Optimization of TEM specimen preparation by double-sided ion beam thinning under low angles

We have developed special specimen holders for double-sided low angle ion beam thinning and have systematically studied possible improvements of the resulting TEM specimens with respect to surface roughness, radiation-induced damage and contamination. The specimen holders are entirely made from graphite and were built for two different ion beam thinners. The design makes it possible to work at angles of incidence of 6 degrees and lower and to precisely position the centre of rotation. The resulting specimen geometry was characterized with respect to a number of critical parameters. The surface topography was measured with atomic force microscopy, defect-induced localized strain was revealed by HRTEM and the thicknesses of contamination layers, carbon coating and amorphous oxide layers were quantitatively determined by electron spectroscopic imaging in an energy filtering TEM. It was found that the surface roughness not only decreases with decreasing angle of incidence, but also when the ion energy is lowered. In the low-angle regime, the thinning mechanism is based on the formation and propagation of ledges. Simultaneous ion bombardment from both sides has been found to be one of the most critical parameters.