Abstract
Fresnel zone plates (FZPs) play an essential role in high spatial resolution x-ray imaging and analysis of materials in many fields. These diffractive lenses are commonly made by serial writing techniques such as electron beam or focused ion beam lithography. Here we show that pinhole diffraction holography has potential to generate FZP patterns that are free from aberrations and imperfections that may be present in alternative fabrication techniques. In this presented method, FZPs are fabricated by recording interference pattern of a spherical wave generated by diffraction through a pinhole, illuminated with coherent plane wave at extreme ultraviolet (EUV) wavelength. Fundamental and practical issues involved in formation and recording of the interference pattern are considered. It is found that resolution of the produced FZP is directly related to the diameter of the pinhole used and the pinhole size cannot be made arbitrarily small as the transmission of EUV or x-ray light through small pinholes diminishes due to poor refractive index contrast found between materials in these spectral ranges. We also find that the practical restrictions on exposure time due to the light intensity available from current sources directly imposes a limit on the number of zones that can be printed with this method. Therefore a trade-off between the resolution and the FZP diameter exists. Overall, we find that this method can be used to fabricate aberration free FZPs down to a resolution of about 10 nm.
Highlights
An Fresnel zone plates (FZPs) is diffraction based optical lens commonly used in the x-ray wavelength region due to the difficulty of using conventional lenses and mirrors in this spectral range
More detailed work has been presented to establish the fact that a Fresnel zone plate is a special type of hologram [5] which was experimentally demonstrated by recording the fringe pattern created by the interference of a spherical and a plane wave obtained from a He-Ne Laser on a photographic emulsion plate [6]
The pinhole is fabricated by electron beam lithography (LION LV-1, Leica, Jena) on a 100 nm thick silicon nitride membrane coated with 23 nm thick chromium film
Summary
An FZP is diffraction based optical lens commonly used in the x-ray wavelength region due to the difficulty of using conventional lenses and mirrors in this spectral range. More detailed work has been presented to establish the fact that a Fresnel zone plate is a special type of hologram [5] which was experimentally demonstrated by recording the fringe pattern created by the interference of a spherical and a plane wave obtained from a He-Ne Laser on a photographic emulsion plate [6]. More recently holographic fabrication of micro-lenses has been demonstrated in deep UV regime [7] by interfering first and second order diffracted waves from an ion etched Fresnel zone plate. First demonstration of zone plate fabrication using EUV radiation was performed by recording diverging and converging diffracted spherical waves generated by two concentric annular parent zone plates [13]. Initial results on holographic lithography at EUV wavelength (13.5 nm) was reported recently [14]
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