Aberration correctors, monochromators, spectrometers

Abstract

After four decades of attempts to correct the primary spherical and chromatic aberrations of electron lenses that led to no improvement in resolution, success was at last achieved in the 1990s with both quadrupole-octupole and sextupole correctors. The successful correctors focused on three aspects of aberration correction: primary aberrations, parasitic aberrations aberrationparasiticparasitic aberration, and overall stability. They quickly demonstrated resolution improvement in the microscopes they were built into, and in the early 2000s, they advanced the attainable resolution to <1Å —a level not achievable by uncorrected electron microscopes. Subsequent generations of correctors included further multipoles and corrected aberrations up to the fifth order, enabling resolution of better than 0.5Å to be reached at 300kV primary voltage, and around 1Å at 30kV. The effect of chromatic aberration was reduced by the use of hybrid quadrupoles or by incorporating a monochromator in the microscope column. After a brief summary of the optics of multipoles, the various types of correctors are examined in detail: quadrupole–octopole correctors quadrupole–octopole corrector, which first improved the performance of a scanning electron microscope and, soon after, that of scanning transmission electron microscopes; and sextupole correctors sextupolecorrector, which first increased the resolving power of conventional (fixed-beam) transmission electron microscopes, and were later used in scanning transmission electron microscopes as well. Ways of combating chromatic aberration are then described, including mirror correctors mirrorcorrector employed in low-energy-electron and photoemission microscopes (LEEM and PEEM). A section is devoted to studies of aberrations beyond the third order and of parasitic aberrations. Electron spectrometers and imaging filters are routine accessories of electron microscopes, and they too must be carefully designed, especially when attached to aberration-corrected instruments. A section covers these devices, and much of the reasoning also applies to monochromators. Separate paragraphs are devoted to post-column and in-column spectrometers and monochromators, and the attainable energy resolution is discussed. Practical aspects of the correction process are described, notably autotuning and aberration measurement. We conclude with a survey of current performance limits and comments on the problems to be overcome if further progress is to be made.