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the idea is that Cu diffuse into Si and oxide , so u need some barrier to use Cu, also Cu is easily oxidized also u have to use additive technique to use Cu interconnects like single or double damascence "cause u cannot RIE the Cu"
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however i think modern echnology tends to use Cu and try to use some techniques to overcome the difficulties in using it
Re: why aluminium is used instead of copper in ic fabricatio
copper diffuses into Si, so you have to put a barrier layer to prevent that (usually TiN)
and after polishing for the surface of copper, another layer is put to totally encapsulate the copper
this technique is called Damascene (taken from the name of the city of damas, the capital of Syria, since they do many metal carvings there)
for aluminium,it also has the diffusion problem for temperatures more than 450° (spiking) and another problem:electromigration
for the first problem, Al can be replaced by AlSi
for the second problem, AlCu can be used and to prevent diffusion then a barrier layer of TiN is used