A Low Detection Limit MEMS Acetylene Gas Sensor and Its Preparation Method

Abstract

The present invention discloses a low-detection-limit MEMS acetylene gas sensor and a preparation method thereof, which includes respectively preparing a SiO2-Si3N4 double-layer composite film on the front and back surfaces of a Si substrate; depositing an insulating layer on the front composite film, using self-assembled monolayer SiO2 microspheres as a template, cooperating with radio frequency sputtering of metal oxide SnO2, and preparing a SnO2 porous film by ultrasonically removing the SiO2 microspheres in acetone. On this basis, Au is evaporated by electron beam evaporation technology, and heat treatment is carried out to form agglomerated particles; a sensitive electrode and a lead pad, a heating electrode and a lead pad are obtained on the front surface of the silicon wafer, and a back groove is etched to obtain a sensor with an adiabatic groove. The Au nanoparticles modified SnO2 porous film has high consistency, fast sensor response speed and low power consumption, meeting the requirements of ultra-low detection limit and volume limitation in acetylene detection.

Description

Technical Field

[0001] The present invention relates to the preparation of a MEMS acetylene gas sensor, in particular to a low-detection-limit MEMS acetylene gas sensor with Au nanoparticles modified SnO2 porous film and a preparation method thereof. Background Art

[0002] Power transformers play a huge role in the entire power system. With the development of the nuclear power industry in recent years, the capacity of power transformers has become larger and the operation time has become longer. The long-term operation of such large-capacity power transformers is likely to cause various problems, ranging from the suspension of power transformer operation to the complete destruction of the power transformer, and even large-scale power outages. Therefore, it is particularly important to avoid major faults in power transformers as much as possible. Major faults are some easily solved faults in the early stage. If relevant problems can be detected and solved in time at the early stage of the fault, the normal operation of the power transformer can be better guaranteed. During early faults, some fault characteristic gases, such as acetylene, will appear in the transformer oil of the power transformer. By detecting these characteristic gases, relevant faults can be detected early. Therefore, the detection of acetylene gas is of great significance for the fault alarm of power transformers.

[0003] At present, there are still many problems with acetylene gas sensors. First, there are problems with volume and power consumption. Existing acetylene gas sensors are relatively large in volume and difficult to be placed in the narrow space of the power transformer system, and the power consumption is relatively high and cannot meet the usage time requirement of more than one year. To solve this problem, the present invention proposes a MEMS acetylene gas sensor, which is sufficient to meet the requirements of volume and power consumption. At the same time, the content of acetylene gas in transformer oil is extremely low. How to achieve the detection of trace acetylene gas is also a major problem that the sensor needs to face. The existing thin films prepared based on the MEMS process are very dense, which affects the diffusion of acetylene gas and is difficult to meet the needs of trace detection. The present invention solves the problem of acetylene gas diffusion by preparing SnO2 porous films, greatly enhancing the sensor response. At the same time, with the modification of Au nanoparticles, a Schottky barrier is constructed between the noble metal Au and the semiconductor SnO2, further enhancing the response and being sufficient to meet the detection limit requirements of trace acetylene gas. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to realize the preparation of a low-detection-limit MEMS acetylene gas sensor and solve the problem of detecting trace acetylene gas in transformer oil. At the same time, the sensitive material, sensitive electrode, heating electrode, silicon substrate, etc. are integrated to prepare a low-detection-limit MEMS acetylene gas sensor.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] On the one hand, the present invention provides a preparation method for a low-detection-limit MEMS acetylene gas sensor, including:

[0007] 1) Prepare SiO2-Si3N4 double-layer composite films on the front and back of the Si substrate respectively;

[0008] 2) Deposit a SiO2 insulating layer and a Si3N4 insulating layer in sequence on the front SiO2-Si3N4 double-layer composite film, and anneal;

[0009] 3) Obtain the pattern of the sensitive material area on the front insulating layer by spin coating and photolithography;

[0010] 4) Prepare a monolayer of SiO2 microspheres on the front of the silicon wafer with the pattern of the sensitive material area, tilt and heat the silicon wafer at a certain temperature, and the SiO2 microspheres self-assemble on the surface to obtain a SiO2 microsphere masking layer;

[0011] 5) Sputter a SnO2 film on the front of the silicon wafer with the SiO2 microsphere masking layer, and strip to obtain a SnO2 sensitive layer with SiO2 microspheres;

[0012] 6) Ultrasonically treat the silicon wafer at a certain ultrasonic power to remove the SiO2 microspheres to obtain a SnO2 porous film;

[0013] 7) On the front of the silicon wafer, obtain the same pattern as in step 3) by spin coating and photolithography, and evaporate a certain thickness of noble metal Au;

[0014] 8) Strip the photoresist on the silicon wafer, perform heat treatment, and the noble metal Au forms agglomerated particles;

[0015] 9) Obtain the patterns of the sensitive electrode and its lead pad, and the heating electrode and its lead pad on the front of the silicon wafer by spin coating and photolithography;

[0016] 10) Evaporate electrode materials Cr and Au on the electrode pattern, and strip to obtain an electrode layer;

[0017] 11) Use spin coating and photolithography process on the SiO2-Si3N4 double-layer composite film on the back of the silicon wafer to obtain the pattern of the back groove window;

[0018] 12) Use the photoresist as a mask, remove the exposed SiO2-Si3N4 double-layer composite film by deep dry etching process, strip and protect the front well, and perform wet etching using the back groove window to obtain a sensor with an adiabatic groove.

