Capacitive micro-machined ultrasonic transducer based on 3D printing and preparation method of capacitive micro-machined ultrasonic transducer

Abstract

The invention belongs to the field of material and electronic device manufacturing, and discloses a capacitive micromachined ultrasonic transducer based on 3D printing and a preparation method thereof.The CMUTs comprise an upper electrode, an upper insulating layer, a vibrating membrane, a supporting column layer, a silicon substrate and a lower electrode which are sequentially connected from top to bottom, and a cavity is formed among the vibrating membrane, the lower insulating layer and a supporting column of the supporting column layer; the method comprises the following steps: printing a pillar layer array on the front surface of a silicon substrate with a functionalized surface by utilizing a nano silicon dioxide composite material and adopting a photocuring 3D printing method; then curing and heat treatment are carried out to obtain a second device; bonding a vibrating diaphragm; printing an insulating layer on the front surface of the vibrating diaphragm; carrying out curing and heat treatment; printing an upper electrode; removing the silicon dioxide on the back surface of the silicon substrate through a wet etching process; printing a lower electrode on the back of the silicon substrate; and removing the photoresist. According to the invention, the consumption of required equipment and materials is reduced, and the production cost is reduced.

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] At present, capacitive micromachined ultrasonic transducers (CMUTs) have been applied to many fields such as audio equipment, fingerprint recognition, bubble monitoring, and stress monitoring. Compared with traditional bulk ultrasonic sensors, CMUTs have the characteristics of low power consumption, small size, wide bandwidth, good impedance matching, and easy integration with integrated circuits (ICs), which have attracted widespread attention from researchers. The CMUTs unit is composed of several parts, including an upper electrode, a vibrating film, an insulating layer, a cavity, a substrate, and a lower electrode. When transmitting ultrasonic waves, the electrostatic force changes generated by the AC voltage between the two plates drive the film to vibrate and emit ultrasonic waves; when receiving, the ultrasonic wave acts on the film to cause changes in capacitance and charge, thereby affecting the voltage in the circuit and realizing ultrasonic detection. The performance of CMUTs depends on the accuracy of each structural dimension and the consistency of the unit size. The literature points out that the cavity height has a greater impact on the transmission sound pressure and receiving sensitivity of CMUTs, but has less impact on the bandwidth. The cavity processing of traditional CMUTs usually adopts etching or sacrificial processes. These chemical processes have high requirements on temperature, humidity, operating equipment, and technical experience of operators. The large surface area of ​​the wafer makes it difficult to maintain consistent processing conditions, which will lead to inconsistent cavity heights and reduce the performance of CMUTs. In addition, the existing CMUTs processing technology is time-consuming and complex, making it difficult to achieve rapid processing. For example, when processing a silicon dioxide insulating layer, it is necessary to first oxidize a layer of silicon dioxide on the surface of the silicon wafer, and then gradually etch it to the designed thickness. A film thickness meter needs to be used for repeated measurements to ensure accuracy. Therefore, there is an urgent need to develop a method that can quickly and accurately prepare CMUTs.

[0003] 3D printing is called rapid prototyping or additive manufacturing. It is a layer-by-layer manufacturing process. There is no material removal, which can simplify the process steps. The printed devices also have the advantages of high dimensional accuracy and low surface roughness. It can be applied to the preparation of CMUTs to solve the above problems. At present, there are few studies on 3D printing CMUTs, but some research institutions have carried out research in related technical fields. The Institute of Microstructure Technology of Karlsruhe Institute of Technology has used stereolithography 3D printers to manufacture transparent fused silica glass components with a resolution of tens of microns, making it possible to 3D print CMUTs silica insulation layers. The French National Institute of Scientific Research uses 3D and 2D inkjet printing technology to print capacitive acoustic resonators. The University of Edinburgh proposed an integrated 3D printing method for rapid manufacturing of MEMS devices. This method successfully manufactured a capacitive force sensor with a relatively complex suspended beam plate structure, with a total time of no more than 11 hours. Carnegie Mellon University uses 3D printing technology to manufacture nanoscale metal electrode column arrays and successfully applies them to bioelectric signal detection.

