Stable heterojunction air quality sensor and preparation method thereof

Abstract

The invention provides a stable heterojunction air quality sensor and a preparation method thereof, belongs to the technical field of MEMS gas sensors, and aims to solve the technical problem of poor gas sensitivity of sensors. The air quality sensor comprises a cover plate, an MEMS chip and a base assembly, the cover plate is in sealed connection with the base assembly through a binder to form a sealed cavity; at least one air inlet hole is formed in the cover plate, so that the outside is communicated with the closed cavity, and the MEMS chip is fixed in the base assembly; the MEMS chip comprises a gas sensitive assembly and a heating assembly, and the gas sensitive assembly is arranged on the heating assembly; the gas-sensitive component comprises a detection electrode and a gas-sensitive material layer covering the detection electrode; and the raw material of the gas sensitive material layer is ZnSnO3 heterojunction. Compared with a traditional device, the air quality sensor has the advantages of high sensitivity, short response recovery time, short preheating time, airflow impact resistance and good repeatability in VOC gas detection.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of MEMS gas sensors, and particularly relates to a heterojunction air quality sensor. BACKGROUND

[0002] VOC or VOCs generally refers to volatile organic compounds (hereinafter referred to as VOC) of the same type. When the concentration of VOC in the atmosphere reaches a certain level, the stability of the atmospheric composition will change. Volatile organic pollutants (VOC) are the main factors causing air pollution and harm to human health in indoor environments, containing various toxic compounds and carcinogens. Long-term exposure of the human body to an atmosphere containing VOCs can cause diseases and even life-threatening. With the rapid development of heavy industry in our country, it has brought earth-shaking changes to our various lives, but at the same time, it has also caused great damage to our living environment. The environmental quality is getting worse and worse. In terms of atmospheric environmental pollution, the coal industry in our country alone emits up to 7 million tons of VOCs into the atmosphere every year, which has a great impact on the living environment of people and endangers human health. Therefore, it is very important to effectively monitor VOCs in indoor and outdoor air through gas sensors to control and reduce the content of VOCs in the air for the governance and pollution prevention of the atmospheric environment. For example, patent publication number CN117571792A, a methane gas sensor based on MEMS technology and its preparation method, belongs to the technical field of MEMS semiconductor gas sensing sensors. The gas sensor includes a substrate and a heating module and a gas sensitive module arranged opposite to the surface of the substrate. The heating module includes a silicon substrate, a support platform, and a heating electrode. A back cavity is etched on the upper part of the silicon substrate, and a heat insulation cavity is provided on the support platform. The gas sensitive module includes an isolation layer and a detection electrode, and a gas sensitive material is deposited on the detection electrode. The gas sensitive material is composed of a sensitive material and a catalytic protective material, containing semiconductor metal oxide, noble metal catalyst, and catalyst carrier, etc.

[0003] Among the numerous types of gas sensors, the resistance-type gas sensor with semiconductor oxide as the sensitive material has the advantages of high sensitivity, low detection limit, good selectivity, fast response and recovery speed, simple manufacturing method, and relatively low cost, etc. It is one of the most widely used gas sensors at present. With the development of nanoscience and technology, the adjustment of gas sensitive materials into nanostructure can greatly improve the specific surface area of the material, increase the active sites, and improve the gas sensitive characteristics.

[0004] ZnSnO3, as a semiconductor material with unique physical and chemical properties, is widely used in the preparation of sensitive materials for semiconductor gas sensors due to its good gas sensing potential. Currently, the research on building MEMS air quality sensors focuses on oxide nanomaterials. However, there are still obvious technical defects in existing air quality sensors based on ZnSnO3 materials: first, the response is low, making it difficult to accurately detect low-concentration VOCs and unable to meet the monitoring needs in complex environments; second, the response and recovery time is too long, and the real-time monitoring performance is insufficient; third, the preheating time is long, affecting the rapid deployment and use efficiency of the sensor. These defects seriously limit the further application and promotion of ZnSnO3-based gas sensors in the VOC monitoring field. Therefore, in view of the shortcomings of the existing technology, it is necessary to optimize the gas sensing properties of ZnSnO3 through material modification. SUMMARY

[0005] In view of the above technical problems, the present application provides an air quality sensor with stable heterojunction and a preparation method thereof, which uses a modification method of ZnSnO3 nanoheterojunction to improve the gas sensing performance of ZnSnO3 material, and then prepares a high-performance VOC detection gas sensor.

