SnO2-Ag-ZnO-based MEMS semiconductor methane sensor and preparation method thereof

Abstract

The preparation method comprises the following steps: selecting a SnO2 target material as a working target material, and forming a SnO2 film on the surface of a micro heating plate substrate; an Ag target material is switched to be a working target material, an Ag film is formed on the surface of the SnO2 film, and a SnO2-Ag composite material is obtained; assembling a ZnO target material on the working target position, and fixing the SnO2-Ag composite material on a rotary sample table; a ZnO thin film is formed on the surface of the SnO2-Ag composite material, and the SnO2-Ag-ZnO sandwich structure sensitive composite film is obtained. The method can effectively solve the problems that a traditional methane sensor is poor in response performance, too high in power consumption and low in response recovery speed.

Description

Technical Field

[0001] The present invention belongs to the technical field of MEMS gas sensors, and particularly relates to a MEMS semiconductor methane sensor based on SnO2 - Ag2O - ZnO and a preparation method thereof. Background Art

[0002] Currently, there are various types of methane sensors, including ceramic tube resistive, thermal conductivity, thermal catalytic, infrared, electrochemistry, optical fiber, etc. However, traditional gas sensors face the problems of low sensitivity and high cost, which to a certain extent limit their wide application in the field of safety monitoring. Compared with traditional gas sensors, MEMS (Micro - Electro - Mechanical System) gas sensing technology (including principles such as electrochemistry, optics, and semiconductor resistance) has advantages such as higher sensitivity, lower power consumption, smaller volume, and lower production cost. In recent years, significant progress has been made in device size and power consumption of MEMS - based MOS gas sensors.

[0003] For the sensitive materials of traditional MEMS semiconductor methane sensors, they are usually first synthesized by solution method and then transferred to the micro - hotplate substrate by drop - coating. This method has the following main disadvantages: 1) The preparation process is cumbersome. Usually, solution synthesis includes multiple different steps, involving many related reactants and solvents, and it is easy to have solvent or reactant residues in the obtained products; 2) Traditional MEMS semiconductor methane sensors transfer the sensitive materials to the middle area of a micro - heating plate of about 100 * 100 microns by manual drop - coating. The controllability and consistency of the process are very low, and the performance differences of MEMS semiconductor methane sensors obtained in different batches or by different operators are huge; 3) The sensitive materials of traditional MEMS semiconductor methane sensors are only added to the surface of the micro - hotplate, and the force between them and the substrate is very weak, with small adhesion, and the sensitive materials are prone to peeling off during vibration or long - term use; 4) Generally, the noble metals used in doped and modified MEMS semiconductor methane sensors, such as platinum and gold, are expensive raw materials, resulting in high costs. Therefore, there is an urgent need to provide a methane sensor and its preparation method with a simple preparation process, good controllability and consistency of the process, good stability, low manufacturing cost, and high detection accuracy. Summary of the Invention

[0004] Aiming at the problems existing in the above - mentioned prior art, the present invention provides a MEMS semiconductor methane sensor based on SnO2 - Ag - ZnO and a preparation method thereof. This methane sensor has a high response sensitivity to CH4 and high measurement accuracy. At the same time, it has good mechanical and chemical stability during long - term use; this method can effectively solve the problems of poor response performance, too high power consumption, and slow response and recovery speed of traditional methane sensors, can significantly improve the gas - sensing performance of methane sensors to methane gas, and thus can obtain excellent measurement accuracy.

[0005] To achieve the above object, the present invention provides a preparation method of a MEMS semiconductor methane sensor based on SnO2-Ag-ZnO, including the following steps:

[0006] Step 1: Assemble the SnO2 target and the Ag target diagonally above the ultraviolet lamp and the air inlet at 45°. At the same time, use a heat-resistant sputtering ceramic substrate to firmly fix the micro-heating plate substrate on the rotary heating table. Then, close the vacuum door and conduct an airtightness inspection to ensure that there is no air leakage.

[0007] Step 2: After connecting the cooling water at the cooling port, turn on the magnetron sputtering system and conduct a vacuum pumping process to reduce the air pressure in the chamber to below 5×10 -3 Pa.

[0008] Step 3: According to the preset gas flow rate, introduce argon into the chamber until the pressure in the chamber stabilizes at about 1 Pa.

[0009] Step 4: Select the SnO2 target as the working target, set the sputtering power and sputtering time, turn on the radio frequency switch, and start the formal coating process to form a SnO2 thin film on the surface of the micro-heating plate substrate.

[0010] Step 5: After completing the deposition of the SnO2 thin film, switch the Ag target to the working target, reset the sputtering power and sputtering time, and start the formal coating process to form an Ag thin film on the surface of the SnO2 thin film.

