JPH109924A - Thermal air flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thermal air flowmeter in which the dynamic range is widened while reducing noise and the reliability of dustproofness is enhanced by providing a pair of heatting elements, respectively, on the upstream side and the downstream side of a temperature measuring element. SOLUTION: In a measuring element 1, upper surface of a semiconductor substrate 2 is coated with an electric insulation film which closes the upper surface side of a cavity 3. A pair of heating elements 5, 6 are arranged, respectively, on the opposite sides of a temperature measuring element 4 in the upstream and downstream of an air flow. The heating elements 5, 6 are fed with currents for heating up the temperature measuring element 4 to a temperature higher by a predetermined level than the temperature of an air temperature measuring element 7. In order to control the temperature measuring element 4 to have a temperature higher by a predetermined level than the temperature of an air temperature measuring element 7, the air flow rate is measured by taking advantage of the fact that the heating current to be fed to the heating elements 5, 6 increase when the air flow rate increases. Direction of the air flow 9 is also detected from the temperature difference between the heating elements 5, 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal air flow meter, and more particularly to a thermal air flow meter suitable for measuring an intake air amount of an internal combustion engine.

2. Description of the Related Art Conventionally, as an air flow meter which is provided in an electronically controlled fuel injection device of an internal combustion engine of an automobile or the like and measures an intake air amount, a thermal air flow meter has become mainstream since it can directly detect a mass air amount. I have. Among them, an air flow meter manufactured by a semiconductor micromachining technique has been particularly noted because it can reduce the cost and can be driven with low power. Such a conventional thermal air flow meter using a semiconductor substrate is disclosed in, for example, Japanese Patent Application Laid-Open No. 60-14 / 1985.
No. 2,268, and JP-A-7-174600. Although the technology described in the above-mentioned publication reduces manufacturing costs to some extent, it has problems such as a narrow flow rate measurement range, large noise, and insufficient dust resistance reliability when measuring the amount of intake air.

The prior art has the following problems. The prior art described in Japanese Patent Application Laid-Open No. Sho 60-142268 will be described with reference to FIG. FIG. 10 is a plan view of a measuring element of a conventional thermal air flow meter, and is a second diagram described in the publication. In the figure, reference numeral 1 denotes a measuring element of a thermal air flow meter, and two bridges 24a, 24a, made of an electrically insulating film bridging a cavity (23a, b, c) formed by anisotropically etching a semiconductor substrate such as silicon. 24b,
The upstream side of the air flow is the bridge 24a, and the downstream side is the bridge 24b. The heating resistor 5 is arranged with the open cavity 23c between the two bridges 24a and 24b, and these bridges 24
a and 24b are provided on the side of the heating resistor 5 respectively.
a and 4b are arranged, and an air temperature measuring resistor 7 for measuring the air temperature is arranged in a part of the electrical insulating film on the upstream side of the cavity 23a. Also, the cavities 23a, 23b and 23
c is anisotropic etching of the semiconductor substrate using the opening of the electric insulating film, so that the bridges 24a and 24 of the electric insulating film are formed.
It is a continuous integral cavity below b. In this air flow meter, the heating resistor 5 is set to a temperature higher than the air temperature determined by the air temperature measuring resistor 7 by a certain temperature.
Is driven by heating. The air flow rate is measured from the temperature difference generated between the upstream temperature measuring element 4b and the downstream temperature measuring element 4a of the flow channel by utilizing the heat transport effect of air. FIG. 11 is a diagram for explaining another example of the conventional thermal air flow meter, and is a plan view of the measuring element shown in FIG. 1 of the above-mentioned Japanese Patent Application Laid-Open No. 7-174600. The measuring element 1 is formed by forming an electric insulating film 10 on a semiconductor substrate 2 in the same manner as in the above-described conventional example, and forming the electric
d, 23e are etched, and the opening 2
The semiconductor substrate 2 is anisotropically etched from 3d and 23e to form cavities 23d and 23e. These cavities 23d, 23
e forms a continuous integral cavity under the bridge 24 of the electric insulating film in the same manner as the conventional example. In this conventional example, a heating resistor 5 and a temperature measuring resistor 4 close to the heating resistor 5 are arranged on the bridge 24 on the upstream side of the air flow. Installed upstream. In this air flow meter, the heating resistor 5 is heated (indirectly heated) so that the temperature of the temperature measuring resistor 4 becomes higher than the air temperature detected by the air temperature measuring resistor 7 by a certain temperature. The air flow rate is measured from a heating current flowing through a heating resistor 5 that indirectly heats the temperature measuring resistor 4 that is cooled as the air flow rate increases. In the conventional example configured as described above, for example, in the measuring element of FIG. 10, the item (00) of the specification of Japanese Patent Application Laid-Open No. 7-174600 is used.
As described in 12), there is a problem in that the output change is small in a large air flow rate region, the linearity is deteriorated, and the measurable flow speed range is narrowed. Figure 1 shows the improvement.
However, in this conventional example, there is a problem that the direction of the air flow cannot be detected. Furthermore,
As a problem common to both the conventional examples of FIGS. 10 and 11,
Output noise is large due to a small effective detection area (in this conventional example, an area in contact with the airflow of the resistance temperature detectors 4, 4a, 4b and the heating resistor 5) which is in contact with the airflow which is important in measuring the airflow. That the cavities 23a, 23b, 23c, 23d,
When the opening 23e is open and used under severe conditions such as an automobile, dust and the like accumulate in the opening, and highly reliable measurement cannot be performed over a long period of time. Accordingly, it is an object of the present invention to solve the problems of the prior art and to provide a thermal air flow meter having a wide dynamic range of flow measurement, low noise, and high dust-proof reliability.

A thermal air flow meter which achieves the above object has a semiconductor substrate having a cavity whose upper surface is closed by an electrical insulating film and which is open at the lower surface, and a portion of the cavity above the cavity. A temperature measuring resistor formed on the electric insulating film and a pair of heating resistors distributed to the upstream and downstream sides of the fluid to be measured in the temperature measuring resistor, and a portion other than on the cavity portion; A measuring element having a fluid temperature measuring resistor formed on an electric insulating film; and heating the heating resistors so as to keep a temperature difference between the temperature measuring resistor and the fluid temperature measuring resistor constant. Heating control means for flowing an electric current, direction detecting means for detecting a flow direction of the fluid to be measured based on a temperature difference between the heating resistors, and a temperature difference between the heating resistors or flowing to the heating resistors. A flow rate detector for detecting a flow rate of the fluid to be measured based on a current value. And it constitutes and means. According to the present invention, one temperature measuring resistor is sandwiched between a pair of heating resistors, and the temperature difference between the pair of heating resistors can be detected largely using the temperature of the temperature measuring resistor as a reference temperature. The dynamic range and noise immunity during flow rate measurement are improved.

