US2243403A - Magnetic objective for electron microscopes - Google Patents

Description

May-27, 1941. M.'VON ARDENNE MAGNETIC OBJECTIVE FOR ELECTRON MICRQSCOPES Filed April 3, 1959 [III/A H II Patented May 27, 1941 OFFICE MAGNETIC OBJECTIVE FOR ELECTRON MIOROSCOPES Manfred von Ardenne, Berlin-Lichterfelde, Germany Application April 3, 1939, Serial No. 265,707 In Germany April 4, 1938 11 Claims.

The present invention relates to a magnetic objective of small focal length for electron microscopes.

One of the most important ways of increasing the resolving power consists in reducing the focal length of microscope objectives. It has been found that in the case of a constant D/f-ratio, both the aperture fault and the chromatic aberration are directly proportional to the focal length. The fault caused by magnetic interfering fields is even proportional for any D/f-ratio to the focal length. Since, as is well known, the maximum resolving power obtainable in practice is determined by the above-mentioned three faults alone, it results that any reduction of the focal length of the objective brings about a cor responding improvement of the resolving power. For the above reasons, the use of a magnetic lens provided with pole pieces, by means of which it is now possible to obtain for electrons of a high voltage small focal lengths for the objectives, is a decided advance in the field of electron microscopes. The known types of magnetic lenses having such pole pieces allow a diminution of the focal length to about 3 mm. for electrons accelerated at a voltage of 40,000 volts. The further reduction of the focal lengths is hampered by constructional difiiculties. These are due to the necessary proportional reduction of the dimensions of the pole pieces, to the production of the extremely small diaphragm apertures required for magnetic lenses, and to the necessity of avoiding a magnetical overstressing of the pole pieces. However, the'greatest difficulty lies in the construction of an object support which at such reduced dimensions of the pole pieces permits arranging the objects in the electron-optically correct plane.

Furthermore, it is indispensable in view of a practical employment that the objects be easily interchangeable in a given plane of intersection. The known types of object supports which make this possible, require an axial extension of several millimeters and thus also prevent reducing the focal length of the lens system below the abovementioned limit of about 3 mm. These known object carriers consist completely of non-magnetic material.

An object of the present invention is to provide means which allow reducing the focal length of electron-optical lenses of the magnetic type to magnitudes smaller than heretofore obtainable in order to decrease the faults of the lens system and/or to increase the resolving power of the electron microscope. An object, more particularly, is to reduce the focal length of a magnetic lens arrangement for electron microscopes to a magnitude of about 1 mm. or less. A further object aimed at by the present invention is to provide object-carrying support means which allow placing the object to be examined in a plane closer to the ferromagnetic parts of a magnetic lens than heretofore feasible.

According to the invention, one of the ferromagnetic pole pieces of a magnetic lens for electron microscopic purposes is removable from the other lens structure and designed as object carrier or as part of such carrier.

Other objects and features of the invention will become a parent from the following description of the embodiments exemplified in the drawing in which Fig. 1 shows diagrammatically an axial cross section through a magnetic objective lens according to the invention,

Fig. 2 shows the essential portion of another lens system according to the invention, also in an axial cross section, while Figs. 3a and. 3b represent in cross section an auxiliary device (Fig. 3a) for attaching a foil carrying the object proper to the removable ferromagnetic pole piece (Fig. 3b) of the lens system.

The magnetic objective lens illustrated in Fig. 1 has its energizing coil 11. enclosed by a magnet body composed of three parts b, c, and cl, covering the exterior and. interior cylindrical surfaces of the coil as well as its upper and lower front faces. The bottom portion of part b forms the upper pole piece of the magnetic lens. The lower pole piece consists of a separate ferromagnetic body a which is detaohably inserted in part d. The two pole pieces are preferably made of Swedish charcoal iron. Incisions e provided in the lower pole piece a render the pole piece sufiicient- 1y resilient to facilitate its proper insertion in and attachment to part (1. A body g of insulating material secures the proper spacing between the two pole pieces. The removable object support or pole piece a is provided with a perforated disk k of non-magnetic material such as brass. The object layer to be examined, or a foil carrying the object proper is designated by I. This object layer covers the aperture 1 of disk It. A cap 2' of non-magnetic material covers the object layer 1.

