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

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 μm), 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 (Au) sensing element was about 2.5–3.5 Ω, 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.