Physics – Optics
Scientific paper
Mar 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002aps..mar.u5005n&link_type=abstract
American Physical Society, Annual APS March Meeting, March 18 - 22, 2002 Indiana Convention Center; Indianapolis, Indiana Meetin
Physics
Optics
Scientific paper
Electron lenses traditionally used in electron microscopes suffer from large aberrations. The most detrimental one is spherical aberration, which is also the optical defect that initially plagued the Hubble space telescope. Unlike with the Hubble telescope, spherical aberration of a round magnetic lens is not due to a design error, but is simply due to the impossibility of shaping magnetic fields arbitrarily. It reduces the resolution of an electron microscope typically 50-100 times compared to what a microscope with no aberrations would achieve. This means that a microscope using 200 kV electrons whose wavelength is 2.5 pm can only achieve 1.3 to 2 angstrom resolution. Equally seriously, the aberration limits the illumination angle used to form small probes to values of the order of 10 mrad, and thereby reduces the current available in a sub-2 angstrom probe to typically a few tens of pA. Correcting the aberration has been a goal of electron optics since the early 1950s. Several microscopes with aberration correctors have become operational in the last 5 years, including several scanning transmission electron microscopes (STEMs) with a corrector made by Nion. These microscopes have improved on their uncorrected resolution by more than two times, reaching all the way to sub-angstrom imaging. They have also increased the probe current to about 200 pA, i.e. 10 times more than previously available. We will outline the principles behind the aberration correctors and explain the difficulties that delayed their full introduction by nearly 50 years. We will then present recent results from Cs-corrected STEMs, and discuss application areas that benefit the most.
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