1. Field of the Invention
This invention relates to a flow sensor that comprises a heating element and a fluid temperature detecting element, which are formed on at least one substrate.
2. Description of the Prior Art
A conventional flow sensor comprises a heater, which is placed within a bypass flow tube connected to a main flow tube. A part of the main flow of fluid is directed to the bypass flow tube in which the fluid is heated by the heater. The flow rate is determined by the fluid-temperature distribution in the direction of the flow in the bypass flow path. A flow sensor of this type can detect the flow rate accurately so that it is widely used as a mass-flow controller for semiconductor gases, or the like. This flow sensor, however, is not suited for miniaturization or mass production because it requires a bypass flow tube, which makes the whole structure of the sensor complicated. Moreover, its production cost is so high that it can only be applied to a limited field.
Another conventional flow sensor comprises a heating element and a fluid temperature detecting element which are placed within a flow path. The heating element heats fluid around it and the amount of heat transferred from the heating element to the fluid is detected by the fluid temperature detecting element. The detected value is used to determine the flow rate of the fluid. In a flow sensor of this type, the difference in temperature between the fluid and the heating element is maintained at a fixed level, so as to compensate for the influence of the fluid temperature on the value of the flow rate to be detected. The output of the flow sensor can be enhanced by setting the above-mentioned difference in temperature at a high level.
In one of the flow sensors of this type, there is a flow sensor comprising a heating transistor and a fluid temperature detecting transistor, both of which are formed on a silicon chip. This flow sensor can be mass-produced by the use of a silicon processing technique, but it is disadvantageous in that the temperature characteristics vary between the sensors. Thus, it is difficult to obtain sensors having the desired temperature characteristics.
Resistor wires made of materials having high melting points such as platinum, tungsten, or the like, are often used as the heating element and the fluid temperature detecting element for this type of flow sensor, but the resistance of the wire made of these materials is small and varies between the elements, so that adjustability of the heating temperature and accuracy of the temperature measurement of the sensor are poor. It is also disadvantageous in that a sensor of this type requires fine wires of platinum, tungsten, or the like, which are extremely difficult to produce and thus cannot be mass-produced.
In order to overcome the above-mentioned problems, a thermal film-type flow sensor comprising a thin-film heating element and a thin-film fluid temperature detecting element that are respectively formed on two separate heat-insulating substrates has been proposed. This type of flow sensor can be miniaturized by the use of a fine patterned thin metal film. Moreover, this flow sensor can be obtained by a process in which a single substrate having a number of thin-film elements formed thereon are cut out into each unit, resulting in a number of units with uniform characteristics at the same time. Thus, the sensor can be readily mass-produced with uniform sensor-characteristics.
FIG. 9 shows the above-mentioned thermal thin-film type flow sensor fixed within a flow path 1. Fluid flows from the direction L toward the direction R within the flow path 1. A fluid temperature detecting unit 2 is disposed upstream of a heating unit 3, so that the fluid temperature detecting unit 2 may not be directly influenced by the heat of the heating unit 3. Both the units 2 and 3 comprise lead terminals, which are mechanically and electrically connected to sockets 5 via supports 4. The sockets 5 are attached to the wall 1a of the flow path 1. In this flow sensor, when the temperature of the wall 1a is different from that of the fluid, heat travels to and from between the wall 1a and the fluid temperature detecting unit 2 via the supports 4. This prevents the accurate measurement of the fluid temperature with use of the fluid temperature detecting unit 2. Also in this sensor, the heat of the heating unit 3 tends to be conducted through the supports 4 to the wall 1a, from which the heat is then transferred to the fluid temperature detecting unit 2. This prevents the fluid temperature detecting unit 2 from detecting the temperature of the fluid with accuracy.
To solve this problem, a flow sensor shown in FIG. 10, in which the units 2 and 3 are directly connected to the socket 5 on the wall 1a without the supports 4, has been proposed. Thus, the substrates, on each of which the thin-film heating element and the thin-film fluid temperature detecting element are respectively formed, are connected to the sockets 5 without the supports 4. In this flow sensor, heat does not travel between the fluid temperature detecting unit 2 and the wall 1a, and heat of the heating unit 3 is not conducted to the wall 1a, because heat-insulation effect is obtained between the heating element and the wall 1a and also between the fluid temperature detecting element and the wall 1a due to the heat-insulating substrates.
As materials for the substrate, materials having high thermal conductivity, i.e., silicon, alumina, or the like are often used. In such a case, in order to obtain sufficient heat-insulation effect between the two elements and the wall 1a, the substrates should be made larger in size because of their high thermal conductivity. In order to make the substrates smaller in size, glass, which has low thermal conductivity, has been proposed as a material for the substrate (Preprints on the sixth symposium "Basics and Application of Sensors" sponsored by Electronic Device Technique Committee of Electrical Society, pp. 95-96). When glass is used as the material for the substrates, they can be made smaller because sufficient heat-insulation effect can be obtained between the elements and the wall 1a even with smaller substrates due to their low thermal conductivity. Glass is also advantageous in that it is inexpensive and the substrates made of it can be used under a relatively wide variety of environmental conditions. Glass, however, has only a small mechanical strength, so that it is difficult to process glass into a substrate, and the substrate made of it is fragile and difficult to handle. Glass does not have sufficient heat-resistance, so that the glass substrate cannot be used in a high-temperature atmosphere, and the sensor comprising the glass substrate cannot be cleaned by being heated by its heating element.