Three-dimensional simulation of microstructure evolution in damascene interconnects: Effect of overburden thickness

Abstract

Microstructure in the damascene interconnects evolves with the overburden layer, an excessive metal layer over trenches. We present the results of threedimensional simulation, which show the effects of overburden thickness on microstructure evolution in a trench. When the thickness of the overburden is less than half of the trench depth, for a trench with the aspect ratio of unity, the microstructure in the trench tends to evolve into a bamboo structure. This effect is discussed in terms of grain sizes in the trench and those in the overburden. The thinner overburden layer would have smaller grains, of which growth is limited by its thickness. Such small-sized grains in the overburden are not likely to grow into the trench, which hardly make grain boundaries in the trench. Meanwhile, the grains from the trench are able to continue growth inside the trench, resulting in a bamboo structure. Overburden thickness affects the reliability and the electrical performance of the damascene copper interconnects. Optimization of overburden thickness is required to minimize these effects.

Current Challenges with Copper Interconnects

“…Even though grain orientation and grain size have been shown to be second order contributors to reliability improvement, (3) grain growth studies are necessary in order to increase the grain size median and distribution. Many studies (17,(21)(22)(23)(24)(25)(26)(27) have been undertaken to quantify microstructure in trenches and models have been developed to understand the grain growth. For narrow lines, the inlaid fabrication method clearly influences the grain growth.…”

“…Larger grains will minimize the grain boundary contribution to resistance, but it is difficult to get Cu grains in a inlaid interconnect line to grow within the thermal budget dictated by the low-K dielectric. Some modeling work is underway to further understand this grain growth (27). Additional sources of scattering, such as line edge roughness, are dictated by the lithography and etch modules.…”

Effect of Overburden Thickness on the Copper Microstructure of Dual-Inlaid Interconnect Structures

“…Simulation results from a 3-D Monte Carlo method [13] showed a different effect of overburden thickness on the characteristics of the trench microstructure, i.e., a bamboo structure evolved in the trench when the overburden was thinner than a transitional thickness. Otherwise, a near-bamboo structure was formed.…”