The effect of thin titanium oxide layers on the wettability of thin‐film nickel metallization

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Abstract
Monolayer thickness TiO2contamination layers are shown to be detrimental to reflow soldering of thin‐film nickel, which is a part of the Al‐Ti‐Ni metallization system used in backplane metallization of silicon chips. Measurements based on real‐time flow of Pb‐Sn(10 wt.%)‐Ag(2 wt%) alloy solder show that the presence of approximately one monolayer of TiO2plus NiO on Ni raises the temperature at which solder begins to flow in the forming gas (10% H2) by 100°C over that of a clean NiO native oxide layer on Ni. Surface chemistry was determined using x‐ray photoelectron spectroscopy and Auger electron spectroscopy. Solder pool area measurements as a function of solder temperature were used to determine qualitatively the wettability of a surface. Poorly wetting surfaces show a negative slope. In this study, the slope was negative at equivalent Ti coverages, as determined by Rutherford back‐scattering analysis, of 0.65 nm or greater. The wettability of the surface appears to be controlled by TiO2at 0.65 nm or above and may be related to the ability of the forming gas to reduce the surface oxide sufficiently at the soldering temperature to permit flow. Therefore, small amounts of Ti that may diffuse through the nickel layer of the three‐metal thin‐film metallization system can have significant effects on the wettability of the Ni surface.