Practical Correction of Three Fold Astigmatism in the Philips CM TEM

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.

Fresnel Diffraction and Lattice Plane Images in a Conventional STEM

1975 ◽  
Vol 33 ◽  
pp. 220-221
Author(s):  
R. H. Geiss ◽  
D. G. Howitt ◽  
H. Arnal
Keyword(s):  
Phase Contrast ◽  
Angular Resolution ◽  
Fresnel Diffraction ◽  
Operating Conditions ◽  
Optical Parameters ◽  
Lattice Plane ◽  
High Brightness ◽  
Brightness Field ◽  
Diffraction Patterns ◽  
Generally True

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.

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

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.

Phase Contrast Images of Periodic and Quasi-Periodic Objects

1967 ◽  
Vol 25 ◽  
pp. 234-235
Author(s):  
T. Komoda
Keyword(s):  
Contrast Effect ◽  
Phase Contrast ◽  
Periodic Structures ◽  
Present Report ◽  
Objective Lens ◽  
Electron Microscopic ◽  
Contrast Transfer Function ◽  
Electron Microscopic Image ◽  
Optical Transfer ◽  
Illuminating Beam

The phase contrast effect due to defocussing of an objective lens is decisive factor for imaging of molecular details in electron microscopy. The effect was intensively investigated by Thon) for amorphous objects. The present report is concerned with the phase contrast effect for the objects having periodic structures such as crystals and that of quasiperiodic ones.The contrast of the electron microscopic image has been theoretically treated by Hanszen) within the frame of the optical transfer theory. Phase contrast images are explained with a phase contrast transfer function, which shows the selective imaging of Fourier spectrum of spacial frequencies (reciprocal of periods) in the potential distribution in the object. The function diminishes under the conventional operating condition of the electron microscope. Fluctuation of focus and angular divergence of the illuminating beam are predominant factors for the diminishing.

The information limit of a 200kv field emission TEM

1995 ◽  
Vol 53 ◽  
pp. 586-587
Author(s):  
T. Kaneyama ◽  
M. Kawasaki ◽  
T. Tomita ◽  
T. Honda ◽  
M. Kersker
Keyword(s):  
Thin Film ◽  
Electron Microscope ◽  
Field Emission ◽  
Phase Contrast ◽  
Mechanical Stability ◽  
Power Supplies ◽  
Objective Lens ◽  
Operating Parameters ◽  
Contrast Transfer Function ◽  
Transmission Electron

The Point resolution of a transmission electron microscope is normally defined by the reciprocal of the spatial frequency of the first zero in the phase contrast transfer function at the Scherzer defocus condition. When a field emission gun (FEG) is used as the electron source, the information limit, that point at which the contrast beyond the first zero goes to zero contrast, becomes equally important. We have investigated the primary microscope parameters that affect the information limit.A 200kV FE-TEM (JEM-2010F) equipped with a ZrO/W shottkey emitter and Gatan Parallel EELS (PEELS) was used for the experiments. The aberration coefficients of the objective lens are Cs = 1mm and Cc = 1.4mm. The specimen used is an evaporated amorphous Ge thin film with small gold islands.The resolution performance of the microscope depends not only on the performance of the objective lens, the high voltage stability, stability of the lens and deflector power supplies, operating parameters of the FEG, and the overall mechanical stability of the microscopes.

Texture details in experimental lattice images from a monolamellar paraffin crystal

1986 ◽  
Vol 44 ◽  
pp. 182-183
Author(s):  
D. L. Dorset ◽  
F. Zemlin ◽  
E. Reuber ◽  
E. Beckmann ◽  
E. Zeitler
Keyword(s):  
Crystal Structure ◽  
Electron Microscope ◽  
Phase Contrast ◽  
Objective Lens ◽  
Direct Visualization ◽  
Routine Procedure ◽  
Contrast Transfer Function ◽  
Lattice Images ◽  
Similar Images ◽  
Beam Damage

The direct visualization of crystal structure at "molecular" (ca 3Å) resolution has become a routine procedure in electron microscopy in the last few years for organic materials which are resistant to electron beam damage by virtue of π-electron derealization or electrical conductivity. More recently, similar images from an aliphatic material, i.e. the paraffin n-tetratetracontane, were published based on work with an electron microscope equipped with a He-cooled superconducting objective lens. Correlation-averaged electron images at 2.5A resolution were shown to correspond well to a theoretical image based on a multislice calculation for the known crystal structure and produced at the phase contrast transfer function of the electron microscope objective lens for the defocus value used in the experiment.

