A porous metal oxide wafer-level micro-nano gas sensor and its preparation method

Abstract

The present invention discloses a porous metal oxide wafer-level micro-nano gas sensor and a preparation method, comprising the following steps: preparing a SiO2-Si3N4 double-layer composite film on the front and back sides of a Si substrate; depositing SiO2 and Si3N4 on the film, and photolithographically obtaining a pattern of a sensitive material area on the front insulating layer; self-assembling a single layer of SiO2 microspheres, dry-etching Si3N4, and removing the SiO2 microspheres by BOE to produce a porous metal oxide film. The photolithography process produces patterns of sensitive electrodes and lead disks, heating electrodes and lead disks, and temperature measuring electrodes and lead disks; wet etching is used to produce insulating grooves, which are then dried to obtain a porous metal oxide wafer-level micro-nano gas sensor. The sensor improves its gas-sensing performance by realizing the production of a porous metal oxide film. The sensitive material of the present invention has submicron-level micropores, extremely high specific surface area, and better gas response characteristics, and can be integrated with MEMS technology, thereby realizing wafer-level chip processing.

Description

Technical Field

[0001] The present invention relates to a porous metal oxide wafer-level micro-nano gas sensor based on a colloidal crystal template method and a preparation method thereof. Background Art

[0002] There is a widespread demand for gas type and concentration detection in many areas of national production and life. In the field of environmental monitoring, there is a need to monitor gases such as nitrogen oxides and sulfur oxides (SO2) caused by the incomplete combustion of fossil fuels; in the field of industrial production, it is necessary to monitor the type and concentration of industrial waste gas; in the field of home life, it is necessary to monitor the concentration of toxic gases such as formaldehyde in newly renovated houses; in the field of traffic management, it is necessary to detect the alcohol concentration in the exhaled breath of drunk drivers. It can be seen that every aspect of our food, clothing, housing and transportation is closely related to gas, and gas detection is of great significance to all aspects of our lives.

[0003] At present, there are still many problems with gas sensors. The first is power consumption. Gas sensors generally need to work at a specific temperature to ensure sufficient sensitivity and response speed, so they need to consume a lot of energy. In order to minimize power consumption, it is necessary to combine the production of gas sensors with MEMS technology, further reduce the size of gas sensors, and produce wafer-level micro-nano gas sensors. However, there are other problems in the production of wafer-level micro-nano gas sensors, namely compatibility and sensitivity. The commonly used spraying method is not well compatible with MEMS technology, which easily leads to insufficient consistency. The use of MEMS-compatible processes such as RF sputtering has a low sensitivity because the sputtered gas-sensitive film is relatively dense, which loses its application value. Therefore, achieving both wafer-level micro-nano gas sensor production and high sensitivity has become the focus of current research. Many researchers have proposed methods such as constructing heterojunctions, doping, and micromorphology control to solve this problem, but they still cannot ensure the compatibility of gas sensors while improving sensitivity. Summary of the Invention

[0004] To address the low sensitivity problem caused by dense thin films in the prior art, the present invention aims to provide a porous metal oxide wafer-scale micro-nano gas sensor and its preparation method. The sensitive material is integrated with a sensitive electrode, a heating electrode, a temperature measuring electrode, and a silicon substrate. Micromorphology control is used to ensure compatibility while improving sensitivity. The sensitive material in the present invention, with its submicron-scale micropores, possesses an extremely high specific surface area and improved gas response characteristics. Furthermore, the entire process can be integrated with MEMS technology, resulting in a wafer-scale, highly sensitive gas sensor.

[0005] The present invention is achieved through the following technical solutions.

[0006] The present invention provides a method for preparing a porous metal oxide wafer-level micro-nano gas sensor, comprising the following steps:

[0007] (1) SiO2-Si3N4 double-layer composite films were prepared on the front and back sides of Si substrates by thermal oxidation and low-pressure chemical vapor deposition, respectively;

[0008] (2) On the front SiO2-Si3N4 double-layer composite film, SiO2 and Si3N4 are sequentially deposited by plasma enhanced chemical vapor deposition and annealed;

[0009] (3) On the front insulating layer, a pattern of the sensitive material area is obtained through a photolithography process;

[0010] (4) obtaining a single layer of SiO2 microspheres on the front side of the silicon wafer with the sensitive material area pattern by a scooping method, and heating to obtain a SiO2 microsphere masking layer;

[0011] (5) Using dry etching of Si3N4 on the front side of the silicon wafer to create submicron-scale micropores;

[0012] (6) Using BOE solution to remove SiO2 microspheres while ultrasonically vibrating;

[0013] (7) sputtering a sensitive material in the sensitive material area on the front side of the submicron microporous silicon wafer to form a porous metal oxide film;

[0014] (8) washing with acetone, ethanol, and deionized water in sequence, and heat-treating in air;

