Doktorarbeit_Mairoser.pdf - OPUS - Universität Augsburg

A. Ergänzungen zur Dünnfilmtechnologie
ew articles | insight
NAture mAterIAls doi: 10.1038/nmat2380
0
–1
Stuttgart
(1.2 MV)
Dietrich
(200 kV)
Aberration-corrected
electron microscope
TEAM/
CREST
NION
Haider
(200 kV)
10 0
10 1
Source
2
3
4
Light
microscope
Electron microscope
Zach, Haider
(1 kV SEM)
Borries and Ruska
Marton, Ardenne
(100 kV)
Amici
Abbe
Ruska (75 kV)
10 2
10 3
10 4
Feature size (Å)
Lenses
5
800
Ross
1850 1900
1950 2000 2050
Year
Sample
ardware advances in imaging microscopies. Optical
y reached its far-field diffraction limit roughly a century ago.
icroscopy, exploiting the reduction in electron wavelength with
beam voltage, showed steady increases in resolving power for
ars, until limited by radiation damage. Recent developments
g electron-optical aberrations have led to improvements in
ower without having to increase the electron beam energy.
till far from their ultimate diffraction limits, corrected electron
es have already demonstrated the ability to resolve submage
features. (Adapted from ref. 114; © 2009 Elsevier.)
ADF
detector
projections increases roughly as the square of the resoluen
improvements in resolution by a factor of two or three
ke a dramatic difference. Even when such small distances
ed to be measured, a smaller probe size allows for higher
mages and improved detectability of single dopant atoms,
studies of a wider range of dopant species and thicker,
istic, substrates.
rticle will explore the limits and opportunities for the
ration of aberration-corrected microscopes. After a brief
n of the STEM itself, the improvements in resolution and
ue to aberration-corrected optics and improved electron
re detailed. Improved energy resolution from monors
and higher-resolution spectrometers has enabled nanosurements
of the photonic properties of materials and
es. Finally, with the improvements in instrumentation, it
ple itself and the radiation dose it can tolerate that seems
the limit to signal and resolution.
ning transmission electron microscope
TEM to include the necessary field-emission source and
um needed to keep samples clean during irradiation was
by Crewe in 1964 (ref. 35), and within a few years they
onstrating images of single heavy atoms 15 and their difoss
thin carbon films 36 . The popular dark-field and high-
-field imaging modes were demonstrated early during the
development, including in the collection of lattice images
oparticles 37 . As the original microscopes were intended for
research, these samples were used more as resolution tests
materials studies — with one exception being the study
t particles 19 . Adoption of the STEM technique was slow
difficulties in sample preparation 190of
bulk materials and a
despread instrument availability as a result of the vacuum
onics requirements.
Electron
spectrometer
© 2009 Macmillan Publishers Limited. All rights reserved
Figure 3 | major elements of a scanning transmission electron
microscope. A high-brightness electron source produces a 100–300- keV
electron beam with an energy spread of 0.3–1 eV, which can be narrowed
to below 100 meV with a monochromator. Round magnetic lenses and
corrective multipole optics focus the beam to a spot size of between
0.05 and ~0.3 nm, which is scanned across an electron-transparent sample.
To a first approximation, when the beam is placed on an atom column,
strong Rutherford-like scattering deflects the transmitted electrons to form
brighter features in an annular dark-field (ADF) image, with less scattering
between the columns. Inelastic scattering is strongly peaked in the forward
direction and is collected simultaneously with the ADF signal. The energy
losses of the transmitted electrons reflect characteristic excitations of
the sample in a frequency range spanning the near-infrared to hard X-ray
regions, allowing electronic and elemental identification from a single
column of atoms.
Abbildung A.2.: Schematische Darstellung der Funktionsweise eines Rastertransmissionselektronenmikroskops.
Von einer Elektronenquelle wird ein kohärenter
Elektronenstrahl emittiert, der durch verschiedene Elektronenlinsen auf
einen Strahldurchmesser von bis zu 0,5 Å fokussiert wird. Während der
Transmission durch die dünne Probe (< 100 nm) wechselwirken die Elektronen
daran. Die elastisch gestreuten Elektronen werden vom ADF-Detektor
registriert, die inelastisch gestreuten werden über ein Elektronenspektrometer
nach ihrer Energie charakterisiert [248]. (Grafik aus [248].)
Almost a decade later, with the development of a commercially
manufactured STEM and better specimen preparation tools such as
ion-milling, there was a renewed interest in the instrument as an
analytical tool for metallurgy and in the semiconductor industry.
Some of these second-generation instruments are still in operation
today. In the mid-1990s, a new generation of commercial fieldemission
TEMs with scanning attachments made STEM techniques
widely available to the materials research community and industry.
The major components of a STEM are shown in Fig. 3. A
field-emission gun provides a high-coherence source of electrons
that is accelerated to between 100 and 300 keV — energies sufficient
to penetrate samples up to 100 nm thick without significant beam
spreading. A series of electron lenses and corrective optics placed
before the sample focuses the beam down to a diameter smaller
nature materials | VOL 8 | APRIL 2009 | www.nature.com/naturematerials
264 11/3/09 11:12:08