Reliability of RF-MEMS switches for space applications

This contribution presents the results achieved under the ESA contract Nr. 20847/07/NL/GLC. Purpose of this project was the realization of a high reliability redundancy switch for space applications. The main challenges of this project were related to the optimization of the switch performances and the identification of the most important failure modes in order to maximize its lifetime. During the first part of the project, several switch typologies were designed and tested, in order to select the most reliable candidates. Dielectric-less switches, in a clamped-clamped configuration where identified as the most promising devices, for both capacitive and ohmic typologies, since they can effectively minimize dielectric charging effects which turned out to be one of the most important failure modes for these devices. During the second phase of the project, a dedicated fabrication run within the FBK RF-MEMS technology was produced to fulfill the specific project requirements, which covered both the design and the fabrication of SPDT devices. In addition, SPST design, corresponding to the building blocks of the SPDTs, were included in the layout in order to study the reliability aspects of the elementary building blocks as well. This material was employed for a series of measurements aimed at testing the suitability for space applications. In particular, RF characterization and environmental testing were performed on the SPDT devices hermetically closed in a dedicated LTCC package, while the long-term actuation measurements, aimed at switch lifetime characterization, were performed on the single SPST units. The study of the long term behavior of the switching units was one of the biggest challenges of this project, and it could be finally established that temperature is the only possible factor that can be used to accelerate failure and therefore predict the switch failure time. Moreover, a more complicated picture of the reliability question was drawn, mostly because of the interplay of different electrical, environmental and mechanical failure mechanisms. As a result, the use of an hermetic package was defined as mandatory to obtain reliable measurements. Preliminary extrapolated lifetimes for specific switch topologies indicated that they can survive several years of continuous actuation, as requested by the project, but the reliability in the long time frame turned out to be deeply affected by mechanical factors, such as wear, inelastic relaxation and creep, which were scarcely investigated during most part of the project. The project is now technically finished but extended additional studies are underway in order to obtain the maximum characterization data output from the available material. Based on the findings and the data analysis, a methodology to implement a standard accelerated lifetime test will be proposed, and the possibilities and the limits in long-term applications for this type of devices will be discussed.