Elucidating the metal-induced crystallization and diffusion behavior of Al/a-Ge thin films

As used in wafer-level bonding in microelectromechanical system (MEMS) devices, the eutectic Al/a-Ge bilayer is characterized by its remarkable hermetic sealing after annealing. For MEMS packaging, this study investigates metal-induced crystallization (MIC) of the amorphous Ge and the layer exchange of Al and Ge, mainly by scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) analysis, and x-ray photoelectron spectroscopy (XPS). A kinetic mechanism to explain the layer exchange of Al and Ge is developed. Experimental results indicate that round-shaped extrusions form on the upper surface of the Ge layer when the bilayer is annealed at 400°C, i.e., close to the Al-Ge eutectic temperature. The morphology and the formation of the extrusions are also discussed. Finally, the bilayer is tested by immersion in red ink, with these results indicating that no red ink penetrates the bonding area of two bonded bilayer films. Therefore, results of this study demonstrate the feasibility of applying the eutectic Al/a-Ge bilayer to MEMS as a hermetic sealing material.