Stable pull-in electrodes for narrow gap actuation

This paper reports the use of movable electrodes that can be electrostatically pulled in to achieve narrow gaps beyond lithography / etch capabilities. Width-extensional resonators with frequencies of 50 MHz and quality factors of 150k are demonstrated with such movable electrodes to have a significantly lower motional impedances when pullin occurs. Sub-ppm stability over 10⁵ pull-in/pull-out cycles is measured using temperature-compensated resonators within the epi-seal epitaxial polysilicon encapsulation process. The pull-in phenomena is reversible, but can be made permanent by electrically welding the pulled-in electrode to a stop.