Optimization of TXRF measurements by variable incident angles

Variable incident angles in TXRF instrumentation open up new possibilities in the field of analytical quality assurance of TXRF measurements as well as the possibility of optimizing the measurement angle with respect to the sample carrier. Measurements on the same sample with different incident angles allow a check to be made on the behavior of the internal standard and the elements under investigation within the sample, which makes quantification more reliable, even for difficult samples. This is demonstrated on the example of standard reference material NIST 1633a comparing the relative fluorescence intensities of the elements K, Ti and Fe obtained from a sample prepared from a suspension and a digestion of the SRM material, respectively. Furthermore, it will be shown how the measurement conditions for different sample carrier materials such as quartz and acrylic glass can be optimized by measuring angular-dependent signal and background intensities.     

Elemental analysis of airborne dust samples with TXRF: Comparison of oxygen-plasma ashing on sapphire carriers and acid digestion for sample preparation

The concentrations of 13 elements (As, Ca, Co, Cd, Cr, Fe, Ga, Ni, Pb, Pt, Ti, V and Zn) were determined in air particulate matter using total reflection X-ray fluorescence spectrometry (TXRF). For silicon analysis synthetic sapphire was chosen as a new sample carrier material – it is silicon-free, resistant to oxygen-plasma, microwaves and concentrated acids. The dust samples were collected on cellulose acetate filters. The decomposition of the filters was carried out by oxidation in a microwave-generated low-pressure oxygen-plasma directly on the TXRF sample carriers. The recovery of the investigated elements was verified with the standard reference material SRM 1648 (urban particulate matter) and ranged from 90 to 97%. The oxygen-plasma method was compared with conventional sample preparation by acid digestion. Received: 9 April 1999 / Revised: 18 May 1999 / Accepted: 3 June 1999     

Elemental analysis of airborne dust samples with TXRF: Comparison of oxygen-plasma ashing on sapphire carriers and acid digestion for sample preparation

The concentrations of 13 elements (As, Ca, Co, Cd, Cr, Fe, Ga, Ni, Pb, Pt, Ti, V and Zn) were determined in air particulate matter using total reflection X-ray fluorescence spectrometry (TXRF). For silicon analysis synthetic sapphire was chosen as a new sample carrier material – it is silicon-free, resistant to oxygen-plasma, microwaves and concentrated acids. The dust samples were collected on cellulose acetate filters. The decomposition of the filters was carried out by oxidation in a microwave-generated low-pressure oxygen-plasma directly on the TXRF sample carriers. The recovery of the investigated elements was verified with the standard reference material SRM 1648 (urban particulate matter) and ranged from 90 to 97%. The oxygen-plasma method was compared with conventional sample preparation by acid digestion. Received: 9 April 1999 / Revised: 18 May 1999 / Accepted: 3 June 1999     

An electrochemical enrichment procedure for the determination of heavy metals by total-reflection X-ray fluorescence spectroscopy

An electrolytic separation and enrichment technique was developed for the determination of trace elements by total-reflection X-ray fluorescence spectroscopy (TXRF). The elements of interest are electrodeposited out of the sample solution onto a solid, polished disc of pure niobium which is used as sample carrier for the TXRF measurement. The electrochemical deposition leads to a high enrichment of the analytes and at the same time to a removal of the matrix. This results in substantially improved detection limits in the lower picogram per gram region. The deposited elements are directly measured by TXRF without any further sample preparation step. The homogeneous thin layer of the analytes is an ideal sample form for TXRF, because scattered radiation from the sample itself is minimized. The proposed sample preparation method is useful particularly for the analysis of heavy metals in liquid samples with for TXRF disturbing matrices, e.g. sea water.     

Nanoliter deposition unit for pipetting droplets of small volumes for Total Reflection X-ray Fluorescence applications

A nanoliter droplet deposition unit was developed and characterized for application of sample preparation in TXRF. The droplets produced on quartz reflectors as well as on wafers show a good reproducibility, also the accuracy of the pipetted volume could be proved by a quantitative TXRF analysis using an external standard. The samples were found to be independent of rotation of the sample carrier. Angle scans showed droplet residue behavior, and the fluorescence signal is relatively invariant of the angle of incidence below the critical angle, which is useful for producing standards for external calibration for semiconductor surface contamination measurements by TXRF. Further it could be demonstrated that the nanoliter deposition unit is perfectly able to produce patterns of samples for applications like the quantification of aerosols collected by impactors.     

