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.

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.

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.

New Aspects in the Design of Electron Optical Magnification Systems

1972 ◽  
Vol 30 ◽  
pp. 606-607
Author(s):  
Willem H.J. Andersen
Keyword(s):  
Electron Microscope ◽  
Imaging System ◽  
Chromatic Aberration ◽  
Research Tool ◽  
Energy Losses ◽  
Objective Lens ◽  
Theoretical Limit ◽  
Optical Magnification ◽  
Chromatic Aberrations ◽  
High Degree

Electron microscope design, and particularly the design of the imaging system, has reached a high degree of perfection. Present objective lenses perform up to their theoretical limit, while the whole imaging system, consisting of three or four lenses, provides very wide ranges of magnification and diffraction camera length with virtually no distortion of the image. Evolution of the electron microscope in to a routine research tool in which objects of steadily increasing thickness are investigated, has made it necessary for the designer to pay special attention to the chromatic aberrations of the magnification system (as distinct from the chromatic aberration of the objective lens). These chromatic aberrations cause edge un-sharpness of the image due to electrons which have suffered energy losses in the object.There exist two kinds of chromatic aberration of the magnification system; the chromatic change of magnification, characterized by the coefficient Cm, and the chromatic change of rotation given by Cp.

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.

Application of the Omega spectrometer TEM

1992 ◽  
Vol 50 (2) ◽  
pp. 1042-1043
Author(s):  
J. L. Lehman ◽  
J. Mayer ◽  
W. Probst
Keyword(s):  
Electron Diffraction ◽  
Inelastic Scattering ◽  
New World ◽  
Chromatic Aberration ◽  
Energy Spread ◽  
Simple Equation ◽  
Objective Lens ◽  
Analytical Tem ◽  
Image Planes ◽  
Thermal Diffuse

The recent development of an analytical TEM with an integrated imaging Omega spectrometer has opened the door to a new world of specimen information. The Omega Spectrometer eliminates some of the information limits which have been imposed by the chromatic aberration effects in thick 3-D specimen imaging, as well as thermal diffuse and inelastic scattering in electron diffraction studies. This benefits both the TEM 3-D imaging and electron diffraction fields.Inelastically scattered electrons are electrons which have lost some of their energy while passing through the specimen. In all conventional TEMs, these inelastically scattered electrons inherently cause the final image to be blurry as the objective lens focuses the different energies into different image planes. The chromatic aberration of the objective lens can be calculated by the simple equation, Δc = Cc .α.ΔE/E, where ΔE is the energy spread of the electrons leaving the specimen, and E is the energy of the electrons entering the specimen. The chromatic aberration of the objective lens can be reduced by increasing the value of E or limiting ΔE.

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.

An objective lens for an electron microscope with compensated axial chromatic aberration

1978 ◽  
Vol 36 (1) ◽  
pp. 36-37
Author(s):  
H. Koops ◽  
W. Bernhard
Keyword(s):  
Test Specimen ◽  
Focal Length ◽  
Chromatic Aberration ◽  
Electric Quadrupole ◽  
Resolving Power ◽  
Objective Lens ◽  
Virtual Image ◽  
Object Plane ◽  
Magnetic Quadrupole ◽  
Chromatic Aberrations

The resolving power of transmission microscopes is limited by diffraction and by spherical and chromatic aberrations of the objective lens.To compensate the axial chromatic aberration, electric quadrupole fields may be used (1,2). We therefore combined a condenser-objective lens of 2 mm focal length with an electron-optical aplanator (3). Earlier experiments showed that the axial chromatic aberration of the aplanator can be compensated by counteracting electric and magnetic quadrupole fields (4,5). High values of these correcting fields yield a negative chromatic error of the aplanator. It compensates the positive chromatic aberration coefficient of 1.5 mm of the objective lens.The specimen is situated near the middle plane of the condenser-objective lens. This lens produces a 60-fold magnified virtual image in a plane 122 mm above the specimen. The aplanator transfers this image 1:1 into the object plane of the intermediate lens and compensates the aberrations (6).Figure 1 shows a micrograph of a resolution test specimen.

Beyond One Angstrom

1990 ◽  
Vol 48 (1) ◽  
pp. 204-205
Author(s):  
Albert. V. Crewe
Keyword(s):  
Electron Microscopy ◽  
Spherical Aberration ◽  
Focal Length ◽  
Chromatic Aberration ◽  
Low Energy ◽  
Objective Lens ◽  
Squeeze Out ◽  
Electron Microscopes

I believe everyone would agree we have just about reached the limit of performance of today's electron microscopes. This is not to say that additional advances will not take place, because there is always one more drop of blood to squeeze out. But it is certainly becoming increasingly apparent that we can not expect more out of the magnetic lenses that we now have. I am sure that everyone who has ever been concerned with this problem has arrived at the same set of conclusions but it may help to set them down one more time.The available resolution in electron microscopy is distressingly poor compared to the wavelength of the electrons. The culprit is always the objective lens. For low energy, say less than 5,000 volts, chromatic aberration is the offending element whereas at high voltages it is the spherical aberration coefficient which we must be concerned with. In both cases, there are some basic restrictions which apply. In the case of chromatic aberration it is always very closely equal with the focal length of the lens and for the spherical aberration coefficient the best we can do is about 1/4 or 1/2 the focal length.

On the Microanalysis of Small Precipitates at Low Voltage with a FE-SEM

1999 ◽  
Vol 5 (S2) ◽  
pp. 308-309
Author(s):  
Raynald Gauvin ◽  
Pierre Hovington
Keyword(s):  
Field Emission ◽  
Secondary Electron ◽  
Low Voltage ◽  
Chromatic Aberration ◽  
Incident Electron Energy ◽  
Objective Lens ◽  
Type I ◽  
Backscattered Electrons ◽  
Electron Microscopes ◽  
Scanning Electron

The observation of microstructural features smaller than 300 nm is generally performed using Transmission Electron Microscopy (TEM) because conventional Scanning Electron Microscopes (SEM) do not have the resolution to image such small phases. Since the early 1990’s, a new generation of microscopes is now available on the market. These are the Field Emission Gun Scanning Electron Microscope with a virtual secondary electron detector. The field emission gun gives a higher brightness than those obtained using conventional electron filaments allowing enough electrons to be collected to operate the microscope with incident electron energy, E0, below 5 keV, with probe diameter smaller than 2.5 nm. Furthermore, what gives FE-SEM outstanding resolution is the combination of new magnetic lenses with a virtual secondary electron (SE) detector. The new lenses are designed to reduce the spherical and chromatic aberration coefficients, giving a smaller probe size. Contrary to the conventional systems, the SE detector is located above the objective lens and it becomes a virtual or through-the-lens (TTL) detector. Therefore, the SE image is mostly made up of all SEs of type I, almost eliminating those of type II and III which are generated by the backscattered electrons inside the specimen as well as in the chamber. It has been shown recently that Nb(CN) precipitates in Fe, as small than 10 nm, can be imaged with a FE-SEM Hitachi S-4500 with the TTL detector.

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