A MEMS thermal flow sensor and its manufacturing method

Abstract

The present invention provides a MEMS thermal flow sensor. Two flow channels are made longitudinally through the substrate inside the substrate, two annular resistors are made in the center of the flow channel to surround the flow channel, and annular thermopiles are placed around the two ends of the flow channel to measure the upstream and downstream temperatures. During operation, since the thermopiles on the upper and lower surfaces of the flow channel are symmetrically distributed with respect to the annular resistors, the output voltage is the same. When the fluid passes through, the symmetrical distribution of the thermal field is changed, causing the output voltage of the thermopiles on the upper and lower surfaces of the flow channel to change, thereby measuring the flow rate and direction of the fluid flowing perpendicular to the substrate. In addition, the sensor has multiple working modes: the two annular resistors can work individually or in series. Correspondingly, the output voltage of the annular thermopiles can be measured individually or in series, realizing the measurement of the fluid in a single flow channel and the simultaneous measurement of the fluid in two flow channels.

Description

Technical Field

[0001] The invention provides a MEMS thermal flow sensor and a preparation method thereof, belonging to the technical field of micro-electromechanical systems (MEMS). Background Art

[0002] Flow measurement is of great significance in the fields of industrial control, living environment monitoring, medical equipment, etc. Due to the requirements of different application scenarios and fluid conditions, a wide variety of fluid sensors have been developed and widely used. According to the measurement principle, flow sensors can be divided into: (1) thermal flow sensors, such as heat loss type, thermal temperature difference type, and thermal pulse type flow sensors; (2) non-thermal flow sensors, such as volumetric type, electromagnetic type, and pressure differential type flow sensors. However, volumetric type and electromagnetic type flow sensors usually have large power consumption and volume, and pressure differential type flow sensors have high installation requirements. In contrast, thermal flow sensors have simple structure, high sensitivity, and low power consumption, and have become the mainstream flow sensors on the market. Among thermal flow sensors, thermal temperature difference type sensors based on the principle of heat diffusion have the advantages of simple structure and high detection accuracy. Thermal temperature difference type flow sensors usually have two temperature sensors and a heat source. When the heat source is working, the temperature is higher than the surrounding environment, and a stable thermal field distribution will be formed around the temperature sensor. When a fluid passes through, the stable thermal field changes. According to the functional relationship between the temperature difference of the thermistor and the flow rate of the medium, the flow rate of the fluid can be calculated. Thermocouples manufactured using MEMS processing technology have become ideal thermal sensors due to their high sensitivity, good linearity, and stability. However, current thermoelectric flow sensors typically detect fluids flowing parallel to the substrate surface, with little research on detecting fluids flowing perpendicular to the substrate. Furthermore, current thermal flow sensors only provide single-channel measurement. Therefore, a new thermal flow sensor is needed that can detect multi-channel fluid flow perpendicular to the substrate. Summary of the Invention

[0003] Technical Problem: To overcome the shortcomings of the existing technology, the present invention provides a MEMS thermal flow sensor. Two flow channels are formed longitudinally through the substrate, with two ring-shaped resistors surrounding the channels in the center. Ring-shaped thermopiles surround each end of the channels to measure upstream and downstream temperatures. The number of flow channels can be increased as needed, along with the number of heating resistors and thermopiles. During operation, the thermopiles on the upper and lower surfaces of the flow channels are symmetrically distributed about the ring-shaped resistors, resulting in the same output voltage. Fluid flow alters the symmetrical distribution of the thermal field, causing the output voltage of the thermopiles on the upper and lower surfaces of the flow channels to change, thereby measuring the flow rate and direction of fluid flowing perpendicular to the substrate. As fluid flows from the upper to lower surface of the substrate, the ring-shaped thermopile surrounding the upper end of the flow channel detects a decrease in temperature and a decrease in output voltage; the ring-shaped thermopile surrounding the lower end detects a rise in temperature and an increase in output voltage. Furthermore, the sensor has multiple operating modes. The two ring-shaped resistors can operate independently or in series. Accordingly, the output voltage of the ring-shaped thermopiles can be measured independently or in series.

