Multielement analysis of biological reference materials by total-reflection X-ray fluorescence and inductively coupled plasma mass spectrometry in the semiquant mode

Summary Total-reflection X-ray fluorescence (TXRF) and inductively coupled plasma mass spectrometry (ICP-MS) were used for the analysis of two biological reference materials. The quantifications were carried out after addition of one single standard element which serves as an internal standard in both cases. The results of TXRF analysis were good to satisfactory. The method is therefore suitable for fast multielement analysis, because it needs no special calibration specimens. The results of ICP-MS analysis are at least in the order of magnitude of the certified values. In some cases recoveries fit very well. For quantitative analysis, however, the use of the standard addition method or the calibration with external standards is required.     

Sapphire sample carriers for silicon determination by total-reflection X-ray fluorescence analysis

Sapphire is presented as a new sample carrier material for total-reflection X-ray fluorescence spectrometry (TXRF). A comparison with conventional sample carrier materials such as quartz glass, Perspex®, glassy carbon and boron nitride demonstrates that sapphire has all the physical and chemical properties required for TXRF micro and trace analysis. Moreover, sapphire sample carriers allow the determination of silicon in many matrices in a comparatively simple way. Especially for airborne particulate matter, acid digestion can be avoided by cool-plasma ashing of suitable filter materials directly on the sample carrier. This technique has been successfully applied to environmental samples.     

Considerations on the ideal sample shape for Total Reflection X-ray Fluorescence Analysis

Total Reflection X-ray Fluorescence analysis (TXRF) is widely used in semiconductor industry for the analysis of silicon wafer surfaces. Typically an external standard is used for the calibration of the spectrometer. This is sensitive to errors in quantification. For small sample amounts the thin film approximation is valid, absorption effects of the exciting and the detected radiation are neglected and the relation between sample amount and fluorescence intensity is linear. For higher total sample amounts deviations from linearity have been observed (saturation effect). These deviations are one of the difficulties for external standard quantification.A theoretical determination of the ideal TXRF sample shape is content of the presented work with the aim to improve the calibration process and therefore the quantification.The fluorescence intensity emitted by different theoretical sample shapes was calculated, whereby several parameters have been varied (excitation energy, density, diameter/height ratio of the sample). It was investigated which sample shape leads to the highest fluorescence intensity and exhibits the lowest saturation effect. The comparison of the different sample shapes showed that the ring shape matches the ideal TXRF sample shape best.     

Applications of total reflection X-ray fluorescence spectrometry in trace element and surface analysis

Total reflection X-ray fluorescence spectrometry (TXRF) is presented as a genuine surface analytical technique. Its low information depth is shown to be the characteristic feature differentiating it from other energy dispersive X-ray fluorescence methods used for layer and surface analysis. The surface sensitivity of TXRF and its analytical capability together with the limitations of the technique are discussed here using typical applications including the contamination control of silicon wafers, thin layer analysis and trace element determination. For buried interfaces and implantation depth profiles in silicon a combination of TXRF and other techniques has been applied successfully. The TXRF method has the particular advantage of being calibrated without the need for standards. This feature is demonstrated for the example of the element arsenic.     

Trace analysis for 300 mm wafers and processes with total reflection X-ray fluorescence

Total reflection X-ray fluorescence (TXRF) is essential for 300-mm silicon wafer production and fabrication of semiconductor devices. The 300-mm TXRF enables non-destructive contamination analysis on wafers for process development and process control. The TXRF system shows a very stable continuous operation, which allows accurate trace and ultra trace analysis on the silicon surface. It is equipped with two excitation sources covering the requirements of very sensitive measurement and wide element range. The TXRF is a key technology for contamination control during wafer reclaim. For this purpose we show that the system is able to examine the wafers during different processing states of reclaim. The system sensitivity is influenced by the surface of the wafer. For important processing steps, e.g. double side polishing, the sensitivity is as good as for measurements on hazefree polished wafers. We show with TXRF that cross-contamination with copper during double side polishing is suppressed.     

