Optical fiber F-P cavity MEMS temperature-pressure composite sensor and preparation method thereof

Abstract

The application discloses a kind of optical fiber F-P cavity MEMS temperature-pressure composite sensor, sensor mainly includes temperature-pressure composite sensitive chip, multimode quartz optical fiber, optical fiber protective sleeve and silica ball head.Wherein, temperature-pressure composite sensitive chip is the silicon glass optical fiber double F-P cavity blind hole structure made of MEMS process based on built-in micro channel, multimode quartz optical fiber end face is obliquely cut 45 ° and is plated with multilayer dielectric reflection-increasing film to form light path turning prism, optical fiber protective sleeve adopts side opening structure, and top is closed by silica ball head.The sensor uses the side opening packaging structure based on micro channel, changes the direction of light path transmission to avoid the cutting and damage to structure when the traditional forward opening structure optical fiber sensor probe is implanted in medical treatment;Meanwhile, by widening F-P air cavity chamber along the lateral direction, the sensor probe packaging size can be effectively reduced while ensuring the sensitivity of the sensor, and it is suitable for intracranial monitoring and related medical fields with strict requirements on size and structure.

Description

Technical Field

[0001] This invention relates to the field of medical high-precision fiber optic sensing technology, and in particular to a fiber optic FP cavity MEMS temperature-pressure composite sensor and its fabrication method. Background Technology

[0002] In clinical medicine, temperature and pressure monitoring is the fastest, most objective, and accurate method for diagnosing diseases of certain organs, and it is also an important tool for guiding clinical medication and improving prognosis, especially for intracranial temperature-pressure monitoring. Intracranial temperature-pressure monitoring is usually performed via implantation, requiring craniotomy beforehand. The size of the craniotomy opening is determined by the outer diameter of the sensor used in the monitoring; increasing the outer diameter increases the invasiveness of the overall structure and the risks of the craniotomy. Currently, intracranial implantable temperature-pressure sensors used clinically mainly include piezoresistive and fiber optic sensors.

[0003] Piezoresistive temperature-pressure composite sensors employ a dual-probe structure combining a pressure sensor based on the piezoresistive effect and a thermistor. By integrating the thermistor and piezoresistive element into a single MEMS chip, the overall chip size can be minimized. Currently, the smallest piezoresistive temperature-pressure composite sensor probe for intracranial measurements can achieve a diameter of less than 800 μm. Furthermore, piezoresistive sensors use metal leads and electrical signal transmission, making them insensitive to signal transmission direction. This allows the pressure-sensing surface to be placed on the side, while the front of the probe is protected with a ball-shaped structure, achieving a minimal outer diameter and minimal invasiveness. However, the drawbacks of piezoresistive sensors lie in their use of electrical signal transmission and metal casing, which introduces eddy currents internally, making them unsuitable for use in strong electromagnetic environments such as brain CT scans and MRI scans, thus limiting their application.

[0004] Fiber optic temperature-pressure composite sensors typically employ a measurement principle combining a fiber optic FP single resonant cavity and a Bragg fiber grating. The fiber optic FP single resonant cavity is sensitive to pressure, while the Bragg fiber grating is sensitive to temperature. Patent 201110308246.1 discloses a miniature intracranial multi-parameter sensor based on fiber optic sensing, which uses the aforementioned measurement method. Through non-metallic material encapsulation and a full optical path transmission structure, the sensor can be used in strong electromagnetic environments. However, due to the sensitivity of optical fibers to signal transmission direction, these sensors cannot be bent at large angles. This currently limits pressure sensing to a forward-facing opening method, increasing the overall packaging size and invasiveness. Furthermore, the sharp edges of the sensor chip may cause damage to human organs during implantation, increasing the difficulty and risk of clinical surgery. Summary of the Invention

[0005] To address the aforementioned problems, this invention aims to provide a fiber optic FP cavity MEMS temperature-pressure composite sensor and its fabrication method. This sensor adopts the fiber optic sensing principle and optical path steering technology, combining the advantages of piezoresistive and fiber optic sensors. It can achieve lateral pressure sensing while ensuring sensor sensitivity and anti-electromagnetic interference capability, reducing the overall package outer diameter size, and minimizing the invasiveness and damage of the sensor probe to the implantation site.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a fiber optic FP cavity MEMS temperature-pressure composite sensor, characterized in that the sensor includes a temperature-pressure composite sensing chip, a multimode silica fiber, a fiber optic protective sleeve, and a silicone ball head, wherein the temperature-pressure composite sensing chip is a cuboid structure composed of a pressure-sensitive diaphragm (ultra-thin single-crystal silicon material with a thickness of 10μm) and a glass substrate (high borosilicate material, preferably BF33 glass sheet), with dimensions of 2000*500*530μm.

