US2272353A

US2272353A - Electronic microscope

Info

Publication number: US2272353A
Application number: US353338A
Authority: US
Inventors: Ruska Ernst
Original assignee: Fides Gesellschaft fuer die Verwaltung und Verwertung von Gewerblichen Schutzrechten mbH
Current assignee: Fides Gesellschaft fuer die Verwaltung und Verwertung von Gewerblichen Schutzrechten mbH
Priority date: 1939-07-22
Filing date: 1940-08-20
Publication date: 1942-02-10
Anticipated expiration: 1959-02-10
Also published as: CH220324A

Description

Feb. 10, 1942. I

E. RUSKA ELECTRONIC MICROSCOPE Filed Aug. 20, 1940 INVENTOR. die/var Ems/(,4.

ATTORNEYS.

Patented Feb. 10, 1942 u ELECTRONIC MICROSCOPE Ernst Buska, Berlin-Spandau, Germany, asslgnor to "Fides Gesellschatt fiir die Verwaltung und Verwertung von gewerblichen Schutzrechten mit beschriinkter Hattung, Berlin, Germany, a corporation of Germany Application August 20, 1940, Serial No. 353,338 In Germany July 22, 1939 7 Claims. (Cl. 250-495) This invention relates to improvements in electronic microscopes.

In many cases it is important that electron optical apparatus be provided with as small an electron emitting source as possible. This is, for instance, the case if the rays are to pass through the objects to be magnified as convergent rays. To this end, the cathode must be of an extremely small size and the rays must be emitted at an angle which is not too small. When employing such a cathode, each point of the object is struck by an approximately needleshaped beam of rays which converges towards the center of the objective diaphragm and passes through a larger zone of the object than corresponds to the irradiated zone of the objective diaphragm. The main object of the invention is to provide electron-optical apparatus, in particular electron microscopes, with a cathode which fulfills all the conditions mentioned. above. According to the invention, the electron source of a microscopic or other electron-optical apparatus consists of a pointed or needle-shaped cold cathode formed, for instance, of a tungsten filament. Aside from the fact that such a cathode dispenses with the heat sources which are otherwise necessary, the arrangement may be so designed that an additional control electrode as well as the means for producing the control voltage are also eliminated.

The invention may be carried into practice in various ways. Thus, for instance, the pointed cathode may be arranged in a support impressed with the same voltage as the cathode proper and so designed that it shields the lines of force which emerge laterally from the pointed cathode which otherwise could pass to the anode. A particuilarly good shielding efifect is attained if an auxiliary electrode is employed which is impressed with a negative voltage as compared with that of the cathode.

A pointed cathode as an electron emitting source of electron-optical apparatus may be employed to advantage in various types of electronic microscopes. For instance, the cathode may be employed in electronic microscopes in which the electron rays emitting from the cathode are converged by means of a condenser lens onto the object and after having passed through the latter are subjected to one or more electron lenses serving to project a magnified image of the object onto a luminescent screen or photographic plate. The invention may also be applied to an arrangement in which the object is arranged more closely to the cathode, for instance directly in front of the cathode. According to another embodiment, the beam of electron rays emitted from the pointed cathode is first concentrated by means of one or more electron optical reduction lenses to a' very fine luminous point, behind or in front of which the object to be magnified is arranged in the path of the rays. In the last-mentioned case a diaphragm with a very fine aperture is placed in the neighborhood of the luminous point for the production of an image with augmented contrasts.

In the accompanying drawing are shown various embodiments of the invention in diagrammatic form.

Figs. 1 to 3 show three different examples of a pointed cathode according to the invention,

Figs. 4 to 6 show three different forms of electronic microscopes.

Referring to the drawing, l denotes a pointed cathode which is held in position by a support 2. 3 denotes the anode and 4- the diaphragm through whose aperture the cathode rays pass to strike the object (not shown). As will be seen from Fig. 1, a comparatively large number of lines of force emerge laterally from the pointed cathode and pass to the anode. An arrangement in which the lines of force are more concentrated on the aperture of the diaphragm 4 is shown in Fig. 2. This effect is brought about by giving the support 6 for the pointed cathode a particular shape so that it contains the needleshaped cathode l within a cup-shaped cavity. A still greater concentration of the lines of force on the diaphragm aperture 4 is obtained if a control electrode 1 according to Fig. 3 is employed and impressed with a negative potential as compared with that of the cathode l.

