A MEMS hydrogen sensor with high signal-to-noise ratio and a preparation method thereof

Abstract

The application belongs to the technical field of MEMS gas sensors, and relates to a MEMS hydrogen sensor with high signal-to-noise ratio and a preparation method thereof, which comprises a heating electrode, a temperature measuring electrode, a hydrogen sensitive electrode, a wire and an electrode pad, the heating electrode, the temperature measuring electrode and the hydrogen sensitive electrode are electrically connected to the electrode pad through the wire respectively; the heating electrode is a double-gate heating structure, and the heating arms on both sides of the heating electrode are in the shape of a Chinese character'men' to enclose the temperature measuring electrode and the hydrogen sensitive electrode; by adopting the symmetrical double-gate heating electrode structure, the application effectively improves the problem that the temperature gradient of the ordinary heating structure is too large and the signal-to-noise ratio of the sensor is low, so that the temperature uniformity of the hydrogen sensitive area of the sensor is higher under the same heating condition, thereby greatly improving the signal-to-noise ratio of the MEMS hydrogen sensor and improving the problem that the signal-to-noise ratio of the traditional MEMS hydrogen sensor is too low and the lower limit of detection is insufficient.

Description

Technical Field

[0001] This invention belongs to the field of MEMS gas sensor technology, specifically relating to a high signal-to-noise ratio MEMS hydrogen sensor and its fabrication method. Background Technology

[0002] In recent years, with the rapid development of microelectromechanical systems (MEMS) technology, miniaturized MEMS hydrogen sensors have gradually become a potential alternative to traditional hydrogen sensors due to their compact structure and high integration. These sensors typically employ nanoscale thin-film structures, detecting changes in the physical quantities of the gas-sensitive material to output a hydrogen concentration signal. However, due to the inherent characteristics of MEMS hydrogen sensors, they suffer from extremely long response times and relatively low signal-to-noise ratios, resulting in weak gas concentration signals and long detection cycles in MEMS-based hydrogen detection systems. This, to some extent, limits the detection sensitivity and response time of MEMS hydrogen sensors. To improve the sensitivity and response time of MEMS hydrogen sensors, existing technologies mainly employ two optimization strategies: first, heating the sensor with a fixed voltage to reach a predetermined temperature; second, reducing the thickness of the gas-sensitive material to shorten the equilibrium time between the material and the gas.

[0003] However, although existing methods can improve the response time of MEMS gas sensors to some extent, due to their inherent limitations, existing technologies still cannot fully meet the needs of high-precision gas detection, especially in application scenarios that require high signal-to-noise ratio signals, such as trace gas analysis or low-concentration gas detection, where their performance is still significantly insufficient.

[0004] Therefore, how to further improve the signal-to-noise ratio of MEMS hydrogen sensors has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention provides the following technical solution: a high signal-to-noise ratio MEMS hydrogen sensor, comprising: Heating electrode, the heating element of the hydrogen sensor, is used to heat the sensor to its operating temperature.

[0006] Temperature sensing electrode, the temperature sensing element of the hydrogen sensor, is used to monitor the temperature of the sensor chip in real time.

[0007] The hydrogen-sensitive electrode is used to directly sense the hydrogen concentration and convert it into a measurable electrical signal. The hydrogen-sensitive electrode includes a reference electrode and a sensitive electrode formed by electron beam evaporation. The reference electrode is used to provide a reference standard to improve the stability and accuracy of the measurement. The sensitive electrode is used to react directly with hydrogen through a sensitive material coated on the electrode that has a specific response to hydrogen. The reference electrode is a serpentine beam on the outer side of the chip surface, and the sensitive electrode is a serpentine beam on the inner side of the chip.

[0008] The wires and electrode pads, the signal transmission lines and end areas of all the aforementioned electrodes, are used for sensor chip signal transmission and signal connection with the printed circuit board pads.

[0009] The heating electrode, temperature measuring electrode, and hydrogen sensitive electrode are electrically connected to the electrode pads via wires. The heating electrode has a double-gate heating structure, with the heating arms on both sides of the heating electrode forming a gate shape that surrounds the temperature measuring electrode and the hydrogen sensitive electrode.

[0010] Preferably, the heating electrode and the temperature measuring electrode are axially symmetrical about the central axis of the hydrogen-sensitive electrode sensor chip; the temperature measuring electrode is located outside the hydrogen-sensitive electrode.

