Gas flow sensor and preparation method

Abstract

The invention discloses a gas flow sensor which comprises a sensing unit and a gas flow channel, the sensing unit is provided with a substrate and a heat insulation medium layer from top to bottom, the flow channel is arranged above the heat insulation medium layer, and a heating resistor, two thermistors and at least two environment temperature measuring resistors are arranged on the upper surface of the heat insulation medium layer. The thermistors and the environment temperature measuring resistor form a Wheatstone bridge, the thermistors and the heating resistor are arranged in a gas flow channel, one thermistor is arranged at the position close to an inlet of the gas flow channel, the other thermistor is arranged at the position close to an outlet of the gas flow channel, and the heating resistor is arranged between the two thermistors. And the environment temperature measuring resistor is arranged on the outer side of the flow channel. According to the gas flow sensor disclosed by the invention, the heat insulation cavity is formed in the surface of the substrate, so that the heat conduction of the silicon substrate is effectively reduced, the heat loss generated by the heating resistor is reduced, the temperature difference between the upstream and downstream temperature measuring resistors is increased, and the sensitivity of the sensor is improved.

Description

Technical Field

[0001] The invention relates to a gas flow sensor and a preparation method thereof, belonging to the technical field of sensors. Background Art

[0002] MEMS flow sensors, with their small chip size, low manufacturing cost, and high performance, are widely used in aerospace, industrial process monitoring, biomedical, and other fields. Flow sensors can be categorized by the flow measurement method, including thermal, differential pressure, Coriolis, cantilever, and biomimetic flow sensors. Thermal flow sensors are further divided into heat loss, thermal temperature difference, and thermal pulse flow sensors. Thermal temperature difference gas flow sensors, based on the principles of heat transfer, utilize gas flow to create a temperature difference near upstream and downstream resistors. The gas flow rate is determined by measuring the temperature difference between the upstream and downstream resistors.

[0003] Existing thermal gas flow sensors are greatly affected by temperature. The thermal conductivity of the substrate causes serious heat loss in the heating resistor. When measuring low flow rates, the sensor output signal is weak and the sensitivity is low.

[0004] Due to the above reasons, it is necessary to conduct in-depth research on existing gas flow sensors to solve the above problems. Summary of the Invention

[0005] In order to overcome the above problems, in-depth research was conducted and a gas flow sensor was proposed, including a sensitive unit and a gas flow channel.

[0006] The sensitive unit has a substrate 11 and a heat-insulating medium layer 12 from top to bottom.

[0007] The flow channel is arranged above the thermal insulation medium layer 12, and a heating resistor, two thermistors and at least two ambient temperature measuring resistors are arranged on the upper surface of the thermal insulation medium layer. The thermistors and the ambient temperature measuring resistors form a Wheatstone bridge. The thermistors and the heating resistors are arranged in the gas flow channel, wherein one thermistor is arranged near the gas flow channel inlet, and one thermistor is arranged near the gas flow channel outlet. The heating resistor is arranged between the two thermistors, and the ambient temperature measuring resistor is arranged outside the flow channel.

[0008] In a preferred embodiment, the substrate is preferably a single crystal silicon substrate.

[0009] In a preferred embodiment, the heating resistor is located between the two thermistors.

[0010] In a preferred embodiment, the environmental temperature measuring resistor is arranged outside the gas flow channel.

[0011] In a preferred embodiment, the environmental temperature measuring resistor adopts a serpentine structure.

[0012] In a preferred embodiment, the thermal insulation medium layer 12 adopts a SiO2 / Si3N4 double-layer film structure.

[0013] In a preferred embodiment, a groove is provided on the upper surface of the substrate, so that a heat-insulating cavity is formed between the substrate and the heat-insulating medium layer.

[0014] In a preferred embodiment, the gas flow sensor further includes an amplification circuit, including an instrument amplifier, a filter, and a ring oscillator.

[0015] The filter is used to filter high-frequency noise of the circuit, and the ring oscillator is used to provide an internal clock frequency of the instrumentation amplifier.

