Abstract. By applying the micro-electro-mechanical-system (MEMS) fabrication technology, we developed a micro-thermal sensor to measure surface shear stress. The heat transfer from a polysilicon heater depends on the normal velocity gradient and thus provides the surface shear stress. However, the sensitivity of the shear-stress measurements in air is less than desirable due to the low heat capacity of air. A unique feature of this micro-sensor is that the heating element, a film 1 µm thick, is separated from the substrate by a vacuum cavity 2 µm thick. The vacuum cavity prevents the conduction of heat to the substrate and therefore improves the sensitivity by an order of magnitude. Owing to the low thermal inertia of the miniature sensing element, this shear-stress micro-sensor can provide instantaneous measurements of small-scale turbulence. Furthermore, MEMS technology allows us make multiple sensors on a single chip so that we can perform distributed measurements. In this study, we use multiple polysilicon sensor elements to improve the dynamic performance of the sensor itself. It is demonstrated that the frequency-response range of a constant-current sensor can be extended from the order of 100 Hz to 100 kHz.

Microhot-film shear-stress sensors have been developed by using surface micromachining techniques. The sensor consists of a suspended silicon-nitride diaphragm located on top of a vacuum-sealed cavity. A heating and heat-sensing element, made of polycrystalline silicon material, resides on top of the diaphragm. The underlying vacuum cavity greatly reduces conductive heat loss to the substrate and therefore increases the sensitivity of the sensor. Testing of the sensor has been conducted in a wind tunnel under three operation modes-constant current, constant voltage, and constant temperature. Under the constanttemperature mode, a typical shear-stress sensor exhibits a time constant of 72 s. [362]

A new MEMS shear stress sensor imager has been developed and its capability of imaging surface shear stress distribution has been demonstrated. The imager consists of multi-rows of vacuum-insulated shear stress sensors with a 300 pm pitch. This small spacing allows it to detect surface flow patterns that could not be directly measured before. The high frequency response (30 kHz) of the sensor under constant temperature bias mode also allows it to be used in high Reynolds number turbulent flow studies. The measurement results in a fully developed turbulent flow agree well with the numerical and experimental results previously published.

Micro hot-film shear-stress sensors have been designed and fabricated by surface micromachining technology compatible with IC technology. A poly-silicon strip, 2kmx80km, is deposited on the top of a thin silicon nitride film and functions as the sensor element. By using sacrificial-layer technique, a cavity (vacuum chamber), 200x200x2pm3, is placed between the silicon nitride film and silicon substrate. This cavity significantly decreases the heat loss to the substrate. For comparison purposes, a sensor structure without a cavity has also been designed and fabricated on the same chip. Theoretical analyses for the two vertical structures with and without a cavity show that the former has a lower frequency response and higher sensitivity than the latter. When the sensor is operated in constant temperature mode, the cut-off frequencies can reach 130 k-Hz and 9 k-Hz respectively for the sensors without and with cavities.