Total Reflection X-ray Fluorescence Spectrometry Research Articles

Utilization of Si atomic steps for Cu nanowire fabrication

Techniques for controlling atomic step position at low-temperature and selective growth of Cu nanowires along the atomic step edges have been studied. By immersing the Si(111) substrates with well-defined step/terrace surfaces in the Cu-contained water with the dissolved oxygen content of less than 1 ppb, selective growth of Cu nanowires along the step edges was successfully achieved. Total reflection X-ray fluorescence spectroscopy (TXRF) revealed that the fabricated nanowires were composed of mono-atomic Cu rows. For step position control, the characteristics of step-flow pinning effect of SiO2 films were investigated. Fine SiO2 line patterns drawn by anodic oxidation using AFM probes enable us to obtain the step-free Si areas predetermined by the patterns.

Remediation for TXRF saturation effects on microdroplet residues from preconcentration methods on semiconductor wafers

In metallic contamination analysis of semiconductor wafers, preconcentration methods such as vapor phase decomposition–droplet collection (VPD-DC) or droplet sandwich etch (DSE) are combined with sensitive analytical techniques, often total-reflection X-ray fluorescence spectrometry (TXRF). Previously we have reported about the occurrence of TXRF saturation effects in the analysis of microdroplet residues from these methods and identified the origin as a mass-absorption effect of primary X-rays. In this study, different approaches to deal with TXRF saturation effects in semiconductor applications are evaluated. Corrective approaches are tested for the application of VPD-DC-TXRF on Si wafers. The application of correction factors resulted in an overestimation of the effect of 20%. Internal standardization approaches did not perform well because of the separation of the internal standard element apart from the analytes during the drying step. We suggest another internal approach using the intensity of scattered primary X-rays as an indicator for accuracy.The parameters in a Ge matrix removal method have been investigated. The application of the method on samples from the DSE method on spin coated Ge wafers resulted in efficiencies at 100% compared to only 40% without matrix removal. Finally, the use of nanoliter deposition or ink-jet spray methods is proposed to extend the dynamic range of TXRF on microdroplet residues. This suggestion is motivated with calculations based on a mass-absorption model and with experimental results from the study of linearity of TXRF on Ni spin coated Si wafers and the related morphology of the surface species.

Nucleation and growth of copper nanoparticles on silicon surfaces

The recent adoption of copper interconnect technology by the semiconductor industry, has led to great interest in understanding the mechanisms of copper metal deposition onto silicon wafer surfaces from ultra pure water (UPW) solutions. We have studied these processes by using total reflection x-ray fluorescence (TXRF) and x-ray absorption near edge spectroscopy (XANES) in a grazing incidence geometry to determine the surface concentration and chemical state of copper atoms on intentionally contaminated Si surfaces. These measurements established that in deoxygenated UPW, copper is deposited on the silicon surface in the form of metallic nanoparticles with sizes up to 16 nm. However, in non-deoxygenated UPW, the copper is incorporated uniformly into the silicon surface oxide as Cu oxide.

Comparison of freshwater biofilms grown on polycarbonate substrata in Lake Velence (Hungary) and Lake Mogan (Turkey)

Freshwater biofilms grown on polycarbonate substrata in Lake Velence (Hungary) and Lake Mogan (Turkey) were studied by applying total reflection X-ray fluorescence spectrometry and taxonomical techniques. Concentration of calcium, which stabilizes the structure of the exopolymer matrix, was identical in the biofilms, independently of the fact that its concentration in the lake waters differs by a factor of 2.5. Enrichment factors calculated as concentration ratios of elements (Ca, K, Fe, Mn, Zn, and Sr) measured in biofilm and water varied from 3×10 4 to 1.5×10 2. The highest enrichment factors were observed for micronutrients of manganese and iron. Although the dominant alga species were different in the Hungarian and Turkish biofilms, the enrichment factors for these two elements differ only slightly. These results demonstrate that the accumulation of these micronutrients is similar within the diatom species. The lowest enrichment factors were measured for potassium.

