A fast response MEMS flow sensor and a manufacturing method thereof

Abstract

The application relates to a fast-response MEMS flow sensor and a manufacturing method thereof, and belongs to the technical field of flow measurement. The sensor comprises a substrate, a laminated support film, a heating element, a temperature sensing element and a protective layer. The substrate is provided with a heat insulation cavity, the upper surface of the substrate is provided with the laminated support film, the laminated support film comprises a lower support film and an upper support film from bottom to top, a gas gap is sealingly arranged at the middle position between the lower support film and the upper support film, the upper surface of the upper support film is provided with the heating element and the temperature sensing element, the temperature sensing element is symmetrically arranged on the two sides of the heating element, the hot end of the heating element and the temperature sensing element is located above the heat insulation cavity, the surface of the heating element and the temperature sensing element is covered with the protective layer, the protective layer is etched with a contact hole corresponding to the electrode position, and the temperature sensing element is connected with the electrode through the contact hole. The structure of the built-in sealing gas gap of the laminated support film realizes the significant acceleration of the response time.

Description

Technical Field

[0001] This invention relates to a fast-response MEMS flow sensor and its fabrication method, belonging to the field of flow measurement technology. Background Technology

[0002] Flow measurement is a basic requirement for industrial production and scientific research. There are many types of flow sensors. Among them, the thermal difference flow sensor based on MEMS technology is widely used because of its many advantages such as simple structure, small size, high accuracy, fast response and low power consumption.

[0003] A typical MEMS thermoelectric flow sensor comprises three key components integrated onto the same suspended membrane structure: a heating element located in the central region, and two temperature-sensing elements symmetrically arranged upstream and downstream of the heating element. The heating element operates to raise the surface temperature of the suspended membrane above the ambient temperature. In the absence of fluid flow, the temperature field is symmetrically distributed around the heating element, and the electrical signals output by the upstream and downstream temperature-sensing elements are essentially identical. When gas flows along the membrane surface, the flowing gas molecules cause a shift in the surface temperature distribution through convective heat transfer, resulting in a difference in the electrical signals from the upstream and downstream temperature-sensing elements. Based on this difference in electrical signals, the gas flow rate through the sensor can be calculated.

[0004] In recent years, the requirements for the response speed of flow sensors have become increasingly stringent in applications such as real-time airflow monitoring in ventilators and high-speed gas control in semiconductor processes. To accurately capture rapid transient changes in flow rate and avoid control deviations or monitoring distortions caused by measurement lag, sensors typically need to have millisecond or even sub-millisecond response capabilities. For suspended diaphragm thermoelectric flow sensors, their response speed is mainly determined by the thermal time constant (τ), which is related to the effective heat capacity (C) and lateral thermal resistance (R) involved in transient heat exchange (τ = R × C).

[0005] Currently, conventional sensor structures mostly employ solid composite films as suspension support membranes. In this structure, the heat generated by the heating element is conducted not only laterally along the film plane but also rapidly along the longitudinal direction across the entire film thickness. Due to the high thermal conductivity of the film materials (such as silicon nitride and silicon oxide) and the tight solid-solid contact between the film layers, the entire solid film layer participates in transient heat exchange as a whole during heating and cooling. This results in a large effective heat capacity for dynamic heating and cooling. Even by reducing the absolute mass through thinner film layers, the effective heat capacity of the solid film is difficult to further reduce due to limitations in mechanical strength and process stability. This causes the thermal time constant of existing sensors to typically be in the tens of milliseconds range, and the dynamic response of the temperature field to fluid changes exhibits a significant lag, urgently requiring further acceleration. Summary of the Invention

[0006] To address the issues of high thermal time constant and limited response speed in existing MEMS thermal differential flow sensors, this invention provides a fast-response MEMS flow sensor and its fabrication method. By using a structure with a built-in sealed air gap in a laminated membrane, the response time is significantly accelerated.

