title: Development of novel in situ electron microscopy techniques and their application to functional thin films
Creators: name: Denninger, Peter
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024

description: Thin films occur in a broad range of daily life applications and they are an integral part of almost any industrial field. The appearance and applications of thin films varies strongly and ranges from simple coatings to protect underlying material up to highly advanced electrical conductive layers with flexible and transparent properties. Due to their wide range of structures and elemental compositions combined with a high accessibility, their application space seems nearly endless. Advances in modern technology are often based on the miniaturization of existing structures to improve and expand the performance. This development can be seen in its entirety in electronic devices, where high-quality structures with excellent properties on very small length scale are required. Besides a profound knowledge about the applied thin film, dedicated manufacturing and characterization tools are needed and have to be improved constantly to support the challenging demands for the ongoing miniaturization. In general, the impact of new investigation techniques and instruments can be enormous and enlighten unknown phenomena. In this work, new characterization techniques are developed and implemented in the Scanning Electron Microscope (SEM) to study a broad variety of thin film structures and their properties. The SEM supports these developments by a simpler setup compared to the Transmission Electron Microscope (TEM). Especially, transmission imaging, known as Scanning Transmission Electron Microscope (STEM), and transmission diffraction are of great importance and the center of this work and corresponding developments. They push the limit of standard SEMs and extend the performance in relation to TEM. Therefore, numerous people could benefit from these developments due to the widespread availability of the instrument in academia and industry. To showcase the power of the developed techniques, a variety of thin film systems are analyzed in great detail. The first developed technique is an aperture-based dark-field (DF) approach in SEM, which allows the selection of any diffraction vector for imaging. The method is based on the existing Low Energy Nano Diffraction (LEND) setup developed for SEM in Erlangen. In contrast to common SEM, which only supports radial integrated DF images by the annular-shaped STEM detector. To illustrate the usefulness of a true DF imaging method, a bilayer graphene sample is investigated with a focus on defects imaging and analyzing. Defects are fundamental and influence the properties of the material. In particular, for low dimensional materials, defects, like dislocations or Moiré patterns, play a key role and they can have a dramatic impact on the overall structure and properties. For instance, the piled up layer stack exhibits superconductivity for a distinct rotation between both layers. Therefore, it is indispensable to investigate and characterize the occurring defects in such a low dimensional material. The DF technique in SEM is not only demonstrated to enable imaging of defects but also compared with common DF imaging in TEM to show that the same contrast types and mechanisms are present. In addition, a 4D-STEM experiment is performed to illustrate the applicability regarding advanced defect characterization in SEM via a virtual approach. 4D-STEM is based on the acquisition of one diffraction pattern (2D image) for each scan point of the real image (2D). The resulting data stack (2D*2D) offers a flexible and nearly unlimited access to sample information via post-processing. However, with respect to integration of dedicated 4D-STEM cameras in SEM together with fast imaging applications, the available solutions quickly reach their capabilities. The second major development tackles a common limitation in electron microscopy (EM), namely the lack of a combined transmission imaging and diffraction setup. During transmission, electrons can interact with the sample or pass through unaffectedly. Regardless of the interaction that occurred, each electron contains specific information about the sample, which can be collected and analyzed via a broad range of different techniques. However, in common instruments, the user has to choose between imaging and diffraction mode. Therefore, only a part of the available information can be acquired, while the rest is lost. 4D-STEM allows to record both information, imaging and diffraction, but this technique is relatively slow due to the cameras available. Especially for in situ experiments, where fast image acquisition is important, 4D-STEM is unsuitable. To eliminate this limitation, a novel approach, which enables simultaneous acquisition of STEM signal and diffraction information is developed and implemented in the SEM. The simultaneous access to STEM and diffraction information enables the investigation of complex reactions and mechanisms. The developed setup is a unique and cost-effective approach based on the existing LEND system and a standard STEM detector. Especially, the STEM detector as an integrating detector allows for fast imaging experiments combined with a large field of view. The continuous monitoring of ongoing phenomena and reactions is a key part of material science, known as in situ microscopy. Therefore, the design of the new setup is focused on in situ experiments with the opportunity for transmission imaging combined with an averaged diffraction signal. Besides the acquisition of STEM and diffraction signal, all other available signals can be also collected. Consequently, the method enables a broad characterization of the present sample system. To showcase the benefits of acquiring real and reciprocal space information simultaneously, an in situ heating experiment of a thin Au film is performed to study the correlation of two important phenomena at elevated temperatures, namely grain growth and texture evolution. In the last part, the power of combining various advanced STEM techniques in SEM and TEM is demonstrated on a member of the metal-induced layer exchange (MILE) system. It is a bilayer thin film system, which consists of an amorphous semiconductor and a poly-crystalline metal layer in the range of nm to µm layer thickness. During heating, two processes occur. On the one hand, both layers change their position, hence it is called layer exchange (LE) mechanism. On the other hand, the amorphous semiconductor crystallizes into a large poly-crystalline thin film well below the actual crystallization temperature of the material. This metal-induced crystallization (MIC) is a well-known phenomenon for a variety of metal – semiconductor couples and an attractive way to produce large-grained poly-crystalline semiconductor films on foreign substrates. However, the underlying processes governing the LE and crystallization are still under debate. Especially, the early stages before the crystallization starts are of great interest and not completely clarified. In this work, the Ag – Si system is studied by correlative in situ microscopy to reveal the complex mechanisms with focus on the early stages. The combination of advanced and scale-bridging techniques provides deep insights into the occurring processes of the Ag – Si system and helps to elucidate unknown phenomena.