Automation the facility has been designed with a high

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Automation The facility has been designed with a high degree of automation in mind to cope with the high throughput that can be envisaged for the years to come. Acknowledgments The realization of the facility has been possible by the dedication of Monique Navizet, the work of Paolo Mangiagalli and Giorgio Apostolo, and the skills of Guy Dumoulin. The important effort on the software aspects of automation has been supported by Emmanuel Papillon. Facility of the PTB at BESSY II Using undulator radiation at electron storage rings, very high photon fluxes of linear polarized radiation for the TXRF excitation can be achieved. The photon energy of the undulator radiation depends on the energy of the stored electrons as well as on both the strength and periodicity of the magnetic field generated by the undulator. At the 1.7 GeV electron storage ring BESSY II in Berlin, Germany, photon energies of undulator radiation range from the VUV well into the soft X-ray regime, up to about 2 keV, thus allowing for the effective inner-shell excitation of light elements. The Physikalisch-Technische Bundesanstalt (PTB), Germany’s national institute for metrology, operates a laboratory [213] at BESSY II, including a plane grating monochromator (PGM) beamline [214] for undulator radiation. Complementary to the work at SSRL and ESRF, the PTB lays emphasis on the methodological develop- ment of soft X-ray TXRF analysis, providing a means for the quantitation of low Z contaminants on silicon wafer surfaces at its PGM beamline. Ini- tial TXRF investigations were carried out in cooperation with the ATI and the semiconductor company Siltronic in order to determine the absolute LOD values of TXRF for some low Z elements [200, 215] such as C, N, Na, Mg and Al, which ranged between 0.3 pg and 1.3 pg with respect to a measuring time of 1000 s. The calculated VPD detection limits, based on the assump- tion that the sample droplets were collected from a 200 mm wafer, ranged from 2 × 10 7 atoms/cm 2 to about 10 8 atoms/cm 2 for Na, Mg, and Al, thus fulfilling current analytical requirements of the semiconductor roadmap. For the explicit purpose of the semiconductor industry, the PTB has extended its TXRF instrumentation [216] and, to date, can handle 200 and 300 mm sili- con wafers. In recent investigations employing again small droplets containing
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Methodological Developments and Applications 547 the elements Na [216] and Al deposited on 200 mm Si wafers considerably improved LOD values of less than 100 fg were achieved by further optimizing the excitation conditions. Compared to the analysis of medium Z elements, such as transition met- als for which excitation energies in the 5 keV to 15 keV range are needed to induce K fluorescence, some drawbacks exist when exciting K fluorescence radiation of low Z elements below 2 keV. The measured fluorescence count rates are proportional to the fluorescence yields when the self-absorption ef- fects are negligible, which is the case when only minute amounts of a low Z element are present. Since the fluorescence yield rapidly decreases as the
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  • Spring '14
  • MichaelDudley

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