[0007] A cavity with a depth of 20 μm is formed on the inner surface of an ultrathin single-crystal silicon pressure-sensitive diaphragm. This cavity is bonded to the surface of the glass substrate through anodizing to form a blind-hole FP air cavity. Unlike the traditional circular diaphragm FP air cavity structure with fixed support on all four sides, this invention increases the pressure sensitivity of the sensor under axial size constraints by widening the FP air cavity chamber laterally.

[0008] Microchannels are etched on the bottom of the glass substrate, and multimode silica optical fiber is inserted and positioned in the middle of the temperature-pressure composite sensitive chip. The circumferential surface of the multimode silica optical fiber and the inner wall of the fiber optic protective sleeve are in contact with the inner wall of the fiber optic protective sleeve.

[0009] The temperature-pressure composite sensitive chip is positioned, embedded, and encapsulated within the optical fiber protective sleeve, and the outer surface of the ultrathin single-crystal silicon pressure-sensitive diaphragm penetrates the sidewall of the optical fiber protective sleeve and communicates with the outside of the optical fiber protective sleeve.

[0010] A light path steering structure is provided on the end face of the multimode quartz fiber located in the microchannel. The light source transmitted from the multimode quartz fiber passes through the temperature-pressure composite sensitive chip and is perpendicularly emitted through the side wall of the fiber protective sleeve. Specifically, the structure is a light path steering prism with an inclined 45° surface.

[0011] Furthermore, the upper surface of the temperature-pressure composite sensing chip is flush with the sidewall of the optical fiber protective sleeve, and a silicone ball head is wrapped around the sidewall of the temperature-pressure composite sensing chip and the side end of the optical fiber protective sleeve.

[0012] The sensor works as follows:

[0013] The light beam is emitted from the SLED white light source and enters the multimode quartz fiber through a 1*2 fiber coupler. When the incident light is transmitted to the 45° inclined structure coated with a metal anti-reflection film, the structure acts as a light path turning prism. The incident light is turned at a 90° angle and exits from the fiber sidewall that is horizontally connected to the glass substrate. It then enters the temperature-pressure composite sensitive chip vertically from the center of the glass substrate.

[0014] For the wavelength range (400-850nm) of the white light source, the glass substrate exhibits optical transparency, while the monocrystalline silicon pressure-sensitive diaphragm exhibits optical absorption. Therefore, the lower and upper surfaces of the glass substrate and the lower surface of the monocrystalline silicon pressure-sensitive diaphragm constitute a dual-FP cavity structure. White light incident on the monocrystalline silicon pressure-sensitive diaphragm is absorbed by the material and does not return. The composite spectral signal modulated by the dual-FP interferometer cavities exits perpendicularly from the lower surface of the glass substrate, is redirected again by a 45° angled optical path deflector, and enters a multimode silica fiber. Finally, the modulated light enters the spectrometer module via a 1*2 fiber coupler beam splitter. By filtering the dual-FP cavity interferometric spectrum and employing an appropriate cavity length demodulation algorithm, the cavity lengths of the dual-FP cavities can be obtained separately.

[0015] Specifically, the lower surface of the monocrystalline silicon pressure-sensitive diaphragm and the upper surface of the glass substrate form a FP air cavity, which exhibits pressure sensitivity based on a large-area suspended thin film; the lower and upper surfaces of the glass substrate form a FP substrate cavity, which exhibits temperature sensitivity based on the thermal expansion of the material. By statically calibrating the pressure and temperature of the sensor, the correspondence between the length of the dual FP cavities and their respective sensitive parameters can be obtained, thus enabling the combined acquisition of temperature and pressure.

[0016] A method for fabricating a fiber optic FP-cavity MEMS temperature-pressure composite sensor, characterized by comprising the following steps:

[0017] (1) Select a clean SOI wafer, which consists of a single crystal silicon device layer, a silicon dioxide buried oxide layer and a single crystal silicon substrate layer. Spin-coat photoresist on the surface of the single crystal silicon device layer, expose and develop it with a photolithography machine to form the chamber etching window of the FP air cavity.

