WO2003052355A1

WO2003052355A1 - Flow sensor

Info

Publication number: WO2003052355A1
Application number: PCT/JP2001/010958
Authority: WO
Inventors: Tsukasa Matsuura; Naoki Yutani; Kazuhiko Tsutsumi; Yuji Ariyoshi
Original assignee: Mitsubishi Denki Kabushiki Kaisha
Priority date: 2001-12-14
Filing date: 2001-12-14
Publication date: 2003-06-26
Also published as: JPWO2003052355A1

Abstract

A flow sensor is constructed to satisfy the relation f = k1/d - k2 where k1 = 19,000, 0 ≤ k2 ≤ 200, f (Hz) is the switching frequency at which heating resistors are alternately heated, d (µm) is the distance between the heating resistor and a temperature sensor, and k1 and k2 are constants. The distance between the heating resistor and the temperature sensor is 1µm or more and 30µm or less. As a result, the output frequency can be set the sampling frequency or higher of an engine control unit for an automobile, and the flow sensor can be applied to automobile.

Description

Description Flow sensor Technical field

The present invention relates to a flow sensor for measuring a gas flow, and more particularly to a flow sensor for an automobile. Background art

Among the flow sensors, especially for in-vehicle flow sensors, those with high detection accuracy, high-speed responsiveness, and a wide dynamic range have been demanded in recent years due to the stricter exhaust gas regulations. As a method to respond to this demand, a thermal type flow sensor that can directly detect the mass air flow has become mainstream. As one type of such a thermal type flow sensor, a thermal type flow sensor that controls two sets of heat generating resistors and a temperature sensor so that the temperatures thereof are equal to each other is disclosed in Japanese Patent Laid-Open Publication No. H11-326600. It is proposed in Japanese Patent Publication No.

A conventional thermal flow sensor disclosed in Japanese Patent Application Laid-Open No. H11-326003 will be described with reference to FIGS.

FIG. 9 is a diagram showing the configuration and operation principle of the flow sensor. In the figure, R1 to R6 are resistors, 100 is a flow tube through which the measurement fluid flows, and resistors R1 to R4 are arranged in the flow tube 20 through which the measurement fluid flows, and are exposed to the flow. ing. Of the resistors R 1 to R 6, R 1 and R 4 are heating resistors, R 2 and R 3 are temperature sensors, heating resistor R 1 and temperature sensor R 2, temperature sensor R 3 and heating resistor R 4 is configured as a pair. Here, Ql and Q2 are switches, 102 is a comparator, and 104 is an inverter.

The operation of the flow sensor having the above configuration will be described below.

When the temperature of the resistors Rl and R2 is higher than the temperature of the resistors R3 and R4, one input voltage of the comparator 102 becomes higher than the + side, so that the output of the comparator 102 becomes one, Switch Q1 is off and switch Q2 is on. As a result, the temperatures of the resistors R 1 and R 2 gradually decrease due to heat radiation. On the other hand, current flows through resistor R4. Therefore, the temperature of the resistors R3 and R4 gradually increases.

After a certain time, when the temperature of the resistors R 3 and R 4 becomes higher than the temperature of the resistors R 1 and R 2, the output of the comparator 102 is inverted, and the switch Q1 is turned on and the switch Q2 is turned off.

Hereinafter, control is performed such that the temperature of the resistors Rl and R2 and the temperature of the resistors R3 and R4 are always equal by the repetition of such switch switching operation, that is, the oscillation of the circuit. The output from the circuit, ie, the sensor output, is a digital output from the comparator 102 as indicated by “info out”.

For example, as shown in Figure 10, the sensor output is high during the time t]. When switch Q1 is on and conducting to resistor R1, while switch Q2 is on and resistor R4 The sensor output is low during the time t2 when power is supplied to the switch. The switching of this circuit occurs spontaneously, and its period t1 + t2 is determined by the thermal time constants of resistors Rl, R2 and resistors R3, R4.

FIG. 11 is a graph showing the flow rate and the temperature distribution on the sensor. The method of measuring the flow rate will be described below with reference to this figure. In the figure, the horizontal axis is position, and the vertical axis is temperature (all are arbitrary units).

