Study of Thermomechanical Damage Mechanism in Al Interconnects in Al-SiO2 structure by High Density Peak Current

This paper concerns the damage mechanism of Al interconnects induced by thermomechanical load due to a exposure to short bursts of high-density electric current in repetition. This damage mechanism is one of the emerging reliability challenges and has been under significant concern while studies have been limited with difficulties in testing and related analysis. Our efforts have enabled us to conduct both experimental and numerical investigations on the mechanism using standard 2-level Al interconnects patterned in Si substate as a model structure. To enable effective testing, a pulse generator circuit was developed to produce microsecond bipolar current pulses with sufficiently long “off” duration for complete heat dissipation. Our numerical analysis finds that Al interconnect is subjected to a temperature spike during “on” period, forcing it be under thermal stress due to the thermal expansion mismatch between Al and SiO2. The repetition of stress by pulsing current was found to induce two types of failure/damage depending on the level of input power during “on” period as well as the interconnect dimensions. The first mode of damage/failure was found when the power per pulse was at excessive level. The failure was associated with cracking in the dielectric layer through which extrusion of Al occurred. At lower power level, the failure/damage mode changed to crack/void formation. The mechanism was found to be related to the repeated plastic deformation, and thus fatigue in Al interconnects. Interestingly, there seems to exist a fatigue limit, below which load, no fatigue failure develops while soft metals like Al are known to have no such limit. Our findings highlight the danger of surge current-driven damage as well as possible mitigation strategies in advanced packaging design.