CN102620878B - Capacitive micromachining ultrasonic sensor and preparation and application methods thereof - Google Patents

Abstract

The present invention provides a capacitive micromachining ultrasonic sensor and its preparation and application method. Its overall structure is as follows from top to bottom: a single crystal silicon vibration film, a silicon dioxide pillar, and a single crystal silicon base, wherein the The middle part of the silicon dioxide pillar is a cavity, the middle part of the monocrystalline silicon vibrating film is heavily doped with boron ions to form an upper electrode, and the middle part of the single crystal silicon base is heavily doped with boron ions to form a lower electrode , the lateral dimension of the upper electrode and the lower electrode is less than or equal to the lateral dimension of the cavity and greater than half of the lateral dimension of the cavity. The sensor of the present invention has simple structure, low processing difficulty, and is suitable for mass production; the vibrating film of the present invention has no integration of force sensitive resistors and circuits, and the thickness and quality of the film can be further reduced, thereby achieving higher sensitivity and smaller range pressure. Measurement; the present invention uses the shift of resonance frequency to measure the change of pressure, thus maintaining a better linear relationship between input and output.

Description

A capacitive micro-machined ultrasonic sensor and its preparation and application method

technical field

The invention belongs to the technical field of MEMS, and relates to a capacitive micro-processing ultrasonic sensor and its preparation and application method.

Background technique

Ultra-micro low-pressure sensors are mainly used for the measurement of tiny pressures. They have urgent needs and wide applications in the fields of industrial control, environmental protection equipment, medical equipment, aerospace and military weapons, so the research on this type of sensors is extremely important. practical value.

At present, silicon micro-pressure sensors based on MEMS (Micro Electro-Mechanical Systems) technology have dominated the field of pressure sensors and have been widely used commercially. According to their working principles, silicon micro-pressure sensors mainly It can be divided into the following three types: piezoresistive, capacitive and resonant. The piezoresistive micro-pressure sensor mainly uses the piezoresistive effect of silicon to measure the pressure through voltage changes. Although its output has a good linear relationship with the input, but The temperature sensitivity of the force sensitive resistor in the silicon film requires that the sensor must implement temperature compensation, which increases the complexity of the measurement. At the same time, the integration of the Wheatstone bridge in the silicon film makes it difficult to further reduce the film thickness under the condition of ensuring the measurement accuracy. It is difficult to further reduce the range and improve the sensitivity. The capacitive silicon micro pressure sensor uses the change of the capacitance pole distance to convert the pressure change into the capacitance change, which has good temperature stability, high sensitivity, low power consumption, and relatively simple further miniaturization. A series of advantages, but the linearity of its output and input is poor.The resonant silicon micro pressure sensor uses the natural frequency of the resonant beam to change with the change of the applied axial force to achieve pressure measurement, although its measurement accuracy and stability and resolution are better than the above two, but the structure is complex and difficult to process. At present, the range of silicon micro pressure sensors is mainly around 1000Pa, and the smallest can reach 300Pa. Due to the limitations of the above structure itself, it is difficult to further realize Ultra-low micro-pressure measurement with lower range and higher sensitivity. Therefore, this paper intends to use a CMUT (Capactive Micromachined Ultrasonic Transducer, capacitive micromachined ultrasonic sensor) based on MEMS technology with more structural and performance advantages for ultra-micro pressure measurement. CMUT It is one of the important research directions of MEMS technology. It has the characteristics of good electromechanical properties, smaller film quality, higher resonance frequency (up to dozens of MHz) and quality factor (up to hundreds of) and so on. It is possible to achieve higher sensitivity and smaller range pressure measurement; its simple structure, easy processing, easy array, easy integration and other characteristics provide many advantages for low cost, short cycle, high-efficiency mass production and complex circuit integration. At present, CMUT It is mainly used in ultrasonic imaging, detection of biochemical substances, etc., and there are no relevant reports in the field of ultra-low and micro-pressure measurement.

Contents of the invention

The technical problem to be solved by the present invention is to provide a capacitive micro-machined ultrasonic sensor and its preparation and application method. The capacitive micro-machined ultrasonic sensor is applied to micro-pressure measurement to solve the current problems in the field of ultra-low micro-pressure measurement, and to achieve a sensitivity higher than 150Hz/Pa and ultra-low micro-pressure measurement with a range below 300Pa.

