Abstract

The effect of the Cu-target pad roughness on the growth mode of electroless Cu from two different Cu baths was investigated. Bath A having a cyanide based-, and bath B a non-cyanide based stabilizer system. Both baths are commonly used in the PCB-industry. In case of bath B two growth modes are observed for an average roughness higher than Ra=300 nm of the target pad. One mode is a copying of the subjacent Cu-substrate morphology, while the other mode forms spherical grains (Cu-nodules) mostly at the exposed sites of the substrate crystals. These Cu-nodules have typically a radius comparable to the plated electroless Cu thickness and contain a high density of nano voids at the base of these features. The related void formation seems relevant to weaken the overall Cu/Cu/Cu-interconnection in the blind micro via. Interestingly, the tendency to form nodules with increasing Cu-base roughness is widely suppressed for the cyanide based bath A, where the deposit is nodule free up to a target pad roughness of Ra=1000 nm.

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