Gas sensitive membrane of a MEMS gas sensor and method for producing the same

Abstract

This invention relates to the field of gas sensor technology, and more particularly to a gas-sensitive film for a MEMS gas sensor and its preparation method. The gas-sensitive film comprises: a SnO2 substrate, an Ag first coating layer, and a Pd second coating layer. The substrate thickness is 300-500 nm, and the Ag and Pd coating layers are both less than 10 nm thick, with both layers exhibiting a dotted distribution. The gas-sensitive film is prepared by magnetron sputtering on a micro-heating plate substrate. The micro-heating plate substrate includes a micro-heating plate coated with a silicon nitride insulating protective layer and a base, with a cavity between the micro-heating plate layer and the base. This invention, through the combined modification of Pd and Ag, achieves a gas-sensitive material with faster response speed, shorter recovery time, and better stability. This method is simple to prepare and has low manufacturing costs. It not only significantly enhances the adhesion between the gas-sensitive film and the substrate but also significantly improves the response rate and stability of the gas-sensitive film, greatly enhancing the sensor's sensitivity and making it suitable for large-scale mass production.

Description

Technical Field

[0001] This invention relates to the field of gas sensor technology, and in particular to a gas-sensitive membrane for a MEMS gas sensor and its preparation method. Background Technology

[0002] In daily life, we are constantly surrounded by various gases, closely connected to our lives and directly or indirectly affecting our lives and health. Methane, a colorless and odorless flammable gas, is not toxic under normal circumstances, but when its concentration is within the explosive limits, it can explode upon contact with an ignition source. Furthermore, high concentrations of methane can displace oxygen in the air, leading to oxygen deficiency and causing asphyxiation, especially in enclosed spaces where methane accumulation can easily lead to oxygen deprivation or explosions, posing serious hazards to personnel and facilities. Carbon monoxide, a colorless and odorless toxic gas, has a much stronger binding affinity to hemoglobin than oxygen, preventing hemoglobin from effectively carrying oxygen and causing poisoning. Prolonged exposure to low concentrations of carbon monoxide can also lead to chronic poisoning, causing serious health problems. In addition, methane, carbon monoxide, and ethylene are all hazardous gases present in large quantities in mines, and their presence can easily lead to dust explosions, gas explosions, and other explosion accidents during mining operations. Therefore, it is very important to be able to detect methane and carbon monoxide gases quickly and accurately. The accurate identification and detection of methane and carbon monoxide is an important topic.

[0003] With the continuous development of gas identification technology, the required equipment must have real-time detection capabilities, be easy to operate, and have good portability to meet the needs of the field environment. Gas sensors are a good match for our needs.

[0004] Based on their working principles and application areas, gas sensors can be categorized into several types, including semiconductor gas sensors, electrochemical gas sensors, optical gas sensors, and contact combustion gas sensors. Among these, semiconductor gas sensors have become the most common and popular type due to their wide applicability and high practical value. This type of gas sensor has been widely used in home gas alarm systems, effectively ensuring home safety.

[0005] Depending on the substrate type, metal-oxide-semiconductor gas sensors can be further classified into ceramic tube type, planar type, and MEMS (Micro-Mechanical Systems) type. Electro Micromechanical systems (MEMS) are characterized by micro-heated plate designs. Among these, MEMS micro-heated plate sensors, with their unique advantages, have become leaders in the field of gas sensors. This technology not only represents the latest development in microelectronics but also represents a cutting-edge research area involving multiple disciplines. Compared to traditional sensors, MEMS sensors exhibit unparalleled advantages due to their small size, lightweight design, economical cost, low power consumption, high reliability, ease of mass production, and ease of integration and intelligentization.

[0006] Metal-oxide-semiconductor (MOS) semiconductors have become a focus of industry attention in recent years due to their superior sensitivity, fast response and recovery speeds, and affordable cost. However, traditional SnO2 gas-sensitive materials generally suffer from problems such as poor tolerance to acidic environments leading to insufficient long-term stability, limited intrinsic sensitivity, slow response / recovery speeds, and difficulty in meeting the batch-to-batch performance consistency requirements for MEMS mass production. While simple noble metal (such as Pd) modification can improve sensitivity, Pd is easily oxidized and deactivated at sensor operating temperatures (>300℃), and excessive noble metal coating can inhibit oxygen adsorption, thereby reducing sensitivity.

[0007] For the commercial application of metal-oxide-semiconductor (MOS) sensors, stability, sensitivity, and mass production capabilities are all issues that the industry needs to address. Therefore, there is an urgent need to provide a sensitive material with fast response speed, short recovery time, good controllability, high detection accuracy, and long service life, which would improve the practicality and stability of the sensor. Summary of the Invention

[0008] The technical problem to be solved by the present invention is: in order to improve the sensitivity, stability and detection accuracy of the sensor for carbon monoxide and methane, the present invention provides a gas-sensitive membrane for a MEMS gas sensor and its preparation method.

