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An industrial SR TXRF facility at ESRF
Nuclear Instruments and Methods in Physics Research B 150 (1999) 538±542
An industrial SR TXRF facility at ESRF F. Comin *, M. Navizet, P. Mangiagalli, G. Apostolo ESRF, BP 220, F-38043 Grenoble, France
Abstract A TXRF industrial facility for the mapping of trace impurities on the surface of 300 mm Silicon wafers is presently under construction at the ESRF, European Synchrotron Radiation Facility, in Grenoble (France) and its commissioning phase will start at the end of 1998. The elements to be detected range from Na to Hg with a target routine detection limit of 108 atoms /cm2 . The facility is the result of a collaboration between the ESRF and some of the major European semiconductor companies in the framework of the MEDEA consortium. Preliminary experiments at ESRF reached a detection limit of 1.7 ´ 108 for Ni atoms (17 fg) in not optimised experimental conditions. The facility will improve the detection limit by a factor of 50. However, this gain in sensitivity will be traded in the possibility of mapping the surface of 300 mm wafer with a resolution of 500 pixels and a throughput of three wafers/h. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Metallic contaminants at the surface of silicon wafers heavily hinder the performances of integrated circuits. The present IC technology of 0.25 lm design rule on 200 mm wafer can tolerate contaminant concentrations between 2 and 5 ´ 1010 at/cm2 that requires a maximum surface detection limits between 2 and 5 ´ 109 at/cm2 . At this level of ultimate detection the existing nondestructive method for measuring metallic surface contamination, Total Re¯ection X-ray Fluorescence (TXRF), approaches its limits. The National Technology Roadmap for Semiconductor published by Sematech [1] has already forecasted an important leap for the TXRF technique in order to *
Corresponding author.
cope with the new industrial requirements: the evolution from laboratory based setups to centralized synchrotron radiation facilities is, for the silicon manufacturing industry, a mandatory step. In synchrotron radiation facilities the orders of magnitudes of increase in brilliance of the X-ray beams can well be exploited to meet the future requirements of IC manufacturer and provide new possibilities in wafer TXRF: mapping capabilities and low Z atom detection. The advantages of synchrotron beams over the isotropic emission of laboratory sources range from the bare intensity and high collimation concepts to the large energy tunability and high linear polarization. Each of these factors carries a speci®c advantage, which has been taken into consideration for the design of the TXRF industrial beamline that is presently under construction at the ESRF.
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 1 0 4 2 - 8
F. Comin et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 538±542
The beamline is the end step of many previous studies carried out by dierent groups in other synchrotron radiation centers like HASYLAB in Hamburg and SSRL in Stanford. Moreover pilot experiments were carried out at ESRF in order to verify the possibility of adopting, opposite to most of the previous experiments, a convenient horizontal geometry for the wafers. 2. Previous results and the test experiments In the early 1990s pioneering work exploiting the characteristic collimation and polarisation properties of synchrotron radiation sources led to a signi®cant enhancement of the minimum detection limit (MDL) for trace element analysis, achieving values of the order of 1010 at/cm2 for transition metals [2]. Several studies [3±5] have been carried out in order to determine the best suitable excitation conditions, in particular with respect to: ± sample / detector geometry ± monochromatic vs. white or pink beam excitation. The use of multilayer crystals on a bending magnet source leads to a MDL of 14 fg for Ni [6] whereas for low Z elements the currently obtained MDLs are in the pg range [7]. The usual geometry adopted for synchrotron radiation TXRF is with the wafer positioned vertically in the X-ray beam: it oers the possibility of closely approaching the detector to the impinging point of the radiation, thus increasing the solid angle of acceptance and the counting rate. However, the increase in angular acceptance leads also to an increase of the elastic scattering and related background. In other terms the counting rate increases but the contrast for impurities decreases. A careful assessment of the optimum conditions at ESRF seemed then necessary to us. The test experiments [8] have been carried out at the beamline ID32 with the speci®c aim of: (i) de®ning the optimum geometry for synchrotron radiation TXRF, (ii) measuring the resulting detection limit and (iii) investigate the possibility of working with wafers kept horizontally considering that the vertical geometry is not appropriate to
539
