Development of hybrid SOI-based micro-gravimetric sensors

A hybrid system consisting of a resonant piezo layer (RPL) and a resonant SOI micromechanical sensor is conceived in this work as highly sensitive gravimetric sensor for applications in various fields. The idea consists in using PZT screen-printed elements, behaving as thickness-mode resonators, coupled to a micromechanical resonator based on a SOI technology. The PZT resonator induces oscillations to the micromechanical device and, if the resonance condition is matched for this latter system, a sensitivity of 5000 Hz/μg can be obtained when a variation of the proof mass occurs. Prototypes of both the mentioned two constitutive parts have been separately realized by the authors showing potentials for batch production. Several different experimental MEMS prototypes, mainly made by a central proof-mass sustained by four compliant beams anchored to its four corners, have been realized. Both Front Side and Back Side DRIE etching procedures have been performed improving the proof mass value with respect to a different set of prototypes realized by using a standard CMOS technology. Even if a low resonance frequency characterize the realized micro-prototypes a drastically improved value of the quality factor allow to obtain very high gravimetric sensitivity then to detect very small changes in the proof mass value due i.e. to chemical or physical compound absorption over the mass surface. Electrical or optical sensing can be adopted, depending on materials embedded into the considered device, as already demonstrated by the authors. Polysilicon strain gauges have been embedded into the springs while optical readout can be addressed by using a novel class of metal-dielectric photonic-band gap materials. In this latter case a process step, which consists of depositing suitable thin films, must be take into account.