[0015] (9) On the rectangular area surrounded by the sensitive material, the same photolithography process as in step (3) is used to obtain the patterns of the sensitive electrode and lead disk, the heating electrode and lead disk, and the temperature measuring electrode and lead disk;

[0016] (10) sputtering a Cr bonding layer on the heating electrode and lead pad and the sensitive electrode and lead pad patterns, and sputtering an Au layer on the bonding layer;

[0017] (11) Using the same process as step (8), remove the photoresist and heat in air;

[0018] (12) adopting the same photolithography process as step (3) to obtain the back side groove window pattern;

[0019] (13) removing the Si3N4-SiO2 layer at the groove window by deep dry etching, and then removing the photoresist by the same process as step (8);

[0020] (14) Spin-coat photoresist on the front of the chip, drip PDMS on the front of the chip, attach the front of the chip to a glass slide, and dry;

[0021] (15) Wet etching is used to make insulating grooves, which are then dried to obtain porous metal oxide wafer-level micro-nano gas sensors.

[0022] Preferably, a SiO2-Si3N4 double-layer composite film is prepared, wherein the SiO2 layer thickness is 450-550nm, and the Si3N4 layer thickness is 130-160nm; the SiO2 layer thickness on the SiO2-Si3N4 double-layer composite film is 450-550nm, and the Si3N4 layer thickness is 200-400nm; and annealing is performed at 550-600℃ for 6-7h.

[0023] Preferably, in step (4), the SiO2 microsphere solution is added dropwise to the water surface to self-assemble into a densely packed single-layer SiO2 microsphere, and the preliminarily assembled single-layer SiO2 microsphere is transferred to the surface of a silicon wafer having a sensitive area pattern by a transfer method; and heated at a temperature of 60-90°C and an inclination angle of 15° to perform secondary self-assembly to obtain a SiO2 microsphere masking layer; the diameter of the SiO2 microsphere is 200-250nm.

[0024] Preferably, for making submicron micropores, the etching gas is SF6, the etching power is 30-60W, and the etching time is 1-2min; for removing SiO2 microspheres, the ultrasonic power is 80-120W, and the ultrasonic time is 8-10s.

[0025] Preferably, a 100 nm layer of sensitive material, including SnO2, TiO2 or ZnO, is sputtered on the sensitive layer pattern, soaked in acetone for 2 hours, soaked in ethanol for 3-5 minutes, rinsed with deionized water, and heat-treated in air at 400-500° C. for 2-4 hours.

[0026] Preferably, a 30-50 nm Cr bonding layer is sputtered on the heating electrode and lead pad and the sensitive electrode and lead pad patterns, and a 200-300 nm Au layer is sputtered on the bonding layer.

[0027] Preferably, the front side of the chip is spin-coated with photoresist and dried at 80-90° C.; the front side of the chip is covered with PDMS and dried at 60-80° C. for 1-2 hours.

[0028] Preferably, the chip and the glass sheet are placed in a 15-25% tetramethylammonium hydroxide solution at 80-90° C. for etching for 15-20 hours; then immersed in acetone and dried at 100-120° C. for 1-2 hours.

[0029] The present invention further provides a porous metal oxide wafer-level micro-nano gas sensor, comprising a Si substrate, a masking layer formed by a SiO2-Si3N4 double-layer thin film composite on the back of the Si substrate, a thermal insulation groove on the back of the Si substrate, an insulating layer formed by a SiO2-Si3N4-SiO2-Si3N4 four-layer thin film composite on the front of the Si substrate, a sensitive material with submicron-level micropores provided on the insulating layer, the sensitive material being located in the center and below the sensitive electrode; a pair of sensitive electrodes and their lead disks, two pairs of heating electrodes and their lead disks, and two pairs of temperature measuring electrodes and their lead disks are provided in the same plane; the sensitive electrodes, the heating electrodes, the temperature measuring electrodes, and the lead disks are made of a Cr-Au thin film with submicron pores.

[0030] Preferably, the heating electrode and the sensitive electrode are arranged in a centrally symmetrical manner, the sensitive electrode is a forked finger structure, and electrode wires are led out from the sensitive electrode in a centrally symmetrical distribution and are led out to the electrode lead disk; the heating electrode is symmetrically distributed on the outer diagonal of the forked finger structure, is a spiral structure and is led to the heating element lead disk, and the temperature measuring electrodes are respectively arranged on both sides.

[0031] The present invention adopts the above technical solution, which has the following beneficial effects:

[0032] 1. By adopting the secondary self-assembly method, a densely packed single-layer SiO2 microspheres is used as a masking layer for dry etching of Si3N4, which can ensure that submicron micropores are etched on Si3N4. Subsequently, metal oxide is sputtered on the surface of Si3N4 with micropores to produce metal oxide with submicron micropores.