[0006] To achieve the above purpose, the technical scheme of the present application is as follows: An air quality sensor with stable heterojunction, comprising a cover plate, a MEMS chip and a base assembly; the cover plate is sealed and connected with the base assembly by an adhesive to form a closed cavity; the cover plate is provided with at least one air inlet hole for communication between the outside and the closed cavity, and the MEMS chip is fixed inside the base assembly; the MEMS chip comprises a gas sensing assembly and a heating assembly, and the gas sensing assembly is arranged on the heating assembly; the gas sensing assembly comprises a detection electrode and a gas sensing material layer covering the detection electrode; the raw material of the gas sensing material layer is ZnSnO3 heterojunction.

[0007] The air inlet hole is provided with three air inlet holes, which are distributed in a triangular shape above the MEMS chip.

[0008] Preferably, the three air inlet holes are arranged in a "pin" shape.

[0009] Preferably, the diameter of the air inlet hole is 0.5-2mm.

[0010] The diameter of the gas sensing material layer is 130-250μm, and the thickness is 2-12μm.

[0011] The heating assembly is a silicon-based micro-hotplate.

[0012] The MEMS chip is connected with the base assembly by a lead wire.

[0013] A preparation method of a stable heterojunction air quality sensor, comprising the following steps: (1) ZnSnO3 nano powder, solvent are mixed and ball milled to obtain a gas sensitive slurry; (2) The gas sensitive slurry is loaded onto a detection electrode in a MEMS chip, and sintering is performed; (3) The sintered MEMS chip is welded, packaged and aged.

[0014] The preparation method is simple, and the prepared MEMS chip has the advantages of small volume, low power consumption, low cost, easy integration, high sensitivity to VOC gas detection, short response recovery time, short preheating time and good repeatability.

[0015] The sintering temperature is 600-850 DEG C, and the time is 1-4h.

[0016] The solvent is any one or two or more of deionized water, methanol, ethanol and isopropanol; the ratio of ZnSnO3 nano powder to solvent is 0.5-10g:3mL. The solid content, viscosity and film forming property of the gas sensitive slurry affect the printing difficulty of the material and the sensing performance of the sensitive chip. For example, if the solid content is too high, the sensitive material layer on the detection electrode is prone to be too thick when spotting, which affects the contact reaction between the gas and the internal sensitive material and reduces the sensing performance. If the solid content is too low, the amount of sensitive material is too small, the reaction with the measured gas is reduced, and the sensing performance is also reduced. The viscosity affects the amount of sensing material attached to the detection electrode, which also causes the problem of the amount of sensing material on the detection electrode. The film forming property affects the porosity of the sensing material and the contact reaction between the material and the gas.

[0017] Specifically, in step (1), 0.5-10g ZnSnO3 nano powder is placed in a agate ball mill tank, ball milling beads are added, 3mL dispersion liquid is added, and ball milling is performed for 30-120min. The particle size D50 distribution of the material is 0.5-3.5um. In step (2), the gas sensitive slurry is loaded onto the detection electrode by screen printing. The aging test is single heating aging or heating-test dual-channel aging.

[0018] The gas sensitive slurry further comprises a catalyst, and the catalyst is any one or two or more of aluminum nitrate hydrate, lanthanum nitrate hydrate, metal platinum, chloroplatinic acid, metal palladium, chloropalladic acid, antimony oxide, acid-washed asbestos, aluminum oxide and silicon dioxide. The addition of the catalyst can make the sensor have higher sensitivity to VOC and shorter response recovery time.

