IN SITU SCANNING ELECTRON MICROSCOPE COMPARISON STUDIES ON ELECTROMIGRATION OF CU AND CU(SN) ALLOYS FOR ADVANCED CHIP INTERCONNECTS

A high‐resolution in situ SEM (scanning electron microscope) has been configured for real time comparison studies of the electromigration characteristics of Cu and Cu(Sn) alloys. Drift velocity test structures were fabricated and used to simulate the Cu line/W via structure in the multilevel interconnects. Electromigration comparison testings were carried out over a temperature range of 250 to 450 °C and current density of 5×105 to 2.1×106 A/cm2. Under these test conditions, the measured electromigration activation energy for Cu, Cu(0.5 wt %Sn), Cu(1.0 wt % Sn), and Cu(2 wt % Sn) are 0.73, 0.95, 1.25, and 1.14 eV, respectively. The measured critical length for Cu and Cu(Sn) alloys are ≂2.5 μm at a current density of 2.1×106 A/cm2. The observed average drift velocity of Cu mass transport in Cu(Sn) alloys changes with the depletion of Sn atoms which were also found to move in the direction of electron current. Eventually, the Cu mass transport rate reaches a value comparable to that in pure copper test stri...