A little background, why this is interesting. Many years ago, when I studied semiconductor engineering we learnt that copper is poison for the CMOS process and that the fab had to be kept free from copper contamination.

The reason for this is that copper, contrary to aluminium, diffuses very quickly in silicon and silicon dioxide.

Of course there are reasons why you might prefer copper over aluminium as an interconnect. So a process has been developed to make this possible and safe. The process is called Damascene and the article is about significant improvements there.