Theoretical approach of the reduction of chromatic and spherical aberrations in an acceleration lens system for hundreds of keV gaseous ion nanobeam

Okubo, Takeru; Ishii, Yasuyuki; Kamiya, Tomihiro

The focused gaseous ion beam system composed of a series of electrostatic lenses, called "acceleration lens system", has been developed to form nanobeams using gaseous ions generated from a plasma ion source. A new all-in-one compact acceleration system including an acceleration tube is now under development to form 300 keV ion nanobeam. Chromatic and spherical aberrations are, however, hindrance to form nanobeams with their smaller sizes in diameter. A deceleration lens, which performs like a defocusing lens, was theoretically introduced to downstream of the present acceleration lens system to reduce the aberrations. Ion beam optics simulations were carried out and we reduced the chromatic aberration coefficient by 26%, the spherical aberration coefficient by 17% and a beam diameter by 17%, with the deceleration energy of 15 keV. The final beam diameter of 103 nm at 300 keV is obtained by the all-in-one acceleration lens system with the total acceleration length of only 650 mm.