This paper presents the stress stability of thin films for MEMS structural and sacrificial layers. The average residual stresses of the thin films were monitored via wafer curvature measurement over a long period of time. Poly-Si, poly-SiGe, poly-Ge and thermally growth SiO2 films are found to be stable in humid environments. Moisture makes LPCVD and TEOS-based PECVD SiO2 films more compressive over time. Multi-layer thin film stress is modeled with the same methodology used to derive the Stoney Equation .