Fabrication of spiral-phase diffractive elements using scanning-electron-beam lithography
Schematic of fabrication process of spiral-phase plates. PMMA is spun on a glass substrate to a thickness corresponding to a phase shift. The electron-beam dose is varied in wedges along the azimuthal direction as depicted. The exposed PMMA is developed in a solution of MIBK and IPA in the ratio of 1:1. Partial development of the PMMA results in a spiral height (phase) variation.
Developed depth vs dose curve for 1:1 MIBK/IPA on silicon and glass substrates in (a) linear and (b) log scales.
(a) A LINNIK interferogram of a fabricated spiral-phase plate (SPP), revealing the height profile of the spiral in PMMA. The phase singularity at the center is clearly visible as a fork in the fringes. (b) Scanning-electron micrograph of the fabricated SPP.
(a) Schematic of the binary spiral zone plate. The black region is phase shifted by with respect to the white region. (b) Schematic of fabrication process. PMMA is spun on top of a glass substrate to a thickness corresponding to a phase shift. The exposed PMMA is developed in a solution of MIBK and IPA in the ratio of 1:3. (c) Scanning-electron micrographs of a spiral zone plate with . The moiré artifacts are a consequence of image formation via scanning.
Characterization of the point-spread function (PSF) of the spiral zone plate designed to focus at a numerical aperture (NA) of 0.7. The experimentally determined PSF is shown by connected circles (in blue). A scanning-electron micrograph of an exposed spot is shown in the inset. Note that both the inner and outer radii of the exposed spots were measured. The theoretical PSF at focus is shown as a dashed line (in black). It was empirically determined that the experimental data were best matched to the theoretical PSF at a defocus of , which is shown as a solid green curve. The theoretical PSFs were calculated using the Fresnel-Kirchoff diffraction theory.
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