Optimized sample preparation procedures for the analysis of solid materials by total-reflection XRF

The total reflection X-ray fluorescence (TXRF) method has been used for the analysis of various types of solid materials of biological, geological and environmental origin. The sample preparation step prior to TXRF measurements has been optimized for the various solid samples, including their decomposition by applying both a microwave oven and a PTFE bomb. Complete procedures for the optimized decomposition from the point of view of speed and completeness of digestion, as well as of the overall precision and accuracy are presented.     

Simultaneous DSC-TG up to 1700 K

A DSC-TG-sample carrier up to 1700 K employs plate-type thermocouples as heat flux sensors. Flat bottomed sample pans ensure an almost ideal thermal contact with these sensors, resulting in high sensitivity and peak resolution. Its performance was tested with standard materials and some polymeric samples.The construction of the sample carrier permits quantitative determinations of specific heat at high temperatures. Results are shown with sapphire, aluminium oxide, graphite and glass samples.     

Simultaneous determination of germanium,arsenic, tin and antimony with total-reflection X-ray fluorescence spectrometry using the hydride generation technique for matrix separation—first steps in the development of a new application

This paper introduces a new perspective for total-reflection X-ray fluorescence analysis (TXRF), that is the simultaneous determination of Ge, As, Sn and Sb in seawater. As is well known from atomic absorption spectroscopy (AAS) and inductively coupled plasma techniques (ICP) compounds of these elements can be reduced by sodium borohydride to their hydrides and thus separated from the matrix. In this work the hydride generation is used for matrix separation in TXRF measurements. For this purpose the following procedures are considered: (1) Preconcentration of hydrides by absorption in solvents, and evaporation of some μl of this solution on the sample carrier. (2) Decomposition of hydrides in a heated thin silica tube, or at rough and/or catalytically active surfaces, e.g. in adequately prepared columns, eluting of the species by acid and evaporation of some μl of this solution on the sample carrier. (3) Decomposition of hydrides directly on the surface of a heated silica sample carrier as a thin amorphous film. (4) Combustion of hydrides in the hydrogen flame and deposition of an elemental film on the sample carrier. Basically, all four ways have been tested and the results are promising. © 1997 Elsevier Science B.V.     

Calibration of reference materials for total-reflection X-ray fluorescence analysis by heavy ion backscattering spectrometry

Total-reflection X-ray fluorescence (TXRF) is widely used for the control of metallic contamination caused by surface preparation processes and silicon materials. At least three companies supply a variety of TXRF systems to the silicon integrated circuit (IC) community, and local calibration of these systems is required for their day to day operation. Differences in local calibration methods have become an issue in the exchange of information between IC manufacturers' different FABs (Fabrication Facility) and also between silicon suppliers and IC FABs. The question arises whether a universal set of fluorescence yield curves can be used by these different systems to scale system sensitivity from a single element calibration for calculation of elemental concentrations. This is emphasized by the variety of experimental conditions that are reported for TXRF data (e.g. different angles of incidence for the same X-ray source, different X-ray sources, etc.). It appears that an instrumental factor is required. We believe that heavy ion backscattering spectrometry (HIBS) provides a fundamental method of calibrating TXRF reference materials, and can be used in calculating this instrumental factor. In this paper we briefly describe the HIBS system at the Sandia National Laboratories HIBS User Facility and its application to the calibration of TXRF reference materials. We will compare HIBS and TXRF mapping capabilities and discuss the issues associated with the restrictions of some older TXRF sample stages. We will also discuss Motorola's cross-calibration of several TXRF systems using different elements as references.     

Electro-deposition as a sample preparation technique for total-reflection X-ray fluorescence analysis

A one-step sample preparation by electro-deposition for total-reflection X-ray fluorescence (TXRF) analysis has been developed using a common three-electrode arrangement with a rotating disc as the working electrode. Several elements such as Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag, Cd, Pb, As and U have been determined simultaneously in saline matrix. A special electrode tip has been constructed as a holder for the TXRF sample carrier, which consists of polished glassy carbon. The influence of parameters such as time, pH value, and trace element concentration on the deposition yield has been examined for 14 elements. From repeatability studies, the uncertainty in deposition yields at the 95% confidence level has been found to be less than 20% for most of these elements. Typical detection limits range from 5 to 20 ng/l under the experimental conditions applied here. By an appropriate choice of the reference element and by calculation of yield factors, reliable quantification can be achieved directly by internal standardization. First results obtained for the standard reference material CRM 505 are presented.     

