C.‐Y. Li

Analysis of thermal stress‐induced grain boundary cavitation and notching in narrow Al‐Si metallizations

Grain boundary voiding and notching have been found to produce failures in narrow metallizations during thermal aging. The nucleation and growth of grain boundary voids are considered to occur as a result of grain boundary sliding and the subsequent stress‐induced mass transport. A proposed model yields the linewidth and temperature dependence of the observed failure rate.

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.

Stress‐migration related electromigration damage mechanism in passivated, narrow interconnects

In passivated metal interconnects, grain boundary sliding during cooldown from high temperature process steps provides the driving force and sites for void nucleation. Furthermore, residual stresses are known to result in appreciable growth of voids during and after cooldown. The current driven coalescence of such voids is shown to constitute an important failure mechanism for the lines during electromigration testing.

Void formation in thin Al films

Al films deposited on Pt layers developed voids after annealing between 200 and 300 °C. Void formation in the Al was also observed when the Pt layer was deposited above the Al film. The amount of Al surrounding the voids increased as the voids grew. The void growth seems to saturate when all the Pt is consumed to form Pt2Al3, and the rate of void growth decreases as the thickness of the initial Al film increases. We believe the controlling mechanism is diffusion along the Al/Pt interface made possible by compound formation there.

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.

Space charge limited ac conduction with non-blocking electrodes

Abstract A model calculation has been performed for space charge limited ac transport at a reversible interface. The continuity equation for current carriers is solved by a numerical method. The calculated frequency dependence of the conductance and the capacitance agrees with the experimental results on solution-silver chloride-solution couples.

EVIDENCE FOR SPACE CHARGE LIMITED IONIC TRANSPORT AT THE SILVER CHLORIDE‐AQUEOUS SOLUTION INTERFACE

Alternating current conductance and capacitance measurements are made on aqueous solution‐silver chloride‐aqueous solution couples as a function of frequency. The frequency dependence of the measured conductance and capacitance is found to agree with the calculated results based on a space‐charge‐limited ac conduction model.

Kinetics of thermal stress induced void growth in narrow aluminum lines

1‐μm‐wide Al lines were passivated at 300 °C and annealed at 400 °C. The thermal stress induced growth of individual voids was monitored during room‐temperature storage. The growth kinetics of voids are analyzed in terms of a grain boundary diffusion controlled model.

Mechanisms of Inelastic Deformation and Stress Relaxation in Thin Metallizations Bonded to Hard Substrates

The yield strength of metallic thin films bonded to hard substrates can be significantly higher than is customary for bulk samples of the same metal. This is related to the constrained nature of the deformation. The constrained deformation, as well as the commonly observed crystallographic texture of thin films, places restrictive conditions on the mechanisms of deformation that produce stress relaxation. In narrow aluminum based metallizations used as interconnects in large scale integrated circuits thermal stress induced voiding provides an effective means for stress relaxation. For these interconnects, the stress state is tensile after excursions to higher temperatures; the stresses relax mainly by dislocation glide and grain boundary sliding during the cooldown, while the longer term relaxation is governed by stress-induced voiding and dislocation creep.