## 3 Calibration Artefacts
### Micro-Contour Standard  
To ensure traceability, especially for high-precision optical measurements at microgeometries, a task-specific micro-contour standard was developed in cooperation between the Alicona Company, IPK-Fraunhofer and PTB. This standard is a further development of a standard presented in [45] and, in contrast to the former standard, is calibrated by tactile probing. The micro-contour standard is made of tungsten carbide, is manufactured by wire-EDM at IPK-Fraunhofer and has a diffusely reflecting surface of \(R\mathrm{z}\approx 1.5~{}\mathrm{\SIUnitSymbolMicro m}\). It is, therefore, well suited for optical measuring tools such as confocal and focus variation instruments. The external size of the standard is 47 mm \(\times\) 15 mm \(\times\) 3 mm with different spherical and prismatic geometric elements on top, as shown in Figure 11a. These elements have dimensions of 0.05 mm up to 5 mm and represent different measurands like radii, angles and step heights. The form deviations of the geometrical elements are in the range of a few tenths of a micrometre.  
Figure 10: Measurement repeatability of mark coordinates measured on a 3D Aztec artefact by the PTB NMM equipped with a boss-membrane probe. The artefact is calibrated in two repeated measurement runs. The difference of the results is shown as vectors: for the \(x\)- and \(y\)-coordinates (**a**); and for the \(z\)-axis (**b**). The scale of the vector is shown in the bottom left region of the plot.  
Figure 9: (**a**) Layout of a 3D Aztec artefact for calibrating micro/nano-CMM; (**b**) detail of (**a**); and (**c**) photo of a fabricated artefact in four-inch wafer size using the photolithography and micromachining technique.  
The micro-contour standards are calibrated by tactile single-point probing, usually as working standards at IPK-Fraunhofer, currently using a Zeiss CMM O-Inspect with a probing sphere of \(\mathcal{O}\) 300 \(\upmu\)m. The uncertainties amount to about 1.2 \(\upmu\)m for radii, 0.2\({}^{\circ}\) for angles and 0.6 \(\upmu\)m for step heights. For some special purposes which require lower uncertainty, the standards are calibrated as reference standards at PTB. For this, a Zeiss micro-CMM F25 is used with a probing sphere of \(\mathcal{O}\) 120 \(\upmu\)m and very low contact forces of approximately 1 mN. The probing deviation of the F25 according to ISO 10360 determined at different reference spheres of \(\mathcal{O}\) 1 mm up to \(\mathcal{O}\) 10 mm amounts to \(PF<\) 0.15 \(\upmu\)m (form error) and \(PS<\) 0.07 \(\upmu\)m (size error). The standards are measured in a horizontal position and the geometric elements are probed in three different traces: 1 mm, 1.5 mm and 2 mm below the front face. Each geometric element in each trace is measured with 50 points, which result in about 10,800 points overall, as shown in Figure 10. The calibration results are determined from the 3D geometrical elements with 3 \(\times\) 50 points each. Figure 10 shows an overview scan result of the standard obtained with an optical focus variation instrument (Alicona InfiniteFocus) with 5\(\times\) magnification. For the testing of optical instruments, the magnification used is adapted to the size of the geometrical element to be measured.  
To test the suitability of using the standard, comparison measurements were carried out between the InfiniteFocus and the F25 at four different standards. The results agree with respect to the measurement uncertainty stated as summarized in Table 1.