The present disclosure relates to a thermal flow rate sensor that detects a flow rate of a fluid.
A thermal flow rate sensor is equipped with a thin film membrane on a substrate, in other words, a diaphragm. In the thermal flow rate sensor, a heater is arranged in the membrane, and temperature sensors are located on an upstream side and a downstream side of a fluid flow through the heater, respectively.
An object of the present disclosure is to provide a thermal flow rate sensor capable of suppressing variation in the amount of heat conduction between upstream and downstream of a fluid flow through the heater.
A thermal flow rate sensor according to one aspect of the present disclosure includes a substrate on which an opening having two opposite sides is formed, a thin film formed on the substrate, and a heat conductive member. The thin film forms a membrane, and between the two opposite sides, the membrane has a heater, an upstream temperature sensor located on one side of the heater, and a downstream temperature sensor located on the opposite side of the upstream temperature sensor across the heater. The heat conductive member promotes heat conduction from the heater to the substrate. In such a configuration, when an end of the opening on the membrane side is defined as an upper end and an end on the side away from the membrane is defined as a lower end, the heat conductive member covers from the upper end to the lower end of the opening in a normal direction of the membrane.
In an assumable example, a thermal flow rate sensor is equipped with a thin film membrane on a substrate, in other words, a diaphragm. In the thermal flow rate sensor, a heater is arranged in the membrane, and temperature sensors are located on an upstream side and a downstream side of a fluid flow through the heater, respectively. In such a configuration, when the heater is heated to a constant temperature, a temperature difference occurs between the upstream and downstream of the heater according to the flow rate of the fluid, and a resistance value of a temperature measuring resistance constituting both temperature sensors is different. Based on this configuration, the thermal flow rate sensor detects the flow rate of the fluid using a signal representing the difference in resistance values as a detection signal.
In such a thermal flow rate sensor, there are variations in the workmanship of the membrane, that is, variations depending on a positional deviation between a formation position of an opening formed in the substrate for forming the membrane and a formation position of the heater or the temperature sensor.
Therefore, a heat conductive member having high thermal conductivity is provided in an edge region of the membrane in each of the upstream and downstream of the fluid flow through the heater and both temperature sensors. By forming the heat conductive member at the same time as the heater and the temperature sensors, it is possible to obtain a highly accurate relative position and suppress the influence of variations in the workmanship of the membrane.
However, it may not be possible to sufficiently suppress the influence of variations in the workmanship of the membrane simply by providing the heat conductive member in the edge region of the membrane.
For example, the opening of the substrate is formed by etching from a surface of the substrate on an opposite side of the membrane, but a side surface of the opening is inclined to some extent. When an end of the opening formed on the substrate on the membrane side is referred to as an upper end and an end on the side away from the membrane is referred to as a lower end, a size of the opening becomes taper from the lower end side to the upper end side. Therefore, the thickness of the substrate gradually increases according to the inclination of the opening toward the outside of the membrane from the outer edge of the membrane.
In the substrate, the thicker the substrate, the larger the heat conduction amount, so that the heat conduction amount becomes smaller at a thin position near the outer edge of the membrane. Therefore, even if the heat conductive member is formed in the outer edge region of the membrane, if it is not formed in the inclined region of the opening, the heat conduction is performed through the thin portion of the substrate. If the formation position of the heat conductive member varies due to the variation in the workmanship of the membrane, the heat conductive member may or may not be formed in the inclined region of the opening between the upstream and downstream of the fluid flow. For this reason, the amount of heat conduction varies between the upstream and downstream of the fluid flow, and the influence of the variation in the workmanship of the membrane cannot be sufficiently suppressed. An object of the present disclosure is to provide the thermal flow rate sensor capable of suppressing variation in the amount of heat conduction between upstream and downstream of a fluid flow through the heater.
A thermal flow rate sensor according to one aspect of the present disclosure includes a substrate on which an opening having two opposite sides is formed, a thin film formed on the substrate, and a heat conductive member. The thin film forms a membrane, and between the two opposite sides, the membrane has a heater, an upstream temperature sensor located on one side of the heater, and a downstream temperature sensor located on the opposite side of the upstream temperature sensor across the heater. The heat conductive member promotes heat conduction from the heater to the substrate. In such a configuration, when an end of the opening on the membrane side is an upper end and an end on the side away from the membrane is a lower end, the heat conductive member covers from the upper end to the lower end of the opening in a normal direction of the membrane.
