Capacitive micro-mechanical ultrasonic transducer for acoustic emission detection

Abstract

The invention discloses a capacitive micromechanical ultrasonic transducer for acoustic emission detection, the ultrasonic transducer is composed of a plurality of infinitesimal elements, each infinitesimal element structurally comprises a metal upper electrode, a vibrating membrane, a high dielectric constant dielectric layer, a lower electrode, an insulating layer and a matching layer, the upper end face of the lower electrode is provided with a spherical groove, and the lower end face of the lower electrode is provided with a spherical surface. The spherical groove is covered with a high-dielectric-constant dielectric layer, the high-dielectric-constant dielectric layer is covered with a horizontal vibrating membrane, a metal upper electrode is arranged in the middle of the upper end face of the vibrating membrane, and a matching layer covering the surfaces of the vibrating membrane and the metal upper electrode is arranged on the lower electrode. A spherical vacuum cavity is formed by a cavity formed by the high-dielectric-constant dielectric layer and the vibrating membrane, and an insulating layer is arranged at the bottom of the lower electrode. According to the method, the sensitivity and the signal-to-noise ratio of the CMUT can be synchronously improved, the CMUT can be applied to positioning of a structural sound source after a vibrating diaphragm of the CMUT is protected, and a feasible way is provided for damage detection of the CMUT in a plate structure.

Description

Technical Field

[0001] The invention belongs to the technical field of acoustic emission application of capacitive micromechanical ultrasonic transducers, and relates to a capacitive micromechanical ultrasonic transducer for acoustic emission detection. Background Art

[0002] Acoustic emission refers to the elastic waves generated by the sudden release of elastic energy inside or on the surface of a material due to deformation or damage. It is also called stress wave emission. The location of structural sound sources is usually completed by detecting acoustic emission signals with piezoelectric transducers, because they have high sensitivity and response speed as transmitters or receivers in the field of non-destructive testing, and can detect tiny sound wave signals. However, usually, the use of piezoelectric transducers to locate structural sound sources requires combining multiple transducers into an array structure, which is difficult to process into a miniaturized transducer array due to its size, so it takes up a large space and is inconvenient to use. In addition, there are limitations such as high manufacturing costs and large temperature effects.

[0003] The emergence of capacitive micromachined ultrasonic transducers (CMUTs) is expected to solve these limitations. CMUTs have the advantages of low mass production costs and miniaturization. Thanks to MEMS technology, capacitive micromachined ultrasonic transducers are easier to process into transducer arrays. The array structure consists of multiple array element structures and can receive acoustic emission signals at the same time, giving it certain advantages in acoustic emission detection. Although CMUTs are currently inferior to piezoelectric transducers in terms of sensitivity and signal-to-noise ratio, many scholars have conducted research on their applications in structural health detection.

[0004] Many scholars have studied the acoustic emission sensing performance of CMUT, confirmed its feasibility for acoustic emission detection applications, and continuously improved the performance of CMUT from different aspects to make it better for acoustic emission detection. However, the current application of CMUT in acoustic emission detection is mainly limited by its low sensitivity. Another important reason limiting its application in acoustic emission detection is the low signal-to-noise ratio. Due to the large difference in acoustic impedance between the metal plate to be tested and the air medium, the ultrasonic energy is greatly attenuated when it is transmitted from the metal plate to the air medium. The acoustic signal received by the CMUT is very weak and may even be covered by noise.

[0005] Therefore, it is necessary to design a capacitive micromachined ultrasonic transducer array with high sensitivity, high signal-to-noise ratio and suitable for acoustic emission detection. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a design method of a capacitive micro-machined ultrasonic transducer for acoustic emission detection in view of the deficiencies of the prior art.

