8. Thermo-Mechanical Stress in Interconnect Layouts

THERMO-MECHANICAL STRESS plays an important role in the development of electromigration failure mechanisms in interconnect layouts. Electromigration is the main reliability factor in integrated circuits, because it can cause a system failure after some undetermined operating time [113]. Electromigration is the mass transport of a metal due to the momentum transfer between conducting electrons and diffusing metal atoms [114]. Electromigration in principle exists, whenever a current flows through a metal wire, because the electrons which flow through a metal wire collide with metal atoms. This collisions produce a force on the metal atoms in the direction of the electron flow (for n-type materials and opposite for p-type materials).

Electromigration is only significant at high current densities (e.g. 10$^6$ A/cm$^2$) in metals [115]. The magnitude of its force is proportional to the current density [116]. Because of its material transport, electromigration leads to void formation and void growth where material is depleted [117]. The void causes a large increase in the electric resistance [118], even up to values that the connection practically fails. The void can also reach so large dimensions that the interconnect is broken [119]. Opposite, in points with material accumulation a cracking of the dielectric and a formation of an extrusion can occur which results in a short between adjacent lines.

In advanced semiconductor manufacturing processes, copper has replaced aluminum as the interconnect material of choice. Despite its greater fragility in the fabrication process [120], copper is preferred for its superior conductivity. It is also intrinsically less susceptible to electromigration [121], but electromigration is still an everpresent challenge for device fabrication. Since copper diffuses into silicon and most dielectrics, copper lines must be encapsulated with metallic (like TaN or TiN) and dielectric (such as SiN or SiC) diffusion barriers in order to prevent corrosion and electrical leakage between adjacent copper leads. Because of the different adjacent materials with its different thermal expansion coefficients, thermo-mechanical stresses are preassigned.

Besides the current density and high temperature, latter one caused by Joule self-heating, thermo-mechanical stress is one of the important electromigration promoting factors [115,122]. For the accurate simulation of the electromigration reliability, the influence of mechanical stress should be taken into account, but state of the art simulators lack this capability. The simulation should predict the time to failure and should locate possible critical points in an interconnect structure. Critical points are locations where electromigration promoting factors like current density, temperature and thermo-mechanical stress have high values. Furthermore, interconnect simulation also includes the prediction of void nucleation, void evolution and resistance change. All these above listed problems about electromigration and its modeling are already described in Chapter 4 in [123]. Therefore, this chapter is only focused on the simulation of thermo-mechanical stress.