[0004] Technical solution: A MEMS thermal flow sensor includes a third substrate, wherein the third substrate includes a first substrate and a second substrate, and the first substrate and the second substrate are silicon-silicon bonded;

[0005] The third substrate includes: a first flow channel and a second flow channel, a first annular resistor and a second annular resistor, the first flow channel and the second flow channel longitudinally passing through the third substrate, a first annular thermopile, a second annular thermopile, a third annular thermopile, and a fourth annular thermopile;

[0006] The first flow channel and the second flow channel are symmetrically distributed on the left and right sides of the third substrate, the first annular resistor surrounds the first flow channel, the second annular resistor surrounds the second flow channel, and the first annular resistor and the second annular resistor are both connected to the first bonding pad via semiconductor connection wires;

[0007] The first flow channel and the second flow channel are respectively surrounded by the first annular thermopile and the second annular thermopile at the openings on the upper surface of the third substrate, and the first annular thermopile and the second annular thermopile are both connected to the second bonding block through metal connecting wires;

[0008] The first flow channel and the second flow channel are respectively surrounded by the third annular thermopile and the fourth annular thermopile at the openings on the lower surface of the third substrate, and the third annular thermopile and the fourth annular thermopile are connected to the third bonding block via metal connecting wires;

[0009] The first annular thermopile, the second annular thermopile, the third annular thermopile, and the fourth annular thermopile each include a plurality of thermocouples connected in series, each of the plurality of thermocouples includes a semiconductor arm and a metal arm, and an insulating layer of a thermoelectric material composed of silicon nitride is provided between the semiconductor arm and the metal arm.

[0010] Preferably, the MEMS thermal flow sensor further includes a first annular thermal insulation groove and a second annular thermal insulation groove, and the first annular thermal insulation groove and the second annular thermal insulation groove are respectively arranged around the first annular resistor and the second annular resistor.

[0011] Preferably, the first annular thermopile, the second annular thermopile, the third annular thermopile, the fourth annular thermopile, the first annular resistor and the second annular resistor, and the first annular thermal insulation groove and the second annular thermal insulation groove are not completely closed.

[0012] Preferably, the MEMS thermal flow sensor has three working modes:

[0013] a. The first annular resistor is electrically heated to measure the flow rate in the first flow channel;

[0014] b. the second annular resistor is electrically heated and the flow rate in the second flow channel is measured;

[0015] c. The first resistor and the second resistor are powered on and heated simultaneously, and the flow rates in the first flow channel and the second flow channel are measured simultaneously.

[0016] Preferably, the first annular resistor and the second annular resistor are doped on the second substrate.

[0017] A method for preparing a MEMS thermal flow sensor, characterized in that it comprises the following steps:

[0018] Step 1): preparing a first heating resistor (4), a second heating resistor (5), a first bonding block (171), and a semiconductor connection line (18) on the second substrate (2);

[0019] Step 2): bonding the first substrate (1) to the second substrate (2) to form a third substrate (3);

[0020] Step 3): preparing a first annular thermopile (10), a second annular thermopile (11), a third annular thermopile (12), a fourth annular thermopile (13), a metal connecting wire (19), a second pressure welding block (172), a third pressure welding block (173), a first flow channel (6), and a second flow channel (7) on a third substrate (3);

[0021] Wherein, step 1) specifically includes:

[0022] Step 1.1): selecting p-type silicon as the second substrate (2);

[0023] Step 1.2): forming a SiO2 dielectric layer on the upper surface of the second substrate (2) by thermal oxidation;

[0024] Step 1.3): coating the second substrate (2) with photoresist, then removing the photoresist at locations where the first heating resistor (4) and the second heating resistor (5) are to be formed; patterning the SiO2 dielectric layer, and then forming the first heating resistor (4) and the second heating resistor (5) by N-type ion implantation;

[0025] Step 1.4): stripping off the remaining SiO2, growing and photolithography SiO2, and performing N-type heavily doped ion implantation to form semiconductor connection lines (18);

[0026] Step 1.5): performing photolithography, Au sputtering, and stripping operations on the second substrate (2) to form a first bonding block (171);

[0027] Step 2) specifically includes:

[0028] Step 2.1): selecting thinned p-type silicon as the first substrate (1), wherein the thickness of the first substrate (1) is smaller than that of the second substrate (2), and the length and width of the first substrate (1) are both smaller than those of the second substrate (2);

[0029] Step 2.2): silicon-silicon bonding: silicon-silicon bonding the first substrate (1) and the second substrate (2) to form a third substrate (3);

[0030] Step 3) specifically includes:

[0031] Step 3.1): After silicon-silicon bonding, the lower surface of the third substrate (3) is thinned by a chemical mechanical polishing (CMP) process, and the first annular resistor (4) and the second annular resistor (5) are ensured to be located at half the height of the third substrate (3);

[0032] Step 3.2): depositing a layer of polysilicon on the upper surface of the third substrate (3) by chemical vapor deposition, and performing N-type ion implantation;

