Sapphire-based suspended film fiber F-P cavity MEMS sound pressure sensor and its preparation method

Abstract

The present invention belongs to the field of optical fiber sensors, and particularly relates to a sapphire-based suspended film optical fiber F-P cavity MEMS sound pressure sensor and a preparation method thereof. Aiming at the problem that the thinning thickness of sapphire-based sensors in the prior art is insufficient and they cannot be used in the high-temperature near-field acoustic measurement field with high sensitivity requirements for sensors, the present invention fills a photoresist in the cavity of the suspended film structure as a mechanical support layer, then uses a precision thinning and polishing process to thin the sapphire wafer to the target thickness, and then removes the photoresist by wet etching to obtain a large-area suspended film structure. The sensor is fabricated by bonding an optical fiber and a ceramic ferrule with a high-temperature resistant ceramic adhesive. The optical fiber F-P cavity MEMS sensor with a large-area ultra-thin suspended film sensitive structure of the present invention, while improving the mechanical sensitivity of the sensor, reduces the damage caused by the lack of mechanical support and the accumulation of grinding stress during precision machining such as grinding of the ultra-thin suspended film, ensures the yield of grinding processing, and can sense sound pressure fluctuations with a sound pressure level greater than 60 dB.

Description

Technical Field

[0001] The present invention belongs to the field of fiber optic sensors, and particularly relates to a sapphire-based F-P cavity fiber optic MEMS sound pressure sensor with a large-area suspended film structure and a preparation method thereof. Background Art

[0002] Engine technology is widely applied in industrial fields such as aviation, aerospace, ships, and vehicles. The design, research, development, and operation monitoring of advanced engines are important industrial technologies that China urgently needs to master. The combustion process stability and sufficiency play a crucial role in the engine performance and operating state. At the same time, for power machinery with complex flow fields and harsh working conditions such as aeroengines, oscillating combustion and unstable combustion are likely to induce deterioration of combustion performance, resulting in increased overall heat load and vibration, leading to engine operation instability. In severe cases, key structures may be ablated and suffer fatigue damage, further causing serious accidents such as engine shutdown and stall. Therefore, combustion diagnosis technology for studying combustion phenomena, evaluating combustion effects, and monitoring combustion states is a key issue of concern in the engine field.

[0003] Combustion noise measurement can provide important quantitative basis for combustion diagnosis technology. Existing fluid and acoustic theories are difficult to quantitatively predict the characteristics under complex conditions of multi-field coupling of heat-flow-sound in the combustion chamber, and real-time measurement must rely on sensors. Currently, noise sensors applied to combustion diagnosis usually use the form of acoustic waveguides for measurement due to limited temperature resistance. The noise signal is transmitted to the far-field noise sensor through the acoustic waveguide, and a transfer function is added step by step based on the acoustic waveguide. This measurement method will cause different degrees of distortion of the noise signal in the far field, reducing the confidence of the signal. Only through in-situ near-field measurement can the most accurate and reliable combustion noise information at the measurement points in the combustion chamber be obtained.

