US2670450A - Electron microscope - Google Patents

Description

Feb. 23, 1954 5, LE PO E 2,670,450

ELECTRON MICROSCOPE Filed May 23, 1952 2 Sheets-Sheet l IIVVE/VTOR Jon Bon Le Poole 1954 J. B. LE POOLE ELECTRON MICROSCOPE -2 Sheets-Sheet 2 Filed May 23, 1952 Patented Feb. 23, 1954 ELECTRON MICROSCOPE Jan Bart Le Poole, Delft, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application May 23, 1952, Serial No. 289,685

Claims priority, application Netherlands June 15, 1951 The invention relates to electron microscopes, more particularly to an improvement in the objective lens of such instruments.

In order to obtain a satisfactory image quality,

the bore of the magnetic electron lens has been made materially wider than is required to allow the electron beam to pass. I The character of the lens is furthermore determined by the spacing between the pole shoes (referred to hereinafter as the pole spacing). By an increase in this spacing the magnetic force which urges the electrons towards the axis becomes operative through a longer distance and the strength of the lens increases, provided that the field strength does not vary. However, if the pole spacing is increased, the maintenance of a definite field strength requires an increase in the number of ampere turns and therefore the pole spacin is chosen to be comparatively large, it is true, but it does not much exceed the diameter of the bore.

An electron microscope according to the invention comprises an objective lens having great field strength and a bore diameter which is smaller with respect to the pole spacing than has hitherto been used. With the construction of this new objective lens the requirements with respect to the constance of the operating voltage and the energizing current are extremely low.

With the conventional electron microscope these electrical values must not vary by more than 0.03 and 0.06% respectively, if a resolving power of 60 A. is desired. Such a constance requires the use of a comparatively complicated, electrontechnically operative precision device. With a microscope according to the invention this may be dispensed with, since the focal distance of the lens is small, owing to the particularly small bore. A small focal distance ensures little chromatic aberration. The field strength required to obtain a sufficiently small focal distance is obtainable owing to the small pole spacing, which in itself is rendered possible by the small bore.

The invention will now be described in connection with the accompanying drawing in which:

Fig. 1 shows, diagrammatically, an electron lens with electron paths for the purpose of illustratin the invention;

Fig. 2 is a curve showing the variation of field strength with distance along the axis in the pole space of an electron lens;

Fig. 3 shows the path of electrons in an electron lens exhibiting the field strength curve shown in Fig. 12;

Fig. .4 shows a cross-sectional view of one sin- 2 Claims. (01. 313-84).

properties of theelectromagnetic lens will now be taken in view.

The focal distance of the electromagnetic lens varies in the first place with the field strength. The increase in field strength is, however, limited by the magnetic saturation in the pole shoes. If this is reached, the field strength can be increased only by means of a large number of ampere turns.

The increase in dimensions, whilst the field strength is maintained does not always give better results either. This applies particularly to an objective lens. If the pole shoes are spaced apart by a greater distance, whilst provisions are made to prevent the field strength from varying, the foci will lie in the active part of the magnetic field with a definite pole spacing. In order to obtain a high amplification, the object is arranged in the proximity of the focus. If this focus is located between the pole shoes, a further increase in the strength of the lens by increasing the pole spacing, the field strength being maintained, Will soon have a negative result.

This is illustrated in Fig. l of the accompanying drawings.

In this figure, the reference numeral l designates the optical axis of a magnetic electron lens and 2 a path of an electron parallel to this axis. The magnetic field of the lens is operative on the strength AB. On this stretch the ray is subjected to a bending force. If the ray 2 enters the field of magnetic force from the left, it is curved towards the axis beyond the point A. It is assumed that the field strength or the distance AB is so great that the ray under consideration intersects the axis I at a point F, situated between the pole shoes. Apart from the spherical aberration each ray which is incident from the left parallel to the axis will intersect this txis at the same point F, which, consequently, is a focal point of the lens. Beyond the point F the ray is curved in the opposite direction and if the portion FB of the stretch AB exceeds the portion AF, the ray emerges from the lens in-a direction towards the axis and intersects the axis again at the point 0. The ray consequently has a straight portion 2, a curved portion 2' and again a straight portion 2". The conditions may, as an alternative, be such that th ray intersects the axis 1 again at a point between F and B.

