A liquid electrolyte-based mems electrochemical gas sensor and a method for manufacturing the same

Abstract

The application provides a liquid electrolyte-based MEMS electrochemical gas sensor and a preparation method thereof, and belongs to the technical field of gas sensors, and aims to solve the technical problem of poor sealing of the MEMS electrochemical gas sensor.The sensor comprises an upper chip and a lower chip, the upper chip is bonded above the lower chip, a microcavity, an air inlet hole and a working electrode, the working electrode has a part of regions in contact with electrolyte in the microcavity and another part of regions in contact with air through the air inlet hole, and a counter electrode, the counter electrode is located on the inner wall of the microcavity, the inner wall where the counter electrode is arranged is provided with a through hole, the counter electrode is arranged on the through hole, the through hole penetrates the upper chip or the lower chip and is in communication with the outside, and the through hole is filled with a metal conductor and is connected with the counter electrode.The MEMS electrochemical gas sensor arranges the lead wire and the pad on the non-bonding surface of the sensor by using the glass through hole or the silicon through hole technology, so that the silicon-glass bonding surface is free of other substances, the bonding is compact, and the sealing performance of the sensor is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of gas sensors, and particularly relates to a MEMS electrochemical gas sensor based on a liquid electrolyte and a preparation method thereof. BACKGROUND

[0002] Nowadays, gas sensors are increasingly widely applied in various fields, and their technology starts to develop in the direction of miniaturization, integration, modularization and intelligentization under the impetus of technology applications such as the Internet of Things. Without doubt, the design scheme of a MEMS gas sensor will become one of the main development directions of future gas sensors. Compared with traditional electrochemical gas sensors, the advantages of a MEMS electrochemical gas sensor are as follows: firstly, the liquid electrolyte can be completely controlled in a microchannel / microcavity, so that the reaction is more accurate and sensitive, and the reaction time is shorter; secondly, since the MEMS sensor has a miniaturized structure, the volume of the liquid electrolyte required is only microliters, thereby greatly saving the cost; in addition, its small volume is also convenient for integration on terminal applications such as mobile phones.

[0003] A most important challenge of a MEMS electrochemical gas sensor using a liquid electrolyte is to ensure its sealing. If the sealing is problematic, liquid leakage will occur, affecting the performance stability of the sensor and even causing the sensor to fail, thereby affecting the service life of the sensor. Patent publication No. CN 113916950A discloses an electrochemical gas sensor based on MEMS and a preparation method thereof, and the proposed electrochemical gas sensor has a main structure which is integrated by glass and silicon through anodic bonding, the lower surface of the glass is provided with an array of working electrodes, and the reference electrode, the counter electrode and the working electrode are located in the electrolyte tank. However, the electrode lead is located on the bonding surface, which will cause the bonding at the position of the lead to be not perfect, a gap exists, and liquid leakage risk is caused. For example, patent publication No. CN 109752428A discloses an electrochemical gas sensor constructed by MEMS manufacturing technology, and the proposed sensor structure is sealed and joined between the bottom and the top through an adhesive. However, the components such as sulfuric acid contained in the liquid electrolyte will corrode the adhesive, and long-term use will also cause the risk of liquid leakage. SUMMARY

[0004] In view of the technical problem of poor sealing of a MEMS electrochemical gas sensor, the application proposes a MEMS electrochemical gas sensor based on a liquid electrolyte and a preparation method thereof. The structure of the MEMS electrochemical gas sensor designed by the application arranges the lead and the pad on the non-bonding surface of the sensor by using glass through-hole (TGV) or silicon through-hole (TSV) technology, so that there is no other substance on the silicon-glass bonding surface, the bonding is ensured to be tight, and the sealing of the sensor is improved.

[0005] In order to achieve the above-mentioned purposes, the technical scheme of the application is as follows:

[0006] A MEMS electrochemical gas sensor based on liquid electrolyte, comprising an upper chip;

[0007] a lower chip, the upper chip is bonded above the lower chip;

[0008] a microcavity, located between the lower chip and the upper chip, the microcavity stores electrolyte;

[0009] an air inlet hole, the air inlet hole is provided on the upper chip and connects the upper chip with the microcavity, and external air communicates with the microcavity through the air inlet hole;

[0010] a working electrode, the working electrode is provided on the upper chip, and a part of the working electrode is in contact with the electrolyte in the microcavity, and another part of the working electrode is in contact with air through the air inlet hole;

[0011] a counter electrode, the counter electrode is located on the inner wall of the microcavity, the inner wall where the counter electrode is provided is provided with a through hole, the counter electrode is arranged on the through hole, the through hole penetrates the upper chip or the lower chip and communicates with the outside, and a metal conductor is filled in the through hole and connected with the counter electrode;

[0012] a working electrode pad and a counter electrode pad, the working electrode pad is arranged on the outer surface of the upper chip and electrically connected with the working electrode, and the counter electrode pad is arranged at the end of the through hole away from the counter electrode and electrically connected with the counter electrode through the metal conductor.

[0013] The working principle is that the measured gas reaches the surface of the working electrode through the air inlet hole, the measured gas, the liquid electrolyte and the working electrode form a gas-liquid-solid three-phase interface, and the measured gas is oxidized or reduced at the electrode to measure the current, so as to obtain the concentration of the measured gas. The sensor generally has two or three electrodes in contact with the electrolyte, and a typical electrode is composed of a large-surface-area noble metal and a porous hydrophobic membrane. The gas diffuses into the working electrode of the sensor through the back of the porous membrane, and the gas is oxidized or reduced here. This electrochemical reaction causes the current flowing through the external circuit.

