Quantum Metrology Division

Technical Highlights

• Three tunable x-ray spectrometers delivered to NASA programs. NASA missions in x-ray astronomy, AXAF (Advanced X-ray Astronomy Facility), XTE (X-ray Timing Explorer) and Astro-E (scheduled for launch by Japan around the year 2000) required widely tunable monochromatic x-radiation for pre-flight calibrations and subsystem development. Division staff used a common baseline monochromator design derived from synchrotron radiation experience to support these programs. The AXAF monochromator, designed to cover the range from 0.3 keV to 12 keV, was installed at Marshall Space Flight Center (MSFC) in late 1995. It uses a MSFC supplied rotating anode source and provides monochromatized beams at the entrance to an 875 meter long vacuum line at the end of which the AXAF telescope will be installed in 1996 for end-to-end preflight calibrations. This system is currently operational and delivering count rates in excess of stated requirements. The large proportional counter array (PCA) for the XTE mission was succesfully launched on December 30, 1995. One of our monochromators (range 1 keV to 60 keV) is currently installed at Goddard Space Flight Center (GSFC) where it is being used to evaluate the performance of a duplicate PCA instrument package. NIST supplied hardware for the Astro-E mission included, in addition to the basic monochromator, a multi-anode target array and electron gun to produce low intensity calibration points from 0.3 keV to 12 keV. All monochromators had dual pentagonal turrets for the diffraction crystals which were individually characterized and in some cases produced by Henins at NIST before delivery. (Staudenmann and Hudson)
• Lattice changes in Si epilayers and Si substrates. Certain high performance microprocessors are fabricated using epitaxially deposited thin Si layers grown on highly doped Si wafers. In at least one case, it was found that material from different vendors gave differing device yields although all sources met stated electrical criteria and appeared consistent using the manufacturer's current metrology toolbox. We examined samples of this material using high resolution lattice comparison techniques developed in the Division. These techniques involved the lattice comparator (delta-d) instrument described above to determine the substrate's lattice parameter relative to that of a high resistivity float-zoned silicon reference. The measurements showed considerable variation (up to 50 ppm) among the sources and even a rather large difference between nominally identically processed samples from 20 cm and 15 cm boules. In a second set of measurements, the lattice constant of the epilayer was measured with respect to that of the substrate using conventional high resolution double crystal diffractometry. Differences obtained in these measurements range from 20 ppm to 100 ppm. The two measurements can be combined to obtain the epilayer lattice parameter relative to the same reference specimen. There is some indication that the lattice parameter differences seen here (somewhat smaller than can be resolved by conventional diffractometry) correlate with device yield although the needed control studies had not yet been undertaken. (Schweppe, Hudson, and Henins)
• A new database for x-ray scattering factors. Through the efforts of a recently departed visiting scientist in the Quantum Metrology Division, Dr. Christopher Chantler, a comprehensive new database for atomic scattering factors has been published (J. Phys. Chem. Ref. Data, 24, 71-643, 1995). This semi-theoretical formulation retains robust empirical connections while offering functional approximations which have good analytic properties over their entire domain. The database provides scattering and absorption coefficients over the very broad range of photon energies from 30 eV to 300 keV. The improved accuracy offered by this database permits more effective modelling of the performance of multilayer optics for x-rays and more efficient analysis of crystal structures. Such modelling and analysis can lead to improved performance of x-ray telescopes for astronomy and x-ray microscopes for biology. Chantler came to the Division from Oxford as a fellowship awardee of the Lindemann Foundation. (Deslattes)
• A new x-ray optics geometry for powder diffraction. The need to recertify powder diffraction standards required realization of accurate powder diffraction measurements with a parallel x-ray beam. In addition, accurate determination of the diffraction angle zero demands an instrument operable in mirror symmetric configurations. These geometric constraints and the accuracy needs led us to construct an entirely new apparatus for the measurement. The restriction to parallel beam geometry leads to a significant loss in signal levels in comparison with conventional (focussing) geometries. One solution would be to use a synchrotron radiation source. This clearly addresses the intensity problem but requires separate determination of the input wavelength and entails establishing a good metrological environment on the floor of an accelerator facility. In a recent development, we have been able to obtain incident beam intensity from a conventional (2 kW) diffraction tube comparable to that available at a synchrotron radiation powder diffraction beamline. The key to this development is a newly realized combination of a graded spacing multilayer paraboloid with a flat multilayer optic having a spacing near the mean value of the graded spacing mirror. The beam from this mirror pair has a divergence of about 10 millidegrees and provides a photon rate of 10 Ghz at the sample. The divergence of this beam in the orthogonal direction is restricted to 10 mrad. Preliminary powder diffraction scans indicate good peak to background ratios, symmetric profiles and counting rates sufficient to proceed with the needed measurements in the more benign environment of the Gaithersburg laboratories. (Staudenmann, Hudson, Henins, and Deslattes)