Method for manufacturing a mems gas sensor and mems gas sensor

Abstract

This application discloses a method for fabricating a MEMS gas sensor and the MEMS gas sensor itself. Specifically, the method for fabricating the MEMS gas sensor includes: depositing a gas-sensitive layer on a MEMS wafer using plasma-enhanced chemical vapor deposition (PECVD); fabricating the gas-sensitive layer into a pre-defined columnar array structure using a patterning process including a hard mask layer and reactive ion etching (RIE); performing post-annealing on the formed columnar array structure; and depositing a catalyst layer on the surface of the columnar array structure using atomic deposition and / or sputtering techniques, ultimately fabricating the MEMS gas sensor. The method for fabricating the MEMS gas sensor and the MEMS gas sensor provided in this application can solve the problems of low sensitivity, poor selectivity, and slow response speed of existing MEMS gas sensors.

Description

Technical Field

[0001] This application relates to the field of gas sensor technology, and more specifically, to a method for fabricating a MEMS gas sensor and a MEMS gas sensor. Background Technology

[0002] In related technologies, MEMS gas sensors (MEMS refers to microelectromechanical systems, and MEMS gas sensors are gas sensors fabricated on micro silicon chips using MEMS technology), especially tin dioxide (SnO2)-based sensors, face numerous technical bottlenecks in traditional fabrication processes, severely restricting the performance improvement and large-scale application of SnO2-based sensors. On the one hand, traditional methods for fabricating SnO2 nanostructures (such as nanowires and nanopillars) mostly rely on wet processes such as hydrothermal methods and microwave-assisted hydrothermal methods. These processes have significant drawbacks, such as severe contamination, inability to pattern, poor wafer uniformity and consistency, and the high-temperature, high-pressure wet processes cannot be integrated into standard vacuum, planar semiconductor process flows, resulting in poor compatibility. Consequently, MEMS gas sensors fabricated using existing technologies exhibit low sensitivity, poor selectivity, and slow response speed. Summary of the Invention

[0003] The main objective of this application is to provide a method for fabricating a MEMS gas sensor and a MEMS gas sensor in order to solve the problems of low sensitivity, poor selectivity and slow response speed of existing MEMS gas sensors.

[0004] According to one aspect of this application, a method for fabricating a MEMS gas sensor is provided, comprising: A gas-sensitive layer is deposited on a MEMS wafer using plasma-enhanced chemical vapor deposition (PECVD). The gas-sensitive layer is fabricated into a pre-defined columnar array structure using a patterning process that includes a hard mask layer and reactive ion etching. The formed columnar array structure is subjected to post-annealing treatment; A catalytic layer is deposited on the surface of the columnar array structure using atomic deposition and / or sputtering techniques to finally prepare the MEMS gas sensor.

[0005] Furthermore, the precursor for the plasma-enhanced chemical vapor deposition process includes tetramethyltin or tin tetrachloride.

[0006] Furthermore, the hard mask layer comprises a chromium metal layer or a ruthenium metal layer; and / or, The thickness of the hard mask layer is 50nm~200nm.

[0007] Furthermore, the post-annealing treatment temperature is 400℃~600℃; and / or, The atmosphere used in the post-annealing process is air or a nitrogen-oxygen mixture.

[0008] Furthermore, the catalyst layer is made of platinum or palladium.

[0009] Furthermore, the nanoparticles of the catalyst layer have a size of 1 nm to 3 nm, and the platinum or palladium nanoparticles are uniformly distributed on the outer periphery of the columnar array structure.

[0010] Furthermore, the verticality of the sidewalls of the columnar array structure where the catalytic layer is deposited is 90°±5°.

[0011] Furthermore, the material of the gas-sensitive layer includes one or more of the following: nano-metal oxides, carbon nanotubes, two-dimensional materials, organic materials, and inorganic-organic composite materials.

[0012] Furthermore, the MEMS wafer integrates micro heaters and measurement electrodes.

[0013] On the other hand, this application also provides a MEMS gas sensor, which is prepared by the above-described method for preparing a MEMS gas sensor.