[0007] The technical solution of the present invention is as follows: A fast-response MEMS flow sensor includes a substrate, a stacked support film, a heating element, a temperature sensing element, and a protective layer. The substrate has a heat insulation cavity, and a stacked support film is disposed on the upper surface of the substrate. The stacked support film includes a lower support film and an upper support film from bottom to top. An air gap is sealed between the lower support film and the upper support film. The stacked support film is located directly above the heat insulation cavity. The heating element and the temperature sensing element are disposed on the upper surface of the upper support film. The temperature sensing element is symmetrically distributed on the upstream and downstream sides of the heating element. The hot ends of the heating element and the temperature sensing element are located above the heat insulation cavity. The surfaces of the heating element and the temperature sensing element are covered with a protective layer. Contact holes are etched in the protective layer at the corresponding electrode positions. Electrodes are connected to the heating element and the temperature sensing element through the contact holes.

[0008] According to a preferred embodiment of the present invention, a sacrificial layer made of polycrystalline silicon is provided on both sides between the lower support film and the upper support film.

[0009] According to a preferred embodiment of the present invention, the substrate is a single-polished or double-polished semiconductor substrate, such as a silicon substrate, a germanium substrate, or an SOI substrate.

[0010] According to a preferred embodiment of the present invention, the lower support film is made of thermally oxidized silicon, LPCVD silicon oxide or PECVD silicon oxide, and has a thickness of 500 nm to 1000 nm. The upper support film is made of LPCVD silicon nitride or PECVD silicon nitride, with a thickness of 200nm~500nm; The air gap thickness is 20nm~200nm, which is a closed microcavity structure formed by sealing after the sacrificial layer is released.

[0011] According to a preferred embodiment of the present invention, the material of the heating element is any one or more combinations of P-type polycrystalline silicon, N-type polycrystalline silicon, platinum, chromium, and tungsten.

[0012] According to a preferred embodiment of the present invention, the temperature sensing element is a thermistor, and the material is one or more combinations of P-type polycrystalline silicon, N-type polycrystalline silicon, platinum, chromium, and tungsten.

[0013] According to a preferred embodiment of the present invention, the protective layer is one or a combination of silicon oxide and silicon nitride.

[0014] According to a preferred embodiment of the present invention, the cross-sectional shape of the heat insulation cavity is rectangular or trapezoidal.

[0015] The fabrication method of the above-mentioned fast-response MEMS flow sensor includes the following steps: S1. Provide a substrate, and deposit a lower support film on the upper surface of the substrate; S2. Prepare a sacrificial layer on the upper surface of the lower support film and pattern it by etching to form a prototype of the built-in air gap; S3. Deposit a support film on the surface of the sacrificial layer and the lower support film, remove the sacrificial layer at the air gap location through a release process to form a sealed air gap, and obtain a stacked support film. S4. Deposit and pattern the heating element and the temperature sensing element on the upper surface of the upper support film; S5. Deposit a protective layer, etch to form contact holes, and expose the electrode positions; S6. Prepare a heat insulation cavity from the back of the substrate, with a stacked support film suspended above the heat insulation cavity.

[0016] According to a preferred embodiment of the present invention, in step S2, a sacrificial layer is first deposited on the surface of the lower support film by LPCVD process. After the deposition is completed, photolithography and etching processes are performed. During etching, the etching slope is controlled to be 30~60° to etch a groove that matches the target built-in sealing air gap. The area between the grooves is the air gap prototype.

[0017] According to a preferred embodiment of the present invention, in step S3, firstly, an upper support film is deposited on the patterned sacrificial layer and the lower support film surface using LPCVD or PECVD processes. During the deposition process, it is ensured that the film layer uniformly covers the inner wall of the groove and the surface of the sacrificial layer to achieve complete encapsulation and sealing of the sacrificial layer. Subsequently, release holes with a diameter of 1~5μm are etched in the area corresponding to the air gap in the upper support film, and XeF2 etching gas is introduced through the release holes to remove the internal sacrificial layer. Finally, a thin layer is deposited using LPCVD to seal the release holes, forming a closed low-pressure internal air gap, resulting in a stacked support film composed of a lower support film, an air gap, and an upper support film.

