The effects of microstructural transitions at width transitions on interconnect reliability

Abstract

Interconnect reliability is usually assessed through lifetime tests on straight stud-to-stud lines with fixed widths. However, real interconnects often have junctions with narrow-to-wide transitions. We have carried out experimental and modeling-based studies of interconnects with narrow-to-wide transitions to test the Korhonen model for electromigration-induced failure and determine parameters for failure. Two-level Al-0.5% Cu via-to-via electromigration test structures with narrow-to-wide width transitions have been studied as a function of the ratio of linewidths and as a function of the ratio of lengths of the wide and narrow line segments. The narrow-to-wide transition was found to be a site of atomic flux divergence due to the discontinuity in diffusivities between the narrow and wide segments, which have bamboo and polygranular microstructures, respectively. Consequently, it was found that the electromigration failure rate increases as the position of width transition is decreased below a critical distance from the electron–source via. This occurs due to an interaction between evolution of the stress field at the electron–source via and the evolution of the stress field at the width transition. Narrow-to-wide transitions with positions greater than a critical distance from the electron–source via have much higher lifetimes which are not a function of the transition location, due to a lack of interaction between the electron–source via and the width transition. Because the mode of failure was identical for all structures (i.e., failure by voiding at the electron–source via), use of experimental results with simulations allows determination that the critical stress range for void-nucleation failure in these structures is 600 MPa±108. It was experimentally found that changing linewidth ratios did not affect interconnect reliability when the transition was 150 μm away from the electron–source via. However, simulations indicate that if the width transition is close to the electron–source via, the lifetime should decrease with the width of the polygranular region. Additional electromigration studies have been conducted on constant-width interconnects in which microstructural transitions were created using a scanned laser annealing. Analyses of the experiments are consistent with those on microstructural transitions caused by width transitions.