Evaluation of residual stress in MEMS structures by means Digital Holography

We investigate digital holography method as metrological tool for inspection and characterization of MEMS structures. The efficiency of the digital holography is demonstrated measuring out of plane deformations due to the intrinsic residual stress. Microstructures under investigation are of two different types: the first are made of a single polysilicon layer, whereas the second are bimorph structures with a thin silicon nitride layer over the polysilicon one. These structures exhibit an out-of-plane deformation owing to residual stresses between the different layers. The characterization of these deformations is instrumental to study and understand the effect of residual stress on the deformation of the single microstructures. To this aim digital holography has been applied as metrological tool in order to obtain the profile of the microstructures. These data are employed in analytical and numerical model to evaluate residual stress inside the investigated structures. Moreover, digital holography has been employed to evaluate MEMS behavior when subjected to thermal load. Profile of cantilevers, with dimensions from 1 to 50 µm, has been measured