[0011] More preferably, the heating arm is provided with a horizontal extension portion and a loop portion; the horizontal extension portion is located on the side of the hydrogen sensitive electrode and is parallel to the central axis of the hydrogen sensitive electrode sensor chip; the loop portion is located on the side of the temperature measuring electrode and is bent and wrapped towards the temperature measuring electrode.

[0012] Preferably, the MEMS hydrogen sensor further includes a device package, which includes a printed circuit board and a limiting bracket. After the printed circuit board and the limiting bracket are assembled, the sensor chip can be fixed on the surface of the printed circuit board. After wire bonding, the bonding pads are encapsulated with epoxy resin.

[0013] More preferably, the wire bonding is gold wire bonding; the printed circuit board is a two-layer board; the surface pads of the printed circuit board are treated with immersion gold; and the limiting bracket is made of nylon material.

[0014] Preferably, the electrode body of the hydrogen-sensitive electrode includes several electrode units, each electrode unit having a "U"-shaped structure, with adjacent electrode units connected at the middle position via an electrode bridge, and the end electrode units connected to electrode terminals.

[0015] Preferably, the heating electrode and the temperature measuring electrode are both made of platinum, the wires and electrode pads are made of gold, and the hydrogen-sensitive electrode is made of palladium-nickel alloy.

[0016] Preferably, an aluminum oxide and silicon dioxide passivation layer is prepared on the upper surface of all electrodes.

[0017] This invention also discloses a method for fabricating a high signal-to-noise ratio MEMS hydrogen sensor. This method is used to fabricate the aforementioned MEMS hydrogen sensor and includes the following steps: Step 1: Quartz substrate pretreatment; clean the quartz substrate with a cleaning solution, and deposit a layer of silicon nitride on the clean quartz substrate using ion-enhanced chemical vapor deposition (PECVD).

[0018] Step 2: Fabrication of heating and temperature measurement structures; using photolithography and lift-off processes, several independent heating and temperature measurement structures are deposited on the treated quartz substrate.

[0019] Step 3: Fabrication of conductor and pad structures; using photolithography and stripping processes, several independent conductor and pad structures are deposited on the treated quartz substrate.

[0020] Step 4: Surface passivation layer deposition; using atomic layer deposition (ALD) process, an alumina passivation layer is deposited on the quartz substrate after the first three steps, and then an ion-enhanced chemical vapor deposition (PECVD) process is used to deposit a silicon dioxide passivation layer on the alumina layer.

[0021] Step 5: Fabrication of hydrogen-sensitive structure; an etching process is used to create windows on the surface passivation layer, followed by photolithography and lift-off processes to deposit several hydrogen-sensitive units at the window openings.

[0022] Step 6: Wire bonding and PCB packaging; Assemble the printed circuit board with the limiting bracket, fix the processed MEMS hydrogen sensor on the limiting bracket, use a gold wire bonding machine to connect the sensor pads to the immersion gold pads of the printed circuit board, and use epoxy resin to encapsulate the gold wire connection.

[0023] The beneficial effects of this invention are: 1. This invention, through a two-dimensional structural design of the heating electrode, not only improves the heating temperature uniformity of the sensor and enhances its signal-to-noise ratio, but also maintains the miniaturization advantage of MEMS hydrogen sensors. By employing a symmetrical dual-gate heating electrode structure, this invention effectively increases the temperature uniformity of the heating electrode, resulting in smaller temperature differences in the device. This leads to a higher signal-to-noise ratio for the sensor at the same input power, thereby significantly improving the gas concentration signal quality and solving the problem of insufficient signal-to-noise ratio in traditional MEMS hydrogen sensors.

[0024] 2. This invention improves performance while maintaining the miniaturization characteristics of the sensor, making it perfectly suited to the compact space requirements of integrated hydrogen sensors. It provides technical support and design solutions for the development of next-generation high-performance miniature hydrogen sensors. This invention employs ALD deposition of an alumina passivation layer and PECVD deposition of a silicon dioxide passivation layer, combined with a symmetrical dual-gate heating electrode structure. This improves the sensor's temperature uniformity by 150% compared to traditional heating structures, raising the detection limit to 10 ppm, while maintaining excellent miniaturization characteristics and thermal stability. It is particularly suitable for the high-sensitivity detection requirements of MEMS hydrogen sensors. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the sensor structure of a high signal-to-noise ratio MEMS hydrogen sensor and its fabrication method according to the present invention. Figure 2 This is a schematic diagram of the planar structure of the sensor of the present invention without the passivation layer; Figure 3 This is a flowchart of the sensor manufacturing process of the present invention; Figure 4 This is a printed circuit board diagram of the sensor of the present invention; Figure 5 This is a diagram of the limiting bracket for the sensor of the present invention; Figure 6 This is a physical image of the sensor of the present invention; Figure 7 This is a graph showing the resistance change rate of the sensor of the present invention at 40 degrees Celsius in response to low concentration hydrogen gas, compared with the hydrogen concentration.

