Research focusing on two topics central to metallization of semiconductor interconnects by copper electroplating is reported. A systematic experimental study of copper electropolishing in phosphoric acid, focusing on current stepping, yielded a model for the process. The model invokes two regimes. The first regime involves the build-up of the copper ion concentration at the dissolving anode. The second regime is characterized by the formation of an insoluble film on the anode when the concentration of the copper phosphate species reaches the solubility limit. Subsequent transport of dissolving copper ions through the film yields the observed high potential drop. This model correlates well with experimental data and explains the various heretofore unexplained phenomena observed in copper electropolishing.
The second focus of the research provides significant improvement to bottom-up fill of copper features by identifying a new class of polyether additives (including polyoxyethylated β-naphthol and polyoxyethylene lauryl ether) which provide stronger inhibition than the currently used poly(ethylene glycol) (PEG). These novel polyether inhibitors extend the present process capabilities to features smaller than the present limit of about 60 nm. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and quartz crystal microbalance (QCM) studies provide insight into the polyether adsorption mechanism.