[0004] In summary, the existing 3D printing technology has achieved high-precision printing at the nanoscale, and can quickly print acoustic resonators and capacitive force sensors, verifying the feasibility of 3D printing CMUTs. However, the current 3D printing CMUTs technology still has the following problems: the research on 3D printing CMUTs is currently mainly focused on 3D printing microstructures of different materials, and few people have used 3D printing technology to completely prepare CMUTs devices, and there is a lack of 3D printing CMUTs process design and structural design. Summary of the invention

[0005] In order to solve the problems existing in the prior art, the purpose of the present invention is to provide a capacitive micromechanical ultrasonic transducer based on 3D printing and a preparation method thereof. The present invention designs a complete set of 3D printed CMUTs process flows, reduces the required equipment and material consumption, and reduces production costs; reduces mask design, development, etching and other processes, thereby simplifying the processing technology, greatly shortening the time from design to finished product, facilitating rapid iteration and product optimization, and improving production rates.

[0006] To achieve the above purpose, the technical solution adopted by the present invention is as follows: A method for preparing a capacitive micromechanical ultrasonic transducer based on 3D printing, wherein the capacitive micromechanical ultrasonic transducer based on 3D printing comprises an upper electrode, an upper insulating layer, a vibration membrane, a support layer, a silicon substrate and a lower electrode connected in sequence from top to bottom, wherein the support layer comprises a lower insulating layer located on the front side of the silicon substrate and a support located on the front side of the lower insulating layer; a cavity is formed between the vibration membrane, the lower insulating layer and the support; 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 with a functionalized surface to obtain a first device; Curing and heat treating the first device to obtain a second device; Bonding a vibration membrane to the front side of the pillar of the second device to obtain a third electrical device; Functionalizing the surface of the vibration membrane of the third electrical device, using nano-silicon dioxide composite materials and a light-curing 3D printing method to print an insulating layer on the front side of the vibration membrane 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 on the surface of the upper insulating layer of the fifth device by 3D printing to obtain a sixth device; 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; Printing a lower electrode on the back side of the silicon substrate in the seventh device by 3D printing to obtain an eighth device; Removing the photoresist on the eighth device to obtain the capacitive micromechanical ultrasonic transducer based on 3D printing; The heat treatment includes a thermal debinding treatment and a sintering treatment after the thermal debinding treatment.

[0007] Preferably, the preparation method of the nano-silicon dioxide composite material comprises: The amorphous nano-silicon dioxide powder is uniformly dispersed in a mixture of hydroxyethyl methacrylate and trimethylolpropane ethoxylated triacrylate, and then degassed to obtain the nano-silicon dioxide 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-silicon dioxide powder is 30%-50% of the mass of the mixture.

[0008] 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; 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-silicon dioxide composite material in the printed support layer array, and after the dissolution, the surface of the first device is blown dry, and then dried to remove water molecules on the surface of the first device, and then the first device is 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. 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.

[0009] Preferably, during the thermal degreasing treatment, the first device or the fourth device is heated to 300-500° C. and kept at this temperature for 3-6 hours to complete the thermal degreasing treatment; During the sintering process, the first device or the fourth device 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.

[0010] Preferably, when the vibrating membrane is bonded to the front side of the pillar of the second device: Align the front side of the support of the second device with the vibrating membrane, 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.

[0011] 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 side of the silicon substrate in the seventh device by 3D printing both include: Add a mixed powder of 10%-20% sodium citrate powder, 5%-15% sodium chloride and 75%-85% metal powder into a 3D printer storage box, wherein the metal powder includes one or a mixture of aluminum powder, gold powder and platinum powder, and add deionized water into the metal 3D printing ink cartridge; Place the front side of the upper insulating layer or the back side of the silicon substrate facing upwards in the metal 3D printer. Spread a layer of mixed powder at the outlet 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. Then, the mixed powder on the surface where the chelation reaction has not occurred is blown off with nitrogen, and then the fifth device or the seventh device is 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.

[0012] Preferably, the method for preparing a capacitive micromechanical ultrasonic transducer based on 3D printing of the present invention further includes the following process: Spin-coat photoresist on the front side of the sixth device, then photolithograph the CMUTs array chip dicing pattern, and use wet etching to etch to the silicon dioxide layer on the front side of the silicon substrate, or etch through the entire silicon substrate to obtain a plurality of chips; 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.

[0013] 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 have a silicon dioxide layer generated by oxidation; 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.

[0014] The present invention also provides a capacitive micromechanical ultrasonic transducer based on 3D printing, and the capacitive micromechanical ultrasonic transducer based on 3D printing is prepared by the preparation method of the capacitive micromechanical ultrasonic transducer based on 3D printing as described above in the present invention.