[0019] For the above technical solution, a further solution also lies in:

[0020] In step 1), first, a SiO2 support layer and a SiO2 masking layer are prepared by a thermal oxidation process, and then a Si3N4 support layer and a Si3N4 masking layer are prepared by a low-pressure chemical vapor deposition method; the thickness of the SiO2 layer thermally oxidized on both sides of the silicon wafer is 500 ± 10 nm, and the thickness of the Si3N4 layer deposited by LPCVD on both sides is 300 ± 10 nm.

[0021] In step 2), a 500 ± 10 nm SiO2 insulating layer and a 300 ± 10 nm Si3N4 insulating layer are prepared by a plasma-enhanced chemical vapor deposition method or a magnetron sputtering process; annealing is performed at 600 °C - 700 °C for 5 - 7 h.

[0022] In step 4), the silicon wafer with the pattern of the sensitive material area is immersed in deionized water, and a SiO2 microsphere - n-butanol solution with a diameter of 300 - 400 nm is dropped to complete the self-assembly of the microspheres on the surface of the deionized water. The silicon wafer is taken out horizontally to complete the transfer of the single-layer SiO2 microspheres. The silicon wafer is heated at 75 - 85 °C with an inclination of 15 - 20° for secondary self-assembly.

[0023] In step 5), the sputtering power of the SnO2 thin film is 50 - 100 W, the time is 20 - 30 min, and the argon gas flow rate is 20 - 30 sccm.

[0024] In step 6), the ultrasonic power is 50 W - 100 W, and the ultrasonic time is 30 - 60 min.

[0025] In step 7), the thickness of the Au thin film evaporated by an electron beam evaporation process is 5 - 8 nm.

[0026] In steps 5), 8) and 12), the silicon wafer is successively immersed in acetone - ethanol - deionized water to soak and strip the photoresist, soaked in acetone for 45 - 60 min, and soaked in ethanol and deionized water for 5 - 10 min each.

[0027] In step 8), heat treatment is performed at 450 - 500 °C for 2 - 3 h.

[0028] In step 11), the SiO2 - Si3N4 double-layer composite film at the groove window is removed by deep dry etching, the etching time power is 200 - 250 w, and the time is 15 - 20 min.

[0029] On the other hand, the present invention provides a low-detection-limit MEMS acetylene gas sensor prepared by the above method, including a SiO2 masking layer and a Si3N4 masking layer provided on the back of the Si substrate and a four-layer composite film provided on the front of the Si substrate. An adiabatic groove is opened on the back of the Si substrate, and a sensitive material, a Cr - Au heating electrode and its lead disk, and a Cr - Au sensitive electrode and its lead disk are provided on the upper surface of the composite film on the front of the Si substrate;

[0030] The sensitive material is distributed at the center of the upper surface of the composite thin film on the front side of the Si substrate, including a SnO2 porous thin film, as well as Au nanoparticles and honeycomb-shaped pores distributed in the SnO2 porous thin film; the Cr-Au heating electrode and the Cr-Au sensitive electrode are symmetrically distributed along the sensitive material, and symmetrically distributed electrode wires are led out from the Cr-Au heating electrode and the Cr-Au sensitive electrode, and the electrode wires of the Cr-Au sensitive electrode have a finger-like structure.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] 1. By using a closely packed single-layer SiO2 microsphere as a masking layer and cooperating with the magnetron sputtering technique, a SnO2 porous thin film can be prepared. Compared with a pure SnO2 thin film, the diffusion depth of acetylene gas in the SnO2 porous thin film is higher, and the gas response characteristics are thus better.

[0033] 2. By means of the electron beam evaporation technique, a Au layer with a certain thickness can be obtained on the porous thin film and heat-treated at a certain temperature. Due to its aggregation effect, Au will form Au nanoparticles at the edges of the pores, and a Schottky barrier is formed between it and the SnO2 porous thin film, which can reduce the detection limit of the sensor from 1 ppm to 0.1 ppm.

[0034] 3. By changing the diameter of the self-assembled single-layer SiO2 microspheres, the pore diameter on the SnO2 porous thin film can be controlled, thereby realizing the control of the microstructure of the sensitive material, with great flexibility. Description of the Drawings

[0035] Figure 1 It is a structural sectional view of the low-detection-limit MEMS acetylene gas sensor of the present invention;

[0036] Figures 2(a), 2(b), and 2(c) are respectively a schematic diagram of the SnO2 porous thin film, a schematic diagram of the SnO2 porous thin film modified with Au nanoparticles, and a three-dimensional diagram of the SnO2 porous thin film modified with Au nanoparticles during the manufacturing process of the low-detection-limit MEMS acetylene gas sensor of the present invention;

[0037] Figure 3 It is a schematic plan view of the sensitive material, heating electrode, and sensitive electrode of the low-detection-limit MEMS acetylene gas sensor of the present invention;

[0038] Figures 4(a)-(p) are the process flow diagrams for the preparation of the low-detection-limit MEMS acetylene gas sensor of the present invention.

[0039] In the figure: 1. Lead wire plate; 2. Si3N4 insulating layer; 3. SiO2 insulating layer; 4. Si substrate; 5. SiO2 masking layer; 6. Adiabatic groove; 7. Si3N4 masking layer; 8. SiO2 support layer; 9. Si3N4 support layer; 10. Cr-Au heating electrode; 11. Cr-Au sensitive electrode; 12. Holes in the sensitive material; 13. SnO2 porous thin film; 14. Au nanoparticles; 15. Sensitive material. Detailed implementation mode

[0040] The invention will be further described in detail below in conjunction with the drawings and embodiments, but it shall not be used as a basis for any limitation to the invention.

