A resonant differential pressure sensor capable of static pressure measurement and a method of manufacture

Abstract

The application provides a resonant differential pressure sensor capable of static pressure measurement and a preparation method. The differential pressure sensor comprises a cover plate structure from top to bottom, a silicon-on-insulator (SOI) structure and a pressure guide structure. The cover plate structure comprises two small cover plates, i.e., a first small cover plate and a second small cover plate, and a large cover plate. The SOI structure comprises a device layer, a buried oxygen layer and a substrate layer. The beam membrane integrated structure has a stress amplification effect, which improves the small static pressure and differential pressure measurement sensitivity in the prior art, and the problems of large hysteresis, poor repeatability and poor precision caused thereby.

Description

Technical Field

[0001] This invention relates to the field of MEMS microsensor technology, and in particular to a resonant differential pressure sensor capable of hydrostatic pressure measurement and its fabrication method. Background Technology

[0002] Differential pressure sensors are sensors that measure the pressure difference between two points and are widely used in aerospace, industrial control, medical electronics, and other fields. Among them, resonant differential pressure sensors are pressure measuring devices that indirectly measure pressure by detecting changes in the resonant frequency of a resonator. They feature high resolution, good stability, and high overall accuracy, and are widely used in medical electronics, industrial control, aerospace, and many other fields. The core structure of a resonant differential pressure sensor typically consists of a pressure-sensitive diaphragm and a resonator fabricated on its surface. The pressure-sensitive diaphragm deforms under the influence of the differential pressure on both sides, which in turn changes the axial stress of the resonator fixed to its surface, ultimately altering the resonator's resonant frequency. By monitoring the change in the resonator's resonant frequency, the differential pressure value across the sensitive diaphragm can be indirectly measured.

[0003] Due to the structural limitations of resonators, resonant differential pressure sensors require sealing the resonator in a high vacuum. This restricts the structural integrity of the pressure-sensitive membrane, further causing nonlinear frequency variations with differential pressure. To address this issue, patent CN115215287A proposes a "design and fabrication method for a resonant differential pressure sensor based on eutectic bonding technology," which involves encapsulating the resonator within the sensitive membrane using eutectic bonding. While this method improves the integrity of the sensitive membrane to some extent, the conversion efficiency between axial stress and differential pressure is low, resulting in low sensitivity. Yokogawa Japan has designed a resonant differential pressure sensor using self-aligned selective epitaxial growth and selective etching techniques, encapsulating the resonator on the surface of the pressure-sensitive membrane. While this method improves the sensitivity of the differential pressure sensor to some extent, it fails to leverage the stress amplification effect of the integrated beam-membrane structure. Furthermore, this technology involves multiple silicon epitaxial growth processes under different conditions, resulting in complex processes, high internal stress, and the selective etching process easily causing resonator adhesion failure. Additionally, the vibration direction is perpendicular to the membrane, leading to modal coupling and increased energy loss from the resonator. Therefore, the key to improving the core performance of differential pressure sensors, such as sensitivity, hysteresis, repeatability, and nonlinearity, lies in how to mount the resonator on the surface of the pressure-sensitive membrane and complete vacuum encapsulation.

[0004] Furthermore, static pressure has a crucial impact on sensor performance during differential pressure measurement, and achieving high-precision static pressure measurement remains a significant technological challenge. Current technical solutions lack a high-precision static pressure sensor solution, although static pressure sensing methods have been explored. For example, patent CN113686483A proposes a "resonant differential pressure sensor with integrated temperature sensor and its fabrication method," which utilizes three resonators and a temperature sensor to achieve static pressure and temperature compensation for the differential pressure sensor. While this can improve the accuracy of differential pressure measurement to some extent, the static pressure sensitivity is extremely low due to the static pressure-sensitive resonator being fabricated at the frame, thus limiting the compensation effect. In addition, this technical solution suffers from a large sensor chip size, failing to meet the requirements for miniaturized differential pressure sensors in industrial applications.

