JPH05223613A - Flow sensor - Google Patents

Abstract

PURPOSE:To achieve a reduction in drifting along with a higher stability of power consumption when a drive current of an exothermic resistor by using for the exothermic resistor a material with a resistance temperature coefficient thereof smaller than that of the material of a resistance thermometer bulb. CONSTITUTION:A thermal oxidation film 4 is formed on a silicon substrate 8 to check damage in a plasma etching and a ground dielectric layer 5 thereon. Then, a Pt thin film made by a sputtering method undergoes a patterning to form an exothermic resistor 3 on the dielectric layer 5. An Ni thin film made by a sputtering method undergoes a patterning to form an upstream side resistance thermometer bulb 1 and a downstream side resistance thermometer bulb 2 separately. Thereafter, an embossing is performed by plasma etching from the rear of the substrate 8 to form a hollow part at a specified dimension. Thus, the use of the material with a resistance temperature coefficient smaller than that of the material of the resistance thermometer bulb for the exothermic resistor enables the prevention of a change in the power consumption as caused by a change in the temperature of a fluid without lowering a sensor output with respect to a flow rate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flow sensor for measuring the flow rate of a fluid because the resistance value of a resistance temperature detector heated by a heating resistor changes as the fluid passes through.

[0002]

2. Description of the Related Art As a flow sensor as described above, there is known a flow sensor in which a fluid is flown into a pipe and a heating resistor and a temperature measuring resistor are placed on the outer wall of the pipe. FIG. 2 shows such a flow sensor, in which a resistance wire is wound on the outer wall of a pipe 22 in which a fluid 21 flows to form an upstream temperature measuring resistor 23, a downstream temperature measuring resistor 24, and a heat generating resistor 25. The bridge circuit detects a difference in resistance value between the resistors 23 and 24 caused by the flow of fluid. However, in order to further increase the sensitivity, a flow sensor has been manufactured in which a heating resistor and a temperature measuring resistor are formed of a metal thin film on a diaphragm made of a dielectric layer having a small heat capacity. In either case, a metal is used as the material of the resistance temperature detector whose resistance value changes due to passage of a fluid in order to increase the temperature difference between the upstream and downstream resistors. For example, as seen in JP-A-60-142268, a resistor is made of a permalloy thin film having a temperature coefficient of resistance of 4000 ppm / ° C.

FIGS. 1 (a) and 1 (b) show the structure of a conventional flow sensor having a thin film resistor.
As described in Japanese Patent Publication No.
The temperature measuring resistor 2 and the heat generating resistor 3 on the downstream side are formed of the same material on the underlying dielectric layer 4 in close proximity to each other on the same plane. When a constant current is applied to the heating resistor 3 via the electrode 5, the temperature of the temperature measuring resistors 1 and 2 rises. Then, the temperature decreases when the fluid flows on the surfaces of the resistance temperature detectors 1 and 2 at right angles to the length direction, and the resistance value change of the resistance temperature detectors 1 and 2 due to the temperature change is passed through the electrode 6. Calculated using a bridge circuit connected by The underlying dielectric layer 5 is the heating resistor 3
It is also intended to reduce the power consumption by suppressing the insulation of the resistance temperature detectors 1 and 2 and the heat radiation from them.

This flow sensor is usually pulse-driven. That is, in order to reduce the power consumption, for example, a pulse current having a width on the order of ms is applied to the heating resistor 3 at regular intervals to reduce the amount of heat generation and enhance the sensitivity. For example, a voltage is applied to 1 in the form of a pulse having a constant width, and the resistance value is detected. Therefore, in order to increase the response speed, it is necessary to make the rising edges of these pulses steep.

In such a flow sensor, the thermal oxide film 4 is removed.
CV on the surface of the silicon substrate 8 formed to a thickness of 4000Å
The SiO ₂ layer 5 is formed by the D method, then a Ni film having a large temperature coefficient is formed on the SiO ₂ layer 5, and the temperature measuring resistors 1 and 2 and the heating resistor 3 are patterned by the photolithography method.
At the same time, the electrodes 6 and 7 are formed, and then the cavity 9 is processed from the back surface of the substrate 8 by plasma etching below the temperature measuring resistors 1 and 2 and the heating resistor 3. When the cavity 9 is processed, the thermal oxide film 4 is removed by plasma, so that only the SiO ₂ layer 5 formed by the CVD method exists below the temperature measuring resistors 1 and 2 and the heat generating resistor 3, and the heat capacity is reduced. Becomes smaller, the temperature changes of the resistance temperature detectors 1 and 2 become sensitive.

