All-fiber transmission pressure sensor based on MEMS

Abstract

The invention relates to an MEMS-based all-fiber transmission pressure sensor, which comprises a sensing unit packaged in a shell, the shell is made of titanium alloy, and the sensing unit comprises a sensing fiber, a reference fiber, an MEMS film and a substrate; wherein a cavity is etched on the back surface of the MEMS film to form a pressure sensitive area, and the sensing optical fiber is directly fixed on the part, corresponding to the pressure sensitive area, of the front surface of the MEMS film through eutectic welding. Circuit packaging can be eliminated through all-fiber transmission, the size is reduced, the MEMS and fiber integrated bonding technology is adopted, and the stability is improved.

Description

Technical field:

[0001] The present invention belongs to the technical field of sensors and relates to a MEMS-based all-fiber transmission pressure sensor. Background technology:

[0002] Pressure sensors are widely used in industrial monitoring, medical equipment (such as intravascular pressure monitoring), aerospace, and ocean exploration. Traditional MEMS piezoresistive sensors detect pressure by changing resistance due to deformation of a silicon membrane. However, these sensors require integrated amplification circuitry, which limits their size (typically >15mm in diameter). Fiber optic sensors (such as FBG sensors) offer strong interference resistance but require complex packaging (such as capillary steel tubing to protect the optical fiber), making them difficult to reduce in size.

[0003] At present, relevant technologies in China use MEMS silicon membranes to couple with fiber Bragg gratings, and stretch the fiber Bragg grating by deforming the silicon membrane to change the reflection wavelength and thus achieve pressure measurement. However, in this method, the silicon membrane and the optical fiber need to be mechanically bonded, which is easy to fall off after long-term use. In addition, the grating area is exposed and easily disturbed by external stress, and the diameter after packaging is greater than 15mm, which exceeds the requirements of miniaturization.

[0004] Therefore, there is an urgent need for an all-fiber transmission pressure sensor based on MEMS technology to achieve the goals of miniaturization, anti-electromagnetic interference, low temperature drift and high sensitivity to solve the above technical problems. Summary of the invention:

[0005] The technical problem to be solved by the present invention is to provide a MEMS-based all-fiber transmission pressure sensor, which eliminates circuit packaging through all-fiber transmission, reduces the volume, and adopts an integrated bonding process of MEMS and optical fiber to improve stability.

[0006] The technical solution of the present invention is to provide a MEMS-based all-fiber transmission pressure sensor, including a sensing unit encapsulated in a housing made of titanium alloy, the sensing unit including a sensing fiber, a reference fiber, a MEMS film and a substrate; wherein,

[0007] A cavity is etched on the back of the MEMS film to form a pressure-sensitive area. The sensing fiber is directly fixed to the corresponding pressure-sensitive area on the front of the MEMS film through eutectic welding (Au-Si bonding).

[0008] The substrate is fixed on the back of the MEMS film, and the reference fiber is fixed on the non-deformed area of ​​the substrate (the thick wall at the edge of the substrate). The reference fiber and the sensing fiber form a spatially separated dual optical path.

[0009] Silicone gel is filled between the housing and the sensing unit to serve as a buffer layer.

[0010] Preferably, the sensing fiber and the reference fiber are single-mode fibers with integrated Bragg gratings (FBGs) on the end faces, and the grating regions are prepared by femtosecond laser direct writing technology.

[0011] Preferably, the pressure sensitive area is circular, the sensing optical fiber is fixed at the center of the pressure sensitive area, and the optical fiber Bragg grating is stretched as the pressure deforms.

[0012] Preferably, the pressure sensitive area adopts the dry etching process in MEMS technology, namely deep reactive ion etching (DRIE) to form a circular film with small diameter and ultra-thin thickness.

[0013] Preferably, the silicone gel is filled into the micron-scale gap (porosity <0.1%) between the sensing unit and the housing by a vacuum infusion process to eliminate local stress concentration.

[0014] Preferably, the surface of the silicone gel is treated with plasma activation, and the bonding strength with the inner wall of the titanium alloy reaches 1.2 MPa.

