CN117346947A - Resonant differential pressure sensor capable of realizing static pressure measurement and preparation method - Google Patents

Abstract

The invention provides a resonance differential pressure sensor capable of realizing static pressure measurement and a preparation method thereof, wherein the differential pressure sensor comprises a cover plate structure, a silicon-on-insulator structure, namely an SOI structure and a pressure guiding structure from top to bottom; the SOI structure comprises a device layer, an oxygen buried layer and a substrate layer. The invention utilizes the beam-film integrated structure to have the effect of amplifying stress, improves the problem of small static pressure and differential pressure measurement sensitivity in the prior art, and solves the problems of large hysteresis, poor repeatability and poor precision caused by the small static pressure and differential pressure measurement sensitivity.

Description

A resonant differential pressure sensor capable of static pressure measurement and its preparation method

Technical field

The invention relates to the technical field of MEMS microsensors, and in particular to a resonant differential pressure sensor that can realize static pressure measurement and a preparation method.

Background technique

Differential pressure sensors are sensors that measure the pressure difference between two ends and are widely used in aerospace, industrial control, medical electronics and other fields. Among them, the resonant differential pressure sensor is a pressure measurement device that indirectly measures pressure by detecting changes in the resonant frequency of the resonator. It has the characteristics of high resolution, good stability and high overall accuracy. It is widely used in medical electronics, Industrial control, aerospace and many other fields. The core structure of a resonant differential pressure sensor usually consists of a pressure-sensitive membrane and a resonator fabricated on its surface. The pressure-sensitive membrane is deformed by the differential pressure on both sides, which in turn changes the axial stress of the resonator fixed on its surface, ultimately changing the resonant frequency of the resonator. By monitoring the change of the resonant frequency of the resonator, the differential pressure value on both sides of the sensitive membrane can be indirectly measured.

Limited by the structural characteristics of the resonator, the resonant differential pressure sensor needs to seal the resonator in a high vacuum. Therefore, the structural integrity of the pressure-sensitive membrane is subject to certain limitations, further causing nonlinear changes in frequency with differential pressure. In order to solve this problem, the patent with publication number CN115215287A proposes "a design and manufacturing method of a resonant differential pressure sensor based on eutectic bonding technology", which proposes a method to wrap the resonator in a eutectic bonding method. Inside the sensitive membrane. Although this method improves the integrity of the sensitive film to a certain extent, the conversion efficiency between the axial stress and differential pressure of the resonator wrapped inside the sensitive film is low, and there is a problem of low sensitivity. Japan's Yokogawa uses self-aligned selective epitaxial growth and selective etching technology to design a resonant differential pressure sensor, wrapping the resonator on the surface of the pressure-sensitive membrane. Although this method will improve the sensitivity of the differential pressure sensor to a certain extent, it cannot exert the stress amplification effect of the beam-membrane integrated structure. In addition, this technology involves multiple silicon epitaxial growths under different conditions. The process is complex and the internal stress is large. The selective etching process can easily cause adhesion failure of the resonator, and the vibration direction is perpendicular to the diaphragm, which is prone to modal coupling. , the problem of increasing resonator energy loss. Therefore, how to make the resonator onto the surface of the pressure-sensitive film and complete vacuum packaging is the key to improving the core performance of the differential pressure sensor such as sensitivity, hysteresis, repeatability, and nonlinearity.

In addition, the impact of static pressure on sensor performance during differential pressure measurement is crucial. How to achieve high-precision measurement of static pressure is a difficult problem faced by current technology. Among the current technical solutions, there is no solution for high-precision static pressure sensors, but static pressure sensitive methods are involved. For example, the patent with publication number CN113686483A proposes "a resonant differential pressure sensor with integrated temperature sensor and its preparation method", in which three resonators and a temperature sensor solution are used to realize the static pressure and temperature compensation of the differential pressure sensor. Although the accuracy of differential pressure measurement can be improved to a certain extent, the compensation effect is limited due to the fact that the static pressure sensitive resonator is manufactured at the frame position, resulting in extremely low static pressure sensitivity. In addition, this technical solution also has the problem of large sensor chip size, which cannot meet the requirements for miniaturized differential pressure sensors in industrial applications.

From the above analysis, it can be seen that the existing resonant differential pressure sensors have problems such as low sensitivity, large hysteresis, poor repeatability, inability to achieve static pressure measurement, and complex processes.

