Microfabrication of a wall shear stress sensor using side-implanted piezoresistive tethers – SPIE Apr 2007

This paper presents the microfabrication process of a MEMS piezoresistive shear stress sensor for direct, quantitative measurement of time-resolved, fluctuating wall shear stress. The sensor structure integrates shadow side-implanted diffused resistors into the sensor tethers for detecting in-plane deflection. Temperature compensation is achieved by integrating a fixed, dummy Wheatstone bridge adjacent to the active shear stress sensor. The device is fabricated from an SOI wafer using an 8-mask process. A phosphorus blanket implantation forms an n-well to ensure P/N junction isolation. Boron implantation forms a heavily doped Ohmic contact. The tethers and floating element are defined by patterning PECVD oxide via RIE and DRIE. The Si is etched vertically to 8 &mgr;m via DRIE to form the trench for the sidewall implant. The scallops formed on the sidewalls during DRIE are smoothed by hydrogen annealing. After a preamorphization implant, boron is implanted at an oblique angle of 54° to achieve a 5 &mgr;m shadow side-wall implantation. The structure is released from the backside using a combination of DRIE and RIE to etch the silicon, oxide, and nitride layers. Finally, the sensors are annealed in forming gas. Preliminary electrical testing indicates linear, junction-isolated resistors.

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