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 (SiNx), 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 SiNx, 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 SiNx. In addition, for the first time, a single-crystal Al1-xScxN/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-xScxN piezoelectric devices.

Tailoring Surface Properties via Functionalized Hydrofluorinated Graphene Compounds.

A new compound material of 2D hydrofluorinated graphene (HFG) is demonstrated whose relative hydrogen/fluorine concentrations can be tailored between the extremes of either hydrogenated graphene (HG) and fluorinated graphene (FG). The material is fabricated through subsequent exposures to indirect hydrogen plasma and xenon difluoride (XeF2 ). Controlling the relative concentration in the HFG compound enables tailoring of material properties between the extremes offered by the constituent materials and in-plane patterning produces micrometer-scale regions with different surface properties. The utility of the technique to tailor the surface wettability, surface friction, and electrical conductivity is demonstrated. HFG compounds display wettability between the extremes of pure FG with contact angle of 95° ± 5° and pure HG with contact angle of 42° ± 2°. Similarly, the HFG surface friction may be tailored between the two extremes. Finally, the HFG electrical conductivity tunes through five orders of magnitude when transitioning from FG to HG. When combined with simulation, the electrical measurements reveal the mechanism producing the compound to be a dynamic process of adatom desorption and replacement. This study opens a new class of 2D compound materials and innovative chemical patterning with applications for atomically thin 2D circuits consisting of chemically/electrically modulated regions.