Disclosure of Invention
The invention aims to provide a stepped FP sensor based on MEMS technology and a preparation method thereof, which realize the multi-resonant frequency of the sensor, have higher sensitivity and signal stability in each frequency response range and simultaneously reduce the production cost of the multi-resonant frequency sensor. The technical scheme adopted by the invention is as follows.
In one aspect, the invention provides a stepped FP sensor based on MEMS technology, comprising a diaphragm, a substrate, a capillary tube and an optical fiber;
The diaphragm comprises a device layer, one surface of the device layer is an induction input end, a plurality of steps with different heights are sequentially arranged in the upper middle area of the device layer, the other surface of the device layer is a plane, and a reflecting layer is arranged on the device layer;
The substrate is a cylinder, the axial center of the substrate is provided with a step-shaped through hole, the periphery of the diaphragm is fixedly connected with the periphery of one end face of the substrate, and the reflecting layer faces the substrate;
One end of the capillary tube is inserted into and fixed in the second fixing hole;
The optical fiber penetrates through and is fixed to the capillary tube, one end of the optical fiber is inserted into the first fixing hole and is fixed, the axial projection of the optical fiber towards the diaphragm is located in the central area of the diaphragm, and an optical transmission interval is arranged between the end part of the optical fiber and the reflecting layer.
In the technical scheme, a plurality of steps with different heights are arranged on the diaphragm to form a multi-resonance structure of the sensor, and a Fabry-Perot interference cavity is formed between the optical fiber and the diaphragm reflecting layer. The optical fiber emits incident light to the reflective layer and receives reflected light reflected by the fabry-perot interference cavity. When sound pressure acts on the surface of the diaphragm, the diaphragm deforms, and the optical fiber fixedly connected with the capillary tube does not displace, so that the cavity length of the FP cavity can be changed, and the ultrasonic sound pressure value can be calculated by utilizing the optical path difference of reflected light caused by the cavity length change of the Fabry-Perot interference cavity. In this process, since the thicknesses of the stepped regions of the diaphragm are different in the present invention, the respective maximum deformation amounts thereof also occur at different frequencies, and a plurality of resonance peaks occur.
Optionally, the area on the membrane sensing input end for setting up a plurality of steps is a circular area taking the center of the membrane as the centre of a circle, the diameter of the capillary is adapted with the diameter of this circular area, makes the circular area projection in the capillary inside. In the embodiment, the FP cavity can better sense the deformation of different resonance areas of the diaphragm caused by external input signals, and the plurality of resonance areas are ensured to have better sensitivity.
Optionally, on the membrane induction input end, the ratio of the area of the circular area where the steps are located to the total area of the membrane and the thickness value of the membrane corresponding to the step areas are designed according to the mutual matching of the resonance peak values required by the step areas:
The larger the required resonance peak, the smaller the area of the circular region where the steps are located, and the larger the step thickness. According to the rule, the FP sensor can have a wider frequency response range due to the reasonable matching of the sensitive area and the step height, and has higher sensitivity in the frequency response range corresponding to each resonance area.
Optionally, on the diaphragm sensing input end, the surface height of the step with the largest height value in the steps is equal to or smaller than the surface height of the peripheral area of the diaphragm. The design considers the convenience of the diaphragm etching process on one hand and realizes a multi-resonance structure with higher sensitivity in the central area of the diaphragm on the other hand.
Optionally, the number of steps on the sensing input end of the diaphragm is designed according to the number of required resonance peaks, and the more the required resonance peaks, the more the number of steps.
Optionally, the width of each step on the sensing input end of the diaphragm is designed according to the sensitivity required by the corresponding frequency range, and for the frequency range corresponding to any step area, if the required sensitivity is larger, the width of the step area is wider.
Optionally, on the membrane induction input end, the width of each step is equal.
Optionally, the heights of the steps from one side of the diaphragm to the other side of the diaphragm increase or decrease sequentially on the diaphragm sensing input.
Further, the heights of the steps from one side of the diaphragm to the other side of the diaphragm on the diaphragm sensing input end are sequentially increased or decreased by the same value.
The design can reduce the calculation complexity in the sensor design and the practical application, and is convenient for the simulation analysis of the sensor design stage.
