Aberration Correction with Electron Mirrors

Abstract

Both the spherical and the chromatic aberration of electron microscope objectives may, in principle, be corrected with the aid of a uniform retarding field acting as a mirror. Such an arrangement has the drawback of requiring a conducting film in the ray path and the insertion of the specimen in a region of high field strength. The employment of concave electron mirrors with concentrated field distribution, forming a real image of approximately unity magnification, is free from this drawback. The formulas for spherical and chromatic aberration, presented in a form suitable for calculation, are applied to a characteristic electron mirror field of this type (Φ = C+tanh(sinhz)). It is found that the aberration coefficients of the mirror are so large, however, that this method of aberration correction encounters serious practical difficulties.

Correction of Chromatic Aberration with an Electron Mirror

“…The history of the mirror was known to Ramberg, and the publication of the list of possibilities for overcoming aberrations articulated by Scherzer may be what led Ramberg to investigate the possibility of utilizing a mirror to improve the resolution of the electron microscope. In 1948 Ramberg [20] proved that the mirror field he studied analytically would have overcorrected chromatic aberration, that is, higher energy electrons would focus closer to the mirror rather than farther away as in the usual case with lenses. In his scheme, the specimen was situated between the objective lens and the mirror.…”

“…Other parameters, associated with the electron optical system to be corrected, include the design and relative scale of the lenses, and their voltage ratios which in turn determine the magnitude of chromatic and spherical error to be corrected. INTRODUCTION Electron mirrors have chromatic and spherical aberration coefficients of opposite sign from those of electron lenses and, in principle, can be used to compensate the spherical and chromatic aberrations of electron lens systems[7,20,22,24]. A method of implementing a hyperbolic electron mirror as an aberration corrector in electron microscopes has been proposed recently[28].…”

Effect of Impurity Aggregation on the Shift of Thermoluminescence Glow Peak of X-Irradiated KCl: Sr2+

“…The idea of using an electron mirror to correct the chromatic and spherical aberration of a round lens dates back more than half a century [16,17]. Extensive studies of electron mirrors have been performed by Kelman [38,39], Rempfer [18][19][20], Shao [21,22], and Rose and co-workers [23,40].…”

“…A third class of aberration corrector system is the so-called electron mirror [16][17][18][19][20][21][22][23]. By introducing a reflection in the electron path using an electron mirror,the electron beam direction reverses and the electron velocity changes sign, thus the Scherzer theorem no longer applies.…”

Analytic solution for a quartic electron mirror

“…In 1990 Gertrude F. Rempfer [1] built on the research of those such as Zworykin et al [2] and Ramberg [3] to lay the theoretical foundations for the hyperbolic electron mirror as a means to counter the spherical and chromatic aberrations of electron lenses since aberrations in the former are of opposite sign to those in the latter. A number of other researchers have also employed such mirrors [4][5][6][7][8].…”