[0014] In this application, by employing a process sequence of first depositing a SnO2 gas-sensitive layer using PECVD, then depositing and patterning a hard mask layer, and finally etching the gas-sensitive layer to form a columnar structure, the wafer-level uniformity of the SnO2 gas-sensitive layer can be ensured, improving the consistency of device performance from the source. It also enables high-precision three-dimensional columnar structure forming of the SnO2 gas-sensitive layer, maximizing the gas-sensitive specific surface area. Furthermore, it avoids deposition contamination and interface damage to the SnO2 gas-sensitive layer caused by the hard mask layer, ensuring the performance of the gas-sensitive layer. Finally, by depositing noble metal materials on the columnar structure, the MEMS sensor is endowed with extremely high specific surface area and catalytic activity, achieving high sensitivity, high selectivity, and fast response of the MEMS gas sensor. Attached Figure Description

[0015] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, are illustrative and descriptive, serving to explain this application and do not constitute an undue limitation thereof. In the drawings: Figure 1 This is a logic diagram of a method for fabricating a MEMS gas sensor disclosed in an embodiment of this application; Figure 2 This is a flowchart illustrating the fabrication process of a MEMS gas sensor disclosed in an embodiment of this application.

[0016] The above figures include the following reference numerals: 10. MEMS wafer; 20. Gas-sensitive layer; 21. Columnar array structure; 30. Hard mask layer; 40. Photoresist; 50. Catalyst layer. Detailed Implementation

[0017] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0018] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0019] Unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps set forth in these embodiments do not limit the scope of the invention. It should also be understood that, for ease of description, the dimensions of the various parts shown in the drawings are not drawn to actual scale. Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of the specification. In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values. It should be noted that similar reference numerals and letters in the following figures denote similar items; therefore, once an item is defined in one figure, it need not be further discussed in subsequent figures.

[0020] As described in the background section, existing MEMS gas sensors often rely on wet processes such as hydrothermal and microwave-assisted hydrothermal methods, resulting in MEMS gas sensors with low sensitivity, poor selectivity, and slow response. Therefore, this application provides a novel method for fabricating a MEMS gas sensor. This method maintains the high specific surface area advantage of the MEMS gas sensor's nanostructure while endowing the sensor with extremely high catalytic activity, resulting in a MEMS sensor with high sensitivity, high selectivity, and fast response speed. The fabrication method of the MEMS gas sensor of this application will be described below with reference to the accompanying drawings.

[0021] See Figure 1 and Figure 2 As shown in the figure, this application provides a method for fabricating a MEMS gas sensor.

[0022] Understandably, MEMS stands for Micro-Electro-Mechanical Systems, an advanced technology that integrates multiple disciplines such as microelectronics, micromachining, materials science, and precision machinery manufacturing. At its core, it consists of miniaturized and integrated mechatronic devices / systems fabricated on microscale substrates such as silicon using semiconductor-compatible processes. The size of these devices is typically in the micrometer to millimeter range.

[0023] In this application, the fabrication method of the MEMS gas sensor includes: The first step is to deposit a gas-sensitive layer 20 on the MEMS wafer 10 using plasma-enhanced chemical vapor deposition.

[0024] Specifically, the MEMS wafer 10 provided in this application integrates a micro heater (not shown in the figure) and a measurement electrode (not shown in the figure), and the MEMS wafer 10 is a silicon-based wafer. The pre-integrated micro heater enables precise and rapid temperature regulation, which can activate the redox reaction between the gas-sensitive layer 20 and the target gas, improving the sensitivity and response speed of the MEMS gas sensor, and also achieve selective detection of different gases through temperature control. Simultaneously, the measurement electrode is directly integrated into the MEMS wafer 10 and aligned with the deposition area of ​​the gas-sensitive layer 20. After the gas-sensitive layer 20 reacts with the target gas, the changes in its electrical properties such as resistance / conductivity can be directly and in real-time acquired through the measurement electrode, eliminating the need for additional electrode fabrication and alignment steps, reducing intermediate steps in signal transmission, and improving the accuracy and response speed of the detection signal. Furthermore, the measurement electrode is integrated through MEMS wafer-level processes, ensuring consistent contact between the measurement electrode and the gas-sensitive layer 20, and improving the sensor yield.

[0025] Furthermore, plasma-enhanced chemical vapor deposition (PECVD) is a mature dry deposition process in the semiconductor / MEMS field. It can directly perform wafer-level large-area deposition on silicon-based MEMS wafers pre-integrated with micro-heaters and measurement electrodes, without the need for wet processes. This avoids the corrosion and contamination caused by wet solutions to the integrated micro-heaters and measurement electrodes, ensuring the integrity of the original integrated structure. At the same time, the PECVD process can be seamlessly integrated with subsequent etching, patterning and other processes, making it suitable for industrialized wafer-level mass production.

[0026] Furthermore, in this application, the PECVD process, through plasma-enhanced reaction, enables the deposition of the gas-sensitive layer 20 at medium to low temperatures (200℃~500℃), avoiding thermal damage to the integrated micro-heater and measurement electrodes caused by high-temperature deposition. Simultaneously, this configuration results in a high film density and uniform thickness for the deposited gas-sensitive layer 20, with strong adhesion to the MEMS wafer 10. This effectively prevents problems such as detachment and cracking of the gas-sensitive layer 20 during long-term operation, thus extending the lifespan of the MEMS gas sensor.

