Mechanism of Ag sulfurization resistance improvement by alloying solutes in Ag-based alloy films

The Ag sulfurization resistance of the pure Ag film and four Ag-based alloy films was evaluated by the reflectance decay after the Ag sulfurization test. Among all Ag-based alloy films, the In alloying solute in the Ag-In alloy film shows the best resistance to the Ag sulfurization. Using x-ray photoelectron spectroscopy analysis, the chemical state of the alloying solutes in the Ag-based alloy films was examined. It is found that, with being alloyed in the Ag matrix phase, the 3d_5/2 core-level peak positions of the alloying solutes (In, Sn, and Pd) shift toward the lower binding energy region, which is defined as a negative chemical shift. The chemical shifts of In, Sn, and Pd alloying solutes are -0.31 eV, -0.23 eV, and -0.2 eV, respectively. The absolute value of the negative chemical shift represents the tendency of Ag atoms losing valence electrons to the alloying solutes. As the Ag atoms lose valence electrons to the alloying solutes, the Ag atoms have a less tendency to provide the valence electrons to form covalent bonding with S atoms, which suppresses the Ag sulfurization reaction (2Ag + S → Ag₂S). Therefore, the larger the absolute value of the negative chemical shift, the higher is the reflectance decay (lesser Ag sulfurization resistance).