[0026] In the diagram, 1 is the heating electrode; 2 is the temperature measuring electrode; 3 is the hydrogen-sensitive electrode; 4 is the wire; 5 is the electrode pad; 11 is the left electrode terminal; 12 is the right electrode terminal; 111 is the left horizontal extension; 121 is the right horizontal extension; 112 is the left loop-shaped part; and 122 is the right loop-shaped part. Detailed Implementation

[0027] The relevant technologies of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0028] like Figures 1-7 As shown, a high signal-to-noise ratio MEMS hydrogen sensor according to this embodiment includes a heating electrode 1, a temperature measuring electrode 2, a hydrogen sensing electrode 3, a wire 4, an electrode pad 5, and a device package.

[0029] The heating electrode 1 is made of platinum metal and is used to heat the sensor to the operating temperature. A silicon dioxide aluminum oxide passivation layer is provided on the upper surface of the electrode 1.

[0030] The temperature measuring electrode 2 is made of platinum metal and is used to monitor the temperature of the sensor chip in real time. A silicon dioxide aluminum oxide passivation layer is provided on the upper surface of the electrode 2.

[0031] The hydrogen-sensitive electrode 3 is made of palladium-nickel alloy material and includes a reference electrode and a sensitive electrode formed by electron beam evaporation. The reference electrode is a serpentine beam on the outer side of the chip surface, and the sensitive electrode is a serpentine beam on the inner side of the chip.

[0032] The wire 4 and electrode pad 5 are made of gold and are used for signal transmission of the sensor chip and signal connection with the printed circuit board pads.

[0033] The device is packaged, including a printed circuit board and a limiting bracket. After assembling the printed circuit board and the limiting bracket, the sensor chip can be fixed on the surface of the printed circuit board. After wire bonding, the bonding pads are encapsulated with epoxy resin. The sensor chip includes a heating electrode 1 and a temperature measuring electrode 2, a hydrogen sensing electrode 3, and wires 4 and electrode pads 5, which are formed in one step.

[0034] The heating electrode 1 includes a left electrode terminal 11 and a right electrode terminal 12 formed in one step. The front end of each of the left and right electrode bodies is designed with a loop-shaped structure. The electrode body includes a left horizontal extension 111, a right horizontal extension 121, a left loop-shaped part 112, and a right loop-shaped part 122. The two loop-shaped parts are bent at a set angle relative to the two horizontal extension parts at a set length.

[0035] In this embodiment, the left loop-shaped portion 112 and the right loop-shaped portion 122 are bent relative to the left horizontal extension portion 111 and the right horizontal extension portion 121 to form a symmetrical gate-shaped heating electrode, which greatly increases the uniformity of heating temperature compared with the heating electrode of the conventional structure.

[0036] like Figure 2 As shown, the heating electrode with a symmetrical gate-shaped structure has its U-shaped part bent three times relative to the horizontal extension part, each time bending 90°.

[0037] A high signal-to-noise ratio (SNR) MEMS hydrogen sensor fabrication process is provided for fabricating the aforementioned high SNR MEMS hydrogen sensor. The process includes the following steps: Step 1: Quartz substrate pretreatment: The quartz substrate was cleaned using a cleaning solution, and then a layer of silicon nitride was deposited on the quartz surface using a PECVD process.

[0038] Specifically, the silicon substrate is ultrasonically cleaned sequentially with acetone, ethanol, and deionized water to remove contaminants from the substrate surface. Subsequently, a silicon nitride layer is deposited on the cleaned quartz substrate surface using plasma-enhanced chemical vapor deposition (PECVD) to increase substrate adhesion and lay the foundation for subsequent processes.

[0039] Step 2, Processing of heating and temperature measuring structures: Platinum metal is deposited on a pretreated quartz substrate using photolithography, sputtering, and lift-off processes to form a heating and temperature measurement unit.

[0040] Specifically, the ultraviolet lithography process is used to spin-coat positive photoresist on the surface of a quartz substrate, and the desired pattern is formed by exposure and development through a mask. Then, the magnetron sputtering (PVD) process is used, with platinum metal as the sputtering source, to sputter 200nm of platinum metal onto the surface pattern formed by the photoresist. Finally, the residual photoresist is removed using a stripping solution to complete the stripping process and leave the complete pattern.

