Chemical reactions of silver and indium at 180°C annealing

Since Ag and In react to form compound at room temperature was investigated, but the chemical reactions during bonding are not well understood. The Ag-In phase diagram shows two dominating intermetallic compounds (IMC), AgIn₂ and Ag₂In. Therefore, based on the following systematic experiments to study the chemical reactions of AgIn system and compound formation. Copper (Cu) substrates were electroplated with 40 μm thick Ag layer, followed by indium layer of 1, 5, 10, and 15 μm, respectively. Thick Ag layer is chosen to prevent In reaction with underlying Cu substrate. The samples were annealed at 180°C in 100 millitorr for 5 minutes to emulate the bonding conditions. The microstructure and the composition on the surface of the samples were evaluated using scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX). X-ray diffraction (XRD) was also employed to identify the IMC on the surface. The samples were then cut in cross section, polished, and studied also by SEM and EDX. The results show that, for the sample with 1μm thick indium, the In layer converts to Ag₂In entirely after annealing. For sample with In layer thickness of 5 μm, AgIn₂, Ag₂In, and silver solid solution (Ag) all exist after annealing. No indium was identified. For sample having 10, and 15μm thick indium, In was detected on the surface, but was not found on the cross section. The polishing process probably bevels the thin In layer on the surface, making it invisible. We next studied what would happen if the Ag was annealed first before plating In. After plating the Ag layer, samples were annealed at 450°C for 3 hours to grow Ag grains. This was followed by plating 10μm In and annealing at 180°C. The result shows AgIn₂, Ag₂In as well as In. Significant In remains on the surface. The thicknesses of AgIn₂ and Ag_{2- /sub>In layers are thinner than the sample whose Ag is not annealed.}