SAW surface acoustic gas sensor based on MEMS and preparation method thereof

Abstract

The invention belongs to the technical field of sensors, and particularly relates to an SAW (surface acoustic wave) based MEMS (micro-electromechanical system) surface acoustic gas sensor and a preparation method thereof, the SAW based MEMS surface acoustic gas sensor comprises a piezoelectric substrate, two reflecting gratings are arranged on the piezoelectric substrate, a resonant cavity area is formed between the two reflecting gratings, and an IDT (interdigital transducer) and a sensitive film are arranged in the resonant cavity area. According to the gas sensor disclosed by the invention, through the piezoelectric substrate, the IDT structure and the sensitive film, effective detection and identification of environmental gas components can be realized on a micro level, so that the gas sensor has higher precision, quicker response and stronger stability, and the sensitivity and accuracy of gas sensing are remarkably improved.

Description

Technical Field

[0001] The present application belongs to the field of sensor technology, and in particular, relates to a SAW-based MEMS surface acoustic gas sensor and a preparation method thereof. Background Art

[0002] In the development of modern science and technology, Micro-Electro-Mechanical Systems (MEMS) technology has broad application prospects in the field of sensor manufacturing due to its miniaturization and integration. Surface Acoustic Wave (SAW) technology, as an important physical effect, has significant advantages in acoustic signal processing, filter design and sensor development.

[0003] Although traditional gas sensors can detect gas components to a certain extent, they still need to be improved in terms of sensitivity, response speed and accuracy. At the same time, the selectivity for specific gases and the ability to work stably for a long time are also important challenges facing current gas sensing technology. Summary of the invention

[0004] The present application provides a SAW-based MEMS surface acoustic gas sensor and a preparation method thereof, which solves the problem of low sensitivity and accuracy of existing gas sensors.

[0005] On the one hand, an embodiment of the present application provides a SAW-based MEMS surface acoustic gas sensor, including a piezoelectric substrate, on which two reflection gratings are arranged, a resonant cavity area is formed between the two reflection gratings, and an interdigital transducer IDT and a sensitive film are arranged in the resonant cavity area.

[0006] Optionally, the interdigital transducer IDT comprises two interdigital electrodes, namely an input interdigital electrode and an output interdigital electrode;

[0007] The input interdigital electrodes are used to excite surface acoustic waves propagating along the surface of the piezoelectric substrate, and the output interdigital electrodes are used to receive the surface acoustic waves propagating along the surface of the piezoelectric substrate excited by the input interdigital electrodes and generate high-frequency alternating voltage signal output.

[0008] Optionally, both the input interdigital electrodes and the output interdigital electrodes are uniform interdigitated electrodes.

[0009] Optionally, the sensitive membrane is prepared from a metal-organic framework-derived metal oxide material.

[0010] On the other hand, the present invention also provides a method for preparing a SAW-based MEMS surface acoustic gas sensor, comprising: a method for manufacturing a sensitive film is:

[0011] Step 21, pre-depositing a metal layer on the surface acoustic wave propagation area of ​​the piezoelectric substrate 1 as a substrate for the sensitive film;

[0012] Step 22, synthesizing metal organic framework particles by a solvothermal method, and calcining the metal organic framework particles in steps of increasing the temperature to obtain a metal organic framework-derived metal oxide material;

[0013] Step 23, ultrasonically dispersing the metal organic framework-derived metal oxide material in a volatile organic solvent to obtain a dispersion;

[0014] Step 24, using electrostatic atomization or spraying to spray the dispersion onto the metal layer in the surface acoustic wave propagation region on the surface of the piezoelectric substrate 1, and obtaining a sensitive film after the dispersion evaporates.

[0015] Optionally, in step 24, the step of depositing a sensitive film on the surface acoustic wave propagation region on the surface of the piezoelectric substrate by a spray method is:

[0016] Step 241, placing the piezoelectric substrate on a heating plate, and using a mask to cover the input interdigital electrodes 31 and the output interdigital electrodes 32 on the surface of the piezoelectric substrate, leaving the area on the surface of the piezoelectric substrate where the sensitive film 4 is pre-deposited;

[0017] Step 242, using a spray gun to spray a pre-prepared dispersion containing a metal organic framework-derived metal oxide material onto the area on the surface of the piezoelectric substrate where the sensitive film 4 is pre-deposited, heating the dispersion sprayed onto the area on the surface of the piezoelectric substrate 1 where the sensitive film 4 is pre-deposited, and volatilizing the solvent in the dispersion to obtain the sensitive film 4.

