Effect of Ni addition and bath temperature on electroless Cu microstructure in microvia preparation for HDI substrate

In this work, we investigated the microstructure evolution of electroless copper (Cu) deposition under different nickel (Ni) additions and reaction temperatures. It was found that the grain structure of electroless Cu is significantly influenced by the addition of Ni due to its inhibitory effect on Cu self-diffusion, while bath temperature had no impact on the electroless Cu grain growth. Additionally. nanovoids were observed in the electroless layer, primarily at the interface between the Cu trace and electroless layer, which is caused by the attachment of hydrogen bubbles during the electroless Cu reaction. These nanovoids do not affect the epitaxial grain growth in the microvia structure as observed through transmission electron microscopy (TEM). The nanovoids in the electroless Cu under different deposition conditions were also investigated through TEM images with the assistance of ImageJ. We found that the dimension of nanovoids initially increases with the reaction rate, as a result of the higher deposition rate introducing large hydrogen bubbles. However, as the reaction rate further increases, the size of the nanovoids stabilizes because larger hydrogen bubbles more easily escape the surface. This work may offer insights into potential approaches to enhance the quality of electroless Cu deposition for microvia preparation.