Microelectromechanical (MEMS) structures for thin film property measurement

Abstract

Microelectromechanical systems (MEMS) are becoming the bases for an important industry with potential applications in numerous fields. The current study explores the use of MEMS-based, electrically-actuated structures to determine thin film mechanical properties, including intrinsic stresses of deposited metal films. An overview of the importance of thin film stress measurement and the advantages of this particular MEMS-­based technique provides motivation for this study. A new analysis of the mechanics in bilayer beams is presented, and is used to relate changes in electrical measurements to the intrinsic stress of films deposited on micromachined beams. Proof-of-principle studies using evaporatively deposited silver films were carried out, and showed agreement in stresses measured using different MEMS-based devices, but the same beam analysis. Stress was measured through experiments using electrical "pull-in" of beams and results are compared both to curvature studies using MEMS-based cantilevers and to standard wafer-based, scanning-laser curvature analysis. Thermal cycling was performed on all the devices, yielding data suggesting different rates of thin film stress relaxation for the films deposited on MEMS-based devices and a film on an oxidized Si wafer. Differences in stress relaxation rates are attributed to the different interfaces involved. These studies point to future work which will lead to optimized MEMS-based devices to be used for high-sensitivity, in-situ, thin film stress measurements.