With the electron microscope the conditions are as described above, with this difference that the electron does not originate from the left, but from the right from point 0, where, for example, the electron source may be arranged.

The point F is the image point associated with the lens formed by the portion of the magnetic field which operates on the stretch FE. Thus F is the image point of the point 0' associated with this lens, which is located in a plane I through point 0 at right angles to the axis I. The figure shows an arbitrary ray, which emanates as a straight line 3" from point 0' and winds as a curved line 3' through the magnetic field in a manner such that it passes through the point F and emerges from the lens as a straight line 3.

Apart from faults of the lens, all rays from point 0 intersect the axis at point F and emerge from the lens parallel to the axis. All rays from point 0' pass through point F and emerge from the lens parallel to the straight line 3. An object arranged at point F thus has its image at definite distance. If it is shifted towards the point P1, so that it is intersected by the ray 2 at point P, a real image is produced, in which the distance PP has increased to p, i. e. the distance between the straight line 2 and the axis I. The nearer the object to the point F, the greater the amplification.

In the portion PB of the stretch AB, consequently, the magnetic field does not contribute to the amplification, but it only constitutes an undesired condenser, which, moreover, requires ampere turns. If the pole spacing of the objective lens is chosen so as not to exceed the distance AF plus the distance FP from the focal point to the object, the maximum obtainable lens stretch is obtained by means of a minimum number of ampere turns.

Since, however, it is difficult to arrange the object exactly at the point where the lens efiect begins, since this would mean that the object should have to be arranged in the plane of the pole shoes or even in the bore of the pole shoes, the pole shoe will have to be drawn back a little more.

For the sake of clearness, all the distances at right angles to the axis are shown in Fig. 1 on an exaggerated scale in proportion to the distances in the direction of the axis.

With a microscope according to the invention, the objective lens has a focal point between the pole shoes and a pole spacing which exceeds the distance AF by so little that the field between the first pole shoe and the focus F has no adverse effeet. This is rendered possible by reducing the diameter of the lens bore to one third or even a smaller part of the pole spacing.

The magnetic electron lens may be assumed to have a focal distance. The tangent of the curved portion 2' of the ray 2, 2', 2" at point F intersects the prolongation of the straight portion 2 of this ray at point S. The distance i between this point of intersection and the plane II is now defined as the focal distance of the lens. In a homogeneous field it is equal at This focal distance is determined not only by the magnetic field strength, but also by the diameter of the bore. In order to restrict the astigmatism the bore has hitherto been chosen so as to be not much smaller than the pole spacing, since the narrower the bore, the more difficult it is to maintain the circular shape of the bore, a non-circular bore giving rise to astigmatism.

With the invention it is taken into account that the field penetrates deeply into the bore, if this is large. The curve representing the variation of the field strength H on the axis then has three portions, a slightly ascending portion, a portion of great field strength having a maximum and a slightly descending portion. Fig. 2 of the accompanying drawings illustrates this.

