Electromigration damage in aluminum-copper films

Abstract

Electromigration (EM) behaviors of pristine Ge2Sb2Te5 (GST), nitrogen-doped GST (N-GST) and cerium-doped GST (Ce-GST) thin-film strips under DC bias are presented. The mean-time-to-failure (MTTF) analysis based on the Black equation found that the EM failure times at room temperature are 1.2 × 104, 40 and 9.2 × 102 years and the activation energies (Ea) of EM process are 1.07, 0.57 and 0.68 eV for GST, N-GST and Ce-GST, respectively. Moreover, the calibration of the current density exponent, n, of Black’s equation found n values are close to 2 for all samples, implying the dominance of grain boundary diffusion during the mass transport of EM process. For doped GSTs, the inferior EM failure lifespans and smaller Ea values were ascribed to the grain refinement effect which increases the number of grain boundaries in such samples. It consequently promoted the short-circuit diffusion and accelerated the EM failure in doped GSTs. The Blech-type tests on GSTs found that the threshold product, i.e., the product of current density and sample length ((j ⋅ L)th), is 200 A/cm for GST, 50 A/cm for N-GST and 66.67 A/cm for Ce-GST. Moreover, the product of diffusivity and effective charge number (i.e., DZ*) for GST, N-GST and Ce-GST was 2.0 × 10−7, 4.5 × 10−6 and 3.8 × 10−6 cm2/sec, respectively. Analytical results illustrated that the electrostatic force effect dominates the EM failure in samples with short strip lengths while the electron-wind force effect dominates the EM failure in samples with long strip lengths. Doping might alleviate the mass segregation in GST; however, its effect was moderate.