The Effect of Mechanical Stress on Electromigration Behavior

Abstract

AbstractThree-dimensional integrated circuit packages with through-silicon vias (TSVs) provide a good solution to the integration of different chips and help achieve high performance. The signals are transmitted to different layers directly through the vias, thereby enabling the high performance of the chips. Plasma-enhanced chemical vapor-deposited SiO2, polyimide, and benzo-cyclo-butene are commonly used as the passivation layer for three-dimensional packages. In the 3D stacked chips, mismatch of the coefficient of thermal expansion between the passivation material and silicon will generate thermal/mechanical stress in the metal trace and the stress affects the behavior of electromigration. However, few studies have examined the relationship among the external mechanical stress and critical product of electromigration. In the present study, external stress is applied to the aluminum thin film by a four-point bending equipment. In order to apply higher external stress in the aluminum thin film, the fracture strength of the silicon substrate should be improved. Reduces the edge chipping of the test sample is a key factor for improving the fracture strength of the silicon substrate and a special cutting approach is employed to obtain higher silicon strength. A two-step cutting method is applied to reduce front side chipping and also a dicing before grinding approachis adopted to reduce backside chipping, the above-mentioned technology can enable more than 275MPa of external stress on the aluminum thin film and can make the critical length effect more visible. The residual stress of the aluminum thin film is at stress-free state after annealing at 300°C for 10h. The critical product is found to be reduced from 1,294A/cm to 1,281A/cm when 120MPa of mechanical tensile stress is applied. It increased to 1,315A/cm under 120MPa of mechanical compressive stress. Clearly, electromigration behavior is enhanced by tensile stress and decreased by compressive stress. In the current research, a modified equation for Blech condition is proposed with a stress-dependent effective charge number, the effective charge number increased when tensile stress was applied and decreased when compressive stress was applied.