[0011] Step 6: Open the chamber and take out the SnO2-Ag composite material.

[0012] Step 7: Assemble the ZnO target at the working target position and fix the SnO2-Ag composite material on the rotary sample stage; turn on the magnetron sputtering system and conduct a vacuum pumping process to reduce the air pressure in the chamber to below 5×10 -3 Pa; according to the preset gas flow rate, introduce argon into the chamber until the pressure in the chamber stabilizes at about 1 Pa; set the sputtering power and sputtering time, turn on the radio frequency switch, and start the formal coating process to form a ZnO thin film on the surface of the SnO2-Ag composite material to obtain a SnO2-Ag-ZnO sandwich structure sensitive composite film.

[0013] Furthermore, to ensure the sputtering accuracy, in Step 1, the diameters of the SnO2 target and the Ag target are 50 mm and the thicknesses are 3 mm.

[0014] Furthermore, to avoid adverse effects of impurities on the coating quality, in Step 1, before assembly, the SnO2 target and the Ag target are first cleaned, and the cleaning process is as follows:

[0015] Place the SnO2 target and the Ag target in an ultrasonic cleaning tank. First, under the condition of ultrapure water, use ultrasonic technology to clean the dust on the surface of the target. Then, under the condition of 75% alcohol, use ultrasonic technology to remove the organic impurities on the surface of the target.

[0016] Further, to ensure the sputtering accuracy, in Step 8, the diameter of the ZnO target is 50 mm and the thickness is 3 mm.

[0017] Further, to avoid the adverse effects of impurities on the coating quality, in Step 8, before assembly, first clean the ZnO target. The cleaning process is as follows:

[0018] Place the ZnO target in an ultrasonic cleaning tank. First, under the condition of ultrapure water, use ultrasonic technology to clean the dust on the surface of the target. Then, under the condition of 75% alcohol, use ultrasonic technology to remove the organic impurities on the surface of the target.

[0019] Further, to obtain a methane sensor with the best response sensitivity, in Step 3, the gas flow rate is 35 sccm; in Step 4, the distance between the working target and the micro-heating plate substrate is 10 cm, the radio frequency power supply power is 50 W, and the sputtering time is 1800 s;

[0020] In Step 5, the distance between the working target and the micro-heating plate substrate is 10 cm, the radio frequency power supply power is 50 W, the sputtering time is 20 s, and the thickness of the formed Ag film is 8 nm;

[0021] In Step 8, the distance between the ZnO target and the micro-heating plate substrate is 10 cm, the gas flow rate is 35 sccm, the radio frequency power supply power is 50 W, the sputtering time is 177 s, and the thickness of the formed ZnO film is 8 nm.

[0022] In a large number of practices, it is found that the thicknesses of the Ag thin film and the ZnO thin film are crucial for improving the response value of the methane sensor. When the Ag thin film is too thin, it cannot effectively reduce the defects and charge traps at the interface of the SnO2 thin film, and the improvement effect on the conductivity and the response value is limited, resulting in the increased cost not being proportional to the obtained response benefit. When the Ag thin film is too thick, the resistance of the sensitive composite film will be too low, and thus the response value will also be lower. Similarly, when the ZnO thin film is too thin, it cannot effectively reduce the defects and charge traps at the interface of the SnO2-Ag composite material, and the improvement effect on the conductivity and the response value is limited. At the same time, it cannot significantly improve the catalytic activity of the SnO2-Ag composite material. When the ZnO thin film is too thick, the response value will be lower, resulting in the increased cost not being proportional to the obtained response benefit. In order to obtain the best response sensitivity, through a large number of experimental combinations and continuous optimization and selection, it is innovatively found that for the SnO2-Ag-ZnO sandwich-structured sensitive composite film, a sandwich structure with the Ag thin film having a thickness of 8 nm and the ZnO thin film having a thickness of 8 nm can obtain a methane sensor with a higher response value.

[0023] As an optimization, the purities of the ZnO target, the Ag target, and the SnO2 target are all above 99.99%.

