Mechanical energy dissipation in room-temperature micromechanical silicon torsional resonators, with resonant frequencies ranging from 2.2 to 23 MHz, is considerably increased when a single monolayer of hydrogen atoms is replaced by 13 Å of silicon oxide (which corresponds to the oxidation of less than 2 silicon bilayers.) Measurements of oxidation-induced resonant frequency shifts show that increased dissipation cannot be attributed to chemically induced changes in the stress or tension of the resonator. The dependence of both the frequency shift and reduction in quality factor on resonator size are consistent with a chemically induced surface loss mechanism. Quantitative analysis shows that even relatively unstressed areas of the surface can contribute considerably to mechanical energy dissipation. The scaling of losses with resonator dimensions suggests that surface effects will become increasingly more important as the sizes of micromechanical devices continue to decrease.

Self-assembled alkyl monolayers that are directly tethered to the silicon surface with a Si–C bond suppress mechanical energy dissipation in megahertz-range micromechanical silicon oscillators as compared to the more common silicon oxide coating. Although not as low loss as freshly prepared H-terminated surfaces, Si–C tethered monolayers are more stable with time. Alkyl monolayers derived from chlorosilanes have much poorer mechanical performance. Both types of monolayers suppress adsorption.

The rate of mechanical energy dissipation in 300-nm-thick, megahertz-range micromechanical silicon resonators is sensitive to single monolayer changes in surface chemistry. Resonators terminated with a single monolayer of methyl groups have significantly higher quality factors (Q's), and thus lower mechanical energy dissipation, than comparable resonators terminated with either long-chain alkyl monolayers (C2H2n+1, n = 2-18) or monolayers of hydrogen atoms. This effect cannot be attributed to mechanical energy dissipation within the alkyl monolayer, as a 9-fold increase in alkyl chain length does not lead to an observable increase in dissipation. Similarly, this effect is not correlated with the chemical structure of the silicon-monolayer interface (e.g., the density of Si-H vs Si-C bonds.) Instead, the chemical trends in resonator quality and stability are consistent with a dissipation mechanism involving the coupling of long-range strain fields to localized, electronically active defects in the monolayer coatings.

The quality factor and long-term stability of megahertz-range micromechanical silicon resonators can be significantly improved by a methyl monolayer directly bonded to the silicon surface. Mechanical energy dissipation in functionalized resonators is shown to be a sensitive function of surface chemistry. At least 18% and 41% of the dissipation in H-terminated and long-chain alkyl-terminated resonators, respectively, is surface related. Surface-induced dissipation is poorly correlated with the mechanical properties of the terminating layer, but may be related to the surface defect density.

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