large size wafers. For the measurements the sample was mounted on a four circle diractometer and a HPGe detector has been used to detect the ¯uorescence signal. From the photons provided by an undulator, a collimated beam of 1011 Ph/s at 10.5 keV has been selected by a Si (1 1 1) monochromator detuned by a factor ®ve in the ®rst harmonic to reject higher harmonics. After a large number of tests, the best suitable geometry was obtained with the detector along the horizontal polarisation direction at a distance of 50 mm from the impinging point and with a 4 mm diameter PTFE collimator to limit the angular acceptance. This geometry allowed to keep the sample in the horizontal position. Under these conditions, not optimised with respect to the incoming photon ¯ux, an ultimate detection limit of 2 ´ 108 at/cm2 has been obtained. More recent Monte Carlo simulations con®rmed these ®ndings [9]. 3. Aims of the facility The ESRF-TXRF facility will provide siliconmanufacturing companies with an analytical centralised TXRF service. It will either make possible to reach ultimate detection limits below 108 at/cm2 , or map the concentration of contaminants at an initial detection limit of 109 at/cm2 in 1999. This last limit is to be pushed down to 108 at/cm2 in the following years. The wafer size can be either 200 or 300 mm and, on the average, the speci®ed mapping resolution is slightly above 1 cm2 , the pixel layout being structured in circular sectors. The project envisages also the detection of low Z elements down to sodium at least, thus requiring excitation beams below the silicon K threshold. 4. Optics The key issues that have been taken into account for the de®nition of the optical layout of the installation can be summarized as follows: · Need for a wide energy range from 1 to 40 keV for optimizing the detection sensitivity for every element. The low energy range below the Si threshold is necessary for the detection of low Z atoms.
540
F. Comin et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 538±542
· Need to reject as much as possible the harmonic content of the X-ray beam to limit detection problems with standard solid state energy dispersive detectors. · Need to preserve the polarization and collimation of the beam for maximum rejection of the elastically scattered radiation at the detector. · Need for a monochromator with ®xed position of the exit beam to avoid realigning the wafer position at each change of energy. In order to ful®ll all these requirements in the shortest possible time, within a reasonable budget and with reliability, a strategy of simpli®cation has been adopted, since the pilot experiments at ID32 have shown that: · No focusing of the beam is necessary to reach the speci®ed detection limits. On the contrary, the vertical extent of the beam at the wafer location, about 1 mm, can be usefully exploited for wetting an entire diameter of a 300 mm wafer positioned at grazing. This geometry is particularly useful for parallel detection. · In principle a high resolving power is not mandatory for TXRF experiments; however, given the results of the test experiment, the choice of a standard Si (1 1 1) monochromator with a resolving power DE/E of 10ÿ4 is retained because of the possibility of performing analysis of the chemical and geometrical surrounding of a contaminant species by scanning the incoming energy through the relevant absorption edge (XANES). 5. The photon source The contamination of the synchrotron beam by the high-energy harmonics produced by the undulator source and transmitted through the monochromator impairs the performances of solid state detectors. At typical beamlines the harmonic content is reduced by slightly detuning the monochromator crystals and/or using expensive and delicate X-ray mirrors as low energy bandpass ®lters. An alternative cheaper and faster way to reduce the harmonic content at the detector is to develop an undulator source with an intrinsic low harmonic content. In these new devices the elec-
trons experience a quasi-periodic magnetic ®eld that makes the emission spectrum ``nonharmonic'', meaning that the undulator characteristic peaks are well displaced from being at energies multiple of the fundamental. In this case they are not transmitted through the crystal monochromator. A ®rst aperiodic device has been manufactured and tested by the Insertion Device group of ESRF [10] and a second one with residual harmonic content of few percents is presently under construction. At high energies further reduction of the harmonic content will be achieved by detuning of the monochromator crystals. For maximum reliability the X-ray monochromator is based on a single rotation motion following well established concepts. The dispersing elements are a Si (1 1 1) ``channel cut'' for the energy range 2.8±40 keV and a multilayer pair for the low energy [11]. In order to have a ®xed-exit beam, the channelcut single crystal walls are machined in such a way that the parallel displacement between the incoming and the outgoing beams is independent of the wavelength. A second advantage of this geometry is that the diracting planes are not parallel to the curved-machined surfaces that at low energy reduce the power density impinging on the ®rst crystal face. A multilayer pair covers the low energy portion of the spectra, from 1 to 3 keV. Here a further harmonic rejection, especially important at low energy, is achieved by using for both crystals two dierent asymmetry parameters ± one strongly rejecting the second harmonic by a factor 2 ´ 10ÿ4 and the other with a rejection factor for the third harmonic of 3 ´ 10ÿ6 . Because of the need to reach energies as low as 1 keV, the 40-meter long beamline has no windows and special care is taken to avoid mutual cross contamination and vacuum accidents. 6. The TXRF set-up The geometry adopted for the TXRF facility is derived from the ®ndings of the test experiments described above.The TXRF end-station of the beamline is located in a shielded hutch of about
F. Comin et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 538±542
541
Fig. 1. Top view of the end station of the TXRF installation at ESRF. The entire structure is embedded in a Class 100 laminar ¯ow hood. (a) wafer cassettes, (b) pre-aligner, (c) transfer robot, (d) load-lock vessel, (e) Si(Li) detectors, (f) high vacuum handler, (g) TXRF chamber.
