Design and manufacturing method of a microflow sensor

Abstract

The present invention discloses a design and manufacturing method of a micro-flow sensor. The core lies in the selection of the thin film material for the heating and temperature measuring elements of the thermal micro-flow sensor, as well as the fabrication of the heat insulation cavity of the sensor. The thin film material of the heating and temperature measuring elements selects indium antimonide thin film material to replace the traditional platinum resistance material. It has higher detection sensitivity and lower manufacturing cost compared with the sensors made of traditional platinum materials, so it is very suitable for the detection of micro-flows.

Description

Technical Field

[0001] The present invention relates to the technical field of sensor manufacturing, and specifically to a design and manufacturing method of a micro-flow sensor. Background Art

[0002] With the rapid development and continuous progress of technology, the application market demand for flow detection is increasing. Currently, traditional flow sensors mainly include turbine flow meters, Roots flow meters, etc. Although such flow meters have mature technologies and their accuracies can meet general requirements, the sensitivity, accuracy, and volume of such mechanical flow meters cannot meet the requirements of fields such as biology, medicine, and automobiles. Therefore, there are great limitations in the measurement of fluid characteristics of microfluids. With the continuous progress of microelectromechanical system technology, MEMS micro-flow sensors have been widely applied in fields such as automobiles, chemical engineering, and medicine. Among them, thermal MEMS micro-flow sensors are widely used due to their advantages such as wide measurement range, high sensitivity, low flow lower limit, small size, low cost, and easy mass production. Thermal MEMS flow sensors are mainly divided into thermal distribution type and heat loss type. The thermal distribution type MEMS flow sensor is based on the principle that when fluid flows through, the temperature field distribution near the heating element is no longer symmetric, the temperature sensed by the upstream temperature measuring element decreases, and the temperature sensed by the downstream temperature measuring element increases. The fluid flow rate is obtained by detecting the temperature difference between the upstream and downstream temperature measuring elements; the heat loss type MEMS flow sensor is based on the heat transfer effect between the fluid and the surface of the heating element (Heater), and uses the temperature measuring element to measure the temperature change caused by the heat loss of the heating element to indirectly measure the change in flow rate. The key components of the thermal MEMS micro-flow sensor are the heating element, the temperature measuring element, and the heat insulation treatment (heat insulation cavity). The tiny heating element and temperature measuring element can improve the response speed of the sensor, and good heat insulation treatment can improve the sensitivity of the sensor.

[0003] There are many choices for the temperature measuring element of the thermal MEMS micro-flow sensor. Commonly used ones are thermistors and thermocouples, and they are both made based on the principle that their physical parameters (such as resistance value, thermoelectromotive force, etc.) change due to temperature changes. However, they each have disadvantages. Among them, the most commonly used thermal flow sensor with a platinum resistance as the temperature measuring element has strong stability, but general sensitivity and high manufacturing cost; while the micro-flow sensor with a thermocouple as the temperature measuring element has low manufacturing cost, but its sensitivity is too low and cold junction compensation is required. Summary of the Invention

[0004] The purpose of the present invention is to provide a design and manufacturing method of a micro-flow sensor to solve the problems raised in the above background art.

[0005] To achieve the above object, the present invention provides the following technical solution: A design and manufacturing method of a micro-flow sensor, including a substrate, a heat insulation cavity, a heating element, and a temperature sensing element. The heating element (Heater) and the temperature sensing element (Temperature sensing element) are formed on a substrate film with a certain thickness, and the film is suspended above the cavity (Cavity).

[0006] Preferably, the substrate 001 is exemplified by silicon (Si), and its crystal orientation can be (100) or (111).

[0007] Preferably, a groove structure 002 is first formed in a specified area on the front surface of the substrate by using photolithography and dry etching processes. The shape and / or depth of the groove can be adjusted according to actual needs. Looking from a top view (not shown), the projection of the groove can be a polygon (including a rectangle), a circle, or obviously other shapes, which will not be elaborated here. The cross-sectional structure of the fabricated groove structure 002 is trapezoidal, and this shape can be achieved by adjusting the dry etching process parameters, which will not be elaborated here. The trapezoidal groove structure is a key protected point of the present invention and is the core of the formation of the heat insulation cavity at the bottom of the groove and the sealing process at the top of the groove.

