Cantilever deflection measurements for thin film stress evaluation for MEMS applications

This experimental work is focused on the study of residual stress in thin films deposited by Chemical Vapor Deposition (CVD) techniques and used for the fabrication of micro-electromechanical systems (MEMS) sensors and actuators. The research activity has been carried out in the STMicroelectronics site located in Agrate Brianza (MB), working within the R&D MEMS process and technology development team. The activity has been focused on the fabrication of MEMS test structures, processed inside the ST clean room on 8” wafers, the stress characterization and the discussion of the results. Residual stress of thin films is a critical aspect which has to be carefully taken into account during both design and manufacturing of MEMS devices. The reference technique in MEMS industry to evaluate the residual stress of thin films is the scanning laser method, which is commonly used in every front-end production line. This method provides the mean residual stress of a thin film from the measurement of the curvature of the wafer by using the Stoney’s equation. The main limitation of this approach is that it does not provide a local measurement and therefore any information on the distribution of the stress variation on the wafer surface. Therefore, it is not possible to study the center-to-edge variability which is an extremely useful information in the framework of semiconductor processing and fabrication. In this work, an alternative method to evaluate the residual stress of thin films has been applied on different dielectric thin films deposited by CVD industrial techniques. This method is based on the characterization of an array of cantilevers which are fabricated on silicon wafers employing a simplified MEMS process flow. The fabrication process of the MEMS cantilevers is thoroughly described along the work with dedicated sections that describe the deposition, patterning and release of the structures. Upon release, cantilevers deflect to partially relieve the unbalanced residual stress in the material. The deflection of the MEMS structures has been measured on the whole wafer by employing visible light interferometry. The residual stress of the thin films integrated in the structure has been calculated from the cantilevers’ deflection. This method differs from wafer level measurement techniques: it enables local analyses of the residual stress in microfabricated structures thus enabling the evaluation of the local and center-to-edge stress variability. The stress distribution on the wafers has been characterized for eight different films; the results have been correlated with the specific deposition technique. This alternative approach to stress evaluation, based on the use of MEMS test structures, paves the way for the study of multi-layers stack.