[0027] Furthermore, the materials of the gas-sensitive layer 20 include one or more of the following: nano-metal oxides, carbon nanotubes, two-dimensional materials, organic materials, and inorganic-organic composite materials. Currently, the mainstream materials for the gas-sensitive layer 20 are metal oxides. Metal oxides have advantages such as low cost, high sensitivity, good stability, and good compatibility with MEMS processes, making them the current mainstream for commercialization. Examples include tin dioxide (SnO2), zinc oxide (ZnO), and tungsten trioxide (WO3). Two-dimensional materials can be transition metal sulfides such as graphene and MoS2. Two-dimensional materials have a large specific surface area, high potential for room temperature operation and low power consumption, but their long-term stability needs improvement. Organic materials or inorganic-organic composite materials, such as conductive polymers and perovskites, can also be used. These materials can operate at room temperature and have strong molecular designability, but they have slow response speeds and poor stability. Materials such as carbon nanotubes have unique electrical / catalytic properties and are often used as sensitizers or to construct heterostructures.

[0028] In this application, the preferred material for the gas-sensitive layer 20 is SnO2, a metal oxide, which possesses high sensitivity and stability. This material also exhibits excellent process compatibility and film quality, and is inexpensive and readily available. Furthermore, the SnO2 film has high mechanical strength, making it less susceptible to damage during packaging or use. The difference in thermal expansion coefficient between SnO2 and the silicon-based MEMS wafer 10 and commonly used measurement electrodes is within an acceptable range, reducing the risk of stress cracking or delamination caused by thermal cycling. Moreover, on the silicon-based MEMS wafer 10, the thickness and morphology of the SnO2 layer can be precisely controlled using photolithography and etching combined with PECVD (e.g., fabricating nanowires, nanopillars, or porous films), thereby maximizing the sensitivity of the MEMS gas sensor.

[0029] In this application, when using the PECVD process, a SnO2 thin film with a thickness of 50nm to 1000nm is deposited on the MEMS wafer 10 as a gas-sensitive layer 20 using a tin organometallic compound or halide as a precursor. When the thickness of the gas-sensitive layer 20 is within the above range, the sensitivity, response speed, power consumption, and mechanical stability of the MEMS gas sensor can be balanced.

[0030] The choice of precursor must be compatible with the PECVD process to ensure low-temperature, high-quality crystallization. Utilizing high-energy electron bombardment from plasma, tin-based organometallic compounds or halides can be effectively decomposed at temperatures ranging from 200°C to 350°C, promoting SnO2 nucleation and growth. This not only protects the underlying circuitry but also allows for grain size control through plasma energy, forming nanocrystalline structures and further enhancing sensitivity. Furthermore, the precursor deposited by vapor deposition possesses excellent fluidity, enabling uniform coating of complex three-dimensional surfaces to form thin films of uniform thickness. Simultaneously, PECVD is a standard process in the semiconductor industry, achieving uniform deposition with minimal thickness deviation on entire silicon-based MEMS wafers (such as 4-inch, 6-inch, 8-inch, or 12-inch wafers). This means that a large number of MEMS gas sensors produced in the same batch exhibit highly consistent performance, solving the problem of high dispersion in traditional coating processes.

[0031] Furthermore, in a plasma environment, the active groups generated by the decomposition of precursors can form stronger chemical bonds or physical anchors with the electrodes (such as Pt, Au) or insulating layers on the surface of silicon-based MEMS wafers. This strong interfacial bonding ensures that the gas-sensitive layer 20 remains firmly attached after repeated thermal shocks to the MEMS gas sensor, without delamination failure.

[0032] Furthermore, the precursors for plasma-enhanced chemical vapor deposition (PECVD) include tetramethyltin (Sn(CH3)4) or tin tetrachloride (SnCl4). It is understood that both tetramethyltin and tin tetrachloride are volatile tin-based compounds that can rapidly vaporize at the PECVD deposition temperature of 200℃~500℃, reacting with oxygen sources (such as O2, H2O) in a plasma atmosphere to generate SnO2 without high-temperature pyrolysis. This low-to-medium temperature deposition characteristic effectively avoids thermal damage to the pre-integrated micro-heaters and measurement electrodes on silicon-based MEMS wafers caused by high temperatures, ensuring the integrity and electrical performance of the original integrated structure.