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

[0008] A capacitive micromechanical ultrasonic transducer for acoustic emission detection, characterized in that: the ultrasonic transducer is composed of multiple microelements, the structure of the microelements includes a metal upper electrode, a vibration membrane, a high dielectric constant dielectric layer, a lower electrode, an insulating layer and a matching layer, a spherical groove is provided on the upper end surface of the lower electrode, the high dielectric constant dielectric layer is covered on the spherical groove, a horizontal vibration membrane is covered on the high dielectric constant dielectric layer, a metal upper electrode is provided in the middle of the upper end surface of the vibration membrane, a matching layer covering the vibration membrane and the metal upper electrode surface is provided on the lower electrode, the cavity formed by the high dielectric constant dielectric layer and the vibration membrane constitutes a spherical vacuum cavity, and an insulating layer is provided at the bottom of the lower electrode.

[0009] Moreover, the micro-elements are distributed in a square array structure, the array is composed of N array elements, and each array element is composed of N micro-elements.

[0010] The N array elements are combined to be used as a receiver, or they can be split into N receivers to receive N signals according to the requirements of the detection method.

[0011] Moreover, the dielectric constant of the high dielectric constant dielectric layer is greater than 20 and the thickness is 1 μm.

[0012] Moreover, the spherical vacuum chamber is a spherical structure.

[0013] Moreover, the material of the matching layer is polydimethylsiloxane.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] 1. Compared with the traditional CMUT micro-element, the capacitive micromachined ultrasonic transducer used for acoustic emission detection has a reduced average height of the structure cavity, which can increase the electrostatic force between the electrodes and the output current value after receiving the signal, thereby improving the emission and receiving sensitivity of the CMUT.

[0016] 2. Compared with the traditional CMUT micro-element, the capacitive micromachined ultrasonic transducer used for acoustic emission detection has a larger dielectric constant value and a thinner thickness of the insulating layer. Under the same DC bias or external pressure, it can generate a larger electrostatic force and output a larger current value, thereby improving the transmission and reception sensitivity of the CMUT.

[0017] 3. The capacitive micromachined ultrasonic transducer used for acoustic emission detection uses a flexible material as the matching layer of the CMUT. On the one hand, it can protect the vibrating membrane of the transducer from being damaged during operation. On the other hand, it can serve as an acoustic impedance matching layer to improve the efficiency of acoustic wave energy transmission and the signal-to-noise ratio of the CMUT, which is crucial for using the CMUT to realize acoustic emission detection.

[0018] 4. The present invention can achieve simultaneous improvement of the sensitivity and signal-to-noise ratio of the CMUT, and after providing protection for the vibration membrane of the CMUT, the CMUT can be applied to the positioning of structural sound sources, providing a feasible way to realize damage detection of the CMUT in the plate structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Schematic diagram of the microelement structure of a traditional capacitive micromechanical ultrasonic transducer;

[0020] Figure 2 It is a schematic diagram of the microelement structure of the capacitive micromechanical ultrasonic transducer of the present invention;

[0021] Figure 3 A comparison diagram of the emitted sound pressures of a conventional capacitive micromachined ultrasonic transducer and the capacitive micromachined ultrasonic transducer of the present invention;

[0022] Figure 4 A comparison diagram of the receiving current of a conventional capacitive micromachined ultrasonic transducer and a capacitive micromachined ultrasonic transducer of the present invention;

[0023] Figure 5 This is a schematic diagram of the CMUT array structure of the present invention;

[0024] Figure 6 It is a schematic diagram of the working principle of the present invention.

