Comparison of two key spectroscopy techniques

Dr Haithem Mansour and Dr Simon Burgess explore the differences between BEX imaging and EDS mapping.

Conventional X-ray mapping, the basis of energy dispersive spectroscopy (EDS), has been around for many years. A backscattered electron image is first generated by the scanning electron microscope (SEM), then an X-ray map is acquired. It requires a prescribed working distance and elevated beam; the map being built up gradually under conditions optimised to ensure accuracy and throughput from a conventional EDS detector.

By contrast, backscattered electron and X-ray (BEX) imaging employs the seamless and simultaneous integration of these two signals into a single image to understand a sample much more quickly and in more detail, and in essence, generates real-time imaging at a wide range of working distances and imaging conditions. The most important signals, in this case, are electrons (for topography and materials contrast) and X-ray (for constituent elements).

If you spend a lot of your SEM time doing all the exploratory work before collecting any of the data you actually want to analyse or report, then you will find the BEX technique liberating. A SEM without BEX is like driving a car without GPS or watching some films in black and white; you can, but its either not as quick nor easy as it should be, or it is missing some critical details to achieve a satisfactory outcome or experience.

A unique new design

To achieve this, Unity, the world’s first BEX detector,  is a radically different type of detector which collects significantly more X-rays and is comparatively unaffected by sample height compared with conventional side mounted EDS detectors, due to being located directly below the microscope pole piece.

The unique design of Unity makes BEX imaging a practical technique by combining electron and X-ray sensors in one unit above the sample and below the pole piece – occupying the traditional backscatter detector position in the microscope. Therefore, its X-ray sensors also benefit from this favourable geometry. The result is a much higher signal by ensuring good line of sight at any sample height. An additional advantage of this overhead view is considerably less sensitivity to topography (Figure 1), and this largely removes the problem of shadowing for rough/highly topographic samples.

Building on conventional EDS

It is the special shape of the Unity sensor head that allows line of site at most working distances. Unity’s signal processing is optimised, so it processes more X-rays faster and more accurately; EDS allows their contribution to BEX imaging to be optimised. Count rates of the EDS are much lower under BEX imaging conditions. This suits more analytical tasks better, like automatic peak identification and low energy X-ray measurement.

By combining Unity with a conventional EDS detector, we achieve the best of all worlds, including an accurate element ID to identify the elements of interest in a sample and optimised light element detection (from EDS); a very fast, low artefact X-ray imaging for the majority of elements (from Unity); and quantitative information from EDS acquired at the same time.

From all this data collected from all other sensors, a single, hyper-spectral image (Figure 2) is created by the AZtec BEX imaging software, seamlessly, in the background. The best data is selected by the software expert algorithms and presented automatically to the user. In this way, EDS is an important signal source that adds more information to the BEX image.

BEX imaging in low vacuum mode

BEX enables analysis under conditions not feasible with EDS, such as low vacuum (LV) mode.  EDS in LV mode suffers from artefacts caused by scattering of the beam and low count rates. By contrast, the electron sensors in Unity work very well in LV mode, and at 50-100Pa, it collects clear, high quality, BEX images, including electron and X-ray information (Figure 3).

X-ray mapping with Unity

X-ray mapping will remain an important function of SEM. Unity is fully compatible with the X-ray mapping software in AZtec and can be used in combination with conventional EDS detectors during X-ray map acquisition, with the capability to compare the X-ray maps collected with each detector.

X-ray mapping is also much more efficient when BEX imaging has been used to identify those regions of the sample that require more detailed analysis by a more detailed and accurate EDS approach.

Dr Haithem Mansour and Dr Simon Burgess are with Oxford Instruments.

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