[0016] The present invention also discloses a method for preparing a gas flow sensor, comprising the following steps:

[0017] S1, growing a layer of SiO2 and Si3N4 on the substrate;

[0018] S2, performing a first photolithography on the SiO2 and Si3N4 layers to generate a growth window for porous silicon;

[0019] S3, growing a layer of porous silicon on the porous silicon window;

[0020] S4, growing a layer of SiO2 and Si3N4 on the porous silicon;

[0021] S5, performing a second photolithography on the SiO2 and Si3N4 to etch the resistor, lead and pad patterns, and fabricate resistors and pads on the silicon wafer surface;

[0022] S6, performing a third photolithography on the porous silicon surface to etch away the grown porous silicon to form a thermal insulation cavity;

[0023] S7, performing a fourth photolithography on the SiO2 and Si3N4 surfaces to etch the flow channel pattern;

[0024] S8, anodic bonding the gas flow channel cover plate to the surface of the silicon wafer;

[0025] S9. Set up an amplifier circuit on the PCB board and connect it to the solder joints to obtain a gas flow sensor.

[0026] In a preferred embodiment, in S2, the growth windows of the porous silicon are in the shape of 6 symmetrically distributed beams.

[0027] The beneficial effects of the present invention include:

[0028] (1) Making a heat-insulating cavity on the substrate surface effectively reduces the heat conduction of the silicon substrate, reduces the heat loss generated by the heating resistor, increases the temperature difference between the upstream and downstream temperature measuring resistors, and improves the sensitivity of the sensor;

[0029] (2) The low-noise programmable instrumentation amplifier can accurately amplify the tiny and low-frequency gas flow sensor signal to meet the subsequent ADC quantization requirements.

[0030] Figure Number

[0031] 11-substrate;

[0032] 111-insulated cavity;

[0033] 12- thermal insulation layer;

[0034] 2-gas flow channel;

[0035] 13-Thermistor;

[0036] 14- Heating resistor;

[0037] 15-environmental temperature measuring resistor;

[0038] 3-Instrumentation amplifier;

[0039] 31- filter;

[0040] 32- digitally controlled resistor;

[0041] 33-Ring oscillator. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 A schematic structural diagram of a gas flow sensor according to a preferred embodiment of the present invention is shown;

[0043] Figure 2 A schematic diagram of step S1 in a method for preparing a gas flow sensor according to a preferred embodiment of the present invention is shown;

[0044] Figure 3 A schematic diagram showing step S2 in a method for preparing a gas flow sensor according to a preferred embodiment of the present invention is shown;

[0045] Figure 4 A schematic diagram showing step S3 in a method for preparing a gas flow sensor according to a preferred embodiment of the present invention is shown;

[0046] Figure 5 A schematic diagram showing step S4 in a method for preparing a gas flow sensor according to a preferred embodiment of the present invention is shown;

[0047] Figure 6A schematic diagram of step S5 in a method for preparing a gas flow sensor according to a preferred embodiment of the present invention is shown;

[0048] Figure 7 A schematic diagram showing step S6 in a method for preparing a gas flow sensor according to a preferred embodiment of the present invention is shown;

[0049] Figure 8 A schematic diagram of step S7 in a method for preparing a gas flow sensor according to a preferred embodiment of the present invention is shown;

[0050] Figure 9 A schematic diagram of step S8 in a method for preparing a gas flow sensor according to a preferred embodiment of the present invention is shown;

[0051] Figure 10 The test results of a gas flow sensor according to a preferred embodiment of the present invention are shown. DETAILED DESCRIPTION

[0052] The present invention will be described in further detail below with reference to the accompanying drawings and examples, through which the features and advantages of the present invention will become more clearly understood.

[0053] The word "exemplary" is used exclusively herein to mean "serving as an example, example, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. Although various aspects of the embodiments are shown in the drawings, the drawings are not necessarily drawn to scale unless otherwise indicated.

[0054] According to the present invention, a gas flow sensor is provided. Figure 1 As shown, it includes a sensitive unit and a gas flow channel 2,

[0055] The sensitive unit has a substrate 11 and a heat-insulating medium layer 12 from top to bottom.

[0056] The flow channel is arranged above the thermal insulation medium layer 12, and a heating resistor 14, two thermistors 13 and at least two environmental temperature measuring resistors 15 are arranged on the upper surface of the thermal insulation medium layer 12. The thermistors and the environmental temperature measuring resistors form a Wheatstone bridge. The thermistors and the heating resistors are arranged in the gas flow channel, wherein one thermistor is arranged near the gas flow channel inlet, and one thermistor is arranged near the gas flow channel outlet. The heating resistor is arranged between the two thermistors, and the environmental temperature measuring resistor is arranged outside the flow channel.

