An ultrasonic probe based on MEMS force feedback and a preparation method thereof

Abstract

The application relates to an ultrasonic probe based on MEMS force feedback and a preparation method thereof. The ultrasonic probe comprises a probe body, an ultrasonic transducer, a MEMS pressure sensor array and a packaging layer. One end surface of the probe body is composed of a circular detection surface located at the center and an annular area located outside the detection surface. The annular area is concentrically arranged with an ultrasonic detection area at the center of the detection surface, and the detection surface protrudes from the annular area. The ultrasonic transducer is fixed inside the probe body, and the detection end of the ultrasonic transducer is attached to the central area of the detection surface. The MEMS pressure sensor array comprises a plurality of capacitive MEMS sensors. The height of the gap cavity of the capacitive MEMS sensor is 1 / 10-1 / 4 of the ultrasonic wave wavelength of the ultrasonic transducer. The packaging layer is covered on the surface of the MEMS pressure sensor array, and the surface of the packaging layer is flush with the central area of the detection surface. The pressure consistency during detection is ensured, and the accuracy and repeatability of the detection result are improved.

Description

Technical Field

[0001] This invention relates to the field of industrial ultrasonic nondestructive testing technology, specifically to an ultrasonic probe with integrated MEMS force feedback function and its fabrication method. It is applicable to ultrasonic testing applications such as defect detection of metallic materials and residual stress detection of basin insulators in power systems, and solves the problem of inconsistent test results caused by inconsistent probe pressure during the testing process. Background Technology

[0002] Ultrasonic testing technology is widely used in industrial defect detection and structural health monitoring due to its non-invasive and high-sensitivity characteristics. However, in practical applications such as defect detection in metallic materials and residual stress detection in basin insulators, the stability of the coupling pressure between the ultrasonic probe and the surface of the object being tested directly affects the sound wave transmittance and the quality of the detection signal. In existing technologies, manual coupling relies on the operator's experience to apply pressure, resulting in large dispersion of coupling pressure and insufficient pass rate for a single test. Typically, 3-5 repeated couplings are required to meet the detection requirements, severely limiting detection efficiency. At the same time, traditional ultrasonic probes lack pressure detection capabilities, making it impossible to sense the pressure magnitude and distribution in real time, and difficult to dynamically adjust the coupling pressure, thus leading to poor consistency of detection results and low defect detection rate.

[0003] While some ultrasonic testing devices related to pressure control exist in existing technologies, they mostly employ external pressure sensors, resulting in complex structures, high costs, poor adaptability, and an inability to accurately detect pressure distribution. The development of MEMS (Micro-Electro-Mechanical Systems) technology has made miniaturized and array-based pressure detection possible. For example, capacitive MEMS sensor arrays offer advantages such as good uniformity, high sensitivity, and ease of integration. However, conventional MEMS lacks acoustic impedance matching packaging, leading to blocked ultrasonic wave transmission and significant signal attenuation. Furthermore, conventional MEMS uses planar / linear array layouts and lacks a dedicated design for a ring-shaped detection surface, making pressure distribution detection impossible. Additionally, conventional MEMS processes do not consider integration with hard alloy probes. Due to these issues, there is currently no technical solution that combines MEMS force feedback structures with industrial ultrasonic probes to achieve real-time detection of pressure magnitude and distribution around the detection surface. Summary of the Invention

[0004] The purpose of this invention is to overcome the problem of inconsistent test results caused by inconsistent pressure in existing industrial ultrasonic testing, and to provide an ultrasonic probe based on MEMS force feedback and its fabrication method. By integrating a MEMS pressure sensor array at the edge of the ultrasonic probe's detection surface, real-time detection of the magnitude and distribution of coupled pressure is achieved, providing data support for pressure closed-loop control, ensuring pressure consistency during the testing process, and improving the accuracy and repeatability of the test results.

[0005] An ultrasonic probe based on MEMS force feedback includes a probe body, an ultrasonic transducer, a MEMS pressure sensor array, and an encapsulation layer;

[0006] One end face of the probe body consists of a circular detection surface located at the center and an annular area located outside the detection surface. The annular area is concentrically arranged with the ultrasonic detection area at the center of the detection surface, and the detection surface protrudes from the annular area. The ultrasonic transducer is fixed inside the probe body, and the detection end of the ultrasonic transducer is in contact with the central area of ​​the detection surface.

[0007] The MEMS pressure sensor array includes several capacitive MEMS sensors, which are uniformly arranged in a ring around the edge of the detection surface. The height of the gap cavity of the capacitive MEMS sensor is 1 / 10 to 1 / 4 of the ultrasonic wavelength of the ultrasonic transducer.

[0008] The encapsulation layer covers the surface of the MEMS pressure sensor array, and the surface of the encapsulation layer is flush with the center area of ​​the detection surface.

[0009] The MEMS pressure sensor array includes at least 7 to 9 uniformly distributed capacitive MEMS pressure sensors, and the uniformity error of the spacing between two adjacent capacitive MEMS pressure sensors does not exceed ±5%.

[0010] Each of the capacitive MEMS pressure sensors consists of, from top to bottom, an upper electrode, a sensitive thin film, a support layer, a lower electrode, an insulating layer, and a substrate;

[0011] The center of the support layer is removed by ICP etching, forming a gap cavity between the sensitive film, the support layer and the lower electrode;

[0012] The sensitive film adopts The composite film has a thickness of 0.5-5 μm; the upper electrode and the lower electrode are parallel and corresponding.

