Numerical Model for Understanding Interconnection Thermal Reliability Mechanism of Cu Via in Back End of Line (BEOL)

The Cu Via structure in the back end of line (BEOL) process was generally made up of heterogeneous materials with different thermal expansion coefficients (Cu, Si substrate, SiCOH dielectric layer, Ta/TaN SiCN barrier layer, etc.), and the thermal mismatch could further lead to the failure of entire interconnection. Since the temperature changed during manufacture would definitely cause internal stress due to thermal mismatch, the thermomechanical stability and long-term usage of Cu Via are no doubt affected. Therefore, it is quite necessary to promote the research of Cu Via thermomechanical stability. The Cu Via prepared through Damascus process presents various geometric characteristics (straight holes, inclined holes, scallop-shaped sidewall holes and holes with pores). The sharp rise of current density resulted from rough via surface would cause quick accumulation of hot spots, which could accelerate the stress concentration of transistors and interconnect lines, as well as shorten the life of the device and interconnect. The high-density BEOL integration based on the Cu Via vertical interconnection significantly reduced the thickness of interconnection and changes the heat source distribution in the module, which could form a new heat transport mode and increase the heat load level per unit volume. As a result, the mechanism of the thermal mismatch effect would be altered thoroughly. In this work, we focused on the influence of structural characteristics and process factors on Cu Via thermomechanical stability. The Cu Via process and structural design guidelines were established while some simulation conclusions have been verified. Meanwhile, since the improvement of Cu Via thermomechanical reliability is essential to further develop the application process of BEOL interconnect miniaturization, we also set up a physical model of an ideal Cu Via structure and analyzed the influence of key structural parameters on the thermomechanical stability under typical temperatures load. Simulation results showed that the Cu Via structure with a ratio of 1.05 (depth and width) and diameter size of 73.91 nm presented maximum thermal stress during the annealing (380 °C) process. Additionally, it was found that the lower annealing temperature of Cu Via could improve thermomechanical reliability. The maximum von Mises stress of Cu Vias was distributed in the corner area of the interface between TaN film and SiCOH film, and the value was approximately 898.11 MPa. Other higher stress concentrated areas were mainly located around the Cu structure connecting to the barrier layer. To effectively reduce the thermal stress of structure, materials with large thermal expansion coefficients should be avoided to the greatest extent. On the one hand, Cu Via with an aspect ratio higher than 1.2 improved the thermomechanical reliability of the heterostructure. On the other hand, further increase of the aspect ratio showed a weaker impact on the thermal stress reduction. Besides, if the aspect ratio was designed to be 1.01.1, the magnitude of the thermal stress would be quite larger. Furthermore, if the working temperature of Cu Via could be limited to under 120 °C, the Cu Via would only undergo elastic deformation, and then the thermal reliability caused by plastic deformation and other issues could be reduced effectively.