Thin film metallic glasses as materials for MEMS and their internal stress control

Thin film metallic glasses (TFMGs) are promising structural materials for microelectromechanical systems (MEMS) because they exhibit amorphous and generally isotropic mechanical characteristics, a high elastic strain limit, and thermoplastic formability in the supercooled liquid region. However, their practical integration into MEMS is strongly affected by internal stress, which influences wafer curvature, release yield, shape stability, and device performance. This review addresses TFMGs for structural MEMS applications from the standpoint of internal stress, rather than biomedical or purely functional thin-film uses. Representative material systems and deposition routes are first surveyed with attention to process integration. The discussion then focuses on three closely related issues: the development of intrinsic stress during deposition and thermal stress during cooling; stress evolution associated with structural relaxation during subsequent thermal history; and the selection of appropriate stress metrology for blanket films, released structures, and device-level evaluation. Representative MEMS applications, including compliant structures and thermoplastic microforming, are also reviewed to demonstrate that stress should be considered not only as an integration issue but as an important design variable. Finally, the principal challenges and future directions for the advancement of TFMG-based MEMS are outlined.