Geometry optimization of uncoated silicon microcantilever-based gas density sensors

In the absence of coating, the only way to improve the sensitivity of silicon microcantilever-based density sensors is to optimize the device geometry. Based on this idea, several microcantilevers with different shapes (rectangular-, U- and T-shaped microstructures) and dimensions have been fabricated and tested in the presence of hydrogen/nitrogen mixtures (H2/N2) of various concentrations ranging from 0.2% to 2%. In fact, it is demonstrated that wide and short rectangular cantilevers are more sensitive to gas density changes than U- and T-shaped devices of the same overall dimensions, and that the thickness does not affect the sensitivity despite the fact that it affects the resonant frequency. Moreover, because of the phase linearization method used for the natural frequency estimation, detection of a gas mass density change of 2 mg/l has been achieved with all three microstructures. In addition, noise measurements have been used to estimate a limit of detection of 0.11 mg/l for the gas mass density variation (corresponding to a concentration of 100 ppm of H2 in N2), which is much smaller than the current state of the art for uncoated mechanical resonators.