[0009] The technical solution adopted by this invention to solve its technical problem is: A gas-sensitive membrane for a MEMS gas sensor, comprising: The substrate is a SnO2 thin film; The first coating layer is disposed on the substrate and is an Ag thin film. The second coating layer is disposed on the side of the first coating layer away from the substrate, and the second coating layer is a Pd thin film.

[0010] Furthermore, the Ag film in the first coating layer exhibits a non-completely continuous silver nano-island structure.

[0011] Furthermore, the Pd film in the second coating layer exhibits a non-completely continuous palladium nanoisland structure.

[0012] Furthermore, the thickness of the base layer is between 300nm and 500nm, and the thicknesses of both the first coating layer and the second coating layer are less than 10nm.

[0013] A method for fabricating a gas-sensitive membrane for a MEMS gas sensor, characterized by comprising the following steps: Step 1: Under vacuum conditions, a SnO2 thin film is formed on the upper surface of the micro-heating plate substrate by magnetron sputtering; Step 2: Sputtering deposition is performed at a sputtering power of 20W to 80W to form an Ag film on the surface of the SnO2 film. Step 2: Sputtering deposition is performed at a sputtering power of 20W to 80W to form a Pd film on the Ag film surface.

[0014] Furthermore, in step one, a combined vacuum pumping operation is performed on the sputtering chamber using a mechanical pump and a molecular pump to achieve a vacuum level of 5 × 10⁻⁶. -3 Below Pa.

[0015] Furthermore, in step one, after evacuation, argon gas with a stable flow rate is introduced into the sputtering chamber until the pressure in the sputtering chamber stabilizes within the range of 0.5 Pa to 1.2 Pa.

[0016] Furthermore, in steps one through three, SnO2, Ag, and Pd targets are respectively mounted on a magnetron sputtering apparatus, and the distance between the target and the micro-heating plate substrate is controlled to be between 5 cm and 15 cm.

[0017] Furthermore, the micro heating plate substrate includes a micro heating plate layer and a substrate, and magnetron sputtering is performed on the micro heating plate layer, with a cavity provided between the micro heating plate layer and the substrate.

[0018] Furthermore, a silicon nitride insulating protective layer is provided on the micro heating plate layer, and the thickness of the silicon nitride insulating protective layer is 300nm to 500nm.

[0019] The beneficial effects of this invention are that, through the co-modification of Pd-Ag, compared to traditional single Ag modification, it can obtain gas-sensitive materials with faster response speed, shorter recovery time, and better stability. This method has a simple preparation process and low manufacturing cost. It not only significantly enhances the bonding force between the gas-sensitive film and the substrate but also significantly improves the response rate and stability of the gas-sensitive film, greatly enhancing the sensitivity of the sensor. It is suitable for large-scale mass production and has good economic and social benefits in industrial production. The Pd-Ag modified SnO2 gas-sensitive material prepared using this invention can be used to manufacture high-performance MEMS sensors. The manufactured sensors not only have high sensitivity, short recovery time, good controllability, and high detection accuracy but also high reusability and good consistency, greatly improving the practicality and stability of the sensors and helping to optimize the overall performance of MEMS gas sensors. Attached Figure Description

[0020] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0021] Figure 1 This is a schematic diagram of the gas-sensitive membrane in this invention.

[0022] Figure 2 This is a comparison of the responses of various sensors to 3000 ppm methane at different temperatures.

[0023] Figure 3 It is a comparison of the responses of various sensors to different concentrations of methane gas.

[0024] Figure 4 This is a graph showing the transient resistance response values ​​of each sensor under different concentrations of methane gas.

[0025] Figure 5 These are the resistance transient response curves of each sensor under different concentrations of carbon monoxide gas.

[0026] Figure 6 These are the resistance transient response curves of each sensor under different concentrations of methane and carbon monoxide mixed gas.

[0027] Figure 7 This is the energy dispersive spectroscopy (EDS) analysis of the SnO2 metal oxide sensitive material loaded with Pd-Ag modified particles of different thicknesses prepared in this invention.

[0028] In the diagram: 1. Base layer; 2. First coating layer; 3. Second coating layer. Detailed Implementation

[0029] The present invention will now be described in further detail with reference to the accompanying drawings. These drawings are simplified schematic diagrams, illustrating only the basic structure of the invention, and therefore only show the components relevant to the invention.

[0030] like Figure 1 The diagram illustrates a gas-sensitive membrane for a MEMS gas sensor, as shown in Embodiment 1 of the present invention. The membrane comprises a base layer, a first coating layer, and a second coating layer sequentially disposed therefrom. The base layer is made of SnO2 thin film, the first coating layer is made of Ag thin film, and the second coating layer is made of Pd thin film. The thickness of the base layer is between 300 nm and 500 nm, and the thicknesses of both the first and second coating layers are less than 10 nm. Furthermore, the films in both the first and second coating layers are distributed in a dotted pattern. The Ag thin film in the first coating layer exhibits a non-completely continuous silver nano-island structure, and the Pd thin film in the second coating layer exhibits a non-completely continuous palladium nano-island structure. The base layer is a continuous layered structure. The dotted distribution of the Pd and Ag films increases the area for interaction with the gas between layers, improving the response value; simultaneously, it does not completely shield the tin dioxide substrate, thus enhancing the gas sensitivity.

