Analysis of Local Strain in Aluminum Interconnects by Convergent Beam Electron Diffraction

Abstract

Energy filtered convergent beam electron diffraction was used to investigate localized strain in aluminum interconnects. By analyzing the position of higher order Laue zone lines, it is possible to measure the three-dimensional lattice strain with high accuracy (∼10⁻⁴) and high spatial resolution (10 to 100 nm). In the present article, important details of the strain analysis procedure are outlined. Subsequently, results of measurements of the local variation of thermal strains in narrow, free-standing interconnects are presented. The strain development in single grains during thermal cycling between −170°C and +100°C was measured in situ and local stress variations along the interconnect were investigated. The interconnects show reversible elastic behavior over the whole temperature range, leading to large stresses at low temperatures. The strain state varies locally within single grains, as well as from grain to grain, by as much as 50% in both types of samples. By comparing the experimental findings with elastic finite element modeling, a detailed understanding of the triaxial strain state could be achieved.