JP3683868B2 - Thermal flow sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a flow sensor used for measuring the flow velocity or flow rate of a fluid flowing in a flow path, and more particularly to a thermal flow sensor.
[0002]
[Prior art]
A thermal flow sensor that measures the flow rate and flow rate of a fluid is installed with a sensor chip equipped with a flow rate detection means parallel to the flow of the fluid to be measured in the pipe. The spatial temperature distribution of the fluid due to the generated heat is biased by the flow, and this is detected by the temperature sensor (indirect heating type), or the change in electric power and resistance due to the heat being taken away by the fluid By detecting the change (self-heating type), the flow velocity or flow rate is measured.
[0003]
9A and 9B are a front view and a sectional view of a conventional thermal flow sensor. The thermal flow sensor 1 includes a flow path forming member 4 that forms a flow path 3 of a fluid 2, a substrate 5 having an outer peripheral edge portion 5a bonded to a front opening 4a of the flow path forming member 4, And a plate 6 fixed to the surface of the substrate 5 by being pressed (bolted) with bolts or the like through an electric insulating film 13, and the central portion of the substrate 5 forms a diaphragm portion 5A to constitute a flow rate detecting means. The body, the two resistors (temperature sensor), and the circuit pattern 7 are formed by a well-known thin film forming technique. The substrate 5 is formed in a thin shape, and the back surface thereof is in contact with the fluid 2 to form a part of the flow path 3 together with the flow path forming member 4. As the material of the flow path forming member 4 and the substrate 5, a material having low heat conductivity and excellent heat resistance and corrosion resistance, for example, SUS304 and SUS316 series stainless steel is used.
[0004]
The plate 6 has a through hole 8 having a size substantially the same as that of the diaphragm portion 5 </ b> A in the center, and an electrode 9 is incorporated in the through hole 8. As the electrode 9, a metal frame 10 having a plurality of terminal pins 11 sealed with hermetic glass 12 is used, and one end of each terminal pin 11 is connected to the circuit pattern 7 by brazing or soldering. .
[0005]
[Problems to be solved by the invention]
However, since the above-described conventional thermal flow sensor 1 is simply bonded to the surface of the thin substrate 5 by fastening the plate 6 with a bolt, the mechanical and thermal connection between the substrate 5 and the plate 6 is achieved. The contact is uncertain and unstable, and the temperature distribution of the diaphragm portion 5A is unstable. Therefore, when the diaphragm portion 5A of the substrate 5 is elastically deformed in the surface direction along with the pressure change of the fluid 2, the contact state between the substrate 5 and the plate 6 changes, and the temperature distribution of the diaphragm portion 5A changes. There was a problem that the flow characteristics and the zero point shifted, and accuracy, reproducibility, reliability and durability were lacking. That is, when the pressure in the flow path 3 becomes negative, the substrate 5 is elastically deformed toward the flow path 3 and only the outer peripheral edge portion 5a is in contact with the plate 6. On the contrary, when the pressure in the flow path 3 is increased, the diaphragm portion Since only 5A is elastically deformed to the sensor side, the outer portion from the diaphragm portion 5A, that is, the entire outer portion from the through hole 8 of the substrate 5 is in contact with the plate 6, and the contact area (heat conduction path) with the plate 6 changes. . When the fixed portion of the substrate 5 changes due to pressure fluctuations in this way, the temperature distribution of the substrate 5 also changes greatly, resulting in a measurement error.
In addition, the number of components such as the plate 6 and the means for pressing the substrate 5 and the plate 6 increases, and the shape is large and complicated.
[0006]
The present invention has been made in order to solve the above-described conventional problems. The object of the present invention is to reduce the change in flow velocity or flow rate characteristics due to a change in fluid pressure, thereby reducing accuracy, reproducibility, reliability, An object of the present invention is to provide a thermal flow sensor capable of improving durability and reducing the number of components.
