Scaling effects on microstructure and resistivity for Cu and Co nano-interconnects

Abstract

The continued scaling of Cu low k technology is facing serious challenges imposed by basic limits from materials, processing and reliability. This has generated great interests recently to further develop Cu nanointerconnects and alternatives, particularly Co and Ru nanointerconnects beyond the 10nm node. In this paper, we investigate the microstructure evolution in Cu and Co nanointerconnects and the effects on resistivity, a key factor contributing to the RC delay and reliability of the nanointerconnects. The scaling effect on microstructure of Cu and Co interconnects was analyzed using a TEM-based high-resolution precession electron diffraction (PED) technique with capabilities to map in detail the orientation and size distribution of individual grains. The results of this study revealed a consistent picture of microstructure evolution with scaling in Cu nano-interconnects. As the (111) texture had been observed to switch from trench bottom to trench width to trench length, a trend which continued to the 22 nm linewidth with the appearance of more small grain aggregates, indicating further dominance of the interface energy in comparison to the strain energy with increasing surface to volume ratios. The amount of twin boundaries continued to decrease with scaling line width as well. The microstructure evolution studies were carried out for Co interconnect using the TEM based high-resolution PED technique with line width of 220nm and 26nm. In contrast to Cu interconnect at line width of 26nm, the Co interconnect did not switch from the trench width direction to the trench length direction. This is an indication that the interfacial energy is not the only controlling factor, whereas the strain energy also plays an important role in texture evolution in Co interconnects, a conclusion supported by the large abnormal grains observed in narrow Co lines. The TEM study was supplemented by Monte Carlo simulation to project the grain growth for future technology nodes based on local energy minimization. In the simulation, the orientation-dependent grain boundary, strain, and interface energies were taken into account in order to examine the effect of scaling and material properties on grain growth for future technology nodes. Finally, the results of the microstructure study by TEM and simulation were used to analyze the scaling effect on the resistivity of Cu and Co nanointerconnects. We are able to account for the scaling effect on resistivity from the contributions of surface and grain boundary scatterings as reported in a recent study