X-ray Reflectometry Data Research Articles

JGIXA — A software package for the calculation and fitting of grazing incidence X-ray fluorescence and X-ray reflectivity data for the characterization of nanometer-layers and ultra-shallow-implants

Grazing incidence XRF (GIXRF) is a very surface sensitive, nondestructive analytical tool making use of the phenomenon of total external reflection of X-rays on smooth polished surfaces. In recent years the method experienced a revival, being a powerful tool for process analysis and control in the fabrication of semiconductor based devices. Due to the downscaling of the process size for semiconductor devices, junction depths as well as layer thicknesses are reduced to a few nanometers, i.e. the length scale where GIXRF is highly sensitive. GIXRF measures the X-ray fluorescence induced by an X-ray beam incident under varying grazing angles and results in angle dependent intensity curves. These curves are correlated to the layer thickness, depth distribution and mass density of the elements in the sample. But the evaluation of these measurements is ambiguous with regard to the exact distribution function for the implants as well as for the thickness and density of nanometer-thin layers. In order to overcome this ambiguity, GIXRF can be combined with X-ray reflectometry (XRR). This is straightforward, as both techniques use similar measurement procedures and the same fundamental physical principles can be used for a combined data evaluation strategy. Such a combined analysis removes ambiguities in the determined physical properties of the studied sample and, being a correlative spectroscopic method, also significantly reduces experimental uncertainties of the individual techniques.In this paper we report our approach to a correlative data analysis, based on a concurrent calculation and fitting of simultaneously recorded GIXRF and XRR data. Based on this approach we developed JGIXA (Java Grazing Incidence X-ray Analysis), a multi-platform software package equipped with a user-friendly graphic user interface (GUI) and offering various optimization algorithms. Software and data evaluation approach were benchmarked by characterizing metal and metal oxide layers on Silicon as well as Arsenic implants in Silicon. The results of the different optimization algorithms have been compared to test the convergence of the algorithms. Finally, simulations for Iron nanoparticles on bulk Silicon and on a W/C multilayer are presented, using the assumption of an unaltered X-ray Standing Wave above the surface.

Determining sample alignment in x-ray reflectometry using thickness and density from GaAs/AlAs multilayer certified reference materials

X-ray reflectometry (XRR) provides researchers and manufacturers with a non-destructive way to determine thickness, roughness and density of thin films deposited on smooth substrates. Due to the nested nature of equations modeling this phenomenon, the inter-relation between instrument alignment and parameter estimation accuracy is somewhat opaque. In this study, we intentionally shift incident angle information from a high-quality XRR data set and refine a series of shifted data sets using an identical structural model to assess the effect this angle misalignment has on parameter estimation. We develop a series of calibration curves relating angle misalignment to variation in layer thickness and density for a multilayer GaAs/AlAs Certified Reference Material on a GaAs substrate. We then test the validity and robustness of several approaches of using known thickness and density parameters from this structure to calibrate instrument alignment. We find the highest sensitivity to and linearity with, measurement misalignment from buried AlAs and GaAs layers, in contrast to the surface layers, which show the most variability. This is a fortuitous result, as buried AlAs and GaAs exhibit the highest long-term stability in thickness. Therefore, it is indeed possible to use reference thickness estimates to validate XRR angle alignment accuracy. Buried layer mass density information also shows promise as a robust calibration approach. This is surprising, as electron density of buried layers is both a highly-correlated phenomenon and a subtle component within the XRR model.

Diagnostics of the atomic structure of multilayer metallic nanoheterostructures from reflectometry data: A new approach to low-contrast systems

A new method for determining the concentration profiles of chemical elements in multilayer metallic nanoheterostructures from X-ray reflectometry data has been developed as applied to low-contrast systems. The method is based on the solution of the Fredholm integral equation of the first kind, which relates the reflection coefficient and the concentration profile of the chemical elements involved in the composition of the sample. The ill-posed inverse problem of the determination of the concentration profile is solved by the regularization method. The efficiency of the proposed method is confirmed by model calculations performed for a four-layer Cr/Gd/Fe/Cr//Si structure with high-contrast Cr/Gd pairs and low-contrast Fe/Cr pairs. The experimental results on the determination of the concentration profile of the surface layer of the epitaxial thin Cr films and three-layer Cr/Fe/Cr structure deposited on the Al2O3 substrate are presented.

