Statistical Analysis of Electromigration Lifetimes and Void Evolution for Cu Interconnects

Abstract

Electromigration (EM) failure statistics and the origin of the lognormal deviation (σ) for Cu interconnects have been investigated by analyzing the lifetime statistics and void size distributions at various stages during EM testing. Experiments were performed on 0.18 μm wide Cu interconnects with tests terminated after specific amounts of resistance increases, or after a specified test time. Void size distributions of resistance-based, as well as time-based EM tests were obtained using focused ion beam (FIB) microscopy. The lifetime and void size distributions were found to follow lognormal distribution functions. The σ values of EM lifetime and time-based void size distributions decrease with higher percentages of resistance increase, reaching an asymptotic value of σ ∼ 0.14. In contrast, σ values of resistance-based void size distributions are significantly smaller and do not show an obvious dependence on time. The statistics of resistance-based void size distributions can mainly be accounted for by geometrical variations of the void shape, while the statistics of time-based void size distributions requires consideration of kinetic aspects of the EM process. The σ values of EM lifetime distributions at long times can be simulated based on measured void size distributions, taking into account geometrical and experimental factors of EM. In contrast, for short times the statistics of initial void formation and the kinetics of interfacial mass transport have to be considered.

Electromigration reliability of open TSV structures

“…At the end an abrupt resistance increase (for the 2:5I 0 case at 4.7Ms) leading to an open circuit failure can be observed. This observations are consistent with the resistance change measured in accelerated EM tests for dual damascene structures [10] and filled TSV structures [11].…”

Interconnect reliability dependence on fast diffusivity paths

“…SiN-and SiC-based films are widely used as capping materials for copper interconnects. It has been shown in several studies [5] …”

Interconnect Lifetime Prediction for Reliability-Aware Systems

“…If the temperature (or ) of the interconnect is time dependent, EM stress build-up can be derived as (detailed derivation can be found in [9], [20]) (5) If we assume that the stress build-up reaches a certain threshold ( ) at which an EM failure occurs, we have (6) where is a constant determined by the critical stress (i.e., ). If an average value of exists, we obtain a closed form for the time to failure (7) where is the expected value for , and is the temperature factor, as defined in (4), having the form , where is a constant.…”

“…Various experiments [6], [26] showed that, in Cu interconnect, voids tend to nucleate at the cathode end (near the via), and void growth is the dominant failure process because the critical mechanical stress for void nucleation in Cu is much smaller than that for aluminum. With spatial thermal gradients in the interconnect, it is possible that the location of void nucleation is no longer at the cathode end.…”

“…The variability of lifetime is strongly dependent on the interconnect structure geometries and weakly dependent on environmental stresses such as current and temperature [6], while median time to failure (MTF) is determined by current and temperature in the interconnect. In this paper, we use MTF as the reliability metric and investigate how it is affected by temporal and spatial thermal gradients.…”