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Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to electrical current. Conduction electrons at high current densities drive out electrons from the electron shell of the lattice atoms, which are thereby ionized and called "activated ions". Through the ionization they receive an increased mobility and recombine in less strained places of the conductor accumulating there. As the structures 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 more than 100 years. The topic became increasingly interesting for the first time in the year 1966 when the first integrated circuits became commercially available. James R. Black is pioneer on this field of research. The Black equation which was named after him created the basis for all research in this area. At this time the interconnects were still about 10 mm wide. Nowadays they are only some µm or even nm in width. Therefore this field of research increasingly gains significance.
Practical implications of electromigration
Electromigration decreases the reliability of ICs. In the worst case it can lead to the total failure of one or several lines and thus to an entirely inoperative circuit.
Fundamentals
Electromigration occurs when some of the momentum of a moving electron is transferred to a nearby activated ion. This causes the ion to move from its original position. Over time this force knocks enough atoms far enough from their original positions. A break or gap can develop in the conducting material, preventing the flow of electricity. In narrow interconnect conductors, such as those linking transistors and other components in integrated circuits, this is known as a void or internal failure open circuit. Electromigration can also cause the the atoms of a conductor to pile up and drift toward other nearby conductors, creating an unintended electrical connection known as a hillock failure or whisker failure (short circuit). Both of these situations can lead to a malfunction of the circuit.
In an ideal conductor, where atoms are arranged in a perfect lattice structure, the electrons moving through it would experience no collisions and electromigration would not occur. In real conductors, defects in the lattice structure and the random thermal vibration of the atoms about their positions causes electrons to collide with the atoms and scatter, which is the source of electrical resistance (at least in metals; see electrical conduction). Normally, the amount of momentum imparted by the relatively low-mass electrons is not enough to permanently displace the atoms. However, in high-power situations (such as with the increasing current draw and decreasing wire sizes in modern VLSI microprocessors), enough electrons bombard the atoms with enough force to become significant.
The two main factors contributing to electromigration are heat and current density. Heat, often arising from the Joule heating of the conductor, accelerates the process of electromigration by causing the atoms of the conductor to vibrate further from their ideal lattice positions, increasing the amount of electron scattering. High current density increases the number of electrons scattering against the atoms of the conductor, and hence the speed at which those atoms are displaced.
In integrated circuits, electromigration does not occur in semiconductors directly, but in the metal interconnects deposited onto them (see semiconductor device fabrication).
See Also: Integrated circuit, semiconductor, electromagnetism, electrical conduction
Weblinks
Literature
- Black, J.R.: Metallization Failures In Integrated Circuits. RADC Technical Report, Vol. TR-68-243, October 1968.
- Black, J.R.: Electromigration-A Brief Survey and Some Recent Results. IEEE Transactions On Electron Devices, Vol. ED-16(No. 4):p. 338 347, april 1969.
- Black, J.R.: Electromigration Failure Modes in Aluminium Metallization for Semiconductor Devices. Proceedings of the IEEE, Vol. 57(No. 9):p. 1587 1594, September 1969.
- Ho, P.S.: Basic problems for EM in VLSI applications. Proc. of the IEEE, IRPS:p. 288 291, 1982.
- Gardner, D.S.: Interconnection and EM scaling theory. IEEE Transaction on electron devices, Vol. ED-34(No. 3), March 1987.
- Wiley Encyclopedia of Electrical and Electronics Engineering. Department of Electrical and Computer Engineering University of Wisconsin Madison, 1999.
- Christou, Aris: Elektromigration and Electronic Device Degradation. John Whiley & Sons, 1994.
- Ghate, P.B.: Electromigration-Induced Failures in VLSI Interconnects. IEEE Conference Publication, Vol. 20:p 292 299, March 1982.
- B.D. Knowlton, C.V. Thompson: Simulation of temperature and current density scaling of the electromigration-limited reliability of near-bamboo interconnects. Material Research Society, Vol. 13(No. 5), 1998.
- Changsup Ryu, Kee-Won, ...: Microstructure and Reliability of Copper Interconnects. IEEE Transactions on Electron Devices, Vol. 46(No. 6):1113 1119, June 1999.
- H.C. Louie Liu, S.P. Murarka: Modeling of Temperature Increase Due to Joule Heating During Elektromigration Measurements. Center for Integrated Electronics and Electronics Manufacturing, Mat. Res. Soc. Symp Proc. Vol. 427:p. 113 119.
- K. Banerjee, A. Mehrotra: Global (Interconnect) Warming. Circuits and Devices, Seiten p 16 32, September 2001.