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. // An ion is an atom or a group of atoms (a chemical substance), or subatomic particle, with a net electric charge. ... In science and engineering, conductors are materials that contain movable charges of electricity. ... The field of electronics is the study and use of systems that operate by controlling the flow of electrons (or other charge carriers) in devices such as thermionic valves and semiconductors. ... Integrated circuit showing memory blocks, logic and input/output pads around the periphery A monolithic integrated circuit (also known as IC, microchip, silicon chip, computer chip or chip) is a miniaturized electronic circuit (consisting mainly of semiconductor devices, as well as passive components) which has been manufactured in the surface...

SEM image of a failure caused by electromigration in a copper interconnect. The passivation has been removed by RIE and HF From the german Wikipedia by Patrick-Emil Zörner File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ... Low temperature SEM magnification series for a snow crystal. ... Reactive ion etching (RIE) is a technology using plasma to etch material deposited on wafers. ... Flash point -37. ... enlarge image

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. 1966 (MCMLXVI) was a common year starting on Saturday (the link is to a full 1966 calendar). ... A micrometre (American spelling: micrometer), symbol µm, is an SI unit of length. ... A nanometre (American spelling: nanometer) is 1. ...

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. Space exploration is the physical exploration of outer space: the technologies, science, and politics regarding space endeavors. ... Armed forces are the military forces of a state. ... An anti-lock braking system (commonly known as ABS, from the German name Antiblockiersystem given to it by its inventors at Bosch) is a system on motor vehicles which prevents the wheels from locking while braking. ... By the mid 20th century humans had achieved a level of technological mastery sufficient to leave the surface of the planet for the first time and explore space. ... Economics (deriving from the Greek words οίκω [okos], house, and νέμω [nemo], rules hence household management) is the social science that studies the allocation of scarce resources to satisfy unlimited wants. ...

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. In mathematics, extrapolation is a type of interpolation. ...

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.) Very-large-scale integration (VLSI) of systems of transistor-based circuits into integrated circuits on a single chip first occurred in the 1980s as part of the semiconductor and communication technologies that were being developed. ... Semiconductor device fabrication is the process used to create chips, the integrated circuits that are present in everyday electrical and electronic devices. ... General Name, Symbol, Number copper, Cu, 29 Chemical series transition metals Group, Period, Block 11, 4, d Appearance metallic brown Atomic mass 63. ... General Name, Symbol, Number aluminium, Al, 13 Chemical series poor metals Group, Period, Block 13, 3, p Appearance silvery Atomic mass 26. ...

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. Electronic design automation (EDA) is the category of tools for designing and producing electronic systems ranging from printed circuit boards (PCBs) to integrated circuits. ...

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. Western Digital Corporation NYSE: WDC (often abbreviated to WD) is a manufacturer of a large proportion of the worlds hard disks, and has a long history in the electronics industry as an IC maker and a storage products company. ...

Overclocking of processors, especially when using higher than nominal voltage, causes electromigration between their transistors and significantly shortens the chips' lifetime. To meet Wikipedias quality standards, this article or section may require cleanup. ...

Electromigration can be a cause of degradation in some power semiconductor devices such as low voltage power MOSFETs, in which the lateral current flow through the source contact metallisation (often aluminium) can reach the critical current densities during overload conditions. The degradation of the aluminium layer causes an increase in on-state resistance, and can eventually lead to complete failure. Power semiconductor devices are electronic semiconductor devices used as switches or rectifiers in high-power electronic circuits (power converters for example). ... Two power MOSFETs in the surface-mount package D2PAK. Each of these component can sustain a blocking voltage of 30 volts and a continuous current of 120 amperes. ...

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. Annealing, in metallurgy and materials science, is a heat treatment wherein the microstructure of a material is altered, causing changes in its properties such as strength and hardness. ... Passivation is the process of making a material passive in relation to another material prior to using the materials together. ... Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ... City lights viewed in a motion blurred exposure. ...

Forces on ions in an electrical field

Two forces affect ionized atoms in a conductor. The direct electrostatic force F_e as a result from the electric field therefore having the same direction. The force from the exchange of momentum with other charge carriers F_p showing toward the flow of charge carriers. In metallic conductors F_p is caused by a so-called "electron wind" or "Ion wind". In physics, a force is anything that causes a free body with mass to accelerate. ... Properties In chemistry and physics, an atom (Greek άτομον meaning indivisible) is the smallest possible particle of a chemical element that retains its chemical properties. ... Electrostatics is the branch of physics that deals with the forces exerted by a static (i. ... Charge carrier denotes in physics a free (mobile, unbound) particle carrying an electric charge. ... The Biefeld-Brown effect is an electrokinetic effect that was discovered by Thomas Townsend Brown (USA) and Dr. Paul Alfred Biefeld (CH). ...