[0018] Optionally, in step 22, the metal organic framework particles are Cu-doped zeolite imidazolate framework material particles, and the zeolite imidazolate framework material is a porous crystalline material self-assembled by coordination of zinc ions and 2-methylimidazole.

[0019] Optionally, the specific process of synthesizing Cu-doped zeolite imidazolate framework material particles by solvothermal method is as follows: dissolve appropriate amounts of hydrated copper nitrate, hydrated zinc nitrate and 2-methylimidazole in an appropriate amount of methanol solution in a molar ratio of 1:3, mix the two, wash them repeatedly with methanol twice and vacuum dry them to obtain Cu-doped zeolite imidazolate framework material particles.

[0020] Beneficial effects of the present invention: The gas sensor of the present application, through the piezoelectric substrate, the interdigital transducer IDT structure and the sensitive film, can effectively detect and identify the components of the ambient gas at the microscopic level, so that the gas sensor has higher accuracy, faster response and stronger stability, and significantly improves the sensitivity and accuracy of gas sensing. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0022] Figure 1 This is a schematic diagram of the internal structure of the gas sensor of this application;

[0023] Figure 2 for Figure 1 Sectional view of AA in the middle;

[0024] Figure 3 A three-dimensional diagram of the gas sensor of the present application;

[0025] Figure 4 A top view of the gas sensor of the present application;

[0026] Figure 5 for Figure 4 Cross-sectional view of BB.

[0027] Explanation of symbols in the figure:

[0028] 1-piezoelectric substrate; 2-reflection grid; 21-first reflection grid; 22-second reflection grid; 3-interdigital transducer IDT; 31. input interdigital electrode; 32. output interdigital electrode; 4-sensitive film; 5-cover; 51-through hole. DETAILED DESCRIPTION

[0029] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0030] A SAW-based MEMS surface acoustic gas sensor provided in an embodiment of the present application is now described. Figure 1 and Figure 2 A SAW-based MEMS surface acoustic gas sensor includes a piezoelectric substrate 1, on which two reflection grids 2 are arranged, a resonant cavity region is formed between the two reflection grids 2, and an interdigital transducer IDT 3 and a sensitive film 4 are arranged in the resonant cavity region. A cover 5 is arranged on the piezoelectric substrate 1, and a through hole 51 is arranged on the cover 5. The cover 5 covers the two reflection grids 2, the interdigital transducer IDT 3 and the sensitive film 4. The piezoelectric substrate 1 is also provided with a plurality of pins, see Figures 3 to 5 .

[0031] Piezoelectric substrate 1, which is a quartz piezoelectric substrate;

[0032] Two reflection grids 2 form a resonant cavity area for reflecting surface acoustic waves;

[0033] Interdigital transducer IDT 3, used for acoustic-to-electric conversion;

[0034] The sensitive film 4 is used for gas detection.

[0035] See also Figure 1 and Figure 2 The two reflection grids 2 are respectively a first reflection grid 21 and a second reflection grid 22, and a resonance area is formed between the first reflection grid 21 and the second reflection grid 22; the setting of the first reflection grid 21 and the second reflection grid 22 can enhance the reflection effect of the surface acoustic wave, thereby increasing the action time of the sound wave on the sensitive film 4, and further improving the detection performance of the gas sensor.

[0036] The interdigital transducer IDT 3 includes two interdigital electrodes, namely an input interdigital electrode 31 and an output interdigital electrode 32. The input interdigital electrode 31 is used to excite the surface acoustic wave propagating along the surface of the piezoelectric substrate 1, and the output interdigital electrode 32 is used to receive the surface acoustic wave propagating along the surface of the piezoelectric substrate 1 excited by the input interdigital electrode 31 and generate a high-frequency alternating voltage signal output; the input interdigital electrode 31 and the output interdigital electrode 32 are both uniform interdigital electrodes;

[0037] A sensitive membrane 4 is arranged between the input interdigital electrode 31 and the output interdigital electrode 32. The sensitive membrane 4 includes a metal oxide material derived from a metal organic framework. When the sensitive membrane reacts with the target gas in the test environment, the physical properties of the sensitive membrane change, causing the propagation state of the surface acoustic wave to change, thereby realizing gas detection.