[0004] The sapphire-based fiber optic F-P cavity interferometric MEMS sensor has significant advantages such as high temperature tolerance, high sensitivity, and low invasiveness. It is an effective means for realizing sensing in high-temperature extreme environments and near-field in-situ measurement. The suspended membrane structure is the most widely used structure in interferometric MEMS sensors based on the F-P cavity. The sapphire suspended thin film structure is usually prepared by a precision thinning and polishing process. The literature "Sapphire Fabry–Perot interferometer for high-temperature pressure sensing" forms a sapphire-based pressure sensor with a suspended membrane sensitive structure by high-temperature bonding of multiple sapphire wafers. The suspended membrane structure is fabricated by double-sided thinning and polishing, enabling dynamic real-time measurement in a high-temperature environment of 1200 °C. The diameter of its sensitive structure is 1.2 mm and the thickness is 40 μm. However, due to insufficient thinning thickness of the suspended membrane sensitive structure, the sensitivity of this sensor can only reach 1.2 nm / MPa, which is only suitable for high-temperature pressure measurement and cannot sense the tiny pressure disturbances caused by changes in sound pressure level. Patent CN107560755A discloses a sapphire-based fiber optic F-P temperature and pressure composite sensor and its preparation method. It thins the sapphire pressure-sensitive thin film and the F-P cavity intermediate layer structure to 50 μm and 20 - 21 μm respectively by chemical mechanical polishing. However, this patent does not elaborate in detail on the chemical mechanical polishing method used and lacks a further thinning scheme for the sensor's sensitive thin film. At this thickness, the film mechanics that the sensor can achieve are in the kilopascal level, and it still cannot be used in the field of high-temperature near-field acoustic measurement where high sensitivity is required for the sensor.

[0005] To further improve the mechanical sensitivity of the suspended membrane structure and thus enhance the resolution of the sapphire fiber optic F-P cavity MEMS sensor for noise signals in the near-field measurement environment of extreme high-temperature environments, the suspended membrane structure needs to meet the characteristics of a larger area and a smaller thickness. Since sapphire has a high Mohs hardness, strong chemical inertness, and is brittle and fragile, it is difficult to process its large-area sensitive thin film (diameter greater than 20 mm) to less than 10 μm. Moreover, due to the internal cavity of the sensor, the connection part between its ultra-thin suspended membrane structure and the cavity is extremely vulnerable to grinding stress during the manufacturing process, resulting in damage and fracture, presenting significant processing difficulties. Summary of the Invention

[0006] In order to meet the high-sensitivity near-field noise measurement requirements in high-temperature extreme environments, the present invention provides a sapphire-based suspended membrane fiber optic F-P cavity MEMS sound pressure sensor and its preparation method to solve the technical problems mentioned in the background art.

[0007] Preparation method of sapphire-based suspended film optical fiber F-P cavity MEMS sound pressure sensor. By filling photoresist in the F-P cavity inside the suspended film to enhance the support during the chemical mechanical polishing process, and releasing the photoresist inside the processed suspended film through wet etching, the local concentrated stress during the precision thinning and polishing process is reduced, avoiding the deficiencies of the sapphire material itself, which is hard and brittle and prone to breakage, providing important technical support for the preparation of a large-area suspended film structure with a thickness less than 10 μm.

[0008] The working principle of the sapphire-based suspended film optical fiber F-P cavity MEMS sound pressure sensor prepared by the method of the present invention is as follows: The incident sound wave causes the vibration of the sound-sensitive film, which in turn causes the change of the cavity length of the F-P cavity. The light beam is transmitted through the sapphire optical fiber and reflected at the end face of the sapphire optical fiber and the bottom surface of the sound-sensitive film to form an F-P interference light beam. The shape of the interference spectrum is related to the cavity length of the F-P cavity. By demodulating the F-P interference spectrum, the cavity length information can be obtained, and thus the sound pressure sensing can be realized. The sapphire-based suspended film optical fiber F-P cavity MEMS sound pressure sensor of the present invention can sense sound pressure fluctuations with a sound pressure level greater than 60 dB.

[0009] The technical solution adopted by the present invention to solve its technical problems is as follows:

[0010] Preparation method of sapphire-based suspended film optical fiber F-P cavity MEMS sound pressure sensor, characterized in that it at least includes the following steps:

[0011] Step 1: Select three sapphire wafers with the same diameter. Ablate a through-hole array with a diameter of 20 mm on the surface of one of them as the sapphire structure layer, process a glue-filling through-hole on the surface of another one to form a sapphire substrate layer, and the remaining one is used as an unprocessed sapphire wafer;

