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Use of Coherency Strains in Epitaxial Multilayers to Inhibit Circuit Failure by Electromigration

IP.com Disclosure Number: IPCOM000079145D
Original Publication Date: 1973-May-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 37K

Publishing Venue

IBM

Related People

Ho, PS: AUTHOR [+2]

Abstract

The passage of large current densities through conducting electrical leads frequently results in void formation. These voids grow, if the passage of current is continued, and ultimately destroy the continuity of the leads. It is believed that the formation of the voids occurs, because the current gives rise to a vacancy excess in certain parts of the leads. Therefore, void formation and thus the failure of leads can be avoided if the build-up of a vacancy excess is prevented.

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Use of Coherency Strains in Epitaxial Multilayers to Inhibit Circuit Failure by Electromigration

The passage of large current densities through conducting electrical leads frequently results in void formation. These voids grow, if the passage of current is continued, and ultimately destroy the continuity of the leads. It is believed that the formation of the voids occurs, because the current gives rise to a vacancy excess in certain parts of the leads. Therefore, void formation and thus the failure of leads can be avoided if the build-up of a vacancy excess is prevented.

Described is a method for preventing such build-up of vacancy excess. It can be applied to both polycrystalline and to single-crystal leads. The method consists of preparing multiple metal layers as illustrated in Fig. 1, which is a section through a stripe made of alternating layers of metals "a" and "b." The choice of "a" and "b" should be carefully made. They must grow epitaxially and coherently (or psuedomorphically) on one another. This means that a grain of "a" should have the same orientation as a grain of "b" which is in contact with it. Also, the two grains should strain elastically to bring their lattices into register at the interface between them. For these conditions to be fulfilled, the lattice parameters of "a" and "b" should differ by less than about 6% and strong bonds should be formed across interfaces between them.

Epitaxial and coherent growth of "a" on "b," does not mean that the layers of "a" and "b" must be single crystals, although they could be. A polycrystalline coherent multilayer is illustrated in Fig. 2, wherein the interfaces of "a" and "b" are perfect matchings of atomic planes which meet the interfaces at right angles. A multilayer as shown in Fig. 2, resists electromigration induced failure by void formation. If the lattice parameters of "a" and "b" differ by a few percent, and there are coherent interfaces between them, there are tensile and compressive stresses in alternate layers, as indicated in Fig. 3.

If a vacancy excess is created by the passage of current, the compressive stress in "a" assists in the nucleation of prismatic dislocation loops in "a", as illustrated in Fig. 4. The loops in "a" form by the absorption of vacancies. They remove part of the excess vacancies present in "a." This removal leads to a flow of vacancies from "b" to "a" and further precipitation of vacancies in "a." The loops therefore remove excess vacancies from both "a" and "b" layers.

The concentration of vacancies that can be removed by this process is very large. It is approximately equal to twice the misfit between the stress-free lattice parameters of the two metals and may be as large as approx.= 0.1.

The precipitation of vacancies at prismatic dislocations does not involve or lead to the formation of a void. Thus, the elimination of a vacancy excess by the process illustrated in Fig. 4 will not lead to failure of the stripe.

A detailed discuss...