Preparation method of MEMS-based vehicle-mounted catalytic combustion type hydrogen sensing chip

Abstract

The invention relates to the technical field of structures and preparation of hydrogen sensor chips, in particular to a preparation method of a vehicle-mounted catalytic combustion type hydrogen sensor chip based on an MEMS (micro-electromechanical system), which comprises the following steps: step 1, growing a layer of SiO2 with low thermal conductivity and insulativity on the front and back surfaces of a Si wafer by adopting a plasma enhanced chemical vapor deposition technology; 2, continuing to grow a SiNX structural layer on the front surface of the Si wafer to form a composite supporting layer with the SiO2 / SiNX micron thickness; 3, patterning is conducted on the surface of the composite film through the photoetching technology, a layer of metal platinum is prepared on the surface of the composite film through the magnetron sputtering technology to serve as a heating electrode and a testing electrode, and the two electrodes are arranged in parallel to form a double-snake-shaped structure which comprises a working area and a welding disc. The catalytic combustion type hydrogen sensor has the advantages of being simple in technological process, low in power consumption and high in response speed, and the response time is 1t; 2s.

Description

Technical Field

[0001] The invention relates to the technical field of structure and preparation of hydrogen sensor chips, and in particular to a method for preparing a vehicle-mounted catalytic combustion type hydrogen sensor chip based on MEMS. Background Art

[0002] With technological breakthroughs and cost reduction in production, storage, transportation, refueling and use, hydrogen energy is expected to become the main power engine for future transportation. However, many characteristics of hydrogen, such as low density, low ignition energy, high combustion heat, wide explosion range, high diffusion coefficient, etc., make hydrogen safety the primary issue that needs to be urgently addressed in the application and promotion of hydrogen energy. Placing hydrogen sensors at key nodes of hydrogen energy devices, establishing regional wireless sensor networks and incorporating them into safety supervision systems can provide timely alarms and interventions for hydrogen leaks to ensure hydrogen safety.

[0003] There are many types of commercial hydrogen sensors, mainly including electrochemical, catalytic combustion, thermal conductivity and metal oxide semiconductor. Among them, the catalytic combustion type has the advantages of good robustness, high sensitivity, long life cycle, and easy miniaturization, which is relatively suitable for mass application scenarios such as hydrogen fuel cell vehicles and hydrogen refueling stations. At present, there are many commercial sensors on the market, but it is difficult to meet the high international demand for hydrogen safety warnings. The difficulties are: 1. The high operating temperature leads to high power consumption (the power consumption is generally between 150 and 1000mW), which limits the layout of sensors in mobile applications and sensor nodes, and there are safety hazards; 2. The response rate is slow (>10s), and it is easy to miss the best intervention time.

[0004] Catalytic combustion hydrogen sensors calibrate H2 concentration by detecting the heat generated by flameless combustion of H2 on the catalyst surface. The response of the sensor increases with the increase of operating temperature and finally reaches a constant value. This sensitivity behavior due to temperature change can be explained by the kinetics of hydrogen oxidation reaction. Therefore, in order to achieve a certain reaction rate, the sensor usually needs to work at a higher temperature. However, the ignition energy of hydrogen is low, and high power will bring safety hazards. And for portable devices, the ideal operating power should be less than 100mW. In addition, for vehicle-mounted hydrogen sensors, the equipment is also required to have a certain ability to resist mechanical vibration. Therefore, it is of great practical significance to develop a miniature hydrogen sensor with low power consumption, high mechanical strength and low cost.

[0005] As sensors develop towards integration and miniaturization, the heating structure and hydrogen-sensitive materials are integrated together to form a micro-hotplate, which has the advantages of small size, low power consumption, fast thermal response and easy integration. The two most important performance parameters of the micro-hotplate are power consumption and temperature uniformity. Although most of the sensors reported in the literature have relatively ideal performance, they are not conducive to large-scale production in terms of the convenience of the preparation process and manufacturing cost.

[0006] Therefore, a method for preparing a vehicle-mounted catalytic combustion hydrogen sensor chip based on MEMS is proposed. Summary of the invention

[0007] The purpose of the present invention is to provide a method for preparing a vehicle-mounted catalytic combustion hydrogen sensor chip based on MEMS, so as to reduce the production cost of the sensor and improve the sensing performance, so as to solve the problems raised in the above background technology.

