low-angle annular dark-field scanning transmission electron microscopy, LAADF-STEM

Low-angle annular dark-field scanning transmission electron microscopy (LAADF-STEM) is a STEM method which receives diffracted or inelastically scattered electrons at low to medium angles (25 to 60 mrad) using an annular dark-field (ADF) detector. A STEM image is acquired by displaying the integrated intensities of the electrons in synchronism with the incident probe position. It is noted that the LAADF image shows diffraction contrast or difference of specimen thickness dependence.
LAADF-STEM is used in place of HAADF-STEM for a specimen composed only of light elements (molecular crystals, two-dimensional crystals, organic materials like macromolecules, biological materials, etc.). This is because the HAADF image does not have a high signal-to-noise ratio due to weak elastic and inelastic scattering at high angles for such light-element specimens. LAADF-STEM enables us to obtain a high-resolution image with a high signal-to-noise ratio whose intensity depends on the atomic number.


STEM images of the cross-sectional thin film of a semiconductor device (an n-channel MOSFET of CPU). (Accelerating voltage: 200 kV, Convergence semi-angle of the incident electron beam: 11 mrad)

Fig.(a) HAADF-STEM image taken at an acceptance semi-angle of the detector of 46 to 208 mrad. As is shown by a blue arrow, only a region where heavy atoms (Ta, W, etc.) exist are observed with bright contrast.
Fig.(b) LAADF-STEM image taken at an acceptance semi-angle of the detector of 14 to 63 mrad.
SiN_x layers in the regions indicated by a red parenthesis are observed brighter than SiO₂ regions indicated by a blue parenthesis. Such layer structures are hardly seen in the HAADF image. In addition, the LAADF image also visualizes lattice defects indicated by red allows. 

Comparison of Ray diagrams of two detectors

Fig.(a) Relationship between the convergence semi-angle of the incident electron beam and acceptance semi-angles of the detector for HAADF-STEM. Typical inner and outer semi-angles of the detector are respectively β1 = ～50 mrad and β2 = ～200 mrad, detecting inelastically scattered electrons at high angles. The value of the convergence semi-angle α is approximately 25 mrad for a 200 kV Cs-corrected TEM. Usually, an ABF detector and a LAADF detector are placed below a HAADF detector.

Fig.(c) Relationship between the convergence semi-angle of the incident electron beam and acceptance semi-angles of the detector for LAADF-STEM. The inner and outer acceptance semi-angles of the detector are respectively taken as that β1 is a little larger than α and β2 is ～60 mrad, where α is the convergence semi-angle. It should be noted that the diffracted waves and inelastically scattered electrons over low to medium angles are used for LAADF-STEM.

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