Modeling of and experiments characterizing electromigration-induced failures in interconnects

Abstract

As interconnect linewidths continue to scale downward, a detailed knowledge of stress evolution and void growth processes enables a determination of the electromigration-induced failure times of these interconnects. This thesis provides this knowledge through the development of an electromigration simulation MIT/EmSim and through experiments. The stress effect on diffusivity and alloying effects were incorporated into the MIT/EmSim model for Al-based interconnects, demonstrating Cu transport effects consistent with experiments by IBM. The void nucleation and growth process was modeled in long and short lines, and current density exponents for Black's equation determined for several failure conditions. Interconnects can also be immortal, either without void nucleation or by resistance saturation. This complex reliability behavior can be catalogued in the form of failure mechanism maps using simulation or analytical approximations, plotting failure or immortality mode as a function of current density and line length. To extend the MIT/EmSim model to Cu interconnects, experimental observation of void phenomenology and measurement of Cu-based electromigration parameters were performed using Cu/Ta interconnects specifically designed and fabricated at MIT. The knowledge gained from these experiments was used to develop an electromigration model for Cu, and incorporated into MIT/EmSim. Simulations comparing void growth in Al and Cu interconnects indicate that differences in failure times of these interconnects observed in accelerated tests may not necessarily be apparent at service conditions.

(cont.) A failure mechanism map constructed for damascene Cu demonstrates the absence of immortality by resistance saturation due to the shunt structure. Finally, proposed damascene designs eliminating the diffusion barrier at the studs may be expected to have an adverse effect on interconnect reliability due to the loss of short length effects. The test structure developed in this thesis provides a simple means of testing the effects of new shunt and barrier layer technologies on the reliability of Cu-based interconnects.Through the use of the simulation, an accurate methodology for predicting the reliability of Al- and Cu-based interconnects in semiconductor chips has been developed. MIT/EmSim is now being used by Motorola and LSI Logic for evaluating interconnect reliability during the design of future Cu interconnects, and has also been used by numerous SRC-companies and universities through EmSim-Web for electromigration research.