Micromachined flow shear-stress sensor based on thermal transfer principles

Micrahot-film sheaf-stress sensors have been developed by using surface mic romachining techniques. The sensor consists of a suspended silicon-nitride diaphragm located on top of a vacuum-sealed cavity. A heating and heat-sens ing element, made of polycrystalline silicon material, resides on top of th e diaphragm. The underlying vacuum cavity greatly reduces conductive heat l oss to the substrate and therefore increases the sensitivity of the sensor. Testing of the sensor has been conducted in a wind tunnel under three oper ation modes-constant current, constant voltage, and constant temperature. U nder the constant-temperature mode, a typical shear-stress sensor exhibits a time constant of 72 mu s. [362].