[0008] Preferably, an oxide layer 003 is grown in the groove 002 by using a thermal oxidation process. A certain proportion of silicon will be consumed during the growth of the oxide layer by the thermal oxidation process. Since the thermal oxidation process is the most common process in semiconductor process technology, its specific process parameters and formation principle will not be elaborated here. At this time, oxide layers will grow on the sidewalls and bottom of the groove 002. Due to the consumption of silicon, the silicon islands 004 between the grooves will gradually shrink, which is related to the time of thermal oxidation (the longer the time, the more silicon is consumed, and the smaller the size of the silicon island). Since the initial cross-sectional morphology of the groove 002 is trapezoidal, the initial cross-sectional morphology of the silicon islands 004 between the corresponding grooves is an inverted trapezoid. In order to facilitate the formation of a cavity at the bottom of the groove later, the present invention needs to obtain silicon islands 004 with an inverted triangular shape after the first-step thermal oxidation mentioned above.

[0009] Preferably, the thermal oxidation reaction will occur in all areas where silicon is exposed, including inside the groove 002 and the front and back surfaces of the substrate; 2) The formation of silicon islands 004 with an inverted triangular cross-section is directly related to the oxidation time (if the oxidation time is too short, the silicon at the bottom of the silicon island cannot be completely consumed, and the cross-sectional morphology of the silicon island 004 is still an inverted trapezoid); 3) The time of the thermal oxidation process is based on the time required to form silicon islands 004 with an inverted triangle. If the thermal oxidation process takes too long, it is not conducive to the control of production costs; 4) To obtain silicon islands 004 with an inverted triangular cross-section within the effective time of thermal oxidation, it is necessary to optimize the combination design of the depth, cross-sectional inclination angle, and spacing of the groove 002.

[0010] Preferably, the oxide layer is removed by using the wet etching process of HF or BOE. The wet etching process of silicon oxide is the most common process in semiconductor process technology, so its specific process parameters will not be elaborated here. After the oxide layer is removed, a gap 005 will be formed at the bottom of the inverted triangular silicon island 004. The final size of the gap 005 is determined by the depth of the groove 002 and the final height of the silicon island 004 at the same time.

[0011] Preferably, the oxide layer is regrown by using the thermal oxidation process. Due to the consumption of silicon, the inverted triangular silicon island 004 will be further reduced. At the same time, the width of the groove 002 will decrease as the thickness of the oxide layer on the sidewall of the groove 002 increases. The smaller the final width of the groove 002, the more conducive it is to the sealing of the top of the groove in the subsequent process. Those skilled in the art should recognize that the final profile of the silicon island 004 is an inverted triangle, but it also includes the case where the silicon island 004 is completely oxidized. The heat insulation effect of the suspended film cavity structure composed of the completely oxidized silicon island 004 and the gap 005 reaches the best.

[0012] Preferably, the silicon oxide film 007 is grown by using the low-pressure chemical vapor deposition method (LPCVD) to seal the groove 002, thereby forming a suspended film that plays a supporting role above the cavity gap. The growth thickness of the silicon oxide 007 is based on the complete sealing of the top of the groove 002. Therefore, the final size of the groove 002 before sealing determines the growth time of the oxide layer 007. Those skilled in the art should recognize that if the growth time of the silicon oxide 007 is too short, the groove 002 cannot be sealed; if the growth time of the silicon oxide 007 is too long, it is not conducive to the control of production costs. Therefore, as described above, it is necessary to optimize the combination design and calculation of the depth / profile inclination angle of the groove 002, the groove spacing, and the thickness of the second thermal oxidation. While effectively controlling the production cost, ensure the formation of the inverted triangular silicon island 004 and the gap 005, and also ensure that the top of the trapezoidal groove 002 is completely sealed to form a sealed heat insulation cavity.