Fig. 4 is a longitudinal sectional view of an electronic microscope in which the rays pass through the object and thence through an objective lens. ll denotes the pointed cold cathode of the electron emitting source. From this cathode a beam of electron rays is emitted at an aperture angle 20:5. The electron rays are converged with the aid of a condenser coil 12 and pass through the diaphragm I3 carrying the object. In the path of ray is also arranged an objective coil It provided with an objective diaphragm |5.- A fluorescent screen or photographic plate for producing an intermediate image is indicated at I5.

a indicates the distance between the electron source and the condenser lens, b the distance between the condenser lens and the objective diaphragm I5, I) the distance between the objective diaphragm and the optical plane of the image produced by the objective lens, and a the distance of the-object from the objective diaphragm. F denotes the focal length of the objective lens. If dk denotes the diameter of the cathode, do the diameter of the object field. and do}! the diameter of the objective diaphragm, the following equations are applicable:

I I d =2g,a,az2%a,f

As will be seen from Fig. 4, the point at which the electron rays are converged by the condenser coil l2 lies in the opening of the objective diaphragm I5.

When using a pointed cathode as the electron emitting source in the arrangement described above, the radius of curvature of the cathode point in the case of a tungsten unicrystal filament amounts, for instance, to 10- mm., 1. e., dk==2 10- mm. The aperture a (free of faults) of a magnet objective amounts to about 10- With a focal length of the objective lens of fo=5 mm. the objective diaphragm should have a diameter doB=2aojo=2'10 5=0.01 mm. For the projection of the cathode in the aperture of the objective diaphragm a magnification must therefore be chosen. If the aperture of the pointed cathode amounts to as=0.2 the aperture of the beam when passing through the objective diaphragm is as follows:

phragm 28 with a fine opening for the production of an image with strong contrasts. The fiuorescent screen or photographic plate is arranged as indicated at 21. Fig. 5 also shows part of the vacuum vessel of the microscope formed of the

vessel portions

29, 30 and 3| and of the metal bodies of

lenses

23 and 24.

A very simple electron optical magnifying device also containing a pointed cathode according to the invention is shown in Fig. 6. The elec tronic rays emitting from the pointed cathode 3| pass directly through the object 32 which is projected on a fluorescent screen 33, or on a photographic plate which may replace the screen.

What is claimed is 1. In an electron-optical apparatus having a source of an electron beam, means for holding an object in the path of said beam, and electronoptical lens means for causing said beam to produce a magnified image of the object, said source havinga cold cathode of pointed shape directed towards the object.

2. In an electron-optical apparatus 'having a source of an electron beam, means for holding an object in the path of said beam, and electronoptical lens means for causing said beam to produce a magnified image of the object, said source having a needle-shaped cathode, an anode diaphragm having an opening opposite the point of said cathode, and a conductive body surrounding the shaft portion of said needle-shaped cathode to shield the lines oi force emerging laterally from said cathode.

3. In an electron-optical apparatus having a source of an electron beam, means for holding an object in the path of said beam, and electronoptical lens means for causing said beam to produce a magnified image of the object, said source having a cold cathode of pointed shape directed towards the object, an anode having an opening opposite the point of said cathode, and a conductive support carrying said cathode and forming a cavity surrounding said cathode to shield said anode from lines of force emerging laterally from said cathode.

4. In an electron-optical apparatus having a source of an electron beam, means for holding an object in the path of said beam. and electronoptical lens means for causing said beam to produce a magnified image of the object, said source having a needle-shaped cold cathode, an anode having an opening'opposite the point of said cathode, and an electrode having a negative potential as regards said cathode and surrounding said cathode to shieldsaid anode from lines of force emerging laterally from said cathode.

5. An electron microscope, comprising a source of an electron beam having a cold cathode oi pointed shape and an anode diaphragm with an opening opposite the point of said cathode, a condensing lens, an object diaphragm, an objective lens having an objective diaphragm, and a condensing lens arranged between said source and said object diaphragm and designed for concentrating said beam so as to taper through said object diaphragm to a focal point lying substantially in the plane of said objective diaphragm.

6. An electron microscope, comprising a source of an electron beam having a cold cathode of pointed shape and an anode diaphragm with an opening opposite the point oi said cathode. diaphragm means for holding an object, said means being arranged immediately in front of said electron source, and electron-optical means for causing the beam to produce a. magnified image of the object.

7. An electron microscope, comprising a source of an electron beam having a cold cathode of pointed shape and an anode diaphragm with an opening opposite the point 01' said cathode, electron-optical reduction lens means for concentrating the beam to form a fine beam point object-holding means arranged so as to place the object in proximity to said beam point, and means arranged behind the obect its magnified image. J for renewing ERNST RUSKA.