Universität Stuttgart

Now showing 1 - 4 of 4

Open Access
Novel X-ray lenses for direct and coherent imaging
(2019) Sanli, Umut Tunca; Schütz, Gisela (Prof. Dr.)
Open Access
Ion beam lithographic and multilayer fresnel zone plates for soft and hard X-rays: nanofabrication and characterization
(2015) Keskinbora, Kahraman; Schütz, Gisela (Prof. Dr.)
X-ray microscopy has become an important analytical characterization method for a plethora of applications in materials science, physics, chemistry and biology, thanks to the emergence of modern synchrotron radiation facilities. These facilities enable high brilliance, energy tunable, variable polarization X-rays which gives access to mass density, elemental, chemical, electronic and magnetic properties of materials. In the soft X-ray energies nearly all elements can be probed by spectromicroscopic methods. Another important property of synchrotron radiation is the time structure in the ns to ps range, which can be utilized for sophisticated time resolution studies. These opportunities can be combined with high spatial resolution which is determined by the focusing method and the optic. Focusing of X-rays has historically been a difficult task due to strong absorption and weak phase shift of X-rays within matter. The required phase shift of X-rays, which depends on the real part of the complex refractive index, differs from 1 (the vacuum refractive index) only on the order of 10^-2 to 10^-6 and conventional lenses do not work. One very successful X-ray optic is the Fresnel Zone Plate (FZP), a diffractive optic that act as a lens under certain conditions and can focus X-rays to nanometer sized spots. The resolution of the FZP depends on the width of the outermost zone and is highly correlated with the smallest feature that can be fabricated. Conventionally, the e-beam lithography (EBL) is used for production FZPs which could resolve up to 10 nm structures with serious limitations. One difficulty of EBL is its ever increasing complexity for many-step fabrication of smaller features or intricate geometries. Therefore, EBL is mostly constrained to planar, binary geometries with moderate efficiencies strongly decreasing with energy and not effective for hard X-rays. Special 3D geometries in the form of kinoform lenses can theoretically have 100 % focusing efficiencies. Attempts to approximate these geometries via EBL increased the number of process steps even further. The smallest FZP feature size even for low aspect ratios achievable via EBL is fundamentally limited due to the proximity effect which is the interaction and spread of electrons within the resist material. We addressed these issues by focusing our research on alternative FZP fabrication techniques as high-speed ion beam lithography (IBL), and gray scale ion lithography to realize efficient kinoforms. Another approach towards full-material multilayer FZPs with infinite aspect ratio was based on atomic layer deposition (ALD) with subsequent ion beam slicing. Each of these three methods targets specific challenges faced by the e-beam lithography based FZP fabrication techniques. All the fabricated FZPs were tested for their resolution and efficiency performances at a state of the art scanning transmission X-ray microscope at BESSY for soft X-rays and/or at optical test stations at ESRF and PETRA III for hard X-rays. Using IBL the rapid preparation of a 110 nm thick Au FZP with 50 µm diameter and 50 nm ∆r in less than 13 minutes is demonstrated. Employed for X-ray microscopy, the FZP clearly resolved 28.5 nm features with a cut-off of 24.3 nm at ~1120 eV. Additional process improvements were made towards smaller zones with higher zone quality. They allowed the preparation of a FZP with 30 nm outermost half-period remarkably, in about 8 min. This FZP was shown to clearly resolve 21 nm features on a multilayer test object with large room for improvement. This high through-put FZP production route is of special interest not only concerning the low cost and easy availability. A large array of these optical components is attractive, for experiments such as one-shot ultra-high brilliance FEL investigations due to the radiation damage or for instance for coded-aperture arrays for high-angle resolving X-ray astronomy. Towards fabrication of kinoforms for high efficiency X-ray focusing, we have performed various materials optimization studies in order to achieve a high surface quality optic. After various trials the materials were finally optimized and the fabricated lenses achieved more than 14 % absolute diffraction efficiency that is almost 90 % compared to the theoretical prediction. This confirms how closely we were able to replicate the ideal three dimensional surface relief structure for the first time. It was possible to carry out imaging