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.

The Reduction of Chromatic Aberrations Due to Instrumental Instabilities

1971 ◽  
Vol 29 ◽  
pp. 14-15
Author(s):  
J. S. Lally ◽  
R. Evans
Keyword(s):  
Electron Microscope ◽  
High Voltage ◽  
Focal Length ◽  
Chromatic Aberration ◽  
Objective Lens ◽  
Usual Practice ◽  
Voltage Electron ◽  
Short Focal Length ◽  
Chromatic Aberrations ◽  
Electron Microscopes

One of the instrumental factors often limiting the resolution of the electron microscope is image defocussing due to changes in accelerating voltage or objective lens current. This factor is particularly important in high voltage electron microscopes both because of the higher voltages and lens currents required but also because of the inherently longer focal lengths, i.e. 6 mm in contrast to 1.5-2.2 mm for modern short focal length objectives.The usual practice in commercial electron microscopes is to design separately stabilized accelerating voltage and lens supplies. In this case chromatic aberration in the image is caused by the random and independent fluctuations of both the high voltage and objective lens current.

Voltage-center COMA-free alignment for high-resolution Electron Microscopy

1994 ◽  
Vol 52 ◽  
pp. 410-411
Author(s):  
K. Ishizuka ◽  
K. Shirota
Keyword(s):  
Electron Microscopy ◽  
High Resolution ◽  
Incident Beam ◽  
Chromatic Aberration ◽  
High Resolution Electron Microscopy ◽  
Beam Deflection ◽  
Objective Lens ◽  
Main Magnetic Field ◽  
Beam Direction ◽  
Resolution Electron

In a conventional alignment for high-resolution electron microscopy, the specimen point imaged at the viewing-screen center is made dispersion-free against a voltage fluctuation by adjusting the incident beam direction using the beam deflector. For high-resolution works the voltage-center alignment is important, since this alignment reduces the chromatic aberration. On the other hand, the coma-free alignment is also indispensable for high-resolution electron microscopy. This is because even a small misalignment of the incident beam direction induces wave aberrations and affects the appearance of high resolution electron micrographs. Some alignment procedures which cancel out the coma by changing the incident beam direction have been proposed. Most recently, the effect of a three-fold astigmatism on the coma-free alignment has been revealed, and new algorithms of coma-free alignment have been proposed.However, the voltage-center and the coma-free alignments as well as the current-center alignment in general do not coincide to each other because of beam deflection due to a leakage field within the objective lens, even if the main magnetic-field of the objective lens is rotationally symmetric. Since all the proposed procedures for the coma-free alignment also use the same beam deflector above the objective lens that is used for the voltage-center alignment, the coma-free alignment is only attained at the sacrifice of the voltage-center alignment.

Toward The Simultaneous Correction of Spherical and Chromatic Aberration in Electron Optics by Means of an Electrostatic Electron Mirror

1990 ◽  
Vol 48 (1) ◽  
pp. 206-207
Author(s):  
Gertrude. F. Rempfer
Keyword(s):  
Chromatic Aberration ◽  
Transverse Magnetic ◽  
Objective Lens ◽  
Ion Imaging ◽  
Corrected Image ◽  
Electric And Magnetic Fields ◽  
Chromatic Aberrations ◽  
Improved Performance ◽  
Electron Microscopes ◽  
Image Planes

Optimum performance in electron and ion imaging instruments, such as electron microscopes and probe-forming instruments, in most cases depends on a compromise either between imaging errors due to spherical and chromatic aberrations and the diffraction error or between the imaging errors and the current in the image. These compromises result in the use of very small angular apertures. Reducing the spherical and chromatic aberration coefficients would permit the use of larger apertures with resulting improved performance, granted that other problems such as incorrect operation of the instrument or spurious disturbances do not interfere. One approach to correcting aberrations which has been investigated extensively is through the use of multipole electric and magnetic fields. Another approach involves the use of foil windows. However, a practical system for correcting spherical and chromatic aberration is not yet available.Our approach to correction of spherical and chromatic aberration makes use of an electrostatic electron mirror. Early studies of the properties of electron mirrors were done by Recknagel. More recently my colleagues and I have studied the properties of the hyperbolic electron mirror as a function of the ratio of accelerating voltage to mirror voltage. The spherical and chromatic aberration coefficients of the mirror are of opposite sign (overcorrected) from those of electron lenses (undercorrected). This important property invites one to find a way to incorporate a correcting mirror in an electron microscope. Unfortunately, the parts of the beam heading toward and away from the mirror must be separated. A transverse magnetic field can separate the beams, but in general the deflection aberrations degrade the image. The key to avoiding the detrimental effects of deflection aberrations is to have deflections take place at image planes. Our separating system is shown in Fig. 1. Deflections take place at the separating magnet and also at two additional magnetic deflectors. The uncorrected magnified image formed by the objective lens is focused in the first deflector, and relay lenses transfer the image to the separating magnet. The interface lens and the hyperbolic mirror acting in zoom fashion return the corrected image to the separating magnet, and the second set of relay lenses transfers the image to the final deflector, where the beam is deflected onto the projection axis.