[0024] The present invention provides a preparation method for a microelectromechanical system semiconductor methane sensor. First, the SnO2 thin film is modified with a metal Ag thin film material, which can effectively reduce the defects and charge traps at the interface, and can effectively reduce the contact resistance between the SnO2 thin film and the subsequent ZnO thin film, which is beneficial to improving the electron transfer efficiency and further improving the conductivity of the overall structure. At the same time, the Ag thin film can enhance the mechanical and chemical stability of the whole structure, prevent the SnO2 thin film from being corroded by environmental factors (such as oxygen, water vapor, etc.), and thus play a certain protective role, which is beneficial to improving the long-term stability and reliability of the methane sensor. In addition, the Ag thin film can improve the sensitivity of the SnO2 thin film, making its response to sensitive substances such as gases more sensitive. Finally, sputtering the ZnO thin film on the surface of the SnO2-Ag composite material can form a better interface contact on the surface of the composite material, further reducing the charge traps, which is beneficial to further improving the conductivity of the overall structure. At the same time, the ZnO thin film can significantly enhance the catalytic activity of the SnO2-Ag composite material, improve the catalytic reduction ability of the obtained SnO2-Ag-ZnO sandwich-structured sensitive composite film to methane gas, and thus improve the selectivity of the sandwich-structured sensitive composite film to methane gas. In this way, during the specific use process, the methane sensor can have an ultra-high response sensitivity to methane gas, and at the same time, it can effectively reduce the influence of other gases on the measurement accuracy of methane gas, significantly improving the accuracy of methane gas.

[0025] The present invention realizes a reliable, fast and simple method for batch preparation of sensitive materials, and at the same time, the gas-sensitive properties of the prepared sensitive materials for methane have been significantly improved. Usually, the preparation of metal thin films generally adopts the DC magnetron sputtering method. Although the preparation rate is fast, it is difficult to accurately control its thickness. The present invention innovatively adopts the RF magnetron sputtering method to prepare metal and oxide thin films, which can accurately regulate the thickness of the metal layer, effectively improve the processing accuracy, and thus can accurately regulate the methane response performance of gas sensors. At the same time, it can achieve the consistency of the batch preparation process and improve the adhesion of the materials. The magnetron sputtering method preparation process is a batch planarization process, and sensitive materials can be batch-prepared within a range of 4 inches at one time, and thousands of MEMS semiconductor methane sensors can be batch-prepared at one time; at the same time, magnetron sputtering is a vacuum preparation process, which can realize continuous thin film preparation, the preparation process is simple and has good continuity; in addition, during the magnetron sputtering process, the atoms of the sensitive materials have a strong interaction with the substrate, and the obtained thin film has good adhesion and stable performance. The sensitive material of the MEMS methane sensor with a tin dioxide-silver- zinc oxide sandwich structure prepared by the present invention using the RF magnetron sputtering method and in-situ heating has improved the uniformity and adhesion of the sensitive thin film, and the precious metals used are inexpensive, saving costs. Compared with ordinary sensors, the present invention realizes the preparation of sensitive materials for micro-heating plate methane sensors with good controllability, high repeatability and good consistency, which is suitable for large-scale preparation.

[0026] The present invention uses the magnetron sputtering technology to prepare a sensitive material thin film based on SnO2. Through the modification of Ag and ZnO thin films, the problems of poor response performance, too high power consumption and slow response and recovery speed of methane sensors are effectively solved, and the gas-sensitive performance of the sensor to methane gas is significantly improved, and thus excellent measurement accuracy can be obtained.

[0027] The present invention provides a MEMS gas sensor for methane mixed gas recognition.

[0028] The precious metal (Ag) as a catalyst provides another method to reduce the decomposition activation energy of CH4 and enhance the CH4 response.

[0029] This method realizes the preparation of sensitive materials for micro-heating plate methane sensors with good accuracy, high repeatability and good consistency, which is suitable for large-scale preparation.

[0030] The present invention also provides a MEMS semiconductor methane sensor based on SnO2-Ag-ZnO, which is prepared by a preparation method of a MEMS semiconductor methane sensor based on SnO2-Ag-ZnO, and includes a ZnO thin film, an Ag thin film, a SnO2 thin film, a micro-heating plate substrate and a ceramic substrate which are distributed in sequence from top to bottom, wherein the thickness of the Ag thin film is 8 nm and the thickness of the ZnO thin film is 8 nm.

[0031] The methane sensor in the present invention is based on the SnO2-Ag-ZnO sandwich structure, which has high response sensitivity to CH4, high measurement accuracy, and good mechanical and chemical stability during long-term use. Brief Description of the Drawings

[0032] Figure 1 is a graph showing the variation of the responses of SnO2, SnO2-Ag 4nm, SnO2-Ag 8nm, and SnO2-Ag 16nm in the present invention to 1000 ppm CH4 with the operating temperature;

[0033] Figure 2 is a dynamic sensing response graph of SnO2-Ag 8nm, SnO2-Ag 8nm–ZnO 5nm, SnO2-Ag 8nm–ZnO 8nm, and SnO2-Ag 8nm–ZnO 20nm to CH4 under the optimal operating temperature conditions in the present invention;

[0034] Figure 3 is a scanning electron microscope (SEM) image of SnO2-Ag 8nm in the present invention;

[0035] Figure 4 is a scanning electron microscope (SEM) image of SnO2-Ag 8nm-ZnO 8nm in the present invention;

[0036] Figure 5 is a scanning electron microscope (SEM) image of SnO2-Ag 8nm-ZnO 20nm in the present invention;

[0037] Figure 6 is the overall flowchart of the present invention. Detailed Description of the Invention

[0038] The present invention will be further described below.