When the flow rate is = 0, a temperature distribution is formed symmetrically between the resistors R]., R2 and the resistors R3, R4 (position = 0) shown in Fig. 9, and the resistor Rl , R 2 and resistors R 3, R 4 have the highest temperature in the vicinity (position = near 0.0002). When the flow rate = 0, the sensor output is t1 = t2, and the duty ratio is 50% (t1 /

(t 1 + t 2) X 100 = 50) pulse output. When the flow rate increases, the temperature on the upstream side drops sharply. The gas heated by the resistor R1 on the upstream side flows into the downstream side, so that the temperature does not decrease as steeply as on the upstream side. As described above, since the temperatures of the resistors R l and R 2 and the resistors R 3 and R 4 are controlled to be equal, the time t 1 during which the current flows to the upstream resistor R 1, and The relationship of the time t2 when the resistor R4 on the downstream side is energized is tl> t2, and the duty ratio of the sensor output changes. Since the duty ratio and the flow rate have a one-to-one relationship, the flow rate can be measured.

FIG. 12 shows an example of the structure of a conventional flow sensor. In the figure, 105 is silicon 106 is a groove formed by etching silicon, and 107 is a bridge supporting resistors R1 to R4. The bridge is made of a thin film and has resistors Rl and R2 and resistors R3 and R4.

The thermal type flow sensor proposed in Japanese Patent Application Laid-Open No. H11-132603 is configured as described above, and the measured flow rate information is digitized by a comparator and output. These outputs are then input to a signal processing circuit, where they are processed as useful data.

For example, the output signal of an automotive flow sensor is input to an engine control unit (ECU) and processed along with data from other sensors, such as air pressure and temperature, to determine the optimal fuel injection quantity. Used to Thus, signal processing circuits such as engine-control units sample the signal from the flow sensor at a certain frequency to make the processed data meaningful. In particular, the higher the response system, the higher the sampling frequency. Therefore, it is meaningless if the digitizing frequency of the sensor signal, that is, the sensor output frequency, is lower than this sampling frequency, and it is necessary that the frequency be at least the same or higher.

In a conventional thermal flow sensor as proposed in Japanese Patent Application Laid-Open No. H11-32603, there is no mention of a method of designing the frequency of the sensor output. Instead, the frequency was random. Therefore, there has been a problem that it cannot be used as it is as a flow sensor that requires high-speed response, such as a flow sensor for an automobile.

The present invention has been made in view of the above-mentioned problems of the related art, and it is possible to appropriately set the output frequency of the flow sensor. In particular, by setting the output frequency to be equal to or higher than the sampling frequency, The purpose is to provide a reliable flow sensor that can be applied to highly responsive systems such as automobiles. Disclosure of the invention

In order to achieve the above object, the present invention provides a silicon substrate, a support film of an insulating thin film formed on the surface of the silicon substrate, and a support film formed on the support film and supported by the support film. It has two sets of heating resistors and a temperature sensor, and the temperature of the heating resistor and the temperature sensor on the upstream side and the temperature of the heating resistor and the temperature sensor on the downstream side in the flow direction of the measurement fluid. So that the upstream and downstream heating resistors are alternately heated so that the flow rate of the measurement fluid is measured based on the ratio of the heating time of the upstream and downstream heating resistors. Where f (H z) is the switching frequency for alternately heating the upstream and downstream heating resistors, d (μm) is the distance between each pair of heating resistors and the temperature sensor, and k 1 is the constant. , K 2, the relational expression f = kl / d−k 2, k 1 = 1900 000, 0≤k 2 ≤200.

With such a configuration, the structure for achieving the output frequency required for the flow sensor, that is, the switching frequency f, can be determined by the distance d between the heating resistor and the temperature sensor. It can be applied to a flow sensor for

The distance d between the heating resistor and the temperature sensor of each set is preferably 1 m or more and 30 μm or less.

Since the sampling frequency of the automotive engine control unit is around 800 Hz, the switching frequency for heating the heating resistors alternately, that is, the output frequency is desirably higher. With the above configuration, the output frequency is set to 80 OHz or more, which is desirable when the flow sensor of the present method is applied to a flow sensor for an automobile. BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A and 1B show a flow sensor according to a first embodiment of the present invention, wherein FIG. 1A is a plan view thereof, and FIG. 1B is a cross-sectional view taken along line Ib-Ib in FIG. is there.