[0019] The beneficial effects of the present application are: (1) The size parameter of the ZnSnO3 gas-sensitive layer is adjusted to a reasonable range, which effectively optimizes the temperature distribution uniformity of the gas-sensitive layer. The size parameter cooperates with the characteristics of ZnSnO3 material, which avoids the problems of uneven temperature distribution and prolonged response recovery time caused by too large gas-sensitive layer pattern, and avoids the defects of increased preparation difficulty and insufficient stability of sensing signal caused by too small pattern. At the same time, by controlling the calcination conditions, the ZnSnO3 material forms a stable heterojunction, fully utilizes the gas-sensitive potential of the material itself, significantly improves the detection sensitivity of the chip to VOC gas, and shortens the response recovery time and preheating time.

[0020] (2) The present application optimizes the specific number and arrangement of the cover plate, which forms a precise cooperation with the specific size ZnSnO3 gas-sensitive layer: the arrangement can avoid the direct alignment of the through hole and the sensor chip, effectively solve the problem of direct blowing of gas flow impact on the gas-sensitive layer, prevent the destruction of the adsorbed oxygen molecular layer and the electron depletion layer on the surface of the ZnSnO3 gas-sensitive layer, and avoid the risk of sensor signal fluctuation and performance decline; at the same time, the arrangement can accurately match the gas exchange demand of the specific size gas-sensitive layer, ensure the smooth and uniform contact of VOC gas molecules with the surface of the gas-sensitive layer, and realize the rapid diffusion and full adsorption of gas molecules with the help of the unique pore structure of ZnSnO3 material, so that the superposition effect of the inner and outer electron depletion layers is fully utilized, and the gas-sensitive response effect is further strengthened. Compared with the performance decline or airflow disturbance caused by other number of holes in the cover plate, the cooperative design of the hole structure and the gas-sensitive layer can balance the gas exchange efficiency and anti-interference ability, and improve the repeatability and long-term stability of the sensor.

[0021] (3) The present application optimizes the gas-sensitive material, the size of the gas-sensitive layer and the number of ventilation holes, forming a stable system of "material characteristics-structure size-environment adaptation". The heterojunction advantage of ZnSnO3 material provides core support for sensing performance, the specific size of the gas-sensitive layer provides structural guarantee for performance, and the specific number of the triangular ventilation hole provides environmental adaptation for performance stability. The three work together to make the chip have high sensitivity, fast response and recovery, short preheating time, excellent repeatability, simple preparation method, small size, low power consumption, low cost and easy integration, effectively solving the problem that sensing performance and preparation cost and integration cannot be balanced in the prior art, and widening the application scenarios of MEMS sensitive chip in VOC gas detection field. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the following embodiment or prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creating any creative labor.

[0023] Figure 1 For Figure 3 Structure diagram of the stable heterojunction air quality sensor of the present application; In the figure, 101, cover plate, 102, adhesive; 103, adhesive; 104, gas sensitive material layer; 105, MEMS chip; 106, base assembly.

[0024] Figure 2 Typical sensitivity characteristic curve of the sensor.

[0025] Figure 3 Typical temperature and humidity characteristic curve of the sensor, Figure 4 Response recovery curve of the sensor.

[0026] Figure 5 Linear characteristic curve of the sensor.

[0027] Figure 6 Repeatability curve of the sensor.

[0028] Figure 7 Repeatability curve of the sensor to hydrogen gas prepared by Example 1 and comparative examples. DETAILED DESCRIPTION

[0029] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0030] A stable heterojunction air quality sensor, the structure is as shown in Figure 1 The stable heterojunction air quality sensor includes a cover plate 101, a MEMS chip 105 and a base assembly 106. The cover plate 101 is sealingly connected with the base assembly 106 through adhesives 103 and 102 to form a closed cavity. The cover plate 101 is provided with at least one air inlet hole to enable the outside to communicate with the closed cavity. The MEMS chip 105 is fixed inside the base assembly 106. The MEMS chip 105 includes a gas sensitive assembly and a heating assembly, and the gas sensitive assembly is arranged on the heating assembly. The gas sensitive assembly includes a detection electrode and a gas sensitive material layer 104 covering the detection electrode. The raw material of the gas sensitive material layer 104 is ZnSnO3 heterojunction.