[0041] As Figure 1 、 Figure 3 shown, the low-detection-limit MEMS acetylene gas sensor of the present invention includes an Si substrate 4. On the back of the Si substrate 4 are successively an SiO2 masking layer 5 and an Si3N4 masking layer 7, and an adiabatic groove 6 is opened on the back of the Si substrate 4. The front of the Si substrate 4 is a support insulating layer composed of four layers of thin films, namely an Si3N4 insulating layer 2, an SiO2 insulating layer 3, an Si3N4 support layer 9, and an SiO2 support layer 8. A sensitive material 15 is arranged on the layer, and a Cr-Au heating electrode 10 and its lead wire plate 1, and a pair of Cr-Au sensitive electrodes 11 and their lead wire plates 1 are arranged in the same plane. The sensitive material 15 is located at the central position and is below the interdigital part of the Cr-Au sensitive electrode 11.

[0042] As shown in Fig. 2(a), holes are presented in a hexagonal honeycomb close-packed distribution on the SnO2 porous thin film. As shown in Fig. 2(b), Au nanoparticle modification points appear on the surface of the SnO2 porous thin film. The Schottky barrier formed between the Au nanoparticle points and SnO2 greatly improves the response of the gas-sensitive material and reduces the detection limit for acetylene. At the same time, Fig. 2(c) more clearly shows the morphology of the Au nanoparticle-modified SnO2 porous thin film through a three-dimensional diagram.

[0043] As Figure 3 shown, the sensitive material 15 is arranged at the central position of the sensor. Electrode wires with a symmetric distribution and an interdigital structure are led out from the Cr-Au sensitive electrode 11 and reach the electrode lead wire plate 1. The interdigital structure can improve the sensitivity and response speed of the sensor; the Cr-Au heating electrode 10 is symmetrically distributed and leads to the heating element lead wire plate 1. The symmetric structure can concentrate the heat on the sensitive material.

[0044] The sensitive material 15 is composed of an SnO2 porous thin film 13, Au nanoparticles 14, and holes 12 in the sensitive material. The support insulating layer and the masking layer are composed of two materials, SiO2 and Si3N4, compounded; the sensitive electrode, the heating electrode, and each lead wire plate are made of Cr-Au thin films.

[0045] Referring to Figure 3 , the heating electrode and the sensitive electrode are both arranged symmetrically, the sensitive electrode is surrounded by the heating electrode, and the sensitive material is located below the interdigital part of the sensitive electrode. Referring to Figure 3 , the inner ring size of the Cr-Au heating electrode is 150μm×200μm, the width of the heating wire is 10μm, and the gap is 15μm. The electrode width of the Cr-Au sensitive electrode is 12μm, the gap is 10μm, and the size of the sensitive material is 100μm×100μm.

[0046] Referring to FIGS. 4(a)-(p), a method for preparing a low-detection-limit MEMS acetylene gas sensor with Au nanoparticle-modified SnO2 porous film according to the present invention includes the following steps:

[0047] S1, as shown in FIG. 4(a), a SiO2-Si3N4 double-layer composite film is prepared on the front and back of the Si substrate 4. First, a SiO2 support layer 8 and a SiO2 masking layer 5 are prepared by a thermal oxidation process, and then a Si3N4 support layer 9 and a Si3N4 masking layer 7 are prepared by a low-pressure chemical vapor deposition method; a 500±10nm SiO2 layer is thermally oxidized on both sides of the silicon wafer, and a 300±10nm Si3N4 layer is deposited by double-sided LPCVD (low-pressure chemical vapor deposition).

[0048] S2, as shown in FIG. 4(b), on the SiO2-Si3N4 double-layer composite film composed of the SiO2 support layer 8 and the Si3N4 support layer 9 on the front, a 500±10nm SiO2 insulating layer 3 and a 300±10nm Si3N4 insulating layer 2 are sequentially deposited by plasma-enhanced chemical vapor deposition, and annealed at 600°C - 700°C for 5 - 7h.

[0049] S3, as shown in FIG. 4(c), on the front of the silicon wafer, a pattern of the sensitive material region is obtained by a spin coating and photolithography process, and the photoresist used is positive photoresist EPG535.

[0050] S4, as shown in FIG. 4(d), a monolayer of SiO2 microspheres is obtained on the front of the silicon wafer with the pattern of the sensitive material region by a fishing method. The silicon wafer with the pattern of the sensitive material region is immersed in deionized water with an area of 120cm 2 deionized water, and 450ul of n-butanol with a mass concentration of SiO2 microspheres of 15 - 25mg / ml is dropped on the surface of the deionized water, where the diameter of the microspheres is 300 - 400nm, so that the SiO2 microspheres complete self-assembly on the surface of the deionized water to form a monolayer, and the silicon wafer is taken out horizontally at a speed of 0.5 - 1cm / s, and the silicon wafer is heated at an inclination of 15 - 20° at 75 - 85°C to ensure secondary self-assembly of the SiO2 microspheres on the surface, and a monolayer of SiO2 microsphere masking layer is obtained.

[0051] S5, as shown in Fig. 4(e), on the front side of the silicon wafer, SnO2 thin film is radio-frequency sputtered by magnetron sputtering. The sputtering power is 50 - 100 W, the time is 20 - 30 min, and the argon gas flow rate is 20 - 30 sccm. The silicon wafer is successively immersed in acetone - ethanol - deionized water for soaking and peeling, soaked in acetone for 45 - 60 min, and soaked in ethanol and deionized water for 5 - 10 min each, to obtain a SnO2 sensitive layer with SiO2 microspheres.

