MICROPROBE OF HELIUM-IONS

A helium field ion source emits intense beams from a (111) plane protruding from a tungsten emitter. Gomer's hopping model was expanded to explain the I-V characteristic of the plane. Our calculations agreed with the experimental results. To make a microprobe of helium ions, we constructed a 100 keV focusing column. It has a gimbal assembly and a phosphor screen with a microchannel plate to adjust the beam axis. Using a measured energy spread of 1 eV, the probe diameter was calculated to be less than 10 nm at a 1 pA probe current. Using scanning ion microscope images obtained with 90 keV helium ions, the actual probe diameter was estimated to be 2-mu-m. Although some improvements are necessary to focus the beam sharply, we showed the basic design of the focusing column for a helium field ion source.