[0012] Step 2: Activate the surfaces of the above three sapphire wafers, align them from top to bottom in the order of the unprocessed sapphire wafer, the sapphire structure layer, and the sapphire substrate layer, and then load pressure in a bonding machine to bond them to form a three-layer sapphire wafer bonding structure;

[0013] Step 3: Immerse the bonded three-layer sapphire wafer bonding structure in photoresist, fill the through-holes with photoresist through the glue-filling through-holes in a vacuum environment, and then heat and bake and harden the film to ensure the strength of the photoresist;

[0014] Step 4: Grind and chemically mechanically polish the upper surface of the unprocessed sapphire wafer until its thickness reaches the set thickness to form a sound-sensitive film;

[0015] Step 5: Remove the photoresist in the sapphire structure layer through wet etching to form the F-P cavity of the suspended film sensitive structure, and then cut the sapphire wafer into small pieces by femtosecond laser ablation to obtain a sound-sensitive chip;

[0016] Step 6: Insert the sapphire optical fiber into the ferrule and install it on the bottom surface of the sapphire substrate layer.

[0017] Preferably, in Step 1, the thickness of the sapphire structure layer is 200 μm, the thicknesses of the unprocessed sapphire wafer and the sapphire substrate layer are both 400 μm, and the diameters of all three are 4 inches.

[0018] Preferably, in Step 2, the method for surface activation treatment is rapid argon ion bombardment, and the loading pressure for bonding in the bonder is 12 Mpa.

[0019] Preferably, in Step 3, it is left standing in the environment of a vacuum chamber for 2 hours. After observing that the photoresist fills the F-P cavity, it is taken out and baked on a hot plate at 120 °C for 5 minutes.

[0020] Preferably, in Step 4, the upper surface of the unprocessed sapphire wafer is ground with single-crystal diamond grinding fluid. The residual thickness of the sapphire wafer is accurately measured by a step gauge. When the thickness is 30 μm, SF1 polishing fluid is used for chemical mechanical thinning and polishing to precisely reduce the film thickness to 10 μm.

[0021] Preferably, in Step 6, the sapphire optical fiber is cut off by an optical fiber cutter to make its cross-section smooth and flat. The sapphire optical fiber is inserted into a zirconia ceramic ferrule and bonded with high-temperature resistant ceramic glue at its tail end.

[0022] Preferably, when the sapphire optical fiber is perpendicular to the acoustic sensor chip by monitoring the reflection spectrum with a spectrometer of a six-axis optical displacement stage, high-temperature resistant ceramic glue is applied and bonded at the contact position between the zirconia ceramic ferrule and the bottom surface of the acoustic sensor chip.

[0023] The present invention also provides a sapphire-based suspended film fiber F-P cavity MEMS acoustic pressure sensor prepared based on the above method, including: an acoustic sensitive film disposed on the top layer; a sapphire structure layer disposed on the middle layer, with a through hole opened thereon; a sapphire substrate layer disposed on the bottom layer, with a glue-filling through hole opened thereon, and a ferrule and a sapphire optical fiber are bonded to its back surface with high-temperature resistant ceramic glue; the acoustic sensitive film, the sapphire structure layer, and the sapphire substrate layer are bonded and connected, and the through hole of the sapphire structure layer forms an F-P cavity between the acoustic sensitive film and the sapphire substrate layer. The diameter of the acoustic sensitive film is greater than 20 mm and the thickness is less than 10 μm.

[0024] The above technical solution of the present invention has the following beneficial effects:

[0025] 1. The method of the present invention adds a supporting photoresist layer in the F-P cavity inside the suspended film made by bonding three-layer sapphire wafers. When the surface sapphire wafer is precisely thinned and polished by chemical mechanical polishing, it can be effectively supported. The photoresist inside the processed suspended film structure is released by wet etching, reducing the local concentrated stress in the precision thinning and polishing process and avoiding the defect that sapphire itself is brittle and fragile, providing an important technical method for the preparation of a large-area suspended film structure with a thickness less than 10 μm.