[0013] The MEMS pressure sensor array also includes a signal transmission module, which includes a data composite cable. The upper / lower electrode leads of each capacitive MEMS pressure sensor communicate with an external control unit through the data composite cable.

[0014] The ultrasonic transducer communicates with an external control unit via a cable.

[0015] It also includes a signal conditioning unit for amplifying, filtering and analog-to-digital conversion of the signal. The signal conditioning unit is connected in series between the control unit and the MEMS pressure sensor array and between the control unit and the ultrasonic transducer to improve the stability and accuracy of signal transmission.

[0016] The encapsulation layer is made of polydimethylsiloxane (PDMS) material, and the acoustic impedance of the encapsulation layer is matched with the acoustic impedance of the coupling medium on the surface of the object under test.

[0017] The width of the annular mounting area is 2-5mm.

[0018] A method for fabricating an ultrasonic probe based on MEMS force feedback includes the following steps:

[0019] Step 1: Pre-processing of the probe body. Select hard alloy material and process it into a hollow cylindrical probe body with an open top. Process concentric detection surfaces and annular areas at the lower end of the probe body. The detection surfaces protrude from the annular areas. Polish the central area of ​​the detection surfaces to Ra≤0.1μm and roughen the annular areas to Ra≤1.2μm. Then, ultrasonically clean the probe body with acetone, ethanol, and deionized water for 15-20 minutes each, and dry it at 110-130℃ for 30-35 minutes.

[0020] Step 2: Fabricate a capacitive MEMS pressure sensor with a gap cavity height of 1 / 10-1 / 4 of the ultrasonic wavelength of the ultrasonic transducer;

[0021] Step 3: Using a coating process, the prepared 7-9 capacitive MEMS pressure sensors are evenly arranged on the annular area around the outer periphery of the probe body's detection surface. They are then fixed with a low-temperature adhesive to ensure that the sensor array is tightly attached to the detection surface, and the spacing error between adjacent sensor units does not exceed ±5%.

[0022] Step 4: Connect the electrode leads of each capacitive MEMS pressure sensor in the MEMS pressure sensor array to the signal conditioning unit. The signal conditioning unit is connected to the external control unit via a data composite cable.

[0023] Step 5: Fix the ultrasonic transducer in the internal cavity of the probe body, ensuring that its detection end is in close contact with the center area of ​​the detection surface and does not interfere with the MEMS pressure sensor array;

[0024] Step 6: Encapsulation process. PDMS encapsulation adhesive is used to encapsulate the MEMS pressure sensor array and electrode leads. During the encapsulation process, the temperature is controlled at 80-100℃ and the curing time is 2-4 hours. After encapsulation, the surface of the encapsulation layer is polished to make it flush with the center area of ​​the detection surface, and the surface roughness Ra≤0.2μm.

[0025] Step 2 is as follows:

[0026] (1) SOI wafer cleaning: Select SOI wafers with a diameter of 4 to 6 inches and a device layer thickness of 3 to 8 μm (the wafer has a three-layer structure: the top layer is the device layer, the middle layer is the insulating layer, and the bottom layer is the substrate), and clean them sequentially with acetone, ethanol, and deionized water by ultrasonic cleaning for 8 to 15 min to remove surface impurities. Then dry them at 110 to 130 °C for 20 to 40 min.

[0027] (2) Photolithography and etching: Photoresist is coated on the upper surface of the device layer of the SOI wafer at a rotation speed of 2500-3500 r / min and a coating thickness of 0.8-1.2 μm. Then, it is dried at a soft baking temperature of 85-95℃ for 50-70 s to remove the photoresist solvent and enhance the adhesion. Photolithography is performed through a mask at an exposure dose of 80-120 mJ / cm². It is then dried at a medium baking temperature of 105-115℃ for 80-100 s to stabilize the photolithographic latent image and eliminate the standing wave effect. It is developed with developer for 1.5-2.5 min and then dried at a hard film temperature of 115-125℃ for 100-140 s to fully cure and harden the photoresist, enhance the resistance to ICP etching and the adhesion of the pattern, fix the sensor structure pattern, and use ICP etching process to perform non-penetrating etching on the device layer to remove excess device layer material, leaving only the outer ring area as a support layer.

[0028] (3) Fabrication of the lower electrode: A thin aluminum metal film is deposited on the upper surface of the insulating layer in the support layer as the lower electrode using vacuum sputtering process. The electrode thickness is 100-200 nm. The lower electrode pattern and lead wire are formed between the support layer and the insulating layer by photolithography and etching process.

[0029] (4) Cavity and sensitive film preparation: A sacrificial layer process is used to fill the area between the support layer and the lower electrode with a polyimide sacrificial layer. Microchannels penetrating to the sacrificial layer are etched in advance at a predetermined position in the support layer. A sensitive film is prepared on the sacrificial layer by low-pressure chemical vapor deposition. The deposition material is polycrystalline silicon and the thickness of the sensitive film is controlled to be 0.5-5 μm. Then, the internal sacrificial layer is removed through the microchannel by ICP etching / plasma ashing to form a gap cavity of a predetermined height. Then, the microchannels on the support layer are sealed by vapor deposition / low-temperature sealing process to obtain a closed gap cavity.

