Umut Tunca Sanli

Multilayer Fresnel zone plates for high energy radiation resolve 21 nm features at 1.2 keV

X-ray microscopy is a successful technique with applications in several key fields. Fresnel zone plates (FZPs) have been the optical elements driving its success, especially in the soft X-ray range. However, focusing of hard X-rays via FZPs remains a challenge. It is demonstrated here, that two multilayer type FZPs, delivered from the same multilayer deposit, focus both hard and soft X-rays with high fidelity. The results prove that these lenses can achieve at least 21 nm half-pitch resolution at 1.2 keV demonstrated by direct imaging, and sub-30 nm FWHM (full-pitch) resolution at 7.9 keV, deduced from autocorrelation analysis. Reported FZPs had more than 10% diffraction efficiency near 1.5 keV.

Comparative study of ALD SiO2 thin films for optical applications

We have investigated the suitability of atomic layer deposition (ALD) for SiO2 optical coatings and applied it to broadband antireflective multilayers in combination with HfO2 as the high refractive index material. SiO2 thin films were successfully grown using tris[dimethylamino]silane (3DMAS), bis[diethylamino]silane (BDEAS) with plasma activated oxygen as precursors, and the AP-LTO330 precursor with ozone, respectively. The amorphous SiO2 films show very low optical losses within a spectral range of 200 nm to 1100 nm. Laser calorimetric measurements show absorption losses of 300 nm thick SiO2 films of about 1.5 parts per million at a wavelength of 1064 nm. The films are optically homogeneous and possess a good scalability of film thickness. The film surface porosity - which correlates to a shift in the transmittance spectra under vacuum and air conditions - has been suppressed by optimized plasma parameters or Al2O3 sealing layers.

High-resolution high-efficiency multilayer Fresnel zone plates for soft and hard x-rays

X-ray microscopy enables high spatial resolutions, high penetration depths and characterization of a broad range of materials. Calculations show that nanometer range resolution is achievable in the hard X-ray regime by using Fresnel zone plates (FZPs) if certain conditions are satisfied. However, this requires, among other things, aspect ratios of several thousands. The multilayer (ML) type FZPs, having virtually unlimited aspect ratios, are strong candidates to achieve single nanometer resolutions. Our research is focused on the fabrication of ML-FZPs which encompasses deposition of multilayers over a glass fiber via the atomic layer deposition (ALD), which is subsequently sliced in the optimum thickness for the X-ray energy by a focused ion beam (FIB). We recently achieved aberration free imaging by resolving 21 nm features with an efficiency of up to 12.5 %, the highest imaging resolution achieved by an ML-FZP. We also showed efficient focusing of 7.9 keV X-rays down to 30 nm focal spot size (FWHM). For resolutions below ~10 nm, efficiencies would decrease significantly due to wave coupling effects. To compensate this effect high efficiency, low stress materials have to be researched, as lower intrinsic stresses will allow fabrication of larger FZPs with higher number of zones, leading to high light intensity at the focus. As a first step we fabricated an ML-FZP with a diameter of 62 μm, an outermost zone width of 12 nm and 452 active zones. Further strategies for fabrication of high resolution high efficiency multilayer FZPs will also be discussed.

Overview of the multilayer-Fresnel zone plate and the kinoform lens development at MPI for Intelligent Systems

The ultimate goal of our research is to develop novel fabrication methods for high efficiency and high resolution X-ray optics. To this end, we have been pursuing the fabrication of several innovative diffractive/refractive optics designs. One such optic is the multilayer type Fresnel zone plate (ML-FZP). Our fabrication process relies on the atomic layer deposition (ALD) of two materials on a smooth glass fiber followed by a focused ion beam (FIB) based slicing and polishing. The ALD process allows much smaller outermost zone widths than the standard electron beam lithography based FZPs, meaning FZPs with potentially higher resolutions. Moreover, by depositing the multilayer on a cm long glass-fiber FZPs with very high optical thicknesses can be fabricated that can efficiently focus harder X-rays as well. A 21 nm half-pitch resolution was achieved using the ML-FZPs. Another optic we have been working on is the kinoform lens, which is a refractive/diffractive optic with a 100 % theoretical focusing efficiency. Their fabrication is usually realized by using approximate models which limit their success. Recently the fabrication of real kinoform lenses has been successfully realized in our lab via gray-scale direct-write ion beam lithography without any approximations. The lenses have been tested in the soft X-ray range achieving up to ~90 % of the calculated efficiency which indicates outstanding replication of the designed profile. Here we give an overview of our research and discuss the future challenges and opportunities for these optics.

Fabrication and X-ray testing of true kinoform lenses with high efficiencies

Kinoform lenses are focusing optics with a 100 % theoretical focusing efficiency. Up to date, the actual continuous 3D surface relief profiles of X-ray kinoform lenses could only be approximately fabricated. Now, we have come up with an effective ion beam lithography fabrication strategy producing first-ever imaging-quality circularly symmetric kinoform lenses which demonstrated reasonably high focusing efficiencies. Here, we will discuss the potential of the fabrication method and the utility of kinoform lenses enabled by it. Special emphases will be placed on materials development including selection and design, efficiency considerations for various energies and possible applications.

