Pinhole formation and its mitigation for Cu electrodeposition on the Ajinomoto build-up film (ABF) with different surface roughnesses

Abstract

In this study, the roughness effects of an Ajinomoto build-up film (ABF) on the crystallographic microstructure and corrosion characteristics of electroplated Cu were systematically investigated via scanning electron microscopy (SEM) in conjunction with electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffraction (XRD), and focused ion beam (FIB) methods. The strong dependences of the Cu crystallographic microstructure and pinhole formation on the ABF roughness were revealed in these investigations. We found that smaller Cu grains with fewer twin boundaries (TBs) and more low-angle grain boundaries (LAGBs) were easily created in the Cu deposits over a low-roughness ABF substrate, which is closely related to pinhole formation because of poor corrosion resistance resulting from numerous grain boundaries (fined grains) and inferior grain boundary (GB) characteristics. Here, we proposed a method that enables the generation of larger Cu grains with more TBs through an appropriate annealing treatment combined with adjusting the plating current density, thereby preventing undesired pinhole formation over a low-roughness ABF substrate. These findings advanced our understanding of the effects of substrate roughness on the Cu microstructure and pinhole formation, which is very beneficial for the application of low-profile substrates for 5G/6G high-frequency signal transmission.