In　ULSI　damascene-fabricated　Cu　lines,　Cu　grain　structure　is　affected　by　the　constraint　of　the　trench　sidewalls　and　then　shows　smaller　grain　size　than　that　of　Cu　blanket　films　with　similar　processing　conditions.　The　grain　decreased　from　80-100　nm　to　30-40　nm,　as　the　linewidth　reduced　from　4　μm　to　0.5　μm.　The　electromigration　(EM)　resistance,　the　activation　energy　and　the　EM　induced　voiding　distribution　obtained　in　the　wide　and　the　narrow　lines　were　studied　as　a　function　of　linewidth　and　various　annealing　conditions.　The　interface　diffusion,　especially　the　sidewalls　of　the　trenches　has　become　an　increasingly　important　issue　with　the　line　miniaturization　and　may　be　in　turn　responsible　for　EM　failures　in　deep　submicron　Cu　lines.