Fresnel Diffraction and Lattice Plane Images in a Conventional STEM

According to the theorem of reciprocity, as related to STEM and CTEM images, identical images should be formed under equivalent operating conditions in the two modes of microscopy. Experimental results have shown this to be generally true for diffraction contrast images. The electron optical parameters are such that higher resolution is usually achieved in CTEM, and the advantages of image processing and increased sample penetration are realized in STEM. Electron diffraction patterns can be obtained by both, with better angular resolution in CTEM and very small selected areas (< 25 Å) attainable in STEM. The equivalence of phase contrast images has been demonstrated by means of Fresnel diffraction and lattice plane images. All the STEM results reported have been obtained using high brightness field emission or LaB6 guns. Incident probe coherence and poor signal/noise have effectively excluded the use of conventional tungsten hair-pin sources.

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Practical Correction of Three Fold Astigmatism in the Philips CM TEM

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100164556 ◽

1996 ◽

Vol 54 ◽

pp. 418-419

Author(s):

Arno J. Bleeker ◽

Mark H.F. Overwijk ◽

Max T. Otten

Keyword(s):

Optical Properties ◽

Phase Contrast ◽

The Other ◽

Objective Lens ◽

Symmetric Part ◽

A Posteriori ◽

Contrast Transfer Function ◽

High Brightness ◽

Rotationally Symmetric ◽

Brightness Field

With the improvement of the optical properties of the modern TEM objective lenses the point resolution is pushed beyond 0.2 nm. The objective lens of the CM300 UltraTwin combines a Cs of 0. 65 mm with a Cc of 1.4 mm. At 300 kV this results in a point resolution of 0.17 nm. Together with a high-brightness field-emission gun with an energy spread of 0.8 eV the information limit is pushed down to 0.1 nm. The rotationally symmetric part of the phase contrast transfer function (pctf), whose first zero at Scherzer focus determines the point resolution, is mainly determined by the Cs and defocus. Apart from the rotationally symmetric part there is also the non-rotationally symmetric part of the pctf. Here the main contributors are not only two-fold astigmatism and beam tilt but also three-fold astigmatism. The two-fold astigmatism together with the beam tilt can be corrected in a straight-forward way using the coma-free alignment and the objective stigmator. However, this only works well when the coefficient of three-fold astigmatism is negligible compared to the other aberration coefficients. Unfortunately this is not generally the case with the modern high-resolution objective lenses. Measurements done at a CM300 SuperTwin FEG showed a three fold-astigmatism of 1100 nm which is consistent with measurements done by others. A three-fold astigmatism of 1000 nm already sinificantly influences the image at a spatial frequency corresponding to 0.2 nm which is even above the point resolution of the objective lens. In principle it is possible to correct for the three-fold astigmatism a posteriori when through-focus series are taken or when off-axis holography is employed. This is, however not possible for single images. The only possibility is then to correct for the three-fold astigmatism in the microscope by the addition of a hexapole corrector near the objective lens.

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Advantages of a Field Emission Gun for a Combined Analytical and High Resolution Transmission Electron Microscope

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s1431927600000313 ◽

1980 ◽

Vol 38 ◽

pp. 72-73

Author(s):

J. Bentley

Keyword(s):

High Resolution ◽

Field Emission ◽

Operating Conditions ◽

Objective Lens ◽

High Brightness ◽

Transmission Electron ◽

Tem Mode ◽

Brightness Field ◽

High Resolution Microscopy ◽

Current Exposure

This paper describes the various areas of analytical and high resolution microscopy which can be greatly improved by the use of a high-brightness field emission gun (FEG). The instrument used was a Philips EM400T equipped with a FEG, 6585 STEM unit, EDAX EDS detector and Kevex 5100 spectrometer. The <111> oriented W tip was supplied by the manufacturer. The brightness β (current density per unit solid angle) normalized to the ac-celeratiang voltage, V0 is defined by β = 4I/πd2α2jV0, where I is the current in a probe of diameter d and divergence αi. Results are presented in Table 1 for three typical operating conditions. Probe currents >10−7 A have been obtained in the TEM mode which is sufficient for work at medium magnifications (20 to 100 K). Probe currents were measured from the screen current/exposure time system which had been calibrated with a purpose built Faraday cup. Probe diameters in TEM were measured from high magnification TEM images and in STEM by imaging the STEM raster in the TEM mode. This is possible because of the symmetric objective lens which can operate at the same excitation in TEM and STEM. An example is shown in Fig. 1. The values in Table 1 should be compared to conventional W hairpin sources for which β ≅ 1.

