A sapphire-based optical fluid wall shear stress sensor and its preparation method

Abstract

The present invention relates to the field of micro-nano manufacturing technology, and more specifically to a sapphire-based optical fluid wall shear stress sensor and a preparation method. The preparation method comprises: preparing a fixed grating and a welding film on a supporting substrate; machining a cavity on a sensitive substrate, machining a groove within the cavity to form a target structure with the same shape as a floating element, and preparing a sensitive grating on the target structure; welding the sensitive substrate and the supporting substrate to form a bonded wafer; and preparing an elastic beam and a floating element to form a shear stress sensor. The present invention fabricates the wall shear stress sensor through high-density multi-angle femtosecond laser etching, magnetorheological and IBE combined with cavity polishing, and laser welding of a sapphire-platinum metal film. The preparation process does not require the entire sapphire wafer to be subjected to a high-temperature and high-pressure bonding environment, and there are no restrictions on the process selection before and after welding, thereby reducing the difficulty of the overall preparation process.

Description

Technical Field

[0001] The present invention relates to the field of micro-nano manufacturing technology, and in particular to a sapphire-based optical fluid wall shear stress sensor and a preparation method thereof. Background Art

[0002] Fluid wall shear stress (also known as viscous friction stress) is one of the most important basic parameters in the field of fluid mechanics. The magnitude, amplitude-frequency characteristics, and distribution information of wall shear stress can directly reflect the development law of the fluid boundary layer, helping to determine whether the fluid in the boundary layer is in a laminar state, transitional state, or turbulent state, and is of great significance for studying the generation and development mechanism of turbulence. Shear stress microsensors based on micro-electro-mechanical systems (MEMS) have the advantages of small structural size, high sensitivity, and good dynamic performance, and have broad application prospects in the field of fine measurement of fluid boundary layers. However, with the development of aerospace technology, the testing environment for wall shear stress has become more severe, and the ambient temperature has even reached above 1000°C. Traditional electrical MEMS shear stress sensors cannot withstand such high temperatures. Optical shear stress sensors based on optical conduction do not have this bottleneck and have great application potential in high-temperature shear stress measurement.

[0003] Currently, the primary material for optical MEMS sensors used in high-temperature environments is sapphire. Compared to silicon, sapphire offers superior optical properties and physical and chemical stability. However, this also presents significant challenges in microstructure fabrication. Firstly, due to sapphire's exceptional physical and chemical stability, bonding between sapphire wafers requires either extreme pressure or extremely high temperatures. Both high temperatures and high pressures reduce chip yield and limit process options. For example, direct sapphire bonding requires temperatures as high as 1300°C, which reaches the melting point of some reflective metals, such as silver and gold. This can cause the reflective metal to melt or aggregate on the surface, reducing reflectivity and ultimately leading to sensor failure. Furthermore, optical MEMS shear stress sensors or fiber-optic pressure sensors require space for the floating structure or pressure-sensitive membrane to move or deform. Therefore, fabricating a microcavity is crucial to providing this space for movement. Furthermore, optical sensors require extremely high roughness within the cavity, as high roughness can cause diffuse reflection of light, leading to sensor signal distortion or even failure. When using ICP to etch sapphire to create cavities, the etching rate is low, making it difficult to etch to depths exceeding 10μm. Furthermore, a micro-mask effect is easily present at the bottom, leading to considerable surface roughening. Using ultrafast lasers to scan and ablate sapphire to create cavities offers high processing efficiency, but due to the uneven distribution of laser light intensity, the sapphire is very rough along the laser scanning path. Furthermore, laser processing involves line overlap and repeated scanning to create the cavity structure. In the overlapped areas, the sapphire exhibits peaks of roughness reaching the μm level, causing the cavity to become extremely rough. The CMP thinning and polishing process, primarily suitable for processing planar surfaces, makes it difficult to effectively polish the planes within the cavity.