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a plan view showing a measuring element (for a thermal air flow meter) of one embodiment according to the present invention. 2 shows a measuring element used in a thermal air flow meter. FIG. 2 is a diagram showing a cross section taken along line AA ′ of FIG.
1 and 2, the measuring element 1
Are a semiconductor substrate 2 made of silicon or the like;
An electric insulating film 10a formed on the upper surface of the substrate and covering the upper surface side of a cavity 3 described later, and the temperature measuring resistor 4 and the temperature measuring resistor 4 formed on the electric insulating film 10a. 4
A pair of heating resistors 5 and 6, which are arranged adjacent to and adjacent to and adjacent to and adjacent to the air flow 9 and arranged upstream and downstream with respect to the air flow 9, for measuring the air temperature. 7, a terminal electrode 8 for extracting a signal from each resistor as the measuring element 1 to an external circuit, an electric insulating film 10b formed to protect each resistor, the terminal electrode 8, and the like; A portion where the heating resistor 4 and the heating resistors 5 and 6 are formed is cut out from the lower surface side (opening 3a) of the semiconductor substrate 2 to the boundary surface of the electric insulating film 10a by anisotropic etching. And a cavity 3 provided. Here, the pair of heating resistors 5 and 6 are electrically connected in series on the electrical insulating film 10a and are drawn out from two terminal electrodes 8 (,) and an intermediate terminal drawn out from the intermediate connection point M. And a terminal electrode 8 () as an electrode. That is, in FIG. 1, the heating resistors 5 and 6 are connected in series, and the terminal electrode 8 () is drawn out from the middle (middle connection point M). However, depending on the case, although there is the complexity that one terminal electrode 8 increases by one, it is also possible to take out the terminal electrodes 8 independently and connect them in series by an external circuit. A pair of the heating resistors 5 and 6 formed on the electric insulating film 10a having a large area (effective detection area) in contact with the air flow 9 has a temperature of the temperature measuring resistor 4 in a flow path of the air flow 9. A heating (indirect heat) current is supplied so that the temperature becomes higher than the temperature of the air temperature measuring resistor 7 disposed at the tip by a certain temperature. The direction of the air flow 9 is distributed to the upstream and downstream sides of the fluid to be measured, with the resistance bulb 4 formed at a position on the cavity 3 and substantially at the center on the electric insulating film 10a as the center. It is detected by comparing each temperature (each resistance value corresponding to the temperature) of the heating resistors 5 and 6, which are preferably formed symmetrically with respect to the temperature measuring resistor 4. That is, when the airflow of the fluid to be measured is zero, the heating resistors 5 and 6 exhibit substantially the same temperature as the temperature of the temperature measuring resistor 4, and no temperature difference occurs. When there is an airflow, in the direction (forward flow) of the airflow 9 in FIG. 1, the heating resistor 5 arranged mainly on the upstream side has a larger airflow 9 than the heating resistor 6 arranged on the downstream side.
In addition, since the heating effect is large, the heating resistors 5 and 6 are connected in series, and the same heating current flows, and both heating values are almost the same.
Is lower than the temperature of the heating resistor 6. Also,
When the airflow is opposite to the direction of the arrow in FIG. 1 (reverse flow), the temperature of the heating resistor 6 on the downstream side is lower than the temperature of the heating resistor 5 on the upstream side. Thus, the heating resistors 5, 6
By comparing each temperature (each resistance value), the direction of the air flow 9 can be detected. On the other hand, the measurement of the air flow rate measures the direction of the air flow and the air flow rate at the same time based on the temperature difference, utilizing the fact that the temperature difference between the heating resistors 5 and 6 increases with an increase in the air flow rate. Alternatively, in order to control the temperature measuring resistor 4 to be higher than the air temperature measuring resistor 7 by a certain temperature, the heating (indirect heat) current value flowing through the heating resistors 5 and 6 increases with an increase in the air flow rate. Measure using. By the way, the heating resistors 5 and 6 are formed on the electric insulating film 10 a on the cavity 3.
The heating resistor area (area of two heating resistors) of the present embodiment is larger than the heating resistor area of the conventional example (area of one heating resistor). Since the configuration has a (large detection effective area), a large air flow rate signal can be taken out and a wide dynamic range can be taken. And,
Since the output becomes an average with respect to the local turbulence of the air flow, in other words, the noise ratio (N / S) to the output becomes small, so that the configuration is strong against noise. Further, as shown in FIG. 1, the air temperature measuring resistor 7 is located at the tip of the substrate 2 and is arranged so as to protrude into the flow path of the air flow 9. In this case, the heaters 5 and 6 are provided at positions not affected by the heated airflow, and the measurement of the airflow can be performed with high accuracy. FIG. 3 is a sectional view showing a thermal air flow meter according to an embodiment of the present invention, on which the measuring element of FIG. 1 is mounted.
For example, it is a cross-sectional view showing an embodiment of a thermal air flow meter mounted in an intake passage of an internal combustion engine such as an automobile. As shown in the figure, the thermal air flow meter includes a measuring element 1, a support 13, and an external circuit 14. Then, the measuring element 1 is arranged in the sub-passage 12 inside the intake passage 11. External circuit 1
4 is electrically connected to the terminal electrode 8 of the measuring element 1 via the support 13. Here, normally, the intake air as the fluid to be measured flows in the direction indicated by the airflow 9,
Depending on the conditions of a certain internal combustion engine, the intake air flows in the direction opposite to the air flow 9 (reverse flow). FIG. 4 is an enlarged view showing the measuring element unit of FIG. Measuring element 1 and support 13 of FIG.
FIG. FIG. 5 is a view showing a BB ′ section of FIG. 4. FIG. 6 is a diagram showing a cross section taken along line CC ′ of FIG.
4 to 6, the measuring element 1 has a support 1 so that the front and back surfaces of the air temperature measuring resistor 7 are directly exposed to the air flow 9.
3b, and an external circuit 14 having a terminal electrode 15 and a signal processing circuit and formed on an electrically insulating substrate such as alumina is similarly fixed on the support 13b. The measurement element 1 and the external circuit 14 are electrically connected between the terminal electrodes 8 and 15 by wire bonding with a gold wire 16 or the like, and then the gold wire 16 and the electrode terminals 8 and 15 are connected.
And the external circuit 14 is hermetically protected by the support 13a. In the measuring element 1 thus mounted,
As shown in FIGS. 4 to 6, the lower surface of the cavity 3
And the upper surface is electrically insulating film 1
By 0, it is substantially isolated (closed) from the air flow 9. Therefore, when the present embodiment as described above is employed, there is no portion where the cavity 3 is open to the air flow 9 as in the conventional example, and therefore, when measuring the air flow rate of the internal combustion engine of an automobile or the like, Dust and the like that cause a problem do not accumulate in the cavity or the opening, and highly reliable measurement can be performed.
Further, in an internal combustion engine such as an automobile, the temperature of the intake passage 11 and the support 13 shown in FIG. 3 rises due to the heat of the internal combustion engine, and this heat is transmitted to the measuring element 1, causing an error in the measurement of the air flow rate. Temperature characteristics may be deteriorated.
On the other hand, in the present embodiment, as shown in FIG. 4, the air temperature measuring resistor 7 is provided at a position farthest from the support 13 and further protrudes from the support 13. . With this arrangement, both the front and back surfaces are exposed by the air flow 9 and sufficient heat radiation is achieved, so that the configuration having excellent temperature characteristics which is hardly affected by the rise in temperature of the intake passage 11 and the support 13 is provided. It has become. Furthermore,
As shown in the BB ′ cross-sectional view of FIG. 5, by making the tip shape of the support 13 b streamlined with respect to the airflow 9,
Even at the position where the air flow 9 reaches the measuring element 1, since the air flow flows uniformly without any disturbance, measurement with less noise is possible. Next, the operation of the thermal air flow meter according to the present invention will be described with reference to FIGS. 7, 8 and 9. FIG. 7 is a circuit diagram showing a thermal air flow meter according to an embodiment of the present invention. Each of the resistors 4, 5, shown in FIG.