[0008] To achieve the above object, the present invention provides the following technical solution: a method for preparing a vehicle-mounted catalytic combustion hydrogen sensor chip based on MEMS, comprising the following steps:

[0009] Step 1: Use plasma enhanced chemical vapor deposition technology to grow a layer of SiO2 with low thermal conductivity and insulation on the front and back sides of the Si wafer;

[0010] Step 2: Continue to grow SiN on the front side of the Si wafer X Structural layer, forming SiO2 / SiN X Micron-thick composite support layer;

[0011] Step 3: Patterning is performed on the surface of the composite film by photolithography, and a layer of metal platinum is prepared on it by magnetron sputtering as a heating electrode and a test electrode. The two electrodes are arranged in parallel in a double serpentine structure, including a working area and a welding pad;

[0012] Step 4: Continue to deposit a layer of SiO2 insulating layer on the Pt heating electrode and the test electrode as a hydrogen barrier layer to prevent hydrogen from affecting the resistance of the Pt electrode, ensure the accuracy of temperature measurement, and maintain the insulation between the Pt electrode and other components;

[0013] Step 5: On the insulating layer of step 4, patterning is performed using a photolithography process, first a Ti adhesion layer is sputtered, and then a PtPd hydrogen sensitive layer is sputtered;

[0014] Step 6: On the silicon wafer obtained in the above steps, tetramethylammonium hydroxide is used as an etching solution to release the suspended structure, and the silicon substrate is removed by etching under specified temperature conditions, so that the suspended structure of the middle heating and testing area of ​​the micro-hotplate is released, forming a flat-bottomed groove structure, and the cavity reaches the bottom of the first silicon oxide support layer.

[0015] Preferably, in step 2, the silicon nitride layer has tension, while the silicon oxide has the opposite, and the thickness ratio of silicon oxide to silicon nitride is 1:2. At this time, the composite support layer has good tensile strength and is used to form the insulating layer and support layer of the Pt heating electrode.

[0016] Preferably, in step 3, the resistance line width is 20 μm and the overall size is 0.28 mm×0.28 mm.

[0017] Preferably, in step 4, the insulating layer covers the entire sensor chip and leaves a corresponding welding pad area, and a metal Ti adhesion layer needs to be sputtered between the Pt electrode and the SiO2 layer to increase the adhesion between the platinum electrode and SiO2.

[0018] Preferably, in step 5, the hydrogen sensitive layer is located directly above the micro-hotplate and is larger than the micro-hotplate. To increase the contact uniformity with the underlying Pt electrode, the hydrogen sensitive layer adopts a spiral structure with a line width of 30 μm.

[0019] Preferably, in step 6, etching and removing the silicon substrate under specified temperature conditions, the specified temperature conditions are 80°C.

[0020] Preferably, in step 5, the SiO2 insulating layer on the surface of the heating electrode and the test electrode should completely cover the chip, and leave a welding pad area for later wiring.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] The catalytic combustion hydrogen sensor of the present invention has the characteristics of simple process flow, low power consumption and fast response speed, and the response time is less than 2s. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0024] Figure 1 This is a flow chart of the sensor chip of the present invention;

[0025] Figure 2 This is a physical diagram of the heating electrode structure of the present invention;

[0026] Figure 3 It is a structural diagram of the test electrode of the present invention;

[0027] Figure 4 This is the response curve of the hydrogen sensor of the present invention in 4% hydrogen. DETAILED DESCRIPTION

[0028] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] See also Figures 1 to 4 , the present invention provides a technical solution:

[0030] The present invention is dedicated to optimizing the structure and preparation method of the hydrogen sensor chip, reducing the production cost of the sensor and improving the sensing performance.

[0031] In order to achieve the above-mentioned purpose, the present invention first reduces the power consumption of the sensor chip by miniaturizing the device based on MEMS micro-hotplate technology. At the same time, due to the scaling law, the substantial reduction in the size of the micro-hotplate also correspondingly shortens the thermal response time. The shorter thermal response time allows the device to be quickly heated and cooled, thereby achieving low duty cycle operation and further reducing power consumption. The structure of the micro-hotplate can be divided into two types: thin film and suspended. The thin film type refers to a micro-heater with a membrane structure in which the heating resistor and the micro-heater substrate are in contact with each other within the boundary of the active area. The suspended type refers to using several cantilever beams to support the active area in the middle part. The active area is in a suspended state and no longer in contact with the substrate, resulting in less heat loss. Due to the particularity of the use scenario of the on-board hydrogen sensor (high-speed movement, bumps, etc.), higher requirements are placed on the mechanical stability of the sensor. However, silicon-based materials are generally brittle and have weak resistance to mechanical vibration. The suspended structure faces problems such as stress concentration on the beam and fragile and easy fracture of the structure. Therefore, the present invention adopts a thin film micro-hotplate structure. The sensor chip is mainly composed of silicon wafers, SiO2 and SiO2 / SiN X The sensor chip is composed of a support layer, a Pt heating electrode, a Pt test electrode and a PtPd hydrogen sensitive layer. The size of the sensor chip is 2mm×2mm×0.5mm.

