Filling the voids in silicon single crystals by precipitation of Cu₃Si

Abstract

This article presents a method to decorate all open volume defects in a silicon single crystal by Cu₃Si precipitation. Si single crystals are being used for the determination of Avogadro's number with sufficient accuracy (1 part in 10⁸) to allow the establishment of a physical standard for the kilogram. The method described here can be used to certify that the open volume in such crystals is sufficiently small. The voids in this work were formed artificially by annealing He-implanted silicon wafers. Annealing after application of a copper nitrate solution to the surface followed by slow cooling produced Cu₃Si precipitates in the η′ phase in the voids. To fill all the voids completely it proved necessary to coat the surfaces, after application of the nitrate, with a pure Cu layer as well. Slow cooling keeps the supersaturation of Cu small but sufficient to precipitate Cu into the voids. Formation of dislocation loops and stacking faults, often seen during Cu silicide precipitation in silicon after fast cooling, can be minimized. Other transition metals, such as Au, Ni, and Fe, were found to be less suitable than Cu for filling voids.

Acknowledgements

This work was supported by the National Science Foundation as a Cooperative Activity in Materials Science between NSF and the European Commission. We are grateful for useful discussion with Joris Proost, and with Paul De Bièvre, Philip Taylor, Christophe Quétel and Vitas Aninkevicius of the Institute for Reference Materials and Measurements, a Joint Research Centre of the European Commission.