Influence of solute doping on nanotwinning, texture, and properties of Co-sputtered Ag–Cu alloy thin films

Abstract

Silver (Ag) and copper (Cu) are widely used in interconnect applications due to their excellent electrical and thermal conductivities. Both metals can form twin structures, which contribute to enhanced mechanical strength, low resistivity, and high thermal stability. To further improve hardness while maintaining functional performance, this study explores the effects of solute doping in Cu-doped Ag and Ag-doped Cu alloy thin films using a combination of density functional theory (DFT) calculations and co-sputtering deposition. The results reveal that doping reduces the stacking fault energy (SFE) in both systems, thereby influencing twin formation and microstructural evolution. In Cu-doped Ag films, increased doping concentrations significantly reduce SFE, promoting the formation of dense nanotwins and a strong (111) crystallographic texture. In contrast, Ag-doped Cu films exhibit more random microstructures and weaker texturing, primarily due to the limited solubility of Ag in Cu. Solute doping also introduces lattice distortions, leading to residual stress variations, grain refinement, and reduced surface roughness. Notably, Cu-doped Ag films with 4.7 at. % Cu exhibit a surface roughness of ∼5 nm and nearly double the hardness of pure Ag, while maintaining a moderate increase in resistivity (∼50 %). These improvements are attributed to solid solution strengthening, twin formation, and inhibited atomic mobility during growth. The combined enhancements in hardness, texture, and surface smoothness make solute-doped Ag–Cu alloy thin films promising candidates for metal-to-metal direct bonding and hybrid bonding in advanced electronic packaging applications.