Jacques Cazaux

Some considerations on the electric field induced in insulators by electron bombardment

Starting from a simple model of the distributions of charge created in an insulator by bombardment with electrons, the components of the electric field are evaluated by using Maxwell’s equations and image effects. The results are applied to the most common experimental situations: a semi‐infinite sample (i) bounded by a vacuum or (ii) covered by a conducting film, and a sample in the form of a film (iii) unsupported or (iv) covering a conducting substrate. The results are compared to some experimental data concerning, for instance, electromigration and electron‐stimulated desorption. In surface analysis the decay of the Auger signal from ions of opposite charges and the opposite behavior of ions of the same charge are explained. Similar effects observed in electron‐probe microanalysis of glasses are also elucidated. The results concern scanning electron microscopy, transmission electron microscopy, and electron‐beam lithography applied to biological objects, polymers, ceramics, minerals, glasses, and elec...

Some considerations on the secondary electron emission, δ, from e− irradiated insulators

The so-called “total yield” approach often fails to explain the measured sign of the surface potential, VS, and the shift of the nominal critical energy EC2∘ (where δ°+η°=1) of electron irradiated insulators. Here, a simple modification of this approach consists in including some extra interactions of the secondary and backscattered electrons with the electron traps generated previously by the irradiation itself. The trends in the evolution of the total yield, δ+η, and of VS as a function of the irradiation time (from their initial values up to their steady values) are then deduced for a wide primary beam energy range (1–50 keV) and for different external collector (or specimen holder) bias. New mechanisms are suggested for the contrasts observed in insulators investigated in scanning electron microscopy (SEM). The present analysis applies for a wide variety of electron beam techniques (SEM, Auger electron spectroscopy, and electron probe microanalysis) operated on a wide variety of insulating specimens a...

Mechanisms of charging in electron spectroscopy

Abstract From the use of physical arguments based on classical electrostatics and elementary solid state physics, the role of the various parameters involved in the charging mechanisms of insulating materials is analysed in detail when these insulating specimens are investigated by surface analytical techniques (mainly XPS and e−AES). The role of the sub-surface composition and structure is outlined and the strong correlations between charging effects and some radiation damage effects are pointed out. Some strategies are also deduced to minimise these effects.

About the charge compensation of insulating samples in XPS

Abstract In electron spectroscopy, charging of insulating specimens is characterised by a normally steady surface potential,V S , which results from the equilibrium between the electron emission into the vacuum, I (out), and various compensation mechanisms contributing to I (in). Continuing from a recent paper (J. Cazaux, J. Electr. Spectrosc. Rel. Phen. 105, 1999, 155), this emission and these compensation mechanisms (including the role of flood guns) are analysed here in more detail for a situation mainly restricted to conventional XPS. In particular, a new graphical representation can be used to explain why the irradiation conditions and the environment of the specimen play a role on the charging shift, V S , and on differential charging, ΔV S , that is more important than its exact chemical composition. A critical analysis of the negative (and positive) biasing of the specimen is also performed. Various practical consequences are deduced from an approach based on I(in)=I(out) at the steady state. Additional electrostatic considerations are given, extending the investigation to small spot XPS and some consequences may also be easily transposed to SEM (scanning electron microscopy) and to AES (Auger electron spectroscopy).

Scenario for time evolution of insulator charging under various focused electron irradiations

The expected time evolution of the secondary electron emission (SEE) yield, δ, and charging of insulators irradiated with keV electron probes of various sizes is deduced from the use of basic laws of electrostatics. Simple models of trapped charge distributions permit, next, quantitative estimates. With respect to the linear increase of δ from 0 towards its nominal value, δ0, the initial phase is characterized by a deficit of δ for incident spots in the submicron range and incident charge of a few 102 primary electrons (PEs). This deficit occurs even when charging is as a whole negative and it results from a partial mirror effect for the less energetic δP secondary electrons (SEs) (directly excited by the PEs) combined to attraction of δBSE (excited by the backscattered electrons) towards the central spot while the more energetic δp SEs are successively focused and next defocused. The next phase starts for incident charges in the pC range and it concerns the evolution of the total yield, δ+η from its nomi...