When in use, the lens is arranged within the vacuum vessel so as to form part of the electronoptical system, i. e. a beam of electron rays in proper electron-optical condition passes from above through the apertures of the pole pieces h and a, thence through the aperture j and the object to be examined, and leaves the aperture 7' of the cover 2' to fall on a recording electrode or a photosensitive surface, as is usual and well known in such microscopes. The air ducts m, shown in Fig. 1, serve to facilitate evacuating the vacuum vessel of the microscope. 4

The magnet field produced between the pole pieces h and a due to the electric energization of magnet coil n, functions as the lens proper and, by virtue of the structure described, has a form which makes it possible to attain an extremely small focal length, for instance of 1 mm.

The embodiment shown in Fig. 2 is represented by the electron-optically essential portion of the lens system, while the coil and the less characteristic other portions of the magnetic enclosure, those elements being similar to the corresponding elements of Fig. 1, are omitted. According to Fig. 2, the upper pole piece h is mounted on a magnetic holder '0 by means of screws 0. The holder 22 is screwed into the part b which, as to arrangement and function, corresponds to part b in Fig. l. The lower pole piece a, having peripheral grooves or incisions c, is detachably inserted into the part 03' which in general corresponds to part at in Fig. 1. g is an insulating spacing body. Screws to secure the individual elements together and are provided with a bore m for facilitating the evacuation. A foil carrying the object proper may be applied, for instance, to the inner portion of the upper surface of the interchangeable object support a. A recording electrode or diaphragmj suitable for the photographic recording is inserted in the object support a, for instance, by means of a lateral slide 1 Such a diaphragm is especially adapted for the so-called scanning microscopes in which the object is scanned by the focal point of an electron beam. The focal length which may be utilized with this system and which may be attained at electron speeds of some 10,- 090 volts is even smaller than 1 mm. For the usual D/f-ratio of 10- an object diaphragm with a diameter of 10- mm. will be necessary when using this small focal length. Perforations of such extremely small diameters require particular methods of production.

Tests have shown that with the aid of the thinnest available steel points (for instance, with the aid of special-type sewing needles) perforations up to 10* mm. may be punched in metal foils arranged on a hard support, for instance, on glass. As metal foils, for instance, duralumin foils of some 10- mm. thickness are particularly suitable. Since these object diaphragms are not subjected to considerable electron loads which might lead to a gradual destruction, there is no objection to using foil diaphragms. In the construction shown in Fig. 2, as a foil diaphragm a band may be employed in which is punched the necessary diaphragm aperture and this band may be moved over the tip of the upper pole piece. With the aid of a guide and of a clamping device indicated in Fig. 2 the band is firmly held in the proper position. It is advisable before taking a number of pictures to examine the diaphragm for freedom from dust with the aid of a light microscope.

To facilitate arranging the object and its supporting foil on the supporting surface of the object support a, the means shown in Figs. 3a and 312 may be employed. The foil is placed over the opening of a bushing g firmly held in a holder or in pliers r. The bushing is then placed onto the inner side of the support a so that the foil adjusts itself to the surface of the support as indicated by the broken line sin Fig. 31).

Since, as shown in Fig. 2, both pole pieces as well as the insulating spacer between them may form parts separate from the other structure of the lens system, it is according to the invention also possible to unite the two pole pieces with its spacer so that the exchangeable object support comprises all of these three elements.

In order that the magnetic field actually begins at the tips of the pole pieces special grades of iron must be employed which possess also with in the range of a high magnetization a sufficiently small magnetic resistance. Such grades of iron employed in pole tips of laboratory electromagnets are well known, for instance, in magnets of the Du-Bois type. The pole shapes and relative dimensions indicated in the above forms of the invention render it possible with a number of ampere turns in the order of magnitude of 1 2x10 to obtain the field concentrations which are necessary for a focal length of the magnitude of 1 mm. at the voltages required in electron microscopes. If considerably smaller focal lengths are desired, the inner resistance of the magnetic circuit increases and the outer magnetic resistance (air gap) decreases to such an extent that it is difficult to attain the necessary high field concentration. The invention also aims at, and provides means for, coping with this difficulty.

According to another feature of the invention the bores of the pole pieces do not have the same diameter as shown in Fig. 2 but different diameters as shown in Fig. 1. The configuration of the field which is presented in the latter case permits attaining a further reduction of the focal length as well as or the aperture fault of the magnetic optical system. This is particularly the case if the diameter of the bore of the pole piece at the side of the object plane is smaller. By this measure the lines of force are more inclined in the electron-optically most-effective portion of the field.

