.............................................................................................................................................................................................. .............................................................................................................................................................................................. .............................................................................................................................................................................................. .............................................................................................................................................................................................. Physical: Full-length Estimation of wave fields of incident beams in a transmission electron microscope by using a small selected-area aperture Shigeyuki Morishita 1, * ,† , Jun Yamasaki 2 and Nobuo Tanaka 2 1 Department of Crystalline Materials Science, Nagoya University, Furo-cho, Nagoya 464-8603, Japan and 2 EcoTopia Science Institute, Nagoya University, Furo-cho, Nagoya 464-8603, Japan *To whom correspondence should be addressed. E-mail: morishita@nagoya-u.jp †Research Fellow of the Japan Society for the Promotion of Science, Japan. Abstract The direction of an electron beam in a nanometer-sized area is measured directly by utilizing a selected-area aperture. By the measurements at several areas in a beam, the wavefront curvature and thus the defocus value of the beam are detected. From the defocus value, the wave field at the specimen plane is also reproduced in consideration of the influences of the condenser aperture and spherical aberration of the illumination lens. The result shows that phase deviation of 2π is caused only at about 10 nm apart from the beam center in a beam with a typical diameter for high-resolution transmission electron microscopy. Based on the defocus value, the convergence angle of the beam is also estimated to be about 6 mrad without being influenced by the partial coherence, that is, inde- pendently of the type of the electron gun. Measuring the defocus values for only two beam diameters enables us to determine geometrical par- ameters peculiar to the illumination system, based on which wave fields of any beam diameters by any condenser aperture sizes can be estimated. The technique proposed in this paper is effective in evaluating the influ- ence of wavefront curvature of incident beams on various kinds of precise measurements conducted in transmission electron microscopes. Keywords incident wave field, wavefront curvature, selected-area diffraction, trans- mission electron microscope, small selected-area aperture, incident angle Received 25 November 2010, accepted 11 January 2011, online 13 February 2011 Introduction Recent improvements in precision of structure ana- lyses using transmission electron microscopes (TEM) have increased the importance of accurate evaluation of incident beam properties such as current density, wavelength, spatial and temporal coherence, and wavefront curvature. In various measurements in TEM such as high-resolution transmission electron microscopy (HRTEM) and selected-area diffraction, incident electron beams are generally assumed to be plane waves. In prac- tice, incident beams are, however, required to be convergent/divergent in order to increase the current density at the specimen plane so that wave- fronts of incident beams around the specimen plane are curved. This means that incident direc- tions differ from position to position in the beam. The wavefront curvature results in a nonuniform phase of the incident wave at the specimen plane. The wavefront curvature should influence results and interpretations in HRTEM with its resolution and data accuracy being improved by aberration cor- rections [1,2]. Also, the results and interpretations obtained by various methods, which deal with ........................................................................................................................................................................................................................................................ Journal of Electron Microscopy 60(2): 101–108 (2011) doi: 10.1093/jmicro/dfr002 ........................................................................................................................................................................................................................................................ © The Author 2011. Published by Oxford University Press [on behalf of Japanese Society of Microscopy]. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com by guest on February 4, 2016 http://jmicro.oxfordjournals.org/ Downloaded from