High-Spatial-Resolution Low-Energy Electron Beam X-Ray Microanalysis

Abstract

Performing X-ray microanalysis at beam energies lower than those conventionally used (`10 keV) is known to signi®cantly improve the spatial resolution for compositional analysis. However, the reduction in the beam energy which reduces the Xray interaction diameter also introduces analytical dif®culties and constraints which can diminish the overall analytical performance. This paper critically assesses the capabilities and limitations of performing low beam energy, high spatial resolution X-ray microanalysis. The actual improvement in the spatial resolution and the reduction in the X-ray yield are explored as the beam energy is reduced. The consequences for spectral interpretation, quantitative analysis and imaging due to the lower X-ray yield and the increased occurrence of X-ray line overlaps are discussed in the context of currently available instrumentation.Conventionally, X-ray microanalysis on scanning electron microscopes (SEM) with energy dispersive spectrometers (EDS) has been performed with relatively high primary energies (b 10 keV). For most samples this results in reasonably good separation of the generated X-ray line series from different elements enabling unambiguous identi®cation and therefore accurate qualitative analysis. Under these circumstances it is widely accepted that quantitative analysis of polished bulk samples is possible on a routine basis with relative errors around 1 ± 57 and detection limits of the order of 0.1 wt7. A new generation of high resolution ®eld emission gun (FEG) SEM instruments which can operate with much improved beam sizes at low beam energies (E P ) down to and below 1 keV has opened a wide range of new applications in surface and materials characterisation. The instruments, which are as straightforward to operate as conventional high vacuum SEMs, provide a new, more detailed and realistic view of surfaces. Additionally, the capability of investigating insulating samples without the requirement of a conductive coating becomes possible by utilising low E P operation.The ability of these instruments to maintain high spatial resolution performance at low E P when providing suf®cient beam current to enable practical microanalysis, in conjunction with the ultra-thin window energy dispersive spectrometers (EDS) has also opened new possibilities for materials characterisation. In particular, the analysis of new advanced materials with thin layers and sub-micron features appear to be realistic goals.In general, according to the literature there are several improvements connected with the application of beam energies below those conventionally used for microanalysis, i.e. E P`1 0 keV: ± The electron range and thus the information depth in X-ray microanalysis signi®cantly decreases with decreasing E P and enters the magnitude of a few 10 nm. ± Owing to the small beam diameters in FEG SEMs and the shrinking excitation volume the lateral resolution for X-ray microanalysis can theoretically be improved by reducing E P . ± The analysis sensitivity of near-surface features such as...

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