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The
solubility limit of Cu in Al is low as shown by the red solvus line on the
phase diagram.
The intermetallic compound Al2Cu, the q-phase, contains a higher atomic percent
of copper. In the wafer processing steps, Cu moves in and out of solution
in the alloy and the q-phase precipitates
at the grain boundaries.
Under operating conditions when electromigration
can occur, the q-phase provides
a reservoir of Cu atoms that can enter the grain boundaries of the aluminum
and decrease Aluminum electro-migration. This process increases chip life
by decreasing the rate of void formation and insulator fracture due to aluminum
"extrusion." |
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