[0018] (2) Using photoresist as an etching mask, the FP air cavity is etched to the cavity length. The etching process adopts the Bosch process, with a cycle of 10 seconds of etching followed by 1 second of passivation. The etching process ensures the verticality of the sidewalls and the roughness of the bottom.

[0019] (3) Select a clean borosilicate glass sheet and use a glue sprayer to spray a thick photoresist onto the surface of the glass sheet;

[0020] A gradient heating method is used to harden the photoresist, and the photoresist is used as an etching mask for deep reactive ions to form microchannels for positioning and inserting multimode quartz optical fibers at the bottom of the glass plate.

[0021] (4) Clean the processed SOI wafer and glass sheet, and use oxygen plasma to activate the single crystal silicon device layer of SOI wafer and the bonding surface of glass sheet. After activation, use a bonding machine to anoly bond SOI wafer and glass sheet. The bonding temperature is 360℃ and the bonding vacuum degree is 0.5mbar.

[0022] (5) After bonding is completed, the single crystal silicon substrate of SOI wafer is wet etched. The wet etching is as follows: the bonded wafer is immersed in TMAH solution at 90℃ for 12h to completely remove the single crystal silicon of the single crystal silicon substrate of SOI wafer until the silicon dioxide buried oxide layer is reached.

[0023] (6) A UV film is attached to the bottom of the glass slide for protection, and then the bonded sheet is immersed in hydrofluoric acid to allow the silicon dioxide buried oxygen layer to fully react with the hydrofluoric acid before removal.

[0024] (7) Use a femtosecond laser to scribe the processed bonding sheet to form the temperature-pressure composite sensitive chip;

[0025] (8) A multimode quartz fiber is cut into the optical path turning prism with a 45° angle, and a multilayer dielectric anti-reflection film is deposited on the cut surface. The dielectric anti-reflection film has a broadband reflectivity of more than 90% in the incident light band.

[0026] (9) After processing, insert the multimode quartz fiber horizontally into the microchannel, adjust the tilt angle of the multimode quartz fiber cutting end face until a stable dual FP cavity signal can be collected at the rear end, and then bond and fix the multimode quartz fiber to the glass plate.

[0027] (10) Make an opening through the temperature-pressure composite sensitive chip on the top side wall of the optical fiber protective sleeve, and insert the temperature-pressure composite sensitive chip with multimode quartz fiber fixed into the optical fiber protective sleeve from the opening until the pressure sensing surface of the temperature-pressure composite sensitive chip is flush with the side wall of the optical fiber protective sleeve, and then bond and fix the temperature-pressure composite sensitive chip to the position of the optical fiber protective sleeve.

[0028] (11) Apply silicone to the radial position at the top of the optical fiber protective sleeve to cover the radial end face of the optical fiber protective sleeve, and form the silicone ball head after curing.

[0029] The beneficial effects of this invention are:

[0030] (1) The temperature-pressure composite sensitive chip of the present invention is packaged using a side opening structure based on microchannels. The side opening of the sensor is realized through the optical path steering system, which can ensure the flatness and smoothness of the overall structure of the sensor probe and prevent the sharp edge of the chip from cutting and damaging human organs during implantation.

[0031] (2) The temperature-pressure composite sensitive chip of the present invention optimizes the sensor for the side-opening probe packaging form. Compared with the traditional four-sided fixed circular diaphragm FP cavity structure, by widening the FP cavity along the side, the radial dimension of the entire probe can be effectively reduced while ensuring the sensor sensitivity, improving the overall packaging structure compactness and reducing sensor implantation trauma.

[0032] (3) Compared with the traditional single-FP cavity probe structure for composite Bragg fiber grating temperature measurement, the dual-FP cavity temperature-pressure composite sensor probe structure of the present invention is characterized by the Bragg fiber grating being wrapped inside the probe, resulting in temperature loss and slow temperature response, which affects the temperature measurement accuracy of the probe. In contrast, the dual-FP cavity temperature-pressure composite sensing chip is directly exposed to the external environment, resulting in faster temperature response and higher temperature measurement accuracy.

[0033] (4) The sensor described in this invention uses full optical path transmission. The probe part does not contain any metal materials or electrical circuits, and has strong anti-electromagnetic interference capability. It can realize uninterrupted continuous monitoring in strong electromagnetic environments such as brain CT and MRI. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the temperature-pressure composite sensor probe structure of the present invention.

