JPH102773A - Thermal air flow meter - Google Patents

Abstract

(57) [Object] To provide a thermal air flow meter which improves the dynamic range and noise of air flow measurement, and is excellent in dust resistance reliability and temperature characteristics. A heating resistor (4a, 4b) and a temperature measuring resistor (5, 6) for measuring the direction and flow rate of an air flow are formed in an electrical insulator (10) on cavities (3a, 3b) formed in a semiconductor substrate (2), respectively. The air temperature measuring resistor 7 is formed on the semiconductor substrate 2 so as to protrude into the air flow, and the electric insulator 10 has no opening for the air flow 9 and has a cavity 3b having a large effective measuring area.
The heating element 4b for measuring the flow rate is formed on the measuring element 1, and the flow rate signals from the temperature measuring resistors 5 and 6 of the measuring element 1 and the flow rate signal of the heating resistor 4b are both output and averaged. .

Description

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 can directly detect a mass air amount and has become mainstream. I have. Among them, an air flow meter manufactured by a semiconductor micromachining technique has attracted particular attention because it can reduce cost and can be driven with low power. Such a conventional thermal air flow meter using a semiconductor substrate is disclosed in, for example, JP-A-60-142268 and JP-A-7-174600. Although the techniques described in these publications have reduced manufacturing costs to some extent, there have been problems in measuring the intake air amount, such as a narrow flow rate measurement range, large noise, and insufficient dust resistance reliability.

[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. 7 is a plan view of a measuring element of a conventional thermal air flow meter, and is a second diagram described in the above-mentioned publication. In the figure, 1 is a measuring element of a thermal air flow meter, and two bridges 24a and 24b made of an electric insulating film bridging the cavities 23a, 23b and 23c formed by anisotropic etching of a semiconductor substrate such as silicon. The upstream side of the air flow is a bridge 24a, and the downstream side is a bridge 24b. The heating resistor 4 is arranged with an open cavity 23c between the two bridges 24a and 24b, and these bridges 24a and 24b are respectively connected to the temperature measuring resistors 5 and 6 on the sides of the heating resistor 4. Is placed,
Further, an air temperature measuring resistor 7 for measuring the air temperature is provided in a part of the electrical insulating film on the upstream side of the cavity 23a.
Further, the cavities 23a, 23b and 23c are continuous and integrated cavities under the bridges 24a and 24b of the electric insulating film because the semiconductor substrate is anisotropically etched using the opening of the electric insulating film.

In this air flow meter, the heating resistor 4 is driven to be heated to a temperature 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 6 and the downstream temperature measuring element 5 of the flow channel by utilizing the heat transport effect of air.

FIG. 8 is an explanatory view of another example of a conventional thermal air flow meter, and is a plan view of a measuring element. In the measuring element 1, the electric insulating film 10 is formed on the semiconductor substrate 2 in the same manner as in the conventional example, and the portions 23d and 23e of the electric insulator 10 are etched by a known photolithography technique. , 23e, the semiconductor substrate 2 is anisotropically etched to form cavities 23d, 23e. The cavities 23d and 23e form a continuous integral cavity under the bridge 24 of the electric insulating film, as in the above-described conventional example. In this conventional example, the heating resistor 4 and the temperature measuring resistor 6 are disposed on the bridge 24 in the vicinity of the heating resistor 4 on the upstream side of the air flow. Installed upstream.

In this air flow meter, the heating resistor 4 is heated (indirectly heated) so that the temperature of the temperature measuring resistor 6 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 the heating resistor 4 that indirectly heats the temperature measuring resistor 6 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. 7, as described in the item (0012) of the specification of Japanese Patent Application Laid-Open No. However, there is a problem that the output change is small, the linearity is deteriorated, and the measurable flow velocity range is narrowed. This is improved by the measuring element shown in FIG. 8, but 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. 7 and 8, a detection effective area in contact with an air flow which is important in measuring an air flow rate (the air flow of the resistance temperature detectors 5 and 6 in this conventional example). Is small, the output noise is large, and the surface of the measuring element in contact with the airflow has a cavity 23.
a, 23b, 23c, 23d, 23e are open,
When used under severe conditions such as in automobiles, dust and the like accumulate in the openings, and highly reliable measurement cannot be performed over a long period of time.

It is an object of the present invention to provide a thermal air flow meter which solves the problems of the prior art, has a wide dynamic range for flow measurement, has low output noise, and has high dust resistance reliability.

