In Situ Localized Annealing for Contamination Resistance and Enhanced Stability in Nickel Micromechanical Resonators

A technique in which a micromechanical resonator is operated at large amplitudes while in situ localized annealed to temperatures exceeding 880 o C is shown to be an effective method for both removal of surface contaminants and for possible " redistribution " of the structural material towards substantially higher quality factor Q and greatly enhanced drift stability. The technique not only provides insight identifying contamination as a dominant mechanism for Q-degradation in nickel-plated micromechanical resonators exposed to uncontrolled environments, but also offers a convenient method for restoring a contaminated device to its original high-Q (Q=14,172) characteristics. I. INTRODUCTION Vibrating micromechanical resonators (" μresona-tors ") with Q's in the tens of thousands and frequencies approaching 100MHz have recently shown great potential for replacing the off-chip high-Q passives used in communication transceivers, with both size and power reduction advantages [1,2]. Thus far, poly-silicon has been the material of choice for mid-VHF applications, with Q's on the order of 8,000 in this range [2]. However, as frequencies rise beyond VHF, losses arising from the finite resistivity of doped poly-silicon become a concern [1]. For this reason, metals are of great interest as structural materials. Unfortunately , the majority of plated or deposited metals used in micromechanical applications have so far shown relatively low Q's (e.g. only 3,000 in nickel [3]), inadequate thermal stability [4], and unacceptable hystere-sis and drift. This work introduces a post-fabrication procedure that significantly improves the performance of nickel μresonators. In particular, a technique in which a μme-chanical resonator is operated at large amplitudes while in situ localized annealed to temperatures exceeding 880 o C has been shown to substantially enhance quality factor Q and drift stability. After first describing the nickel plating process used to realize resonators in Section II, this paper details the in situ localized annealing technique, providing operational specifics and experimental data in Sections III and IV, respectively, then concluding with a discussion of possible mechanisms behind observed enhancements.