Construction of a windowless Si-anode X-ray tube for a more efficient excitation of low Z elements on Si-wafer surfaces in total reflection fluorescence analysis

To improve the achievable detection limits of low Z element with TXRF, a commercially available 2 kW X-ray tube (SEIFERT Type SF 60, Ahrensburg) with a 40 μm×8 mm fine focus has been modified. A windowless X-ray tube has been realized by removing the Be window out of the tube. The original Cu anode block has been changed to Al, because of sputtering reasons. A 4–6 μm thick pure silicon layer has been sputtered on the Al substrate. The geometry of the anode has been constructed in a specific way in order to optimize the photon flux of the X-ray beam concerning self-absorption and brilliance. Direct vacuum tight coupling to the measuring chamber and operation at 10⁻⁶ mbar vacuum was successfully shown. First measurements have been perfomed with a detector suitable for the detection of low energy photons in total reflection XRF geometry. Sodium has been analyzed on a Si-wafer surface and detection limits of 36 pg (corresponds to 3E9 atoms/cm²) have been achieved and are 10 times better than the detection limits for Na excited with a 1.3 kW Cr standard tube of 330 pg. With this developed X-ray tube the detection limits required by the Semiconductor industry for Si wafer surface contamination quality control are fulfilled.     

Analysis of low Z elements in various environmental samples with total reflection X-ray fluorescence (TXRF) spectrometry

Recently there is a growing interest in low Z elements such as carbon, oxygen up to sulphur and phosphorus in biological specimen. Total reflection X-ray fluorescence (TXRF) spectrometry is a suitable technique demanding only very small amounts of sample. On the other side, the detection of low Z elements is a critical point of this analytical technique. Besides other effects, self absorption may occur in the samples, because of the low energy of the fluorescence radiation. The calibration curves might be not linear any longer. To investigate this issue water samples and samples from human cerebrospinal fluid were used to examine absorption effects. The linearity of calibration curves in dependence of sample mass was investigated to verify the validity of the thin film approximation. The special requirements to the experimental setup for low Z energy dispersive fluorescence analysis were met by using the Atominstitute's TXRF vacuum chamber. This spectrometer is equipped with a Cr-anode X-ray tube, a multilayer monochromator and a SiLi detector with 30 mm² active area and with an ultrathin entrance window. Other object on this study are biofilms, living on all subaqueous surfaces, consisting of bacteria, algae and fungi embedded in their extracellular polymeric substances (EPS). Many trace elements from the water are bound in the biofilm. Thus, the biofilm is a useful indicator for polluting elements. For biomonitoring purposes not only the polluting elements but also the formation and growth rate of the biofilm are important. Biofilms were directly grown on TXRF reflectors. Their major elements and C-masses correlated to the cultivation time were investigated. These measured masses were related to the area seen by the detector, which was experimentally determined. Homogeneity of the biofilms was checked by measuring various sample positions on the reflectors.     

Total Reflection X-ray Fluorescence attachment module modified for analysis in vacuum

Based on the design of the low cost Total Reflection X-Ray Fluorescence attachment module available since 1986 from Atominstitut (WOBRAUSCHEK-module) which can be attached to existing X-ray equipment, a new version was developed which allows the analysis of samples in vacuum. This design was in particular possible as the Peltier cooled light weight Silicon Drift Detector is following all adjustment procedures for total reflection as angle rotation and linear motion. The detector is mounted through a vacuum feed and O-ring tightening to the small vacuum chamber. The standard 30 mm round quartz, Si-wafer or Plexiglas reflectors are used to carry the samples. The reflectors are placed on the reference plane with the dried sample down looking facing in about 0.5 mm distance the up looking detector window. The reflectors are resting on 3 steel balls defining precisely the reference plane for the adjustment procedure. As the rotation axis of the module is in the plane of the reflector surface, angle dependent experiments can be made to distinguish between film and particulate type contamination of samples. Operating with a Mo anode at 50 kV and 40 mA with a closely attached multilayer monochromator and using a 10 mm² KETEK silicon drift detector with 8 μm Be window, a sensitivity of 70 cps/ng for Rb was measured and detection limits of 2 pg were obtained.     

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.     

Adaptation of a commercial total reflection X-ray fluorescence system for wafer surface analysis equipped with a new generation of silicon drift detector

Within the framework of a collaborative project, it is shown that commercial total reflection X-ray fluorescence (TXRF) systems used in laboratories can easily be upgraded with a silicon drift detector (SDD). SDDs have advantages when used with fully automatized wafer analyzers working under cleanroom conditions, because no liquid nitrogen is required as they are electrically cooled. The goal of this work was the integration of a KETEK 10 mm² SDD in an ATOMIKA 8030W wafer analyzer with special attention to maintain the high degree of automation of the system. An electronic device was designed to establish communication between the SDD and the TXRF electronic control system. The adapted system was tested and compared with the original setup using an 80 mm² Si(Li) detector. Multielement droplet samples on silicon wafers were analyzed and the results showed two times better detection limits for the Si(Li) detector for 1000 pg Ni in comparison to the SDD. Additionally, a RADIANT 50 mm² SDD (VORTEX) was tested which showed identical detection limits compared to the 80 mm² Si(Li) detector.     

