Re: CPU interconnects: beyond copper?

Dear Ed Powell:

You have asked about the use of copper for electrical interconnects on 
chips, and why not use gold or silver. Let us look at the 
electrical resistivity of the metals gold, silver, aluminum, and copper:
    
    Metal          Resistivity (Micro-Ohm Centimeters)@300Deg K
    Gold:............2.271       Ref: Handbook of Physics and 
    Silver...........1.629            Chemistry-CRC Publishing 1998
    Aluminum.........2.733
    Copper...........1.725

We want the smallest resistivity possible.  Note that Copper is smaller 
than gold and aluminum; and is almost as small as silver.  The 
difference between copper and silver is very small, but copper offers
the advantage of being les expensive, easier to work with metallurgically,
and so it is the natural selection which has been made.  

In addition silver suffers from serious electromigration-problems.  This 
means that when a silver conductor is carrying large electrical currents 
the atoms tend to become mobile and the grains physically move causing
mechanical problems with the metal connections.  Copper suffers much less 
from this problem.  Therefore copper has been selected as the best choice.
[Reference: Handbook for Multilevel Metallization; Wilson, Tracy, and 
Freeman; Noyes Pubilication 1993].

Richard Bersin......