresist 12. Then, hydrofluoric acid is used to remove the silicon dioxide on the back of the substrate.

[0137] 8. 3D printing the lower electrode 6 on the back of the silicon substrate 5

[0138] (1) Place the silicon wafer with the back side facing up in the metal 3D printer, spread a layer of mixed powder on the discharge port and compact it with a roller, and spray deionized water along the predetermined trajectory with the inkjet nozzle to activate the chelation reaction until the lower electrode 6 array is printed.

[0139] (2) Take out the silicon wafer and blow away the powder on the surface that has not undergone chelation reaction with nitrogen. Then place the silicon wafer in a humidifier with a water spray rate of 35 ml / h and humidify for 30 minutes to allow the mixed powder to completely trigger the chelation reaction.

[0140] (3) Place the treated silicon wafer in a vacuum tube furnace for heat treatment.

[0141] (4) Remove the silicon wafer and complete the printing of the lower electrode 6.

[0142] 9. Remove the glue and take out the CMUTs array 10

[0143] (1) Remove the photoresist 12 spin-coated on the front side of the chip by dry method and take out the CMUTs array 10.

[0144] The above is only one embodiment of the present invention, not all or the only embodiment. Any equivalent transformation of the technical solution of the present invention made by ordinary technicians in this field after reading the specification of the present invention is covered by the claims of the present invention.

Claims

1. A method for preparing a capacitive micromachined ultrasonic transducer based on 3D printing, characterized in that: The capacitive micromechanical ultrasonic transducer based on 3D printing comprises an upper electrode (1), an upper insulating layer (2), a vibrating membrane (3), a support layer (4), a silicon substrate (5), and a lower electrode (6) connected in sequence from top to bottom, wherein the support layer (4) comprises a lower insulating layer (9) located on the front side of the silicon substrate (5) and a support (8) located on the front side of the lower insulating layer (9); a cavity (7) is formed between the vibrating membrane (3), the lower insulating layer (9), and the support (8); The preparation method comprises the following steps: Using nano-silicon dioxide composite materials and a light-curing 3D printing method, a pillar layer array is printed on the front side of a silicon substrate (5) with a functionalized surface, thereby obtaining a first device; curing and heat-treating the first device to obtain a second device; Bonding a vibration membrane (3) to the front surface of the support (8) of the second device to obtain a third electrical device; Functionalizing the surface of the vibration membrane (3) of the third electrical device, and using a nano-silicon dioxide composite material and a light-curing 3D printing method to print an insulating layer (2) on the front surface of the vibration membrane (3) of the third electrical device, thereby obtaining a fourth device; curing and heat-treating the fourth device to obtain a fifth device; Printing an upper electrode (1) on the surface of the upper insulating layer (2) of the fifth device by 3D printing to obtain a sixth device; Removing silicon dioxide on the back side of the silicon substrate (5) in the sixth device by a wet etching process to obtain a seventh device; Printing a lower electrode (6) on the back side of the silicon substrate (5) in the seventh device by 3D printing to obtain an eighth device; Removing the photoresist on the eighth device to obtain the 3D printing-based capacitive micromachined ultrasonic transducer; The heat treatment includes a thermal debinding process and a sintering process after the thermal debinding process.

2. The method for preparing a capacitive micromachined ultrasonic transducer based on 3D printing according to claim 1, characterized in that: The preparation method of the nano-silicon dioxide composite material comprises: Amorphous nano-silica powder is uniformly dispersed in a mixture of hydroxyethyl methacrylate and trimethylolpropane ethoxylated triacrylate, and then degassed to obtain the nano-silica composite material; in terms of mass percentage, the mixture contains 60%-70% of hydroxyethyl methacrylate and 30%-40% of trimethylolpropane ethoxylated triacrylate, and the mass of the amorphous nano-silica powder accounts for 30%-50% of the mass of the mixture.

3. The method for preparing a capacitive micromachined ultrasonic transducer based on 3D printing according to claim 1, characterized in that: The functionalization process includes: washing the surface of the silicon device with acidic methanol to remove organic matter on the surface of the device, and then immersing the device in a dimethylchlorosilane solution to functionalize the surface of the device, wherein the device is a silicon substrate (5) before printing the support layer array or a vibration membrane (3) of the third electrical device; The first device is placed in a mixed solution of methanol and water prepared in a volume ratio of (1-3):1, and ultrasonic vibration is used to dissolve the uncured nano-silica composite material in the printed pillar layer array. After dissolution, the surface of the first device is blown dry, and then dried to remove water molecules on the surface of the first device. The first device is then finally cured by ultraviolet light; The fourth device is placed in a mixed solution of methanol and water prepared in a volume ratio of (1-3):1, and ultrasonic vibration is used to dissolve the uncured nano-silicon dioxide composite material in the printed upper insulating layer (2). After dissolution, the surface of the fourth device is blown dry, and then dried to remove water molecules on the surface of the fourth device. The fourth device is then finally cured by ultraviolet light.