Efficient Isotope Ratio Analysis of Uranium Particles in Swipe Samples by Total-Reflection X-ray Fluorescence Spectrometry and Secondary Ion Mass Spectrometry

A new particle recovery method and a sensitive screening method were developed for subsequent isotope ratio analysis of uranium particles in safeguards swipe samples. The particles in the swipe sample were recovered onto a carrier by means of vacuum suction-impact collection method. When grease coating was applied to the carrier, the recovery efficiency was improved to 48±9%, which is superior to that of conventionally-used ultrasoneration method. Prior to isotope ratio analysis with secondary ion mass spectrometry (SIMS), total reflection X-ray fluorescence spectrometry (TXRF) was applied to screen the sample for the presence of uranium particles. By the use of Si carriers in TXRF analysis, the detection limit of 22 pg was achieved for uranium. By combining these methods with SIMS, the isotope ratios of 235U/238U for individual uranium particles were efficiently determined.

Determination of trace and minor elements in Slovenian honey by total reflection X-ray fluorescence spectroscopy

Trace and minor elements in Slovenian honey were analysed by total reflection X-ray fluorescence spectroscopy. Upto 16 elements (K, Cl, S, P, Ca, Mn, Rb, Cu, Fe, Ni, Cr, Br, Ti, Pb, Sr and As) were detected, in a range of average content from 1.24 mg kg −1 for Sr to 2590 mg kg −1 for K. Statistically significant differences were established between different types of honey (acacia, floral, lime, chestnut, spruce, fir, forest and Metcalfa pruinosa honeydew honey). The highest content of elements was determined in forest honey and the lowest in acacia honey. Honeys were also separated according to their botanical origin as nectar honey or honeydew honey. Total elemental content and the content of S, Cl, K and Rb in honeydew honey was statistically significantly higher than in nectar honey. Chestnut honey differed in statistically its significantly higher contents of Rb and Ca from nectar and honeydew honeys. The year of honey production proved to have no statistically significant influence on elemental content. A comparison of our data with the literature data showed a relatively large diversity.

Comparison of total-reflection X-ray fluorescence, static and portable energy dispersive X-ray fluorescence spectrometers for art and archeometry studies

Abstract In this paper, a Total-reflection X-ray Fluorescence (TXRF), a static and a portable Energy Dispersive X-ray Fluorescence (EDXRF) spectrometers are described. Both the equipments and the techniques employed in the field of the art and archeometry are compared. Some applications in this area are presented as well. The aim of the work is to know which spectrometer is the best suited depending on the work of art and the problem treated. The conclusion reached from the experience is that the portable EDXRF spectrometer is advisable to make “in situ” and online analysis in a multidisciplinary environment, the static EDXRF equipment is good to perform analysis on paper and metal pieces and the TXRF spectrometry is the best technique for very sensitive analysis of trace elements from micro-samples. Within the last technique, we propose to analyze the cottons used by the restorers in the different steps of the restoration process since it could be a good tool to study the composition of different layers and zones of the work of art.

EDXRF and TXRF analysis of elemental size distributions and environmental mobility of airborne particles in the city of Riga, Latvia

AbstractAirborne particles were investigated in the central part of Riga during October 2000. Mass, black carbon and elemental concentrations of airborne particles were measured on Teflon filters from a dichotomous impactor, which samples fine (<2.5 µm) and coarse (2.5–10 µm) fractions of particles. In order to obtain more detailed information on the size distributions of different elements, a seven‐stage Batelle cascade impactor was used, in which quartz plates treated with silicone grease were utilized as backing for the different stages. Total reflection x‐ray fluorescence (TXRF) and energy‐dispersive x‐ray fluorescence (EDXRF) spectrometry were used for elemental analysis on the quartz plates and Teflon filters. The environmentally mobile part of the fine particle elements in the aerosol was determined by subtraction of x‐ray spectra measured before and after sequential leaching of the aerosol filters. The results of the different measurements show that naturally generated street dust and soil particles are dominant in coarse particles, whereas particles generated by human activities are dominant in the size fraction <0.5 µm. Copyright © 2004 John Wiley & Sons, Ltd.

