Radius and roundness measurement of micro spheres based on a set of AFM surface scans

Micro coordinate measuring machines have been developed for the traceable characterization of small complex parts, due to the demand in research and industry. These machines require geometrically well characterized probing spheres of ever smaller radii. Currently, there is no established procedure for the measurement of such spheres below radii of 500 mu m. In this paper we, therefore, propose and investigate an approach which is based on a set of atomic force microscope (AFM) surface scans in conjunction with a stitching algorithm. The strategy was implemented on a nano measuring machine and investigated on a ruby sphere with a radius of 150 mu m. Although the strategy can generally be applied to the characterization of a full sphere, we limit ourselves to the measurement of one great circle (equator). The technique enables the measurement of micro spheres with a high lateral and vertical resolution. The mean radius of the ruby sphere was measured with a standard deviation of 3.7 nm over six repetitions. As our experiments have shown, the measurement procedure is at the moment mainly influenced by the shape of the AFM tip which requires further attention.