[0009]

According to the present invention, there is provided a semiconductor substrate having an electric insulating film on an upper surface and having first and second cavities extending from a boundary surface of the electric insulating film to a lower surface; A first heating resistor formed on the electrical insulating film on the first cavity; and a first heating resistor on the electrical insulating film on the first cavity and close to the first heating resistor. A pair of resistance temperature detectors formed symmetrically in the upstream and downstream directions of the measurement fluid,
A second heating resistor formed on the electric insulating film on the second cavity, and a fluid temperature measuring resistor formed on the electric insulating film other than the first and second cavities; And a control means for keeping the temperature difference between the first and second heat generating resistors and the fluid temperature measuring resistor constant, and measuring the temperature of the fluid to be measured based on the temperature difference between the pair of temperature measuring resistors. Means for detecting the flow direction, and means for detecting the flow rate of the fluid to be measured from the temperature difference between the pair of resistance temperature detectors and the value of the current flowing through the second heating resistor; and And the second heating resistor is configured to be electrically connected in series on the electric insulating film, and furthermore, a temperature difference between the pair of temperature measuring resistors and a current flowing through the second heating resistor. Means for detecting and outputting the flow rate of the fluid to be measured independent of Or the flow rate of the fluid to be measured is detected from the temperature difference between the pair of resistance temperature detectors and the current value flowing through the second heating resistor, and averaged from each of the obtained flow rate values. It is configured by means for outputting the processed flow rate value, in which the electric resistance of the first heating resistor is made smaller than the electric resistance of the second heating resistor, and further, the first cavity is It was formed smaller than the second cavity.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described.
This will be described with reference to the drawings.

FIG. 1 is a plan view showing a measuring element 1 of a thermal air flow meter according to an embodiment of the present invention, and FIG. 2 is a sectional view of the measuring element 1 taken along line AA 'and BB' of FIG. .

In FIG. 1 and FIG. 2, a measuring element 1 is a first cavity formed by anisotropic etching from a lower surface of a semiconductor substrate 2 such as silicon to a boundary surface between electric insulating films 10a and 10b. 3a, the second cavity 3b, the first heating resistor 4a, the second heating resistor 4b and the first heating resistor 4a formed on the electric insulating film 10a. Temperature measuring resistors 6 and 5 arranged downstream respectively, air temperature measuring resistors 7 for measuring air temperature,
It comprises a terminal electrode 8 for connecting a signal of the measuring element 1 to an external circuit, and an electric insulating film 10b for protecting each resistor.

Here, the first cavity 3a and the resistor 4
a, 5 and 6 are formed sufficiently small with respect to the second cavity 3b and the resistor 4b, and the first heating resistor 4a is set to a sufficiently small electric resistance value with respect to the second heating resistor 4b. Is done. First heating resistor 4a and second heating resistor 4
b is electrically connected in series. The temperature of the second heat generating resistor 4b having a large area (detection effective area) mainly in contact with the air flow 9 and a large resistance value is determined by the temperature of the air temperature measuring resistor 7 arranged at the flow path end of the air flow 9. The temperature is controlled to be higher than the temperature by a certain value. Along with this, the first heating resistor 4 a is also set at a certain temperature higher than the air temperature measuring resistor 7.

The direction of the air flow 9 is determined by the first heating resistor 4a.
Is detected by comparing the temperatures (resistance values) of the resistance temperature detectors 5 and 6 formed symmetrically with respect to. That is,
Temperature measuring resistor 5 indirectly heated by first heating resistor 4a
Numeral 6 indicates the same temperature when the air flow is zero. In the direction (forward flow) of the air flow 9 in FIG. 1, the temperature measuring resistor 6 arranged mainly on the upstream side has the temperature measuring resistor arranged on the downstream side. Since the cooling effect of the airflow 9 is greater than that of the body 5, the temperature of the temperature measuring resistor 6 on the upstream side becomes lower than the temperature of the temperature measuring resistor 5. On the other hand, when the air flow is opposite to the direction of FIG. 1 (reverse flow), the temperature of the downstream temperature measuring resistor 5 becomes lower than the temperature of the upstream temperature measuring resistor 6. As described above, by comparing the temperatures (resistance values) of the resistance temperature detectors 5 and 6, the air flow 9 is determined.
Direction can be detected.

