We studied the feasibility of using vacuum ultraviolet (VUV) treatment as a surface improvement technique with Au-Au flip-chip bonding. For finepitch electrical interconnections in three-dimensional (3D) stack applications, robust and reliable bonding is desirable; in this case, surface modification treatment is needed before the bonding process. A VUV surface treatment was used to remove organic contaminants. Samples of electroplated Au pads were examined by X-ray photoelectron spectroscopy (XPS) to evaluate the chemical composition of the Au surfaces. The XPS results revealed that the carbon-based contaminants on the surface were removed by the VUV treatment. The shear strength of the bonded sample was also improved. The average shear strength of a bump with VUV treatment is 1.6 times larger than that of a bump without VUV treatment. Cross-sectional scanning electron microscopy (SEM) images of the bonded samples confirmed the absence of voids and cracks. The results show that VUV treatment has clear effects on Au-Au flip-chip bonding. © 2010 The Japan Society of Applied Physics.