[0005] The above analysis shows that existing resonant differential pressure sensors have problems such as low sensitivity, large hysteresis, poor repeatability, inability to perform static pressure measurement, and complex manufacturing process. Summary of the Invention

[0006] The purpose of this invention is to address the aforementioned problems in existing technologies by proposing a resonant differential pressure sensor with static pressure measurement and its fabrication method. This technology utilizes the stress amplification effect of the integrated beam-membrane structure to improve the low sensitivity of static pressure and differential pressure measurements in existing technologies, as well as the resulting problems of large hysteresis, poor repeatability, and low accuracy.

[0007] This invention proposes a resonant differential pressure sensor capable of static pressure measurement. The differential pressure sensor includes a top-to-bottom cover plate structure, a silicon-on-insulator (SOI) structure, and a pressure-conducting structure. The cover plate structure includes two small cover plates, namely a first small cover plate and 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.

[0008] This invention also proposes a method for fabricating a resonant differential pressure sensor capable of static pressure measurement, the specific steps of which are as follows:

[0009] a) Clean the SOI silicon wafer;

[0010] b) Etch a pressure-sensitive film onto the substrate layer of the SOI silicon wafer;

[0011] c) Etch resonators, leads, and other device layer structures into the device layer of the SOI silicon wafer;

[0012] d) Resonator release;

[0013] e) Clean the silicon wafers;

[0014] f) Grow silicon oxide on the upper and lower surfaces of the silicon wafer;

[0015] g) Etching cavity structures on a silicon wafer;

[0016] h) Bonding silicon wafers to SOI silicon wafers;

[0017] i) Remove excess portion of the silicon wafer to form a cover plate structure;

[0018] j) Growing silicides within an isolation tank to form a vacuum package;

[0019] k) Clean the glass slides;

[0020] l) Create through holes in the glass slide;

[0021] m) Bonding the glass sheet to the bonded silicon-SOI composite sheet;

[0022] n) Electrode preparation.

[0023] The present invention has the following beneficial effects:

[0024] 1) The beam-membrane integrated structure is adopted, and the resonators are all located on the surface of the pressure-sensitive membrane, which improves the sensitivity of the resonators;

[0025] 2) The small package cover design of the differential pressure resonator ensures the integrity of the sensitive film structure where the differential pressure resonator is located and reduces the influence of static pressure on differential pressure measurement.

[0026] 3) The static pressure large cover plate design reduces the influence of differential pressure at the static pressure resonator on the measurement of static pressure;

[0027] 4) By adopting a composite sensing method of static pressure and differential pressure and a 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 realized at the same time.

[0028] 5) The use of silicon-silicon bonding technology reduces the manufacturing stress of the sensor;

[0029] 6) Vacuum encapsulation is achieved by using a silicide deposition method, which reduces the difficulty of vacuum encapsulation of silicon-silicon bonding. Attached Figure Description

[0030] Figure 1 Schematic diagram of the overall structure of the sensor;

[0031] Figure 2 : A schematic diagram of the sensor with the cover plate structure omitted;

[0032] Figure 3 Cross-sectional view of the overall sensor structure;

[0033] Figure 4 Schematic diagram of differential pressure resonator;

[0034] Figure 5 Schematic diagram of a static pressure resonator;

[0035] Figure 6 Schematic diagram of sensor manufacturing process;

[0036] Figure 7 Schematic diagram of different layouts of the overall sensor structure. Detailed Implementation

[0037] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings 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 should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a schematic diagram of the multilayer structure of the sensor. From top to bottom, it includes a cover plate structure, a silicon-on-insulator (SOI) structure, and a voltage-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.

[0039] Figure 2 This is a schematic diagram omitting the cover plate structure. Three resonators 210, 220, and 230 are fabricated on the device layer 200 of the SOI structure. Resonators 210 and 220 have high differential pressure sensitivity and low static pressure sensitivity, while resonator 230 has high static pressure sensitivity and low differential pressure sensitivity. Around each resonator corresponding to 210, 220, and 230 are distributed bias electrodes 211, 221, and 231, drive electrodes 212, 222, and 232, and detection electrodes 213, 223, and 233, respectively. The positions of the drive electrodes and detection electrodes can be interchanged. An alternating electrical signal is applied to the drive electrode. The drive electrode and the resonator are very close together; the portion directly opposite each other can be considered a capacitor. Under the action of the drive electrode, the resonator operates at its resonant frequency. Simultaneously, the capacitance between the resonator and the detection electrode on the other side changes; the resonant frequency of the resonator is detected by this change in capacitance. The resonator and its associated electrodes are fixed to the pressure-sensitive membrane 401 of the substrate layer 400 through the buried oxide layer 300. There is also an isolation trench 240 that penetrates the device layer 200 between each group of resonators.