In this way, in the normal direction of the membrane, each heat conductive member is arranged so as to cover from the upper end to the lower end of the opening. Therefore, high thermal conductivity can be maintained by the heat conductive member even on the side surface of the opening whose thickness changes. Therefore, even if the workmanship of the membrane varies, its influence can be sufficiently suppressed. Therefore, it is possible to suppress the variation in the amount of heat conduction between the upstream and downstream of the fluid flow through the heater, reduce the variation in the responsiveness, and detect the flow rate of the fluid with high accuracy.
Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each embodiment described below, same or equivalent parts are designated with the same reference numerals.
A first embodiment will be described. A thermal flow rate sensor according to the present embodiment is applied as, for example, an air flow sensor provided in an intake pipe of an engine in a vehicle, and is used for measuring an air flow rate for adjusting an intake air amount so that an air-fuel ratio is suitable for an operating state of the engine. Although not shown here, the airflow sensor includes a housing in which an air introduction pipe is formed, and is installed so that the air introduction pipe is exposed to the intake pipe of the engine. A part of the air flowing through the intake pipe is introduced into the air introduction pipe, and the air introduction pipe is branched in the housing, and the air flow sensor is installed on the branch path side, so that a main air flow is prevented from reaching the air flow sensor directly. Therefore, an influence of dust contained in the intake air is suppressed, and the amount of intake air can be detected accurately.
Hereinafter, the configuration of the thermal flow rate sensor of the present embodiment will be described with reference to
As shown in
As shown in
Further, in the case of the present embodiment, a side surface of the opening 100a is in an inclined state. Hereinafter, an end of the opening 100a on the membrane 10 side is referred to as an upper end 100b, and an end on the side away from the membrane 10 is referred to as a lower end 100c.
As shown in
The heater 20 is laid out in a meandering shape at a center position of the membrane 10 with a direction orthogonal to the fluid flow direction indicated by the arrow in the drawing (hereinafter, simply referred to as an orthogonal direction) as a longitudinal direction, and the pull-out wiring 21 is pulled out below a paper surface in
The upstream temperature sensor 30 is arranged on one side of the membrane 10 with the heater 20 as a center, that is, on the upstream side of the fluid flow. The upstream temperature sensor 30 is also laid out in a meandering shape with the orthogonal direction as the longitudinal direction. Further, the downstream temperature sensor 40 is arranged on the opposite side of the membrane 10 from the upstream temperature sensor 30 with the heater 20 as a center, that is, on the downstream side of the fluid flow. Therefore, the upstream temperature sensor 30, the heater 20, and the downstream temperature sensor 40 are arranged side by side with the fluid flow direction as the arrangement direction. The downstream temperature sensor 40 is also laid out in a meandering shape with the orthogonal direction as the longitudinal direction.
The upstream temperature sensor 30 and the downstream temperature sensor 40 may each be composed of one resistance temperature detector. However, in the case of the present embodiment, the upstream temperature sensor 30 and the downstream temperature sensor 40 form the Wheatstone bridge circuit shown in
Specifically, the upstream temperature sensor 30 has a first resistance temperature detector 30a and a second resistance temperature detector 30b, and has a meandering shape so that the first resistance temperature detector 30a and the second resistance temperature detector 30b are arranged side by side. The pull-out wiring 31a and the pull-out wiring 31b are pulled out from each of them. In the Wheatstone bridge circuit shown in
Similarly, the downstream temperature sensor 40 has a first resistance temperature detector 40a and a second resistance temperature detector 40b, and is arranged in a meandering shape so that the first resistance temperature detector 40a and the second resistance temperature detector 40b are arranged side by side. The pull-out wiring 41a and the pull-out wiring 41b are pulled out from each of them. In the Wheatstone bridge circuit shown in
Then, in the Wheatstone bridge circuit of
Although an outer part of the membrane 10 is omitted for each of the pull-out wiring 31a, 31b, 41a, and 41b, these pull-out wirings are appropriately connected so as to form the Wheatstone bridges of
Further, two heat conductive members 50 are provided along the two opposite sides 11 and 12 of the membrane 10, specifically, the two sides 11 and 12 which are opposed to each other in the fluid flow direction and extend in the orthogonal direction. The heat conductive member 50 is preferably made of a material having a higher thermal conductivity than that of the substrate 100, but may be made of a material having a thermal conductivity similar to that of the substrate 100 to assist the heat conduction of the substrate 100. In the case of the present embodiment, one heat conductive member 50 covers the entire side 11 and the other heat conductive member 50 covers the entire side 12. More specifically, as shown in
For example, in the flow direction of the fluid, the width W1 of the heat conductive member 50 is set to several μm or more and several hundred μm or less, and the width W2 from the upper end 100b to the lower end 100c is set to be larger than 0 and several hundred μm or less. The width W1 needs to be larger than the width W2, and has a dimension that allows for a manufacturing error when forming the heat conductive member 50. The width W2 is determined by the angle (hereinafter referred to as the taper angle) of the side surface of the tapered opening 100a and the thickness of the substrate 100. Therefore, the width W1 is determined in consideration of the taper angle of the opening 100a, the thickness of the substrate 100, the width of the opening 100a, and the formation error of the heat conductive member 50.
