X-ray Reflectometry Measurements Research Articles

Millisecond X-ray reflectometry and neural network analysis: unveiling fast processes in spin coating.

X-ray reflectometry (XRR) is a powerful tool for probing the structural characteristics of nanoscale films and layered structures, which is an important field of nanotechnology and is often used in semiconductor and optics manufacturing. This study introduces a novel approach for conducting quantitative high-resolution millisecond monochromatic XRR measurements. This is an order of magnitude faster than in previously published work. Quick XRR (qXRR) enables real time and in situ monitoring of nanoscale processes such as thin film formation during spin coating. A record qXRR acquisition time of 1.4 ms is demonstrated for a static gold thin film on a silicon sample. As a second example of this novel approach, dynamic in situ measurements are performed during PMMA spin coating onto silicon wafers and fast fitting of XRR curves using machine learning is demonstrated. This investigation primarily focuses on the evolution of film structure and surface morphology, resolving for the first time with qXRR the initial film thinning via mass transport and also shedding light on later thinning via solvent evaporation. This innovative millisecond qXRR technique is of significance for in situ studies of thin film deposition. It addresses the challenge of following intrinsically fast processes, such as thin film growth of high deposition rate or spin coating. Beyond thin film growth processes, millisecond XRR has implications for resolving fast structural changes such as photostriction or diffusion processes.

Investigation of the Solid Electrolyte Interphase of Silicon Wafers Using a Fluorine-Free Electrolyte

Lithium-ion (Li-ion) batteries have become an important solution for energy storage owing to their high gravimetric and volumetric energy density. The electrolyte solution of the battery has a large effect on the stability and performance of the cell. During the first cycles of a battery, the solid electrolyte interphase (SEI) layer is formed due to the degradation of the electrolyte. Hence the composition of this layer, and the stability, are dependent on the electrolyte composition.The most commonly used electrolyte for Li-ion batteries is the salt lithium hexafluorophosphate LiPF6 dissolved in organic solvents together with additives, both containing fluorinated compounds.1 However these fluorinated compounds are susceptible to release HF which is toxic and corrosive which could be detrimental for the cell performance but also hinder the recycling process. Therefore, fluorine-free electrolytes could be a promising alternative. While this type of electrolytes has been investigated with graphite anodes, when moving to higher energy densities, such as silicon, their performance and degradation mechanism is not yet clear.2 Previous research has shown increased stability for Li-ion batteries using a fluorine-free electrolyte based on the salt lithium bis(oxalato) borate (LiBOB).2 Furthermore, operando X-ray reflectometry (XRR) measurements on silicon wafers half-cells with the same electrolyte have shown the build-up of multiple layers on the anode, providing the thicknesses and densities of the different layers. This work presents the chemical composition of these different layers formed on the silicon wafers at different potentials when using a fluorine-free electrolyte based on LiBOB in ethylene carbonate and methyl ethyl carbonate. These results are also compared with the effect of adding vinylene carbonate VC as an additive. The different components in these layers are correlated to the previous XRR data to explain the SEI-formation mechanism with fluorine-free electrolytes. Xu, K. Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chem Rev 104, (2004).Hernández, G. et al. Elimination of fluorination: The influence of fluorine-free electrolytes on the performance of LiNi1/3Mn1/3Co1/3O2/silicon-graphite li-ion battery cells. ACS Sustain Chem Eng 8, (2020).

Protection of Cu from Oxidation by Ta Capping Layer

X-ray reflectometry (XRR) and X-ray photoelectron spectroscopy (XPS) measurements (core levels and valence bands) were made of Cu thin films that were prepared and coated by capping Ta layers with different thicknesses (5, 10, 15, 20, and 30 Å), and are presented. The XRR and XPS Ta 4f-spectra revealed a complete oxidation of the protective layer up to a thickness of 10 Å. From the thickness of the capping layer of 15 Å, a pure Ta-metal line appeared in the XPS Ta 4f-spectrum, the contribution of which increased up to 30 Å. The XPS Cu 2p-spectra of the underlying copper layer revealed the oxidation with the formation of CuO up to a thickness of the Ta-layer of 10 Å. Starting from a thickness of 15 Å, the complete protection of the Cu layer against oxidation was ensured during exposure to the ambient atmosphere.