In order to solve the above technical problems, the present invention adopts the following technical solutions:

A capacitive micro-machined ultrasonic sensor, its overall structure from top to bottom is: a single crystal silicon vibration film, a silicon dioxide pillar, and a single crystal silicon base, wherein the middle part of the silicon dioxide pillar is a cavity , the middle part of the monocrystalline silicon vibrating film is heavily doped with boron ions to form an upper electrode, the middle part of the single crystal silicon base is heavily doped with boron ions to form a lower electrode, and the lateral dimensions of the upper electrode and the lower electrode are less than Or equal to the transverse dimension of the cavity and greater than half of the transverse dimension of the cavity.

As a preferred embodiment of the present invention, the thickness of the single crystal silicon vibrating film is 0.06-0.12um;

As a preferred embodiment of the present invention, the lateral size of the effective vibration film of the single crystal silicon vibration film is 5 μm to 15 μm, and the effective vibration film is the part of the vibration film above the cavity;

As a preferred embodiment of the present invention, the lateral dimension of the cavity is equal to that of the effective vibrating film, which is 5-15 μm, and the height of the cavity is equal to the silica pillar, which is 0.08-0.15 μm;

As a preferred embodiment of the present invention, the resistivity of the upper electrode and the lower electrode is less than 10 ⁻³ Ω·cm.

A method for preparing a capacitive micromachined ultrasonic sensor, comprising the following steps:

(1) Take <111> crystal silicon, and use local ion implantation technology to implant boron ions in the middle of the single crystal silicon, so that the resistivity is less than 10 ^-3 Ω·cm. Among them, the heavily doped single crystal silicon part forms the lower electrodes, and the rest are CMUT bases;

(2) Plasma-enhanced chemical vapor deposition Is the English abbreviation of this technology needed here: PECVD technology deposits a silicon dioxide layer on the CMUT base, and then photolithographically etches the silicon dioxide layer to form a cavity pattern window, and then uses The buffer etching solution etches away the silicon dioxide layer exposed in the graphics window, and the remaining silicon dioxide layer forms the silicon dioxide pillar. Finally, the upper surface of the silicon dioxide pillar is polished by chemical mechanical polishing CMP technology to form the first part ;

(3) Take the SOI wafer, dry oxidize the upper surface of the top monocrystalline silicon slice of the SOI wafer by dry oxidation technology to form a silicon dioxide layer, and the thickness of the silicon dioxide layer formed by oxidation is the same as that of the silicon dioxide layer formed The ratio of the thickness of the monocrystalline silicon flakes consumed during the process is 1:0.44, and the unoxidized part is called the monocrystalline silicon layer;

(4) Etch the silicon dioxide layer formed in step (3) with a buffer etchant to expose the unoxidized single crystal silicon layer, and then use local ion implantation technology to heavily dope the middle of the single crystal silicon layer Boron ions to make the resistivity less than 10 ^-3 Ω·cm, wherein the heavily doped single crystal silicon part forms the upper electrode, and finally chemical mechanical polishing is performed on the upper surface of the single crystal silicon layer to form the second part;

(5) In a vacuum environment, the first part obtained in step (2) and the second part obtained in step (4) are anodically bonded, wherein the upper surface of the silicon dioxide pillar of the first part is connected to the top of the second part The upper surface of the unoxidized single crystal silicon layer is bonded; because the SOI wafer is divided into the bottom substrate silicon, the buried silicon dioxide and the top single crystal silicon slice, here only the second part of the single crystal silicon layer is It is not possible to distinguish between the substrate monocrystalline silicon and the top monocrystalline silicon layer.

(6) The device obtained in step (5) is wet-etched from top to bottom to remove the substrate monocrystalline silicon and 80% of the buried silicon dioxide of the SOI wafer, and then etch the remaining silicon dioxide with a buffer etching solution. 20% buried silicon dioxide.

