A study of optical properties and current emission processes of gas phase field ionization sources

Abstract

Field emission ion sources are extremely important for producing high-resolution ion beams essential for several fields of research and especially for semiconductor manufacturing. Although most sources used are based on liquid metals they cannot produce beams of H, He or other noble gases, so that gas field ionization sources (GFIS) could have great utility for microscopy or applications sensitive to metal contamination (such as in-line processing). This dissertation explores the properties of the gas field ionization source with the goal of providing a resource to the ion column designer. For the first time a detailed treatment of the optics of the gas field ionization source is derived. Also, the first theoretical analysis of the current generation mechanism is presented that explains both the current-voltage characteristic and the total current of the GFIS with reasonable agreement with experiment.

The optical properties in the emission diode region are derived from the ray equation. For the evaluation of the spherical and chromatic aberrations, two new aberration integrals are derived, which are applicable to the diode region and are appropriate for numerical calculations. The results show that, regarding the aberration coefficients, essential differences exist between the field ionization and field electron emission.

The virtual source size is evaluated in two ways. First, by the algorithm of addition in quadrature (A.I.Q.) of the contributions from the Gaussian source size, the spherical and chromatic aberrations, and the diffraction effect. The dependence of the virtual source size on the emitter radius, the beam limiting aperture and the tip temperature are analyzed. As an alternative, the method of direct ray tracing is used, taking into account the energy distribution of ions. The results from these two methods are compared.

The current emission process of GFIS is studied using a relatively simple model based on the mechanism of gas material supply into the ionization zone. Although a complete solution from the first principle was not possible due to the complexity of the gas-surface interactions, the results obtained agree reasonably well with experimental values both in magnitude and in the shape of the current-voltage characteristic.