[0033] Furthermore, the PECVD process using tetramethyltin and tin tetrachloride as precursors can produce SnO2 gas-sensitive layers with uniform thickness, high density, and strong adhesion to silicon substrates / electrodes. The composition of the SnO2 gas-sensitive layer is controllable, with no obvious impurities or defects. Simultaneously, during the PECVD process, the oxygen vacancy content in the SnO2 gas-sensitive layer can be precisely controlled by adjusting the flow ratio of the precursors (tetramethyltin / tin tetrachloride) to the oxygen source, thereby improving the response sensitivity of the MEMS gas sensor to target gases.

[0034] Furthermore, tetramethyltin is an organotin precursor, resulting in a SnO2 gas-sensitive layer with low carbon impurity content and a smoother surface, making it suitable for applications requiring high-precision morphology of the gas-sensitive layer 20 (such as high-precision columnar structure etching). Tin tetrachloride, on the other hand, is an inorganic tin precursor with lower raw material costs, easier storage, higher reactivity, and faster deposition rates, making it suitable for wafer-level mass production. In actual production, the appropriate precursor can be selected based on specific needs.

[0035] Furthermore, both tetramethyltin and tin tetrachloride are common precursors in semiconductor processes and can be directly integrated into PECVD wafer-level deposition equipment without modifying existing MEMS / CMOS process lines. This enables large-area, uniform deposition on 8 / 12-inch wafers, ensuring consistent SnO2 gas-sensitive layer performance across all sensor units on the wafer and significantly improving device yield. Compared to stannous chloride and tin nitrate precursors commonly used in sol-gel methods, tetramethyltin / tin tetrachloride eliminates the need for wet solutions, preventing corrosion and contamination of the MEMS wafer's measurement electrodes and microheaters. It also enables dry vapor deposition, avoiding the poor process compatibility and low consistency issues associated with wet processes.

[0036] The second step involves using a patterning process that includes a hard mask layer 30 and reactive ion etching (RIE) to fabricate the gas-sensitive layer 20 into a pre-defined columnar array structure 21. In this embodiment, by using the hard mask layer 30 in conjunction with reactive ion etching (RIE) technology, a columnar array structure 21 with a high aspect ratio and steep sidewalls can be achieved.

[0037] Specifically, in this application, a hard mask layer 30 with a thickness of 50 nm to 200 nm is deposited on the SnO2 gas-sensitive layer 20 by sputtering. This hard mask layer 30 includes a chromium (Cr) metal layer or a ruthenium (Ru) metal layer. In this application, the reactive ion etching system of SnO2 (CF4 / Ar, BCl3 / Cl2 / Ar) has an extremely high etching selectivity (greater than 30), meaning that during the etching of SnO2, the etching rate of Ru or Cr is much lower than that of SnO2 itself. It can be understood that etching selectivity refers to the ratio of the relative etching rates of the etched material to the masking material or adjacent layer material under the same etching conditions. Furthermore, the chromium and ruthenium hard masks can withstand long-term etching, ensuring the high consistency of the SnO2 columnar array structure 21. Simultaneously, chromium and ruthenium have high melting points and will not soften or deform during reactive ion etching, ensuring pattern fidelity. Chromium and ruthenium can be uniformly deposited by sputtering with controllable thickness, making them suitable for wafer-level processing. After etching, the hard mask layer 30 can be selectively removed by wet etching without damaging the underlying SnO2 columnar structure. Of course, in other embodiments of this application, if the material of the gas-sensitive layer 20 changes, the material selection of the hard mask layer 30 will be adaptively chosen according to the change in the gas-sensitive layer 20.

[0038] Meanwhile, in this application, a hard mask layer 30 is sputtered and deposited on the gas-sensitive layer 20. The hard mask patterning process can achieve precise alignment with the micro heaters and measurement electrodes on the MEMS wafer 10 below, ensuring that the columnar structure is located in the sensitive area and improving the consistency of the MEMS gas sensor.

[0039] Further, after sputtering a hard mask layer 30 onto the gas-sensitive layer 20, photoresist 40 is spin-coated onto the hard mask layer 30. After exposure and development, a columnar array pattern is formed. Then, reactive ion etching is used to transfer the columnar array pattern onto the hard mask layer 30. Subsequently, the photoresist 40 is removed. Using the patterned hard mask layer 30 as a mask, a dry etching process based on CF4 / Ar or BCl3 / Cl2 / Ar is employed to etch the SnO2 thin film, forming a columnar array structure 21. Finally, the hard mask layer 30 is removed using a wet process. Of course, in other embodiments of this application, if the material of the gas-sensitive layer 20 is different, the etching system may also be different; the appropriate system can be selected based on the actual situation.