Porphyrin adsorption and ligand exchange at the (110)-TiO[formula omitted] / organic liquid interface

Highly efficient dye-sensitized solar cells (DSSCs) have been prepared on the basis of porphyrin dyes with significant success. Recent studies have demonstrated a correlation between the solar cell efficiency and the configuration of the adsorbed porphyrin molecules at the surface of the semiconducting support material. The preparation of dense self assembled monolayers (SAMs) of upright standing porphyrin molecules on TiO2 surfaces was achieved from toluene solutions only when using a molecular exchange reaction. Here, an exchange study of an oleic acid (OLA) SAM on a <110> TiO2 surface by porphyrin molecules is presented. The exchange reaction has been studied by ex-situ as well as in-situ X-ray reflectometry (XRR) measurements. It was found that an almost complete exchange of OLA with porphyrins can be achieved. With increasing coating times the orientation of the porphyrin molecules relative to the substrate surface changed from flat lying to upright standing.

Impact of the pre amorphization by Ge implantation on Ni0.9Pt0.1 silicide

The impact of the Pre Amorphization by Ge Implantation (PAI) on Ni0.9Pt0.1 silicide is studied. Reactions between a 10 nm thick Ni0.9Pt0.1 film and Si (100) substrate are analyzed as a function of the induced amorphous-Si thicknesses. In view of being compatible with the integration constraint of the 28 nm CMOS technologies, the Ge implantation dose is fixed at a low level. The relative position of the amorphous-Si/crystalline-Si interface and the silicide growth front is defined for each sample. Then, in-situ XRD analyses, X-Ray Reflectometry (XRR) and Sheet resistance (Rs) measurements are achieved to provide a deep study of silicide growth kinetics and silicide properties. First, a clear relationship is established between the silicide growth rate and the amorphous-Si thickness. Secondly, an easier NiSi nucleation and a decrease of its resistivity is observed when NiSi nucleates at the θ-Ni2Si/a-Si interface. These observations are discussed considering the impact of the amorphous-Si layer on the driving force, the nucleation barrier, the lateral growth rate, and NiSi roughness.

Effect of Periodic Modulation of Tungsten Oxide Magnetron Deposition Angle upon the Obtained Layer Structure according to X-Ray Reflectometry Measurements

Effect of Periodic Modulation of Tungsten Oxide Magnetron Deposition Angle upon the Obtained Layer Structure according to X-Ray Reflectometry Measurements

Structure and Oligonucleotide Binding Efficiency of Differently Prepared Click Chemistry-Type DNA Microarray Slides Based on 3-Azidopropyltrimethoxysilane.

The structural characterization of glass slides surface-modified with 3-azidopropyltrimethoxysilane and used for anchoring nucleic acids, resulting in the so-called DNA microarrays, is presented. Depending on the silanization conditions, the slides were found to show different oligonucleotide binding efficiency, thus, an attempt was made to correlate this efficiency with the structural characteristics of the silane layers. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and X-ray reflectometry (XRR) measurements provided information on the surface topography, chemical composition and thickness of the silane films, respectively. The surface for which the best oligonucleotides binding efficiency is observed, has been found to consist of a densely-packed silane layer, decorated with a high-number of additional clusters that are believed to host exposed azide groups.

The anisotropy in the optical constants of quartz crystals for soft X-rays.

