Fundamental electrochemical aspects of chemical mechanical planarization of aluminum thin films

Abstract

The chemical mechanical planarization (CMP) process is critical in fabricating ultra large scale integrated (ULSI) circuit devices in semiconductor industry. In a typical aluminum damascene interconnect scheme, Al is usually blanket deposited over a liner layer to fill contact holes and vias. After deposition, the excess of Al is removed by CMP, leaving Al only in the holes and vias to form interconnects. Since the slurries used for aluminum CMP typically contain an oxidant and other chemical additives, the electrochemical behavior of Al and the liner may be expected to affect the polishing rates. In addition, when the excess of Al is removed, a surface transition from Al to liner occurs. Since Al and the liner may exhibit different electrochemical behaviors in the slurry, galvanic coupling between Al and the underlayer is a possibility. Such a coupling may lead to localized corrosion or rate control problems. The objective of this research was to characterize the fundamental electrochemical behavior of thin aluminum-0.5%copper, titanium and aluminum/titanium stack films before, during and after abrasion in a commercially available alumina based slurry containing iodate as an oxidant. A special apparatus in which electrochemical tests can be carried out during polishing was fabricated and used for this research. It was found that the electrochemical corrosion rates during abrasion were much smaller than the actual polishing rates obtained with the simulated CMP apparatus, indicating that the mechanism of Al removal by the iodate based slurry may not be dominated by electrochemical factors. A sharp rise in corrosion potential (Ecorr) during the transition from Al to Ti film was measured during polishing of the Al/Ti film stack. This potential change during transition was of the order of 1V on the Al film deposited at room temperature. The transition was much sharper with the low-temperature (25°C) Al film than the high-temperature (475°C) Al thin film. The slower transition in OCP in high-temperature films is most likely due to a Ti-Al intermetallic compound formed at the Al/Ti interface. The galvanic corrosion between Al and Ti during polishing and Al post-polishing corrosion issues were also investigated. It was found that the galvanic corrosion rate between Al and Ti is 6*10⁻⁴ A/cm² and the corrosion potential is -0.24 V. Also, the corrosion current density for Al after abrasion and immersion in de-ionized water is lower than that in the slurry. In addition, the post polishing corrosion of Al in after abrasion in the iodate based alumina slurry was also investigated. It was found that the corrosion of Al in DI water after abrasion was insignificant.