Electromigration

Notes

The dominant back end of line wear-out limit; the industry moved from aluminium to copper largely to buy electromigration margin.

Overview

Electromigration is the gradual transport of metal atoms along a conductor, driven by the momentum of the electrons flowing through it, which over time creates voids that break wires and hillocks that can short them to their neighbours. It is one of the main wear-out mechanisms in chip interconnect, and it sets how much current each wire and via may carry over a product's lifetime.

How it works

In a wire carrying current, electrons collide with metal atoms and push them, on average, in the direction of electron flow. At normal current densities the effect is negligible, but in the narrow wires of modern chips densities are high, and over years atoms pile up downstream and leave gaps upstream. Atoms move most easily along grain boundaries and interfaces, which is why capping layers on copper lines matter.

Lifetime falls steeply as current density and temperature rise. Engineers use empirical models, most famously Black's equation from the late 1960s, which relate median time to failure to current density and an activation energy, and run accelerated tests at high current and temperature to measure the parameters for each metal layer and via. AEC-Q100 lists electromigration among its die fabrication reliability tests for automotive qualification.

The results become design rules: maximum current per unit width for each layer, adjusted for temperature and lifetime target. Short wires can benefit from a back-stress effect that resists migration. As wires shrink, and in advanced packages where solder bumps and redistribution layers carry high currents, electromigration remains an active reliability concern.

Related terms

• Black's equation
• Stress migration
• HTOL
• Activation energy
• Copper interconnect
• Cobalt interconnect

Where it appears on the Hub

Used on 42 pages of the Hub. In each group, the pages that mention it most come first.