[0032] The resistance temperature coefficient of platinum metal has good linearity, and the physical and chemical properties of platinum are stable, with excellent thermal and electrical conductivity, and good experimental repeatability. Therefore, platinum metal is selected as the heating resistor. The test electrode also uses Pt metal, and the resistance-temperature characteristics of Pt are used to track the heat generated by the hydrogen sensitive layer on its surface in real time. Then, by detecting the change of Pt resistance and performing a series of data processing, the concentration value of hydrogen in the environment is finally obtained.

[0033] The traditional micro-hot plate sensor chip structure is arranged in a vertical direction between layers, and is composed of a substrate, a supporting layer, a heating electrode, an insulating layer, a test electrode, and a hydrogen sensitive layer from bottom to top. Since the micro-hot plate and the test electrode of this vertical structure MEMS hydrogen sensor are located at different levels, a parasitic electric field will be generated in the vertical direction, which will cause certain interference to the detection signal of the gas sensor and reduce the performance of the gas sensor. In addition, when processing, the vertical structure MEMS sensor needs to prepare the test electrode and the heating electrode respectively through multiple lithography, sputtering, and stripping processes, which makes its process flow relatively complicated, which is not conducive to reducing the production cost of the device and improving the yield of batch product processing. Therefore, the present invention adopts a coplanar design, and the heating electrode and the test electrode are placed in the same plane. Both electrodes are Pt metal and can be prepared synchronously, which greatly simplifies the process flow. In order to improve the temperature uniformity of the micro-hot plate, the heating electrode and the test electrode adopt a double serpentine structure arranged in parallel.

[0034] The hydrogen sensitive layer is made of PtPd alloy material. The high selectivity of Pd to H2 improves the detection accuracy of the hydrogen sensitive material and reduces the response to impurity gases. The hydrogen sensitive layer adopts a spiral structure and the hydrogen sensitive layer should completely cover the test electrode.

[0035] Finally, the back side of the substrate is etched by wet etching to form a cavity structure. During back lithography, the release hole needs to be placed completely below the working area.

[0036] It is worth pointing out that in order to reduce heat loss and maintain insulation between the components of the device, a layer of SiO2 insulation layer needs to be deposited on the surfaces of the heating electrode and the test electrode. At the same time, a layer of Ti metal needs to be sputtered between the electrode and the SiO2 layer to improve the adhesion between the two.

[0037] A method for preparing a vehicle-mounted catalytic combustion hydrogen sensor chip based on MEMS includes the following detailed steps:

[0038] Step 1: Use plasma enhanced chemical vapor deposition technology to grow a layer of SiO2 with low thermal conductivity and good insulation on the front and back surfaces to prevent heat from being lost quickly through the silicon wafer with good thermal conductivity, so that the micro-hotplate has better heating performance.

[0039] Step 2: Continue to grow SiN on the front side X Structural layer, forming SiO2 / SiN X The silicon nitride layer has tension, while the silicon oxide has the opposite, so the appropriate ratio of silicon nitride to silicon oxide helps to reduce stress and deformation, and is conducive to enhancing mechanical stability. The present invention adopts a thickness ratio of silicon oxide to silicon nitride of 1:2, at which time the composite support layer has good tensile strength and is used to form an insulating layer and a support layer of a Pt heating electrode.

[0040] Step 3: Use photolithography to pattern the surface of the composite film, and prepare a layer of metal platinum on it by magnetron sputtering as a heating electrode and a test electrode. The two electrodes are arranged in parallel as a double serpentine structure, including a working area and a welding pad, see Figure 2 The resistor line width is 20μm and the overall size is approximately 0.28mm×0.28mm.

[0041] Step 4: Continue to deposit a layer of SiO2 insulation layer on the Pt heating electrode and the test electrode as a hydrogen barrier layer to prevent hydrogen from affecting the resistance of the Pt electrode, ensure the accuracy of temperature measurement, and maintain the insulation between the Pt electrode and other components. The insulation layer covers the entire sensor chip and leaves a corresponding welding pad area. It should be noted that a metal Ti adhesion layer needs to be sputtered between the Pt electrode and the SiO2 layer to increase the adhesion between the platinum electrode and SiO2.

[0042] Step 5: On the insulating layer of step 4, a photolithography process is used for patterning. First, a Ti adhesion layer is sputtered, and then a PtPd hydrogen sensitive layer is sputtered. The hydrogen sensitive layer is located directly above the micro-hotplate and is slightly larger than the micro-hotplate. In order to increase the contact uniformity with the lower Pt electrode, the hydrogen sensitive layer adopts a spiral structure with a line width of 30μm ( Figure 3 ).

[0043] Step 6: On the silicon wafer obtained in the above steps, tetramethylammonium hydroxide is used as an etching solution to release the suspended structure, and the silicon substrate is removed by etching at 80°C, so that the suspended structure in the middle heating and testing area of ​​the micro-hotplate is released, forming a flat-bottomed groove structure, and the cavity reaches the bottom of the first silicon oxide support layer.

