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Because of the small dimensions of the connections
in an MOS-structure, current densities are very high and the electron flux
can enhance atomic diffusion.
·
For the Al-Cu alloy, copper atoms are moved towards
the anode by the electron flux. Copper transport is along grain boundaries
and interfaces, the lowest energy paths.
· As
copper atoms are depleated from the alloy, the aluminum atoms begin to
move creating voids in the connection and "extruding" aluminum into the
passivating SiO2.
·
These processes cause premature device failure. |
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