[0040] During operation, the resonant frequencies of the three resonators are affected by the pressures P1 and P2 on both sides of the sensitive membrane and the temperature T. Therefore, the relationship between the resonant frequencies and P1, P2, and T can be established as follows:

[0041]

[0042] f1, f2, and f3 are the resonant frequencies of the first, second, and third resonators, respectively; F1, F2, and F3 are polynomial functions of the frequencies of the first, second, and third resonators with respect to the pressures P1 and P2 on both sides of the pressure-sensitive membrane and the temperature T, respectively.

[0043] Through function transformation, the relationships between P1, P2, and T and the three resonant frequencies can be obtained as follows:

[0044]

[0045] G1, G2, and G3 are compensation functions for the pressures P1 and P2 and the temperature T on both sides of the pressure-sensitive membrane.

[0046] Therefore, the static pressure P can be obtained. S Differential pressure P d The expression for temperature T is as follows:

[0047]

[0048] Figure 3 This is a cross-sectional view of the overall sensor structure. Three sets of resonators and their associated electrodes are fixed to the pressure-sensitive membrane 401 of the substrate layer 400 via a buried oxide layer 300. A through-hole 501 is formed in the center of the pressure-conducting structure 500, and it is sealed to the surrounding area of ​​the substrate layer 400, excluding the pressure-sensitive membrane. One end of the differential pressure to be measured acts on the pressure-sensitive membrane through the through-hole 501 in the pressure-conducting structure 500, while the other end acts on the pressure-sensitive membrane through the exposed device layer 200 and the surfaces of the three cover plates 101, 102, and 103. The resonators 210 and 220 are located under the small cover plates 101 and 102.

[0049] Resonators 210 and 220 are primarily subjected to the resultant force of the pressures on both sides, and are used to measure differential pressure. Taking cover plate 101 as an example, as... Figure 4 As shown, a small resonant cavity 121 is fabricated on the cover plate 101, within which the resonator can vibrate. Simultaneously, this cover plate design is only slightly larger than the resonator to ensure the integrity of the pressure-sensitive membrane at the resonator's sensitive point. The resonator 210 is fabricated on the device layer 200, with driving electrodes 212 and detection electrodes 213 on the left and right sides, connected together by a buried oxide layer 300 and the pressure-sensitive membrane 401. The oxide layer beneath the resonator is removed through a release process, allowing the resonator to vibrate within the resonant cavity formed by the cover and the substrate. Figure 5 This is a partial cross-sectional view of the resonator 230. A large resonant cavity 123 is fabricated on the cover plate 103, so the pressure applied to 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.

[0050] Figure 6This is a schematic diagram of the sensor fabrication process. The SOI process includes fabricating the pressure-sensitive film 401 on the substrate layer, and fabricating the resonator and electrodes on the device layer 200, as well as the resonator release mechanism. The cover plate process is mainly for completing the vacuum encapsulation of the resonator. On the one hand, it isolates the resonator from the outside world, preventing the influence of external contaminants on the resonator. On the other hand, encapsulating the resonator in a near-vacuum environment can improve the resonator's quality factor. The pressure-conducting structure process is mainly for fabricating the pressure-conducting structure. The specific steps are as follows:

[0051] a) Clean the SOI silicon wafer;

[0052] b) Etch a pressure-sensitive film onto the substrate layer of the SOI silicon wafer;

[0053] c) Etch resonators, leads, and other device layer structures into the device layer of the SOI silicon wafer;

[0054] d) Resonator release;

[0055] e) Clean the silicon wafers;

[0056] f) Grow silicon oxide on the upper and lower surfaces of the silicon wafer;

[0057] g) Etching cavity structures on a silicon wafer;

[0058] h) Bonding silicon wafers to SOI silicon wafers;

[0059] i) Remove excess portion of the silicon wafer to form a cover plate structure;

[0060] j) Growing silicides within an isolation tank to form a vacuum package;

[0061] k) Clean the glass slides;

[0062] l) Create through holes in the glass slide;

[0063] m) Bonding the glass sheet to the bonded silicon-SOI composite sheet;

[0064] n) Electrode preparation.

