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

Abstract

Polycrystalline copper films have recently received considerable interest as a semiconductor interconnection metallization due to coppers 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 0 of 10-13 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. © 1991 The American Physical Society.