Failure Criterion Setting for the Wafer Level Isothermal Electromigration Test

The lifetime prediction has been performed on the Al-Cu metal line using the wafer level isothermal electromigration test. A 8000-Å-thick Al-Cu (0.5%wt Cu) metal line of 800 mm long and 0.4 μmm wide is stacked with a 500 Å Ti/TiN bottom layer and a 400 Å Ti-TiN top layer. The resistance change ratio ΔR/R
_i
increases rapidly in the beginning (phase I), reaches a linear rate region (phase II), and finally abruptly increases (phase III). Self-heating, electromigration, and melting occur, respectively, in three phases I, II, and III. The (ΔR/R
_i
0) value can be extracted from the ΔR/R
_i
vs. stress time curve, which defines the ΔR/R
_i
value at the beginning of electromigration and is nearly at ~1% in our cases. The failure criterion is set at 1.2(ΔR/R
_i
)0 , which is an important value for lifetime prediction of the metal interconnect. The lifetime derived from the wafer level ISO-EM test is 2.9 times longer than that from the package level EM test.