[0026] 2. The sapphire-based suspended film fiber F-P cavity MEMS sound pressure sensor prepared by the method of the present invention through the combination of cavity filling photoresist and precision thinning and polishing makes use of the advantages of high temperature tolerance and high sensitivity of sapphire materials, can realize dynamic real-time measurement in high-temperature environments, and the sensor prepared by this method is a large-area suspended film F-P cavity structure with a diameter greater than 20 mm and a thickness less than 10 μm, which can sense the minute pressure disturbances caused by changes in sound pressure level and can realize the measurement of the internal combustion chamber noise sound pressure level with a resolution greater than 60 dB. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below.

[0028] Figure 1 It is a schematic structural diagram of a sapphire-based suspended film fiber F-P cavity MEMS sound pressure sensor prepared by the method of the present invention;

[0029] Figure 2 It is a top view of the three-layer sapphire wafer bonding of the present invention;

[0030] Figure 3 It is an isometric exploded view of the three-layer sapphire wafer bonding of the present invention;

[0031] Figure 4 It is a schematic diagram of a single sound-sensitive chip unit of the present invention;

[0032] Figure 5 It is that the present invention injects photoresist into the F-P cavity through a through hole, and then bakes and hardens the film;

[0033] Figure 6 It is that the present invention precisely thins the upper sapphire wafer to the designed thickness by precision thinning and polishing;

[0034] Figure 7 It is that the present invention removes the photoresist in the cavity by wet etching and dices the wafer to obtain the sound-sensitive chip;

[0035] Figure 8 It is that the present invention bonds the sapphire optical fiber and the ferrule on the sound-sensitive chip of the sensor;

[0036] In the figure: 1. Three-layer sapphire wafer bonding structure; 2. Acoustic sensor chip; 3. Acoustic sensitive film; 4. Sapphire structure layer; 5. Sapphire substrate layer; 6. Glue filling through-hole; 7. F-P cavity; 8. Photoresist; 9. Ferrule; 10. Sapphire optical fiber; 11. High-temperature resistant ceramic glue. Specific implementation mode

[0037] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some but not all of the embodiments of the present invention.

[0038] Refer to Figure 1-8 , the present invention provides a sapphire-based suspended film fiber F-P cavity MEMS acoustic pressure sensor and a preparation method thereof. Using sapphire as the sensitive film and substrate material, a fiber F-P cavity MEMS acoustic pressure sensor based on a large-area suspended film structure is prepared by direct surface activation bonding of sapphire, which can perform acoustic measurements in high-temperature extreme environments; Figure 2-8 For the preparation process of the method of the present invention, the acoustic sensor chip is obtained by dicing the wafer, where Figure 4-8 Taking Figure 3 a single acoustic sensor chip unit in the black dashed box in

[0039] as an example to show the processing flow of the entire sapphire wafer.

[0040] The specific preparation method includes the following steps:

[0040] Step 1: Select two double-polished sapphire wafers with a thickness of 400 μm and a diameter of 4 inches, perform standard cleaning, process glue filling through-holes 6 on the surface of one of them as the sapphire substrate layer 5, and the other as the unprocessed sapphire wafer; select another double-polished sapphire wafer with a thickness of 200 μm and a diameter of 4 inches, perform standard cleaning, and ablate a through-hole array with a diameter of 20 mm on it by femtosecond laser as the sapphire structure layer 4.

[0041] Step 2: Activate the surfaces of the three sapphire wafers by rapid argon ion bombardment, and align them from top to bottom in the order of the unprocessed sapphire wafer, the sapphire structure layer 4, and the sapphire substrate layer 5, and then perform bonding in a bonding machine with a loading pressure of 12 Mpa to form a three-layer sapphire wafer bonding structure 1.

