Homo/heterogeneous bonding of Cu, SiO2, and polyimide by low temperature vapor-assisted surface activation method

This paper presents high feasibility of homo/heterogeneous bonding of Cu, SiO₂, and polyimide by means of the vapor-assisted surface activation method at 150°C at atmospheric pressure. Such a bonding technology is expected to have high practical value when three-dimensional integration of thin and flexible bumpless structures, which are made of diverse materials including organic substrate, is considered, because the flattened surfaces of metal electrodes and insulation layer should be bonded at the same time for the sake of low process complexity. In order to obtain sufficient binding energy on the surfaces of mixed materials and a good electrical conduction at the metal-metal interface, a bridging layer that is applicative to metal, ionic-bond material, and polymeric material, has to be developed regardless of the difference in bond mechanisms. For Cu and SiO₂, we created the bridging layers based on Cu hydroxide hydrate and silanol group, respectively, by introducing water vapor onto the atomically clean surfaces. With this process, it was proven that considerably low contact resistance was obtained at the Cu-Cu interface with a controlled layer thickness. This technique was considered effective also to the polyimide surface since an ultrathin layer of molecular-bound water would be available once the oxo anion in the main chain is dissociated and substituted with hydroxyl. Therefore, it was necessary to specify: 1) The condition of surface cleaning by the Ar fast atom beam (Ar-FAB); and, 2) The change in chemical binding state of the outmost surface through the adsorption of water molecules. We carried out the X-ray photoelectron spectroscopy (XPS) analyses for the polyimide surface as well as Cu and SiO₂, after their atomically clean surfaces were exposed to nitrogen gas at different absolute humidity. The change in atomic concentration ratio taken from C1s, N1s and O1s spectra of polyimide indicated that the oxo anion w- - as removed preferentially during the beam bombardment rather than the ring opening at the main chain end. The angle-resolved and depth profiling results showed that the formation of hydroxyl, which would induce the adsorption of water molecules in ambient condition, occurred with the thickness increasing concomitantly with the number of water molecules in collision with the clean surface. In the bonding experiments, such hydrophilic surfaces were proven to make tight bridges with the surfaces of Cu and SiO₂. Transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) analyses provided the results that nominally voidless interfaces were obtained in the combinations of polyimide-Cu, polyimide-SiO₂, and Cu-Cu at 150°C through oxygen-rich amorphous bridging layers, when the absolute humidity was limited to 8 g/m³ to control the interfacial layer thickness to be less than 15 nm.