[0038] The input interdigital electrodes 31 are used to excite the surface acoustic wave propagating along the surface of the piezoelectric substrate 1, and the output interdigital electrodes 32 are used to receive the surface acoustic wave propagating along the surface of the piezoelectric substrate excited by the input interdigital electrodes 31 and generate a high-frequency alternating voltage signal output. After the surface acoustic wave propagating along the surface of the piezoelectric substrate reaches the output interdigital electrodes 32, the output interdigital electrodes 32 generate a high-frequency alternating voltage signal through the positive piezoelectric effect and output it;

[0039] The control system is used to input a first signal to the input interdigital electrode 31 and receive a second signal output by the output interdigital electrode 32. The second signal output by the output interdigital electrode 32 monitors the concentration of the target gas in the test environment to determine the concentration of the target gas in the test environment.

[0040] The sensitive film 4 is located between the input interdigital electrode 31 and the output interdigital electrode 32. The sensitive film 4 is located between the first reflective grating 21 and the second reflective grating 22. The sensitive film 4 is deposited on the surface of the piezoelectric substrate 1 in the surface acoustic wave propagation region. The sensitive film 4 includes a metal organic framework derived metal oxide material. When the sensitive film 4 reacts with the target gas in the test environment, the physical properties of the sensitive film 4 change.

[0041] The sensitive film 4 is located in the surface acoustic wave propagation area on the surface of the piezoelectric substrate 1. When the sensitive film 4 reacts with the target gas in the test environment, the physical properties of the sensitive film 4 (determined by the mass load effect, the conductivity load effect, and the elastic load effect) will change. Since the sensitive film 4 is located on the surface of the piezoelectric substrate 1, the change in the physical properties of the sensitive film 4 will affect the propagation state of the surface acoustic wave, which is reflected in the change in the speed and amplitude of the surface acoustic wave, thereby causing the voltage signal output by the output interdigital electrode 32 to change, and then the concentration of the target gas in the environment can be detected based on the voltage signal output by the output interdigital electrode 32. After the reaction, the physical properties of the sensitive film 4 are manifested as changes in mass, conductivity, Young's modulus, etc.

[0042] The gas sensor can effectively excite and receive surface acoustic waves propagating along the surface of the piezoelectric substrate, providing a basis for subsequent gas detection.

[0043] The preparation method of the interdigitated electrode is as follows:

[0044] Step 11, cleaning of piezoelectric substrate: Before processing, the piezoelectric substrate must be cleaned to ensure that the surface of the piezoelectric substrate is clean. Use acetone, isopropyl alcohol and deionized water to perform ultrasonic cleaning in sequence, then blow dry with a nitrogen gun, and finally put the piezoelectric substrate into a drying oven to ensure that the quartz surface is clean and dry.

[0045] Step 12, EBL lithography: electronic resist ZEP110 and negative photoresist are coated on the piezoelectric substrate, a layer of aluminum is covered as a conductive sacrificial layer by electron beam evaporation, and a photolithography mask is used to photolithographically obtain an interdigitated photoresist pattern of input interdigitated electrodes and output interdigitated electrodes with an inverted trapezoidal cross-section on the surface of the piezoelectric substrate.

[0046] Step 13, sputtering coating: using a magnetron sputtering method to sputter deposit a metal layer on the piezoelectric substrate.

[0047] Step 14, stripping: float away the conductive sacrificial layer, then develop it in a xylene developer, and fix it in isopropyl alcohol; put the piezoelectric substrate after EBL lithography and sputtering coating into a stripping solution for ultrasonic vibration washing, remove the photoresist and the metal layer deposited on the photoresist pattern, and obtain the interdigitated metal layer closely attached to the surface of the piezoelectric substrate, thereby stripping to obtain the input interdigitated electrode and the output interdigitated electrode.

[0048] In step 12, an electron beam is used to evaporate a metal, and the evaporated metal includes gold, chromium, and chromium. The piezoelectric substrate on which the metal has been evaporated is immersed in butanone, and the metal deposited on the piezoelectric substrate is retained, while the metal deposited on the electronic resist is detached from the piezoelectric substrate as it dissolves, thereby obtaining an interdigitated electrode. After the etching is completed, the piezoelectric substrate is taken out, washed with deionized water, blown dry with a nitrogen gun, and then placed in an oven for drying.