In addition to the design, the method is used as another embodiment of the multiple steps of the induction input end of the diaphragm, wherein the heights of the multiple steps are symmetrically distributed from one side of the diaphragm to the other side of the diaphragm.
Preferably, the height of the steps on the two sides of the step is sequentially reduced or increased.
Optionally, the membrane further comprises a supporting layer and a connecting layer, the supporting layer and the connecting layer are annular, are connected to the periphery of the reflecting end of the device layer and are projected to the periphery of the step areas, the upper surface and the lower surface of the connecting layer are respectively and fixedly connected with the supporting layer and the device layer, and the end face of one end of the substrate is fixedly connected with the supporting layer.
Optionally, the optical fiber adopts a single-mode optical fiber, and the reflecting layer is made of gold material sputtered on the surface of the reflecting end of the membrane by vacuum ions. Other metal materials with high reflection performance can be used as the material of the reflecting layer.
In the technical scheme, the supporting layer can realize the mechanical supporting function on the membrane device layer, and an FP cavity is formed between the membrane and the optical fiber.
In a second aspect, the present invention provides a method for preparing the stepped FP sensor based on MEMS technology according to the first aspect, including preparing the diaphragm, where a preparation process of the diaphragm includes:
Coating a photoresist layer on the cleaned SOI wafer and baking;
Aligning the SOI wafer coated with the photoresist layer and baked with a mask plate, then placing the SOI wafer into an exposure machine for exposure and development treatment, and then performing post-baking treatment on the SOI wafer subjected to exposure and development treatment, wherein the mask plate comprises an upper mask plate and a lower mask plate which are respectively used for etching a silicon substrate layer and a silicon substrate layer of the SOI wafer, the pattern on the upper mask plate corresponds to the surface shape of the sensing input end of the diaphragm, and the pattern on the lower mask plate corresponds to the shape of the area to be etched of the reflecting end of the diaphragm;
Etching the exposed area of the silicon substrate layer of the SOI wafer by adopting a deep reactive ion etching mode until the silicon oxide layer is exposed, continuing to etch and clean the silicon oxide layer, and performing metal coating treatment on the exposed area after the silicon oxide layer is cleaned to obtain the reflecting layer, etching the silicon substrate layer of the SOI wafer by adopting a plurality of times of deep reactive ion etching processes, and etching the silicon substrate layer of the SOI wafer layer by layer according to different heights of a plurality of steps until the step areas are obtained, thereby completing the preparation of the membrane.
The layer-by-layer etching may be shallow-to-deep layer-by-layer etching or deep-to-shallow layer-by-layer etching.
As an embodiment of etching operation, optionally, the number of the upper mask plates is multiple, and the upper mask plates are respectively manufactured according to the shapes of the multiple step areas;
Etching the silicon substrate layer of the SOI wafer by adopting a plurality of deep reactive ion etching processes, and etching layer by layer according to different heights of a plurality of steps until a plurality of step areas are obtained, wherein the method comprises the following steps:
dividing the etching process of the silicon substrate layer into a plurality of etching operation flows with a plurality of height grades according to the difference of the heights of the steps;
Sequentially utilizing corresponding mask plates according to the sequence of the heights of the step surfaces from high to low or from low to high, and executing etching operation flows of each height level on the silicon substrate layer of the SOI crystal to sequentially obtain a plurality of step areas;
And for each height grade, coating photoresist on the surface of the current silicon substrate layer, transferring a pattern to be etched to the photoresist layer by using a mask plate corresponding to the current height grade, controlling etching depth according to the surface height difference between the step area to be etched and the adjacent surface height step area, and completing the etching of the current height grade to obtain a corresponding step area.