[0041] Step 3, Fabrication of wire and pad structures: Using photolithography, sputtering, and lift-off processes, metallic gold is deposited on a quartz substrate that has undergone the above steps to form wires and pads.

[0042] Specifically, the ultraviolet lithography process is used to spin-coat positive photoresist on the surface of a quartz substrate, and the desired pattern is formed by exposure and development through a mask. Then, the magnetron sputtering (PVD) process is used, with metallic gold as the sputtering source, to sputter 450nm metallic gold onto the surface pattern formed by the photoresist. Finally, the residual photoresist is removed using a stripping solution to complete the stripping process and leave the complete pattern.

[0043] Step 4: Surface passivation layer deposition: An alumina layer was deposited using atomic layer deposition (ALD) and a silicon dioxide layer was deposited using plasma-enhanced chemical vapor deposition (PECVD) to form a surface passivation layer.

[0044] Specifically, using an atomic layer deposition (ALD) device, the precursor and oxide washing were completed sequentially, and alumina was deposited layer by layer until a thickness of 12 nm was reached. Then, a plasma-enhanced chemical vapor deposition (PECVD) device was used. First, the heating device heated the sample to the set temperature and introduced a specific process gas. The gas underwent various chemical reactions under the action of plasma, and finally a 1.5 μm thick solid silicon dioxide film was generated on the sample surface.

[0045] Step 5, Fabrication of the hydrogen-sensitive structure: Using photolithography, vapor deposition, and stripping processes, metallic gold is deposited on a quartz substrate that has undergone the above steps to form wires and pads.

[0046] Specifically, the ultraviolet lithography process is used to spin-coat positive photoresist onto the surface of a quartz substrate, and the desired pattern is formed by exposure and development through a mask. Then, the electron beam evaporation (PVD) process is used, with a fixed ratio of palladium and nickel as the evaporation source, to deposit a 50nm thick palladium-nickel alloy layer on the surface pattern formed by the photoresist. Finally, the residual photoresist is removed using a stripping solution to complete the stripping process and leave the complete pattern.

[0047] Step 6, Wire bonding and PCB packaging: Assemble the printed circuit board with the limiting bracket, fix the finished MEMS hydrogen sensor on the limiting bracket, use a gold wire bonding machine to connect the sensor pads to the immersion gold pads of the printed circuit board, and encapsulate the gold wire connection with epoxy resin.

[0048] Specifically, the printed circuit board and the limiting bracket are designed according to the size of the sensor chip. After assembling the printed circuit board and the nylon limiting bracket, the sensor chip is fixed on the limiting bracket using soft electronic encapsulant. After the adhesive dries, the sensor pads are connected to the immersion gold pads of the printed circuit board using a gold wire soldering machine. The gold wire connection of the pads is then sealed with epoxy resin potting compound to complete the encapsulation.

[0049] This embodiment, based on MEMS fabrication technology, optimizes the structural design of the sensor heating module, improving heating temperature uniformity and enhancing the signal-to-noise ratio (SNR) of the hydrogen sensor output while maintaining the miniaturization advantages of MEMS hydrogen sensors. The invention employs a symmetrical dual-gate heating electrode structure design, effectively addressing the problem of excessive temperature gradients and low SNR in conventional heating structures. This results in higher temperature uniformity in the hydrogen-sensitive area of ​​the sensor under the same heating conditions, significantly improving the SNR of the MEMS hydrogen sensor and overcoming the issues of low SNR and insufficient detection limit in traditional MEMS hydrogen sensors. The symmetrical dual-gate heating structure design of this embodiment enhances performance while maintaining the miniaturization characteristics of the MEMS hydrogen sensor, making it perfectly suited for hydrogen detection requirements in compact spaces.

[0050] In summary, this invention effectively improves the temperature uniformity of the sensor through a symmetrical dual-gate heating electrode structure design. Combined with ALD and PECVD composite passivation layer technology, it significantly enhances the signal-to-noise ratio while maintaining miniaturization characteristics, reducing the detection limit to 10 ppm, thus providing a reliable technical solution for high-precision hydrogen detection. Its fabrication process is clearly defined, involving quartz substrate pretreatment, multi-step electrode structure processing, surface passivation layer deposition, and wire bonding encapsulation, achieving efficient integration of various sensor modules. The resulting sensor exhibits excellent resistivity change rate response to low-concentration hydrogen at 40 degrees Celsius, fully verifying the significant advantages of this invention in improving the performance of MEMS hydrogen sensors.

