Capping layer or layers on a surface of a copper interconnect wiring layer for use in interconnect structures for integrated circuits and methods and apparatus for forming improved integration interconnection structures for integrated circuits by the application of gas-cluster ion-beam processing. Reduced copper diffusion and improved electromigration lifetime result and the use of selective metal capping techniques and their attendant yield problems are avoided. Various cluster tool configurations including gas-cluster ion-beam processing modules for copper capping, cleaning, etching, and film formation steps are disclosed.