Quantitative analysis of the Ge concentration in a SiGe quantum well: comparison of low-energy RBS and SIMS measurements

The germanium concentration and the position and thickness of the quantum well in molecular beam epitaxy (MBE)-grown SiGe were quantitatively analyzed via low-energy Rutherford backscattering (RBS) and secondary ion mass spectrometry (SIMS). In these samples, the concentrations of Si and Ge were assumed to be constant, except for the quantum well, where the germanium concentration was lower. The thickness of the analyzed quantum well was about 12 nm and it was situated at a depth of about 60 nm below the surface. A dip showed up in the RBS spectra due to the lower germanium concentration in the quantum well, and this was evaluated. Good depth resolution was required in order to obtain quantitative results, and this was obtained by choosing a primary energy of 500 keV and a tilt angle of 51° with respect to the surface normal. Quantitative information was deduced from the raw data by comparing it with SIMNRA simulated spectra. The SIMS measurements were performed with oxygen primary ions. Given the response function of the SIMS instrument (the SIMS depth profile of the germanium delta (δ) layer), and using the forward convolution (point-to-point convolution) model, it is possible to determine the germanium concentration and the thickness of the analyzed quantum well from the raw SIMS data. The aim of this work was to compare the results obtained via RBS and SIMS and to show their potential for use in the semiconductor and microelectronics industry. The detection of trace elements (here the doping element antimony) that could not be evaluated with RBS in low-energy mode is also demonstrated using SIMS instead.     

Nondestructive analysis of single crystals of selenide spinels by X-ray spectrometry techniques

The paper presents possibilities and difficulties in nondestructive analysis of small multielement single crystals performed by means of X-ray spectrometry techniques: micro-X-ray fluorescence spectrometry (μ-XRF), energy-dispersive electron probe microanalysis (ED-EPMA), and X-ray photoelectron spectroscopy (XPS). The capability of the X-ray spectroscopy techniques in elemental analysis is demonstrated with the single crystals of selenide spinels of the general formula M _x N _y Cr _z Se₄ (M⁺² and N⁺³ are, for example, Zn⁺², V⁺³, Ga⁺³, Cd⁺², In⁺³, and Sb⁺³). The results of the nondestructive analyses (μ-XRF, ED-EPMA, and XPS) are compared with those obtained by inductively coupled plasma optical emission spectrometry (ICP-OES) and wavelength-dispersive X-ray spectrometry (WDXRF) following sample digestion. The present study shows satisfactory agreement between the results of μ-XRF analysis performed using the standardless fundamental parameter method and the results obtained with the WDXRF and ICP-OES analyses. If the measured single crystal is precisely positioned, the difference between μ-XRF and wet analysis (WDXRF and ICP-OES) does not exceed 5% rel. The reliable results of ED-EPMA can be obtained only if the measured area is sufficiently large, i.e., of 200 × 300 μm. Even if this condition is fulfilled, the relative difference between the ED-EPMA and the wet analysis may reach 10% rel. In case of the XPS analysis, the accuracy of results depends on the proper preparation of the sample surface. It should be free of contamination that can be obtained by scraping in situ in ultrahigh vacuum. The ion etching, commonly used for cleaning the surface, leads to preferential sputtering; therefore, the reliable results cannot be obtained.     

Potential of Total Reflection and Grazing Incidence XRF for Contamination and Process Control in Semiconductor Fabrication

 The actual detection limits of total reflection X-ray fluorescence (TXRF) are determined and compared to those of destructive physical analytical methods like secondary ion mass spectrometry (SIMS) and chemical methods like vapour phase decomposition in combination with inductively coupled plasma-mass spectrometry (VPD-ICP-MS). The elements Ca, Ti, Cr, Fe, Cu were analyzed on a Si wafer with 10 nm thermal oxide using TXRF and VPD-ICP-MS. The deviation of the TXRF and the VPD-ICP-MS results is less than 30%. The thickness, composition and density of a Co/Ti two-layer stack were determined using angle dependent total reflection and grazing incidence X-ray fluorescence (A-TXRF). The obtained data were compared with X-ray reflectometry (XRR) and energy filtered transmission electron microscopy (EFTEM). The agreement between TEM and A-TXRF is excellent for the determination of the thickness of the metal layers. From these results we conclude, that A-TXRF permits the accurate determination of composition, thickness and density of thin metallic layers. The results are discussed regarding detection efficiency, acquisition time, accuracy and reproducibility.     