25 m2 embedded in a class 100 laminar ¯ow. The station encompasses a loading robot that transfers the wafers from standard cassettes to a load-lock vessel and a high vacuum handler that transfers the wafer from the load-lock to the measuring position in the main TXRF chamber. The layout of this arrangement is shown in Fig. 1. The TXRF vacuum chamber is sketched in Fig. 2: a large, rugged hexapode actuator aligns the entire structure in the X-ray beam. In the center of the chamber the wafer sits, almost horizontally, on a chuck that is aligned and kept in position by an external smaller hexapode through a ¯exible bellow, and rotated by a goniometer through a differentially pumped feedthrough. The wafer is aligned at grazing incidence on the X-ray beam to illuminate a diametrical strip. A linear array of Si(Li) detectors parallel to the impinging beam acquires in parallel the ¯uorescence data from 14 independent segments 10 mm in length each. The rotation of the wafer around its vertical axis enables to explore and map the entire surface. The detector array is split into two independent units of 7 elements each for servicing/ reliability purposes. 7. Time schedule
Fig. 2. View of the TXRF measuring chamber: (a) vacuum enclosure, (b) wafer, (c) wafer alignment hexapode, (d) Si(Li) detector arrays, (e) goniometer and dierential pumping rotary feedthrough, (f) main alignment hexapode.
The commissioning phase of the facility will start early 1999 with the ®rst tests on real 300 mm wafer. This will lead to an experimental assessment
of the speci®cations attained and further tests are already envisaged to ascertain which other opportunities can be explored by the instrument.
542
F. Comin et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 538±542
Two directions seem particularly interesting, i.e. the possibility of performing XANES spectroscopy in selected atomic species for chemical and structural identi®cation of the contaminant environment and detection of carbon species via electron spectroscopies. Acknowledgements This work has been carried out thanks to a grant from the MEDEA program Package T618. The authors gratefully acknowledge Eric Ziegler for the prompt preparation and characterization of the multilayer crystals. References [1] http://notes.sematech.org/97melec.htm. [2] A. Iida, Adv. X-ray Anal. 35 (1992) 795.
[3] R. Rieder et al., Nucl. Instr. and Meth. Phys. Res. A 355 (1995) 648. [4] P. Pianetta et al., Rev. Sci. Instrum. 66 (1995) 1293. [5] R. Goergl et al., X-Ray Spectr. 26 (1997) 189. [6] Wobrauschek et al., Spectrochimica Acta Part B 52 (1997) 901. [7] C. Streli et al., Nucl. Instr. and Meth. Phys. Res. A 345 (1994) 399. [8] L. Ortega, F. Comin, V. Formoso, A. Stierle, Journal of Synch. Radiation 5 (1998) 1064. [9] L. Vincze, K. Janssens, F. Adams, F. Comin, L. Ortega. Book of Abstracts, European Conference on Energy Dispersive X-ray Spectrometry 1998 (EDXRS-98), 1998, p. 108. [10] J. Chavanne, P. Elleaume, P. Van Vaerenbergh. Proceedings of the European Accelerator Conference 1998, June 1998, Stockholm. www.cern.ch/accelconf/e98/PAPERS/ MOP07F.PDF. [11] F. Comin, M. Navizet, P. Mangiagalli, A. Freund, Proc. Intern. Symp. On Optical Science, Eng., and Instr. Conference 3448: Crystal and Multilayer Optics. San Diego, July 1998.
