The Gas Field Ion Source for Finely Focused Ion Beam Systems
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ABSTRACT 9 2 The Gas Field IonSource, GFIS, promises a 10 A/(cm str) brightness, small beam sizes, and inert gas ion species. If this performance could be demonstrated on a commercial system, the GFIS might replace the liquid metal ion source as the standard source for FIB applications. Recent work at the Max-Planck-Institut fUr Kernphysik (MPI-K) in Heidelberg, Germany has shown that a GFIS with a 'Super Tipped' emitter can be reliably fabricated and can be run with stable helium beam current for more than 200 hours. However, this GFIS source must operate in a bakable UHV chamber, at cryogenic temperatures, and at high voltages with low vibration. A GFIS is now being integrated with high resolution ion optics and a vacuum chamber designed for studying GFIS image quality and ion induced chemistry.
INTRODUCTION Definition A gas field ion source consists of a sharpened tungsten needle in an inert gas ambient held at cryogenic temperatures. The source is biased around 10 kV positive with respect to a downstream extraction aperture which produces an electric field strong enough to ionize the gas atoms in the vicinity of the tungsten needle. The major components of a GFIS source are shown schematically in figure 1.
G1EIS Hist=r The technology of the gas field ion source, or field ion microscope, was first explored by Muller et al' during the 1950's. Muller's original field ion microscope produced a high magnification image of the tungsten needle. It was the first occasion that one actually saw crystal structure information on an atomic scale. Several researchers have refined Muller's original field ion source techniques to build gas field ionization sources appropriate for focusing. 3 More recently, research at Cornell2 and the Max Planck Institute has shown that it is possible to sculpt the tungsten needle so that a small boss is generated on the surface of a larger sphere at the end of the needle, as is shown in figure 1. The intent of this 'Super Tip', as the Max Planck team has called it, is to have a very high electric field strength concentrated at the tip for ionizing the atoms supplied by the abundant gas volume concentrated around the larger sphere. This method increases the gas supply to the high field region surrounding the super tip and thus increases the output current density. With this method it is possible to obtain ion 2 beams with brightness approaching 10' A/(cm str). In addition to higher brightness, the spread, high angular current intensity, and a energy 'Super Tip' has other advantages; low
687 Mat. Res. Soc. Symp. Proc. Vol. 396 ©1996 Materials Research Society
small virtual source size. One further advantage of a gas field ion source is the availability of ion species such as H, He, Ne, Ar, or 0 which will not interfere with the electrical properties of any sample being imaged with these ions.
Figure 1: A Gas Field Ion Source assembly shown schematically. The main components are a tungsten needle and its high voltage cryogenic mounting hardware, a set of electrodes for ion extraction, focu
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