[0031] A method for preparing Pd-Ag modified SnO2 gas-sensitive material for MEMS gas sensors includes the following steps: Step 1: Fix the micro-heating plate substrate onto the sample holder of the magnetron sputtering instrument; To improve thermal insulation performance and also help regulate the thermal response characteristics of the material, the micro heating plate substrate includes a micro heating plate layer and a substrate, with the micro heating plate layer located above the substrate and a cavity provided between the micro heating plate layer and the substrate. A platinum metal heating layer is covered on the micro heating plate layer.

[0032] To ensure the heating efficiency of the micro heating plate layer and to enhance its stability, the micro heating plate layer is also covered with a silicon nitride insulating protective layer with a thickness of 300nm to 500nm.

[0033] Step 2: Install the SnO2 target on the target position of the magnetron sputtering instrument, and control the distance between the SnO2 target and the micro-heating plate substrate to be between 5 and 15 cm; Step 3: The sputtering chamber is evacuated sequentially using a mechanical pump and a molecular pump, until the vacuum level in the sputtering chamber is reduced to 5 × 10⁻⁶. -3 For pressures below Pa, specifically, first use a mechanical pump to evacuate the sputtering chamber to 1 Pa–10 Pa; then use a molecular pump to evacuate the sputtering chamber to 5 × 10 Pa. -3 Below Pa.

[0034] Step 4: Introduce argon gas with a stable flow rate into the sputtering chamber and control the flow rate of the argon gas to be 10 sccm to 50 sccm until the pressure in the sputtering chamber stabilizes in the range of 0.5 Pa to 1.2 Pa; preferably, the flow rate of the argon gas is 30 sccm and the pressure in the sputtering chamber is stabilized at about 1 Pa. Step 5: Perform the formal sputtering deposition operation under conditions of sputtering power of 20-80W and minimal reflection power, depositing for 0.5-1.5 hours until a SnO2 film with a thickness of 300nm-500nm is formed on the surface of the micro-heated plate substrate. The SnO2 film is formed on the silicon nitride insulating protective layer. Preferably, the sputtering power is 50W and the formal sputtering time is 0.5 hours. Step Six: Replace the working target in Step Two with an Ag target and repeat the atmosphere control process in Step Three; perform the formal sputtering coating operation under the condition of sputtering power of 20W to 80W, and deposit for 5 to 50 seconds to form an Ag film on the SnO2 film surface to obtain an Ag-modified SnO2 sensitive film; preferably, the flow rate of argon gas is 30 sccm, and the pressure in the sputtering chamber is stabilized at about 1 Pa, the sputtering power is 30W, and the formal sputtering time is 10 seconds or 20 seconds; Step 7: Replace the working target in Step 2 with a Pd target and repeat the atmosphere control process in Step 3; perform the formal sputtering coating operation under the condition of sputtering power of 20W to 80W, deposit for 5 to 50 seconds, and form a Pd film on the Ag film surface to obtain a Pd-Ag modified SnO2 sensitive film; preferably, the flow rate of argon gas is 30 sccm, and the pressure of the sputtering chamber is stabilized at about 1 Pa, the sputtering power is 30W, and the formal sputtering time is 5 seconds and 10 seconds.

[0035] To ensure the gas-sensitive material produced has the best performance, in step four, the argon gas flow rate is 30 sccm, and the pressure in the sputtering chamber is stabilized at approximately 1 Pa. To ensure the effectiveness and efficiency of the vacuuming process, the vacuuming operation in step three is combined as follows: As a preferred option, the purity of SnO2 target, Ag target and Pd target are all above 99.99%.

[0036] This invention creatively proposes a Pd-Ag composite nanocrystal synergistic modification of SnO2 gas-sensitive material and a controllable magnetron sputtering process. First, a SnO2 thin film is uniformly deposited on the surface of a micro-heated plate substrate. Then, a Pd-Ag thin film is coated on the SnO2 film, thus constructing a Pd-Ag modified SnO2 composite thin film structure. The gas-sensitive material is prepared using a magnetron sputtering process. During the preparation process, argon gas is introduced, and the impact of argon ions on the target material enhances the adhesion of the coating, improving the stability and reliability of the gas-sensitive material. Pd or Ag itself has electronic and catalytic effects; by controlling the order and thickness, the sensor's response to different gases can be better controlled, resulting in better selectivity. This application uses radio frequency magnetron sputtering, which has a relatively slower rate, thus allowing for better control of the Pd or Ag thickness.

[0037] Optical gas-sensitive materials obtained by simply doping with Pd typically have a short lifespan and long recovery time, increasing the cost of gas detection and reducing its stability. Simultaneous doping with Pd and Ag can significantly improve the activity, selectivity, and stability of SnO2-based catalysts through the synergistic effect and distribution changes among multiple noble metals.

