Al-based high-aspect-ratio microscale structures (HARMS) are basic building blocks for all-Al microdevices. Bonding of Al-based HARMS is essential for device assembly. In this paper, bonding of Al-based HARMS to flat Al plates using Al-Ge thin film intermediate layers is investigated. The structure of sputter codeposited Al-Ge thin films was studied by high-resolution transmission electron microscopy as a function of the average film composition. The structure of the interface region between Al-based HARMS bonded to flat Al plates is studied by combining focused ion beam sectioning and scanning electron microscopy. An extended bonding interface region, ∼100 μm in width, is observed and suggested to result from liquidus/solidus reactions as well as diffusion of Ge in solid Al at the bonding temperature of 500 °C. The extended interface region is suggested to be beneficial to Al-Al bonding via Al-Ge intermediate layers.