Investigation of element distribution and homogeneity of TXRF samples using SR-micro-XRF to validate the use of an internal standard and improve external standard quantification

Total reflection X-ray fluorescence analysis (TXRF) offers a nondestructive qualitative and quantitative analysis of trace elements. Due to its outstanding properties TXRF is widely used in the semiconductor industry for the analysis of silicon wafer surfaces and in the chemical analysis of liquid samples. Two problems occur in quantification: the large statistical uncertainty in wafer surface analysis and the validity of using an internal standard in chemical analysis. In general TXRF is known to allow for linear calibration. For small sample amounts (low nanogram (ng) region) the thin film approximation is valid neglecting absorption effects of the exciting and the detected radiation. For higher total amounts of samples deviations from the linear relation between fluorescence intensity and sample amount can be observed. This could be caused by the sample itself because inhomogeneities and different sample shapes can lead to differences of the emitted fluorescence intensities and high statistical errors. The aim of the study was to investigate the elemental distribution inside a sample. Single and multi-element samples were investigated with Synchrotron-radiation-induced micro X-ray Fluorescence Analysis (SR-μ-XRF) and with an optical microscope. It could be proven that the microscope images are all based on the investigated elements. This allows the determination of the sample shape and potential inhomogeneities using only light microscope images. For the multi-element samples, it was furthermore shown that the elemental distribution inside the samples is homogeneous. This justifies internal standard quantification.     

The use of a portable total reflection X-ray fluorescence spectrometer for field investigation

A newly developed, portable total reflection X-ray fluorescence (TXRF) spectrometer was tested during a field campaign on Chilean lakes and a German river in January 2002. The field measurements were compared with laboratory measurements carried out on a stationary instrument in the German laboratory. For method validation certified reference material (NIST SRM 1640 Trace elements in natural water) and water samples from different freshwater sources were analyzed with both techniques and evaluated statistically. Based on these preliminary results, it could be concluded that the portable TXRF is a useful technique for the quantitative elemental screening of freshwater samples during field campaigns. Future tests with biological samples (e.g. biofilms and zooplankton), and suspended matter will provide information about the suitability of the portable TXRF for these materials.     

Determination of ultra trace contaminants on silicon wafer surfaces using total-reflection X-ray fluorescence TXRF ‘state-of-the-art’

In a well balanced system of highly motivated, well trained personnel and automated equipment, pure reagents and bulk media, cleanrooms and integrated data management, total-reflection X-ray fluorescence (TXRF) can and must contribute to quality assurance and process stability, support and canalize creative engineering by continuous learning about materials and processes. TXRF has the advantage of controlled one-point calibration, a linear dynamic range of three orders of magnitude, high grade of automation in operation and data management, high up-time, and a simple control of data plausibility.     

Application of total reflection X-ray fluorescence in angle scan mode to establish the location of arsenic contamination in silicon and silicon oxide,respectively

Experiments have been carried out using total reflection X-ray fluorescence (TXRF) to determine the location of arsenic cross-contamination on or in silicon and silicon oxide, respectively, caused during argon-implantation. TXRF has been applied at varying angles of incidence — the so-called angle scan mode. By comparing the angle scan curves of implanted samples with those of a wafer, spin-coated with arsenic, at which arsenic is certainly located on top of the silicon surface, clear differences are observed. This indicates the presence of arsenic embedded in the subsurface. These observations are confirmed by Rutherford backscattering measurements, by modeling As-implantation profiles for low implantation energies as well as by step-by-step oxide etching followed by standard TXRF analysis. This fast and non-destructive application of TXRF angle scan appears a useful method for qualitative depth profiling.     

Application of X-ray fluorescence analytical techniques in phytoremediation and plant biology studies

Phytoremediation is an emerging technology that employs the use of higher plants for the clean-up of contaminated environments. Progress in the field is however handicapped by limited knowledge of the biological processes involved in plant metal uptake, translocation, tolerance and plant–microbe–soil interactions; therefore a better understanding of the basic biological mechanisms involved in plant/microbe/soil/contaminant interactions would allow further optimization of phytoremediation technologies. In view of the needs of global environmental protection, it is important that in phytoremediation and plant biology studies the analytical procedures for elemental determination in plant tissues and soil should be fast and cheap, with simple sample preparation, and of adequate accuracy and reproducibility. The aim of this study was therefore to present the main characteristics, sample preparation protocols and applications of X-ray fluorescence-based analytical techniques (energy dispersive X-ray fluorescence spectrometry—EDXRF, total reflection X-ray fluorescence spectrometry—TXRF and micro-proton induced X-ray emission—micro-PIXE). Element concentrations in plant leaves from metal polluted and non-polluted sites, as well as standard reference materials, were analyzed by the mentioned techniques, and additionally by instrumental neutron activation analysis (INAA) and atomic absorption spectrometry (AAS). The results were compared and critically evaluated in order to assess the performance and capability of X-ray fluorescence-based techniques in phytoremediation and plant biology studies. It is the EDXRF, which is recommended as suitable to be used in the analyses of a large number of samples, because it is multi-elemental, requires only simple preparation of sample material, and it is analytically comparable to the most frequently used instrumental chemical techniques. The TXRF is compatible to FAAS in sample preparation, but relative to AAS it is fast, sensitive and multi-elemental. The micro-PIXE technique requires rather expensive instrumentation, but offers multi-elemental analysis on the tissue and cellular level.     

