The trajectories of secondary electrons in the scanning electron microscope

Abstract

Summary: Three-dimensional simulations of the trajectories of secondary electrons (SE) in the scanning electron microscope have been performed for plenty of real configurations of the specimen chamber, including all its basic components. The primary purpose was to evaluate the collection efficiency of the Everhart-Thornley detector of SE and to reveal fundamental rules for tailoring the set-ups in which efficient signal acquisition can be expected. Intuitive realizations about the easiness of attracting the SEs towards the biased front grid of the detector have shown themselves likely as false, and all grounded objects in the chamber have been proven to influence the spatial distribution of the signal-extracting field. The role of the magnetic field penetrating from inside the objective lens is shown to play an ambiguous role regarding possible support for the signal collection.

High‐Contrast SEM Imaging of Supported Few‐Layer Graphene for Differentiating Distinct Layers and Resolving Fine Features: There is Plenty of Room at the Bottom

“…If the collection efficiency (CE) is defined as the ratio of the number of SE1 plus SE2 collected by the E‐T SE detector to that emitted from the sample, then this hindering effect can be characterized. It has been reported that CE can reach as low as 0.05−0.2 for WD = 10 mm, indicating serious hindering effect at routine WD. Therefore, higher CE for SE1 plus SE2 is expected provided that this hindering effect can be diminished.…”

“…SE1 plus SE2 emitted from a sample are attracted and accelerated toward the side‐attached E‐T SE detector by the collector grid, which is generally positively biased with a +300 V voltage. However, acceleration of SE1 plus SE2 toward the detector could be hindered due to the presence of the pole piece, as illustrated in Figure a. If the collection efficiency (CE) is defined as the ratio of the number of SE1 plus SE2 collected by the E‐T SE detector to that emitted from the sample, then this hindering effect can be characterized.…”

Collection of secondary electrons in scanning electron microscopes

“…The detector is usually side‐positioned in the specimen chamber, just below the objective lens (OL). Only weak electrostatic field attracts the low energy SEs (Konvalina & Müllerová, 2006) and nearly no magnetic field exists in the specimen region in the traditional configuration.…”

On some contrast reversals in SEM: Application to metal/insulator systems

“…When a lateral detector is operated, the field effects pushing electrons emitted from a negative surface potential towards a positively biased grid may also lead to a possible increase of the apparent brightness of the pre-irradiated area. The collection efficiency of a lateral detector is normally very large so that the field amplification effect is expected to be less than for a coaxial detector but detailed calculations of trajectories are complicated because they involve working distance, z1, voltage drop between the detector's grid and the specimen surface as well as distance between the specimen and the detector [12]. For short working distances and large distances between the specimen and the grid, the SE trajectories would be mainly governed by the field between the specimen and the pole pieces and the amplification effect may be less than unity for the lateral detector to the advantage of electrons going towards the pole pieces.…”

“…Then this formation may be considered as a combination of a physical process, SE emission from a specimen, and of an instrumental process, SE detection. In this field, the abundant literature often reflects this sharing with, on the one hand, much work and from a long time on theory and experiments of electron emission from solids [4][5][6][7][8][9] but without attention to SE detection and, on the other , excellent research articles dealing with many important aspects of SE detection [10][11][12][13] but often postulating an isotropic or a Lambert (cosine) angular SE emission from the specimen or dealing with yield curves, d ¼ f(E1), restricted to an approximate sketch without any attention to specimen composition.…”