Optionally, the preparation method further comprises the steps of assembling the sensor, and specifically comprises the following steps:
cutting optical fibers with required lengths, removing coating layers of the optical fibers and cleaning bare fibers;
Etching to form a step-shaped through hole in the substrate;
penetrating the capillary into a second fixing hole of the step-type through hole of the substrate, and fixing the capillary and the substrate through UV glue;
penetrating an optical fiber into the capillary, penetrating the optical fiber into a first fixing hole of the step-shaped through hole from the capillary until the end part of the optical fiber slightly protrudes out of the first fixing hole, and fixing the optical fiber, the capillary and the first fixing hole through UV glue;
Performing laser cutting and cleaning treatment on the end part of the optical fiber protruding out of the first fixing hole to enable the plane of the end part of the optical fiber to be flush with the surface of the substrate;
and the prepared reflective end of the diaphragm faces the substrate and is fixedly connected with the substrate through UV glue, so that the centers of areas where a plurality of steps are positioned on the diaphragm are positioned on the axial extension line of the optical fiber, and the stepped FP sensor based on the MEMS technology is obtained.
Advantageous effects
The stepped FP sensor based on the MEMS technology has the advantages that the stepped structure is introduced to realize the multiband measurement performance of the optical fiber sensor by gradually changing the height or thickness of the sensing input end of the diaphragm, the working frequency range of the sensor is widened, the limitation that the traditional sensor only depends on a single resonant frequency is avoided, meanwhile, the overall sensitivity of the sensor and the sensitivity in each frequency range are improved, and the flexibility, the accuracy and the response speed of the sensor and the application capability in complex environments are greatly improved. The method can be widely applied to the fields of machinery, optics and sensors, and promotes the development of the optical fiber sensor technology in more diversified application scenes.
In addition, the structural design of the invention not only improves the performance of the sensor, but also simplifies the manufacturing process and reduces the cost. The preparation method is used for designing an etching operation flow according to the structural characteristics of the diaphragm, and realizing multi-level step area layer-by-layer etching by matching with different mask plates, so that the realization of the characteristics of high sensitivity and wide frequency response range of the sensor can be ensured. Meanwhile, the nested protection design of the substrate, the optical fiber and the capillary tube and the like ensure the center positioning precision and assembly of the sensor diaphragm by the preparation method.
Detailed Description
Further description is provided below in connection with the drawings and the specific embodiments. The examples are presented as merely exemplary descriptions of the invention and are not to be construed as limiting the invention. In the absence of conflict, the various embodiments of the present invention and features thereof may be combined with one another.
Example 1
The embodiment introduces a stepped structure Fabry-Perot sensor based on MEMS technology, and referring to FIG. 1, the sensor comprises a diaphragm 1, a substrate 2, a capillary tube 3 and an optical fiber 4;
Referring to fig. 1 to 5, the membrane 1 includes a device layer 11, which may be formed by processing a silicon substrate layer of SOI crystal, wherein one surface of the device layer 11 is an inductive input end, and a middle area of the inductive input end is provided with a plurality of steps with different heights, which are sequentially arranged;
the substrate 2 is a column, the axial center of the substrate 2 is provided with a step-shaped through hole, the periphery of the diaphragm 1 is fixedly connected with the periphery of one end face of the substrate 2, and the reflecting layer 14 faces the substrate;
One end of the capillary tube 3 is inserted into and fixed in the second fixing hole;
The optical fiber 4 penetrates through and is fixed on the capillary tube 3, one end of the optical fiber is inserted into the first fixing hole and is fixed, the axial projection of the optical fiber towards the diaphragm is located in the central area of the diaphragm, and an optical transmission interval is arranged between the end of the optical fiber and the reflecting layer.
In this embodiment, the diaphragm is provided with a plurality of steps with different heights, so as to form a multi-resonant Fabry-Perot structure of the sensor. The substrate is used for installing and positioning the capillary tube and the optical fiber, the capillary tube is used for protecting and positioning the optical fiber penetrating into the capillary tube, the alignment of the optical fiber and the central area of the diaphragm is ensured, and an accurate optical fiber interferometry path is realized. A Fabry-Perot interference cavity is formed between the end face of the optical fiber and the surface of the reflecting layer in the center of the diaphragm and is used for detecting acoustic signals.
When sound pressure acts on the surface of the diaphragm, the diaphragm can deform, and as the thicknesses of the resonance areas corresponding to the step areas of the diaphragm are different, the maximum deformation of the resonance areas also occurs at different frequencies, a plurality of resonance peaks can appear in the frequency response of the diaphragm, and the stepped induction input ends formed by the steps can effectively enhance the optical signal detection capability of the sensor, so that the whole constant sensor has higher sensitivity.
