Gold-based thin multilayers for ohmic contacts in RF-MEMS switches

In RF-MEMS switches many reliability issues are related to the metal contacts in the switching area. The quality of this contact strongly influences switching failures due to wear, adhesion and stiction. Gold is widely used as contact material, but is a soft metal. Contact hardness can be improved preserving good conductivity and chemical inertness alternating gold layers with thin layers of other metals. A detailed study of mechanical, electrical and morphological properties of gold–chromium, gold–platinum and gold–palladium multilayers is presented and discussed. Annealing treatments modify hardness values, but a careful choice of the alloying metal is essential because hardness is reduced upon exposure to plasma ashing, a very frequent microfabrication process. This treatment also increases contact roughness, deteriorating the contact surface. Platinum is the only metal tested that is unaffected by plasma oxidation, and it also reduces the diffusion of chromium adhesion layer on the contact surface.