[0033] Step 3.3): applying photoresist, removing the photoresist in areas other than the semiconductor arms (15) of the first annular thermopile (10) and the second annular thermopile (11), etching the polysilicon, and then removing the photoresist;

[0034] Step 3.4): depositing a thermoelectric material insulating layer (20) composed of silicon nitride, and patterning the thermoelectric material insulating layer (20) composed of silicon nitride to isolate the semiconductor arm (15) from the metal arm (16);

[0035] Step 3.5): depositing a layer of Al and patterning it to form metal arms (16) of the first annular thermopile (10) and the second annular thermopile (11), metal connecting wires (19) of the thermopile, and preliminarily forming a second bonding block (172);

[0036] Step 3.6): performing photolithography, sputtering Au, and peeling on the third substrate 3 to form a second bonding block (172);

[0037] Step 3.7): depositing a layer of silicon nitride to form a passivation layer;

[0038] Step 3.8): removing the silicon nitride at the position of the second bonding pad (172);

[0039] Step 3.9): Repeat the process of steps 3.2) to 3.8) on the lower surface of the third substrate (3);

[0040] Step 3.10): Use a deep trench etching process to etch out the first flow channel (6) and the second flow channel (7).

[0041] A method for preparing a MEMS thermal flow sensor comprises the following steps:

[0042] Step 1): preparing a first heating resistor (4), a second heating resistor (5), a first bonding block (171), and a semiconductor connection line (18) on the second substrate (2);

[0043] Step 2): preparing a first annular thermal insulation groove (8) and a second annular thermal insulation groove (9), and bonding the first substrate (1) and the second substrate (2) to form a third substrate (3);

[0044] Step 3): preparing a first annular thermopile (10), a second annular thermopile (11), a third annular thermopile (12), a fourth annular thermopile (13), a metal connecting wire (19), a second pressure welding block (172), a third pressure welding block (173), a first flow channel (6), and a second flow channel (7) on a third substrate (3);

[0045] Wherein, step 1) specifically includes:

[0046] Step 1.1): selecting p-type silicon as the second substrate (2);

[0047] Step 1.2): forming a SiO2 dielectric layer by thermal oxidation;

[0048] Step 1.3): coating the second substrate (2) with photoresist, then removing the photoresist at the locations where the first heating resistor (4) and the second heating resistor (5) are to be formed; and then forming the first heating resistor (4) and the second heating resistor (5) by N-type ion implantation;

[0049] Step 1.4): stripping off the remaining SiO2, growing and photolithography SiO2, and performing N-type heavily doped ion implantation to form semiconductor connection lines (18);

[0050] Step 1.5): performing photolithography, Au sputtering, and stripping operations on the second substrate (2) to form a first bonding block (171);

[0051] Step 2) specifically includes:

[0052] Step 2.1): using a deep trench etching process, etching a first annular thermal insulation groove (8) and a second annular thermal insulation groove (9) with openings on the upper surface of the second substrate (2), wherein the first annular thermal insulation groove (8) surrounds the first annular resistor (4), and the second annular thermal insulation groove (9) surrounds the second annular resistor (5);

[0053] Step 2.2): Select thinned p-type silicon as the first substrate (1), requiring that the thickness of the first substrate (1) is smaller than that of the second substrate (2), and the length and width of the first substrate (1) are smaller than those of the second substrate (2);

[0054] Step 2.3): using a deep groove etching process, etching an annular deep groove with an opening on the lower surface of the first substrate (1), the position of which corresponds to the position of the deep groove of the second substrate (2), and the depth of the groove is the same as the depth of the groove etched on the second substrate (2);

[0055] Silicon-silicon bonding: bonding the first substrate (1) and the second substrate (2) to form a third substrate (3);

[0056] Step 3) specifically includes:

[0057] Step 3.1): After silicon-silicon bonding, the lower surface of the third substrate (3) is thinned by a chemical mechanical polishing (CMP) process, and the first annular resistor (4) and the second annular resistor (5) are ensured to be located at half the height of the third substrate (3);

[0058] Step 3.2): depositing a layer of polysilicon on the upper surface of the third substrate (3) by chemical vapor deposition, and performing N-type ion implantation;

[0059] Step 3.3): applying photoresist, removing the photoresist in areas other than the semiconductor arms (15) of the first annular thermopile (10) and the second annular thermopile (11), etching the polysilicon, and then removing the photoresist;

