MEMS (Micro Electro Mechanical System) heat conduction type gas sensor and preparation method thereof

Abstract

The invention provides an MEMS heat-conducting gas sensor and a preparation method thereof. The MEMS heat-conducting gas sensor comprises a silicon-based structure layer, a first silicon nitride layer, a first composite metal layer and a second silicon nitride layer which are sequentially arranged from bottom to top, wherein the silicon-based structure layer is provided with a through heat conduction cavity which is used for accommodating external gas for heat exchange; the first silicon nitride layer, the first composite metal layer and the second silicon nitride layer of the corresponding parts above the heat conduction cavity form a suspended silicon nitride supporting beam, and a metal heating electrode in the first composite metal layer is located in the middle of the interior of the silicon nitride supporting beam. The heat exchanger is used for exchanging heat with external gas in the heat conduction cavity so as to detect the concentration of the target to-be-detected gas. The detection sensitivity of the thermal conductivity type gas sensor can be improved, and the response time can be shortened.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of testing or analyzing materials by measuring physical properties of the materials, or the technical field of devices or systems of microstructure, and in particular to a MEMS thermal conductivity gas sensor and a preparation method thereof. BACKGROUND

[0002] As a key carrier of future energy system, hydrogen energy attracts much attention in the field of safety monitoring technology. However, the highly volatile and flammable characteristics (explosion limit 4vol%-75vol%) make hydrogen concentration monitoring a key link to ensure industrial safety. In application scenarios such as chemical synthesis, oil refining process and fuel cell vehicles, hydrogen leakage may cause catastrophic explosion accidents, directly threatening the safety of operating personnel. The existing technology mainly monitors the hydrogen concentration in the environment by using thermal conductivity gas sensors.

[0003] Traditional thermal conductivity gas sensors use thick-walled glass or metal cavity structures, which have problems such as large volume, high power consumption, and slow response. Existing MEMS thermal conductivity sensors mostly use 2-5μm thick silicon nitride support films, which have large heat capacity and limited sensitivity, and are difficult to meet the needs of low concentration scenarios. When the thickness of the support beam decreases, the solid heat transfer can be significantly reduced, and the thermal efficiency can be improved, so the ultra-thin support beam can significantly reduce the detection limit. However, the thinnest silicon nitride support structure recorded in the prior art is 500nm, and the concentration detection limit for gases such as hydrogen is 2000ppm (parts per million), which has not yet broken through the process bottleneck of sub-micron scale. SUMMARY

[0004] The present application provides a MEMS thermal conductivity gas sensor and a preparation method thereof. By depositing a first silicon nitride layer through an LPCVD (Low-Pressure Chemical Vapor Deposition) process, depositing a second silicon nitride layer through a PECVD (Plasma-Enhanced Chemical Vapor Deposition) process, and combining deep silicon etching and wet etching trimming processes, a suspended silicon nitride support beam with a total thickness of 120-500nm is prepared, breaking through the thickness limit of existing silicon nitride support structures, reducing the detection lower limit for hydrogen concentration to 500ppm, improving the detection sensitivity of the thermal conductivity gas sensor, and shortening the response time.

[0005] In a first aspect, the present application provides a MEMS thermal conductivity gas sensor, comprising a silicon-based structure layer, a first silicon nitride layer, a first composite metal layer and a second silicon nitride layer arranged in order from bottom to top; wherein, The silicon-based structural layer has a through-hole thermally conductive cavity for accommodating external gas for heat exchange. The first silicon nitride layer, the first composite metal layer, and the second silicon nitride layer above the thermally conductive cavity form a suspended silicon nitride support beam. The metal heating electrode in the first composite metal layer is located in the middle of the silicon nitride support beam and is used to exchange heat with the external gas in the thermally conductive cavity to detect the concentration of the target gas.

[0006] In some embodiments, the first composite metal layer includes the metal heating electrode and the metal pad connector, the metal heating electrode being located at the center of the first silicon nitride layer, the metal pad connector being located at the edge of the first silicon nitride layer, the first composite metal layer being presented with a first preset pattern, and the first composite metal layer including a first adhesion layer and a first metal layer sequentially disposed on the first silicon nitride layer.

[0007] In some embodiments, the silicon nitride support beam is provided with a plurality of through holes that penetrate the first silicon nitride layer and the second silicon nitride layer and surround the metal heating electrode, and the plurality of through holes are in communication with the thermally conductive cavity.

[0008] In some embodiments, a second composite metal layer is disposed on the surface of the metal pad connector, the second composite metal layer includes gold pads, the second silicon nitride layer covers the second composite metal layer, and the gold pads are exposed on the surface of the second silicon nitride layer; the second composite metal layer is presented in a second preset pattern, and the second composite metal layer includes a second adhesion layer and a second metal layer disposed sequentially.

[0009] In some embodiments, the metal pads are connected to an external circuit and are energized to heat the metal heating electrode through the metal pad connector.

[0010] In some embodiments, the materials of the first adhesive layer and the second adhesive layer include at least one of the following: titanium, chromium, and nickel; the material of the first metal layer includes at least one of the following: nickel, gold, and platinum; and the material of the second metal layer is gold. The thickness of the first adhesive layer and the second adhesive layer is 3-20 nm, the thickness of the first metal layer is 20-500 nm, and the thickness of the second metal layer is 100-500 nm. The thickness of the silicon nitride material in the first silicon nitride layer is 100-500 nm, the thickness of the silicon nitride material in the second silicon nitride layer is 20-500 nm, and the thickness of the silicon nitride support beam is 120 nm-500 nm.

[0011] Secondly, embodiments of this application provide a method for fabricating a MEMS thermally conductive gas sensor, applicable to the MEMS thermally conductive gas sensor described in the first aspect of this application. The method includes: depositing a first silicon nitride layer on a silicon-based structural layer at 650°C using an LPCVD process, ensuring that the thin film stress of the first silicon nitride layer is less than 100 MPa; sequentially fabricating a first composite metal layer and a second composite metal layer on the first silicon nitride layer; covering the surface of the first composite metal layer, the surface of the second composite metal layer, and a localized area of ​​the first silicon nitride layer not covered by the first and second composite metal layers using a PECVD process to cover the second silicon nitride layer with the second silicon nitride layer; etching the silicon substrate surface of the silicon-based structural layer and the surface of the second silicon nitride layer to form a through thermally conductive cavity and a silicon nitride support beam containing vias on the silicon-based structural layer, and exposing the gold pads in the second composite metal layer on the surface of the second silicon nitride layer.

[0012] In some embodiments, the process involves spin-coating photoresist onto the surface of the first silicon nitride layer and performing a first photolithography; fabricating the first composite metal layer on the first silicon nitride layer according to a first preset pattern using magnetron sputtering, and washing away excess photoresist; spin-coating photoresist onto the surface of the first silicon nitride layer and performing a second photolithography; and fabricating the second composite metal layer on the first silicon nitride layer according to a second preset pattern using magnetron sputtering, and washing away excess photoresist. The second composite metal layer is disposed on the surface of the metal pad connector in the first composite metal layer.