An industrial SR TXRF facility at ESRF F. Comin *, M. Navizet, P. Mangiagalli, G. Apostolo ESRF, BP 220, F-38043 Grenoble, France
Abstract A TXRF industrial facility for the mapping of trace impurities on the surface of 300 mm Silicon wafers is presently under construction at the ESRF, European Synchrotron Radiation Facility, in Grenoble (France) and its commissioning phase will start at the end of 1998. The elements to be detected range from Na to Hg with a target routine detection limit of 108 atoms /cm2 . The facility is the result of a collaboration between the ESRF and some of the major European semiconductor companies in the framework of the MEDEA consortium. Preliminary experiments at ESRF reached a detection limit of 1.7 ´ 108 for Ni atoms (17 fg) in not optimised experimental conditions. The facility will improve the detection limit by a factor of 50. However, this gain in sensitivity will be traded in the possibility of mapping the surface of 300 mm wafer with a resolution of 500 pixels and a throughput of three wafers/h. Ó 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Metallic contaminants at the surface of silicon wafers heavily hinder the performances of integrated circuits. The present IC technology of 0.25 lm design rule on 200 mm wafer can tolerate contaminant concentrations between 2 and 5 ´ 1010 at/cm2 that requires a maximum surface detection limits between 2 and 5 ´ 109 at/cm2 . At this level of ultimate detection the existing nondestructive method for measuring metallic surface contamination, Total Re¯ection X-ray Fluorescence (TXRF), approaches its limits. The National Technology Roadmap for Semiconductor published by Sematech [1] has already forecasted an important leap for the TXRF technique in order to *
Corresponding author.
cope with the new industrial requirements: the evolution from laboratory based setups to centralized synchrotron radiation facilities is, for the silicon manufacturing industry, a mandatory step. In synchrotron radiation facilities the orders of magnitudes of increase in brilliance of the X-ray beams can well be exploited to meet the future requirements of IC manufacturer and provide new possibilities in wafer TXRF: mapping capabilities and low Z atom detection. The advantages of synchrotron beams over the isotropic emission of laboratory sources range from the bare intensity and high collimation concepts to the large energy tunability and high linear polarization. Each of these factors carries a speci®c advantage, which has been taken into consideration for the design of the TXRF industrial beamline that is presently under construction at the ESRF.
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 8 ) 0 1 0 4 2 - 8
F. Comin et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 538±542
The beamline is the end step of many previous studies carried out by dierent groups in other synchrotron radiation centers like HASYLAB in Hamburg and SSRL in Stanford. Moreover pilot experiments were carried out at ESRF in order to verify the possibility of adopting, opposite to most of the previous experiments, a convenient horizontal geometry for the wafers. 2. Previous results and the test experiments In the early 1990s pioneering work exploiting the characteristic collimation and polarisation properties of synchrotron radiation sources led to a signi®cant enhancement of the minimum detection limit (MDL) for trace element analysis, achieving values of the order of 1010 at/cm2 for transition metals [2]. Several studies [3±5] have been carried out in order to determine the best suitable excitation conditions, in particular with respect to: ± sample / detector geometry ± monochromatic vs. white or pink beam excitation. The use of multilayer crystals on a bending magnet source leads to a MDL of 14 fg for Ni [6] whereas for low Z elements the currently obtained MDLs are in the pg range [7]. The usual geometry adopted for synchrotron radiation TXRF is with the wafer positioned vertically in the X-ray beam: it oers the possibility of closely approaching the detector to the impinging point of the radiation, thus increasing the solid angle of acceptance and the counting rate. However, the increase in angular acceptance leads also to an increase of the elastic scattering and related background. In other terms the counting rate increases but the contrast for impurities decreases. A careful assessment of the optimum conditions at ESRF seemed then necessary to us. The test experiments [8] have been carried out at the beamline ID32 with the speci®c aim of: (i) de®ning the optimum geometry for synchrotron radiation TXRF, (ii) measuring the resulting detection limit and (iii) investigate the possibility of working with wafers kept horizontally considering that the vertical geometry is not appropriate to
539