[0031] The sensing mechanism of the ZnSnO3 heterojunction to VOC gas can be explained by the adsorption mechanism of gas molecules and the structure of the material. When ZnSnO3 is exposed to air, oxygen molecules in the air will be adsorbed on the surface of the material to form adsorbed oxygen molecules. During gradual heating, the adsorbed oxygen molecules will capture more electrons from the conduction band of the material, which forms an electron depletion layer on the surface of the material, causing the resistance to increase. When the material is placed in a VOC gas atmosphere, the VOC gas molecules undergo a redox reaction with the adsorbed oxygen ions, and the captured electrons will be released back into the conduction band of the material, causing the electron depletion layer to shrink, thereby reducing the resistance. In addition, the unique pore structure of the material is beneficial for the diffusion and adsorption of gas molecules. Oxygen molecules can enter the interior of the material through the pores on the surface of ZnSnO3, forming another electron depletion layer. The superposition of the inner and outer electron depletion layers makes ZnSnO3 have excellent gas sensing performance.

[0032] Specifically, the sensor of the embodiment preferably adopts a three-hole cover plate 101, and the three holes are arranged in a triangular shape. This arrangement can avoid the through hole directly facing the sensor chip, which can not only realize gas exchange during the use of the sensor and ensure the sensitivity, response time and recovery time of the sensor, but also solve the problem of signal fluctuation and performance decline caused by direct airflow impact on the sensor chip during use. Comparative tests show that when a 1-hole cover plate 101 is used, the performance of the sensor will decrease significantly; when a 5-hole or 9-hole cover plate 101 is used, the sensor signal will be disturbed by airflow to varying degrees, and cannot meet the optimal use requirements.

[0033] The detection electrode is completely covered by the gas sensitive material layer 104, and the gas sensitive material layer 104 needs to be designed in patterns if necessary to accurately control its diameter size. The specific parameter control of the gas sensitive material layer 104 is as follows: the diameter is controlled to be 130-250 μm, the film thickness is controlled to be 2-12 μm, and the film thickness deviation is controlled to be 1-4 μm. If the pattern of the gas sensitive material layer 104 is too large, it will cause uneven temperature distribution of the material, thereby prolonging the response recovery time of the sensor; if the pattern is too small, although it can make the material temperature distribution more uniform and the response recovery time of the sensor shorter, it needs to be comprehensively regulated in combination with the material thickness. In summary, in order to obtain a sensor with shorter response recovery time and shorter preheating time, the thickness and diameter parameters of the gas sensitive material layer 104 need to be controlled.

[0034] A preparation method of a stable heterojunction air quality sensor, comprising the following steps: (1) mixing ZnSnO3 nano powder and a solvent to ball mill to obtain a gas sensitive slurry; (2) loading the gas sensitive slurry on the detection electrode in the MEMS chip 105 and sintering; (3) welding, packaging and aging testing the sintered MEMS chip 105.

[0035] The solvent is any one or two or more of deionized water, methanol, ethanol and isopropanol; the ratio of the ZnSnO3 nanopowder to the solvent is 0.5-10 g:3 mL. The solid content, viscosity and film-forming property of the gas-sensitive slurry affect the printing difficulty of the material and the sensing performance of the sensitive chip. For example, if the solid content is too high, the sensitive material layer on the detection electrode is prone to be too thick when spotting, which affects the contact reaction between the gas and the internal sensitive material and reduces the sensing performance. If the solid content is too low, the amount of the sensitive material is too small, which reduces the reaction with the gas to be detected and also leads to a decrease in the sensing performance. The viscosity affects the amount of the sensing material attached to the detection electrode, which also leads to the problem of the amount of the sensing material on the detection electrode. The film-forming property affects the porosity of the sensing material and the contact reaction between the material and the gas.