[0014] If a two-electrode structure is adopted, the working electrode needs to be arranged to be in direct contact with the measured gas and to form a gas-liquid-solid three-phase interface with the measured gas and the liquid electrolyte; the counter electrode needs to be arranged to be in contact with the liquid electrolyte and not in contact with the measured gas; and the working electrode and the counter electrode generally adopt noble metal materials such as platinum (Pt), gold (Au) and silver (Ag) which have catalytic effect and stable properties. In the design, appropriately increasing the electrode area helps to increase the generated current, thereby improving the sensitivity of the sensor.

[0015] If a three-electrode structure is adopted, a reference electrode needs to be arranged in the same plane beside the above-mentioned counter electrode, and the reference electrode generally adopts refractory salt materials such as silver chloride (AgCl), silver oxide (Ag2O) and mercury oxide (HgO).

[0016] The upper chip is a silicon wafer, which comprises a top silicon layer away from the lower chip and a bottom silicon layer close to the lower chip, and a silicon dioxide layer is arranged between the top silicon layer and the bottom silicon layer; the material of the lower chip is glass.

[0017] The bottom silicon layer is provided with a cavity, and the gas inlet hole is arranged in the area of the upper chip provided with the cavity, and the cavity opening is sealed to form a microcavity after the upper chip is bonded to the lower chip.

[0018] The working electrode is arranged on the inner wall of the cavity and the side wall of the gas inlet hole, and extends to the outer surface of the upper chip.

[0019] The working electrode and the contact area of the upper chip are provided with a porous membrane.

[0020] The porous membrane is generally aluminum oxide or other porous materials that can be processed by atomic layer deposition, which can uniformly cover the surface and the side wall, and the thickness is generally 5-50 nm, and can also be waterproof and breathable porous materials such as polytetrafluoroethylene emulsion after solidification.

[0021] The counter electrode is arranged on one side of the lower chip inside the microcavity, the through hole is a glass through hole, and the glass through hole penetrates the lower chip, and the counter electrode pad is arranged on the glass through hole of the outer surface of the lower chip.

[0022] A preparation method of a MEMS electrochemical gas sensor based on a liquid electrolyte, comprising the following steps:

[0023] (1) An air inlet hole is made on the surface of the top silicon layer of the silicon wafer by photolithography and dry etching;

[0024] (2) A layer of 2-3 μm thick silicon dioxide is formed on the top silicon layer, the bottom silicon layer and the side wall of the silicon wafer by thermal oxidation;

[0025] (3) A wet etching window is processed on the surface of the bottom silicon layer of the silicon wafer by photolithography and RIE etching, and then the bottom silicon layer is etched by wet etching to obtain a cavity structure;

[0026] (4) The silicon dioxide on the surface of the top silicon layer and the side wall of the air inlet hole is etched away by dry etching;

[0027] (5) A 5-50 nm thick porous membrane is first formed on the surface of the top silicon layer, the side wall of the air inlet hole and the side wall of the cavity by atomic layer deposition, and then a 5-50 nm thick working electrode is formed on the porous membrane;

[0028] (6) A 50-150 nm thick working electrode pad is deposited on the surface of the top silicon layer;

[0029] (7) The silicon dioxide on the surface of the bottom silicon layer is removed by wet etching, and the processing on the upper chip is completed;

[0030] (8) glass through holes with a diameter of 30-100 μm are prepared on the glass sheet, and then metal conductors are plated to fill the glass through holes; after filling, the metal conductors on the surface of the glass sheet are removed by CMP to leave only the copper metal conductors in the glass through holes;

[0031] (9) a 50-150 nm thick counter electrode and a 50-150 nm thick counter electrode pad are respectively processed on the glass through holes on the two sides of the glass sheet by photolithography, sputtering and stripping processes to complete the processing on the lower chip;

[0032] (10) the processed upper chip and lower chip are bonded together by anode bonding to complete wafer processing, and then the entire wafer is divided into a plurality of sensors by cutting.

[0033] The material of the upper chip is glass, and the lower chip is a silicon wafer.

[0034] The lower chip is provided with a recessed cavity, and the recessed cavity is sealed to form a microcavity after the upper chip and the lower chip are bonded.

[0035] The working electrode is located on the side wall of the gas inlet hole and extends to the outer surface of the upper chip.

[0036] The working electrode and the contact area of the upper chip are provided with a porous membrane.

[0037] The counter electrode is arranged on the side of the lower chip inside the microcavity, the through hole is a silicon through hole, the silicon through hole penetrates the lower chip, and the counter electrode pad is arranged on the silicon through hole on the outer surface of the lower chip.

[0038] A preparation method of a MEMS electrochemical gas sensor based on a liquid electrolyte, comprising the following steps:

[0039] (1) a recessed cavity structure is prepared on the front surface of a silicon wafer by photolithography and dry etching;

[0040] (2) a silicon through hole is prepared in the recessed cavity of the silicon wafer, and then a metal conductor is plated to fill the silicon through hole; after filling, the metal conductor on the surface of the silicon wafer is removed by CMP to leave only the metal conductor in the silicon through hole;

[0041] (3) a silicon through hole on the two sides of the silicon wafer is respectively processed by photolithography, sputtering and stripping to form a layer of counter electrode and a layer of counter electrode pad to complete the processing on the lower chip;

[0042] (4) a gas inlet hole is prepared on the front surface of a glass sheet;

[0043] (5) a porous membrane is first formed on the front surface of the glass substrate and the side wall of the gas inlet hole by atomic layer deposition, and then a working electrode is formed on the porous membrane;

[0044] (6) Prepare the working electrode pad on the working electrode of the front surface of the glass sheet, complete the processing on the upper chip;

[0045] (7) Bond the processed upper chip and lower chip together by anodic bonding, complete wafer processing, and then cut the entire wafer into several sensors by cutting.

[0046] The upper chip is provided with a liquid injection port and a liquid outlet port, and the liquid injection port and the liquid outlet port are in communication with the microcavity. The electrolyte flows into the microcavity through the liquid injection port and flows out through the liquid outlet port.