[0038] Meanwhile, by adding a Pd-Ag modification layer to SnO2, a chemical passivation protective layer is formed, which can effectively resist the corrosion of acidic gases and improve long-term stability. Through the catalytic effect of the noble metal itself, the activation energy of the surface chemical reaction of methane and carbon monoxide can be reduced, thereby increasing their chemical adsorption ratio. On the other hand, through the Schottky barrier effect of the noble metal-semiconductor interface, the width of the electron depletion layer on the SnO2 surface is expanded, which significantly enhances the resistance change value during gas response. This is beneficial to the electron and ion transport between the catalyst and the substrate material during the gas response process, significantly increasing the resistance change value after gas response, and giving the material a high overall response speed.

[0039] like Figure 2 As shown in the figure, extensive experiments have demonstrated that the material can achieve a gas-sensitive response value of up to 2.65 for 3000ppm methane at an operating temperature of 353~600 ℃, with a response time of only 15 seconds. It combines high sensitivity, fast response and excellent anti-interference ability, making it suitable for high-efficiency and high-consistency mass production of MEMS gas sensors.

[0040] Furthermore, in this invention, the SnO2 thin film is formed on the micro-heating plate substrate by sputtering, and the Ag thin film is formed on the SnO2 thin film by sputtering. Simultaneously, the Pd thin film is also formed on the Ag thin film by sputtering. Therefore, the prepared gas-sensitive material has good surface roughness, and the uniformity of Ag and Pd doping in SnO2 is ideal, further promoting electron and ion transport between the catalyst and the substrate material during gas compliance, thereby further improving the gas response speed. This invention uses SnO2 as the base material and, through magnetron sputtering, forms a Pd-Ag element modification on the SnO2 thin film, effectively ensuring the material's gas sensitivity. Simultaneously, the Pd-Ag coating forms a protective layer on the SnO2 surface, effectively preventing corrosion from acidic gases and SnO2, further improving the stability of the gas-sensitive material. This method, through precise magnetron sputtering technology, simultaneously overcomes the core challenges of SnO2-based gas sensors in terms of stability, sensitivity, response speed, and mass production consistency.

[0041] Compared to traditional single Ag modification, the co-modification of Pd and Ag yields gas-sensitive materials with faster response speeds, shorter recovery times, and better stability. This method is simple to prepare and has low manufacturing costs. It not only significantly enhances the adhesion between the gas-sensitive film and the substrate but also significantly improves the response rate and stability of the gas-sensitive film, greatly enhancing the sensitivity of the gas sensor. It is suitable for large-scale mass production and has good economic and social benefits in industrial production. The Pd-Ag modified SnO2 gas-sensitive material prepared using this invention can be used to fabricate high-performance MEMS sensors. These sensors not only have high sensitivity, short recovery time, good controllability, and high detection accuracy but also high reusability and good consistency, greatly improving the practicality and stability of gas sensors and contributing to the optimization of the overall performance of MEMS gas sensors.

[0042] To improve thermal insulation performance and also help regulate the material's thermal response characteristics, the micro-heating plate substrate includes a micro-heating plate layer and a substrate, with the micro-heating plate layer located above the substrate and a cavity provided between the micro-heating plate layer and the substrate. This design aims to enhance the material's thermal insulation performance while also helping to regulate its thermal response characteristics.

[0043] This invention effectively addresses the technical bottlenecks of traditional SnO2-based gas-sensitive materials, such as insufficient stability, low sensitivity, and slow response speed. Based on SnO2, this invention utilizes the co-modification of Pd and Ag to obtain a Pd-Ag composite nanocrystal-modified SnO2 metal oxide sensitive material. Compared to traditional single Ag modification, this significantly expands the width of the electron depletion layer on the SnO2 surface, thereby greatly enhancing the resistance change after gas response. This significantly improves the material's response performance when detecting gases, achieving a high response value and contributing to the optimization of the overall performance of MEMS gas sensors.

[0044] This gas-sensitive material has a fast response rate, short recovery time, good controllability, and high detection accuracy, which is beneficial to improving the practicality and stability of gas sensors.

[0045] Example 1: The SnO2 metal oxide sensitive material Pd 5-Ag20 / SnO2 loaded with Pd-Ag modified particles in this embodiment is prepared by the following steps: First, prepare the magnetron sputtering instrument according to standard procedures (check valves, connect cooling water, and turn on power). After introducing a stable argon gas flow, load the micro-heated plate substrate and SnO2 target, and close the chamber door. Evacuate to 5*10 -3 Below Pa, argon gas was introduced at a flow rate of 30 sccm, and the gas pressure in the sputtering chamber was stabilized to approximately 1 Pa. The RF sputtering power supply (50W) was turned on, and a SnO2 thin film was sputtered onto the micro-heated plate substrate for 0.5 hours. After SnO2 film deposition, the RF power supply and gas path (flow meter, inlet valve, pressure regulator switch) were turned off, and the target was replaced with a silver (Ag) target. The vacuum was then re-evacuated to 5*10 Pa. -3 Below Pa, argon gas was introduced at a flow rate of 30 sccm, and the gas pressure in the sputtering chamber was stabilized to approximately 1 Pa. The RF sputtering power supply (30W) was turned on, and an Ag thin film was sputtered and deposited on the SnO2 surface for 20 seconds. Subsequently, the RF power supply and gas path were turned off again, and the target material was replaced with palladium (Pd). The vacuum was then re-evacuated to 5*10 Pa. -3 Below Pa, argon gas was introduced at a flow rate of 30 sccm, and the gas pressure in the sputtering chamber was stabilized to about 1 Pa. The RF sputtering power supply (30W) was turned on, and a Pd thin film was sputtered and deposited on the Ag / SnO2 surface for 5 seconds.

