24.7: Summary
- Page ID
- 32722
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This TLP has covered several points:-
- The reliability of microelectronic devices is important, as our lifestyles depend on the current technology and we expect continuing improvements in performance. Electromigration reduces device reliability, which is a problem for the electronic industry.
- Electromigration is the transport of material in a conductor under the influence of an applied electric field.
- Electromigration phenomena occur in all conductors on application of an electric field. Damage is commonly observed within narrow metallization lines in integrated circuits, as they are exposed to particularly high current densities.
- The electromigration force, which causes atomic migration, is a combination of two forces – the direct force and the wind force, related by:
\[F_{\text {net }}=F_{\text {wind }}+F_{\text {direct }}=Z^{*} ejp\]
where Z* = effective charge; j = current density (A m-2); and ρ = resistivity (Ω m).
Mass atomic transport towards the cathode occurs as the net force biases the net diffusion direction.
- Damage induced by electromigration results in hillock and void formation. Electromigration-induced damage is caused by divergences in atomic flux, themselves caused by place-to-place variation in:
- Microstructure
- Material
- Temperature
- Choosing the best materials for use in an integrated circuit, the most suitable processing methods and good device design can assist in reducing the resultant electromigration damage.
- The device lifetime can be roughly estimated using Black’s Law to extrapolate median time to failure at service temperature from accelerated test conditions:
\[t_{50}=c j^{-n} e^{\frac{E_{e}}{k T}}\]
- where t50 = median time to failure of metal lines subjected to electromigration (hrs); c = constant based on the metal line properties (units depend on exponent n); j = current density (A m-2); n = value between 1 and 7 (though commonly 2); Ea = activation energy (J) [within the range 0.5–0.7 for Al]; k = Boltzmann constant (1.38×10-23 J K-1); and T = temperature (K).
- Due to the increased miniaturization of microelectronic devices, there is a need for new materials and processes to be used for metallization lines. One such development is the change from Al-based to Cu-based metallization in high performance devices. Further speed, reliability and miniaturization requirements are spurring on the search for the use of new materials within integrated circuits.
Going further
Books and Papers:
- Reliability and Failure of Electronic Materials and Devices by Milton Ohring, Academic Press, San Diego, 1998.
Provides a good overall explanation on the topic of electromigration
- Review Article: Electromigration in integrated circuit conductors by J R Lloyd (J. Phys. D: Appl. Phys. 32 (1999) R109-R118).
A good summary on electromigration in integrated circuit conductors
- VLSI Technology by S.M. Sze (editor), Mc-Graw-Hill Book Company, New Jersey, 1988.
Contains a good section on metallization and its choice of use for different materials