[0047] The working electrode is arranged on one side of the upper chip on the inner wall of the microcavity. The inner wall where the working electrode is arranged is provided with a glass through hole. The working electrode is arranged on the glass through hole. The through hole penetrates the upper chip and communicates with the outside. The glass through hole is filled with a metal conductor connected with the working electrode. The working electrode pad is arranged on the glass through hole on the outer surface of the upper chip. Inside the microcavity, the working electrode covers at least a part of the gas inlet hole, so that the working electrode can contact with the outside air.

[0048] The side wall of the gas inlet hole is provided with a hydrophobic layer.

[0049] The counter electrode is also arranged on one side of the upper chip on the inner wall of the microcavity, which is on the same side as the working electrode. The through hole is a glass through hole. The glass through hole penetrates the upper chip. The counter electrode pad is arranged on the glass through hole on the outer surface of the upper chip.

[0050] A preparation method of a MEMS electrochemical gas sensor based on liquid electrolyte, comprising the following steps:

[0051] (1) Make a concave cavity structure on a silicon wafer, complete the processing on the lower chip;

[0052] (2) Prepare two groups of glass through holes on the front surface of a glass sheet. After filling the holes, use CMP to remove the metal conductor on the surface of the glass sheet, leaving only the metal conductor in the glass through hole;

[0053] (3) Prepare a liquid injection port, a liquid outlet port and a gas inlet hole on the glass sheet;

[0054] (4) Fill the gas inlet hole with waterproof and breathable material by spraying, and form a hydrophobic layer after solidification.

[0055] (5) Prepare a working electrode pad and a counter electrode pad on the two groups of glass through holes on the front surface of the glass sheet by photolithography, sputtering and stripping process respectively. A layer of working electrode and counter electrode is also processed on the back surface of the glass sheet by photolithography, sputtering and stripping process, complete the processing on the upper chip;

[0056] (6) Bond the processed upper chip and lower chip together by anodic bonding, complete wafer processing, and then cut the entire wafer into several sensors by cutting.

[0057] The beneficial effects of the present application are:

[0058] (1) The MEMS electrochemical gas sensor structure designed in the present application applies glass through-hole (TGV) or silicon through-hole (TSV) technology to arrange the lead wire and the pad on the non-bonding surface of the sensor, so that the silicon-glass bonding surface is free of other substances, ensuring the tightness and reliability of the bonding, improving the sealing property of the sensor, and avoiding the risk of liquid leakage of the sensor.

[0059] (2) The MEMS electrochemical gas sensor designed in the present application deposits a layer of catalytic metal on the surface of the porous membrane as the working electrode. The prepared working electrode is a porous structure with high specific surface area, and the sidewalls of the gas inlet holes are also uniformly covered with catalytic metal. The sensitivity of the sensor is improved by increasing the gas-liquid-solid three-phase interface formed by the measured gas, the liquid electrolyte and the working electrode, and by filling the gas inlet holes with waterproof and breathable materials to increase the three-phase interface.

[0060] (3) The liquid electrolyte used in the present application can be pre-processed to form a liquid injection port and a liquid outlet port on the surface of the sensor. Then, the liquid electrolyte is injected from the liquid injection port by a micro-syringe, and the gas is extracted from the liquid outlet port to ensure that the liquid electrolyte fills the micro-cavity. Alternatively, the liquid electrolyte can be placed in a beaker or other container, and the sensor is placed in the container. Then, the container is placed in a vacuum box and pumped to a low pressure state for 5 minutes. Then, the vacuum pump is turned off, and the pressure gradually returns to normal. The liquid electrolyte fills the micro-cavity by utilizing the pressure difference. The composition of the liquid electrolyte used in the present application depends on the specific measured gas, including but not limited to acidic solution, alkaline solution, solidifiable polymer solution and water. BRIEF DESCRIPTION OF DRAWINGS

[0061] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort based on these drawings.

[0062] Figure 1 It is a sectional view of the MEMS electrochemical gas sensor structure of the present application.

[0063] Figure 2 It is a schematic diagram of the top silicon surface of the upper chip in the MEMS electrochemical gas sensor of the present application.

[0064] Figure 3 It is a schematic diagram of the bottom silicon surface of the upper chip in the MEMS electrochemical gas sensor of the present application.

[0065] Figure 4The lower chip glass two-side structure of the MEMS electrochemical gas sensor.

[0066] Figure 5 The structure sectional view of the MEMS electrochemical gas sensor in Example 3.

[0067] Figure 6 The structure sectional view of the MEMS electrochemical gas sensor in Example 4.

[0068] In the figure, 1 is an upper chip; 101 is a top silicon layer; 102 is a silicon dioxide layer; 103 is a bottom silicon layer; 2 is a lower chip; 3 is a microcavity; 4 is an air inlet hole; 5 is a working electrode; 51 is a working electrode pad; 6 is a counter electrode; 61 is a counter electrode pad; 7 is a through hole; 8 is a porous membrane; 9 is a recessed cavity; 10 is a liquid injection port; 11 is a liquid outlet port; and 12 is a reference electrode. DETAILED DESCRIPTION

[0069] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative effort belong to the scope of protection of the present application.

[0070] Example 1

[0071] A MEMS electrochemical gas sensor based on liquid electrolyte, as shown in FIG. 1, comprises an upper chip 1. Figure 1

[0072] A lower chip 2, the upper chip 1 is bonded above the lower chip 2 to connect the two into one whole; the upper chip 1 and the lower chip 2 can be selected from silicon wafers or glass wafers respectively.

[0073] A microcavity 3, located between the lower chip 2 and the upper chip 1, the microcavity 3 stores electrolyte; the microcavity 3 can be arranged in the lower chip 2, the upper chip 1, or both the upper chip 1 and the lower chip 2.

