Electromigration
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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. 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.
SEM image of a failure caused by electromigration in a copper interconnect. The passivation has been removed by RIE and HF |
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History
The phenomenon of electromigration has been known for over 100 years. 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 for whom the Black equation is named. At the time the metal interconnects in ICs were still about 10 mm 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 loss of one or more connections and causes the 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.
Using the Black equation the life span of interconnects in integrated circuits tested under "stress", that is external heating and increased current density, can be extrapolated to the life span under real conditions. Due to the relatively high life span of interconnects it would be impractical to analyse electromigration under real conditions while at the same time produce state-of-the-art chips.
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. Due to the increasing utilisation of copper instead of aluminium as interconnect material this field of research becomes ever more important for the chip industry. Because of difficulties in the technological process copper is relatively new as conductive material in the chip production process. It is however a better conductor and not as susceptible to electromigration as aluminium. Therefore the EM research for copper interconnects is still a relatively new field.
A reduction of the structure (scaling) by a factor k increases the power density proportional to k and the current density increases by k2 whereby EM is clearly strengthened.
Fundamentals
The material properties of the metal interconnects have a strong influence on the life span. The characteristics are predominantly the composition of the metal alloy and the dimensions of the conductor. The shape of the conductor, the crystallographic orientation of the grains in the metal, procedures for the layer deposition, heat treatment or annealing, characteristics of the passivation and the interface to other materials also affect the durability of the interconnects. There are also grave differences with time dependent current: direct current or different alternating current forms cause different effects.
Forces on ions in an electrical field
Two forces affect ionized atoms in a conductor. The direct electrostatic force Fe as a result from the electric field therefore having the same direction. The force from the exchange of momentum with other charge carriers Fp showing toward the flow of charge carriers. In metallic conductors Fp is caused by a so-called "electron wind".
The resulting force Fres on as activated ion in the electrical field is
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 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.
Failure mechanisms
Diffusion mechanisms
Thermal effects
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.
Joule heating
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).
Electromigration is exacerbated by high current densities and the Joule heating of the conductor (see electrical resistance), and can lead to eventual failure of electrical components.
See also: Integrated circuit, semiconductor, electromagnetism, electrical conduction
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.
External links
- [1] (http://www.csl.mete.metu.edu.tr/Electromigration/emig.htm) What is Electromigration?, Computer Simulation Laboratory, Middle East Technical University.
- [2] (http://www.techonline.com/community/ed_resource/feature_article/20421) Electromigration for Designers: An Introduction for the Non-Specialist, J.R. Lloyd, TechOnLine.de:Elektromigration