A sophisticated FIB system with a new very low energy ion gun for nm-finishing

A compact very low energy ion gun has been developed for finishing a cross-sectional sample prepared with a focused ion beam (FIB) for transmission electron microscopic observation. This ion gun enables the damaged layers of the cross-sectional sample to be reduced below nanometer thickness. The system allows us to irradiate a sample with FIB and a very low energy ion beam simultaneously and/or alternatively. The ion gun consists of a permanent magnet aided electron impact type ionization cell, extractor, cylindrical retarding immersion lens, and einzel objective lens system. The ions extracted from the ionization cell by the potential differences of 1~1.5 kV to ensure a high current intensity are decelerated through the cylindrical immersion lens and focused onto a specimen surface by the einzel objective lens. The intensities of Ar⁺ ion beams are ~1 μA at 250 eV and ~2.5 μA at 500 eV. The beam spots for 250 eV and 500 eV ions are ~2.7 mm and ~1.9 mm in diameter, respectively, at a working distance of 40 mm. The damaged layers of ~25 nm thickness on a Si sample prepared with 25 keV Ga⁺ ions were reduced by finishing with 250 eV and 500 eV Ar⁺ ions to ~2.1 nm and ~2.8 nm thickness, respectively. High resolution TEM observations of the cross-sectional samples with damaged layers reduced to a thickness of a few nm are also presented