title: Nonlinear intensity attenuation with increasing thickness and quantitative TEM tomography of micron-sized materials.
abstract: Nowadays three-dimensional (3D) analyses of nanometer-sized and sub-micron-sized objects have been widely achieved by tomography in transmission electron microscopes (TEM). One of the next methodological targets should be quantitative 3D reconstructions in which not only the shape but also the internal density is correctly reproduced. This is, however, generally hindered by the nonlinearity between projection mass-thickness and image intensity. In the case of BF-TEM images, the ideal exponential attenuation with increasing thickness is disturbed by multiple scatterings. The nonlinearity in the tilt series should induce an inaccurate density distribution in the reconstructed volume.In the present study, the nonlinear attenuation effect has been analyzed using amorphous carbon microcoils (CMCs). Their well-defined shapes and compositional homogeneity are quite useful to estimate the mass-thickness. For measurements over a wide range of acceleration voltages (400, 600, 800 and 1000 kV), we used a high-voltage electron microscope (JEOL: JEM-1000KRS). The intensity attenuation with increasing thickness was measured in the BF-TEM images taken with various sizes of objective apertures (OAs). The results have shown that using a smaller OA and a lower voltage enhances the nonlinearity in the intensity attenuation.It is considered that such nonlinear attenuation should induce failures in conversion from intensity to thickness and thus inhibits correct 3D reconstructions. The influence of the nonlinearity on the tomographic reconstructions has been examined using a specially-developed 360°-tilt sample holder for elimination of the missing-wedge effect [1]. Figure 1 shows the reconstruction results from the tilt series taken under three imaging conditions; 1000 kV with the smallest or the largest OA, and 400 kV with the largest OA. At a glance, the 3D shape of the CMC has been reconstructed well regardless of the differences in the imaging conditions. However, in the sliced image in (a), the interior region is so blurred that the inner walls are difficult to identify. If the reconstructed density is examined by the profiles between x and y in the sliced images, it is clear that the internal density is not uniform but has gradient from the center in (b). Moreover, there is a slight increase of the vacuum level at the inside region of the coil. The uniform densities in the material and vacuum have been reconstructed only in (c) taken at 1000 kV with the largest OA, which is the imaging condition achieving the perfect linearity in the intensity attenuation.jmicro;63/suppl_1/i5/DFU044F1F1DFU044F1Fig. 1.3D reconstructions of a carbon microcoil taken at (a) 1000 kV with the largest objective aperture (OA), (b) 400 kV with the largest OA and (c) 1000 kV with the smallest OA Based on analysis of the nonlinearity under other various imaging conditions, we found that the linearity tends to collapse when the electron transmittance falls below about 2/3 [1]. This information should be beneficial in actual tomography experiments because one can foresee quality of the reconstruction before acquiring numerous images for the tilt series.
authors.affiliation: Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, Ibaraki, Osaka 567-0047, Japan EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan yamasaki@uhvem.osaka-u.ac.jp.
authors.forename: Jun
authors.initials: J
authors.lastname: Yamasaki