When detecting, when the optical fiber emits incident light, the cavity length of the Fabry-Perot interference cavity changes along with the deformation of the diaphragm, the change of the cavity length causes the change of the optical path difference of the reflected light, and the sound pressure values of different frequencies can be calculated by detecting the optical path difference of the reflected light of the interference cavity, so that the high-precision measurement of the multi-band acoustic signals is realized.
In the step structure of the diaphragm sensing input end, the number and the width of steps can be adjusted according to the needs, and the adjusting rules comprise:
If the number of the resonance peaks needed by the sensor is larger, the number of steps is larger;
for the frequency range corresponding to any step region, the larger the required sensitivity is, the wider the step region is.
Example 2
Based on embodiment 1, as shown in fig. 2 to 4, in the step-type structural fabry-perot sensor of this embodiment, the area on the sensing input end of the diaphragm for setting a plurality of steps is a circular area taking the center of the diaphragm as the center of a circle, namely, a step-like resonance sensing area 5, the diameter of the capillary tube is adapted to the diameter of the circular area, so that the circular area projects inside the capillary tube, the FP cavity can better sense the deformation of the resonance areas corresponding to different steps of the diaphragm caused by external input signals, and the plurality of resonance areas are ensured to have better sensitivity respectively.
The ratio of the area of the circular area where the steps are positioned to the total area of the diaphragm and the thickness value of the diaphragm corresponding to the step areas can be designed according to the mutual matching of the resonance peak values required by the step areas, and specifically, if the required resonance peak value is larger, the area of the circular area where the steps are positioned is smaller, and the thickness of the steps is larger. According to the rule, the FP sensor can have a wider frequency response range due to the reasonable matching of the sensitive area and the step height, and has higher sensitivity in the frequency response range corresponding to each resonance area.
In this embodiment, as shown in fig. 3 and 4, the surface height of the step with the largest height value among the steps on the diaphragm sensing input end is equal to or smaller than the height of the peripheral area of the diaphragm. The design considers the convenience of the diaphragm etching process on one hand and realizes a multi-resonance structure with higher sensitivity in the central area of the diaphragm on the other hand.
The number, width and height of steps on the membrane can be designed according to the design rules described in embodiment 1 and the practical application needs, for example, the steps are as follows:
the width of each step on the sensing input end of the diaphragm is equal or unequal;
Or the heights of a plurality of steps from one side to the other side of the diaphragm are sequentially increased or decreased, or are arranged at intervals as required;
Or the heights of the steps are symmetrically distributed from one side to the other side of the diaphragm, for example, the height of the steps is designed to be the highest or the lowest on one diameter of a circular area where the steps are positioned, and the heights of the steps at two sides of the steps are sequentially reduced or increased.
Referring to fig. 2, in this embodiment, the sensing input end of the diaphragm has 4 steps, and the steps are sequentially arranged according to the height, the height difference of any two adjacent steps is equal, and the widths of all the steps are also equal.
In addition, the shape of the area where the steps are located can be square and other shapes, but the shape is more friendly to the consistency of deformation response of the multi-resonance area, and the simulation and the logic operation during detection are facilitated. The number of the plurality of resonance areas can be adjusted according to the requirement so as to adapt to the detection requirements of scenes in different frequency ranges.
Example 3
In order to facilitate the assembly of the whole step-type structural Fabry-Perot sensor, the whole structure of the optical fiber sensor is more stable, an FP interference cavity is realized, and the diaphragm has better resonance response performance. The membrane as a whole may be designed in a circular or other shape.
Specifically, based on embodiment 1 or embodiment 2, referring to fig. 1, 3 and 4, the step-type fabry-perot sensor of this embodiment includes a membrane, a substrate, a capillary tube and an optical fiber, where the membrane includes a device layer 11, a connection layer 12 and a support layer 13, the support layer and the connection layer are annular, connected to a periphery of a reflection end of the device layer and projected to a periphery of a step area, an upper surface and a lower surface of the connection layer are respectively and fixedly connected to the support layer and the device layer, and an end surface of one end of the substrate is fixedly connected to the support layer. The support layer can realize mechanical support function to the membrane device layer and form an FP cavity between the membrane and the optical fiber.
