The Optimum CS Condition for High-Resolution Transmission Electron Microscopy

Abstract

High-resolution electron microscopists are familiar with the concept of an “optimum defocus” for obtaining highresolution transmission electron microscope images. Scherzer recognized that it is possible to balance the phase changes imposed by the spherical aberration of the TEM objective lens by adjustments to lens defocus. Selection of this focus condition maximizes the same-phase transfer of structural information carried by electrons scattered from the specimen. The upper limit of spatial frequencies transferred with the same phase change determines the resolution of the microscope. The resolution and “optimum defocus” depend only on the electron wavelength of the microscope and the spherical aberration coefficient, CS, of its objective lens. Reduction of Cs is the major route to improved resolution.With the advent of electron-optical systems able to generate negative spherical aberration (usually called “Cs correctors“), it has now become feasible to zero-out objective lens Cs in the high-resolution transmission electron microscope.