[0044] Figure 1 The tape-out process of the sensor chip mainly includes eight steps, which are to prepare the support layer, Pt heating electrode and test electrode, insulating layer, and hydrogen sensitive layer in sequence, and finally release the suspended structure. The heating electrode, test electrode and hydrogen sensitive layer are all prepared by magnetron sputtering. The special structure of the electrode and hydrogen sensitive layer is first patterned by photoresist and mask shielding, and then magnetron sputtering is used to deposit metal, and then acetone is used to dissolve the photoresist pattern and strip off the excess metal material to form a special structure;

[0045] Figure 2 This is a photo of the actual object after the Pt heating electrode and the test electrode are prepared in step 3. The electrodes are in a double serpentine structure, and the two electrodes include a working area and two upper and lower welding plates respectively;

[0046] Figure 3This is a real picture after the PtPd hydrogen sensitive layer is prepared in step 5, showing a spiral structure. The SiO2 insulating layer on the surface of the heating electrode and the test electrode should completely cover the chip, leaving a welding pad area for later wiring.

[0047] Figure 4 This is the test result of the sensor chip in a 4% H2 / air environment, with a response time of <2s.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a vehicle-mounted catalytic combustion hydrogen sensor chip based on MEMS, characterized in that: The steps include: Step 1: Use plasma enhanced chemical vapor deposition technology to grow a layer of SiO2 with low thermal conductivity and insulation on the front and back sides of the Si wafer; Step 2: Continue to grow SiN on the front side of the Si wafer X Structural layer, forming SiO2 / SiN X Micron-thick composite support layer; Step 3: Patterning is performed on the surface of the composite film by photolithography, and a layer of metal platinum is prepared on it by magnetron sputtering as a heating electrode and a test electrode. The two electrodes are arranged in parallel in a double serpentine structure, including a working area and a welding pad; Step 4: Continue to deposit a layer of SiO2 insulating layer on the Pt heating electrode and the test electrode as a hydrogen barrier layer to prevent hydrogen from affecting the resistance of the Pt electrode, ensure the accuracy of temperature measurement, and maintain the insulation between the Pt electrode and other components; Step 5: On the insulating layer of step 4, patterning is performed using a photolithography process, first a Ti adhesion layer is sputtered, and then a PtPd hydrogen sensitive layer is sputtered; Step 6: On the silicon wafer obtained in the above steps, tetramethylammonium hydroxide is used as an etching solution to release the suspended structure, and the silicon substrate is removed by etching under specified temperature conditions, so that the suspended structure of the middle heating and testing area of ​​the micro-hotplate is released, forming a flat-bottomed groove structure, and the cavity reaches the bottom of the first silicon oxide support layer.

2. The method for preparing a MEMS-based vehicle-mounted catalytic combustion hydrogen sensor chip according to claim 1, characterized in that: In the step 2, the silicon nitride layer has tension, while the silicon oxide has the opposite, and the thickness ratio of silicon oxide to silicon nitride is 1:

2. At this time, the composite support layer has good tensile strength and is used to form the insulating layer and support layer of the Pt heating electrode.

3. The method for preparing a MEMS-based vehicle-mounted catalytic combustion hydrogen sensor chip according to claim 1, characterized in that: In the step 3, the resistance line width is 20 μm and the overall size is 0.28 mm×0.28 mm.

4. The method for preparing a MEMS-based vehicle-mounted catalytic combustion hydrogen sensor chip according to claim 1, characterized in that: In step 4, the insulating layer covers the entire sensor chip and leaves a corresponding welding pad area. A metal Ti adhesion layer needs to be sputtered between the Pt electrode and the SiO2 layer to increase the adhesion between the platinum electrode and SiO2.

5. The method for preparing a MEMS-based vehicle-mounted catalytic combustion hydrogen sensor chip according to claim 1, characterized in that: In the step 5, the hydrogen sensitive layer is located directly above the micro-hotplate and is larger than the micro-hotplate. To increase the contact uniformity with the underlying Pt electrode, the hydrogen sensitive layer adopts a spiral structure with a line width of 30 μm.

6. The method for preparing a MEMS-based vehicle-mounted catalytic combustion hydrogen sensor chip according to claim 1, characterized in that: The step 6 is to remove the silicon substrate by etching under a specified temperature condition, wherein the specified temperature condition is 80°C.

7. The method for preparing a vehicle-mounted catalytic combustion hydrogen sensor chip based on MEMS according to claim 1, characterized in that: In step 5, the SiO2 insulating layer on the surface of the heating electrode and the test electrode should completely cover the chip, and leave a welding pad area for later wiring.

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