RF MEMS electrical contact resistance calculation using mechanical contact simulations and analytical models

The testing and development of contact material or topology can be addressed with a dedicated experimental set up for monitoring test structures. However, it is difficult to perform the tests under realistic conditions. Moreover several works have already been published about the different theories describing rough mechanical contact. But they often ignore interaction between asperities, bulk deformation or elastoplastic deformations. In order to tackle these issues advanced simulation tools are needed. These tools for finite element analysis allow us to model assembly structures quickly and accurately with a minimal amount of effort. We have developed an original reverse engineering method for generating rough surfaces on ANSYS platform, by using the actual shape of the contact surface.We used this method to predict the real contact area between rough surfaces as a function of the applied force using the augmented Lagrangian method. The number of asperities in contact, their sizes and their distribution allow us to discriminate the more appropriate electric contact model in diffusive or ballistic electron transport. MEM test structures with gold-to-gold electric contacts are fabricated and tested with an experimental set up in NovaMEMS/CNES lab and will allow to validate the new methodology. The contact resistance is monitored during all experiments, to correlate the mechanical and electrical behavior of the structure. The measurements are in progress. We can already expect to some discrepancies due to the difficulty to measure accurately contact material properties and to the potential contamination around the metal contact area. Yet this application is a major concern in RF MEMS ohmic switches and shows an original approach to extract a guideline in choosing a design, materials and process flow to minimize the contact resistance.