[0057] Preferably, the substrate is a single crystal silicon substrate, more preferably <100> Crystal orientation double-sided polished single crystal silicon wafer.

[0058] In a preferred embodiment, the heating resistor is located between the two thermistors to avoid static temperature difference (bridge output is non-zero when there is no airflow) and eliminate errors caused by structural asymmetry.

[0059] According to the present invention, air entering the gas flow channel is heated by a heating resistor. Two thermistors are used to measure the temperature difference between the inlet and outlet channels, respectively. The relationship between the temperature difference and the bridge output voltage is used to determine the gas flow rate. Furthermore, an ambient temperature resistor serves as the fixed reference arm of the bridge to eliminate interference from ambient temperature fluctuations.

[0060] Preferably, the ambient temperature measuring resistor is arranged outside the gas flow channel to ensure that it only responds to the ambient temperature and avoids its temperature compensation function being affected by the air flow velocity, for example, avoiding local temperature changes caused by gas flow.

[0061] In a preferred embodiment, the resistor is made of platinum metal material. The resistance value of platinum is highly linear with temperature, which simplifies the calibration process and improves measurement accuracy.

[0062] In a preferred embodiment, the ambient temperature measuring resistor adopts a serpentine structure. The serpentine structure makes the heat distribution more uniform by extending the resistance path, reduces the interference of local temperature mutations on the measurement, and ensures the stability of the ambient temperature signal; and compared with the linear structure, the distribution characteristics of the serpentine resistor can reduce the current density per unit area, thereby reducing the temperature drift error caused by self-heating.

[0063] In a preferred embodiment, the thermal insulation medium layer 12 adopts a SiO2 / Si3N4 double-layer film structure. The SiO2 / Si3N4 double-layer film structure synergistically reduces the interface state density and suppresses leakage current, and its stability is stronger.

[0064] In a preferred embodiment, a groove is provided on the upper surface of the substrate, so that a heat-insulating cavity 111 is formed between the substrate and the heat-insulating medium layer 12 .

[0065] In the present invention, by providing a heat-insulating cavity, the influence of heat conduction below the device on the air temperature in the gas flow channel is further avoided, thereby improving the measurement accuracy.

[0066] Preferably, the gas flow sensor further includes an amplifying circuit for amplifying the weak voltage signal output by the bridge circuit.

[0067] Preferably, the amplifier circuit uses an instrumentation amplifier 3. More preferably, the amplifier circuit is provided with a filter 31 for filtering high-frequency noise of the circuit, preferably a low-pass filter.

[0068] Furthermore, a ring oscillator 33 is provided in the amplifying circuit for providing an internal clock frequency of the instrumentation amplifier.

[0069] More preferably, a digitally controlled resistor 32 is provided in the amplifier circuit, and the amplifier circuit gain is adjusted by programming to compensate for the signal amplitude change caused by differences in gas type, temperature or flow rate, thereby ensuring that the output signal is linearly related to the flow rate.

[0070] In a preferred embodiment, the cross section of the gas flow channel is square, which reduces the friction resistance and local vortex between the gas and the flow channel.

[0071] The present invention also discloses a method for preparing a gas flow sensor, comprising the following steps:

[0072] S1. Grow a layer of SiO2 and Si3N4 on the substrate, such as Figure 2 As shown;

[0073] S2, the SiO2 and Si3N4 layers are subjected to the first photolithography to generate a growth window for porous silicon, such as Figure 3 As shown;

[0074] S3, growing a layer of porous silicon on the porous silicon window, such as Figure 4 As shown;

[0075] S4, growing a layer of SiO2 and Si3N4 on the porous silicon, such as Figure 5 As shown;

[0076] S5, perform a second photolithography on the SiO2 and Si3N4 to etch the resistor, lead and solder joint patterns, and make resistors and solder joints on the surface of the silicon wafer, such as Figure 6 As shown;

[0077] S6, the porous silicon surface is subjected to a third photolithography to etch away the grown porous silicon to form a heat-insulating cavity, such as Figure 7 As shown;

[0078] S7, perform a fourth photolithography on the SiO2 and Si3N4 surfaces to etch the flow channel pattern, such as Figure 8 As shown;

[0079] S8. Anodic bonding is used to bond the gas flow channel cover to the silicon wafer surface, such as Figure 9 As shown;

[0080] S9. Set up an amplifier circuit on the PCB board and connect it to the solder joints to obtain a gas flow sensor.

