LabAdviser/314/Microscopy 314-307/SEM/Nova/Transmission Kikuchi diffraction: Difference between revisions
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= On-axis Transmission Kikuchi diffraction = | = On-axis Transmission Kikuchi diffraction = | ||
A development of the EBSD technique, proposed in 2012, led to a signi | |||
�cant improvement in spatial resolution. The new technique, introduced | |||
by Keller and Geiss [40], is based on the collection of Kikuchi patterns from | |||
electron transparent samples using a conventional EBSD detector. In the | |||
new geometry the sample, to improve the detector electron collection ef- | |||
�ciency, is tilted away from the EBSD detector by 20°. In this way, the | |||
collected di�raction pattern is projected from the lower surface of the sample. | |||
This technique has been given the name transmission EBSD (t-EBSD) | |||
at �rst [41]. However, in this case the term electron backscatter di�raction is | |||
inaccurate, because the collected di�racted electrons are not backscattered, | |||
but transmitted through the sample instead. In order to deal with that, a | |||
more appropriate name, transmission Kikuchi di�raction (TKD), has been | |||
introduced in later publications | |||
This mechanism leads to two main e�ects: (1) for TKD an electron | |||
transparent sample is required, resulting in more intensive sample preparation | |||
compared to EBSD. This also results in both a smaller region being | |||
available for analysis and additional stress release considerations, as there | |||
are two free surfaces rather than one; (2) the acquisition of TKD patterns | |||
from relatively thick samples can be done (as soon as they are electron | |||
transparent at that thickness), but the e�ects of beam broadening due to | |||
electron scattering inside the sample become stronger in increasingly thick | |||
specimens and lead to degradation of the lateral spatial resolution. | |||
The last step in the evolution of TKD has been carried out by Fundenberger, | |||
together with Bruker Nano, in 2016 [46]. They presented a new | |||
con�guration for the SEM-based mapping with the detector located perepndicularly | |||
beneath the electron transparent sample on the optical axis of | |||
the microscope, obtaining an instrument resolution almost the same as that | |||
using Kikuchi patterns in TEM. A schematic illustration of the detector is | |||
shown in Fig. 3.8. | |||
Moving the detector from a high-angle to the on-axis position permits | |||
to reduce the probe current and size to record a solvable pattern. Kikuchi | |||
patterns are more intense at small scattering angles (i.e., near the direction | |||
of the optical axis) than at higher angles. Therefore, the intensity of the | |||
incident electron beam, and thus the probe size needed to record a solvable | |||
pattern is smaller when the detector is moved from a high-angle to the onaxis | |||
position. The probe size in combination with the beam broadening | |||
a�ect the total interaction volume, and the width of the interaction volume | |||
is directly linked to the lateral resolution. Further information about TKD | |||
can be found in the detailed review presented by Sneddon [47]. All the | |||
experiments and results presented in the next section were obtained using | |||
the on-axis detector con�guration, which has already become the standard | |||
geometry for TKD measurements. | |||