P. Bo

Microstructure based statistical model of electromigration damage in confined line metallizations in the presence of thermally induced stresses

Probability distributions are evaluated for electromigration induced open failures in narrow, passivated interconnects with a near‐bamboo grain structure. Void formation is initiated at the cathode ends of the polycrystalline line segments or ‘‘grain clusters.’’ If these clusters are longer than a critical size Lc, they can accommodate enough atoms for voids to reach the critical size to fail the line. Obviously, the critical size Lc depends on the thermal stress: a cluster under a tensile stress is able to incorporate more atoms from the void than an unstressed cluster. In the case the clusters are shorter than Lc, atoms from the voids must be distributed also to bamboo sections, outside the clusters, in order for the voids to induce open failures. Based on this physical picture, failure probabilities are evaluated as a function of time. The predicted failure distributions and parametric dependencies compare well with the experiments.

Thermal-stress-induced voiding in narrow, passivated Cu lines

Copper is being considered as an alternative to aluminum‐based metallizations in microelectronic circuits, both because copper is a better conductor and because it is expected to be more resistant to thermal stress and electromigration induced failure. However, thermal stresses are found to cause significant voiding in passivated copper lines, in a manner very similar to that commonly observed for passivated aluminum lines.

Do thermal spikes contribute to the ion-induced mixing of Ni into Zr, Ti, and Pd

Low‐temperature ion beam mixing rates for Ni‐Ti, Zr‐Ni, and Pd‐Ni bilayers significantly exceeded binary collision estimates, and appeared quite sensitive to thermodynamic driving forces. In the absence of a temperature dependence such a behavior is commonly ascribed to interdiffusion within thermal spikes. However, the Ni‐Ti mixing rate was seen to vary linearly with nuclear damage energy for irradiation with 600 keV Xe, Kr, or Ar, 300 keV Ne or N, or 200 keV N ions, or 1 MeV Au ions (literature value). This excludes overlapping thermal spikes. An expression was derived for mixing due to nonoverlapping thermal spikes, but this could also not explain our results.

Stress‐induced nucleation of voids in narrow aluminum‐based metallizations on silicon substrates

This work investigates thermal stress‐induced voiding, and void nucleation in particular, in narrow, passivated aluminum‐based metallizations on silicon substrates. After excursions to a higher temperature, the thermal stress is tensile, and increases during cooldown to room temperature, after which it relaxes with time. Experiments conducted on two aluminum alloy metallizations suggest that stress‐induced void nucleation is a one shot phenomenon during cooldown from the heat treatment temperature. Further, the high thermal stresses present, and the strong constraints against deformation provided by the substrate and passivation layer, make void nucleation unique in narrow passivated metallizations. Finally, voids always appear to be connected to grain boundaries. The above experimental evidence and theoretical considerations together suggest that grain boundary sliding is the main mechanism facilitating void nucleation in passivated aluminum alloy metallizations.

On the validity of a thermal spike mixing model for low‐Z metals

Low temperature ion beam mixing rates for Cu‐Ti, Ni‐Ti, and Fe‐Ti layers have been found to be significantly lower than predicted by a popular semi‐empirical thermal spike model. It has been proposed that the unavoidable hydrogen contamination of the as‐deposited Ti films may have reduced the mixing rates, but the measurement of even lower mixing rates for Fe‐V and Fe‐Co bilayers shows the discrepancy to be more fundamental. Still, a systematic dependence on heat of mixing suggests that some sort of diffusional (thermal spike?) mechanism is involved.

Electromigration‐induced failure in passivated aluminum‐based metallizations−The dependence on temperature and current density

A new dynamic picture of electromigration‐induced failure in passivated narrow lines allows the prediction of the variation of lifetimes with temperature and current density. According to the model, damage is usually nucleated by thermal stress‐induced voiding. Small voids are trapped and grow at grain and phase boundaries. After reaching a critical size, voids then begin to migrate and coalesce, eventually leading to line severance. In most cases this leads to lifetimes varying approximately as the square of the current density j for low and moderate j, and faster for large j. The temperature dependence is determined by a combination of bulk, surface, and grain boundary diffusivities.

Stress‐induced voiding and electromigration

Thermal stress induced voiding provides an effective nucleation mechanism for the electromigration induced damage and failure of narrow, passivated metal lines. Small voids are trapped at grain and phase boundaries, where they grow under a current. Growth rates are determined by the local flux divergencies and the current induced stress distribution. After reaching a critical size, some voids begin to migrate and coalesce, eventually leading to line failure. A model is outlined, which is capable of explaining a large number of experimental observations and offers a basis for the prediction of failure statistics.

Observation of multiple precipitate layers in MeV Au++‐implanted silicon

Room‐temperature MeV Au++ implantation into silicon with energies above 1.8 MeV shows a splitting of the Au concentration profile in the Rutherford backscattering spectrometry (RBS) spectra. Cross‐section transmission electron microscopy micrographs show two distinct regions of Au precipitates corresponding to the peaks in the RBS spectra. The double peaks can be explained by the segregation of Au into the highly damaged region near the end of the implant range and Au segregation along a dislocation network. These dislocations arise from dynamic beam annealing during the implant and act as paths for rapid diffusion. Precipitation occurs when the Au concentration exceeds the solubility limit. Lower energy implants resulted in the expected Gaussian distributions.

Au‐Ag ion mixing rate—disagreement with theory resolved

Measurements of the low‐temperature mixing of several 5d–4d bilayers by 600 keV Xe ions have strongly supported the assumption of a thermal spike mechanism. Quite disturbingly, however, the Au‐Ag mixing rate appeared to exceed theoretical predictions by about a factor of 3. A closer examination of this system shows the discrepancy to be caused by the formation of a strongly nonuniform Au surface structure during irradiation. An improved value for the mixing rate is in reasonable agreement with predictions.

Stress evolution during stress migration and electromigration in passivated interconnect lines

Differences between the coefficients of thermal expansion of a narrow metal line and surrounding chip and passivation lead to the establishment of very large tensile stresses, which can only be effectively relaxed through the formation and growth of voids when the passivation remains intact. Furthermore, an electromigration induced atomic flux will lead to simultaneous void growth and accumulation of atoms at appropriate flux divergences. For a given current and microstructures (including void locations) a unique relationship can be established between the time, stress distributions and the corresponding void volumes. We model the stress evolution in passivated near‐bamboo lines to account for the synergistic effects of stress migration and electromigration on void growth and open failure under various combinations of temperature, current, microstructure, and the presence of W‐studs/vias.