FIG. 2 is a process chart for explaining a method of manufacturing the flow sensor of FIG.

FIG. 3 is a control circuit diagram of the flow sensor of FIG.

FIG. 4 is an output waveform diagram of the flow sensor of FIG.

FIG. 5 is a graph showing the relationship between the distance between the heating resistor and the temperature sensor and the output frequency in the flow sensor of FIG.

Figure 6 shows the output frequency, heating resistor, and temperature sensor of the flow sensor in Figure 1. It is a graph which shows the relationship with distance.

7A and 7B show a flow sensor according to a second embodiment of the present invention, wherein FIG. 7A is a plan view thereof, and FIG. 7B is a cross-sectional view taken along line VEb-VHb in FIG.

FIG. 8 shows a flow sensor according to a third embodiment of the present invention, (a) is a plan view thereof, and (b) is a cross-sectional view taken along line Mb-Mb in (a).

FIG. 9 is a control circuit diagram of a conventional flow sensor.

FIG. 10 is an output waveform diagram of the conventional flow sensor of FIG.

FIG. 11 is a graph showing the temperature distribution on the surface of the conventional flow sensor of FIG. FIG. 12 is a longitudinal sectional view of the conventional flow sensor of FIG. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

Embodiment 1.

FIG. 1 shows a flow sensor S1 according to the first embodiment of the present invention.

As shown in FIG. 1, the flow sensor S 1 includes an upstream temperature sensor 1, an upstream heating resistor 2, a downstream heating resistor 3, and a downstream heating resistor arranged in order from the upstream side to the downstream side of the flow. A side temperature sensor 4 is provided, and all of these resistors 1 to 4 are supported on a supporting film 6 which is made of a silicon nitride film and is a diaphragm-like insulating thin film. Each of the resistors 1 to 4 is formed of a platinum thin film having a line width of 5 / im and a thickness of 2 m, and a line interval of 5 / m. The minimum distance between the upstream-side temperature sensor 1 and the upstream-side heating resistor 2 and the minimum distance between the downstream-side heating resistor 3 and the upstream-side temperature sensor 4 are d and the deviation is d.

Figure]. The dimensions of the diaphragm indicated by broken lines in (a) are 0.75 mm or 1.5 mm in width a, 2 mm in height b, and 3 μπι in thickness t. Further, as shown in FIG. 1B, the support film 6 is supported on the silicon substrate 5 via the thermal oxidation film 9. Reference numeral 8 denotes a bonding pad for wire bonding. Next, a manufacturing process of the flow rate sensor S1 according to the present invention will be described with reference to FIG.

First, as shown in Fig. 2 (a), a thermal oxide film 9 with a thickness of 0.5 m is formed on both sides. A silicon wafer 5 having a plane orientation (100) and a thickness of 380 / m is prepared. Next, as shown in FIG. 2 (b), a silicon nitride film 11 having a thickness of about 2 / im was formed on one of the thermal oxide films 9 by sputtering. Then, a platinum film 7 having a thickness of about 0.2 / zm is further formed on the silicon nitride film 11 by sputtering. Then, as shown in FIGS. 1 (a) and 2 (d), the platinum film 7 was patterned using photolithography, and the temperature sensors 1, 4, the heating resistors 2, 3, and the wiring portion were formed. Form 10 (the wiring 10 is not shown in FIG. 2 (d)). Next, as shown in FIG. 2 (e), a silicon nitride film having a thickness of about 1 im is formed as a protective film by sputtering, and then, although not shown, a pad portion for wire bonding is formed by dry etching. Open. Further, as shown in FIG. 2 (f), the thermal oxide film 9 on the back surface is patterned and the silicon substrate 5 is anisotropically etched from the back surface using this as a mask. As the etchant, for example, an alkaline etching solution such as TM AH (tetra'methylammonium'hydroxide) is used. Finally, as shown in FIG. 2 (g), the thermal oxide film 9 on the back surface of the support film 6 and the back surface of the silicon substrate 5 is removed by wet etching to complete a series of processes.