An application method of a capacitive micro-machined ultrasonic sensor, the capacitive micro-machined ultrasonic sensor is used to achieve ultra-low micro-pressure measurement with a sensitivity higher than 150 Hz/Pa and a range lower than 300 Pa, the specific method is: jointly determine through simulation analysis and experimental means The best operating point of the CMUT is the bias DC voltage. At the same time, determine the natural resonant frequency and the AC signal at this frequency point. The sum of the voltage amplitude of the AC signal and the bias voltage should be less than the collapse voltage of the CMUT. The principle is to maximize the coupling coefficient ; Under the excitation of the determined bias DC voltage and the AC signal of the resonant frequency, the CMUT resonates, and it is placed in a micro-pressure environment. Because the pressure acts on the CMUT vibration film, the vibration state of the CMUT is changed and the resonance is lost. At this time, the adjustment The frequency of the AC signal causes the CMUT to resonate again, record the resonant frequency, calculate the difference Δf between the natural resonant frequency and the resonant frequency under the pressure, and then use the functional relationship between pressure and frequency shift P=Δf/k to obtain Get the measured pressure value to realize pressure measurement.

The CMUT-based ultra-low micro-pressure sensor and preparation method thereof of the present invention have at least the following advantages:

(1) Compared with the piezoresistive micro-pressure sensor, the CMUT vibrating film of the present invention has no integration of force-sensing resistors and circuits, and the thickness and quality of the film can be further reduced, so that the measurement of higher sensitivity and smaller range pressure can be realized.

(2) Compared with the capacitive micro-pressure sensor, the present invention uses the shift of the resonant frequency to measure the change of the pressure, thus maintaining a better linear relationship between the input and output.

(3) Compared with the resonant micro-pressure sensor, the present invention has a simple structure, less difficulty in processing, is suitable for mass production, and is easy to integrate.

(4) Since the CMUT structure of the present invention is only composed of two measurement materials, single crystal silicon and silicon dioxide, and the expansion coefficients of these two materials are the same, the structure is suitable for high temperature environments, and its thermal expansion coefficients are very similar (wherein the thermal expansion of silicon dioxide The coefficient is 2.3×10 ^-6 /℃, single crystal silicon is 2.6×10 ^-6 /℃), so this structure can also be used for the measurement of micro pressure in high temperature environment.

Description of drawings

Fig. 1 is the structural representation of the ultra-low pressure sensor based on CMUT of the present invention;

Fig. 2 is a flow chart of the preparation process of the sensor of the present invention.

Detailed ways

A capacitive micromachining ultrasonic sensor of the present invention and its preparation and application method are described in detail below in conjunction with the accompanying drawings:

Please refer to accompanying drawing 1, the overall structure of a capacitive micromachined ultrasonic sensor of the present invention is as follows from top to bottom: single crystal silicon diaphragm 1, silicon dioxide pillar 2, cavity 6 and single crystal silicon base 3. Among them, the upper electrode 4 is formed in the middle part of the monocrystalline silicon vibration film 1 through the heavily doped region of boron ions, that is, the vibration film 1 is also used as the upper electrode of the CMUT, and the upper electrode 4 and the vibration film 1 are an integrated structure; the single crystal silicon base The middle part of the CMUT is heavily doped with boron ions to form the lower electrode 5 of the CMUT, that is, the monocrystalline silicon base 3 is also used as the lower electrode, and the lower electrode 5 and the base 3 are of an integrated structure.

The single crystal silicon vibration film 1 is not only used for the CMUT vibration film, but also used for the upper electrode. On the one hand, the thickness of the film should be as small as possible to reduce the mass of the film, thereby improving the sensitivity of the CMUT and realizing a smaller range of pressure measurement; on the other hand, too small a thickness will increase the series resistance and affect the conductivity of the upper electrode 4; Should be 0.06 ~ 0.12um. In addition, according to the theoretical calculation formula of the vibration frequency of the membrane, it can be known that to obtain a high resonance frequency, the surface radius should be reduced as much as possible. Therefore, the effective vibration membrane lateral dimension of the vibration membrane 1 in the present invention ranges from 5 μm to 15 μm, and the effective vibration membrane area is It is above the cavity 6 of the vibration film 1 or the part of the film that is not bonded to the silicon dioxide support 2 . The lateral size of the upper electrode 4 should be smaller than or equal to the lateral size of the cavity 6, but should be equal to or larger than half of the lateral size, so as to reduce the parasitic capacitance and increase the coupling coefficient as the design principle.

The cavity 6 is circular or polygonal, its lateral dimension is the same as that of the effective vibrating film, its size ranges from 5 μm to 15 μm, and its height is the same as that of the silicon dioxide pillar 2 .

The height of the silicon dioxide pillar 2 is as small as possible to reduce the cavity height, increase the coupling coefficient, and further improve the sensitivity, and the height range is 0.08-0.15 μm.