[0039] As Figures 1 to 6 shown, the present invention provides a preparation method for a MEMS semiconductor methane sensor based on SnO2-Ag-ZnO, including the following steps:

[0040] Step 1: Assemble the SnO2 target and the Ag target on different magnetron sputtering targets of the magnetron sputtering instrument respectively. Specifically, they can be assembled at 45° above the ultraviolet lamp and the air inlet diagonally. At the same time, use a high-temperature-resistant sputtering ceramic substrate to firmly fix the micro-heating plate substrate on the rotary heating table. Connect high-purity argon gas to the air inlet of the magnetron sputtering instrument, then close the vacuum door, the air release valve, and the flowmeter, and conduct a airtightness check to ensure that there is no air leakage;

[0041] Step 2: After connecting the cooling water at the cooling port, turn on the magnetron sputtering system, start the mechanical pump, open the isolation valve, and start the vacuum pumping process. When the air pressure in the vacuum chamber drops below 10 Pa, turn on the molecular pump working button. After the molecular pump stabilizes, reduce the air pressure in the chamber to below 5×10-3 Pa;

[0042] Step 3: According to the preset gas flow rate, introduce argon gas into the chamber until the pressure in the chamber stabilizes at about 1 Pa;

[0043] Step 4: Select the SnO2 target as the working target, set the sputtering power and sputtering time, turn on the radio frequency switch, and start the formal coating process to form a SnO2 thin film on the surface of the micro-heating plate substrate;

[0044] Step 5: After completing the deposition of the SnO2 thin film, switch the Ag target to the working target, reset the sputtering power and sputtering time, and start the formal coating process to form an Ag thin film on the surface of the SnO2 thin film;

[0045] Step 6: Open the chamber and take out the SnO2-Ag composite material;

[0046] Step 7: Assemble the ZnO target at the working target position and fix the SnO2-Ag composite material on the rotating sample stage; turn on the magnetron sputtering system and perform vacuum pumping to reduce the air pressure in the chamber to below 5×10-3 Pa; according to the preset gas flow rate, introduce argon gas into the chamber until the pressure in the chamber stabilizes at about 1 Pa; set the sputtering power and sputtering time, turn on the radio frequency switch, and start the formal coating process to form a ZnO thin film on the surface of the SnO2-Ag composite material to obtain a SnO2-Ag-ZnO sandwich-structured sensitive composite film.

[0047] To ensure sputtering accuracy, in Step 1, the diameters of the SnO2 target and the Ag target are 50 mm and the thicknesses are 3 mm.

[0048] To avoid adverse effects of impurities on the coating quality, in Step 1, before assembly, clean the SnO2 target and the Ag target first. The cleaning process is as follows:

[0049] Place the SnO2 target and the Ag target in an ultrasonic cleaning tank. First, under the condition of ultrapure water, use ultrasonic technology to clean the dust on the surface of the target, and then under the condition of 75% alcohol, use ultrasonic technology to remove the organic impurities on the surface of the target.

[0050] To ensure sputtering accuracy, in Step 8, the diameter of the ZnO target is 50 mm and the thickness is 3 mm.

[0051] To avoid adverse effects of impurities on the coating quality, in Step 8, before assembly, the ZnO target is first cleaned, and the cleaning process is as follows:

[0052] Place the ZnO target in an ultrasonic cleaning tank. First, under the condition of ultrapure water, use ultrasonic technology to clean the dust on the surface of the target, and then under the condition of 75% alcohol, use ultrasonic technology to remove the organic impurities on the surface of the target.

[0053] To obtain a methane sensor with the best response sensitivity, in Step 3, the gas flow rate is 35 sccm; in Step 4, the distance between the working target and the micro-heating plate substrate is 10 cm, the radio frequency power supply power is 50 W, and the sputtering time is 1800 s;

[0054] In Step 5, the distance between the working target and the micro-heating plate substrate is 10 cm, the radio frequency power supply power is 50 W, the sputtering time is 20 s, and the thickness of the formed Ag film is 8 nm;

[0055] In Step 8, the distance between the ZnO target and the micro-heating plate substrate is 10 cm, the gas flow rate is 35 sccm, the radio frequency power supply power is 50 W, the sputtering time is 177 s, and the thickness of the formed ZnO film is 8 nm.