[0074] An air inlet hole 4, the air inlet hole 4 is arranged on the upper chip 1 and penetrates the upper chip 1 to connect with the microcavity 3, and the air inlet hole 4 is located directly above the microcavity 3, forming an air passage perpendicular to the surface of the upper chip 1, and the external air communicates with the microcavity 3 through the air inlet hole 4; the air inlet hole 4 is provided with a plurality of air inlet hole 4 arrays.

[0075] ​Working electrode 5, working electrode 5 is arranged on the upper chip 1, working electrode 5 has a part of the area in contact with the electrolyte in the microcavity 3, that is, at least a part of the working electrode 5 is located inside the microcavity 3; working electrode 5 also has a part of the area in contact with air through the air inlet hole 4, that is, at least a part of the working electrode 5 is located in or covers the air inlet hole 4;

[0076] Counter electrode 6, counter electrode 6 is located on the inner wall of the microcavity 3, counter electrode 6 is in contact with the electrolyte but not in contact with the working electrode 5; the inner wall where the counter electrode 6 is arranged is provided with a through hole 7, the counter electrode 6 is arranged on the through hole 7 to cover the through hole 7, the through hole 7 penetrates the upper chip 1 or the lower chip 2 and is in communication with the outside, when the counter electrode 6 is arranged on the inner wall of the microcavity 3 close to the upper chip 1, the through hole 7 is arranged in the upper chip 1, and vice versa, the through hole 7 is arranged in the lower chip 2. The metal conductor in the through hole 7 is filled and connected with the counter electrode 6, the metal conductor in the through hole 7 acts as a wire to realize the electrical connection of the counter electrode 6 with the outside; similarly, the through hole 7 can be arranged in an array to increase the electrical connection path of the counter electrode 6.

[0077] Working electrode pad 51 and counter electrode pad 61, working electrode pad 51 is arranged on the outer surface of the upper chip 1 and is in electrical communication with the working electrode 5, the working electrode pad 51 can be in direct contact with the working electrode 5 or realize electrical connection with the working electrode 5 through a lead. The counter electrode pad 61 covers the through hole 7 away from the counter electrode 6, and the counter electrode pad 61 is in electrical communication with the counter electrode 6 through a metal conductor. The counter electrode pad 61 is arranged on the outer surface of the sensor, and is arranged on the outer surface of the upper chip 1 or the lower chip 2 according to the position of the counter electrode 6. The working electrode pad 51 and the counter electrode pad 61 are used to connect the external circuit.

[0078] The working principle of the MEMS electrochemical gas sensor is that the measured gas reaches the surface of the working electrode 5 through the air inlet hole 4, the measured gas, the liquid electrolyte and the working electrode 5 form a gas-liquid-solid three-phase interface, and the measured gas is oxidized or reduced at the working electrode 5 to measure the current, so as to obtain the concentration of the measured gas. The sensor generally has two or three working electrodes 5 in contact with the electrolyte, a typical working electrode 5 is composed of a large surface area of noble metal and a porous hydrophobic membrane, the gas diffuses into the working electrode 5 of the sensor through the back of the porous membrane 8, and the gas is oxidized or reduced here, and such electrochemical reaction causes the current flowing through the external circuit.

[0079] The above embodiment features a two-electrode structure. The working electrode 5 is positioned to directly contact the gas to be measured and form a gas-liquid-solid three-phase interface with the gas and liquid electrolyte. The counter electrode 6 is positioned to contact the liquid electrolyte but not the gas to be measured. Both the working electrode 5 and the counter electrode 6 are generally made of noble metals such as platinum (Pt), gold (Au), and silver (Ag), which have catalytic properties and are stable. Appropriately increasing the electrode area during the design helps to increase the generated current, thereby improving the sensor sensitivity.

[0080] Furthermore, a three-electrode structure can be adopted, requiring only a reference electrode to be arranged coplanarly next to the aforementioned counter electrode 6. Its arrangement and connection to the external circuit are the same as those of the counter electrode 6. The reference electrode is generally made of refractory salt materials such as silver chloride (AgCl), silver oxide (Ag2O), and mercury oxide (HgO).

[0081] Example 2

[0082] A MEMS electrochemical gas sensor based on liquid electrolyte, such as Figure 1 As shown, it includes the upper chip 1;

[0083] Lower chip 2, upper chip 1 is bonded above lower chip 2;

[0084] Microcavity 3 is located between lower chip 2 and upper chip 1, and electrolyte is stored inside microcavity 3;

[0085] Air inlet 4 is provided on the upper chip 1 and the upper chip 1 is connected to the micro cavity 3 through it. External air is connected to the micro cavity 3 through the air inlet 4.

[0086] Working electrode 5 is disposed on the upper chip 1. Part of the working electrode 5 is in contact with the electrolyte in the microcavity 3, and another part of the working electrode 5 is in contact with air through the air inlet 4.

[0087] Counter electrode 6 is located on the inner wall of microcavity 3. A through hole 7 is provided on the inner wall where counter electrode 6 is located. Counter electrode 6 is set on through hole 7. Through hole 7 penetrates the lower chip 2 and communicates with the outside. Through hole 7 is filled with a metal conductor connected to counter electrode 6.

[0088] The working electrode pad 51 and the counter electrode pad 61 are provided. The working electrode pad 51 is located on the outer surface of the upper chip 1 and is electrically connected to the working electrode 5. The counter electrode pad 61 is located at the end of the through hole 7 away from the counter electrode 6 and is electrically connected to the counter electrode 6 through a metal conductor.

[0089] Specifically, the upper chip 1 is a silicon wafer (silicon on insulator, SOI), the SOI includes a top silicon layer 101 away from the lower chip 2 and a bottom silicon layer 103 close to the lower chip 2, and a silicon dioxide layer 102 is arranged between the top silicon layer 101 and the bottom silicon layer 103; the material of the lower chip 2 is glass.