[0030] (5) Fabrication of the upper electrode: A chromium / gold composite metal film is deposited on the upper surface of the sensitive film as the upper electrode; the thickness of the upper electrode is 100-200 nm; the upper electrode lead is formed by photolithography and etching processes.

[0031] Beneficial effects

[0032] 1. This invention integrates a MEMS pressure sensor array at the edge of the ultrasonic probe's detection surface, enabling real-time detection of the magnitude and distribution of coupling pressure. This provides precise data support for pressure closed-loop control, effectively solving the problem of inconsistent detection results caused by manual coupling pressure, and improving the accuracy and repeatability of the detection results.

[0033] 2. Employing a capacitive MEMS sensor array design, the sensor units exhibit good uniformity and high spacing uniformity, and the sensitive thin film utilizes... The composite membrane ensures the sensitivity and stability of pressure detection, while the gap cavity height is matched with the ultrasonic wavelength to avoid mutual interference between pressure detection and ultrasonic detection.

[0034] 3. The MEMS pressure sensor array adopts a coating and integration method, which is compact and small in size. It does not change the overall shape and installation method of the ultrasonic probe, has strong adaptability, and is compatible with existing industrial ultrasonic testing systems.

[0035] 4. The fabrication method adopts mature MEMS technology and low-temperature packaging technology, which has strong process controllability, low cost, and is suitable for mass production. In addition, the acoustic impedance matching between the packaging layer material and the coupling medium ensures the effective transmission of ultrasonic signals.

[0036] 5. It is applicable to various industrial ultrasonic testing scenarios such as internal defect detection of metal materials and residual stress detection of basin insulators, and can meet the pressure detection needs of different test objects, thus broadening the application scope of ultrasonic testing technology. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the technical description are briefly introduced.

[0038] Figure 1 is a schematic diagram of the overall structure of the ultrasonic probe based on MEMS force feedback according to the present invention;

[0039] Figure 2 is a schematic cross-sectional view of the MEMS pressure sensor unit of the present invention;

[0040] Figure 3 is a process flow diagram of the preparation method of the present invention. Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] This invention provides an ultrasonic probe based on MEMS force feedback, comprising a probe body 1, an ultrasonic transducer 2, a MEMS pressure sensor array, a signal transmission module, and an encapsulation layer;

[0043] The probe body 1 is a hollow cylindrical structure with one end open and the other end closed. The closed end has a circular detection surface located in the center and an annular area located outside the detection surface. The ultrasonic transducer 2 is fixed inside the probe body 1, and the detection end of the ultrasonic transducer 2 is in contact with the central area of ​​the detection surface for transmitting and receiving ultrasonic signals.

[0044] The MEMS pressure sensor array consists of 7 to 9 capacitive MEMS sensors, which are uniformly distributed in a ring around the edge of the detection surface using a coating method to form a ring array. The width of the ring area is 2-5 mm. The ring area is concentric with the ultrasonic detection area at the center of the detection surface to ensure that each sensor unit can comprehensively collect pressure data from different positions when the detection surface is in contact with the object being measured. The uniformity error of the spacing between adjacent sensor units does not exceed ±5%. It is used to collect the magnitude and distribution data of the coupling pressure between the detection surface and the object being measured in real time.

[0045] Each capacitive MEMS pressure sensor includes an upper electrode 31, a sensitive thin film 32, a support layer 33, a lower electrode 34, an insulating layer 35, and a substrate 36. The sensitive thin film 32 employs... The composite film, with a thickness of 0.5-5μm, ensures the sensitivity and stability of pressure detection. The upper electrode 31 and the lower electrode 34 are parallel and corresponding. The center of the support layer is removed by ICP etching, forming a gap cavity 37 between the sensitive film 32, the support layer and the lower electrode. The height of the gap cavity 37 is 1 / 10-1 / 4 of the ultrasonic wavelength, ensuring that pressure detection and ultrasonic detection do not interfere with each other.

[0046] The signal transmission module includes a data composite cable. The upper and lower electrode leads of each capacitive MEMS pressure sensor communicate with the external control unit through the data composite cable. The ultrasonic transducer 2 also communicates with the external control unit through a cable to realize the transmission of pressure signals.

[0047] The encapsulation layer is made of polydimethylsiloxane (PDMS) material and covers the surface of the MEMS pressure sensor array. The surface of the encapsulation layer is flush with the center area of ​​the detection surface, and the acoustic impedance of the encapsulation layer is matched with the acoustic impedance of the coupling medium on the surface of the object being measured to avoid affecting the ultrasonic transmission.

[0048] Preferably, it also includes a signal conditioning unit, which is connected in series between the control unit and the MEMS pressure sensor array and between the control unit and the ultrasonic transducer 2, for amplifying, filtering and analog-to-digital conversion of the pressure signal to improve the stability and accuracy of signal transmission.

[0049] This invention also provides a method for fabricating an ultrasonic probe based on MEMS force feedback, comprising the following steps:

[0050] Step 1: Pre-processing of the probe body. Select hard alloy material and process it into a hollow cylindrical probe body 1 with an open top. Process concentric detection surfaces and annular areas at the lower end of the probe body. The detection surfaces protrude from the annular areas. Polish the central area of ​​the detection surfaces to Ra≤0.1μm and roughen the annular areas to Ra≤1.2μm. Then, ultrasonically clean the probe body with acetone, ethanol, and deionized water for 15-20 minutes each, and dry it at 110-130℃ for 30-35 minutes.