The monocrystalline silicon base 3 is not only used as the base of the entire CMUT structure, but also serves as the lower electrode 5 after the middle part is heavily doped with boron ions. The lower electrode 5 and the single crystal silicon base 3 are an integrated structure. The lateral size of the lower electrode 5 should be smaller than or equal to the lateral size of the cavity 6, but should be equal to or larger than half of the lateral size, so as to reduce the parasitic capacitance and increase the coupling coefficient as the design principle.

After the upper electrode 4 and the lower electrode 5 are heavily doped with boron ions, the resistivity should be less than 10 ⁻⁴ Ω·cm, so as to reduce series resistance and reduce power consumption.

The upper electrode 4 , the lower electrode 5 and the cavity 6 have the same shape, are coaxial and symmetrical about the central axis.

The material of the vibrating membrane 1 and the monocrystalline silicon base 3 is monocrystalline silicon, and the material of the pillar 2 is silicon dioxide. The linear thermal expansion coefficients of these two materials are very close, so this structure meets the requirements of micro pressure measurement in high temperature environment .

Sensor of the present invention, its main structural parameters are as follows:

Cavity height: 0.08～0.15μm

Thickness of vibration film: 0.06～0.12um

Effective vibrating film lateral size: 5μm ~ 15μm.

Below in conjunction with accompanying drawing 2, the preparation technological process of a kind of capacitive micromachining ultrasonic sensor of the present invention is described in detail:

(1) Take <111> crystal silicon, and use local ion implantation technology to implant boron ions in the middle of the single crystal silicon, so that the resistivity is less than 10 ^-3 Ω·cm, and the heavily doped single crystal silicon part forms the lower The electrode 5 and the bottom electrode 5 form the CMUT base 3 together with the rest of the undoped single crystal silicon; take the SOI wafer, and the thickness of the top single crystal silicon slice is 150nm.

(2) A silicon dioxide layer is deposited on the CMUT base 3 by using plasma enhanced chemical vapor deposition (Plasma Enhanced Chemical Vapor Deposition, PECVD for short), and the thickness of the silicon dioxide layer is strictly controlled to precisely control the formation of silicon dioxide. The height of the cavity pillars, and then accurately control the height of the entire structural cavity; dry oxidation technology is used to dry the upper surface of the single crystal silicon slice 8 on the top of the SOI wafer to form a silicon dioxide layer, and the single crystal silicon slice 8 on the top of the SOI wafer The single crystal silicon layer 10 is formed from the unoxidized single crystal silicon layer, wherein the ratio of the thickness of the silicon dioxide layer formed by oxidation to the thickness of the single crystal silicon flakes consumed when forming the silicon dioxide layer is 1:0.44.

(3) The silicon dioxide layer on the upper layer of the CMUT base 3 is photolithographically formed to form a cavity pattern window, and the silicon dioxide layer exposed to the pattern window is completely etched away in the shortest time with a 20:1 buffer etching solution, and the remaining The silicon dioxide layer forms the silicon dioxide pillars 2; at the same time, the silicon dioxide layer on the right SOI wafer is etched away in the shortest time with a 20:1 buffer etchant, exposing the upper surface of the single crystal silicon layer 10.

(4) The upper surface of the silicon dioxide pillar 2 on the left is polished using chemical mechanical polishing technology; the middle part of the monocrystalline silicon layer 10 on the right is heavily doped with boron ions using local ion implantation technology to make the resistivity less than 10 ^{− 3} Ω·cm, in which the heavily doped single crystal silicon part forms the upper electrode 4, and the upper electrode 4 forms the CMUT vibration film 1 together with the rest of the undoped single crystal silicon, and then the upper surface of the film 1 is chemically mechanically polished.

(5) In a vacuum environment, perform anodic bonding on the upper surface of the silicon dioxide pillar 2 on the left side and the upper surface of the vibrating membrane 1 on the right side, wherein the SOI wafer is on top and the base 3 is on the bottom.

(6) Use wet etching to remove the SOI substrate monocrystalline silicon and 80% of the buried silicon dioxide from top to bottom, and then use a 20:1 buffer etchant to etch the remaining 20% in the shortest time. % buried silicon dioxide to ensure the surface smoothness of the vibrating membrane 1.