[0036] The ZnSnO3 nanopowder can be prepared by a hydrothermal method. For example, 2 g of zinc acetate dihydrate and 2 g of sodium stannate tetrahydrate are weighed and each is made into a 50 mL solution. The sodium stannate solution is added dropwise to the zinc acetate solution while stirring, and the mixture is uniformly mixed and adjusted to a pH of 6-8 before use. The mixed solution is moved into a hydrothermal reaction kettle, which is sealed and reacted at a set temperature of 240℃ for 24 h. After the hydrothermal reaction is completed, the mixture is naturally cooled and filtered, and then washed repeatedly with deionized water.

[0037] Specifically, in step (1), 0.5-10 g of ZnSnO3 nanopowder is placed in a maroon ball mill jar, ball milling beads are added, 3 mL of a dispersion liquid is added, and the mixture is ball milled for 30-120 min. A particle size analyzer is used to test the particle size D50 distribution of the material, which is in the range of 0.5-3.5 μm.

[0038] In step (2), the gas-sensitive slurry is loaded onto the detection electrode by screen printing.

[0039] Specifically, in step (2), the loading step includes dipping a cotton swab into the ball-milled slurry and applying it to the detection electrode of the MEMS chip 105, or using a syringe to suck the ball-milled slurry and drop it onto the detection electrode, or using screen printing to load the slurry onto the detection electrode. Preferably, in step (2), the loading method is selected to be screen printing to print the slurry onto the detection electrode.

[0040] In step (2), the MEMS chip 105 is placed in a muffle furnace and annealed at 640℃ for 1-4 h at a heating rate of 1℃ / min. The annealing process can make the ZnSnO3 material construct a stable heterojunction under the action of temperature, thereby improving the performance of the sensor. When the annealing temperature is 640℃, the sensitivity to VOC is the highest. When the annealing temperature is lower or higher than 640℃, the heterojunction in the material is affected in different ways, and the sensitivity to VOC is significantly reduced.

[0041] The gas-sensitive slurry further comprises a catalyst, which is any one or two or more of aluminum nitrate hydrate, lanthanum nitrate hydrate, metallic platinum, chloroplatinic acid, metallic palladium, chloropalladic acid, antimony oxide, acid-washed asbestos, aluminum oxide and silicon dioxide. The addition of the catalyst can make the sensor more sensitive to VOCs and have a shorter response recovery time.

[0042] Specifically, the step (3) is: fixing the prepared MEMS chip 105 and the base assembly 106 to ensure that the chip is stably fixed on the base assembly 106; wire bonding the fixed chip to connect the MEMS chip 105 and the base assembly 106 and ensure normal electrical signal transmission; packaging the base assembly 106 and the cover plate 101 using epoxy resin glue to obtain a MEMS gas sensor; aging the prepared sensor to stabilize the performance of the sensor; powering off and cooling the sensor for 1-7 days after aging for 3-7 days, and testing the gas-sensitive performance to ensure the stability of the sensor signal.

[0043] Specifically, the fixed position of the MEMS chip 105 is selected at the center position of the base assembly 106, so that the gas enters and exits the sensor more uniformly and the response recovery time is faster. The bonding of the sensor chip and the base uses gold wire to maximize the stability of signal transmission and minimize the contact resistance.

[0044] The aging and testing can be single-heating aging and heating-test double-channel aging.

[0045] Single-heating aging: only the heating end is powered during the aging process of the sensor, and the heating end power voltage is adjusted to maintain the sensor at the rated working temperature (i.e., normal working temperature) when detecting the target gas, so as to realize the performance stability of the heating layer; Heating-test double-channel aging: the heating end and the test end are powered at the same time - the heating end power is used to maintain the rated working temperature of the sensor, and the test end applies the rated working voltage when the sensor detects, so that the sensor simulates the complete normal working state in the actual application scenario, and synchronously completes the aging and working state calibration; preferably, the heating-test double-channel aging method is adopted, which can accelerate the activity stability of the sensitive material (ZnSnO3 nano heterojunction) and the baseline drift calibration by synchronously simulating the actual working condition of the sensor, reduce the data fluctuation in the subsequent detection process, and significantly improve the detection accuracy and long-term working stability of the sensor.

