Creep as a reliability problem in MEMS

Creep is expected to be one of the major reliability problems for certain MEMS. In particular RF-MEMS switches with metal bridges will be endangered, especially at high RF power, if the bridge material is not selected appropriately. In this paper, substrate curvature measurements are used to study creep properties of Al, Al98.3Cu1.7, Al9.93V0.2Pd0.1 and Al93.5Cu4.4Mg1.5Mn0.6 films using isothermal tensile stress relaxation. For all aluminum compositions dislocation glide describes the relaxation data very well up to 110 degreesC, However a large difference in creep sensitivity is measured: Al93.5Cu4.4Mg1.5Mn0.6 is the alloy that is most resistive to creep. We propose a very simple way of characterizing Al alloy thin films by two creep parameters: activation energy DeltaF and athermal flow stress c. We also show a direct relation between the parameters and the coherence, size and spacing of precipitates observed by SEM and TEM in the alloys. In addition we report that neither heating rates nor maximum temperatures (above 200 degreesC) have an influence on stress in pure Al films. (C) 2004 Elsevier Ltd. All rights reserved.