Si drift detector in comparison to Si(Li) detector for total reflection X-ray fluorescence analysis applications

TXRF is routinely used and suited to inspect Si wafer surfaces for possible impurities of metallic elements at the level of pg and below. Lightweight, compact sized, high-resolution Silicon drift detectors (FWHM=148 eV at 5.9 keV) electically cooled and with high throughput are ideally as the new spectrometer and for clean room application. A KETEK 5 mm² Si drift detector was compared with a NORAN 80 mm² SiLi in a previously commercially available ATOMIKA 8010 wafer analyzer. Results are presented and show that almost the same detection limits for both detector types were achieved analyzing a droplet sample containing 1 ng Ni on a Si wafer. Also, the performance to detect low Z elements like Na, excited with monochromatic Cr K radiation in a vacuum chamber was tested and detection limits of 600 pg obtained.     

Laboratory and synchrotron radiation total-reflection X-ray fluorescence: new perspectives in detection limits and data analysis

Having established detection limits for transition elements exceeding current requirements of the semiconductor industry, our recent efforts at the Stanford Synchrotron Radiation Laboratory (SSRL) have focused on the improvement of the detection sensitivity for light elements such as Al. Data analysis is particularly challenging for Al, due to the presence of the neighboring Si signal from the substrate. Detection limits can be significantly improved by tuning the excitation energy below the Si–K absorption edge. For conventional TXRF systems this can be done by using a W–Mα fluorescence line (1.78 keV) for excitation. At a synchrotron radiation facility energy tunability is available. However, in both cases this results in a substantial increase in background due to resonant X-ray Raman scattering. This scattering dominates the background under the Al Kα fluorescence line, and consequently limits the achievable sensitivity for the detection of Al surface contaminants. In particular, we find that for a precise determination of the achievable sensitivity, the specific shape of the continuous Raman background must be taken into account in the data analysis. The data deconvolution presented here opens a new perspective for conventional TXRF systems to mitigate this background limitation. This results in a minimum detection limit of 2.4×10⁹ atoms/cm² for Al. Based on these results it will also be demonstrated that by improving the detector resolution, the minimum detection limit can be improved significantly. For a detector resolution of 15 eV as predicted for novel superconducting tunnel junction detectors, an improvement in minimum detection limit of approximately a factor of 3 can be estimated.     

Trends,applications and results in X-ray fluorescence analysis

X-ray fluorescence (XRF) has reached a mature state and represents a powerful analytical tool for the qualitative and quantitative determination of almost all the chemical elements in a sample. Standard XRF methods as well as special techniques to improve the detection limits will be presented, emphasizing the versatility of the method. With modern instrumentation the detectable number of elements ranges from Be to U. The minimum detectable quantities under optimizied excitation and detection conditions for medium Z elements are a few hundred femtograms. Other features like rapid analysis because of the multielement capability and in some cases the nondestruction of the sample are advantageous for a wide field of applications.     

Recent and future developments in low power total reflection X-ray fluorescence spectroscopy

Today, energy-dispersive X-ray fluorescence (EDXRF) spectroscopy with low power X-ray tubes and thermoelectrically cooled detectors provide analytical performance which in the past was restricted to systems with kW power X-ray tubes and liquid nitrogen cooled Si(Li) detectors. Especially for low power TXRF spectrometers, the sensitivity could be improved by an order of one magnitude in the last 5 years. This progress was mainly based on improvements in quantum efficiency of all components and less on high power excitation sources.Recent developments caused further improvements in the analytical sensitivity as well as in the analytical performance.The introduction of a micro-focus X-ray tube increased the excitation power by a factor of 1.3. An additional improvement could be achieved by optimizing the detector window design. By optimizing the detector entrance geometry, the solid detection angle of a silicon drift detector (SDD) was increased by a factor of 1.8. In addition, the recent development of a new generation of silicon drift detectors increased the active detector area by a factor of three enhancing the peak to background ratio by a factor of two. Furthermore, the high-energy efficiency of this new detector type was significantly improved.As a result of all these improvements the detection limit for nickel could be decreased to a value of 1 pg.     

Quo Vadis total reflection X-ray fluorescence?

