Surface science, MEMS and NEMS: Progress and opportunities for surface science research performed on, or by, microdevices

Impact of oxygen and argon plasma exposure on the roughness of gold film surfaces

A systematic comparison between several pairs of contact materials based on an innovative methodology early developed at NOVA MEMS is hereby presented. The technique exploits a commercial nanoindenter coupled with electrical measurements, and test vehicles specially designed in order to investigate the underlying physics driving the surface-related failure modes. The study provides a comprehensive understanding of micro-contact behavior with respect to the impact of low- to medium levels of electrical current. The decrease of the contact resistance, when the contact force increases, is measured for contact pairs of soft material (Au/Au contact), harder materials (Ru/Ru and Rh/Rh contacts) and mixed configuration (Au/Ru and Au/Ni contacts). The contact temperatures have been calculated and compared to the theoretical values of softening temperature for each couple of contact materials. This threshold temperature is reached for gold, ruthenium and rhodium material, with different levels of current intensity. In spite of that, no softening behavior has been observed for mixed contact at the theoretical softening temperature of both materials. Hence, considering the sensitivity to power handling and the related failure mechanisms, namely the contact adhesion, the enhanced resilience of the bimetallic contacts Au/Ru and Au/Ni was demonstrated. Finally focusing on the temperature distribution around the hottest levels on the surface contact interface, these results have been theoretically investigated. Keywords-microcontact; MEMS; micro-switch; contact temperature; contact materials; adhesion; creep.

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Multi-Physical Characterization of Micro-Contact Materials for MEMS Switches

This paper presents an electrostatically actuated microelectromechanical systems (MEMS) relay with a stacked-electrode structure having meshed drain electrode and a soft dielectric layer under the contact material to achieve high contact force and low hardness simultaneously, with the aim of providing ultralow contact resistance. In particular, a novel method for laying benzocyclobutene polymer under the contact layer to enlarge the contact area by reducing the effective hardness is proposed and theoretically analyzed. This could reduce the contact resistance by more than half in the case of an Au-Au contact. The fabricated devices have pull-in voltage of 32-43 V, switching time of 25 μs, and current driving capability of 350 mA in a hot-switching condition. Furthermore, the achieved minimum contact resistance is as low as 4 mΩ, which, to our knowledge, is the lowest value reported to date. In addition, negligible variation of contact resistance was observed during 1.4 × 106 hot-switching cycles in a 100-mA current level. The fabricated MEMS relay in the relatively low current of 1 mA was able to operate for more than 100 million cycles without failure before the test was stopped.

An Extremely Low Contact-Resistance MEMS Relay Using Meshed Drain Structure and Soft Insulating Layer

This paper reports on a novel electrostatically actuated microelectromechanical systems (MEMS) relay for use in power-switching applications. It features a levering and torsional spring to enhance the stand-off voltage and contact endurance by means of an active-opening scheme. The proposed relay is based on a unique stacked-electrode structure and a soft insulating layer under the contact material that make it possible to obtain extremely low contact resistance, resulting in high current driving capability and reliable contact endurance. The fabricated relay demonstrated actuation voltages under 40 V, a switching time of 230 μs, and a maximum stand-off voltage of 360 V, which is the highest level among electrostatically actuated MEMS relays reported to date. The contact resistance was under 5 mΩ at 40 V of applied voltage, and more than 1 A could be carried. The contact reliability in a hot-switching condition was investigated for various dc current levels. At a current of 10 mA, the relay operated for more than 107 cycles before the test was stopped. In addition, the permanent contact stiction during switching operation at a 200-mA current level was overcome with a pull-off (active-opening) voltage of 90 V by the levering and torsional spring. Using this healing process, a device that failed at about 104 switching cycles in the 200-mA hot-switching mode was revived and reoperated with negligible contact resistance variation, lasting up to 4.9 ×105 cycles, constituting an order-of-magnitude enhancement in the lifetime even after failure.

An Electrostatically Actuated Stacked-Electrode MEMS Relay With a Levering and Torsional Spring for Power Applications

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.

https://iopscience.iop.org/article/10.1088/0960-1317/18/10/105010/pdf

Tae-Sik Kim

Papers

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