Hollow-cone illumination in simulated ADF STEM

1989 ◽  
Vol 47 ◽  
pp. 124-125
Author(s):  
R. F. Loane ◽  
J. Silcox
Keyword(s):  
Stationary Phase ◽  
Spherical Aberration ◽  
Image Interpretation ◽  
Incident Beam ◽  
Bragg Angle ◽  
Hollow Cone ◽  
Fringe Contrast ◽  
Convergence Angle ◽  
Probe Size ◽  
Overlap Region

The overlap regions between CBED disks are a source of lattice fringe contrast in ADF STEM images of crystals. For sufficiently small lattice spacings, the Scherzer focus convergence angle is too small for the CBED disks to overlap, and the lattice is not visible. By increasing the convergence angle beyond the Scherzer angle to create an overlap region, and by using stationary phase defocus, smaller lattices can be imaged However, with larger convergence angles, lens aberrations spread the incident beam and small aperiodic structures, such as adatoms, become difficult to image. If the specimen Bragg angle is known, an annular objective aperture can be designed to pass the overlap regions of the incident beam, while blocking the central, non-overlap region which produces a large constant background. The annular probe size is minimized by choosing an annulus width and defocus which balance the spherical aberration across the annulus (close to stationary phase defocus). Simulations indicate hollow cone illumination may allow both adatoms and smaller lattices to be imaged simultaneously. This approach presumes knowledge of the specimen, and must be used in conjunction with standard methods for correct image interpretation. An incorrectly applied annulus will produce image artifacts.

On parallel illumination conditions for ALCHEMI

1986 ◽  
Vol 44 ◽  
pp. 622-623
Author(s):  
K. K. Christenson ◽  
J. A. Eades
Keyword(s):  
Crystal Structure ◽  
Incident Beam ◽  
Bragg Angle ◽  
Angular Range ◽  
Convergence Angle ◽  
X Ray ◽  
Atomic Species

ALCHEMI is a technique for locating atomic species within a crystal structure. It depends on the fact that, near low index planes or zone axes, the ratio of the x-ray signals from different sublattices vary as a function of the angle between the incident beam and the crystal structure.In order to see the changes in these ratios, it is necessary that the illumination be close to parallel in order to minimize the angular range of the measurement. In simple cases it is sufficient to make the convergence angle smaller than the operating Bragg angle. This requirement can often be met by the conditions of a focussed probe.

Resolution Attainable With the Present Day STEM

1973 ◽  
Vol 31 ◽  
pp. 230-231 ◽  
Author(s):  
J. S. Wall ◽  
J. P. Langmore ◽  
H. Isaacson ◽  
A. V. Crewe
Keyword(s):  
Spherical Aberration ◽  
Focal Length ◽  
Incident Beam ◽  
Scanning Transmission Electron Microscope ◽  
Aperture Size ◽  
Magnetic Lens ◽  
Peak Intensity ◽  
Transmission Electron ◽  
Scanning Transmission ◽  
Half Angle

The scanning transmission electron microscope (STEM) constructed by the authors employs a field emission gun and a 1.15 mm focal length magnetic lens to produce a probe on the specimen. The aperture size is chosen to allow one wavelength of spherical aberration at the edge of the objective aperture. Under these conditions the profile of the focused spot is expected to be similar to an Airy intensity distribution with the first zero at the same point but with a peak intensity 80 per cent of that which would be obtained If the lens had no aberration. This condition is attained when the half angle that the incident beam subtends at the specimen, 𝛂 = (4𝛌/Cs)¼

Experimental study on the restoration from a tilt-azimuth series for improvement in resolution

1995 ◽  
Vol 53 ◽  
pp. 636-637
Author(s):  
Norio Baba ◽  
Norihiko Ichise ◽  
Syunya Watanabe
Keyword(s):  
Experimental Study ◽  
Spherical Aberration ◽  
Incident Beam ◽  
Optical Parameters ◽  
Beam Tilting ◽  
Illumination Method ◽  
Restoration Method ◽  
Illumination Mode

