Gerhard G. Paulus

XUV coherence tomography for cross-sectional nanoscale imaging

Friedrich Schiller University Jena / Jena, Germany

F. Wiesner^a, J. Reinhard^a,b, J. J. Abel^a, M. Wünsche^a, J. Nathanael, S. Skruszewicz^ab, S. Fuchs^a,b, G. G. Paulus^a,b

^aInstitute of Optics and Quantum Electronics, Friedrich Schiller University, 07743 Jena, Germany
^bHelmholtz Institute Jena, 07743 Jena, Germany

Optical coherence tomography (OCT) is an established method for non-invasive cross-sectional imaging of biological samples using visible and near infrared light. The axial resolution of OCT only depends on the coherence length, l_c=λ_0^2/λ_FWHM, with the central wavelength λ_0 and the spectral width λ_FWHM of the light source. For OCT the axial resolution is in the range of a few micrometers.

XUV coherence tomography (XCT) extends OCT into the extreme ultraviolet and soft x-ray range. The significant reduction of the coherence length of a broadband XUV source allows nanoscale axial resolution. The usable spectral bandwidth in XCT is mainly limited by absorption edges of the sample under investigation. For example, the so-called silicon transmission window allows cross-sectional imaging of silicon-based circuits with resolutions better than 20 nm.

**Figure 1** 3D Tomogram of thin gold layers embedded in silicon. The remarkable sensitivity of XCT is apparent in the detection of an ≈2nm thin SiO₂layer that had unintentionally been created during the production of the sample.

A laboratory-based XCT setup has been implemented by using XUV radiation from a laser-driven high harmonic source. The harmonics are focused onto the sample and the reflected radiation is recorded. Interferences due to reflections at structures in different depths result in modulations in the measured spectra that can be used to reconstruct the axial (i.e. depth) structure of the sample. An axial resolution of 24 nmhas been achieved, see Fig. 1.

Phase-reconstruction not only allows the elimination of certain artifacts that are typical for the OCT geometry used. Rather, quantitative data on the spectral reflectivity and even the dispersion can be obtained. Comparison with respective data tabulated in the literature permits clear information on the material that is imaged inside bulk samples. Fig. 2 displays an example where layers of different materials are buried in a silicon sample.

**Figure 2** a) The Fourier transform of the measured intensity spectrum (blue) contains autocorrelation artifacts. The artifact-free structure (red) is obtained through phase retrieval. The peak amplitudes now contain the information of single layer reflectivities. b)-e) The effective reflectivity of single layers is obtained through a Fourier transform of the corresponding depth region. The spectral behavior enables the identification of, e.g., titanium.

References

S. Fuchs et al., Scientific Reports, 2016, 6, 20658.
M. Wünsche et al., Optics Express, 2017, 25, 6936.
S. Fuchs et al., Optica, 2017, 4, 903.