Early detection of the metallization quality using moderately accelerated electromigration stress conditions

In this work we use a wafer-level, High Resolution Resistance Measuring Technique (HRRMT) to detect fabrication faults of Al-Cu interconnections. Experiments have been performed on two distinct sets of metal lines. The first set includes two lots of 4 μm wide lines which, once tested at moderately accelerated stress conditions, gave largely different life times. A microstructural analysis confirmed a major defect of the lot with shorter life time. An accurate examination of the early resistance variations revealed the presence of two distinct and subsequent phases, namely an initial pseudo-parabolic resistance increase followed by a linear resistance drop. Significant differences between the resistance behaviour of the two lots were detected during the first stage, lasting a few hours. Measurable differences could even be detected in the first few minutes. A second group of experiments was launched in order to assess the capability of HRRMT as in-line monitors. Samples from four wafers, one reference wafer and three wafers with intentional process variations, have been tested using our HRRMT at constant temperature and current, simulating an in-line production test. The standard life time of the four wafers have also been collected. Preliminary measurements highlight that a change of life time due to process variation corresponds to changes of the resistance behaviour in the first hours of test. These results pave the way for a new application of high resolution methods to assess the quality of a metallization system in a reasonable amount of time.