A comparison of AES, SIMS, ISS and RBS analysis of SixNy layers

Summary Si _x N _y , thin layers were analyzed by AES, SIMS, ISS and RBS. A comparison of the experimental results is made, and the artifacts and characteristics for each experimental method are discussed. It seems that of all these methods, only RBS is suitable for analysing silicon nitride layers.

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Vergleich von AES, SIMS, ISS und RBS für die Analyse von Si _x N _y -Schichten

     

Accuracy of film thickness determination in electron probe microanalysis

The thickness of copper films (100–450 nm) on silicon substrates was determined by electron probe microanalysis (EPMA) applying (z) procedures of Pouchou and Pichoir. Film thickness was calculated from experimental k-ratios analyzed with electron energies between 6 and 30 keV using commercial software (LAYERF distributed by CAMECA). The influence of the incident electron energy and X-ray line chosen for analysis on the results was investigated. Accuracy of film thickness determination was evaluated by comparison with Rutherford backscattering spectroscopy (RBS) and secondary ion mass spectrometry (SIMS). The difference between layer thicknesses determined with EPMA and RBS is in general less than 2%, if EPMA measurements are performed with various electron energies. Layer thicknesses determined with Cu-L are mostly closer to values obtained by RBS than those derived from Cu-K radiation. Preliminary SIMS measurements yielded inconsistent results and, thus, cannot be used in this case to determine the layer thickness of Cu films on Si accurately.     

Characterization of the stoichiometry of coevaporated FeSix films by AES, EDX, RBS, and electron microscopy

The composition of FeSi_x films on Si coevaporated from separate sources of Fe and Si was analyzed comparatively by AES, EDX, RBS, and electron microscopy. Cross-checks between EDX and RBS reveal systematic errors originating from the spectra background subtraction in RBS and from the thickness correction of the EDX signals. PCA (principal component analysis) assisted AES was successfully applied to the characterization of different Si bonding states in nonstoichiometric FeSi_x films. For the growth of β-FeSi₂ films by means of molecular beam epitaxy (MBE) the adjustment of the atomic beam intensities is reported in order to illustrate the capabilities of the various techniques.     

Characterization of Two-Component Metal Coatings (Al/Sn) with SIMS

 Vacuum deposited films of immiscible metal–metal systems can be applied as tribological coatings for plain bearings in high performance diesel engines. These industrially manufactured coatings show higher lifetimes than conventional electroplated coatings. For our investigations we used aluminum-tin coatings of 1 μm thickness on a glass substrate produced by sequential deposition from two separate targets under working gas pressure of 0.4 Pa. The tough Al matrix takes high mechanical loads and the soft inclusions of Sn act as solid lubricant. While Sn is deposited it migrates on the Al surface and grows as islands. We characterized the conformation and the distribution of the Sn islands on the surface and to the interstitial area between the islands with 3-D secondary ion mass spectroscopy (SIMS). The existence of a Sn layer between the islands (“wetting layer”) has been detected by SIMS and verified by measurements with Auger electron spectroscopy (AES).     

Advantages of combined μ-XRF and μ-XRD for phase characterization of Ti–B–C ceramics compared with conventional X-ray diffraction

Design and processing of new materials with improved high-temperature properties is one of the most challenging tasks of modern engineering. Among such materials, nonoxidic ceramics hold an important place. When optimizing the synthesis conditions of these new materials in an largely empirical manner, the use of analytical methods that can fully document the resulting phase compositions is of great importance. In this paper, we demonstrate the advantages of using combined microbeam X-ray diffraction and X-ray fluorescence over conventional X-ray diffraction as the characterization method in the specific case of Ti–B–C ceramics. Ceramic samples were synthesized by the pulse plasma method starting from high-purity powders of titanium, boron, graphite, and nickel. Different mixtures were heated in a pulsed fashion and sintered by combustion synthesis at various temperatures and time durations, as is the case during empirical optimization of a synthesis procedure. Conventional X-ray diffraction showed the presence of two phases at the end of the sintering process, TiB₂ and TiC, irrespective of the conditions employed. Scanning μ-XRF/μ-XRD on the other hand allowed one to detect and visualize the distribution of additional phases present in the sintering products, during which a dependence on sintering conditions was apparent. The μ-XRD results showed that three phases (TiB₂, TiC, and TiB) instead of two were present in samples sintered during a short time interval. The addition of metallic Ni to the initial mixture as a sintering facilitator resulted in the formation of a Ni₃B phase. All phases proved to have strongly heterogeneous distributions above the 15-μm level with the presence of TiB₂ anticorrelated to that of TiC and TiB, emphasizing the necessity of the use of laterally resolved methods of characterization.     

