Researchers from the Paul Scherrer Institut and the École Polytechnique Fédérale de Lausanne in Switzerland have unveiled the next best thing in X-ray microscopy by combining the advantages of two well-established techniques.
In scanning transmission X-ray microscopy, a sample is raster scanned and the transmitted X-ray intensity at each point is recorded to build up an absorption image. However, the resolution of the approach is limited to the spot size of the focused X-ray beam.
On the other hand, a family of techniques under the rubric of coherent diffraction imaging have been developed, in which a number of overlapping diffraction patterns of an illuminated sample are deconvoluted to provide an image. While the resolution of the approach is higher, the data analysis is difficult.
The new approach, dubbed scanning X-ray diffraction microscopy or SXDM, uses the core ideas of both methods by raster scanning a sample and collecting tens of thousands of diffraction patterns [Thibault et. al., Science (2008) 321, 379].
At the heart of the experiment is the Megapixel Pilatus detector, developed at the Paul Scherrer Institut. It is the world's first array detector that counts single photons with no readout noise, acquiring images with frame rates up to 100 Hz.
With step sizes smaller than the size of the focused X-ray beam, the image is thus oversampled, and the team have developed a novel image reconstruction method to make sense of both the phase and the intensity in the thousands of diffraction patterns.
The team ran the microscope through its paces by imaging a Fresnel zone plate buried beneath a gold layer. They employed a 6.8 keV X-ray source focused to a 300 nm spot size, collecting a 201 by 201 array of diffraction patterns. While conventional scanning electron microscopy yields a high-resolution surface image, the SXDM image reveals the structure beneath in great detail.
The authors note that the approach is noninvasive and can be carried out at ambient conditions using both hard and soft X-rays, depending on the sample. The team intends to extend the method to 3-dimensional imaging, and expects that with coming improvements in coherent X-ray sources and focusing optics, the resolution limit of the approach will reach 10 nm.