1 shows the configuration of a measuring element 1 composed of 6, 7 or the like, and an external circuit 14 (a circuit as a heating control means, a direction detecting means, and a flow rate detecting means) for processing a signal from the measuring element 1. . In the figure, 17a, 17b, 17c, 17d
Is a differential amplifier, 18 is a transistor for passing a heating (indirectly heated) current to the heating resistors 5 and 6, 19 is a power supply, 22a and 2a
2b and 22c are resistors, 20 is an air flow obtained from a differential amplifier 17d from a signal output C corresponding to an air flow obtained from a potential of the resistor 22a proportional to a heating current flowing through the heating resistors 5 and 6. (Forward) based on the direction signal F of
Alternatively, it is a switching circuit for obtaining an output signal G converted to minus (backflow). Here, a bridge circuit composed of the temperature measuring resistor 4, the air temperature measuring resistor 7, and the resistors 22b and 22c is an air temperature measuring resistor whose temperature (resistance value) corresponds to the air temperature. Each resistance value is set so as to be higher than the temperature (resistance value) 7 by a certain value (for example, 150 ° C.). If the temperature of the resistance bulb 4 is lower than the set value, a difference occurs between the potentials H and I at the middle point of the bridge circuit, and the transistor 18 is turned on by the output J of the differential amplifier 17a, and the heating resistor Heating current flows through 5,6. Heating resistors 5, 6
When the temperature of the resistance bulb 4 heated by the indirect heat reaches the set value, the output J of the differential amplifier 17a causes the transistor 18 to output.
Is turned off, and the heating current is cut off. As described above, the feedback control is performed so that the temperature of the temperature measuring resistor 4 becomes constant at the set value, and the heating current value (corresponding to the potential C of the resistor 22a) flowing through the heating resistors 5 and 6 at this time is adjusted. Air flow rate. On the other hand, the direction of the air flow is detected from the temperature difference between the heating resistors 5 and 6. FIG. 9 schematically shows the temperature distribution of the heating resistors 5 and 6 and the temperature measuring resistor 4 when the air flow is a forward flow and a reverse flow. In the figure, the resistance temperature detector 4 is set at a certain reference temperature as described above.
Since the heating resistors 5 and 6 are connected in series and have a configuration in which the same heating current flows, when the air flow is a forward flow,
The temperature is lowered because the heating resistor 5 on the upstream side is deprived of heat by the airflow. On the other hand, when the air flow is the reverse flow, the temperature of the heating resistor 6 decreases. That is, FIG.
As shown in (2), the direction of the air flow can be detected by comparing the temperatures (resistance values) of the heating resistors 5 and 6. This will be described in more detail below. FIG. 9 is a diagram showing a cross section AA ′ of the measuring element of FIG. 1 and an operation principle. FIG.
As shown in FIG. 2, the configuration of the present invention in which one temperature measuring resistor 4 is sandwiched between a pair of heat generating resistors 5 and 6 and the temperature of the temperature measuring resistor 4 is set to a reference temperature is employed. The temperature difference between the bodies 5 and 6 can be detected with a sufficiently large value. On the other hand, in the conventional example shown in FIG. 10, on the contrary, one heating resistor is sandwiched between a pair of temperature measuring resistors, and the central heating resistor is set to the reference temperature. As shown in the temperature distribution of the conventional example in FIG. 9, the temperatures of the pair of resistance temperature detectors are both lower than the reference temperature, so that the temperature difference does not increase so much. As in the present invention, the ability to detect a large temperature difference means that the signal level increases,
A wide dynamic range is obtained, and it is advantageous with respect to noise characteristics. Returning to FIG. 7, in the external circuit 14,
Comparison of both temperatures (both resistance values) of the heating resistors 5 and 6 is performed based on the potentials at both ends of each resistor connected in series. The potential difference between points A and B in FIG. 7 corresponds to the temperature of the heating resistor 5 on the upstream side, and the potential difference between B and C corresponds to the temperature of the heating resistor 6 on the downstream side. Therefore, the differential amplifier 17b
The output D corresponds to the temperature of the heating resistor 5 and the output E of the differential amplifier 17c corresponds to the temperature of the heating resistor 6, and the outputs D and E are compared by the differential amplifier 17d. The direction can be detected as the output F. The output signal C of the resistor 22a corresponding to the above-described air flow and the switching circuit 20 which operates with the output F of the differential amplifier 17d, which is the direction signal of the air flow, provide an air flow signal G ( (For forward flow, negative for forward and reverse flow). FIG. 8 is a circuit diagram showing a thermal air flow meter according to another embodiment of the present invention. FIG. 5 shows another type of resistors 4, 5, 6, 7 and an external circuit 14 for signal processing. 7 is different from the configuration of FIG.
That is, In this method, the temperature difference between the heating resistors 5 and 6 is output as the direction and flow rate of the air flow. As shown in FIG. 9, the temperature difference (ΔT) between the heating resistors 5 and 6 can detect the direction of the air flow by its sign, and the absolute value corresponds to the air flow. Therefore, the temperature difference (ΔT) between the heating resistors 5 and 6
Is equivalent to the resistance value difference (ΔR). Therefore, if the resistance value difference (ΔR) is output, the air flow rate is measured. That is, in FIG. 8, the potentials at both ends of the heating resistors 5 and 6 are input to the differential amplifier 17d as in the operation in FIG. 7, and the potential difference is output to the division circuit 21 as F.
Since the potential difference F is the product of the resistance value difference (ΔR) between the heating resistors 5 and 6 and the heating current flowing through the heating resistors 5 and 6, the potential difference F is determined by the potential of the resistor 22a corresponding to the heating current value. By dividing by C, the resistance difference (ΔR) becomes
1 as an output G. The direction of the air flow is measured from the sign of the output G, and the air flow rate is measured from the absolute value of the output G. Next, a specific example of the measuring element of the thermal air flow meter according to the present invention will be described with reference to FIGS. First, an electric insulator 10a is formed on a silicon semiconductor substrate 2.
To form silicon dioxide, silicon nitride, etc. with a thickness of about 0.5 micron by a method such as thermal oxidation or CVD. Further, as a resistor 4, 5, 6, 7 by a method such as sputtering. Form platinum 0.2 microns thick. Then, after a resist is formed in a predetermined shape by a known photolithography technique, platinum is patterned by a method such as ion milling. Next, after the terminal electrode 8 is formed by gold plating or the like, a portion other than the terminal electrode 8 is used as a protective film, and the electrical insulator 1 having a thickness of about 0.5 μm is formed as described above.
0b is formed. Finally, the cavity 3 is formed by anisotropic etching from the back surface of the silicon substrate 2 using silicon dioxide or the like as a mask material, and cut into chips to obtain the measuring element 1. Here, the electric insulating film 10 on the cavity 3 has a detection effective area (about 0.2 mm × 1) of the conventional example.
mm: Japanese Patent Application Laid-Open No. 7-174600, “002”
8)), in this embodiment, 1.5 mm × 1.
The detection effective area was 5 mm, and the size was about 10 times the size of the conventional example. Further, since the heating resistors 5 and 6 are formed so as to surround the temperature measuring resistor 4, the area occupied by the heating resistors can be increased. As a result, the dynamic range of the air flow rate signal of the heating resistors 5 and 6 on the cavity 3 and the noise immunity are greatly improved as compared with the conventional example. In addition, even when the cavity 3 is enlarged, the size of the measuring element 1 of the present embodiment is about 2.5 mm × 5 mm, and the conventional example (about 3 mm
× 3 mm: JP-A-7-174600, specification “0”
028 ”)). In addition, since there is no opening exposed to the airflow 9 as in the conventional example, the reliability of dust resistance is improved.

