UC Berkeley

The proliferation of information and communication devices over the past few decades has been enabled by continual advancement of semiconductor manufacturing technology to steadily miniaturize semiconductor switching devices – most notably, metal-oxide-semiconductor field effect transistors (MOSFETs) – to increase the number of transistors in the most advanced integrated circuit (IC) products, at a pace set by Moore’s Law, for enhanced chip functionality and performance. In recent years, however, the incremental benefit of transistor scaling has diminished largely because the Boltzmann energy distribution of electrons in a semiconductor results in switching steepness (subthreshold swing) proportional to the thermal voltage (kT/q), which does not scale. As a result, conventional MOSFETs cannot switch ON/OFF more abruptly than 60 mV/decade at room temperature, which limits the extent to which the transistor threshold voltage (VT) can be reduced for a given OFF-state leakage current specification (IOFF). As the operating voltage (VDD) of a digital IC is reduced with increasing transistor density to meet power density constraints (set by chip cooling limitations), then, the gate overdrive voltage (VDD – VT) is disproportionately reduced, limiting the transistor ON-state current and hence IC performance. With the advent of the Internet of Things, the need for more energy-efficient electronics has emerged; alternative switching devices that can be operated at much lower voltage than the MOSFET will be required. Micro-electro-mechanical (MEM) relays are promising candidate switching devices for low-voltage digital ICs, since they can achieve immeasurably low IOFF and abrupt switching behavior across a wide range of operating temperatures. Since MEM relays exhibit hysteretic switching behavior (i.e., the value of the control/gate voltage at which a relay switches ON is different than that at which it switches OFF) the hysteresis voltage sets a lower limit for their operating voltage.

This dissertation discusses approaches and challenges for realizing milli-Volt MEM relay technology for energy-efficient computing. First the application of self-assembled molecular (SAM) anti-stiction coatings to reduce contact adhesive force and thereby the hysteresis voltage is investigated, and stable sub-50 mV operation is demonstrated. Next the issue of variability in relay performance parameters over many switching cycles and from device to device is systematically studied, and SAM coating is found to improve stability. Then the effects of contacting electrode design and body-biased operation on relay ON-state resistance (RON) are investigated. The direct source/drain contact design provides for lowest and least variable RON. Ultra-low-voltage relay operation facilitated by body-biasing results in lower contact velocity, which mitigates the need for a wear-resistant contacting electrode material while necessitating a contacting electrode material that is not susceptible to oxidation.