Silicon Nodules: Cause of Aluminum Stress Cracking and Bond Lifting

Silicon Nodules

Silicon Nodules 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:

1) 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.

2) 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.

3) As the metal is cooled down after the alloying process, more silicon atoms separate from and come out of the aluminum solution.

4) The 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.

5) Silicon nodules grow bigger with long exposure to elevated temperatures. Studies have shown that silicon nodules can attain diameters greater than 1 micron.

6) The 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.'

7) Aluminum stress cracking is aggravated by factors other than silicon nodules. During 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.

8) 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.

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