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.     

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.     

TXRF法测定松花粉中的9种生命元素

建立了微波消解前处理,全反射X射线荧光法(TXRF)同时测定松花粉中K、Ca、Ti、Mn、Fe、Ni、Cu、Zn和Rb9种生命元素含量的分析方法.松花粉原料经过微波消解前处理后,采用全反射X射线荧光光谱净计数、QXAS分析软件解谱和单一内标法进行定量分析.比较了干灰化法、湿消解法和微波消解法3种前处理方法的效果,并确立微波消解法作为样品前处理方法.用微波消解- TXRF法测定了花粉标准物质中的上述9种元素,并计算得到其仪器检出限(LLD)为0.002～0.054 mg/L,方法检出限(LDM)为0.004～0.122 mg/kg.TXRF法测定各元素的相对标准偏差(RSDs)为1.0％～5.5％.该方法操作简单、样品用量少、检出限低,对实际样品松花粉的测定结果与ICP - MS法无显著性差异.     

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     

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.     

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.     

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.     

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.     

A method for trace element determination of marine periphyton communities on discs of float glass (without sample preparation) using total-reflection X-ray fluorescence spectrometry

A quick method for trace element determination of marine periphyton communities on soda float glass discs is presented. After addition of an internal standard, the community is measured by total-reflection X-ray fluorescence (TXRF) spectrometry. No sample preparation is required except a gentle wash with distilled water. The soda glass disc on which the periphyton community grows is used directly as the sample reflector in TXRF. The method was evaluated by the analysis of a certified reference material of plankton (CRM 414) and by comparison to a wet digestion method. Recovery rates for 13 and 130 μg-samples of CRM 414 are reasonable: between 0.6 and 1.4 for the elements K, Ca, Mn, Fe, Ni, Cu, Zn, As, Rb and Sr. Relative standard deviations for 130 μg-samples are 10% or less for most of these elements. In the comparison to wet digestion, natural periphyton samples were used and the two methods showed a good agreement.The different steps used in the quantification, such as accounting for the contribution from the glass to the TXRF spectrum, and the calculation of the sample mass from the spectrum, are described. It is shown that complicating factors, such as the required water wash and the influence of an inhomogeneous spatial distribution of the periphyton on the glass disc, do not adversely affect the quantification.     

An X-ray refractive lens comprising two sections cut from a gramophone record for a portable total reflection X-ray fluorescence spectrometer

An X-ray refractive lens is assembled from two sections cut from a gramophone record. The refractive lens is placed in a portable total reflection X-ray fluorescence (TXRF) spectrometer, and it is used for collimation of the incident X-ray beams. A TXRF spectrum measured with the refractive lens is compared with that measured with a waveguide. Compared with the refractive lens, the waveguide enhances the intensities of the X-rays illuminating an analyte. Therefore, fluorescent X-ray intensities increase when using the waveguide. On the other hand, the vertical angular divergence of the incident X-ray beams is smaller when using the refractive lens, and the smaller angular divergence results in a reduction of the scattering of the incident X-rays from a sample holder. Therefore, the spectral background is reduced when using the refractive lens, resulting in an increase of the signal to background ratios of the fluorescent X-rays. Detection limits for 3d transition metals obtained with the refractive lens are sub-nanograms to a few nanograms, and the detection limits are similar to those obtained with the waveguide.     

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.     

Total reflection X-ray fluorescence analysis of fly ash from Bulgarian coal-fired power plants

The contents of Cl, Ca, K, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Rb, Sr, Ba and Pb in raw coal fly ash from five Bulgarian power plants were determined by total reflection X-ray fluorescence (TXRF), using gallium as the internal standard. The samples were analysed as in slurry form in Triton? X-114. The experimental parameters, such as grain size, concentrations of fly ash slurry and excitation time were optimised. For validation of the method, the certified reference material BCR-176R fly ash was used. The precision of the results obtained is characterised by a relative standard deviation of approximately 10%. The resulting data confirm the suitability of TXRF for the simultaneous determination of major, minor and trace elements in coal fly ash samples. Further advantages provided by TXRF are easy sample preparation (no sample dissolution) and the small sample amount required for analysis.     

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.     

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.     

Thin film characterization by total reflection x-ray fluorescence

Sensitive and accurate characterization of films thinner than a few nm used in nanoelectronics represents a challenge for many conventional production metrology tools. With capabilities in the 10¹⁰ at/cm², methods usually dedicated to contamination analysis appear promising, especially Total-reflection X-Ray Fluorescence (TXRF). This study shows that under usual configuration for contamination analysis, with incident angle smaller than the critical angle of the substrate, TXRF signal saturation occurs very rapidly for dense films (below 0.5 nm for HfO₂ films on Si wafers using a 9.67 keV excitation at 0.5°). Increasing the incident angle, the range of linear results can be extended, but on the other hand, the TXRF sensitivity is degraded because of a strong increase of the measurement dead time. On HfO₂ films grown on Si wafers, an incident angle of 0.32° corresponding to a dead time of 95% was used to achieve linear analysis up to 2 nm. Composition analysis by TXRF, and especially the detection of minor elements into thin films, requires the use of a specific incident angle to optimize sensitivity. Although quantitative analyses might require specific calibration, this work shows on Co–based films that the ratio between minor elements (W, P, Mo) and Co taking into account their relative sensitivity factors is a good direct reading of the composition.     

A road map to assess critical materials content in boron industrial wastes using sustainable micro-X-ray fluorescence and total reflection X-ray fluorescence instrumentation

Turkey is the leading country in the world in terms of boron production and sale. Increasing boron production goes along with an increasing generation of boron wastes. The pollution of the soil and the air around the waste piles, as well as the occupation of several square kilometers of ground, are major environmental problems. It is, therefore, very important to make use of the wastes to both protect the environment and create revenue. This work presenteda road map for fast screening of boron waste for critical elements followed by determination of the elements using small footprint low power instrumentation. The sample preparation was kept to a minimum. A procedure that allowed an assessment of critical materials in industrial production waste with minimal consumption of hazardous acids, energy, and time was presented. The samples were first screened for valuable and hazardous elements by micro-X-ray fluorescence (XRF). Samples with considerable contents of Cs, Rb, and Aswere then prepared as slurries for the total reflection XRF (TXRF) measurement. To evaluate the TXRF procedure, a standard reference material was analyzed. As a result, Rb and Cs in concentrations up to 420 ± 70 and 1500 ± 200 mg/kg were detected in some of the waste forms. The time savings were in order of a factor of 3 when comparing the prescreening combined micro-XRF and TXRF approach to an all TXRFanalysis approach.     

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.     

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.     

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.