[0042] Step 3: Immerse the bonded three-layer sapphire wafer bonding structure 1 in the photoresist 8, let it stand in a vacuum chamber for 2 hours, fill the through-holes of the sapphire structure layer 4 with the photoresist 8 through the glue filling through-holes 6. After observing that the photoresist 8 fills the F-P cavity 7, take it out and bake the glue on a hot plate at 120 °C for 5 minutes to ensure the strength of the photoresist 8.

[0043] Step 4: Grind the upper surface of the unprocessed sapphire wafer with single crystal diamond grinding fluid, accurately measure the residual thickness of the upper sapphire wafer through a profilometer, and use SF1 polishing fluid for chemical mechanical thinning and polishing when the thickness is 30 μm, and precisely thin the film thickness of the upper sapphire wafer to 10 μm to form the acoustic sensitive film 3.

[0044] Step 5: Remove the photoresist 8 in the sapphire structural layer 4 through wet etching to form the F-P cavity 7 of the suspended film sensitive structure, and use femtosecond laser ablation to cut the three-layer sapphire wafer bonding structure 1 into small pieces to obtain the acoustic sensitive chip 2.

[0045] Step 6: Cut off the sapphire optical fiber 10 with an optical fiber cutting machine and make its cross-section smooth and flat. Insert the sapphire optical fiber 10 into the zirconia ceramic ferrule 9 and bond it with high-temperature resistant ceramic glue 11 at its tail end. Prepare two six-axis optical displacement stages, install the above-mentioned fixed sapphire optical fiber 10 and zirconia ceramic ferrule 9 on the six-axis optical displacement stage, fix the acoustic sensitive chip 2 on the other six-axis optical displacement stage, and when the sapphire optical fiber 10 is perpendicular to the acoustic sensitive chip 2 by monitoring the reflection spectrum with a spectrometer, apply high-temperature resistant ceramic glue 11 at the contact position between the zirconia ceramic ferrule 9 and the bottom surface of the acoustic sensitive chip 2 for bonding.

[0046] The sapphire-based suspended film optical fiber F-P cavity MEMS acoustic pressure sensor prepared by the above method includes: an acoustic sensitive film 3 provided on the top layer, a sapphire structural layer 4 provided on the middle layer with a through hole formed thereon, a sapphire substrate layer 5 provided on the bottom layer with a glue filling through hole 6 formed thereon, and a ferrule 9 and a sapphire optical fiber 10 bonded on its back surface with high-temperature resistant ceramic glue 11; the acoustic sensitive film 3, the sapphire structural layer 4, and the sapphire substrate layer 5 are bonded and connected, and the through hole of the sapphire structural layer 4 forms an F-P cavity 7 between the acoustic sensitive film 3 and the sapphire substrate layer 5. The diameter of the acoustic sensitive film 3 is greater than 20 mm and the thickness is less than 10 μm.

[0047] The above is only the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention and without creative labor, several improvements, retouches and substitutions can be made, and these improvements, retouches and substitutions should also be regarded as the protection scope of the present invention.

Claims

1. Preparation method of sapphire-based suspended film fiber F-P cavity MEMS sound pressure sensor, Characterized in that, It includes at least the following steps: Step 1: Select three sapphire wafers with the same diameter. On the surface of one of them, use femtosecond laser ablation to form a through-hole array with a diameter of 20 mm as the sapphire structure layer (4). On the surface of another one, process glue-filling through-holes (6) to form a sapphire substrate layer (5). The remaining one is used as an unprocessed sapphire wafer; Step 2: Activate the surfaces of the above three sapphire wafers, and then align and bond them in a bonder under pressure from top to bottom in the order of the unprocessed sapphire wafer, the sapphire structure layer (4), and the sapphire substrate layer (5) to form a three-layer sapphire wafer bonding structure (1); Step 3: Immerse the bonded three-layer sapphire wafer bonding structure (1) in photoresist (8), and fill photoresist (8) into it through the glue-filling through-holes (6) in a vacuum environment, and then bake and harden the film; Step 4: Grind and chemically mechanically polish the upper surface of the unprocessed sapphire wafer to make its thickness reach the set thickness to form a sound-sensitive film (3); Step 5: Remove the photoresist (8) in it by wet etching to form a suspended film sensitive F-P cavity (7), and cut the three-layer sapphire wafer bonding structure (1) into small pieces to obtain a sound-sensitive chip (2); Step 6: Insert the sapphire optical fiber (10) into the ferrule (9) and install it on the bottom surface of the sapphire substrate layer (5).

