Laboratory setup for extreme ultraviolet coherence tomography driven by a high-harmonic source

Abstract

We present a laboratory beamline dedicated to nanoscale subsurface imaging using extreme ultraviolet coherence tomography (XCT). In this setup, broad-bandwidth extreme ultraviolet (XUV) radiation is generated by a laser-driven high-harmonic source. The beamline is able to handle a spectral range of 30–130 eV and a beam divergence of 10 mrad (full width at half maximum). The XUV radiation is focused on the sample under investigation, and the broadband reflectivity is measured using an XUV spectrometer. For the given spectral window, the XCT beamline is particularly suited to investigate silicon-based nanostructured samples. Cross-sectional imaging of layered nanometer-scale samples can be routinely performed using the laboratory-scale XCT beamline. A depth resolution of 16 nm has been achieved using the spectral range of 36–98 eV which represents a 33% increase in resolution due to the broader spectral range compared to previous work.

Coherence tomography with broad bandwidth extreme ultraviolet and soft X-ray radiation

“…The experiments with the synchrotron radiation offer the most versatile controlling of the wavelength. By its [27], (7) [41,42], and (8) e.g., ΔE = 20 − 280 eV [36], ΔE = 250 − 3000 eV [43], XCT approach Δ (nm) Axial (3) resolution Photon flux (ph/s) Comments High flux widely tunable energy range 4.43 -2.34 7 (5) nature, the synchrotron radiation typically delivers monochromatic radiation ranging from VUV to hard X-ray at high flux ( 10 11 ph/s).…”

“…However, their limited accessibility and the large costs severely restrict the applicability of synchrotron-based XUV/SXR imaging. A standard table-top HHG laser source delivers 10 9 ph/s [27] in the XUV range. Although extending the cutoff of the HHG spectrum towards SXR range can be realized using mid-infrared wavelength, it happens at unavoidable costs of the photon flux.…”

“…A schematic representation of a common-path Fourierdomain XCT interferometer: broad bandwidth XUV/SXR radiation is focused onto the sample mounted on a motorized positioner [27]. The reflected light from the sample is detected by an XUV spectrometer or by a photodiode.…”

Photon counting of extreme ultraviolet high harmonics using a superconducting nanowire single-photon detector

“…By focusing the linearly polarized laser pulses from the OPA into an argon gas jet, the HHG process is triggered, resulting in a mixture of EUV radiation with the typical harmonic comb structure and remaining infrared laser light. EUV photons up to an energy of ∼ 100 eV are produced [41]. They are separated from the laser radiation by thin metal foils.…”