[0040] Removing the photoresist 40 clears the process obstacles for the subsequent reactive ion etching of the SnO2 gas-sensitive layer, preventing photoresist 40 residue from affecting the performance of the MEMS gas sensor. Specifically, the photoresist 40 can be removed by dry removal (plasma ashing) or wet removal (acetone, stripping solution, etc.).

[0041] Understandably, in this embodiment, photoresist 40 is first spin-coated and patterned on the hard mask layer 30 to provide a high-precision pattern template for the hard mask layer 30. After exposure to ultraviolet light, the chemical properties of the exposed / unexposed areas of the photoresist 40 undergo selective changes. The photoresist 40 in the target area can be precisely removed by the developing solution, forming a columnar array photoresist pattern with smooth edges, precise dimensions, and a regular array arrangement. The pattern size deviation can be controlled at the micrometer level. Furthermore, the spin-coating method can form a photoresist film layer with uniform thickness, no pinholes, and no bubbles on the surface of the hard mask layer 30. The photoresist thickness can be precisely controlled by adjusting the spin-coating speed and time, ensuring the consistency of the photoresist pattern of all sensor units on the MEMS wafer 10, laying the foundation for wafer-level uniformity in subsequent hard mask pattern transfer. The spin-coated photoresist 40 can adhere tightly to the metal surface of the hard mask layer 30 without obvious gaps, thus preventing plasma from penetrating through gaps during subsequent reactive ion etching of the hard mask layer 30 and causing non-target etching of the hard mask layer 30, ensuring the accuracy of pattern transfer.

[0042] After removing the photoresist 40, a patterned chromium / ruthenium hard mask layer is used as the sole etching barrier layer. A dedicated CF4 / Ar or BCl3 / Cl2 / Ar etching system is used to replicate the hard mask pattern in proportion to achieve high-precision forming with regular array arrangement. Furthermore, the high aspect ratio design of the columnar array structure 21 significantly increases the specific surface area of ​​the gas-sensitive layer 20, enhancing the contact efficiency with the target gas.

[0043] Specifically, in the CF4 / Ar system, the F produced by the decomposition of CF4 - Ar reacts with SnO2 in the gas phase to generate volatile SnF4, achieving efficient etching of SnO2. + The physical bombardment ensures the anisotropy of the etching, resulting in steeper sidewalls for the columnar structure. In the BCl3 / Cl2 / Ar system, Cl2 decomposes into Cl... - The reaction with SnO2 produces volatile SnCl2, and BCl3 can effectively inhibit the redeposition of etching byproducts, preventing impurity residues from forming on the surface of the columnar array structure 21 of SnO2. Meanwhile, Ar... + The physical bombardment further improves the etching precision.

[0044] Compared to existing technologies that first deposit a hard mask layer and pattern it on the MEMS wafer 10 before finally depositing the target gas-sensitive layer, this application uses a process sequence of first depositing the SnO2 gas-sensitive layer via PECVD, then depositing and patterning the hard mask layer 30, and finally etching the gas-sensitive layer 20 to form a columnar structure. This process ensures the wafer-level uniformity of the SnO2 gas-sensitive layer, improving device performance consistency from the source; it also enables high-precision three-dimensional columnar structure forming of the SnO2 gas-sensitive layer, maximizing the gas-sensitive specific surface area; and it avoids deposition contamination and interface damage to the SnO2 gas-sensitive layer caused by the hard mask layer 30, ensuring the performance of the gas-sensitive layer 20.

[0045] Meanwhile, this embodiment primarily uses dry etching, supplemented by wet etching to remove the mask layer 30. The wet etching selectively removes the hard mask layer 30, avoiding any impact on the SnO2 columnar structure. In other words, this application avoids contamination of SnO2 by metal ions and alkali metal ions in the solution, as well as corrosion of the SnO2 gas-sensitive layer and the pre-integrated micro-heater / measuring electrode, ensuring the integrity of the original integrated structure of the MEMS wafer 10. The wet etching used to remove the hard mask layer 30 is selective etching, employing a specialized etching solution that reacts only with Cr / Ru and not with SnO2 (e.g., a nitric acid-hydrofluoric acid mixture for Cr, and a precious metal-specific etching solution for Ru). This precisely removes the hard mask layer 30 without causing any corrosion or damage to the SnO2 columnar array structure 21. After wet removal of the mask layer 30, a simple cleaning process can achieve zero residue of the hard mask layer 30. The surface and sidewalls of the SnO2 columnar structure remain clean, free of any metal or photoresist impurities. This preserves complete surface active sites for subsequent post-annealing to optimize oxygen vacancies and deposit noble metal catalyst layers, avoiding impurity masking that could lead to a decrease in gas-sensing performance.

