Metallization of Advanced Interconnects by Cu Electroplating in Alkaline Media

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Copper electroplating in acid media was developed for interconnect metallization since the late 1990s, when the semiconductor industry replaced aluminum with copper for higher conductivity. To connect transistors with an ever-growing density, more Cu layers are added over time, while feature size shrinks from microscale to tens of nanometers. The downscaling of feature size has posed challenge in the extension of Cu plating to each new node since plating processes require a continuous Cu seed on features with an opening large enough for void-free fill. For years, the semiconductor industry has found ways to improve the Cu seed. First, PVD Cu deposition followed by sputtering helped direct Cu atoms to the sidewalls of features and keep them open. Later, a metal layer was added between barrier and Cu seed to improve the wetting of Cu; this so-called liner layer has evolved from Ta to Co and recently RuCo. Furthermore, barriers prepared by atomic layer deposition were another innovation to help extend the Cu plating by leaving more space for seed and plating. Equally important was the continuous improvement in plating chemistry, where a stronger suppressor could promote nucleation, protect against seed dissolution, and/or enhance bottom-up growth. All this has made Cu plating in acid work seamless over the years. Cu plating in acid electrolyte has challenges when feature size decreases below critical dimensions (e.g., 25 nm) and Cu seed becomes thin on new barrier liner schemes (Ta-Ru-Co). Before wafers can be transported to a plating tool, oxidation can impact the thin Cu seed in some spots and possibly reach the barrier. Under this circumstance, the process window for void-free fill by Cu plating is rather tight, since Cu and Co oxides dissolve instantly upon contact with the acid. A loss of Co liner in some schemes can lead to poor sidewall integrity and void formation. We have demonstrated that Cu electroplating in alkaline media can address challenges facing the thin Cu seed by reducing native Cu oxides back to metal, which minimizes the seed loss. This talk will cover how ligands stabilize Cu ions in alkaline Cu electrolytes to enable the reduction of Cu oxides to preserve sidewall integrity. More importantly, we will report on how bottom-up growth in alkaline electrolytes might be achieved. Fig. 1 shows a cyclic voltammogram (CV) from an alkaline bath. The hysteresis in CV suggests that plating in such a bath can give rise to on and off states on the electrode surface. To enhance the bottom-up growth, an additive can be added to the bath. The injection study in Fig. 2 shows that a galvanostatic curve undergoes depolarization when such an additive is added to the baseline bath, which is indicative of the displacement of adsorbed molecules by the additive on a growing Cu surface. In such an alkaline bath with the additive, a bottom-up and void-free fill has been achieved on narrow features, as shown in Fig. 3 . Overall, Cu plating in alkaline for interconnect metallization is a field where literature is still limited. By reporting the advancement in this area, we hope to stimulate further work from the community to shed more light on alkaline plating. Figure 1

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