[0004] In existing sapphire direct bonding techniques, to avoid damaging the metal reflective grating during the bonding process, the sensor does not use highly reflective metal film materials, relying solely on the sapphire's inherent reflection. This results in extremely low sensor sensitivity and resolution. Using an intermediate layer for bonding, such as gold-on-gold bonding or hydrophilic bonding, can significantly impact sensor performance due to the significant difference in thermal expansion coefficient between these intermediate layer materials and sapphire. This can lead to significant thermal stress in high-temperature environments, causing sensor failure. Furthermore, the temperature tolerance of the intermediate layer material limits the sensor's operating temperature. For example, the bond strength of gold-on-gold bonding decreases significantly above 400°C. In existing techniques, to create smooth, deep cavities, a through-hole sapphire layer is typically bonded to another, intact sapphire wafer. This method, while requiring two sapphire wafers, is costly, bonding the two sapphires together increases process complexity and reduces chip yield.

[0005] In summary, the manufacturing process of existing high-temperature sapphire-based optical MEMS shear stress sensors has problems such as the difficulty of bonding, which limits process selection, and the difficulty in preparing deep smooth cavities. These problems greatly limit the manufacturing and application of sapphire-based high-temperature optical MEMS shear stress sensors.

[0006] Therefore, it is necessary to provide a sapphire-based optical fluid wall shear stress sensor and a preparation method to solve the above problems. Summary of the Invention

[0007] Aiming at the problems of difficulty in high-temperature bonding of sapphire and difficulty in preparing deep and smooth cavities during the process of manufacturing existing high-temperature optical shear stress sensor chips, the present invention provides a sapphire-based optical fluid wall shear stress sensor and a preparation method to solve the existing problems.

[0008] The sapphire-based optical fluid wall shear stress sensor of the present invention adopts the following technical solution, including:

[0009] a supporting base layer on which a sensitive structural layer is disposed;

[0010] Among them, the supporting base layer includes: a supporting base, a central array on the upper surface of which is provided with multiple fixed gratings, and a welding film is provided on the supporting base outside the multiple fixed gratings; the sensitive structure layer includes: a sensitive base, the sensitive base is connected to the supporting base, and a through groove is provided on the sensitive base, a movable structure is provided in the through groove, and a plurality of sensitive gratings are arranged in an array on the side of the movable structure facing the supporting base, and the sensitive gratings and the fixed gratings are arranged correspondingly and form moiré fringes; the supporting base and the sensitive base are both made of sapphire material.

[0011] Preferably, the movable structure includes: a floating element, which is connected to the inner wall of the through slot via a plurality of elastic beams so as to be suspended in the through slot.

[0012] Preferably, the period of the fixed grating is 10-50 μm, and the duty cycle is 30%-70%.

[0013] Preferably, the period of the sensitive grating is 10-50 μm, and the duty cycle is 30%-70%.

[0014] Preferably, the thickness of the welding film is 200-500 nm.

[0015] Preferably, the welding film includes a titanium metal film disposed on a supporting substrate, and a platinum metal film is disposed on the titanium metal film.

[0016] A method for preparing a sapphire-based optical fluid wall shear stress sensor, comprising:

[0017] Photolithography is performed on the top surface of the cleaned and dried support substrate to form a patterned area of ​​a fixed grating and a welding film, and magnetron sputtering metal is used to form a fixed grating and a welding film in the corresponding patterned area;

[0018] A femtosecond laser is used to ablate a cavity on the bottom surface of a cleaned and dried sensitive substrate, and the cavity is polished. A groove with the same shape as the through-slot is ablated on the bottom surface of the polished cavity, and the inner area surrounded by the groove forms a target structure with the same shape as the floating element. A patterned area of ​​the sensitive grating is photoetched on the target structure, and a metal is magnetron sputtered to form a sensitive grating in the corresponding patterned area.