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.     

Some aspects of the high-temperature behavior of bismuth,strontium and barium on silicon surfaces studied by total reflection X-ray fluorescence spectrometry

Thin films of novel dielectric and ferroelectric materials, such as barium strontium titanate (BST) and strontium bismuth tantalate (SBT), which are scheduled for short-term implementation into standard microelectronic device technology, contain elements like Bi, Sr and Ba which may involve risks with regard to device yield and reliability. Therefore, the high-temperature behavior of bismuth, strontium and barium impurities on Si (100) substrates was studied. Intentionally contaminated Si substrates were annealed at 1000°C under different ambient (inert, oxidizing) by rapid thermal annealing (RTA) or in a furnace and analyzed by total reflection X-ray fluorescence spectrometry (TXRF), vapor phase decomposition/TXRF (VPD/TXRF) and electrolytic metal tracer (Elymat) technique. Ba and Sr are incorporated in the existing or growing oxide during rapid thermal annealing (RTA). Cross-contamination due to gas phase transport may occur in the case of Bi, in particular under N₂ atmosphere, but is of no concern in the case of Ba and Sr. All three contaminants do not exert an influence on the minority carrier lifetime on their own. The results illustrate that TXRF and VPD/TXRF are appropriate techniques for such studies.     

Total reflection X-ray fluorescence analysis with chemical microchip

A chemical microchip, which has a flat region on the surface, was recently designed for total reflection X-ray fluorescence (TXRF) analysis. A sample solution was introduced from an inlet by a microsyringe and flowed into a microchannel. Finally it overflowed from the well-type microchannel on the flat region. The sample solution on this region was dried, and then measured by TXRF. The TXRF spectra could be measured with a low background level. This preliminary result indicated that the edge of the well-type channel would not cause a serious problem for TXRF analysis. In addition, a good linear relationship was obtained for Zn Kα in Zn standard solution. This suggests that quantitative analysis by TXRF is feasible in combination with a chemical microchip.     

Characterization of nuclear materials by total reflection X-ray fluorescence spectrometry

Nuclear energy is one of the available energy options for long term energy security of world. In order to produce electricity using this mode of energy generation in an efficient and safe manner, it is necessary that the materials used for such energy generation comply with the specifications assigned. The major and trace composition of these materials is an important specification for their quality control. Different analytical techniques are used for such quality control. Total reflection X-ray fluorescence (TXRF) is a comparatively new technique having several features well suited for trace and major element determinations in nuclear materials. However, this technique has not been used so far extensively for characterization of nuclear materials. The present paper gives a brief introduction of TXRF, its suitability for nuclear material characterization and some details of the TXRF studies made in our laboratory for the characterization of nuclear materials.     

Element Analysis of Oolong Teas from the Fujian Province by TXRF and HG-AAS

Oolong tea from the Fujian Province is very popular at home and abroad, especially in Taiwan and Southeast Asia. In order to elucidate the possibilities to compensate the need state of people to essential elements by the national drink,a set of samples of oolong tea from the Fujian Province were analyzed by total reflection X-ray fluorescence (TXRF), a fairly new efficient method in the field of multielement analysis. After addition of a single internal standard element (e.g. gallium) to a solution of tea leaves digested by acid or to an acidified tea infusion, a portion of 5 μ1 was dried on a sample carrier and then analyzed by TXRF.     

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.     

Total-reflection X-ray fluorescence imaging

A new experimental technique for surface imaging using total-reflection X-ray fluorescence (TXRF) is described. Although TXRF has so far been used to analyze the average chemical composition of rather large sample areas in the order of centimeters squared, a new opportunity to obtain spatial information has arisen through the combination of conventional TXRF and position-sensitive measurement using a collimator and a CCD camera. The most significant point here is that the extremely close detector sample geometry of TXRF measurement fits very well with the present imaging procedure. Scanning of the sample and/or incident beam is not necessary, and therefore the exposure time is reasonably short, typically 3–10 min. The number of pixels is approximately 1 million, and the spatial resolution obtained was several tens of microns in the present preliminary case. The selective-excitation capability of tunable monochromatic synchrotron radiation enhances the present imaging technique. Changing the energy of incident photons makes it possible to distinguish the elements, and one can obtain a surface image of the specific elements.     