[0033] 2. By varying the diameter of the self-assembled monolayer of SiO2 microspheres, the pore size of the submicron pores in the sensitive material can be controlled, thereby enabling control of the sensitive material's microstructure. Due to the submicron-scale pores, the sensitive material possesses an extremely high specific surface area and improved gas response characteristics. The response value of porous metal oxide gas-sensitive films can be increased to more than twice that of pure metal oxide gas-sensitive films. Furthermore, the entire process can be integrated with MEMS technology, enabling wafer-level chip processing and greater flexibility.

[0034] 3. The self-assembly method can achieve large-area dense production of single-layer SiO2 microspheres. At the same time, this method can also ensure the uniformity of the produced single layer and meet the requirements of mass production.

[0035] 4. Placing the sensitive material under the electrode not only protects the sensitive material, but also has the biggest advantage of ensuring direct planar contact between the sensitive material and the substrate, which can ensure that the sensitive material is heated evenly and at a higher heating rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The drawings described herein are used to provide a further understanding of the present invention, constitute a part of this application, and do not constitute an improper limitation of the present invention. In the drawings:

[0037] Figure 1(a) is a structural cross-sectional view of the wafer-level high-sensitivity micro-nano gas sensor of the present invention. Figure 1(b) and Figure 1(c) are schematic diagrams of a single-layer SiO2 microsphere and a metal oxide sensitive material with submicron-level micropores during the fabrication process of the wafer-level high-sensitivity micro-nano gas sensor of the present invention, respectively.

[0038] Figure 2 The present invention provides a planar structure of a heating electrode, a sensitive electrode, and a temperature measuring electrode of a wafer-level high-sensitivity micro-nano gas sensor.

[0039] Figure 3 (a)- Figure 3 (o) is a flow chart of the preparation process of the wafer-level high-sensitivity micro-nano gas sensor of the present invention.

[0040] In the figure: 1. Lead disk; 2. Si3N4 insulation layer I; 3. SiO2 insulation layer I; 4. Si substrate; 5. SiO2 masking layer; 6. Insulation groove; 7. Si3N4 masking layer; 8. SiO2 insulation layer II; 9. Si3N4 insulation layer II; 10. Temperature measuring electrode; 11. Cr-Au heating electrode; 12. Cr-Au sensitive electrode; 13. Metal oxide nanofilm; 14. Submicron pores; 15. Sensitive material. DETAILED DESCRIPTION

[0041] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. The exemplary embodiments and descriptions of the present invention are used to explain the present invention but are not intended to limit the present invention.

[0042] As shown in Figure 1(a), Figure 2 As shown, on one hand, the present invention provides a wafer-level high-sensitivity micro-nano gas sensor structure, including a Si substrate 4, the back of the Si substrate 4 is sequentially provided with a SiO2 masking layer 5 and a Si3N4 masking layer 7, and the masking layer is composed of a SiO2-Si3N4 double-layer thin film composite; and an insulation groove 6 is opened on the back of the Si substrate 4, and the front of the Si substrate 4 is an insulating layer composed of a Si3N4 insulating layer I2, a SiO2 insulating layer I3, a Si3N4 insulating layer II9 and a SiO2 insulating layer II8. A sensitive material 15 is provided on the insulating layer; and two pairs of temperature measuring electrodes 10 and their lead disks 1, two pairs of Cr-Au heating electrodes 11 and their lead disks 1, and a pair of Cr-Au sensitive electrodes 12 and their lead disks 1 are provided in the same plane. The sensitive material 15 is located in the center and below the interdigitated portion of the Cr-Au sensitive electrode 12. The sensitive material 15 is composed of a metal oxide nanofilm 13 and submicron pores 14.

[0043] As shown in FIG1(b), the result of the SiO2 microsphere assembly presents a honeycomb-like dense distribution, which serves as a masking layer for the subsequent production of a sensitive material 15 with submicron pores. As shown in FIG1(c), the formed sensitive material 15 with submicron pores 14 has triangular submicron pores, which greatly increases the specific surface area of ​​the sensitive material.

[0044] like Figure 2 As shown, the sensitive material 15 is arranged at the center of the sensor, and electrode lines are led out from the Cr-Au sensitive electrode 12 in a centrally symmetrical distribution and are led out to the electrode lead disk 1; the Cr-Au sensitive electrode 12 is an interdigitated structure, and the interdigitated part is located above the sensitive material 15, which can ensure full contact with the sensitive layer without damaging the structure of the sensitive material 15.

[0045] Reference Figure 2 The Cr-Au heating electrode 11 and the Cr-Au sensitive electrode 12 are arranged in a centrally symmetrical manner. The sensitive electrode is surrounded by the heating electrode, and the sensitive material 15 is located below the interdigitated portion of the Cr-Au sensitive electrode 12. The Cr-Au heating electrode 11 is also centrally symmetrically distributed around the diagonal periphery of the interdigitated structure, and adopts a spiral structure and is led to the heating element lead disk 1. The temperature measuring electrodes 10 are respectively arranged on both sides, each electrode has an independent lead disk, distributed on both sides. All electrodes and lead disks of the sensor are arranged in a centrally symmetrical manner.

