Proceedings Article | 6 August 2014
KEYWORDS: Ion beams, Tungsten, Ion beam finishing, Optical fabrication, Lenses, Ions, Electrons, Particles, Interferometers, 3D modeling
Ion beam Figuring (IBF) is believed to be one of the best ways that can fabricate lenses to surface error less than Root Mean Square (RMS) value 1 nanometer. There are several aspects that will influence the fabricate efficiency and accuracy, for example, stability of ion beam, measuring errors of the lenses’ surfaces, dwell time algorithm, fabrication path, etc. Lots of works have been done and papers are published. Since the ion beam generated from argon plasma is electropositive, the lens’ surface will also be electropositive when it is stroked by the ion particles, so that the fabricating efficiency will be decreased because of the repelling force with the same kind electric charges. To avoid the accumulation of charges on lens’ surface, ion beam is neutralized by electrons excited by electric gun or heated tungsten filament. But sometimes the neutralization is not well done. In this paper, fabricating influence of electrified ion beam was analysed, and experiments were done. First, the shape of ion beam without neutralization was tested by Faraday cup, and its removal volumes with different sputtering time were calculated by subtracting lens’ surface error before and after it were fabricated by the ion beam. Then by adjusting the current of the tungsten filament, the excited electrons’ number was changed and the ion beam was neutralized partly, the shapes of ion beams were recorded and the corresponding removal volumes were also tested. Finally, a lens with diameter about 64mm was fabricated. The results showed that the fabrications were either excessed or not enough when using ion beams that were not neutralized perfectly. When neutralized perfectly, the convergent rate of the fabrication will be very fast, in a single iteration, the initial surface error with RMS value more than 7nm might decrease to less than 1nm.