[0025] Description of reference numerals:

[0026] 1-metal upper electrode, 2-silicon vibration membrane, 3-high dielectric constant dielectric layer, 4-matching layer, 5-lower electrode, 6-insulating layer, 7-external excitation connection terminal, 8-microelement. DETAILED DESCRIPTION

[0027] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0028] The capacitive micromechanical ultrasonic transducer for acoustic emission detection has the following innovations: the ultrasonic transducer is composed of a plurality of microelements 8, the structure of the microelements includes a metal upper electrode 1, a silicon vibration membrane 2, a high dielectric constant dielectric layer 3, a matching layer 4, a lower electrode 5 and an insulating layer 6, a spherical groove is provided on the upper end surface of the lower electrode, a high dielectric constant dielectric layer is covered on the spherical groove, a horizontal vibration membrane is covered on the high dielectric constant dielectric layer, a metal upper electrode is provided in the middle of the upper end surface of the vibration membrane, a matching layer covering the surface of the vibration membrane and the metal upper electrode is provided on the lower electrode, a cavity formed by the high dielectric constant dielectric layer and the vibration membrane constitutes a spherical vacuum cavity, and an insulating layer is provided at the bottom of the lower electrode.

[0029] The present invention improves on the traditional CMUT structure. The metal upper electrode and the silicon vibration membrane are both circular thin layers. The metal upper electrode is located in the central area above the vibration membrane. The shape of the cavity is designed to be spherical according to actual needs. The interior is a vacuum environment. The upper part of the cavity corresponding to the lower surface of the vibration membrane is circular, and the lower part of the cavity corresponding to the upper surface of the silicon substrate is spherical. In order to meet the needs of different vibration membrane shapes and different cavity depths, the circle above the cavity corresponding to the lower surface of the vibration membrane is not specified as the cross-section at the center of the spherical cavity, that is, the lower surface of the vibration membrane can be a horizontal cross-section at any position of the spherical cavity. Different from the traditional structure, a silicon dioxide insulating layer is not formed on the surface of the lower surface of the cavity by thermal oxidation, but a dielectric layer with a high dielectric constant is deposited on the silicon insulating layer on the lower surface of the cavity.

[0030] Combined with the application scenario of the transducer, a matching layer is covered on the surface of the CMUT vibration membrane. By adding the matching layer, the transmission efficiency of ultrasonic energy is improved, laying the foundation for realizing the sound source positioning of CMUT in metal plates.

[0031] like Figure 2 As shown in the figure, the combination of the spherical vacuum cavity and the high dielectric constant dielectric layer can effectively improve the sensitivity of the CMUT. The transducer array is composed of N×N micro-elements, including N array elements, each of which can be used individually or flexibly combined into an array to adapt to different application scenarios.

[0032] like Figure 5 As shown, it is a schematic diagram of an N×N array. For example, when N=6, every 6 micro-elements can form an array element. The micro-elements are connected to the metal upper electrodes through metal wires and connected to the external excitation connection terminal 7. A total of 6 array elements form a CMUT array. The 6×6 array is a case of an embodiment of the present invention, but the present invention is not limited to a 6×6 array structure.

[0033] The parallel plate model is used to simplify the analysis of the CMUT of the present invention, and the electrostatic force applied to the electrode is expressed as:

[0034]

[0035] Where ε0 is the vacuum dielectric constant, g is the initial vacuum cavity spacing, A is the relative area of ​​the electrodes, V is the voltage applied across the electrodes, k is the relative dielectric constant of the dielectric layer, and t d is the thickness of the dielectric layer.

[0036] When the CMUT is in the transmitting state, it can be seen from the above formula that if Figure 2The spherical vacuum cavity shown replaces the cavity structure of the traditional CMUT to reduce the average spacing of the cavities. After the spacing is reduced, the electrostatic force generated between the upper and lower electrodes becomes larger. At this time, when the same AC voltage is applied, the force exerted on the vibration membrane is greater than the force exerted on the vibration membrane of the traditional CMUT, and its emission capability is also stronger and the emission sensitivity is high.

[0037] Therefore, the present invention adopts Figure 2 The spherical structure shown can effectively reduce the average spacing of the vacuum cavity and improve the emission sensitivity of the CMUT. In addition, from the above formula, it can be seen that if a larger electrostatic force is to be generated in the emission state, a larger dielectric constant k and a smaller thickness t should be selected. d of dielectric material.

