Subsurfaces Seen Sharply

The high penetration power of x-rays has been combined with diffraction imaging to create a novel x-ray microscope that makes it possible, for the first time, to view the detailed interiors of semiconductor devices and cellular structures.

The microscope was developed by a team of researchers from the Paul Scherrer Institut (PSI) and EPFL (Ecole Polytechnique Fédérale de Lausanne) in Switzerland, and uses a detector, a megapixel PILATUS (pixel apparatus for the Swiss Light Source), that could be considered the younger brother of the one that will soon be detecting collisions from CERN's Large Hadron Collider outside Geneva. CERN is the European Organization for Nuclear Research.

"Researchers have been working on such superresolution microscopy concepts for electrons and x-rays for many years," said EPFL professor and team leader Franz Pfeiffer. "Only the construction of a dedicated multimillion Swiss-franc instrument at PSI's Swiss Light Source allowed us to achieve the stability that is necessary to implement our novel method in practice."

The megapixel PILATUS can count millions of single x-ray photons over a large area, a key feature that makes it possible to record detailed diffraction patterns while the sample is raster-scanned through the focal spot of the beam. In contrast, conventional x-ray (or electron) scanning microscopes measure only the total transmitted intensity. These diffraction data are then treated with an algorithm conceived by the Swiss team.

"We developed an image reconstruction algorithm that deals with the several tens of thousands of diffraction images and combines them into one superresolution x-ray micrograph," said PSI researcher Pierre Thibault, first author on a paper on the microscope appearing in the online edition of the journal Science. "In order to achieve images of the highest precision, the algorithm not only reconstructs the sample but also the exact shape of the light probe resulting from the x-ray beam."

Conventional electron scanning microscopes can provide high-resolution images, but usually only for the surface of the specimen, and the samples must be kept in vacuum. The Swiss team's new superresolution microscope bypasses these requirements, meaning that scientists will now be able to look deeply into semiconductors or biological samples without altering them. It can be used to nondestructively characterize nanometer defects in buried semiconductor devices and to help improve the production and performance of future semiconductor devices with subhundred-nanometer features.

Another promising application of the technique is in high-resolution life science microscopy, where the penetration power of x-rays can be used to investigate embedded cells or subcellular structures. Finally, the approach can also be transferred to electron or visible laser light, and help in the design of new and better light and electron microscopes.

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