In the membrane 10, the width W3 in the fluid flow direction and the width W4 in the orthogonal direction are both set to 300 μm to 700 μm. In
As described above, the thermal flow rate sensor of the present embodiment is configured. The thermal flow rate sensor configured in this way detects the flow rate of the fluid flowing in the direction of the arrow in
At this time, when the fluid flows over the upstream temperature sensor 30 and the downstream temperature sensor 40, the temperature of the upstream temperature sensor 30 decreases and the temperature of the downstream temperature sensor 40 increases according to the flow rate. Then, the resistance values of the resistance temperature detectors 30a, 30b, 40a, and 40b constituting the upstream temperature sensor 30 and the downstream temperature sensor 40 change with the temperature change. For example, the resistance values of the first resistance temperature detector 30a and the second resistance temperature detector 30b constituting the upstream temperature sensor changes as shown in Equation 1, and the resistance values of the first resistance temperature detector 40a and the second resistance temperature detector 40b constituting the downstream temperature sensor 40 changes as shown in Equation 2. In Equations 1 and 2, R0 represents a resistance value at 0° C., a represents a temperature coefficient of resistance, and ΔT represents an amount of temperature change. When the resistance temperature detectors 30a, 30b, 40a, and 40b are made of metal, the resistance value increases as the temperature rises.
Therefore, the potential difference between the midpoint voltage of the resistance element RU2 and the resistance element RD2 and the midpoint voltage of the resistance element RD1 and the resistance element RU1 changes based on the temperature changes of the upstream temperature sensor 30 and the downstream temperature sensor 40 according to the flow rate of the fluid. The potential difference is input to the control unit from the Wheatstone bridge circuit as a differential output, and the control unit detects the flow rate of the fluid based on the differential output.
When performing such an operation, if the thermal conductivity from the heater 20 varies between the upstream temperature sensor 30 side and the downstream temperature sensor 40 side due to the variation in the workmanship of the membrane 10, it is impossible to detect the flow rate of the fluid accurately. That is, a responsiveness variation occurred between the upstream temperature sensor 30 and the downstream temperature sensor 40 in the upstream and downstream of the heater 20, in other words, a time constant of heat conduction varies, and the flow rate detection cannot be performed accurately. Further, a heat capacity varies between the upstream and downstream of the heater due to the variation in the workmanship of the membrane 10, and the responsiveness also varies due to the variation in the workmanship, so that more accurate flow rate detection cannot be performed. However, since the heat conductive member 50 is formed along the two opposite sides 11 and 12 of the membrane 10, heat conduction is promoted by the heat conductive member 50, and even if the workmanship of the membrane 10 varies, such the effect can be suppressed.
However, even if the heat conductive member 50 is formed on the outer edge of the membrane 10, as shown in
On the other hand, in the thermal flow rate sensor of the present embodiment, each heat conductive member 50 is arranged so as to overlap the entire side surface of the opening 100a from the upper end 100b to the lower end 100c in the normal direction of the membrane 10. Therefore, high thermal conductivity can be maintained by the heat conductive member 50 even on the side surface of the opening 100a whose thickness changes. Therefore, even if the workmanship of the membrane 10 varies, its influence can be sufficiently suppressed. Therefore, it is possible to suppress the variation in the amount of heat conduction between the upstream and downstream of the fluid flow through the heater 20, and it is possible to detect the flow rate of the fluid with high accuracy.
Further, as shown in
Further, the thermal flow rate sensor configured as described above is formed as follows. First, the first silicon nitride film 101 and the first silicon oxide film 102 are formed on the substrate 100, and then a resistor material for forming the pattern layer 103 is formed. Then, a mask is placed on the resistor material to open the positions where the heater 20, the upstream temperature sensor 30, the downstream temperature sensor 40, the heat conductive member 50, etc. are to be formed, and the resistor material is etched to form the pattern layer 103. As a result, the heater 20, the upstream temperature sensor 30, the downstream temperature sensor 40, the heat conductive member 50, and the like are patterned. Further, the second silicon oxide film 104 and the second silicon nitride film 105 are formed in order so as to cover the pattern layer 103. Then, after arranging a mask on the back surface side of the substrate 100 at which the planned formation position of the opening 100a opens, the substrate 100 is etched by dry etching or the like to form the opening 100a. In this way, the thermal flow rate sensor is manufactured.
