Cryogenic etching and characterization of nano-sized silicon metadevice

The dielectric materials were usually deposited as hard masks for etching in the fabrication of dielectric metasurfaces, which increased fabrication steps and the distortion of the devices. We present a simplified process for preparing nano-sized silicon structure. The samples after photolithography were cooled and directly etched without deposited the hard mask. The cryogenic etching process reduce the fabrication steps and distortion caused by the transfer of photolithographic patterns. Here, a transmission middle-wavelength infrared (MWIR) metalens array was designed and fabricated as a demonstration. The optimal temperature for cryogenic etching was proved and found at 163 K by comparing the etching effects at different temperatures. The side walls of metalens prepared by the proposed process are steep, and the depth-width ratio meets the requirements. The metalens with operating at λ=4μm, can focus the light to a spot with a full-width at half-maximum (FWHM) of 20.308μm at the focal plane. The converging efficiency of the metalens reaches 95%, and the focal length is 385μm. Considering the increasingly complex and finer size of metasurface, this process may have great potentials in the preparation of nanostructures.