<title>Immersion lenses for low-voltage SEM and LEEM</title>

Abstract

ABSTRACT Spherical and chromatic aberration coefficients Cs and C of various immersion lenses forlow voltage SEM and LEEM are calculated. The minimun values of magnetic immersion lens are C5Cl mm. For the combined electrostatic and magnetic lenses, those values are at most C5=1 mm and CO.7 mm, when the specimen is free from the electrostatic field. Whenthe specimen is immersed in the electrostatic field, those values reduce to C50.2 mm and Cc=o.1 mm at 1 kV. Key words: SEM, LEEM, Objective lens, Spherical aberration, Chromatic aberration, Immersion lens, Cathode lens, Computer aided design 1. INTRODUCTION Importance of low accelerating voltage in scanning electron microscopy (SEM) increasesrecently for semiconductor and biological applications. A main difficulty of low voltage SEM(LVSEM) is its low spacial resolution due to the increase of the wave length and the increaseof AVIV0 ratio where AV is the energy spread and V0 the incident energy. The immersionobjective lens as well as the field emission electron gun is very useful in realizing the highresolution LVSEM. Low energy electron microscopy (LEEM) instruments are very useful forsurface studies of metals and semiconductors. The electrostatic immersion objective lensis essentially necessary for LEEM to get a low landing voltage and low aberrations.There are three kind of immersion lenses: the magnetic immersion lens, the retardingelectrostatic immersion lens and the combination of immersion electrostatic and magneticlenses. The latter two lenses are reviewed by Mullerova and Lenc (1992)1. Here we have to