Different interfacial structures of Cu/In obtained by surface activated bonding (SAB) in vacuum and vapor-assisted vacuum ultraviolet (V-VUV) at atmospheric pressure

Abstract

A difference in interfacial structure derived from a difference in binding mechanisms, was assessed to obtain a robust Cu/In interface with low-temperature fluxless solder bonding. As examples of typical bonding methods, Ar fast atom beam (Ar-FAB) surface activation in high vacuum and vapor-assisted vacuum ultraviolet irradiation (V-VUV) method were adopted for Cu and In. The X-ray photoelectron spectroscopy (XPS) results show that atomically clean surfaces were created at room temperature in high vacuum using the Ar-FAB method, whereas ultrathin metal hydrate bridge layers were created in ambient in V-VUV method. Both methods created tight voidless bond interfaces: at room temperature in Ar-FAB and at 50 °C in V-VUV. However, transmission electron microscopy (TEM) observations revealed an oxide-based 5-nm-thick interfacial layer obtained via the V-VUV method, which was attributable to dehydration of the hydrate bridge layer. Moreover, this interfacial layer was regarded as effective to recess the heavy interfacial reaction between Cu and In, causing formation of less Cu-consuming inter-metallic compound (IMC).

Keywords

• Bonding
• Low temperature solder
• Surface analysis
• 3D packaging