At PTB, atomically smooth silicon surfaces have been produced which are also suited to be used as reference standards for surface metrology. As they are, in addition, surrounded by mono-atomically high, but micrometre-wide steps, they can also be used for the precise calibration of the topography measuring axis of surface measuring devices.
In the case of surface measuring devices, smooth surfaces can serve to measure deviations in the "image" – in the case of tactile measuring instruments they can, for example, measure the guiding errors of the movement axes and, in the case of interference microscopes, the quality of the reference surface. Corresponding surfaces can, for example, be manufactured for gauge blocks, optical flats or silicon wafers with different polishing methods. Over large areas and on the atomic scale, these surfaces are, however, rather rough.
In the case of crystalline material without dislocations, it would be ideal to produce faces as large as possible, consisting of one single lattice plane. If these atomically smooth areas are, however, surrounded, in addition, by mono-atomic steps with large terrace widths, a precise calibration of the topography measuring axis of surface measuring instruments for highest resolutions could be achieved in addition to this flatness. As a material, silicon with atomic lattice distances – i.e. step heights of 0.13 nm for Si(001) and 0.31 nm for Si(111) planes – would be well suited.
Although silicon can be manufactured without dislocations, many images which have been recorded with scanning probe microscopes show a great number of steps with terrace widths of less than 300 nm. This is due to the limited accuracy when the wafers are aligned – before cutting – with respect to their crystalline orientation (> 0.02°).
At PTB, surfaces with larger stepfree areas could be produced by means of annealing silicon wafers in the ultrahigh vacuum. These areas are surrounded by a series of mono-atomic steps with terrace widths of up to a few micrometres. Through a suitable treatment of the surface after annealing in vacuum, a – very thin and homogeneous – oxide layer grows on the silicon wafer which passivates these areas at the same time. Surfaces which have been produced in this way are stable over months (also in air) and can be used as reference standards for surface metrology.
These surfaces were used to determine guiding errors of the moving axes and effects of the optical detection systems for the determination of the cantilever deflection in the case of scanning force microscopes and, thus, to further optimize the scanning force microscopes. In the case of optical microscopes for topography determination, the resolving power of the topography measuring axis could be checked. In the case of an adequately exact calibration, the surface steps could, in future, also be used as a standard for the traceability of microscopes.