The refractive index of a y-cut SiO2 crystal surface is reconstructed from orientation-dependent soft X-ray reflectometry measurements in the energy range from 45 to 620 eV. Owing to the anisotropy of the crystal structure in the (100) and (001) directions, a significant deviation of the measured reflectance at the Si L 2,3 and O K absorption edges is observed. The anisotropy in the optical constants reconstructed from these data is also confirmed by ab initio Bethe-Salpeter equation calculations for the O K edge. This new experimental data set expands the existing literature data for quartz crystal optical constants significantly, particularly in the near-edge regions.

Electrolyte Transfer at a Liquid-Solid Polypyrrole Interface Quantified By in Situ X-Ray Reflectometry

In situ characterization of the electrochemical (EC) intercalation mechanism of ions in conductive polymer films is one of the most important aspects in the development of novel organic supercapacitors. Compared to atomic force microscopy and quartz microbalance measurements, synchrotron-based in situ X-ray reflectometry (XRR) allows deeper and more detailed insights into electrically controlled transfer processes of electrolytes across liquid-solid interfaces. The low roughness of the layer system to be investigated, which is required for XRR, is a particular challenge: Electrochemically deposited polypyrrole (PPy) films naturally possess a rough surface, the typical cauliflower structure. In the present study, PPy was deposited on a silicon crystal with a fast-varying voltage to obtain a PPy film with a very smooth surface, which led in parallel to a significantly denser film, which also increases the XRR contrast. This 30 nm thin PPy film was measured in situ in a 0.1M HClO4 environment with XRR at different voltage levels between -0.2 and +0.5V vs Ag/AgCl and the film thickness and electron density were determined. A linear and reversible change in thickness and electron density was measured, which is typical for the potential-dependent intercalation process of anions. The number of electrons transferred across the electrolyte-PPy interface was determined from the change in thickness and electron density, from which the transport of matter was derived. The correlation with the electrically measured transferred charge resulted in a ratio that was significantly below the expected value for exclusive anion exchange. Considering all present and available matter, it can be deduced that proton transport partially compensates for the charge and that 2-3 water molecules per incorporated perchlorate anion are removed from the film. Fast XRR measurements on the same PPy film during applied cyclovoltammetry showed two intervals of different behavior of the PPy film: In the higher voltage range anion intercalation dominates, the PPy film shows a very high pseudo-capacity due to its high density, while in the lower voltage range proton exchange starts and reduces the pseudo-capacity. Electrochemical impedance spectroscopy confirmed these results: Above the transition of these two voltage ranges, the conductivity of the PPy film increases dramatically due to the intercalation of perchlorate anions. With the presented synchrotron-based, in situ EC-XRR setup the film thickness, the electron density, and the change of both, as well as the pseudo-capacitance and conductivity as a function of the applied voltage can be measured simultaneously on ultra-thin electrochemical polymer films. Especially for the development of new electrochemical systems this is a highly potent measurement technique for in situ characterization.

Development of x-ray reflectivity measurement system under N2 to prevent surface contamination.

In this study, a method to prevent surface contamination on an SiO2 film/Si substrate system was used to improve thickness determination by x-ray reflectometry (XRR). XRR profiles can be significantly affected by the growth of a surface contamination layer, originating from the organic matter present in the measurement environment. An N2 spray method that enables XRR measurement under high-purity N2 has been developed to keep the surface free of contaminants. The method was adopted due to its high applicability to an existing XRR instrument and the ease of system construction. The system mainly consists of an outlet for N2 spray that is positioned in front of the sample and an N2 gas purifier. The high-purity N2 is sprayed on the sample until the measurements are complete. It was revealed that the measured XRR profiles were stable for 115 h, and the evaluated thicknesses were obtained with high reproducibility (±0.05 nm for 10 nm thickness) by adopting the analysis model for clean surfaces.

