Micro Scale Experiments and Modeling of MEMS RF-Switches

Northwestern University, Evanston IL

Investigators

Abstract

0120866 Espinosa A GOALI award supports study of materials properties and problems relevant to the integration of micro-electro-mechanical systems (MEMS) with IC components for wireless applications. These systems offer unique advantages over other technologies because of their low power consumption, high sensitivity, physical size, and low cost. The technology is ideal for next generation of cell phones, base stations for wireless systems, and highly-efficient software-controlled digital radio for military communications. In these applications the MEMS devices are typically used as filters or switches. Filters with Q-factors of 94,000 can be employed to pre-select a communication band and a specific channel within that band. Switches with insertion losses bellow 0.1 dB per switch can be used for electromagnetic beam steering in radar antennas, accomplished through phase shifting. This feature enables antennas to transmit and receive signals without the need for physical reorientation. However, despite the large industrial interest, the technology is not yet commercially viable because a number of technical obstacles have to be overcome first. Key among these are packaging and mechanical modeling of MEMS materials at the micron scale. For instance, in the case of RF-switches, the effect of the environment can result in stiction of the membranes due to humidity or other sources. This requires the development of a cheap hermetic package. From a reliability standpoint, it is necessary to consider the plasticity limit and its temperature dependence with the materials involved. Temperatures as low as minus 50 degrees Celcius can be reached in satellite and airplane wireless applications while temperatures of a few hundred degrees can be present during device packaging. Another important failure mechanism is fatigue caused by excessive actuation cycles. Most of these devices are actuated trillions cycles pushing the design envelope and our current knowledge of material behavior beyond known parameters. In this project the reliability of capacitive RF-MEMS switch materials and components will be investigated. The effect of temperature and number of cycles on material degradation as a result of defects generation and evolution will be examined for aluminum alloy and doped nanocrystalline diamond films. Likewise, the evolution of the built-in stresses will be identified with a nanoindentation technique for use in the SEM, recently developed in the PI's laboratory. Experiments will consist of the deflection of freestanding films, to assess the elastic and inelastic properties of the films, and fatigue analysis by means of successive electrostatic actuation. Evolution of built-in stresses and material microstructure will be assessed as a function of the number of actuation cycles. Modeling of the experiments will be performed at several length scales, from ab initio calculations and molecular dynamics to discrete dislocation and continuously distributed dislocation networks; extensive simulations will link modeling and experiment. ***