[0052] S6, as shown in Fig. 4(f), the silicon wafer is placed in an acetone solution and ultrasonically treated at an ultrasonic power of 50 - 100 W for 30 - 60 min to remove the SiO2 microspheres, obtaining a SnO2 porous thin film;

[0053] S7, as shown in Fig. 4(g), spin coating and photolithography operations are performed on the processed silicon wafer to obtain the same sensitive area pattern as S3. On the front side of the silicon wafer, noble metal Au is evaporated by electron beam evaporation process, and the thickness of the evaporated Au layer is controlled to be 5 - 8 nm.

[0054] S8, as shown in Fig. 4(h), the silicon wafer is successively immersed in acetone - ethanol - deionized water to soak and peel off the photoresist, soaked in acetone for 45 - 60 min, and soaked in ethanol and deionized water for 5 - 10 min each.

[0055] S9, as shown in Fig. 4(i), the silicon wafer after peeling is heat-treated at 450 - 500 °C for 2 - 3 h, so that the noble metal Au can form agglomerated particles.

[0056] S10, as shown in Fig. 4(j), the same spin coating and photolithography process as S3 is adopted to obtain the patterns of the sensitive electrodes and lead frames, and the heating electrodes and lead frames.

[0057] S11, as shown in Fig. 4(k), on the patterns of the sensitive electrodes and lead frames, and the heating electrodes and lead frames, a 30 - 40 nm Cr adhesive layer is evaporated at a rate of 0.5 Å / s, and then a 200 - 300 nm Au conductive layer is evaporated at a rate of 2 Å / s. Here, Cr is used to ensure the adhesion between the Au electrode and the bottom surface.

[0058] S12, as shown in Fig. 4(l), the same process as S8 is adopted to remove the photoresist, and then it is heated in air at 300 - 350 °C for 10 - 20 min to make the Cr - Au electrode layer and the sensitive material combine more tightly.

[0059] S13, as shown in Fig. 4(m), on the SiO2 - Si3N4 double - layer composite film composed of the SiO2 masking layer 5 and the Si3N4 masking layer 7 on the back side of the silicon wafer, the same spin coating and photolithography process as step S3 is adopted to obtain the back - side groove window pattern.

[0060] S14, as shown in Figure 4(n), use photoresist as a masking layer, and do a good job in front protection. Remove the SiO2-Si3N4 double-layer composite film at the groove window by deep dry etching. The etching time power is 200 - 250w, and the time is 15 - 20min. Then use the same process as in step S8 to remove the photoresist.

[0061] S15, spin-coat photoresist on the front side of the silicon wafer. After drying at 90 - 95°C, drop PDMS on the front side of the silicon wafer drop by drop until the front side is full. Dry at 70 - 80°C for 1 - 2h. As shown in Figure 4(o), stick the front side of the silicon wafer on the glass slide, and drop a circle of PDMS at the edge of the back side of the silicon wafer to firmly stick the silicon wafer on the glass slide.

[0062] S16, put the silicon wafer and the glass slide together into a TMAH (tetramethylammonium hydroxide with a concentration of 25 - 30%) solution, the temperature is 80 - 90°C, and etch for 24 - 48h to form an insulating groove. Gently tear off the PDMS, soak it in acetone for 3h, remove the photoresist and residual PDMS, and dry at 120 - 130°C for 15 - 20min. The obtained sensor is shown in Figure 4(p).

[0063] The modification of Au nanoparticles in the present invention is achieved by using an electron beam evaporation process on the SnO2 porous film to obtain a certain thickness of Au film and cooperating with heat treatment at a certain temperature. The prepared Au nanoparticle-modified SnO2 porous film is fully compatible with the MEMS process. The SiO2 microsphere template in the preparation of the porous film can be uniformly realized on a large area of the substrate. Therefore, the obtained porous film is uniform on the substrate, and the preparation of Au nanoparticles is realized through a typical MEMS process, making the final obtained Au nanoparticle-modified SnO2 porous film highly consistent.

[0064] Due to its porous structure, the sensitive material in the present invention improves the diffusion depth of acetylene gas in the sensitive material, thereby enhancing the response. And the Au nanoparticles further enhance the response through the Schottky barrier formed between them and SnO2. At the same time, the catalytic effect of the Au nanoparticles themselves is beneficial to reducing the working temperature of the sensor. The entire acetylene gas sensor integrates the Au nanoparticle-modified SnO2 porous film with the MEMS process, meeting the requirements of ultra-low detection limit and volume limitation in acetylene detection. This sensor can meet the requirements of ultra-low detection limit and volume limitation detection of the fault characteristic gas acetylene in transformer oil. The preparation of the Au nanoparticle-modified SnO2 porous film is the main means to achieve a low detection limit, and the compatibility with the MEMS process is the main method to control the volume of the sensor.

[0065] The preparation of the sensor of the present invention will be further described below through different embodiments.

[0066] Example 1

[0067] S1. On the front and back sides of the Si substrate 4, first use the thermal oxidation process to prepare the SiO2 support layer 8 and the SiO2 masking layer 5, and then use the low-pressure chemical vapor deposition method to prepare the Si3N4 support layer 9 and the Si3N4 masking layer 7; thermally oxidize a 500-nm SiO2 layer on both sides of the silicon wafer, and deposit 300 nm of Si3N4 by double-sided LPCVD (low-pressure chemical vapor deposition).

[0068] S2. On the double-layer film composed of the front SiO2 support layer 8 and the Si3N4 support layer 9, sequentially deposit a 490-nm SiO2 insulating layer 3 and a 290-nm Si3N4 insulating layer 2 on the front using plasma-enhanced chemical vapor deposition, and anneal at 600 °C for 7 h.