[0081] Preferably, in S1, the substrate is <100> Crystal orientation double-sided polished single crystal silicon wafer.

[0082] Preferably, in S1 and S4, the growth is performed by using a PECVD method.

[0083] Preferably, in S2, the growth windows of the porous silicon are in the shape of six symmetrically distributed beams.

[0084] Preferably, in S5, a thin film resistor and a pressure welding point are manufactured by a sputtering method, the resistor is a Ti / Pt thin film resistor, and the electric welding point is a Pt pressure welding point.

[0085] In the present invention, a heat-insulating cavity is made on the substrate surface, which effectively reduces the heat conduction of the silicon substrate, reduces the heat loss generated by the heating resistor, increases the temperature difference between the upstream and downstream temperature measuring resistors, and improves the sensitivity of the sensor.

[0086] In S8, the gas flow channel cover plate is a square cover plate, which is fastened to the surface of the silicon wafer to form a gas flow channel.

[0087] Example

[0088] Example 1

[0089] The production of gas flow sensor includes the following steps:

[0090] S1. Grow a layer of SiO2 and Si3N4 on the substrate, such as Figure 2 As shown;

[0091] S2, the SiO2 and Si3N4 layers are subjected to the first photolithography to generate a growth window for porous silicon, such as Figure 3 As shown;

[0092] S3, growing a layer of porous silicon on the porous silicon window, such as Figure 4 As shown;

[0093] S4, growing a layer of SiO2 and Si3N4 on the porous silicon, such as Figure 5 As shown;

[0094] S5, perform a second photolithography on the SiO2 and Si3N4 to etch the resistor, lead and solder joint patterns, and make resistors and solder joints on the surface of the silicon wafer, such as Figure 6 As shown;

[0095] S6, the porous silicon surface is subjected to a third photolithography to etch away the grown porous silicon to form a heat-insulating cavity, such as Figure 7 As shown;

[0096] S7, perform a fourth photolithography on the SiO2 and Si3N4 surfaces to etch the flow channel pattern, such as Figure 8 As shown;

[0097] S8. Anodic bonding is used to bond the gas flow channel cover to the silicon wafer surface, such as Figure 9 As shown;

[0098] S9. Set up an amplifier circuit on the PCB board and connect it to the solder joints to obtain a gas flow sensor.

[0099] The gas flow sensor includes a sensitive unit and a gas flow channel.

[0100] The sensitive unit has a substrate 11 and a heat-insulating medium layer 12 from top to bottom.

[0101] The flow channel is arranged above the thermal insulation medium layer 12. A heating resistor, two thermistors and two ambient temperature measuring resistors are arranged on the upper surface of the thermal insulation medium layer 12. The two thermistors and the two ambient temperature measuring resistors form a Wheatstone bridge, wherein one thermistor is arranged near the inlet of the gas flow channel, and one thermistor is arranged near the outlet of the gas flow channel. The heating resistor is arranged between the two thermistors and located in the gas flow channel, and the ambient temperature measuring resistor is arranged outside the flow channel. The heating resistor is located between the two thermistors, and the ambient temperature measuring resistor is arranged outside the gas flow channel, adopting a serpentine structure. The resistor is made of platinum metal material, and the thermal insulation medium layer adopts a SiO2 / Si3N4 double-layer membrane structure. A groove is provided on the upper surface of the substrate, so that an insulating cavity is formed between the substrate and the thermal insulation medium layer. The cross-section of the gas flow channel is square.

[0102] The gas flow sensor further includes an amplifying circuit, which includes an instrument amplifier, a filter, a ring oscillator and a digitally controlled resistor.

[0103] The manufactured gas flow sensor is tested, and the testing device includes a gas flow generating device, a gas flow control device and a gas flow testing device.

