Development of a Versatile High-Brightness Electron Impact Ion Source for Nano-Machining, Nano-Imaging and Nano-Analysis

High brightness low energy spread (ΔE) ion sources are needed for focused ion beam nano-applications in order to get a high lateral resolution while having sufficiently high ion beam currents to obtain reasonable erosion rates and large secondary electron/ion yields. The objectives of this thesis are: the design of an electron impact ion source, a reduced brightness B_r of 10³ - 10⁴ A m^-2 sr^-1 V^-1 with an energy distribution spread ΔE ∼≤ 1 eV and a versatile ion species choice. In a first evaluated concept an electron beam is focused in between two parallel plates spaced by ≤ 1 mm. A micron sized ionisation volume is created above an extraction aperture of a few tens of μm. By using a LaB₆ electron emitter and the ionisation region with a pressure around 0.1 mbar, B_r is close to 2x10² A m^-2 sr^-1 V^-1 with source sizes of a few μm, ionic currents of a few nA for Ar⁺/Xe⁺/O₂⁺ and the energy spread being ΔE < 0.5 eV. The determined B_r value is still below the minimum targeted value and furthermore the main difficulty is that the needed operation pressure for the LaB₆ emitter cannot be achieved across the compact electron column and therefore a prototype has not been constructed. The second evaluated source concept is based on stopping a 100 eV high current electron beam within a 1 V/mm retarding field ionisation region kept at a pressure of 0.1 mbar. Ion currents up to hundreds of μA are created within an ionisation volume of mm dimensions. In order to obtain an efficient ion extraction, which is done coaxially to the electron beam, and keep the ion beam source size in the μm range a radio frequency ion carpet is used to funnel the ions through a central 1 mm opening followed by an extraction aperture of 100 μm in diameter. The determined B_r is in the lower 10² A m^-2 sr^-1 V^-1 range for Ar⁺ ions with ΔE ≅ 3 eV. The reduced brightness is lower than the original goal but a few μA of ion beam current are available and ΔE is in the same order of magnitude as the objective. Experimental prototype tests have shown that the argon ion extraction is clearly raised by using the radio frequency ion carpet. Unfortunately, the measured ion currents are still one or two orders of magnitude below the μA range. Further optimisations to further enhance the source performance have been identified and will be implemented in the future. (author)