[0046] Furthermore, the formed columnar array structure 21 undergoes a post-annealing process. The post-annealing temperature is 400℃~600℃, and exemplarily, the temperature can be 400℃, 500℃, or 600℃. The atmosphere during the post-annealing process is air or a nitrogen-oxygen mixture. The post-annealing time is 30 min~120 min.

[0047] Specifically, after wet removal of the mask layer 30, the surface of the SnO2 columnar array structure is clean and unobstructed by the mask. During post-annealing, the air or nitrogen-oxygen mixture atmosphere can fully contact the surface and sidewalls of the columnar array structure 21, achieving precise control of oxygen vacancy concentration and full optimization of crystallinity, thus improving the intrinsic response sensitivity of the gas-sensitive layer 20. Setting the post-annealing temperature within the aforementioned range allows SnO2 atoms to obtain sufficient energy for rearrangement, recrystallization, and grain growth. Highly crystallized SnO2 exhibits more stable electrical properties and higher carrier mobility. For gas-sensitive materials, grain boundaries are the primary sites for gas adsorption and reaction. Appropriate grain size control can maximize the depletion layer effect, thereby significantly improving sensitivity. Annealing in air or a nitrogen-oxygen mixture (oxygen-containing atmosphere) ensures that sufficient oxygen molecules are adsorbed on the SnO2 surface and dissociated into oxygen ions, capturing conduction band electrons to form a depletion layer. Furthermore, temperatures between 400℃ and 600℃ can maintain oxygen vacancy balance. Compared to pure air, adjusting the N2 / O2 ratio allows for more precise control of the oxidation rate, preventing excessive oxidation on the surface that forms an overly thick inactive layer, while ensuring adequate retention of oxygen vacancies within the cell, thus achieving the optimal balance between sensitivity and stability.

[0048] Furthermore, the gas-sensitive layer 20 accumulates huge internal stress during the deposition and patterning etching process. Within the above temperature range, the internal stress can be released, which makes the geometry and physical properties of the columnar array structure 21 tend to be stable, ensuring that the MEMS sensor has small zero-point drift and long lifespan during subsequent long-term use.

[0049] Furthermore, the post-annealing time is maintained between 30 min and 120 min, for example, 30 min, 60 min, 90 min, and 120 min. Maintaining the post-annealing time within this range provides a sufficient time window, allowing Sn and O atoms ample time for long-range diffusion, completing the transformation from an amorphous / defect state to a stable polycrystalline state, while simultaneously achieving a dynamic equilibrium between surface oxygen adsorption / desorption. This also allows for precise control of grain size, preventing structural collapse and maintaining the shape stability of the columnar array structure 21. In other words, appropriate time control ensures that the columnar array structure 21 maintains its independent columnar morphology while releasing stress, preserving the designed high specific surface area advantage.

[0050] Finally, a catalyst layer 50 is deposited on the surface of the columnar array structure 21 using atomic deposition and / or sputtering techniques, thus fabricating the MEMS gas sensor. The catalyst layer 50 is made of platinum (Pt) or palladium (Pb), with platinum or palladium nanoparticles having a size of 1 nm to 3 nm, and these nanoparticles are uniformly distributed on the outer periphery of the columnar array structure 21. Of course, in other embodiments of this application, the catalyst layer 50 can also be made of other materials among noble metals, and the appropriate material can be selected according to actual needs during production.

[0051] Understandably, atomic deposition, based on self-confined surface reactions, enables sub-nanometer thickness control, forming a uniform, continuous, and pinhole-free catalytic layer 50 on the surface (including sidewalls and top) of the high aspect ratio columnar array structure 21. This ensures that every inch of the surface of each columnar structure possesses catalytic activity, maximizing the utilization of specific surface area. Sputtering can deposit high-purity metal thin films, and by adjusting parameters such as sputtering power, time, and gas pressure, the size and distribution density of nanoparticles can be precisely controlled.

