A tungsten based SOI CMOS MEMS wall shear stress sensor

In this work we report, for the first time, a silicon on insulator (SOI) complementary metal oxide semiconductor (CMOS) MEMS thermal wall shear stress sensor that uses CMOS tungsten metallization as sensing element, supported by a composite membrane comprising of silicon oxide and silicon nitride. The sensor was fabricated using a commercial 1 μm SOI CMOS process. The CMOS tungsten metallization was used to create a hot film element with size 200 μm × 2 μm × 0.3 μm. Post-CMOS, the wafers were back-etched in a single Deep Reactive Ion Etching (DRIE) step to create a 250 μm diameter circular membrane comprising silicon oxide and silicon nitride layers under the hot-film sensor. The sensor exhibits a high Temperature Coefficient of Resistance (TCR) (0.21 %/°C), and very effective thermal isolation from substrate evident from its thermal resistance (20,435 °C/Watt, or ~ 6mW for temperature rise of 100 °C). The sensor has been calibrated in constant temperature (CT) mode in a 2-D laminar flow wind tunnel for a wall shear stress range of 0-1.6 Pa to show an average sensitivity of 35 mV/Pa at an Over Heat Ratio (OHR) of 1.0.