Fabrication of Diffractive Optical Elements by Electron Beam Lithography

Abstract

Advanced optical ffounctions, such as optical interconnects, beam splitters, fan-out elements, read-out optics, device displays, miniature arrays of microlenses and holograms can be realised by means of Diffractive Optical Elements (DOE). Many DOEs exploit micrometer and sub-micrometer surface relief patterns to modulate the phase of the incoming radiation and to perform the redistribution of the electromagnetic field arising from the DOE itself.1 Computer generated phase profiles are directly transferred onto the optical active media by means of standard microfabrication technologies borrowed from microelectronics. Many of the surface details of a DOE are comparable in size with those of current microelectronics integrated circuits (ICs), and several technologies used in IC production can be adapted for DOE fabrication. Lithography is the first and fundamental step of any microfabrication process; by means of lithography the fine and often complex features composing a DOE are delineated. Electron Beam Lithography (EBL) is ideally suited as a pattern generation technique in the fabrication of DOE. Its intrinsic flexibility, along with its exceptional resolution, makes EBL the natural choice for the pattern generation of a wide range of DOE. In this paper, EBL is applied to the fabrication of DOE for a wavelength range spanning from the infra-red to x-rays. EBL is used both for binary shaping, and for more complex element generation. The process cal have developed allows the fabrication of nanometer size diffractive elements and, in combination with a custom electron scattering algorithm and proximity effect correction package, permits the shaping of a suitable resist in just one shot of exposure (continuous profiling).