Multi-elemental EDXRF mapping of polluted soil from former horticultural land

The distribution of major and trace elements was systematically investigated by use of energy dispersive X-ray fluorescence spectrometry (EDXRF) on a former horticultural soil. The purpose of the study was to combine mapping of soil element concentration levels with multivariate statistics for characterisation of soil metal pollution in relation to previous and present land use. A 1-ha study site was chosen from a former horticulture where a previous preliminary survey indicated increased concentration levels of toxic elements. The soil was sampled from the top 20 cm of the soil surface in a 10×10-m grid-like pattern covering the 1-ha study area. In addition, three soil profiles were studied. The elemental composition of the soil samples was investigated by EDXRF while the composition of aqueous soil extracts was determined by total reflection X-ray fluorescence spectrometry (TXRF). Based on mapping and multivariate statistically analysis of the data obtained by EDXRF, most elements were found in almost constant concentration levels in the top soil throughout the investigated site. However, the contents of the toxic elements Zn, Cu, As, and Pb were found to vary significantly within the area. Hence, the samples with high accumulations of As also contained relatively high amounts of Zn, Cu, and Pb, which indicates that toxic-element-containing pesticides have been applied to the soil surface in the area of the former green houses at the study site. The Pb/As mass ratio in the soil indicates that PbHAsO 3 was the preferential lead arsenate used for pest management at the investigated site, while Cu as Bordeaux liquid (CuSO 4) and Zn were applied to minimize the leaf damaging effect from the former compounds. Calculations indicated that As annually was applied to the soil in the former greenhouses in doses up to 4 kg As/ha while Pb had been annually applied in doses up to 12 kg Pb/ha. The enrichment of Zn, Cu, As and Pb was greatest in the top 20 cm of the soil and no anthropogenic enrichment of these elements occurred below a depth of 50 cm, indicating that the toxic elements are rather immobile in this soil. The results of this investigation suggest that EDXRF used in combination with multivariate statistics is a strong tool for multi-element mapping of elemental contents, sources and mobility in the terrestrial environment.

A hydride generation flow system for determination of arsenic and selenium by total reflection X-ray fluorescence spectrometry

In this work, a preconcentration and separation system based on continuous flow hydride generation is proposed to improve the determination of As and Se by total reflection X-ray fluorescence spectrometry. The generated hydrides are continuously separated from the liquid phase and collected in a chamber containing 250 μl of HCl/HNO 3 1:1 (v/v) solution. Hydride generation conditions and collection of the hydrides were evaluated. Under optimised conditions, enrichment factors of 55 for As and 82 for Se were attained. Detection limits of 0.3 μg l −1 for As and Se were obtained when 20 ml of sample was used. Analysis of a natural water standard reference material from National Institute of Standard and Technology (SRM-1640) was in agreement with the certified values at the 95% confidence level.

Removal of copper and nickel contaminants from Si surface by use of cyanide solutions

The cleaning method using cyanide solutions has been developed to remove heavy metals such as copper (Cu) and nickel (Ni) from Si surfaces. Immersion of Si wafers with both Cu and Ni contaminants in potassium cyanide (KCN) solutions of methanol at room temperature decreases these surface concentrations below the detection limit of total reflection X-ray fluorescence spectroscopy of ∼3×10 9 atoms/cm 2. UV spectra of the KCN solutions after cleaning of the Cu-contaminated Si surface show that stable copper-cyanide complexes are formed in the solution, leading to the prevention of the re-adsorption of copper in the solutions. From the complex stability constants, it is concluded that the Cu(CN) 4 3− is the most dominant species in the KCN solutions.

Validation of vapor phase decomposition–droplet collection–total reflection X-ray fluorescence spectrometry for metallic contamination analysis of silicon wafers

The combination of vapor phase decomposition–droplet collection (VPD–DC) with total reflection-X-ray fluorescence spectrometry (TXRF) is a well-established method for metallic contamination analysis of Si wafers. However, the efficiency of the methodology is not fully quantitatively understood. This study aims for the identification of the sources of error in the VPD–DC–TXRF process. From a study of systematic recovery rate (RR) on standard wafers, it is concluded that the VPD–DC method is very efficient in collecting impurities from Si wafers. TXRF saturation effect on the micro-droplet residue limits the accuracy of quantification to the levels below 3×10 13 atoms of metallic contamination. This represents a homogenous single element surface contamination of 1×10 11 at/cm 2 on a 200-mm wafer. Implications of this finding are more serious for multi-element contaminations and for larger wafer dimensions. The origin of this saturation effect is discussed and solutions are evaluated. Further, we suggest a monitoring of the scattered X-rays from VPD–DC residues as an indicator for TXRF accuracy.

