PTB News 1/2008 (1.2 MB) PTB News 1/2008, English edition, Issue May 2008In the scope of the international Avogadro Project, an almost perfect silicon sphere is to be characterised with highest accuracy. In addition, the thickness of the unavoidable oxide layer of several nanometres on the surface of the silicon sphere has to be measured too. Measuring the thickness of the layer is based on X-ray reflectometry with synchrotron radiation of suitable photon energy as well as additional ellipsometric measurements.
Layer thicknesses in the nm range can be measured by means of X-ray reflectometry. Hereby, the fact is used that when varying the angle of incidence, in the reflected radiation, interference effects occur which result from the two reflections on the outer layer surface and on the inner interface. Commercial devices which use copper Kα radiation with a photon energy of 8048 eV at almost grazing incidence are, however, less suitable here because at this photon energy, the reflection on the SiO2/Si interface and therefore also the interference effects, are only weak. Clear interference effects appear, in contrast, in the area of the silicon K edge at 1840 eV and at the oxygen K edge at 540 eV, where strongly curved layers can also be measured with steep angles of incidence of up to 50°. Corresponding measurements were carried out with the UHV Xray reflectometer at the undulator beamline in the PTB laboratory of the Berlin electron storage ring BESSY. Measurements at different photon energies in the environment of the oxygen K edge now allow the thicknesses of the SiO2 layer as well as the carbonaceous contamination layer to be determined separately and the uncertainty of measurement of currently 0.2 nm to be reduced to the 0.1 nm demanded. The SiO2 layer thicknesses determined by means of X-ray reflectometry at several points on the surface, will be used as reference values for ellipsometric measurements of the variations in layer thickness along great circles across the surface of the sphere.