The tilted beam illumination method is used to improve the resolution comparing with the axial illumination mode. Using this advantage, a restoration method of several tilted beam images covering the full azimuthal range was proposed by Saxton, and experimentally examined. To make this technique more reliable it seems that some practical problems still remain. In this report the restoration was attempted and the problems were considered. In our study, four problems were pointed out for the experiment of the restoration. (1) Accurate beam tilt adjustment to fit the incident beam to the coma-free axis for the symmetrical beam tilting over the full azimuthal range. (2) Accurate measurements of the optical parameters which are necessary to design the restoration filter. Even if the spherical aberration coefficient Cs is known with accuracy and the axial astigmatism is sufficiently compensated, at least the defocus value must be measured. (3) Accurate alignment of the tilt-azimuth series images.

Chromatic aberration and low-voltage SEM

1987 ◽  
Vol 45 ◽  
pp. 546-547
Author(s):  
Zhifeng Shao ◽  
A.V. Crewe
Keyword(s):  
Electron Energy ◽  
Low Voltage ◽  
Spherical Aberration ◽  
Chromatic Aberration ◽  
Primary Electron ◽  
Probe Size ◽  
Non Local ◽  
Optical Treatment ◽  
Electron Microscopes ◽  
Scanning Electron Microscopes

For scanning electron microscopes, it is plausible that by lowering the primary electron energy, one can decrease the volume of interaction and improve resolution. As shown by Crewe /1/, at V0 =5kV a 10Å resolution (including non-local effects) is possible. To achieve this, we would need a probe size about 5Å. However, at low voltages, the chromatic aberration becomes the major concern even for field emission sources. In this case, δV/V = 0.1 V/5kV = 2x10-5. As a rough estimate, it has been shown that /2/ the chromatic aberration δC should be less than ⅓ of δ0 the probe size determined by diffraction and spherical aberration in order to neglect its effect. But this did not take into account the distribution of electron energy. We will show that by using a wave optical treatment, the tolerance on the chromatic aberration is much larger than we expected.

Probe size and 5th-order aberration

1987 ◽  
Vol 45 ◽  
pp. 134-135 ◽  
Author(s):  
Zhifeng Shao
Keyword(s):  
Electron Probe ◽  
Spherical Aberration ◽  
Wave Length ◽  
Cylindrical Symmetry ◽  
Electron Wave ◽  
Third Order ◽  
The Third ◽  
Probe Radius ◽  
Small Probe ◽  
Probe Size

A small electron probe has many applications in many fields and in the case of the STEM, the probe size essentially determines the ultimate resolution. However, there are many difficulties in obtaining a very small probe.Spherical aberration is one of them and all existing probe forming systems have non-zero spherical aberration. The ultimate probe radius is given byδ = 0.43Csl/4ƛ3/4where ƛ is the electron wave length and it is apparent that δ decreases only slowly with decreasing Cs. Scherzer pointed out that the third order aberration coefficient always has the same sign regardless of the field distribution, provided only that the fields have cylindrical symmetry, are independent of time and no space charge is present. To overcome this problem, he proposed a corrector consisting of octupoles and quadrupoles.

Ultra-high resolution SEMI-in-lens type FE-SEM, JSM-6320F, with strong magnetic-field lens with built-in secondary-electron detector

1994 ◽  
Vol 52 ◽  
pp. 484-485
Author(s):  
T. Miyokawa ◽  
H. Kazumori ◽  
S. Nakagawa ◽  
C. Nielsen
Keyword(s):  
High Resolution ◽  
Secondary Electron ◽  
Spherical Aberration ◽  
Incident Beam ◽  
Chromatic Aberration ◽  
Magnetic Flux Density ◽  
Objective Lens ◽  
Electron Detector ◽  
High Resolution Images ◽  
Working Distance