[0060] Step 3.4): depositing a thermoelectric material insulating layer (20) composed of silicon nitride, and patterning the thermoelectric material insulating layer (20) composed of silicon nitride to isolate the semiconductor arm (15) from the metal arm (16);

[0061] Step 3.5): depositing a layer of Al and patterning it to form metal arms (16) of the first annular thermopile (10) and the second annular thermopile (11), metal connecting wires (19) of the thermopile, and preliminarily forming a second bonding block (172);

[0062] Step 3.6): performing photolithography, sputtering Au, and peeling on the third substrate (3) to form a second bonding block (172);

[0063] Step 3.7): depositing a layer of silicon nitride to form a passivation layer;

[0064] Step 3.8): removing the silicon nitride at the position of the second bonding pad (172);

[0065] Step 3.9): Repeat the process of steps 3.2) to 3.8) on the lower surface of the third substrate (3);

[0066] Step 3.10): Use a deep trench etching process to etch out a first longitudinal flow channel (6) and a second flow channel (7) located inside the substrate.

[0067] Beneficial effects:

[0068] (1) The flow sensor uses doped resistors instead of polysilicon / metal resistors as heating resistors. The heating resistors are in the same plane as the substrate, which reduces stress and enhances device reliability.

[0069] (2) The flow sensor realizes multi-channel measurement and has multiple measurement modes. It can measure the flow rate and direction of the fluid in one flow channel alone, or measure the flow rate and direction of the fluid in multiple flow channels at the same time.

[0070] (3) The flow sensor has a thermally symmetrical structure in the longitudinal direction, and the flow channel is located inside the substrate, which can measure the flow rate and direction of the fluid passing perpendicular to the substrate.

[0071] (4) The heating resistor and thermopile structure of the flow sensor surround the flow channel and are sensitive to temperature changes caused by the passage of fluid. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] The following detailed description of the specific embodiments of the present invention in conjunction with the accompanying drawings will make the technical solutions and beneficial effects of the present invention apparent.

[0073] Figure 1 FIG2 is a schematic top view of the MEMS thermal flow sensor of the present invention;

[0074] Figure 2 FIG2 is a cross-sectional view of the MEMS thermal flow sensor according to the first embodiment of the present invention taken along line AA;

[0075] Figure 3 FIG2 is a top view of the second substrate of the MEMS thermal flow sensor according to the first embodiment of the present invention;

[0076] Figure 4 FIG2 is a cross-sectional view of the MEMS thermal flow sensor according to the second embodiment of the present invention taken along line AA;

[0077] Figure 5 FIG2 is a top view of the second substrate of the MEMS thermal flow sensor according to the second embodiment of the present invention.

[0078] The parts numbers in the figure are as follows:

[0079] First substrate 1, second substrate 2, third substrate 3, first annular resistor 4, second annular resistor 5, first flow channel 6, second flow channel 7, first thermal insulation groove 8, second thermal insulation groove 9, first annular thermopile 10, second annular thermopile 11, third annular thermopile 12, fourth annular thermopile 13, thermocouple 14, semiconductor arm 15, metal arm 16, first bonding block 171, second bonding block 172, third bonding block 173, semiconductor connecting wire 18, metal connecting wire 19, and thermoelectric material insulating layer 20. Specific implementation plan

[0080] The specific implementation scheme of the MEMS thermal flow sensor of the present invention is as follows:

[0081] The MEMS thermal flow sensor of the present invention uses thinned p-type silicon (Si) as a first substrate 1 and unthinned p-type silicon as a second substrate 2. The first substrate 1 and the second substrate 2 are silicon-silicon bonded and processed to form a third substrate 3. The surface and body of the third substrate 3 are provided with: a first flow channel 6 and a second flow channel 7 that longitudinally penetrate the third substrate; a first annular resistor 4 and a second annular resistor 5; a first annular thermopile 10, a second annular thermopile 11, a third annular thermopile 12, a fourth annular thermopile 13; optional first annular thermal insulation grooves 8 and 9; a bonding pad 17; a semiconductor connecting wire 18; a metal connecting wire 19; and a thermoelectric material insulating layer 20. Each of the thermopiles is formed by a series connection of a thermocouple 14 consisting of a semiconductor arm 15 and a metal arm 16. The semiconductor arm 15 and the metal arm 16 are separated by a thermoelectric material insulating layer 20 composed of silicon nitride.