[0051] Step 2: Fabricate a capacitive MEMS pressure sensor 3 with a gap cavity height equal to 1 / 10-1 / 4 of the ultrasonic wavelength of the ultrasonic transducer 2.

[0052] SOI wafers with a diameter of 4–6 inches and a device layer thickness of 3–8 μm (the wafer has an inherent three-layer structure: the top layer is the device layer, the middle layer is the insulating layer, and the bottom layer is the substrate) are selected and ultrasonically cleaned with acetone, ethanol, and deionized water for 8–15 min in sequence to remove surface impurities. Then they are dried at 110–130 °C for 20–40 min.

[0053] (2) Photolithography and etching: Photoresist is coated on the upper surface of the device layer of the SOI wafer at a rotation speed of 2500-3500 r / min and a coating thickness of 0.8-1.2 μm. Then, it is dried at a soft baking temperature of 85-95℃ for 50-70 s to remove the photoresist solvent and enhance the adhesion. Photolithography is performed through a mask at an exposure dose of 80-120 mJ / cm². It is then dried at a medium baking temperature of 105-115℃ for 80-100 s to stabilize the photolithographic latent image and eliminate the standing wave effect. It is developed with developer for 1.5-2.5 min and then dried at a hard film temperature of 115-125℃ for 100-140 s to fully cure and harden the photoresist, enhance the resistance to ICP etching and the adhesion of the pattern, fix the sensor structure pattern, and use ICP etching process to perform non-penetrating etching on the device layer to remove excess device layer material, leaving only the outer ring area as a support layer.

[0054] (3) Fabrication of the lower electrode: A thin aluminum metal film is deposited on the upper surface of the insulating layer in the support layer as the lower electrode using vacuum sputtering process. The electrode thickness is 100-200 nm. The lower electrode pattern and lead wire are formed between the support layer and the insulating layer by photolithography and etching process.

[0055] (4) Cavity and sensitive film preparation: A sacrificial layer process is used to fill the area between the support layer and the lower electrode with a polyimide sacrificial layer. Microchannels penetrating to the sacrificial layer are etched in advance at a predetermined position in the support layer. A sensitive film is prepared on the sacrificial layer by low-pressure chemical vapor deposition. The deposition material is polycrystalline silicon and the thickness of the sensitive film is controlled to be 0.5-5 μm. Then, the internal sacrificial layer is removed through the microchannel by ICP etching / plasma ashing to form a gap cavity of a predetermined height. Then, the microchannels on the support layer are sealed by vapor deposition / low-temperature sealing process to obtain a closed gap cavity.

[0056] (5) Fabrication of the upper electrode: A chromium / gold composite metal film is deposited on the upper surface of the sensitive film as the upper electrode; the thickness of the upper electrode is 100-200 nm; the upper electrode lead is formed by photolithography and etching processes.

[0057] Step 3: Transfer and fixation of MEMS pressure sensor array. The prepared MEMS pressure sensor is transferred to the annular area of ​​the probe body detection surface using a coating process, and fixed with a low-temperature adhesive to ensure that the sensor array is tightly attached to the detection surface, and the spacing error between adjacent sensor units does not exceed ±5%.

[0058] Step 4: Connect the signal transmission module. Connect the electrode leads of each capacitive MEMS pressure sensor 3 to the signal conditioning unit. The signal conditioning unit is connected to the external interface through a data composite cable.

[0059] Step 5: Ultrasonic transducer installation. Fix the ultrasonic transducer 2 inside the probe body 1, ensuring that its detection end is in close contact with the center area of ​​the detection surface and does not interfere with the MEMS pressure sensor array.

[0060] Step 6: Encapsulation process. The MEMS pressure sensor array and signal lead-out area are encapsulated using PDMS material. During the encapsulation process, the temperature is controlled at 80-100℃ and the curing time is 2-4 hours. After encapsulation, the surface of the encapsulation layer is polished to make it flush with the center area of ​​the detection surface, and the surface roughness Ra≤0.2μm.

[0061] Step 7: Performance testing. Perform pressure detection accuracy testing, ultrasonic signal transmission efficiency testing, and stability testing on the prepared ultrasonic probe to ensure that the pressure detection range of the MEMS pressure sensor array is 0-50N, the detection accuracy error is ≤±2%, and the ultrasonic signal transmission efficiency is not less than 90%.

[0062] Example 1

[0063] An ultrasonic probe based on MEMS force feedback is provided. The height of the gap cavity of the capacitive MEMS sensor 3 is 1 / 10 of the ultrasonic wavelength of the ultrasonic transducer 2. The MEMS pressure sensor array has 9 capacitive MEMS sensors 3.

[0064] The preparation method includes the following steps:

[0065] 1. Pre-treatment of probe body: Select cemented carbide rod and machine it into a columnar structure with a diameter of 20mm and a length of 80mm. Machining a circular detection surface 6 with a diameter of 20mm at the front end, polishing the central 10mm area with a diamond wheel to Ra=0.1μm, and sandblasting the surrounding annular area (width 2mm) to Ra=1.2μm. Then, ultrasonically clean it with acetone, ethanol, and deionized water for 15min each, and dry it at 120℃ for 30min.