[0038] Therefore, selecting a dielectric material with a larger dielectric constant and changing the cavity structure to a spherical shape can effectively improve the emission sensitivity of the CMUT.

[0039] like Figure 3 As shown, the emission sound pressure of two structures (traditional capacitive micromachined ultrasonic transducer and capacitive micromachined ultrasonic transducer of the present invention) under the same conditions are compared through finite element simulation analysis. It can be seen that the emission sensitivity of the CMUT of the present invention is significantly improved.

[0040] When the CMUT is in the receiving state, when the vibration displacement of the membrane caused by external ultrasonic waves is x, the current output by the CMUT is:

[0041]

[0042] Where v is the vibration velocity of the diaphragm, V DC is the DC bias voltage, and g0 is the initial vacuum cavity spacing.

[0043] The results show that in order to generate a larger receiving current in the receiving mode, a dielectric with a high dielectric constant k and a thickness t should be used. d Thinner dielectric material. At the same time, the reduction of cavity spacing g can also increase the output current value.

[0044] Therefore, selecting a dielectric material with a larger dielectric constant and changing the cavity structure to a spherical shape can effectively improve the receiving sensitivity of the CMUT.

[0045] like Figure 4 As shown, it is a comparison of the output current values ​​of two structures after receiving the acoustic signal under the same conditions obtained by finite element simulation analysis. It can be seen that the receiving sensitivity of the CMUT of the present invention is significantly improved.

[0046] In summary, the sensitivity of the CMUT can be effectively improved by changing the cavity structure to a sphere and selecting a dielectric layer material with a high dielectric constant.

[0047] Sensitivity is an important factor that limits the application of CMUT in acoustic emission detection, and determines whether CMUT can detect small signal changes. Another important factor that limits it is the signal-to-noise ratio, which is a measure of the relative strength of signal quality and noise, and describes the ratio between useful information (signal) and interference information (noise) in the output signal of the transducer. The signal-to-noise ratio of traditional CMUT is usually low. When used for acoustic emission detection, acoustic emission signals with very small signal intensity will be covered by noise and cannot be detected. By selecting a suitable matching layer, the vibration membrane on the surface of the CMUT can be well protected, and the energy transmission efficiency of the acoustic emission signal can be improved, thereby improving the signal-to-noise ratio of the CMUT.

[0048] When selecting matching layer materials, it is necessary to consider that when ultrasonic waves encounter media with different acoustic properties during propagation, transmission and reflection will occur, and the magnitude of the transmitted sound pressure and the reflected sound pressure depends on the acoustic impedance of different media. Acoustic impedance is the basic acoustic property of acoustic materials, which is defined as the ratio of the sound pressure in the material to the speed of the sound wave propagation body, that is, Z = P / v = ρc, where Z represents the mass impedance of a point in the medium, P represents the sound pressure of the particle, v represents the vibration speed of the particle, ρ represents the density of the propagation medium, and c represents the sound propagation speed in the medium.

[0049] Consider that when a sound wave is emitted perpendicular to the interface between two different media, the acoustic impedances of the two media are Z1 and Z2 respectively. At the interface between the two media, part of the energy is reflected back to the original medium, which is called the reflected wave energy, and part of the energy passes through the interface into the other medium, which is called the incident wave energy. The proportional relationship between the two energies depends on the acoustic impedance of the two media, which can be expressed as:

[0050] Reflection coefficient

[0051] Transmission coefficient

[0052] The reflection coefficient R represents the ratio of the sound pressure of the reflected wave to the sound pressure of the incident wave, and the transmission coefficient D represents the ratio of the sound pressure of the transmitted wave to the sound pressure of the incident wave. 7 ~4×10 7 Pa·s / m, the characteristic impedance of air is 415Pa·s / m, which shows that the characteristic impedance of sound waves in metal plates is about 2×10 5times. Considering that the sound wave enters the air interface from the metal plate, at this time Z1>>Z2, R≈-1, D≈0, the amplitude of the reflected wave sound pressure is approximately equal to the amplitude of the incident wave sound pressure, but the phase of the reflected wave sound pressure jumps 180°, so the total pressure on the interface is equal to zero. It can be seen that the impedance of the medium has a great influence on the reflection of the wave. The greater the difference in the characteristic impedance of the medium, the greater the reflection coefficient; the smaller the difference, the smaller the reflection coefficient, the weaker the reflected wave, and the incident sound wave can better enter the second medium.

