Formula omitted] noise and electromigration in multilayered via structures

Abstract

Al-Cu-Si TiW multilayered via electromigration behavior was researched using the low frequency noise method. Noise measurements were performed with ambient temperature ranging from 100 to 200°C and applied current density from 5.3 × 10 5 to 3.3 × 10 6 A cm −2, corresponding to sample temperatures up to 276°C. The noise spectral density was found to be in the form of 1 f γ with 0.95≤ γ≤1.15, which corresponds to 1 f noise. The noise magnitude was independent of the electron flow direction at relatively low current densities and temperatures. However, for J ≥ 2.6 × 10 6 A cm −2 and T ambient ≥ 180°C ( T sample≥228°C), the noise for electron flow out of the via was higher than that for electron flow into the via. At low current densities, the noise spectrum showed a quadratic dependence on current for both electron flow directions. However, at high current densities, the current exponent depended on the direction of electron flow. The average activation energy, E a, was found to be 0.11 eV when noise measurements were done on the same via sample for both current directions. When different samples were used for different current directions, a 0.23 eV activation energy value was observed. We interpret the first case as a very low-frequency pseudo-a.c. measurement, and the second case as a true d.c. measurement. In both cases, the activation energy did not depend on the direction of the electron flow. These activation energy values are small compared to the results obtained by the median time-to-failure method. The results are interpreted in terms of the electron mobility fluctuations due to fluctuations in the number of migrating vacancies.