Contents of the invention

The purpose of the present invention is to propose a resonant differential pressure sensor with static pressure measurement and a preparation method thereof in view of the above-mentioned problems existing in the prior art. This technology utilizes the beam-membrane integrated structure to have a stress amplification effect and improve the existing technology's problems of low sensitivity in static pressure and differential pressure measurement, as well as the resulting problems of large hysteresis, poor repeatability, and poor accuracy.

The present invention proposes a resonant differential pressure sensor that can realize static pressure measurement. The differential pressure sensor includes a top-down cover structure, a silicon-on-insulator structure, that is, an SOI structure, and a pressure guiding structure; wherein the cover structure It includes two small cover plates, namely a first small cover plate, a second small cover plate and a large cover plate. The SOI structure includes a device layer, a buried oxide layer and a substrate layer.

The present invention also proposes a method for preparing a resonant differential pressure sensor that can realize static pressure measurement. The specific steps are as follows:

a) Clean SOI silicon wafer;

b) Etch a pressure-sensitive film on the substrate layer of the SOI silicon wafer;

c) Etch resonators, leads and other device layer structures on the device layer of the SOI silicon wafer;

d)Resonator release;

e) Clean the silicon wafer;

f) Growth of silicon oxide on the upper and lower surfaces of the silicon wafer;

g) Etching the cavity structure on the silicon wafer;

h) Bonding of silicon wafer and SOI silicon wafer;

i) Remove the excess part of the silicon wafer to form a cover structure;

j) Grow silicide in the isolation trench to form a vacuum package;

k) Clean the glass pieces;

l) Make through holes in the glass sheet;

m) The glass sheet is bonded to the bonded silicon-SOI composite sheet;

n) Electrode preparation.

The present invention has the following beneficial effects:

1) Using a beam-membrane integrated structure, the resonators are all on the surface of the pressure-sensitive membrane, which improves the sensitivity of the resonator;

2) The small package cover design of the differential pressure resonator is used to ensure the integrity of the sensitive film structure where the differential pressure resonator is located and reduce the impact of static pressure on differential pressure measurement;

3) The design of a large static pressure cover plate reduces the impact of the differential pressure at the static pressure resonator on the measurement of static pressure;

4) Using static pressure and differential pressure composite sensitivity methods and multi-resonator design, in-situ static pressure compensation and differential pressure compensation can be achieved, while high-precision static pressure and differential pressure measurement can be achieved;

5) The silicon-to-silicon bonding process is used to reduce the manufacturing stress of the sensor;

6) The method of depositing silicide is used to achieve vacuum packaging, which reduces the difficulty of vacuum packaging for silicon-silicon bonding.

Description of drawings

Figure 1: Schematic diagram of the overall structure of the sensor;

Figure 2: Schematic diagram of the sensor with the cover structure omitted;

Figure 3: Cross-sectional view of the overall structure of the sensor;

Figure 4: Sensitive diagram of differential pressure resonator;

Figure 5: Sensitive diagram of static pressure resonator;

Figure 6: Schematic diagram of sensor processing process;

Figure 7: Schematic diagram of different layouts of the overall sensor structure.

Detailed ways

In order to explain the technical solutions of the embodiments of the present invention more clearly, the drawings required to be used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and therefore do not It should be regarded as a limitation of the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without exerting creative efforts.

Figure 1 is a schematic diagram of the multi-layer structure of the sensor. From top to bottom, it includes a cover structure, a silicon-on-insulator structure, that is, an SOI structure, and a pressure-conducting structure 500 . The cover plate structure includes two small cover plates 101 and 102 and a large cover plate 103. The SOI structure includes a device layer 200, a buried oxide layer 300 and a substrate layer 400.

