High aspect ratio optical gratings in diamond

In this paper we describe a process for fabricating high aspect ratio gratings in single- or polycrystalline diamond with the high precision required by micro-optics. Nanoimprint lithography with a soft stamp and several layers of hard masks allowed for rapid and accurate replication of patterns written by e-beam or laser into thick Al masks on diamond substrates as large as 2 cm in diameter. Vertical sidewalls in the mask were crucial for avoiding microvilli formation during diamond plasma etching and were achieved by etching and oxidizing the Al mask in cycles. Circularly symmetric half-wave plates for wavelength bands around 4 and 11 μm were fabricated with deep circular gratings on one side and antireflective gratings on the other.

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► High aspect ratio gratings were etched in single- and polycrystalline diamond.
► The sub-wavelength gratings form circularly symmetric half-wave plates for mid-IR.
► Mask thickness and edge profile were critical for good results.
► Our process allows fabrication of high precision gratings over large substrates.