At this time, an error when forming the heat conductive member 50 may occur due to a mask shift when patterning the pattern layer 103 or a mask shift when forming the opening 100a. Further, there is a possibility that an error in forming the width W3 due to the variation in the lateral etching when the opening 100a is formed by etching may occur, and the side surface of the opening 100a may be tapered. However, the width W1 of the heat conductive member 50 is set in consideration of these formation errors, the taper angle of the side surface of the opening 100a, and the thickness of the substrate 100. Therefore, in the normal direction of the membrane 10, each heat conductive member 50 can be arranged so as to overlap the entire side surface of the opening 100a from the upper end 100b to the lower end 100c.
Then, if the heat conductive member 50 is formed together with the heater 20, the upstream temperature sensor 30, and the downstream temperature sensor 40 as a part of the pattern layer 103 as in the present embodiment, these components can be formed without misalignment. Therefore, the distance from the heater 20 to the heat conductive member 50 can be set without error, and it is possible to further suppress the variation in the amount of heat conduction between the upstream and downstream of the fluid flow through the heater 20.
(Modification of First Embodiment)
In the first embodiment, the case where the side surface of the opening 100a is tapered is described as an example. However, when the opening 100a is formed by dry etching, as shown in
Therefore, the width W1 of the heat conductive member 50 may be set in consideration with the formation error of the heat conductive member 50 due to the mask deviation when patterning the pattern layer 103 and the mask deviation when forming the opening 100a, and the formation error of the width W3.
Further, here, the heat conductive member 50 is configured as a part of the pattern layer 103 that constitutes the heater 20, the upstream temperature sensor 30, and the downstream temperature sensor 40. However, this configuration is only an example, and the heat conductive member 50 may be made of a material different from the pattern layer 103. For example, the pattern layer 103 may be made of Pt, and the heat conductive member 50 may be made of another material such as Mo.
Further, regardless of whether or not the heat conductive member 50 is formed as a part of the pattern layer 103, the thickness of the heat conductive member 50 is set to be different from that of the heater 20, the upstream temperature sensor 30, and the downstream temperature sensor 40. For example, it is preferable that the heat conductive member 50 is thicker than the heater 20, the upstream temperature sensor 30, and the downstream temperature sensor 40 because the amount of heat conduction can be increased. Such a configuration can be realized by, for example, forming the heat conductive member 50 as a part of the pattern layer 103, and further stacking the materials of the heat conductive member 50 by using a mask that opens only the part of the heat conductive member 50. Further, when the heat conductive member 50 is made of a material different from the pattern layer 103, the thickness of the material may be made thicker than that of the pattern layer 103 from the beginning.
A second embodiment will be described. In the present embodiment, the layout of the heat conductive member 50 is changed with respect to the first embodiment, and the other parts are the same as those in the first embodiment. Therefore, the parts different from the first embodiment will be mainly described.
As shown in
With such a configuration, when etching the opening 100a, the width W3 of the membrane 10 can be confirmed by transmitting from the upper surface side of the membrane 10 using an optical microscope, an electron microscope, or the like. For example, when the optical microscope is used, when light is irradiated from the substrate 100 side, the method of transmitting light differs between the membrane 10 and its surroundings, so that the width W3 can be confirmed based on this way.
Therefore, if the width W3 of the membrane 10 can be set to a desired value by controlling the etching conditions of the opening 100a, but the time constant of heat conduction does not reach the desired value, the etching amount can be adjusted by confirming the width W3. As a result, the time constant of heat conduction can be corrected to a desired value, and the flow rate of the fluid can be detected more accurately.
(Modification of Second Embodiment)
In the second embodiment, the heat conductive member 50 is arranged only at the inner part separated from both ends of the sides 11 and 12 of the membrane 10 by a predetermined distance. However, it is sufficient that at least a part of the sides 11 and 12 of the membrane 10 is not covered with the heat conductive member 50. For example, only two adjacent corners of the four corners of the membrane 10 in the fluid flow direction need not be covered with the heat conductive member 50. However, in the structure of the second embodiment, each heat conductive member 50 is axisymmetric with respect to the center line of the membrane 10 passing through the centers of the sides 11 and 12, and the heat conduction to the upstream temperature sensor and the downstream temperature sensor 40 can be made uniform.
Here, the length of the heat conductive member 50 in the orthogonal direction is arbitrary, but as shown in
A third embodiment will be described. In the present embodiment as well, as in the second embodiment, a part of the sides 11 and 12 of the membrane 10 can be confirmed, but the structure of the heat conductive member 50 for confirming the sides 11 and 12 is changed with respect to the second embodiment. Since the other configuration is the same as that of the first embodiment, only portions different from those of the first embodiment will be described.
