Investigation of the electrical contact behaviors in Au-to-Au thin-film contacts for RF MEMS switches

Abstract

This paper presents the electrical contact behaviors of gold-to-gold thin-film contacts under high current conditions and describes the major factors that influence these contact behaviors for radio frequency microelectromechanical system switches. The fundamental phenomena in the contact resistance versus contact force curve were investigated with a contact measurement setup, which we devised. Based on the experimental results, the contact resistance behavior with increasing contact force can be divided into three regions: first, an unstable contact regime before the minimum contact force is reached; second, a stable contact regime where the plastic deformation of the asperities explains the behavior and, finally, a saturated regime where the thickness effect of the film on a hard substrate becomes evident. The minimum contact force required for a stable contact depended on the roughness of the surface and the current flow through the contact. The smooth surface of the Au film and the metal softening caused by a high current flow of 100 mA had the effect of lowering the minimum contact force. After the minimum contact force is exceeded, the contact resistance initially follows the asperity plastic deformation behavior, where the contact resistance is related to the resistivity and hardness of the contact material. In the higher contact force regime, the decrease in the contact resistance becomes almost negligible. With increasing contact force, the effective contact area is limited by the effect of the film thickness on a hard substrate. From these results, we found the principal factors influencing the contact behaviors of the Au-to-Au contacts to be the resistivity, hardness and softening temperature of the contact materials and the design factors such as the surface roughness, the film thickness and the substrate.

Methodology to analyze failure mechanisms of ohmic contacts on MEMS switches

“…In addition, the contact temperature (Tc) of contact a-spots could be linked with the contact force depending on the contact voltage [5]. Indeed the plastic deformation of the asperities during the contact formation proceeds more rapidly when the softening temperature is reached [2].…”

Comparative study of RF MEMS micro-contact materials

“…The best compromise between mechanical and electrical performances has to be found to reach reliable operations. The material must have good electrical conductivity to avoid losses, high melting point to handle power, appropriate hardness to avoid stiction, and chemical inertness to avoid oxidation [7]. This information can be partly found in the literature, but it does not replace a direct quantitative characterization of the MUT, as it is done by means of the setup presented in next subsection.…”

Observation and Understanding of the Initial Unstable Electrical Contact Behaviors

“…11 Recent work on MEMS switches where simple hemisphere-on-flat systems are to simulate MEMS switch behavior, have shown that the Au-to-Au contact metal failure remains an important issue at the micrometer and nanometer scales. [12][13]14,15 Admittedly, macroscopic contact bounce phenomena inevitably occur during the contacts breaking. They are generally attributed to the mechanical reaction due to high force and high velocity applied to the mobile contact part during contact closure.…”