Patterson analysis for layer profile determination by neutron or X-ray reflectometry. II

A data analysis technique based on the Patterson function is tested with simulated X-ray reflectometry data. In comparison to the case for neutron reflectometry, additional data manipulation is required because the scattering-length density of materials is generally much higher for X-rays than for neutrons. With this extra step duly performed, the technique yields a model layer profile accurate enough for least-squares refinement.

Problems in measurements of parameters of elements and structures in modern micro- and nanoelectronics considering TiN/Ti diffusion barrier structures as an example

Results of a comprehensive analysis of process variables of TiN/Ti diffuse barrier structures used in modern microelectronics are presented. To provide reliable spectral ellipsometry results, which is a common technique for postprocess control of these structures during the testing stage of the manufacturing process, a comprehensive approach is suggested that consists in consistent processing of transmission electron microscopy and X-ray reflectometry data. The resulting data were used to calculate the variances of optical coefficients of spectral ellipsometry, which depend on features of the manufacturing process and are required for analysis of the parameters of manufactured diffuse barrier structures.

Investigation of the structure and luminescence properties of CdF2-CaF2 : Eu superlattices on Si(111)

A complex investigation of CdF2-CaF2 : Eu superlattices with different bilayer thicknesses (2.0–17.5 nm) grown by molecular beam epitaxy on Si(111) has been carried out. The structural perfection of the layers and interfaces of the superlattices have been estimated from the X-ray diffractometry and reflectometry data. The possibility of producing short-period pseudomorphic superlattices with a period of approximately 2 nm has been established. It has been shown that these superlattices are characterized by a larger root-mean-square roughness amplitude of the interfaces as compared to the long-period superlattices. The specific features of cathodoluminescence spectra have been analyzed as a function of the superlattice period. It has been revealed that, with a decrease in the superlattice period, the intrinsic luminescence intensity of fluorides increases in comparison with the intensity of the luminescence associated with the emission of Eu2+ impurity ions; in this case, several Eu3+ luminescence bands appear in the spectrum. The possibilities of electron probe microanalysis for determining the ratio of thicknesses of individual layers in short-period superlattices have been demonstrated.

X-ray reflectometry study of diamond-like carbon films prepared by plasma enhanced chemical vapor deposition in a low pressure inductively coupled plasma

In this work we probe the structural properties of amorphous hydrogenated carbon thin films prepared by plasma-enhanced chemical vapor deposition in a low pressure inductively coupled plasma using X-ray reflectometry in order to study the effect of varying the ion energy on the density of these films. The ion energy is varied by varying the RF power used to bias the substrate. It is shown that a very low ion energy is already sufficient to obtain a dense diamond-like carbon (DLC) film, in contrast with other deposition techniques where much higher ion energies are required to obtain a dense DLC film. The results of this study are corroborated by Raman spectroscopy and ellipsometry measurements. The X-ray reflectometry data analysis is detailed in order to highlight some methodological problems encountered during the data fitting which could lead to an incorrect interpretation of the measured curves.