Quantitative analysis of W-C:H coatings by EPMA,RBS (ERD) and SIMS

Several analytical techniques have been used to characterize homogeneous films of tungsten-containing hydrogenated carbon (W-C: H), deposited on Si with a film thickness of 1–1.5 m. Electron probe microanalysis (EPMA) enables one to determine the major components W (3–43 at %) and C, impurities (< 2 at %) of Ar and O, and the mass thickness (300–1800 g/cm²) of the films. The agreement between the results of EPMA and the data (W-content, mass thickness) provided by Rutherford backscattering spectrometry (RBS) is 5–10% relative. Quantitative analysis of hydrogen in W-C:H films (1–16 at %) is carried out by the technique of elastic recoil detection (ERD). A suitable scheme for the determination of H in W-C: H films by SIMS is proposed, based on monitoring the intensity ratio of HCs⁺/CCs⁺ secondary ions.     

Characterisation of Radioactive Particles by SIMS

 Secondary ion mass spectrometry (SIMS) was optimised for characterisation of uranium- and plutonium-containing particles in soils, swipes and forensic samples. This was done by analysing in-house produced spherical UO₂-particles. Screening techniques as α-autoradiography together with SIMS analysis were employed to detect UO₂-particles in a soil sample from Chernobyl. The use of SIMS was exploited for the identification of uranium- and plutonium-containing particles and for the determination of their isotopic composition. The particles collected on swipe samples were transferred to a special adhesive support for the analysis by SIMS. Particles containing highly enriched uranium with diameters up to 10 μm were also detected in a forensic sample. For the measurements of the isotopic ratios a mass resolution of 1000 was used. At this resolution flat-top peaks were obtained which greatly improve the accuracy of the measurement. The isotopic composition of the particles was measured with a typical accuracy and precision of 0.5%. Statistically meaningful results can be obtained, for instance, from a specimen containing as few as 10¹⁰ atoms/μm³ of uranium in particles of UO₂ weighing a few picograms.     

Stepwise self-assembly of ordered supramolecular assemblies based on coordination chemistry

Ultrathin multilayers based on transition metal complexes have been prepared by successive deposition and self-assembly. Dendrimer layers were deposited onto SiO2 wafers by alternately immersing the substrate into a solution of terpyridyl (tpy)-pendant poly(amido amine) (PAMAM) dendrimers (dend-n-tpy; n = 8, 16) dissolved in CH2Cl2, followed by the interfacial coordination reaction with cobalt (Co2+) from aqueous solution. The films derived from this simple assembly method have been characterized by electrochemical methods, synchrotron-based X-ray reflectivity (XRR), and X-ray fluorescence (XRF) recorded under grazing incidence. XRR analysis revealed a linear thickness dependence with an increase of 9.9 +/- 0.5 angstroms and 10.8 +/- 0.4 angstroms per growth cycle for both dend-8-tpy and dend-16-tpy, respectively, indicative of a layer-by-layer (LbL) growth of single dendrimer/Co2+ layers. XRF and electrochemical results showed that the amount of Co2+ increases linearly as more layers are deposited, and that the Co2+ concentration (mol/L) in dend-8-tpy/Co2+ films decays slowly as the number of growth cycles (l) increases. Moreover, a preliminary kinetics analysis indicated that the growth of a dendrimer layer in a deposition cycle is a self-limiting process.     

Specific features of X-ray fluorescence analysis techniques using capillary lenses and synchrotron radiation

This paper discusses the features of an application of two versions of X-ray fluorescence analysis (XRF), commonly used at present, namely XRF using synchrotron radiation to excite the fluorescence in the sample investigated (SRXRF), and XRF using capillary X-ray optics. The operational characteristics of different models of micro-XRF spectrometers are considered. The general differences between conventional XRF and SRXRF and their influence on the choice of the analytical procedure are also presented. Examples of the typical errors resulting from the use of some classical analytical procedures in several applications are illustrated.     

Determination of amount of substance for nanometre‐thin deposits: consistency between XPS,RBS and XRF quantification

