Optical properties of a low energy focused ion beam apparatus for direct deposition

We designed and constructed a low energy focused ion beam apparatus for direct deposition. The optical properties of our lens system were calculated to obtain its optimum shape and arrangement for a low energy (50–100 eV) and fine focused (<1 μm) ion beams. We evaluated magnification, chromatic aberration, and spherical aberration. Using the apparatus based on our design, we deposited a focused ion beam and deduced the beam diameter from linewidth measurement of the deposited film. The diameter of 50–200 eV Au+ beams could be tuned between 0.4–7 μm corresponding to beam currents of 40 pA–10 nA. The current density was constant at about 30 mA/ cm2. At lower currents, the minimum beam diameter was limited to 0.35 μm. These experimental results agree with calculated results qualitatively, but quantitative differences exist. Assumptions, based on Ga+ ion sources, seem to cause the differences. If we adopt our measured energy dispersion (30 eV) and angular current density (10 μA/sr) and assume the virtual source size to be 0.13 μm, the measured beam diameter and current density can be reproduced by calculation. Ways to reduce beam diameter and increase current density are discussed.