Key improvements of the MEMS switch lifetime thanks to a dielectric-free design and contact reliability investigations in hot/cold switching operations

Abstract

Dielectric charging and contact degradations are the two major challenges to improve the lifetime of series ohmic electrostatic MEMS switches. This paper details our approach and our main results to overcome the failures due to dielectric charging. We introduce a time predictive model for charge accumulation that we use as a design tool for reliability. The key parameters are listed and a new switch design is proposed to reduce the charging: the dielectric materials between the actuation electrodes are removed as much as possible. This dielectric-free design gives remarkable results in terms of dielectric charging sensitivity: the pull-in voltage decreases a little bit at initial times, and remains steady for longer time. The second major challenge, that remains to be considered, deals with the contact reliability. For that purpose, gold and ruthenium contacts have been investigated under several operating conditions. Gold contacts give good results in cold switching conditions whereas ruthenium suffers from surface contamination which increases quickly the contact resistance. In hot switching conditions, both materials are sensitive to material transfer mechanism, the failure rate being proportional to the open circuit voltage. Nevertheless, ruthenium seems more resistant to a material transfer than gold, and offers performances acceptable for some specific applications. A hermetic packaging at wafer level should allow to improve the contact reliability thanks to an efficient management of the surface contamination issue.