Seeing strain at the nanometer level with new TEM technique

Material strain is a critical factor in the electron transport properties of a material, which directly affect the lifetime and performance of electronic devices. However, the ongoing miniaturization of electronic devices demands spatial resolutions most investigation techniques struggle to provide.

Guzzinati et al. turned towards transmission electron microscopy (TEM) for its high spatial resolution in measuring strain, and modified conventional electron diffraction techniques to produce the most accurate and precise measurements.

Electron diffraction experiments commonly suffer from multiple scattering caused by the strong interaction between the electron beam and the sample, which causes non-homogenous diffraction discs to form, masking the structural information and complicating the analysis process. This can sometimes be solved by averaging diffraction patterns taken using different beam incidence directions to create a diffraction pattern with uniform discs, but the process is complicated and cumbersome.

The authors developed an alternative approach using a hollow-cone illumination where rays from different directions are present simultaneously, resulting in a diffraction pattern made of rings. Using computational models and experimental data acquired on a sample of layered Silicon/Germanium alloys, they developed a protocol to analyze the diffraction patterns. According to their results, the new approach can measure deformations as small as 1 part per 4000 with a spatial resolution down to a nanometer, meaning the technique is sensitive to changes down to half a picometer.

Even though the technique requires some modification to the optics of the TEM, including a custom-made aperture to create the necessary hollow-cone illumination, it requires no additional alignment of the microscope itself. The analysis code is freely available from the authors.

Source: “Electron Bessel beam diffraction for precise and accurate nanoscale strain mapping,” by Giulio Guzzinati, Wannes Ghielens, Christoph Mahr, Armand Béché, Andreas Rosenauer, Toon Calders, and Jo Verbeeck. Applied Physics Letters, (2019). The article can be accessed at https://doi.org/10.1063/1.5096245 .