[0082] The first flow channel 6 and the second flow channel 7 are symmetrically distributed on the left and right sides of the third substrate 3 for fluid passage. At half the depth of the third substrate 3, a first annular resistor 4 surrounds the first flow channel 6, and a second annular resistor 5 surrounds the second flow channel 7. The first and second annular resistors 4 and 5 are connected to a bonding pad 17 via semiconductor wires 18. A first annular thermal insulation groove 8 surrounds the first annular resistor 4, and a second annular thermal insulation groove 9 surrounds the second annular resistor 5, thereby reducing the mutual influence between the two annular resistors. The annular thermal insulation grooves can be manufactured as needed. The first and second flow channels 6 and 7, respectively, are surrounded by a first annular thermopile 10 and a second annular thermopile 11 on the upper surface of the third substrate 3. The thermopile is connected to the bonding pad 17 via a metal wire 19. The first and second flow channels 6 and 7, respectively, are surrounded by a third annular thermopile 12 and a fourth annular thermopile 13 on the lower surface of the third substrate 3. The thermopile is connected to the bonding pad 17 via a metal wire 19. The annular thermopiles are used to measure temperature and convert it into a voltage output. The shapes of the annular thermopile, annular resistor and annular thermal insulation groove are not completely closed, but there are gaps of a certain length for the layout of the leads.

[0083] During operation, there are three modes: (1) the first annular resistor 4 is powered on for heating, and the flow rate in the first flow channel 6 is measured; (2) the second annular resistor 5 is powered on for heating, and the flow rate in the second flow channel 7 is measured; (3) the first resistor 4 and the second resistor 5 are powered on for heating at the same time, and the flow rates in the first flow channel 6 and the second flow channel 7 are measured. The stable thermal field generated by the resistance heating is symmetrically distributed about the annular resistors. Since the first annular thermopile 10 and the third annular thermopile 12 are thermally symmetrical about the first annular resistor 4, and the second annular thermopile 11 and the fourth annular thermopile 13 are thermally symmetrical about the second annular resistor 5, the voltages output by the corresponding thermopiles at both ends of the same flow channel are equal. When the fluid passes through the flow channel, the fluid will disrupt the originally symmetrically distributed thermal field, and the temperature upstream of the fluid flow direction will be lower than the temperature downstream, making the voltages output by the annular thermopiles corresponding to the same flow channel unequal. By measuring the magnitude and trend of the voltage change, the volume and direction of the fluid passing through over a period of time can be obtained. If the fluid flows from the upper surface to the lower surface of the third substrate 3 , the voltage of the upper surface thermopile decreases, while the voltage of the lower surface thermopile increases.

[0084] This sensor has two implementation examples. Compared to Example 1, Example 2 adds a thermal insulation groove surrounding the annular resistor. The process flow of Example 1 is divided into three parts. The first part completes the structure of the first heating resistor 4, the second heating resistor 5, the bonding pad 17, and the semiconductor connecting wire 18 on the second substrate 2. The second part bonds the first substrate 1 to the second substrate 2 to form the third substrate 3. The third part processes the third substrate and completes the structure of the first annular thermopile 10, the second annular thermopile 11, the third annular thermopile 12, the fourth annular thermopile 13, the metal connecting wire 19, the bonding pad 17, and the first and second flow channels 6 and 7 on the third substrate 3.

[0085] The first part of the process is as follows:

[0086] (1) p-type silicon is selected as the second substrate 2;

[0087] (2) forming a SiO2 dielectric layer by thermal oxidation;

[0088] (3) Photolithography of SiO2 and N-type implantation: Apply photoresist and remove the photoresist where the ring resistor is to be made.

[0089] By N-type ion implantation, ring-shaped doped resistors 4 and 5 with a resistance value of 500-500 kΩ are formed;

[0090] (4) stripping off the remaining SiO2, growing and photolithography SiO2, and performing N-type heavily doped ion implantation to form semiconductor connection lines 18;

[0091] (5) Photolithography, Au sputtering, and lift-off are performed on the second substrate to form the bonding pad 17.

[0092] The second part of the process is as follows:

[0093] (1) Preparing a silicon substrate 1: Select thinned p-type silicon as the first substrate 1. The thickness of the first substrate 1 is required to be smaller than that of the second substrate 2, and the length and width of the first substrate 1 are required to be smaller than those of the second substrate 2.

[0094] (2) Silicon-silicon bonding: The first substrate 1 and the second substrate 2 are silicon-silicon bonded to form a third substrate 3.