On the other hand, the measurement of air flow rate
The direction of the air flow and the air flow rate are simultaneously measured based on the temperature difference by utilizing the fact that the temperature difference 6 becomes larger as the air flow rate increases. Further, in order to expand the dynamic range at the time of flow rate measurement, which is a problem of the conventional example, and to reduce noise, the second heating resistor 4b having a larger effective detection area as compared with the conventional example is connected to the air temperature measuring device. Direct heating is performed at a certain temperature higher than that of the resistor 7, and measurement is performed based on a heating current value that increases with an increase in the air flow rate. Since the air flow rate measured by the second heating resistor 4b has a large detection effective area, a large air flow rate signal can be taken out, and an average output can be obtained for local turbulence in the air flow. Therefore, the configuration is strong against noise and a wide dynamic range can be obtained. As described above, by utilizing the temperature difference between the temperature measuring resistors 5 and 6 and the air flow rate signal of the second heating resistor 4b having a wide detection effective area in a suitable air flow region, the dynamic range is wide and the noise is wide. It is possible to measure the air flow rate with less.

In FIG. 1, the first heating resistor 4a and the second heating resistor 4b are connected in series. In some cases, the number of terminal electrodes 8 increases, but the connection is made in parallel. The heating resistors 4a and 4b are independently driven (in this case, two air temperature measuring resistors 7 are configured), and a signal is taken out, and then a signal from an external circuit is averaged. Effects can be obtained. 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. Heating resistor 4
They are arranged so as not to be affected by the heated airflows a and 4b, and can measure the airflow rate with high accuracy.

FIG. 3 is a sectional view showing an embodiment of a thermal air flow meter on which the measuring element 1 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. And the sub-passage 1 inside the intake passage 11
The measuring element 1 is arranged at 2. The external circuit 14 is the support 1
3 is electrically connected to the terminal electrode 8 of the measuring element 1. Here, the intake air normally flows in the direction indicated by 9, and the intake air flows in the opposite direction (backflow) to 9 depending on the condition of a certain internal combustion engine.

FIG. 4 shows the measuring element 1 and the support 13 of FIG.
FIG. FIG. 5 is a cross-sectional view taken along lines CC ′ and D
It is -D 'sectional drawing. As shown in FIGS. 4 and 5, the measuring element 1 is fixed on a support 13b such that the front and back surfaces of an air temperature measuring resistor 7 are directly exposed to an air flow 9, and furthermore, an electrical insulating material such as alumina is used. An external circuit 14 having a terminal electrode 15 and a signal processing circuit formed on a substrate is similarly fixed on a support 13b. This measuring element 1 and external circuit 1
Reference numeral 4 denotes a state in which the terminal electrodes 8 and 15 are electrically connected to each other by wire bonding using a gold wire 16 or the like.
6. In order to protect the electrode terminals 8, 15 and the external circuit 14, they are hermetically protected by the support 13a.

As shown in FIGS. 4 and 5, the measuring element 1 thus mounted has a first cavity 3a and a second cavity 3b each having a lower surface provided by a support 13b and an upper surface provided by an electric insulator. Due to 10 it is substantially isolated from the air flow 9. Therefore, unlike the conventional example, since the cavity has no opening in the air flow, dust and the like which are problematic when measuring the air flow rate of an internal combustion engine such as an automobile do not accumulate in the cavity or the opening. Reliable measurement becomes possible.

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 the heat of the internal combustion engine, and this heat is transmitted to the measuring element 1 to cause an error in the measurement of the air flow rate. May be caused to deteriorate the temperature characteristics. On the other hand, in the present embodiment, as shown in FIG. 4, the air temperature measuring resistor 7 is disposed at a position farthest from the support 13 and further protrudes from the support 13 so that the air temperature Since the front and back surfaces are exposed by the flow 9 and the heat is sufficiently released, the configuration is excellent in the temperature characteristic which is hardly affected by the temperature rise of the intake passage 11 and the support 13.

Further, as shown in the sectional view along the line DD 'in 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 embodiment of the present invention will be described with reference to FIG. FIG. 6 shows the resistors 4a, 4b, 5, 6, and 7 of the measuring element 1 of FIG. 1 and an external circuit 14 for signal processing. In the figure, reference numeral 17 denotes a drive circuit formed by a bridge circuit for measuring the absolute value of the air flow, and 18 denotes a drive circuit formed by a bridge circuit for measuring the direction of the air flow (forward or reverse flow) and the air flow. The driving circuit 19 adds (forward) or minus (backflow) to the signal output A corresponding to the air flow rate obtained from the driving circuit 17 by the direction signal E of the air flow obtained from the driving circuit 18.
Switching circuit for obtaining an output signal F converted to
Is a power supply, 21a, 21b, 21c and 21d are resistors constituting a bridge circuit, and 22a and 22b are differential amplifiers.

