Performance of a 20–200 kV focused-ion-beam system with a new optical design concept

Abstract

A computer controlled focused-ion-beam system (JIBL-200S) with a new optical design concept has been developed. The JIBL-200S has good focusing properties over a wide-accelerating voltage range. A double-mode optical system has been provided in order to improve the focusing properties for the low-accelerating voltage region. In this system the two einzel lenses are operated in the decel mode (central electrode positive) for beam accelerating voltages above 100 kV and in the accel mode (central electrode negative) below 100 kV. This reduces a chromatic aberration for the low energy beams. The beam diameter for a Ga ion was measured using a knife edge and Faraday cup. As a result, compared to the conventional mode of operation, the beam size is improved to 0.09 from 0.15 μm at 75 kV and to 0.13 μm from 0.20 μm at 50-kV-accelerating voltage by changing the operating mode. The writing performance was also evaluated by the use of a vernier pattern writer on a Si substrate coated with chloromethylated polymethylstyrene resist exposed with 250-keV-Si++ ions. Overlay accuracies of 0.183 μm in the X axis and 0.243 μm in the Y axis 3σ were obtained.