[0049] EBL lithography (electron beam lithography) uses a high-energy electron beam to expose the photoresist to obtain a structural pattern. Since its de Broglie wavelength is about 0.004 nanometers. The interdigital transducer IDT is a regular, periodically distributed dense pattern. Therefore, the geometric size adjustment method is used to optimize the size of the pattern to be exposed. It is simple and effective to obtain the ideal design size after development. Specifically, the EBL lithography method is: first, several geometric corrections are made in advance based on experience, and exposure is performed at different doses; then, the produced pattern is observed and measured by SEM, and the appropriate pattern correction value and exposure dose are selected.

[0050] The piezoelectric substrate selected for the piezoelectric substrate is an ST-X quartz substrate.

[0051] On the other hand, the present invention also provides a method for preparing a SAW-based MEMS surface acoustic gas sensor, comprising: a method for manufacturing a sensitive film 4 is as follows:

[0052] Step 21, pre-depositing a metal layer on the surface acoustic wave propagation area of ​​the piezoelectric substrate 1 as a substrate for the sensitive film;

[0053] Step 22, synthesizing metal organic framework particles by a solvothermal method, and calcining the metal organic framework particles in steps of increasing the temperature to obtain a metal organic framework-derived metal oxide material;

[0054] Step 23, ultrasonically dispersing the metal organic framework-derived metal oxide material in a volatile organic solvent to obtain a dispersion;

[0055] Step 24, spray the dispersion liquid on the metal layer in the surface acoustic wave propagation area on the surface of the piezoelectric substrate 1 in a direction, and obtain a sensitive film after the dispersion liquid evaporates.

[0056] The dispersion prepared in step 23 is extracted by a syringe, and the metal connector of the syringe is connected to the high voltage end of the high voltage power supply, and the metal layer in the surface acoustic wave propagation area of ​​the piezoelectric substrate 1 is connected to the ground end of the high voltage power supply. The dispersion flowing out of the syringe can form a stable Taylor cone jet phenomenon under the action of the electrostatic field, so that the dispersion will be sprayed and dragged by the electrostatic field force and atomized and deposited on the metal layer in the surface acoustic wave propagation area of ​​the piezoelectric substrate surface, and the sensitive film is obtained after the solvent in the dispersion on the metal layer evaporates, that is, the sensitive film 4 is formed by directional patterned deposition on the metal layer, and the sensitive film 4 can be thin, dense, flat and low in roughness.

[0057] Specifically, in step 21, the piezoelectric substrate 1 is placed on a heating plate, and a mask is used to cover the input interdigital electrodes 31 and the output interdigital electrodes 32 on the surface of the piezoelectric substrate 1, exposing the area on the surface of the piezoelectric substrate 1 where the sensitive film 4 is pre-deposited; a metal layer is pre-deposited in the surface acoustic wave propagation area on the surface of the piezoelectric substrate 1. The area where the metal layer is located surrounds the area where the sensitive film 4 is to be deposited. The metal layer can be a single metal layer or a composite metal layer, as long as the material and thickness of the deposited metal layer do not affect the device performance. The metal layer is pre-sputtered and deposited in the area on the surface of the piezoelectric substrate 1 where the sensitive film 4 is to be deposited.

[0058] In step 22, the metal organic framework particles are Cu-doped zeolite imidazolate framework material particles, and the zeolite imidazolate framework material is a porous crystalline material self-assembled by coordination of zinc ions and 2-methylimidazole. The specific process of synthesizing Cu-doped zeolite imidazolate framework material particles by solvothermal method is as follows: dissolving appropriate amounts of hydrated copper nitrate, hydrated zinc nitrate and 2-methylimidazole in an appropriate amount of methanol solution at a molar ratio of 1:3, mixing the two, repeatedly washing with methanol twice and vacuum drying to obtain Cu-doped zeolite imidazolate framework material particles.

[0059] In order to ensure that the subsequent Cu / zeolite imidazolate framework material-derived CuO / ZnO material has a small particle size and a small relative molecular mass, 2-methylimidazole is selected as the organic ligand.

