Making the diamond vortex phase masks for the METIS instrument

Direct observation of exoplanets and proto-planetary disks with the METIS instrument at the Extremely Large Telescope will provide new insights into the processes of planet formation and exoplanet atmospheres. This will be possible thanks to a powerful vector vortex coronagraph that can suppress the starlight to reveal faint signals around it. Here we present the process of making the phase masks at the heart of the coronagraph. These annular groove phase masks consist of deep sub-wavelength gratings in diamond that are etched using inductively coupled oxygen plasma with a strong bias. The METIS instrument requires a wider bandwidth than such components have previously been demonstrated for, leading to a grating design with higher aspect ratio and more vertical walls. To achieve this, the etch mask used for diamond etching was changed from aluminium to silicon and the plasma power was increased. We also improved on our method for reducing the grating depth of finished components to fine-tune them. Together with improved optical testing, this allowed us to produce the best vortex phase masks so far demonstrated for the astronomical N-band. [Display omitted] •The METIS instrument will use vortex phase masks to make extrasolar planets visible.•Broadband starlight rejection requires deep circular gratings with vertical walls.•Gratings were etched in diamond using high power plasma and a silicon mask.•Mid-infrared tests were done on a cryogenic test bench with five QCL lasers.•Optical test results were used to tune the gratings for best performance.