[0019] aligning the top surface of the supporting substrate and the bottom surface of the sensitive substrate and welding them to form a bonded wafer;

[0020] The top surface of the sensitive substrate in the bonded wafer is thinned and polished, and the bottom surface of the groove is ablated to form an elastic beam that penetrates the bottom of the groove to form a through groove. The target structure and the groove are separated to form a floating element, thus obtaining a shear stress sensor with the floating element and the elastic beam integrally processed.

[0021] Preferably, the steps of cleaning and drying are:

[0022] The supporting substrate and the sensitive substrate were spray-cleaned in deionized water for 60 seconds and then blown dry with high-purity nitrogen.

[0023] Preferably, the step of polishing the cavity is:

[0024] The cavity is polished for the first time using magnetorheological polishing, and the roughness inside the cavity is ensured to be less than 3nm during the first polishing;

[0025] The concave cavity polished for the first time is polished for the second time using ion beam etching, and the second polishing ensures that the roughness inside the concave cavity is less than 1 nm.

[0026] Preferably, the steps of aligning the top surface of the supporting substrate and the bottom surface of the sensitive substrate and welding them to form a bonded wafer are:

[0027] The top surface of the supporting substrate and the bottom surface of the sensitive substrate are cleaned and activated, and then aligned, the supporting substrate and the sensitive substrate are bonded together using van der Waals force, and vacuum pre-bonded;

[0028] By irradiating the sensitive substrate with a femtosecond laser, the sensitive substrate and the welding film on the supporting substrate are melted together to form a strong bonding layer to obtain a bonded wafer.

[0029] The beneficial effects of the present invention are:

[0030] By combining high-density, multi-angle femtosecond laser polishing with magnetorheological and IBE (ion beam etching) polishing techniques, deep, smooth cavities on sapphire substrates are fabricated. This simplifies the traditional method of bonding multiple sapphire layers, reducing process costs and improving yield. Furthermore, femtosecond laser bonding is used to bond the sapphire substrates corresponding to the support and sensitive substrates through a welding film, enabling inter-sapphire wafer bonding. The bonding layer, formed by laser melting, is a high-temperature-resistant compound, exceeding 1000°C compared to bonding through an intermediate layer. Furthermore, the thermal impact zone of the two wafers bonded through the welding film is minimized to micrometers. Compared to traditional direct sapphire bonding, this method eliminates the need to subject the entire sapphire wafer to high temperatures (1300°C) and high pressure (above 100kPa). This eliminates the need for pre- and post-bonding process options, reducing overall process complexity, allowing for a wide range of optical materials and types, and enabling a diverse range of processing techniques. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0032] Figure 1 This is a schematic structural diagram of a fluid wall shear stress sensor chip according to an embodiment of the present invention;

[0033] Figure 2 A top view of the supporting base layer in an embodiment of the present invention;

[0034] Figure 3 A bottom view of the sensitive structure layer in an embodiment of the present invention;

[0035] Figure 4 is a top view of the sensitive structure layer in an embodiment of the present invention;

[0036] Figure 5 1 is a step diagram of a method for preparing a fluid wall shear stress sensor according to an embodiment of the present invention;

[0037] Figure 6 is a structural diagram corresponding to each process in the manufacturing process of the sensor in the embodiment of the present invention; wherein, Figure 6 (a) Cross-sectional view of the structure after photolithography to fix the grating and pattern the area of ​​the welding film; Figure 6 (b) A cross-sectional view of the structure after sputtering a metal film to form a fixed grating and welding the film; Figure 6(c) is a cross-sectional view of the structure after laser ablation of the cavity; Figure 6 (d) is a cross-sectional view of the structure after polishing the cavity; Figure 6 (e) is a cross-sectional view of the structure of the laser ablated groove; Figure 6 (f) is a cross-sectional view of the structure of the patterned area where the photolithography sensitive grating is formed; Figure 6 (g) is a cross-sectional view of the structure of a sensitive grating formed by sputtering a metal film; Figure 6 (h) is a cross-sectional view of the structure of the bonded wafer obtained by laser welding sapphire wafer A and sapphire wafer B; Figure 6 (i) is a cross-sectional view of the bonded wafer after polishing and thinning; Figure 6 (j) Structural cross-sectional view of the shear stress sensor obtained by laser ablation grooves to form elastic beams.