Figure 2 is a schematic diagram with the cover plate structure omitted. On the device layer 200 of the SOI structure, three resonators 210, 220 and 230 are fabricated. The resonator 210 and the resonator 220 have greater differential pressure sensitivity and lower static pressure sensitivity, while the resonator 230 has greater static pressure sensitivity and smaller differential pressure sensitivity. Bias electrodes 211, 221, 231, driving electrodes 212, 222, 232 and detection electrodes 213, 223, 233 are distributed around each resonator corresponding to the resonators 210, 220 and 230 in sequence, wherein the driving electrodes and the detection electrodes The position can be changed. There is an alternating electrical signal passing through the driving electrode. The distance between the driving electrode and the resonator is very close, and the part facing them can be regarded as a capacitor. Under the action of the driving electrode, the resonator works at its own resonant frequency. At the same time, the capacitance between the resonator and the detection electrode on the other side changes, and the resonant frequency of the resonator is detected through the change in capacitance. The resonator and its associated electrodes are fixed to the pressure sensitive film 401 of the substrate layer 400 through the buried oxide layer 300. And there are isolation trenches 240 penetrating the device layer 200 between each group of resonators.

During operation, the resonant frequencies of the three resonators are affected by the pressures P ₁ , P ₂ and temperature T on both sides of the sensitive membrane. Therefore, the relationship between the resonant frequencies and P ₁ , P ₂ and T can be established as follows:

f1, f2, and f3 are the resonant frequencies of the first, second, and third resonators respectively; F1, F2, and F3 are the frequencies of the first, second, and third resonators respectively and the pressures P1, P2, and Polynomial function of temperature T.

Through function conversion, the relationship between P ₁ , P ₂ and T and the three resonant frequencies can be obtained as follows:

G1, G2, and G3 are the compensation functions of the pressure P1, P2 and temperature T on both sides of the pressure-sensitive membrane.

Therefore, the expressions of static pressure _PS , differential pressure P _d and temperature T can be obtained as follows:

Figure 3 is a cross-sectional view of the overall structure of the sensor. The three groups of resonators and their associated electrodes are fixed to the pressure sensitive film 401 of the substrate layer 400 through the buried oxide layer 300 . A through hole 501 is formed in the center of the pressure guiding structure 500 and is sealed and connected to the surrounding area of the substrate layer 400 except for the pressure sensitive film. The pressure at one end of the differential pressure to be measured acts on the pressure sensitive membrane through the through hole 501 on the pressure guide structure 500, and the pressure at the other end acts on the pressure sensitive film by acting on the exposed device layer 200 and the surfaces of the three cover plates 101, 102 and 103. on the membrane. The corresponding resonators 210 and 220 are under the small covers 101 and 102.

The resonators 210 and 220 are mainly affected by the resultant force of the pressure on both sides and are used to measure the differential pressure. Take cover 101 as an example, as shown in Figure 4 . A smaller resonant cavity 121 is made on the cover plate 101, and the resonator can vibrate in the resonant cavity. At the same time, the cover plate of this design is only slightly larger than the resonator, so as to ensure the integrity of the pressure-sensitive membrane at the sensitive part of the resonator as much as possible. The resonator 210 is fabricated on the device layer 200, with driving electrodes 212 and detection electrodes 213 on the left and right, connected together through the buried oxide layer 300 and the pressure sensitive film 401. The oxide layer below the resonator is removed through a release process, so that the resonator can Vibration occurs in the resonant cavity formed by the cover and substrate. FIG. 5 is a partial cross-sectional view of the location of resonator 230. A larger resonant cavity 123 is made on the cover plate 103, so the pressure exerted on the upper surface of the cover plate 103 cannot act on the pressure sensitive membrane 401 and is mainly affected by the pressure at the lower end, so it can be used to measure static pressure.

Figure 6 is a schematic diagram of the sensor preparation process. The SOI process includes fabricating a pressure-sensitive film 401 part on the substrate layer, fabricating structures such as resonators and electrodes on the device layer 200, and releasing the resonator. The cover plate process is mainly to complete the vacuum packaging of the resonator. On the one hand, it can isolate the resonator from the outside world and avoid the impact of external pollutants on the resonator. On the other hand, it can package the resonator in a near-vacuum environment. The quality factor of the resonator can be improved. The pressure-guiding structure process is mainly used to make the pressure-guiding structure. Specific steps are as follows:

a) Clean SOI silicon wafer;

b) Etch a pressure-sensitive film on the substrate layer of the SOI silicon wafer;

c) Etch resonators, leads and other device layer structures on the device layer of the SOI silicon wafer;

d)Resonator release;

e) Clean the silicon wafer;

f) Growth of silicon oxide on the upper and lower surfaces of the silicon wafer;

g) Etching the cavity structure on the silicon wafer;

h) Bonding of silicon wafer and SOI silicon wafer;

i) Remove the excess part of the silicon wafer to form a cover structure;

j) Grow silicide in the isolation trench to form a vacuum package;

k) Clean the glass pieces;

l) Make through holes in the glass sheet;

m) The glass sheet is bonded to the bonded silicon-SOI composite sheet;

n) Electrode preparation.

