A feasibility study towards traceable calibration of size and form of microspheres by stitching AFM images using ICP point-to-plane algorithm

Abstract

We present a new method for traceable calibration of size and form error of microspheres, which was realised by stitching a series of atomic force microscopic (AFM) images measured at different orientations of microspheres using the metrological large range AFM of the PTB. The stitching algorithm is achieved using an iterative closest point (ICP) point-to-plane algorithm. As the AFM tip geometry is one of the most significant error sources for the developed method, it was traceably calibrated to a line width standard (type IVPS100-PTB), whose feature geometry was calibrated with a traceable route to the lattice constant of crystal silicon. Measurement setup, scan strategy, and data evaluation processes have been detailed in the paper. Measurement results show high stability and robustness of the developed method. For instance, the standard deviation of four repeated measurements reaches 5 nm, indicating promising performance.

Influence of the reference surface and AFM tip on the radius and roundness measurement of micro spheres

“…The loss of information due to the associated filter effect cannot be fully recovered. Another emerging technique is based on a set of surface scans in conjunction with a suitable evaluation strategy [12,[15][16][17][18]. These use either optical [12] or tactile [15][16][17][18] surface sensors.…”

“…In order to correct for the geometric filter effect of the tip, its shape needs to be known. Strategies to characterise the shape of the tip are based on a known reference surface such as [18,23] or on comparisons between a set of unknown tips (e.g. a three cantilever test [24][25][26]).…”

“…An implementation of such an approach has been published in [18,23]. The shape of the tip is characterised by a traceable reference surface.…”

“…Although strategies based on a set of tactile surface scans are promising for the precise and traceable characterisation of micro spheres given recent experimental results [15,18], estimating the uncertainty of these measurements, therefore, remains an unresolved challenge. That is especially the case if one considers the whole measurement process starting at the reference surface.…”

“…Our investigations thus cover tips typically used by AFMs and stylus instruments. Although the sampling of a full sphere has recently been demonstrated [16,18], we still limit ourselves to the equator of the sphere for these investigations as it makes them more efficient. The measurand is, thus, the local radius r as a function of the polar angle φ (figure 1).…”