Evolution of chemical configuration of the Al/PI interface with storage at room temperature. (a) EELS Al L2,3-edge from the Al matrix, and C- and O-edges from the PI matrix after hybrid bonding. (b) EELS Al L2,3- and O-edges from the center of the Al/PI interface after storage at room temperature for 3 and 6 months. (c) EELS C-edge from the center of the interfacial region (i.e. a position ca. 10 nm away from the native alumina) after storage at room temperature for 6 months (upper), and that obtained at a position ca. 5 nm away from the native alumina (lower). The ratio (R) marked at the upper right corner in each figure represents the intensity ratio Iσ*/(Iσ* + Iπ*), where Iσ* and Iπ* are the signal intensity of σ* and π* bonds in each C-K spectrum, respectively.