[0046] The insulating layer and the shielding layer are made of SiO2 and Si3N4 composite materials; the sensitive electrode, the heating electrode, the temperature measuring electrode and each lead plate are made of Cr-Au film.

[0047] In one embodiment, the inner circle size of the Cr-Au heating electrode 11 is 155 μm × 155 μm, the width of the Cr-Au heating electrode 11 is 8 μm, and the gap is 15 μm. The electrode width of the Cr-Au sensing electrode 12 is 10 μm, the gap is 10 μm, and the size of the sensitive material 15 is 100 μm × 100 μm.

[0048] like Figure 3 (a)-(o), the method for preparing a wafer-level high-sensitivity micro-nano gas sensor of the present invention is as follows:

[0049] (1) On the front and back sides of the Si substrate, a SiO2-Si3N4 double-layer composite film was prepared by thermal oxidation and low-pressure chemical vapor deposition, respectively; the thickness of the SiO2 layer was 450-550nm, and the thickness of the Si3N4 layer was 130-160nm.

[0050] (2) On the front SiO2-Si3N4 double-layer composite film, SiO2 and Si3N4 are sequentially deposited by plasma enhanced chemical vapor deposition, with the SiO2 layer thickness being 450-550nm and the Si3N4 layer thickness being 200-400nm; and annealed at 550-600℃ for 6-7h.

[0051] (3) On the front insulating layer, a pattern of the sensitive material area is obtained through a photolithography process;

[0052] (4) A single layer of SiO2 microspheres is obtained on the front side of the silicon wafer with the sensitive material area pattern by a scooping method, that is, a single layer of SiO2 microspheres is obtained by self-assembly to form a dense pattern, and the preliminarily assembled single layer of SiO2 microspheres is transferred to the surface of the silicon wafer with the sensitive area pattern by a transfer method; the SiO2 microspheres are heated at a temperature of 60-90°C and an inclination angle of 15° to perform secondary self-assembly to obtain a SiO2 microsphere masking layer; the diameter of the SiO2 microspheres is 200-250nm.

[0053] (5) Etching Si3N4 on the front side of the silicon wafer using a dry etching method to produce submicron micropores; the etching gas is SF6, the etching power is 30-60W, and the etching time is 1-2min;

[0054] (6) Use BOE solution to remove SiO2 microspheres while ultrasonically oscillating; the ultrasonic power is 80-120W, and the ultrasonic time is 8-10s.

[0055] (7) Sputtering 100 nm of SnO2, TiO2 or ZnO as a sensitive material in the sensitive material area on the front side of the submicron microporous silicon wafer to form a porous metal oxide film;

[0056] (8) Clean with acetone-ethanol-deionized water in sequence, soak in acetone for 2 hours, soak in ethanol for 3-5 minutes, rinse with deionized water, and heat treat in air at 400-500°C for 2-4 hours.

[0057] (9) On the rectangular area surrounded by the sensitive material, the same photolithography process as in step (3) is adopted to obtain the patterns of the sensitive electrode and lead disk, the heating electrode and lead disk, and the temperature measuring electrode and lead disk.

[0058] (10) A 30-50 nm Cr bonding layer is sputtered on the heating electrode and lead pad and the sensitive electrode and lead pad patterns, and a 200-300 nm Au layer is sputtered on the bonding layer.

[0059] (11) Using the same process as step (8), remove the photoresist and heat in air at 250-300°C for 10-20 minutes.

[0060] (12) The same photolithography process as in step (3) is used to obtain the back groove window pattern.

[0061] (13) Remove the Si3N4-SiO2 layer at the groove window by deep dry etching, and then remove the photoresist using the same process as step (8).

[0062] (14) Spin-coat photoresist on the front of the chip and dry it at 80-90°C; drip PDMS on the front of the chip, stick the front of the chip on a glass sheet, and dry it at 60-80°C for 1-2 hours.

[0063] (15) Wet etching is used to make an insulating groove. The chip and the glass sheet are placed in a 15-25% tetramethylammonium hydroxide solution at 80-90°C for 15-20 hours; then soaked in acetone and dried at 100-120°C for 1-2 hours to obtain a porous metal oxide wafer-level micro-nano gas sensor.

[0064] The present invention is further illustrated below by means of specific examples.

[0065] Example 1

[0066] (1) Figure 3 As shown in (a), a SiO2-Si3N4 double-layer composite film was prepared on the front and back sides of the Si substrate by thermal oxidation and low-pressure chemical vapor deposition, respectively; a 500nm SiO2 layer was thermally oxidized on both sides of the silicon wafer, and a 150nm Si3N4 layer was deposited on both sides by LPCVD (low-pressure chemical vapor deposition).

[0067] (2) Figure 3 As shown in (b), on the front SiO2-Si3N4 double-layer composite film, 500nm SiO2 and 300nm Si3N4 were deposited in sequence by PECVD on the front side and annealed at 550℃ for 7h.