[0013] In some embodiments, etching the silicon substrate surface of the silicon-based structural layer and the surface of the second silicon nitride layer includes: spin-coating photoresist onto the silicon substrate surface of the silicon-based structural layer and performing a third photolithography to draw the etching area; etching the silicon-based structural layer in the etching area using a DRIE deep silicon etching process to form a thin silicon-based structural layer; spin-coating photoresist onto the surface of the second silicon nitride layer and performing a fourth photolithography to draw a first contour of the silicon nitride support beam and a second contour of the gold pad; etching the second silicon nitride layer according to the first contour and the second contour to form a silicon nitride support beam including through-holes, and exposing the gold pads on the surface of the second silicon nitride layer; and etching away the thin silicon-based structural layer using a wet etching process to form a through thermally conductive cavity on the silicon-based structural layer.

[0014] In some embodiments, after etching the silicon substrate surface of the silicon-based structural layer and the surface of the second silicon nitride layer, the method further includes: placing the etched thermally conductive gas sensor in a nitrogen atmosphere and performing a first annealing process at 300°C for 30 minutes to eliminate metal layer stress; performing a second annealing process at 450°C for 15 minutes to enhance interface adhesion; and performing a third annealing process at 800°C for 30-180 minutes to eliminate instability between metal atoms.

[0015] As can be seen from the embodiments of this application, the MEMS thermal conductivity gas sensor includes a silicon-based structural layer, a first silicon nitride layer, a first composite metal layer, and a second silicon nitride layer arranged sequentially from bottom to top. The silicon-based structural layer has a through-hole for accommodating external gas for heat exchange. Above the thermal conductivity cavity, the corresponding portion of the first silicon nitride layer, the first composite metal layer, and the second silicon nitride layer forms a suspended silicon nitride support beam. The metal heating electrode in the first composite metal layer is located in the middle of the silicon nitride support beam, used for heat exchange with the external gas inside the thermal conductivity cavity to detect the concentration of the target gas. Thus, the MEMS thermal conductivity gas sensor provided by this application includes a suspended silicon nitride support beam with a total thickness of 120-500 nm, breaking through the thickness limit of existing silicon nitride support structures, reducing the detection limit for hydrogen concentration to 500 ppm, improving the detection sensitivity of the thermal conductivity gas sensor, and shortening the response time. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a cross-sectional view of a MEMS thermal conductivity gas sensor provided in an embodiment of this application; Figure 2 This is a front view of a MEMS thermal conductivity gas sensor provided in an embodiment of this application. Figure 3 This is a schematic diagram of the back structure of a MEMS thermal conductivity gas sensor provided in an embodiment of this application; Figure 4 This is a simulation diagram of the thermal field distribution of a MEMS thermal conductivity gas sensor provided in an embodiment of this application; Figure 5This is a schematic diagram of the overall process of a method for fabricating a MEMS thermally conductive gas sensor provided in an embodiment of this application; Figure 6 This is a flowchart illustrating the steps involved in fabricating a MEMS thermally conductive gas sensor, as provided in an embodiment of this application. Figure 7 This is a flowchart illustrating the steps involved in fabricating a metal layer in a MEMS thermally conductive gas sensor, as provided in an embodiment of this application. Figure 8 This is a flowchart illustrating the etching process for a MEMS thermally conductive gas sensor, as provided in an embodiment of this application.

[0018] Explanation of reference numerals in the attached figures: 1-MEMS thermally conductive gas sensor, 10-silicon-based structural layer, 20-first silicon nitride layer, 30-first composite metal layer, 40-second composite metal layer, 50-second silicon nitride layer, 110-thermally conductive cavity, 120-silicon substrate surface, 210-silicon nitride support beam, 211-through hole, 310-metal heating electrode, 320-metal pad connector, 410-gold pad. Detailed Implementation

[0019] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application.

[0020] The terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish different objects, not to describe a specific order. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or apparatuses.

[0021] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0022] In the embodiments of this application, "and / or" describes the relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists alone; A and B exist simultaneously; B exists alone. Among them, A and B can be singular or plural.

[0023] In this embodiment, the symbol " / " can indicate that the preceding and following objects are in an "or" relationship. Alternatively, the symbol " / " can also represent a division sign, i.e., performing a division operation. For example, A / B can mean A divided by B.

[0024] In the embodiments of this application, "at least one item" or its similar expression refers to any combination of these items, including any combination of a single item or a plurality of items. "One or more" means one or more, while "multiple" means two or more. For example, "at least one item" of a, b, or c can represent the following seven cases: a, b, c; a and b; a and c; b and c; a, b, and c. Each of a, b, and c can be an element or a set containing one or more elements.

[0025] In the embodiments of this application, "equal to" can be used with "greater than" and is applicable to technical solutions used when "greater than" is used; it can also be used with "less than" and is applicable to technical solutions used when "less than" is used. When "equal to" is used with "greater than", it is not used with "less than"; when "equal to" is used with "less than", it is not used with "greater than".

[0026] Traditional thermal conductivity gas sensors employ thick-walled glass or metal cavity structures, resulting in large size, high power consumption, and slow response. Existing MEMS thermal conductivity sensors mostly utilize 2-5 μm thick silicon nitride support films, which have high heat capacity, limited sensitivity, and are difficult to meet the requirements of low-concentration applications. Reducing the thickness of the support beam can significantly decrease solid-state heat transfer and improve thermal efficiency; therefore, ultrathin support beams can significantly lower the detection limit. However, the thinnest silicon nitride support structure currently recorded is 500 nm, with a detection limit of 2000 ppm, indicating that the submicron-scale process bottleneck has not yet been overcome.

[0027] To address the aforementioned issues, this application provides a MEMS thermal conductivity gas sensor and its fabrication method. The embodiments of this application will be described in detail below with reference to the accompanying drawings.

[0028] Please see Figure 1 , Figure 1 This is a cross-sectional view of a MEMS thermal conductivity gas sensor provided in an embodiment of this application, as shown below. Figure 1As shown, the MEMS thermal conductivity gas sensor 1 includes a silicon-based structural layer 10, a first silicon nitride layer 20, a first composite metal layer 30, and a second silicon nitride layer 50 arranged sequentially from bottom to top. The silicon-based structural layer 10 has a through-hole thermal conductivity cavity 110 for accommodating external gas for heat exchange. Above the thermal conductivity cavity 110, the corresponding portions of the first silicon nitride layer 20, the first composite metal layer 30, and the second silicon nitride layer 50 form a suspended silicon nitride support beam 210. A metal heating electrode 310 in the first composite metal layer 30 is located in the middle of the silicon nitride support beam 210 and is used to exchange heat with the external gas inside the thermal conductivity cavity 110 to detect the concentration of the target gas.

[0029] The silicon-based structural layer 10 can be made of silicon or other materials combined with silicon.

[0030] It is understood that a first silicon nitride layer 20, a first composite metal layer 30, and a second silicon nitride layer 50 are sequentially disposed above the silicon-based structural layer 10. One or more composite metal layers or other components may be disposed between the first silicon nitride layer 20 and the second silicon nitride layer 50, with the silicon nitride layer serving a sealing function. The thermally conductive cavity 110 specifically manifests as a through-hole in the silicon-based structural layer 10, which can be a cylindrical through-hole, a cubic through-hole, etc. The portion of the composite structural layer corresponding to the area above the through-hole of the thermally conductive cavity 110 constitutes a suspended silicon nitride support beam 210, while the remaining non-suspended portion of the composite structural layer is disposed on the top end face of the silicon-based structural layer 10.

