Joseph Yahalom

Planarization of Copper Thin Films by Electropolishing in Phosphoric Acid for ULSI Applications

Electropolishing of thin films poses additional challenges in comparison to hulk material polishing. The existence of a resistive anode/electrolyte boundary layer is crucial for achieving polishing. A finite amount of copper is required to he anodically dissolved to create the boundary layer of the appropriate thickness for effective electropolishing of a given hillock. This is a significant consideration in the application of electropolishing for planarization of thin films where the disparity in the topography is significant in proportion to the thickness of the film. Here electropolishing is shown to effectively remove the bulk of electrodeposited copper layers used in ultralarge scale integration (ULSI) metallization schemes without application of mechanical force and to planarize local topography. Efficient polishing can be achieved under galvanostatic conditions (i.e., constant current between the wafer and a counter electrode). Anodic transient studies indicated that the mechanism of formation of the boundary layer (in mass-transport controlled regime) is determined by the diffusive transport of an acceptor species to the anode/electrolyte Interface. Effects of changes in current density and rotational speed of wafer on the extent of planarization have heen determined. Under optimal galvanostatic and hydrodynamic conditions, the disparity in the topography over wide trenches adjacent to dense features decreased by 60%.

Electrodeposition of copper–tin alloy thin films for microelectronic applications

The continuing shrink in device size has generated great interest to create interconnects with low resistivity and superior resistance to electromigration (EM) and stress migration (SM) in comparison to the existing Al or Al-alloy interconnections. Copper has become the metal of choice to meet the needs of present and future generation devices. In order to improve the intrinsic resistance of copper to EM/SM induced failure, alloying elements can be added into copper metallurgy. In the present investigation, we discuss a method to co-deposit an alloy of copper and tin in sub-microscopic features with high aspect ratio using a sulfate bath. It is observed that a small amount tin begins to co-deposit at potentials smaller than the equilibrium reduction potential. Under activation control regime, the composition is not affected by current density. The results of this study conclude that substantial tin deposition occurs upon onset of mass-transport limitation. It is found that a finite amount of time is required before electrolysis is controlled by mass-transfer. The transition time and hence, the composition of the plated film is affected by the hydrodynamic conditions, current density, and electrolyte composition. These factors must be taken into account in order to control the composition profile of tin in vias and trenches.