Surface morphology and oxide phases of Cu pads in vias under thermal annealing in air

Thermal annealing of Cu, including oxidation and grain growth, plays a crucial role in the reliability performance of microelectronic devices. We present an experimental analysis of Cu oxide growth in dielectric-confined Cu pads in vias, essential for the development of a reliable Cu/oxide hybrid bonding process. Thermal oxidation of Cu pads grown by electroplating and processed by dual damascene was performed at a fixed annealing temperature (Ta) from 200 to 350 °C for 30 min under ambient air. As the temperature increases, the Cu pad expands and bulges above the dielectric surface. The surface morphology and roughness of Cu pad were examined by atomic force microscopy. The evolution of Cu oxide phases versus Ta was examined by Raman spectroscopy. At low Ta, the Cu2O phase dominates. As Ta increases, the CuO phase appears at 250 °C and coexists with the Cu2O phase. The energy dispersive x-ray spectroscopy shows that Cu oxide phases exist in the bulged-out Cu pad and can be detached from the unoxidized part of the Cu pad beneath the dielectric surface. The focused ion beam cross sections of Cu pads show a boundary region with gaps and void between oxidized Cu and unoxidized Cu pads. This understanding is relevant for the reliability of hybrid bonding in microelectronic Cu interconnects.