Thin-film air flow sensor using temperature-biasing resistive element

Claim 1

A fluid flow sensor system having a silicon substrate, a doped silicon boundary means on one surface of the substrate defining an enclosed area on the substrate and providing thermal isolation between the substrate and the enclosed area; a dielectric layer extending over the one surface of the substrate and the doped silicon boundary means, the layer having two layers of silicon oxide sandwiching a layer of silicon nitride, one of the silicon oxide layers on the one surface of the substrate; window means extending through the substrate bounded by the boundary means and enclosed at one end by the layer forming a diaphragm over the window means; the sensor system characterized in that a thin film, self-heating, transfer heating element formed on the diaphragm, said heating element being connected with a constant current source to heat and maintain the diaphragm at a predetermined temperature above ambient; a thin film, self-heating, sensor heating element formed on the diaphragm, said thin film sensor heating element being at substantially the same temperature as said predetermined temperature due to the heat conducted from said heating element; and feedback current supplying means connected to said sensor element to supply a first current to bring said sensor element to said predetermined temperature in the absence of fluid flow across said sensor and to supply a second current to said element to raise its temperature to said predetermined temperature in the presence of fluid across said sensor element, said second current is proportional to the amount of fluid flow across said sensor.