The multielement trace analytical method ‘total reflection X-ray fluorescence’ (TXRF) has become a successfully established method in the semiconductor industry, particularly, in the ultra trace element analysis of silicon wafer surfaces. TXRF applications can fulfill general industrial requirements on daily routine of monitoring wafer cleanliness up to 300 mm diameter under cleanroom conditions. Nowadays, TXRF and hyphenated TXRF methods such as ‘vapor phase decomposition (VPD)-TXRF’, i.e. TXRF with a preceding surface and acid digestion and preconcentration procedure, are automated routine techniques (‘wafer surface preparation system’, WSPS). A linear range from 10⁸ to 10¹⁴ [atoms/cm²] for some elements is regularly controlled. Instrument uptime is higher than 90%. The method is not tedious and can automatically be operated for 24 h/7 days. Elements such as S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Br, Sn, Sb, Ba and Pb are included in the software for standard peak search. The detection limits of recovered elements are between 1×10¹¹ and 1×10⁷ [atoms/cm²] depending upon X-ray excitation energy and the element of interest. For the determination of low Z elements, i.e. Na, Al and Mg, TXRF has also been extended but its implementation for routine analysis needs further research. At present, VPD-TXRF determination of light elements is viable in a range of 10⁹ [atoms/cm²]. Novel detectors such as silicon drift detectors (SDD) with an active area of 5 mm², 10 mm² or 20 mm², respectively, and multi-array detectors forming up to 70 mm² are commercially available. The first SDD with 100 mm² (!) area and integrated backside FET is working under laboratory conditions. Applications of and comparison with ICP-MS, HR-ICP-MS and SR-TXRF, an extension of TXRF capabilities with an extremely powerful energy source, are also reported.     

Determination of carbon in natural freshwater biofilms with total reflection X-ray fluorescence spectrometry

There is a growing interest in determination of low Z elements, i.e., carbon to phosphorus, in biological samples. Total reflection X-ray fluorescence spectrometry (TXRF) has been already established as suitable trace element analytical method with low sample demand and quite good quantification limits. Recently, the determinable element range was extended towards Z = 6 (carbon).Biofilms can be used for biomonioring purposes in the aquatic environment. Besides the trace metals, especially the determination of the carbon content is important for the better understanding of the early stage of biofilm formation. For this, an ATI low Z spectrometer equipped with Cr-anode X-ray tube, multilayer monochromator, vacuum chamber, and a Si(Li) detector with ultra thin window was used. Biofilms were grown on two different artificial supports (granite and plexiglass), freeze dried, suspended in high purity water and analyzed. As an internal standard the natural titanium content of the biofilms was used. The accuracy of the method was checked by total carbon measurement using a combusting carbon analyzer.     

Determination of technetium by total reflection X-ray fluorescence

The measurement of technetium in inorganic solutions is reported for the first time using total reflection X-ray fluorescence (TXRF). Sodium pertechnetate solutions eluted from decayed ⁹⁹Mo generators were efficiently excited with a silver-anode X-ray tube in a standard configuration. The technique has been developed to aid establishing the extent of stoichiometric relations between Tc and a ligand in organo-metallic compounds synthesized with diagnostic purposes for nuclear medicine. The precision attained was 5% and the detection limit achieved for Tc in inorganic solutions by TXRF at 1000 s was 0.039 μg/ml.     

Determination of depth profiling of metal trace impurities on Si surface using total reflection X-ray fluorescence

Summary Total reflection X-ray fluorescence (TXRF) is used for non-destructive determination of depth profiling. A numerical processing is presented as impurity quantification in the continuum excitation TXRF without using standards. Dependences of concentration of impurities on depths ranging from a few tens to thousands Angströms are given for Fe and Cu on Si-wafer. The detection limits are in the range of 10¹⁰ atoms/cm². The method was checked with Secondary Ion Mass Spectrometry (SIMS) and the agreement is reasonably good.     

Applicability of direct total reflection X-ray fluorescence analysis in the case of human blood serum samples

Total Reflection X-Ray Fluorescence (TXRF) is a well-established method, mainly applied in the analysis of liquid samples, offering very low detection limits in most of the cases. Direct application of the TXRF method is not so efficient in blood serum analysis, since the high content of the organic matrix increases significantly the background due to Compton scattering. Chemical treatment of the blood serum samples and related preconcentration techniques have been suggested in the literature, but they are time consuming and increase the possibility of adding contaminants in the sample. In this paper, the applicability of direct TXRF analysis in blood serum samples is examined. The insertion of a Mo filter, after the cut-off reflector, has been found to improve significantly the peak-to-background ratio, especially for the elements of interest such as Cu, Zn, Se and Br. The influence of self-absorption phenomena in the quantification procedure was also investigated with respect to the internal standard used and the sample mass analyzed. Precision and accuracy in the analysis was found to be approximately 4% over the whole atomic number range.     