[0013] Preferably, the heating and temperature - measuring element patterns of the micro - flow sensor are fabricated by semiconductor thin - film evaporation process, photolithography process, etching process or lift - off process. Another key protected point of the present invention is to use an indium antimonide alloy thin - film material to fabricate the heating element and temperature - measuring element of the micro - flow sensor. Since the thermal MEMS flow sensor detects the flow change based on the principle of temperature or temperature - field change, its temperature - measuring element usually has the characteristic that its physical properties change with temperature. The indium antimonide thin - film material selected in the present invention has the characteristic that the resistance value changes decrease with the increase of temperature and shows a negative exponential relationship with temperature, having a negative temperature coefficient. As can be seen from the figure, in the temperature range of - 40°C to 40°C, for the indium antimonide thin - film, as the temperature increases, its resistance value drops very rapidly, and the absolute value of the temperature coefficient is relatively large, with very high sensitivity; while in the temperature range of 40°C to 125°C, the resistance decreases slowly with the increase of temperature, and the absolute value of the temperature coefficient is relatively small, and the sensitivity decreases. Therefore, according to different application environments, the micro - flow sensor can be designed to specifically meet the measurement of different flow ranges. Those skilled in the art should recognize that the thin - film materials selected in the present invention are not limited to indium antimonide, but also include indium antimonide - indium or indium antimonide thin - film materials doped with other elements, and their resistance must have the characteristic of significant change with temperature.

[0014] Preferably, the passivation protection layer 013 is grown by plasma - enhanced chemical vapor deposition (PECVD). The protection layer generally selects a silicon nitride and / or silicon oxynitride and / or silicon nitride oxide composite thin - film. Then, the passivation layer 013 is etched through photolithography and dry - etching processes to form a metal PAD window 14. Finally, the top - layer metal is formed in the PAD window 14 by photolithography and etching processes. The top - layer metal material generally selects the common aluminum alloy in semiconductor processes.

[0015] Preferably, the series of micro - release windows are further etched downward by silicon deep reactive ion etching process, so that the series of micro - release windows are etched to the required reference thermal insulation cavity.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] A method for fabricating the heating element and temperature - measuring element of a micro - flow sensor with an indium antimonide alloy thin - film material. Due to its good temperature characteristics and extremely high detection sensitivity, it is very suitable for the detection of micro - flows. At the same time, the present invention proposes a method for fabricating a thermal insulation cavity, which can further reduce the chip size and production cost while ensuring good thermal insulation effect of the sensor, and is suitable for mass production. Therefore, the thermal film - type micro - flow sensor involved in the present invention has many advantages such as high sensitivity, fast response speed, strong stability, good reliability and low price. Description of the Drawings

[0018] Figure 1 is a distribution diagram of the structure of the micro-flow sensor of the present invention;

[0019] Figure 2 is a cross-sectional view of the substrate of the present invention;

[0020] Figure 3 is a cross-sectional view of the groove of the present invention;

[0021] Figure 4 is a cross-sectional view of the oxidized groove of the present invention;

[0022] Figure 5 is a schematic diagram of the inverted triangular silicon island structure of the present invention;

[0023] Figure 6 is a schematic diagram of the silicon island structure after oxidation of the present invention;

[0024] Figure 7 is a schematic diagram of the formation of the heat insulation cavity of the present invention;

[0025] Figure 8 is a schematic diagram showing the exponential relationship between the temperature and resistance of the thin film material of the present invention;

[0026] Figure 9 is a schematic diagram of the formation of the passivation protection layer of the present invention.

[0027] Figure 10 is a schematic diagram of the formation of the top metal PAD of the present invention

[0028] In the figure: 001, substrate; 002, groove; 003, oxide layer; 004, inverted triangular silicon island; 005, gap; 007, silicon oxide film; 013, passivation protection layer. Specific embodiments

[0029] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0030] In the description of the present invention, it should be noted that the orientation or positional relationship indicated by the terms "upper", "lower", "inner", "outer", "front end", "rear end", "both ends", "one end", "the other end", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation of the present invention. In addition, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0031] In the description of the present invention, it should be noted that unless otherwise clearly specified and defined, terms such as "installation", "provided with", "connection", etc. should be understood in a broad sense. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two components. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations.

