Paper PS-WeM11
Surface Cleaning for Enhanced Adhesion to Packaging Surfaces: Plasma and Free Radical Chemistries

Wednesday, October 30, 2013, 11:20 am, Room 104 C

In microelectronics device packaging, the removal of adventitious carbon and other contaminants, and the formation of a hydrophilic surface, are key steps to ensure adequate bonding to epoxy. This surface cleaning is commonly accomplished by oxygen plasma treatment, but the effects of such treatment on contaminant removal, surface composition and surface hydrophilicity are not well-understood. In-situ x-ray photoelectron spectroscopy (XPS) indicates that He, O₂ and NH₃ capacitively-coupled plasmas are equally effective at removing adventitious carbon from silicon nitride (SiN_x) and Si oxynitride (SiO_xN_y) surfaces. O₂ or He plasma treatment, however, results in initial oxidation of SiN_x or SiO_xN_y surfaces, and eventual formation of a SiO₂-like overlayer. In contrast, exposure of either surface to thermal atomic O yields carbon removal, but only surface oxidation of SiN_x, and no further change in SiO_xN_y surface composition. The data demonstrate that silica overlayer growth involves reaction of background O₂ or H₂O with reactive sites induced by ions or vacuum ultraviolet photons present in the plasma. In contrast, the exposure to NH₃ plasma results in negligible surface oxidation of the SiN_x or SiO_xN_y surface, but with effective removal of the adventitious carbon. There is no formation of a SiO₂ overlayer even at higher exposure times. This indicates that the presence of H passivates or reduces reactive sites for SiO₂ formation at the Si_xN or SiO_xN_y surfaces. Ex-situ contact angle measurements show that SiN_x and SiO_xN_y surfaces exposed to oxygen plasma are initially more hydrophilic than surfaces exposed to NH₃ plasma, indicating that the O₂ plasma-induced SiO₂ overlayer is highly reactive towards ambient. At longer ambient exposures (> ~ 10 hours), however, surfaces treated by either method exhibit similar steady state contact angles, correlated with rapid uptake of adventitious C, as determined by XPS. These data demonstrate that O₂ or even He plasma cleaning of SiN_x or SiO_xN_y surfaces results in unintended SiO₂ overlayer formation and that the use of NH₃ plasma can clean the substrates efficiently without the SiO₂ formation. The data also demonstrate that the hydrophilicity of such surfaces decreases rapidly upon ambient exposure, suggesting the potential advantage of in-situ surface passivation following plasma cleaning.

Acknowledgement: This work was supported by the Semiconductor Research Corporation under Task ID 2071.016. The authors thank Texas Instruments for providing the samplesand the contact angle measurement capability, and thank Andy Burnett and Varughese Matthew for stimulating discussions.