[0056] In a large number of practices, it is found that the thicknesses of the Ag film and the ZnO film are crucial for improving the response value of the methane sensor. When the Ag film is too thin, it cannot effectively reduce the defects and charge traps at the interface of the SnO2 film, and the improvement effect on the conductivity and response value is limited, making the increased cost not proportional to the obtained response benefit. When the Ag film is too thick, it will cause the resistance of the sensitive composite film to be too low, and then the response value will also be lower. Similarly, when the ZnO film is too thin, it cannot effectively reduce the defects and charge traps at the interface of the SnO2-Ag composite material, and the improvement effect on the conductivity and response value is limited. At the same time, it cannot greatly improve the catalytic activity of the SnO2-Ag composite material. When the ZnO film is too thick, the response value will be lower, making the increased cost not proportional to the obtained response benefit. To obtain the best response sensitivity, through a large number of experimental combinations and continuous optimization and selection, it is innovatively found that for the SnO2-Ag-ZnO sandwich structure sensitive composite film, a sandwich structure with a thickness of 8 nm for the Ag film and a thickness of 8 nm for the ZnO film can obtain a methane sensor with a higher response value.

[0057] As an optimization, the purities of the ZnO target, the Ag target, and the SnO2 target are all above 99.99%.

[0058] The present invention provides a preparation method of a microelectromechanical system semiconductor methane sensor. First, the SnO2 thin film is modified with a metal Ag thin film material, which can effectively reduce defects and charge traps at the interface and can effectively reduce the contact resistance between the SnO2 thin film and the subsequent ZnO thin film, facilitating the improvement of the electron transport efficiency and thus enhancing the conductivity of the overall structure. At the same time, the Ag thin film can enhance the mechanical and chemical stability of the entire structure, prevent the SnO2 thin film from being corroded by environmental factors (such as oxygen, water vapor, etc.), and thus play a certain protective role, which is conducive to improving the long-term stability and reliability of the methane sensor. In addition, the Ag thin film can improve the sensitivity of the SnO2 thin film, making its response to sensitive substances such as gases more sensitive. Finally, sputtering a ZnO thin film on the surface of the SnO2-Ag composite material can form a better interface contact on the surface of the composite material, further reducing charge traps and facilitating further improvement of the conductivity of the overall structure. At the same time, the ZnO thin film can significantly enhance the catalytic activity of the SnO2-Ag composite material, improve the catalytic reduction ability of the obtained SnO2-Ag-ZnO sandwich structure sensitive composite film to methane gas, thereby improving the selectivity of the sandwich structure sensitive composite film to methane gas. In this way, during the specific use process, the methane sensor can have an ultra-high response sensitivity to methane gas. At the same time, it can also effectively reduce the influence of other gases on the measurement accuracy of methane gas, significantly improving the accuracy of methane gas.

[0059] The present invention realizes a reliable, fast and simple method for batch preparation of sensitive materials. Meanwhile, the gas-sensitive properties of the prepared sensitive materials towards methane have been significantly improved. Generally, metal thin films are usually prepared by DC magnetron sputtering. Although the preparation rate is fast in this way, it is difficult to precisely control their thickness. The present invention innovatively adopts radio frequency (RF) magnetron sputtering to prepare metal and oxide thin films, which can precisely regulate the thickness of the metal layer, effectively improving the processing accuracy. Thus, the methane response performance of gas sensors can be accurately regulated. At the same time, the consistency of the batch preparation process can be achieved, and the adhesion of the materials can be improved. The process of preparing by magnetron sputtering is a batch planarization process. Sensitive materials can be batch-prepared within a range of 4 inches at one time, and thousands of MEMS semiconductor methane sensors can be batch-prepared at one time. Meanwhile, magnetron sputtering is a vacuum preparation process, which can realize continuous thin film preparation, with a simple preparation process and good continuity. In addition, during the magnetron sputtering process, the atoms of the sensitive materials have a strong interaction with the substrate, and the obtained thin film has good adhesion and stable performance. The sensitive materials of the SnO₂-Ag-ZnO sandwich structure MEMS methane sensor prepared by the present invention using RF magnetron sputtering and in-situ heating improve the uniformity and adhesion of the sensitive thin film, and the precious metals used are inexpensive, saving costs. Compared with ordinary sensors, the present invention realizes the preparation of sensitive materials for micro-heating plate methane sensors with good controllability, high repeatability and good consistency, which is suitable for large-scale preparation.

[0060] The present invention uses magnetron sputtering technology to prepare sensitive material thin films based on SnO₂. Through the modification of Ag and ZnO thin films, the problems of poor response performance, too high power consumption and slow response and recovery speed of methane sensors are effectively solved, and the gas-sensitive performance of the sensors towards methane gas is significantly improved, and thus excellent measurement accuracy can be obtained.

