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Hybrid metrology technology for nanostructured surfaces


Figure 1: Schematic representation of the measurement principle used when combining GIXRF and the FEM-Maxwell solver to calculate the electric field strength distribution

Figure 2: Click on the picture to start picture animation

The significant challenges posed by the increasing complexity of nanostructured surfaces used in technological applications include the characterization of such surfaces. The importance of complex and multidimensional nanostructures consisting of several materials is particularly noticeable in the semiconductor industry. At PTB's laboratory at the BESSY II electron storage ring, synchrotron radiation has been used to perform a material-sensitive dimensional reconstruction of a nanostructured surface by means of X-ray spectroscopy-based methods such as a reference-free grazing-incidence X-ray fluorescence (GIXRF) analysis.

X-ray fluorescence analysis is a procedure in which electrons from inner atomic shells are excited by incident X-ray photons. When these excited states later decay, element-specific X-ray fluorescence is emitted. Quantitative conclusions concerning the composition of the material examined can be made based on an energy-selective measurement of the X-ray fluorescence intensities, for which calibrated instruments are used. Due to the grazing incidence of the exciting X-rays hitting the sample, even depth-dependent element distributions can be determined.

To date, various scattering methods have been developed in PTB's laboratory at BESSY II to determine dimensional parameters of a nanostructured surface. By analyzing a nanoscale grating structure made of Si3N4 (40 nm linewidth), it has recently been demonstrated that GIXRF can also be used for this purpose. Used at grazing incidence, the exciting X-rays generate a standing wave field surrounding the grating structure. By rotating the lattice structures on two axes around the incident X-ray beam, the interference between the incident beam and the reflected beam is exploited to vary the locations of the maximum electric field strength within the structure and thus to "scan" the nanostructure with sub-nanometer resolution. The intensity of the fluorescence radiation generated by this process is proportional to the exciting electric field strength in the medium. For this reason, assessing the data requires numeric modelling of the spatial field strength distribution; this modelling took place based on a finite element method (FEM) used to solve Maxwell's equations for the nanostructure. Combining GIXRF and the FEM Maxwell solver allows a quantitative interpretation of the fluorescence intensities emitted to be made, and thus allows conclusions to be drawn concerning the number of atoms involved and their position within the nanostructure.

In the medium to long term, it will be possible to combine this approach, which is based on experiments as well as on theory, with conventional grazing-incidence small-angle X-ray scattering (GISAXS). Analyzing the elastically scattered photons allows a complementary reconstruction of nanostructure geometries to be made that is largely independent of the material used. In addition, a hybrid methodological approach allows the spatial distribution of certain elements to be determined that are difficult to differentiate using GISAXS. This approach will also be useful in meeting future metrological requirements in the field of nanotechnology.  


P. Hönicke, 7.24, E-Mail: Opens window for sending emailPhilipp.Hoenicke(at)ptb.de

V. Soltwisch, 7.12, E-Mail: Opens window for sending emailVictor.Soltwisch(at)ptb.de


Victor Soltwisch, Philipp Hönicke, Yves Kayser, Janis Eilbracht, Jürgen Probst, Frank Scholze and Burkhard Beckhoff, Element sensitive reconstruction of nanostructured surfaces with finite elements and grazing incidence soft X-ray fluorescence, NANOSCALE, 2018, 10, 6177-6185