Collection of secondary electrons in scanning electron microscopes

Abstract

SummaryCollection of the secondary electrons in the scanning electron microscope was simulated and the results have been experimentally verified for two types of the objective lens and three detection systems. The aberration coefficients of both objective lenses as well as maximum axial magnetic fields in the specimen region are presented. Compared are a standard side-attached secondary electron detector, in which only weak electrostatic and nearly no magnetic field influence the signal trajectories in the specimen vicinity, and the side-attached (lower) and upper detectors in an immersion system with weak electrostatic but strong magnetic field penetrating towards the specimen. The collection efficiency was calculated for all three detection systems and several working distances. The ability of detectors to attract secondary electron trajectories for various initial azimuthal and polar angles was calculated, too. According to expectations, the lower detector of an immersion system collects no secondary electrons I and II emitted from the specimen and only backscattered electrons and secondary electrons III form the final image. The upper detector of the immersion system exhibits nearly 100% collection efficiency decreasing, however, with the working distance, but the topographical contrast is regrettably suppressed in its image. The collection efficiency of the standard detector is low for short working distances but increases with the same, preserving strong topographical contrast.

Material contrast in SEM: Fermi energy and work function effects

“…In a SEM, the point sources are scarcely surrounded by concentric potential surfaces and calculations of SE trajectories are basically a problem of electron optics in the vacuum gap where the role of magnetic fields or of biased electrodes has been taken into account [2,43]. Such calculations are specific to each instrument and operating mode but an isotropic emission is often postulated for evaluating the detected intensities.…”

“…(2), I e corresponds to the total collection of the emitted SE, O1=2p, and d d is I d /I1 when O1o2p. The goal of the present subsection is to evaluate k a for Au and Si using analytical expressions describing their angular and energy distributions.…”

“…In such a situation, the work function difference, limited to a few eV, is negligible for larger biases but the material dependence of k f remains, via the specific SE angular distribution and the collection of low energy SEs being emitted along directions oblique to the field direction, but experiencing large deflections towards this field direction. Despite some possible simplifications, such a global approach is obviously a challenging task, and comparison to SEM images is complicated by the initial assumption being scarcely satisfied [2]: only true SEs are collected by detectors.…”

“…11. A more precise evaluation requires the use of electron optical calculations, specific to the sample surroundings of each experimental arrangement [2,43], e.g. type of microscope, working distance, position, calibration and response function of detectors, etc.…”

“…The first question concerns the nature of the signal being detected: Is the image only formed with true secondary electrons (SE)? Details of the answer have been indicated elsewhere [2]. In SEM instruments, all the emitted SEs cannot be collected and only a fraction of them, kd, are.…”