A one-step pattern transferring process for freestanding diffractive spiral photon sieves in extreme ultraviolet

Spiral zone plates in the form of photon sieves are one of the important phase modulation devices, being well developed in both visible lights and X-rays for mode conversion from Gaussian beam to vortex one. However, in extreme ultraviolet (EUV) region as a diffractive focusing lens, spiral photon sieves in freestanding SiNx membrane with through-holes to avoid significant absorptions of EUV light by the membrane material, still remain a big challenge in manufacturing. In this work, a simplified nanofabrication process using electron beam lithography directly followed by reactive ion etch, was developed. The fabricated spiral photon sieves involve two layers of SiNx/ZEP520A with the focusing efficiency estimated to be 23%. Optical characterization of the converted vortex beam by using a typical imprint technique was carried out using a 46.9 nm EUV laser. Donut-shaped ablation pattern by EUV laser was observed on the exposed PMMA (polymethyl methacrylate), indirectly proving the phase modulation functionality of the fabricated freestanding spiral photon sieves. The developed fabrication process greatly simplifies the manufacturing of freestanding zone plates/photon sieves as well as other freestanding nanometer structures, and should be applicable in the production in large volume.