[0069] S3. On the front side of the silicon wafer, use the positive photoresist EPG535 photoresist to obtain the pattern of the sensitive material area through the spin coating and photolithography process.

[0070] S4. Immerse the silicon wafer with the pattern of the sensitive material area in deionized water with an area of 120 cm 2 Drop 450 μl of n-butanol with a mass concentration of 25 mg / ml of SiO2 microspheres on the surface of the deionized water, where the diameter of the microspheres is 400 nm, so that the SiO2 microspheres complete self-assembly on the surface of the deionized water to form a single layer. Take out the silicon wafer horizontally at a speed of 0.5 cm / s and heat the silicon wafer at 85 °C with a 15° inclination to ensure secondary self-assembly of the SiO2 microspheres on the surface, and a single-layer SiO2 microsphere masking layer is obtained.

[0071] S5. Use the magnetron sputtering method to radiofrequency sputter the SnO2 thin film on the front side of the silicon wafer, with a sputtering power of 80 W, a time of 25 min, and an argon flow rate of 20 sccm. Immerse the silicon wafer in acetone-ethanol-deionized water in sequence for soaking and stripping, soak in acetone for 50 min, and soak in ethanol and deionized water for 10 min each to obtain the SnO2 sensitive layer with SiO2 microspheres.

[0072] S6. Place the silicon wafer in an acetone solution and ultrasonically process it at an ultrasonic power of 75 W for 45 min to remove the SiO2 microspheres. Perform spin coating and photolithography operations on the processed silicon wafer to obtain the same pattern of the sensitive area as in step (3).

[0073] S7. Evaporate the noble metal Au on the front side of the silicon wafer using the electron beam evaporation process, and control the thickness of the evaporated Au layer to be 6 nm.

[0074] S8. Immerse the silicon wafer in acetone-ethanol-deionized water in sequence for soaking and stripping the photoresist, soak in acetone for 45 min, and soak in ethanol and deionized water for 10 min each.

[0075] S9. Heat-treat the peeled silicon wafer at 500 °C for 2 h so that the noble metal Au can form agglomerated particles.

[0076] S10. Adopt the same spin coating and lithography process as in S3 to obtain the patterns of the sensitive electrodes and lead pads, and the heating electrodes and lead pads.

[0077] S11. Evaporate a 30-nm Cr adhesive layer at a rate of 0.5 Å / s on the patterns of the sensitive electrodes and lead pads, and the heating electrodes and lead pads, and then evaporate a 250-nm Au conductive layer at a rate of 2 Å / s.

[0078] S12. Adopt the same process as in S8 to remove the photoresist, and then heat it in air at 300 °C for 15 min to make the Cr-Au electrode layer and the sensitive material bond more tightly.

[0079] S13. Adopt the same spin coating and lithography process as in S3 on the double-layer film composed of the SiO2 masking layer 5 and the Si3N4 masking layer 7 on the back of the silicon wafer to obtain the back groove window pattern.

[0080] S14. Use the photoresist as a masking layer to remove the Si3N4-SiO2 layer at the groove window by deep dry etching. The etching power is 200 w and the time is 20 min. Then use the same process as in S8 to remove the photoresist.

[0081] S15. Spin coat photoresist on the front of the silicon wafer. After drying at 95 °C, drop PDMS onto the front of the silicon wafer drop by drop until the front is full. Dry it at 70 °C for 1 h, stick the front of the silicon wafer onto a glass slide, and drop a circle of PDMS at the edge of the back of the silicon wafer to firmly stick the silicon wafer onto the glass slide.

[0082] S16. Put the silicon wafer and the glass slide together into a TMAH (25% tetramethylammonium hydroxide) solution at a temperature of 80 °C and etch for 48 h to form an insulating groove. Gently tear off the PDMS, soak it in acetone to remove the photoresist and residual PDMS, and dry it at 120 °C for 20 min to obtain the sensor.

[0083] Place the chip in the gas to be measured, and apply a certain voltage across the heating wire, which can enable the sensitive material to quickly reach the working temperature. The lower sputtering power results in a smaller material thickness. Combining with the porous structure, it can ensure that 90% of the gas is completely diffused in the gas-sensitive material. At the same time, the Schottky barrier between the Au nanoparticles and SnO2 further improves the response, making the response of this gas-sensitive material more than 3 times that of the pure SnO2 thin film. At the same time, due to the catalytic effect of the Au nanoparticles, the working temperature of the sensitive material is reduced by 100 - 120 °C compared with common acetylene gas sensors, and the existence of the suspended film structure can reduce heat loss, greatly reducing the power consumption of the sensor. Based on the reduction of the working temperature and the reduction of heat loss, the power consumption of this sensor is reduced by 90% compared with the acetylene gas sensor without the suspended film structure.

[0084] Example 2

[0085] S1. On the front and back of the Si substrate 4, first use the thermal oxidation process to prepare the SiO2 support layer 8 and the SiO2 masking layer 5, and then use the low-pressure chemical vapor deposition method to prepare the Si3N4 support layer 9 and the Si3N44 masking layer 7; thermally oxidize a 490 nm SiO2 layer on both sides of the silicon wafer, and deposit 290 nm Si3N4 by double-sided LPCVD (low-pressure chemical vapor deposition).

[0086] S2. On the double-layer film composed of the front SiO2 support layer 8 and the Si3N4 support layer 9, on the front, use plasma-enhanced chemical vapor deposition to sequentially deposit a 500 nm SiO2 insulating layer 3 and a 300 nm Si3N4 insulating layer 2, and anneal at 700 °C for 5 h.

[0087] S3. On the front of the silicon wafer, use the positive photoresist EPG535 photoresist to obtain the pattern of the sensitive material area through the spin coating and photolithography process.

