Investigation of copper oxide ring formation during post chemical mechanical polishing cleaning of Cu interconnect

The formation of a ring-shaped residue after the chemical mechanical polishing (CMP) process of a copper (Cu) interconnect was investigated. The chemical composition of the residue was analyzed and found to be copper oxide (CuO). In the present study, the mechanism of CuO formation was investigated using interdisciplinary methods. First, the wafer surface was analyzed by electrical charge mapping and chemical composition analysis. These analyses revealed that the organic residue had not been removed in the region where the ring shaped CuO residue formed. Then, with the support of the analysis data, a hypothesis on residue formation was proposed. This hypothesis includes the nozzle configuration of the cleaning tool used in CMP. Investigations by experimental observation, as well as a simulation of chemical fluid motion, were conducted. It was shown that, owing to the configuration of the cleaning tool, there exists a specific region where the concentration of cleaning chemicals is too low for removing organic residue. Then, cross-contamination occurs without chemical fluid lubrication; in other words, there is direct solid-solid contact between the wafer and cleaning brush. Finally, the organic contamination on the wafer surface forms an electrical circuit, which results in the formation of CuO residue.