[0068] (3) Figure 3 As shown in (c), on the front insulating layer, the pattern of the sensitive material area is obtained by the photoresist process, and the positive photoresist EPG535 is used.

[0069] (4) Figure 3 As shown in (d), a single layer of SiO2 microspheres was obtained by scooping on the front side of the silicon wafer with the sensitive material area pattern. The SiO2 microspheres were heated at a temperature of 80°C and an inclination angle of 15° to ensure that the SiO2 microspheres self-assembled on the surface, thereby obtaining a SiO2 microsphere masking layer with a diameter of 200nm.

[0070] (5) Figure 3(e) Si3N4 is etched on the front side of the silicon wafer using dry etching to produce submicron-level micropores. The etching gas is SF6, and the etching power and time are 50W and 1min respectively.

[0071] (6) Figure 3 As shown in (f), BOE solution is used to remove SiO2 microspheres, and ultrasonic oscillation is used to ensure that the SiO2 microspheres are completely removed. The ultrasonic power and time are 100W and 10s respectively.

[0072] (7) Figure 3 As shown in (g), 100nm of SnO2 is sputtered on the sensitive layer pattern.

[0073] (8) Figure 3 As shown in (h), the photoresist is removed by cleaning with acetone-ethanol-deionized water in sequence. The acetone soaking time is 2 hours, the ethanol soaking time is 5 minutes, and deionized water is used for rinsing. Then, the film is heat-treated at 450°C in air for 3 hours.

[0074] (9) Figure 3 As shown in (i), on the rectangular area surrounded by the sensitive material, the same photolithography process as step (3) is adopted to obtain the patterns of the sensitive electrode and lead disk, the heating electrode and lead disk, and the temperature measuring electrode and lead disk.

[0075] (10) Figure 3 As shown in (j), a Cr bonding layer is sputtered on the heating electrode and lead pad and the sensitive electrode and lead pad patterns, and then an Au layer is sputtered on the bonding layer; 50nm Cr and 250nm Au are sputtered in sequence using a sputtering machine, where Cr is used to ensure the adhesion between the Au electrode and the bottom surface.

[0076] (11) Figure 3 As shown in (k), the same process as step (8) is adopted to remove the photoresist, and then heated in air at 300°C for 10 minutes to allow the electrode and the sensitive material to be tightly bonded.

[0077] (12) Figure 3 As shown in (l), the same photolithography process as step (3) is adopted to obtain the back groove window pattern.

[0078] (13) Figure 3 As shown in (m), using photoresist as a masking layer, the Si3N4-SiO2 layer at the recess window is removed by deep dry etching, and the etching time is 13 minutes. The photoresist is then removed using the same process as step (8).

[0079] (14) Spin-coat photoresist on the front of the chip, dry it at 90°C, and then drop PDMS onto the front of the chip until the front is full. Dry it at 70°C for 1 hour. Figure 3As shown in (n), the front side of the chip is attached to the glass slide, and a circle of PDMS is applied to the outer edge of the back side of the chip to ensure that the chip is firmly attached to the glass slide.

[0080] (15) Place the chip and glass slide in a TMAH (25% tetramethylammonium hydroxide) solution at 85°C for 16 hours to form an insulating groove. Gently peel off the PDMS, soak it in acetone to remove the photoresist and residual PDMS, and dry it at 110°C for 1 hour. The resulting sensor is as shown below. Figure 3 (o) shown.

[0081] The gas response characteristics of the obtained chip were tested, and the chip was placed in the gas to be tested to test its sensitivity. It was found that the chip's response value to 50ppm ethanol gas was as high as 8.4, which is 4.2 times that of the sensor chip made by the ordinary sputtering thin film process. At the same time, the response time of this chip to 50ppm ethanol was 18s, which was 10s shorter than the sensor chip made by the ordinary sputtering thin film process.

[0082] Example 2

[0083] (1) Figure 3 As shown in (a), a SiO2-Si3N4 double-layer composite film was prepared on the front and back sides of the Si substrate by thermal oxidation and low-pressure chemical vapor deposition, respectively; a 450nm SiO2 layer was thermally oxidized on both sides of the silicon wafer, and a 160nm Si3N4 layer was deposited on both sides by LPCVD (low-pressure chemical vapor deposition).

[0084] (2) Figure 3 As shown in (b), on the front SiO2-Si3N4 double-layer composite film, 450nm SiO2 and 400nm Si3N4 were deposited in sequence by PECVD on the front side and annealed at 580℃ for 6.5h.

[0085] (3) Figure 3 As shown in (c), on the front insulating layer, the pattern of the sensitive material area is obtained by the photoresist process, and the positive photoresist EPG535 is used.

