Surface Metrology on Nanostructures

Investigation of crystalline surfaces

Investigations of crystalline surfaces for the applicability in dimensional metrology are of interest [M. Aketagawa et al., Meas. Sci. Technol. 18 (2007) 342 - 349]. A sufficient traceability of crystalline surfaces [annual report in 2006] and definition of the necessary conditions for stable crystal surfaces would be inevitably for this. For the traceability of such stable surfaces measurements by x-ray interferometry may be used [A Yacoot et al., Meas. Sci. Technol. 12 (2001) 1660 - 1665]. On the base of such crystalline lattices standards should be developed which are suitable for other high-resolution techniques (SEM, SFM).

Idea and development of a "Nanostandard" / atomic lattice standard

	As a basis, the known crystalline structure of some surfaces, which are already used for calibration in scanning probe microscopy, is used. In the case of silicon, the lattice constant a0 of these samples is known with extreme accuracy (10-8) from X-ray interferometers (Avogadro constant). However, some microscopes are not in a position to resolve this atomic structure; they require structures with diameters in the range from 2 to 20 nm.
Figure 1: Basic representation of a nanostandard on the basis of a crystalline surface
	In the nanostandard project, such structures shall be generated with a special scanning tunnelling microscope on the surface of single-crystalline samples. During this process, the atomic structure of the base and of the imaging tip must be maintained. If this is guaranteed, the size and the distance of the generated structures can be indicated in units of the lattice constant of the substrate. The figure 2 shows five structures which have been produced on a silicon surface. The atomic structure of the surface can be clearly seen. The stated distances and angles can be derived from the lattice constant and the crystal structure.
Figure 2: Five structures which have been produced on a silicon surface. Plotted is the unit cell of the surface lattice. The lengths of the distances asy and asx can be derived from it.