[0061] The present invention also provides a MEMS semiconductor methane sensor based on SnO₂-Ag-ZnO, which is prepared by a preparation method of a MEMS semiconductor methane sensor based on SnO₂-Ag-ZnO. It includes a ZnO thin film, an Ag thin film, an SnO₂ thin film, a micro-heating plate substrate and a ceramic substrate which are distributed in sequence from top to bottom. Among them, the thickness of the Ag thin film is 8 nm, and the thickness of the ZnO thin film is 8 nm.

[0062] The methane sensor in the present invention is based on the SnO₂-Ag-ZnO sandwich structure, which has high response sensitivity and high measurement accuracy towards CH₄. At the same time, it has good mechanical and chemical stability during long-term use.

[0063] In order to further elaborate on the present invention in depth, the present invention will be described below through examples and several comparative examples:

[0064] Example:

[0065] The SnO2 target and the Ag target are respectively assembled at a 45° angle above the ultraviolet lamp and the air inlet. At the same time, the micro-heating plate substrate is firmly fixed on the rotary heating table by using a high-temperature-resistant sputtering ceramic substrate. Then, the vacuum door is closed and an airtightness check is carried out to ensure that there is no air leakage. After connecting the cooling water at the cooling port, the magnetron sputtering system is turned on and vacuum pumping is carried out to reduce the air pressure in the chamber to below 5×10-3 Pa. According to the preset gas flow rate, argon gas is introduced into the chamber until the pressure in the chamber stabilizes at about 1 Pa.

[0066] Select the SnO2 target as the working target, set the sputtering power to 50 W, set the sputtering time to 1800 s, turn on the radio frequency switch, and start the formal coating process to form a SnO2 thin film on the surface of the micro-heating plate substrate. Repeat the above process four times to prepare four composite materials.

[0067] After the deposition of the SnO2 thin film is completed, switch the Ag target to the working target and set the sputtering power to 50 W. For three of the composite materials, sputtering operations are carried out with sputtering times of 10 s, 20 s, and 30 s respectively. Start the formal coating process to form an Ag thin film on the surface of the SnO2 thin film, and obtain the SnO2-Ag composite material. Open the chamber and take out the SnO2-Ag composite material. Thus, four different gas-sensitive materials are obtained, namely a pure SnO2 sample with a sputtering time of 1800 s, a SnO2-Ag1 composite material with a sputtering time of 10 s and an Ag thin film thickness of 4 nm, a SnO2-Ag2 composite material with a sputtering time of 20 s and an Ag thin film thickness of 8 nm, and a SnO2-Ag3 composite material with a sputtering time of 30 s and an Ag thin film thickness of 16 nm, as shown in Table 1.

[0068] Table 1: Parameters set by the magnetron sputtering instrument

[0069]

[0070]

[0071] The operating temperature determines the adsorption and desorption equilibrium of gases on the surface of the sensing material, as well as the effective mobility of carriers between different phases in the heterostructure. At the optimal operating temperature, the metal oxide semiconductor can obtain the highest carrier concentration, and at this time, the surface activation and adsorption of the material particle layer reach the peak. At the same time, the adsorption of oxygen and the catalytic reaction of reducing gases are significantly enhanced. Therefore, it is first necessary to optimize the operating temperature of the gas-sensitive thin film. Figure 1The gas response of pristine SnO2, SnO2-Ag4nm, SnO2-Ag 8nm and SnO2-Ag 16nm composites to 1000ppm CH4 in the temperature range of 200℃-400℃ is shown.

[0072] pass Figure 1 It can be seen that the response of SnO2 and SnO2-Ag4nm increases with the increase of working temperature, reaches the maximum value at 375℃ and then decreases, while the working temperature required for the SnO2-Ag8nm and SnO2-Ag16nm composite materials to reach the maximum value drops to 350℃. Due to the catalytic effect of Ag, the activation energy required for the reaction process between CH4 molecules and adsorbed oxygen ions is reduced. Therefore, 350℃ was selected as the optimal working temperature for gas sensing tests. With the increase of Ag content, the response of CH4 first increases and then decreases, and SnO2-Ag8nm shows a high sensing response in the entire temperature range, and its response value to 1000ppm CH4 reaches 1.42. This is due to the formation of a pn junction between SnO2 and Ag, which increases the width of the space charge layer and improves the response value. However, the thickness of Ag is the key. If it is too thin, the space charge layer effect is weak, and if it is too thick, it completely covers the tin dioxide surface below, reducing the response value. After a large number of experimental studies, it was determined that SnO2-Ag8nm has the highest response to methane.

[0073] Among them, the SnO2-Ag8nm scanning electron microscope (SEM) image is as follows Figure 3 As shown, the energy spectrum analysis (EDS) table corresponding to the sensor is shown in Table 2.