[0046] The aging is performed for 3-7 days, then the sensor is powered off and cooled for 1-7 days, and the performance is tested to ensure stable output signal of the sensor. The aging and cooling process of the sensor stabilizes the catalyst material in the ZnSnO3 gas-sensitive element. The temperature and time of aging and cooling have a great influence on the performance of the sensor.

[0047] Specifically, power aging at 1.5-3.5V for 3-7 days, and after the sensor signal is stable, cold storage for 1-7 days.

[0048] Example 1 A stable heterojunction air quality sensor comprises a cover plate, a MEMS chip and a base assembly; the cover plate is sealed and connected with the base assembly by epoxy resin glue to form a closed cavity; the cover plate is provided with three air inlet holes, which are arranged in a "pin" shape to make the outside and the closed cavity communicate, and the diameter of the air inlet hole is 1mm; the MEMS chip is fixed in the inside of the base assembly and connected with the base assembly through gold wires; the MEMS chip comprises a gas sensitive assembly and a silicon-based micro hot plate, and the gas sensitive assembly is arranged on the silicon-based micro hot plate; the gas sensitive assembly comprises a detection electrode and a gas sensitive material layer covering the detection electrode; the raw material of the gas sensitive material layer is ZnSnO3 heterojunction.

[0049] The preparation method of the above-mentioned stable heterojunction air quality sensor comprises the following steps: (1) Put 5g of ZnSnO3 nano powder into a maroon ball mill jar, add ball milling beads, add 3mL of ethanol, and ball mill for 60min to obtain a gas sensitive slurry, and use a particle size instrument to test the material particle size D50 distribution in 0.5-3.5μm; (2) The gas sensitive slurry is loaded onto the detection electrode in the MEMS chip by screen printing, with a diameter of 180μm and a thickness of 8μm, and the thickness deviation is controlled in 1-4μm. After printing, the MEMS chip is placed in a muffle furnace, the heating rate is 1℃ / min to 640℃, and the temperature is kept for 2h, and the diameter of the gas sensitive material layer is; (3) The prepared MEMS chip and base assembly are fixed, and the chip is stably fixed on the base assembly; the fixed chip is wire bonded for connecting the MEMS chip and the base assembly to ensure normal electrical signal transmission; the cover plate and the base assembly are packaged with epoxy resin glue to obtain a MEMS gas sensor; (4) The prepared sensor is aged by heating and testing in two channels: power aging at 2.5V for 7 days, and after the sensor signal is stable, cold storage for 7 days.

[0050] Comparative Example 1 The difference from Example 1 is the number of air inlet holes, and only one air inlet hole is arranged on the cover plate, which is located directly above the MEMS chip, and the other structures and preparation methods are the same as those of Example 1.

[0051] Comparative Example 2 The difference from Example 1 is the number of air inlet holes, and five air inlet holes are arranged on the cover plate, four of which are arranged at the four vertices of a square, and one is arranged at the center, and the other structures and preparation methods are the same as those of Example 1.

[0052] Comparative Example 3 The difference from Example 1 is the number of air inlets. The cover plate has only nine air inlets, arranged in a "well" shape. The other structures and preparation methods are exactly the same as in Example 1.

[0053] The air quality sensor prepared according to the above embodiments was subjected to performance testing, such as... Figures 2-6 As shown: Figure 2 In the figure, Rs represents the resistance value of the sensor in gases of different concentrations; R0 represents the resistance value of the sensor in clean air. All tests in the figure were performed under standard test conditions. Standard test conditions: temperature: 20℃±2℃, humidity: 55%RH±5%RH. Figure 2 The results show that the sensor has different sensitivities to different gases, and its linearity is better for ethanol compared to other gases.

