Detection of Topographic Contrast in the Scanning Electron Microscope at Low and Medium Resolution by Different Detectors and Detector Systems

Origins of topographic contrast in the scanning electron microscope (SEM) are different at different resolution levels. At low resolution, tilt contrast of large features dominates; at medium resolution, diffusion contrast of features smaller than an interaction volume of primary electrons dominates. The secondary electron (SE) signal, commonly used in the SEM, does not give a good tilt contrast; better contrast can be obtained with backscattered electron (BSE) signal of a converter and a sector-shaped ring detector. For obtaining topographic images from signals containing topographic and material contrast, signals of detector systems containing two or more detectors are mixed. Detector systems containing BSE detectors give more reproducible signals with a more uniform dependence on tilt angles than systems containing SE detectors. Tilt contrast of specimens coated with thin layers of heavy metals is similar to the contrast of uncoated specimens in the case of an SE detector, and better tilt contrast can be obtained with a sector-shaped ring BSE detector. Diffusion contrast dominates at medium resolution. Contrast obtained with three selected detectors: SE detector, sector-shaped ring BSE detector, annular top BSE detector, is also discussed. The contrast is lowest for the top BSE detector and highest in the case of SE detection, especially for materials of low density. In the case of coated specimens, the SE detector and the sector-shaped ring BSE detector give good contrast and both are suitable for medium resolution studies. The discussion in the paper concerns untilted or slightly tilted specimens

Divergent-Beam X-Ray Diffraction in the Scanning Electron Microscopy and its Use for the Study of the Semiconductor Epitaxial Layers

Among the X-ray techniques belonging to the family of Kassel methods the divergent-beam diffraction method gives the best contrast for semiconductor specimens. The technique has been accomplished in the scanning electron microscope (SEM) in a back reflection configuration. Epitaxial layers of GaAsSb and GaAsSbP on GaAs [100] oriented substrates were investigated. The diffraction lines from lattice planes giving only high Bragg angles were used. For the purpose of the layer strain analysis, the diffraction experiments were carried out for specimens placed horizontally (the lines {711} type recorded) and for tilted 45° (the lines {551) type recorded). A Cu foil was used as a target material

A Ring Scintillation Detector for Detection of Backscattered Electrons in the Scanning Electron Microscope

A backscattered electron detector with a cylindrical detecting surface has been constructed and installed in a scanning electron microscope. The detector surrounds the specimen and accepts electrons emitted into a specific range of zenith angles. In the case of untilted specimens it collects electrons emerging from the specimen surface at low exit angles relating to it. This enables us to obtain a good resolution of images of untilted specimens. Moreover, the detect or gives very high level of topographic contrast and good three-dimensional impression of the specimen shape

Signal Mixing Technique for Backscattered Electrons in the Scanning Electron Microscope

Signal mixing technique using asymetrically placed backscattered electron detectors in a scanning electron microscope is presented in this paper. Two types of detectors have been used: a low-take off angle ring scintillation detector (placed around the specimen) and a wide-angle semiconductor detector (placed above the specimen). It has been shown that the discussed configuration gives good real topography in all directions on the specimen surface and also reduces significantly the pseudo-topography effect of flat grain boundaries

Detection System for Scanning Electron Microscope

The kind of information and the quality of the backscattered electron (BSE) images depend upon numerous features of the detector. Therefore, various types of detectors should be used simultaneously to obtain as much of information as possible. The detection system presented here contains a large area semiconductor detector and a BSE to secondary electrons (SE) converter system. These two different kinds of detectors give different BSE images. After subtracting the signal of a semiconductor detector from that of a converter system, an image with good topography and reduced material contrast can be achieved

Influence of the cooling bypass on the aircraft nozzle outflow

UL-39 ultralight aircraft has been developed at the Department of Aerospace Engineering, Czech Technical University in Prague [3]. This aircraft is powered by unique propulsion system, which consists of a fan driven by piston engine. Various concepts of the cooling system with various radiator positions were studied. Radiator in the bypass channel behind fan is used at the flying prototype of UL-39. Test bed with the model fan was build in order to conduct test and experiments for the development of this propulsion system. Model of the UL-39 outlet duct is placed behind the fan, see Fig. 1. Radiator pressure loss is simulated by various inserts including clear frame (i.e., without pressure loss), honeycomb and screen. Bypass channel is is on the upper side of the outlet duct, i.e., on the opposite side than at the real UL-39 aircraft. Flow field behind the nozzle in the plane of symmetry is measured for each case by PIV (Particle Image Velocimetry) method for each case. Measurements were done in the wind tunnel at the Department of Aerospace Engineering. Fan RPM between 20,000 and 35,000 were used

Nucleotide sequence analysis of the inversion termini located within IS3 elements α3β3 and β5α5 of Escherichia coli K-12

This paper presents the first detailed structural analysis of termini of an inversion mediated by recombination between Escherichia coli native IS elements. The complete nucleotide sequence of the inversion termini in the lactose region of Escherichia coli K-12 confirms our previous suggestion that the inversion occurred by homologous recombination between α3β3 and β5α5 IS3 elements (D.J. Savic, J. Bacteriol. 140:311-319, 1979; D.J. Savic, S. Romac, and S.D. Ehrlich, J. Bacteriol. 155:943-946, 1983). The data show a slight structural divergence of α3β3 and β5α5 elements, but they do not reveal new sequences within recomhined IS3 elements that could influence the expression of nearby genes

Deformed General Relativity and Torsion

We argue that the natural framework for embedding the ideas of deformed, or doubly, special relativity (DSR) into a curved spacetime is a generalisation of Einstein-Cartan theory, considered by Stelle and West. Instead of interpreting the noncommuting "spacetime coordinates" of the Snyder algebra as endowing spacetime with a fundamentally noncommutative structure, we are led to consider a connection with torsion in this framework. This may lead to the usual ambiguities in minimal coupling. We note that observable violations of charge conservation induced by torsion should happen on a time scale of 10^3 s, which seems to rule out these modifications as a serious theory. Our considerations show, however, that the noncommutativity of translations in the Snyder algebra need not correspond to noncommutative spacetime in the usual sense.Comment: 20 pages, 1 figure, revtex; expanded sections 3 and 4 for clarity, moved material to appendix B, corrected a few minor error

Chd1 protects genome integrity at promoters to sustain hypertranscription in embryonic stem cells

Stem and progenitor cells undergo a global elevation of nascent transcription, or hyper- transcription, during key developmental transitions involving rapid cell proliferation. The chromatin remodeler Chd1 mediates hypertranscription in pluripotent cells but its mechanism of action remains poorly understood. Here we report a novel role for Chd1 in protecting genome integrity at promoter regions by preventing DNA double-stranded break (DSB) accumulation in ES cells. Chd1 interacts with several DNA repair factors including Atm, Parp1, Kap1 and Topoisomerase 2βand its absence leads to an accumulation of DSBs at Chd1-bound Pol II-transcribed genes and rDNA. Genes prone to DNA breaks in Chd1 KO ES cells are longer genes with GC-rich promoters, a more labile nucleosomal structure and roles in chromatin regulation, transcription and signaling. These results reveal a vulnerability of hypertranscribing stem cells to accumulation of endogenous DNA breaks, with important implications for developmental and cancer biology