Tuning the optical, electrical resistivity and structural properties of DC magnetron sputtered aluminum zinc oxide films by changing the oxygen flow rate

Thin films of aluminum zinc oxide (AZO) were fabricated by DC reactive magnetron sputtering from 50 wt% Al/Zn metallic target. The structure and optical constants of AZO films were controlled by changing the oxygen (O2) flow rate. X-Ray diffraction revealed that the addition of oxygen transforms the crystalline nature of the metallic Al/Zn to completely amorphous film at 4.5 sccm. The film density, surface roughness and deposition rate were evaluated from X-ray reflectometry measurements and they were found to decrease strongly upon transformation from metallic to oxidic sputtering mode. The optical constants were extracted from the ellipsometry measurements. As O2 flow increased from 4.5 to 30 sccm, the optical band gap increased from 4.74 to 5.33 eV whereas the refractive index decreased simultaneously. Low resistivity films were obtained as O2 flow increased from 0 to 4 sccm whereas insulating films were obtained for O2 flows above 4 sccm. The results indicated that by regulating the oxygen flow diverse ZAO films with diverse chemical stoichiometries and properties can be tuned.

Structural Changes in Adsorbed Cytochrome c upon Applied Potential Characterized by Neutron Reflectometry.

The structural behavior of an electron-transfer protein, cytochrome c, at the 316L stainless steel electrode/aqueous interface was investigated over a range of applied potentials using neutron reflectometry supported by solution depletion isotherms, X-ray reflectometry, and quartz crystal microbalance measurements. A custom-made electrochemical cell allowed in situ observation of the adsorbed protein across a range of applied potentials; models fitted to the NR data showed a compact inner protein layer at the metal/electrolyte interface and a further thicker but highly diffuse layer that could be removed by rinsing. The overall amount adsorbed was found to be strongly dependent on the applied potential and buffer pH. Subtle but significant changes in the structure of the adsorbed protein layer were seen as the potential was swept between ±0.40 V, reflecting changing attractive/repulsive interactions between the protein's charged side groups and the surface. At greater applied potentials, irreversible changes in the stainless steel film structure were also observed and attributed to deuterium absorption into the metal.

Evaluation of different dispersion models for correlation of spectroscopic ellipsometry and X-ray reflectometry.

The importance of proper modeling of the materials' optical properties for interpretation of spectroscopic ellipsometry (SE) data is pretty much impossible to underestimate. In this study, the thickness-dependent titanium nitride (TiN) optical properties were represented by the multiple-oscillator Drude-Lorentz, Forouhi-Bloomer, and Lorentz optical dispersions with different numbers of parameters. The dielectric function of thin TiN films with intermediate behavior can be appropriately expressed in terms of 9-13 model parameters. Using X-ray reflectometry (XRR) as a reference technique and taking into account surface roughness of TiN films, it has been shown that three-term Lorentz dispersion model provides not only the best fit quality for the nominal thicknesses ranging from 125 Å to 350 Å but also an extremely good SE-XRR linear correlation with slope 1.05 ± 0.01, intercept -9.88 ± 2.06 Å, and R2 = 0.9998. Use of other dispersion models results in a somewhat worse correlation with XRR measurements. Thus, an appropriate modeling of the film optical properties is one of the factors needed to be considered to establish well-grounded and credible SE and XRR correlation in case of using XRR as a reference technique, i.e., calibrating SE using x-ray reflectivity.

Influence of contamination layer on thickness evaluation by X‐ray reflectometry

In this study, the effect of allowing for a contamination layer on a SiO2 film/Si substrate system on thickness determination by X‐ray reflectometry (XRR) was investigated. The calculated XRR profiles obtained using a calculation model that utilizes a contamination layer for analysis showed good agreement with measured profiles. Further, the obtained physical structures were promising and within acceptable limits. Where the existence of a contamination layer was ignored in the calculation process, a part of the thickness of the contamination layer was incorporated into the determined thickness of the SiO2 layer. In that case, the evaluated thickness was proportional to the density ratio between the contaminated and SiO2 layers. The results of investigation of the effect of X‐ray energy on layer thickness determination indicated that the effects of contamination also depend on the X‐ray energy used for XRR measurements. These effects increase in the case of experiments that employ X‐ray energy with a high contrast for the contamination.