2. The preparation method of the sapphire-based suspended film fiber F-P cavity MEMS sound pressure sensor according to claim 1, Characterized in that, In step 1, the thickness of the sapphire structure layer (4) is 200 μm, the thicknesses of the unprocessed sapphire wafer and the sapphire substrate layer (5) are both 400 μm, and the diameters of all three are 4 inches.

3. The preparation method of the sapphire-based suspended film fiber F-P cavity MEMS sound pressure sensor according to claim 2, Characterized in that, In step 2, the method of surface activation treatment is rapid argon ion bombardment, and the bonding pressure loaded in the bonder is 12 MPa.

4. The preparation method of the sapphire-based suspended film fiber F-P cavity MEMS sound pressure sensor according to claim 3, Characterized in that, In step 3, let it stand in the environment of a vacuum chamber for 2 hours. After observing that the photoresist (8) fills the F-P cavity (7), take it out and bake the film on a hot plate at 120 °C for 5 minutes.

5. The preparation method of the sapphire-based suspended film fiber F-P cavity MEMS sound pressure sensor according to claim 4, Characterized in that, In step 4, grind the upper surface of the unprocessed sapphire wafer with single crystal diamond grinding fluid, measure the residual thickness of the sapphire wafer with a step gauge, and use SF1 polishing fluid for chemical mechanical thinning and polishing when the thickness is 30 μm, and precisely thin the film thickness to 10 μm.

6. The preparation method of the sapphire-based suspended film fiber F-P cavity MEMS sound pressure sensor according to claim 5, Characterized in that, In step 6, the sapphire optical fiber (10) is cut by an optical fiber cutting machine to make its cross-section smooth and flat. The sapphire optical fiber (10) is inserted into the ferrule (9), and is bonded at its tail end with a high-temperature resistant ceramic adhesive (11).

7. The preparation method of the sapphire-based suspended film optical fiber F-P cavity MEMS sound pressure sensor according to claim 6, characterized in that, when the sapphire optical fiber (10) is perpendicular to the acoustic sensitive chip (2) by monitoring the reflection spectrum with a spectrometer of a six-axis optical displacement stage, a high-temperature resistant ceramic adhesive (11) is applied and bonded at the contact position between the ferrule (9) and the bottom surface of the acoustic sensitive chip (2).

8. The sapphire-based suspended film optical fiber F-P cavity MEMS sound pressure sensor prepared by the preparation method according to any one of claims 1-7, characterized in that, it includes: an acoustic sensitive film (3), arranged on the outermost layer; a sapphire structural layer (4), arranged in the middle layer, with a through hole opened thereon; a sapphire substrate layer (5), arranged on the bottom layer, with a glue filling through hole (6) opened thereon, and a ferrule (9) and a sapphire optical fiber (10) are bonded to its back surface through a high-temperature resistant ceramic adhesive (11); the acoustic sensitive film (3), the sapphire structural layer (4), and the sapphire substrate layer (5) are bonded and connected, and the through hole of the sapphire structural layer (4) forms an F-P cavity (7) between the acoustic sensitive film (3) and the sapphire substrate layer (5).

9. The sapphire-based suspended film optical fiber F-P cavity MEMS sound pressure sensor according to claim 8, characterized in that, the diameter of the acoustic sensitive film (3) is greater than 20 mm and the thickness is less than 10 μm.

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