Quiescent bilayers at the mica–water interface

Despite extensive studies with many experimental techniques, the morphology and structure of the self-assembled aggregates of quaternary alkyl ammonium bromides (CnTABs; where n denotes the number of hydrocarbons in the surfactant tail) at the solid–liquid interface remains controversial, with results from atomic force microscopy (AFM) imaging pointing to a variety of surface aggregates such as cylinders and surface micelles, whilst surface force measurements and neutron reflectivity (NR) measurements reporting bilayer structures. Using a home-built liquid cell that employs the “bending mica” method, we have performed unprecedented synchrotron X-ray reflectometry (XRR) measurements to study the adsorption behaviour of a CnTAB series (n = 10, 12, 14, 16 and 18) at the mica–water interface at different surfactant concentrations. We find that our XRR data cannot be described by surface aggregates such as cylindrical and spherical structures reported by AFM studies. In addition we have observed that the bilayer thickness, surface coverage and the tilt angle all depend on the surfactant concentration and surfactant hydrocarbon chain length n, and that the bilayer thickness reaches a maximum value at approximately the critical micellisation concentration (∼1 cmc) for all the CnTABs investigated. We propose that CnTABs form disordered bilayer structures on mica at concentrations below cmc, whilst at ∼1 cmc they form more densely packed bilayers with the tails possibly tilted at an angle θt ranging from ∼40 to 60° with respect to the surface normal in order to satisfy the packing constraints due to the mica lattice charge, i.e. so that the cross-section area of the tilted chain would match that of the area of the lattice charge (As ≅ 46.8 Å2). As the surfactant concentration further increases, we find that the bilayer thickness decreases, and we ascribe this to the desorption of surfactant molecules, which recovers certain disorder and fluidity in the chain and thus leads to interdigitated bilayers again. In light of our XRR results, previously unattainable at the mica–water interface, we suggest that the surface aggregates observed by AFM could be induced by the interaction between the scanning probe and the surfactant layer, thus representing transient surface aggregation morphologies; whereas the CnTAB bilayers we observe with XRR are intrinsic structures under quiescent conditions. The suggestion of such quiescent bilayers will have fundamental implications to processes such as lubrication, self-assembly under confinement, detergency and wetting, where the morphology and structure of surfactant layers at the solid–liquid interface is an important consideration.

Double layer structure of ZnO thin films deposited by RF-magnetron sputtering on glass substrate

Transparent ZnO films are synthesized by RF-magnetron sputtering (1.78MHz) onto glass substrates, using a mild-pressed ZnO powder target. The depositions were carried at three inert argon pressures (0.25Pa, 0.30Pa, and 0.45Pa) at two substrate temperatures (100°C and 400°C). The role of the sputtering conditions on ZnO thin films nanostructuring, optical properties and morphology is investigated by X-ray diffraction (XRD), X-ray reflectometry (XRR) and Spectroscopic ellipsometry (SE). XRD investigations revealed that ZnO films show a (00l) texture with nanosized crystallites. Right-angle asymmetry of the (002) diffraction peak is observed. The peak profile analysis using pseudo-Voigt functions unveils a double overlapped peak structure with different coherent zone size values. A double layer structure is evidenced by analyzing the XRR data. Samples prepared at 0.3Pa at a temperature of 400°C have a ∼4nm bottom layer consisting of highly depleted in oxygen ZnO1−x structure, continued by a 53nm top layer of textured ZnO. Electrical measurements show that the temperature dependence of the conductivity is well described by the Mott variable range hopping (VRH) law. The samples obtained at 400°C have a significantly lower resistivity.

Patterson analysis for layer profile determination by neutron or X-ray reflectometry

The possibility of using Patterson analysis to interpret neutron or X-ray reflectometry data has been critically examined in terms of a hypothetical case study. The technique yields a highly accurate layer profile model provided the data are measured with a beam geometry that gives rise to unit reflectivity atQ= 0 only,i.e.Qc= 0. For those cases where a plateau of unit reflectivity extends out to finiteQ,i.e.Qc> 0, a distorted model is obtained. A strategy to significantly improve this initial estimate of the model is proposed and demonstrated.

Stable, ordered multilayers of partially fluorinated bolaamphiphiles at the air–water interface

The article presents systematic research on Langmuir films of partially fluorinated bolaamphiphiles of different shapes. Such films exhibit a layering transition from a monolayer to a trilayer during compression on the air–water interface. Further compression gives different results depending on the shape and degree of fluorination of the molecules. Partially fluorinated compounds form well defined multilayers in a reversible process. The balance between rigidity and flexibility of the molecules, adjusted by the fluorination and shape of the molecules, seems to be the key factor in avoiding irreversible aggregation of the molecules and creating ordered multilayer structures. Anchor-shaped bolaamphiphiles form a trilayer and, subsequently, a 9-layer film due to a double roll-over mechanism. In contrast, when trilayer films of X-shaped bolaamphiphiles are compressed, 5- and 7-layer films are created according to a different mechanism. Films of thickness of up to nine layers were transferred from the water surface to solid substrates in a single step procedure without any distortion in the structure of the layers. X-ray reflectometry (XRR) was used to measure the thickness of the layers. Perfect fits of the XRR data to theoretical equations allowed for a conclusion that the multilayers are well-ordered lamellar structures. These investigations lead to an improvement in the general understanding of trilayer and multilayer formation and indicate that only in exceptional cases it happens due to a roll-over process.

