The Electrochemical Effects of Immersion Au on Electroless Nickel and Its Consequences on the Hermetic Reliability of a Semiconductor Device

We have analyzed the effects of the thicknesses of immersion Au (i-Au) and electroless nickel (e-Ni) on the reliability of a semiconductor device that has special constraints on the absolute thicknesses of Ni and Au layers. The electrochemical reactions involved in the deposition of i-Au over Ni involve a substitution process by which Ni atoms are replaced by Au atoms. We demonstrate that, in some cases, this reaction changes the microstructure of the Ni near the perimeter of the pad where the Ni layer overlaps the passivation layer of a semiconductor die, forming a mechanical seal to the passivation. This seal is extremely important for hermetic applications where one wants to keep moisture and contaminants away from the active pad area of the semiconductor die. We have found that extended exposure of this Ni-to-passivation interface to the Au plating chemicals damages this interface, leading to reliability issues. In this paper, we analyze the phenomena by changing the thicknesses of Ni and Au layers and by observing its effects on metal-oxide-semiconductor field-effect transistors that are subjected to accelerated reliability stress testing in an autoclave.