The Effect of Ultrasonic Agitation on the Seedless Growth of Copper on Ruthenium–Tungsten Thin Films ^†

Ruthenium (Ru) has attracted considerable attention as a candidate to replace tantalum nitride (TaN) as a diffusion barrier layer for copper (Cu) interconnect metallisation. Its high melting point, chemical inertness, and strong adhesion to Cu make Ru an interesting material for interconnect linings. However, its effectiveness as a diffusion barrier falls short of that of TaN. The addition of tungsten (W) improves the diffusion barrier properties of Ru, but appears to weaken the adhesion strength between the barrier and Cu, as well as the direct (seedless) electroplating behaviour. Although Cu can be directly electroplated onto near equimolar Ru-W thin films, a complete substrate coverage has not been obtained. To fully understand the Cu electro-crystallisation on Ru-W, it is essential to develop methods of fabricating thin, continuous and adherent films for advanced interconnect metallisation. Focusing on achieving full substrate coverage, this work studies the effect of ultrasonic agitation on the direct electro-crystallisation of Cu on Ru-W, namely, the impact on substrate coverage. The film structure, morphology and chemical composition were evaluated by digital microscopy, scanning and transmission electron microscopies, as well as X-ray diffraction. The results confirm that when no electrolyte agitation is applied, the substrate coverage is incomplete with exposed substrate around the larger Cu particles, regardless of the current density. However, when ultrasonic agitation is applied, complete substrate coverage is achieved for intermediate current densities. A mechanism involving Cu particle detachment and hydrogen evolution is proposed to mediate the process. On the other hand, the use of ultrasonic agitation may impair adhesion strength.