are silicon aggregates that come out of silicon-doped aluminum metal
lines, causing the device to fail in several ways. Here are some
key points about silicon nodules:
The aluminum metal lines used in die circuits are doped with silicon
atoms in a very controlled manner to enhance their properties. A
typical process involves sintering or alloying at 400-450 deg C, wherein
the aluminum lines are doped with about 1-2% silicon.
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During this alloying process, not all of the silicon dopants are
dissolved in the aluminum metal lines. Instead of going into the
solution, some Si atoms remain as silicon precipitates. Only about
0.4% silicon dissolves in the aluminum solution.
the metal is cooled down after the alloying process, more silicon atoms
separate from and come out of the aluminum solution.
elemental silicon precipitates existing in the metal (as discussed in #
2) act as nucleation sites for silicon atoms that emerge from the
solution during the cool-down phase. The silicon atoms that
nucleate eventually form larger aggregates of silicon that are known as
Silicon nodules grow bigger with long exposure to elevated temperatures.
Studies have shown that silicon nodules can attain diameters greater
than 1 micron.
growth of silicon nodules to large diameters exert stress on the metal
lines. In fact, narrow metal lines, i.e., those whose widths are
less than 3 microns, can fracture and become open in the presence of
silicon nodules with diameters greater than 1 micron. This
phenomenon is often referred to as
Aluminum stress cracking is aggravated by factors other than silicon
sputter-deposition of the aluminum, for instance, nitrogen
may be trapped within the layer, producing additional strain on the
aluminum. Differences among the coefficients of thermal expansion
of silicon, silicon dioxide, and aluminum also result in stresses
within the die circuit that can aggravate aluminum cracking.
Aside from aluminum stress cracking, the formation of silicon nodules on
bond pads also impede wire bonding. As a result, excessive silicon
nodule formation on bond pads has been confirmed to cause
ball bond lifting
issues as well.
Die-related Failure Mechanisms;
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