We have developed a strongly excited objective lens with a built-in secondary electron detector to provide ultra-high resolution images with high quality at low to medium accelerating voltages. The JSM-6320F is a scanning electron microscope (FE-SEM) equipped with this lens and an incident beam divergence angle control lens (ACL).The objective lens is so strongly excited as to have peak axial Magnetic flux density near the specimen surface (Fig. 1). Since the speciien is located below the objective lens, a large speciien can be accomodated. The working distance (WD) with respect to the accelerating voltage is limited due to the magnetic saturation of the lens (Fig.2). The aberrations of this lens are much smaller than those of a conventional one. The spherical aberration coefficient (Cs) is approximately 1/20 and the chromatic aberration coefficient (Cc) is 1/10. for accelerating voltages below 5kV. At the medium range of accelerating voltages (5∼15kV). Cs is 1/10 and Cc is 1/7. Typical values are Cs-1.lmm. Cc=l. 5mm at WD=2mm. and Cs=3.lmm. Cc=2.9 mm at WD=5mm. This makes the lens ideal for taking ultra-high resolution images at low to medium accelerating voltages.

Atomic Resolution in Crystal Aperture Stem

1990 ◽  
Vol 48 (1) ◽  
pp. 236-237
Author(s):  
J T Fourie
Keyword(s):  
Optical System ◽  
Spherical Aberration ◽  
Three Dimensional ◽  
Transmission Function ◽  
Optical Systems ◽  
Bragg Angle ◽  
Electron Optical System ◽  
Intimate Contact ◽  
Diffraction Effects ◽  
Design Aspect

The attempts at improvement of electron optical systems to date, have largely been directed towards the design aspect of magnetic lenses and towards the establishment of ideal lens combinations. In the present work the emphasis has been placed on the utilization of a unique three-dimensional crystal objective aperture within a standard electron optical system with the aim to reduce the spherical aberration without introducing diffraction effects. A brief summary of this work together with a description of results obtained recently, will be given.The concept of utilizing a crystal as aperture in an electron optical system was introduced by Fourie who employed a {111} crystal foil as a collector aperture, by mounting the sample directly on top of the foil and in intimate contact with the foil. In the present work the sample was mounted on the bottom of the foil so that the crystal would function as an objective or probe forming aperture. The transmission function of such a crystal aperture depends on the thickness, t, and the orientation of the foil. The expression for calculating the transmission function was derived by Hashimoto, Howie and Whelan on the basis of the electron equivalent of the Borrmann anomalous absorption effect in crystals. In Fig. 1 the functions for a g220 diffraction vector and t = 0.53 and 1.0 μm are shown. Here n= Θ‒ΘB, where Θ is the angle between the incident ray and the (hkl) planes, and ΘB is the Bragg angle.

Stresses in Multilayered Thin Films

MRS Bulletin ◽  
1999 ◽  
Vol 24 (2) ◽  
pp. 34-38 ◽  
Author(s):  
Robert C. Cammarata ◽  
John C. Bilello ◽  
A. Lindsay Greer ◽  
Karl Sieradzki ◽  
Steven M. Yalisove
Keyword(s):  
Thin Films ◽  
Angular Displacement ◽  
Incident Beam ◽  
Two Dimensions ◽  
Bragg Angle ◽  
Radius Of Curvature ◽  
Wave Source ◽  
Contour Maps ◽  
Film Substrate ◽  
Image Position

Almost all thin films deposited on a substrate are in a state of stress. Fifty years ago pioneering work concerning the measurement of thin-film stresses was conducted by Brenner and Senderoff. They electroplated a metal film onto a thin metal substrate strip fixed at one end and measured the deflection of the free end of the substrate with a micrometer. Using a beam-bending analysis, they were able to calculate a residual stress from the measured deflection of the bimetallic film-substrate system. A variety of other, more sensitive methods of measuring the curvature of the surface of a film-substrate system have since been developed using, for example, capacitance measurements and interferometry techniques.When a monochromatic x-ray beam is incident onto a curved single crystal, the diffraction condition is satisfied only for regions of the crystal where the inclination angle with respect to the incident beam exactly matches the Bragg angle. When a parallel beam plane-wave source is used, the diffracted beam from a particular set of (hkl) planes gives rise to a single narrow-contour band. If the crystal is rocked by an angle ω, the contour band will move by a certain distance D. The radius of curvature R of the crystal lattice planes is given bywhere θ is the Bragg angle. Equal rocking angles produce equivalent D values for uniform curvature, or varied D values for nonuniform curvature. Using this procedure, detailed contour maps of the angular displacement field of the crystal can be mapped in two dimensions.