Quantitative determination of cadmium in polyethylene using total reflection X-ray fluorescence (TXRF) spectroscopy

A new application of total reflection X-ray fluorescence (TXRF) spectroscopy was tested for its use in the quantitative determination of element mass fractions (Z ≥ 12) in solid polyethylene samples. The experiments were conducted with four polyethylene reference materials from the Institute for Reference Materials and Measurements (IRMM) with cadmium mass fractions in the range from 40 to 400 mg/kg (VDA 001–004). Samples from each reference material were transferred as thin films straight onto quartz glass discs commonly used for TXRF analysis. Precision, accuracy and capability of the method are discussed on the basis of the current results and recent experiences with real samples. The method appears to be suitable for tasks in forensic science and polymer analysis.     

Improvement of total reflection X-ray fluorescence analysis of low Z elements on silicon wafer surfaces at the PTB monochromator beamline for undulator radiation at the electron storage ring BESSY II

Several different total reflection X-ray fluorescence (TXRF) experiments were conducted at the plane grating monochromator beamline for undulator radiation of the Physikalisch-Technische Bundesanstalt (PTB) at the electron storage ring BESSY II, which provides photon energies between 0.1 and 1.9 keV for specimen excitation. The lower limits of detection of TXRF analysis were investigated for some low Z elements such as C, N, O, Al, Mg and Na in two different detection geometries for various excitation modes. Compared to ordinary XRF geometries involving large incident angles, the background contributions in TXRF are drastically reduced by the total reflection of the incident beam at the polished surface of a flat specimen carrier such as a silicon wafer. For the sake of an application-oriented TXRF approach, droplet samples on Si wafer surfaces were prepared by Wacker Siltronic and investigated in the TXRF irradiation chamber of the Atominstitut and the ultra-high vacuum TXRF irradiation chamber of the PTB. In the latter, thin C layer depositions on Si wafers were also studied.     

Study on deposition kinetics of high-K materials by X-ray fluorescence techniques

X-ray fluorescence (XRF) techniques have been used to study, on different pretreated substrates, deposition kinetics of HfO₂ and Al₂O₃, two of the possible high-K materials under evaluation for future integration in microelectronic devices.X-ray fluorescence (XRF) and total X-ray fluorescence (TXRF) measurements demonstrate their capability to give useful information on the very initial growing cycles of deposition and on carbon and chlorine inclusion in the film. Moreover, XRF signal shows a good linear correlation with layer thickness for thick samples of both materials.     

Application of vapor phase decomposition/total reflection X-ray fluorescence in the silicon semiconductor manufacturing environment

Total reflection X-ray fluorescence (TXRF), in combination with vapor phase decomposition (VPD), provides an efficient method for analyzing trace metal contaminants on silicon wafer surfaces. The progress made in applying these techniques to the analysis of silicon wafers in a wafer fabrication cleanroom environment is reported. Methods of standardization are presented, including the preparation and characterization of VPD standards. While the VPD wafer preparation process increases the sensitivity of the TXRF measurement by at least one order of magnitude, inherent uncertainties associated with the VPD technique itself are apparent. Correlation studies between VPD/TXRF and VPD/inductively coupled plasma mass spectrometry (ICP-MS) are presented.     

Beijing synchrotron radiation total-reflection X-ray fluorescence analysis facility and its applications on trace element study of cells

The feasibility of total-reflection X-ray fluorescence (TXRF) analysis excited by synchrotron radiation applied to trace element analysis of biological cells is investigated. The Beijing synchrotron radiation TXRF facility and the related experimental method are also described. The elemental minimum detection limits of some standard reference materials are determined. The elemental compositions of a cluster of small intestine cells of a small white mouse are given, and hence the average trace element contents of the single small intestine cell are also obtained. With this technique, the changes of some trace elements in the cells of lung and cervix cancer before and after apoptosis are also preliminarily studied.     