Total reflection X-ray fluorescence spectrometric determination of element inlets from mining activities at the upper Tisza catchment area, Hungary

Natural freshwater and biofilm samples collected at seven sites at the upper catchment area of the Tisza river system in Hungary were analyzed by total reflection X-ray fluorescence (TXRF) spectrometry. On the basis of the results, it could be established that there are elevated trace element concentrations in the river Szamos compared to the upper course of the river Tisza. A portable TXRF spectrometer was also used for in-field water analysis. Using the portable TXRF spectrometer, a rapid screening of trace elements was possible just before the confluence of both rivers. The in-field analysis confirmed the statement above. Since biofilms are very good trace element scavengers, the elevated trace element concentrations were also found in the naturally grown biofilms.

Wet chemical cleaning process of GaAs substrate for ready-to-use

We investigate the wet chemical cleaning process of GaAs substrate for ready-to-use. To prepare epi-ready GaAs wafer, we used NH 4OH mixture to remove particles and metals as a first step of wet-chemical cleaning and acidic peroxide mixture to remove residual metallic contaminants and control the components of oxide layer as a second step of wet-chemical cleaning. The particle-free, metal-free, and thin As-oxide rich surface is achieved by the proposed ratio of NH 4OH mixture followed by HCl mixture among the second step of wet-chemical cleaning solution used in this experiment. Metal-contamination was analyzed by total reflection X-ray fluorescence spectroscopy. Characterization of the oxide composition was carried out by X-ray photoelectron spectroscopy. Surface morphology and thickness of oxide layer were observed by atomic force microscopy and ellipsometer, which show very flat surface roughness of 0.95 nm, and uniform oxide thickness of about 2 nm, respectively.

Saturation effects in TXRF on micro-droplet residue samples

Total reflection X-ray fluorescence spectrometry (TXRF) is a well-accepted technique for ultra-trace analysis of ultra-pure reagents and silicon wafers. Nevertheless, the technique’s linear range is not well characterized. In this paper, the upper limits of the linear range of TXRF on micro-droplet residues are identified and the origin of the non-linear effect is investigated. It is observed experimentally that a systematic decrease in the accuracy occurs as a function of the metallic content, starting from amounts above 3–10 ng, depending on the sample composition. A mass-absorption model for thin films is re-formulated for the micro-droplet residue samples and the model parameters are tested on the experimental observations. The calculations are in good agreement with the experimental data. Finally, solutions to deal with these saturation effects are discussed and a method to extend the dynamic range of TXRF is proposed.

Copper Diffusion Behavior in SiO2/Si Structure During 400°C Annealing

The behavior of copper atoms diffusing from the back surface to the front surface of a silicon wafer during 400°C annealing was examined using total reflection X-ray fluorescence spectrometry. The behavior strongly depended on the back-surface copper concentration and on the front-surface SiO2 thickness. The higher the back-surface copper concentration, the harder it was for the copper to diffuse into the silicon. The number of copper atoms that diffused from the back surface to the front surface was limited by the copper solubility in silicon at 400°C. When the front SiO2 surface was thinner than approximately 3 nm, the atoms easily diffused to the SiO2 surface, but they did not when it was thicker than approximately 3 nm. This difference in the copper diffusion behavior apparently results from whether the SiO2 surface is electrically active or not and indicates that the effect of back-surface copper contamination on the electrical characteristics of semiconductor devices is very small during a back-end process.

