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.

Image:Leiterbahn ausfallort elektromigration.jpg

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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Contents

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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.

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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 electromigation 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 aluminum 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 fabircation, and therefore the EM research for copper interconnects is ongoing (albeit being 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.

In modern consumer electronic devices, ICs rarely fail due to electromigration effects. This is because proper semiconductor design practices incorporate the effects of electromigration into the IC's layout. Nearly all IC design houses use automated EDA tools to check and correct electromigration problems at the transistor layout-level. When operated within the manufacturer's specified temperature and voltage range, a properly designed IC-devce is more likely to fail from other (environmental) causes, such as cumulative damage from gamma-ray bombardment.

Nevertheless, there have been documented cases of product failures due to electromigration. In the late 1980s, one line of Western Digital's desktop drives suffered widespread, predictable failure 12-18 months after field usage. Using forensic analysis of the returned bad units, engineers identified improper design-rules in a third-party supplier's IC controller. By replacing the bad component with that of a different supplier, WD was able to correct the flaw, but not before significant damage to the company's reputation.

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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.

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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

<math>F_{res}=F_e-F_p=q\cdot Z^*\cdot E=q\cdot Z^*\cdot j\cdot \rho </math>


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.

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Failure mechanisms

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Diffusion mechanisms


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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. or, 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

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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.
  • Tarik Omer Ogurtani, Ersin Emre Oren: Irreversible thermodynamics of triple junctions during the intergranular void motion under the electromigration forces, Int. J. Solids Struct. 42 (13): 3918-3952, June 2005.
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Reference standards

  • EIA/JEDEC Standard EIA/JESD61: Isothermal Electromigration Test Procedure.
  • EIA/JEDEC Standard EIA/JESD63: Standard method for calculating the electromigration model parameters for current density and temperature.
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External links

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