Bayesian determination of the effect of a deep eutectic solvent on the structure of lipid monolayers.

In this work, we present the first example of the self-assembly of phospholipid monolayers at the interface between air and an ionic solvent. Deep eutectic solvents are a novel class of environmentally friendly, non-aqueous, room temperature liquids with tunable properties, that have wide-ranging potential applications and are capable of promoting the self-assembly of surfactant molecules. We use a chemically-consistent Bayesian modelling of X-ray and neutron reflectometry measurements to show that these monolayers broadly behave as they do on water. This method allows for the monolayer structure to be determined, alongside the molecular volumes of the individual monolayer components, without the need for water-specific constraints to be introduced. Furthermore, using this method we are able to better understand the correlations present between parameters in the analytical model. This example of a non-aqueous phospholipid monolayer has important implications for the potential uses of these solvents and for our understanding of how biomolecules behave in the absence of water.

Effect of RF power on the structural and optical properties of ZnS thin films prepared by RF-sputtering

Zinc sulphide (ZnS) thin films have grown on glass and Si substrates by reactive cathodic radio frequency (RF) sputtering. The RF power was varied in the range of 100 to 250 W, while the deposition time is set at 75 min. The optical, structural, and morphological properties of these thin films have been studied. The optical properties (mainly thickness, refractive index, absorption coefficient, and optical band gap) were investigated by optical transmittance measurements in the wavelength range of ultraviolet-visible-near infrared spectroscopy and spectroscopy infrared with Fourier transform. Fourier (FT-IR) and XRD analysis indicated that all sputtering ZnS films had a single-phase with a preferred orientation along the (111) plane of the zinc sphalerite phase (ZB). The crystallite size ranged from 11.5 to 48.5 nm with RF power getting a maximum of 200 W. UV–visible measurements exhibited that the ZnS film had more than 80% transmission in the visible wavelength region. In addition, it has been observed that the band gap energy of ZnS films is decreased slightly from 3.52 to 3.29 eV, and as the RF power is increased, the film thickness increases with the speed of deposit growth. Scanning electron microscopy observations revealed the types of smooth-surfaced films. The measurements (FT-IR) revealed at wave number 1118 and 465.02 cm−1 absorption bands corresponding to the symmetrical and asymmetric vibration of the Zn–S stretching mode. X-ray reflectometry measurements of ZnS films have shown that the density of the films is (3.9 g/cm3) close to that of solid ZnS.

Evidence of germanium segregation in gold thin films

Using X-ray Reflectometry (XRR) and X-ray Diffraction (XRD) measurements, we investigated the crystallinity of ∼35 nm-thick gold films deposited on glass substrates with 2 nm-thick germanium wetting interlayer. Such wetting decreases the average grain size in the Au film and introduces gradient growth. With X-ray Photoelectron Spectroscopy (XPS) we measured the atomic concentration of Ge as a function of subsurface depth. Our results show the highest concentration of Ge atoms near the surface and the lowest at the Au/substrate interface, which is most likely the result of segregation. Ellipsometric measurements demonstrate that although presence of Ge atoms segregated in the system introduces additional plasmonic bands in the permittivity spectrum, the intensity of these bands is low in comparison to the intensity of the Drude term, which is greater than for the pure Au film. Annealing the Ge-wetted Au films immediately after the deposition decreases the value of the Dude term, however still not to the level of the nonwetted films, which suggests that some kind of mixing still occurs.

Lipid Monolayer Formation and Lipid Exchange Monitored by a Graphene Field-Effect Transistor.