[0095] The third part of the process is as follows

[0096] (1) After bonding, the lower surface of the third substrate 3 is thinned by a chemical mechanical polishing (CMP) process to ensure that the first annular resistor 4 and the second annular resistor 5 are located at half the height of the third substrate 3;

[0097] (2) Depositing polysilicon and performing N-type ion implantation: depositing a layer of polysilicon on the upper surface of the third substrate 3 by chemical vapor deposition, and performing N-type ion implantation;

[0098] (3) Photolithography of polysilicon: applying photoresist, removing the photoresist outside the semiconductor arms 15 of the first annular thermopile 10 and the second annular thermopile 11, etching the polysilicon, and then removing the photoresist;

[0099] (4) depositing a layer of silicon nitride 19 and patterning it to isolate the two thermoelectric materials;

[0100] (5) depositing a layer of Al and patterning it to form the metal arms 16 of the first annular thermopile 10 and the second annular thermopile 11, the metal connecting wires 19 of the thermopile, and preliminarily forming the bonding pads 17;

[0101] (6) Photolithography, Au sputtering, and peeling are performed on the third substrate 3 to form the thermopile bonding pad 17;

[0102] (7) depositing a layer of silicon nitride to form a passivation layer;

[0103] (8) Removing the silicon nitride at the position of the bonding pad 17;

[0104] (9) Repeat the process (2) to (8) on the lower surface of the third substrate 3;

[0105] (10) Using a deep trench etching process, a first longitudinal flow channel 6 and a second flow channel 7 located inside the substrate are etched.

[0106] Example 2:

[0107] The main difference between Example 2 and Example 1 lies in the addition of a first annular thermal insulation groove 8 and a second annular thermal insulation groove 9 between the annular resistors. The annular thermal insulation grooves are formed by separately etching the first substrate 1 and the second substrate 2 and then bonding them together. Compared to Example 1, this example reduces the mutual influence between the annular resistors and improves the independence of each flow channel measurement. Since the other processes are identical to Example 1, only the process flow of the second part of Example 1 is supplemented here to explain how the thermal insulation grooves are manufactured.

[0108] Part 1:

[0109] Same as the first part of Example 1.

[0110] Part II:

[0111] (1) Using a deep trench etching process, an annular deep trench with an opening is etched on the upper surface of the second substrate, wherein the first annular thermal insulation groove 8 surrounds the first annular resistor 4, and the second annular thermal insulation groove 9 surrounds the second annular resistor 5;

[0112] (2) Preparing a silicon substrate: Select thinned p-type silicon as the first substrate 1. The thickness of the first substrate 1 is required to be smaller than that of the second substrate 2, and the length and width of the first substrate 1 are required to be smaller than those of the second substrate 2.

[0113] (3) using a deep trench etching process, an annular deep trench with an opening is etched on the lower surface of the first substrate 1, the position of which corresponds to the position of the deep trench of the second substrate 2, and the depth of the trench is the same as the depth of the trench etched on the second substrate 2;

[0114] (4) Silicon-silicon bonding: The first substrate 1 and the second substrate 2 are silicon-silicon bonded to form a third substrate 3.

[0115] Part III:

[0116] Same as the third part of Example 1.

[0117] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A MEMS thermal flow sensor, characterized in that: Comprising a third substrate (3), the third substrate (3) comprising a first substrate (1) and a second substrate (2), wherein the first substrate (1) and the second substrate (2) are silicon-silicon bonded; The third substrate (3) comprises: a first flow channel (6) and a second flow channel (7), a first annular resistor (4) and a second annular resistor (5), the first flow channel (6) and the second flow channel (7) longitudinally passing through the third substrate (3), a first annular thermopile (10), a second annular thermopile (11), a third annular thermopile (12), and a fourth annular thermopile (13); The first flow channel (6) and the second flow channel (7) are symmetrically distributed on the left and right sides of the third substrate (3); the first annular resistor (4) surrounds the first flow channel (6); the second annular resistor (5) surrounds the second flow channel (7); and the first annular resistor (4) and the second annular resistor (5) are both connected to the first bonding block (171) via a semiconductor connection line (18); The first flow channel (6) and the second flow channel (7) are respectively surrounded by the first annular thermopile (10) and the second annular thermopile (11) at the openings on the upper surface of the third substrate (3); the first annular thermopile (10) and the second annular thermopile (11) are both connected to the second bonding block (172) via a metal connecting wire (19); The first flow channel (6) and the second flow channel (7) are respectively surrounded by the third annular thermopile (12) and the fourth annular thermopile (13) at the openings on the lower surface of the third substrate (3); the third annular thermopile (12) and the fourth annular thermopile (13) are connected to the third bonding block (173) via a metal connecting wire (19); The first annular thermopile (10), the second annular thermopile (11), the third annular thermopile (12), and the fourth annular thermopile (13) all include a plurality of thermocouples (14) connected in series, and the plurality of thermocouples (14) all include a semiconductor arm (15) and a metal arm (16), and an insulating layer (20) of a thermoelectric material composed of silicon nitride is provided between the semiconductor arm (15) and the metal arm (16).