On the stretch b the magnetic field has a much greater refractive power than on the stretches a and c. The strength of a lens element included between two planes at right angles to the axis, spaced apart by a distance ds is proportional to H ds with a given number of ampere turns, i. c. with a given value of the surface included between the curve Ic and the axis the portions a and c are restricted as much as possible, that is to say, if the surface is concentrated as much as possible on the stretch b, a stronger lens is obtained by means of the same number of ampere turns. In order to improve the lens, efforts will therefore have to be made to vary thecurve k in a manner such that it approximates the rectangle r, the sum of the cross-hatched surfaces (1 and e being equal to the cross-hatched surface 9. The curve r could represent the variation of the field strength along a straight line parallel to the axis, connecting two points of the pole surfaces of the same lens, spaced apart by a distance b. If, Fig. 2 relates to a lens, the rays of which are incident from the right-hand. the por' tion 0 does not contribute to the amplification, if it is located in front of the object, as is evident from the description with reference to Fig. 1. The portion it produces an after-effect like a weak lens behind the strong lens, which operates on the stretch b. This results in a reduction in lens strength owing to the portion 11 instead of resulting in an increase. A similar phenomenon is known from optics. A weak lens which is spaced apart from a stronger lens by more than a definite distance has the effect that the focal distance of the combination exceeds that of the Strong lens alone. For thin lenses this definite distance is the focal distance of the weaker lens.

Fig. 3 of the accompanying drawings illustrates the adverse eifect of the offshoot of the curve K shown in Fig. 2. Fig. 3 shows the path 5 of an electron entering the magnetic field at A1. This field is first weak, so that on the stretch a the ray (51) is curved only slightly. Beyond this stretch the electron path is curved more strongly and it intersects the axis at point F1. The focal distance is indicated in this case by ii,

If the magnetic field had concentrated its refractive power in the space beyond the stretch a, so that the ray would have been curved only after point A2 (for the two cases we assume the magnetomotive force to be the same), the ray (A2) would be curved more strongly, so that it would intersect the axis at point F2 at a larger angle to the axis. The focal distance would have been smaller, i. e. f2.

Owing to the effect of the offshoot of the field, the distance of the ray from the axis, at the area where the ray enters the strong field, is smaller and to this distance the radius of the curvature of the path is inversely proportional.

' This is the reason why it is'useless to increase the number of ampere turns in excess of the number required for the saturation of the iron. Although thus the field strength is increased, this increase is neutralized by the reduction of the distance of the rays from the axis, at their entrance into the central part, since owing to the saturation, a strong offshoot of the field is produced in the bore of the pole shoe.

, The thesis that reduction of the diameter of the bore increases astigmatism owing to the unavoidable deformation of the circular bore, is correct, if the magnetic field penetrates comparatively deeply into the bore of the pole shoe. However, the bore of the objective lens in an electron microscope according to the invention is so narrow as compared with the pole spacing that the ofishoots of the field no longer attenuate the lens to an appreciable extent, nor produce serious astigmatism.

This must presumably be explained as follows. In the annular space between the two (parallel) pole surfaces the magnetic field is substantially homogeneous, but at the edge of the bore the magnetic lines of force are concentrated more strongly than beyond this area. Consequently, at this edge the iron is saturated sooner than in the further parts of the pole surfaces. A less strong concentration of the magnetic lines of force at this area permits a stronger energization and hence a greater operative field strength. This reduction in concentration of the lines of force is obtained by narrowing the bore. Then the lines of force diverge less strongly, so that the inhomogeneity of the field is reduced. The more the homogeneous field is approximated, the less influence has the shape of the bore diameter since the inhomogeneous parts of the field then contribute the less to the total strength of the lens and hence to the astigmatism. The nearer the pole shoes are to one another, the stronger becomes the concentration of the lines of force at the edges. Therefore the advantages of the small pole spacing of the objective lens cannot be obtained without rendering the bore comparatively small.

The objective lens of an electron microscope according to the invention thus has a combination of properties, resulting ultimately in an improved quality of the lens; these properties may be summarized as follows:

1. The maximum field strength is so great that the magnetisation of the iron touches the limit of saturation. The value of the maximum field strength varies with the quality of the magnetic material. For the conventional iron it amounts to 21,400 Gauss, for an alloy made substantially of iron and cobalt in a ratio of 3:2 it may be increased to 23,000 Gauss.

