The scanning electron microscope (SEM) continues to play an essential role in the imaging, characterization and metrology of nanoscale materials and structures. While the SEM does not have the ultimate imaging resolution and analytical capabilities of the transmission electron microscope (TEM), it nevertheless offers benefits in terms of ease of sample preparation and the ability to image surface details and three-dimensional structures, especially at low beam voltages where reduced electron range and interaction volume provides highly surface sensitive information [1, 2].
Major challenges for extending the performance of the low voltage SEM are the inherent aberrations that limit the beam resolution. This chapter reviews these limitations and discusses developments that have helped extend SEM into an extreme high-resolution (XHR) regime, where sub-nanometer resolution can be achieved at low beam voltages. In particular, it will describe the implementation of a source monochromator, which reduces the effects of chromatic aberrations at low voltage, resulting in a more tightly focused electron beam. This capability, first available in the Magellan™ XHR SEM in 2008 [3–6], but since extended to other FEI microscopes (including dual-beam systems combining SEM with focused ion beam: FIB), provides significant ...