Reliability of Metal-Dielectric Structures Under Intermittent Current Pulsing

Design rules for specification of current densities in lines are driven by consideration of possible failure mechanisms. Under large amplitude intermittent current pulses, thermal stresses are the dominant cause for failure, with chief failure mechanisms being cyclic fatigue and plastic ratcheting of the metal line or dielectric fracture. In this study, a BEOL structure with aluminum or copper interconnects and silicon dioxide dielectric under well-separated current pulses is studied numerically through finite element simulations. The effects of varying duty cycles, peak current densities and line widths are studied. Line fatigue is found to be the dominant mode of failure in the absence of package structure. The results are characterized by the temperature rise in the line and the duty cycle of current pulses. A threshold is found to exist for the per cycle temperature rise below which there is no observable fatigue. A comparison of reliability between aluminum and copper lines is made and it is observed the copper lines permit a lower threshold temperature rise, but induce a lower dielectric stress. The results of the study may be used to develop peak current density specifications for dies.