An apparatus for the study of low energy ion backscattering from solid targets

Abstract

When energetic ions strike the surface of a solid target, a fraction, depending on the ion energy and other parameters, penetrates the surface, whilst the remainder are reflected. Experimental studies of ion reflection have shown that this phenomenon is extremely sensitive to both the structure and composition of the surface and measurements of the energy and angular distributions of backscattered ions, particularly at high energies, have been extensively employed to investigate such features as the composition of the surface and near surface layers of a solid, the location of impurity atoms in a lattice, and the orientation and crystallinity of a solid. In the incident ion energy range below about 5 keV, in which the backscattering phenomena are most sensitive to the condition of the surface, the considerable influence of experimental parameters such as target preparation technique and residual gas pressure in the target environment makes meaningful study difficult and the present paper describes an apparatus that has been constructed to carry out a comprehensive study of the energy and angular distributions of low energy (50 eV-2 keV) rare gas ions incident on the surfaces of solid targets. This type of study requires that a well-collimated, mono-energetic, mass analysed ion beam should be incident on an atomically clean and smooth target surface and that both the target and detector should be movable inside the ultra-high vacuum scattering chamber. This paper, in addition to briefly reviewing the surface probing techniques currently in use, describes in detail the ion gun, manipulation systems and vacuum apparatus that have been used in the construction of the low energy ion backscattering system.