[0088] S4. Immerse the silicon wafer with the pattern of the sensitive material area in deionized water with an area of 120 cm 2 Deionized water, and drop 450 μl of n-butanol with a mass concentration of 20 mg / ml of SiO2 microspheres on the surface of the deionized water. The diameter of the microspheres is 300 nm, so that the SiO2 microspheres complete self-assembly on the surface of the deionized water to form a single layer. Take out the silicon wafer horizontally at a speed of 1 cm / s, and heat the silicon wafer at 75 °C with a 20° tilt to ensure the secondary self-assembly of the SiO2 microspheres on the surface, and then a single-layer SiO2 microsphere masking layer is obtained.

[0089] S5. On the front side of the silicon wafer, the SnO2 thin film is radio frequency sputtered by magnetron sputtering. The sputtering power is 100 W, the time is 30 min, and the argon gas flow rate is 25 sccm. The silicon wafer is successively immersed in acetone - ethanol - deionized water for soaking and stripping. It is soaked in acetone for 45 min, and soaked in ethanol and deionized water for 5 min each, obtaining the SnO2 sensitive layer with SiO2 microspheres.

[0090] S6. Place the silicon wafer in the acetone solution and ultrasonically treat it for 60 min at an ultrasonic power of 50 W to remove the SiO2 microspheres. Perform spin coating and photolithography operations on the treated silicon wafer to obtain the same sensitive area pattern as in S3.

[0091] S7. On the front side of the silicon wafer, deposit the noble metal Au by electron beam evaporation process, and control the thickness of the deposited Au layer to be 5 nm.

[0092] S8. Immerse the silicon wafer successively in acetone - ethanol - deionized water for soaking and stripping the photoresist. It is soaked in acetone for 60 min, and soaked in ethanol and deionized water for 5 min each.

[0093] S9. Heat - treat the stripped silicon wafer at 450 °C for 3 h so that the noble metal Au can form agglomerated particles.

[0094] S10. Adopt the same spin coating and photolithography process as in S3 to obtain the patterns of the sensitive electrodes and lead frames, heating electrodes and lead frames.

[0095] S11. Deposit a 35 - nm - thick Cr adhesion layer at a rate of 0.5 Å / s on the patterns of the sensitive electrodes and lead frames, heating electrodes and lead frames, and then deposit a 200 - nm - thick Au conductive layer at a rate of 2 Å / s.

[0096] S12. Adopt the same process as in S8 to remove the photoresist, and then heat it in air at 350 °C for 10 min to make the Cr - Au electrode layer and the sensitive material bond more tightly.

[0097] S13. On the double - layer film composed of the SiO2 masking layer 5 and the Si3N4 masking layer 7 on the back side of the silicon wafer, adopt the same spin coating and photolithography process as in S3 to obtain the back - side groove window pattern.

[0098] S14. Use the photoresist as a masking layer, remove the Si3N4 - SiO2 layer at the groove window by deep dry etching. The etching power is 250 w and the time is 15 min. Then use the same process as in S8 to remove the photoresist.

[0099] S15. Spin-coat photoresist on the front side of the silicon wafer. After drying at 90 °C, dropwise add PDMS onto the front side of the silicon wafer until it is fully covered, then dry at 80 °C for 1 h. Attach the front side of the silicon wafer to a glass slide, and apply a circle of PDMS at the edge of the back side of the silicon wafer to firmly attach the silicon wafer to the glass slide.

[0100] S16. Immerse the silicon wafer together with the glass slide into a TMAH (tetramethylammonium hydroxide with a concentration of 25 - 30%) solution at a temperature of 85 °C for 36 h to form an insulating groove. Gently tear off the PDMS, soak it in acetone to remove the photoresist and residual PDMS, and then dry at 130 °C for 15 min to obtain the sensor.

[0101] Place the chip in the gas to be measured, apply a certain voltage across the heating wire ends, which can enable the sensitive material to quickly reach the working temperature. A higher sputtering power not only increases the material thickness but also improves the material stability. Combined with the porous structure, it can ensure that 80% of the gas is fully diffused in the gas-sensitive material. At the same time, the Schottky barrier between the Au nanoparticles and SnO2 further enhances the response, making the response of this gas-sensitive material more than 2.6 times that of a pure SnO2 thin film. Meanwhile, due to the catalytic effect of the Au nanoparticles, the working temperature of the sensitive material is reduced by 80 - 100 °C compared to common acetylene gas sensors, and the existence of the suspended film structure can reduce heat loss, greatly reducing the power consumption of the sensor. Based on the reduction of the working temperature and heat loss, the power consumption of this sensor is reduced by 85% compared to an acetylene gas sensor without a suspended film structure.

[0102] Example 3

[0103] S1. On the front and back sides of the Si substrate 4, first use the thermal oxidation process to prepare the SiO2 support layer 8 and the SiO2 masking layer 5, and then use the low-pressure chemical vapor deposition method to prepare the Si3N4 support layer 9 and the Si3N44 masking layer 7; thermally oxidize a 510 nm SiO2 layer on both sides of the silicon wafer, and deposit 310 nm Si3N4 by LPCVD (low-pressure chemical vapor deposition) on both sides.

[0104] S2. On the double-layer film composed of the front-side SiO2 support layer 8 and the Si3N4 support layer 9, sequentially deposit a 510 nm SiO2 insulating layer 3 and a 310 nm Si3N4 insulating layer 2 on the front side by plasma-enhanced chemical vapor deposition, and anneal at 65 °C for 6 h.

[0105] S3. On the front side of the silicon wafer, use the positive photoresist EPG535 photoresist to obtain the pattern of the sensitive material area through the spin-coating and lithography process.