[0031] Furthermore, the first composite metal layer 30 includes a metal heating electrode 310 and a metal pad connector 320. The metal heating electrode 310 is located at the center of the first silicon nitride layer 20, and the metal pad connector 320 is located at the edge of the first silicon nitride layer 20. The first composite metal layer 30 is presented with a first preset pattern. The first composite metal layer 30 includes a first adhesion layer and a first metal layer sequentially disposed on the first silicon nitride layer.

[0032] In one embodiment, the material of the first adhesive layer includes at least one of the following: titanium, chromium, and nickel, and the material of the first metal layer includes at least one of the following: nickel, gold, and platinum. The thickness of the first adhesive layer is preferably 5-10 nm.

[0033] In one embodiment, the thickness of the first adhesive layer is 3-20 nm, and the thickness of the first metal layer is 20-500 nm. Preferably, the thickness of the first adhesive layer is 5-10 nm, and the thickness of the first metal layer is 50-150 nm.

[0034] The metal heating electrode 310 is used for heating and thermal signal sensing. When energized, it can stably generate heat, forming a thermal field concentrated inside the silicon nitride support beam 210. When the gas to be tested in the thermally conductive cavity 110 exchanges heat with the electrode, the difference in gas thermal conductivity will cause the electrode temperature (or resistance) to change. The electrode converts the gas thermal conductivity characteristics into a detectable electrical signal through this parameter change, providing the core basis for subsequent concentration calculation. At the same time, its position in the middle of the silicon nitride support beam 210 maximizes the contact area with the gas, reduces the ineffective heat loss to the edge, and ensures detection sensitivity.

[0035] The metal pad connector 320 is used for power transmission and structural connection, efficiently transferring the electrical energy input from the external circuit to the metal heating electrode 310 to provide energy support for its heating. In addition, its edge distribution design keeps it away from the high-temperature area of ​​the metal heating electrode 310, avoiding heat loss due to its own heat conduction, reducing power loss, and ensuring the stability of the electrode power supply.

[0036] The first preset pattern refers to the planar layout shape of the first composite metal layer 30 defined by photolithography. Its design must meet the following conditions: (1) Thermal field distribution requirements: The metal heating electrode 310 is usually designed as a serpentine, spiral or other tortuous structure to increase the effective heating length within a limited area, improve the heat density and make the thermal field distribution more uniform; (2) Conductive path optimization: The metal pad connector 320 needs to be designed as a wide line or fan-shaped structure, extending from both ends of the metal heating electrode 310 to the edge, ensuring low resistance while avoiding spatial interference with other structures; (3) Process compatibility: The pattern needs to be adapted to the coverage area of ​​the subsequent second silicon nitride layer 50 and the position of the via 211 to ensure that the metal heating electrode 310 is effectively wrapped by the first silicon nitride layer 20 and the second silicon nitride layer 50, while reserving a precise stacking area for the deposition of the second composite metal layer 40.

[0037] Specifically, the metal heating electrode 310 and the metal pad connector 320 are both partial areas of the first composite metal layer 30. They have different functions, and the material of the first metal layer selected needs to be determined according to the process and application scenario, whether the same material or different materials are used.

[0038] From the perspective of basic structure, both the metal heating electrode 310 and the metal pad connector 320 follow the double-layer structure of the first composite metal layer, namely "first adhesive layer and first metal layer". The material of the first adhesive layer is usually completely consistent. The adhesive layer is used to solve the interface bonding problem between the metal layer and the first silicon nitride layer 20, ensuring that the first metal layer can be stably attached and avoiding interlayer peeling.

[0039] From the perspective of core functions, the first metal layers corresponding to the metal heating electrode 310 and the metal pad connector 320 can be made of the same material, and deposition can be completed in one magnetron sputtering, simplifying the process and reducing costs. As long as the material can meet the basic requirements of high temperature resistance of the heating electrode and low resistance of the connector, it can be adapted to the function and is suitable for scenarios that focus on process efficiency. Alternatively, the first metal layers corresponding to the metal heating electrode 310 and the metal pad connector 320 can be made of different materials. For example, the metal heating electrode 310 can be made of high temperature resistant and stable temperature coefficient of resistance materials such as platinum and nickel, while the metal pad connector 320 can be made of low resistance and high conductivity materials such as gold. This can accurately match their respective functions and optimize performance, but requires two photolithography and sputtering processes, which are more complex and are suitable for high-performance scenarios that pursue low detection limits and fast response speeds.

[0040] It should be noted that this application does not limit the specific materials of the first metal layer corresponding to the metal heating electrode 310 and the metal pad connector 320. The relevant processes can be adjusted according to actual needs and usage scenarios. Also, this application does not limit the metal heating electrode 310 to be placed inside the silicon nitride support beam 210. It can also extend from inside the silicon nitride support beam 210 to the surface of the second silicon nitride layer 50. It can be achieved by controlling the thickness of the second silicon nitride layer 50.

[0041] In one embodiment, the silicon nitride support beam 210 is provided with a plurality of through holes 211 that penetrate the first silicon nitride layer 20 and the second silicon nitride layer 50 and surround the metal heating electrode 310, and the plurality of through holes 211 are connected to the thermally conductive cavity 110.

[0042] Understandably, by setting multiple through holes 211 on the silicon nitride support beam 210, penetrating the first silicon nitride layer 20 and the second silicon nitride layer 50 and surrounding the metal heating electrode 310, and communicating with the thermally conductive cavity 110, it can provide a multi-path efficient flow channel for external gas, accelerate the speed at which gas reaches the detection area, and improve the sensor response speed. On the other hand, it can enhance the thermal convection effect, making the heat exchange between the gas and the metal heating electrode 310 more efficient, improving the detection sensitivity. At the same time, it can also disperse thermal stress, enhance the thermal shock resistance of the silicon nitride support beam 210, and its patterned design matches the structural process of the metal layers, ensuring the spatial matching degree of each layer and the connectivity of the subsequent conductive path.

[0043] Specifically, the through-hole 211 can be square, trapezoidal, circular, or other shapes. The number of through-holes 211 needs to be designed according to the actual requirements and structural conditions of the sensor, and there are usually multiple through-holes 211. It should be noted that when setting through-holes 211 on the silicon nitride support beam 210, the metal heating electrode 310 should be avoided to prevent damage to the heating component.

[0044] Furthermore, such as Figure 1As shown, a second composite metal layer 40 is provided on the surface of the metal pad connector 320. The second composite metal layer 40 includes gold pads 410. A second silicon nitride layer 50 covers the second composite metal layer 40, and the gold pads 410 are exposed on the surface of the second silicon nitride layer 50. The second composite metal layer 40 is presented with a second preset pattern. The second composite metal layer 40 includes a second adhesion layer and a second metal layer disposed sequentially.