Quantification on the nanometre scale is a key task in quality control and for the development of new materials in nanotechnology. In this paper we have studied the consistency in the determination of the amount of substance found by XPS peak shape analysis, Rutherford backscattering spectroscopy (RBS) and x‐ray fluorescence spectrometry (XRF). To this end, ZnO was deposited by plasma‐enhanced chemical vapour deposition on the three substrates. Four different sets of samples were produced, with the amount of ZnO deposited in the range 1–10 nm. From XPS analysis it is found that ZnO grows in the form of islands on all three substrates. For each system, the analysis was done independently with two XPS peaks from the overlayer with widely different kinetic energy and one XPS peak from the substrate. The growth mechanism found from analysis of each of the three peaks was consistent and the total amount of determined ZnO material was identical to within 15%. The root‐mean‐square deviation from the XPS quantification of the relative AOS was 20% for XRF and 16% for RBS. Because the absolute amount of substance determined from analysis of the three XPS peaks for each sample was consistent, it is concluded that the energy dependence of the applied inelastic mean free paths (taken here from the empirical TPP‐2M formula) is correct. It was found that the absolute amounts of substance determined by RBS and XRF are consistently factors of 2.1 and 1.5 lower than the that determined by XPS peak shape analysis. It is suggested that the main reason for this large discrepancy is inaccuracy in the applied ‘effective’ inelastic electron mean free path. Copyright © 2003 John Wiley & Sons, Ltd.     

Review on the thickness measurement of ultrathin oxide films by mutual calibration method

Mutual calibration was suggested as a method to determine the absolute thickness of ultrathin oxide films. It was motivated from the large offset values in the reported thicknesses in the Consultative Committee for Amount of Substance (CCQM) pilot study P-38 for the thickness measurement of SiO₂ films on Si(100) and Si(111) substrates in 2004. Large offset values from 0.5 to 1.0 nm were reported in the thicknesses by ellipsometry, X-ray reflectometry (XRR), medium-energy ion scattering spectrometry (MEIS), Rutherford backscattering spectroscopy (RBS), nuclear reaction analysis (NRA), and transmission electron microscopy (TEM). However, the offset value for the thicknesses by X-ray photoelectron spectroscopy (XPS) was close to zero (−0.013 nm). From these results, the mutual calibration method was reported for the thickness measurement of SiO₂ films on Si(100) by combination of TEM and XPS. The mutual calibration method has been applied for the thickness measurements of hetero oxide films such as Al₂O₃ and HfO₂. Recently, the effect of surface contamination was reported to be critical to the thickness measurement of HfO₂ films by XPS. On the other hand, MEIS was proved to be a powerful zero offset method which is not affected by the surface contamination. As a result, the reference thicknesses in the CCQM pilot study P-190 for the thickness measurement of HfO₂ films on Si(100) substrate were determined by mutual calibration method from the average XRR data and MEIS analysis. Conclusively, the thicknesses of ultrathin oxide films can be traceably certified by mutual calibration method and most thickness measurement methods can be calibrated from the certified thicknesses.     

Observation of surface contamination layer by X‐ray reflectometry (XRR) analyses

Ultra‐thin HfO₂ films of 3.5, 5.0, and 8.0 nm nominal thicknesses were prepared, respectively, on silicon substrates by using atomic layer deposition method. Through the analyses of X‐ray reflectometry (XRR), X‐ray photoelectron spectroscopy, and transmission electron microscopy for HfO₂ films with and without sample cleaning, the effects of surface contamination on XRR curve and film thickness were investigated, and contamination layer was observed and the thickness of the layer was determined. X‐ray photoelectron spectroscopy results indicated that the amount of surface contamination varied considerably because of the surface cleaning. XRR curve shapes and the positions of thickness fringes changed and the thickness from Fourier analyses of the curves were different for the same sample due to the different surface contamination. Contamination layer of about 1 nm thickness was observed by Fourier analysis of XRR curve. Simulation for XRR curve showed the best fit to data when contamination layer of about 1 nm thickness was considered, and the result was consistent with that of the Fourier analysis. Copyright © 2016 John Wiley & Sons, Ltd.     

Elemental Analyses Using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) of Geological Samples Fused with Li2B4O7 and Calibrated Without Matrix-Matched Standards

 Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was used as a complementary technique to X-ray fluorescence (XRF), for multi-element analysis of geological samples fused with lithium-tetraborate Li₂B₄O₇). Different calibration strategies using external non-matrix matched reference materials were investigated. Various internal standards were tested, including the use of Li from the flux, and the use of the naturally occurring internal standards, Si or Ca. The use of a naturally occurring standard is not as efficient as this required a prior analysis of the samples using XRF. The obtained values for the analysis of geological reference materials were compared with consensus literature values, and satisfactory agreement was found. Laser pits, which were formed, had a diameter of 80 μm and 3–5 replicates on each fused disc were measured. The reproducibility of the method was better than 10% for concentrations above 1 μg/g and better than 15% for lower concentrations. The use of Li as the internal standard offers the possibility of multi-element determinations in geological samples, which have an unknown composition when the laser ablation analysis is carried out. However, using the calculated stoichiometric composition of the lithium-tetraborate for the calculation of the Li concentration leads to a constant deviation from the recommended values. Therefore, it was necessary to determine the Li concentration within each sample batch using at least one lithium tetraborate fused geological reference material. This resulting Li concentration in the beads was then used for all subsequent samples in a run. Limits of detection, reproducibility, deviation from reference values indicate the potential of LA-ICP-MS for such bulk analysis without matrix matched calibration standards.     