Theoretical And Practical Design Considerations for Ultra-High Resolution Electron Lenses

1980 ◽  
Vol 38 ◽  
pp. 266-269
Author(s):  
Y. Harada ◽  
K. Tsuno ◽  
Y. Arai
Keyword(s):  
Optical Properties ◽  
High Resolution ◽  
Chromatic Aberration ◽  
Point Of View ◽  
Pole Piece ◽  
Axial Field ◽  
Design Considerations ◽  
Resolution Electron ◽  
Practical Design ◽  
Peak Flux

Magnetic objective lenses, from the point of view of pole piece geometry, can he roughly classified into two types, viz., symmetrical and asymmetrical. In the case of the former, the optical properties have been calculated by several authors1-3) and the results would appear to suggest that, in order to reduce the spherical and chromatic aberration coefficients, Cs and Cc, it is necessary to decrease the half-width value of the axial field distribution and to increase the peak flux density. The expressions for either minimum Cs or minimum Cc were presented in the form of ‘universal’ curves by Mulvey and Wallington4).

Electron Microscopic Observation of Crystal Lattice Images

1972 ◽  
Vol 30 ◽  
pp. 548-549
Author(s):  
Tsutomu Komoda
Keyword(s):  
Electron Microscope ◽  
Spherical Aberration ◽  
Electron Microscopic Observation ◽  
Chromatic Aberration ◽  
Operating Conditions ◽  
Optimum Operating ◽  
Objective Lens ◽  
Electron Microscopic ◽  
Electron Microscopes ◽  
Lattice Resolution

Electron microscope images of crystal lattices have been observed by many authors since the first achievement by Menter in 1956. During these years, the optimum operating conditions with electron microscopes have been investigated for the high resolution lattice imaging. Finally minute lattice spacings around 1 Å have been resolved by several authors by using contemporary instruments. The major lattice planes with low index in crystals are almost within a range of spacing capable to be resolved by electron microscopy (1-3 Å). Therefore, the observing techniques are now essential for practical studies in the area of crystallography as well as metal physics.Although the point to point resolution of the electron microscope is restricted due to the spherical aberration of the objective lens in addition to the diffraction, the lattice resolution is mainly limited due to the chromatic aberration under the normal illumination.

Mirror-hexapole corrector with compact beam splitter for eliminating the chromatic and spherical aberration of low-voltage EMs

1992 ◽  
Vol 50 (1) ◽  
pp. 94-95
Author(s):  
H. Rose
Keyword(s):  
Beam Splitter ◽  
Low Voltage ◽  
Spherical Aberration ◽  
Chromatic Aberration ◽  
Objective Lens ◽  
Deflection System ◽  
Magnetic Deflection ◽  
Electron Microscopes ◽  
Electrostatic Mirror ◽  
Illuminating Beam

To significantly improve the performance of electron microscopes it is necessary to enlarge the usable aperture. At low voltages this requirement can only be met if the chromatic and the spherical aberration are corrected simultaneously. For imaging surfaces with reflected electrons (LEEM) a magnetic deflection system separating the illuminating beam from the image-forming beam must be incorporated in the region above the objective lens. Since the use of an electrostatic mirror for the correction of the chromatic aberration also necessitates such a system, it would be extremely helpful if the beam splitter can be designed in such a way that it also separates the parts of the image-forming beam heading toward and away from the mirror.

The twin lens system - A high resolution objective lens at 300 kV

1986 ◽  
Vol 44 ◽  
pp. 610-613
Author(s):  
B. Bormans ◽  
P. Hagemann
Keyword(s):  
Chromatic Aberration ◽  
Objective Lens ◽  
Finite Element Program ◽  
Medium Voltage ◽  
Voltage Electron ◽  
System A ◽  
Element Program ◽  
Electron Microscopes ◽  
Single Field ◽  
Better Than

Point resolution of better than 0.2 nm is today achievable in medium voltage electron microscopes. Crucial for this achievement is the objective lens design which has to provide sufficiently low spherical and chromatic aberration figures. In addition to the lens itself, its integration into the overall microscope optics has to ensure a reproducible and accurate alignment.Cleaver has shown that symmetrical lenses have lower aberrations than their asymmetrical counterparts by a factor of 1.5 or 2. Theoretical optimization of this lens at 300 kV can be carried out using a first-order finite element program, as developed by Munro. The symmetrical single-field condenser objective with saturated pole-piece tips produces particularly good values with Cs = 1.0 mm and Cc = 1.4 mm at 300 kV. In this lens the specimen is placed in the centre of the magnetic field.

