Thermal, residual stress and contact reliability of RF MEMS: Fundamental physics and improvements.

摘要

Radio frequency micro-electromechanical systems (RF MEMS) have been envisioned to be ideal devices for future wireless applications over a decade; however, they still suffer from their limited reliability. This results from inadequate understandings to the governing phenomena. The device performance is normally limited by three main problems: self-heating, residual stress, and contact reliability. This thesis studies the associated dominant phenomena using representative devices/structures, and proposes technological solutions.; The thesis first discovers that skin-effect self-heating causes premature buckling to air-suspended (or microshield) transmission line structures. A new analytical model and experimental technique are established. Hence, increasing the thickness from 1 to 3 microns of a typical transmission line can enhance a 25-fold power handling.; Secondly, the thesis addresses the residual stress problem in fabrication. Sputtering is commonly-used in deposit structural film deposition; nonetheless, residual stress makes thick film sputtering impossible. A new low stress sputtering technique is developed. Thus, inserting 6 cooling intervals in sputtering mitigates gold residual stress from 203 MPa to 40 MPa.; Thirdly, the thesis studies the contact failures of direct contact switches: unstable contact resistance, slow switching time, and low power handling at certain lifetime. A switch operational condition and a new contact micro-structure are proposed. Also, some new phenomena discovered include gold nanowire formation and multiple-to-single electrical conduction transition at the contacts.; Incorporating these three reliability studies, a new direct contact RF MEMS switch with Ball-Grid-Array (BGA) dimples has been designed, mass-fabricated and evaluated in DC and RF performance. To our knowledge, it is the first RF MEMS direct contact switch using conventional fabrication process (1-micron UV lithography, 99.99% sputtered gold, 160 °C process temperature), having fast opening of 2 micro-seconds, and closing of 50 micro-seconds, low contact resistance of 0.7 O, and low insertion loss (DC to 20 GHz) of -0.1 dB, at >1W RF power handling for >100 million hot-switching cycles!; The specific devices studied here, i.e., transmission lines and direct contact switches, are the building blocks for many RF MEMS applications such as reconfigurable antenna. Therefore, this fundamental knowledge can well provide a general insight into reliability improvement for RF MEMS technology development.