Using encapsulated MEMS resonators to measure evolution in thin film stress

Encapsulated resonators can offer excellent long-term frequency stability, because the encapsulation provides protection from external environmental variations. Encapsulated double-anchored flexural-mode resonators exhibit a sensitive coupling between changes in stresses applied to the chip and changes in the resonant frequency while still isolated from other external variations. In this work, this sensitivity is exploited to observe stress relaxation in thin films deposited on the outside of the encapsulated resonator. Because of the high sensitivity to stress, and the insensitivity to other factors, these devices provide the first opportunity to observe film stress relaxation with nano-strain resolution. We have begun a study of film stress relaxation in LPCVD and TEOS oxides and aluminum-all of which are commonly used as structural layers in MEMS devices. Theoretical analysis, finite element analysis (FEA), as well as preliminary data on different film materials are presented herein.