[0035] Figure 2 This is a three-dimensional schematic diagram of the temperature-pressure composite sensitive chip structure of the present invention.

[0036] Figure 3 This is a flowchart of the MEMS process for the temperature-pressure composite sensitive chip of the present invention.

[0037] Figure 4 This is the sensor spectrum of the present invention when the film thickness is 10 μm.

[0038] Figure 5 This is the sensor spectrum of the present invention after the film thickness is reduced to 8 μm.

[0039] Figure 6 This is a diagram showing the relationship between the widening dimension of the FP cavity and the displacement of the pressure-sensitive diaphragm under unit pressure according to the present invention.

[0040] In the figure: 1-Temperature-pressure composite sensitive chip; 11-Pressure sensitive diaphragm; 101-Cavity; 12-Glass substrate; 2-Multimode quartz optical fiber; 3-Optical fiber protective sleeve; 4-Silicone ball head; 5-Single crystal silicon device layer; 6-Silica buried oxide layer; 7-Single crystal silicon substrate layer; 8-Glass sheet; 9-Microchannel; 10-Multilayer dielectric antireflective coating. Detailed Implementation

[0041] To enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0042] A fiber optic FP-cavity MEMS temperature-pressure composite sensor, such as Figure 1 As shown, the sensor consists of a temperature-pressure composite sensing chip 1, a multimode quartz optical fiber 2, an optical fiber protective sleeve 3, and a silicone ball head 4, wherein:

[0043] The temperature-pressure composite sensitive chip 1 is formed by anodic bonding of an ultra-thin single-crystal silicon pressure-sensitive diaphragm (hereinafter referred to as pressure-sensitive diaphragm) 11 and a high borosilicate glass substrate (hereinafter referred to as glass substrate) 12. The structural dimensions are 2000*500*530μm. A cavity 101 with a depth of 20μm and a cavity length of 20μm and a blind hole structure is formed on the inner surface of the pressure-sensitive diaphragm 11. The cavity 101 and the surface of the glass substrate 12 form an FP air cavity.

[0044] The thickness of the pressure-sensitive diaphragm 11 at the position corresponding to the cavity 101 is 10 μm. The sensor spectrum at this size is shown below. Figure 4 As shown, the sensor's spectral interference signal is good at 10μm, and the FP cavity contrast is obvious.

[0045] When the thickness of the pressure-sensitive diaphragm 11 is less than this thickness and reduced to 8 μm, the sensor spectrum is as follows: Figure 5 As shown, the white light incident into the pressure-sensitive diaphragm 11 cannot be completely absorbed by the single-crystal silicon material due to insufficient penetration depth. It will be reflected by the upper surface of the pressure-sensitive diaphragm 11 and interfere with the sensor signal. The contrast of the FP cavity interference signal is poor and the signal cannot be effectively demodulated.

[0046] The relationship between sensor pressure sensitivity and diaphragm thickness can be characterized by the sensor sensitivity formula:

[0047]

[0048] Where S is the sensor pressure sensitivity, E and v are the Young's modulus and Poisson's ratio of the sensitive diaphragm, R is the equivalent radius of the sensor FP cavity, and h is the thickness of the sensitive diaphragm.

[0049] Therefore, when the thickness of the pressure-sensitive diaphragm 11 is greater than 10 μm, the pressure sensitivity of the sensor decreases exponentially with the increase of the thickness of the sensitive diaphragm, which cannot meet the actual use requirements.

[0050] Compared to the traditional circular diaphragm FP air cavity structure with fixed sides, the recessed cavity 101 within the pressure-sensitive diaphragm 11 is widened laterally, which can improve the pressure sensitivity of the sensor while reducing the radial dimension of the sensor. Widening the FP cavity laterally increases the equivalent radius of the FP cavity, resulting in a larger displacement of the pressure-sensitive diaphragm under a unit change in external pressure, thus corresponding to an increase in sensor sensitivity. Figure 6 The figure shows the relationship between the expansion dimension of the FP cavity and the displacement of the pressure-sensitive diaphragm under unit pressure.