[0018] According to a preferred embodiment of the present invention, in step S6, the heat insulation cavity is prepared by anisotropic wet etching or DRIE dry etching.

[0019] The beneficial effects of this invention are as follows: 1. The introduction of a sealing air gap in this invention creates a high thermal resistance insulating interface between the multiple supporting films. During transient heating and cooling processes, this interface effectively blocks the rapid transfer of heat to the lower supporting films, thus shielding the thermal inertia of the lower supporting films. Therefore, only the upper supporting film and the components it supports participate in rapid heat exchange as the main heat capacity, significantly reducing the system's equivalent dynamic heat capacity and consequently shortening the thermal time constant, thereby significantly accelerating the sensor's response time.

[0020] 2. The high thermal resistance of the air gap in this invention forces the heat generated by the heating element to be concentrated and confined within the upper supporting film layer, reducing energy dissipation to the lower film. This allows the central region of the upper supporting film to reach thermal equilibrium temperature more quickly, and the establishment of the upstream and downstream temperature difference signal caused by fluid flow is more rapid. While achieving rapid response, the detection sensitivity of the sensor is effectively guaranteed.

[0021] 3. The lower support membrane of this invention provides mechanical support for the main body, while the upper support membrane carries the functional components. The two are thermally decoupled through an internal air gap, but mechanically they are still an integral composite membrane, which ensures the mechanical reliability of the sensor and the yield of the finished product.

[0022] 4. The fabrication process of this invention is compatible with standard MEMS micromachining processes, requires no special equipment, and has good manufacturability. Attached Figure Description

[0023] Figure 1 This is a flowchart of the MEMS flow sensor fabrication method of the present invention; Figure 2 This is a schematic diagram of the product in step S1 of the present invention; Figure 3 This is a schematic diagram of the product in step S2 of the present invention; Figure 4 This is a schematic diagram of the product after etching the release hole in step S3 of the present invention; Figure 5 This is a schematic diagram of the product after the sacrificial layer is removed in step S3 of the present invention; Figure 6 This is a schematic diagram of the product after the release hole is sealed in step S3 of the present invention; Figure 7 This is a schematic diagram of the product after the release hole is sealed in step S4 of the present invention; Figure 8 This is a schematic diagram of the product after the release hole is sealed in step S5 of the present invention; Figure 9 This is a schematic diagram of the product after the release hole is sealed in step S6 of the present invention; In the figure, 10 is the substrate; 20 is the stacked support film; 30 is the heating element; 40 is the temperature sensing element; and 50 is the protective layer. 101. Insulated cavity; 21. Lower support membrane; 22. Sacrificial layer; 23. Upper support membrane; 221. Groove; 222. Air gap; 231. Release hole; 501. Contact hole. Detailed Implementation

[0024] The present invention will be further described below with reference to the embodiments and accompanying drawings, but is not limited thereto.

[0025] Example 1: This embodiment provides a fast-response MEMS flow sensor, including a substrate 10, a stacked support film 20, a heating element 30, a temperature sensing element 40, and a protective layer 50. The substrate 10 is provided with a heat insulation cavity 101. The stacked support film 20 is disposed on the upper surface of the substrate 10. The stacked support film 20 includes a lower support film 21 and an upper support film 23 from bottom to top. An air gap 222 is sealed at the middle position between the lower support film 21 and the upper support film 23. The stacked support film 20 is located directly above the heat insulation cavity 101. The heating element 30 and the temperature sensing element 40 are disposed on the upper surface of the upper support film 23. The temperature sensing element 40 is symmetrically distributed on the upstream and downstream sides of the heating element 30. The hot ends of the heating element 30 and the temperature sensing element 40 are located above the heat insulation cavity 101. The surfaces of the heating element 30 and the temperature sensing element 40 are covered with a protective layer 50. The protective layer 50 has contact holes 501 etched at the corresponding electrode positions. The heating element 30 and the temperature sensing element 40 are connected to electrodes through the contact holes 501.