[0066] 2. Fabrication of MEMS pressure sensor array:

[0067] (1) SOI wafer cleaning: Select an SOI wafer with a diameter of 4 inches and a device layer thickness of 5 μm (the SOI wafer has an inherent three-layer structure: the top layer is the device layer, the middle layer is the insulating layer, and the bottom layer is the substrate), and clean it by ultrasonication with acetone, ethanol, and deionized water for 10 min in sequence, and then dry it at 120℃ for 30 min.

[0068] (2) Photolithography and etching: A positive photoresist is coated on the SOI wafer device layer at a rotation speed of 3000 r / min and a coating thickness of 1 μm. The soft baking temperature is 90℃ and the time is 60s. The photomask is used for alignment and exposure, with an exposure dose of 100 mJ / cm² and a medium baking temperature of 110℃ and a time of 90s. The photoresist is developed for 2 min with a hardening solution at a hardening temperature of 120℃ and a time of 120s. The photoresist is fully cured and hardened to enhance the resistance to ICP etching and the adhesion of the pattern, and to fix the sensor structure pattern. The device layer is etched using the ICP etching process to remove excess device layer material, leaving only the outer ring area as a support layer.

[0069] (3) Electrode fabrication: A thin aluminum metal film is deposited on the upper surface of the insulating layer in the support layer as the lower electrode using vacuum sputtering process. The electrode thickness is 200 nm. The lower electrode pattern and lead wires are formed between the support layer and the insulating layer by photolithography and etching process.

[0070] (4) Cavity and sensitive film preparation: A sacrificial layer process is used to fill the area between the support layer and the lower electrode with a polyimide sacrificial layer. Microchannels penetrating to the sacrificial layer are etched in advance at a predetermined position in the support layer. A sensitive film is prepared on the sacrificial layer by low-pressure chemical vapor deposition. The deposition material is polycrystalline silicon and the thickness of the sensitive film is controlled to be 0.5 μm. Then, the internal sacrificial layer is removed through the microchannel by ICP etching / plasma ashing to form a gap cavity of a predetermined height. Then, the microchannels on the support layer are sealed by vapor deposition / low-temperature sealing process to obtain a closed gap cavity.

[0071] (5) Fabrication of the upper electrode: A chromium / gold composite metal film is deposited on the upper surface of the sensitive film as the upper electrode; the thickness of the upper electrode is 200 nm; the upper electrode lead is formed by photolithography and etching processes.

[0072] 3. The prepared 9-capacitor MEMS pressure sensor is uniformly arranged on the annular area on the outer periphery of the detection surface of the probe body 1 using a coating process. It is then fixed with a low-temperature adhesive to ensure that the sensor array is tightly attached to the detection surface and the spacing error between adjacent sensor units is controlled within ±0.05mm.

[0073] 4. Signal transmission module connection: Connect the electrode leads of each capacitive MEMS pressure sensor 3 in the MEMS pressure sensor array to the signal conditioning unit. The signal conditioning unit is connected to the external control unit through a data composite cable.

[0074] 5. Ultrasonic transducer installation: Fix the ultrasonic transducer 2 in the internal cavity of the probe body 1, ensuring that its detection end is in close contact with the center area of ​​the detection surface and does not interfere with the MEMS pressure sensor array;

[0075] 6. Encapsulation process: Prepare PDMS encapsulation adhesive (base adhesive to curing agent mass ratio 10:1), and uniformly coat it on the surface of the MEMS pressure sensor array and the signal lead-out area, with a thickness of 50μm, and cure at 80℃ for 3h; after curing, use a diamond tool to grind the surface of the encapsulation layer 5 so that it is flush with the central area of ​​the detection surface 6, with a roughness Ra=0.15μm;

[0076] 7. Performance Testing: The prepared ultrasonic probe was tested. The pressure detection range of the MEMS pressure sensor array is 0-50N, the detection accuracy error is ±1.5%, and the ultrasonic signal transmission efficiency is 92%, which meets the requirements of industrial ultrasonic testing.

[0077] Example 2

[0078] An ultrasonic probe based on MEMS force feedback is provided. The height of the gap cavity of the capacitive MEMS sensor 3 is 1 / 7 of the ultrasonic wavelength of the ultrasonic transducer 2. The MEMS pressure sensor array has 8 capacitive MEMS sensors 3.

[0079] The preparation method includes the following steps:

[0080] 1. Pre-treatment of probe body: Select cemented carbide rod and machine it into a columnar structure with a diameter of 20mm and a length of 80mm. Machining a circular detection surface 6 with a diameter of 20mm at the front end, polishing the central 10mm area with a diamond wheel to Ra=0.08μm, and sandblasting the surrounding annular area (3mm wide) to Ra=1.1μm. Then, ultrasonically clean it with acetone, ethanol and deionized water for 20min each, and dry it at 110℃ for 35min.

[0081] 2. Fabrication of MEMS pressure sensor array:

[0082] (1) SOI wafer cleaning: Select an SOI wafer with a diameter of 5 inches and a device layer thickness of 3 μm (the SOI wafer has an inherent three-layer structure: the top layer is the device layer, the middle layer is the insulating layer, and the bottom layer is the substrate), and clean it by ultrasonic cleaning with acetone, ethanol, and deionized water for 8 min in sequence, and then dry it at 110℃ for 40 min.