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Observation of Phase Contrast Images in STEM and CTEM Modes by Using 100 kV Field Emission Electron Gun

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100051967 ◽

1974 ◽

Vol 32 ◽

pp. 390-391

Author(s):

Y. Harada ◽

T. Goto ◽

H. Koike ◽

T. Someya

Keyword(s):

Field Emission ◽

Phase Contrast ◽

Image Formation ◽

Fresnel Diffraction ◽

Experimental Results ◽

Electron Gun ◽

Scanning Microscopy ◽

Emission Electron ◽

Lattice Images

Since phase contrasts of STEM images, that is, Fresnel diffraction fringes or lattice images, manifest themselves in field emission scanning microscopy, the mechanism for image formation in the STEM mode has been investigated and compared with that in CTEM mode, resulting in the theory of reciprocity. It reveals that contrast in STEM images exhibits the same properties as contrast in CTEM images. However, it appears that the validity of the reciprocity theory, especially on the details of phase contrast, has not yet been fully proven by the experiments. In this work, we shall investigate the phase contrast images obtained in both the STEM and CTEM modes of a field emission microscope (100kV), and evaluate the validity of the reciprocity theory by comparing the experimental results.

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A Stable Field Emission Electron Source

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100077918 ◽

1977 ◽

Vol 35 ◽

pp. 58-59

Author(s):

W.R. Bottoms ◽

G.B. Haydon

Keyword(s):

Field Emission ◽

Signal To Noise Ratio ◽

High Brightness ◽

Electron Sources ◽

Brightness Field ◽

Current Densities ◽

Properties Of Materials ◽

Stable Field ◽

Stable Current ◽

Careful Design

There is great interest in improving the brightness of electron sources and therefore the ability of electron optical instrumentation to probe the properties of materials. Extensive work by Dr. Crew and others has provided extremely high brightness sources for certain kinds of analytical problems but which pose serious difficulties in other problems. These sources cannot survive in conventional system vacuums. If one wishes to gather information from the other signal channels activated by electron beam bombardment it is necessary to provide sufficient current to allow an acceptable signal-to-noise ratio. It is possible through careful design to provide a high brightness field emission source which has the capability of providing high currents as well as high current densities to a specimen. In this paper we describe an electrode to provide long-lived stable current in field emission sources.The source geometry was based upon the results of extensive computer modeling. The design attempted to maximize the total current available at a specimen.

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Partially coherent Fresnel diffraction by a slit aperture. III. Fresnel diffraction patterns

Optics Communications ◽

10.1016/0030-4018(73)90175-2 ◽

1973 ◽

Vol 8 (1) ◽

pp. 33-36 ◽

Cited By ~ 5

Author(s):

T. Asakura

Keyword(s):

Fresnel Diffraction ◽

Partially Coherent ◽

Diffraction Patterns

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The lamina splendens of articular cartilage is an artefact of phase contrast microscopy

Proceedings of the Royal Society of London Series B Biological Sciences ◽

10.1098/rspb.1979.0094 ◽

1979 ◽

Vol 206 (1162) ◽

pp. 109-113 ◽

Cited By ~ 15

Keyword(s):

Surface Layer ◽

Articular Cartilage ◽

Phase Contrast ◽

Fresnel Diffraction ◽

Refractive Indices ◽

Phase Contrast Microscopy ◽

Bright Line ◽

Contrast Microscopy ◽

Diffraction Effects

The so-called lamina splendens of articular cartilage is shown to be a characteristic of phase contrast microscopy; this technique provides no evidence for an anatomically distinct surface layer. Fresnel diffraction occurs at edges separating regions of different refractive indices. These diffraction effects, when viewed under phase contrast, lead to the appearance of a bright line along the edge.

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