The Otto Scherzer Memorial Symposium on
Aberration-Corrected Electron Microscopy
David J. Smith
1
* and Uli Dahmen
2
1
Department of Physics, Arizona State University, Tempe, AZ 85287-1504
2
National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, CA 94720
* david.smith@asu.edu
Introduction
electron microscope can be visualized in terms of a magnetic
The year 2009 marks the centenary of the birth of Otto
field that transforms the incoming plane wave into a highly
Scherzer, one of the early pioneers of electron microscopy.
irregular shape. This shape can be described as a power series
Scherzer, shown in Figure 1, was the originator of the famous
of waves, each with its own symmetry (2-fold, 3-fold, 4-fold)
microscopy theorem that the spherical and chromatic
and order (1
st
order, 2
nd
order, 3
rd
order, etc.). This leads to a
aberrations of rotationally symmetric electron lenses were
veritable “zoo” of aberrations of different symmetry and order,
unavoidable [1]. In honor of this centennial occasion,
which in turn could limit the microscope performance. Latest
we organized a special memorial symposium during the
aberration correctors take care of aberrations up to 5
th
order,
Microscopy & Microanalysis 2009 meeting, which was held in
and these have succeeded in improving the resolution limits of
Richmond, Virginia, in late July. The introductory talks of the
the transmission electron microscope (TEM) to about 50pm
symposium presented a fascinating mix of firsthand accounts
for both fixed-beam and scanning-beam TEMs.
about working with Scherzer in Darmstadt and descriptions
Emergence of ACEM
of the correction concepts and the early corrector prototypes
The early days of electron microscope construction and
that emerged from his group. Placed in this historical context,
development were mostly spent struggling to overcome the
the latest advances in aberration correction for scanning and
many serious limitations to instrument performance that
fixed-beam instruments that were presented in this symposium
were caused by mechanical and/or electrical defects. Although
were all the more impressive and conveyed a vivid sense of
resolution was already much better than that achievable with a
history in the making. Representative applications of aberration
light microscope, it was still far removed from the theoretical
correction to a broad range of materials were also highlighted in
limit, and even further from the electron wavelength. Little
platform and poster presentations. Here we give a short account
of the emergence of aberration-corrected electron microscopy
(ACEM) and very briefly summarize some of the prospects and
challenges for this burgeoning field. Further information about
these developments, including details of applications, will be
found in selected papers from the symposium, which will be
published in a forthcoming issue of the journal Microscopy and
Microanalysis due to appear in mid-2010.
Aberration Correction
Spherical aberration is a focusing defect that is inherent
to circularly symmetric electron lenses, and it prevents
off-axis electrons from all being focused to the same point.
Aberration correction makes it possible to “tune” spherical
aberration (Cs) by making its value vanishingly small or even
negative [2]. Because the Scherzer resolution limit—as given
by d~0.7l
3/4
C
1/4
S
, where l is the electron wavelength—is
proportional to the spherical aberration, one might reasonably
ask why the microscope resolution can’t be made arbitrarily
good by making the Cs value arbitrarily small. The problem
is that there are many lens aberrations: spherical aberration is
only the tip of the iceberg. This situation is similar to the case
for the human eye where spherical aberration leads to blurred
vision, astigmatism leads to distorted vision, and chromatic
aberration leads to color blurring. Once the spherical
aberration of the eye is corrected with a pair of glasses or
contacts, then other aberrations such as astigmatism become
Figure 1: Image of Otto Scherzer (courtesy of Dieter Typke).
important. The distortions present in the objective lens of the
10 doi: 10.1017/S1551929509000303
www.microscopy-today.com • 2009 September
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