Virtual Objective Aperture (VOA) in TEM/STEM
- Practical Electron Microscopy and Database -
- An Online Book -  

This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.


In SEMs and TEMs, the beam-limiting aperture confines the convergence angle of the electron beam to control the aberrations of the final lens. In conventional systems, the real objective aperture locates near the poorer vacuum of the specimen chamber, so it is easy to contaminate the aperture. However, in many modern SEMs and STEMs the beam-limiting aperture is located after the last condenser and before the objective lens, called as "virtual objective aperture" (VOA). This type of apertures is required for the immersion and the snorkel objective lenses in some SEMs and it stays further from the specimen chamber and thus is clean. The VOA limits the beam angle in a similar manner to a real objective aperture. However, for the same beam convergence angle the VOA opening required is equal to the size of the equivalent real aperture divided by the demagnification of the objective. In STEMs, the useful part of the Ronchigram is selected by the VOA, or called “condenser aperture”.




The book author (Dr. Liao) welcomes your comments, suggestions, and corrections, please click here for submission. You can click How to Cite This Book to cite this book. If you let Dr. Liao know once you have cited this book, the brief information of your publication will appear on the “Times Cited” page. This appearance can help advertise your publication.

Copyright (C) 2006 GlobalSino, All Rights Reserved