WEB In-situ study of copper fatigue induced by high strain rate thermo-mechanical pulsing
When a multilayer system is thermally cycled it undergoes a corresponding stress/strain cycle due to the different coefficients of thermal expansion of the respective materials. For thin metal films on silicon substrates it is known that the heating rate used for such a thermo-mechanical treatment has a significant influence on the system’s mechanical response. For microelectronic applications, it is indispensable to replicate application-typical heating conditions, which are characteristic pulses of sub-millisecond duration. In this study, the thermo-mechanical fatigue behavior of copper on silicon is investigated under repetitive thermal pulsing. In order to achieve extreme thermal conditions, special microheaters with a copper metallization are used. Due to the low thermal mass, the microheaters allow quasi-adiabatic heating with heating rates on the order of 10^(6) K/s reaching peak temperatures above 400°C. This corresponds to relative strain rates >10 s^(-1) in the Cu films. A novel in-situ setup is used to actuate the devices inside a scanning electron microscope to allow one to study the gradual deformation of the metallization on a microscopic scale, while applying several thousands of thermal pulses. In-situ electrical resistance monitoring and intermittent roughness measurements serve as additional means for characterizing degradation and deformation. For a more comprehensive understanding, tests have been performed with pulses of different temperature spans.