[0052] Specifically, in this application, atomic deposition technology alone or sputtering technology alone can be used, or atomic deposition technology can be used first, followed by sputtering technology, to deposit the catalyst layer 50. In this application, atomic deposition and sputtering technology are preferred for the synergistic deposition of the catalyst layer 50. Atomic deposition is used for the initial layer, and sputtering technology is used for the subsequent layering. Specifically, atomic deposition technology is first used, with trimethylplatinum ((MeCp)PtMe3) and oxygen as precursors, to deposit on a columnar SnO2 surface for 1 to 20 cycles at 100°C to 300°C, forming a highly dispersed platinum nanoparticle layer or palladium nanoparticle layer. This provides uniform nucleation sites for subsequent deposition. Then, through low-power, short-time sputtering, Pt / Pd particles are grown on the nucleation layer. Simultaneously, the plasma bombardment effect of sputtering allows Pt / Pd atoms to form a dual physical and chemical bond with the SnO2 surface. Compared to the chemical adsorption of atomic deposition alone, the film adhesion is improved several times, completely avoiding the problem of catalyst layer 50 detachment in MEMS gas sensors operating at high temperatures. Furthermore, in this process, the atomic-level nucleation layer of atomic deposition guides the uniform growth of sputtered Pt / Pd atoms, avoiding particle agglomeration and localized thick films that are prone to occur in direct sputtering deposition, ensuring a uniform distribution of the catalyst layer across the entire periphery of the columnar array. Through the synergistic effect of these two processes, the catalyst layer 50 achieves an optimal structure with high dispersion, high catalytic activity, and high stability.

[0053] Furthermore, in this application, the size of the nanoparticles in the catalyst layer 50 after atomic deposition and sputtering is between 1 nm and 3 nm. Within this range, Pt / Pd nanoparticles exhibit a high surface atomic ratio and quantum size effect, resulting in the highest catalytic activity. This maximizes catalytic efficiency while ensuring stability, thereby enhancing the sensitivity and response speed of the MEMS gas sensor. Simultaneously, in this application, the high specific surface area advantage of the columnar structure can only be fully realized through uniform coverage of the outer periphery (including the sidewalls and top).

[0054] Furthermore, in this application, the verticality of the sidewalls of the columnar array structure 21 with the deposited catalyst layer 50 is 90°±5°. Here, 90° represents the ideal vertical sidewall, allowing the sidewall area of ​​the columnar structure to reach its theoretical maximum value. This ensures the target gas can fully contact the entire sidewall area and undergo redox reactions with the oxygen vacancy active sites on the SnO2 surface, significantly improving gas contact efficiency. 85° and 95° represent the upper and lower limits of the process, balancing structural formability and specific surface area. Compared to inclined sidewalls outside this range, sidewall verticality within the aforementioned range avoids a significant reduction in effective contact area due to excessive sidewall inclination, and also prevents the columnar array from experiencing "top adhesion and gap blockage" problems. This ensures that the gas can freely diffuse within the array gaps, allowing both the surface and sidewalls of the columnar structure to participate in the gas-sensitive reaction, fully leveraging the specific surface area advantage of the three-dimensional micro / nano structure.

[0055] Meanwhile, the near-vertical sidewalls allow precursors from atomic deposition technology to penetrate unimpeded into the gaps and the entire sidewall region of the columnar array structure 21, achieving atomically uniform nucleation and deposition. Simultaneously, sputtered Pt / Pd atoms adhere uniformly to the sidewall surface, preventing deposition defects such as "thicker at the top, thinner at the bottom" or "localized exposure" due to sidewall tilt. The uniform distribution of the catalyst layer 50 on the near-vertical sidewalls ensures that every gas-sensitive reaction area has a catalytic site. Activated target gas molecules can directly react with the oxygen vacancy active sites of SnO2, achieving a full-area gas-sensing-catalytic synergistic effect. This avoids signal drift caused by localized lack of catalysis, improving the detection accuracy and response consistency of the MEMS gas sensor. Furthermore, the near-vertical sidewalls with a verticality of 90°±5° provide better mechanical support for the columnar structure and offer a flat, continuous substrate for the catalyst layer 50, preventing catalyst layer detachment and aggregation due to structural tilt or sidewall irregularities, thus ensuring the long-term operational stability of the MEMS gas sensor.

[0056] In other words, limiting the verticality of the sidewalls of the columnar array structure 21 after depositing the catalyst layer 50 to 90°±5° represents the optimal balance between structural design, process feasibility, and gas-sensing performance. This verticality not only maximizes the specific surface area and ensures unimpeded gas diffusion, laying the structural foundation for the gas-sensing reaction, but also provides a good substrate for the uniform deposition of the catalyst layer 50, allowing the synergistic effect of gas sensing and catalysis to be fully realized. At the same time, this range is within the range that the MEMS dry etching process can stably achieve, ensuring the consistency and yield of wafer-level mass production, and also improving the mechanical stability of the columnar structure and the adhesion of the catalyst layer 50. Ultimately, this achieves a comprehensive performance improvement in the MEMS gas sensor, characterized by high sensitivity, high response speed, high consistency, and high reliability.

