Electron imaging of dielectrics under simultaneous electron–ion irradiation

Abstract

We demonstrate that if charging caused by electron irradiation of an insulator is controlled by a defocused flux of soft-landing positive ions, secondary electron ͑SE͒ images can contain contrast due to lateral variations in ͑i͒ changes in the SE yield caused by subsurface trapped charge and ͑ii͒ the SE-ion recombination rate. Both contrast mechanisms can provide information on microscopic variations in dielectric properties. We present a model of SE contrast formation that accounts for localized charging and the effects of gas ions on the SE emission process, emitted electrons above the sample surface, and subsurface trapped charge. The model explains the ion flux dependence of charge-induced SE contrast, an increase in the sensitivity to surface contrast observed in SE images of charged dielectrics, and yields procedures for identification of contrast produced by localized sample charging.

Electron beam charging of insulators: A self-consistent flight-drift model

“…Incident electrons (so-called primary electrons PE) with initial energy E 0 and current density j 0 penetrate the insulator target up to the maximum range R(E 0 ) and create a mainly positive-negative spatial charge distribution (x) where the positive charge beneath the surface is due to secondary electron (SE) escape into vacuum (sample A) or into an electrode upon the surface (sample C). Moreover, a surface coverage by positive ions (sample B) becomes also possible when irradiating the insulating sample in residual gas pressure as we have already mentioned in context with environmental scanning electron microscopy (ESEM) techniques 28,29 in the introduction. According to the electrode arrangements A, B, or C the charges will create different field and potential distributions F (x) and The generated secondary electrons (SE) and holes (H) will be redistributed by the respective fields F (x) and potentials V (x) maintaining the selfconsistent charge transport in a planar (1-dimensional) model:…”

“…It leads to a dynamical equillibrium of charging-discharging at the surface and a resulting less negative surface potential of only about V 0 = −500 Volt, calculated and measured e.g. by M. Todt et al 28,29 . Thus we use this value for our simulation according to Fig.…”

“…There the ballistic flight of excited electrons and holes is followed by their drift and respective recombination and/or trapping in shallow and deep traps. Three types of non-conductive samples have been taken into account: A -the insulating bulk sample with an open surface to the vacuum, B -the bulk sample with a positive ion-covered surface as it is given in environmental secondary electron microscopy (ESEM) under a certain gas pressure (some Torr) and gas ionization 28,29 , and C -the conducting and grounded surface coated by a metal or carbon layer in order to prevent charging of the bulk insulator. Nevertheless, in the two latter cases strong internal electric fields will appear within the bulk insulator beneath the surface.…”

Charge contrast imaging of biomaterials in a variable pressure scanning electron microscope

“…As such, CCI is thought to be related to 'charge-trapping' at the sample surface and is in some way associated with the gaseous environment and electron-ion-sample interactions. The complex theory behind how these unique contrast variations are generated is largely unknown, but has been the topic of recent discussion and modelling (see Toth et al, 2002b;Stokes, 2003;Cazaux, 2004;Robertson et al, 2004;Thiel et al, 2004), and as such will not be discussed here.…”

Secondary electron imaging of nonconductors with nanometer resolution

“…In low vacuum SEM, positive gas ions neutralize ͑and hence generate holes͒ at the sample surface. 8 Hence, dq + / dt has a component corresponding to the ion neutralization cur-…”