[0038] In the figure: 1. Support base layer; 2. Sensitive structure layer; 3. Welding film; 4. Fixed grating; 5. Through slot; 6. Movable structure; 7. Sensitive grating; 8. Elastic beam; 9. Floating element. DETAILED DESCRIPTION

[0039] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0040] An embodiment of a sapphire-based optical fluid wall shear stress sensor of the present invention includes: a supporting base layer 1 , on which a sensitive structural layer 2 is provided.

[0041] like Figure 1 and Figure 2 As shown, the supporting base layer 1 includes: a supporting base, a central array on the upper surface of which is provided with four fixed gratings 4, and a welding film 3 is provided on the supporting base outside the four fixed gratings 4. In this embodiment, the supporting base is made of sapphire material, and the thickness of the supporting base is 200-500μm; the four fixed gratings 4 are distributed in a field shape, the material is metal, and the thickness is 100-300nm. The period and duty cycle of the four fixed gratings 4 are the same. Specifically, the period of the fixed grating 4 is 10-50μm, and the duty cycle is 30%-70%.

[0042] like Figure 3 and Figure 4As shown, the sensitive structure layer 2 includes: a sensitive substrate, which is connected to a supporting substrate and is provided with a through groove 5 on the sensitive substrate, in which an active structure is provided, and four sensitive gratings 7 are arranged in an array on the side of the active structure facing the supporting substrate. The period of the sensitive grating 7 is 10-50 μm, and the duty cycle is 30%-70%. The sensitive gratings 7 and the fixed gratings 4 are arranged correspondingly to form moiré fringes; the period and duty cycle of the four sensitive gratings 7 are the same. In this embodiment, the sensitive substrate is made of sapphire material, and the thickness of the sensitive substrate is 30-150 μm; the depth of the through groove 5 is 20-50 μm; the four sensitive gratings 7 are distributed in a field shape and correspond one to one to the four fixed gratings 4, the material of the sensitive gratings 7 is metal, the thickness is 100-300 nm, the period of the sensitive gratings 7 is 10-50 μm, and the duty cycle is 30%-70%.

[0043] It should be noted that the period of the sensitive grating 7 and the fixed grating 4 have a period difference of less than or equal to 1 μm, and in this embodiment, it is 0.5 μm.

[0044] For example, in a specific embodiment, the movable structure includes: a floating element 9 , which is connected to the inner wall of the through-slot 5 via a plurality of elastic beams 8 so as to be suspended in the through-slot 5 .

[0045] For example, in a specific embodiment, the thickness of the welding film 3 is 200-500 nm, wherein the welding film 3 includes a titanium metal film arranged on a supporting substrate, and a platinum metal film is arranged on the titanium metal film. It should be noted that the titanium metal film is used as an adhesion layer for adhering the platinum metal film.

[0046] like Figure 5 As shown, a method for preparing a sapphire-based optical fluid wall shear stress sensor includes:

[0047] S1. Preparing a fixed grating and welding film on a supporting substrate;

[0048] Specifically, a patterned area of ​​a fixed grating and a welding film is photoetched on the top surface of the cleaned and dried supporting substrate, and a fixed grating and a welding film are formed in the corresponding patterned area by magnetron sputtering metal.

[0049] For example, in a specific embodiment, the steps of cleaning and drying the support substrate are as follows: spray-cleaning the support substrate in deionized water for 60 seconds, and then drying it with high-purity nitrogen.

