Mechanical dissipation in tetrahedral amorphous carbon

We have fabricated micromechanical oscillators from tetrahedrally coordinated amorphous carbon (ta-C) in order to study mechanical dissipation mechanisms in this material. Cantilever oscillators with either in-plane or out-of-plane dominant transverse vibrational modes and free-free beam oscillators with in-plane modes were fabricated with critical dimensions ranging from 75 nm to over 1 mm. The resonant frequency and quality factor were measured for all oscillators. The resonant frequencies ranged from a few kilohertz to several megahertz, while the quality factor remained nearly constant at approximately 2-4x10(3). Possible dissipation mechanisms were evaluated for these oscillators, and it was found that the observed dissipation was not limited by mechanical clamping losses, air damping, thermoelastic dissipation, or dissipation due to phonon-mechanical vibration interactions. However, an extrinsic dissipation mechanism in which dissipation is limited by a spectrum of defects in ta-C was found to be consistent with the observed behavior. Assuming that the mechanical relaxation associated with the dissipative defects is thermally activated, we derive a defect distribution that is relatively flat with activation energies ranging from about 0.35 to over 0.55 eV. (C) 2005 American Institute of Physics.