Logo of the Physikalisch-Technische Bundesanstalt

At the surface of the new kilogram

New instrumentation to analyze the surface of silicon spheres

PTB-News 1.2017
Especially interesting for

fundamental research in physics

materials research

non-destructive analyzing

In order to become independent of the international prototype of the kilogram kept in Paris, the definition of the SI unit of mass, the kilogram, has to be revised. For this purpose, the number of silicon atoms in a monocrystalline silicon sphere has been determined at PTB within the scope of the Avogadro proj´ect. It is therefore necessary to measure the surface properties with the highest possible metrological accuracy. To this end, PTB has set up ultra-high vacuum (UHV) instrumentation with combined X-ray fluorescence spectroscopy and photoelectron spectroscopy.

UHV instrumentation for combined X-ray fluorescence and photoelectron spectroscopy on surfaces of silicon spheres

Surface effects, such as the forming of oxide and contamination layers, have a notable influence on the mass and the volume of the silicon spheres used (the surface layers altogether have a mass of approx. 100 µg).

By combining X-ray fluorescence and photoelectron spectroscopy, it is pos-sible to determine both the chemical composition and the mass deposition at the surface of the sphere(s) by means of a single apparatus. In this way, PTB can characterize the surface with the highest accuracy.

The core piece of this apparatus is a sample manipulator which only has minimum contact (three-point contact) with the sphere and allows measurement at any point on the surface. The instrumentation is equipped with a monochromatic Al X-ray source. By means of silicon oxide reference samples, it is possible to determine the oxygen mass deposition and thus the silicon oxide layer thickness over the total surface of the sphere with extremely small uncertainties. In addition, it becomes possible to determine contaminations which may occur during the production process or when handling the spheres.


Michael Kolbe
Department 7.1 Radiometry with Synchrotron Radiation
Phone: +49 (0)30 3481-7131

Scientific publication

R. Fliegauf, B. Beckhoff, E. Beyer, E. Darlatt, I. Holfelder, P. Hönicke, G. Ulm, M. Kolbe: Surface characterization of silicon spheres by combined XRF and XPS analysis for determination of the avogadro constant. Proc. of 2016 Conference on Precision Electromagnetic Measurements (CPEM 2016), DOI: 10.1109/CPEM.2016.7540797