Residual Stress Anisotropy in Thin-Film Lithium Niobate for Stress-Managed Mems

In this work, we present the first experimental study of residual stress and post-release beam deflection in$128^{\circ} \mathrm{Y}$cut thin-film lithium niobate (TFLN) on Si, revealing pronounced stress anisotropy with in-plane orientation. Using optical profilometry with curvature fitting, we extract gradient stress ($\sigma_{1}$) to build orientation-resolved stress maps across multiple film thicknesses (100, 220, 460$\text{nm})$. For films in the$220-460 ~\text{nm}$range, we identify stressfree in-plane orientations near$\sim 55^{\circ}$and 125°, enabling extremely flat suspended beams. In contrast, ultra-thin 100 nm films exhibit shifted stress-free orientations near$\sim 20^{\circ}$and 160°. Leveraging these orientations, we demonstrate extremely long suspended beams up to 2 cm in length, 10$\mu \mathrm{m}$in width, and 460 nm in thickness without collapse. These results establish in-plane stress anisotropy and thickness selection in TFLN as a practical design lever for mechanically stable, scalable, and stress-managed microelectromechanical systems (MEMS).