Also, heat that can respond to practical needs such as semiconductor manufacturing equipment that wants to perform zero point adjustment (correction) when the flow path is under negative pressure or vacuum and perform flow measurement under pressure It is in providing a flow sensor.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, a first invention is a thermal flow sensor comprising a substrate that forms a flow path of a fluid to be measured together with a flow path forming member, and the substrate includes a thin portion and surrounds the thin portion. a fixing portion of the thick formed integrally, the channel side of the thin portion is provided with a flow rate detection means on the opposite side.
[0008]
In the first invention, since the substrate is integrally formed of the thick fixed portion and the thin portion, the position of the fixed end of the thin portion does not change even if the thin portion is elastically deformed by a change in fluid pressure. .
[0009]
According to a second invention, in the first invention, the flow path forming member and the substrate are integrally formed.
[0010]
In the second invention, since the flow path forming member and the substrate are formed by one member, the fluid does not leak and the number of parts can be reduced.
[0011]
According to a third aspect of the present invention, a sensor mounting hole is formed in a pipe through which a fluid to be measured flows, and a substrate that covers the sensor mounting hole is disposed so as to contact the fluid to be measured. parts are integrally formed with the fixed portion of the thick surrounding the, and the surface contacting the fluid to be measured of the thin portion is provided with a flow rate detection means on the opposite side.
[0012]
In the third invention, since the substrate is directly attached to the sensor attachment hole of the pipe, the flow path forming member becomes unnecessary. Further, since it can be easily attached to a large-diameter pipe, a large flow rate can be measured.
[0013]
According to a fourth invention, in the first, second, or third invention, the substrate is formed of any one of stainless steel, sapphire, and ceramics.
[0014]
In the fourth invention, stainless steel, sapphire or ceramics is used as the substrate material. Since stainless steel is a conductive material, an electrical insulating film is formed, and sapphire and ceramics are insulating materials, so that is not necessary.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings.
FIG. 1 is a cross-sectional view showing a first embodiment of a thermal flow sensor according to the present invention, and FIG. 2 is a front view of a sensor unit. In these drawings, the thermal flow sensor generally indicated by reference numeral 20 is composed of a sensor chip 21 and a flow path forming member 22 that forms the flow path 3 of the fluid 2 together with the sensor chip 21.
[0016]
The sensor chip 21 includes a substrate 24 and a flow velocity detection means 25 formed at the center of the surface of the substrate 24.
[0017]
The substrate 24 is formed in a thin and elongated rectangular plate shape, the outer peripheral edge portion forms a thick fixing portion 24A, and is bonded to the surface of the flow path forming member 22. The central portion of the substrate 24 forms a thin portion 24B by forming an oval concave portion 26 on the back side. The thin part 24B has a film thickness of about 50 to 150 μm and forms a sensor part having a diaphragm structure. In addition, the length (width) of the flow direction (arrow A direction) and the perpendicular direction (short direction) of the thin portion 24B is preferably about 1 to 3 mm in terms of strength (pressure resistance). Moreover, although the recessed part 26 was made into the ellipse, not only this but circular, a rectangle, etc. may be sufficient.
[0018]
As the material of the substrate 24, a material having a lower thermal conductivity than silicon and having high heat resistance, corrosion resistance and rigidity, such as stainless steel, sapphire, or ceramics, is used. In this case, in the present embodiment, an example is shown in which the substrate 24 is formed of a thin plate of stainless steel (particularly SUS316L) having a plate thickness of about 0.5 to 3 mm.
[0019]
When the substrate 24 is made of stainless steel, it is not preferable that the thickness of the thin portion 24B constituting the sensor portion is 50 μm or less because the strength is lowered. Further, when the thickness is 150 μm or more, the heat conduction efficiency in the direction parallel to the surface of the substrate 24 is reduced while the heat conduction efficiency between the thickness direction of the substrate 24, that is, between the fluid 2 and the flow velocity detection means 25 is decreased. Loss) increases, which is not preferable. Note that the fixing portion 24A of the substrate 24 plays a role of maintaining the shape of the thin portion 24B and a heat sink.
