DE2915207A1 - ELECTRON MICROSCOPE WITH STIGMATOR - Google Patents

Description

291520?

1.11. 1978 ¹ ■ PHN 9091

Electron microscope with stigmator.

The invention relates to a

Electron microscope with an electron gun, an electron optical lens system, a stigmator and an electron detector.

. In electron microscopes with

rotationally symmetrical lenses are always somewhat spherical Aberration on. Since the spherical aberration of rotationally symmetrical electron-optical lenses always has the same sign, the in lioht-

™ optical systems usual compensation can be applied.

To reduce the spherical

Aberration for all azimuth directions is described in "Optik", 5 (19 ^ 9), p. 490 ... k96, a combination of a number of multi-pole lenses with a rotationally symmetrical lens. The conditions that must be placed on the mutual centering of the different lenses are so high that they cannot be met in a satisfactory manner in practical electron microscopes.

, In "Optik", 26 (1967-1968),

P. 569 «« «573j is a correction method using a single eight-pole field and a rotationally symmetrical one Lens. This is the disadvantage of the high Centering conditions sufficiently eliminated, however

90984 3/0917

- 291520?

11/1/1978 X tj PHN 9091

instead, there is the disadvantage that the spherical aberration only occurs in two mutually perpendicular directions is completely eliminated, whereby anisotropy occurs in the image.

In some cases this anisotropy is considered to be sufficiently small, but most of the time it is felt to be extremely disturbing. For measuring the phase contrast, for example, in an electron microscope operating in transmission raster mode, as described in "Optik", 39 (1974), p. Ki6. . Λ36, is indicated, such anisotropy is undesirable and could only be avoided with an extremely complicated detector geometry.

The invention is based on the object to delete this task, to do this is one Electron microscope of the type mentioned at the outset is achieved in that the stigmator is at least eight to one another contains symmetrically arranged coils, the detector from parts with a relation to at least one mirror line composed of mirror-symmetrical geometry adapted to the exposing electron beam and an im Stigmator to be generated lens correction field is adaptable to the direction of the mirror lines in the detector.

Because in an electron microscope According to the invention, the detector is split mirror-inverted and the direction of the dividing line between detector halves on the geometry of a recorded in the passage for the exposing electron beam, not is adapted to a rotationally symmetrical diaphragm, can be measured with an adapted orientation of the octopole field in this way be that in the final picture there is no anisotropy perturbation is obtained.

In a preferred embodiment, the electron microscope is in transmission raster mode with a detection arrangement for generating phase contrast signals from difference signals of the equipped with both detector halves.

909843/0917

ORIGINAL INSPECTED

1.11.1978 / _ PHN 9091

In a further preferred embodiment is an electromagnetic Stigmator designed in such a way that in it by suitable selection of the excitation of the different coils a combination of a four-pole field and an eight-pole field can be generated.

In a further preferred embodiment, are in the electron microscope Means for perceiving an electron beam to be generated by the electron beam have been added. Through the Setting a symmetrical electron spot, for example visually, an optimal centering of the coil system with respect to can be achieved in a simple manner the corresponding lens can be obtained.

Some preferred embodiments according to the invention are given below with reference to FIG Drawing explained in more detail. Show it:

Fig. 1 is a schematic

Section through an electron microscope according to the invention,

20th

Fig. 2 is a plan view in two

symmetrical half of distributed electron detectors for an electron microscope according to the invention.

An electron microscope after

Fig. 1 contains an electron gun 1 with an anode 2, a Strahlrichtsys; system 3 (beam alignment) and a diaphragm 4, a condenser system with a first condenser lens 5 »a second condenser lens 6 and one Condenser diaphragm 7j an objective with a first objective pole 8 and a second lens pole 9 »a beam deflector

30th

scanning system 10, an object space 11, a diffraction lens 12 with a diffraction diaphragm 13 and an intermediate lens 14 Projektinnssystem with a first projection lens 15 and a second projection lens 16, a film camera 17 and an electron detector 18. All of these parts are in one

Housing 20 with a feed line 21 for the electron gun and a viewing window 22 was added. To the Housing can be an optical binocular 23, a

§09843 / 0917 PAGINAL INSPECTgD

23.15.207

11/1/1978 X PHN 9091

Vent assembly 24 and a plate camera 25 connected be.

^! In a preferred embodiment

Approximate shape of such an electron microscope.