[0065] It should be noted that the etching process described above is a preferred solution; in actual processes, it can be achieved using silicon deep reactive ion etching, wet etching, etc. Steps h) to j) can be replaced by silicon-silicon vacuum bonding. Step j) can be replaced by metal deposition. Step l) can be achieved by mechanical drilling, laser processing, or sandblasting. The metal / composite metal deposited in step n) includes, but is not limited to, Al, Cr / Au, Ti / Pt / Au, Ni / Pd / Au, etc.

[0066] This invention discloses a resonant pressure sensor with combined static and differential pressure sensitivity. It fully utilizes the stress amplification characteristics of the integrated beam-membrane structure and improves the sensitivity of the resonator through the design of cover plates of different sizes, simultaneously achieving the measurement of static and differential pressure. To make the objectives, technical solutions, and advantages of this invention clearer, the following detailed description, in conjunction with specific embodiments and accompanying drawings, further illustrates the invention.

[0067] A resonant differential pressure sensor capable of static pressure measurement and its fabrication method, comprising:

[0068] 1) The sensor includes two differential pressure resonators and one static pressure resonator;

[0069] 2) Differential pressure resonators are packaged with small cover plates, while static pressure resonators are packaged with large cover plates.

[0070] 3) All resonators are located on the surface of the pressure-sensitive diaphragm;

[0071] 4) Due to the small cover plate, the pressure on the upper and lower surfaces of the differential pressure resonator is distributed on both sides of the sensitive diaphragm during operation, which reduces the impact of static pressure on differential pressure measurement.

[0072] 5) Because the cover plate of the static pressure resonator is large, the pressure on the upper surface only acts on the surface of the large cover plate during operation, and has little effect on the pressure-sensitive diaphragm. Therefore, the deformation of the pressure-sensitive diaphragm is mainly affected by the pressure on the lower surface, which reduces the influence of differential pressure on static pressure sensitivity.

[0073] 6) Multi-resonators can achieve in-situ static pressure and temperature compensation;

[0074] 7) The vacuum packaging of the resonator is achieved using silicon-silicon bonding;

[0075] 8) The resonator preferably uses electrostatic excitation and capacitance detection as the driving and detection methods.

[0076] It should be noted that:

[0077] 1) In this technical solution, the cover plate can be replaced by a glass plate / quartz plate, the pressure guiding structure can be made of a silicon plate / quartz plate, and the SOI silicon plate can be replaced by cavity-SOI, multilayer silicon-silicon structure / quartz structure / glass structure;

[0078] 2) The resonator in this technical solution is not targeted at any specific structure, but is applicable to the resonator structures commonly used by professionals in the field, including but not limited to long straight beams, H-shaped beams, and ring beams;

[0079] 3) The electrostatic excitation / electrostatic detection of the resonator used in this technical solution is the preferred solution and is applicable to the driving / detection methods commonly used by professionals in this field, including but not limited to electrostatic driving / capacitive detection, electrostatic driving / piezoresistive detection, electromagnetic driving / electromagnetic detection, electromagnetic driving / piezoresistive detection, electrothermal excitation / piezoresistive detection, etc.

[0080] 4) In this technical solution, the position and orientation of the resonator on the pressure-sensitive membrane can be arranged in different ways, and the resonator can be distributed at different angles with the pressure-sensitive membrane;

[0081] 5) In this technical solution, the location of the static pressure resonator cover is not limited to the edge or one side of the sensor; it can be located at different positions, such as the center of the pressure-sensitive diaphragm. Figure 7 The cap of the static pressure resonator shown is located in the center.

