A COMPARATIVE STUDY INTO SURFACE ELECTROMIGRATION OF SPUTTERED COPPER PATTERNED USING MILLING OR CMP

摘要

Electromigration characteristics of sputtered copper patterned both by milling and, more recently, by chemical mechanical polishing (CMP) have been studied. Comparison was also made with the electromigration characteristics of sputtered aluminium. Unpassivated copper lines, stressed at 300℃ and 2 MA/cm~2 in a N_2 ambient, exhibited a mean time to failure (MTF) of 91 hours, compared with only 22 hours for aluminium lines of similar dimensions. However, copper samples with a slightly higher oxygen content had a MTF of 46 hours, whilst deliberate introduction of oxygen during stress testing led to failure within a few hours. It is believed that oxidation of the copper retards surface diffusion, thereby reducing its capacity to continuously resupply copper to areas depleted by electromigration. A passivating tantalum overlayer further increased the MTF to greater than 100 hours. To date, the best MTF obtained for copper lines patterned by CMP is only approximately 50 hours. This is attributed to a reduction in surface diffusion caused by scratches from the alumina abrasive particles. Benzotriazole (BTA), used as a passivating agent in the polish process, is also effective as passivation against copper oxidation during electromigration testing.