[0015] Preferably, the shape of the upper electrode is circular, square, triangular or regular polygonal; 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 faces the cavity directly. 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.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The preparation method of the capacitive micromechanical ultrasonic transducer based on 3D printing of the present invention reduces the required equipment and material consumption, reduces the production cost; reduces the mask design, development, etching and other processes, thereby simplifying the processing technology, greatly shortening the time from design to finished product, and facilitating rapid iteration and product optimization, and improving the production rate; the cavity shape, insulating layer thickness, and electrode shape of the CMUTs device can be customized according to specific needs, thereby improving the 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 less loss, can be recycled and reused, reducing material waste, and having higher material utilization and environmental benefits. The preparation method of the capacitive micromechanical ultrasonic transducer proposed in the present invention can be applied to nanoscale printing of various structures, and the printed structure has the characteristics of low surface roughness, low porosity, and high compatibility with the MEMS preparation process, wherein the chelating agent used for the 3D printed metal electrode is a non-toxic and environmentally friendly material, which improves the safety of printing and environmental protection. It can be seen from the process of the preparation method of the present invention that the method of the present invention has the advantages of simple process, low processing cost, short preparation cycle, high design freedom, and environmental friendliness.

[0017] In the capacitive micromechanical ultrasonic transducer based on 3D printing of the present invention, the upper electrode of the designed CMUTs unit only covers the cavity area of ​​the CMUTs unit and the upper electrode is insulated from the single crystal silicon by a silicon dioxide insulating layer, which can reduce the parasitic capacitance caused by the electrode covering the pillar area, and can appropriately increase the width of the pillar and the width of the array outer bonding area to improve the bonding strength without increasing the parasitic capacitance; the front of the silicon substrate is designed with a silicon dioxide layer (i.e., the lower insulating layer) and covers the entire front of the silicon substrate. This design can ensure that when the vibrating film breaks, collapses, etc., the vibrating film will not directly contact the silicon substrate and short-circuit; the CMUTs unit has a simple structure, reduces the difficulty of processing, and improves the device performance and yield rate. The capacitive micromechanical 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

[0018] Figure 1 is a structural diagram of a CMUTs unit provided in an embodiment of the present invention; Figure 2 is a 1 / 4 cross-sectional view of a CMUTs unit provided by an embodiment of the present invention; Figure 3 This is a chip diagram of CMUTs before dicing provided by an embodiment of the present invention; Figure 4 is a diagram of a square array of CMUTs provided by an embodiment of the present invention; Figure 5 It is a flow chart of the CMUTs preparation method provided in an embodiment of the present invention.

[0019] Figure numerals: 1-upper electrode, 2-upper insulating layer, 3-vibration membrane, 4-support layer, 5-silicon substrate, 6-lower electrode, 7-cavity, 8-support, 9-lower insulating layer, 10-CMUTs array, 11-CMUTs unit, 12-photoresist. DETAILED DESCRIPTION

[0020] In order to make the purpose, technical solution and advantages of the technical solution more clear, the technical solution is further described in detail below 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 the technical solution.

[0021] In order to solve the problems existing in the subtractive preparation of CMUTs, the present invention proposes a capacitive micromachined ultrasonic transducer based on 3D printing and a corresponding preparation method, which directly constructs the key structure of CMUTs, thereby significantly shortening the preparation cycle, reducing costs, and increasing production output. The materials used in this method are all green and environmentally friendly materials, and the existing process is optimized to reduce the porosity and surface roughness of the 3D printed structure.

[0022] See also Figure 1 , Figure 2 and Figure 5 The capacitive micromechanical 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 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 vibration membrane 3, the lower insulating layer 9 and the support 8, and each structure is tightly connected to each other.

[0023] As a preferred embodiment of the present invention, in this embodiment, the unit in the capacitive micromechanical ultrasonic transducer based on 3D printing of the present invention is the smallest unit of the array. The actual CMUTs are composed of multiple units, and each unit can independently control its reception and transmission of ultrasonic waves. This embodiment uses an array size of 5mm×5mm, and the unit array is an 8X8 linear array. The units are connected in series to ensure that the transmission and reception control of each unit can be controlled simultaneously. After arraying, the ratio of effective capacitance to parasitic capacitance can be improved, and the difficulty of signal detection can be reduced.

[0024] As a preferred embodiment, the shape of the upper electrode 1 is circular, square, triangular or regular polygonal.

[0025] As a preferred embodiment of the present invention, in this embodiment, the upper electrode 1 is circular in shape and the cavity 7 is cylindrical in shape. 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.

