CUBES: prototyping the high efficiency UV transmission grating

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.