A High Resolution Scanning Microscope

1968 ◽  
Vol 26 ◽  
pp. 356-357
Author(s):  
A. V. Crewe ◽  
J. Wall ◽  
L. M. Welter
Keyword(s):  
High Resolution ◽  
Field Emission ◽  
Spherical Aberration ◽  
Focal Length ◽  
Spot Size ◽  
Emission Source ◽  
Field Of View ◽  
Scanning Microscope ◽  
Magnetic Lens ◽  
High Quality

A scanning microscope using a field emission source has been described elsewhere. This microscope has now been improved by replacing the single magnetic lens with a high quality lens of the type described by Ruska. This lens has a focal length of 1 mm and a spherical aberration coefficient of 0.5 mm. The final spot size, and therefore the microscope resolution, is limited by the aberration of this lens to about 6 Å.The lens has been constructed very carefully, maintaining a tolerance of + 1 μ on all critical surfaces. The gun is prealigned on the lens to form a compact unit. The only mechanical adjustments are those which control the specimen and the tip positions. The microscope can be used in two modes. With the lens off and the gun focused on the specimen, the resolution is 250 Å over an undistorted field of view of 2 mm. With the lens on,the resolution is 20 Å or better over a field of view of 40 microns. The magnification can be accurately varied by attenuating the raster current.

High Resolution Scanning Microscopy

1969 ◽  
Vol 27 ◽  
pp. 172-173 ◽  
Author(s):  
A. V. Crewe ◽  
J. Wall
Keyword(s):  
High Resolution ◽  
Field Emission ◽  
Focal Length ◽  
Emission Source ◽  
Scanning Microscopy ◽  
Objective Lens ◽  
Scanning Microscope ◽  
Current Point ◽  
Single Objective

We have previously reported on the development of a scanning microscope with a resolution of 20 Å. This instrument has now been improved so that the current point resolution is about 5 Å at 18 Kv.The microscope consists of a field emission gun followed by a single objective lens (see Fig. 1). The gun produces an image (real or virtual) of the field emission source which is then demagnified by the lens. The focal length of this lens has been shortened from 1 mm to 0.6 mm to produce a second beam crossover at the exit of the lens.

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

Towards 1-Ångstrom-resolution STEM

1993 ◽  
Vol 51 ◽  
pp. 996-997
Author(s):  
H.S. von Harrach ◽  
D.E. Jesson ◽  
S.J. Pennycook
Keyword(s):  
High Resolution ◽  
Field Emission ◽  
Phase Contrast ◽  
Spherical Aberration ◽  
A Priori ◽  
Dark Field ◽  
Image Resolution ◽  
Objective Lens ◽  
Annular Dark Field ◽  
First Results

Phase contrast TEM has been the leading technique for high resolution imaging of materials for many years, whilst STEM has been the principal method for high-resolution microanalysis. However, it was demonstrated many years ago that low angle dark-field STEM imaging is a priori capable of almost 50% higher point resolution than coherent bright-field imaging (i.e. phase contrast TEM or STEM). This advantage was not exploited until Pennycook developed the high-angle annular dark-field (ADF) technique which can provide an incoherent image showing both high image resolution and atomic number contrast.This paper describes the design and first results of a 300kV field-emission STEM (VG Microscopes HB603U) which has improved ADF STEM image resolution towards the 1 angstrom target. The instrument uses a cold field-emission gun, generating a 300 kV beam of up to 1 μA from an 11-stage accelerator. The beam is focussed on to the specimen by two condensers and a condenser-objective lens with a spherical aberration coefficient of 1.0 mm.

A Spherical-Aberration Corrector Using Electrontransparent Conducting Foils

1973 ◽  
Vol 31 ◽  
pp. 262-263
Author(s):  
M. G. R. Thomson
Keyword(s):  
Spherical Aberration ◽  
Focal Length ◽  
Chromatic Aberration ◽  
Field Of View ◽  
Objective Lens ◽  
Electric Field Gradients ◽  
Field Gradients ◽  
Short Focal Length ◽  
Successful Approach ◽  
Fifth Order

One of the problems associated with building any aberration-corrected electron microscope objective lens lies in the difficulty of obtaining a sufficiently short focal length. A number of systems have focal lengths in the 1cm. range, and these are more suitable for microprobe work. If the focal length can be made short enough, the chromatic aberration probably does not need to be corrected, and the design is much simplified. A corrector device which can be used with a conventional magnetic objective lens of short focal length (Fig. 1) must either have dimensions comparable to the bore and gap of that lens, or have very large magnetic or electric field gradients. A successful approach theoretically has been to use quadrupoleoctopole corrector units, although these suffer from very large fifth order aberrations and a limited field of view.

