ELASTOPLASTIC CREEP ANALYSIS OF THERMAL-STRESS AND STRAIN IN ALUMINUM INTERCONNECTS OF LSIS

To study a thermally induced open-failure mechanism of aluminum interconnects of LSIs, a three-dimensional (3-D) simulation using a finite element method was carried out. In this calculation, a creep strain increasing with time in addition to an elastoplastic strain was taken into account. Further, the effect of a manufacturing process on a stress and a strain induced in an aluminum layer was examined using a simulation for a subsequent stacking of oxide-layer/aluminum film/protective layer on an Si chip. The condition for a 3-D plasticity to occur was calculated in which case a strain was represented using the von Mises criteria. As a result, after the manufacturing process is completed, the stress is found to decrease significantly with time, while a plastic strain is kept constant. However, the creep strain responsible fdr the failure is found to increase. The temperature dependence of the creep strain also was studied. The three different results previously reported, i.e., the lifetime becomes shorter as the temperature rises, there is no temperature dependence, and the shortest lifetime is at about 150 degrees C are all found to be possible. The calculation was applied to the multilayered interconnects and the creep strain is shown to decrease by more than a few tens percent in an aluminum layer,