[0051] The glass substrate 12 has a thickness of 500μm, which matches the width of the cuboid structure formed by the temperature-pressure composite sensitive chip 1, which is 500μm. A microchannel 9 is built into the bottom of the glass substrate 12, extending from one side of the substrate to its center. The width and depth of the channel are both 130μm. The microchannel 9 is formed by reactive ion dry etching, and its bottom is smooth and flat, which can be used as an optical reflective surface.

[0052] The end face of the multimode silica fiber 2 adopts a 45° bevel structure and is horizontally connected to the glass substrate 12 of the temperature-pressure composite sensing chip 1 through the microchannel 9, and inserted into the center of the glass substrate 12 for bonding and fixation. The 45° bevel end face of the multimode silica fiber 2 is coated with a multilayer dielectric anti-reflection film 10, and the 45° bevel end face faces away from the microchannel 9, forming a vertical optical path steering prism system for the light beam entering from the multimode silica fiber 2.

[0053] The incident light is directed through the optical path prism (45° end face) and is perpendicularly incident on the temperature-pressure composite sensitive chip 1. For the incident light, the lower surface of the pressure-sensitive diaphragm 11 and the upper surface of the glass substrate 12 form an FP air cavity, which is pressure sensitive based on a large-area suspended thin film; the lower surface and the upper surface of the glass substrate 12 form an FP substrate cavity, which is temperature sensitive based on the thermal expansion of the material.

[0054] The top sidewall of the fiber optic protective sleeve 3 (outer diameter 900μm, inner diameter 700μm) has a small hole that matches the outer diameter of the temperature-pressure composite sensing chip 1. The temperature-pressure composite sensing chip 1 is inserted through the small hole and bonded to the fiber optic protective sleeve 3. The upper surface of the temperature-pressure composite sensing chip 1 is flush with the sidewall of the fiber optic protective sleeve, and the bottom surface of the multimode quartz fiber 2 is in contact with the inner bottom surface of the fiber optic protective sleeve 3.

[0055] The radial position at the top of the fiber optic protective sleeve 3 is covered with silicone to form a silicone ball head 4 structure. The overall probe structure covered by the fiber optic protective sleeve 3 is required to be smooth and free of burrs and sharp edges with small angles.

[0056] The set cavity length of 20μm is based on the overall size design of the sensor. Further reducing the cavity length would result in insufficient available interference period when demodulating the FP cavity interference spectrum, affecting the demodulation accuracy. On the other hand, further increasing the cavity length would increase the longitudinal dimension of the temperature-pressure composite sensitive chip 1, causing the upper surface of the chip to be higher than the currently set outer diameter of the fiber optic protective sleeve. It would be necessary to further expand the outer diameter of the fiber optic protective sleeve 3 to completely enclose the sensor chip, which would lead to an increase in the overall package outer diameter of the sensor probe.

[0057] For FP cavity interference spectra, we have:

[0058]

[0059] Where δ is the interference period of the FP cavity, n is the refractive index of the medium inside the FP cavity, L is the cavity length of the FP cavity, λ is the wavelength of light, ν is the frequency of light, and c is the speed of light.

[0060] As shown in the above equation, n and c are generally constants. However, when the light source is fixed, the upper and lower limits corresponding to λ and ν are also determined. When the FP cavity length decreases, a wider wavelength and frequency range of light is required to complete a full δ interference period. That is, within the wavelength range covered by the light source, the interference spectrum has fewer interference periods. Commonly used FP cavity length demodulation employs a multi-peak demodulation algorithm, which demodulates the cavity length by capturing multiple interference peaks in the FP cavity interference spectrum. The more peaks captured, the more accurate the demodulation. When the FP cavity length is too small, and the available interference period of the interference spectrum is insufficient, the demodulation accuracy of the sensor cannot be guaranteed.

[0061] In summary, this sensor utilizes a 45° angled multimode quartz fiber 2, coated with a multilayer dielectric antireflective film 10, and a side-aperture fiber optic protective sleeve 3 to achieve lateral sensing of temperature and pressure. Combined temperature and pressure measurement is achieved through the acquisition and demodulation of the dual FP cavity composite spectrum. This design effectively reduces the sensor probe's outer diameter and packaging size while maintaining sensor sensitivity, making it suitable for intracranial monitoring and related medical fields with strict requirements on size and structure.

