Paper MN+PS-WeA7
Sub-100nm Thin Polycrystalline Diamond Nanomechanical Torsional Resonators

Wednesday, November 12, 2014, 4:20 pm, Room 301

We report experimental demonstration of high-frequency (HF) torsional nanomechanical resonators based on nanoscale polycrystalline diamond thin films. We fabricate devices with tethers as thin as 100nm×50nm in cross section, measure their multi-mode resonances with frequency (f_res) into the HF band (up to ~10MHz, while most existing sensitive torsional devices are at kHz or low-MHz), and quality (Q) factors exceeding 2000 at room temperature. We also perform temperature-varying measurements, and observe strikingly different temperature coefficients of frequency (TCf) between the torsional and flexural resonant modes.

Diamond is particularly interesting for micro/nanoelectromechanical systems (MEMS/NEMS), because of its exceptional mechanical properties (Young’s modulus greater than 10¹²Pa), relatively low mass density (3500kg/m³), very high thermal conductivity (22W/(cm·K)), and excellent wear/corrosion resistivity¹. Especially, its high sound velocity is attractive for making high frequency mechanical resonators². Resonators based on diamond thin films from microwave plasma chemical vapor deposition have been demonstrated, showing mechanical properties comparable to single crystal. However, torsional resonators based on diamond thin films showing resonance in HF band and exceptional force and torque sensitivities have not been explored. While we demonstrated torsional resonators using 1.2µm-thick SiC film³, much thinner and smaller devices are required for higher sensitivities.

Here we fabricate torsional resonators on 50 to 100nm thin polycrystalline diamond films with focused ion beam. We perform Raman spectroscopy to confirm the nanocrystalline diamond nature of the membrane. The mechanical resonances are measured by driving the mechanical motion with a modulated laser (405nm), and detecting the resonant motion with laser interferometry (633nm). These devices show force sensitivity down to the sub-fN/√Hz range, and torque sensitivity on the order of 10^-22 (N·m)/√Hz, which is similar to the best reported results in other materials⁴. This opens up the possibility for fabricating ultrasensitive devices for force/torque, inertia, and thermal sensing, based on nanocrystalline diamond platform. TCf measurement shows clear and intriguing anti-crossing behavior, which vividly illustrates cross-mode mechanical coupling.