Examples of electrostatic electron optics: The Farrand and Elektros microscopes and electron mirrors

Abstract

The role of Gertrude Rempfer in the design of the Farrand and Elektros microscopes is evoked. The study of electron mirror optics, aberration correction using mirrors and the development of microscopes employing electron mirrors are recapitulated, accompanied by a full bibliography, of earlier publications in particular.

Introduction

Sporadic attempts have been made to build electron microscopes with electrostatic lenses, with commercial exploitation in mind. Since electrostatic lenses turn into electron mirrors if the potential barrier is high enough, the idea of constructing an electron microscope incorporating an electron mirror soon followed. As we shall see below, the AEG Forschungsinstitut in Berlin was a pioneer in this area, as we might expect since their earliest work was concerned with a related field, electron emission microscopy. In the early years, it was not clear whether magnetic or electrostatic lenses would dominate electron microscope design and among those who believed in the future of the electrostatic electron microscope was C.L. Farrand. Gertrude Rempfer and her husband were deeply involved in Farrand's project and, in Section 2 of this article, the story of the Farrand instrument is recounted; a few words on the Elektros microscope conclude this part.

In the third part, we turn to the optics of electron mirrors, for this was a major preoccupation of Gertrude Rempfer after she left Farrand and moved to Portland. The optics of mirrors is not a straightforward extension of the optics of lenses since one of the assumptions on which the derivation of the paraxial equation of motion is based is no longer valid: at the turning point, where the axial component of the electron velocity falls to zero and changes sign, the gradient of the rays is no longer small. Some other form of equation than the familiar paraxial ray equation must be found. The various ways of achieving this and hence of establishing the corresponding aberration integrals are listed in Section 3.

Electron mirrors are not covered by Scherzer's “theorem” and can hence be used to correct the spherical and chromatic aberrations of round lenses. Section 4 is devoted to this approach to aberration correction, which was another of Gertrude Rempfer's interests

Finally, we provide some pointers for future historians of the mirror electron microscope, both the laboratory instruments that go back to the Mahl–Pendzich design of 1943 and the rare attempts to market commercial models, notably by Litton Instruments in the USA and by JEOL, who launched their JEM–M1 model in 1968. Although a number of reviews of mirror microscopy have appeared, none of these mentions all the different projects (and no doubt I too have overlooked some). I hope that this arid bibliography of the subject may provide a starting point for a thorough re-examination of this somewhat neglected subject. The most recent wide-ranging review is that published by Luk'yanov and Spivak in 1973!