Focused Ion Beam Fabrication Research Articles

Focused ion beam fabrication of submicron gold structures

Because of their ability to both mill and deposit material with submicron resolution, focused ion beams are now used to repair photolithography masks and are of increasing technological interest in the repair of x-ray lithography masks and in integrated circuit restructuring. With the latter two applications in mind, we have fabricated milled and deposited Au features with linewidths of ≤0.1 μm using a 40 keV Ga focused ion beam. In addition, we present the results of a study parameterizing focused ion beam induced Au deposition under conditions of practical interest. Milling is accomplished by simple physical sputtering. Examples of milled microfeatures include a grating with a 210 nm period milled through a 5000 Å thick evaporated Au film. Deposition is accomplished by ion bombarding a SiO2 substrate on which a precursor gas, dimethyl gold hexafluoro acetylacetonate, is continuously being adsorbed. Examples of deposited Au features include a 3×3 μm patch 1-μm-thick with steep sidewalls. The deposition rate was measured at room temperature as a function of ion and precursor flux, and a simple model of the process is fitted to the data. Ion beam induced deposition efficiency is shown to depend critically on the time averaged beam current density and only weakly on the precursor flux. The maximum achievable growth rate is shown to be ∼10 Å/s. Deposited Au films contain 30–60 at. % carbon and have conductivities 200–600 times less than that of bulk Au. Those films formed using lower organometallic pressures or higher ion beam current densities are characterized by greater purity with more continuous microstructure.

Focused ion beam technology and applications

Ions of kiloelectron volt energies incident on a solid surface produce a number of effects: several atoms are sputtered off, several electrons are emitted, chemical reactions may be induced, atoms are displaced from their equilibrium positions, and ions implant themselves in the solid, altering its properties. Some of these effects, such as sputtering and implantation are widely used in semiconductor device fabrication and in other fields. Thus the capability to focus a beam of ions to submicrometer dimensions, i.e., dimensions compatible with the most demanding fabrication procedures, is an important development. The focused ion beam field has been spurred by the invention of the liquid metal ion source and by the utilization of focusing columns with mass separation capability. This has led to the use of alloy ion sources making available a large menu of ion species, in particular the dopants of Si and GaAs. The ability to sputter and to also induce deposition by causing breakdown of an adsorbed film has produced an immediate application of focused ion beams to photomask repair. The total number of focused ion beam fabrication systems in use worldwide is about 35, about 25 of them in Japan. In addition, there are many more simpler focused ion beam columns for specialized uses. The interest is growing rapidly. The following range of specifications of these systems has been reported: accelerating potential 3 to 200 kV, ion current density in focal spot up to 10 A/cm2, beam diameters from 0.05 to 1 μm, deflection accuracy of the beam over the surface ±0.1 μm, and ion species available Ga, Au, Si, Be, B, As, P, etc. Some of the applications which have been demonstrated or suggested include: mask repair, lithography (to replace electron beam lithography), direct, patterned, implantation doping of semiconductors, ion induced deposition for circuit repair or rewiring, scanning ion microscopy, and scanning ion mass spectroscopy.