[0006]

As described above, in the conventional photolithography, since the heating resistor and the temperature measuring resistor of the sensor section are made of the same material having a large temperature coefficient,
The formation process was simple, but when the drive current of the heating resistor was turned on, the temperature coefficient was high until the thermal insulation part where the sensor part was formed reached a certain temperature distribution according to the heat capacity. Since the amount of increase in the resistance value of the heating resistor is large, the power consumption fluctuates and the rise time becomes long. As described above, since the signal detection of the resistance temperature detector which measures the temperature difference when the fluid passes is performed in the ms order, if the time to reach the steady temperature distribution is long, extra power consumption is required. This also applies to the flow sensor using the resistance wire as shown in FIG.

An object of the present invention is to eliminate the above-mentioned drawbacks, improve the stability of power consumption when the drive current of the heating resistor is turned on, and reduce the drift when detecting the ms order. To provide.

[0008]

In order to achieve the above object, the present invention is directed to an upstream temperature measuring resistor in the direction of fluid flow,
In a flow sensor in which a heating resistor and a downstream temperature measuring resistor are arranged in order, the heating resistor is made of a material having a resistance temperature coefficient smaller than that of the material of the temperature measuring resistor. The heating resistor and the temperature measuring resistor may be a thin film formed on a diaphragm made of a dielectric layer, or may be a wire wound on a fluid conduit. The heating resistor is made of one of Nb, Pt, Ir, and Ta or its alloy, and the resistance temperature detector is Ni, Fe, Co, Be, W, Rh, Zr, Cu, M.
It is effective to use one of o and Ti or its alloy. Alternatively, if the heating resistor is W, Rh, Zr, Cu, Mo,
Made of one of Ti or its alloy,
It is effective to consist of one of Ni, Fe, Co and Be or an alloy thereof. In particular, the heating resistor is made of Pt and the resistance temperature detector is made of one of Ni, W, Rh, Zr, Mo, Ti and Ni-Fe alloy, or the heating resistor is made of W and It is effective that the temperature resistor is made of one of Ni, Fe, and Co.

[0009]

[Function] A metal resistance wire or metal thin film resistor having a small temperature coefficient of resistance is used for the heating resistor, and the resistance temperature detector is formed by a metal resistance wire or metal thin film resistor having a resistance temperature coefficient larger than that of the material of the heating resistor. By doing so, it is possible to prevent the power consumption from fluctuating due to changes in the temperature of the fluid without lowering the sensor output with respect to the flow rate, and to stabilize the power consumption when the drive current is turned on to the heating resistor. The time can be shortened, and the detection speed of the resistance temperature detector can be increased.

[0010]

EXAMPLE The structure of the flow sensor of one example of the present invention is the same as that of FIG. 1 and was manufactured as follows. That is, the thermal oxide film 4 that suppresses damage during plasma etching is formed on the silicon substrate 8, and the underlying dielectric layer 5 is formed thereon. Next, the Pt thin film formed by sputtering on the underlying dielectric layer 5 is patterned to form the heating resistor 3.
Further, the Ni thin film formed by the sputtering method is patterned to form the upstream temperature measuring resistor 1 and the downstream temperature measuring resistor 2, respectively. After that, recess processing is performed from the back surface of the base body 8 by plasma etching to form a cavity portion 9 of 800 μm square. In this embodiment, when the upstream resistance temperature detector 1 and the downstream resistance temperature detector 2 are formed after forming the heating resistor 3,
A suitable etchant can be used for patterning, but if it is difficult to select the etchant due to the combination of metals, after forming the heating element, cover it with another dielectric layer and then form the resistance temperature detector. Is also possible.

FIG. 3 compares the time during which the power consumption is constant when the heating resistor is turned on, with the temperature coefficient being large or small. As a result, it was found that the one having a smaller temperature coefficient of resistance α was in a steady state in a shorter time. Figure 4
The figure shows the fluctuation of the power consumption of the heating resistor when the room temperature and 70 ° C. are changed as the environmental conditions at a cycle of t ₀ = 30 seconds. As a result, it was found that the heating resistor having a smaller temperature coefficient has more stable power consumption.

