The Optics Laboratory

Group of Hans Hallen, North Carolina State University Physics Department

Electromigration

Electromigration can cause the formation of voids in metal current carrying wires by the transport of metal atoms in the high current density environment. Once the process begins, it is unstable, since the damaged portion of the wire has a lower cross section so even higher current density. Various schemes for controlling the electromigration including adding foreign atoms to the metal and passivating the surface.

We study electromigration of a very different type here: oxygen electromigration in the high temperature superconductor YBa₂Cu₃O₇ (YBCO). In this system, there are no structural changes. Rather, some of the oxygen atoms are weakly bound, and can move when given energy. The energy can be thermal (heating) or kinetic, as in electromigration. The oxygen moves along the Cu-O planes parallel to the ab-planes in this material:

(a) Tetragonal YBa₂Cu₃O₆ (b) orthorhombic YBa₂Cu₃O₇From The Role of Oxygen in YBa₂Cu₃O_7- by John B. Goodenough and A. Manthiram.

We are interested in the mechanism of oxygen electromigration. Possible mechanisms are that of classical electromigration,

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in which the atom is bombarded with a very large number of (low momentum) collisions with the moving electrons in an ‘electron wind’ mechanism; of an ‘electric force’ mechanism, in which the charged atom (ion) is attracted to the electrode of opposite charge through a Coulomb force,

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and by a few larger momentum transfer collisions with relatively high energy electrons in a ‘hot electron’ mechanism,

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In the latter mechanism, the energetic electrons may be capable of breaking bonds (exciting the electron in the bond through the collision) to aid the motion of atom. The latter two mechanisms are sometimes referred to as electron-induced-motion (EIM) rather than electro-migration (EM) for historical reasons.

Much work has been done on EM in YBCO in the past by Moeckley, Buhrman and collaborators. They showed that when electromigration occurs in lithographically-defined constrictions, that the electric force mechanism dominates. We have found that for the case of tunnel-electron induced migration, the hot electron mechanism wins. The direction of migration, the limiting size of the region effected by the electromigration, and other measurements (we can hold our probe out of tunneling range, apply a high bias, and show that we get no electric-field induced migration) help to pinpoint the mechanism in each case.

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