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Taking a closer look into nanostructures

Reconstructing nanostructured surfaces by means of complementary experiments and theoretical modeling

PTBnews 1.2019
Especially interesting for

the semiconductor industry


The increasing complexity of nanostructured surfaces used in technological applications makes their metrological characterization challenging. The importance of highly complex nanostructures consisting of several materials is particularly noticeable in the semiconductor industry. At PTB's laboratory at the BESSY II electron storage ring, it has now been demonstrated on a Si3N4 nanostructure on a silicon substrate that a material-sensitive dimensional reconstruction of a nanostructured surface is possible by modeling X-ray fluorescence experiments with synchrotron radiation.

Schematic representation of the combination of the FEM calculation of the electric field strength distribution and the GIXRF measurements carried out on a Si3N4 grating structure on a silicon substrate.

X-ray fluorescence analysis is a procedure in which incident X-ray photons excite electrons in inner atomic shells. When these excited states subsequently decay, element-specific X-ray fluorescence is emitted. Quantitative conclusions concerning the composition of the material examined can be drawn based on an energy- selective measurement of the X-ray fluorescence intensities, for which calibrated instruments are used.

Under grazing incidence of the exciting X-ray beam (grazing-incidence X-ray fluorescence, GIXRF), a standing wave field occurs through interference with the fraction of radiation reflected by the sample. This fraction of radiation penetrates locally into the surface structure. By rotating the sample on two axes around the incident X-ray beam, the locations of the maximum electric field strength – and thus the fluorescence emission – can be varied in a targeted way and the structure can be scanned with sub-nanometer resolution.

To assess the data, exact numerical modeling of the spatial field strength distribution is required. To this end, an approach was chosen that had already been developed for experiments on grazingincidence small-angle X-ray scattering (GISAXS) and that is based on a finiteelement method (FEM) to solve the Maxwell equations.

Combining GIXRF with the FEM calculation as well as the quantitative modeling of the emitted fluorescence intensities allows conclusions to be drawn concerning the number and position of atoms of a specific element within the nanostructure.

For future requirements of nanometrology, it is envisaged to combine this experimental theoretical approach with data obtained by means of GISAXS. These data may not contain any information related to the specific material, but they allow a very accurate dimensional reconstruction of the nanostructures.


Victor Soltwisch
Department 7.1
Radiometry with Synchrotron Radiation
Phone: +49 (0)30 3481-7129
Opens window for sending emailvictor.soltwisch(at)ptb.de

Philipp Hönicke
Department 7.2
X-ray Metrology with Synchrotron
Phone: +49 30 3481-7174
Opens window for sending emailphilipp.hoenicke(at)ptb.de

Scientific publication

V. Soltwisch, P. Hönicke, Y. Kayser, J. Eilbracht, J. Probst, F. Scholze, B. Beckhoff: Element sensitive reconstruction of nanostructured surfaces with finite elements and grazing incidence soft X-ray fluorescence. Nanoscale 10, 6177–6185 (2018)