Evaluation of depth resolution with time-of-flight medium energy backscattering

The energy and depth resolution of Vanderbilt's time-of-flight medium energy backscattering (TOF-MEBS) system has been evaluated as a function of target tilt and beam energy. The target tilt and beam energy for optimizing depth resolution of He+ scattering at a depth of 55 A was determined. Calculations were performed with DEPTH to determine how various energy spread factors contributed to depth resolution. The influence of detector position on depth resolution was also calculated. These results showed that depth resolution improved as the angle between the incident beam and detector decreased. However, the optimum target tilt angle did not change with detector position. Depth resolution measurements indicated an optimum target tilt angle of 6 deg. For more grazing angles, depth resolution degrades due to increased energy spread from multiple scattering, geometric spread and energy straggling. Due to the energy dependence of the TOF spectrometer, the beam energy for optimizing depth resolution is much lower than the stopping maximum of He+ ions in Al: ∼170 keV as opposed to ∼500 keV