[0074] Table 2: SnO2-Ag8nm spectrum analysis table

[0075] Element Wt% Wt% Sigma O 23.31 Si 6.07 0.07 Zn 0.00 0.10 Ag 1.51 0.13 Sn 46.84 0.22 Pt 22.27 0.22 Total: 100.00

[0076] When preparing the SnO2--Ag8nm-ZnO heterostructured composite sensor, the magnetron sputtering method is still used. On the basis of the SnO2-Ag8nm composite material, a high-purity ZnO thin film is sputtered and coated again on its surface. The specific operation process is as follows: Assemble the ZnO target on the working target position, and fix the SnO2-Ag composite material on the rotating heating table; Turn on the magnetron sputtering system and perform vacuum pumping to reduce the air pressure in the chamber to below 5×10-3 Pa; According to the preset gas flow rate, introduce argon gas into the chamber until the pressure in the chamber stabilizes at about 1 Pa; Set the sputtering power and sputtering time, turn on the radio frequency switch, and start the formal coating process to form a ZnO thin film on the surface of the SnO2-Ag composite material, obtaining the SnO2-Ag-ZnO sandwich-structured sensitive composite film. The above operation process is carried out three times respectively, and the sputtering operations are carried out with sputtering times of 104 s, 177 s, and 374 s respectively. Thus, four different gas-sensitive materials are obtained, namely the SnO2-Ag8nm gas-sensitive material with a sputtering time of 0 s, the SnO2-Ag8nm–ZnO5nm gas-sensitive material with a sputtering time of 104 s and a ZnO thin film thickness of 5 nm, the SnO2-Ag8nm–ZnO8nm gas-sensitive material with a sputtering time of 177 s and a ZnO thin film thickness of 8 nm, and the SnO2-Ag8nm–ZnO20nm gas-sensitive material with a sputtering time of 374 s and a ZnO thin film thickness of 20 nm.

[0077] By adjusting the sputtering time to sputter ZnO thin films with different thicknesses on the surface of SnO2--Ag8nm, the parameter settings of the magnetron sputtering instrument are shown in Table 3.

[0078] Table 3: Parameters set by the magnetron sputtering instrument

[0079]

[0080] The real-time response curves of each gas sensor to different concentrations of CH4 at the optimal working temperature are as Figure 2 shown. As the gas concentration increases, the response curves of each gas sensor to CH4 increase significantly in a stepped manner. The results show that depositing ZnO on the SnO2-Ag8nm composite material can significantly improve the CH4 response, and at all CH4 concentrations, when the Zn loading is 5 wt%, that is, the sensing response of the SnO2-Ag8nm-ZnO8nm composite material is the highest. The response value of SnO2-Ag8nm-ZnO8nm to 2000 ppm CH4 is 2.03, which is 1.79 and 1.43 times that of the original SnO2 and SnO2-Ag8nm respectively.

[0081] Compared with SnO2-Ag8nm, the SnO2-Ag8nm-ZnO8nm composite material exhibits a higher sensing response, which may be due to the formation of an n-n type heterojunction between SnO2-Ag8nm-ZnO8nm. Through its interfacial effect, the adsorption and reaction of gases are promoted, thereby enhancing the sensitivity to CH4. Although the sensitivity change of SnO2-Ag8nm-ZnO8nm to CH4 is limited, considering that the preparation of metal oxide semiconductor gas sensors by magnetron sputtering can deposit uniform and dense thin films on a large-area substrate, with good crystallinity, low defect density, and good reproducibility, it is suitable for mass production. This makes this method very suitable for the manufacture of large-scale gas sensors and reduces the production cost.

[0082] Among them, the scanning electron microscope (SEM) image of SnO2-Ag8nm-ZnO8nm is as Figure 4 shown, and the corresponding energy dispersive spectroscopy (EDS) table of this sensor is shown in Table 4. The scanning electron microscope (SEM) image of SnO2-Ag8nm-ZnO20nm is as Figure 5 shown, and the corresponding energy dispersive spectroscopy (EDS) table of this sensor is shown in Table 5.

[0083] Table 4: Energy Dispersive Spectroscopy Table of SnO2-Ag8nm-ZnO8nm

[0084] Element Wt% Wt% Sigma O 22.55 Si 5.15 0.05 Zn 4.16 0.09 Ag 1.41 0.09 Sn 43.75 0.16 Pt 22.98 0.16 Total: 100.00

[0085] Table 5 Energy Dispersive Spectroscopy Table of SnO2-Ag8nm-ZnO20nm

[0086] Element Wt% Wt% Sigma O 22.26 Si 4.56 0.04 Zn 7.44 0.10 Ag 1.48 0.09 Sn 43.55 0.16 Pt 20.71 0.16 Total: 100.00