It should be pointed out that the etching process described above is the preferred solution, and in actual processes, it can be achieved by means of silicon deep reactive ion etching, wet etching and other means. Steps h) to j) can be replaced by a silicon-to-silicon vacuum bonding process. Step j) can be replaced by depositing metal. Step l) can be achieved by mechanical drilling, laser processing, and sandblasting processes. The metal/composite metal deposited in step n) includes, but is not limited to, Al, Cr/Au, Ti/Pt/Au, Ni/Pd/Au, etc.

This technical solution invents a resonant pressure sensor that is sensitive to static pressure and differential pressure, making full use of the stress amplification characteristics of the beam-membrane integrated structure. Through the design of cover plates of different sizes, the sensitivity of the resonator is improved respectively, and the static pressure and differential pressure are simultaneously realized. pressure measurement. In order to make the purpose, technical solutions and advantages of this technical solution more clear, the present invention will be further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

A resonant differential pressure sensor capable of static pressure measurement and a preparation method thereof, including:

1) The sensor includes 2 differential pressure resonators and 1 static pressure resonator;

2) The differential pressure resonator is packaged with a small cover plate, and the static pressure resonator is packaged with a large cover plate;

3) The resonators are all on the surface of the pressure-sensitive membrane;

4) Due to the small cover plate of the differential pressure resonator, the pressure on the upper and lower surfaces during operation is all distributed on the upper and lower sides of the sensitive membrane, which reduces the impact of static pressure on differential pressure measurement;

5) Since the cover plate of the static pressure resonator is large, the upper surface pressure only acts on the surface of the large cover plate during operation, and has little impact on the pressure sensitive membrane. Therefore, the deformation of the pressure sensitive membrane is mainly affected by the lower surface pressure, which reduces the difference. Pressure is sensitive to static pressure;

6) Multiple resonators can achieve in-situ static pressure and temperature compensation;

7) The vacuum packaging of the resonator is realized by silicon-to-silicon bonding;

8) The resonator is preferably driven and detected by electrostatic excitation and capacitance detection.

It should be noted:

1) In this technical solution, the cover plate can be replaced by glass sheet/quartz sheet, the pressure guiding structure can be made of silicon wafer/quartz sheet, and the SOI silicon wafer can be replaced by cavity-SOI, multi-layer silicon silicon structure/quartz structure/glass structure ;

2) The resonator in this technical solution is not targeted at a specific structure and is suitable for the resonator structures commonly used by professionals and technicians in the current field, including but not limited to long straight beams, H-shaped beams, ring beams, etc.;

3) The resonator electrostatic excitation/electrostatic detection used in this technical solution is the preferred solution and is suitable for the current driving/detection methods commonly used by professionals in this field, including but not limited to electrostatic drive/capacitive detection, electrostatic drive/piezoresistive detection, electromagnetic Drive/electromagnetic detection, electromagnetic drive/piezoresistive detection, electrothermal excitation/piezoresistive detection, etc.;

4) In this technical solution, the position and direction of the resonator on the pressure-sensitive film can be in different layouts, and the resonator can be distributed at different angles to the pressure-sensitive film;

5) In this technical solution, the position of the static pressure resonator cover is not limited to the edge or one side of the sensor. It can be located in different positions such as the center of the pressure sensitive film. As shown in Figure 7, the static pressure resonator cover is located in the center.

It is easy for those skilled in the art to understand that the above descriptions are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements, etc., made within the spirit and principles of the present invention, All should be included in the protection scope of the present invention.

Claims (8)

1. The resonant differential pressure sensor capable of realizing static pressure measurement is characterized by comprising a cover plate structure, a silicon-on-insulator structure, namely an SOI structure and a pressure guiding structure from top to bottom; the SOI structure comprises a device layer, an oxygen buried layer and a substrate layer.

