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 mu 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 degrees 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 mu m in a 9 mm(2) area of 4-mu m-thick silicon membranes on < 110 > SOI wafers.