Crystallographic structure of aluminum oxide bonding interfaces prepared via room‐temperature surface-activated bonding

The structure and chemistry of the aluminum oxide bonding interface formed via room-temperature surface-activated bonding were investigated using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). The results revealed that Al atoms at the interface of sapphire (α-Al₂O₃)–sapphire bonding occupied both octahedral and tetrahedral sites. The sapphire–Al₂O₃ film bonding interface also exhibited localized formation of a γ-Al₂O₃ phase, whereas the Al₂O₃–Al₂O₃ film bonding showed no distinct reaction layer. Fast Fourier transform (FFT) analyses of high-resolution TEM images from the interlayer region at the sapphire–sapphire bonding interfaces revealed lattice diffraction patterns similar to those of the sapphire substrate. A corresponding FFT diffraction pattern was observed at the sapphire–Al₂O₃ film bonding interface in spite of the fact that the atomic structure was not clearly visible. Changes in the coordination state of the Al atoms on the sapphire surface during activation significantly affect these aluminum oxide bonds. This study contributes to the understanding of the bonding mechanism in direct bonding of aluminum oxides at room temperature.