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D. McDavitt, J. Ge, J. Bernecker, S. Miller (Penn State University)
We present new results in the development of large silicon grisms (~ 25x25 mm2). Using photolithography, new etching techniques and post-processing steps we have obtained greatly improved results. Experiments were performed adding ammonium persulfate to the TMAH etching solution. This new method reduced the surface roughness up to ~30% (from rms 32 nm to 22 nm), eliminated all hillock formations and helped maintain constant etch rates. This combined with improved lithography processes has reduced the large-scale defects to less than a few per square inch. Other experiments in post-processing were also carried out. Thin layers of dry silicon dioxide were repeatedly added to and removed from the grating surface producing an additional improvement of ~20% to the surface roughness (from rms roughness of 22 nm to 16 nm) or a ~50% total improvement over previous results. Optical testing of a grating with rms roughness of 16 nm shows less than 1% integrated scattered light at 0.6328 um. We are applying the new techniques in etching an 80x40 mm2 grating on a ~ 30 mm thick substrate to make an anamorphic silicon immersion grating, which can provide a diffraction-limited spectral resolution (R = 200,000) at 2.2 micron.
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