Comparison of isothermal, constant current and SWEAT wafer level EM testing methods

We present data from three wafer level electromigration test techniques, isothermal (ISOT), constant current (CI), and standard wafer level electromigration accelerated test (SWEAT) and compare various aspects of the data. ISOT keeps the test line at a constant temperature. CI applies a constant current of the same magnitude to all lines, without making any adjustments for individual geometric differences. SWEAT holds the time to failure constant by using Black´s equation to determine the applied current needed to bring about the chosen failure time. Six different line widths ranging from 0.15 to 4.29 μm of AlCu metal were stressed by all three methods, at temperatures from 300 to 380°C in 10°C steps. Nine wafers were used; one for each temperature, and 17 chips per wafer were tested for each algorithm. The structures were 857 μm long, single-level stripes (to avoid thermal anomalies introduced by W studs and vias). Mean initial current values from ISOT were used as the input for the CI tests. Similarly, mean times to failure from ISOT were used as the target median times to failure (τ₅₀) for the SWEAT tests. Values for activation energy ΔH and Black constant (A) were calculated from the ISOT results using current density exponent n=1.7 from package level. τ₅₀ ´s are therefore comparable for all three methods