J. R. Lloyd

A model for conductor failure considering diffusion concurrently with electromigration resulting in a current exponent of 2

A model was constructed for electromigration failure where both Fickian diffusion and mass transport due to the electromigration driving force are considered concurrently. The solution to the resulting diffusion equation yields a current exponent of 2 and an activation energy consistent with grain‐boundary self‐diffusion. A modification of the standard median time to failure equation first proposed by Black is suggested.

Relationship between interfacial adhesion and electromigration in Cu metallization

A relationship between the adhesion of a Cu conductor to its surrounding medium, the electromigration drift velocity, and lifetime in a conventional electromigration test has been demonstrated. Lifetime measurements demonstrate that the drift velocity and the activation energy for mass transport along an interface correlate with the measured adhesion energy of the interface as determined by a four-point bending test. The results indicate that a linear relationship is expected between the electromigration activation energy and the intrinsic work of adhesion, which is consistent with a simple model relating the two. The data indicate that interfacial cleanliness and bond character at the interface (i.e., metal/dielectric versus metal/metal bonding) have a significant impact on all measured parameters.

Electromigration in copper conductors

Abstract The electromigration performance of copper-based conductors is reviewed critically. From the available literature it appears that electromigration in copper proceeds via an interfacial diffusion path with an activation energy significantly less than that of grain boundary diffusion. As a result the advantage of copper over Al/Cu is not as great as originally anticipated, but still may be significant. Unfortunately, there are no literature data available suitable for a comparison of reliability. Because of the large relative interfacial area in state-of-the-art conductors and the substantial difference in the activation energies for interface and lattice diffusion, the “cross-over temperature” where lattice -dominated diffusion would predominate is high enough not be a factor in reliability experiments.

Simple model for time-dependent dielectric breakdown in inter- and intralevel low-k dielectrics

A simple physical model is applied to time-dependent dielectric breakdown failure in ultralow-k(k=2.3) interlevel dielectrics. The model assumes that failure depends on the probability that an electron will have enough energy to damage the dielectric as it is accelerated in an electric field. It is seen that the characteristic form of the dependence of failure time on voltage or electric field is primarily dependent on the probability of having sufficient energy and not on the precise physical mechanism causing damage. An argument for a log-normal-like failure distribution is also presented.

Copper metallization reliability

Abstract The electromigration performance of Cu metalization has been reviewed with an explanation as to why the advantage of Cu over Al alloys in the fine line regime is not as great as anticipated. Cu metalization testing procedures are described and the dangers of “overstressing” are explained in some detail.

Electromigration and adhesion

It has been demonstrated that, in those instances where electromigration-induced mass transport is dominated by interfacial diffusion, the adhesion at the interface where mass transport is primarily taking place is related to the electromigration flux. Furthermore, it is shown that the cohesive energy of the interface is directly related to the activation energy for diffusion.

Electromigration-induced microstructure evolution in tin studied by synchrotron x-ray microdiffraction

Under constant current electromigration, white tin exhibited a resistance drop of up to 10%. It has a body-centered-tetragonal structure, and the resistivity along the a and b axes is 35% smaller than along the c axis. Microstructure evolution under electromigration could be responsible for the resistance drop. Synchrotron radiation white beam x-ray microdiffraction was used to study this evolution. Grain-by-grain analysis was obtained from the diffracted Laue patterns about the changes of grain orientation before and after electromigration. We observed that high-resistance grains reorient with respect to the neighboring low-resistance grains, most likely by grain growth of the latter. A different mechanism of grain growth under electromigration from the normal grain growth is proposed and discussed.

The role of metal and passivation defects in electromigration-induced damage in thin film conductors

Abstract The effect of both metal defects (nicks and scratches) and passivation defects (Griffith cracks or pinholes) was theoretically and experimentally investigated. Flux divergences due to temperature and stress gradients are considered. It was observed that failure near a severe metal defect or at a defect in the passivation layer can be a major contributor to electromigration-induced lifetime degradation.

The effect of Cu diffusion on the TDDB behavior in a low-k interlevel dielectrics

Abstract The effect of Copper on TDDB failure in a structure incorporating a low-k interlevel dielectric was studied theoretically and experimentally. Interdigitated comb capacitor structures were prepared with and without Cu metallization and stressed at 4.0 to 6.6 MV/cm at 150C. The samples without Cu did not fail to over 1800 hours at 4 MV/cm whereas the samples with Cu exhibited a median time to failure (t50) of ∼45 minutes. At 6.6 MV/cm, the t50 was ∼1.8 hours for the Cu free samples. This experiment demonstrated the importance of Cu in the TDDB behaviour of low-k dielectrics, but also demonstrated that the presence of Cu was not a necessary condition for failure. The effect of Cu diffusion on TDDB behaviour was studied in the context of the “Impact Damage” model. Both field assisted ionic diffusion and diffusion of neutral Cu was considered. It is seen that the behaviour at low fields near use conditions may have little relationship to the predictions obtained from the results of typical TDDB testing.

Line edge roughness and spacing effect on low-k TDDB characteristics

The study of low-k TDDB line space scaling is important for assuring robust reliability for new technologies. Although spacing effects due to line edge roughness (LER) on low-k TDDB lifetime were reported previously (Chen et al., 2007; Lloyd et al., 2007; Kim et al., 2007), there has been a lack of an analytical model with which to link line edge roughness to experimental TDDB data in a simple quantitative format. This work reports a thorough investigation into the low-k SiCOH line LER effect on low-k TDDB covering both experimental results and finite element modeling (FEM) simulations. The maximum electric field intensity as a result of sidewall LER bump was found to depend on the bump curvature. The decrease of low-k line spacing that resulted in a shorter TDDB lifetime even under the same applied electric field was then carefully analyzed. A simple analytical model of the effect of line edge roughness on TDDB failure time reduction is presented. This model was verified by experimental results. Additionally, a method to electrically quantify an overall line edge roughness is introduced.