Physikalisch-Technische Bundesanstalt

By means of nearly perfect crystals such as, e.g., silicon, germanium or sapphire and of the x-ray interferometers built on this basis, it is possible to realise measurement systems meeting the extreme precision requirements placed on nanometrology length measurements in science and technology. The measurement uncertainties of such systems are no larger than a millionth of an atomic spacing.

The atomic length scale has been derived from the SI length unit with highest accuracy by means of metrological fundamental experiments (e.g. with X-ray interferometers). Such experiments show new paths to link definitions of units of measurement - one of PTB's most important tasks - with the fundamental constants occurring in fundamental physical theories. Fundamental constants for whose determination the lattice parameter of silicon is of particular importance are the Sommerfeld fine-structure constant and the Avogadro constant.

Due to great problems, full-scale application of this X-ray method in industrial practice is not possible yet; a solution to these problems is, however, being investigated, also within the scope of international cooperation. The aims are, amongst others, the extension of the measuring ranges of X-ray interferometers and better processing techniques for crystals used in the production of complex elements for metrological purposes.

The unit of mass is the only base unit which has - until today - been embodied by the kilogram prototype. All other base units can be traced back to physical fundamental constants. Working Group "X-ray Optics" therefore also pursues the aim to fulfil the pre-condition for a redefinition of the unit of mass on the basis of atomic masses. For this purpose, it coordinates partial investigations which have been carried out by different departments. One of these investigations is, for example, the determination of the number of atoms in a single crystal of 1 kg mass. The method of measurement is based on an extremely precise determination of the Avogadro constant N_A by means of selected, high-purity silicon crystals; on this basis, it is possible to derive further tasks of research and development for length and angle measurements of atom-sized quantities. This project provides industry with services in the form of crystal orientation measurements on rods and wafers.

• Metrological application of X-ray interferometry
  
• X-ray Fabry-Perot interferometer
• Silicon processing and investigation of crystal structure errors (X-ray topography)
  
• Crystal orientation measurements (off-angle by means of wafers, crystal orientation device)