Mechanical stiffness and dissipation in ultrananocrystalline diamond microresonators

Abstract

We have characterized mechanical properties of ultrananocrystalline diamond (UNCD) thin films grown using the hot filament chemical vapor deposition (HFCVD) technique at $680\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$, significantly lower than the conventional growth temperature of $\ensuremath{\sim}800\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. The films have $\ensuremath{\sim}4.3%$ $s{p}^{2}$ content in the near-surface region as revealed by near edge x-ray absorption fine structure spectroscopy. The films, $\ensuremath{\sim}1\text{ }\ensuremath{\mu}\text{m}$ thick, exhibit a net residual compressive stress of $370\ifmmode\pm\else\textpm\fi{}1\text{ }\text{MPa}$ averaged over the entire 150 mm wafer. UNCD microcantilever resonator structures and overhanging ledges were fabricated using lithography, dry etching, and wet release techniques. Overhanging ledges of the films released from the substrate exhibited periodic undulations due to stress relaxation. This was used to determine a biaxial modulus of $838\ifmmode\pm\else\textpm\fi{}2\text{ }\text{GPa}$. Resonant excitation and ring-down measurements in the kHz frequency range of the microcantilevers were conducted under ultrahigh vacuum (UHV) conditions in a customized UHV atomic force microscope system to determine Young's modulus as well as mechanical dissipation of cantilever structures at room temperature. Young's modulus is found to be $790\ifmmode\pm\else\textpm\fi{}30\text{ }\text{GPa}$. Based on these measurements, Poisson's ratio is estimated to be $0.057\ifmmode\pm\else\textpm\fi{}0.038$. The quality factors $(Q)$ of these resonators ranged from 5000 to 16000. These $Q$ values are lower than theoretically expected from the intrinsic properties of diamond. The results indicate that surface and bulk defects are the main contributors to the observed dissipation in UNCD resonators.