The present invention is a capacitive micro-machined ultrasonic sensor, which is used in the field of pressure measurement. Its working principle is: under the action of working voltage (bias DC voltage), the CMUT film is deformed due to electrostatic attraction and moves closer to the substrate. When an AC signal of a certain frequency is applied, the CMUT film will generate forced vibrations with the same frequency as the signal, changing the frequency of the AC signal. When the input frequency is equal to the natural frequency of the CMUT structure itself, the CMUT will resonate. When there is no pressure, the resonance frequency of the CMUT is the natural frequency of the structure itself. When there is pressure on the film, the resonance frequency of the CMUT will shift accordingly. Different pressures correspond to different frequency shifts. In a certain range Internal pressure and frequency shift maintain a good linear relationship: Δf=kP or P=Δf/k. Among them, Δf is the frequency shift of the resonant frequency f of the CMUT after pressurization relative to the natural resonant frequency f ₀ , that is, Δf=f ₀ -f, and its unit is Hz; k is the sensor sensitivity, and the unit is Hz/Pa; P is the measured Pressure value, the unit is Pa. Therefore, the corresponding pressure value can be calculated by changing the resonance frequency of the CMUT.

When a capacitive micromachining ultrasonic sensor of the present invention is applied to the field of pressure measurement, its specific application method is: jointly determine the optimal operating point of the CMUT through simulation analysis and experimental means, that is, the bias DC voltage (generally 80% of the collapse voltage ~90%), and at the same time determine the natural resonance frequency and the AC signal at this frequency point, the sum of the voltage amplitude of the AC signal and the bias voltage should be less than the CMUT collapse voltage, and the principle is to maximize the coupling coefficient. Under the excitation of the determined bias DC voltage and the AC signal of the resonant frequency, the CMUT resonates, and it is placed in a micro-pressure environment. Because the pressure acts on the CMUT vibration film, the vibration state of the CMUT is changed and the resonance is lost. At this time, the AC signal is adjusted. Frequency, so that the CMUT resonates again, record the resonant frequency, calculate the difference Δf between the natural resonant frequency and the resonant frequency under the pressure, and then use the functional relationship P=Δf/k between the pressure and the frequency shift to obtain the obtained Measure the pressure value to realize the pressure measurement.

The present invention is not limited to the above-described embodiments. Under conditions and technology permitting conditions, the monocrystalline silicon vibration film 1 can not use the method of thinning the thickness of the top monocrystalline silicon slice through the oxidation process of the SOI wafer, and directly select the top single crystal silicon slice. An SOI wafer that meets the thickness requirements of the vibrating membrane 1; secondly, the material of the vibrating membrane 1 can also be polysilicon, in order to improve the mechanical properties of the membrane and reduce the thickness and quality. The shape of the cavity 6, the upper electrode 4, and the lower electrode 5 can be flexibly selected according to the actual application in addition to the above-mentioned shapes, and the design principles are to increase the effective capacitance, improve the coupling coefficient and sensitivity. When used in a high-temperature environment, since the conductivity of heavily doped single crystal silicon changes according to temperature, an appropriate working voltage should be used according to different temperature conditions to ensure accurate measurement of small pressure by the sensor. In addition, there is no electrical isolation layer between the upper electrode 4 and the lower electrode 5 in this structure, so a corresponding overcurrent protection circuit should be installed in the subsequent measurement circuit to prevent damage to the structure itself and other related equipment after the film collapses.

The main performance index of a kind of ultramicro pressure sensor based on CMUT of the present invention is as follows:

Measuring range: 0～100Pa

Measurement accuracy: better than 2%FS

Sensitivity: 180Hz/Pa

Working temperature: -50～300℃

The above is only one embodiment of the present invention, not all or the only embodiment. Any equivalent transformation of the technical solution of the present invention adopted by those of ordinary skill in the art by reading the description of the present invention is the right of the present invention. covered by the requirements.