[0062] A method for fabricating a fiber optic FP-cavity MEMS temperature-pressure composite sensor includes the following steps:

[0063] (1) A clean SOI wafer is selected, which consists of a single-crystal silicon device layer 5, a silicon dioxide buried oxide layer 6, and a single-crystal silicon substrate layer 7. The thickness of the single-crystal silicon device layer 5 is 30 μm. Photoresist is spin-coated onto the surface of the single-crystal silicon device layer 5, and exposed and developed using a photolithography machine to form the cavity etching window of the FP air cavity, such as... Figure 3 As shown in a.

[0064] (2) Using photoresist as an etching mask, the FP air cavity was etched to a length of 20 μm using a high-density plasma dry etching process. The etching process employed the Bosch process, with a cycle of 10 seconds of etching followed by 1 second of passivation. The etching process ensured a sidewall perpendicularity of 90° ± 1° and a bottom roughness Ra < 0.5 nm. Figure 3 As shown in b.

[0065] (3) Take a clean BF33 glass plate 8 with a thickness of 500 μm; use a spray gun to spray a thick photoresist onto the surface of the glass plate 8, with a thickness greater than 30 μm; use a gradient heating method to harden the photoresist and improve its etching resistance; use the photoresist as an etching mask to perform deep reactive ion etching on the bottom microchannels 9 of the glass plate 8, with an etching depth of 130 μm. When the etching depth is close to the target depth, reduce the power of the upper and lower electrodes of the etching machine and the gas flow rate to ensure that the bottom roughness Ra < 3 nm. Figure 3 As shown in c.

[0066] (4) The processed SOI wafer and BF33 glass sheet 8 were cleaned using the RCA standard cleaning process. Oxygen plasma was used to activate the single-crystal silicon device layer 5 of the SOI wafer and the bonding surface of the glass sheet. After activation, anodizing bonding was performed on the SOI wafer and glass sheet 8 using a bonding machine at a bonding temperature of 360℃ and a bonding vacuum of 0.5 mbar. Figure 3 As shown in d.

[0067] (5) After bonding is completed, wet etching is performed. Specifically, the bonded wafer is immersed in a TMAH solution at 90℃ for 12 hours to completely remove the single-crystal silicon of the SOI wafer's single-crystal silicon substrate layer 7, stopping etching at the silicon dioxide buried oxide layer position. Figure 3 As shown in e.

[0068] (6) A UV film is attached to the bottom of the BF33 glass slide 8 for protection. The bonded sheet is then immersed in hydrofluoric acid for approximately 30 seconds to allow the silicon dioxide embedded oxide layer 6 to fully react with the hydrofluoric acid before removal. Figure 3 As shown in f.

[0069] (7) The processed bonded wafer is diced using a femtosecond laser to form a shape like... Figure 2 The temperature-pressure composite sensitive chip 1 structure is shown; the chip dicing size is 2000μm*500μm, and the dicing process uses dicing marks to ensure that the FP air cavity is located at the center of the temperature-pressure composite sensitive chip 1.

[0070] (8) The end face of the multimode quartz fiber 2 is cut with a special fiber cleaver at a 45° angle, and a multilayer dielectric anti-reflection film 10 is deposited on the 45° angled end face of the multimode quartz fiber 2 using a fiber coating machine, so that the broadband reflectivity of the surface is greater than 90% in the incident light band. The processed multimode quartz fiber 2 is horizontally inserted into the microchannel 9 until the 45° angled end face is at the center of the temperature-pressure composite sensitive chip 1 substrate. After adjusting the end face tilt angle to the point where a stable dual FP cavity signal can be collected at the rear end, the multimode quartz fiber 2 and the substrate are fixed with biocompatible epoxy resin.

[0071] (9) After processing, the multimode quartz fiber is horizontally inserted into the microchannel 9. The tilt angle of the cutting end face of the multimode quartz fiber 2 is adjusted until a stable dual FP cavity signal can be collected at the rear end. Then, the multimode quartz fiber 2 is bonded and fixed to the glass substrate 8 with biocompatible epoxy resin.

[0072] (10) A fiber optic protective sleeve 3 made of biocompatible polypropylene resin (PLA) material is used. The outer diameter of the sleeve is 900 μm and the inner diameter is 700 μm. An opening (small hole) is made on the top side wall of the fiber optic protective sleeve 3 based on the cut size of the temperature-pressure composite sensitive chip 1. The temperature-pressure composite sensitive chip 1, which is fixed with multimode quartz fiber 2, is inserted into the fiber optic protective sleeve 3 from the opening until the pressure-sensing surface of the temperature-pressure composite sensitive chip 1 is flush with the side wall of the fiber optic protective sleeve 3. The temperature-pressure composite sensitive chip 1 and the fiber optic protective sleeve 3 are fixed in position using biocompatible epoxy resin.

