PTB News 3/2002 (148 kB) PTB News 3/2002, English edition, Issue December 2002PTB has combined a scanning tunnelling microscope (as the actual measurement instrument) and an x-ray interferometer (as a length standard) in one experimental set up. Standards – even containing atom size lateral structures – can be calibrated with such an apparatus.
In nanotechnology atomic structures are commonly investigated with a scanning tunnelling microscope (STM). In the new experimental set up an STM is applied to determine the topography of the sample surface as well. An x-ray interferometer produced by PTB is used as a high precision adjusting mechanism. The interferometer is made of dislocation-free, single crystal silicon. The samples are adjusted in steps of 0,192 nm, the spacing between the silicon (220) planes. In the last years this spacing has been traced very accurately to the SI base unit one meter. A further advantage of the construction: the periodic sequence of the atomic planes is free of non-linearities, which have to be taken into account when measuring structures of a few fractions of the applied optical wavelength in size, for instance, as in case of a laser-based interferometer.
The experimental stand at PTB demonstrated an extremely high stability and, in correspondence, a very good reproducibility of results. First investigations were carried out on a two-dimensional grating with several smallpits etched into the surface with a nominal lateral spacing of 100 nm (so-called 2D100-lattice). For this distance the calibration with the silicon grating scale produced a value of (100,12 ± 0,36 nm) over a measurement path of 0,8 µm.
As a result of these investigations which were carried out in cooperation with the National Physical Laboratory (NPL), Teddington, U. K., PTB is planning the installation of a permanent in-house measurement station.