Operational Data of a Simple Microfocus Gun Using an EHD-Type Indium Ion Source

Abstract

When a liquid conductor (metal) is exposed to a high electrostatic field the liquid surface, under the action of surface tension and electrostatic polarization force, assumes an equilibrium shape, viz. a cone with a total apex angle of 98.6° (“Taylor cone” [1,2]). When the metal is positively polarized with respect to its surroundings, intense positive ion emission from the cone apex is observed [3,4]. The mechanism of ion emission is not yet fully understood; according to GOMER [5], ion emission appears to be initiated by field desorption from the liquid apex and, at higher emission currents, changes over to field ionization of thermally evaporated atoms in front of the cone apex. We can identify several critical parameters for ion emission: the “cone forming voltage” ΔV is to be applied between liquid metal and extractor electrode before a Taylor cone can develop; if both the microscopic cone apex and the extractor are assumed to be of ideal shape (according to the “sphere-on-cone”, SOC-model) ΔV is given by [5] $$\Delta {{V}_{threshold}}=4.53\cdot {{10}^{5}}{{({{\gamma }_{0}})}^{1/2}}$$ (V) where γ is the surface tension of the metal (N/m) and Ro the apex-extractor distance (m). The electrostatic field at the microscopic cone apex has to exceed the value $${{F}_{vap}}=6.96\cdot {{10}^{8}}{{\left( {{E}_{vap}}+{{E}_{i}}-\Phi -0.63 \right)}^{2}}$$ (V/m) before ions can be field desorbed [3]; here Evap is the evaporation voltage of a metal atom, Ei the ionization voltage and Φ the voltage of the work function of the metal, all given in volt. Once ion emission is initiated, the emission current I+ seems to be space charge limited and follows a characteristic voltage dependence [3], $${{I}^{+}}=K\centerdot {{\left( \Delta V/\Delta {{V}_{threshold}} \right)}^{3/2}}-1for\Delta V\ge \Delta {{V}_{threshold}}$$ where ΔV is the tip/extractor voltage difference and k a constant, somewhat depending on geometry.