[0032] Embodiment: Please refer to Figure 1- Figure 9 An embodiment provided by the present invention: The present invention relates to a design and manufacturing method of a micro-flow sensor, and its principle is a device that measures the size of fluid flow by using the change of temperature field caused by fluid flow. The following further illustrates the present invention in combination with specific embodiments and drawings. More details are elaborated in the following description for a full understanding of the present invention. However, the present invention can obviously be implemented in many other ways different from this description. Those skilled in the art can make similar generalizations and deductions according to the actual application situation without violating the connotation of the present invention. Therefore, the protection scope of the present invention should not be limited by the content of this specific embodiment.

[0033] It includes a heat insulation cavity (Cavity), a heating element (Heater), and a temperature sensing element (Temperature sensing element) on the substrate. To better illustrate the structure of the micro-flow sensor, a cross-sectional structure schematic diagram of the above sensor structure is made, as Figure 1b shown.

[0034] The heating element (Heater) and the temperature sensing element (Temperature sensing element) are formed on a substrate film with a certain thickness, and the film is suspended above the cavity (Cavity).

[0035] Those skilled in the art should recognize that, here, it should also be understood that they can arbitrarily arrange and adjust the positions of the internal units of the micro-flow sensor structure according to the principle of the thermal micro-flow sensor and application requirements, which are all within the protection scope of this application of the present invention. In addition, the shapes and / or sizes of the heating element and the temperature sensing element involved in the present invention can also be arbitrarily adjusted according to needs.

[0036] The following further illustrates the manufacturing process of the thermal micro-flow sensor structure of the embodiment of the present invention in combination with specific embodiments and drawings.

[0037] Figures 2 to 10Schematic cross-sectional structure diagram of the manufacturing process of a thermal micro flow sensor according to an embodiment of the present invention. It should be noted that these and subsequent other drawings are only examples, and they are not drawn under equal proportion conditions and should not be used to limit the actual protection scope required by the present invention.

[0038] As Figure 2 and Figure 3 shown, in this case, the substrate 001 is silicon (Si) as an example, and its crystal orientation can be (100) or (111). Obviously, the selection of the substrate crystal orientation is not limited to this, and those skilled in the art can make corresponding adjustments according to actual needs. First, a groove structure 002 is formed in a specified area on the front surface of the substrate by using photolithography and dry etching processes, where the shape and / or depth of the groove can be adjusted according to actual needs. From a top view (not shown), the projection of the groove can be a polygon (including a rectangle) or a circle, and obviously, it can also be other shapes, which will not be elaborated here. As Figure 2 shown, the cross-sectional structure of the fabricated groove structure 002 is trapezoidal, and this shape can be achieved by adjusting the dry etching process parameters, which will not be elaborated here. The trapezoidal groove structure is a key protected point of the present invention and is the core of the formation of the heat insulation cavity at the bottom of the groove and the sealing process at the top of the groove.

[0039] As Figure 4 shown, an oxide layer 003 is grown in the groove 002 by using a thermal oxidation process. A certain proportion of silicon will be consumed during the growth of the oxide layer by the thermal oxidation process. Since the thermal oxidation process is the most common process in semiconductor process technology, its specific process parameters and formation principle will not be elaborated here. At this time, oxide layers will grow on the sidewalls and bottom of the groove 002. Due to the consumption of silicon, the silicon islands 004 between the grooves will gradually shrink, which is related to the time of thermal oxidation (the longer the time, the more silicon is consumed, and the smaller the size of the silicon island). Since the initial cross-sectional morphology of the groove 002 is trapezoidal, the initial cross-sectional morphology of the silicon islands 004 between the corresponding grooves is an inverted trapezoid. In order to facilitate the formation of a cavity at the bottom of the groove later, the present invention needs to obtain silicon islands 004 with an inverted triangular shape as shown in Figure 4 after the first thermal oxidation step described above. Those skilled in the art should recognize the following points: 1) The thermal oxidation reaction will occur in all areas where silicon is exposed, including inside the groove 002 and the front and back surfaces of the substrate; 2) The formation of the silicon islands 004 with an inverted triangular cross-section is directly related to the oxidation time (if the oxidation time is too short, the silicon at the bottom of the silicon island cannot be completely consumed, and the cross-sectional morphology of the silicon island 004 is still an inverted trapezoid); 3) The time of the thermal oxidation process is based on the time to form the inverted triangular silicon islands 004. If the thermal oxidation process takes too long, it is not conducive to cost control; 4) To obtain silicon islands 004 with an inverted triangular cross-section within the effective time of thermal oxidation, it is necessary to optimize the combination design of the depth, cross-sectional inclination angle, and spacing of the groove 002.