Semiconductor applications of nanoliter droplet methodology with total reflection x-ray fluorescence analysis

In this study, the nanoliter dried spot method was applied to semiconductor contamination analysis to enhance vapor phase decomposition processes with total reflection X-ray fluorescence detection. Nanoliter-sized droplets (10 and 50 nl) were deposited onto native silicon oxide wafer surfaces in a clean room environment from both single and multielemental standards containing various concentrations of iron in different matrices. Direct comparisons were made to droplets formed by conventional VPD with similar iron standards. Nanoliter dried spots could be reproducibly deposited and dried in air with typical drying times ranging from 20 s to 2 min depending on the nanoliter volume deposited, compared to VPD spots which have drying times ranging from tens of minutes to several hours. Both types of residues showed a linear relationship between Fe intensity and mass deposited. Variable angle experiments showed that both nanoliter and VPD deposits of single element standards were film-like in character, while residues formed from much more complex matrices and higher mass loadings were particulate in character. For the experimental conditions used in this study (30 kV, 100 mA), typical TXRF spectral Fe limits of detection were calculated to be on the order of picograms or ∼1×10¹⁰ atoms/cm² for a 0.8 cm² X-ray excitation beam area for both nanoliter dried spots and VPD spots prepared from single elemental standards. Calculated Fe detection limits for 200 mm diameter silicon wafers used in this study were in the ∼1×10⁸ atoms/cm² range. By using nanoliter sized droplets, the required sample volume is greatly reduced resulting in higher sample throughput than with conventional VPD methods.     

Synchrotron radiation induced total reflection X-ray fluorescence of low Z elements on Si wafer surfaces at SSRL — comparison of excitation geometries and conditions

The analysis of low Z elements, like Na and Al at ultra trace levels (<10¹⁰ atoms/cm²) on Si wafer surfaces is required by the semiconductor industry. Synchrotron radiation induced total reflection X-ray fluorescence analysis (SR-TXRF) is a promising method to fulfill this task, if a special energy dispersive detector with an ultra thin window is used. Synchrotron radiation is the ideal excitation source for TXRF of low Z elements due to its intense, naturally collimated and linearly polarized radiation with a wide spectral range down to low energies even below 1 keV. TXRF offers some advantages for wafer surface analysis such as non-destructive analysis and mapping capabilities. Experiments have been performed at the Stanford Synchrotron Radiation Lab (SSRL) using Beamline 3-4 (BL 3-4), a bending magnet beamline using white (<3 keV) and monochromatic radiation, as well as Beamline 3-3 (BL 3-3), using a crystal monochromator as well as a multilayer monochromator. A comparison of excitation–detection geometry was performed, using a side-looking detector with a vertically positioned wafer as well as a down-looking detector with a horizontally arranged wafer. The advantages and disadvantages of the various geometrical and excitation conditions are presented and the results compared. Detection limits are in the 100-fg range for Na, as determined with droplet samples on Si wafer surfaces.     

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.     

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.     

The application of long wavelength radiation in X-ray analysis

The authors used long wavelength X-ray radiation excited by low energy electrons for analytical purposes. Low energy electron excitation was achieved with an open window tube. The authors used a type that was developed in Philips laboratories. The electron source was a glow discharge from which the electrons ( 15 keV) are extracted and directed towards the anode. A photon spectrum of “Bremsstrahlung” and characteristic peaks is generated at a gold anode. In addition about half of the impinging electrons is reflected and may, thanks to the open window, be used to irradiate a specimen in spite of a partial loss of energy due to the collisions.The authors mention the following attractive characteristics: the tube is simple to operate and may easily be exchanged for a closed tube; no high vacuum is needed, a feature which, however, at the same time inhibits the determination of traces of O, N, C; about 7 cm² of the specimen, which may be an insulator, is irradiated. A discussion of applications illustrates the usefulness of the tube.     

Detection limit calculations for the total reflection techniques of X-ray fluorescence analysis

In this work, theoretical calculations of detection limits for different total reflection techniques of X-ray fluorescence analysis are presented. Calculations include grazing incidence (TXRF) and gracing emission (GEXRF) conditions. These calculations are compared with detection limits calculated for conventional X-ray fluorescence (XRF). In order to compute detection limits, Shiraiwa and Fujino's model was used to calculate X-ray fluorescence intensities. This model makes certain assumptions and approximations to achieve the calculations, especially in the case of the geometrical conditions of the sample, and the incident and takeoff beams. Nevertheless, the calculated data of detection limits for conventional XRF and total-reflection XRF show a good agreement with previous results. The model proposed here allows us to analyze the different sources of background and the influence of the excitation geometry, which contribute to a better understanding of the physical processes involved in the XRF analysis by total reflection. Finally, a comparison between detection limits in total-reflection analysis at grazing incidence and at grazing emission is carried out. Here, a good agreement with the theoretical predictions of the Reciprocity Theorem is found, showing that, in theory, detection limits are similar for both techniques.