2. A MEMS thermal flow sensor according to claim 1, characterized in that: The MEMS thermal flow sensor further comprises a first annular thermal insulation groove (8) and a second annular thermal insulation groove (9), wherein the first annular thermal insulation groove (8) and the second annular thermal insulation groove (9) are respectively arranged around the outside of the first annular resistor (4) and the second annular resistor (5).

3. A MEMS thermal flow sensor according to claim 2, characterized in that: The first annular thermopile (10), the second annular thermopile (11), the third annular thermopile (12), the fourth annular thermopile (13), the first annular resistor (4), the second annular resistor (5), the first annular thermal insulation groove (8), and the second annular thermal insulation groove (9) are not completely closed.

4. The MEMS thermal flow sensor according to claim 1, wherein: The MEMS thermal flow sensor has three operating modes: (a) the first annular resistor (4) is energized for heating, and the flow rate in the first flow channel (6) is measured; (b) the second annular resistor (5) is electrically heated and the flow rate in the second flow channel (7) is measured; (c) The first annular resistor (4) and the second annular resistor (5) are energized and heated simultaneously, and the flow rates in the first flow channel (6) and the second flow channel (7) are measured simultaneously.

5. The MEMS thermal flow sensor according to claim 1, wherein: The first annular resistor (4) and the second annular resistor (5) are doped on the second substrate (2).

6. The method for preparing a MEMS thermal flow sensor according to claim 1, wherein: The following steps are involved: Step 1): preparing a first annular resistor (4), a second annular resistor (5), a first bonding block (171), and a semiconductor connecting wire (18) on the second substrate (2); Step 2): Bonding the first substrate (1) to the second substrate (2) to form a third substrate (3); Step 3): preparing a first annular thermopile (10), a second annular thermopile (11), a third annular thermopile (12), a fourth annular thermopile (13), a metal connecting wire (19), a second pressure welding block (172), a third pressure welding block (173), a first flow channel (6) and a second flow channel (7) on a third substrate (3); Wherein, step 1) specifically includes: Step 1.1): Select p-type silicon as the second substrate (2); Step 1.2): forming a SiO2 dielectric layer on the upper surface of the second substrate (2) by thermal oxidation; Step 1.3): Coating a photoresist on the second substrate (2), then removing the photoresist at the locations where the first annular resistor (4) and the second annular resistor (5) are to be formed; patterning the SiO2 dielectric layer, and then forming the first annular resistor (4) and the second annular resistor (5) by N-type ion implantation; Step 1.4): Stripping the remaining SiO2, growing and photolithography SiO2, and performing N-type heavy doping ion implantation to form semiconductor connecting wires (18); Step 1.5): performing photolithography, sputtering Au, and stripping operations on the second substrate (2) to form a first bonding block (171); Step 2) specifically includes: Step 2.1): Select thinned p-type silicon as the first substrate (1), wherein the thickness of the first substrate (1) is smaller than that of the second substrate (2), the length of the first substrate (1) is smaller than that of the second substrate (2), and the width of the first substrate (1) is smaller than that of the second substrate (2); Step 2.2): Silicon-silicon bonding: silicon-silicon bonding the first substrate (1) and the second substrate (2) to form a third substrate (3); Step 3) specifically includes: Step 3.1): After silicon-silicon bonding, the lower surface of the third substrate (3) is thinned by a chemical mechanical polishing (CMP) process, and the first annular resistor (4) and the second annular resistor (5) are ensured to be located at half the height of the third substrate (3); Step 3.2): depositing a layer of polysilicon on the upper surface of the third substrate (3) by chemical vapor deposition, and performing N-type ion implantation; Step 3.3): coating photoresist, removing the photoresist in the area other than the semiconductor arms (15) of the first annular thermopile (10) and the second annular thermopile (11), etching the polysilicon, and then removing the photoresist; Step 3.4): depositing a thermoelectric material insulating layer (20) composed of silicon nitride, and patterning the thermoelectric material insulating layer (20) composed of silicon nitride to isolate the semiconductor arm (15) from the metal arm (16); Step 3.5): depositing a layer of Al and patterning it to form metal arms (16) of the first annular thermopile (10) and the second annular thermopile (11), metal connecting wires (19) of the thermopile, and preliminarily forming a second bonding block (172); Step 3.6): performing photolithography, sputtering Au, and peeling on the third substrate 3 to form a second bonding block (172); Step 3.7): Deposit a layer of silicon nitride to form a passivation layer; Step 3.8): removing the silicon nitride at the position of the second bonding block (172); Step 3.9): Repeat the process of steps 3.2) to 3.8) on the lower surface of the third substrate (3); Step 3.10): Use a deep trench etching process to etch out the first flow channel (6) and the second flow channel (7).