[0020]
The concave portion 26 of the substrate 24 is formed by a photolithography technique and an etching technique, an end mill, or a composite technique thereof. In the case of photolithography and etching techniques, first, a resist is applied to the entire back surface of a stainless steel wafer by spin coating or the like, or a resist film is pasted, and ultraviolet (or electron beam) is irradiated to form a mask pattern on the resist. Is transferred and exposed. Next, the exposed resist is developed with a developer, and unnecessary portions of the resist are removed. A negative resist or a positive resist is selected depending on whether the exposed portion is left or removed. The wafer is exposed at the portion where the resist is removed, and the exposed portion is removed by wet etching or dry etching until the thickness becomes about 50 to 150 μm. Then, when the remaining resist is peeled off, removed, and washed, the thin portion 24B and the concave portion 26 are formed. In the case of wet etching, it is immersed or sprayed in an etching solution and gradually dissolved. In the case of dry etching, it can be formed by irradiating the back surface of the wafer with ions or electrons by sputtering, plasma, or the like and scraping it little by little. In the case where the substrate 24 is made of ceramics, the substrate 24 may be manufactured by firing with the concave portions 26 from the beginning.
[0021]
The surface (surface) opposite to the fluid 2 side of the thin portion 24B is mirror-polished, and the electrical insulating film 13 is formed over the entire surface. Further, the flow rate detecting means 25 and the ambient temperature detecting means 34 including the plurality of electrode pads 30 (30a to 30f) and the wiring metal thin film 31 are formed on the surface of the electrical insulating film 13 by a known thin film forming technique. ing. For example, it is formed by depositing a material such as platinum on the surface of the electrical insulating film 13 and etching it into a predetermined pattern. The flow velocity detecting means 25 and the ambient temperature detecting means 34 are connected to the electrode pad 30 via the metal thin film 31 for wiring. Are each electrically connected. Further, each electrode pad 30 is connected to an electrode terminal of a printed wiring board 35 provided above the substrate 24 via a spacer 36 via a bonding wire (not shown).
[0022]
The electrical insulating film 13 is formed of, for example, a thin silicon oxide (SiO ₂ ) film, silicon nitride film, alumina film, polyimide film or the like having a thickness of about several thousand angstroms to several microns. The silicon oxide film can be formed by, for example, sputtering, CVD, or SOG (spin on glass). The silicon nitride film can be formed by sputtering, CVD, or the like.
[0023]
The flow velocity detection means 25 and the ambient temperature detection means 34 will be described in more detail with reference to FIG. 2. The flow velocity detection means 25 includes one heating element 32 (resistance heater) and two temperature sensors 33A and 33B. A thermal flow rate detecting means is formed. The heating element 32 is formed so as to be positioned at the approximate center of the thin portion 24B. The two temperature sensors 33A and 33B are formed so as to be respectively located on the upstream side and the downstream side in the flow direction of the fluid 2 with the heating element 32 interposed therebetween.
[0024]
The ambient temperature detection means 34 is used to compensate for a change in ambient temperature, that is, the temperature of the fluid 2, and is further upstream than the upstream temperature sensor 33A near the outer periphery of the thin portion 24B. Is formed. The pattern width of the heating element 32 is preferably 10 to 50 μm, and the pattern widths of the temperature sensors 33A and 33B and the ambient temperature detecting means 34 are preferably about 5 to 20 μm. When the ambient temperature detection unit 34 is affected by heat from the heating element 32, the ambient temperature detection unit 34 is not a thin portion 24B of the substrate 24, but a thick portion (fixed portion 24A) or the like. It is formed in another location that is optimal for detecting ambient temperature. An external temperature sensor may be used instead.
