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The meter that came out of the crystal

Alternative access to length measurements in the nanometer range

PTBnews 2.2020
Especially interesting for

metrology institutes

manufacturers and users of scanning probe microscopes

In the course of the revision of the International System of Units (SI), which entered into force on 20 May 2019, the international Metre Convention, upon a proposal from the Consultative Committee for Length (CCL), endorsed the use of the lattice parameter of the silicon crystal (i.e. the spacing between the atomic layers) as a basis of the secondary realization of the meter in the nanometer range. In high-resolution microscopy, uncertainties in the picometer range are thus achievable for measurements of step heights up to 10 nm. For this purpose, PTB has manufactured crystalline silicon samples whose high quality is reproducible.

Probe mit atomaren Stufenhöhen, aufgenommen mit einem konfokalen Laserscanning-Mikroskop. Jede der Terrassen ist eine atomar glatte Netzebene des Silizium (111)-Kristalls, mit einer konstanten Stufenhöhe von h ≈ 0,314 nm.

The exact measurement of lengths in the nanometer range is a particular challenge. As the meter has, to date, been realized mainly by means of lasers in the visible spectral range, the journey to the nanometer range has been long and arduous. In practice, this has often implied both large uncertainties and high costs for users.

Within the scope of a mise en pratique, the Consultative Committee for Length (CCL) of the international Metre Convention endorsed the lattice parameter of silicon as a secondary realization of the meter for dimensional measurements in nanometrology. This provides the possibility of achieving substantial progress in the dimensional calibration of atomic force microscopes, since the lattice parameter in the bulk of the Si crystal is available with relative uncertainties smaller than 10-8.

Within the scope of the European Metrology Research Project titled Crystal, crystalline silicon samples were manufactured at PTB with reproducible high quality. In these samples, the height of a monolayer of silicon amounts to 0.314 nm. Comparison measurements performed at diverse metrology institutes have demonstrated that these crystalline standards for the measurement of small step heights on the nanometer scale allowed an expanded uncertainty of 10 pm to be achieved. Compared with previous methods, this is an improvement by more than one order of magnitude.

These standards are manufactured by means of a self-organizing process in ultra-high vacuum on silicon wafers with (111)-orientation. A special processing method allows the crystal lattice of the wafer to be reconstructed at the surface of the sample without any disturbance. This ensures that the atoms at the surface are arranged just as regularly as in the depths of the crystal.

In a new preparation chamber, this procedure is to be extended to yet more complex application cases.

Scientific publication

Bureau International des Poids et Mesures (Ed.): Mise en pratique for the definition of the metre in the SI. SI Brochures, 9th edition (2019) Appendix 2


Ingo Busch
Department 5.1
Surface Metrology
Phone: +49 531 592-6136
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