Electron Backscattering from Real and In-Situ Treated Surfaces

Abstract

Signi®cant differences in backscattered electron (BSE) yields exist between the surfaces cleaned by methods used in electron microscopy and spectroscopy. These differences have been observed for Au, Cu and Al specimens, and are interpreted on the basis of simulated BSE yields. Composition and thickness of the surface contamination layers, responsible for the differences, are estimated. The results (7 nm of carbon on Au or 3 nm of oxide on Al) remain within expectation and indicate that the BSE yield measurements and BSE images should be interpreted cautiously. Peculiar results are obtained for Cu, perhaps due to a different cleaning procedure. A new concept of an information depth for the BSE signal is introduced as a depth within which the total BSE yield can be modelled as composed of the yields of layers proportional to their thickness weighted by the escape depths. This concept proved satisfactory for thin surface layers and brought the information depth values 2 to 4 times smaller than ®rst estimated, i.e. half the penetration depth.

Scanning Electron Microscopy with Samples in an Electric Field

“…One is the size of interaction volume achieving units of micrometers at tens of keV. Although the SE are emitted only from a shallow subsurface layer the thickness of which is governed by the rate of absorption of hot electrons and is therefore significantly thicker for nonconductors but still not exceeding 10 to 20 nm, the BSE can escape from substantial part of the interaction depth that approaches 50% of this depth at low keV [ 21 ]. Moreover, the lateral diffusion of primary electrons extends the information spot dimension far behind the primary spot dimension so the “real” resolution is generally much worse than the nominal one [ 22 ].…”

About the information depth of backscattered electron imaging

“…Information depth has then been simply taken as a fraction of this penetration depth. Most usually this fraction has been set at 0.5, but some authors state a fraction much smaller such as 0.2 (Richards & ap Gwynn, ; Richards et al ., ), around 0.25 and decreasing with growing energy (Rau & Reimer, ), or between 0.12 and 0.25 (Frank et al ., ). At http://www.emal.engin.umich.edu/courses/sem_lecturecw/SEM_bse2.html, we found the following Eq.…”

“…One study compared landing energy dependences of the total BSE yield from several atomically clean (in situ cleaned) materials and the same materials in the asinserted status, that is, with spontaneously generated surface coatings. The thickness of coatings was then estimated according to Monte Carlo simulations of the BSE yields from both clean and contaminated samples (Frank et al, 2000). Alternatively, intentionally deposited surface coatings of tens of nm in thickness were observed and their thickness was shown to project itself in the energy spectra of BSE released by 10, 20 and 30 keV primary electrons as the ID-indicating parameter (Schlichting et al, 1999).…”

Scanning electron microscopy of partially de-hydrogenated MgH2 powders

“…The contrast among these phases in an SEM observation is based on their relative electron yields. Considering the backscattered electron signal, we can notice that the corresponding yield is a function of the average atomic number of the target and it is classically described by the following equation [34,35], holding for a relatively high value of the primary electron energy [36]:…”