[0106] S4. Immerse the silicon wafer with the pattern of the sensitive material area into an area of 120 cm 2In deionized water, 450 μl of n-butanol with a mass concentration of 15 mg / ml of SiO2 microspheres was dropped onto the surface of the deionized water. The diameter of the microspheres was 350 nm, enabling the SiO2 microspheres to self-assemble on the surface of the deionized water to form a single layer. The silicon wafer was horizontally taken out at a speed of 0.8 cm / s and heated at 80 °C with an inclination of 18° to ensure the secondary self-assembly of the SiO2 microspheres on the surface, thus obtaining a single-layer SiO2 microsphere masking layer.

[0107] S5, On the front side of the silicon wafer, a SnO2 thin film was radio frequency sputtered by magnetron sputtering. The sputtering power was 50 W, the time was 20 min, and the argon gas flow rate was 30 sccm. The silicon wafer was successively immersed in acetone - ethanol - deionized water for soaking and stripping, immersed in acetone for 60 min, and immersed in ethanol and deionized water for 8 min each, obtaining a SnO2 sensitive layer with SiO2 microspheres.

[0108] S6, The silicon wafer was placed in an acetone solution and ultrasonically treated at an ultrasonic power of 100 W for 30 min to remove the SiO2 microspheres. The processed silicon wafer was subjected to spin coating and photolithography operations to obtain the same sensitive area pattern as in S3.

[0109] S7, On the front side of the silicon wafer, a noble metal Au was evaporated by electron beam evaporation process, and the thickness of the evaporated Au layer was controlled to be 8 nm.

[0110] S8, The silicon wafer was successively immersed in acetone - ethanol - deionized water to soak and strip the photoresist, immersed in acetone for 50 min, and immersed in ethanol and deionized water for 7 min each.

[0111] S9, The peeled silicon wafer was heat-treated at 450 °C for 2 - 3 h to enable the noble metal Au to form agglomerated particles.

[0112] S10, The same spin coating and photolithography process as in S3 was adopted to obtain the patterns of the sensitive electrodes and lead pads, and the heating electrodes and lead pads.

[0113] S11, On the patterns of the sensitive electrodes and lead pads, and the heating electrodes and lead pads, a 40 nm Cr adhesive layer was evaporated at a rate of 0.5 Å / s, and then a 300 nm Au conductive layer was evaporated at a rate of 2 Å / s.

[0114] S12, The same process as in S8 was adopted to remove the photoresist, and then it was heated in air at 300 °C for 20 min to make the Cr - Au electrode layer and the sensitive material combine more tightly.

[0115] S13, On the double-layer film composed of the SiO2 masking layer 5 and the Si3N4 masking layer 7 on the back side of the silicon wafer, the same spin coating and photolithography process as in S3 was adopted to obtain the back groove window pattern.

[0116] S14. Using photoresist as a masking layer, the Si3N4-SiO2 layer at the groove window is removed by deep dry etching. The etching power is 220 w and the time is 18 min. Then, the photoresist is removed using the same process as in S8.

[0117] S15. Spin-coat photoresist on the front side of the silicon wafer. After drying at 95 °C, drop PDMS onto the front side of the silicon wafer one by one until the front side is fully covered. Dry at 70 °C for 2 h. Stick the front side of the silicon wafer onto a glass slide, and apply a circle of PDMS to the edge of the back side of the silicon wafer to firmly attach the silicon wafer to the glass slide.

[0118] S16. Put the silicon wafer together with the glass slide into a TMAH (tetramethylammonium hydroxide with a concentration of 25%) solution at a temperature of 90 °C for 24 h to form an insulating groove. Gently tear off the PDMS, soak it in acetone to remove the photoresist and residual PDMS, and dry at 125 °C for 15 min to obtain the sensor.

[0119] Place the chip in the gas to be measured. Apply a certain voltage across the heating wire ends, which can enable the sensitive material to quickly reach the operating temperature. The lower sputtering power results in a smaller material thickness. Combined with the porous structure, it can ensure complete gas diffusion. At the same time, the Schottky barrier between Au nanoparticles and SnO2 further improves the response, making the response of this gas-sensitive material more than 3.2 times that of a pure SnO2 thin film. At the same time, due to the catalytic effect of Au nanoparticles, the operating temperature of the sensitive material is reduced by 110 - 130 °C compared to common acetylene gas sensors. Moreover, the existence of the suspended film structure can reduce heat loss and greatly reduce the power consumption of the sensor. Based on the reduction of the operating temperature and heat loss, the power consumption of this sensor is reduced by 92% compared to an acetylene gas sensor without a suspended film structure.

[0120] The acetylene gas sensor compatible with the MEMS process prepared by the method of the present invention has excellent performance of high response, low power consumption, high yield and batch production. Compared with the prior art, its unique porous structure combined with the Schottky barrier formed between Au and SnO2 makes the sensor response increase by no less than 2.6 times, and the combination of the catalytic effect of the noble metal Au and the suspended film structure makes the sensor power consumption reduce by no less than 85%. Therefore, the method of the present invention is an excellent method for preparing a high-response acetylene gas sensor compatible with the MEMS process.

[0121] The present invention is not limited to the above embodiments. Based on the technical solutions disclosed in the present invention, those skilled in the art can make some substitutions and deformations of some technical features without creative labor according to the disclosed technical content, and these substitutions and deformations are all within the protection scope of the present invention.

