Design and fabrication of ultra-coherent nanomechanical oscillators using phononic crystals and strain engineering

Ultra-coherent mechanical oscillators are promising candidates for studying the quantum behavior of macroscopic objects. A necessary condition for such tests of quantum mechanics is to have sufficiently low dissipation to perform several coherent oscillations at. In high-stress Si₃N₄ thin films, so-called ‘dissipation dilution’—where the stiffness of the material is increased without added loss—enables anomalously high quality factors.
Here, we will present design techniques to further increase the quality factors of Si₃N₄ nanobeams, and thereby allow up to hundred coherent oscillations at room temperature. Through fabrication of non-uniform phononic crystal patterns on nanobeams, we co-localize the strain and flexural motion of a Si₃N₄ nanobeam. Fabrication of levitated nanobeams with thicknesses ranging from 20nm to 100nm and lengths varying from 3mm to 8mm is demanding due to the extreme aspect ratios and requires a set of specialized techniques. The measurement of mechanical quality factors in the hundreds of millions at MHz frequencies must be performed in high vacuum, and are here performed using an optical interferometer. Using these techniques we have realized nanobeams with Qf products exceeding 10^14 Hz at frequencies ranging from 1 to 6 MHz.