[0060] The Cu-doped zeolite imidazolate framework material particles synthesized by the solvent thermal method have a dodecahedral structure. The prepared metal organic framework particles are placed in a preset container, the preset container is heated at a first preset heating rate, and after heating to a first preset temperature, the first preset temperature is maintained for a first preset time, and then the preset container is heated at a second preset heating rate, and after heating to a second preset temperature, the second preset temperature is maintained for a second preset time, and then the temperature is lowered to room temperature to obtain a metal organic framework-derived metal oxide material; the dried Cu-doped zeolite imidazolate framework material particles are placed in a muffle furnace, and then cooled to room temperature to obtain a Cu / zeolite imidazolate framework material-derived CuO / ZnO material with complete oxide conversion and good dispersibility. If the calcination temperature of the metal organic framework particles is too high or the heating rate is too fast, the product may agglomerate, which is not conducive to the subsequent preparation of the metal organic framework-derived metal oxide sensitive film. A step-by-step heating method is required to prepare a metal organic framework-derived metal oxide material with complete oxide conversion and good dispersibility. If the prepared metal organic framework derived metal oxide material is large in size and relative molecular mass, it will not be able to be ultrasonically dispersed in an organic solvent to form a stable nanoparticle dispersion and will settle quickly, and it will also not be able to be uniformly deposited using spraying and electrostatic atomization.

[0061] Further, in step 22, the preparation method of the metal organic framework derived metal oxide material is:

[0062] The metal organic framework derived metal oxide material is ultrasonically dispersed in a preset volatile organic solvent with low boiling point, low surface tension and low density to obtain a dispersion containing the metal organic framework derived metal oxide material.

[0063] In step 23, the dispersion is prepared by ultrasonically dispersing the metal organic framework-derived metal oxide material in a preset volatile organic solvent with low boiling point, low surface tension and low density to obtain a dispersion containing the metal organic framework-derived metal oxide material.

[0064] In step 24, the dispersion is sprayed onto the metal layer in a directionally manner by using an electrostatic atomization method or a spraying method.

[0065] The steps of preparing the sensitive film by electrostatic atomization method are as follows:

[0066] In one embodiment, the propagation loss caused by the sensitive film prepared by the electrostatic atomization method in the surface acoustic wave sensor is proportional to the deposition time of the sensitive film, wherein the attenuation amplitude caused by the thickness of the sensitive film 4 is analogous to the thickness of the sensitive film. The thickness of the metal organic framework derived metal oxide sensitive film prepared by the electrostatic atomization method is approximately linearly related to the deposition time of the sensitive film, especially in a shorter deposition time. Since the thickness of the sensitive film is thin, the thickness of the sensitive film can be controlled by controlling the deposition time of the sensitive film deposited by the electrostatic atomization method, thereby forming a light and thin sensitive film.

[0067] Since a metal layer is deposited in advance in the surface acoustic wave propagation area on the surface of the piezoelectric substrate, the final surface acoustic wave gas sensor also has a metal layer between the sensitive film and the piezoelectric substrate surface, but the metal layer does not affect the performance of the surface acoustic wave gas sensor.

[0068] The deposition is performed by electrostatic atomization, thereby ensuring the uniformity and stability of the sensitive film 4 and enhancing the adsorption capacity and reaction sensitivity to the target gas.

[0069] Furthermore, the steps of preparing the sensitive film by the spray method are:

[0070] Step 241, placing the piezoelectric substrate on a heating plate, and using a mask to cover the input interdigital electrodes 31 and the output interdigital electrodes 32 on the surface of the piezoelectric substrate, leaving the area on the surface of the piezoelectric substrate where the sensitive film 4 is pre-deposited;

[0071] Step 242, use a spray gun to spray the dispersion onto the area on the surface of the piezoelectric substrate where the sensitive film 4 is pre-deposited. Under the action of the heating plate, the solvent in the dispersion sprayed onto the area on the surface of the piezoelectric substrate 1 where the sensitive film 4 is pre-deposited evaporates to obtain the sensitive film 4.

[0072] Specifically, a fixed or mobile spray gun is used to intermittently spray a dispersion containing a metal organic framework-derived metal oxide material onto the area on the surface of the piezoelectric substrate 1 where the sensitive film is pre-deposited. The dispersion can be atomized into extremely fine droplets by adjusting the air pressure of the spray gun. The vertical distance between the nozzle of the spray gun and the mask can be between 8cm and 10cm. Due to the high temperature of the heating plate, the volatile organic solvent in the dispersion sprayed onto the area on the surface of the piezoelectric substrate where the sensitive film is pre-deposited evaporates, leaving metal organic framework-derived metal oxide particles to form a sensitive film. By changing the spraying duration of the spray gun, metal organic framework-derived metal oxide sensitive films of different thicknesses can be obtained.

