(A) study on novel approaches of enhancing contact reliability of MEMS switch

Abstract

Mircoelectromechanical systems (MEMS) switches (relays) have attracted a lot of attention in recent years owing to their amazing advantages of high isolation, low on-resistance, low power consumption, and abrupt switching characteristics. Under current electronics trends of miniaturization and portability which require small device size and high power efficiency, these strengths make the MEMS relays to be one of promising switching devices for the next-generation. In spite of the outstanding performance of the MEMS relays, there has been considerable difficulty in using these relays in power applications, owing to relatively poor reliability in high power (current) levels. To solve the contact endurance issue (the most severe problem), various novel approaches have been suggested in this dissertation. 1. As a structural approach, an electrostatically actuated MEMS relay with a stacked-electrode structure including a soft insulating layer and a levering and torsional spring is proposed, designed, and demonstrated. Very low contact resistance, reliable contact endurance, and high current driving capability can be achieved by means of the stacked-electrode structure (for high contact force) and the soft dielectric material under the contact material (for low effective hardness). Furthermore, the levering and torsional spring revives the switch itself after failure by stiction, resulting in a tenfold increase (up to $4.9 \times 10^5$ cycles at 200 mA of current level) of its lifetime, for the first time. 2. A simple method to make field-less or arc-less condition for the longer-lasting switching operations is presented, as a circuitry approach. It employs a drain voltage-sustaining capacitor, which maintains the drain voltage at the switch “opening” instant, thereby eliminating the voltage difference between two contacts. This drain voltage-sustaining capacitor could result in significantly enhanced lifetime in an Au-Au contact MEMS relay, by an order of magnitude, in hot switching condition. 3. As a material approach, a complementary dual-contact switch, which is composed of two sub-switches with respective hard and soft contact materials to acquire both high contact endurance and low contact resistance, is suggested. The dual-contact switch is designed to make the hard contact material sub-switch turn on first and turn off last. Under the umbrella operation of a reliable hard contact material sub-switch, the soft contact material sub-switch could obtain more enhanced contact endurance and aptly utilizes its low contact resistance characteristic. In turn, due to the low contact resistance of the soft contact material sub-switch, the hard contact material sub-switch made up for its high contact resistance. 4. A new type of complementary dual-contact “zipping” switch is also proposed to solve two critical issues in the previous dual-contact concept. By means of this “zipping” technique, the dual-contact sequence can now be realized by only a single micro-switch while preserving the complementary switching performance (smaller footprint). Furthermore, the zipping actuation could allow dual-contact sequence to be automatically realized even by a normal square gate voltage pulse, and thus a special waveform of the gate signal is not required to achieve dual-contact sequence (simpler actuation). 5. By being inspired from these three approaches, a novel “contact-refresh” approach is suggested to employ full apparent contact area, thus resulting in very reliable contact endurance. The concept is just accomplished by adding a lateral motion of the upper contact part to a conventional vertically-actuated switching device for making use of even unused contact spots. In this way, increased contact resistance by damaged contact spots during switching operations could be refreshed (decreased) when the switch is re-operated after slight movement of the upper contact part. Also, the estimated lifetime can be calculated to be $10^9$ cycles at 100 mA of current level in hot switching condition, which is about 3-order higher than the average value of the measured lifetimes in a single positions.