[0090] The bottom silicon layer 103 is provided with a cavity 9, and the gas inlet hole 4 is arranged in the area (the bottom of the cavity 9) of the upper chip 1 provided with the cavity 9 and penetrates the top silicon layer 101 and the silicon dioxide layer 102. After the upper chip 1 and the lower chip 2 are bonded, the cavity 9 is sealed to form a microcavity 3, and the gas can enter the microcavity 3 through the gas inlet hole 4.

[0091] The working electrode 5 is arranged on the inner wall of the cavity 9 and the side wall of the gas inlet hole 4 and extends to the outer surface of the upper chip 1. The working electrode 5 on the inner wall of the cavity 9 is directly in contact with the electrolyte, and the working electrode 5 covers the entire inner wall of the cavity 9, and the side wall of the gas inlet hole 4 is also uniformly covered with the working electrode 5. This arrangement increases the gas-liquid-solid three-phase interface formed by the measured gas, the liquid electrolyte and the working electrode 5, which helps to improve the sensitivity of the sensor.

[0092] Further, the working electrode 5 and the contact area of the upper chip 1 are provided with a porous film 8. The porous film 8 is generally aluminum oxide or other porous materials that can be processed by atomic layer deposition, which can uniformly cover the surface and the side wall, and the thickness is generally 5-50 nm. The gas diffuses into the working electrode 5 of the sensor through the back of the porous film 8, which helps to improve the sensitivity of the sensor.

[0093] The counter electrode 6 is arranged on one side of the lower chip 2 in the inner wall of the microcavity 3 and does not directly contact the working electrode 5 in the microcavity 3; the through hole 7 is a glass through hole, the glass through hole penetrates the lower chip 2 (the lower chip 2 is glass), and the counter electrode pad 61 is arranged on the glass through hole of the outer surface of the lower chip 2. The glass through hole is filled with copper, which seals the glass through hole and also serves as a lead wire, so that the counter electrode 6 in the microcavity 3 can be connected to the external circuit through the copper in the glass through hole and the counter electrode pad 61.

[0094] The preparation method of the above-mentioned MEMS electrochemical gas sensor based on liquid electrolyte includes the following steps:

[0095] (1) An array structure of gas inlet holes 4 is formed on the surface of the top silicon layer 101 of the SOI wafer by photolithography and dry etching Figure 2 );

[0096] (2) After the glue is removed and cleaned, a layer of 2-3 μm thick silicon dioxide is formed on the top silicon layer 101, the bottom silicon layer 103 and the side wall by thermal oxidation;

[0097] (3) Etching a wet etching window (1002) on the bottom silicon layer 103 surface by lithography and RIE etching, then etching the bottom silicon layer 103 by KOH solution to obtain the cavity 9 structure; Figure 3

[0098] (4) Protecting the back surface structure of the SOI wafer by sticking a tape on the bottom silicon layer 103 surface, then etching the silicon dioxide on the top silicon layer 101 surface and the side wall of the gas inlet hole 4 by dry etching (HF gas), and etching the silicon dioxide in the SOI wafer at the end of the gas inlet hole 4, so that the gas inlet hole 4 is connected to the cavity 9;

[0099] (5) Forming a 5-50 nm thick porous film 8 (aluminum oxide or other porous materials that can be processed by atomic layer deposition) on the top silicon layer 101 surface, the side wall of the gas inlet hole 4, and the side wall of the cavity 9, and then forming a 5-50 nm thick working electrode 5 (generally made of platinum or other noble metal materials with catalytic effect);

[0100] (6) Depositing a 50-150 nm thick gold layer on the top silicon layer 101 surface as the working electrode pad 51 by sputtering, evaporation, or other methods;

[0101] (7) Removing the protective tape and then removing the silicon dioxide on the bottom silicon layer 103 surface by wet etching to complete the processing of the upper chip 1;

[0102] (8) Forming a glass via hole with a diameter of 30-100 μm on the glass wafer by laser-induced wet etching or laser micro-processing, then plating copper to fill the glass via hole, and removing the copper on the surface of the glass wafer by CMP after filling to leave only the copper in the glass via hole;

[0103] (9) Processing a 50-150 nm thick counter electrode 6 structure (generally made of platinum or other noble metal materials with catalytic effect) and a 50-150 nm thick gold counter electrode pad 61 (1003) on both sides of the glass wafer by lithography, sputtering, and stripping to complete the processing of the lower chip 2; Figure 4

[0104] (10) Bonding the processed upper chip 1 and lower chip 2 together by anode bonding to complete the wafer processing, and then cutting the entire wafer into several sensors.

[0105] Example 3

[0106] A MEMS electrochemical gas sensor based on liquid electrolyte, as shown in FIG. 1, includes an upper chip 1; Figure 5

[0107] A lower chip 2, and the upper chip 1 is bonded above the lower chip 2;

[0108] ​​​A microcavity 3 is located between the lower chip 2 and the upper chip 1, and an electrolyte is stored in the microcavity 3;

[0109] An air inlet hole 4 is provided on the upper chip 1 and penetrates the upper chip 1 to connect with the microcavity 3, and external air communicates with the microcavity 3 through the air inlet hole 4;

[0110] A working electrode 5 is provided on the upper chip 1, and a part of the working electrode 5 contacts the electrolyte in the microcavity 3, and another part of the working electrode 5 contacts air through the air inlet hole 4;

[0111] A counter electrode 6 is located on the inner wall of the microcavity 3, and a through hole 7 is provided on the inner wall where the counter electrode 6 is located. The counter electrode 6 is arranged on the through hole 7, the through hole 7 penetrates the upper chip and communicates with the outside, and the through hole 7 is filled with a metal conductor connected with the counter electrode 6;

[0112] A working electrode pad 51 is provided on the outer surface of the upper chip 1 and is in electrical connection with the working electrode 5, and a counter electrode pad 61 is provided on the end of the through hole 7 away from the counter electrode 6 and is in electrical connection with the counter electrode 6 through the metal conductor.