[0086] (4) Figure 3 As shown in (d), a single layer of SiO2 microspheres was obtained by scooping on the front side of the silicon wafer with the sensitive material area pattern. The SiO2 microspheres were heated at a temperature of 60°C and an inclination angle of 15° to ensure that the SiO2 microspheres self-assembled on the surface, thus obtaining a SiO2 microsphere mask. The diameter of the SiO2 microspheres was 250nm.

[0087] (5) Figure 3 (e) Si3N4 is etched on the front side of the silicon wafer using dry etching to produce submicron-level micropores. The etching gas is SF6, and the etching power and time are 60W and 1min respectively.

[0088] (6) Figure 3 As shown in (f), BOE solution is used to remove SiO2 microspheres, and ultrasonic oscillation is used to ensure that the SiO2 microspheres are completely removed. The ultrasonic power and time are 120W and 8s respectively.

[0089] (7) Figure 3 As shown in (g), 100nm of TiO2 is sputtered on the sensitive layer pattern.

[0090] (8) Figure 3 As shown in (h), the photoresist is removed by cleaning with acetone-ethanol-deionized water in sequence. The acetone soaking time is 2 hours, the ethanol soaking time is 4 minutes, and deionized water is used for rinsing. Then, the film is heat-treated in air at 400°C for 4 hours.

[0091] (9) Figure 3 As shown in (i), on the rectangular area surrounded by the sensitive material, the same photolithography process as step (3) is adopted to obtain the patterns of the sensitive electrode and lead disk, the heating electrode and lead disk, and the temperature measuring electrode and lead disk.

[0092] (10) Figure 3 As shown in (j), a Cr bonding layer is sputtered on the heating electrode and lead pad and the sensitive electrode and lead pad patterns, and then an Au layer is sputtered on the bonding layer; 30nm Cr and 200nm Au are sputtered in sequence using a sputtering machine.

[0093] (11) Figure 3 As shown in (k), the same process as step (8) is adopted to remove the photoresist, and then heated in air at 280°C for 10 minutes to allow the electrode and the sensitive material to be tightly bonded.

[0094] (12) Figure 3 As shown in (l), the same photolithography process as step (3) is adopted to obtain the back groove window pattern.

[0095] (13) Figure 3 As shown in (m), using photoresist as a masking layer, the Si3N4-SiO2 layer at the recess window is removed by deep dry etching for 15 minutes. The photoresist is then removed using the same process as step (8).

[0096] (14) Spin-coat photoresist on the front of the chip, dry it at 80°C, and then drop PDMS onto the front of the chip until the front is full. Dry it at 80°C for 1 hour. Figure 3 As shown in (n), the front of the chip is attached to the glass slide, and a circle of PDMS is applied to the back of the chip to ensure that the chip is firmly attached to the glass slide.

[0097] (15) Place the chip and glass sheet in a TMAH (20% tetramethylammonium hydroxide) solution at 80°C for 20 hours to form an insulating groove. Gently peel off the PDMS, soak it in acetone to remove the photoresist and residual PDMS, and dry it at 100°C for 2 hours. The resulting sensor is as shown in the figure. Figure 3 (o) shown.

[0098] The gas response characteristics of the obtained chip were tested, and the chip was placed in the gas to be tested to test the sensitivity of the chip. It was found that the chip's response value to 50ppm ethanol gas was as high as 6.6, which is three times that of the sensor chip made by the ordinary sputtering thin film process. At the same time, the response time of this chip to 50ppm ethanol is 30s, which is 23s shorter than the sensor chip made by the ordinary sputtering thin film process.

[0099] Example 3

[0100] (1) Figure 3 As shown in (a), a SiO2-Si3N4 double-layer composite film was prepared on the front and back sides of the Si substrate by thermal oxidation and low-pressure chemical vapor deposition, respectively; a 550nm SiO2 layer was thermally oxidized on both sides of the silicon wafer, and a 130nm Si3N4 layer was deposited on both sides by LPCVD (low-pressure chemical vapor deposition).

[0101] (2) Figure 3 As shown in (b), on the front SiO2-Si3N4 double-layer composite film, 550nm SiO2 and 200nm Si3N4 are deposited in sequence on the front using PECVD, and annealed at 550℃ for 7h.

[0102] (3) Figure 3 As shown in (c), on the front insulating layer, the pattern of the sensitive material area is obtained by the photoresist process, and the positive photoresist EPG535 is used.

[0103] (4) Figure 3 As shown in (d), a single layer of SiO2 microspheres was obtained by scooping on the front side of the silicon wafer with the sensitive material area pattern. The SiO2 microspheres were heated at a temperature of 90°C and an inclination angle of 15° to ensure that the SiO2 microspheres self-assembled on the surface, thus obtaining a SiO2 microsphere mask. The diameter of the SiO2 microspheres was 220nm.

[0104] (5) Figure 3 (e) Si3N4 is etched on the front side of the silicon wafer using dry etching to produce submicron-level micropores. The etching gas is SF6, and the etching power and time are 30W and 2min respectively.