[0046] Experimental results showed that the sensor made from the Pd-Ag modified SnO2 gas-sensitive film had a good response to 3000 ppm methane at an operating temperature of 353~600 ℃.

[0047] Figure 7This is a transient resistance response curve of the SnO2 metal oxide sensitive material Pd 5-Ag20 / SnO2 loaded with Pd-Ag modified particles, as described in this invention, to different concentrations of methane gas at an operating temperature of 480°C. Figure 3-6 It is known that the gas-sensitive element has a response time of 15s and a recovery time of 10s for 3000ppm methane at 425℃. This ultra-fast response rate enables rapid detection of methane leaks. The response time is defined as the time required from the start of contact between the gas and the gas-sensitive element until the element's resistance drops to 90% of its stable resistance. The recovery time is defined as the time required for the element's resistance to recover to 90% of its initial value after the gas no longer contacts the gas-sensitive element. Figure 7 The table shows the energy dispersive spectroscopy (EDS) analysis of SnO2 metal oxide sensitive materials loaded with Pd-Ag modified particles of different thicknesses prepared in this invention. The elements are shown in Table 1.

[0048] Table 1: Elemental content of Pd-Ag modified particles in SnO2 metal oxide sensitive material Pd5-Ag20-SnO2 (representing Pd thin film sputtering deposition for 5 seconds and Ag thin film sputtering deposition for 20 seconds).

[0049] To evaluate the sensor's performance, the sensing characteristic parameters of similar gas sensors were compared with the relevant data in Table 2 of this invention. The results show that the SnO2 gas sensor prepared using the co-decoration process exhibits excellent response performance and rapid response characteristics in gas detection. On one hand, this technology, with its high-precision thin-film control capabilities and rich material compatibility, enables precise control over the microstructure and chemical composition of sensitive materials, providing reliable technical support for the fabrication of high-performance gas sensors. On the other hand, by rationally designing and optimizing the composite decoration sequence, the selectivity and sensitivity of the sensor can be significantly adjusted. This discovery provides a novel design approach and technical strategy for constructing sensor arrays suitable for various detection scenarios.

[0050] Table 2: Comparison of methane response performance of different SnO2-based gas sensors

[0051] Comparative Example 1: The Ag / SnO2 metal oxide sensitive material loaded with Ag-modified particles in this comparative example is prepared by the following steps: First, prepare the magnetron sputtering instrument according to standard procedures (check valves, connect cooling water, and turn on power). After introducing a stable argon gas flow, load the micro-heated plate substrate and SnO2 target, and close the chamber door. Evacuate to 5*10 -3Below Pa, argon gas was introduced at a flow rate of 30 sccm, and the gas pressure in the sputtering chamber was stabilized to approximately 1 Pa. The RF sputtering power supply (50W) was turned on, and a SnO2 thin film was sputtered onto the micro-heated plate substrate for 0.5 hours. After SnO2 film deposition, the RF power supply and gas path (flow meter, inlet valve, pressure regulator switch) were turned off, and the target was replaced with a silver (Ag) target. The vacuum was then re-evacuated to 5*10 Pa. -3 Argon gas was introduced at a flow rate of 30 sccm below Pa, and the gas pressure in the sputtering chamber was stabilized to approximately 1 Pa. The RF sputtering power supply (30W) was turned on, and an Ag thin film was sputtered onto the SnO2 surface for 20 seconds. After the Ag thin film deposition was completed, the RF power supply and gas path (flow meter, inlet valve, pressure regulator switch) were turned off, and the Ag-modified SnO2 material was obtained.

[0052] Experimental results showed that the sensor made from the Ag-modified SnO2 gas-sensitive thin film had a gas-sensitive response value of 1.25 to 3000 ppm methane at an operating temperature of 480℃.

[0053] A comparison of the results from Example 1 and Comparative Example 1 revealed that the SnO2 metal oxide sensitive material loaded with Pd-Ag modified particles exhibited a faster response time and a higher response value than the SnO2 metal oxide sensitive material loaded only with Ag modified particles. When multiple noble metals are simultaneously doped, a synergistic effect occurs, meaning the presence of one noble metal can enhance the response of another to gases.