[0026] As a preferred embodiment of the present invention, in this embodiment, the upper insulating layer 2 covers the entire surface of the vibration membrane to ensure that the upper electrode 1 does not directly contact the vibration membrane 3 to prevent a short circuit.

[0027] 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. in contact with the substrate.

[0028] 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.

[0029] 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 conductivity.

[0030] As a preferred embodiment of the present invention, the materials of the upper electrode 1 and the lower electrode 6 adopted 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.

[0031] As a preferred implementation scheme 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 to ensure that the upper electrode does not directly contact the vibration membrane, thereby preventing circuit short circuit and increasing the collapse voltage.

[0032] 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.

[0033] like Figure 5 As shown, the present invention also proposes a method for preparing the above-mentioned capacitive micromechanical ultrasonic transducer based on 3D printing, which specifically includes the following steps: Step 1: Prepare the substrate silicon wafer (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 for identification of the substrate silicon wafer. The silicon wafer is then subjected to standard cleaning, air showering, and drying to complete the substrate preparation.

[0034] Step 2: 3D print the silicon dioxide pillar layer on the base silicon wafer (1) Dispersing amorphous nano-silica powder in a beaker containing dispersants of hydroxyethyl methacrylate and trimethylolpropane ethoxylated triacrylate, stirring the mixture using a laboratory dissolving stirrer, and degassing the mixture using ultrasound to prepare a nano-silica composite material, which is then added to a photocurable 3D printer. In terms of mass percentage, the dispersant contains 60%-70% hydroxyethyl methacrylate and 30%-40% trimethylolpropane ethoxylated triacrylate, and the mass of the amorphous nano-silica powder is 30%-50% of the mass of the dispersant.

[0035] (2) The substrate silicon surface is rinsed with acidic methanol to remove surface organic matter and increase surface activity, and then immersed in a dimethylchlorosilane solution to functionalize the substrate silicon surface.

[0036] (3) The processed base silicon wafer is placed with its back side facing downward in a photocuring 3D printer, and a three-dimensional pillar layer array is printed on the base silicon wafer using photocuring 3D printing technology.

[0037] (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°C to dry for 1-2 hours to ensure that there are no excess water molecules on the surface.

[0038] (5) Placing the dried substrate silicon wafer in a UV curing chamber to further cure the nanocomposite material and reduce the void content.

[0039] (6) The base silicon wafer is placed in an ashing furnace for thermal debinding, 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.

[0040] (7) Take out the base silicon wafer that has been cooled to room temperature, let it stand for 1 hour, and then perform the bonding operation.

[0041] Step 3: Top silicon wafer preparation (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 showering, and drying are performed to complete the preparation of the vibration membrane 3.

[0042] Step 4: Directly bond the vibration membrane to the upper surface of the support layer at low temperature (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.

[0043] Step 5: 3D print an insulating layer on the vibration membrane (1) The bonded vibrating membrane is rinsed with acidic methanol to remove surface organic matter and increase surface activity, and then immersed in a dimethylchlorosilane solution to functionalize its surface.

[0044] (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.

[0045] (3) Place the printed silicon wafer 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°C to dry for 1-2 hours to ensure that there are no excess water molecules on the surface.

[0046] (4) Placing the dried silicon wafer in a UV curing chamber to further cure the nanocomposite material and reduce the void content.

[0047] (5) The silicon wafer is placed in an ashing furnace for thermal debinding, 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.

[0048] (6) Take out the silicon wafer that has cooled to room temperature, let it stand for 1 hour, and then print the upper electrode.

[0049] Step 6: 3D print the upper electrode on the upper insulating layer (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.

[0050] (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 on 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.

[0051] (3) Take out the silicon wafer and use nitrogen to blow away the powder on the surface that has not undergone chelation reaction. 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.

[0052] (4) The treated silicon wafer is placed in a tube furnace under vacuum conditions 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.

[0053] (5) Remove the silicon wafer and complete the upper electrode printing.

[0054] Step 7: Slice Etching (1) Spin-coat photoresist, photolithography the CMUTs array chip dicing pattern, and use wet etching to etch the silicon dioxide layer on the front side 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 sides).

[0055] (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.

[0056] Step 8: 3D print the lower electrode 6 on the back of the silicon substrate 5 (1) Place the silicon wafer with the back side facing up in the metal 3D printer, spread a layer of mixed powder on 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.

[0057] (2) Take out the silicon wafer and use nitrogen to blow away the powder on the surface that has not undergone chelation reaction. 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.