[0073] (11) Apply silicone to the radial position of the top of the optical fiber protective sleeve 3 to cover the radial end face of the optical fiber protective sleeve 3. After curing, the silicone ball head 4 is formed. The multimode quartz optical fiber 2 is connected to the back-end demodulation part. The back-end demodulation part includes a 1*2 optical fiber coupler, an SLED white light source, a spectrometer module, and a computer signal processing system.

[0074] The principle of this invention is as follows: the light beam is emitted from the SLED white light source and enters the multimode quartz fiber 2 through the 1*2 fiber coupler beam splitter; when the incident light is transmitted to the inclined 45° structure coated with a metal anti-reflection film, the structure acts as a light path turning prism, and the incident light is turned at a 90° angle and exits from the fiber sidewall that is horizontally connected to the glass substrate 12, and is perpendicularly incident on the temperature-pressure composite sensitive chip 1 from the center of the glass substrate 12.

[0075] For the wavelength range (400-850nm) of the white light source, the glass substrate 12 is optically transparent while the monocrystalline silicon pressure-sensitive diaphragm 11 is optically absorptive. Therefore, the lower and upper surfaces of the glass substrate 12 and the lower surface of the monocrystalline silicon pressure-sensitive diaphragm 11 constitute a dual-FP cavity structure. The white light incident on the monocrystalline silicon pressure-sensitive diaphragm 11 is absorbed by the material and does not return. The composite spectral signal modulated by the dual-FP interferometric cavity exits perpendicularly from the lower surface of the glass substrate 12, is redirected again by a 45° angled optical path turning prism, and enters the multimode silica fiber 2. Finally, the modulated light enters the spectrometer module through a 1*2 fiber coupler beam splitter. By filtering the dual-FP cavity interferometric spectrum and using an appropriate cavity length demodulation algorithm, the cavity lengths of the dual-FP cavities can be obtained separately.

[0076] Specifically, the lower surface of the monocrystalline silicon pressure-sensitive diaphragm 11 and the upper surface of the glass substrate 12 form an FP air cavity, which exhibits pressure sensitivity based on a large-area suspended thin film; the lower and upper surfaces of the glass substrate 12 form an FP substrate cavity, which exhibits temperature sensitivity based on the thermal expansion of the material. By statically calibrating the pressure and temperature of the sensor, the correspondence between the length of the dual FP cavities and their respective sensitive parameters can be obtained, thus enabling the combined acquisition of temperature and pressure.

[0077] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Various changes and modifications can be made to the present invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A fiber optic FP-cavity MEMS temperature-pressure composite sensor, characterized in that, The sensor includes a temperature-pressure composite sensing chip (1), a multimode quartz fiber (2), an optical fiber protective sleeve (3), and a silicone ball head (4). The temperature-pressure composite sensing chip (1) is composed of a pressure-sensitive diaphragm (11) and a glass substrate (12). A cavity (101) is formed on the inner surface of the pressure-sensitive diaphragm (11), and the cavity (101) and the surface of the glass substrate (12) form an FP air cavity. The temperature-pressure composite sensitive chip (1) is positioned and embedded in the optical fiber protective sleeve (3), and the outer surface of the pressure sensitive diaphragm (11) penetrates the side wall of the optical fiber protective sleeve (3) and communicates with the outside of the optical fiber protective sleeve (3). One end of the multimode quartz fiber (2) is inserted into the fiber optic protective sleeve (3) and positioned at the middle position of the bottom side of the temperature-pressure composite sensitive chip (1). The circumferential surface of the multimode quartz fiber (2) that is close to the inner wall of the fiber optic protective sleeve (3) is in contact with the inner wall surface of the fiber optic protective sleeve (3). The multimode quartz fiber (2) is inserted into the end face of the temperature-pressure composite sensitive chip (1), and a light path deflection structure is provided for the light source transmitted from the multimode quartz fiber (2) to be emitted vertically through the temperature-pressure composite sensitive chip (1) and its side wall that penetrates the fiber protective sleeve (3). The silicone ball head (4) is connected to the end side of the fiber optic protective sleeve (3) on the side away from the multimode quartz fiber (2); the optical path turning structure is to set the inner end face of the multimode quartz fiber (2) as an optical path turning prism facing a 45° angle with the side wall of the fiber optic protective sleeve (3). The temperature-pressure composite sensitive chip (1) has a structural size of 2000*500*530μm, the FP air cavity has a length of 20μm, and the pressure sensitive diaphragm (11) has a thickness of 10μm at the corresponding position of the FP air cavity.