The resulting force F_res 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. In classical mechanics momentum (pl. ... Open circuit can mean:- In electronics, where there is nothing connected to a load and no current can flow. ... For alternate meanings see Short circuit (disambiguation) A short circuit (sometimes known as simply a short) is a fault whereby electricity moves through a circuit in an unintended path, usually due to a connection forming where none was expected. ...

Failure mechanisms

Diffusion mechanisms

In a homogeneous crystalline structure, because of the uniform lattice structure of the metal ions, there is hardly any momentum transfer between the conduction electrons and the metal ions. However, this symmetry does not exist at the grain boundaries and material interfaces, and so here momentum is transferred much more vigorously. Since the metal ions in these regions are bonded more weakly than in a regular crystal lattice, once the electron wind has reached a certain strength, atoms become separated from the grain boundaries and are transported in the direction of the current. This direction is also influenced by the grain boundary itself, because atoms tend to move along grain boundaries.

Diffusion processes caused by electromigration can be divided into grain boundary diffusion, bulk diffusion and surface diffusion. In general, grain boundary diffusion is the major electromigration process in aluminum wires, whereas surface diffusion is dominant in copper interconnects.

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. The ordinary meaning of lattice is the basis for several technical usages A cherry lattice pastry A mathematical lattice that is a type of partially ordered set. ... In particle physics, scattering is a class of phenomena by which particles are deflected by collisions with other particles. ... Electrical conduction is the movement of electrically charged particles through matter. ... Mass is a property of a physical object that quantifies the amount of matter and energy it contains. ... Very-large-scale integration (VLSI) of systems of transistor-based circuits into integrated circuits on a single chip first occurred in the 1980s as part of the semiconductor and communication technologies that were being developed. ... Microprocessors, including an Intel 80486DX2 and an Intel 80386. ... The ordinary meaning of lattice is the basis for several technical usages A cherry lattice pastry A mathematical lattice that is a type of partially ordered set. ... In particle physics, scattering is a class of phenomena by which particles are deflected by collisions with other particles. ... In electricity, current refers to electric current, which is the flow of electric charge. ...

In integrated circuits, electromigration does not occur in semiconductors directly, but in the metal interconnects deposited onto them (see semiconductor device fabrication). A semiconductor is a material with an electrical conductivity that is intermediate between that of an insulator and a conductor. ... Nasas Glenn Research Center clean room. ...

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. Localized increase of current density is known as current crowding. In electricity, current refers to electric current, which is the flow of electric charge. ... In electronics, and in physics more broadly, Joule heating refers to the increase in temperature of a conductor as a result of resistance to an electrical current flowing through it. ... Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... Current crowding (also current crowding effect, or CCE) is a nonhomogenous distribution of current density through a conductor or semiconductor, especially at the vicinity of the contacts and over the PN junctions. ...

Electromigration-aware design

Electromigration reliability of a wire (Black's equation)

At the end of the 1960s J. R. Black developed an empirical model to estimate the MTTF (mean time to failure) of a wire, taking electromigration into consideration: MTTF = A (J^-n) e^(E_a / (k T)). Here A is a constant based on the cross-sectional area of the interconnect, J is the current density, E_a is the activation energy (e.g. 0.7 eV for grain boundary diffusion in aluminum), k is the Boltzmann constant, T is the temperature and n a scaling factor (usually set to 2 according to Black). It is clear that current density J and (less so) the temperature T are deciding factors in the design process that affect electromigration.

The temperature of the conductor appears in the exponent, i.e. it strongly affects the MTTF of the interconnect. For an interconnect to remain reliable in rising temperatures, the maximum tolerable current density of the conductor must necessarily decrease.

Wire material

It is known that pure copper used for Cu-metallization is more electromigration-robust than aluminum. Copper wires can withstand approximately five times more current density than aluminum wires while assuming similar reliability requirements. This is mainly due to the higher electromigration activation energy levels of copper caused by its superior electrical and thermal conductivity as well as its higher melting point.