[0045] In one embodiment, the material of the second adhesive layer includes at least one of the following: titanium, chromium, nickel, and the material of the second metal layer is gold.

[0046] In one embodiment, the thickness of the second adhesive layer is 3-20 nm, and the thickness of the second metal layer is 100-500 nm. Preferably, the thickness of the second adhesive layer is 5-10 nm, and the thickness of the second metal layer is 150-300 nm.

[0047] In one embodiment, the gold pad 410 is connected to an external circuit and is heated by electricity supplied to the metal heating electrode 310 through the metal pad connector 320.

[0048] The gold pad 410 serves to physically contact and conduct electricity with the external circuit. The core function of the second silicon nitride layer 50 is to protect the internal structures, such as the metal heating electrode 310 and the metal pad connector 320, from direct exposure to the external environment and prevent contamination and oxidation. The gold pad 410 is exposed on the surface of the second silicon nitride layer 50 to ensure that the external circuit can directly act on the gold pad 410, thus achieving a complete conductive path between the external circuit, the gold pad 410, the second composite metal layer 40, the metal pad connector 320, and the metal heating electrode 310.

[0049] The second preset pattern is the planar shape of the second composite metal layer 40 defined by photolithography. It needs to adapt to both connection requirements and spatial constraints. Common forms include: square or rectangular pads, "L" or "U" shaped pads, and pads with "leads".

[0050] It should be noted that the second composite metal layer 40 is disposed on the metal pad connector 320, rather than inside the silicon nitride support beam 210. The reason is that the core of the second composite metal layer 40 is the gold pad 410, which needs to be located at the edge of the device for easy connection to external circuits. The metal pad connector 320 extends to the edge, and the combination of the two can shorten the conductive path and avoid interference with the silicon nitride support beam 210.

[0051] like Figure 1As shown, the second composite metal layer 40 is disposed on the metal pad connector 320 and is completely adapted to the surface structure of the metal pad connector 320. However, this does not mean that the first preset pattern and the second preset pattern are exactly the same, nor does it mean that the second composite metal layer 40 can exactly cover the surface of the metal pad connector 320. The pattern shape of the second composite metal layer 40 needs to be designed based on comprehensive considerations such as conductivity and sensor performance. Furthermore, the gold pad 410 is a partial area of ​​the metal layer in the second composite metal layer 40, exposed on the surface of the second silicon nitride layer 50 for connection with external circuitry. Figure 1 As shown, the gold pads 410 are symmetrically distributed on the left and right sides of the silicon nitride support beam 210. This application does not limit the position of the gold pads 410 on the surface of the second silicon nitride layer 50. This application only provides one example.

[0052] Understandably, the silicon-based structural layer 10 serves as a basic support, providing a stable substrate; the first silicon nitride layer 20 not only acts as an insulator but also provides a suitable surface for the subsequent preparation of the metal layer; the metal heating electrode 310 in the first composite metal layer 30 is a key component for realizing thermal conductivity detection; and the second silicon nitride layer 50 provides protection and further insulation for the internal structure.

[0053] In one embodiment, the thickness of the silicon nitride material in the first silicon nitride layer 20 is 100-500 nm, the thickness of the silicon nitride material in the second silicon nitride layer 50 is 20-500 nm, and the thickness of the silicon nitride support beam 210 is 120 nm-500 nm. Preferably, the thickness of the silicon nitride material in the first silicon nitride layer 20 is 300 nm, and the thickness of the silicon nitride material in the second silicon nitride layer 50 is preferably 100-200 nm.

[0054] Understandably, the first silicon nitride layer 20 (100-500nm) serves as the main support layer for the supporting beam. The lower limit of 100nm ensures the mechanical strength of the suspended structure, while the upper limit of 500nm avoids excessive thickness that could lead to increased heat loss. The second silicon nitride layer 50 (20-500nm) focuses on protection and insulation. The lower limit of 20nm achieves basic coverage, while the upper limit adapts to the protection requirements of different scenarios. The two layers are stacked to form a supporting beam of 120-500nm, breaking through the submicron bottleneck while ensuring structural reliability.

[0055] Furthermore, the first silicon nitride layer 20 is preferably 300nm, which can provide sufficient support without significantly increasing thermal conductivity loss due to excessive thickness, thus balancing strength and thermal efficiency; the second silicon nitride layer 50 is preferably 100-200nm, with a moderate thickness, which is more resistant to environmental corrosion and protects the internal metal layer than the basic value of 20nm, and is thinner than the upper limit of 500nm, avoiding increasing the overall weight and stress of the support beam, so that the support beam achieves the optimal state between "strong protection" and "low load", which is suitable for the performance requirements of most gas detection scenarios.

[0056] As can be seen, in this embodiment, a layered architecture consisting of a silicon-based structural layer 10, a double silicon nitride layer, and a double composite metal layer is adopted to reduce the thickness of the silicon nitride support beam 210 to 120-500nm, thereby breaking through the submicron process bottleneck. At the same time, through holes 211 surrounding the metal heating electrode 310 are designed on the support beam. This, combined with the functional partitioning of the metal heating electrode 310 and metal pad connector 320 located in the first composite metal layer 30 and the gold pad 410 in the second composite metal layer 40, not only enhances gas flow and heat exchange efficiency, but also ensures the reliability of external connections through the exposure of the gold pad 410. Furthermore, the precise matching of the materials and thicknesses of each layer improves structural stability and process compatibility, thereby achieving improved sensor sensitivity, shortened response time, and lower detection limit.

[0057] Please see Figure 2 , Figure 2 This is a front structural diagram of a MEMS thermal conductivity gas sensor provided in an embodiment of this application, as shown below. Figure 2 As shown, the gold pads 410 in the second composite metal layer 40 are clearly exposed on the surface of the second silicon nitride layer 50. They are usually distributed in a square or rectangular array in the edge area of ​​the device, serving as a direct interface for connection with external circuits. The outline of the silicon nitride support beam 210 is visible in the central area. Multiple through holes 211 surrounding the metal heating electrode 310 are arranged in a regular pattern and penetrate the silicon nitride support beam 210, forming windows for gas flow. Overall, the area presents a clear partition with the gold pads 410 at the edges and the detection area in the center.

[0058] Among them, the silicon nitride support beam 210 is presented as a central suspended frame on the front. Its outline corresponds to the projection of the thermally conductive cavity 110 of the silicon-based structural layer 10 below, ensuring that the silicon nitride support beam 210 can accurately cover the thermally conductive cavity 110, providing a stable suspended carrier for the metal heating electrode 310, while avoiding the edge of the silicon nitride support beam 210 from exceeding the silicon base range, which would cause the structure to become fragile.

