Surface chemical control of mechanical energy losses in micromachined silicon structures

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 Angstrom 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.