[0083] (2) Photolithography and etching: A positive photoresist is coated on the SOI wafer device layer at a rotation speed of 2500 r / min and a coating thickness of 1.2 μm. The soft baking temperature is 85℃ and the time is 69s. The photomask is used for alignment and exposure, with an exposure dose of 80 mJ / cm² and a medium baking temperature of 108℃ and a time of 100s. The photoresist is developed for 1.6 min with a hardening solution at a hardening temperature of 115℃ and a time of 139s. The photoresist is fully cured and hardened to enhance the resistance to ICP etching and the adhesion of the pattern, and to fix the sensor structure pattern. The device layer is etched using the ICP etching process to remove excess device layer material, leaving only the outer ring area as a support layer.

[0084] (3) Electrode fabrication: A thin aluminum metal film is deposited on the upper surface of the insulating layer in the support layer as the lower electrode using vacuum sputtering process. The electrode thickness is 100 nm. The lower electrode pattern and lead wires are formed between the support layer and the insulating layer by photolithography and etching process.

[0085] (4) Cavity and sensitive film preparation: A sacrificial layer process is used to fill the area between the support layer and the lower electrode with a polyimide sacrificial layer. Microchannels penetrating to the sacrificial layer are etched in advance at a predetermined position in the support layer. A sensitive film is prepared on the sacrificial layer by low-pressure chemical vapor deposition. The deposition material is polycrystalline silicon and the thickness of the sensitive film is controlled to be 0.5 μm. Then, the internal sacrificial layer is removed through the microchannel by ICP etching / plasma ashing to form a gap cavity of a predetermined height. Then, the microchannels on the support layer are sealed by vapor deposition / low-temperature sealing process to obtain a closed gap cavity.

[0086] (5) Fabrication of the upper electrode: A chromium / gold composite metal film is deposited on the upper surface of the sensitive film as the upper electrode; the thickness of the upper electrode is 200 nm; the upper electrode lead is formed by photolithography and etching processes.

[0087] 3. MEMS pressure sensor array transfer and fixation: The prepared 8 capacitive MEMS pressure sensors are uniformly arranged on the annular area on the outer periphery of the detection surface of the probe body 1 using a coating process. They are then fixed with a low-temperature adhesive to ensure that the sensor array is tightly attached to the detection surface, and the spacing error between adjacent sensor units is controlled within ±0.05mm.

[0088] 4. Signal transmission module connection: Connect the electrode leads of each capacitive MEMS pressure sensor 3 in the MEMS pressure sensor array to the signal conditioning unit. The signal conditioning unit is connected to the external control unit through a data composite cable.

[0089] 5. Ultrasonic transducer installation: Fix the ultrasonic transducer 2 in the internal cavity of the probe body 1, ensuring that its detection end is in close contact with the center area of ​​the detection surface and does not interfere with the MEMS pressure sensor array;

[0090] 6. Encapsulation process: Prepare PDMS encapsulation adhesive (base adhesive to curing agent mass ratio 10:1), and uniformly coat it on the surface of the MEMS pressure sensor array and the signal lead-out area, with a thickness of 50μm, and cure at 90℃ for 4h; after curing, use a diamond tool to grind the surface of the encapsulation layer 5 so that it is flush with the central area of ​​the detection surface 6, with a roughness Ra=0.2μm;

[0091] 7. Performance Testing: The prepared ultrasonic probe was tested. The pressure detection range of the MEMS pressure sensor array is 0-50N, the detection accuracy error is ±1.5%, and the ultrasonic signal transmission efficiency is 92%, which meets the requirements of industrial ultrasonic testing.

[0092] Example 2

[0093] An ultrasonic probe based on MEMS force feedback is provided. The height of the gap cavity of the capacitive MEMS sensor 3 is 1 / 4 of the ultrasonic wavelength of the ultrasonic transducer 2. The MEMS pressure sensor array has 7 capacitive MEMS sensors 3.

[0094] The preparation method includes the following steps:

[0095] 1. Pre-treatment of probe body: Select cemented carbide rod and machine it into a columnar structure with a diameter of 20mm and a length of 80mm. Machining a circular detection surface 6 with a diameter of 20mm at the front end, polishing the central 10mm area with a diamond wheel to Ra=0.05μm, and sandblasting the surrounding annular area (width 5mm) to Ra=0.9μm. Then, ultrasonically clean it with acetone, ethanol, and deionized water for 18min each, and dry it at 130℃ for 32min.

[0096] 2. Fabrication of MEMS pressure sensor array:

[0097] (1) SOI wafer cleaning: Select an SOI wafer with a diameter of 6 inches and a device layer thickness of 8 μm (the SOI wafer has a three-layer structure: the top layer is the device layer, the middle layer is the insulating layer, and the bottom layer is the substrate), and clean it by ultrasonic cleaning with acetone, ethanol, and deionized water for 15 min in sequence, and dry it at 130℃ for 20 min.

[0098] (2) Photolithography and etching: A positive photoresist is coated on the SOI wafer device layer at a rotation speed of 3500 r / min and a coating thickness of 0.8 μm. The soft baking temperature is 95℃ and the time is 50s. The photomask is used for alignment and exposure, with an exposure dose of 120 mJ / cm² and a medium baking temperature of 115℃ and a time of 80s. The photoresist is developed for 2.5 min with a hardening solution at a hardening temperature of 125℃ and a time of 100s. The photoresist is fully cured and hardened to enhance the resistance to ICP etching and the adhesion of the pattern, and to fix the sensor structure pattern. The device layer is etched using the ICP etching process to remove excess device layer material, leaving only the outer ring area as a support layer.

