In recent years copper-based metalization systems for interconnects have been introduced. Typically, copper interconnect lines are formed using a Damascene process which starts with etching trenches in the intralevel dielectric. A thin layer of refractory barrier metal, such as Ta, TaN, or TiN, is sputter-deposited, along with a thin copper ``seed'' layer. A much thicker copper layer is than deposited by electroplating. The excess copper overburden is removed by chemical mechanical polishing back to the surface of the dielectric to form the interconnect line. A thin silicon nitride barrier and the interlevel dielectric are deposited to isolate and encapsulate the copper line. While silicon dioxide is commonly used as intralevel dielectric, new materials with lower permittivity are being actively investigated as replacements in particular to improve high-frequency performance.
The development of intrinsic voids which leads to interconnect failure goes through two distinctive phases [55,52,8,53]. These phases exhibit different influence on the operating ability of the interconnect, and they are also based on different physics. The first phase is the void nucleating phase. In this phase no electromigration generated voids are present and there is no significant resistance change.
The second phase begins when a void is nucleated and is already visible in SEM pictures.
This is the rapid phase of the failure development, when the void expands from its initial position (nucleation site) to a size which can significantly change or completely sever the interconnect line.
Estimating the time to failure of the interconnect line demands an accurate prediction of the duration of both phases of the failure development. Thus both of these phases have to be investigated in subsequent cycles.
By now, Sarychev [70] proposed a comprehensive electromigration model which appears due to it's generality to be adequate for rigorous application. The model connects vacancy diffusion with the local stress evolution for an arbitrary interconnect layout.
By solving the Sarychev equation system, time dependent stress and vacancy concentration distributions can be obtained. The main information contained in this picture is about the critical positions in the interconnect where the vacancy concentration exceeds some configurable threshold, and the time which is needed for reaching this value.
It is obviously necessary to integrate both models describing a period up to void nucleation and models describing the evolution of the nucleated void in order to enable prediction of electromigration stressed interconnect behavior.