Characterization of a miniature thermal shear-stress sensor with backside connections

Abstract

A miniature thermal shear-stress sensor was fabricated with bulk micromachining techniques and its dynamic performance was evaluated using a rotating disk calibration device. The calibration device consisted of an upper rotating disk and a lower stationary disk separated by a small gap (275 mu m), with the shear-stress sensor flush-mounted on the lower disk. A novel backside connection scheme compatible to the present sensor fabrication process was devised. Rotation of the upper disk caused a circular Couette flow to be generated in the fluid between the disks. The resistance of the fabricated gold (An) sensing element was about 2.5-3.5 Omega, which is suitable for interfacing with a commercial constant temperature anemometer (CTA). The static sensitivity was in the range of 200-800 mV/Pa at an overheat ratio of 0.2. The transient response characteristics of the sensor indicated that natural convection is significant in the low-frequency range. The dynamic frequency response was tested up to 400 Hz; its gain was about -30 dB/decade of the frequency around 400 Hz. The unusual high gain observed in the low-frequency range seems to be due to the natural convection effect. The shear-stress sensor was found to have a large signal to noise ratio (SNR) in the frequency range around 400 Hz, and it expected to have a discernible SNR values over 1 kHz. (c) 2006 Elsevier B.V. All rights reserved.