[0099] (3) Electrode fabrication: A thin aluminum metal film is deposited on the upper surface of the insulating layer in the support layer as the lower electrode using vacuum sputtering process. The electrode thickness is 150 nm. The lower electrode pattern and lead wires are formed between the support layer and the insulating layer by photolithography and etching process.

[0100] (4) Cavity and sensitive film preparation: A sacrificial layer process is used to fill the area between the support layer and the lower electrode with a polyimide sacrificial layer. Microchannels penetrating to the sacrificial layer are etched in advance at a predetermined position in the support layer. A sensitive film is prepared on the sacrificial layer by low-pressure chemical vapor deposition. The deposition material is polycrystalline silicon and the thickness of the sensitive film is controlled to be 0.5 μm. Then, the internal sacrificial layer is removed through the microchannel by ICP etching / plasma ashing to form a gap cavity of a predetermined height. Then, the microchannels on the support layer are sealed by vapor deposition / low-temperature sealing process to obtain a closed gap cavity.

[0101] (5) Fabrication of the upper electrode: A chromium / gold composite metal film is deposited on the upper surface of the sensitive film as the upper electrode; the thickness of the upper electrode is 150 nm; the upper electrode lead is formed by photolithography and etching processes.

[0102] 3. MEMS pressure sensor array transfer and fixation: The prepared 7-capacitor MEMS pressure sensors are uniformly arranged on the annular area on the outer periphery of the detection surface of the probe body 1 using a coating process. They are then fixed with a low-temperature adhesive to ensure that the sensor array is tightly attached to the detection surface, and the spacing error between adjacent sensor units is controlled within ±0.05mm.

[0103] 4. Signal transmission module connection: Connect the electrode leads of each capacitive MEMS pressure sensor 3 in the MEMS pressure sensor array to the signal conditioning unit. The signal conditioning unit is connected to the external control unit through a data composite cable.

[0104] 5. Ultrasonic transducer installation: Fix the ultrasonic transducer 2 in the internal cavity of the probe body 1, ensuring that its detection end is in close contact with the center area of ​​the detection surface and does not interfere with the MEMS pressure sensor array;

[0105] 6. Encapsulation process: Prepare PDMS encapsulation adhesive (base adhesive to curing agent mass ratio 10:1), and uniformly coat it on the surface of the MEMS pressure sensor array and the signal lead-out area, with a thickness of 50μm, and cure at 100℃ for 2h; after curing, use a diamond tool to grind the surface of the encapsulation layer 5 so that it is flush with the central area of ​​the detection surface 6, with a roughness Ra=0.1μm;

[0106] 7. Performance Testing: The prepared ultrasonic probe was tested. The pressure detection range of the MEMS pressure sensor array is 0-50N, the detection accuracy error is ±1.5%, and the ultrasonic signal transmission efficiency is 92%, which meets the requirements of industrial ultrasonic testing.

Claims

1. An ultrasonic probe based on MEMS force feedback, characterized in that, It includes a probe body (1), an ultrasonic transducer (2), a MEMS pressure sensor array, and a packaging layer; The probe body (1) consists of a circular detection surface at the center and an annular area outside the detection surface. The annular area is concentrically set with the ultrasonic detection area at the center of the detection surface, and the detection surface protrudes from the annular area. The ultrasonic transducer (2) is fixed inside the probe body, and the detection end of the ultrasonic transducer (2) is in contact with the central area of ​​the detection surface. The MEMS pressure sensor array includes several capacitive MEMS sensors (3), which are evenly arranged in a ring around the edge of the detection surface. The height of the gap cavity of the capacitive MEMS sensor (3) is 1 / 10 to 1 / 4 of the ultrasonic wavelength of the ultrasonic transducer (2). The encapsulation layer covers the surface of the MEMS pressure sensor array, and the surface of the encapsulation layer is flush with the center area of ​​the detection surface.

2. The ultrasonic probe based on MEMS force feedback according to claim 1, characterized in that, The MEMS pressure sensor array includes at least 7 to 9 uniformly distributed capacitive MEMS pressure sensors (3), and the uniformity error of the spacing between two adjacent capacitive MEMS pressure sensors (3) does not exceed ±5%.

3. An ultrasonic probe based on MEMS force feedback according to claim 1 or 2, characterized in that, Each of the capacitive MEMS pressure sensors (3) consists of, from top to bottom, an upper electrode (31), a sensitive film (32), a support layer (33), a lower electrode (34), an insulating layer (35), and a substrate (36); The center of the support layer is removed by ICP etching, forming a gap cavity (37) between the sensitive film (32), the support layer and the lower electrode; The sensitive film (32) adopts The composite film has a thickness of 0.5-5 μm; the upper electrode (31) and the lower electrode (34) are parallel and corresponding.

4. The ultrasonic probe based on MEMS force feedback according to claim 3, characterized in that, The MEMS pressure sensor array also includes a signal transmission module, which includes a data composite cable. The upper / lower electrode leads of each capacitive MEMS pressure sensor (3) communicate with an external control unit through the data composite cable.

5. The ultrasonic probe based on MEMS force feedback according to claim 1, characterized in that, The ultrasonic transducer (2) communicates with an external control unit via a cable.

