Use of a two-frequency ac bridge to measure 1/fnoise in copper films

Polycrystalline copper films have recently received considerable interest as a semiconductor interconnection metallization due to copper’s low resistivity and low susceptibility to electromigration. In this paper the 1/f noise in polycrystalline copper films was measured over a temperature range of 300 to 600 K using a two-frequency ac bridge. Two lock-in amplifiers were used to demodulate the voltage across the bridge, resulting in two estimates of R(t) for the film, each measured at a different modulation frequency. The averaged cross-correlation spectrum of the two R(t) signals is a measure of the resistance fluctuation of the sample only, with no contribution from either the samples’ Johnson noise and/or preamplifier noise; hence, no subtraction is necessary. With this technique we measured the noise magnitude and slope versus temperature and frequency for copper films. The temperature dependence of the noise magnitude showed a weak peak at 490 K. Using the conventional value of ${\mathrm{\tau }}_0$ of ${10}^{\mathrm{-}13}$ s, characteristic of point-defect diffusion, and the Dutta-Horn model for 1/f noise in metal films, we measured the mean activation enthalpy for grain boundary migration to be ∼1.1 eV. This was, subsequently, equated to the expected electromigration activation energy for copper films.