Aberration-Corrected Electron Microscopy
electron gun (FEG) was developed [15]. The clear visibility
of individual Si atomic columns in reconstructed phase and
amplitude images was an early validation of the efficacy of
this approach [12]. High-resolution electron holography has
since been shown to be facilitated, and phase sensitivity greatly
improved, when online correction hardware has been used
for compensation of most aberrations before commencing
detailed holography observations [16]. Reconstruction of the
exit-surface wavefunction in the fixed-beam TEM combines
images recorded at several different defocus values to avoid any
loss of information due to zero crossovers in the objective-lens
transfer function—and image interpretability is pushed out as
far as the microscope information limit, which is defined by
incoherent effects or by the envelope functions that are due
either to beam divergence (spatial coherence) or focal spread
(temporal coherence).
Online corrector systems involve various combinations
of multipole elements that serve to overcome the symmetry
constraints recognized in Scherzer’s original 1936 theorem. In
his later 1947 paper, Scherzer had suggested using quadrupoles
to distort the beam from its rotational symmetry and octopoles
to cancel the aberrations [2]. Based on this concept, the original
corrector for the scanning TEM consisted of a combination of
Ga 63 pm
six quadrupole-octopole elements that preceded the normal
Figure 3: Aberration-corrected STEM image of [211] GaN recorded with the
objective lens [10]. Ronchigram analysis and beam-induced TEAM 0.5 ACEM, showing clear resolution of individual Ga atomic columns with
image shift were used for alignment of the corrector elements
separations of 0.63Å (from Ref. 17).
and for aberration measurement, which was then followed by
the correction of third-order spherical aberration, axial coma, applications of ACEM, many of which were also represented in
and astigmatism (two-fold and three-fold). For fixed-beam the Scherzer symposium.
TEM, the first corrector combined two hexapoles and two There are additional benefits of aberration correction
additional round-lens doublets that were inserted into the that have yet to be fully appreciated by the broader materials
microscope lens column as a single unit immediately below community. For example, one simple advantage of using a
the objective lens [8]. The correction procedure then consisted corrected TEM is the complete removal of the displacement
of diffractogram analysis, followed by computer-controlled error present in selected-area diffraction patterns for higher-
feedback to the various corrector, stigmator, and lens focusing order diffracted beams that is caused by spherical aberration.
coils. More complicated corrector systems have since been Another advantage is that axial coma is markedly reduced
developed to correct the prevailing higher-order aberrations, so that slight beam tilts by electronic controls rather than
but the basic symmetry-breaking principles remain in place. crystal tilting by mechanical means can be used to achieve
more accurate alignment of the incident-beam direction with
Benefits of ACEM the required crystal zone axis. A further benefit is that TEM
The successful online correction of spherical aberration in imaging with a small but slightly negative Cs value, combined
the electron microscope is an exciting and praiseworthy feat with a slight overfocus lens setting, can greatly enhance the
that has deservedly attracted much attention. Representative visibility of oxygen atomic columns that are closely adjacent
examples of aberration-corrected TEM and aberration- to much heavier metal-atom columns [2]. An additional
corrected STEM images are shown in Figures 2 and 3, advantage is the potential for depth sectioning due to the
respectively. Direct TEM image interpretation has been pushed reduced depth of field that results from the larger convergence
into the sub-Ångstrom regime, and aberration-corrected probes angles achievable with aberration corrected probes [18].
promise a veritable revolution for atomic-scale microanalysis. Furthermore, as already pointed out, the off-line holography
However, these improvements in resolution per se for imaging and exit-wave reconstruction methods are greatly simplified
and analysis are not the only important contributions of because determination of the aberration phase plate, which
aberration correction; the improved signal quality for dynamic is an essential step for these techniques, is an integral part of
studies in aberration-corrected images, the greater current the correction procedure. In the case of scanning TEM, much
available for analysis with the corrected probe, and the higher larger collection angles can be used for bright-field (BF)
sensitivities to variations in atomic position or chemical imaging because aberration correction reduces the impact
composition are all significant for materials investigations. of the spatial coherence envelope: signal intensities can be
These advances have already resulted in a number of striking increased by factors of 100 or more, and aberration-corrected
12
www.microscopy-today.com • 2009 September
Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76