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Finding missing Si atoms with Cu F. Spaepen, C.Y. Wen, J. Proost Harvard University

Finding missing Si atoms with Cu F. Spaepen, C.Y. Wen, J. Proost Harvard University C. Quétel, V. Aninkevicius, K. Filee, P. Taylor EC - JRC - IRMM. How perfect are the Si crystals used for the determination of the Avogadro Constant ?. Dr/r= 4x10 -6.

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Finding missing Si atoms with Cu F. Spaepen, C.Y. Wen, J. Proost Harvard University

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  1. Finding missing Si atoms with Cu F. Spaepen, C.Y. Wen, J. Proost Harvard University C. Quétel, V. Aninkevicius, K. Filee, P. Taylor EC - JRC - IRMM

  2. How perfect are the Si crystals used for the determination of the Avogadro Constant ?

  3. Dr/r= 4x10-6 Do even the best float-zone Si crystals contain voids ? 4x10-6 deficit, equivalent to 1.7 mm3/kg Si How perfect are all these crystals ?

  4. Experiments with Cu in Si

  5. Diffusivity Diffusion of Cu in Si Solubility of Cu in Si 550 oC 10 ppb Cu: fastest diffuser in Si moves interstitially through the lattice Cu in solution is interstitial

  6. Cu source Measuring the number of missing Si atoms Heat to 900 oC Saturate with Cu Cool at 1 K/min Excess Cu precipitates as silicide and fills up voids Clean off surface Cu Anneal at 400 oC Diffuse interstitial Cu to surface Clean off surface Cu Repeat previous step until interstial concentration is negligible Digest sample Determine number of Cu atoms

  7. Observing filling of the voids by transmission electron microscopy Wafer surface 200 nm 200 nm Plan view Voids created by implantation (30 keV He4 nuclei, 3 x 1016 cm-2) and annealing (30 min at 700 oC) of a Si wafer Cross-section All voids are filled with Cu-silicide after heat treatment and cooling

  8. Identification of the silicide b-Cu5Si Si Electron diffraction pattern [100] direction in the body-centered cubic structure Lattice parameter: 0.2854 nm Two atoms per unit cell, chemically disordered High-resolution electron micrograph showing the lattice planes in the silicide and the Si matrix

  9. Cu-Si phase diagram b-phase forms on cooling Anneal at 900oC

  10. IRMM • Ultra Clean Chemical Laboratory • know-how on inorganic analytical chemistry at low level Quantitative determination of the amount of Cu in the precipitates and in solution: a challenge ! • Very small amounts of Cu==> minimize blank issues on Si digestion • Use Isotope Dilution Mass Spectrometry (IDMS)==> implement sound protocol at very low content level

  11. Application in the Avogadro project Boule Avogadro sphere Certify that all test samples have fewer that 10 ppb missing atoms

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