Modified diffusion bonding for both Cu and SiO2 at 150 °C in ambient air

This paper presents the vapor-assisted bonding method that is applicable to Cu and SiO₂ at 150°C at atmospheric pressure. Considering a future three-dimensional integration of highly flattened substrates, such a bonding technology is expected to be effective in generating high binding energy on both the metal electrodes and the insulation layer at the same time. To interconnect the materials with different bond mechanisms (metallic-bond and ionic-bond) by single process in ambient air-like condition, the structure of adsorbate on the surface had to be well controlled. In this study, it was proven that the water molecules in collision with the atomically clean surfaces of Cu and SiO₂ provoked the formation of bridging layers. Therefore, oxygen gas with controlled humidity was used as the process atmosphere. In order to ensure a good bond quality, first we had to understand the influence of water molecules on the growth behavior of adsorbate layers created after the surface cleaning by Ar fast atom beam. The X-ray photoelectron spectroscopy (XPS) observations were carried out on the samples exposed to nitrogen gas with controlled humidity. Analysis results identified that the bridging layers for Cu and SiO₂ included oxide, hydroxide hydrate, and silanol groups, respectively. These layers were considered to contain extra intermolecular bound water, which inferred the additional adsorption of water molecules in ambient air-like atmosphere. In addition, the angle resolved testing showed that thickness of bridging layers increased concomitantly with absolute humidity. Upon heating at the temperatures higher than 100°C, high die-shear strength was obtained at the interfaces of Cu-Cu, Cu-SiO₂, and SiO₂-SiO₂ due to the dissociation of water molecules. The transmission electron microscopy (TEM) observations showed that, in particular at the Cu-Cu interfaces, the amorphous interfacial layers were created in increasing thickness with absolute humidity. In such amorphous layers, recrystallization of CuO grains was observed during the storage in ambient air at room temperature. Since thin CuO interfacial layers had turned out to be tolerant of bondability in previous studies, high feasibility is expected with this vapor-assisted bonding method when a good control of layer thickness is provided.