[0050] S2, machining a cavity on a sensitive substrate, machining a groove in the cavity to form a target structure having the same shape as the floating element, and preparing a sensitive grating on the target structure;

[0051] Specifically, a femtosecond laser is used to ablate a cavity on the bottom surface of the cleaned and dried sensitive substrate, and the cavity is polished. A groove with the same shape as the through groove is ablated on the bottom surface of the polished cavity. The internal area surrounded by the groove forms a target structure with the same shape as the floating element. A patterned area of ​​the sensitive grating is photoetched on the target structure, and magnetron sputtering metal is used to form a sensitive grating in the corresponding patterned area.

[0052] For example, in a specific embodiment, the steps of cleaning and drying the sensitive substrate are: spray-cleaning the sensitive substrate in deionized water for 60 seconds, and then drying it with high-purity nitrogen.

[0053] Exemplarily, in a specific embodiment, the steps of polishing the cavity are: using magnetorheological polishing to perform a first polishing on the cavity, and the first polishing ensures that the roughness inside the cavity is less than 3nm; using ion beam etching to perform a second polishing on the cavity that has been polished for the first time, and the second polishing ensures that the roughness inside the cavity is less than 1nm.

[0054] It should be noted that in this embodiment, only a cavity needs to be processed on the sensitive substrate, and no additional intermediate layer needs to be bonded to realize the cavity for accommodating the floating unit, thereby reducing the processing difficulty and improving the processing efficiency.

[0055] S3, welding the sensitive substrate and the supporting substrate to form a bonded wafer;

[0056] Specifically, the top surface of the supporting substrate and the bottom surface of the sensitive substrate are aligned and welded to form a bonded wafer.

[0057] Illustratively, in a specific embodiment, the steps of aligning the top surface of the supporting substrate and the bottom surface of the sensitive substrate and welding them to form a bonded wafer are as follows: aligning the top surface of the supporting substrate and the bottom surface of the sensitive substrate after cleaning and activation treatment, bonding the supporting substrate and the sensitive substrate together using van der Waals force, and performing vacuum pre-bonding; irradiating the sensitive substrate from above with a femtosecond laser to melt the welding films on the sensitive substrate and the supporting substrate together to form a strong bonding layer, thereby obtaining a bonded wafer.

[0058] S4, preparing an elastic beam and a floating element to form a shear stress sensor;

[0059] Specifically, the top surface of the sensitive substrate in the bonded wafer is thinned and polished, the bottom surface of the groove is ablated to form an elastic beam and penetrates the bottom of the groove to form a through groove, and the target structure and the groove are separated to form a floating element, that is, a shear stress sensor with a floating element and an elastic beam processed as one.

[0060] The following further describes the preparation method of a sapphire-based optical fluid wall shear stress sensor in conjunction with the accompanying drawings and specific parameters. The preparation process of this embodiment is as follows:

[0061] S1: Prepare fixed grating and welding film on supporting substrate;

[0062] S101: Select a sapphire A wafer as a support substrate with a thickness of 300 μm, spray clean it in deionized water for 60 seconds, and then blow dry it with high-purity nitrogen.

[0063] S102: Figure 6 As shown in (a), a 3μm thick photoresist is spin-coated on the front side of the sapphire A wafer by the spin-on method, and then exposed by photolithography to form a patterned area for fixing the grating and the welding film.

[0064] S103: Using magnetron sputtering technology, a 10nm titanium metal film is sputtered on the patterned area of ​​the fixed grating and the welding film, and a 200nm platinum metal film is sputtered on the titanium metal film, and then the photoresist is washed away with acetone to form a Figure 6 (b) The welded film and fixed grating shown, where the fixed grating has a period of 30 μm and a duty cycle of 60%.

[0065] S2: machining a cavity on a sensitive substrate, machining a groove in the cavity to form a target structure having the same shape as the floating element, and preparing a sensitive grating on the target structure;

[0066] S201: The steps of machining grooves on a sensitive substrate are:

[0067] S1021: Select a sapphire B wafer with a thickness of 300 μm as the sensitive substrate, spray clean it in deionized water for 60 seconds, and then blow dry it with high-purity nitrogen.