In this embodiment, the distance d between the heating resistors 2 and 3 and the temperature sensors 1 and 4 is changed to 10 m, 14 μm, 25 μπι 50 / xm, and the width a of the diaphragm is 0.75 mm to 1 We fabricated a sample with a diameter of 5 mm and confirmed the correlation with the switching frequency. FIG. 3 is a view showing the operation principle of the flow sensor S1 according to the present invention.

As shown in FIG. 3, the flow sensor S1 includes a plurality of (in this case, six) resistors R1 to R6, and among the resistors R1 to R6, the resistors R1 to R4. Is arranged in a flow tube 20 through which the measurement fluid flows, and is exposed to the flow of the measurement fluid. Also, resistor R 1 is the upstream temperature sensor 1, resistor R 2 is the upstream heating resistor 2, resistor R 3 is the downstream heating resistor 3, and resistor R 4 is the downstream temperature sensor 4. It is. Further, the resistors R5 and R6 are resistors forming a bridge circuit together with the resistors R1 and R4.

The resistors R2 and R3 are turned on / off by switches Q1 and Q2, respectively, which are composed of transistors. The bridge circuit is connected to the comparator 22. Have been. The output of the comparator 22 is directly input to the switch Q1 and is also input to the switch Q2 via the inverter 24, and is also output to, for example, an engine control unit (ECU) or an output monitor. .

The operation of the flow sensor S1 having the above configuration will be described below.

When the temperature of the resistors R l and R 2 is higher than the temperature of the resistors R 3 and R 4, one input voltage of the comparator 22 becomes higher than the + side, so that the output of the comparator 22 becomes 1 and the switch Q 1 is off, switch Q 2 is on. As a result, the temperatures of the resistors R 1 and R 2 gradually decrease due to heat radiation. On the other hand, since current flows through resistor R4, the temperature of resistors R3 and R4 gradually increases.

After a certain time, when the temperature of the resistors R3 and R4 becomes higher than the temperature of the resistors R1 and R2, the output of the comparator 22 is inverted, and the switch Q1 is turned on and the switch Q2 is turned off.

Hereinafter, control is performed so that the temperature of the resistors Rl and R2 and the temperature of the resistors R3 and R4 are always equal by the repetition of such a switch switching operation, that is, the oscillation of the circuit. The output from the circuit, ie, the sensor output, is a digital output from the comparator 22.

For example, as shown in Figure 4, the sensor output is high during time t1 when switch Q1 is on and resistor R2 is energized, while switch Q2 is on and resistor R3 The sensor output is low during the time t2 during which power is supplied to the sensor. The switching of this circuit is performed spontaneously, and its period T = t1 + t2 is determined by the thermal time constant τ of the resistors R1, R2 and the resistors R3, R4. The switching frequency f for alternately heating the heating resistors is the reciprocal of the period Τ, f = 1 ノ.

In the flow rate sensor S1, since such a circuit is configured, the temperature of the resistors R1, R2 and the temperatures of the resistors R3, R4 are controlled to be always equal. When the flow for measurement occurs in the direction of the arrow shown in the figure, the resistors R l and R 2 on the upstream side take more heat than the resistors R 3 and R 4 on the downstream side. The relationship between the time t1 when the resistor R1 is energized and the time t2 when the resistor R4 on the downstream side is energized is t1> t2, and the duty ratio of the sensor output changes. When the duty ratio and the flow rate have a one-to-one relationship, the force, the flow rate, and the like can be measured. Experiments have shown that the switching frequency for heating the heating resistors alternately, that is, the output frequency f, is linear with the reciprocal 1 / d of the distance between the heating resistors and the temperature sensor.

Fig. 5 is a graph showing the relationship between the reciprocal 1 / d of the distance between the heating resistor and the temperature sensor and the frequency f of the flow sensor output. The four points are on a straight line, and it can be seen that the smaller the d (the larger the lZd), the higher the output frequency f of the flow sensor. The straight line has a relationship of f = 1900 O / d—k2, where k2 is 200 when the diaphragm width a is 0.75 mm and 100 when the diaphragm width a is 1.5 mm. k 2 takes a larger value as the amount of heat escaping from the heating resistor to the silicon substrate through the diaphragm, that is, as the heat loss increases. Therefore, the larger the tire flam, the smaller the value of k2, and in an ideal case without heat loss, k2 = 0, and the straight line of f = 1900 O / d shown by the broken line in Fig. 5 Become.

