An anti-oxidant Cu layer was achieved by remote mode N2 plasma. Remote mode plasma treatment offers the advantages of having no defect formation, such as pinholes, by energetic ions. In this study, an activated Cu surface by Ar plasma chemically reacted with N free radicals to evenly form Cu nitride passivation over the entire Cu surface. According to chemical state analysis using XPS, Cu oxidation was effectively prevented in air, and the thickness of the Cu nitride passivation was within 3 nm. Based on statistical analysis using the DOE technique with N2 plasma variables, namely, RF power, working pressure, and plasma treatment time, we experimentally demonstrated that a lower RF power is the most effective for forming uniform Cu nitride passivation because of a lower plasma density. When the N2 plasma density reached approximately 109 cm−3 in which the remote mode was generated, high energy electrons in the plasma were significantly reduced and the amount of oxygen detected on the Cu surface was minimized. Finally, low temperature (300 °C) Cu–Cu bonding was performed with a pair of the anti-oxidant Cu layers formed by the remote mode N2 plasma. Cu atomic diffusion with new grains was observed across the bonded interface indicating significantly improved bonding quality over bare Cu–Cu bonding.
The technology trends of next generation electronic packaging are moving toward heterogeneous 3D packaging systems. One of the key processes of 3D packaging system is Cu-to-Cu bonding, which is highly dependent on the planarized, activated, and oxygen-free Cu surface. A two-step plasma treatment is studied to form a Cu surface that does not react with oxygen and improves the Cu bonding interface quality at low bonding temperature (300 °C). In this study, the effects of two-step plasma treatment on both sputtered and electroplated Cu surfaces were evaluated through structural, chemical, and electrical analysis. The Cu bonding interface was studied by scanning acoustic tomography analysis after the thermocompression bonding process. Both sputtered and electroplated Cu thin films had the preferred orientation of (111) plane, but sputtered Cu exhibited larger grains than the electroplated Cu. As a result, the roughness of sputtered Cu was lower, and the resistivity was higher than that of electroplated Cu. Based on X-ray photoelectron spectroscopy analysis, the sputtered Cu formed more copper nitrides and fewer copper oxides than the electroplated Cu. A significant improvement in bonding quality at the Cu bonded interface was observed in sputtered Cu.
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