Infrared grisms using anisotropic etching of silicon to produce a highly asymmetric groove profile

Grisms are an important tool for astronomical spectroscopy because they allow for very compact, straight-through spectrometer designs in systems that can double as imagers. In the infrared, silicon grisms offer the advantage of superior resolving power for a given beam size and opening angle, when compared to grisms made of low refractive index materials. Silicon grisms with symmetric profiles and a blaze angle of 54.7degrees, the natural result of anistropic etching of silicon substrates oriented with the (100) crystal plane exposed, are relatively easy to produce. Low-order grisms, however, must be blazed at much shallower angles and will therefore have highly asymmetric groove profiles. In order to achieve these shallow blaze angles, the silicon surface must be precisely oriented at a bias from the (100) plane before cutting and polishing the substrate. Production of gratings with blaze angles as small as 6degrees is more difficult than production of unbiased gratings because it is very sensitive to changes in the etching process parameters. In this paper, we discuss our techniques for etching highly biased surfaces in silicon wafers, along with the first results of our production and testing of highly biased silicon gratings, including SEM groove profile pictures and optical testing in reflection at 632 nm and in transmission at 1523 nm.