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A new silicon flow sensor with a robust thermal isolation structure has been developed. The thermal isolation structure is mainly made of a 20-µm-thick oxidized porous silicon membrane. This thermal isolation structure makes it possible for the sensor to have a fast-response characteristic and an on-chip fluid temperature sensing element design. The sensor can be used in liquid flow as well as gas flow. Its operation is based on heat transfer from the heated sensor to a moving fluid. It has two platinum thin-film resistors, a heating element, and a fluid temperature sensing element on the chip. The sensing element is thermally isolated from the heating element. The external circuit of the sensor maintains a constant temperature difference between the heating element and the fluid. The sensor chip characteristics were evaluated theoretically by heat transfer analysis during the chip design. Measurements were made for oil flow velocity of 0-30 cm/s and air flow velocity of 0-14 m/s. Response time was below 100 ms, and a compensated output for fluid temperature change was obtained.
Date of Publication: Mar 1986