Low-Energy ion Beam Reduces Damage
Figure 3: Final thinned sample ready for HR-TEM.
atomic structure on the thin lamella to be seen directly in the Figure 4: C
s
-corrected HR-TEM image of LaB
6
with model of the cubic structure
HR-TEM images.
(green = boron, yellow = lanthanum) and power spectrum of the image.
Because of the extreme thinness of the sample, it behaves
like a “weak phase object,” and the atoms are imaged in Conclusion
dark contrast. Th e clearly apparent positions of the boron Th is case study on the thermal electron emitter material
octahedron and the lanthanum atoms demonstrate the power
of LaB
6
demonstrates some signifi cant advances in the
of C
s
-corrected TEM and DualBeam sample preparation to
preparation of thin sections for high-resolution S/TEM
visualize atomic structure. Th e power spectrum of the image in
(scanning/transmission electron microscopy) and the progress
the upper-left corner demonstrates that information below 100
that has been made in atomic resolution imaging using a
pm is transferred in the image.
To quantify the lamella thickness, some image simulation
with variation in focus and sample thickness was calculated
(Figure 5). In the simulation, an area is marked indicating the
region of focus and thickness where the experimental image
best fi ts the simulation. Th e result gives an estimated local
thickness below 10nm for the LaB
6
lamella.
Th e complex electron wave exiting the sample contains
information about the material in both its amplitude and its
phase. Any single image contains only the intensity distribution
(amplitude) and cannot reveal all the information potentially
available from HR-TEM analysis. However, focus series
reconstruction, which uses a series of images acquired at
diff erent focus values, can recover the phase information and
reconstruct the full complex form of the exit wave. An example
of the phase part of the LaB
6
image is shown in Figure 6. From
the complex wave, residual aberrations can be compensated to
improve the image quality even further or to access information
about the occupancy of single columns in the beam direction.
A sample of the tip region of the thermal emitter was
obtained using the site-specifi c sample preparation capability
of a DualBeam and subsequently imaged with C
s
-corrected
HR-TEM. In the TEM images, small rectangular areas (approx.
2 x 2 nm) are visible where the crystalline structure of the LaB
6
Figure 5: Image simulation of the HR-TEM image depending on the focus of
is destroyed (Figure 7). Th is gives a new insight into the aging
the objective lens and thickness of the LaB
6
crystal. The dotted region closely
process of a thermal emitter tip in an electron microscope.
matches the experimental image in Figure 3.
2009 September •
www.microscopy-today.com 43
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