Focused ion beam microlithography using an etch-stop process in gallium-doped silicon

We have found that silicon, when implanted with doses of gallium in excess of 1013 ions/cm2, experiences little or no etching in aqueous caustic solutions. By exploiting a finely focused 40–50 keV gallium ion beam (0.05–0.1 μm diameter) in our scanning ion microscope, we have shown that it is possible to fabricate structures with submicrometer (0.1 μm) features. The silicon behaves as a negative resist with a sensitivity of about 1 μC/cm2. This etch-stop process is largely insensitive to crystallographic orientation, except for the highly insoluble (111) plane in which damage-promoted etching occurs.