Actuation and internal friction of torsional nanomechanical silicon resonators

We report on the actuation and mechanical properties of silicon resonators with nanometer-scale supporting rods operating in the 3–20 MHz range. The symmetrically designed paddles can be excited both in their flexural and torsional modes of motion. Fabrication imperfections as small as 10–20 nm provide enough asymmetry to allow such torsional excitation. We also report on internal friction studies in these systems. Thin Al overlayers contribute to the room temperature internal losses, as quality factor drops from 3300 to 380 for 160 Å thick film. A temperature dependence of internal friction has a broad peak in the $\text{T=160–190\hspace{0.05em}}\mathrm{K}$ range, and attributed to the Debye relaxation and thermally activated friction mechanisms. Analysis shows that the peak shifts to higher temperatures with increasing resonator frequency.