The prototype sensor was actually operated, and power consumption and heat loss were measured. The power consumption at a flow rate of 8 m / s (30 g / s in flow rate) was 61 mW. At this time, the air temperature was 23 ° C, and the temperature of the heating resistor was 143 ° C.

At this time, the amount of heat Q that escapes from the heating resistor through the diaphragm to the silicon substrate, that is, the value of heat loss, can be calculated as follows. In this method, switching is performed, so it is only necessary to consider the heat loss from either the upstream heating resistor or the downstream heating resistor.

For example, it is considered that most of the heat escaping from the upstream heating resistor 2 escapes to the silicon substrate on the left side in FIG. 1 (a). The length L of the upstream heating resistor 2 is 1 mm, the thickness t of the support film 6 is 3 μm, and the distance L ′ (m) from the upstream heating resistor 2 to the left silicon substrate 5 is the width of the diaphragm a. When is 0.75mm, it is 250 / im. Assuming that the thermal conductivity k of the silicon nitride film 11 is 2.79 W / mK, the temperature Th of the upstream heating resistor 2 is 143 ° C, and the temperature T s of the silicon substrate 5 is 23 ° C, The cross-sectional area A is

A = L 'X t = 1 X 10- 3 X 3 X 10 _6 = 3 X 1 0- 9 (m 2)

Therefore, the heat loss Q escaping from the upstream heating resistor to the left silicon substrate 5 through the heat flow path is Q = A- k-(T hT s) size L '= 4.0 (mW)

It is. This means that the power consumption at a flow rate of 8 m / s (SOg / s in flow rate) is 6.5% of 61 mW.

In practice, the heat loss under the above operating conditions must be less than 10%, and above that, the normal operation of the flow sensor may not be performed. The heat loss of the flow sensor tested this time is 6.5%, and there is a margin of 3.5%, but it can be considered to be almost the upper limit considering the margin of the structure and usage. Therefore, it can be said that the width a of the diaphragm must be at least 0.75 mm. If the left end of the diaphragm approaches the upstream temperature sensor 1 and the right end of the diaphragm approaches the downstream temperature sensor 4, a = 0.59mm <0.75mm, which is of course not applicable.

Incidentally, the heat loss of a sensor with a diaphragm width a of 1.5 mm is 1.6 mW, and the power consumption is equivalent to 2.6% of 61 mW, so there is no problem in characteristics.

From the above results, the lower limit of the diaphragm width is 0.75 mm, and the relational expression between the output frequency f and the distance d between the heating resistor and the temperature sensor at that time is ί = 1900 O / d — 200, The value of k 2 is in the range 0≤k 2 200. That is, the flow rate sensor shown in the present embodiment has a relation between the distance d between the heating resistor and the temperature sensor and the output frequency f. As shown in FIG. 5, 力 = 19000 / d and f = 1900 OZd-200 It is designed to enter a straight line question.

Now, as shown in Fig. 3, the output signal from the flow sensor is input to an external engine-control unit (ECU) and processed together with data from other sensors, such as air pressure and temperature, to optimize Used to determine the fuel injection amount. Since the sampling frequency of the engine control unit for automobiles is mainly around 80 OHz, the output frequency must be 800 Hz or more.

Figure 5 shows the relationship between the output frequency f and the distance d between the heating resistor and the temperature sensor.

As can be seen from Fig. 5, when the output frequency f is 800 Hz or more, d = 25; um when there is almost no heat loss (the curve shown by the broken line in the figure), and usually less than that. Need to be designed with d. As mentioned above, Currently, the sampling frequency of the control unit is currently around 800 Hz, but there are also low-frequency ones.Therefore, as a flow sensor applied to automobiles, a heating resistor and a temperature sensor are used. The distance should be d ^ 30 im, preferably d≤25 im. Since the minimum gap between the heating resistor and the temperature sensor is manufactured using photolithography, the lower limit of d is 1 // m. Therefore, the distance d between the heating resistor and the temperature sensor is 1 111 to 30 / im, preferably 1 x rn to 25 / im.

