MEMS resonant sensors for real-time thin film shear stress monitoring

This paper presents MEMS resonant stress sensors for real-time monitoring of induced shear stress during formation of a thin film on a micro-diaphragm. The device is comprised of a thin silicon membrane coupled to a thermal-piezoresistive resonator. Membrane deflection due to the stress resulting from deposition or growth of a thin film deforms the resonator, thus changing its resonance frequency. Such stress sensors can be used in different media without perturbing the resonator operation due to the isolation provided by the membrane. Response of the fabricated devices due to thermal stress induced in 400nm thick Ni and Al layers deposited on the membranes was measured demonstrating sensitivities as high as 1.7 Hz/Pa. Real-time characterization of the fabricated sensors was also performed demonstrating a sensitivity of ~1000ppm/MPa while etching a 2μm SiO2 layer underneath the membrane over the time. A finite element Simulation and a mathematical modeling were employed to calculate the stress in the center of the membrane resulting from different thicknesses of SiO2 layer. The modeling suggests the experimental frequency shift of 7kHz corresponds to the stress of ~7×106 N/m2 caused by formation of 2μm thick oxide layer underneath the membrane.