Charge Implantation Measurement on Electron-Irradiated Insulating Materials by Means of a SEM Technique

Abstract

The goal of this article is first to review the charging effects occurring when an insulating material is subjected to electron irradiation in a scanning electron microscope (SEM) and next their consequences from both scanning electron microscopy and electron probe microanalysis (EPMA) points of view. When bare insulators are observed, the so-called pseudo mirror effect leads to an anomalous contrast and also to an erroneous surface potential, VS, measurement when a Duane–Hunt limit (DHL) method is used. An alternative possibility is to use an electron toroidal spectrometer (ETS), specially adapted to a SEM, which directly gives the VS value. In the case of a bulk specimen coated with a grounded layer, although the layer prevents external effects of the trapped charge, the electric field beneath the coating is reinforced and leads to loss of ionizations that reduces the number of generated X-ray photons. To take into account both effects mentioned above, whether the studied insulator is coated or not, a method is proposed to deduce the trapped charge inside the insulator and the corresponding internal or external electric field.

Differential cross section for reflected electrons measured by electron mirror method

“…When nonconductive samples, such as PVC, 1 a-SiO 2 , 2 polyester, 3 and a polyethylene terephthalate (PET) irradiated by electron radiation with sufficiently high energy, the scanning electron microscope analysis fails. [4][5][6] When charged particles impinge on a solid they can impart energy, exciting electrons within the material. Due to the localization of the injected electrons and secondary emission as well as defects produced during electron irradiation, the sign of the trapped charge and the resulting macroscopic surface potential may locally change between positive and negative.…”

Charge regulation mechanism of grounded-coated insulators

“…This fraction of trapped charge may disturb the micro beam analysis and leads to loss of ionizations that reduces the number of generated X-ray photons. It is worth noticing that only negative charges are implanted below the surface, leading to a negative trapped charge density [1]. To take into account these effects, an original method is proposed for such a dynamical investigation of the trapping proprieties of coated insulating samples submitted to electron irradiation in a scanning electron microscope (SEM).…”

Observation of the ion‐mirror effect during microscopy of insulating materials

“…The EME has been under investigation by many SEM users both to understand its working principles and how to avoid it, because it actually impairs the conventional imaging of the sample. The effort to understand its working principles has led to the following picture (Clarke & Stuart, 1970; Vallayer et al , 1999; Wintle, 1999; Jbara et al , 2004).…”

“…In such conditions the sample acquires a net negative charge, that is a number of electrons remains trapped within the sample itself. The trapped charge Q e ( t ) shows exponential dynamics that can be expressed (Belhaj et al , 2000b; 2001b; Jbara et al , 2002, 2004) by in which Q ∞ e ( t ) is the saturation charge limit for very large irradiation time, while τ e is the time constant of electron charging, given by the relation in which, σ (Δ V s ) is the secondary electron yield depending on the accelerating scanning potential Δ V s and I e is the electron current.…”

“…If the two latter quantities are known, as well as the electron current, the secondary electron yield can be calculated. For example, in reference Jbara et al , 2004 a sample is irradiated with Δ V s = 24 kV electrons with a current of I e = 2 nA for a total time of Δ t = 60 s providing a saturated charge of Q ∞ e ≈ 2.9 nC and a characteristic time of τ e ≈ 12 s . From these data and , the secondary electron yield turns out to be σ (Δ V s ) = 0.88, which is less than unity, as expected.…”