3D Nanofabrication of High-Resolution Multilayer Fresnel Zone Plates

Abstract Focusing X‐rays to single nanometer dimensions is impeded by the lack of high‐quality, high‐resolution optics. Challenges in fabricating high aspect ratio 3D nanostructures limit the quality and the resolution. Multilayer zone plates target this challenge by offering virtually unlimited and freely selectable aspect ratios. Here, a full‐ceramic zone plate is fabricated via atomic layer deposition of multilayers over optical quality glass fibers and subsequent focused ion beam slicing. The quality of the multilayers is confirmed up to an aspect ratio of 500 with zones as thin as 25 nm. Focusing performance of the fabricated zone plate is tested toward the high‐energy limit of a soft X‐ray scanning transmission microscope, achieving a 15 nm half‐pitch cut‐off resolution. Sources of adverse influences are identified, and effective routes for improving the zone plate performance are elaborated, paving a clear path toward using multilayer zone plates in high‐energy X‐ray microscopy. Finally, a new fabrication concept is introduced for making zone plates with precisely tilted zones, targeting even higher resolutions.

High-throughput synthesis of modified Fresnel zone plate arrays via ion beam lithography

Fresnel zone plates (FZP) are diffractive photonic devices used for high-resolution imaging and lithography at short wavelengths. Their fabrication requires nano-machining capabilities with exceptional precision and strict tolerances such as those enabled by modern lithography methods. In particular, ion beam lithography (IBL) is a noteworthy method thanks to its robust direct writing/milling capability. IBL allows for rapid prototyping of high-resolution FZPs that can be used for high-resolution imaging at soft X-ray energies. Here, we discuss improvements in the process enabling us to write zones down to 15 nm in width, achieving an effective outermost zone width of 30 nm. With a 35% reduction in process time and an increase in resolution by 26% compared to our previous results, we were able to resolve 21 nm features of a test sample using the FZP. The new process conditions are then applied for fabrication of large arrays of high-resolution zone plates. Results show that relatively large areas can be decorated with nanostructured devices via IBL by using multipurpose SEM/FIB instruments with potential applications in FEL focusing, extreme UV and soft X-ray lithography and as wavefront sensing devices for beam diagnostics.

3D nanoprinted plastic kinoform x-ray optics

High-performance focusing of X-rays requires the realization of very challenging 3D geometries with nanoscale features, sub-millimeter-scale apertures, and high aspect ratios. A particularly difficult structure is the profile of an ideal zone plate called a kinoform, which is manufactured in nonideal approximated patterns, nonetheless requires complicated multistep fabrication processes. Here, 3D fabrication of high-performance kinoforms with unprecedented aspect ratios out of low-loss plastics using femtosecond two-photon 3D nanoprinting is presented. A thorough characterization of the 3D-printed kinoforms using direct soft X-ray imaging and ptychography demonstrates superior performance with an efficiency reaching up to 20%. An extended concept is proposed for on-chip integration of various X-ray optics toward high-fidelity control of X-ray wavefronts and ultimate efficiencies even for harder X-rays. Initial results establish new, advanced focusing optics for both synchrotron and laboratory sources for a large variety of X-ray techniques and applications ranging from materials science to medicine.

Comparative study of ALD SiO_2 thin films for optical applications

We have investigated the suitability of atomic layer deposition (ALD) for SiO2 optical coatings and applied it to broadband antireflective multilayers in combination with HfO2 as the high refractive index material. SiO2 thin films were successfully grown using tris[dimethylamino]silane (3DMAS), bis[diethylamino]silane (BDEAS) with plasma activated oxygen as precursors, and the AP-LTO330 precursor with ozone, respectively. The amorphous SiO2 films show very low optical losses within a spectral range of 200 nm to 1100 nm. Laser calorimetric measurements show absorption losses of 300 nm thick SiO2 films of about 1.5 parts per million at a wavelength of 1064 nm. The films are optically homogeneous and possess a good scalability of film thickness. The film surface porosity - which correlates to a shift in the transmittance spectra under vacuum and air conditions - has been suppressed by optimized plasma parameters or Al2O3 sealing layers.

Comparative study of ALD SiO 2 thin films for optical applications

We have investigated the suitability of atomic layer deposition (ALD) for SiO2 optical coatings and applied it to broadband antireflective multilayers in combination with HfO2 as the high refractive index material. SiO2 thin films were successfully grown using tris[dimethylamino]silane (3DMAS), bis[diethylamino]silane (BDEAS) with plasma activated oxygen as precursors, and the AP-LTO330 precursor with ozone, respectively. The amorphous SiO2 films show very low optical losses within a spectral range of 200 nm to 1100 nm. Laser calorimetric measurements show absorption losses of 300 nm thick SiO2 films of about 1.5 parts per million at a wavelength of 1064 nm. The films are optically homogeneous and possess a good scalability of film thickness. The film surface porosity - which correlates to a shift in the transmittance spectra under vacuum and air conditions - has been suppressed by optimized plasma parameters or Al2O3 sealing layers.