A small angle incident X-ray photoelectron spectroscopy (XPS) instrument was developed to perform high sensitivity analyses on silicon surfaces by reducing background noise. Silicon surfaces were treated by a new wet cleaning process based on ultra clean technology. Cleaning effectiveness for this process was determined by small angle incident XPS measurements and spectra analyses, At each cleaning step, no elements other than silicon, oxygen, and carbon atoms were detected. The thickness of the SiO2 film formed during each step was evaluated. Based on the relative O-1s and Si-2p intensities, too much oxygen was found on the surface to attribute only to SiO2