[0053] Therefore, the present invention improves the acoustic wave transmission efficiency and the signal-to-noise ratio of CMUT by adding a flexible material as a matching layer. Although the characteristic impedance of the flexible material is still quite different from that of the metal plate, it has been greatly improved compared with the air medium. At the same time, it should be considered that the selected material must be able to have good contact with the metal plate to be tested and not damage the CMUT diaphragm and the metal plate to be tested. Therefore, the present invention selects polydimethylsiloxane (PDMS) material as the matching layer, and its acoustic impedance value is about 1.03×10 6 Pa·s / m.

[0054] like Figure 6 As shown, it is a schematic diagram of the working principle of the present invention. When the CMUT designed by the present invention is used for acoustic emission detection, it is necessary to apply a DC bias voltage between the upper and lower electrodes. When the external sound pressure acts on the upper surface of the vibrating membrane to vibrate the vibrating membrane, the electrode spacing changes, thereby causing the CMUT capacitance value to change. At this time, an electrical signal that changes synchronously with the displacement of the vibrating membrane will be output to the external circuit to realize the conversion of mechanical energy into electrical energy. For example, when the surface of an aluminum plate is subjected to an external impact signal, an acoustic signal will be propagated in the plate. When the signal propagates to the transducer, it will be sensitively captured, and the vibrating membrane of the transducer will vibrate and cause a change in capacitance, outputting an electrical signal to the outside to realize the detection of acoustic emission signals. The sound source positioning can be realized by collecting multi-channel signals through multiple arrays or multiple array elements, which provides a feasible way to realize damage detection of CMUT in plate structures.

[0055] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will appreciate that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A capacitive micromachined ultrasonic transducer for acoustic emission detection, characterized in that: The ultrasonic transducer is composed of multiple microelements, and the structure of the microelement includes a metal upper electrode, a vibration membrane, a high dielectric constant dielectric layer, a lower electrode, an insulating layer and a matching layer. A spherical groove is provided on the upper end surface of the lower electrode, and a high dielectric constant dielectric layer is covered on the spherical groove. A horizontal vibration membrane is covered on the high dielectric constant dielectric layer. A metal upper electrode is provided in the middle of the upper end surface of the vibration membrane. A matching layer covering the vibration membrane and the metal upper electrode surface is provided on the lower electrode. The cavity formed by the high dielectric constant dielectric layer and the vibration membrane constitutes a spherical vacuum cavity, and an insulating layer is provided at the bottom of the lower electrode.

2. A capacitive micromachined ultrasonic transducer for acoustic emission detection according to claim 1, characterized in that: The micro-elements are distributed in a square array structure. The array is composed of N array elements, and each array element is composed of N micro-elements.

3. A capacitive micromachined ultrasonic transducer for acoustic emission detection according to claim 1, characterized in that: The N array elements are used as one receiver when combined, and can be used as N receivers to receive N signals when separated.

4. A capacitive micromachined ultrasonic transducer for acoustic emission detection according to claim 1, characterized in that: The dielectric constant of the high dielectric constant dielectric layer is greater than 20 and the thickness is 1 μm.

5. A capacitive micromachined ultrasonic transducer for acoustic emission detection according to claim 1, characterized in that: The spherical vacuum chamber is a spherical structure.

6. A capacitive micromachined ultrasonic transducer for acoustic emission detection according to claim 1, characterized in that: The material of the matching layer is polydimethylsiloxane.

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