The Effect of Cu Target Pad Roughness and Solution Flow on the Growth Mode and Void Formation in Electroless Cu Films

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Abstract The effect of the Cu target pad roughness on the growth mode of electroless Cu from two different Cu baths was investigated, with bath A having a cyanide based, and bath B, a non-cyanide-based stabilizer system. Both baths are commonly used within the PCB industry. In the case of bath B, for an average target pad roughness higher than Ra = 300 nm, two growth modes are observed. The first mode is a copying of the subjacent Cu substrate morphology, whereas the second forms spherical grains (Cu-nodules) predominantly at the exposed sites of the substrate crystals. These Cu nodules typically have a radius comparable to that of the plated electroless Cu thickness and contain a high density of nanovoids toward their base. The related void formation seems relevant to weaken the overall Cu/Cu/Cu interconnection in the blind microvia. 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 approximately Ra=1,000 nm. When solution delivery and exchange were investigated, it is apparent that a low exchange rate has a negative impact on the electroless Cu deposition, and results with undesirable nodules and voids, even if the surface roughness values would suggest otherwise, could be expected.