Total reflection X-ray fluorescence spectrometers for multielement analysis: status of equipment

Multielement analysis by total reflection X-ray fluorescence spectrometry has evolved during two decades. At present commercial equipment is available for chemical analysis of all types of biological and mineral samples. The electronic industry has also benefited from scientific and technological developments in this field due to new instrumentation to determine contamination on the surface of silicon wafers (the equipment will not be covered in this paper). The basic components of the spectrometers can be summarized as follows: (a) excitation source; (b) geometric arrangement (optics) for collimation and monochromatization of the primary radiation; (c) X-ray detector; and (d) software for operation of the instrument, data acquisition and spectral deconvolution to determine the concentrations of the elements (quantitative analysis). As an optional feature one manufacturer offers a conventional 45° geometry for direct excitation. Personal communications of the author and commercial brochures available have allowed us to list the components used in TXRF for multielement analysis. Excitation source: high-power sealed X-ray tubes, output from 1300 to 3000 W, different mixed alloy anodes Mo/W are used but molybdenum, tungsten and copper are common; single anode metal ceramic low power X-ray tubes, output up to 40 W. Excitation systems can be customized according to the requirements of the laboratory. Detector: silicon–lithium drifted semiconductor detector liquid nitrogen cooled; or silicon solid state thermoelectrically cooled detector (silicon drift detector SDD and silicon-PIN diode detector). Optics: multilayer monochromator of silicon–tungsten, nickel–carbon or double multilayer monochromator. Electronics: spectroscopy amplifier, analog to digital converter adapted to a PC compatible computer with software in a Windows environment for the whole operation of the spectrometer and for qualitative/quantitative analysis of samples are standard features in the production of this instrument. The detection limits reported in the literature are presented; pricing, analytical capability, ease of operation, calibration and optical alignment as well as technical support are also discussed.     

Sample pretreatment strategies for total reflection X-ray fluorescence analysis: A tutorial review

In the last years, there has been a revival of total reflection X-ray fluorescence spectrometry (TXRF), which was firstly applied for analytical purposes in the late 80s. The aim of this work is to discuss and compare the current approaches for sample pretreatment including in situ microdigestion, slurry preparation, acid digestion, extraction, etc. prior to TXRF analysis. Advantages and drawbacks inherent to each of those procedures are considered. A comprehensive revision in the period January 2008–July 2013 about different sample preparation strategies prior to TXRF analysis apart from early pioneering reports dealing with sample pretreatment are included in the review. Non-conventional sample pretreatment approaches such as microflow online preconcentration, lab-on-a-chip, etc., are also discussed.Finally, future prospects in sample preparation prior to TXRF analysis are outlined.     

Examination of steel samples, provided as reference materials, by instrumental analyses

Summary Seven steel samples considered as reference materials have been analysed by atomic emission spectrometry with inductively coupled plasma (ICP-AES), atomic absorption spectrometry with flame and graphite furnace atomization (FAAS, GFAAS), differential pulse anodic stripping voltammetry (DPASV), X-ray fluorescence analysis with total reflection sample carrier (TXRF), and instrumental neutron activation analysis (INAA). Totally, 18 elements were determined. Over 90% of all results were in good agreement with the values determined by wet analytical methods. The deviations can be explained by systematical errors for special elements in distinct methods and by statistical errors, mainly at very low concentrations because some elements, in the low ng/g region, are not homogeneously distributed in the given material.formerly: Amt für Standardisierung, Meßwesen und Warenprüfung, Große Steinernetischstrasse 4, DDR-3010 Magdeburg     

Heavy metal analysis of rainwaters: A comparison of TXRF and ASV analytical capabilities

The aim of this work consisted on the implementation of sufficiently accurate and sensitive analytical procedure for the analysis of metal concentration in rainwater. Different sample preparation procedures were tested to achieve the required concentration prior to direct total reflection X-ray fluorescence (TXRF) analysis. TXRF and anodic stripping voltammetry (ASV) were compared in regard to achieved detection limits, precision and accuracy.