Enhanced Bonding by Applied Current in Cu-to-Cu Joints Fabricated Using 20 mu m Cu Microbumps

Abstract

The effect of applied current in enhancing bonding was studied in Cu-to-Cu direct bonding using Cu microbumps. A daisy-chain structure of electroplated Cu microbumps (20 mu m x 20 mu m) was fabricated on Si wafer. Cu-to-Cu bonding was performed in ambient atmosphere at 200-300 degrees C for 10 min under 260 MPa, during which direct current of 0-10 A (2.5 x 10(6) A/cm(2)) was applied. With increasing applied current, the contact resistance decreased and the shear strength in the Cu-to-Cu joints increased. The enhanced bonding imparted by the application of current was ascribed to Joule heating and electromigration effects. Subsequently, the joint temperature was calibrated to isolate the electromigration effects for study. In Cu-to-Cu joints joined at the same adjusted temperature, increasing the current caused unbonded regions to decrease and regions of cohesive failure to increase. The enhanced diffusion across the Cu/Cu interfaces under the applied current was the main mechanism whereby the quality of the Cu-to-Cu joints was improved.