A compact focused gaseous ion beam system (gas-FIB) is under development to form proton microbeams of a few hundreds of keV. An experiment to form proton beams was carried out to show the gas-FIB had feasibility for 3D proton beam writing (3D-PBW) with a size and a position of a microbeam independent of energy. Proton microbeams with an approximately 6 m diameter were formed at the same position over an energy range of 100-140 keV when a ratio of the kinetic energy of the object side to that of the image side was kept constant. In addition, these characteristics of the system were also numerically demonstrated in an energy range of 100-1000 keV. Those results show that the 1 MeV system will enable 3D-PBW with variable penetration depth in a sample by varying the kinetic energy.
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