6. The ultrasonic probe based on MEMS force feedback according to claim 1, characterized in that, It also includes a signal conditioning unit for amplifying, filtering and analog-to-digital conversion of the signal. The signal conditioning unit is connected in series between the control unit and the MEMS pressure sensor array and between the control unit and the ultrasonic transducer (2) to improve the stability and accuracy of signal transmission.

7. The ultrasonic probe based on MEMS force feedback according to claim 1, characterized in that, The encapsulation layer is made of polydimethylsiloxane (PDMS) material, and the acoustic impedance of the encapsulation layer is matched with the acoustic impedance of the coupling medium on the surface of the object under test.

8. The ultrasonic probe based on MEMS force feedback according to claim 1, characterized in that, The width of the annular mounting area is 2-5mm.

9. A method for fabricating an ultrasonic probe based on MEMS force feedback, characterized in that, Includes the following steps: Step 1: Pre-treatment of probe body (1) Select hard alloy material to process into a hollow columnar probe body (1) with an open top. Process concentric detection surface and annular area at the lower end. The detection surface protrudes from the annular area. Polish the central area of ​​the detection surface to Ra≤0.1μm and roughen the annular area to Ra≤1.2μm. Then, ultrasonically clean with acetone, ethanol and deionized water for 15-20min each, and dry at 110-130℃ for 30-35min. Step 2: Prepare a capacitive MEMS pressure sensor with a gap cavity height of 1 / 10-1 / 4 of the ultrasonic wavelength (2) of the ultrasonic transducer (3); Step 3: The prepared 7~9 capacitive MEMS pressure sensors are evenly arranged on the annular area on the outer periphery of the detection surface of the probe body (1) using a coating process, and fixed with a low-temperature adhesive to ensure that the sensor array is tightly attached to the detection surface and the spacing error between adjacent sensor units does not exceed ±5%; Step 4: Connect the electrode leads of each capacitive MEMS pressure sensor (3) in the MEMS pressure sensor array to the signal conditioning unit. The signal conditioning unit is connected to the external control unit through a data composite cable. Step 5: Fix the ultrasonic transducer (2) inside the cavity of the probe body (1) to ensure that its detection end is in close contact with the center area of ​​the detection surface and does not interfere with the MEMS pressure sensor array; Step 6: Encapsulation process. PDMS encapsulation adhesive is used to encapsulate the MEMS pressure sensor array and electrode leads. During the encapsulation process, the temperature is controlled at 80-100℃ and the curing time is 2-4 hours. After encapsulation, the surface of the encapsulation layer is polished to make it flush with the center area of ​​the detection surface, and the surface roughness Ra≤0.2μm.

10. The method for fabricating an ultrasonic probe based on MEMS force feedback according to claim 9, characterized in that, Step 2 is as follows: (1) SOI wafer cleaning: Select SOI wafers with a diameter of 4 to 6 inches and a device layer thickness of 3 to 8 μm (the wafer has a three-layer structure: the top layer is the device layer, the middle layer is the insulating layer, and the bottom layer is the substrate), and clean them sequentially with acetone, ethanol, and deionized water by ultrasonic cleaning for 8 to 15 min to remove surface impurities. Then dry them at 110 to 130 °C for 20 to 40 min. (2) Photolithography and etching: Photoresist is coated on the upper surface of the device layer of the SOI wafer at a rotation speed of 2500-3500 r / min and a coating thickness of 0.8-1.2 μm. Then, it is dried at a soft baking temperature of 85-95℃ for 50-70 s to remove the photoresist solvent and enhance adhesion. Photolithography is performed through a mask at an exposure dose of 80-120 mJ / cm². It is then dried at a medium baking temperature of 105-115℃ for 80-100 s to stabilize the photolithographic latent image and eliminate the standing wave effect. It is developed with developer for 1.5-2.5 min and then dried at a hard film temperature of 115-125℃ for 100-140 s to fully cure and harden the photoresist, enhance the resistance to ICP etching and the adhesion of the pattern, fix the sensor structure pattern, and use ICP etching process to perform non-penetrating etching on the device layer to remove excess device layer material, leaving only the outer ring area as a support layer. (3) Fabrication of the lower electrode: A thin aluminum metal film is deposited on the upper surface of the insulating layer in the support layer as the lower electrode using vacuum sputtering process. The electrode thickness is 100-200 nm. The lower electrode pattern and lead wire are formed between the support layer and the insulating layer by photolithography and etching process. (4) Cavity and sensitive film preparation: A sacrificial layer process is used to fill the area between the support layer and the lower electrode with a polyimide sacrificial layer. Microchannels penetrating to the sacrificial layer are etched in advance at a predetermined position in the support layer. A sensitive film is prepared on the sacrificial layer by low-pressure chemical vapor deposition. The deposition material is polycrystalline silicon and the thickness of the sensitive film is controlled to be 0.5-5 μm. Then, the internal sacrificial layer is removed through the microchannel by ICP etching / plasma ashing to form a gap cavity of a predetermined height. Then, the microchannels on the support layer are sealed by vapor deposition / low-temperature sealing process to obtain a closed gap cavity. (5) Fabrication of the upper electrode: A chromium / gold composite metal film is deposited on the upper surface of the sensitive film as the upper electrode; the thickness of the upper electrode is 100-200 nm; the upper electrode lead is formed by photolithography and etching processes.

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