The relationship of surface chemistry and mechanical energy dissipation in micromechanical resonators

Micro- and nanomechanical resonant devices could have a considerable impact on commercial applications. They have been proposed for applications as diverse as scanned probe microscopy, calorimetry, and biosensing. To be effective these applications require ultra small resonant devices with high quality factors (i.e. low mechanical energy losses). Research has focused primarily on the challenges of manufacturing and detecting small resonant devices; however, progress has been slowed by the persistently low quality factors of these devices. Although the previously observed scaling of mechanical energy losses suggests that surface effects may dominate losses in micro- and nanomechanical resonators, the relationship of surface chemistry to mechanical energy dissipation has not previously been investigated.; The quality factor of micromechanical, MHz-range silicon resonators can be dramatically improved by replacing a thin layer of surface oxide with a single monolayer of hydrogen. The increased mechanical energy losses associated with this thin layer of oxide (∼13 A) cannot be attributed to oxide-induced changes in the stress or tension of the resonator, nor can they be attributed to size-dependent changes in the bulk elastic properties of silicon. Quantitative analysis of the oxidation-induced mechanical energy losses successfully attributes losses to just three energy loss pathways: the environment, bulk silicon, and the surface. Importantly this model also predicts that the importance of surface losses will increase as the device surface-atom-to-bulk-atom ratio increases.; Unfortunately, the high quality factor of hydrogen-terminated devices is unstable even at high vacuum (10-8 Torr). Analysis of the frequency shifts of hydrogen-terminated devices suggests that the nature of the instability is surface chemical. Protection of devices with alkyl self-assembled monolayers bound to the silicon surface through a silicon-carbon covalent bond results in devices with stable quality factors and frequencies. In particular, methyl-terminated resonators have quality factors that are higher than hydrogen-terminated resonators and quite stable even at atmospheric pressure. The mechanism of surface-induced mechanical energy dissipation is uncertain; however, experimental measurements of quality factor are consistent with the involvement of surface electronic defects.