BS ISO 16531:2013 pdf download – Surface chemical analysis — Depth profiling — Methods for ion beam alignment and the associated measurement of current or current density for depth profiling in AES and XPS
5 Ion beam alignment methods
5.1 General This International Standard describes not all but seven simple methods for ion beam alignment, easily applied. These methods and a summary of their advantages are set out in Table 1. Also indicated are which methods are best for ion beam current or current density measurement. Each method has different advantages and requires different instrumental capabilities. The analyst needs to select the method based on requirements and equipment capabilities. Some issues depend on the raster size of the ion beam. A small raster is good, since little material is consumed or sputter deposited in the spectrometer. Additionally, for industrial samples, the material to be profiled may only occupy a small area. A very small raster is possible in AES where the electron beam is small and some users may deliberately use higher ion beam energies where ion beams tend to be better focused to obtain small sputtered areas with a faster sputtering rate. In these cases, and for systems with small-area XPS analysis, particular care needs to be taken with alignment. For broader ion beams, such as for some XPS instruments, the alignment accuracy may be more relaxed. If more than one method is suitable, tests with each will show which is most convenient for the sputtering conditions intended. The effects of good and poor ion beam alignment in sputter depth profiling are illustrated in Annex A. General precautions are given in 5.2. If analysts wish to align the beam and measure the ion beam current or current density, or change the ion beam energy, they can choose one of the two methods that use a Faraday cup. The alignment methods specified in 5.3 and 5.4 are those using Faraday cups with a circular aperture and an elliptical aperture, respectively; whereas Annex B introduces a method using co-axial electrodes giving measurements proportional to the ion current or current density. If analysts wish to align the beam and not measure the ion current or current density, they can align the beam using images from secondary electrons or ions excited by ions or primary electrons, or an optical image, or by ion-induced luminescence, using the methods specified in 5.5, 5.6, 5.7 and 5.8, respectively. The method chosen depends on the capability and facility of the instrument used. Clause 6 describes when to conduct the ion beam alignment.
5.2 Important issues to be considered prior to ion beam alignment
5.2.1 For consistent, high quality analysis, the analytical probe beam, whether stationary or rastered over an area, and the electron energy analyser axis shall be aligned at the analysis position. The intersection of these two axes with the specimen surface shall also define the centre for the sputtered area for sputter depth profiling.
5.2.2 It is important that the analysis area be located in the central, uniform region of the ion beam irradiation area. This is shown in Figure 1.  It is useful to know the sputtering rate for the ion gun and sample as a function of sputtering parameters such as the ion beam energy, beam current, raster size, and so on or their equivalent instrumental control settings in order to choose the best settings for the alignment. The two most important aspects for the analyst are to ensure that, through alignment of the ion beam, the analysis area coincides with the central uniform region of the ion beam irradiation area and also that an appropriate ion beam current density and raster size can be set. Ion beam currents and current densities may be measured using a Faraday cup using the methods specified in 5.3 and 5.4, as summarized in Table 1. Some design details and the accurate measurement of both electron and ion beam currents using Faraday cups are given in References and .