In situ scanning electron microscope comparison studies on electromigration of Cu and Cu(Sn) alloys for advanced chip interconnects

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Abstract
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 stripes. The measured resistivity values of the Cu(0.5 wt % Sn) and Cu(1 wt % Sn) using Van der Pauw test structures are 2.4 and 2.9 μΩ cm, respectively. In comparison with Cu and Al(Cu) device interconnects, Cu(Sn) alloys exhibits higher electromigration activation energy, good resistance to hillocks and void formation and comparable resistivity. These characteristics clearly indicate the fact that Cu(Sn) alloys are potentially good candidate for advanced device interconnect applications where high‐current density and good electromigration resistance are required.