Self-Rolled-Up Aluminum Nitride-Based 3D Architectures Enabled by Record-High Differential Stress

Aluminum nitride (AlN) continues to kindle considerable interest in various microelectromechanical system (MEMS)related fields because of its superior optical, mechanical, thermal, and piezoelectric properties. In this study, we use magnetron sputtering to tailor intrinsic stress in AlN thin films from highly compressive (-1200 MPa) to highly tensile (+700 MPa), with a differential stress of 1900 MPa. By monolithically combining the compressive and tensile ultrathin AlN bilayer membranes (20-60 nm) during deposition, perfectly curved three-dimensional (3D) architectures are spontaneously formed upon dry-releasing from the substrate via a 3D MEMS approach: the complementary metal-oxide-semiconductor (CMOS)-compatible strain-induced self-rolled-up membrane (S-RuM) method. The thermal stability of the AlN 3D architectures is examined, and the curvature of S-RuM microtubes and helical structures as a function of the cumulative membrane thickness and stress are characterized experimentally and simulated using a finite-element physiomechanic method. By combining AlN with various materials such as metal (Cu) and silicon nitride (SiN_x), AlN-based hybrid S-RuM microtubes with diameters as small as ∼6 μm are demonstrated with a near-unity yield (∼99%). Compared with other stressed thin films for S-RuMs, including PECVD SiN_x, magnetron-sputtered AlN-based S-RuMs show better structural controllability and versatility, probably due to the high Young’s modulus and stress uniformity. This work establishes the sputtered AlN thin film as a superior stress-configurable S-RuM shell material for high-performance applications in miniaturizing and integrating electronic components beyond those based on other materials such as SiN_x. In addition, for the first time, a single-crystal Al₁-_xSc_xN/AlN bilayer grown by molecular beam epitaxy is successfully rolled-up with the diameter varying from ∼9 to 14 μm, paving the way for 3D tubular Al₁-_xSc_xN piezoelectric devices.