[0058] (3) Place the processed silicon wafer in a tube furnace under vacuum conditions for heat treatment.

[0059] (4) Remove the silicon wafer and complete the printing of the lower electrode.

[0060] Step 9: Remove the glue and take out the CMUTs array (1) Dry-remove the photoresist spin-coated on the front side of the chip and remove the CMUTs array.

[0061] Example 1 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 vibration membrane 3, a support layer 4, a substrate 5, and a lower electrode 6. The upper electrode 1 and the lower electrode 6 are made of aluminum, the upper insulating layer 2 and the support layer 4 are made of silicon dioxide, the vibration membrane 3 and the substrate material 5 are made of silicon, and the structures are closely connected to each other.

[0062] 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 be cleaned in a standard manner. The lower electrode 6 covers the entire surface of the substrate 5 to improve the mechanical strength of the chip.

[0063] 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 be independently controlled to receive and transmit ultrasonic waves. The present invention selects a linear CMUTs array 10 with an array size of 5mm×5mm and an array of 8X8 CMUTs units 11. Each unit is connected in series to ensure that the transmission and reception control of each unit can be controlled simultaneously. After arraying, the ratio of effective capacitance to parasitic capacitance can be improved, and the difficulty of signal detection can be reduced.

[0064] like Figure 5 As shown, the 3D printed CMUTs array preparation method of this embodiment specifically includes the following steps: 1. Preparation of substrate silicon wafer (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 cleaned, air-dried and dried in a standard manner to complete the preparation of the silicon substrate 5.

[0065] 2. 3D printing of silicon dioxide support layer 4 on silicon substrate 5 silicon wafer (1) Amorphous nano-silica powder is dispersed in a beaker containing dispersants hydroxyethyl methacrylate and trimethylolpropane ethoxylated triacrylate, the mixture is stirred evenly using a laboratory dissolving stirrer, and degassed using ultrasound to prepare a nano-silica composite material, which is then added to a light-curing 3D printer. In terms of mass percentage, the dispersant contains 65% hydroxyethyl methacrylate and 35% trimethylolpropane ethoxylated triacrylate, and the mass of the amorphous nano-silica powder is 35% of the mass of the dispersant.

[0066] (2) The substrate silicon surface is rinsed with acidic methanol to remove surface organic matter and increase surface activity, and then immersed in a dimethylchlorosilane solution to functionalize the substrate silicon surface.

[0067] (3) The processed base silicon wafer is placed with its back side facing downward in a photocuring 3D printer, and a three-dimensional pillar layer 4 array is printed on the base silicon wafer using photocuring 3D printing technology.

[0068] (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.

[0069] (5) The dried base silicon wafer 5 is placed in a UV curing chamber to further cure the nanocomposite material and reduce the void content.

[0070] (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.

[0071] (7) Take out the silicon substrate 5 that has been cooled to room temperature, let it stand for 1 hour, and then perform the bonding operation.

[0072] 3. Top Silicon Wafer Preparation (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 standard cleaning, air showering, and drying are performed to complete the preparation of the vibration membrane 3.

[0073] 4. Low-temperature direct bonding of the vibration membrane 3 and the upper surface of the support layer 4 (1) Place the silicon substrate 5 silicon wafer and the top silicon wafer 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; after bonding, annealing was performed at 350°C for 6 hours to further improve the bonding strength.

[0074] 5. 3D print the insulating layer 2 on the vibration membrane 3 (1) The upper surface of the bonded vibration membrane 3 is rinsed with acidic methanol to remove surface organic matter and improve surface activity, and then it is immersed in a dimethylchlorosilane solution to functionalize its surface.

[0075] (2) The processed silicon wafer is correctly placed in the light-curing 3D printer, and a layer of silicon dioxide insulation layer 2 is printed on the vibration membrane using light-curing 3D printing technology.

[0076] (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.

[0077] (4) Placing the dried silicon wafer in a UV curing chamber to further cure the nanocomposite material and reduce the void content.

[0078] (5) The silicon wafer is placed in an ashing furnace for thermal debinding, and then the silicon wafer is placed in a high-temperature tube furnace for sintering to evaporate molecular water and surface-bound silanol groups.

[0079] (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.

[0080] 6. 3D print the upper electrode 1 on the upper insulating layer 2 (1) Mix 20% sodium citrate powder, 5% sodium chloride powder, and 75% aluminum powder with a mesh size of 40 or less evenly, screen them, and add them into the metal 3D printer material storage box. At the same time, add deionized water into the metal 3D printing ink cartridge.

