Most ICs use aluminum lines with a small amount of
copper (~ 1at%) for improved electromigration. When you include
the effects of the metallic liner (eg. TiW) and grain structure,
the effective resistivity for the overall line is approximately
3.5 - 4.0 micro-ohm - cm (rather than the ideal value of 2.8-2.9.
With copper lines as recently introduced in some products,
the electromigration capability is not enhanced by alloying
(copper has 1-2 orders of magnitude higher electromigration
capability compared to aluminum alloys), but the metallic liner
(eg. Ta and/or TaN) and the grain structure result in a
resistivity of approximately 1.9-2.1 micro-ohm - cm (rather
than the ideal value of 1.7).
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