The driving circuits 17 and 18 are independent circuits. The drive circuit 17 connected to the power supply 20 includes a second heating resistor 4b for measuring the air flow and a first heating resistor 4a for indirectly heating the temperature measuring resistors 5 and 6 to a constant temperature. This is a bridge circuit which is connected in series and further includes an air temperature measuring resistor 7 and resistors 21a and 21b. The differential amplifier 22a adjusts the heating current flowing through the heating resistors 4a and 4b so that the potential difference at the bridge midpoint becomes zero. With this configuration, the temperature (resistance value) of the heating resistor 4b is controlled to be higher than the temperature (resistance value) of the air temperature measuring resistor 7 corresponding to the air temperature by a certain fixed value (for example, 150 ° C.). At this time, the signal corresponding to the air flow rate by the heating resistor 4b is the potential at point A in the drawing (corresponding to the heating current).

On the other hand, the drive circuit 18 is connected to an external power supply voltage (Vref), and is a bridge circuit composed of the temperature measuring resistors 5 and 6 and the resistors 21c and 21d. The resistance temperature detectors 5 and 6 have the same resistance value when the air flow rate is zero (in this case, the potential at the point C becomes 0.5 Vref).
Is set to As described above, when the air flow 9 is in the forward flow, the temperature measuring resistor 6 located upstream is further cooled and the temperature (resistance value) decreases, while the temperature measuring resistor 5 located downstream is heated (resistance value). Rises, the potential at point C becomes 0.5 Vre
lower than f. On the other hand, when the air flow 9 flows backward, the operation is the reverse of that when the air flow 9 flows forward, and the potential at the point C becomes larger than 0.5 Vref. At this time, the potential at point C (resistance temperature detectors 5, 6)
(Corresponding to the temperature difference) corresponds to the air flow rate at which the temperature difference increases as the air flow rate increases, and the air flow rate is measured as the amount of deviation from the reference potential (0.5 Vref) at point C. . Accordingly, the air flow rate is detected together with the direction of the air flow 9 from the potential at the point C, and is output as the potential at the point D of the bridge circuit set to a predetermined value and the output signal E amplified by the differential amplifier 22b. .

The switching circuit 19 outputs an air flow signal F considering the direction of the air flow from the output A of the driving circuit 17 and the output E of the driving circuit 18. Therefore, in the configuration of FIG. 6, the air flow rate signal taking into account the direction of the air flow is output as the signal F related to the second heating resistor 4b and the signal E related to the temperature measuring resistors 5 and 6, respectively. Will be.

The configuration shown in FIG. 6 can be applied in various forms, and is not limited to the configuration shown in FIG. For example, as described above, when the heating resistors 4a and 4b are connected in parallel, the driving circuit 17 includes the first heating resistor 4a, the second heating resistor 4b, and the two air temperature measuring resistors 7. Thus, two independent driving circuits are formed. Although not shown in FIG. 6, a circuit for performing an averaging process to reduce output noise from the air flow rates of the output signals F and E may be further added to the circuit of FIG. is there. Here, as the averaging process, a simple average of the output signals F and E, a weighted average in which the output signal F of the second heating resistor 4b having a large effective detection area is weighted, or A method of selecting the output signals F and E according to the airflow region may be used. In either case, the dynamic range and noise are improved in the flow measurement.

Next, a specific example of the measuring element of the thermal air flow meter of the present invention will be described with reference to FIGS. First, silicon dioxide, silicon nitride or the like is formed as an electrical insulator 10a on the silicon semiconductor substrate 2 to a thickness of about 0.5 μm by a method such as thermal oxidation or CVD.
a, 4b, 5, 6 and 7, platinum is formed to a thickness of about 0.2 μm by a method such as sputtering, a resist is formed in a predetermined shape by a known photolithography technique, and then platinum is formed by a method such as ion milling. Is patterned. Next, after the terminal electrode 8 is formed by gold plating or the like, the portion other than the terminal electrode 8 is formed as a protective film, and the electric insulator 10b is formed to a thickness of about 0.5 μm as before. Finally, the cavities 3a and 3b are 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 cavity 3a is formed smaller than the detection effective area of the conventional example (about 0.2 mm × 1 mm: described in the specification (0028) of JP-A-7-174600), while the cavity 3b is formed. Is about 1 mm x 2 mm and the effective area is about 10
Doubled the size. As a result, the dynamic range and noise of the air flow rate signal of the second heating resistor 4b on the cavity 3b are greatly improved as compared with the conventional example. Even when the cavity 3b is enlarged as described above, the size of the measuring element 1 is about 2.5 mm × 5 mm, which is about 3 mm × 3 mm, which is about 3 mm × 3 mm described in the paragraph (0028) of JP-A-7-174600. 1.
Only four times, and because there is no opening exposed to the airflow 9 as in the conventional example, the reliability of dust resistance is improved.