[0105] (6) Figure 3As shown in (f), BOE solution is used to remove SiO2 microspheres, and ultrasonic oscillation is used to ensure that the SiO2 microspheres are completely removed. The ultrasonic power and time are 80W and 9s respectively.

[0106] (7) Figure 3 As shown in (g), 100nm of ZnO is sputtered on the sensitive layer pattern.

[0107] (8) Figure 3 As shown in (h), the photoresist is removed by cleaning with acetone-ethanol-deionized water in sequence. The acetone soaking time is 2 hours, the ethanol soaking time is 3 minutes, and deionized water is used for rinsing. Then, the film is heat-treated at 500°C in air for 2 hours.

[0108] (9) Figure 3 As shown in (i), on the rectangular area surrounded by the sensitive material, the same photolithography process as step (3) is adopted to obtain the patterns of the sensitive electrode and lead disk, the heating electrode and lead disk, and the temperature measuring electrode and lead disk.

[0109] (10) Figure 3 As shown in (j), a Cr bonding layer is sputtered on the heating electrode and lead pad and the sensitive electrode and lead pad patterns, and then an Au layer is sputtered on the bonding layer; 40nm Cr and 300nm Au are sputtered in sequence using a sputtering machine.

[0110] (11) Figure 3 As shown in (k), the same process as step (8) is adopted to remove the photoresist, and then heated in air at 300℃ for 15 minutes to allow the electrode and sensitive material to be tightly bonded.

[0111] (12) Figure 3 As shown in (l), the same photolithography process as step (3) is adopted to obtain the back groove window pattern.

[0112] (13) Figure 3 As shown in (m), using photoresist as a masking layer, the Si3N4-SiO2 layer at the recess window is removed by deep dry etching for 12 minutes. The photoresist is then removed using the same process as step (8).

[0113] (14) Spin-coat photoresist on the front of the chip, dry it at 85°C, and then drop PDMS onto the front of the chip until the front is full. Dry it at 60°C for 2h. Figure 3 As shown in (n), the front of the chip is attached to the glass slide, and a circle of PDMS is applied to the back of the chip to ensure that the chip is firmly attached to the glass slide.

[0114] (15) Place the chip and glass sheet in a TMAH (15% tetramethylammonium hydroxide) solution at 90°C for 15 hours to form an insulating groove. Gently peel off the PDMS, soak it in acetone to remove the photoresist and residual PDMS, and dry it at 120°C for 1 hour. The resulting sensor is as shown below. Figure 3 (o) shown.

[0115] The gas response characteristics of the obtained chip were tested, and the chip was placed in the gas to be tested to test its sensitivity. It was found that the chip's response value to 50ppm ethanol gas was as high as 8.8, which is 2.2 times that of the sensor chip made by the ordinary sputtering thin film process. At the same time, the response time of this chip to 50ppm ethanol was 15s, which was 8s shorter than the sensor chip made by the ordinary sputtering thin film process.

[0116] The porous metal oxide wafer-level micro-nano gas sensor prepared by the method of the present invention has a response value of not less than 6 to 50ppm of ethanol gas, which is more than twice the response value of the sensor chip produced by the ordinary sputtering thin film process, and the response time is less than 30s, and the response speed is relatively fast. At the same time, it can be seen from the schematic diagram of Figure 1(c) that the triangular porous side walls increase the surface area, so that the material has an extremely high specific surface area. Therefore, the method of the present invention is a method for preparing porous metal oxide wafer-level micro-nano gas sensors with excellent performance.

[0117] The present invention is not limited to the above-mentioned embodiments. On the basis of the technical solutions disclosed in the present invention, those skilled in the art can make some substitutions and modifications to some of the technical features therein according to the disclosed technical content without creative labor, and these substitutions and modifications are all within the protection scope of the present invention.

Claims

1. A method for preparing a porous metal oxide wafer-level micro-nano gas sensor, characterized in that: The following steps are involved: (1) SiO2-Si3N4 double-layer composite films were prepared on the front and back sides of Si substrates by thermal oxidation and low-pressure chemical vapor deposition, respectively; (2) On the front SiO2-Si3N4 double-layer composite film, SiO2 and Si3N4 are sequentially deposited by plasma enhanced chemical vapor deposition and annealed; (3) On the front insulating layer, a pattern of the sensitive material area is obtained through a photolithography process; (4) obtaining a single layer of SiO2 microspheres on the front side of the silicon wafer with the sensitive material area pattern by a scooping method, and heating to obtain a SiO2 microsphere masking layer; (5) Using dry etching of Si3N4 on the front side of the silicon wafer to create submicron-scale micropores; (6) Using BOE solution to remove SiO2 microspheres while ultrasonically vibrating; (7) sputtering a sensitive material in the sensitive material area on the front side of the silicon wafer with submicron-scale micropores to form a porous metal oxide film; (8) washing with acetone, ethanol, and deionized water in sequence, and heat-treating in air; (9) On the rectangular area surrounded by the sensitive material, the same photolithography process as in step (3) is used to obtain the patterns of the sensitive electrode and lead disk, the heating electrode and lead disk, and the temperature measuring electrode and lead disk; (10) sputtering a Cr bonding layer on the heating electrode and lead pad and the sensitive electrode and lead pad patterns, and sputtering an Au layer on the bonding layer; (11) Using the same process as step (8), remove the photoresist and heat in air; (12) adopting the same photolithography process as step (3) to obtain the back side groove window pattern; (13) removing the Si3N4-SiO2 layer at the groove window by deep dry etching, and then removing the photoresist by the same process as step (8); (14) Spin-coat photoresist on the front of the chip, drip PDMS on the front of the chip, attach the front of the chip to a glass slide, and dry; (15) Wet etching is used to make insulating grooves, which are then dried to obtain porous metal oxide wafer-level micro-nano gas sensors.

