Multicusp ion souces

Schematic diagram of a multicusp ion source

The above is a schematic diagram of a typical multicusp ion source. The external surface of the source chamber is surrounded by colums of permanent magnets (consisting of samarium-cobalt) which generate longitudinal line-cusp magnetic fields that can confine the primary ionizing electrons efficiently. The polarity of these magnets is alternated for such purpose. One end of the chamber is terminated by an end flange which is covered with rows of permanent magnets to complete the line-cusp arrangement. The open end of the chamber is where the energy analysis takes place.

Axial Ion Energy Spread

In many applications, an ion source that can provide low longitudinal or axial energy spread is required. This is especially true when ion beams must be transported, manipulated, analyzed and applied in very low energy applications. For instance, in Ion Projection Lithography (IPL), projection of sub 0.20 micron patterns from a stencil mask onto a wafer substrate is desired. In order to maintain the chromatic aberrations below 25 nm, an ion source which delivers a beam with an energy spread of less than 3 eV is required. In the production of radioactive ion beams for nuclear physics experiments, an ion source with axial energy spread less than 1 eV is needed to perform isobaric separation with a magnetic deflection spectrometer. In low energy (<100eV) ion beam deposition processes, very low energy spread is required in order to separate and focus the ions properly. Low energy (<500eV) mass spectrometers have been proposed for analyzing nuclear and chemical wastes. In order to achieve good mass resolution, an ion source that has low longitudinal energy spread is also needed.

Applications

For IPL (Ion Projection Lithography) applications, multicusp plasma generators are considered. The multicusp source is capable of producing large volumes of uniform, quiescent and high-density plasmas with high gas and electrical efficiencies. They are now being used in neutral beam injectors for fusion devices, particle accelerators, ion implantation systems, neutron tubes for oil-well logging and proton theraphy machines.

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