Determining depth profiles from angle dependent x-ray photoelectron spectroscopy: The effects of analyzer lens aperture size and geometry

Abstract

Lens apertures that reduce the solid angle of photoelectrons directed to the detector can be used to better resolve the variations in x-ray photoelectron spectroscopy (XPS) signals which occur with electron takeoff angle. The effects of aperture size and geometry on angle-dependent XPS data, and on the depth profiles calculated from the data, have been investigated. Both theoretical calculations and angle-dependent XPS experiments have been used in this study to determine the effects of apertures. Circular and slit geometry apertures varying in size from a 6° solid collection angle to a 30° solid collection angle were studied. Samples consisting of a substrate covered by a thin, uniform overlayer were examined. At low photoelectron takeoff angles, the differences between the apertures were insignificant. At high photoelectron takeoff angles, decreasing the size of the aperture improved surface sensitivity. Decreasing the width of the aperture also sharpened the transitions between layers in a generated depth profile. Even data collected through a 30° solid angle circle, however, can be used to determine useful information about the concentration depth profile of a sample. The effects of the aperture size on the estimated depth profile are similar to the effects of random noise, so improvements in angular resolution must be balanced against the signal-to-noise ratio. Slit geometry apertures offer a good compromise between increased resolution and a high signal-to-noise ratio.