As shown in
In this way, even if the recess 50a in which a part of the heat conductive member 50 is recessed is formed, the same effect as that of the second embodiment can be obtained. The recess 50a of the heat conductive member 50 may be formed so as to recess on the side of the upstream temperature sensor 30 and the downstream temperature sensor 40, and the above effect can be obtained. However, if the recess 50a of the heat conductive member 50 is formed on the opposite side of the upstream temperature sensor 30 and the downstream temperature sensor 40 as in the present embodiment, the side of the heat conductive member 50 on the side of the upstream temperature sensor 30 and the downstream temperature sensor 40 can be made linear. Therefore, it is possible to make the heat conduction uniform over the entire area in the orthogonal direction, and it is possible to measure the flow rate of the fluid more accurately.
The location of the recess 50a is arbitrary, but in the present embodiment, the recess 50a is formed at a center position of the membrane 10 in the orthogonal direction, that is, on the center line of the membrane 10. The portion located on the center line of the membrane 10 of the upstream temperature sensor 30 and the downstream temperature sensor 40 is a portion that particularly contributes to temperature measurement. Further, in this portion, the variation of the width W3 due to etching is most likely to occur. Therefore, by measuring the width W3 of the membrane 10 in this portion, the width W3 can be measured in the portion that further contributes to temperature measurement and in which the etching variation is reflected. This makes it possible to measure the flow rate of the fluid more accurately.
A fourth embodiment will be described. In the present embodiment, the configuration of the heat conductive member 50 is changed from that of the first embodiment, and the remaining configurations are the same as those of the first embodiment, and therefore, only portions different from the first embodiment will be described.
As shown in
As described above, even if the heat conductive member 50 is formed so as to cover the entire sides 11 and 12, if the heat conductive member 50 is made of the translucent material, the width W3 can be confirmed from above the heat conductive member 50. Even in this way, it is possible to obtain the same effect as that of the second embodiment.
A fifth embodiment will be described. In the present embodiment, the configuration of the heat conductive member 50 is changed from that of the first to fourth embodiments, and the remaining configurations are the same as those of the first to fourth embodiments, and therefore, only portions different from the first to fourth embodiments will be described. Here, in the present embodiment, the case where the shape of the heat conductive member 50 is the one of the first embodiment will be described as an example, but the shape of the heat conductive members 50 of the second to fourth embodiments may be used.
As shown in
Although the present disclosure has been described in accordance with the above-described embodiments, the present disclosure is not limited to the above-described embodiments, and encompasses various modifications and variations within the scope of equivalents. In addition, while various combinations and configurations, which are preferred, other combinations and configurations including further only a single element, more or less, are also within the spirit and scope of the present disclosure.
For example,
Further, in the second embodiment, the recess 50a is formed as the opening that allow the sides 11 and 12 to protrude from the heat conductive member 50 in order to confirm the width W3, but the opening has another shape. For example, as shown in
Further, in each of the above embodiments, an example is given in which the openings 100a forming the two opposite sides 11 and 12 have a quadrangular shape. However, this is also only an example, and the opening 100a may have another shape, for example, a polygonal shape. In that case, the upstream temperature sensor 30 and the downstream temperature sensor 40 interpose the heater 20 and they are arranged on both sides of the heater 20 between two opposite sides of the polygonal shape.
The constituent element(s) of each of the above embodiments is/are not necessarily essential unless it is specifically stated that the constituent element(s) is/are essential in the above embodiment, or unless the constituent element(s) is/are obviously essential in principle. A quantity, a value, an amount, a range, or the like referred to in the description of the embodiments described above is not necessarily limited to such a specific value, amount, range or the like unless it is specifically described as essential or understood as being essential in principle. Furthermore, a shape, positional relationship or the like of a structural element, which is referred to in the embodiments described above, is not limited to such a shape, positional relationship or the like, unless it is specifically described or obviously necessary to be limited in principle.
Number | Date | Country | Kind |
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2019-127165 | Jul 2019 | JP | national |
This application is a continuation application of International Patent Application No. PCT/JP2020/026553 filed on Jul. 7, 2020, which designated the U.S. and based on and claims the benefits of priority of Japanese Patent Application No. 2019-127165 filed on Jul. 8, 2019. The entire disclosure of all of the above applications is incorporated herein by reference.
Number | Date | Country | |
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Parent | PCT/JP2020/026553 | Jul 2020 | US |
Child | 17565049 | US |