P. Wobrauschek

A method for quantitative X-ray fluorescence analysis in the nanogram region

Abstract Application of X-ray total reflexion is used to reduce the background and to improve the ratio between signal and background so that measurements in the nanogram region can be made.

Low Z total reflection X-ray fluorescence analysis — challenges and answers ☆

Low Z elements, like C, O, ... Al are difficult to measure, due to the lack of suitable low-energy photons for efficient excitation using standard X-ray tubes, as well as difficult to detect with an energy dispersive detector, if the entrance window is not thin enough. Special excitation sources and special energy dispersive detectors are required to increase the sensitivity and to increase the detected fluorescence signal and so to improve the detection limits. Synchrotron radiation, due to its features like high intensity and wide spectral range covering also the low-energy region, is the ideal source for TXRF, especially of low-Z elements. Experiments at a specific beamline (BL 3-4) at SSRL, Stanford, designed for the exclusive use of low-energy photons has been used as an excitation source. Detection limits <100 fg for Al, Mg and Na have been achieved using quasimonochromatic radiation of 1.7 keV. A Ge(HP) detector with an ultra-thin NORWAR entrance window is used. One application is the determination of low-Z surface contamination on Si-wafers. Sodium, as well as Al, are elements of interest for the semiconductor industry, both influencing the yield of ICs negatively. A detection capacity of 1010 atoms/cm2 is required which can be reached using synchrotron radiation as excitation source. Another promising application is the determination of low-Z atoms implanted in Si wafers. Sodium, Mg and Al were implanted in Si-wafers at various depths. From measuring the dependence of the fluorescence signal on the glancing angle, characteristic shapes corresponding to the depth profile and the relevant implantation depth are found. Calculations are compared with measurements. Finally, aerosols sampled on polycarbonate plates in a Battelle impactor were analyzed with LZ-TXRF using multilayer monochromatized Cr-Kα radiation from a 1300-W fine-focus tube for excitation. Results are presented.

Analysis of Ni on Si-wafer surfaces using synchrotron radiation excited total reflection X-ray fluorescence analysis

Abstract Total Reflection X-Ray Fluorescence Analysis excited with synchrotron radiation (SR-TXRF) monochromatized by a multilayer (ML) has been used for the analysis of Ni on Si-wafer surfaces. Intentionally contaminated wafers using droplet samples have been used to determine the detection limits. Two different kinds of the geometrical arrangement of sample and detector have been compared, one of them resulting in detection limits of 13 fg for Ni. Experiments have been performed at Hasylab, Beam L using a bending magnet radiation.

TXRF with synchrotron radiation Analysis of Ni on Si-wafer surfaces

Abstract SR-TXRF (Synchrotron Radiation excited Total Reflection X-ray Fluorescence Analysis) with monoenergetic radiation produced by a W C multilayer monochromator has been applied to the analysis of Ni on a Si-wafer surface. An intentionally contaminated wafer with 100 pg has been used to determine the detection limits. 13 fg have been achieved for Ni at a beam current of 73 mA and extrapolated to 1000 s. This technique simulates the sample preparation technique of Vapour Phase Decomposition (VPD) on a wafer surface.

Total Reflection X‐Ray Fluorescence Analysis Excited by Synchrotron Radiation (SR‐TXRF): Variation of Excitation Conditions and Sample Geometries

The use of synchrotron radiation as the excitation source for total reflection x-ray fluorescence analysis (SR-TXRF) in combination with a multilayer structure for monochromatization led to detection limits in the femtogram range for medium-Z elements. Experiments were performed at the Hasylab Beamline L using bending magnet radiation. Different methods for the modification of the spectral distribution, viz. multilayer monochromatization, insertion of a high-energy cut-off and a simple filter technique, were compared. Also, several possible types of geometrical arrangement of the sample and detector were examined to establish whether an improvement in excitation or detection conditions is possible. Samples were of evaporated droplets of aqueous or acidic solutions of several elements at various concentrations. Spectra were evaluated and the data used to extrapolate the detection limits (DL). Monochromatization of the synchrotron radiation in combination with a sample carrier positioned in the vertical plane and a side-looking detector in the plane of polarization turned out to give the best results with a DL of 15 fg for Ni.

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☆

Abstract 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 analysis using special X-ray sources

Abstract The parameter variations of exciting radiation, like spectral distribution, intensity, brilliance, polarization and the phenomenon of X-ray total reflection, leads to improved lower limits of detection (LLD) in XRF. Observations and results from experiments performed with different X-ray tubes such as fine focus Cu and Mo anodes, a specially designed Au anode operated with 100 kV and high power rotating anodes are reported. Results from measurements with monochromatic X-rays tuned with a multilayer structure as well as the use of polarized X-rays from the synchrotron will be shown. All developed measuring devices will be described in terms of their recent design features showing the possible geometric arrangements denned by the beam-reflector-detector position. The extrapolated detection limits for the K -shell excitation of rare earth elements are in the region of 0.3 ng, for medium Z elements in the pg range and for optimized conditions, with a rotating Cu anode, 170 fg for Mn are achieved corresponding to the pg g −1 (ppt) concentration level.

Total reflection X-ray fluorescence analysis with synchrotron radiation monochromatized by multilayer structures

Abstract To achieve lowest detection limits in total reflection X-ray fluorescence analysis (TXRF) synchrotron radiation has been monochromatized by a multilayer structure to obtain a relative broad energy band compared to Bragg single crystals for an efficient excitation. The energy has been set to 14 keV, 17.5 keV, 31 keV and about 55 keV. Detection limits of 20 fg and 150 fg have been achieved for Sr and Cd, respectively.

Total reflection X-ray fluorescence analysis of light elements using synchrotron radiation

Synchrotron radiation from SSRL (Stanford Synchrotron Radiation Laboratory) Beam Line III-4 was used as excitation source for Total Reflection X-ray Fluorescence Analysis (TXRF) of light elements (Z < 14). Due to the outstanding features of synchrotron radiation this source is best suited for the photon induced excitation of light elements offering a large number of photons in the energy range of interest. The high intensity and the natural collimation of the beam is ideal for total reflection geometry. The experiments were performed with a special TXRF spectrometer using an ultrathin window Ge(HP) detector for the energy dispersive measurement of light elements. BL III-4 is equipped with a cut-off mirror (Ecut = 3 keV) for white beam measurements, and a double multilayer monochromator can be inserted. Detection limits of some hundred femtograms have been obtained for Mg with the white beam excitation. Boron could be detected with monochromatic excitation.

A new X-ray tube for efficient excitation of low-Z-elements with total reflection X-ray fluorescence analysis

Abstract Total reflection X-ray fluorescence analysis has turned out to be a suitable method for the analysis of light elements using a Si(Li)-detector with an ultra-thin window. To improve the excitation conditions of the low-Z-elements, a special windowless tube was built. Two different anode configurations are checked, especially the influence of the take-off angle on the spectral distribution of the bremsstrahlung. Influence on detection limits and technical details are presented.