A PRECISE METHOD TO DETERMINE THE STRESS CURRENT TO BE APPLIED TO ELECTROMIGRATION TEST STRUCTURES

The density of the stress current flowing through metal stripes for el ectromigration (EM) median time to failure (MTF) tests should vary as little as possible among all the structures under test. This paper pro poses an alternative method to the procedure described in ASTM F 1260, to determine the stress currents for these tests, resulting in smalle r stress current density differences among different test structures. This method uses the average resistivity of the metal film to calculat e the stress current for each individual test structure. In the standa rd method (ASTM F 1260), the average film thickness is used to calcula te the cross-sectional area, which is used to calculate the stress cur rent. The resistivity of the deposited film (almost always) varies les s over a wafer than the film thickness. The alternative method results in an improved precision of the stress current density. A comparison between the two methods is made and two experiments are used to illust rate the difference. For Al-lat.% Si layers deposited in our sputterin g system, the precision of the stress current density is improved by a factor of three. Copyright (C) 1996 Published by Elsevier Science Ltd