The whole membrane 1 in this embodiment can be made based on SOI crystals, the device layer, the connection layer and the support layer corresponding to the silicon substrate layer, the silicon oxide layer and the silicon substrate layer of the SOI crystal, respectively.
The optical fiber adopts a single-mode optical fiber, and the reflecting layer is made of gold material sputtered on the surface of the reflecting end of the diaphragm by vacuum ions. Other metal materials with high reflection performance can be used as the material of the reflecting layer.
Example 4
Referring to fig. 4, this embodiment describes a method for manufacturing a fabry-perot sensor with a ladder structure according to embodiment 1 to embodiment 3, where the method includes manufacturing the membrane, and a manufacturing process of the membrane includes:
Coating a photoresist layer on the cleaned SOI wafer and baking;
Aligning the SOI wafer coated with the photoresist layer and baked with a mask plate, then placing the SOI wafer into an exposure machine for exposure and development treatment, and then performing post-baking treatment on the SOI wafer subjected to exposure and development treatment, wherein the mask plate comprises an upper mask plate and a lower mask plate which are respectively used for etching a silicon substrate layer and a silicon substrate layer of the SOI wafer, the pattern on the upper mask plate corresponds to the surface shape of the sensing input end of the diaphragm, and the pattern on the lower mask plate corresponds to the shape of the area to be etched of the reflecting end of the diaphragm;
and etching the silicon substrate layer of the SOI wafer by adopting a multi-time deep reactive ion etching process, and etching layer by layer according to different heights of a plurality of steps until the step areas are obtained, thereby completing the preparation of the membrane.
Because a plurality of step areas with different thicknesses are needed to be prepared on the silicon substrate layer, the number of the upper mask plates needed during etching the silicon substrate layer of the SOI wafer is suitable for the number of steps of the step structure, and each mask plate is designed according to the width and the shape of different steps.
The layer-by-layer etching may be a deep-to-shallow layer-by-layer etching or a shallow-to-deep layer-by-layer etching. Specific:
Etching the exposed area of the silicon substrate layer of the SOI wafer by adopting a plurality of times of deep reactive ion etching processes, and etching the exposed area layer by layer according to different heights of a plurality of steps until the step areas are obtained, wherein the method comprises the following steps:
dividing the etching process of the silicon substrate layer into a plurality of etching operation flows with a plurality of height grades according to the difference of the heights of the steps;
Sequentially utilizing corresponding mask plates according to the sequence of the heights of the step surfaces from high to low or from low to high, and executing etching operation flows of each height level on the silicon substrate layer of the SOI crystal to sequentially obtain a plurality of step areas;
And for each height grade, coating photoresist on the surface of the current silicon substrate layer, transferring a pattern to be etched to the photoresist layer by using a mask plate corresponding to the current height grade, controlling etching depth according to the surface height difference between the step area to be etched and the adjacent surface height step area, and completing the etching of the current height grade to obtain a corresponding step area.
After the preparation of the membrane, the membrane was assembled with the substrate, the capillary tube and the optical fiber according to the sensor structures described in examples 1 to 3, to obtain the stepped structure FP sensor.