[0025]
The flow path forming member 22 is made of a long and narrow stainless steel metal plate, like the substrate 24, and has a convex portion 22A projecting from the center of the surface substantially equal to the substrate 24 and two through holes 40 and 41. The fixing portion 24A of the substrate 24 is joined to the upper surface of the convex portion 22A, and the through holes 40 and 41 and the concave portion 26 of the substrate 24 communicate with each other to form the flow path 3 of the fluid 2. . The shape of the flow path 3 may not be oval in the concave portion 26, but a shape in which the flow direction of the fluid 2 is clear and flows smoothly is preferable. When such a flow path forming member 22 is formed of stainless steel, which is the same material as the substrate 24, both members can be welded by using YAG laser welding or the like without using different metals. The flow path forming member 22 may be aluminum, ceramics, or the like. In this case, the O-ring and bolts are used for joining. Of course, even when the flow path forming member 22 is made of stainless steel, it may be joined using an O-ring and a bolt.
[0026]
FIG. 3 is a diagram showing a constant temperature difference circuit of the thermal flow sensor 20. In the figure, a heating element 32, an ambient temperature detecting means 34 and three fixed resistors R1, R2, R3 form a bridge circuit, and this and an operational amplifier (OP1) constitute a constant temperature difference circuit. OP1 takes the midpoint voltage of the bridge circuit, resistor R1 and heating element 32 as an inverting input, and the midpoint voltage of resistors R2 and R3 as a non-inverting input. The output of OP1 is commonly connected to one ends of resistors R1 and R2. The resistance values of the resistors R1, R2, and R3 are set so that the heating element 32 is always higher than the ambient temperature sensor 34 by a constant temperature.
[0027]
FIG. 4 is a diagram showing a sensor output circuit of the thermal flow sensor 20. In the figure, two temperature sensors 33A and 33B and two fixed resistors R4 and R5 form a bridge circuit, and this and OP2 constitute a sensor output circuit.
[0028]
In such a thermal flow sensor 20, the fluid 2 is heated to a certain higher temperature than the ambient temperature by energizing the bridge circuit of the constant temperature difference circuit shown in FIG. When flowing in the direction, the thin portion 24B is deprived of heat by the fluid 2 in proportion to its flow velocity, so that the heating element 32 is also deprived of heat and the resistance value decreases. For this reason, although the equilibrium state of the bridge circuit is broken, a voltage corresponding to the voltage generated between the inverting input and the non-inverting input is applied to the bridge circuit by OP1, so heat is generated so as to compensate for the heat taken away by the fluid 2. The calorific value of the body 32 increases. As a result, when the resistance value of the heating element 32 increases, the bridge circuit returns to the equilibrium state. Therefore, a voltage corresponding to the flow velocity is applied to the bridge circuit in an equilibrium state. As a method of using the constant temperature difference circuit of FIG. 3, if the sensor is shared by the heater, it is possible to output the voltage between the terminals of the heating element 32 as a voltage output among the voltages applied to the bridge circuit.
[0029]
When the temperature distribution in the vicinity of the heating element 32 is destroyed due to the flow of the fluid 2, a temperature difference is generated between the temperature sensor 33A located on the upstream side of the heating element 32 and the temperature sensor 33B located on the downstream side. The voltage difference or resistance value difference is detected by the sensor output circuit. The temperature difference between the two temperature sensors 33A and 33B is proportional to the flow velocity of the fluid 2. Therefore, if the relationship between the flow path cross-sectional average flow velocity or flow rate and the temperature difference, that is, the voltage difference or resistance value difference detected by the sensor output circuit is calibrated in advance, the actual flow rate is calculated from the voltage difference or resistance value difference. The road section average flow velocity or flow rate can be measured. In addition, the structure of the flow velocity detection means 25 and the ambient temperature detection means 34 is not limited to the above-described embodiment, and various changes can be made. In addition, the ambient temperature detection means 34 is arranged where the fluid temperature can be detected without being affected by the heat from the heating element 32.