According to the present invention, a pair of heating resistors 5 and 6 are formed on the electrical insulator 10 above and below the temperature measuring resistor 4 on the cavity 3 having a large effective detection area formed on the semiconductor substrate. ,
In addition, since the heating resistors 5 and 6 are connected in series and the direction and the flow rate of the air flow are measured from the temperature difference and the heating current of the heating resistors 5 and 6, the dynamic range at the time of measuring the air flow rate is obtained. In addition, the noise immunity is improved. In addition, since there is no opening for the air flow 9, the reliability of dust resistance is improved, and further, the air temperature measuring resistor 7 is configured to protrude into the air flow. There is an effect that a thermal air flowmeter with improved temperature characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a plan view showing a measuring element (for a thermal air flow meter) of one embodiment according to the present invention.

FIG. 2 is a diagram showing a cross section taken along line A-A 'of FIG.

FIG. 3 is a sectional view showing a thermal air flow meter according to an embodiment of the present invention, on which the measuring element of FIG. 1 is mounted.

FIG. 4 is an enlarged view showing a measuring element unit of FIG. 3;

FIG. 5 is a view showing a B-B ′ section of FIG. 4;

FIG. 6 is a view showing a cross section taken along line C-C ′ of FIG. 4;

FIG. 7 is a circuit diagram showing a thermal air flow meter according to an embodiment of the present invention.

FIG. 8 is a circuit diagram showing a thermal air flow meter according to another embodiment of the present invention.

9 is a diagram showing an AA ′ cross section and an operation principle of the measuring element of FIG. 1;

FIG. 10 is a plan view illustrating a measuring element of a conventional thermal air flow meter.

FIG. 11 is a plan view illustrating a measuring element of another conventional thermal air flow meter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Measurement element, 2 ... Semiconductor substrate, 3 ... Cavity, 3a ... Opening part, 4 ... Temperature measuring resistor, 5,6 ... Heat-generating resistor, 7 ... Air temperature measuring resistor, 8,15 ... Terminal electrode, 9 ... air flow, 10
a, 10b: electric insulating film, 11: intake passage, 12: sub passage, 13, 13a, 13b: support, 14: external circuit,
16: gold wire, 17a, 17b, 17c, 17d: differential amplifier, 18: transistor, 19: power supply, 20: switching circuit, 21: division circuit, 22a, 22b, 22c: resistor

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[Procedure amendment]

[Submission date] July 1, 1996

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Full text

[Correction method] Change

[Correction contents]

[Document Name] Statement

[Title of the Invention] Thermal air flow meter

[Claims]

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal air flow meter, and more particularly to a thermal air flow meter suitable for measuring an intake air amount of an internal combustion engine.

