Tribology of thin film applied to ohmic contact MEMS micro-relays

Since the 1980s, the use of MicroElectroMechanical Systems (MEMS) has become increasingly widespread: micro-mirrors for video projectors, accelerometers for airbags and gyroscopes for game consoles. Thanks to their performance and compatibility with silicon microtechnology, new functions are being targeted, such as relays and switches.

The ohmic MEMS microrelay is nothing more than a switch of the order of a hundred microns in size, made up of metal contacts. A moving structure closes or opens this contact. In this way, the electrical signal can pass through or be interrupted. The production of a first demonstrator has demonstrated their performance in terms of pass frequencies and miniaturization, ahead of conventional silicon microtechnology variants (diodes or transistors).

However, after many opening-closing cycles of the electrical contact, performance deteriorates. The characteristics of electrical contact on a submicrometer scale are complex, due to the many interdependent phenomena involved. When the ohmic MEMS microrelay closes, the rough surfaces come into contact, deform and an electric current flows through them. Thus, topography, in conjunction with the local mechanical properties and chemical composition of the contact surfaces, modulates the electrical contact resistance (CR). The contact surface can also be thermally affected by current flow and/or arcing. What's more, these properties change over the lifetime of the ohmic MEMS microrelay (several million cycles).

In this context, mastering the tribology of the thin layers making up the contact surfaces of ohmic MEMS microrelays will be a source of progress. This article focuses on the mechanical aspect of contact, in close connection with the evolution of topography. After a general introduction to ohmic MEMS microrelays, the various modes of degradation are presented. A multiscale approach is then applied to account for topographical evolutions through a statistical then discrete description of the nanorugosities or asperities constituting the surface of these thin films. The mechanical aspect of contact is addressed in particular at low forces, in the case of the operation and actuation of ohmic MEMS microrelays. This approach will be based on experimental methods using nanoindenters and atomic force microscopy (AFM), as well as discrete modeling of rough contact.