Multi-step electron beam technology for the fabrication of high performance diffractive optical elements

摘要

Optical performance, i.e. the diffraction efficiency, numerical aperture and wave front quality is of great importance for further successful application of diffractive optical elements in a wide range of research and commercial applications. For the fabrication process, simultaneously large numerical aperture and high diffraction efficiency result in a pattern with sub-wavelength resolution and a multi-level phase profile of nanometer accuracy. To fulfill such high requirements, a multi-step fabrication technique is proposed, in which the electron beam is used both for pattern writing and layout alignment. This method enables the fabrication of a structure with the L = 2~n discrete phase levels in n sequential lithographic steps. In each step, the pattern written in a thin, high contrast resist layer is transferred after development into the substrate by reactive ion etching to form the phase profile. In comparison to the multi-mask binary optics technology, the proposed method offers a higher resolution and reduces the alignment errors to insignificant values. On the other hand, in comparison to the direct write analog technique, our approach allows for a much better control of the phase profile and ensures larger process tolerances. The method has been successfully applied in the fabrication of several types of diffractive elements, including micro-Fresnel lens arrays, phase sampling filters and diffraction gratings on quartz and GaAs wafers. The diffraction efficiencies of these elements were found to be up to 92%. The lens exhibited diffraction-limited focusing characteristics and an insignificant wave front aberration (rms 0.01 λ).