Note that the output frequency of the flow sensor hardly changes depending on the thickness t of the insulating film 6, and is a function of only the distance w. This is because the thermal resistance R between the heating resistor and the temperature sensor is proportional to the reciprocal 1 / t of the thickness t of the insulating film 6, but the thermal capacitance C between the heating resistor and the temperature sensor is proportional to the thickness t Therefore, the thermal time constant τ = RC is independent of the film thickness.

In this embodiment, platinum is used as the heating resistor, but the material is nickel, nickel-iron, nickel-aluminum, tungsten, iron-palladium, nickel silicide, molybdenum silicide, titanium silicide, low-resistance A metal, alloy, silicide, silicon, or the like such as silicon or polysilicon may be used, and has the same effect as the above embodiment. Further, in this embodiment, a platinum resistor was used as the temperature sensor, but the material may be the above-mentioned metal, alloy, silicide, silicon, or the like, or a thermistor-thermocouple. This has the same effect as the above embodiment.

Further, in this embodiment, the silicon nitride film is used as the support base material. However, the material may be any material as long as the portion in contact with the heating resistor and the temperature sensor is an insulator, and may be a material other than the silicon nitride film. . Furthermore, although the temperature control circuit is shown in FIG. 3 in the present embodiment, a control method that alternately heats the heating resistors so that the temperature of the upstream heating resistor and the temperature of the downstream heating resistor become the same is used. However, other circuit configurations may be used, and the same effects as in the above embodiment can be obtained. Embodiment 2

FIG. 7 shows a flow sensor S2 according to the second embodiment of the present invention. Figure 7 In this case, the components with the same reference numerals as those in FIG. 1 are the same or equivalent. In this embodiment, the support film 6 is formed not in a diaphragm shape but in a bridge shape. The bridge can be formed by forming an opening in the support film 6 by dry etching and performing anisotropic etching of the silicon substrate 5 from both the support film side and the back surface. According to the present embodiment, the heat loss from the heat generating resistor to the silicon substrate 5 can be reduced, and the same effect as in Embodiment 1 can be obtained by increasing the size of the diaphragm. Embodiment 3.

FIG. 8 shows a flow sensor S3 according to the third embodiment of the present invention. In FIG. 8, the components denoted by the same reference numerals as those in FIG. 1 are the same or equivalent components. In this embodiment, the support film 6 is formed not in a diaphragm shape but in a bridge shape, and the support film 6 is supported on the cavities 12 of the silicon substrate 5 as shown in FIG. 1 (b). . The cavities 12 can be formed by forming an opening in the support film 6 by dry etching and performing anisotropic etching of the silicon substrate 5 from the support film side. According to this embodiment, can heat loss reduction from the heating resistor to the silicon ^ substrate 5, there same effect force ^s and to increase the size of the diaphragm in the first embodiment. Industrial applicability

As described above, since the output frequency of the flow sensor according to the present invention can be appropriately set to be equal to or higher than the sampling frequency, the flow sensor is suitable for use as a flow sensor for an automobile requiring high-speed response.

Claims

The scope of the claims

1. a silicon substrate, a supporting film of an insulating thin film formed on the surface of the silicon substrate, and two sets of heating resistors and a temperature sensor formed on the supporting film and supported by the supporting film. So that the temperature of the heating element and the temperature sensor on the upstream side in the direction of the flow of the measurement fluid are the same as the temperature of the heating element and the temperature sensor on the downstream side. This is a flow rate sensor that heats the resistors alternately and measures the flow rate of the measurement fluid based on the ratio of the heating time of the upstream and downstream heating resistors. Assuming that the switching frequency for heating is f (H z), the distance between each set of heating resistor and the temperature sensor is d (β τη), and the constants are kl and k2, the relational expression f = kl / d—k2, A flow sensor characterized by satisfying kl = 19 000, 0 ≤ k 2 ≤ 200.

2. The flow sensor according to claim 1, wherein the distance d between the heat generating resistor and the temperature sensor of each set is not less than 1/2 m and not more than 30 μm.