Topography and field effects in the quantitative analysis of conductive surfaces using ToF-SIMS

A detailed study is presented of the effects of surface topography on ToF-SIMS images, using experimental results from model conducting fibres, consisting of gold wires of diameters in the range of 33–125 μm mounted onto silicon wafers, and ion optics simulations using SIMION. Chemical analysis over the whole of the fibre surface is not possible, and typically only the central 15 μm of the fibre can be observed. Signals from the substrate are also shadowed by the presence of the fibre. Both effects are caused by the distortion of the extraction field around these conducting samples. We provide clear guidance to practical analysts for the reduction of the topographic field effects, by the use of a lower extractor voltage and an extraction delay. In particular, the effects of extraction delay on ion intensities, mass resolution and time-of-flight are studied in detail. In addition, for large incidence angles of ≥55°, a fraction of the Bi+ primary ions are scattered from the fibre target, emitting secondary ions from the substrate which are recorded at the location of the fibre. A method to diagnose this effect is given.