[0104] The gas flow generator is a nitrogen cylinder, providing the nitrogen required for testing. The gas flow control device, consisting of a two-way solenoid valve and a standard flow controller, controls the gas flow rate within a range of 0 to 250 sccm. A voltage source, current source, and digital multimeter are connected to the gas flow sensor to form a Wheatstone bridge circuit, forming the gas flow test device. The voltage source provides the output voltage of the Wheatstone bridge, the current source provides the input current across the heating resistor, and the digital multimeter measures the circuit output voltage. The gas flow test was conducted at a temperature of 29°C and a humidity of 44% RH. The current supplied to the heating resistor was 20 mA, and the bridge voltage was 5 V.

[0105] During the test, the gas flow rate test range is 0-250sccm, the step size is 50sccm, and the multimeter data is recorded when the gas flow rate value is stable. The forward and reverse stroke experiments are repeated three times.

[0106] The experimental results show that the gas flow sensor can measure the gas flow rate in the range of 0-250sccm. The output characteristic curve of the gas flow sensor is as follows: Figure 10 As shown in the figure, the bridge output voltage increases with increasing gas flow rate, with good linearity. Using experimental data, the sensitivity of the gas microflow sensor chip is calculated to be 12.556 μV / sccm. The test results show that the designed microflow sensor can measure gas flow rates in the range of 0 to 250 sccm.

[0107] The present invention has been described above with reference to preferred embodiments, but these embodiments are merely exemplary and serve only as illustrations. On this basis, various replacements and improvements can be made to the present invention, all of which fall within the scope of protection of the present invention.

Claims

1. A gas flow sensor, characterized in that: Including sensitive unit and gas flow channel, The sensitive unit has a substrate and a heat-insulating medium layer from top to bottom. The flow channel is arranged above the thermal insulation medium layer, and a heating resistor, two thermistors and at least two ambient temperature measuring resistors are arranged on the upper surface of the thermal insulation medium layer. The thermistors and the ambient temperature measuring resistors form a Wheatstone bridge. The thermistors and the heating resistors are arranged in the gas flow channel, wherein one thermistor is arranged near the gas flow channel inlet, and one thermistor is arranged near the gas flow channel outlet, the heating resistor is arranged between the two thermistors, and the ambient temperature measuring resistor is arranged outside the flow channel.

2. The gas flow sensor according to claim 1, characterized in that The substrate is preferably a single crystal silicon substrate.

3. The gas flow sensor according to claim 1, wherein: The heating resistor is located between the two thermistors.

4. The gas flow sensor according to claim 1, characterized in that The environmental temperature measuring resistor is arranged outside the gas flow channel.

5. The gas flow sensor according to claim 1, wherein: The environmental temperature measuring resistor adopts a serpentine structure.

6. The gas flow sensor according to claim 1, characterized in that The thermal insulation medium layer 12 adopts a SiO2 / Si3N4 double-layer film structure.

7. The gas flow sensor according to claim 1, wherein: A groove is provided on the upper surface of the substrate, so that a heat-insulating cavity is formed between the substrate and the heat-insulating medium layer.

8. The gas flow sensor according to claim 1, wherein: The gas flow sensor further includes an amplifying circuit, including an instrument amplifier, a filter and a ring oscillator. The filter is used to filter high-frequency noise of the circuit, and the ring oscillator is used to provide an internal clock frequency of the instrumentation amplifier.

9. A method for preparing a gas flow sensor, characterized in that: The following steps are involved: S1, growing a layer of SiO2 and Si3N4 on the substrate; S2, performing a first photolithography on the SiO2 and Si3N4 layers to generate a growth window for porous silicon; S3, growing a layer of porous silicon on the porous silicon window; S4, growing a layer of SiO2 and Si3N4 on the porous silicon; S5, performing a second photolithography on the SiO2 and Si3N4 to etch the resistor, lead and pad patterns, and fabricate resistors and pads on the silicon wafer surface; S6, performing a third photolithography on the porous silicon surface to etch away the grown porous silicon to form a thermal insulation cavity; S7, performing a fourth photolithography on the SiO2 and Si3N4 surfaces to etch the flow channel pattern; S8, anodic bonding the gas flow channel cover plate to the surface of the silicon wafer; S9. Set up an amplifier circuit on the PCB board and connect it to the solder joints to obtain a gas flow sensor.

10. The method for preparing a gas flow sensor according to claim 9, wherein: In S2, the growth windows of the porous silicon are in the shape of six symmetrically distributed beams.

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