Generation of small-diameter ion beam for high-precision optical fabrication (Structural optimization of quadrupole magnetic lens using permanent magnets by numerical simulation)

Abstract

Ion beam etching is effectively used for the fabrication of high-precision optics. The main application of ion beam etching is to figure large optical surfaces to correct shape errors remaining on polished surfaces. To figure small or medium-size optical surfaces, the generation of an ion beam with a smaller diameter and a higher ion current is required. In this study, we designed a magnetic lens with quadrupole magnets using neodymium magnets to obtain an ion beam with a small diameter. The magnetic lens is installed between the outlet of an ion gun and a chamber, which enables the trajectory of the ion beam in the chamber to be changed. The trajectories of the ion beam and the ion particle distribution on a workpiece surface when a doublet or a triplet magnetic lens was used were simulated. Simulations were also conducted for ion beams with large and small emittances entering a magnetic lens. In the simulations, the ion beam without a magnetic lens was approximately 30 mm in diameter on a workpiece surface. The simulations showed that the doublet magnetic lens can converge the ion beam on a workpiece surface to an area of approximately 6 mm × 10 mm when the emittance is small. However, this lens is less capable of converging the ion beam in one direction when the emittance is large: the ion beam was converged to an area of approximately 6 mm × 27 mm on a workpiece surface for the large emittance. On the other hand, the triplet magnetic lens can converge the ion beam for both large and small emittances: the ion beams with large and small emittances converged to areas of approximately 4 mm × 4 mm and 10 mm × 5 mm, respectively.

Keywords

• optics
• fabrication
• figuring
• ion beam
• magetic lens
• quadrupole lens
• ion gun
• ion source