XuM: Image below the surface and add another dimension to microscopy using the SEM
By Julie Sheffield-Parker

At M&M in August this year, Gatan launched the XuM, a novel x-ray microscope which is an accessory to a scanning electron microscope. Unlike most imaging and analytical techniques available on the SEM, the XuM allows the user to look inside the sample structure rather than just examine the surface or near-surface structure. This ability to image internal structure means that many samples can be analysed completely intact without the need for cross-sectioning which would both destroy the sample and would raise the possibility of introducing sample preparation artefacts.

The XuM uses a special target placed under the electron beam as its source of x-rays. X-rays generated from the target are transmitted through the sample and projected onto a highly sensitive CCD camera to form an image. This image is similar in nature to a typical medical x-ray containing image contrast due to the effect of x-ray absorption within the sample; however images acquired using the XuM also contain an added contrast mechanism referred to as phase contrast.

Phase contrast is a phenomenon that exploits the wave properties of x-rays. It arises from the refraction (rather than absorption) of x-rays by the sample and is therefore sensitive to changes in refractive index which typically occur at boundaries and edges. As well as providing this edge enhancement, phase contrast allows us to see objects which are either too thin or are insufficiently dense to produce any significant absorption contrast. Under these conditions the phase contrast component is much stronger than the absorption component so even biological materials can be imaged without the need for heavy metal staining.

Several imaging modes are possible using the XuM. In its most simple operation, 2D images can provide a high resolution transmission view through the thickness of the sample. When looking at complex multi-layered structures a simple 2D image can be difficult to interpret with so much information superimposed into one view. For samples such as these, stereo pairs, acquired by taking two images at different rotations may be more easily interpretable as they provide a depth perspective to the combined image. Where full 3-dimensional information is required 3D micro-tomography provides the ultimate information by creating 3D solid models of the sample reconstructed from rotational datasets. A wide range of visualization and analysis tools can then be used to digitally “slice open” the sample to reveal the internal structure.

The XuM has already been used to provide new information about a wide range of sample types from both the biological sciences and materials science. Such applications include the study of electromigration, delamination and other defects in semiconductors and micro-electronic device, corrosion of aluminium alloys, internal structure of wood and paper products, fish embryos, perforations in polymers, defects in diamonds and minerals to name but a few. The opportunity to study an even wider range of sample types will also be available soon with the installation of an XuM application and demonstration facility at Gatan’s headquarters in Pleasanton, CA.

Add the 3rd dimension to your SEM with x-ray microscopy using the XuM.

The benefits include:

• The ability to look at the internal structure of optically opaque materials
• High resolution x-ray imaging with enhanced image detail using both absorption and phase contrast effects
• Large depth of field allowing stereo imaging and full 3D micro-tomography.

Example: 2D image (left) of a composite aerospace material consisting of fibres embedded in a polymer material. Air bubbles can be clearly seen as well as de-bonding of fibres within the polymer matrix.

Example: 3D tomographic reconstruction (right) of a micro-composite constructed from thin metallic wires in a polymer matrix. Volumetric datasets created in this way can be analysed to determine both the spatial arrangement of the wires and the volumetric proportion of each material. This would be very difficult to perform by cross-sectioning methods due to the very different mechanical properties of the wire and polymer components.

(Micro-composite data courtesy of Dr Sherry Mayo, CSIRO CMIT, Clayton, Australia).