Study of electromigration in thin tin film using edge displacement method

Threshold current density and other electromigration parameters of pure Sn films were measured using edge displacement method. Sn film with a thickness of 5000 Å was evaporated on a 1200-Å -thick Ti film on a Si substrate. Electromigration behavior was investigated under the current densities of 2.5× 104 -1.5× 105 A cm2 at room temperature (RT 27-32 °C), 50, 75, and 100 °C. Both needle-type and hillock-type whiskers grew in the anode end when the films were stressed at RT and 50 °C, but only hillock-type whiskers were observed when they were stressed at 75 and 100 °C. The electromigration rate increased linearly with the applied current density for the four stressing temperatures. The threshold current density (Jc) was measured to be 1.93× 104, 9.65× 103, 9.57× 103, and 7.93× 103 A cm2 for RT, 50, 75, and 100 °C, respectively. The measured activation energy was 0.32 eV. In addition, the measured critical length of the Sn film was 18 μm at RT and the products of D Z* were 1.95× 10-10, 4.84× 10-10, 1.27× 10-9, and 1.99× 10-9 cm2 s for RT, 50, 75, and 100 °C, respectively. These results are fundamental to electromigration in Pb-free solders, since most of their matrices consist of almost pure Sn.