[0059] It is important to note that the silicon nitride support beam 210 can cover most of the hollow area corresponding to the thermally conductive cavity 110, or it can cover only a small portion of the hollow area corresponding to the thermally conductive cavity 110. This depends on the location and size of the through holes 211. For example, if the through holes 211 are scattered and have small diameters, the remaining solid area of ​​the silicon nitride support beam 210 is sufficient to cover most of the hollow area corresponding to the thermally conductive cavity 110. This ensures gas flow through the through holes 211 while enhancing the stability of the suspended structure by relying on the complete support surface, thus avoiding local stress concentration. If the through holes 211 are concentrated and have large diameters, the solid area of ​​the support beam is reduced, and it can only cover a small portion of the hollow area corresponding to the thermally conductive cavity 110. In this case, it is necessary to optimize the arrangement of the through holes 211 to ensure efficient gas flow while ensuring that the silicon nitride support beam 210 can still stably support the metal heating electrode 310, thus preventing structural collapse due to insufficient coverage.

[0060] Among them, the metal heating electrodes 310 are distributed in a serpentine or spiral shape in the central region of the silicon nitride support beam 210, thereby increasing the heat density by increasing the effective heating length.

[0061] It should be noted that the front side of the MEMS thermal conductivity gas sensor 1 should be covered by the second silicon nitride layer 50, with only the gold pads 410 and vias 211 exposed on the surface of the second silicon nitride layer 50. Therefore, Figure 2 The metal heating electrode 310 shown is actually hidden under the second silicon nitride layer 50 on the front of the sensor.

[0062] As can be seen, in this embodiment, the exposed design of the gold pads 410 in the edge region provides physical contact points, ensuring that after the external current is input through the gold pads 410, it is efficiently conducted to the central metal heating electrode 310 via the lower metal pad connector 320 (hidden under the second silicon nitride layer 50); the combination of the through hole 211 in the central region and the silicon nitride support beam 210 ensures that the gas to be tested quickly enters the thermally conductive cavity 110 and exchanges heat with the metal heating electrode 310, so that the electrical energy input by the gold pads 410 is efficiently converted into the thermal signal required for detection.

[0063] Please see Figure 3 , Figure 3 This is a schematic diagram of the back structure of a MEMS thermal conductivity gas sensor provided in an embodiment of this application, as shown below. Figure 3 As shown, the back of the MEMS thermally conductive gas sensor 1 has a thermally conductive cavity 110.

[0064] The silicon substrate 120 is etched to form a through opening that corresponds to the projection of the silicon nitride support beam 210 on the front side. This opening is the back channel of the thermally conductive cavity 110. Its outline usually matches the size of the silicon nitride support beam 210 on the front side, presenting an overall structure of "hollowed-out in the center and supported by silicon substrate at the edges".

[0065] Specifically, the opening of the thermally conductive cavity 110 on the back of the MEMS thermally conductive gas sensor 1 is formed by a combination of deep silicon etching (DRIE) and wet etching processes. The edge smoothness and dimensional accuracy of this cavity directly determine the suspension stability of the silicon nitride support beam 210 on the front. If the back opening is too large, the support area of ​​the silicon-based structural layer 10 for the silicon nitride support beam 210 will be insufficient; if the opening is too small, it will restrict gas flow. Therefore, Figure 3 The design of the opening on the back side needs to match the size of the front detection area and ensure the mechanical strength of the silicon-based structural layer 10. It is a direct reflection of the balance between "structural stability" and "gas flow".

[0066] As can be seen, in this embodiment, the front of the MEMS thermally conductive gas sensor 1 is suspended detection area based on the silicon nitride support beam 210, and the back is formed by silicon-based etching to form a gas inlet and outlet channel, so that the gas to be measured can enter the thermally conductive cavity 110 from the back opening, and then be conducted through the through hole 211 on the silicon nitride support beam 210 on the front. The gas is heated by the metal heating electrode 310, forming a bidirectional gas flow path with the back inlet and the front outlet, avoiding the gas retention problem caused by the traditional single-end opening, and providing structural support for shortening the response time.

[0067] For further details, please refer to Figure 4 , Figure 4 This is a simulation diagram of the thermal field distribution of a MEMS thermal conductivity gas sensor provided in an embodiment of this application, such as... Figure 4 As shown, a simulation diagram of the thermal field distribution of the MEMS thermally conductive gas sensor 1 is presented. The temperature spatial distribution of the metal heating electrode 310 during operation is intuitively presented through color gradients (such as red-orange representing the high temperature area and blue-violet representing the low temperature area).

[0068] The high-temperature region is concentrated at the metal heating electrode 310 in the center of the silicon nitride support beam 210. The temperature gradually decreases from the electrode to the surrounding area, forming a "thermal field gradient" centered on the metal heating electrode 310. The boundary of the thermal field coincides with the outline of the thermally conductive cavity 110 and the silicon nitride support beam 210.

[0069] Understandably, the centrally located heating electrode design concentrates heat in the core detection area, reducing ineffective conduction to the peripheral silicon substrate. The low thermal conductivity of the silicon nitride support beam (compared to the silicon substrate) further suppresses heat loss, keeping the high-temperature zone stably confined above the cavity and ensuring efficient heat exchange with the gas to be measured. While the through holes in the support beam accelerate gas flow, they do not disrupt the concentration of the thermal field, ensuring both heat exchange efficiency and avoiding a decrease in detection sensitivity due to excessive temperature diffusion.

[0070] As can be seen, in this embodiment, simulations can verify the matching degree between the heating electrode power and the thermal field range (e.g., ensuring that the high-temperature zone completely covers the cavity) and the influence of the support beam thickness on heat loss (e.g., the thermal field concentration of a 120nm thin support beam). This guides the adjustment of parameters such as the metal layer pattern and the number of vias, ensuring that the sensor can accurately reflect the difference in gas thermal conductivity through changes in the thermal field. This provides thermal design verification support for reducing the detection limit of gases such as hydrogen concentration to 500ppm. Furthermore, it verifies that the MEMS thermal conductivity gas sensor provided by this solution, through a suspended silicon nitride support beam with a total thickness of 120-500nm, can break through the thickness limit of the existing thinnest silicon nitride support structure of 500nm, reducing the detection limit from 2000ppm to 500ppm, improving the detection sensitivity of the thermal conductivity gas sensor, and shortening the response time.

[0071] Please see Figure 5 , Figure 5 This is a schematic diagram of the overall process of fabricating a MEMS thermal conductivity gas sensor according to an embodiment of this application, as shown below. Figure 5 As shown, the transformation process from silicon-based structural layer 10 to the complete MEMS thermally conductive gas sensor 1 is intuitively demonstrated.

[0072] in, Figure 5 (a) shows the appearance of the silicon-based structural layer 10 after the first silicon nitride layer 20 has been deposited: the silicon base serves as the supporting substrate, and the surface is covered with a uniform first silicon nitride layer 20. This stage provides a flat and insulating substrate for the subsequent metal layer preparation and is the basis of the "multilayer stacked structure".

[0073] in, Figure 5 (b) to Figure 5 (c) in the text usually corresponds to the preparation process of the composite metal layer: Figure 5 (b) shows the patterned state of the first composite metal layer 30 (first adhesion layer, first metal layer) after photolithography and magnetron sputtering. The center is the pattern corresponding to the metal heating electrode 310, and the edge is the pattern corresponding to the metal pad connector 320, clearly presenting the layout of the "first preset pattern". Figure 5 Image (c) shows the deposition effect of the second composite metal layer 40 on the surface of the metal pad connector 320, clearly indicating that it only covers the edge area of ​​the metal pad connector 320, reserving space for the subsequent exposure of the gold pad 410. Further, a second silicon nitride layer 50 is deposited on the surface of the second composite metal layer 40 for encapsulation.