Possibilities and limitations of the total reflection X-ray fluorescence spectrometry for the determination of low Z elements in biological samples

The analytical capability of the laboratory scale vacuum total reflection X-ray fluorescence (TXRF) spectrometer (Wobistrax) was studied for the determination of the Z elements (Na, Mg, P, S, K and Ca) in different biological matrices represented by the following certified reference materials: MURST-ISS-A2 Antarctic krill, IAEA-331 spinach, NIST 1577a bovine liver, and SERONORM™ Trace Elements Serum Level 1.First, the stability of the response factors (relative sensitivity) against Ti internal standard was checked in the concentration range of 1 to 1000 mg/L in a diluted nitric acid matrix. It has been found that the upper limit of the analytical concentration range for K and Ca can be as high as 1000 mg/L; on the other hand, the remaining elements cannot be determined above a concentration of some tens mg/L.The established response factors were used for the elemental analysis of the four certified reference materials after normal-volume microwave assisted acid digestion. In the case of the serum sample, different preparation methods were compared as follow: direct analysis, microwave assisted acid digestion in normal-volume and micro-vessels, as well as the vapor-phase digestion directly on the TXRF carrier plates.On the basis of the results, the normal-volume digestion results in rather high dilution of the samples; thus, elements at low concentration could not be detected in some of the samples. On the other hand, this method offers the highest rate of both organic matrix decomposition and inorganic matrix dilution; thus, the background and the standard deviation of the results were the lowest. In general, this method was found to be useful for the analysis of samples with high dissolved (organic + inorganic) content if the analytes are present at a concentration considerable above the quantification limit.In the case of the microscale and the vapor-phase digestion, both the organic and inorganic matters remain at elevated concentration; thus, higher background and self-absorption of the fluorescent radiation occurred, deteriorating the analytical performance.     

Comparison of synchrotron radiation total reflection X-ray fluorescence excitation–detection geometries for samples with differing matrices

The linear polarization of synchrotron radiation (SR) in the orbital plane leads to a background reduction in total reflection X-ray fluorescence (TXRF) analysis if a side-looking detector is used. The optimum orientation of the sample carrier in a SR-TXRF experiment, however, is determined by a trade-off between the exploitation of the linear polarization, the efficiency of excitation and the solid angle of detection and depends on the nature and size of the sample. SR-TXRF measurements on different sample types and using different reflector orientations have been carried out at the Hamburger Synchrotronstrahlungslabor bending magnet beamline L. A NIST standard water sample, a steel sample and an oil standard were analyzed with both a horizontal and a vertical sample carrier orientation. Strongly scattering samples led to lower detection limits with a horizontal reflector whereas weakly scattering samples showed lower detection limits with a vertical reflector configuration. On an intentionally contaminated wafer absolute detection limits of 6.6 fg for Ni could be extrapolated.     

Trends in total reflection X-ray fluorescence spectrometry for metallic contamination control in semiconductor nanotechnology

The development of transistor manufacturing into the nanoscale regime is accompanied by a continuous awareness concern for contamination control. The ever-increasing demands for analysis sensitivity (in the sub-10⁹ at/cm² regime) combined with the introduction of new materials (i.e. non-silicon based) put severe challenges on the application of analytical techniques for atomic level contamination monitoring. Since many years, total reflection X-ray fluorescence spectrometry (TXRF) has developed as a preferred technique, ideally suitable due to the excellent reflectivity and flatness of the starting Si substrates. Driven by performance enhancement requirements, many new materials are being introduced at the substrate level (Ge, III/V compounds for higher mobility), gate stack (alternative dielectric materials and gate electrodes for capacitance scaling) and interconnect level (low-k and copper for faster switching). This paper reviews some recent developments in the state-of-the-art TXRF developments for semiconductor applications. Among the focus areas are the expansion of the elemental range (through multi-excitation line selection or multi-excitation source to excite low Z as well as high Z elements in one analysis sweep) and dynamic range (by pre-concentration techniques, synchrotron radiation analysis and detector developments). Further, emphasis is also focused towards quantification issues—whereby the three methodologies (micro-droplet, film and bulk type standards) are critically reviewed. Also, a recent development of sweeping TXRF, suitable for fast screening of large surface areas is being discussed. The applications of TXRF in a semiconductor environment are being reviewed. Finally, the performance of TXRF for the various semiconductor applications is assessed with respected to competitive techniques.