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

Abstract

Electropolishing of thin films poses additional challenges in comparison to bulk material polishing. The existence of a resistive anode/electrolyte boundary layer is crucial for achieving polishing. A finite amount of copper is required to be 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 been determined. Under optimal galvanostatic and hydrodynamic conditions, the disparity in the topography over wide trenches adjacent to dense features decreased by 60%. © 2002 The Electrochemical Society. All rights reserved.