Correlation between the x-ray induced and the electron-induced electron emission yields of insulators

Similarly to the constant loss model used for expressing the electron-induced secondary electron yield from solids, δ, a simple model is used for expressing the x-ray-induced electron emission yield, δX, from uncharged insulators. The two yields involve common mechanisms for the transport and the escape of the secondary electrons and are sensitive to the crystalline state, to the temperature and to the surface composition of the investigated insulator. For practical purposes, it is thus possible to estimate δX of the insulator of interest from the knowledge of δ and vice versa. In particular, these two yields may be related to each other by an expression of the form: δX/δ=μR hν/E0 (with μ linear absorption coefficient of the material of interest for incident photons of energy hν) when the range, R, of incident electrons (of energy E0) is far larger than the escape depth, s, of the secondary electrons. Deduced from some published δ(E0) curves, the illustrations concern the evolution in the photon energy ra...

Reflectivity of very low energy electrons (< 10 eV) from solid surfaces: Physical and instrumental aspects

The impact of very low energy electrons (VLEE) on solid surfaces plays an important role in various fields of modern technology. Plasma physics, space research and particle-accelerators and progress in these fields are based partly on investigation of VLEE emission and reflection properties as obtained from laboratory measurements. Here the influence of the material composition and of the angle of incidence on the reflectivity, R, of VLEE is derived by the use of simple quantum mechanical arguments showing a rapid decrease of R from 100% when the incident energy of electrons increases from 0 eV while the surface sensitivity increases. The measurements depend significantly on the potential referencing between the electron source, the sample, and the detector, as well as of the energy spread of the incident electrons. VLEE thin film transmission is briefly considered and various practical consequences of the contrasts (crystalline, topographic, doping) as reported in scanning low energy electron microscopy ...

A new experimental approach for characterizing the internal trapped charge and electric field build up in ground-coated insulators during their e− irradiation

Abstract An original method is proposed to investigate the dynamical trapping properties of bulk insulators during their irradiation by keV electrons when they are coated with a grounded metallic film. This method is based on the measurement of the displacement current and it allows to evaluate time constants for charging and discharging the dielectric as well as to evaluate the electric field build up and trapped charge density below the coating. This method is illustrated by the estimate of the charging and discharging time constants in e − irradiated PMMA and the estimate of the magnitude of the electric field which drives the migration of the mobile ions in e − irradiated glasses.

Calculated effects of work function changes on the dispersion of secondary electron emission data: Application for Al and Si and related elements

The published secondary electron yield (SEY) data, δ = f(E°), are characterized by a poor level of agreement, rarely more than 25% and lower for a common element such as Al. Some possible sources of discrepancies are related to sample preparation, leading to differences in surface composition (contamination and oxidation). This is theoretically explored by a quantitative estimate of a change of the work function, φ (or electron affinity, χ), on the escape probability, A, of secondary electrons (SEs) and consequently on the SEY data of clean and oxidized samples. An electron-stimulated reduction effect may also explain the change of the SEY with the incident electron dose (fluence). Deduced from analytical expressions for the SE angular and energy distributions, ∂δ/∂α and ∂δ/∂Ek, respectively, the present investigation also includes instrumental effects due to work function differences when a partial angular collection of SEs is conducted. Although it is illustrated here for Al and Si, the present methodol...

A random walk model for the crystallite size effect on the secondary electron yield from insulators

A recent model for the random walk of low energy electrons is applied to the crystallite-size effect on the secondary electron yield δ from insulators. The corresponding fitting procedure is successfully applied to the experimental results previously obtained by Ushio et al. on various MgO films (Thin Solid Films 167 (1988) 299). The values of the transport mean free path and of the maximum path length r for the SE in this type of MgO specimen are deduced for the room temperature. The difference between crystalline size effects and the influence of other parameters such as topographic effects or surface composition is established. Applications of the same model to X-ray induced electron emission yield δX from insulators are also illustrated for MgO. Some other practical consequences, such as a new type of (crystallite size) contrast in scanning electron microscopy, are suggested.