Fabrication of 200 nm period blazed transmission gratings on silicon-on-insulator wafers

The authors report on the fabrication of 200 nm period blazed transmission gratings on silicon-on-insulator SOI wafers. These critical angle transmission CAT gratings require 3–5 m 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 574 nm 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 50 wt % 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 200 nm period CAT gratings with support mesh periods of 25 and 40 m in a 9 mm2 area of 4m-thick silicon membranes on 110 SOI wafers. © 2008 American Vacuum Society. DOI: 10.1116/1.2968613