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Electron microscopy by specimen design : application to strain measurements

Published online on the 29 of September in Scientific Reports

par Evelyne Prevots - publié le

Researchers from CEMES-CNRS have proposed a procedure for preparing “double lamella” samples that allows the formation of controlled moiré patterns for general monocrystalline structures in cross-section. By applying moiré image analysis algorithms, they demonstrated the possibility of strain measurements with nanometer resolution and 10-4 precision. The technique is complementary to dark-field electron holography (DFEH) as it offers an even greater field of view and only needs the most basic conventional transmission electron microscope for the observations. Imaging and diffraction techniques in other fields may in turn benefit from this technique.

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(a)-(c) Principe de la formation de franges de moiré ; (d) Méthode de préparation d’échantillons à doubles lames par tripode qui permettra la formation de moirés avec des distances interfranges prédéfinies pour des structures monocristallines en coupe transversale.
© CEMES-CNRS

A bewildering number of techniques have been developed for transmission electron microscopy (TEM), involving the use of ever more complex combinations of lens configurations, apertures and detector geometries. In parallel, the developments in the field of ion beam instruments have modernized sample preparation and enabled the preparation of various types of materials. However, the desired final specimen geometry is always almost the same : a thin foil of uniform thickness. Most of the ingenuity and effort has been applied for the development of microscopy and imaging techniques. We explored changing this paradigm. The idea was to prepare the specimen in such a way as the electron microscopy itself becomes almost trivial and leads directly to the measurement we wish to perform. The specimen becomes the technique.

For this, we proposed a preparation method of “double lamella” samples that allow the formation of controlled moiré patterns for general monocrystalline structures in cross-section. The principal technique is based on tripod specimen preparation. A cross-sectional lamella is prepared in the desired zone axis and carefully cut into two pieces that are put one upon the other. The upper lamella is displaced laterally until the substrate is superposed with the region of interest and rotated. The art relies in precisely controlling the parallelism of the two adjacent surfaces during superimposition and the in-plane rotation angle that determines the spatial resolution of the technique. The double lamella sample is thinned mechanically then by Ar ions until reaching a thickness transparent for electrons. Finally, we developed moiré image analysis algorithms using an absolute correction of projection lens distortions of a TEM that allows strain measurements and mapping with a nanometer resolution and 10-4 precision. We have developed also a site-specific method of preparation by FIB which allows a study of devices and localized structures. We anticipate that the principle of specimen design can be extended to other types of measurement and to other types of epitaxial samples such as ferroelectric oxides, metals and ceramics.

These results were published in Scientific Reports.

 

Reference

N. Cherkashin, T. Denneulin, M. J. Hÿtch, “Electron microscopy by specimen design : application to strain measurements”, Scientific Reports 7, 12394 (2017). doi:10.1038/s41598-017-12695-8

 

Contact

Nikolay Cherkashin, CEMES-CNRS
nikolay.cherkashin chez cemes.fr, +33 5 62 25 79 04