[0040] As Figure 5 , the oxide layer is removed by using the HF or BOE wet etching process. The wet etching process of silicon oxide is the most common process in semiconductor process technology, so its specific process parameters will not be elaborated here. After the oxide layer is removed, a gap 005 will be formed at the bottom of the inverted triangular silicon island 004. The final size of the gap 005 is determined by both the depth of the groove 002 and the final height of the silicon island 004.

[0041] As Figure 6 , the oxide layer is regrown by using the thermal oxidation process. Due to the consumption of silicon, the inverted triangular silicon island 004 will be further reduced. At the same time, the width of the groove 002 will decrease as the thickness of the oxide layer on the sidewall of the groove 002 increases. The smaller the final width of the groove 002 (i.e., the silicon island spacing), the more beneficial it is for the subsequent sealing of the top of the groove. Those skilled in the art should recognize that the final cross-sectional morphology of the silicon island 004 is an inverted triangle, but it also includes the case where the silicon island 004 is completely oxidized. The heat insulation effect of the suspended film cavity structure composed of the completely oxidized silicon island 004 and the gap 005 reaches the best.

[0042] As Figure 7 , the silicon oxide film 007 is grown by using the low-pressure chemical vapor deposition method (LPCVD) and the groove 002 is sealed to form a suspended film that supports the upper part of the cavity gap. The growth thickness of the silicon oxide 007 is based on the complete sealing of the top of the groove 002. Therefore, the final size of the groove 002 before sealing determines the growth time of the oxide layer 007. Those skilled in the art should recognize that if the growth time of the silicon oxide 007 is too short, the groove 002 cannot be sealed; if the growth time of the silicon oxide 007 is too long, it is not conducive to the control of production costs. Therefore, as described above, it is necessary to optimize the combination design and calculation of the depth / profile inclination angle of the groove 002, the groove spacing, and the thickness of the second thermal oxidation. While ensuring the effective control of production costs, ensure the formation of the inverted triangular silicon island 004 and the gap 005, and also ensure that the top of the trapezoidal groove 002 is completely sealed to form a sealed heat insulation cavity.

[0043] As Figure 8a , the patterns of the heating and temperature measuring elements of the micro flow sensor are fabricated by semiconductor thin film evaporation process, photolithography process, etching process or lift-off process. Another key protected point of the present invention is to use an indium antimonide alloy thin film material to fabricate the heating element and the temperature measuring element of the micro flow sensor. Since the thermal MEMS flow sensor detects the flow rate change based on the principle of temperature or temperature field change, its temperature measuring element usually has the characteristic that its physical properties change with temperature. As Figure 8b, the indium antimonide thin film material selected in the present invention has a resistance value that decreases with increasing temperature and shows a negative exponential relationship with temperature, having a negative temperature coefficient. As can be seen from the figure, in the temperature range of -40°C to 40°C, the resistance value of the indium antimonide thin film drops very rapidly with increasing temperature, the absolute value of the temperature coefficient is relatively large, and the sensitivity is very high; while in the range of 40°C to 125°C, the resistance decreases slowly with increasing temperature, and the absolute value of the temperature coefficient is relatively small, and the sensitivity decreases. Therefore, according to the different application environments, a micro flow sensor can be designed to specifically meet the measurement requirements of different flow ranges. Those skilled in the art should recognize that the thin film material selected in the present invention is not limited to indium antimonide, but also includes indium antimonide - indium or indium antimonide thin film materials doped with other elements, and its resistance must have a significant change with temperature (showing an exponential or linear relationship, having a negative or positive temperature coefficient), such as Figure 8b .

