Fabrication and testing of a silicon immersion grating for infrared spectroscopy

Abstract

recent advances in silicon micromachining techniques allow the fabrication of very coarse infrared echelle gratings. When used in immersion mode the dispersion is increased proportionally to the refractive index. This permits a very significant reduction in the overall size of a spectrometer while maintaining the same resolution. We have fabricated a right triangular prism from silicon with a grating etched into the face of the hypotenuse. The grating covers an area of 32 mm by 64 mm and has a 97.5 micrometers periodicity with a blaze angle of 63.4 degree(s). The groove surfaces are very smooth with a roughness of a few nm. Random defects in the silicon are the dominant source of grating scatter. We measure a grating ghost intensity of 1.2%. The diffraction peak is quite narrow, slightly larger than the Airy disc diameter at F/12. However, due to wavefront aberrations, perhaps 15-20% of the diffracted power is in the peak with the rest distributed in a diameter roughly five times the airy disc.