GB2240627A - Microbridge flow sensor - Google Patents

Abstract

In a flow sensor comprising a bridge 5' carrying temperature-measuring resistive elements 7, 8 and a heater 7 (centre), heat loss by thermal conduction from the bridge 5' to a supporting semiconductor substrate 1 is minimised by providing cut- outs 12, 13, 14, 15 or narrowings (as in figures 2 and 3) at the ends of the bridge. The remaining end areas of the bridge are just sufficient to accommodate electrical connections to the elements 7,8 and heater 7. <IMAGE>

Description

Microbridge Flow Sensor 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. 5(a) and 5(b). This sensor has many advantageous features, e.g., a very high response speed, high sensitivity, low power consumption, and good mass productivity.

Figs. 5(a) and 5(b) show an arrangement of a microbridge flow sensor. Fig. 5(a) is a perspective view of the sensor. Fig. 5(b) is a sectional view taken along a line B - B' in Fig. 5(a). Referring to Figs. 5(a) and 5(b), a through hole 4 is formed in the central portion of a semiconductor 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 semiconductor substrate 1 in the form of a bridge. As a result, the bridge portion 5 is thermally isolated from the semiconductor 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 semiconductor 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 B in Fig. 5(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. 5(c).

In the above-described conventional microbridge flow sensor, however, the bridge portion 5 is thermally isolated from the semiconductor substrate 1 as shown in Fig. 6, which shows an enlarged plan view of a main part of the flow sensor, and end portions 5A and SB of the bridge portion 5 which are connected to the semiconductor substrate 1 receive a large amount of heat, thus posing problem in terms of heat loss.

It is a principal object of the present invention to provide a microbridge flow sensor which has high sensitivity and precision in measurement of flow speed and allows a decrease in power to be supplied to heating elements.

In order to achieve the object of the present invention, there is provided a microbridge flow sensor including an integral bridge portion formed above a surface of a semiconductor substrate, that is, thermally isolated from the semiconductor substrate and define a through hole communicating with right and left openings, and a detecting portion constituted on the bridge portion by a heating element and temperature-measuring elements which are also thermally isolated from the semiconductor substrate but have thermal conductance along the bridge portion to the semiconductor substrate, the microbridge flow sensor being adapted to detect a flow speed on the basis of changes in resistances of the temperature-measuring elements, wherein a width of a connecting portion of the bridge portion which is connected to the semiconductor substrate is set to be smaller than that of a middle portion of the bridge portion so as to reduce greatly thermal conductance along the bridge portion to the substrate.

Fig. 1 is an enlarged plan view showing a main part of a microbridge flow sensor according to an embodiment of the present invention; Figs. 2 to 4 are enlarge plan views respectively showing main parts of microbridge flow sensors according to other embodiments of the present invention; Fig. 5(a) is a perspective view showing an arrangement of a conventional microbridge flow sensor; Fig. 5(b) is a sectional view taken along a line B - B' in Fig. 5(b); Fig. 5(c) is a graph showing a relationship between a voltage output and a flow speed; and Fig. 6 is an enlarged plan view showing a main part of an arrangement of a detecting portion of the conventional microbridge flow sensor.

Fig. 1 shows a microbridge flow sensor according to an embodiment of the present invention. The same reference numerals in Fig. 1 denote the same parts as in Figs. 5(a), 5(b), and 6. Referring to Fig. 1, a bridge portion 5' is formed above a through hole 4 of a semiconductor substrate 1 in the form a bridge. In the bridge portion 5', openings 12 and 13, and 14 and 15 each communicating with the through hole 4 are respectively formed in end portions 5A and SB connected to the semiconductor substrate 1, and the lateral width of each of the end portions 5A and 5B of the bridgetportion 5' is set to be substantially smaller than that of a middle portion of the bridge portion 5'. These openings 12 to 15 are formed at the same time when openings 2 and 3 are formed.

According to such an arrangement, since the openings 12 and 13, and 14 and 15 are respectively formed in the end portions 5A and 5B of the bridge portion 5' which are connected to the semiconductor substrate 1, the connecting width of each of the end portions 5A and 5B of the bridge portion 5' is reduced to a value large enough to allow extraction of the leads of a heater element 7 and temperature-measuring resistive elements 8 and 9.

Therefore, heat generated by the heater element 7 is not easily conducted to the semiconductor substrate 1. Hence, heat loss can be minimized, and thermal insulating properties can be improved. That is, the bridge portion 5' is thermally isolated.

Figs. 2 to 4 respectively show microbridge flow sensors according to other embodiments of the present invention. The same reference numerals in Figs. 2 to 4 denote the same parts as in Fig. 1. Referring to Fig. 2, a bridge portion 5" is designed such that the widths of end portions 5A and 5B connected to a semiconductor substrate 1 are reduced by increasing the sizes of openings 2 and 3 so as to be smaller than the width of the middle-portion of the bridge portion 5". Referring to Fig. 3, a bridge portion 5" is designed such that end portions SA and 5B are further reduced in width by extending one end of each of openings 2 and 3.Referring to Fig. 4, a bridge portion 5" is designed such that end portions 5A and 5B are further reduced in width by forming openings 16 and 17 in connecting portions between the end portions and a semiconductor substrate 1 so as to communicate with a through hole 4.

In such arrangements, the respective bridge portions 5" can be thermally isolated.

As has been described above, according to the present invention, since the width of each connecting portion of a bridge portion which is connected to a semiconductor substrate is set to be smaller than that of the middle portion of the bridge portion, the bridge portion is thermally isolated, and the thermal isolating properties can be improved. Therefore, excellent effects can be obtained. For example, the sensitivity and precision of the flow sensor in measurement of flow speed can be improved. In addition, the power to be supplied t the heating elements can be reduced.

Claims (4)

What is claimed is:

1. A microbridge flow sensor including an integral bridge portion formed above a surface of a semiconductor substrate so as to be spatially isolated from said semiconductor substrate and define a through hole communicating with right and left openings, and a detecting portion constituted on said bridge portion by a heating element and temperature-measuring elements which are also thermally isolated from said semiconductor substrate but have thermal conductance along said bridge portion to said semiconductor substrate, said microbridge flow sensor being adapted to detect a flow speed on the basis of changes in resistances of said temperature-measuring elements, wherein a width of a connecting portion of said bridge portion which is connected to said semiconductor substrate is set to be smaller than that of a middle portion of said bridge portion so as to reduce greatly thermal conductance along said bridge portion to said substrate.

2. A sensor according to claim 1, wherein the width of said connecting portion is reduced by forming at least one additional opening in said connecting portion so as to communicate with the through hole, said at least one additional opening being formed simultaneously with said right and left openings.

3. A sensor according to claim 1, wherein the width of said connecting portion is reduced by increasing at least one of said right and left openings in size so as to be smaller than the width of the middle portion.

4. A microbridge flow sensor substantially as described herein with reference to Figs. 1 to 4 of the accompanying drawings.