with these lenses with half-pitch resolutions down to 60 nm. The kinoform lenses were tested at the soft X-ray range where a significant absorption is present in materials. These results also potentially pave the way for very high efficiency hard X-ray focusing which can in principle be utilized in laboratory based X-ray sources, X-ray astronomy and the new rising field of X-ray ptychography. To fabricate high resolution ML-FZPs, Al2O3/Ta2O5multilayers, deposited on a smooth glass optical fiber via atomic layer deposition using non-dedicated instruments were carefully cut-out, sliced and polished to a high quality surface finish using focused ion beams. Following the transfer of the slice to a TEM grid as holder the slices were polished to a high surface finish quality, also via a focused ion beam. Fabricated ML-FZPs were synchrotron tested using an in-house constructed 2-axis tilt stage specially designed for aligning ML-FZP with respect to the X-ray optical axis. The results showed that it was possible to resolve 21 nm features in direct imaging at 1200 eV and sub-30 nm focusing at 8 keV. This is the highest demonstrated resolving power for a multilayer type FZP, to date to the best of our knowledge. Results exhibit the potential for high-resolution hard X-ray focusing where this type of optics are especially efficient. For ultra-high resolution hard and soft X-ray imaging, with potentially achievable ∆r of a few nm is well below what can be achieved through any lithography method available today.
Open Access
Deposition and characterization of multi-functional, complex thin films using atomic layer deposition for copper corrosion protection
(2022) Dogan, Gül; Schütz, Gisela (Prof. Dr.)
This thesis focuses on ALD thin film protection properties against corrosion of copper to develop an understanding of material interface properties and to develop novel thin films processes. This understanding is then applied to enhance materials with potential use in semiconductor devices. The main research objectives are listed below: Understanding corrosion protection properties of ALD thin films: - Development of protective thin films by combining different oxide layers - To characterize the protection properties at high temperatures and in aggressive environments, - To understand the interaction of copper and ALD protection layers when exposed to high temperatures, - Finding the optimum deposition parameters to achieve defect-free thin layers for best corrosion protection Application of ALD oxide thin films for copper corrosion protection in semiconductor devices: - Structuring the ALD thin films to make reliable interface for copper-copper interconnects with micromachining methods such as laser drilling and plasma etching - To remove ALD layers in a localized, selective way without degradation of the underlying copper layer
Open Access
Interfacial mechanics and liquid crystal structure of liquid gallium
(2021) Yunusa, Muhammad; Sitti, Metin (Prof. Dr.)
This dissertation aims to shed light on the fundamental aspects of supercooled liquid gallium. First, the mechanical properties of the oxide skin encapsulating the supercooled liquid gallium droplet is investigated. By leveraging the supercooling behavior of gallium and the formation of its oxide skin, we characterized the emergence of wrinkling at the interface and adhesion energy between the liquid gallium and rigid substrate. An interfacial energy of 0.238 ± 0.008 J m-2 was measured between gallium droplet and flat glass. Second, seed induced crystallization of supercooled liquid gallium on the different substrate materials revealed the dependence of the kinetics of crystallization on thermal behavior of the substrate. Such approach further elucidates on the importance of interface temperature during crystallization with a preferred orientation. The third part of the dissertation reports an unprecedented liquid crystal structure in supercooled liquid gallium. By the use of techniques applied in liquid crystal (LC) research, observation of LC texture in supercooled liquid gallium was realized. Reflective polarized optical microscopy (R-POM) on liquid gallium sandwiched between glasses treated with rubbed polymers reveals the onset of an anisotropic reflection of possible dimer molecules or clusters at 120°C that increases on cooling and persists down to room temperature or below. On the other hand, when gallium is sandwiched between substrates that align conventional liquid crystal molecules normal to the surface, the reflection is isotropic. This observation of LC structure of a highly electrically conductive supercooled liquid gallium provides an unexpected new field of materials science and liquid crystal research.

Universität Stuttgart

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