Using the temperature coefficient of the resistance (TCR) as early reliability indicator for stressvoiding risks in Cu interconnects

Comprehensive investigations were carried out to study the correlation between the temperature coefficient of resistance (TCR) and both the microstructure and the stressvoiding behavior of copper dual damascene (DD) interconnects. TCR values have been investigated with regard to median grain size and crystal lattice defects. For narrow submicron lines the TCR was found to decrease exponentially with line width as the grains become smaller. For wide lines the value decreases with the plating thickness since the median grain size in thinner films becomes smaller as well. A change of grain size was observed in samples subjected to different post-plating anneal temperatures. Samples treated at lower temperature contain smaller grains resulting in reduced TCR values and are affected by stress-induced voiding as a consequence of vacancy generation due to recrystallization and grain growth. In contrast, no stressvoiding was observed in the case of high temperature anneals, where the samples show considerable larger grains and thus higher TCR values. A clear correlation was found between lattice defects, the measured TCR and stressvoiding behavior. Samples with a large number of crystal defects are affected by stressvoids due to the recovery of the defects. They show significantly smaller TCR values compared to samples with an intact crystal lattice which are not susceptible to stress-induced voiding. The results demonstrate the capability of TCR measurements to reveal microstructural differences such as median grain size or the density of crystal defects, as the result of different interconnect geometries or process-related influences. The measurement of the temperature coefficient of resistance is not only a powerful method to characterize the microstructural properties of copper interconnects but also shows potential as an early indicator of stressvoiding risks, e.g. during in-line monitoring.