Micro thermal shear stress sensor with and without cavity underneath

Abstract

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.

The dynamic response of a hot-wire anemometer: III. Voltage-perturbation versus velocity-perturbation testing for near-wall hot-wire/film probes

“…Moen and Schneider (1993) also found that, for their hotfilm sensors mounted on glass, polyamide-coated aluminium and alumina substrates, f S distinctly tends to increase with thermally more conducting materials. Similarly, Huang et al (1995) observed that their micro-hot-film shear-stress sensor without a cavity underneath performs much better than does its counterpart with a cavity such that f S is almost 14 times better at about 128 kHz. Invariably, the presence of cavity underneath with a vacuum seal translates into a substrate that overall is much less thermally conducting.…”

“…see Davis (1970)) or 'high-order behaviour observed in the system' as put forth by Moen and Schneider (1994). Yet, as observed in all previous squarewave perturbation tests for hot-film sensors like those of Moen andSchneider (1993, 1994), Huang et al (1995) and Kreplin and Eckelmann (1979), to name a few, the 'tail' effect is routinely disregarded by researchers in deference to figure 4 and equation ( 5) for the evaluation of f S without further explanation. In this work, we shall adopt this widely accepted practice for evaluating f S since it also provides an avenue for comparison with all previously published works.…”

“…Here, τ S corresponds to the time interval at the voltage level 'midway between the minimum and maximum of the resonance' after the initial peak reckoned by Freymuth (1981); Moen andSchneider (1993, 1994), evaluated τ S as the time interval from the voltage rise until the first minimum and the difference is not expected to be very significant and limited to about 10% variation. Huang et al (1995) used the same definition for τ S as that used by Moen and Schneider, but instead modified the expression for the cut-off frequency to f S = 1/(1.5τ S ).…”

Micro thermal shear stress sensor based on vacuum anodic bonding and bulk-micromachining

“…In comparison with pressure measurement techniques, thermal sensors can be used in a wide variety of flow conditions and present minimal disturbance to the flow itself because of the surface flush mount. Huang et al [6] compared the performances of micro thermal shear stress sensors with and without cavity underneath. They found that the former has a higher sensitivity than the latter.…”

Flexible MEMS Shear Stress Sensor with Improved Performance for Wind Tunnel Measurements

“…13,14 The flexible sensors also have lower thermal inertia and production cost, quicker response than those of conventional rigid sensors. 15 In recent years, researchers have emphasized the investigation of flexible MEMS shear stress sensors with a thermistor on polyimide substrate. 16,17 The heat transfer from a resistively heated thermistor to the flow is measured to figure out the shear stress.…”