large size wafers. For the measurements the sample was mounted on a four circle diractometer and a HPGe detector has been used to detect the ¯uorescence signal. From the photons provided by an undulator, a collimated beam of 1011 Ph/s at 10.5 keV has been selected by a Si (1 1 1) monochromator detuned by a factor ®ve in the ®rst harmonic to reject higher harmonics. After a large number of tests, the best suitable geometry was obtained with the detector along the horizontal polarisation direction at a distance of 50 mm from the impinging point and with a 4 mm diameter PTFE collimator to limit the angular acceptance. This geometry allowed to keep the sample in the horizontal position. Under these conditions, not optimised with respect to the incoming photon ¯ux, an ultimate detection limit of 2 ´ 108 at/cm2 has been obtained. More recent Monte Carlo simulations con®rmed these ®ndings [9]. 3. Aims of the facility The ESRF-TXRF facility will provide siliconmanufacturing companies with an analytical centralised TXRF service. It will either make possible to reach ultimate detection limits below 108 at/cm2 , or map the concentration of contaminants at an initial detection limit of 109 at/cm2 in 1999. This last limit is to be pushed down to 108 at/cm2 in the following years. The wafer size can be either 200 or 300 mm and, on the average, the speci®ed mapping resolution is slightly above 1 cm2 , the pixel layout being structured in circular sectors. The project envisages also the detection of low Z elements down to sodium at least, thus requiring excitation beams below the silicon K threshold. 4. Optics The key issues that have been taken into account for the de®nition of the optical layout of the installation can be summarized as follows: · Need for a wide energy range from 1 to 40 keV for optimizing the detection sensitivity for every element. The low energy range below the Si threshold is necessary for the detection of low Z atoms.
540
F. Comin et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 538±542
· Need to reject as much as possible the harmonic content of the X-ray beam to limit detection problems with standard solid state energy dispersive detectors. · Need to preserve the polarization and collimation of the beam for maximum rejection of the elastically scattered radiation at the detector. · Need for a monochromator with ®xed position of the exit beam to avoid realigning the wafer position at each change of energy. In order to ful®ll all these requirements in the shortest possible time, within a reasonable budget and with reliability, a strategy of simpli®cation has been adopted, since the pilot experiments at ID32 have shown that: · No focusing of the beam is necessary to reach the speci®ed detection limits. On the contrary, the vertical extent of the beam at the wafer location, about 1 mm, can be usefully exploited for wetting an entire diameter of a 300 mm wafer positioned at grazing. This geometry is particularly useful for parallel detection. · In principle a high resolving power is not mandatory for TXRF experiments; however, given the results of the test experiment, the choice of a standard Si (1 1 1) monochromator with a resolving power DE/E of 10ÿ4 is retained because of the possibility of performing analysis of the chemical and geometrical surrounding of a contaminant species by scanning the incoming energy through the relevant absorption edge (XANES). 5. The photon source The contamination of the synchrotron beam by the high-energy harmonics produced by the undulator source and transmitted through the monochromator impairs the performances of solid state detectors. At typical beamlines the harmonic content is reduced by slightly detuning the monochromator crystals and/or using expensive and delicate X-ray mirrors as low energy bandpass ®lters. An alternative cheaper and faster way to reduce the harmonic content at the detector is to develop an undulator source with an intrinsic low harmonic content. In these new devices the elec-
trons experience a quasi-periodic magnetic ®eld that makes the emission spectrum ``nonharmonic'', meaning that the undulator characteristic peaks are well displaced from being at energies multiple of the fundamental. In this case they are not transmitted through the crystal monochromator. A ®rst aperiodic device has been manufactured and tested by the Insertion Device group of ESRF [10] and a second one with residual harmonic content of few percents is presently under construction. At high energies further reduction of the harmonic content will be achieved by detuning of the monochromator crystals. For maximum reliability the X-ray monochromator is based on a single rotation motion following well established concepts. The dispersing elements are a Si (1 1 1) ``channel cut'' for the energy range 2.8±40 keV and a multilayer pair for the low energy [11]. In order to have a ®xed-exit beam, the channelcut single crystal walls are machined in such a way that the parallel displacement between the incoming and the outgoing beams is independent of the wavelength. A second advantage of this geometry is that the diracting planes are not parallel to the curved-machined surfaces that at low energy reduce the power density impinging on the ®rst crystal face. A multilayer pair covers the low energy portion of the spectra, from 1 to 3 keV. Here a further harmonic rejection, especially important at low energy, is achieved by using for both crystals two dierent asymmetry parameters ± one strongly rejecting the second harmonic by a factor 2 ´ 10ÿ4 and the other with a rejection factor for the third harmonic of 3 ´ 10ÿ6 . Because of the need to reach energies as low as 1 keV, the 40-meter long beamline has no windows and special care is taken to avoid mutual cross contamination and vacuum accidents. 6. The TXRF set-up The geometry adopted for the TXRF facility is derived from the ®ndings of the test experiments described above.The TXRF end-station of the beamline is located in a shielded hutch of about
F. Comin et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 538±542
541
Fig. 1. Top view of the end station of the TXRF installation at ESRF. The entire structure is embedded in a Class 100 laminar ¯ow hood. (a) wafer cassettes, (b) pre-aligner, (c) transfer robot, (d) load-lock vessel, (e) Si(Li) detectors, (f) high vacuum handler, (g) TXRF chamber.