[0068] S1022: Using a femtosecond laser to ablate a cavity on the front side of a sapphire B wafer in a high-density, multi-angle direct writing method, the cavity depth is 30μm and the overall roughness is less than 1μm. Figure 6 As shown in (c), tiny peaks of varying sizes are formed at the bottom of the cavity due to the ablation characteristics of the laser.

[0069] S1023: Use magnetorheological polishing to polish the cavity and remove tiny peaks, so that the roughness inside the cavity is less than 3nm.

[0070] S1024: Use ion beam etching to further polish the cavity so that the roughness inside the cavity is less than 1nm. Finally, the cavity processed on the sapphire B wafer is as follows: Figure 6 (d) shown.

[0071] S202: The steps of machining a groove in the cavity to form a target structure having the same shape as the floating element, and preparing a sensitive grating on the target structure are as follows:

[0072] S2021: Figure 6As shown in (e), a femtosecond laser is used to burn a groove with a depth of 50 μm in the cavity of the sapphire B wafer. The groove has the same shape as the through groove, and the area surrounded by the groove forms a target structure. The target structure has the same shape as the floating element, so as to reduce the difficulty of ablating a complete active structure in one go.

[0073] S2022: Figure 6 As shown in (f), ultrasonic spraying photoresist technology is used to spray 3μm thick photoresist on the cavity, groove and target structure of the sapphire B wafer, and photolithography exposure is performed to form a patterned area of ​​the sensitive grating on the target structure.

[0074] S2023: Figure 6 As shown in (g), titanium metal is sputtered in the patterned area using magnetron sputtering technology to form a 10nm titanium metal film, and a 200nm platinum metal film is sputtered on the titanium metal film. The photoresist is then washed away with acetone to form a sensitive grating, wherein the period of the sensitive grating is 29.5μm and the duty cycle is 60%.

[0075] Step S3: welding the sensitive substrate and the supporting substrate to form a bonded wafer;

[0076] S301: Clean and activate the top surface of the sapphire wafer A processed in step S1 and the bottom surface of the sapphire wafer B processed in step S2, and align them, with the sapphire wafer B on top and the sapphire wafer A on the bottom. Use van der Waals forces to bond the wafers together, and then perform vacuum pre-bonding on a bonding machine.

[0077] S302: If Figure 6 As shown in (h), a femtosecond laser is used to irradiate the sapphire B wafer from above, melting the welding films of the sapphire B wafer and the sapphire A wafer together to form a strong bonding layer. At this point, a bonded wafer is obtained.

[0078] S4: preparing elastic beams and floating elements to form shear stress sensors;

[0079] S401: Polishing the bonded wafers after welding by CMP thinning and polishing process, that is, in this embodiment, the top surface of the sapphire B wafer in the bonded wafer is thinned to 150 μm. The bonded wafer after thinning is as follows: Figure 6 (i) shown.

[0080] S402: On the top surface of the thinned sapphire wafer B, a femtosecond laser is used to ablate the elastic beam 8 in the groove ablated in sub-step S201 and ablate through the sapphire wafer B. Figure 6As shown in (j), after burning through the connection between the groove and the target structure on the sapphire B crystal, the groove forms a through groove, and the target structure is separated from the groove to form a floating element with a thickness of 120μm. An elastic beam is formed between the floating element and the inner wall of the through groove, that is, a shear stress sensor with a floating element and an elastic beam processed as one.