4. The method for preparing a capacitive micromachined ultrasonic transducer based on 3D printing according to claim 1, characterized in that: During the thermal debinding process, the first component or the fourth component is heated to 300-500°C and kept at this temperature for 3-6 hours to complete the thermal debinding process; During the sintering process, the first component or the fourth component after the thermal debinding process is heated to 600-900° C., kept at this temperature for 10-20 hours, and then cooled to complete the sintering process.

5. The method for preparing a capacitive micromachined ultrasonic transducer based on 3D printing according to claim 1, characterized in that: When the vibration membrane (3) is bonded to the front side of the support (8) of the second device: Align the front side of the support (8) of the second device with the vibration membrane (3), pressurize the bonding pressure to 20-60KN, and evacuate to 10 -5 mbar, and then continue bonding at 300-500° C. for 2-4 hours, and then anneal at 350-600° C. for 5-8 hours to obtain a third electrical device.

6. The method for preparing a capacitive micromachined ultrasonic transducer based on 3D printing according to claim 1, characterized in that: The process of printing an upper electrode (1) on the surface of the upper insulating layer (2) of the fifth device by 3D printing, and printing a lower electrode (6) on the back surface of the silicon substrate (5) in the seventh device by 3D printing both includes: A mixed powder of 10%-20% sodium citrate powder, 5%-15% sodium chloride, and 75%-85% metal powder is added to a 3D printer storage box. The metal powder includes one or more of aluminum powder, gold powder, and platinum powder. Deionized water is added to the metal 3D printing cartridge. The front side of the upper insulating layer (2) or the back side of the silicon substrate (5) is placed upward in a metal 3D printer, a layer of mixed powder is spread on the discharge port of the metal 3D printer and compacted with a roller, and deionized water is sprayed along a predetermined trajectory using a precision inkjet nozzle to activate the chelation reaction; The mixed powder that has not undergone chelation reaction is then blown off with nitrogen gas, and the fifth device or the seventh device is then placed in a humidifier with a water spray rate of 20-40 ml / h for 30-60 minutes to completely trigger the chelation reaction of the mixed powder. Then, the fifth device or the seventh device is placed in a tube furnace under vacuum conditions for heat treatment to obtain a sixth device or an eighth device accordingly.

7. The method for preparing a capacitive micromachined ultrasonic transducer based on 3D printing according to claim 1, characterized in that: It also includes the following processes: Spin-coat photoresist on the front of the sixth device, then photoetch a CMUTs array chip dicing pattern, and use wet etching to etch to the silicon dioxide layer on the front of the silicon substrate (5), or etch through the entire silicon substrate (5) to obtain a plurality of chips; Then, photoresist is spin-coated on the front of the sixth device or the obtained chip, and the sixth device or the obtained chip is baked at 70-100° C. for 30-60 minutes, and the silicon dioxide on the back of the silicon substrate (5) in the sixth device or chip is removed by hydrofluoric acid to obtain the seventh device.

8. The method for preparing a capacitive micromachined ultrasonic transducer based on 3D printing according to claim 1, characterized in that: The silicon substrate (5) and the vibration membrane (3) both adopt N-type <100> Low resistance silicon wafer, the N-type <100> The front and back sides of the low-resistance silicon wafer both have a silicon dioxide layer generated by oxidation; Before bonding the vibration membrane (3) to the front surface of the pillar (8) of the second device, the silicon dioxide layer on the surface of the vibration membrane (3) is first removed and processed to a predetermined thickness.

9. A capacitive micromachined ultrasonic transducer based on 3D printing, characterized in that: The capacitive micromachined ultrasonic transducer based on 3D printing is manufactured by the method for manufacturing a capacitive micromachined ultrasonic transducer based on 3D printing according to any one of claims 1 to 8.

10. The capacitive micromachined ultrasonic transducer based on 3D printing according to claim 9, characterized in that: The shape of the upper electrode (1) is circular, square, triangular or regular polygonal; The shape of the cavity (7) is cylindrical. When the shape of the upper electrode (1) is circular, the diameter of the upper electrode (1) is the same as the diameter of the cavity (7), and the electrode (1) and the cavity (7) are directly opposite. The upper insulating layer (2) covers the entire surface of the vibration membrane (3), and the lower insulating layer (9) covers the entire surface of the silicon substrate (5).

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