[0015] As a preferred method, fiber laser (wavelength 1070nm) is used to weld the shell seams, and the weld tightness reaches 1×10 - 9 Pa·m 3 / s (helium mass spectrometer leak detection), the inner cavity of the shell is polished, the surface roughness Ra<0.2μm, reducing silicone gel filling defects.

[0016] Preferably, the fiber outlets of the sensing optical fiber and the reference optical fiber are sealed with a ceramic ferrule, which is a ZrO2 ceramic ferrule, sealed to the housing by active brazing (Ag-Cu-Ti solder), and has a pressure resistance of 100 MPa.

[0017] Preferably, the non-deformed region of the substrate is located at the thick wall of the substrate edge.

[0018] Preferably, a U-shaped microgroove is etched on the edge of the substrate using a DRIE process, and the reference optical fiber is placed in the microgroove after removing the coating section. A 0.5N preload is applied, and the surface is coated with low-shrinkage UV glue and fixed by UV irradiation for at least 60 seconds.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] The present invention adopts an integrated structure of eutectic welding of MEMS film and optical fiber, eliminates circuit packaging through all-fiber transmission, reduces the volume, and achieves the miniaturization requirement of less than 15mm in diameter; and adopts an integrated bonding process of MEMS and optical fiber to improve stability; and designs a dual-optical path differential structure to suppress temperature drift. Description of the drawings:

[0021] Figure 1 Schematic diagram of the sensing area of ​​the present invention. Specific implementation method:

[0022] The present invention will be further described below with reference to the accompanying drawings:

[0023] A MEMS-based all-fiber transmission pressure sensor, such as Figure 1 As shown, it includes a sensing unit encapsulated in a housing made of titanium alloy. The sensing unit includes a sensing optical fiber 1, a reference optical fiber 6, a MEMS film and a substrate 5.

[0024] A cavity is etched on the back of the MEMS film to form a pressure-sensitive region 2. The sensing fiber is directly fixed to the corresponding pressure-sensitive region on the front of the MEMS film through eutectic welding (Au-Si bonding). The eutectic welding point 4 is located at the center of the pressure-sensitive region 2.

[0025] The substrate 5 is fixed on the back of the MEMS film, and the reference optical fiber 6 is fixed on the non-deformed area of ​​the substrate 5. The reference optical fiber 6 and the sensing optical fiber 1 form a spatially separated dual optical path;

[0026] Silicone gel is filled between the housing and the sensing unit to serve as a buffer layer.

[0027] Specifically, in terms of structural design, for the MEMS pressure-sensitive unit, a dry etching process in MEMS technology, deep reactive ion etching (DRIE), is used to etch a cavity on the back of the MEMS film to form a pressure-sensitive region, thereby forming a circular film region with a diameter not exceeding 10 mm and a thickness on the order of μm. The sensing fiber 1 and reference fiber 6 are single-mode optical fibers with integrated Bragg gratings on the end faces. The grating regions are prepared using femtosecond laser direct writing technology. In this example, the sensing fiber Bragg grating region 3 is located on the pressure-sensitive region 2. The sensing fiber 1 is directly fixed to the front of the MEMS film corresponding to the pressure-sensitive region via eutectic welding (Au-Si bonding) to prevent adhesive aging, while the reference fiber 6 is fixed to the non-deformed region of the substrate 5. In this embodiment, the non-deformed region is located at the thick wall edge of the substrate 5.

[0028] Since the reference optical path formed by the reference optical fiber 6 is fixed in the non-deformed area of ​​the substrate, and the sensing optical path formed by the sensing optical fiber 1 is fixed in the center of the pressure-sensitive area, the fiber Bragg grating is stretched with pressure deformation, thereby forming a dual-path differential design.

[0029] As for the packaging structure, the shell is made of titanium alloy and filled with silicone gel. Silicone gel is an organic silicone elastomer with the following key properties:

[0030] Ultra-low modulus (0.5MPa): much lower than titanium alloy (110GPa), can absorb mechanical impact energy and reduce the stress transmitted to the MEMS film;

[0031] High elastic deformation (compression rate ≥ 50%): can still restore to its original shape under deep sea high pressure (such as 50MPa) to avoid permanent deformation;

[0032] Wide temperature adaptability (-60~250℃): matches the thermal expansion requirements of the titanium alloy shell and avoids interface peeling caused by temperature cycling.

