Aberration-Corrected Imaging in Transmission Electron Microscopy: An Introduction

Aberration-Corrected Imaging in Transmission Electron Microscopy: An Introduction, Rolf ErniImperial College Press, London, 2010. $88.00 (354 pp.). ISBN 978-1-84816-536-6

Aberration-Corrected Imaging in Transmission Electron Microscopy: An Introduction is a discussion of the successful approaches that have emerged in the past few years for correcting spherical aberration and the higher-order electron-optic aberrations that remain when spherical aberration is eliminated. “Microscopists who used to deal with spherical aberration and defocus have started to adopt the optical concepts which were previously mostly relevant for experts working in electron optics,” says author Rolf Erni, who heads the Electron Microscopy Center of the Swiss Federal Laboratories for Materials Testing and Research in Zürich.

In 1936 Otto Scherzer proposed his famous theorem: For stationary, round electron lenses that are free of charges, the constant of spherical aberration, C₃, and the constant of chromatic aberration, C_c, are positive finite. Positive C₃ is an optical defect in which off-axis beams are focused too sharply; positive C_c causes lower-energy electrons to be focused more strongly than higher-energy electrons. In 1949 Scherzer proposed a theoretical limit to the optical resolution, but it was not until the 1970s that minimization of instabilities, vibrations, and off-axis aberrations allowed instruments to approach it.

Then, for 30 years, the figures of merit for commercial instruments were the point resolution, determined by the electron wavelength and C₃ of the objective lens, and the information limit, determined by the spread in electron energies emerging from the gun and C_c of the same lens. It was some 60 years after Scherzer’s original theorem before practical, non-round lens systems or lens combinations could be developed to create zero or negative values of C₃. Correction of C_c is still a work in progress, but correction of C₃ is a remarkable advance. Nanoscience and technology researchers in universities and national labs around the world are clamoring for aberration-corrected electron microscopes, despite costs that easily exceed $2.5 million.

Though Erni doesn’t explicitly say so, it is readily apparent that Aberration-Corrected Imaging in Transmission Electron Microscopy is directed at professionals who spent their careers in the era of spherical-aberration-limited instruments and are looking to the very near future when C₃ can be adjusted by nonspecialists for routine imaging. The book begins with a concise review of established concepts in high-resolution transmission electron microscopy and moves to an area that is more likely to be unfamiliar to applied microscopists—fundamentals of charged-particle optics.

The author presents a descriptive analysis of the approximations that are useful for electron optics in an electron microscope, not a grand mathematical treatise of the kind found in Grundlagen der Elektronenoptick (Springer, 1952) by Walter Glaser or Principles of Electron Optics (3 volumes, Academic Press, 1996) edited by Peter Hawkes and Erich Kasper. Throughout Aberration-Corrected Imaging in Transmission Electron Microscopy, the optics is front and center; this is not a work that discusses potential applications in biology, materials science, or nanotechnology. In his last major section, Erni describes the two major aberration-correction systems that have been commercialized, establishes the hierarchy of residual optical aberrations that become important when C₃ is set to very low magnitudes, and works through the theory of image optimization when it is necessary to consider higher-order axial aberrations.

The book’s main technical fault is that it somewhat neglects practical issues beyond those concerning the objective lens. The source of electrons affects the brightness, energy spread, and effective source size. Erni describes in detail how partial coherence can be resolution limiting for electron microscopes, but he has much less to say about establishing a highly coherent beam. Similarly, he does not make clear how much the aberration correctors, with their stacks of optical elements, reduce beam intensity. He mentions methods to detect and measure aberrations, but does not say anything about the algorithms that are employed to align the microscopes. He also calls for rigorous control of vibrations, instabilities in the power supplies, and electromagnetic fields but does not provide many helpful suggestions. After reading this book, microscopists may suspect that they are revisiting Scherzer’s era, when chromatic aberration and a multitude of minor practical concerns precluded microscopes from achieving their theoretical resolution.

Aberration-Corrected Imaging in Transmission Electron Microscopy is impeccably edited. The schematic diagrams are accurate and informative. The equations presented are necessary and sufficient. Erni has an admirable habit of summarizing previous chapters at the onset of a new chapter. His only fault as a writer is excessive wordiness. Sometimes he plows ahead with pages of immaculate English when a schematic diagram and a half-page description might have done. Despite its wordiness, however, the book was a delight to read.

Practical aberration-corrected instruments have been a long time coming but are still very expensive. Researchers who are fortunate enough to acquire one should also acquire a copy of Aberration-Corrected Imaging in Transmission Electron Microscopy.