The effect of microstructure on resistivity and reliability in copper interconnects

Abstract

The resistivities of Cu interconnects with thicknesses of 0.05-0.14 μm and varied line widths were measured for standard Damascene processed electroplated Cu lines along with lines that had intentionally varied Cu microstructures. For standard processing, the resistivity increased with decreasing Cu cross-sectional area and the resistivity could be fitted with to (Cu area)-1 relationship. The wide line resistivity remained constant at 2.0 ± 0.1 μΩcm but increased to greater than 3.0 μΩcm for sub-60 nm wide lines. The Cu grain size did not greatly affect the Cu resistivity unless the median grain diameter was near or below the dimension of the mean free path for electron scattering in Cu, 30nm at 27°C. The major source of the resistivity increase as line dimensions decrease was probably from increased surface scattering. Resistivity determined by direct measurements of resistance and Cu area produced lower resistivity values than a method based on the temperature coefficient of resistance (TCR), especially for fine lines. The difference could be due to incorrect assumptions in the TCR method or projection errors in the microscopy based Cu area measurements. Interconnects with no anneal before chemical mechanical polish and with line widths ≤ 90 nm had multi-grained microstructures and reduced median electromigration lifetimes due to the addition of a grain boundary fast diffusion path. From electromigration testing, the electromigration activation energy for grain boundary diffusion in Cu was determined to be 0.79 eV. © 2008 Materials Research Society.