[0029]

According to the present invention, two heating resistors 4 are provided.
a, 4b and temperature measuring resistors 5, 6 for measuring the direction and flow rate of air flow are formed in the electric insulator 10 on the cavities 3a, 3b formed in the semiconductor substrate, respectively, and the cavity 3b having a large effective detection area. The heating resistor 4b for measuring the flow rate is formed on the upper side, and the flow rate signal from the temperature measuring resistors 5 and 6 and the flow rate signal of the heating resistor 4b are both output and averaged, so that the air is obtained. The dynamic range and noise at the time of measuring the flow rate are improved, and since there is no opening for the air flow 9, the reliability of dust resistance is improved. By adopting a configuration in which the body 7 protrudes into the air flow, a thermal air flow meter with improved temperature characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a plan view of a measuring element of a thermal air flow meter according to an embodiment of the present invention.

FIG. 2 is a sectional view taken along line AA ′ and line BB ′ of the measuring element of FIG. 1;

FIG. 3 is a cross-sectional view of a thermal air flowmeter on which the measuring element of FIG. 1 is mounted.

FIG. 4 is a plan view of the measuring element unit of FIG. 3;

FIG. 5 is a cross-sectional view taken along CC ′ and DD ′ in FIG. 4;

FIG. 6 shows resistors 4a, 4b, 5, 6, 7 and an external circuit 14.
FIG.

FIG. 7 is a plan view of a measuring element of a conventional thermal air flow meter.

FIG. 8 is a plan view of a measuring element of a conventional thermal air flow meter.

[Explanation of symbols]

1 ... measuring element, 2 ... semiconductor substrate, 3a, 3b ... cavity,
4, 4a, 4b: heating resistor, 5, 6, temperature measuring resistor, 7
... air temperature measuring resistor, 8 ... terminal electrode, 9 ... air flow.

Claims (6)

[Claims] 1. A semiconductor substrate having an electric insulating film on an upper surface and having first and second cavities extending from a boundary surface of the electric insulating film to a lower surface, and formed on the electric insulating film on the first cavity. A first heating resistor, and a pair formed on the electrical insulating film on the first cavity and symmetrically formed in the upstream and downstream directions of the fluid to be measured close to the center of the first heating resistor. A temperature measuring resistor, a second heating resistor formed on the electric insulating film on the second cavity, and formed on the electric insulating film other than the first and second cavities. A measuring element comprising a fluid temperature measuring resistor, a control means for keeping a temperature difference between the first and second heating resistors and the fluid temperature measuring resistor constant, and the pair of temperature measuring resistors. Means for detecting the flow direction of the fluid to be measured from the temperature difference of the body, and the temperature difference between the pair of resistance temperature detectors and Serial second thermal air flow meter, characterized in that it consists of means for detecting the flow rate of the fluid to be measured from the value of the current flowing in the heating resistor. 2. The thermal air flowmeter according to claim 1, wherein said first and second heating resistors are electrically connected in series on said electrical insulating film. 3. A means for detecting and outputting a flow rate of the fluid to be measured, which is independent of a temperature difference between the pair of temperature measuring resistors and a current value flowing through the second heating resistor, respectively. Thermal air flow meter equipped with. 4. The method according to claim 1, wherein a flow rate of the fluid to be measured is detected and obtained from a temperature difference between the pair of temperature measuring resistors and a current value flowing through the second heating resistor. A thermal air flow meter provided with a means for outputting a flow value averaged from each flow value. 5. The thermal air flow meter according to claim 1, wherein the electric resistance of the first heating resistor is smaller than the electric resistance of the second heating resistor. 6. The method of claim 1, 2, 3, 4, or 5,
A thermal air flowmeter wherein the first cavity is formed smaller than the second cavity.