The formation of an interfacial AlN layer was observed in an Al/Si3N4thin‐film system immediately after electron beam deposition of Al, employing Auger electron spectroscopy, x‐ray diffraction, and transmission electron microscopy. Heat treatments up to 600 °C resulted in growth of this layer. The Si liberated by the direct reaction between Al and Si3N4was found to crystallize into small islands of peculiar fractal‐like shape. The AlN layer acted as a diffusion barrier for diffusion of Al into Si3N4.