AC electromigration characterization and modeling of multilayered interconnects

Abstract

The ac electromigration of multilayered interconnect systems, consisting of TiW/AlSiCu and CVD-W/AlSiCu, is studied under stress of repetitive dual-pulse current waveforms at 2 MHz. By using a general ac waveform in which the peak current density of the first pulse jl is fixed and the second is varied from +jl to -j1, we obtain a continuous electromigration spectrum from dc through pulsed dc to pure ac conditions. Although an average current model fits the data well, there is a singularity in the median time-tofailure at pure-ac conditions. To avoid this singularity, a novel modification of the average current model, called the Average Current Recovery (ACR) model, is developed. It heuristically accounts for the degree of damage recovery during oppositepolarity pulses through a single recovery parameter.

Atomic flux divergence-based AC electromigration model for signal line reliability assessment

“…For the second problem, by taking advantage of 3D distributed RC model, the total atomic flux can be orthogonally decomposed into x, y and z directions, as shown in (6). Then the sum of atomic fluxes in different directions can be directly calculated.…”

“…Unlike pulsed DC case, a simple conversion from AC to DC using the average current or root mean square (RMS) current either obtains too optimistic or too pessimistic MTTF. To fit the measured data, the concept of healing factor was proposed to facilitate conversion from AC to DC [6]. The EM reliability equivalent current density for AC case is then expressed as (2), where jAC, avg+ and jAC, avg-denote the time average current density including only positive or negative pulses and  is the healing factor.…”

Overcoming Variations in Nanometer-Scale Technologies

“…This can cause the wire to have increased resistance as it is thinned, or even become an open-circuit, and is therefore a serious reliability problem. This problem is witnessed most notably in supply (power and ground) wires [131], [132], where the flow of current is mostly unidirectional, but AC electromigration is also seen in signal wires [133]. Amelioration strategies for electromigration are primarily built in by ensuring that the current density on a wire never exceeds a specified threshold.…”

Interconnect Lifetime Prediction for Reliability-Aware Systems

“…As for ac current, an EM effective current is used by the average current recovery (ACR) model [18], [19]. In this paper, we do not distinguish between these two cases.…”