[0074] in, Figure 5(d) focuses on the key steps of the second silicon nitride layer 50 covering and etching, showing the state formed by photolithography etching after the second silicon nitride layer 50 completely covers the first composite metal layer 30, the second composite metal layer 40, and the exposed first silicon nitride layer 20. At this time, the outline of the silicon nitride support beam 210 is visible, and part of the silicon substrate surface 120 of the silicon-based structure layer 10 is about to be etched through. Only a thin layer of silicon-based structure layer 10 remains in the etched area, still attached to the first silicon nitride layer 20, forming an unconnected thermally conductive cavity 110. The remaining thin layer of silicon-based structure layer 10 in the etched area is used to support the etching of the gold pad 410, the via 211, and the second silicon nitride layer 50 on the front side of the sensor.

[0075] in, Figure 5 (e) shows the final state of the sensor: the silicon substrate 120 is formed by DRIE deep silicon etching and wet etching to form a through thermally conductive cavity 110, the silicon nitride support beam 210 is suspended above the cavity, and the gold pad 410 is fully exposed on the surface of the second silicon nitride layer 50. The whole structure shows the complete stacking between "silicon substrate, silicon nitride, and metal layer" and the corresponding functional structure of "cavity, support beam, through hole, and pad".

[0076] As can be seen, in this embodiment, the MEMS thermally conductive gas sensor 1 includes a silicon-based structural layer 10, a first silicon nitride layer 20, a first composite metal layer 30, and a second silicon nitride layer 50 arranged sequentially from bottom to top. The silicon-based structural layer 10 has a through thermally conductive cavity 110 for accommodating external gas for heat exchange. The portion of the first silicon nitride layer 20, the first composite metal layer 30, and the second silicon nitride layer 50 above the thermally conductive cavity 110 forms a suspended silicon nitride support beam 210. The metal heating electrode 310 in the first composite metal layer 30 is located in the middle position inside the silicon nitride support beam 210 and is used to exchange heat with the external gas inside the thermally conductive cavity 110 to detect the concentration of the target gas. Thus, the MEMS thermal conductivity gas sensor 1 provided in this application includes a suspended silicon nitride support beam 210 with a total thickness of 120-500nm, which breaks through the thickness limit of existing silicon nitride support structures, reduces the detection limit of hydrogen concentration to 500ppm, improves the detection sensitivity of the thermal conductivity gas sensor, and shortens the response time.

[0077] Please see Figure 6 , Figure 6 This is a flowchart illustrating the steps involved in fabricating a MEMS thermally conductive gas sensor, as provided in an embodiment of this application. Figure 6As shown, the method includes: S601, depositing a first silicon nitride layer on a silicon-based structural layer at 650°C using an LPCVD process to ensure that the film stress of the first silicon nitride layer is less than 100 MPa; S602, sequentially preparing a first composite metal layer and a second composite metal layer on the first silicon nitride layer; S603, covering the surface of the first composite metal layer, the surface of the second composite metal layer, and local areas of the first silicon nitride layer not covered by the first and second composite metal layers using a PECVD process to cover the second silicon nitride layer; S604, etching the silicon substrate surface of the silicon-based structural layer and the surface of the second silicon nitride layer to form a through-hole thermally conductive cavity and a silicon nitride support beam containing vias on the silicon-based structural layer, and exposing the gold pads in the second composite metal layer on the surface of the second silicon nitride layer.

[0078] Specifically, a first silicon nitride layer of 100-500 nm is deposited at 650 °C using LPCVD process, with a film thickness uniformity of ≤2% and a refractive index of 1.95±0.02; a second silicon nitride layer of 20-500 nm (100-200 nm is preferred) is deposited using PECVD process to form a protective layer for the composite metal layer.

[0079] The principle of LPCVD (Low-Pressure Chemical Vapor Deposition) is as follows: Under sub-atmospheric pressure, gaseous reactant materials are transported to a reaction chamber, where they undergo a chemical reaction or decomposition on the heated substrate (such as a silicon-based structural layer), generating solid materials that are deposited on the substrate surface to form a thin film. Taking the deposition of the first silicon nitride layer in the fabrication of a MEMS thermally conductive gas sensor as an example, silane and ammonia are typically used as reactant gases. At a high temperature of 650°C, silane and ammonia react to generate silicon nitride, which is then deposited on the silicon-based structural layer.

[0080] Specifically, by precisely controlling the film stress to <100MPa using the LPCVD process, on the one hand, the silicon nitride layer can be prevented from warping and cracking due to excessive stress, ensuring its structural integrity as a support and insulating layer and preventing components such as suspended heating electrodes from losing stable support; on the other hand, appropriate tensile stress can optimize the interfacial bonding force between the silicon nitride layer and the silicon-based and metal layers, reducing the risk of interlayer delamination, and ensuring that the various structural layers of the sensor work stably and collaboratively during long-term thermal cycling and gas detection, maintaining the accuracy and reliability of thermal conductivity detection.

[0081] The principle of PECVD (Plasma-Enhanced Chemical Vapor Deposition) is as follows: under relatively low pressure, reactive gases are excited by energy sources such as radio frequency or microwaves, causing them to form plasma. The active particles in the plasma (such as ions, atoms, and free radicals) have higher reactivity, enabling rapid chemical reactions at lower temperatures and deposition of a thin film on the substrate surface. In the fabrication of MEMS thermally conductive gas sensors, commonly used reactive gases include silane and ammonia to coat the surfaces of the first and second composite metal layers with a second silicon nitride layer.

[0082] Specifically, please refer to Figure 7 , Figure 7 This is a flowchart illustrating the steps involved in fabricating a metal layer in a MEMS thermally conductive gas sensor, as provided in an embodiment of this application. Figure 7 As shown, the step of sequentially preparing a first composite metal layer and a second composite metal layer on a first silicon nitride layer includes: S701, spin-coating photoresist onto the surface of the first silicon nitride layer and performing a first photolithography; S702, preparing a first composite metal layer on the first silicon nitride layer according to a first preset pattern using a magnetron sputtering process, and washing away excess photoresist; S703, spin-coating photoresist onto the surface of the first composite metal layer and the surface of the first silicon nitride layer and performing a second photolithography; S704, preparing a second composite metal layer on the first silicon nitride layer according to a second preset pattern using a magnetron sputtering process, and washing away excess photoresist.

[0083] Specifically, a photoresist is spin-coated onto the surface of the first silicon nitride layer for the first photolithography. The photoresist can be ROL1733, AZ4562, AZ5214, AZGXR601, etc. The first composite metal layer is sputtered by magnetron sputtering. A first adhesion layer (Ti, Cr, Ni, etc.) of 3-20nm (5-10nm is preferred) and a first metal layer Ni / Au / Pt (20-500nm, 50-150nm is preferred) are sequentially deposited on the surface of the first silicon nitride layer. Then, a lift-off process is performed to wash away the excess photoresist, leaving only the first composite metal layer presented according to the first preset pattern, thereby realizing the patterning of the metal layer.

