Improvement of freestanding CMOS-MEMS through detailed stress analysis in metallic layers

A freestanding cross-shaped structure designed as a planar rotation stress sensor [1], [2], [3] is manufactured using standard CMOS technology (Complementary Metal-Oxide-Semiconductor). The fabrication process induces thermal residual stresses which result in out-of-plane bending, which degrades the device reliability and precision. To control such movements, the design was studied under stress compensation using a bilayered aluminum (Al) / titanium nitride (TiN) structure. Likewise, a single layer of aluminum was studied, to determine a technological solution, with better compatibility. Fabrication stresses have been measured using Stoney´s formula based on bending of full-wafer coatings. The Finite Element Method (FEM) is used to model the effect of these stresses on the geometry after release, and the results are compared with measurements. For this purpose, a comb-shaped structure has been designed to relate residual stress in a freestanding Al-TiN bi-layered structure with its bending. Based on this, conservation or elimination of TiN layer is judged, so that the design remains planar after release. The model is then applied to the movement of the cross-shaped sensor after release, and a second optimization variable is studied for maximum sensitivity: the shape of the hinge between the two arms of the cross.