On-Line Aberration Measurement and Correction in STEM

1997 ◽  
Vol 3 (S2) ◽  
pp. 1171-1172 ◽  
Author(s):  
Ondrej L. Krivanek ◽  
Niklas Dellby ◽  
Andrew J. Spence ◽  
Roger A. Camps ◽  
L. Michael Brown
Keyword(s):  
Electron Microscopy ◽  
Electron Microscope ◽  
Spherical Aberration ◽  
Computer Control ◽  
Beam Current ◽  
Scanning Transmission Electron Microscope ◽  
Transmission Electron ◽  
Probe Size ◽  
Scanning Transmission ◽  
On Line

Aberration correction in electron microscopy is a subject with a 60 year history dating back to the fundamental work of Scherzer. There have been several partial successes, such as Deltrap's spherical aberration (Cs) corrector which nulled Cs over 30 years ago. However, the practical goal of attaining better resolution than the best uncorrected microscope operating at the same voltage remains to be fulfilled. Combining well-known electron-optical principles with stable electronics, versatile computer control, and software able to diagnose and correct aberrations on-line is at last bringing this goal within reach.We are building a quadrupole-octupole Cs corrector with automated aberration diagnosis for a VG HB5 dedicated scanning transmission electron microscope (STEM). A STEM with no spherical aberration will produce a smaller probe size with a given beam current than an uncorrected STEM, and a larger beam current in a given size probe.

Elektronenoptischer Phasenkontrast

1969 ◽  
Vol 24 (3) ◽  
pp. 377-389 ◽  
Author(s):  
L. Reimer
Keyword(s):  
Scattering Amplitudes ◽  
Spherical Aberration ◽  
Error Function ◽  
Atomic Layer ◽  
Bragg Angle ◽  
Continuous Transition ◽  
Single Atoms ◽  
Crystal Films ◽  
Finite Aperture ◽  
Atomic Image

Complex atomic scattering amplitudes calculated by the WKB-method and verified experimentally in earlier work are used to compute the decrease in intensity if imaging single atoms, plane arrays of atoms or atomic monolayers, especially of platinum atoms. By calculating the decrease of intensity in the centre of an atomic image for a large number of different objective apertures α and defocussing parameters Δf one gets a better survey for the limits of optimal underfocus. We used constants of spherical aberration Cö = 0.5, 1, 2 and 4 mm. The energy of the electrons is 100 keV. Some calculations are done up to 1200 keV. The use of complex scattering amplitudes does not justify to distinguish between amplitude and phase contrast. Both limiting cases are obtatined in the used theory. Two platinum atoms in nearest distance can be resolved in the optimal underfocus only for Cö= 0 .5 and 1 mm. But for larger Cö there exists a region at larger α with a much better resolution. These results show that there is a continuous transition from the resolution of an atomic pair up to that of the lattice planes in crystal films. Disks of 7 and 19 atoms in a two-dimensional dense packing are calculated and some deviations in optimal focus are found compared to the results of single atoms. At low α beyond the Bragg angle of the dense packed atomic rows no large difference exist if varying Cö and α. The single atoms are not resolved, but the decrease of intensity should be large enough for observation. Further calculations have been done about the contrast of a mono-atomic layer considering the influence of a vacancy too. The influence of a finite aperture of illumination and the energy distribution of the electrons are considered by folding the results with a distribution like a Gaussian error function.

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