When a metal conductor is driven by an AC current of frequency ω, its resistance will vary at a frequency 2ω, and a third harmonic voltage will be generated. If the conductor is also under a DC bias, a second harmonic voltage will arise as well. The magnitude of these harmonic voltages increases with damage because of nonlinear increases in the resistance caused by localized Joule heating. We use the harmonic technique to evaluate the damage in metal lines. The correlation between deliberately introduced defects of different sizes and the amplitude of the harmonic voltages has been studied experimentally. The harmonic technique shows higher damage sensitivity than the commonly used resistance method. Additionally, it has a better rejection to noise and resistance drifting.
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