[0033] As an implementation method, silicone gel uses a vacuum infusion process to fill the micron-scale gap between the sensing unit and the housing (porosity <0.1%) to eliminate local stress concentration;

[0034] At the same time, the bonding interface is treated: the surface of the silicone gel is treated with plasma activation, and the bonding strength with the inner wall of the shell reaches 1.2MPa.

[0035] As an implementation method, laser micro welding is used when encapsulating the shell: a fiber laser (wavelength 1070nm) is used to weld the shell seams, and the weld airtightness reaches 1×10 -9 Pa·m 3 / s (helium mass spectrometer leak detection); the inner cavity is polished to a surface roughness of Ra < 0.2μm to reduce silicone gel filling defects; and ceramic ferrule integration: the fiber outlet uses a ZrO2 ceramic ferrule, which is sealed to a titanium alloy through active brazing (Ag-Cu-Ti solder) and has a pressure resistance of 100MPa. In other words, the fiber outlets of the sensing fiber and the reference fiber are sealed with ceramic ferrules.

[0036] The reference optical fiber of the present invention is fixed to the non-deformed area of ​​the substrate (the thick wall at the edge of the substrate), forming a spatially separated dual optical path with the sensing optical fiber (fixed in the center of the circular thin film area). A U-shaped microgroove is etched at the edge of the substrate using a DRIE process. The reference optical fiber (with the coating removed) is placed into the microgroove, a 0.5N preload is applied, and the surface is coated with low-shrinkage UV adhesive, followed by UV irradiation for 60 seconds to secure the fiber.

[0037] The working principle of the sensor of the present invention is as follows:

[0038] Pressure acts on the film → film deformation → sensing optical path fiber Bragg grating wavelength shift Δλ sense ;

[0039] The reference optical path is not deformed and the wavelength remains Δλ ref ;

[0040] By demodulating Δλ sense -Δλ ref The pressure value is calculated by the difference between the two values, while offsetting the temperature effect.

[0041] The basic grating wavelength shift formula is as follows:

[0042] Bragg wavelength λ of fiber Bragg grating (FBG)B Determined by the following formula:

[0043] λ B =2n eff Λ

[0044] Where: n eff is the effective refractive index of the optical fiber, and Λ is the grating period.

[0045] When the grating is subjected to strain (ε) and temperature change (ΔT), the wavelength shift Δλ can be expressed as:

[0046] Δλ=λ B ·(K ε ·ε+K T ·ΔT)

[0047] Where: K ε is the strain sensitivity coefficient (typical value: 1.2pm / με1.2pm / με); K T is the temperature sensitivity coefficient (typical value: 10pm / ℃10pm / ℃).

[0048] The dual optical path differential model is as follows:

[0049] Assume that the wavelength offsets of the sensing optical path and the reference optical path are:

[0050]

[0051] The differential output is the difference between the two signals:

[0052] Δλ diff =Δλ sense -Δλ ref =λ B ·K ε ·ε

[0053] It can be seen that the temperature term K T ΔT is completely cancelled, leaving only the strain term; while the pressure is shifted by the strain ε and the wavelength Δλ difff Directly related.

[0054] The following is the derivation of the relationship between strain and pressure:

[0055] The strain ε of the MEMS film is caused by the pressure P. According to the small deflection theory of thin plates:

[0056]

[0057] Where: E is the Young's modulus of silicon (substitute 160 GPa for the calculation below), v 2is the Poisson's ratio (substitute 0.22 for the following calculations), h is the film thickness (substitute 50 μm for the following calculations), and r is the film radius (substitute 4 mm for the following calculations)

[0058] Substituting into the difference formula, we get the relationship between pressure and wavelength shift:

[0059]

[0060] Can be simplified to:

[0061]

[0062] It can be seen that the pressure P is related to ·Δλ diff It is a linear relationship, and the proportional coefficient is determined by the material and structural parameters. The pressure sensitivity of the sensor can be expressed as:

[0063]

[0064] The following is the verification of the temperature drift suppression effect

[0065] Assume that the temperature change ΔT = 50°C:

[0066] Single optical path temperature drift:

[0067] Δλ single =λ B ·K T ·ΔT=1550(nm)×10(pm / ℃)×50(℃)

[0068] =775pm

[0069] Dual optical path temperature drift:

[0070] Δλ diff =0pm (ideally)

[0071] Actual residuals come from:

[0072] Grating temperature coefficient K of the two optical paths T Not completely consistent (manufacturing error <1%)

[0073] Residual temperature drift:

[0074] Δλ residual =λ B ·K T ·ΔT=1550(nm)×0.1(pm / ℃)×500(℃)

[0075] =7:75pm

[0076] Temperature drift suppression rate:

[0077]

[0078] Through the above example calculation, the pressure sensor in this embodiment can theoretically effectively produce a temperature drift reduction effect of not less than 90%.