2. The sensor according to claim 1, characterized in that: The upper surface of the temperature-pressure composite sensitive chip (1) is flush with the side wall of the optical fiber protective sleeve (3).

3. The method for fabricating a fiber optic FP-cavity MEMS temperature-pressure composite sensor according to claim 2, characterized in that, Includes the following steps: (1) Select a clean SOI wafer, which consists of a single crystal silicon device layer (5), a silicon dioxide buried oxide layer (6) and a single crystal silicon substrate layer (7). Spin-coat photoresist on the surface of the single crystal silicon device layer (5), expose and develop it with a photolithography machine to form the chamber etching window of the FP air cavity. (2) Using photoresist as an etching mask, the FP air cavity is etched to the cavity length. Etching and passivation are a cycle. The etching process ensures the sidewall verticality and bottom roughness. (3) Select a clean borosilicate glass sheet (8) and use a glue sprayer to spray a thick photoresist onto the surface of the glass sheet (8); The photoresist was hardened by gradient heating and the photoresist was used as an etching mask to form a microchannel (9) for positioning and inserting multimode quartz optical fiber (2) at the bottom of the glass plate (8). (4) Clean the processed SOI wafer and glass sheet (8), and use oxygen plasma to activate the bonding surface of the SOI wafer's single crystal silicon device layer (5) and glass sheet (8). After activation, use a bonding machine to anoly bond the SOI wafer and glass sheet (8). (5) After bonding is completed, the single crystal silicon substrate layer (7) of the SOI wafer is subjected to wet etching; (6) A UV film is attached to the bottom of the glass slide (8) for protection. Then the bonded sheet is immersed in hydrofluoric acid to allow the silicon dioxide buried oxide layer (6) to react fully with the hydrofluoric acid before removal. (7) Use a femtosecond laser to cut the processed bonding sheet to form the temperature-pressure composite sensitive chip (1); (8) A multimode quartz fiber (2) cuts the optical path turning prism with a 45° angled surface and deposits a multilayer dielectric anti-reflection film on the cut surface (10). (9) After processing, insert the multimode quartz fiber (2) horizontally into the microchannel (9), adjust the tilt angle of the cutting end face of the multimode quartz fiber (2) until a stable dual FP cavity signal can be collected at the rear end, and then bond and fix the multimode quartz fiber (2) to the glass plate (8). (10) An opening is made on the top side wall of the fiber optic protective sleeve (3) to pass through the temperature-pressure composite sensitive chip 1, and the temperature-pressure composite sensitive chip (1) fixed with multimode quartz fiber (2) is inserted into the fiber optic protective sleeve (3) from the opening until the pressure sensing surface of the temperature-pressure composite sensitive chip (1) is flush with the side wall of the fiber optic protective sleeve (3), and the temperature-pressure composite sensitive chip (1) and the fiber optic protective sleeve (3) are bonded and fixed. (11) Apply silicone to the radial position at the top of the optical fiber protective sleeve (3) to cover the radial end face of the optical fiber protective sleeve (3), and form the silicone ball head (4) after curing.

4. The preparation method according to claim 3, characterized in that: In step (2), the etching process adopts the Bosch process, with a cycle of 10 seconds of etching followed by 1 second of passivation.

5. The preparation method according to claim 4, characterized in that, In step (4), the bonding conditions include: bonding temperature 360°C and bonding vacuum degree 0.5 mbar.

6. The preparation method according to claim 5, characterized in that: In step (5), wet etching involves immersing the bonded wafer in a TMAH solution at 90°C for 12 hours to completely remove the single-crystal silicon of the single-crystal silicon substrate layer (7) of the SOI wafer, and stopping the etching at the position of the buried oxide layer (6) of silicon dioxide.

7. The preparation method according to claim 6, characterized in that: In step (8), the reflectivity of the dielectric antireflective film (10) is greater than 90% in the wavelength band of the incident light.

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