X-ray microscopy has the potential for imaging thick samples with high resolution owing to the high penetration power of X-rays and their short wavelength.1
To fully exploit the capabilities of X-ray microscopy, it is necessary to efficiently control X-ray beams. Among a variety of techniques to manipulate the X-rays (e.g.
for focusing or sample illumination), diffractive optical elements are some of the most versatile. For instance, Fresnel zone plates (FZPs) can routinely focus soft X-rays into spots smaller than 50 nm and even resolve sub-10 nm features (Vila-Comamala et al.
). The resolution performance of FZPs is proportional to the width of the outermost ring or ‘zone’. To produce FZPs for high-resolution applications, high-resolution lithography methods are generally used. The method of choice is typically electron-beam lithography (EBL) which is, in principle, capable of sub-10 nm patterning. However, patterning FZPs by EBL with the smallest outermost zone width is not sufficient because the efficiency of FZPs at a given photon energy, that is the ratio of the focused to the incident intensity, depends on the height of the zones, as well as the material from which they are made. Ideally, the zones must be sufficiently tall to provide a phase shift close to π for the highest diffraction efficiency (DE). For soft X-rays the required heights are typically of the order of a few hundreds of nanometers, while for the hard X-rays the optimal zone height would be several micrometers even for high-Z
materials with high refractive index (e.g.
Au). Thus, to provide high-efficiency focusing of hard X-rays into a small spot, FZPs must have nanostructured zones with extremely high aspect ratios. The conventional low-energy EBL is capable of patterning only shallow nanostructures owing to the strong scattering of electrons and must, therefore, be followed by post-processing steps, e.g.
several etchings and electroplating (Schneider et al.
; Jefimovs et al.
; Feng et al.
; Lindblom et al.
), to transfer the low-aspect-ratio structures produced by EBL in thin layers of photoresist into taller metallic nanostructures. The fabrication of FZPs with high-aspect-ratio zones is, therefore, challenging. A much faster and more reliable approach is to use 100 keV EBL to directly write high-aspect-ratio nanostructures in thick layers of resist, which can be used as a mold for electroplating to produce metallic FZPs (Lo et al.
; Chen et al.
; Gorelick et al.
). Unlike low-energy electron beams (<30 keV) of conventional e-beam writers, high-energy, 100 keV, electrons are able to penetrate several micrometers into the resist with little scattering, making them suitable for exposure of high-aspect-ratio nanostructures in polymethyl-methacrylate (PMMA) resist. Such a direct-write approach eliminates the need of optimization of several consecutive nanofabrication steps, thus increasing the reliability of the fabrication process and its yield. Using this direct-write approach we fabricated FZPs made from Au by filling the PMMA molds with Au by electroplating.
In addition to the height of the zone, the pattern accuracy is a prerequisite of the high-efficiency focusing by a FZP. Thus, if the line-to-space ratio of a FZP is not optimal (e.g.
0.5 for the outermost zones), the photons are rechanneled from the first diffraction order2
to other orders, thus decreasing the first-order focus efficiency (Kirz, 1974
). We optimized the fabrication parameters (exposure, development time and line shrinkage with respect to the target line width) such that the almost optimal pattern dimensions were realised in fabricated FZPs.
The efficiencies of several fabricated FZPs were measured over a wide range of energies (2.8–13.2 keV). To the best of our knowledge this is the first measurement of the efficiencies of various FZPs in such a wide range of photon energies. The measured DEs are relatively close to the theoretical values, reflecting the good quality of the FZPs. Such measurements are important for assessment of the quality of the FZPs and further optimization of the fabrication parameters.