[0051] It should be emphasized that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A high signal-to-noise ratio MEMS hydrogen sensor, characterized in that, include: Heating electrodes are used to heat the sensor to its operating temperature; Temperature sensing electrodes are used to monitor the temperature of the sensor chip in real time. A hydrogen-sensitive electrode is used to directly sense hydrogen concentration and convert it into a measurable electrical signal. The hydrogen-sensitive electrode includes a reference electrode and a sensitive electrode formed by electron beam evaporation. The reference electrode is used to provide a reference standard to improve the stability and accuracy of the measurement. The sensitive electrode is used to react directly with hydrogen through a sensitive material coated on the electrode that has a specific response to hydrogen. The reference electrode is a serpentine beam on the outer side of the chip surface, and the sensitive electrode is a serpentine beam on the inner side of the chip. Wires and electrode pads are used for signal transmission of the sensor chip and signal connection with the printed circuit board pads; The heating electrode, temperature measuring electrode, and hydrogen-sensitive electrode are electrically connected to the electrode pads via wires. The heating electrode has a double-gate heating structure, and the heating arms on both sides of the heating electrode are arranged in a gate shape to surround the temperature measuring electrode and the hydrogen sensing electrode. The heating electrode and the temperature measuring electrode are symmetrically distributed about the central axis of the hydrogen-sensitive electrode sensor chip; the temperature measuring electrode is located outside the hydrogen-sensitive electrode. The heating arm is provided with a horizontal extension portion and a loop-shaped portion; the horizontal extension portion is located to the side of the hydrogen sensitive electrode and is parallel to the central axis of the hydrogen sensitive electrode sensor chip; the loop-shaped portion is located to the side of the temperature measuring electrode and is bent and wound towards the temperature measuring electrode.

2. The high signal-to-noise ratio MEMS hydrogen sensor according to claim 1, characterized in that, The MEMS hydrogen sensor also includes a device package, which includes a printed circuit board and a limiting bracket.

3. A high signal-to-noise ratio MEMS hydrogen sensor according to claim 2, characterized in that, The MEMS hydrogen sensor uses gold wire bonding for its leads; the printed circuit board is a two-layer board; the surface pads of the printed circuit board are treated with immersion gold; and the limiting bracket is made of nylon material.

4. A high signal-to-noise ratio MEMS hydrogen sensor according to claim 1, characterized in that, The electrode body of the hydrogen-sensitive electrode includes several electrode units, each electrode unit having a "U"-shaped structure. The middle positions of adjacent electrode units are connected by an electrode bridge, and the end electrode units are connected to electrode terminals.

5. A high signal-to-noise ratio MEMS hydrogen sensor according to claim 1, characterized in that, The heating electrode and temperature measuring electrode are both made of platinum, the wires and electrode pads are made of gold, and the hydrogen-sensitive electrode is made of palladium-nickel alloy.

6. A high signal-to-noise ratio MEMS hydrogen sensor according to claim 1, characterized in that, All electrodes have an aluminum oxide and silicon dioxide passivation layer on their upper surface.

7. A method for fabricating a high signal-to-noise ratio MEMS hydrogen sensor, characterized in that, The preparation method is used to prepare the MEMS hydrogen sensor according to any one of claims 1 to 6, and the preparation method includes the following steps: Step 1: Quartz substrate pretreatment; The quartz substrate is cleaned, and silicon nitride is deposited on the quartz substrate using ion-enhanced chemical vapor deposition. Step 2: Fabrication of heating and temperature measurement structures; Several independent heating and temperature measurement structures are deposited on a quartz substrate using photolithography and lift-off processes. Step 3: Fabrication of conductor and pad structures; using photolithography and stripping processes, several independent conductor and pad structures are deposited on a quartz substrate. Step 4: Surface passivation layer deposition; using atomic layer deposition (ALD) to deposit an alumina passivation layer on the quartz substrate after the first three steps, followed by ion-enhanced chemical vapor deposition (IECVD) to deposit a silicon dioxide passivation layer on top of the alumina layer. Step 5: Fabrication of hydrogen-sensitive structure; an etching process is used to create windows on the surface passivation layer, followed by photolithography and lift-off processes to deposit several hydrogen-sensitive units at the window openings; Step 6: Wire bonding and PCB packaging; Assemble the printed circuit board with the limiting bracket, fix the finished MEMS hydrogen sensor on the limiting bracket, connect the sensor pads to the immersion gold pads of the printed circuit board, and encapsulate the gold wire connection with epoxy resin.

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