Therefore, as in the above embodiment, the resistance temperature detectors 1 and 2 are made of Ni having a resistance temperature coefficient of 6750 ppm / ° C.,
By forming the heating resistor 3 with Pt having a temperature coefficient of resistance of 3920ppm / ° C, the rise time of the temperature of the heating resistor 3 and the power consumption when driving current is reduced, and the uniformity of the sensor surface temperature is improved. did. Moreover, the fluctuation of the power consumption with respect to the temperature change of the fluid was small, and the fluctuation of the sensitivity was also small.

Various examples can be considered as a combination of materials of the heating resistor and the temperature measuring resistor. Table 1 shows metals that can be used as resistors and have a melting point of 1000 ° C. or more and a temperature coefficient of 3500 ppm / ° C. or more in the temperature range of 0 to 20 ° C.

[0014]

[Table 1]

For example, as the material of the heating resistor 3, one of the metals of group III or its alloy is selected, and as the material of the resistance temperature detectors 1 and 2, one of the metals of group I or group II or its alloy is selected. Choose. When the heating resistance single crystal 3 is made of Pt, the resistance temperature detectors 1 and 2 are set to Ni, W, Rh, Zr, Mo, Ti or Ni-Fe.
It is made of alloy (Permalloy). As another combination, a metal of group II or one of its alloys is selected as the material of the heating resistor 3, and a metal of group I or one of its alloys is selected as the material of the resistance temperature detectors 1 and 2. When the heating resistor 3 is made of W, the temperature measuring resistors 1 and 2 are made of Ni, Fe or Co. However, you may select and combine a thing with a small resistance temperature coefficient and a thing with a large resistance temperature coefficient in the same group.

Such a combination of materials can be applied when the resistor is a thin film or a wire. The temperature coefficient in Table 1 is for bulk, and for thin films it is about 40% smaller than this value.

[0017]

According to the present invention, it is possible to stabilize the power consumption of the heating resistor without lowering the output sensitivity by using a wire or thin film of a material having a resistance temperature coefficient smaller than that of the resistance temperature detector for the heating resistor. It is possible to shorten the time until it is done and to improve the stability of the power consumption of the heating resistor against the ambient temperature. As a result, it was possible to obtain a flow sensor in which the output detection time was short, the power consumption of the drive circuit was small, and the influence of changes in environmental temperature on the output was small.

[Brief description of drawings]

FIG. 1 shows an example of a thin film resistor type flow sensor according to the present invention, in which (a) is a plan view and (b) is a sectional view taken along line AA of (a).

FIG. 2 is a side view showing an example of a wire resistor type flow sensor according to the present invention.

FIG. 3 is a diagram showing a change in power consumption from the time when the drive current of the heating resistor is started to flow.

FIG. 4 is a change diagram of the power consumption of the heating resistor when the environmental temperature changes.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Upstream temperature measuring resistor 2 Downstream temperature measuring resistor 3 Heating resistor 5 Underlying dielectric layer 8 Silicon substrate 9 Cavity 23 Upstream temperature measuring resistor 24 Downstream temperature measuring resistor 25 Heat generating resistor

Claims (7)

[Claims] 1. An upstream temperature measuring resistor, a heat generating resistor, and a downstream temperature measuring resistor are sequentially arranged in a fluid flow direction, wherein the heat generating resistor has a resistance temperature coefficient higher than that of the material of the temperature measuring resistor. A flow sensor characterized by being made of a small material. 2. The heating resistor and the temperature measuring resistor are thin films formed on a diaphragm made of a dielectric layer.
The described flow sensor. 3. The flow sensor according to claim 1, wherein the heating resistor and the temperature measuring resistor are wires wound on a fluid conduit. 4. The heating resistor is made of one of niobium, platinum, iridium and tantalum or an alloy thereof, and the resistance temperature detector is nickel, iron, cobalt, beryllium, tungsten, rhodium, zirconium, copper, molybdenum and The flow sensor according to claim 1, which is made of one of titanium or an alloy thereof. 5. The flow sensor according to claim 4, wherein the heating resistor is made of platinum, and the temperature measuring resistor is made of one of nickel, tungsten, rhodium, zirconium, molybdenum, titanium and nickel, and an iron alloy. 6. The heating resistor is made of one of tungsten, rhodium, zirconium, copper, molybdenum and titanium or an alloy thereof, and the resistance temperature detector is nickel, iron,
The flow sensor according to claim 1, 2 or 3, which is made of one of cobalt and beryllium or an alloy thereof. 7. The flow sensor according to claim 6, wherein the heating resistor is made of tungsten and the temperature measuring resistor is made of one of nickel, iron and cobalt.