Single‐Step 3D Nanofabrication of Kinoform Optics via Gray‐Scale Focused Ion Beam Lithography for Efficient X‐Ray Focusing

Abstract

material is achieved, kinoform lenses exhibit focusing effi ciencies of 100% [ 10 ] and concentrate all the incident light into a single focus. This very desirable property is of crucial importance for many applications such as laboratory based X-ray microscopy where every single photon is important due to limited source brightness. Unfortunately, to date the fabrication of kinoform lenses for X-ray microscopy mainly relies on approximations, which reduce their potential effi ciency. Approximations based on step profi les have been developed including a variety of routes such as optical gray scale lithography, [ 2b,c ] multilevel/multilayer quasi-Kinoforms, [ 11 ] consecutive overlaid e-beam lithographic steps, [ 10 ] direct ion beam writing of steps profi les, [ 12 ] 2D approximations, [ 13 ] and stacking of two binary FZPs. [ 14 ] Recently, progress was made in structuring of continuous linear and circular blazed profi les via combining focused ion beam implantation and reactive ion etching, [ 15 ] and diamond tool based micro-machining [ 16 ] sometimes also in combination with reactive ion etching. [ 17 ] A few attempts toward Fresnel lenses using direct write gray scale ion beam lithography (IBL) were already reported but with rather large feature sizes, small apertures and dedicated mostly to the optical wavelength range, [ 18 ] while infrared applications were also reported. [ 19 ] Overall, the preparation of the fabled circularly symmetric, continuous kinoform 3D surface profi le remains a daunting task especially for X-ray applications and was recently considered impossible with current fabrication methods. [ 14a ] Here, for the fi rst time, we demonstrate a simple and singlestep fabrication strategy for X-ray kinoform lenses approaching the ideal profi le and we show their performance for X-ray microscopy (XRM) supported with imaging and effi ciency measurements in a state of the art X-ray microscope (STXM) located at BESSY II synchrotron radiation facility. [ 20 ] XRM has developed as a powerful analytical tool used in various research fi elds ranging from biology-biochemistry and materials science/engineering to information technology with applications including for instance, imaging of ultracellular structure, [ 21 ] in situ and in operando examination of chemical reactions via spectroscopy and imaging, [ 22 ] and investigation of ultrafast A kinoform lens is a highly effi cient photonic device capable of focusing a wide range of electromagnetic radiation. Unfortunately, its realization without the need of any approximation remained elusive for a very long time. A direct, simple and precise fabrication method was still missing. Here, an effi cient preparation scheme is presented for the fi rst time. Two kinoform lenses made out of polycrystalline gold and nano-crystalline PdSi, are successfully fabricated, thoroughly characterized and tested for their ultimate focusing performances at soft X-ray energies. The fabrication is made possible by means of gray-scale direct-write ion beam lithograph...

Overview of the multilayer-Fresnel zone plate and the kinoform lens development at MPI for intelligent systems

“…Although the resultant film was confirmed not to be amorphous by TEM investigations, due to various other process requirements [39], the nano-crystalline nature of the film exhibited a virtually isotropic response to the incident ions throughout the lens aperture which lead to very smooth and high quality surface. Also, the major component of the alloy, 80 at.…”

“…However, when the absorption within the lens is taken into the account, which applies for any real lens, especially in the soft X-ray or EUV energies where absorption is strong, the ideal picture is not accurate [39]. The consequences are that the optimum thickness is smaller than predicted by the equation (1) and also that the focusing efficiency is not 100 % anymore.…”

“…The fact that the optimum thickness is lower as absorption increases is actually useful as it leads to lower aspect ratios, which relaxes the fabrication requirements. We demonstrated that even in the presence of absorption, the optimum theoretical diffraction efficiency of a kinoform lens can be significantly larger than the 40 % limit of the ideal phase reversal zone plate [39]. Figure 3.…”

“…A gold kinoform structure, that was fabricated using 10 pA of current, is shown in Figure 5a, with very promising results. However, the differential milling in polycrystalline gold leads to a rough surface which limits the focusing efficiency [39] by introducing random phase variations. This sort of fabrication errors lead to appearance of diffraction effects such as higher order foci, thus reducing the focusing efficiency.…”

“…Until very recently, the surface relief profile of kinoforms has been difficult to reproduce lithographically, leading researchers to resort to step [6,11,35,36] or planar [34,37,38] approximations. Recently, we have succeeded in reproducing surface profiles of the kinoform lenses almost perfectly by using a grayscale direct-write ion beam lithography process (IBL) [39] where the beam dosage is dynamically modulated during milling to fabricate the complex surface relief profile of the kinoform lens. The fabricated kinoform lenses were used for efficient soft X-ray focusing and imaging.…”