Manufacturing of a highly efficient gold coated echelle grating for NIR on a thick silicon substrate with small straylight and wavefront error

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.