Claims (7)

1. A capacitive micro-machined ultrasonic sensor, characterized in that: its overall structure from top to bottom is: monocrystalline silicon vibration film (1), silicon dioxide pillar (2), and monocrystalline silicon base (3) , wherein the middle part of the silicon dioxide pillar (2) is a cavity (6), the middle part of the single crystal silicon vibrating film (1) is heavily doped with boron ions to form an upper electrode, and the single crystal silicon The middle part of the base (3) is heavily doped with boron ions to form a lower electrode, and the lateral size of the upper electrode and the lower electrode is less than or equal to the lateral size of the cavity (6) and greater than half of the lateral size of the cavity (6). 2. A capacitive micro-machined ultrasonic sensor according to claim 1, characterized in that: the thickness of the single crystal silicon diaphragm (1) is 0.06-0.12um. 3. A capacitive micro-machined ultrasonic sensor according to claim 1, characterized in that: the effective vibration film lateral size of the single crystal silicon vibration film (1) is 5 μm to 15 μm, and the effective vibration film is a cavity (6) Above the diaphragm part. 4. A capacitive micro-machined ultrasonic sensor according to claim 3, characterized in that: the transverse dimension of the cavity (6) is equal to the transverse dimension of the effective vibrating film, which is 5-15 μm, and the cavity (6) The height is equal to that of the silica pillar (2), and is 0.08-0.15 μm. 5. A capacitive micromachined ultrasonic sensor according to claim 1, characterized in that: the resistivity of the upper electrode and the lower electrode is less than 10 ^-3 Ω·cm. 6. A method for preparing a capacitive micromachining ultrasonic sensor, which is characterized in that it comprises the following steps: (1) Take <111> oriented monocrystalline silicon, and implant boron ions in the middle of the monocrystalline silicon by using local ion implantation technology to make it The resistivity is less than 10 ^-3 Ω·cm, wherein the heavily doped single crystal silicon part forms the lower electrode (5), and the rest is the CMUT base (3); (2) Deposit a silicon dioxide layer on the CMUT base (3) by using plasma-enhanced chemical vapor deposition technology, and then photoetch the silicon dioxide layer to form a cavity pattern window, and then etch away the exposed A silicon dioxide layer is formed in the graphics window, and the remaining silicon dioxide layer forms a silicon dioxide pillar (2), and finally the upper surface of the silicon dioxide pillar (2) is polished by chemical mechanical polishing technology to form the first part; (3) Take the SOI wafer, dry oxidize the upper surface of the top monocrystalline silicon slice of the SOI wafer by dry oxidation technology to form a silicon dioxide layer, and the thickness of the silicon dioxide layer formed by oxidation is the same as that of the silicon dioxide layer formed The ratio of the thickness of the monocrystalline silicon flakes consumed during the process is 1:0.44, and the unoxidized part is called the monocrystalline silicon layer; (4) Etch the silicon dioxide layer formed in step (3) with a buffer etchant to expose the unoxidized single crystal silicon layer, and then use local ion implantation technology to heavily dope the middle of the single crystal silicon layer Boron ions to make the resistivity less than 10 ^-3 Ω·cm, wherein the heavily doped single crystal silicon part forms the upper electrode (4), and finally chemical mechanical polishing is performed on the upper surface of the single crystal silicon layer to form the second part; (5) In a vacuum environment, the first part obtained in step (2) and the second part obtained in step (4) are anodically bonded, wherein the upper surface of the silicon dioxide pillar (2) of the first part is connected to the second part The upper surface of part of the monocrystalline silicon layer is bonded; (6) Remove the substrate monocrystalline silicon and 80% of the buried layer silicon dioxide of the SOI wafer by wet etching from top to bottom, and then etch the remaining silicon dioxide with buffer etching solution. 20% buried silicon dioxide. 7. An application method of a capacitive micro-processed ultrasonic sensor, characterized in that: the capacitive micro-processed ultrasonic sensor is used to realize ultra-low micro-pressure measurement with a sensitivity higher than 150Hz/Pa and a range lower than 300Pa, the specific method is: through simulation Analytical and experimental means jointly determine the optimal operating point of the CMUT, that is, the bias DC voltage, and at the same time determine the natural resonant frequency and the AC signal at this frequency point. The sum of the voltage amplitude of the AC signal and the bias voltage should be less than the CMUT collapse voltage. The principle is to maximize the coupling coefficient; under the excitation of the determined bias DC voltage and the AC signal of the resonant frequency, the CMUT resonates, and it is placed in a micro-pressure environment. Since the pressure acts on the CMUT vibration film, the vibration state of the CMUT is changed. When the resonance is lost, adjust the frequency of the AC signal to make the CMUT resonate again, record the resonance frequency, calculate the difference Δf between the natural resonance frequency and the resonance frequency under the pressure, and then use the functional relationship between pressure and frequency shift P=Δf /k, the measured pressure value can be obtained to realize pressure measurement.

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