The Unipotential Foil Lens

1975 ◽  
Vol 33 ◽  
pp. 142-143
Author(s):  
N. D. Wittels ◽  
E. H. Jacobsen
Keyword(s):  
Optical Properties ◽  
Fourier Series ◽  
Spherical Aberration ◽  
Equations Of Motion ◽  
Fourier Series Expansion ◽  
Considerable Effort ◽  
Magnetic Lens ◽  
High Quality ◽  
Center Electrode ◽  
Paraxial Ray

We are investigating theoretically and experimentally the optical properties of an electrostatic unipotential lens whose center electrode is an aperture and whose outer electrodes are electron-transparent foils. This lens is potentially useful as a spherical aberration corrector for a high-quality magnetic lens.The electrostatic field inside the lens is calculated analytically from a Bessel-Fourier series expansion of the potential. This method gives results that are accurate in all regions of the lens, including those near the conducting foils, and readily allows the lens boundaries and excitation to be varied. The equations of motion are integrated numerically to determine the lens' optical properties. Since we are specifically interested in the aberrations of this lens, considerable effort has been made to assure the validity of the non-paraxial ray calculations.

Development of an Ultra - High - Resolution Low Voltage(LV) SEM with an Optimized “in-lens” Design

1990 ◽  
Vol 48 (1) ◽  
pp. 388-389
Author(s):  
Takashi Nagatani ◽  
Mitsugu Sato ◽  
Masako OSUMI
Keyword(s):  
High Resolution ◽  
Low Voltage ◽  
Collection Efficiency ◽  
Spherical Aberration ◽  
Chromatic Aberration ◽  
Spot Size ◽  
Objective Lens ◽  
Tilting Angle ◽  
Working Distance ◽  
Better Than

An “in-lens” type FESEM, Hitachi S-900, developed as an ultra high resolution SEM having 0.7nm resolution at 30kV(Nagatani et al 1986), was modified for better performance at low beam energy(about 5kV or below) with small aberrations of ths objective lens and dual specimen position design. This is in responce to the recent upsurge of interest in using the LVSEM, which enables us hopefully to observe the surface topography of uncoated samples directly with maximum fidelity(Pawley 1987).The actual visibility of the minute topographical details depends upon not anly the spot size of the scanning beam but also physics of interaction between impinging electrons and solid sample(Joy 1989). However, the resolution can never be better than the spot size. Then, it would seem logical to specify the spot size first when designing a high resolution SEM. As discussed earlier(Crewe 1985; Nagatani et al 1987), the spot size of the beam is mainly limited by spherical aberration of the objective lens and diffraction at high voltage(about 10 kV and above). On the other hand, chromatic aberration and diffraction are the dominant factors at low voltages(about 5kV or below). Source size of a cold field emission is so small that we could neglect it for simplicity.In general, chromatic aberration can be smaller at higher excitation of a narrow gap objective pole-piece, which also made the working distance short. Therefore, some compromise is necessary among minimized aberrations, required specimen size, stage traverse and tilting angle etc. In practice, tolerable distortion of the image at low magnification and collection efficiency of the secondary electrons are another factors to be considered in designing the instrument. By taking these factors in simulation, an optimized objective lens was designed as shown in Table 1.

A Field Emission Probe Forming System with a Magnetic Pre-Accelerator Lens

1976 ◽  
Vol 34 ◽  
pp. 522-523 ◽  
Author(s):  
Huei Pei Kuo ◽  
Benjamin M. Siegel
Keyword(s):  
Field Emission ◽  
Power Dissipation ◽  
Spot Size ◽  
Beam Current ◽  
Magnetic Lens ◽  
Razor Blade ◽  
Computer Optimization ◽  
Exciting Coil ◽  
Blade Assembly ◽  
Magnetic Transfer

A field emission electron probe forming system with a magnetic preaccelerator lens has been developed.The magnetic lens has been designed using computer optimization for best compromise between high lens quality, ease of tip manipulation and tolerable power dissipation of the exciting coil. Figure 1 shows the configuration of the magnetic lens and the field emission tip assembly. The optical properties of the magnetic lens are plotted in Figure 2.The performance of the magnetic pre-accelerator lens was evaluated with a prototype electron optical bench developed by Veneklasen. It consists of a two cylinder accelerator, a magnetic transfer lens and a projector lens followed by a razor blade assembly for measurement of the spot size and the beam current.

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