Overlay performance on tungsten CMP layers using the ATHENA alignment system

ATHENA is the new alignment system designed to minimize the alignment offset due to mark-deformation as induced by a WCMP process. We report the results of its evaluation at STMicroelectronics for a 0.25 mu m-device which employs six metal-interconnect layers (metal layer thicknesses ranging from 4000 A to 8000A) obtained by a W-CMP process. ATHENA's sensitivity to process variations on resist coating, WCMP pad-aging, metal-deposition effect, oxide thickness variation, and differences in the alignment scheme has been investigated. Results of the characterization were implemented on volume production to verify the overlay-performances under presence of natural process-fluctuations of a W-CMP backend-process. Final results show the robustness of ATHENA system: The process-induced contribution to the overlay-error is below 15 nm. A stable performance is obtained with a maximum overlay-error below 60 nm over a period of 4 months under manufacturing conditions.