Ion beam figuring machine for ultra-precision silicon spheres correction

In the framework of the Avogadro project, isotopically enriched 28Si spheres had been manufactured as artifacts for the assessment of various physical quantities including the sphere volume which finally leads to a very accurate determination of the Avogadro constant NA. The Avogadro constant is an important input datum for the redefinition of the unit of mass, the kilogram, on the basis of fundamental physical constants. During the recent measurement campaign, it has turned out that one of the main contributions to the overall uncertainty of NA is the sphericity error and consequently the interferometric volume measurement. Since chemical-mechanical polishing has reached its limits with respect to form accuracy due to the sensitivity of material removal rate to crystal orientation, it has been proposed to use ion beam figuring for further reduction of sphericity error from currently 50 nm PV to values <10 nm PV. In this paper, a new concept and realization of a multi-axis ion beam figuring machine dedicated for deterministic correction of silicon spheres is presented. Aspects of long term tool stability and alignment procedures in order to relate the ion beam footprint to the sphere surface are covered. Furthermore, a process dwell time calculation and tool path generation method dedicated for spheres manufacturing will be presented and discussed.