Nanoscale metrology using coherent extreme-ultraviolet (EUV) or soft x-ray (SXR) light has unique advantages for a broad range of science and technology. Short EUV/SXR wavelengths have high sensitivity to small features, elemental composition, as well as electronic and magnetic orders. Tabletop high harmonic sources (HHG) have high spatial and temporal coherence, enabling precise phase-sensitive measurements of nanoscale functional properties (e.g. transport and mechanical), as well as diffraction-limited imaging with both amplitude and phase contrast. However, to date, most EUV HHG measurements were performed on hard materials, where damage is not significant concern.
STROBE scientists demonstrated that EUV HHG can rapidly and nondestructively characterize dose-sensitive materials such as polymer-based structures, with higher spatial resolution than visible light, and with far less damage than electron imaging. They collaborated with scientists from 3M to characterize polymer metamaterials, that have 2D periodic features less than the wavelength of visible light. Using HHG scatterometry, they extracted layer thicknesses, densities and top-surface geometry, without the need to coat or cut the sample. In contrast, SEM imaging of this polymer metamaterial requires that the sample be coated, and the high-energy electron beam can cause shrinking (see Fig.). Here, the significantly lower photon energy (~42eV) of EUV HHG compared with electron beams (~1-30keV) is key to lowering the dose, while maintaining high spatial resolution (<60nm transverse and <nm axial). Finally, correlative electron imaging, which requires highly specialized sample preparation to avoid sample damage, was implemented to validate the EUV HHG findings.