[0084] A second photolithography is performed by spin-coating photoresist onto the surface of the first silicon nitride layer. Photoresist options include ROL1733, AZ4562, AZ5214, and AZGXR601. A second composite metal layer is deposited using magnetron sputtering. A second adhesion layer (Ti, Cr, Ni, etc.) of 3-20 nm (5-10 nm is preferred) and a second metal layer (Au) of 100-500 nm (150-300 nm is preferred) are sequentially deposited on the surface of the first silicon nitride layer. Then, a lift-off process is performed to pattern the metal layer and form gold pads, which facilitates subsequent chip bonding.

[0085] The second composite metal layer is disposed on the surface of the metal pad connector in the first composite metal layer, rather than on the entire surface of the first composite metal layer.

[0086] Magnetron sputtering is a physical vapor deposition (PVD) technique widely used in semiconductor manufacturing and MEMS device fabrication. In MEMS thermal conductivity gas sensor fabrication, it is used to deposit thin film materials such as metal layers. Specifically, under vacuum, a high-voltage electric field ionizes inert gases such as argon to generate plasma. Argon ions are accelerated and bombard the cathode target, sputtering target atoms. These atoms, fueled by energy, fly towards the anode substrate to deposit a film. Simultaneously, the magnetic field confines electrons, increasing plasma density and improving sputtering efficiency.

[0087] Lift-off technology, also known as the lift-off process, is an important method for patterning thin film materials in semiconductor and micro / nano fabrication. It is often used in conjunction with thin film deposition processes such as magnetron sputtering and is used to form precise patterns of metal layers in the fabrication of MEMS thermally conductive gas sensors. Specifically, the lift-off process first spin-coates photoresist onto a substrate and photolithographically prints the target pattern. Then, a thin film is deposited to cover the entire surface (including the photoresist). Finally, the photoresist is removed with a solvent, and the thin film on its surface is peeled off. Ultimately, only the areas of the substrate not covered by photoresist retain the thin film, forming the desired pattern.

[0088] Further, please refer to Figure 8 , Figure 8 This is a flowchart illustrating the etching steps for a MEMS thermal conductivity gas sensor, as provided in an embodiment of this application. Figure 8As shown, the etching on the silicon substrate surface of the silicon-based structural layer and the surface of the second silicon nitride layer includes: S801, spin-coating photoresist on the silicon substrate surface of the silicon-based structural layer and performing a third photolithography to draw the etching area; S802, using DRIE deep silicon etching process to etch the silicon-based structural layer in the etching area to form a thin silicon-based structural layer; S803, spin-coating photoresist on the surface of the second silicon nitride layer and performing a fourth photolithography to draw the first contour of the silicon nitride support beam and the second contour of the gold pad; S804, etching the second silicon nitride layer according to the first and second contours to form a silicon nitride support beam including through holes, and exposing the gold pad on the surface of the second silicon nitride layer; S805, etching away the thin silicon-based structural layer by wet etching process to form a through thermally conductive cavity on the silicon-based structural layer.

[0089] Specifically, a third photolithography step is performed on the silicon substrate of the silicon-based structural layer by spin-coating photoresist to mark the etching area. Photoresist options include AZ4562, AZ5214, and AZGXR601. Then, a deep silicon etching (DRIE) process is used to etch deep silicon in the etched area, removing 450-490 μm of silicon, leaving only a thin silicon-based structural layer to support etching on the front side of the sensor. Next, a fourth photolithography step is performed on the front side of the sensor, i.e., the surface of the second silicon nitride layer, by spin-coating photoresist to mark the first contour of the silicon nitride support beam (including the contour of the vias) and the second contour of the gold pads. Reactive ion etching (RIE) is then performed based on these contours, such as using a photoresist mask to achieve silicon nitride RIE etching. The etching gas can be selected from various options. , , This process involves creating windows in the second silicon nitride layer and exposing the gold pads; further, a wet etching process is used to etch away the thin silicon-based structural layer, forming a through-hole thermally conductive cavity on the silicon-based structural layer.

[0090] In one embodiment, wet etching includes etching the silicon substrate at 70-90°C using a KOH (20-40%) or TMAH (5-25%) solution to ensure that the suspension film breakage rate of the first silicon nitride layer is <0.1%.

[0091] Among them, DRIE deep silicon etching technology is a high-precision deep trench etching technology for materials such as silicon. Its core achieves high aspect ratio structures by alternating "etching" and "passivation" steps: the etching stage uses plasma (such as...) Etching the silicon surface; passivation stage using A fluorocarbon polymer protective layer is formed, covering the sidewalls but not affecting the bottom surface; alternating cycles can achieve vertical etching up to hundreds of micrometers deep, with high precision and good sidewall perpendicularity, making it suitable for fabricating thermally conductive cavities (through-openings on the back of silicon substrates) for MEMS sensors, and can precisely control the cavity size and the width of the silicon substrate edge support.

[0092] Among them, silicon nitride RIE etching is a dry etching technique for silicon nitride layers, utilizing reactive gases, such as... , The active ions generated in the plasma react chemically with silicon nitride to produce volatile products, achieving selective etching. Its etching rate is controllable, and it has a high selectivity for materials such as metal layers. It is suitable for etching through-holes in silicon nitride support beams and exposing gold pad areas in sensors. It can precisely preserve the support beam structure while avoiding damage to the underlying metal layer, ensuring gas channels and electrical connections.

[0093] In some embodiments, after etching the silicon substrate surface of the silicon-based structural layer and the surface of the second silicon nitride layer, the method further includes: placing the etched thermally conductive gas sensor in a nitrogen atmosphere and performing a first annealing process at 300°C for 30 minutes to eliminate metal layer stress; performing a second annealing process at 450°C for 15 minutes to enhance interface adhesion; and performing a third annealing process at 800°C for 30-180 minutes to eliminate instability between metal atoms.

[0094] Annealing addresses potential issues in sensor fabrication and ensures long-term stability through controlled heating and cooling. Its functions include eliminating residual stress in the metal layer after sputtering and etching, preventing subsequent cracking and warping that could damage the support beam; promoting atomic diffusion at the interfaces of each layer through thermal activation, enhancing the bonding force between the metal, silicon nitride, and silicon substrate, and preventing interlayer delamination; and driving metal atom rearrangement and grain growth, eliminating atomic disorder and grain boundary defects, ensuring the stability of the metal layer's conductivity and high-temperature resistance, and avoiding affecting the accuracy of the detection thermal field.

[0095] It should be noted that this application divides the annealing process into three stages, each under a nitrogen atmosphere, with different temperatures and durations for each stage. This application only provides one example. In practice, the gas atmosphere can be adjusted according to time requirements and sensor performance, such as using inert gases like argon or a mixture of hydrogen and nitrogen. The annealing temperature and duration for each stage also need to be adjusted based on the specific circumstances.

