Soft X-ray Reflection Spectroscopy for Nano-Scaled Layered Structure Materials

Absolute EUV reflectivity measurements using a broadband high-harmonic source and an in situ single exposure reference scheme

We present a tabletop setup for extreme ultraviolet (EUV) reflection spectroscopy in the spectral range from 40 to 100 eV by using high-harmonic radiation. The simultaneous measurements of reference and sample spectra with high energy resolution provide precise and robust absolute reflectivity measurements, even when operating with spectrally fluctuating EUV sources. The stability and sensitivity of EUV reflectivity measurements are crucial factors for many applications in attosecond science, EUV spectroscopy, and nano-scale tomography. We show that the accuracy and stability of our in situ referencing scheme are almost one order of magnitude better in comparison to subsequent reference measurements. We demonstrate the performance of the setup by reflective near-edge x-ray absorption fine structure measurements of the aluminum L_2/3 absorption edge in α-Al₂O₃ and compare the results to synchrotron measurements.

Three-dimensional chemical-state imaging with reflection-mode soft x-ray absorption spectroscopy

In this study, a method for reflection-mode soft x-ray absorption spectroscopy was developed to realize three-dimensional chemical-state imaging. Soft x rays from a pinhole were reflected by the sample, and the magnified image was observed with a two-dimensional detector. This technique was applied to a Co film with an Au-island-covered surface to obtain the surface chemical state images with a spatial resolution of several tens of micrometers. Furthermore, the soft x-ray reflection spectra within and outside the Au layer were extracted from the images by changing the photon energy. Distinct differences were observed at the Co absorption edge. By considering anomalous x-ray scattering around the Co L-edges in the simulation, the reflection spectrum near the absorption edge in the nm depth resolution was reproduced. In the region without the Au layer, the results were well reproduced, assuming that 4 nm CoO was formed at the surface. These results demonstrate the feasibility of three-dimensional imaging of the chemical states in multilayer films.

Angstrom-Resolved Interfacial Structure in Buried Organic-Inorganic Junctions

Charge transport processes at interfaces play a crucial role in many processes. Here, the first soft x-ray second harmonic generation (SXR SHG) interfacial spectrum of a buried interface (boron-Parylene N) is reported. SXR SHG shows distinct spectral features that are not observed in x-ray absorption spectra, demonstrating its extraordinary interfacial sensitivity. Comparison to electronic structure calculations indicates a boron-organic separation distance of 1.9 Å, with changes of less than 1 Å resulting in easily detectable SXR SHG spectral shifts (ca. hundreds of milli-electron volts).

Understanding of stress and its correlation with microstructure near the layer continuous limit in nano-scaled multilayers

The evolution of residual stress and its correlation with microstructure are investigated systematically in nano-scaled periodic W/B4C multilayers (MLs) as a function of individual layer thicknesses at the ultra-thin limit (∼0.4–3 nm). Details of the microstructure are accessed through hard X-ray reflectivity and X-ray diffuse scattering (rocking scan) measurements. To understand the contributions of stresses in the layers of each type of material to the total stress in ML films, both the total stress in MLs and the stress in nanocrystalline W layers are analyzed and correlated. It is observed that the physical properties of the materials as well as their interfacial morphology undergo significant modification as the layer thickness varies from the continuous to the quasi-discontinuous regime. A non-monotonic variation of compressive total residual stress in the MLs is observed as a function of thicknesses of W and B4C and explained using a model of the mechanism of film growth. The observed value of in-plane total compressive residual stress of W/B4C MLs is less than the residual stress in W layers in the MLs, which indicates that the net combined stress from B4C layers and interfaces is tensile in nature. The observed compressive stress and the increase of lattice spacing with respect to the stress-free structure in W layers provide evidence of a peening effect. The observed higher surface density of grains with smaller average size and phase formation also provide high compressive stress in W layers.