Microstructure analysis of sputtered copper thin film in packaging substrate

The feature size of transistors is continuously decreasing while the wafer size is increasing, which presents new challenges to the interconnection between the chips and substrates. Currently, substrate has become the most influential factor of the cost in electronic packaging. In packaging substrate, the most commonly used conducting metal is copper, which is generally fabricated by electroplating process. Before electroplating, it typically requires a seed film of sputtered copper film formed as the electric conductor of the subsequent process. Because sputtered copper films are generally thinner than 1um, their polycrystalline microstructures are significantly different from that of the electro-plated copper on them. Therefore, it is worth further exploring whether the sputtered copper thin film of ultra-fine grains has expected microstructure and properties, and how it changes and affects the reliability of substrate in subsequent process. In this paper, Focused ion beam (FIB) was adopted to observe the microstructure of the sputtered thin copper film, then we used Image J (a public domain, Java-based image processing program) to measure and statistically analyze the grain size. Finally, we achieved a simple and reliable method to characterize and quantitatively analyze the microstructure of sputtered copper thin film.