2. The pole distance is suificiently large to arrange the object between the focus located between the pole shoes and the first pole shoe, but it is so small that the magnetic forces operating in front of the focus have no adverse effect. It has been found that for this purpose it need not be chosen greater than the value dmax, which is expressed in millimetres by the formula:

E E max mms.

where E designates the voltage in kvs., by which the electrons are accelerated.

Since with a lens having a large bore the pole shoes cannot be approached to one another to the same small extent as with a lens having a small bore without producing an appreciable field concentration at the edge of the bore, the following condition, indicated by 3, must be fulfilled.

in order to fulfil condition 2.

3. The diameter of the bore is not more than one third of the pole spacing.

Fig. 4 of the accompanying drawings shows one embodiment of an objective lens for use in an electron microscope according to the invention in a sectional view taken in a plane through the axis. In this figure ll designates part of the iron wallof the microscope. An annular chamber,-'formed by the two yoke plates. l2. and I3 and the intermediate piece [4, all three of ferromagnetic material, comprises a coil cylinder [5 of brass. The winding space of this coil cylinder is divided into two portions I! and 18 by an intermediate flange l6, each of these portions comprising one excitation coil.

To the tongues I 9 and 20 of the yoke plates are secured pole shoes 23 and 24 by means of the nuts 2| and 22. They are made of an alloy substantially of iron and cobalt.

The plates I2 and I3 and the pole shoes 23 and 24 have a central bore, so that continuous channels 25 and 26 are formed, opening into the object chamber 21.

The intermediate flange l 6 has a hole 28, which accommodates an object holder (not shown). The channels 25 and 26 and the object chamber 21 together form a space which may be evacuated. This space is shut in an airtight manner from the open air by means of stufling boxes 29 and 30 and the rings 3| and 32 of rubber. The penetration of air between the object holder and the wall of the hole 28 may furthermore be prevented by means of stufiing material. An electron ray is passed through the evacuated space exactly along the axis, i. e. from the channel 26 across the object chamber 21 to the channel 25.

The front surface of the pole shoes is shaped in the form of a truncated cone. Their fiat parts are spaced apart by adistance of 1.4 mms. With a bore in the pole shoes of a diameter of 0.3 mm. at the surface the lens has a focal distance of 0.7 mm. with an electron velocity obtained by means of an acceleration voltage of kvs. This requires a number of ampere turns of about 3000. The distance of the first focus from the front surface of the pole shoe 24 is, in this case, 0.3 mm.

Fig. 5 of the accompanying drawings is a detail view on an enlarged scale. Here the object surface 34 is indicated by a broken line. It may be located at a distance of 0.2 mm. from the front surface 35 of the pole shoe 24, i. e. at a distance of 0.1 mm. from the focus 36. At a distance of 10 cms. the lens described above may produce an amplification of times.

With a variation of the operating voltage of A2 to 1% and of the energizing current of A to /z% (these values vary slightly with the contrast sharpness of the image) this lens still has a resolving power of 60 A., owing to the small focal distance of 0.7 mm., which is obtained by the suppression of the offshoots of the field. This has the important advantage that the supply device for the microscope can be considerably simpler, since the electron-technical precision device for stabilisation is dispensed with.

What I claim is:

1. An electron microscope comprising an electromagnetic objective lens including a pair of pole shoes each of which have a bore and including means to produce a magnetomotive force which increases the magnetisation in the pole 7 shoes to'the limit otsaturation, said pole slices being spaced apart a. distance at which the foci of the lens are located between the: pole shoes and which does not exceed a value dmax, which is expressed in millimetres hy the formula:

where E designates the voltage in kvs., by means of which the electrons are accelerated, the diameter of the bore being not more than onethird the spacing between the pole shoes.

2. An electron microscope as claimed in claim 1, in which the pole spacing is approximately 1.4 mms. and the bore diameter is approximately 0.3 mm.

JAN BART LE POOLE.

References. Cited in the file of this patent UNITED STATES PATENTS

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