[0113] Specifically, the material of the upper chip 1 is glass, and the lower chip 2 is a silicon wafer.

[0114] The surface of the lower chip 2 is provided with a recess, and the recess is sealed to form a microcavity 3 after the upper chip 1 and the lower chip 2 are bonded, and the air inlet hole 4 is provided in the upper chip 1 above the microcavity 3.

[0115] The working electrode 5 is located on the side wall of the air inlet hole 4 and extends to the outer surface of the upper chip 1, and the working electrode pad 51 is arranged above the working electrode 5 on the outer surface.

[0116] A porous membrane 8 is arranged between the contact area of the working electrode 5 and the upper chip 1. The porous membrane 8 is generally aluminum oxide or other porous materials that can be processed by atomic layer deposition, and can uniformly cover the surface and the side wall, and the thickness is generally 5-50nm. The gas diffuses into the working electrode 5 of the sensor through the back of the porous membrane 8, which helps to improve the sensitivity of the sensor.

[0117] The counter electrode 6 is arranged on one side of the lower chip 2 on the inner wall of the microcavity 3, and the through hole 7 is a through silicon via (TSV hole), which penetrates the lower chip 2, and the counter electrode pad 61 is arranged on the through silicon via on the outer surface of the lower chip 2. The through silicon via is filled with copper, which seals the through silicon via and acts as a lead wire, so that the counter electrode 6 in the microcavity 3 can be connected with the external circuit through the copper in the through silicon via and the counter electrode pad 61.

[0118] The preparation method of the above-mentioned MEMS electrochemical gas sensor based on liquid electrolyte includes the following steps:

[0119] (1) Fabricate the cavity structure on the front side of the silicon substrate by photolithography and dry etching;

[0120] (2) Protect the front side of the silicon substrate with adhesive tape or other means, and prepare the TSV hole on the back side by photolithography and dry etching, then fill the via hole 7 with copper plating, and remove the copper on the surface of the silicon after filling the hole by CMP method, leaving only the copper in the via hole 7;

[0121] (3) Remove the protective tape, and process a layer of counter electrode 6 structure and a layer of gold pad on both sides of the silicon wafer by photolithography, sputtering and stripping process; In other three-electrode structure embodiments, the processing of the reference electrode 12 can be to deposit silver first and then form silver oxide or silver chloride through chemical reaction, and the processing on the lower chip 2 is completed;

[0122] (4) Prepare the gas inlet hole 4 on the front side of the glass substrate by laser-induced wet etching method or laser microprocessing method, and protect the back side with adhesive tape or other means;

[0123] (5) Use atomic layer deposition (ALD) to form a 5-50 nm thick porous film 8 (aluminum oxide or other porous materials that can be processed by atomic layer deposition) on the front side of the glass substrate and the sidewall of the gas inlet hole 4, and then form a 5-50 nm thick working electrode 5 (usually made of platinum or other noble metal materials with catalytic effect);

[0124] (6) Deposit a 50-150 nm thick gold layer on the front side of the glass substrate as a working electrode pad 5 using sputtering, evaporation or other methods, and complete the processing on the upper chip 1;

[0125] (7) Bond the processed upper chip 1 and lower chip 2 together by anodic bonding, complete wafer processing, and then cut the entire wafer into several sensors by cutting.

[0126] Example 4

[0127] A MEMS electrochemical gas sensor based on liquid electrolyte, as shown in Figure 6 , includes an upper chip 1;

[0128] a lower chip 2, the upper chip 1 is bonded above the lower chip 2;

[0129] a microcavity 3 located between the lower chip 2 and the upper chip 1, the microcavity 3 stores an electrolyte;

[0130] a gas inlet hole 4, the gas inlet hole 4 is provided on the upper chip 1 and connects the upper chip 1 with the microcavity 3, and the external air communicates with the microcavity 3 through the gas inlet hole 4;

[0131] The working electrode 5 is arranged on the upper chip 1, and has a part of the area in contact with the electrolyte in the microcavity 3 and another part of the area in contact with the air through the air inlet hole 4.

[0132] The counter electrode 6 is arranged on the inner wall of the microcavity 3, and the inner wall where the counter electrode 6 is arranged is provided with the through hole 7. The counter electrode 6 is arranged on the through hole 7, the through hole 7 penetrates the upper chip 1 and is in communication with the outside, and the through hole 7 is filled with a metal conductor to connect the counter electrode 6.

[0133] The working electrode pad 51 is arranged on the outer surface of the upper chip 1 and is in electrical communication with the working electrode 5, and the counter electrode pad 61 is arranged at the end of the through hole 7 away from the counter electrode 6 and is in electrical communication with the counter electrode 6 through the metal conductor.

[0134] Specifically, the material of the upper chip 1 is glass, and the lower chip 2 is a silicon wafer or organic silicon PDMS.

[0135] The upper chip 1 is provided with a liquid injection port 10 and a liquid outlet 11, and the liquid injection port 10 and the liquid outlet 11 are in communication with the microcavity 3. The electrolyte flows into the microcavity 3 through the liquid injection port 10 and flows out through the liquid outlet 11. Preferably, the liquid injection port 10 and the liquid outlet 11 are arranged at both ends of the microcavity 3 to ensure that the electrolyte quickly fills the microcavity 3.