[0073] The gas sensor of the present application, through the piezoelectric substrate, interdigital transducer IDT structure and sensitive film, can effectively detect and identify the components of environmental gas at the microscopic level, so that the gas sensor has higher precision, faster response and stronger stability, and significantly improves the sensitivity and accuracy of gas sensing.

[0074] In the description of the present application, it should be understood that terms such as "length", "width", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating orientation or positional relationships are based on the orientation or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0075] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0076] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A SAW-based MEMS surface acoustic gas sensor, comprising a piezoelectric substrate, characterized in that: Two reflection grids are arranged on the piezoelectric substrate, a resonant cavity region is formed between the two reflection grids, and an interdigital transducer IDT and a sensitive film are arranged in the resonant cavity region.

2. A SAW-based MEMS surface acoustic gas sensor as claimed in claim 1, characterized in that: The interdigital transducer IDT comprises two interdigital electrodes, namely an input interdigital electrode and an output interdigital electrode; The input interdigital electrodes are used to excite surface acoustic waves propagating along the surface of the piezoelectric substrate, and the output interdigital electrodes are used to receive the surface acoustic waves propagating along the surface of the piezoelectric substrate excited by the input interdigital electrodes and generate high-frequency alternating voltage signal output.

3. A SAW-based MEMS surface acoustic gas sensor as claimed in claim 2, characterized in that: The input interdigital electrodes and the output interdigital electrodes are both uniformly interdigitated.

4. A SAW-based MEMS surface acoustic gas sensor according to any one of claims 1 to 3, characterized in that: The sensitive film is prepared from a metal oxide material derived from a metal organic framework.

5. A method for preparing a SAW-based MEMS surface acoustic gas sensor as claimed in claim 1, characterized in that: include: The manufacturing method of the sensitive film is: Step 21, pre-depositing a metal layer on the surface acoustic wave propagation area of ​​the piezoelectric substrate 1 as a substrate for the sensitive film; Step 22, synthesizing metal organic framework particles by a solvothermal method, and calcining the metal organic framework particles in steps of increasing the temperature to obtain a metal organic framework-derived metal oxide material; Step 23, ultrasonically dispersing the metal organic framework-derived metal oxide material in a volatile organic solvent to obtain a dispersion; Step 24, using electrostatic atomization or spraying to spray the dispersion onto the metal layer in the surface acoustic wave propagation region on the surface of the piezoelectric substrate 1, and obtaining a sensitive film after the dispersion evaporates.

6. The method for preparing a SAW-based MEMS surface acoustic gas sensor according to claim 5, characterized in that: In step 24, the steps of depositing a sensitive film on the surface acoustic wave propagation region of the piezoelectric substrate surface by using a spray method are as follows: Step 241, placing the piezoelectric substrate on a heating plate, and using a mask to cover the input interdigital electrodes 31 and the output interdigital electrodes 32 on the surface of the piezoelectric substrate, leaving the area on the surface of the piezoelectric substrate where the sensitive film 4 is pre-deposited; Step 242, using a spray gun to spray a pre-prepared dispersion containing a metal organic framework-derived metal oxide material onto the area on the surface of the piezoelectric substrate where the sensitive film 4 is pre-deposited, heating the dispersion sprayed onto the area on the surface of the piezoelectric substrate 1 where the sensitive film 4 is pre-deposited, and volatilizing the solvent in the dispersion to obtain the sensitive film 4.

7. The method for preparing a SAW-based MEMS surface acoustic gas sensor according to claim 5, characterized in that: In step 22, the metal organic framework particles are Cu-doped zeolite imidazolate framework material particles, and the zeolite imidazolate framework material is a porous crystalline material self-assembled by coordination of zinc ions and 2-methylimidazole.

8. The method for preparing a SAW-based MEMS surface acoustic gas sensor according to claim 7, characterized in that: The specific process of synthesizing Cu-doped zeolite imidazolate framework material particles by solvothermal method is as follows: dissolve appropriate amounts of hydrated copper nitrate, hydrated zinc nitrate and 2-methylimidazole in an appropriate amount of methanol solution in a molar ratio of 1:3, mix the two, wash them repeatedly with methanol twice and vacuum dry them to obtain Cu-doped zeolite imidazolate framework material particles.

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