Gradient Residual Stress in Transferred Thin-Film Lithium Niobate and Its Compenstation Using Periodically Poled Piezoelectric Bilayers

TL;DR

This study investigates the gradient residual stress in transferred thin-film lithium niobate, its dependence on orientation and thickness, and proposes bilayer structures for stress compensation to improve MEMS device stability.

Contribution

It provides experimental insights into stress gradients in TFLN and introduces a bilayer design to mitigate residual stress effects.

Findings

Stress gradient strongly depends on crystallographic orientation and film thickness.

Finite element simulations agree well with experimental stress measurements.

Bilayer structures with opposite orientations reduce residual stress and deformation.

Abstract

In this work, we experimentally investigate the gradient stress (sigma1) in 128 deg Y-cut transferred thin film lithium niobate (TFLN) films with thicknesses from 100 to 460 nm using cantilever curvature analysis. The results reveal a strong dependence of sigma1 on both crystallographic orientation and film thickness, with stress-free orientations at approximately 55 deg and 125 deg for 220-460 nm films, shifting to approximately 20 deg and 160 deg for 100 nm films. The extracted normalized sigma1 ranges from -0.1 to 3.4 MPa/nm (100 nm), -0.8 to 0.34 MPa/nm (220 nm), and -0.12 to 0.08 MPa/nm (460 nm), indicating a pronounced thickness-dependent through-thickness stress gradient. Finite element simulations show excellent agreement with the measurements, validating the curvature-based extraction method and confirming that sigma1 originates from an orientation-dependent residual stress gradient. To mitigate this effect, a bilayer TFLN structure with opposite crystallographic orientations, forming a periodically poled piezoelectric film (P3F), is investigated, enabling partial cancellation of sigma1. A 90/110 nm P3F bilayer reduces the equivalent normalized sigma1 to -0.4 to -0.04 MPa/nm, resulting in significantly reduced deformation. These results establish gradient stress engineering through orientation, thickness, and bilayer design as an effective strategy for achieving mechanically stable and scalable TFLN microelectromechanical systems (MEMS) devices.