[0054] Comparative Example 2: The preparation method of the SnO2 metal oxide sensitive material Pd-Ag-SnO2 loaded with Pd-Ag modified particles in this embodiment specifically includes the following steps: First, prepare the magnetron sputtering instrument according to standard procedures (check valves, connect cooling water, and turn on power). After introducing a stable argon gas flow, load the micro-heated plate substrate and SnO2 target, and close the chamber door. Evacuate to 5*10 -3 Below Pa, argon gas was introduced at a flow rate of 30 sccm, and the gas pressure in the sputtering chamber was stabilized to approximately 1 Pa. The RF sputtering power supply (50W) was turned on, and a SnO2 thin film was sputtered onto the micro-heated plate substrate for 0.5 hours. After SnO2 film deposition, the RF power supply and gas path (flow meter, inlet valve, pressure regulator switch) were turned off, the target was replaced with an Ag target, and the vacuum was re-evacuated to 5*10 Pa. -3 Below Pa, argon gas was introduced at a flow rate of 30 sccm, and the gas pressure in the sputtering chamber was stabilized to approximately 1 Pa. The RF sputtering power supply (30W) was turned on, and an Ag thin film was sputtered and deposited on the SnO2 surface for 20 seconds. Subsequently, the RF power supply and gas path were turned off again, and a palladium (Pd) target was replaced. The vacuum was then re-evacuated to 5*10 Pa. -3Below Pa, argon gas is introduced at a flow rate of 30 sccm, and the gas pressure in the sputtering chamber is stabilized to approximately 1 Pa. The RF sputtering power supply (30W) is turned on, and a Pd thin film is sputtered and deposited on the Ag-SnO2 surface for 10 seconds. The final product, Pd10-Ag20-SnO2 gas-sensitive film, is thus obtained (indicating that the Pd film was sputtered and deposited for 10 seconds, and the Ag film was sputtered and deposited for 20 seconds).

[0055] Experimental results showed that the sensor made from the Pd-Ag modified SnO2 gas-sensitive film had a response value of 1.35 to 3000ppm methane at an operating temperature of 425℃.

[0056] A comparison of the results from Example 1 and Comparative Example 2 revealed that, with changes in the Pd doping time, the material doped with 5sPd exhibited a better response than the material doped with 10sPd. Specifically, the SnO2 metal oxide sensing material (Pd5-Ag20-SnO2) loaded with Pd-Ag modified particles in Example 1 showed a better response than the sensing material Pd10-Ag20-SnO2 in Comparative Example 2. Changing the Pd doping time affected the SnO2 gas sensor's response to gases, primarily due to the enhanced catalytic activity, improved electrical properties, altered chemisorption capacity, and changes in spillover and chemisensitization effects of Pd. Careful selection and optimization of the Pd concentration are necessary to obtain optimal gas sensing performance.

[0057] Comparative Example 3: The preparation method of the SnO2 metal oxide sensitive material in this comparative example specifically includes the following steps: First, prepare the magnetron sputtering instrument according to standard procedures (check valves, connect cooling water, and turn on power). After introducing a stable argon gas flow, load the micro-heated plate substrate and SnO2 target, and close the chamber door. Evacuate to 5*10 -3 Below Pa, argon gas was introduced at a flow rate of 30 sccm, and the gas pressure in the sputtering chamber was adjusted to stabilize at about 1 Pa. The RF sputtering power supply (50W) was turned on, and a SnO2 thin film was sputtered and deposited on the micro-heated plate substrate for 0.5 hours to obtain a SnO2 gas-sensitive thin film.

[0058] Experimental results show that the sensor made from this pure SnO2 thin film has a gas-sensitive response value of approximately 0.45 to 3000ppm methane at an operating temperature of 353-600℃.

[0059] A comparison of the results from Example 1 and Comparative Example 3 revealed that the response was significantly improved when doped with noble metals Pd and Ag. Doping with noble metals Pd and Ag can significantly enhance the gas sensing performance of the SnO2 sensor. The spillover effect and chemisensitization generated by noble metal particles on the SnO2 surface play a key role in improving the gas sensing performance.

[0060] In summary, the gas sensor provided by this invention, which uses Pd-Ag alloy nanocrystals as a catalyst, is specifically designed for the field of MEMS gas sensors. The Pd-Ag alloy nanocrystals selected are used as a catalyst, and the sensitivity of the gas sensor can be improved by changing the proportion of Pd-Ag alloy. This significantly improves stability and sensitivity, and the sensor exhibits unparalleled advantages in terms of ease of mass production, integration, and intelligence.

[0061] This invention creatively proposes a Pd-Ag composite nanocrystal synergistic modification of SnO2 into a sensitive material and a controllable magnetron sputtering preparation process. First, a SnO2 thin film is uniformly deposited on the surface of a micro-heated plate substrate. Then, a Pd-Ag thin film is coated onto the SnO2 film, thus constructing a Pd-Ag modified SnO2 composite thin film structure. The gas-sensitive material is prepared using a magnetron sputtering process. During the preparation process, argon gas is introduced, and the impact of argon ions on the target material enhances the adhesion of the coating, improving the stability and reliability of the gas-sensitive material. By simultaneously doping with Pd and Ag, the activity, selectivity, and stability of the SnO2-based catalyst can be significantly improved through the synergistic effect and distribution variation among multiple noble metals. Furthermore, by adding a Pd-Ag modification layer to SnO2, compared to pure SnO2, the Ag and Pd particles act as electronic catalysts, and their introduction modulates the reactivity of the SnO2 material surface, exhibiting a higher gas-sensitive response.

