In this paper, we presented is a four-terminal piezoresistive sensor commonly referred to as a van der Pauw (VDP) structure for its application to MEMS pressure sensing. In a recent study, our team has determined the relation between the biaxial stress state and the piezoresistive response of a VDP structure by combining the VDP resistance equations with the equations governing silicon piezoresistivity and has proposed a new piezoresistive pressure sensor. It was observed that the sensitivity of the VDP sensor is over three times higher than the conventional filament type Wheatstone bridge resistor. To check our theoretical findings, we fabricated several (100) silicon diaphragms with both the VDP sensors and filament resistor sensors on the same wafer so both the sensor elements have same doping concentration. The diaphragms were subjected to known pressures, and the pressure sensitivities of both types of sensors were measured using an in-house built calibration setup. It was found that the VDP devices had a linear response to pressure as expected, and were more sensitive than the resistor sensors. Also, the VDP sensors provided a number of additional advantages, such as its size independent sensitivity and simple fabrication steps due to its simple geometry.
We have considered two configurations of cantilever beam for characterization of rheological properties (density and viscosity) of viscous material. Both the beams are made of thin steel foils, having different free end condition. The beams were vibrated in various viscous fluids such as water and lubrication oils under sweeping modes, and the frequency response of the beam was measured using Polytec laser vibrometer. For both beam configurations, the resonance frequency decreased while the width of the resonance peak (related to Q-factor) increased with increasing viscosity and density. Thus, the viscosity and the density of a liquid can be determined simultaneously using the experimentally measured resonance frequency and peak width.
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