Decreasing step coverage of self-ionized plasma sputtered copper seed layer with target lifetime

Self-ionized plasma (SIP) sputtering has become a widely accepted solution for the copper barrier and seed deposition in narrow trenches typical of the backend of line structures starting from the 90 nm technology node and beyond. Process performances of a thin (70 nm) copper seed layer sputtered by SIP were studied throughout the target life (75 kW h) to highlight possible drifts in thickness, resistance and step coverage on 100 nm trenches with aspect ratio 2:1. While film thickness can be kept stable by a predictable correction of deposition time, a remarkable step coverage variation across wafer radius (∼15%) and a not recoverable worsening (up to ∼8%) of the same parameter with target life were detected by TEM; the latter anomaly associated with the reduction of plasma energy with target erosion. Considering the advantage in decreasing the copper seed thickness without compromising the film continuity to facilitate the electroplating filling in very narrow structures, the obtained results help assuring adequate film coverage at any wafer position and target lifetime.