TOTAL-REFLECTION PIXE (TPIXE) AND RBS FOR SURFACE-ANALYSIS

Proton beams with an energy of 2.5 MeV and alpha beams with an energy of 4.8 MeV at small incident angles (0-35 mrad) were used to analyse surfaces by detecting x-rays (PIXE) and backscattered protons (RBS). The specimens consisted of a flat substrate, such as Si wafers or quartz, with a thin evaporated coating. PIXE detection limits were investigated as a function of angle. A minimum detection limit for Au, using L x-rays, was found to be 15% of an atomic layer using an incident angle of 16 mrad and 2.5 MeV protons. For angles between 4 and 20 mrad, both the PIXE and RBS yields drop drastically owing to reflection of the incoming ions. This was confirmed by ion trajectory calculations. At very small angle sof a few mrad, was observed that the RBS yield drops faster than the x-ray yield, which indicates surface channelling.