Correlation of cryo-electron microscopic and x-ray data and compensation of the contrast transfer function

1992 ◽  
Vol 50 (2) ◽  
pp. 996-997
Author(s):  
R. Holland Cheng
Keyword(s):  
Transfer Function ◽  
Phase Contrast ◽  
Objective Lens ◽  
Biological Macromolecules ◽  
Electron Microscopic ◽  
Contrast Transfer Function ◽  
Image Reconstruction Techniques ◽  
X Ray ◽  
Mass Distributions ◽  
Spatial Frequencies

Cryo-electron microscopy (cryoEM) along with image reconstruction techniques can produce vivid images of biological macromolecules in their “native” state, although objective interpretation of these images is influenced by the fact that the contribution of phase contrast greatly exceeds that of amplitude contrast in such weakly scattering objects. The microscope contrast transfer function (CTF), which is strongly dependent on the defocus level of objective lens, modulates images of the object density distribution as a function of spatial frequency. Compensation for the effects of phase contrast transfer is important because underweighting of the low spatial frequencies usually causes difficulties in evaluating absolute mass distributions in objects.Correct compensation for the CTF is difficult to achieve. This is due, in part, to ambiguities in measuring the exact defocus level in noisy micrographs, and in knowing the relative contributions of amplitude and phase contrast, beam coherence, and inelastic scattering. The availability of atomic resolution determinations for a few viruses allows one to determine empirically how to correct the cryoEM images to best fit the x-ray data.

Design of objective lens for 200-kV high-resolution electron microscopes

1983 ◽  
Vol 41 ◽  
pp. 314-315
Author(s):  
K. Tsuno ◽  
T. Honda ◽  
Y. Harada ◽  
M. Naruse
Keyword(s):  
Optical Properties ◽  
Computer Technology ◽  
Axial Magnetic Field ◽  
Chromatic Aberration ◽  
Objective Lens ◽  
Resolution Electron ◽  
Correction Of Astigmatism ◽  
Three Stages ◽  
Electron Microscopes ◽  
Stage 1

Developement of computer technology provides much improvements on electron microscopy, such as simulation of images, reconstruction of images and automatic controll of microscopes (auto-focussing and auto-correction of astigmatism) and design of electron microscope lenses by using a finite element method (FEM). In this investigation, procedures for simulating the optical properties of objective lenses of HREM and the characteristics of the new lens for HREM at 200 kV are described.The process for designing the objective lens is divided into three stages. Stage 1 is the process for estimating the optical properties of the lens. Firstly, calculation by FEM is made for simulating the axial magnetic field distributions Bzc of the lens. Secondly, electron ray trajectory is numerically calculated by using Bzc. And lastly, using Bzc and ray trajectory, spherical and chromatic aberration coefficients Cs and Cc are numerically calculated. Above calculations are repeated by changing the shape of lens until! to find an optimum aberration coefficients.

A Stable Field Emission Electron Source

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.

Characteristics of an Electrostatic Phase Plate

1972 ◽  
Vol 30 ◽  
pp. 618-619
Author(s):  
William Krakow ◽  
Benjamin Siegel
Keyword(s):  
Phase Contrast ◽  
Objective Lens ◽  
Phase Plate ◽  
Net Charge ◽  
Phase Plates ◽  
Beam Currents ◽  
Contrast Image ◽  
Bright Phase ◽  
Additional Phase Shift ◽  
Additional Phase

Unwin has used a metallized non-conducting thread in the back focal plane of the objective lens that stops out a portion of the unscattered beam, takes on a localized positive charge and thus produces an additional phase shift to give a different transfer function of the lens. Under the particular conditions Unwin used, the phase contrast image was shifted to bright phase contrast for optimum focus.We have investigated the characteristics of this type of electrostatic phase plate, both analytically and experimentally, as functions of the magnitude of charge and defocus. Phase plates have been constructed by using Wollaston wire to mount 0.25μ diameter platinum wires across apertures ranging from 50 to 200μ diameter and vapor depositing SiO and gold on the mounted wires to give them the desired charging characteristics. The net charge was varied by adjusting only the bias on the Wehnelt shield of the gun, and hence the beam currents and effective size of the source.

Optical Properties of HVEM Accelerators

1975 ◽  
Vol 33 ◽  
pp. 180-181
Author(s):  
A. Strojnik ◽  
J.W. Scholl ◽  
V. Bevc
Keyword(s):  
Optical Properties ◽  
Electron Accelerator ◽  
Systematic Investigation ◽  
Electron Source ◽  
High Brightness ◽  
The Past ◽  
Wide Range ◽  
Optical Behavior

The electron accelerator, as inserted between the electron source (injector) and the imaging column of the HVEM, is usually a strong lens and should be optimized in order to ensure high brightness over a wide range of accelerating voltages and illuminating conditions. This is especially true in the case of the STEM where the brightness directly determines the highest resolution attainable. In the past, the optical behavior of accelerators was usually determined for a particular configuration. During the development of the accelerator for the Arizona 1 MEV STEM, systematic investigation was made of the major optical properties for a variety of electrode configurations, number of stages N, accelerating voltages, 1 and 10 MEV, and a range of injection voltages ϕ0 = 1, 3, 10, 30, 100, 300 kV).

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