Abstract.
An integrated structure of capacitive pressure sensor for biomedical applications achieved by surface micromachining technology, consisting in the pressure sensor and a testing component is presented. The test structure permitted to apply the pull-in voltage method for the evaluation of the residual stress in the polysilicon layer on the basis of a new form of the set of two equations describing the beam pull-in voltage effect. The investigation of the microstructure morphology and the doping properties by secondary ion mass spectroscopy and by spreading resistance profiling allowed to elaborate a doping-restructuring model of the polysilicon layers, operational during the phosphorus diffusion from a liquid POCl3 source. Such a model points out the contribution of the silicon self-interstitial atoms injected by the phosphorus diffusion and the phosphosilicate glass growth to the doping-restructuring process. The role of the oxygen atoms for the SiOx precipitation in the polysilicon layers is also discussed, showing the influence on the internal stress. Both the integrated structure and the characterization of the polysilicon layers allowed the optimization of the phosphorus diffusion process to obtain low stress polysilicon membranes of the capacitive pressure sensors for biomedical applications. The device characterization was realized by an automatical system suitable to determine the capacitance-pressure characteristics. The performances of the capacitive pressure sensors shows a high sensitivity on a large range of the pressure values and a good behavior with respect to the frequency of the measurements.