[0082] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A resonant differential pressure sensor capable of static pressure measurement, characterized in that, The differential pressure sensor includes a top-to-bottom cover plate structure, a silicon-on-insulator (SOI) structure, and a pressure-conducting structure; the cover plate structure includes two small cover plates, namely a first small cover plate and 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. Three resonators—a first resonator, a second resonator, and a third resonator—are fabricated on the device layer of the SOI structure. The first and second resonators exhibit high differential pressure sensitivity and low static pressure sensitivity, while the third resonator exhibits high static pressure sensitivity and low differential pressure sensitivity. Around each of the first, second, and third resonators are distributed first, second, and third bias electrodes, first, second, and third drive electrodes, and first, second, and third detection electrodes. These first, second, and third bias electrodes, first, second, and third drive electrodes, and first, second, and third detection electrodes constitute the auxiliary electrodes of the resonator. An alternating electrical signal is applied to the drive electrode, and the portion of the drive electrode and the resonator facing each other forms a capacitor. Under the action of the drive electrode, the resonator operates at its resonant frequency. Simultaneously, the capacitance between the resonator and the detection electrode on the other side changes, and the resonant frequency of the resonator is detected by this capacitance change. The resonators and their auxiliary electrodes are fixed to the pressure-sensitive film on the substrate layer through a buried oxide layer. An isolation trench penetrating the device layer exists between each group of resonators.

2. The resonant differential pressure sensor according to claim 1, characterized in that, During operation, the resonant frequencies of the three resonators are affected by the pressure across the pressure-sensitive diaphragm. P 1 , P 2 and temperature T The influence of the pressure on both sides is considered, therefore the resonant frequency is established. P 1 , P 2 and temperature T The relationship is as follows: ； f 1, f 2, f 3 represents the resonant frequencies of the first, second, and third resonators, respectively; F 1, F 2, F 3 represents the frequencies of the first, second, and third resonators, and the pressures on both sides of the pressure-sensitive membrane, respectively. P 1 , P 2 and temperature T polynomial functions; Through function transformation, we obtain P 1 , P 2 and T The relationship with the three resonant frequencies is as follows: ； G 1, G 2, G 3 represents the pressure across the pressure-sensitive membrane. P 1 , P 2 and temperature T The compensation function; Obtain static pressure P S Differential pressure P d and temperature T The expression is as follows: 。 3. The resonant differential pressure sensor according to claim 1, characterized in that, Three sets of resonators and their associated electrodes are fixed to the pressure-sensitive membrane of the substrate through a buried oxide layer; a through hole is made in the center of the pressure-conducting structure and is sealed and connected to the surrounding area of ​​the substrate except for the pressure-sensitive membrane; the pressure of one end of the differential pressure to be measured is applied to the pressure-sensitive membrane through the through hole in the pressure-conducting structure, and the pressure of the other end is applied to the pressure-sensitive membrane through the exposed device layer and the surface of the three cover plates. The first and second resonators are located under the two small cover plates, respectively.

4. The resonant differential pressure sensor according to claim 1, characterized in that, The first and second resonators are subjected to the combined force of the pressure on both sides and are used to measure differential pressure.

5. The resonant differential pressure sensor according to claim 1, characterized in that, The first small cover plate has a small first resonant cavity, in which the first resonator can vibrate. The first small cover plate is only slightly larger than the first resonator. The first resonator is fabricated on the device layer, with the first driving electrode and the first detection electrode on the left and right sides, which are connected together by a buried oxide layer and a pressure-sensitive membrane. The buried oxide layer under the first resonator is removed by a release process.

6. The resonant differential pressure sensor according to claim 1, characterized in that, The third resonator, influenced by the pressure at its lower end, is used to measure static pressure.

7. A method for fabricating a resonant differential pressure sensor capable of static pressure measurement as described in any one of claims 1-6, characterized in that, The specific steps are as follows: a) Cleaning SOI silicon wafers; b) Etch a pressure-sensitive film onto the substrate layer of the SOI silicon wafer; c) Etch resonators, leads, and other device layer structures into the device layer of the SOI silicon wafer; d) Resonator release; e) Clean the silicon wafers; f) Growing silicon oxide on the upper and lower surfaces of the silicon wafer; g) Etching cavity structures onto the silicon wafer; h) Bonding silicon wafers to SOI silicon wafers; i) Remove excess portions of the silicon wafer to form a cover plate structure; j) Growing silicides within an isolation tank to form a vacuum package; k) Clean the glass slides; l) Create through holes in the glass slide; m) Bonding the glass sheet to the bonded silicon-SOI composite sheet; n) Electrode preparation.

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