2. The resonant differential pressure sensor of claim 1, wherein three resonators, namely a first resonator, a second resonator, and a third resonator, are fabricated on the device layer of the SOI structure; wherein the first resonator and the second resonator have a large differential pressure sensitivity and a small static pressure sensitivity, and the third resonator has a large static pressure sensitivity and a small differential pressure sensitivity; the first, second and third bias electrodes, the first, second and third drive electrodes, the first, second and third detection electrodes, the first, second and third bias electrodes, the first, second and third drive electrodes, the first, second and third detection electrodes form auxiliary electrodes of the resonators, alternating electric signals are communicated to the drive electrodes, and a capacitor is formed at the opposite part between the drive electrodes and the resonators; under the action of the driving electrode, the resonator works at the resonance frequency of the resonator; meanwhile, the capacitance between the resonator and the detection electrode at the other side is changed, and the resonance frequency of the resonator is detected through the change of the capacitance; the resonator and the auxiliary electrode thereof are fixed on the pressure sensitive film of the substrate layer through the oxygen burying layer; and there is an isolation trench between each set of resonators that extends through the device layer.

3. The resonant differential pressure sensor of claim 2, wherein in operation, 3 resonancesPressure P on two sides of pressure sensitive film of resonant frequency of resonator ₁ 、P ₂ And the influence of temperature T, thus establishing the resonant frequency and the pressure P on both sides ₁ 、P ₂ The relationship with temperature T is as follows:

f ₁ ，f ₂ ，f ₃ the resonant frequencies of the first, second and third resonators, respectively; f (F) ₁ ，F ₂ ，F ₃ The frequency of the first resonator, the second resonator and the third resonator are respectively equal to the pressure P on two sides of the pressure sensitive film ₁ 、P ₂ And a polynomial function of temperature T;

by function conversion, P is obtained ₁ 、P ₂ And T is related to the three resonant frequencies as follows:

G ₁ ，G ₂ ，G ₃ for pressure P on both sides of the pressure-sensitive membrane ₁ 、P ₂ And a compensation function for temperature T;

obtaining static pressure P _S Differential pressure P _d The expression of temperature T is as follows:

4. the resonant differential pressure sensor of claim 1, wherein the three sets of resonators and their accompanying electrodes are secured to the pressure sensitive membrane of the substrate layer by a buried oxide layer; the center of the pressure guide structure is provided with a through hole and is in sealing connection with the peripheral area of the substrate layer except the pressure sensitive film; one end pressure of the differential pressure to be measured acts on the pressure sensitive film through the through hole on the pressure guiding structure, and the other end pressure acts on the pressure sensitive film through the exposed device layer and the surfaces of the three cover plates; and the first resonator and the second resonator are respectively corresponding to the two small cover plates.

5. The resonant differential pressure sensor of claim 1, wherein the first and second resonators are subjected to a resultant of two side pressures for measuring differential pressure.

6. The resonant differential pressure sensor of claim 1, wherein the first small cover plate is provided with a smaller first resonant cavity, the first resonator can vibrate in the first resonant cavity, and the first small cover plate is only slightly larger than the first resonator; the first resonator is manufactured on the device layer, the left and right parts are a first driving electrode and a first detecting electrode, the first driving electrode and the first detecting electrode are connected together through the oxygen burying layer and the pressure sensitive film, and the oxygen burying layer below the first resonator is removed through a release process.

7. The resonant differential pressure sensor of claim 1, wherein the third resonator is affected by a lower end pressure for measuring static pressure.

8. A preparation method of a resonant differential pressure sensor capable of realizing static pressure measurement is characterized by comprising the following specific steps:

a) Cleaning an SOI silicon wafer;

b) Etching a pressure sensitive film on a substrate layer of the SOI silicon wafer;

c) Etching a resonator, a lead and other device layer structures on a device layer of the SOI silicon wafer;

d) Releasing the resonator;

e) Cleaning a silicon wafer;

f) Silicon oxide grows on the upper surface and the lower surface of the silicon wafer;

g) Etching a cavity structure on a silicon wafer;

h) Bonding a silicon wafer with an SOI silicon wafer;

i) Removing redundant parts of the silicon wafer to form a cover plate structure;

j) Growing silicide in the isolation groove to form vacuum package;

k) Cleaning the glass sheet;

l) making a through hole in the glass sheet;

m) bonding the glass sheet to the bonded silicon-SOI composite sheet;

n) electrode preparation.