[0081] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A sapphire-based optical fluid wall shear stress sensor, characterized in that: include: a supporting base layer on which a sensitive structural layer is disposed; The supporting substrate layer includes: a supporting substrate, a central array of a plurality of fixed gratings is arranged on the upper surface of the supporting substrate, and a welding film is arranged on the supporting substrate outside the plurality of fixed gratings; the sensitive structure layer includes: a sensitive substrate, the sensitive substrate is connected to the supporting substrate, and a through slot is provided on the sensitive substrate, a movable structure is provided in the through slot, and a plurality of sensitive gratings are arranged in an array on the side of the movable structure facing the supporting substrate, the sensitive gratings and the fixed gratings are arranged correspondingly and form moiré fringes; the supporting substrate and the sensitive substrate are both made of sapphire material; the welding film includes a titanium metal film provided on the supporting substrate, and a platinum metal film is provided on the titanium metal film; The thickness of the supporting substrate is 200-500μm, the thickness of the fixed grating is 100-300nm, the period of the fixed grating is 10-50μm, and the duty cycle is 30%-70%; the thickness of the sensitive substrate is 30-150μm; the depth of the through groove is 20-50μm; the thickness of the sensitive grating is 100-300nm, the period of the sensitive grating is 10-50μm, and the duty cycle is 30%-70%; the period of the sensitive grating 7 is less than or equal to 1μm from that of the fixed grating 4.

2. The sapphire-based optical fluid wall shear stress sensor according to claim 1, characterized in that: The movable structure comprises a floating element which is connected to the inner wall of the through slot through a plurality of elastic beams so as to be suspended in the through slot.

3. The sapphire-based optical fluid wall shear stress sensor according to claim 1, characterized in that: The materials used for fixing the grating are titanium and platinum.

4. The sapphire-based optical fluid wall shear stress sensor according to claim 1, characterized in that: The sensitive grating is made of titanium and platinum.

5. A method for preparing a sapphire-based optical fluid wall shear stress sensor, characterized in that: include: Photolithography is performed on the top surface of the cleaned and dried support substrate to form a patterned area of ​​a fixed grating and a welding film, and magnetron sputtering metal is used to form a fixed grating and a welding film in the corresponding patterned area; A femtosecond laser is used to ablate a cavity on the bottom surface of a cleaned and dried sensitive substrate, and the cavity is polished. A groove with the same shape as the through-groove is ablated on the bottom surface of the polished cavity. The inner area surrounded by the groove forms a target structure with the same shape as the floating element. A patterned area of ​​a sensitive grating is photoetched on the target structure, and a metal is magnetron sputtered to form a sensitive grating in the corresponding patterned area. The steps of polishing the cavity are as follows: a first polishing of the cavity is performed using magnetorheological polishing, and the first polishing ensures that the roughness of the cavity is less than 3nm; a second polishing of the cavity is performed using ion beam etching, and the second polishing ensures that the roughness of the cavity is less than 1nm. aligning the top surface of the supporting substrate and the bottom surface of the sensitive substrate and welding them to form a bonded wafer; The steps of aligning the top surface of the supporting substrate and the bottom surface of the sensitive substrate and welding them to form a bonded wafer are as follows: cleaning and activating the top surface of the supporting substrate and the bottom surface of the sensitive substrate, aligning them, bonding the supporting substrate and the sensitive substrate together using van der Waals force, and performing vacuum pre-bonding; irradiating the sensitive substrate from above with a femtosecond laser to melt the welding films on the sensitive substrate and the supporting substrate together to form a strong bonding layer, thereby obtaining a bonded wafer; The top surface of the sensitive substrate in the bonded wafer is thinned and polished, and the bottom surface of the groove is ablated to form an elastic beam that penetrates the bottom of the groove to form a through groove. The target structure and the groove are separated to form a floating element, thus obtaining a shear stress sensor with the floating element and the elastic beam integrally processed.

6. The method for preparing a sapphire-based optical fluid wall shear stress sensor according to claim 5, characterized in that: The steps for cleaning and drying are: The supporting substrate and the sensitive substrate were spray-cleaned in deionized water for 60 seconds and then blown dry with high-purity nitrogen.

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