[0030]
According to the thermal flow sensor 20 having such a structure, the outer peripheral portion of the substrate 24 is joined to the surface of the flow path forming member 22 as a thick fixed portion 24A, and the central portion is a thin-walled portion 24B having a diaphragm structure. Since the flow velocity detection means 25 and the ambient temperature detection means 34 are formed on the surface side where the fluid 2 of the thin portion 24B does not contact, there is no need to press the plate 6 shown in FIG. Even if the thin-walled portion 24B is elastically deformed due to the pressure change, there is no part to be peeled off. Therefore, compared to the conventional flow sensor 1 shown in FIG. Can be maintained. In particular, since the shift of the zero point is small, high measurement accuracy can be obtained, and the reliability and durability of the sensor can be improved.
[0031]
FIG. 5 is a cross-sectional view showing a second embodiment of the present invention, and FIG. 6 is a plan view of the substrate.
This embodiment is applied to a thermal flow sensor of a type called a header type. The header type thermal flow sensor 50 is inserted from the outside into a sensor mounting hole 52 provided in a pipe wall 51 through which the fluid 2 flows, and is fixed by welding or an O-ring and a bolt. A container is constituted by 54 and a printed circuit board 55 is accommodated therein.
[0032]
The bracket 53 is formed in a cylindrical shape whose both ends are opened by stainless steel, fitted into the sensor mounting hole 52 from the outside, and a flange 53A is joined to the outer peripheral surface of the pipe 51. On the other hand, the sensor chip 54 is joined to the inner end surface of the bracket 53, that is, the opening end surface opposite to the flange 53A side.
[0033]
The sensor chip 54 includes a substrate 56 formed of stainless steel or the like as in the above embodiment. The substrate 56 is joined to the inner end surface of the bracket 53, covers the sensor mounting hole 52 of the pipe 51 in an airtight manner, and first and second recesses 57a, 57b are formed on the surface 56a on the bracket 53 side. A surface 56 b opposite to the surface 56 a forms a contact surface with the fluid 2 flowing in the pipe 51. The portions of the substrate 56 where the concave portions 57a and 57b are formed form thin-walled portions 56B1 and 56B2 having a diaphragm structure, and other portions form a fixing portion 56A and are joined to the inner end surface of the bracket 53. Yes.
[0034]
The first recess 57a is formed in the approximate center of the substrate 56, and the second recess 57b is formed on the upstream side of the first recess 57a. Electrical insulating films 13 are formed on the bottom surfaces of the first and second recesses 57a and 57b, respectively, and the flow velocity detecting means 25 and the ambient temperature detecting means 34 are formed thereon. That is, in this embodiment, in order to prevent the heat generation of the heating element 32 (FIG. 1) of the flow velocity detection means 25 from affecting the ambient temperature detection means 34, two recesses 57a and 57b are provided. The flow velocity detection means 25 and the ambient temperature detection means 34 are separately arranged. Each of the recesses 57a and 57b is preferably a circle having a diameter of about 1 to 3 mm from the viewpoint of strength (pressure resistance), but may have other shapes.
[0035]
Such a sensor chip 54 is manufactured in the same manner as in the above-described embodiment. In this case, photolithography for forming a pattern on the surfaces of the thin-walled portions 56B1 and 56B2 located at the bottoms of the recesses 57a and 57b. For this, a projection exposure apparatus or a direct drawing apparatus is used. Alternatively, a resistor or conductor pattern is directly formed using a jet printing system. As a modification, one concave portion 57 may be formed at the center of the substrate 56 as shown in FIG. 7, and the ambient temperature detecting means 34 may be formed on the fixed portion 56A.
[0036]
It will be apparent that the thermal flow sensor 50 having such a structure can achieve the same effects as those of the first embodiment.
[0037]
FIGS. 8A and 8B are a cross-sectional view and a cross-sectional view taken along line AA showing a third embodiment of the present invention.
In this embodiment, the substrate constituting the sensor chip is formed by a stainless steel pipe 61, and the center hole of the pipe 61 is used as the flow path 3 of the fluid 2. For this reason, the flow path forming member 22 in the first embodiment described above is unnecessary, and the sensor chip itself also serves as the flow path forming member. In other words, the sensor chip of the thermal flow sensor 60 and the flow path forming member are integrally formed by the pipe 61. The pipe 61 is not limited to a circular cross section, and may be a non-circular shape such as a rectangle or an ellipse.