[0044] Such as Figures 9 to 10 As shown, a passivation protection layer 013 is grown by plasma enhanced chemical vapor deposition (PECVD). Generally, a combination film of silicon nitride and / or silicon oxynitride and / or silicon nitride oxide is selected as the protection layer. Then, the passivation layer 013 is etched through photolithography and dry etching processes to form a metal PAD window 14. Finally, a top metal is formed in the PAD window 14 using photolithography and etching processes. Generally, the top metal material is aluminum alloy (aluminum, aluminum silicon, aluminum copper, or aluminum silicon copper, etc.) most commonly used in semiconductor processes.

[0045] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and without departing from the spirit or basic characteristics of the present invention, the present invention can be implemented in other specific forms. Therefore, from any point of view, the embodiments should be regarded as exemplary and non - restrictive. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention. Any reference signs in the claims should not be regarded as limiting the claims involved.

Claims

1. A design and manufacturing method of a micro - flow sensor, comprising a substrate, a heat - insulating cavity, a heating element and a temperature - measuring element, characterized in that: The heating element and the temperature measuring element are formed on a substrate film with a certain thickness, and the substrate film is suspended above the cavity; The manufacturing method is as follows: First, a groove structure is formed in a specified area on the front surface of the substrate by using photolithography and dry etching processes; the shape and / or depth of the groove are adjusted according to actual needs, the projection of the groove is a polygon or a circle; the cross-sectional structure of the fabricated groove structure is trapezoidal; An oxide layer is grown in the groove by using a thermal oxidation process, and the initial cross-sectional morphology of the silicon island between the corresponding grooves is an inverted trapezoid; The oxide layer is removed by using an HF or BOE wet etching process. After the oxide layer is removed, a gap is formed at the bottom of the inverted triangular silicon island; The oxide layer is grown again by using a thermal oxidation process, and the final cross-sectional morphology of the silicon island is an inverted triangle; Silicon oxide film is grown by using low-pressure chemical vapor deposition method and the groove is sealed to form a suspended film that supports above the cavity gap; The patterns of the heating and temperature measuring elements of the micro flow sensor are fabricated by semiconductor thin film evaporation process, photolithography process, etching process or lift-off process; A passivation protection layer is grown by using plasma enhanced chemical vapor deposition method.

2. The design and manufacturing method of a micro-flow sensor according to claim 1, characterized in that: The substrate is silicon for example, and its crystal orientation is [100] or [111].

3. The design and manufacturing method of a microflow sensor according to claim 1, characterized in that: In the step of removing the oxide layer by using the HF or BOE wet etching process, after the oxide layer is removed, the final size of the gap formed at the bottom of the inverted triangular silicon island is determined by both the depth of the groove and the final height of the silicon island.

4. A design and manufacturing method of a micro-flow sensor according to claim 1, characterized in that: In the step of growing the oxide layer again by using the thermal oxidation process, the heat insulation effect of the suspended film cavity structure composed of the fully oxidized silicon island and the gap reaches the best.

5. A design and manufacturing method of a micro-flow sensor according to claim 1, characterized in that: When growing the silicon oxide film by using the low-pressure chemical vapor deposition method and sealing the groove to form a suspended film that supports above the cavity gap, the growth thickness of the silicon oxide is based on completely sealing the top of the groove.

6. The design and manufacturing method of a microflow sensor according to claim 1, characterized in that: In the step of growing the passivation protection layer by using the plasma enhanced chemical vapor deposition method, the protection layer is selected as a silicon nitride and / or silicon oxynitride and / or silicon nitride oxide composite film, and then the passivation layer is etched by photolithography and dry etching processes to form a metal PAD window, and finally the top metal is formed in the PAD window by photolithography and etching processes, and the top metal material is selected as aluminum alloy.

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