[0054] Figure 3 Rs represents the resistance value at various temperatures and humidity levels with 50 ppm ethanol; Rs0 represents the resistance value at 20℃ and 65%RH with 50 ppm ethanol. It can be seen that the sensor changes linearly with temperature and humidity.

[0055] Figure 4 The output voltage is the load resistor (R) connected in series with the sensor. L The voltage across the sensor is shown in the figure. The test was performed under standard conditions, with an alcohol concentration of 50 ppm. The sensor's response to 50 ppm ethanol differs by approximately 0.9 V, with a response time T90 < 30 seconds and a recovery time T90 < 40 seconds.

[0056] Figure 5 The output voltage is the load resistor (R) connected in series with the sensor. L The voltage on the sensor is shown in the figure. All tests in the figure were performed under standard conditions. The voltage exhibits a linear trend typical of semiconductor sensors.

[0057] Figure 6 All tests were conducted under standard experimental conditions. The horizontal axis represents the number of tests, with a 5-minute interval between each test. It can be seen that the sensor's zero point and response value remain stable after 10 tests.

[0058] Figure 7 The sensors prepared in Example 1 and Comparative Examples 1-3 were tested under standard experimental conditions. In the figure, Rs represents the sensor's resistance in 50 ppm ethanol gas; R0 represents the sensor's resistance in clean air. The horizontal axis represents the number of tests, with a 5-minute interval between each test. The figure shows that the sensor cover plate with 3 holes performs significantly better than those with other hole numbers.

[0059] Embodiment 2 A stable heterojunction air quality sensor comprises a cover plate, a MEMS chip and a base assembly; the cover plate is sealed and connected with the base assembly by epoxy resin glue to form a closed cavity; three air inlet holes are arranged on the cover plate and arranged in a "pin" shape to make the outside and the closed cavity communicate, and the diameter of the air inlet hole is 1 mm; the MEMS chip is fixed in the inside of the base assembly and connected with the base assembly by gold wires; the MEMS chip comprises a gas sensitive assembly and a silicon-based micro hot plate, and the gas sensitive assembly is arranged on the silicon-based micro hot plate; the gas sensitive assembly comprises a detection electrode and a gas sensitive material layer covering the detection electrode; the raw material of the gas sensitive material layer is ZnSnO3 heterojunction.

[0060] The preparation method of the above-mentioned stable heterojunction air quality sensor comprises the following steps: (1) 9 g of ZnSnO3 nano powder is placed in a agate ball mill jar, ball milling beads are added, 3 mL of deionized water is added, and ball milling is carried out for 45 min to obtain a gas sensitive slurry, and a particle size instrument is used to test the material particle size D50 distribution in 0.5-3.5 μm; (2) The gas sensitive slurry is loaded onto the detection electrode in the MEMS chip by screen printing, the diameter is 180 μm, the thickness is 8 μm, and the thickness deviation is controlled in 1-4 μm. After printing, the MEMS chip is placed in a muffle furnace, the heating rate is 1 ℃ / min to 640 ℃, and the temperature is kept for 2 h; (3) The prepared MEMS chip and the base assembly are fixed, so that the chip is stably fixed on the base assembly; the fixed chip is wire bonded to connect the MEMS chip and the base assembly, so as to ensure normal electrical signal transmission; the cover plate and the base assembly are packaged by using epoxy resin glue, and a MEMS gas sensor is obtained; (4) The prepared sensor is aged by heating and testing in two channels: powered at 3.3 V for 5 days, and then cooled for 5 days after the sensor signal is stable.

[0061] Embodiment 3 A stable heterojunction air quality sensor comprises a cover plate, a MEMS chip and a base assembly; the cover plate is sealed and connected with the base assembly by epoxy resin glue to form a closed cavity; three air inlet holes are arranged on the cover plate and arranged in a "pin" shape to make the outside and the closed cavity communicate, and the diameter of the air inlet hole is 1 mm; the MEMS chip is fixed in the inside of the base assembly and connected with the base assembly by gold wires; the MEMS chip comprises a gas sensitive assembly and a silicon-based micro hot plate, and the gas sensitive assembly is arranged on the silicon-based micro hot plate; the gas sensitive assembly comprises a detection electrode and a gas sensitive material layer covering the detection electrode; the raw material of the gas sensitive material layer is ZnSnO3 heterojunction.

