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Register a new userGrazing incidence X-ray fluorescence based characterization of nanostructures for element sensitive profile reconstruction
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Anna Andrle
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For the reliable fabrication of the current and next generation of nanostructures it is essential to be able to determine their material composition and dimensional parameters. Using the grazing incidence X-ray fluoresence technique, which is taking advantage of the X-ray standing wave field effect, nanostructures can be investigated with a high sensitivity with respect to the structural and elemental composition. This is demonstrated using lamellar gratings made of Si$_3$N$_4$. Rigorous field simulations obtained from a Maxwell solver based on the finite element method allow to determine the spatial distribution of elemental species and the geometrical shape with sub-nm resolution. The increasing complexity of nanostructures and demanded sensitivity for small changes quickly turn the curse of dimensionality for numerical simulation into a problem which can no longer be solved rationally even with massive parallelisation. New optimization schemes, e.g. machine learning, are required to satisfy the metrological requirements. We present reconstruction results obtained with a Bayesian optimization approach to reduce the computational effort.
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The characterization of nanostructured surfaces with sensitivity in the sub-nm range is of high importance for the development of current and next generation integrated electronic circuits. Modern transistor architectures for e.g. FinFETs are realized by lithographic fabrication of complex, well ordered nanostructures. Recently, a novel characterization technique based on X-ray fluorescence measurements in grazing incidence geometry has been proposed for such applications. This technique uses the X-ray standing wave field, arising from an interference between incident and the reflected radiation, as a nanoscale sensor for the dimensional and compositional parameters of the nanostructure. The element sensitivity of the X-ray fluorescence technique allows for a reconstruction of the spatial element distribution using a finite-element method. Due to a high computational time, intelligent optimization methods employing machine learning algorithms are essential for a timely provision of results. Here, a sampling of the probability distributions by Bayesian optimization is not only fast, it also provides an initial estimate of the parameter uncertainties and sensitivities. The high sensitivity of the method requires a precise knowledge of the material parameters in the modeling of the dimensional shape provided that some physical properties of the material are known or determined beforehand. The unknown optical constants were extracted from an unstructured but otherwise identical layer system by means of soft X-ray reflectometry. The spatial distribution profiles of the different elements contained in the grating structure were compared to scanning electron and atomic force microscopy and the influence of carbon surface contamination on the modeling results were discussed.
The optics of a number of future X-ray telescopes will have very long focal lengths (10 - 20 m), and will consist of a number of nested/stacked thin, grazing-incidence mirrors. The optical quality characterization of a real mirror can be obtained via profile metrology, and the Point Spread Function of the mirror can be derived via one of the standard computation methods. However, in practical cases it can be difficult to access the optical surfaces of densely stacked mirror shells, after they have been assembled, using the widespread metrological tools. For this reason, the assessment of the imaging resolution of a system of mirrors is better obtained via a direct, full-illumination test in X-rays. If the focus cannot be reached, an intra-focus test can be performed, and the image can be compared with the simulation results based on the metrology, if available. However, until today no quantitative information was extracted from a full-illumination, intra-focal exposure. In this work we show that, if the detector is located at an optimal distance from the mirror, the intensity variations of the intra-focal, full-illumination image in single reflection can be used to reconstruct the profile of the mirror surface, without the need of a wavefront sensor. The Point Spread Function can be subsequently computed from the reconstructed mirror shape. We show the application of this method to an intra-focal (8 m distance from mirror) test performed at PANTER on an optical module prototype made of hot-slumped glass foils with a 20 m focal length, from which we could derive an expected imaging quality near 16 arcsec HEW.
We have developed an experimental system to simultaneously observe surface structure, morphology, composition, chemical state, and chemical activity for samples in gas phase environments. This is accomplished by simultaneously measuring X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray scattering (GIXS) in gas pressures as high as the multi-Torr regime, while also recording mass spectrometry. Scattering patterns reflect near-surface sample structures from the nano- to the meso-scale. The grazing incidence geometry provides tunable depth sensitivity while scattered X-rays are detected across a broad range of angles using a newly designed pivoting-UHV-manipulator for detector positioning. At the same time, XPS and mass spectrometry can be measured, all from the same sample spot and in ambient conditions. To demonstrate the capabilities of this system, we measured the chemical state, composition, and structure of Ag-behenate on a Si(001) wafer in vacuum and in O$_2$ atmosphere at various temperatures. These simultaneous structural, chemical, and gas phase product probes enable detailed insights into the interplay between structure and chemical state for samples in gas phase environments. The compact size of our pivoting-UHV-manipulator makes it possible to retrofit this technique into existing spectroscopic instruments installed at synchrotron beamlines. Because many synchrotron facilities are planning or undergoing upgrades to diffraction limited storage rings with transversely coherent beams, a newly emerging set of coherent X-ray scattering experiments can greatly benefit from the concepts we present here.
Grazing Incidence X-ray Diffraction (GIXD) studies of monolayers of biomolecules at the air-water interface give quantitative information of in-plane packing, coherence lengths of the ordered diffracting crystalline domains and the orientation of hydrocarbon chains. Rheo-GIXD measurements revel quantitative changes in the monolayer under shear. Here we report GIXD studies of monolayers of Alamethicin peptide, DPPC lipid and their mixtures at the air-water interface under the application of steady shear stresses. The Alamethicin monolayer and the mixed monolayer show flow jamming transition. On the other hand, pure DPPC monolayer under the constant stress flows steadily with a notable enhancement of area/molecule, coherence length, and the tilt angle with increasing stress, suggesting fusion of nanocrystallites during flow. The DPPC-Alamethicin mixed monolayer shows no significant change in the area/DPPC molecule or in the DPPC chain tilt but the coherence length of both phases (DPPC and Alamethicin) increases suggesting that the crystallites of individual phases are merging to bigger size promoting more separation of phases in the system during flow. Our results show that Rheo-GIXD has the potential to explore in-situ molecular structural changes under rheological conditions for a diverse range of confined biomolecules at the interfaces.
Grazing incidence interferometry has been applied to rough planar and cylindrical surfaces. As suitable beam splitters diffractive optical phase elements are quite common because these allow for the same test sensitivity for all surface points. But a rotational-symmetric convex aspheric has two curvatures which reduces the measurable region to a meridian through the vortex of the aspheric, which is in contrast to cylindrical surfaces having a one-dimensional curvature which allows the test of the whole surface in gracing incidence. The meridional limitation for rotational-symmetric aspherics nevertheless offers the possibility to measure single meridians in a one-step measurement. An extension to the complete surface can be obtained by rotating the aspheric around its vortex within the frame of the test interferometer.
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