Example 5
Based on example 4, taking the step-structure fabry-perot sensor shown in fig. 1 as a preparation target, referring to fig. 4, the preparation process of this example includes the following steps:
S1, selecting or preparing SOI crystals with proper area size and shape, such as a circular SOI wafer substrate shown in the working procedure (a) in FIG. 4;
S2, manufacturing mask plates for step etching with different thicknesses;
S3, as in the step (a) in fig. 4, cleaning the SOI crystal, removing surface dirt and ensuring that the surface of the wafer is free of any impurities. In this process, the wafer is typically rinsed with deionized water and chemical solutions, after which it is placed in an oven for drying. Then placing the SOI wafer on a spin coater, uniformly coating a photoresist layer on the silicon substrate, pre-baking the photoresist layer at 90-100 ℃ for 1-2 minutes, removing the solvent in the photoresist, and enhancing the stability of the photoresist in the exposure process;
s4, accurately aligning the silicon substrate layer of the wafer covered with the photoresist with the lower mask plate, then placing the wafer into an exposure machine, irradiating the transparent area of the mask plate by ultraviolet light to enable the surface of the photoresist to generate photochemical reaction so as to form projection of a mask plate pattern, removing the reacted photoresist area by using a developing solution after exposure, reserving the pattern structure of an unexposed area, and then performing post baking to enhance the adhesiveness and the tolerance of the developed photoresist layer so as to ensure that the required pattern area is effectively protected in the deep etching process;
S5, as in the process (b) in FIG. 4, for the silicon substrate layer of the SOI wafer, adopting a deep reactive ion etching DRIE technology to gradually etch until the silicon oxide layer is exposed, continuing to etch and clean the silicon oxide layer as in the process (c) in FIG. 4, and after the silicon oxide layer is cleaned, accurately transferring the pattern structure required by the reflecting end of the sensor onto the wafer to form a target microstructure of the reflecting end of the diaphragm;
S6, as shown in the process (d) in FIG. 4, carrying out fine processing on the silicon substrate layer of the SOI crystal by adopting the multi-time deep reactive ion etching DRIE process described in the embodiment 4, and realizing the accurate structure of areas with different thicknesses through step-by-step photoetching and etching, for example, firstly, coating photoresist on the surface of the silicon substrate layer and carrying out photoetching by using an upper mask pattern to define etching areas of each layer, then sequentially forming peripheral areas with decreasing thickness by controlling etching depth, and after each layer of etching is finished, replacing the upper mask and carrying out photoetching again to define the etching depth of the next layer. The step structure is positioned in the central area of the device layer and provided with a plurality of steps;
S7, as in the step (e) in FIG. 4, the bottom surface of the silicon substrate layer, namely the surface of the silicon substrate layer with the exposed reflective end of the diaphragm is subjected to metal coating treatment by adopting a vacuum sputtering technology. Sputtering a metal target in a vacuum environment to uniformly deposit gold atoms on the surface of the membrane to form a high-reflection gold membrane layer, so that the precise processing of the membrane with the asymmetric multi-resonance structure is realized;
S8, cutting the optical fiber with the required length, removing the coating layer, and lightly wiping the optical fiber by using the dust-free paper stained with alcohol to thoroughly remove the coating residues;
penetrating the capillary into a second fixing hole of the step-type through hole of the substrate, and fixing the capillary and the substrate through UV glue;
The method comprises the steps of penetrating a bare fiber into a capillary tube, penetrating the bare fiber into a first fixing hole of the step-shaped through hole from the capillary tube until the end part of the optical fiber slightly protrudes out of the first fixing hole, fixing the optical fiber between the capillary tube and the first fixing hole through UV glue, cutting and leveling the end surface of the optical fiber by using a precise cutting tool, ensuring smooth and defect-free, providing a good optical interface for subsequent assembly, ensuring that the end surface of the optical fiber is flush with the surface of a substrate, and finishing the primary fixing of the optical fiber, the capillary tube and the substrate;
S9, utilizing the three-dimensional adjusting frame and the microscope to assist, ensuring accurate alignment of all parts so as to realize an effective acoustic conduction path, forming a Fabry-Perot interference cavity between the end face of the single-mode fiber and the gold-plated surface of the diaphragm, attaching the diaphragm to the end face of the substrate, and enabling the reflecting end of the diaphragm to face the substrate. And (3) irradiating for 10 minutes at 360 degrees by using an ultraviolet curing lamp, so that the bonding and fixing of the diaphragm and the substrate are completed, and the FP sensor with the stepped structure is obtained.
As shown in fig. 5, experiments prove that the stepped structure Fabry-Perot sensor manufactured according to the invention can effectively improve the frequency response capability and the sensitivity of signal detection. Particularly in the scene that a plurality of frequency band signals need to be detected simultaneously, the sensor can provide more flexible and high-precision performance, and the limitation that the traditional optical fiber sensor only depends on a single resonant frequency is overcome.
The embodiments of the present invention have been described above with reference to the accompanying drawings, but the present invention is not limited to the above-described embodiments, which are merely illustrative and not restrictive, and many forms may be made by those having ordinary skill in the art without departing from the spirit of the present invention and the scope of the claims, which are all within the protection of the present invention.