Focused Ion Beam Microscopy and Micromachining

The FIB Instrument The basic functions of the FIB, namely, imaging and sputtering with an ion beam, require a highly focused beam. A consistent tenet of any focused beam is that the smaller the effective source size, the more current that can be focused to a point. Unlike the broad ion beams generated from plasma sources, high-resolution ion beams are defined by the use of a field ionization source with a small effective source size on the order of 5 nm, therefore enabling the beam to be tightly focused. The ion source type used in all commercial systems and in the majority of research systems designed with micromachining applications in mind is the liquidmetal ion source (LMIS).6,7 Of the existing ion source types, the LMIS provides the brightest and most highly focused beam (when connected to the appropriate optics). There are a number of different types of LMIS sources, the most widely used being a Ga-based blunt needle source. Ga has decided advantages over other LMIS metals such as In, Bi, Sn, and Au because of its combination of low melting temperature (30∞C), low volatility, and low vapor pressure. The low melting temperature makes the source easy to design and operate, and because Ga does not react with the material defining the needle (typically W) and evaporation is negligible, Ga-based LMISs are typically more stable than other LMIS metals. During operation, Ga flows from a reservoir to the needle tip (with an end radius of about 10 mm), where it is extracted by field emission. A large negative potential between the needle and an extraction electrode generates an electric field of magnitude 1010 V/m at the needle tip. The balance between the electrostatic forces and the Ga surface tension wetting the tapered W needle geometry results in the formation of a single Taylor cone at the needle tip. For typical emission currents used in FIB microscopes (~2 mA), a cusp forms at the tip of the Taylor cone with a tip radius of approximately 5 nm. The simplest and most widely used ion beam columns consist of two lenses (a condenser and objective lens) to define the beam and then focus it on the sample, beam-defining apertures to select the beam diameter and current, deflection plates to raster the beam over the sample surface, stigmation poles to ensure a spherical beam profile, and a high-speed beam blanker to quickly deflect the beam