According to the invention, the electron detector 18 is divided into at least two mirror-symmetrical halves and one Detection arrangement 26 assigned to the separate signals of one of the detector halves via electrical conductors 17 are supplied. The detection arrangement 26 is both

M, for example, a television monitor 28 for visual reproduction assigned to the images to be generated from the detector signals. Such an electron microscope can, for example used to generate a differential phase contrast image according to "Optik", 4i (1974, pp. 452 ... 456)

'* will. In images obtained with this method an object structure is shown anisotropically and the shown Differentiation gives images that Exterior of a relief exposed by shear; because there, too, the contrast is anisotropic, and at most in Direction of exposure. Anisotropy from a corrective eight-pole system is therefore not undesirable per se and can increase the visual value of the images to be generated. Another advantage here is that can be worked with a relatively simple detector geometry.

Calculations result in reduced form for the aberration ¥ of a wavefront in an electron optical system with an eight-pole field as a function of the distance R from the optical axis,

the strength g of the eight-pole field, the azimuth angle and a defocusing / S ^ the equation ¥ = - (1 - g cos 4 oC ) r

ι Q

+ 2 / \ R. The radius R of a rotationally symmetrical diaphragm assigned to this system has a value R ″ if the aberration ¥ is subject to the condition that it varies in the diaphragm over a maximum of TT / 2. In the absence of an eight-pole field, g = 0 and a value of χ / 21Γ applies to the maximum permissible center distance R of the diaphragm

909843/0917

OR »O! NAL INSPECTS)

11/1/1978 J> \ PHN 9091

for Δ = V ΤΤ / 2 ^

With the addition of the octopole, the spherical aberration is minimized if 1 - g cos

r-γγ

(X> = 0, so for ς / ^ = γ arc cos l / g + η -.

Elimination of the spherical aberration therefore applies to four azimuth directions if g \ 1, and for two azimuth directions if g = 1. In a preferred embodiment with g = 1 and Δ = Ο, this results in a contour for an additional diaphragm, as shown in FIG. 2 indicated with a dashed line 30, in which figure a circle 31 is the maximum permissible size of the diaphragm without the use of an eight-pole field. Since the separating lines of the detector coincide with the lines of symmetry in the projection of the diaphragm, a low-aberration phase image can be generated with such a lens when it is recorded in an electron microscope operating in a transmission raster. A particularly advantageous embodiment here is an electron microscope of a type in which an electron spot to be generated by the electron beam can be visually observed. The centering of the coil system of the octopole lens can be carried out by forming a symmetrical spot. From the comparison of the spherical aberrations

4 tion it follows that, since the coefficient of R is a function of the azimuth ς / ^, there is an optimal defocusing A for every yert of oC. With a value of Δ that is independent of the azimuth, optimal defocusing is only achieved for a few values of the azimuth and the diaphragm can only be enlarged for these directions. For all other directions, the radius of the diaphragm just has to be reduced. A solution for this is given in a preferred embodiment according to the invention in that a four-pole field, for which a defocusing / λ sin 2 oC applies, is superimposed on the eight-pole field. In an octopole lens, for example, as described in "Journal Phys. D. Appl. Phys," Vol. 7, (1974), pp. 805 ... 814 ,. can a deraifcLg

909843 / Θ917

- ORIGINAL INSPECTED

11/1/1978 ^ PHN 9091

set lens field can be generated by a suitable number of excitation of the different coil pairs. A preferred setting again with k = and j / \ = V f * / 2. results in an optimal focus for all values within two quadrants and the value of R at this point can be increased to a value that is 30 $ higher. A diaphragm to be used for this purpose is indicated by a line 32 in FIG. 2-.

9098A3 / 0917

PJQiWAL INSPECTED

Claims (2)

11/1/1978 ^] PHN 9091 PATENT CLAIMS Λ Λ electron microscope with a Electron gun, an electron optical lens system with a stigmator and an electron detector, thereby characterized in that the stigmator contains at least eight coils arranged symmetrically to one another, the Electron detector (18) made up of parts with at least one mirror line of an exposing electron beam adapted mirror-symmetrical geometry and a lens correction field to be generated in the stigmator is adaptable to the direction of the mirror line in the detector (18). 2. Electron microscope after Claim 1, characterized in that it is provided with a detector system for generating phase contrast images Equipped from differential signals from detector parts is. 3 «Electron microscope according to claim 1 or 2, characterized in that the stigmator contains a magnetically excitable octopole lens and a supply source is provided with a presetting for generating a quadruple field in the octopole lens. H. Electron microscope after "a of the preceding claims, characterized in that it is provided with an electron target 909843/0917 2Si5207 11/1/1978 2 PHN 9091 the one electron impact area on symmetrical Geometry is adjustable. 5 ·: Electron-optical system with an octopole lens according to one of the preceding 5 claims. ORIGINAL INSPECTS) 909843/0917