[0136] The working electrode 5 is arranged on the inner wall of the microcavity 3 on one side of the upper chip 1. The inner wall where the working electrode 5 is arranged is provided with a glass through hole, the working electrode 5 is arranged on the glass through hole, the glass through hole penetrates the upper chip 1 and is in communication with the outside, the glass through hole is filled with a metal conductor to connect the working electrode 5, and the working electrode pad 51 is arranged on the glass through hole on the outer surface of the upper chip 1. Inside the microcavity 3, the working electrode 5 covers at least a part of the air inlet hole 4, so that the working electrode 5 can be in contact with the outside air. In this embodiment, the working electrode 5 is completely arranged inside the microcavity 3 and connected to the external circuit through the glass through hole as a lead. At the same time, one side of the working electrode 5 contacts the electrolyte, and the other side contacts the external air through the air inlet hole 4, so as to ensure that the gas-liquid-solid three-phase interface can be formed.

[0137] The side wall of the air inlet hole 4 is provided with a hydrophobic layer, such as polytetrafluoroethylene or the like.

[0138] The counter electrode 6 is also arranged on the inner wall of the microcavity 3 on one side of the upper chip 1, which is the same side as the working electrode 5 and does not contact the working electrode 5 and the air inlet hole 4. The through hole 7 is a glass through hole, and the glass through hole penetrates the upper chip 1. The counter electrode pad 61 is arranged on the glass through hole on the outer surface of the upper chip 1.

[0139] The preparation method of the above-mentioned MEMS electrochemical gas sensor based on liquid electrolyte includes the following steps:

[0140] (1) Fabricate the cavity structure on the front side of the silicon substrate by photolithography and dry etching, or use the PDMS reverse mode to fabricate the cavity structure, complete the processing on the lower chip 2;

[0141] (2) Prepare the TGV hole (two groups, respectively on the back of the working electrode 5 and the counter electrode 6) on the front side of the glass substrate by laser-induced humidification etching method or laser microprocessing method, then plate copper to fill the via 7, after filling the hole, use the CMP method to remove the copper on the surface of the silicon, only leave the copper in the via 7;

[0142] (3) Process the liquid injection port 10, liquid outlet 11 and air inlet hole 4 by laser microprocessing method;

[0143] (4) Fill the waterproof and breathable material such as polytetrafluoroethylene emulsion in the air inlet hole 4 using spraying method and solidify;

[0144] (5) Process a layer of gold as the working electrode pad 51 and the counter electrode pad 61 on the front side of the glass substrate by photolithography, sputtering and stripping process, and process a layer of working electrode 5 and counter electrode 6 on the back of the glass substrate using the same photolithography, sputtering and stripping process, complete the processing on the upper chip 1;

[0145] (6) Bond the processed upper chip 1 and lower chip 2 together by anodic bonding (the material of the lower chip 2 is silicon) or plasma bonding (the material of the lower chip 2 is PDMS), complete the wafer processing, and then cut the entire wafer into several sensors by cutting method.

[0146] The above only describes the preferred embodiments of the present application and is not used to limit the present application, any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A MEMS electrochemical gas sensor based on a liquid electrolyte, characterized in that, The application relates to a chip structure. An upper chip (1); A lower chip (2) on which the upper chip (1) is bonded; A microcavity (3) between the lower chip (2) and the upper chip (1), the microcavity (3) storing an electrolyte; An air inlet hole (4) arranged on the upper chip (1) and connecting the upper chip (1) with the microcavity (3), external air communicating with the microcavity (3) through the air inlet hole (4); A working electrode (5) arranged on the upper chip (1), the working electrode (5) having a part of an area in contact with the electrolyte in the microcavity (3) and another part of an area in contact with air through the air inlet hole (4); A counter electrode (6) arranged on the inner wall of the microcavity (3), the inner wall where the counter electrode (6) is arranged being provided with a through hole (7), the counter electrode (6) being arranged on the through hole (7), the through hole (7) penetrating the upper chip (1) or the lower chip (2) and communicating with the outside, and the through hole (7) being filled with a metal conductor and connected with the counter electrode (6); A working electrode pad (51) arranged on the outer surface of the upper chip (1) and electrically connected with the working electrode (5), and a counter electrode pad (61) arranged on the end of the through hole (7) away from the counter electrode (6) and electrically connected with the counter electrode (6) through the metal conductor. The upper chip (1) is a silicon wafer, the silicon wafer comprises a top silicon layer (101) away from the lower chip (2) and a bottom silicon layer (103) close to the lower chip (2), a silicon dioxide layer (102) being arranged between the top silicon layer (101) and the bottom silicon layer (103); the material of the lower chip (2) is glass; the bottom silicon layer (103) is provided with a recess (9), the air inlet hole (4) is arranged in the region of the upper chip (1) provided with the recess (9) on the back surface, and the recess (9) is sealed to form the microcavity (3) after the upper chip (1) is bonded with the lower chip (2); the working electrode (5) is arranged on the inner wall of the recess (9) and the side wall of the air inlet hole (4) and extends to the outer surface of the upper chip (1); a porous membrane (8) is arranged between the working electrode (5) and the contact area of the upper chip (1); the counter electrode (6) is arranged on one side of the inner wall of the microcavity (3) and the lower chip (2), the through hole (7) is a glass through hole, the glass through hole penetrates the lower chip (2), and the counter electrode pad (61) is arranged on the glass through hole on the outer surface of the lower chip (2); Or the material of the upper chip (1) is glass, and the lower chip (2) is a silicon wafer; the lower chip (2) is provided with a recess (9), and the recess (9) is sealed to form the microcavity (3) after the upper chip (1) is bonded with the lower chip (2); the working electrode (5) is arranged on the side wall of the air inlet hole (4) and extends to the outer surface of the upper chip (1); a porous membrane (8) is arranged between the working electrode (5) and the contact area of the upper chip (1); the counter electrode (6) is arranged on one side of the inner wall of the microcavity (3) and the lower chip (2), the through hole (7) is a silicon through hole, the silicon through hole penetrates the lower chip (2), and the counter electrode pad (61) is arranged on the silicon through hole on the outer surface of the lower chip (2).

