We present the design, manufacturing and characterization results of a customized high-resolution echelle grating. The grating was manufactured at Fraunhofer IOF and delivered to the NIRPS (Near Infrared Planet Searcher Instrument) consortium. The technology workflow for the manufacturing of the echelle grating is relying on wet-chemical etching, applied to crystalline silicon substrates, which enables the creation of highly determined micro-facets and surfaces over macroscopic dimensions. The echelle’s grating period and plateau size within one period are established based on electron-beam lithography. A binary pattern in a hard mask material is performed by dry-reactive ion etching while transferring the pattern in the silicon substrate is achieved by wet-chemical etching with potassium hydroxide. The grating is designed to operate at a blaze angle of 76° in a wavelength band of 0.9μm – 1.8μm. A gold coating is applied to increase the diffraction efficiency to about 70%; verified at wavelengths of 1030nm and 1640nm, respectively. The overall grating size is 78mm x 284mm providing a WFE of less than 70nm (RMS) measured throughout the full aperture. In this article we present the manufacturing workflow and structural inspection results of the manufactured echelle grating, having a critical eye on the impact of sub-surface defects of the initial silicon crystal. Moreover, we present optical performance test results covering diffraction efficiency, PSF, WFE and spectral ghosts. It is concluded that the imaging properties of the manufactured grating are as good as those of a plane gold mirror reference. Additional presentation content can be accessed on the supplemental content page. Additional presentation content can be accessed on the supplemental content page.
The development of a small size prototype of a UV-grating for the CUBES instrument of ESO’s VLT is presented. It has a line density of 3600 l/mm and is manufactured on a fused-silica substrate using electron-beam lithography, reactive ion etching and atomic layer deposition. In the ideal case the grating has a pure lamellar profile with a groove width in the range of 100nm only. To achieve a high polarization independent diffraction efficiency the grating depth is required to be in the range of 700nm and the duty cycle needs to be met with an accuracy in the nm-range. To achieve this high aspect ratio with sufficient accuracy a trimming process based on a conformal overcoating by ALD is performed.
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/Δλ_FWHM, with the central wavelength λ_0 and the spectral width Δλ_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 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 samples like semiconductors.
A laboratory-based XCT setup has been implemented by using XUV radiation from a laser-driven high-harmonic source. By averaging harmonic combs generated by different fundamental wavelengths, a quasi-supercontinuous spectrum, which is well-suited for XCT, is generated.
The radiation is focused onto the sample and the reflected radiation is recorded. Interferences due to reflections at structures in different depths result in a modulated spectra that can be used to reconstruct the axial structure of the sample. Experimentally, we achieve an axial resolution of 24 nm.
In the XUV range, focusing with high numerical aperture (NA) is extremely expensive. Therefore, XCT uses low-NA optics, which limits the lateral resolution to the micrometer range. A combination of XCT with coherent diffraction imaging would provide improved lateral resolution. We present first results a proof-of-concept experiment at a synchrotron source.
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