[0096] As can be seen, in this embodiment, the MEMS thermal conductivity gas sensor includes a silicon-based structural layer, a first silicon nitride layer, a first composite metal layer, and a second silicon nitride layer arranged sequentially from bottom to top. The silicon-based structural layer has a through-hole for accommodating external gas for heat exchange. Above the thermal conductivity cavity, the corresponding portions of the first silicon nitride layer, the first composite metal layer, and the second silicon nitride layer form a suspended silicon nitride support beam. The metal heating electrode in the first composite metal layer is located in the middle of the silicon nitride support beam, used for heat exchange with the external gas inside the thermal conductivity cavity to detect the concentration of the target gas. Thus, the MEMS thermal conductivity gas sensor provided in this application includes a suspended silicon nitride support beam with a total thickness of 120-500 nm, breaking through the thickness limit of existing silicon nitride support structures, reducing the detection limit for hydrogen concentration to 500 ppm, improving the detection sensitivity of the thermal conductivity gas sensor, and shortening the response time.

[0097] In this application, the terms "embodiment" and "implementation" mean that a specific feature, structure, or characteristic described in connection with an embodiment can be included in at least one embodiment of this application. The appearance of these phrases in various locations throughout the specification does not necessarily refer to the same embodiment, nor are they independent or alternative embodiments mutually exclusive with other embodiments. Those skilled in the art will understand, explicitly and implicitly, that the embodiments described in this application can be combined with other embodiments. Furthermore, it should be understood that the features, structures, or characteristics described in the various embodiments of this application can be arbitrarily combined to form another embodiment that does not depart from the spirit and scope of the technical solution of this application, provided there is no contradiction between them.

[0098] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of this application should not depart from the spirit and scope of the technical solutions of this application.

Claims

1. A MEMS thermal conductivity gas sensor, characterized in that, It includes, from bottom to top, a silicon-based structural layer, a first silicon nitride layer, a first composite metal layer, and a second silicon nitride layer; wherein, The silicon-based structural layer has a through-hole thermally conductive cavity for accommodating external gas for heat exchange. The first silicon nitride layer, the first composite metal layer, and the second silicon nitride layer above the thermally conductive cavity form a suspended silicon nitride support beam. The metal heating electrode in the first composite metal layer is located in the middle of the silicon nitride support beam and is used to exchange heat with the external gas in the thermally conductive cavity to detect the concentration of the target gas.

2. The MEMS thermal conductivity gas sensor according to claim 1, characterized in that, The first composite metal layer includes the metal heating electrode and the metal pad connector. The metal heating electrode is located at the center of the first silicon nitride layer, and the metal pad connector is located at the edge of the first silicon nitride layer. The first composite metal layer is presented in a first preset pattern. The first composite metal layer includes a first adhesion layer and a first metal layer sequentially disposed on the first silicon nitride layer.

3. The MEMS thermal conductivity gas sensor according to claim 2, characterized in that, The silicon nitride support beam is provided with a plurality of through holes that penetrate the first silicon nitride layer and the second silicon nitride layer and surround the metal heating electrode, and the plurality of through holes are connected to the thermally conductive cavity.

4. The MEMS thermal conductivity gas sensor according to claim 2 or 3, characterized in that, The surface of the metal pad connector is provided with a second composite metal layer, the second composite metal layer includes gold pads, the second silicon nitride layer covers the second composite metal layer, and the gold pads are exposed on the surface of the second silicon nitride layer; The second composite metal layer is presented in a second preset pattern, and the second composite metal layer includes a second adhesive layer and a second metal layer disposed sequentially.

5. The MEMS thermal conductivity gas sensor according to claim 4, characterized in that, The metal pads are connected to an external circuit, and the metal heating electrodes are heated by electricity through the metal pad connectors.

6. The MEMS thermal conductivity gas sensor according to claim 4, characterized in that, The materials of the first adhesive layer and the second adhesive layer include at least one of the following: titanium, chromium, and nickel; the material of the first metal layer includes at least one of the following: nickel, gold, and platinum; and the material of the second metal layer is gold. The thickness of the first adhesive layer and the second adhesive layer is 3-20 nm, the thickness of the first metal layer is 20-500 nm, and the thickness of the second metal layer is 100-500 nm; The thickness of the silicon nitride material in the first silicon nitride layer is 100-500 nm, the thickness of the silicon nitride material in the second silicon nitride layer is 20-500 nm, and the thickness of the silicon nitride support beam is 120 nm-500 nm.

7. A method for preparing a MEMS thermal conductivity gas sensor as described in any one of claims 1-6, characterized in that, Includes the following steps: The first silicon nitride layer was deposited on the silicon-based structural layer at 650°C using LPCVD process, ensuring that the film stress of the first silicon nitride layer was less than 100 MPa; A first composite metal layer and a second composite metal layer are sequentially prepared on the first silicon nitride layer; A second silicon nitride layer is applied to the surface of the first composite metal layer, the surface of the second composite metal layer, and a localized area of ​​the first silicon nitride layer not covered by the first and second composite metal layers using a PECVD process. Etching is performed on the silicon substrate surface of the silicon-based structural layer and the surface of the second silicon nitride layer to form a through thermally conductive cavity and a silicon nitride support beam containing vias on the silicon-based structural layer, and to expose the gold pads in the second composite metal layer on the surface of the second silicon nitride layer.

8. The method according to claim 7, characterized in that, The step of sequentially fabricating a first composite metal layer and a second composite metal layer on the first silicon nitride layer includes: Photoresist was spin-coated onto the surface of the first silicon nitride layer and a first photolithography was performed. The first composite metal layer is prepared on the first silicon nitride layer according to the first preset pattern using a magnetron sputtering process, and excess photoresist is washed away. Photoresist was spin-coated onto the surface of the first silicon nitride layer, and a second photolithography was performed. The second composite metal layer is prepared on the first silicon nitride layer according to the second preset pattern using a magnetron sputtering process, and excess photoresist is washed away. The second composite metal layer is disposed on the surface of the metal pad connector in the first composite metal layer.

9. The method according to claim 7 or 8, characterized in that, The etching of the silicon substrate surface of the silicon-based structural layer and the surface of the second silicon nitride layer includes: Photoresist is spin-coated onto the silicon substrate surface of the silicon-based structural layer and a third photolithography is performed to draw the etched area. The silicon-based structural layer is etched in the etching area using the DRIE deep silicon etching process to form a thin silicon-based structural layer. Photoresist is spin-coated onto the surface of the second silicon nitride layer and a fourth photolithography is performed to draw the first outline of the silicon nitride support beam and the second outline of the gold pad. Etching is performed on the second silicon nitride layer according to the first and second contours to form a silicon nitride support beam containing through holes, and to expose the gold pads on the surface of the second silicon nitride layer. The thin silicon-based structural layer is etched away using a wet etching process to form a through thermally conductive cavity on the silicon-based structural layer.

10. The method according to claim 9, characterized in that, After etching the silicon substrate surface of the silicon-based structural layer and the surface of the second silicon nitride layer, the method further includes: The etched thermal conductivity gas sensor was placed in a nitrogen atmosphere and subjected to a first annealing process at 300°C for 30 minutes to eliminate stress in the metal layer; and, A second annealing process was performed at 450°C for 15 minutes to enhance interfacial adhesion; and, A third annealing process is performed at 800℃ for 30-180 minutes to eliminate instabilities between metal atoms.

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