When transistors are formed on a silicon wafer, traditional manufacturing methods join them in an electrical circuit by depositing tiny aluminum tracks between them. The semiconductor industry has used aluminum wiring on chips for over 30 years. The shrinking size of semiconductors has made aluminum more and more problematic, since it resists the flow of electricity, as wires are made ever thinner and narrower.
Copper would be the ideal electrical conductor to use, but up to now it has been notoriously difficult to control the precise position of copper atoms during the manufacture of silicon chips. This is because copper and silicon atoms basically do not like each other. When copper atoms are deposited onto silicon, they refuse to stay put and instead run about the surface of the silicon, ending up just about everywhere other than where you planned to place them.
Not only does copper rapidly diffuse into the silicon substrate in which the transistors are formed (like ink on wet paper), but it changes the electrical properties of silicon in such a way as to prevent the transistors from functioning. In traditional manufacturing copper (along with gold and silver) was considered to be a killer to semiconductor devices. (Hence the fact that in most chip fabrication plants jewelry is strictly forbidden!) For this reason, aluminum has traditionally been used, since it obediently stays where it’s told.