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.
This paper presents the modeling and design optimization of a micromachined floating element piezoresistive shear stress sensor for the time-resolved, direct measurement of fluctuating wall shear stress in a turbulent flow. The sensor structure integrates side-implanted diffused resistors into the silicon tethers for piezoresistive detection. A theoretical nonlinear mechanical model is combined with a piezoresistive model to determine the electromechanical sensitivity. Lumped element modeling (LEM) is used to estimate the resonant frequency. Finite element modeling is employed to verify the mechanical models and LEM results. Two dominant noise sources, 1/f noise and thermal noise, were considered to determine the noise floor. These models were then leveraged to obtain optimal sensor designs for several sets of specifications. The cost function is the minimum detectable shear stress that is formulated in terms of sensitivity and noise floor. This cost function is subjected to the constraints of geometry, linearity, bandwidth, power and resistance. The results indicate the possibility of designs possessing dynamic ranges of greater than 85dB.
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