7. The method for preparing a MEMS thermal flow sensor according to claim 2, wherein: The following steps are involved: Step 1): preparing a first annular resistor (4), a second annular resistor (5), a first bonding block (171), and a semiconductor connecting wire (18) on the second substrate (2); Step 2): preparing a first annular thermal insulation groove (8) and a second annular thermal insulation groove (9), bonding the first substrate (1) and the second substrate (2) to form a third substrate (3); Step 3): preparing a first annular thermopile (10), a second annular thermopile (11), a third annular thermopile (12), a fourth annular thermopile (13), a metal connecting wire (19), a second pressure welding block (172), a third pressure welding block (173), a first flow channel (6) and a second flow channel (7) on a third substrate (3); Wherein, step 1) specifically includes: Step 1.1): Select p-type silicon as the second substrate (2); Step 1.2): Forming a SiO2 dielectric layer by thermal oxidation; Step 1.3): coating the second substrate (2) with photoresist, then removing the photoresist at the locations where the first annular resistor (4) and the second annular resistor (5) are to be formed; and then forming the first annular resistor (4) and the second annular resistor (5) by N-type ion implantation; Step 1.4): Stripping the remaining SiO2, growing and photolithography SiO2, and performing N-type heavy doping ion implantation to form semiconductor connecting wires (18); Step 1.5): performing photolithography, sputtering Au, and stripping operations on the second substrate (2) to form a first bonding block (171); Step 2) specifically includes: Step 2.1): using a deep trench etching process, etching an annular deep trench with an opening on the upper surface of the second substrate (2), wherein the first annular thermal insulation groove (8) surrounds the first annular resistor (4), and the second annular thermal insulation groove (9) surrounds the second annular resistor (5); Step 2.2): Select thinned p-type silicon as the first substrate (1), requiring that the thickness of the first substrate (1) is smaller than that of the second substrate (2), the length of the first substrate (1) is smaller than that of the second substrate (2), and the width of the first substrate (1) is smaller than that of the second substrate (2); Step 2.3): using a deep groove etching process, etching an annular deep groove with an opening on the lower surface of the first substrate (1), the position of which corresponds to the position of the deep groove of the second substrate (2), and the depth of the groove is the same as the depth of the groove etched on the second substrate (2); Silicon-silicon bonding: bonding the first substrate (1) and the second substrate (2) to form a third substrate (3); Step 3) specifically includes: Step 3.1): After silicon-silicon bonding, the lower surface of the third substrate (3) is thinned by a chemical mechanical polishing (CMP) process, and the first annular resistor (4) and the second annular resistor (5) are ensured to be located at half the height of the third substrate (3); Step 3.2): depositing a layer of polysilicon on the upper surface of the third substrate (3) by chemical vapor deposition, and performing N-type ion implantation; Step 3.3): coating photoresist, removing the photoresist in the area other than the semiconductor arms (15) of the first annular thermopile (10) and the second annular thermopile (11), etching the polysilicon, and then removing the photoresist; Step 3.4): depositing a thermoelectric material insulating layer (20) composed of silicon nitride, and patterning the thermoelectric material insulating layer (20) composed of silicon nitride to isolate the semiconductor arm (15) from the metal arm (16); Step 3.5): depositing a layer of Al and patterning it to form metal arms (16) of the first annular thermopile (10) and the second annular thermopile (11), metal connecting wires (19) of the thermopile, and preliminarily forming a second bonding block (172); Step 3.6): performing photolithography, sputtering Au, and peeling on the third substrate (3) to form a second bonding block (172); Step 3.7): Deposit a layer of silicon nitride to form a passivation layer; Step 3.8): removing the silicon nitride at the position of the second bonding block (172); Step 3.9): Repeat the process of steps 3.2) to 3.8) on the lower surface of the third substrate (3); Step 3.10): Using a deep trench etching process, a first longitudinal flow channel (6) and a second flow channel (7) located inside the substrate are etched.

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