HREM autotuning on crystalline materials

1995 ◽  
Vol 53 ◽  
pp. 24-25
Author(s):  
M. Pan ◽  
O.L. Krivanek
Keyword(s):  
High Resolution ◽  
Amorphous Materials ◽  
Optical Axis ◽  
Amorphous Layer ◽  
Objective Lens ◽  
Crystalline Materials ◽  
Crystalline Sample ◽  
Resolution Electron ◽  
High Resolution Electron Microscope ◽  
Electron Microscopes

Complete autotuning of a high resolution electron microscope has been well established. It performs the following tasks: align the electron beam along the true electron-optical axis of objective lens (autoalignment), correct the astigmatism (autostigmation), and set the defocus to a user defined value (autofocus). It can also characterize the coefficient of 3-fold astigmatism while performing the autoalignment. Based on diffractogram analysis current HREM autotuning algorithm only works on amorphous materials. In reality, however, most of the HREM practice is performed on crystalline materials. Therefore it is highly desirable to extend the current HREM autotuning algorithm to crystalline specimens. In this abstract we report preliminary studies on attempting to analyze diffractograms from a mix of crystalline and amorphous materials.For crystalline specimens observed in most high resolution electron microscopes, except under UHV conditions, there is typically a thin layer of amorphous contamination due to either sample preparation or poor vacuum conditions. This amorphous layer can be easily seen at the edge of a crystalline sample in the microscope.

Computer Simulation of the Imaging of Atomic Defects in Metals

1977 ◽  
Vol 35 ◽  
pp. 14-16 ◽  
Author(s):  
P. M. Fields ◽  
J. M. Cowley
Keyword(s):  
Local Strain ◽  
Dark Field ◽  
Objective Lens ◽  
Heavy Atoms ◽  
Dark Field Imaging ◽  
Resolution Electron ◽  
Localized Defects ◽  
Computer Calculations ◽  
Electron Microscopes ◽  
Diffraction Patterns

Localized defects in metals, such as vacancies and interstitials, should have scattering powers for electrons comparable with those of the single heavy atoms which have been imaged in recent years by use of high resolution electron microscopes. It should therefore be possible to obtain appreciable contrast from such defects with current lOOkV electron microscopes. However in order to distinguish the intensity variations due to these defects from those due to specimen surface structure, contamination or other causes, it may be necessary to use the improved resolution of high voltage microscopes so that the characteristic shapes of the defects and their local strain fields can be recognized.In order to test these speculations, we have made computer calculations of the images and diffraction patterns to be expected from split interstitials in thin crystals of gold and aluminum for bright-field and dark-field imaging with a lOOkV microscope with an objective lens having Cs = 1.8mm and for a l000kV microscope, Cs = 1.8mm.

An Automated Procedure for On-Line Measurement of Spherical-Aberration Coefficient of High-Resolution Electron Microscopes

1996 ◽  
Vol 54 ◽  
pp. 420-421 ◽  
Author(s):  
M. Pan ◽  
O.L. Krivanek
Keyword(s):  
High Resolution ◽  
Structural Information ◽  
Spherical Aberration ◽  
Amorphous Material ◽  
Ccd Camera ◽  
Objective Lens ◽  
Resolution Electron ◽  
Line Measurement ◽  
On Line ◽  
Electron Microscopes

Spherical aberration coefficient (Cs) of the objective lens and electron wavelength ultimately determine the point-resolution of a high resolution electron microscope (HREM). Accurate measurement of Cs has become increasingly critical for reconstruction of structural information well beyond the point-resolution by means of either electron holography or focal series methods with a field emission gun (FEG) microscope. There are two main existing procedures for Cs measurement, i.e. (1) using diffractograms from a thin amorphous material, and (2) using beam-tilt-induced image displacement (BID). Since these procedures generally involve intensive data measurement, it is highly desirable to have an automated procedure. With an image pickup system such as CCD camera and appropriate software, we have developed an automated procedure for on-line Cs measurement. The procedure is based on analyzing diffractograms from a thin amorphous material such as amorphous carbon or germanium. The use of CCD camera allows for on-line measurement, and also for magnification to be calibrated with high precision, which is critical in Cs measurement.

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