Practical Electron Microscopy and Database

To perform strain analysis using Nanobeam Electron Diffraction (NBED), a near-parallel electron beam is used to obtain diffraction patterns from a region of interest. These diffraction patterns provide information on local strain by detecting shifts in diffraction spots relative to a reference. TEM NBD is the most favorable technique for routine strain analysis due to its simple experimental setup, nanometer spatial resolution, and high measurement sensitivity (with a standard deviation of approximately 0.1%). However, for strain measurements, NBD is typically performed in STEM line-scanning mode, which can make strain mapping over large sample areas time-consuming. 

When the NBD technique is operated in scanning mode, it can be used to map strain across the specimen by recording diffraction patterns directly on a two-dimensional (2D) detector at each probe position. The recorded NBD patterns contain contributions from both elastic and inelastically scattered electrons, but the sharpness of the NBD signal can be significantly improved by selecting the elastic scattering with an energy filter. 

Although this technique is straightforward, the data collection process can be labor-intensive, as using NBD (or convergent beam electron diffraction, CBED) in TEM mode for strain distribution is not convenient, requiring manual data acquisition. The quality of NBD data also heavily depends on the operator's skill. 

 Precession diffraction can mitigate some of these challenges by providing more accurate strain maps with improved spatial resolution, allowing for the analysis of strain with high precision​

Figure 2615 illustrates the basic process of NBED for strain measurement. In part (a), it shows a near-parallel electron beam, typically generated using a 200 kV TEM, being focused on a {110} silicon specimen. The beam’s full width at half maximum (FWHM) is approximately 6 nm, as depicted in part (b). As the electron beam is scanned across the sample in STEM mode, diffraction patterns are recorded at each position. These diffraction patterns contain shifts in diffraction spots, which can be analyzed to determine the local strain. The analysis can be performed using software, which automatically fits the diffraction spots, as described in part (c). This process enables the extraction of strain data across the scanned area, providing detailed information on strain distribution in the sample​. The sample structure shown in Figure 2615 (c) consists of a layered semiconductor material. It includes a 150-nm-thick capping layer at the top, followed by four 10-nm-thick SiGe layers with varying germanium (Ge) concentrations of 45%, 38%, 31%, and 20%. These SiGe layers are separated by 30 nm of pure silicon (Si) and are grown epitaxially on a silicon substrate.

[1] Cooper D, Rouviere JL. Strain measurement with nanometer resolution by transmission electron microscopy. Adv Mater Res., 996:3‐7. https://doi.org/10.4028/www.scientific.net/amr.996.3, 2014.