Design and characterization of microfabricated piezoresistive floating element-based shear stress sensors

This paper presents the design, fabrication, and characterization of unique piezoresistive microfabricated shear stress sensors for direct measurements of shear stress underwater. Sidewall-implanted piezoresistors measure lateral force (integrated shear stress) and traditional top-implanted piezoresistors detect normal forces. In addition to the oblique-implant technique, the fabrication process includes a hydrogen anneal step to smooth scalloped silicon sidewalls left by the deep reactive ion etch (DRIE) process. This step was found to reduce the 1/f noise level by almost an order of magnitude for the sidewall-implanted piezoresistors. Lateral sensitivity was characterized using a microfabricated silicon cantilever force sensor. Out-of-plane sensitivity was evaluated by laser Doppler vibrometry and resonance of the plate element. In-plane sensitivity and out-of-plane crosstalk were characterized, as well as hysteresis and repeatability of the measurements. TSUPREM-4 simulations were used to investigate the discrepancies between the theoretical and experimental values of sidewall-implanted piezoresistor sensitivity. The sensors are designed to be used underwater for studies of hydrodynamic flows.