[0026] Sacrificial layers 22 are provided on both sides between the lower support membrane 21 and the upper support membrane 23.

[0027] The air gap 222 has a thickness of 20nm and is a closed microcavity structure formed by sealing after the sacrificial layer is released.

[0028] The protective layer 50 is made of silicon oxide.

[0029] The cross-sectional shape of the heat insulation cavity 101 is rectangular or trapezoidal.

[0030] The above-mentioned method for fabricating a fast-response MEMS flow sensor, such as Figure 1 As shown, it includes the following steps: S1. A substrate 10 is provided, and a lower support film 21 is deposited on the upper surface of the substrate 10, such as... Figure 2 As shown; Substrate 10 is a silicon substrate; lower support film 21 is PECVD silicon oxide with a thickness of 500 nm; S2. A sacrificial layer 22 is prepared on the upper surface of the lower support film 21, and a patterned etching is performed to form a prototype of the built-in air gap, such as... Figure 3 As shown; First, polycrystalline silicon is deposited on the surface of the lower support film 21 as a sacrificial layer 22 using LPCVD process. After deposition, photolithography and etching processes are performed. During etching, the etching slope is controlled to be 30~60° to etch grooves 221 that match the target built-in sealing air gap. The area between the grooves 221 is the air gap prototype.

[0031] S3. A support film 23 is deposited on the surface of the sacrificial layer 22 and the lower support film 21. The sacrificial layer at the air gap position is removed by the release process to form a sealed air gap 222, and a stacked support film 20 is obtained. First, silicon nitride is deposited as an upper support film 23 with a thickness of 200 nm on the surface of the patterned sacrificial layer 22 and the lower support film 21 using PECVD process. During the deposition process, it is ensured that the film uniformly covers the inner wall of the groove and the surface of the sacrificial layer to achieve complete encapsulation and sealing of the sacrificial layer 22. Subsequently, release holes 231 with a diameter of 1~5 μm are etched in the region of the upper support film 23 corresponding to the air gap rudiment. Figure 4 As shown, XeF2 etching gas is introduced through the release hole to remove the internal sacrificial layer 22, as... Figure 5 As shown; finally, a thin layer of silicon nitride is deposited using LPCVD technology to seal the release pores, forming a closed, low-pressure internal air gap, as shown. Figure 6 As shown, a stacked support membrane consisting of a lower support membrane 21, an air gap 222, and an upper support membrane 23 is obtained.

[0032] S4. Deposit and pattern the heating element 30 and the temperature sensing element 40 on the upper surface of the upper support film 23, such as... Figure 7 As shown; S5. Deposit protective layer 50, etch to form contact hole 501, expose electrode location, such as Figure 8 As shown; S6. From the back side of substrate 10, fabricate the heat insulation cavity using anisotropic wet etching or DRIE dry etching, such as... Figure 9 As shown, the laminated support membrane 20 is suspended above the heat insulation cavity 101.

[0033] Example 2: A fast-response MEMS flow sensor, with the structure described in Example 1, except that the substrate is a germanium substrate; the lower support film 21 is made of LPCVD silicon oxide with a thickness of 1000 nm; The upper support film 23 is made of PECVD silicon nitride with a thickness of 500 nm; The thickness of air gap 222 is 200 nm; Both the heating element 30 and the temperature sensing element 40 are made of N-type polycrystalline silicon; The protective layer 50 is made of silicon nitride.

[0034] Furthermore, it is understood that although the present invention has been disclosed above with reference to preferred embodiments, these embodiments are not intended to limit the present invention. For any person skilled in the art, many possible variations and modifications can be made to the technical solutions of the present invention based on the disclosed technical content, or equivalent embodiments can be modified accordingly, without departing from the scope of the present invention. Therefore, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the present invention shall still fall within the scope of protection of the present invention.