[0002]

2. Description of the Related Art Conventionally, as an air flow meter which is provided in an electronically controlled fuel injection device of an internal combustion engine of an automobile or the like and measures an intake air amount, a thermal air flow meter has become mainstream since it can directly detect a mass air amount. I have. Among them, an air flow meter manufactured by a semiconductor micromachining technique has been particularly noted because it can reduce the cost and can be driven with low power. Such a conventional thermal air flow meter using a semiconductor substrate is disclosed in, for example, Japanese Patent Application Laid-Open No. 60-14 / 1985.
No. 2,268, and JP-A-7-174600. Although the technology described in the above-mentioned publication reduces manufacturing costs to some extent, it has problems such as a narrow flow rate measurement range, large noise, and insufficient dust resistance reliability when measuring the amount of intake air.

[0003]

The prior art has the following problems. The prior art described in Japanese Patent Application Laid-Open No. Sho 60-142268 will be described with reference to FIG. FIG. 10 is a plan view of a measuring element of a conventional thermal air flow meter, and is a second diagram described in the publication. In the figure, reference numeral 1 denotes a measuring element of a thermal air flow meter, and two bridges 24a, 24a, made of an electrically insulating film bridging a cavity (23a, b, c) formed by anisotropically etching a semiconductor substrate such as silicon. 24b,
The upstream side of the air flow is the bridge 24a, and the downstream side is the bridge 24b. The heating resistor 5 is arranged with the open cavity 23c between the two bridges 24a and 24b, and these bridges 24
a and 24b are provided on the side of the heating resistor 5 respectively.
a and 4b are arranged, and an air temperature measuring resistor 7 for measuring the air temperature is arranged in a part of the electrical insulating film on the upstream side of the cavity 23a. Also, the cavities 23a, 23b and 23
c is anisotropic etching of the semiconductor substrate using the opening of the electric insulating film, so that the bridges 24a and 24 of the electric insulating film are formed.
It is a continuous integral cavity below b.

In this air flow meter, the heating resistor 5 is driven to be heated to a temperature that is higher than the air temperature determined by the air temperature measuring resistor 7 by a certain temperature. The air flow rate is measured from the temperature difference generated between the upstream temperature measuring element 4b and the downstream temperature measuring element 4a of the flow channel by utilizing the heat transport effect of air.

FIG. 11 is a view for explaining another example of a conventional thermal air flow meter.
FIG. 2 is a plan view of the measuring element described in FIG.
The measuring element 1 includes a semiconductor substrate 2 as in the above-described conventional example.
An electric insulating film 10 is formed thereon, the portions 23d and 23e of the electric insulator 10 are etched by a known photolithography technique, and the semiconductor substrate 2 is further anisotropically etched from the openings 23d and 23e to form a cavity 23d. , 23e. These cavities 23d and 23e form continuous and continuous cavities under the bridge 24 of the electric insulating film, as in the above-described conventional example. In this conventional example, a heating resistor 5 and a temperature measuring resistor 4 close to the heating resistor 5 are arranged on the bridge 24 on the upstream side of the air flow. Installed upstream.

In this air flow meter, the heating resistor 5 is heated (indirectly heated) so that the temperature of the temperature measuring resistor 4 becomes higher than the air temperature detected by the air temperature measuring resistor 7 by a certain temperature. The air flow rate is measured from a heating current flowing through a heating resistor 5 that indirectly heats the temperature measuring resistor 4 that is cooled as the air flow rate increases.

In the conventional example configured as described above, for example, in the measuring element of FIG. 10, as described in the item (0012) of the specification of Japanese Patent Application Laid-Open No. 7-174600, the air flow rate is large. There is a problem that the output change is small in the range, the linearity is deteriorated, and the measurable flow velocity range is narrowed. The measurement element shown in Fig. 11 improved this.
In this conventional example, there is a problem that the direction of the air flow cannot be detected. Further, as a problem common to both of the conventional examples shown in FIGS. 10 and 11, there is a detection effective area in contact with an air flow which is important for measuring the air flow rate (in this conventional example, the resistance temperature detectors 4, 4a, 4b and the heat generation). The output noise is large due to the small area of the resistor 5 in contact with the air flow, and the cavity 23 is formed on the surface of the measuring element in contact with the air flow.
a, 23b, 23c, 23d, 23e are open,
When used under severe conditions, such as in an automobile, dust and the like accumulate in the opening, and highly reliable measurement cannot be performed over a long period of time.

Accordingly, it is an object of the present invention to solve the problems of the prior art, and to provide a thermal air flow meter having a wide dynamic range of flow measurement, small noise, and high dust resistance and reliability. is there.

[0009]

A thermal air flow meter which achieves the above object has a semiconductor substrate having a cavity whose upper surface is closed by an electrical insulating film and which is open at the lower surface, and a portion of the cavity above the cavity. A temperature measuring resistor formed on the electric insulating film and a pair of heating resistors distributed to the upstream and downstream sides of the fluid to be measured in the temperature measuring resistor, and a portion other than on the cavity portion; A measuring element having a fluid temperature measuring resistor formed on an electric insulating film; and heating the heating resistors so as to keep a temperature difference between the temperature measuring resistor and the fluid temperature measuring resistor constant. Heating control means for flowing an electric current, direction detecting means for detecting a flow direction of the fluid to be measured based on a temperature difference between the heating resistors, and a temperature difference between the heating resistors or flowing to the heating resistors. A flow rate detector for detecting a flow rate of the fluid to be measured based on a current value. And it constitutes and means.