[0057] On the other hand, this application also provides a MEMS gas sensor, which is fabricated using the aforementioned MEMS gas sensor fabrication method. Therefore, the MEMS gas sensor provided in this embodiment includes all the technical effects of the aforementioned MEMS gas sensor fabrication method. Since the technical effects of the MEMS gas sensor fabrication method have been described in detail above, they will not be repeated here.

[0058] As can be seen from the above description, the embodiments of the present invention achieve the following technical effects: (1) This application uses columnar crystal structure formed directly by PECVD combined with deposited noble metal material Pt or Pd to endow MEMS gas sensor with extremely high specific surface area and catalytic activity, thereby achieving high sensitivity, high selectivity and fast response.

[0059] (2) In the fabrication method of the MEMS gas sensor of this application, the entire process is free from wet pollution and can be seamlessly integrated with existing CMOS / MEMS process lines.

[0060] (3) This application can achieve precise positioning and morphology control of SnO2 columnar structure through photolithography and reactive ion etching, ensuring perfect alignment with the measurement electrode on the MEMS wafer.

[0061] (4) The fabrication method of the MEMS gas sensor in this application ensures the uniformity of sensor performance through wafer-level processing, which is suitable for large-scale production.

[0062] For ease of description, spatial relative terms such as "above," "on top of," "on the upper surface of," "above," etc., are used herein to describe the spatial positional relationship of a device or feature as shown in the figures to other devices or features. It should be understood that spatial relative terms are intended to encompass different orientations in use or operation beyond the orientation of the device as described in the figures. For example, if the device in the figures were inverted, a device described as "above" or "on top of" other devices or structures would subsequently be positioned as "below" or "under" other devices or structures. Thus, the exemplary term "above" can include both "above" and "below." The device may also be positioned in other different ways (rotated 90 degrees or in other orientations), and the spatial relative descriptions used herein will be interpreted accordingly.

[0063] Furthermore, it should be noted that the use of terms such as "first" and "second" to define components is merely for the purpose of distinguishing the corresponding components. Unless otherwise stated, the above terms have no special meaning and therefore should not be construed as limiting the scope of protection of this invention.

[0064] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for fabricating a MEMS gas sensor, characterized in that, include: A gas-sensitive layer (20) is deposited on the MEMS wafer (10) using plasma-enhanced chemical vapor deposition. The gas-sensitive layer (20) is fabricated into a preset columnar array structure (21) using a patterning process that includes a hard mask layer (30) and reactive ion etching. The formed columnar array structure (21) is subjected to post-annealing treatment; A catalytic layer (50) is deposited on the surface of the columnar array structure (21) using atomic deposition and / or sputtering techniques, and the MEMS gas sensor is finally prepared.

2. The method for fabricating a MEMS gas sensor according to claim 1, characterized in that, The precursors for the plasma-enhanced chemical vapor deposition process include tetramethyltin or tin tetrachloride.

3. The method for fabricating a MEMS gas sensor according to claim 1, characterized in that, The hard mask layer (30) includes a chromium metal layer or a ruthenium metal layer; and / or, The thickness of the hard mask layer (30) is 50nm~200nm.

4. The method for fabricating a MEMS gas sensor according to claim 1, characterized in that, The post-annealing treatment temperature is 400℃~600℃; and / or, The atmosphere used in the post-annealing process is air or a nitrogen-oxygen mixture.

5. The method for fabricating a MEMS gas sensor according to claim 1, characterized in that, The catalyst layer (50) is made of platinum or palladium.

6. The method for fabricating a MEMS gas sensor according to claim 5, characterized in that, The nanoparticles of the catalyst layer (50) have a size of 1 nm to 3 nm, and the platinum or palladium nanoparticles are uniformly distributed on the outer periphery of the columnar array structure (21).

7. The method for fabricating a MEMS gas sensor according to claim 1, characterized in that, The columnar array structure (21) has a sidewall verticality of 90°±5° where the catalyst layer (50) is deposited.

8. The method for fabricating a MEMS gas sensor according to claim 1, characterized in that, The gas-sensitive layer (20) is made of one or more of the following materials: nano-metal oxides, carbon nanotubes, two-dimensional materials, organic materials, and inorganic-organic composite materials.

9. The method for fabricating a MEMS gas sensor according to any one of claims 1 to 8, characterized in that, The MEMS wafer (10) integrates a micro heater and a measurement electrode.

10. A MEMS gas sensor, characterized in that, The MEMS gas sensor is prepared using the method described in any one of claims 1 to 9.

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