[0038]
The pipe 61 has a concave portion 64 formed at the center in the longitudinal direction of the outer peripheral surface, and a thin portion between the concave portion 64 and the inner periphery of the pipe 61 forms a thin portion 65. The concave portion 64 is formed by etching, machining such as an end mill or a press, or a composite technique thereof.
[0039]
The surface of the thin portion 65 opposite to the surface in contact with the fluid 2 is mirror-finished and covered with the electrical insulating film 13. Further, the flow velocity detecting means 25 and the ambient temperature detecting means 34 including the plurality of electrode pads 30 and the wiring metal thin film 31 shown in FIG. 2 are formed at the center of the surface of the electrical insulating film 13 by a known thin film forming technique. Has been. When the pipe 61 is an insulator such as ceramics, the above-described electrical insulating film 13 is not necessary. Further, the ambient temperature detection means 34 may be formed at a place optimal for temperature detection. An external sensor may be substituted.
[0040]
According to the thermal flow sensor 60 having such a structure, since one pipe 61 serves as both the flow path forming member and the sensor chip substrate, there is no junction and the fluid 2 does not leak. Since the number of points is small, a highly reliable thermal flow sensor can be manufactured.
[0041]
【The invention's effect】
As described above, according to the thermal type flow sensor of the present invention, the change in flow velocity or flow rate characteristic of the sensor chip due to the change in fluid pressure is small, and the measurement accuracy, reproducibility, reliability and durability of the sensor are improved. In addition, the number of parts can be reduced.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view showing a first embodiment of a thermal flow sensor according to the present invention.
FIG. 2 is a front view of a sensor unit.
FIG. 3 is a diagram showing a constant temperature difference circuit of a thermal type flow sensor.
FIG. 4 is a diagram showing a sensor output circuit.
FIG. 5 is a cross-sectional view showing a second embodiment of the present invention.
FIG. 6 is a plan view of a substrate.
FIG. 7 is a cross-sectional view showing another embodiment of a sensor chip.
FIGS. 8A and 8B are a cross-sectional view and a cross-sectional view taken along line AA showing a third embodiment of the present invention. FIGS.
FIG. 9 is a cross-sectional view showing a conventional example of a thermal flow sensor.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 2 ... Fluid, 3 ... Channel, 13 ... Electrical insulating film, 20 ... Thermal type flow sensor, 21 ... Sensor chip, 22 ... Channel formation member, 24 ... Substrate, 24A ... Fixed part, 24B ... Thin part, 25 ... Flow rate detecting means, 26 ... concave portion, 30 ... electrode pad, 31 ... metal thin film for wiring, 32 ... heating element (heater), 33A, 33B ... temperature sensor, 34 ... ambient temperature detecting means, 50 ... header type thermal flow sensor 51 ... Piping wall, 60 ... thermal flow sensor, 61 ... pipe.

Claims (4)

In a thermal flow sensor comprising a substrate that forms a flow path of a fluid to be measured together with a flow path forming member,
The substrate, the heat and the thin portion, characterized in that the the fixing portion of the thick surrounding the thin portion formed integrally, and the flow path side of the thin portion provided with the flow rate detection means on the opposite side Type flow sensor. The thermal flow sensor according to claim 1,
A thermal flow sensor, wherein a flow path forming member and a substrate are integrally formed. A sensor mounting hole is formed in a pipe through which the fluid to be measured flows, and a substrate that covers the sensor mounting hole is disposed so as to contact the fluid to be measured, and the substrate has a thin portion and a thick wall that surrounds the thin portion. a fixing portion formed integrally, the thermal type flow sensor, characterized in that a flow rate detection means on the opposite side to the surface contacting the fluid to be measured of the thin portion. The thermal flow sensor according to claim 1, 2, or 3,
A thermal flow sensor characterized in that the substrate is formed of any one of stainless steel, sapphire, and ceramics.

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