Impact of Creep and Softening Mechanisms on the Contact Resistance of RF MEMS Ohmic Switches: Study of the Current and Time Effects on Au-to-Au Microcontacts in Static Contact Conditions

This paper presents test results and electrothermal modeling on Au-to-Au microcontacts and brings up new elements for the comprehension of the physics of contact in RF MEMS switches. The experiments were performed directly on MEMS switches using an original method to calculate precisely the contact resistance based on a specific test sample. The contact resistance was carefully characterized as a function of time and current in static conditions. The time dependence of the contact resistance is allocated to creep, a well-known phenomenon which increases the real contact area. The current dependence shows a sudden contact resistance decrease at a contact voltage value significantly lower than the softening voltage of bulk gold. This behavior is explained by the fact that the current can increase the contact temperature, because the heat produced by Joule heating can not be sufficiently dissipated outside the contact region. In this case, softening conditions can be activated and lead to a decrease of the contact resistance. In conclusion, this work shows that the classical electrical contact theory due to size effects is not directly applicable to the micro-scale.