[0079] The present invention adopts an integrated structure of eutectic welding of MEMS film and optical fiber, eliminates circuit packaging through all-fiber transmission, reduces the volume, and can achieve miniaturization requirements with a diameter of less than 15mm; and adopts an integrated bonding process of MEMS and optical fiber to improve stability; and designs a dual-optical path differential structure to suppress temperature drift; and the present invention realizes all-fiber transmission, and since there is no circuit, it can achieve resistance to electromagnetic interference.

[0080] The above description is only for the preferred embodiment of the present invention, which should not be understood as limiting the claims. Any equivalent process changes made using the present invention description are included in the patent protection scope of the present invention.

Claims

1. A MEMS-based all-fiber transmission pressure sensor, comprising a sensing unit encapsulated in a housing made of titanium alloy, characterized in that: The sensing unit includes a sensing optical fiber, a reference optical fiber, a MEMS film and a substrate; wherein, A cavity is etched on the back of the MEMS film to form a pressure-sensitive area, and the sensing fiber is directly fixed to the corresponding pressure-sensitive area on the front of the MEMS film through eutectic welding; The substrate is fixed on the back of the MEMS film, and the reference optical fiber is fixed on the non-deformed area of ​​the substrate. The reference optical fiber and the sensing optical fiber form a spatially separated dual optical path; Silicone gel is filled between the housing and the sensing unit to serve as a buffer layer.

2. The MEMS-based all-fiber transmission pressure sensor according to claim 1, characterized in that: The sensing fiber and reference fiber are single-mode fibers with integrated Bragg gratings on the end faces. The grating area is prepared by femtosecond laser direct writing technology.

3. The MEMS-based all-fiber transmission pressure sensor according to claim 1, characterized in that: The pressure sensitive area is circular, the sensing optical fiber is fixed at the center of the pressure sensitive area, and the fiber Bragg grating is stretched as the pressure deforms.

4. The MEMS-based all-fiber transmission pressure sensor according to claim 3, characterized in that: The pressure sensitive area is formed by deep reactive ion etching technology.

5. The MEMS-based all-fiber transmission pressure sensor according to claim 1, characterized in that: Silicone gel is filled in the gap between the sensing unit and the housing using a vacuum infusion process to eliminate local stress concentration.

6. The MEMS-based all-fiber transmission pressure sensor according to claim 5, characterized in that: The surface of the silicone gel is treated with plasma activation, and the bonding strength with the inner wall of the titanium alloy reaches 1.2MPa.

7. The MEMS-based all-fiber transmission pressure sensor according to claim 1, characterized in that: Fiber laser is used to weld the shell seams, and the weld airtightness reaches 1×10 -9 Pa·m 3 / s, the inner cavity of the shell is polished, and the surface roughness Ra<0.2μm is used to reduce silicone gel filling defects.

8. The MEMS-based all-fiber transmission pressure sensor according to claim 1, characterized in that: The fiber outlets of the sensing fiber and the reference fiber are sealed with a ZrO2 ceramic ferrule, which is sealed to the shell by active brazing and has a pressure resistance of 100 MPa.

9. The MEMS-based all-fiber transmission pressure sensor according to claim 1, characterized in that: The non-deformed area of ​​the substrate is located at the thick wall of the substrate edge.

10. The MEMS-based all-fiber transmission pressure sensor according to claim 1, characterized in that: A U-shaped microgroove was etched on the edge of the substrate using the DRIE process. The reference optical fiber was placed in the microgroove after removing the coating section. A preload of 0.5N was applied. After low-shrinkage UV adhesive was applied to the surface, it was fixed by UV irradiation for at least 60 seconds.

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