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

AFM Linewidth Metrology

Working Group 5.23

Accurate and traceable nanoscale calibration

This research task is being continuously carried out at PTB since 1990s. It aims to develop accurate and traceable metrology methods needed for calibrating the geometric properties of almost all kind of microscopes including e.g. 

  • Amplification (i.e. scaling factors) of x-, y- and z-axes;
  • Nonlinearity of the axes;
  • Flatness;
  • Squareness;

Our research activities in this field consist of:

  • advanced instrumentation of metrology AFMs;
  • measurement/calibration strategies;
  • data evaluation algorithm/software;
  • estimation of measurement uncertainty;
  • calibration services with the highest level of accuracy;
  • international comparisons;
  • documentary standards.

A representative metrology tool applied for this task is the metrological large range AFM. It is developed based on the nanomeasuring and nanopositioning machine (NMM) collaborated with the technical university Ilmenau and the SIOS company.

The metrology tool has the following unique features:

  • direct large range AFM scanning on a surface area with a size of up to 25 mm x 25 mm x 5 mm without the need of imaging stitching;
  • displacements of scanner along the x-, y-, z-axes are directly measured by laser interferometers with a resolution of 0.08 nm in fully compliant with the Abbe measurement principle. Thus, the measurement results are directly traceable to the SI unit of the length – the “metre”;
  • ultrahigh measurement accuracy. The state-of-the-art calibration accuracy is, U = ~1 nm (k=2) for the step height calibration; U = ~ 0.01 nm (k=2) for the pitch calibration of 1D/2D gratings with a nominal pitch of 100 nm;
  • versatile probing technique available, including the contact-mode, intermittent contact-mode and noncontact mode AFM, stylus profilometer, micro-CMM probes and optical focus sensors;
  • workhorse at PTB for nanometrology;
  • outstanding metrology performance confirmed by several international comparisons, e.g. the Nano2 and Nano5.

Figure 1. (a) Photo of the Met. LR-AFM located at the clean room centre of PTB. The tool is developed based on the nanomeasuring and nanopositioning machine (NMM) of the company SIOS;

(b) Noise of the z-axis measured at a sampling frequency of 6.25 kHz, showing as the raw data without any filtering. The noise level of the tool before and after its upgrade in the year 2011 are compared in the plot.

For more details of this research task, please refer to some selected publications listed below:

  • Gaoliang Dai, Frank Pohlenz, Hans-Ulrich Danzebrink, et al. 2004 Metrological large range scanning probe microscope, Rev. Sci. Instrum., Vol. 75, No. 4
  • Gaoliang Dai,Helmut Wolff, Frank Pohlenz et al. 2009 A metrological large range atomic force microscope with improved performance, Review of Scientific Instruments 80, 043702
  • Gaoliang Dai, Frank Pohlenz, Min Xu et al. 2006, Accurate and traceable measurement of nano- and microstructures, Meas. Sci. Technol. 17, pp. 545–552
  • Gaoliang Dai, Ludger Koenders, Frank Pohlenz et al. 2005 Accurate and traceable calibration of one-dimensional gratings, Meas. Sci. Technol. 16, pp.1241–1249
  • Gaoliang Dai, Frank Pohlenz, Thorsten Dziomba et al. 2007 Accurate and traceable calibration of two-dimensional gratings, Meas. Sci. Technol. 18:  415–421