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




As a scanning probe method, the low measurement speed remains as a major shortcoming of the SPM technique today. Taking a typical Atomic Force Microscope (AFM) with a tip scanning speed of 30 µm/s as an example, it will take approximately 1 hour to finish an image with a size of 100 µm x 100 µm by 1024 x 1024 pixels. This situation becomes more challenging for a metrological large range microscope which has a capable measurement volume of 25 mm x 25 mm x 5 mm (x, y, z). The low speed leads not only to a low measurement throughput, but also to a significant drift over the long measurement time (up to hours or even days).
To overcome this problem, a metrological high-speed large range AFM (Met. HS-LRAFM) with a scanning speed up to 1 mm/s has been recently developed at PTB. Both measurement performances concerning the speed and the accuracy are stressed in this development, as they are essential to realise fast and accurate nanometrology.
Several important design concepts have been implemented to realise both high measurement speed and high metrology performance:

  • The contact AFM mode is applied instead of the intermittent and non-contact modes, which offers shorter AFM response time and larger AFM sensing range.    
  • During measurements, the sample is scanned in the xy-plane solely by the NMM (such a motion usually has a constant velocity. Therefore, high dynamics of the xy-scanner is not needed.). Meanwhile, a high dynamic z motion of the sample is realised by a combined piezo stage and the z-sage of the NMM controlled in parallel.
  • The AFM output signal is combined with the position readouts of the piezo stage and the NMM to derive the surface topography. The combination of these readouts offers a large bandwidth of measurement signal, thus provides high speed measurement capability.
  • Two important means are taken to reduce the distortion of measured profiles, namely (a) the time delay of sensor signal is corrected; (b) the position sensors of the AFM and piezo stage are traceably calibrated to the z-interferometer of the NMM in situ.

To demonstrate the metrology performance of the high speed Met.LR-AFM, Fig. 1 shows surface profiles at the same location measured at different speeds (from 10 μm/s up to 500 μm/s). It can be seen the quality of measured profile is very similar although the scanning speed has been increased by 50 times.

Surface profiles taken on a PTB roughness standard (RN 505) by the high-speed Met. LR-AFM, measured at the same location with different speeds from 10 μm/s up to 500 μm/s.

For more details of this research task, please refer to some selected publications listed below:
[1] Gaoliang Dai et al. High-speed metrological large range AFM, Meas. Sci. Technol. 26 (2015) 095402
[2] Gaoliang Dai et al. Fast and accurate: high-speed metrological large range AFM for surface and nanometrology Meas. Sci. Technol. 29 (2018) 054012
[3] Timo Strahlendorff et al. Tip wear and tip breakage in high-speed atomic force microscopes, Ultramicroscopy 201 (2019) 28–37