Anionic and cationic lipids are key molecules involved in many cellular processes; their distribution in biomembranes is highly asymmetric, and their concentration is well-controlled. Graphene solution-gated field-effect transistors (SGFETs) exhibit high sensitivity toward the presence of surface charges. Here, we establish conditions that allow the observation of the formation of charged lipid layers on solution-gated field-effect transistors in real time. We quantify the electrostatic screening of electrolyte ions and derive a model that explains the influence of charged lipids on the ion sensitivity of graphene SGFETs. The electrostatic model is validated using structural information from X-ray reflectometry measurements, which show that the lipid monolayer forms on graphene. We demonstrate that SGFETs can be used to detect cationic lipids by self-exchange of lipids. Furthermore, SGFETs allow measuring the kinetics of layer formation induced by vesicle fusion or spreading from a reservoir. Because of the high transconductance and low noise of the electrical readout, we can observe characteristic conductance spikes that we attribute to bouncing-off events of lipid aggregates from the SGFET surface, suggesting a great potential of graphene SGFETs to measure the on-off kinetics of small aggregates interacting with supported layers.

Growth model and structure evolution of Ag layers deposited on Ge films.

We investigated the crystallinity and optical parameters of silver layers of 10–35 nm thickness as a function 2–10 nm thick Ge wetting films deposited on SiO2 substrates. X-ray reflectometry (XRR) and X-ray diffraction (XRD) measurements proved that segregation of germanium into the surface of the silver film is a result of the gradient growth of silver crystals. The free energy of Ge atoms is reduced by their migration from boundaries of larger grains at the Ag/SiO2 interface to boundaries of smaller grains near the Ag surface. Annealing at different temperatures and various durations allowed for a controlled distribution of crystal dimensions, thus influencing the segregation rate. Furthermore, using ellipsometric and optical transmission measurements we determined the time-dependent evolution of the film structure. If stored under ambient conditions for the first week after deposition, the changes in the transmission spectra are smaller than the measurement accuracy. Over the course of the following three weeks, the segregation-induced effects result in considerably modified transmission spectra. Two months after deposition, the slope of the silver layer density profile derived from the XRR spectra was found to be inverted due to the completed segregation process, and the optical transmission spectra increased uniformly due to the roughened surfaces, corrosion of silver and ongoing recrystallization. The Raman spectra of the Ge wetted Ag films were measured immediately after deposition and ten days later and demonstrated that the Ge atoms at the Ag grain boundaries form clusters of a few atoms where the Ge–Ge bonds are still present.

SiGe layer thickness effect on the structural and optical properties of well-organized SiGe/SiO2 multilayers

In this work, we report on the production of regular (SiGe/SiO2)20 multilayer structures by conventional RF-magnetron sputtering, at 350 °C. Transmission electron microscopy, scanning transmission electron microscopy, raman spectroscopy, and x-ray reflectometry measurements revealed that annealing at a temperature of 1000 °C leads to the formation of SiGe nanocrystals between SiO2 thin layers with good multilayer stability. Reducing the nominal SiGe layer thickness (tSiGe) from 3.5–2 nm results in a transition from continuous SiGe crystalline layer (tSiGe ∼ 3.5 nm) to layers consisting of isolated nanocrystals (tSiGe ∼ 2 nm). Namely, in the latter case, the presence of SiGe nanocrystals ∼3–8 nm in size, is observed. Spectroscopic ellipsometry was applied to determine the evolution of the onset in the effective optical absorption, as well as the dielectric function, in SiGe multilayers as a function of the SiGe thickness. A clear blue-shift in the optical absorption is observed for tSiGe ∼ 2 nm multilayer, as a consequence of the presence of isolated nanocrystals. Furthermore, the observed near infrared values of n = 2.8 and k = 1.5 are lower than those of bulk SiGe compounds, suggesting the presence of electronic confinement effects in the nanocrystals. The low temperature (70 K) photoluminescence measurements performed on annealed SiGe/SiO2 nanostructures show an emission band located between 0.7–0.9 eV associated with the development of interface states between the formed nanocrystals and surrounding amorphous matrix.