By designing according to the invention, one or both pole pieces in the form of an interchangeable object support, it is possible to insert this object support in an accurately fitting mounting of a light microscope in which the position of the object zone to be examined may be accurately measured. The object support is then inserted into the magnetic objective of the electron microscope and the microscope is electrically ad.- justed in accordance with the desired magnification factor, whereafter the magnified images are observed or photographically recorded in the usual manner.

Tests have been made with object supports as above described, in order to determine to what extent a point of the object support maintains its original position relative to the three axes of a spacial coordinate system after a repeated insertion of the object support in the mounting of a light microscope. It has been found that in the case of a proper mechanical design the changes of position remain below 10- mm. Therefore, the error in the important Z-direction is smaller than 10- mm. so that when employing the means according to the invention, it is p0..- sible to focus an electron microscope and to magnify a selected area previously spotted by means of a light-optical microscope, if the Z-coordinate of the object or area is determined and if the electron microscope is electrically adjusted accordingly on the basis of a previous electrical u ing.

What is claimed is:

1. In a magnetic lens of short focal length for electron microscopes, having a body of ferromagnetic material forming a central passage for the electron beam, said body comprising a detachable central pole section of ferromagnetic material, and said pole section forming a support for the object to be examined.

2. In a magnetic lens of short focal length for electron microscopes, having a body of ferromagnetic material provided with two central pole pieces spaced axially from each other and having a central aperture, one of said pole pieces being exchangeably attached to said body and forming a support for the object to be examined.

3. In a magnetic lens of short focal length for electron microscopes, having a body of ferromagnetic material provided with two central pole pieces spaced axially from each other and having a central aperture, one of said pole pieces being detachable from said body and having a frontal surface portion facing said other pole piece designed for supporting the object to be examined.

4. In a magnetic lens of short focal length for electron microscopes, in particular scanning microscopes, having a body of ferromagnetic material forming a central passage for the electron beam, said body comprising a detachable central pole piece of ferromagnetic material forming an object support and having a central aperture, and an electrode consisting of a diaphragm mounted on said detachable pole piece and covering said aperture so as to be exposed to the electrons after their passing through the object to be examined.

5. In a magnetic lens of short focal length for electron microscopes, in particular scanning microscopes, having a body of ferromagnetic material provided with two central pole pieces spaced axially from each other and having a central aperture, one of said pole pieces being exchangeably detachable from said body and having its side facing said other pole piece designed as an object carrier, an electrode diaphragm arranged at the other side of said de radially movable slide associated with said detachable pole piece for holding said diaphragm.

6. In a magnetic lens of short focal length for electron microscopes, having a body of ferromagnetic material forming a central passage for the electron beam, said body comprising a detachable central pole piece of ferromagnetic material forming an object holder and a centrally perforated diaphragm foil covering the central portion of said support for carrying the object to be examined.

7. In a magnetic lens structure according to claim 6, the aperture of said foil having a diameter in the order of magnitude of 10 mm.

8. In a magnetic lens of short focal length for electron microscopes, having a magnet body, two central pole pieces mounted on said body and spaced axially from each other, one of said pole pieces being detachable from said body and forming an exchangeable object holder, each of said pole pieces having a central conical aperture.

9. In a magnetic lens of short focal length for electron microscopes, having a magnet body, two central pole pieces magnetic-ally connected with said body and spaced axially from each other, one of said pole pieces being detachable from said body and forming an exchangeable object holder, each of said pole pieces having a central conical bore with its smallest diameter facing the plane of the object.

10. In a magnetic lens structure for electron microscopes, having a magnet body provided with two centrally perforated pole pieces of ferromagnetic material spaced axially from each other, in combination a body of non-magnetic material mounted on one of said pole pieces, said non-magnetic body also having a central aperture, and holding means for securing an object to be examined to said non-magnetic body so as to cover said aperture of said body.

11. In an electron microscope, in combination, a magnetic lens structure including a magnetic body surrounding the electron-optical axis, an object support having a central aperture in the electron-optical axis, said object support consisting of ferromagnetic material and being magnetically connected with said body to form a pole of said lens structure.

MANFRED VON ARDENNE.

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