Towards sub-Å electron beams
N. Dellby 1 and O.L. KRIVANEK 1,2
e-mail: krivanek@u.washington.edu
1 Colibris Co., 1102 8th St., Kirkland, WA 98033
2 Department of Materials Science and Engineering, box 352120, U. of Washington, Seattle WA 98195, USA
A scanning transmission electron microscope (STEM) giving an electron beam with a diameter of less than 1 Å would permit many new types of experiments, including elemental mapping with single atom detection capability for many elements in the periodic table. The beam would need to contain a significant electron current (>0.1 nA), and the beam profile would need to be free of spurious tails.
The recent success of an aberration corrector incorporated in a dedicated STEM1 shows that this type of performance is now within reach. The ideal instrument will probably operate at around 200 kV, use an optimized cold field emission gun, have an aberration corrector preceding its objective lens, and flexible condenser and post-sample lens systems. Because successful aberration correctors have only been introduced recently, their design is likely to evolve substantially in the next few years. Factors such as correction precision, stability and repeatability, as well as ease-of-use are likely to become at least as important as the peak optical performance. The various design choices will be discussed and their strengths and weaknesses put into perspective.
It will be shown that an optimized STEM corrector will be able to reach about 0.5 Å resolution through accurate Cs correction (even without Cc correction), and be readily feasible through a simple extension of existing technologies.
1 O.L. Krivanek, N. Dellby, A.J. Spence, R.A. Camps and L.M. Brown "Aberration correction in the STEM" EMAG '97 IoP Conference Series No 153 (Ed. J M Rodenburg, 1997) 35-40