Surface structure of a “non-amphiphilic” protic ionic liquid

The nanostructure of the ethanolammonium nitrate (EtAN)-air surface has been investigated using X-ray reflectometry (XRR), vibrational sum frequency spectroscopy (VSFS) and neutral impact collision ion scattering spectroscopy (NICISS). The XRR data decays more rapidly than expected for a perfectly sharp interface, indicating a diffuse electron (scattering length) density profile. Modelling of the XRR data using three different fitting routines produced consistent interfacial profiles that suggest the formation of interfacial EtAN clusters. Consistent with this, VSFS reveals that the EtAN surface is predominantly covered by -CH(2)- moieties, with the -NH(3)(+) and -OH groups of the cation buried slightly deeper in the interface. The elemental profiles determined using NICISS also show enrichment of carbon relative to nitrogen and oxygen in the outermost surface layer, which is consistent with the surface cation orientation deduced from VSFS, and with the presence of EtAN aggregates at the liquid surface.

Performance of the genetic algorithm in X-ray reflectometry data analysis

The effect of the width of the search range of solutions on genetic algorithm performance in the interpretation of X-ray reflectograms has been analyzed. The search algorithm has been optimized, based on the results.

Study of roughness in multilayer Mo–Si mirrors

AbstractCombined approach based on X‐ray reflectometry (XRR), atomic‐force microscopy (AFM), and small‐angle X‐ray scattering (SAXS) has been applied to two series of multilayer Mo/Si mirrors grown by magnetron sputtering on standard and specially smoothed substrates under different technological conditions (A and B). Simulation of XRR data, using a model of two layers in period, can distinguish the mirror quality in terms of the total root mean square roughness. The quality is better for mirrors grown on smoother substrate, but it also depends on technological conditions. In addition, use of a model with transition layers reveals an asymmetry of Mo/Si and Si/Mo interfaces. Analysis of surface roughness PSD functions obtained by AFM shows that the surface diffusion is the dominant smoothing mechanism for all the mirrors studied. The low‐frequency roughness is conformal to that of the substrate for better mirrors (series A), but increases for poorer quality mirrors (series B). Simulation of SAXS data provides roughness parameters of the internal interfaces. For the better mirrors, the lateral correlation length of the roughness does not exceed 13 nm. However, this length is about 50 nm for the poorer quality mirrors, in agreement with AFM data.

DEoptim: AnRPackage for Global Optimization by Differential Evolution

This article describes the R package <b>DEoptim</b>, which implements the differential evolution algorithm for global optimization of a real-valued function of a real-valued parameter vector. The implementation of differential evolution in <b>DEoptim</b> interfaces with C code for efficiency. The utility of the package is illustrated by case studies in fitting a Parratt model for X-ray reflectometry data and a Markov-switching generalized autoregressive conditional heteroskedasticity model for the returns of the Swiss Market Index.

Growth kinetics of intermetallic alloy phase at the interfaces of a Ni/Al multilayer using polarized neutron and x-ray reflectometry

Al-Ni-based binary alloys offer several intermetallic phases of immense technological importance. We have attempted to understand the growth of interface alloy in an Al-Ni multilayer sample as a function of annealing time using primarily x-ray reflectometry (XRR) and polarized neutron reflectometry (PNR). Powder x-ray diffraction was also used to determine various crystallographic phases in the sample. The multilayer showed remarkable stability with respect to annealing time, following an initial alloy formation at the interface. Stability of such multilayers is important for their applicability as corrosion resistant coatings as well as metallization layers in microelectronic devices. Using XRR and PNR data we have identified the interface layer as ${\text{Al}}_{3}\text{Ni}$ intermetallic phase. Magnetic depth profile obtained from PNR shows that the interface alloy layer is magnetically dead. From the Bragg peak intensities of polarized neutron reflectivity measurements, we have estimated the diffusion lengths after annealing at $160\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ for 1--8 h.

