Logo of the Physikalisch-Technische Bundesanstalt
Production sequence of Si-spheres and interferometrical determination of the sphere volume





Proper calibration of an instrument using a set of reference material measures is a widely applied method to ensure the correctness of the measurement. For the last years international efforts have been increased in creating the ISO 25178 series of standards, which describe the nominal characteristics and calibration methods of areal surface topography measuring instruments. Today, such calibration in terms of amplification, linearity, squareness, flatness as well as noise using reference material measures are well addressed.  Prior to their usage, however, these reference material measures must be calibrated accurately and traceably, for instance, the high-speed metrological large-range atomic force microscopy (AFM) and the 3D-AFM.
The metrological performance of areal surface measurement tools needs to be evaluated over the surface spectrum, i.e. the instrument transfer function (ITF) needs to be known.  However, both the traceable reference metrology and suitable material measures for the characterization of ITF are yet unavailable.  
Recently a novel circular chirp sample is developed at PTB. It is applicable for characterizing the ITF of areal surface topography measuring instruments as well as its angular-dependent asymmetries. Two main innovative ideas are implemented in the sample design. Firstly, the sample is based on circular structure patterns. Such rotational symmetric patterns are preferred for characterising the ITF features in different angular directions to detect angular-dependent asymmetries. Secondly, the feature pattern of the sample is generated by rotating a horizontal linear chirp profile around the z-axis. The chirp pattern can well present a surface with a predefined band spectrum (λmin … λmax) with a quasi-flat amplitude. In this study, a prototype sample made of Niobium on silicon and with patterns in a spectral range λmin = 1 μm … λmax = 16.0 μm was fabricated by electron-beam lithography combined with reactive ion etching technique. It has non-transparent and reflecting surfaces, thus it is ideal for characterising optical measurement devices. The material composition of the top and bottom surfaces of the features are the same, thus avoiding the influence of the phase jump issue of optical reflection.

Topography data map of the circular chirp pattern (R of 60 µm) measured by a commercial laser scanning confocal microscopy (LSCM) with a 100× objective.

The profiles of the image shown above along the orientation of 0.0° and 90.0°. The fast scanning direction of the LCSM is horizontal.

For more details of this research task, please refer to some selected publications listed below:
[1] Gaoliang Dai et al. Development of a novel material measure for characterising instrument transfer function (ITF) considering angular-dependent asymmetries of areal surface topography measuring instruments, Proceeding of euspen’s 19th International Conference &Exhibition, Bilbao, ES, June 2019.
[2] Gaoliang Dai et al. Fast and accurate: high-speed metrological large range AFM for surface and nanometrology Meas. Sci. Technol. 29 (2018) 054012