Quantitative Thickness Mapping In High-angle Annular Dark-field (haadf) Scanning Transmission Electron Microscopy (stem)

Only a few methods are currently available for the measurement of sample thicknesses in Transmission Electron Microscopy (TEM). These methods, Convergent-Beam Electron Diffraction (CBED) and thickness mapping in Energy-Filtered TEM (EFTEM), are either elaborate or complex. In this present work, I have investigated and come up with a simple straight-forward method to measure the local thickness of a TEM sample with the atomic number (Z-contrast) imaging using High-Angle Annular Dark Field (HAADF) Scanning Transmission Electron Microscopy (STEM). HAADF STEM shows atomic number contrast for high scattering angles of the electrons, owing to predominant electron scattering at the potential of the nucleus similar to Rutherford scattering. The characterization of materials by STEM helps to identify microstructures and nanostructures within a sample and to analyze defects in samples. HAADF STEM imaging is capable of resolving atomic structures with better than 2 Å lateral resolution. However, HAADF STEM has so far not been systematically used to measure sample thicknesses. In Z-contrast imaging, it was known that the intensity of the electrons scattered to high angles increases with increase in the atomic number (Z) of the element/compound with increasing thickness of the sample based on the equation, I ~ t.Zα Where t, is the thickness and α, is a parameter between 1 and 2. This project was started with this simple approach, but the experimental results within the thesis show that the relation between the intensity and the atomic number is not well described by this equation. A more reliable parameter, σZ, the interaction coefficient of the material was calculated. Samples containing Ag2Al platelets in Al matrix were used for calibration purposes. Additional samples containing layers of known elements/compounds were obtained from TriQuint Semiconductors and from the Physics department of UCF to calculate σ for various elements/compounds. These experimental values were used to measure the local thicknesses in nanoparticles and also the total volume of the nanoparticles. This quantitative HAADF STEM analysis represents a new method, which can be added to the list of methods used for the purpose of measurement of the local thickness of a sample in the TEM. This method is especially useful for the thickness measurement of nanoparticles. The other two methods, CBED and thickness maps in EFTEM are strongly affected by the sample orientation and therefore not appropriate for the study of nanoparticle thicknesses, whereas orientation effects are negligible for the conditions used in this HAADF STEM analysis.