Der Widerstand eines Atoms : Elektromigration im Miniaturformat

Mit dem immer weiter voranschreitenden Trend, Computer bei gleichzeitig zunehmender Leistungsfähigkeit schrumpfen zu lassen, wachsen die Herausforderungen an ihre Mini-Bauelemente. So sind die Milliarden von Transistoren auf einem Chip mit elektrischen Leiterbahnen aus Metallen wie Aluminium oder Kupfer verbunden. Aufgrund ihres schrumpfenden Querschnitts sind diese durch die so genannte Elektromigration gefährdet, die durch Streuung der Leitungselektronen an rauen Oberflächen zusätzlich verstärkt wird.

The electric resistance of very thin wires or films is affected by their surface. With decreasing dimensions electrons are not only scattered at bulk defects and lattice vibrations but also from defects at the surface as roughness, adsorbates or even single atoms. This effect becomes dominant when the diameter of a wire or the thickness of a film becomes smaller than the mean free path of the electrons between two scattering events which determines the electric resistance of a thick wire. We used ultra thin crystalline films of Silver on quasi insulating Silicon substrates in ultra high vacuum to study these effects under well controlled conditions. During additional deposition at temperatures below -100°C the surface becomes very rough and the resistance increases due to enhanced scattering of the electrons. This effect is even more drastic when we use Gold atoms, which forms an alloy with the silver film where electrons are strongly scattered. On ultra thin Bismuth films even the deposition of single isolated Bi atoms gave rise to a strong increase of the film resistance. This experiment allows determining the precise cross section of scattering for single atoms.



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