The semiconductor industry is now struggling with an integrated-circuit “chip” power density crisis due to the non-scalability of the thermal voltage (kBT/q), which sets the minimum subthreshold swing (SS) of a metal-oxide-semiconductor transistor and hence limits reductions in transistor threshold voltage and hence chip operating voltage. In contrast to electronic switches, mechanical switches (“relays”) operate by making/breaking physical contact and therefore offer the ideal characteristics of zero off-state leakage current and abrupt transition between on/off states, which provide for zero static power dissipation and (in principle) lower operating voltage, so that they potentially can provide a means for overcoming this crisis. In order to fully realize their promise, however, miniaturized relays must operate with sufficient reliability to be viable for ultra-low-power digital logic applications.

In this work, the reliability of micro-relays designed for digital logic applications is systematically investigated. Contact resistance (Ron) instability is identified as the limiting factor for micro-relay endurance. Due to surface oxidation, Ron of prototype relays with tungsten (W) contacting electrodes increases with the number of operating cycles. This phenomenon is affected by relay operating conditions, including switching frequency and contact force. Larger contact force is found to be beneficial for stable operation, possibly due to breakdown of the insulating oxide layers. An alternative contact electrode material, ruthenium (Ru), is demonstrated to ameliorate the problem of metal surface oxidation; however, friction polymer formation and material transfer become dominant contact reliability issues, which eventually degrade Ron. An inkjet-printed micro-shell encapsulation process is used to provide an isolated ambient environment for the logic relays, and demonstrated to result in improved Ron stability by 100× as compared to devices tested in atmospheric conditions. In complementary logic circuits, slower switch turn-off than turn-on results in undesirable "crowbar” current, causing transient power dissipation during signal transitions and potential reliability issues. The effects of the contact electrode material mechanical properties, relay design parameters, and relay operating conditions on contact detachment delay (τCD) are experimentally studied and theoretically explained. Specifically, τCD is compared for logic relays with tungsten, ruthenium or nickel contacting electrode materials, and tungsten is found to provide for the smallest τCD.




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