[0062] In this invention, the SnO2 thin film is formed on a micro-heating plate substrate by sputtering, the Ag thin film is formed on the SnO2 thin film by sputtering, and the Pd thin film is also formed on the Ag thin film by sputtering. Therefore, the prepared gas-sensitive material has good surface roughness, and the uniformity of Ag and Pd doping in SnO2 is ideal, thereby further improving the gas response speed. This invention uses SnO2 as the base material and, through magnetron sputtering, forms a Pd-Ag element modification on the SnO2 thin film, effectively ensuring the material's gas sensitivity. Simultaneously, the Pd-Ag coating forms a protective layer on the SnO2 surface, effectively preventing corrosion from acidic gases and SnO2, further improving the stability of the gas-sensitive material. This method overcomes the core challenges of gas sensors in terms of stability, sensitivity, response speed, and mass production consistency through precise magnetron sputtering technology.

[0063] In summary, this invention, through the combined modification of Pd and Ag, achieves gas-sensitive materials with faster response speeds, shorter recovery times, and better stability compared to traditional single Ag modification. The method is simple to prepare and has low manufacturing costs. It not only significantly enhances the adhesion between the gas-sensitive film and the substrate but also significantly improves the response rate and stability of the gas-sensitive film, greatly enhancing the sensor's sensitivity. It is suitable for large-scale mass production and offers significant economic and social benefits in industrial manufacturing. The Pd-Ag modified SnO2 gas-sensitive material prepared using this invention can be used to fabricate high-performance MEMS sensors. These sensors not only exhibit high sensitivity, short recovery time, good controllability, and high detection accuracy but also high reusability and good consistency, greatly improving the sensor's practicality and stability and contributing to the optimization of the overall performance of MEMS gas sensors.

[0064] The present invention also provides a Pd-Ag modified SnO2 sensitive material for MEMS gas sensors, which is prepared by a method comprising, from top to bottom, a Pd-Ag modified layer, a SnO2 thin film layer, a micro heating plate layer and a substrate, wherein the thickness of the Pd thin film in the Pd-Ag modified layer is 1 nm to 10 nm, the thickness of the Ag thin film is 1 nm to 10 nm, and the thickness of the SnO2 thin film layer is 300 nm to 500 nm.

[0065] To improve thermal insulation performance and also help regulate the thermal response characteristics of the material, the micro heating plate substrate includes a micro heating plate layer and a substrate, with the micro heating plate layer located above the substrate and a cavity provided between the micro heating plate layer and the substrate.

[0066] The sensitive material in this invention employs a unique multi-layer structure design, consisting of a Pd-Ag composite nanocrystal modification layer, a SnO2 thin film layer, an integrated micro-heating plate layer, and a substrate, arranged from top to bottom. A Pd-Ag composite nanocrystal layer is constructed on the SnO2 surface using magnetron sputtering technology. This forms a chemical passivation protective layer, effectively resisting the corrosion of acidic gases and improving long-term stability. Furthermore, the Schottky barrier effect at the noble metal-semiconductor interface expands the width of the electron depletion layer on the SnO2 surface, significantly enhancing the resistance change during gas response. Extensive experiments demonstrate that this material achieves a response value (Ra / Rg) of 2.65 to 3000 ppm methane gas with a response time of only 15 seconds, exhibiting high sensitivity, rapid response, and excellent anti-interference capabilities, making it suitable for high-efficiency, high-consistency mass production of MEMS gas sensors. This invention effectively solves the technical bottlenecks of insufficient stability, low sensitivity, and slow response speed in traditional SnO2-based gas-sensitive materials. Based on SnO2, this invention obtains a SnO2 metal oxide sensitive material modified with Pd-Ag composite nanocrystals through the co-modification of Pd-Ag. Compared with the traditional single Ag modification, it can significantly expand the width of the electron depletion layer on the SnO2 surface, thereby greatly enhancing the resistance change value after gas response. This significantly enhances the material's response performance when detecting gases, thus achieving a high response value and helping to optimize the overall performance of MEMS gas sensors.

[0067] This gas-sensitive material has a fast response rate, short recovery time, good controllability, and high detection accuracy, which is beneficial to improving the practicality and stability of gas sensors.