[0062] The preparation method of the stable heterojunction air quality sensor comprises the following steps: (1) 0.5 g of ZnSnO3 nano powder is placed in a agate ball mill tank, ball milling beads are added, 3 mL of deionized water is added, and ball milling is performed for 45 min to obtain a gas sensitive slurry, and a particle size instrument is used to test the particle size D50 distribution of the material in the range of 0.5-3.5 μm; (2) The gas sensitive slurry is loaded onto the detection electrode in the MEMS chip by screen printing, the diameter is 130 μm, and the thickness is 2 μm. After printing, the MEMS chip is placed in a muffle furnace, the heating rate is 1 ℃ / min to 800 ℃, and the temperature is kept for 1 h; (3) The prepared MEMS chip and base assembly are fixed, and the chip is stably fixed on the base assembly; the fixed chip is wire bonded to connect the MEMS chip and the base assembly, and normal electrical signal transmission is ensured; the cover plate and the base assembly are packaged using epoxy resin glue, and a MEMS gas sensor is obtained; (4) The prepared sensor is aged by single heating: power aging at 3.3 V for 3 days, and after the sensor signal is stable, cold storage is performed for 1 day.

[0063] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. An air quality sensor with a stable heterojunction, characterized in that, The device includes a cover plate, a MEMS chip, and a base assembly. The cover plate is sealed to the base assembly with an adhesive to form a closed cavity. The cover plate has at least one air inlet to allow communication between the outside and the closed cavity. The MEMS chip is fixed inside the base assembly. The MEMS chip includes a gas-sensitive component and a heating component, with the gas-sensitive component disposed on the heating component. The gas-sensitive component includes a detection electrode and a gas-sensitive material layer covering the detection electrode. The raw material of the gas-sensitive material layer is a ZnSnO3 heterojunction.

2. The stable heterojunction air quality sensor according to claim 1, characterized in that, The device has three air intakes arranged in a triangle above the MEMS chip.

3. The stable heterojunction air quality sensor according to claim 2, characterized in that, The diameter of the gas-sensitive material layer is 130-250 μm and the thickness is 2-12 μm.

4. The stable heterojunction air quality sensor according to claim 3, characterized in that, The heating component is a silicon-based micro-heating plate.

5. The stable heterojunction air quality sensor according to claim 4, characterized in that, The MEMS chip is connected to the base assembly via leads.

6. A method for preparing an air quality sensor with a stable heterojunction as described in any one of claims 1-5, characterized in that, Includes the following steps: (1) A gas-sensitive slurry was prepared by ball milling ZnSnO3 nanopowder and solvent; (2) The gas-sensitive slurry is loaded onto the detection electrode in the MEMS chip and then sintered; (3) The sintered MEMS chip is welded, packaged and aged.

7. The method for fabricating a stable heterojunction air quality sensor according to claim 6, characterized in that, The sintering temperature is 600-850℃, and the time is 1-4h.

8. The method for fabricating a stable heterojunction air quality sensor according to claim 7, characterized in that, The solvent is any one or two or more of deionized water, methanol, ethanol and isopropanol; the ratio of ZnSnO3 nanopowder to solvent is 0.5-10g:3mL.

9. The method for preparing a stable heterojunction air quality sensor according to claim 8, characterized in that, In step (2), the gas-sensitive paste is loaded onto the detection electrode by screen printing; the aging test is either single-heat aging or heating-test dual-channel aging.

10. The method for fabricating a stable heterojunction air quality sensor according to claim 6, characterized in that, The gas-sensitive slurry also contains a catalyst, which is any one or two or more of the following: aluminum nitrate hydrate, lanthanum nitrate hydrate, platinum metal, chloroplatinic acid, palladium metal, chloropalladium acid, antimony oxide, acid-washed asbestos, alumina, and silica.

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