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 micro-mechanical 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.
This work deals with the development of integrated relative humidity dew point sensors realized by adopting standard CMOS technology for applications in various fields. The proposed system is composed by a suspended plate that is cooled by exploiting integrated Peltier cells. The cold junctions of the cells have been spread over the plate surface to improve the homogeneity of the temperature distribution over its surface, where cooling will cause the water condensation. The temperature at which water drops occur, named dew point temperature, is a function of the air humidity. Measurement of such dew point temperature and the ambient temperature allows to know the relative humidity. The detection of water drops is achieved by adopting a capacitive sensing strategy realized by interdigited fixed combs, composed by the upper layer of the adopted process. Such a capacitive sensor, together with its conditioning circuit, drives a trigger that stops the cooling of the plate and enables the reading of the dew point temperature. Temperature measurements are achieved by means of suitably integrated thermocouples. The analytical model of the proposed system has been developed and has been used to design a prototype device and to estimate its performances. In such a prototype, the thermoelectric cooler is composed by 56 Peltier cells, made by metal 1/poly 1 junctions. The plate has a square shape with 200 μm side, and it is realized by exploiting the oxide layers. Starting from the ambient temperature a temperature variation of ΔT = 15 K can be reached in 10 ms thus allowing to measure a relative humidity greater than 40%.