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

The electrical reliability of multilayer High Density Interconnection Printed Circuit boards (HDI PCBs) is mainly affected by the thermo-mechanical stability of stacked micro via interconnections. In this regard, a critical failure mode is the stress related crack between the electrolytically filled via and the target pad, commonly known as target pad separation. The junction includes two Cu-Cu-interfaces, one between the target Cu pad and the thin electroless Cu layer and the second between electroless Cu and electrolytic Cu. To ensure a clean and defect-free target pad/electroless Cu interface, elimination of organic residues (after desmear) and melted Cu (from laser drilling) is a prerequisite. Therefore strong etch cleaners, mostly based on sodium persulfate, are used to remove the top $1-2\ \mu \mathrm{m}$ Cu from the target-pad surface. According to the applied type of etch cleaner and etching intensity, a specific target pad roughness remains as the plating base for the subsequent electroless Cu process. In this context, we investigated the impact of the Cu-base roughness on the growth mode of two different electroless Cu baths. Bath A has a cyanide based and bath B a non-cyanide based stabilizer system, both baths are commonly used in the PCB industry. We found that for a Cu-base roughness at about $\mathrm{R}_{\mathrm{a}}=300\ \text{nm}$ , two growth modes are observed for electroless Cu bath B. One mode is copying 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 typically have a radius comparable to, or even higher than the plated electroless Cu thickness and are easily detectable via standard SEM. FIB/SEM-microsections through these Cu-nodules have shown a high density of nano-voids at the base of these features. The voids have typical diameters of some ten to hundred nanometers. A further increase of the Cu-base roughness to $\mathrm{R}_{\mathrm{a}}=1000\ \text{nm}$ results in a substantially higher nodule density for electroless Cu bath B. 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 sporadically nodules are not formed until a roughness of $\mathrm{R}_{\mathrm{a}}=1000\ \text{nm}$ is achieved. To understand the different impacts of the stabilizer systems A and B, the nucleation of nodules during the first stage of electroless Cu growth was investigated: In the first seconds of plating, the agglomeration of nano-sized Cu crystals at the exposed sites of the substrate Cu-grains (peaks and edges of crystals) is observable for bath B, whereas for bath A these surface sites seem efficiently poisoned by the stabilizer system and no Cu-agglomeration takes place. These nano-porous Cu-agglomerations are the nucleation sites of nodules and cause the high void density. From the results of this study we conclude, that the target pad roughness, predefined by the applied etch-cleaning process, has a crucial impact on the void density of the subsequently deposited electroless Cu layer. It is recommended to adjust the target pad roughness, e.g. by an appropriate pretreatment, in a range, where a nodule free deposition of electroless Cu is assured. On the other hand electroless Cu baths with cyanide based stabilizer systems seem to provide a broader process window, compared to the tested non-cyanide based bath type. In the latter case a precise adjustment of single stabilizer components is mandatory to ensure nodule-free deposits also for rough target pad surfaces.