Cu Recrystallization and the Formation of Epitaxial and Non-Epitaxial Cu/Cu/Cu Interfaces in Stacked Blind Micro Via Structures

A key factor in achieving high electrical reliability in multilayer HDI PCBs is the integrity of the target pad - electroless Cu - electrolytic Cu interface (Cu/Cu/Cu-interface), especially in stacked blind micro via interconnections (BMVs). In the present paper it is shown, that aside from a low density of nanovoids, the formation of epitaxial Cu/Cu/Cu interfaces is important in satisfying the highest reliability requirements. Using example microstructures that are typical to stacked BMVs, the post-plating formation of epitaxial and non-epitaxial interfaces are described through the concept and mechanisms of “bottom-up” and “top-down” recrystallization. Epitaxial Cu/Cu/Cu interfaces typically form through unhindered “bottom-up” recrystallization across both interfaces and penetrate deep into the electrolytically deposited Cu layer (ECD Cu). The ability of thin electroless Cu layers (< 200 nm) to recrystallize in a “bottom-up” fashion is generally high. However, if there is an appreciable pre-existing contamination of the target pad surface, due to smear residues or a persistent organic layer etc, then such contaminants are determined to be critical factors as they can inhibit this “bottom-up” recrystallization across the target pad - electroless Cu interface. When this occurs, the electroless Cu layer is said to recrystalise in a “top-down” manner, as recrystallisation is determined to initiate on the ECD Cu side, and a non-epitaxial electroless Cu/ECD Cu interface results. Such occurrences can typically be avoided though the use of a horizontal “wet-to-wet” plating sequence, otherwise a more common observation is one of a non-epitaxial electroless Cu/ECD Cu interface. Additionally, it has been found that a high codeposition of the ECD-plating additives within the first
$1\ \mu\mathrm{m}$
of plated ECD Cu, greatly reduces the ability for “bottom-up” recrystallization to occur, and typically gives rise to an alignment of grain boundaries in this region. Failure analysis carried out after reliability testing has shown that such alignment of grain boundaries can be causative for a premature crack formation, and based on these findings, a fatigue-like crack formation mechanism is proposed, and recommendations given to improve the epitaxy of Cu/Cu/Cu interconnections.