Systematic efficiency study of line-doubled zone plates

Line-doubled Fresnel zone plates provide nanoscale, high aspect ratio structures required for efficient high resolution imaging in the multi-keV x-ray range. For the fabrication of such optics a high aspect ratio HSQ resist template is produced by electron-beam lithography and then covered with Ir by atomic layer deposition (ALD).The diffraction efficiency of a line-doubled zone plate depends on the width of the HSQ resist structures as well as on the thickness of the deposited Ir layer. It is very difficult to measure these dimensions by inspection in a scanning electron microscope (SEM) without performing laborious and destructive cross-sections by focus ion beams (FIB). On the other hand, a systematic measurement of the diffraction efficiencies using synchrotron radiation in order to optimize the fabrication parameters is not realistic, as access to synchrotron radiation is sparse.We present a fast and reliable method to study the diffraction efficiency using filtered radiation from an x-ray tube with a copper anode, providing an effective spectrum centered around 8 keV. A large number of Fresnel zone plates with varying dimensions of the resist structures and the ALD coating were measured in an iterative manner. Our results show an excellent match with model calculations. Moreover, this systematic study enables us to identify the optimum fabrication parameters, resulting in a significant increase in diffraction efficiency compared to Fresnel zone plates fabricated earlier without having feedback from a systematic efficiency measurement.

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