OPTICAL-PROPERTIES OF A LOW-ENERGY FOCUSED ION-BEAM APPARATUS FOR DIRECT DEPOSITION

We designed and constructed a low energy focused ion beam apparatus fo r direct deposition. The optical properties of our lens system were ca lculated to obtain its optimum shape and arrangement for a low energy (50-100 eV) and fine focused (<1 mu m) ion beams. We evaluated magnifi cation, chromatic aberration, and spherical aberration. Using the appa ratus based on our design, we deposited a focused ion beam and deduced the beam diameter from linewidth measurement of the deposited film. T he diameter of 50-200 eV Au+ beams could be tuned between 0.4-7 mu m c orresponding to beam currents of 40 pA-10 nA. The current density was constant at about 30 mA/cm(2). At lower currents, the minimum beam dia meter was limited to 0.35 mu m. 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 mu A/sr) and assume the virtual source size to be 0.13 mu m, the measured beam diameter and current density can be reproduced b y calculation. Ways to reduce beam diameter and increase current densi ty are discussed. (C) 1997 American Institute of Physics.