According to the present invention, one temperature measuring resistor is sandwiched between a pair of heating resistors so that a large temperature difference between the pair of heating resistors can be detected using the temperature of the temperature measuring resistor as a reference temperature. Therefore, the dynamic range and noise immunity at the time of flow rate measurement are improved.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a plan view showing a measuring element (for a thermal air flow meter) of one embodiment according to the present invention. 2 shows a measuring element used in a thermal air flow meter. FIG. 2 is a diagram showing a cross section taken along line AA ′ of FIG. 1 and 2, a measuring element 1 includes a semiconductor substrate 2 made of silicon or the like, and an electric insulating film 10a formed on the upper surface of the semiconductor substrate 2 so as to cover the upper surface side of a cavity 3 described later. And the temperature measuring resistor 4 formed on the electric insulating film 10a and arranged between the upstream side and the downstream side of the airflow 9 with the temperature measuring resistor 4 interposed therebetween and adjacent to the vicinity thereof. A pair of heating resistors 5 and 6, an air temperature measuring resistor 7 for measuring air temperature, a terminal electrode 8 for extracting a signal from each resistor as the measuring element 1 to an external circuit, The portion where the temperature measuring resistor 4 and the heating resistors 5 and 6 are formed to protect the resistor, the terminal electrode 8 and the like, and the portion where the temperature measuring resistor 4 and the heating resistors 5 and 6 are formed (Opening 3a) Electrical insulation by anisotropic etching The semiconductor substrate 2 is bored to a boundary surface of 10a
And the cavity 3 provided in the first position.

Here, the pair of heating resistors 5 and 6 are electrically connected in series on the electrical insulating film 10a and are drawn out from the two terminal electrodes 8 (,) and the intermediate connection point M. And a terminal electrode 8 () as an intermediate terminal electrode. That is, in FIG. 1, the heating resistors 5 and 6 are connected in series, and the terminal electrode 8 () is drawn out from the middle (middle connection point M). However, depending on the case, although there is the complexity that one terminal electrode 8 increases by one, it is also possible to take out the terminal electrodes 8 independently and connect them in series by an external circuit.

The temperature of the temperature measuring resistor 4 is applied to the pair of heating resistors 5 and 6 formed on the electric insulating film 10a having a large area (effective detection area) in contact with the air flow 9.
The heating (indirect heat) current is passed so that the temperature becomes higher than the temperature of the air temperature measuring resistor 7 arranged at the end of the flow path.

Further, the direction of the air flow 9 is set such that the temperature measuring resistor 4 formed at a position on the cavity 3 and substantially at the center on the electric insulating film 10a is located in the upstream and downstream of the fluid to be measured. It is detected by comparing each temperature (each resistance value corresponding to the temperature) of the heating resistors 5 and 6 desirably symmetrically arranged with respect to the temperature measuring resistor 4 distributed to the side. You. That is, when the airflow of the fluid to be measured is zero, the heating resistors 5 and 6 exhibit substantially the same temperature as the temperature of the temperature measuring resistor 4, and no temperature difference occurs.

If there is an air flow, the direction of the air flow 9 in FIG.
In (forward flow), the heating resistor 5 arranged mainly on the upstream side has a greater cooling effect by the airflow 9 than the heating resistor 6 arranged on the downstream side. 5 and 6 are connected in series, and since the same heating current flows and both calorific values are almost the same, the heating resistor 5 on the upstream side
Is lower than the temperature of the heating resistor 6. Also,
When the airflow is opposite to the direction of the arrow in FIG. 1 (reverse flow), the temperature of the heating resistor 6 on the downstream side is lower than the temperature of the heating resistor 5 on the upstream side. Thus, the heating resistors 5, 6
By comparing each temperature (each resistance value), the direction of the air flow 9 can be detected.

On the other hand, the measurement of the air flow rate utilizes the fact that the temperature difference between the heating resistors 5 and 6 increases with an increase in the air flow rate. measure. Alternatively, in order to control the temperature measuring resistor 4 to be higher than the air temperature measuring resistor 7 by a certain temperature, the heating (indirect heat) current value flowing through the heating resistors 5 and 6 increases with an increase in the air flow rate. Measure using.

The heating resistors 5 and 6 are formed on the electric insulating film 10a above the cavity 3, and the area of the heating resistor (the area of the two heating resistors) of the present embodiment is smaller than that of the conventional example. Larger than the heating resistor area (area of one heating resistor)
Since the configuration is such that the detection effective area is large, a large air flow rate signal can be taken out and a wide dynamic range can be obtained. In addition, an average output is obtained with respect to local turbulence of the air flow, in other words, a noise ratio (N /
Since S) is small, the configuration is strong against noise.

Further, as shown in FIG. 1, the air temperature measuring resistor 7 is located at the tip of the substrate 2 and is arranged so as to protrude into the flow path of the air flow 9 so that the air flow 9 is directed in the forward or reverse flow. In any case, the heating resistors 5 and 6 are provided at positions not affected by the heated airflow, and the measurement of the airflow can be performed with high accuracy.

FIG. 3 is a sectional view showing a thermal air flow meter according to an embodiment of the present invention, on which the measuring element of FIG. 1 is mounted.
For example, it is a cross-sectional view showing an embodiment of a thermal air flow meter mounted in an intake passage of an internal combustion engine such as an automobile. As shown in the figure, the thermal air flow meter includes a measuring element 1, a support 13, and an external circuit 14. Then, the measuring element 1 is arranged in the sub-passage 12 inside the intake passage 11. External circuit 1
4 is electrically connected to the terminal electrode 8 of the measuring element 1 via the support 13. Here, normally, the intake air as the fluid to be measured flows in the direction indicated by the airflow 9,
Depending on the conditions of a certain internal combustion engine, the intake air flows in the direction opposite to the air flow 9 (reverse flow).

FIG. 4 is an enlarged view showing the measuring element section of FIG. FIG. 4 is an enlarged view of a measuring element 1 and a support 13 of FIG. 3. FIG. 5 is a view showing a BB ′ section of FIG. 4. FIG.
FIG. 5 is a view showing a cross section taken along line CC ′ of FIG. 4. 4 to 6, the measuring element 1 is fixed on a support 13b such that the front and back surfaces of the air temperature measuring resistor 7 are directly exposed to the air flow 9, and further includes a terminal electrode 15 and a signal processing circuit. External circuit 14 formed on an electrically insulating substrate such as alumina
Are also fixed on the support 13b.

After the measuring element 1 and the external circuit 14 are electrically connected between the terminal electrodes 8 and 15 by wire bonding with a gold wire 16 or the like, the gold wire 16, the electrode terminals 8, 15 and the external circuit 14 are connected. 14 to protect 14
Is sealed and protected. In the measuring element 1 mounted as described above, as shown in FIGS. 4 to 6, the lower surface of the cavity 3 is mostly closed by the support 13 b, and the upper surface is 9 is almost isolated (closed). Therefore, when the present embodiment as described above is employed, there is no portion where the cavity 3 is open to the air flow 9 as in the conventional example, and therefore, when measuring the air flow rate of the internal combustion engine of an automobile or the like, Dust and the like that cause a problem do not accumulate in the cavity or the opening, and highly reliable measurement can be performed.

