The Schottky electron emitter is a predominant electron-emitting source in today’s electron beam equipment. This book comprehensively covers the Schottky emitter, dealing with its theoretical as well as practical aspects. The main questions that are addressed in this book are: what is the Schottky electron emitter? How does it work? And how do its properties affect the performance of electron beam equipment?
The focus is on the direct link between the operating conditions of the source and the properties of the beam at the target level. This coupling is made clear by discussing the effect of the operating conditions and the geometry of the source and gun on the emission properties of the emitting surface, the effect of Coulomb interactions on the brightness and energy spread in the first few millimeters of the beam path, and the effect of the operating conditions and the shape of the emitter on the consequences of the beam at the target. The final chapter combines all these effects to demonstrate that there is a trade-off to be made between brightness, energy spread, and shape stability.
Physics of Schottky Electron Sources Theory and Optimum Operation Merijn Bronsgeest
Physics of Schottky Electron Sources
Physics of Schottky Electron Sources Theory and Optimum Operation
editors
Merijn Bronsgeest
Preben Maegaard Anna Krenz Wolfgang Palz
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Physics of Schottky Electron Sources: Theory and Optimum Operation Copyright © 2014 by Pan Stanford Publishing Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the publisher.
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Contents
Preface Introduction
ix
1
1. Electron Emission from a Surface 7 1.1 The Potential Energy Barrier at a Surface 7 1.2 Emission by Heating 9 1.3 The Effect of an Electric Field on the Potential Energy Barrier at a Surface 12 1.4 Emission by Heating and Applying an Electric Field 16 1.4.1 Escape Probability 16 1.4.2 Current Density 22 1.4.3 Energy Distributions 25 2. Emission from a Schottky Emitter 29 2.1 Work Function Variations across the Emitter Surface 30 2.2 Applying a Bias 32 2.3 Applying a Heating Current 38 2.3.1 Temperature 38 2.3.2 Tip Protrusion (Field Strength) 42 2.3.3 Surface Properties (Work Function) 44 2.4 Total Emission Current 45 3. Emission from the End Facet 49 3.1 The Facet Extractor Lens 50 3.1.1 At the Facet 50 3.1.2 Lens Properties 52 3.1.3 Behind the Extractor 55 3.1.3.1 The angular intensity of the source 55 3.1.3.2 The full facet emission pattern 57 3.2 The Effect of the Voltage Settings 59 3.2.1 Different Options 59
vi
Contents
3.3 3.4 3.5
3.2.2 The Effect of Changing the Extraction Voltage 60 3.2.2.1 From the facet toward the extractor 62 The