Geometric tuning of stress in silicon nitride beam resonators

Silicon nitride is a material with high intrinsic stress, ensuring very high quality factors in micro- and nanomechanical resonators. Typically, the film stress can only be changed via the deposition process. We control the SiN beam stress via the geometry. Our “S”-shaped beams are displaced in the center and thus longer than the distance between the two clamping points. When releasing the beams, they straighten and partially relax their stresses, which influences the dynamics of the devices. We used an integrated Mach-Zehnder interferometer to optically sense the driven motion of the suspended beams and studied the stress-dependence of the eigenfrequencies and quality factors. A clear dependence of the resonance frequency on the initial displacement and length is observed, and the results match finite element simulations. Finally, we studied the influence of different beam shapes on the mechanical properties. These devices will provide more insight in the role of stress in the damping of SiN resonators, and adds geometrically-tunable stress as a new degree of freedom for a variety of experiments in the field of optomechanics, such as synchronization.