On the use of highly accelerated electromigration tests (SWEAT) on copper

Electromigration (EM) tests on copper metallization have been performed in a wide range of stress conditions using both, highly accelerated wafer level (WL) test and standard iso-current package level (PL) tests in order to determine the kinetics and the acceleration limits of highly accelerated tests. Similar activation energies and current density exponents were found for via-line structures tested with both stress methods, implying that the same mass transport mechanisms lead to the EM fail. Using the experimentally determined activation energy and current density exponent to extrapolate the data of moderate PL and highly accelerated WL tests to operation conditions similar life times are obtained. As a consequence, fast WL tests can not only be used for monitoring but also to quantify the reliability of the metallization. A detection of an early fail mode (as might occur for some via-line configurations in copper interconnects) is not yet proven with highly accelerated tests, since thermal inhomogeneities caused by higher stress conditions can hide certain failure modes.