The present invention relates to a microbridge flow sensor for detecting the flow speed of a very small amount of gas.
A conventional microbridge flow sensor of this type is a flow sensor chip having a thin-film bridge structure having a very small heat capacity, which is formed by a thin-film forming technique and an anisotropic etching technique, as shown in FIGS. 3(a) to 3(b). This sensor has many advantageous features, e.g., a very high response speed, high sensitivity, low power consumption, and good mass productivity.
FIGS. 3(a) and 3(b) show an arrangement of a microbridge flow sensor. FIG. 3(a) is a perspective view of the sensor. FIG. 3(b) is a sectional view taken along a line B-B' in FIG. 3(a). Referring to FIGS. 3(a) and 3(b), a through hole 4 is formed in the central portion of a substrate 1 by anisotropic etching so as to communicate with left and right openings 2 and 3. A bridge portion 5 is integrally formed above the through hole 4 so as to be spatially isolated from the substrate 1 in the form of a bridge. As a result, the bridge portion 5 is thermally insulated from the substrate 1. A thin-film heater element 7 and thin-film temperature-measuring resistive elements 8 and 9 are arranged on the upper surface of the bridge portion 5 such that the element 7 is located between the elements 8 and 9. These elements are covered with a protective film 6. In addition, a peripheral thin-film temperature-measuring resistive element 10 is formed on a corner portion of the substrate 1.
In this arrangement, if the heater element 7 is controlled at a temperature higher than ambient temperature by a predetermined temperature, the temperature distribution near the thin-film bridge portion becomes symmetrical about the heater element 7. If, for example, a gas moves from a direction indicated by an arrow 11 in FIG. 3(a), the upstream side temperature-measuring resistive element 8 is cooled, and heat conduction from the heater element 7 to the downstream side temperature-measuring resistive element 9 is promoted through the flow of the gas as a medium. As a result, the temperature of the element is increased, and a difference in temperature between the elements 8 and 9 appears. If the temperature-measuring resistive elements 8 and 9 formed on both the sides of the heater element 7 are incorporated in a Wheatstone bridge circuit, the temperature difference can be converted into a voltage, and a voltage output corresponding to a flow speed can be obtained. Hence, the flow speed of the gas can be detected, as shown in FIG. 3(c).
In the above-described conventional microbridge flow sensor, however, since the temperature-measuring resistive elements 8 and 9 are arranged on both the sides of the heater element 7 as shown in FIG. 3(a), the following problem is posed when the flow speed of a gas is to be measured by incorporating these elements 8 and 9 into a Wheatstone bridge circuit. As indicated by a characteristic curve A in FIG. 3(c), since the output voltage is increased with an increase in flow speed, a sufficient voltage can be obtained when the flow speed is high. However, when the flow speed is low, the output voltage is decreased, and satisfactory sensitivity cannot be obtained.