Claims

1. A preparation method of a low-detection-limit MEMS acetylene gas sensor, characterized in that, Including: 1) On the front and back of the Si substrate, SiO2-Si3N4 double-layer composite films are respectively prepared; 2) On the front SiO2-Si3N4 double-layer composite film, a SiO2 insulating layer and a Si3N4 insulating layer are sequentially deposited and annealed; 3) On the front insulating layer, a pattern of the sensitive material area is obtained through spin coating and photolithography; 4) On the front of the silicon wafer with the pattern of the sensitive material area, a monolayer of SiO2 microspheres is prepared. The silicon wafer is heated obliquely at a certain temperature, and the SiO2 microspheres are self-assembled on the surface to obtain a SiO2 microsphere masking layer; 5) On the front of the silicon wafer with the SiO2 microsphere masking layer, a SnO2 film is sputtered and peeled to obtain a SnO2 sensitive layer with SiO2 microspheres; 6) The silicon wafer is ultrasonically treated at a certain ultrasonic power to remove the SiO2 microspheres, obtaining a SnO2 porous film; 7) On the front of the silicon wafer, the same pattern as in step 3) is obtained through spin coating and photolithography, and a precious metal Au with a certain thickness is evaporated; 8) The photoresist on the silicon wafer is peeled off and heat-treated, and the precious metal Au forms agglomerated particles; 9) On the front of the silicon wafer, patterns of the sensitive electrode and its lead pad, and the heating electrode and its lead pad are obtained through spin coating and photolithography; 10) On the electrode pattern, electrode materials Cr and Au are evaporated and peeled off to obtain an electrode layer; 11) On the SiO2-Si3N4 double-layer composite film on the back of the silicon wafer, a back groove window pattern is obtained by using the spin coating and photolithography process; 12) Using the photoresist as a mask, the exposed SiO2-Si3N4 double-layer composite film is removed by using the deep dry etching process. After peeling off, the front is protected and wet etching is carried out using the back groove window to obtain a sensor with an adiabatic groove.

2. The preparation method of the low-detection-limit MEMS acetylene gas sensor according to claim 1, characterized in that, In step 1), a SiO2 support layer and a SiO2 masking layer are first prepared by using the thermal oxidation process, and then a Si3N4 support layer and a Si3N4 masking layer are prepared by using the low-pressure chemical vapor deposition method; the thickness of the SiO2 layer thermally oxidized on both sides of the silicon wafer is 500±10nm, and the thickness of the Si3N4 layer deposited by LPCVD on both sides is 300±10nm.

3. The preparation method of the low-detection-limit MEMS acetylene gas sensor according to claim 1, characterized in that, In step 2), a 500±10nm SiO2 insulating layer and a 300±10nm Si3N4 insulating layer are prepared by using the plasma-enhanced chemical vapor deposition method or the magnetron sputtering process; annealed at 600°C - 700°C for 5 - 7h.

4. The preparation method of the low-detection-limit MEMS acetylene gas sensor according to claim 1, characterized in that, In step 4), the silicon wafer with the pattern of the sensitive material area is immersed in deionized water, and a SiO2 microsphere - n-butanol solution with a diameter of 300 - 400nm is dropped, so that the microspheres are self-assembled on the surface of the deionized water. The silicon wafer is taken out horizontally to complete the transfer of the monolayer of SiO2 microspheres, and the silicon wafer is heated obliquely at 75 - 85°C by 15 - 20° for secondary self-assembly.

5. The preparation method of the low-detection-limit MEMS acetylene gas sensor according to claim 1, wherein, In step 5), the sputtering power of the SnO2 film is 50 - 100W, the time is 20 - 30min, and the argon flow rate is 20 - 30sccm.

6. The preparation method of the low-detection-limit MEMS acetylene gas sensor according to claim 1, characterized in that, In step 6), the ultrasonic power is 50W - 100W, and the ultrasonic time is 30 - 60min; In step 7), the thickness of the Au film evaporated by using the electron beam evaporation process is 5 - 8nm.

7. The preparation method of the MEMS acetylene gas sensor with low detection limit according to claim 1, characterized in that In steps 5), 8), and 12), the silicon wafer is successively immersed in acetone - ethanol - deionized water to strip the photoresist, immersed in acetone for 45 - 60 min, and immersed in ethanol and deionized water for 5 - 10 min each.

8. The preparation method of the MEMS acetylene gas sensor with low detection limit according to claim 1, characterized in that, In step 8), heat treatment is carried out at 450 - 500 °C for 2 - 3 h.

9. The preparation method of the low-detection-limit MEMS acetylene gas sensor according to claim 1, characterized in that, In step 11), the SiO2 - Si3N4 double - layer composite film at the groove window is removed by deep dry etching, the etching power is 200 - 250 w, and the time is 15 - 20 min.

10. A low-detection-limit MEMS acetylene gas sensor prepared by the method according to any one of claims 1-9, characterized in that, It includes a SiO2 masking layer and a Si3N4 masking layer provided on the back of the Si substrate and a four - layer composite film provided on the front of the Si substrate. An adiabatic groove is opened on the back of the Si substrate, and a sensitive material, a Cr - Au heating electrode and its lead disk, and a Cr - Au sensitive electrode and its lead disk are provided on the upper surface of the composite film on the front of the Si substrate; The sensitive material is distributed in the center of the upper surface of the composite film on the front of the Si substrate, including a SnO2 porous film, and Au nanoparticles and honeycomb - shaped pores distributed in the SnO2 porous film; the Cr - Au heating electrode and the Cr - Au sensitive electrode are symmetrically distributed along the sensitive material. Electrode lines are led out from the Cr - Au heating electrode and the Cr - Au sensitive electrode and are symmetrically distributed. The electrode line of the Cr - Au sensitive electrode has a finger - like structure.

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