Bamboo structure and metal slotting

It is obvious that a wider wire results in smaller current density and, hence, less likelihood of electromigration. However, if you reduce wire width to below the average grain size of the wire material, the resistance to electromigration increases, despite an increase in current density. This apparent contradiction is caused by the position of the grain boundaries, which in such narrow wires as in a bamboo structure lie perpendicular to the width of the whole wire. Because the grain boundaries in these so-called “bamboo structures” are at right angles to the current flow, the boundary diffusion factor is excluded, and material transport is correspondingly reduced.

However, the maximum wire width possible for a bamboo structure is usually too narrow for signal lines of large-magnitude currents in analog circuits or for power supply lines. In these circumstances, slotted wires are often used, whereby rectangular holes are carved in the wires. Here, the widths of the individual metal structures in between the slots lie within the area of a bamboo structure, while the resulting total width of all the metal structures meets power requirements.

Blech length

There is a lower limit for the length of the interconnect that will allow electromigration to occur. It is known as “Blech length”, and any wire that has a length below this limit will not fail by electromigration. Here, a mechanical stress buildup causes a reversed migration process which reduces or even compensates the effective material flow towards the anode. The Blech length must be considered when designing test structures for electromigration.

Via arrangements and corner bends

Particular attention must be paid to vias and contact holes, because generally the ampacity of a (tungsten) via is less than that of a metal wire of the same width. Hence multiple vias are often used, whereby the geometry of the via array is very significant: Multiple vias must be organized such that the resulting current flow is distributed as evenly as possible through all the vias.

Attention must also be paid to bends in interconnects. In particular, 90-degree corner bends must be avoided, since the current density in such bends is significantly higher than that in oblique angles (e.g., 135 degrees).

See also: Integrated circuit, semiconductor, electromagnetism, electrical conduction Integrated circuit showing memory blocks, logic and input/output pads around the periphery A monolithic integrated circuit (also known as IC, microchip, silicon chip, computer chip or chip) is a miniaturized electronic circuit (consisting mainly of semiconductor devices, as well as passive components) which has been manufactured in the surface... A semiconductor is a material with an electrical conductivity that is intermediate between that of an insulator and a conductor. ... Electromagnetism is the physics of the electromagnetic field: a field, encompassing all of space, which exerts a force on those particles that possess the property of electric charge, and is in turn affected by the presence and motion of such particles. ... Electrical conduction is the movement of electrically charged particles through matter. ...

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.
• J. Lienig, G. Jerke: Current-Driven Wire Planning for Electromigration Avoidance in Analog Circuits. Proceedings of the 8th Asia and South Pacific Design Automation Conference (ASP-DAC), 2003, pp. 783-788.
• G. Jerke, J. Lienig: Hierarchical Current Density Verification in Arbitrarily Shaped Metallization Patterns of Analog Circuits. IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems, Vol. 23, No. 1, pp. 80-90, January 2004.
• 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.
• J. Lienig: Introduction to Electromigration-Aware Physical Design. (Download Paper) Proceedings of the International Symposium on Physical Design (ISPD) 2006, pp. 39-46, April 2006.

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.

EIA may refer to the following: Electronic Industries Alliance Energy Information Administration Environmental Impact Assessment Enzyme-Linked Immunosorbent Assay Equity Indexed Annuity Exercise-induced anaphylaxis This is a disambiguation page, a list of pages that otherwise might share the same title. ... JEDEC stands for Joint Electron Device Engineering Council and is the semiconductor engineering standardization body of the Electronic Industries Alliance (EIA), a trade association that represents all areas of the electronics industry. ... EIA may refer to the following: Electronic Industries Alliance Energy Information Administration Environmental Impact Assessment Enzyme-Linked Immunosorbent Assay Equity Indexed Annuity Exercise-induced anaphylaxis This is a disambiguation page, a list of pages that otherwise might share the same title. ... JEDEC stands for Joint Electron Device Engineering Council and is the semiconductor engineering standardization body of the Electronic Industries Alliance (EIA), a trade association that represents all areas of the electronics industry. ...

External links

• [1] What is Electromigration?, Computer Simulation Laboratory, Middle East Technical University.
• [2] Electromigration for Designers: An Introduction for the Non-Specialist, J.R. Lloyd, TechOnLine.
• Semiconductor electromigration in-depth at DWPG.Com