The authors report on the fabrication of 200nm period blazed transmission gratings on silicon-on-insulator (SOI) wafers. These critical angle transmission (CAT) gratings require 35μm tall freestanding grating bars with a very high aspect ratio (>100) and smooth sidewalls. In order to meet the challenging geometrical requirements, they modified and improved our previously reported process for the fabrication of a CAT grating prototype with 574nm period. They have used potassium hydroxide (KOH) solutions to fabricate high aspect ratio gratings on ⟨110⟩ SOI wafers. The KOH etching process was improved to minimize the lateral undercut through precise grating alignment to ⟨111⟩ planes within ±0.05° and a room temperature etch process with 50wt% KOH. In addition, an image-reversal technique with a high silicon content spin-on polymer was applied to increase process latitude with a high duty cycle nitride mask. A surfactant was also added to the KOH solution to promote hydrogen bubble release. With the improved process, they achieved a high etch anisotropy of above 300 on a ⟨110⟩ silicon wafer. They successfully fabricated 200nm period CAT gratings with support mesh periods of 25 and 40μm in a 9mm2 area of 4-μm-thick silicon membranes on ⟨110⟩ SOI wafers.

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