Claims

1. A fast-response MEMS flow sensor, characterized in that, It includes a substrate, a multilayer support film, a heating element, a temperature sensing element, and a protective layer, wherein: The substrate has a heat insulation cavity, and a stacked support film is disposed on the upper surface of the substrate. The stacked support film includes a lower support film and an upper support film from bottom to top. An air gap is sealed in the middle between the lower support film and the upper support film. A heating element and a temperature sensing element are disposed on the upper surface of the upper support film. The temperature sensing elements are symmetrically distributed on both sides of the heating element. The hot ends of the heating element and the temperature sensing element are located above the heat insulation cavity. The surfaces of the heating element and the temperature sensing element are covered with a protective layer. Contact holes are etched on the protective layer at the corresponding electrode positions. Electrodes are connected to the heating element and the temperature sensing element through the contact holes.

2. The fast-response MEMS flow sensor as described in claim 1, characterized in that, Sacrificial layers, made of polycrystalline silicon, are provided on both sides between the lower and upper support films.

3. The fast-response MEMS flow sensor as described in claim 1, characterized in that, The substrate is a single-polished or double-polished semiconductor substrate.

4. The fast-response MEMS flow sensor as described in claim 1, characterized in that, The lower support film is made of thermally oxidized silicon, LPCVD silicon oxide or PECVD silicon oxide, with a thickness of 500nm~1000nm; The upper support film is made of LPCVD silicon nitride or PECVD silicon nitride, with a thickness of 200nm~500nm; The air gap thickness is 20nm~200nm.

5. The fast-response MEMS flow sensor as described in claim 1, characterized in that, The heating element is made of any one or more combinations of P-type polycrystalline silicon, N-type polycrystalline silicon, platinum, chromium, and tungsten. The temperature sensing element is a thermistor, and the material is one or more combinations of P-type polycrystalline silicon, N-type polycrystalline silicon, platinum, chromium, and tungsten; The protective layer is made of one or more combinations of silicon oxide and silicon nitride.

6. The fast-response MEMS flow sensor as described in claim 1, characterized in that, The cross-sectional shape of the heat insulation cavity is rectangular or trapezoidal.

7. The method for fabricating a fast-response MEMS flow sensor as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Provide a substrate, and deposit a lower support film on the upper surface of the substrate; S2. Prepare a sacrificial layer on the upper surface of the lower support film and pattern it by etching to form a prototype of the built-in air gap; S3. Deposit a support film on the surface of the sacrificial layer and the lower support film, remove the sacrificial layer at the air gap location through a release process to form a sealed air gap, and obtain a stacked support film. S4. Deposit and pattern the heating element and the temperature sensing element on the upper surface of the upper support film; S5. Deposit a protective layer, etch to form contact holes, and expose the electrode positions; S6. Prepare a heat insulation cavity from the back of the substrate, with a stacked support film suspended above the heat insulation cavity.

8. The method for fabricating a fast-response MEMS flow sensor as described in claim 7, characterized in that, In step S2, a sacrificial layer is first deposited on the surface of the lower support film using LPCVD process. After deposition, photolithography and etching processes are performed. During etching, the etching slope is controlled to be 30~60° to etch grooves that match the target built-in sealing air gap. The area between the grooves is the air gap prototype.

9. The method for fabricating a fast-response MEMS flow sensor as described in claim 7, characterized in that, In step S3, an upper support film is first deposited on the patterned sacrificial layer and the lower support film surface using LPCVD or PECVD technology. Then, release holes with a diameter of 1~5μm are etched in the region corresponding to the air gap in the upper support film, and XeF2 etching gas is introduced through the release holes to remove the internal sacrificial layer. Finally, a thin layer is deposited using LPCVD technology to seal the release holes and form a closed low-pressure internal air gap, resulting in a stacked support film composed of the lower support film, the air gap, and the upper support film.

10. The method for fabricating a fast-response MEMS flow sensor as described in claim 7, characterized in that, In step S6, the heat insulation cavity is prepared by anisotropic wet etching or DRIE dry etching.

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