[0081] (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 a predetermined trajectory with an inkjet nozzle to activate the chelation reaction until the electrode array is printed.

[0082] (3) Take out the silicon wafer and use nitrogen to blow away the powder on the surface that has not undergone chelation reaction. 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.

[0083] (4) The treated silicon wafer is placed in a tube furnace under vacuum conditions 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.

[0084] (5) Remove the silicon wafer and complete the printing of the upper electrode 1.

[0085] 7. Scribe Etching (1) Spin-coat the photoresist 12, photolithograph the CMUTs array chip dicing pattern, and use wet etching to etch to the silicon dioxide layer of the substrate 5.

[0086] (2) Glue is applied to the front side 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 a furnace at 80°C for 40 minutes to enhance the acid resistance of the photoresist 12, and then hydrofluoric acid is used to remove the silicon dioxide on the back side of the substrate.

[0087] 8. 3D printing the lower electrode 6 on the back of the silicon substrate 5 (1) Place the silicon wafer with the back side facing up in the metal 3D printer, spread a layer of mixed powder on the outlet and compact it with a roller, and spray deionized water along a predetermined trajectory with the inkjet nozzle to activate the chelation reaction until the lower electrode 6 array is printed.

[0088] (2) Take out the silicon wafer and use nitrogen to blow away the powder on the surface that has not undergone chelation reaction. 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.

[0089] (3) Place the processed silicon wafer in a tube furnace under vacuum conditions for heat treatment.

[0090] (4) Take out the silicon wafer and complete the printing of the lower electrode 6.

[0091] 9. Remove the glue and take out the CMUTs array 10 (1) Remove the photoresist 12 spin-coated on the front side of the chip by dry method and take out the CMUTs array 10.

[0092] The above is only one embodiment of the present invention, not all or the only embodiment. Any equivalent changes made to the technical solution of the present invention by ordinary technicians in this field after reading the specification of the present invention are covered by the claims of the present invention.

Claims

1. A method for preparing a capacitive micromechanical 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 supporting layer (4), a silicon substrate (5) and a lower electrode (6) which are connected in sequence from top to bottom, the supporting layer (4) comprising a lower insulating layer (9) located on the front side of the silicon substrate (5) and a supporting column (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 supporting column (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 side 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 side 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 capacitive micromechanical ultrasonic transducer based on 3D printing; The heat treatment includes a thermal debinding treatment and a sintering treatment after the thermal debinding treatment.

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: The amorphous nano-silicon dioxide powder is uniformly dispersed in a mixture of hydroxyethyl methacrylate and trimethylolpropane ethoxylated triacrylate, and then degassed to obtain the nano-silicon dioxide 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-silicon dioxide powder is 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-silicon dioxide composite material in the printed support layer array, and after the dissolution, the surface of the first device is blown dry, and then dried to remove water molecules on the surface of the first device, and then the first device is 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 the 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 treatment, the first device or the fourth device is heated to 300-500° C. and kept at this temperature for 3-6 hours to complete the thermal debinding treatment; During the sintering process, the first device or the fourth device 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: The front side of the support (8) of the second device is aligned with the vibration membrane (3), the bonding pressure is increased to 20-60 kN, and the vacuum is drawn 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 an upper insulating layer (2) of the fifth device by means of 3D printing, and the process of printing a lower electrode (6) on the back surface of a silicon substrate (5) of the seventh device by means of 3D printing both include: Add a mixed powder of 10%-20% sodium citrate powder, 5%-15% sodium chloride and 75%-85% metal powder into a 3D printer storage box, wherein the metal powder includes one or a mixture of aluminum powder, gold powder and platinum powder, and add deionized water into the metal 3D printing ink 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 outlet 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; Then, the mixed powder on the surface where the chelation reaction has not occurred is blown off with nitrogen, and then the fifth device or the seventh device is 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 side of the sixth device, then photolithograph a CMUTs array chip dicing pattern, and use wet etching to etch to the silicon dioxide layer on the front side 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 side 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 side 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) are both N-type <100> Low resistance silicon wafer, the N-type <100> The front and back sides of the low-resistance silicon wafer have a silicon dioxide layer generated by oxidation; Before bonding the vibration membrane (3) to the front side 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 micromechanical ultrasonic transducer based on 3D printing, characterized in that: The capacitive micromechanical ultrasonic transducer based on 3D printing is manufactured by the method for manufacturing a capacitive micromechanical ultrasonic transducer based on 3D printing as described in 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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