Utility of Certain σ and π-Acceptors for the Spectrophotometric Determination of Sildenafil Citrate (Viagra)

 The molecular interaction between sildenafil citrate as electron donor and each of iodine; 7,7,8,8-tetracyanoquinodimethane (TCNQ); 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ); tetracyanoethylene (TCNE); 2,4,7-trinitro-9-fluorenon (TNF); chloranilic acid (CLA); chloranil (CL) and bromanil (BL) as acceptors have been investigated spectrophotometrically. Different variables affecting the reaction were studied and optimized. Beer’s law was obeyed in a concentration limit of 10–260 μg/mL for sildenafil citrate. For more accurate analysis, Ringbom optimum concentration range was found to be between 20–240 μg/mL. The limits of detection and determination were calculated and found to be 1.5 and 5.2 μg/mL, respectively. The standard deviations were calculated for different concentrations of sildenafil citrate using various acceptors. A Job’s plot of the absorbance versus the molar ratio of the sildenafil citrate to each of acceptors under consideration indicated (1:1) ratio. The proposed methods were found to be rapid, accurate, precise and sensitive and could be applied for determination of sildenafil citrate in pharmaceutical dosage forms (Viagra) without interferences from common additives encountered. Received August 30, 2000. Revision January 5, 2001.     

Metal content determination in polymerization catalysts by direct methods

Metal contents in polymerization catalysts were comparatively determined by Rutherford backscattering spectrometry (RBS), X-ray photoelectron spectroscopy (XPS) and X-ray fluorescence (XRF) spectroscopy. Catalysts were prepared by grafting metallocene onto bare silica or onto silica chemically modified with methylaluminoxane (MAO). Catalysts were compressed as self-supporting pellets (RBS and XRF), or mounted on adhesive copper tape (XPS). The proximity of the mass of the atomic nuclei did not allow resolution by RBS of the signals corresponding to Zr and Nb, nor Si and Al in catalyst systems such as (nBuCp)₂ZrCl₂/Cp₂NbCl₂/MAO/SiO₂. On the other hand, Zr, Nb, Si and Al lines were completely resolved in an XRF spectrum. For supported metallocenes on bare silica, XPS measurement was ca. 40% higher than that obtained by RBS. Silica-supported zirconocene showed good agreement in Zr content determination by XRF and RBS.     

Structural characterization of 4-bromostyrene self-assembled monolayers on si(111)

Organic functionalization of silicon holds promise for a variety of applications ranging from molecular electronics to biosensing. Because the performance and reliability of organosilicon devices will be intimately tied to the detailed structure of the organic adlayers, it is imperative to develop systematic strategies for forming and characterizing self-assembled monolayers (SAMs) on silicon with submolecular spatial resolution. In this study, we use 4-bromostyrene for the photochemical growth of Br-terminated SAMs on Si(111). A variety of experimental and theoretical techniques including atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray reflectivity (XRR), X-ray standing waves (XSW), X-ray fluorescence (XRF), and density functional theory (DFT) have been employed to determine the coverage and conformation of the 4-bromostryene molecules within the SAM. In particular, AFM verifies a continuous and atomically flat SAM, and the XRR data indicate a SAM thickness of 8.50 A and a molecular coverage of 46% of the surface silicon atoms. Because the DFT calculations indicate a molecular length of 8.89 A, the measured XRR thickness implies a molecular tilt angle of approximately 17 degrees. The XRR analysis also suggests that the Br atoms are preserved on top of the SAM in agreement with XPS measurements that show bromine bound solely to carbon and not to silicon. XRF reveals a Br atomic coverage of 50%, again in close agreement to that found by XRR. Single-crystal Bragg diffraction XSW is used to generate a three-dimensional map of the Br distribution within the SAM, which in conjunction with the XRR result suggests that the 4-bromostyrene molecules are tilted such that the Br atoms are located over the T4 sites at a height of 8.50 A above the top bulklike Si(111) layer. The direction of molecular tilt toward the T4 sites is consistent with that predicted by the DFT calculation. Overall, through this unique suite of complementary structural characterization techniques, it is concluded that the Br functional handle is preserved at the top of the SAM and is available for further substitutional chemistry.