2. The method for preparing a porous metal oxide wafer-level micro-nano gas sensor according to claim 1, characterized in that: Prepare a SiO2-Si3N4 double-layer composite film, with the SiO2 layer thickness of 450-550nm and the Si3N4 layer thickness of 130-160nm; the SiO2 layer thickness on the SiO2-Si3N4 double-layer composite film is 450-550nm, and the Si3N4 layer thickness is 200-400nm; anneal at 550-600℃ for 6-7h.

3. The method for preparing a porous metal oxide wafer-level micro-nano gas sensor according to claim 1, characterized in that: In step (4), the SiO2 microsphere solution is added dropwise to the water surface to self-assemble into a densely packed single-layer SiO2 microsphere. The preliminarily assembled single-layer SiO2 microsphere is transferred to the surface of a silicon wafer having a sensitive area pattern by a transfer method; and the solution is heated at a temperature of 60-90°C and an inclination angle of 15° to perform secondary self-assembly to obtain a SiO2 microsphere masking layer. The diameter of the SiO2 microsphere is 200-250nm.

4. The method for preparing a porous metal oxide wafer-level micro-nano gas sensor according to claim 1, characterized in that: To make submicron micropores, the etching gas is SF6, the etching power is 30-60W, and the etching time is 1-2min; to remove SiO2 microspheres, the ultrasonic power is 80-120W, and the ultrasonic time is 8-10s.

5. The method for preparing a porous metal oxide wafer-level micro-nano gas sensor according to claim 1, characterized in that: Sputter 100nm of sensitive material, including SnO2, TiO2 or ZnO, on the sensitive layer pattern, soak in acetone for 2 hours, soak in ethanol for 3-5 minutes, rinse with deionized water, and heat treat in air at 400-500℃ for 2-4 hours.

6. The method for preparing a porous metal oxide wafer-level micro-nano gas sensor according to claim 1, characterized in that: A 30-50 nm Cr bonding layer is sputtered on the heating electrode and lead pad and the sensitive electrode and lead pad patterns, and a 200-300 nm Au layer is sputtered on the bonding layer.

7. The method for preparing a porous metal oxide wafer-level micro-nano gas sensor according to claim 1, characterized in that: Spin-coat photoresist on the front of the chip and dry it at 80-90°C; then drip PDMS all over the front of the chip and dry it at 60-80°C for 1-2 hours.

8. The method for preparing a porous metal oxide wafer-level micro-nano gas sensor according to claim 1, characterized in that: The chip and the glass sheet are placed in a 15-25% tetramethylammonium hydroxide solution at 80-90°C for 15-20 hours; then, the solution is soaked in acetone and dried at 100-120°C for 1-2 hours.

9. The porous metal oxide wafer-level micro-nano gas sensor prepared by the method according to any one of claims 1 to 8, characterized in that: The invention comprises a Si substrate, a masking layer formed by a SiO2-Si3N4 double-layer film composite on the back of the Si substrate, a heat-insulating groove on the back of the Si substrate, an insulating layer formed by a SiO2-Si3N4-SiO2-Si3N4 four-layer film composite on the front of the Si substrate, a sensitive material with submicron-level micropores arranged on the insulating layer, the sensitive material being located in the center and below the sensitive electrode; a pair of sensitive electrodes and their lead disks, two pairs of heating electrodes and their lead disks, and two pairs of temperature measuring electrodes and their lead disks are arranged in the same plane; the sensitive electrodes, heating electrodes, temperature measuring electrodes and each lead disk are made of Cr-Au thin film.

10. The porous metal oxide wafer-level micro-nano gas sensor according to claim 9, characterized in that: The heating electrode and the sensitive electrode are arranged in a centrally symmetrical manner. The sensitive electrode has an interdigitated structure, and electrode wires are led out of the sensitive electrode in a centrally symmetrical distribution and are led out to the electrode lead disk; the heating electrode is symmetrically distributed on the outer diagonal of the interdigitated structure, has a spiral structure and is led to the heating element lead disk, and the temperature measuring electrodes are respectively arranged on both sides.

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