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Fluxless bonding can be used for fine-pitch low-solder-volume interconnections for three-dimensional large-scale integrated-circuit (3D-LSI) applications. Surface treatments with hydrogen radicals, formic acid, vacuum ultraviolet (VUV), and Ar plasma were evaluated as candidate methods for fluxless bonding. Three-μm-thick Sn solders were evaluated for intermetallic-compound (IMC) bonding of 3D integration as a target material for fluxless bonding. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), time-of-flight secondary ion mass spectrometry (TOF-SIMS), a scanning electron microscope (SEM), and a focused ion beam scanning ion microscope (FIB-SIM) were used to examine the samples. The experiments shows solder oxides and organic contaminants on the surfaces of the micro-bumps were most effectively eliminated without flux by hydrogen radical treatment among various treatments we evaluated. Bonding strength was also improved by the hydrogen radical treatment, since the shear strength was more than 50 times stronger than that of the untreated samples.