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

Abstract

Key words. anomalous contrast, focused ion beam microscopy, insulators, mirror effect, scanning electron microscopy, trapped charge. SummaryUtilizing the ion beam of a focused ion beam (FIB)/scanning electron microscope (SEM) microscope to investigate nonconductive samples, we observe a mirror image very much similar to the one that is commonly obtained with the electron beam and the same samples. To our knowledge this is the first observation of what can be called 'Ion-Mirror Effect'. This effect is produced by a positive charging of the sample obtained by rastering with high-energy ions (30 kV) and a subsequent imaging with low energy ones (5 kV). The proposed explanation is that first a positive charge is trapped within the sample and eventually the lower energy ions are deflected back by the latter, and hit the surface of the microscope chamber very similar to what happens in the 'Electron-Mirror Effect'. The mirror image is produced after detection of the electrons produced by the interaction between ions and the chamber materials.

New way to overcome on the mirror effect for imaging the surface of insulator

“…If the sample is observed later (in a second phase), with a lower energy electron beam, the electric field can be strong enough to deflect the probing electron in the same manner as a convex mirror dose with light [11]. Recently a similar effect involving ions in a focused ion beam microscope (FIB) has been observed by [5].The electron mirror phenomenon in a scanning electron microscope (SEM) system has been reported by electron microscopists since the 1970s. For seemingly inexplicable reasons, instead of imaging a surface of interest, an image of the interior of the SEM appears.…”

“…In contrast, in a non-conductive sample, an excess charge is spatially trapped within the sample generating distortions in the beam path. In this condition an anomalous contrast can be detected [5]. The study of insulator charging effect in a SEM has led to several interesting observations.…”

“…The determination of charge number that is trapped on the surface of insulator may lead to important information about the mirror effect phenomena and the insulator properties [16,17]. The starting point for computing the trapped charges Q t is the following relation [5]:…”

“…For seemingly inexplicable reasons, instead of imaging a surface of interest, an image of the interior of the SEM appears. Traditionally, much effort has been extended to make sure this effect does not occur [5]. (10) II.…”

The Beam Current Considerations in SEM Accordance to Mirror Effect Phenomenon

“…Substantially, this phenomenon arises due to the accumulation of charges, electrons in SEM [7] and ions in FIB [8], at the sample surface during the irradiation process. Consequently, an electric potential begin to growth over the irradiated region up to the state of saturation [9].…”