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Production sequence of Si-spheres and interferometrical determination of the sphere volume

Characterization of the AFM tip shape with an innovative stylus tip testing standard

16.12.2019

Based on a recently developed probe tip testing standard (TSPN) [1] for stylus instruments and for microprobes, which consists of rectangular grooves of different widths with sharp edges, a non-destructive measuring method for the reconstruction of the 2D and 3D form of tips was developed. The precise knowledge of tip form is important for high precision tactile reference measuring methods and for all kinds of nanomechanical measurements.

The method to determine the 2D tip radius and opening angle can be applied for every stylus instrument, scanning microprobe or AFM. The 3D method uses several 2D scans over the TSPN in different directions (see angle θ in Fig. 1(a)). The scanning direction for all these scans with the rotated TSPN has always to be perpendicular to the grooves. Since this requirement is not met on many instruments, we tested the new method on an AFM which had this functionality.

Atomic force microscopy (AFM) using nm-sized probing tips as indenters has become one of the important instruments for determining the mechanical properties of nanomaterials. In AFM-based nanomechanical measurements of soft materials, in particular the indentation modulus and hardness, the typical penetration depth hc can be in the order of a few micrometers. The measurement uncertainty of nanomechanical AFM measurements, such as nanoindentation and contact resonance (CR-AFM), depends strongly on the uncertainty of the contact area Ap of the AFM tip. In the case of deep indentation, i.e. hc >> 100 nm, not only the tip radius R, but also the semi-apex angle γ must be determined to calculate the tip area function Ap(hc).

As shown in Fig. 1 (a), the AFM tip to be measured is first used to measure a profile over the probe tip testing standard. Always one half of the probe tip is imaged on each of the very sharp edges of the standard. The two half images of the AFM tip can be combined to form an image of the probe tip using a tip reconstruction algorithm developed in-house (see Fig. 1(a) top right).

The 3D topography of the AFM tip can also be measured. For this purpose, a series of tip profile measurements are performed at different angles θ (see Fig. 1(a) top left). The reconstructed 2D tip images can then be used to determine the 3D tip form (see Fig. 1(b)). This can then be used to derive the tip radius and the semi-apex angle of the tip and from this in turn the tip area function.

This low-cost measurement method for characterizing 3D tips was validated by comparison with high-resolution SEM images. A further comparison between this presented method and tip calibration methods using other commercial tip characterization standards, is currently under investigation.



Fig. 1(a) Scheme for the determination of the AFM tip form using a stylus tip testing standard with sharp-edged grooves and for the analytical derivation of the tip surface function Ap(h).


Fig. 1(b) Reconstructed 3D topography of an AFM diamond tip (Adama Innovations Ltd.).

The results in this paper partly come from EMPIR 17IND05 MicroProbes, an EU-funded project. MicroProbes has received funding from the EMPIR program co-financed by the EU participating states and from the European Union’s Horizon 2020 research and innovation program.

References

[1] U. Brand, M. Xu et al., "Long Slender Piezo-Resistive Silicon Microprobes for Fast Measurements of Roughness and Mechanical Properties inside Micro-Holes with Diameters below 100 µm", Sensors 2019, 19 (6), 1410 ; https://doi.org/10.3390/s19061410

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