Aberration-Corrected Electron Microscopy
thought was probably given to the possibility of
actually correcting lens aberrations. Th e situation
changed dramatically following the publication in
1947 of another seminal article by Scherzer [3],
which pointed out several possible avenues for
overcoming spherical and chromatic aberration.
Most workers in the fi eld of electron optics pursued
Scherzer’s suggestion of using asymmetric imaging
correctors based on multipole elements, but scant
success with aberration correction was achieved
over the following years despite many valiant
attempts. Although mechanical imperfections
and electrical instabilities were undoubtedly
A B
major contributors to the lack of progress [4], the
absence of any systematic procedure for carrying
out electrical and mechanical adjustments based
solely on image appearance meant that correction
of spherical aberration came to be regarded as a
task so complex that it was well beyond the skills
of an unaided human operator [5]. Something
much faster and more routine was needed before
online aberration correction could become a
reality.
Online computer control of microscope
parameters was fi rst implemented for the
scanning electron microscope [6], and iterative
C D
procedures based on image contrast analysis that
Figure 2: Four sequentially recorded TEM lattice images of gold [110] nanobridge connecting
were suitable for online focusing and stigmating in two grains that are rotated relative to each other by 90 degrees around [110] axis. The four images
high-resolution electron microscopy soon followed
shown are part of a 15-member focal series, recorded in time intervals of 1.5 s. Black arrows:
[7]. Th e development of automated diff ractogram
(a,b) 2-atom column and (c,d) single atom. Red arrows: thirteen 2-atom columns, some of which
disappear in (d). Turquoise arrows: Rearrangement of atom columns at the intersection of a
analysis, sometimes termed “autotuning,” was
dissociated grain boundary with the surface. The focus difference on both sides of the bridge is
made possible by the quantitative recording
negligible because the fi lm was grown onto a fl at single crystal substrate. Images from Kisielowski
capability of the slow-scan CCD camera [8],
et al., Microsc. Microanal. 14, 469–477, 2008 (courtesy of the Microscopy Society of America).
and this technique led to rapid and reproducible
adjustment of focus, beam tilt, and image astigmatism. Similar
in turn limiting image interpretability [11]. Th ese additional
to the more recent procedures that are applicable to aberration
aberrations can be measured accurately using either the
correction, this autotuning approach for the fi xed-beam TEM
diff ractogram or Ronchigram approaches, and they can
mode relied on the presence of a small region of amorphous
then be easily accounted for during the process of aberration
material in or near the fi eld of view. Th is specimen requirement
correction.
obviously represents a serious restriction when investigating Approaches to ACEM
certain types of materials. By analyzing a systematic set, or Aberration correction can be achieved nowadays in several
“tableau,” of diff ractograms taken from images recorded with diff erent ways. Online approaches in either fi xed-beam TEM
axial and tilted-beam illumination, aberration coeffi cients can [9] or scanning-beam STEM [10] follow the principles outlined
be determined to the high degree of accuracy that is necessary by Scherzer and use hardware corrector systems installed on
for subsequent correction of aberrations [9]. An alternative the instrument, whereas off -line correction techniques use
approach to aberration assessment, based on the acquisition special soft ware programs to reconstruct off -axis electron
and analysis of far-fi eld images, also termed Ronchigrams, is holograms [12] or exit-surface wavefunctions [13]. All of these
used during the aberration correction procedure normally approaches involve computer-aided analysis to produce a phase
applied in the scanning TEM mode [10]. plate that eff ectively combines the eff ects of all prevailing lens
Initially, spherical aberration was the primary target aberrations as a function of scattering angle. An inverse phase
for correction eff orts because it was widely perceived as plate for either off -line or online correction purposes can then
the dominant factor that predetermined the microscope be easily generated.
resolution. However, as the impact of incoherent eff ects Th e electron holography technique was initially proposed
such as vibrations, noise, and stray fi elds was progressively by Gabor in 1949 as a path towards improving microscope
reduced, thereby extending resolution limits, it slowly came resolution beyond the traditional spherical-aberration limit
to be realized that other diffi cult-to-measure lens aberrations [14], but little practical progress towards this goal was made
such as misalignment coma and three-fold astigmatism were until the high-brightness, high-coherence, fi eld-emission
2009 September • www.microscopy-today.com 11
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