Analysis of electron range versus energy relationship of insulators in low‐voltage scanning electron microscopy

Abstract

A theoretical analysis is made concerning the question of whether or not an insulator's surface is observable without the unstable disturbance due to negative charge-up in the secondary electron mode of a low-voltage scanning electron microscope. Introducing a simple modification into the elementary theory of secondary electron emission from solid materials, the threshold condition as to observability is formulated as a function of the energy E,, and the incident angle Qp of the primary beam. It is shown that for insulators the material's constant n, which appears in the standard formula of the electron range ( R ) versus energy (Ep) relationship R E,", can be determined through experimental investigations into observability of the surface. Careful consideration is also given to the effectiveness of the present theoretical analysis. and Ikeda 1983), since electron irradiation at a lower accelerating voltage causes less damage to specimens. In addition, at a low accelerating voltage it is not uncommon for SEM operators to observe the insulator surface in the secondary electron (SE) mode without the unstable disturbance inherent in negative charge-up. They know that observability depends also on conditions of the energy El, and the incident angle @, , of PEs.Uchikawa and co-workers (Sugiyama et al. 1986, Uchikawa and Ikeda 1983 carried out systematic experiments as to the observability of SiOz-passivated devices, and worked out a simple empirical formula of conditions for observability without the unstable disturbance, such as

Scanning electron microscopy of nonconductive specimens at critical energies in a cathode lens system

“…Sugiyama et al (1986) verified this dependence experimentally on microelectronic devices coated with a nonconductive passivation layer. The formula is easy to derive under the assumption that the charge-building process is primarily dependent on the penetration depth of primary electrons so that quantitatively it is governed by the normal component vcosθ of the primary beam velocity v. Ishibashi et al (1992) studied a threshold energy E th = E P cos 2 θ , below which an insulator can be observed without unstable image disturbances. They found E th generally not constant; for example, for Si 3 N 4 , a relation E th~ (cosθ ) -0.5 was obtained, which corresponds to even stronger E II -θ dependence than that mentioned above.…”