Claims

1. A preparation method of an SnO2-Ag-ZnO-based MEMS semiconductor methane sensor, characterized in that, It includes the following steps: Step 1: Assemble the SnO2 target and the Ag target respectively at 45° above the ultraviolet lamp and the air inlet obliquely. At the same time, use the sputtering ceramic substrate to firmly fix the micro-heating plate substrate on the rotary heating table, then close the vacuum door and conduct an airtightness inspection to ensure there is no air leakage; Step 2: After connecting the cooling water at the cooling port, turn on the magnetron sputtering system and perform a vacuum pumping process to reduce the air pressure in the chamber to below 5×10 -3 Pa; Step 3: According to the preset gas flow rate, introduce argon gas into the chamber until the pressure in the chamber stabilizes at about 1 Pa; Step 4: Select the SnO2 target as the working target, set the sputtering power and sputtering time, turn on the radio frequency switch, and start the formal coating process to form a SnO2 thin film on the surface of the micro-heating plate substrate; Step 5: After the SnO2 thin film is deposited, switch the Ag target to be the working target, and reset the sputtering power and sputtering time, then start the formal coating process to form an Ag thin film on the surface of the SnO2 thin film; Step 6: Open the chamber and take out the SnO2-Ag composite material; Step 7: Assemble the ZnO target on the working target position, and fix the SnO2-Ag composite material on the rotating sample stage; turn on the magnetron sputtering system and perform vacuum pumping to reduce the air pressure in the chamber to below 5×10 -3 Pa; according to the preset gas flow rate, introduce argon gas into the chamber until the pressure in the chamber stabilizes at about 1 Pa; set the sputtering power and sputtering time, turn on the radio frequency switch, and start the formal coating process to form a ZnO thin film on the surface of the SnO2-Ag composite material, obtaining the SnO2-Ag-ZnO sandwich-structured sensitive composite film.

2. The preparation method of a MEMS semiconductor type methane sensor based on SnO2-Ag-ZnO according to claim 1, characterized in that, In Step 1, the diameters of the SnO2 target and the Ag target are 50 mm, and the thicknesses are 3 mm.

3. The preparation method of a MEMS semiconductor type methane sensor based on SnO2-Ag-ZnO according to claim 1, characterized in that, In Step 1, before assembly, first clean the SnO2 target and the Ag target. The cleaning process is as follows: Place the SnO2 target and the Ag target in an ultrasonic cleaning tank. First, under the condition of ultrapure water, use ultrasonic technology to clean the dust on the surface of the target, and then under the condition of 75% alcohol, use ultrasonic technology to remove the organic impurities on the surface of the target.

4. The preparation method of a MEMS semiconductor methane sensor based on SnO2-Ag-ZnO according to claim 1, characterized in that, In Step 8, the diameter of the ZnO target is 50 mm, and the thickness is 3 mm.

5. The preparation method of a MEMS semiconductor type methane sensor based on SnO2 - Ag - ZnO according to claim 1, characterized in that, In Step 8, before assembly, first clean the ZnO target. The cleaning process is as follows: Place the ZnO target in an ultrasonic cleaning tank. First, under the condition of ultrapure water, use ultrasonic technology to clean the dust on the surface of the target, and then under the condition of 75% alcohol, use ultrasonic technology to remove the organic impurities on the surface of the target.

6. The preparation method of a MEMS semiconductor methane sensor based on SnO2-Ag-ZnO according to claim 1, characterized in that, In Step 3, the gas flow rate is 35 sccm; in Step 4, the distance between the working target and the micro-heating plate substrate is 10 cm, the radio frequency power supply power is 50 W, and the sputtering time is 1800 s; In Step 5, the distance between the working target and the micro-heating plate substrate is 10 cm, the radio frequency power supply power is 50 W, the sputtering time is 20 s, and the thickness of the formed Ag thin film is 8 nm; In Step 8, the distance between the ZnO target and the micro-heating plate substrate is 10 cm, the gas flow rate is 35 sccm, the radio frequency power supply power is 50 W, the sputtering time is 177 s, and the thickness of the formed ZnO thin film is 8 nm.

7. The preparation method of a MEMS semiconductor methane sensor based on SnO2-Ag-ZnO according to claim 1, characterized in that The purities of the ZnO target, the Ag target, and the SnO2 target are all above 99.99%.

8. A MEMS semiconductor methane sensor based on SnO2-Ag-ZnO, characterized in that, Prepared by using the preparation method of a SnO2-Ag-ZnO-based MEMS semiconductor methane sensor according to any one of claims 1 to 8, which includes a ZnO thin film, an Ag thin film, a SnO2 thin film, a micro-heating plate substrate, and a ceramic substrate distributed in sequence from top to bottom. Among them, the thickness of the Ag thin film is 8 nm, and the thickness of the ZnO thin film is 8 nm.

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