Creep characterization of Al alloy thin films for use in MEMS applications

Abstract

Creep is expected to be a major reliability problem in some MEMS, as for example RF-MEMS switches, especially at high RF powers. For this reason it should be avoided to use creep sensitive materials in these devices. In this paper we report on creep studies on Al-alloys, materials that are often used for the bridge in RF MEMS switches. Substrate curvature measurements were used to study Al98.3Cu1.7, Al99.7V0.2Pd0.1 and Al93.5Cu4.4Mg1.5Mn0.6 films during isothermal tensile stress relaxation. It is shown that to all studied aluminum compositions dislocation glide describes the relaxation data well for temperatures up to 110 °C. The paper discusses a very simple way of characterizing creep in thin Al-alloy films by the activation energy ΔF and the athermal flow stress τ. A large difference in creep sensitivity of the studied alloys is measured: Al–Cu–Mg–Mn is found to be very creep resistant while Al–Cu is creep sensitive. It is shown that there is a direct relation between the measured creep parameters and the coherency, size and spacing of precipitates observed by TEM and SEM in the alloys. There is also a direct link between the creep-sensitivity and the obstacle strength of the precipitates.