Approaching perfection in the manufacturing of silicon immersion gratings

Silicon immersion gratings make near-IR spectrographs compact and allow them to have continuous wavelength coverage over a large bandwidth. We have produced an exceptional silicon immersion grating that approaches optical perfection in terms of surface error. This grating has a peak-to-valley error of 79 nm over a 25 mm beam, which exceeds the 85 nm requirement to have λ/4 peak-to-valley error at the shortest wavelength where silicon immersion gratings can be used. In order to reduce the level of large-scale errors we have honed our contact printing method by optimizing our UV exposure system, introducing additional process checks and inspections and carefully evaluating large-scale errors in the gratings produced.