Temperature effects on the mechanical reliability of MEMS structures: experimental study on creep and thermal fatigue

Abstract

The reliability of micro electro-mechanical systems (MEMS) became a fundamental topic of investigation as a result of their widespread application in many day-to-day life devices. From the structural viewpoint the most relevant sources of lifetime reduction are mechanical fatigue and high temperatures. The typical working conditions of MEMS include repetitive cyclic loads and high operative frequencies; examples are represented by some of the most common applications: RF devices (switches, varactors, resonators, filters), inertial sensors, optical components, etc. These operative conditions activate that processes commonly associated to fatigue, like crack initiation and propagation and the self-heating of the material accompanied to the progressive degradation of mechanical properties. The fatigue associated to MEMS structures was investigated by the authors in some previous studies [1] providing experimental results in good agreement with the literature. High temperatures are responsible of additional mechanisms of material degradation leading to a reduction of components lifetime and devices reliability. The most diffused scenarios associate high temperature to: a) static loads and b) alternate loads. In the first case creep effect appears: the temperature is responsible of progressively increasing deflections of the structure under the applied load; the deformation is usually plastic (and so, permanent) and large. In the second case the cause of material damaging is actually mechanical fatigue, but the presence of high levels of temperature modifies the S-N curve obtained at ambient temperature and generally determines faster failures at lower load levels. This work is focused on the reliability of metallic MEMS structures subjected to static and alternate loads in presence of increasing levels of temperature. Some works in the literature were focused on the study of creep in MEMS that appears when static loads are associated to medium and high temperatures: Modlinski et al. [2] characterized the bridge of a capacitive switch made by an Al alloy, Larsen et al. [3] presented the design for a test structure devoted to investigation on creep for electroplated nickel, Zhang and Dunn [4] studied creep on thin gold-polysilicon bilayer films subjected to thermal loading. It was observed that long-term deformation of metal parts of microdevices may play a key role in the lifetime of a device, making creep a very important issue in the reliability of MEMS. Apart effects associated with diffusion, all plastic deformation occurs as a consequence of the migration of atomic planes in the crystal lattice of the material (namely dislocations); this effect is deeply emphasized by the coexistence of increasing temperature and stress. If alternate loads act on the component, the effect of temperature produces an acceleration of material damaging due to mechanical fatigue process; local plasticized area can be detected on the surface. © Owned by the authors, published by EDP Sciences, 2010 DOI:10.1051/epjconf/20100606001 EPJ Web of Conferences 6, 06001 (2010)