2. The liquid-electrolyte-based MEMS electrochemical gas sensor according to claim 1, wherein A reference electrode (12) is also included, which is arranged near the counter electrode (6).

3. The liquid-electrolyte-based MEMS electrochemical gas sensor of claim 1, wherein, The upper chip (1) is provided with a liquid injection port (10) and a liquid outlet (11), which are in communication with the micro-cavity (3). Electrolyte flows into the micro-cavity (3) through the liquid injection port (10) and flows out through the liquid outlet (11). The working electrode (5) is arranged on one side of the inner wall of the micro-cavity (3) in the upper chip (1). The inner wall where the working electrode (5) is arranged is provided with a glass through hole. The working electrode (5) is arranged on the glass through hole. The glass through hole penetrates the upper chip (1) and is in communication with the outside. The glass through hole is filled with a metal conductor which is connected with the working electrode (5). The working electrode pad (51) is arranged on the glass through hole on the outer surface of the upper chip (1). Inside the micro-cavity (3), the working electrode (5) covers at least a part of the air inlet hole (4), so that the working electrode (5) can be in contact with the outside air. The side wall of the air inlet hole (4) is provided with a hydrophobic layer.

4. The liquid-electrolyte-based MEMS electrochemical gas sensor according to claim 3, wherein The counter electrode (6) is also arranged on one side of the inner wall of the micro-cavity (3) in the upper chip (1), which is on the same side as the working electrode (5). The through hole (7) is a glass through hole which penetrates the upper chip (1). The counter electrode pad (61) is arranged on the glass through hole on the outer surface of the upper chip (1).

5. The method of claim 1, wherein the liquid electrolyte-based MEMS electrochemical gas sensor is prepared by the steps of: The method comprises the following steps: (1) An air inlet hole (4) is formed on the surface of the top silicon layer (101) of the silicon wafer by photolithography and dry etching; (2) A layer of 2-3 μm thick silicon dioxide is formed on the top silicon layer (101), the bottom silicon layer (103) and the side wall by thermal oxidation; (3) A wet etching window is processed on the surface of the bottom silicon layer (103) of the silicon wafer by photolithography and RIE etching, and then the bottom silicon layer (103) is etched by wet etching to obtain a recess cavity (9) structure; (4) The silicon dioxide on the surface of the top silicon layer (101) and the side wall of the air inlet hole (4) is removed by dry etching; (5) A 5-50 nm thick porous film (8) is first formed on the surface of the top silicon layer (101), the side wall of the air inlet hole (4) and the side wall of the recess cavity (9) by atomic layer deposition, and then a 5-50 nm thick working electrode (5) is formed on the porous film (8); (6) A 50-150 nm thick working electrode pad (51) is deposited on the surface of the top silicon layer (101); (7) The silicon dioxide on the surface of the bottom silicon layer (103) is removed by wet etching to complete the processing on the upper chip (1); (8) A glass through hole with a pore size of 30-100 μm is prepared on a glass sheet, and a metal conductor is plated to fill the glass through hole. After filling, the metal conductor on the surface of the glass sheet is removed by CMP method, leaving only the metal conductor in the glass through hole; (9) A 50-150 nm thick counter electrode (6) and a 50-150 nm thick counter electrode pad (61) are processed on the glass through hole on both sides of the glass sheet by photolithography, sputtering and stripping processes to complete the processing on the lower chip (2); (10) The processed upper chip (1) and lower chip (2) are bonded together by anodic bonding to complete wafer processing, and then the entire wafer is divided into several sensors by cutting.

6. The method of claim 1, wherein the liquid electrolyte-based MEMS electrochemical gas sensor is prepared by the steps of: The method comprises the following steps: (1) A concave cavity (9) structure is made on the front surface of a silicon wafer by photolithography and dry etching; (2) A through-silicon via is made in the concave cavity (9) of the silicon wafer, and then a metal conductor is plated to fill the through-silicon via. After filling, the metal conductor on the surface of the silicon wafer is removed by CMP, leaving only the metal conductor in the through-silicon via; (3) A counter electrode (6) and a counter electrode pad (61) are processed on the two surfaces of the silicon wafer by photolithography, sputtering, and stripping, completing the processing of the lower chip (2); (4) An air inlet hole (4) is made on the front surface of a glass wafer; (5) A porous film (8) is first formed on the front surface of the glass wafer and the sidewall of the air inlet hole (4) using atomic layer deposition, and then a working electrode (5) is formed on the porous film (8); (6) A working electrode pad (51) is made on the working electrode (5) on the front surface of the glass wafer, completing the processing of the upper chip (1); (7) The processed upper chip (1) and lower chip (2) are bonded together by anodic bonding, completing wafer processing, and then the entire wafer is divided into several sensors by cutting.

7. The method of claim 3, wherein the liquid electrolyte-based MEMS electrochemical gas sensor is prepared by the steps of: The steps include: (1) A concave cavity (9) structure is made on the silicon wafer, completing the processing of the lower chip (2); (2) Two groups of glass vias are made on the front surface of a glass wafer. After filling, the metal conductor on the surface of the glass wafer is removed by CMP, leaving only the metal conductor in the glass via; (3) A reagent inlet (10), a liquid outlet (11), and an air inlet hole (4) are made on the glass wafer; (4) A waterproof and breathable material is filled in the air inlet hole (4) using spraying, and a hydrophobic layer is formed after solidification; (5) A working electrode pad (51) and a counter electrode pad (61) are made on the two groups of glass vias on the front surface of the glass wafer by photolithography, sputtering, and stripping, respectively. A working electrode (5) and a counter electrode (6) are also processed on the back surface of the glass wafer using photolithography, sputtering, and stripping, completing the processing of the upper chip (1); (6) The processed upper chip (1) and lower chip (2) are bonded together by anodic bonding, completing wafer processing, and then the entire wafer is divided into several sensors by cutting.

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