Interim report on the fabrication and characterisation of the surface of silicon artefacts used in the Avogadro project

A new definition of the kilogram is being sought. A possibility is to relate the kilogram to an atomic mass, through the accurate measurement of the Avogadro constant. This approach is realised through spheres manufactured from extremely pure, single crystal silicon. One parameter to be determined is the bulk density of the silicon crystal. However, the measured density will include a contribution due to the surface oxide and any contaminants that are adsorbed onto a sphere's surface. Furthermore, the scale and structure of damage in the near surface region of the crystal (arising from the manufacturing process) may also significantly change the measured density. NPL has a project to manufacture and characterise the surface of such artefacts. Three complementary techniques, namely Variable Angle Spectroscopic Ellipsometry (VASE), Rutherford Backscattering Spectroscopy (RBS) and X-ray Photoelectron Spectroscopy (XPS), are being used to determine the structure, composition and thickness of surface layers. Atomic Force Microscopy (AFM) is used to determine the morphology of the surface and to monitor the effective removal of surface contaminants arising from the manufacturing stages. Large discrepancies have been found when determining oxide thickness using VASE, RBS and XPS. However, these values are also strongly correlated. Damage to the crystal structure has been identified and quantified using RBS. The scale of the damage suggests an amorphized region with a depth of up to 50 Angstroms. The presence of a carbonaceous overlayer has been detected using XPS. The work reported here describes the current state of this project and future experimental stages. (author)SIGLEAvailable from British Library Document Supply Centre-DSC:6029.28076(CMAM 64) / BLDSC - British Library Document Supply CentreGBUnited Kingdo