Proposal for Inverse-Phase Composite Zone Plate for Deeper Depth of Focus

Zone plates (ZPs), including their derivations, and total reflection mirrors have facilitated the realization of sub-100-nm spatial resolution, and the demand for localized analysis of practical samples using X-ray microscopy is rapidly increasing. According to the optical principle in a focusing optical system, the smaller the focused beam size is, the shallower is the depth of focus (DoF). Usually, diffraction limit in optics means that the product of a spatial resolution ( res. ) and the numerical aperture (NA) cannot be smaller than the wavelength, i.e., res. NA . Because DoF is also determined by NA (DoF 1/NA 2 ), smaller res. and deeper DoF cannot be simultaneously attained, which is another viewpoint of the diffraction limit. This limit restricts the analysis of thicker samples than DoF and is a bottleneck in highspatial-resolution observation of practical samples using X-ray microscopy. We attempt to relax this limit using a wave-optical approach. In this paper, a novel type of ZP, called inverse-phase composite ZP (IP-CZP), is proposed and examined to make the DoF deeper with little vitiation in the spatial resolution. We must mention that a fractal ZP was also proposed for deep focus