ANALYTICAL AND LAB EQUIPMENT
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
Figure 3: BEX image of an uncoated shell
collected in VP mode; note the image quality and level of elemental information revealed from this highly topographic area
Figure 1: Battery cathode sample, comparison of outputs: A) EDS map B) BEX element images from the X-ray detectors in the Unity detector; note the significantly higher intensity and data from the shadowed parts of the sample in A, C) Complete BEX image where X-ray and backscatter images from Unity are combined
Figure 2: Multi-signal dataset combined together to produce a detailed picture of the sample
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. n
Dr Haithem Mansour and Dr Simon Burgess are with Oxford Instruments.
nano.oxinst.com
www.scientistlive.com 23
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