[0068] Compared with existing gas-sensitive materials, this invention has the following advantages: (1) This invention provides an innovative technical solution based on SnO2-based materials, which significantly improves material performance through synergistic modification with Pd-Ag elements. In terms of material protection, the Pd-Ag coating on the SnO2 surface forms a protective barrier, effectively resisting acidic gas corrosion and greatly enhancing the chemical stability of the material. The core of this invention lies in utilizing the Pd-Ag composite nanostructure to achieve a breakthrough improvement in the gas-sensing performance of SnO2 through bandgap modulation and catalysis. Based on the higher work function characteristics of Pd (work function 5.12 eV) and Ag (work function 4.74 eV) compared to SnO2 (3.37 eV), when noble metals come into contact with SnO2, electrons undergo directional transfer, spontaneously forming a Schottky barrier at the interface. This process induces a bandgap bending effect, promoting the formation of an enhanced electron depletion layer on the SnO2 surface, significantly increasing the initial resistance value of the material. The higher initial resistance value provides greater room for improvement in the rate of resistance change, thereby achieving significant optimization of sensing sensitivity. Regarding catalysis and gas-sensitive response mechanisms, noble metal surfaces respond to oxygen anions (O2).2- The catalytic adsorption of O through the spillover effect promotes O 2- Uniform distribution on the SnO2 surface effectively enhances the generation density and migration ability of chemically adsorbed oxygen ions. The resulting uniform nanoparticles significantly increase the effective reaction area of ​​the material, especially under high-temperature conditions, effectively promoting the formation of O2 ions. 2- The generation of ions further enhances the gas-sensitive response performance. Furthermore, Ag nanoparticle modification exhibits a unique antioxidant regulatory effect on Pd nanoparticles. The synergistic antioxidant mechanism based on electronic structure regulation proposed in this invention breaks through the limitations of traditional noble metal catalysts that rely on alloying strategies to improve high-temperature stability, providing a novel theoretical framework and technical path for the autonomous antioxidant design of single-atom-layer catalysts.

[0069] (2) The thickness of the Pd / Ag nanoparticles was optimized (typically ranging from 1 nm to 10 nm) to avoid a single electron accumulation layer caused by excessive noble metal coverage (which would inhibit the electron capture ability of oxygen molecules). A co-modification process was used to achieve uniform dispersion of Pd / Ag nanoparticles, ensuring maximum exposure of active sites. Pd and Ag layers were sequentially deposited on the SnO2 film surface using magnetron sputtering to form uniform nanoparticles.

[0070] (3) This invention utilizes magnetron sputtering technology, which enhances the adhesion of SnO2 coatings by bombarding the target material with argon ions. At the same time, multiple targets can be placed at one time during the magnetron sputtering process and rotated at a uniform speed within the instrument, ensuring that the coating of each target material is more uniform and dense, reducing costs, and making it particularly suitable for large-scale production; (4) By applying the Pd-Ag modified SnO2 gas-sensitive material of the present invention, high-performance MEMS sensors can be manufactured. These sensors not only have precise control, but also high repeatability and good consistency, which greatly improves their practicality.

[0071] In summary, this invention, with its simple process, economical cost, and potential for large-scale production, demonstrates broad application prospects in industrial production.

[0072] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A gas sensitive membrane of a MEMS gas sensor, characterized in that include: The substrate is a SnO2 thin film; The first coating layer is disposed on the substrate and is an Ag discontinuous thin film. The second coating layer is disposed on the side of the first coating layer away from the substrate, and the second coating layer is a Pd discontinuous thin film.

2. The gas sensitive membrane of the MEMS gas sensor according to claim 1, characterized in that: The Ag film in the first coating layer has a non-completely continuous silver nano-island structure.

3. The gas-sensitive membrane of the MEMS gas sensor according to claim 1, characterized in that: The Pd film in the second coating layer has a non-completely continuous palladium nano-island structure.

4. The gas-sensitive membrane of the MEMS gas sensor according to claim 1, characterized in that: The thickness of the base layer is between 300nm and 500nm, and the thickness of both the first coating layer and the second coating layer is less than 10nm.

5. A method for preparing a gas-sensitive membrane for a MEMS gas sensor as described in any one of claims 1-4, characterized in that, Includes the following steps: Step 1: Under vacuum conditions, a SnO2 thin film is formed on the upper surface of the micro-heating plate substrate by magnetron sputtering; Step 2: Sputtering deposition is performed at a sputtering power of 20W to 80W to form an Ag film on the surface of the SnO2 film. Step 2: Sputtering deposition is performed at a sputtering power of 20W to 80W to form a Pd film on the Ag film surface.

6. The gas-sensitive membrane of the MEMS gas sensor according to claim 5, characterized in that: In the step one, the sputtering chamber is vacuumed by a mechanical pump-molecular pump combination, so that the vacuum degree of the sputtering chamber is 5x10 -3 Pa or below.

7. The method for preparing the gas-sensitive membrane of a MEMS gas sensor according to claim 5, characterized in that: In step one, after evacuation, argon gas with a stable flow rate is introduced into the sputtering chamber until the pressure in the sputtering chamber stabilizes within the range of 0.5 Pa to 1.2 Pa.

8. The method for preparing the gas-sensitive film of the MEMS gas sensor according to claim 5, characterized in that: In steps one through three, SnO2, Ag, and Pd targets are respectively mounted on a magnetron sputtering instrument, and the distance between the target and the micro-heating plate substrate is controlled to be between 5 cm and 15 cm.

9. The method for preparing the gas-sensitive membrane of the MEMS gas sensor according to claim 5, characterized in that: The micro heating plate substrate includes a micro heating plate layer and a substrate. Magnetron sputtering is performed on the micro heating plate layer, and a cavity is provided between the micro heating plate layer and the substrate.

10. The method for preparing the gas-sensitive membrane of the MEMS gas sensor according to claim 9, characterized in that: The micro heating plate layer is provided with a silicon nitride insulating protective layer, the thickness of which is 300nm to 500nm.

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