25 m2 embedded in a class 100 laminar ¯ow. The station encompasses a loading robot that transfers the wafers from standard cassettes to a load-lock vessel and a high vacuum handler that transfers the wafer from the load-lock to the measuring position in the main TXRF chamber. The layout of this arrangement is shown in Fig. 1. The TXRF vacuum chamber is sketched in Fig. 2: a large, rugged hexapode actuator aligns the entire structure in the X-ray beam. In the center of the chamber the wafer sits, almost horizontally, on a chuck that is aligned and kept in position by an external smaller hexapode through a ¯exible bellow, and rotated by a goniometer through a differentially pumped feedthrough. The wafer is aligned at grazing incidence on the X-ray beam to illuminate a diametrical strip. A linear array of Si(Li) detectors parallel to the impinging beam acquires in parallel the ¯uorescence data from 14 independent segments 10 mm in length each. The rotation of the wafer around its vertical axis enables to explore and map the entire surface. The detector array is split into two independent units of 7 elements each for servicing/ reliability purposes. 7. Time schedule
Fig. 2. View of the TXRF measuring chamber: (a) vacuum enclosure, (b) wafer, (c) wafer alignment hexapode, (d) Si(Li) detector arrays, (e) goniometer and dierential pumping rotary feedthrough, (f) main alignment hexapode.
The commissioning phase of the facility will start early 1999 with the ®rst tests on real 300 mm wafer. This will lead to an experimental assessment
of the speci®cations attained and further tests are already envisaged to ascertain which other opportunities can be explored by the instrument.
542
F. Comin et al. / Nucl. Instr. and Meth. in Phys. Res. B 150 (1999) 538±542
Two directions seem particularly interesting, i.e. the possibility of performing XANES spectroscopy in selected atomic species for chemical and structural identi®cation of the contaminant environment and detection of carbon species via electron spectroscopies. Acknowledgements This work has been carried out thanks to a grant from the MEDEA program Package T618. The authors gratefully acknowledge Eric Ziegler for the prompt preparation and characterization of the multilayer crystals. References [1] http://notes.sematech.org/97melec.htm. [2] A. Iida, Adv. X-ray Anal. 35 (1992) 795.
[3] R. Rieder et al., Nucl. Instr. and Meth. Phys. Res. A 355 (1995) 648. [4] P. Pianetta et al., Rev. Sci. Instrum. 66 (1995) 1293. [5] R. Goergl et al., X-Ray Spectr. 26 (1997) 189. [6] Wobrauschek et al., Spectrochimica Acta Part B 52 (1997) 901. [7] C. Streli et al., Nucl. Instr. and Meth. Phys. Res. A 345 (1994) 399. [8] L. Ortega, F. Comin, V. Formoso, A. Stierle, Journal of Synch. Radiation 5 (1998) 1064. [9] L. Vincze, K. Janssens, F. Adams, F. Comin, L. Ortega. Book of Abstracts, European Conference on Energy Dispersive X-ray Spectrometry 1998 (EDXRS-98), 1998, p. 108. [10] J. Chavanne, P. Elleaume, P. Van Vaerenbergh. Proceedings of the European Accelerator Conference 1998, June 1998, Stockholm. www.cern.ch/accelconf/e98/PAPERS/ MOP07F.PDF. [11] F. Comin, M. Navizet, P. Mangiagalli, A. Freund, Proc. Intern. Symp. On Optical Science, Eng., and Instr. Conference 3448: Crystal and Multilayer Optics. San Diego, July 1998.