Microfabricated silicon-on-Pyrex passive wireless wall shear stress sensor

This paper presents the design, fabrication, and characterization of a passive wireless sensor for the measurement of wall shear stress. A micromachined variable-capacitor shear stress transducer is realized using a silicon-on-Pyrex microfabrication process. The design features a diamond-shaped 2.25 mm² silicon floating-element to accommodate more comb fingers for improved capacitive transduction in a smaller die area. The variable-capacitor device is connected to a fixed inductor on a printed circuit board to enable passive wireless sensing. The nominal resonant frequency of the device is 168 MHz with a quality factor of 8.6. Calibrations of static shear stress in a flow cell show a linear response to over 4 Pa, with a frequency-shift sensitivity of 474 kHz/Pa (1.1% full scale). Theoretically a minimum detectable shear stress of 4.1 mPa can be resolved giving a 61.7 dB dynamic range.