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10 July 2018 Process and metrology developments in the production of immersion gratings
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Silicon immersion gratings take advantage of the high index of refraction of silicon (3.4) to significantly improve the performance and reduce the volume of near-infrared spectrographs. The immersion gratings we discuss here are produced by contact photolithography. Lithography is followed by plasma etching of a silicon nitride hard mask, which defines the pattern for wet etching of the silicon v-grooves in potassium hydroxide that form the blazed grating. We have shown that interference fringes between the photomask and the polished silicon nitride on silicon substrate produce a phase error in the completed grating. With our standard process, the lines in photoresist formed during the lithography step have a slope with an additional “foot” at the base of the line. The thickness of this foot can vary and may be partially etched away causing a shift in the position of the line during etching. To reduce the effect of the foot, we have added a plasma etch step designed to remove the foot prior to completing the silicon nitride etch. We have also found that thinning the photoresist to better control the profile formed during contact printing and subsequent etching results in very uniform gratings over a 125 mm grating length. We will also describe a method to predict the phase uniformity at the patterning stage, which allows us to pattern and evaluate the potential grating before etching, saving both time and material costs.
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Cynthia B. Brooks, Benjamin Kidder, Michelle Grigas, and Daniel T. Jaffe "Process and metrology developments in the production of immersion gratings", Proc. SPIE 10706, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III, 1070654 (10 July 2018);

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