There exist a range of applications for ion beams particularly in the semiconductor industry. For example, the fabrication and correction of lithography masks involves sub-micron etching capability. This is currently achieved using medium-energy particle beams (10-50 kilo-electron-Volts (keV)), commonly referred to as Focused Ion Beams (FIB). To enable sub-micron feature creation, the FIB must be capable of focus down to a nanometer scale spot size. This requires the extraction of very high brightness beams in excess of 105 Angstroms per steradian per meter squared (A/sr/m2).
Liquid Metal Ion Source (LMIS) technology has been capable of this level of brightness for many years. The technology exploits the capillary effect of liquid Gallium to cover a sharp Tungsten needle onto which a strong electric field is applied, thereby removing ions. The effect of the field is strongest at the needle point and so a beam of ions is created that appears to diverge from a nanometer spot. The beam is then accelerated and focused onto the target where it sputters the surface by collision processes.
Though LMIS technology may present nanometer scale milling capability, it produces unwanted doping effects by introducing Gallium into a substrate or target. To avoid this, a high brightness beam of inert ions would be preferable. Inert ions could be extracted from an inert ion gas plasma. But this has proved difficult and much research is devoted to improvement of extraction mechanisms to extract the ions from the plasma in the form of a beam. For example, electrode extraction optimization by adjustment of aperture ratios and electrode spacing is described by J. R. Copeland, et. al., “A study of the ion beam intensity and divergence obtained from a single aperture three electrode extraction system”, Rev. Sci. Instrum., 44(9):1258, 1973. Other references describing shaping of electrodes include D. E. Radley, “The theory of the pierce type electric gun” J. Electron. Control, 4:125, 1957. E. R. Harrison, “Approximate electrode shapes for a cylindrical electron beam” Brit. J. Appl. Phys., 5:40, 1953, and P. N. Daykin, “Electrode shapes for a cylindrical electron beam” Brit. J. Appl. Phys., 6:248, 1955. Despite these efforts brightness in excess of 105 A/sr/m2 has not been achieved with a plasma ion source. Thus, in the field of extracting an ion beam from a plasma, design of electrodes for extracting a beam of high brightness is desired.
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