Interconnects

Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. more...

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The effect is only important in applications where high direct current densities are used, such as in microelectronics and related structures. As the structure size in electronics such as integrated circuits (ICs) decreases, the practical significance of this effect increases.

History

The phenomenon of electromigration has been known for over 100 years, having been discovered by an obscure French scientist called Gerardin. The topic first became of practical interest in 1966 when the first integrated circuits became commercially available. Research in this field was pioneered by James R. Black, who set the basis for all research in this area and after whom Black's law (a. k. a. Black's equation) is named. At the time the metal interconnects in ICs were still about 10 micrometres wide. Currently interconnects are only micrometers or nanometers in width making research in electromigration increasingly important.

Practical implications of electromigration

Electromigration decreases the reliability of ICs. In the worst case it leads to the eventual loss of one or more connections and intermittent failure of the entire circuit. Since the reliability of interconnects is not only of great interest in the field of space travel and for military purposes but also with civilian applications like for example the anti-lock braking system of cars, high technological and economic values are attached to this effect.

Due to the relatively high life span of interconnects and the short product lifecycle of most consumer ICs, it is not practical to characterize a product's electromigration under real operating conditions. A mathematical equation, the Black equation, is commonly used to predict the life span of interconnects in integrated circuits tested under "stress", that is external heating and increased current density, and the model's results can be extrapolated to the device's expected life span under real conditions.

Although electromigration damage ultimately results in failure of the affected IC, the first symptoms are intermittent glitches, and are quite challenging to diagnose. As some interconnects fail before others, the circuit exhibits seemingly random errors, which may be indistinguishable from other failure mechanisms (such as ESD damage.) In a laboratory setting, electromigration failure is readily imaged with an electron microscope, as interconnect erosion leaves telltale visual markers on the metal layers of the IC.

With increasing miniaturization the probability of failure due to electromigration increases in VLSI and ULSI circuits because both the power density and the current density increase. In advanced semiconductor manufacturing processes, copper has replaced aluminium as the interconnect material of choice. Despite its greater fragility in the fabrication process, copper is preferred for its superior conductivity. It is also intrinsically less susceptible to electromigration. However, electromigration continues to be an everpresent challenge to device fabrication, and therefore the EM research for copper interconnects is ongoing (albeit being a relatively new field.)

Read more at Wikipedia.org