Fabrication of Smooth SAC305 Thin Films via Magnetron Sputtering and Evaluations of Microstructure, Creep, and Electrical Resistivity

Abstract

SAC305 (96.5%Sn-3%Ag-0.5%Cu) is the leading alternative to the traditional Sn-Pb solder eutectic alloy owing to its low melting temperature, better compatibility with other components, and excellent mechanical/structural properties. In the realm of modern electronics, where devices are increasingly miniaturized, the design and characterization of thin solder joints become paramount. The orientation and size of the grains within the solder can influence its ability to withstand mechanical stresses. However, research on SAC thin films remains sparse, and these films present unique challenges and characteristics compared to their bulk counterparts, influenced by factors like interfaces, stresses, thickness, microstructure, and the nature of the underlying substrates. This study investigated a method to improve the surface morphology of SAC305 thin films by depositing on various substrates, varying annealing processes modulating the substrate, and various magnetron sputtering parameters to produce smooth and continuous SAC305 thin film. A comprehensive annealing treatment was carried out in a vacuum furnace to investigate the surface morphology of the films. Various deposition parameters were tested using DC and RF power sources at different pressures and powers. This study investigates the surface morphology of SAC305 thin films deposited on four substrates: Silicon, Gallium Arsenide, Sapphire, and Glass. The most optimal surface morphology, with an average grain size of ~1 μm and a thickness of ~2 μm, was accomplished at a pressure of 2.4 mTorr and 200-watt power deposited on Si substrate. After polishing, a uniform thickness of 1500 nm with a mean roughness (Ra) of 14.9 nm was obtained. The samples contained polycrystalline β-Sn grains at (200) diffraction planes with a preferred orientation 2θ of 30.70°. However, the XRD pattern did not indicate any Ag peaks; weak peaks of Ag3Sn were observed at 2θ of 37.60° and 39.59°, corresponding to diffraction planes (020) and (211), respectively. The electrical resistivity of the SAC305 thin film deposited on the SiO2 substrate and of the bulk SAC305 samples were measured as 19.6 μΩ·cm and 13.7 μΩ·cm, respectively. The reported average (m) value ≈ 0.10 ± 0.02 of SAC305 thin films and bulk sample agrees with the previous literature data.