High Sensitivity Microbridge for Molecular Sensing Applications

This paper presents the design and simulation of a SiO2 microbridge which measures the differential surface stress induced by the adsorption of molecules. Materials with smaller Young's modulus would be appropriate to measure the small variation of surface stress on the microbridge and therefore SiO2 is chosen as the material for microbridge. The SiO2 microbridge could be a potential application for the glucose detection if the enzyme glucose oxidase was immobilized on a top surface. The microbridge can be constructed on Silicon on insulator (SOI) substrate using the bulk micromachining technology. The surface stress causes displacement which is measured using the piezoresistors placed on top surface of the microbridge. The piezoresistors are arranged in a Wheatstone bridge configuration to measure the change resistance in terms of voltage. The two piezoresistors are placed at the edges where they experience the maximum stress. The obtained sensitivity, ΔR/R of SiO2 microbridge is 6.15x10-4 which is four times higher than that of existing work [1]. The mechanical behavior of the SiO2 microbridge and the electrical response of the piezoresistors are analyzed using the Finite Element Method (FEM). The dimensions for the microbridge are optimized by considering both the sensitivity and nonlinearity with respect to the deflection. The placement location of the piezoresistors and the effect of geometrical parameters of the piezoresistors on the sensitivity SiO2 microbridge are studied.