In an internal combustion engine of an automobile or the like, the temperature of the intake passage 11 and the support 13 shown in FIG. 3 rises due to heat of the internal combustion engine.
An error may occur in the measurement of the air flow rate and the temperature characteristics may be deteriorated. On the other hand, in the present embodiment, as shown in FIG. 4, the air temperature measuring resistor 7 is provided at a position farthest from the support 13 and further protrudes from the support 13. . With this arrangement, both the front and back surfaces are exposed by the air flow 9 and sufficient heat radiation is achieved, so that the configuration having excellent temperature characteristics which is hardly affected by the rise in temperature of the intake passage 11 and the support 13 is provided. It has become.

Further, as shown in the cross-sectional view taken along the line BB 'of FIG. 5, the tip of the support 13b is made streamlined with respect to the air flow 9, so that the air flow 9 reaches the measuring element 1. Even at the position, since the air flow is uniform without any turbulence, measurement with less noise is possible.

Next, the operation of the thermal air flow meter according to the present invention will be described with reference to FIGS. 7, 8 and 9. FIG. 7 is a circuit diagram showing a thermal air flow meter according to an embodiment of the present invention. A measuring element 1 including resistors 4, 5, 6, and 7 shown in FIG. 1 and an external circuit 14 for processing a signal from the measuring element 1 (heating control means, direction detecting means, and flow rate detecting means). Circuit).

In the figure, 17a, 17b, 17c and 17d are differential amplifiers, 18 is a transistor for passing a heating (indirect heat) current to the heating resistors 5 and 6, 19 is a power supply, 22a and 22
b and 22c are resistors, 20 is an air flow obtained from the differential amplifier 17d from a signal output C corresponding to an air flow obtained from the potential of the resistor 22a proportional to the heating current flowing through the heating resistors 5 and 6. Is a switching circuit for obtaining an output signal G converted to plus (forward flow) or minus (reverse flow) based on the direction signal F.

Here, a bridge circuit composed of the temperature measuring resistor 4, the air temperature measuring resistor 7, and the resistors 22b and 22c provides an air temperature measuring device in which the temperature (resistance value) of the temperature measuring resistor 4 corresponds to the air temperature. Each resistance value is set so as to be higher by a certain value (for example, 150 ° C.) than the temperature (resistance value) of the temperature resistor 7. If the temperature of the resistance bulb 4 is lower than the set value, a difference occurs between the potentials H and I at the middle point of the bridge circuit, and the transistor 18 is turned on by the output J of the differential amplifier 17a, and the heating resistor Heating current flows through 5,6. When the temperature of the resistance bulb 4 heated by the heating resistors 5 and 6 reaches the set value, the transistor 18 is turned off by the output J of the differential amplifier 17a and the heating current is cut off. Thus, the resistance temperature detector 4
The feedback current is controlled so that the temperature of the heating resistor becomes constant. The heating current value (corresponding to the potential C of the resistor 22a) flowing through the heating resistors 5 and 6 at this time becomes the air flow rate.

On the other hand, the direction of the air flow depends on the heating resistors 5 and 6.
Is detected from the temperature difference. FIG. 9 schematically shows the temperature distribution of the heating resistors 5 and 6 and the temperature measuring resistor 4 when the air flow is a forward flow and a reverse flow. In the figure, the resistance temperature detector 4 is set at a certain reference temperature as described above. Since the heating resistors 5 and 6 are connected in series and have a configuration in which the same heating current flows, when the airflow is a forward flow, the heating resistor 5 on the upstream side is more deprived of heat by the airflow. The temperature decreases. On the other hand, when the air flow is the reverse flow, the temperature of the heating resistor 6 decreases. That is, as shown in FIG. 9, the direction of the air flow can be detected by comparing the temperatures (resistance values) of the heating resistors 5 and 6. This will be described in more detail below.

FIG. 9 is a view showing the AA ′ cross section and the operating principle of the measuring element of FIG. As shown in FIG. 9, by adopting a configuration of the present invention in which one temperature measuring resistor 4 is sandwiched between a pair of heating resistors 5 and 6 and the temperature of the temperature measuring resistor 4 is set to a reference temperature. The temperature difference between the heating resistors 5 and 6 can be sufficiently large to detect. On the other hand, in the conventional example shown in FIG. 10, on the contrary, one heating resistor is sandwiched between a pair of temperature measuring resistors, and the central heating resistor is set to the reference temperature. As shown in the temperature distribution of the conventional example in FIG. 9, the temperatures of the pair of resistance temperature detectors are both lower than the reference temperature, so that the temperature difference does not increase so much. The fact that a large temperature difference can be detected as in the present invention increases the signal level, provides a wide dynamic range, and is advantageous in terms of noise immunity.

Returning to FIG. 7, in the external circuit 14, the two temperatures (both resistance values) of the heating resistors 5 and 6 are compared based on the potentials at both ends of each resistor connected in series. The potential difference between the points A and B in FIG. 7 corresponds to the temperature of the heating resistor 5 on the upstream side, and BC corresponds to the temperature of the heating resistor 6 on the downstream side.
Potential difference between the two. Therefore, the output D of the differential amplifier 17b
Corresponds to the temperature of the heating resistor 5, and the output E of the differential amplifier 17 c corresponds to the temperature of the heating resistor 6. By comparing the outputs D and E with the differential amplifier 17 d, the direction of the air flow is output. F can be detected. A switching circuit 2 that operates with an output signal C of the resistor 22a corresponding to the above-described air flow and an output F of the differential amplifier 17d, which is a direction signal of the air flow.
0, the air flow signal G taking into account the direction of the air flow
(Forward flow is negative for forward / reverse flow).