Specific Ion versus Electrostatic Effects on the Construction of Polyelectrolyte Multilayers

Self-assembled multilayers of a strong polyanion, poly(sodium 4-styrenesulfonate) (PSS), and a strong polycation, poly[(diallyl-dimethyl-ammonium chloride)-stat-(N-methyl-N-vinyl acetamide)] (P(DADMAC-stat-NMVA)), are fabricated on silicon substrates. This article addresses the effect of electrostatics versus ion specificity. Therefore, multilayer formation and growth are investigated as a function of the charge density of the polycation, the type of salt in the polyelectrolyte dipping solution, and its ionic strength. This study focuses on monovalent ions (Li(+), Na(+), K(+), Cs(+), Rb(+), F(-), Cl(-), Br(-), and ClO(3)(-)). Ellipsometry and X-ray reflectometry data indicate that anions have a significantly larger effect on the thickness of the multilayer, but contrary to other studies on ion-specific effects, the influence of the type of cation is not negligible at higher salt concentrations. Larger ions, with smaller hydration shells, are highly polarizable and consequently interact strongly with charged polyelectrolytes, resulting in thicker and rougher multilayers. AFM studies confirm a higher roughness of the multilayer prepared from larger anions. The substrate can mask ion-specific effects over a distance of about 10 nm. Ion-specific effects become important above an ionic strength of 0.1 M in the case of anions and above an ionic strength of 0.25 M for cations. At lower ionic strengths, electrostatic interactions between and within the polyelectrolyte chains are dominating. Reducing the degree of polymer charge down to 75% does not shift this threshold of ionic strength. It is shown that a combination of ionic strength, polymer charge, and type of ion is a suitable tool for tuning the mobility and stability of polyelectrolyte multilayers.

Solution of the inverse problem for reconstructing the real structure of materials from the data of different X-ray methods

A software program has been developed for joint solution of the inverse problem on the basis of the X-ray diffractometry and reflectometry data. A system of basic objects supporting some functions of automatic data flow processing in the program and oriented to mathematical calculations has been created. It is shown by the example of experimental data for an InxGa1 − xAs-InyAl1 − yAs/InP(001) sample that joint fitting of X-ray diffractometry and reflectometry curves makes it possible to reconstruct the model parameters of a multilayer with smaller rms errors. The data were processed taking into account the angular dependences of the background intensity and the aperture factors stemming from the geometry of the experiment.

Thermal grafting of organic monolayers on amorphous carbon and silicon (111) surfaces: A comparative study

Thermally-assisted (160 °C) liquid phase grafting of linear alkene molecules has been performed simultaneously on amorphous carbon (a-C) and hydrogen passivated crystalline silicon Si(111):H surfaces. Atomically flat a-C films with a high sp 3 average surface hybridization, sp 3 / (sp 2 + sp 3) = 0.62, were grown using pulsed laser deposition (PLD). Quantitative analysis of X-ray photoelectron spectroscopy, X-ray reflectometry and spectroscopic ellipsometry data show the immobilization of a densely packed (> 3 × 10 14 cm − 2) single layer of organic molecules. In contrast with crystalline Si(111):H and other forms of carbon films, no surface preparation is required for the thermal grafting of alkene molecules on PLD amorphous carbon. The molecular grafted a-C surface is stable against ambient oxidation, in contrast with the grafted crystalline silicon surface.

X-ray reflectivity and total reflection x-ray fluorescence study of surface oxide evolution in a GaAs/AlAs multilayer system

Modifications of GaAs/air interface under x-ray exposure have been investigated by means of x-ray reflectometry (XRR). For this purpose a GaAs/AlAs multilayer system was continuously monitored for 90 h while acquiring XRR profiles. Three different starting models for the oxide/contaminants layer were tested for the analysis of XRR data and discussed. The main effect of x-ray exposure, revealed by the three different approaches, is an increase in the thickness of the gallium and arsenic oxides until a saturation value (about 3.0 nm) is reached after about 50 h. Total reflection x-ray fluorescence analysis performed on a couple of twin samples, either irradiated and nonirradiated, confirms that the oxidation process is promoted by x-ray exposure and indicates the presence of a richer As oxide phase at the surface of the x-ray irradiated samples.