Strain Characterization Using Scanning Transmission Electron Microscopy and Moiré Interferometry

Abstract

The characterization of the material’s deformation is nowadays common in transmission electron microscopy. The ability to resolve the crystalline lattice enables the strain to be linked with the deformation of the crystal unit cells. Imaging the crystal unit cells imposes the sampling scheme to oversample the resolved crystal periodicities and, thus, limits the field of view (FOV) of the micrograph. Therefore, alternative methods were developed (electron diffraction and holography) to overcome the FOV limitation. The method presented in this thesis is part of the large FOV challenge. Its principle is based on the coherent interference of the sampling grid with the crystalline lattices of the material in scanning transmission electron microscopy (STEM). The interference results to a set of Moiré fringes embedding the structural properties of the material such as a strain field. The STEM Moiré hologram (SMH) formation can be elegantly described using the concept of Moiré sampling in STEM imaging. The STEM Moiré fringes reveals to be undersampling artefacts commonly known as aliasing artefacts. The SMH is, therefore, violating the sampling theorem and is not a proper representation of the crystal unit cells. However, an oversampled representation can be recovered from the SMH using a set of prior knowledge. The SMH becomes suitable to characterize the 2D strain field giving birth to a new dedicated method, called STEM Moiré GPA (SMG), that is using the Geometric Phase Analysis method on the SMH directly. After detailing the theory of SMG, the technique is validated experimentally by comparing it to other strain characterization methods and to Finite Element Method simulations. The characteristics of SMG (resolution, precision and accuracy) and its limits are then detailed. Finally, the SMG method is applied on semiconductor devices to highlight the typical capabilities of the technique.