FIG. 8 is a circuit diagram showing a thermal air flow meter according to another embodiment of the present invention. FIG. 5 shows another type of resistors 4, 5, 6, 7 and an external circuit 14 for signal processing. The difference from the configuration of FIG. 7 is that a division circuit 21 is provided instead of the switching circuit 20. In this method, the temperature difference between the heating resistors 5 and 6 is output as the direction and flow rate of the air flow. As shown in FIG. 9, the temperature difference between the heating resistors 5 and 6
(ΔT) is such that the direction of the air flow can be detected by its sign, and the absolute value corresponds to the air flow rate. Therefore, the temperature difference (ΔT) between the heating resistors 5 and 6 is equal to the resistance value difference (ΔR).
Therefore, if this resistance value difference (ΔR) is output, the air flow rate is measured.

That is, in FIG. 8, the potentials at both ends of the heating resistors 5 and 6 are input to the differential amplifier 17d in the same manner as in the operation in FIG.
Is output to Since the potential difference F is the product of the resistance value difference (ΔR) between the heating resistors 5 and 6 and the heating current flowing through the heating resistors 5 and 6, the potential difference F is determined by the potential of the resistor 22a corresponding to the heating current value. By dividing by C, the resistance value difference (ΔR)
Is obtained as the output G of the division circuit 21. The direction of the air flow is measured from the sign of the output G, and the air flow rate is measured from the absolute value of the output G.

Next, a specific example of the measuring element of the thermal air flow meter according to the present invention will be described with reference to FIGS.
First, silicon dioxide, silicon nitride, or the like having a thickness of about 0.5 μm is formed as an electrical insulator 10a on the silicon semiconductor substrate 2 by a method such as thermal oxidation or CVD. In Steps 6 and 7, platinum having a thickness of about 0.2 μm is formed by a method such as sputtering. Then, after a resist is formed in a predetermined shape by a known photolithography technique, platinum is patterned by a method such as ion milling.

Next, after the terminal electrode 8 is formed by gold plating or the like, an electric insulator 10b having a thickness of about 0.5 μm is formed in the same manner as described above, using a portion other than the terminal electrode 8 as a protective film. Finally, the cavity 3 is formed by anisotropic etching from the back surface of the silicon substrate 2 using silicon dioxide or the like as a mask material, and cut into chips to obtain the measuring element 1.

Here, the electric insulating film 10 on the cavity 3 has a detection effective area (about 0.2 mm × 1 mm:
In this example, the detection effective area was 1.5 mm × 1.5 mm, which was about 10 times larger than that of the conventional example. Also,
Since the heating resistors 5 and 6 are formed so as to surround the temperature measuring resistor 4, the occupied area of the heating resistor can be increased.

As a result, the dynamic range and noise immunity of the air flow rate signals of the heating resistors 5 and 6 on the cavity 3 are greatly improved as compared with the conventional example. In addition, even when the cavity 3 is enlarged, the size of the measuring element 1 of the present embodiment is about 2.5 mm × 5 mm, and the conventional example (about 3 mm
× 3 mm: JP-A-7-174600, specification “0”
028 ”)). In addition, since there is no opening exposed to the airflow 9 as in the conventional example, the reliability of dust resistance is improved.

[0036]

According to the present invention, a pair of heating resistors 5 and 6 are formed on the electrical insulator 10 above and below the temperature measuring resistor 4 on the cavity 3 having a large effective detection area formed on the semiconductor substrate. ,
In addition, since the heating resistors 5 and 6 are connected in series and the direction and the flow rate of the air flow are measured from the temperature difference and the heating current of the heating resistors 5 and 6, the dynamic range at the time of measuring the air flow rate is obtained. In addition, the noise immunity is improved.

Further, since there is no opening for the air flow 9, the reliability of dust resistance is improved, and the air temperature measuring resistor 7 is projected into the air flow. Thus, there is an effect that a thermal air flowmeter with improved temperature characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a plan view showing a measuring element (for a thermal air flow meter) of one embodiment according to the present invention.

FIG. 2 is a diagram showing a cross section taken along line A-A 'of FIG.

FIG. 3 is a sectional view showing a thermal air flow meter according to an embodiment of the present invention, on which the measuring element of FIG. 1 is mounted.

FIG. 4 is an enlarged view showing a measuring element unit of FIG. 3;

FIG. 5 is a view showing a B-B ′ section of FIG. 4;

FIG. 6 is a view showing a cross section taken along line C-C ′ of FIG. 4;

FIG. 7 is a circuit diagram showing a thermal air flow meter according to an embodiment of the present invention.

FIG. 8 is a circuit diagram showing a thermal air flow meter according to another embodiment of the present invention.

9 is a diagram showing an AA ′ cross section and an operation principle of the measuring element of FIG. 1;

FIG. 10 is a plan view illustrating a measuring element of a conventional thermal air flow meter.

FIG. 11 is a plan view illustrating a measuring element of another conventional thermal air flow meter.

[Explanation of Symbols] 1 ... measuring element, 2 ... semiconductor substrate, 3 ... cavity, 3 a ... opening, 4 ... temperature measuring resistor, 5, 6 ... heating resistor, 7 ... air temperature measuring resistor, 8, 15 ... Terminal electrode, 9 ... Air flow, 10
a, 10b: electric insulating film, 11: intake passage, 12: sub passage, 13, 13a, 13b: support, 14: external circuit,
16: gold wire, 17a, 17b, 17c, 17d: differential amplifier, 18: transistor, 19: power supply, 20: switching circuit, 21: division circuit, 22a, 22b, 22c: resistor

Claims (2)

[Claims] 1. A semiconductor substrate having a cavity whose upper surface is closed by an electric insulating film and opened to the lower surface, a temperature measuring resistor formed on the electric insulating film at a position on the cavity, and A pair of heating resistors distributed to the upstream and downstream sides of the fluid to be measured with the temperature resistor inside, and a fluid temperature measuring resistor formed on the electric insulating film in a portion other than on the cavity. A measuring element having: heating control means for supplying a heating current to each of the heating resistors so as to keep a temperature difference between the temperature measuring resistor and the fluid temperature measuring resistor constant; and a temperature difference between the heating resistors. Direction detecting means for detecting the flow direction of the fluid to be measured based on the flow rate, and flow rate detection for detecting the flow rate of the fluid to be measured based on the temperature difference between the respective heating resistors or the current value flowing through each of the heating resistors. And a thermal air flow meter. 2. The terminal according to claim 1, wherein said pair of heating resistors are two terminal electrodes which are electrically connected in series on said electric insulating film and are drawn out, and an intermediate terminal which is drawn out from an intermediate connection point. A thermal air flow meter comprising: an electrode;