Particle size distributions and compositions of aerosols produced by near-IR femto- and nanosecond laser ablation of brass

Particle size distributions and compositions of primary aerosols produced by means of near-IR femtosecond laser ablation (λ = 775 nm) of brass in He or Ar at atmospheric pressure have been measured. Aerosols were characterized using a 13-stage low-pressure impactor covering a size range from 5 nm up to 5 μm and subsequently analyzed applying total reflection X-ray fluorescence spectrometry. The results indicate, that for femtosecond laser ablation in the low-fluence regime (<5 J cm−2) ultra-fine aerosols (mean diameter dp ≈ 10 nm/peak width wp ≈ 35 nm) are produced. Furthermore, the total Cu/Zn ratio of these aerosols corresponds to the composition of the bulk material. In contrast, ablation above 10 J cm−2 results in the formation of polydisperse, bimodal aerosols, which are distributed around dp1 ≈ 20 nm (wp1 ≈ 50 nm) and dp2 ≈ 1 μm (wp2 ≈ 5 µm), respectively, and whose total Cu/Zn ratio slightly deviates from the bulk composition. In order to examine the influence of pulse duration on particle size distribution and aerosol composition, comparative measurements by means of near-IR nanosecond ablation were also performed. The data show that nanosecond ablation generally leads to an intensified formation of particles in the micrometer range. Moreover, the composition of these aerosols strongly departs from the stoichiometry of the bulk. Aspects concerning the formation of particles during ablation as well as implications for the element-selective analysis by inductively coupled plasma spectrometry are discussed.

Efficient Isotope Ratio Analysis of Uranium Particles in Swipe Samples by Total-Reflection X-ray Fluorescence Spectrometry and Secondary Ion Mass Spectrometry

Efficient Isotope Ratio Analysis of Uranium Particles in Swipe Samples by Total-Reflection X-ray Fluorescence Spectrometry and Secondary Ion Mass Spectrometry

Cold plasma ashing improves the trace element detection of single Daphnia specimens by total reflection X-ray fluorescence spectrometry

The recently developed dry method for the element determination of single freshwater microcrustacean specimens ( Daphnia) using total reflection X-ray fluorescence (TXRF) spectrometry showed that inhomogeneities of the biological material on the glass carriers resulted in some cases in high background and hampered the detection of certain trace elements (e.g. Cr, Ni). The aim of this study was to test how inhomogeneities of the biological material can be reduced using cold plasma ashing (CPA) techniques. For that, single specimens of the microcrustacean Daphnia pulex prepared according to the dry method were measured by TXRF before and after CPA. To determine the efficiency of the removal of organic matrix, the background and signal-to-background relationship of 28 samples were analyzed. The results showed (1) a highly significant reduction of the background by CPA fluctuating between 26 and 46% (all elements) and (2) a significant increase of the signal-to-background relationship by the factor 1.5–2.5 (all elements) and a much better detection of Cr, Pb, As and Se. The element concentrations (with exception of Cr, Ni and Pb) after ashing were in the same range or slightly higher than that before ashing. No significant differences between the two treatments were observed for Mn, As, Pb, Se (November), Sr (November), Cr (March) and Pb (March). The element concentration of P, K, Ca, Cu, Zn, Cr (November), Fe and Rb were significantly higher after ashing. In general, they increased by 1.5–13.6% and were highest for Rb (March) and P (November). In contrast, the element concentration of Ni and Cr (only March) decreased significantly after ashing (Ni: 91.6–92.1%, Cr: 91.3%). We recommend the use of CPA for biological material in the microgram-range as a routine method for TXRF analysis, especially when trace elements in minute concentrations are of interest.

The use of Si carriers for aerosol particle collection and subsequent elemental analysis by total-reflection X-ray fluorescence spectrometry

The adoption of polished Si carriers was studied for the sensitive elemental analysis of aerosol particles using total-reflection X-ray fluorescence (TXRF) spectrometry. The surface roughness of the Si carrier measured by atomic force microscopy was found to be smaller than those of glassy carbon and quartz glass carriers, which are commonly used for TXRF analysis. The detection limits of elements for the Si carrier were superior to those for the glassy carbon and the quartz glass carriers, presumably due to its smaller surface roughness. For example, the detection limit of Sr for the Si carrier was 9 pg, which was 100 times and 3 times lower than those for the glassy carbon and the quartz glass carriers, respectively. The Si carriers could be successfully applied to the direct aerosol particle collection by impaction and the subsequent elemental analysis by TXRF. From the results of the elemental analysis of aerosol particles, the variations in the concentrations of K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn Sr and Pb with time could be clarified.